CN116770185A - Cold-rolled high-strength plastic product medium manganese steel plate for thin-specification automobile and manufacturing method thereof - Google Patents
Cold-rolled high-strength plastic product medium manganese steel plate for thin-specification automobile and manufacturing method thereof Download PDFInfo
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Abstract
The invention relates to a cold-rolled high-strength-plastic medium manganese steel plate for a thin-specification automobile and a manufacturing method thereof, wherein the steel comprises the following chemical components: 0.15 to 0.40 percent of Mn:3.0 to 10.0 percent, si:0.5 to 2.0 percent of Al:1.5 to 3.0 percent, V:0.10 to 0.50 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; and (3) adding: less than or equal to 1.0 percent of Ni, less than or equal to 1.0 percent of Cr, less than or equal to 0.5 percent of Mo, less than or equal to 1.0 percent of Cu, less than or equal to 10.0 percent of Mn+Ni+Cr+Mo+Cu, less than or equal to 0.04 percent of Ti, less than or equal to 0.04 percent of Nb, less than or equal to 0.005 percent of B, less than or equal to 0.005 percent of Ca, less than or equal to 0.005 percent of REM, and the balance of Fe and impurities. The invention adopts the technical route of continuous casting, hot rolling, pickling, cold rolling and annealing to manufacture the medium manganese steel sheet with yield strength of more than 600MPa, tensile strength of more than 980MPa, elongation of more than 30 percent, strength-plastic product of more than 30GPa percent and hole expansion rate of more than 30 percent, and the product is easy to realize industrialized production.
Description
Technical Field
The invention relates to the technical field of automobile steel production, in particular to a cold-rolled high-strength-product medium manganese steel plate for a thin automobile and a manufacturing method thereof.
Background
At present, the development of automobile light weight in the direction of steel materials is roughly divided into two aspects of high-strength thinning and low-density thinning, and the development path of the high-strength thinning is more close to the large environment of the current automobile plate production in terms of difficulty. Just as the 'Ji Pagang' proposes, the unavoidable current situation of sacrificing the comprehensive performance of forming and the like after the high strengthening of the automobile steel plate is faced, the application performance is optimized according to a certain aspect, and the user solution of personalized requirements is achieved, such as the plastic upgrading of the dual-phase steel, the reaming performance upgrading of the quenching distribution steel, the toughness upgrading of the hot forming steel and the like. With the continuous increase of the application proportion of high-strength steel car bodies above 980MPa and the continuous maturing of the development and production of high-strength steel, japanese steel material research and development universities represented by Kong's steel, xin-ai iron and JFE are more general to propose future steel performance systems based on high-strength plastic grades, namely, the tensile strength multiplied by the elongation after break is 1000MPa multiplied by 35%, 1200MPa multiplied by 30% and 1500MPa multiplied by 20%. In this view, the current Ji Pagang is developed to the performance index of future steel, and a long path is taken in the middle.
And analyzing the structure and the strong plastic matching condition of the existing high-strength steel of the automobile, wherein the performance of the medium manganese steel sheet is more in line with the development concept of steel for the future automobile. The main phase composition of the medium manganese steel is ferrite and reverse austenite, the transformation stage depends on the reverse austenite to generate TRIP effect, and the transformation is performed to martensite, so that the strength and the plasticity of the steel plate are improved. The earliest proposal of medium manganese steel is that two students of Krupitzer and Heimbuch put forward the development concept of third-generation steel, namely the strength is more than 1GPa and the plasticity is more than 30 percent, and put forward the concept of strong plastic product of 30 GPa. The Dong Han teaching team of China iron and steel institute takes medium manganese steel with 5 percent of Mn content as a research object, deeply explores key problems of tissue constitution, strengthening mechanism, austenite stability behavior, manganese element diffusion and the like of the medium manganese steel, and prepares a hot rolled product of the medium manganese steel in the test of Taiyuan iron and steel in 2010, and has objective performance. The cold-rolled product of the medium manganese steel is industrially produced by the 2015 Bao steel, the strength of the cold-rolled product meets 980MPa, the plasticity of the cold-rolled product is ensured to be more than 30%, and the industrialization of the steel with the concentration of 30GPa is realized.
Seemingly, the newly developed 30 GPa% steel meets the requirements of the third generation steel, but a plurality of problems still exist in careful pushing. Firstly, the steel plate is produced by adopting cover annealing, so that the yield of the steel plate is greatly reduced; secondly, longer Lv Des bands exist on the surface of the steel plate, and the surface quality of the steel plate is seriously affected; in addition, the Mn content of about 7 percent is greatly increased compared with the conventional steel plate in both smelting difficulty and subsequent process difficulty, and is not beneficial to the welding of the whole vehicle.
The existence of the industrial production obstacle of the 30GPa percent steel is indistinguishable from the composition and process design of the medium manganese steel. In general, mn content in the medium manganese steel is more than 5%, so that the content and stability of austenite are ensured. However, addition of Mn to 5% or more will cause serious C/Mn segregation during continuous casting, while improper slow cooling will cause hot cracking of the cast slab. These reduce the medium manganese steel yield from the raw end. Furthermore, the performance of medium manganese steel is required to be obtained through ART annealing (austenite reverse transformation annealing), and according to various documents, the annealing time of most of cold-rolled medium manganese steel is required to be at least 0.5 hour so as to ensure the diffusion of Mn atoms and improve the stability of austenite phase, but obviously cannot be matched with the length of a continuous annealing production line.
