A 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel and its preparation method

Abstract

The present invention relates to a 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel and a preparation method thereof, and belongs to the technical field of steel for bridge structures. The existing 550MPa bridge steel has poor matching of strength, low-temperature fracture toughness and weldability, and cannot have the problem of high strength, high fracture toughness and welding performance at the same time. A 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel, the chemical composition is calculated by mass percentage: C 0.06-0.11%, Si 0.20-0.40%, Mn 0.90-1.50%, Cr 0.40-0.60%, Ni 0.3-1.1%, Cu 0.30-0.45%, Mo 0.20-0.40%, Nb 0.02-0.04%, Ti 0.010-0.020%, and the rest is Fe and unavoidable impurities. The steel prepared by the present invention has excellent mechanical properties, fracture toughness and weldability.

Description

Technical Field

[0001] The present invention relates to the technical field of steel for bridge structures, and in particular to a 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel and a preparation method thereof. Background Art

[0002] Bridge steel is a steel plate specially used for building railway or highway bridges. It is required to have high strength and toughness, and be able to withstand the load and impact of locomotives and vehicles. It must also have good fatigue resistance, certain low-temperature toughness and atmospheric corrosion resistance.

[0003] Fracture is one of the most dangerous failure forms of bridge steel. The fracture failure of bridge steel is affected by many environmental factors, among which temperature has a more significant effect on it; most metal materials will undergo a ductile-brittle transition, that is, as the temperature drops, the material gradually transitions from ductility to brittleness. As bridge steel increases from a yield strength of 345MPa to a yield strength of 550MPa or higher strength levels and the thickness continues to develop from less than 32mm to more than 80mm, the low-temperature fracture behavior of bridge steel and measures to improve fracture performance have attracted widespread attention. Optimizing alloy composition and rolling process is an effective means to achieve high weather resistance and low-temperature high toughness of high-strength bridge steel plates. However, the increase in alloy elements will increase welding crack sensitivity while improving weather resistance and toughness, thereby deteriorating welding performance. In addition, as the thickness of bridge steel plates increases, the matching of rolling process and heat treatment process needs to be considered simultaneously during the preparation process design.

[0004] The existing 550MPa bridge steel and the bridge steel prepared by the preparation method thereof have poor matching in strength, low-temperature fracture toughness and weldability, and cannot have good welding performance while meeting the requirements of high strength and high fracture toughness. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel and a preparation method thereof, so as to solve the problem that the strength, low-temperature fracture toughness and weldability of the bridge steel prepared by the existing 550 MPa bridge steel and its preparation method are poorly matched, and the steel cannot have good welding performance while meeting the requirements of high strength and high fracture toughness.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The invention provides a 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel, the chemical composition of which is calculated by mass percentage as follows: C 0.06-0.11%, Si 0.20-0.40%, Mn 0.90-1.50%, Cr 0.40-0.60%, Ni 0.3-1.1%, Cu0.30-0.45%, Mo 0.20-0.40%, Nb 0.02-0.04%, Ti 0.010-0.020%, and the rest is Fe and inevitable impurities.

[0008] Furthermore, when the steel plate thickness H of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 24≤H<36mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies: 1.5%≤Mn+Cr+Mo≤2.0% and 0.6%≤Ni≤1.1%.

[0009] Furthermore, when the steel plate thickness of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 36mm≤H≤80mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies 2.0%<Mn+Cr+Mo≤2.6%.

[0010] Furthermore, when the steel plate thickness of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 80mm≤H≤120mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies 2.0%<Mn+Cr+Mo≤2.6% and 0.6%≤Ni≤1.1%.

[0011] Furthermore, the microstructure of the 550MPa grade high-strength and high fracture toughness weather-resistant bridge steel includes proeutectoid ferrite, lath bainite, martensite and residual austenite, or intercritical ferrite, lath bainite, martensite and residual austenite.

[0012] The present invention also provides a method for preparing 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel, which is used to prepare the above 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel, comprising the following steps: smelting, controlled rolling, controlled cooling and heat treatment;

[0013] When the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 24≤H<36mm, the heat treatment process is tempering;

[0014] When the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 36mm≤H≤120mm, the heat treatment process is critical zone quenching and tempering.

[0015] Furthermore, the controlled rolling process adopts two-stage rolling of rough rolling and finish rolling, the starting rolling temperature of rough rolling is ≥1120°C, and the starting rolling temperature of finish rolling is 850-870°C.

