A dynamic soft reduction process for continuous casting of s2 alloy tool steel bloom

Abstract

The application discloses a dynamic soft reduction process for S2 alloy tool steel continuous casting bloom, in which the reduction amount is controlled by the change of the center solid phase rate fs of the casting blank in the continuous casting reduction area, and the relationship between the reduction rack reduction amount and the center solid phase rate of the casting blank is as follows: no reduction is performed when fs is less than 0.40; the reduction amount is 0.5 mm when fs is 0.4-0.45; the reduction amount is 1 mm when fs is 0.50-0.59; the reduction amount is 5 mm when fs is 0.72-0.78; the reduction amount is 6 mm when fs is 0.87-0.92; the reduction amount is 0.5 mm when fs is 0.96-0.999; and no reduction is performed when fs is greater than 0.999. The method mixes linear difference and step method to control, guarantees the reduction quality, prolongs the service life of the equipment, can reduce the center porosity and center shrinkage hole of the casting blank, and can also improve the uniformity of the composition of the continuous casting blank.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical continuous casting technology, and in particular to a dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel. Background Technology

[0002] S2 steel is an impact-resistant alloy tool steel conforming to the American AISI / SAE S2 and UNS T41902 standards. It is renowned for its excellent wear resistance, heat resistance, and high toughness, and its "hard yet non-brittle" characteristics make it an ideal choice for high-end manual and power tools. The production quality requirements for S2 steel are extremely stringent. Due to its high carbon and alloy content, it experiences large temperature variations during solidification, a long solidification gap, and a wide pasty zone. Therefore, it is prone to severe compositional segregation during solidification, which is difficult to effectively eliminate during subsequent heating and rolling processes. Severe compositional segregation increases the sensitivity to heat treatment, widens the microhardness difference between segregation bands, and affects contact fatigue life. Furthermore, because large billet continuous casting machines produce large billet cross-sections, the solidification time is longer, increasing the tendency for central defects such as central porosity and central shrinkage cavities. Therefore, researching and developing a reasonable S2 continuous casting reduction process to effectively solve serious technical problems such as component segregation, central shrinkage, and central porosity in continuous casting billets is crucial for reducing energy loss in subsequent processes, improving the hot working performance of products, and producing high-quality and high-value-added alloy tool steels. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to provide a dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel that effectively improves the quality of the cast billets.

[0004] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows: In the continuous casting reduction zone, the reduction amount is controlled by the change in the solid fraction fs at the center of the billet. The relationship between the reduction amount of the reduction stand and the solid fraction at the center of the billet is as follows: when fs < 0.40, no reduction is performed; when fs is 0.4 to 0.45, the reduction amount is 0.5 mm; when fs is 0.50 to 0.59, the reduction amount is 1 mm; when fs is 0.72 to 0.78, the reduction amount is 5 mm; when fs is 0.87 to 0.92, the reduction amount is 6 mm; when fs is 0. When fs is between 0.96 and 0.999, the reduction is 0.5 mm; when fs > 0.999, no reduction is performed; when the solid fraction at the center of the billet in each stand is between 0.45 < fs < 0.50 or 0.78 < fs < 0.87, the reduction corresponding to that stand is calculated using the linear difference method based on the above solid fraction; when the solid fraction at the center of the billet in each stand is between 0.59 < fs < 0.72 or 0.92 < fs < 0.96, the solid fraction range is divided into 3 to 6 steps using the step method, and the reduction corresponding to that stand is determined based on the above solid fraction.

[0005] Furthermore, the stepwise control method is as follows: when 0.59 < fs ≤ 0.62, the compression amount is 1.5 mm; when 0.62 < fs ≤ 0.65, the compression amount is 2 mm; when 0.65 < fs ≤ 0.68, the compression amount is 2.5 mm; when 0.68 < fs ≤ 0.70, the compression amount is 3 mm; when 0.70 < fs < 0.72, the compression amount is 4 mm; when 0.92 < fs ≤ 0.93, the compression amount is 5 mm; when 0.93 < fs ≤ 0.94, the compression amount is 4 mm; when 0.94 < fs ≤ 0.95, the compression amount is 3 mm; and when 0.95 < fs < 0.96, the compression amount is 1 mm.

