CN116815068A

CN116815068A - Steel for hot forming and preparation method thereof

Info

Publication number: CN116815068A
Application number: CN202310742616.5A
Authority: CN
Inventors: 徐德超; 朱国森; 王松涛; 张博明; 韩赟; 滕华湘; 李学涛; 姜军; 张士杰; 黄�俊; 王振鹏; 罗星; 刘华赛; 徐海卫; 于孟
Original assignee: Shougang Group Co Ltd; Shougang Jingtang United Iron and Steel Co Ltd
Current assignee: Shougang Group Co Ltd; Shougang Jingtang United Iron and Steel Co Ltd
Priority date: 2023-06-21
Filing date: 2023-06-21
Publication date: 2023-09-29

Abstract

The application relates to the technical field of steel preparation, in particular to steel for hot forming and a preparation method thereof. The steel matrix of the steel comprises the following chemical components: C. mn, si, S, P, al, nb, ti, V, N, ni, cr, mo, B, O and Fe; wherein, the content of C is 0.05-0.15% by weight, the content of Mn is 0.6-2.0%, the content of Si is less than or equal to 0.20%, the content of S is less than or equal to 0.005%, the content of P is less than or equal to 0.020%, the content of Al is less than or equal to 0.08%, the content of Nb is 0.01-0.60%, the content of Ti is 0.02-0.07%, the content of V is 0.002-0.05%, the content of N is less than or equal to 0.01%, the content of Ni is less than or equal to 0.100%, the content of Cr is 0.1-0.3%, the content of Mo is less than or equal to 0.100%, the content of B is 0.0003-0.003%, and the content of O is less than or equal to 0.003%; and simultaneously satisfies: 1.2 percent or less of [ Cr ] + [ Mn ] < 2.0 percent or less, 0.04 percent or less of [ Ti ] + [ Nb ] + [ V ] < 0.15 percent or less. The application solves the technical problem that the existing steel for hot forming is difficult to simultaneously combine excellent mechanical property and excellent cold bending property.

Description

Steel for hot forming and preparation method thereof

Technical Field

The application relates to the technical field of steel preparation, in particular to steel for hot forming and a preparation method thereof.

Background

With the rapid development of the automobile industry, light weight and safety become main directions of the development of the automobile industry. The use of hot formed steel is currently the most effective means of improving the collision safety of automobiles and is also an important approach to weight reduction. The hot stamping forming technology utilizes the characteristics of increased plasticity and reduced forming resistance of the steel plate at high temperature, and after the plate material with lower initial strength is heated at high temperature, the plate material is rapidly stamped, formed, quenched and cooled in a die with a cooling system, so that the ultrahigh-strength part can be obtained, and the problems of easiness in cold forming, serious rebound and the like can be well solved. At present, the low-carbon Mn-B series steel plate has uniform martensite structure after quenching, and the tensile strength is 1300-2000 MPa. For some parts, in the collision process, the important parts of the automobile not only need to have ultrahigh strength to resist impact deformation to a large extent, but also need to have good toughness to absorb collision energy and reduce collision acceleration, so that the protection of the automobile passengers is improved. Therefore, in designing an automobile safety part, a hot-formed steel having a high tensile strength level is used as a whole, and is partially softened to improve the collision energy absorbing performance of the whole part. For example, the upper part of the b-pillar is a collision supporting unit, and the lower part is an impact absorbing unit. The current laser welding technology can weld together the steel plate for the soft zone and the ultra-high strength hot forming steel plate, and then carry out hot stamping to realize the requirements of different strength of different areas of the part.

Therefore, there is a need to develop a steel for hot forming that combines excellent mechanical properties with excellent cold bending properties to meet the above requirements.

Disclosure of Invention

The application provides steel for hot forming and a preparation method thereof, which are used for solving the technical problem that the existing steel for hot forming is difficult to simultaneously achieve excellent mechanical properties and excellent cold bending properties.

