Heat treatment process for improving overall quality of SJB-4 forge piece

Abstract

The invention discloses a heat treatment process for improving the overall quality of an SJB-4 forge piece, which comprises the following steps: loading the forge piece into a high-temperature heating furnace for primary high-temperature normalizing, then loading the forge piece into the high-temperature heating furnace for secondary high-temperature normalizing, then loading the forge piece into the high-temperature heating furnace for quenching, and then loading the forge piece into a tempering heating furnace for high-temperature tempering, the temperature difference between the surface and the core is reduced through stepped heating, so that the heating stress is reduced, and deformation or cracking caused by overlarge stress is avoided; tissue homogenization is promoted by adopting a two-time high-temperature normalizing mode, rapid cooling is performed after normalizing heating, surface and core pearlite tissue transformation is promoted, and mixed crystals or coarse crystal grains brought by non-equilibrium tissues caused by tissue heredity are cut off; through rapid heating and repeated austenitizing, pearlite with small interlamellar spacing is promoted to be converted into an austenite structure, the nucleation rate is increased, and the grain size is refined; and the quenching cooling strength is enhanced to guarantee the hardenability depth, the austenite-martensite structure from the surface to the core of the forging is promoted, and the quenching uniformity of the forging is improved.

Description

Technical Field

[0001] The invention relates to a heat treatment process for solving the problem of coarse grain or mixed grain caused by structural inheritance of Ni-containing steel and improving comprehensive mechanical properties such as strength, plasticity and toughness after quenching and tempering to improve the overall quality of SJB-4 forgings. Background Art

[0002] In actual production, since the SJB-4 material contains a high amount of Ni, reaching 2.5% to 4%, the Ni steel with this content is prone to organizational inheritance during production, forming coarse grains or mixed grains, and at the same time forming directional needle-shaped ferrite group undesirable organization. The above problems are more prominent for SJB-4 material forgings with a diameter or wall thickness of more than 300 mm.

[0003] For example, when producing large-sized forgings made of SJB-4 material with a wall thickness of 500mm, due to the large number of forging times, the forging blank is repeatedly heated and kept warm at high temperature, resulting in tissue inheritance in the forging blank, forming a directional coarse structure and grain size, and the grain size is basically between level 0 and level 3. If the conventional production process is followed: normalizing + tempering or tempering, it is difficult to effectively improve the coarse grain size or mixed crystal phenomenon caused by tissue inheritance, and it is difficult to match the strength, plasticity, toughness and hardness well, and it is impossible to meet the use requirements, resulting in certain waste and affecting the delivery progress. Therefore, there is an urgent need for a heat treatment process that can improve the coarse grain size or mixed crystal of SJB-4 material and improve the overall mechanical properties.

[0004] Patent application number: 201310258738.3, invention name: 30CrNi3MoV forging heat treatment method, the method is through the first normalizing, the first supercooling, the second normalizing, the second supercooling, the complete annealing and the tempering treatment, that is, the 30CrNi3MoV heat treatment method is through the process of two normalizing, two supercooling and complete annealing after forging, completely cut off the tissue inheritance, achieve the effect of grain refinement, make the material potential fully utilized, completely cut off the tissue inheritance, and thus achieve the ideal effect of eliminating mixed crystals. It does not disclose that through two high-temperature normalizing, the rapid cooling method is adopted after the furnace is taken out to promote the transformation of the pearlite structure on the surface and the core, cut off the tissue inheritance caused by the high Ni content, and then through the tempering process, strengthen the cooling effect, promote the transformation of the austenite from the surface to the core of the forging to the martensite structure, improve the overall quality of the forging, and achieve a better match between strength and plasticity, toughness and hardness. This application also cannot eliminate the problems of mixed crystals and coarse grains caused by tissue inheritance, and cannot shorten the process time and reduce the manufacturing cost. Summary of the invention

[0005] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a heat treatment process method for improving the overall quality of SJB-4 forgings, which can solve the coarse grain or mixed grain problem caused by the structural inheritance of Ni-containing steel and improve the comprehensive mechanical properties such as strength, plasticity, toughness, etc. after quenching and tempering, thereby improving the overall quality of the product.

