A skt4 die steel forging and a preparation method thereof

Abstract

The application provides a SKT4 die steel forge piece and a preparation method thereof, and relates to the technical field of metal materials. The preparation method of the SKT4 die steel forge piece comprises the following steps: smelting an electroslag ingot; forging the steel ingot; and performing post-forging heat treatment on the forged forge piece to obtain the SKT4 die steel forge piece. The reversing-upsetting is adopted, the upsetting direction is different in each fire, the overall quality of the forge piece is more uniform, the anisotropy of the blank is improved, the influence of the anisotropy on the quality of the forge piece is reduced, the end protrusion of the square plate type forge piece is effectively avoided, the utilization rate of the steel ingot of the forge piece is improved, the direction of the square flattening and the lengthening is changed after the upsetting in each fire, the steel ingot axis direction is changed, the defects in the central part of the steel ingot can be dispersed, the number of the defects in the unit area is greatly reduced, the deformation of the central part of the steel ingot is larger after the direction changing forging, the defects are dispersed, the product quality is improved, and the finished product rate detected by the ultrasonic flaw detection is higher.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal materials, in particular to a SKT4 die steel forge piece and a preparation method thereof. BACKGROUND

[0002] At present, the forging method of square plate type forgings is mainly single direction square deformation forging, which makes it difficult to forge the core defects of the forgings and easily produces fibrous structure, thereby causing anisotropy problem. In addition, the SKT4 die steel forgings have high requirement for isotropy, and the single direction square deformation forging method cannot meet the requirement of the SKT4 die steel forgings for isotropy. Moreover, the piece has thin wall and large cross section, and usually needs to be produced by using a large steel ingot, and the technical requirement indicates that an electroslag remelted steel ingot is used for production. However, the electroslag ingot of 20T or more has quality risk, and the above reasons result in the failure to use the ingot production. SUMMARY

[0003] The present application solves the problem of how to solve the problem of anisotropy of the forgings and difficulty in using the ingot production of the SKT4 die steel forgings.

[0004] To solve the above problems, the present application provides a SKT4 die steel forge piece and a preparation method thereof.

[0005] In the first aspect, the present application provides a preparation method of a SKT4 die steel forge piece, which comprises the following steps:

[0006] S1: taking pig iron, sequentially performing medium frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, and demolding to obtain a steel ingot;

[0007] S2: preheating: preheating the steel ingot, heating to 600-650℃ for ≥10 hours, and heating to 1250±10℃ for 7-10 hours;

[0008] First fire forging is performed on the steel ingot, pre-drawing is performed using an upper flat and lower V anvil, the forging ratio is 1.1-1.3, the water riser ends are removed by gas cutting, a first down material blank is obtained, and the first down material blank is reheated to 1250±10℃ for 9-15 hours;

[0009] Second fire forging is performed on the first down material blank, upsetting is performed between the upper flat and the lower platform, upper and lower wide flat anvil is used to draw flat, the forging ratio is 1.6-1.8, a second down material blank is obtained, and the second down material blank is reheated to 1230±10℃ for 5-15 hours;

[0010] carrying out third hot forging on the second blank, upsetting the second blank along the length direction of the second blank, and elongating the second blank along the width direction of the second blank, with a forging ratio of 1.5 to 1.7, to obtain a third blank, and returning to 1200±10 DEG C for 5 to 15 hours for holding;

[0011] carrying out fourth hot forging on the third blank, upsetting the third blank along the length direction of the third blank, and elongating the third blank along the width direction of the third blank, with a forging ratio of 1.2 to 1.4, to obtain a fourth blank, and returning to 1200±10 DEG C for 5 to 15 hours for holding;

[0012] carrying out fifth hot forging on the fourth blank, upsetting the fourth blank along the length direction of the fourth blank, and elongating the fourth blank along the width direction of the fourth blank, with a forging ratio of 1.4 to 1.5, to obtain a forged piece;

[0013] S3: carrying out post-forging heat treatment on the forged piece to obtain an SKT4 die steel forged piece.

