Production process of large hot work die steel

Abstract

The invention relates to a production process of large-scale hot work die steel, which comprises the following steps: carrying out homogenization heat treatment on a cast steel ingot, eliminating dendritic segregation in the steel ingot, and promoting the healing of micro cracks in the steel ingot. And the steel ingot obtained after homogenization heat treatment is subjected to forging treatment, and the as-cast structure in the steel ingot is broken. And after-forging heat treatment is conducted on the forge piece obtained through forging treatment, the orientation relation between original crystal grains and new nucleation crystal grains is broken, recrystallized austenite in the forge piece is homogenized, the crystal grains are refined, and the internal structure state of the forge piece is stable. And quenching and tempering heat treatment is carried out on the forged piece subjected to post-forging heat treatment, the forged piece is rapidly cooled, carbide precipitation in the forged piece in a high-temperature state is prevented, thermal stress of the forged piece is reduced, and cracking is prevented. The hot die steel prepared through the method has good thermal mechanical fatigue strength performance, and the service life is prolonged.

Description

Technical Field

[0001] The present invention relates to the field of production of hot work die steel, and particularly to a production process for large-sized hot work die steel. Background Art

[0002] In recent years, with the rapid development of China's high-end equipment manufacturing industry, high requirements have been put forward for the dimensional accuracy, forming quality and mechanical properties of key load-bearing components. The die steel for forming these components is required to have the characteristics of long-term stable service under high-temperature and high-load conditions. Therefore, higher requirements are put forward for the thermal mechanical fatigue strength and service life of this type of hot work die steel. At present, China is unable to completely independently produce this type of high-performance die steel, mainly relying on imports, with high costs and various restrictions. Therefore, it is necessary to solve this problem and break through the technical bottleneck restricting the quality improvement of the manufacturing industry. Summary of the Invention

[0003] The object of the present invention is to provide a production process for large-sized hot work die steel, which can improve the thermal mechanical fatigue strength and service life of the die steel.

[0004] The technical solution adopted by the present invention is as follows.

[0005] A production process for large-sized hot work die steel, characterized by including the following operations.

[0006] S10: Perform homogenization heat treatment on the cast steel ingot to eliminate dendritic segregation in the steel ingot and promote the healing of microcracks in the steel ingot.

[0007] S20: Perform forging treatment on the steel ingot after homogenization heat treatment to break the as-cast structure in the steel ingot.

[0008] S30: Perform post-forging heat treatment on the forgings obtained by forging treatment to interrupt the orientation relationship between the original grains and the newly nucleated grains, make the recrystallized austenite in the forgings uniform and refine the grains, and make the internal tissue state of the forgings stable.

[0009] S40: Perform quenching and tempering heat treatment on the forgings after post-forging heat treatment, rapidly cool the forgings to prevent carbide precipitation in the forgings at high temperature, reduce the thermal stress of the forgings and prevent cracking.

[0010] The composition of the prepared die steel components by weight percentage includes: 0.54% - 0.58% of C, 0.10% - 0.30% of Si, 0.80% - 0.85% of Mn, 1.10% - 1.20% of Cr, 1.70% - 1.80% of Ni, 1.00% - 1.10% of Mo, 0.65% - 0.75% of V, 0.030% - 0.040% of Nb, 0.015% - 0.025% of Al, and the contents of the following elements are controlled: S ≤ 0.005%, P ≤ 0.008%, H ≤ 0.0002%, O ≤ 0.0020%, N ≤ 0.006%, and the balance is Fe and impurities.

[0011] The preferred composition of the die steel group is: 0.56% of C, 0.10% of Si, 0.83% of Mn, 1.18% of Cr, 1.80% of Ni, 1.10% of Mo, 0.73% of V, 0.038% of Nb, 0.025% of Al, 0.008% of P, 0.005% of S, 0.00015% of H, 0.0015% of O, 0.006% of N, and the balance is Fe and impurities.

[0012] The above technical solution provided by the present invention starts from two aspects of adjusting the component composition and manufacturing process, so as to effectively improve the performance of the die steel, expand the scope of application and meet the requirements of the high-end equipment manufacturing industry.

