METHOD FOR CONTROLLING BRITTLE INCLUSIONS IN CORD STEEL

Abstract

A method for controlling brittle inclusions in cord steel. The method includes the following steps: adding alloys at the moment of tapping in a furnace primary smelting stage and adding silicon carbide and synthetic slag to a top of ladle slag at the end of tapping to form slag; adding the alloys in a refining stage and feeding carbon wires; and adding lime, silicon carbide, the synthetic slag, and performing electrification to slag, where the slag composition meets the following conditions: CaO/SiO.sub.2=0.9-1.2, Al.sub.2O.sub.35%, MgO 4-8%, [MnO+T.Math.Fe]2-5%.

Claims

1. A method for controlling brittle inclusions in cord steel, wherein chemical components of the cord steel comprise the following components in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, less than or equal to 0.015% of P, less than or equal to 0.01% of S, less than or equal to 0.0008% of Al in total, less than or equal to 0.003% of N, less than or equal to 0.002% of O, and the balance of Fe and other inevitable impurities; and the method comprises the following steps performed in order: in a furnace primary smelting stage, performing deoxidation alloying on molten steel, paving 0.5-1 kg/t silicon carbide and a 40-60% low-nitrogen carburant at a bottom of a ladle where the molten steel is carried before tapping, sequentially adding alloys for alloying in a tapping process, completing addition of all alloys when 75% of molten steel is tapped, starting to add the remaining 40-60% low-nitrogen carburant at a rate of 200-300 kg/min when 80% of the molten steel is tapped, finishing tapping after the low-nitrogen carburant is completely dissolved in the molten steel, adding the 0.5-1 kg/t silicon carbide to a top of ladle slag, and adding 3-5 kg/t synthetic slag to form slag; in a refining stage, feeding the molten steel subjected to furnace primary smelting to a ladle furnace for refining and detecting the temperature, the chemical components, and the content of the molten steel, adding the alloys according to the detected chemical components and content of the molten steel and feeding carbon wires to adjust the chemical components of the molten steel to meet the following conditions in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, less than or equal to 0.015% of P, less than or equal to 0.01% of S, less than or equal to 0.0008% of Al in total, less than or equal to 0.003% of N, less than or equal to 0.002% of O, and the balance of Fe and other inevitable impurities; and adding lime, the silicon carbide, and the synthetic slag and performing electrification to slag, adjusting the temperature of the molten steel to 1510-1535 C., making the slag composition meet the following conditions in percent by mass: CaO/SiO.sub.2=0.9-1.2, Al.sub.2O.sub.35%, MgO 4-8%, [MnO+T.Math.Fe]2-5%, and the balance of inevitable impurities, then adjusting ladle bottom blowing to a soft stirring pattern, wherein soft stirring time is longer than 20 min, and performing tapping; and in a billet casting stage, conveying the molten steel after tapping in the refining stage to a continuous casting platform to stand for 15 min or more, importing stuffing sand into a slag receiving bucket at the start of ladle casting, and performing protective casting on the molten steel to a tundish to form a continuous casting billet; wherein the alloys comprise ferrosilicon, manganese metal, and chromium iron, and wherein Al in the ferrosilicon is less than or equal to 0.035%, Al in the manganese metal is less than or equal to 0.015%, Al in the chromium iron is less than or equal to 0.020% and C in the chromium iron is less than or equal to 0.15%, and N in the low-nitrogen carburant is less than or equal to 0.015%.

2. The method for controlling brittle inclusions in cord steel according to claim 1, wherein bottom bricks, molten pool bricks and closer bricks of the ladle all are magnesia carbon bricks, and in the magnesia carbon bricks, Al.sub.2O.sub.3 is less than or equal to 3%; slag line bricks and air bricks of the ladle are magnesium-zirconium-carbon bricks, and in the magnesium-zirconium-carbon bricks, Al.sub.2O.sub.3 is less than or equal to 3%; and a long nozzle of the ladle is a long corundum nozzle, an inner wall of the long nozzle is coated with an SiO.sub.2 coating, and the thickness of the SiO.sub.2 coating is 3-8 mm.

3. The method for controlling brittle inclusions in cord steel according to claim 1, wherein the stuffing sand is silicochromium stuffing sand, and wherein Al.sub.2O.sub.3 is less than or equal to 5%.

