CN114807549A - A thermal deformation method for refining grains of hot work die steel - Google Patents

Abstract

The invention discloses a thermal deformation method for refining grains of hot-work die steel, which comprises the steps of heating and insulating the hot-work die steel, then rapidly cooling to a thermal deformation temperature, thermally deforming the hot-work die steel at a specific strain rate, rapidly cooling to room temperature after true strain is achieved, and completing thermal deformation treatment of the hot-work die steel; the method has simple process control, and completely and dynamically recrystallizes the hot die steel by controlling the deformation parameters, thereby achieving the purposes of refining crystal grains and adjusting the form and distribution of the crystal grains, effectively improving the coarse structure of the hot die steel and improving the mechanical property of the hot die steel.

Description

A thermal deformation method for refining grains of hot work die steel

technical field

The invention belongs to the field of metal material thermal processing technology, and particularly provides a thermal deformation method for refining the grains of hot working die steel.

Background technique

Hot work die steel is an alloy tool steel suitable for manufacturing solid metal or high temperature liquid metal forming dies above the recrystallization temperature, such as hammer forging dies, die casting dies, hot extrusion dies, etc. The service conditions of hot work dies are harsh, and they need to withstand high temperature, high load, heating and cooling for a long time. Large hammer forging dies need to withstand an impact force of up to hundreds of MN. 500-800℃. Therefore, in addition to high thermoplastic deformation resistance, high temperature strength, high temperature hardness and good impact toughness, hot work die steel is required to have good fatigue resistance and anti-tempering stability in order to meet the needs of special service. conditions of work demand. It is an effective way to improve the service life of the mold by adopting a reasonable hot deformation process to improve the uniformity of the structure of the hot work die steel, adjust the grain shape, and improve the coarse grain phenomenon.

The commonly used method for grain refinement is to thermally deform the material to recrystallize and nucleate and grow inside the material to form uniform and fine equiaxed crystals, so as to refine the grains, adjust the grain shape, and eliminate thermal deformation. Deformation defects in the process. This method is relatively simple, but it is difficult to determine the appropriate austenite recrystallization temperature and holding time due to the comprehensive influence of various factors (deformation temperature, strain rate, strain amount) during thermal deformation. When the austenite recrystallization temperature is too low, the incubation period is long and the nucleation rate is low. With the prolongation of the holding time, the grains grow and cause coarse grains; and when the recrystallization temperature is too high, the incubation period short, the grains grow rapidly, which will also cause the recrystallized grains to be coarse. When the material undergoes incomplete dynamic recrystallization, a large number of dislocation groups will gather at the grain boundaries to form small equiaxed recrystallized grains, and a typical "necklace-like structure" will appear; after complete dynamic recrystallization, continuous heat preservation will cause grains to appear. Coarse grains caused by growth. Therefore, it is necessary to determine the appropriate thermal deformation process parameters in combination with the thermal processing map, and achieve the purpose of stable performance by refining the grains. The thermal processing map is a new method to study the thermal deformation behavior of materials based on the dynamic material model (DMM) and dissipation theory, which superimposes the power dissipation map and the instability map. , to determine and optimize the thermal deformation process parameters of the refined hot work die steel grains to provide theoretical guidance for actual production.

At present, there are no patents related to combining the thermal deformation method and the thermal processing map to refine the grains of hot work die steel, to determine the optimal thermal deformation process parameters for refining the grains of hot work die steel, and to improve its mechanical properties. Most of the patents for refining the grains of other forgings use the heat treatment method of normalizing + quenching + tempering, but this type of process is more complicated, the process time is long, and the efficiency is low.

SUMMARY OF THE INVENTION

The invention provides a thermal deformation method for refining the grains of hot work die steel. By controlling the deformation parameters, the hot work die steel can be completely recrystallized dynamically, so as to achieve the purpose of refining grains and adjusting the shape of grains, thereby effectively improving the The coarse structure of hot work die steel is improved, and the mechanical properties of hot work die steel are improved. The invention utilizes the hot deformation method to make the crystal grains of the hot work die steel uniform and fine, the average grain size is less than 10 μm, the operation process is easy to control, the procedure is simple, and the processing cycle is short, and the grain size of the hot work die steel can be greatly improved. The efficiency of refinement, combined with the comparative analysis of the instability zone and the microstructure, determines and optimizes the thermal deformation process parameters of the refined hot work die steel grains to provide guidance for actual production.

The specific technical scheme of the present invention is as follows:

A thermal deformation method for refining the grains of hot work die steel. The hot work die steel is heated and kept warm, so that the temperature inside and outside the hot work die steel is uniform, and then rapidly cooled to the hot deformation temperature, and the hot work die steel is subjected to a specific strain rate. Carry out thermal deformation, and then quickly cool to room temperature after reaching the true strain, and complete the thermal deformation treatment of hot work die steel.

