Low cost production process of low c die steel plate

Abstract

This invention provides a low-cost production process for low-C mold steel plates. The chemical composition of the steel plate, by weight percentage, is C=0.31%~0.41%, Si=0.2%~0.4%, Mn=1.3%~1.7%, Cr=1.5%~2.0%, Mo=0.20%~0.35%, Nb=0.10%~0.30%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable trace chemical elements. The manufacturing process employs differential temperature rolling, with two-stage water cooling pre-cooling of the cast billet before rolling; the initial rolling temperature is 870±30℃, and the final rolling temperature is 820±30℃; after rolling, it is ultra-rapidly cooled to 500℃, with an initial cooling temperature of 780~800℃, a reddening temperature of 500~550℃, and then air-cooled to room temperature. The resulting mold steel plate exhibits excellent uniformity in cross-sectional hardness, with a hardness difference between the core and the edge of HRC≤2.0, tensile strength≥1150MPa, and transverse impact energy≥35 J. It also possesses excellent wear resistance, impact resistance, and good machinability.

Description

Technical Field

[0001] This invention belongs to the field of mold steel preparation technology, specifically relating to a low-cost production process for low-C mold steel plates. Background Technology

[0002] With the rapid development of industries such as automotive, electronics, aerospace, and home appliances, the demand for mass production has driven continuous improvements in mold steel to meet requirements for high strength, wear resistance, and fatigue resistance. The emergence of advanced metallurgical technologies and heat treatment processes, such as vacuum smelting, powder metallurgy, and thermomechanical controlled machining (TMCP), has continuously improved the quality and performance of mold steel. However, with the increasing complexity of mold designs, traditional mold steels can no longer meet new high-performance requirements. The use of new materials such as plastics and aluminum alloys necessitates that mold steels possess better wear resistance, uniform hardness, and machinability. The trend towards lower carbon content and lower cost in attached mold steel plates has led to the development of low-cost, low-carbon mold steel plates with simplified production processes, which has become a key research area, and related patents already exist.

[0003] CN202410752984.2, "An H13 mold steel and its production method," discloses a process involving hot metal pretreatment, converter steelmaking, LF refining, RH vacuum treatment, continuous casting, and slow cooling in a pit. This method produces H13 mold steel via a converter, resulting in low production costs and a short production cycle. It also solves the problems of alloys being difficult to melt and phosphorus reversion in molten steel. However, a drawback is that its composition contains 0.8%–1.2% V, which increases production costs.

[0004] CN202311183668.X, entitled "A Large-Size 3Cr17 Plastic Mold Steel and Its Production Method," discloses a process involving heating an electroslag ingot, upsetting and drawing, heat treatment, circulating water cooling, air cooling, tempering, and cooling to room temperature. This solves the ferrite problem in thick-sized plastic mold modules and achieves low-cost pre-hardening treatment for large 3Cr17 modules, significantly improving polishability and corrosion resistance. However, its drawbacks include a Cr content of 16.35-17.0% and the addition of 0.050%-0.1% Ni, along with the need for multiple heat treatment processes, further increasing costs and resulting in lower production efficiency.

[0005] CN202410838542.X, entitled "A Plastic Mold Steel and its Manufacturing Process," discloses a process involving smelting, vacuum degassing, heat preservation, forging, normalizing, upsetting, drawing, and heat treatment. The resulting plastic mold steel exhibits uniform hardness distribution along its cross-section, strong corrosion resistance, and good sawing performance. However, its drawbacks include the presence of 0.5%–1.3% Ni in its composition, and the need for forging and normalizing processes, which increases production cycle and cost.

[0006] As can be seen from the above-disclosed patents, the current production of low-C mold steel has the following main shortcomings: First, the steel has a large number of alloying elements and a high content, which increases the production cost; second, the heat treatment process after the steel plate is formed is more complicated, which further extends the production cycle. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a production process for low-C mold steel plates with low production cost and short production cycle.

