CN110983181B

CN110983181B - 700 MPa-grade hot rolled steel and preparation method and application thereof

Info

Publication number: CN110983181B
Application number: CN201911294765.XA
Authority: CN
Inventors: 李晓林; 艾矫健; 缪成亮; 肖宝亮; 徐永先; 杨孝鹤; 张震; 高智慧; 富晓航; 安瑞东; 杜倩; 王志鹏; 张大伟
Original assignee: Shougang Corp
Current assignee: Shougang Corp
Priority date: 2019-12-16
Filing date: 2019-12-16
Publication date: 2021-04-16
Anticipated expiration: 2039-12-16
Also published as: CN110983181A

Abstract

The invention discloses 700 MPa-grade hot rolled steel and a preparation method and application thereof, wherein the hot rolled steel comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.02 percent; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities. The hot rolled steel prepared by the preparation method provided by the invention has the advantages of low internal stress, good plate shape in the opening and using processes and high precision.

Description

700 MPa-grade hot rolled steel and preparation method and application thereof

Technical Field

The invention belongs to the technical field of metallurgy, and particularly relates to 700 MPa-grade hot rolled steel, and a preparation method and application thereof.

Background

With the acceleration of the industrialization process, the demand for high reinforcement of steel materials is more and more obvious. However, the higher the strength of the steel, the greater the internal stress, which leads to the easy occurrence of plate shape problems during the flattening process after rolling and the use process, such as warping, wave shape, camber and the like. Especially for the mechanical industry with high requirements on the dimensional accuracy and the assembly accuracy of each part, the plate shape requirement after cutting and blanking of the steel plate is very strict, the flatness of the steel plate is required to be less than or equal to 2mm/m after plasma or laser cutting, after the steel plate is longitudinally cut into (500 + 600mm) × 10m steel plates by plasma cutting, the camber is less than or equal to 2mm/10m, no waves or middle waves exist, and the defects of distortion, camber, bulging, misalignment and the like cannot occur after the steel plate is bent.

At present, 700MPa high-strength steel with the thickness of 4.0-14.0 mm is mainly produced by TMCP or TMCP plus tempering process. Because TMCP technological process is short, delivery cycle is short, it is the most common mode of production in domestic hot rolled coil steel mill, but 700MPa hot rolled coil produced by TMCP is produced by controlled rolling and controlled cooling mode, it is easy to cause uneven distribution of residual stress, the apparent plate type problem shown is wave shape, bow back or C warp, after straightening and leveling, it can improve the apparent plate type, but the hidden plate type problem, such as camber, warp after cutting, etc., still can't be evaluated quantitatively, the plate type quality problem is unstable, and affects normal use. The production mode of TMCP + tempering process is that the hot rolled coil produced by TMCP is made into steel plate by means of flattening and transverse cutting, and then is tempered in heat treatment furnace, and the delivery mode of said production process is single, and if the required specification is more and more complicated, the organization and production of steel factory are more difficult, and the steel factory adopts steel coil to deliver, and its downstream production flexibility is higher, and the delivery period of steel factory also can be shortened.

From the above, the hot rolled steel prepared by the existing production process has high internal stress, so that the plate type problem is easy to occur in the flattening process and the using process after rolling.

The patent application with the application number of CN201010162933.2 discloses a residual stress relieving method for hot-rolled high-strength steel, which aims at hot-rolled steel plate products, the material strength of which is 600MPa and is lower than that of the patent application. In addition, the slow cooling treatment is carried out after the 8.0mm steel coil is off-line, so that the working procedures are increased, and the production cost and the delivery period are increased.

The patent application with the application number of CN201510396674.2 discloses a method for preparing low-stress hot-rolled high-strength steel with the thickness of 8.0-20.0 mm, and the patent mainly aims at hot-rolled steel plate products.

The patent application with the application number of CN201410593263.8 discloses a steel plate for engineering machinery and a manufacturing method thereof, and the patent application with the application number of CN201810293804.3 applies a ferrite-based tempered steel plate with the yield strength of 900MPa and a production method thereof. Both of the above applications are directed to hot rolled cross-cut steel sheet products.

Disclosure of Invention

Aiming at the defects of the prior art, the invention provides 700 MPa-grade hot rolled steel and a preparation method and application thereof, and aims to solve the problem that the plate shape is easy to appear in the flattening process and the using process after rolling due to high internal stress in the hot rolled steel in the prior art.

