Production process of large-thickness high-internal-quality die steel plate

Abstract

According to the production process of the large-thickness high-internal-quality die steel plate, the thickness of the produced steel plate is 120-220 mm, and steel comprises the following chemical components in percentage by mass: 0.35%-0.45% of C, 0.20%-0.40% of Si, 1.30%-1.60% of Mn, less than or equal to 0.015% of P, less than or equal to 0.002% of S, 1.80%-2.00% of Cr, 0.15%-0.25% of Mo, 0.90%-1.20% of Ni and the balance of Fe and inevitable impurities. The production process route is BOF-LF-VD-continuous casting-plate blank diffusion annealing-rolling-heat treatment, the diffusion annealing rolling process is adopted in the steel plate rolling process, the difference between the microhardness of a core segregation zone and a base body is smaller than or equal to 50 HV1, and the internal quality of the steel plate meets the NB / T47013.3 I-level flaw detection requirement.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgy and relates to a production process of a die steel plate with large thickness and high internal quality. Background Art

[0002] 2738H, a typical high-grade, mirror-finished, pre-hardened plastic mold steel, boasts a high alloy content and excellent pre-hardening properties, making it widely used in the market. This steel possesses excellent polishing and etching properties and is primarily used in mold cores, requiring high material density and uniform internal hardness. The thickness of billet-rolled steel in the international market is generally ≤120mm. A few companies can produce 2738H mold steel in sizes from 120 to 260mm, but this production process is complex, resulting in low efficiency and high steelmaking requirements.

[0003] Chinese patent CN202211607361.3 discloses a method for producing large-thickness SM3Cr2Ni1Mo mold steel, but it uses water-cooled ingots, has high continuous casting superheat, and is difficult to control internal segregation and porosity. It also has a low yield and high cost.

[0004] Chinese patent CN201910580856.3 discloses a method for producing ultra-thick alloy mold steel plates from continuous casting slabs, and a manufacturing method thereof. Chinese patent CN201710269705.7 discloses a 100-260mm thick, quenched and tempered, pre-hardened plastic mold steel plate, and a manufacturing method thereof. However, the Ni content in these two mold steels is ≤0.30%, making them unsuitable for the production of high-Ni mirror-finish mold steels. Furthermore, the rolling process employs a "differential temperature + high reduction rolling" method, placing high demands on the mill's capacity.

[0005] Chinese patent CN201811201427.2 discloses a continuous casting production process for SM4Gr2MnNi mold steel plates with low reduction ratio and high flaw detection requirements. It focuses on improving the internal quality of the slab from the perspective of continuous casting, has extremely high requirements for the steelmaking and continuous casting processes, and adopts two-fire slab production, supplemented by a high-temperature, low-speed, large-reduction process. The rolling process is complex and cannot completely improve the hardness of the core segregation zone. Summary of the Invention

[0006] The present invention aims to provide a production process for thick die steel plates with low compression ratio and high internal quality. Without modifying existing steelmaking and continuous casting equipment, the present invention improves the rolling process to produce thick 27.8H mirror-finished die steel, thereby obtaining high-internal-quality plastic die steel plates with a maximum thickness of 240 mm and high flaw detection requirements. The steel plates have excellent internal density and hardness uniformity, are tolerant to the quality requirements of the core of the slab, and have high production efficiency.

[0007] The technical solution of the present invention: A production process for a mold steel plate with large thickness and high internal quality. The production process route of the steel plate is BOF - LF - VD - continuous casting - slab diffusion annealing - rolling - heat treatment. The thickness of the produced steel plate is 120 - 220 mm, and the reduction ratio is ≥2.0. The chemical composition of the steel in mass percentage is C = 0.35% - 0.45%, Si = 0.20% - 0.40%, Mn = 1.30% - 1.60, P ≤ 0.015%, S ≤ 0.002%, Cr = 1.80% - 2.00%, Mo = 0.15% - 0.25%, Ni = 0.90% - 1.20%, and the balance is Fe and inevitable impurities. The key process steps include: 1) Steelmaking: The production process is BOF - LF - VD. The residual elements P ≤ 0.015% and S ≤ 0.002%. The vacuum treatment process meets the requirement that the [H] at tapping is ≤1.5 ppm. 2) Continuous casting: It is produced by a 450 - mm large - thickness continuous caster. Low - superheat steel pouring is adopted, and the superheat is controlled at 15 - 30°C. Large reduction with the assistance of convex - roll reduction technology is used in continuous casting. The macrostructure quality of the billet meets the requirements that the center segregation is ≤ class B 1.5 or class C ≤ 2.0; the center porosity is ≤ 1.5, and there are no triangular - zone cracks. After the slab is taken off the production line, it is stacked to release stress and hydrogen, and the stacking cooling time is ≥24 h. 3) Slab heating: The slab is heated and diffusion - annealed in a uniform furnace. For slabs with segregation ≤ class C 1.0, the required diffusion - annealing time is ≥12 h; for slabs with segregation in class C 1.5 - 2.0 and class B ≤ 1.5, the diffusion - annealing time is ≥24 h. 4) Steel plate rolling: The diffusion - annealing rolling process is adopted. The reduction per pass in rough rolling is ≥40 - 60 mm. After rolling, the steel plate is taken off the production line and stacked for slow cooling to release stress and hydrogen, and the stacking cooling time is ≥48 h. 5) Heat treatment: Normalizing + tempering heat treatment is carried out on the steel plate. The normalizing process temperature is 880 - 900°C, and the heating time is 1.8 - 2.2 min / mm; the tempering process temperature is 550 - 600°C, and the heating time is 1.8 - 2.2 × plate thickness (mm) × min / mm.

