Post-forging heat treatment method for improving structure uniformity of hot work die steel

Abstract

The invention discloses a post-forging heat treatment method for improving the structure uniformity of hot-work die steel, which comprises the following steps: (1) returning the forged hot-work die steel to a furnace, supplementing the temperature to 980-1020 DEG C, preserving the heat for 4-6 hours, discharging the hot-work die steel, alternately cooling the hot-work die steel to 150-300 DEG C by water and air, and loading the hot-work die steel into a heating furnace; (2) the temperature is increased to 650-750 DEG C along with the furnace, heat preservation is conducted for 2-4 h, and the mixture is discharged out of the furnace, air-cooled to 150-300 DEG C and loaded into a (3) the temperature is increased to 800-850 DEG C along with the furnace for preheating and heat preservation, and the heat preservation time is 2-4 Raising the temperature to 1020-1050 DEG C, and keeping the temperature for 4-6 hours; and discharging from the furnace, alternately cooling to 150-300 DEG C by water and air, loading into a heating furnace, and carrying out isothermal spheroidizing annealing. According to the method, the forging structure can be refined, the forging coarse grain and mixed crystal defects are eliminated, chain carbide precipitation of the annealing structure is inhibited, and the uniformity of the annealing structure and the transverse core impact toughness of the material can be remarkably improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a heat treatment technology, in particular to a post-forging heat treatment method for improving the uniformity of hot work die steel. BACKGROUND

[0002] Medium-carbon medium-alloy hot work die steel mainly includes Cr series and Cr-Mo series two product series, and the best use temperature range is 400-650℃. Because of the good comprehensive mechanical properties, high thermal stability and thermal fatigue resistance, good wear resistance and resistance to molten metal erosion performance, it is the most widely used hot work die steel. Among them, the typical grades of Cr series hot work die steel are 4Cr5MoSiV and 4Cr5MoSiV1, etc.; the typical grades of Cr-Mo series hot work die steel are 4Cr5Mo2V, 4Cr5Mo3V, 4Cr3Mo3SiV, 3Cr3Mo3V, etc.

[0003] The alloy content of medium-carbon medium-alloy hot work die steel is 7-10%, and during solidification, due to the selection of crystallization, there is serious dendritic segregation in the core of the ingot, and a large amount of high-melting-point block eutectic carbide is enriched between the dendrites. High-temperature homogenization and multi-directional forging are effective processes to eliminate composition segregation and uniform structure, but in the production process, mixed crystals, coarse grain defects caused by poor homogenization effect and uneven forging deformation will inevitably occur, accompanied by the aggregation of annealing structure chain carbide, resulting in too low impact toughness and poor thermal fatigue performance of the material.

[0004] In order to solve the above-mentioned mixed crystal and coarse grain defects, the medium-carbon medium-alloy hot work die steel is usually normalized before annealing to promote the uniformity of the structure and the dissolution of the carbide, thereby refining the grains and improving the distribution of the carbide. However, in actual production process, for serious composition and structure segregation, the conventional normalizing pretreatment cannot completely eliminate the structure inheritance, and the structure inheritance will still have adverse effects on the precipitation of annealing structure carbide and the mechanical properties of the quenched and tempered state. SUMMARY

[0005] The present application aims to provide a post-forging heat treatment method for improving the uniformity of hot work die steel, which can effectively block the mixed crystal, coarse grain and network carbide of hot work die steel forging material, and improve the uniformity of hot work die steel.

[0006] In order to achieve the above-mentioned application purpose, the technical scheme adopted by the present application is: A post-forging heat treatment method for improving the uniformity of hot work die steel, comprising the following steps: (1) The hot work die steel after forging is reheated to 980-1020℃, and the temperature is kept for 4-6h, and then the temperature is cooled to 150-300℃ by water-air alternation, and then the hot work die steel is loaded into a heating furnace; (2) furnace temperature is raised to 650-750 DEG C, and the temperature is kept for 2-4 hours, the furnace is discharged, and the forged blank is air-cooled to 150-300 DEG C, and then is put into a heating furnace; (3) the furnace temperature is raised to 800-850 DEG C, and the temperature is kept for 2-4 hours, then the temperature is raised to 1020-1050 DEG C, and the temperature is kept for 4-6 hours, the furnace is discharged, and the forged blank is cooled by water and air alternately to 150-300 DEG C, and then is put into a heating furnace, and isothermal spheroidizing annealing is carried out.

