Bridge steel with excellent plasticity and toughness and production method thereof

Abstract

The present invention discloses a bridge steel with excellent plasticity and toughness and a production method thereof. The chemical composition of the bridge steel, measured by mass percentage, includes: C: 0.11-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.39%, Nb: 0.011-0.019%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P ≤ 0.0151%, S ≤ 0.0080%, N ≤ 0.0045%, with the remainder being iron and unavoidable impurities. The steel has a thickness of 6-64 mm, an impact energy KV2 of 280 J or greater at 0°C, 260 J at -20°C, and 250 J at -40°C, an elongation of 30% or greater, and a yield strength ratio of 0.75 or less.

Description

Technical Field

[0001] The invention belongs to the technical field of steel alloy materials and relates to a bridge steel with excellent plasticity and toughness and a production method thereof. Background Art

[0002] Structural steel for bridges is mainly used in key projects such as the construction of railways, highways, cross-sea and cross-river bridges. Due to the particularity of its use environment, it has strict requirements on comprehensive performance.

[0003] In existing manufacturing, high alloying content of precious metals such as Ni, Mo, and Cr is often used in chemical compositions to improve toughness and ductility. However, this leads to poor weldability, impacting the steel's application in bridge engineering. It also increases production difficulties, for example, requiring post-rolling heat treatment and resulting in long production cycles. Furthermore, the excessive addition of alloying content significantly increases steel production costs. Summary of the Invention

[0004] The object of the present invention is to provide a bridge steel with excellent plasticity and toughness and a production method thereof.

[0005] To achieve the above-mentioned purpose, one embodiment of the present invention provides a bridge steel with excellent plasticity and toughness. The chemical composition of the bridge steel comprises, by mass percentage, the following: C: 0.11-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.39%, Nb: 0.011-0.019%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P ≤ 0.0151%, S ≤ 0.0080%, N ≤ 0.0045%, and the remainder is iron and unavoidable impurities;

[0006] The thickness of the bridge steel is 6 to 64 mm, the impact energy KV2 at 0°C is ≥280J, the impact energy KV2 at -20°C is ≥260J, the impact energy KV2 at -40°C is ≥250J, the elongation is ≥30%, and the yield strength ratio is ≤0.75.

[0007] As a further improvement of one embodiment, the carbon equivalent CEV of the chemical composition of the bridge steel satisfies 0.328 to 0.371.

[0008] As a further improvement of one embodiment, the chemical composition of the bridge steel includes, in mass percentage, P: 0.0101-0.0151%, S: 0.0020-0.0080%, and N: 0.0025-0.0045%.

[0009] As a further improvement of one embodiment, the bridge steel has a duplex structure of deformation induced ferrite + polygonal ferrite + acicular ferrite + bainite.

[0010] As a further improvement of one embodiment, the area of ​​deformation-induced ferrite in the metallographic sampling surface accounts for approximately 35% to 45%, the area of ​​polygonal ferrite in the metallographic sampling surface accounts for approximately 35% to 45%, the area of ​​acicular ferrite in the metallographic sampling surface accounts for approximately 10% to 20%, and the area of ​​bainite in the metallographic sampling surface accounts for approximately 0% to 10%.

[0011] As a further improvement of one embodiment, the yield strength of the bridge steel is ≥345 MPa, and the tensile strength is ≥510 MPa.

[0012] To achieve the above-mentioned purpose, one embodiment of the present invention provides a method for producing the bridge steel. The method comprises:

[0013] Heat the steel billet in the heating furnace to a temperature above T1+10℃ and below T2, where T1 is T NbC 、T NbN and T TiC The maximum value of T2 is T1+50℃, T TiN -150℃, the larger value; where T NbC 、T NbN 、T TiC and T TiN are the complete solution temperatures of NbC, NbN, TiC and TiN respectively;