[0016] Furthermore, the controlled cooling process includes water inlet laminar cooling and air cooling. During laminar cooling, the water inlet temperature is 780-800°C, cooled to 450-500°C, and then cooled to room temperature by air outlet.

[0017] Furthermore, the critical zone quenching temperature is 760-820° C., and the heat preservation time after through-heat preservation is 30-60 minutes.

[0018] Furthermore, the tempering temperature is 500-550°C, and the heat preservation time after the heat preservation is 30-60 minutes. Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0019] 1. The 550MPa grade bridge steel of the present invention ensures low welding crack sensitivity and plastic toughness of the bridge steel by proposing an alloy design of low C + high Ni + (Mn + Mo) + (Cr-Cu) + (Ti-Nb) and accurately controlling the process parameters in the preparation process, and the welding crack sensitivity index Pcm (%) <0.25; while the weather resistance index is relatively high, low welding crack sensitivity is guaranteed, and the strength, low-temperature fracture toughness and weldability of the prepared bridge steel are well matched. While meeting the requirements of high strength and high fracture toughness, it has good welding performance, thereby realizing the design of highly weather-resistant and easy-to-weld bridge steel materials.

[0020] 2. The present invention provides an optimized alloy element design and preparation process for bridge steel plates of different thicknesses, so that the prepared bridge steel has a yield strength of ≥550Mpa, a tensile strength of ≥700MPa, an elongation after fracture of ≥21.0%, and a yield strength ratio of ≤0.84; at the same time, it has good low-temperature toughness, -40℃ V-type impact energy ≥120J, and a ductile-brittle transition temperature of <-60℃ (such as -92℃~-68℃); at the same time, the steel plate has good weldability, and the -40℃ crack opening displacement value CTOD>0.25mm.

[0021] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. In the entire drawings, the same reference symbols represent the same components;

[0023] Figure 1 This is the microstructure diagram of the bridge steel of Example 1;

[0024] Figure 2 is the tensile curve diagram of the bridge steel of Example 2;

[0025] Figure 3 This is a schematic diagram of the ductile-brittle transition temperature of the bridge steel in Example 2;

[0026] Figure 4 This is the microstructure diagram of the bridge steel of Example 3;

[0027] Figure 5 This is the impact fracture morphology of the bridge steel of Example 4;

[0028] Figure 6 This is the morphology of the crack propagation area in the fracture toughness test of the bridge steel in Example 5. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0030] The invention provides a 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel. The chemical composition of the 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is as follows by mass percentage: C 0.06-0.11%, Si 0.20-0.40%, Mn0.90-1.50%, Cr 0.40-0.60%, Ni 0.3-1.1%, Cu 0.30-0.45%, Mo 0.20-0.40%, Nb 0.02-0.04%, Ti 0.010-0.020%, and the rest is Fe and unavoidable impurities.

[0031] The reasons for limiting the composition of the ingot of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel and its preparation method in the present invention are explained. Only % is used to represent the mass percentage in the composition.

[0032] C: significantly affects the welding performance, strength and toughness of bridge steel. When the carbon content is high, the strength of the steel is higher, but the increase in carbon content increases the welding crack sensitivity of the steel and deteriorates the welding performance of the bridge steel. On the other hand, it reduces the toughness of the bridge steel and cannot meet the requirements of the bridge steel for low temperature and high toughness. In the present invention, the C content range is 0.06-0.11%.

[0033] Si: It is a deoxidizer in the steelmaking process and has a high solid solubility in steel. It mainly improves the strength of steel in a solid solution manner. The present invention adds silicon to ensure the uniformity of the strength of the thick plate at the edge and the core, but excessive Si will accelerate high-temperature delamination and deteriorate the toughness and welding performance of the steel. The Si content range in the present invention is 0.20-0.40%.

[0034] Mn: It significantly affects the structural transformation products of steel and improves the hardenability of the element. Increasing the manganese content can improve the hardenability of thick plates and ensure the uniformity of the microstructure from the surface to the core of thick steel plates; however, too high a manganese content is prone to macrosegregation and deteriorates welding performance; the range of the Mn content in the present invention is 0.90-1.50%.

