Boron microalloyed high-strength ductile cold-rolled medium manganese steel and preparation method thereof

Abstract

The invention discloses a boron microalloyed high-strength plastic cold-rolled medium manganese steel and a preparation method thereof. The chemical composition mass percentage is C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and inevitable impurities. The medium manganese steel is prepared by smelting, forging, hot rolling, softening annealing, pickling, cold rolling, and two-phase critical annealing. By utilizing the competitive segregation relationship between boron and manganese elements at austenite grain boundaries, the grain boundary brittleness of the steel is improved, and the medium manganese steel with a tensile strength of 954-1166MPa, an elongation after fracture of 31.4-49.4%, and a strength-plasticity product of 36.6-47.1GPa·% is obtained, which meets the performance indicators of the third generation of automotive high-strength steel.

Description

Technical Field

[0001] The invention relates to the technical field of automobile high-strength steel preparation, and in particular to a boron microalloyed high-strength plastic cold-rolled medium manganese steel and a preparation method thereof. Background Art

[0002] With the rapid development of the automobile industry, the requirements for lightweight and collision resistance of automobile bodies are becoming higher and higher, and the proportion of advanced high-strength steel plates used in automobile bodies is gradually increasing. Low cost and high strength and plasticity are the basic directions for the development of automobile steel in the future. Compared with high manganese steel (Mn>15%), the third-generation advanced high-strength steel with a manganese content of 4-12wt% - medium manganese steel has significant advantages such as high strength and toughness, low alloy content and low manufacturing cost, and has a very broad application prospect. At present, the typical process flow for producing medium manganese steel is mainly hot rolling-cold rolling-critical annealing. In recent years, scholars' research areas have mainly focused on the reverse phase transformation annealing (ART) heat treatment process of medium manganese steel. The ART process requires that the medium manganese steel be heated to the single-phase region for austenitization, quenched to form a martensitic structure, and finally annealed to cause austenite reverse transformation. However, in order to obtain as much austenite as possible, it is necessary to keep it in the two-phase region for a long time, which is not suitable for large-scale industrial production.

[0003] Mn is the most important alloying element in medium manganese steel and will be enriched in austenite during heat treatment. A large amount of retained austenite and ultrafine ferrite can be obtained through the austenite reversal process. Properly increasing the manganese content in steel can effectively increase the volume fraction and stability of retained austenite and improve the strength and plasticity of steel. However, the Mn element is prone to segregation at grain boundaries and phase interfaces during steel heat treatment, resulting in the initiation of brittle cracks and causing grain boundary brittleness. The higher the concentration of Mn at the grain boundaries in steel, the easier it is to produce intergranular brittle fracture.

[0004] In the prior art, the patent with publication number CN110408861B discloses a cold-rolled high-strength and high-ductility medium manganese steel with a low Mn content and a preparation method thereof, wherein the chemical composition weight percentages are: C: 0.15-0.6%, Mn: 3.0-6.0%, Al: 1.0-3.0%, Si: 0.0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, and the balance is Fe and unavoidable impurities. After hot rolling + cold rolling + continuous annealing, the tensile strength can reach 1061MPa, and the strength-ductility product is 67%. However, the medium manganese steel prepared by this system uses more precious alloy elements such as nickel and chromium, and the preparation cost is relatively high.

[0005] In view of this, it is necessary to design a boron microalloyed high-strength and ductile cold-rolled medium manganese steel and a preparation method thereof to solve the above problems. Summary of the invention

[0006] The object of the present invention is to provide a boron microalloyed high-strength and plastic cold-rolled medium manganese steel and a preparation method thereof, which can effectively improve the strength and plasticity of the medium manganese steel through a reasonable component ratio and a heat treatment process.

[0007] To achieve the above-mentioned purpose of the invention, the present invention provides a boron microalloyed high-strength and ductile cold-rolled medium manganese steel. The chemical composition of the medium manganese steel includes, by mass percentage: C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and unavoidable impurities.

[0008] As a further improvement of the present invention, the microstructure of the boron microalloyed high-strength ductility cold-rolled medium manganese steel is a ferrite+austenite dual-phase structure, and the volume percentage of the austenite content is 22-33.5%.

