CN115216688A - 800 MPa-grade hot-rolled low-alloy high-strength steel and steel matrix and preparation method thereof - Google Patents

Abstract

The invention particularly relates to 800 MPa-grade hot-rolled low-alloy high-strength steel, a steel matrix thereof and a preparation method, belonging to the technical field of steel preparation, wherein the steel matrix comprises the following chemical components in percentage by mass: 0.10 to 0.15 percent of C, 0.1 to 0.3 percent of Si, 1.7 to 2.3 percent of Mn, 0.03 to 0.06 percent of Al, 0.05 to 0.1 percent of Cr0, 0 to 0.05 percent of Mo, 0 to 0.05 percent of V, 0.1 to 0.14 percent of Ti, 0 to 0.008 percent of P, 0 to 0.001 percent of S, 0 to 0.005 percent of N and the balance of Fe; by adopting V, ti microalloying, the yield strength reaches 800MPa or above, the hole expansion rate is more than or equal to 70 percent, the minimum bending core diameter R of a 90-degree V-shaped bending test is less than or equal to 0.5T (T is the thickness of high-strength steel), the ultrahigh strength and excellent local forming performance are achieved, and the requirements of lightweight in the automobile industry and complex forming of automobile body parts are met.

Description

800 MPa-grade hot-rolled low-alloy high-strength steel and steel matrix and preparation method thereof

Technical Field

The invention belongs to the technical field of steel preparation, and particularly relates to 800 MPa-grade hot-rolled low-alloy high-strength steel, a steel matrix thereof and a preparation method.

Background

The hot-rolled low-alloy high-strength steel has good local forming performances such as bending, reaming and the like, and is a preferred material for manufacturing automobile chassis parts such as various beams, fender supporting plates and the like.

Currently, the highest strength grade hot rolled low alloy high strength steel in commercial use is 700MPa. With the gradual improvement of the requirements of the automobile industry on light weight, energy conservation and emission reduction, the development of automobile steel with higher strength level and higher local forming capability is an inevitable trend of light weight development.

In addition, the current hot-rolled low-alloy high-strength steel is mainly supplied in the states of a hot-rolled bare plate and a hot-rolled pickled plate, and a layer of primer is coated on the surface of the steel plate through the processes of pickled plate and electrophoresis or pickled plate, hot-dip galvanizing and electrophoresis to achieve the purpose of corrosion prevention. However, due to the harsh environment in which the chassis operates, a thicker zinc layer or paint layer is often required, which presents challenges to both stamping and welding. More importantly, with the diversification of the service environment of automobiles, the electrophoretic paint film is often peeled and scratched due to stone impact damage, so that the steel plate matrix is exposed and rusted. It can be seen that the conventional hot-rolled or acid-washed low-alloy high-strength steel cannot meet the requirement of high corrosion resistance of chassis parts. Therefore, there is a need for a low-alloy high-strength steel having a higher strength level, better local formability, and better corrosion resistance.

Disclosure of Invention

The application aims to provide 800 MPa-grade hot-rolled low-alloy high-strength steel, a steel matrix thereof and a preparation method thereof, so as to solve the problem that the yield strength of the conventional high-strength steel is insufficient.

The embodiment of the invention provides a steel matrix of 800 MPa-grade hot-rolled low-alloy high-strength steel, which comprises the following chemical components in percentage by mass: c:0.10% -0.15%, si: 0.1-0.3%, mn:1.7% -2.3%, al:0.03% -0.06%, cr: 0.05-0.1%, mo:0 to 0.05%, V:0 to 0.05%, ti:0.1% -0.14%, P:0 to 0.008 percent, S:0 to 0.001%, N:0 to 0.005 percent, and the balance of Fe and inevitable impurities.

Optionally, the metallographic structure of the steel matrix includes, in terms of area ratio: 80 to 95 percent of ferrite and 5 to 20 percent of martensite/austenite island, wherein the equivalent grain diameter of the ferrite is less than 7 mu m, and the equivalent grain diameter of the martensite/austenite island is less than 3 mu m.

