Continuous casting process of a large thickness die steel
By controlling the Ti and Al content, using low-superheat casting and heavy-pressure techniques, combined with a dual-loop gas cooling system and straightening treatment, the problems of corner cracking and segregation in the continuous casting process of thick mold steel were solved, the flaw detection pass rate was improved, and high-quality mold steel production was achieved.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- HEBEI PUYANG NEW MATERIAL IND CO LTD
- Filing Date
- 2023-10-12
- Publication Date
- 2026-07-10
AI Technical Summary
Defects such as corner cracks, segregation, and central porosity exist in the continuous casting process of thick die steel, resulting in a low pass rate for flaw detection. In particular, under the compression ratio limitation in the steel rolling workshop, it is difficult to extend the thickness specification to more than 150mm.
By rationally designing the composition and continuously casting process, including low superheat casting, dual-loop gas cooling system, heavy pressure and straightening machine treatment, controlling the Ti and Al content, and using the slab external arc chamfering crystallizer described in CN202222981081.0, combined with straightening force and angle control, the plastic deformation capacity and density of the cast slab are significantly improved.
It significantly reduces defects such as corner cracks, segregation, and central porosity, and improves the pass rate of flaw detection for thick mold steel, especially reaching 99-96% for 400mm thickness specifications.
Abstract
Description
Technical Field
[0001] This invention relates to the field of mold steel preparation technology, and in particular to a continuous casting process for thick mold steel. Background Technology
[0002] Mold steel is a high-quality steel used to manufacture various molds, characterized by high strength, high hardness, high wear resistance, high toughness, and high stability. Continuous casting technology for mold steel is a technique that directly pours molten steel into billets of the desired shape and size, offering advantages such as high production efficiency, low energy consumption, low cost, and high quality.
[0003] However, due to the special characteristics of mold steel, its continuous casting process presents many technical challenges, such as corner cracks, segregation, and central porosity, which affect product quality and flaw detection pass rates. Particularly for thick mold steel, the flaw detection pass rate drops sharply due to the compression ratio limitations in the rolling mill. To extend the thickness specifications of mold steel to over 150mm, the billet size must be over 400mm. However, due to the high strength, high hardness, and poor plasticity and toughness of mold steel, continuous casting of billets over 400mm in thickness easily causes corner cracks.
[0004] Therefore, how to solve the technical challenges of continuous casting of thick mold steel in order to improve the pass rate of flaw detection of thick mold steel is an urgent technical problem to be solved in the current mold steel production field. Summary of the Invention
[0005] To address the aforementioned problems, this invention provides a continuous casting process for thick mold steel. By combining a reasonable composition design with an appropriate continuous casting process, the flaw detection pass rate of thick mold steel is significantly improved.
[0006] The continuous casting process for thick mold steel described in this invention includes the following steps:
[0007] S1. The molten steel after heating and melting the raw materials is refined through deoxidation, desulfurization, denitrification and other refining processes before being cast.
[0008] S2. After exiting the crystallizer, the billet enters the secondary cooling zone, which is equipped with a dual-loop gas cooling system. The billet is subjected to heavy pressure 1m before the end of the secondary cooling zone.
[0009] S3. After the billet exits the secondary cooling zone, it enters the straightening machine. The billet is then cut by the cutting machine and sent to the heating furnace for homogenization heat treatment. After the heat treatment is completed, thick mold steel is obtained.
[0010] Furthermore, the Ti content in the molten steel is 0.03wt%-0.04wt%, and the Al content is <0.02wt%.
[0011] Titanium has a stronger affinity for nitrogen than aluminum, thus titanium preferentially forms stable titanium nitride (TiN), reducing the free nitrogen content in the billet. Simultaneously, reducing the aluminum content decreases the formation of aluminum nitride (AlN), thereby lowering the AlN content in the billet. During continuous casting of mold steel, due to its high strength, hardness, and poor plasticity and toughness, continuous casting of billets thicker than 400mm easily leads to corner cracks. The main cause of corner cracks is the bending and tensile stresses experienced by the billet during straightening, resulting in insufficient plastic deformation capacity at the corners and subsequent fracture. AlN in the billet reduces its plastic deformation capacity, especially in the second brittle zone. Therefore, in the continuous casting process of the mold steel of this invention, by controlling the content of titanium and aluminum, the plastic deformation capacity of the billet in the second brittle zone can be effectively improved, preventing corner cracks during straightening.