Overall, the performance of medium manganese steel is adequate for most of the complex vehicle body structural members, but how to solve the technical bottlenecks of medium manganese steel is a key to determine whether medium manganese steel can be truly applied.
The Chinese patent with the publication number of CN109680130B discloses a high-strength plastic product cold-rolled medium manganese steel and a preparation method thereof, wherein the steel plate comprises the following components in percentage by weight: c:0.2%, mn: 7-9%, al:1.5%, zr:0.08 to 0.10 percent, less than or equal to 0.008 percent of P, less than or equal to 0.008 percent of S, and the balance of Fe and other unavoidable impurities. The preparation method comprises the working procedures of smelting, hot rolling, pickling, cold rolling, annealing and low-temperature tempering, and the cold-rolled annealed steel plate with the steel plate performance of more than 1200MPa and the elongation of more than 50-56% is obtained. The steel plate can obviously be qualified for various complex vehicle body parts by virtue of excellent mechanical properties, but the steel plate cannot be matched with the existing production line conditions from component design to process design, i.e. industrial production cannot be realized. For example, the annealing temperature of 660-680 ℃ can only be performed through the hood-type annealing, and the temperature difference of 20 ℃ cannot be matched with the fluctuation range of the hood-type annealing temperature, which can lead to the great reduction of the yield and the great increase of the production cost. In addition, an annealing time of 10 to 20 minutes cannot be realized in industrial production.
The Chinese patent with the publication number of CN110117755B discloses a method for preparing 980 MPa-grade low-yield-ratio cold-rolled medium manganese steel, wherein the alloy composition of the steel is C: 0.09-0.12%, si:0.1 to 0.3 percent, mn:4.8 to 7.20 percent of Al: 0.02-0.05%, P is less than or equal to 0.02%, S is less than or equal to 0.003%, and the balance is Fe and other unavoidable impurities. The preparation method comprises the working procedures of smelting, hot rolling, acid washing, cold rolling and continuous annealing, and the steel plate with the tensile strength of 980MPa or more is obtained. However, as can be seen from examples, the elongation of the steel sheet was about 23% and less than 30%. Compared with the QP980 cold-rolled or galvanized steel sheet which is mature in technology and realizes industrial production, the method has no advantage in plastic index.
The Chinese patent application with the publication number of CN109778075B discloses a preparation method of a medium manganese steel material with high yield ratio and continuous yield, wherein the steel alloy comprises the following components: 0.05 to 0.20 percent, si:1.0 to 2.0 percent, mn:7.0 to 11.0 percent, al:1.0 to 3.0 percent, P is less than or equal to 0.005 percent, S is less than or equal to 0.005 percent, and the balance is Fe and other unavoidable impurities. Preparing the high-strength 980MPa grade cold-rolled steel plate. The steel plate has excellent performance and is suitable for complex automobile parts. However, the steel is added by adopting an alloy with 7-11% of Mn, and is not suitable for industrial smelting; meanwhile, the continuous annealing temperature of about 650 ℃ is difficult to realize in the current industrial production.
Disclosure of Invention
The invention provides a cold-rolled high-strength plastic product medium manganese steel plate for a thin-specification automobile and a manufacturing method thereof, wherein the medium manganese steel plate with yield strength of more than 600MPa, tensile strength of more than 980MPa, elongation of more than 30%, plastic product of more than 30GPa percent and hole expansion rate of more than 30% is manufactured by adopting a continuous casting-hot rolling-pickling-cold rolling-annealing technical route, and the product is easy to realize industrial production.
In order to achieve the above purpose, the invention is realized by adopting the following technical scheme:
the cold-rolled high-strength and high-ductility medium-manganese steel plate for the thin-specification automobile comprises the following chemical components in percentage by mass: 0.15 to 0.40 percent of Mn:3.0 to 10.0 percent, si:0.5 to 2.0 percent of Al:1.5 to 3.0 percent, V:0.10 to 0.50 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; and (3) adding: less than or equal to 1.0 percent of Ni, less than or equal to 1.0 percent of Cr, less than or equal to 0.5 percent of Mo, less than or equal to 1.0 percent of Cu, less than or equal to 10.0 percent of Mn+Ni+Cr+Mo+Cu, less than or equal to 0.04 percent of Ti, less than or equal to 0.04 percent of Nb, less than or equal to 0.005 percent of B, less than or equal to 0.005 percent of Ca, less than or equal to 0.005 percent of REM, and the balance of Fe and unavoidable impurities; the steel plate has the following properties: the yield strength is more than or equal to 600MPa, the tensile strength is more than or equal to 980MPa, the elongation is more than or equal to 30 percent, the strength-plastic product is more than or equal to 30GPa, and the reaming ratio is more than or equal to 30 percent.
Further, the medium manganese steel plate is an L-MnTRIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.25 to 0.40 percent, mn:3 to 5 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.1 to 0.2 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; in addition, ni, cr, mo and Cu are added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 5.0%; the steel plate structure is composed of ferrite, austenite, bainite and 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 less than or equal to 5%.
