阅读说明：本技术 一种屈服强度800MPa级热轧钢板及其制造方法 (Hot rolled steel plate with yield strength of 800MPa and manufacturing method thereof ) 是由 严立新 梁亮 李光辉 汪净 刘浩 肖爱达 汪宏兵 刘旭辉 徐德强 谢世正 于 2020-04-07 设计创作，主要内容包括：本申请属于钢铁冶金技术领域,尤其涉及一种屈服强度800MPa级热轧钢板,以质量百分数计包括：0.08wt％≤C≤0.15wt％；0.10wt％≤Si≤0.40wt％；1.30wt％≤Mn≤2.00wt％；0.20wt％≤Mo≤0.45wt％；0＜P≤0.015wt％；0＜S≤0.005wt％；0.02wt％≤Als≤0.07wt％；0.02wt％≤Nb≤0.06wt％；0.10wt％≤Ti≤0.20wt％；其余为铁和其他不可避免的杂质；其中,所述屈服强度800MPa级热轧钢板的碳当量控制为≤0.48wt％；其可焊性较好。(The application belongs to the technical field of ferrous metallurgy, especially relates to a yield strength 800MPa level hot rolled steel plate, includes by mass percent: c is between 0.08 and 0.15 percent by weight; si is more than or equal to 0.10 weight percent and less than or equal to 0.40 weight percent; mn is more than or equal to 1.30 wt% and less than or equal to 2.00 wt%; mo is between 0.20 and 0.45 percent by weight; p is more than 0 and less than or equal to 0.015 wt%; s is more than 0 and less than or equal to 0.005 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.07 wt%; nb is more than or equal to 0.02 wt% and less than or equal to 0.06 wt%; ti is more than or equal to 0.10 wt% and less than or equal to 0.20 wt%; the balance of iron and other inevitable impurities; wherein the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.48 wt%; the solderability is better.)

1. A hot rolled steel plate with the yield strength of 800MPa, which is characterized by comprising the following components in percentage by mass: c is between 0.08 and 0.15 percent by weight; si is more than or equal to 0.10 weight percent and less than or equal to 0.40 weight percent; mn is more than or equal to 1.30 wt% and less than or equal to 2.00 wt%; mo is between 0.20 and 0.45 percent by weight; p is more than 0 and less than or equal to 0.015 wt%; s is more than 0 and less than or equal to 0.005 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.07 wt%; nb is more than or equal to 0.02 wt% and less than or equal to 0.06 wt%; ti is more than or equal to 0.10 wt% and less than or equal to 0.20 wt%; the balance of iron and other inevitable impurities; wherein the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.48 wt%.

2. The hot rolled steel sheet with the yield strength of 800MPa according to claim 1, wherein the hot rolled steel sheet with the yield strength of 800MPa comprises the following chemical compositions in percentage by weight: c is between 0.10 and 0.12 percent by weight; si is more than or equal to 0.20 weight percent and less than or equal to 0.30 weight percent; mn is more than or equal to 1.50 wt% and less than or equal to 1.80 wt%; mo is between 0.25 and 0.40 percent by weight; p is more than 0 and less than or equal to 0.010 wt%; s is more than 0 and less than or equal to 0.002 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.05 wt%; nb is more than or equal to 0.03 weight percent and less than or equal to 0.05 weight percent; ti is more than or equal to 0.12 wt% and less than or equal to 0.15 wt%; the balance of iron and other inevitable impurities.

3. The hot rolled steel sheet with the yield strength of 800MPa according to claim 1, wherein the hot rolled steel sheet with the yield strength of 800MPa comprises the following chemical composition and weight percentages thereof as follows: 0.15 wt%; si: 0.26 wt%; mn: 1.70 wt%; mo: 0.30 wt%; p: 0.008 wt%; s: 0.002 wt%; and Als: 0.03 wt%; nb: 0.04 wt%; ti: 0.12 wt%; the balance of iron and other inevitable impurities.

4. The hot rolled steel sheet having a yield strength of 800MPa or more according to claim 1, wherein the carbon equivalent of the hot rolled steel sheet having a yield strength of 800MPa or less is controlled to 0.40 wt% or less.