The chemical composition and mass percentage content of the hot work die steel: C: 0.5-0.6%, Cr: 0.8-1.1%, Mn: 0.5-0.8%, Mo: 0.35-0.5%, Ni: 1.4-1.8%, V : 0.1-0.3%, Si: ≤ 0.35%, S: ≤ 0.03%, P: ≤ 0.03%, and the balance is Fe.

The heating rate is determined according to the size of the hot work die steel and the capacity of the heating equipment, and is generally not higher than 2°C/s.

The holding temperature is 50-110°C above AC ₃ of hot work die steel.

The holding time is not less than 10min, so that the temperature difference between the core and the surface is less than 10°C.

The heat preservation section is rapidly cooled to the deformation temperature and the cooling rate of the rapid cooling to room temperature after the thermal deformation reaches the true strain is not less than 5°C/s.

The deformation temperature is at or below the austenitizing temperature, and most of the hot rolling and hot deformation are deformed above the austenitizing temperature, and the increase in temperature easily leads to grain growth; Combined analysis of microstructure shows that the deformation temperature range of grain refinement hot work die steel is 800-870℃, the strain rate range is 0.001-0.05s ^-1 , and the true strain is 1.1.

Compared with the prior art, the present invention has the following advantages and effects:

(1) In the present invention, by controlling the deformation temperature, strain rate and deformation amount, the hot work die steel can be completely recrystallized dynamically, so as to achieve the purpose of refining the grains and adjusting the grain shape, and effectively improve the unevenness of the hot work die steel. coarse tissue.

(2) Dynamic recrystallization can be completed during the hot deformation process of the present invention, so as to achieve the purpose of refining grains, reducing the secondary heat treatment process, simplifying the process, shortening the heat treatment time, and greatly improving the refinement of hot work die steel. grain efficiency.

(3) The general hot rolling and hot deformation processes are all performed above the austenitizing temperature, and the deformation temperature used in the present invention is all at or below the austenitizing temperature, which can prevent grain growth to a certain extent.

(4) The thermal deformation process of the hot work die steel of the present invention is always at a constant temperature, which avoids the temperature gradient caused by large heating and cooling, prevents the inhomogeneity of the recrystallized structure, and avoids the phenomenon of mechanical instability.

(5) The present invention combines the hot working diagram and the microstructure analysis to determine the optimal thermal deformation process range for refining the grains of the hot work die steel, providing technical support for improving the quality of the hot work die and prolonging its service life, Provide theoretical guidance for actual production.

Description of drawings

Figure 1 shows the original microstructure of hot work die steel without refining treatment;

Fig. 2 is a thermal deformation process diagram of refining hot work die steel grains;

Fig. 3 is the microstructure of the hot work die steel refined in Comparative Example 1;

Fig. 4 is the microstructure of hot work die steel refined in Comparative Example 2;

Fig. 5 is the microstructure of hot work die steel refined in Embodiment 1;

Fig. 6 is the hot work die steel microstructure of embodiment 2 refinement treatment;

Fig. 7 is the hot work die steel microstructure of embodiment 4 refinement treatment;

FIG. 8 is the microstructure of the hot work die steel refined in Example 5. FIG.

Detailed ways

The specific embodiments of the present invention will be described in detail below in conjunction with comparative examples and examples.

The chemical composition and mass percentage content of the unrefined hot work die steel used in the embodiment of the present invention: C: 0.5-0.6%, Cr: 0.8-1.1%, Mn: 0.5-0.8%, Mo: 0.35 -0.5%, Ni: 1.4-1.8%, V: 0.1-0.3%, Si: ≤ 0.35%, S: ≤ 0.03%, P: ≤ 0.03%, the balance is Fe, and its original microstructure is shown in Figure 1 , the grains are coarse and uneven, and the average grain size is about 21.20 μm; it is processed according to the thermal deformation process shown in FIG. 2 , and the specific process is as follows.

Comparative Example 1

The hot work die steel was heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel was 770°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature was 40°C/s. The cooling rate was lowered to 750 °C, followed by thermal deformation at a strain rate of 0.005s ^-1 , the temperature was kept constant during the deformation, and the true strain was 1.1. After the deformation, it was rapidly cooled to room temperature at a cooling rate of 40 °C/s. The deformed microstructure is shown in Figure 3, and a typical "necklace-like" structure appears.

Comparative Example 2

The hot work die steel was heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel was 770°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature was 40°C/s. The cooling rate was lowered to 750°C, followed by thermal deformation at a strain rate of 0.05s ^-1 . The temperature remained unchanged during the deformation, and the true strain was 1.1. After the deformation, the cooling rate was 40°C/s. The deformed microstructure is shown in Fig. 4, and there are local deformation bands formed by many elongated coarse grains.