[0035] Cr, Cu: Adding Cr and Cu elements can improve the corrosion resistance of bridge steel plates; the addition of Cr and Cu elements can form a dense oxide film, and the composite addition of Cr and Cu can further hinder the invasion of corrosive media and improve the corrosion resistance of steel plates; but too high Cr and Cu content will cause the deterioration of steel welding performance and increase alloy costs. In the present invention, the Cr and Cu content ranges are 0.40-0.60% and 0.30-0.45%, respectively.

[0036] Ni: After adding Ni to steel, on the one hand, Ni can refine the structure of steel and increase the interface that is conducive to hindering crack propagation, thereby improving toughness; on the other hand, adding Ni to steel can promote dislocation slip and improve the plastic deformation ability of steel. During the impact process, good plastic deformation ability can consume more energy, thereby improving impact and low-temperature fracture toughness; in addition, Ni can effectively improve the density, stability and bonding strength of the rust layer with the matrix, thereby improving the corrosion resistance of bridge steel. In the present invention, the Ni content range is 0.3-1.1%.

[0037] Mo: significantly improves the hardenability and atmospheric corrosion resistance of steel, reduces temper brittleness; is conducive to obtaining bainite, and thus is conducive to the control of the microstructure of the surface and core of thick bridge steel plates. However, Mo is a rare and precious metal, and excessive addition increases the preparation cost. In the present invention, the Mo content ranges from 0.2-0.4%.

[0038] Nb: It forms NbC phase with elements such as C in steel. The undissolved NbC particles during rolling and heating can prevent the growth of austenite grains. In addition, during the tempering process of Nb-containing bridge steel, nano-Nb-containing phases are precipitated, which significantly improves the strength of the steel. In the present invention, the range of Nb content is 0.020-0.04%.

[0039] Ti: After adding a small amount of Ti to the steel, small and stable Ti-containing particles are easily formed, which can effectively inhibit the coarsening of the original austenite grains in the pre-rolling soaking and welding heat-affected zones. In the present invention, the Ti content ranges from 0.010 to 0.020%.

[0040] It should be noted that when the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 24≤H<36mm, the chemical composition of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel must also meet the following requirements: 1.5%≤Mn+Cr+Mo≤2.0%, 0.6%≤Ni≤1.1%; this is because the steel plate thickness is relatively thin, a large compression ratio rolling can be achieved, and a consistent microstructure can be obtained from the surface to the core during the cooling process, so a lower alloy cost design is adopted;

[0041] When the steel plate thickness of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 36mm≤H≤80mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel must also meet the following requirements: 2.0%<Mn+Cr+Mo≤2.6%; among them, when the steel plate thickness is 80mm≤H≤120mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel must also meet the following requirements: 0.6%≤Ni≤1.1%, which can ensure that when the steel plate thickness is thicker, a higher low-temperature toughness can still be obtained.

[0042] Preferably, a 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is provided, wherein the chemical composition of the 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is as follows by mass percentage: C 0.06-0.1%, Si 0.20-0.35%, Mn 1.1-1.46%, Cr 0.40-0.51%, Ni 0.4-0.95%, Cu 0.32-0.40%, Mo 0.25-0.31%, Nb 0.02-0.025%, Ti 0.011-0.013%, and the rest is Fe and unavoidable impurities.

[0043] The present invention also provides 550MPa grade high strength and high fracture toughness weathering bridge steel, comprising the following steps: smelting, controlled rolling, controlled cooling and heat treatment;

[0044] Specifically, the smelting process includes batching according to chemical composition, converter smelting, refining outside the furnace, and continuous casting to obtain a continuous casting slab;

[0045] Specifically, the controlled rolling process is as follows: the controlled rolling process adopts two-stage rolling of rough rolling and finishing rolling, the starting rolling temperature of rough rolling is ≥1120°C, and the starting rolling temperature of finishing rolling is 850~870°C (for example, the starting rolling temperature of finishing rolling is 852°C, 854°C, 856°C, 858°C, 860°C, 862°C, 864°C, 865°C, 866°C, 868°C); it should be noted that the starting rolling temperature of rough rolling is ≥1120°C, which ensures rolling in the austenite recrystallization zone, allows the austenite to fully recrystallize, and refines the austenite grains through repeated rolling and recrystallization. The starting rolling temperature of finishing rolling is 850~870°C, and a fine microstructure is obtained during the cooling process after rolling. It should be noted that before the two-stage rolling, the continuous casting slab needs to be heated to improve the plasticity of the steel, reduce the deformation resistance, and facilitate subsequent rolling. The continuous casting slab rolling heating temperature is 1200-1250℃, and the insulation time is 2-3h.

