A preparation method of cold-rolled medium manganese QP steel based on Mn inhomogenization regulation

Abstract

The present invention discloses a preparation method of cold-rolled medium manganese QP steel based on Mn inhomogenization regulation, belonging to the field of preparation of medium manganese QP steel. The method comprises the following steps: increasing the Mn content of traditional QP steel to obtain an experimental steel; performing smelting, forging and rolling on the experimental steel to obtain a cold-rolled plate; pre-treating the cold-rolled plate to obtain initial lath martensite; sequentially performing critical region annealing treatment and rapid austenitization treatment on the steel plate containing the initial lath martensite to obtain a steel plate with non-uniform distribution of Mn element; and performing quenching and partitioning treatment on the steel plate with non-uniform distribution of Mn element to form a medium manganese QP steel with a core-shell structure.

Description

Technical Field

[0001] The present invention belongs to the technical field of medium manganese QP steel preparation, and in particular relates to a method for preparing cold-rolled medium manganese QP steel based on Mn non-uniformity control. Background Art

[0002] QP steel is a representative of the third generation of advanced high-strength steels. However, the Mn content in conventional QP steels is less than 2%, resulting in low retained austenite content and low plasticity in the experimental steels. As a key alloying element, Mn plays a vital role in medium-manganese QP steels. By regulating the distribution of Mn through inhomogeneous treatment, the stability and phase transformation behavior of austenite can be significantly influenced, thereby improving the mechanical properties of the material. The diffusion and enrichment of Mn have a significant impact on the stability of retained austenite and are key factors in optimizing material properties.

[0003] Although the research on medium manganese QP steel has made certain progress, the existing technology still has some defects: the patent with publication number CN110129680A discloses a medium manganese lightweight Q&P steel and its preparation method, which adds lightweight elements such as Al and Si through reasonable chemical composition design, reduces the density of steel while ensuring the mechanical properties of steel, but the technical solution lacks the enhancement of the stability of retained austenite. The patent with publication number CN101638749 discloses a low-cost, high-strength and high-plasticity automotive steel and its preparation method. The alloy ratio in this invention is relatively high, and the heat treatment process adopts a long critical zone annealing, which usually requires a long critical zone isothermal time to obtain a high proportion of retained austenite. In addition, serious segregation problems and weldability problems are easily generated during production, which increases the difficulty of industrial production and has no obvious process cost advantage. At the same time, the temperature fluctuation of the hood annealing seriously affects the mechanical properties of the steel plate, greatly reducing the yield rate. Therefore, based on the current technical problems, the present invention discloses a preparation method of cold-rolled medium manganese QP steel based on Mn non-uniformity regulation. Summary of the Invention

[0004] In order to solve the above technical problems, the present invention proposes a method for preparing cold-rolled medium manganese QP steel based on Mn non-uniformity control to solve the problems existing in the above-mentioned prior art.

[0005] To achieve the above objectives, the present invention provides a method for preparing cold-rolled medium manganese QP steel based on Mn non-uniformity control, comprising:

[0006] The experimental steel was obtained by increasing the Mn content of the conventional QP steel;

[0007] The experimental steel is smelted, forged and rolled to obtain a cold-rolled plate;

[0008] Pre-treating the cold-rolled sheet to obtain initial lath-shaped martensite;

[0009] The steel plate containing initial lath martensite is successively subjected to critical annealing treatment and rapid austenitization treatment to obtain a steel plate with non-uniform distribution of Mn element;

[0010] The steel plate with non-uniform distribution of Mn element is subjected to quenching and partitioning treatment to form medium-Mn QP steel with a core-shell structure.

[0011] Optionally, in the chemical composition of the experimental steel, C: 0.1-0.4%, Mn: 3-6%, Si: 0.5-2%, Al: 0-1%, Nb: 0-0.05%, V: 0-0.1%.

[0012] Optionally, the process of smelting, forging and rolling the experimental steel to obtain a cold-rolled plate includes:

[0013] Smelting the experimental steel based on a vacuum melting furnace and casting to obtain an ingot;

[0014] Forging the ingot into a billet;

[0015] Insulating the billet in a high-temperature resistance furnace and then rolling it into a hot-rolled plate using a hot rolling mill;

[0016] Successively subjecting the hot-rolled plate to tempering heating, pickling and several times of rolling to obtain a cold-rolled plate.

