Steel for hot extrusion die and method for manufacturing the same

Abstract

The application provides a kind of hot extrusion die steel and its preparation method, belongs to material hot processing field.The hot extrusion die steel described in the application is obtained by sequentially passing through the following processes: ESR ingot production, forging, solid solution, spheroidizing annealing;In the forging process, the ESR ingot is heated before forging to promote high-temperature diffusion homogenization, the high-temperature diffusion temperature is 1260-1310 DEG C, and the time is 28-32 h;When forging, the forging ratio is 4-6;In the solid solution process, quenching is carried out in the form of water cooling and air cooling alternately.The preparation method of the application can directly prepare by forging the H13ESR ESR ingot, obtain φ960 H13ESR hot extrusion die material, and the material indicators are qualified, without using small size round steel (φ550) to reforge and heat treatment for secondary production to obtain φ960 large size round steel, which can save cost (4000 yuan / ton) and shorten the production cycle.

Description

Technical Field

[0001] This invention belongs to the field of hot working of materials, and specifically relates to a steel for hot extrusion dies and its preparation method. Background Technology

[0002] With the increasing size of industrial products, higher requirements are being placed on H13ESR die materials. These materials need to be of high quality, and the die sizes are also gradually increasing. Previously, the largest forged round bar specification for H13ESR hot extrusion dies used for aluminum profiles was φ550, but now the market demands φ960 round bars. To meet the demand for φ960 round bars, it is necessary to use φ550 forged round bars, then upset forging them into φ960 round bars, followed by heat treatment (solution + spheroidizing treatment). This process has the following technical defects and shortcomings:

[0003] 1. The addition of re-forging and heat treatment processes (solution treatment + spheroidizing treatment) increases costs by 4,000 yuan / ton and also requires a longer production cycle, adding approximately 45 days. Furthermore, the quality of products re-forged and heat-treated by non-professional manufacturers cannot meet usage requirements, affecting mold life;

[0004] 2. H13ESR round steel with a diameter of φ960 was found to have unqualified products due to the presence of liquid carbide precipitation.

[0005] 3. The metallographic structure is unqualified and does not meet the requirements of AS1 to AS9 grades of the "NADCA207~2011 North American Die Casting Mold Metallographic Standard Atlas" standard;

[0006] 4. The impact energy is unqualified and does not meet the standards and requirements.

[0007] Therefore, how to provide a large and high-quality steel for hot extrusion dies and its preparation method still needs to be explored. Summary of the Invention

[0008] The present invention aims to provide a steel for hot extrusion dies and a method for preparing the same, in order to solve the above-mentioned problems and meet the needs for steel for large and high-quality hot extrusion dies.

[0009] In a first aspect, the present invention provides a method for preparing steel for hot extrusion dies, wherein the steel for hot extrusion dies undergoes the following steps in sequence: electroslag ingot production, forging, solution treatment, and spheroidizing annealing to obtain the steel.

[0010] The forging process includes heating before forging to promote high-temperature diffusion homogenization of the electroslag ingot, with a high-temperature diffusion temperature of 1260-1310°C and a time of 28-32 hours; during forging, the forging ratio is 4-6.

[0011] The solution treatment process includes quenching using alternating water cooling and air cooling.

[0012] Furthermore, the high-temperature diffusion temperature is 1270–1300°C.

[0013] Furthermore, the forging ratio is 5.5.

[0014] Furthermore, the aspect ratio of the electroslag ingot is less than 3.0.

[0015] Further, the quenching process involves three water cooling cycles interspersed with two air cooling cycles; wherein the duration of each water cooling cycle is equal or unequal, ranging from 20 to 90 minutes; and the duration of each air cooling cycle is equal or unequal, ranging from 15 to 20 minutes; preferably, the quenching process involves: first water cooling for 60 to 90 minutes, then air cooling for 20 minutes, followed by a second water cooling cycle of 30 to 60 minutes, then a second air cooling cycle of 15 minutes, and finally a third water cooling cycle of 20 to 40 minutes to complete the quenching.

[0016] Furthermore, the electroslag ingot production process includes the following steps:

[0017] S11. Preparation of base material casting rods and slag: LF refining, VD vacuum degassing, casting rods and first annealing to obtain base material casting rods, while preparing binary pre-melted slag with protective function.

[0018] S12, Electroslag Remelting: Using argon protection, the base material casting rod and the binary pre-melted slag are remelted together to obtain an electroslag ingot;

[0019] Preferably, the diameter of the base material casting rod is Ф920mm~930mm, more preferably, the specification of the base material casting rod is Φ920*4000mm;

[0020] Preferably, the amount of the binary premelted slag is 640-660 kg;

[0021] Preferably, the primary annealing temperature is 900-950℃, and the holding time is 40-50h;

[0022] Preferably, the binary premelted slag is a mixture of Al2O3 and CaF2 in a mass ratio of 3:7;

[0023] Preferably, the melting rate of the electroslag remelting is set to 780-960 kg / h, the electrode is pre-loaded with 1200-1500 kg to begin feeding, and the mold cooling time is 200-240 min.

[0024] Preferably, the diameter of the electroslag ingot is Ф1150mm~1300mm, and more preferably, the specifications of the electroslag ingot are Ф1150mm*2500mm.

[0025] Further, the pre-forging heating includes: first heating the electroslag ingot to 830-870°C and holding it for 10-12 hours; then heating it to 1260-1310°C and holding it for 28-32 hours for high-temperature diffusion homogenization; then cooling it to 1200-1220°C and holding it for 4-5 hours before forging begins; preferably, the heating rate of the first stage is no more than 60°C / h, and the heating rate of the second stage is no more than 100°C / h.

[0026] Furthermore, the forging process includes: forging the electroslag ingot after high-temperature diffusion homogenization by upsetting and drawing, and cooling after forging. Preferably, when the surface temperature of the electroslag ingot is lower than 850°C during forging, it needs to be reheated in the furnace, and the heating time for each forging is 3 to 4 hours. During the final forging, the temperature at the corners of the electroslag ingot is greater than 850°C, and the temperature at other positions is greater than 900°C.

[0027] Furthermore, the forging process includes: during the drawing process on a flat anvil, the first pass involves light pressure of 10-20 mm on both sides, and the subsequent pass involves a reduction of 15%-18% of the billet thickness during each drawing pass on both sides. The amount of material fed into the anvil is 1 / 2 to 3 / 4 of the anvil width. The forging stop temperature is >900℃, and the material is returned to the furnace for heat preservation for 3-4 hours after drawing is completed.

[0028] Furthermore, during the final drawing, the billet size is 100-200mm larger than the finished product size; the double-sided pressing amount is 30-50mm per pass, the anvil feed amount is 1 / 4-1 / 2 of the anvil width, leaving a 20mm allowance for rounding; the final finishing temperature is >850℃, and mist cooling is performed after completion.

[0029] Furthermore, the post-forging cooling includes: first mist cooling to 350°C, then air cooling to 200-300°C, and then furnace annealing; preferably, the secondary annealing includes first heating to 850-870°C and holding for 25-28 hours; then cooling to 730-750°C and holding for 32-36 hours, and then cooling with the furnace, and removing from the furnace when the temperature does not exceed 250°C.

[0030] Further, the solution treatment process includes: first heating the forged electroslag ingot in a third stage to 830-870°C and holding it at that temperature for 6-7 hours; then heating it in a fourth stage to 1040-1060°C and holding it at that temperature for 15-18 hours; then immersing it in water for alternating water cooling and air cooling quenching; preferably, the heating rate of the third stage is no greater than 60°C / h, and the heating rate of the fourth stage is no greater than 80°C / h; the water temperature is below 30°C when it is first immersed in water.