Optionally, the metallographic structure of the steel matrix further includes, in terms of area ratio: 1.4 to 2.0 percent of carbide distributed at the ferrite matrix and the grain boundary, and the average equivalent diameter of the carbide is between 10 and 80nm.

Based on the same inventive concept, the embodiment of the invention also provides a preparation method of the steel matrix of the 800 MPa-grade hot-rolled low-alloy high-strength steel, which comprises the following steps:

smelting molten iron, and then carrying out continuous casting to obtain a plate blank;

and carrying out hot rolling on the plate blank to obtain a steel matrix.

Optionally, the final rolling temperature of the hot rolling is 880-920 ℃, the cooling of the hot rolling adopts an air cooling and laminar flow water cooling mode, the final temperature of the air cooling is more than or equal to 800 ℃, the cooling speed of the laminar flow water cooling is 10-20 ℃/s, and the coiling temperature of the hot rolling is 400-500 ℃.

Based on the same inventive concept, the embodiment of the invention also provides 800 MPa-grade hot-rolled low-alloy high-strength steel, which comprises a steel matrix and a plating layer attached to the steel matrix, wherein the steel matrix is the steel matrix.

Optionally, the plating layer is a zinc-aluminum-magnesium alloy plating layer, the thickness of the zinc-aluminum-magnesium alloy plating layer is 8 μm to 20 μm, and the chemical components of the zinc-aluminum-magnesium alloy plating layer include, by mass: al:5% -7%, mg:2 to 4 percent, and the balance of Zn and inevitable impurities.

Based on the same inventive concept, the embodiment of the invention also provides a preparation method of the 800 MPa-grade hot-rolled low-alloy high-strength steel, which comprises the following steps:

leveling a steel substrate, and then pickling to obtain a pickled plate;

and carrying out continuous hot galvanizing on the pickled plate to obtain the high-strength steel.

Optionally, the flattening rolling force is 2000 kN-2500 kN.

Optionally, the pickled plate is subjected to continuous hot galvanizing of aluminum and magnesium to obtain high-strength steel, and the method specifically comprises the following steps:

preheating the pickling plate to obtain a preheating plate;

heating the preheating plate to obtain a heating plate;

cooling the heating plate to obtain a cooling plate;

putting the cooling plate into a pot for galvanizing aluminum magnesium to obtain plated steel;

carrying out air knife blowing on the plated steel, and then carrying out gas jet cooling to obtain high-strength steel;

wherein the preheating heating rate is 2-5 ℃/s, and the temperature of the preheating plate is 210-230 ℃; the heating rate is 10-25 ℃/s, the heating soaking and heat preservation temperature is 640-720 ℃, and the heating soaking and heat preservation time is 40-180 s; the cooling rate is 5-20 ℃/s, and the temperature of the cooling plate is 450-470 ℃.

One or more technical solutions in the embodiments of the present invention have at least the following technical effects or advantages:

the steel matrix of the 800 MPa-level hot-rolled low-alloy high-strength steel provided by the embodiment of the invention adopts V, ti micro-alloying, the yield strength reaches 800MPa or above, the hole expansion rate is more than or equal to 70%, the minimum bending core diameter R of a 90-degree V-shaped bending test is less than or equal to 0.5T (T is the thickness of the high-strength steel), the ultrahigh strength and excellent local forming performance are achieved, and the requirements of lightweight automobile industry and complex forming of automobile body parts are met.

The above description is only an overview of the technical solutions of the present invention, and the present invention can be implemented in accordance with the content of the description so as to make the technical means of the present invention more clearly understood, and the above and other objects, features, and advantages of the present invention will be more clearly understood.

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 are briefly introduced below, and it is obvious that the drawings in the following description are some embodiments of the present invention, and it is obvious for those skilled in the art to obtain other drawings based on the drawings without creative efforts.