[0046] Specifically, the controlled cooling process is: water inlet laminar cooling + air cooling, the water inlet temperature during laminar cooling is 780-800°C (exemplarily, the laminar cooling water inlet temperature is 782°C, 784°C, 786°C, 788°C, 790°C, 792°C, 794°C, 796°C, 798°C), cooled to 450-500°C (exemplarily, cooled to 455°C, 460°C, 465°C, 470°C, 475°C, 480°C, 485°C, 490°C, 495°C), and then the water outlet air is cooled to room temperature.

[0047] Specifically, when the steel plate thickness H of the 550MPa grade high-strength and high fracture toughness weathering bridge steel is 24≤H<36mm, the heat treatment process is: tempering, the tempering treatment temperature is 500-550℃ (exemplarily, the tempering treatment temperature is 505℃, 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, 540℃, 545℃), and the insulation time after through insulation is 30-60min (exemplarily, the insulation time is 32min, 34min, 35min, 36min, 37min, 38min, 40min, 42min, 44min, 46min, 48min, 50min, 52min, 54min, 56min, 58min), and the tempering temperature and time meet the requirements to eliminate quenching stress and obtain nano-precipitation.

[0048] When the steel plate thickness H of the 550MPa grade high-strength and high fracture toughness weathering bridge steel is 36mm≤H≤120mm, the heat treatment process is: critical zone quenching and tempering, the tempering temperature is 500-550℃ (exemplarily, the tempering temperature is 505℃, 510℃, 515℃, 520℃, 525℃, 530℃, 535℃, 540℃, 545℃), the insulation time after through insulation is 30-60min (exemplarily, the insulation time is 32min, 34min, 35min, 36min, 37min, 38min, 40min, 42min, 44min, 46min, 48min, 50min, 52min, 54min, 56min, 58min), the critical zone quenching temperature is 760-820℃ (exemplarily, the critical zone quenching temperature is 765℃, 770℃, 77 5℃, 780℃, 785℃, 790℃, 795℃, 800℃, 805℃, 810℃, 815℃), the holding time after through-holding is 30-60min (exemplarily, the holding time is 32min, 34min, 35min, 36min, 38min, 40min, 42min, 44min, 46min, 48min, 50min, 52min, 54min, 56min, 58min), and the steel plate is water-cooled to 500℃ or below after being taken out of the furnace, and then air-cooled; It should be noted that after controlled rolling and controlled cooling, the steel plate is reheated to the two-phase region of austenite and ferrite, and after holding for 30-60min, it is water-cooled to a temperature of 500℃ or below, and then air-cooled to room temperature to obtain a microstructure of proeutectoid ferrite, lath bainite, martensite and retained austenite or critical region ferrite, lath bainite, martensite and retained austenite.

[0049] The microstructure of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel described in the present invention includes proeutectoid ferrite, lath-like bainite, martensite and residual austenite or critical zone ferrite, lath-like bainite, martensite and residual austenite; it should be noted that the martensite structure in the steel has a relatively high hardness, which ensures the strength of the bridge steel; the hardness of ferrite and austenite is relatively low, which ensures the plasticity and toughness of the steel; in addition, the presence of ferrite, on the one hand, increases the large-angle grain boundaries of the steel and improves the fracture toughness, and on the other hand, improves the coordinated deformation ability of the steel and further improves the fracture toughness. The present invention adopts an alloy design of low C + high Ni + (Mn + Mo) + (Cr-Cu) + (Ti-Nb), and a low carbon component design to ensure low welding crack sensitivity and plastic toughness of bridge steel, with a welding crack sensitivity index Pcm (%) <0.25; a high Ni component design to improve low-temperature fracture toughness; an Mn + Mo design to expand the cooling rate range of bainite phase transformation to ensure that the full-thickness microstructure is mainly bainite during the cooling process of the steel plate after rolling and during the quenching cooling process in the critical zone; a Cr-Cu main corrosion-resistant alloy element design and matching of Ni, Mo, etc. to ensure the corrosion resistance of the bridge steel and ensure that the weathering index I of the bridge steel is greater than 6.5; and Nb-Ti composite microalloying is used to improve the strength of the steel to ensure the strength of the bridge steel and the core strength of the thick steel plate.