[0014] First, the heated billet is rolled at a rolling temperature of T3 or higher and T3+40℃ or lower to a thickness of 4 to 6.8 times that of the finished product to obtain the first intermediate plate; then, the billet is rolled at a rolling temperature of T4-50℃ or higher and T4+20℃ or lower to a thickness of 1.8 to 3.2 times that of the finished product to obtain the second intermediate plate; finally, the billet is rolled at a rolling temperature of T5-55℃ or higher and T5-15℃ or lower to the thickness of the finished product; T3≥T4≥T5, T5=910-310[C]-80[Mn]-15[Cr], -15.2[Ni]+44.7[Si];

[0015] The steel plate is air-cooled at a cooling rate of 1 to 10°C / s to a temperature between T6-50°C and T6-30°C, then water-cooled at a cooling rate of 8 to 30°C / s to a temperature between T7+90°C and T7+130°C, and then naturally cooled to room temperature to obtain a finished steel plate; T5 ≥ T6 ≥ T7, T7 = 539-423[C]-17.7[Ni]-12.1[Cr]-11[Si]-30.4[Mn],

[0016] T6=830-270[C]-90[Mn]-37[Ni]-70[Cr];

[0017] Among them, [C], [Nb], [Ti], [Al], [Si], [Ni], [Mn], and [Cr] are the mass percentages of C, Nb, Ti, Al, Si, Ni, Mn, and Cr in the steel billet, respectively.

[0018] As a further improvement of one embodiment, the steel billet stays at a temperature above T1+10°C and below T2 for 0.07-0.13 min / mm, and the total time in the heating furnace is 1.15-1.35 min / mm.

[0019] As a further improvement of one embodiment, the steel billet adopts a five-stage process in the heating furnace, including a heat recovery section, a preheating section, a first heating section, a second heating section and a soaking section. The temperature of the first heating section is 1120±15°C, the temperature of the second heating section is above T1+10°C and below T2, the temperature of the soaking section is lower than the temperature of the second heating section, and the temperature of the soaking section is 20~40°C lower than the temperature of the second heating section.

[0020] As a further improvement of one embodiment, the steel billet adopts a three-stage process in the heating furnace, including a preheating section, a heating section and a soaking section. The temperature of the heating section is above T1+10°C and below T2, the temperature of the soaking section is lower than the temperature of the heating section, and the temperature of the soaking section is 20 to 40°C lower than the temperature of the heating section.

[0021] As a further improvement of one embodiment, when rolling to obtain the first intermediate plate, the starting rolling temperature is greater than T3+20°C and less than T3+40°C, and the finishing rolling temperature is greater than T3 and less than T3+20°C;

[0022] When rolling to obtain the second intermediate plate, the starting rolling temperature is above T4 and below T4+20°C, and the finishing rolling temperature is above T4-50°C and below T4-30°C;

[0023] When the second intermediate plate is rolled to the finished thickness, the starting rolling temperature is higher than T5-30°C and lower than T5-15°C, and the finishing rolling temperature is higher than T5-55°C and lower than T5-40°C.

[0024] Compared with the prior art, the beneficial effects of one embodiment are:

[0025] (1) Compared with the existing high-content Ni, Mo, and Cr components, the content of these alloys is greatly reduced. Through the design of chemical composition, the low alloy content eliminates the defects of poor welding performance caused by high alloy content while still ensuring the high plasticity and toughness of the steel. In addition, the production difficulty of the steel is low;

[0026] (2) On the basis of chemical composition, combined with specific process control of heating, rolling and cooling, precise control of microstructure is achieved, ensuring high plasticity and toughness of steel (that is, the plasticity and toughness brought by these elements are not reduced due to low element content as commonly known). At the same time, the welding performance is improved, and ultimately the comprehensive improvement of steel plates in terms of low yield ratio, easy welding, high strength and toughness, and good plate shape is achieved. In addition, the production cost is low, the process is short, and the difficulty is small. The resulting steel plates are suitable for bridge engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 FIG. 4 is a metallographic structure diagram of a steel plate according to an embodiment of the present invention. DETAILED DESCRIPTION

[0028] One embodiment of the present invention provides a bridge steel having a thickness of 6 to 64 mm, an impact energy KV2 of ≥280 J at 0°C, an impact energy KV2 of ≥260 J at -20°C, an impact energy KV2 of ≥250 J at -40°C, an elongation of ≥30%, a yield strength ratio of ≤0.75, and excellent plasticity and toughness.

