CN114410893B - Ultra-fine grain structure heat treatment process for annealed hot work die steel - Google Patents
Ultra-fine grain structure heat treatment process for annealed hot work die steel Download PDFInfo
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Abstract
A heat treatment process for an ultra-fine grained structure of annealed hot work die steel belongs to the technical field of heat treatment of hot work die steel. The heat treatment material is an annealing state hot work die blank, and a plurality of thermocouples for detecting the temperature are arranged at the center part and the surface of the die; the method comprises the steps of primary high-temperature homogenization, primary air/water alternate quenching, primary high-temperature tempering, secondary high-temperature quenching after tempering, stress relief annealing, die quenching and tempering and the like. The beneficial effects are as follows: the heat treatment process is completely utilized for regulation and control, liquated carbide, martensite structure and austenite grains can be simultaneously refined, and the impact toughness is improved while the strength of the material is maintained. The grain size can be improved from 6-7 grade to 8-10 grade, the transverse impact toughness can be improved by more than 15%, and the room temperature plasticity is also improved.
Description
Technical Field
The invention belongs to the technical field of heat treatment of hot work die steel, and particularly relates to a heat treatment process for an ultrafine grained structure of annealed hot work die steel.
Background
The die-casting is a production technology with high mechanization degree and production efficiency, is an advanced few-cutting-free process, and is considered to be the most effective production process for producing castings with high surface quality in a large scale. H13 (4 Cr5MoSiV 1), 4Cr5Mo1V and 4Cr5Mo2V are generally adopted as hot die steel for metal die-casting forming dies of copper, aluminum and the like, thermal fatigue cracks (hereinafter referred to as hot cracks) are one of the most main failure modes influencing the service life of the metal die-casting dies of copper, aluminum and the like, the mechanical stress change induced by heat is an important reason for generating the hot cracks on the surface of the die, and the plastic strain plays a key role in the whole die failure process and the surface stress state change process. As the die-casting die has the characteristic of multidirectional stress in the use process, in order to prevent catastrophic accidents caused by die cracking and crack propagation, the die-casting die steel is required to have higher transverse impact toughness and fracture toughness, and the impact toughness is considered as a key index for determining the quality of hot-working die steel such as H13 and the like and is also used as a key index for judging the grade in the NADCA207-2016 standard. Some stress concentration points in the hot die steel are positions where cracks are easily initiated, such as segregation zones in a strip-shaped segregation structure, liquated carbides, non-metallic inclusions, coarse grain boundaries and the like, and these structural defects are main factors influencing the toughness of the steel. Under the existing equipment smelting technical level, the cleanliness of the material is improved through high-purity smelting, the component segregation and the strip segregation are controlled through high-temperature homogenization diffusion, and the coarse liquated carbide is refined through large-forging ratio forging, so that the manufacturing process commonly used in China is realized, and the improvement of the toughness of the material is difficult to make a major breakthrough. However, after the die leaves the factory, the die is controlled by an effective heat treatment process before being used, so that the grain refining of the material is still the most effective method for simultaneously improving the strength and toughness, and the method has important significance for further improving the mechanical property of the die steel and further prolonging the service life of the die under the existing equipment system.
Disclosure of Invention
The invention aims to provide a heat treatment process for an ultra-fine grain structure of annealed hot work die steel, which solves the problem that the toughness of steel is influenced by structural defects such as a segregation zone, liquated carbides, non-metallic inclusions, coarse grain boundaries and the like in the conventional banded segregation structure.
