Method for producing high-strength high-toughness bridge steel having low yield ratio and good weldability

Abstract

A method for producing a high-strength high-toughness bridge steel having a low yield ratio and good weldability. The steel comprises the chemical components of: C: 0.08-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.47%, Nb: 0.011-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr≤0.13%, and Ni≤0.13%. The method comprises: first, rolling a heated steel slab to a thickness of (4-6.8)t at a temperature of T3 to T3+40°C, then rolling same to a thickness of (1.8-3.2)t at a temperature of T4-50°C to T4+20°C, and finally rolling same to a thickness of t at a temperature of T5-55°C to T5; and air-cooling same to T6-50°C to T6+20°C at a cooling rate of 1-10°C / s, and then water-cooling same to T7-30°C to T7+130°C at a cooling rate of 5-30°C / s, so as to obtain a finished steel plate product.

Description

Production method of low yield ratio easy-to-weld high toughness bridge steel

[0001] The present application claims priority to the Chinese patent application with the application date of 2024-08-13, the application number of 202411103094.5, and the invention name of "Production method of low yield ratio easy-to-weld high toughness bridge steel", the whole content of which is incorporated herein by reference. TECHNICAL FIELD

[0002] The present application belongs to the technical field of steel alloy materials, and relates to a production method of low yield ratio easy-to-weld high toughness bridge steel. BACKGROUND

[0003] With the rapid development of expressways and high-speed railways, the steel bridge manufacturing industry has developed rapidly. Steel bridges are developing towards multifunctionality, multi-lane, heavy load, large span, etc. while meeting the functionality. Bridge structural steel is mainly used in key projects such as railway, highway, sea-crossing, and river-crossing bridge construction. Due to the particularity of its use environment, it has strict requirements on comprehensive performance, such as low yield ratio, good weldability, high toughness, etc.

[0004] In the existing manufacturing of bridge steel plates, in order to improve the mechanical properties (toughness, strength, yield ratio, etc.), the chemical composition system usually adopts high alloy content of noble metals such as Ni, Mo, and Cr, but this will cause the defect of poor weldability, affecting the application effect of the steel plate in bridge engineering, and also causing the production difficulty to increase, for example, it must rely on post-rolling heat treatment, and the production cycle is long. Moreover, with the excessive addition of high alloy content, the production cost of the steel material is also greatly increased. SUMMARY

[0005] In order to solve the problem that the existing bridge steel plate cannot balance the mechanical properties, the purpose of the present application is to provide a production method of steel.

[0006] To achieve the above-mentioned purpose of the application, an embodiment of the present application provides a production method of steel. The production method comprises the steps of:

[0007] A steel billet is obtained by molten steel smelting and casting, and the chemical composition of the steel billet includes, in terms of mass percentage, C: 0.08-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.47%, Nb: 0.011-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr≤0.13%, Ni≤0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the rest is iron and unavoidable impurities;

[0008] The steel billet is heated to above T1+10℃ and below T2 in a heating furnace, T1 is TNbC , T NbN and the maximum value among T TiC , T TiN -150℃; wherein, T NbC , T NbN , T TiC and T TiN are complete solid solution temperatures of NbC, NbN, TiC and TiN respectively;

[0009] After the heating furnace, the billet is firstly rolled to 4-6.8 times of the finished product thickness at a rolling temperature of T3 or above and T3+40℃ or below, to obtain a first intermediate plate; then rolled to 1.8-3.2 times of the finished product thickness at a rolling temperature of T4-50℃ or above and T4+20℃ or below, to obtain a second intermediate plate; finally rolled to the finished product thickness at a rolling temperature of T5-55℃ or above and T5 or below;

[0010] The rolled steel plate is air-cooled to T6-50℃ or above and T6+20℃ or below at a cooling rate of 1-10℃ / s, then water-cooled to T7-30℃ or above and T7+130℃ or below at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature, to obtain the finished steel plate;

[0011] T3≥T4≥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], T5=910-310[C]-80[Mn]-15[Cr], [C], [Nb], [Ti], [Al], [Si], [Ni], [Mn], [Cr] are mass percentages of C, Nb, Ti, Al, Si, Ni, Mn, Cr in the billet respectively.

[0012] Preferably, the chemical composition of the billet comprises, in mass percentage: P: 0.0101-0.0151%, S: 0.0020-0.0080%, N: 0.0025-0.0045%.

[0013] Preferably, the carbon equivalent CEV of the chemical composition of the billet satisfies 0.318-0.380.

[0014] Preferably, the chemical composition of the billet includes, in 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 balance being iron and inevitable impurities.

[0015] Preferably, the chemical composition of the billet includes, in mass percentage: C: 0.10-0.13%, Si: 0.11-0.18%, Mn: 1.34-1.42%, Nb: 0.014-0.022%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

[0016] Preferably, the chemical composition of the billet includes, in mass percentage: C: 0.09-0.12%, Si: 0.11-0.18%, Mn: 1.37-1.45%, Nb: 0.024-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

[0017] Preferably, the chemical composition of the billet includes, in mass percentage: C: 0.08-0.11%, Si: 0.11-0.18%, Mn: 1.39-1.47%, Ni: 0.07-0.13%, Nb: 0.021-0.029%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr: 0.07-0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

[0018] Preferably, the billet stays at a temperature above T1+10℃ 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] Preferably, the billet is heated in the heating furnace by a five-stage process including a heat recovery stage, a preheating stage, a first adding stage, a second adding stage, and a soaking stage, the temperature of the second adding stage is above T1+10℃ and below T2, the temperature of the soaking stage is lower than that of the second adding stage, and the temperature of the soaking stage is 20-40℃ lower than that of the second adding stage.

[0020] Alternatively, the billet is heated in a heating furnace using a three-stage process including a preheating stage, a heating stage and a soaking stage, the temperature of the heating stage is above T1+10℃ and below T2, the temperature of the soaking stage is lower than the temperature of the heating stage, and the temperature of the soaking stage is 20-40℃ lower than the temperature of the heating stage.