[0007] Further, the heat treatment method of the present application, wherein the temperature rising rate of each stage is 50-120 DEG C / h.

[0008] Further, the heat treatment method of the present application, wherein the water and air alternately cooling process of step (1) and step (3) is as follows: after the first quenching, the surface temperature of the forged blank is 470-570 DEG C, after the water is discharged and the temperature is raised for 10-15 minutes, the surface temperature is controlled to be 550-650 DEG C, then the second quenching is carried out, after the second quenching, the surface temperature of the forged blank is 100-200 DEG C, after the water is discharged and the temperature is raised for 1-1.5 hours, the surface temperature is controlled to be 150-300 DEG C.

[0009] Further, the hot work die steel of the present application is a medium-carbon medium-alloy Cr or Cr-Mo hot work die steel.

[0010] Further, the hot work die steel obtained by the method of the present application has a better core transverse annealing structure than AS5 grade.

[0011] Further, the hot work die steel obtained by the method of the present application has a transverse V-notch impact energy ≥15 J after quenching and tempering heat treatment.

[0012] Further, the quenching and tempering heat treatment process of the present application is as follows: 1030 DEG C x 40 min, oil quenching; 595-610 DEG C x 2 h, air cooling and second tempering.

[0013] The beneficial effects produced by the above technical solution are as follows: (1) the hot work die steel of the present application is reheated to 980-1020 DEG C after forging, and the temperature is kept for 4-6 hours, so as to avoid the second carbide from precipitating along the crystal in the range of 700-900 DEG C of the final forging temperature, and to refine the forging grain size; the forged blank is cooled by water and air alternately to 150-300 DEG C, so as to make the forged blank obtain martensite + a small amount of bainite structure, and to avoid the local chain-like precipitation of carbide.

[0014] (2) the forged blank of the present application is raised to 650-750 DEG C, and the temperature is kept for 2-4 hours, and then is air-cooled to 150-300 DEG C after being discharged from the furnace, so as to obtain tempered sorbite structure, to avoid the direct austenitizing of martensite or bainite and the like unbalanced structure, to destroy the strict crystallographic orientation relationship between the new phase and the parent phase in the structure transformation, to inhibit the structure inheritance, and to further make good preparation for the second high-temperature normalizing.

[0015] (3) the present application is in the forging billet after one normalizing + high temperature tempering treatment, 1020-1050℃, 4~6h, furnace water cooling to 150~300℃ of the second high temperature normalizing treatment, completely eliminate or block the forging billet coarse grain, mixed crystal and other organization genetic, thereby obtaining the fine grain, carbide dispersed distribution of martensite + a small amount of bainite organization, and then isothermal spheroidizing annealing, because the spheroidizing mechanism of martensite and bainite is the same, ensure that the size of the carbide is uniform, the particle roundness is high, and the distribution of pearlite is dispersed. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is the heat treatment process chart in the specific embodiment of the present application; Figure 2 is the annealed metallographic structure chart of 4Cr5MoSiV1 steel of example 1; Figure 3 is the annealed metallographic structure chart of 4Cr5Mo2V steel of example 2; Figure 4 is the annealed metallographic structure chart of 4Cr5MoSiV1 steel of comparative example 1; Figure 5 is the annealed metallographic structure chart of 4Cr5Mo2V steel of comparative example 2. DETAILED DESCRIPTION

[0017] The present application will be further described in detail below in combination with specific embodiments. Example 1

[0018] The cross-sectional area of 4Cr5MoSiV1 module product is 240mm×810mm, the forging after heat treatment method for improving the uniformity of 4Cr5MoSiV1 steel organization is as shown in Figure 1 , and the specific steps are as follows: (1) the 4Cr5MoSiV1 steel after forging is reheated to 1000℃, and the temperature is kept for 5h, and then the furnace is cooled to 248℃, wherein the surface temperature of the forging billet after the first water quenching is 562℃, the surface temperature of the forging billet after the water temperature is returned to 12min is 644℃, the second water quenching is carried out, the surface temperature of the forging billet after the second water quenching is 193℃, the surface temperature of the forging billet after the water temperature is returned to 1h is 248℃, and then the heating furnace is loaded.

[0019] (2) the temperature is raised to 720℃ at a rate of 80℃ / h, the temperature is kept for 3h, and then the furnace is cooled to 281℃, and then the heating furnace is loaded.

