HIGH-STRENGTH AND HIGH-DUCTILITY STEEL SHEET AND METHOD OF MANUFACTURING THE SAME
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.
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 INVENTION1. Field of the Invention
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.
2. Description of Related Art
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 of methods for improving vehicle fuel efficiency have been proposed in response to environmental issues such as air pollution. In particular, as a lighter weight of a vehicle is considered important to improve fuel efficiency, automotive steel sheets are required to have not only high workability but also high strength.
In addition, the necessity of commercializing high-strength steel is significantly increasing since some vehicle parts, such as bumper stiffeners or shock absorbers within the vehicle doors, are directly related to the safety of the occupants. For example, the use of ultra-high-strength steel having a high tensile strength of typically 780 MPa or higher and a large elongation is required.
Examples of such high-strength automotive steel include dual phase (DP) steel, transformation induced plasticity (TRIP) steel and twin induced plasticity (TWIP) steel.
First, DP steel is produced by lowering the cooling end temperature below the martensite start temperature (Ms) in the process of quenching hot-rolled steel to room temperature so that part of austenite transforms to martensite. Consequently, the dual phases of martensite transformed from austenite and ferrite exist at room temperature. Such DP steel can have a variety of mechanical properties due to an adjustment in the proportions of martensite and ferrite.
TRIP steel has improved workability, and is produced by partially forming retained austenite in a structure, followed by the martensitic transformation of austenite during the machining of components. 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.
The hardening mechanism of either DP steel or TRIP steel is typically based on martensite, the hard phase. Martensite exhibits a significant increase in hardening during plastic deformation, thereby enabling the manufacture of high-strength hot-rolled steel. However, the extremely low ductility of martensite makes it difficult to achieve an elongation of 30% or more, which is problematic.
On the other hand, TWIP steel contains a large amount of Mn and has a single austenite phase, which is stable at room temperature and allows mechanical twins to form in the austenite structure during the machining of component, thereby increasing the level of work hardening. That is, TWIP steel has an austenite structure instead of a ferrite structure as a matrix and has improved elongation through additional work hardening by continuously generating mechanical twins in austenite grains to obstruct movement of the dislocations during plastic deformation. Further, TWIP steel may have large elongation and high tensile strength due to the mechanical twins causing a high level of work hardening. In particular, TWIP steel has elongation 50% larger than that of conventional DP steel or TRIP steel and is thus preferably applied to steel sheets for automobiles.
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. Moreover, there is a need for the development of TWIP steel having a low Mn content in order to avoid an additional increase in manufacturing costs caused by volatilization of Mn or temperature decrease during a steel manufacturing process or continuous casting process. Furthermore, in terms of mechanical properties, since currently developed TWIP steel has a low yield strength of about 300 MPa and a tensile strength of 1 GPa or less, there is a need for steel sheets which have a higher strength without deteriorating elongation.
The information disclosed in the Background of the Invention section is provided only for better understanding of the background of the invention and should not be taken as an acknowledgment or any form of suggestion that this information forms a prior art that would already be known to a person skilled in the art.
BRIEF SUMMARY OF THE INVENTIONVarious aspects of the present invention provide a steel sheet able to overcome the problems of dual phase (DP) steel, transformation induced plasticity (TRIP) steel and twin induced plasticity (TWIP) steel of the related art.
Also provided is a steel sheet able to achieve both high strength and high ductility while reducing the content of Mn.
In an aspect of the present invention, provided is a high-strength and high-ductility steel sheet having a composition including, by weight, 0.8 to 1.4% C, 5.0 to 10.0% Mn, 2.0 to 8.0% Cr and the balance Fe, and unavoidable impurities.
The steel sheet may be formed of twinning-induced plasticity steel.
The steel sheet may exclude chromium carbides.
The steel sheet may be a hot-rolled steel sheet heat-treated at a temperature of 1100° C. or higher.