[0031] Furthermore, after the quenching is completed, the surface steel temperature of the electroslag ingot is warmed up to 180-250°C, preferably 180-200°C, after being removed from the water, and finally air-cooled to 200-150°C before being put into the furnace for spheroidizing annealing.

[0032] Further, the spheroidizing annealing process includes: first heating the electroslag ingot after solution treatment in the fifth stage to 850-870°C and holding it at that temperature for 25-28 hours; then cooling it down to 730-750°C and holding it at that temperature for 32-36 hours; then cooling it in the furnace and removing it from the furnace when the temperature does not exceed 250°C; preferably, the heating rate in the fifth stage is not greater than 60°C / h, and the cooling rate is not greater than 40°C / h.

[0033] Secondly, the present invention provides a hot extrusion die steel obtained by the aforementioned preparation method, wherein the chemical composition of the hot extrusion die steel comprises: C 0.37-0.43wt%, Si 0.8-1.2wt%, Mn 0.2-0.5wt%, P≤0.015wt%, S≤0.005wt%, Cr 5-5.5wt%, Mo 1.3-1.5wt%, V 0.8-1.2wt%, Ni≤0.25wt%, and Cu≤0.25wt%.

[0034] Furthermore, the steel used for the hot extrusion die is φ960 round steel with a tolerance of +0 to +5 and a hardness of 44 to 46 HRC.

[0035] Samples were taken from the core of the round steel and examined for metallographic structure at 500x magnification. The metallographic structure was evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas and reached AS1-AS9 level.

[0036] Banded segregation was examined at 50x magnification and evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, reaching the SA1-SA4 level.

[0037] The average value of the transverse unnotched impact energy of multiple specimens is ≥220J, and the minimum value of a single specimen is ≥170J.

[0038] Ultrasonic testing shall be performed in accordance with GB / T6402-2008, meeting Level 4, with single-point defects ≤ Φ3.0mm flat-bottom hole equivalent and continuous defects ≤ Φ2.0mm flat-bottom hole equivalent.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] 1. The preparation method of the present invention can directly forge H13ESR electroslag ingots to obtain φ960 H13ESR hot extrusion die material, and the material has qualified all indicators. It does not require secondary production of φ960 large-size round steel by forging and heat treatment of small-size round steel (φ550), which can save costs (4000 yuan / ton) and shorten the production cycle.

[0041] 2. The preparation method of this invention has been successfully applied to the production of multiple batches of large-scale, high-quality hot-extruded die steel products, with nearly 1,000 tons delivered. The metallographic structure, impact energy, and liquid carbide content of the products have been greatly improved, and the quality level is superior to the technical standards, gaining market recognition. Attached Figure Description

[0042] Figure 1 This is a temperature-time curve of the forging heating process during the preparation of the hot extrusion die material of this invention;

[0043] Figure 2 This is a temperature-time curve of solid solution preparation during the preparation of the hot extrusion die material of this invention;

[0044] Figure 3 This is a specific temperature-time curve of spheroidizing annealing during the preparation of hot extrusion die materials for this invention;

[0045] Figure 4 The metallographic structure (500x) of the Φ960 hot extrusion die material obtained in Example 1 is shown in the SEM image.

[0046] Figure 5 The image shows a banded segregation (50x magnification) SEM image of the Φ960 hot extrusion die material obtained in Example 1.

[0047] Figure 6 The metallographic structure (500x) of the Φ960 hot extrusion die material obtained in Comparative Example 1 is shown in the SEM image.

[0048] Figure 7 The image shows a banded segregation pattern (50x magnification) of the Φ960 hot extrusion die material obtained in Comparative Example 1.

[0049] Figure 8 The image shows a 500x SEM image of the carbide liquid precipitation of the Φ960 hot extrusion die material obtained in Comparative Example 1.

[0050] Figure 9 The image shows the cracking caused by continuous water cooling during the solution treatment of the Φ960 hot extrusion die material obtained in Comparative Example 6.

[0051] Figure 10 The image shows the metallographic structure (500x magnification) of the Φ960 hot extrusion die material obtained in Comparative Example 7. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. The embodiments of this invention are implemented based on the technical solutions of this invention, and detailed implementation methods and processes are given. However, the scope of protection of this invention is not limited to the following embodiments. Those skilled in the art should understand that the embodiments are merely helpful in understanding this invention and should not be considered as specific limitations on this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0053] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0054] In this invention, unless otherwise specified and / or stated, all numerical values ​​relating to component amounts are by weight throughout. Process parameters in the following examples, unless otherwise specified, are generally performed under conventional conditions. The raw materials described in the following examples are all available from publicly available commercial sources.

[0055] In the first aspect, a method for preparing steel for hot extrusion dies involves the following steps: electroslag ingot production, forging, solution treatment, spheroidizing annealing, and rough turning, to obtain steel for hot extrusion dies.

[0056] The forging process includes pre-forging heating to promote high-temperature diffusion homogenization of the steel ingot. The high-temperature diffusion temperature is 1260–1310℃ (e.g., 1261℃, 1263℃, 1265℃, 1267℃, 1269℃, 1271℃, 1275℃, 1280℃, 1285℃, 1290℃, 1295℃, 1300℃, 1305℃, 1307℃, 1309℃), and the time is 28–32 hours (e.g., 28.5 hours, 29 hours, 29.5 hours, 30 hours, 30.5 hours, 31 hours, 31.5 hours). During forging, the forging ratio is 4–6 (e.g., 4.1, 4.3, 4.5, 4.7, 4.9, 5.1, 5.3, 5.5, 5.7, 5.9).

[0057] In the solution treatment process, quenching is performed by alternating between water cooling and air cooling.

[0058] The preparation method of this invention can directly forge H13ESR electroslag ingots with the largest possible diameter, such as φ1150 electroslag ingots, to obtain φ960 H13ESR hot extrusion die material. The material meets all the required indicators. It eliminates the need for secondary production by forging and heat treatment of small-diameter round steel (φ550) to obtain φ960 large-diameter round steel, thus saving costs (4000 yuan / ton) and shortening the production cycle.

[0059] Due to the characteristics of steel ingots, H13ESR steel ingots exhibit dendritic segregation and liquid carbide precipitation. Liquid carbide precipitation is generated in the interdendritic region due to solidification segregation and poses a certain hazard, potentially causing cracks to form at the liquid carbide sites during product service. Furthermore, the higher the level of liquid carbide precipitation in H13ESR steel, the greater the reduction in impact energy, severely degrading the material's properties and service life.

[0060] High-temperature diffusion during forging heating is the primary means of improving the precipitated carbides in H13RSR steel. Exploring effective and reasonable high-temperature diffusion processes (temperature and time) is essential. Precipitated carbides are mainly of the MC, M6C, M7C3, or M23C6 type; depending on their composition, they can be classified as Mo-rich, V-rich, and Ti / Nb-rich types. These carbides are distributed in point-like or chain-like patterns, are hard and brittle, and severely detach from the matrix, acting as stress concentration points and fatigue crack propagation initiations within the material. They are difficult to melt at high temperatures (below 1230℃) and are generally difficult to eliminate with heat treatment. Currently, many manufacturers only use the traditional forging heating process (i.e., heating the steel ingot to 1200–1230℃ and holding for ≤24 hours) before forging the finished product. This process cannot improve the precipitated carbides and dendrite segregation, leading to substandard products. The preparation method of this invention involves high-temperature diffusion of large-diameter H13ESR electroslag ingots at 1260–1310℃ for 28–32 hours. This process eliminates liquid carbides, significantly improves impact energy, enhances the banded structure, improves the quality of the product material, and ensures that all product material indicators meet the required standards.

