WO2017092104A1 - Steel material for stamping forming, formed member and heat treatment method thereof - Google Patents

Abstract

Provided is a steel material for stamping forming, comprising, by weight percent, 0.22-0.48% of C, 5-9.5% of Mn, 0.5-3.0% of Si + Al, the balance being Fe and inevitable impurities, wherein the steel material is a hot rolled, hot rolled acid washed, or hot rolled acid washed coated steel coil or steel sheet, or a cold rolled annealed or cold rolled coated steel coil or steel sheet. The stamping forming process and the stamping formed member based on the steel material is mainly, without limitation, for use in automobile structural members. The steel coil or steel sheet or formed member is obtained by being heated to 650-780ºC for 0.5-60 minutes, and comprises more than 30% (by volume fraction) of retained austenite and achieve mechanical properties of 0.5-1.2 GPa of yield strength, 1.0-1.5 GPa of tensile strength, and more than 25 GPa% of product of strength and elongation (tensile strength * elongation rate). Also provided are a formed member and a heat treatment method.

Description

Steel for stamping forming, forming member thereof and heat treatment method Technical field

The present invention relates to a steel material for press forming, and a press forming member and a heat treatment method based on the above steel material.

Background technique

Energy saving and emission reduction has become an important indicator of automobiles, and an important means to achieve these goals is the lightweight of automobiles. Reducing vehicle weight can increase fuel efficiency and reduce emissions. In order to reduce safety while ensuring safety, the use of high-strength steel becomes an inevitable trend. At present, first-generation advanced high-strength steels such as dual-phase (DP) steel, multi-phase (CP) steel, phase change induced plasticity (TRIP) steel, martensitic steel (the strong plastic product, ie tensile strength × total elongation) It has been widely used in the automotive field and has played an important role in reducing the weight of automobiles. However, the molding performance and strength of the first generation of advanced high-strength steel still needs to be improved to meet the increasing demand for automotive design. The second-generation advanced high-strength steel represented by twin-induced plasticity (TWIP) can reach 60GPa% and has been industrialized. However, due to the large amount of alloying elements, its manufacturing cost is high and production is difficult. And a large amount of alloying elements have deteriorated the welding properties of steel, and thus have not been widely used. The main purpose of developing the third generation of advanced high-strength steel is to strike a balance between the cost of materials and the performance of materials. The strong plastic product is generally above 25GPa%.

According to recent research progress, advanced high-strength steel with a manganese content of 5wt% to 30wt% will be the next-generation automotive steel with a strength of more than 1GPa. The level of manganese in steel and the plastic deformation mechanism of steel divide manganese steel into three categories: Mn content is 4-8 wt%, generally low carbon content, and its structure is double of retained austenite and ferrite. Phase organization, the strengthening mechanism is generally TRIP effect, the phase transformation of retained austenite during deformation, resulting in high work hardening rate and elongation of about 30%; Mn content is 12-22% by weight, such steel the initial structure of austenite, typically stacking fault energy 20mJ / m ^2, since it is a strengthening mechanism of TWIP (twinning induced plasticity), such steel has excellent work hardening rate and is generally about 60% elongation; The Mn content is 22-30% by weight, and the strengthening mechanism is MBIP (Microband-induced plasticity). The steel has high carbon content and aluminum content, and the stacking fault energy is 80 mJ/m ² . The formation of a microshear band during the deformation results in a high work hardening rate and a generally 60% elongation.

Combining the current requirements of energy saving and emission reduction and the third generation of advanced high-strength steel, medium-manganese steel with manganese content of 3-10% has been widely concerned. The hot-rolled microstructure of medium-manganese steel is generally martensite structure, and then the appropriate annealing temperature is selected, and the two-phase region is kept for a sufficient time to obtain a two-phase structure of austenite and ferrite, and is completed in the process. The diffusion of carbon and manganese into austenite stabilizes the austenite to room temperature and gives more austenite structure. Higher strength and elongation are obtained by utilizing the deformation of retained austenite and hindering crack propagation. Generally, in order to enlarge the temperature range of the two-phase region, higher aluminum is added, and the addition of more aluminum causes an increase in the annealing temperature, and the crystal grains are coarse, resulting in a decrease in yield strength. Generally, the design of medium manganese TRIP steel has a low carbon content, generally below 0.2%, and its tensile strength is generally about 1000 MPa, the yield strength is below 600 MPa, and the elongation is below 30%.

