A method for manufacturing a marine low-temperature steel plate using explosive technique

Abstract

A kind of low-temperature steel plate for ship is provided with the following chemical components by mass percentage: C=0.05%~0.09%, Si=0.10%~0.30%, Mn=1.20%~1.80%, P≤0.008%, S≤0.002%, Al=0.05%~0.08%, Ti≤0.005%, Nb≤0.020%, N=0.0050%~0.0080%, B=0.0010%~0.0025%, and the balance is Fe and inevitable impurities.The steel plate is produced by TMCP process to obtain fine equiaxed ferrite and a small amount of pearlite structure, which has good strength and toughness: yield strength≥355MPa, tensile strength≥490MPa, elongation≥21%, and impact energy at-60℃≥180J.The microstructure of the steel plate after heat treatment is mainly composed of fine lath bainite, granular bainite and tempered sorbite with fine lath or short rod ferrite as the matrix, and the strength and low-temperature impact toughness are not lower than those of the base material, which fully meets the requirements of low-temperature steel and heat treatment for shipbuilding such as LPG ship.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel processing, and relates to a pyrotechnic process method for a marine low-temperature steel plate. BACKGROUND

[0002] In recent years, marine low-temperature steels are continuously challenged to serve in more extreme low-temperature environments, such as LPG marine low-temperature steels that need to withstand at least minus 60 degrees Celsius. For such steels, stable control of performance is a great challenge, especially in the process of shipbuilding, the steel plate often needs to be corrected and bent by pyrotechnics, or to eliminate adverse stress concentration, but the strength and impact toughness of the steel plate after pyrotechnics often decrease significantly. Therefore, the composition and organization design of such extreme environment serving marine low-temperature steels and the control of pyrotechnic process are crucial.

[0003] Chinese patent CN114807762A discloses a 300MPa grade low-temperature steel with excellent low-temperature toughness, the chemical composition of which is as follows in terms of percentage by weight: C: 0.10%~0.15%, Si: 0.20%~0.40%, Mn: 1.4%~1.7%, Ni: 0.30%~0.80%, Al: 0.015%~0.030%, S: ≤0.005%, P: ≤0.008%, the balance being Fe and impurities; wherein 10C+Mn: 2.45%~3%. The steel meets the low-temperature toughness requirement at-80℃, but the steel does not involve pyrotechnic performance, and the yield strength thereof is only 300MPa grade. Chinese patent CN110629111A discloses a marine low-temperature steel plate with excellent pyrotechnic performance and a manufacturing method thereof, the main chemical components of which are as follows in terms of mass percentage: C: 0.04%~0.08%, Si: 0.15%~0.25%, Mn: 0.80%~1.80%, P ≤0.012%, S ≤0.002%, Als: 0.045%~0.070%, N ≤0.0040%, Ti: 0.005%~0.020%, the balance being Fe and inevitable impurities. The steel is produced by TMCP process to obtain a refined ferrite + a small amount of pearlite structure, the yield strength thereof before and after pyrotechnics is above 355MPa, and the-60℃ impact absorption energy is above 120J, but to ensure the performance of the steel plate, the number of pyrotechnics is controlled to be within 2~3 times. SUMMARY

[0004] The present patent aims to provide a pyrotechnic process method for a marine low-temperature steel plate, through the composition and organization design of the marine low-temperature steel, the strength and low-temperature impact toughness thereof after pyrotechnics are not less than those of the base material by using the pyrotechnic method.

[0005] The technical scheme of the present application is as follows:

[0006] A heat treatment process for marine low-temperature steel plates, wherein the steel's chemical composition (mass percentage) is: C = 0.05%~0.09%, Si = 0.10%~0.30%, Mn = 1.20%~1.80%, P ≤ 0.008%, S ≤ 0.002%, Al = 0.05%~0.08%, Ti ≤ 0.005%, Nb ≤ 0.020%, N = 0.0050%~0.0080%, B = 0.0010%~0.00 25%, with the balance being Fe and unavoidable impurities; the steel plate is produced using the TMCP process, with the billet heating temperature at 1100~1150℃ and the heating time within this temperature range ≤120min, controlling the cumulative deformation in both the recrystallization and non-recrystallization zones to be ≥90%, controlling the final rolling temperature at 780±20℃, and then accelerating cooling to 650±20℃ at a cooling rate of 3~5℃ / s after rolling, followed by air cooling to room temperature; the aforementioned fire-working process includes the following steps:

[0007] (1) Determine the heating area of ​​the workpiece and mark the boundary line; then prepare two suction welding torches, the heating nozzle diameter of the No. 1 welding torch is 3.0mm and the heating nozzle diameter of the No. 2 welding torch is 1.5mm; at the same time prepare a water pipe with a water flow rate of 80~120ml / s.

[0008] (2) First, use welding torch No. 1 to heat the area, using acetylene as fuel and a neutral flame. During the heating process, control the welding torch tilt angle to be no less than 60° and the flame core distance to be ≤3mm. At the same time, use an infrared thermometer to monitor the flame channel temperature. When the surface temperature of the heated area reaches 800±10℃, stop increasing the temperature and allow it to cool slowly. When the surface temperature of the area cools down to 700±10℃, use welding torch No. 2 to reheat the area, using propane as fuel and a neutral or lightly carburized flame. During the heating process, the welding torch tilt angle should be controlled at 30~60° and the flame core distance to be 2~3mm until the target temperature is reached. The target temperature should be higher than 800℃ but not higher than 950℃. Then stop heating and start air cooling.

[0009] (3) After heating, deform it to meet the target shape requirements; if it cannot meet the requirements, repeat step (2) until the deformation meets the requirements.

[0010] (4) Heat the original flame channel again according to step (2). After heating to the target temperature, air cool for a period of time, and then use a water pipe to cool it down; control the water-fire distance between 50 and 80 mm.

[0011] (5) After cooling to room temperature, use No. 2 welding torch to heat the original flame channel again. The fuel is propane. Use a neutral flame, control the welding torch tilt angle between 20 and 40°, and the flame core distance is 3 to 4 mm. Heat to 650±20℃, and then air cool to room temperature.

[0012] The principle of this invention:

[0013] The marine low-temperature steel described in this invention employs a low-carbon, low-phosphorus, and low-sulfur composition, with a higher addition of aluminum and nitrogen, and a design that limits titanium and niobium while adding boron. The low carbon content results in a matrix structure primarily composed of fine equiaxed ferrite after the steel plate is produced using the TMCP process. Furthermore, the matrix structure obtained after the steel plate undergoes the fire-forming process described in this invention is also primarily composed of fine lath-like or short rod-like ferrite. This fine ferrite plays a crucial role in achieving high low-temperature toughness. Low phosphorus content improves the steel's cold brittleness, while low sulfur content reduces strip-shaped MnS inclusions and improves toughness. The control of trace elements such as aluminum, nitrogen, titanium, niobium, and boron is designed to form a larger number of dispersed BN and AlN precipitates. These particles play a vital role in controlling grain coarsening during the fire-forming process, thereby ensuring the strength and toughness of the low-temperature steel after fire-forming.

[0014] The marine low-temperature steel plate described in this invention is produced using the TMCP process. This process employs a lower heating temperature and avoids prolonged heating, while ensuring sufficient deformation in the recrystallization zone. This is to obtain uniform and fine initial austenite grains, which limits the size of any abnormal grain growth that may occur during subsequent heat treatment. Controlling the final rolling temperature and ensuring the cumulative deformation in the non-recrystallization zone, along with accelerated cooling after rolling, is to obtain a fine ferrite structure through deformation-induced ferrite phase transformation, thereby improving the strength and toughness of the steel plate.