[0013] The specific principle of performance improvement is as follows: Optimizing and adjusting the content of Mo element can improve the hardenability of the steel and reduce the temper brittleness of the steel; Optimizing and adjusting the V element, the combination of V and C can precipitate fine carbides, which can hinder the movement of dislocations and improve the strength and toughness, but the content of V should not be too high, otherwise it is easy to cause embrittlement; Adding the rare element Nb is mainly used to refine the grains, but its content should be reasonably controlled, otherwise segregation and liquid segregation carbides will be caused; Adding the new element Al can refine the grains, improve the strength, and form nitrogen-aluminum compounds to pin dislocations and improve the toughness.

[0014] At the same time, reliable treatment is carried out on the steel billet by adopting processes such as homogenization heat treatment, forging treatment, post-forging heat treatment, quenching and tempering heat treatment, etc. that match the above component composition. Through homogenization heat treatment: before forging deformation of the steel ingot, dendritic segregation in the steel ingot is eliminated, and the healing of microcracks in the steel ingot is promoted. Through forging treatment, the as-cast structure is broken, and the module forming size required by the process is formed. In the post-forging heat treatment, the forgings are successively subjected to a combined treatment of two high-temperature normalizations + two high-temperature temperings. Among them, the first normalizing heats the forgings to 880 - 920 °C to break the orientation relationship between the original grains and the newly nucleated grains, making the recrystallized austenite in the forgings uniform; the second normalizing heats the forgings to 760 - 780 °C to obtain finer grains in the forgings; then through two temperings (640 - 660 °C, 660 - 680 °C), the internal structure of the forgings is transformed into a relatively stable state. The quenching and tempering heat treatment consists of high-temperature oil quenching + low-temperature air cooling + low-temperature tempering + air cooling, which can not only quickly cool down the forgings to prevent the precipitation of carbides in the forgings at high temperatures, but also reduce the thermal stress of the forgings and prevent cracking.

[0015] Thus, the prepared hot die steel has good thermal mechanical fatigue strength and an extended service life. Brief Description of the Drawings

[0016] Figure 1 It is the metallographic structure diagram of the hot die steel prepared in Example 1.

[0017] Figure 2 It is the metallographic structure diagram of the hot die steel prepared in Example 2.

[0018] Figure 3 It is the metallographic structure diagram of the hot die steel prepared in Example 3. Detailed Embodiments

[0019] In order to make the purpose and advantages of the present invention more clear and understandable, the present invention is specifically described below in conjunction with embodiments. It should be understood that the following text is only used to describe one or several specific implementation manners of the present invention, and does not strictly limit the specific protection scope claimed by the present invention.

[0020] A production process of a large hot working die steel. The composition of the die steel by weight percentage includes: 0.54% - 0.58% of C, 0.10% - 0.30% of Si, 0.80% - 0.85% of Mn, 1.10% - 1.20% of Cr, 1.70% - 1.80% of Ni, 1.00% - 1.10% of Mo, 0.65% - 0.75% of V, 0.030% - 0.040% of Nb, 0.015% - 0.025% of Al, and the contents of the following elements are controlled: S ≤ 0.005%, P ≤ 0.008%, H ≤ 0.0002%, O ≤ 0.0020%, N ≤ 0.006%, and the balance is Fe and impurities. The production process includes the following operations.

[0021] S10: Perform homogenization heat treatment on the ingot to eliminate dendritic segregation in the ingot and promote the healing of microcracks in the ingot, specifically as follows.

[0022] S11: Heat the ingot in the furnace to 600 - 650 °C and hold for 6 - 8 h.

[0023] S12: Continue to heat up to 800 - 850 °C and hold for 6 - 8 h.

[0024] S13: Continue to heat up to 1350 °C and hold for T10 hours, then cool slowly in the furnace to 1100 °C naturally. T10 = k10 × d10, k10 = [0.0075, 0.0125], and d10 is the current maximum diameter of the ingot (unit: mm, the same below).

[0025] During the processes of steps S11 - S13, control the heating rate not to exceed 80 °C / h.