[0012] In this invention, excessive Ti content leads to the formation of too much TiN in the steel. These titanium nitrides are distributed in the steel as fine particles, affecting the steel's compactness, grain refinement, and reduction of aging sensitivity and cold brittleness. Simultaneously, excessive titanium nitride also reduces the steel's weldability and machinability. Conversely, insufficient TiN formation results in inadequate TiN formation. These titanium nitrides cannot effectively fix nitrogen, leading to an increase in free nitrogen content in the billet. Free nitrogen increases the steel's aging sensitivity and cold brittleness, reducing its toughness and weldability. Therefore, this invention strictly controls the Ti content in the molten steel to ensure it forms stable titanium nitride with nitrogen, reducing free nitrogen content in the billet, while avoiding excessive titanium nitride formation that could affect other properties. Furthermore, this invention also strictly limits the Al content in the molten steel. Excessive Al content leads to the formation of too much AlN in the steel. These aluminum nitrides are distributed in the steel as hard and brittle flakes, affecting the steel's plastic deformation capacity, especially in the second brittle region. Within this temperature range, the billet is prone to corner cracking during straightening. Therefore, the Al content should be controlled below 0.02 wt% to both form stable aluminum nitride with nitrogen, reducing the free nitrogen content in the billet, and avoid the formation of excessive aluminum nitride, which would affect the steel's plastic deformation capacity.
[0013] Furthermore, the casting adopts low superheat casting, with a superheat of 15℃-20℃.
[0014] This invention employs a low-superheat casting process and strictly controls the superheat to reduce the oxidation tendency of molten steel, decrease the formation of oxide inclusions, and prevent oxide inclusions from reducing the toughness, strength, and wear resistance of the steel, thus preventing castings from failing during use due to fractures or cracks. Simultaneously, it promotes the formation of numerous primary crystal nuclei in the molten steel, inhibits dendrite development, and results in a spherical or near-spherical distribution of the primary phase, improving the microstructure and homogenization of the casting. This, in turn, enhances the uniformity, toughness, and fatigue resistance of the mold steel, reducing stress concentration and crack propagation within the casting. Finally, it shortens the solidification time of the molten steel, reduces the solidification shrinkage rate, and decreases internal stress and deformation in the casting, preventing defects such as shrinkage cavities, porosity, and cracks caused by internal stress and deformation during cooling or processing.
[0015] Furthermore, the chamfered crystallizer is the slab chamfered crystallizer for external arc as described in CN202222981081.0.
[0016] Furthermore, the length of the second cooling zone is 8-12m, the gas pressure at the corner is 0.6MPa, and the gas pressure on the surface of the billet is 1.2MPa, which reduces the cooling intensity at the corner and avoids corner cracks due to poor corner toughness during straightening.
[0017] Furthermore, the speed under heavy pressure is 1 mm / s.
[0018] This invention employs heavy pressure after low-superheat casting to refine the grains in the center of the billet, increasing the proportion of equiaxed crystals and suppressing columnar crystal growth, thereby reducing segregation. Heavy pressure also forces out or disperses the residual liquid phase in the center of the billet, reducing the accumulation of gases, inclusions, and low-melting-point components, thus decreasing porosity. Finally, heavy pressure closes or fills shrinkage cavities or porosity in the center of the billet, improving its density and integrity. Ultimately, this reduces segregation and central porosity, improving the pass rate of flaw detection.
[0019] Furthermore, the straightening machine has a straightening force of 400-600kN and a straightening angle of 1°-2°.
[0020] The present invention also provides a thick die steel prepared according to the above method.
[0021] The continuous casting process provided by this invention is also applicable to the production of mold steel with a thickness of less than 400 mm.
