A 250-300mm thick 3cr2mo pre-hardened die steel plate and a manufacturing method thereof

Abstract

The present application relates to a kind of 250-300mm thick 3Cr2Mo pre-hard type die steel plate manufacturing method, belong to metallurgical technical field.The chemical composition of steel mass percent is, C:0.30~0.40%, Si:0.40~0.80%, Mn:0.60~1.00%, P≤0.013%, S≤0.002%, Cr:1.60~2.00%, Mo:0.30~0.55%, V+Ti+Al:0.02~0.25%, the rest is iron and other inevitable impurities.Through low segregation continuous casting slab after rolling on-line quenching plus tempering realizes the pre-hardening of extra-thick large size 3Cr2Mo die steel plate, the process involved obtains extra-thick large size die steel plate, hardness 280-330HBW, cross section hardness difference is within 20HBW, flaw detection NB / T47013.3 TⅠ is qualified.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical technology, specifically relating to a pre-hardened mold steel plate and its manufacturing method. Background Technology

[0002] 3Cr2Mo mold steel, due to the addition of Cr and Mo alloying elements, has high hardenability, allowing for uniform hardness even in larger cross-sections. It also exhibits good polishability, resulting in a high surface finish for molds. This makes it suitable for manufacturing large plastic molds such as those for television sets, large audio equipment casings, and washing machine panels. With the increasing size of molds, the demand for extra-thick and wide steel plates is growing. However, as the cross-sectional dimensions of the steel increase, the tendency for compositional segregation increases, and the difficulty in controlling cooling uniformity increases. This leads to uneven steel microstructure and hardness, affecting the polishability of the steel and consequently reducing the surface finish of the molds.

[0003] Chinese patent publication number CN 106350643 A, published on November 16, 2018, entitled "A Heat Treatment Method for Large P20 Plastic Mold Steel," discloses a heat treatment method for large P20 plastic mold steel. The method includes: quenching heating at 860–920℃ for 9–24 hours; quenching cooling consisting of first air cooling for 2–3 minutes after removal from the furnace, followed by spray cooling for 2–4 minutes, then water cooling to a surface temperature of 700℃, followed by air cooling for 2–3 minutes, then continued water cooling to 400℃, followed by air cooling for 3–5 minutes, then continued water cooling to 200℃, and finally air cooling after removal from the heat treatment tank. The quenching cooling process is complex and suitable for heat treatment of forged modules, but difficult to implement in steel plate heat treatment processes.

[0004] Chinese Patent Publication No. CN 107081412 A, published on August 22, 2017, entitled "Preparation Method of High-Quality Plastic Mold Steel Extra-Thick Plate Continuous Casting Master Billet," discloses a method for preparing a high-quality plastic mold steel extra-thick plate continuous casting master billet. This method employs dynamic heavy pressure technology at the end of solidification to improve the internal quality of the billet, thereby mitigating center segregation and center porosity defects. It effectively improves the internal quality of thick plates with low compression ratios under the target of producing 120 mm extra-thick plates. However, the thinness of the steel plate cannot meet the requirements of large-size molds.

[0005] Chinese Patent Publication No. CN 112481546 A, published on March 12, 2021, entitled "A Production Method of P20 Steel Plate for Extra-Thick Plastic Molds," discloses a method for producing P20 steel plates for extra-thick plastic molds. The steel plate is 200-300mm thick. The method employs a series of processes including water-cooled casting, ingot cleaning and heating, rolling, rapid cooling after rolling, quenching in a water tank, stacking cooling, and tempering heat treatment. The resulting steel plate achieves internal quality comparable to forgings, possessing high hardness and a hardness difference within 40HB. However, quenching in a water tank can easily lead to the formation of an air film on the surface of the steel plate at high temperatures, affecting cooling uniformity. Furthermore, water-cooled ingot rolling results in a low yield and high manufacturing costs.

