1900MPa-grade cold-rolled medium manganese steel and preparation method thereof

Abstract

The invention provides 1900MPa-grade cold-rolled medium manganese steel and a preparation method thereof, and relates to the technical field of high-strength and high-toughness automobile steel. The thickness of the 1900 MPa-grade cold-rolled medium manganese steel ranges from 1.3 mm to 1.5 mm, and the 1900 MPa-grade cold-rolled medium manganese steel is prepared from, by mass, 0.22%-0.25% of C, 6.5%-7.5% of Mn, 1.2%-1.7% of Al, 0.7%-0.9% of Si and Plt. 0.005%, Slt; 0.02% of the total weight; and the balance of Fe and inevitable impurities. The preparation method comprises the steps of raw material preparation, smelting, casting, forging, hot rolling, two-phase region annealing, tempering, cold rolling, quenching and baking treatment. According to the invention, by optimizing the two-phase region annealing process and the quenching temperature, ultrafine-grained ferrite, austenite and martensite microstructures are formed, the strength of the medium manganese steel is greatly improved, the enough percentage elongation after fracture is ensured, and the performance requirements of automobile anti-collision structural components are met.

Description

Technical Field

[0001] The invention relates to the technical field of ferritic stainless steel, in particular to a 1900MPa grade cold-rolled medium manganese steel and a preparation method thereof. Background Art

[0002] The contradiction between automobile lightweighting and safety has always been one of the problems faced by the automobile manufacturing industry. Therefore, ultra-high-strength steel plates are increasingly used in the manufacture of automobile structural parts. Currently, advanced high-strength steel has developed to the third generation, which usually exhibits an excellent combination of strength and elongation.

[0003] Among them, medium manganese steel has become one of the third generation advanced high strength steels with the most development potential due to its moderate cost, excellent strength and plasticity. The excellent combination of strength and plasticity of medium manganese steel is widely believed to be achieved by the TRIP effect or TWIP effect of metastable austenite in medium manganese steel. The strength level of many medium manganese steels is not high enough (1000-1500MPa), but the elongation can reach 20-30%, or even higher.

[0004] However, for automotive anti-collision structural parts, high strength is generally more important than high elongation. Many medium manganese steels sacrifice too much strength to obtain higher elongation, thereby obtaining a higher strength-plasticity product. In particular, there are currently few methods for preparing ultra-high strength medium manganese steels, and for steel plates greater than 1800MPa, the C content is usually high, which will sharply deteriorate weldability. Therefore, it is of great significance to prepare an ultra-high strength medium manganese steel under the premise of controlling the C content.

[0005] Chinese patent CN108330406A discloses a 44GPa·% high-strength and plasticity cold-rolled medium manganese steel and its preparation method, which is obtained not only by adding alloy elements but also by improving the preparation method; although it helps to improve strength and plasticity to a certain extent, the tensile strength is still not high and cannot reach the 1900MPa level. It is easy to break when subjected to tensile force, and the high-cost alloy elements increase the production cost.

[0006] Chinese patent CN109680130A discloses a high-strength and high-plasticity cold-rolled medium manganese steel and its preparation method, which is prepared by smelting + forging + hot rolling + two-phase zone annealing + cold rolling + annealing + low-temperature tempering, and at the same time utilizes microalloying technology of adding trace Zr elements to make the cold-rolled medium manganese TRIP steel with low C and Al contents achieve high strength and high plasticity; however, the tensile strength can still reach 1900MPa level; and the two-phase zone annealing is directly cold rolled without quenching and baking.

