A controlled rolling and controlled cooling process applied to 1960mpa grade spring steel 55sicr

Abstract

The application discloses a controlled rolling and controlled cooling process applied to 1960MPa-grade spring steel 55SiCr, which comprises a heating and dephosphorization stage and a controlled cooling stage; the surface and core temperature of the spring steel is ensured to be uniform when entering and discharging from a finishing mill through water cooling, the high point temperature of the rolled piece in the rolling process is controlled by controlling the temperature of the entering and discharging finishing mill, and the rolling load of the equipment and the low-temperature rolling requirement are balanced; the strength and toughness of the material is maximally improved by controlling the cooling speed of the air cooling line in the wire rod temperature rising stage; the spring steel finished product is uniformly organized, the austenite grain size is not less than 8, the tensile strength is controlled in the range of 1050-1100MPa, the surface reduction is in the range of 45-60%, the pearlite interlamellar spacing is controlled in the range of 200nm, the above process can balance the equipment load, and the industrialized batch stable production is realized.

Description

Technical Field

[0001] This invention relates to a controlled cooling process for high-strength spring steel, specifically a controlled rolling and controlled cooling process applied to 1960MPa grade spring steel 55SiCr. Background Technology

[0002] With the development of lightweight automotive springs, higher requirements have been placed on the strength and toughness of materials. Generally, the strength of 55SiCr automotive suspension springs after heat treatment is required to be in the range of 1860MPa to 1950MPa, with a shrinkage of area of ​​more than 40%. High-strength automotive suspension springs, on the other hand, are required to have a strength in the range of 1960MPa to 2050MPa after heat treatment, with a shrinkage of area of ​​more than 40%. This places higher demands on the strength and toughness of materials, especially the grain size of rolled material, tensile strength in hot-rolled state, shrinkage of area, and pearlite lamellar spacing.

[0003] To meet the performance requirements of high-strength spring materials, low-temperature rolling is necessary during production to control grain growth and achieve fine-grained strengthening. However, low-temperature rolling significantly increases the load on the rolling mill, increasing the risk of accidents such as roll box burnout. Therefore, one of the main research directions is how to refine the grains through low-temperature rolling while reducing equipment load to achieve stable mass production quality. Simultaneously, controlling the air-cooling rate in the air-cooling line to reduce the pearlite lamellar spacing to within 200 nm is crucial for improving the material's tensile strength and area reduction in the hot-rolled state. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a controlled rolling and cooling process for high-strength spring steel 55SiCr that meets the performance requirements for high-strength spring materials. This process requires strict control of rolling temperature during production, balancing equipment load to achieve stable batch production, and controlling the cooling rate in the air-cooling line to improve the tensile strength and area shrinkage of the hot-rolled material. The specific solution is as follows: A controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr involves implementing controlled rolling and controlled cooling on the rolled piece, comprising the following three stages. During the heating and descaling stage, the temperature of the heating furnace is controlled, the temperature of the soaking zone in the furnace is controlled in the range of 1020 to 1070℃, the initial rolling temperature of the rolled piece is controlled in the range of 910 to 960℃, and the descaling pressure is controlled in the range of 15 to 18MPa. In the controlled cooling stage, before entering the finishing mill, the temperature of the rolled piece is controlled between 900 and 920°C by water cooling process, so that the rolled piece enters the finishing mill in the austenite non-recrystallization region. After the rolled piece exits the finishing mill, the temperature is controlled between 920 and 950°C, and then the rolled piece is cooled to a uniform temperature of 830 to 860°C by water cooling process before entering the wire drawing machine. During the wire rod reheating stage, after the rolled wire is produced, the wire rod temperature is rapidly reduced to 680-700℃ using the Stellmore cooling control process, and then the wire rod enters the wire rod reheating stage. During the reheating stage, the temperature is controlled to cool down to 650-680℃ before entering the heat preservation stage. The cooling rate inside the heat preservation hood is in the range of 1±0.5℃ / S, so that the pearlite lamellar spacing of the rolled wire is controlled within the range of 200nm.

[0005] Furthermore, during the wire rod warming stage, the insulation cover is closed after the wire rod temperature reaches the range of 650-680°C, and the insulation cover is fully opened before this temperature range.