[0006] Chinese patent publication number CN 117758149 A, published on March 26, 2024, entitled "A 280mm Thick Low-Cost High-Polishability P20 Plastic Mold Steel and Its Production Method," discloses a 280mm thick low-cost, high-polishability P20 plastic mold steel and its production method, containing the following chemical composition (unit, %) by mass fraction: C 0.35~0.38, Si 0.20~0.40, Mn 1.5~1.6, P 0.013~0.020, S≤0.002, Cr 1.8~1.9, Mo 0.25~0.30, Ti 0.020~0.040, Al 0.030~0.060, B 0.0008~0. The balance of .0010 is Fe and unavoidable impurities. 300g of rare earth silicon alloy (containing Ce) is added in batches of 100kg. This invention refines grains and reduces dendritic and regional segregation by controlling the water-cooling intensity during the water-cooled ingot casting process, thereby improving the low-magnification quality of the steel plate. The resulting structure is uniform after rolling, with a significant grain refinement effect, particularly a marked improvement in the polishability of the steel plate. However, rolling from ingots results in a low yield and high manufacturing cost.

[0007] In summary, continuing to strengthen the research and development of large-size 3Cr2Mo steel plates and improving the manufacturing methods of pre-hardened steel plates remains a research direction for those skilled in the art. Summary of the Invention

[0008] To address the technical challenges of existing production technologies for extra-thick, large-size mold steel plates, this invention provides a new process for producing 250-300mm thick 3Cr2Mo pre-hardened mold steel plates. This process involves rolling extra-thick continuously cast slabs into finished products, followed by appropriate heating and rolling, post-rolling heat quenching, and a strict tempering process. This yields large-size steel plates with a thickness of 250-300mm, a width of over 2200mm, a hardness of 280-330HBW, a cross-sectional hardness difference of less than 20HBW, and passing the NB / T 47013.3 TⅠ flaw detection.

[0009] The technical solution adopted in this invention is a method for manufacturing a 250-300mm thick 3Cr2Mo pre-hardened steel plate for molds. The chemical composition of the steel by mass percentage is as follows: C: 0.30-0.40%, Si: 0.40-0.80%, Mn: 0.60-1.00%, P≤0.013%, S≤0.002%, Cr: 1.60-2.00%, Mo: 0.30-0.55%, V+Ti+Al: 0.02-0.25%, with the remainder being iron and other unavoidable impurities.

[0010] The roles of the components and their contents in the steel in this invention: Carbon (C) is an important element for ensuring the hardness of mold steel plates. Its solid solution strengthening effect is significant, and an appropriate carbon content helps to improve the hardness of steel. However, excessive carbon content will reduce the toughness of the steel plate, which is detrimental to the subsequent processing and use of mold steel. This invention controls the C content to be 0.30-0.40%.

[0011] Si is an important deoxidizing element in steel, typically used for solid solution strengthening to improve steel strength. Appropriate amounts of silicon help increase the yield point and tensile strength of steel. In this invention, the Si content is controlled at 0.40–0.80%.

[0012] Mn is an element that improves the hardenability of steel and has a solid solution strengthening effect, making it an important element for increasing the strength of steel plates. In this invention, the Mn content is controlled at 0.6–1.00%.

[0013] Cr: This element improves the hardenability of steel, enhancing the hardness, wear resistance, and corrosion resistance of mold steel. Furthermore, an appropriate chromium content can inhibit the formation of pearlite and polygonal ferrite, promoting the transformation of bainite or martensite in low-temperature structures. In this invention, its content is controlled at 1.60–2.00%.

[0014] Mo (Mo) can significantly improve the hardenability and strength of steel. Molybdenum can improve the hardenability, hot strength, and red hardness of steel. An appropriate molybdenum content allows mold steel to maintain sufficient strength and creep resistance at high temperatures. However, Mo is an expensive alloy, and a high Mo content increases production costs. In this invention, the Mo content is controlled between 0.30% and 0.55%.

[0015] P: Phosphorus is an impurity element in steel, which easily forms intergranular segregation, adversely affecting the toughness and hot workability of steel plates. Its content should be minimized. This invention controls P ≤ 0.013%.

[0016] S: Sulfur is a harmful impurity element in steel. It easily forms segregation and inclusions, reducing the ductility, toughness and polishability of steel. Its content should be minimized. In this invention, S is controlled to be ≤0.002%.

