CN114000064B

CN114000064B - Ultrahigh-strength hot-rolled steel strip with thickness less than 4mm and production method thereof

Info

Publication number: CN114000064B
Application number: CN202111274414.XA
Authority: CN
Inventors: 熊雪刚; 张开华; 陈述; 汪创伟; 宋裕; 胡云凤
Original assignee: Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd
Current assignee: Pangang Group Panzhihua Steel and Vanadium Co Ltd; Pangang Group Panzhihua Iron and Steel Research Institute Co Ltd; Pangang Group Xichang Steel and Vanadium Co Ltd
Priority date: 2021-10-29
Filing date: 2021-10-29
Publication date: 2023-03-31
Anticipated expiration: 2041-10-29
Also published as: CN114000064A

Abstract

The invention relates to an ultra-high strength hot rolled steel strip with the thickness less than 4mm and a production method thereof, wherein the steel strip comprises the following components: 0.07 to 0.14 percent of C, 1.70 to 1.90 percent of Mn1, 0.05 to 0.40 percent of Si, less than or equal to 0.015 percent of P, less than or equal to 0.005 percent of S, 0.010 to 0.060 percent of Als0, less than or equal to 0.0040 percent of N, less than or equal to 0.0020 percent of O, 0.13 to 0.17 percent of Ti0.13, 0.15 to 0.20 percent of Mo0, 0.08 to 0.12 percent of V, and the balance of Fe and inevitable impurities, wherein the total content is less than or equal to 0.9 ([ Ti ] -48[ N ]/14 to 96[ S ]/32)/([ C ] -12[ S ]/32) ], and less than or equal to 1.2. The method comprises the steps of desulfurization, converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, hot rolling, laminar cooling and curling. The casting blank homogenization, the liquid precipitation TiN control and the TiC precipitation phase regulation are carried out, the process is optimized, and the cost is reduced.

Description

Ultrahigh-strength hot-rolled steel strip with thickness less than 4mm and production method thereof

Technical Field

The invention relates to an ultrahigh-strength hot-rolled steel strip with the thickness less than 4mm and a production method thereof, belonging to the technical field of steel strip rolling.

Background

In recent years, more and more attention is paid to the aspects of resource saving, energy conservation and emission reduction. The method is obvious in the automobile manufacturing industry, for example, increasingly strict emission standards are provided in the passenger vehicle industry, the development of new energy automobiles is greatly promoted, and the method is embodied in the aspects of light weight and weight reduction in the commercial vehicle industry. At present, 700 MPa-level tensile strength hot-rolled high-strength steel is generally applied to commercial vehicle frames, and achieves a remarkable light-weight effect, but the steel for the carriage mainly has a low strength level, and the light-weight level needs to be improved. Therefore, there is still a great need to develop high-strength hot rolled steel to promote the high-strength and light-weight process of commercial vehicles.

A high-strength thin steel sheet and a method for manufacturing the same are disclosed in the national patent publication No. CN 104254632B. The components by weight percentage are as follows: c:0.08-0.20%, mn:0.1-3.0%, si:0.3% or less, al:0.10% or less, P:0.10% or less, S:0.030% or less, N:0.010%, V:0.20-0.80%, and one or two groups of Ti, nb/Mo, B and Cr/Ni, and the rest is Fe and inevitable impurities. The steel with the components is heated, rough rolled, finish rolled and annealed by plating to obtain a thin steel plate with yield strength of more than 1000MPa, the microstructure of the thin steel plate is more than 95 percent of ferrite, the grain diameter of a precipitated phase is less than 10nm, and the number density is 1.0 multiplied by 105/um < 3 >. VN microalloying is adopted in the method, proper strength is obtained through precipitation strengthening, but N is easy to form N2 to cause bubbles under the skin of a casting blank, and the yield fluctuation of V in the finish rolling process is large, so that the steel has certain limitation, and meanwhile, the annealing heat treatment can also increase the process cost.