5. A method for producing a hot-rolled steel sheet having a yield strength of 800MPa according to any one of claims 1 to 4,

hot rolling the steel billet into a steel coil, wherein the hot rolling comprises coiling, and the coiling temperature is 500-580 ℃;

the steel coil is sequentially flattened and tempered to obtain a hot rolled steel plate with the yield strength of 800MPa, wherein the tempering temperature is 600-680 ℃, and the tempering heat preservation time is 20-40 min.

6. The method for manufacturing a hot rolled steel sheet having a yield strength of 800MPa according to claim 5, wherein the step of hot rolling the steel slab into a steel coil comprises:

the steel billet is heated and then rough rolled into an intermediate billet;

the intermediate blank is sequentially cooled for the first time, finish rolled and cooled for the second time, and then coiled into a steel coil; wherein the step of first cooling comprises:

the intermediate blank is cooled at a speed of 25-100 ℃/s, and the surface temperature of the intermediate blank is controlled to be 850-950 ℃.

7. The method for manufacturing the hot-rolled steel plate with the yield strength of 800MPa according to claim 6, wherein in the heating step, the heating temperature is 1180-1300 ℃ and the holding time is 20-240 min.

8. The method for manufacturing a hot-rolled steel sheet having a yield strength of 800MPa according to claim 6, wherein the total reduction ratio of the steel slab in the rough rolling step is 70 to 85%.

9. The method for manufacturing a hot-rolled steel sheet having a yield strength of 800MPa according to claim 6, wherein the hot rolling further comprises a finish rolling, the steel slab has a total reduction ratio of 70% to 97% in the finish rolling step, a finish rolling entrance temperature of 950 ℃ to 1050 ℃, a finish rolling finishing temperature of 820 ℃ to 900 ℃, and a thickness of the steel sheet formed by the finish rolling is 2.0mm to 16.0 mm.

10. The method for manufacturing a hot rolled steel sheet having a yield strength of 800MPa according to claim 6, wherein the second cooling step includes:

cooling the steel billet at a cooling speed of 100-300 ℃/s, carrying out laminar cooling at a cooling speed of 10-25 ℃/s, and cooling to 500-580 ℃.

Technical Field

The application belongs to the technical field of steel smelting, and particularly relates to a hot rolled steel plate with yield strength of 800MPa and a manufacturing method thereof.

Background

The steel plate for the crane boom is generally made of high-strength steel, so that on one hand, high strength is required to realize light weight, and on the other hand, low internal stress is required to meet deformation in the machining process. In order to ensure the strength and the low-temperature impact toughness during production of the traditional high-strength steel, a large amount of alloy elements such as Nb, V, Ti, Mo and the like are usually added, so that the welding carbon equivalent is often improved, and the weldability of welding is reduced.

Disclosure of Invention

The invention aims to provide a hot rolled steel plate with the yield strength of 800MPa and a manufacturing method thereof, and aims to solve the problem of poor weldability in the prior art.

In order to achieve the above purpose, an aspect of the embodiments of the present application provides a hot rolled steel sheet with a yield strength of 800MPa, where the hot rolled steel sheet with a yield strength of 800MPa comprises the following chemical components in percentage by weight: c is between 0.08 and 0.15 percent by weight; si is more than or equal to 0.10 weight percent and less than or equal to 0.40 weight percent; mn is more than or equal to 1.30 wt% and less than or equal to 2.00 wt%; mo is between 0.20 and 0.45 percent by weight; p is more than 0 and less than or equal to 0.015 wt%; s is more than 0 and less than or equal to 0.005 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.07 wt%; nb is more than or equal to 0.02 wt% and less than or equal to 0.06 wt%; ti is more than or equal to 0.10 wt% and less than or equal to 0.20 wt%; the balance of iron and other inevitable impurities; wherein the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.48 wt%.

Optionally, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical compositions in percentage by weight: c is between 0.10 and 0.12 percent by weight; si is more than or equal to 0.20 weight percent and less than or equal to 0.30 weight percent; mn is more than or equal to 1.50 wt% and less than or equal to 1.80 wt%; mo is between 0.20 and 0.40 weight percent; p is more than 0 and less than or equal to 0.010 wt%; s is more than 0 and less than or equal to 0.002 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.05 wt%; nb is more than or equal to 0.03 weight percent and less than or equal to 0.05 weight percent; ti is more than or equal to 0.12 wt% and less than or equal to 0.15 wt%; the balance of iron and other inevitable impurities.