The control of retained austenite has an important influence on the strong plasticity of steel. There are many factors affecting the mechanism of residual austenite plastic deformation, including mechanical stability and austenite stacking fault energy. Among them, carbon is an inexpensive solid solution strengthening element and austenite stabilizing element, and carbon element has an influence on the stacking fault energy and changes the grain size of the steel by influencing the heat treatment process.

The stacking fault energy has an important influence on the deformation mechanism of austenite. With the decrease of stacking fault energy, the plastic deformation mechanism is followed by dislocation slip mechanism, deformation twinning and martensite transformation. The factors affecting the stacking fault energy are mainly the composition and grain size of austenite. Carbon, manganese and aluminum will increase the austenite stacking fault energy. A small amount of silicon can increase the austenite stacking fault energy and can act as a solid solution strengthening to increase the steel yield strength and tensile strength, while the chromium decreases. Austenitic stacking fault energy. The austenite grain transformation provides more nucleation sites and shear band interactions, but at the same time causes the elastic strain energy of the lattice trimming and the strain energy of the shear deformation to decrease. The phase transformation of the body. Austenite grains or slabs are small in size, which increases the stacking fault energy and affects the plastic deformation mechanism.

Summary of the invention

An object of the present invention is to provide a steel material for press forming having high toughness and high energy absorbing property, and a press forming member and a heat treatment method based on such a steel material. The heat treatment of the steel of the invention is heat-treated in the two-phase region, which is advantageous for achieving precise temperature control, high carbon and manganese content, good hardenability and low requirement for cooling rate. The mechanical properties of the heat-treated steel sheet are as follows: yield strength 0.5 to 1.2 GPa, tensile strength 1.0 to 1.5 GPa, and strong plastic product (tensile strength × elongation) of 25 GPa% or more. It is stamped and formed into a member on a mold, and is mainly used for, but not limited to, an automobile structural member.

According to a first aspect of the invention, there is provided a steel material for press forming comprising 0.22 to 0.48% of C, 5 to 9.5% of Mn, 0.5 to 3.0% of Si+Al and the balance by weight percent a quantity of Fe and unavoidable impurities, wherein the steel is one of a hot rolled coil (plate), a hot rolled pickled coil (plate), a cold rolled annealed coil (plate), a cold rolled coated coil (plate), Heat treatment before heating to 650-780 ° C (including 680, 700, 720, 750 ° C, etc.) for 0.5 to 60 minutes (5, 10, 20, 30, 40, 50, etc.) And get. The changes of manganese, carbon and other alloying elements in austenite during annealing equilibrium of two-phase region were calculated by thermodynamic softness. The ratio of equilibrium then the steel of austenite and martensite transformation KM-based _{formula, f M = 1-exp (} -0.011 (Ms-QT) ( where QT = room temperature, Ms of elements change according austenite Calculate), calculate the corresponding annealing temperature of the largest retained austenite, and heat at this temperature for 0.5 minutes to 1 hour according to different alloy compositions and manufacturing processes. Combine the composition design and process selection to obtain the volume of retained austenite. Martensitic (ferritic) structure with a fraction of 30% to 60%, volume fraction of 40% to 70%, and 0 to 3% of carbides. The diffusion rate of carbon and manganese in austenite is higher than that of austenite. It is several orders of magnitude slower in martensite, which causes a localized region of austenite to be enriched with more carbon and manganese. The final measured residual austenite has a carbon content of 0.5% or more and a manganese content of more than 7%. The grain of austenite is less than 1 μm. During the deformation process, the martensite transformation and deformation twinning are generated inside the austenite, which is beneficial to improve the energy absorption and elongation of the steel sheet.