[0050] It should be noted that the weathering index of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel prepared by the present invention is I = 26.01Cu+ 3.88Ni+ 1.20Cr+ 1.49Si+ 17.28P- 7.29Cu×Ni- 9.10Ni×P-33.39Cu 2 >6.5, welding crack sensitivity index Pcm(%) = C+Si / 30+(Mn+Cu+Cr) / 20+Mo / 15+Ni / 60+V / 10+5B<0.25; while the weathering index is high, low welding crack sensitivity is guaranteed. The strength, low-temperature fracture toughness and weldability of the prepared bridge steel are well matched. While meeting the requirements of high strength and high fracture toughness, it has good welding performance, realizing the design of high weathering resistance and easy-to-weld bridge steel materials.

[0051] The bridge steel prepared by the present invention has a yield strength of ≥550MPa (such as 560~673MPa), a tensile strength of ≥700MPa (such as 707~816MPa), an elongation after fracture of ≥21.0%, and a yield strength ratio of ≤0.84 (such as 0.78~0.84); at the same time, it has good low-temperature toughness, -40℃ V-type impact energy ≥120J, and a ductile-brittle transition temperature of <-60℃ (such as -92℃~-68℃); at the same time, the steel plate has good weldability, and the -40℃ crack opening displacement value CTOD is greater than 0.25mm (such as 0.27-0.80mm).

[0052] The advantages of the precise control of the chemical composition, content and preparation process parameters of the present invention will be demonstrated below with specific examples and comparative examples.

[0053] Example

[0054] This embodiment discloses five types (Examples 1-5) of 550 MPa grade high-strength and high-fracture toughness weather-resistant bridge steels, and selects three types (Comparative Examples 1-3) of steels as comparative steels.

[0055] Examples 1-5 all use the same preparation process: smelting, controlled rolling, controlled cooling and heat treatment;

[0056] The smelting process includes batching according to chemical composition, converter smelting, refining outside the furnace, and continuous casting to obtain continuous casting slabs;

[0057] The controlled rolling process is: the controlled rolling process adopts two-stage rolling of rough rolling and finishing rolling, the starting rolling temperature of rough rolling is ≥1120℃, and the starting rolling temperature of finishing rolling is 850~870℃. It should be noted that before the two-stage rolling, the continuous casting slab needs to be heated to improve the plasticity of the steel, reduce the deformation resistance, and facilitate the subsequent rolling. The continuous casting slab rolling heating temperature is 1200-1250℃, and the insulation time is 2-3h.

[0058] The controlled cooling process is: water inlet laminar cooling + air cooling, the water inlet temperature is 780-800°C during laminar cooling, cooled to 450-500°C, and then the water outlet is air cooled to room temperature.

[0059] When the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 24≤H<36mm, the heat treatment process is: tempering, the tempering treatment temperature is 500~550℃, and the time is 30~60min.

[0060] When the steel plate thickness H of the 550MPa grade high-strength, high-fracture toughness weather-resistant bridge steel is 36mm≤H≤120mm, the heat treatment process is: critical zone quenching and tempering, the tempering treatment temperature is 500~550℃, the time is 30~60min, the critical zone quenching temperature is 760~820℃, the time is 30~60min, and the furnace is water-cooled to 500℃ or below, and then air-cooled.

[0061] Example 1 The thickness of the bridge steel is 24 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 1.9%, and the content of Ni is 0.8% (when the thickness of the bridge steel plate is 24-36 mm, 1.5%≤Mn+Cr+Mo≤2.0%, 0.6%≤Ni≤1.1%);

[0062] The thickness of the bridge steel in Example 1 is 24 mm, the heat treatment is tempering, the tempering temperature is 510° C., and the time is 30 min, which meets the requirements of the present invention.

[0063] Example 2 The thickness of the bridge steel is 36 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 2.2% (when the thickness of the steel plate is ≥36 mm, 2.0%<Mn+Cr+Mo≤2.6%);

[0064] The thickness of the bridge steel in Example 2 is 36 mm, and the heat treatment is critical zone quenching and tempering. The tempering temperature is 550°C and the time is 40 min. The critical zone quenching temperature is 810°C and the time is 35 min. It is water-cooled to 490°C after being taken out of the furnace and then air-cooled, which meets the requirements of the present invention.

[0065] Example 3 The thickness of the bridge steel is 60 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 2.26% (when the thickness of the steel plate is ≥36 mm, 2.0%<Mn+Cr+Mo≤2.6%);

[0066] The thickness of the bridge steel in Example 3 is 60 mm, and the heat treatment is critical zone quenching and tempering. The tempering temperature is 530°C and the time is 37 min. The critical zone quenching temperature is 800°C and the time is 30 min. It is water-cooled to 460°C after being taken out of the furnace and then air-cooled, which meets the requirements of the present invention.

