METHOD FOR MANUFACTURING HIGH-CHROMIUM (CR) MOLTEN STEEL, METHOD FOR MANUFACTURING CAST PIECE, AND METHOD FOR MANUFACTURING PRESS ROLL

Abstract

An embodiment of the present invention relates to a method for manufacturing high-chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt % to 5.5 wt %, and the method may comprise the operations of: inserting molten steel into a steel converter used in the process of manufacturing stainless steel; and inputting chromium (Cr)-containing chromium steel alloy into the steel converter such that the content of chromium (Cr) in the molten steel reaches 4.5 wt % to 5.5 wt %. Therefore, according to the embodiments of the present invention, high-chromium (Cr) molten steel can be manufactured by using a steel converter that is used in the steelmaking process of other steel types, without contaminating same.

Claims

1. A method for manufacturing high-chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt % to 5.5 wt %, the method comprising: charging molten steel into a steel converter used in a process of manufacturing stainless steel; and inputting a chromium (Cr)-containing chromium alloy steel into the steel converter so that a content of chromium (Cr) in the molten steel reaches 4.5 wt % to 5.5 wt %.

2. The method of claim 1, further comprising: performing oxygen blowing to remove carbon (C) from the molten steel by blowing oxygen into the steel converter into which the chromium alloy steel is charged; removing oxygen (O) from the molten steel by inputting a deoxidizer to the steel converter; and reducing chromium oxide contained in slag floating on a bath surface of the molten steel to chromium (Cr) by inputting a reducing agent to the steel converter.

3. The method of claim 1, further comprising: discharging the molten steel from the steel converter to a ladle; and controlling a temperature to heat the molten steel received from the ladle by using a ladle furnace (LF), which is a heating device used in a carbon steelmaking process.

4. The method of claim 3, wherein, in the controlling of the temperature, the temperature of the molten steel is controlled to 1,560 C. to 1,600 C.

5. The method of claim 3, further comprising performing degassing to remove hydrogen (H.sub.2) and nitrogen (N.sub.2) from the molten steel received in the ladle using a Rheinstaal Huttenwerke und Heraus (RH) device, which is a vacuum processing device used in the carbon steelmaking process, wherein the degassing is performed after the temperature control is completed.

6. The method of claim 3, wherein, in the discharging of the molten steel from the steel converter into the ladle, the molten steel inside the steel converter is discharged into the ladle in the carbon steelmaking process.

7. A method for manufacturing a slab, the method comprising: preparing molten steel containing 4.5 wt % to 5.5 wt % of chromium (Cr); performing a casting process of supplying the molten steel into a mold of a casting device to solidify the molten steel inside the mold, thereby manufacturing a slab containing non-solidified molten steel; drawing the slab manufactured in the casting process to a lower side of the mold; and performing a solidification process of applying a magnetic field to the slab drawn to the lower side of the mold to solidify non-solidified molten steel contained in the slab while allowing the non-solidified molten steel contained in the slab to flow, thereby manufacturing the slab.

8. The method of claim 7, wherein the solidification process comprises heating a distal end of the slab drawn to the lower side of the mold.

9. The method of claim 7, wherein the casting process comprises applying a magnetic field to the mold to allow the molten steel inside the mold to flow.

10. The method of claim 7, wherein, in the drawing of the slab to the lower side of the mold, the drawing is performed at a speed of 0.04 m/min or less.

11. The method of claim 7, wherein the solidification process comprises injecting cooling water onto the slab drawn to the lower side of the mold, and when injecting the cooling water onto the slab, the injecting is performed so that a surface temperature of the slab becomes 800 C. to 900 C.

12. The method of claim 7, wherein, in the drawing process, the slab is drawn from the mold in a direction perpendicular to the ground, and in the solidification process, the slab is solidified in a state in which the slab is disposed in the direction perpendicular to the ground.

13. A method for manufacturing a press roll, the method comprising: heating the slab manufactured by the method for manufacturing the slab as set forth in claim 7; forging the heated slab to mold the slab into a shape of a press roll; and heating the press roll manufactured in the molding to remove hydrogen (H2) from the press roll, wherein the heating of the slab comprises heating the slab by rising a temperature of the slab to a target temperature in multiple stages.

14. The method of claim 13, wherein the heating of the slab to the target temperature comprises: heating the slab to a first temperature of 250 C. to 350 C.; heating the slab to a second temperature of 450 C. to 550 C.; heating the slab to a third temperature of 650 C. to 750 C.; and heating the slab to the target temperature of 1,100 C. to 1,250 C.

15. The method of claim 14, wherein, in the heating of the slab to the first to third temperatures and the target temperature, the slab is maintained at the first temperature for 3 hours to 5 hours, the slab is maintained at the second temperature for 5 hours to 7 hours, the slab is maintained at the third temperature for 3 hours to 5 hours, and the slab is maintained at the target temperature for 14 hours to 18 hours.

16. The method of claim 15, wherein, in the forging of the slab, the slab heated to the temperature of 1,100 C. to 1,250 C. is pressed down to be forged, and the slab is pressed down so that a thickness of the slab is reduced by 250 mm to 350 mm whenever the slab is pressed down once using a press-down device.

17. The method of claim 13, further comprising, in the removing of hydrogen (H.sub.2) from the press roll, heating the press roll to a temperature of 200 C. to 400 C.

18. The method of claim 13, wherein, in the heating of the press roll to the temperature of 200 C. to 400 C., the press roll is heated for 48 hours or more.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] FIG. 1 is a flowchart illustrating a method for manufacturing a press roll through a method according to an embodiment of the present invention.

[0029] (a) to (g) of FIG. 2 are process diagrams sequentially illustrating a process of preparing molten steel according to an embodiment of the present invention.

[0030] (a) to (d) of FIG. 3 are views sequentially illustrating operations of a casting device according to an embodiment of the present invention.

[0031] (a) to (c) of FIG. 4 are process diagrams sequentially illustrating a method for manufacturing a press roll using a slab manufactured by the method according to an embodiment of the present invention.

[0032] (a) and (b) of FIG. 5 are views illustrating results obtained by confirming whether or more coarse segregation occurs by cutting and etching a cross-section of the press roll.

MODE FOR CARRYING OUT THE INVENTION

[0033] Hereinafter, specific embodiments will be described in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. In the figures, the dimensions of layers and regions are exaggerated for clarity of illustration. Like reference numerals refer to like elements throughout.

[0034] FIG. 1 is a flowchart illustrating a method for manufacturing a press roll through a method according to an embodiment of the present invention.

[0035] Referring to FIG. 1, a method for manufacturing a press roll includes a process (S100) of preparing molten steel, a process (S200) of solidifying the molten steel to manufacture a slab, a process (S300) of heating the slab, a process (S400) of forging the heated cast steel to manufacture a press roll, and a process (S500) of heating the press roll.

[0036] The press roll is made of alloy steel with a high content of chromium (Cr). More specifically, the press roll is made of high chromium (Cr) alloy steel containing a high content of chromium (Cr) of 4.5 wt % to 5.5 wt % throughout the entire press roll. More specifically, the press roll is made of alloy steel containing, in its entirety, 4.5 wt % to 5.5 wt % of chromium (Cr), 0.75 wt % to 0.95 wt % of carbon (C), 0.2 wt % to 0.5 wt % of silicon (Si), 0.2 wt % to 0.5 wt % of manganese (Mn), 0.4 wt % to 0.65 wt % of molybdenum (Mo), 0.5 wt % or less (0 wt % or more) of nickel (Ni), 0.2 wt % or less (0 wt % or more) of copper (Cu), 0.0025 wt % or less (0 wt % or more) of phosphorus (P), 0.015 wt % or less (0 wt % or more) of sulfur (S), and iron (Fe) as a residual content (91.16 wt % to 93.15 wt %).

[0037] The content of chromium (Cr) is controlled to 4.5 wt % to 5.5 wt %, and the content of carbon (C) is controlled to 0.75 wt % to 0.95 wt % with respect to the entire press roll to ensure hardness and processability. For example, if the content of chromium (Cr) in the press roll is less than 4.5 wt %, or the content of carbon (C) is less than 0.75 wt %, a problem of low hardness of the press roll may occur. Conversely, if the content of chromium (Cr) in the press roll exceeds 5.5 wt %, or the content of carbon (C) exceeds 0.95 wt %, the processability of the press roll is low, and the processing of the manufactured press roll may be difficult as necessary.

[0038] In addition, the content of phosphorus (P) in the press roll is controlled to 0.025 wt % or less, and the content of sulfur (S) is controlled to 0.015 wt % or less to allow an organization of the press roll to be uniform and suppress or prevent an occurrence of cracks. That is, if the content of phosphorus (P) in the press roll exceeds 0.025 wt %, or the content of sulfur (S) exceeds 0.015 wt %, the organization may become uneven due to segregation, etc., inside the press roll. Thus, when pressing an object to be rolled (hereinafter, referred to as a rolled object) by using the press roll, a problem of cracks occurring in the press roll may occur. Thus, the content of phosphorus (P) in the press roll is controlled to 0.025 wt % or less, and the content of sulfur (S) is controlled to 0.015 wt % or less.

[0039] In an embodiment of the present invention, the press roll to be manufactured may be a roll applied to the rolling device that applies force to the rolled object, such as a slab or steel plate, to roll the object.

[0040] In the process (S100) of preparing the molten steel, the molten steel for manufacturing the press roll is prepared. That is, in the process (S100) of preparing the molten steel, the molten steel having a composition content that has to be contained in the press roll is prepared. That is, high chromium (Cr) molten steel having a chromium (Cr) content of 4.5 wt % to 5.5 wt % is prepared. More specifically, in preparing the molten steel, molten steel having contents of 4.5 wt % to 5.5 wt % of chromium (Cr), 0.75 wt % to 0.95 wt % of carbon (C), 0.2 wt % to 0.5 wt % of silicon (Si), 0.2 wt % to 0.5 wt % of manganese (Mn), 0.4 wt % to 0.65 wt % of molybdenum (Mo), 0.5 wt % or less (0 wt % or more) of nickel (Ni), 0.2 wt % or less (0 wt % or more) of copper (Cu), 0.0025 wt % or less (0 wt % or more) of phosphorus (P), 0.015 wt % or less (0 wt % or more) of sulfur (S), and iron (Fe) as a residual content (91.16 wt % to 93.15 wt %), is manufactured. This may be achieved by sequentially performing each process of preparing the molten steel described below.

[0041] (a) to (g) of FIG. 2 are process diagrams sequentially illustrating the process of preparing the molten steel according to an embodiment of the present invention.

[0042] Referring to FIG. 2, the process (S100) of preparing the molten steel M may include a process (S110) of processing the molten steel M using a steel converter 11 (hereinafter, a steel converter processing process), a process (S120) of discharging the molten steel M inside the steel converter 11 into a ladle 20, a temperature control process (S130) of controlling a temperature of the molten steel M discharging into the ladle 20 using a temperature control device 30, and a degassing process (S140) of removing a gas from the molten steel M using a vacuum processing device 40.

[0043] The steel converter processing process (S110) is a process of controlling the composition of the molten steel M while charging the molten steel M into the steel converter 11. The steel converter 11 used in the steel converter processing process (S110) is a steel converter used in a steelmaking process (hereinafter, stainless steelmaking process) of preparing the molten steel for manufacturing stainless steel (hereinafter, molten steel for manufacturing stainless steel). That is, in preparing the molten steel according to the embodiment, the steel converter 11 used in the stainless steelmaking process is utilized.