The steel material of the present invention is designed based on the composition of manganese in high carbon, having a carbon content of between 0.22 and 0.48%, preferably between 0.25 and 0.45%, and a manganese content of between 5 and 9.5%, preferably between 6 and 8%. Both carbon and manganese are austenite stabilizing elements, which can strongly reduce the austenitizing temperature of steel and the phase transition temperature of martensite. During the annealing heat treatment, the austenite/ferrite slab alternates. The structure, and the carbon and manganese are distributed into the austenite to stabilize the austenite to below room temperature, and the TRIP effect gradually changes to martensite during the deformation process, thereby improving the strength and ductility of the steel; in particular, the steel of the present invention Optimized composition design and annealing process steel, the retained austenite has higher carbon and manganese, some austenite has higher stacking fault energy, and deformation twinning is formed during deformation, which can further improve the work hardening rate and at the same time Improve material strength and ductility. When the content of carbon and manganese is low, in order to obtain more austenite, the annealing temperature is preferably increased, resulting in coarser austenite grains, resulting in poor austenite stability during deformation. The toughness of steel is reduced. When the carbon content is high, the eutectoid structure may be formed, and more coarse carbides are formed to deteriorate the mechanical properties of the steel. The applicant found that the Mn content was controlled at 5 to 9.5%, and the carbon content was controlled between 0.22 and 0.48%, and good strong plasticity was obtained.

According to a preferred embodiment of the present invention, the steel material further comprises at least one of the following components: Cr: 0.001% to 5%; Mo: 0.001% to 2.0%; W: 0.001% to 2.0%; Ti: 0.0001% ～0.4%; Nb: 0.0001% to 0.4%; Zr: 0.0001% to 0.4%; V: 0.0001% to 0.4%; Cu: 0.0005% to 2%; Ni: 0.0005% to 3.0%; B: 0.0001% to 0.005 %. By combining at least one of these components with the above-mentioned basic components, the ultra-high strength and toughness matching of the stamping member can be further ensured, so that the mechanical properties thereof are: yield strength of 0.5 to 1.2 GPa, tensile strength of 1.0 to 1.5 GPa, and strong plastic product. (Tensile strength × elongation) 25 GPa% or more.

According to a preferred embodiment of the invention, the steel material comprises a hot rolled steel sheet, a cold rolled steel sheet, or a steel sheet with a coated layer. The steel plate with a coating layer may be a zinc-coated steel sheet which is a hot-rolled steel sheet or a cold-rolled steel sheet on which a metal zinc layer is formed. The zinc-coated steel sheet includes one selected from the group consisting of hot dip galvanizing (GI), galvannealing (GA), zinc plating, or zinc-iron plating (GE). The steel sheet with a coating layer is a hot-rolled steel sheet or a cold-rolled steel sheet on which an aluminum-silicon layer is formed, or a steel sheet of an organic plating layer or a steel sheet with another alloy-plated layer.

According to an aspect of the invention, there is provided a press forming process comprising providing a steel material having the above-described composition, which is subjected to a heat treatment process after being heated to 650 to 780 ° C for 0.5 to 60 minutes, and then subjected to press forming. Alternatively, the steel material is directly subjected to cold stamping or hot stamping, and is subjected to a heat treatment process heated to 650 to 780 ° C for 0.5 to 60 minutes to obtain a desired stamping. With the increase of carbon and manganese content, the hardenability of steel will be greatly improved, so that the whole martensite structure can be obtained under air-cooling conditions after hot rolling, so it has high strength and causes cold rolling difficulties. . In order to solve the problem of cold rolling difficulties, the applicant proposed a solution in combination with the production process. The steel has a carbide formation temperature of 630 to 750 ° C, which is consistent with the softening annealing process conventionally used in industrial hood furnaces to precipitate carbides and reverse phase transformation of austenite in the steel, which reduces the strength of the steel. It also increases the elongation of steel for cold rolling.

According to another aspect of the present invention, there is provided a press-formed member which is produced by the press-forming steel material of the present invention by the press forming process of the present invention.