[0061] Furthermore, the inventors discovered that increasing the forging ratio during forging allows for sufficient deformation of the round steel core, breaking down coarse grains and carbide networks. When the forging ratio increases from 2 to 4, the longitudinal impact toughness improves significantly. However, when the forging ratio exceeds 6, the improvement in longitudinal impact toughness slows down, and isotropy decreases, but costs increase substantially. Considering both production costs and the mechanical properties of the steel, and based on practical application experience, a forging ratio between 4 and 6 is deemed most suitable. This ensures thorough forging and compaction of the round steel, facilitates the breaking down of eutectic carbides, and guarantees satisfactory flaw detection and impact strength.

[0062] To achieve good microstructure and impact toughness, H13ESR hot work die steel requires solution treatment followed by spheroidizing annealing. Manufacturers often use oil cooling or water mist cooling during solution treatment to prevent cracking during quenching. However, inadequate cooling can easily lead to substandard microstructure and impact toughness. If oil quenching or air cooling is used during solution treatment, the slower the cooling rate, the more likely the carbide particles are to form angular and irregular shapes, resulting in uneven size distribution and poor microstructure and impact toughness. The method described in this invention uses water quenching (alternating water and air quenching) during solution treatment. This ensures thorough core hardening and the formation of a martensitic structure without cracking, and yields a more ideal annealed microstructure after spheroidizing annealing.

[0063] In a preferred embodiment of the present invention, the high-temperature diffusion temperature is 1270-1300℃ (e.g., 1271℃, 1273℃, 1275℃, 1277℃, 1279℃, 1281℃, 1283℃, 1285℃, 1287℃, 1289℃, 1291℃, 1293℃, 1295℃, 1297℃, 1299℃).

[0064] In a preferred embodiment of the present invention, the forging ratio is 5.5.

[0065] In the above technical solution, a forging ratio of 5.5 is selected, that is, a 3-upsetting and 3-drawing forging process is used, which can ensure that the round steel is forged through and compacted, and is conducive to breaking eutectic carbides, ensuring the flaw detection qualification and impact energy.

[0066] In a preferred embodiment of the present invention, the height-to-diameter ratio of the electroslag ingot is less than 3.0 (e.g., 2.9, 2.8, 2.7, 2.6, 2.5, 2.4, 2.3, 2.2, 2.1, 2.0).

[0067] In the above technical solution, controlling the height-to-diameter ratio of the electroslag ingot to be less than 3.0 can not only ensure the large size requirements of the electroslag ingot, but also prevent the electroslag ingot from bending during upsetting.

[0068] As an optional embodiment of the present invention, the quenching is performed by interspersing three water cooling cycles with two air cooling cycles; wherein the water cooling time for each cycle is equal or unequal, ranging from 20 to 90 minutes (e.g., 25 minutes, 30 minutes, 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes, 60 minutes, 65 minutes, 70 minutes, 75 minutes, 80 minutes, 85 minutes, 90 minutes); and the air cooling time for each cycle is equal or unequal, ranging from 15 to 20 minutes (e.g., 16 minutes, 17 minutes, 18 minutes, 19 minutes).

[0069] The above technical solution uses alternating water and air cooling during solution treatment, which prevents the product from cracking and ensures that the core is thoroughly quenched and martensitic structure is obtained. The more uniform the carbide distribution, the more ideal the annealed structure is obtained after spheroidizing annealing, and the metallographic structure and impact energy are qualified.

[0070] In a preferred embodiment of the present invention, the quenching is performed as follows: first, water cooling for 60-90 minutes (e.g., 65 minutes, 70 minutes, 75 minutes, 80 minutes, 85 minutes), then air cooling for 20 minutes, followed by a second water cooling for 30-60 minutes (e.g., 35 minutes, 40 minutes, 45 minutes, 50 minutes, 55 minutes), followed by a second air cooling for 15 minutes, and finally a third water cooling for 20-40 minutes (e.g., 25 minutes, 30 minutes, 35 minutes) to complete the quenching.

[0071] In the above technical solution, the water-air alternating quenching (water cooling for 60-90 min + air cooling for 20 min + water cooling for 30-60 min + air cooling for 15 min + water cooling for 20-40 min) can ensure sufficient quenching and avoid quenching cracks, thus ensuring controllable quality.

[0072] As an optional embodiment of the present invention, the electroslag ingot production process includes the following steps:

[0073] S11. Preparation of base material casting rods and slag: LF refining, VD vacuum degassing, casting rods, and annealing at temperatures of 900-950℃ (e.g., 905℃, 910℃, 915℃, 920℃, 925℃, 930℃, 935℃, 940℃, 945℃) and holding times of 40-50h (e.g., 41h, 43h, 45h, 47h, 49h) to obtain base material casting rods. At the same time, prepare 640-660kg (e.g., 641kg, 643kg, 645kg, 647kg, 649kg, 651kg, 653kg, 655kg, 657kg, 659kg) of binary pre-melted slag with protective function. The binary pre-melted slag is a mixture of Al2O3 and CaF2 with a mass ratio of 3:7.

[0074] S12. Electroslag Remelting: Under argon protection, the base material casting rod and the binary pre-melted slag are remelted together to obtain an electroslag ingot. The melting rate setting during electroslag remelting is: 780~960kg / h (e.g., 790kg / h, 800kg / h, 810kg / h, 820kg / h, 830kg / h, 840kg / h, 850kg / h, 860kg / h, 870kg / h, 880kg / h, 890kg / h, 900kg / h, 910kg / h, 920kg / h, 9...). 30kg / h, 940kg / h, 950kg / h), electrode allowance of 1200-1500kg (e.g., 1210kg, 1250kg, 1290kg, 1310kg, 1350kg, 1390kg, 1410kg, 1450kg, 1490kg) to begin shrinkage compensation, mold cooling time: 200-240min (e.g., 205min, 210min, 215min, 220min, 225min, 230min, 235min).

[0075] In the above technical solution, the diameter of the base material casting rod is Ф920mm~930mm (e.g. Ф921mm, Ф923mm, Ф925mm, Ф927mm, Ф929mm), preferably, the specification of the base material casting rod is Φ920*4000mm; the diameter of the resulting electroslag ingot is Ф1150mm~1300mm (e.g. Ф1160mm, Ф1180mm, Ф1200mm, Ф1220mm, Ф1240mm, Ф1260mm, Ф1280mm). Electroslag ingots within this diameter range can ensure both electroslag metallurgical quality and ease of forging. Specifically, a large-size electroslag ingot with a weight of 20t and a diameter of Ф1150mm*2500mm can be obtained for direct production of φ960 H13ESR hot extrusion die steel.

[0076] As an optional embodiment of the present invention, the pre-forging heating includes: firstly heating the electroslag ingot at a heating rate not exceeding 60℃ / h (e.g., 60℃ / h, 58℃ / h, 56℃ / h, 54℃ / h, 52℃ / h, 50℃ / h, 48℃ / h, 46℃ / h) to raise the temperature to 830-870℃ (e.g., 835℃, 840℃, 845℃, 850℃, 855℃, 860℃, 865℃) and holding it at that temperature for 10-12 hours (e.g., 10.5 hours, 11 hours, 11.5 hours); then heating the ingot at a heating rate not exceeding 100℃ / h (e.g., 100℃ / h, 98℃ / h, 96℃ / h, 94℃ / h, 92℃ / h, 90℃ / h, 88℃ / h, 86℃ / h) to raise the temperature to 830-870℃ (e.g., 835℃, 840℃, 845℃, 855℃, 860℃, 86℃ / h). Set the temperature to 1260–1310℃ (e.g., 1261℃, 1263℃, 1265℃, 1267℃, 1269℃, 1271℃, 1275℃, 1280℃, 1285℃, 1290℃, 1295℃, 1300℃, 1305℃, 1307℃, 1309℃) and hold for 28–32 hours (e.g., 28.5 hours, 29 hours, 29.5 hours). The temperature is first heated for 30 hours, 30.5 hours, 31 hours, and 31.5 hours to achieve high-temperature diffusion homogenization; then cooled to 1200–1220℃ (e.g., 1201℃, 1203℃, 1205℃, 1207℃, 1209℃, 1211℃, 1213℃, 1215℃, 1217℃, 1219℃), held at that temperature for 4–5 hours (e.g., 4.5 hours), and then forging begins. The specific temperature-time curve for pre-forging heating is shown in the figure. Figure 1 As shown.