[0017] Optionally, the conditions for insulating the billet in a high-temperature resistance furnace include: the temperature of the high-temperature resistance furnace is 1100-200 °C, and the insulation time is 1-2 hours;

[0018] The conditions for successively subjecting the hot-rolled plate to tempering heating include: the heating temperature is 400 °C - Ac3, and the tempering time is 0.5-2 hours.

[0019] Optionally, the process of pre-treating the cold-rolled plate to obtain initial lath martensite includes: subjecting the cold-rolled plate to full austenitization quenching to obtain initial lath martensite.

[0020] Optionally, the process of successively subjecting the steel plate containing initial lath martensite to critical annealing treatment and rapid heating full austenitization treatment to obtain a steel plate with non-uniform distribution of Mn element includes:

[0021] Successively subjecting the steel plate containing initial lath martensite to critical annealing treatment to obtain lath-shaped Mn-rich austenite and lath-shaped Mn-poor ferrite;

[0022] Performing rapid full austenitization and short-time holding on the steel plate to retain the non-uniform distribution of Mn element; wherein, the short-time holding time is 0-6 s, and the heating rate is 10-100 °C / s.

[0023] Optionally, the process of quenching and partitioning based on the non-uniform distribution morphology of Mn element includes:

[0024] Obtaining Mn-rich retained austenite based on the lath-shaped Mn-rich austenite;

[0025] Rapidly austenitizing the lath-shaped Mn-poor ferrite to obtain lath-shaped Mn-poor austenite;

[0026] Performing quenching and partitioning treatment on the lath-shaped Mn-poor austenite to generate hard-phase martensite;

[0027] The hard-phase martensite wraps the adjacent Mn-rich retained austenite to form a core-shell structure.

[0028] Optionally, the microstructure of the medium-Mn QP steel includes: lath-shaped tempered martensite, lath-shaped fresh martensite, and lath-shaped retained austenite; the content of lath-shaped retained austenite is higher than 20%.

[0029] Compared with the prior art, the present invention has the following advantages and technical effects:

[0030] The traditional QP process obtains austenite with uniform Mn element distribution. When the experimental steel is cooled, there is only one Ms point. The FAQ (Flash Austenitization Quenching) process realizes the non-uniform distribution of Mn. The Mn-poor austenite and Mn-rich austenite undergo phase transformation at different quenching temperatures, generating two Ms points.

[0031] The QP heat treatment process experimental steel combining critical region annealing and rapid heating combines critical region annealing and rapid heating to achieve non-uniform distribution of Mn in austenite. The Mn concentration gradient plays a role in dividing austenite. At the same time, rapid austenitization and short-time heat preservation refine austenite, and the structure after QP treatment is refined.

[0032] The structure of the Mn inhomogenized QP steel is refined, the volume fraction of retained austenite is high, and the hard-phase martensite generated by the phase transformation of Mn-poor austenite wraps the adjacent Mn-rich retained austenite. The formed "core-shell" structure improves its mechanical stability. The FAQP experimental steel has a significant fine grain strengthening effect, obvious TRIP effect and fine grain plasticizing effect. The strength reaches 1460 MPa, the elongation is 21.2%, and the strength-ductility product exceeds 30 GPa%, which is significantly better than the experimental steel of the traditional QP. Description of the Drawings

[0033] The drawings constituting a part of this application are used to provide a further understanding of this application. The schematic embodiments of this application and their descriptions are used to explain this application and do not constitute an improper limitation to this application. In the drawings:

[0034] Figure 1Schematic diagram of the FAQP process based on Mn non-uniformity regulation according to an embodiment of the present invention;

[0035] Figure 2 Thermal expansion curve of the FAQ experimental steel according to an embodiment of the present invention;

[0036] Figure 3 Microstructure of the FAQP experimental steel according to an embodiment of the present invention;

[0037] Figure 4 IQ diagram, phase diagram and residual austenite grain size distribution diagram of the FAQP experimental steel according to an embodiment of the present invention;

[0038] Figure 5 Engineering stress-strain curve of the FAQP experimental steel according to an embodiment of the present invention;

[0039] Figure 6 Distribution law of martensite grain size in the FAQP experimental steel according to an embodiment of the present invention. Detailed implementation manners

[0040] It should be noted that, without conflict, the embodiments in the present application and the features in the embodiments may be combined with each other. The present application will be described in detail below with reference to the drawings and in combination with the embodiments.

