CN111519104B

CN111519104B - Thin-gauge hot-rolled low-alloy high-strength steel and preparation method thereof

Info

Publication number: CN111519104B
Application number: CN202010398208.9A
Authority: CN
Inventors: 张衍; 郭子峰; 牛涛; 武巧玲; 郭佳; 李秋寒; 冯军; 陈斌; 李玉鹏; 吕宝锋; 赵青; 尼兴; 范然然; 杨业; 周娜; 徐伟; 张嘉琪; 宋春华; 陈波
Original assignee: Beijing Shougang Co Ltd; Shougang Corp
Current assignee: Beijing Shougang Co Ltd; Shougang Corp
Priority date: 2020-05-12
Filing date: 2020-05-12
Publication date: 2021-06-15
Anticipated expiration: 2040-05-12
Also published as: CN111519104A

Abstract

The invention discloses thin-gauge hot-rolled low-alloy high-strength steel and a preparation method thereof, wherein the high-strength steel comprises the following chemical components in percentage by mass: 0.03-0.1%, Si: 0.1-0.6%, Mn: 0.5-1.8%, P is less than or equal to 0.02%, S is less than or equal to 0.006%, Al: 0.015-0.05%, V: 0.01-0.07%, N: 0.01-0.03%, the balance being Fe and unavoidable impurities. The thin-gauge high-strength steel prepared by adopting the component design can reduce the rolling force and improve the production stability; the heating temperature can be reduced, and the energy consumption is saved; can improve the uniformity of product performance and surface quality.

Description

Thin-gauge hot-rolled low-alloy high-strength steel and preparation method thereof

Technical Field

The invention belongs to the technical field of steel preparation, and particularly relates to thin hot-rolled low-alloy high-strength steel and a preparation method thereof.

Background

The low-alloy high-strength steel is prepared by adding a small amount of Mn, Si and trace amounts of Nb, V or Ti and other alloy elements on the basis of carbon structural steel, thereby improving the strength, and can be widely applied to bridges, ships, vehicles, oil and gas pipelines, large steel structures and the like with high requirements on the strength. Particularly, in the application of the automobile industry, in order to ensure safety and reduce oil consumption and emission, the low-alloy high-strength steel is required to have high strength, improve safety and reduce thickness so as to reduce self weight and realize energy conservation and emission reduction.

After the hot-rolled low-alloy high-strength steel is subjected to cold rolling, the thickness of the hot-rolled low-alloy high-strength steel is thin, the strength of the hot-rolled low-alloy high-strength steel is high, and the requirements for safety and light weight can be met simultaneously. However, compared to cold-rolled steel sheets, hot-rolled steel sheets are less expensive because of the reduction of processes such as cold rolling, and are favored by the automobile industry. The trend of 'replacing cold with heat' of thin-specification hot-rolled low-alloy high-strength steel is increasingly obvious, and the thin-specification hot-rolled low-alloy high-strength steel is widely applied to structural members such as automobile chassis, automobile bodies, seats, spokes and the like in the states of finished products such as hot-rolled bare plates, hot-rolled pickled galvanized plates and the like.

However, the thin hot-rolled low-alloy high-strength steel has high strength due to a small thickness in the hot rolling process, and has large rolling force and unstable rolling in the hot rolling process.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides thin hot-rolled low-alloy high-strength steel and a preparation method thereof, and aims to solve the problems of large rolling force and unstable rolling in the hot rolling process due to thin thickness and high strength in the rolling process of the thin hot-rolled low-alloy high-strength steel in the prior art.

The invention realizes the purpose through the following technical scheme:

in one aspect, the invention provides thin gauge hot rolled low alloy high strength steel, the high strength steel consisting of the following chemical components in mass fraction, C: 0.03-0.1%, Si: 0.1-0.6%, Mn: 0.5-1.8%, P is less than or equal to 0.02%, S is less than or equal to 0.006%, Al: 0.015-0.05%, V: 0.01-0.07%, N: 0.01-0.03%, the balance being Fe and unavoidable impurities.

Further, the metallographic structure of the high-strength steel comprises ferrite and pearlite, the volume fraction of the ferrite is 90-95%, and the volume fraction of the pearlite is 5-10%.