[0029] The chemical composition of the bridge steel includes, by mass percentage, C: 0.11-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.39%, Nb: 0.011-0.019%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the rest is iron and unavoidable impurities.

[0030] The role and control of each chemical component are described in detail below.

[0031] C: Carbon is the most economical strengthening element in steel. It has a significant solid solution strengthening effect and can effectively reduce the yield strength ratio. At the same time, it forms carbides with Nb, Ti, Cr, Mo, etc., which has a precipitation strengthening effect. When the carbon content is lower than 0.08%, the effect of increasing strength and reducing the yield strength ratio is not obvious. When the carbon content is higher than 0.11%, it will lead to deterioration of toughness and welding performance.

[0032] Si: Silicon has a solid solution strengthening effect in steel. When the Si content is lower than 0.11%, the solid solution strengthening effect is not obvious; when the Si content exceeds 0.18%, Fe2SiO4 is easily generated on the surface of the continuous casting billet, which is difficult to remove during descaling and is detrimental to the surface quality of the steel plate.

[0033] Mn: Manganese plays a role in solid solution strengthening in steel and is the most economical strengthening element besides carbon. When the Mn content is lower than 1.39%, the solid solution strengthening effect is not obvious; when the Mn content exceeds 1.47%, it is easy to form segregation in the center of the slab, reducing the low-temperature toughness of the steel plate and at the same time being detrimental to the welding performance.

[0034] Nb: Niobium is an important grain-refining element in steel. When the Nb content exceeds 0.021%, niobium carbonitride precipitates are formed during the rolling process, refining the recrystallized grains. During the cooling process, niobium can continue to precipitate in the form of carbonitrides, refining the microstructure after phase transformation and improving strength and toughness. However, when the Nb content is greater than 0.029%, the alloy cost will increase, and precipitation strengthening will be detrimental to the yield strength ratio.

[0035] Ti: Titanium is a nitrogen-fixing element in steel. It can form dispersed titanium nitride particles, which inhibit the coarsening of austenite grains during billet heating and rolling. However, when the Ti content is less than 0.009%, Ti cannot fully play its role. When the Ti content is greater than 0.017%, titanium carbonitride precipitation is easily formed in the core of the ingot, affecting the low-temperature toughness of the steel plate core.

[0036] Al: Aluminum is a deoxidizing element in steel. When the content is lower than 0.026%, Al cannot effectively achieve its deoxidation effect. When the content is higher than 0.046%, the Al2O3 inclusions in the steel increase, affecting the low-temperature toughness of the steel.

[0037] P, S, and N: impurity elements. However, S can also form MnS inclusions with Mn, reducing the low-temperature toughness of the steel. In this application, P ≤ 0.02%, S ≤ 0.0080%, and N ≤ 0.0045%. In some embodiments, the P content is preferably 0.0101-0.0151%, the N content is preferably 0.0025-0.0045%, and the S content is preferably 0.0020-0.0080%.

[0038] Thus, the chemical composition of the present invention greatly reduces the content of these alloys compared to existing compositions with high contents of Ni, Mo, and Cr. Through the design of the chemical composition, with a low alloy content, the defects of poor welding performance caused by high alloy content are eliminated while still ensuring the high plasticity and toughness of the steel. In addition, the production difficulty of the steel is low.

[0039] Furthermore, the bridge steel has a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite. Here, the metallographic structure of the steel can be obtained by conducting a structure test in accordance with GB / T 15125-2009 "Specimens and test methods for metallographic examination of metallic materials".