FIG. 1 is a flow chart of a method provided by an embodiment of the present invention;

fig. 2 is a microstructure diagram of a high-strength steel provided by an example 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 general idea of the embodiment of the application is as follows:

according to an exemplary embodiment of the present invention, there is provided a steel matrix of hot-rolled low-alloy high-strength steel of 800MPa grade, the chemical composition of the steel matrix comprises, in mass fraction: c:0.10% -0.15%, si: 0.1-0.3%, mn:1.7% -2.3%, al:0.03% -0.06%, cr: 0.05-0.1%, mo:0 to 0.05%, V:0 to 0.05%, ti:0.1% -0.14%, P:0 to 0.008%, S:0 to 0.001%, N:0 to 0.005 percent, and the balance of Fe and inevitable impurities.

The control principle of the chemical components of the invention is as follows:

c is the main strengthening element in steel. In the low-alloy high-strength steel, C improves the hardness and the strength of a matrix structure through interstitial solid solution strengthening, and forms carbide with micro-alloy elements such as V, ti and the like to precipitate on a ferrite matrix to play a role in precipitation strengthening. However, the C content should not be too high, otherwise the weldability is impaired. In order to ensure the yield strength of the high-strength steel of the invention above 800MPa, the content of C is controlled to be 0.10-0.15%.

Si is a ferrite forming element, has a strong solid solution strengthening effect, can improve the hardness and strength of ferrite, reduces the hardness difference between the ferrite and the martensite/austenite island, and is beneficial to improving the local forming performance. However, the high content of Si can cause the surface of the hot rolled coil to have iron scales, and the iron scales are difficult to clean in the acid cleaning stage, which easily causes the defects of plating leakage, dezincification and zinc flow lines. The invention comprehensively considers the function of Si and controls the content of Si to be 0.1-0.3%.

Mn is a solid solution strengthening element, and in order to ensure that the low-alloy high-strength steel obtains yield strength and tensile strength of more than 800MPa, the Mn content is not less than 1.7 percent. However, if the Mn content is too high, hardenability is improved, a hard phase structure such as bainite and martensite is likely to be excessively formed, segregation is likely to be formed, the structure is not uniform, and formability is impaired, so that the Mn content in the steel is set to not more than 2.3%.

In the present invention, al is mainly present as a deoxidizing element, and therefore the Al content is set to 0.03% to 0.06%.

Cr is an element for improving hardenability and a strengthening element, and is beneficial to improving the strength of the low-alloy high-strength steel. However, during the annealing galvanization, cr tends to be externally oxidized on the steel sheet surface, and surface skip plating is easily caused. Therefore, the Cr content is set to 0.05% to 0.1%.

Like Cr, mo is an element that improves hardenability and is a strong strengthening element. However, mo is an expensive alloying element, and excessive addition increases the alloy cost, so the upper limit of the Mo content is set to 0.05% in the present invention.

V is a stronger carbonitride forming element. In the continuous hot galvanizing aluminum magnesium plating process of the high-strength steel, soaking annealing is carried out for 40-180 s at 640-720 ℃, carbide of V is precipitated in the temperature range, good precipitation strengthening effect can be obtained, the yield strength and yield ratio of the low-alloy high-strength steel are further improved, and the improvement on local forming performances such as hole expansion and bending is facilitated. In addition, unlike Nb, V does not significantly increase the recrystallization temperature, can avoid or reduce the formation of a fibrous structure along the rolling direction during hot rolling, and is beneficial to reducing the difference in the transverse and longitudinal structures and mechanical properties, thereby achieving higher uniformity of the structure.

Ti is similar to V and is a stronger carbonitride forming element, and in the hot rolling process, the carbonitride of Ti has an obvious grain refining effect, so that the strength of the low-alloy high-strength steel can be improved, and the local forming performance can be improved. In addition, in the annealing soaking process, ti carbide can be further precipitated, the hardness and the strength of a ferrite structure are improved, and the yield strength of more than 800MPa can be obtained. However, since the precipitation effect is saturated and the cost is increased when the Ti content is too high, the Ti content is limited to 0.1 to 0.14% in the present invention.

P as interstitial solid solution atoms can suitably improve the strength of the steel sheet, but tends to segregate at grain boundaries to deteriorate plasticity and formability, so the upper line of the P content is set to 0.008%.