4. The method for controlling brittle inclusions in cord steel according to claim 1, wherein an inner wall of the tundish is coated with a magnesium coating, and in the magnesium coating, Al.sub.2O.sub.3 is less than or equal to 2%; a retaining wall of the tundish is a magnesium-zirconium-carbon retaining wall, and in the magnesium-zirconium-carbon retaining wall, Al.sub.2O.sub.3 is less than or equal to 5%; and an upper nozzle and a submersed nozzle of the tundish both are magnesium-carbon nozzles, and in the magnesium-carbon nozzles, Al.sub.2O.sub.3 is less than or equal to 5%.

5. The method for controlling brittle inclusions in cord steel according to claim 1, wherein the furnace primary smelting stage is performed in a converter or an electric furnace, the temperature of the molten steel at a smelting end-point is equal to or higher than 1650 C., C is equal to or greater than 0.10%, and O is less than or equal to 0.03%.

6. The method for controlling brittle inclusions in cord steel according to claim 1, wherein in the furnace primary smelting stage, a ladle bottom blowing flow in the initial tapping stage and the alloying process is 100-200 NL/min; the ladle bottom blowing flow in the process of starting to add the remaining 40-60% low-nitrogen carburant when 80% of the steel is tapped is 600-800 NL/min; and the ladle bottom blowing flow in the process of finishing tapping and adding the silicon carbide and the synthetic slag to the top of the ladle slag to form the slag is 300-500 NL/min.

7. The method for controlling brittle inclusions in cord steel according to claim 1, wherein in the refining stage, the ladle bottom blowing flow during the time when the temperature, the chemical components and the content of the molten steel are detected is 100-150 NL/min; the ladle bottom blowing flow in the process of adding the alloys and feeding the carbon wires is 300-400 NL/min; the ladle bottom blowing flow in the process of performing electrification to slag is 200-300 NL/min; and the ladle bottom blowing flow in the soft stirring process is 30-80 NL/min.

8. The method for controlling brittle inclusions in cord steel according to claim 1, wherein from finishing of tapping in the furnace primary smelting stage to completion of adjustment of the chemical components of the molten steel in the refining stage, the basicity of slag of the ladle is less than or equal to 0.6; and in the refining stage, 1-2 kg/t lime, 1-1.5 kg/t silicon carbide, and 8-15 kg/t synthetic slag are added to make the basicity of slag of the ladle be 0.9-1.2.

9. The method for controlling brittle inclusions in cord steel according to claim 1, wherein chemical components of the silicon carbide comprise the following components in percent by mass: equal to or greater than 99.3% of SiC and the balance of inevitable impurities; and chemical components of the synthetic slag comprise the following components in percent by mass: 35-45% of CaO, 45-55% of SiO.sub.2, 3-8% of MgO, less than or equal to 2% of Al.sub.2O.sub.3 and the balance of inevitable impurities.

10. The method for controlling brittle inclusions in cord steel according to claim 1, wherein in the billet casting stage, in the molten steel in the tundish, Als is less than or equal to 0.0005%; and in the inclusions of the molten steel, the content of the Al.sub.2O.sub.3 inclusions is less than or equal to 10%, sizes of the Al.sub.2O.sub.3 inclusions and magnesia-alumina spinel inclusions are less than 5 m, and densities of the Al.sub.2O.sub.3 inclusions and the magnesia-alumina spinel inclusions with the sizes of 1-5 m are less than or equal to 0.0005/mm.sup.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 shows distribution of inclusions in a cord steel prepared by an existing production process in an SiO.sub.2MnOAl.sub.2O.sub.3 system ternary phase diagram;

[0022] FIG. 2 shows fracture morphology of a finer wire of the cord steel prepared by the existing production process as a result of inclusion of Al.sub.2O.sub.3 at a core of a coil rod;

[0023] FIG. 3 shows fracture morphology of the finer wire of the cord steel prepared by the existing production process as a result of inclusion of Al.sub.2O.sub.3 at an edge of the coil rod;

[0024] FIG. 4 shows fracture morphology of the finer wire of the cord steel prepared by the existing production process as a result of inclusion of magnesia-alumina spinel (MgO.Math.Al.sub.2O.sub.3) at the core of the coil rod; and

[0025] FIG. 5 shows distribution of the inclusions in the cord steel prepared in an implementation of the disclosure in the SiO.sub.2MnOAl.sub.2O.sub.3 system ternary phase diagram.