[0044] For a more specific explanation, it may briefly explain the steelmaking process that utilizes the steel converter during the stainless steelmaking process. The stainless steelmaking process includes a process of charging the molten steel into the steel converter, a process of inputting alloy steel containing chromium (Cr) and alloy steel containing nickel (Ni) into the steel converter, and an oxygen annealing process of removing carbon (C) and phosphorus (P) by injecting oxygen into the steel converter using a lance. When adding chromium (Cr)-containing iron into the steel converter, the content of chromium (Cr) in the molten steel is input to be 10.5 wt % to 11 wt %. Thus, the molten steel prepared in the steelmaking process using the steel converter during the stainless steelmaking process contains 10.5 wt % to 11 wt % of chromium.

[0045] For comparison, a steelmaking process using the steel converter is briefly described in a steelmaking process (hereinafter, a carbon steelmaking process) that provides molten steel for manufacturing carbon steel (hereinafter, molten steel for manufacturing carbon steel). The carbon steelmaking process includes a process of charging the molten steel into the steel converter and an oxygen annealing process of removing carbon (C) and phosphorus (P) by injecting oxygen into the steel converter using the lance. Carbon steel is steel having a low content of chromium (Cr). Thus, in the carbon steelmaking process, chromium (Cr) is not input to the steel converter, and the content of chromium (Cr) in the molten steel is controlled to be low at 0.1 wt % to 1.0 wt %.

[0046] As described above, in the steel converter for the stainless steelmaking process, the molten steel is prepared at a high chromium (Cr) content of 10.5 wt % to 11 wt %. In other words, the inside of the steel converter used in the stainless steelmaking process contains molten steel at a high chromium (Cr) content of 10.5 wt % to 11 wt %. However, the inside of the steel converter used in the carbon steelmaking process contains molten steel at a low chromium (Cr) content of 0.1 wt % to 1.0 wt %.

[0047] Thus, when preparing the molten steel for manufacturing the press roll having a high chromium (Cr) content of 4.5 wt % to 5.5 wt %, if the steel converter used in the carbon steelmaking process is used, the steel converter for manufacturing the carbon steel may be contaminated. That is, if the molten steel having a high chromium (Cr) content of 4.5 wt % to 5.5 wt % is placed in the steel converter for manufacturing the carbon steel, even if the molten steel is casted, a large amount of chromium (Cr) may remain inside the steel converter for manufacturing the carbon steel. For example, a large amount of chromium (Cr) may be attached or adhered to an inner wall of the steel converter for manufacturing the carbon steel. Thus, the steel converter for manufacturing the carbon steel is contaminated with chromium (Cr). In addition, when the steel converter for manufacturing carbon steel containing a large amount of residual chromium (Cr) is used in the carbon steelmaking process, the molten steel contains a large amount of chromium (Cr), and the chromium (Cr) acts as an impurity that deteriorates quality of the carbon steel.

[0048] However, when using the steel converter used in stainless steelmaking process to prepare high chromium (Cr) steel having a high chromium (Cr) content of 4.5 wt % to 5.5 wt %, the steel converter used in stainless steelmaking process is not contaminated. That is, even if the molten steel having a high chromium (Cr) content of 4.5 wt % to 5.5 wt % is put into the steel converter 11 for manufacturing the stainless steel and then discharged, and a large amount of chromium (Cr) remains inside the steel converter for manufacturing the stainless steel, the chromium remaining inside the steel converter does not act as the impurity. This is because the steel converter for manufacturing the stainless steel is already a means of containing the molten steel at a high chromium content of 10.5 wt % to 11 wt %. In addition, this is because the chromium content of the molten steel put into the steel converter for manufacturing the stainless steel is higher than that of the molten steel to be manufactured in an embodiment. Thus, even if the molten steel for the press roll manufacturing is placed in the steel converter for manufacturing the stainless steel, the steel converter for manufacturing the stainless steel is not contaminated. Thus, when manufacturing the molten steel for the stainless steel again using the steel converter for manufacturing the stainless steel in which the molten steel according to an embodiment is contained, no problem occurs due to the chromium (Cr) in the molten steel that is previously contained.

[0049] Thus, in an embodiment, when processing molten steel using the steel converter 11 (S110), the steel converter 11 used in the stainless steelmaking process as described above is used. Thus, the steel converter 11 may be used without causing the contamination of the steel converter 11 due to chromium (Cr). In addition, the steel converter used to prepare the molten steel according to an embodiment containing 4.5 wt % to 5.5 wt % of chromium (Cr) may be used to prepare the molten steel for manufacturing the stainless steel. That is, the steel converter for manufacturing the stainless steel may be used interchangeably in an operation for preparing the molten steel for manufacturing the stainless steel and an operation for preparing the molten steel according to an embodiment containing 4.5 wt % to 5.5 wt % of chromium (Cr).

[0050] The temperature control device 30 used in the temperature control process and the vacuum processing device 40 used in the degassing process use a ladle furnace (LF) and a Rheinstaal Huttenwerke und Heraus (RH) device used in the steelmaking process (hereinafter, carbon steelmaking process) that prepares molten steel for manufacturing carbon steel (hereinafter, molten steel for manufacturing carbon steel). The reasons for using the ladle furnace and the RH device in the carbon steelmaking process will be described later.

[0051] Hereinafter, with reference to FIG. 2, a process for preparing the molten steel according to an embodiment of the present invention will be described in more detail.

[0052] As described above, the molten steel preparation process (S100) includes a process (S110) of processing molten steel M using the steel converter 11, a process (S120) of discharging the molten steel M inside the steel converter 11 into a ladle, a process (S130) of controlling a temperature of the molten steel M using the temperature control device 30, and a degassing process (S140) of removing a gas from the molten steel M using the vacuum processing device 40.

[0053] Referring to (a) to (d) of FIG. 2, the process (S110) of processing the molten steel M using the steel converter 11 may include a process (S111) of inputting alloy steel (hereinafter, chromium (Cr) alloy steel) containing chromium (Cr) into the steel converter 11 ((a) of FIG. 2), an oxygen annealing process (S112) of injecting oxygen into the molten steel inside the steel converter 11 ((b) of FIG. 2), a deoxidation process (S113) of removing oxygen (O) contained in the molten steel M ((c) of FIG. 2), and a process (S114) of reducing chromium oxide contained in slag floating on a surface of the molten steel M to chromium (Cr) ((d) of FIG. 2).

[0054] In addition, although not shown, the molten steel preparation process may include a preliminary refining process of removing sulfur (S), phosphorus (P), and silicon (Si), which are contained in the molten steel before charging the molten steel into the steel converter. First, the preliminary refining process will be briefly described. The preliminary refining process may include a first preliminary refining process of removing sulfur (S) from the molten steel, and a second preliminary refining process of removing phosphorus (P) and silicon (Si) from the molten steel. In the first preliminary refining process, a content of sulfur (S) in the molten steel is controlled to 0.04 wt % or less. In addition, in the second preliminary refining process, a content of silicon (Si) in the molten steel is controlled to 0.05 wt % or more, and a content of phosphorus (P) is controlled to 0.03 wt % or less. In performing the first and second preliminary refining processes, quicklime (CaO) and fluorite (CaF.sub.2) are input to the ladle containing the molten steel to remove sulfur (S), silicon (Si), and phosphorus (P) from the molten steel. In addition, the first and second preliminary refining processes may be performed in a hot metal preprocessing station (HMPS).

[0055] After the first and second preliminary refining processes are completed, the molten steel M is charged into the steel converter 11. Here, the steel converter 11 for charging the molten steel M is used, and as described above, the steel converter 11 used in the stainless steelmaking process is used.

[0056] When the molten steel M is charged into the steel converter 11, chromium (Cr) alloy steel is input into the steel converter 11 as illustrated in (a) of FIG. 2. Thus, the chromium (Cr) alloy steel input into the steel converter 11 is melted by heat of the molten steel M. Here, the input amount of chromium (Cr) alloy steel is controlled so that the content of chromium (Cr) in the entire molten steel M is 4.5 wt % to 5.5 wt %. Thus, high molten steel M with a chromium (Cr) content of 4.5 wt % to 5.5 wt % is manufactured.

[0057] Next, oxygen annealing is performed by blowing or injecting oxygen into the molten steel M inside the steel converter 11 as illustrated in (b) of FIG. 2. That is, after inserting a portion of the lance into the inside of the steel converter 11, oxygen (O) is supplied into the lance 12. Thus, oxygen (O) is blown or injected from the lance 12. Oxygen (O) blown to the inside of the steel converter 11 using the lance 12 reacts with carbon (O) contained in the molten steel M (C+O->CO (gas)). Here, carbon (O) in the molten steel M reacts with oxygen (O) to become a carbon monoxide (CO) gas, and the generated carbon monoxide (CO) gas is discharged outside the steel converter 11. Thus, the content of carbon (C) contained in the molten steel M decreases. That is, decarburization in which the carbon (C) content in the molten steel M decreases occurs. Here, at least one of the oxygen blowing flow rate or time is controlled so that the carbon (C) content in the molten steel M becomes 0.75 wt % to 0.95 wt %.

[0058] As described above, when the decarbonization is performed by blowing oxygen into the steel converter 11, reaction by-products provided as metal oxides are generated in addition to the carbon monoxide (CO) gas. In addition, the generated reaction by-products ascend toward a top surface of the molten steel M, that is, a bath surface of the molten steel M to float on an upper portion of the bath surface of the molten steel M. The reaction by-products floating on the bath surface of the molten steel is slag SL.

[0059] When the carbon (C) content in the molten steel M reaches 0.75 wt % to 0.95 wt %, and the decarburization is completed, or when the carbon (C) content in the molten steel M decreases to approach 0.95 wt % and reach a final decarburization stage, a dephosphorizing agent is input into the steel converter 11. Here, oxygen is blown to the steel converter 11 using the lance 12, and simultaneously, the dephosphorizing agent is input, and the dephosphorizing agent may be a material including, for example, quicklime (CaO). Here, phosphorus (P) in molten steel M reacts with the dephosphorizing agent and oxygen (O) (3CaO+2P+5O->3CaO.Math.P.sub.2O.sub.5). In addition, 3CaO.Math.P.sub.2O.sub.5, which is the reaction by-products, ascends and is absorbed into the slag SL floating on the bath surface of the molten steel M. That is, phosphorus (P) separated from the molten steel M is absorbed into the slag SL floating on the bath surface of the molten steel M. Thus, the content of phosphorus (P) contained in the molten steel M decreases. That is, dephosphorization in which the phosphorus (P) content in molten steel M decreases occurs. Here, at least one of a dephosphorizing agent inputting amount, an oxygen blowing flow rate, or an upper sintering injection time is controlled so that the phosphorus (P) content in the molten steel M becomes 0.025 wt % or less.

[0060] After performing oxygen annealing on the molten steel M to perform the decarburization and dephosphorization, the oxygen (O) content in the molten steel M increases. Oxygen (O) in molten steel M causes pin holes in the slab. In addition, oxygen (O) in the molten steel reacts with a metal contained in the molten steel and becomes a factor in generating metal oxide inclusions. The inclusions contained in the molten steel may cause defects such as cracks in the cast steel.