S is easily combined with Mn to form coarse MnS inclusions, which deteriorate formability of the steel sheet such as hole expansion and flanging, so that the upper limit of the S content is set to 0.001%.

N is inevitably present in steel and is generally a harmful impurity element, the binding force of N to Ti is stronger than that of C, and the content of N is controlled to be 0.005% or less because Ti is excessively consumed due to the high content of N, resulting in a decrease in the amount of carbide formation of Ti.

In some embodiments, the metallographic structure of a steel substrate comprises, in area fraction: 80 to 95 percent of ferrite, 5 to 20 percent of martensite/austenite island, and 1.4 to 2.0 percent of carbide distributed at the ferrite matrix and the crystal boundary, wherein the equivalent grain diameter of the ferrite is less than 7 mu m, the equivalent grain diameter of the martensite/austenite island is less than 3 mu m, and the average equivalent diameter of the carbide is between 10nm and 80nm.

Ferrite is a matrix structure, has relatively low hardness and strength, is easy to deform, and is an important composition phase for ensuring the plasticity and the formability of the low-alloy high-strength steel. The martensite-austenite-martensite structure is a structure obtained by incomplete transformation from austenite to martensite, and has higher strength and hardness than ferrite and poorer plasticity and toughness than ferrite. When the ferrite proportion is less than 80% and the martensite/austenite proportion exceeds 20%, the formability of the steel sheet is deteriorated; when the ferrite proportion exceeds 95%, it is difficult to secure a yield strength of 800MPa or more.

The grain refinement can not only improve the strength of the steel plate, but also improve the plasticity and the formability. In addition, the smaller the crystal grain size, the more the crystal grain boundary, the stronger the inhibition effect on the crack propagation in the deformation process, and the better the local forming performance such as hole expansion, bending and the like of the steel plate, so the invention limits the equivalent crystal grain diameter of the ferrite to less than 7 μm, and the equivalent crystal grain diameter of the martensite/austenite island to less than 3 μm.

The carbides include both cementite and microalloy V, ti. The cementite is formed in the annealing soaking process and is decomposed from part of the martensite-austenite island; v, ti carbide is formed during hot rolling coiling and annealing soaking. The carbides are precipitated at a ferrite matrix and a grain boundary, so that the strength of ferrite is remarkably improved, the hardness difference between the ferrite and the martensite/austenite island is reduced, and the formation and the expansion of cracks in the deformation process are inhibited, thereby improving the hole expanding and bending performance. When the total amount of carbides is less than 1.4%, the effect of strengthening the yield strength is insufficient; since V, ti has a limited amount of carbide precipitation, when the total amount of carbide is more than 2.0%, it is proved that the martensite/austenite islands are excessively decomposed, which results in insufficient tensile strength. Therefore, the carbide area fraction is limited to 1.4% to 2.0%.

Carbides increase yield strength through interaction with dislocations. When the equivalent diameter of the carbide is less than 10nm, the interaction between the carbide and dislocation is generally a cut-through mechanism, and in this case, the smaller the equivalent diameter, the poorer the strengthening effect is; when the equivalent diameter of the carbide reaches 80nm, most of the action mechanisms with dislocation are bypass mechanisms, and the larger the equivalent diameter is, the poorer the strengthening effect is. Therefore, in order to obtain a good strengthening effect, the average equivalent diameter of the carbide is preferably limited to 10nm to 80nm.

According to another exemplary embodiment of the present invention, there is provided a method of manufacturing a steel substrate of a hot-rolled low-alloy high-strength steel of 800MPa class as described above, the method including:

s1, smelting molten iron, and then carrying out continuous casting to obtain a plate blank;

and S2, carrying out hot rolling on the plate blank to obtain a steel matrix.

In some embodiments, the final rolling temperature of the hot rolling is 880-920 ℃, the cooling of the hot rolling adopts an air cooling and laminar flow water cooling mode, the final temperature of the air cooling is more than or equal to 800 ℃, the cooling speed of the laminar flow water cooling is 10-20 ℃/s, and the coiling temperature of the hot rolling is 400-500 ℃.