[0009] The present invention also provides a method for preparing boron microalloyed high-strength ductility cold-rolled medium manganese steel, comprising the following steps:

[0010] S1. The chemical composition of medium manganese steel is as follows: C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and unavoidable impurities; the steel is added into a vacuum induction furnace for smelting and forging to obtain a slab;

[0011] S2, cutting the slab prepared in step S1 into pieces and then hot rolling it, and then air cooling it to room temperature to obtain a hot-rolled plate;

[0012] S3, performing softening annealing on the hot-rolled sheet prepared in step S2, and then cooling to room temperature to obtain an annealed sheet;

[0013] S4, pickling the annealed sheet prepared in step S3 and then cold rolling it to obtain a cold rolled sheet;

[0014] S5, subjecting the cold-rolled sheet prepared in step S4 to critical annealing at a certain temperature for a certain period of time to obtain boron microalloyed high-strength and plastic cold-rolled medium manganese steel.

[0015] As a further improvement of the present invention, in step S5, the critical annealing heat treatment temperature is 675-725° C., and the holding time is 10-60 min.

[0016] As a further improvement of the present invention, in step S1, the forging process is to heat to 1200°C and keep it for 2 hours, the final forging temperature is not less than 900°C, and the slab is forged into a 30 mm thick slab, and then air-cooled to room temperature.

[0017] As a further improvement of the present invention, in step S2, the hot rolling process is to heat to 1200°C and keep warm for 2 hours, the initial rolling temperature is 1150-1250°C, the final rolling temperature is ≥800°C, and a thin plate with a thickness of 4-6 mm is rolled through 5 passes, and then air-cooled to room temperature after rolling.

[0018] As a further improvement of the present invention, in step S3, the softening annealing temperature is 630-670° C., and the holding time is 2-12 hours.

[0019] As a further improvement of the present invention, in step S4, the pickling solution is a mixed solution of hydrochloric acid and water in a volume ratio of 1:3.

[0020] As a further improvement of the present invention, in step S4, the thickness of the cold-rolled plate is 1.8 to 2.5 mm.

[0021] As a further improvement of the present invention, the boron microalloyed high-strength and ductile cold-rolled medium manganese steel prepared according to the preparation method of the boron microalloyed high-strength and ductile cold-rolled medium manganese steel has a tensile strength of 954 to 1166 MPa, an elongation after fracture of 31.4 to 49.4%, and a strength-ductility product of 36.6 to 47.1 GPa·%.

[0022] The beneficial effects of the present invention are:

[0023] 1. The present invention improves the hardenability of steel by utilizing boron to segregate at grain boundaries, fill defects, and reduce grain boundary energy. At the same time, the contents of manganese and boron are limited, and the competitive segregation relationship between boron and manganese at austenite grain boundaries is utilized to effectively reduce the segregation of Mn at grain boundaries, improve the grain boundary brittleness of steel, and avoid the formation of boron phase with carbon, nitrogen and other elements in steel during heat treatment due to a large deviation in the use ratio of manganese and boron, thereby deteriorating the toughness and plasticity of steel. At the same time, critical annealing treatment at a certain temperature and a certain time is utilized to effectively enhance the strength and plasticity of medium manganese steel, so as to obtain medium manganese steel with a tensile strength of 954-1166MPa, an elongation after fracture of 31.4-49.4%, and a strength-plasticity product of 36.6-47.1GPa·%, which meets the performance indicators of the third generation of automotive high-strength steel.

[0024] 2. The present invention adopts a reasonable component ratio and heat treatment process, and can prepare medium manganese steel that meets the performance indicators of the third-generation automotive high-strength steel on the basis of reducing part of the manganese content in the steel, thereby effectively reducing the production cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1The SEM microstructure images of the medium manganese steels prepared in Examples 1(a), 2(b), 3(c) and 4(d) of the present invention.

[0026] Figure 2 1, 2, 3 and 4 are XRD patterns of the samples of Examples 1, 2, 3 and 4 of the present invention.