[0026] S20: Perform forging treatment on the ingot after homogenization heat treatment to break the as-cast structure in the ingot; specifically as follows.

[0027] S21: Heat the ingot after homogenization treatment in the furnace to 1250 °C and hold for T21 hours. After the holding ends, remove the ingot from the heating furnace and quickly transfer it to the press platform for clamping mouth and chamfering. T21 = k20 × d21, k20 = [0.01, 0.015], and d21 is the maximum diameter of the ingot during the current holding.

[0028] S22: Heat the ingot after S21 treatment in the furnace to 1250 °C and hold for T22 hours. After the holding ends, remove the ingot from the heating furnace and quickly transfer it to the press platform for the first upsetting and square forging. T22 = k20 × d22, and d22 is the maximum diameter of the ingot during the current holding. Upset the ingot to a height of H1 and a diameter of R1 for the first time, and forge it to the size of D1 × D1 × L1 with the first wide anvil.

[0029] S23: Heat the ingot processed in S22 in the furnace to 1250 °C and hold for T23 hours. After the heat preservation ends, remove the ingot from the heating furnace and quickly transfer it to the press platform for the second upsetting and square drawing. T23 = k20 × d23, where d23 is the maximum diameter of the ingot during the current heat preservation. Upset the ingot for the second time to a height of H2 and a diameter of R2, and perform square drawing with a wide anvil for the second time to a size of D2 × D3 × L2.

[0030] S24: Heat the ingot processed in S23 in the furnace to 1250 °C and hold for T24 hours. After the heat preservation ends, remove the ingot from the heating furnace and quickly transfer it to the press platform for blanking and trimming according to the preset size; T24 = k20 × d24, where d24 is the maximum diameter of the ingot during the current heat preservation.

[0031] S25: After the treatment in S24, remove the ingot from the press table and place it in the open air to air-cool to room temperature.

[0032] During the processes of steps S21 - S25, when the furnace temperature is higher than 600 °C, the heating rate does not exceed 80 °C / h.

[0033] S30: Perform post-forging heat treatment on the forgings obtained by forging treatment to break the mutual relationship between the original grains and the newly nucleated grains, homogenize and refine the recrystallized austenite in the forgings, and make the internal tissue state of the forgings stable, as follows.

[0034] S31: First, heat the forgings obtained by forging treatment to 600 - 650 °C and hold for 6 - 8 hours.

[0035] S32: Then heat up to 880 - 920 °C and hold for T31 hours, where T31 = k30 × d30, k30 = [0.0125, 0.02], and d30 is the thickness of the forging.

[0036] S33: Subsequently, cool in the furnace to 250 - 300 °C and hold for 20 hours.

[0037] S34: Heat up to 660 - 680 °C and hold for 6 - 8 hours.

[0038] S35: Continue to heat up to 760 - 780 °C and hold for T32 hours, where T32 = k30 × d30, k30 = [0.0125, 0.02].

[0039] S36: Subsequently, cool in the furnace to 640 - 660 °C and hold for 16 hours.

[0040] S37: Continue to cool in the furnace to 250 - 300 °C, and then take the forgings out of the furnace and air-cool to room temperature.

[0041] During the processes of steps S31 to S37, in the temperature range of 200 - 600 °C, the heating rate of the furnace does not exceed 50 °C / h, and above 600 °C, the heating rate of the furnace does not exceed 80 °C / h. The cooling rate of the furnace does not exceed 20 °C / h.

[0042] S40: Perform quenching and tempering heat treatment on the forgings after forging heat treatment. Rapidly cool the forgings to prevent the precipitation of ferrite in the forgings at high temperatures, reduce the thermal stress of the forgings, and prevent cracking, as follows.

[0043] S41: Keep the forgings after forging heat treatment at 600 - 650 °C for 6 - 8 h.

[0044] S42: Then heat up to 840 - 860 °C and keep it for T41 hours, where T41 = k40 × d40, k40 = [0.0125, 0.02], and d40 is the thickness of the forging.