Further, the thickness of the high-strength steel is 1.4-2.0 mm.

Furthermore, iron scale is attached to the surface of the high-strength steel, and the thickness of the iron scale is less than or equal to 10 microns.

In another aspect, the present invention provides a method for preparing a thin gauge hot-rolled low alloy high strength steel, the method comprising,

continuously casting the smelted molten steel to obtain a plate blank; the slab comprises the following chemical components in percentage by mass: 0.03-0.1%, Si: 0.1-0.6%, Mn: 0.5-1.8%, P is less than or equal to 0.02%, S is less than or equal to 0.006%, Al: 0.015-0.05%, V: 0.01-0.07%, N: 0.01-0.03%, the balance being Fe and inevitable impurities;

and sequentially heating, rough rolling, finish rolling, laminar cooling and coiling the plate blank to obtain the high-strength steel.

Further, the heating temperature is 1160-1230 ℃.

Further, the rough rolling is performed on a rough rolling mill, the rough rolling mill comprises a first rough rolling mill and a second rough rolling mill which are sequentially arranged along a rolling direction, in the rough rolling, the rolling frequency of the first rough rolling mill is 0, the rolling frequency of the second rough rolling mill is 5, or the rolling frequency of the first rough rolling mill is 1, the rolling frequency of the second rough rolling mill is 3, and the rough rolling termination temperature is 1040-1100 ℃.

Further, the thickness of the rough-rolled plate blank is 32-38 mm.

Further, the finish rolling termination temperature is 870-940 ℃.

Further, the coiling temperature is 570-640 ℃.

The beneficial effects of the invention at least comprise:

the invention provides thin-gauge hot-rolled low-alloy high-strength steel and a preparation method thereof, wherein the high-strength steel comprises the following chemical components in percentage by mass: 0.03-0.1%, Si: 0.1-0.6%, Mn: 0.5-1.8%, P is less than or equal to 0.02%, S is less than or equal to 0.006%, Al: 0.015-0.05%, V: 0.01-0.07%, N: 0.01-0.03%, the balance being Fe and unavoidable impurities. The method adopts a reasonable component system of C-Si-Mn-V-N, fully utilizes precipitation strengthening of vanadium carbonitride to obtain the low-alloy high-strength steel, vanadium element mainly plays a role in precipitation strengthening in the cooling and coiling processes after finish rolling, nitrogen element promotes the precipitation of vanadium carbonitride, and the rolling force of rough rolling and finish rolling is reduced while the strength is improved, so that the obtained thin-specification low-alloy high-strength steel is reduced in rolling difficulty, and the rolling stability is improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the drawings needed to be used in the description of the embodiments will be briefly introduced below, and it is obvious that the drawings in the following description are only some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on these drawings without creative efforts.

FIG. 1 is a process step diagram of a method for preparing thin gauge hot-rolled low-alloy high-strength steel according to an embodiment of the invention.

Detailed Description

The present invention will be described in detail below with reference to specific embodiments and examples, and the advantages and various effects of the present invention will be more clearly apparent therefrom. It will be understood by those skilled in the art that these specific embodiments and examples are for the purpose of illustrating the invention and are not to be construed as limiting the invention.

Throughout the specification, unless otherwise specifically noted, terms used herein should be understood as having meanings as commonly used in the art. Accordingly, unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. If there is a conflict, the present specification will control.

Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.

In order to solve the technical problems, the technical scheme in the embodiment of the invention has the following general idea:

In the invention, the C element is a main element of the reinforced steel, the strength requirement cannot be met due to too low C content, and the toughness and the weldability are adversely affected due to too high C content, so that the C content is more reasonable from 0.03 to 0.1 percent.

The content of the Si element is 0.1-0.6%, the solid solution strengthening effect is mainly achieved, but the surface quality is affected by the defects of red scales and the like which are difficult to remove and are easily formed on the surface of the steel plate due to the excessively high Si element.

The Mn content is 0.5-1.8%, the strengthening mode is mainly solid solution strengthening, and excessive Mn elements are easy to form segregation.