The invention completely utilizes the regulation and control of the heat treatment process on the basis of the existing hot-work die steel of a factory annealing state H13 (4 Cr5MoSiV 1), 4Cr5Mo1V, 4Cr5Mo2V and the like in a steel mill, can simultaneously refine liquated carbide, martensite structure and austenite grains, and improves the impact toughness while maintaining the material strength. The technical scheme is as follows:
the heat treatment material is an annealing state hot work die blank, and a plurality of thermocouples for detecting the temperature are arranged at the center and the surface of the die; the specific steps and the technical parameters of the control are as follows:
(1) High-temperature homogenization for the first time: the working procedure comprises secondary preheating, namely heating the hot working die to be thermally treated to 500-600 ℃ at a heating rate of less than or equal to 80 ℃/h, and keeping the temperature for 1h, and heating to 800-850 ℃ at a heating rate of less than or equal to 80 ℃/h, and keeping the temperature for 0.9-1.1h; heating to 1050-1100 ℃ at a heating rate of less than or equal to 80 ℃/h, and keeping the temperature for 0.9-1.1h when the core temperature is 5-10 ℃ lower than the set temperature;
(2) First air/water alternate quenching: taking the die out of the furnace for air cooling after the heat preservation according to the step (1), monitoring the surface temperature of the die to 900-950 ℃, immediately cooling the surface temperature to be lower than 400 ℃, taking out the die, placing the die in air for cooling, performing secondary water cooling when the surface temperature of the die returns to 450 ℃, after the water cooling is finished, controlling the surface temperature of the die steel to be lower than 200 ℃, starting air cooling to room temperature, checking that the surface of the die is dry and has no water vapor, and immediately putting the die into a tempering heat treatment furnace;
(3) High-temperature tempering for the first time: setting the tempering temperature to 750-780 ℃, before charging, keeping the temperature of the tempering heat treatment furnace less than or equal to 200 ℃, rapidly heating the die along with the furnace, keeping the temperature for 1.5-2 h when the core temperature is 5-10 ℃ lower than the set tempering temperature, discharging the die, and air cooling until the surface temperature of the die is lower than 100 ℃;
(4) Secondary high-temperature quenching after tempering: the process does not need preheating, the temperature of the mold is heated to 900-950 ℃ before the mold is put into the furnace, the mold is charged into the furnace at the temperature, when the core temperature is 5-10 ℃ lower than the set temperature, the temperature is kept for 0.5h, the mold is cooled by water until the surface temperature of the mold is lower than 200 ℃, and the mold is cooled by air to the room temperature;
(5) Stress relief annealing: the furnace temperature before the annealing in the furnace is less than or equal to 200 ℃, the temperature of the die and the furnace is raised to 600-650 ℃, the temperature is kept for 1-2 h, and the quenching stress is removed;
(6) Quenching and tempering the die: the temperature of the mold entering the furnace is lower than 200 ℃, the temperature of the mold entering the furnace is preheated and preserved for 3.3 to 4 hours at the temperature of 800 to 850 ℃, the temperature of the furnace is raised to the austenitizing temperature of the mold steel, when the core temperature is 5 to 10 ℃ lower than the set austenitizing temperature, the temperature is preserved for 0.4 to 0.6 hours, the mold entering the furnace is quenched to 200 ℃ by vacuum gas, and the mold is taken out of the furnace and air-cooled; and after the die is quenched, a tempering process is carried out for three times at 580-600 ℃, the die is cooled to room temperature in an air mode, and the subsequent surface finish machining of the die is carried out to finish the die manufacturing.
Preferably, the annealed hot work die steel is 4Cr5MoSiV1 hot work die steel, 4Cr5Mo1V hot work die steel or 4Cr5Mo2V hot work die steel.
Preferably, in the step (1), the temperature rise rate is 50 ℃/h to 80 ℃/h.
Preferably, in the step (6), two-stage preheating and heat preservation treatment is adopted, wherein the first stage preheating is carried out for 0.9-1.1h at 500-600 ℃, and the second stage preheating is carried out for 2.3-3 h at 800-850 ℃.