[0021] Preferably, in the five-stage process, the temperature of the first stage is 1120±15℃;

[0022] wherein T1, T2, T3, T4 and T5 are calculated by lg([Nb]×[N])=3.70-10800 / (T+273.15), lg([Nb]×[C])=2.26-6770 / (T+273.15), lg([Ti]×[C])=5.33-10475 / (T+273.15), lg([Ti]×[N])=0.32-8000 / (T+273.15), and lg([C]×[N])=2.48-7000 / (T+273.15) respectively, and [Nb], [N], [C], [Ti] are the mass percentages of Nb, N, C, Ti in the billet respectively. NbN NbC TiC TiN NbN NbC TiC TiN

[0023] Preferably, the second intermediate plate is rolled to the finished thickness at a rolling temperature of above T5-55℃ and below T5-15℃;

[0024] The rolled steel plate is air-cooled to above T6-50℃ and below T6-30℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+90℃ and below T7+130℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate; or the rolled steel plate is air-cooled to above T6-30℃ and below T6-10℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+50℃ and below T7+90℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate.

[0025] Preferably, the second intermediate plate is rolled to the finished thickness at a rolling temperature of above T5-40℃ and below T5;

[0026] The rolled steel plate is air-cooled to above T6-10℃ and below T6+10℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+10℃ and below T7+50℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate; or the rolled steel plate is air-cooled to above T6 and below T6+20℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7-30℃ and below T7+10℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate.​​​​​​​​

[0027] Preferably, when the first intermediate plate is rolled, the rolling starting temperature is above T3+20℃ and below T3+40℃, and the rolling ending temperature is above T3 and below T3+20℃;

[0028] When the second intermediate plate is rolled, the rolling starting temperature is above T4 and below T4+20℃, and the rolling ending temperature is above T4-50℃ and below T4-30℃;

[0029] When the second intermediate plate is rolled to the finished thickness, the rolling starting temperature is above T5-30℃ and below T5-15℃, and the rolling ending temperature is above T5-55℃ and below T5-40℃, or the rolling starting temperature is above T5-15℃ and below T5, and the rolling ending temperature is above T5-40℃ and below T5-25℃.

[0030] Compared with the prior art, the embodiment has the beneficial effect that, compared with the existing high content of Ni, Mo and Cr, the content of these alloys is greatly reduced, through the design of the chemical composition and the specific process control of heating, rolling and cooling, while eliminating the defect of poor welding performance caused by high alloy content, the mechanical properties such as toughness, strength and yield ratio are still ensured (i.e. the reduction of the mechanical properties caused by the reduction of the content of these elements such as Ni, Mo and Cr does not occur), so that in the case of low alloy content, low production difficulty and short production process, the comprehensive improvement of the steel plate in terms of low yield ratio, easy welding, high strength and toughness, and good plate shape is realized, and the obtained steel plate is suitable for bridge engineering. BRIEF DESCRIPTION OF DRAWINGS

[0031] Fig. 1 is a flow chart of the production method of the steel according to the present application;

[0032] Fig. 2 is a metallographic structure diagram of the steel plate according to embodiment 1 of the present application;

[0033] Fig. 3 is a metallographic structure diagram of the steel plate according to embodiment 2 of the present application;

[0034] Fig. 4 is a metallographic structure diagram of the steel plate according to embodiment 3 of the present application;

[0035] Fig. 5 is a metallographic structure diagram of the steel plate according to embodiment 4 of the present application. DETAILED DESCRIPTION

[0036] The embodiment of the present application provides a production method of a steel, which is improved from two aspects of chemical composition and preparation process, and compared with the prior art, a steel plate product with low yield ratio, easy welding and high strength and toughness is prepared in the case of low alloy content, low production difficulty and short production cycle.

[0037] In terms of chemical composition, the chemical composition of the steel of an embodiment of the present application includes, in terms of mass percentage: C: 0.08-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.47%, Nb: 0.011-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr≤0.13%, Ni≤0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

[0038] The detailed description of the effects and control of each chemical component is as follows.

[0039] C: Carbon is the most economical strengthening element in steel, has a significant solid solution strengthening effect, can effectively reduce the yield ratio, and can form carbides with Nb, Ti, Cr, Mo, etc., to play a precipitation strengthening role; when the carbon content is lower than 0.08%, the improvement effect on improving the strength and reducing the yield ratio is not obvious; when the carbon content is higher than 0.14%, the toughness and welding performance will be poor. In some embodiments, the C content is preferably 0.11-0.14%, 0.10-0.13%, 0.09-0.12% or 0.08-0.11%.

[0040] Si: Silicon in steel plays a solid solution strengthening effect, 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 produced on the surface of the continuous casting billet, which is not easy to remove during descaling, and is not conducive to the surface quality of the steel plate.

[0041] Mn: Manganese in steel plays a solid solution strengthening role, and is the most economical strengthening element besides carbon, when the Mn content is lower than 1.31%, the solid solution strengthening effect is not obvious; when the Mn content exceeds 1.47%, segregation is easily formed in the center of the slab, which reduces the low-temperature toughness of the steel plate, and is not conducive to the welding performance. In some embodiments, the Mn content is preferably 1.31-1.39%, 1.34-1.42%, 1.37-1.45% or 1.39-1.47%.

[0042] Nb: Niobium is an important grain refining element in steel, when the Nb content exceeds 0.011%, niobium carbonitride precipitates are formed during rolling, which refines the recrystallized grains, and niobium can continue to precipitate in the form of carbonitride during cooling, which refines the structure after phase transformation, improves the strength and toughness; however, when Nb is greater than 0.032%, it will increase the alloy cost, and the precipitation strengthening is not conducive to the yield ratio. In some embodiments, the Nb content is preferably 0.011-0.019%, 0.014-0.022%, 0.024-0.032% or 0.021-0.029%.