[0077] In the above technical solution, after high-temperature diffusion at 1260-1310℃, the temperature is cooled to 1200-1220℃ and held for 4-5 hours before forging, in order to reduce the risk of forging cracks.

[0078] As an optional embodiment of the present invention, the forging process includes: forging the electroslag ingot after high-temperature diffusion homogenization by upsetting and drawing, and cooling after forging. When the surface temperature is lower than 850°C (e.g., 849°C, 847°C, 845°C, 843°C, 841°C, 839°C, 837°C) during forging, it needs to be reheated in the furnace. The heating time for each forging is 3 to 4 hours (e.g., 3.5 hours). The final forging temperature is greater than 850°C (e.g., 860°C, 870°C, 880°C, 890°C, 900°C, 910°C, 920°C, 930°C, 940°C, 950°C).

[0079] In the above technical solution, the temperature of the edges and corners should be controlled to be greater than 850℃ during the final forging, and greater than 900℃ in other locations. The product should be returned to the furnace for reheating in time to avoid corner cracks.

[0080] As an optional embodiment of the present invention, the forging process includes: during the drawing process on a flat anvil, the first pass involves light pressure on both sides for 10-20 mm (e.g., 11 mm, 13 mm, 15 mm, 17 mm, 19 mm), and the subsequent pass involves a reduction of 15%-18% (e.g., 15.5%, 16%, 16.5%, 17%, 17.5%) of the billet thickness during each drawing pass. The anvil feed is 1 / 2 to 3 / 4 of the anvil width, and the forging stop temperature is >900℃ (e.g., 905℃, 910℃, 915℃, 920℃, 925℃, 930℃, 935℃, 940℃, 945℃, 950℃). After the drawing process is completed, the billet is returned to the furnace for heat preservation for 3-4 hours (e.g., 3.5 hours).

[0081] In the above technical solution, the flat anvil is a double-sided flat anvil with a width of 700mm.

[0082] As an optional embodiment of the present invention, the forging process includes: during the final drawing, the billet size is 100-200mm larger than the finished product size (e.g., 110mm, 130mm, 150mm, 170mm, 190mm); the double-sided reduction per pass is 30-50mm (e.g., 31mm, 33mm, 35mm, 37mm, 39mm, 41mm, 43mm, 45mm, 47mm, 49mm), the anvil feed is 1 / 4 to 1 / 2 of the anvil width, leaving a 20mm allowance for rounding; the final finishing temperature is >850℃ (e.g., 860℃, 870℃, 880℃, 890℃, 900℃, 910℃, 920℃, 930℃, 940℃, 950℃), and mist cooling is performed after completion.

[0083] As an optional embodiment of the present invention, the post-forging cooling includes: first mist cooling to 350°C, then air cooling to 200-300°C (e.g., 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C) followed by furnace secondary annealing. Preferably, the secondary annealing includes first heating to 850-870°C (e.g., 851°C, 853°C, 855°C, 857°C, 859°C, 861°C, 863°C, 865°C, 867°C, 869°C), and holding at that temperature for 25-28 minutes. h (e.g., 25.5h, 26h, 26.5h, 27h, 27.5h); then cool down to 730-750℃ (e.g., 731℃, 733℃, 735℃, 737℃, 739℃, 741℃, 743℃, 745℃, 747℃, 749℃), hold for 32-36h (e.g., 33h, 34h, 35h), then cool with the furnace, and remove from the furnace when the temperature does not exceed 250℃ (e.g., 250℃, 248℃, 246℃, 244℃, 242℃, 240℃). After removing from the furnace, the black skin is finished and inspected for flaws.

[0084] As an optional embodiment of the present invention, the solution treatment process includes: first, subjecting the forged electroslag ingot to a third-stage heating at a heating rate not exceeding 60℃ / h (e.g., 60℃ / h, 58℃ / h, 56℃ / h, 54℃ / h, 52℃ / h, 50℃ / h, 48℃ / h, 46℃ / h) and heating it to 830-870℃ (e.g., 835℃, 840℃, 845℃, 850℃, 855℃, 860℃, 865℃), and holding it at that temperature for 6-7 hours (e.g., 6.5 hours); then holding it at a rate not exceeding 80℃ / h (e.g., 80℃ / h, 78℃ / h, 76℃ / h) The solution is heated in the fourth stage at heating rates of 74℃ / h, 72℃ / h, and 70℃ / h, reaching 1040–1060℃ (e.g., 1041℃, 1043℃, 1045℃, 1047℃, 1049℃, 1051℃, 1053℃, 1055℃, 1057℃, and 1059℃), and held at this temperature for 15–18 hours (e.g., 15.5 hours, 16 hours, 16.5 hours, 17 hours, and 17.5 hours). Afterward, it is immersed in water for alternating water and air quenching, with the initial water temperature below 30℃ (e.g., 29℃, 28℃, 27℃, 26℃, and 25℃). The specific temperature-time curves for solution treatment are shown below. Figure 2 As shown.

[0085] As an optional embodiment of the present invention, after quenching, the surface steel temperature of the electroslag ingot after water removal is warmed up to 180-250℃ (e.g., 185℃, 190℃, 195℃, 200℃, 205℃, 210℃, 215℃, 220℃, 225℃, 230℃, 235℃, 240℃, 245℃), preferably 180-200℃ (e.g., 181℃, 183℃, 185℃, 187℃, 189℃, 191℃, 193℃, 195℃, 197℃, 199℃), and finally air-cooled to 200-150℃ (e.g., 195℃, 190℃, 185℃, 180℃, 175℃, 170℃, 165℃, 160℃, 155℃) before being put into the furnace for spheroidizing annealing.

[0086] In the above technical solution, the surface steel temperature of the electroslag ingot is controlled at 180-250℃ after rinsing, which can ensure sufficient cooling and good quality, and also avoid cooling cracking and scrapping.

[0087] As an optional embodiment of the present invention, the spheroidizing annealing process includes: first, subjecting the solution-treated electroslag ingot to a fifth-stage heating at a heating rate not exceeding 60℃ / h (e.g., 60℃ / h, 58℃ / h, 56℃ / h, 54℃ / h, 52℃ / h, 50℃ / h, 48℃ / h, 46℃ / h) and heating it to 850-870℃ (e.g., 851℃, 853℃, 855℃, 857℃, 859℃, 861℃, 863℃, 865℃, 867℃, 869℃), and holding it at this temperature for 25-28 hours (e.g., 25.5 hours, 26 hours, 26.5 hours, 27 hours, 27 hours). 0.5h); then, at a cooling rate not exceeding 40℃ / h (e.g., 40℃ / h, 38℃ / h, 36℃ / h, 34℃ / h, 32℃ / h, 30℃ / h), cool to 730-750℃ (e.g., 731℃, 733℃, 735℃, 737℃, 739℃, 741℃, 743℃, 745℃, 747℃, 749℃), hold at this temperature for 32-36h (e.g., 33h, 34h, 35h); then, cool with the furnace, removing from the furnace and inspecting for flaws when the temperature does not exceed 250℃ (e.g., 250℃, 248℃, 246℃, 244℃, 242℃, 240℃). The specific temperature-time curve for spheroidizing annealing is shown below. Figure 3 As shown.

