A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition

Abstract

The present invention discloses a method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite components, which belongs to the technical field of metal material preparation. The preparation method includes the following steps: S1, hot rolling in austenite region; S2, heating in two-phase region; S3, cold rolling; S4, variable temperature treatment. The present invention realizes effective regulation of the distribution of chemical heterogeneity of austenite by variable temperature treatment. Under this process, austenite grains nucleate and grow at different temperatures, thereby forming a gradient distribution of solute elements (such as Mn) from the crystal to the grain boundary. This heterogeneous construction not only realizes the differentiated regulation of the stability of different austenite grains, but also improves the comprehensive mechanical properties of medium manganese steel.

Description

Technical Field

[0001] The invention belongs to the technical field of metal material preparation, and in particular relates to a method for preparing a medium-manganese steel with high strength and plasticity and chemical heterogeneity of austenite components. Background Art

[0002] With the rapid development of the automobile industry, the requirements for lightweight and high-strength steel materials for automobiles are increasing. As a new type of steel material, medium manganese steel is considered to be a strong candidate for the third generation of advanced high-strength automotive steel due to its excellent mechanical properties. The Mn content of medium manganese steel is usually between 5% and 12%, which gives it good hardenability and makes it easy to form a martensitic structure during the cooling process after hot rolling. Through subsequent room temperature cold rolling and intercritical annealing processes, a dual-phase structure composed of ferrite and residual austenite can be obtained. This organizational structure gives medium manganese steel high tensile strength (800-1300MPa) and high elongation (30%-60%), meeting the needs of lightweight and safety of automobiles.

[0003] The preparation process of medium manganese steel mainly includes smelting, hot rolling, cold rolling and two-phase zone annealing. Among them, the two-phase zone annealing is carried out at a constant temperature to form the desired microstructure. However, the chemical heterogeneity (i.e., the non-uniform distribution of chemical composition) of the preparation of medium manganese steel in the prior art usually adopts flash heating technology. This technology achieves rapid heating through a heating rate greater than 30°C / min, so that the elements in the austenite do not have time to fully diffuse during the heating process, thereby forming chemical heterogeneity. Although this technology can obtain the desired material properties, it has high requirements on equipment and the cost is relatively high. In addition, flash heating technology is not suitable for the heating of large steel plates, and it is difficult to apply it on the existing cold-rolled strip production line, which limits its wide application in industrial production. Summary of the invention

[0004] In view of the problems and shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition, by which the heterogeneous construction of the gradient distribution of solute elements from the intragranular to the grain boundary in the austenite is achieved through a variable temperature (continuous heating / cooling) critical zone annealing process, thereby achieving differentiated regulation of the stability of different austenite grains and improving the comprehensive mechanical properties of the medium manganese steel.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0006] A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition comprises the following steps:

[0007] S1. Hot rolling in austenite zone: the medium manganese steel is placed in a high-temperature box-type resistance furnace and heated to a certain temperature, and kept warm at this temperature, and hot-rolled into a billet plate with a thickness of 5 mm by a high-rigidity two-roll hot rolling experimental rolling mill, and air-cooled to room temperature;

[0008] S2, two-phase zone heating: the blank plate obtained in step S1 is cut into 10 cm long plates by electric spark wire cutting, the cut plates are placed in a high temperature furnace, and kept at this temperature for 0.5-3 hours, and then air-cooled to room temperature;

[0009] S3, cold rolling: cold rolling the sheet obtained in step S2 into a plate, and cutting the cold rolled plate into a dog bone shape by electric spark wire cutting to obtain a stretched part;

[0010] S4, temperature change treatment: the drawn part obtained in step S3 is placed in a high temperature furnace at 650°C-750°C for temperature change treatment, the drawn part is taken out, and water quenched to room temperature.

[0011] Preferably, the manganese steel material in step S1 includes two or more elements of C, Mn, Al, Zr, Nb, and V.

[0012] Further, preferably, the manganese steel in step S1 includes one or two of C-Mn-Al medium manganese steel, C-Mn-Al-Zr medium manganese steel, C-Mn-Al-Nb medium manganese steel, C-Mn-Al-V medium manganese steel, and medium manganese steel.

[0013] Preferably, the chemical composition of the medium manganese steel is as follows by mass percentage: C: 0.1%-0.5%, Mn: 5%-10%, Al: 0.5%-3%, Zr+Nb+V≤0.5%, and the remainder is Fe and unavoidable impurities.