[0045] S43: After taking the forgings out of the furnace, put them into an oil bath and cool them in oil until the surface temperature reaches 400 °C. During the oil cooling process, control the oil temperature not to exceed 25 °C.

[0046] S44: Remove the forgings from the oil bath and continue to air-cool them until the surface temperature of the forgings reaches 200 °C.

[0047] S45: Put the forgings into the furnace and heat them up with the furnace to 540 - 560 °C and keep it for T42 hours. After the heat preservation is completed, take them out of the furnace and air-cool them to room temperature. T42 = k40 × d40, k40 = [0.0125, 0.02], and d40 is the thickness of the forging.

[0048] During the processes of steps S41 to S45, in the temperature range of 200 - 600 °C, the heating rate of the furnace does not exceed 50 °C / h, and above 600 °C, the heating rate of the furnace does not exceed 80 °C / h.

[0049] In the hot work die steel provided by the present invention, the content of Si is reduced to prevent embrittlement of forgings caused by the combination of Si and P elements; in order to improve the hardenability of the hot work die steel and suppress the high-temperature brittle-ductile transition, the content of Ni in the die steel is increased. The content of Mo element is increased to improve the hardenability of the steel and can reduce the temper brittleness of the steel; the rare element Nb is introduced. On the one hand, it can refine the grains, and on the other hand, it can form stable carbides with carbon elements, which can improve both strength and toughness at the same time. And the content of Nb is controlled appropriately to avoid that too high content of Nb will aggravate segregation of the steel and produce more liquid segregation carbides, causing embrittlement or microcracks. The introduced Al element can refine the grains, improve the strength, and can form aluminum nitride compounds to pin dislocations and improve the toughness; at the same time, the contents of harmful elements P, S, and gas elements H, O, N in the steel are restricted, and according to the content of N element in the steel after restriction, the content of Al element is controlled within an appropriate range, so as to not only ensure the strengthening effect of Al element and AlN on the low-temperature performance of the steel, but also avoid the influence of too high N content in the steel on the performance of the steel. In addition, through the implementation of the above-mentioned specific homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment processes, the thermal mechanical fatigue strength and service life performance of the hot work die steel are significantly improved.

[0050] The following further specifically describes the present invention through specific embodiments. Example 1

[0051] A 52.5-ton steel ingot was obtained by a double-vacuum (vacuum melting + vacuum casting) method. The steel ingot was successively subjected to homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment according to the above operations. The operation parameters of the homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment are shown in Tables 1 and 2, and a hot die steel with dimensions of 680×2000×3600 mm was obtained. Samples were taken from the middle part of the steel ingot for chemical composition detection. The preferred composition of the die steel group is 0.56% C, 0.10% Si, 0.83% Mn, 1.18% Cr, 1.80% Ni, 1.10% Mo, 0.73% V, 0.038% Nb, 0.025% Al, 0.008% P, 0.005% S, 0.00015% H, 0.0015% O, 0.006% N, and the balance is Fe and impurities. Example 2

[0052] A 74.5-ton steel ingot was obtained by the double-vacuum method (vacuum melting + vacuum casting). The steel ingot was successively subjected to homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment according to the above operations. The operating parameters of the homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment are shown in Tables 1 and 2, and a hot die steel with dimensions of 930×2250×3100 mm was obtained. Samples were taken from the middle part of the steel ingot for chemical composition detection. The preferred composition of the die steel group is 0.56% C, 0.10% Si, 0.83% Mn, 1.18% Cr, 1.80% Ni, 1.10% Mo, 0.73% V, 0.038% Nb, 0.025% Al, 0.008% P, 0.005% S, 0.00015% H, 0.0015% O, 0.006% N, and the balance is Fe and impurities. Example 3

[0053] A 126-ton steel ingot was obtained by the double-vacuum method (vacuum melting + vacuum casting). The steel ingot was successively subjected to homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment according to the above operations. The operating parameters of the homogenization treatment, forging treatment, post-forging heat treatment, and quenching and tempering heat treatment are shown in Tables 1 and 2, and a hot die steel with dimensions of 1090×2350×4000 mm was obtained. Samples were taken from the middle part of the steel ingot for chemical composition detection. The preferred composition of the die steel group is 0.56% C, 0.10% Si, 0.83% Mn, 1.18% Cr, 1.80% Ni, 1.10% Mo, 0.73% V, 0.038% Nb, 0.025% Al, 0.008% P, 0.005% S, 0.00015% H, 0.0015% O, 0.006% N, and the balance is Fe and impurities.