[0088] Secondly, a hot extrusion die steel obtained by the above preparation method, wherein the chemical composition of the hot extrusion die steel includes: C 0.37-0.43wt% (e.g., 0.38wt%, 0.39wt%, 0.40wt%, 0.41wt%, 0.42wt%), Si 0.8-1.2wt% (e.g., 0.85wt%, 0.9wt%, 0.95wt%, 1wt%, 1.05wt%, 1.1wt%, 1.15wt%), Mn 0.2–0.5 wt% (e.g., 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%), P ≤ 0.015 wt% (e.g., 0.014 wt%, 0.013 wt%, 0.012 wt%, 0.011 wt%, 0.01 wt%, 0.009 wt%, 0.008 wt%), S ≤ 0.005 wt% (e.g., 0.004 wt%, 0.003 wt%, 0.002 wt%, 0.001 wt%, 0.0009 wt%, 0.0008 wt%), Cr 5–5.5 wt% (e.g., 5.1 wt%, 5.2 wt%, 5.3 wt%, 5.4 wt%), Mo 1.3–1.5 wt% (e.g., 1.35 wt%, 1.37 wt%, 1.4 wt%, 1.45 wt%, 1.47 wt%), V 0.8–1.2 wt% (e.g., 0.85 wt%, 0.9 wt%, 0.95 wt%, 1 wt%, 1.05 wt%, 1.1 wt%, 1.15 wt%), Ni ≤ 0.25 wt% (e.g., 0.24 wt%, 0.23 wt%, 0.22 wt%, 0.21 wt%, 0.2 wt%, 0.19 wt%, 0.18 wt%), Cu ≤ 0.25 wt% (e.g., 0.24 wt%, 0.23 wt%, 0.22 wt%, 0.21 wt%, 0.2 wt%, 0.19 wt%, 0.18 wt%).

[0089] As an optional embodiment of the present invention, the steel used for the hot extrusion die is φ960 round steel with a tolerance of +0 to +5 and a hardness of 44 to 46 HRC.

[0090] Samples were taken from the core of the round steel and the annealed microstructure was examined at 500x magnification. The microstructure was evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas and reached AS1-AS9 level.

[0091] Banded segregation was examined at 50x magnification and evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, reaching the SA1-SA4 level.

[0092] The average value of the transverse unnotched impact energy of multiple specimens is ≥220J, and the minimum value of a single specimen is ≥170J.

[0093] Ultrasonic testing shall be performed in accordance with GB / T6402-2008, meeting Level 4, with single-point defects ≤ Φ3.0mm flat-bottom hole equivalent and continuous defects ≤ Φ2.0mm flat-bottom hole equivalent.

[0094] In this invention, the technical requirements for the H13ESR hot extrusion die material are as follows:

[0095] 1.1. Chemical composition:

[0096] The large hot extrusion die is made of H13ESR, and its chemical composition requirements are shown in the table below:

[0097]

[0098] 1.2 Forged round steel dimensions and tolerances: Φ960mm after rough machining, tolerance +0 / +5.

[0099] 1.3 Smelting method: Electric furnace (LF+VD) + electroslag remelting

[0100] 1.4 Microstructure requirements:

[0101] 1.4.1 Annealing Structure

[0102] Samples were taken from the core of the round steel and the annealed microstructure was examined at 500x magnification. The microstructure was evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, and the microstructure should reach AS1-AS9 level.

[0103] 1.4.2 Banded segregation

[0104] The bands were examined at 50x magnification and evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, aiming to reach SA1-SA4 level.

[0105] 1.4.3 Carbide liquid precipitation

[0106] The presence of obvious carbide precipitation is not allowed.

[0107] 1.5 Lateral Impact Test

[0108] 1.5.1 Samples were taken from the center of the cross section of the round steel body. The sample blank was austenitized at 1030±10℃, held for 30 min, oil quenched, and tempered at least twice to ensure that the final hardness was between 44 and 46 HRC.

[0109] 1.5.2 In accordance with NADCA#207 standard, unnotched specimens of 7mm×10mm×55mm shall be used, with dimensional deviations of 7±0.100mm, 10±0.100mm, and 55±1.00mm. The transverse unnotched impact energy shall be calculated as the arithmetic mean of a set of three specimens: ≥220J, with a minimum single value of ≥170J allowed for one specimen.

[0110] 1.6 Ultrasonic Flaw Detection

[0111] The steel shall be subjected to ultrasonic testing in accordance with GB / T6402-2008, and shall meet the requirements of level 4. Single point defects ≤ Φ3.0mm flat bottom hole equivalent and continuous defects ≤ Φ2.0mm flat bottom hole equivalent shall be qualified products.

[0112] In this invention, since the forged round steel bar is φ960mm, far exceeding the general round steel bar specification of φ550mm, large-diameter electroslag ingots are prioritized to ensure the smooth upsetting of the 60MN press and the quality of the electroslag ingots. Currently, the largest electroslag ingot in China is 20 tons. A 20T electroslag furnace can produce electroslag ingots with a diameter of 1150mm, a length of 2500mm, and a single weight of 20 tons. Therefore, the production process planning and route are as follows:

[0113] Electric furnace (LF+VD) → casting rod → annealing → electroslag remelting (Ф1150mm*2500mm) → hot conveying → 60MN forging into finished product → annealing → ultrasonic testing → solution treatment + spheroidizing annealing → ultrasonic testing → inspection → rough turning → ultrasonic testing → warehousing.

[0114] The present invention will now be described in further detail with reference to specific embodiments.

[0115] Example 1

[0116] A steel for hot extrusion dies and its preparation method, the preparation method comprising the following steps:

[0117] 1. Production of 20t electroslag ingots

[0118] 1.1 Slag and related preparations: Φ920*4000mm base material casting rod, 650kg of binary pre-melted slag.

[0119] 1.2 Electroslag remelting: Argon gas protection is used, melting rate is set to 850 kg / h, electrode pre-reserved 1350 kg to start feeding, mold cooling time: 220 min.

[0120] The Φ920*4000mm base material casting rod was prepared according to the designed composition, refined by LF, vacuum degassing by VD, cast into a rod, and then annealed once; subsequently, it was electroslag remelted to obtain 20t electroslag ingots (Φ1150mm*2500mm). The composition design objectives are shown in the table below:

[0121] Brand Chemical elements C Si Mn P S Cr Mo V Ni Cu Al H13ESR Ingredient design goals 0.39 1.00 0.38 ≤0.012 ≤0.003 5.30 1.40 1.00 ≤0.10 ≤0.10 0.020

[0122] 2. Forging

[0123] The process of forging a 20t steel ingot (electroslag ingot, Ф1150mm*2500mm) into finished products includes the following steps:

[0124] (1) The steel ingot adopts a high-temperature homogenization diffusion heating process. First, the temperature is raised to 850℃ at a heating rate of 55℃ / h and held for 10h. Then, the temperature is raised to the high-temperature diffusion temperature at a heating rate of 80℃ / h and held for a period of time. The high-temperature diffusion temperature is 1280~1300℃ and the high-temperature diffusion time for 20t ingot is 30h. After that, the temperature is lowered to 1200~1210℃ and held for 4h before forging begins.

