Heat treatment process for improving coarse grain size of Cr16Ni6 stainless steel

Abstract

The application relates to a heat treatment process for improving the grain size of Cr16Ni6 stainless steel, wherein the grain size is obviously improved to 4-6 levels after the heat treatment, and the product quality is improved; the process scheme is as follows: after forging, cooling treatment, one-time strengthening treatment, one-time tempering, two-time strengthening treatment and two-time tempering; the grain size is effectively improved; the forging organization is stabilized and part of the martensite organization is transformed through the after-forging cooling treatment, internal stress is reduced, and workpiece cracking is avoided; through the after-forging cooling treatment and the strengthening treatment, austenite is repeatedly transformed into fine lath martensite, nucleation is ensured in the subsequent high-temperature heating process, nucleation rate is increased, grain size is refined, and the grain size is improved from the original coarse 1-3 levels to the fine 4-6 levels.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a heat treatment process for improving the coarse grain size of Cr16Ni6 stainless steel after heat treatment, with the grain size level reaching 4 to 6, thereby improving product quality. Background Technology

[0002] Cr16Ni6 series stainless steel, with a Cr content of 15.0%~17.5% and a Ni content of 5.0%~7.0%, is suitable for manufacturing high-strength and high-toughness products. However, the high Ni content of this steel makes it prone to developing stable coarse grains in the forged billet due to repeated high-temperature heating during forging. The grain size of the forged billet is generally between 0 and 3. Furthermore, Cr16Ni6 series stainless steel is a transitional medium-temperature high-strength stainless steel, requiring strengthening treatment during heat treatment to induce the austenite-to-martensite transformation. Therefore, conventional heat treatment processes of quenching and tempering are insufficient to form fine grains and improve the grain size. After heat treatment, the grain size is generally between 1 and 3, resulting in scrap due to the low grain size and impacting delivery schedules. To improve product quality and meet market demands, there is an urgent need for a heat treatment process that can effectively improve Cr16Ni6 series stainless steel. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the existing technology and provide a heat treatment process to improve the coarse grain size of Cr16Ni6 stainless steel, which effectively improves the coarse grain size of Cr16Ni6 stainless steel and thus improves the quality of Cr16Ni6 stainless steel products.

[0004] To achieve the above-mentioned objectives, the present invention adopts the following technical solution:

[0005] A heat treatment process for improving the coarse grain size of Cr16Ni6 stainless steel is carried out according to the following process:

[0006] Step 1) Post-forging cooling treatment: After forging, if the surface temperature of the forging is ≥900℃, perform mist cooling or air cooling, rotating it 90 degrees every half hour. When the surface temperature of the forging is ≤40℃ after mist cooling or air cooling, perform air cooling for 24h~100h. After air cooling, transfer the forging to a low-temperature cooling furnace for cold treatment at a cooling temperature of -10℃~-30℃. Hold at this temperature for 7h~30h. After holding, remove it from the furnace and air cool until the surface temperature of the forging reaches room temperature.

[0007] Step 2) After completing Step 1), perform the first strengthening treatment in Step 2): The furnace temperature is ≤400℃. Increase the temperature to 550℃~700℃ at a rate of ≤80℃ / h and hold for 3~8 hours. After holding, increase the temperature to 1000℃~1060℃ at a rate of 50℃~100℃ / h and hold for 5~20 hours. Then remove the forging from the furnace and cool it. The air cooling time should be ≤100 seconds. Place the forging in a water tank. Water cooling is performed for 10 to 100 minutes, with the water temperature controlled at 0℃ to 25℃. Then, the forging is lifted out and air-cooled for 30 to 300 seconds. The forging is then placed in a water tank and water-cooled for 10 to 50 minutes. After that, the forging is lifted out and air-cooled to room temperature. Once it reaches room temperature, it is transferred to a low-temperature furnace at -60℃ to -100℃ and held for 5 to 20 hours. After removing it from the furnace, the forging is reheated until the surface temperature reaches 20℃ to 40℃.