[0061] Thus, after the oxygen annealing is completed, the deoxidation is performed to remove oxygen (O) from the molten steel M. For this, a deoxidizer containing at least one of silicon (Si) or aluminum (Al) is input into the inside of the steel converter 11. The deoxidizer may use at least one of an alloy containing silicon (Si) or an alloy containing aluminum (Al). Here, the alloy containing silicon (Si) may be an FeSi alloy containing silicon (Si) and iron (Fe), containing 70 wt % to 80 wt % of silicon (Si) and 20 wt % to 30 wt % of iron (Fe). In addition, the alloy containing aluminum (Al) may be an alloy close to pure aluminum (Al) containing 90 wt % to 95 wt % of aluminum (Al).

[0062] When the deoxidizer is input, the oxygen (O) contained in the molten steel M reacts with at least one of silicon (Si) or aluminum (Al) contained in the deoxidizer. At least one of silicon oxide such as SiO.sub.2 or aluminum oxide such as Al.sub.2O.sub.3 is generated. In addition, at least one reaction by-product of silicon oxide and aluminum oxide is absorbed into the slag SL floating on the bath surface of the molten steel M. Thus, the content of oxygen (O) contained in the molten steel M decreases (deoxidation). Here, the input amount of deoxidizer is controlled so that the content of oxygen (O) in the molten steel M becomes 0.001 wt % or less.

[0063] As described above, at least one of the alloy containing silicon (Si) or the alloy containing aluminum (Al) is used as the deoxidizer. Here, it is preferable to use the alloy containing silicon (Si) and the alloy containing aluminum (Al) together as the deoxidizer, when compared to using the alloy containing silicon (Si) as the deoxidizer or the alloy containing aluminum (Al) as the deoxidizer.

[0064] When the content of oxygen (O) in the molten steel exceeds 0.001 wt %, pin hole defects may occur in the cast steel due to oxygen (O), and cracks may occur due to a large amount of inclusions. Thus, during the deoxidation process, the content of oxygen (O) in the molten steel M is controlled to be 0.001 wt % or less.

[0065] As described above, when the oxygen annealing is performed to remove carbon (C) and phosphorus (P) from the molten steel M, chromium (Cr) input into the molten steel M (S110) is oxidized to become chromium oxide. In addition, the generated chromium oxide is absorbed into the slag SL floating on the bath surface of the molten steel M. Thus, the content of chromium (Cr) in the molten steel decreases. Thus, it is necessary to reduce the chromium oxide contained in the slag SL to chromium and then supply the reduced chromium again to the molten steel M. For this, a reducing agent is input into the steel converter 11 in which the deoxidation has been completed. More specifically, the slag SL is input as the reducing agent. Here, the reducing agent may be a material containing silicon (Si), and more specifically, may be alloy steel containing silicon (Si). As a more specific example, the reducing agent may contain 70 wt % to 80 wt % of silicon (Si) and 20 wt % to 30 wt % of iron (Fe).

[0066] When the reducing agent is input into the steel converter 11, the chromium oxide contained in the slag SL and the silicon (Si) contained in the reducing agent react with each other. Thus, chromium oxide contained in the slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel. Thus, the content of chromium (Cr) in the molten steel increases. Here, the input amount of reducing agent is controlled so that the content of chromium (Cr) in the molten steel is 4.5 wt % to 5.5 wt %.

[0067] After the chromium reduction is completed, composition contents and temperature of the molten steel M are confirmed. For example, the molten steel M inside the steel converter 11 is collected or sampled, and the composition contents and temperature of the collected molten steel are measured. In addition, it is confirmed whether the composition content of the measured molten steel M is included in a target composition content, and it is confirmed whether the temperature of the measured molten steel M is included in a target temperature. Here, the target temperature may be, for example, 1,650 C. to 1, 750 C.

[0068] To describing a more specific example, whether each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is included in the target content is confirmed. Here, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is included in the target content, and the temperature of the molten steel is included in the target temperature, the slag floating on the upper portion of the bath surface of the molten steel is excluded (not shown). That is, the slag is removed from the bath surface of the molten steel. However, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is not within the target content, or the temperature of the molten steel is not within the target temperature, the process for controlling the composition content or temperature is performed again. That is, if each of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) included in the molten steel is not within the target content, at least one of the processes of inputting of the chromium (Cr) alloy steel, the decarburization, the dephosphorization, the desulfurization, the deoxidation, and the chromium reduction is performed. In addition, if the temperature of the molten steel M is lower than 1,650 C., the molten steel M is heated so that the temperature of the molten steel becomes higher than 1,650 C. Conversely, if the temperature of the molten steel exceeds 1,750 C., the temperature of the molten steel is reduced.

[0069] If the content of each composition included in the molten steel exceeds the target content, a problem of deterioration in quality of the slab S may occur. That is, if at least one of the contents of chromium (Cr), carbon (C), phosphorus (P), sulfur (S), and oxygen (O) is deviated from the target content, hardness of the manufactured cast steel may be low, or a problem in which the pinholes and cracks occur in a surface or inside of the cast steel may occur This may lower the hardness of the press roll and cause the pinholes and cracks in the surface or inside of the cast steel.

[0070] In addition, if the temperature of the molten steel M is less than 1,650 C., it may be difficult to raise the temperature of the molten steel M to the target temperature (LF starting target temperature) in the subsequent temperature control process. Thus, when supplying the molten steel M through a tundish, the temperature of the molten steel M may not reach the casting target temperature. In this case, a nozzle that supplies the molten steel of the tundish to a mold may become clogged. Conversely, if the temperature of the molten steel M exceeds 1, 750 C., a refractory constituting the steel converter may be damaged by erosion due to high heat. Thus, the temperature of the molten steel M in the steel converter processing process is controlled to be between 1,650 C. and 1,750 C.

[0071] When the content of each composition in the molten steel M is within the target content, and the temperature of the molten steel M is within the target temperature, the slag floating on the top surface of the molten steel M is removed (not shown).

[0072] After removing the slag SL inside the steel converter 11, the molten steel M inside the steel converter 11 is discharged into the ladle 20. Here, it is desirable to discharge the molten steel M using the ladle 20 used in the carbon steelmaking process. This is because the device used in the process performed later uses the ladle 20 used in the carbon steelmaking process. That is, in the temperature control process, the ladle furnace LF is used as the temperature control device 30, and in the degassing process, the Rheinstaal Huttenwerke und Heraus (RH) device is used as the vacuum processing device 40. Here, the ladle furnace LF and the RH devices are devices used in the carbon steelmaking process, and are devices coupled to the ladle used in the carbon steelmaking process.

[0073] Also, the stainless steelmaking process does not use the ladle furnace LF. In addition, the vacuum processing device is used in the stainless steelmaking process, but this is a vacuum tank degasser (VTD) device, which is different from the RH device used in the carbon steelmaking process.

[0074] In discharging the molten steel inside the steel converter 11 into the ladle, in the embodiment, the molten steel is discharging into the ladle 20 used in the carbon steelmaking process. That is, in order to use the device used in the carbon steelmaking process in the subsequent temperature control process and degassing process, the molten steel is discharged into the ladle 20 used in the carbon steelmaking process.

[0075] As described above, in the embodiment, the ladle furnace is used as the temperature control device 30, and the RH device is used as the vacuum processing device 40. Hereinafter, for convenience of explanation, the temperature control device and the ladle furnace are referred to by the same reference symbol 30, and the vacuum processing device and the RH device are referred to by the same reference symbol 40.

[0076] When the discharging of the molten steel M into the ladle 20 is completed, the ladle 20 containing the molten steel M is moved to the temperature control device 30 as illustrated in (f) of FIG. 2, and the ladle 20 is connected to the temperature control device 30. That is, the ladle 20 is moved to the ladle furnace 30, which is the used temperature control device 30 in the carbon steelmaking process, and the ladle 20 is connected to the ladle furnace 30. The ladle furnace may include a cover 32 that covers an upper opening of the ladle 20 as illustrated in (f) of FIG. 2, and an electrode rod 31 that is inserted into the ladle 20 through the cover 32. In addition, in this ladle furnace 30, the desulfurization is performed to remove sulfur (S) from the molten steel, and the contents of carbon (C) and chromium (Cr) are controlled, and then, the temperature of the molten steel is controlled.

[0077] First, the desulfurization process using the ladle furnace 30 will be described. When the ladle 20 arrives at the ladle furnace 30, the cover 32 of the ladle furnace 30 is attached to the upper portion of the ladle 20. Then, the desulfurizing agent is input into the ladle 20. Here, the desulfurizing agent may pass through the opening provided in the cover 32 and then input into the ladle 20. It is preferable that the desulfurizing agent use at least one of a first material including quicklime (CaO) and alumina (Al.sub.2O.sub.3) and a second material including fluorite (CaF.sub.2). When the desulfurizing agent is input into the ladle 20, the desulfurizing agent reacts with sulfur (S) in the molten steel to generate a reaction by-product containing sulfur (S), and the generated reaction by-product is absorbed into the slag SL of the bath surface of the molten steel M. As a result, the sulfur (S) content in the molten steel decreases (desulfurization). Here, the content of sulfur (S) in the molten steel is set to 0.015 wt % or less, and this may be controlled by controlling the input amount of desulfurizing agent.

[0078] When the desulfurization is completed, the molten steel M contained in the ladle 20 is sampled to measure the contents of carbon (C) and chromium (Cr). In addition, depending on the contents of carbon (C) and chromium (Cr), the contents of carbon (C) and chromium (Cr) in the molten steel M are controlled. For example, when the carbon (C) content is low to be less than 0.75 wt %, a carbon (C)-containing carbon material is input into the ladle 20. In addition, the input amount of carbon dioxide is controlled so that the carbon (C) content in the molten steel M is 0.75 wt % or more. Conversely, when the measured carbon (C) content exceeds 0.95 wt %, solid oxygen, for example iron ore such as FeO, is input into the ladle 20. Here, the input amount of solid oxygen is controlled so that the carbon (C) content in the molten steel M is 0.95 wt % or less.

[0079] As another example, when the measured chromium (Cr) content is low to be less than 4.5 wt %, alloy steel containing chromium (Cr) is input into the t ladle 20. Conversely, when the measured chromium (Cr) content exceeds 5.5 wt %, solid oxygen, for example iron ore such as FeO, is input into the ladle 20. Here, the input amount of solid oxygen is controlled so that the content of chromium (Cr) in the molten steel is 5.5 wt % or less.

[0080] In the above, it was explained that solid oxygen is input into the ladle 20 coupled to the ladle furnace 30 when the carbon (C) content exceeds 0.95 wt %, or the chromium (Cr) content exceeds 5.5 wt %. However, it is not limited thereto, and oxygen may be blown to the molten steel M in the vacuum processing device 40 implemented thereafter to control the carbon (C) content to 0.95 wt % or less, or the chromium (Cr) content to 5.5 wt % or less.

[0081] When the molten steel M is discharged into the ladle 20, the ladle 20 is moved for the next process. However, while the ladle 20 moves, the temperature of the molten steel M gradually decreases. That is, even if the temperature of the molten steel M is controlled to a temperature of 1, 650 C. to 1, 750 C. before the molten steel from the steel converter 11 is discharged into the ladle 20, the temperature of the molten steel M is lowered during the process in which the ladle 20 moves to the next process.

[0082] Thus, before moving the ladle 20 to the next process, i.e., the vacuum processing device 40, it is necessary to heat the molten steel M inside the ladle 20. For this, power is supplied to the electrode rod 31 to generate arc and heat from the electrode rod 31, and the molten steel is heated by the generated arc and heat. Here, the molten steel M is heated so that the temperature becomes the starting target temperature, and the starting target temperature may be 1,560 C. to 1,600 C. Here, the starting target temperature means a target temperature of the molten steel M contained in the ladle 20 when the ladle 20 separated from the ladle furnace 30 departs for the next process. The temperature of the molten steel M may be controlled to become the starting target temperature by controlling at least one of an intensity of the power supplied to the electrode rod 31 and a heating time of the molten steel using the electrode rod 31.