In order to avoid the formation of a fibrous structure along the rolling direction and the deterioration of the forming performance of the low-alloy high-strength steel caused by the fact that hot rolling enters a non-recrystallization area, the final rolling temperature is not lower than 880 ℃; when the finishing rolling temperature is higher than 920 ℃, austenite grains are relatively coarse, and the sizes of ferrite grains and martensite/austenite grains formed in the cooling and coiling processes are increased, so that the strong plasticity and the hole expansion performance of the steel plate are not facilitated.

The air cooling speed is relatively low, and ferrite phase change and microalloy carbide precipitation are easy to occur in the air cooling process. When the air cooling end point temperature is lower than 800 ℃, microalloy carbides are gradually precipitated, and the carbides precipitated prematurely are coarsened greatly in the subsequent coiling and soaking annealing processes so as to weaken the precipitation strengthening effect. Therefore, in order to allow more microalloying elements to remain until precipitation during annealing soaking, it is necessary to suppress precipitation during hot rolling cooling. For the above purpose, the present invention sets the hot rolling air cooling temperature not lower than 800 ℃.

The purpose of water cooling is to make the hot rolled steel plate enter the bainite transformation zone quickly and further inhibit the precipitation of microalloy carbide during cooling. When the water cooling speed is less than 10 ℃/s, ferrite structure is excessively generated, and the final tensile strength is insufficient; when the water cooling speed is more than 20 ℃/s, the coiling temperature is not easy to be accurately controlled.

The coiling temperature affects the texture and surface quality of the steel sheet. When the coiling temperature is lower than 400 ℃, the steel plate enters a martensite phase transformation area, and martensite belongs to a hard and brittle phase and is unfavorable for the local forming performance of the steel plate; when the coiling temperature is higher than 500 ℃, excessive V, ti carbide is precipitated and grown, and the precipitation strengthening effect in the annealing soaking stage is weakened. In addition, too high coiling temperature easily causes the surface of the hot rolled plate to generate iron scale, and influences the surface quality of subsequent galvanizing.

According to another exemplary embodiment of the present invention, there is provided a hot-rolled low-alloy high-strength steel of the 800MPa class, the high-strength steel comprising a steel substrate and a plating layer attached to the steel substrate, the steel substrate being as described above.

In some embodiments, the coating is a zinc-aluminum-magnesium alloy coating, the thickness of the zinc-aluminum-magnesium alloy coating is 8 μm to 20 μm, and the chemical composition of the zinc-aluminum-magnesium alloy coating comprises the following components in percentage by mass: al:5% -7%, mg:2 to 4 percent, and the balance of Zn and inevitable impurities.

According to another exemplary embodiment of the present invention, there is provided a method for preparing the hot-rolled low-alloy high-strength steel of 800MPa class as described above, the method including:

s1, smelting molten iron, and then carrying out continuous casting to obtain a plate blank;

s2, carrying out hot rolling on the plate blank to obtain a steel matrix;

s3, flattening the steel substrate, and then carrying out acid pickling to obtain a pickled plate;

in some embodiments, the temper rolling force for temper rolling is 2000kN to 2500kN.

When the temper rolling force is lower than 2000kN, the strip steel has poor shape, and the deviation of the strip steel in the annealing galvanizing furnace can be caused by the existing wave-shaped defects; when the temper rolling pressure is higher than 2500kN, the surface of the strip steel has transverse roll mark defects and is inherited to galvanized finished products, so that the appearance quality of the products is poor.

S4, continuously hot-dip galvanizing aluminum magnesium on the pickled plate to obtain high-strength steel;

in some embodiments, the continuous hot-dip galvanization aluminum magnesium process is: preheating the strip steel to 210-230 ℃ at the speed of 2-5 ℃/s, then heating to 640-720 ℃ at the speed of 10-25 ℃/s, soaking and preserving heat for 40-180 s, then cooling to 450-470 ℃ at the speed of 5-20 ℃/s, plating zinc aluminum magnesium, discharging the strip steel out of a zinc pot, blowing by an air knife to cool to 420 ℃, and finally spraying and cooling by nitrogen/hydrogen mixed gas to room temperature.