[0043] Ti: Titanium is a nitrogen-fixing element in steel, which can form dispersed titanium nitride particles to inhibit austenite grain coarsening during billet heating and rolling. When the content of Ti is less than 0.009%, Ti cannot fully play its role, and when the content of Ti is greater than 0.017%, titanium carbonitride is prone to precipitate in the center of the cast blank, affecting the low-temperature toughness of the center of the steel plate.

[0044] Al: Aluminum is a deoxidizing element in steel. When the content of Al is less than 0.026%, Al cannot effectively realize its deoxidizing effect, and when the content of Al is greater than 0.046%, the content of Al2O3 inclusions in steel increases, affecting the low-temperature toughness of the steel.

[0045] P, S, N: Impurity elements; but at the same time, S element can also form MnS inclusions with Mn to reduce the low-temperature toughness of the steel. Optionally, P≤0.0151%, S≤0.0080%, N≤0.0045%, but not limited to this. In some embodiments, the content of P is preferably 0.0101-0.0151%, the content of N is preferably 0.0025-0.0045%, and the content of S is preferably 0.0020-0.0080%.

[0046] Ni: Nickel in steel plays a solid solution strengthening role and is beneficial to low-temperature toughness. Therefore, in the present application, Ni element can be selectively not added or added in a certain amount, for example, when the content of Ni is 0.07-0.13%, the low-temperature toughness can be greatly improved.

[0047] Cr: Chromium in steel plays a solid solution strengthening role, and can delay pearlite transformation. Therefore, in the present application, Cr element can be selectively not added or added in a certain amount, for example, when the content of Cr is 0.07% or more, the low-temperature toughness can be greatly improved, and if the content of Cr exceeds 0.13%, it is not conducive to the welding performance. Therefore, in an embodiment, the content of Cr is preferably 0.07-0.13%.

[0048] Further, in terms of chemical composition, the carbon equivalent CEV of the steel satisfies 0.328-0.371.

[0049] wherein the carbon equivalent CEV = [C] + [Mn] / 6 + ([Cr] + [Mo] + [V]) / 5 + ([Cu] + [Ni]) / 15, in the formula, [C], [Mn], [Cr], [Mo], [V], [Cu], [Ni] represent the mass percentage of C, Mn, Cr, Mo, V, Cu, Ni and other elements in steel, for example, the mass percentage of C in steel is 0.09%, then the mass percentage of C is 0.09, which is brought into the formula of CEV.

[0050] In addition, in the present application, the content of each element in the steel can be determined by sampling the steel in solid form such as a billet or a steel sheet product and measuring it.

[0051] In a preferred embodiment, the chemical composition of the steel comprises, in mass percent: 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%, the remainder being iron and unavoidable impurities; and the carbon equivalent CEV satisfies 0.328-0.371.

[0052] In a preferred embodiment, the chemical composition of the steel comprises, in mass percent: C: 0.10-0.13%, Si: 0.11-0.18%, Mn: 1.34-1.42%, Nb: 0.014-0.022%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, the remainder being iron and unavoidable impurities; and the carbon equivalent CEV satisfies 0.323-0.366.

[0053] In a preferred embodiment, the chemical composition of the steel comprises, in mass percent: C: 0.09-0.12%, Si: 0.11-0.18%, Mn: 1.37-1.45%, Nb: 0.024-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, the remainder being iron and unavoidable impurities; and the carbon equivalent CEV satisfies 0.318-0.361.

[0054] In a preferred embodiment, the chemical composition of the steel comprises, in mass percent: C: 0.08-0.11%, Si: 0.11-0.18%, Mn: 1.39-1.47%, Ni: 0.07-0.13%, Nb: 0.021-0.029%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr: 0.07-0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, the remainder being iron and unavoidable impurities; and the carbon equivalent CEV satisfies 0.333-0.380.

[0055] Further, in the production process, the production method of an embodiment of the present application includes the following steps.

[0056] Step 1, steel casting step

[0057] That is, by molten steel smelting, casting to get billets, the chemical composition as described above.

[0058] Specific molten steel smelting technology, casting technology can be disclosed in the art to be implemented. For example, KR desulfurization, converter smelting, RH refining, LF refining, continuous casting technology, preparation to get the chemical composition of the above-described steel billets.

[0059] Step 2, heating step

[0060] That is, the billet in the heating furnace for heating, the highest temperature T1+10℃ or more, T2 below. That is, the billet in the heating furnace to T1+10℃ or more, T2 below. Usually, the heating furnace set temperature, that is, it is believed that the billet in the heating furnace to the temperature, and do not need to measure the temperature element to the billet direct contact / measurement to determine the temperature of the billet.

[0061] T1 value is T NbC , T NbN and T TiC the maximum value.

[0062] Where, T NbC , T NbN , T TiC and T TiN respectively NbC, NbN, TiC and TiN complete solid solution temperature.

[0063] In an alternative embodiment, can be calculated by lg([Nb]x[N])=3.70-10800 / (T NbN +273.15), lg([Nb]x[C])=2.26-6770 / (T NbC +273.15), lg([Ti]x[C])=5.33-10475 / (T TiC +273.15), lg([Ti]x[N])=0.32-8000 / (T TiN +273.15) T NbN , T NbC , T TiC and T TiN , [Nb], [N]), [C], [Ti] are Nb, N, C, Ti in the billet mass percentage.

[0064] Of course, in the variant embodiment, T NbC , T NbN , T TiC and T TiNIt can also be obtained in a manner known in the art, for example, industry experience value.

[0065] T2 is T1+50℃, T TiN Maximum value in 150℃.

[0066] Thus, by controlling the temperature in the heating furnace to be above T1+10℃ and below T2, the Nb precipitates in the steel are completely dissolved, the Ti carbides are completely dissolved, and the Ti nitrides are retained, preventing abnormal growth of austenite grains, and further ensuring the comprehensive performance of the finished steel plate.

