A 620mpa grade ultra-thick bridge steel plate and a production method thereof

Abstract

The present application relates to a kind of 620MPa grade ultra-thick bridge steel plate and its production method, the bridge steel comprises the following weight percentage content components: C:0.06~0.10%, Si:0.10~0.30%, Mn:1.5~1.7%, P≤0.012%, S≤0.003%, Cr:0.24~0.34%, Ni:0.15~0.30%, Cu:0.15%~0.30%, Mo:0.2~0.40%, Als:0.020~0.045%, the rest is Fe and inevitable impurity element;And the content of above-mentioned component satisfies the following relationship formula: 0.40%≤(Cu+Cr)+4Mo / 3≤1.0%;Its production method includes: converter smelting→LF refining→RH vacuum refining→slab continuous casting→ casting blank heating→rolling→straightening→on-line quenching→tempering→inspection, package finishing;The finished steel plate prepared by the present application has the thickness of 60~80mm, yield strength ≥620MPa, tensile strength ≥720MPa, thickness direction section strength difference is not higher than 30MPa, elongation ≥16%, longitudinal impact energy at-40℃≥180J, with high strength, high toughness and excellent welding performance, can satisfy the production requirement of bridge welded steel structure.

Description

Technical Field

[0001] This invention relates to the field of ultra-thick bridge steel plate manufacturing technology, specifically to a 620MPa grade ultra-thick bridge steel plate and its production method. Background Technology

[0002] With the rapid development of infrastructure construction in my country and the continuous increase in the mileage of high-speed railways and highways, long-span dual-purpose railway and highway bridges have become key nodes in the high-speed railway networks along the Yangtze River and the coast. Among these large bridges with long spans and heavy loads, the application ratio of steel structure bridges is increasing. The span of railway bridges has increased from 128m to 1092m, and the strength grade of steel has increased from 235MPa to 500MPa. With the increase in bridge span, new requirements have been placed on the high strength, high toughness, and weldability of the steel plates used. However, the highest grade of Q690 bridge steel in China, due to its strength improvement of three quality grades, has only been demonstrated in the Wuhan Jianghan Seventh Bridge and has not been further promoted. In the existing technology, when using offline quenching and tempering heat treatment to produce ultra-thick bridge steel with a yield strength of 620MPa and a thickness greater than 60mm, there are obvious problems such as excessive alloy addition, uneven properties in the thickness direction, more processes, higher costs, sensitivity to welding cracks, higher carbon equivalent, and greater welding difficulty. Its widespread application is still in the exploratory stage.

[0003] Therefore, developing an ultra-thick bridge steel with a strength of 620MPa to meet the construction needs of large-span bridge steel in the current and future periods has become an urgent problem to be solved in the industry. Summary of the Invention

[0004] The purpose of this invention is to provide a 620MPa grade ultra-thick bridge steel and its production method, solving the problems of existing technologies that require offline quenching and tempering heat treatment to produce ultra-thick bridge steel with a yield strength of 620MPa and a thickness greater than 60mm, which suffers from excessive alloy additions, uneven properties in the thickness direction, and numerous processing steps. The ultra-thick bridge steel plate produced by this invention has a yield strength ≥620MPa, tensile strength ≥720MPa, a thickness-direction section strength difference not exceeding 30MPa, elongation ≥16%, and longitudinal impact energy ≥180J at -40℃. It possesses high strength, high toughness, and excellent weldability, meeting the production requirements of welded steel structures for bridges.

[0005] The present invention discloses a 620MPa grade ultra-thick bridge steel comprising the following components by weight percentage: C: 0.06-0.10%, Si: 0.10-0.30%, Mn: 1.5-1.7%, P: ≤0.012%, S≤0.003%, Cr: 0.24-0.34%, Ni: 0.15-0.30%, Cu: 0.15%-0.30%, Mo: 0.2-0.40%, Als: 0.020-0.045%, with the remainder being Fe and unavoidable impurity elements; and the content of the above components satisfies the following relationship: 0.40%≤(Cu + Cr) + 4Mo / 3≤1.0%.

[0006] The steel plate also includes the following chemical composition in weight percentage: one, two or three of the following: Nb: 0.025-0.044%, V: 0.016-0.035%, Ti: 0.013-0.017%, with Nb+V+Ti≤0.06%.

[0007] The finished steel plate obtained by this invention has a thickness of 60-80mm, a yield strength ≥620MPa, a tensile strength ≥720MPa, a cross-sectional strength difference in the thickness direction not higher than 30MPa, an elongation ≥16%, and a longitudinal impact energy ≥180J at -40℃.