[0027] Figure 3 1 and 2 are stress-strain curves of the samples of Examples 1, 2, 3 and 4 of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical solutions and advantages of the present invention more clear, the present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0029] It should also be noted that, in order to avoid obscuring the present invention due to unnecessary details, only structures and / or processing steps closely related to the scheme of the present invention are shown in the drawings, while other details that are not closely related to the present invention are omitted.

[0030] In addition, it should be noted that the terms "comprises", "includes" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or apparatus.

[0031] The invention provides a boron microalloyed high-strength and plastic cold-rolled medium manganese steel. The chemical composition of the medium manganese steel comprises, by mass percentage, C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and inevitable impurities; the microstructure of the boron microalloyed high-strength and plastic cold-rolled medium manganese steel is a ferrite+austenite dual-phase structure, and the volume percentage of the austenite content is 22-33.5%.

[0032] The present invention also provides a method for preparing boron microalloyed high-strength ductility cold-rolled medium manganese steel, comprising the following steps:

[0033] S1. The chemical composition of medium manganese steel is as follows: C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and unavoidable impurities; the steel is added into a vacuum induction furnace for smelting and forging to obtain a slab with a thickness of 30 mm, and then air-cooled to room temperature; wherein the forging process is heating to 1200°C and keeping warm for 2 hours, and the final forging temperature is not less than 900°C;

[0034] S2, cutting the slab prepared in step S1 into pieces and then hot rolling it, and then air cooling it to room temperature to obtain a hot-rolled plate;

[0035] S3, performing soft annealing on the hot-rolled sheet prepared in step S2 at an annealing temperature of 630-670° C. and a holding time of 2-12 h, and then cooling to room temperature to obtain an annealed sheet;

[0036] S4, pickling the annealed sheet obtained in step S3 with a mixed solution of hydrochloric acid and water in a volume ratio of 1:3, and then cold rolling to obtain a cold-rolled sheet with a thickness of 1.8 to 2.5 mm;

[0037] S5. The cold-rolled sheet prepared in step S4 is subjected to intercritical annealing treatment at a temperature of 675-725° C. and a holding time of 10-60 min to obtain a boron microalloyed high-strength and plastic cold-rolled medium manganese steel.

[0038] Specifically, in step S2, the hot rolling process is to heat to 1200°C and keep warm for 2h, the initial rolling temperature is 1150-1250°C, the final rolling temperature is ≥800°C, and the sheet is rolled into a thin plate with a thickness of 4-6mm through 5 passes, and then air-cooled to room temperature after rolling.

[0039] Specifically, the boron microalloyed high-strength and ductile cold-rolled medium manganese steel prepared according to the preparation method of the boron microalloyed high-strength and ductile cold-rolled medium manganese steel has a tensile strength of 954-1166 MPa, an elongation after fracture of 31.4-49.4%, and a strength-ductility product of 36.6-47.1 GPa·%.

[0040] The boron microalloyed high-strength and ductile cold-rolled medium manganese steel and the preparation method thereof provided by the present invention are described below in conjunction with specific embodiments.

[0041] Example 1

[0042] This embodiment provides a method for preparing boron microalloyed high-strength ductility cold-rolled medium manganese steel, comprising the following steps:

[0043] S1. The chemical composition of medium manganese steel is as follows: C: 0.25%, Mn: 5.68%, Al: 1.15%, Si: 0.39%, P: 0.003%, S: 0.003%, N: 0.0012%, B: 0.0027%, and the balance is Fe and unavoidable impurities; the steel is added into a vacuum induction furnace for smelting and forging to obtain a slab with a thickness of 30 mm, and then air-cooled to room temperature; wherein the forging process is heating to 1200°C and keeping warm for 2 hours, and the final forging temperature is not less than 900°C;

[0044] S2, the slab prepared in step S1 is cut into pieces and then Hot rolling was carried out on a multifunctional two-roll experimental rolling mill, and then air-cooled to room temperature to obtain a hot-rolled plate. The hot rolling process was to heat the forging billet to 1200°C and keep it for 2 hours, and then hot-roll it into a steel plate with a thickness of 5 mm through 5 passes. The initial rolling temperature was 1150°C and the final rolling temperature was about 800°C.