[0067] Example 4 The thickness of the bridge steel is 80 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 2.25%, and the content of Ni is 0.95% (when the thickness of the steel plate is ≥80 mm, 2.0%<Mn+Cr+Mo≤2.6%, 0.6%≤Ni≤1.1%);

[0068] The thickness of the bridge steel in Example 4 is 80 mm, and the heat treatment is critical zone quenching and tempering. The tempering temperature is 530°C and the time is 40 min. The critical zone quenching temperature is 770°C and the time is 30 min. It is water-cooled to 480°C after being taken out of the furnace and then air-cooled, which meets the requirements of the present invention.

[0069] The thickness of the bridge steel of Example 5 is 120 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 2.13%, and the content of Ni is 0.95% (when the thickness of the steel plate is ≥80 mm, 2.0%<Mn+Cr+Mo≤2.6%, 0.6%≤Ni≤1.1%);

[0070] The thickness of the bridge steel in Example 5 is 120 mm, and the heat treatment is critical zone quenching and tempering. The tempering temperature is 520°C and the time is 40 min. The critical zone quenching temperature is 780°C and the time is 30 min. It is water-cooled to 450°C after being taken out of the furnace and then air-cooled, which meets the requirements of the present invention.

[0071] The thickness of the bridge steel of Comparative Example 1 is 24 mm, and the content of each element in the chemical composition meets the requirements of the present invention, but the content of Mn+Cr+Mo is 2.29%, and the content of Ni is 0.44% (when the thickness of the bridge steel plate is 24-36 mm, 1.5%≤Mn+Cr+Mo≤2.0%, 0.6%≤Ni≤1.1%) is not met);

[0072] The thickness of the bridge steel in Comparative Example 1 is 24 mm, the heat treatment is tempering, the tempering temperature is 510° C., and the time is 30 min, but the laminar cooling temperature during the post-rolling cooling process is 400° C., which does not meet the requirements of the present invention.

[0073] The thickness of the bridge steel in Comparative Example 2 is 60 mm, and Ni is not added in the chemical composition, which does not meet the requirements of the present invention;

[0074] The thickness of the bridge steel in Comparative Example 2 is 60 mm, and the heat treatment is critical zone quenching + tempering. The tempering temperature is 530°C and the time is 37 min. The critical zone quenching temperature is 810°C and the time is 30 min. It is water-cooled to 460°C after being taken out of the furnace and then air-cooled, which meets the requirements of the present invention.

[0075] The thickness of the bridge steel of Comparative Example 3 is 80 mm. In the chemical composition, the content of each element meets the requirements of the present invention, and the content of Mn+Cr+Mo is 2.28%, and the content of Ni is 0.98% (when the thickness of the steel plate is ≥80 mm, 2.0%<Mn+Cr+Mo≤2.6%, 0.6%≤Ni≤1.1%).

[0076] The thickness of the bridge steel in Comparative Example 3 is 80 mm, and its heat treatment method should be a heat treatment process of: critical zone quenching + tempering, but the heat treatment method used in Comparative Example 3 is tempering, which does not meet the requirements of the present invention.

[0077] Table 1 is a table of chemical compositions of the bridge steels of the embodiments and comparative examples; Table 2 is a table of preparation process parameters of the bridge steels of the embodiments and comparative examples; Table 3 is a table of properties of the bridge steels of the embodiments and comparative examples; Table 4 is a table of microstructures of the bridge steels of the embodiments and comparative examples.

[0078]

[0079]

[0080]

[0081]

[0082] In Examples 1-5, the steel component design and preparation process parameters all meet the requirements of the present invention; the steel component design and laminar cooling temperature of Comparative Example 1 do not meet the requirements of the present invention, the steel component design of Comparative Example 2 does not meet the requirements of the present invention, and the steel component design of Comparative Example 3 meets the requirements of the present invention, but the heat treatment method during the preparation process does not meet the requirements of the present invention. As can be seen from Table 3, the performance of the bridge steel in the examples is better than that of the comparative examples.