[0125] (2) Forging is performed on a 60MN press using upper and lower flat anvils with a width of 700 mm. To ensure the product passes flaw detection, the steel ingot must be fully forged through to weld internal porosity and other defects. The total forging ratio in the process design is 5.5, with the steel ingot being forged into finished product through 3 upsetting and 3 drawing operations. Since this steel is a high-alloy die steel, it is prone to forging cracks, especially corner cracks. The final forging temperature must be controlled to be greater than 900℃ (corner temperature greater than 850℃). If the surface temperature is below 850℃ during forging, it must be reheated in the furnace promptly, with each heating cycle lasting 3-4 hours. The forging process and forging deformation steps are as follows:

[0126]

[0127] (3) Cooling after forging:

[0128] After forging, the metal is mist-cooled to 350°C, then air-cooled to 260°C before furnace annealing. After exiting the furnace, the black skin is finished, flawed, and then solution-treated and spheroidizing annealed.

[0129] 3. Solid solution:

[0130] 1. First, heat the temperature to 850℃ at a heating rate of 55℃ / h and hold for 6 hours; then heat the temperature to the solution temperature of 1040~1060℃ at a heating rate of 70℃ / h and hold for 16 hours; then immerse the product in water and perform alternating water cooling and air cooling quenching. The water temperature should be below 30℃ when the product is first immersed in water.

[0131] Alternating water and air quenching is adopted, specifically: water cooling for 75 min + air cooling for 20 min + water cooling for 45 min + air cooling for 15 min + water cooling for 30 min.

[0132] After water-air alternating quenching, the surface steel temperature is controlled at 180-190℃ after exiting the water, and then air-cooled to 180℃ before entering the furnace for spheroidizing.

[0133] 4. Spheroidizing annealing:

[0134] First, heat the furnace to 860℃ at a heating rate of 55℃ / h and hold for 26 hours; then cool the furnace to 730-740℃ at a cooling rate of 35℃ / h and hold for 34 hours; then cool the furnace with the furnace and remove the furnace for inspection when the temperature does not exceed 250℃.

[0135] 5. Rough car

[0136] After rough machining, a φ960mm round steel bar is obtained with a tolerance of +0 / +5.

[0137] Example 2

[0138] A steel for hot extrusion dies and its preparation method differ from Example 1 in the following ways:

[0139] 1. Production of 20t electroslag ingots

[0140] 1.1 Slag and related preparations: Φ920*4000mm base material casting rod, 640kg of binary pre-melted slag.

[0141] 1.2 Electroslag remelting: Argon gas protection is used, melting rate is set at 780 kg / h, electrode pre-reserved 1200 kg to start feeding, mold cooling time: 200 min.

[0142] The Φ920*4000mm base material casting rod was prepared according to the designed composition, refined by LF, vacuum degassing by VD, cast into a rod, and then annealed once; subsequently, it was electroslag remelted to obtain 20t electroslag ingots (Φ1150mm*2500mm). The composition design objectives are shown in the table below:

[0143] Brand Chemical elements C Si Mn P S Cr Mo V Ni Cu Al H13ESR Ingredient design goals 0.40 1.10 0.30 ≤0.012 ≤0.003 5.00 1.30 0.80 ≤0.20 ≤0.20 0.020

[0144] 2. Forging

[0145] The process of forging a 20t steel ingot (electroslag ingot, Ф1150mm*2500mm) into finished products includes the following steps:

[0146] (1) The steel ingot adopts a high-temperature homogenization diffusion heating process. First, the temperature is raised to 830℃ at a heating rate of 60℃ / h and held for 12h. Then, the temperature is raised to the high-temperature diffusion temperature at a heating rate of 90℃ / h and held for a period of time. The high-temperature diffusion temperature is 1260~1280℃, and the high-temperature diffusion time for a 20t ingot is 28h. After that, the temperature is lowered to 1200~1208℃ and held for 4h before forging begins. The specific temperature-time curve is as follows: Figure 1 As shown.

[0147] (3) Cooling after forging:

[0148] After forging, the metal is mist-cooled to 350°C, then air-cooled to 200°C before furnace annealing. After exiting the furnace, the black skin is finished, flawed, and then solution-treated and spheroidizing annealed.

[0149] 3. Solid solution:

[0150] 2. First, heat the temperature to 830℃ at a heating rate of 60℃ / h and hold for 6 hours; then heat the temperature to the solution temperature of 1040~1050℃ at a heating rate of 80℃ / h and hold for 15 hours; then immerse the product in water and perform alternating water cooling and air cooling quenching. The water temperature should be below 30℃ when the product is first immersed in water.

[0151] Alternating water and air quenching is adopted, specifically: water cooling for 60 min + air cooling for 20 min + water cooling for 30 min + air cooling for 15 min + water cooling for 20 min.

[0152] After water-air alternating quenching, the surface steel temperature is controlled at 200-210℃ after exiting the water, and then air-cooled to 195℃ before entering the furnace for spheroidizing.

[0153] 4. Spheroidizing annealing:

[0154] First, the temperature is increased to 850℃ at a heating rate of 60℃ / h and held for 25 hours; then, it is decreased to 730–735℃ at a cooling rate of 40℃ / h and held for 32 hours; afterwards, it is cooled in the furnace, and removed from the furnace at a temperature not exceeding 250℃ for flaw detection. The specific temperature-time curve for spheroidizing annealing is as follows: Figure 3 As shown.

[0155] 6. Rough car

[0156] After rough machining, a φ960mm round steel bar is obtained with a tolerance of +0 / +5.

[0157] Example 3

[0158] A steel for hot extrusion dies and its preparation method differ from Example 1 in the following ways:

[0159] 1. Production of 20t electroslag ingots

[0160] 1.1 Slag and related preparations: Φ920*4000mm base material casting rod, 660kg of binary pre-melted slag.

[0161] 1.2 Electroslag remelting: Argon gas protection is used, melting rate is set to 960 kg / h, electrode pre-reserved 1500 kg to start feeding, mold cooling time: 240 min.

[0162] The Φ920*4000mm base material casting rod was prepared according to the designed composition, refined by LF, vacuum degassing by VD, cast into a rod, and then annealed once; subsequently, it was electroslag remelted to obtain 20t electroslag ingots (Φ1150mm*2500mm). The composition design objectives are shown in the table below:

[0163] Brand Chemical elements C Si Mn P S Cr Mo V Ni Cu Al H13ESR Ingredient design goals 0.42 1.15 0.25 ≤0.012 ≤0.003 5.50 1.30 1.10 ≤0.10 ≤0.10 0.020

[0164] 2. Forging

[0165] The process of forging a 20t steel ingot (electroslag ingot, Ф1150mm*2500mm) into finished products includes the following steps:

[0166] (1) The steel ingot adopts a high-temperature homogenization diffusion heating process. First, the temperature is raised to 870℃ at a heating rate of 50℃ / h and held for 10h. Then, the temperature is raised to the high-temperature diffusion temperature at a heating rate of 95℃ / h and held for a period of time. The high-temperature diffusion temperature is 1300~1310℃ and the high-temperature diffusion time for 20t ingot is 32h. After that, the temperature is lowered to 1210~1220℃ and held for 4h before forging begins.

[0167] (3) Cooling after forging:

[0168] After forging, the metal is mist-cooled to 350°C, then air-cooled to 300°C before furnace annealing. After exiting the furnace, the black skin is finished, flawed, and then solution-treated and spheroidizing annealed.

[0169] 3. Solid solution:

[0170] 3. First, heat the temperature to 870℃ at a heating rate of 50℃ / h and hold for 6 hours; then heat the temperature to the solution temperature of 1050~1060℃ at a heating rate of 75℃ / h and hold for 18 hours; then immerse the product in water and perform alternating water cooling and air cooling quenching. The water temperature should be below 30℃ when the product is first immersed in water.