[0041] Embodiment 1

[0042] In this embodiment, a preparation method of cold-rolled medium manganese QP steel based on Mn non-uniformity regulation is provided. Without adding a large amount of precious alloying elements, the present invention only increases the Mn content of the traditional QP steel to about 4% to increase the content of retained austenite, thereby improving plasticity. Compared with blocky retained austenite, lath-shaped retained austenite has higher stability. Therefore, an initial structure of lath-shaped martensite is obtained after full austenitization quenching, and then critical region annealing is carried out to form lath-shaped Mn-rich austenite and Mn-poor ferrite. Then, rapid heating is carried out for full austenitization and short-time holding to retain the Mn non-uniform distribution morphology formed during critical region annealing. Finally, QP treatment is carried out to obtain a duplex structure of lath-shaped retained austenite and martensite. The hard-phase martensite formed by quenching the Mn-poor austenite wraps the Mn-rich soft-phase retained austenite. This "core-shell" structure can effectively enhance the stability of the retained austenite, fully exert the TRIP effect, and improve plasticity. The microstructure regulation based on the non-uniform distribution of Mn can also refine the microstructure, achieve the synergistic effect of grain refinement strengthening and TRIP effect strengthening, achieve a good match between high strength and high plasticity, meet the performance requirements of the third-generation automotive steel, and avoid the addition of a large amount of alloying elements, realizing the balanced control of cost and performance. By obtaining this "core-shell" structure to increase the stability of the retained austenite and refine the microstructure, a good match between high strength and high plasticity can be achieved.

[0043] Specifically, it includes the following steps: increasing the Mn content of traditional QP steel to obtain experimental steel; smelting, forging and rolling the experimental steel to obtain a cold-rolled sheet; pre-treating the cold-rolled sheet to obtain initial lath martensite; successively performing critical annealing treatment and rapid heating treatment on the steel sheet containing the initial lath martensite to obtain a steel sheet with non-uniform Mn element distribution; performing quenching and partitioning treatment on the steel sheet with non-uniform Mn element distribution to form medium-Mn QP steel with a core-shell structure.

[0044] In the chemical composition of the experimental steel, C: 0.1 - 0.4%, Mn: 3 - 6%, Si: 0.5 - 2%, Al: 0 - 1%, Nb: 0 - 0.05%, V: 0 - 0.1%. As a specific solution of this embodiment, the composition of the experimental steel is shown in Table 1. The process of smelting and forging the experimental steel to obtain a cold-rolled sheet includes: smelting the experimental steel based on a vacuum melting furnace and casting to obtain an ingot; forging the ingot into a billet; keeping the billet in a high-temperature resistance furnace and then rolling it into a hot-rolled sheet using a hot rolling mill; successively performing tempering heating, pickling and several times of rolling on the hot-rolled sheet to obtain a cold-rolled sheet. Among them, the conditions for keeping the billet in the high-temperature resistance furnace include: the temperature of the high-temperature resistance furnace is 1100 - 200 °C, and the holding time is 1 - 2 hours; the conditions for successively performing tempering heating on the hot-rolled sheet include: the heating temperature is 400 °C - Ac3, and the tempering time is 0.5 - 2 hours.

[0045] As a specific solution of this embodiment, it is smelted using a vacuum melting furnace and then cast into an ingot. The ingot is forged into a 40-mm-thick billet, then kept in a 1200 °C high-temperature resistance furnace for 2 hours, and then rolled into a 4.0-mm-thick sheet using a hot rolling mill, and finally air-cooled to room temperature. The hot-rolled sheet is first tempered in a heating furnace (550 °C) for 1 hour to soften the martensite matrix and avoid edge cracking during rolling. The scale is removed by pickling, and then multi-pass rolling is performed on a four-high cold rolling mill to finally obtain a 1.5-mm-thick cold-rolled sheet.

[0046] Table 1

[0047]

[0048] The cold-rolled sheet is subjected to QP heat treatment (Flash Austenitization Quenching and Partitioning, FAQP) combining critical annealing and rapid heating after complete austenitization using a continuous annealing simulation tester. The FAQP process is as Figure 1 shown.

[0049] As Figure 2As shown, the experimental steel of Flash Austenitization Quenching (FAQ) undergoes rapid heating austenitization treatment. The diffusion rate of C element is relatively fast and is evenly distributed in the austenite, while the diffusion rate of Mn element is relatively slow, which does not match the rapid migration of the ferrite-austenite interface during the rapid heating process. Therefore, the austenite generated by the FAQ process retains the Mn element concentration difference generated during the critical region annealing. During the quenching process, the Mn-depleted austenite has poor thermal stability and martensite transformation occurs first. The Ms point is 295 °C, and the second Ms point is observed when cooled to 136 °C.