The P element is an inevitable impurity element, is easy to be segregated in a grain boundary, causes phosphorus brittleness and is controlled below 0.02 percent. The S element is also an inevitable impurity element, and easily forms MnS impurities with Mn, affecting the performance, and is controlled to be 0.006% or less.

Al element plays a role in deoxidation in the steel making process, and adding too much Al element will increase unnecessary cost.

The content of V element is 0.01-0.07%. V plays a main role in precipitation strengthening in the present invention. Generally, Nb, Ti, and V are microalloy elements commonly used in low-alloy high-strength steel. On one hand, Nb element is precipitated on the austenitic grain boundary to play a role in precipitation pinning, so that the grain growth is strongly hindered, and the fine grain strengthening effect is obvious. Ti element is separated out in the form of TiN in the continuous casting stage, and austenite grains are refined. The main strengthening processes of the two are all in the austenite stage, the deformation resistance in the rolling process can be increased while the steel plate is strengthened, and particularly for thin-specification high-strength steel with the thickness of less than 2mm, the rolling force is greater than that of thick-specification high-strength steel, so that the rolling difficulty is higher, and the rolling is unstable. On the other hand, the Nb element can inhibit austenite recrystallization and raise the recrystallization temperature, thereby increasing the rolling proportion of a non-recrystallization region, so that crystal grains are elongated, the anisotropy is obvious, and the formability of the steel plate is not facilitated. On one hand, the V element has weaker strengthening effect before the finish rolling is finished, and mainly plays a role in precipitation strengthening during the cooling and coiling processes after the finish rolling, namely the ferrite stage, and can effectively reduce the rolling force compared with Nb and Ti elements. On the other hand, the V element replaces the Nb element, the recrystallization temperature is low, the rolling proportion of a non-recrystallization region is low, the isotropy is better, and the formability of the steel plate is improved.

The content of N element is 0.01-0.03%. The N element can reduce the plastic toughness and increase the cracking risk of the plate blank, but not too high. However, the proper amount of N causes precipitation of V element in solid solution in the form of VN or V (CN), promotes precipitation strengthening of V element, and improves strength.

Ferrite has low carbon content, good plasticity and toughness, but low strength and hardness. Pearlite is a structure with alternate ferrite and cementite lamellar layers, the performance of the pearlite is between that of ferrite and cementite, the toughness is good, the strength is high, and the hardness is moderate.

Further, the thickness of the high-strength steel is 1.4-2.0 mm.

The high-strength steel is thin, and can be directly applied to the automobile industry without cold rolling.

The thickness of the scale on the surface of the high-strength steel is thin and is not more than 10 mu m, if the hot rolled plate is subjected to subsequent pickling, the scale is easy to pickle, the acid consumption is saved, and the surface quality of the pickled steel plate is good and the defects of pits, pockmarks, color difference and the like are not easy to appear.

In another aspect, the present invention provides the above method for preparing the thin gauge hot-rolled low alloy high strength steel, and fig. 1 is a process step diagram of a method for preparing the thin gauge hot-rolled low alloy high strength steel according to an embodiment of the present invention, and with reference to fig. 1, the method includes,

s1, continuously casting the smelted molten steel to obtain a plate blank; the slab comprises the following chemical components in percentage by mass: 0.03-0.1%, Si: 0.1-0.6%, Mn: 0.5-1.8%, P is less than or equal to 0.02%, S is less than or equal to 0.006%, Al: 0.015-0.05%, V: 0.01-0.07%, N: 0.01-0.03%, the balance being Fe and inevitable impurities;

the molten steel smelting process comprises blast furnace ironmaking, molten iron pretreatment, converter steelmaking and refining, and can also be electric furnace smelting, and all smelting methods capable of obtaining qualified molten steel can be applied to the application.

And S2, sequentially heating, roughly rolling, finely rolling, laminar cooling and coiling the plate blank to obtain the high-strength steel.

Further, the heating temperature is 1160-1230 ℃.