[0040] Among them, the area of ​​deformation-induced ferrite in the metallographic sampling surface accounts for approximately 35% to 45%, the area of ​​polygonal ferrite in the metallographic sampling surface accounts for approximately 35% to 45%, the area of ​​acicular ferrite in the metallographic sampling surface accounts for approximately 10% to 20%, and the area of ​​bainite in the metallographic sampling surface accounts for approximately 0% to 10%.

[0041] As previously mentioned, the bridge steel is a plate with a thickness of 6-64 mm, a yield strength ≥ 345 MPa, a tensile strength ≥ 510 MPa, an elongation ≥ 30%, and a yield strength ratio ≤ 0.75. Mechanical property testing can be performed in accordance with GB / T 228.1-2021, "Tensile Tests on Metallic Materials Part 1: Room Temperature Test Methods."

[0042] In terms of low-temperature toughness, the steel has an impact energy KV2 of ≥280 J at 0°C, ≥260 J at -20°C, and ≥250 J at -40°C. Low-temperature toughness testing can be performed in accordance with GB / T 229-2007, "Metallic Materials Charpy Pendulum Impact Test Method."

[0043] Furthermore, in terms of chemical composition, the carbon equivalent CEV of the steel satisfies 0.328 to 0.371.

[0044] Among them, carbon equivalent CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15, where [C], [Mn], [Cr], [Mo], [V], [Cu], and [Ni] represent the mass percentages of elements such as C, Mn, Cr, Mo, V, Cu, and Ni in the steel. For example, when the mass percentage of C in the steel is 0.09%, the mass percentage of C is 0.09, and this value is substituted into the CEV formula.

[0045] The steel has excellent welding performance and excellent plate shape, with unevenness ≤1 mm / m, where the unit mm / m refers to mm per unit length of the steel plate, and the unit length is calculated in m.

[0046] Furthermore, an embodiment of the present invention also provides a method for producing the bridge steel.

[0047] The production method includes the following processes: heating-controlled rolling-controlled cooling and the like.

[0048] Specifically, in the heating process, the continuous casting billet having the chemical composition consistent with the above-mentioned chemical composition of the steel is heated in a heating furnace.

[0049] The continuous casting billet can be prepared using known technologies in the industry, generally using KR desulfurization, converter smelting, RH refining, LF refining, and continuous casting technologies.

[0050] During the heating process, the maximum temperature of the heating furnace is set to be greater than T1+10°C and less than T2. ​​In other words, the billet is heated in the heating furnace to a temperature greater than T1+10°C and less than T2. ​​Generally, the set temperature of the heating furnace is considered to represent the temperature reached by the billet within the heating furnace. It is not necessary to directly contact or measure the billet with a temperature measuring element to determine the billet's temperature.

[0051] T1 is T NbC 、T NbN and T TiC The maximum value in .

[0052] Among them, T NbC 、T NbN 、T TiC and T TiN are the complete solution temperatures of NbC, NbN, TiC and TiN respectively.

[0053] In an optional embodiment, the method can be implemented by lg([Nb]×[N])=3.70-10800 / (T NbN +273.15), lg([Nb]×[C])=2.26-6770 / (T NbC +273.15), lg([Ti]×[C])=5.33-10475 / (T TiC +273.15), lg([Ti]×[N])=0.32-8000 / (T TiN +273.15) and calculate T NbC 、T NbN 、T TiC and T TiN , [Nb], [N]), [C], and [Ti] are the mass percentages of Nb, N, C, and Ti in the steel billet, respectively.

[0054] Of course, in a variant embodiment, T NbC 、T NbN 、T TiC and T TiN It can also be obtained using methods known in the art, such as industry experience values.

[0055] T2 is T1+50℃, T TiN -150℃, the larger value.

[0056] In this way, by controlling the temperature in the heating furnace to be above T1+10℃ and below T2, it can be ensured that the Nb precipitates in the steel are completely dissolved, the Ti carbides are completely dissolved, and the Ti nitrides are retained, thereby preventing the abnormal growth of austenite grains and ultimately ensuring the comprehensive performance of the finished steel plate.