[0008] The present invention discloses a method for producing 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: 300-360mm thick slabs are obtained by casting through a slab continuous casting machine; the slabs are stacked and slow-cooled for no less than 48 hours after being removed from the line; (2) Heating of billet: The temperature of the soaking zone is controlled at 1220~1250℃, and the soaking time T≥(h / 10+15)min, where h is the billet thickness in mm; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet exits the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1160-1180℃. The thickness of roughing is more than 2.0 times the thickness of the finished product. The starting temperature of finishing is ≤900℃. The rolling process is fast and uninterrupted. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 830-860℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.5m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 770~800℃ and the cooling rate is 10~20℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 30~70%. The red temperature of the steel plate after cooling is not higher than 350℃. Then it is quickly removed from the line and stacked for slow cooling to room temperature. (5) Tempering: The tempering temperature is 480-520℃ and the tempering time is 20-50min.

[0009] The steel plate of this invention features a rationally designed composition. To refine the grain size, the total amount of Nb, V, and Ti is controlled to not exceed 0.06% when added. To achieve excellent weldability, the mass percentages of Cr, Ni, and Cu in the steel must meet the requirements of the weldability strengthening index: 0.40% ≤ (Cu + Cr) + 4Mo / 3 ≤ 1.0%, ensuring a proper balance of Cr and Cu. By setting the weldability strengthening index, two objectives are achieved: first, to improve the hardenability of ultra-thick steel plates while ensuring excellent weldability and low-temperature impact toughness; and second, to achieve uniformity in the thickness direction by utilizing Cu precipitation, thereby reducing the thickness effect of the steel plate.

[0010] The technical solution provided by this invention, through composition optimization and online quenching process, increases the strength of the steel plate, improves the uniformity of performance in the thickness direction, reduces the thickness effect of high-performance bridge steel plates, and possesses excellent low-temperature impact toughness, thus obtaining an ultra-thick bridge steel plate with a yield strength of 620MPa. The main principle of the production method of this invention is: by controlling the final rolling temperature at 830-860℃ and the post-rolling roller conveying speed ≥4.5m / s, the temperature of the steel plate before cooling is controlled at 770-800℃. On the one hand, this minimizes the rolling temperature of the steel plate, bringing the final rolling temperature as close as possible to the austenite-ferrite transformation temperature, reducing the austenite grain size, and completing the austenite grain elongation and refinement. On the other hand, it prevents the austenite-ferrite transformation before the steel plate enters water cooling, maintaining refined deformed austenite grains, thereby ensuring that online quenching quickly transforms all austenite into a fine and uniform bainite structure, avoiding the formation of proeutectoid ferrite structure that reduces the strength of the steel plate.

[0011] The finished steel plate produced by this invention has a thickness of 60-80mm, a yield strength ≥620MPa, a tensile strength ≥720MPa, a cross-sectional strength difference in the thickness direction not exceeding 30MPa, an elongation ≥16%, and a longitudinal impact energy ≥180J at -40℃. It possesses high strength, high toughness, and excellent weldability, meeting the production requirements of welded steel structures for bridges. This invention solves the problems of low strength, significant thickness limitations, and unstable low-temperature impact toughness in existing low-carbon bainitic ultra-thick steel plates. Attached Figure Description

[0012] Figure 1 This is a metallographic image of the steel plate obtained in Example 1 of the present invention, magnified to 20 μm. Figure 2 This is a metallographic image of the steel plate obtained in Example 3 of the present invention, magnified to 20 μm. Detailed Implementation

[0013] To better explain the technical solution of the present invention, the technical solution of the present invention will be further described below with reference to specific embodiments. The following embodiments are merely illustrative of the technical solution of the present invention and do not limit the present invention in any way.

[0014] Table 1 below lists the chemical composition values ​​of the steel plates produced in various embodiments of the present invention; Table 2 below lists the test results of the main mechanical properties of the steel plates produced in various embodiments of the present invention.

[0015] Table 1. List of chemical composition values ​​(wt, %) of steel plates produced in various embodiments of the present invention Example 1 The chemical composition and weight percentage of a 620MPa grade ultra-thick bridge steel plate of this embodiment are shown in Table 1.

[0016] This embodiment describes a production method for 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: A 360mm thick slab is obtained by casting through a slab continuous casting machine; the slab is stacked and slowly cooled for 50 hours after being removed from the line; (2) Billet heating: control the temperature of the soaking zone at 1228℃ and the soaking time at 55min; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet leaves the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1176℃ and the thickness at the end of roughing is 130mm. The starting temperature of finishing is 894℃. The rolling process is fast and uninterrupted. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated by 0.3mm within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 845℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥5.0m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 786℃ and the cooling rate is 16.7℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 40%. The steel plate turns red after cooling at 280℃. Then it is quickly removed from the line and stacked to cool slowly to room temperature. (5) Tempering: The tempering temperature is 500℃ and the tempering time is 20min.