[0008] Step 3) After completing Step 2), perform a tempering in Step 3): The furnace temperature is ≤400℃, and the temperature is increased to 400℃~500℃ at a heating rate of ≤50℃ / h~100℃ / h. Hold the temperature for 5h~40h, and then air cool to room temperature after removing from the furnace.

[0009] Step 4) After completing Step 3), perform the secondary strengthening treatment in Step 4): The furnace temperature is ≤400℃. Increase the temperature to 550℃~700℃ at a rate of ≤80℃ / h and hold for 3~8 hours. After holding, increase the temperature to 1000℃~1060℃ at a rate of 50℃~100℃ / h and hold for 5~20 hours. Then remove the forging from the furnace and cool it. The air cooling time should be ≤100 seconds. Place the forging in water. The forging is cooled in water for 10 to 100 minutes, with the water temperature controlled at 0℃ to 25℃. Then, the forging is lifted out and air-cooled for 30 to 300 seconds. The forging is then placed back into the water tank and cooled in water for 10 to 50 minutes. The forging is then lifted out and air-cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -60℃ to -100℃ and held for 5 to 20 hours. After being taken out of the furnace, the forging is reheated until the surface temperature of the forging reaches 20℃ to 40℃.

[0010] Step 5) After completing Step 4), perform the second tempering in Step 5): The furnace temperature is ≤400℃, and the temperature is increased to 400℃~500℃ at a heating rate of ≤50℃ / h~100℃ / h. Hold the temperature for 5h~40h, and then air cool to room temperature after removing from the furnace.

[0011] Compared with the prior art, the process of this invention has the following advantages:

[0012] 1. This invention is the first to combine post-forging cooling treatment, primary strengthening treatment, primary tempering, secondary strengthening treatment, and secondary tempering process, which effectively improves the phenomenon of coarse grain size.

[0013] 2. First, use cold treatment after forging to promote the stability of the forging structure and cause partial martensitic transformation, reduce internal stress and avoid cracking of the workpiece.

[0014] 3. Through post-forging cooling and strengthening treatment, austenite is repeatedly transformed into fine lath martensite, thereby ensuring re-nucleation during subsequent high-temperature heating, increasing the nucleation rate, and promoting grain refinement. Cr16Ni6 series stainless steel forgings produced according to this invention show a significant improvement in grain size, from the original coarse grade 1-3 grains to fine grade 4-6, meeting market demands. Detailed Implementation

[0015] A heat treatment process for improving the coarse grain size of Cr16Ni6 stainless steel is carried out according to the following process:

[0016] Step 1) Post-forging cooling treatment: After forging, if the surface temperature of the forging is ≥900℃, the forging is subjected to mist cooling or air cooling, rotating 90 degrees every half hour. When the surface temperature of the forging is ≤40℃ after mist cooling or air cooling, it is subjected to air cooling for 24h~100h. After air cooling, the forging is transferred to a low-temperature cooling furnace for cold treatment at a cooling temperature of -10℃~-30℃. It is held at this temperature for 7h~30h. After the holding is completed, it is removed from the furnace and air-cooled until the surface temperature of the forging reaches room temperature. Then the forging is loaded into a heating furnace. After completing Step 1), it is ready to perform a strengthening treatment.

[0017] Step 2) Primary strengthening treatment: The furnace temperature is ≤400℃. The temperature is increased to 550℃~700℃ at a rate of ≤80℃ / h and held for 3~8 hours. After holding, the temperature is increased to 1000℃~1060℃ at a rate of 50℃~100℃ / h and held for 5~20 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is ≤100 seconds. The forging is then placed in a water bath for water cooling for 10min~100min, with the water temperature controlled at [temperature range missing]. At 0℃~25℃, the forging is then lifted out and air-cooled for 30S~300S. Then, the forging is placed in a water tank for water cooling for 10min~50min. After that, the forging is lifted out and air-cooled to room temperature. Once it reaches room temperature, it is transferred to a low-temperature furnace at -60℃~-100℃ and held for 5h~20h. After being taken out of the furnace, the forging is reheated until the surface temperature of the forging reaches 20℃~40℃. Then, the forging is loaded into a heating furnace. After completing step 2), it is ready to undergo tempering.

