Large-thickness, low-yield-ratio and high-toughness 620 MPa bridge steel plate and production method thereof

Abstract

The invention discloses a large-thickness, low-yield-ratio and high-toughness 620 MPa bridge steel plate which comprises the following chemical components in percentage by mass: 0.08-0.12% of C, 0.20-0.40% of Si, 1.2-1.5% of Mn, less than or equal to 0.012% of P, less than or equal to 0.005% of S, 0.30-0.65% of Ni, 0.40-0.60% of Cr, 0.25-0.45% of Mo, 0.30-0.40% of Cu, 0.005-0.035% of Nb, 0.040-0.060% of V, 0.010-0.020% of Ti, 0.0005-0.0015% of B, 0.020-0.040% of Alt and the balance of Fe and inevitable impurities, and a production method of the steel plate comprises the working procedures of smelting, continuous casting, heating, rolling, cooling, off-line quenching Through economical component design and matched controlled rolling and controlled cooling, off-line quenching and off-line tempering, the bridge steel plate has the advantages of low yield ratio, high low-temperature toughness and high ductility.

Description

Technical Field

[0001] The invention relates to a 620MPa bridge steel plate with large thickness, low yield strength ratio and high toughness and a preparation method thereof. Background Art

[0002] As bridges develop towards large spans, heavy loads, and fully welded structures, the requirements for the safety and reliability of bridge structures are becoming increasingly stringent. This places higher demands on designers, and also places higher standards on the quality of steel plates, requiring not only high strength to meet the needs of lightweight structures, but also excellent plasticity, low-temperature toughness, and weldability to meet the conditions of larger bridge spans and heavier loads. The application of high-strength, high-toughness, and high-plasticity bridge steel can reduce the deadweight of bridge structures and reduce the difficulty of bridge engineering design, manufacturing, and construction.

[0003] However, as the strength of steel increases, the yield strength ratio of steel will generally increase, and can even reach above 0.93. Due to the high yield strength ratio, once the component is overloaded, such as under conditions such as earthquakes, the ultimate strength of the steel grade will be quickly reached, leading to accidents. Therefore, the high yield strength ratio limits the application of high-strength structural steel in bridge engineering. The frequent occurrence of earthquakes and the catastrophic consequences they cause have attracted great attention to the seismic resistance of bridges at home and abroad, and relevant regulations have been made in some structural design specifications. Therefore, low yield strength ratio, high strength, high toughness, and high plasticity bridge steel is the development trend of bridge construction. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a large-thickness, low-yield-to-strength ratio, high-strength and toughness 620MPa bridge steel plate and a production method thereof in view of the shortcomings of the above-mentioned prior art. Through scientific component design and a matching manufacturing method of controlled rolling and controlled cooling + offline quenching + offline tempering, a bridge steel with low yield ratio, high toughness and high ductility is obtained, with a yield strength of more than 620MPa and a tensile strength of more than 720MPa. The yield ratio of the finished steel is less than or equal to 0.85, the KV2 at -20℃ is more than 200J, the fiber cross-section rate is more than 80%, and the elongation is ≥19%.

[0005] The technical solution of the present invention to solve the above technical problems is: a thick, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, the chemical composition and mass percentage of the steel plate are: C: 0.08-0.12%, Si: 0.20-0.40%, Mn: 1.2-1.5%, P≤0.012%, S≤0.005%, Ni: 0.30-0.65%, Cr: 0.40-0.60%, Mo: 0.25-0.45%, Cu: 0.30-0.40%, Nb: 0.005-0.035%, V: 0.040-0.060%, Ti: 0.010-0.020%, B: 0.0005-0.0015%, Alt: 0.020-0.040%, and the balance is Fe and unavoidable impurities.