The national patent, publication No. CN 103194676B, discloses 1000MPa super ferrite steel and a preparation method thereof, the components of which are calculated by weight percentage: c:0.085-0.09%, mn:1.7-1.74%, si:0.21-0.23%, P:0.053-0.0080%, S:0.0050 to 0.0078%, N:0.0041-0.0050%, ti:0.14 to 0.19%, mo:0.37-0.55%, and the balance of Fe and inevitable impurities. The components are smelted, cast and forged into billets, the billets are heated at 1250-1300 ℃, heat is preserved for 1-2h, then the billets are rolled by 80-90% of deformation, the final rolling temperature is 880-950 ℃, then water cooling is carried out, the cooling speed is 30-50 ℃/s, the cooling is carried out to 600-700 ℃, then coiling is carried out, the coiling temperature is 600-700 ℃, the temperature is preserved for 1-2h, and then the steel is cooled to room temperature along with a furnace, so that the super ferrite steel with the tensile strength of 950-1050MPa, the elongation A50 of 18-21% and the hole expansion rate of more than 50% is obtained. The invention is that the steel smelted in a laboratory is rolled after being forged into a billet, the size of a sample is smaller, and the uniformity of the steel smelted in the laboratory by an electric furnace is better, and when an industrial trial-manufacturing route of continuous casting, heating and rolling is adopted, the size of the billet is larger, the homogenization of a casting blank, the homogenization of a microstructure and the control difficulty of large-size liquated TiN are higher, so the invention has technical difficulty in being popularized to the industrial trial-manufacturing.

The patent of this country, publication No. CN 109023111B discloses a 1000MPa grade hot-rolled automobile beam steel and a production method thereof, and the components of the hot-rolled automobile beam steel are calculated by weight percentage: c:0.10-0.20%, mn:1.5-1.7%, si is less than or equal to 0.10%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, nb:0.045-0.055%, ti:0.08-0.10%, N is not more than 40ppm, als:0.025-0.06%, H is less than or equal to 0.002%, and the balance of Fe and inevitable impurities. By adopting the production process of deep desulfurization pretreatment, converter smelting, LF refining, RH refining, continuous casting, slab heating, rough rolling, finish rolling and ultra-fast cooling, the finished steel with yield strength of more than or equal to 750MPa, tensile strength of more than or equal to 1000MPa, elongation of more than or equal to 10 percent, qualified 180-degree cold bending d =4a and martensite proportion of more than or equal to 80 percent is obtained. According to the invention, nb-Ti composite microalloying is adopted, and a rapid cooling process is combined to obtain the steel with most martensite of matrix structure, and the steel is not subjected to tempering heat treatment, so that the steel is expected to have low toughness, poor formability and limited application field.

In summary, most of the prior patents adopt a heat treatment process or form martensite to improve the strength and sacrifice the toughness so as to obtain 1000 MPa-grade ultrahigh-strength hot rolled steel, and have the defects of high production cost, insufficient comprehensive mechanical properties and difficulty in wide application through industrial production.

Disclosure of Invention

The invention aims to solve the technical problems that the production cost is higher and the comprehensive mechanical property is insufficient when the ultrahigh-strength hot rolled steel is produced by the existing process.

The technical scheme adopted by the invention for solving the technical problem is as follows: an ultra-high strength hot rolled steel strip with the thickness less than 4mm comprises the following components in percentage by weight: c:0.07-0.14%, mn:1.70-1.90%, si:0.05-0.40%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, als:0.010-0.060%, N is less than or equal to 0.0040%, O is less than or equal to 0.0020%, ti:0.13-0.17%, mo0.15-0.20%, V:0.08 to 0.12%, and satisfies 0.9 ≦ ([ Ti ] -48N ]/14-96, [ S ]/32)/([ C ] -12, [ S ]/32) ≦ 1.2, and the balance of Fe and inevitable impurities.