Optionally, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical composition and weight percentage: 0.15 wt%; si: 0.26 wt%; mn: 1.70 wt%; mo: 0.30 wt%; p: 0.008 wt%; s: 0.002 wt%; and Als: 0.03 wt%; nb: 0.04 wt%; ti: 0.12 wt%; the balance of iron and other inevitable impurities.

Optionally, the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.40 wt%.

In another aspect, an embodiment of the present application provides a method for manufacturing a hot rolled steel sheet with a yield strength of 800MPa, including the following steps:

hot rolling the steel billet into a steel coil, wherein the hot rolling comprises coiling, and the coiling temperature is 500-580 ℃;

the steel coil is sequentially flattened and tempered to obtain a hot rolled steel plate with the yield strength of 800MPa, wherein the tempering temperature is 600-680 ℃, and the tempering heat preservation time is 20-40 min.

Optionally, the step of hot rolling the steel slab into a steel coil includes: the steel billet is heated and then rough rolled into an intermediate billet;

the intermediate blank is sequentially cooled for the first time, finish rolled and cooled for the second time, and then coiled into a steel coil; wherein the step of first cooling comprises:

the intermediate blank is cooled at a speed of 25-100 ℃/s, and the surface temperature of the intermediate blank is controlled to be 850-950 ℃.

Optionally, in the heating step, the heating temperature is 1180-1300 ℃, and the heat preservation time is 20-240 min.

Optionally, the total reduction rate of the steel billet in the rough rolling step is 70% to 85%.

Optionally, the hot rolling further comprises finish rolling, the total reduction rate of the steel billet in the finish rolling step is 70% -97%, the inlet temperature of the finish rolling is 950 ℃ -1050 ℃, the finish rolling temperature of the finish rolling is 820 ℃ -900 ℃, and the thickness of the steel plate formed after the finish rolling is 2.0-16.0 mm.

Optionally, the step of second cooling comprises:

cooling the steel billet at a cooling speed of 100-300 ℃/s, carrying out laminar cooling at a cooling speed of 10-25 ℃/s, and cooling to 500-580 ℃.

Compared with the prior art, the method has the following beneficial effects:

according to the embodiment of the application, a low-C Nb-Ti component system and a controlled rolling and cooling process system are adopted, the improvement effect of the process on the toughness of the steel plate is fully utilized through optimization among alloy element proportions, the high-toughness steel plate with the lower carbon equivalent CEV (CeV) being less than or equal to 0.48% is produced, and the microstructure of a microalloy precipitated phase with the grain diameter of about 5nm is dispersedly released on an ultrafine crystal ferrite matrix with the grain size of less than 5 mu m can be obtained, so that the yield strength of the steel plate can reach more than 800MPa, the elongation is more than or equal to 15%, and the Charpy impact power AKv (-40 ℃) is more than or equal to 100J.

In the embodiment of the application, the precipitation of carbide particles TiC is avoided by controlling the low coiling temperature, and the uniform precipitation and growth of the carbide particles TiC are controlled by tempering treatment, so that the aim of low internal stress and even no internal stress is fulfilled; the tempering treatment can control the size of carbide particles such as TiC particles, so that the size of precipitated particles can be controlled to be about 5nm, a good strengthening effect can be achieved, the addition amount of the alloy can be reduced, and the alloy cost is greatly reduced.

The embodiment of the application can realize differential temperature infiltration rolling by controlling the quick cooling process of the intermediate billet, the core temperature of the intermediate billet is higher than the surface temperature, thereby ensuring the large infiltration rolling of the core in the finish rolling process, greatly reducing the phenomenon of central segregation and tissue unevenness of a steel plate, and further improving the low-temperature impact toughness of the steel plate.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or the prior art descriptions will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments of the present application, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without inventive exercise.

FIG. 1 is a schematic flow chart of a method for manufacturing a hot rolled steel plate with a yield strength of 800MPa according to an embodiment of the present application.

Detailed Description

In order to make the technical problems, technical solutions and advantageous effects to be solved by the present invention more clearly apparent, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

On one hand, the embodiment of the application provides a hot rolled steel plate with the yield strength of 800 MPa.