Further, the medium manganese steel plate is an H-MnTRIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.15 to 0.25 percent of Mn:5 to 8 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.15 to 0.25 percent of Ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 8.0 percent; the steel plate structure is composed of ferrite, austenite and martensite, wherein the ferrite content is 40-50%, the austenite content is 40-50%, and the martensite content is less than or equal to 10%; the steel plate has the following properties: the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 1080MPa, the elongation is more than or equal to 40%, the strength-plastic product is more than or equal to 40GPa, and the reaming ratio is more than or equal to 30%.
Further, the medium manganese steel plate is an H-MnTRIP/TWIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.30 to 0.40 percent, mn:8 to 10 percent of Si, 1.5 to 2.0 percent of Al:2.0 to 3.0 percent, V:0.20 to 0.50 percent, ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 10 percent; the steel plate structure is ferrite and austenite, wherein the ferrite content is 30-40%, and the austenite content is 60-70%; the steel plate has the following properties: the yield strength is more than or equal to 800MPa, the tensile strength is more than or equal to 1180MPa, the elongation is more than or equal to 50%, the strength-plastic product is more than or equal to 60GPa, and the reaming ratio is more than or equal to 30%.
The manufacturing approach of the manganese steel sheet in cold rolling high-strength plastic product for thin specification car, including smelting, continuous casting, hot rolling, cover annealing and pickling, cold rolling, first continuous annealing and pickling, second continuous annealing, finishing process; the specific control process is as follows:
1) Smelting and continuous casting: smelting according to the set chemical components, wherein the casting temperature is 1580-1620 ℃;
2) And (3) hot rolling: heating at 1230-1280 deg.c for 150-300 min; the rough rolling temperature is 1150-1200 ℃; the finish rolling adopts two-stage rolling, the first stage rolling temperature is 1070-1130 ℃, the second stage rolling temperature is 960-1050 ℃, and the finish rolling finishing temperature is above 920 ℃; the coiling temperature is 700-760 ℃;
3) Hood annealing and pickling: the cover annealing temperature is 680-750 ℃ and the annealing time is 18-25 h;
4) Cold rolling: the rolling reduction is controlled to be 46.7 to 48.6 percent;
5) Primary continuous annealing and acid washing: heating the cold-rolled steel plate to 800-900 ℃, carrying out isothermal treatment for 120-240 s, then slowly cooling to 720-760 ℃ at a cooling speed of 1.2-3.6 ℃/s, and finally carrying out quenching treatment, wherein the quenching water temperature is controlled to be more than 80 ℃;
6) And (3) secondary continuous annealing: heating the quenched steel plate to 730-780 ℃, and carrying out isothermal treatment for 120-240 s; then slowly cooling to 720-760 ℃ at a cooling rate of 1.2-3.6 ℃/s, and then cooling to below 230 ℃ at a cooling rate of 10-18 ℃/s for overaging treatment, wherein the overaging time is 60-300 s.
Further, the thickness of the continuous casting blank is 170-230 mm, and the thickness of the rough rolling intermediate blank is 50-80 mm; the thickness of the hot rolled plate is 2.8-3.5 mm, and the specification of the cold rolled plate is 1.4-1.8 mm.
Further, after primary annealing, the steel plate structure is ferrite, martensite and retained austenite, wherein the ferrite content is 20% -40%, the martensite content is 40% -55%, and the retained austenite content is 8% -21%.
Compared with the prior art, the invention has the beneficial effects that:
(1) The invention realizes the industrial composition and process design of the medium manganese steel cold-rolled sheet and provides a feasible industrial production path of the medium manganese steel sheet for the automobile;
(2) The invention divides the medium manganese steel cold-rolled sheet product into three types according to manganese content, analyzes the structure constitution of each type of medium manganese steel, and researches and analyzes the related structure evolution behavior and strengthening mechanism;
(3) The medium manganese steel cold-rolled sheet product produced by the method has excellent performance, is suitable for complex vehicle body structural members, and can completely replace products such as DP590 and 420 LA.
Drawings
FIG. 1 is an SEM image of a manganese steel sheet in L-MnTRIP according to the present invention.
FIG. 2 is an SEM image of the manganese steel sheet in H-MnTRIP according to the present invention.
FIG. 3 is an SEM image of the manganese steel sheet in H-MnTRIP/TWIP according to the present invention.
Detailed Description
The invention relates to a cold-rolled high-strength-product medium-manganese steel plate for a thin-specification automobile, which comprises the following chemical components in percentage by mass: 0.15 to 0.40 percent of Mn:3.0 to 10.0 percent, si:0.5 to 2.0 percent of Al:1.5 to 3.0 percent, V:0.10 to 0.50 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; and (3) adding: less than or equal to 1.0 percent of Ni, less than or equal to 1.0 percent of Cr, less than or equal to 0.5 percent of Mo, less than or equal to 1.0 percent of Cu, less than or equal to 10.0 percent of Mn+Ni+Cr+Mo+Cu, less than or equal to 0.04 percent of Ti, less than or equal to 0.04 percent of Nb, less than or equal to 0.005 percent of B, less than or equal to 0.005 percent of Ca, less than or equal to 0.005 percent of REM, and the balance of Fe and unavoidable impurities; the steel plate has the following properties: the yield strength is more than or equal to 600MPa, the tensile strength is more than or equal to 980MPa, the elongation is more than or equal to 30 percent, the strength-plastic product is more than or equal to 30GPa, and the reaming ratio is more than or equal to 30 percent.