[0045] S3, the hot-rolled sheet prepared in step S2 is subjected to softening annealing in a KF1400 box furnace at a temperature of 650° C. for a holding time of 12 h, and then cooled to room temperature to obtain an annealed sheet;

[0046] S4, the annealed plate prepared in step S3 is pickled with a mixed solution of hydrochloric acid and water in a volume ratio of 1:3. Cold rolling was carried out on a multifunctional two-roll experimental rolling mill to obtain a cold-rolled plate with a thickness of 1.8 mm;

[0047] S5. The cold-rolled sheet prepared in step S4 is subjected to intercritical annealing in a KF1400 box furnace at a temperature of 675° C. for 30 min, and then air-cooled to room temperature to obtain a boron microalloyed high-strength and plastic cold-rolled medium manganese steel.

[0048] Embodiments 2 to 4

[0049] Examples 2 to 4 respectively provide a method for preparing a boron microalloyed high-strength and ductile cold-rolled medium manganese steel. Compared with Example 1, Examples 2 to 4 adjust the critical annealing temperature and time to 700°C, 30 min, 700°C, 60 min and 725°C, 10 min, respectively.

[0050] Comparative Examples 1 to 3

[0051] Comparative Examples 1 to 3 respectively provide a method for preparing a boron microalloyed high-strength and ductile cold-rolled medium manganese steel. The chemical composition of the medium manganese steel is shown in Table 1. The remaining steps are consistent with those in Example 2 and are not repeated here.

[0052] Table 1 Chemical composition (wt%) of boron microalloyed high strength ductility cold rolled medium manganese steel of Comparative Examples 1 to 3

[0053] element C Mn Al Si P S N B Fe Comparative Example 1 0.23 9.75 1.38 0.40 0.003 0.003 0.002 0.0031 Bal. Comparative Example 2 0.23 3.86 1.40 0.39 0.004 0.003 0.002 0.003 Bal. Comparative Example 3 0.22 5.53 1.43 0.38 0.003 0.004 0.002 / Bal.

[0054] The mechanical properties of the boron microalloyed high-strength ductility cold-rolled medium manganese steels prepared in Examples 1 to 4 and Comparative Examples 1 to 3 were tested, and the relevant test results are shown in Table 2. The SEM microstructure morphology of the boron microalloyed high-strength ductility cold-rolled medium manganese steels prepared in Examples 1 to 4 is as follows: Figure 1 The XRD characterization results are shown in Figure 2 The engineering stress-strain curve is shown in Figure 3 shown.

[0055] Table 2 Mechanical properties test results of boron microalloyed high strength and ductility cold rolled medium manganese steel prepared in Examples 1 to 4 and Comparative Examples 1 to 3

[0056]

[0057] Depend on Figure 1-Figure 3 It can be seen that the morphology of the medium manganese steel prepared in Examples 1 to 4 has an obvious concave-convex structure, in which the protruding bright white structure is austenite (γ), and the concave gray structure is ferrite (α). With the increase of annealing temperature and the extension of annealing time, the austenite grains gradually increase and the size continues to increase, presenting an equiaxed shape. In Example 1, due to the low annealing temperature, there are finely dispersed carbides, and as the temperature rises, the carbides continue to dissolve until they disappear completely. The higher strength comes from the mechanical instability of the retained austenite during the stretching process, the TRIP effect generates martensite, and the chemical instability of the two-phase region reversely transforming austenite during the annealing cooling process generates martensite, and the stress-strain curves of Examples 1-4 are jagged, with good strength and plasticity.

[0058] It can be seen from Table 2 that, compared with Example 2, when the Mn content increases, resulting in an imbalance in the relative proportion of Mn and B, the total elongation and strength-ductility product of the medium manganese steel prepared in Comparative Example 1 are significantly reduced, and the plasticity is poor, which cannot meet the use requirements of the third-generation automotive high-strength steel; and when the Mn content decreases, resulting in an imbalance in the relative proportion of Mn and B, the strength of the medium manganese steel prepared in Comparative Example 2 is lower; when B is not added, the strength-ductility product is still somewhat different from the standard of the third-generation automotive high-strength steel.

