SKT4 die steel forge piece and preparation method thereof

Abstract

The invention provides an SKT4 die steel forge piece and a preparation method thereof, and relates to the technical field of metal materials, and the preparation method of the SKT4 die steel forge piece comprises the following steps: smelting an electroslag ingot; and forging the steel ingot, and carrying out post-forging heat treatment on the forged piece to obtain the SKT4 die steel forged piece. Reversing upsetting is adopted, and the upsetting direction of each heating number is different, so that the overall quality of the forge piece is more uniform, the anisotropy of a blank is improved, the influence of the anisotropy on the quality of the forge piece is reduced, the phenomenon that raised heads at the ends of square plate forge pieces occupy materials is effectively avoided, and the steel ingot utilization rate of the forge piece is increased; after each heating number of upsetting, variable-direction flattening and drawing-out are carried out, and the axial direction of the steel ingot is changed, so that defects in the center of the steel ingot can be scattered, the number of defects in unit area is greatly reduced, the deformation of the center of the steel ingot is larger after variable-direction forging, the defects are scattered, the product quality is improved, and the yield of ultrasonic flaw detection is higher.

Description

Technical Field

[0001] The present invention relates to the technical field of metal materials, and in particular to an SKT4 die steel forging and a preparation method thereof. Background Art

[0002] At present, the forging method of square plate forgings is mostly single-direction square drawing deformation forging, which makes it difficult to forge the core defects of the forgings, and easily produces fiber tissues, resulting in anisotropy problems. In addition, SKT4 die steel forgings have high requirements for isotropy, and the single-direction square drawing deformation forging method cannot meet the isotropy requirements of SKT4 die steel forgings. In addition, this section has thin walls and large cross-sections, and usually requires multiple pieces to be combined into a large ingot for production. The technical requirements indicate that electroslag remelting ingots should be used for production, but our factory's electroslag ingots above 20T have quality risks. The above reasons make it impossible to use combined ingots for production. Summary of the invention

[0003] The problem solved by the invention is how to solve the problem that forgings are anisotropic and electroslag ingot SKT4 die steel forgings are difficult to produce using combined ingots.

[0004] In order to solve the above problems, the present invention provides an SKT4 die steel forging and a preparation method thereof.

[0005] In a first aspect, the present invention provides a method for preparing a SKT4 die steel forging, comprising the following steps:

[0006] S1: Take pig iron and carry out medium frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, demoulding to obtain steel ingot;

[0007] S2: Heating before forging: preheating the steel ingot, keeping the ingot at 600 to 650°C for ≥10 hours, heating it to 1250±10°C for 7 to 10 hours;

[0008] The steel ingot is subjected to first fire forging, and is pre-drawn using an upper flat and lower V anvil, with a forging ratio of 1.1 to 1.3, and waste materials at both ends of the water riser are removed by gas cutting to obtain a first blank, and the blank is returned to the furnace and kept at 1250±10° C. for 9 to 15 hours;

[0009] The first blank is subjected to a second fire forging, and is subjected to upsetting between the upper flat upsetting cover plate and the lower platform, and is flattened by using upper and lower wide flat anvils, with a forging ratio of 1.6 to 1.8, to obtain a second blank, and is returned to the furnace to be kept at 1230±10°C for 5 to 15 hours;

[0010] The second blank is subjected to third fire forging, the second blank is upset along the length direction of the second blank steel ingot, and the second blank is flattened along the width direction of the second blank, the forging ratio is 1.5 to 1.7, and a third blank is obtained, and the blank is returned to the furnace and kept at 1200±10° C. for 5 to 15 hours;

[0011] The third blank is subjected to fourth fire forging, the third blank is upset along the length direction of the third blank, and the third blank is stretched along the width direction of the third blank, with a forging ratio of 1.2 to 1.4 to obtain a fourth blank, and the blank is returned to the furnace for heat preservation at 1200±10° C. for 5 to 15 hours;

[0012] The fourth blank is subjected to fifth fire forging, the fourth blank is upset along the length direction of the fourth blank, and the fourth blank is stretched along the width direction of the fourth blank, and the forging ratio is 1.4 to 1.5, to obtain a forged forging;

[0013] S3: performing post-forging heat treatment on the forged part to obtain a SKT4 die steel forging.