[0083] When the temperature of the molten steel M (hereinafter, the starting temperature) is less than 1,560 C. or more than 1,600 C. when the ladle 20 separated from the ladle furnace 30 starts for the next process, the temperature of the molten steel M may exceed a casting target temperature when the ladle 20 reaches the turndish of the casting device. Here, the casting target temperature refers to the target temperature of the molten steel received in the tundish, and the casting target temperature may be 1,520 C. to 1,550 C. If the starting temperature is less than 1,560 C., the temperature of the molten steel M may be less than 1,520 C. when the ladle reaches the tundish. In addition, if the starting temperature is less than 1,600 C., the temperature of the molten steel may exceed 1,550 C. when the ladle reaches the tundish.

[0084] When the ladle 20 reaches the tundish, if the temperature of the molten steel M is less than 1,520 C., a problem in which the nozzle supplying the molten steel M from the tundish to the mold becomes clogged may occur In addition, when the temperature of the molten steel exceeds 1,600 C. when the ladle 20 reaches the tundish, break out in which a solidification shell bursts, and the molten steel therein is discharged out when the molten steel is solidified in the casting device. Thus, the starting temperature of the molten steel M in the ladle furnace 30 is controlled to be 1,560 C. to 1,600 C., so that when the ladle 20 reaches the tundish, the temperature of the molten steel is 1,520 C. to 1,550 C.

[0085] The reason for using the ladle furnace LF used in the carbon steelmaking process in performing the temperature control process (S130) is that it effectively increases in temperature without contaminating the molten steel M. That is, in the stainless steelmaking process, when raising the temperature of the molten steel, a metal such as aluminum (Al) is input into the molten steel to cause an exothermic reaction and raise the temperature. That is, in the stainless steelmaking process, a separate heating device is not used to raise the temperature of the molten steel. Thus, in an embodiment, the temperature of the molten steel M is controlled by using the ladle furnace 30 used to increase in temperature of the molten steel in the carbon steelmaking process. Thus, the temperature of the molten steel M may increase without inputting other materials to the molten steel M. That is, the temperature of the molten steel may increase without changing the composition of the molten steel M.

[0086] When the composition and temperature control in the ladle furnace 30 are completed, the ladle 20 is separated from the ladle furnace 30. Next, the ladle 20 separated from the ladle furnace 30 is moved to the vacuum processing device 40. In addition, degassing is performed (S140) to remove nitrogen (N.sub.2) and hydrogen (H.sub.2) contained in the molten steel M using the vacuum processing device 40.

[0087] The vacuum processing device 40 is the Rheinstaal Huttenwerke und Heraus (RH) device 40, and is the RH device 40 used in the carbon steelmaking process. Referring to (g) of FIG. 2, the RH device 40 may include a vessel 41 having an internal space, a pump (not shown) connected to the vessel 41 to control a pressure inside the vessel 41, a pair of reflux tubes 42a and 42b disposed in parallel at left and right sides so as to be inserted into the ladle 20 and connected to a lower portion of the vessel 41, and a lance 43 inserted into an upper side of the vessel 41 to blow oxygen. Here, one 42a of the pair of reflux tubes 42a and 42b is an ascending tube through which the molten steel inside the ladle 20 ascends, and the other reflux tube 42b is a descending tube through which the molten steel ascending from the ascending tube descends toward the ladle 20.

[0088] The process of removing a gas from the molten steel M using the RH device 40 as described above is described. First, the ladles 20 are disposed at the lower sides of the pair of reflux tubes 42a and 42b and are coupled to each other to be sealed. Then, the inside of the vessel 41 is decompressed to a vacuum pressure of, for example, 0.2 torr or less, and then, the ladle 20 ascends to immerse the pair of reflux tubes 42a and 42b into the molten steel inside the ladle 20. Thereafter, an inert gas, such as argon (Ar), is blown to the pair of reflux tubes 42a and 42b to circulate the molten steel M inside the vessel 41. The circulating molten steel is exposed to a vacuum atmosphere inside the vessel 41, and nitrogen (N.sub.2) and hydrogen (H.sub.2) gases in the molten steel are discharged outside the vessel 41. Thus, the contents of nitrogen (N) and hydrogen (H.sub.2) in the molten steel M are reduced. Here, the content of nitrogen (N.sub.2) is made to be 0.015 wt % or less, and the content of hydrogen (H.sub.2) is made to be 0.0005 wt % or less.

[0089] Hereinafter, the reason why the content of hydrogen (H.sub.2) is controlled to 0.0005 wt % or less in the degassing process (S140) will be described.

[0090] In manufacturing the press roll, it is necessary to control the content of hydrogen (H.sub.2) contained in the press roll to 0.0002 wt % or less. This is because hydrogen (H.sub.2) contained inside the press roll is easily accumulated in the inclusions and the segregation contained in the press roll, and this may become a factor in causing internal cracks in the press roll when the press roll is actually used. Here, the actual use of the press roll may refer to a case in which the manufactured press roll is used to roll the rolled object, such as the slab. Thus, the content of hydrogen (H.sub.2) contained in the press roll is controlled to 0.0002 wt % or less. Here, when the content of hydrogen (H.sub.2) contained in the press roll is 0.0002 wt % or less, the cracks do not occur or are almost non-existent inside the press roll due to hydrogen (H.sub.2). In addition, since it is realistically difficult to completely remove hydrogen from the press roll, the hydrogen content is controlled to be 0.0002 wt % or less.

[0091] As described above, in order for the hydrogen (H.sub.2) content of the press roll to be 0.0002 wt % or less, it is desirable to reduce the hydrogen (H.sub.2) content in the molten steel to 0.0005 wt % or less in the degassing process (S140). That is, if the content of hydrogen (H.sub.2) in the molten steel M is not controlled to 0.0005 wt % or less in the degassing process (S140), it is difficult to control the content of hydrogen (H.sub.2) to 0.0002 wt % or less in the subsequent process of removing hydrogen by heating the press rolls ((c) of FIG. 4). In other words, it is difficult to manufacture the press roll in which the hydrogen (H.sub.2) content is controlled to 0.0002 wt % or less. Thus, in the degassing process (S140), the content of hydrogen (H.sub.2) in the molten steel is controlled to 0.0005 wt % or less.

[0092] In addition, in the degassing process (S140), the content of nitrogen (N.sub.2) in the molten steel M is controlled to 0.015 wt % or less. If the content of nitrogen (N.sub.2) in molten steel M exceeds 0.015 wt %, precipitates due to nitrogen (N.sub.2) are generated when the cast steel is manufactured, to cause a large number of cracks in the cast steel. Thus, the content of nitrogen (N.sub.2) in molten steel M is controlled to 0.015 wt % or less through the degassing process (S130).

[0093] As described above, the content of hydrogen (H.sub.2) in the molten steel M may be controlled to 0.0005 wt % or less, and the content of nitrogen (N.sub.2) may be controlled to 0.015 wt % or less by controlling the pressure and processing time of the vessel 41 of the RH device 40. That is, the pressure of the vessel 41 may be controlled to 0.2 torr or less and maintained at the pressure for 20 minutes or more to process the molten steel to manufacture the molten steel having a hydrogen (H.sub.2) content of 0.0005 wt % or less and a nitrogen (N.sub.2) content of 0.015 wt % or less.

[0094] When performing the degassing processing using the vacuum processing device 40, the reason for using the RH device 40 as the vacuum processing device 40 is that other types of vacuum processing devices have relatively low pressure control capabilities. That is, the RH device 40 used in the carbon steelmaking process may control a pressure inside the vessel 41 to 2 torr or less, but a vacuum oxygen decarburization (VOD) device, which is the vacuum processing device used in the stainless steelmaking process, may only lower the pressure to a maximum of 3 torr to 4 torr. In addition, in removing hydrogen (H.sub.2) and nitrogen (N.sub.2) in the molten steel, the lower the pressure, that is, the higher the vacuum, the more advantageous it is. Thus, the RH device 40 used for degassing in the carbon steelmaking process is used. Thus, hydrogen (H.sub.2) and nitrogen (N.sub.2) may be easily removed from the molten steel M, and hydrogen (H.sub.2) and nitrogen (N.sub.2) may be removed more effectively than when using other vacuum processing devices, such as the VOD.

[0095] In this embodiment, in the steel converter processing process (S110), the steel converter 11 used in the stainless steelmaking process may be used. That is, in order to prepare the molten steel for manufacturing the press roll at a high chromium (Cr) content, the steel converter 11 used in the stainless steelmaking process is used. Thus, it is possible to prepare the molten steel for manufacturing the press roll at the high chromium (Cr) content without contaminating the steel converter for other types of steel. In addition, the when preparing molten steel for manufacturing stainless steel again using the steel converter for manufacturing the stainless steel that is used to prepare the molten steel for manufacturing the press roll, the chromium remaining in the steel converter for manufacturing the stainless steel does not act as an impurity.

[0096] In addition, the ladle furnace 30 used in the carbon steelmaking process is used as a device for adjusting the composition and raising the temperature of the molten steel M discharged from the steel converter 11. That is, the temperature of the molten steel M may increase without inputting other materials to the molten steel. Thus, the temperature of the molten steel may increase without changing the composition of the molten steel.

[0097] In addition, when performing the degassing process using the vacuum processing device 40, the RH device 40 used in the carbon steelmaking process is used. That is, the degassing is performed using the RH device 40 that is capable of controlling the pressure of the vessel to 0.2 torr or lower. Thus, nitrogen and hydrogen (H.sub.2) may be effectively removed from the molten steel.

[0098] In addition, in preparing the molten steel M for manufacturing the press roll, a new steel converter, a heating device, and a vacuum processing device are not separately prepared, but the stainless steelmaking steel converter 11, the carbon steelmaking ladle furnace 30, and the RH device 40 are used. Thus, there is no need to prepare a separate device so as to prepare the molten steel for manufacturing the press roll, and thus, there is an effect of reducing costs.

[0099] (a) to (d) of FIG. 3 are views sequentially illustrating operations of the casting device according to an embodiment of the present invention.

[0100] First, the casting device 100 will be described with reference to (a) to (d) of FIG. 3.

[0101] Referring to (a) to (d) of FIG. 3, the casting device 100 is a vertical casting device 100 that draws the slab S vertically with respect to the ground when drawing the slab from the mold 120. The casting device according to an embodiment may be a device capable of manufacturing a thick slab (i.e., a thick plate) having a thickness of, for example, about 700 mm.

[0102] Referring to (a) to (d) of FIG. 3, the casting device 100 includes a mold 120 capable of solidifying the molten steel M supplied therein, a support part 171 that is capable of being inserted into the mold 120 to move up and down, a driving part 172 connected to the support part 171 to provide moving force, a first magnetic field generating part 150a disposed laterally outside the mold 120, a second magnetic field generating part 150b disposed laterally outside the mold 120 below the mold 120, and a heating part 160 disposed laterally outside the mold 120 between the mold 120 and the second magnetic field generating part 150b.