The microstructure of the steel material of the present invention or its press-formed member by volume includes: 30% or more of retained austenite, 70% or less of martensite (ferrite) structure, or 3% or less of carbides, thereby The mechanical properties are obtained: yield strength of 0.5 to 1.2 GPa, tensile strength of 1.0 to 1.5 GPa, and strong plastic product (tensile strength x elongation) of 25 GPa% or more. Preferably, this The microstructure of the invented steel or its stamped and formed member after heat treatment to 650 to 780 ° C for 0.5 to 60 minutes, the microstructure in terms of area includes: 30% to 60% of retained austenite, 40% to 70% The body-centered cubic crystal structure of martensite or ferrite, less than 3% of carbides. Preferably, the retained austenite and martensite or ferrite may be in the form of lath-like phases or have a slab thickness of between 10 and 300 nm. Preferably, the austenite may include Mn of 7% or more and C of 0.5% or more in weight percentage. Preferably, in the steel material of the present invention or the stamped and formed member thereof, the retained austenite induces plasticity by phase transformation during the deformation process, or simultaneously assists the twinning induced plastic effect, thereby improving the matching of strength and ductility, so that the mechanical properties are achieved. The yield strength is 0.5 to 1.2 GPa, the tensile strength is 1.0 to 1.5 GPa, and the strong plastic product is ≥25 GPa%.

According to a preferred embodiment of the present invention, the stamped forming member can be used for at least one of an automotive safety structural member, a reinforcing structural member, and a high strength automotive structural member. More specifically, the forming member may be used for at least one of a B-pillar reinforcement, a bumper, and a door impact beam, a wheel spoke. Of course, the forming member can also be used in any other land vehicle where a lightweight high strength and high ductility member is required.

According to the technical solution of the present invention, the following advantages can be obtained:

1. By increasing the carbon content and controlling the reasonable manganese content, there is a large amount of retained austenite with high stability in the steel. The volume fraction of austenite content is above 30%, and the TRIP effect occurs during the deformation process. Strong plasticity

2. In the preferred case, the partially stabilized austenite undergoes the TWIP effect during the deformation process, assisting the deformation, and is more conducive to improving the strong plasticity of the steel;

3. Compared with medium Mn-TRIP steel, the yield strength is obviously improved, and the matching between tensile strength and elongation is also obviously improved;

4. The production process is simple, and the requirements on the cooling rate are low, which is beneficial to the control of the microstructure of the steel and the stability of the mechanical properties.

In particular, compared with Patent Document CN102021472B (which refer to the process of FIG. 7), the higher the present invention because the addition of Si and Al, increase the material temperature Ac _1, cold-rolled to make a softening annealing temperature is increased to 630-750 prepared °C (different from the softening annealing temperature of 450-630 °C according to the claims), its elongation and rolling deformation ability are improved compared with the comparative patent, which is consistent with the softening annealing process conventionally used in industrial hood furnaces, which is beneficial to the production organization. . Moreover, the present invention can obtain the desired performance by directly annealing at 650-780 ° C for 0.5-60 min using a hot rolled sheet. The invention adopts optimized C and Mn alloy design and optimizes the heat treatment design to make the high-volume fraction retained austenite (30%-60%) with reasonable stability, and therefore has a tensile strength of 1000 MPa or more, and is strong. Plastic product is more than 25GPa%. For components that have been subjected to direct stamping or hot stamping, the desired properties can be obtained after 650-780 ° C for 0.5-60 min.

Compared with the patent document CN101638749B, the optimized design of the present invention allows the final microstructure to be obtained by annealing at 650-780 ° C for more than one time of Ac ₁ or more for 0.5 to 60 minutes (the subsequent process path is required to further 100 ° C below Ac _{1 ).} The complex process of holding to 1 to 10 hours at Ac ₁ temperature is different), the steel of the present invention can directly obtain the desired structure and properties after heat treatment in an industrial continuous annealing furnace. For a comparison of the process of the present invention and the comparative patent, reference may be made to FIG.

CN101638749B austenite is at a reverse transformation temperature 100 ℃ to the Ac _{1 1} Ac 1-10 hours. The alloying element treatment of the hot rolled sheet is the same as the austenite reverse phase transformation treatment.