Further, the medium manganese steel plate is an L-MnTRIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.25 to 0.40 percent, mn:3 to 5 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.1 to 0.2 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; in addition, ni, cr, mo and Cu are added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 5.0%; the steel plate structure is composed of ferrite, austenite, bainite and 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 less than or equal to 5%.
Further, the medium manganese steel plate is an H-MnTRIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.15 to 0.25 percent of Mn:5 to 8 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.15 to 0.25 percent of Ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 8.0 percent; the steel plate structure is composed of ferrite, austenite and martensite, wherein the ferrite content is 40-50%, the austenite content is 40-50%, and the martensite content is less than or equal to 10%; the steel plate has the following properties: the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 1080MPa, the elongation is more than or equal to 40%, the strength-plastic product is more than or equal to 40GPa, and the reaming ratio is more than or equal to 30%.
Further, the medium manganese steel plate is an H-MnTRIP/TWIP medium manganese steel plate; the steel comprises the following chemical components in percentage by mass: 0.30 to 0.40 percent, mn:8 to 10 percent of Si, 1.5 to 2.0 percent of Al:2.0 to 3.0 percent, V:0.20 to 0.50 percent, ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 10 percent; the steel plate structure is ferrite and austenite, wherein the ferrite content is 30-40%, and the austenite content is 60-70%; the steel plate has the following properties: the yield strength is more than or equal to 800MPa, the tensile strength is more than or equal to 1180MPa, the elongation is more than or equal to 50%, the strength-plastic product is more than or equal to 60GPa, and the reaming ratio is more than or equal to 30%.
The invention relates to a manufacturing method of a cold-rolled high-strength and medium-strength manganese steel plate for a thin-specification automobile, which comprises the processes of smelting, continuous casting, hot rolling, hood annealing and pickling, cold rolling, primary continuous annealing and pickling, secondary continuous annealing and finishing; the specific control process is as follows:
1) Smelting and continuous casting: smelting according to the set chemical components, wherein the casting temperature is 1580-1620 ℃;
2) And (3) hot rolling: heating at 1230-1280 deg.c for 150-300 min; the rough rolling temperature is 1150-1200 ℃; the finish rolling adopts two-stage rolling, the first stage rolling temperature is 1070-1130 ℃, the second stage rolling temperature is 960-1050 ℃, and the finish rolling finishing temperature is above 920 ℃; the coiling temperature is 700-760 ℃;
3) Hood annealing and pickling: the cover annealing temperature is 680-750 ℃ and the annealing time is 18-25 h;
4) Cold rolling: the rolling reduction is controlled to be 46.7 to 48.6 percent;
5) Primary continuous annealing and acid washing: heating the cold-rolled steel plate to 800-900 ℃, carrying out isothermal treatment for 120-240 s, then slowly cooling to 720-760 ℃ at a cooling speed of 1.2-3.6 ℃/s, and finally carrying out quenching treatment, wherein the quenching water temperature is controlled to be more than 80 ℃;
6) And (3) secondary continuous annealing: heating the quenched steel plate to 730-780 ℃, and carrying out isothermal treatment for 120-240 s; then slowly cooling to 720-760 ℃ at a cooling rate of 1.2-3.6 ℃/s, and then cooling to below 230 ℃ at a cooling rate of 10-18 ℃/s for overaging treatment, wherein the overaging time is 60-300 s.
Further, the thickness of the continuous casting blank is 170-230 mm, and the thickness of the rough rolling intermediate blank is 50-80 mm; the thickness of the hot rolled plate is 2.8-3.5 mm, and the specification of the cold rolled plate is 1.4-1.8 mm.
Further, after primary annealing, the steel plate structure is ferrite, martensite and retained austenite, wherein the ferrite content is 20% -40%, the martensite content is 40% -55%, and the retained austenite content is 8% -21%.
The invention relates to a cold-rolled high-strength and high-strength manganese steel plate for a thin-specification automobile, which is divided into the following three types according to the addition content of Mn element and the difference of austenite work hardening modes: the low manganese series TRIP medium manganese steel plate is an L-MnTRIP medium manganese steel plate, the high manganese series TRIP medium manganese steel plate is an H-MnTRIP medium manganese steel plate, and the high manganese series TRIP/TWIP medium manganese steel plate is an H-MnTRIP/TWIP medium manganese steel plate. The three types of medium manganese steel plates all achieve the performances of yield strength of more than 600MPa, tensile strength of more than 980MPa, elongation of more than 30 percent, strength-plastic product of more than 30GPa percent and hole expansion rate of more than 30 percent. Wherein, the manganese steel plate in the L-MnTRIP realizes the performances of over 600MPa of yield strength, over 980MPa of tensile strength, over 30% of elongation, over 30 GPa% of strength-plastic product and over 30% of hole expansion rate; the H-MnTRIP medium manganese steel plate further realizes the performances of more than 700MPa of yield strength, more than 1080MPa of tensile strength, more than 40% of elongation, more than 40 GPa% of strength-plastic product and more than 30% of hole expansion rate; the H-MnTRIP/TWIP medium manganese steel sheet further realizes the performances of more than 800MPa of yield strength, more than 1180MPa of tensile strength, more than 50% of elongation, more than 60 GPa% of strength-plastic product and more than 30% of hole expansion rate.