In a first aspect, the present application provides a steel for hot forming, the steel comprising the chemical composition of the steel matrix:

C. mn, si, S, P, al, nb, ti, V, N, ni, cr, mo, B, O and Fe; wherein, the content of C is 0.05-0.15% by weight, the content of Mn is 0.6-2.0%, the content of Si is less than or equal to 0.20%, the content of S is less than or equal to 0.005%, the content of P is less than or equal to 0.020%, the content of Al is less than or equal to 0.08%, the content of Nb is 0.01-0.60%, the content of Ti is 0.02-0.07%, the content of V is 0.002-0.05%, the content of N is less than or equal to 0.01%, the content of Ni is less than or equal to 0.100%, the content of Cr is 0.1-0.3%, the content of Mo is less than or equal to 0.100%, the content of B is 0.0003-0.003%, and the content of O is less than or equal to 0.003%;

and simultaneously satisfies: 1.2 percent or less of [ Cr ] + [ Mn ] < 2.0 percent or less, 0.04 percent or less of [ Ti ] + [ Nb ] + [ V ] < 0.15 percent;

wherein [ Cr ] represents the weight fraction of Cr, [ Mn ] represents the weight fraction of Mn, [ Ti ] represents the weight fraction of Ti, [ Nb ] represents the weight fraction of Nb, and [ V ] represents the weight fraction of V.

Optionally, the internal microstructure of the steel comprises: ferrite, martensite, and bainite; wherein the content of the microstructure is judged according to the chemical components.

Optionally, the determining the content of the microstructure according to the chemical composition includes:

if [ C ] + [ Mn ]/6+ [ Cr ]/5 is less than or equal to 0.35% and 0.0003% < B > < 0.001%, the volume fraction of the martensite is

Less than or equal to 30 percent, and the volume fraction of the bainite is less than or equal to 20 percent;

if [ C ] + [ Mn ]/6+ [ Cr ]/5 > 0.35% and 0.001% < [ B ]. Ltoreq.0.003%, the volume fraction of ferrite is

Less than or equal to 20 percent, and the volume fraction of the bainite is less than or equal to 30 percent;

wherein [ C ] represents the weight fraction of C, [ Mn ] represents the weight fraction of Mn, [ Cr ] represents the weight fraction of Cr, and [ B ] represents the weight fraction of B.

Optionally, the yield strength of the steel is more than or equal to 350MPa, the tensile strength of the steel is more than or equal to 550MPa, the elongation of the steel is more than or equal to 6%, and the ultimate cold bending performance of the steel is more than or equal to 90 degrees.

In a second aspect, the present application provides a method for producing a steel for hot forming, for producing the steel according to any one of the embodiments of the first aspect, the method comprising:

continuously casting the molten steel, and controlling technological parameters of continuous casting to obtain a casting blank;

heating the casting blank, rolling and coiling to obtain a hot rolled coil; wherein the temperature of the heated casting blank, the final rolling temperature of the rolling and the coiling temperature are controlled;

cold rolling the hot rolled coil, and controlling the rolling reduction of the cold rolling to obtain cold-rolled strip steel;

annealing the cold-rolled strip steel to obtain a steel plate; wherein, according to the way of the annealing, the technological parameters of the annealing are controlled;

and performing tailor welding on the steel plate, and performing hot stamping treatment and quenching treatment to obtain the steel for hot forming.

Optionally, the annealing mode is used for controlling the annealing process parameters, including:

if the annealing mode is continuous annealing, the annealing process parameters include: the annealing temperature is 720-860 ℃;

if the annealing mode is hood annealing, the annealing process parameters include: the annealing temperature is 600-720 ℃, and the heat preservation time is more than or equal to 8 hours.

Optionally, annealing the cold-rolled strip steel to obtain a steel plate; wherein, according to the annealing mode, the process parameters of the annealing are controlled, and the method comprises the following steps:

annealing the cold-rolled strip steel, and then performing hot dip galvanizing or aluminum-silicon to obtain a steel plate; wherein, according to the way of the annealing, the technological parameters of the annealing are controlled;

the thickness of the zinc coating of the hot dip galvanizing is 5-20 mu m, and the thickness of the aluminum silicon coating of the hot dip aluminum silicon plating is 8-40 mu m.

