A processing method to improve the performance of large-size DC53 cold-working die steel

Abstract

The invention discloses a processing method for improving the performance of large-size DC53 cold-working die steel, comprising the following steps: S1. spray forming: spray forming the refined DC53 steel liquid by high-pressure inert gas; S2. forging and annealing: first forging and forming the spray-deposited ingot, and then annealing to obtain a module; S3. quenching: keeping the module at 1010-1050 DEG C for 1-4 hours in a vacuum atmosphere, and then quenching by high-pressure inert gas; S4. cryogenic treatment: performing cryogenic treatment at -120-180 DEG C for 4-8 hours; S5. tempering: heating to 500-580 DEG C at a heating rate of ≤300 DEG C / h, keeping the temperature for 4-8 hours, taking out of the furnace for cooling, and tempering 2-3 times. After being treated by the treatment method of the present invention, the carbides of DC53 cold working die steel are dispersed and refined, the content of residual austenite is small, the unevenness of eutectic carbides is within level 1, the toughness of DC53 steel is greatly improved, it has excellent hardness and wear resistance, the deformation after heat treatment is small, and the comprehensive performance is improved compared with DC53 steel made by traditional processes.

Description

Technical Field

[0001] The invention belongs to the technical field of die steel manufacturing, and in particular relates to a processing method for improving the performance of large-size DC53 cold-working die steel. Background Art

[0002] DC53 die steel is a cold working die steel improved on the basis of SKD11 steel. It mainly contains alloy elements such as chromium and molybdenum and a high carbon content. The hardness after heat treatment is usually 56-62HRC. It can be used for stamping automobiles, home appliances, electronics and other products. In the prior art, when solidifying, molybdenum, chromium, vanadium and other elements in steel combine with carbon to easily generate large-sized eutectic carbides, which affects the toughness of the die steel. At the same time, since the generated large-sized eutectic carbides consume more alloy elements, it will affect the generation of fine nano-carbides in the alloy during the heat treatment process, thereby affecting the hardness and wear resistance of the die steel.

[0003] At present, DC53 steel is mainly produced by traditional technology. The main process is: smelting → refining → pouring electrode → electroslag remelting → forging / rolling → annealing. When producing modules with a thickness of more than 150mm, a large number of large-sized eutectic carbides exist in DC53 steel (see below). Figure 1 ), according to GB / T14979-1994 "Evaluation Method for Eutectic Carbide Inhomogeneity of Steel", the eutectic carbide inhomogeneity reaches level 3 or even level 4-5. The eutectic carbide in the steel is large in size, which seriously affects the toughness and wear resistance of DC53 mold steel and reduces the service life of the mold. Summary of the invention

[0004] In view of the deficiencies in the prior art, the present invention provides a processing method for improving the performance of large-size DC53 cold working die steel, which solves the problems of poor toughness, poor wear resistance and low hardness of traditional DC53 die steel, and improves the comprehensive performance of the die steel.

[0005] The purpose of the present invention is achieved by the following technical solutions:

[0006] A processing method for improving the performance of large-size DC53 cold-working die steel comprises the following steps:

[0007] S1. Spray forming: The refined DC53 molten steel is spray-formed by high-pressure inert gas to obtain a spray-deposited ingot;

[0008] S2. Forging and annealing: The spray-deposited ingot is first forged and then annealed to obtain a DC53 module;

[0009] S3. Quenching: The DC53 module is heated to 550-650°C at a heating rate of ≤350°C / h in a vacuum atmosphere and then kept warm for 2-3h, then heated to 800-900°C for 2-4h, then heated to 1010-1050°C at a heating rate of ≤250°C / h for 1-4h, and then quenched by high-pressure inert gas after the end of the insulation;

[0010] S4. Cryogenic treatment: The DC53 module after quenching in step S3 is cryogenically treated at -120 to -180 ° C for 4 to 8 hours;

[0011] S5. Tempering: heating to 500-580°C at a heating rate of ≤300°C / h, keeping the temperature for 4-8h, then cooling out of the furnace, tempering 2-3 times, and obtaining the DC53 cold working die steel.