[0017] Ti, V, and Al are microalloying elements. Titanium is an important nitrogen-fixing element; the formed TiN can effectively prevent the coarsening of austenite grains during high-temperature heating. Vanadium forms carbonitrides, which have unique advantages in precipitation strengthening; appropriate vanadium content can improve the strength and hardness of steel plates. Aluminum mainly plays a role in nitrogen fixation and deoxidation; AlN, formed by the combination of Al and N, can effectively refine grains. The combined use of these elements helps improve the overall performance of mold steel. This invention simultaneously adds V, Ti, and Al, controlling the total V+Ti+Al content to be between 0.02% and 0.25%.

[0018] The key process steps are as follows: (1) Smelting: Clean steel smelting method is adopted to control the phosphorus, sulfur, oxygen, hydrogen content and oxide inclusions in the molten steel to the design requirements. After LF furnace refining and RH vacuum degassing, the hydrogen content in the molten steel is not greater than 1.0 ppm, the oxygen content is not greater than 15 ppm, and the total of inclusions of type A, B, C and D is not greater than grade 3.0.

[0019] Continuous casting: Thick slabs are continuously cast using a straight-arc continuous casting machine. By precisely controlling the superheat of molten steel within 20°C, the casting speed fluctuation within ±0.01m / min, and the final reduction at the solidification end greater than 15mm, the center segregation of extra-thick slabs is optimized, meeting the Class C 0.5 grade of low-magnification center segregation of slabs.

[0020] (3) Heating: The billet is heated in a walking beam furnace. The temperature of the soaking zone is 1180-1260℃ and the soaking time is 4-6 min / cm. The core temperature of the billet after exiting the furnace is not lower than 1180℃. The long soaking time promotes the high-temperature diffusion of carbon in the steel, further improves the component segregation formed by selective crystallization during the solidification process of the billet, and achieves uniform composition of the steel plate.

[0021] (4) Rolling: The rolling process employs a directional rolling widening-vertical roll edge trimming-longitudinal high-reduction rolling. The initial rolling temperature is 1050-1200℃, using a high-temperature high-reduction rolling process with a maximum reduction of not less than 60mm per pass. The final rolling temperature is 850-900℃, utilizing residual heat after rolling for quenching. The steel plate is then subjected to reciprocating water cooling after rolling, with a water-to-water ratio of 2:1, and the steel plate exiting the water at a temperature less than 350℃. The cooling rate of the steel plate is greater than 0.5℃ / s. Post-rolling water cooling increases the cooling rate of the steel plate, improves the uniformity of the cross-sectional structure, inhibits pearlite transformation, and completes bainite transformation.

[0022] (5) Tempering: High-temperature tempering is adopted, with a tempering temperature of 500-600℃ and a holding time of 4-6 min / mm. After exiting the furnace, the steel plate is air-cooled. The tempering time is guaranteed to fully eliminate the stress in the steel plate structure. At the same time, the precipitation of carbides such as Mo2C occurs, the hardness of the steel plate decreases slightly, and the plasticity and toughness are improved to meet the performance requirements of the mold. The final steel plate structure is a bainitic tempered structure with no low-hardness pearlite structure in the center of the thickness.

[0023] 3Cr2Mo pre-hardened thick steel plate was obtained.

[0024] Compared with the prior art, the advantages of the present invention are as follows: This invention employs clean steel smelting, controls the content of easily segregated impurity elements P and S, and improves C segregation through continuous casting constant casting speed and large reduction technology at the end of solidification. The segregation at the center of the billet reaches the optimal level of C class 0.5.

[0025] During the hot rolling process, the soaking time is 4-6 min / cm. The prolonged soaking time promotes high-temperature diffusion of carbon in the steel, further improving the compositional uniformity caused by selective crystallization during slab solidification. After rolling, online residual heat quenching is performed, with the steel plate exiting the water at a temperature below 350℃. Online water cooling avoids the lower hardness caused by the pearlite transformation in the core of air-cooled steel plates and the uneven hardness caused by the cooling film on the surface of quenched steel plates in the quenching tank, ensuring uniform hardness across the cross-section of large-size die steel and achieving high polishability. Simultaneously, the process is shortened, and energy consumption is reduced.

[0026] The 3Cr2Mo pre-hardened thick steel plate produced by the manufacturing method of this invention has a thickness of 250-300mm, a width greater than 2200mm, a hardness of 280-330HBW, a cross-sectional hardness difference within 20HBW, and passes the flaw detection standard NB / T 47013.3 TⅠ.