In the embodiment of the application, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical components in percentage by weight: c is between 0.08 and 0.15 percent by weight; si is more than or equal to 0.10 weight percent and less than or equal to 0.40 weight percent; mn is more than or equal to 1.30 wt% and less than or equal to 2.00 wt%; mo is between 0.20 and 0.45 percent by weight; p is more than 0 and less than or equal to 0.015 wt%; s is more than 0 and less than or equal to 0.005 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.07 wt%; nb is more than or equal to 0.02 wt% and less than or equal to 0.06 wt%; ti is more than or equal to 0.10 wt% and less than or equal to 0.20 wt%; the balance of iron and other inevitable impurities; wherein the carbon equivalent CEV of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.48 wt%.

In one embodiment, the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.45 wt%.

In one embodiment, the carbon equivalent of the hot rolled steel plate with the yield strength of 800MPa is controlled to be less than or equal to 0.40 wt%.

The carbon equivalent is calculated by the formula CEV (%) ═ C + Mn/6+ (Mo + Cr + V)/5+ (Ni + Cu)/15. In order to optimize C, Mn and other element contents, the relation between C and Mn and other elements is set in the embodiment of the application so as to ensure that excellent performance is obtained by adopting proper component proportion; the alloying equivalent AEQ considers the influence of different alloying elements and the interaction thereof on the toughness under the condition of proper carbon equivalent CEV. The constant term before the alloying element is related to the influence of the alloying element on the toughness. The alloying equivalent simultaneously considers the influence of the Nb and Ti composite addition on the mechanical property of the steel plate. Meanwhile, if the alloying equivalent is too low, a steel plate meeting the mechanical property requirement cannot be produced, and if the alloying equivalent is too high, the carbon equivalent is increased, and the welding property is deteriorated. In the embodiment of the application, the carbon equivalent CEV is controlled to be less than or equal to 0.48 wt%, so that the weldability of welding can be ensured.

In one embodiment, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical compositions in percentage by weight: c is between 0.10 and 0.12 percent by weight; si is more than or equal to 0.20 weight percent and less than or equal to 0.30 weight percent; mn is more than or equal to 1.50 wt% and less than or equal to 1.80 wt%; mo is between 0.25 and 0.40 percent by weight; p is more than 0 and less than or equal to 0.010 wt%; s is more than 0 and less than or equal to 0.002 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.05 wt%; nb is more than or equal to 0.03 weight percent and less than or equal to 0.05 weight percent; ti is more than or equal to 0.12 wt% and less than or equal to 0.15 wt%; the balance of iron and other inevitable impurities.

In one embodiment, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical compositions in percentage by weight: c is between 0.10 and 0.11 percent by weight; si is more than or equal to 0.25 weight percent and less than or equal to 0.28 weight percent; mn is more than or equal to 1.60 wt% and less than or equal to 1.80 wt%; mo is between 0.30 and 0.35 weight percent; p is more than 0 and less than or equal to 0.008 wt%; s is more than 0 and less than or equal to 0.002 wt%; als is more than or equal to 0.02 wt% and less than or equal to 0.04 wt%; nb is more than or equal to 0.03 weight percent and less than or equal to 0.04 weight percent; ti is more than or equal to 0.12 wt% and less than or equal to 0.14 wt%; the balance of iron and other inevitable impurities.

As a specific example, the hot rolled steel plate with the yield strength of 800MPa comprises the following chemical composition in percentage by weight: 0.15 wt%; si: 0.26 wt%; mn: 1.70 wt%; mo: 0.32 wt%; p: 0.008 wt%; s: 0.002 wt%; and Als: 0.03 wt%; nb: 0.04 wt%; ti: 0.12 wt%; the balance of iron and other inevitable impurities.

In the embodiment of the application, the addition principle of each chemical element is as follows:

c, carbon C: the different C content has important influence on the phase change of the steel plate in the cooling process: steel grades with higher C content are easy to form structures with higher strength, such as bainite or martensite, in the cooling process under the same cooling condition; however, if the content of C is too high, a brittle structure is formed, and the low-temperature impact toughness of the steel sheet is lowered, whereas if the content of C is too low, a structure having low strength such as ferrite is easily formed. Proper amount of C, Ti and Nb can form stable MC nano-scale precipitates such as TiC and/or NbC, the MC nano-scale precipitates can generate strong precipitation strengthening effect and fine crystal strengthening effect, and the strength of the steel plate is greatly improved; in order to achieve yield strength of above 800MPa and integrate other mechanical properties, processability and other considerations, the content of C is controlled to be 0.08-0.15 wt% in the embodiments of the present application.