[0059] In summary, the boron microalloyed high-strength and plastic cold-rolled medium manganese steel and the preparation method thereof disclosed in the present invention improve the hardenability of the steel by utilizing boron segregation at the grain boundary, filling defects, and reducing the grain boundary energy, and at the same time, the contents of manganese and boron are limited, and the competitive segregation relationship between the boron element and the manganese element at the austenite grain boundary is utilized to effectively reduce the segregation of Mn at the grain boundary, improve the grain boundary brittleness of the steel, and avoid the formation of a boron phase with elements such as carbon and nitrogen in the steel during the heat treatment process due to a large deviation in the use ratio of manganese and boron, thereby deteriorating the toughness and plasticity of the steel. At the same time, critical annealing treatment at a certain temperature and a certain time is utilized to effectively enhance the strength and plasticity of the medium manganese steel, and a medium manganese steel with a tensile strength of 954-1166MPa, an elongation after fracture of 31.4-49.4%, and a strength-plasticity product of 36.6-47.1GPa·% is obtained, which meets the performance indicators of the third generation of automotive high-strength steel. The present invention adopts a reasonable component ratio and a heat treatment process, and can prepare medium manganese steel that meets the performance index of the third generation of automotive high-strength steel on the basis of reducing part of the manganese content in the steel, thereby effectively reducing the production cost.

[0060] 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 the technical solution of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. A boron microalloyed high-strength ductility cold-rolled medium manganese steel, characterized in that: The chemical composition of the medium manganese steel includes, by mass percentage, C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and unavoidable impurities; The method for preparing the boron microalloyed high-strength ductility cold-rolled medium manganese steel comprises the following steps: S1. The chemical composition of medium manganese steel is as follows: C: 0.15-0.25%, Mn: 4.5-6%, Al: 1.0-1.5%, Si: 0.3-0.4%, P: <0.005%, S: <0.005%, N: <0.004%, B: 0.001-0.005%, and the balance is Fe and unavoidable impurities; the steel is added into a vacuum induction furnace for smelting and forging to obtain a slab; S2, cutting the slab prepared in step S1 into pieces and then hot rolling it, and then air cooling it to room temperature to obtain a hot-rolled plate; S3, performing softening annealing on the hot-rolled sheet prepared in step S2, and then cooling to room temperature to obtain an annealed sheet; S4, pickling the annealed sheet prepared in step S3 and then cold rolling it to obtain a cold rolled sheet; S5. The cold-rolled sheet prepared in step S4 is critically annealed at a certain temperature for a certain time to obtain boron microalloyed high-strength and plastic cold-rolled medium manganese steel; the critical annealing heat treatment temperature is 675-725° C., and the holding time is 10-60 min.

2. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: The microstructure of the boron microalloyed high-strength plastic cold-rolled medium manganese steel is a ferrite+austenite dual-phase structure, and the volume percentage of the austenite content is 22-33.5%.

3. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: In step S1, the forging process is to heat to 1200°C and keep it for 2 hours, the final forging temperature is not less than 900°C, forging into a slab with a thickness of 30 mm, and then air cooling to room temperature.

4. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: In step S2, the hot rolling process is to heat to 1200°C and keep warm for 2h, the initial rolling temperature is 1150-1250°C, the final rolling temperature is ≥800°C, and the sheet is rolled into a thin plate with a thickness of 4-6mm through 5 passes, and then air-cooled to room temperature after rolling.

5. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: In step S3, the softening annealing temperature is 630-670° C., and the holding time is 2-12 hours.

6. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: In step S4, the pickling solution is a mixed solution of hydrochloric acid and water in a volume ratio of 1:

3.

7. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 6, characterized in that: The thickness of the cold-rolled sheet is 1.8 to 2.5 mm.

8. The boron microalloyed high-strength and ductile cold-rolled medium manganese steel according to claim 1, characterized in that: The boron microalloyed high-strength and ductile cold-rolled medium manganese steel prepared according to the preparation method of the boron microalloyed high-strength and ductile cold-rolled medium manganese steel has a tensile strength of 954-1166 MPa, an elongation after fracture of 31.4-49.4%, and a strength-ductility product of 36.6-47.1 GPa%.

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