[0067] Step 3, controlled rolling step

[0068] After exiting the heating furnace, the billet is first rolled to 4-6.8 times the finished thickness at a rolling temperature of above T3 and below T3+40℃ to obtain a first intermediate plate; then rolled to 1.8-3.2 times the finished thickness at a rolling temperature of above T4-50℃ and below T4+20℃ to obtain a second intermediate plate; and finally rolled to the finished thickness at a rolling temperature of above T5-55℃ and below T5.

[0069] That is, in an embodiment of the present application, the temperature during rolling of the billet to 4-6.8 times the finished thickness, the temperature during rolling of 4-6.8 times the finished thickness to 1.8-3.2 times the finished thickness, and the temperature during rolling of 1.8-3.2 times the finished thickness to completion are controlled, so that, on the one hand, sufficient recrystallization and refined recrystallized grains are ensured, on the other hand, the recrystallized equiaxed grains are deformed and flattened, and on the other hand, rolling deformation is used to induce ferrite transformation, ferrite transformation occurs in the deformation zone or at the grain boundaries, and fine ferrite grains are obtained, thereby ensuring the yield strength and good toughness of the steel plate.

[0070] Wherein:

[0071] T3≥T4≥T5;

[0072] T5=910-310[C]-80[Mn]-15[Cr];

[0073] In the formulas, [C], [Nb], [Ti], [Al], [Si], and [Ni] are the mass percentages of C, Nb, Ti, Al, Si, and Ni in the billet, respectively.

[0074] Step 4, controlled cooling step

[0075] The rolled steel plate is air-cooled to above T6-50℃ and below T6+20℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7-30℃ and below T7+130℃ at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature, to obtain a steel plate product.

[0076] That is, in an embodiment of the present application, the hot-rolled steel plate is divided into three stages with different cooling rates and cooling modes during cooling. In the first cooling stage, ferrite phase transformation occurs, and the presence of ferrite ensures low yield strength of the steel plate. In the second stage, the carbon-rich supercooled austenite phase in the steel plate is rapidly cooled to transform into acicular ferrite / bainite structure, and the acicular ferrite / bainite transformation is complete through the control of the final cooling temperature. In addition, the control of the cooling rate effectively avoids the generation of martensite phase or pearlite phase. Furthermore, the control of the above temperatures can also improve the plate shape of the steel plate product. In summary, the above aspects ensure high tensile strength, low yield ratio, excellent low-temperature toughness, excellent plate shape, and good welding performance of the steel plate.

[0077] wherein:

[0078] T5≥T6≥T7;

[0079] T7 = 539-423[C]-17.7[Ni]-12.1[Cr]-11[Si]-30.4[Mn];

[0080] T6 = 830-270[C]-90[Mn]-37[Ni]-70[Cr];

[0081] In the above formulas, [C], [Si], [Ni], [Mn], and [Cr] represent the mass percentages of C, Si, Ni, Mn, and Cr in the billet, respectively.

[0082] Therefore, the steel plate product prepared in an embodiment of the present application has a microstructure of deformed induced ferrite + polygonal ferrite + acicular ferrite + bainite. Here, the microstructure of the steel can be obtained by microstructure detection according to the standard GB / T15125-2009 "Metallic Materials - Preparation of Samples and Test Methods for Metallographic Examination".

[0083] The thickness of the steel plate product is 6-64mm, the yield strength of the steel plate is ≥360MPa, the tensile strength is ≥510MPa, the elongation is ≥25%, and the yield ratio is ≤0.75. Here, the mechanical properties can be detected according to GB / T 228.1-2021 "Metallic Materials - Tensile Testing - Part 1: Room Temperature Test Methods".

[0084] The impact energy KV2 of the finished steel plate at -20℃ is greater than or equal to 260 J, and the impact energy KV2 at -40℃ is greater than or equal to 240 J. At this point, the low-temperature toughness can be detected according to GB / T 229-2007 "Metallic Materials Charpy Pendulum Impact Test Method".

[0085] The unevenness of the finished steel plate is less than or equal to 1 mm / m, where the unit mm / m refers to millimeters per unit length of the steel plate, and the unit length is calculated in meters.

[0086] In addition, the finished steel plate has excellent weldability.

[0087] Further preferably, in step 2, the billet stays in the heating furnace at a temperature greater than or equal to T1+10℃ and less than or equal to T2 for a time of 0.07-0.13 min / mm, and the total time in the heating furnace is 1.15-1.35 min / mm.

[0088] Here, the unit min / mm refers to minutes per unit thickness of the billet, and the unit thickness is calculated in millimeters.

[0089] More specifically, in an optional embodiment, the billet can adopt a five-stage process including a heat recovery stage, a preheating stage, a first heating stage, a second heating stage, and a soaking stage in the heating furnace.

[0090] In the five-stage process, the temperature of the second heating stage defines the maximum temperature of the billet, i.e., the temperature of the second heating stage is set to be greater than or equal to T1+10℃ and less than or equal to T2, and the billet stays in the second heating stage for a time of 0.07-0.13 min / mm.

[0091] In the five-stage process, the temperature of the first heating stage is 1120±15℃.

[0092] In addition, in the five-stage process, preferably, the temperature of the soaking stage is lower than the temperature of the second heating stage, and the temperature of the soaking stage is 20-40℃ lower than the temperature of the second heating stage. In this way, the toughness of the finished steel plate can be further improved.

[0093] In an optional variant embodiment, the billet adopts a three-stage process including a preheating stage, a heating stage, and a soaking stage in the heating furnace.

[0094] In the three-stage process, the temperature of the heating stage defines the maximum temperature of the billet, i.e., the temperature of the heating stage is set to be greater than or equal to T1+10℃ and less than or equal to T2, and the billet stays in the heating stage for a time of 0.07-0.13 min / mm.