[0012] Preferably, the chemical composition of the DC53 cold working die steel includes, by mass percentage, C 0.9-1.2%, Si 0.8-1.2%, Mn 0.2-0.5%, Cr 7.0-9.0%, Mo 1.5-2.5%, V 0.2-0.5%, and the remainder is Fe and unavoidable impurity elements.

[0013] Preferably, the refined DC53 molten steel in step S1 is obtained by sequentially melting in an induction furnace or an electric furnace, refining in an LF furnace and refining in a VD furnace.

[0014] Preferably, before the refined DC53 molten steel in step S1 is spray-formed, it is first poured into a preheated tundish, the preheating temperature of the tundish is 750-850°C, and the superheat of the molten steel in the tundish is ensured to be 80-150°C.

[0015] Preferably, the carbide non-uniformity in the spray-deposited ingot obtained in step S1 does not exceed level 1.

[0016] Preferably, the inert gas pressure in step S1 is 0.8-1.8 MPa.

[0017] Preferably, the pretreatment temperature before forging in step S2 is 1180-1250°C; the deformation amount of the forging process is greater than 4, and the forging module size is 410-710 mm in width × 200-400 mm in thickness; and the annealing is performed at 820-860°C for spheroidizing annealing.

[0018] Preferably, the inert gas pressure in step S3 is 0.5-1.0 MPa.

[0019] Preferably, the rate of cooling to -120 to -180°C during deep cooling in step S4 is ≤50°C / h.

[0020] Preferably, the furnace cooling method in step S5 is air cooling, and the next tempering is started after cooling to room temperature.

[0021] After being treated by the treatment method of the present invention, the DC53 cold-working die steel has dispersed and refined carbides, less residual austenite content, and the unevenness of eutectic carbides is within level 1. The toughness of DC53 steel is greatly improved, and it has excellent hardness and wear resistance. The deformation after heat treatment is small, and the comprehensive performance is improved compared with DC53 steel made by traditional processes. At the same time, since the eutectic carbide is small in size and easy to dissolve into the matrix, the austenitization holding time during quenching can be appropriately shortened to improve production efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 The microstructure of the conventional DC53 die steel after annealing obtained in Comparative Example 1 of the present invention;

[0023] Figure 2 This is the annealing microstructure diagram of DC53 steel after being processed in Example 2 of the present invention;

[0024] Figure 3 This is the microstructure diagram of DC53 steel after tempering after being treated in Example 2 of the present invention;

[0025] Figure 4 The impact energy of DC53 steel at different hardnesses after being treated in Example 1 of the present invention. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention is further described in detail in conjunction with the following embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0027] The present invention provides a processing method for improving the performance of large-size DC53 cold-working die steel, comprising the following steps:

[0028] S1. Spray forming: The refined DC53 steel liquid is sprayed with a high-pressure inert gas to obtain a spray-deposited ingot; the inert gas is preferably nitrogen, which is widely used in industry and is relatively cheap; other inert gases such as argon may also be used;

[0029] S2. Forging and annealing: The spray-deposited ingot is first forged and then annealed to obtain a DC53 module;

[0030] S3. Quenching: The DC53 module is heated to 550-650℃ at a heating rate of ≤350℃ / h in a vacuum atmosphere, and then kept warm for 2-3h, then heated to 800-900℃ and kept warm for 2-4h, then heated to 1010-1050℃ at a heating rate of ≤250℃ / h and kept warm for 1-4h, and quenched by high-pressure inert gas after the end of the insulation; the inert gas is preferably nitrogen; it has been found that when the quenching temperature is lower than 1010℃, the alloy and carbon elements in the carbide cannot be fully dissolved into the matrix, so that the quenching hardness is low, and at the same time, the fine carbide precipitation is insufficient during tempering, which reduces the tempering hardness, thereby affecting the wear resistance and strength of DC53 steel; when the quenching temperature is higher than 1050℃, increasing the quenching temperature cannot continue to increase the quenching hardness. At this time, the grains may grow, reducing the plasticity and strength of the steel, resulting in mold cracking;

[0031] S4. Cryogenic treatment: The DC53 module after quenching in step S3 is cryogenically treated at -120 to -180°C for 4 to 8 hours. It has been found that when the cryogenic temperature is higher than -120°C, the transformation of the residual austenite is insufficient and the content is high, which will affect the dimensional stability and wear resistance of the material after heat treatment. When the cryogenic temperature is lower than -180°C, the consumption of system energy and cryogenic medium (such as liquid nitrogen) will increase. If the cryogenic temperature is too low or the cooling is too fast, the risk of cracking of the material will increase.