[0006] Furthermore, during the controlled cooling stage, the water flow rate of the finishing mill stand is controlled to be 80-90% of the total flow rate.

[0007] Furthermore, during the wire rod warming-up stage, the wire rod temperature drops rapidly at a rate of 4–12℃ / s after spinning to reach the warming-up stage.

[0008] Furthermore, during the wire rod warming stage, the cooling rate is controlled at -2 to 2℃ / s.

[0009] Furthermore, the temperature of the rolled piece after exiting the insulation cover is controlled below 600℃.

[0010] Compared with the prior art, the advantages of the present invention are as follows: 1. This invention controls the temperature in the heating and descaling stages and the controlled cooling stage, so that the rolled piece is rolled in the non-recrystallized austenite region, without having to be rolled in the lower-temperature two-phase region, thus reducing the load on the equipment and achieving stable industrial-scale mass production.

[0011] 2. This invention controls the cooling rate of the wire rod during the reheating stage by using a Stellmore air-cooling line, ensuring that "the wire rod temperature drops rapidly at a rate of 4-12℃ / s after coiling to the reheating stage, with the cooling rate controlled at -2 to 2℃ / s during the reheating stage, and the temperature enters the heat preservation stage after reaching 650-680℃." This greatly improves the strength and toughness of the material in the hot-rolled state. By controlling the material cooling rate, the pearlite lamellar spacing is controlled within 200nm, the austenite grain size is not lower than grade 8, and the strength and toughness of the material also increase as the pearlite lamellar spacing decreases, with tensile strength controlled at 1050-1100MPa and area reduction reaching 45-60%. Attached Figure Description

[0012] Figure 1 Photographs showing tissue uniformity; Figure 2 A photograph showing austenite grain size of level 8; Figure 3 This is a photograph taken when the interlayer spacing of pearlite is controlled within the range of 200nm. Implementation

[0013] The present invention will be described in more detail below with reference to examples. These examples are merely descriptions of the best embodiments of the invention and do not limit the scope of the invention in any way. Example

[0014] like Figures 1 to 3 As shown, a controlled rolling and cooling process for 1960MPa grade spring steel 55SiCr is described. The furnace temperature is controlled at 1020℃, the initial rolling temperature is 910℃, and the descaling pressure is controlled at 15MPa. Before entering the finishing mill, the workpiece is cooled by water cooling. Specifically, the workpiece is cooled below two sets of water tanks located before the finishing mill, controlling the temperature of the workpiece entering the finishing mill at 900℃. This ensures the workpiece enters the finishing mill within the non-recrystallized austenite region. The water flow rate in the finishing mill stand is controlled at 80% of the total flow rate. The temperature of the workpiece exiting the finishing mill is controlled at 900℃. At 20℃, the rolled piece passes under five sets of water tanks located between the finishing mill and the wire rod mill. The water flow of the five sets of water tanks is evenly distributed, so that the rolled piece is uniformly cooled to 830℃. After the rolled piece is wire rod, the Stellmore cooling control process rapidly cools the wire rod to 700℃ at a rate of 4-10℃ / s. Then the wire rod enters the warming stage, and the cooling rate is controlled at -2 to 2℃ / s. After the wire rod temperature reaches 670℃, it enters the heat preservation stage under the heat preservation hood. After the wire rod temperature reaches 670℃, the heat preservation hood is closed. Before this temperature range, the heat preservation hood is fully open. The cooling rate inside the heat preservation hood is in the range of 0.5℃ / s, and the temperature outside the heat preservation hood is controlled below 600℃.

