A thin gauge cold rolled high strength product of specific plasticity medium manganese steel plate for automobile and a manufacturing method thereof

Abstract

The present application relates to a kind of thin gauge cold rolled high strength plastic product of middle manganese steel plate for automobile and its manufacturing method, the chemical composition in steel is C:0.15%~0.40%, Mn:3.0%~10.0%, Si:0.5%~2.0%, Al:1.5%~3.0%, V:0.10%~0.50%, P≤0.01%, S≤0.005%;In addition, add: Ni≤1.0%, Cr≤1.0%, Mo≤0.5%, Cu≤1.0%, and Mn+Ni+Cr+Mo+Cu≤10.0%, Ti≤0.04%, Nb≤0.04%, B≤0.005%, Ca≤0.005%, REM≤0.005%, the balance is Fe and impurity.The present application uses the technical route of continuous casting-hot rolling-pickling-cold rolling-annealing, and the yield strength of more than 600MPa, tensile strength 980MPa above, elongation 30% above, strength plastic product 30GPa·% above, hole expansion ratio 30% above middle manganese steel sheet is manufactured, and product is easy to realize industrial production.

Description

Technical Field

[0001] This invention relates to the field of automotive steel production technology, and in particular to a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles and its manufacturing method. Background Technology

[0002] Currently, the development of automotive lightweighting in the steel materials sector can be broadly divided into two aspects: "high strength and thinning" and "low density." In terms of difficulty, the "high strength and thinning" development path is closer to the current environment of automotive sheet metal production. As exemplified by the concept of "GMP steel," given the unavoidable limitations of high-strength automotive steel sheets in terms of sacrificial forming and other comprehensive performance characteristics, optimization of specific application performance is crucial to achieving personalized user solutions. Examples include upgrading the plasticity of duplex steel, improving the pore-expanding performance of quenched and partitioned steel, and enhancing the toughness of hot-formed steel. With the increasing application ratio of high-strength steel (above 980MPa) in vehicle bodies and the maturing of high-strength steel research and production, Japanese steel materials research institutions, represented by Kobe Steel, Nippon Steel, and JFE, have boldly proposed future steel performance systems based on high strength and plasticity levels: tensile strength × elongation after fracture of 1000MPa × 35%, 1200MPa × 30%, and 1500MPa × 20%. Therefore, there is still a long way to go in developing from the current performance indicators of GMP steel to the performance indicators of steel used in the future.

[0003] Based on the analysis of the microstructure and strength-ductility matching of existing high-strength automotive steels, the performance exhibited by medium-manganese steel sheets is more in line with the future development concept of automotive steel. The main phase composition of medium-manganese steel is ferrite + reversed austenite. During deformation, the reversed austenite undergoes a TRIP effect, transforming into martensite, thereby improving the strength and ductility of the steel sheet. The concept of medium-manganese steel was first proposed at the 2007 International Automotive Steel Development Symposium, where scholars Krupitzer and Heimbuch proposed the development concept of third-generation steel, namely, strength greater than 1 GPa and ductility greater than 30%, and proposed the concept of a 30 GPa·% strength-ductility product. Professor Dong Han's team at the China Iron & Steel Research Institute used 5% Mn content medium-manganese steel as the research object, deeply exploring key issues such as the microstructure, strengthening mechanism, austenite stability behavior, and manganese diffusion. In 2010, they trial-produced hot-rolled medium-manganese steel products at Taiyuan Iron & Steel, with relatively objective performance. At the end of 2015, Baosteel began industrial-scale trial production of cold-rolled products of medium manganese steel, with a strength of 980MPa and a plasticity of over 30%, achieving the industrialization of 30GPa·% steel.

[0004] On the surface, the newly developed 30GPa·% steel meets the requirements of third-generation steel, but upon closer examination, many problems remain. First, the production of this type of steel plate requires bell-type annealing, which greatly reduces the yield of the steel plate; second, the steel plate surface has long Lüders bands, which seriously affect the surface quality of the steel plate; in addition, the addition of about 7% Mn content greatly increases the difficulty of both smelting and subsequent processes compared to conventional steel plates, which is not conducive to the welding of the whole vehicle.

[0005] The aforementioned obstacles to the industrial production of 30GPa·% steel are inextricably linked to the composition and process design of medium manganese steel. Generally, medium manganese steel contains more than 5% Mn to ensure the content and stability of austenite. However, adding more than 5% Mn will cause severe C / Mn segregation during continuous casting, and improper slow cooling will lead to hot cracking of the billet. These factors reduce the yield of medium manganese steel from the raw material end. Furthermore, the properties of medium manganese steel require ART annealing (austenite reversal annealing). According to numerous literature reports, the annealing time for most cold-rolled medium manganese steels is at least 0.5 hours to ensure the diffusion of Mn atoms and improve the stability of the austenite phase. However, this is clearly unsuitable for the length of continuous annealing production lines.

[0006] Overall, medium manganese steel is capable of handling most complex body structural components, but how to solve the many technical bottlenecks of medium manganese steel is the key to whether it can be truly applied.