DETAILED DESCRIPTION

[0026] An implementation of the disclosure provides a method for controlling brittle inclusions in cord steel, where chemical components of the cord steel include the following components in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, less than or equal to 0.015% of P, less than or equal to 0.01% of S, less than or equal to 0.0008% of Al in total, less than or equal to 0.003% of N, less than or equal to 0.002% of O, and the balance of Fe and other inevitable impurities.

[0027] The method for controlling brittle inclusions in cord steel provided by the disclosure is obtained according to a plenty of experimental studies. The method for controlling brittle inclusions in cord steel is further described below in combination with specific examples.

[0028] The method for controlling brittle inclusions in cord steel includes the following steps performed in order:

(1) A Furnace Primary Smelting Stage

[0029] performing smelting in a converter or an electric furnace and performing deoxidation alloying on molten steel, where the temperature of the molten steel at a smelting end-point is equal to or higher than 1650 C., C is equal to or greater than 0.10%, and O is less than or equal to 0.03%; pushing off the slag to tap at the smelting end-point, paving 0.5-1 kg/t silicon carbide and a 40-60% low-nitrogen carburant at a bottom of a ladle where the molten steel is carried before tapping, sequentially adding alloys for alloying in a tapping process, completing addition of all alloys when 75% of molten steel is tapped, starting to add the remaining 40-60% low-nitrogen carburant at a rate of 200-300 kg/min when 80% of the molten steel is tapped, finishing tapping after the low-nitrogen carburant is completely dissolved in the molten steel, adding the 0.5-1 kg/t silicon carbide to a top of ladle slag, and adding 3-5 kg/t synthetic slag to form slag.

[0030] Preferably, the alloys include ferrosilicon, manganese metal, and chromium iron, where Al in the ferrosilicon is less than or equal to 0.035%, Al in the manganese metal is less than or equal to 0.015%, Al in the chromium iron is less than or equal to 0.020% and C in the chromium iron is less than or equal to 0.15%, and N in the low-nitrogen carburant is less than or equal to 0.015%.

[0031] Preferably, bottom bricks, molten pool bricks and closer bricks of the ladle all are magnesia carbon bricks, and in the magnesia carbon bricks, Al.sub.2O.sub.3 is less than or equal to 3%; slag line bricks and air bricks of the ladle are magnesium-zirconium-carbon bricks, and in the magnesium-zirconium-carbon bricks, Al.sub.2O.sub.3 is less than or equal to 3%; and a long nozzle of the ladle is a long corundum nozzle, an inner wall of the long nozzle is coated with an SiO.sub.2 coating, and the thickness of the SiO.sub.2 coating is 3-8 mm.

[0032] More preferably, an upper limit of the number of use of the bottom bricks, the molten pool bricks, and the closer bricks of the ladle is 35-45; and an upper limit of the number of use of the slag line bricks and the air bricks of the ladle is 15-25. Thus, the stability of the quality of the refractory material of the ladle may be ensured, so that a situation that because the refractory material of the ladle is subjected to increasing severe erosion with extension of the service time, addition of the brittle inclusions such as Al.sub.2O.sub.3 inclusions and magnesia-alumina spinel in the finally prepared cord steel as a large amount of Al.sub.2O.sub.3 and MgO in the refractory material of the ladle enter the molten steel is avoided.

[0033] Preferably, a ladle bottom blowing flow in the initial tapping stage and the alloying process is 100-200 NL/min; the ladle bottom blowing flow in the process of starting to add the remaining 40-60% low-nitrogen carburant when 80% of the steel is tapped is 600-800 NL/min; and the ladle bottom blowing flow in the process of finishing tapping and adding the silicon carbide and the synthetic slag to the top of the ladle slag to form the slag is 300-500 NL/min.

[0034] Preferably, chemical components of the silicon carbide include the following components in percent by mass: equal to or greater than 99.3% of SiC and the balance of inevitable impurities; and chemical components of the synthetic slag include the following components in percent by mass: 35-45% of CaO, 45-55% of SiO.sub.2, 3-8% of MgO, less than or equal to 2% of Al.sub.2O.sub.3 and the balance of inevitable impurities.