[0050] The microstructure of the FAQP experimental steel is as Figure 3 shown. The microstructure consists of tempered martensite, fresh martensite and retained austenite. Among them, the tempered martensite is the product of the first quenching. Tempering occurs during the subsequent partitioning stage, which reduces the internal dislocation density and weakens the stress concentration. Moreover, the increase in temperature leads to an enhanced thermal activation ability of C in the tempered martensite, and C atoms diffuse into the untransformed austenite or precipitate as carbides. These two factors make the tempered martensite easy to be etched, with a weak secondary electron reflection ability and a darker tissue color, showing a concave state. The fresh martensite is formed by the phase transformation of the untransformed austenite during the second quenching and is relatively large in size. Its high internal dislocation density and C atom content make it difficult to be etched. In addition, the retained austenite with a high carbon content is also not easily etched. The bright white areas in the figure represent fresh martensite and retained austenite.

[0051] The microstructure of the FAQP experimental steel was characterized by EBSD detection Figure 4 Figs. are the IQ map, phase map and retained austenite grain size distribution map of the FAQP experimental steel. According to the quality of the Kikuchi diffraction pattern, it can be judged that the light gray in the IQ map represents primary martensite and the dark gray represents fresh martensite. In the phase map, the red area is the face-centered cubic structure, representing retained austenite; while the green area is the body-centered cubic structure, representing martensite. The black solid line represents the large-angle grain boundary with an orientation difference greater than 15°. The martensite lath size in the FAQP steel is significantly refined. The Mn-depleted austenite transforms into primary martensite, and a large amount of Mn-rich austenite remains to form retained austenite, which is wrapped by adjacent martensite. The retained austenite is mainly in the form of laths. Some retained austenite with small sizes are mainly distributed in the tempered martensite. By quantitatively detecting the volume fraction of retained austenite in the FAQP experimental steel, it is 25.6%, and the proportion of retained austenite grains with a size below 1 μm in the FAQP experimental steel is 65%, and the average grain size of the retained austenite is 0.86 μm.

[0052] The engineering stress-strain curve of the experimental steel and as Figure 5As shown, the yield strength is 1235 MPa, the tensile strength is 1460 MPa, the elongation is 21.2%, and the product of strength and plasticity is 30.9 GPa%.

[0053] According to the EBSD results, the statistical distribution law of martensite grain size in the experimental steel is as Figure 6 shown. The proportion of martensite grains larger than 2 μm in the FAQP experimental steel is 13%.

[0054] The process of quenching and partitioning based on the non-uniform distribution morphology of Mn element includes: obtaining Mn-rich retained austenite based on lath Mn-rich austenite; rapidly austenitizing lath Mn-poor ferrite to obtain lath Mn-poor austenite; quenching and partitioning the lath Mn-poor austenite to generate hard-phase martensite; the above-mentioned hard-phase martensite wraps the adjacent Mn-rich retained austenite to form a core-shell structure.

[0055] Among them, the quenching temperature is 50 - 150 °C, the holding time is 0 - 20 s, the partitioning temperature is 200 - 550 °C, and the partitioning time is 10 - 1800 s.

[0056] Based on the above implementation process, the following conclusions are obtained:

[0057] The FAQ process realizes the non-uniform distribution of Mn. Mn-poor austenite and Mn-rich austenite undergo phase transformation at different quenching temperatures, generating two Ms points of 295 °C and 136 °C.

[0058] The room-temperature microstructure of the FAQP experimental steel is composed of tempered martensite, retained austenite and fresh martensite. The volume fraction of retained austenite is 25.6%, and the average grain size of retained austenite is 0.86 μm. The FAQP experimental steel combines critical region annealing and rapid heating to achieve non-uniform distribution of Mn in austenite. The Mn concentration gradient plays a role in dividing austenite. At the same time, rapid austenitization and short-time holding refine austenite, and the microstructure after QP treatment is significantly refined.

[0059] The microstructure of the FAQP experimental steel is refined, the volume fraction of retained austenite is high, and the hard-phase martensite formed by the phase transformation of Mn-poor austenite wraps the adjacent Mn-rich retained austenite, and the formed "core-shell" structure improves its mechanical stability. The FAQP experimental steel has a significant fine-grain strengthening effect, obvious TRIP effect and fine-grain plasticizing effect, with a strength of 1460 MPa, an elongation of 21.2%, and a product of strength and plasticity of 30.9 GPa%, which is significantly better than that of the conventional quenching and partitioning (CQP) experimental steel.