The implementation of the invention comprises the following technical effects:
the quenching temperature of the traditional H13 hot work die steel is 1010-1030 ℃, the heat treatment process realizes the dissolution of carbide and the homogenization of tissues by the first high-temperature homogenization temperature which is higher than the quenching temperature of the traditional H13 hot work die steel, only part of undissolved VC carbide is remained in the tissues after the first high-temperature homogenization heat treatment, the average size of the carbide is reduced, and the alloy carbide is favorably dispersed and precipitated in a fine and uniform state after the first high-temperature tempering. Because the adopted homogenization temperature is higher, if the water-cooling quenching is directly carried out on a large-scale die, the risk of die cracking is increased, therefore, the air/water alternative quenching heat treatment is designed, the air cooling with a slower cooling speed is firstly adopted, the temperature of the die is reduced to 900-950 ℃, the cooling stress is reduced, meanwhile, the temperature of the die is ensured to be above the critical temperature of die steel, and then the rapid water cooling is adopted, so that the martensite transformation of the structure is realized. The structure after the first high-temperature homogenization quenching is an unbalanced structure, carbides are precipitated again in the first high-temperature tempering heating process, when carbon in the quenched martensite is fully precipitated, but the recrystallization of the martensite phase is not generated, and the lath martensite characteristic still exists. The heating temperature of the second high-temperature quenching is set to be slightly higher than the critical point temperature, new needle-shaped austenite crystal nuclei are formed in the area with carbides at the lath martensite interface, and the size of the new needle-shaped austenite crystal nuclei is equivalent to the size of the lath martensite. In the alpha → gamma conversion process, the austenite grains generate multiple times of plastic deformation accumulation, the volume change causes the newly generated austenite grains to be processed and hardened to form recrystallization, and the recrystallization austenite grains are not coarsened in time due to the low temperature and short heat preservation time in the second quenching process, so the austenitizing process is finished, and the grain refining structure is formed. After the secondary high-temperature quenching, a stress relief process is added to the large-scale die to prevent thermal stress cracking, then the traditional quenching and tempering processes are carried out, the steel core position of the original die is sampled, and the sample and the test die are subjected to quenching and tempering heat treatment in the same furnace, under the condition that the hardness and the strength are basically unchanged, the grain size can be improved from 6-7 grade to 8-10 grade, the transverse impact toughness can be improved by more than 15 percent, and the room-temperature plasticity is also improved.
Detailed Description
The present invention will be described in detail with reference to the following examples, which are intended to facilitate the understanding of the present invention and should not be construed as limiting in any way.
Example 1
According to the superfine grain structure heat treatment process of the annealed hot work die steel, the heat treatment material is the annealed H13 (4 Cr5MoSiV 1) hot work die steel (meeting the standard requirement of GB/T1299-2014), the die steel is processed into a die blank according to the requirement, the machining allowance is kept between 1.5mm and 2mm according to the size of the die, and a plurality of thermocouples for detecting the temperature are arranged in different areas such as the center, the surface and the like of the die; the method comprises the following steps:
(1) High-temperature homogenization for the first time: the working procedure comprises secondary preheating, namely heating the hot working die to be subjected to heat treatment to 500 ℃ at a heating rate of 80 ℃/h, and keeping the temperature for 1h, and heating to 800 ℃ at a heating rate of 80 ℃/h, and keeping the temperature for 1h; heating to 1050 ℃ at a heating rate of 80 ℃/h, and keeping the temperature for 1h when the core temperature is 5 ℃ lower than the set austenitizing temperature; round steel or square or flat steel with nominal diameter or thickness less than 200mm can omit the first stage of preheating. In this step, the internal stress can be avoided from being too high by secondary preheating. The high-temperature homogenization temperature of the embodiment is higher than the quenching temperature of traditional hot-work die steel such as H13, the dissolution of carbide and the homogenization of tissues are realized, only part of undissolved VC carbide is remained in the tissues after the first high-temperature homogenization heat treatment, the average size of the carbide is reduced, and the alloy carbide is dispersed and precipitated in a fine and uniform state after the first high-temperature tempering is facilitated.
(2) First air/water alternate quenching: taking the die out of the furnace for air cooling after the heat preservation according to the step (1), monitoring the surface temperature of the die to about 900 ℃, immediately cooling the die to the surface temperature of 380 ℃, taking out the die, placing the die in air for cooling, performing secondary water cooling when the surface of the die returns to 450 ℃, after the water cooling is finished, keeping the surface temperature of the die steel to 190 ℃, starting air cooling to the room temperature, checking that the surface of the die is dry and has no water vapor, and immediately putting the die into a tempering heat treatment furnace; round steel or square steel or flat steel with nominal diameter or thickness less than 200mm can be directly cooled by water until the surface temperature of the die is 190 ℃, and then cooled by air to room temperature so as to ensure stronger cooling speed. Because the adopted homogenization temperature is higher, if the water-cooling quenching is directly carried out on a large-scale die, the risk of die cracking is increased, therefore, the air/water alternative quenching heat treatment is designed, the air cooling with a slower cooling speed is firstly adopted, the temperature of the die is reduced to 900 ℃, the cooling stress is reduced, meanwhile, the temperature of the die is ensured to be above the critical temperature of die steel, and then the rapid water cooling is adopted, so that the martensite transformation of the structure is realized.