[0007] Chinese patent CN113862579A discloses an ultra-low carbon medium manganese steel and its preparation method, in which the manganese content in the composition is increased and high-cost alloy elements are added, all of which increase the cost; and although the preparation method is simple, the tensile strength and elongation are low, and the impact resistance is poor; the surface hardness is also poor, and the material is prone to breakage during impact. Summary of the invention

[0008] In order to solve the technical defects of the prior art in the preparation of medium manganese steel for automobiles, which makes it have low tensile strength, high elongation and high strength-plasticity product, although this technical defect can improve the impact resistance, it is easy to break during the stretching process, making it difficult to effectively use medium manganese steel for automobiles in the mechanical processing process; and in order to meet the above-mentioned needs, the prior art adjusts the composition and increases the process steps by adding high-cost multiple types of alloy elements. These methods increase the production cost and cannot increase the tensile strength level to 1900MPa, reducing the scope of application of medium manganese steel. The present invention proposes a 1900MPa grade cold-rolled medium manganese steel and a preparation method that can solve the above-mentioned defects. The technical scheme is as follows:

[0009] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.3-1.5mm, and the composition contents are as follows by mass percentage: C 0.22-0.25%, Mn 6.5-7.5%, Al 1.2-1.7%, Si 0.7-0.9%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0010] Optionally, the microstructure of the 1900MPa grade cold-rolled medium manganese steel consists of 2-5% by volume of ultrafine-grained austenite + 20-25% by volume of ferrite + 67-76% by volume of martensite + 1-2% by volume of nano-Cr-rich carbide.

[0011] Optionally, in the 1900MPa grade cold-rolled medium manganese steel, the ultrafine-grained austenite is block-shaped with an average grain size of 1.5-3μm; the ferrite is spherical with an average grain size of 1-2μm; the martensite is lath-shaped with an average lath width of 1-2μm; and the nano-Cr-rich carbide is spherical with an average size of 100-200nm.

[0012] Optionally, the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1850-1950MPa, a yield strength of 1380-1450MPa, and an elongation after fracture of 11-13%.

[0013] Optionally, the 1900MPa grade cold-rolled medium manganese steel has a yield strength ratio of 0.730-0.738 and a strength-ductility product of 21.900-25.500GPa·%.

[0014] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0015] S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, melting and casting the weighed raw materials to obtain ingots of medium manganese steel;

[0016] S2, forging: forging the medium manganese steel ingot in S1 to obtain a forged billet of medium manganese steel;

[0017] S3, hot rolling: removing the surface oxide scale of the medium manganese steel forging billet in S2, and then hot rolling to obtain a hot-rolled medium manganese steel slab;

[0018] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering to obtain a tempered medium manganese steel hot-rolled plate;

[0019] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling to obtain a cold-rolled medium manganese steel plate;

[0020] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate to obtain a quenched medium manganese steel plate;

[0021] S7, baking: baking the S6 quenched medium manganese steel plate to obtain 1900MPa grade cold rolled medium manganese steel.

[0022] Optionally, the melting temperature of the manganese steel in S1 is 1600-1650°C, the casting temperature is 1500-1550°C, the ingot shape is cylindrical, and the size is

[0023] Optionally, the forging temperature in S2 is 900-950°C, and the shape of the forging blank of the medium manganese steel is a rectangular parallelepiped with a cross-sectional size of 40×80 mm.

[0024] Optionally, S3 needs to be kept at 1200±30℃ for 2-3h before hot rolling, the starting rolling temperature of hot rolling is 1250±30℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 2.8-3.2mm through 7 passes. After rolling, it is air-cooled to 450±20℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the reduction of each pass is 30-33%.

[0025] Optionally, the temperature of S4 two-phase region annealing is 680-720°C, the holding time is 50-70 minutes, and air cooling is performed to room temperature; the temperature of tempering is 160-180°C, and the holding time is 15-20 minutes.

[0026] Optionally, the S5 cold rolling adopts a large reduction at one time, with a total reduction of 50-57%, and the thickness of the cold-rolled medium manganese steel plate is 1.3-1.5 mm.

[0027] Optionally, the temperature of S6 quenching is 820-900°C, the holding time is 4-6min, and the quenching depth is 0.65-0.75mm.

[0028] Optionally, the baking temperature in S7 is 160-180° C., and the holding time is 15-20 min; the yield strength of the medium manganese steel plate after baking is increased by 8-10% and the elongation after fracture is increased by 200-300% compared with the medium manganese steel plate before baking.