The invention realizes the purpose through the following technical scheme:

on one hand, the invention provides 700 MPa-grade hot rolled steel which comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities.

Further, the thickness of the hot rolled steel is 4.0-14.0 mm.

On the other hand, the embodiment of the invention also provides a preparation method of the 700 MPa-grade hot rolled steel, which comprises the following steps,

carrying out molten iron pretreatment, full-three-strip converter smelting, RH refining, LF refining and continuous casting on molten iron in sequence to obtain a plate blank; the slab comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities;

sequentially heating, rough rolling, finish rolling, ultra-fast cooling and coiling the plate blank to obtain a hot-rolled coil;

and sequentially performing cover annealing and cooling on the hot rolled coil to obtain hot rolled steel.

Further, in the heating, the heating temperature is 1250-1300 ℃, the discharging temperature is 1250-1300 ℃, the heating time is 3-5 hours, and the soaking time is 30-40 min.

Further, the rough rolling is 6-pass rolling, and the deformation amount of the rough rolling is 76-86%.

Further, the inlet temperature of finish rolling is 950-1060 ℃, the finish rolling termination temperature is 820-880 ℃, the deformation amount of the finish rolling is 70-80%, the finish rolling is 7-pass rolling, and the reduction rate of the finish rolling final pass is 10-15%.

Further, the cooling rate of the ultra-fast cooling is 40-60 ℃/s.

Further, the coiling temperature is 500-550 ℃.

Further, the cap annealing includes,

sequentially carrying out primary heating and primary heat preservation on the hot-rolled coil;

carrying out secondary heating and secondary heat preservation on the hot-rolled coil subjected to primary heat preservation in sequence; the secondary temperature rise time is 8.5-11.5 h, the secondary heat preservation time is 2-4 h, and the secondary heat preservation temperature is 570-630 ℃;

and (4) carrying out slow cooling and discharging on the hot rolled coil subjected to secondary heat preservation in sequence.

On the other hand, the embodiment of the invention also provides application of 700 MPa-grade hot rolled steel as a crane boom, a modified truck girder, an automobile girder, an ATM and a safety box.

The beneficial effects of the invention at least comprise:

the invention provides 700 MPa-grade hot rolled steel and a preparation method and application thereof, wherein the hot rolled steel comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities. A C-Mn-Nb-Ti-V component system is adopted, and a rolling process of hot rolling, ultra-fast cooling and annealing is matched, so that the solid solution strengthening of C, Mn, the fine grain strengthening of Nb and the precipitation strengthening of TiC are exerted, the material is ensured to obtain a ferrite structure with uniform and fine grains, the precipitation of microalloy elements such as Nb, Ti and V is inhibited in the cooling and coiling processes, the hot rolled coil adopts a cover annealing process, the precipitation strengthening effect of the elements such as Nb, Ti and V is exerted on the basis of eliminating the residual stress of the steel coil, the hot rolled steel with high strength and low residual stress is obtained, and the plate shape defect is eliminated.

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 preparation method of 700MPa hot rolled steel according to an embodiment of the invention;

FIG. 2 is a metallographic structure diagram of a 700MPa hot rolled steel according to an embodiment of the present invention;

FIG. 3 is a diagram of a 700MPa hot rolled steel bell annealing process according to an embodiment of the present invention;

FIG. 4 is a precipitated phase diagram of a 700MPa grade hot rolled steel according to an embodiment of the present 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:

on one hand, the embodiment of the invention provides 700 MPa-grade hot rolled steel, which comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities.

The function of each element in the invention is as follows:

c: the solid solution strengthening element, however, too high C content easily forms a band-like structure during slab smelting and rolling, the severe band-like structure adversely affects cold formability and fatigue properties of the steel sheet, and the C content is high, which is disadvantageous in weldability of the material.

Si: si is a traditional solid solution strengthening element, but excessively high Si addition has negative effects on the plasticity, toughness and surface quality of the material, and can also reduce the diffusion rate of carbon in austenite, which is not beneficial to the formation of ferrite structure.