[0007] Chinese invention patent CN109680130B discloses "A high-strength, high-ductility cold-rolled medium-manganese steel and its preparation method." The steel plate's components by weight percentage are: C: 0.2%, Mn: 7-9%, Al: 1.5%, Zr: 0.08-0.10%, P≤0.008%, S≤0.008%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, annealing + low-temperature tempering processes, yielding a cold-rolled annealed steel plate with a tensile strength greater than 1200 MPa and an elongation greater than 50-56%. This steel plate, with its excellent mechanical properties, is clearly suitable for many complex vehicle body parts. However, its composition and process design are incompatible with existing production line conditions, making industrial production impossible. For example, annealing temperatures of 660–680°C can only be achieved through bell-type annealing, and a temperature difference of 20°C cannot match the temperature fluctuation range of bell-type annealing. This will result in a significant reduction in yield and a substantial increase in production costs. Furthermore, annealing times of 10–20 minutes cannot be achieved in industrial production.

[0008] Chinese invention patent CN110117755B discloses a method for preparing a 980MPa grade low yield strength ratio cold-rolled medium manganese steel. The steel's alloy composition is: C: 0.09–0.12%, Si: 0.1–0.3%, Mn: 4.8–7.20%, Al: 0.02–0.05%, P≤0.02%, S≤0.003%, with the balance being Fe and other unavoidable impurities. The preparation method includes smelting, hot rolling, pickling, cold rolling, and continuous annealing processes to obtain a steel plate with a tensile strength of over 980MPa. However, examples show that the elongation of this steel plate is only around 23%, below 30%. Compared to the mature, industrially produced QP980 cold-rolled or galvanized steel plate, it lacks advantages in plasticity.

[0009] Chinese patent application CN109778075B discloses a method for preparing a medium-manganese steel material with high yield strength ratio and continuous yield. The steel's alloy composition is: C: 0.05–0.20%, Si: 1.0–2.0%, Mn: 7.0–11.0%, Al: 1.0–3.0%, P ≤ 0.005%, S ≤ 0.005%, with the balance being Fe and other unavoidable impurities. High-strength 980 MPa cold-rolled steel sheets are produced. The steel sheets have excellent properties and are suitable for complex automotive parts. However, this steel uses 7–11% Mn alloying, making it unsuitable for industrial smelting; furthermore, the continuous annealing temperature of around 650℃ is difficult to achieve in current industrial production. Summary of the Invention

[0010] This invention provides a thin-gauge cold-rolled high-strength, high-ductility medium-manganese steel sheet for automobiles and its manufacturing method. It adopts a technical route of continuous casting-hot rolling-pickling-cold rolling-annealing to produce a medium-manganese steel sheet with a yield strength of over 600 MPa, a tensile strength of over 980 MPa, an elongation of over 30%, a strength-ductility product of over 30 GPa·%, and a hole expansion rate of over 30%. The product is easy to industrialize.

[0011] To achieve the above objectives, the present invention employs the following technical solution:

[0012] A thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automotive applications, comprising, by mass percentage: C: 0.15%–0.40%, Mn: 3.0%–10.0%, Si: 0.5%–2.0%, Al: 1.5%–3.0%, V: 0.10%–0.50%, P≤0.01%, S≤0.005%; additionally added: Ni≤1.0%, Cr≤1.0%, Mo≤0.5%, Cu≤1.0%, and Mn+Ni+Cr+Mo+Cu≤10.0%, Ti≤0.04%, Nb≤0.04%, B≤0.005%, Ca≤0.005%, REM≤0.005%, with the balance being Fe and unavoidable impurities; the steel sheet properties are: yield strength ≥600MPa, tensile strength ≥980MPa, elongation ≥30%, strength-ductility product ≥30GPa·%, and expansion rate ≥30%.

[0013] Furthermore, the medium-manganese steel plate is an L-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, contains C: 0.25%–0.40%, Mn: 3%–5%, Si: 0.5%–1.5%, Al: 1.5%–2.0%, V: 0.1%–0.2%, P≤0.01%, S≤0.005%; additionally, Ni, Cr, Mo, and Cu are added, and Mn+Ni+Cr+Mo+Cu≤5.0%; the microstructure of the steel plate is ferrite + austenite + bainite + martensite, wherein the ferrite content is 50%–60%, the austenite content is 30%–40%, the bainite content is 5%–10%, and the martensite content is ≤5%.

[0014] Furthermore, the medium-manganese steel plate is an H-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.15%~0.25%, Mn: 5%~8%, Si: 0.5%~1.5%, Al: 1.5%~2.0%, V: 0.15%~0.25%, with the addition of Ni, Cr, Mo and Cu, and Mn+Ni+Cr+Mo+Cu≤8.0%; the microstructure of the steel plate is ferrite + austenite + martensite, wherein the ferrite content is 40%~50%, the austenite content is 40%~50%, and the martensite content is ≤10%; the steel plate properties are: yield strength ≥700MPa, tensile strength ≥1080MPa, elongation ≥40%, strength-ductility product ≥40GPa·%, and expansion rate ≥30%.