[0014] Further, preferably, the mass percentages of C, Mn and Al in C-Mn-Al medium manganese steel are 0.1-0.5:3-10:0.5-3; the mass percentages of C, Mn, Al and Zr in C-Mn-Al-Zr medium manganese steel are 0.1-0.5:3-10:0.5-3:0-0.5; the mass percentages of C, Mn, Al and Nb in C-Mn-Al-Nb medium manganese steel are 0.1-0.5:3-10:0.5-3:0-0.5; the mass percentages of C, Mn, Al and V in C-Mn-Al-V medium manganese steel are 0.1-0.5:3-10:0.5-3:0-0.5.

[0015] Preferably, in step S1, the medium manganese steel is heated to 1050-1200° C. and kept at this temperature for 0.5-3 h.

[0016] Preferably, in step S2, the temperature of the high temperature furnace is 550-650° C., and the temperature is maintained for 0.5-3 hours.

[0017] Preferably, in step S3, the sheet is cold rolled to a thickness of 1.6 mm.

[0018] Preferably, in step S4, the temperature-changing treatment is one of a furnace-raising treatment or a furnace-lowering treatment of the stretched part.

[0019] Preferably, in step S4, the stretched part obtained in step S3 is placed in a 650°C high temperature furnace, and the temperature is raised to 670-750°C at a rate of 0.33-10°C / min. The stretched part is taken out and quenched with water to room temperature.

[0020] Preferably, in step S4, the stretched part obtained in step S3 is placed in a 670°C-750°C high-temperature furnace, cooled to 650°C at a cooling rate of 0.33°C / min, and the stretched part is taken out and quenched with water to room temperature.

[0021] The present invention also provides a medium manganese steel material obtained by the above-mentioned preparation method.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The present invention provides a method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite components, discloses a variable temperature critical zone annealing process, and realizes effective regulation of the distribution of austenite chemical heterogeneity by continuous heating / cooling. Under this process, austenite grains nucleate and grow at different temperatures, thereby forming a gradient distribution of solute elements (such as Mn) from the crystal to the grain boundary. This heterogeneous construction not only realizes the differentiated regulation of the stability of different austenite grains, but also improves the comprehensive mechanical properties of medium manganese steel.

[0024] The variable temperature critical zone annealing process of the present invention is not only an innovative austenite chemical heterogeneity distribution preparation technology, but also effectively improves the comprehensive mechanical properties of medium manganese steel, including strength, plasticity and deformation resistance, by realizing the gradient distribution of solute elements from the crystal to the grain boundary, thus providing a technical basis for the wide application of medium manganese steel in high-performance application fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 It is a temperature change treatment process diagram of the present invention, wherein: Figure 1 a in the figure is the heat treatment process diagram of the continuous heating critical zone. Figure 1 b in the figure is the heat treatment process diagram of the continuous cooling critical zone;

[0026] Figure 2 This is an EDS line scan of the high strength and plasticity austenite chemically heterogeneous medium manganese steel provided in Example 2;

[0027] Figure 3The mechanical property curves of the medium manganese steel with high strength and plasticity and chemically heterogeneous distribution of austenite components prepared by temperature-dependent critical zone annealing provided in Example 2 and the mechanical property curves of the medium manganese steel with relatively uniform distribution of austenite chemical components prepared by isothermal critical zone annealing provided in Comparative Example 1. DETAILED DESCRIPTION

[0028] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0029] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0030] Embodiment 1 A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition comprises the following steps:

[0031] S1. Austenite hot rolling: Fe-0.4C-7Mn-2Al-0.5Zr (mass percentage) medium manganese steel was placed in a high temperature box-type resistance furnace and heated to 1200°C and kept at this temperature for 1 hour, and then hot-rolled into a 5 mm thick billet plate by a high rigidity two-roll hot rolling experimental rolling mill, and then air-cooled to room temperature;

[0032] S2 two-phase zone heating: the blank plate obtained in step S1 was cut into 10cm long plates by wire EDM, the cut plates were placed in a 650℃ high temperature furnace, kept warm for 1h, and air cooled to room temperature;

[0033] S3 cold rolling: The sheet obtained in step S2 is cold rolled to a thickness of 1.6 mm, and the cold rolled sheet is cut into a dog bone shape by wire electric discharge cutting to obtain a stretched piece;

[0034] S4. Variable temperature heating: The drawn part obtained in step S3 is placed in a 650°C high temperature furnace, and the temperature is rapidly increased to 670°C at a heating rate of 1°C / min. The drawn part is taken out and quenched with water to room temperature to obtain a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition.