A production method of an ultrahigh-strength hot-rolled steel strip with the thickness less than 4mm comprises the following steps: the method comprises the steps of molten iron pre-desulfurization, converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, hot rolling, laminar cooling and coiling.

Wherein, the continuous casting billet in the method adopts low-temperature constant-speed casting, the temperature of the tundish is 1535-1545 ℃, the soft reduction is more than or equal to 5mm, the pulling speed is 0.9-1.2m/min, and the pulling speed fluctuation within 10s is not more than 0.05m/min.

In the method, the continuously cast steel billet directly enters a slab heating furnace through hot conveying and hot charging or is loaded into the slab heating furnace through stacking and slow cooling.

Further, in the method, the discharging temperature of the steel billet in the slab heating furnace is 1230-1270 ℃, and the total furnace time during hot delivery and hot charging is more than or equal to 180-320min; the total furnace time of furnace charging after slow cooling is 200-320min, and the stage heating time of the billet temperature being more than or equal to 1200 ℃ is more than or equal to 120min.

Further, the width of the reheated steel billet in the method is adjusted by a fixed width press, and the side pressure of the fixed width is less than or equal to 150mm.

The method comprises the following steps of rolling a steel billet in two stages, sending the steel billet into a roughing mill for roughing, adopting 5-pass or 6-pass rolling, wherein the first-pass deformation is more than or equal to 19 percent, the other-pass single-pass deformation is more than or equal to 20 percent, and the thickness of an intermediate billet obtained after roughing is 36-42mm.

Further, in the method, the intermediate billet is conveyed into a hot coil box or a heat preservation cover for heat preservation, the intermediate billet is conveyed into a finishing mill for finish rolling, 6-pass or 7-pass rolling is adopted, and the finishing temperature is 860-940 ℃.

Further, in the method, the steel plate after finish rolling is subjected to laminar cooling, concentrated cooling is carried out at a cooling rate of 30-60 ℃/s, the final cooling temperature is 620-670 ℃, then coiling is carried out, the coiling temperature of 10m at the tail part of the steel coil is 650-700 ℃, and the coiling temperature of the rest positions is 580-640 ℃.

Further, the coiled steel coil is sent into a slow cooling pit for slow cooling in the method, and the slow cooling time is more than or equal to 36 hours.

The invention has the beneficial effects that:

1. the steel provided by the method does not need to be subjected to heat treatment, and has the characteristics of low production cost, good equipment adaptability and capability of being produced on a common hot continuous rolling line.

2. The microstructure of the steel provided by the method is ferrite plus a precipitated phase, the strength and toughness are considered, and the forming performance is more excellent compared with high-strength steel with a martensite structure.

3. The method provides the measures of dendritic crystal segregation and component segregation of the steel, microstructure uniformity, liquid precipitation TiN control and precipitated phase regulation and control, and provides a strong toughness control scheme of 1000 MPa-level ultrahigh-strength hot rolled steel.

4. The method promotes the development and popularization of new hot-rolled products, promotes the development of hot-rolled high-strength steel, and meets the requirements of the national automobile manufacturing industry development. Meanwhile, the generation efficiency of each ton of steel is estimated to be 1000 yuan, and the generation efficiency is estimated to be 100 ten thousand yuan according to the yield of 1000 tons per year.

Drawings

FIG. 1 is a schematic view of the microstructure of example 1 of the present invention;

FIG. 2 is a schematic view of the microstructure of example 2 of the present invention;

FIG. 3 is a schematic microstructure of example 3 of the present invention;

FIG. 4 is a schematic microstructure view of example 4 of the present invention;

FIG. 5 is a schematic view of the present invention showing liquating out TiN;

FIG. 6 is a schematic view of the morphology of TiC precipitates of the present invention;

FIG. 7 is a schematic view of the microstructure of comparative example 1 of the present invention.