Controlling the soaking temperature to be 640-720 ℃; when the soaking temperature is lower than 640 ℃, the carbide precipitation driving force of the microalloy V, ti is insufficient, the good precipitation strengthening effect cannot be realized, and the chemical action between the plating solution and the surface of the steel plate is weak at the temperature, so that the surface defects such as plating leakage, zinc flow marks and the like are easy to appear. When the soaking temperature is higher than 720 ℃, the growth and coarsening of carbides are obvious, the precipitation strengthening effect is obviously weakened, and the structure is recovered and softened, so that the yield strength and the tensile strength of the low-alloy high-strength steel cannot reach 800MPa or more.

The soaking time is controlled to be 40-180 s, and when the soaking time is less than 40s, the carbide precipitation quantity of the microalloy V, ti is small, the size is small, and the precipitation strengthening effect is limited; when the soaking time exceeds 180s, V, ti carbide grows and coarsens, and the martensite/austenite island is excessively decomposed, which is not beneficial to improving the yield strength and the tensile strength.

In some embodiments, the method further comprises the step of: and S5, performing surface finishing on the high-strength steel.

The finishing elongation is controlled to be 0.4-0.8%, the low-alloy high-strength steel is characterized in that a yield platform often exists in a tensile curve, a Luders strip appears on the surface of a steel plate during stamping forming, and the improvement of the finishing elongation is a feasible method for eliminating the yield platform. In addition, the coating uniformity can be improved by improving the finishing elongation. For the steel plate, the finishing elongation is lower than 0.4 percent, and the yield platform cannot be completely eliminated; when the finishing elongation is more than 0.8%, the steel plate is seriously hardened in a processing way, the yield strength is too high, a finishing roll mark is formed on the surface, and the appearance quality is influenced.

The hot-rolled low-alloy high-strength steel of 800MPa grade of the present application, and the steel substrate and the manufacturing method thereof will be described in detail below with reference to examples, comparative examples and experimental data.

Examples and comparative examples

(1) Molten steels of examples 1 to 8 and comparative examples 1 to 4 were prepared and cast into slabs having chemical compositions as shown in table 1.

TABLE 1 mass percents of chemical components of examples and comparative examples

(2) And carrying out hot rolling on the plate blank to obtain a hot rolled coil with the thickness of 2.0-4.0 mm. The hot rolling finishing temperature is 880-920 ℃, the hot rolling cooling adopts an air cooling and laminar flow water cooling mode, the air cooling finishing temperature is more than or equal to 800 ℃, the laminar flow water cooling speed is 10-20 ℃/s, and the coiling temperature is 400-500 ℃.

(3) And uncoiling the hot-rolled plate coil, flattening, carrying out acid pickling, and removing hot-rolled iron scales to obtain the pickled plate. The flattening rolling force of the flattening procedure is 2000 kN-2500 kN.

Specific process parameters of each example and comparative example are shown in table 2.

TABLE 2 Hot Rolling and temper rolling Process parameters for the examples and comparative examples

(4) The pickled plate is annealed and galvanized with aluminum magnesium on a continuous hot-dip galvanized aluminum magnesium production line, and the specific process comprises the following steps: preheating the strip steel to 210-230 ℃ at the speed of 2-5 ℃/s, then heating to 640-720 ℃ at the speed of 10-25 ℃/s, soaking and preserving heat for 40-180 s, then cooling to 450-470 ℃ at the speed of 5-20 ℃/s, plating zinc aluminum magnesium, discharging the strip steel out of a zinc pot, blowing by an air knife to cool to 420 ℃, and finally spraying and cooling by nitrogen/hydrogen mixed gas to room temperature.

(5) And (3) performing surface finishing on the galvanized aluminum-magnesium steel plate, wherein the finishing elongation controlled by the finishing machine is 0.4-0.8%.