[0027] Compared with existing technologies, the advantages of this invention are as follows: Pre-hardening of extra-thick, large-size 3Cr2Mo die steel plates is achieved through online quenching and tempering after rolling of low-segregation continuously cast slabs. The extra-thick, large-size die steel plates obtained by this process have a hardness of 280-330 HBW, a cross-sectional hardness difference within 20 HBW, and pass the NB / T 47013.3 TⅠ flaw detection. Furthermore, the yield of rolled steel plates from continuously cast slabs reaches over 90%, far exceeding the yield of rolled plates from die-cast ingots, thus reducing production costs. Attached Figure Description

[0028] Figure 1 This is a typical metallographic structure at 1 / 2 of a 300mm thick plate according to an embodiment of the present invention. The entire structure is tempered bainite. Figure 2 This is a typical metallographic structure diagram of 1 / 4 of a 300mm thick plate according to an embodiment of the present invention. The structure is entirely tempered bainite. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the accompanying drawings. The embodiments described are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Example 1

[0030] The extra-thick, large-size plastic mold steel plate involved in this embodiment has a thickness of 260mm, and its chemical composition by mass percentage is: C: 0.36%, Si: 0.51%, Mn: 0.89%, P: 0.012%, S: 0.001%, Cr: 1.90%, Mo: 0.46%, V+Ti+Al: 0.06%, with the balance being Fe and unavoidable impurity elements.

[0031] The manufacturing process of this steel plate is as follows: Production process flow: Converter steelmaking → Refining → Vacuum degassing → Continuous casting billet → Heating → Rolling → Rolling residual heat quenching → Tempering → Flaw detection → Finishing → Inspection → Warehousing.

[0032] The specific operations of the main processes are as follows: The molten steel undergoes refining and vacuum degassing to reduce harmful elements such as P and S. The casting speed fluctuation range of the continuous casting billet is 0.01m / s, and the solidification end reduction is 17mm, which ensures the internal quality of the extra-thick continuous casting billet. The low magnification center segregation of the slab is Class C 0.5.

[0033] The billet is heated in a walking beam furnace with a soaking temperature of 1220℃ and a holding time of 5.1 min / cm; the core temperature of the billet after exiting the furnace is 1201℃.

[0034] The steel billet is rolled at high temperature, with an initial rolling temperature of 1061℃. It is then widened to 2390mm by directional rolling, followed by a 40mm reduction using vertical rolls, and then longitudinal rolling with a maximum reduction of 60mm per pass, reaching a final rolling temperature of 893℃. After rolling, the steel plate undergoes online water quenching, with a reciprocating water cooling cycle of 2:1 (top to bottom water ratio), a cooling rate of 0.56℃ / s, and an outlet water temperature of 342℃.

[0035] The steel plate was subjected to tempering heat treatment using a bogie hearth furnace at a tempering temperature of 580℃ and a holding time of 4.5 min / mm.

[0036] The 250mm thick and 2350mm wide steel plate produced by the above manufacturing process has no low-hardness structure at 1 / 4 and 1 / 2 of the thickness. It is entirely bainitic tempered structure with uniform hardness throughout the plate and excellent comprehensive performance. It passes the flaw detection standard NB / T 47013.3 TⅠ. Its main properties are detailed in Table 1. Example 2

[0037] The extra-thick, large-size plastic mold steel plate involved in this embodiment has a thickness of 300mm, and its chemical composition by mass percentage is: C: 0.38%, Si: 0.62%, Mn: 0.91%, P: 0.011%, S: 0.001%, Cr: 1.95%, Mo: 0.52%, V+Ti+Al: 0.11%, with the balance being Fe and unavoidable impurity elements.

[0038] The manufacturing process of this steel plate is as follows: Production process flow: Converter steelmaking → Refining → Vacuum degassing → Continuous casting billet → Heating → Rolling → Rolling residual heat quenching → Tempering → Flaw detection → Finishing → Inspection → Warehousing.

[0039] The specific operations of the main processes are as follows: The molten steel undergoes refining and vacuum degassing to reduce harmful elements such as P and S. The continuous casting billet casting speed fluctuation range is 0 m / s, and the solidification end reduction is 18 mm, ensuring the internal quality of the extra-thick continuous casting billet. The slab exhibits low-magnification center segregation of 0.5% (C type).