[0020] (3) Heat up the forging blank with the furnace at a heating rate of 80℃ / h to 850℃ for preheating and holding for 2h; then heat up to 1050℃ at a heating rate of 100℃ / h and hold for 4h; cool it to 163℃ alternately in water and air after being taken out of the furnace. The surface temperature of the forging blank is 545℃ after the first quenching, and 628℃ after 15min of water return. The surface temperature of the forging blank is 122℃ after the second quenching, and 163℃ after 1.2h of water return; load it into the heating furnace and carry out isothermal spheroidizing annealing.

[0021] Isothermal spheroidizing annealing process of 4Cr5MoSiV1 steel: keep at 880℃ for 4h, cool to 740℃ at a cooling rate of ≤50℃ / h, keep for 6h, cool to ≤500℃ and air cool out of the furnace.

[0022] The core of the 4Cr5MoSiV1 steel in this embodiment has a transverse annealing structure of AS2. Figure 2 As shown in the figure, a sample was taken from the core of a 4Cr5MoSiV1 steel forging billet and tempered using a heat treatment process of oil quenching at 1030°C for 40 minutes followed by air cooling and secondary tempering at 595°C for 2 hours. The sample had a tempered hardness of 45.6 HRC and was machined into a standard Charpy impact specimen of 10 mm × 10 mm × 55 mm, with a transverse impact energy of 17.4 J. Example 2

[0023] Taking the 4Cr5Mo2V module product with a cross-sectional area of ​​310mm×910mm as the treatment object, the post-forging heat treatment method to improve the uniformity of the 4Cr5Mo2V steel structure is as follows: Figure 1 The specific process parameters are as follows: (1) The forged 4Cr5Mo2V steel was returned to the furnace and heated to 1020℃, kept warm for 4 hours, and then cooled to 300℃ by alternating water and air. The surface temperature of the forging blank was 536℃ after the first quenching, and 627℃ after 15 minutes of returning to the water. The second quenching was carried out, and the surface temperature of the forging blank was 198℃ after the second quenching, and 300℃ after 1.5 hours of returning to the water. Then the forging blank was loaded into the heating furnace.

[0024] (2) Heat the sample to 700°C at a heating rate of 90°C / h, keep the temperature for 3 hours, remove the sample from the furnace and air-cool it to 154°C, then place it in the heating furnace.

[0025] (3) Heat the forging blank to 800℃ with the furnace at a heating rate of 90℃ / h and preheat it for 4h; then heat it to 1040℃ at a heating rate of 120℃ / h and keep it for 5h; cool it to 180℃ alternately in water and air after it is taken out of the furnace, where the surface temperature of the forging blank is 470℃ after the first quenching, and 552℃ after 10min after it is taken out of the water and returned to the temperature; the surface temperature of the forging blank is 134℃ after the second quenching, and 180℃ after it is taken out of the water and returned to the temperature for 1h; load it into the heating furnace and carry out isothermal spheroidizing annealing.

[0026] 4Cr5Mo2V steel isothermal spheroidizing annealing process: 860℃ for 4h, cooling to 720℃ at a rate of ≤50℃ / h, holding for 6h, furnace cooling to ≤500℃, and air cooling after discharging.

[0027] The core transverse annealing structure of the 4Cr5Mo2V steel in this embodiment is AS4 level, as shown in Figure 3 The core of the 4Cr5Mo2V steel forging blank was sampled, and the sample was quenched at 1030℃ for 40min and then oil-cooled, and then tempered at 610℃ for 2h and air-cooled, to perform a quenching and tempering treatment according to the heat treatment process, the tempering hardness was 46.2HRC, and the sample was processed into a standard Charpy impact sample with a size of 10mm×10mm×55mm, and the transverse impact energy was 22.3J. Example 3

[0028] The 4Cr5MoSiV1 module product with a cross-sectional area of 280mm×910mm was taken as the processing object, and the post-forging heat treatment method for improving the uniformity of the 4Cr5MoSiV1 steel structure is as shown in Figure 1 and is as follows: (1) The 4Cr5MoSiV1 steel after forging was reheated to 1020℃, and held for 4h, and then discharged and cooled by water and air alternately to 206℃, wherein the surface temperature of the forging blank after the first water quenching was 545℃, the surface temperature of the forging blank after the reheating for 15min was 611℃, the surface temperature of the forging blank after the second water quenching was 129℃, the surface temperature of the forging blank after the reheating for 1h was 206℃, and then the forging blank was loaded into the heating furnace; (2) The forging blank was heated to 750℃ at a heating rate of 60℃ / h, held for 2h, and then discharged and air-cooled to 178℃, and then loaded into the heating furnace.