The chemical elements and the content of the manganese steel plate in the cold-rolled high-strength product for the thin-specification automobile are selected as follows:
C：0.15％～0.40％。
c is one of important elements in the steel, is a common interstitial solid solution atom, and is dissolved in a matrix to improve solid solution strength by causing lattice distortion. In the invention, the addition of C can ensure the stabilizing behavior of austenite in the continuous annealing isothermal stage, promote the nucleation of the reversed austenite and ensure the content of austenite in the critical region; in addition, an effective enrichment of C in the room temperature reversed austenite will ensure the stability of the austenite, ensuring an effective progression of the TRIP or TRIP/TWIP effect in the work hardening stage of the austenite. In the three types of medium manganese steel plates, the content range of C is 0.25 to 0.4 percent, 0.15 to 0.25 percent and 0.30 to 0.40 percent respectively.
Mn：3.0％～10.0％。
The Mn element is one of the important elements in the steel of the present invention, and Mn atoms strengthen the solid solution in such a manner that substitutional solid solution causes lattice distortion. In the medium manganese steel plate, the addition of Mn element is a guarantee for realizing the performance, sufficient Mn content can promote the nucleation of the critical region of austenite, and the thermal stability of the austenite is improved; meanwhile, mn atoms are fully enriched in the reversed austenite, so that the stability of the reversed austenite phase at room temperature is improved. In addition, the change of Mn content directly influences the level of the stacking fault energy of austenite, thereby influencing the work hardening mode of the austenite in the deformation process. In the three types of medium manganese steel plates, the Mn content ranges are 3.0% -5.0%, 5.0% -8.0% and 8.0% -10.0% respectively.
Si：0.5％～2.0％。
Si element is one of important elements in the steel, and sufficient Si is added to ensure the matrix strength of ferrite; meanwhile, the addition of Si can improve the AC3 point of the steel plate, effectively adjust the annealing process window of the continuous annealing stage and ensure proper ferrite and austenite proportion of the critical zone at the industrialized continuous annealing temperature; in addition, the addition of Si with sufficient content can inhibit the formation of carbide in the overaging stage and avoid the degradation of the performance of the steel plate due to carbide precipitation. However, too high Si addition causes embrittlement of the steel sheet after rolling, thereby degrading cold workability of the steel sheet. Therefore, the content of Si element is controlled to be 0.5-2.0%. In the three types of medium manganese steel plates, the Si content ranges are respectively 0.5% -1.5%, 1.0% -1.5% and 1.5% -2.0%.
Al：1.5％～3.0％。
Al has a major role in conventional steels as a deoxidizer in the smelting process. In the present invention, the addition of Al consists in adjusting the process window of the critical section. The optimal annealing temperature of the traditional medium manganese steel is kept between 650 and 700 ℃, which is difficult to match with the existing continuous annealing production line. According to the invention, 1.0-1.5% of Al is added, the annealing temperature is increased to more than 750 ℃, and the lower temperature limit of a continuous annealing production line is met.
V：0.10％～0.50％。
The V element is properly added into the steel, so that the precipitation strengthening effect in the coiling stage can be enhanced, the dislocation self-recovery phenomenon in the cold rolling process is inhibited, the retention of deformation energy storage is improved, and the recrystallization behavior in the continuous annealing stage is promoted; meanwhile, VC is precipitated in ferrite in the continuous annealing isothermal stage, so that the effect of precipitation strengthening can be achieved.
P≤0.01％。
The P element is an impurity element in the steel, is extremely easy to be biased to gather at a grain boundary, and is easy to form Fe when the P content in the steel is higher 2 P particles reduce the plasticity and toughness of the steel, so the lower the content is, the better. In the invention, the content of the P element is controlled below 0.01 percent.
S≤0.005％。
The S element is an impurity element in steel, and is easily combined with Mn to form MnS inclusions, so that the plasticity of the steel sheet is deteriorated, and the lower the S element content is, the better the S element content is. In the invention, the content of S element is controlled below 0.005%.
Additional added elements in the steel of the invention:
Ni≤1.0％。
ni is a solid solution strengthening element, and can improve the stability of austenite as C, mn; meanwhile, ni can improve the corrosion resistance of the steel plate to a certain extent. In the invention, a proper amount of the additive can be added to improve the corrosion resistance of the steel plate.
Cr≤1.0％，Mo≤1.0％。
Cr and Mo are solid solution strengthening elements and play a role in strengthening the steel plate. In the invention, cr and Mo can improve the hardenability of the steel plate, delay the formation of pearlite and bainite in the cooling stage and promote the formation of martensite; meanwhile, cr and Mo can change the type of iron scale in the coiling process, limit the progress of oxidation in the steel plate and improve the surface quality of the steel plate.
Cu≤1.0。
The Cu element itself is solid-dissolved in austenite to improve the strength of the steel sheet. In the continuous annealing process, the simple substance Cu is separated out from austenite to play a certain role in separation strengthening. In addition, the addition of Cu has a certain effect on the corrosion resistance of the steel sheet.