The thin low-alloy high-strength steel with the thickness not more than 2mm has the advantages of small thickness, less heat storage and quick heat dissipation, so the temperature drop is large in the rolling process. In order to ensure that the subsequent rough rolling temperature and the finish rolling temperature meet the design requirements, the rolling of the thin-specification high-strength steel needs higher furnace tapping temperature, the furnace tapping temperature is generally above 1240 ℃, the energy consumption is high due to the fact that the furnace tapping temperature is higher, meanwhile, the thickness of the iron oxide scales generated on the surface of the thin-specification high-strength steel is increased, and the probability of occurrence of defects such as iron oxide scale pressing in is increased. Thus, if the hot rolled plate with thick iron oxide scale is subsequently subjected to acid cleaning, the defects of pits, pockmarks, color difference and the like are easily generated. If the galvanized steel plate after acid cleaning is galvanized, pockmarks and color difference defects are more obvious and can not meet the use requirements. The invention adopts lower heating temperature and is matched with a subsequent 0+5 or 1+3 mode rough rolling process, namely the first rough rolling mill does not roll, the second rough rolling mill rolls 5 times, or the first rough rolling mill rolls 1 time, and the second rough rolling mill rolls 3 times, so that the rolling procedure passes are reduced, the temperature drop is reduced, the low furnace outlet temperature can be adopted for thin rolling, the energy consumption is reduced, the thickness of the iron sheet is reduced, and the surface quality of high-strength steel is improved.

Generally, a rough rolling mode adopts a 1+5 or 3+3 rolling mode, namely a first rough rolling mill rolls 1 pass, a second rough rolling mill rolls 5 passes, or the first rough rolling mill rolls 3 passes, and the second rough rolling mill rolls 3 passes, the rolling mode rolls thin high-strength steel, the temperature drop is large, the rough rolling mode adopts a 0+5 mode or a 1+3 mode, namely the first rough rolling mill does not roll, the second rough rolling mill rolls 5 passes, or the first rough rolling mill rolls 1 pass, the second rough rolling mill rolls 3 passes, and the rough rolling termination temperature is controlled to be 1040 and 1100 ℃, so that the mode can reduce the rolling passes from 6 to 5 or 4 passes, reduce the rolling passes and the rolling time, reduce the cooling times, reduce the process temperature drop and reduce the heating temperature in the slab heating furnace; secondly, the V element is adopted as a precipitation strengthening element, and the V element precipitation mainly occurs in the cooling and coiling processes after the finish rolling, so that the rolling force of the rough rolling and the finish rolling is reduced, the rough rolling mode of 0+5 or 1+3 can meet the requirements of rolling thickness and strength, and an intermediate billet with a thinner thickness can be obtained.

Further, the thickness of the rough-rolled plate blank is 32-38 mm.

According to the method, V element precipitation strengthening is matched with heating and rough rolling processes, so that the rolling force is low, the plate blank after rough rolling, namely the intermediate blank, is thin, the reduction and the rolling force of a post-pass finish rolling procedure can be reduced, and the stability of the finish rolling process is favorably improved.

Further, the finish rolling termination temperature is 870-940 ℃.

The traditional thin-specification low-alloy high-strength steel is strengthened by adopting the Nb element, the Nb element can inhibit austenite recrystallization, and the recrystallization temperature is increased, so that the rolling proportion of a non-recrystallization region is increased, grains are elongated, the anisotropy is obvious, and the formability of a steel plate is not facilitated. The steel plate is strengthened by adopting the V element, the recrystallization temperature is reduced, so that the rolling proportion of a non-recrystallization region is low, the isotropy is better, and the processing formability of the steel plate is improved. On the other hand, the higher finish rolling temperature is adopted to ensure that the finish rolling is completely carried out in an austenite region, so that the precipitation of vanadium carbonitride before the finish rolling is finished is avoided, and the early precipitation of vanadium carbonitride can increase the rolling force of a plurality of stands after the finish rolling.

Further, the coiling temperature is 570-640 ℃. The higher coiling temperature is adopted to reduce the cooling speed of laminar cooling, so that V (CN) is more fully precipitated, the precipitation strengthening effect is enhanced, and the strength of the high-strength steel is improved.

Further, the yield strength rp0.2 of the high strength steel: 420-680 MPa, tensile strength Rm: 480-760 MPa, and the difference of the transverse and longitudinal mechanical properties at the same position is less than or equal to 30 MPa.

Further, the post-fracture elongation a80 of the high-strength steel: 12 to 30 percent.