Example 1

The hot work die steel was heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel was 770°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature was 40°C/s. The cooling rate was lowered to 800°C, followed by thermal deformation at a strain rate of 0.001s ^-1 , the temperature remained unchanged during the deformation, and the true strain was 1.1. After the deformation, the cooling rate was 40°C/s. The microstructure after deformation is shown in Figure 5, the grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

Example 2

The hot work die steel is heated at a heating rate of 1°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel is 765°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature is 40°C/s. The cooling rate was lowered to 800 °C, followed by thermal deformation at a strain rate of 0.001 s ⁻¹ . The temperature was kept constant during the deformation, and the true strain was 1.1. After the deformation, it was rapidly cooled to room temperature at a cooling rate of 40 °C/s. The microstructure after thermal deformation was shown in Figure 6. The grain size was refined and the structure was uniform. Improved, the average grain size is less than 10 μm.

Example 3

The hot work die steel was heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel was 770°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature was 40°C/s. The cooling rate was lowered to 820°C, followed by thermal deformation at a strain rate of 0.001s ^-1 . During the deformation, the temperature was kept constant, and the true strain was 1.1. After the deformation, it was rapidly cooled to room temperature at a cooling rate of 40°C/s. The grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

Example 4

The hot work die steel is heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel is 770°C, heated to 830°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature is 40°C/s. The cooling rate was lowered to 800°C, followed by thermal deformation at a strain rate of 0.001s ^-1 , the temperature remained unchanged during the deformation, and the true strain was 1.1. After the deformation, the cooling rate was 40°C/s. The microstructure after deformation is shown in Figure 7, the grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

Example 5

The hot work die steel was heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel was 770°C, heated to 870°C, and kept for 10 minutes to make the temperature inside and outside the material uniform. The temperature was 20°C/s. The cooling rate was lowered to 800°C, followed by thermal deformation at a strain rate of 0.001s ^-1 . During the deformation, the temperature was kept constant, and the true strain was 1.1. After the deformation, the cooling rate was 20°C/s. The microstructure after deformation is shown in Figure 8, the grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

Example 6

The hot work die steel is heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel is 770°C, heated to 820°C, and kept for 12 minutes to make the temperature inside and outside the material uniform. The temperature is 20°C/s. The cooling rate was cooled to 800 °C, followed by thermal deformation at a strain rate of 0.005s ^-1 , the temperature was kept constant during the deformation, and the true strain was 1.1. After the deformation, the cooling rate was 20 °C/s. Rapid cooling to room temperature, the crystal The grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

Example 7

The hot work die steel is heated at a heating rate of 2°C/s. At this heating rate, the AC ₃ temperature of the corresponding die steel is 770°C, heated to 880°C, and kept for 10 minutes to make the temperature inside and outside the material uniform, with a temperature of 5°C/s. The cooling rate was cooled to 870 °C, and then thermal deformation was carried out at a strain rate of 0.05s ^-1 . During the deformation, the temperature was kept unchanged, and the true strain was 1.1. After the deformation, it was rapidly cooled to room temperature at a cooling rate of 5 °C/s. The grain size is refined, the uniformity of the structure is improved, and the average grain size is less than 10 μm.

The above-mentioned specific comparative cases and implementation cases describe in detail the technical problems, technical solutions and beneficial effects solved by the present invention. It should be understood that the above are only specific embodiments of the present invention and are not intended to limit it. In the present invention, all technical solutions formed by adopting equivalent transformations in terms of structure, method or function according to the spirit of the present invention are within the protection scope of the present invention.

Claims (7)

1. a thermal deformation method of refining hot work die steel grain, is characterized in that, hot work die steel is heated and kept warm, then rapidly cooled to the thermal deformation temperature, and hot work die steel is thermally deformed with a specific strain rate , after reaching the true strain, it is rapidly cooled to room temperature to complete the thermal deformation treatment of the hot work die steel. 2. The hot deformation method for refining hot work die steel grains according to claim 1, wherein the chemical composition and mass percentage content of the hot work die steel: C: 0.5-0.6%, Cr: 0.8- 1.1%, Mn: 0.5-0.8%, Mo: 0.35-0.5%, Ni: 1.4-1.8%, V: 0.1-0.3%, Si: ≤ 0.35%, S: ≤ 0.03%, P: ≤ 0.03%, the remainder The amount is Fe. 3 . The thermal deformation method for refining hot work die steel grains according to claim 1 , wherein the heating rate is not higher than 2° C./s. 4 . 4 . The thermal deformation method for refining hot work die steel grains according to claim 1 , wherein the holding temperature is 50-110° C. above AC ₃ of hot work die steel. 5 . 5. The thermal deformation method for refining hot work die steel grains according to claim 1, wherein the holding time is not less than 10min. 6. the thermal deformation method of refining hot work die steel grains according to claim 1, is characterized in that, the cooling rate of fast cooling to room temperature after the heat preservation section is rapidly cooled to the thermal deformation temperature and reaching true strain is not less than 5 ℃ /s. 7 . The thermal deformation method for refining hot work die steel grains according to claim 1 , wherein the thermal deformation temperature is 800-870° C., the strain rate is 0.001-0.05s ⁻¹ , and the true strain is 1.1. 8 .

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