[0095] In addition, in the three-stage process, preferably, the temperature of the soaking stage is lower than the temperature of the heating stage, and the temperature of the soaking stage is 20-40℃ lower than the temperature of the heating stage. In this way, the toughness of the finished steel plate can be further improved.

[0096] In step 3, the reduction per pass during rolling the bloom into the first intermediate plate is > 20 mm; the reduction per pass during rolling the first intermediate plate into the second intermediate plate is > 12 mm; and the reduction per pass during rolling the second intermediate plate to the finished thickness is > 4 mm.

[0097] Preferably, the rolling temperature during rolling the bloom into the first intermediate plate is > T3 + 20°C and < T3 + 40°C, and the finishing temperature is > T3 and < T3 + 20°C.

[0098] Further preferably, the rolling temperature during rolling the first intermediate plate into the second intermediate plate is > T4 and < T4 + 20°C, and the finishing temperature is > T4 - 50°C and < T4 - 30°C.

[0099] Further, in step 3, the second intermediate plate is rolled to the finished thickness at a rolling temperature > T5 - 55°C and < T5 - 15°C. For example, in some embodiments, the rolling temperature is > T5 - 30°C and < T5 - 15°C, and the finishing temperature is > T5 - 55°C and < T5 - 40°C.

[0100] Alternatively, the second intermediate plate is rolled to the finished thickness at a rolling temperature > T5 - 40°C and < T5. For example, in some embodiments, the rolling temperature is > T5 - 15°C and < T5, and the finishing temperature is > T5 - 40°C and < T5 - 25°C.

[0101] Further, in the case where the second intermediate plate is rolled to the finished thickness at a rolling temperature > T5 - 30°C and < T5 - 15°C, and a finishing temperature > T5 - 55°C and < T5 - 40°C, in step 4:

[0102] In some preferred embodiments, the rolled steel plate is air-cooled to above T6-50℃ and below T6-30℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+90℃ and below T7+130℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature, to obtain a steel plate product, and the resulting microstructure is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling plane is about 35%-45%, the area ratio of polygonal ferrite in the metallographic sampling plane is about 35%-45%, the area ratio of acicular ferrite in the metallographic sampling plane is about 10%-20%, and the area ratio of bainite in the metallographic sampling plane is about 0%-10%; the thickness of the steel plate product is 6-64mm, the yield strength is ≥360MPa, the tensile strength is ≥510MPa, the elongation is ≥30%, the yield strength ratio is ≤0.75, the 0℃ impact energy KV2 is ≥280J, the -20℃ impact energy KV2 is ≥260J, the -40℃ impact energy KV2 is ≥250J, the unevenness of the steel plate is ≤1mm / m, and the welding performance is excellent;

[0103] In some variant embodiments, the rolled steel plate is air-cooled to above T6-30℃ and below T6-10℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+50℃ and below T7+90℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature, to obtain a steel plate product, and the resulting microstructure is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling plane is about 35%-45%, the area ratio of polygonal ferrite in the metallographic sampling plane is about 10%-30%, the area ratio of acicular ferrite in the metallographic sampling plane is about 15%-25%, and the area ratio of bainite in the metallographic sampling plane is about 10%-30%; the thickness of the steel plate product is 6-64mm, the yield strength is ≥390MPa, the tensile strength is ≥530MPa, the elongation is ≥28%, the yield strength ratio is ≤0.75, the 0℃ impact energy KV2 is ≥280J, the -20℃ impact energy KV2 is ≥260J, the -40℃ impact energy KV2 is ≥250J, the unevenness of the steel plate is ≤1mm / m, and the welding performance is excellent.

[0104] Further, in the case of rolling the second intermediate plate to the finished thickness with a rolling temperature of above T5-15℃ and below T5, and a finish rolling temperature of above T5-40℃ and below T5-25℃, in step 4:

[0105] In some preferred embodiments, the rolled steel plate is air-cooled at a cooling rate of 1-10℃ / s to a temperature of T6-10℃ or above and T6+10℃ or below, then water-cooled at a cooling rate of 8-30℃ / s to a temperature of T7+10℃ or above and T7+50℃ or below, and then naturally cooled to room temperature, to obtain a steel plate product, and the obtained microstructure is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of the deformation-induced ferrite in the metallographic sampling plane is about 10%-25%, the area ratio of the polygonal ferrite in the metallographic sampling plane is about 10%-25%, the area ratio of the acicular ferrite in the metallographic sampling plane is about 20%-30%, and the area ratio of the bainite in the metallographic sampling plane is about 30%-50%; the thickness of the steel plate product is 6-64mm, the yield strength is ≥440MPa, the tensile strength is ≥585MPa, the elongation is ≥26%, the yield strength ratio is ≤0.75, the impact energy KV2 at -20℃ is ≥270J, the impact energy KV2 at -40℃ is ≥250J, the impact energy KV2 at -60℃ is ≥200J, the unevenness of the steel plate is ≤1mm / m, and the welding performance is excellent;

[0106] In some variant embodiments, the rolled steel plate is air-cooled at a cooling rate of 1-10℃ / s to a temperature of T6℃ or above and T6+20℃ or below, then water-cooled at a cooling rate of 8-30℃ / s to a temperature of T7-30℃ or above and T7+10℃ or below, and then naturally cooled to room temperature, to obtain a steel plate product, and the obtained microstructure is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of the deformation-induced ferrite in the metallographic sampling plane is about 5%-10%, the area ratio of the polygonal ferrite in the metallographic sampling plane is about 5%-10%, the area ratio of the acicular ferrite in the metallographic sampling plane is about 5%-10%, and the area ratio of the bainite in the metallographic sampling plane is about 70%-85%; the thickness of the steel plate product is 6-64mm, the yield strength is ≥500MPa, the tensile strength is ≥670MPa, the elongation is ≥25%, the yield strength ratio is ≤0.75, the impact energy KV2 at -20℃ is ≥260J, the impact energy KV2 at -40℃ is ≥240J, the impact energy KV2 at -60℃ is ≥200J, the unevenness of the steel plate is ≤1mm / m, and the welding performance is excellent.