[0103] In addition, the casting device 100 includes a cooling part 140 extending in a direction perpendicular to the ground from the lower side of the mold 120 and injecting cooling water onto the slab S drawn to the lower side of the mold 120 to solidify the slab S, a rotating part 190 disposed at a lower side of the second magnetic field generating part 150b to receive and rotate the slab S supported on the support part 171, and a moving part 180 disposed to the rotating part 190 at the lower side of the second magnetic field generating part 150b to push and move the slab S supported on the support part 171 toward the rotating part 190.

[0104] In addition, the casting device 100 may include a tundish 110 disposed at an upper side of the mold 120 to supply the molten steel M to the mold 120 and a nozzle 130 connected to a lower portion of the tundish 110 to supply the molten steel M to the mold 120.

[0105] For convenience of explanation, in FIG. 3, the rotating part 190 is illustrated only in (d) of FIG. 3, and is omitted in (a) to (c) of FIG. 3. However, even in case such as (a) to (c) of FIG. 3, the rotating part 190 is disposed to face the moving part 180 at the lower side of the cooling part.

[0106] The mold 120 is a means for receiving the liquid molten steel M from the tundish 110 to primarily solidify the molten steel M into a certain shape. The mold 120 may have a cooling water pipe (not shown) provided inside through which the cooling water is circulated. When the molten steel is supplied to the mold 120 and solidified primarily, the molten steel M becomes a reaction solid state in which a solidified area A and a non-solidified area B coexist.

[0107] The support part 171 is inserted into the mold 120 to close a lower opening of the mold 120 before supplying the molten steel M to the mold 120. In addition, when the molten steel is supplied into the mold 120 while the support part 171 closes the lower opening of the mold 120, the molten steel M begins to be solidified on the support part 171. Thus, the slab S in a semi-solidified state is supported on the support part 171. In addition, when the driving part 172 operates to lower the support part 171, the support part 171 is lowered to the lower side of the mold 120 while supporting the slab S in the semi-solidified state. That is, the slab S inside the mold 120 is drawn to the lower side of the mold 120 by lowering the support part 171. The support part may have a plate shape.

[0108] The driving part 172 may be a means for elevating the support part 171 and may be connected to the lower portion of the support part 171. The driving part 172 may include a power source 172-1 that provides driving force for elevating, and a driving part 172-2 that connects the power source 172-1 to the support part 171 to enable the elevation by the driving force transmitted from the power source 172-1.

[0109] The power source 172-1 may be a device including a hydraulic piston. Alternatively, the power source 172-1 is not limited to the above-described example and any means that is capable of elevating the driving part 172-2 may be used.

[0110] The driving part 172-2 has one end connected to the power source 172-1 and the other end connected to the support part 171. The driving part 172-2 may have a shape extending, for example, in the vertical direction. In addition, the driving part 172-2 may be provided so that a height of one end connected to the power source 172-1 is fixed, and a height of the other end connected to the support part 171 may ascend or descend by the operation of the power source 172-1.

[0111] As described above, the driving part 172 is a means for elevating the support part 171 and may control a speed at which the support part 171 is elevated. Particularly, the speed at which the support part 171 descends may be controlled to control a speed at which the slab S supported on the support part 171 is drawn downward from the mold 120. Here, the speed at which the slab S is drawn to the lower side of the mold 120 may refer to a casting speed. The driving part 172 controls the descending speed of the support part 171 by controlling its operation to descend at a speed of 0.04 m/min or less. More specifically, the movement is controlled to descend at a speed of 0.01 m/min to 0.04 m/min. In other words, the driving part 172 controls the descending speed of the support part so that the casting speed is 0.04 m/min or less, more specifically, 0.01 m/min to 0.04 m/min. As described above, the controlling of the casting speed to 0.04 m/min or less is intended to suppress or prevent segregation from occurring in the slab S and to suppress or prevent a production rate from being deteriorated. That is, if the casting speed exceeds 0.04 m/min, the non-solidified molten steel M inside the slab S may not be sufficiently solidified, resulting in segregation. In addition, when the casting speed is less than 0.01 m/min, there is a problem that the production rate of the slab S is reduced. Thus, it is desirable to control the casting speed to 0.01 m/min to 0.04 m/min.

[0112] The cooling part 140 includes a plurality of rolls 141 arranged in a direction in which the support part 171 is elevated at the lower side of the mold 120 and a nozzle (not shown) disposed between the plurality of rolls 141 to inject the cooling water onto the slab drawn toward the lower side of the mold.

[0113] The plurality of rolls 141 may be disposed in a vertical direction relative to the ground. In addition, each of the plurality of rolls 141 is provided so that the slab S rotates by the force of the slab S descending downward by the support part 171. Thus, the slab S drawn downward from the mold 120 descends in a direction perpendicular to the ground by the lowering of the support part 171 and the plurality of rolls 141.

[0114] The nozzle for injecting the cooling water is disposed between the plurality of rolls 141. Thus, the slab S drawn to the lower side of the mold 120 descends by the support part 171 and secondarily cooled by the cooling water injected from the nozzle.

[0115] In the process of secondarily solidifying the slab S by injecting the cooling water onto the slab S drawn to the lower side of the mold 120, the injection flow rate of cooling water is controlled so that a surface temperature of the slab S becomes 800 C. to 900 C. That is, the injection flow rate of cooling water is controlled so that the surface of the entire slab S at the lower side of the mold 120 becomes 800 C. to 900 C. More specifically, the injection flow rate of cooling water is controlled so that the surface temperature from an upper portion to a lower portion of the slab S is uniform at 800 C. to 900 C. For this, in the plurality of nozzles arranged in the vertical direction, the injection amount of cooling water may be controlled so that the nozzles disposed at the lower side decrease.

[0116] The purpose of controlling the surface temperature of the slab S to 800 C. to 900 C. is to suppress or prevent an occurrence of surface cracks in the slab S and to prevent an occurrence of bulging. That is, when the temperature of the surface of the slab S is less than 800 C., the cracks may occur in the surface of the slab S due to overcooling. Conversely, if the temperature of the cast surface exceeds 900 C., strength of the solidification shell, which is the surface of the slab S, may be low, causing the bulging, in which the solidification shell is swelled. Thus, the surface temperature of the slab S drawn out of the mold 120 is controlled to 800 C. to 900 C.

[0117] If the temperature of the molten steel M supplied to the mold 120 is low, the molten steel M solidifies and stagnates to act as a factor that causes the segregation in the slab S. Thus, in order to suppress the decrease in molten steel temperature, mold flux, which is an insulating agent, is applied on the bath surface of the molten steel supplied to the mold 120. However, only mold plus is not enough to suppress the decrease in molten steel temperature.

[0118] Thus, in order to allow the molten steel M inside the mold 120 to flow so as to suppress the temperature drop of the molten steel M, the first magnetic field generating part 150a is installed outside the mold 120. The first magnetic field generating part 150a is disposed at the lateral outer side of the mold 120 to generate a magnetic field. Here, the lateral outer side of the mold 120 may mean the outside of an outer surface, which is an opposite surface of an inner surface of the mold 120 with which the molten steel M comes into contact. That is, the first magnetic field generating part 150a is installed outside the mold 120 to face the outer surface of the mold 120. In other words, the first magnetic field generating part 150a may be installed to surround the mold 120 from the lateral outer side of the mold 120. For this, the first magnetic field generating part 150a may be provided in a hollow shape extending along an outer surface of the mold 120. This first magnetic field generating part 150a may include a coil installed on a body and inside the body and generating the magnetic field by the applied power.

[0119] When the magnetic field is generated in the first magnetic field generating part 150a, the generated magnetic field allows the molten steel M inside the mold 120 to flow. Thus, the molten steel M inside the mold 120 flows by the magnetic field, and the decrease of the temperature due to the flow of the molten steel M may be suppressed or prevented. When the molten steel M flows, the compositions contained in the molten steel M are mixed evenly or uniformly. As a result, the occurrence of the segregation, in which specific compositions are formed by being accumulated or gathered on a certain area within the slab, may be suppressed or prevented.

[0120] The magnetic field generated from the first magnetic field generating part 150a may vary depending on an intensity of current according to power or voltage applied to the first magnetic field generating part 150a. Thus, the power or voltage supplied to the first magnetic field generating part 150a is controlled so that the current having the target intensity flows through the coil of the first magnetic field generating part 150a. The current may flow through the first magnetic field generating part 150a as described above, and the magnetic field may be generated in the first magnetic field generating part 150a. In addition, the molten steel M inside the mold 120 may flow by the magnetic field generated from the first magnetic field generating part 150a to suppress prevent a temperature of the molten steel M from decreasing, and the mold flux on the bath surface of the molten steel M may be suppressed or prevented from being mixed into the molten steel.

[0121] If the current flowing in the first magnetic field generating part 150a is too small, the molten steel M inside the mold 120 may not flow, and as a result, the temperature of the molten steel M may decrease to cause the segregation in the slab S. Conversely, if the current flowing in the first magnetic field generating part 150a is too large, the molten steel flow rate inside the mold 120 may be very fast to cause the mold flux to be mixed into the molten steel. The mold flux mixed into the molten steel M may be an impurity that causes the cracks in the cast steel. Thus, the intensity of the current flowing to the first magnetic field generating part 150a is controlled by controlling the intensity of the power or voltage supplied to the first magnetic field generating part 150a so that the magnetic field capable of allowing the molten steel inside the mold 120 to flow at an appropriate flow rate is generated.

[0122] When the molten steel M is solidified inside the mold 120, the entire molten steel M supplied into the inside of the mold 120 is not solidified at the same time. That is, the molten steel M is solidified sequentially from an edge in width and length directions of the mold 120 toward a center area. Thus, the edge in the width and length directions inside the mold 120 are solidified with molten steel to become a solidified shell or solid state, but the center area in the width and length directions of the mold 120 is in a non-solidified state in which the molten steel M exists in a liquid state. In other words, the cast steel manufactured by partially solidifying the molten steel inside the mold 120 is in a semi-solidified state in which a solidified area A and a non-solidified area B exist.

[0123] In addition, when the slab S in the semi-solidified state is drawn to the lower side of the mold 120, the slab S is secondarily cooled by the cooling water injected from the nozzle of the cooling part 140. Here, since the cooling part 140 is disposed in a lateral direction of the slab S, the molten steel is solidified sequentially from the edge in the width and length direction of the slab S toward the center area. Thus, the slab S drawn to the lower portion of the mold 120 may be in the semi-solidified state in which the solidified area A and the non-solidified area B exist. In addition, as the molten steel M on the non-solidified area B is solidified over time, the completely solidified cast steel without the non-solidified area B is manufactured.

[0124] In addition, in the slab S drawn to the lower portion of the mold 120, the accumulated solidification time is shorter toward the upper portion near the mold 120, and the accumulated solidification time is longer toward the lower portion. Thus, in the slab S drawn to the lower portion of the mold 120, a surface area of the non-solidified area B becomes wider as it goes to the upper portion near the mold 120, and the surface area of the non-solidified area B becomes smaller as it goes to the lower portion. In other words, in the slab S drawn to the lower portion of the mold 120, the surface area of the solidification area A becomes smaller as it goes to the upper portion near the mold 120, and the surface area of the solidification area A becomes larger as it goes to the lower portion.

[0125] As described above, there is the non-solidified area B in the upper portion of the slab S drawn downward from the mold 120 in which the molten steel M is not solidified. That is, the slab S contains non-solidified molten steel M. In addition, when the slab S is drawn to the lower side of the mold 120, the slab S is solidified by the cooling water injected from the cooling part 140. However, if the non-solidified molten steel M exists in the slab S is solidified without the stagnation, the segregation may occur inside the slab S.