[0057] Preferably, the steel billet's residence time in the heating furnace at a temperature above T1+10°C and below T2 is 0.07 to 0.13 min / mm, and its total residence time in the heating furnace is 1.15 to 1.35 min / mm. Here, "min / mm" refers to one minute per unit thickness of the steel billet, with unit thickness measured in mm.

[0058] More specifically, in an optional embodiment, the steel billet may be heated in the heating furnace using a five-stage process comprising a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage.

[0059] In this five-stage process, the temperature of the heat recovery section is ≤900℃, the temperature of the preheating section is ≤1000℃, the temperature of the first heating section is 1120±15℃, and the temperature of the second heating section limits the maximum temperature of the steel billet, that is, the temperature of the second heating section is set to above T1+10℃ and below T2, and the residence time of the steel billet in the second heating section is 0.07~0.13min / mm.

[0060] Preferably, in the five-stage process, the temperature of the soaking stage is lower than that of the second heating stage, and the temperature of the soaking stage is 20-40° C. lower than that of the second heating stage. This arrangement can further improve the toughness of the finished steel plate.

[0061] In an optional variation, the steel billet is heated in the heating furnace using a three-stage process comprising a preheating section, a heating section, and a soaking section.

[0062] Among them, in this three-stage process, the temperature of the preheating section is ≤1000℃, and the temperature of the heating section limits the maximum temperature of the steel billet, that is, the temperature of the heating section is set to above T1+10℃ and below T2, and the residence time of the steel billet in the heating section is 0.07~0.13min / mm.

[0063] Preferably, in the five-stage process, the temperature of the soaking stage is lower than that of the heating stage, and the temperature of the soaking stage is 20-40° C. lower than that of the heating stage. This arrangement can further improve the toughness of the finished steel plate.

[0064] Next, in the controlled rolling process:

[0065] First, the steel billet after exiting the heating furnace is rolled at a rolling temperature above T3 and below T3+40°C to a thickness 4 to 6.8 times that of the finished product to obtain a first intermediate plate; preferably, the reduction in each pass is ≥20 mm;

[0066] Then rolling at a rolling temperature of T4-50°C or higher and T4+20°C or lower to a thickness of 1.8 to 3.2 times that of the finished product to obtain a second intermediate plate; preferably, the reduction in each pass is ≥12 mm;

[0067] Finally, the product is rolled at a rolling temperature of above T5-55°C and below T5-15°C to a finished thickness of, for example, 6 to 64 mm. Preferably, the reduction in each pass is ≥4 mm.

[0068] Among them, T3≥T4≥T5, T5=910-310[C]-80[Mn]-15[Cr],

[0069] [C], [Nb], [Ti], [Al], [Si], and [Ni] are the mass percentages of C, Nb, Ti, Al, Si, and Ni in the steel billet, respectively.

[0070] That is to say, one embodiment of the present invention controls the temperature of the steel billet during rolling to 4 to 6.8 times the thickness of the finished product, the temperature during rolling from 4 to 6.8 times the thickness of the finished product to 1.8 to 3.2 times the thickness of the finished product, and the temperature during rolling from 1.8 to 3.2 times the thickness of the finished product to completion. In this way, on the one hand, sufficient recrystallization and refinement of the recrystallized grains can be ensured first, and on the other hand, the recrystallized equiaxed grains can be deformed and flattened. On the other hand, rolling deformation can be used to induce ferrite phase transformation, and ferrite phase transformation occurs on the deformation band or in the grain boundary to obtain fine ferrite grains, thereby ensuring the yield strength and good toughness of the steel plate.

[0071] More specifically, during rolling the steel slab into the first intermediate plate, the start rolling temperature is greater than or equal to T3+20°C and less than or equal to T3+40°C, and the finish rolling temperature is greater than or equal to T3 and less than or equal to T3+20°C.