(3) High-temperature tempering for the first time: the process does not need preheating, before tempering, the temperature of a tempering heat treatment furnace is 190 ℃, the temperature of a die is rapidly increased along with the furnace, the temperature is kept for 1.5 hours when the core temperature is 5 ℃ lower than the set tempering temperature (750 ℃), and the die is taken out of the furnace and air-cooled until the surface temperature of the die is 95 ℃; the structure after the first high-temperature homogenization quenching is an unbalanced structure, carbides are precipitated again in the first high-temperature tempering heating process, when carbon in the quenched martensite is fully precipitated, but the recrystallization of the martensite phase is not generated, and the lath martensite characteristic still exists.
(4) And (3) secondary high-temperature quenching after tempering: the process does not need preheating, the temperature of the mold is heated to 900 ℃ before the mold is put into the furnace, the mold is charged when the temperature reaches 5 ℃ lower than the set temperature, the temperature is kept for 0.5h, the mold is cooled by water until the surface temperature of the mold is 180 ℃, and air cooling is started to the room temperature; the heating temperature of the second high-temperature quenching is set to be slightly higher than the critical point temperature, new needle-shaped austenite crystal nuclei are formed in the area with carbides at the lath martensite interface, and the size of the new needle-shaped austenite crystal nuclei is equivalent to the size of the lath martensite. In the alpha → gamma conversion process, the austenite grains generate multiple times of plastic deformation accumulation, the volume change causes the newly generated austenite grains to be processed and hardened to form recrystallization, and the recrystallization austenite grains are not coarsened in time due to the low temperature and short heat preservation time in the second quenching process, so the austenitizing process is finished, and the grain refining structure is formed.
(5) Stress relief annealing: the furnace temperature is 200 ℃ before the annealing, the temperature of the die is raised to 600 ℃ along with the furnace, the temperature is kept for 1h, and the quenching stress is removed; the stress relief annealing process can be omitted for round steel or square or flat steel with nominal diameter or thickness less than 200 mm. After the secondary high-temperature quenching, the large-scale die is added with a stress relief process to prevent thermal stress cracking.
(6) Quenching and tempering the die: the temperature of the mold in the furnace is 180 ℃, the temperature is preserved for 3.3h by adopting two-stage preheating (the first stage preheating is carried out for 500 ℃ and the temperature is preserved for 1h, the second stage preheating is carried out for 800 ℃ and the temperature is preserved for 2.3 h), the temperature of the furnace is raised to the austenitizing temperature of the mold steel, when the core temperature is 5 ℃ lower than the set austenitizing temperature, the temperature is preserved for 0.5h, the mold is quenched to 200 ℃ by adopting vacuum gas, and the mold is taken out of the furnace and is cooled by air; immediately tempering the die after quenching, adopting a tempering process for three times at 580 ℃, air-cooling the die to room temperature, and enabling the surface hardness of the die to be between 44 and 46 HRC; and performing subsequent surface finish machining on the die to finish the die manufacturing. The austenitizing temperature is set in the hot work die steel grade of the actual heat treatment. The step carries out traditional quenching and tempering thermal refining treatment, the grain size can be improved from 6-7 grade to 8-10 grade under the condition of basically unchanged hardness and strength by sampling the steel core part of the original die and carrying out thermal refining treatment in the same furnace with the test die, the transverse impact toughness can be improved by more than 15 percent, and the room-temperature plasticity is also improved.
