High-strength and high-ductility steel sheet and method of manufacturing the same

Abstract

A high-strength and high-ductility steel sheet having a composition including, by weight, 1.0 to 1.4% C, 5.0 to 9.0% Mn, 2.0 to 8.0% Cr and the balance Fe, and unavoidable impurities. The steel sheet has an austenite structure formed at room temperature, and stacking fault energy is effectively controlled by the addition of Cr and N2. Mechanical twins are formed during the plastic deformation of the steel, thereby leading to high levels of work hardening, tensile strength and workability.

Description

CROSS REFERENCE TO RELATED APPLICATION[0001]The present application claims priority from Korean Patent Application Number 10-2013-00125214 filed on Oct. 21, 2013, the entire contents of which are incorporated herein for all purposes by this reference.BACKGROUND OF THE INVENTION[0002]1. Field of the Invention[0003]The present invention relates to a high-strength and high-ductility steel sheet, and more particularly, to an automotive steel sheet for which high workability is required, a high manganese (Mn) steel sheet applicable as a shock absorber such as a vehicle bumper stiffener, and a method of manufacturing the same.[0004]2. Description of Related Art[0005]Steel sheets applicable for vehicle bodies typically require high workability. In order to satisfy this requirement, ultra-low carbon steel has been mainly used for automotive steel sheets in the related art regardless of its low tensile strength ranging from 200 to 300 MPa, since it has high workability. Recently, a variety o...

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Benefits of technology

[0015]Also provided is a steel sheet able to achieve both high strength and high ductility while reducing the content of Mn.

[0033]At a Mn content less than 5.0% by weight, Fe—Mn binary alloys have ε martensite or α′ martensite partially formed instead of a single austenite phase at room temperature. In order to form a single austenite phase structure at room by overcoming this problem, C can be desirably added as an inexpensive and highly effective austenite stabilizing element. At a C content less than 0.8% by weight, it is difficult to obtain a single austenite phase during cooling after hot rolling since austenite stability is still insufficient. Even if the single austenite phase is obtained at room temperature, phase transformation occurs from austenite to martensite during plastic deformation to form TRIP steel, due to insufficient austenite stability. Consequently, TWIP steel intended in the present invention cannot be obtained. On the other hand, at a C content exceeding 1.4% by weight, stable austenite can be obtained at room temperature, but cementite precipitation occurs, thereby decreasing elongation and reducing weldability. Even if the cooling rate is controlled after annealing heat treatment, it is still difficult to control the precipitation of carbides. Since C increases stacking fault energy, a large C content makes it difficult to form mechanical twins during deformation. Accordingly, it is preferable that the C content is limited to the range from 1.0 to 1.4% by weight.

[0036]Although it is known that Cr decreases stacking fault energy in stainless steel like Mn. In contrast, in Fe—Mn—C ternary alloys, Cr increases stacking fault energy. If the Cr content exceeds 8.0% by weight, the stacking fault energy of austenite becomes excessively high. Hardening is caused by simple perfect dislocation movement instead of mechanical twins during plastic deformation, making it difficult to achieve either high strength or high ductility. In addition, since Cr is a ferrite stabilizing element, the Cr content exceeding 8.0% by weight may cause partial formation of ferrite during hot rolling. Furthermore, the use of a large amount Cr significantly increases manufacturing costs. Therefore, it is preferable that the Cr content is limited to the range from 2.0 to 8.0% by weight.

[0038]As set forth above, the steel sheet according to the present invention has the austenite structure formed at room temperature while containing a small amount of Mn. In addition, the stacking fault energy is effectively controlled. Therefore, mechanical twins are formed during the plastic deformation of steel, leading to high levels of work hardening, tensile strength and workability. That is, in the steel sheet according to the present invention, the product of the tensile strength and the total elongation (TS×El) has a very large value of 30,000MPa % or greater. The product of the tensile strength and the total elongation is substantially the same and manufacturing costs are significantly reduced comparing to those of related-art TWIP steel having a Mn content of about 20% by weight.

Problems solved by technology

Steel sheets applicable for vehicle bodies typically require high workability.

Although TRIP steel has the advantage of high strength caused by significant hardening through the martensitic transformation, its elongation is too short, which is problematic.

However, the extremely low ductility of martensite makes it difficult to achieve an elongation of 30% or more, which is problematic.

Further, TWIP steel may have large elongation and high tensile strength due to the mechanical twins causing a high level of work hardening.

However, current TWIP steel has a high Mn content ranging from about 18% to about 30% in order to guarantee austenite stability and adjust stacking fault energy, and requires the addition of large amounts of aluminum or silicon together with manganese, causing a significant increase in material and manufacturing costs.

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