(2) A Refining Stage

[0035] Feeding the molten steel subjected to furnace primary smelting to a ladle furnace for refining and detecting the temperature, the chemical components, and the content of the molten steel, adding the alloys according to the detected chemical components and content of the molten steel and feeding carbon wires to adjust the chemical components of the molten steel to meet the following conditions in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, less than or equal to 0.015% of P, less than or equal to 0.01% of S, less than or equal to 0.0008% of Al in total, less than or equal to 0.003% of N, less than or equal to 0.002% of O, and the balance of Fe and other inevitable impurities; and adding lime, the silicon carbide, and the synthetic slag and performing electrification to slag, adjusting the temperature of the molten steel to 1510-1535 C., making the slag composition meet the following conditions in percent by mass: CaO/SiO.sub.2=0.9-1.2, Al.sub.2O.sub.35%, MgO 4-8%, [MnO+T.Math.Fe]2-5%, and the balance of inevitable impurities, then adjusting ladle bottom blowing to a soft stirring pattern, where soft stirring time is longer than 20 min, and performing tapping; and [0036] where [MnO+T.Math.Fe] represents a sum of mass percent of MnO and T.Math.Fe.

[0037] Preferably, the ladle bottom blowing flow during the time when the temperature, the chemical components and the content of the molten steel are detected is 100-150 NL/min; the ladle bottom blowing flow in the process of adding the alloys and feeding the carbon wires is 300-400 NL/min; the ladle bottom blowing flow in the process of performing electrification to slag is 200-300 NL/min; and the ladle bottom blowing flow in the soft stirring process is 30-80 NL/min.

[0038] Preferably, from finishing of tapping in the furnace primary smelting stage to completion of adjustment of the chemical components of the molten steel in the refining stage, the basicity of slag of the ladle is less than or equal to 0.6.

[0039] Preferably, in the refining stage, 1-2 kg/t lime, 1-1.5 kg/t silicon carbide, and 8-15 kg/t synthetic slag are added to make the basicity of slag of the ladle be 0.9-1.2.

[0040] Preferably, chemical components of the silicon carbide include the following components in percent by mass: equal to or greater than 99.3% of SiC and the balance of inevitable impurities; and chemical components of the synthetic slag include the following components in percent by mass: 35-45% of CaO, 45-55% of SiO.sub.2, 3-8% of MgO, less than or equal to 2% of Al.sub.2O.sub.3 and the balance of inevitable impurities.

(3) A Billet Casting Stage

[0041] Conveying the molten steel after tapping in the refining stage to a continuous casting platform to stand for 15 min or more, importing stuffing sand into a slag receiving bucket at the start of ladle casting, and performing protective casting on the molten steel to a tundish to form a continuous casting billet.

[0042] Preferably, the stuffing sand is silicochromium stuffing sand, where Al.sub.2O.sub.3 is less than or equal to 5%.

[0043] Preferably, an inner wall of the tundish is coated with a magnesium coating, and in the magnesium coating, Al.sub.2O.sub.3 is less than or equal to 2%; a retaining wall of the tundish is a magnesium-zirconium-carbon retaining wall, and in the magnesium-zirconium-carbon retaining wall, Al.sub.2O.sub.3 is less than or equal to 5%; and an upper nozzle and a submersed nozzle of the tundish both are magnesium-carbon nozzles, and in the magnesium-carbon nozzles, Al.sub.2O.sub.3 is less than or equal to 5%.

[0044] After detection, in the molten steel in the tundish, the acid soluble aluminum Als is less than or equal to 0.0005%; and in the inclusions of the molten steel, the content of the Al.sub.2O.sub.3 inclusions is less than or equal to 10%, sizes of the Al.sub.2O.sub.3 inclusions and magnesia-alumina spinel inclusions are less than 5 m, and densities of the Al.sub.2O.sub.3 inclusions and the magnesia-alumina spinel inclusions with the sizes of 1-5 m are less than or equal to 0.0005/mm.sup.2.

[0045] To make the objects, technical solutions and advantages of an implementation of the disclosure more clearly, the method for controlling brittle inclusions in cord steel in the implementation of the disclosure will be further described in combination with examples 1-2 of the implementation. Apparently, the described examples 1-2 are merely a part of examples of the disclosure, rather than, all the examples.

Example 1

(1) A Furnace Primary Smelting Stage

[0046] Smelting is performed in a 135 t converter, where the temperatures of the molten steel, the contents of C, and the contents of O at smelting end-points of the converters with heat numbers (1)-(4) are respectively shown in table 1.

