Control process for carbon segregation of high-carbon tool steel continuous casting billet

Abstract

The invention discloses a control technology for carbon segregation of a high-carbon tool steel continuous casting billet. The control technology comprises the steps of molten iron pretreatment, LF refining and continuous casting. The superheat degree of continuous casting and tundish molten steel is controlled to be 46-55 DEG C, a weak cooling mode is adopted in secondary cooling, the specific water flow is 0.15-0.20 L / kg, the throwing speed is controlled to be 0.70-0.80 m / min, a solidification tail end soft reduction process is adopted, and reduction is started when the solid fraction fs is 0.4-0.5; in the molten iron pretreatment, molten iron is subjected to desulfurization pretreatment, and the S content of the molten iron entering a furnace is controlled to be smaller than or equal to 0.005%; according to the LF refining, a high-alkalinity slag system is adopted, the final slag alkalinity (CaO / SiO2) is 7-10, the content of Al2O3 is controlled to be 20-25%, and the content of S in a finished product is controlled to be smaller than or equal to 0.002%. By means of the process, the center carbon segregation index of the high-carbon tool steel casting blank can be controlled to be smaller than or equal to 1.10.

Description

Technical Field

[0001] The invention belongs to the field of iron and steel metallurgy, and in particular relates to a control process for carbon segregation of high-carbon tool steel continuous casting billets. Background Art

[0002] The reasons for failure of high carbon tool steel wire rod during production or use are mainly related to segregation control issues. High carbon tool steel T12A is a hypereutectoid high carbon steel (carbon content up to 1.2%). With severe carbon segregation, the carbide network of the wire rod becomes more serious, which can easily lead to intergranular fracture, reduce the mechanical properties of the product, and affect wear resistance. Common high carbon steel products, such as bearing steel, cord steel, bridge cable steel, and spring steel, have a liquidus temperature difference of 100°C to the solidus temperature, while the liquidus temperature difference of high carbon tool steel T12A reaches 140-150°C, which also makes it more difficult to control the center carbon segregation of T12A continuous casting billets. In order to solve the fracture and fatigue problems caused by carbon segregation in high carbon steel, the common control method in the industry is to control the carbon segregation index of the continuous casting billet by means of low superheat, electromagnetic stirring, light pressure reduction, heavy pressure reduction, etc.; but for some special steel grades, such as T12A with a higher carbon content, there may be situations where the optimization of process parameters cannot achieve the expected results. Summary of the invention

[0003] The object of the present invention is to provide a control process for carbon segregation of high carbon tool steel continuous casting billet, which can control the carbon segregation index of the billet center to be less than or equal to 1.10.

[0004] To achieve the above purpose, the technical solution of the present invention is as follows: A control process for carbon segregation of high-carbon tool steel continuous casting billet, comprising molten iron pretreatment, LF refining and continuous casting; in the continuous casting, the superheat of molten steel in the tundish is controlled at 46-55°C, the secondary cooling adopts a weak cooling mode, the specific water volume is 0.15-0.20L / kg, the billet drawing speed is controlled at 0.70-0.80m / min, a soft reduction process is adopted at the end of solidification, and the reduction starts when the solid phase fraction fs is 0.4-0.5.

[0005] Furthermore, in the process for controlling carbon segregation of high carbon tool steel continuous casting billet of the present invention, the molten iron is pretreated, the molten iron is pretreated with desulfurization, and the S content of the molten iron entering the furnace is controlled to be ≤0.005%.

[0006] Furthermore, the control process of carbon segregation in the high carbon tool steel continuous casting billet of the present invention, wherein the LF refining adopts a high basicity slag system, the final slag basicity (CaO / SiO2) is 7-10, the Al2O3 content is controlled at 20-25%, the S content of the finished product is controlled to be ≤0.002%, and the degree of central carbon segregation of the high carbon steel continuous casting billet is suppressed.

[0007] Furthermore, the present invention provides a control process for carbon segregation of high carbon tool steel continuous casting billet, wherein, in the continuous casting, the crystallizer adopts a side-opening water inlet + strong electromagnetic stirring to uniformly distribute the temperature of the crystallizer, and the electromagnetic stirring current and frequency are 400-500A and 2-3Hz respectively; the total reduction of the continuous casting billet is 16-20mm, the number of reduction passes is 6, and the reduction of a single pass is 2-5mm.

[0008] Furthermore, the high carbon tool steel of the present invention is T12A steel.

[0009] Furthermore, the process of the present invention can control the central carbon segregation index of the high carbon tool steel ingot to be ≤1.10.