[0014] Optionally, the S1 includes:

[0015] S11: Medium frequency furnace smelting, add pig iron into the medium frequency furnace to smelt molten steel, and when the medium frequency furnace is equipped with returned scrap steel, ensure that the chemical composition meets the process requirements; it is allowed to use the medium frequency furnace directly; when the medium frequency furnace is directly heated, attention should be paid to the control of Mn; when the medium frequency furnace lining has a lot of slag hanging on it and a large amount of slag is added directly, the refining furnace must turn over the slag after deoxidation and alloy recovery;

[0016] S12: LF furnace refining is performed. After all the electric lifting slag is melted, Al powder and appropriate amount of C powder are used for diffusion deoxidation. The white slag retention time is required to be ≥30 minutes, so that the main chemical composition of the steel refined by the LF furnace is within the internal control range;

[0017] S13: Perform VD vacuum degassing operation, vacuum degree ≤ 2mbar, holding time ≥ 20 minutes, tapping temperature 1550 to 1560°C for plate 1, 1560 to 1570°C for plate 2; take gas sample for analysis before tapping, make the hydrogen content internally controlled ≤ 0.00015%, if the hydrogen content is greater than 0.00015%, multiple vacuum removals are required until the hydrogen content is ≤ 0.00015%;

[0018] S14: Casting electrode: casting the electrode under argon protection environment, the casting temperature is 1530 to 1550°C, the mold temperature is controlled at 120 to 200°C, the electrode blank is hood-cooled or hot-sent annealed after demoulding;

[0019] S15: A slag system consisting of CaF2: 70%, Al2O3: 25% and MgO: 5% by mass is used, and coke is used as a slag-forming arc-starting agent. The mold cooling time is 3.5 hours, and hot delivery is immediately performed after demolding.

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

[0021] Optionally, the medium frequency furnace is a medium frequency induction furnace.

[0022] Optionally, before the first fire forging of the steel ingot in S2, the steel ingot is preheated to 600 to 650°C and kept warm for ≥10 hours, and then heated to 1250±10°C and kept warm for 7-15 hours.

[0023] Optionally, the S3 includes:

[0024] S31: the forged forging is kept at a normalizing temperature of 840 to 860° C. for 2 to 4 hours, the forged forging is suspended and air-cooled to 350 to 450° C., placed in an annealing furnace and sealed, and kept at a tempering temperature of 630 to 650° C. for 7 to 9 hours, and after cooling to below 400° C., the temperature is then cooled to below 180° C. at a rate of ≤15° C. / h;

[0025] S32: performing a performance heat treatment on the forged piece, keeping the normalizing temperature at 850 to 870° C. for 3 to 5 hours, hanging the forged piece for circulating oil cooling for 70 to 90 minutes, keeping the tempering temperature at 610 to 630° C. for 5 to 7 hours, cooling the temperature to below 400° C., cooling the furnace to below 300° C. and taking the steel billet out of the furnace;

[0026] S33: According to the operating procedures, the steel billet is machined to perform finishing steel processing so that the steel billet meets the corresponding use requirements.

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

[0028] In a second aspect, the present invention provides a SKT4 die steel forging, which is made by the preparation method of the SKT4 die steel forging as described in any one of the above items.

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

[0030] The beneficial effects of the SKT4 die steel forging and the preparation method thereof of the present invention are as follows: by adopting reversing upsetting, the upsetting direction of each fire is different, so that the overall quality of the forging is more uniform, which not only improves the anisotropy of the blank and reduces the influence of anisotropy on the quality of the forging, but also effectively avoids the convex head at the end of the square plate forging occupying the material, thereby improving the utilization rate of the forging steel ingot; after each fire upsetting, the direction is changed to flatten the square and stretch, and the axial direction of the steel ingot is changed. Between adjacent upsetting and stretching processes, the axial direction is 90°. For example, the width direction of the upsetting in the previous fire is used as the length direction of the stretching in this fire, which can break up the defects in the center of the steel ingot, so that the inclusion defects are no longer gathered but scattered around, and the number of defects per unit area is greatly reduced. Moreover, after the reversing forging, the deformation of the core of the steel ingot is larger, the looseness can be improved, the defects are dispersed, the product quality is improved, and the yield rate through ultrasonic flaw detection is higher. In order to improve product quality, homogenization treatment is added before forging, and the furnace is returned to keep warm for 5 to 15 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a flow chart of a method for preparing a SKT4 die steel forging according to an embodiment of the present invention;