[0015] Furthermore, the medium-manganese steel plate is an H-MnTRIP / TWIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.30%–0.40%, Mn: 8%–10%, Si: 1.5%–2.0%, Al: 2.0%–3.0%, V: 0.20%–0.50%, with the addition of Ni, Cr, Mo, and Cu, and Mn+Ni+Cr+Mo+Cu≤10%; the microstructure of the steel plate is ferrite + austenite, wherein the ferrite content is 30%–40% and the austenite content is 60%–70%; the steel plate properties are: yield strength ≥800MPa, tensile strength ≥1180MPa, elongation ≥50%, strength-ductility product ≥60GPa·%, and expansion rate ≥30%.

[0016] A method for manufacturing thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheets for automobiles includes smelting, continuous casting, hot rolling, bell-type annealing and pickling, cold rolling, one-time continuous annealing and pickling, two-time continuous annealing, and finishing processes; the specific process control is as follows:

[0017] 1) Smelting and continuous casting: Smelting is carried out according to the set chemical composition, and the casting temperature is 1580~1620℃;

[0018] 2) Hot rolling: Heating temperature is 1230~1280℃, furnace time is 150~300min; roughing temperature is 1150~1200℃; finishing rolling adopts two-stage rolling, the first stage rolling temperature is 1070~1130℃, the second stage rolling temperature is 960~1050℃, the finishing rolling temperature is above 920℃; coiling temperature is 700~760℃;

[0019] 3) Bell-type annealing and pickling: The bell-type annealing temperature is 680~750℃, and the annealing time is 18~25h;

[0020] 4) Cold rolling: The rolling reduction is controlled between 46.7% and 48.6%;

[0021] 5) One-time continuous annealing and pickling: Heat the cold-rolled steel plate to 800-900℃, hold it at the same temperature for 120-240s, then slowly cool it to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and finally quench it. The quenching water temperature is controlled above 80℃.

[0022] 6) Secondary annealing: Heat the quenched steel plate to 730-780℃ and hold it at a constant temperature for 120-240s; then cool it slowly to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and then cool it down to below 230℃ at a cooling rate of 10-18℃ / s for over-aging treatment, with an over-aging time of 60-300s.

[0023] Furthermore, the thickness of the continuously cast slab is 170–230 mm, the thickness of the rough-rolled intermediate slab is 50–80 mm, the thickness of the hot-rolled plate is 2.8–3.5 mm, and the thickness of the cold-rolled plate is 1.4–1.8 mm.

[0024] Furthermore, after a single annealing, the microstructure of the steel plate consists of ferrite, martensite, and retained austenite, with ferrite content ranging from 20% to 40%, martensite content ranging from 40% to 55%, and retained austenite content ranging from 8% to 21%.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] (1) This invention realizes the industrial composition and process design of medium manganese steel cold-rolled thin sheet, and proposes a practical and feasible industrial production path for medium manganese steel thin sheet for automobiles.

[0027] (2) This invention classifies medium manganese steel cold-rolled thin plate products into three types according to manganese content, analyzes the microstructure of each type of medium manganese steel, and studies and analyzes the related microstructure evolution behavior and toughening mechanism.

[0028] (3) The medium manganese steel cold-rolled sheet produced according to the method of the present invention has excellent performance and is suitable for complex body structure parts. It can completely replace products such as DP590 and 420LA. Attached Figure Description

[0029] Figure 1 This is a scanning electron microscope (SEM) image of the manganese steel plate in L-MnTRIP as described in this invention.

[0030] Figure 2 This is a scanning electron microscope (SEM) image of the manganese steel plate in the H-MnTRIP of this invention.

[0031] Figure 3 This is a scanning electron microscope (SEM) image of the manganese steel plate in the H-MnTRIP / TWIP of the present invention. Detailed Implementation

[0032] The present invention discloses a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles. The chemical composition of the steel, by mass percentage, contains: C: 0.15%–0.40%, Mn: 3.0%–10.0%, Si: 0.5%–2.0%, Al: 1.5%–3.0%, V: 0.10%–0.50%, P≤0.01%, S≤0.005%; additionally added: Ni≤1.0%, Cr≤1.0%, Mo≤0.5%, Cu≤1.0%, and Mn+Ni+Cr+Mo+Cu≤10.0%, Ti≤0.04%, Nb≤0.04%, B≤0.005%, Ca≤0.005%, REM≤0.005%, with the balance being Fe and unavoidable impurities. The steel sheet properties are: yield strength ≥600MPa, tensile strength ≥980MPa, elongation ≥30%, strength-ductility product ≥30GPa·%, and expansion rate ≥30%.

[0033] Furthermore, the medium-manganese steel plate is an L-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, contains C: 0.25%–0.40%, Mn: 3%–5%, Si: 0.5%–1.5%, Al: 1.5%–2.0%, V: 0.1%–0.2%, P≤0.01%, S≤0.005%; additionally, Ni, Cr, Mo, and Cu are added, and Mn+Ni+Cr+Mo+Cu≤5.0%; the microstructure of the steel plate is ferrite + austenite + bainite + martensite, wherein the ferrite content is 50%–60%, the austenite content is 30%–40%, the bainite content is 5%–10%, and the martensite content is ≤5%.