The invention provides thin-gauge hot-rolled low-alloy high-strength steel and a preparation method thereof, wherein a C-Si-Mn-V-N reasonable component system is adopted, vanadium carbonitride precipitation strengthening is fully utilized to obtain the low-alloy high-strength steel, vanadium element mainly plays a role in precipitation strengthening in the cooling and coiling processes after finish rolling, and nitrogen element promotes the precipitation of vanadium carbonitride, so that the strength is improved, the rolling force of rough rolling and finish rolling is reduced, the rolling difficulty of the obtained thin-gauge low-alloy high-strength steel is reduced, and the rolling stability is improved; the addition of the element V can reduce the recrystallization temperature of the material, so that the rolling proportion of the material in a non-recrystallization region is low, the isotropy is better, and the processing formability of the steel plate is improved; the low heating temperature is adopted to be matched with the rough rolling process and the finish rolling process in a 0+5 or 1+3 mode, so that the temperature drop is reduced while the energy consumption is reduced, the thickness of the iron sheet is reduced, and if the iron sheet is used for acid pickling treatment, the surface defects of pockmarks, pockmarks or color difference can be reduced, and the surface quality of high-strength steel is improved.

The technical solution of the present application will be further described with reference to specific examples.

TABLE 1

Item	C，％	Si，％	Mn，％	P，％	S，％	Al，％	Nb，％	V，％	N，％	Ti，％
											Example 1	0.07	0.1	1.00	0.015	0.005	0.025	0	0.05	0.015	0
Example 2	0.07	0.1	1.00	0.015	0.005	0.025	0	0.05	0.015	0
											Example 3	0.07	0.1	1.30	0.015	0.005	0.025	0	0.06	0.02	0
Example 4	0.07	0.1	1.30	0.015	0.005	0.025	0	0.06	0.02	0
											Example 5	0.07	0.15	1.4	0.015	0.005	0.025	0	0.065	0.025	0
Comparative example 1	0.07	0.1	1.00	0.015	0.005	0.025	0.04	0	0	0
											Comparative example 2	0.07	0.1	1.00	0.015	0.005	0.025	0.02	0	0	0.02
Comparative example 3	0.06	0.1	1.10	0.015	0.005	0.025	0.04	0	0	0

Examples

Examples 1 to 5 provide a method for manufacturing thin gauge hot-rolled low alloy high strength steel, by continuously casting qualified molten steel obtained by blast furnace ironmaking, molten iron pretreatment, converter smelting and refining to obtain a slab, the chemical composition of which is shown in table 1. And sequentially heating, rough rolling, finish rolling, laminar cooling and coiling the plate blank to obtain the high-strength steel. The process control of the heating, rough rolling, finish rolling, laminar cooling and coiling processes is shown in table 2, the rolling force of each stand in the finish rolling process is shown in table 3, and the volume ratio of each metallographic structure of the high-strength steel and the thickness of the iron scale are shown in table 4.

Comparative example

Comparative examples 1 and 2 provide a method for manufacturing thin gauge hot-rolled low-alloy high-strength steel, by continuously casting molten steel which is qualified by smelting, a slab is obtained, and the chemical composition of the slab is shown in table 1. And sequentially heating, rough rolling, finish rolling, laminar cooling and coiling the plate blank to obtain the high-strength steel. The process control of the heating, rough rolling, finish rolling, laminar cooling and coiling processes is shown in table 2, the rolling force of each stand in the finish rolling process is shown in table 3, and the volume ratio of each metallographic structure of the high-strength steel and the thickness of the iron scale are shown in table 4.

Comparative example 3 provides a cold-rolled sheet, the qualified molten steel of smelting is continuously cast to obtain a plate blank, the chemical components of the plate blank are shown in table 1, the plate blank is heated, roughly rolled, finish rolled and coiled to obtain a hot-rolled steel coil with the thickness of 3mm, the hot-rolled steel coil is sequentially subjected to acid cleaning and cold rolling to obtain a cold-rolled sheet with the thickness of 1.6mm, and the mechanical properties of the cold-rolled sheet are shown in table 4.

