CN110343970B - Hot-rolled high-strength-ductility medium manganese steel with lower Mn content and preparation method thereof - Google Patents

Hot-rolled high-strength-ductility medium manganese steel with lower Mn content and preparation method thereof Download PDF

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CN110343970B
CN110343970B CN201910679886.XA CN201910679886A CN110343970B CN 110343970 B CN110343970 B CN 110343970B CN 201910679886 A CN201910679886 A CN 201910679886A CN 110343970 B CN110343970 B CN 110343970B
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manganese steel
medium manganese
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CN110343970A (en
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阳锋
韩赟
姜英花
刘华赛
谢春乾
邱木生
潘丽梅
白雪
滕华湘
陈斌
曹杰
章军
朱国森
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Shougang Corp
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Abstract

The invention provides a hot-rolled high-strength-plastic-product medium manganese steel with lower manganese content and a preparation method thereof, wherein the hot-rolled high-strength-plastic-product medium manganese steel comprises the following chemical components in percentage by weight: c: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, and the balance of Fe and inevitable impurities. The preparation method of the hot-rolled high-strength-ductility medium manganese steel comprises the following steps: and smelting the molten steel in a converter, and then obtaining a continuous casting blank in a continuous casting mode, heating the continuous casting blank in a heating furnace, then carrying out hot rolling to obtain a hot rolled coil, and annealing the hot rolled coil in a bell-type furnace to obtain a hot rolled high-strength-plasticity product medium manganese steel finished product. The invention ensures that the hot-rolled medium manganese steel has higher product of strength and elongation on the premise of ensuring good toughness, and has good practical application significance.

Description

Hot-rolled high-strength-ductility medium manganese steel with lower Mn content and preparation method thereof
Technical Field
The invention belongs to the technical field of automobile steel, and particularly relates to hot-rolled high-strength-ductility medium manganese steel with lower Mn content and a preparation method thereof.
Background
With the gradual improvement of the requirements of the automobile industry on energy conservation, emission reduction and collision safety, the medium manganese steel for automobiles with high product of strength and elongation becomes the focus of people's attention. At present, the product of strength and elongation of the hot-rolled medium manganese steel can reach about 60 GPa%. After annealing in the two-phase region, the medium manganese steel mainly consists of fine ferrite and metastable austenite, wherein the ferrite has very low C content and the chemical composition of the ferrite is similar to that of Fe-Mn alloy. It is known that Fe-Mn alloys tend to be brittle, and the higher the grain boundary Mn concentration in the alloy, the more brittle fracture along the grain is generated in the Fe-Mn alloy. The Mn concentration of the crystal boundary is mainly influenced by two factors, namely the Mn content of a matrix and a heat treatment process. If the content of the matrix Mn is higher, the concentration of the grain boundary Mn is also higher; if the cooling rate after austenitizing is slower, Mn tends to be more easily segregated toward ferrite grain boundaries during cooling, and the ferrite grain boundary concentration tends to be higher. Earlier studies show that when the Mn content in the Fe-Mn alloy is 7% (mass percent, the same applies below), the Fe-Mn alloy is subjected to austenitization and is cooled to room temperature at any cooling speed, and the Fe-Mn alloy is subjected to brittle fracture when an impact test is carried out at the temperature of-40 ℃ due to the fact that the concentration of Mn in a grain boundary is relatively high. When the Mn content of the matrix is reduced to 5 percent, the Fe-Mn alloy is in ductile fracture at minus 40 ℃ after austenitizing and then cooling to room temperature in a quenching or air cooling mode; after the alloy is cooled to room temperature by furnace cooling, Mn is promoted to be segregated to the grain boundary in the slow cooling process, so that the Fe-Mn alloy is brittle and broken. Therefore, the Mn content in the medium manganese steel cannot be too high, which would otherwise lead to embrittlement of the ferrite of one of the constituent phases, and thus to a risk of embrittlement of the medium manganese steel as well. In the annealing process of the medium manganese steel coil in the bell-type furnace, because the soaking time is long (generally more than 1 hour), Mn in ferrite has sufficient time to diffuse into austenite, and finally the Mn content in the ferrite is close to the Mn content under the equilibrium state and is lower than that of a medium manganese steel matrix. Thus, the Mn content of the matrix of the medium manganese steel subjected to the bell-type furnace annealing treatment can be properly increased. In the current patents on medium manganese steels, there is general attention on how to achieve higher product of strength and elongation by adjusting the alloy composition or heat treatment process, and no attention has been paid to the adverse effect of Mn content on the ferrite toughness of one of the medium manganese steel constituent phases.