[0034] Furthermore, the medium-manganese steel plate is an H-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.15%~0.25%, Mn: 5%~8%, Si: 0.5%~1.5%, Al: 1.5%~2.0%, V: 0.15%~0.25%, with the addition of Ni, Cr, Mo and Cu, and Mn+Ni+Cr+Mo+Cu≤8.0%; the microstructure of the steel plate is ferrite + austenite + martensite, wherein the ferrite content is 40%~50%, the austenite content is 40%~50%, and the martensite content is ≤10%; the steel plate properties are: yield strength ≥700MPa, tensile strength ≥1080MPa, elongation ≥40%, strength-ductility product ≥40GPa·%, and expansion rate ≥30%.

[0035] Furthermore, the medium-manganese steel plate is an H-MnTRIP / TWIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.30%–0.40%, Mn: 8%–10%, Si: 1.5%–2.0%, Al: 2.0%–3.0%, V: 0.20%–0.50%, with the addition of Ni, Cr, Mo, and Cu, and Mn+Ni+Cr+Mo+Cu≤10%; the microstructure of the steel plate is ferrite + austenite, wherein the ferrite content is 30%–40% and the austenite content is 60%–70%; the steel plate properties are: yield strength ≥800MPa, tensile strength ≥1180MPa, elongation ≥50%, strength-ductility product ≥60GPa·%, and expansion rate ≥30%.

[0036] The present invention discloses a method for manufacturing thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheets for automobiles, comprising smelting, continuous casting, hot rolling, bell-type annealing and pickling, cold rolling, one-time continuous annealing and pickling, two-time continuous annealing, and finishing processes; the specific control process is as follows:

[0037] 1) Smelting and continuous casting: Smelting is carried out according to the set chemical composition, and the casting temperature is 1580~1620℃;

[0038] 2) Hot rolling: Heating temperature is 1230~1280℃, furnace time is 150~300min; roughing temperature is 1150~1200℃; finishing rolling adopts two-stage rolling, the first stage rolling temperature is 1070~1130℃, the second stage rolling temperature is 960~1050℃, the finishing rolling temperature is above 920℃; coiling temperature is 700~760℃;

[0039] 3) Bell-type annealing and pickling: The bell-type annealing temperature is 680~750℃, and the annealing time is 18~25h;

[0040] 4) Cold rolling: The rolling reduction is controlled between 46.7% and 48.6%;

[0041] 5) One-time continuous annealing and pickling: Heat the cold-rolled steel plate to 800-900℃, hold it at the same temperature for 120-240s, then slowly cool it to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and finally quench it. The quenching water temperature is controlled above 80℃.

[0042] 6) Secondary annealing: Heat the quenched steel plate to 730-780℃ and hold it at a constant temperature for 120-240s; then cool it slowly to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and then cool it down to below 230℃ at a cooling rate of 10-18℃ / s for over-aging treatment, with an over-aging time of 60-300s.

[0043] Furthermore, the thickness of the continuously cast slab is 170–230 mm, the thickness of the rough-rolled intermediate slab is 50–80 mm, the thickness of the hot-rolled plate is 2.8–3.5 mm, and the thickness of the cold-rolled plate is 1.4–1.8 mm.

[0044] Furthermore, after a single annealing, the microstructure of the steel plate consists of ferrite, martensite, and retained austenite, with ferrite content ranging from 20% to 40%, martensite content ranging from 40% to 55%, and retained austenite content ranging from 8% to 21%.

[0045] This invention discloses a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automotive applications. Based on the Mn content and austenitic work hardening method, it is divided into three types: low-manganese TRIP medium-manganese steel sheet (L-MnTRIP), high-manganese TRIP medium-manganese steel sheet (H-MnTRIP), and high-manganese TRIP / TWIP medium-manganese steel sheet (H-MnTRIP / TWIP). All three types of medium-manganese steel sheets achieve yield strength above 600 MPa, tensile strength above 980 MPa, elongation above 30%, strength-ductility product above 30 GPa·%, and porosity above 30%. Among them, L-MnTRIP manganese steel plate achieves a yield strength of over 600MPa, tensile strength of over 980MPa, elongation of over 30%, strength-ductility product of over 30GPa·%, and porosity of over 30%; H-MnTRIP manganese steel plate further achieves a yield strength of over 700MPa, tensile strength of over 1080MPa, elongation of over 40%, strength-ductility product of over 40GPa·%, and porosity of over 30%; H-MnTRIP / TWIP manganese steel sheet further achieves a yield strength of over 800MPa, tensile strength of over 1180MPa, elongation of over 50%, strength-ductility product of over 60GPa·%, and porosity of over 30%.

[0046] The reasons for selecting the chemical elements and their contents in the thin-gauge cold-rolled high-strength plastic steel sheet for automobiles described in this invention are as follows:

[0047] C: 0.15%–0.40%.

[0048] Carbon (C) is one of the important constituent elements in the steel of this invention. It is a common interstitial solid solution atom, and C dissolved in the matrix increases the strength of the solid solution by causing lattice distortion. In this invention, the addition of C ensures the stabilization behavior of austenite during the continuous annealing isothermal stage, promotes the nucleation of reversed austenite, and ensures the content of austenite in the critical region. In addition, the effective enrichment of C in room temperature reversed austenite ensures the stability of austenite and the effective execution of the TRIP or TRIP / TWIP effect during the austenite work hardening stage. In the three types of medium manganese steel plates of this invention, the C content ranges from 0.25% to 0.4%, 0.15% to 0.25%, and 0.30% to 0.40%, respectively.