[0015] The rolled material produced by this process has a uniform microstructure, with austenite grain size of not less than grade 8, tensile strength controlled at 1050 MPa, area shrinkage of 45%, and pearlite lamellar spacing of 198 nm. After quenching and tempering heat treatment, the wire rod has a tensile strength of 1970 MPa and an area shrinkage of 42%, meeting the user's strength requirements of 1960 MPa to 2050 MPa. Example

[0016] like Figures 1 to 3As shown, a controlled rolling and cooling process for 1960MPa grade spring steel 55SiCr is described. The furnace temperature is controlled at 1040℃ in the soaking zone, the initial rolling temperature is 940℃, and the descaling pressure is controlled at 16MPa. The temperature of the rolled piece entering the finishing mill is controlled at 905℃, the water flow rate of the finishing mill stand is controlled at 84% of the total flow rate, and the temperature of the rolled piece exiting the finishing mill is controlled at 930℃. After uniform cooling in a water tank, the temperature is reduced to 840℃ before wire drawing. A Stellmore cooling control process is used to rapidly reduce the wire rod temperature to 685℃ at a rate of 6–12℃ / s. After the wire rod warms up, the cooling rate is controlled at -2–2℃ / s. Once the wire rod temperature reaches 660℃, it enters the insulation hood insulation stage. After the wire rod temperature reaches 660℃, the insulation hood is closed. Before this temperature range, the insulation hood is fully open, and the cooling rate inside the insulation hood is within the range of 0.8℃ / s. The temperature exiting the insulation hood is controlled below 600℃.

[0017] The rolled material produced by this process has a uniform microstructure, with an austenite grain size of not less than grade 8, a tensile strength controlled at 1070 MPa, a shrinkage of 50%, and a pearlite lamellar spacing of 188 nm. After quenching and tempering heat treatment, the wire rod has a tensile strength of 1990 MPa and a shrinkage of 46%, meeting the user's strength requirements of 1960 MPa to 2050 MPa. Example

[0018] like Figures 1 to 3 As shown, a controlled rolling and cooling process for 1960MPa grade spring steel 55SiCr is described. The furnace temperature is controlled at 1060℃ in the soaking zone, the initial rolling temperature is 950℃, and the descaling pressure is controlled at 17MPa. The temperature of the rolled piece entering the finishing mill is controlled at 915℃, the water flow rate in the finishing mill stand is controlled at 88% of the total flow rate, and the temperature of the rolled piece exiting the finishing mill is controlled at 945℃, then uniformly cooled to 850℃ in a water tank. After the wire is drawn, the wire rod is cooled to 695°C at a uniform rate of 8-12°C / s using the Stellmore cooling control process. After entering the wire rod warming stage, the cooling rate is controlled at -2 to 2°C / s. When the wire rod temperature reaches 670°C, it enters the insulation stage. After the wire rod temperature reaches 670°C, the insulation hood is closed. Before this temperature range, the insulation hood is fully open. The cooling rate inside the insulation hood is in the range of 1.2°C / s, and the temperature outside the insulation hood is controlled below 600°C.

[0019] The rolled material produced by this process has a uniform microstructure, with an austenite grain size of not less than grade 8, a tensile strength controlled at 1090 MPa, a shrinkage of area of ​​55%, and a pearlite lamellar spacing of 180 nm. After quenching and tempering heat treatment, the wire rod has a tensile strength of 2010 MPa and a shrinkage of area of ​​48%, meeting the user's strength requirements of 1960 MPa to 2050 MPa. Example

[0020] like Figures 1 to 3As shown, a controlled rolling and cooling process for 1960MPa grade spring steel 55SiCr is described. The furnace temperature is controlled at 1070℃ in the soaking zone, the initial rolling temperature is 960℃, and the descaling pressure is controlled at 18MPa. The temperature of the rolled piece entering the finishing mill is controlled at 920℃, the water flow rate in the finishing mill stand is controlled at 90% of the total flow rate, and the temperature of the rolled piece exiting the finishing mill is controlled at 950℃. After uniform cooling in a water tank, the temperature is reduced to 860℃ before being discharged. After the wire is rolled out, the wire rod is cooled to 700°C at a constant rate of 8-12°C / s using the Stellmore cooling control process. After entering the wire rod warming stage, the cooling rate is controlled at -2 to 2°C / s. After the wire rod temperature drops to 680°C, it enters the insulation stage. The insulation hood is closed after the wire rod temperature reaches 680°C. Before this temperature range, the insulation hood is fully open. The cooling rate inside the insulation hood is in the range of 1.5°C / s. The temperature outside the insulation hood is controlled below 600°C.