[0032] S5. Tempering: Heat to 500-580℃ at a heating rate of ≤300℃ / h, keep warm for 4-8h, then cool out of the furnace, temper 2-3 times to obtain DC53 cold working die steel.

[0033] In the production of traditional DC53 die steel, the refined molten steel is generally cast into electrodes and electroslag remelting methods to obtain electroslag ingots. After forging and annealing, the electroslag ingots are quenched and tempered to improve hardness and wear resistance. When this method produces small-sized DC53 steel (i.e., thickness <150mm), the product performance can still meet the standard requirements. However, when the thickness is ≥150mm, a large number of large-sized eutectic carbides are present in the obtained product steel, and the unevenness of the eutectic carbides reaches level 3 or even level 4 to 5, which seriously affects the toughness and wear resistance of the DC53 die steel and reduces the service life of the die. Moreover, the risk of deformation of DC53 steel after quenching and tempering heat treatment is high, especially for large-sized DC53 steel molds, which may even be distorted after heat treatment. After research, the applicant found that the use of injection molding + deep cryogenic treatment can significantly improve the above problems. Specifically, the refined molten steel is spray-formed by high-pressure nitrogen, which can significantly reduce the size and generation of eutectic carbides compared to casting electrodes and electroslag remelting, so that the unevenness of eutectic carbides does not exceed one level. After spray forming, conventional forging and annealing are performed to obtain a large-size module. The module has a low hardness and needs to be quenched to meet the mold's demand for high hardness. Since the size of the eutectic carbides in the module is small, the uniformity is high, and it is easy to dissolve into the matrix, the austenitization insulation time during quenching can be appropriately shortened (the traditional austenitization insulation time is affected by the quality, specification, furnace loading, furnace loading method, etc. of the mold steel, and generally requires at least 2 hours, especially the greater the thickness, the longer the insulation time, and the present application is for conventional furnace loading, even if the thickness is ≥150mm, generally insulation within 2 hours, for special cases such as super-large specifications and large furnace loading, the insulation time is extended to 4 hours), and production efficiency is improved. However, if only quenching and tempering are performed, the performance of the product obtained is not fully utilized and there is a greater risk of deformation. Therefore, this application performs deep cryogenic treatment after quenching, which can increase the toughness by more than 2 times and greatly improve the mold accuracy.

[0034] Therefore, the present application adopts the processing method of refining → spray forming → forging → annealing to produce DC53 die steel modules, and then performs vacuum quenching, cryogenic treatment and tempering, so that the DC53 die steel after heat treatment has higher hardness and dimensional stability, and the toughness index is increased by more than 2 times, and the service life of the DC53 die steel is greatly improved. At the same hardness (≥58HRC), the DC53 steel obtained by the present invention has a toughness increase of more than 2 times compared with the DC53 steel obtained by the traditional process, less residual austenite, better wear resistance and dimensional stability.

[0035] Preferably, the chemical composition of DC53 cold working die steel refers to the national standards GB / T 1299-2014 "Tool Steel" and GB / T 34564.1-2017 "Cold Working Die Steel Part 1: High Toughness and High Wear Resistance Steel", as shown in Table 1.

[0036] Table 1

[0037]

[0038] Based on Table 1, the present application has made further optimization and improvement for large-size DC53 steel. The specific composition is as follows, including by mass percentage: C 0.9-1.2%, Si 0.8-1.2%, Mn 0.2-0.5%, Cr 7.0-9.0%, Mo1.5-2.5%, V 0.2-0.5%, and the balance is Fe and unavoidable impurity elements.