Silicon Si: the Si element is dissolved in the steel sheet, and the strength of the steel sheet can be improved. Too high Si content inhibits the formation of cementite, while higher Si content deteriorates the weldability of the steel sheet. Therefore, the Si content in the examples of the present application is controlled to 0.10 wt% to 0.40 wt%.

Manganese Mn: mn element is a weak carbide-forming element, and is usually dissolved in a steel sheet to exert a solid solution strengthening effect; in the high-strength steel plate produced by adopting the controlled rolling and cooling mode, Mn element dissipates free energy by crossing a diffusion interface, inhibits the diffusion control growth of the end surface of a lamellar phase, and forms a refined lamellar bainite lath, thereby improving the comprehensive properties of the steel plate, such as strength, toughness and the like. The high Mn content can increase the cracking tendency of the plate blank, easily form longitudinal cracks and other defects in the plate blank production process, and the low Mn content has small contribution to the strength, so that C element or other precious alloy elements such as Mo element and the like need to be added to ensure the strength of the steel plate. However, the addition of C element deteriorates the weldability of the steel plate, and the addition of other noble elements increases the cost of the steel plate. Therefore, in order to provide good toughness to the steel sheet, the content of the Mn element in the examples of the present application is controlled to be 1.30 wt% to 2.0 wt%.

Molybdenum Mo: mo element is solid-dissolved in steel during austenitizing, has a certain austenite recrystallization delaying effect in the hot rolling process, improves the defect density in the deformed austenite, and can realize the refining of the final structure by inhibiting the movement of a diffusion interface in the cooling process. The free energy dissipated by the Mo element to the dragging action of the diffusion interface is about 3 times that of the Mn element, and the Mo element and Ti are separated out together in the coiling process to form a nano (Ti, Mo) C composite separated phase, so that the obvious precipitation strengthening effect is generated. When the content of Mo is less than 0.20 wt%, the above effect is small, and when the content exceeds 0.45 wt%, the effect is saturated, and the content of Mo element in the embodiment of the application is controlled to be 0.20 wt% to 0.45 wt%.

Titanium Ti: ti and N form TiN at a high temperature, and the TiN inhibits austenite grains from growing when the slab is heated to austenitize. In the hot rolling process, Ti and C form nano TiC at a lower temperature range, fine TiC particles have obvious precipitation strengthening and fine grain strengthening effects, the strength and the low-temperature impact performance of the steel plate are improved, and meanwhile, Ti and Nb are separated out together in the coiling process to form a nano (Ti, Nb) C composite precipitated phase. However, when the Ti content is too high, on one hand, coarse square TiN is precipitated, and stress is concentrated near TiN particles when the steel plate is stressed, so that the TiN particles become a nucleation growth source of micro-cracks, and the fatigue performance of the steel plate is reduced. On the other hand, Ti is difficult to be dissolved in a solid solution manner in the heating process of the continuous casting billet because the solid solubility product of TiC is small, and the corresponding effect cannot be achieved. In summary, the content of Ti element in the embodiment of the present application is controlled to be 0.10 wt% to 0.20 wt%.

Niobium Nb: the steel plate can form a large number of defects such as dislocation and the like in the rolling process. Austenite recrystallizes under the action of defect energy, wherein the recrystallization process comprises nucleation and growth of new austenite grains, and the Nb element improves the recrystallization temperature of the steel plate by inhibiting the movement of an austenite interface. The addition of a certain amount of Nb allows for a two-stage rolling with the non-recrystallization zone rolling at a lower temperature to increase the internal dislocation density of the austenite, forming a refined structure during subsequent cooling. Nb is the most effective element for achieving non-recrystallization rolling and obtaining a final fine grain structure. The higher Nb content can form coarse NbC precipitation in the tempering process, thereby reducing the low-temperature impact energy of the steel plate. Therefore, in order to control the microstructure and the mechanical property of the steel plate, the content of the Nb element is controlled to be 0.02 wt% to 0.06 wt% in the embodiment of the application.

Aluminum Al: al element forms fine AlN precipitation at high temperature, and austenite grains are inhibited from growing when a plate blank is heated to austenitize, so that the aims of refining the austenite grains and improving the toughness of the steel at low temperature are fulfilled. Too high an Al content results in the formation of large Al oxides, which degrades the low-temperature impact properties and flaw detection properties of the steel sheet. Therefore, in order to refine grains, improve the toughness of the steel plate and ensure the welding performance thereof, the content of the Al element in the embodiment of the present application is controlled to be 0.02 wt% to 0.07 wt%.

