Method for manufacturing forged steel roll

Abstract

A method for manufacturing a forged steel roll comprises: casting, by the ESR method, a steel ingot which contains, by mass %, C: 0.3% or more, Si: 0.2% or more, Cr: 2.0-13.0% and Mo: 0.2% or more, and further contains Bi at 10-100 ppm by mass; and forging the steel ingot to manufacture the roll. According to this method, since freckle defects can be sealed near the center of the steel ingot, the roll can be stably used over a long period of time.

Claims

1. A method for manufacturing a forged steel roll, comprising: casting, by an electroslag remelting method, a steel ingot which contains, by mass %, C: 0.3% or more, Si: 0.2% or more, Cr: 2.0-13.0% and Mo: 0.2% or more, and further contains Bi at 10-38 ppm by mass; and forging the steel ingot into a roll.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a longitudinal sectional view of a general ingot obtained by the ingot-making method.

(2) FIG. 2 is a longitudinal sectional view of a general steel ingot obtained by the ESR method.

(3) FIG. 3 is a schematic view showing, in the method for manufacturing a forged steel roll of the present invention, one example of casting of a steel ingot used as the material by the ESR method.

(4) FIG. 4 is a view showing the relationship between Bi content and dendrite primary arm spacing.

(5) FIG. 5 is a view showing the relationship between the radial distance from steel ingot surface and the dendrite primary arm spacing.

(6) FIG. 6 is a view showing the relationship between the radial distance from steel ingot surface and the value of Ra/Ra.sub.0.

DESCRIPTION OF EMBODIMENTS

(7) The method for manufacturing a forged steel roll of the present invention is characterized by: casting, by the ESR method, a steel ingot which contains C: 0.3% or more, Si: 0.2% or more, Cr: 2.0-13.0% and Mo: 0.2% or more, and further contains Bi at 10-100 ppm; and forging the steel ingot to manufacture the roll.

(8) The reasons to specify the method for manufacturing a forged steel roll of the present invention as described above and preferred embodiments thereof will be then described.

(9) 1. Casting of Steel Ingot by ESR Method

(10) FIG. 3 is a schematic view showing, in the method for manufacturing a forged steel roll of the present invention, one example of a state for casting a steel ingot used as the material by the ESR method.

(11) As shown in this figure, in the ESR method, a stub 4 is connected by welding to the upper end of a cylindrical consumable electrode 2 that is a base metal of a steel ingot 1, and the electrode is moved down in accordance with the lowering of the stub 4 by a raising and lowering mechanism not shown. A molten slag 7 is held in a casting mold (water-cooled copper mold) 6 within a chamber 5, and energization is performed with the consumable electrode 2 being immersed in the molten slug 7, whereby electricity is carried to the molten slug 7, and the molten slug 7 generates heat. The consumable electrode 2 is successively molten from the lower end by the Joule heat of the molten slug 7. The molten consumable electrode 2 settles out through the molten slug 7 as droplets, and solidifies in layers while being retained as a pool of molten steel 3 within the casting mold 6. The consumable electrode 2 is successively molten up to the upper end, and the molten steel 3 is successively solidified in the casting mold 6, whereby the steel ingot 1 for the forged steel roll is obtained.

(12) In the present invention, since the steel ingot 1 obtained by the ESR method contains a predetermined amount of Bi, the molten steel 3 must be caused to contain Bi in the process of casting by the ESR method. As a method therefor, Bi may be added to the molten steel 3 in a casting stage by the ESR method, or Bi may be added, at a stage prior to the casting by the ESR method or in the stage of producing the consumable electrode 2 that is the base metal by the ingot-making method, to the molten steel of the electrode.

(13) When Bi is added to the molten steel 3 in the casting stage by the ESR method as the former, the addition of Bi can be attained by supplying a Bi wire 8 containing Bi to the molten steel 3 as shown in FIG. 3. Besides, it can be attained also by preliminary welding the Bi wire to the side surface of the consumable electrode 2 along the axial direction.