[0035] Embodiment 2 A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition comprises the following steps:

[0036] S1. Austenite hot rolling: Fe-0.4C-7Mn-2Al (mass percentage) medium manganese steel is placed in a high temperature box-type resistance furnace and heated to 1200°C and kept at this temperature for 1 hour, and then hot-rolled into a 5 mm thick billet plate by a high rigidity two-roll hot rolling experimental rolling mill, and then air-cooled to room temperature;

[0037] S2 two-phase zone heating: the blank plate obtained in step S1 was cut into 10cm long plates by wire EDM, the cut plates were placed in a 650℃ high temperature furnace, kept warm for 1h, and air cooled to room temperature;

[0038] S3 cold rolling: The sheet obtained in step S2 is cold rolled to a thickness of 1.6 mm, and the cold rolled sheet is cut into a dog bone shape by wire electric discharge cutting to obtain a stretched piece;

[0039] S4. Variable temperature heating: The drawn part obtained in step S3 is placed in a 650°C high temperature furnace, and the temperature is rapidly increased to 670°C at a heating rate of 1°C / min. The drawn part is taken out and quenched with water to room temperature to obtain a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition.

[0040] Example 3

[0041] A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition comprises the following steps:

[0042] S1. Austenite hot rolling: Fe-0.4C-7Mn-2Al-0.5Zr (mass percentage) medium manganese steel was placed in a high temperature box-type resistance furnace and heated to 1200°C and kept at this temperature for 1 hour, and then hot-rolled into a 5 mm thick billet plate by a high rigidity two-roll hot rolling experimental rolling mill, and then air-cooled to room temperature;

[0043] S2 two-phase zone heating: the blank plate obtained in step S1 was cut into 10cm long plates by wire EDM, the cut plates were placed in a 650℃ high temperature furnace, kept warm for 1h, and air cooled to room temperature;

[0044] S3 cold rolling: The sheet obtained in step S2 is cold rolled to a thickness of 1.6 mm, and the cold rolled sheet is cut into a dog bone shape by wire electric discharge cutting to obtain a stretched piece;

[0045] Variable temperature heating: the drawn part obtained in step S3 is placed in a 670°C high temperature furnace, and the temperature is lowered to 650°C at a cooling rate of 1°C / min. The drawn part is taken out and quenched in water to room temperature.

[0046] Example 4

[0047] A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition comprises the following steps:

[0048] S1. Austenite hot rolling: Fe-0.4C-7Mn-2Al (mass percentage) medium manganese steel is placed in a high temperature box-type resistance furnace and heated to 1200°C and kept at this temperature for 1 hour, and then hot-rolled into a 5 mm thick billet plate by a high rigidity two-roll hot rolling experimental rolling mill, and then air-cooled to room temperature;

[0049] S2 two-phase zone heating: the blank plate obtained in step S1 was cut into 10cm long plates by wire EDM, the cut plates were placed in a 650℃ high temperature furnace, kept warm for 1h, and air cooled to room temperature;

[0050] S3 cold rolling: The sheet obtained in step S2 is cold rolled to a thickness of 1.6 mm, and the cold rolled sheet is cut into a dog bone shape by wire electric discharge cutting to obtain a stretched piece;

[0051] S4. Variable temperature heating: Place the drawn part obtained in step S3 in a 670°C high temperature furnace, cool it to 650°C at a cooling rate of 1°C / min, take out the drawn part, and quench it with water to room temperature.

[0052] Comparative Example 1

[0053] S1. Austenite hot rolling: Fe-0.4C-7Mn-2Al (mass percentage) medium manganese steel is placed in a high temperature box-type resistance furnace and heated to 1200°C and kept at this temperature for 1 hour, and then hot-rolled into a 5 mm thick billet plate by a high rigidity two-roll hot rolling experimental rolling mill, and then air-cooled to room temperature;

[0054] S2 two-phase zone heating: the blank plate obtained in step S1 was cut into 10cm long plates by wire EDM, the cut plates were placed in a 650℃ high temperature furnace, kept warm for 1h, and air cooled to room temperature;

[0055] S3 cold rolling: The sheet obtained in step S2 is cold rolled to a thickness of 1.6 mm, and the cold rolled sheet is cut into a dog bone shape by wire electric discharge cutting to obtain a stretched piece;

[0056] S4. Variable temperature heating: The drawn part obtained in step S3 is placed in a 650°C high temperature furnace and kept warm for 1 hour. The drawn part is taken out and quenched with water to room temperature to obtain a medium manganese steel with a relatively uniform distribution of austenite chemical composition.