FIG. 8 is a schematic view of the microstructure of comparative example 2 of the present invention;

FIG. 9 is a schematic view of the microstructure of comparative example 3 of the present invention;

FIG. 10 is a schematic view of the microstructure of comparative example 4 of the present invention.

Detailed Description

The invention is further illustrated by the following examples and figures.

As shown in fig. 1 to 10, the ultra-high strength hot rolled steel strip with a thickness of less than 4mm according to the present invention comprises the following components by weight: c:0.07-0.14%, mn:1.70-1.90%, si:0.05-0.40%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, als:0.010-0.060%, N is less than or equal to 0.0040%, O is less than or equal to 0.0020%, ti:0.13-0.17%, mo0.15-0.20%, V:0.08 to 0.12%, and satisfies 0.9 ≦ ([ Ti ] -48N ]/14-96, [ S ]/32)/([ C ] -12, [ S ]/32) ≦ 1.2, and the balance of Fe and inevitable impurities. As will be understood by those skilled in the art, C is mainly responsible for interstitial solid solution strengthening and forms carbide with V, ti, mo, etc., thereby performing precipitation strengthening, and thus a certain C content must be maintained, but if the C content is too high, pearlite or grain boundary cementite is easily formed, the carbide precipitation phase is reduced, and the strength of the steel is reduced, so that the C content is controlled to 0.07 to 0.14%.

Mn and Si are solid solution strengthening elements, and too high Mn easily causes center segregation and dendrite segregation and improves plasticity, so that the uniformity of the structure is influenced, and the obdurability is reduced, so that the content of Mn is controlled to be 1.70-1.90%. Si can also improve the transformation temperature and inhibit the formation of cementite, but the plasticity of the steel is easily reduced due to the excessively high Si content, so the Si content is controlled to be 0.05-0.40%.

V can play a role in precipitation strengthening and refining original austenite grains by promoting austenite recrystallization. The content of V is controlled to be 0.08-0.12%.

The main functions of Mo are to improve the hardenability of steel, to suppress pearlite transformation, and to promote the fine dispersion of the second phase of V and Ti, thereby improving the toughness of steel. The content of Mo is controlled to be 0.15-0.20%.

Ti is the most main precipitation strengthening original in the test steel, but because the chemical property of Ti is more active, S and N elements in the steel are easy to form large-particle inclusions such as Ti4C2S2, liquated TiN and the like, the content of N and S elements in the steel must be controlled, the liquated TiN is easy to grow up due to overhigh content of Ti, the content of N is controlled to be less than or equal to 0.0040%, the content of S is controlled to be less than or equal to 0.0050%, and the content of Ti is controlled to be 0.13-0.17%. Meanwhile, in order to fully exert the precipitation strengthening effect of TiC, the elements meet the following ideal chemical proportion: 0.9 ≦ ([ Ti ] -48 ]/14-96, ([ C ] -12 ]/32) ≦ 1.2.

A production method of an ultrahigh-strength hot-rolled steel strip with the thickness less than 4mm comprises the following steps: the method comprises the following steps of molten iron pre-desulfurization, converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, hot rolling, laminar cooling and coiling. It can be understood by those skilled in the art that the billet having a thickness of 200 to 250mm of the above chemical composition is preferably obtained through the flow of molten iron pre-desulfurization → converter smelting → LF refining → RH vacuum refining → calcium treatment → continuous casting. And simultaneously, the chemical components of the steel strip are limited, and the steel strip is subjected to molten iron pre-desulfurization, converter smelting, LF refining, RH vacuum refining, calcium treatment, continuous casting, hot rolling, laminar cooling and coiling to obtain a product with yield strength of more than or equal to 800MPa, tensile strength of 950-1050MPa, elongation of more than 14%, qualified 180-degree cold bending test d =2a, microstructure of the product is ferrite structure with more than 95%, grain size of 13.5-14.0 grade, size of liquated TiN of less than 5 micrometers, and size of second-phase precipitates of less than 10 nanometers.