[0083] By comparison, it can be seen that the present invention achieves excellent mechanical properties of the steel plate through the synergistic effect of reasonable steel composition design and strict control of process parameters. The yield strength is ≥550MPa, the tensile strength is ≥700MPa, the elongation after fracture is ≥21.0%, and the yield strength ratio is ≤0.84; at the same time, it has good low-temperature toughness, -40℃ V-type impact energy is ≥120J, and the ductile-brittle transition temperature is <-60℃; at the same time, the steel plate has good weldability, and the -40℃ crack opening displacement value CTOD is >0.25mm.

[0084] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel, characterized in that: The process includes the following steps: smelting, controlled rolling, controlled cooling and heat treatment; When the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 24≤H<36mm, the heat treatment process is tempering; When the steel plate thickness H of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is 36mm≤H≤120mm, the heat treatment process is intercritical quenching and tempering; The controlled rolling process adopts two-stage rolling of rough rolling and finishing rolling, the starting rolling temperature of rough rolling is ≥1120°C, and the starting rolling temperature of finishing rolling is 852~870°C; The controlled cooling process includes water inlet laminar cooling and air cooling. During laminar cooling, the water inlet temperature is 780-800°C, cooled to 450-500°C, and then cooled to room temperature by air outlet. The critical zone quenching temperature is 760-820°C, and the heat preservation time after through-heat preservation is 30-48 minutes; The tempering treatment temperature is 500-550°C, and the heat preservation time after through-heat treatment is 30-60 minutes; The microstructure of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel includes proeutectoid ferrite, lath-shaped bainite, martensite and retained austenite, or intercritical ferrite, lath-shaped bainite, martensite and retained austenite; The 550MPa grade high strength and high fracture toughness weather-resistant bridge steel has a crack opening displacement value CTOD>0.25mm at -40°C; The chemical composition of the 550MPa grade high-strength and high-fracture toughness weather-resistant bridge steel is as follows, by mass percentage: C 0.06-0.11%, Si 0.20-0.40%, Mn 0.90-1.50%, Cr 0.40-0.60%, Ni 0.3-1.1%, Cu 0.30-0.45%, Mo0.20-0.40%, Nb 0.02-0.04%, Ti 0.010-0.020%, and the rest is Fe and unavoidable impurities.

2. The preparation method according to claim 1, characterized in that: The controlled rolling process adopts two-stage rolling of rough rolling and finishing rolling, the starting rolling temperature of rough rolling is ≥1130°C, and the starting rolling temperature of finishing rolling is 854-868°C.

3. The preparation method according to claim 1, characterized in that: The controlled cooling process includes water inlet laminar cooling and air cooling. During laminar cooling, the water inlet temperature is 782-798° C., cooled to 455-495° C., and then the water outlet is air-cooled to room temperature.

4. The preparation method according to claim 1, characterized in that: The critical zone quenching temperature is 765-815° C., and the continued heat preservation time after through-heat preservation is 32-46 minutes.

5. The preparation method according to claim 1, characterized in that: The tempering treatment temperature is 505-545° C., and the heat preservation time after through-heat preservation is 32-58 minutes.

6. A 550MPa grade high strength and high fracture toughness weathering bridge steel, prepared by the preparation method according to any one of claims 1 to 5, characterized in that: The chemical composition by mass percentage is: C 0.06-0.11%, Si 0.20-0.40%, Mn0.90-1.50%, Cr 0.40-0.60%, Ni 0.3-1.1%, Cu 0.30-0.45%, Mo 0.20-0.40%, Nb 0.02-0.04%, Ti 0.010-0.020%, and the rest is Fe and unavoidable impurities.

7. The 550MPa grade high strength and high fracture toughness weather-resistant bridge steel according to claim 6 is characterized in that: When the steel plate thickness H of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 24≤H<36mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies: 1.5%≤Mn+Cr+Mo≤2.0% and 0.6%≤Ni≤1.1%.

8. The 550MPa grade high strength and high fracture toughness weather-resistant bridge steel according to claim 6, characterized in that: When the steel plate thickness of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 36mm≤H≤80mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies 2.0%<Mn+Cr+Mo≤2.5%.

9. The 550MPa grade high strength and high fracture toughness weathering bridge steel according to claim 6, characterized in that: When the steel plate thickness of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel is 80mm≤H≤120mm, the chemical composition of the 550MPa grade high-strength and high-fracture-toughness weather-resistant bridge steel satisfies 2.0%<Mn+Cr+Mo≤2.5% and 0.6%≤Ni≤1.1%.

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