[0006] Furthermore, the solidification rate of the billet in the pressing region is 30% to 100%.

[0007] Furthermore, the total reduction amount of the light pressing is 6.5 to 13 mm, the maximum reduction amount of a single pressing frame is ≤6 mm, and the maximum reduction rate is ≤4.5 m / m.

[0008] Furthermore, the casting speed in the continuous casting process is 0.70–0.85 m / min, the superheat of the molten steel in the tundish is 20–35°C, and the surface temperature of the billet in the pressing zone is controlled at 760–960°C.

[0009] Furthermore, in the continuous casting process, the maximum reheating rate of the billet surface in the secondary cooling zone is ≤20℃ / m, and the maximum cooling rate is ≤15℃ / m.

[0010] The beneficial effects of adopting the above technical solution are as follows: This invention controls the reduction amount according to the solidity of the billet center at each straightening roller position, and adopts a mixed control of linear difference and step method. While ensuring the continuity and accuracy of the reduction quality, it can also avoid frequent and large-scale adjustments of the reduction roller, thus protecting electrical components. Through the mixed control of linear difference and step method, this invention not only ensures the reduction quality but also extends the service life of the equipment. It can reduce defects such as central porosity and central shrinkage cavity in the billet, and at the same time improve the uniformity of the composition of the continuously cast billet, stabilize and improve the mechanical properties and service performance of S2 alloy tool steel, and meet the technical requirements for manufacturing various high-end tools. The resulting billet has a carbon segregation index ≤1.01, central porosity ≤1.0 grade, and central shrinkage cavity ≤0.5 grade.

[0011] This invention significantly reduces or eliminates defects such as component segregation, central porosity, and central shrinkage cavities by controlling factors that affect the position of the solidification end of the billet, such as continuous casting speed, molten steel superheat, and continuous casting cooling intensity. The tool steel produced can meet the technical requirements for manufacturing various measuring tools, cutting tools, impact-resistant tools, cold and hot molds, and some special-purpose tools. Detailed Implementation

[0012] The present invention will now be described in further detail with reference to specific embodiments.

[0013] This process describes a dynamic light reduction method for continuously casting large square billets of S2 alloy tool steel. The composition (wt) of the S2 alloy tool steel is: C 0.40%–0.55%, Si 0.90%–1.20%, Mn 0.30%–0.50%, V ≤0.50%, Mo 0.30%–0.50%, with the balance being Fe and unavoidable impurities. The cross-sectional dimensions of the continuously cast large square billet are not less than 250mm × 250mm.

[0014] The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel includes the following control processes: (1) Continuous casting speed and tundish molten steel temperature control system: continuous casting speed 0.70~0.85m / min, tundish molten steel superheat 20~35℃, tundish molten steel temperature controlled at 1503~1518℃. Ensure that the solidification end of the billet is located in the reduction zone, the distance from the reduction zone to the meniscus of the crystallizer is within the range of 16187mm~24649mm, and the total length of the reduction zone is 8462mm. It can flexibly adjust the length of the reduction zone and the reduction amount to adapt to changes in continuous casting process such as steel composition, continuous casting speed, superheat, cooling intensity, etc.

[0015] (2) Secondary cooling zone control system: The continuous casting specific water volume is 0.18~0.28L / kg, the surface temperature of the billet in the pressing zone is 760~960℃, to ensure that the billet in the pressing zone of continuous casting has good high-temperature ductility, and control the maximum temperature recovery rate of the billet surface in the secondary cooling zone to ≤20℃ / m, and the maximum cooling rate during water cooling to ≤15℃ / m.

[0016] (3) Dynamic light reduction regime: In the continuous casting reduction zone, the reduction amount is controlled by the change in the solid fraction fs at the center of the billet. A light reduction is applied in the early stage of solidification to force the formed dendrites to break and reduce compositional segregation; while a larger reduction is applied in the middle and late stages of solidification to force the mushy area formed in the core to the molten steel area, which not only continues to improve compositional segregation but also significantly improves core defects such as central porosity and central shrinkage cavities. The solidification rate of the billet in the reduction zone is 30% to 100%, the total reduction is 6.5 to 13 mm, the maximum reduction of a single reduction stand is ≤6 mm, and the maximum reduction rate of a single stand is ≤4.5 mm / m. The relationship between the reduction amount of the reduction stand and the solid fraction at the center of the billet is shown in Table 1.