Compared with the patent document CN104846274A, in the present invention, the steel material is subjected to one-step treatment, that is, annealing at 650-780 ° C for 0.5 to 60 minutes, and the austenite content formed is much higher than the comparative patent technology, achieving a residual of 30% to 60%. Cylinder, so it has better strength and ductility matching, strong plastic product of more than 25GPa%; its stamped and formed components are annealed at 650-780 °C for 0.5 to 60 minutes, the heat treatment process and microstructure are different from the contrast The patented technology has higher ductility, the elongation is higher than 18%, and the strong plastic product is above 25GPa%.

The initial hot-rolled structure of the steel of the present invention is a lath-like martensite structure, which is characterized by the use of martensite laths. Annealing in the two-phase region completes the partitioning of carbon and manganese, so that the reverse phase transformation of austenite preserves the morphology of the martensite lath, and has sufficient carbon content and manganese content to enable it at room temperature. Stable existence, using the coordinated deformation of retained austenite and annealed martensite to increase the elongation of steel.

The enrichment of a large amount of carbon in the austenite and austenite with fine grain or lath size caused by the reverse phase causes the austenite to increase the stacking fault energy, causing the austenite to undergo TWIP/ during the deformation process. The TRIP effect increases the work hardening rate of steel, suppresses the yielding platform, and improves the ductility of steel.

The present invention obtains a hot rolled structure into a martensite structure by increasing the carbon content in the steel and controlling the content of manganese. Then in the two-phase zone, the two-phase structure of austenite and ferrite is obtained, and during this period, the carbon and manganese in the steel are enriched on the austenite, based on the design of appropriate carbon and manganese, at room temperature. Retained austenite having a higher carbon content and a manganese content is obtained.

DRAWINGS

1 is a heat treatment process diagram of an embodiment of the present invention, wherein HR represents hot rolling; CR represents cold rolling;

2 is an engineering stress-strain curve of No. 1 steel after softening annealing at 650 ° C for 10 h according to an embodiment of the present invention;

3 is a microstructure of a parallel section of a No. 1 steel after being stretched and deformed by softening annealing at 650 ° C for 10 h according to an embodiment of the present invention;

Figure 4 is the engineering stress-strain curve of the No. 5 steel cold-rolled sheet No. C treatment process at normal temperature;

Figure 5 is the microstructure of the No. 5 steel cold-rolled sheet No. C treatment process;

Figure 6 is an X-ray diffraction pattern before and after deformation of the No. 5 steel cold-rolled sheet after the treatment process of the label C;

Figure 7 is a heat treatment process diagram of the comparative patent document CN102021472B;

Figure 8 is a comparison diagram of the process of the present invention and the comparative patent CN102021472B.

detailed description

The invention will be described in more detail below with reference to examples. The examples are intended to explain the exemplary embodiments of the invention, and the invention is not limited to the embodiments.

The reason why the chemical composition (in percent by weight) of the steel of the present invention is limited is as follows:

C: 0.22 to 0.48%

Carbon is the cheapest strengthening element and can strongly increase the strength of steel through gap solid solution. In addition, an increase in the carbon content strongly reduces Ac ₃ , thereby lowering the heating temperature and saving energy. However, excessively high carbon content may result in poor weldability of the steel, and may cause excessive strength of the sheet, damage the mold during the hot forming process, and reduce the life of the mold. For this reason, the upper limit of carbon is set to 0.48%. In order to ensure enrichment of sufficient carbon in the austenite, the lower limit of the carbon content is 0.2%, preferably 0.25 to 0.45%.

Mn: 5% to 9.5%

Manganese is an important element for improving the stability of austenite. Manganese austenite region can be enlarged, reduced temperature Ac _3. Manganese has an excellent effect of suppressing the transformation of austenite to ferrite and improving the hardenability of steel. In order to ensure that sufficient austenite is stably present at room temperature, the lower limit of manganese is limited to 5%. The increase in manganese reduces the corrosion resistance of the steel and reduces the weldability, and it is easy to form a twinned martensite to reduce the ductility of the material during quenching and cooling. Therefore, the upper limit of manganese is set to 9.5%.