P, S, O, N: are harmful impurity elements in steel, remarkably lower ductility and weldability of steel, and therefore, the content of the impurity elements should be reduced as much as possible.

In the embodiment of the application, the carbon content is 0.07 wt% -0.14 wt%, and the carbon content is neither very low nor very high, so that the requirement of a steelmaking process can be met, and the subsequent requirement on welding performance of a steel plate can be ensured. The content of the added Ca and S ensures that the Ca/S is 0.5-2.0, so that sulfides are completely spheroidized or approximate to a spindle shape, and the transverse impact property and the cold bending property of the steel plate can be improved; when Nb, Ti and Mo are added into the steel plate, the Nb, Ti and Mo are not close to the upper limit or the lower limit at the same time, so that the strength and the carbon equivalent of the steel plate can be ensured. By properly controlling the elements, the steel plate can have better comprehensive properties such as mechanics, welding and the like by using lower alloy cost, accurate component proportion and simple steel-making, rolling and cooling processes.

On the other hand, the embodiment of the application provides a manufacturing method of a hot rolled steel plate with the yield strength of 800 MPa. Referring to fig. 1, fig. 1 is a schematic flow chart of a manufacturing method of a hot rolled steel plate with a yield strength of 800MPa according to an embodiment of the present application. In the embodiment of the application, the manufacturing method of the hot rolled steel plate with the yield strength of 800MPa comprises the following steps:

s10, hot rolling the steel billet into a steel coil;

the method comprises the steps of smelting blast furnace molten iron in a converter, blowing argon and RH vacuum treatment at L F stations and performing calcium treatment in sequence, then pouring the molten iron into an oxygen top-bottom composite blowing converter for smelting such as a 210t stage, performing argon blowing, vacuum and calcium treatment on qualified molten steel for smelting, wherein the chemical composition of the molten steel for smelting is the same as that of a finished hot-rolled steel plate with the yield strength of 800MPa, and pouring the molten steel subjected to argon blowing, vacuum and calcium treatment into a steel billet, wherein the steel billet can be continuously cast by a continuous casting machine, and the thickness of the steel billet can be 230 mm.

The hot rolling step specifically comprises the steps of roughly rolling the steel billet into an intermediate billet after the steel billet is subjected to heating treatment, and coiling the intermediate billet into a steel coil after primary cooling, finish rolling and secondary cooling in sequence;

the heating step specifically comprises the steps of feeding the continuously cast steel billet into a soaking pit furnace or a heating furnace for heating, and keeping the temperature for more than or equal to 20min after the steel billet is heated to 1180-1300 ℃. The heating temperature and the holding time can homogenize the austenite structure in the billet, the carbides of Nb, Ti and the like in the billet are fully dissolved, and TiN is partially dissolved to grow the original austenite grains of the structure.

The rough rolling step specifically comprises the step of carrying out two-stage rolling on the heated steel billet, specifically, the heated steel billet is sent into a rough rolling mill set for rough rolling, the rough rolling can be carried out for 5-7 passes, the single-pass reduction rate of the rough rolling mill is more than or equal to 15%, and the total reduction rate of the steel billet on the rough rolling mill is 70% -85%; the steel billet becomes an intermediate billet after rough rolling.

The first cooling is that the intermediate blank after rough rolling is rapidly cooled, and the surface temperature of the intermediate blank is controlled to be 850-950 ℃; preferably 850 deg.C, 860 deg.C, 870 deg.C, 880 deg.C, 890 deg.C, 900 deg.C, 910 deg.C, 920 deg.C, 930 deg.C, 940 deg.C or 950 deg.C; the method adopts an intermediate billet cooling mode to realize differential temperature penetration rolling, utilizes an intermediate billet rapid cooling device to rapidly cool the surface temperature of the intermediate billet after rough rolling to 850-950 ℃, and simultaneously the core temperature of the intermediate billet is higher than the surface temperature, thereby ensuring the large penetration rolling of the core in the finish rolling process, greatly reducing the phenomena of center segregation and uneven structure of the steel plate, further improving the low-temperature impact toughness of the steel plate and ensuring that the Charpy impact energy AKv (-40 ℃) of the final steel plate is more than or equal to 100J.