Optionally, the temperature of the heated casting blank is 1150-1350 ℃, the final rolling temperature of rolling is 850-950 ℃, and the coiling temperature is 500-700 ℃.

Optionally, the cold rolling reduction is 30% -80%.

Optionally, the technological parameters of continuous casting include: the superheat degree of molten steel in the tundish is 30-70 ℃, and the continuous casting drawing speed is 0.8-3.0m/min.

Compared with the prior art, the technical scheme provided by the embodiment of the application has the following advantages:

the steel for hot forming provided by the embodiment of the application has the following purposes by controlling chemical components: the high-strength and high-toughness parts are obtained and are applied to splice welded plates, so that the collision energy absorption effect is facilitated. For a thickness range of about 0.5-2.5mm, the part has both higher strength and better elongation at break after hot forming or quenching in the tool, with a cold bend angle of greater than or equal to 90 °. The strength and elongation of the respective portions of the steel for hot forming do not vary greatly even if the degree of local deformation or the local cooling rate thereof is not uniform during the hot stamping manufacturing. The steel plate can be used for the pressed and hardened part with higher ductility in the deformation area to fully obtain the collision energy absorption effect. The steel plate can be used as an energy-absorbing crumple zone for manufacturing automobile structural members, safety members and the like, can enable an automobile to absorb energy and impact force better when an accident happens while realizing the weight reduction of the automobile body, and protects the safety of drivers and passengers in the automobile.

Drawings

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments consistent with the application and together with the description, serve to explain the principles of the application.

In order to more clearly illustrate the embodiments of the application or the technical solutions of the prior art, the drawings which are used in the description of the embodiments or the prior art will be briefly described, and it will be obvious to a person skilled in the art that other drawings can be obtained from these drawings without inventive effort.

FIG. 1 is a metallographic microstructure of a steel for hot forming according to example 1 of the present application;

FIG. 2 is a metallographic microstructure of a steel for hot forming according to example 2 of the present application;

fig. 3 is a schematic flow chart of a method for preparing steel for hot forming according to an embodiment of the present application.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the embodiments of the present application more apparent, the technical solutions of the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application, and it is apparent that the described embodiments are some embodiments of the present application, but not all embodiments of the present application. All other embodiments, which can be made by those skilled in the art based on the embodiments of the application without making any inventive effort, are intended to be within the scope of the application.

Various embodiments of the application may exist in a range of forms; it should be understood that the description in a range format is merely for convenience and brevity and should not be construed as a rigid limitation on the scope of the application; it is therefore to be understood that the range description has specifically disclosed all possible sub-ranges and individual values within that range. For example, it should be considered that a description of a range from 1 to 6 has specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5, and 6, wherever applicable. In addition, whenever a numerical range is referred to herein, it is meant to include any reference number (fractional or integer) within the indicated range.

In the present application, unless otherwise specified, terms such as "upper" and "lower" are used specifically to refer to the orientation of the drawing in the figures. In addition, in the description of the present specification, the terms "include", "comprising" and the like mean "including but not limited to". Relational terms such as "first" and "second", and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Herein, "and/or" describing an association relationship of an association object means that there may be three relationships, for example, a and/or B, may mean: a alone, a and B together, and B alone. Wherein A, B may be singular or plural. Herein, "at least one" means one or more, and "a plurality" means two or more. "at least one", "at least one" or the like refer to any combination of these items, including any combination of single item(s) or plural items(s). For example, "at least one (individual) of a, b, or c," or "at least one (individual) of a, b, and c," may each represent: a, b, c, a-b (i.e., a and b), a-c, b-c, or a-b-c, wherein a, b, c may be single or multiple, respectively.

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

The positive effect of controlling the content of C to be 0.05-0.15 percent: c is the most effective and least expensive solid solution strengthening element, and can effectively ensure the strength grade of steel for hot stamping, while C is an austenite stabilizing element, and can most effectively stabilize austenite. If the content of C is too high, the toughness and plasticity are not easily obtained, and the impact energy absorption effect cannot be effectively met due to the too high strength; if the content of C is too low, enough strengthening effect is not formed, so that stable high dislocation martensite and bainite are obtained, and the strength is improved. Specifically, the C content may be 0.05%, 0.10%, 0.15%, etc.