The parameters of the continuous hot dip galvanizing Al-Mg and the surface finishing process for each example and comparative example are shown in Table 3.

TABLE 3 continuous hot-dip galvanization of Al-Mg alloy for examples and comparative examples and surface finishing process parameters

Examples of the experiments

The microstructure of the low-alloy high-strength steel is analyzed by a Zeiss Ultra-55 scanning electron microscope and a transmission electron microscope, the yield strength, the tensile strength and the elongation after fracture of the low-alloy high-strength steel are detected by a ZWICK/Roell Z100 tensile testing machine, the hole expanding rate is detected by a ZWICK BUP1000 forming testing machine, and the minimum relative bending core diameter is determined by a microcomputer control electro-hydraulic servo bending testing machine.

The microstructure and mechanical properties of each of the examples and comparative examples are shown in Table 4.

TABLE 4 microstructure and mechanical Properties of the examples and comparative examples

From the above table, the yield strength of the high-strength steel prepared by the method provided by the embodiment of the application is more than or equal to 800MPa, the tensile strength is more than or equal to 850MPa, the hole expansion ratio is more than or equal to 70%, and the minimum bending core diameter R of the 90-degree V-shaped bending test is less than or equal to 0.5T (T is the thickness of the high-strength steel). In the comparative example 1, the Mn content is out of the range of the embodiment of the invention, the yield strength of the prepared low-alloy high-strength steel is only 744MPa, the tensile strength is only 808MPa, and the hole expansion rate is only 72 percent; in the comparative example 2, the Ti content is out of the range of the embodiment of the invention, the carbide proportion of the prepared low-alloy high-strength steel is only 1.2 percent, the yield strength is only 768MPa, and the hole expansion rate is only 66 percent; in the comparative example 3, the coiling temperature is out of the range of the embodiment of the invention, the Malaysia/Australian ratio of the prepared low-alloy high-strength steel is lower than 5 percent, the equivalent diameter of carbide reaches 92nm, the yield strength is only 727MPa, and the tensile strength is only 813MPa; in comparative example 4, the soaking temperature is out of the range of the embodiment of the invention, the yield strength of the prepared low-alloy high-strength steel is only 652MPa, the hole expansibility is only 45%, and the minimum bending core diameter R exceeds 0.5T.

Detailed description of the drawings fig. 2:

as shown in fig. 2, a microstructure of the high-strength steel provided in the examples of the present application is shown, and it can be seen that the microstructure of the steel is mainly composed of ferrite, martensite/austenite, and carbide.

One or more technical solutions in the embodiments of the present invention at least have the following technical effects or advantages:

(1) The steel matrix provided by the embodiment of the invention adopts V, ti microalloying, the yield strength reaches 800MPa or above, the hole expansion rate is more than or equal to 70%, the minimum bending core diameter R of a 90-degree V-shaped bending test is less than or equal to 0.5T (T is the thickness of high-strength steel), the ultrahigh strength and excellent local forming performance are achieved, and the requirements of lightweight automobile industry and complex forming of automobile body parts are met;

(2) The high-strength steel provided by the embodiment of the invention has a novel zinc-aluminum-magnesium alloy coating on the surface, and compared with hot-rolled and pickled low-alloy high-strength steel without a coating, the corrosion resistance of the low-alloy high-strength steel is greatly improved, and the problem that the steel for an automobile chassis is easy to rust is solved. In addition, the corrosion resistance of the zinc-aluminum-magnesium coating is more than 6 times of that of a pure zinc coating, and the thickness of the electrophoretic primer can be properly reduced by applying the zinc-aluminum-magnesium coating, so that the effect of saving cost is achieved;

(3) The preparation method of the high-strength steel provided by the embodiment of the invention belongs to a surface coating process of a hot-rolled pickled plate, has short process flow, can realize the replacement of cold by heat, reduces energy consumption and reduces emission.