[0054] Table 1 Holding times at each stage in Examples 1 to 3 （h） T10 T21 T22 T23 T24 T31 T32 T41 T42 Example 1 23.75 19 19 23.95 23.35 7.3 7.3 7.3 7.3 Example 2 28.75 23 23 25.8 25.4 10.8 9.3 9.3 9.3 Example 3 36.25 29 29 31.8 31 12.1 12.1 12.1 12.1 Table 2 Dimensions of the workpieces after upsetting and square drawing in Examples 1 to 3 (mm) H1 R1 D1 L1 H2 R2 D2 D3 L2 Example 1 1195 2395 1430 2630 1195 2335 1730 730 4050 Example 2 1440 2580 1550 3130 1440 2540 2250 930 3480 Example 3 1590 3180 1860 3650 1590 3100 2390 1210 4140 Example 4

[0055] The properties of the hot die steels obtained in Examples 1 to 3 were tested, and each example was tested twice. Using a scanning electron microscope (Zeiss sigma300), the detected metallographic structure is as Figures 1 to 3 shown. A universal testing machine was used to test the mechanical properties of the forgings at room temperature, and a thermal fatigue testing machine was used to conduct thermal mechanical fatigue tests. The test results are shown in Table 3.

[0056] Table 3 Performance test results of the hot work die steels prepared in Examples 1 to 3

[0057] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention. The structures, devices, and operation methods not specifically described and explained in the present invention are implemented according to the conventional means in the art without special instructions and limitations.

Claims

1. A production process for large hot working die steel, characterized in that: The following operations are included: S10: Performing homogenization heat treatment on the steel ingot to eliminate dendrite segregation in the steel ingot and promote the healing of micro cracks in the steel ingot; S20: forging the steel ingot after homogenization heat treatment to break the cast structure in the steel ingot; S30: performing post-forging heat treatment on the forgings obtained by forging treatment, interrupting the orientation relationship between the original grains and the newly nucleated grains, homogenizing the recrystallized austenite in the forgings and refining the grains, and stabilizing the internal structure of the forgings; S40: Perform quenching and tempering heat treatment on the forgings after heat treatment, quickly cool the forgings to prevent carbides from precipitating at high temperatures in the forgings, reduce thermal stress of the forgings, and prevent cracking; The prepared mold steel components include, by weight percentage: 0.54% to 0.58% C, 0.10% to 0.30% Si, 0.80% to 0.85% Mn, 1.10% to 1.20% Cr, 1.70% to 1.80% Ni, 1.00% to 1.10% Mo, 0.65% to 0.75% V, 0.030% to 0.040% Nb, 0.015% to 0.025% Al, and the contents of the following elements are controlled: S≤0.005%, P≤0.008%, H≤0.0002%, O≤0.0020%, N≤0.006%, and the balance is Fe and impurities.

2. The production process of large hot working die steel according to claim 1, characterized in that: The components of mold steel are: 0.56% C, 0.10% Si, 0.83% Mn, 1.18% Cr, 1.80% Ni, 1.10% Mo, 0.73% V, 0.038% Nb, 0.025% Al, 0.008% P, 0.005% S, 0.00015% H, 0.0015% O, 0.006% N, and the balance is Fe and impurities.

3. The production process of large hot working die steel according to claim 1, characterized in that: Homogenization heat treatment includes the following operations: S11: heating the steel ingot to 600-650°C with the furnace and keeping it warm for 6-8h; S12: Continue heating to 800-850°C and keep warm for 6-8h; S13: Continue heating to 1350°C, keep warm for T10 hours, then cool naturally to 1100°C with the furnace, T10=k10×d10, k10=[0.0075, 0.0125], d10 is the current maximum diameter of the ingot.