(14) In the casting by the ESR method, the temperature of molten steel exceeds 1,600 C. On the other hand, the pure boiling point of Bi is only 1,564 C. which falls below the molten steel temperature. Therefore, when the Bi wire is composed of Bi single body, Bi cannot be effectively retained in the molten steel since Bi is evaporated during casting. Thus, the Bi wire is appropriately composed of an alloy of Bi with Ni or the like. The inclusion of Ni or the like leads to an apparent rise of the boiling point of Bi. When NiBi series is selected as the alloy, the content of Bi in the Bi wire is preferably set to 20 to 70 mass % so that Bi is present in a liquid phase state in the molten steel.

(15) When Bi is added to the molten steel in the stage of producing the consumable electrode 2 as the latter, Bi can be added in prospect of the evaporation amount of Bi during the casting by the ESR method.

(16) 2. Component Composition of Forged Steel Roll and Determination Reason Thereof

(17) C: 0.3% or more

(18) C enhances the hardenability of steel. C also enhances the wear resistance of steel by bonding to Cr or V to form a carbide. Therefore, the content of C is set to 0.3% or more, more preferably to 0.5% or more, further preferably to 0.85% or more. The upper limit of the C content is not particularly limited, but when C is excessively contained, sufficient hardness particularly as forged steel rolls for cold rolling cannot be secured, and the toughness and machinability of steel are deteriorated due to uneven distribution of the carbide. Thus, the content of C is preferably set to 1.3% or less, more preferably to 1.05% or less.

(19) Si: 0.2% or more

(20) Si is an element effective for deoxidizing steel. Si also enhances the resistance to temper softening of steel and enhances the hardness of steel by being solid-dissolved in the steel. Therefore, the content of Si is set to 0.2% or more, more preferably to 0.3% or more. Although the upper limit of Si content is not particularly limited, the cleanliness of steel is deteriorated when Si is excessively contained. Thus, the Si content is preferably set to 1.1% or less, more preferably to 0.85% or less, further preferably to 0.6% or less.

(21) Cr: 2.0-13.0%

(22) Cr enhances the hardenability of steel. Cr also enhances the wear resistance of steel by forming a carbide. On the other hand, when Cr is excessively contained, the ductility or toughness of steel is deteriorated due to uneven distribution of the carbide. Thus, the content of Cr is set to 2.0 to 13.0%, more preferably to 2.5 to 10.0%.

(23) Mo: 0.2% or more

(24) Mo enhances the hardenability of steel. Mo also enhances the resistance to temper softening. Therefore, the content of Mo is set to 0.2% or more, more preferably to 0.3% or more. The upper limit of the Mo content is not particularly limited. However, when Mo is excessively contained, the ductility or toughness of steel is deteriorated due to formation of a carbide. Thus, the Mo content is set preferably to 1.0% or less, more preferably 0.7% or less.

(25) Bi: 10-100 ppm

(26) Since C and Si are light elements, freckle defects tend to occur when 0.2% or more Si is contained in high-carbon carbon steel having a C content of 0.3% or more. However, Bi is contained in molten steel in the process of casting by the ESR method to set the content of Bi to 10 ppm or more, as will be described below, whereby the generation of freckle defects can be suppressed. When the content of Bi exceeds 100 ppm, the embrittlement becomes problematic, even if it is a trace amount, in forming a roll by forging. Therefore, the Bi content is set to 100 ppm or less.

(27) The forged steel roll can further contain the following elements, in addition to the above-mentioned essential elements.

(28) Mn: 0.4-1.5%

(29) Mn enhances the hardenability of steel. Further, Mn is an element effective for deoxidizing steel. When Mn is excessively contained, the crack resistance of steel is deteriorated. Therefore, when Mn is aggressively contained, the content thereof is set to 0.4 to 1.5%.