[0017] Table 1: Correspondence between reduction and solid fraction at the center of the billet

[0018] When the solid fraction at the center of the billet is less than 0.40 and greater than 0.999, no reduction is performed. When the solid fraction at the center of the billet of each stand is between the two data segments in the table above: between interval 2 and interval 3 and between interval 4 and interval 5, the reduction amount corresponding to that stand is calculated using the linear difference method based on its solid fraction; between interval 3 and interval 4 and between interval 5 and interval 6, the step method is used; the step control between interval 3 and interval 4 is shown in Table 2, and the step control between interval 5 and interval 6 is shown in Table 3.

[0019] Table 2: Stepped control between intervals 3 and 4

[0020] Table 3: Stepped control between interval 5 and interval 6

[0021] In the aforementioned dynamic light pressing process, the pressing transition range uses a linear difference calculation method for pressing control in the range where the pressing amount fluctuation is small, which can ensure the pressing quality; while in the range where the pressing amount span is large, a step control is adopted to avoid frequent adjustment of the pressing roller during the production process while ensuring the pressing quality, thus protecting the electrical components such as relays of the pressing equipment.

[0022] (4) Electromagnetic stirring process of crystallizer: the current intensity is controlled at 400A, the frequency is 2.4Hz, and the rotation mode is unidirectional rotation.

[0023] The following embodiments were implemented on a continuous casting machine equipped with a 7-stand straightening machine, the distance from the pressing area to the meniscus of the crystallizer was in the range of 16187mm to 24649mm, and the total length of the pressing area was 8462mm. Example 1

[0024] In this embodiment, the cross-sectional dimensions of the S2 alloy tool steel continuously cast large square billet are 280mm × 325mm. The main chemical composition of the casting batch is: C 0.48%, Si 1.00%, Mn 0.43%, P 0.018%, S 0.003%, V 0.14%, Mo 0.38%, with the balance being Fe and unavoidable impurities.

[0025] The main process parameters for continuous casting in the casting furnace are as follows: casting speed 0.70 m / min, tundish steel temperature 1518℃, superheat 35℃; continuous casting specific water volume 0.22 L / kg, billet surface temperature in the reduction zone 850℃, maximum reheating rate of billet surface in the secondary cooling zone 16℃ / m, maximum cooling rate 12℃ / m; current intensity controlled at 400A, frequency at 2.4Hz, and rotation mode unidirectional rotation.

[0026] The solid fraction, reduction amount, and total reduction amount of each frame in the pressing area are shown in Table 4.

[0027] Table 4: Parameters under dynamic light pressure

[0028] After adopting the above process, the carbon segregation index of the cast billet obtained in this embodiment is ≤1.01, the central porosity is ≤1.0 grade, and the central shrinkage cavity is ≤0.5 grade. Example 2

[0029] In this embodiment, the cross-sectional dimensions of the S2 alloy tool steel continuously cast large square billet are 280mm × 325mm. The main chemical composition of the casting batch is: C 0.47%, Si 1.06%, Mn 0.43%, P 0.015%, S 0.001%, V 0.15%, Mo 0.38%, with the balance being Fe and unavoidable impurities.

[0030] The main process parameters for continuous casting in the casting furnace are as follows: casting speed 0.78 m / min, tundish steel temperature 1510℃, superheat 27℃; continuous casting specific water volume 0.18 L / kg, billet surface temperature in the reduction zone 960℃, maximum reheating rate of billet surface in the secondary cooling zone 20℃ / m, maximum cooling rate 10℃ / m; current intensity control 400A, frequency 2.4Hz, and rotation mode unidirectional rotation.

[0031] The solid fraction, reduction amount, and total reduction amount of each frame in the pressing area are shown in Table 5.

[0032] Table 5: Parameters under dynamic light pressure

[0033] After adopting the above process, the carbon segregation index of the cast billet obtained in this embodiment is ≤1.01, the central porosity is ≤1.0 grade, and the central shrinkage cavity is ≤0.5 grade. Example 3

[0034] In this embodiment, the cross-sectional dimensions of the S2 alloy tool steel continuously cast large square billet are 280mm × 325mm. The main chemical composition of the casting batch is: C 0.50%, Si 1.09%, Mn 0.44%, P 0.016%, S 0.003%, V 0.15%, Mo 0.38%, with the balance being Fe and unavoidable impurities.