[0014] Optionally, the S1 comprises:

[0015] S11: smelting in a medium-frequency furnace, smelting molten steel by adding pig iron into the medium-frequency furnace, in the case of returning scrap steel, ensuring that the chemical composition meets the process requirements; allowing direct receiving in the medium-frequency furnace; when directly receiving in the medium-frequency furnace and hot charging, attention should be paid to the control of Mn; when a large amount of slag is charged after direct receiving with a large amount of slag due to a large amount of slag hanging on the lining of the medium-frequency furnace, the refining furnace must be treated by slag turning after alloy recovery by deoxidization;

[0016] S12: carrying out LF furnace refining, after the slag of the electric furnace is completely melted, using Al powder and appropriate C powder for diffusion deoxidization, and keeping the white slag for more than or equal to 30 minutes, so that the main chemical components of the LF furnace refined steel enter the internal control range;

[0017] S13: carrying out VD vacuum degassing operation, with a vacuum degree of less than or equal to 2 mbar and a holding time of more than or equal to 20 minutes, with a tapping temperature of 1550 to 1560 DEG C in the first disc and 1560 to 1570 DEG C in the second disc; before tapping, a gas sample is taken for analysis, so that the hydrogen content is controlled to be less than or equal to 0.00015%; if the hydrogen content is greater than 0.00015%, multiple vacuum operations are required until the hydrogen content is less than or equal to 0.00015%;

[0018] S14: casting an electrode: casting into an electrode in an argon protection environment, with a casting temperature of 1530 to 1550 DEG C and a mold temperature controlled at 120 to 200 DEG C, and the electrode blank is hot sent for annealing after cover cooling or demolding;

[0019] S15: using a slag system consisting of CaF2: 70%, Al2O3: 25% and MgO: 5% by mass fraction, and adopting coke as a slagging arc agent, the mold cooling time is 3.5 hours, and the hot delivery is performed immediately after demolding.

[0020] Optionally, the internal control components and ranges of the main chemical components of the refined steel include, by mass fraction: C: 0.51-0.53%, Si: 0.24-0.26%, Mn: 0.88-0.92%, P: ≤0.015%, S: ≤0.003%, Cr: 1.28-1.32%, Ni: 1.78-1.82%, Mo: 0.38-0.42%, V: 0.16-0.20%, and Al: 0.02%.

[0021] Optionally, the intermediate frequency furnace is an intermediate frequency induction furnace.

[0022] Optionally, before the first hot forging of the ingot in S2, the ingot is preheated to 600-650℃ and kept for ≥10H, and then heated to 1250±10℃ and kept for 7-15H.

[0023] Optionally, S3 includes:

[0024] S31: the forged workpiece is kept at a normalizing temperature of 840-860℃ for 2-4 hours, the forged workpiece is hung down and air-cooled to 350-450℃, placed into an annealing furnace and sealed, the tempering temperature is kept at 630-650℃ for 7-9 hours, and after cooling to below 400℃, the temperature is decreased to below 180℃ at a rate of ≤15℃ / h;

[0025] S32: the forged workpiece is subjected to performance heat treatment, kept at a normalizing temperature of 850-870℃ for 3-5 hours, the forged workpiece is hung down and cyclically oil-cooled for 70-90 minutes, kept at a tempering temperature of 610-630℃ for 5-7 hours, and after cooling to below 400℃, furnace-cooled to below 300℃ and taken out of the furnace, to obtain a billet;

[0026] S33: the billet is subjected to finishing steel treatment according to the operation rules, so that the billet meets the corresponding use requirements.

[0027] Optionally, the total forging ratio of S2 is ≥6.

[0028] In a second aspect, the application provides a SKT4 die steel workpiece, which is prepared by the preparation method of the SKT4 die steel workpiece as described in any one of the above.

[0029] Optionally, the cross-sectional size of the SKT4 die steel workpiece is not less than 2000mm×2400mm×300mm, and the weight of the SKT4 die steel workpiece is not less than 10000kg.