[0006] To achieve the above-mentioned purpose, the present invention adopts the following technical solution: a heat treatment process for improving the overall quality of SJB-4 forgings, the process steps of which are as follows:

[0007] Step 1), the forging is loaded into a high-temperature heating furnace, and the forging is subjected to a high-temperature normalizing: the temperature of the heating furnace is ≤450°C when the furnace is loaded, and the temperature is increased to 600°C-700°C at a heating rate of 10°C / h-100°C / h, and the temperature is increased to 900°C-920°C at full power after being kept for 2h-5h, and the temperature is kept for 5h-20h, and the forging is taken out of the furnace for air cooling after being kept for 2h-5h. During the air cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air cooling, and air cooling is performed after the air cooling is completed, and the surface temperature of the forging is cooled to ≤100°C;

[0008] Step 2), after step 1) is completed with air cooling, the forging is loaded into a high-temperature heating furnace and subjected to secondary high-temperature normalizing: the temperature of the heating furnace is ≤450°C when loaded, and the temperature is increased to 600°C-700°C at a heating rate of 10°C / h-100°C / h, and then the temperature is increased to 880°C-895°C at full power for 5h-20h after being kept at a temperature for 2h-5h, and then the forging is taken out of the furnace for air cooling for 2h-5h. During the air cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air cooling, and air cooling is performed after the air cooling is completed until the surface temperature of the forging is ≤100°C;

[0009] Step 3), after step 2) air cooling is completed, the forging is loaded into a high-temperature heating furnace for quenching: when loading the furnace, the temperature of the heating furnace is ≤450°C, and the temperature is increased to 600°C~700°C at a heating rate of 10°C / h~100°C / h. After keeping the temperature for 2h~5h, the temperature is increased to 855°C~875°C at full power, and after keeping the temperature for 5h~20h, the forging is taken out of the furnace for quenching and cooling. Precooling is performed for 100s~200s, and then the forging is placed in a water tank at a water temperature of 20°C~30°C for cooling for 300s~800s. After the water cooling is completed, the forging is placed in an oil tank at an oil temperature of 30°C~70°C for oil cooling for 60min~200min;

[0010] Step 4), after the quenching and cooling in step 3) is completed, the forging is loaded into the tempering heating furnace and high-temperature tempering is performed: the heating furnace temperature is ≤450°C when loading the furnace, and the temperature is increased to 550°C~650°C at a heating rate of 10°C / h~100°C / h. After keeping the temperature for 10h~50h, the forging is taken out of the furnace and air-cooled to room temperature.

[0011] Compared with the prior art, the present invention has the following advantages:

[0012] According to the present invention, a heat treatment process for improving the overall quality of SJB-4 forgings has the following characteristics:

[0013] 1. Through step heating, the temperature difference between the surface and the core is reduced, thereby reducing the heating stress and avoiding deformation or cracking caused by excessive stress.

[0014] 2. Use two high-temperature normalizing methods to promote organizational homogenization. After normalizing heating, perform appropriate rapid cooling to promote the transformation of pearlite organization on the surface and in the core, and cut off the mixed crystal or coarse grain phenomenon caused by the unbalanced organization caused by organizational inheritance.

[0015] 3. Through rapid heating and repeated austenitization, the pearlite with smaller interlamellar spacing is promoted to transform into austenite structure, the nucleation rate is increased, and the grain size is refined.

[0016] 4. Strengthen the quenching cooling intensity, ensure the hardenability depth, promote the transformation of austenite from the surface to the core of the forging to martensite, and improve the overall quenching uniformity of the forging.

[0017] 5. High temperature tempering ensures the overall stability of the forgings, so that the strength, plasticity, toughness and hardness are well matched, meeting the use requirements.

[0018] 6. The process cycle of the present invention is short, which not only reduces the manufacturing cost but also improves the production efficiency. The SJB-4 forgings produced according to the present invention not only improve the overall quality but also speed up the production efficiency to meet the market demand.