[0126] Thus, the second magnetic field generating part 150b is installed at the lower side of the mold 120 to allow the non-solidified molten steel M inside the slab S drawn to the lower side of the mold 120 to flow. The second magnetic field generating part 150b is disposed at the lower side of the mold 120 and disposed at the lateral outer side of the mold 120 to generate the magnetic field. More specifically, the second magnetic field generating part 150b may be disposed to face the first magnetic field generating part 150a in the vertical direction at the lower side of the mold 120. More specifically, the second magnetic field generating part 150b may be disposed at the lateral outer side of the cooling part 140 at the lower side of the mold 120. For this, the second magnetic field generating part 150b may be provided in a hollow shape. In addition, the second magnetic field generating part 150b may be provided with the same configuration as the first magnetic field generating part 150a. That is, the second magnetic field generating part 150b may include a coil installed on the body and inside the body and generating the magnetic field by the applied power.

[0127] When the magnetic field is generated in the second magnetic field generating part 150b, the generated magnetic field is applied to the slab. Thus, the liquid steel inside the slab S flows due to the magnetic field. Thus, the molten steel M inside the slab S may be solidified while flowing to suppress or prevent the segregation from occurring within the slab S.

[0128] The magnetic field generated from the second magnetic field generating part 150b may vary depending on the intensity of the current according to the power or voltage applied to the second magnetic field generating part 150b. Thus, the power or voltage supplied to the second magnetic field generating part 150b is controlled so that the current having the target intensity flows through the coil of the second magnetic field generating part 150b. The current may flow through the second magnetic field generating part 150b as described above, and the magnetic field may be generated in the second magnetic field generating part 150b. In addition, the molten steel inside the slab S may be made to flow by the magnetic field generated from the second magnetic field generating part 150b, and the mold flux may be suppressed or prevented from being mixed into the molten steel M.

[0129] If the current flowing to the second magnetic field generating part 150b is too small, the molten steel M inside the slab S may not flow or may flow insufficiently to cause the segregation inside the slab S. Conversely, if the current flowing to the first magnetic field generator 150a is too large, the flow rate of the molten steel M may be very fast to cause the mold flux to be mixed into the molten steel. The mold flux mixed into the molten steel M may cause the cracks in the slab S as the impurity. Thus, when generating the magnetic field using the second magnetic field generating part 150b, the current flowing in the second magnetic field generating part 150b is controlled by controlling the intensity of the power or voltage supplied to the second magnetic field generating part 150b so that the magnetic field capable of causing the molten steel M inside the slab S to flow at an appropriate velocity is generated.

[0130] In addition, if a time taken to generate the magnetic field by operating the second magnetic field generating part 150b is too short, the slab S may not be sufficiently solidified. That is, the casting may be completed in the state in which the non-solidified molten steel existing inside the slab S remains. Conversely, if the time taken to generate the magnetic field by operating the second magnetic field generating part 150b is too long, the magnetic field may be applied to an area on which there is no non-solidified molten steel, i.e., up to the lower portion of the slab. That is, the magnetic field may be applied to an area on which the application of the magnetic field is unnecessary. Thus, when generating the magnetic field using the second magnetic field generating part 150b, a time taken to apply the magnetic field to the slab is controlled so that the non-solidified molten steel existing inside the slab S is sufficiently solidified while the magnetic field is not unnecessarily applied to the solidified area.

[0131] As explained above, the slab drawn to the lower side of the mold 120 has a larger surface area of the non-solidified area as it goes upward. Here, the lowermost end of the slab S is an area that is drawn first from the mold 120, and the uppermost end of the slab S is an area that is drawn last from the mold 120. Thus, in the slab drawn to the lower side of the mold 120, the uppermost end of the slab S refers to a distal end of the slab S.

[0132] When the slab S drawn out to the lower side of the mold 120 is solidified, the non-solidified molten steel at the distal end of the slab is solidified firstly compared to the molten steel M at the lower side of the distal end of the slab. As described above, the molten steel M at the distal end of the slab is solidified firstly compared to the molten steel M at the lower end of the slab, resulting in pipe defects caused by solidification and shrinkage at the distal end of the slab. In addition, since the distal end of the cast in which the pipe defects occur may not be used as a product, the distal end of the cast in which the pipe defects occur is cut off after the casting is completed. Thus, there is a problem that casting yield decreases by a length of the cut distal end, and since the cut distal end is discarded, there is a problem in terms of cost as the material equivalent to the cut length is consumed for each operation.

[0133] Thus, in order to suppress or prevent the solidification and shrinkage of the distal end of the slab, the heating part 160 capable of heating the distal end of the slab is installed at the lower side of the mold 120. The heating part 160 is disposed on the lateral outer side of the mold 120 between the mold 120 and the second magnetic field generating part 150b. More specifically, the heating part 160 may be disposed to face the first magnetic field generating part 150a and the second magnetic field generating part 150b in the vertical direction at the lower side of the mold 120. In addition, the heating part 160 may be disposed at the lateral outer side of the cooling part 140. For this, the heating part 160 may be provided in a hollow shape. This heating part 160 may be installed on the body and inside the body and may include a heating element that generates heat by the applied power.

[0134] When the heating part 160 operates, and heat is generated, the heat heats or inductively heats the molten steel M inside the distal end of the slab. This can delay the solidification of the molten steel M at the distal end of the slab. That is, the molten steel at the distal end of the slab may be controlled to be solidified later than the molten steel at the lower side of the slab. Thus, it is possible to suppress or prevent the occurrence of the pipe defects at the distal end of the slab.

[0135] The heat generated in the heating part 160 may vary depending on the power or voltage applied to the heating part 160. Thus, the power applied to the heating part 160 is controlled so that the heat of the target temperature is generated in the heating part 160. The power may be applied to the heating part 160 as described above to generate heat, and the distal end of the slab may be heated. Thus, the solidification and shrinkage of the distal end of the slab may be suppressed or prevented.

[0136] If the power supplied to the heating part 160 is too small, the temperature of the heat generated from the heating part 160 is low, so that the molten steel M at the distal end of the slab is not be sufficiently heated, and as a result, the solidification and shrinkage may occur at the distal end of the slab, resulting in the pipe defects. Conversely, if the power supplied to the heating part 160 is too large, the temperature of the heat generated from the heating part 160 is too high, so there is a problem of remelting by heating the lower portion of the distal end in addition to the distal end of the slab. Thus, when heating the distal end of the slab using the heating part 160, the power supplied to the heating part 160 is controlled so that the distal end of the slab is sufficiently heated, and thus, the power is controlled to prevent the lower portion of the distal end of the slab from being heated and re-melted.

[0137] In addition, if the time taken to heat the distal end of the slab using the heating part 160 is too short, the distal end of the slab may not be sufficiently heated. Thus, the molten steel M at the distal end of the slab may be solidified firstly compared to the molten steel M at the lower portion of the slab to cause the solidification and shrinkage at the distal end of the slab, resulting in the pipe defects. Conversely, if the time taken to heat the distal end of the slab using the heating part 160 is too long, there is no need to heat the distal end of the slab using the heating part because the molten steel at the distal end of the slab has already completed solidification. Thus, when heating the distal end of the slab using the heating part 160, the heating time of the distal end of the slab is controlled so that the distal end of the slab is sufficiently heated while not heating the distal end of the slab that has already completed solidification.

[0138] In this embodiment, when solidifying the molten steel inside the mold 120, the first magnetic field generating part 150a is used to allow the molten steel inside the mold 120 to flow. Thus, it is possible to suppress or prevent the temperature of the molten steel M inside the mold 120 from decreasing, and thus, it is possible to suppress or prevent the segregation from occurring in the slab S.

[0139] In addition, when the slab drawn to the lower side of the mold 120 is solidified, the non-solidified molten steel inside the slab S is made to flow using the second magnetic field generating part 150b disposed at the lower side of the mold 120. Thus, when the cast steel is solidified outside the mold 120, the non-solidified molten steel inside the slab S may be solidified while flowing without stagnating. Thus, it is possible to suppress or prevent the segregation in the slab S due to the stagnation of the non-solidified molten steel.

[0140] In order to prevent the segregation from occurring in the ingot for manufacturing the press roll, the electro slag remelting (ESR) method was used. Hereinafter, the electro slag remelting (ESR) method will be briefly explained. First, the molten steel of which the upper portion is controlled for manufacturing the press roll is solidified to be manufactured into the electrode rod. Then, arc was generated using the manufactured electrode rod to remelt the electrode rod, and the remelted molten steel was solidified by dropping the molten steel downward in the shape of a droplet to manufacture an ingot again. Although this process suppresses or prevents the segregation in the ingot, it requires a complex process of manufacturing the ingot as described above into the electrode rod, remelting the electrode rod, and re-solidifying the remelted molten steel. Thus, there is a significant problem in terms of the time and cost consumed in the process to suppress the segregation.

[0141] On the other hand, in this embodiment, the first and second magnetic field generating parts M provided in the casting device 100 are used to allow the molten steel in the mold 120 and the non-solidified molten steel M in the slab S to flow, thereby suppressing or preventing the segregation from occurring inside the slab S. Thus, compared to manufacturing the ingot using the conventional electro slag remelting (ESR) method, when manufacturing the slab using the method according to this embodiment has an effect of simplifying the process and reducing time and cost. In addition, compared to the intensity of the power applied to generate the arc in the electrode rod in the conventional electro slag remelting (ESR) method, the intensity of the power applied to operate the first and second magnetic field generating parts in an embodiment is small. Thus, compared to manufacturing the ingot using the conventional electro slag remelting (ESR) method, there is an effect of reducing an amount of electric energy consumed when manufacturing the slab using the method according to an embodiment.

[0142] In addition, in an embodiment, the heating part 160 may be heated using the distal end of the slab, and thus, the solidification of the molten steel M at the cast end may be delayed. Thus, it is possible to suppress or prevent the occurrence of the solidification and shrinkage at the distal end of the slab, and thus, to suppress the occurrence of the pipe defects at the distal end of the slab.

[0143] Returning to FIG. 3, the moving part 180 and the rotating part 190 will be described.

[0144] The moving part 180 is a means for pushing the slab S drawn to the lower side of the mold 120 from one side to move the slab S toward the rotating part 190. That is, the moving part 180 is drawn downward from the mold 120, is vertically supported on the support part 171, and pushes the solidified slab S to transfer the slab S to the rotating part 190. The moving part 180 may be, for example, a means driven horizontally with respect to the ground, and for example, the moving part may include a hydraulic or pneumatic cylinder.

[0145] The rotating part 190 receives the slab S separated from the support part 171 by the moving part 180 to rotate the received slab S. That is, the rotating part 190 receives the slab supported in a direction perpendicular to the ground on the support part 171 to rotate the received slab parallel to the ground.

[0146] The rotating part 190 is disposed horizontally to face the moving part 180. The rotating part 190 may include a rotating table 191 that receives and supports the slab S separated from the support part 171 to rotate and a rotating member 192 connected to the rotating table 191 so that the rotating table 191 rotates.

[0147] The rotating table 191 includes a first table 191-1 that extends in one direction and a second table 191-2 that extends in a direction intersecting the extension direction of the first table 191-1 and has one end connected to the first table 191-1.

