Method for improving surface quality of 20CrNiMo forging

Abstract

The application relates to a method for improving the surface quality of 20CrNiMo forgings, which can effectively improve the surface color difference problem of the 20CrNiMo forgings and improve the product quality. Through high-temperature heating, the composition homogenization is effectively promoted, and the composition segregation phenomenon is reduced. The two roughing + thinning methods and the interchanging of the thickness and width dimensions of the forging are adopted to effectively ensure the compaction effect of the forgings, sufficiently break the coarse organization of the steel ingot, and break the internal dendritic segregation of the steel ingot in the one roughing + thinning deformation + high-temperature homogenization + two roughing + thinning deformation mode for the first time, shorten the diffusion distance between the dendrites, and further promote the organization and composition homogenization, so that the surface color difference quality problem of the forgings is effectively improved. Through the heat treatment after forging, the forging organization is not only refined, but also the internal stress of the forging is removed, so that the mechanical property result reaches the standard requirement.

Description

Technical Field

[0001] This invention belongs to the field of heat treatment technology for metallic materials, and specifically relates to a method for improving the surface quality of 20CrNiMo forgings, thereby improving product quality and meeting market demands. Background Technology

[0002] In actual production, due to the low alloy content of 20CrNiMo steel, coarse interdendritic segregation easily forms during the cooling process after casting. Conventional forging deformation methods such as direct upsetting followed by flat anvil drawing or direct flat anvil drawing cannot effectively break up the coarse interdendritic segregation of the steel ingot. As a result, when the surface quality of the forgings is inspected after rough machining, surface color difference is found, and the surface quality is unqualified, resulting in scrap. In particular, 20CrNiMo round bars or stepped shafts with an outer diameter of φ200mm~φ500mm are more prone to surface color difference after rough machining, and the proportion is relatively high, which has seriously affected product production. Therefore, for 20CrNiMo round bars or stepped shafts with an outer diameter of φ200mm~φ500mm, there is an urgent need for a method to effectively improve the surface color difference of forgings and improve the surface quality of forgings. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a process technology to improve the surface quality of 20CrNiMo forgings. This effectively improves the color difference problem on the surface of 20CrNiMo forgings, enhances product quality, and meets market demands.

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

[0005] A method for improving the surface quality of 20CrNiMo forgings is carried out according to the following steps:

[0006] Step 1) High temperature heating: The steel ingot is placed at 600℃~750℃ and heated to 1200℃~1250℃ at a heating rate of 50℃ / h~100℃ / h. After holding at this temperature for 2h~30h, it is taken out of the furnace for forging and deformation.

[0007] Step 2) Forging Deformation: Use a 5000t hydraulic press or a 3150t hydraulic press. Utilize the upper and lower platforms for upsetting to 1 / 2H height of the ingot. Then, use a 500mm or 800mm anvil to draw it to a flat shape with a width of 2 / 3R and a thickness of 1 / 2R. After flattening, return it to the furnace for heating to 1200℃~1260℃ and hold for 15h~30h. Then, use a 5000t or 3150t hydraulic press for upsetting to 1 / 2H height. Then, use a 500mm or 800mm anvil to draw it to a flat shape with a width of 2 / 3R and a thickness of 1 / 2R. Exchange the width and thickness of the flat shape with the first upsetting and drawing. After flattening, forge it back to square, then turn it into an octagon. Finally, forge it on a precision forging machine. Control the reduction in each pass to 40mm~80mm to produce the finished product.

[0008] Step 3) Annealing: After forging, place the forging in an air-cooling zone and air-cool it until the surface temperature of the forging is 500~600℃. Then put it into a heating furnace at 400℃~600℃ and heat it to 600℃~700℃ at a heating rate of 30℃ / h~100℃ / h. Hold it at this temperature for 2h~10h and then take it out of the furnace and air-cool it until the surface temperature of the forging is 280~320℃. Put it into a heating furnace at 280℃~350℃ and hold it for 2h~25h. Then heat it to 600℃~700℃ at a heating rate of 30℃ / h~100℃ / h and hold it for 20h~60h. Then cool it to 400℃~450℃ at a cooling rate of ≤50℃ / h and then cool it to ≤200℃ at a cooling rate of ≤30℃ / h. Take it out of the furnace and air-cool it to room temperature.