[0018] Step 3) First tempering: The furnace temperature is ≤400℃. The temperature is increased to 400℃~500℃ at a heating rate of ≤50℃ / h~100℃ / h. The temperature is held for 5h~40h. The forging is then air-cooled to room temperature. After completing Step 3), the forging is ready to undergo a second strengthening treatment.

[0019] Step 4) Secondary strengthening treatment: The furnace temperature is ≤400℃. The temperature is increased to 550℃~700℃ at a rate of ≤80℃ / h and held for 3~8 hours. After holding, the temperature is increased to 1000℃~1060℃ at a rate of 50℃~100℃ / h and held for 5~20 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is ≤100 seconds. The forging is then placed in a water bath for water cooling for 10min~100min, with the water temperature controlled at [temperature range missing]. At 0℃~25℃, the forging is then lifted out and air-cooled for 30S~300S. Then, the forging is placed in a water tank for water cooling for 10min~50min. After that, the forging is lifted out and air-cooled to room temperature. Once it reaches room temperature, it is transferred to a low-temperature furnace at -60℃~-100℃ and held for 5h~20h. After being taken out of the furnace, the forging is reheated until the surface temperature of the forging reaches 20℃~40℃. Then, the forging is loaded into a heating furnace. After completing step 4), it is ready to perform a second tempering.

[0020] Step 5) Secondary tempering: The furnace temperature is ≤400℃. The temperature is increased to 400℃~500℃ at a rate of ≤50℃ / h~100℃ / h. The temperature is held for 5h~40h. The furnace is then removed and air-cooled to room temperature.

[0021] Example 1: A heat treatment process for improving the coarse grain size of Cr16Ni6 stainless steel.

[0022] Steel grade: 06Cr16Ni6; Chemical composition: C=0.074%, Si=0.43%, Mn=0.42%, Cr=16.29%, Ni=5.72%, S=0.0021%, P=0.018%, Mo=0.31%, Al=0.014%; Specifications: 530mm*Φ580mm*1650mm.

[0023] The heat treatment process is as follows:

[0024] Step 1) Post-forging cooling treatment: After forging, the surface temperature of the forging is 980℃. The forging is then mist-cooled, rotating 90 degrees every half hour. When the surface temperature of the forging is 37℃ after mist cooling, it is air-cooled. After air cooling for 40 hours, the forging is transferred to a low-temperature cooling furnace for cold treatment at a cooling temperature of -20℃. It is then held at this temperature for 10 hours. After holding, it is removed from the furnace and air-cooled until the surface temperature of the forging reaches room temperature. Then, the forging is loaded into a heating furnace.

[0025] Step 2) After Step 1) is completed, perform a strengthening treatment: the furnace temperature is 200℃, and the temperature is increased to 650℃ at a rate of 80℃ / h and held for 3 hours. After the holding period, the temperature is increased to 1020℃ at a rate of 80℃ / h and held for 10 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is 60 seconds. The forging is placed in a water tank for water cooling for 60 minutes (the water temperature is controlled at 21℃). Then, the forging is lifted out and air cooled for 80 seconds. The forging is then placed in a water tank for water cooling for 40 minutes. The forging is then lifted out and air cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -78℃ and held for 10 hours. The forging is then removed from the furnace and reheated until the surface temperature of the forging reaches 28℃. Finally, the forging is loaded into the heating furnace.

[0026] Step 3) After step 2) is completed, perform a tempering process: the furnace temperature is 220℃, and the temperature is increased to 420℃ at a rate of 80℃ / h. The temperature is held for 15 hours, and the forging is then air-cooled to room temperature. Finally, the forging is loaded into the heating furnace.