[0148] In the slab S supported vertically on the support part 171, the first table 191-1 is a means for supporting a side surface of the slab S, and the second table 191-2 is a means for supporting a bottom surface of the slab S. The first table 191-1 may be provided with a longer extension length than the second table 191-2. In addition, it is preferable that the second table 191-2 is provided to have a surface area equal to or greater than that of the bottom surface of the slab S. A plurality of rotatable rollers 191-3 may be installed on the first table 191-1, and the plurality of rollers 191-3 are arranged in a direction in which the first table 191-1 extends.

[0149] One end of the first table 191-1 and one end of the second table 191-2 may be interconnected by the rotating member 192. In addition, the rotating table 191 may rotate or be tilted by the rotating member 192 as illustrated in (d) of FIG. 3. That is, when the slab S is supported vertically on the support part 171, the first table 191-1 of the rotating part is disposed perpendicular to the ground, and the second table 191-2 is arranged parallel to the ground. In addition, when the slab S supported on the support part 171 is transferred to the rotating part 190, the rotating part 190 rotates or is tilted. That is, the first table 191-1 of the rotating part 190 rotates to be parallel to the ground, and the second table 191-2 rotates to be perpendicular to the ground.

[0150] In the above description, the manufacturing of the slab using the vertical casting device has been described. However, the present invention is not limited thereto, and the casting device having various forms, which includes a first magnetic field generating part 150a installed at the lateral outer side of the mold 120, a second magnetic field generating part 150b installed at the lower side of the mold 120, and a heating part 160, may be used to manufacture the slab S.

[0151] (a) to (c) of FIG. 4 are process diagrams sequentially illustrating the method for manufacturing the press roll using the slab manufactured by the method according to an embodiment of the present invention.

[0152] When the slab S is manufactured, the manufactured slab S is heated and softened (S300). For this, the slab S is charged into a first heating device 200. The first heating device 200 may include a steel converter having an internal space and a heater for heating the steel converter. Here, the heater is installed, for example, inside or outside a wall forming the steel converter and may include a heating element capable of generating heat by applied power. As another example, the heater may include a burner that burns fuel to generate heat.

[0153] In heating the slab S in the first heating device 200, the slab is heated to a temperature of 1,100 C. to 1,250 C., preferably 1,140 C. to 1,240 C. Here, instead of charging the slab into the steel converter heated to 1,100 C. to 1,250 C., the slab is charged into the steel converter controlled to a lower temperature, and then, the temperature inside the steel converter gradually increases to 1,140 C. to 1,240 C.

[0154] To explain more specifically, first, the inside of the steel converter is heated to 250 C. to 350 C. (first temperature) and maintained at the first temperature for 3 hours to 5 hours, preferably 3 hours and 30 minutes to 4 hours and 30 minutes. Thereafter, the inside of the steel converter is heated to 450 C. to 550 C. (second temperature) and maintained at the second temperature for 5 to 7 hours, preferably 5 hours and 30 minutes to 6 hours and 30 minutes. Next, the inside of the steel converter is heated to 650 C. to 750 C. (third temperature) and maintained at the third temperature for 3 to 5 hours, preferably 3 hours and 30 minutes to 4 hours and 30 minutes. Next, the inside of the steel converter is heated to 1,100 C. to 1,250 C. (fourth temperature) and maintained at the fourth temperature for 14 hours to 18 hours, preferably 15 hours and 30 minutes to 17 hours and 30 minutes. In addition, it is preferable that each of a time taken to heat the inside of the steel converter from the first temperature to the second temperature (first heating time) and a time taken to heat the inside of the steel converter from the second temperature to the third temperature is 3 hours to 4 hours, and it is preferable that a time taken to heat the inside of the steel converter from the third temperature to the fourth temperature is 9 hours to 10 hours. As described above, the heating of the inside of the steel converter to the first to fourth temperatures and maintaining the inside of the steel converter at the first to fourth temperatures means that the temperature of the slab charged into the inside of the steel converter is heated to the first to fourth temperatures, and the temperature of the slab is maintained at the first to fourth temperatures.

[0155] The reason for gradually rising the temperature of the slab S to 1,100 C. to 1, 250 C. as described above is that the slab S containing a high amount of chromium of 4.5 wt % or more has high hardness, and thus cracks easily occur during heating and cooling. Thus, in the increasing of the temperature of the slab S to 1,100 C. to 1, 250 C. in the manner described above, the cracks may be suppressed or prevented from occurring in the slab S.

[0156] When the process of heating or thermally processing the slab S in the first heating device 200 is completed, the slab S is forged using a forging device 300 to manufacture the press roll. The forging device 300 may include, for example, an upper press-down part and a lower press-down part, which are spaced apart from each other in the vertical direction, and a driving part connected to the upper press-down part and the lower press-down part to apply predetermined force. Here, the driving part may be a cylinder driven hydraulically or pneumatically.

[0157] Hereinafter, a method for forging the slab S using the forging device 300 is described. First, the slab is disposed between the upper and lower press-down parts of the forging device 300. In addition, the driving part operates to allow the upper press-down part to descend and allow the lower press-down part to ascend, thereby reducing a distance between the upper press-down part and the lower press-down part. Next, the driving part operates to apply more downward force to the upper press-down part and more upward force to the lower press-down part. Here, as time passes after the upper and lower press-down parts are in contact with the slab S, the force with which the upper and lower press-down parts press the slab increases. Thus, the slab S disposed between the upper press-down part and the lower press-down part is pressed to reduce a thickness of the slab. Then, when the thickness of the slab is reduced to a target thickness, the upper and lower press-down parts are separated from the slab. Here, the target thickness may be 250 mm to 350 mm. In addition, this process is repeated multiple times.

[0158] As described above, force may be applied by allowing the upper and lower press-down parts in contact with the slab S, and when the thickness of the slab S is reduced to the target thickness, the upper and lower press-down parts are separated from the slab S. This series of processes is defined as one pass. Here, in an embodiment, the target thickness is set to 250 mm to 350 mm. In addition, the reduction thickness of the slab S per pass is controlled to 250 mm to 350 mm. In addition, the above-described one pass is repeated multiple times to manufacture the press roll.

[0159] If the reduction thickness of the slab S per pass is less than 250 mm, the force may not be sufficiently applied to the center area of the thickness direction of the slab S, and thus, pores in the center area of the slab may not be removed. In addition, if the pores at the center of the thickness direction of the slab S are not removed, this may be a factor that reduces the hardness of the press roll. Conversely, if the thickness of the slab S is reduced by more than 350 mm per pass, the cracks may occur in the surface of the slab to cause defects in the press roll.

[0160] When the forging is completed, and the press roll 141 is manufactured, the press roll 141 is heated to remove hydrogen (H.sub.2). For this, the press roll 141 is loaded into the second heating device 400, and the press roll 141 is heated using the second heating device 400. Here, the second heating device 400 may be the same as or different from the first heating device 200 described above.

[0161] When heating the press roll 141 using the second heating device 400, the press roll 141 is heated to 200 C. to 400 C., more preferably 250 C. to 350 C. In addition, the temperature of the press roll 141 is maintained at a temperature of 200 C. to 400 C. for 48 hours or longer, more preferably 48 hours or longer and 55 hours or shorter.

[0162] As described above, the press roll 141 is heated to 200 C. to 400 C., e preferably 250 C. to 350 C., and maintained at this temperature for 48 hours or more, while hydrogen (H.sub.2) contained or remaining in the press roll 141 is diffused to become a hydrogen gas, and the hydrogen gas is discharged to the outside. Thus, the hydrogen content in the press roll 141 decreases. Here, it is desirable to make the hydrogen content in the press roll 141 at 0.0002 wt % or less (0 wt or more), and this may be controlled by controlling at least one of the temperature or heat processing time for heating the press roll 141.

[0163] The hydrogen contained inside the press roll 141 is easily accumulated in inclusions and segregation contained in the press roll 141, and this may become a factor in causing internal cracks in the press roll 141 when the press roll 141 is actually used. Here, the actual use of the press roll may mean a case in which the manufactured press roll 141 is used to roll a rolled object, for example, the slab S.

[0164] Thus, the manufactured press roll 141 is heated at 200 C. to 400 C. for 48 hours or more to remove hydrogen so that the content of hydrogen contained in the press roll 141 becomes 0.0002 wt % or less. Here, when the content of hydrogen contained in the press roll 141 is 0.0002 wt % or less, the cracks do not occur inside the press roll 141 due to the hydrogen. In addition, since it is realistically difficult to completely remove the hydrogen from the press roll 141, the hydrogen content is set to 0.0002 wt % or less.

[0165] Hereinafter, a method for manufacturing the press roll according to an embodiment of the present invention will be collectively described with reference to FIGS. 1 to 4. Here, any content that overlaps the above-described contents will be omitted or briefly explained.

[0166] First, the molten steel M for manufacturing the press roll is prepared (S100). For this, first, the molten steel M is charged into a stainless steelmaking steel converter 11. In addition, as illustrated in (a) of FIG. 2, a chromium (Cr) alloy steel is input into the steel converter 11. Here, an input amount of chromium (Cr) alloy steel is controlled so that the content of chromium (Cr) in the molten steel inside the steel converter 11 is 4.5 wt % to 5.5 wt %.

[0167] Next, oxygen is blown to the inside of the steel converter 11 using a lance 12 as illustrated in (b) of FIG. 2. That is, oxygen is blown to the molten steel M in the steel converter 11 to remove carbon (C) and phosphorus (P). Here, the content of carbon (C) in the molten steel M is set to 0.75 wt % to 0.95 wt %, and the content of phosphorus (P) is set to 0.025 wt % or less.

[0168] Next, as illustrated in (c) of FIG. 2, a deoxidizer is input into the inside of the steel converter 11 to remove oxygen (O) from the molten steel M. Here, at least one of an alloy containing silicon (Si) or an alloy containing aluminum (Al) may be used as a deoxidizer. In addition, deoxidation is performed so that the content of oxygen (O) in the molten steel M becomes 0.001 wt % or less.

[0169] When the deoxidation is completed, a reducing agent, i.e., the alloy steel containing silicon (Si), is input into the steel converter 11 as illustrated in (d) of FIG. 2. Thus, chromium oxide contained in slag SL and silicon (Si) contained in the reducing agent react with each other. Thus, chromium oxide contained in slag SL is reduced to chromium (Cr), and the generated chromium (Cr) is absorbed or supplied to the molten steel M. Thus, the content of chromium (Cr) in the molten steel M increases. Here, the input amount of reducing agent is controlled so that the content of chromium (Cr) in the molten steel M is 4.5 wt % to 5.5 wt %.

[0170] When the chromium reduction is completed, the molten steel M inside the steel converter 11 is discharged into the ladle 20 as illustrated in (e) of FIG. 2. Here, the molten steel M is discharged into the ladle 20 for making carbon steel.

[0171] Then, the ladle 20 is moved to the temperature control device 30, i.e., the ladle furnace 30, and the ladle 20 is connected to the cover 32 of the ladle furnace 30 ((f) of FIG. 2). Next, a desulfurizing agent is input to the ladle 20 to remove sulfur (S) from the molten steel. Here, the content of sulfur (S) in the molten steel is set to 0.015 wt % or less.

[0172] When the desulfurization is completed, the molten steel M contained in the ladle 20 is sampled to measure the contents of carbon (C) and chromium (Cr). In addition, depending on the measured carbon (C) and chromium (Cr) contents, the carbon (C) and chromium (Cr) contents in the molten steel M are controlled. Here, depending on the measured carbon (C) and chromium (Cr) contents, at least one of a carbonizing agent or alloy steel containing chromium (Cr) is input, or solid oxygen such as iron ore is input, so that the measured carbon (C) content of the molten steel becomes 0.75 wt % to 0.95 wt %, and the chromium (Cr) content becomes 4.45 wt % to 5.5 wt %.