The positive effect of controlling the Mn content to be 0.6-2.0 percent: mn is used for increasing an austenite region, reducing austenitizing temperature, improving hardenability, and reducing toughness due to excessive Mn content. Specifically, the Mn content may be 0.6%, 1.0%, 2.0%, etc.

The positive effect of controlling the content of Si to be less than or equal to 0.20 percent: is favorable for improving the surface quality, reducing the surface problems of chromatic aberration, poor coating quality and the like. Specifically, the content of Si may be 0.20%, 0.15%, or the like.

The positive effect of controlling the S content to be less than or equal to 0.005 percent is that: s is an unavoidable impurity, and the formation of MnS inclusions and segregation at grain boundaries deteriorate the toughness of steel, thereby lowering the toughness and plasticity of steel and increasing the hydrogen-induced delayed fracture sensitivity. Specifically, the content of S may be 0.005%, 0.004%, or the like.

The positive effect of controlling the content of P to be less than or equal to 0.020 percent is that: p is easy to form micro segregation when molten steel is solidified, and then is biased to grain boundary when heated at a temperature after austenite, so that the brittleness of steel is obviously increased, and the hydrogen-induced delayed fracture sensitivity is increased.

The positive effect of controlling the content of Al to be less than or equal to 0.08 percent: the production difficulty is reduced. Specifically, the content of Al may be 0.08%, 0.07%, or the like.

The positive effect of controlling the content of N to be less than or equal to 0.01 percent is that: n combines with Al, ti, nb, V and the like to form a compound, thereby refining grains and reducing hydrogen induced delayed fracture sensitivity, but also biasing grain boundaries to reduce grain boundary strength. Specifically, the content of N may be 0.01%, 0.008%, 0.006%, or the like.

The positive effects of controlling the Nb content to be 0.01-0.60%, the Ti content to be 0.02-0.07%, and the V content to be 0.002-0.05% are that the product of 0.04% is less than or equal to [ Ti ] + [ Nb ] + [ V ] < 0.15%): nb, ti, V and C, N are combined to form a precipitate, and the precipitate is mainly used for refining austenite grains, wherein V element with specific components is added into alloy components of the austenite grains, so that a certain amount of VC or (V, ti/Nb) C composite carbide is precipitated at a grain boundary in the hot stamping process (the total austenitizing heating temperature range is 850-950 ℃), and the prior austenite grains can be refined due to the fact that 0.005-0.05% of V is added into the material, and second phase particles are effectively pinned with the austenite grains. If the value of [ Ti ] + [ Nb ] + [ V ] is too high, coarsening of a precipitated phase may be caused, which is unfavorable for obtaining a nano-scale dispersion-distributed precipitated phase, and in addition, the precipitation strengthening effect is not obviously improved and the cost is increased; if the value of [ Ti ] + [ Nb ] + [ V ] is too low, enough precipitated phases are formed, the effects of precipitation strengthening and fine grain strengthening are obtained, and the number of H traps formed by the precipitates is also insufficient, so that the product is not beneficial to use. Specifically, the Ti content may be 0.02%, 0.05%, 0.07%, etc., the Nb content may be 0.01%, 0.10%, 0.30%, 0.60%, etc., the V content may be 0.002%, 0.01%, 0.03%, 0.05%, etc., and the value of [ Ti ] + [ Nb ] + [ V ] may be 0.04%, 0.10%, 0.15%, etc.

The positive effect of controlling the Ni content to be less than or equal to 0.100 percent is that: the hardenability is effectively improved and the low-temperature toughness is improved, but the Ni content is too high, so that the cost is increased. Specifically, the Ni content may be 0.100%, 0.090%, 0.080%, or the like.

The positive effect of controlling the Cr content to be 0.1-0.3 percent: cr can obviously increase hardenability and alleviate severe oxidation on the high-temperature surface, is favorable for the use of non-plating products, but promotes the formation of bainite, and is not suitable for being too high. Specifically, the content of Cr may be 0.1%, 0.2%, 0.3%, or the like.