[0017] The steel plate manufactured using the chemical composition content and production process steps of this embodiment has a thickness of 60mm. The mechanical property test results of the finished steel plate are shown in Table 2.

[0018] Example 2 The chemical composition and weight percentage of a 620MPa grade ultra-thick bridge steel plate of this embodiment are shown in Table 1.

[0019] This embodiment describes a production method for 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: 300mm thick slabs are obtained by casting through a slab continuous casting machine; the slabs are stacked and slow-cooled for 50 hours after being removed from the line; (2) Heating of billet: The temperature of the soaking zone is controlled at 1235℃ and the soaking time is 47min; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet leaves the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1163℃ and the thickness at the end of roughing is 145mm. The starting temperature of finishing is 863℃. The rolling process is fast and continuous. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated by 0.26mm within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 834℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.8m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 772℃ and the cooling rate is 13.5℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 32%. The steel plate turns red after cooling at 303℃. Then it is quickly removed from the line and stacked to cool slowly to room temperature. (5) Tempering: The tempering temperature is 488℃ and the tempering time is 30min.

[0020] The steel plate manufactured using the chemical composition content and production process steps of this embodiment has a thickness of 70mm. The mechanical property test results of the finished steel plate are shown in Table 2.

[0021] Example 3 The chemical composition and weight percentage of a 620MPa grade ultra-thick bridge steel plate of this embodiment are shown in Table 1.

[0022] This embodiment describes a production method for 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: A 360mm thick slab is obtained by casting through a slab continuous casting machine; the slab is stacked and slowly cooled for 48 hours after being removed from the line; (2) Heating of billet: The temperature of the soaking zone is controlled at 1248℃ and the soaking time is 58min; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet leaves the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1175℃ and the thickness at the end of roughing is 170mm. The starting temperature of finishing is 868℃. The rolling process is fast and uninterrupted. The final pass of finishing uses plate shape flatness control. The reduction is compensated by 0.34mm within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 850℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.5m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 782℃ and the cooling rate is 10.4℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 30%. The steel plate turns red after cooling at 338℃. Then it is quickly removed from the line and stacked to cool slowly to room temperature. (5) Tempering: The tempering temperature is 515℃ and the tempering time is 50min.

[0023] The steel plate manufactured using the chemical composition content and production process steps of this embodiment has a thickness of 80mm. The mechanical property test results of the finished steel plate are shown in Table 2.

[0024] Example 4 The chemical composition and weight percentage of a 620MPa grade ultra-thick bridge steel plate of this embodiment are shown in Table 1.

[0025] This embodiment describes a production method for 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: A 360mm thick slab is obtained by casting through a slab continuous casting machine; the slab is stacked and slowly cooled for 52 hours after being removed from the line; (2) Billet heating: control the temperature of the soaking zone at 1220℃ and the soaking time at 53min; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet leaves the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1160℃ and the thickness at the end of roughing is 160mm. The starting temperature of finishing is 845℃. The rolling process is fast and uninterrupted. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated by 0.28mm within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 830℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.5m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 770℃ and the cooling rate is 10.8℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 60%. The steel plate turns red after cooling at 256℃. Then it is quickly removed from the line and stacked to cool slowly to room temperature. (5) Tempering: The tempering temperature is 505℃ and the tempering time is 30min.

[0026] The steel plate manufactured using the chemical composition content and production process steps of this embodiment has a thickness of 76mm. The mechanical property test results of the finished steel plate are shown in Table 2.

[0027] Example 5 The chemical composition and weight percentage of a 620MPa grade ultra-thick bridge steel plate of this embodiment are shown in Table 1.

[0028] This embodiment describes a production method for 620MPa grade ultra-thick bridge steel, comprising: converter smelting → LF refining → RH vacuum refining → slab continuous casting → slab heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; wherein: (1) Slab continuous casting: A 360mm thick slab is obtained by casting through a slab continuous casting machine; the slab is stacked and slowly cooled for 48 hours after being removed from the line; (2) Heating of billet: control the temperature of the soaking zone at 1250℃ and the soaking time at 60min; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet leaves the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1190℃ and the thickness at the end of roughing is 150mm. The starting temperature of finishing is 894℃. The rolling process is fast and uninterrupted. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated by 0.3mm within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 858℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.6m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 795℃ and the cooling rate is 16.3℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 50%. The steel plate turns red after cooling at 305℃. Then it is quickly removed from the line and stacked to cool slowly to room temperature. (5) Tempering: The tempering temperature is 500℃ and the tempering time is 25min.