[0029] Compared with the prior art, the above technical solution has at least the following beneficial effects:

[0030] The above scheme, the present invention proposes a 1900MPa grade cold-rolled medium manganese steel and a preparation method, which can solve the technical defects of the prior art in the preparation of medium manganese steel for automobiles, such as low tensile strength, high elongation, and high strength-plasticity product. Although this technical defect can improve the impact resistance, it is easy to break during the stretching process, making it difficult to effectively use the medium manganese steel for automobiles in the mechanical processing process; and in order to meet the above-mentioned needs, the prior art adjusts the composition and increases the process steps by adding high-cost multiple types of alloy elements. These methods increase the production cost and cannot increase the tensile strength level to 1900MPa, reducing the scope of application of the medium manganese steel.

[0031] The present invention does not add high-cost multiple types of alloy elements to adjust the composition. Instead, under the condition of controlling the C element content, Cr-rich carbides are precipitated through two-phase zone annealing before cold rolling to reduce the yield strength of the hot-rolled plate and reduce the rolling force requirement; tempering is used to reduce the interface stress between the carbide and the matrix to prevent the occurrence of microcracks.

[0032] The present invention produces local chemical composition differences by quenching the cold-rolled plate for a short time, and realizes gradient TRIP effect by adjusting local stacking fault energy, thereby avoiding plasticity reduction caused by excessive concentration of TRIP effect.

[0033] The present invention obtains a microstructure composed of ultrafine-grained austenite+ferrite+martensite+nano Cr-rich carbide after quenching, and can produce bake hardening after baking treatment, and the yield strength and elongation can be further improved.

[0034] The 1900MPa cold-rolled medium manganese steel prepared by the present invention can maintain the same level of mechanical properties after quenching in a larger temperature range.

[0035] The 1900MPa grade cold-rolled medium manganese steel of the present invention is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1850-1950MPa, a yield strength of 1380-1450MPa, and an elongation after fracture of 11-13%.

[0036] In summary, compared with other traditional methods, the method of the present invention obtains medium manganese steel with improved comprehensive properties such as strength, plasticity and impact resistance through process steps such as two-phase zone annealing + tempering before cold rolling, short-time quenching treatment of cold-rolled plates, and baking treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, wide in application range, and conducive to large-scale industrial production and promotion. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0038] Figure 1 It is a comparison diagram of engineering stress-strain curves of a 1900MPa grade cold-rolled medium manganese steel plate tensile test according to Examples 1-3 of the present invention;

[0039] Figure 2 This is the EBSD phase distribution diagram of a 1900MPa grade cold-rolled medium manganese steel in Example 1 of the present invention, in which: red is BCC, blue is FCC, and yellow is M 7 C 3 ;

[0040] Figure 3 This is the EBSD phase distribution diagram of a 1900MPa grade cold-rolled medium manganese steel in Example 2 of the present invention, in which: red is BCC, blue is FCC, and yellow is M 7 C 3 ;

[0041] Figure 4 This is the EBSD phase distribution diagram of a 1900MPa grade cold-rolled medium manganese steel in Example 3 of the present invention, in which: red is BCC, blue is FCC, and yellow is M 7 C 3 . DETAILED DESCRIPTION

[0042] The technical solution of the present invention is described below in conjunction with the accompanying drawings.

[0043] In the embodiments of the present invention, words such as "exemplarily" and "for example" are used to indicate examples, illustrations or explanations. Any embodiment or design described as "example" in the present invention should not be interpreted as being more preferred or more advantageous than other embodiments or designs. Specifically, the use of the word "example" is intended to present the concept in a specific way. In addition, in the embodiments of the present invention, the meaning expressed by "and / or" can be both, or it can be either of the two.

[0044] In the embodiments of the present invention, "image" and "picture" may sometimes be used interchangeably. It should be noted that when the distinction between them is not emphasized, the meanings they intend to express are consistent.

[0045] In the embodiments of the present invention, sometimes the subscripts such as W 1 It may be written in non-subscript form such as W1. When the difference is not emphasized, the meaning is the same.

[0046] In order to make the technical problems, technical solutions and advantages to be solved by the present invention more clear, a detailed description will be given below with reference to the accompanying drawings and specific embodiments.