[0030] The beneficial effects of the SKT4 die steel forging and the preparation method thereof are as follows: reversing and upsetting are adopted, the upsetting direction of each fire is different, the overall quality of the forging is more uniform, the anisotropy of the blank is improved, the influence of the anisotropy on the quality of the forging is reduced, the end protrusion of the square plate type forging is avoided, and the utilization rate of the steel ingot of the forging is improved; after the upsetting of each fire, the square is flattened and elongated in the direction, the direction of the axis of the steel ingot is changed, the directions of the axes between adjacent upsetting and elongating processes are 90°, for example, the width direction of the upsetting of the upper fire is used as the length direction of the elongation of the fire, the defects in the central part of the steel ingot can be dispersed, the inclusion defects are no longer gathered but spread to the periphery, the number of defects in the unit area is greatly reduced, and the deformation of the core of the steel ingot is larger after the direction changing forging, the porosity condition is improved, the defects are dispersed, the product quality is improved, and the finished product rate is higher through ultrasonic flaw detection. In order to improve the product quality, homogenization treatment is added before forging, and the furnace is kept for 5 to 15 hours. BRIEF DESCRIPTION OF DRAWINGS

[0031] Figure 1 The flow chart of the preparation method of the SKT4 die steel forging of the embodiment of the present application is shown in the figure.

[0032] Figure 2 The direction diagram of the upsetting forging of the second blanking steel ingot in the third fire forging of the SKT4 die steel forging of the present embodiment is shown in the figure.

[0033] Figure 3 The direction diagram of the elongation of the second blanking steel ingot in the third fire forging of the SKT4 die steel forging of the present embodiment is shown in the figure. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below with reference to the drawings. Although some embodiments of the present application are shown in the drawings, it should be understood that the present application can be implemented in various forms, and should not be interpreted as being limited to the embodiments described herein, on the contrary, these embodiments are provided for a more thorough and complete understanding of the present application. It should be understood that the drawings and embodiments of the present application are only for illustrative purposes, and are not intended to limit the scope of protection of the present application.

[0035] Unless otherwise defined, all technical and scientific terms used in the present application have the same meanings as understood by those skilled in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments, and are not intended to limit the present application;

[0036] The term "includes" and its variants are open-ended, meaning that "includes but is not limited to"; the term "based on" means "based, at least in part, on"; the term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment"; the term "some embodiments" means "at least some embodiments"; the term "optional" means "optional in at least some embodiments". Related definitions are given throughout the description. It is noted that the concepts of "first", "second", etc. mentioned in the present application are used to distinguish different objects, and are not used to describe a specific order or primary and secondary relationship. In addition, the terms "first", "second" are only for description purposes, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more features.

[0037] To solve the problems in the related art, the embodiment provides an SKT4 die steel forging and a preparation method thereof.

[0038] As shown in Figure 1 The preparation method of the SKT4 die steel forging provided by the embodiment includes the following steps:

[0039] S1: Taking pig iron, sequentially performing intermediate frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, and demolding to obtain a steel ingot;

[0040] S2: Preheating the steel ingot, heating to 600-650 DEG C for ≥10H, and then heating to 1250±10 DEG C for 7-10H;

[0041] First fire forging is performed on the steel ingot, pre-drawing is performed using an upper flat and a lower V anvil, the forging ratio is 1.1 to 1.3, the water riser ends are removed by gas cutting, a first down material blank is obtained, and the first down material blank is reheated to 1250±10 DEG C for 9 to 15 hours;

[0042] Second fire forging is performed on the first down material blank, upsetting is performed between the upper flat surface and the lower platform, and the upper and lower wide flat anvil is used to draw a square, the forging ratio is 1.6 to 1.8, a second down material blank is obtained, and the second down material blank is reheated to 1230±10 DEG C for 5 to 15 hours;

