Martensitic stainless steel spheroidizing annealing process for refrigeration compressor valve plate

Abstract

The invention provides a martensitic stainless steel spheroidizing annealing process for a refrigeration compressor valve plate, and belongs to the technical field of stainless steel material heat treatment. According to the process, carbide spheroidization and structure homogenization are achieved through staged temperature control annealing for the martensitic stainless steel hot-rolled black coil with the carbon content of 0.36-0.43% and the chromium content of 12.00-15.00%. The method specifically comprises the following steps: heating to 480 DEG C at a full speed, heating to 860 DEG C at a constant speed of less than or equal to 25 DEG C / h, and preserving heat for 26 hours; quickly cooling (more than or equal to 50 DEG C / h) to 760 DEG C, and preserving heat for 16 hours to promote formation of a ferrite and carbide equilibrium structure; and slowly cooling to 500 DEG C, quickly cooling to less than or equal to 180 DEG C, and discharging. By optimizing the heating rate, high-temperature rapid cooling and two-phase region isothermal processes, dendritic segregation and string-shaped carbides are effectively eliminated, the carbides are dispersed and distributed, the banded segregation defect and performance instability existing in martensitic stainless steel for the refrigeration compressor valve plate are overcome, and the obdurability and high-cycle fatigue strength of the material are guaranteed.

Description

Technical Field

[0001] The invention belongs to the technical field of heat treatment of stainless steel materials, and in particular relates to a spheroidizing annealing process of martensitic stainless steel for refrigeration compressor valve plates. Background Art

[0002] Martensitic stainless steel is a type of stainless steel whose properties are modified by adjusting the carbon, chromium, and other components of the steel and the heat treatment process. After spheroidizing annealing, its main structure is ferrite + carbide; after high-temperature austenitization and air cooling, the main structure is martensite.

[0003] In particular, the martensitic stainless steel involved in the present invention is mainly used for the production of refrigeration compressor valve plates. After multiple hot rolling and cold rolling and heat treatment, it is processed through a series of processes such as stamping, quenching and tempering to produce compressor valve plates with a thickness of only 0.1-0.3 mm and a weight of 2-5 g. The compressor valve plate is to the compressor what the valve is to the human heart, and is a key component of the compressor system. For some high-frequency variable-frequency refrigeration compressors, the frequency conversion is as high as 150HZ / S or more. Under high-frequency operation, the compressor valve plate not only needs to withstand alternating stress and bending stress, but also needs to withstand very high suction and exhaust pressures to accelerate the circulation of the refrigerant. It is the part of the compressor that is most subject to impact and must have ultra-high toughness, resistance to high-cycle cycles and high impact fatigue strength. Uniform structure is the greatest guarantee of uniform material properties.

[0004] The martensitic stainless steel involved in the present invention has a carbon content of about 0.4%, which is medium-carbon martensite. Chromium is a strong carbide-forming element. During the solidification process of molten steel, component segregation easily occurs between dendrites, and string-like primary carbides are precipitated. If it is not properly treated before cold rolling, it will be inherited to the finished product, causing the strength and toughness of the material to deteriorate, seriously affecting the high-cycle fatigue service life and comprehensive mechanical properties of the product. In severe cases, it will cause the compressor to fail and reduce the service life of the refrigeration system. Summary of the Invention

[0005] The purpose of the present invention is to provide a spheroidizing annealing process for martensitic stainless steel used in refrigeration compressor valve plates, so as to solve the banded segregation structure defect existing in martensitic stainless steel used in refrigeration compressor valve plates.

[0006] The technical solution of the present invention is: a spheroidizing annealing process of martensitic stainless steel for refrigeration compressor valve plates, comprising the following steps: (1) Heating stage: Place the hot-rolled black coils of martensitic stainless steel used for refrigeration compressor valve plates into a full hydrogen bell-type furnace and stack them vertically. First, heat them at full speed, then heat them at a uniform speed and heat them to 860°C for insulation. The annealing process in the heating stage is as follows: full-speed heating stage, heating time ≤ 120 minutes, heating temperature 480°C; uniform heating stage, heating time 18 hours, heating rate ≤ 25°C / h; annealing holding stage, holding time 26 hours, holding temperature 860°C; (2) Rapid cooling stage: After the 860℃ insulation is completed, the heating cover is removed and replaced with a cooling cover for rapid cooling. After cooling to 760℃, the heating cover is replaced again for insulation; The process system for the rapid cooling stage is as follows: rapid cooling, cooling rate ≥ 50℃ / h; after cooling to 760℃, heat preservation with a heating cover for 16 hours at a temperature of 760℃; (3) Slow cooling stage: After the insulation is completed, turn off the burner and continue to cool slowly with the heating cover. After cooling to 500℃, replace the cooling cover. After rapid cooling to the furnace temperature, take it out of the furnace. After cooling naturally to 80℃, send it to cold rolling for pickling.