(30) Ni: 2.5% or less

(31) Ni enhances the toughness of steel. Ni also enhances the hardenability of steel. On the other hand, when Ni is excessively contained, hydrogen cracking tends to occur after heat treatment. Since Ni is an austenite forming element, the hardness of steel is deteriorated when Ni is excessively contained. Therefore, when Ni is aggressively contained, the content of Ni is set to 2.5% or less, more preferably to 0.8% or less.

(32) V: 1.0% or less

(33) V enhances the wear resistance of steel by forming a carbide. However, when V is excessively contained, the ductility or toughness of steel is deteriorated due to formation of the carbide. Therefore, when V is aggressively contained, the content thereof is set to 1.0% or less, preferably to 0.2% or less.

(34) In steel ingots having the above-mentioned composition, the dendrite structure becomes fine by casting by the ESR method. Therefore, in forged steel rolls manufactured by forging these steel ingots as the material, freckle detects are perfectly suppressed, or the freckle defects are sealed near the center of the steel ingots, compared with a case in which no Bi is contained, so that no segregation lines are exposed even when the surface of the forged steel rolls is repeatedly repaired by cutting, and the forged steel rolls can be thus stably used also as recycled rolls.

(35) 3. Effects of Inclusion of Bi

(36) The present inventors found, by the following unidirectional solidification test, that the dendrite structure can be miniaturized to suppress the generation of freckle defects by causing molten steel to contain Bi in the process of casting by the ESR method so that a resulting steel ingot contains a trace amount (10 ppm or more) of Bi.

(37) 3-1. Test Condition

(38) A test was performed for casting of a columnar steel ingot having a diameter of 15 mm and a height of 50 mm by the ESR method. In that regard, steel ingots having Bi contents of 10 ppm, 21 ppm and 38 ppm were produced respectively by adding Bi to molten steels, and a steel ingot free from Bi was also produced without addition of Bi. The cooling rate was set to 5 to 15 C./min in accordance with the condition of real operation.

(39) With respect to each of the obtained steel ingots, spacings each between about 10 primary arms extending substantially in parallel to the axial direction in a longitudinal section passing through the center were measured, and an arithmetic average value thereof was taken as the dendrite primary arm spacing of each steel ingot.

(40) 3-2. Test Result

(41) FIG. 4 is a view showing the relationship between Bi content and dendrite primary arm spacing. In this figure, dendrite primary arm spacing (d) was shown in the vertical axis as the ratio (d/d.sub.B) to dendrite primary arm spacing (d.sub.B) of Bi-free steel ingot. It is found from this figure that as the Bi content is higher, the dendrite primary arm spacing of carbon steel is narrower, and the dendrite structure is finer. This is attributed to that Bi is an element having an effect to reduce the interface energy of solid-liquid interface of the carbon steel, and shows an effect on the miniaturization of dendrite primary arm spacing even if its content is trace. If the Bi content is 10 ppm or more, the generation of freckle defects can be effectively suppressed, as shown in examples to be described later.

(42) 4. Index of Freckle Defect Generation

(43) The present inventors focused attention on the use of Ra number as a index of freckle defect generation. The Ra number is a dimensionless number indicating a convective flow in temperature field, or a product of Pr number (Prandtl number) and Gr number (Grashof number), and is represented by the following equation (1).
Ra=Pr.Math.Gr=g(TsT.sub.)L.sup.3/(1)

(44) In the equation, g [m/s.sup.2]: gravity acceleration, [1/K]: volume expansion coefficient, Ts [K]: object surface temperature, T.sub. [K]: temperature of fluid, [m.sup.2/s]: kinetic viscosity coefficient, [m.sup.2/s]: thermal diffusivity, and L [m]: typical length.

(45) The Ra number is considered physically to be a ratio of buoyancy that is flow-driving force to flow-resisting force, and is proportional to the cube of typical length as shown in the above-mentioned equation (1). If the criticality of freckle defect generation is contemplated, the typical length in the Ra number should be set to the magnitude of micro-segregation between dendrite trees. Since micro-segregation molten steel is filled between dendrite trees in the early state of generation, the magnitude of micro-segregation can be regarded as the dendrite primary arm spacing. Accordingly, the typical length in the Ra number can be set to the dendrite primary arm spacing. Thus, the Ra number can be said to be proportional to the cube of the dendrite primary arm spacing.