The step of finish rolling specifically comprises the steps of putting the intermediate blank after rough rolling into a finishing tandem mill group for finish rolling, wherein the single-stand reduction rate of the finishing tandem mill group is more than or equal to 10 percent, and the total reduction rate of the intermediate blank in the finishing tandem mill group is 70 to 97 percent, and can be preferably 75 to 85 percent;

in the second stage of rolling, the finishing temperature of the intermediate billet in the finishing continuous rolling mill set can be controlled to be 820-900 ℃; at this stage, austenite forms elongated austenite without recrystallization, a large number of deformation zones exist in elongated austenite crystals, and solid solution atoms such as niobium Nb and titanium Ti are precipitated as carbides and carbonitrides by strain induction. After the final rolling in the non-recrystallization zone, the structure of the steel is a deformed austenite structure. The thickness of the steel plate after the intermediate billet is finely rolled by the fine continuous rolling mill group can be 2.0 mm-16.0 mm.

The steel plate after the secondary cooling is finish rolling can be subjected to ultra-fast cooling at a cooling speed of 100 ℃/S-300 ℃/S, and then the steel plate is cooled to 500-580 ℃ by a laminar cooling method at a speed of 10 ℃/S-25 ℃/S; the steel plate is coiled into a steel coil under the condition of 500-580 ℃.

S20, the steel coil is sequentially flattened and tempered to obtain a hot rolled steel plate with the yield strength of 800 MPa;

stacking and cooling the coil of the coil which is off-line or slowly cooling the coil of the coil to room temperature; then flattening the steel plate at the temperature lower than 70 ℃ to obtain the steel plate, preferably, the flattening temperature can be 40-60 ℃, such as 40 ℃, 45 ℃, 50 ℃, 55 ℃ or 60 ℃, and after flattening, tempering the steel plate at the temperature of 600-680 ℃, wherein the tempering and heat preservation time is controlled to be 20-40 min; the thickness of the steel plate is controlled to be 2.0 mm-16.0 mm. Preferably, the tempering temperature can be 600 ℃, 610 ℃, 620 ℃, 630 ℃, 640 ℃, 650 ℃, 660 ℃, 670 ℃ or 680 ℃, and the tempering heat preservation time can be 20min, 25min, 30min, 35min or 40 min.

In the embodiment of the application, on one hand, a Ti strengthening process is adopted, and because the sensitivity of the titanium compound to temperature is very high when the titanium compound is precipitated, the coiling temperature can be controlled to be 500-580 ℃ during hot rolling, the process is a low-temperature coiling process, carbide particles such as TiC are not precipitated basically during the hot rolling, the hot-rolled steel plate structure is mainly ferrite, bainite and a small amount of pearlite, and the hot-rolled steel plate structure is about 0.5-1 grade finer than the grain size coiled at high temperature, so the hot-rolled steel plate structure has higher low-temperature impact power ratio and can reach more than 100J at-40 ℃;

the tempering temperature is controlled to be 600-680 ℃, carbide particles such as TiC and TiN which are not precipitated in the hot rolling process can be fully precipitated in the tempering process, and bainite is partially decomposed, so that the strength of the tempered steel plate can be greatly improved, and the low-temperature toughness of the tempered steel plate is still high. The low-temperature impact energy of the finished product after the tempering treatment is still more than 100J at the temperature of minus 40 ℃.

On the other hand, by controlling the tempering temperature of 600-680 ℃ and the tempering heat preservation time of 20-40 min, on the one hand, the precipitated carbide particles can not grow up, the size of the carbide particles is ensured to be below 10nm, the size of the carbide particles is controlled to be about 5nm, the precipitation strengthening of the steel plate can be obviously improved, and the size and the number of the carbide precipitated particles at each position of the steel plate are not greatly different due to good temperature uniformity in the tempering process of the steel plate, so that the performance fluctuation of the steel plate can be controlled within 50MPa, and the aim of narrow performance control is fulfilled; in the second aspect, by controlling the tempering temperature and the tempering heat preservation time, the dislocation density in the hot rolling process can be greatly reduced, so that the internal stress of the steel plate can be greatly reduced; therefore, the increment of the precipitation strength can be ensured and higher low-temperature impact toughness can be ensured by controlling the tempering process parameters.