Under the principle, the content of alloy elements such as C, Al in the hot-rolled medium manganese steel is reasonably designed, and the high-strength-ductility medium manganese steel with the strength-ductility product of 45-60 GPa% can be obtained on the premise of ensuring the good toughness of the hot-rolled medium manganese steel.
Disclosure of Invention
In view of the above, the invention provides a hot-rolled high-product-of-strength-and-elongation medium manganese steel with a lower Mn content and a preparation method thereof, wherein the content of alloy elements such as C, Al in the hot-rolled medium manganese steel is reasonably designed on the premise of the lower Mn content, so that the high-product-of-strength-and-elongation medium manganese steel with a product of strength and elongation of 45-60 GPa% can be obtained on the premise of ensuring the good toughness of the hot-rolled medium manganese steel.
The invention provides hot-rolled high-strength-ductility medium manganese steel with lower Mn content, wherein a room-temperature structure is a fine ferrite and metastable austenite dual-phase structure, the volume fraction of metastable austenite is between 15 and 60 percent, the metastable austenite size is less than 1 mu m, and the fine ferrite size is not more than 5 mu m. The chemical components comprise: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0 to 3.0 percent of the total weight of the mixture,
si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, and the balance of Fe and inevitable impurities.
The actions and the limited ranges of the main alloy elements are explained in detail as follows:
manganese Mn: the main austenitizing element, the content is critical, and too high a content embrittles the matrix. Research shows that when the Mn content in the Fe-Mn alloy is 7% (mass percentage, the same below), the Fe-Mn alloy is subjected to austenitization and is cooled to room temperature at any cooling speed, and the Fe-Mn alloy is subjected to brittle fracture when an impact test is carried out at the temperature of minus 40 ℃ due to the fact that the concentration of Mn in a grain boundary is relatively high. When the Mn content of the matrix is reduced to 5 percent, the Fe-Mn alloy is in ductile fracture at minus 40 ℃ after austenitizing and then cooling to room temperature in a quenching or air cooling mode; after the alloy is cooled to room temperature by furnace cooling, Mn is promoted to be segregated to the grain boundary in the slow cooling process, so that the Fe-Mn alloy is brittle and broken. Therefore, the Mn content in the medium manganese steel cannot be too high, which would otherwise lead to embrittlement of the ferrite of one of the constituent phases, and thus to a risk of embrittlement of the medium manganese steel as well. Theoretical calculation and experimental determination prove that the Mn content should be controlled to be 3.0-6.0%.
C, carbon C: the contents of main austenitizing elements and interstitial solid solution strengthening elements are important. A lower content may result in an insufficient content of metastable austenite; higher contents lead to deviations from the lath-like morphology of the rolled structure and thus to lower metastable austenite stability. The carbon content should be controlled to 0.19-0.5%.
Aluminum Al: in addition to deoxidation and grain refinement, the aluminum element may also inhibit precipitation of carbides during coiling and annealing. More importantly, the two-phase region of the aluminum is expanded on one hand, so that the selectable annealing temperature range is expanded; on the other hand, the aluminum can also increase the stacking fault energy of the metastable austenite, and the reasonable aluminum content can ensure that the stacking fault energy of the metastable austenite is in a range capable of simultaneously generating a TWIP effect and a TRIP effect, thereby achieving the purpose of improving the matrix product of strength and elongation; in addition, aluminum is a ferrite-forming element, and can inhibit the generation of austenite, and the austenite with reduced volume fraction can absorb more alloying elements such as carbon, manganese and the like from the matrix, thereby improving the stability of metastable austenite. However, the aluminum content should not be too high, otherwise the production is difficult and coarse delta ferrite is easily generated, which is disadvantageous in tensile strength. The invention controls the content of aluminum at 1.0-3.0%.