Therefore, the limit is 1.2 percent or less and the limit is 2.0 percent or less of [ Cr ] + [ Mn ], the requirement of hardenability is met, and meanwhile, the plasticity is not reduced due to the excessively high condition.

The positive effect of controlling the content of Mo to be less than or equal to 0.100 percent is that: reduces the alloy cost and obtains better strength. Specifically, the Mo content may be 0.100%, 0.090%, 0.080%, or the like.

The positive effect of controlling the content of B to be 0.0003-0.003 percent: a small amount of B ensures a sufficiently good hardenability. Specifically, the content of B may be 0.0003%, 0.001%, 0.002%, 0.003%, etc.

The positive effect of controlling the content of O to be less than or equal to 0.003 percent is that: o is a harmful gas and affects the hydrogen induced delayed fracture sensitivity, and may form coarse alumina inclusions with aluminum, deteriorating toughness of steel. Specifically, the content of O may be 0.003%, 0.002%, 0.0025%, etc.

In some embodiments, the internal microstructure of the steel comprises: ferrite, martensite and bainite, see fig. 1 and 2; wherein the content of the microstructure is judged according to the chemical components.

In some embodiments, the determining the content of the microstructure according to the chemical composition includes:

In the embodiment of the application, the dispersed fine precipitated phase can be used as an H trap to capture H atoms, so that the toughness is improved.

Less than or equal to 30 percent, the volume fraction of the bainite is less than or equal to 20 percent, and the positive effects are that: ensuring sufficient plasticity. Specifically, the volume fraction of the martensite may be 30%, 28%, 26%, etc., and the volume fraction of the bainite may be 20%, 18%, 16%, etc.

Less than or equal to 20 percent, the volume fraction of the bainite is less than or equal to 30 percent, and the positive effects are that: the best match of strength and toughness is obtained by adjusting the martensite fraction. Specifically, the volume fraction of the bainite may be 30%, 28%, 26%, etc., and the volume fraction of the ferrite may be 20%, 18%, 16%, etc.

In some embodiments, the yield strength of the steel is greater than or equal to 350MPa, the tensile strength of the steel is greater than or equal to 550MPa, the elongation of the steel is greater than or equal to 6%, and the cold bending performance of the steel is greater than or equal to 90 °.

The steel for hot forming provided by the embodiment of the application can simultaneously realize excellent mechanical properties and excellent cold bending property.

In a second aspect, the present application provides a method for preparing steel for hot forming, please refer to fig. 3, for preparing steel according to any one of the embodiments of the first aspect, the method comprising:

s1, continuously casting molten steel, and controlling technological parameters of continuous casting to obtain a casting blank;

s2, heating the casting blank, rolling and coiling to obtain a hot rolled coil; wherein the temperature of the heated casting blank, the final rolling temperature of the rolling and the coiling temperature are controlled;

s3, cold rolling the hot rolled coil, and controlling the rolling reduction of the cold rolling to obtain cold-rolled strip steel;

s4, annealing the cold-rolled strip steel to obtain a steel plate; wherein, according to the way of the annealing, the technological parameters of the annealing are controlled;

and S5, performing tailor welding on the steel plate, and performing hot stamping treatment and quenching treatment to obtain the steel for hot forming.

In step S5, it includes: and (3) finishing and straightening the steel plate, and blanking, welding, heating and quenching. The blanking material sheet is connected with other grades of hot stamping steel by adopting a laser welding mode, then is integrally put into a heating furnace with the temperature of 860-980 ℃, the atmosphere in the furnace is air, nitrogen or mixed gas of nitrogen and methane, and then is preserved for 3-15 min in the furnace to obtain a complete austenitic structure; transferring the steel plate to a hot stamping forming die in air, wherein the transfer process is less than 15s, and the temperature after transfer is 600-850 ℃; the steel plate is formed by hot stamping in a die with a cooling system, is in a pressure maintaining state, has a pressure of 300-1500 tons, and is quenched to a target temperature at an average cooling speed of more than or equal to 20 ℃/s. Wherein, the proper improvement of the mold opening temperature and the coating baking tempering process is favorable for greatly improving and optimizing the problem of hydrogen to delayed fracture, and the specific technological parameters can be seen in Table 3.