[0060] Based on the characteristic that the diffusion rates of C and Mn elements in steel are different, lean Mn ferrite and rich Mn austenite are obtained through critical region annealing. It is rapidly heated to complete austenitization for instantaneous heat preservation (1 s) so that Mn has no time to diffuse, and two types of austenite, lean Mn and rich Mn, are formed within the structure. The lean Mn austenite is quenched to form a hard-phase martensite that wraps the rich Mn soft-phase retained austenite, forming a "core-shell" structure. The technical preparation of medium-Mn QP steel based on the tissue characteristics of this "core-shell" structure includes: the tensile strength reaches above 1450 MPa, the elongation rate is above 20%, and the product of strength and plasticity exceeds 30 GPa%.

[0061] In the present invention, the non-uniform distribution of Mn is obtained by regulation and control. The Mn concentration gradient plays a role in dividing austenite. At the same time, rapid austenitization and short-time heat preservation refine austenite, and the combined action refines the structure.

[0062] The above is only the preferred specific implementation manner of this application, but the protection scope of this application is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in this application should be covered by the protection scope of this application. Therefore, the protection scope of this application should be subject to the protection scope of the claims.

Claims

1. A preparation method of cold-rolled medium manganese QP steel based on Mn non-uniformity regulation, characterized in that Including: The experimental steel is obtained by increasing the Mn content of traditional QP steel; in the chemical composition of the experimental steel, C: 0.1 - 0.4%, Mn: 3 - 6%, Si: 0.5 - 2%, Al: 0 - 1%, Nb: 0 - 0.05%, V: 0 - 0.1%; The experimental steel is smelted, forged and rolled to obtain a cold-rolled sheet; The process of smelting, forging and rolling the experimental steel to obtain a cold-rolled sheet includes: smelting the experimental steel based on a vacuum melting furnace and casting to obtain an ingot; forging the ingot into a blank; insulating the blank in a high-temperature resistance furnace and then rolling it into a hot-rolled sheet using a hot rolling mill; subjecting the hot-rolled sheet to tempering heating, pickling and several passes of rolling to obtain a cold-rolled sheet; The cold-rolled sheet is pretreated to obtain initial lath martensite; The steel sheet containing initial lath martensite is successively subjected to critical region annealing treatment and rapid austenitization treatment to obtain a steel sheet with non-uniform distribution of Mn element; The process of successively subjecting the steel sheet containing initial lath martensite to critical region annealing treatment and rapid heating complete austenitization treatment to obtain a steel sheet with non-uniform distribution of Mn element includes: successively subjecting the steel sheet containing initial lath martensite to critical region annealing treatment to obtain lath Mn-rich austenite and lath Mn-poor ferrite; performing rapid complete austenitization and short-time holding on the steel sheet to retain the non-uniform distribution of Mn element; wherein, the short-time holding time is 0 - 6 s and the heating rate is 10 - 100 °C / s; The process of quenching and partitioning treatment based on the morphology of non-uniform distribution of Mn element includes: obtaining Mn-rich retained austenite based on the lath Mn-rich austenite; rapidly austenitizing the lath Mn-poor ferrite to obtain lath Mn-poor austenite; performing quenching and partitioning treatment on the lath Mn-poor austenite to generate hard-phase martensite; the hard-phase martensite wraps the adjacent Mn-rich retained austenite to form a core-shell structure; The microstructure of the medium-Mn QP steel includes: lath tempered martensite, lath fresh martensite and lath retained austenite; the content of lath retained austenite is higher than 20%; The steel sheet with non-uniform distribution of Mn element is subjected to quenching and partitioning treatment to form a medium-Mn QP steel with a core-shell structure.

2. The preparation method of cold-rolled medium-Mn QP steel based on Mn non-uniformity regulation according to claim 1, characterized in that The conditions for insulating the blank in a high-temperature resistance furnace include: the temperature of the high-temperature resistance furnace is 1100 - 1200 °C and the insulation time is 1 - 2 hours; The conditions for successively subjecting the hot-rolled sheet to tempering heating include: the heating temperature is 400 °C - Ac3 and the tempering time is 0.5 - 2 hours.

3. The preparation method of the cold-rolled medium manganese QP steel based on Mn non-uniformity regulation according to claim 1, wherein, The process of pretreating the cold-rolled sheet to obtain initial lath martensite includes: performing complete austenitization quenching on the cold-rolled sheet to obtain initial lath martensite.

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