[0047] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.3-1.5mm, and the composition contents are as follows by mass percentage: C 0.22-0.25%, Mn 6.5-7.5%, Al 1.2-1.7%, Si 0.7-0.9%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0048] In particular, the organizational structure of the 1900MPa grade cold-rolled medium manganese steel consists of 2-5% by volume ultrafine grain austenite + 20-25% by volume ferrite + 67-76% by volume martensite + 1-2% by volume nano Cr-rich carbide.

[0049] In particular, in the 1900MPa grade cold-rolled medium manganese steel, the ultrafine-grained austenite is block-shaped with an average grain size of 1.5-3μm; the ferrite is spherical with an average grain size of 1-2μm; the martensite is lath-shaped with an average lath width of 1-2μm; and the nano-Cr-rich carbide is spherical with an average size of 100-200nm.

[0050] In particular, the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1850-1950MPa, a yield strength of 1380-1450MPa, and an elongation after fracture of 11-13%.

[0051] Optionally, the 1900MPa grade cold-rolled medium manganese steel has a yield strength ratio of 0.730-0.738 and a strength-ductility product of 21.900-25.500GPa·%.

[0052] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0053] S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, melting and casting the weighed raw materials to obtain ingots of medium manganese steel;

[0054] S2, forging: forging the medium manganese steel ingot in S1 to obtain a forged billet of medium manganese steel;

[0055] S3, hot rolling: removing the surface oxide scale of the medium manganese steel forging billet in S2, and then hot rolling to obtain a hot-rolled medium manganese steel slab;

[0056] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering to obtain a tempered medium manganese steel hot-rolled plate;

[0057] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling to obtain a cold-rolled medium manganese steel plate;

[0058] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate to obtain a quenched medium manganese steel plate;

[0059] S7, baking: baking the S6 quenched medium manganese steel plate to obtain 1900MPa grade cold rolled medium manganese steel.

[0060] In particular, the melting temperature of S1 manganese steel is 1600-1650℃, the casting temperature is 1500-1550℃, the ingot shape is cylindrical, and the size is

[0061] In particular, the forging temperature in S2 is 900-950°C, and the forging billet shape of the medium manganese steel is a rectangular parallelepiped with a cross-sectional size of 40×80 mm.

[0062] In particular, S3 needs to be kept at 1200±30℃ for 2-3h before hot rolling, the starting rolling temperature of hot rolling is 1250±30℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 2.8-3.2mm through 7 passes. After rolling, it is air-cooled to 450±20℃ and then water-cooled to room temperature. Hot rolling adopts multiple passes with small reduction, and the reduction of each pass is 30-33%.

[0063] In particular, the temperature of S4 two-phase region annealing is 680-720°C, the holding time is 50-70min, and air cooling to room temperature; the temperature of tempering is 160-180°C, and the holding time is 15-20min.

[0064] In particular, S5 cold rolling adopts a large reduction at one time, with a total reduction of 50-57%, and the thickness of the cold-rolled medium manganese steel plate is 1.3-1.5 mm.

[0065] In particular, the S6 quenching temperature is 820-900°C, the holding time is 4-6min, and the quenching depth is 0.65-0.75mm.

[0066] In particular, the S7 baking temperature is 160-180°C, and the holding time is 15-20min; the yield strength of the baked medium manganese steel plate is increased by 8-10% and the elongation after fracture is increased by 200-300% compared with the medium manganese steel plate before baking.

[0067] Example 1

[0068] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0069] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0070] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0071] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0072] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1200±30℃ for 2h, the starting rolling temperature of hot rolling is 1250℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 450℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0073] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 720°C, the holding time is 60 minutes, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20 minutes; and the tempered medium manganese steel hot-rolled plate is obtained;

[0074] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0075] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate at a quenching temperature of 820° C., a holding time of 5 min, and a quenching depth of 0.75 mm to obtain a quenched medium manganese steel plate;

[0076] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0077] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 2% by volume of ultrafine-grained austenite + 20% by volume of ferrite + 67% by volume of martensite + 1% by volume of nano-Cr-rich carbide.