[0040] The billet is heated in a walking beam furnace with a soaking zone temperature of 1210℃ and a holding time of 5.3 min / cm; the core temperature of the billet after exiting the furnace is 1196℃.

[0041] The billet is rolled at high temperature without widening. The initial rolling temperature is 1056℃. It is then rolled to widen to 2340mm, then rolled down by vertical rolls by 40mm, and then rolled longitudinally. The maximum reduction per pass is 62mm. The final rolling temperature is 881℃. The billet is then water-quenched online. After rolling, the steel plate is oscillating and water-cooled with a water-to-water ratio of 2:1. The cooling rate of the steel plate is greater than 0.55℃ / s, and the water outlet temperature is 321℃.

[0042] The steel plate was subjected to tempering heat treatment using a bogie hearth furnace at a tempering temperature of 560℃ and a holding time of 5.0 min / mm.

[0043] The 300mm thick and 2300mm wide steel plate produced by the above manufacturing process has no low-hardness structure (such as pearlite) in the 1 / 4 and 1 / 2 thickness, and is entirely bainitic tempered structure. The steel plate has uniform hardness and excellent comprehensive performance. The flaw detection NB / T47013.3 TⅠ is qualified. Its main properties are detailed in Table 1.

[0044] Table 1 shows the hardness of the steel plates produced in each embodiment.

Claims

1. A method for manufacturing a 250-300mm thick 3Cr2Mo pre-hardened mold steel plate, characterized in that: The steel's chemical composition by mass percentage is as follows: C: 0.30–0.40%, Si: 0.40–0.80%, Mn: 0.60–1.00%, P≤0.013%, S≤0.002%, Cr: 1.60–2.00%, Mo: 0.30–0.55%, V+Ti+Al: 0.02–0.25%, with the remainder being iron and other unavoidable impurities. The manufacturing process includes the following steps: Smelting: Clean steel smelting method is adopted, and the content of phosphorus, sulfur, oxygen, hydrogen and oxide inclusions in molten steel are strictly controlled. After refining in LF furnace and vacuum degassing, the hydrogen content of molten steel is not greater than 1.0 ppm, the oxygen content is not greater than 15 ppm, and the total amount of inclusions of categories A, B, C and D is not greater than 3.

0. Continuous casting: Extra-thick slabs are continuously cast using a straight-arc continuous casting machine, and the continuous casting process parameters are controlled to ensure that the low-magnification center segregation of the slab is grade C of 0.

5. Heating: Heating the continuously cast billet, controlling the temperature of the soaking zone to 1180-1260℃, the soaking zone time to 4-6 min / cm, and the core temperature of the billet exiting the furnace not lower than 1180℃; Rolling: Set the initial rolling temperature to 1050-1200℃, adopt a high temperature and large reduction rolling process, with a maximum reduction of not less than 60mm per pass, and a final rolling temperature of 850-900℃. Utilize the residual heat after rolling for quenching, and ensure that the water-cooled steel plate exits at a temperature of less than 350℃ after rolling to complete the transformation of bainite and martensite. Tempering: High-temperature tempering is adopted, with a tempering temperature of 500-600℃ and a holding time of 4-6 min / mm. After exiting the furnace, the steel plate is air-cooled to fully eliminate the structural stress of the steel plate. At the same time, Mo2C carbides precipitate, and the final steel plate structure is bainitic tempered structure.

2. The method for manufacturing a 250-300mm thick 3Cr2Mo pre-hardened mold steel plate according to claim 1, characterized in that: The steel plate is quenched using residual heat after rolling, and then oscillated and water-cooled after rolling with a water-to-water ratio of 2:

1. The water outlet temperature of the steel plate is less than 350℃, and the cooling rate of the steel plate is greater than 0.5℃ / s.

3. The method for manufacturing a 250-300mm thick 3Cr2Mo pre-hardened mold steel plate according to claim 1, characterized in that: The process involves widening by directional rolling, edge trimming by vertical rolls, and longitudinal heavy reduction rolling. The directional widening rolling is completed to a thickness of over 2300mm, followed by vertical rolls for 30-50mm, and then longitudinal heavy reduction rolling.

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