[0039] Preferably, the DC53 molten steel after the refining in step S1 can be produced by conventional methods. Specifically, it can be obtained by sequentially smelting in an induction furnace or an electric furnace, and refining in an LF furnace and a VD furnace.

[0040] Preferably, the refined DC53 molten steel in step S1 is first poured into a preheated tundish before spray forming, and the preheating temperature of the tundish is 750-850°C, and the superheat of the molten steel in the tundish is ensured to be 80-150°C. The tundish is mainly used to store molten steel and provide liquid molten steel for the spray forming device. The liquid level in the tundish remains basically unchanged during the spray forming process. At the same time, the tundish can be heated to ensure a suitable superheat to meet the requirements of the spray forming process; and the inclusions in the tundish can partially float up to improve the purity of the molten steel.

[0041] Preferably, the inert gas pressure in step S1 is 0.8-1.8 MPa, where the inert gas is used to break the molten steel into small droplets. If the pressure is too small, the droplet size is large, which is not conducive to controlling the solidification segregation. If the pressure is too large, the porosity of the sprayed ingot may be increased, affecting the solidification quality. Large-sized DC53 is generally produced by sprayed ingots with large ingot weight, so the preferred sprayed ingot weight is 2.5t or more.

[0042] Preferably, the pretreatment temperature before forging in step S2 is 1180-1250°C; the deformation amount during the forging process is greater than 4, and the size of the forged module is 410-710mm in width × 200-400mm in thickness; the large specifications of this application are mainly for the module thickness. Annealing is performed at 820-860°C for spheroidizing annealing. Heating pretreatment before forging can promote the dissolution of carbides and facilitate forging. If the heating temperature is too low, it is easy to cause less carbide dissolution and difficulty in forging; if the temperature is too high, it is easy to cause high energy consumption, coarsening of grains, easy cracking during forging, and poor steel performance. A certain amount of deformation can evenly refine the structure after forging and improve the performance of steel. The greater the deformation, the more energy is consumed and the higher the cost, while the organizational properties do not change much or even decrease.

[0043] Preferably, the inert gas pressure in step S3 is 0.5-1.0 MPa. The greater the inert gas pressure, the faster the cooling, which is beneficial to improving the structure after quenching, but may aggravate quenching deformation and cracking tendency, so the inert gas pressure needs to be maintained within a suitable range.

[0044] Preferably, the cooling rate during deep cooling in step S4 is ≤50°C / h. Deep cooling is to reduce the content of retained austenite and improve the wear resistance and dimensional stability of the steel. Too high a cooling rate will cause excessive thermal stress in the steel, resulting in cracking during the cooling process.

[0045] Preferably, the cooling method of step S5 is air cooling, and the next tempering is started after cooling to room temperature.

[0046] Example 1

[0047] The elements in DC53 cold working die steel are as follows by mass percentage: C 0.95%, Si 0.89%, Mn 0.46%, Cr7.94%, Mo 2.34%, V 0.28%, and the remainder is Fe and unavoidable impurity elements.

[0048] S1. Production of DC53 steel by spraying: After reasonable batching of scrap steel and alloy, the above-mentioned metal solution is obtained by smelting in an induction furnace, and then pure DC53 steel liquid is obtained by LF and VD refining. Then the refined DC53 steel liquid is poured into a tundish preheated to 780°C, and the superheat of the steel liquid is 80°C. After flowing out of the guide pipe of the tundish, it is atomized into fine droplets by 1.0MPa high-pressure nitrogen in the spray forming equipment, and then a 2.8t spray ingot is prepared on a deposition plate; the spray deposition ingot is forged after being kept at 1220-1240°C, with a forging ratio of 6 and a final forging temperature of not less than 850°C, and then a 410mm×200mm module is obtained after being kept at 840°C for spheroidizing annealing;

[0049] S2. Quenching: The DC53 steel module of step S1 is cut according to actual needs, and then placed in a vacuum furnace for heating, heated to 550°C at a heating rate of 300°C / h and kept warm for 2h, then heated to 820°C and kept warm for 3h, and then heated to 1030°C at 200°C / h and kept warm for 1.5h. After the insulation is completed, it is quenched by 0.8Mpa high-pressure nitrogen, cooled to about 50°C and taken out of the furnace;

[0050] S3. Cryogenic treatment: The quenched DC53 steel was placed in a cryogenic box for cryogenic treatment at a cooling rate of 50°C / h, and the temperature was reduced to -150°C and the holding time was 5h;

[0051] S4. Tempering: Put the deep-cooled DC53 steel into a box furnace, heat it to 500-520℃ at a heating rate of 250℃ / h and keep it at that temperature for 5h, then take it out of the furnace and air-cool it, and temper it twice.