[0078] In the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 2.5μm; the ferrite is spherical with an average grain size of 1.8μm; the martensite is lath-shaped with an average lath width of 1.4μm; the nano-Cr-rich carbide is spherical with an average size of 135nm.

[0079] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1939MPa, a yield strength of 1431MPa, a yield strength ratio of 0.738, an elongation after fracture of 11.5%, and a strength-ductility product of 22.299GPa·%.

[0080] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 8.3% and the elongation after fracture is increased by 270% compared with the medium manganese steel plate before baking.

[0081] Example 2

[0082] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0083] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0084] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0085] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0086] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1210℃ for 2h, the starting rolling temperature of hot rolling is 1250℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 450℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0087] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 700°C, the holding time is 60 minutes, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20 minutes; and the tempered medium manganese steel hot-rolled plate is obtained;

[0088] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0089] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate at a quenching temperature of 840° C., a holding time of 5 min, and a quenching depth of 0.75 mm to obtain a quenched medium manganese steel plate;

[0090] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0091] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 4.5% by volume of ultrafine-grained austenite + 22% by volume of ferrite + 72% by volume of martensite + 1.5% by volume of nano-Cr-rich carbide.

[0092] In the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 2.3μm; the ferrite is spherical with an average grain size of 1.2μm; the martensite is lath-shaped with an average lath width of 1.6μm; the nano-Cr-rich carbide is spherical with an average size of 164nm.

[0093] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1891MPa, a yield strength of 1432MPa, a yield strength ratio of 0.757, an elongation after fracture of 12.3%, and a strength-ductility product of 23.259GPa·%.

[0094] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 8.3% and the elongation after fracture is increased by 235% compared with the medium manganese steel plate before baking.

[0095] Example 3

[0096] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0097] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0098] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0099] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0100] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1200±30℃ for 2.5h, the starting rolling temperature of hot rolling is 1240℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 460℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0101] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 680°C, the holding time is 60 minutes, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20 minutes; and the tempered medium manganese steel hot-rolled plate is obtained;

[0102] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0103] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate at a quenching temperature of 860° C., a holding time of 5 min, and a quenching depth of 0.75 mm to obtain a quenched medium manganese steel plate;

[0104] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0105] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 2% by volume of ultrafine-grained austenite + 20% by volume of ferrite + 77% by volume of martensite + 1% by volume of nano-Cr-rich carbide.

[0106] In the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 1.7μm; the ferrite is spherical with an average grain size of 2.1μm; the martensite is lath-shaped with an average lath width of 1.3μm; the nano-Cr-rich carbide is spherical with an average size of 194nm.

[0107] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1873MPa, a yield strength of 1373MPa, a yield strength ratio of 0.733, an elongation after fracture of 11.7%, and a strength-ductility product of 21.914GPa·%.

[0108] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 9.5% and the elongation after fracture is increased by 221% compared with the medium manganese steel plate before baking.

[0109] Example 4

[0110] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0111] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0112] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0113] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0114] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1200℃ for 2h, the starting rolling temperature of hot rolling is 1260℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 455℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0115] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 700°C, the holding time is 60 minutes, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20 minutes; and the tempered medium manganese steel hot-rolled plate is obtained;

[0116] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0117] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate at a quenching temperature of 880° C., a holding time of 5 min, and a quenching depth of 0.75 mm to obtain a quenched medium manganese steel plate;

[0118] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0119] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 3.6% by volume of ultrafine-grained austenite + 23% by volume of ferrite + 72.2% by volume of martensite + 1.2% by volume of nano-Cr-rich carbide.

[0120] In the 900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 1.9μm; the ferrite is spherical with an average grain size of 1.6μm; the martensite is lath-shaped with an average lath width of 1.8μm; the nano-Cr-rich carbide is spherical with an average size of 129nm.

[0121] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1924MPa, a yield strength of 1412MPa, a yield strength ratio of 0.734, an elongation after fracture of 12.6%, and a strength-ductility product of 24.242GPa·%.

[0122] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 9.9% and the elongation after fracture is increased by 272% compared with the medium manganese steel plate before baking.