[0006] The functions of each component in the present invention are: C: Carbon has a significant effect on the yield, tensile strength and welding performance of steel. Carbon can significantly improve the strength of steel plates through interstitial solid solution. However, too high a carbon content will affect the welding performance and toughness of steel. Therefore, C is controlled at 0.08-0.12%; Si: It acts as a reducing agent and deoxidizer in the steelmaking process. It can also play a role in solid solution strengthening. However, when it exceeds 0.4%, it will reduce the toughness of the steel and reduce the welding performance of the steel. Mn: Manganese is low in cost and can increase the toughness, strength and hardness of steel, improve the hardenability of steel, and improve the hot working performance of steel; but if the manganese content is too high, center segregation is likely to occur in thick steel plates, so it is controlled at 1.2-1.5%.

[0007] P, S: In general, phosphorus and sulfur are harmful elements in steel, increasing the brittleness of steel. Phosphorus deteriorates welding performance and reduces plasticity; sulfur reduces the ductility and toughness of steel and causes cracks during rolling. Therefore, the content of phosphorus and sulfur in steel should be reduced as much as possible, so P≤0.012%, S≤0.005%; Al: Aluminum is a commonly used deoxidizer in steel. Adding a small amount of aluminum to steel can refine the grains and improve impact toughness. Aluminum also has antioxidant and corrosion resistance. Nb: Niobium can promote the grain refinement of steel rolling microstructure and improve strength and toughness at the same time. Niobium can effectively refine the microstructure by inhibiting austenite recrystallization during controlled rolling and strengthen the matrix by precipitation; V: Its main function is interphase precipitation during γ-α transformation and precipitation strengthening in ferrite; Ti: Titanium is a strong deoxidizer in steel. It can make the internal structure of steel dense, refine the grains, reduce aging sensitivity and cold brittleness, and improve welding performance; Ni: Its main function is to ensure the low temperature toughness of the steel plate; Cr: can improve hardenability and solid solution strengthening, can improve the strength and hardness of steel in the heat treatment state, but also reduce the toughness of the steel plate and increase the temper brittleness. Therefore, the reasonable Cr content can be determined according to the strength and toughness requirements. Therefore, the Cr content of the present invention is 0.40-0.60%; Mo: It can make ferrite precipitate from austenite and increase the stability of austenite, has a strong inhibitory effect on the formation of pearlite, and promotes the transformation of bainite structure; Cu: Copper is an element that expands the austenite phase, but its solubility is not high. It can play a precipitation strengthening role and improve the corrosion resistance of steel. B: It is cheap. Adding B to steel can significantly improve the hardenability of steel and save precious alloy elements such as Ni, Cr, and Mo.

[0008] The present invention further defines the scheme: Preferably, the structure of the steel plate is bainite+ferrite, and the thickness of the steel plate is 40-100 mm.

[0009] Preferably, the steel plate has a Ceq of 0.50 to 0.55%, a yield strength of 630 to 690 MPa, a tensile strength of 720 to 850 MPa, a yield strength ratio of 0.81 to 0.85, an impact energy of ≥200 J at -20°C, and an elongation of ≥19%.

[0010] The present invention also provides a method for producing a large-thickness, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, which specifically includes smelting, continuous casting, heating, rolling, cooling, off-line quenching and off-line tempering processes, wherein: In the heating process, the maximum heating temperature of the steel billet is 1180-1200°C, the soaking temperature is 1150-1180°C, the total heating time is ≥460min, and the soaking time in the furnace is ≥60min; The rolling process adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 960-1100℃, the single-pass reduction is 10-15%, and the cumulative reduction rate is 40-60%; the rolling temperature of the second stage is 790-840℃, and the cumulative reduction rate is 30-50%; The cooling process is to carry out laminar cooling after the final rolling, and the red-return temperature is controlled at 350-550°C, and then air-cooled to room temperature; Quenching process, quenching temperature 810 ~ 860 ℃, and keep the steel plate thickness * 2min / mm at this temperature, then naturally cool to room temperature heating time coefficient 2min / mm; Tempering process, tempering temperature 450 ~ 600 ℃, and keep the steel plate thickness * 2min / mm at this temperature, and then cool naturally to room temperature.

[0011] Preferably, the continuous casting process is performed by stack cooling the continuous casting billet for more than 72 hours, the thickness of the continuous casting billet is 370-460 mm, and the temperature uniformity of the continuous casting billet in the soaking process is less than 15°C.