[0072] Furthermore, preferably, during rolling the first intermediate plate into the second intermediate plate, the start rolling temperature is greater than T4 and less than T4+20°C, and the finish rolling temperature is greater than T4-50°C and less than T4-30°C.

[0073] When the second intermediate plate is rolled to the finished thickness, the starting rolling temperature is higher than T5-30°C and lower than T5-15°C, and the finishing rolling temperature is higher than T5-55°C and lower than T5-40°C.

[0074] Next, in the controlled cooling process, the steel plate is air-cooled to above T6-50°C and below T6-30°C at a cooling rate of 1-10°C / s, then water-cooled to above T7+90°C and below T7+130°C at a cooling rate of 8-30°C / s, and then naturally cooled to room temperature to obtain a finished steel plate.

[0075] Among them, T5≥T6≥T7, T7=539-423[C]-17.7[Ni]-12.1[Cr]-11[Si]-30.4[Mn], T6=830-270[C]-90[Mn]-37[Ni]-70[Cr], [C], [Si], [Ni], [Mn], [Cr] are the mass percentages of C, Si, Ni, Mn, and Cr in the steel billet, respectively.

[0076] That is, in one embodiment of the present invention, the cooling of a hot-rolled steel plate is divided into three stages with different cooling rates and cooling methods. Thus, in the first cooling stage, a ferrite phase transformation occurs, and the presence of ferrite ensures the low yield strength of the steel plate. Furthermore, in the second stage, the carbon-rich, supercooled austenite phase in the steel plate is rapidly cooled, causing it to transform into an acicular ferrite / bainite structure. The acicular ferrite / bainite phase transformation is completed by controlling the final cooling temperature. Furthermore, controlling the cooling rate effectively avoids the formation of martensite or pearlite phases. Furthermore, the above temperature control can also improve the shape of the finished steel plate. Taking all of the above aspects into consideration, the steel plate ultimately achieves high tensile strength, low yield ratio, excellent low-temperature toughness, excellent shape, and good weldability.

[0077] Specifically, cooling can be carried out in an ultra-fast cooling system. For example, the cooling water manifolds in the front section of the system can be closed and the roller speed can be 0.5 to 2.2 m / s, so as to cool at a low cooling rate; while in the rear section of the system, 5 to 24 groups of cooling water manifolds can be opened and the roller speed can be 0.6 to 2.3 m / s, so as to achieve rapid cooling.

[0078] In summary, the beneficial effects of the present invention are:

[0079] (1) Compared with the existing high-content Ni, Mo, and Cr components, the content of these alloys is greatly reduced. Through the design of chemical composition, the low alloy content eliminates the defects of poor welding performance caused by high alloy content while still ensuring the high plasticity and toughness of the steel. In addition, the production difficulty of the steel is low;

[0080] (2) On the basis of chemical composition, combined with specific process control of heating, rolling and cooling, precise control of microstructure is achieved, ensuring high plasticity and toughness of steel (that is, the plasticity and toughness brought by these elements are not reduced due to low element content as commonly known). At the same time, the welding performance is improved, and ultimately the comprehensive improvement of steel plates in terms of low yield ratio, easy welding, high strength and toughness, and good plate shape is achieved. In addition, the production cost is low, the process is short, and the difficulty is small. The resulting steel plates are suitable for bridge engineering.

[0081] Some specific embodiments are described below. In these embodiments, the chemical composition of the steel is shown in Table 1.

[0082] [Table 1]

[0083]

[0084] The metallographic structure of the steel in each example is a duplex structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite. The deformation-induced ferrite accounts for approximately 35% to 45% of the metallographic sampling surface area, the polygonal ferrite accounts for approximately 35% to 45% of the metallographic sampling surface area, the acicular ferrite accounts for approximately 10% to 20% of the metallographic sampling surface area, and the bainite accounts for approximately 0% to 10% of the metallographic sampling surface area.

[0085] Furthermore, the thickness and properties of the steel of each embodiment are shown in Table 2. In addition, the steel plate roughness of each embodiment is ≤1 mm / m, and the welding performance is excellent.