Wherein, the continuous casting billet in the method adopts low-temperature constant-speed casting, the temperature of the middle package is 1535-1545 ℃, the soft reduction is more than or equal to 5mm, the pulling speed is 0.9-1.2m/min, and the pulling speed fluctuation within 10s is not more than 0.05m/min. As can be understood by those skilled in the art, in order to control the quality of the steel billet, the method actually prefers that the continuous casting steel billet is cast at a low temperature and a constant speed, the temperature of a middle ladle is 1535-1545 ℃, the soft reduction is more than or equal to 5mm, the drawing speed is 0.9-1.2m/min, and the fluctuation of the drawing speed within 10s is not more than 0.05m/min.

In the method, the continuously cast steel billet directly enters a slab heating furnace through hot conveying and hot charging or is loaded into the slab heating furnace through stacking and slow cooling. As will be appreciated by those skilled in the art,

further, in the method, the discharging temperature of the steel billet in the slab heating furnace is 1230-1270 ℃, and the total furnace time during hot delivery and hot charging is more than or equal to 180-320min; the total furnace time of furnace charging after slow cooling is 200-320min, and the stage heating time of the billet temperature being more than or equal to 1200 ℃ is more than or equal to 120min. As can be understood by those skilled in the art, the main function of slab reheating is to make alloy elements solid-dissolve, if the slab heating temperature is lower, the alloy elements such as V, ti, mo and the like in the finished steel are not fully solid-dissolved, and the subsequent second phase precipitation is reduced, so that the strength is lower. Meanwhile, the problems of dendritic crystal segregation, component segregation and the like caused by the casting blank can be easily reduced by the higher heating temperature of the plate blank, so that the tapping temperature is set to be 1230-1270 ℃, and the stage heating time of the steel blank temperature being more than or equal to 1200 ℃ is required to be more than or equal to 120min.

Further, the width of the steel billet reheated in the method is adjusted by a fixed width press, and the side pressure of the fixed width is less than or equal to 150mm. As can be appreciated by those skilled in the art, in order to facilitate the subsequent hot rolling, the method preferably adjusts the width of the reheated slab by a constant width press, and the constant width side pressure is less than or equal to 150mm.

The method comprises the following steps of rolling a steel billet in two stages, sending the steel billet into a roughing mill for roughing, adopting 5-pass or 6-pass rolling, wherein the first-pass deformation is more than or equal to 19 percent, the other-pass single-pass deformation is more than or equal to 20 percent, and the thickness of an intermediate billet obtained after roughing is 36-42mm. As can be understood by those skilled in the art, the main effect of rough rolling is to refine grains through austenite dynamic recrystallization, and the single-pass deformation of the rough rolling must be larger than the critical deformation, otherwise, the central deformation of the steel plate is easy to be insufficient, so that the first-pass deformation of the rough rolling is controlled to be more than or equal to 19%, and the single-pass deformation of the rest passes is controlled to be more than or equal to 20%. Meanwhile, the adoption of the lower thickness of the intermediate blank is beneficial to improving the deformation amount of rough rolling, promoting the high-temperature precipitation of a recrystallization zone and refining austenite grains during finish rolling, thereby achieving the purpose of refining finished steel grains. Therefore, the present invention sets the thickness of the intermediate billet to 36 to 42mm.

Further, in the method, the intermediate billet is conveyed into a hot coil box or a heat preservation cover for heat preservation, the intermediate billet is conveyed into a finishing mill for finish rolling, 6-pass or 7-pass rolling is adopted, and the finishing temperature is 860-940 ℃. As will be understood by those skilled in the art, the primary function of finish rolling is to provide nucleation energy and nucleation sites for subsequent transformation by non-recrystallization deformation of austenite, wherein the finish rolling reduction and rolling temperature are key process parameters, the thickness of the intermediate blank is set to 36-42mm, the thickness of the final product is 1.8-4.0mm, and the finish rolling reduction is 9 at the lowest, which is sufficient to fully refine austenite during finish rolling. Meanwhile, the steel plate is thin, the whole steel coil is long, the temperature of the tail part of the steel plate is obviously reduced, and a hot coil box or a heat-insulating cover is adopted for heat insulation before finish rolling for reducing the temperature reduction of the tail part of the steel coil. In addition, in order to prevent the strength of the finished steel from being lowered by reducing the strain-induced precipitation during the finish rolling when the rolling temperature is too low, the finish rolling temperature is controlled to a high level, that is, to a range of 860 to 940 ℃.