Mn+Ni+Cr+Mo+Cu≤10.0％。
As described above, the alloy elements such as Ni, cr, mo, cu areThe main effect of the Mn-supplementing substitute elements in the invention is to improve the stability of austenite and supplement the stability of austenite. However, increasing the ratio of alloying elements tends to change the overall stacking fault energy variation in the steel. Related studies indicate that when the stacking fault energy of austenite is less than 18mJ/m 2 When in austenite deformation, TRIP effect only occurs; when the stacking fault energy is between 18 and 25mJ/m 2 During the process, austenite simultaneously carries out TRIP effect and TWIP effect, and belongs to a composite phase change mechanism; when the stacking fault energy is more than 25mJ/m 2 When austenite grains are too stable, the TRIP or TWIP effect does not occur, and the work hardening mode is mainly dislocation strengthening. The main mode of work hardening of the steel plate is TRIP or TRIP+TWIP, thus controlling Mn+Ni+Cr+Mo+Cu to be less than or equal to 10.0 percent so as to ensure that the stacking fault is not more than 25mJ/m 2 。
Ti≤0.04％，Nb≤0.04％。
Ti and Nb are microalloy strengthening elements. Ti combines with impurity element N in steel to form TiN, and free N atoms in the steel exist in the steel to deteriorate the toughness of the steel plate, so that the formation of TiN has an effect of fixing N; in addition, ti forms Ti (C, N) with C, N, and plays a role of refining prior austenite grains. However, too much Ti content will cause excessive size of TiN, deteriorating the steel plate properties. Nb is mainly subjected to strain induction precipitation in a hot rolling recrystallization zone to form Nb (C, N) which plays a role in refining original austenite grains. However, too much Nb addition decreases the strength of the hot rolled steel sheet too high, and increases the cold rolling load. Therefore, the invention limits the addition amount of Ti and Nb in the steel to be within 0.04 percent.
B≤0.005％。
In the invention, the addition of B can supplement the hardenability of the steel plate and ensure the formation of martensite in the rapid cooling stage in the continuous annealing process. And too much B addition will increase the brittleness of the steel sheet, deteriorating the workability of the steel sheet.
Ca≤0.005％。
The morphology of the inclusion can be controlled by adding a proper amount of Ca, so that the quality of the casting blank steel plate is improved.
REM≤0.005％。
The rare earth element is added to form a high-melting-point rare earth compound with S, O element, so that the impurity content is reduced, meanwhile, the addition of the rare earth can promote grain refinement, and the strength and the plasticity of the steel plate are integrally improved.
The invention relates to a preparation method of a cold-rolled high-strength and high-ductility medium-manganese steel plate for a thin-specification automobile, which comprises the following steps: smelting and continuous casting, hot rolling, hood annealing and pickling, cold rolling, primary continuous annealing and pickling, secondary continuous annealing, finishing and the like. The process and parameter selection reasons are as follows:
1. smelting and continuous casting: industrial smelting and continuous casting are carried out according to the set chemical components, the casting temperature is 1580-1620 ℃, and the thickness of the casting blank is 170-230 mm.
2. And (3) hot rolling: the heating temperature is between 1230 and 1280 ℃, the furnace time is 150 to 300min, the rough rolling temperature is 1150 to 1200 ℃, and the thickness of the intermediate billet is 50 to 80mm; the finish rolling is divided into two stages of rolling, wherein the first stage rolling temperature is 1070-1130 ℃, the second stage rolling temperature is 960-1050 ℃, the final rolling temperature is above 920 ℃, and the coiling temperature is 700-760 ℃. The thickness of the hot rolled plate is between 2.8 and 3.5 mm.
Principle of: the heating temperature is controlled between 1230 and 1280 ℃ and the furnace time is 150 to 300 minutes, so as to promote the alloy to be fully dissolved and control the banded structure caused by segregation. The purpose of the finish rolling stage in two stages is to promote the recrystallization behavior of the prior austenite grains and to inhibit coarsening of unrecrystallized austenite grains. The purpose of the coiling temperature being controlled at 700-760 ℃ is to promote the formation of proeutectoid ferrite to a certain extent and prevent the formation of the coiling structure by the full martensite.
3. Hood annealing and pickling: the cover annealing temperature is 680-750 ℃ and the annealing time is 18-25 h; then the surface of the hot rolled and coiled steel plate is treated with FeO and Fe 2 O 3 、Fe 3 O 4 Iron oxide existing in different forms is removed by acid washing.
Principle of: the steel plate after hot rolling and coiling is a small amount of proeutectoid ferrite and martensite, and cold rolling cannot be directly performed, so that the hot rolling and coiling steel plate is subjected to cover annealing at 680-750 ℃ to ensure austenite transformation nucleation, and the cover annealing structure is a ferrite+austenite structure.
4. Cold rolling: the specification of the cold-rolled product is 1.4-1.8 mm, for example, a cold-rolled plate with the thickness of 1.4mm corresponds to a hot-rolled plate with the thickness of 2.8mm, cold-rolled plates with the thicknesses of 1.6mm and 1.8mm correspond to a hot-rolled plate with the thickness of 3.0-3.5 mm, and the cold rolling reduction is controlled to be 46.7-48.6%.
Principle of: too low rolling reduction cannot ensure enough cold rolling deformation energy storage, resulting in insufficient ferrite recrystallization effect in the continuous annealing stage; too high rolling reduction greatly increases the load of the cold rolling mill, and the achievement of the target thickness cannot be ensured.
5. Primary continuous annealing and acid washing: heating the cold-rolled steel plate to 800-900 ℃, carrying out isothermal treatment for 120-240 s, then slowly cooling to 720-760 ℃ at a cooling speed of 1.2-3.6 ℃/s, and then quenching the plate blank, wherein the quenching water temperature is controlled to be more than 80 ℃; and then the quenched steel plate is subjected to acid washing to remove surface oxides formed in the primary continuous annealing stage.