[0049] Mn: 3.0%–10.0%.

[0050] Manganese (Mn) is one of the key elements in the steel of this invention. Mn atoms strengthen the solid solution by inducing lattice distortion through substitutional solid solution. In the medium-manganese steel plate of this invention, the addition of Mn is a guarantee of performance. Sufficient Mn content can promote the nucleation of austenite in the critical region and improve the thermal stability of austenite. At the same time, the sufficient enrichment of Mn atoms in reverse austenite improves the stability of the room-temperature reverse austenite phase. In addition, changes in Mn content directly affect the stacking fault energy of austenite, and thus affect the work hardening mode of austenite during deformation. In the three types of medium-manganese steel plates of this invention, the Mn content ranges are 3.0%–5.0%, 5.0%–8.0%, and 8.0%–10.0%, respectively.

[0051] Si: 0.5%–2.0%.

[0052] Si is one of the key elements in the steel of this invention. Sufficient Si addition ensures the matrix strength of the ferrite. Simultaneously, Si addition increases the AC3 point of the steel plate, effectively adjusting the annealing process window during continuous annealing and ensuring an appropriate ratio of ferrite and austenite in the critical region at industrial continuous annealing temperatures. Furthermore, sufficient Si addition can suppress the formation of carbides during the over-aging stage, preventing the steel plate from experiencing performance degradation due to carbide precipitation. However, excessive Si addition will cause embrittlement of the steel plate after rolling, thereby reducing its cold workability. Therefore, the Si content in this invention is controlled between 0.5% and 2.0%. In the three types of medium-manganese steel plates of this invention, the Si content ranges are 0.5%–1.5%, 1.0%–1.5%, and 1.5%–2.0%, respectively.

[0053] Al: 1.5%–3.0%.

[0054] In conventional steel, Al primarily functions as a deoxidizer during the smelting process. In this invention, the addition of Al is used to adjust the process window of the critical zone. Traditionally, the optimal annealing temperature for manganese steel is maintained at 650–700°C, which is difficult to match with existing continuous annealing lines. This invention adds 1.0–1.5% Al, raising the annealing temperature to above 750°C, which meets the lower limit of the temperature range for continuous annealing lines.

[0055] V: 0.10%~0.50%.

[0056] Appropriate addition of VC to the steel of this invention can enhance precipitation strengthening during the coiling stage, suppress dislocation self-recovery during cold rolling, improve the retention of deformation energy, and promote recrystallization behavior during the continuous annealing stage. At the same time, VC precipitates in ferrite during the continuous annealing isothermal stage, which can play a role in precipitation strengthening.

[0057] P≤0.01%.

[0058] Phosphorus (P) is an impurity element in steel that readily agglomerates at grain boundaries. High P content in steel easily leads to the formation of Fe₂P particles, reducing the steel's plasticity and toughness; therefore, a lower P content is better. In this invention, the P content is controlled below 0.01%.

[0059] S≤0.005%.

[0060] Sulfur (S) is an impurity element in steel. It readily combines with manganese (Mn) to form MnS inclusions, which deteriorates the plasticity of the steel plate. Therefore, the lower its content, the better. In this invention, the S content is controlled below 0.005%.

[0061] Additional elements added to the steel of this invention:

[0062] Ni ≤ 1.0%.

[0063] Ni is a solid solution strengthening element, and like C and Mn, it can improve the stability of austenite; at the same time, Ni can improve the corrosion resistance of steel plates to a certain extent. In this invention, it can be added in appropriate amounts to improve the corrosion resistance of the steel plates.

[0064] Cr ≤ 1.0%, Mo ≤ 1.0%.

[0065] Cr and Mo are solid solution strengthening elements, which strengthen the steel plate. In this invention, Cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite during the cooling stage, and promote the formation of martensite; at the same time, Cr and Mo can change the type of iron oxide scale during the coiling process, limit the oxidation inside the steel plate, and improve the surface quality of the steel plate.

[0066] Cu≤1.0.

[0067] Cu, when dissolved in austenite, can increase the strength of steel plates. During continuous annealing, the precipitation of elemental Cu in austenite plays a role in precipitation strengthening. Furthermore, the addition of Cu can improve the corrosion resistance of steel plates.

[0068] Mn+Ni+Cr+Mo+Cu≤10.0%.

[0069] As mentioned earlier, alloying elements such as Ni, Cr, Mo, and Cu are all substitutes for Mn, and their main role in this invention is to improve austenite stabilization and supplement its stability. However, increasing the proportion of alloying elements inevitably alters the overall stacking fault energy in the steel. Related studies indicate that when the stacking fault energy of austenite is less than 18 mJ / m 2 At this time, only the TRIP effect occurs during the deformation of austenite; when the stacking fault energy is between 18 and 25 mJ / m 2 During this period, austenite undergoes both the TRIP and TWIP effects simultaneously, constituting a complex phase transformation mechanism; when the stacking fault energy exceeds 25 mJ / m... 2 When austenite grains are too stable to undergo TRIP or TWIP effects, work hardening primarily occurs through dislocation strengthening. The work hardening mechanism of the steel plate involved in this invention is mainly TRIP or TRIP+TWIP; therefore, Mn+Ni+Cr+Mo+Cu is controlled to ≤10.0% to ensure that stacking faults do not exceed 25 mJ / m. 2 .