Silicon Si: the silicon element can inhibit the precipitation of carbide in the coiling and annealing processes, can also enlarge a two-phase region, and can improve the ferrite strength by dissolving silicon in a solid solution in ferrite. However, too high silicon content can increase the red rust defect on the hot rolling surface and reduce the surface quality of the finished product. Therefore, the addition amount of silicon depends on the application, and the silicon content of the invention is 0-2.0%.
Chromium Cr: the chromium element can inhibit the precipitation of carbide to a certain degree and can also improve the corrosion resistance of the matrix. The addition amount of chromium depends on the application, and the chromium content adopted by the invention is 0-5.0%.
Nickel Ni: the nickel element can increase the stability of austenite and effectively improve the toughness of the matrix. The nickel content depends on the application, and the nickel content adopted by the invention is 0-2.0%.
Preferably, on the basis of the chemical compositions, similar mechanical properties can be obtained by adding one or more alloy compositions. I.e. the chemical composition further comprises one or more of the following alloy compositions: v, Nb, Ti, Cu, B, Mo, Zr, W, Co or Re.
The invention provides a preparation method of the hot-rolled high-strength-plastic-product medium manganese steel with lower Mn content, which comprises the following specific steps:
s1, producing a casting blank by molten steel continuous casting or producing an ingot by die casting, wherein the molten steel comprises the following chemical components: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, the balance being Fe and unavoidable impurities;
s2, cogging the casting blank or the ingot obtained in the step S1, then carrying out hot rolling, coiling after rolling, and air cooling to room temperature;
and S3, annealing the hot rolled plate or the coiled plate obtained in the step S2, and then cooling to room temperature.
Based on the reasonable design of the chemical components, the invention further designs a preparation process, including smelting, continuous casting, hot rolling and bell-type furnace annealing, especially an annealing process, so that the medium manganese steel contains a large amount of metastable austenite with moderate stability, and the metastable austenite can continuously generate TRIP or TWIP effect in the deformation process, and finally the hot-rolled high strength-elongation medium manganese steel with low Mn content is obtained.
Preferably, in step S2, the hot rolling process includes: heating the steel billet to 1120-.
More preferably, in step S2, the final rolling temperature of the multi-pass hot rolling is 820-.
Preferably, in step S2, the curling temperature is 500-600 ℃.
Preferably, in step S3, the annealing is performed in a hood-type annealing furnace, and the annealing process includes: and annealing at 660-740 ℃ for 1-30 h to realize the recovery and recrystallization of ferrite tissues and the reversion of metastable austenite, and then obtaining the medium manganese steel with high product of strength and elongation.
In step S3, the tensile strength of the obtained hot-rolled high-strength-product medium manganese steel is more than or equal to 800MPa, the yield strength is more than or equal to 550MPa, the elongation A80 is more than or equal to 45%, and the strength-product is 45-60 GPa%.
Compared with the prior art, the invention has the following advantages: according to the invention, the high-strength-ductility hot-rolled medium manganese steel is obtained on the premise of reducing the Mn content of a matrix and reducing the embrittlement risk of the medium manganese steel, metastable austenite with the volume fraction of 15% -60% is obtained by increasing the C content on the basis of narrowing the Mn content, meanwhile, the stability and the stacking fault energy of the metastable austenite are adjusted by Al element, and by matching with other chemical composition design and process design, a large amount of stable metastable austenite in the medium manganese steel continuously generates TRIP or TWIP effect in the deformation process, so that the hot-rolled high-strength-ductility medium manganese steel with lower Mn content is prepared, the tensile strength is 800-1200 MPa, the yield strength is not less than 550MPa, the elongation is 45-60%, and the strength-ductility can reach about 60GPa ·, and the hot-rolled high-strength-ductility medium manganese steel has a good practical application prospect.