[0029] (3) The forging blank was preheated to 850℃ at a heating rate of 60℃ / h, held for 2h, then heated to 1030℃ at a heating rate of 120℃ / h, held for 5h, and then discharged and cooled by water and air alternately to 259℃, wherein the surface temperature of the forging blank after the first water quenching was 534℃, the surface temperature of the forging blank after the reheating for 12min was 608℃, the surface temperature of the forging blank after the second water quenching was 178℃, the surface temperature of the forging blank after the reheating for 1.5h was 259℃, and then the forging blank was loaded into the heating furnace to perform isothermal spheroidizing annealing.

[0030] The isothermal spheroidizing annealing process of the 4Cr5MoSiV1 steel is as follows: 880℃ for 4h, cooling to 740℃ at a rate of ≤50℃ / h, holding for 6h, furnace cooling to ≤500℃, and air cooling after discharging.

[0031] The core of the 4Cr5MoSiV1 steel in Example 4 was transversely annealed to have an AS3 grade structure. The core of the 4Cr5MoSiV1 steel forging blank was sampled, and the sample was quenched at 1030°C for 40 min and then oil-cooled, and then the sample was tempered at 595°C for 2 h and then air-cooled, to perform a quenching and tempering treatment according to a heat treatment process, the tempering hardness was 45.2 HRC, and the sample was processed into a standard Charpy impact sample with a size of 10 mm x 10 mm x 55 mm, and the transverse impact energy was 16.7 J. Example 4

[0032] A 4Cr5Mo2V module product with a cross-sectional area of 340 mm x 910 mm was taken as a processing object, and a post-forging heat treatment method for improving the structure uniformity of the 4Cr5Mo2V steel was as shown in Figure 1 The specific process parameters are as follows: (1) The 4Cr5Mo2V steel after forging was reheated to 980°C, and the temperature was maintained for 6 h, and then the temperature was alternately cooled to 150°C by water and air, wherein the surface temperature of the forging blank after the first water quenching was 554°C, the surface temperature of the forging blank was 619°C after the forging blank was reheated for 10 min after the water quenching, the surface temperature of the forging blank was 102°C after the second water quenching, the surface temperature of the forging blank was 150°C after the forging blank was reheated for 1 h after the water quenching, and then the forging blank was loaded into a heating furnace.

[0033] (2) The temperature was increased to 650°C at a rate of 50°C / h, the temperature was maintained for 4 h, the forging blank was unloaded and air-cooled to 296°C, and then the forging blank was loaded into a heating furnace.

[0034] (3) The temperature was increased to 820°C at a rate of 50°C / h, the temperature was maintained for 3 h, the temperature was increased to 1020°C at a rate of 90°C / h, the temperature was maintained for 6 h, the temperature was alternately cooled to 285°C by water and air, wherein the surface temperature of the forging blank after the first water quenching was 497°C, the surface temperature of the forging blank was 574°C after the forging blank was reheated for 15 min after the water quenching, the surface temperature of the forging blank was 200°C after the second water quenching, the surface temperature of the forging blank was 285°C after the forging blank was reheated for 1 h after the water quenching, and then the forging blank was loaded into a heating furnace to perform isothermal spheroidizing annealing.

[0035] The isothermal spheroidizing annealing process of the 4Cr5Mo2V steel was as follows: the temperature was maintained at 860°C for 4 h, the temperature was cooled to 720°C at a rate of ≤50°C / h, the temperature was maintained for 6 h, the temperature was cooled to ≤500°C in the furnace, and the forging blank was unloaded and air-cooled.

[0036] The core of the 4Cr5Mo2V steel in Example 4 was transversely annealed to have an AS3 grade structure. The core of the 4Cr5MoSiV1 steel forging blank was sampled, and the sample was quenched at 1030°C for 40 min and then oil-cooled, and then the sample was tempered at 595°C for 2 h and then air-cooled, to perform a quenching and tempering treatment according to a heat treatment process, the tempering hardness was 45.2 HRC, and the sample was processed into a standard Charpy impact sample with a size of 10 mm x 10 mm x 55 mm, and the transverse impact energy was 16.7 J.