The thickness of the scale of examples 1 to 5, comparative example 1 and comparative example 2 was measured by taking 1 sample at the head, middle and tail widths 1/4, 1/2 and 3/4, respectively, of the high strength steel, measuring the thickness of a minimum of 3 points for each sample, and averaging all the thicknesses to obtain the scale thickness of the high strength steel. The mechanical property detection is that samples are taken at the positions of 10m close to the tail end and 1/4 wide of the high-strength steel, the samples are respectively sampled along the rolling direction (longitudinal direction) and the width direction (transverse direction), the yield strength, the tensile strength and the elongation are detected, the detection results are shown in table 5, and table 6 is the difference value of the transverse mechanical property and the longitudinal mechanical property.

TABLE 2

TABLE 3

Numbering	F1，KN	F2，KN	F3，KN	F4，KN	F5，KN	F6，KN	F7，KN
								Example 1	18322	17168	16117	15228	13211	11890	10942
Example 2	18138	17011	16034	15103	13119	11680	10798
								Example 3	22637	17463	16441	15858	14171	12903	11384
Example 4	21578	18123	16544	15923	14103	12759	11124
								Example 5	23748	18312	17249	16478	14884	13292	12403
Comparative example 1	19772	18255	17257	16597	14279	12882	11811
								Bielli 2	19693	18234	17213	16523	14215	12809	11789

TABLE 4

TABLE 5

TABLE 6

Numbering	Yield strength (MPa)	Tensile strength (MPa)	Elongation A80 (%)
				Example 1	6	7	0
Example 2	11	9	0.5
				Example 3	6	8	0.5
Example 4	5	4	0.5
				Example 5	5	9	2
Comparative example 1	37	24	2
				Comparative example 2	26	28	2.5

The mechanical property test results of examples 1 to 5 show that the examples have satisfactory mechanical properties, and table 6 shows the difference between the transverse mechanical properties and the longitudinal mechanical properties, and it can be seen from table 6 that the differences between the transverse yield strength and the longitudinal yield strength, the tensile strength and the elongation of examples 1 to 5 are smaller and the isotropy is better than those of comparative examples 1 and 2, and the steel sheet processing formability is improved. Since examples 1 and 2 are of the same strength grade as comparative examples 1 and 2, the rolling force of examples 1 and 2 of the present application is greatly reduced compared to comparative examples 1 and 2, the rolling stability is improved, and the above-mentioned precipitation of V element mainly occurs in the cooling and coiling processes after the finish rolling, and the rolling force of the finish rolling is reduced. Compared with the comparative example 1 and the comparative example 2, the thicknesses of the finished iron sheets of the examples 1 to 5 are obviously thinner, the energy consumption of the heating furnace is reduced in different proportions, and the high-strength steel provided by the application is easier to pickle and lower in cost.

Finally, it should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus.

While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.

It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims

1. The thin-gauge hot-rolled low-alloy high-strength steel is characterized by comprising the following chemical components in percentage by mass

，

The balance of Fe and inevitable impurities;

the surface of the high-strength steel is adhered with an iron scale, and the thickness of the iron scale

The thickness of the high-strength steel is 1.4-2.0 mm;

the preparation method of the thin-gauge hot-rolled low-alloy high-strength steel comprises the following steps,

continuously casting the smelted molten steel to obtain a plate blank; the slab consists of the following chemical components in mass fraction,

，

the balance of Fe and inevitable impurities;

sequentially heating, rough rolling, finish rolling, laminar cooling and coiling the plate blank to obtain high-strength steel;

the heating temperature is

；

The rough rolling is carried out on a rough rolling mill which comprises a plurality of rough rolling mills arranged in sequence along the rolling directionThe first roughing mill and the second roughing mill, in the roughing, the rolling frequency of the first roughing mill is 0, the rolling frequency of the second roughing mill is 5, or the rolling frequency of the first roughing mill is 1, the rolling frequency of the second roughing mill is 3, and the roughing end temperature is

；

The thickness of the rough rolled plate blank is

The finish rolling finish temperature is

；

The coiling temperature is

。

2. The thin gauge hot rolled low alloy high strength steel as claimed in claim 1, wherein the metallographic structure of the high strength steel includes ferrite and pearlite in a volume fraction of ferrite

The volume fraction of pearlite being

。