[0009] Step 4) High-temperature normalizing: The forging is placed in a heating furnace with a furnace temperature ≤500℃ and heated to 550℃~650℃ at a heating rate of 50℃ / h~100℃ / h. After holding at this temperature for 2h~10h, the temperature is increased to 900℃~930℃ at a heating rate of 50℃ / h~100℃ / h. After holding at this temperature for 5h~30h, the forging is removed from the furnace and air-cooled, or air-cooled, or mist-cooled. The forging is cooled to a surface temperature of 500℃~600℃ and then placed in a heating furnace to prepare for quenching.

[0010] Step 5) Quenching: When loading the forging into the furnace, the furnace temperature is controlled at 500℃~600℃. After holding at this temperature for 2h~10h, the temperature is increased to 880℃~900℃ at a rate of 50℃ / h~100℃ / h. After holding at this temperature for 5h~30h, the forging is removed from the furnace and quenched. The pre-cooling time after removing the forging is ≤80s. Then, the forging is placed in a water tank with an initial water temperature of ≤28℃ and cooled with a cooling intensity of 4R~15R, where R is the outer diameter or wall thickness. During cooling, the forging moves up and down at least 3 times per minute. The forging is cooled to a surface temperature of 80℃~150℃ before being loaded into the furnace.

[0011] Step 6) First high-temperature tempering: When the forging is loaded into the furnace, the furnace temperature is controlled at ≤250℃. The temperature is increased to 250℃~300℃ at a heating rate of ≤50℃ / h. After holding at this temperature for 2h~10h, the temperature is increased to 590℃~650℃ at full power. After holding at this temperature for 5h~30h, the forging is removed from the furnace and air-cooled. After air-cooling, a second high-temperature tempering is performed.

[0012] Step 7) Secondary high-temperature tempering: When loading the forgings into the furnace, the furnace temperature is controlled at ≤450℃, and the temperature is raised to 550℃~630℃ at full power. After holding at this temperature for 5h~30h, the forgings are removed from the furnace and air-cooled.

[0013] Compared with the prior art, the process of this invention has the following advantages:

[0014] According to the present invention, a process for improving the surface quality of 20CrNiMo forgings includes: 1. High-temperature heating effectively promotes compositional homogenization and reduces compositional segregation. 2. For the first time, a two-stage upsetting and flattening process, along with interchangeable forging thickness and width dimensions, effectively ensures the compaction effect of the forging and fully breaks down the coarse structure of the ingot. 3. For the first time, a process of one-stage upsetting + flattening deformation + high-temperature homogenization + two-stage upsetting + flattening deformation is used to break down dendrite segregation within the ingot, shortening the diffusion distance between dendrites and further promoting the homogenization of the microstructure and composition, thereby effectively improving the surface color difference quality of the forging. 4. Post-forging heat treatment not only refines the microstructure of the forging but also removes internal stress, ensuring that its mechanical properties meet standard requirements. The 20CrNiMo forgings produced according to the present invention exhibit significantly improved surface quality, meeting market demands. Detailed Implementation

[0015] Example 1: A process for improving the surface quality of 20CrNiMo forgings.

[0016] Steel grade: 20CrNiM; Chemical composition: C=0.21%, Si=0.35%, Mn=0.86%, Cr=0.65%, Ni=0.49%, S=0.002%, P=0.013%, Mo=0.21%; Specifications: Hollow tube with diameter Φ360*Φ180mm.