[0010] The beneficial effect of adopting the above technical scheme is that: the present invention aims at the problem that the carbon content of high carbon tool steel T12A is high, the temperature difference between the liquidus line temperature and the solidus line temperature is large (the temperature difference is 140-150°C), and the center carbon segregation of the continuous casting billet is difficult to control. By controlling the S content, the degree of center carbon segregation of the high carbon steel continuous casting billet is suppressed, and a high tundish molten steel superheat and a weak secondary cooling mode are adopted, which is suitable for normal drawing speed, and the solid phase rate fs of the solidification center of the continuous casting billet at different reduction roll positions is reduced. The reduction is started when the solid phase rate fs is 0.4-0.5, and the reduction range is expanded to match the 6-roll light reduction process at the end of solidification. Compared with the conventional low superheat matching 4-roll or 5-roll light reduction, the total reduction is increased from 10-12mm to 16-20mm, which greatly improves the carbon segregation index of the high carbon tool steel continuous casting billet and provides a good basis for the control of wire rod network carbides. DETAILED DESCRIPTION

[0011] The present invention will be further described in detail below in conjunction with the embodiments. Example 1

[0012] The production process of high carbon tool steel T12A is: hot metal desulfurization pretreatment → converter smelting → LF refining → RH refining → bloom continuous casting → blooming → rolling; the process of controlling carbon segregation of high carbon tool steel T12A ingot in this embodiment is specifically as follows: (1) Hot metal pretreatment: The hot metal is pretreated with desulfurization, and the sulfur content of the hot metal entering the furnace is 0.003%.

[0013] (2) LF refining: After the converter is finished tapping, lime, bauxite or pre-melted refined synthetic slag is added to make slag. In the LF refining process, alloys are added to fine-tune the chemical composition. The amount of lime and bauxite added is adjusted according to the final slag composition. The final slag basicity (CaO / SiO2) is 7.2, the Al2O3 content is 20%, and the S content of the finished product is 0.002%.

[0014] (3) Continuous casting: The superheat of the molten steel in the tundish is 55°C, the crystallizer adopts a side-opening water inlet + strong electromagnetic stirring (400A, 3Hz), the secondary cooling adopts a weak cooling mode (specific water volume 0.15L / kg), the billet drawing speed is 0.70m / min, and the treated molten steel is cast into a large square billet with an end face size of 280mm×325mm. The light reduction process at the end of solidification is adopted. The reduction starts when the solid phase ratio fs is 0.4. The total reduction of the continuous casting billet is 20mm, the number of reduction passes is 6, and the reduction of a single pass is 2-5mm.

[0015] According to the test, the central carbon segregation index of the high carbon tool steel T12A ingot in this embodiment is 1.09. Example 2

[0016] The production process of high carbon tool steel T12A is: hot metal desulfurization pretreatment → converter smelting → LF refining → RH refining → bloom continuous casting → blooming → rolling; the process of controlling carbon segregation of high carbon tool steel T12A ingot in this embodiment is specifically as follows: (1) Hot metal pretreatment: The hot metal is pretreated with desulfurization, and the sulfur content of the hot metal entering the furnace is 0.005%.

[0017] (2) LF refining: After the converter is finished tapping, lime, bauxite or pre-melted refined synthetic slag is added to make slag. In the LF refining process, alloys are added to fine-tune the chemical composition. The amount of lime and bauxite added is adjusted according to the final slag composition. The final slag basicity (CaO / SiO2) is 9.7, the Al2O3 content is 25%, and the S content of the finished product is 0.001%.

[0018] (3) Continuous casting: The superheat of the molten steel in the tundish is 46°C. The crystallizer adopts a side-opening water inlet + strong electromagnetic stirring (500A, 2Hz). The secondary cooling adopts a weak cooling mode (specific water volume 0.18L / kg). The billet drawing speed is 0.80m / min. The treated molten steel is cast into a large square billet with an end face size of 280mm×325mm. The light reduction process at the end of solidification is adopted. The reduction starts when the solid phase ratio fs is 0.5. The total reduction of the continuous casting billet is 16mm, the number of reduction passes is 6, and the reduction of a single pass is 2-5mm.

[0019] According to the test, the central carbon segregation index of the high carbon tool steel T12A ingot in this embodiment is 1.07. Example 3

[0020] The production process of high carbon tool steel T12A is: hot metal desulfurization pretreatment → converter smelting → LF refining → RH refining → bloom continuous casting → blooming → rolling; the process of controlling carbon segregation of high carbon tool steel T12A ingot in this embodiment is specifically as follows: (1) Hot metal pretreatment: The hot metal is pretreated with desulfurization, and the sulfur content of the hot metal entering the furnace is 0.004%.