[0123] Example 5

[0124] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0125] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0126] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0127] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0128] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1200℃ for 3h, the starting rolling temperature of hot rolling is 1220℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 430℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0129] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 690°C, the holding time is 60min, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20min; and the tempered medium manganese steel hot-rolled plate is obtained;

[0130] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0131] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate at a quenching temperature of 890° C., a holding time of 5 min, and a quenching depth of 0.75 mm to obtain a quenched medium manganese steel plate;

[0132] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0133] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 3% by volume ultrafine-grained austenite + 22% by volume ferrite + 73% by volume martensite + 2% by volume nano-Cr-rich carbide.

[0134] In the 900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 2.2μm; the ferrite is spherical with an average grain size of 1.8μm; the martensite is lath-shaped with an average lath width of 1.7μm; the nano-Cr-rich carbide is spherical with an average size of 146nm.

[0135] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1871MPa, a yield strength of 1392MPa, a yield strength ratio of 0.744, an elongation after fracture of 12.9%, and a strength-ductility product of 24.136GPa·%.

[0136] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 8.9% and the elongation after fracture is increased by 271% compared with the medium manganese steel plate before baking.

[0137] Example 6

[0138] A 1900MPa grade cold-rolled medium manganese steel, wherein the thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.5mm, and the composition contents are as follows by mass percentage: C 0.25%, Mn 7.2%, Al 1.5%, Si 0.8%, P<0.005%, S<0.02%; the rest are Fe and unavoidable impurities.

[0139] A method for preparing the 1900MPa grade cold-rolled medium manganese steel, the method for preparing the 1900MPa grade cold-rolled medium manganese steel comprises the following steps:

[0140] S1. Weighing and smelting raw materials: weigh raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, and melt and cast the weighed raw materials. The smelting temperature of the medium manganese steel is 1600℃, and the casting temperature is 1500℃ to obtain an ingot of medium manganese steel; the ingot is cylindrical in shape and has a size of

[0141] S2, forging: forging the medium manganese steel ingot of S1, the forging temperature of the ingot is 900°C, and obtaining a forged billet of medium manganese steel; the forged billet of medium manganese steel is in the shape of a rectangular parallelepiped, and the cross-sectional size is 40×80 mm;

[0142] S3, hot rolling: remove the surface oxide scale of the S2 medium manganese steel forging billet, and then perform hot rolling. Before hot rolling, it needs to be kept at 1200℃ for 3h, the starting rolling temperature of hot rolling is 1250℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 3mm through 7 passes. After rolling, it is air-cooled to 450℃ and then water-cooled to room temperature; hot rolling adopts multiple passes with small reduction, and the total reduction is 92.5% to obtain a hot-rolled medium manganese steel slab;

[0143] S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering, the temperature of the two-phase annealing is 710°C, the holding time is 60 minutes, and the slab is air-cooled to room temperature; the tempering temperature is 170°C, and the holding time is 20 minutes; and the tempered medium manganese steel hot-rolled plate is obtained;

[0144] S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling, the cold rolling adopts a large reduction at one time, and the total reduction is greater than 50%, to obtain a cold-rolled medium manganese steel plate; the thickness of the cold-rolled medium manganese steel plate is 1.5mm;

[0145] S6, quenching: quenching the S5 cold-rolled medium manganese steel plate, the quenching temperature is 900° C., the holding time is 5 min, the quenching depth is 0.75 mm, and a quenched medium manganese steel plate is obtained;

[0146] S7, baking: baking the S6 quenched medium manganese steel plate at a temperature of 170° C. for a holding time of 20 min to obtain 1900 MPa grade cold-rolled medium manganese steel.

[0147] The microstructure of the 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment consists of 2% by volume of ultrafine-grained austenite + 20% by volume of ferrite + 67% by volume of martensite + 1% by volume of nano-Cr-rich carbide.

[0148] In the 900MPa grade cold-rolled medium manganese steel prepared in this embodiment, the ultrafine-grained austenite is block-shaped with an average grain size of 2.1μm; the ferrite is spherical with an average grain size of 1.6μm; the martensite is lath-shaped with an average lath width of 1.9μm; the nano-Cr-rich carbide is spherical with an average size of 198nm.