[0032] Figure 2 It is a schematic diagram of the direction of upsetting forging the second blank steel ingot in the third fire forging of the SKT4 die steel forging of this embodiment 1;

[0033] Figure 3 It is a schematic diagram of the direction of drawing the second blank steel ingot during the third fire forging of the SKT4 die steel forging of this Example 1. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings. Although certain embodiments of the present invention are shown in the accompanying drawings, it should be understood that the present invention 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 to provide a more thorough and complete understanding of the present invention. It should be understood that the drawings and embodiments of the present invention are only for exemplary purposes and are not intended to limit the scope of protection of the present invention.

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

[0036] The term "including" and its variations used in this article are open inclusions, that is, "including but 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 other embodiment"; the term "some embodiments" means "at least some embodiments"; the term "optionally" means "optional embodiments". The relevant definitions of other terms will be given in the following description. It should be noted that the concepts of "first", "second", etc. mentioned in the present invention are used to distinguish different objects, rather than to describe a specific order or a primary and secondary relationship. In addition, the terms "first" and "second" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Therefore, the features defined as "first" and "second" may explicitly or implicitly include one or more of the features.

[0037] In view of the problems existing in the above-mentioned related technologies, this embodiment provides an SKT4 die steel forging and a preparation method thereof.

[0038] like Figure 1 As shown, the present embodiment provides a method for preparing a SKT4 die steel forging, comprising the following steps:

[0039] S1: Take pig iron and carry out medium frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, demoulding to obtain steel ingot;

[0040] S2: Heating before forging: preheat the steel ingot, keep it at 600-650℃ for ≥10H, heat it to 1250±10℃ and keep it for 7-10H;

[0041] The steel ingot is subjected to first fire forging, and is pre-drawn using an upper flat and lower V anvil, with a forging ratio of 1.1 to 1.3, and waste materials at both ends of the water riser are removed by gas cutting to obtain a first blank, and the blank is returned to the furnace and kept at 1250±10° C. for 9 to 15 hours;

[0042] The first blank is subjected to a second fire forging, and is subjected to upsetting between the upper flat upsetting cover plate and the lower platform, and is flattened by using upper and lower wide flat anvils, with a forging ratio of 1.6 to 1.8, to obtain a second blank, and is returned to the furnace to be kept at 1230±10°C for 5 to 15 hours;

[0043] The second blank is subjected to third fire forging, the second blank is upset along the length direction of the second blank steel ingot, and the second blank is flattened along the width direction of the second blank, the forging ratio is 1.5 to 1.7, and a third blank is obtained, and the blank is returned to the furnace and kept at 1200±10° C. for 5 to 15 hours;

[0044] The third blank is subjected to fourth fire forging, the third blank is upset along the length direction of the third blank, and the third blank is stretched along the width direction of the third blank, with a forging ratio of 1.2 to 1.4 to obtain a fourth blank, and the blank is returned to the furnace for heat preservation at 1200±10° C. for 5 to 15 hours;

[0045] The fourth blank is subjected to fifth fire forging, the fourth blank is upset along the length direction of the fourth blank, and the fourth blank is stretched along the width direction of the fourth blank, and the forging ratio is 1.4 to 1.5, to obtain a forged forging;

[0046] S3: performing post-forging heat treatment on the forged part to obtain a SKT4 die steel forging.