[0019] The heat treatment process technology of the present invention can effectively improve the problem of coarse grain size or mixed crystals. After heat treatment, the grain size reaches level 5 to 7, the mechanical property strength is above 1000Mpa, the impact energy toughness at room temperature is above 80J, and the impact energy at -40℃ low temperature is above 42J. Not only is the product quality improved and the waste loss reduced, but it also meets market demand and creates economy. DETAILED DESCRIPTION

[0020] Example 1: A heat treatment process for improving the overall quality of SJB-4 forgings, steel type: SJB-4; chemical composition: C = 0.31%, Si = 0.17%, Mn = 0.53%, Cr = 1.26%, Ni = 3.48%, S = 0.005%, P = 0.013%, V = 0.08%, Mo = 0.51%; specification: square billet of 580mm*520mm*1610mm; heat treatment process is as follows:

[0021] Step 1), the forging is loaded into a high-temperature heating furnace, and the forging is subjected to a high-temperature normalizing: the furnace temperature is 270°C when the furnace is loaded, and the temperature is increased to 640°C to 660°C at a heating rate of 80°C / h to 100°C / h. After keeping the temperature for 3 hours, the temperature is increased to 900°C to 920°C at full power and kept for 12 hours. After keeping the temperature, the forging is taken out of the furnace for air cooling for 2 hours. During the air cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air cooling. After the air cooling is completed, air cooling is performed to cool the forging surface to 73°C to 80°C;

[0022] Step 2) After step 1) is completed with air cooling, the forging is loaded into a high-temperature heating furnace and subjected to secondary high-temperature normalizing: the temperature of the heating furnace is 315°C when loaded, and the temperature is increased to 640°C to 660°C at a heating rate of 80°C / h to 100°C / h. After being kept at this temperature for 3 hours, the temperature is increased to 880°C to 890°C at full power and kept at this temperature for 12 hours. After being kept at this temperature, the forging is taken out of the furnace and air-cooled for 3 hours. During the air-cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air-cooling. After the air-cooling is completed, air-cooling is performed to cool the forging surface to 65°C to 80°C.

[0023] Step 3) After step 2) is completed with air cooling, the forging is loaded into a high-temperature heating furnace for quenching: the furnace temperature is 230°C when loaded, and the temperature is increased to 640°C to 660°C at a heating rate of 80°C / h to 100°C / h. After keeping the temperature for 3 hours, the temperature is increased to 860°C to 870°C at full power. After keeping the temperature for 12 hours, the forging is taken out of the furnace for quenching and cooling. Pre-cool for 120 seconds, then place the forging in a water tank at a water temperature of 25°C for 700 seconds. After the water cooling is completed, place the forging in an oil tank at an oil temperature of 36°C for oil cooling for 105 minutes.

[0024] Step 4) After the quenching in step 3) is completed, the forging is placed in a tempering heating furnace for high-temperature tempering: the furnace temperature is 277°C when the furnace is loaded, and the temperature is increased to 600°C~610°C at a heating rate of 80°C / h~100°C / h. After keeping the temperature for 23 hours, the forging is taken out of the furnace and air-cooled to room temperature.

[0025] After production according to the above heat treatment process, the test results are shown in Table 1:

[0026] Table 1 Test results

[0027]

[0028] After the heat treatment process method for improving the overall quality of SJB-4 forgings is used, the mechanical properties and grain size test results meet the requirements.

[0029] Example 2: A heat treatment process for improving the overall quality of SJB-4 forgings; Steel type: SJB-4; Chemical composition: C = 0.31%, Si = 0.26%, Mn = 0.57%, Cr = 1.12%, Ni = 3.41%, S = 0.005%, P = 0.013%, V = 0.08%, Mo = 0.56%; Specification: 580mm*520mm*1610mm square billet; Heat treatment process is as follows:

[0030] Step 1), primary high-temperature normalizing: the furnace temperature is 269°C when the furnace is loaded, and the temperature is increased to 640°C to 660°C at a heating rate of 80°C / h to 100°C / h. After keeping the temperature for 3 hours, the temperature is increased to 900°C to 920°C at full power and kept for 12 hours. After keeping the temperature, the forging is taken out of the furnace for air cooling for 2 hours. During the air cooling, the forging should be rotated 90 degrees every half an hour to ensure uniform air cooling. After the air cooling is completed, air cooling is performed to cool the forging surface temperature to 71°C to 85°C;