[0070] Ti≤0.04%, Nb≤0.04%.

[0071] Ti and Nb are microalloying strengthening elements. Ti combines with N, an impurity element in steel, to form TiN. The presence of free N atoms in the steel deteriorates the toughness of the steel sheet, so the formation of TiN effectively solidifies N. Furthermore, Ti combines with C and N to form Ti(C,N), refining the original austenite grains. However, excessive Ti content will lead to excessively large TiN sizes, worsening the steel sheet's properties. Nb mainly precipitates in the hot-rolled recrystallization zone due to strain, forming Nb(C,N), which also refines the original austenite grains. However, excessive Nb addition leads to excessively high strength in hot-rolled coiled steel sheets, increasing the cold-rolling load. Therefore, this invention limits the addition of Ti and Nb in steel to less than 0.04%.

[0072] B≤0.005%.

[0073] In this invention, the addition of boron (B) can improve the hardenability of the steel plate and ensure the formation of martensite during the rapid cooling stage of continuous annealing. However, excessive addition of boron will increase the brittleness of the steel plate and deteriorate its machinability.

[0074] Ca ≤ 0.005%.

[0075] The quality of cast steel plates can be improved by adding an appropriate amount of Ca to control the morphology of inclusions.

[0076] REM ≤ 0.005%.

[0077] The addition of rare earth elements is intended to form high-melting-point rare earth compounds with S and O elements, reducing the impurity content. At the same time, the addition of rare earth elements can promote grain refinement and improve the overall strength and plasticity of the steel plate.

[0078] The present invention discloses a method for preparing thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheets for automobiles, comprising the following steps: smelting and continuous casting, hot rolling, bell-type annealing and pickling, cold rolling, single continuous annealing and pickling, double continuous annealing, and finishing. The rationale for the selection of the process steps and parameters is as follows:

[0079] 1. Smelting and continuous casting: Industrial smelting and continuous casting are carried out according to the set chemical composition. The casting temperature is 1580~1620℃ and the billet thickness is 170~230mm.

[0080] 2. Hot rolling: Heating temperature is between 1230 and 1280℃, furnace time is 150 to 300 minutes, roughing temperature is 1150 to 1200℃, and intermediate billet thickness is 50 to 80 mm; finishing rolling is divided into two stages: the first stage rolling temperature is between 1070 and 1130℃, the second stage rolling temperature is between 960 and 1050℃, the final rolling temperature is above 920℃, and the coiling temperature is between 700 and 760℃. The thickness of hot-rolled plate is between 2.8 and 3.5 mm.

[0081] Principle: The heating temperature is controlled at 1230–1280℃, and the furnace time is 150–300 min. The purpose is to promote full solid solution of the alloy and control the banded structure caused by segregation. The two-stage rolling in the finishing rolling stage aims to promote the recrystallization of the original austenite grains and inhibit the coarsening of the unrecrystallized austenite grains. The purpose of controlling the coiling temperature at 700–760℃ is to promote the formation of proeutectoid ferrite to a certain extent and prevent the formation of a coiled structure from full martensite.

[0082] 3. Bell-type annealing and pickling: The bell-type annealing temperature is 680~750℃ and the annealing time is 18~25h; then the iron oxides present on the surface of the hot-rolled coiled steel plate in different forms such as FeO, Fe2O3, Fe3O4 are removed by pickling.

[0083] Principle: The microstructure of hot-rolled coiled steel sheets consists of a small amount of proeutectoid ferrite and martensite, which cannot be directly cold-rolled. Therefore, hot-rolled coiled steel sheets are subjected to bell-type annealing at a temperature of 680-750℃ to ensure austenite transformation nucleation. The bell-type annealed microstructure is ferrite + austenite.

[0084] 4. Cold rolling: The specifications of cold-rolled products are 1.4 to 1.8 mm. For example, a 1.4 mm thick cold-rolled plate corresponds to a 2.8 mm thick hot-rolled plate, and 1.6 mm and 1.8 mm thick cold-rolled plates correspond to 3.0 to 3.5 mm thick hot-rolled plates. The cold rolling reduction is controlled at 46.7% to 48.6%.

[0085] Principle: Too low a rolling reduction rate cannot guarantee sufficient cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization during the continuous annealing stage; too high a rolling reduction rate will significantly increase the load on the cold rolling mill and cannot guarantee the achievement of the target thickness.

[0086] 5. Continuous annealing and pickling: The cold-rolled steel plate is heated to 800-900℃ and isothermaled for 120-240s, then slowly cooled to 720-760℃ at a cooling rate of 1.2-3.6℃ / s. The slab is then quenched, with the quenching water temperature controlled above 80℃. After quenching, the steel plate is pickled to remove the surface oxides formed during the continuous annealing stage.