TABLE-US-00001 TABLE 1 Temperature of the molten Heat number steel, C. C, % O, % (1) 1655 0.24 0.021 (2) 1650 0.31 0.012 (3) 1661 0.10 0.030 (4) 1657 0.19 0.027

[0047] The slag was pushed off to tap at the smelting end-point, silicon carbide and a 40-60% low-nitrogen carburant were paved at a bottom of a ladle where the molten steel was carried in advance before tapping, alloys for alloying were sequentially added in a tapping process, addition of all alloys was completed when 75% of molten steel was tapped, it was started to add the remaining 40-60% low-nitrogen carburant at a rate of 200-300 kg/min when 80% of the molten steel was tapped, tapping was finished after the low-nitrogen carburant was completely dissolved in the molten steel, the silicon carbide was added to a top of ladle slag, and the synthetic slag was added to form slag, where the basicity of slag of the ladle was less than or equal to 0.6. The quantities of the silicon carbide paved at the bottoms of the ladles in the converters with the heat numbers (1)-(4), the silicon carbide added into the tops of the slag of the ladles when tapping is finished, and the synthetic slag refer to table 2.

TABLE-US-00002 TABLE 2 Silicon carbide Silicon carbide paved at the added when Synthetic Heat bottom of the tapping is slag, number ladle, kg/t finished, kg/t kg/t (1) 0.9 0.5 3.0 (2) 0.5 0.9 3.3 (3) 1.0 0.6 5.0 (4) 0.7 1.0 4.5

[0048] The ladle bottom blowing is started in the full tapping process. In the converters with the heat numbers (1)-(4), in the initial tapping stage and the alloying process, the process of starting to add the remaining 40-60% low-nitrogen carburant when 80% of the steel is tapped, and the process of finishing tapping and adding silicon carbide and synthetic slag to the top of the ladle slag to form slag, the ladle bottom blowing flows are respectively shown in table 3.

TABLE-US-00003 TABLE 3 Initial tapping Process of adding stage and the remaining Finishing Heat alloying process, low-nitrogen tapping, number NL/min carburant, NL/min NL/min (1) 180 600 300 (2) 100 750 360 (3) 200 680 500 (4) 150 800 420

(2) A Refining Stage

[0049] The molten steel subjected to furnace primary smelting was fed to a ladle furnace for refining and the temperature, the chemical components, and the content of the molten steel were detected, the alloys were added according to the detected chemical components and content of the molten steel and carbon wires were fed to adjust the chemical components of the molten steel to meet the following conditions in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, 0.007-0.015% of P, 0.006-0.01% of S, 0.0005-0.0008% of Al in total, 0.0015-0.0030% of N, 0.001-0.002% of O, and the balance of Fe and other inevitable impurities; and lime, the silicon carbide, and the synthetic slag were added and electrification was performed to slag, the temperature of the molten steel was adjusted to 1510-1535 C., making the slag composition meet the following conditions in percent by mass: CaO/SiO.sub.2=0.9-1.2, Al.sub.2O.sub.35%, MgO 4-8%, [MnO+T.Math.Fe]2-5%, and the balance of inevitable impurities, then ladle bottom blowing was adjusted to a soft stirring pattern, where soft stirring time was longer than 20 min, and tapping was performed. The quantities of the lime, the silicon carbide, and the synthetic slag added into the ladle furnaces with the heat numbers (1)-(4) and the basicity of the slag in the ladles refer to table 4.

TABLE-US-00004 TABLE 4 Synthetic Silicon Heat Lime, slag, carbide, Basicity number kg/t kg/t kg/t of slag (1) 2.0 8.0 1.3 1.2 (2) 1.7 10.6 1.4 1.1 (3) 1.0 15.0 1.5 0.9 (4) 1.3 14.7 1.0 1.0

[0050] In the ladle furnaces with the heat numbers (1)-(4), the ladle bottom blowing flows during the time of detecting the temperature, the chemical components, and the content of the molten steel, in the process of adding the alloys and feeding the carbon wires, in the process of performing electrification to slag and in the soft stirring process, and the soft stirring time are respectively shown in table 5.

TABLE-US-00005 TABLE 5 Ladle bottom blowing flow, NL/min During the time of detecting the temperature, the In the process chemical of adding the In the process components, and alloys and of performing Soft Heat the content of the feeding the electrification Soft stirring stirring number molten steel carbon wires to slag process time, min (1) 140 390 200 80 20 (2) 100 400 260 55 28 (3) 120 300 300 30 35 (4) 150 350 280 70 30

[0051] The alloys include ferrosilicon, manganese metal, and chromium iron, where the Al contents in the ferrosilicon, the manganese metal, and the chromium iron are shown in table 6. Besides, C in the chromium iron is less than or equal to 0.15%, and N in the low-nitrogen carburant is less than or equal to 0.015%.