[0007] The process system of the slow cooling stage is as follows: the heating cover burner is not turned on during the slow cooling process, the slow cooling target temperature is 500℃, and after the slow cooling is completed, the cooling cover is replaced for full speed cooling, and the furnace temperature is ≤180℃.

[0008] The chemical composition of the martensitic stainless steel is as follows by mass percentage: C 0.36-0.43%, Cr 12.00-15.00%, Si≤1.0%, Mn≤1.0%, P≤0.02%, S≤0.005%, Mo≤1.5%, V≤0.30%, N≤0.2%, and the balance is Fe and residual elements.

[0009] The beneficial effects of the present invention are as follows: 1. Martensitic stainless steel will undergo phase transformation during the annealing and heating process. The uniformity of the microstructure after the phase transformation has an important impact on product performance. By controlling the uniform heating rate to ≤25℃ / h, the phase transformation of the martensitic stainless steel used in refrigeration compressor valves can be ensured to be more complete, greatly improving the uniformity of the microstructure after the phase transformation.

[0010] 2. Martensite has a low thermal conductivity, and slow, uniform heating helps to evenly distribute the temperature inside the annealed steel coil, controlling the temperature difference between hot and cold spots to ≤10°C, avoiding large temperature differences that could cause performance differences between the surface and core of the material. It also prevents thermal stress from being generated in the material due to rapid heating, preventing the crystal structure from being destroyed and causing hidden cracks or crack sources, thus providing a solid guarantee for the material's plasticity and toughness.

[0011] 3. According to tissue genetics theory, for steel containing austenite-stabilizing elements such as chromium and molybdenum, rapid cooling at high temperatures can accelerate the transformation of austenite to ferrite, while also promoting the rapid precipitation of carbides and increasing the nucleation rate. This increase in nucleation points also leads to finer and more evenly dispersed carbides, effectively improving the material's fatigue strength and toughness. Therefore, after spheroidizing annealing and holding, rapid cooling to 760°C and holding for 16 hours promotes a balanced structure of ferrite and carbides, contributing to improved material strength and toughness.

[0012] 4. According to the equilibrium phase diagram analysis, at 760°C, the martensitic stainless steel used for compressor valve plates is in the ferrite + carbide two-phase region. Since the diffusion rate of atoms in ferrite is much higher than that in austenite, long-term high temperature at this temperature can effectively eliminate or reduce the element segregation produced during the solidification process and prevent uneven performance caused by segregation.

[0013] 5. Compared with ordinary martensitic stainless steel, the present invention increases the austenitizing temperature and adds isothermal spheroidizing annealing and slow cooling after rapid cooling, which can effectively improve the banded segregation structure and string-like carbide distribution, improve the degree of carbide spheroidization and the uniformity of dispersion distribution, ensure the subsequent quenching and tempering mechanical properties, and especially play a special role in improving the strength and toughness of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 is the annealing process curve of the present invention; Figure 2 It is the ordinary annealing process curve; Figure 3 This is a carbide structure distribution diagram of the finished product obtained in specific embodiment 1 of the present invention; Figure 4 This is the carbide structure distribution diagram of the finished product obtained by ordinary annealing process. DETAILED DESCRIPTION

[0015] Example 1 The chemical element composition and mass percentage of the martensitic stainless steel continuous casting billet are: carbon: 0.36%, silicon: 1.0%, manganese: 1.0%, phosphorus: 0.02%, sulfur: 0.005%, chromium: 12.00%, molybdenum: 1.5%, vanadium: 0.30%, nitrogen: 0.2%, and the rest are iron and residual elements.

[0016] (1) The martensitic stainless steel hot-rolled black coils off the production line after hot rolling are placed in a full hydrogen bell furnace and stacked vertically. They are first heated at full speed to 450°C for 116 minutes. Then they are heated at a uniform speed for 18 hours. After heating to 860°C, they are kept warm for 26 hours. (2) After the 860℃ holding period is completed, the heating cover is removed and replaced with a cooling cover for rapid cooling (cooling rate 50℃ / h). After cooling to 760℃, the cooling cover is removed and replaced with a heating cover, and the burner is turned on to hold the temperature at 760℃ for 16 hours. (3) After the insulation is completed, turn off the burner and continue to cool slowly with the heating cover. After cooling to 500℃, replace the cooling cover. After rapid cooling to 160℃, take it out of the furnace. After natural cooling to 80℃, send it for cold rolling and pickling.