[0052] After treatment, the hardness of the DC53 mold steel of the present invention after deep cooling is tested to be 65-66HRC, the hardness after tempering is 62HRC, the residual austenite is 1.6%, the deformation of the template after tempering is within 0.3μm / mm, the maximum size of eutectic carbide is 8μm, the unevenness of eutectic carbide is level 1, and the notch-free impact energy after tempering is 49J.

[0053] Example 2

[0054] The elements in DC53 cold working die steel are as follows by mass percentage: C 1.08%, Si 0.85%, Mn 0.42%, Cr8.06%, Mo 2.25%, V 0.32%, and the remainder is Fe and unavoidable impurity elements.

[0055] S1. Production of DC53 steel by spraying: After reasonable batching of scrap steel and alloy, the above-mentioned metal solution is obtained by smelting in an electric arc furnace, and then pure DC53 steel liquid is obtained by LF and VD refining. Then the refined DC53 steel liquid is poured into a tundish preheated to 800°C, and the superheat of the steel liquid is 100°C. After flowing out of the guide pipe of the tundish, it is atomized into fine droplets by 1.3MPa high-pressure nitrogen in the spray forming equipment, and then a 3.9t spray ingot is prepared on a deposition plate; the spray deposition ingot is forged after being kept at 1220-1240°C, with a forging ratio of 6 and a final forging temperature of not less than 850°C, and then a 610mm×280mm module is obtained after being kept at 840°C for spheroidizing annealing;

[0056] S2. Quenching: The DC53 steel module of step S1 is cut according to actual needs, and then placed in a vacuum furnace for heating, heated to 600°C at a heating rate of 250°C / h and kept warm for 3h, then heated to 850°C and kept warm for 4h, and then heated to 1020°C at 180°C / h and kept warm for 2h. After the insulation is completed, it is quenched by 0.6Mpa high-pressure nitrogen, cooled to about 50°C and taken out of the furnace;

[0057] S3. Cryogenic treatment: The quenched DC53 steel was placed in a cryogenic box for cryogenic treatment at a cooling rate of 40°C / h, and the temperature was reduced to -160°C and the holding time was 6h;

[0058] S4. Tempering: Put the deep-cooled DC53 steel into a box furnace, heat it to 540-560℃ at a heating rate of 250℃ / h and keep it at that temperature for 6h, then take it out of the furnace and air-cool it, and temper it 3 times.

[0059] After treatment, the hardness of the DC53 die steel of the present invention after deep cooling is tested to be 65-66HRC, the hardness after tempering is 60HRC, the residual austenite is 1.1%, the deformation of the die after tempering is within 0.25μm / mm, the maximum size of the eutectic carbide is 7.5μm, the unevenness of the eutectic carbide is 0-1 level, and the notch-free impact energy after tempering is 57J.

[0060] Comparative Example 1

[0061] The refined DC53 steel liquid in Example 2 is cast into consumable electrodes under argon protection conditions, and is electroslag remelted to produce DC53 electroslag ingots. Then, the same forging and annealing process as in Example 2 is used to produce DC53 modules of the same specifications, with a size of 610 mm×280 mm, and the same quenching temperature is used for heating. The specific heat treatment process is as follows.

[0062] (1) Quenching: The DC53 steel module is cut according to actual needs, and then placed in a vacuum furnace for heating. The temperature is raised to 600°C at a heating rate of 250°C / h and then kept at this temperature for 3h. The temperature is then raised to 850°C and kept at this temperature for 4h. The temperature is then raised to 1020°C at a heating rate of 180°C / h and kept at this temperature for 2.5h. After the holding period, the module is quenched by high-pressure nitrogen at 0.6Mpa and cooled to about 50°C before being taken out of the furnace.