[0012] Preferably, after the first stage of rolling in the rolling process, the intermediate billet is kept at a temperature of 850°C to 920°C, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage of rolling.

[0013] The beneficial effects of the present invention are as follows: the present invention obtains a 40-100 mm thick 620 MPa grade low yield ratio, high strength and high toughness bridge steel with low yield ratio, high toughness and high ductility through scientific component design and a matching controlled rolling and controlled cooling + offline quenching + offline tempering manufacturing method; its yield strength is above 620 MPa, the tensile strength is above 720 MPa, the yield ratio of the finished steel is less than or equal to 0.85, the -20°C KV2 is above 200 J, and the fiber cross-section rate is above 80%, the elongation is ≥19%, the steel has good performance uniformity and excellent plate shape, the comprehensive production cost is low, and it is suitable for application in bridge structures. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 This is the optical metallographic structure of the product in Example 3 at 500 times magnification. DETAILED DESCRIPTION Example

[0015] The present embodiment provides a 620MPa bridge steel plate with large thickness, low yield ratio and high toughness, which includes the following components by mass percentage: C: 0.09%, Si: 0.22%, Mn: 1.30%, P: 0.010%, S: 0.0030%, Ni: 0.35%, Cr: 0.42%, Mo: 0.28%, Cu: 0.32%, Nb: 0.025%, V: 0.042%, Ti: 0.015%, B: 0.0006, Alt: 0.026%, and the remainder is Fe and unavoidable impurities.

[0016] This embodiment provides a method for producing a large-thickness, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, specifically comprising: S1 raw materials are smelted, refined, alloyed, and calcium treated to obtain molten steel; S2 slab continuous casting on molten steel, the thickness of the slab is 460mm, and the continuous casting slab is pile-cooled for more than 72 hours; S3: Heat the bad material at 1180℃, with the temperature uniformity less than 15℃, and then remove phosphorus after heating for 480min; S4 adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 1050℃, the single-pass reduction is 15%, and the cumulative reduction rate is 55%; after the first stage rolling, the intermediate billet is kept at a temperature of 870℃, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage rolling; the second stage rolling temperature is 835℃, the cumulative reduction rate is 32%, and the final product thickness reaches 40mm; S5 cooling process, which is laminar cooling after final rolling, the red-return temperature is controlled at 550°C, and then air-cooled to room temperature; S6 performs offline quenching on the steel plate at a quenching temperature of 850°C and keeps at this temperature for 10 minutes; After S7 offline quenching, offline tempering is carried out at a tempering temperature of 450°C and kept at this temperature for 20 minutes.

[0017] The microstructure type of the steel plate produced above is "tempered bainite" + "ferrite", the yield strength of the material is 672MPa, the tensile strength is 806MPa, the yield strength ratio of the finished steel is 0.83, the -40℃ KV2 is 243J, the cross-sectional fiber ratio is 95%, and the elongation A is 20%. Example

[0018] The present embodiment provides a 620MPa bridge steel plate with large thickness, low yield ratio and high toughness, which includes the following components by mass percentage: C: 0.10%, Si: 0.25%, Mn: 1.35%, P: 0.010%, S: 0.0030%, Ni: 0.45%, Cr: 0.51%, Mo: 0.28%, Cu: 0.32%, Nb: 0.025%, V: 0.042%, Ti: 0.015%, B: 0.0006, Alt: 0.026%, and the remainder is Fe and unavoidable impurities.