Finally, it should be further 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 (10)

1. A steel matrix of 800MPa grade hot-rolled low-alloy high-strength steel, characterized in that the chemical composition of the steel matrix comprises by mass fraction: c:0.10% -0.15%, si: 0.1-0.3%, mn:1.7% -2.3%, al: 0.03-0.06%, cr: 0.05-0.1%, mo:0 to 0.05%, V:0 to 0.05%, ti:0.1% -0.14%, P:0 to 0.008 percent, S:0 to 0.001%, N:0 to 0.005 percent, and the balance of Fe and inevitable impurities.

2. The steel substrate of 800MPa grade hot rolled low alloy high strength steel according to claim 1, characterized in that the metallographic structure of the steel substrate comprises in area fraction: 80 to 95 percent of ferrite and 5 to 20 percent of martensite/austenite island, wherein the equivalent grain diameter of the ferrite is less than 7 mu m, and the equivalent grain diameter of the martensite/austenite island is less than 3 mu m.

3. The steel substrate of 800MPa grade hot rolled low alloy high strength steel according to claim 2, wherein the metallographic structure of the steel substrate further comprises in area ratio: 1.4 to 2.0 percent of carbide distributed at the ferrite matrix and the grain boundary, and the average equivalent diameter of the carbide is between 10 and 80nm.

4. A method of producing a steel substrate of a hot rolled low alloy high strength steel of 800MPa class according to any one of claims 1 to 3, characterized in that the method comprises:

smelting molten iron, and then carrying out continuous casting to obtain a plate blank;

and carrying out hot rolling on the plate blank to obtain a steel matrix.

5. The method for preparing the steel matrix of the 800 MPa-grade hot-rolled low-alloy high-strength steel according to claim 4, wherein the finish rolling temperature of the hot rolling is 880-920 ℃, the cooling of the hot rolling adopts an air cooling and laminar flow water cooling mode, the finish temperature of the air cooling is more than or equal to 800 ℃, the cooling speed of the laminar flow water cooling is 10-20 ℃/s, and the coiling temperature of the hot rolling is 400-500 ℃.

6. A hot-rolled low-alloy high-strength steel of 800MPa grade, comprising a steel substrate and a coating layer attached to the steel substrate, the steel substrate being as defined in any one of claims 1 to 3.

7. The 800MPa grade hot rolled low alloy high strength steel according to claim 6, wherein the coating is a zinc aluminum magnesium alloy coating, the thickness of the zinc aluminum magnesium alloy coating is 8-20 μm, and the chemical composition of the zinc aluminum magnesium alloy coating comprises, in mass fraction: al:5% -7%, mg:2 to 4 percent, and the balance of Zn and inevitable impurities.

8. A method for preparing 800MPa grade hot rolled low alloy high strength steel according to any one of claims 6 to 7, characterized in that the method comprises:

leveling a steel substrate, and then pickling to obtain a pickled plate;

and carrying out continuous hot galvanizing on the pickled plate to obtain the high-strength steel.

9. The method for preparing 800MPa grade hot rolled low alloy high strength steel according to claim 8, wherein the temper rolling force is 2000 kN-2500 kN.

10. The preparation method of 800 MPa-level hot-rolled low-alloy high-strength steel according to claim 8, wherein the pickled plate is subjected to continuous hot-dip galvanizing of aluminum and magnesium to obtain the high-strength steel, and the preparation method specifically comprises the following steps:

preheating the pickling plate to obtain a preheating plate;

heating the preheating plate to obtain a heating plate;

cooling the heating plate to obtain a cooling plate;

putting the cooling plate into a pot for galvanizing aluminum magnesium to obtain plated steel;

carrying out air knife blowing on the plated steel, and then carrying out gas jet cooling to obtain high-strength steel;

wherein the preheating heating rate is 2-5 ℃/s, and the temperature of the preheating plate is 210-230 ℃; the heating rate is 10-25 ℃/s, the heating soaking and heat preservation temperature is 640-720 ℃, and the heating soaking and heat preservation time is 40-180 s; the cooling rate is 5-20 ℃/s, and the temperature of the cooling plate is 450-470 ℃.

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