[0043] Third fire forging is performed on the second down material blank, the second down material blank is upset along the length direction of the second down material steel ingot, and the second down material steel ingot is drawn along the width direction of the second down material blank, the forging ratio is 1.5 to 1.7, a third down material blank is obtained, and the third down material blank is reheated to 1200±10 DEG C for 5 to 15 hours;

[0044] carrying out fourth hot forging on the third blank, upsetting the third blank along the length direction of the third blank, elongating the third blank along the width direction of the third blank, a forging ratio being 1.2 to 1.4, to obtain a fourth blank, and returning to the furnace at 1200±10℃ for 5 to 15 hours;

[0045] carrying out fifth hot forging on the fourth blank, upsetting the fourth blank along the length direction of the fourth blank, elongating the fourth blank along the width direction of the fourth blank, a forging ratio being 1.4 to 1.5, to obtain a forged piece;

[0046] S3: carrying out post-forging heat treatment on the forged piece to obtain an SKT4 die steel forged piece.

[0047] In the embodiment, the upsetting direction is changed every time, so that the overall quality of the forged piece is more uniform, the anisotropy of the blank is improved, the influence of the anisotropy on the quality of the forged piece is reduced, the end protrusion of the square plate type forged piece is avoided, and the utilization rate of the steel ingot is improved; after each time of upsetting, the square is flattened and elongated in the direction, the direction of the axis of the steel ingot is changed, the directions of the axes between adjacent upsetting and elongating processes are 90°, for example, the width direction of the upsetting of the previous time is used as the length direction of the elongation of the current time, the defects in the center of the steel ingot are dispersed, the defects are not gathered but spread to the periphery, the number of defects in a unit area is greatly reduced, and the deformation of the center of the steel ingot is larger after the change of the direction, the porosity is improved, the defects are dispersed, the product quality is improved, and the yield of the finished product detected by ultrasonic testing is higher. In order to improve the product quality, homogenization treatment is added before forging, and the furnace is returned to the temperature for 5 to 15 hours.

[0048] Optionally, step S1 comprises:

[0049] S11: intermediate frequency furnace smelting: smelting molten steel by adding pig iron into the intermediate frequency furnace, when the return scrap is added into the induction furnace, the chemical composition must meet the process requirements; the induction furnace can be directly received; when the induction furnace is directly received and the slag is added, the Mn control should be paid attention to; when a large amount of slag is added after the induction furnace is directly received and the slag is hung on the furnace lining, the slag must be treated after the refining furnace is deoxidized and the alloy is recovered;

[0050] S12: LF furnace refining, after the slag of the electric furnace is completely melted, Al powder and appropriate C powder are used for diffusion deoxidization, the white slag is kept for more than or equal to 30 minutes, and the main chemical composition of the LF furnace refined steel enters the internal control range;

[0051] S13: VD vacuum degassing operation is performed, vacuum degree is less than or equal to 2 mbar, holding time is greater than or equal to 20 minutes, tapping temperature is 1550-1560°C for the first ladle and 1560-1570°C for the second ladle; before tapping, a gas sample is taken for analysis, [H] internal control is less than or equal to 0.00015%, and when [H] is greater than 0.00015%, secondary vacuum is needed;

[0052] S14: Casting electrodes: pouring into electrodes is performed under an argon protective environment, pouring temperature T is 1530-1550°C, mold temperature is controlled at 120-200°C, and the electrode blank is hot sent for annealing after cover cooling or demolding;

[0053] S15: CaF2: 70%, Al2O3: 25%, MgO: 5% slag system is used, and coke is used as a slag-making arc initiator, mold cooling time is 3.5 hours, and hot sending is performed immediately after demolding.

[0054] Optionally, the internal control components and ranges of the main chemical components of the refined tapping steel include, by mass fraction: C: 0.51-0.53%, Si: 0.24-0.26%, Mn: 0.88-0.92%, P: less than or equal to 0.015%, S: less than or equal to 0.003%, Cr: 1.28-1.32%, Ni: 1.78-1.82%, Mo: 0.38-0.42%, V: 0.16-0.20%, and Al: 0.02%.