Mn: mn has a solid solution strengthening effect and can improve the hardenability of the material, but the content of Mn is too high, and a strip-shaped structure is easy to appear in the smelting and rolling processes of the plate blank due to the segregation effect of Mn elements.

S and P: too high S and P elements can adversely affect the plasticity, toughness and fatigue properties of the material. The invention limits the S content to be controlled within 0.005 percent and the P content to be controlled within 0.015 percent.

Ti: ti is a strong carbonitride forming element, a certain amount of Ti is added into the steel, and the Ti and N are combined to form TiN or Ti (C, N) in a high-temperature austenite zone to inhibit the growth of austenite grains. And low-temperature coiling is adopted in the cooling process, and TiC is controlled to be precipitated in the coiling process. Adopting 570-630 ℃ annealing, the annealing temperature interval is that TiC precipitation strengthening effect is best, and the strength is prevented from decreasing by the TiC precipitation strengthening effect in the annealing process, the invention controls the Ti content as follows: 0.01 to 0.05 percent.

Nb: nb is a strong carbonitride forming element, is precipitated in an austenite region, can refine grains, improves the strength and toughness of the material, and can ensure the refinement of a weld joint structure and improve the strength after welding.

Al: al is one of effective deoxidizing elements, and can form nitrides to refine grains. Too high Al content impairs the toughness of the steel and also deteriorates the toughness of the weld heat-affected zone.

V: the V element is a strong carbide precipitation element, and is combined with elements such as Ti, Nb and the like in the annealing process to form a composite precipitation phase, so that the strength of the material is ensured. In addition, in the welding process, V and C are combined in the cooling process of the heat affected zone to form a VC precipitated phase, the strength of the heat affected zone is improved, the heat affected zone is prevented from being softened, but the low-temperature toughness of the steel is obviously deteriorated when the V content is higher, particularly the toughness of the welding heat affected zone, and the V content is controlled as follows: 0.01 to 0.10 percent.

Further, the thickness of the hot rolled steel is 4.0-14.0 mm.

In another aspect, the embodiment of the present invention provides the above method for preparing 700 MPa-grade hot rolled steel, and fig. 1 is a process step diagram of the method for preparing 700 MPa-grade hot rolled steel according to the embodiment of the present invention, with reference to fig. 1, the method includes,

s1, sequentially carrying out molten iron pretreatment, full-three-strip converter smelting, RH refining, LF refining and continuous casting on the molten iron to obtain a plate blank; the slab comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities.

The molten iron is pretreated by KR desulfurized molten iron and smelted by a full-three-desulfurizing converter, so that S and P harmful elements of the molten steel can be controlled, and the molten steel with required components can be obtained by RH and LF double refining treatment.

And S2, sequentially heating, roughly rolling, finely rolling, ultrafast cooling and coiling the plate blank to obtain a hot-rolled coil.

Further, in the heating, the heating temperature is 1250-1300 ℃, the discharging temperature is 1250-1300 ℃, the heating time is 3-5 hours, and the soaking time is 30-40 min. The higher heating temperature and the longer heating time aim to ensure the full dissolution of the carbonitride of the alloy elements and the homogenization of austenite, and provide conditions for realizing fine grain strengthening and precipitation strengthening.

Further, the rough rolling is 6-pass rolling, and the deformation amount of the rough rolling is 76-86%. And rolling the plate blank into an intermediate blank meeting the finish rolling requirement through rough rolling.

Further, the inlet temperature of finish rolling is 950-1060 ℃, the finish rolling termination temperature is 820-880 ℃, the deformation amount of the finish rolling is 70-80%, the finish rolling is 7-pass rolling, and the reduction rate of the finish rolling final pass is 10-15%. By controlling the temperature of the finish rolling process, finish rolling is carried out in an austenite region, and the ultra-fast cooling process and the coiling process are matched, so that a large amount of (Nb, Ti and V) C composite precipitated phases can be prevented from being formed in steel, and the precipitation strengthening after annealing is guaranteed.

Further, the cooling rate of the ultra-fast cooling is 40-60 ℃/s. The carbide of elements such as Ti, Nb, V and the like can be effectively inhibited from being precipitated in the phase change and cooling processes at a high cooling rate, and a guarantee is provided for precipitation strengthening after annealing. In addition, a finer and more uniform ferrite structure is obtained through rapid cooling (see fig. 2), and the refinement of the crystal grains can improve the strength and the toughness of the material and ensure the low-temperature toughness of the material.