[0086] [Table 2]

[0087]

[0088]

[0089] The preparation process of each of the above embodiments is as follows: preparing a continuous casting billet through steelmaking and continuous casting; preparing a finished steel plate by adopting a five-stage heating, controlled rolling, and controlled cooling process route for the obtained continuous casting billet; wherein, during the controlled cooling process, first air cooling to a first cooling temperature at a cooling rate of 1 to 10°C / s, then water cooling to a second cooling temperature at a cooling rate of 5 to 30°C / s, and then naturally cooling to room temperature.

[0090] Among them, the important process parameters of each embodiment are shown in Table 3 and Table 4.

[0091] [Table 3]

[0092]

[0093] [Table 4]

[0094]

Claims

1. A bridge steel with excellent plasticity and toughness, characterized in that: The chemical composition of the bridge steel includes, by mass percentage, C: 0.11-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.39%, Nb: 0.011-0.019%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the remainder is iron and unavoidable impurities; The production method of the bridge steel comprises: Heat the steel billet in the heating furnace to a temperature above T1+10℃ and below T2, where T1 is T NbC 、T NbN and T TiC The maximum value of T2 is T1+50℃, T TiN -150℃, the larger value; where T NbC 、T NbN 、T TiC and T TiN are the complete solution temperatures of NbC, NbN, TiC and TiN respectively; First, the heated billet is rolled at a rolling temperature of T3 or higher and T3+40℃ or lower to a thickness of 4 to 6.8 times the finished product thickness to obtain the first intermediate plate; then, the billet is rolled at a rolling temperature of T4-50℃ or higher and T4+20℃ or lower to a thickness of 1.8 to 3.2 times the finished product thickness to obtain the second intermediate plate; finally, the billet is rolled at a rolling temperature of T5-55℃ or higher and T5-15℃ or lower to the finished product thickness; T3≥T4≥T5, T3=887+464[C]+6445[Nb]-644 +890[Ti]+363[Al]-357[Si], T5=910-310[C]-80[Mn]-15[Cr], T4=910-203 -15.2[Ni]+44.7[Si]; The steel plate is air-cooled at a cooling rate of 1-10°C / s to a temperature between T6-50°C and T6-30°C, then water-cooled at a cooling rate of 8-30°C / s to a temperature between T7+90°C and T7+130°C, and then naturally cooled to room temperature to obtain a finished steel plate; T5 ≥ T6 ≥ T7, T7 = 539-423[C]-17.7[Ni]-12.1[Cr]-11[Si]-30.4[Mn], T6 = 830-270[C]-90[Mn]-37[Ni]-70[Cr]; Wherein, [C], [Nb], [Ti], [Al], [Si], [Ni], [Mn], and [Cr] are the mass percentages of C, Nb, Ti, Al, Si, Ni, Mn, and Cr in the steel billet, respectively; The thickness of the bridge steel is 6~64mm, the impact energy KV2 at 0℃ is ≥280J, the impact energy KV2 at -20℃ is ≥260J, the impact energy KV2 at -40℃ is ≥250J, the elongation is ≥30%, and the yield strength ratio is ≤0.

75.

2. The bridge steel with excellent plasticity and toughness according to claim 1, characterized in that: The carbon equivalent CEV of the chemical composition of the bridge steel satisfies 0.328~0.

371.

3. The bridge steel with excellent plasticity and toughness according to claim 1, characterized in that: The chemical composition of the bridge steel includes, by mass percentage, P: 0.0101-0.0151%, S: 0.0020-0.0080%, and N: 0.0025-0.0045%.

4. The bridge steel with excellent plasticity and toughness according to claim 1, characterized in that: The bridge steel has a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area of ​​the deformation-induced ferrite in the metallographic sampling surface accounts for 35% to 45%, the area of ​​the polygonal ferrite in the metallographic sampling surface accounts for 35% to 45%, the area of ​​the acicular ferrite in the metallographic sampling surface accounts for 10% to 20%, and the area of ​​the bainite in the metallographic sampling surface accounts for 0% to 10%.