[0027] Step 4) After step 3) is completed, perform a secondary strengthening treatment: the furnace temperature is 310℃, and the temperature is increased to 650℃ at a rate of 80℃ / h and held for 3 hours. After the holding period, the temperature is increased to 1020℃ at a rate of 80℃ / h and held for 10 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is 55 seconds. The forging is placed in a water tank for water cooling for 60 minutes (the water temperature is controlled at 23℃). Then, the forging is lifted out and air cooled for 80 seconds. The forging is then placed in a water tank for water cooling for 40 minutes. The forging is then lifted out and air cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -78℃ and held for 10 hours. The forging is then removed from the furnace and reheated until the surface temperature of the forging reaches 30℃. Finally, the forging is loaded into the heating furnace.

[0028] Step 5) After step 4) is completed, perform a second tempering: the furnace temperature is 308℃, and the temperature is increased to 420℃ at a rate of 80℃ / h. The temperature is held for 15 hours, and the furnace is then air-cooled to room temperature.

[0029] The test results after production according to the above heat treatment process are shown in Table 1:

[0030] Table 1 Test Results

[0031] Technical Requirements Yield strength ≥ 1000 MPa Tensile strength ≥1200 MPa Elongation ≥14% Area shrinkage rate / Impact energy AKV≥70J Grain size ≥ 4 Actual testing 1040 1310 15 49 97 / 91 Levels 4-6

[0032] After production using the heat treatment process of the present invention to improve the coarse grain size of Cr16Ni6 stainless steel, the performance and grain size test results meet the requirements.

[0033] Example 2: A heat treatment process for improving the coarse grain size of Cr16Ni6 stainless steel.

[0034] Steel grade: 06Cr16Ni6; Chemical composition: C=0.071%, Si=0.44%, Mn=0.43%, Cr=16.33%, Ni=5.71%, S=0.0023%, P=0.017%, Mo=0.31%, Al=0.014%; Specifications: 530mm*Φ580mm*1650mm.

[0035] The heat treatment process is as follows:

[0036] Step 1) Post-forging cooling treatment: After forging, the surface temperature of the forging is 955℃. The forging is then mist-cooled, rotating 90 degrees every half hour. When the surface temperature of the forging is 32℃ after mist cooling, it is air-cooled. After air cooling for 40 hours, the forging is transferred to a low-temperature cooling furnace for cold treatment at a cooling temperature of -20℃. It is then held at this temperature for 10 hours. After holding, it is removed from the furnace and air-cooled until the surface temperature of the forging reaches room temperature. Then, the forging is loaded into a heating furnace.

[0037] Step 2) After Step 1) is completed, perform a strengthening treatment: the furnace temperature is 190℃, and the temperature is increased to 650℃ at a rate of 80℃ / h and held for 3 hours. After the holding period, the temperature is increased to 1020℃ at a rate of 80℃ / h and held for 10 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is 65 seconds. The forging is placed in a water tank for water cooling for 60 minutes (the water temperature is controlled at 21.5℃). Then, the forging is lifted out and air cooled for 83 seconds. The forging is then placed in a water tank for water cooling for 40 minutes. The forging is then lifted out and air cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -78℃ and held for 10 hours. The forging is then removed from the furnace and reheated until the surface temperature of the forging reaches 35℃. Finally, the forging is loaded into the heating furnace.

[0038] Step 3) After step 2) is completed, perform a tempering: the furnace temperature is 270℃, and the temperature is increased to 420℃ at a rate of 80℃ / h. Hold the temperature for 15h, remove the forging from the furnace and air cool it to room temperature. Then, put the forging into the heating furnace.

[0039] Step 4) After step 3) is completed, perform a secondary strengthening treatment: the furnace temperature is 350℃, and the temperature is increased to 650℃ at a rate of 80℃ / h and held for 3 hours. After the holding period, the temperature is increased to 1020℃ at a rate of 80℃ / h and held for 10 hours. Then, the forging is removed from the furnace and cooled. The air cooling time is 55 seconds. The forging is placed in a water tank for water cooling for 60 minutes (the water temperature is controlled at 20.8℃). Then, the forging is lifted out and air cooled for 85 seconds. The forging is then placed in a water tank for water cooling for 40 minutes. The forging is then lifted out and air cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -78℃ and held for 10 hours. The forging is then removed from the furnace and reheated until the surface temperature of the forging reaches 31℃. Finally, the forging is loaded into the heating furnace.