[0047] In this embodiment, reversing upsetting is adopted, and the upsetting direction of each fire is different, so that the overall quality of the forging is more uniform, which not only improves the anisotropy of the blank and reduces the influence of anisotropy on the quality of the forging, but also effectively avoids the convex head at the end of the square plate forging to occupy the material, and improves the utilization rate of the forging steel ingot; after each fire upsetting, the direction is changed to flatten the square and stretch, and the axis direction of the steel ingot is changed. Between adjacent upsetting and stretching processes, the axis direction is 90°. For example, the width direction of the upsetting in the previous fire is used as the length direction of the stretching in this fire, which can break up the defects in the center of the steel ingot, so that the inclusion defects no longer gather but spread around, and the number of defects per unit area is greatly reduced. After the reversing forging, the deformation of the core of the steel ingot is larger, which can improve the looseness, disperse the defects, improve the product quality, and the yield rate through ultrasonic flaw detection is higher. In order to improve the product quality, homogenization treatment is added before forging, and the furnace is returned to the furnace for 5 to 15 hours of heat preservation.

[0048] Optionally, step S1 includes:

[0049] S11: Medium frequency furnace smelting: Pig iron is added to the medium frequency furnace to smelt molten steel. When the induction furnace is equipped with returned scrap steel, the chemical composition must meet the process requirements; it is allowed to use the induction furnace directly; when the induction furnace is directly heated, attention should be paid to the control of Mn; when the induction furnace lining has a lot of slag hanging on it and a large amount of slag is added after direct heating, the refining furnace must turn over the slag for alloy recovery after deoxidation;

[0050] S12: LF furnace refining is performed. After all the electric lifting slag is melted, Al powder and appropriate amount of C powder are used for diffusion deoxidation. The white slag retention time is required to be ≥30 minutes, so that the main chemical composition of the steel refined by the LF furnace is within the internal control range;

[0051] S13: Perform VD vacuum degassing operation, vacuum degree ≤ 2mbar, holding time ≥ 20 minutes, tapping temperature 1550-1560℃ for the first plate, 1560-1570℃ for the second plate; take gas sample analysis before tapping, [H] internal control ≤ 0.00015%, and secondary vacuum is required when [H]>0.00015%;

[0052] S14: Casting electrode: casting into electrode under argon protection environment, casting temperature T = 1530-1550 ° C, mold temperature controlled at 120-200 ° C, electrode blank is hood-cooled or hot-sent annealing after demoulding;

[0053] S15: Use CaF2: 70%, Al2O3: 25%, MgO: 5% slag system, and use coke as slag-making and arc-starting agent, mold cooling time: 3.5 hours, and hot delivery immediately after demolding.

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

[0055] Optionally, the medium frequency furnace is a medium frequency induction furnace.

[0056] Optionally, before the first fire forging of the steel ingot in step S2, the steel ingot is preheated to 600 to 650°C and kept warm for ≥10 hours, and then heated to 1250±10°C and kept warm for 7-15 hours.

[0057] In this optional embodiment, the steel ingot is preheated to allow the steel ingot to diffuse at high temperature.

[0058] Optionally, step S3 includes:

[0059] S31: the forged forging is kept at a normalizing temperature of 840 to 860° C. for 2 to 4 hours, the forged forging is suspended and air-cooled to 350 to 450° C., placed in an annealing furnace and sealed, and kept at a tempering temperature of 630 to 650° C. for 7 to 9 hours, and after cooling to below 400° C., the temperature is then cooled to below 180° C. at a rate of ≤15° C. / h;

[0060] S32: performing a performance heat treatment on the forged piece, keeping the normalizing temperature at 850 to 870° C. for 3 to 5 hours, hanging the forged piece for circulating oil cooling for 70 to 90 minutes, keeping the tempering temperature at 610 to 630° C. for 5 to 7 hours, cooling the temperature to below 400° C., cooling the furnace to below 300° C. and taking the steel billet out of the furnace;

[0061] S33: According to the operating procedures, the steel billet is machined to perform finishing steel processing so that the steel billet meets the corresponding use requirements.

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

[0063] Specifically, air cooling of forgings after hanging forging refers to the process of hanging the heated forgings from the heating equipment during the heat treatment process and then cooling them naturally in the air. This process is particularly important in the normalizing treatment of forgings, and aims to improve the internal structure and performance of the workpiece through natural cooling.