Principle of: the isothermal temperature of the primary continuous annealing is 800-900 ℃, and the aim is to form an austenite structure with a larger proportion of critical areas, and the higher isothermal temperature can promote the recrystallization behavior of ferrite in the critical areas, so as to provide more austenite transformation nuclear points; meanwhile, the higher isothermal temperature can promote Mn atoms in ferrite to diffuse into austenite, promote high-proportion austenite to be rich in Mn, and improve the stabilizing behavior of austenite in a critical region. In addition, the higher isothermal temperature promotes the sufficient dissolution of carbide and the precipitation of VC, which is beneficial to strengthening the steel plate. The aim of quenching to room temperature is to provide a good structure matrix for secondary continuous annealing, the quenched structure is ferrite, a small part of reverse austenite and a certain proportion of martensite, a small part of austenite is enriched by a certain amount of C, mn and a smaller grain size is reserved, and most of austenite structures obtained in critical areas are transformed into martensite structures after quenching due to a larger grain size.
After primary annealing, the structure is ferrite, martensite and retained austenite, wherein the ferrite content is 20% -40%, the martensite content is 40% -55%, and the retained austenite content is 8% -21%.
6. And (3) secondary continuous annealing: heating the quenched steel plate to 730-780 ℃, and carrying out isothermal treatment for 120-240 s; then slowly cooling to 720-760 ℃ at a cooling rate of 1.2-3.6 ℃/s, and then cooling to below 230 ℃ at a cooling rate of 10-18 ℃/s for overaging treatment, wherein the overaging time is 60-300 s.
Principle of: the isothermal temperature of secondary annealing is 730-780 ℃ and is used for matching with the component design of the steel plate, ideal reversed austenite content and better austenite stability can be obtained through secondary annealing at the isothermal temperature, the effective occurrence of austenite work hardening in the subsequent deformation stage is ensured, and better strong plastic matching is further ensured. In the process, the quenched martensite structure is subjected to austenite transformation again to form lath-shaped austenite, and the original retained austenite is formed at a ferrite grain boundary to be small in a block shape due to the higher Mn-rich degree and the retention of a fine lath structure, so that the Mn enrichment is continuously carried out in a secondary isothermal stage, and the transformation stability of the lath-shaped austenite is also improved. The aim of controlling the cooling speed to be 10-18 ℃/s is to prevent excessive bainite from being formed at a low cooling speed, and simultaneously prevent a small amount of austenite from generating martensitic transformation at a high cooling speed to influence the performance of the steel plate; the aim of controlling the overaging temperature below 230 ℃ is to prevent spinodal austenite decomposition at excessive temperatures.
The following examples are given by way of illustration of detailed embodiments and specific procedures based on the technical scheme of the present invention, but the scope of the present invention is not limited to the following examples.
[ example ]
The chemical compositions of the example steels are shown in Table 1, the continuous casting and hot rolling process parameters of the example steels are shown in Table 2, the cold rolling and continuous annealing process parameters of the example steels are shown in Table 3, the structure composition of the example steels are shown in Table 4, and the mechanical properties of the example steels are shown in Table 5. Fig. 1 to 3 are SEM scanning electron microscope images of three types of manganese steel sheets.
TABLE 1 chemical composition of the example steels, wt%
Table 2 hot rolling process parameters of the steels of each example
TABLE 3 Cold-rolled annealing process parameters for steels of examples
| Examples | T 1 /℃ | Ts 1 /℃ | t 1 /s | T 2 /℃ | Ts 2 /℃ | t 2 /min | SP/(℃/s) | T OA /℃ | 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 |
In Table 3, T 1 : primary annealing temperature; t (T) s1 : primary annealing and slow cooling temperature; t is t 1 : primary annealing isothermal time; t (T) 2 : secondary annealing temperature; t (T) s2 : secondary annealing and slow cooling temperature; t is t 2 : secondary annealing isothermal time; sP: fast cooling speed; t (T) OA : overaging temperature; t is t OA : overaging time
TABLE 4 structure composition of steels of examples
In table 4, F: ferrite; m martensite; b: bainite; RA: retained austenite
TABLE 5 force Properties of the steels of the examples
In table 5, YS: yield strength; TS: tensile strength; EL: elongation percentage; PSE: strong plastic accumulation; lambda: reaming value
The foregoing is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art, who is within the scope of the present invention, should make equivalent substitutions or modifications according to the technical scheme of the present invention and the inventive concept thereof, and should be covered by the scope of the present invention.
Claims (8)
1. The cold-rolled high-strength and high-ductility medium-manganese steel plate for the thin-specification automobile is characterized by comprising the following chemical components in percentage by mass: 0.15 to 0.40 percent of Mn:3.0 to 10.0 percent, si:0.5 to 2.0 percent of Al:1.5 to 3.0 percent, V:0.10 to 0.50 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; and (3) adding: less than or equal to 1.0 percent of Ni, less than or equal to 1.0 percent of Cr, less than or equal to 0.5 percent of Mo, less than or equal to 1.0 percent of Cu, less than or equal to 10.0 percent of Mn+Ni+Cr+Mo+Cu, less than or equal to 0.04 percent of Ti, less than or equal to 0.04 percent of Nb, less than or equal to 0.005 percent of B, less than or equal to 0.005 percent of Ca, less than or equal to 0.005 percent of REM, and the balance of Fe and unavoidable impurities; the steel plate has the following properties: the yield strength is more than or equal to 600MPa, the tensile strength is more than or equal to 980MPa, the elongation is more than or equal to 30 percent, the strength-plastic product is more than or equal to 30GPa, and the reaming ratio is more than or equal to 30 percent.