[0008] During the steel plate rolling process, the difference in micro - hardness between the center segregation area and the matrix is ≤50 HV1, and the internal quality of the steel plate meets the requirements of level I flaw detection in NB / T47013.3.

[0009] Advantages of the present invention: Based on the analysis of internal porosity and segregation of 2738H mold steel, the diffusion - annealing + large - reduction rolling technology is adopted during the rolling process, effectively improving the internal quality of the large - thickness 2738H mirror - type mold steel, and solving the problems of internal hardness uniformity and internal quality of large - thickness molds. The process adjustment scheme involved in the present invention has a high tolerance for the macrostructure quality of the slab, and the production process is simple, suitable for industrial mass production. Brief Description of the Drawings

[0010] Figure 1 It is a metallographic photo of the core of 2738H steel plate produced by ordinary rolling process.

[0011] Figure 2 It is a micro-porosity photo of the core of 2738H steel plate produced by ordinary rolling process.

[0012] Figure 3 It is a metallographic photo of the core of 2738H steel plate produced by the rolling process of the present invention.

[0013] Figure 4 It is a micro-porosity photo of the core of 2738H steel plate produced by the rolling process of the present invention. Detailed Description of the Invention

[0014] Examples 1 to 3: A production process for large-thickness high-internal-quality die steel plates. The production process route of the steel plate is BOF - LF - VD - continuous casting - slab diffusion annealing - rolling - heat treatment. The thickness of the produced steel plate is 120 - 220 mm, and the reduction ratio is ≥2.0. The chemical composition of the steel by mass percentage is shown in Table 1, and the balance is Fe and unavoidable impurities. The key process steps include: 1) Steelmaking: The production process is BOF - LF - VD. VD treatment is used for vacuum, and the holding time is ≥15 min; 2) Continuous casting: Continuous casting is carried out using a 450 mm large-thickness continuous caster. The superheat control, continuous casting reduction, and macrostructure production conditions are shown in Table 2. The stacking cooling time requirement for the slab is ≥24 h; 3) Slab heating: The slab is heated and diffusion annealed in a uniform furnace. For slabs with segregation ≤ Class C 1.0 level, the diffusion annealing time requirement is ≥12 h; for slabs with segregation in Class C 1.5 - 2.0 and Class B ≤ 1.5 level, the diffusion annealing time is ≥24 h. The slab heating process in the examples is carried out as shown in Table 3; 4) Steel plate rolling: Heavy reduction process is used for rough rolling. After the rolled steel plate is taken off the production line, it is subjected to stacking cooling treatment, and the stacking cooling time requirement is ≥48 h to avoid the occurrence of microcracks in the core. The rolling process parameters are shown in Table 3; 5) Heat treatment: The 2738H large-thickness die steel is produced by normalizing + tempering process. The heat treatment process execution parameters are shown in Table 3;

[0015] Final product inspection: After heat treatment, the 2738H steel plate is subjected to hardness inspection. The hardness uniformity of the whole thickness is good. The hardness difference between the core segregation area HV1 and the matrix hardness is ≤50 HV1. The flaw detection meets the requirements of Class I flaw detection in NB / T47013.3, and the internal quality is good. The inspection results are shown in Table 4.

[0016] Table 1 Smelting Composition of Examples (wt%) .

[0017] Table 2 Continuous casting process and macrostructure rating of examples .

[0018] Table 3 Rolling process and heat treatment process of examples .

[0019] Table 4 Hardness uniformity (HV1) and flaw detection results of examples .

Claims

1. A production process for a mold steel plate with large thickness and high internal quality, characterized in that: The produced steel plate has a thickness of 120 - 220 mm and a compression ratio of ≥2.

0. The mass percentages of the chemical components of the steel are as follows: C = 0.35% - 0.45%, Si = 0.20% - 0.40%, Mn = 1.30% - 1.60, P ≤ 0.015%, S ≤ 0.002%, Cr = 1.80% - 2.00%, Mo = 0.15% - 0.25%, Ni = 0.90% - 1.20%, and the balance is Fe and inevitable impurities. The production process route of the steel plate is BOF - LF - VD - continuous casting - slab diffusion annealing - rolling - heat treatment. The key process steps include: 1) Steelmaking: The production process is BOF - LF - VD. The residual elements are P ≤ 0.015% and S ≤ 0.002%. The vacuum treatment process meets the requirement that the [H] content at tapping is ≤1.5 ppm. 2) Continuous casting: It is produced by a 450 - mm large - thickness continuous caster. The steel is poured with a low superheat, and the superheat is controlled at 15 - 30°C. The continuous casting adopts large reduction supplemented by convex roll reduction technology. The macro - structure quality of the billet meets the requirements that the center segregation is ≤class B 1.5 or class C ≤2.0; the center porosity is ≤1.5, and there are no triangular - zone cracks. After the slab is taken off the production line, it is stacked to release stress and hydrogen, and the stacking cooling time is ≥24 h. 3) Slab heating: The slab is heated and diffusion - annealed in a uniform furnace. For slabs with segregation ≤class C 1.0, the required diffusion - annealing time is ≥12 h; for slabs with segregation in class C 1.5 - 2.0 and class B ≤1.5, the diffusion - annealing time is ≥24 h. 4) Steel - plate rolling: The diffusion - annealing rolling process is adopted. The reduction per pass in rough rolling is ≥40 - 60 mm. After rolling, the steel plate is taken off the production line and stacked for slow cooling to release stress and hydrogen, and the stacking cooling time is ≥48 h. 5) Heat treatment: The steel plate is subjected to normalizing + tempering heat treatment. The normalizing process temperature is 880 - 900°C, and the heating time is 1.8 - 2.2 min / mm; the tempering process temperature is 550 - 600°C, and the heating time is 1.8 - 2.2 × plate thickness (mm) × min / mm.

Images