[0055] Optionally, the intermediate frequency furnace is an intermediate frequency induction furnace.

[0056] Optionally, before the first hot forging of the ingot in step S2, the ingot is preheated to 600-650°C and held for greater than or equal to 10 hours, and then heated to 1250±10°C and held for 7-15 hours.

[0057] In this optional embodiment, the ingot is preheated to allow the ingot to undergo high-temperature diffusion.

[0058] Optionally, step S3 includes:

[0059] S31: The forged workpiece is held at a normalizing temperature of 840-860°C for 2-4 hours, the forged workpiece is hung down and air-cooled to 350-450°C, placed in an annealing furnace and sealed, tempered at a temperature of 630-650°C for 7-9 hours, cooled to below 400°C, and then cooled to below 180°C at a rate of less than or equal to 15°C / h;

[0060] S32: The forged workpiece is subjected to performance heat treatment, held at a normalizing temperature of 850-870°C for 3-5 hours, the forged workpiece is hung down and cycle oil-cooled for 70-90 minutes, tempered at a temperature of 610-630°C for 5-7 hours, cooled to below 400°C, and then furnace-cooled to below 300°C and taken out of the furnace, to obtain a billet;

[0061] S33: According to the operation rules, the billet is machined for finishing steel treatment to meet the corresponding use requirements.

[0062] In this optional embodiment, the purpose of post-forging heat treatment is to homogenize the structure, refine the grains, eliminate residual stress, and improve the metal structure and mechanical properties, and to prepare for the final performance heat treatment and finishing treatment, reduce hardness, facilitate machining, and prevent the formation of white spots inside the forged part.

[0063] Specifically, the air cooling of the forged part after being lifted out refers to the process of lifting the heated forged part out of the heating equipment and then naturally cooling it in the air during the heat treatment process. This process is particularly important in the normalizing treatment of the forged part, aiming to improve the internal structure and performance of the workpiece through natural cooling.

[0064] Reasons and importance of air cooling of forged parts after being lifted out:

[0065] (1) Uniform cooling: The lifting operation can ensure that all parts of the workpiece are uniformly cooled, avoiding the difference in cooling speed caused by uneven temperature distribution in the furnace, which in turn affects the internal structure and performance of the workpiece.

[0066] (2) Prevent deformation: During the cooling process, internal stress of the workpiece changes, which may cause deformation. The lifting operation helps to reduce this risk, as natural cooling in the air can gradually release stress, avoiding stress concentration caused by sudden temperature changes.

[0067] (3) Improve efficiency: The lifting operation can shorten the production cycle, as the workpiece needs to wait for the furnace temperature to decrease when cooling in the furnace, while cooling in the air after lifting can greatly shorten this time.

[0068] Optionally, the total forging ratio of step S2 is ≥ 6.

[0069] The SKT4 die steel forging provided in this embodiment is made by the preparation method of the SKT4 die steel forging described in any of the above.

[0070] Optionally, the cross-sectional size of the SKT4 die steel forging is not less than 2000mm x 2400mm x 300mm, and the weight of the SKT4 die steel forging is not less than 10000kg.

[0071] In this optional embodiment, the production of thin-walled large-section SKT4 die steel forgings is difficult, and the preparation method of the SKT4 die steel forging of this embodiment can produce thin-walled large-section SKT4 die steel forgings.

[0072] The present application will be further described in conjunction with specific embodiments.

[0073] Example 1: Preparation of a thin-walled large-section SKT4 die steel forging with a weight of 10480 kg and a total forging ratio requirement > 6, using a 16T electroslag ingot to produce one piece per ingot.