[0017] The specific steps are as follows:

[0018] Step 1) High temperature heating: The steel ingot is placed at 700℃ and heated to 1230℃ at a heating rate of 80℃ / h. It is held at this temperature for 5.5h and then forged and deformed.

[0019] Step 2) After Step 1) is completed, perform forging deformation: Use a 3150t hydraulic press, use the upper and lower platforms to uplift to H=650mm, use 500mm upper and lower flat anvils to draw to 750mm*530mm, beveling, and then heat in the furnace to 1250℃ and hold for 16 hours; use the upper and lower platforms to uplift to H=650mm, use 500mm upper and lower flat anvils to draw to 530mm*750mm (interchange with the width and thickness of the first uplift and drawing), straighten and draw to 500mm, beveling to 520mm, and control the reduction of each pass to 50mm to finish the forging;

[0020] Step 3) After step 2) is completed, the forging is loaded into the heating furnace for annealing: After forging, the forging is first placed in the air cooling zone and air-cooled until the surface temperature of the forging is 550℃. Then it is loaded into a 600℃ heating furnace and heated to 650℃ at a heating rate of 50℃ / h. After holding at this temperature for 3 hours, it is removed from the furnace and air-cooled until the surface temperature of the forging is 300℃. Then it is loaded into a 320℃ heating furnace and held for 7 hours. Then it is heated to 650℃ at a heating rate of 50℃ / h and held for 30 hours. Then it is cooled to 410℃ at a cooling rate of 40℃ / h and then cooled to 190℃ at a cooling rate of 20℃ / h. Finally, it is removed from the furnace and air-cooled to room temperature.

[0021] Step 4) After step 3) is completed, the forging is loaded into the heating furnace and subjected to high-temperature normalizing: the forging is loaded into the heating furnace at a furnace temperature of 400℃, heated to 600℃ at a heating rate of 80℃ / h, held for 3 hours, heated to 910℃ at a heating rate of 80℃ / h, held for 6 hours, and then removed from the furnace and air-cooled until the surface temperature of the forging is 550℃.

[0022] Step 5) After step 4) is completed, the forging is loaded into the heating furnace and quenched: When the forging is loaded into the furnace, the furnace temperature is controlled at 550℃. After holding for 2 hours, the temperature is increased to 900℃ at a heating rate of 80℃ / h. After holding for 6 hours, the forging is taken out of the furnace for quenching. After taking out of the furnace, the pre-cooling time is 60s. Then, it is placed in a water tank with an initial water temperature of 25℃ and cooled with a cooling intensity of 10.5R wall thickness. During cooling, it moves up and down 4 times per minute until the surface temperature of the forging is 130℃.

[0023] Step 6) After step 5) is completed, the forging is loaded into the heating furnace for a first high-temperature tempering: when the forging is loaded into the furnace, the furnace temperature is controlled at 230℃, and the temperature is increased to 270℃ at a heating rate of 50℃ / h. After holding at the temperature for 2 hours, the temperature is increased to 620℃ at full power. After holding at the temperature for 10 hours, the forging is removed from the furnace and air-cooled. After air-cooling, a second high-temperature tempering is performed.

[0024] Step 7) After step 6) is completed, the forging is loaded into the heating furnace for a second high-temperature tempering: the furnace temperature is controlled at 350℃ when the forging is loaded into the furnace, and the temperature is raised to 600℃ at full power. After holding at the temperature for 10 hours, the forging is taken out of the furnace and air-cooled.

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

[0026] Table 1 Test Results

[0027] After production using the process method of the present invention to improve the surface quality of 20CrNiMo forgings, the performance and surface quality test results meet the requirements.

[0028] Example 2: A process for improving the surface quality of 20CrNiMo forgings.

[0029] Steel grade: 20CrNiMo; Chemical composition: C=0.21%, Si=0.35%, Mn=0.86%, Cr=0.65%, Ni=0.49%, S=0.002%, P=0.013%, Mo=0.21%; Specifications: Hollow tube with diameter Φ350*Φ160mm.