[0021] The rolled material produced by this process has a uniform microstructure, with austenite grain size of not less than grade 8, tensile strength controlled at 1100 MPa, area reduction of 60%, and pearlite lamellar spacing within 170 nm. After quenching and tempering heat treatment, the wire rod has a tensile strength of 2020 MPa and an area reduction of 50%, meeting the strength requirements of 1960 MPa to 2050 MPa. Example

[0022] In the existing technology, the heating system of the heating furnace is controlled as follows: the temperature of the soaking zone is controlled at 1100℃, the initial rolling temperature of the rolled piece is 990℃, and the descaling pressure is controlled at 15MPa; the temperature entering the finishing mill is 950℃, the water volume of the finishing mill stand is unlimited, the temperature exiting the finishing mill is controlled at 1050℃, and the water tank in the area from the finishing mill to the wire rod spinning machine is not limited; the temperature of the wire rod spinning machine is controlled at 830℃, and the fan is controlled to reduce the temperature of the wire rod at a uniform speed of 4-8℃ / S to 680℃. After entering the wire rod reheating stage, the fan stops blowing, and the wire rod reheats to about 750℃. When the wire rod temperature reaches 740℃, it enters the heat preservation stage under the heat preservation hood. The cooling rate inside the heat preservation hood is 0.5℃ / S, and the temperature exiting the heat preservation hood is controlled below 650℃.

[0023] The austenite grain size obtained through this process is grade 6, the tensile strength is controlled at 950-1050 MPa, the area shrinkage is 35-45%, and the pearlite lamellar spacing is about 280 nm. After quenching and tempering heat treatment, the wire rod has a tensile strength of 1900 MPa and an area shrinkage of 40%, which only meets the requirement of 1860 MPa-1950 MPa.

[0024] Compared with the above-mentioned solutions, the present invention has the advantages of realizing stable industrial-scale mass production, greatly improving the strength and toughness of the material in the hot-rolled state, avoiding the formation of martensite and other structures, controlling the tensile strength to 1050-1100MPa, reducing the area by 45-60%, and controlling the pearlite lamellar spacing to within 200nm.

[0025] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr, characterized in that, The controlled rolling and controlled cooling process for the rolled pieces includes the following three stages: heating and descaling stage, in which the temperature of the heating furnace is controlled, the temperature of the soaking zone in the furnace is controlled in the range of 1020-1070℃, the initial rolling temperature of the rolled piece is controlled in the range of 910-960℃, and the descaling pressure is controlled in the range of 15-18MPa; in the controlled cooling stage, before entering the finishing mill, the temperature of the rolled piece is controlled between 900-920℃ by water cooling process, so that the rolled piece enters the finishing mill in the austenite non-recrystallization region, and the temperature of the rolled piece after exiting the finishing mill is controlled between 920-950℃, and then the rolled piece is cooled evenly to the range of 830-860℃ by water cooling process before entering the wire drawing machine; In the wire rod reheating stage, after the rolled wire is wired, the wire rod temperature is rapidly reduced to 680-700℃ through the Stellmore cooling control process, and then the wire rod enters the wire rod reheating stage. In the reheating stage, the temperature is controlled to be cooled to 650-680℃ and then enters the heat preservation stage. The cooling rate in the heat preservation hood is in the range of 1±0.5℃ / S, so that the pearlite lamellar spacing of the rolled wire is controlled within the range of 200nm. During the reheating stage of the wire rod, the cooling rate is controlled between -2 and 2℃ / s.

2. The controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr according to claim 1, characterized in that, During the wire rod warming stage, the insulation cover is closed after the wire rod temperature reaches the range of 650-680°C, and the insulation cover is fully opened before this temperature range.

3. The controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr according to claim 1, characterized in that, During the controlled cooling stage, the water flow rate of the finishing mill stand is controlled at 80-90% of the total flow rate.

4. The controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr according to claim 1, characterized in that, During the reheating stage of the wire rod, the temperature of the wire rod drops rapidly at a rate of 4-12℃ / s after spinning to reach the reheating stage.

5. The controlled rolling and controlled cooling process for 1960MPa grade spring steel 55SiCr according to claim 1, characterized in that, The temperature of the rolled piece after exiting the insulation cover is controlled below 600℃.

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