[0074] S11: Induction furnace smelting: pig iron is added to the induction furnace to smelt molten steel. When return scrap is added to the induction furnace, the chemical composition must meet the process requirements. Direct induction is allowed. When a large amount of slag is added after direct induction of the induction furnace lining with a large amount of slag, the refining furnace must be treated by slag turning after alloy recovery by deoxidation;

[0075] S12: Pig iron and molten steel are added to the LF refining furnace for refining. After the pig iron slag is completely melted, Al powder and C powder are added for diffusion deoxidation. The white slag formed by diffusion deoxidation is kept for not less than 30 minutes, so that the main chemical composition of the steel refined in the LF refining furnace enters the internal control range. The internal control composition range in the refining furnace is: C: 0.51-0.53%, Si: 0.24-0.26%, Mn: 0.88-0.92%, P: ≤0.015%, S: ≤0.003%, Cr: 1.28-1.32%, Ni: 1.78-1.82%, Mo: 0.38-0.42%, V: 0.16-0.20%, Al: 0.02%.

[0076] S13: VD vacuum degassing operation is performed to reduce the hydrogen content (i.e. [H]) of the molten steel to ≤0.00015%, wherein the vacuum degree is ≤2mbar and the holding time is ≥20 minutes. If the hydrogen content [H] is >0.00015%, vacuum degassing needs to be performed again until [H] is ≤0.00015%.

[0077] S14: The electrode is cast into the mold under an argon protective environment, the mold temperature is 120 to 200℃, and the casting temperature is 1530 to 1550℃; the electrode blank is cooled or hot sent for annealing after demolding, and one electrode per furnace is selected from the steelmaking plant to analyze the chemical composition of the water gap and the riser, including gas content.

[0078] S15: Electroslag remelting is performed using a slag system composed of 70% CaF2, 25% Al2O3 and 5% MgO by mass fraction, and coke as a slag-making arc starter, and the mold is cooled, wherein the cooling time is 3.5 hours, demolding is performed, and a steel ingot is obtained. 50x50x100mm test blocks are taken from the water gap and riser scrap of the steel ingot for chemical composition analysis, including gas content.

[0079] S2: Preheat the steel ingot to 600 to 650℃ and keep for ≥10H, then heat to 1250±10℃ and keep for 15H.

[0080] The steel ingot was forged using a 4500T hydraulic press. It was pre-drawn to a diameter of Φ1100mm and a length of 2100mm using a 400mm upper flat and lower V anvil, with a forging ratio of 1.21. The sprue and riser were removed by gas cutting, and 180mm of waste material was discarded at both ends to obtain the first blank. The blank was then returned to the furnace and held at 1250±10℃ for 15 hours.

[0081] The forging process before the final finishing heat was entirely carried out using a 4500T hydraulic press. The forging fixtures consisted of an upper flat upsetting cover plate, a lower platform upsetting plate, and a flattening and drawing process between a wide upper anvil and a large lower platform. The first blank was then subjected to a second heat forging, using a flat anvil and a drawing tool to upset and draw the blank to a width of 1200mm, a height of 700mm, and a length of 1930mm, with a forging ratio of 1.67, resulting in the second blank. The blank was then returned to the furnace and held at 1230±10℃ for 15 hours.

[0082] The second blank is then forged a third time, such as... Figure 2 and Figure 3 As shown, along the length direction of the second blank (e.g.) Figure 2 The second blank is upset to a height (e.g., in the Z direction) in the middle Z direction. Figure 2 The Z-direction dimension is 1400mm, along the width direction of the second blank (e.g., Figure 3 The second blank (in the Y direction) is drawn to a width of 1400mm, a height of 550mm, and a length of 2080mm, with a forging ratio of 1.61, to obtain the third blank, which is then reheated at 1200±10℃ for 15 hours; wherein along the width direction of the second blank (e.g., in the Y direction) Figure 3 After being drawn out in the Y direction, the width direction of the second blank (e.g., in the Y direction) Figure 3 The Y-direction then becomes the length direction of the third blank, and so on. The width direction of the third blank becomes the length direction of the fourth blank.

[0083] The third blank is forged in a fourth pass along the length of the third blank (e.g., Figure 3 (In the Y direction) the third blank is upset to a height of 1600mm, and then drawn along the width direction of the third blank to a width of 1600mm, a height of 450mm, and a length of 2190mm, with a forging ratio of 1.33, to obtain the fourth blank, which is then reheated at 1200±10℃ for 15 hours.