Further, in the method, the steel plate after finish rolling is subjected to laminar cooling, concentrated cooling is carried out at a cooling rate of 30-60 ℃/s, the final cooling temperature is 620-670 ℃, then coiling is carried out, the coiling temperature of 10m at the tail part of the steel coil is 650-700 ℃, and the coiling temperature of the rest positions is 580-640 ℃. As can be understood by those skilled in the art, phase transformation and interphase precipitation can occur during laminar cooling, and supersaturation precipitation of ferrite can occur during subsequent coiling and slow cooling. In order to avoid forming coarse proeutectoid ferrite, pearlite, grain boundary cementite and other abnormal structures in the cooling process, rapid concentrated cooling is carried out by adopting a rapid cooling rate and a low final cooling temperature, so that the austenite structure is converted into a full ferrite structure. The cooling rate and the finish cooling temperature are preferably controlled to 30 to 60 ℃/s and 620 to 670 ℃, respectively, and at this finish cooling temperature, it is also advantageous to promote interphase precipitation. In addition, the coiling temperature is mainly set by considering that the nose point temperature of the supersaturated ferrite precipitation is positioned near 600 ℃, the coiling temperature is controlled to be 580-640 ℃ to be beneficial to promoting the supersaturated ferrite precipitation, and meanwhile, the higher coiling temperature is adopted at the tail part of the steel coil to reduce the temperature loss in the steel coil in the coiling process.

Further, the coiled steel coil is fed into a slow cooling pit for slow cooling in the method, and the slow cooling time is more than or equal to 36h. As can be understood by those skilled in the art, the method preferably sends the coiled steel coil into a slow cooling pit for slow cooling, and the final product can be obtained when the slow cooling time is more than or equal to 36 hours.

Table 1 shows the compositions of examples and comparative examples, table 2 shows hot rolling process parameters of examples and comparative examples, and Table 3 shows mechanical properties of steels of examples and comparative examples.

TABLE 1

TABLE 2

TABLE 3

In the method, the V-Ti-Mo component route is adopted in the embodiment 1 to the embodiment 4, the finished steel plate is obtained through hot continuous rolling and rapid cooling, and the mechanical property meets the qualified requirements of the tensile strength of 950-1050MPa, the yield strength of more than 800MPa, the elongation of more than 14 percent and the cold bending test d =2a at 180 degrees. The TiN can be controlled below 5 micrometers as can be seen by combining the attached figures 5 and 6, and the precipitated phase of the nano TiC is dispersed and distributed in the matrix.

The chemical components of the comparative example 1 have low Mn and Ti contents, the heating temperature of the plate blank is more than or equal to 1200 ℃, the time is short, the alloy elements are not fully dissolved, and the tensile strength of the finished steel is low. In comparative example 2 ([ Ti ] -48, [ N ]/14-96, [ S ]/32)/([ C ] -12, [ S ]/32) =0.69, the stoichiometry is not satisfied, indicating that C has a margin in addition to the formation of the MC precipitated phase, and that the final cooling temperature is high due to the low cooling rate of comparative example 2, coarse proeutectoid ferrite and pearlite structure transformation are easily promoted, resulting in a decrease in the strength of the finished steel. The chemical components of the comparative example 3 meet the requirements, but the cooling speed is too high, the final cooling temperature is lower, the second phase is inhibited from being separated out, the strength of the finished steel is reduced, and the microstructure of the finished steel is bainite. The content of N in chemical components of the comparative example 4 is higher, the probability of forming liquated TiN is increased, the temperature of the ladle is higher, the secondary cooling water adopts a weak cooling mode, dendritic crystal segregation and component segregation of a casting blank are easily caused, the laminar cooling rate is lower, the final cooling temperature is higher, the microstructure is further large, and the strength and the toughness of steel are reduced.