In some embodiments, the controlling the process parameters of the annealing according to the annealing mode includes:

if the annealing mode is hood annealing, the annealing process parameters include: the annealing temperature is 600 ℃ to ultra-high

The heat preservation time is more than or equal to 8 hours at 720 ℃.

The positive effect of controlling the continuous annealing temperature to be 720-860℃: and the recrystallization is finished, so that uncoiling blanking and tissue uniformity control are facilitated.

Specifically, the continuous annealing temperature is 720 ℃, 760 ℃, 820 ℃, 860 ℃, etc.

The cover type annealing temperature is controlled to be 600-720 ℃, and the heat preservation time is more than or equal to 8 hours, which has the positive effects that: too low is unfavorable for recrystallization, and too high causes coarse structure. Specifically, the temperature of the hood-type annealing is 600 ℃, 650 ℃, 700 ℃, 720 ℃ and the like, and the heat preservation time is 8 hours, 9 hours, 10 hours and the like.

In some embodiments, the annealing the cold-rolled strip steel results in a steel sheet; wherein, according to the annealing mode, the process parameters of the annealing are controlled, and the method comprises the following steps:

The positive effect of controlling the zinc coating thickness of the hot dip galvanization to be 5-20 mu m or the aluminum silicon coating thickness of the hot dip aluminum silicon to be 8-40 mu m is that: ensuring sufficient corrosion resistance and avoiding LME problems caused by excessively thick plating. Specifically, the zinc plating layer thickness may be 5 μm, 10 μm, 15 μm, 20 μm, etc., and the aluminum silicon plating layer thickness may be 8 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, etc.

In some embodiments, the temperature of the heated billet is 1150 ℃ to 1350 ℃, the final rolling temperature of the rolling is 850 ℃ to 950 ℃, and the temperature of the coiling is 500 ℃ to 700 ℃.

The positive effects of controlling the above-mentioned technological parameters of heating, rolling and coiling are: the quality of the steel plate is ensured. Specifically, the temperature of the heated cast slab may be 1150 ℃, 1200 ℃, 1250 ℃, 1300 ℃, 1350 ℃, etc.; the final rolling temperature of the rolling may be 850 ℃, 900 ℃, 950 ℃, etc., and the coiling temperature may be 500 ℃, 600 ℃, 700 ℃, etc.

In some embodiments, the cold rolling reduction is 30% to 80%.

The positive effect of controlling the cold rolling reduction to be 30% -80%: a sufficient deformation is obtained and the structure is refined. Specifically, the cold rolling reduction may be 30%, 50%, 70%, 80%, or the like.

In some embodiments, the process parameters of continuous casting include: the superheat degree of molten steel in the tundish is 30-70 ℃, and the continuous casting drawing speed is 0.8-3.0m/min.

The positive effect of controlling the superheat degree of molten steel in the tundish to be 30-70℃ is that: the weight of the casting blank is ensured. Specifically, the degree of superheat of the molten steel may be 30 ℃, 40 ℃, 50 ℃, 60 ℃, 70 ℃, or the like.

The positive effect of controlling the continuous casting drawing speed to be 0.8-3.0m/min is that: improving the banding tissue and segregation problems. Specifically, the continuous casting drawing speed may be 0.8m/min, 1.0m/min, 2.0m/min, 3.0m/min, etc.

The preparation method of the steel for hot forming is realized based on the steel for hot forming, and the chemical components of the steel for hot forming can refer to the embodiment, and because the preparation method of the steel for hot forming adopts part or all of the technical schemes of the embodiment, the steel for hot forming has at least all the beneficial effects brought by the technical schemes of the embodiment, and the description is omitted herein.

The application will be further illustrated with reference to specific examples. It is to be understood that these examples are illustrative of the present application and are not intended to limit the scope of the present application. The experimental procedures, which are not specified in the following examples, are generally determined according to national standards. If the corresponding national standard does not exist, the method is carried out according to the general international standard, the conventional condition or the condition recommended by the manufacturer.

One steel for hot forming according to the present application will be described in detail with reference to examples, comparative examples and experimental data.