2. The cold-rolled high-strength product medium-manganese steel sheet for a thin-gauge automobile according to claim 1, wherein the medium-manganese steel sheet is an L-MnTRIP medium-manganese steel sheet; the steel comprises the following chemical components in percentage by mass: 0.25 to 0.40 percent, mn:3 to 5 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.1 to 0.2 percent, P is less than or equal to 0.01 percent, and S is less than or equal to 0.005 percent; in addition, ni, cr, mo and Cu are added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 5.0%; the steel plate structure is composed of ferrite, austenite, bainite and 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 less than or equal to 5%.
3. The cold-rolled high-strength product medium-manganese steel sheet for a thin-gauge automobile according to claim 1, wherein the medium-manganese steel sheet is an H-MnTRIP medium-manganese steel sheet; the steel comprises the following chemical components in percentage by mass: 0.15 to 0.25 percent of Mn:5 to 8 percent of Si, 0.5 to 1.5 percent of Al:1.5 to 2.0 percent, V:0.15 to 0.25 percent of Ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 8.0 percent; the steel plate structure is composed of ferrite, austenite and martensite, wherein the ferrite content is 40-50%, the austenite content is 40-50%, and the martensite content is less than or equal to 10%; the steel plate has the following properties: the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 1080MPa, the elongation is more than or equal to 40%, the strength-plastic product is more than or equal to 40GPa, and the reaming ratio is more than or equal to 30%.
4. The cold rolled high strength product medium manganese steel sheet for a thin gauge automobile according to claim 1, wherein the medium manganese steel sheet is an H-MnTRIP/TWIP medium manganese steel sheet; the steel comprises the following chemical components in percentage by mass: 0.30 to 0.40 percent, mn:8 to 10 percent of Si, 1.5 to 2.0 percent of Al:2.0 to 3.0 percent, V:0.20 to 0.50 percent, ni, cr, mo and Cu are additionally added, and Mn+Ni+Cr+Mo+Cu is less than or equal to 10 percent; the steel plate structure is ferrite and austenite, wherein the ferrite content is 30-40%, and the austenite content is 60-70%; the steel plate has the following properties: the yield strength is more than or equal to 800MPa, the tensile strength is more than or equal to 1180MPa, the elongation is more than or equal to 50%, the strength-plastic product is more than or equal to 60GPa, and the reaming ratio is more than or equal to 30%.
5. The method for manufacturing the cold-rolled high-strength and high-plasticity medium-manganese steel plate for the thin-gauge automobile according to any one of claims 1 to 4, which is characterized by comprising the processes of smelting, continuous casting, hot rolling, hood annealing and pickling, cold rolling, primary continuous annealing and pickling, secondary continuous annealing and finishing; wherein the following processes are controlled:
1) Primary continuous annealing and acid washing: heating the cold-rolled steel plate to 800-900 ℃, carrying out isothermal treatment for 120-240 s, then slowly cooling to 720-760 ℃ at a cooling speed of 1.2-3.6 ℃/s, and finally carrying out quenching treatment, wherein the quenching water temperature is controlled to be more than 80 ℃;
2) And (3) secondary continuous annealing: heating the quenched steel plate to 730-780 ℃, and carrying out isothermal treatment for 120-240 s; then slowly cooling to 720-760 ℃ at a cooling rate of 1.2-3.6 ℃/s, and then cooling to below 230 ℃ at a cooling rate of 10-18 ℃/s for overaging treatment, wherein the overaging time is 60-300 s.
6. The method for manufacturing a cold rolled high strength and elongation manganese steel sheet for thin gauge automobile according to claim 5, wherein the following process is additionally controlled:
1) Smelting and continuous casting: smelting according to the set chemical components, wherein the casting temperature is 1580-1620 ℃;
2) And (3) hot rolling: heating at 1230-1280 deg.c for 150-300 min; the rough rolling temperature is 1150-1200 ℃; the finish rolling adopts two-stage rolling, the first stage rolling temperature is 1070-1130 ℃, the second stage rolling temperature is 960-1050 ℃, and the finish rolling finishing temperature is above 920 ℃; the coiling temperature is 700-760 ℃;
3) Hood annealing and pickling: the cover annealing temperature is 680-750 ℃ and the annealing time is 18-25 h;
4) Cold rolling: the rolling reduction is controlled between 46.7% and 48.6%.
7. The method for manufacturing the cold-rolled high-strength and high-ductility medium-manganese steel plate for the thin-gauge automobile according to claim 5, wherein the thickness of the continuous casting blank is 170-230 mm, and the thickness of the rough rolling intermediate blank is 50-80 mm; the thickness of the hot rolled plate is 2.8-3.5 mm, and the specification of the cold rolled plate is 1.4-1.8 mm.
8. The method for manufacturing a cold rolled high strength and elongation medium manganese steel sheet for thin gauge automobile according to claim 5, wherein the steel sheet structure after primary annealing is ferrite + martensite + retained austenite, wherein the ferrite content is 20% to 40%, the martensite content is 40% to 55%, and the retained austenite content is 8% to 21%.
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