Further, the coiling temperature is 500-550 ℃. Coiling in the temperature range can effectively inhibit the carbide of elements such as Ti, Nb, V and the like from being precipitated in the coiling process, and ensure the precipitation strengthening effect in the annealing process.

And S3, sequentially performing cover annealing and cooling on the hot rolled coil to obtain hot rolled steel.

Further, fig. 3 is a diagram of a 700 MPa-grade hot rolled steel hood annealing process according to an embodiment of the invention, and in combination with fig. 3, the hood annealing includes,

sequentially carrying out primary heating (0-t 1) and primary heat preservation (t 1-t 2) on the hot rolled coil;

sequentially carrying out secondary temperature rise (t 2-t 3) and secondary heat preservation (t 3-t 4) on the hot rolled coil subjected to the primary heat preservation; the secondary temperature rise time is 8.5-11.5 h, the secondary heat preservation time is 2-4 h, and the secondary heat preservation temperature is 570-630 ℃;

and (4) slowly cooling the hot rolled coil subjected to secondary heat preservation (t 4-t 5) and discharging the hot rolled coil from the furnace in sequence.

The hot rolled coil is a ferrite structure before annealing, secondary heat preservation is carried out at the temperature range of 570-630 ℃, the structure type is not changed, the matrix structure recovers and grows, partial dislocation in crystal grains recovers, merges and disappears, the internal stress of the hot rolled steel matrix structure is more uniform, and the internal residual stress of the hot rolled coil can be effectively eliminated. On the other hand, the precipitation strengthening effect of the microalloy elements such as Nb, Ti and V in the temperature range is optimal, a large amount of precipitated (Ti, Nb and V) C has the particle size of about 10nm and strong thermal stability, does not easily grow up within the heat preservation time of 2-4 hours, ensures the precipitation strengthening effect (see figure 4), and effectively makes up for the strength loss caused by tissue softening.

On the other hand, the embodiment of the invention also provides application of the 700 MPa-grade hot rolled steel as a crane boom, a modified truck girder, an automobile girder, an ATM and a safety box.

The 700MPa hot rolled steel is prepared by adopting a TMCP + annealing process, a C-Mn-Nb-Ti-V component system design is adopted, the solid solution strengthening of C, Mn, the fine grain strengthening of Nb and the precipitation strengthening of TiC are exerted, an ultra-fast cooling and low-temperature coiling process is adopted in the rolling process (TMCP), the material is ensured to obtain a ferrite structure with uniform and fine grains, the precipitation of microalloy elements such as Nb, Ti, V and the like in the cooling and coiling processes is inhibited, then the steel coil adopts a cover type annealing process, the secondary heat preservation temperature is 570-.

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

Embodiments 1 to 5 of the invention provide 700 MPa-grade hot rolled steel, a preparation method and application thereof, wherein the method comprises the following steps,

and S1, sequentially carrying out molten iron pretreatment, full-three-strip converter smelting, RH refining, LF refining and continuous casting on the molten iron to obtain a plate blank.

And S2, sequentially heating the plate blank, roughly rolling for 6 times, finely rolling for 7 times, ultra-fast cooling and coiling to obtain a hot rolled coil.

The chemical components of the hot rolled steel smelted by the method are shown in table 1, the process parameters of the smelting process are controlled to be shown in tables 2 and 3, the mechanical property of the hot rolled coil in the step S2 is detected, the mechanical property of the hot rolled steel in the step S3 is detected after the hot rolled coil is annealed, and the results are shown in table 4.

Examples 1 to 5 the 700MPa grade hot rolled steel prepared by the above method is respectively used as a crane boom, a modified car girder, an automobile girder, an ATM machine and a safe.

TABLE 1

TABLE 2

TABLE 3

TABLE 4

TABLE 5 plate measurement

Item	Flatness, mm/m	Camber, mm/10m
			Example 1	0	0
Example 2	0	0
			Example 3	0	0
Example 4	0	0
			Example 5	0	0
Comparative example 1	15.0	25

Mechanical property tests were performed on the hot rolled coils obtained in the step S2 and the hot rolled steel obtained in the step S3 of the production methods of examples 1 to 5, and the test results are shown in table 4; and (4) flattening the hot rolled steel obtained in the step S3 to obtain a flattened plate, and measuring the flatness, the C-camber and the camber of the flattened plate, wherein the detection results are shown in Table 5.