5. The bridge steel with excellent plasticity and toughness according to claim 1, characterized in that: The yield strength of the bridge steel is ≥345MPa, and the tensile strength is ≥510MPa.

6. A method for producing bridge steel with excellent plasticity and toughness according to claim 1, characterized in that: include: Heat the steel billet in the heating furnace to a temperature above T1+10℃ and below T2, where T1 is T NbC 、T NbN and T TiC The maximum value of T2 is T1+50℃, T TiN -150℃, the larger value; where T NbC 、T NbN 、T TiC and T TiN are the complete solution temperatures of NbC, NbN, TiC and TiN respectively; First, the heated billet is rolled at a rolling temperature of T3 or higher and T3+40℃ or lower to a thickness of 4 to 6.8 times the finished product thickness to obtain the first intermediate plate; then, the billet is rolled at a rolling temperature of T4-50℃ or higher and T4+20℃ or lower to a thickness of 1.8 to 3.2 times the finished product thickness to obtain the second intermediate plate; finally, the billet is rolled at a rolling temperature of T5-55℃ or higher and T5-15℃ or lower to the finished product thickness; T3≥T4≥T5, T3=887+464[C]+6445[Nb]-644 +890[Ti]+363[Al]-357[Si], T5=910-310[C]-80[Mn]-15[Cr], T4=910-203 -15.2[Ni]+44.7[Si]; The steel plate is air-cooled at a cooling rate of 1-10°C / s to a temperature between T6-50°C and T6-30°C, then water-cooled at a cooling rate of 8-30°C / s to a temperature between T7+90°C and T7+130°C, and then naturally cooled to room temperature to obtain a finished steel plate; T5 ≥ T6 ≥ T7, T7 = 539-423[C]-17.7[Ni]-12.1[Cr]-11[Si]-30.4[Mn], T6 = 830-270[C]-90[Mn]-37[Ni]-70[Cr]; Among them, [C], [Nb], [Ti], [Al], [Si], [Ni], [Mn], and [Cr] are the mass percentages of C, Nb, Ti, Al, Si, Ni, Mn, and Cr in the steel billet, respectively.

7. The method for producing bridge steel with excellent plasticity and toughness according to claim 6, characterized in that: The residence time of the steel billet at a temperature above T1+10℃ and below T2 is 0.07~0.13min / mm, and the total time in the heating furnace is 1.15~1.35min / mm.

8. The method for producing bridge steel with excellent plasticity and toughness according to claim 6, characterized in that: The steel billet adopts a five-stage process in the heating furnace, including a heat recovery section, a preheating section, a first heating section, a second heating section and a soaking section. The temperature of the first heating section is 1120±15℃, the temperature of the second heating section is above T1+10℃ and below T2, the temperature of the soaking section is lower than that of the second heating section, and the temperature of the soaking section is 20~40℃ lower than that of the second heating section.

9. The method for producing bridge steel with excellent plasticity and toughness according to claim 6, characterized in that: The steel billet adopts a three-stage process in the heating furnace, including a preheating section, a heating section and a soaking section. The temperature of the heating section is above T1+10℃ and below T2. The temperature of the soaking section is lower than that of the heating section, and the temperature of the soaking section is 20~40℃ lower than that of the heating section.

10. The method for producing bridge steel with excellent plasticity and toughness according to claim 6, characterized in that: When rolling to obtain the first intermediate plate, the starting rolling temperature is above T3+20°C and below T3+40°C, and the finishing rolling temperature is above T3 and below T3+20°C; When rolling to obtain the second intermediate plate, the starting rolling temperature is above T4 and below T4+20°C, and the finishing rolling temperature is above T4-50°C and below T4-30°C; When the second intermediate plate is rolled to the finished thickness, the starting rolling temperature is higher than T5-30°C and lower than T5-15°C, and the finishing rolling temperature is higher than T5-55°C and lower than T5-40°C.

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