(46) As described above, since freckle defects are more likely to be coarsened as the dendrite structure is coarser, the freckle defects are considered to more easily occur as the Ra number is larger. If generation results of freckle defects in actual steel ingots are compared with the Ra number, the Ra number can be taken as an index for the criticality of freckle defect generation. Since the Ra number is proportional to the cube of the dendrite primary arm spacing even if the reduction of the dendrite primary arm spacing by containing a trace amount of Bi in steel ingots is relatively small, the inclusion of Bi in the steel ingots is effective for the reduction in Ra number, and thus extremely effective for suppressing the generation of freckle defects.

EXAMPLES

(47) The effects of the present invention were evaluated by a preliminary test performed actually using steel ingots and a simulation by numerical calculation.

(48) 1. Preliminary Test

(49) A casting test of a steel ingot 800 mm in diameter by the ESR method was performed as the preliminary test. As the object steel, a high-carbon steel of 0.87% C-0.30% Si-0.41% Mn-0.10% Ni-4.95% Cr-0.41% Mo-0.01% V (Bi-free) was adopted. The liquidus-line temperature of this steel is 1460 C., and the solidus-line temperature thereof is 1280 C. As the casting conditions, a molten steel scale of 9 t(ton) and a steel ingot length of 2.3 m were adopted.

(50) As a result, no freckle defects were generated up to a position 133 mm radially inward from the steel ingot surface, and freckle defects were generated on the inner side thereof. Namely, the critical point of freckle defect generation was the position 133 mm radially inward from the steel ingot surface. The dendrite primary arm spacing and Ra number at this freckle defect generation critical point were represented by d.sub.0 and Ra.sub.0, respectively, and used as reference values of the following simulation by numerical calculation.

(51) 2. Simulation by Numerical Calculation

(52) Evaluation conditions of the numerical calculation simulation were set as follows. The object steel has the same composition as the above-mentioned preliminary test of 0.87% C-0.30% Si-0.41% Mn-0.10% Ni-4.95% Cr-0.41% Mo-0.01% V, with the content of Bi being 0 ppm (Bi-free), 10 ppm, 21 ppm, and 38 ppm. The diameter of the object steel ingot was set to 800 mm similarly to the preliminary test.

(53) In the above-mentioned evaluation conditions, the solidification rate and cooling rate of each part of the steel ingot were calculated by radial unidimensional non-steady heat transfer analysis of the steel ingot, and distribution of dendrite primary arm spacings in the radial direction from the surface of the steel ingot was calculated by the following equation (2) (Solidification of Iron and Steel, The Iron and Steel Institute of Japan-Iron and Steel Basic Joint Research, Division of Solidification, 1997, Appendix-4). The equation (2) is an experimental expression of dendrite primary arm spacing d (m) using solidification rate V (cm/min) and temperature gradient G ( C./cm) as parameters in a case that a CrMo steel is adopted.
d=1620V.sup.0.2G.sup.0.4(2)

(54) FIG. 5 is a view showing the relationship between the radial distance from the steel ingot surface and the dendrite primary arm spacing. Dendrite primary arm spacing (d.sub.B) in the Bi-free case, shown in this figure, was calculated from the above-mentioned equation (2). Dendrite primary arm spacing (d) in the Bi-containing case was calculated by multiplying the ratio (d/d.sub.B) of dendrite primary arm spacing with respect to each Bi content (10 ppm, 21 ppm and 38 ppm) shown in the above-mentioned FIG. 4 by the value of d.sub.B which was calculated from the equation (2).