[0084] The fifth fire forging is a finished product fire time, a 6000T water press is used, the forging auxiliary tool uses an upper plane upsetting cover plate, a lower platform and an upper and lower flat anvil, the heating temperature of the fifth fire forging is lower than that of the first fire forging to the fourth fire forging, so that pits are not left on the forged piece during the forming process due to high temperature, so that the finished product size is not met, and the purpose of refining grains is achieved. The fourth blank is subjected to the fifth fire forging, the fourth blank is upset to a height of 1950mm along the length direction of the fourth blank, the fourth blank is elongated to a finished product size of 2000mm in width, 2400mm in length and 300mm in height along the width direction of the fourth blank, the forging ratio is 1.42, the flatness is to the finished product size, and a forged piece after forging is obtained;

[0085] The 4500T oil press and the 6000T water press are used for linkage production, the production process is greatly shortened, and the forging energy consumption is also reduced compared with the production by using a ten-thousand-ton press.

[0086] S3: the forged piece after forging is subjected to post-forging heat treatment.

[0087] S31: the forged piece after forging is subjected to normalizing at a temperature of 840 to 860 DEG C for 3 hours, the forged piece after forging is hung down and air-cooled to 400 DEG C, is put into an annealing furnace and sealed, is tempered at a temperature of 630 to 650 DEG C for 8 hours, is cooled to 400 DEG C, and is cooled to 180 DEG C at a rate of ≤15 DEG C / h;

[0088] S32: the forged piece after forging is subjected to performance heat treatment, is subjected to normalizing at a temperature of 850 to 870 DEG C for 4 hours, is hung down and is subjected to circulating oil cooling for 80 minutes, is tempered at a temperature of 610 to 630 DEG C for 6 hours, is cooled to 400 DEG C, is furnace-cooled to 300 DEG C and is taken out of the furnace, and a billet is obtained;

[0089] S33: according to the operation rules, the billet is subjected to finishing machining, so that the billet meets the corresponding use requirements.

[0090] Although the present application is disclosed as above, the protection scope of the present application is not limited to this. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present application, and these changes and modifications will fall within the protection scope of the present application.

Claims

1. A method of making a SKT4 die steel forging, characterized in that, The method comprises the following steps: S1: taking pig iron and sequentially performing intermediate frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, and demolding to obtain a steel ingot; S2: preheating: preheating the steel ingot, heating to 600-650 DEG C for ≥10 hours, and then heating to 1250±10 DEG C for 7-10 hours; first fire forging is performed on the steel ingot, pre-drawing is performed using an upper flat and a V-shaped anvil, the forging ratio is 1.1-1.3, the water riser is removed by gas cutting, and a first down material blank is obtained, which is reheated to 1250±10 DEG C for 9-15 hours; second fire forging is performed on the first down material blank, upsetting is performed between the upper flat and the cover plate, and the upper and lower wide flat anvil is used to draw the square, the forging ratio is 1.6-1.8, and a second down material blank is obtained, which is reheated to 1230±10 DEG C for 5-15 hours; third fire forging is performed on the second down material blank, the second down material blank is upset along the length direction of the second down material blank, and the second down material blank is drawn along the width direction of the second down material blank, the forging ratio is 1.5-1.7, and a third down material blank is obtained, which is reheated to 1200±10 DEG C for 5-15 hours; fourth fire forging is performed on the third down material blank, the third down material blank is upset along the length direction of the third down material blank, and the third down material blank is drawn along the width direction of the third down material blank, the forging ratio is 1.2-1.4, and a fourth down material blank is obtained, which is reheated to 1200±10 DEG C for 5-15 hours; fifth fire forging is performed on the fourth down material blank, the fourth down material blank is upset along the length direction of the fourth down material blank, and the fourth down material blank is drawn along the width direction of the fourth down material blank, the forging ratio is 1.4-1.5, and a forged workpiece is obtained; S3: the forged workpiece is subjected to post-forging heat treatment to obtain an SKT4 die steel workpiece.