Comparative example 1 provides a method of manufacturing a hot rolled steel, which is: and (3) sequentially carrying out molten iron pretreatment, full-three-strip converter smelting, RH refining, LF refining and continuous casting on the molten iron to obtain a plate blank. And sequentially heating the plate blank, carrying out 6-pass rough rolling, 7-pass finish rolling, carrying out ultra-fast cooling and coiling to obtain the hot rolled steel. The hot rolled steel prepared in comparative example 1 was subjected to measurement of mechanical properties, flatness and camber after flattening, as shown in tables 4 and 5.

As is apparent from the results of the tests before and after annealing in examples 1 to 5 in Table 4, the yield strength and tensile strength after annealing in examples 1 to 5 were higher than those before annealing. It is thus understood that the annealing process of the present invention does not reduce the strength of the hot rolled coil, but also precipitates a large amount of (Ti, Nb, Mo, V) C during the annealing process, thereby improving the precipitation strengthening effect and the strength of the hot rolled steel.

Observing the data in table 4 for examples 1 to 5 after annealing and comparative example 1, the yield strength, tensile strength and-20 ℃ impact energy after annealing for examples 1 to 5 are comparable to the levels of comparative example 1, or higher than the strength of comparative example 1. The hot rolled steels prepared in examples 1 to 5 were measured to have flatness of 0 and camber of 0 after flattening; the hot rolled steel prepared in comparative example 1 was measured to have a flatness of 15mm/m and a camber of 25mm/10m after flattening. Compared with the comparative example 1, the flatness and the camber of the hot rolled steel prepared in the examples 1 to 5 are greatly improved after flattening, and the flatness and the camber are improved due to the reduction of the internal stress of the hot rolled steel.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention and are intended to be equivalent substitutions are included in the scope of the present invention.

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 preparation method of the 700 MPa-grade hot rolled steel is characterized by comprising the steps of sequentially carrying out molten iron pretreatment, full-three-strip converter smelting, RH refining, LF refining and continuous casting on molten iron to obtain a plate blank; the slab comprises the following chemical components in percentage by mass: 0.08-0.30% of C; 0.10 to 0.25 percent of Si; mn: 1.0-2.5%; p is less than or equal to 0.020%; s is less than or equal to 0.010 percent; 0.02-0.35% of Al; 0.010-0.10% of Nb; 0.05 to 0.20 percent of Ti; 0.01 to 0.10 percent of V; the balance of Fe and inevitable impurities; sequentially heating, rough rolling, finish rolling, ultra-fast cooling and coiling the plate blank to obtain a hot-rolled coil; sequentially performing cover annealing and cooling on the hot rolled coil to obtain hot rolled steel; the thickness of the hot rolled steel is 4.0-14.0 mm;

in the heating process, the heating temperature is 1250-1300 ℃, the tapping temperature is 1250-1300 ℃, the heating time is 3-5 hours, and the soaking time is 30-40 min;

the rough rolling is 6-pass rolling, and the deformation of the rough rolling is 76-86%;

the inlet temperature of finish rolling is 950-1060 ℃, the finish rolling termination temperature is 820-880 ℃, the deformation of finish rolling is 70-80%, the finish rolling is 7-pass rolling, and the final-pass reduction rate of finish rolling is 10-15%;

the cover annealing comprises the steps of sequentially carrying out primary heating and primary heat preservation on the hot rolled coil; carrying out secondary heating and secondary heat preservation on the hot-rolled coil subjected to primary heat preservation in sequence; the secondary temperature rise time is 8.5-11.5 h, the secondary heat preservation time is 2-4 h, and the secondary heat preservation temperature is 570-630 ℃; carrying out slow cooling and discharging on the hot rolled coil subjected to secondary heat preservation in sequence;

the cooling rate of the ultra-fast cooling is 40-60 ℃/s;

the coiling temperature is 500-550 ℃.

2. The 700MPa grade hot rolled steel prepared by the preparation method of 700MPa grade hot rolled steel according to claim 1 is applied to crane booms, modified car girders, automobile girders, ATM machines and safe cases.