Example 2
According to the superfine grain structure heat treatment process of the annealed hot work die steel, the heat treatment material is the annealed H13 (4 Cr5MoSiV 1) hot work die steel (meeting the standard requirement of GB/T1299-2014), the die steel is processed into a die blank according to the requirement, the machining allowance is kept between 1.5mm and 2mm according to the size of the die, and a plurality of thermocouples for detecting the temperature are arranged in different areas such as the center, the surface and the like of the die; the method comprises the following steps:
(1) High-temperature homogenization for the first time: the working procedure comprises secondary preheating, namely heating the hot working die to be thermally treated to 550 ℃ at a heating rate of 70 ℃/h, and preserving heat for 1h, and heating to 830 ℃ at a heating rate of 70 ℃/h, and preserving heat for 1h; heating to 1080 ℃ at a heating rate of 70 ℃/h, and preserving heat for 1h when the core temperature is 8 ℃ lower than the set austenitizing temperature; round steel or square or flat steel with nominal diameter or thickness less than 200mm can omit the first stage of preheating. In this step, the internal stress can be avoided from being too high by secondary preheating. The high-temperature homogenization temperature of the embodiment is higher than the quenching temperature of traditional hot-work die steel such as H13, the dissolution of carbide and the homogenization of tissues are realized, only part of undissolved VC carbide is remained in the tissues after the first high-temperature homogenization heat treatment, the average size of the carbide is reduced, and the alloy carbide is dispersed and precipitated in a fine and uniform state after the first high-temperature tempering is facilitated.
(2) First air/water alternate quenching: taking the die out of the furnace for air cooling after the heat preservation according to the step (1) is finished, monitoring the surface temperature of the die to about 930 ℃, immediately cooling the die by water until the surface temperature is lower than 400 ℃, taking out the die, placing the die in air for cooling, performing secondary water cooling when the surface temperature of the die returns to 450 ℃, after the water cooling is finished, keeping the surface temperature of the die steel at 150 ℃, starting air cooling to room temperature, checking that the surface of the die is dry and has no water vapor, and immediately putting the die into a tempering heat treatment furnace; round steel or square steel or flat steel with nominal diameter or thickness less than 200mm can be directly cooled to 150 ℃ on the surface of the die by water, and then cooled to room temperature by air so as to ensure stronger cooling speed. Because the adopted homogenization temperature is higher, if the water-cooling quenching is directly carried out on a large-scale die, the risk of die cracking is increased, therefore, the air/water alternative quenching heat treatment is designed, the air cooling with a slower cooling speed is firstly adopted, the temperature of the die is reduced to 930 ℃, the cooling stress is reduced, meanwhile, the temperature of the die is ensured to be above the critical temperature of die steel, and then the rapid water cooling is adopted, so that the martensite transformation of the structure is realized.
(3) High-temperature tempering for the first time: the process does not need preheating, before tempering, the temperature of a tempering heat treatment furnace is 180 ℃, the temperature of the die is rapidly increased along with the furnace, when the core temperature is 8 ℃ lower than the set tempering temperature (770 ℃), the temperature is kept for 1.7 hours, and the die is discharged from the furnace and air-cooled until the surface temperature of the die is 90 ℃; the structure after the first high-temperature homogenization quenching is a non-equilibrium structure, carbides are re-precipitated in the first high-temperature tempering heating process, and when carbon in quenched martensite is fully precipitated but the recrystallization of a martensite phase is not generated, the lath martensite characteristic still exists.
(4) Secondary high-temperature quenching after tempering: the process does not need preheating, the temperature of the mold is heated to 930 ℃ before entering the furnace, the mold is charged to the temperature, when the temperature of the core reaches 8 ℃ lower than the set temperature, the temperature is kept for 0.5h, the mold is cooled by water until the surface temperature of the mold is 150 ℃, and air cooling is started to the room temperature; the heating temperature of the second high-temperature quenching is set to be slightly higher than the critical point temperature, new needle-shaped austenite crystal nuclei are formed in the area with carbides at the lath martensite interface, and the size of the new needle-shaped austenite crystal nuclei is equivalent to the size of the lath martensite. In the alpha → gamma conversion process, the austenite grains generate multiple times of plastic deformation accumulation, the volume change causes the newly generated austenite grains to be processed and hardened to form recrystallization, and the recrystallization austenite grains are not coarsened in time due to the low temperature and short heat preservation time in the second quenching process, so the austenitizing process is finished, and the grain refining structure is formed.
(5) Stress relief annealing: the furnace temperature before the annealing in the furnace is 170 ℃, the temperature of the die and the furnace is raised to 630 ℃, the temperature is kept for 1.5h, and the quenching stress is removed; the stress relief annealing process can be omitted for round steel or square or flat steel with nominal diameter or thickness less than 200 mm. After the secondary high-temperature quenching, the large-scale die is added with a stress relief process to prevent thermal stress cracking.