TABLE-US-00006 TABLE 6 Heat Manganese Chromium number Ferrosilicon, % metal, % iron, % (1) 0.035 0.008 0.013 (2) 0.017 0.015 0.008 (3) 0.026 0.010 0.009 (4) 0.011 0.011 0.020

[0052] Chemical components of the silicon carbide include the following components in percent by mass: equal to or greater than 99.3% of SiC and the balance of inevitable impurities; and chemical components and mass percent of the synthetic slag are shown in table 7.

TABLE-US-00007 TABLE 7 Balance of Heat inevitable number CaO, % SiO.sub.2, % MgO, % Al.sub.2O.sub.3, % impurities (1) 40 45 8 2.0 Bal (2) 35 55 5 1.0 Bal (3) 39 50 3 1.8 Bal (4) 45 48 3 0.7 Bal

(3) A Billet Casting Stage

[0053] The molten steel after tapping in the refining stage was conveyed to a continuous casting platform to stand for 15 min or more, stuffing sand was imported into a slag receiving bucket at the start of ladle casting, and protective casting was performed on the molten steel to a tundish to form a continuous casting billet.

[0054] Bottom bricks, molten pool bricks and closer bricks of the ladle all are magnesia carbon bricks, and the numbers of use do not exceed 35-45; slag line bricks and air bricks of the ladle are magnesium-zirconium-carbon bricks, and the numbers of use do not exceed 15-25; a long nozzle of the ladle is a long corundum nozzle, an inner wall of the long nozzle is coated with an SiO.sub.2 coating, and the thickness of the SiO.sub.2 coating is 3-8 mm; the stuffing sand is silicochromium stuffing sand; an inner wall of the tundish is coated with a magnesium coating, a retaining wall of the tundish is a magnesium-zirconium-carbon retaining wall, and an upper nozzle and a submersed nozzle of the tundish all are magnesium-carbon nozzles.

[0055] The Al.sub.2O.sub.3 contents of the bottom bricks, molten pool bricks, closer bricks, slag line bricks, and air bricks of the ladle, the stuffing sand, and the magnesium coating, the retaining wall, the upper nozzle and the submersed nozzle of the tundish are shown in table 8.

TABLE-US-00008 TABLE 8 Molten Slag Heat Bottom pool Closer line Air stuffing Magnesium Retaining Upper Submersed number brick/% brick/% brick/% brick/% brick/% sand/% coating/% wall/% nozzle/% nozzle/% (1) 1.2 1.4 2.8 3.0 0.8 3.7 1.5 3.9 2.9 1.7 (2) 0.7 1.8 1.6 1.2 1.7 5.0 0.7 5.0 5.0 3.8 (3) 2.9 0.9 0.5 2.3 3.0 2.6 2.0 2.2 4.3 3.6 (4) 1.5 3.0 1.7 1.9 2.5 4.1 0.9 4.8 2.7 5.0

[0056] A molten steel sample in the tundish is scanned with a Zeiss scanning electron microscope and inclusions with sizes larger than 1 m are counted. The scanning area is 1000 mm.sup.2. The Al.sub.2O.sub.3 content in the inclusions is less than or equal to 8.5%. Al.sub.2O.sub.3 and magnesia-alumina spinel inclusions of 5 m or more are not found. The densities of the Al.sub.2O.sub.3 and magnesia-alumina spinel inclusions with the sizes of 1-5 m are less than or equal to 0.0004/mm.sup.2.

Example 2

(1) A Furnace Primary Smelting Stage

[0057] Smelting is performed in a 100 t electric furnace, where the temperatures of the molten steel, the contents of C, and the contents of O at smelting end-points of the converters with heat numbers (1)-(4) are respectively shown in table 9.