2. The method of making a SKT4 die steel forging according to claim 1, wherein, The S1 comprises: S11: intermediate frequency furnace smelting, pig iron is added into the intermediate frequency furnace to smelt molten steel, under the condition that the returned scrap steel is added into the intermediate frequency furnace, the chemical composition is ensured to meet the process requirements; the induction furnace is allowed to be directly received; when the intermediate frequency furnace is directly received and the slag is added, the slag on the furnace lining is removed, and after the alloy is recovered by deoxidization in the refining furnace, the slag must be treated by turning over; S12: LF furnace refining is performed, after the electric lifting furnace slag is completely melted, Al powder and appropriate C powder are used for diffusion deoxidization, the white slag is required to be kept for ≥30 minutes, and the main chemical components of the LF furnace refined steel are made to enter the internal control range; S13: VD vacuum degassing operation is performed, the vacuum degree is ≤2 mbar, the holding time is ≥20 minutes, the tapping temperature is 1550-1560 DEG C in the first disc and 1560-1570 DEG C in the second disc; before tapping, a gas sample is taken for analysis, so that the hydrogen content is controlled to be ≤0.00015%; if the hydrogen content is >0.00015%, the vacuum is required to be increased until the hydrogen content is ≤0.00015%. S14: Casting the electrode: Pouring into the electrode under the protection of argon gas environment, the pouring temperature is 1530-1550℃, the mold temperature is controlled at 120-200℃, the electrode blank is covered with cold or hot sent after demolding for annealing; S15: Using the slag system composed of mass fraction of CaF2: 70%, Al2O3: 25% and MgO: 5%, and using coke as the slag forming arc agent, the mold cooling time is 3.5 hours, and the hot sending is immediately after demolding.

3. The method of making a SKT4 die steel forging according to claim 2, wherein, The main chemical components of the refined steel include: C: 0.51-0.53%, Si: 0.24-0.26%, Mn: 0.88-0.92%, P: ≤0.015%, S: ≤0.003%, Cr: 1.28-1.32%, Ni: 1.78-1.82%, Mo: 0.38-0.42%, V: 0.16-0.20% and Al: 0.02% by mass fraction.

4. The method of making a SKT4 die steel forging of claim 2, wherein, The intermediate frequency furnace is an intermediate frequency induction furnace.

5. The method of making a SKT4 die steel forging of claim 1, wherein, Before the first fire forging of the ingot in S2, the ingot is preheated to 600-650℃ and kept for ≥10H, and then heated to 1250±10℃ and kept for 7-15H.

6. The method of making a SKT4 die steel forging of claim 1, wherein, The S3 includes: S31: The forged piece is heated to normalizing temperature 840-860℃ and kept for 2-4 hours, the forged piece is hung down and air cooled to 350-450℃, put into the annealing furnace and sealed, the tempering temperature is kept at 630-650℃ for 7-9 hours, and then cooled to below 400℃, and then cooled to below 180℃ at a rate of ≤15℃ / h; S32: The forged piece is subjected to performance heat treatment, heated to normalizing temperature 850-870℃ and kept for 3-5 hours, the forged piece is hung down and cyclically oil cooled for 70-90 minutes, the tempering temperature is kept at 610-630℃ for 5-7 hours, and then cooled to below 400℃, and then furnace cooled to below 300℃ and taken out of the furnace, to obtain a billet; S33: The billet is subjected to finishing steel treatment according to the operation procedure, so that the billet meets the corresponding use requirements.

7. The method of making a SKT4 die steel forging of claim 1, wherein, The total forging ratio of S2 is ≥6.

8. A SKT4 die steel forging, characterized in that, The SKT4 die steel forging is made by the preparation method of any one of claims 1-7.

9. The SKT4 die steel forging of claim 8, wherein, The cross-sectional size of the SKT4 die steel forging is not less than 2000mm×2400mm×300mm, and the weight of the SKT4 die steel forging is not less than 10000kg.

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