[0064] Reasons and importance of air cooling for forgings after hanging forging:

[0065] (1) Uniform cooling: The hanging operation can ensure that all parts of the workpiece can be cooled evenly, avoiding the difference in cooling rate 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, the internal stress of the workpiece will change, which may cause deformation. Suspending operation helps to reduce this risk because natural cooling in the air can gradually release the stress and avoid stress concentration caused by sudden temperature changes.

[0067] (3) Improve efficiency: Suspending operation can shorten the production cycle, because the workpiece needs to wait for the furnace temperature to drop when cooling in the furnace, while cooling in the air after being suspended 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 manufactured by the preparation method of the SKT4 die steel forging as described in any one of the above items.

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

[0071] In this optional embodiment, thin-walled large-section SKT4 die steel forgings are difficult to produce. The preparation method of SKT4 die steel forgings in this embodiment can prepare thin-walled large-section SKT4 die steel forgings.

[0072] The present invention is further described below in conjunction with specific embodiments.

[0073] Example 1: Preparation of thin-walled large-section SKT4 die steel forgings weighing 10480 kg and requiring a total forging ratio of >6, using 16T electroslag ingots to produce one piece.

[0074] S11: Medium frequency furnace smelting: Pig iron is added to the medium frequency furnace to smelt molten steel. When the induction furnace is equipped with returned scrap steel, the chemical composition must meet the process requirements; it is allowed to use the induction furnace directly; when the induction furnace is directly heated, attention should be paid to the control of Mn; when the induction furnace lining has a lot of slag hanging on it and a large amount of slag is added after direct heating, the refining furnace must turn over the slag for alloy recovery after 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 retention time of white slag formed by diffusion deoxidation is not less than 30 minutes, so that the main chemical composition of the steel refined by the LF refining furnace enters the internal control range. The internal control composition range of 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: Perform VD vacuum degassing operation to reduce the hydrogen content (i.e. [H]) in the molten steel to ≤ 0.00015%, wherein the vacuum degree is ≤ 2 mbar and the holding time is ≥ 20 minutes; if the hydrogen content [H] is greater than 0.00015%, vacuum degassing is required again until [H] ≤ 0.00015%.

[0077] S14: Casting electrodes into molds under argon protection environment, mold temperature is 120 to 200°C, casting temperature is 1530 to 1550°C; electrode blanks are hood cooled or hot annealed after demoulding, and the steel mill selects one electrode from each furnace to perform chemical composition analysis at the nozzle and riser, including gas content.

[0078] S15: Use a slag system consisting of 70% CaF2, 25% Al2O3 and 5% MgO by mass, and use coke as a slag-forming arc-starting agent to perform electroslag remelting, cool the mold, wherein the cooling time is 3.5 hours, demold, and obtain a steel ingot, and take 50×50×100mm test blocks from the nozzle and riser discards of the steel ingot for chemical composition analysis, including gas content.

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

[0080] Use a 4500T hydraulic press to perform the first fire forging on the steel ingot, use a 400mm upper flat lower V anvil to pre-draw it to a diameter of Φ1100mm and a length of 2100mm, with a forging ratio of 1.21, and gas cut to remove the waste material of 180mm at both ends of the nozzle and riser to obtain the first blank, and return it to the furnace to keep it at 1250±10℃ for 15 hours;

[0081] The forging process before the subsequent finished product fire is all done with a 4500T hydraulic press. The forging aids use an upper flat upsetting cover plate, a lower platform upsetting, and an upper wide flat anvil and a lower large platform for flattening and drawing. The first blank is forged in the second fire, and the upper and lower flat anvils are used to upset and draw to a width of 1200mm, a height of 700mm, and a length of 1930mm. The forging ratio is 1.67, and the second blank is obtained. It is returned to the furnace and kept at 1230±10℃ for 15 hours;