[0107] In summary, the present application has the following advantages:

[0108] Compared with the existing high content of Ni, Mo and Cr, the content of these alloys is greatly reduced. Through the design of chemical composition, combined with the specific process control of heating, rolling and cooling, the poor welding performance caused by high alloy content is eliminated, while the mechanical properties such as toughness, strength and yield ratio are still guaranteed (that is, the reduction of the content of these elements such as Ni, Mo and Cr does not lead to the reduction of the mechanical properties brought by these elements), so that in the case of low alloy content, low production difficulty and short production process, the comprehensive improvement of the steel plate in low yield ratio, easy welding, high strength and toughness, good plate shape and the like is realized, and the obtained steel plate is suitable for bridge engineering.

[0109] Some specific embodiments will be introduced below to supplement the beneficial effects of the present application.

[0110] Example 1

[0111] A continuous casting billet was prepared by steelmaking and continuous casting, and the chemical composition of the continuous casting billet included, by mass percent: C: 0.13%, Si: 0.14%, Mn: 1.33%, P: 0.0131%, S: 0.0040%, Nb: 0.014%, Ti: 0.014%, Al: 0.036%, N: 0.0035%, CEV = 0.352, and the balance being Fe and unavoidable impurities.

[0112] A three-stage process was adopted, the temperature of the preheating stage was ≤1000℃, the temperature of the heating stage was 1198℃, the residence time in the heating stage was 0.07-0.13min / mm, the temperature of the soaking stage was 1168℃, and the total time in the heating furnace was 1.15-1.35min / mm.

[0113] After leaving the heating furnace, the billet was first rolled to 156mm at a roughing temperature of 967±10℃ and a finishing temperature of 947±10℃ to obtain a first intermediate plate; then rolled to 72mm at a roughing temperature of 853±10℃ and a finishing temperature of 803±10℃ to obtain a second intermediate plate; and finally rolled to 24mm at a roughing temperature of 731-746℃ and a finishing temperature of 706-721℃ to obtain a hot-rolled steel plate.

[0114] The hot-rolled steel plate was air-cooled to 635±10℃ at a cooling rate of 1-10℃ / s, then water-cooled to 552±20℃ at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature to obtain a steel plate product.

[0115] The steel plate product was sampled and detected to obtain:

[0116] (1) The metallographic structure is shown in Figure 2, which is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling surface is about 40%, the area ratio of polygonal ferrite in the metallographic sampling surface is about 40%, the area ratio of acicular ferrite in the metallographic sampling surface is about 15%, and the area ratio of bainite in the metallographic sampling surface is about 5%.

[0117] (2) The yield strength is 365 MPa, the tensile strength is 521 MPa, the yield strength ratio is 0.70, and the elongation is 41%;

[0118] (3) The 0℃ impact energy KV2 is 320J, 331J, 326J, the -20℃ impact energy KV2 is 295J, 298J, 290J, and the -40℃ impact energy KV2 is 260J, 255J, 262J;

[0119] (4) The unevenness is ≤1mm / m, and the welding performance is excellent.

[0120] Example 2

[0121] A continuous casting billet is prepared by steelmaking and continuous casting, and the chemical composition of the continuous casting billet includes, by mass percent: C: 0.11%, Si: 0.14%, Mn: 1.38%, P: 0.0128%, S: 0.0056%, Nb: 0.018%, Ti: 0.0013%, Al: 0.034%, N: 0.0031%, CEV = 0.340, and the balance being Fe and unavoidable impurities.

[0122] A five-stage process is adopted, the temperature of the heat recovery stage is ≤900℃, the temperature of the preheating stage is ≤1000℃, the temperature of the first adding stage is 1120℃, the temperature of the second adding stage is 1188℃, the residence time in the second adding stage is 0.07-0.13min / mm, the temperature of the soaking stage is 1158℃, and the total time in the heating furnace is 1.15-1.35min / mm.

[0123] After coming out of the heating furnace, the billet is first rolled to 160mm at an opening rolling temperature of 972±10℃ and a final rolling temperature of 952±10℃ to obtain a first intermediate plate; then rolled to 84mm at an opening rolling temperature of 859±10℃ and a final rolling temperature of 809±10℃ to obtain a second intermediate plate; and finally rolled to 28mm at an opening rolling temperature of 733-748℃ and a final rolling temperature of 708-723℃ to obtain a hot-rolled steel plate.

[0124] The hot-rolled steel plate is air-cooled to 656±10℃ at a cooling rate of 1-10℃ / s, then water-cooled to 519±20℃ at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature to obtain a steel plate finished product.

[0125] The steel plate finished product is sampled and detected to obtain:

[0126] (1) The metallographic structure is shown in Figure 3, which is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling surface is about 40%, the area ratio of polygonal ferrite in the metallographic sampling surface is about 20%, the area ratio of acicular ferrite in the metallographic sampling surface is about 20%, and the area ratio of bainite in the metallographic sampling surface is about 20%.

[0127] (2) The yield strength is 410 MPa, the tensile strength is 570 MPa, the yield strength ratio is 0.72, and the elongation is 39%;

[0128] (3) The impact energy KV2 at 0℃ is 292J, 288J, 295J, the impact energy KV2 at -20℃ is 272J, 269J, 273J, and the impact energy KV2 at -40℃ is 271J, 259J, 263J;

[0129] (4) The unevenness is ≤1mm / m, and the welding performance is excellent.

[0130] Example 3

[0131] A continuous casting billet was prepared by steelmaking and continuous casting, and the chemical composition of the continuous casting billet included, by mass percent: C: 0.10%, Si: 0.13%, Mn: 1.41%, P: 0.0132%, S: 0.0053%, Nb: 0.028%, Ti: 0.0015%, Al: 0.031%, N: 0.0031%, CEV = 0.335, and the balance being Fe and unavoidable impurities.