[0031] Step 2), after step 1) is completed with air cooling, the forging is loaded into a high-temperature heating furnace and subjected to secondary high-temperature normalizing: the furnace temperature is 315°C when loaded, and the temperature is increased to 640°C to 660°C at a heating rate of 80°C / h to 100°C / h. After keeping the temperature for 3 hours, the temperature is increased to 880°C to 890°C at full power and kept for 12 hours. After keeping the temperature, the forging is taken out of the furnace for air cooling for 3 hours. During the air cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air cooling. After the air cooling is completed, air cooling is performed to cool the forging surface to 55°C to 86°C;

[0032] Step 3), after step 2) air cooling is completed, the forging is loaded into a high-temperature heating furnace for quenching: the furnace temperature is 230°C when loading, and the temperature is increased to 640°C~660°C at a heating rate of 80°C / h~100°C / h. After keeping warm for 3 hours, the temperature is increased to 860°C~870°C at full power. After keeping warm for 12 hours, the forging is taken out of the furnace for quenching and cooling. Pre-cooling is performed for 119 seconds, and then the forging is placed in a water tank with a water temperature of 25°C for cooling for 700 seconds. After the water cooling is completed, the forging is placed in an oil tank with an oil temperature of 36°C for oil cooling for 105 minutes.

[0033] Step 4), after the quenching in step 3) is completed, the forging is loaded into the tempering heating furnace for high-temperature tempering: the furnace temperature is 256°C when loading, and the temperature is increased to 600°C~610°C at a heating rate of 80°C / h~100°C / h. After keeping the temperature for 23 hours, it is taken out of the furnace and air-cooled to room temperature.

[0034] After production according to the above heat treatment process, the test results are shown in Table 2:

[0035] Table 2 Test results

[0036]

[0037] After the heat treatment process method for improving the overall quality of SJB-4 forgings is used, the mechanical properties and grain size test results meet the requirements.

Claims

1. A heat treatment process for improving the overall quality of SJB-4 forgings, characterized by: The process steps are as follows: Step 1), load the forging into a high-temperature heating furnace, and perform high-temperature normalizing: when loading the furnace, the furnace temperature is ≤450℃, and the temperature is increased to 600℃~700℃ at a heating rate of 10℃ / h~100℃ / h. After keeping the temperature for 2h~5h, the temperature is increased to 900℃~920℃ at full power and kept for 5h~20h. After keeping the temperature, take the forging out of the furnace for air cooling, which lasts for 2h~5h. During the air cooling, the forging should be rotated 90 degrees every half an hour to ensure uniform air cooling. After the air cooling, air cooling is performed until the surface temperature of the forging is ≤100℃; Step 2), after step 1) is completed with air cooling, the forging is loaded into a high-temperature heating furnace and subjected to secondary high-temperature normalizing: the heating furnace temperature is ≤450℃ when loaded, and the temperature is raised to 600℃~700℃ at a rate of 10℃ / h~100℃ / h, and then heated to 880℃~895℃ at full power for 5h~20h after being kept at temperature for 2h~5h. After being kept at temperature, the forging is taken out of the furnace for air cooling for 2h~5h. During air cooling, the forging is rotated 90 degrees every half an hour to ensure uniform air cooling. After air cooling, air cooling is performed until the surface temperature of the forging is ≤100℃; Step 3), after step 2) air cooling is completed, the forging is loaded into a high-temperature heating furnace for quenching: when loading the furnace, the temperature of the heating furnace is ≤450℃, and the temperature is increased to 600℃~700℃ at a heating rate of 10℃ / h~100℃ / h. After keeping the temperature for 2h~5h, the temperature is increased to 855℃~875℃ at full power. After keeping the temperature for 5h~20h, the forging is taken out of the furnace for quenching and cooling. Precool for 100s~200s, and then put the forging into a water tank at a water temperature of 20℃~30℃ for cooling for 300s~800s. After the water cooling is completed, the forging is placed in an oil tank at an oil temperature of 30℃~70℃ for oil cooling for 60min~200min; Step 4) After the quenching and cooling in step 3) is completed, the forging is loaded into the tempering heating furnace for high temperature tempering: the furnace temperature is ≤450℃ when loading, and the temperature is increased to 550℃~650℃ at a heating rate of 10℃ / h~100℃ / h. After keeping the temperature for 10h~50h, it is taken out of the furnace and air-cooled to room temperature.

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