[0008] The content of this invention is implemented as follows: A low-cost production process for low-C mold steel plates, wherein the chemical composition of the steel plate, by weight percentage, is C=0.31%~0.41%, Si=0.2%~0.4%, Mn=1.3%~1.7%, Cr=1.5%~2.0%, Mo=0.20%~0.35%, Nb=0.10%~0.30%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable trace chemical elements; the process steps include: (1) Smelting and casting: The steel with the above composition is smelted by conventional vacuum smelting to obtain molten steel, and the molten steel is cast into billets, which are then smelted by electric arc furnace and refined by LF+RH. (2) Heating and heat preservation: The billet is sent into the heating furnace for uniform heating. The heating temperature is controlled between 1150±30℃ and the heat preservation time is more than 2 hours. (3) Pre-cooling: The billet is pre-cooled in two continuous stages: the first stage uses a small amount of water and a slow roller speed for cooling; the second stage uses a large amount of water and a fast roller speed for cooling. (4) Differential temperature rolling: The pre-cooled steel billet is rolled by the rolling mill. The initial rolling temperature is controlled at 870±30℃, the final rolling temperature is 820±30℃, and the reduction rate per pass is 25%~35%; (5) Cooling after rolling: The rolled steel plate is cooled to 500°C at ultra-fast temperature, the initial cooling temperature is 780-800°C, the red temperature is 500-550°C, and then air-cooled to room temperature.

[0009] The steel plate obtained by the above method has uniform cross-sectional hardness, with a Rockwell hardness HRC difference between the core and the edge of ≤2.0, and meets the requirements of tensile strength Rm≥1150MPa and transverse impact energy KV2≥35 J.

[0010] The following is a detailed analysis and explanation of the role and selection range of the components contained in the low-cost, low-C mold steel plate of the present invention.

[0011] Carbon (C) is one of the main elements affecting the hardness and strength of steel. With increasing C content, the hardness and strength of steel generally increase. Mold steel, under high hardness, can better resist wear and maintain its shape and dimensions, thus extending the service life of the mold. The addition of C also helps to form harder C compounds, further improving the wear resistance of the steel. In this invention, the weight percentage of C is controlled at 0.31% to 0.41%.

[0012] The addition of Si can significantly improve the high-temperature oxidation resistance of steel and can form a solid solution strengthening effect. In this invention, the weight percentage of Si is controlled at 0.20% to 0.40%.

[0013] Mn enhances the strength and hardness of steel through solid solution strengthening, enabling the steel to maintain good performance under high temperature and high stress conditions. Especially in mold steel, Mn helps to improve the wear resistance and pressure resistance of the steel. In this invention, the weight percentage of Mn is controlled at 1.30% to 1.70%.

[0014] Mo can significantly enhance the hardenability of steel, promote the formation of bainite, reduce temper brittleness, and improve the steel's resistance to delayed fracture. Simultaneously, the addition of Mo enhances the stability of carbon compounds and inhibits their coarsening tendency. In this invention, the weight percentage of Mo is controlled at 0.20%–0.35%.

[0015] Nitrogen (Nb), as a potent microalloying element, primarily functions as a grain refiner and precipitation strengthener in steel. During heating and rolling, undissolved Nb(C,N) particles effectively pin grain boundaries, significantly inhibiting austenite grain coarsening during these processes and laying the foundation for a fine room-temperature microstructure. Under controlled rolling and cooling conditions, Nb dissolved in austenite can precipitate as nano-sized NbC during phase transformation or in ferrite, producing a significant precipitation strengthening effect and further enhancing the steel's strength and toughness. This invention controls the Nb weight percentage to be between 0.10% and 0.30%.

[0016] Cr enhances the hardness and strength of steel through solid solution strengthening and promotes martensite formation, thus playing an important role in improving the hardness of die steel after quenching. Cr combines with C to form chromium carbides (Cr3C2, Cr7C3, etc.), and these hard phases can enhance the wear resistance of steel. In this invention, the weight percentage of Cr is controlled at 1.50% to 2.00%.

[0017] P and S are unavoidable harmful impurity elements in steel. P atoms can enter the grain boundaries of steel, damaging their strength and making the steel prone to brittle fracture under stress or impact, especially at low temperatures, affecting the service life and reliability of molds. S combines with iron in steel to form iron sulfides, which are distributed along the grain boundaries, leading to a significant decrease in the toughness and plasticity of the steel. Therefore, the P and S content in steel should be strictly limited. This invention controls the weight percentage of P to ≤0.010% and the weight percentage of sulfur to ≤0.005%.

[0018] The beneficial effects of this invention are as follows: By optimizing the alloy system and reducing the content of precious metal elements, combined with a unique differential temperature rolling process, a reasonable temperature gradient is formed in the thickness direction of the die steel plate. This process increases the deformation resistance of the surface layer due to the low temperature, forcing the rolling deformation force to be more effectively transferred and penetrated to the high-temperature, high-plasticity core region, thereby achieving deeper and more uniform plastic deformation along the thickness direction. Ultimately, a die steel plate with excellent comprehensive performance can be obtained without complex heat treatment after rolling. This steel plate not only has good wear resistance and impact resistance, but more importantly, its cross-sectional hardness uniformity is significantly improved, and the hardness difference between the core and the edge can be controlled within 2.0 HRC. At the same time, it ensures a tensile strength ≥1150MPa and a transverse impact energy ≥35 J, and has good processing performance, achieving a balance between low cost and high performance. Attached Figure Description

[0019] Figure 1 This is a process flow diagram of an embodiment of the present invention. Detailed Implementation

[0020] The present invention will be further described below through specific embodiments.