[0087] Principle: The isothermal temperature of the first continuous annealing is 800–900℃, aiming to form a larger proportion of austenite in the critical region. The higher isothermal temperature also promotes the recrystallization of ferrite in the critical region, thus providing more austenite transformation nucleation sites. Simultaneously, the higher isothermal temperature promotes the diffusion of Mn atoms from ferrite into austenite, resulting in a high proportion of Mn-rich austenite and improved stabilization behavior of austenite in the critical region. Furthermore, the higher isothermal temperature promotes the complete dissolution of carbides and the precipitation of VC, which is beneficial for strengthening the steel plate. The subsequent quenching to room temperature aims to provide a good microstructure matrix for the second continuous annealing. After quenching, the microstructure consists of ferrite + a small portion of reversed austenite + a certain proportion of martensite. The small portion of austenite is retained due to a certain amount of C and Mn enrichment and a small grain size. Most of the austenite obtained in the critical region, due to its larger grain size, transforms into martensite after quenching.

[0088] After one annealing, the microstructure consists of ferrite, martensite, and retained austenite, with ferrite content ranging from 20% to 40%, martensite content ranging from 40% to 55%, and retained austenite content ranging from 8% to 21%.

[0089] 6. Secondary annealing: Heat the quenched steel plate to 730-780℃ and hold it at a constant temperature for 120-240s; then slowly cool it to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and then cool it down to below 230℃ at a cooling rate of 10-18℃ / s for over-aging treatment, with an over-aging time of 60-300s.

[0090] Principle: The isothermal temperature of 730–780℃ for secondary annealing is designed to match the composition of the steel plate described in this invention. At this temperature, secondary annealing achieves an ideal reverse austenite content and good austenite stability, ensuring effective austenite work hardening during subsequent deformation stages, thereby guaranteeing a good strength-plasticity balance. During this process, the quenched martensite undergoes a second austenite transformation to form lath austenite. Due to the high Mn enrichment and fine lath structure retained to room temperature, the previously retained austenite forms in small chunks at the ferrite grain boundaries, continuously accumulating Mn during the secondary isothermal stage, which also improves its transformation stability. The cooling rate is controlled at 10–18℃ / s to prevent excessive bainite formation at low cooling rates and to prevent a small amount of austenite from undergoing martensitic transformation at high cooling rates, thus affecting the steel plate's properties. The over-aging temperature is controlled below 230℃ to prevent the decomposition of metastable austenite at excessively high temperatures.

[0091] The following embodiments are implemented based on the technical solution of the present invention, and provide detailed implementation methods and specific operation processes. However, the scope of protection of the present invention is not limited to the following embodiments.

[0092]

Example

[0093] Table 1 lists the chemical composition of the steels in each embodiment, Table 2 lists the continuous casting and hot rolling process parameters of the steels in each embodiment, Table 3 lists the cold rolling and continuous annealing process parameters of the steels in each embodiment, Table 4 lists the microstructure of the steels in each embodiment, and Table 5 lists the mechanical properties of the steels in each embodiment. Figures 1-3 These are SEM images of manganese steel plates from the three types.

[0094] Table 1. Chemical composition of the steel in each embodiment, wt%.

[0095]

[0096] Table 2 Hot rolling process parameters of steel in each embodiment

[0097]

[0098] Table 3 Cold rolling and annealing process parameters for steels in each embodiment

[0099] Example <![CDATA[T1 / ℃]]> <![CDATA[Ts1 / ℃]]> <![CDATA[t1 / s]]> <![CDATA[T2 / ℃]]> <![CDATA[Ts2 / ℃]]> <![CDATA[t2 / min]]> SP / (℃ / s) <![CDATA[T OA / ℃]]> <![CDATA[t OA / s]]> 1 875 735 120 735 720 160 16.50 228 120 2 842 748 240 775 741 180 17.30 224 60 874 752 160 744 732 200 16.50 228 60 4 889 722 180 779 722 240 10.60 227 120 5 900 726 200 758 726 180 10.20 226 180 6 862 728 240 748 721 160 11.70 214 240 7 842 735 180 752 732 180 12.60 213 60 8 856 728 160 736 722 240 11.60 225 60 9 867 745 180 776 720 180 13.40 228 120 10 842 756 200 741 734 160 14.20 226 60 11 896 758 240 755 722 120 14.60 223 60 12 874 748 180 753 741 240 13.80 227 120 13 842 758 160 758 740 160 17.90 224 120 14 825 759 180 746 721 180 18.00 218 180 15 864 725 200 743 723 180 10.90 219 240 16 875 732 240 775 758 200 10.60 216 60 17 845 741 200 742 723 240 13.50 217 180 18 865 752 240 746 730 200 12.90 225 240 19 802 722 180 739 731 240 14.80 228 60 20 832 726 160 768 743 180 14.60 224 120 21 854 723 180 748 720 200 15.40 223 180 22 854 745 200 736 723 240 15.80 217 240 23 869 760 240 766 756 180 10.40 219 60 24 863 722 180 744 720 160 10.90 229 60

[0100] In Table 3, T1 represents the primary annealing temperature; T s1 t1: First annealing slow cooling temperature; t2: First annealing isothermal time; T3: Second annealing temperature; T4: ... s2 t2: Secondary annealing slow cooling temperature; sP: Secondary annealing isothermal time; T: Rapid cooling rate; OA : Over-aging temperature; t OA Expiration time

[0101] Table 4. Microstructure of steel in each embodiment

[0102]

[0103] In Table 4, F represents ferrite; M represents martensite; B represents bainite; and RA represents retained austenite.