[0173] After the controlling of the carbon (C) and chromium (Cr) compositions are completed, the molten steel M is heated to increase its temperature. That is, arc and heat are generated from the electrode rod 31 to heat the molten steel contained in the ladle 20. Here, the temperature of the molten steel has to be between 1,560 C. and 1,600 C.

[0174] When the temperature of the molten steel M reaches 1,560 C. to 1,600 C., the ladle 20 is separated from the ladle furnace 30. In addition, the ladle 20 is moved to a vacuum processing device, i.e., an RH device 40, as illustrated in (g) of FIG. 2. In addition, the ladle 20 is disposed below a pair of reflux tubes 42a and 42b, and the pair of reflux tubes 42a and 42b are connected to each other to be sealed. Next, the pump operates to reduce a pressure inside the vessel 41 to a vacuum pressure of, for example, 0.2 torr or less, and then the ladle 20 ascends to immerse the pair of reflux tubes 42a and 42b into the molten steel inside the ladle 20. Thereafter, an argon (Ar) gas is blown to the pair of reflux tubes 42a and 42b to circulate the molten steel inside the vessel 41. The circulating molten steel is exposed to a vacuum atmosphere inside the vessel 41, and nitrogen (N.sub.2) and hydrogen (H.sub.2) gases in the molten steel are discharged outside the vessel 41. Thus, the contents of nitrogen (N) and hydrogen (H.sub.2) in the molten steel are reduced. Here, the content of nitrogen (N.sub.2) is made to be 0.015 wt % or less, and the content of hydrogen (H.sub.2) is made to be 0.0005 wt % or less.

[0175] When performing the processes (a) to (g) of FIG. 2 described above, the molten steel M for manufacturing the press roll is prepared.

[0176] When the molten steel M is prepared, the ladle 20 is moved to the tundish 110 of the casting device 100, and the molten steel M is supplied to the tundish 110 to initiate casting. For this, first, a lower opening of the mold 120 is closed using a support part 171 of the casting device 100. In addition, as illustrated in (a) of FIG. 3, the molten steel M of the tundish 110 is supplied to the mold 120 (S210). Thus, the molten steel M supplied to the mold 120 is solidified to begin to be solidified from an upper portion of the support part 171 (casting process) (S210). In addition, while continuously supplying the molten steel M from the tundish 110 to the mold 120, the driving part 172 operates to allow the support part 171 to descend. Thus, as illustrated in (b) of FIG. 3, the slab S inside the mold 120 is gradually drawn downward into the mold (S220).

[0177] As described above, while continuously supplying the molten steel M to the mold 120, the first magnetic field generating part 150a disposed at the outside of the mold 120 operates to generate a magnetic field. Thus, the magnetic field generated from the first magnetic field generating part 150a is applied to the inside of the mold 120, and as a result, the molten steel M inside the mold 120 flows due to the magnetic field. Thus, it is possible to suppress or prevent a temperature in the molten steel M inside the mold 120 from decreasing, and to suppress or prevent the mold flux on a bath surface of the molten steel M from being mixed into the molten steel.

[0178] While continuously supplying the molten steel into the mold 120, the driving part 172 operates to allow the support part 171 to gradually descend to the lower side of the mold 120. Here, a speed at which the support part 171 descends downward is controlled to be 0.04 m/min or less, and more specifically, controlled to be 0.01 m/min to 0.04 m/min. That is, the casting speed is controlled to 0.04 m/min or less, and more specifically, to 0.01 m/min to 0.04 m/min.

[0179] When the slab is drawn to the lower side of the mold 120 as illustrated in (b) of FIG. 3, the drawn slab S is secondarily solidified by cooling water injected from a nozzle of a cooling part 140 (solidification process) (S220). In addition, the tundish 110 continuously supplies the molten steel M to the mold, and the support part 171 continuously descends, and thus, a length of the slab drawn to the lower side of the mold 120 gradually increases. When the slab of the target length is drawn to the lower side of the mold 120, the supply of the molten steel to the mold 120 is stopped. In addition, as illustrated in (c) of FIG. 3, the uppermost end of the slab, that is, a distal end, is drawn downwards of the mold (S230). In addition, when the slab is drawn downward to the distal end of the mold, the second magnetic field generating part 150b operates to generate a magnetic field. Thus, the non-solidified molten steel existing inside the slab S drawn to the lower side of the mold 120 flows due to the magnetic field generated from the second magnetic field generating part 150b. This may suppress or prevent the mold flux from being mixed into the molten steel.

[0180] In the above, it was explained that when the slab S is drawn downward to the distal end of the mold, the second magnetic field generating part 150b operates to generate the magnetic field. However, it is not limited thereto, and when the slab begins to be drawn toward the lower side of the mold 120, the second magnetic field generating part 150b may operate to generate the magnetic field.

[0181] When the distal end of the slab S is drawn toward the lower side of the mold, the heating part 160 operates to heat the distal end of the slab. Thus, the solidification of the non-solidified molten steel M at the distal end of the slab S is delayed. This may suppress or prevent solidification and shrinkage at the distal end of the slab and prevent remelting of areas other than the distal end of the slab.

[0182] When the solidification is completed up to the distal end of the slab S, the moving part 180 operates as illustrated in (d) of FIG. 3 to push the slab S placed vertically on the support part 171 toward the rotating part 190. Thus, the slab S of the support part 171 is transferred to the rotating part 190. Here, the side of the slab S is supported on a first table 191-1 of the rotating part 190, and a bottom surface of the slab is supported on a second table 191-2. Next, the rotating part 190 rotates, i.e., is tilted (S240). More specifically, the rotating part 190 rotates so that the first table 191-1 is parallel to the ground, and the second table 191-2 is horizontal to the ground. Thus, the slab S rotates to be placed horizontally with respect to the ground. The slab placed horizontally on the rotating part 190 is transferred to the next process.

[0183] When the slab S is manufactured, the slab is heated using the first heating device 200 (S300). Here, a temperature inside the steel converter in which the slab S is charged gradually increases to 1,140 C. to 1,240 C. (target temperature) to heat the slab. To explain more specifically, first, the slab S is heated to 250 C. to 350 C. (first temperature) and maintained for 3 hours to 5 hours. Thereafter, the slab S is heated to 450 C. to 550 C. (second temperature) and maintained for 5 hours to 7 hours. Next, the slab S is heated to 650 C. to 750 C. (third temperature) and maintained at the third temperature for 3 hours to 5 hours. Next, the slab S is heated to the target temperature of 1,100 C. to 1,250 C. (fourth temperature) and maintained for 14 hours to 18 hours. As described above, the temperature gradually increases from the first to fourth temperatures as described above while heating the slab S to 1,100 C. to 1,250 C. (the fourth temperature), it is possible to suppress or prevent the cracks in the cast steel containing a large amount of chromium (Cr) of 4.5 wt % or more from occurring due to the heat.

[0184] When the process of heating the slab S in the first heating device 200 is completed, the slab is forged using the forging device 300 to manufacture the press roll (S400). Here, the reduced thickness of the slab S per pass is set to 250 mm to 350 mm, and this is repeated multiple times to manufacture the press roll 141.

[0185] When the press roll R is manufactured, the press roll 141 is heated using the second heating device 400 to remove hydrogen (H.sub.2). Here, the press roll 141 is heated at a temperature of 200 C. to 400 C. for 48 hours or more. Thus, the hydrogen content in the press roll 141 may be controlled to 0.0002 wt % or less (0 wt or more).

[0186] Table 1 is an evaluation table for the press roll manufactured by the method according to an embodiment of the present invention. In order to evaluate quality, a portion of the press roll manufactured by the method according to the example were cut and used as a test specimen.

[0187] Ultrasonic testing (UT) quality evaluation is a quality evaluation method used to detect the internal defects such as the pores and the cracks. That is, it is an evaluation method that detects the presence and occurrence of the pores and cracks inside the specimen by transmitting ultrasonic waves into the specimen and using an energy amount of ultrasonic waves reflected from discontinuities inside the specimen and the propagation time of the ultrasonic waves.

[0188] Macro quality evaluation is intended to determine the presence and amount of coarse organization. After etching the specimen using corrosion for the evaluation, the internal organization of the specimen was observed using an optical microscope.

[0189] The purpose of inclusion quality evaluation is to determine quantity and size of inclusions present in the specimen. After polishing a portion of the specimen for the evaluation, the size and quantity of the inclusions were detected using the optical microscope.

[0190] Hardness was measured using a Brinell hardness tester as the hardness of the specimen.

TABLE-US-00001 TABLE 1 Evaluation Quality evaluation items results Evaluation standard UT(Ultrasonic testing) quality Pass JIS Grade 1 inspection evaluation standard (Presence and amount of pores and cracks) Macro quality evaluation Pass ASTM S1. R1. C2 (Presence and amount of coarse standard organization) Quality evaluation of inclusion Pass ASTM E45 inspection (content of inclusion) standard Hardness quality evaluation Pass HB255 or less

[0191] Referring to Table 1, the specimen manufactured by the method according to an embodiment has acceptable UT quality, macro quality, inclusion quality, and hardness quality. That is, the press roll manufactured by the method according to an embodiment passed the evaluation results for UT quality, macro quality, inclusion quality, and hardness quality. As a result, it is seen that the press roll using the method according to an embodiment is manufactured, and thus, the press roll having a small number of pores, cracks, and inclusions, a uniform organization, and high hardness may be manufactured.

[0192] (a) and (b) of FIG. 5 are views illustrating results obtained by confirming whether or more coarse segregation occurs by cutting and etching a cross-section of the press roll.

[0193] Here, (a) of FIG. 5 is a cross-section of the press roll manufactured by the method according to an embodiment, and (a) of FIG. 5 is a cross-section of the press roll manufactured by a method according to a Comparative Example. In addition, the press roll manufactured by the method according to Comparative Example is a press roll manufactured by an electro slag remelting (ESR) method.

[0194] When comparing (a) and (b) of FIG. 5, no coarse segregation occurred in either of the press rolls according to an embodiment or Comparative Example. That is, the press roll according to an embodiment has a uniform organization equivalent to that of the press roll according to Comparative Example. As a result, it is seen that even if the press roll is manufactured using a method similar to that according to an embodiment without using the conventional electro slag remelting (ESR) method, the occurrence of segregation is sufficiently suppressed. That is, even if the slab is casted by using the first and second magnetic field generating parts to allow the molten steel to flow in the casting process as in an embodiment, it is seen that the occurrence of the segregation is suppressed to the same level as the conventional electro slag remelting (ESR) method.

[0195] In addition, compared to the conventional electro slag remelting (ESR) method, the process of suppressing the segregation by using the first and second magnetic field generating parts in the casting process as in an embodiment is simplified and takes a short time. That is, in the case of an embodiment, while the occurrence of the segregation is suppressed to a level equivalent to that of the prior art, and also, there is an advantage in that the process required for the occurrence of the segregation is short, and the process time is short.

INDUSTRIAL APPLICABILITY

[0196] According to the embodiments of the present invention, the high-chromium (Cr) molten steel may be manufactured by using the steel converter that is used in the steelmaking process of other steel types, without contaminating the steel converter. In addition, when raising the temperature of high-chromium (Cr) molten steel or performing degassing to discharge gas, the present invention enables efficiently increasing the temperature of the molten steel and improves degassing efficiency.