[0149] The 1900MPa grade cold-rolled medium manganese steel prepared in this embodiment is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1891MPa, a yield strength of 1383MPa, a yield strength ratio of 0.731, an elongation after fracture of 11.9%, and a strength-ductility product of 22.503GPa·%.

[0150] The yield strength of the medium manganese steel plate after baking in this embodiment is increased by 8.3% and the elongation after fracture is increased by 257% compared with the medium manganese steel plate before baking.

[0151] The above scheme, the present invention proposes a 1900MPa grade cold-rolled medium manganese steel and a preparation method, which can solve the technical defects of the prior art in the preparation of medium manganese steel for automobiles, such as low tensile strength, high elongation, and high strength-plasticity product. Although this technical defect can improve the impact resistance, it is easy to break during the stretching process, making it difficult to effectively use the medium manganese steel for automobiles in the mechanical processing process; and in order to meet the above-mentioned needs, the prior art adjusts the composition and increases the process steps by adding high-cost multiple types of alloy elements. These methods increase the production cost and cannot increase the tensile strength level to 1900MPa, reducing the scope of application of the medium manganese steel.

[0152] The present invention does not add high-cost multiple types of alloy elements to adjust the composition. Instead, under the condition of controlling the C element content, Cr-rich carbides are precipitated through two-phase zone annealing before cold rolling to reduce the yield strength of the hot-rolled plate and reduce the rolling force requirement; tempering is used to reduce the interface stress between the carbide and the matrix to prevent the occurrence of microcracks.

[0153] The present invention produces local chemical composition differences by quenching the cold-rolled plate for a short time, and realizes gradient TRIP effect by adjusting local stacking fault energy, thereby avoiding plasticity reduction caused by excessive concentration of TRIP effect.

[0154] The present invention obtains a microstructure composed of ultrafine-grained austenite+ferrite+martensite+nano Cr-rich carbide after quenching, and can produce bake hardening after baking treatment, and the yield strength and elongation can be further improved.

[0155] The 1900MPa cold-rolled medium manganese steel prepared by the present invention can maintain the same level of mechanical properties after quenching in a larger temperature range.

[0156] The 1900MPa grade cold-rolled medium manganese steel of the present invention is characterized in that the 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1850-1950MPa, a yield strength of 1380-1450MPa, and an elongation after fracture of 11-13%.

[0157] In summary, compared with other traditional methods, the method of the present invention obtains medium manganese steel with improved comprehensive properties such as strength, plasticity and impact resistance through process steps such as two-phase zone annealing + tempering before cold rolling, short-time quenching treatment of cold-rolled plates, and baking treatment; the method is simple and easy to operate, green and environmentally friendly, low in cost, short in process, high in efficiency, wide in application range, and conducive to large-scale industrial production and promotion.

[0158] It should be understood that the term "and / or" in this article is only a description of the association relationship of associated objects, indicating that there can be three relationships. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. A and B can be singular or plural. In addition, the character " / " in this article generally indicates that the associated objects before and after are in an "or" relationship, but it may also indicate an "and / or" relationship. Please refer to the context for specific understanding.

[0159] In the present invention, "at least one" means one or more, and "more than one" means two or more. "At least one of the following" or similar expressions refers to any combination of these items, including any combination of single or plural items. For example, at least one of a, b, or c can be represented by: a, b, c, ab, ac, bc, or abc, where a, b, c can be single or multiple.

[0160] It should be understood that in various embodiments of the present invention, the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present invention.

[0161] The above is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art can easily think of changes or substitutions within the technical scope disclosed by the present invention, which should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention should be based on the protection scope of the claims.

Claims

1. A 1900MPa grade cold-rolled medium manganese steel, characterized in that: The thickness of the 1900MPa grade cold-rolled medium manganese steel is 1.3-1.5mm, and the composition content is calculated by mass percentage as follows: C 0.22-0.25%, Mn 6.5-7.5%, Al 1.2-1.7%, Si0.7-0.9%, P<0.005%, S<0.02%; the rest is Fe and unavoidable impurities.