[0082] The second blank is subjected to the third fire forging, such as Figure 2 and Figure 3 As shown, along the length direction of the second blank (such as Figure 2 The second blank is upset to a height (such as Figure 2 The Z-direction dimension is 1400 mm, and the width direction of the second blank is 1400 mm. Figure 3 The second blank is drawn to a width of 1400 mm, a height of 550 mm, a length of 2080 mm, and a forging ratio of 1.61 to obtain a third blank, which is then returned to the furnace at 1200 ± 10 ° C for 15 hours; Figure 3 After the second blank is drawn in the Y direction, the width direction (such as Figure 3 The Y direction in the middle) 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 subjected to a fourth fire forging, and the third blank is subjected to a fourth fire forging along the length direction (such as Figure 3 The third blank is upset to a height of 1600 mm along the width direction of the third blank, and the third blank is stretched to a width of 1600 mm, a height of 450 mm, and a length of 2190 mm, with a forging ratio of 1.33, to obtain a fourth blank, and the blank is returned to the furnace at 1200±10°C for 15 hours;

[0084] The fifth forging is to produce the finished product, using a 6000T hydraulic press. The forging aids include an upper flat upsetting cover plate, a lower platform, and upper and lower flat anvils. The heating temperature of the fifth forging is lower than that of the first to fourth forgings, so as to avoid leaving pits on the forgings due to high temperature during the forming process, resulting in failure to meet the finished product size, while achieving the purpose of grain refinement. The fourth blank is subjected to the fifth forging, and the fourth blank is upset to a height of 1950mm along the length direction of the fourth blank, and is stretched 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, and it is flattened to the finished product size to obtain the forged forging;

[0085] The use of 4500T oil press and 6000T water press for joint production can greatly shorten the production process, and can also reduce forging energy consumption compared with the use of 10,000-ton press for ingot production.

[0086] S3: Perform post-forging heat treatment on the forged parts.

[0087] S31: Keep the forged parts at a normalizing temperature of 840 to 860°C for 3 hours, hang the forged parts down and air cool them to 400°C, put them into the annealing furnace and seal the furnace, keep the tempering temperature at 630 to 650°C for 8 hours, cool down to 400°C, and then cool down to 180°C at a rate of ≤15°C / h;

[0088] S32: The forged forgings are subjected to a performance heat treatment, with a normalizing temperature of 850 to 870°C for 4 hours, the forged forgings are hung for circulating oil cooling for 80 minutes, the tempering temperature is 610 to 630°C for 6 hours, the temperature is lowered to 400°C, the furnace is cooled to 300°C and the steel billet is obtained;

[0089] S33: According to the operating procedures, the steel billet is machined for finishing steel processing to make the steel billet meet the corresponding use requirements.

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

Claims

1. A method for preparing SKT4 die steel forgings, characterized in that: The following steps are involved: S1: Take pig iron and carry out medium frequency furnace smelting, LF furnace refining, VD vacuum degassing, casting electrode and electroslag remelting, demoulding to obtain steel ingot; S2: Heating before forging: preheating the steel ingot, keeping the ingot at 600 to 650°C for ≥10 hours, heating to 1250±10°C for 7 to 10 hours; The steel ingot is subjected to first fire forging, and is pre-drawn using an upper flat and lower V anvil, with a forging ratio of 1.1 to 1.3, and waste materials at both ends of the water riser are removed by gas cutting to obtain a first blank, and the blank is returned to the furnace and kept at 1250±10° C. for 9 to 15 hours; The first blank is subjected to a second fire forging, and is subjected to upsetting between the upper flat upsetting cover plate and the lower platform, and is flattened by using upper and lower wide flat anvils, with a forging ratio of 1.6 to 1.8, to obtain a second blank, and is returned to the furnace to be kept at 1230±10°C for 5 to 15 hours; The second blank is subjected to third fire forging, the second blank is upset along the length direction of the second blank, and the second blank is flattened along the width direction of the second blank, the forging ratio is 1.5 to 1.7, and a third blank is obtained, and the blank is returned to the furnace and kept at 1200±10° C. for 5 to 15 hours; The third blank is subjected to fourth fire forging, the third blank is upset along the length direction of the third blank, and the third blank is stretched along the width direction of the third blank, with a forging ratio of 1.2 to 1.4 to obtain a fourth blank, and the blank is returned to the furnace for heat preservation at 1200±10° C. for 5 to 15 hours; The fourth blank is subjected to fifth fire forging, the fourth blank is upset along the length direction of the fourth blank, and the fourth blank is stretched along the width direction of the fourth blank, and the forging ratio is 1.4 to 1.5, to obtain a forged forging; S3: performing post-forging heat treatment on the forged part to obtain a SKT4 die steel forging.