(55) FIG. 6 is a view showing the relationship between the radial distance from the steel ingot surface and the value of Ra/Ra.sub.0. With respect to the Ra number (Ra) in each Bi content, Ra/Ra.sub.0 can be said to be the cube of d/d.sub.0, as shown in the following equation (3) derived from the above-mentioned equation (1). The Ra/Ra.sub.0 shown in this figure was calculated based on the equation (3).
Ra/Ra.sub.0=(d/d.sub.0).sup.3(3)

(56) In the equation, Ra/Ra.sub.0 is the ratio of Ra number (Ra) in each Bi content to basic Ra number (Ra.sub.0 determined in the above-mentioned preliminary test), and d/d.sub.0 is the ratio of dendrite primary arm spacing d of each Bi-containing steel ingot to dendrite primary arm spacing d.sub.0 at freckle defect generation critical point of the Bi-free steel ingot.

(57) It is found from the above-mentioned FIG. 5 that the dendrite primary arm spacing d.sub.0 at freckle defect generation critical point of the Bi-free steel ingot is about 400 m. In the inside of the steel ingot in which the dendrite primary arm spacing d is larger than d.sub.0, freckle defects are generated. On the other hand, when Bi is contained in trace amounts (10 ppm, 21 ppm, and 38 ppm), the dendrite primary arm spacing d becomes smaller than the above-mentioned arm spacing at critical point d.sub.0 almost over the whole area extending radially from the steel ingot surface. In this case, or when d/d.sub.0<1 is satisfied, the generation of freckle defects is suppressed. Since d/d.sub.0<1 corresponds to Ra/Ra.sub.0<1 from the above-mentioned equation (3), when rephrased using the Ra number, it can be said that the generation of freckle defects is suppressed in the case satisfying Ra/Ra.sub.0<1.

(58) According to the above-mentioned FIG. 6, since Ra/Ra.sub.0<1 is satisfied up to a rather deep portion (the vicinity of the center of the steel ingot) from the surface of the steel ingot in the Bi-containing case, it was indicated that freckle defects can be sealed not only in the vicinity of the surface of the steel ingot but also near the center, or the generation of freckle defects can be perfectly suppressed.

(59) From the above results, if the content of Bi is 10 ppm or more, the generation of freckle defects can be surely suppressed.

(60) Further, it is supposed from the above-mentioned FIG. 6 that the area where Ra/Ra.sub.0 is smaller than 1 in the Bi-containing case is extended closer to the center side of the steel ingot than in the Bi-free case. Therefore, it is quite possible that the purpose to keep the generation position of freckle defects away from the steel ingot surface as much as possible can be attained in optional sizes of steel ingots. However, since actual cooling of steel ingots is not necessarily performed evenly, but is frequently unevenly performed, it is assumable that the dendrite primary arm spacing is partially extended. From this, it is important to set the Bi content to 10 ppm or more.

(61) In addition, when the same preliminary test and simulation were performed by selecting, as the object steel, a high-carbon steel of 1.30% C-0.24% Si-0.32% Mn-0.51% Ni-9.75% Cr-0.50% Mo-0.11% V, the same results were obtained.

(62) As seen from the above, the possible effect by inclusion of a trace amount (10 ppm or more) of Bi in steel ingots was proved.

(63) As mentioned above, since the embrittlement becomes problematic in formation of rolls by forging if the content of Bi exceeds 100 ppm, the Bi content is up to 100 ppm.

(64) Although the shape of the steel ingot was a cylindrical shape in the above-mentioned examples, it is obvious that the same effects can be obtained even when it is a square columnar shape.

INDUSTRIAL APPLICABILITY

(65) According to the method for manufacturing a forged steel roll of the present invention, freckle defects that are a macro segregation generated during casting of steel ingots can be sealed nearer the center in relation to than the surface of the steel ingot. Therefore, cracks starting from the segregation in heat treatment of the steel ingots can be suppressed, and the rolls can be stably used over a long period of time since segregation lines of freckle defects are hardly exposed even when the roll surface is repaired by cutting for reuse.

REFERENCE SIGNS LIST

(66) 1. Steel ingot 2. Consumable electrode 3. Molten steel 4. Stub 5. Chamber 6. Casting mold 7. Molten slag 8. Bi wire