A forging method for high Cr heat-resistant alloy steel round billet

Abstract

The present invention relates to the field of high Cr heat-resistant alloy steel round billet forging. A forging method for a high Cr heat-resistant alloy steel round billet is controlled in four links. The specific control conditions of each link are as follows: in the first link, a step-by-step slow heating is adopted to ensure the uniformity of the heating temperature; in the second link, each end face of the high Cr heat-resistant alloy steel round billet is locally spot welded; in the third link, a low-speed, small-pressure segmented upsetting is adopted during upsetting; in the fourth link, a "head-forging" forging process of forging the head first is adopted for elongation after upsetting. After production using the forging process of the present invention, the eccentricity problem of the round billet forging is significantly improved, and the flaw detection and inspection pass rate of the finished forgings meet the user's use requirements.

Description

Technical Field

[0001] The invention relates to the field of high-Cr heat-resistant alloy steel round billet forging. Background Art

[0002] High Cr heat-resistant alloy steel is mainly used for key components of super (ultra) critical thermal power units and for making high temperature and high pressure pipeline materials for thermal power generation. Representative grades include P91, P92, etc. This series of varieties belongs to high alloy steel, with an average Cr content of about 9% and Mo content of about 1%. It is an alloy steel with poor thermal conductivity. If the heating temperature is improper, it is very easy to cause internal and external temperature gradients during forging, forming stress concentration and leading to forging cracks. The tube billet manufacturing process generally adopts die-cast steel ingots or continuous casting billet forging production. According to the normal free forging process requirements, the height-to-diameter ratio of varieties with an alloy content of more than 5% during upsetting is less than 2.5. When it is higher than 2.5, not only is the upsetting deformation resistance large, but it is also very easy to bend or even crack during upsetting. In some special needs, the height-to-diameter of the high Cr heat-resistant alloy steel round billet reaches more than 3.7, which is 1.5 times the conventional height-to-diameter ratio, which is a great challenge to the forging process requirements.

[0003] The process flow of producing high Cr heat-resistant forged tube billets is: continuous casting round billet → heating → forging upsetting → drawing and forming. In a certain demand, the maximum diameter of high Cr heat-resistant alloy steel round billet (continuous casting round billet) is 690 rounds, and the finished tube billet is required to weigh more than 8 tons. Therefore, the height-to-diameter ratio of high Cr heat-resistant forged tube billets is more than 4, which is far more than the 2.5 height-to-diameter ratio required by the free forging process. Such a large height-to-diameter ratio will not only cause severe bending during forging, but also may cause cracking defects due to uneven deformation, making the product unable to be forged and scrapped or downgraded.

[0004] In view of the above situation, in order to prevent the center eccentricity or cracking of the high Cr heat-resistant tube billet after forging, it is necessary to study and improve the upsetting and drawing process under the condition of ultra-large aspect ratio. Summary of the invention

[0005] The technical problem to be solved by the present invention is: how to improve the upsetting and drawing forging process of high Cr heat-resistant alloy steel round billet (height-to-diameter ratio of more than 4, diameter greater than 650mm) and solve the problems of upsetting and drawing cracking and eccentricity.

[0006] The technical solution adopted by the present invention is: a forging method for a high-Cr heat-resistant alloy steel round billet. For a high-Cr heat-resistant alloy steel round billet with a diameter greater than 650mm and a height-to-diameter ratio greater than 4, during forging, a study is conducted on how to avoid the eccentricity during the upsetting process causing the center line of the tube billet to deviate, resulting in the inability to forge through the inside. Considering that the raw material height-to-diameter ratio is greater than 4, it far exceeds the 2.5 height-to-diameter ratio requirement for free forging upsetting, and the normal process upsetting cannot be performed. Severe bending will occur during the upsetting process, and due to the high alloy content, there is a central crack inside the round billet itself. The two end faces are easily oxidized during the heating process. The two end faces cannot be welded during the upsetting process and the central crack will further expand due to the large force. At the same time, the central crack will extend inward during subsequent drawing.

[0007] Control is carried out in four links. The specific control situation of each link is as follows

[0008] In the first stage, a step-by-step slow heating method is used to ensure uniform heating temperature.

[0009] In the second step, each end face of the high Cr heat-resistant alloy steel round billet is locally spot welded;

[0010] The third step is to use low speed and small pressure for segmented upsetting during upsetting;

[0011] The fourth step is to forge the steel piece by using the “baotou” forging process, which forges the head first, after upsetting.