[0019] This embodiment provides a method for producing a large-thickness, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, specifically comprising: S1 raw materials are smelted, refined, alloyed, and calcium treated to obtain molten steel; S2 slab continuous casting on molten steel, the thickness of the slab is 460mm, and the continuous casting slab is pile-cooled for more than 72 hours; S3: Heat the bad material at 1180℃, with the temperature uniformity less than 15℃, and then remove phosphorus after heating for 480min; S4 adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 980℃, the single pass reduction is 15%, and the cumulative reduction rate is 55%; after the first stage rolling, the intermediate billet is kept at 870℃, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage rolling; the second stage rolling temperature is 829℃, the cumulative reduction rate is 32%, and the final product thickness reaches 60mm; S5 cooling process, which is laminar cooling after final rolling, the red-return temperature is controlled at 530°C, and then air-cooled to room temperature; S6 performs offline quenching on the steel plate at a quenching temperature of 819°C and keeps at this temperature for 10 minutes; After S7 offline quenching, offline tempering is carried out at a tempering temperature of 450°C and kept at this temperature for 20 minutes.

[0020] The microstructure type of the steel plate produced above is "tempered bainite" + "ferrite", the yield strength of the material is 675MPa, the tensile strength is 823MPa, the yield strength ratio of the finished steel is 0.82, the -40℃ KV2 is 229J, the cross-sectional fiber ratio is 93%, and the elongation A is 21%. Example

[0021] The present embodiment provides a 620MPa bridge steel plate with large thickness, low yield ratio and high toughness, which includes the following components by mass percentage: C: 0.09%, Si: 0.32%, Mn: 1.32%, P: 0.010%, S: 0.0030%, Ni: 0.40%, Cr: 0.49%, Mo: 0.35%, Cu: 0.32%, Nb: 0.025%, V: 0.042%, Ti: 0.015%, B: 0.0006, Alt: 0.026%, and the remainder is Fe and unavoidable impurities.

[0022] This embodiment provides a method for producing a large-thickness, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, specifically comprising: S1 raw materials are smelted, refined, alloyed, and calcium treated to obtain molten steel; S2 slab continuous casting on molten steel, the thickness of the slab is 460mm, and the continuous casting slab is pile-cooled for more than 72 hours; S3: Heat the bad material at 1180℃, with the temperature uniformity less than 15℃, and then remove phosphorus after heating for 480min; S4 adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 1000℃, the single-pass reduction is 15%, and the cumulative reduction rate is 55%; after the first stage rolling, the intermediate billet is kept at 870℃, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage rolling; the second stage rolling temperature is 820℃, the cumulative reduction rate is 32%, and the final product thickness reaches 80mm; S5 cooling process, which is laminar cooling after final rolling, the red-return temperature is controlled at 537°C, and then air-cooled to room temperature; S6 performs offline quenching on the steel plate at a quenching temperature of 819°C and keeps at this temperature for 10 minutes; After S7 offline quenching, offline tempering is carried out at a tempering temperature of 450°C and kept at this temperature for 20 minutes.

[0023] The microstructure type of the steel plate produced above is "tempered bainite" + "ferrite", the yield strength of the material is 659MPa, the tensile strength is 798MPa, the yield strength ratio of the finished steel is 0.83, the -40℃ KV2 is 225J, the cross-sectional fiber ratio is 93%, and the elongation A is 21%. Example

[0024] The present embodiment provides a 620MPa bridge steel plate with large thickness, low yield ratio and high toughness, which includes the following components by mass percentage: C: 0.10%, Si: 0.22%, Mn: 1.46%, P: 0.010%, S: 0.0030%, Ni: 0.55%, Cr: 0.42%, Mo: 0.28%, Cu: 0.33%, Nb: 0.025%, V: 0.042%, Ti: 0.015%, B: 0.0006, Alt: 0.026%, and the remainder is Fe and unavoidable impurities.

[0025] This embodiment provides a method for producing a large-thickness, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, specifically comprising: S1 raw materials are smelted, refined, alloyed, and calcium treated to obtain molten steel; S2 slab continuous casting on molten steel, the thickness of the slab is 460mm, and the continuous casting slab is pile-cooled for more than 72 hours; S3: Heat the bad material at 1180℃, with the temperature uniformity less than 15℃, and then remove phosphorus after heating for 480min; S4 adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 960℃, the single pass reduction is 20%, and the cumulative reduction rate is 50%; after the first stage rolling, the intermediate billet is kept at 870℃, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage rolling; the second stage rolling temperature is 836℃, the cumulative reduction rate is 32%, and the final product thickness reaches 100mm; S5 cooling process, which is laminar cooling after final rolling, the red-return temperature is controlled at 528℃, and then air-cooled to room temperature; S6 performs offline quenching on the steel plate at a quenching temperature of 830°C and keeps at this temperature for 10 minutes; After S7 offline quenching, offline tempering is carried out at a tempering temperature of 450°C and kept at this temperature for 20 minutes.