[0015] The pyrotechnic process described in this invention employs two different welding torches for heating. The first stage of heating utilizes a higher flame energy (e.g., using acetylene as fuel and a large-diameter welding nozzle) and a larger torch tilt angle to concentrate the flame. Therefore, the workpiece temperature rises rapidly in the early stages, increasing heating efficiency and facilitating subsequent deformation. The second stage of heating is the opposite, with lower heating efficiency, resulting in a slower heating rate during the ferrite-to-austenite transformation. This avoids microstructural inheritance and promotes the acquisition of fine, uniform austenite grains. Furthermore, heating to the austenite region first, followed by slow cooling in the two-phase region and reheating to the austenite region effectively increases the nucleation rate of the austenite blank and reduces the tendency for austenite grain growth, further contributing to the acquisition of fine, uniform austenite grains. Following this heating method, even with repeated heating to meet deformation requirements, severe coarsening of the austenite grains will not occur. After the workpiece is deformed, it is heated again to austenitize it, and then water-cooled to cause the transformation of lath bainite and lath martensite. After that, it is reheated for tempering to transform it into supersaturated ferrite with almost no carbon. The carbides are also distributed in fine granular or thin film form. This multiphase structure has high strength and toughness.

[0016] The beneficial effects of this invention are as follows: The marine low-temperature steel described in this invention is carbon-manganese steel, requiring no addition of expensive alloying elements such as Ni, Mo, and Cr, resulting in low alloy cost. By controlling microalloying elements and employing the TMCP process, the steel plate exhibits excellent strength and toughness: yield strength ≥355MPa, tensile strength ≥490MPa, elongation ≥21%, and impact absorption energy at -60℃ ≥180J. Using the flame-working process described in this invention, performance tests were conducted on samples taken from test plates according to the CB 1371 standard. The longitudinal tensile strength of the flame channel was: yield strength ≥420MPa, tensile strength ≥530MPa, and elongation ≥21%; the transverse tensile strength of the flame channel was: yield strength ≥420MPa, tensile strength ≥530MPa, and elongation ≥21%; the impact absorption energy at the center of the flame channel was: -60℃ ≥220J; and the impact absorption energy at the boundary of the flame channel was: -60℃ ≥180J. It is evident that the strength and low-temperature impact toughness of the marine low-temperature steel after flame-working are not lower than those of the base material. The steel composition and the TMCP and pyrotechnic processes used in this invention are easy to implement, simple to operate, low in cost, and high in production efficiency, fully meeting the requirements of low-temperature steel and pyrotechnic processing for the manufacture of ships such as LPG carriers. Attached Figure Description

[0017] Figure 1 This is a metallographic photograph of the flame channel of the test plate in Example 1. Implementation

[0018] The following examples further illustrate the content of the present invention. Example

[0019] A heat treatment process for marine low-temperature steel plates, wherein the chemical composition of the steel, by mass percentage, is: C=0.07%, Si=0.15%, Mn=1.45%, P=0.007%, S=0.002%, Al=0.065%, Ti=0.002%, Nb=0.003%, N=0.0058%, B=0.0012%, with the balance being Fe and unavoidable impurities; the steel plates are produced using the TMCP process, with the billet heating temperature at 1120±10℃, and the heating time within the temperature range of 1100~1150℃ being [not specified]. After 60 minutes of rolling, the cumulative deformation in the recrystallization zone was 113.9%, and the cumulative deformation in the non-recrystallization zone was 131.1%. The final rolling temperature was 780±20℃. After rolling, the temperature was accelerated to 650±20℃ at a cooling rate of approximately 4.0℃ / s, and then air-cooled to room temperature. The steel plate thickness was 16mm. A test plate of 16mm×400mm (rolling direction, width)×1000mm (length) was taken for a flame test. First, the test plate was mechanically bent at approximately 10° along the centerline of its length. The goal was to straighten it into a flat plate through flame treatment. The flame treatment process included the following steps:

[0020] (1) Draw two parallel lines symmetrical to the bending line and 30mm apart along the inner surface of the test plate as flame channels and mark the boundary lines; then prepare two injection welding torches, the heating nozzle diameter of the No. 1 welding torch is 3.0mm and the heating nozzle diameter of the No. 2 welding torch is 1.5mm; at the same time prepare a water pipe with a water flow rate of 90±10ml / s;

[0021] (2) First, use welding torch No. 1 to heat the area, using acetylene as fuel and a neutral flame. During the heating process, control the welding torch tilt angle within the range of 60~70° and the flame core distance ≤3mm. At the same time, use an infrared thermometer to monitor the flame channel temperature. When the surface temperature of the heated area reaches 800±10℃, stop increasing the temperature and allow it to cool slowly. When the surface temperature of the area cools down to 700±10℃, use welding torch No. 2 to reheat the area, using propane as fuel and a neutral flame. During the heating process, control the welding torch tilt angle within the range of 35~45° and the flame core distance 2~3mm until the target temperature of 900±10℃ is reached. Then stop heating and begin air cooling.