The chemical compositions shown in table 1 are adopted in the examples 1-7 and the comparative examples 1-3 respectively, and the alloy composition ratios designed in table 1 are adopted for smelting and casting to obtain casting blanks; table 1 weight fractions of chemical components for each example and comparative example.

TABLE 1 chemical composition (wt%) of steel for hot forming, balance Fe and unavoidable impurities

The casting blank is subjected to hot rolling, cold rolling, annealing, conventional finishing and withdrawal straightening, and specific process parameters are shown in table 2.

Table 2 preparation process parameters of steel for hot forming

And heating the blanking and splice welded material sheets, quenching, and air-cooling the product component obtained by the hot stamping quenching to room temperature. The holding temperature, time and quenching end temperature (mold opening temperature) time at the time of heating are shown in table 3.

Table 3 hot stamping treatment and quenching treatment process parameters of steel for hot forming

Sequence number	Thermal insulationTemperature, DEG C	Holding time, min	Mold opening temperature, DEG C
				Example 1	920	5	130
Example 2	930	4	130
				Example 3	910	4	140
Example 4	910	5	160
				Example 5	900	5	145
Example 6	910	5	160
				Example 7	910	5	160
Comparative example 1	930	5	140
				Comparative example 2	940	5	140

The properties of the steel for hot forming obtained in the final group are shown in Table 4.

TABLE 4 mechanical Properties of Steel for thermoforming and volume fraction of each Structure

From tables 1 to 4, it is understood that the hot forming steel prepared by the method of the present application can achieve both excellent mechanical properties and excellent cold bending properties. In contrast, in comparative example 1, the steel sheet for hot stamping obtained was less than 350MPa in yield strength, without Nb, V, ni and Mo elements, and with Cr content much lower than 2%. In comparative example 2, the value of ti+nb+v was less than 0.05%, and the steel sheet for hot stamping obtained was high in strength and poor in cold bending property.

The foregoing is only a specific embodiment of the application to enable those skilled in the art to understand or practice the application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the application. Thus, the present application is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A steel for hot forming, characterized in that the steel matrix of the steel comprises the chemical components:

2. The steel of claim 1, wherein the internal microstructure of the steel comprises: ferrite, martensite, and bainite; wherein the content of the microstructure is judged according to the chemical components.

3. The steel according to claim 1, wherein said determining the content of said microstructure according to said chemical composition comprises:

if [ C ] + [ Mn ]/6+ [ Cr ]/5 is less than or equal to 0.35% and 0.0003% < B is less than or equal to 0.001%, the volume fraction of the martensite is less than or equal to 30%, and the volume fraction of the bainite is less than or equal to 20%;

if [ C ] + [ Mn ]/6+ [ Cr ]/5 > 0.35% and 0.001% < [ B ] is less than or equal to 0.003%, the volume fraction of ferrite is less than or equal to 20%, and the volume fraction of bainite is less than or equal to 30%;

4. The steel according to claim 1, characterized in that the yield strength of the steel is not less than 350MPa, the tensile strength of the steel is not less than 550MPa, the elongation of the steel is not less than 6%, and the cold bending performance of the steel is not less than 90 °.

5. A method for producing a steel for hot forming, characterized by being used for producing the steel according to any one of claims 1 to 4, the method comprising:

6. The method of claim 5, wherein said controlling process parameters of said annealing in accordance with the manner of said annealing comprises:

7. The method according to claim 5 or 6, wherein the annealing of the cold-rolled strip steel results in a steel sheet; wherein, according to the annealing mode, the process parameters of the annealing are controlled, and the method comprises the following steps:

8. The method according to claim 5, wherein the temperature of the heated slab is 1150 ℃ to 1350 ℃, the finishing temperature of the rolling is 850 ℃ to 950 ℃, and the temperature of the coiling is 500 ℃ to 700 ℃.

9. The method of claim 5, wherein the cold rolling reduction is 30% to 80%.

10. The method according to claim 5, wherein the process parameters of the continuous casting include:

the superheat degree of molten steel in the tundish is 30-70 ℃, and the continuous casting drawing speed is 0.8-3.0m/min.