Drawings
FIG. 1 is a scanning electron microscope image of the structure of hot-rolled high-strength-ductility medium manganese steel with lower Mn content after 1h of bell-type furnace annealing in the embodiment 1 of the invention.
FIG. 2 is an engineering stress-strain curve of hot-rolled high-strength-plastic-product medium manganese steel with lower Mn content after 1h of bell-type furnace annealing in example 1 of the invention.
Detailed Description
In order to facilitate understanding of the present invention, the present invention will be described in more detail with reference to the following examples, but the scope of the present invention is not limited to the following specific examples.
Unless otherwise defined, all terms of art used hereinafter have the same meaning as commonly understood by one of ordinary skill in the art. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the present invention. For example, the room temperature may be a temperature within a range of 10 to 35 ℃.
Unless otherwise specifically stated, various raw materials, reagents, instruments, equipment and the like used in the present invention are commercially available or can be prepared by existing methods.
The invention provides a hot-rolled high-strength-ductility medium manganese steel with lower Mn content, which comprises the following chemical components: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, and the balance of Fe and inevitable impurities. The chemical composition can also comprise one or more of the following alloy compositions: v, Nb, Ti, Cu, B, Mo, Zr, W, Co or Re.
The invention provides a preparation method of the hot-rolled high-strength-plastic-product medium manganese steel with lower Mn content, which comprises the following steps:
s1, producing a casting blank by molten steel continuous casting or producing an ingot by die casting, wherein the molten steel comprises the following chemical components: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, the balance being Fe and unavoidable impurities;
s2, cogging the casting blank or ingot obtained in the step S1, and then carrying out hot rolling, wherein the hot rolling process comprises the following steps: heating the steel billet to 1120-;
s3, putting the hot rolled plate or the coiled plate obtained in the step S2 into a bell-type furnace for annealing, wherein the annealing process comprises the following steps: annealing at 660-740 ℃ for 1-30 h, and then cooling to room temperature, wherein the tensile strength of the obtained hot-rolled high-strength-plastic-product medium manganese steel is not less than 800MPa, the yield strength is not less than 550MPa, the elongation A80 is not less than 45%, and the strength-plastic-product is 45-60 GPa%.
The hot rolled high product of strength and elongation medium manganese steel having a low Mn content and the method for manufacturing the same provided by the present application will be described in detail with reference to specific examples.
Example 1
The embodiment provides a hot-rolled high-strength-plastic-product medium manganese steel with low Mn content, which comprises the following chemical components in percentage by weight: 0.45%, Mn: 6.0%, Al: 2.0%, Si: 1.0%, Cr: 0.7%, Ni: 0.4%, and the balance of Fe and inevitable impurities.
The preparation method of the hot-rolled high-strength-ductility medium manganese steel with lower Mn content comprises the following steps:
heating the steel billet to 1200 ℃, soaking for about 2 hours, carrying out multi-pass hot rolling to obtain a steel plate with the thickness of 4mm, carrying out final rolling at the temperature of 820 ℃, then coiling at the temperature of 550 ℃, and carrying out air cooling to room temperature.
The hot rolled steel coil is put into a cover type heating furnace for annealing, and is kept at 700 ℃ for 1h, and then is cooled to room temperature.
The structure scanning electron microscope picture of the hot-rolled medium manganese steel with high strength-elongation product is shown as the attached figure 1.
And processing the annealed hot-rolled medium manganese steel into a standard tensile sample according to GB/T228-2002 'metal material room temperature tensile test method', and performing quasi-static tensile according to the test standard. The mechanical properties results are shown in table 1, and a typical tensile curve is shown in fig. 2.
TABLE 1 mechanical Properties of Hot rolled high Strength product Medium manganese steels with lower Mn content
Yield strength (MPa) Tensile strength (MPa) Total elongation (%) Product of strength and elongation (GPa%) Ductile-brittle transition temperature (. degree. C.)
530 972 62 60 ≤-40
Example 2
The embodiment provides a hot-rolled high-strength-plastic-product medium manganese steel with low Mn content, which comprises the following chemical components in percentage by weight: 0.38%, Mn: 3.0%, Al: 2.1%, Si: 0.8%, Cr: 3.1%, Ni: 1.8%, V: 0.2%, B: 0.2%, and the balance of Fe and inevitable impurities.