[0017] It can be seen from Table 1 that the performance of the finished product obtained by the process embodiment 1 is significantly better than that of the conventional annealing process. Figure 3 and Figure 4 From the comparison of the carbide organization distribution diagram of , it can be seen that the carbide distribution of the finished product obtained by the process embodiment 1 is relatively uniform, no string-like carbides are found in the field of view, and the banded segregation basically disappears.

[0018] From the comparison of the data in Table 1, it can be seen that the finished product obtained by the production process of the present invention is superior to the finished product obtained by the original process in terms of yield strength, tensile strength, elongation, HV hardness and other indicators, and is also greatly improved compared with the industry standard. The process method reasonably controls the heating process system to achieve uniform heating temperature during the heating process of the continuous casting billet, uniform internal structure without stress deformation, fine and evenly distributed carbides, and no chain-shaped segregation bands and hidden cracks and other structural defects. The finished product has the advantages of high strength and toughness, high flatness, and excellent precision rolling performance, and is suitable for the production of high-end green and energy-saving refrigeration compressor ultra-thin valve plates.

Claims

1. A spheroidizing annealing process for martensitic stainless steel for refrigeration compressor valve plates, characterized in that: The following steps are involved: (1) Heating stage: After the martensitic stainless steel hot-rolled black coils are placed in a full hydrogen bell-type furnace and stacked vertically, they are first heated at full speed, then heated at a uniform speed, and then heated to 860°C and kept warm; The annealing process in the heating stage is as follows: full-speed heating stage, heating time ≤ 120 minutes, heating temperature 480°C; uniform heating stage, heating time 18 hours, heating rate ≤ 25°C / h; annealing holding stage, holding time 26 hours, holding temperature 860°C; (2) Rapid cooling stage: After the 860℃ insulation is completed, rapid cooling is carried out. After cooling to 760℃, the heating cover is replaced for insulation; The process system for the rapid cooling stage is as follows: rapid cooling with a cooling hood, cooling rate ≥ 50℃ / h; after cooling to 760℃, heat preservation with a heating hood, heat preservation time 16 hours, and heat preservation temperature 760℃; (3) Slow cooling stage: After the insulation is completed, the heating cover is slowly cooled, and the cooling cover is replaced after cooling to 500 ° C. The furnace is quickly cooled to the furnace temperature and then taken out of the furnace. After cooling naturally to 80 ° C, it is sent to cold rolling for pickling; The process system of the slow cooling stage is as follows: the heating cover burner is not turned on during the slow cooling process, the slow cooling target temperature is 500℃, and after the slow cooling is completed, the cooling cover is replaced for full speed cooling, and the furnace temperature is ≤180℃.

2. The spheroidizing annealing process for martensitic stainless steel for refrigeration compressor valve plates according to claim 1, characterized in that: The chemical composition of the martensitic stainless steel is as follows by mass percentage: C 0.36-0.43%, Cr 12.00-15.00%, Si≤1.0%, Mn≤1.0%, P≤0.02%, S≤0.005%, Mo≤1.5%, V≤0.30%, N≤0.2%, and the balance is Fe and residual elements.

3. The spheroidizing annealing process for martensitic stainless steel for refrigeration compressor valve plates according to claim 1, characterized in that: The heating rate in the uniform heating stage is ≤25°C / h, and the temperature difference between hot and cold spots inside the steel coil is controlled to be ≤10°C.

4. The spheroidizing annealing process for martensitic stainless steel for refrigeration compressor valve plates according to claim 1, characterized in that: The cooling rate in the rapid cooling stage is ≥50°C / h to promote rapid precipitation of carbides; and the composition segregation is eliminated by atomic diffusion in the ferrite phase during the 760°C holding stage.

5. The spheroidizing annealing process for martensitic stainless steel for refrigeration compressor valve plates according to claim 1, characterized in that: During the slow cooling stage, the burner is closed, and the temperature is lowered by relying on the residual heat of the heating hood and the natural heat dissipation of the furnace body, thereby avoiding stress cracks caused by a sudden drop in temperature.

6. The process according to any one of claims 1 to 5, characterized in that: The yield strength of the final product is greater than 1535MPa, the tensile strength is greater than 1815MPa, the elongation is greater than 5.5%, the HV hardness is greater than 560, and the average particle size of the finished carbide is ≤1μm and is dispersed without string or chain segregation.

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