[0012] In the first step, a step-by-step slow heating method is used to ensure the uniformity of the heating temperature, which means that the temperature is first added to 400°C and kept warm for 3 hours, then the temperature is increased to 950±10°C in the organizational transformation zone at a rate of less than or equal to 100°C / h, and kept warm for 3 hours, and the temperature is increased to the highest temperature zone of 1250±10°C at a rate of less than or equal to 120°C / h, and kept warm for more than 4 hours.

[0013] The second step, performing local spot welding on each end face of the high Cr heat-resistant alloy steel round billet, means that before forging, a welding joint is formed by spot welding on each end face of the high Cr heat-resistant alloy steel round billet, and the projection of the welding joint on each end face covers a circle with a diameter of 100 mm and the center of the end face as the center.

[0014] The third step, using low-speed, small-pressure segmented upsetting during upsetting, means that upsetting is carried out in the form of upsetting + straightening + flat head + upsetting. Each time the upsetting is performed, the downset is 50-60mm. After upsetting to a height of 10%, the flat anvil is used for lengthening and straightening. The upsetting height of a single fire is not more than 40% of the total height. During the upsetting process, the high-Cr heat-resistant alloy steel round billet is heated and kept warm again when the temperature is equal to 1000℃. During heating, the temperature is increased to 1250±10℃ at a rate of less than or equal to 120℃ / h, and then upsetting is performed again until the target diameter is reached.

[0015] After each upsetting, if there is obvious bending in the high Cr heat-resistant alloy steel round billet, it will not be stretched first. Instead, it will be returned to the furnace and heated to 1250±10℃ at a rate of less than or equal to 120℃ / h. After keeping warm for 1 hour, it will be straightened by flat anvil stretching after being taken out of the furnace. After straightening, the round billet will be stood up and the upper and lower end faces will be pressed down before the next upsetting.

[0016] The fourth step, the upsetting followed by drawing, adopts the "head-wrapping" forging process of forging the head first, which means that the heads at both ends are forged first, and after the heads are rounded, they are drawn from the middle to form, and the final forging temperature is greater than 980°C.

[0017] The beneficial effects of the present invention are as follows: after the forging process of the present invention is adopted, the eccentricity problem of round billet forging is significantly improved, and the flaw detection and inspection pass rate of finished forgings meet the use requirements of users. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is the process curve of the step-by-step slow heating process of the present invention. DETAILED DESCRIPTION

[0019] Example 1

[0020] The forging of P92 heat-resistant alloy steel forgings was carried out on a 3150-ton hydraulic press. The raw material was ￠690*3000mm, and the forged product specification was ￠480 round. After the forging process was optimized and improved, the round billet had no eccentricity and had good surface quality.

[0021] A forging method for a high-Cr heat-resistant alloy steel round billet, wherein the high-Cr heat-resistant alloy steel round billet has a diameter of 690 mm, a length of 3000 mm, and a height-to-diameter ratio greater than 4.

[0022] During forging, research was conducted on how to avoid eccentricity during upsetting, which would cause the center line of the tube billet to shift and the inability to forge through the inside. Considering that the raw material height-to-diameter ratio is greater than 4, it is far beyond the 2.5 height-to-diameter ratio requirement for free forging upsetting, and it is impossible to upset according to the normal process. Severe bending will occur during the upsetting process, and due to the high alloy content, there is a center crack inside the round billet itself. The two end faces are easily oxidized during the heating process. The two end faces cannot be welded during the upsetting process, and the center crack will further expand due to the large force. At the same time, the center crack will extend inward during subsequent drawing.

[0023] Control is carried out in four links. The specific control situation of each link is as follows

[0024] In the first stage, a step-by-step slow heating method is used to ensure uniform heating temperature.

[0025] like Figure 1As shown, first add it to 400℃, keep it warm for 3 hours, then heat it to the structure transformation zone of 950±10℃ at a rate of less than or equal to 100℃ / h, keep it warm for 3 hours, heat it to the highest temperature zone of 1250±10℃ at a rate of less than or equal to 120℃ / h, and keep it warm for 4-6 hours.

[0026] In the second step, local spot welding is performed on each end face of the high-Cr heat-resistant alloy steel round billet to alleviate the problem of flame heating and oxidation along the central crack during the heating process.

[0027] Spot welding is performed locally on each end face of the high Cr heat-resistant alloy steel round billet, which means that before forging, a welding joint is formed by spot welding on each end face of the high Cr heat-resistant alloy steel round billet, and the projection of the welding joint on each end face covers a circle with a diameter of 100 mm and the center of the end face as the center.