TABLE-US-00009 TABLE 9 Temperature of the molten Heat number steel, C. C, % O, % (1) 1658 0.20 0.023 (2) 1670 0.10 0.030 (3) 1663 0.14 0.026 (4) 1650 0.25 0.019

[0058] The slag was pushed off to tap at the smelting end-point, silicon carbide and a 40-60% low-nitrogen carburant were paved at a bottom of a ladle where the molten steel was carried in advance before tapping, alloys for alloying were sequentially added in a tapping process, addition of all alloys was completed when 75% of molten steel was tapped, it was started to add the remaining 40-60% low-nitrogen carburant at a rate of 200-300 kg/min when 80% of the molten steel was tapped, tapping was finished after the low-nitrogen carburant was completely dissolved in the molten steel, the silicon carbide was added to a top of ladle slag, and the synthetic slag was added to form slag, where the basicity of slag of the ladle was less than or equal to 0.6. The quantities of the silicon carbide paved at the bottoms of the ladles in the electric furnaces with the heat numbers (1)-(4), the silicon carbide added into the tops of the slag of the ladles when tapping is finished, and the synthetic slag refer to table 10.

TABLE-US-00010 TABLE 10 Silicon carbide Silicon carbide paved at the added when Heat bottom of the tapping is Synthetic number ladle, kg/t finished, kg/t slag, kg/t (1) 0.8 1.0 2.7 (2) 0.5 0.6 5.0 (3) 0.7 0.8 4.1 (4) 1.0 0.9 3.0

[0059] The ladle bottom blowing is started in the full tapping process. In the electric furnaces with the heat numbers (1)-(4), in the initial tapping stage and the alloying process, the process of starting to add the remaining 40-60% low-nitrogen carburant when 80% of the steel is tapped, and the process of finishing tapping and adding silicon carbide and synthetic slag to the top of the ladle slag to form slag, the ladle bottom blowing flows are respectively shown in table 11.

TABLE-US-00011 TABLE 11 Initial tapping Process of adding stage and the remaining Finishing Heat alloying process, low-nitrogen tapping, number NL/min carburant, NL/min NL/min (1) 150 650 380 (2) 200 800 430 (3) 190 720 500 (4) 100 600 300

(2) A Refining Stage

[0060] The molten steel subjected to furnace primary smelting was fed to a ladle furnace for refining and the temperature, the chemical components, and the content of the molten steel were detected, the alloys were added according to the detected chemical components and content of the molten steel and carbon wires were fed to adjust the chemical components of the molten steel to meet the following conditions in percent by mass: 0.70-0.95% of C, 0.15-0.45% of Si, 0.25-0.80% of Mn, 0.10-0.45% of Cr, 0.01-0.015% of P, 0.008-0.01% of S, 0.0005-0.0008% of Al in total, 0.0019-0.0030% of N, 0.0011-0.0020% of O, and the balance of Fe and other inevitable impurities; and lime, the silicon carbide, and the synthetic slag were added and electrification was performed to slag, the temperature of the molten steel was adjusted to 1510-1535 C., making the slag composition meet the following conditions in percent by mass: CaO/SiO.sub.2=0.9-1.2, Al.sub.2O.sub.35%, MgO 4-8%, [MnO+T.Math.Fe]2-5%, and the balance of inevitable impurities, then ladle bottom blowing was adjusted to a soft stirring pattern, where soft stirring time was longer than 20 min, and tapping was performed. The quantities of the lime, the silicon carbide, and the synthetic slag added into the ladle furnaces with the heat numbers (1)-(4) and the basicity of the slag in the ladles refer to table 12.

TABLE-US-00012 TABLE 12 Synthetic Silicon Heat Lime, slag, carbide, Basicity number kg/t kg/t kg/t of slag (1) 1.0 8.0 1.5 1.1 (2) 2.0 14.8 1.4 1.2 (3) 1.9 15.0 1.0 0.9 (4) 1.6 10.3 1.3 1.1

[0061] In the ladle furnaces with the heat numbers (1)-(4), the ladle bottom blowing flows during the time of detecting the temperature, the chemical components, and the content of the molten steel, in the process of adding the alloys and feeding the carbon wires, in the process of performing electrification to slag and in the soft stirring process, and the soft stirring time are respectively shown in table 13.

TABLE-US-00013 TABLE 13 Ladle bottom blowing flow, NL/min During the time of detecting the temperature, the In the process chemical of adding the In the process components, and alloys and of performing Soft Heat the content of the feeding the electrification Soft stirring stirring number molten steel carbon wires to slag process time, min (1) 140 390 270 70 28 (2) 100 400 300 80 33 (3) 120 300 200 40 30 (4) 150 350 220 30 20

[0062] The alloys include ferrosilicon, manganese metal, and chromium iron, where the Al contents in the ferrosilicon, the manganese metal, and the chromium iron are shown in table 14. Besides, C in the chromium iron is less than or equal to 0.15%, and N in the low-nitrogen carburant is less than or equal to 0.015%.