[0037] Comparative Example 1 The 4Cr5MoSiV1 forging module product with cross-sectional area of 240 mm x 810 mm was selected and isothermally spheroidized annealing treatment was performed.

[0038] The isothermal spheroidizing annealing process was as follows: 880℃ for 4h, cooled to 740℃ at a rate of ≤50℃ / h, 6h holding, furnace cooling to ≤500℃, and air cooling after discharging.

[0039] The 4Cr5MoSiV1 module core transverse annealing structure of the present comparative example was AS5 grade, as shown in Figure 4 The 4Cr5MoSiV1 steel forging blank was sampled at the core, and the sample was quenched at 1030℃ x 40min with oil cooling + 595℃ x 2h air cooling for secondary tempering, and the tempering hardness was 45.2HRC. The sample was processed into a standard charpy impact sample with a size of 10mm x 10mm x 55mm, and the transverse impact energy was 12.2J.

[0040] Comparative Example 2 The 4Cr5Mo2V forging module product with cross-sectional area of 310 mm x 910 mm was selected and isothermally spheroidized annealing treatment was performed.

[0041] The isothermal spheroidizing annealing process of the 4Cr5Mo2V steel was as follows: 860℃ for 4h, cooled to 720℃ at a rate of ≤50℃ / h, 6h holding, furnace cooling to ≤500℃, and air cooling after discharging.

[0042] The 4Cr5Mo2V module core transverse annealing structure of the present comparative example was AS7 grade, as shown in Figure 5 The 4Cr5Mo2V steel forging blank was sampled at the core, and the sample was quenched at 1030℃ x 40min with oil cooling + 610℃ x 2h air cooling for secondary tempering, and the tempering hardness was 45.2HRC. The sample was processed into a standard charpy impact sample with a size of 10mm x 10mm x 55mm, and the transverse impact energy was 16.9J.

Claims

1. A post-forging heat treatment method for improving the uniformity of the structure of hot work die steel, characterized by, The heat treatment method comprises the following steps: (1) the hot work die steel after forging is reheated to 980-1020 DEG C, and is kept for 4-6 hours, is alternately cooled to 150-300 DEG C by water and air, and is loaded into a heating furnace; (2) the furnace is heated to 650-750 DEG C, and is kept for 2-4 hours, is cooled to 150-300 DEG C by air, and is loaded into a heating furnace; (3) the furnace is heated to 800-850 DEG C for preheating and keeping for 2-4 hours, is heated to 1020-1050 DEG C, and is kept for 4-6 hours, is alternately cooled to 150-300 DEG C by water and air, and is loaded into a heating furnace for isothermal spheroidizing annealing.

2. The post-forgmg heat treatment method for improving the microstructure uniformity of hot work die steel according to claim 1, characterized by, The heating rate of each stage is 50-120 DEG C / h.

3. The post-forgmg heat treatment method for improving the microstructure uniformity of hot work die steel according to claim 1, characterized by, In the water-air alternate cooling process of step (1) and step (3), the surface temperature of the forged blank is 470-570 DEG C after first quenching, the surface temperature is controlled to 550-650 DEG C after water-out and reheating for 10-15 minutes, the surface temperature of the forged blank is 100-200 DEG C after second quenching, and the surface temperature is controlled to 150-300 DEG C after water-out and reheating for 1-1.5 hours.

4. The post-forgmg heat treatment method for improving the microstructure uniformity of hot work die steel according to any one of claims 1 to 3, characterized by, The hot work die steel is a medium-carbon medium-alloy Cr or Cr-Mo hot work die steel.

5. The post-forgmg heat treatment method for improving the microstructure uniformity of hot work die steel according to any one of claims 1 to 3, characterized in that, The hot work die steel obtained by the method has better transverse annealing structure than AS5 grade.

6. The post-forging heat treatment method for improving the microstructure uniformity of hot working die steel according to any one of claims 1 to 3, characterized in that: The hot work die steel obtained by the method has transverse V-notch impact energy ≥15 J after quenching and tempering heat treatment.

7. The post-forgmg heat treatment method for improving the microstructure uniformity of hot work die steel according to claim 6, characterized by, The quenching and tempering heat treatment process is: 1030 DEG C x 40 min, oil quenching; 595-610 DEG C x 2 h, air cooling and second tempering.

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