[0026] The microstructure type of the steel plate produced above is "tempered bainite" + "ferrite", the yield strength of the material is 683MPa, the tensile strength is 812MPa, the yield strength ratio of the finished steel is 0.84, the -40℃ KV2 is 263J, the cross-sectional fiber ratio is 95%, and the elongation A is 20%.

[0027] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A thick, low yield-to-strength ratio, high-toughness 620MPa bridge steel plate, characterized by: The chemical composition and mass percentage of the steel plate are: C: 0.08-0.12%, Si: 0.20-0.40%, Mn: 1.2-1.5%, P≤0.012%, S≤0.005%, Ni: 0.30-0.65%, Cr: 0.40-0.60%, Mo: 0.25-0.45%, Cu: 0.30-0.40%, Nb: 0.005-0.035%, V: 0.040-0.060%, Ti: 0.010-0.020%, B: 0.0005-0.0015%, Alt: 0.020-0.040%, and the balance is Fe and unavoidable impurities.

2. According to claim 1, a thick, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate, characterized in that: The structure of the steel plate is bainite+ferrite, and the thickness of the steel plate is 40-100 mm.

3. The thick, low yield-to-strength ratio, high-toughness 620MPa bridge steel plate according to claim 1, characterized in that: The steel plate has a Ceq of 0.50 to 0.55%, a yield strength of 630 to 690 MPa, a tensile strength of 720 to 850 MPa, a yield strength ratio of 0.81 to 0.85, an impact energy of ≥200 J at -20°C, and an elongation of ≥19%.

4. The method for producing a thick, low-yield-to-strength ratio, high-toughness 620MPa bridge steel plate according to any one of claims 1 to 3, characterized in that: Specifically, it includes smelting, continuous casting, heating, rolling, cooling, offline quenching and offline tempering processes, among which: In the heating process, the maximum heating temperature of the steel billet is 1180-1200°C, the soaking temperature is 1150-1180°C, the total heating time is ≥460min, and the soaking time in the furnace is ≥60min; The rolling process adopts a two-stage controlled rolling and controlled cooling process of recrystallization zone + non-recrystallization zone. The rolling temperature of the first stage is 960-1100℃, the single-pass reduction is 10-15%, and the cumulative reduction rate is 40-60%; the rolling temperature of the second stage is 790-840℃, and the cumulative reduction rate is 30-50%; The cooling process is to carry out laminar cooling after the final rolling, and the red-return temperature is controlled at 350-550°C, and then air-cooled to room temperature; Quenching process, quenching temperature 810 ~ 860 ℃, and keep the steel plate thickness * 2min / mm at this temperature, then naturally cool to room temperature heating time coefficient 2min / mm; Tempering process, tempering temperature 450 ~ 600 ℃, and keep the steel plate thickness * 2min / mm at this temperature, and then cool naturally to room temperature.

5. The thick, low yield-to-strength ratio, high-toughness 620MPa bridge steel plate according to claim 4, characterized in that: The continuous casting process is performed by stack cooling the continuous casting billet for more than 72 hours, the thickness of the continuous casting billet is 370-460 mm, and the temperature uniformity of the continuous casting billet in the soaking process is less than 15°C.

6. The thick, low yield-to-strength ratio, high-toughness 620MPa bridge steel plate according to claim 4, characterized in that: After the first stage of rolling in the rolling process, the intermediate billet is kept at a temperature of 850°C to 920°C, and the waiting thickness is more than 3 times the thickness of the finished product. After reaching the temperature, the non-recrystallization zone is rolled for the second stage of rolling.

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