TABLE-US-00014 TABLE 14 Heat Manganese Chromium number Ferrosilicon, % metal, % iron, % (1) 0.030 0.007 0.009 (2) 0.015 0.010 0.008 (3) 0.035 0.012 0.020 (4) 0.018 0.015 0.014

[0063] Chemical components of the silicon carbide include the following components in percent by mass: equal to or greater than 99.3% of SiC and the balance of inevitable impurities; and chemical components and mass percent of the synthetic slag are shown in table 15.

TABLE-US-00015 TABLE 15 Balance of Heat inevitable number CaO, % SiO.sub.2, % MgO, % Al.sub.2O.sub.3, % impurities (1) 43 45 7 2.0 Bal (2) 40 50 6 1.0 Bal (3) 36 55 3 1.8 Bal (4) 35 49 8 0.7 Bal

(3) A Billet Casting Stage

[0064] The molten steel after tapping in the refining stage is conveyed to a continuous casting platform to stand for 15 min or more, stuffing sand was imported into a slag receiving bucket at the start of ladle casting, and protective casting was performed on the molten steel to a tundish to form a continuous casting billet.

[0065] Bottom bricks, molten pool bricks and closer bricks of the ladle all are magnesia carbon bricks, and the numbers of use do not exceed 35-45; slag line bricks and air bricks of the ladle are magnesium-zirconium-carbon bricks, and the numbers of use do not exceed 15-25; a long nozzle of the ladle is a long corundum nozzle, an inner wall of the long nozzle is coated with an SiO.sub.2 coating, and the thickness of the SiO.sub.2 coating is 3-8 mm; the stuffing sand is silicochromium stuffing sand; an inner wall of the tundish is coated with a magnesium coating, a retaining wall of the tundish is a magnesium-zirconium-carbon retaining wall, and an upper nozzle and a submersed nozzle of the tundish all are magnesium-carbon nozzles.

[0066] The Al.sub.2O.sub.3 contents of the bottom bricks, molten pool bricks, closer bricks, slag line bricks, and air bricks of the ladle, the stuffing sand, and the magnesium coating, the retaining wall, the upper nozzle and the submersed nozzle of the tundish are shown in table 16.

TABLE-US-00016 TABLE 16 Molten Slag Heat Bottom pool Closer line Air stuffing Magnesium Retaining Upper Submersed number brick/% brick/% brick/% brick/% brick/% sand/% coating/% wall/% nozzle/% nozzle/% (1) 1.8 2.6 2.2 2.6 3.0 1.7 1.6 2.7 1.7 5.0 (2) 0.9 1.3 1.1 1.4 2.7 5.0 2.0 2.0 3.4 4.4 (3) 2.3 3.0 0.6 3.0 0.6 3.3 1.0 3.1 5.0 2.5 (4) 1.2 2.4 1.0 1.3 1.5 4.1 0.7 5.0 2.3 3.0

[0067] A molten steel sample in the tundish is scanned with a Zeiss scanning electron microscope and inclusions with sizes larger than 1 m are counted. The scanning area is 1000 mm.sup.2. The Al.sub.2O.sub.3 content in the inclusions is less than or equal to 10%. Al.sub.2O.sub.3 and magnesia-alumina spinel inclusions of 5 m or more are not found. The densities of the Al.sub.2O.sub.3 and magnesia-alumina spinel inclusions with the sizes of 1-5 m are less than or equal to 0.0005/mm.sup.2.

[0068] Besides, FIG. 5 shows distribution of the inclusions in the cord steel prepared in an implementation of the disclosure in the SiO.sub.2MnOAl.sub.2O.sub.3 system ternary phase diagram. It may be known by comparing FIG. 5 with FIG. 1 that the inclusions in the cord steel provided by the disclosure are mainly SiO.sub.2MnO inclusions. Compared with the cord steel prepared by the existing production process, the brittle inclusions such as Al.sub.2O.sub.3 and magnesia-alumina spinel (MgO.Math.Al.sub.2O.sub.3) are greatly reduced, so that the drawing wire breakage ratio caused by the brittle inclusions may be greatly reduced.