[0021] Examples 1 to 4: A low-cost production process for low-C mold steel plates is described, with the chemical composition and mass percentage shown in Table 1. The process steps include: (1) Smelting and casting: The steel with the above composition is smelted by conventional vacuum smelting to obtain molten steel, and the molten steel is cast into billets, which are then smelted by electric arc furnace and refined by LF+RH. (2) Heating and heat preservation: The billet is sent into the heating furnace for uniform heating. The heating temperature is controlled between 1150±30℃ and the heat preservation time is more than 2 hours. (3) Pre-cooling: The billet is pre-cooled in two continuous stages: the first stage uses a small amount of water and a slow roller speed for cooling; the second stage uses a large amount of water and a fast roller speed for cooling. (4) Differential temperature rolling: The pre-cooled steel billet is rolled by the rolling mill. The initial rolling temperature is controlled at 870±30℃, the final rolling temperature is 820±30℃, and the reduction rate per pass is 25%~35%; (5) Cooling after rolling: The rolled steel plate is cooled to 500°C at ultra-fast temperature, the initial cooling temperature is 780-800°C, the red temperature is 500-550°C, and then air-cooled to room temperature.

[0022] The process parameters for the example are shown in Table 2, and the performance test results of the steel are shown in Table 3.

[0023] Comparative Examples 1 and 2 are plastic mold steels with different element contents. The specific chemical composition and mass percentage are shown in Table 1; the process parameters are shown in Table 2; and the performance test results of the steel are shown in Table 3.

[0024] Table 1 Chemical composition (wt.%) of the examples and comparative examples .

[0025] Table 2. Main process parameters of the examples and comparative examples .

[0026] Table 3 shows the performance testing methods for the steel. The room temperature tensile strength of the obtained mold steel was tested according to the testing methods in GB / T 228.1-2021 "Metallic Materials - Tensile Testing"; the transverse impact energy of the obtained mold steel was tested at room temperature (20℃) according to the testing methods in GB / T 229-2020 "Metallic Materials - Charpy Pendulum Impact Test"; and the hardness of the obtained mold steel was tested according to the testing methods in GB / T230.1-2008 "Metallic Materials - Rockwell Hardness Test".

[0027] Table 3 Performance test results of steel produced in the examples and comparative examples .

Claims

1. A low-cost production process for low-C mold steel plates, characterized in that: The chemical composition of the steel plate, by weight percentage, is C=0.31%~0.41%, Si=0.2%~0.4%, Mn=1.3%~1.7%, Cr=1.5%~2.0%, Mo=0.20%~0.35%, Nb=0.10%~0.30%, P≤0.010%, S≤0.005%, with the remainder being iron and unavoidable trace chemical elements; the process steps include: (1) Smelting and casting: The steel with the above composition is smelted by conventional vacuum smelting to obtain molten steel, and the molten steel is cast into billets, which are then smelted by electric arc furnace and refined by LF+RH. (2) Heating and heat preservation: The billet is sent into the heating furnace for uniform heating. The heating temperature is controlled between 1150±30℃ and the heat preservation time is more than 2 hours. (3) Pre-cooling: The billet is pre-cooled in two continuous stages: the first stage uses a small amount of water and a slow roller speed for cooling; the second stage uses a large amount of water and a fast roller speed for cooling. (4) Differential temperature rolling: The pre-cooled steel billet is rolled by the rolling mill. The initial rolling temperature is controlled at 870±30℃, the final rolling temperature is 820±30℃, and the reduction rate per pass is 25%~35%; (5) Cooling after rolling: The rolled steel plate is cooled to 500°C at ultra-fast temperature, the initial cooling temperature is 780-800°C, the red temperature is 500-550°C, and then air-cooled to room temperature.

2. The low-cost production process for low-C mold steel plates according to claim 1, characterized in that: The steel plate has uniform cross-sectional hardness, with a Rockwell hardness HRC difference between its core and edge ≤2.0, and meets the requirements of tensile strength Rm≥1150MPa and transverse impact energy KV2≥35 J.

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