[0022] (3) After heating, use a small hammer to straighten the test plate to make it flat; repeat step (2) 3 to 5 times to achieve the desired result;

[0023] (4) Then, heat the original flame channel again according to step (2). After heating to 900±10℃, first air cool for a period of time, and then use a water pipe to cool it down; control the water-fire distance to be 60±5mm.

[0024] (5) After cooling to room temperature, use No. 2 welding torch to heat the original flame channel again. The fuel is propane. Use a neutral flame, control the welding torch tilt angle between 20 and 40°, and the flame core distance is 3 to 4 mm. Heat to 650±20℃, and then air cool to room temperature.

[0025] Sampling of the steel plate was subjected to microstructural observation and performance testing. The microstructure of the steel plate was mainly composed of fine equiaxed ferrite and a small amount of pearlite. Its yield strength was 390 MPa, tensile strength was 510 MPa, elongation was 35%, and impact absorption energy at -60℃ was 195 J. After tempering, the microstructure of the flame channel of the test plate was observed. It was mainly composed of lath bainite, granular bainite, and tempered sorbite with fine lath or short rod-shaped ferrite as the matrix. Figure 1As shown. Performance tests were conducted on samples taken from the test plates according to CB 1371 standard. The longitudinal tensile strength of the flame channel was 476 MPa, tensile strength was 562 MPa, and elongation was 25%; the transverse tensile strength of the flame channel was 481 MPa, tensile strength was 565 MPa, and elongation was 24%; the impact energy absorbed at the center of the flame channel at -60℃ was 235 J; the impact energy absorbed at the boundary of the flame channel at -60℃ was 200 J. It can be seen that the marine low-temperature steel of Example 1 has higher strength after heat treatment, but the low-temperature impact toughness has not decreased. Example

[0026] A heat treatment process for marine low-temperature steel plates, wherein the chemical composition of the steel, by mass percentage, is: C=0.08%, Si=0.22%, Mn=1.60%, P=0.005%, S=0.0015%, Al=0.068%, Ti=0.004%, Nb=0.013%, N=0.0067%, B=0.0015%, with the balance being Fe and unavoidable impurities; the steel plates are produced using the TMCP process, with the billet heating temperature at 1120±10℃, and within the range of 1100~115℃. The heating time within the 0℃ temperature range was 90 min. The cumulative deformation in the recrystallization zone was 116.9%, and the cumulative deformation in the non-recrystallization zone was 92.2%. The final rolling temperature was 780±20℃. After rolling, the temperature was accelerated to 650±20℃ at a cooling rate of approximately 4.0℃ / s, and then air-cooled to room temperature. The steel plate thickness was 32mm. A test plate of 32mm×400mm (rolling direction, width)×1000mm (length) was taken for a fire test. The goal was to simulate the elimination of stress concentration. The fire test process included the following steps:

[0027] (1) Draw two parallel lines symmetrical to the center line and 30mm apart along the length of the test plate as flame channels and mark the boundary lines; then prepare two injection welding torches, the heating nozzle diameter of the No. 1 welding torch is 3.0mm and the heating nozzle diameter of the No. 2 welding torch is 1.5mm; at the same time prepare a water pipe with a water flow rate of 100±10ml / s;

[0028] (2) First, use welding torch No. 1 to heat the flame, using acetylene as fuel and a neutral flame. During the heating process, control the welding torch tilt angle within the range of 70~80° and the flame core distance ≤3mm. At the same time, use an infrared thermometer to monitor the flame channel temperature. When the surface temperature of the heated area reaches 800±10℃, stop increasing the temperature and allow it to cool slowly. When the surface temperature of the area cools down to 700±10℃, use welding torch No. 2 to reheat the flame, using propane as fuel and a slightly carburizing flame. During the heating process, control the welding torch tilt angle within the range of 45~55° and the flame core distance 2~3mm until the target temperature is reached, which is 940±10℃. Then stop heating and begin air cooling.