Claims

1. A production method of an ultrahigh-strength hot-rolled steel strip with the thickness less than 4mm is characterized by comprising the following steps: pre-desulfurizing molten iron, smelting in a converter, LF refining, RH vacuum refining, calcium treatment, continuous casting, hot rolling, laminar cooling and coiling; the steel strip comprises the following components in percentage by weight: c:0.07-0.14%, mn:1.70-1.90%, si:0.05-0.40%, P is less than or equal to 0.015%, S is less than or equal to 0.005%, als:0.010-0.060%, N is less than or equal to 0.0040%, O is less than or equal to 0.0020%, ti:0.13-0.17%, mo0.15-0.20%, V:0.08 to 0.12%, and satisfies 0.9 or less ([ Ti ] -48N ]/14-96, [ S ]/32)/([ C ] -12, [ S ]/32) ≦ 1.2, and the balance of Fe and inevitable impurities; the method comprises the following steps of rolling a steel billet in two stages, sending the steel billet into a rough rolling mill for rough rolling, adopting 5-pass or 6-pass rolling, wherein the first-pass deformation is more than or equal to 19 percent, the other-pass single-pass deformation is more than or equal to 20 percent, and the thickness of an intermediate billet obtained after rough rolling is 36-42mm; the intermediate blank is sent into a hot coil box or a heat preservation cover for heat preservation, the intermediate blank is sent into a finishing mill for finish rolling, 6-pass or 7-pass rolling is adopted, and the final rolling temperature is 860-940 ℃; and carrying out laminar cooling on the finish-rolled steel plate, carrying out concentrated cooling at a cooling rate of 30-60 ℃/s at a final cooling temperature of 620-670 ℃, and then coiling, wherein the coiling temperature of 10m at the tail part of the steel coil is 650-700 ℃, and the coiling temperature of the rest positions is 580-640 ℃.

2. A method of producing ultra high strength hot rolled steel strip having a thickness < 4mm as claimed in claim 1 wherein: the continuous casting billet is cast at a low temperature and a constant speed, the temperature of a tundish is 1535-1545 ℃, the soft reduction is more than or equal to 5mm, the drawing speed is 0.9-1.2m/min, and the drawing speed fluctuation within 10s is not more than 0.05m/min.

3. A method of producing ultra high strength hot rolled steel strip having a thickness < 4mm as claimed in claim 1 wherein: the continuously cast steel billet directly enters a slab heating furnace through hot conveying and hot charging or is loaded into the slab heating furnace after being stacked and slowly cooled.

4. A method of producing ultra high strength hot rolled steel strip having a thickness < 4mm as claimed in claim 3 wherein: the discharging temperature of the billet in the slab heating furnace is 1230-1270 ℃, and the total in-furnace time during hot delivery and hot charging is more than or equal to 180-320min; the total furnace time of furnace charging after slow cooling is 200-320min, and the stage heating time of the billet temperature being more than or equal to 1200 ℃ is more than or equal to 120min.

5. A method of producing ultra high strength hot rolled steel strip having a thickness < 4mm as claimed in claim 4 wherein: the width of the reheated billet is adjusted by a fixed width press, and the side pressure of the fixed width is less than or equal to 150mm.

6. A method of producing ultra high strength hot rolled steel strip having a thickness < 4mm as claimed in claim 1 wherein: and feeding the coiled steel coil into a slow cooling pit for slow cooling, wherein the slow cooling time is more than or equal to 36h.