[0104] Table 5 Mechanical properties of steel in each embodiment

[0105]

[0106] In Table 5, YS represents yield strength; TS represents tensile strength; EL represents elongation; PSE represents strength-ductility product; and λ represents the pore size.

[0107] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for manufacturing thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles, characterized in that, The chemical composition of the steel, by mass percentage, contains: C: 0.15%–0.40%, Mn: 3.0%–10.0%, Si: 0.5%–2.0%, Al: 1.5%–3.0%, V: 0.10%–0.50%, P≤0.01%, S≤0.005%; Ni≤1.0%, Cr≤1.0%, Mo≤0.5%, Cu≤1.0%, and Mn+Ni+Cr+Mo+Cu≤10.0%, Ti≤0.04%, Nb≤0.04%, B≤0.005%, Ca≤0.005%, REM≤0.005%, with the balance being Fe and unavoidable impurities. The steel plate properties are: yield strength ≥600MPa, tensile strength ≥980MPa, elongation ≥30%, strength-ductility product ≥30GPa·%, and expansion rate ≥30%. The manufacturing method of the thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles includes smelting, continuous casting, hot rolling, bell-type annealing and pickling, cold rolling, one-time continuous annealing and pickling, two-time continuous annealing and finishing processes; wherein the following processes are controlled: 1) One-time continuous annealing and pickling: The cold-rolled steel plate is heated to 800-900℃, isothermal for 120-240s, and then slowly cooled to 720-760℃ at a cooling rate of 1.2-3.6℃ / s. Finally, it is quenched, and the quenching water temperature is controlled above 80℃. 2) Secondary annealing: The quenched steel plate is heated to 730-780℃ and isothermaled for 120-240s; then it is slowly cooled to 720-760℃ at a cooling rate of 1.2-3.6℃ / s, and then cooled to below 230℃ at a cooling rate of 10-18℃ / s for over-aging treatment, with an over-aging time of 60-300s.

2. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that, The medium-manganese steel plate is an L-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, contains C: 0.25%~0.40%, Mn: 3%~4.8%, Si: 0.5%~1.5%, Al: 1.5%~2.0%, V: 0.1%~0.2%, P≤0.01%, S≤0.005%; Mn+Ni+Cr+Mo+Cu≤5.0%; the microstructure of the steel plate is ferrite+austenite+bainite+martensite, wherein the ferrite content is 50%~60%, the austenite content is 30%~40%, the bainite content is 5%~10%, and the martensite content is ≤5%.

3. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that, The medium-manganese steel plate is H-MnTRIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.15%~0.25%, Mn: 5%~8%, Si: 0.5%~1.5%, Al: 1.5%~2.0%, V: 0.15%~0.25%, Mn+Ni+Cr+Mo+Cu≤8.0%; the microstructure of the steel plate is ferrite + austenite + martensite, wherein the ferrite content is 40%~50%, the austenite content is 40%~50%, and the martensite content is ≤10%; the steel plate properties are: yield strength ≥700MPa, tensile strength ≥1080MPa, elongation ≥40%, strength-ductility product ≥40GPa·%, and expansion rate ≥30%.

4. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that, The medium-manganese steel plate is an H-MnTRIP / TWIP medium-manganese steel plate; the chemical composition of the steel, by mass percentage, is C: 0.30%~0.40%, Mn: 8%~10%, Si: 1.5%~2.0%, Al: 2.0%~3.0%, V: 0.20%~0.50%, Mn+Ni+Cr+Mo+Cu≤10%; the microstructure of the steel plate is ferrite + austenite, of which the ferrite content is 30%~40% and the austenite content is 60%~70%; the steel plate properties are: yield strength ≥800MPa, tensile strength ≥1180MPa, elongation ≥50%, strength-ductility product ≥60Gpa·%, and expansion rate ≥30%.

5. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that: 1) Smelting and continuous casting: Smelting is carried out according to the set chemical composition, and the casting temperature is 1580~1620℃; 2) Hot rolling: Heating temperature is 1230~1280℃, furnace time is 150~300min; roughing temperature is 1150~1200℃; finishing rolling adopts two-stage rolling, the first stage rolling temperature is 1070~1130℃, the second stage rolling temperature is 960~1050℃, and the finishing rolling temperature is above 920℃; coiling temperature is 700~760℃; 3) Bucket annealing and pickling: The bucket annealing temperature is 680-750℃, and the annealing time is 18-25h; 4) Cold rolling: The rolling reduction is controlled at 46.7% to 48.6%.

6. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that, The thickness of continuously cast billets is 170–230 mm, the thickness of rough-rolled intermediate billets is 50–80 mm, the thickness of hot-rolled plates is 2.8–3.5 mm, and the thickness of cold-rolled plates is 1.4–1.8 mm.

7. The method for manufacturing a thin-gauge cold-rolled high-strength, high-ductility, medium-manganese steel sheet for automobiles according to claim 1, characterized in that, After one annealing, the microstructure of the steel plate is ferrite + martensite + retained austenite, with ferrite content of 20% to 40%, martensite content of 40% to 55%, and retained austenite content of 8% to 21%.

Images