[0028] The third step is to use low speed and small pressure to perform segmented upsetting during upsetting to prevent severe bending caused by upsetting once, which may cause the center line of the billet to deviate or the center crack to further crack. The low speed here refers to the minimum speed of the forging machine during forging.

[0029] The low-speed, small-pressure segmented upsetting during upsetting means that upsetting is carried out in the form of upsetting + straightening + flat head + upsetting. Each time the upsetting is performed, the downset is 50-60mm. After upsetting to a height of 10%, the flat anvil is used for lengthening and straightening. The upsetting height of a single fire is not more than 40% of the total height. During the upsetting process, the high-Cr heat-resistant alloy steel round billet is heated and kept warm again when the temperature is equal to 1000℃. During heating, the temperature is increased to 1250±10℃ at a rate of less than or equal to 120℃ / h, and then upsetting is performed again until the target diameter is reached.

[0030] After each upsetting, if there is obvious bending in the high Cr heat-resistant alloy steel round billet, it will not be stretched first. Instead, it will be returned to the furnace and heated to 1250±10℃ at a rate of less than or equal to 120℃ / h. After keeping warm for 1 hour, it will be straightened by flat anvil stretching after being taken out of the furnace. After straightening, the round billet will be stood up and the upper and lower end faces will be pressed down before the next upsetting.

[0031] The fourth step is to use the "head-wrapped" forging process to forge the head first after upsetting (forging the two ends first and then the middle during forming and drawing, and the conventional process is to forge from the middle to both ends). This prevents internal cracks from extending from the head to the inside during the drawing process.

[0032] The forging process of forging the head first after upsetting is called "baotou" forging process, which means that the heads at both ends are forged first, and then the heads are rounded, and then the drawing and forming begins from the middle. The final forging temperature is 1000℃.

[0033] By adopting a step-by-step slow heating system, optimizing the high alloy round billet forging process, adding the raw material end face welding process, and adopting the upsetting + flat head + upsetting and "head-wrapping" forging methods, the problem of tube billet forging eccentricity is solved, and the stable production of this series of products is achieved. It meets the user's control requirements for the quality of the core of large-size high Cr heat-resistant alloy steel tube billets.

Claims

1. A forging method for a high Cr heat-resistant alloy steel round billet, characterized in that: For high Cr heat-resistant alloy steel round billet with a diameter greater than 650mm and a height-to-diameter ratio greater than 4, four links are controlled during forging. The specific control conditions of each link are as follows: The first step is to use step-by-step slow heating to ensure uniform heating temperature. The step-by-step slow heating to ensure uniform heating temperature means first heating to 400°C, keeping warm for 3 hours, then heating to the organizational transition zone of 950±10°C at a rate of less than or equal to 100°C / h, keeping warm for 3 hours, and then heating to the highest temperature zone of 1250±10°C at a rate of less than or equal to 120°C / h, keeping warm for more than 4 hours. The second step is to perform local spot welding on each end face of the high Cr heat-resistant alloy steel round billet. The local spot welding on each end face of the high Cr heat-resistant alloy steel round billet means that before forging, a welding joint is formed by spot welding on each end face of the high Cr heat-resistant alloy steel round billet. The projection of the welding joint on each end face covers a circle with a diameter of 100 mm and the center of the end face as the center of the circle. The third link is to use low-speed, small-pressure segmented upsetting during upsetting. The low-speed, small-pressure segmented upsetting during upsetting means that upsetting is carried out by upsetting + straightening + flat head + upsetting. Each time upsetting, the downset is 50-60mm. After upsetting to 10% height, the flat anvil is stretched and straightened. The upsetting height of a single fire is not more than 40% of the total height. During the upsetting process, the high-Cr heat-resistant alloy steel round billet is heated and kept warm again when it is equal to 1000°C. During heating, the temperature is raised to 1250±10°C at a rate of less than or equal to 120°C / h, and then upsetting is carried out again until the target diameter is reached. The fourth step is to use the "Baotou" forging process to forge the head first after upsetting. The "Baotou" forging process to forge the head first after upsetting means that the heads at both ends are forged first, and then the heads are rounded, and then drawn from the middle to form the final forging temperature is greater than 980°C.

2. The forging method of a high Cr heat-resistant alloy steel round billet according to claim 1, characterized in that: After each upsetting, if there is obvious bending in the high Cr heat-resistant alloy steel round billet, it is not stretched first, but returned to the furnace and heated to 1250±10°C at a rate of less than or equal to 120°C / h. After keeping warm for 1 hour, it is taken out of the furnace and straightened by flat anvil stretching. After straightening, the round billet is stood up and the upper and lower end faces are pressed down first before the next upsetting.

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