Si+Al: 0.5% to 3.0%

Both silicon and aluminum can inhibit the formation of carbides. Silicon and aluminum can increase the temperature of Ac ₁ and increase the temperature of softening treatment of steel sheets before cold rolling. This is consistent with the softening annealing process conventionally used in industrial hood furnaces, and can suppress large sizes. Cementite is produced. During the cooling of the steel to room temperature, silicon and aluminum can inhibit the precipitation of cementite. And silicon can play the role of solid solution strengthening, improve the yield strength of steel, aluminum can significantly expand the temperature range of the two-phase region, and expand the process window, so the lower limit of Si + Al is 0.5%. In industrial production, too much Al will block the nozzle during continuous casting, increasing the difficulty of continuous casting. High silicon content can cause more impurities in the steel. Therefore, the upper limit of Si+Al is set to 3.0%.

Cr, Mo, W: 0.001% to 2.0%

Cr, Mo, and W improve the hardenability of steel and can effectively increase the strength of steel. Cr can make the structure uniform and increase the tempering stability. The alloy cementite formed by Cr and C can hinder the austenite grain growth and improve the toughness of the steel. In addition, Mo can prevent temper brittleness, increase remanence and coercivity, and corrosion resistance in certain media. Mo can reduce the tendency of carbides to be distributed at grain boundaries. W has a similar effect to Mo, which can reduce the heat sensitivity of steel, increase hardenability and increase hardness. When the concentration is less than 0.001%, sufficient hardenability cannot be obtained, and even if it is more than 2.0%, no additional effect can be obtained, but the cost is increased.

Ti, Nb, Zr, V: 0.0001% to 0.4%

Ti, Nb, Zr, and V refine the grain of the steel, increase the strength, and obtain good heat treatment characteristics. Too low a concentration of Ti, Nb, Zr, and V does not work, and more than 0.4% increases unnecessary cost.

Cu, Ni: 0.0005% to 2.0%

Cu improves strength and toughness, especially atmospheric corrosion. When the content of Cu is more than 2.0%, workability may be deteriorated. Ni improves the strength of steel while maintaining good ductility and toughness. When the content of Ni is less than 0.0005%, the desired effect is not obtained, and when the concentration of Ni is more than 2.0%, the cost is increased.

B: 0.0001% to 0.005%

B segregates at the austenite grain boundary, prevents the nucleation of the ferrite, and can strongly improve the hardenability of the steel, and can significantly increase the strength of the steel after the heat treatment. When the content of B is less than 0.0001%, it does not function. A B content higher than 0.005% does not significantly improve the function.

One object of the present invention is to produce a yield strength of 500 to 1200 MPa, tensile strength 980 ~ 1480MPa, its strong plastic product (tensile strength × elongation) ≥ 25GPa% of the steel plate. The steel sheet includes a hot rolled steel sheet, a cold rolled steel sheet, and a galvanized steel sheet or other plated steel sheet. The microstructure by volume after heat treatment includes: 30% to 70% of retained austenite, and 30% to 70% of body-centered cubic structure (martensite or ferrite). Its austenite has Mn ≥ 7% by weight percentage, and C ≥ 0.5%.

The experiment conducted with the steel sheet of the present invention is described below. The steel materials of the components specified in Table 1 were homogenized at 1200 to 1500 ° C for 5 to 20 hours, and then held at 1000 ° C to 1200 ° C for 0.5 to 2 hours, followed by hot rolling to form a hot rolled sheet. Referring to Figure 1, the hot rolled sheet is treated by heating the hot rolled sheet to an annealing temperature for 0.5 to 60 minutes. In order to facilitate cold rolling, the hot rolled sheet is subjected to softening annealing treatment before cold rolling, and the annealed hot rolled sheet is pickled, cold rolled to 1.5 mm, and kept at an annealing temperature for 0.5 to 60 minutes. The hot-rolled sheet and the cold-rolled sheet of No. 1 steel in Table 1 were subjected to one-step annealing treatment after being subjected to flat-plate stamping and forming after austenitizing treatment at 780 °C.