[0029] The steel plate manufactured using the chemical composition content and production process steps of this embodiment has a thickness of 68mm. The mechanical property test results of the finished steel plate are shown in Table 2.

[0030] Table 2. List of test results of the main mechanical properties of the steel plates produced in various embodiments of the present invention.

[0031] As can be seen from Table 2, the ultra-thick bridge steel plates with a yield strength of 620MPa obtained in Examples 1-5 of the present invention have a yield strength ≥620MPa at 1 / 4 and 1 / 2 of the thickness, a tensile strength ≥720MPa, a cross-sectional strength difference in the thickness direction not higher than 30MPa, an elongation ≥16%, and a longitudinal impact energy ≥180J at -40℃. They have high strength, high toughness and excellent weldability, and can meet the manufacturing requirements of welded steel structures for bridges.

[0032] Figure 1 This is a metallographic diagram of the steel plate obtained in Example 1 of the present invention. As can be seen from the diagram, the microstructure is a uniform bainite + ferrite structure. Figure 2 This is a metallographic diagram of the steel plate obtained in Example 3 of the present invention. As can be seen from the diagram, the microstructure consists of uniform bainite, martensite and a small amount of ferrite.

[0033] The above embodiments are merely specific examples exemplified to explain the present invention and do not limit the present invention in any way. Any non-substantial changes made by any person based on the above content and form that do not depart from the scope of protection of the claims of the present invention should be considered to fall within the scope of protection of the claims of the present invention. The present invention is not limited to the specific embodiments described above.

Claims

1. A 620MPa grade ultra-thick bridge steel, comprising the following components by weight percentage: C: 0.06–0.10%, Si: 0.10–0.30%, Mn: 1.5–1.7%, P≤0.012%, S≤0.003%, Cr: 0.24–0.34%, Ni: 0.15–0.30%, Cu: 0.15%–0.30%, Mo: 0.2%–0.40%, Als: 0.020–0.045%, with the remainder being Fe and unavoidable impurity elements; and the content of the above components satisfies the following relationship: 0.40%≤(Cu + Cr) + 4Mo / 3≤1.0%; The finished steel plate has a thickness of 60-80mm, a yield strength ≥620MPa, a tensile strength ≥720MPa, a cross-sectional strength difference in the thickness direction not higher than 30MPa, an elongation ≥16%, and a longitudinal impact energy ≥180J at -40℃. The steel plate also contains one, two, or three of the following chemical components by weight percentage: Nb: 0.025–0.044%, V: 0.016–0.035%, and Ti: 0.013–0.017%, with Nb+V+Ti ≤ 0.06%; The production method of the 620MPa grade ultra-thick bridge steel includes: Converter smelting → LF refining → RH vacuum refining → slab continuous casting → billet heating → rolling → straightening → online quenching → tempering → inspection, packaging and finishing; among which: (1) Slab continuous casting: 300-360mm thick slabs are obtained by casting through a slab continuous casting machine; the slabs are stacked and slow-cooled for no less than 48 hours after being removed from the line; (2) Heating of billet: The temperature of the soaking zone is controlled at 1220~1250℃, and the soaking time T≥(h / 10+15)min, where h is the billet thickness in mm; (3) Rolling: The two-stage controlled rolling process of roughing and finishing is adopted. After the billet exits the heating furnace, it is descaled and then roughed. The reduction of the longitudinal rolling pass in the roughing stage is reduced in a progressive manner. The reduction rate of each pass is controlled at 8-10%. The starting temperature of roughing is 1160-1180℃. The thickness of roughing is more than 2.0 times the thickness of the finished product. The starting temperature of finishing is ≤900℃. The rolling process is fast and uninterrupted. The final pass of finishing is controlled by plate shape flatness. The reduction is compensated within 1.5m of the beginning and end of the steel plate to make the rolled steel plate straight. The final rolling temperature is controlled at 830-860℃. (4) Online quenching: After rolling, the steel plate is quickly conveyed to the pre-straightening device at a roller speed of ≥4.5m / s. After leveling, it enters the cooling device and adopts the DQ cooling method. The initial cooling temperature is 770~800℃ and the cooling rate is 10~20℃ / s. In order to ensure the uniformity of cooling, the edge is blocked within 100mm. The cooling water volume within 1m of the head and tail is reduced by 30~70%. The red temperature of the steel plate after cooling is not higher than 350℃. Then it is quickly removed from the line and stacked for slow cooling to room temperature. (5) Tempering: The tempering temperature is 480-520℃ and the tempering time is 20-50min.

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