2. The method for preparing SKT4 die steel forgings according to claim 1, characterized in that: The S1 includes: S11: smelting in medium frequency furnace, adding pig iron into medium frequency furnace to smelt molten steel, when the medium frequency furnace is equipped with returned scrap steel, the chemical composition is guaranteed to meet the process requirements; it is allowed to use induction furnace directly; when the medium frequency furnace is directly heated, attention should be paid to the control of Mn; when the lining of the medium frequency furnace is directly heated with a lot of slag, the slag must be turned over after the refining furnace deoxidizes and recovers the alloy; S12: LF furnace refining is performed. After all the electric lifting slag is melted, Al powder and appropriate amount of C powder are used for diffusion deoxidation. The white slag retention time is required to be ≥30 minutes, so that the main chemical composition of the steel refined by the LF furnace is within the internal control range; S13: Perform VD vacuum degassing operation, vacuum degree ≤ 2mbar, holding time ≥ 20 minutes, tapping temperature 1550 to 1560°C for plate 1, 1560 to 1570°C for plate 2; take gas sample for analysis before tapping, and make the hydrogen content internally controlled ≤ 0.00015%. If the hydrogen content is greater than 0.00015%, multiple vacuum degassing is required until the hydrogen content is ≤ 0.00015%; S14: Casting electrode: casting the electrode under argon protection environment, the casting temperature is 1530 to 1550°C, the mold temperature is controlled at 120 to 200°C, and the electrode blank is hood-cooled or hot-sent annealed after demoulding; S15: A slag system consisting of CaF2: 70%, Al2O3: 25% and MgO: 5% by mass is used, and coke is used as a slag-making and arc-starting agent. The mold cooling time is 3.5 hours, and hot delivery is immediately performed after demolding.

3. The method for preparing SKT4 die steel forgings according to claim 2, characterized in that: The internal controlled components and ranges of the main chemical components of the refined steel include, by mass, C: 0.51 to 0.53%, Si: 0.24 to 0.26%, Mn: 0.88 to 0.92%, P: ≤0.015%, S: ≤0.003%, Cr: 1.28 to 1.32%, Ni: 1.78 to 1.82%, Mo: 0.38 to 0.42%, V: 0.16 to 0.20% and Al: 0.02%.

4. The method for preparing SKT4 die steel forgings according to claim 2, characterized in that: The medium frequency furnace is a medium frequency induction furnace.

5. The method for preparing SKT4 die steel forgings according to claim 1, characterized in that: Before the first fire forging of the steel ingot in S2, the steel ingot is preheated to 600 to 650°C and kept warm for ≥10 hours, and then heated to 1250±10°C and kept warm for 7-15 hours.

6. The method for preparing SKT4 die steel forgings according to claim 1, characterized in that: The S3 includes: S31: the forged forging is kept at a normalizing temperature of 840 to 860° C. for 2 to 4 hours, the forged forging is suspended and air-cooled to 350 to 450° C., placed in an annealing furnace and sealed, and kept at a tempering temperature of 630 to 650° C. for 7 to 9 hours, and after cooling to below 400° C., the temperature is then cooled to below 180° C. at a rate of ≤15° C. / h; S32: performing a performance heat treatment on the forged piece, keeping the normalizing temperature at 850 to 870° C. for 3 to 5 hours, hanging the forged piece for circulating oil cooling for 70 to 90 minutes, keeping the tempering temperature at 610 to 630° C. for 5 to 7 hours, cooling the temperature to below 400° C., cooling the furnace to below 300° C. and taking the steel billet out of the furnace; S33: According to the operating procedures, the steel billet is machined to perform finishing steel processing so that the steel billet meets the corresponding use requirements.

7. The method for preparing SKT4 die steel forgings according to claim 1, characterized in that: The total forging ratio of S2 is ≥6.

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

9. The SKT4 die steel forging according to claim 8, characterized in that: The cross-sectional dimensions of the SKT4 die steel forging are not less than 2000mm×2400mm×300mm, and the weight of the SKT4 die steel forging is not less than 10000kg.

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