2. The 1900MPa grade cold-rolled medium manganese steel according to claim 1, characterized in that: The organizational structure of the 1900MPa grade cold-rolled medium manganese steel consists of 2-5% by volume ultrafine grain austenite + 20-25% by volume ferrite + 67-76% by volume martensite + 1-2% by volume nano Cr-rich carbide.

3. The 1900MPa grade cold-rolled medium manganese steel according to claim 1, characterized in that: In the 1900MPa grade cold-rolled medium manganese steel, the ultrafine-grained austenite is block-shaped with an average grain size of 1.5-3μm; the ferrite is spherical with an average grain size of 1-2μm; the martensite is lath-shaped with an average lath width of 1-2μm; and the nano-Cr-rich carbide is spherical with an average size of 100-200nm.

4. The 1900MPa grade cold-rolled medium manganese steel according to claim 1, characterized in that: The 1900MPa grade cold-rolled medium manganese steel has a tensile strength of 1850-1950MPa, a yield strength of 1380-1450MPa, and an elongation after fracture of 11-13%.

5. A method for preparing 1900MPa grade cold-rolled medium manganese steel according to any one of claims 1 to 4, characterized in that: The preparation method of the 1900MPa grade cold-rolled medium manganese steel comprises the following steps: S1. Weighing and smelting raw materials: weighing raw materials according to the chemical composition content of 1900MPa grade cold-rolled medium manganese steel, melting and casting the weighed raw materials to obtain ingots of medium manganese steel; S2, forging: forging the medium manganese steel ingot in S1 to obtain a forged billet of medium manganese steel; S3, hot rolling: removing the surface oxide scale of the medium manganese steel forging billet in S2, and then hot rolling to obtain a hot-rolled medium manganese steel slab; S4, two-phase annealing + tempering: the S3 hot-rolled medium manganese steel slab is subjected to two-phase annealing and tempering to obtain a tempered medium manganese steel hot-rolled plate; S5, cold rolling after pickling: pickling the medium manganese steel hot-rolled plate after S4 tempering, and then cold rolling to obtain a cold-rolled medium manganese steel plate; S6, quenching: quenching the S5 cold-rolled medium manganese steel plate to obtain a quenched medium manganese steel plate; S7, baking: baking the S6 quenched medium manganese steel plate to obtain 1900MPa grade cold rolled medium manganese steel.

6. The method for preparing 1900MPa grade cold-rolled medium manganese steel according to claim 5, characterized in that: Before hot rolling, S3 needs to be kept at 1200±30℃ for 2-3h. The starting temperature of hot rolling is 1250±30℃, the final rolling temperature is higher than 900℃, and it is rolled into a plate with a thickness of 2.8-3.2mm through 7 passes. After rolling, it is air-cooled to 450±20℃ and then water-cooled to room temperature. Hot rolling adopts multiple passes with small reduction, and the reduction of each pass is 30-33%.

7. The method for preparing 1900MPa grade cold-rolled medium manganese steel according to claim 5, characterized in that: The temperature of S4 two-phase annealing is 680-720℃, the holding time is 50-70min, and air cooling to room temperature; the temperature of tempering is 160-180℃, and the holding time is 15-20min.

8. The method for preparing 1900MPa grade cold-rolled medium manganese steel according to claim 5, characterized in that: S5 cold rolling adopts a large reduction at one time, with a total reduction of 50-57%, and the thickness of the cold rolled medium manganese steel plate is 1.3-1.5mm.

9. The method for preparing 1900MPa grade cold-rolled medium manganese steel according to claim 5, characterized in that: The S6 quenching temperature is 820-900℃, the holding time is 4-6min, and the quenching depth is 0.65-0.75mm.

10. The method for preparing 1900MPa grade cold-rolled medium manganese steel according to claim 5, characterized in that: The baking temperature of S7 is 160-180°C, and the holding time is 15-20min; the yield strength of the baked medium manganese steel plate is increased by 8-10% and the elongation after fracture is increased by 200-300% compared with the medium manganese steel plate before baking.

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