[0132] A three-stage process was used, the temperature of the preheating stage was ≤1000℃, the temperature of the heating stage was 1220℃, the residence time in the heating stage was 0.07-0.13min / mm, the temperature of the soaking stage was 1190℃, and the total time in the heating furnace was 1.15-1.35min / mm.

[0133] After coming out of the heating furnace, the billet was first rolled to 180mm at a roughing temperature of 1014±10℃ and a finishing temperature of 994±10℃ to obtain a first intermediate plate; then rolled to 80mm at a roughing temperature of 862±10℃ and a finishing temperature of 812±10℃ to obtain a second intermediate plate; and finally rolled to 36mm at a roughing temperature of 748-763℃ and a finishing temperature of 723-738℃ to obtain a hot-rolled steel plate.

[0134] The hot-rolled steel plate was air-cooled to 676±10℃ at a cooling rate of 1-10℃ / s, then water-cooled to 482±20℃ at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature to obtain a steel plate product.

[0135] The steel plate product was sampled and detected to obtain:

[0136] (1) The metallographic structure is shown in Figure 4, which is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling surface is about 18%, the area ratio of polygonal ferrite in the metallographic sampling surface is about 16%, the area ratio of acicular ferrite in the metallographic sampling surface is about 26%, and the area ratio of bainite in the metallographic sampling surface is about 40%.

[0137] (2) The yield strength is 460 MPa, the tensile strength is 630 MPa, the yield strength ratio is 0.73, and the elongation is 32%;

[0138] (3) The impact energy KV2 at -20℃ is 289J, 291J, 286J, the impact energy KV2 at -40℃ is 269J, 272J, 277J, and the impact energy KV2 at -60℃ is 232J, 245J, 233J;

[0139] (4) The unevenness is ≤1mm / m, and the welding performance is excellent.

[0140] Example 4

[0141] A continuous casting billet was prepared by steelmaking and continuous casting, and the chemical composition of the continuous casting billet included, by mass percent: C: 0.10%, Si: 0.15%, Mn: 1.43%, P: 0.0120%, S: 0.0030%, Cr: 0.12%, Ni: 0.12%, Nb: 0.025%, Ti: 0.015%, Al: 0.033%, N: 0.0032%, CEV = 0.370, and the balance being Fe and unavoidable impurities.

[0142] A five-stage process was used, the temperature of the heat recovery stage was ≤900℃, the temperature of the preheating stage was ≤1000℃, the temperature of the first adding stage was 1120℃, the temperature of the second adding stage was 1215℃, the residence time in the second adding stage was 0.07-0.13min / mm, the temperature of the soaking stage was 1185℃, and the total time in the heating furnace was 1.15-1.35min / mm.

[0143] After coming out of the heating furnace, the billet was first rolled to 200mm at a roughing temperature of 995±10℃ and a finishing temperature of 975±10℃ to obtain a first intermediate plate; then rolled to 100mm at a roughing temperature of 861±10℃ and a finishing temperature of 811±10℃ to obtain a second intermediate plate; and finally rolled to 50mm at a roughing temperature of 745-760℃ and a finishing temperature of 720-735℃ to obtain a hot-rolled steel plate.

[0144] The hot-rolled steel plate was air-cooled to 672±10℃ at a cooling rate of 1-10℃ / s, then water-cooled to 538±20℃ at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature to obtain a steel plate finished product.

[0145] The finished steel plate is sampled and detected, and the results are as follows:

[0146] (1) The metallographic structure is shown in Figure 5, which is a complex phase structure of deformation-induced ferrite + polygonal ferrite + acicular ferrite + bainite, wherein the area ratio of deformation-induced ferrite in the metallographic sampling surface is about 7%, the area ratio of polygonal ferrite in the metallographic sampling surface is about 8%, the area ratio of acicular ferrite in the metallographic sampling surface is about 9%, and the area ratio of bainite in the metallographic sampling surface is about 76%.

[0147] (2) The yield strength is 520 MPa, the tensile strength is 695 MPa, the yield strength ratio is 0.75, and the elongation is 30%;

[0148] (3) The impact energy KV2 at -20℃ is 278J, 284J, 285J, the impact energy KV2 at -40℃ is 262J, 258J, 265J, and the impact energy KV2 at -60℃ is 244J, 250J, 239J;

[0149] (4) The unevenness is ≤1mm / m, and the welding performance is excellent.

Claims

1. A method of producing steel, characterized by, The method comprises the steps of: obtaining a billet by molten steel smelting and casting, the chemical composition of the billet comprising, in percentage by mass: C: 0.08-0.14%, Si: 0.11-0.18%, Mn: 1.31-1.47%, Nb: 0.011-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr≤0.13%, Ni≤0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the rest being iron and inevitable impurities; The steel billet is heated in a heating furnace to a temperature above T1+10℃ and below T2, where T1 is taken as T NbC T NbN and T TiC The maximum value in the range, T2 takes the value of T1+50℃, T TiN The maximum value at -150℃; where, T NbC T NbN T TiC and T TiN The complete solution temperatures of NbC, NbN, TiC, and TiN are respectively. after exiting the heating furnace, the billet is first rolled to 4-6.8 times the finished product thickness at a rolling temperature T3 above and T3+40℃ below, to obtain a first intermediate plate; then rolled to 1.8-3.2 times the finished product thickness at a rolling temperature T4-50℃ above and T4+20℃ below, to obtain a second intermediate plate; finally rolled to the finished product thickness at a rolling temperature T5-55℃ above and T5 below; the rolled steel plate is air-cooled to T6-50℃ above and T6+20℃ below at a cooling rate of 1-10℃ / s, then water-cooled to T7-30℃ above and T7+130℃ below at a cooling rate of 5-30℃ / s, and then naturally cooled to room temperature, to obtain the finished steel plate product; wherein T3≥T4≥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], T5 = 910 - 310[C] - 80[Mn] - 15[Cr], [C], [Nb], [Ti], [Al], [Si], [Ni], [Mn], [Cr] are the mass percentages of C, Nb, Ti, Al, Si, Ni, Mn, Cr in the billet respectively.