[0063] (2) Tempering: The quenched DC53 steel is placed in a box furnace and heated to 520-540°C at a heating rate of 250°C / h for 6 hours, then taken out of the furnace and air-cooled, and tempered three times.

[0064] The maximum size of the eutectic carbide of the DC53 steel in the test of this comparative example is 64μm, the eutectic carbide unevenness is level 4, the quenching hardness of the mold steel is 63-64HRC, the hardness after tempering is 60HRC, the residual austenite is 3.8%, the mold deformation after tempering is about 1μm / mm, and the unnotched impact energy is 16J.

[0065] Comparative Example 2

[0066] Compared with Example 2, this comparative example does not adopt cryogenic treatment, and the remaining steps are the same as those of Example 2.

[0067] The performance results of DC53 steel obtained in Example 2 and Comparative Examples 1-2 are shown in Table 2.

[0068] Table 2

[0069]

[0070] Although preferred embodiments of the present invention have been described, additional changes and modifications may be made to these embodiments by those skilled in the art once the basic inventive concepts are known. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention. Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims

1. A processing method for improving the performance of large-size DC53 cold-working die steel, characterized in that: The method is used to produce modules with a thickness exceeding 150 mm; the processing method comprises the following steps: S1. Spray forming: The refined DC53 molten steel is spray-formed by high-pressure inert gas to obtain a spray-deposited ingot, the weight of which is more than 2.5t; the pressure of the inert gas is 0.8~1.8MPa; before the refined DC53 molten steel is spray-formed, it is first poured into a preheated tundish, the preheating temperature of the tundish is 750~850℃, and the superheat of the molten steel in the tundish is ensured to be 80~150℃; S2. Forging and annealing: The spray-deposited ingot is first forged and then annealed to obtain a DC53 module; S3. Quenching: The DC53 module is heated to 550-650°C at a heating rate of ≤350°C / h in a vacuum atmosphere, and then heated to 800-900°C for 2-4h, and then heated to 1010-1050°C at a heating rate of ≤250°C / h for 1-4h. After the insulation, it is quenched by high-pressure inert gas; the inert gas pressure is 0.5-1.0MPa; S4. Cryogenic treatment: The DC53 module after quenching in step S3 is cryogenically treated at -120~-180℃ for 4~8h; the rate of cooling to -120~-180℃ during cryogenic treatment is ≤50℃ / h; S5. Tempering: heating to 500-580°C at a heating rate of ≤300°C / h, keeping the temperature for 4-8h, then cooling out of the furnace, tempering 2-3 times, and obtaining the DC53 cold working die steel.

2. The method for improving the performance of large-size DC53 cold-working die steel according to claim 1, characterized in that: The chemical composition of the DC53 cold working die steel is as follows by mass percentage: C 0.9-1.2%, Si 0.8-1.2%, Mn 0.2-0.5%, Cr 7.0-9.0%, Mo 1.5-2.5%, V 0.2-0.5%, and the remainder is Fe and unavoidable impurity elements.

3. The method for improving the performance of large-size DC53 cold-working die steel according to claim 1, characterized in that: The refined DC53 steel liquid in step S1 is obtained by melting in an induction furnace or an electric furnace, refining in an LF furnace and refining in a VD furnace in sequence.

4. The method for improving the performance of large-size DC53 cold-working die steel according to claim 1, characterized in that: The carbide non-uniformity in the spray-deposited ingot obtained in step S1 does not exceed level 1.

5. The method for improving the performance of large-size DC53 cold-working die steel according to claim 1, characterized in that: The pretreatment temperature before forging in step S2 is 1180-1250°C; the deformation amount of the forging process is greater than 4, and the forging module size is 410-710 mm in width × 200-400 mm in thickness; the annealing is performed at 820-860°C for spheroidizing annealing.

6. The method for improving the performance of large-size DC53 cold-working die steel according to claim 1, characterized in that: The cooling method after the furnace is taken out of step S5 is air cooling, and the next tempering is started after cooling to room temperature.

Images