[0171] Alternating water and air quenching is adopted, specifically: water cooling for 90 min + air cooling for 20 min + water cooling for 60 min + air cooling for 15 min + water cooling for 40 min.

[0172] After water-air alternating quenching, the surface steel temperature is controlled at 190-200℃ after exiting the water, and then air-cooled to 150℃ before entering the furnace for spheroidizing.

[0173] 4. Spheroidizing annealing:

[0174] First, heat the furnace to 870℃ at a heating rate of 50℃ / h and hold for 28 hours; then cool the furnace to 740-750℃ at a cooling rate of 35℃ / h and hold for 36 hours; then cool the furnace with the furnace and remove the furnace for inspection when the temperature does not exceed 250℃.

[0175] 7. Rough car

[0176] After rough machining, a φ960mm round steel bar is obtained with a tolerance of +0 / +5.

[0177] Comparative Example 1

[0178] A hot extrusion die steel and its preparation method differ from Example 1 only in that, during forging heating, the high-temperature diffusion temperature is 1200–1230°C, and the high-temperature diffusion holding time is 24 hours. All other settings are the same as in Example 1.

[0179] Comparative Example 2

[0180] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that the total forging ratio is 2 during forging. All other settings are the same as in Example 1.

[0181] Comparative Example 3

[0182] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that the total forging ratio is 7 during forging. All other settings are the same as in Example 1.

[0183] Comparative Example 4

[0184] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that water mist cooling is used instead of water cooling and air cooling during solution treatment. All other settings are the same as in Example 1.

[0185] Comparative Example 5

[0186] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that oil cooling is used instead of water cooling and air cooling during solution treatment. All other settings are the same as in Example 1.

[0187] Comparative Example 6

[0188] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that continuous water cooling is used during solution treatment. All other settings are the same as in Example 1.

[0189] Comparative Example 7

[0190] A hot extrusion die steel and its preparation method are disclosed, differing from Example 1 only in that continuous air cooling is used during solution treatment. All other settings are the same as in Example 1.

[0191] test

[0192] The microstructure of the materials obtained after spheroidizing annealing of the three samples obtained in Example 1, and the samples obtained in Examples 2, 3, and Comparative Examples 1-7 was tested as follows:

[0193] 1. Metallographic structure

[0194] Samples were taken from the core of the round steel and the annealed metallographic structure was examined at 500x magnification. The structure was evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, and should reach the AS1-AS9 level.

[0195] 2. Banded segregation

[0196] The bands were examined at 50x magnification and evaluated according to the NADCA207-2011 North American Die Casting Mold Metallographic Standard Atlas, aiming to reach SA1-SA4 level.

[0197] 3. Carbide liquid precipitation

[0198] No obvious carbide segregation is present, which is qualified.

[0199] 4. Transverse impact test

[0200] 4.1. Samples are taken from the core of the round steel along the cross-section. The test specimen blanks are austenitized at 1030 ± 10 °C, held for 30 min, oil quenched, and tempered twice. The hardness required by the standard is between 44 and 46 HRC.

[0201] 4.2. According to the NADCA #207 standard, non-notch specimens with dimensions of 7 mm × 10 mm × 55 mm are used. The specimen size deviations are processed as 7 ± 0.100 mm, 10 ± 0.100 mm, and 55 ± 1.00 mm. The transverse non-notch impact energy is tested with a set of three specimens. The qualified standard requirements are: the average impact energy ≥ 220 J, and it is allowed that the single minimum value of one specimen ≥ 170 J.

[0202] 5. Ultrasonic flaw detection

[0203] The steel is subjected to ultrasonic flaw detection in accordance with GB / T 6402 - 2008. It is required to meet level 4, with single-point defects ≤ Φ3.0 mm flat-bottom hole equivalent and continuous defects ≤ Φ2.0 mm flat-bottom hole equivalent as qualified products.

[0204] Using the above inspection methods, the test results are shown in the following table:

[0205] project Metallographic structure Banded segregation Carbide liquid precipitation hardness Lateral unnotched impact energy Ultrasonic flaw detection Does the solution crack upon cooling? Example 1 AS4 SA2 qualified qualified qualified qualified no Example 2 AS4 SA2 qualified qualified qualified qualified no Example 3 AS4 SA2 qualified qualified qualified qualified no Comparative Example 1 AS10 SB3 Unqualified qualified Unqualified qualified no Comparative Example 2 AS4 SA2 qualified qualified Unqualified Unqualified no Comparative Example 3 AS4 SA2 qualified qualified Unqualified qualified no Comparative Example 4 AS10 SA2 qualified qualified Unqualified qualified no Comparative Example 5 AS10 SA2 qualified qualified Unqualified qualified no Comparative Example 6 AS4 SA2 qualified qualified qualified qualified yes Comparative Example 7 SA15 SA2 qualified qualified Unqualified qualified no

[0206] It can be seen from the test results that the test results of the specimens obtained in Examples 1 - 3 are all qualified, while there are unqualified items in the test results of the specimens obtained in Comparative Examples 1 - 7. Among them, the metallographic structure of the three Φ960 hot extrusion die material specimens obtained in Example 1 after solution + spheroidizing annealing reaches level AS4, the band segregation reaches level SA2, no obvious carbide segregation is present, the ultrasonic flaw detection is qualified, the non-notch impact energies are 341 J, 309 J, and 322 J, far superior to the standard. And the measured hardness is 44.5 HRC, 45 HRC, and 45 HRC, meeting the hardness requirement of 44 - 46 HRC in the standard. All indicators are qualified, and the production cycle is short and the cost is low. Among them, the SEM image of the metallographic structure (500 times) of one specimen in Example 1 is shown in Figure 4 , and the SEM image of the band segregation (50 times) is shown in Figure 5 .

[0207] A comparison of the tests conducted in Example 1 and Comparative Example 1 shows that, in the method of this invention, the 20t electroslag ingot, using a high-temperature diffusion temperature of 1280-1300℃ and a diffusion time of 30h, produces a product with all indicators meeting standards, no liquid-precipitated carbides, and effectively improved banded structure. In contrast, the metallographic structure of Comparative Example 1 is AS10, which is unqualified. Its metallographic structure (500x SEM image) is shown below. Figure 6 Its banded segregation grade is SB3, which is unacceptable. The SEM image of its banded segregation (50x magnification) is shown below. Figure 7 Furthermore, there is obvious carbide precipitation, which is unacceptable. See details below. Figure 8 SEM image of carbide liquid chromatography (500x magnification).

[0208] A comparison of the tests conducted in Example 1 with those in Comparative Examples 2 and 3 reveals that the method in this embodiment of the invention, which forges Φ960 large-diameter round steel using a 60MN press (700mm wide anvil), employs three upsetting and drawing processes on a 20t electroslag ingot, with a forging ratio of 5.5, and produces a forged product of Φ960mm, effectively penetrates the internal porosity and other defects of the steel ingot, achieving satisfactory flaw detection and transverse impact strength. In contrast, Comparative Examples 2 and 3 exhibit issues with transverse unnotched impact strength and / or ultrasonic flaw detection failures. Furthermore, the larger forging ratio in Comparative Example 3 increases production costs by 500 yuan / ton.

[0209] A comparison of tests conducted in Example 1 and Comparative Examples 4-7 shows that, in the method of this invention, the solution heating temperature for the Φ960 H13ESR large-size hot extrusion die material is 1040-1060℃, and alternating water and air quenching is used. This ensures thorough quenching and avoids quenching cracks, resulting in controllable quality. However, Comparative Examples 4 and 5 showed defects in metallographic structure (level AS10) and ultrasonic testing; Comparative Example 6 exhibited cracking and scrapping due to continuous water cooling during solution treatment. (See details...) Figure 9 Comparative Example 7 showed defects in metallographic structure (level AS15) and ultrasonic testing. The SEM image of its metallographic structure (500x magnification) is shown below. Figure 10 .