2. The method of producing steel according to claim 1, characterized in that, The chemical composition of the billet comprises, in percentage by mass: P: 0.0101-0.0151%, S: 0.0020-0.0080%, N: 0.0025-0.0045%.

3. The method of producing steel according to claim 1, characterized in that, The carbon equivalent CEV of the chemical composition of the billet satisfies 0.318-0.

380.

4. The method of producing steel according to claim 3, characterized in that, The chemical composition of the billet comprises, in percentage by mass: 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 being iron and inevitable impurities.

5. The method of producing steel according to claim 4, characterized in that, The carbon equivalent CEV of the chemical composition of the billet satisfies 0.328-0.

371.

6. The method of producing steel according to claim 3, characterized in that, The chemical composition of the billet comprises, in percentage by mass: C: 0.10-0.13%, Si: 0.11-0.18%, Mn: 1.34-1.42%, Nb: 0.014-0.022%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the rest being iron and inevitable impurities.

7. The method of producing steel according to claim 6, characterized in that, The carbon equivalent CEV of the chemical composition of the billet satisfies 0.323-0.

366.

8. The method of producing steel according to claim 3, characterized in that, The chemical composition of the billet includes, in mass percent: C: 0.09-0.12%, Si: 0.11-0.18%, Mn: 1.37-1.45%, Nb: 0.024-0.032%, Ti: 0.009-0.017%, Al: 0.026-0.046%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

9. The method of producing steel according to claim 8, characterized in that, The carbon equivalent CEV of the chemical composition of the billet satisfies 0.318-0.

361.

10. The method of producing steel according to claim 3, characterized in that, The chemical composition of the billet includes, in mass percent: C: 0.08-0.11%, Si: 0.11-0.18%, Mn: 1.39-1.47%, Ni: 0.07-0.13%, Nb: 0.021-0.029%, Ti: 0.009-0.017%, Al: 0.026-0.046%, Cr: 0.07-0.13%, P≤0.0151%, S≤0.0080%, N≤0.0045%, and the balance being iron and inevitable impurities.

11. The method of producing steel according to claim 10, characterized in that, The carbon equivalent CEV of the chemical composition of the billet satisfies 0.333-0.

380.

12. The method of producing steel according to claim 1, characterized in that, The billet stays at a temperature above T1+10℃ 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.

13. The method of producing steel according to claim 1, characterized in that, The billet is heated in the heating furnace by a five-stage process including a heat recovery stage, a preheating stage, a first heating stage, a second heating stage and a soaking stage, the temperature of the second heating stage is above T1+10℃ and below T2, 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℃ lower than that of the second heating stage. Alternatively, the billet is heated in the heating furnace by a three-stage process including a preheating stage, a heating stage and a soaking stage, the temperature of the heating stage is above T1+10℃ and below T2, the temperature of the soaking stage is lower than that of the heating stage, and the temperature of the soaking stage is 20-40℃ lower than that of the heating stage. In the five-stage process, the temperature of the first heating stage is 1120±15℃.

14. The method of producing steel according to claim 13, characterized in that, The second intermediate plate is rolled to the finished thickness at a rolling temperature above T5-55℃ and below T5-15℃. Among them, 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) respectively calculate T NbN T NbC T TiC and T TiN [Nb], [N]), [C], and [Ti] represent the mass percentages of Nb, N, C, and Ti in the steel billet, respectively.

15. The method of producing steel according to claim 1, characterized in that, The rolled steel plate is air-cooled to above T6-50℃ and below T6-30℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+90℃ and below T7+130℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate; or the rolled steel plate is air-cooled to above T6-30℃ and below T6-10℃ at a cooling rate of 1-10℃ / s, then water-cooled to above T7+50℃ and below T7+90℃ at a cooling rate of 8-30℃ / s, and then naturally cooled to room temperature to obtain the finished steel plate. The second intermediate plate is rolled to the finished thickness at a rolling temperature above T5-40℃ and below T5.

16. The method of producing steel according to claim 1, characterized in that, ​ The rolled steel plate is air-cooled at a cooling rate of 1-10 ℃ / s to a temperature of T6-10 ℃ or above and T6+10 ℃ or below, then water-cooled at a cooling rate of 8-30 ℃ / s to a temperature of T7+10 ℃ or above and T7+50 ℃ or below, and then naturally cooled to room temperature to obtain a finished steel plate; or the rolled steel plate is air-cooled at a cooling rate of 1-10 ℃ / s to a temperature of T6 or above and T6+20 ℃ or below, then water-cooled at a cooling rate of 8-30 ℃ / s to a temperature of T7-30 ℃ or above and T7+10 ℃ or below, and then naturally cooled to room temperature to obtain a finished steel plate.

17. The method of producing steel according to claim 1, characterized in that, When the first intermediate plate is rolled, the rolling starting temperature is T3+20 ℃ or above and T3+40 ℃ or below, and the final rolling temperature is T3 or above and T3+20 ℃ or below; When the second intermediate plate is rolled, the rolling starting temperature is T4 or above and T4+20 ℃ or below, and the final rolling temperature is T4-50 ℃ or above and T4-30 ℃ or below; When the second intermediate plate is rolled to the finished thickness, the rolling starting temperature is T5-30 ℃ or above and T5-15 ℃ or below, and the final rolling temperature is T5-55 ℃ or above and T5-40 ℃ or below, or the rolling starting temperature is T5-15 ℃ or above and T5 or below, and the final rolling temperature is T5-40 ℃ or above and T5-25 ℃ or below.

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