[0057] The effects of the high strength and plasticity austenite chemically heterogeneous medium manganese steel prepared in Example 2 and Comparative Example 1 were verified by the following tests:

[0058] 1. The high strength and plasticity austenite chemically heterogeneous medium manganese steel obtained in Example 2 was subjected to EDS line scanning to analyze the microstructure of the medium manganese steel (heated from 650°C to 670°C) after annealing in the critical temperature region and the distribution of Mn in austenite. Figure 2 shown.

[0059] Depend on Figure 2 It can be seen that the Mn content inside the austenite grains is high and the Mn content at the grain boundaries is low. The strength-ductility product of the medium manganese steel annealed in the isothermal critical zone is 64GPa%, and the strength-ductility product of the solute element heterogeneous medium manganese steel prepared by variable temperature critical zone annealing is 74GPa%. The upper and lower yield point difference and the Luders strain of the chemically heterogeneous medium manganese steel prepared by variable temperature critical zone annealing are both smaller than those of the isothermal critical zone annealed medium manganese steel. It is shown that the variable temperature (continuous heating / cooling) critical zone annealing process proposed by the present invention is an effective preparation technology for the chemical heterogeneity distribution of austenite, and can improve the comprehensive mechanical properties of medium manganese steel.

[0060] 2. The high strength and plasticity austenite chemically heterogeneous medium manganese steel provided in Example 2 and Comparative Example 1 was subjected to a tensile test: the tensile test was carried out on the specimens at room temperature using an Instron 3382 universal material testing machine, the tensile rate was 0.0075 mm / s, the total length of the specimen was 60 mm, and the gauge length was 14 mm.

[0061] Depend on Figure 3 It can be seen that the difference between the upper and lower yield strengths of the medium manganese steel with high strength and plasticity austenite composition chemical heterogeneity after temperature variation treatment in Example 2 is lower than that of the medium manganese steel in Comparative Example 1, and the yield platform of Example 2 is shorter than that of Comparative Example 1, indicating that the heat treatment process in the critical zone of temperature variation reduces the Luders strain and alleviates the expansion of the Luders band; the number of PLC bands of the medium manganese steel in Example 2 is more than that in Comparative Example 1, and the types of PLC bands after temperature variation treatment are significantly different, which is related to the changes in strain rate and temperature, and its PLC band also shows stress serrations (up to tens of MPa).

[0062] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for preparing a medium manganese steel with high strength and plasticity and chemical heterogeneity of austenite composition, characterized in that: The steps include: S1. Hot rolling in austenite zone: the medium manganese steel is placed in a high-temperature box-type resistance furnace and heated to a certain temperature, and kept warm at this temperature, and hot-rolled into a billet plate with a thickness of 5 mm by a high-rigidity two-roll hot rolling experimental rolling mill, and air-cooled to room temperature; S2, two-phase zone heating: the blank plate obtained in step S1 is cut into 10 cm long plates by electric spark wire cutting, the cut plates are placed in a high temperature furnace, and kept at this temperature for 0.5-3 hours, and then air-cooled to room temperature; S3, cold rolling: cold rolling the sheet obtained in step S2 into a plate, and cutting the cold rolled plate into a dog bone shape by electric spark wire cutting to obtain a stretched part; S4, temperature change treatment: placing the drawn part obtained in step S3 in a high temperature furnace at 650°C-750°C for temperature change treatment, taking out the drawn part, and quenching it with water to room temperature; The chemical composition of the manganese steel material in step S1 is as follows by mass percentage: C: 0.1%-0.5%, Mn: 5%-10%, Al: 0.5%-3%, Zr+Nb+V≤0.5%, and the remainder is Fe and unavoidable impurities; In step S1, the medium manganese steel is heated to 1050-1200° C. and kept at this temperature for 0.5-3 h; In step S2, the temperature of the high temperature furnace is 550-650°C, and the temperature is kept at this temperature for 0.5-3 hours; In step S3, the sheet is cold rolled to a thickness of 1.6 mm; In step S4, the temperature change treatment is to subject the stretched part to a temperature increase treatment or a temperature decrease treatment in the furnace; In step S4, the drawn part obtained in step S3 is placed in a 650°C high temperature furnace, and the temperature is raised to 670-750°C at a rate of 0.33-10°C / min. The drawn part is taken out and quenched in water to room temperature; In step S4, the drawn part obtained in step S3 is placed in a 670-750°C high temperature furnace, and the temperature is lowered to 650°C at a cooling rate of 0.33-10°C / min. The drawn part is taken out and quenched with water to room temperature.

2. A medium manganese steel material, characterized in that: Obtained by the preparation method described in claim 1.

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