4. The production process of large hot working die steel according to claim 1, characterized in that: The forging process includes the following operations: S21: The homogenized steel ingot is heated to 1250°C and kept at this temperature for T21 hours. After the heat preservation, the steel ingot is removed from the heating furnace and quickly transferred to the press platform for jaw pressing and chamfering. T21=k20×d21, k20=[0.01, 0.015], d21 is the maximum diameter of the steel ingot during the current heat preservation; S22: The steel ingot treated with S21 is heated to 1250℃ and kept warm for T22 hours. After the heat preservation, the steel ingot is removed from the heating furnace and quickly transferred to the press platform for the first upsetting and square drawing. T22=k20×d22, d22 is the maximum diameter of the steel ingot during the current heat preservation; S23: The forgings after S22 treatment are heated to 1250℃ and kept warm for T23 hours. After the end of the heat preservation, the forgings are removed from the heating furnace and quickly transferred to the press platform for the second upsetting and square drawing. T23=k20×d23, d23 is the maximum diameter of the steel ingot during the current heat preservation; S24: The forgings after S23 treatment are heated to 1250℃ and kept warm for T24 hours. After the heat preservation, the forgings are removed from the heating furnace and quickly transferred to the press platform for blanking and trimming according to the preset size; T24=k20×d24, d24 is the maximum diameter of the forgings during the current heat preservation; S25: After S24 treatment, the forging is removed from the press table and placed on an open space to be air cooled to room temperature.

5. The production process of large hot working die steel according to claim 1, characterized in that: Post-forging heat treatment includes the following operations: S31: first heat the forging obtained by forging at 600-650°C for 6-8h; S32: Then heat up to 880-920℃ and keep warm for T31 hours, T31=k30×d30, k30=[0.01, 0.02], d30 is the thickness of the forging; S33: The furnace is then cooled to 250-300°C and kept at this temperature for 20 hours; S34: heating to 660-680°C and keeping the temperature for 6-8h; S35: Continue to raise the temperature to 760-780°C and keep warm for T32 hours, T32=k30×d30, k30=[0.01, 0.02]; S36: The furnace is then cooled to 640-660°C and kept at this temperature for 16 hours; S37: Continue furnace cooling to 250-300°C, then take the forging out of the furnace and air cool to room temperature.

6. The production process of large hot working die steel according to claim 1, characterized in that: The quenching and tempering heat treatment includes the following operations: S41: heat-treat the forgings after forging at 600-650°C for 6-8h; S42: then heat to 840-860°C and keep warm for T41 hours, T41=k40×d40, k40=[0.01, 0.02], d40 is the thickness of the forging; S43: After the forging is taken out of the furnace, it is placed in an oil pool for oil cooling to a surface temperature of 400°C. During the oil cooling process, the oil temperature is controlled not to exceed 25°C; S44: The forging is removed from the oil pool and air-cooled until the surface temperature of the forging reaches 200°C; S45: Put the forging into the furnace and heat it to 540-560℃, keep it warm for T42 hours, take it out of the furnace and air cool it to room temperature, T42=k40×d40, k40=[0.01, 0.02], d40 is the thickness of the forging.

7. The production process of large hot working die steel according to claim 3, characterized in that: During steps S11 to S13, the heating rate is controlled not to exceed 80° C. / h.

8. The production process of large hot working die steel according to claim 5, characterized in that: During steps S31 to S37, in the range of 200-600°C, the furnace heating rate does not exceed 50°C / h, above 600°C the furnace heating rate does not exceed 80°C / h, and the furnace cooling rate does not exceed 20°C / h.

9. The production process of large hot working die steel according to claim 6, characterized in that: In step S43, during the oil cooling process, the oil temperature is controlled not to exceed 25°C.

10. The production process of large hot working die steel according to claim 4, characterized in that: During steps S21 to S25, the furnace heating rate does not exceed 80°C / h when the temperature is higher than 600°C; during steps S41 to S45, the furnace heating rate does not exceed 50°C / h in the range of 200-600°C, and does not exceed 80°C / h when the temperature is higher than 600°C.

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