A method of manufacturing the shaped member will be described below. The hot rolled sheet or the cold rolled sheet or other plated steel sheet is heated to 650 to 780 ° C for heat preservation before stamping. The cooling method can be air cooling or cooling to room temperature by other cooling methods. After cooling, the steel sheet is transferred into a mold for press forming to obtain a shaped member of a desired shape. Alternatively, the obtained steel sheet may be directly subjected to cold stamping or hot stamping, and then kept at 650 to 780 ° C, and cooled to room temperature by air cooling or other cooling. In the above manufacturing method, the steel sheet is heated to 650 to 780 ° C to maintain the structure of the face-centered cubic structure and the body-centered cubic structure, and the carbon and manganese in the martensite (ferrite) are diffused to the austenite. Increase the stability of austenite and store it at room temperature. During the deformation of the shaped member, the sufficiently stable austenite undergoes phase change induced plasticity (TRIP, in particular, in some cases, partial austenite undergoes twinning induced plasticity (TWIP), thereby increasing the plasticity of the material. Energy absorption. The microstructure of the stamped and formed member of the present invention by volume includes: 30% to 60% of retained austenite, 40% to 70% of body-centered cubic crystal structure of martensite or ferrite. Mn ≥ 7% by weight percentage in austenite, C ≥ 0.5%.

Table 1 chemical composition of steel

	C	Mn	P	S	other
1	0.3	6.66	0.012	0.008	1.05Si 0.04Ti 0.005B
2	0.28	5.75	0.99	0.002	1.05Si
3	0.4	4	0.006	0.008	1Al 1.5Si 0.5Mo
4	0.2	9.5	0.99	0.002	2Al 1Si
5	0.28	6.3	0.99	0.008	1.5Si

Steel plate heating and heat treatment experiment

Heating and holding experiments were performed on hot rolled or cold rolled sheets in a resistive box furnace with nitrogen protection. The specific experimental process is shown in Table 2.

Table 2 different heat treatment processes

The mechanical properties of the steel sheets obtained by different heat treatment processes are shown in Table 3.

Table 3 tensile mechanical properties of steel plates

Note:

YS represents the yield strength, the stress value of 0.2% residual deformation is its yield limit; TS is the tensile strength; TE is the total elongation.

The tensile specimens in Table 3 were ASTM standard specimens with a gauge length of 50 mm; the strain rate for the tensile mechanical properties test was 6.67 × 10 ^-4 s ^-1 .

As can be seen from the mechanical property data of Table 3, the steel sheet having the composition of the present invention can obtain a molded member excellent in strength and elongation comprehensive performance by the heat treatment process of the present invention. Specifically, a steel sheet having a yield strength of 500 to 1200 MPa, a tensile strength of 980 to 1480 MPa, and a strong plastic product (tensile strength × elongation) of ≥ 30 GPa% can be realized.

The tensile curves and microstructures of the steel sheet of the present invention after softening treatment are shown in Figures 2 and 3. Microstructure analysis of the uniform deformation zone of the invention steel, a small amount of carbides exist in the steel, the structure is distributed in a strip shape, the austenitic slab is around 200 nm, and there is a strip structure in the austenite, which may be ε - Martensite or austenite twins, which are mechanically twinned during deformation by calibration of the austenite.

Figure 4 shows the tensile curve of the No. 5 steel cold-rolled sheet held at 670 ° C for 10 minutes. The yield strength, tensile strength and elongation meet the requirements of the invention, and the steel of the present invention remains consistent. High work hardening rate, no yielding platform and Ryders belt. According to the microstructure characterization of Fig. 5, the microstructure after cold rolling annealing still maintains the slab state distribution, and the grain size is below 1 micrometer, which not only improves the yield strength of the steel, but also improves the austenite layer. The wrong energy is beneficial to the mechanical twinning during the deformation process, thereby increasing the elongation of the steel and the work hardening rate. Fig. 6 is an X-ray diffraction pattern before and after deformation in this process, and the volume fractions before and after deformation of the austenite were calculated to be 40% and 6%, respectively. During the deformation process, the retained austenite undergoes a TWIP or TRIP effect, thereby ensuring the required strength and ductility of the steel of the present invention.