(6) Quenching and tempering the die: the temperature of the mold entering the furnace is 150 ℃, the temperature is preserved for 4 hours by adopting two-stage preheating (the first stage preheating is carried out for preserving heat for 1 hour at 550 ℃, and the second stage preheating is carried out for preserving heat for 3 hours at 830 ℃), the temperature of the furnace is raised to the austenitizing temperature of the mold steel, when the core temperature is 5-10 ℃ lower than the set austenitizing temperature, the temperature is preserved for 0.5 hour, the temperature is quenched to 200 ℃ by adopting vacuum gas, and the mold is taken out of the furnace and cooled by air; immediately tempering the die after quenching, adopting a three-time tempering process at 590 ℃, air-cooling the die to room temperature, and controlling the surface hardness of the die to be between 44 and 46 HRC; and performing subsequent surface finish machining on the die to finish the die manufacturing. The austenitizing temperature is set in the hot die steel grade of the actual heat treatment. The traditional quenching and tempering thermal refining treatment is carried out in the step, the steel core position of the original die is sampled, and the sample and the test die are subjected to thermal refining and thermal refining treatment in the same furnace, so that the grain size can be improved from 6-7 grade to 8-10 grade under the condition that the hardness and the strength are basically unchanged, the transverse impact toughness can be improved by more than 15 percent, and the room-temperature plasticity is also improved.
Example 3
According to the superfine grain structure heat treatment process of the annealed hot work die steel, the heat treatment material is the annealed H13 (4 Cr5MoSiV 1) hot work die steel (meeting the standard requirement of GB/T1299-2014), the die steel is processed into a die blank according to the requirement, the machining allowance is kept between 1.5mm and 2mm according to the size of the die, and a plurality of thermocouples for detecting the temperature are arranged in different areas such as the center, the surface and the like of the die; the method comprises the following steps:
(1) High-temperature homogenization for the first time: the working procedure comprises secondary preheating, namely heating the hot work die to be thermally treated to 600 ℃ at a heating rate of 50 ℃/h, and keeping the temperature for 1h, and heating to 850 ℃ at a heating rate of 50 ℃/h, and keeping the temperature for 1h; heating to 1100 deg.C at a heating rate of 50 deg.C/h, and maintaining for 1h when the core temperature is 10 deg.C lower than the set austenitizing temperature; round steel or square or flat steel with nominal diameter or thickness less than 200mm can omit the first stage of preheating. In this step, the internal stress can be avoided from being too high by secondary preheating. The high-temperature homogenization temperature of the embodiment is higher than the quenching temperature of traditional hot-work die steel such as H13, the dissolution of carbide and the homogenization of tissues are realized, only part of undissolved VC carbide is remained in the tissues after the first high-temperature homogenization heat treatment, the average size of the carbide is reduced, and the alloy carbide is dispersed and precipitated in a fine and uniform state after the first high-temperature tempering is facilitated.
(2) First air/water alternate quenching: taking the die out of the furnace for air cooling after the heat preservation according to the step (1), monitoring the surface temperature of the die to about 950 ℃, immediately cooling the die to 350 ℃ by water, taking out the die, placing the die in air for cooling, performing secondary water cooling when the surface of the die returns to 450 ℃, after the water cooling is finished, controlling the surface temperature of the die steel to be 120 ℃, starting air cooling to room temperature, checking that the surface of the die is dry and has no water vapor, and immediately putting the die into a tempering heat treatment furnace; round steel or square steel or flat steel with nominal diameter or thickness less than 200mm can be directly cooled by water until the surface temperature of the die is 120 ℃, and then cooled by air to room temperature so as to ensure stronger cooling speed. Because the homogenization temperature is higher, if the large-scale die is quenched by direct water cooling, the risk of die cracking is increased, and therefore, the air/water alternative quenching heat treatment is designed, the air cooling with a slow cooling speed is firstly adopted, the temperature of the die is reduced to 950 ℃, the cooling stress is reduced, meanwhile, the temperature of the die is ensured to be higher than the critical temperature of die steel, and then the rapid water cooling is adopted, so that the martensite transformation of the structure is realized.