[0022] Compared with the prior art, the beneficial technical effects of the present invention are as follows:
[0023] This invention significantly reduces defects such as corner cracks, segregation, and central porosity in thick (400mm) mold steel by using reasonable component design, a dual-loop gas cooling system, a low superheat casting process, and heavy pressure before solidification, thereby significantly improving its flaw detection pass rate. Detailed Implementation
[0024] This invention provides a continuous casting process for thick die steel, specifically including the following steps:
[0025] S1. The molten steel after heating and melting the raw materials is refined through deoxidation, desulfurization, denitrification and other refining processes before being cast.
[0026] S2. After exiting the crystallizer, the billet enters the secondary cooling zone, which is equipped with a dual-loop gas cooling system. The billet is subjected to heavy pressure 1m before the end of the secondary cooling zone.
[0027] S3. After the billet exits the secondary cooling zone, it enters the straightening machine. The billet is then cut by the cutting machine and sent to the heating furnace for homogenization heat treatment. After the heat treatment is completed, thick mold steel is obtained.
[0028] In one embodiment, the Ti content in the molten steel is 0.03wt%-0.04wt%, and the Al content is <0.02wt%.
[0029] In one embodiment, the casting is performed using low superheat casting, with a superheat of 15°C-20°C.
[0030] In one embodiment, the chamfered crystallizer is the slab chamfered crystallizer for external arc as described in CN202222981081.0.
[0031] In one embodiment, the length of the second cooling zone is 8-12m, the corner gas pressure is 0.6MPa, and the billet surface gas pressure is 1.2MPa, which reduces the corner cooling intensity and avoids corner cracks due to poor corner toughness during straightening.
[0032] In one embodiment, the velocity under heavy pressure is 1 mm / s.
[0033] In one embodiment, the straightening force of the straightening machine is 400-600kN, and the straightening angle is 1°-2°.
[0034] The technical solution provided by the present invention will be further described below with reference to the embodiments.
[0035] In this embodiment of the invention, the chamfered crystallizer is the slab chamfered crystallizer for external arc as described in CN202222981081.0.
[0036] Example 1
[0037] The chemical composition of the molten steel is: C 0.35%, Si 0.4%, Mn 1.5%, Cr 1.9%, Mo 0.2%, Ni 0.85%, Ti 0.03%, Al 0.015%, N 0.02%, with the balance being Fe and unavoidable impurities.
[0038] Molten steel is heated to 1600℃ and, after refining treatment including deoxidation, desulfurization, and denitrification, is poured using low superheat casting (15℃). The crystallizer of the casting machine has an inner diameter of 500mm × 500mm, a vibration frequency of 120Hz, and an amplitude of 3mm. After exiting the crystallizer, the billet enters the secondary cooling zone, which is 12m long and equipped with a dual-loop gas cooling system. The gas pressure at the corners is 0.6MPa, and the gas pressure on the billet surface is 1.2MPa. The billet is subjected to heavy pressure for 1m before the end of the secondary cooling zone, at a speed of 1mm / s. After exiting the secondary cooling zone, the billet enters a straightening machine with a straightening force of 600kN and a straightening angle of 2°. The billet is then cut into 6m long segments by a cutting machine and sent to a heating furnace for homogenization treatment.
[0039] The continuous casting process of this invention produces a 400mm×400mm thick mold steel billet free of defects such as corner cracks, segregation, and central porosity, with a flaw detection pass rate of 99%.
[0040] Example 2
[0041] The chemical composition of the molten steel is: C 0.32%, Si 0.4%, Mn 1.4%, Cr 2.1%, Mo 0.3%, Ni 1.2%, Ti 0.04%, Al 0.01%, N 0.03%, with the balance being Fe. The molten steel is heated to 1620℃ and, after refining treatment including deoxidation, desulfurization, and denitrification, is cast using low superheat casting (20℃). The crystallizer of the casting machine has an inner diameter of 500mm × 500mm, a vibration frequency of 100Hz, and an amplitude of 2mm. After exiting the crystallizer, the billet enters the secondary cooling zone, which is 10m long and equipped with a dual-loop gas cooling system. The gas pressure at the corners is 0.6MPa, and the gas pressure on the billet surface is 1.2MPa. The billet is subjected to heavy pressure 1m before the end of the secondary cooling zone at a rate of 0.8mm / s. After exiting the secondary cooling zone, the billet enters a straightening machine with a straightening force of 500 kN and a straightening angle of 1.5°. The billet is then cut into 5-meter-long segments by a cutting machine and sent to a heating furnace for homogenization treatment.