[0029] (3) To more thoroughly eliminate stress concentration, repeat step (2) 4 times;

[0030] (4) Then, heat the original flame channel again according to step (2). After heating to 940±10℃, first air cool for a period of time, and then use a water pipe to cool it down; control the water-fire distance to be 70±5mm.

[0031] (5) After cooling to room temperature, use No. 2 welding torch to heat the original flame channel again. The fuel is propane. Use a neutral flame, control the welding torch tilt angle between 20 and 40°, and the flame core distance is 3 to 4 mm. Heat to 650±20℃, and then air cool to room temperature.

[0032] Performance tests were conducted on samples of the steel plate. The yield strength was 375 MPa, tensile strength was 495 MPa, elongation was 36%, and impact energy absorbed at -60℃ was 210 J. After heat treatment, performance tests were performed on samples of the test plates according to CB 1371 standard. For longitudinal tensile testing of the flame channel: yield strength was 459 MPa, tensile strength was 546 MPa, and elongation was 25%; for transverse tensile testing of the flame channel: yield strength was 460 MPa, tensile strength was 552 MPa, and elongation was 25%; for impact testing at the center of the flame channel: impact energy absorbed at -60℃ was 226 J; for impact testing at the boundary of the flame channel: impact energy absorbed at -60℃ was 192 J. It can be seen that the strength of the marine low-temperature steel in Example 2 was improved after heat treatment, while the low-temperature impact toughness remained basically the same.

Claims

1. A fire-forming process for marine low-temperature steel plates, characterized in that: The steel's chemical composition by mass percentage is: C = 0.05%~0.09%, Si = 0.10%~0.30%, Mn = 1.20%~1.80%, P ≤ 0.008%, S ≤ 0.002%, Al = 0.05%~0.08%, Ti ≤ 0.005%, Nb ≤ 0.020%, N = 0.0050%~0.0080%, B = 0.0010%~0.0025%, with the balance being Fe and unavoidable impurities; The steel plate is produced using the TMCP process. The billet is heated to 1100~1150℃ for ≤120min within this temperature range. The cumulative deformation in both the recrystallization and non-recrystallization zones is controlled to be ≥90%. The final rolling temperature is controlled at 780±20℃. After rolling, the plate is accelerated to 650±20℃ at a cooling rate of 3~5℃ / s, and then air-cooled to room temperature. The heat treatment process includes the following steps: (1) Determine the heating area of ​​the workpiece and mark the boundary line; then prepare two suction welding torches, the heating nozzle diameter of the No. 1 welding torch is 3.0mm and the heating nozzle diameter of the No. 2 welding torch is 1.5mm; at the same time prepare a water pipe with a water flow rate of 80~120ml / s. (2) First, use welding torch No. 1 to heat the area, using acetylene as fuel and a neutral flame. During the heating process, control the welding torch tilt angle to be no less than 60° and the flame core distance to be ≤3mm. At the same time, use an infrared thermometer to monitor the flame channel temperature. When the surface temperature of the heated area reaches 800±10℃, stop increasing the temperature and allow it to cool slowly. When the surface temperature of the area cools down to 700±10℃, use welding torch No. 2 to reheat the area, using propane as fuel and a neutral or lightly carburized flame. During the heating process, the welding torch tilt angle should be controlled at 30~60° and the flame core distance to be 2~3mm until the target temperature is reached. The target temperature should be higher than 800℃ but not higher than 950℃. Then stop heating and start air cooling. (3) After heating, deform it to meet the target shape requirements; if it cannot meet the requirements, repeat step (2) until the deformation meets the requirements. (4) Heat the original flame channel again according to step (2). After heating to the target temperature, air cool for a period of time, and then use a water pipe to cool it down; control the water-fire distance between 50 and 80 mm. (5) After cooling to room temperature, use No. 2 welding torch to heat the original flame channel again. The fuel is propane. Use a neutral flame, control the welding torch tilt angle between 20 and 40°, and the flame core distance is 3 to 4 mm. Heat to 650±20℃, and then air cool to room temperature.

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