[0040] Step 5) After step 4) is completed, perform a second tempering: the furnace temperature is 278℃, and the temperature is increased to 420℃ at a rate of 80℃ / h. The temperature is held for 15 hours, and the furnace is then air-cooled to room temperature.

[0041] The test results after production according to the above heat treatment process are shown in Table 2:

[0042] Table 2 Test Results

[0043] Technical Requirements Yield strength ≥ 1000 MPa Tensile strength ≥1200 MPa Elongation ≥14% Area shrinkage rate / Impact energy AKV≥70J Grain size ≥ 4 Actual testing 1020 1290 16 58 117 / 117 Levels 4-6

[0044] After production using the heat treatment process of the present invention to improve the coarse grain size of Cr16Ni6 stainless steel, the performance and grain size test results meet the requirements.

Claims

1. A heat treatment process for improving the coarse grain size of Cr16Ni6 series stainless steel, characterized in that: It is carried out according to the following process: Step 1) Post-forging cooling treatment: After forging, if the surface temperature of the forging is ≥900℃, the forging is subjected to mist cooling or air cooling, rotating 90 degrees every half hour. When the surface temperature of the forging is ≤40℃ after mist cooling or air cooling, it is subjected to air cooling for 24h~100h. After air cooling, the forging is transferred to a low-temperature cooling furnace for cold treatment at a cooling temperature of -10℃~-30℃. It is held at this temperature for 7h~30h. After holding, it is removed from the furnace and air-cooled until the surface temperature of the forging reaches room temperature. Step 2) After completing Step 1), perform the first strengthening treatment in Step 2): The furnace temperature is ≤400℃. Increase the temperature to 550℃~700℃ at a rate of ≤80℃ / h and hold for 3~8 hours. After holding, increase the temperature to 1000℃~1060℃ at a rate of 50℃~100℃ / h and hold for 5~20 hours. Then remove the forging from the furnace and cool it. The air cooling time should be ≤100 seconds. Place the forging in a water tank. Water cooling is performed for 10 to 100 minutes, with the water temperature controlled at 0℃ to 25℃. Then, the forging is lifted out and air-cooled for 30 to 300 seconds. The forging is then placed in a water tank and water-cooled for 10 to 50 minutes. The forging is then lifted out and air-cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -60℃ to -100℃ and held for 5 to 20 hours. After being taken out of the furnace, the forging is reheated until the surface temperature of the forging reaches 20℃ to 40℃. Step 3) After completing Step 2), perform a tempering in Step 3): The furnace temperature is ≤400℃, and the temperature is increased to 400℃~500℃ at a heating rate of 50℃ / h~100℃ / h. Hold the temperature for 5h~40h, and then air cool to room temperature after removing from the furnace. Step 4) After completing Step 3), perform the secondary strengthening treatment in Step 4): The furnace temperature is ≤400℃. Increase the temperature to 550℃~700℃ at a rate of ≤80℃ / h and hold for 3~8 hours. After holding, increase the temperature to 1000℃~1060℃ at a rate of 50℃~100℃ / h and hold for 5~20 hours. Then remove the forging from the furnace and cool it. The air cooling time should be ≤100 seconds. Place the forging in water. The forging is cooled in water for 10 to 100 minutes, with the water temperature controlled at 0℃ to 25℃. Then, the forging is lifted out and air-cooled for 30 to 300 seconds. The forging is then placed back into the water tank and water-cooled for 10 to 50 minutes. The forging is then lifted out and air-cooled to room temperature. After reaching room temperature, it is transferred to a low-temperature furnace at -60℃ to -100℃ and held for 5 to 20 hours. After being taken out of the furnace, the forging is reheated until the surface temperature of the forging reaches 20℃ to 40℃. Step 5) After completing Step 4), perform the second tempering in Step 5): The furnace temperature is ≤400℃, and the temperature is increased to 400℃~500℃ at a heating rate of 50℃ / h~100℃ / h. Hold the temperature for 5h~40h, and then air cool to room temperature after removing from the furnace.