The preparation method of the hot-rolled high-strength-ductility medium manganese steel with lower Mn content comprises the following steps:
heating the steel billet to 1180 ℃, soaking for about 2 hours, carrying out multi-pass hot rolling to obtain a steel plate with the thickness of 5mm, carrying out final rolling at 850 ℃, then coiling at 525 ℃ and carrying out air cooling to room temperature.
And (3) putting the hot rolled steel coil into a cover type heating furnace for annealing, preserving the heat for 3h at the temperature of 670 ℃, and then cooling to room temperature to obtain the hot rolled medium manganese steel with high strength-plastic product, wherein the mechanical property of the hot rolled medium manganese steel is basically consistent with that of the example 1.
Example 3
The embodiment provides a hot-rolled high-strength-plastic-product medium manganese steel with low Mn content, which comprises the following chemical components in percentage by weight: 0.29%, Mn: 3.5%, Al: 1.5%, Si: 1.3%, Cr: 2.0%, Ni: 0.8%, Mo: 0.5%, and the balance of Fe and inevitable impurities.
The preparation method of the hot-rolled high-strength-ductility medium manganese steel with lower Mn content comprises the following steps:
heating the steel billet to 1190 ℃, soaking for about 2 hours, carrying out multi-pass hot rolling to obtain a steel plate with the thickness of 5mm, carrying out final rolling at 860 ℃, then coiling at 565 ℃, and carrying out air cooling to room temperature.
And (3) putting the hot rolled steel coil into a cover type heating furnace for annealing, preserving the heat for 2 hours at 700 ℃, and then cooling to room temperature to obtain the hot rolled medium manganese steel with high strength-plastic product, wherein the mechanical property of the hot rolled medium manganese steel is basically consistent with that of the medium manganese steel in the example 1.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including preferred embodiments and all such alterations and modifications as fall within the scope of the invention.
It will be apparent to those skilled in the art that various changes and modifications may be made in the present invention without departing from the spirit and scope of the invention. Thus, if such modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include such modifications and variations.

Claims (1)

1. A hot-rolled high-strength-ductility medium manganese steel with lower Mn content is characterized in that: the chemical components comprise: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, the balance being Fe and unavoidable impurities;
the room temperature structure is a fine ferrite and metastable austenite dual-phase structure, the volume fraction of metastable austenite is between 15 and 60 percent, the size of metastable austenite is less than 1 mu m, and the size of fine ferrite is not more than 5 mu m; the chemical components also comprise one or more of V, Nb, Ti, Cu, B, Mo, Zr, W, Co or Re; the preparation method of the hot-rolled high-strength-ductility medium manganese steel with low Mn content comprises the following steps:
s1, producing casting blanks by continuous casting of molten steel or producing casting ingots by die casting, wherein the molten steel comprises the following chemical components: c, by weight percentage: 0.19 to 0.5%, Mn: 3.0-7.0%, Al: 1.0-3.0%, Si: 0-2.0%, Cr: 0-5.0%, Ni: 0-2.0%, the balance being Fe and unavoidable impurities;
s2, cogging the casting blank or the ingot obtained in the step S1, then carrying out hot rolling, coiling after rolling, and air cooling to room temperature;
s3, annealing the hot rolled plate or the coiled plate obtained in the step S2, and then cooling to room temperature;
in step S2, the hot rolling process includes: heating the steel billet to 1120-;
in the step S2, the final rolling temperature of the multi-pass hot rolling is 820-900 ℃;
in step S2, the curling temperature is 500-600 ℃;
in step S3, the annealing is performed in a hood-type annealing furnace, and the process includes: annealing at 660-740 ℃ for 1-30 h;
in step S3, the tensile strength of the obtained hot-rolled high-strength-product medium manganese steel is more than or equal to 800MPa, the yield strength is more than or equal to 550MPa, the elongation A80 is more than or equal to 45%, and the strength-product is 45-60 GPa%.
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