(3) High-temperature tempering for the first time: preheating is not needed in the working procedure, before tempering, the temperature of a tempering heat treatment furnace is less than or equal to 200 ℃, the temperature of a die is rapidly increased along with the furnace, the temperature is kept for 2 hours when the core temperature is 10 ℃ lower than the set tempering temperature (780 ℃), and the die is taken out of the furnace and air-cooled until the surface temperature of the die is 80 ℃; the structure after the first high-temperature homogenization quenching is an unbalanced structure, carbides are precipitated again in the first high-temperature tempering heating process, when carbon in the quenched martensite is fully precipitated, but the recrystallization of the martensite phase is not generated, and the lath martensite characteristic still exists.
(4) Secondary high-temperature quenching after tempering: the process does not need preheating, the temperature of the mold is heated to 950 ℃ before entering the furnace, the mold is charged at the temperature, when the core temperature reaches 10 ℃ lower than the set temperature, the temperature is kept for 0.5h, the mold is cooled by water until the surface temperature of the mold is 150 ℃, and air cooling is started to the room temperature; the heating temperature of the second high-temperature quenching is set to be slightly higher than the critical point temperature, new needle-shaped austenite crystal nuclei are formed in the area with carbides at the lath martensite interface, and the size of the new needle-shaped austenite crystal nuclei is equivalent to the size of the lath martensite. In the alpha → gamma conversion process, the austenite grains generate multiple times of plastic deformation accumulation, the volume change causes the newly generated austenite grains to be processed and hardened to form recrystallization, and the recrystallization austenite grains are not coarsened in time due to the low temperature and short heat preservation time in the second quenching process, so the austenitizing process is finished, and the grain refining structure is formed.
(5) Stress relief annealing: the furnace temperature before the annealing in the furnace is 150 ℃, the temperature of the die is raised to 650 ℃ along with the furnace, the temperature is kept for 2h, and the quenching stress is removed; the stress relief annealing process can be omitted for round steel or square or flat steel with nominal diameter or thickness less than 200 mm. After the secondary high-temperature quenching, the large-scale die is added with a stress relief process to prevent thermal stress cracking.
(6) Quenching and tempering the die: the temperature of the mold in the furnace is 150 ℃, the temperature is preserved for 4h by adopting two-stage preheating (the first stage preheating is carried out for 600 ℃ and the heat preservation is carried out for 1h, the second stage preheating is carried out for 850 ℃ and the heat preservation is carried out for 3 h), the temperature of the furnace is raised to the austenitizing temperature of the mold steel, when the core temperature is 10 ℃ lower than the set austenitizing temperature, the temperature is preserved for 0.5h, the temperature is quenched to 200 ℃ by adopting vacuum gas, and the mold is taken out of the furnace and cooled; immediately tempering the die after quenching, adopting a three-time tempering process at 600 ℃, air-cooling the die to room temperature, and ensuring that the surface hardness of the die is between 44 and 46 HRC; and performing subsequent surface finish machining on the die to finish the die manufacturing. The austenitizing temperature is set in the hot work die steel grade of the actual heat treatment. The traditional quenching and tempering thermal refining treatment is carried out in the step, the steel core position of the original die is sampled, and the sample and the test die are subjected to thermal refining and thermal refining treatment in the same furnace, so that the grain size can be improved from 6-7 grade to 8-10 grade under the condition that the hardness and the strength are basically unchanged, the transverse impact toughness can be improved by more than 15 percent, and the room-temperature plasticity is also improved.
The ultrafine grain structure heat treatment process of the annealed hot die steel is suitable for H13 (4 Cr5MoSiV 1) hot die steel, 4Cr5Mo1V hot die steel or 4Cr5Mo2V hot die steel, H13 (4 Cr5MoSiV 1) is taken as an example in examples 1 to 3, the core position of the original die steel is sampled, the sample and the test die are subjected to quenching and tempering heat treatment in the same furnace, and finally, the grain size, the tempering hardness, the transverse impact energy, the tensile strength, the yield strength, the elongation, the surface shrinkage and other tests are carried out, and the test results are shown in Table 1.