[0030] The specific steps are as follows:

[0031] Step 1) High temperature heating: The steel ingot is placed at 700℃ and heated to 1230℃ at a heating rate of 80℃ / h. It is held at this temperature for 5.5h and then forged and deformed.

[0032] Step 2) After Step 1) is completed, perform forging deformation: Use a 3150t hydraulic press, use the upper and lower platforms to uplift to H=650mm, use 500mm upper and lower flat anvils to draw to 750mm*530mm, slightly beveling, and then heat in the furnace to 1250℃ and hold for 16 hours; use the upper and lower platforms to uplift to H=650mm, use 500mm upper and lower flat anvils to draw to 530mm*750mm (interchange with the width and thickness of the first uplift and drawing), straighten and draw to 500mm square, beveling to 520mm octagon, and control the reduction of each pass to 50mm to finish the forging;

[0033] Step 3) After step 2) is completed, the forging is loaded into the heating furnace for annealing: After forging, the forging is first placed in the air cooling zone and air-cooled until the surface temperature of the forging is 555℃. Then it is loaded into a 600℃ heating furnace and heated to 650℃ at a heating rate of 50℃ / h. After holding at this temperature for 3 hours, it is removed from the furnace and air-cooled until the surface temperature of the forging is 305℃. Then it is loaded into a 310℃ heating furnace and held for 7 hours. Then it is heated to 650℃ at a heating rate of 50℃ / h and held for 30 hours. Then it is cooled to 405℃ at a cooling rate of 40℃ / h and then cooled to 195℃ at a cooling rate of 20℃ / h. Finally, it is removed from the furnace and air-cooled to room temperature.

[0034] Step 4) After step 3) is completed, the forging is loaded into the heating furnace and subjected to high-temperature normalizing: the forging is loaded into the heating furnace at a furnace temperature of 350℃, heated to 600℃ at a heating rate of 80℃ / h, held for 3 hours, heated to 910℃ at a heating rate of 80℃ / h, held for 6 hours, and then removed from the furnace and air-cooled until the surface temperature of the forging is 560℃.

[0035] Step 5) After step 4) is completed, the forging is loaded into the heating furnace and quenched: When the forging is loaded into the furnace, the furnace temperature is controlled at 530℃. After holding for 2 hours, the temperature is increased to 900℃ at a heating rate of 80℃ / h. After holding for 6 hours, the forging is taken out of the furnace for quenching. After taking out of the furnace, the pre-cooling time is 55s. Then, it is placed in a water tank with an initial water temperature of 26℃ and cooled with a cooling intensity of 10.5R wall thickness. During cooling, it moves up and down 4 times per minute until the surface temperature of the forging is 128℃.

[0036] Step 6) After step 5) is completed, the forging is loaded into the heating furnace for a first high-temperature tempering: when the forging is loaded into the furnace, the furnace temperature is controlled at 240℃, and the temperature is increased to 270℃ at a heating rate of 50℃ / h. After holding at the temperature for 2 hours, the temperature is increased to 620℃ at full power. After holding at the temperature for 10 hours, the forging is removed from the furnace and air-cooled. After air-cooling, a second high-temperature tempering is performed.

[0037] Step 7) After step 6) is completed, the forging is loaded into the heating furnace for a second high-temperature tempering: the furnace temperature is controlled at 350℃ when the forging is loaded into the furnace, and the temperature is raised to 600℃ at full power. After holding at the temperature for 10 hours, the forging is taken out of the furnace and air-cooled.

[0038] The test results after production according to the above heat treatment process are shown in Table 2:

[0039] Table 2 Test Results

[0040] After production using the process method of the present invention to improve the surface quality of 20CrNiMo forgings, the performance and surface quality test results meet the requirements.