[0042] The continuous casting process of this invention produces a 400mm×400mm thick mold steel billet free of defects such as corner cracks, segregation, and central porosity, with a flaw detection pass rate of 98%.
[0043] Example 3
[0044] The chemical composition of the molten steel is: C 0.38%, Si 0.3%, Mn 1.6%, Cr 2%, Mo 0.5%, Ni 1%, Ti 0.02%, Al 0.02%, N 0.01%, with the balance being Fe. The molten steel is heated to 1640℃ and, after refining treatment including deoxidation, desulfurization, and denitrification, is cast using low superheat casting (15℃). The crystallizer of the casting machine has an inner diameter of 500mm × 500mm, a vibration frequency of 80Hz, and an amplitude of 1.5mm. After exiting the crystallizer, the billet enters the secondary cooling zone, which is 8m long and equipped with a dual-loop gas cooling system. The gas pressure at the corners is 0.6MPa, and the gas pressure on the billet surface is 1.2MPa. The billet is subjected to heavy pressure 1m before the end of the secondary cooling zone at a rate of 0.5mm / s. After exiting the secondary cooling zone, the billet enters a straightening machine with a straightening force of 400 kN and a straightening angle of 1°. The billet is then cut into 4-meter-long segments by a cutting machine and sent to a heating furnace for homogenization treatment.
[0045] The continuous casting process of this invention produces a 400mm×400mm thick mold steel billet free of defects such as corner cracks, segregation, and central porosity, with a flaw detection pass rate of 96%.
[0046] Comparative Example 1
[0047] Same as Example 1, except that: low superheat casting is used, with a superheat of 10°C.
[0048] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was found to be 89%.
[0049] Comparative Example 2
[0050] Same as Example 1, except that: low superheat casting is used, with a superheat of 30°C.
[0051] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was 90%.
[0052] Comparative Example 3
[0053] Same as Example 1, except that: low superheat casting was not used.
[0054] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was 73%.
[0055] Comparative Example 4
[0056] Same as Example 1, except that: Ti 0.1wt%.
[0057] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was found to be 84%.
[0058] Comparative Example 5
[0059] Same as Example 1, except that: Ti 0.01wt%.
[0060] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was found to be 81%.
[0061] Comparative Example 6
[0062] Same as Example 1, except that Al is 0.05 wt%.
[0063] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was found to be 80%.
[0064] Comparative Example 7
[0065] Same as Example 1, except that the continuous casting secondary cooling system does not use a dual-loop gas cooling system.
[0066] The flaw detection pass rate of the 400mm×400mm thick mold steel billet was 93%.
[0067] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.
Claims
1. A continuous casting process for thick mold steel, characterized in that, Includes the following steps: S1. The molten steel after heating and melting the raw materials undergoes refining treatments such as deoxidation, desulfurization, and denitrification before being poured. S2. After exiting the crystallizer, the billet enters the secondary cooling zone, which is equipped with a dual-loop gas cooling system. The billet is subjected to heavy pressure 1m before the end of the secondary cooling zone. S3. After the billet exits the second cooling zone, it enters the straightening machine. The billet is cut by the cutting machine and then sent to the heating furnace for homogenization heat treatment. After the heat treatment is completed, thick mold steel is obtained. The molten steel has a Ti content of 0.03wt%-0.04wt% and an Al content of <0.02wt%. The casting process employs low superheat casting, with a superheat of 15℃-20℃. The length of the secondary cooling zone is 8-12 m, the gas pressure at the corner is 0.6 MPa, and the gas pressure on the surface of the billet is 1.2 MPa. This reduces the cooling intensity at the corner and avoids corner cracks due to poor corner toughness during straightening.
2. The continuous casting process for thick mold steel according to claim 1, characterized in that, The velocity under heavy pressure is 1 mm / s.
3. The continuous casting process for thick mold steel according to claim 1, characterized in that, The straightening machine has a straightening force of 400-600 kN and a straightening angle of 1°-2°.