TABLE 1 comparison of the properties of the mold obtained by the heat treatment process of this example with those of a conventional heat treatment mold
As can be seen from Table 1, compared with the mold obtained by the heat treatment process of the ultrafine grain structure of the hot die steel of the invention, the hot die steel obtained by the heat treatment process of the invention has the same tempering hardness, tensile strength and yield strength, and simultaneously has the grain size increased by 2-3 grades, the transverse impact energy increased by more than 15% and the room temperature plasticity also increased. Further improving the mechanical property of the die steel and further prolonging the service life of the die.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the protection scope of the present invention, although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.
Claims (3)
1. A heat treatment process for an ultrafine grain structure of annealed hot work die steel, wherein the annealed hot work die steel is 4Cr5MoSiV1 hot work die steel, 4Cr5Mo1V hot work die steel or 4Cr5Mo2V hot work die steel; the heat treatment material is an annealing state hot work die blank, and a plurality of thermocouples for detecting the temperature are arranged at the center part and the surface of the die; the method is characterized by comprising the following steps:
(1) High-temperature homogenization for the first time: the working procedure comprises secondary preheating, namely heating the hot work die to be thermally treated to 500-600 ℃ at a heating rate of less than or equal to 80 ℃/h, and keeping the temperature for 1h, and heating to 800-850 ℃ at a heating rate of less than or equal to 80 ℃/h, and keeping the temperature for 0.9-1.1h; heating to 1050-1100 ℃ at a heating rate of less than or equal to 80 ℃/h, and preserving heat for 0.9-1.1h when the core temperature is 5-10 ℃ lower than the set temperature;
(2) First air/water alternate quenching: taking the die out of the furnace for air cooling after the heat preservation according to the step (1), monitoring the surface temperature of the die to 900-950 ℃, immediately cooling the die by water until the surface temperature is lower than 400 ℃, taking out the die, placing the die in air for cooling, performing secondary water cooling when the surface temperature of the die returns to 450 ℃, after the water cooling is finished, keeping the surface temperature of the die steel lower than 200 ℃, starting air cooling to room temperature, checking that the surface of the die is dry and has no water vapor, and immediately putting the die into a tempering heat treatment furnace;
(3) High-temperature tempering for the first time: setting the tempering temperature to 750-780 ℃, before charging, keeping the temperature of the tempering heat treatment furnace less than or equal to 200 ℃, rapidly heating the die along with the furnace, keeping the temperature for 1.5-2 h when the core temperature is 5-10 ℃ lower than the set tempering temperature, discharging from the furnace, and air cooling until the surface temperature of the die is lower than 100 ℃;
(4) Secondary high-temperature quenching after tempering: the process does not need preheating, the furnace temperature is increased to 900-950 ℃ before the die is put into the furnace, the furnace is charged at the temperature, when the core temperature is 5-10 ℃ lower than the set temperature, the temperature is kept for 0.5h, the water is cooled until the surface temperature of the die is lower than 200 ℃, and the air cooling is started to the room temperature;
(5) Stress relief annealing: the furnace temperature before the annealing in the furnace is less than or equal to 200 ℃, the temperature of the die and the furnace is raised to 600-650 ℃, the temperature is kept for 1-2 h, and the quenching stress is removed;
(6) Quenching and tempering the die: the temperature of the mold entering the furnace is lower than 200 ℃, the temperature of the mold entering the furnace is preheated and preserved for 3.3 to 4 hours at the temperature of 800 to 850 ℃, the temperature of the furnace is raised to the austenitizing temperature of the mold steel, when the core temperature is 5 to 10 ℃ lower than the set austenitizing temperature, the temperature is preserved for 0.4 to 0.6 hours, the mold entering the furnace is quenched to 200 ℃ by vacuum gas, and the mold is taken out of the furnace and air-cooled; after the die is quenched, a tempering process is carried out for three times at 580-600 ℃, and the die is cooled to room temperature by air.
2. The process of claim 1, wherein the heat treatment comprises the steps of: in the step (1), the temperature rise speed is 50-80 ℃/h.
3. The process of claim 1, wherein the heat treatment comprises the steps of: in the step (6), two-stage preheating and heat preservation treatment is adopted, and the first-stage preheating is carried out for 0.9-1.1h at the temperature of 500-600 ℃.
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