[0035] The main process parameters for continuous casting in the casting furnace are as follows: casting speed 0.85 m / min, tundish steel temperature 1503℃, superheat 20℃; continuous casting specific water volume 0.28 L / kg, billet surface temperature in the reduction zone 760℃, maximum reheating rate of billet surface in the secondary cooling zone 15℃ / m, maximum cooling rate 15℃ / m; current intensity controlled at 400A, frequency at 2.4Hz, and rotation mode unidirectional rotation.

[0036] The solid fraction, reduction amount, and total reduction amount of each frame in the pressing area are shown in Table 6.

[0037] Table 6: Parameters under dynamic light pressure

[0038] After adopting the above process, the carbon segregation index of the cast billet obtained in this embodiment is ≤1.01, the central porosity is ≤1.0 grade, and the central shrinkage cavity is ≤0.5 grade.

[0039] Statistical Case: Before adopting this method, using conventional methods for continuous casting and light reduction, statistics showed that 76% of the billets had a carbon segregation index ≤1.01, 48% had central porosity ≤1.0, and 65% had central shrinkage cavities ≤0.5. After adopting this method for continuous casting and light reduction, statistics from 50 batches showed that all billets had a carbon segregation index ≤1.01, 100% had central porosity ≤1.0, and 100% had central shrinkage cavities ≤0.5.

Claims

1. A dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel, characterized in that: In the continuous casting reduction zone, the reduction amount is controlled by the change in the solid fraction fs at the center of the billet. The relationship between the reduction amount of the reduction stand and the solid fraction at the center of the billet is as follows: when fs < 0.40, no reduction is performed; when fs is 0.4–0.45, the reduction amount is 0.5 mm; when fs is 0.50–0.59, the reduction amount is 1 mm; when fs is 0.72–0.78, the reduction amount is 5 mm; when fs is 0.87–0.92, the reduction amount is 6 mm; when fs is 0.96–0.999, the reduction amount is... The reduction is 0.5 mm; when fs > 0.999, no reduction is performed; when the solid fraction at the center of the billet in each stand is 0.45 < fs < 0.50 or 0.78 < fs < 0.87, the reduction corresponding to that stand is calculated using the linear difference method based on the above solid fraction; when the solid fraction at the center of the billet in each stand is 0.59 < fs < 0.72 or 0.92 < fs < 0.96, the solid fraction range is divided into 3 to 6 steps using the step method, and the reduction corresponding to that stand is determined based on the above solid fraction.

2. The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel according to claim 1, characterized in that, The stepwise control method is as follows: when 0.59 < fs ≤ 0.62, the compression amount is 1.5 mm; when 0.62 < fs ≤ 0.65, the compression amount is 2 mm; when 0.65 < fs ≤ 0.68, the compression amount is 2.5 mm; when 0.68 < fs ≤ 0.70, the compression amount is 3 mm; when 0.70 < fs < 0.72, the compression amount is 4 mm; when 0.92 < fs ≤ 0.93, the compression amount is 5 mm; when 0.93 < fs ≤ 0.94, the compression amount is 4 mm; when 0.94 < fs ≤ 0.95, the compression amount is 3 mm; and when 0.95 < fs < 0.96, the compression amount is 1 mm.

3. The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel according to claim 1, characterized in that: The solidification rate of the billet in the pressing zone is 30% to 100%.

4. The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel according to claim 1, characterized in that: The total reduction under light pressing is 6.5 to 13 mm, and the maximum reduction of a single pressing frame is ≤6 mm and the maximum reduction rate is ≤4.5 m / m.

5. The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel according to any one of claims 1-4, characterized in that: The continuous casting process has a casting speed of 0.70–0.85 m / min, a tundish superheat of 20–35°C, and a billet surface temperature controlled at 760–960°C in the pressing zone.

6. The dynamic light reduction process for continuous casting large square billets of S2 alloy tool steel according to claim 5, characterized in that: In the continuous casting process, the maximum reheating rate of the billet surface in the secondary cooling zone is ≤20℃ / m, and the maximum cooling rate is ≤15℃ / m.

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