Claims

1. A method for improving the surface quality of a 20CrNiMo forging, characterized in that: Is carried out according to the following steps: Step 1), high temperature heating: the ingot is heated at 600-750 DEG C, and is heated to 1200-1250 DEG C at a heating rate of 50-100 DEG C / h, and is kept for 2-30 h before being taken out for forging deformation; Step 2), forging deformation: a 5000 t oil press or a 3150 t oil press is selected, and the upper and lower platforms are used for roughing, and the ingot is roughed to 1 / 2H height, and then a 500 mm upper and lower flat anvil or an 800 mm upper and lower flat anvil is used for elongation to a flat square of 2 / 3R width*1 / 2R thickness, and after the flat square is drawn, the furnace is heated to 1200-1260 DEG C, and is kept for 15-30 h, and then the furnace is taken out, a 5000 t oil press or a 3150 t oil press is selected, and the upper and lower platforms are used for roughing, and the roughing is to 1 / 2H height, and then a 500 mm upper and lower flat anvil or an 800 mm upper and lower flat anvil is used for elongation to a flat square of 2 / 3R width*1 / 2R thickness, and the width and thickness are interchanged with the size after the first roughing and elongation, and the flat square is forged back to a square, and an inverted octagonal square is formed, and then a fine forging machine is used for forging, and the reduction per pass is controlled to be 40-80 mm, and the finished product is fine forged out; Step 3), annealing: after forging, the forged piece is first placed in an air cooling zone, and is air cooled to a surface temperature of 500-600 DEG C, and is loaded into a 400-600 DEG C heating furnace, and is heated to 600-700 DEG C at a heating rate of 30-100 DEG C / h, and is kept for 2-10 h before being taken out for air cooling, and is air cooled to a surface temperature of 280-320 DEG C, and is loaded into a 280-350 DEG C heating furnace and kept for 2-25 h, and then is heated to 600-700 DEG C at a heating rate of 30-100 DEG C / h, and is kept for 20-60 h, and is cooled to 400-450 DEG C at a cooling rate of ≤50 DEG C / h, and is cooled to ≤200 DEG C at a cooling rate of ≤30 DEG C / h, and is taken out for air cooling to room temperature; Step 4), high temperature normalizing: the forged piece is loaded into a heating furnace with a furnace temperature ≤500 DEG C, and is heated to 550-650 DEG C at a heating rate of 50-100 DEG C / h, and is kept for 2-10 h, and is heated to 900-930 DEG C at a heating rate of 50-100 DEG C / h, and is kept for 5-30 h, and is taken out for air cooling or air cooling or mist cooling, and is cooled to a surface temperature of 500-600 DEG C, and is loaded into a heating furnace for quenching; Step 5), quenching: when the forged piece is loaded into the furnace, the heating furnace is controlled at a temperature of 500-600 DEG C, and is kept for 2-10 h, and is heated to 880-900 DEG C at a heating rate of 50-100 DEG C / h, and is kept for 5-30 h before being taken out for quenching, and after being taken out, the precooling time is ≤80 s, and then the forged piece is placed in a water tank with an initial water temperature ≤28 DEG C for cooling, and is moved up and down at least 3 times per minute during cooling, and is cooled to a surface temperature of 80-150 DEG C, and is loaded into a heating furnace; Step 6), one high temperature tempering: when the forging is loaded into the furnace, the furnace temperature is controlled at ≤250℃, the temperature is raised to 250℃~300℃ at a temperature rising speed of ≤50℃ / h, after 2h~10h of heat preservation, the temperature is raised to 590℃~650℃ at full power, after 5h~30h of heat preservation, the forging is taken out of the furnace and air cooled, and after the air cooling is completed, secondary high temperature tempering is carried out; Step 7), secondary high temperature tempering: when the forging is loaded into the furnace, the furnace temperature is controlled at ≤450℃, the temperature is raised to 550℃~630℃ at full power, after 5h~30h of heat preservation, the forging is taken out of the furnace and air cooled.

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