[0021] (2) LF refining: After the converter is finished tapping, lime, bauxite or pre-melted refined synthetic slag is added to make slag. In the LF refining process, alloys are added to fine-tune the chemical composition. The amount of lime and bauxite added is adjusted according to the final slag composition. The final slag basicity (CaO / SiO2) is 8.5, the Al2O3 content is 23%, and the S content of the finished product is 0.002%.

[0022] (3) Continuous casting: The superheat of the molten steel in the tundish is 50°C, the crystallizer adopts a side-opening water inlet + strong electromagnetic stirring (400A, 2.4Hz), the secondary cooling adopts a weak cooling mode (specific water volume 0.20L / kg), the billet drawing speed is 0.75m / min, and the treated molten steel is cast into a large square billet with an end face size of 280mm×325mm. The light reduction process at the end of solidification is adopted. The reduction starts when the solid phase ratio fs is 0.45. The total reduction of the continuous casting billet is 18mm, the number of reduction passes is 6, and the reduction of a single pass is 2-5mm.

[0023] According to the test, the central carbon segregation index of the high carbon tool steel T12A ingot in this embodiment is 1.06. Comparative Example

[0024] The production process of high carbon tool steel T12A is: hot metal desulfurization pretreatment → converter smelting → LF refining → RH refining → bloom continuous casting → blooming → rolling; the process of controlling carbon segregation of high carbon tool steel T12A ingot in this comparative example is as follows: (1) Hot metal pretreatment: The hot metal is pretreated with desulfurization, and the sulfur content of the hot metal entering the furnace is 0.010%.

[0025] (2) LF refining: After the converter is finished tapping, lime, bauxite or pre-melted refined synthetic slag is added to make slag. In the LF refining process, alloys are added to fine-tune the chemical composition. The amount of lime and bauxite added is adjusted according to the final slag composition. The final slag basicity (CaO / SiO2) is 4, the Al2O3 content is 22%, and the S content of the finished product is 0.005%.

[0026] (3) Continuous casting: The superheat of the molten steel in the tundish is 30°C, the crystallizer adopts straight-through water inlet + strong electromagnetic stirring (500A, 2Hz), the secondary cooling adopts weak cooling mode (specific water volume 0.20L / kg), the billet drawing speed is 0.70m / min, and the treated molten steel is cast into large square billets with end face dimensions of 280mm×325mm. The light reduction process at the end of solidification is adopted. The reduction starts when the solid phase ratio fs is 0.60. The total reduction of the continuous casting billet is 12mm, the number of reduction passes is 4, and the reduction of each pass is 2-3mm.

[0027] According to the test, the central carbon segregation index of the high carbon tool steel T12A ingot in this comparative example is 1.22.

Claims

1. A process for controlling carbon segregation in continuous casting billets of high carbon tool steel, characterized in that: It includes molten iron pretreatment, LF refining and continuous casting; in the continuous casting, the superheat of the molten steel in the tundish is controlled at 46-55°C, the secondary cooling adopts a weak cooling mode, the specific water volume is 0.15-0.20L / kg, the billet drawing speed is controlled at 0.70-0.80m / min, and a light reduction process is adopted at the end of solidification, and the reduction starts when the solid phase fraction fs is 0.4-0.

5.

2. The process for controlling carbon segregation in continuous casting billets of high carbon tool steel according to claim 1, characterized in that: The molten iron is pretreated, and the molten iron is pretreated with desulfurization, and the sulfur content of the molten iron entering the furnace is controlled to be ≤0.005%.

3. The process for controlling carbon segregation in continuous casting billets of high carbon tool steel according to claim 1, characterized in that: The LF refining adopts a high basicity slag system, the final slag basicity (CaO / SiO2) is 7-10, the Al2O3 content is controlled at 20-25%, the S content of the finished product is controlled to be ≤0.002%, and the degree of central carbon segregation of high carbon steel continuous casting billets is suppressed.

4. The process for controlling carbon segregation in continuous casting billets of high carbon tool steel according to claim 1, characterized in that: The continuous casting crystallizer adopts side opening water inlet + strong electromagnetic stirring to even the crystallizer temperature distribution, and the electromagnetic stirring current and frequency are 400-500A and 2-3Hz respectively; the total reduction of the continuous casting billet is 16-20mm, the reduction passes are 6, and the reduction of a single pass is 2-5mm.

5. The process for controlling carbon segregation in continuous casting billets of high carbon tool steel according to claim 1, characterized in that: The process can control the central carbon segregation index of the high carbon tool steel ingot to be ≤1.10.