Claims (14)

1. A steel material for press forming, characterized in that the steel material comprises 0.22 to 0.48% of C, 5 to 9.5% of Mn, 0.5 to 3.0% of Si + Al, and the balance of Fe and not by weight. Avoid impurities.
2. The steel material according to claim 1, further comprising at least one of the following components:

   Cr: 0.001% to 2%;

   Mo: 0.001% to 2.0%;

   W: 0.001% to 2.0%;

   Ti: 0.0001% to 0.4%;

   Nb: 0.0001% to 0.4%;

   Zr: 0.0001% to 0.4%;

   V: 0.0001% to 0.4%;

   Cu: 0.0005% to 2%;

   Ni: 0.0005% to 3.0%;

   B: 0.0001% to 0.005%.
3. The steel material according to claim 1, wherein the steel material is hot rolled, hot rolled pickling, hot rolled pickled coated steel coil or steel sheet, or cold rolled annealed, cold rolled coated steel coil or steel sheet, wherein the steel layer is coated. Steel coils or steel sheets include zinc coated steel coils or steel or other coated steel coils or steel sheets.
4. The steel material according to claim 3, wherein the zinc-coated steel sheet is a hot-rolled steel sheet or a cold-rolled steel sheet on which a metal zinc layer is formed.
5. The steel material according to claim 4, wherein the zinc-coated steel sheet comprises one selected from the group consisting of hot dip galvanizing, galvanizing annealing, zinc plating, or zinc-iron plated steel.
6. The steel material according to claim 3, wherein said other coated steel sheet is a hot rolled steel sheet or a cold rolled steel sheet on which an aluminum silicon layer is formed, or another alloyed plating layer or organic plating layer is formed thereon. Steel plate.
7. The steel material according to any one of claims 1 to 6, wherein the steel material is subjected to heat treatment by heating to 650 to 780 ° C for 0.5 to 60 minutes.
8. The steel material according to claim 7, wherein after said heat treatment, said microstructure of said steel material by area comprises: 30% to 60% of retained austenite, and 40% to 70% of body-centered cubic crystal structure of martensite or ferrite, less than 3% of carbide.
9. The steel material according to claim 8, wherein the retained austenite comprises, by weight percentage, Mn of 7% or more and C of 0.5% or more.
10. The steel material according to claim 9, wherein during the forced deformation, the retained austenite induces plasticity by phase transformation, or simultaneously assists twinning to induce plasticity, thereby improving the matching of strength and ductility, thereby The mechanical properties are as follows: the yield strength is 0.5-1.2 GPa, the tensile strength is 1.0-1.5 GPa, and the strong plastic product is ≥25 GPa%.
11. A forming member characterized in that the forming member is formed by a press forming process of the steel material according to any one of claims 7 to 10, the forming member comprising an automobile structural member.
12. A forming member characterized in that the forming member is formed by a direct press forming process or a hot stamping forming process of the steel material according to any one of claims 1 to 6, the forming member comprising an automobile structural member.
13. The forming member according to claim 12, wherein the forming member is heated to 650 to 780 ° C for 0.5 to 60 minutes, and the microstructure of the member in terms of area includes: 30% to 60% of residual a body, 40% to 70% of body-centered cubic crystal structure of martensite or ferrite, less than 3% of carbides, including 7% or more by weight of Mn in the retained austenite and For C greater than or equal to 0.5%, during the deformation process, the austenite induces plasticity through phase transformation, and can assist the twinning induced plastic effect, improve the matching of strength and ductility, and achieve the mechanical properties: yield strength 0.5 ~1.2GPa, tensile strength 1.0 ~ 1.5GPa, strong plastic product ≥ 25GPa%.
14. A heat treatment method comprising the steps of:

    (A) providing a steel material according to any one of claims 1 to 6 or a formed member formed by direct stamping or hot stamping of the steel material;

    (B) The steel or the formed member is heated to 650 to 780 ° C for 0.5 to 60 minutes.

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