[0210] In summary, the preparation method of this invention solves the material quality problem of Φ960 H13ESR hot extrusion die, with all indicators meeting the requirements. It eliminates the need for re-forging and heat treatment with small-diameter round steel (Φ550), saving production costs (4000 yuan / ton) and production cycle.

[0211] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention shall be within the scope of protection of the pending claims of the present invention.

Claims

1. A method for producing a steel for hot extrusion dies, characterized in that, The hot extrusion die steel sequentially passes through the following processes: electroslag ingot production, forging, solid solution, spheroidizing annealing, and the like. The electroslag ingot production process comprises the following steps: S11, mother material casting rod and slag preparation: LF refining, VD vacuum degassing, casting rod, and mother material casting rod after one annealing, and preparation of binary pre-melted slag with protective effect; S12, electroslag remelting: remelting the mother material casting rod and the binary pre-melted slag together under argon protection to obtain an electroslag ingot; The diameter of the mother material casting rod is Ф920mm~930mm; The melting rate of the electroslag remelting is set to 780~960kg / h, the electrode is reserved for 1200~1500Kg to start supplementing, and the mold cooling time is 200~240min; The diameter of the electroslag ingot is Ф1150mm~1300mm; In the forging process, the electroslag ingot is heated before forging to promote high-temperature diffusion homogenization; during forging, the forging ratio is 4~6; The heating before forging comprises: first heating and raising the temperature of the electroslag ingot to 830~870℃ in the first section, and keeping the temperature for 10-12h; then heating and raising the temperature to 1260~1310℃ in the second section, and keeping the temperature for 28~32h to perform high-temperature diffusion homogenization; and then lowering the temperature to 1200~1220℃, keeping the temperature for 4-5h, and then starting forging; In the solid solution process, the electroslag ingot after forging is first heated and raised to 830~870℃ in the third section, and kept for 6-7h; then heated and raised to 1040~1060℃ in the fourth section, and kept for 15~18h; and then quenched by water cooling and air cooling alternately; The quenching is: three times of water cooling with two times of air cooling in between; wherein, each time of water cooling is equal or unequal, and is 20~90min; each time of air cooling is equal or unequal, and is 15~20min; The spheroidizing annealing process comprises: first heating and raising the temperature of the electroslag ingot to 850~870℃ in the fifth section, and keeping the temperature for 25~28h; then lowering the temperature to 730~750℃, and keeping the temperature for 32~36h; and then furnace cooling, and taken out of the furnace when the temperature is not higher than 250℃; The hot extrusion die steel is H13ESR.

2. The production method according to claim 1, wherein The high-temperature diffusion temperature is 1270~1300℃; And / or, the forging ratio is 5.5; And / or, the height-diameter ratio of the electroslag ingot is less than 3.

0.

3. The production method according to claim 1, wherein The quenching is: first water cooling for 60~90min, then air cooling for 20min, then second water cooling for 30~60min, then second air cooling for 15min, and finally third water cooling for 20~40min to complete the quenching.

4. The production method according to claim 1, wherein The specification of the mother material casting rod is Φ920 4000 mm.

5. The production method according to claim 1, wherein The amount of the binary pre-melted slag is 640~660kg.

6. The production method according to claim 1, wherein The one annealing temperature is 900-950℃, and the keeping time is 40-50h.

7. The production method according to claim 1, wherein The binary pre-melted slag is a mixture of Al2O3 and CaF2 with a mass ratio of 3:

7.

8. The production method according to claim 1, wherein The size of the electroslag ingot is Ф1150mm 2500mm.

9. The production method according to claim 1, wherein The heating rate of the first heating is not greater than 60℃ / h, and the heating rate of the second heating is not greater than 100℃ / h; And / or, the forging process includes: the high-temperature diffusion homogenization after the electroslag ingot is forged in the form of upsetting and elongation and cooled after forging, and the electroslag ingot surface temperature is lower than 850 DEG C when forging, and the electroslag ingot is reheated every fire heating time is 3~4 h; the temperature of the electroslag ingot corner position is greater than 850 DEG C, and the temperature of the remaining position is greater than 900 DEG C.

10. The production method according to claim 9, wherein The forging includes: when elongating on the flat anvil, the first time 10~20mm is pressed on both sides, and the subsequent each time 15%~18% of the blank thickness is pressed on both sides, the anvil amount is 1 / 2~3 / 4 anvil width, the stop forging temperature is greater than 900 DEG C, and the elongation is completed and the reheating is preserved for 3~4h; And / or, the last elongation, the blank size is 100~200mm larger than the finished material size; the double-side pressing amount is 30~50mm, the anvil amount is 1 / 4~1 / 2 anvil width, and 20mm is left for round forming; the last fire finishing temperature is greater than 850 DEG C, and the mist cooling is carried out after finishing; And / or, the cooling after forging includes: first mist cooling to 350 DEG C, then air cooling to 200~300 DEG C, and then entering the furnace for secondary annealing; the secondary annealing includes first rising to 850~870 DEG C, preserving for 25~28h; then reducing to 730~750 DEG C, preserving for 32~36h, and then cooling with the furnace, and the temperature is not higher than 250 DEG C when out of the furnace.

11. The production method according to claim 1, wherein The heating rate of the third segment is not greater than 60 DEG C / h, the heating rate of the fourth segment is not greater than 80 DEG C / h; the water temperature is lower than 30 DEG C when entering water for the first time; And / or, after the quenching is completed, the surface steel temperature of the electroslag ingot is reheated to 180~250 DEG C after out of water, and the final air cooling is to 200~150 DEG C to enter the furnace to execute spheroidizing annealing process.

12. The production method according to claim 11, wherein After the quenching is completed, the surface steel temperature of the electroslag ingot is reheated to 180~200 DEG C after out of water.

13. The production method according to claim 1, wherein The heating rate of the fifth segment is not greater than 60 DEG C / h, and the cooling rate of the cooling is not greater than 40 DEG C / h.

14. A steel for hot extrusion dies, obtainable by the production process according to any one of claims 1 to 13, characterized in that The chemical composition of the hot extrusion die steel includes: C 0.37~0.43wt%, Si 0.8~1.2wt%, Mn 0.2~0.5wt%, P≤0.015wt%, S≤0.005wt%, Cr 5~5.5wt%, Mo 1.3~1.5wt%, V 0.8~1.2wt%, Ni≤0.25wt%, Cu≤0.25wt%.

15. Steel for hot extrusion dies according to claim 14, characterized in that, The hot extrusion die steel is φ960 round steel, and the tolerance is +0~+5; the hardness is 44~46HRC; The round steel core is sampled, the metallographic structure is inspected under 500 times, the evaluation is carried out according to the standard of "NADCA207-2011 North American Die Casting Die Metallographic Standard Atlas", and AS1-AS9 level is reached; The banding segregation is inspected under 50 times, the evaluation is carried out according to the standard of "NADCA207-2011 North American Die Casting Die Metallographic Standard Atlas", and SA1-SA4 level is reached; The average value of multiple samples of transverse unnotched impact energy is greater than or equal to 220J, and the minimum value of single sample is greater than or equal to 170J; Ultrasonic testing according to GB / T6402-2008, meet 4 level, single point defect ≤Φ3.0mm flat bottom hole equivalent, continuous defect ≤Φ2.0mm flat bottom hole equivalent.

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