On the other hand, compared with the process of tempering first and then flattening, the process of tempering first and then flattening cannot achieve the uniformity of the temperature of the inner ring, the outer ring and the middle of the steel coil, and the steel plate can have the redistribution of internal stress during flattening, and the distribution of the internal stress is not uniform; compared with whole-pack tempering, the heat absorption and heat dissipation processes of the steel plates at the upper and lower positions and the steel plate at the middle position are inconsistent, the heat absorption rate and the heat dissipation rate of the steel plate at the middle position are slower than those of the steel plate close to the surface, and the performances and the internal stress of the steel plates at the upper and lower positions and the steel plate at the middle position are very uneven; in the embodiment of the application, after the steel coil is uncoiled, a single steel plate is tempered, so that the internal stress of the steel plate can be further eliminated, and even the state without the internal stress can be achieved by combining the control of tempering treatment process parameters.

According to the embodiment of the application, a low-C Nb-Ti component system is adopted, proper Mo is added, no noble element V element is added, a controlled rolling and cooling process system is adopted, the improvement effect of the process on the toughness of the steel plate is fully utilized through the optimization of the proportion of alloy elements, the high-toughness steel plate with lower carbon equivalent CEV (CeV) less than or equal to 0.48% is produced, the microstructure of a microalloy precipitated phase with the grain size of about 5nm dispersed and released on an ultrafine crystal ferrite matrix with the grain size of less than 5 mu m can be obtained, the yield strength of the steel plate can reach more than 800MPa, the elongation is more than or equal to 15%, and the Charpy impact energy AKv (-40 ℃) is more than or equal to 100J.

The embodiment of the application avoids the precipitation of carbide particles such as TiC by controlling the low coiling temperature, and controls the uniform precipitation and growth of the carbide particles such as TiC by tempering treatment, thereby achieving the purpose of low internal stress and even no internal stress.

The tempering treatment can control the size of carbide particles such as TiC particles, so that the size of precipitated particles can be controlled to be about 5nm, a good strengthening effect can be achieved, the addition amount of the alloy can be reduced, and the alloy cost is greatly reduced.

Wherein, in the step S20, after cooling, the steel sheet is coiled into a steel coil, preferably, the finish-rolled steel sheet is ultrafast-cooled at a cooling rate of 100 ℃/S to 300 ℃/S, and then the steel sheet is cooled to 520 to 580 ℃ by a laminar cooling method of 10 ℃/S to 25 ℃/S; the steel plate is coiled into a steel coil at the temperature of 520-580 ℃. Preferably, the steel plate is cooled to 520 ℃; coiling the steel plate into a steel coil at 520 ℃; cooling the steel plate to 530 ℃; coiling the steel plate into a steel coil at 530 ℃; cooling the steel plate to 540 ℃; coiling the steel plate into a steel coil at 540 ℃; cooling the steel plate to 550 ℃; coiling the steel plate into a steel coil at 550 ℃; cooling the steel plate to 560 ℃; coiling the steel plate into a steel coil at the temperature of 560 ℃; cooling the steel plate to 570 ℃; coiling the steel plate into a steel coil at 570 ℃; cooling the steel plate to 580 ℃; coiling the steel plate into a steel coil at 580 ℃;

if the final cooling temperature, namely the temperature after laminar cooling, is higher than 580 ℃, a large amount of TiC particles can be separated out in the coiling process, and the head, the middle and the tail of the steel plate are cooled inconsistently due to inconsistent cooling of steel coils, the separation amount of the head, the middle and the tail of the steel plate can be greatly different, the TiC particles can be separated out again on the steel plate at a low temperature part in the subsequent tempering process, the separated TiC particles can grow up, the size of the separated particles can be obviously coarsened, the strength is obviously reduced, and the impact toughness is reduced.

Preferably, the coiling temperature can be 510 ℃ to 550 ℃; the tempering temperature is 620-660 ℃, and the tempering heat preservation time is 20-40 min. So as to more accurately control the uniform precipitation of TiC particles and ensure the size uniformity of the TiC particles, and the size of the TiC particles is about 5 nm.

The hot rolled steel plate with the yield strength of 800MPa and the thickness of 2.0-16.0 mm manufactured in the embodiment can be widely applied to crane booms and structural parts, and has good welding performance and bending performance.

The invention is further illustrated by the following specific examples