The hot-rolled steel may be manufactured by heating the steel sheet at a temperature of 1100° C. or higher, hot-rolling the heated steel sheet at a temperature of 900° C. or higher, and subsequently cooling the hot-rolled steel sheet by air cooling or forced cooling.
The steel sheet may be a cold-rolled annealed steel manufactured by cold-rolling the hot-rolled steel.
The cold-rolled annealed steel may be manufactured by cold-rolling the cooled steel sheet with a thickness reduction ratio of 30% or greater at room temperature, annealing the cold-rolled steel sheet at a temperature of 800° C. or higher, and subsequently cooling the annealed steel sheet by air cooling or forced cooling.
In the steel sheet, a value obtained by multiplying the tensile strength and the total elongation of the steel sheet may be 30,000 MPa % or greater.
The amount of the Cr may range from 4.0 to 7.0% by weight.
The composition may further include, by weight, 0.1 to 2.0% Al.
In an aspect of the present invention, provided is a method of manufacturing a high-strength and high-ductility steel sheet. The method includes the following operations of: preparing a steel sheet having a composition comprising, by weight, 0.8 to 1.4% carbon, 5.0 to 10.0% manganese, 2.0 to 8.0% chromium and the balance iron, and unavoidable impurities; heating the steel sheet at a temperature of 1100° C. or higher; hot-rolling the heated steel sheet at a temperature of 900° C. or higher; and cooling the hot-rolled steel sheet by air cooling or forced cooling.
The method may further include the operations of: cold-rolling the cooled steel sheet with a thickness reduction ratio of 30% or greater at room temperature; annealing the cold-rolled steel sheet at a temperature of 800° C. or higher; and cooling the annealed steel sheet by air cooling or forced cooling.
The reasons why the composition of the steel sheet according to the invention is limited as above will be described as follows.
Mn: 5.0 to 10.0 wt %
Twin induced plasticity (TWIP) steel must have an austenite phase at room temperature after being hot rolled, since mechanical twins are formed in an austenite matrix at room temperature during plastic deformation. Mn is an austenite stabilizing alloying element that allows austenite, i.e. a high-temperature phase in the Fe—C binary phase diagram, to form at room temperature. At a Mn content less than 5% by weight, the austenite phase becomes remarkably unstable. After hot rolling, a ferrite or martensite phase is formed in the austenite structure during cooling, whereby no mechanical twins can be formed during plastic deformation. Accordingly, the Mn content is required to be 5.0% by weight or greater.
At a Mn content exceeding 10.0% by weight, it is possible to produce a single austenite phase and form mechanical twins at room temperature. However, there are no significant differences from related-art TWIP steel. Thus, some problems, such as expensive manufacturing costs, degraded weldability and inserts, may still occur. Therefore, according to the invention, the Mn content is limited to the range from 5.0 to 9.0% by weight.
C: 0.8 to 1.4 wt %
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.
Cr: 2.0 to 8.0 wt %
Cr has been mainly used in stainless steel since it improves corrosion resistance. Cr not only functions as a ferrite stabilizing element, but also stabilizes austenite by lowering martensite transformation temperature when added to austenite steel. When Cr is added to the Fe—Mn—C ternary system, Cr can control martensite transformation to promote the formation of mechanical twins in the austenite matrix. In contrast, at a Cr content less than 2.0% by weight, austenite stability is insufficient, and strain induced martensite is formed instead of mechanical twins during plastic deformation, thereby producing TRIP steel.
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.
The composition of the steel sheet according to the present invention may selectively include Al. Hydrogen permeation into the steel sheet according to the invention may cause problems involving hydrogen embrittlement. These problems can be effectively overcome by the addition of Al. Although the Al content is not specifically limited, Al is typically added at an amount of 2.0% or less by weight.
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.
In addition, the high manganese nitrogen-containing steel sheet can be implemented as a variety of steel sheets, such as a hot-rolled steel sheet and a cold-rolled annealed steel sheet.
The methods and apparatuses of the present invention have other features and advantages that will be apparent from, or are set forth in greater detail in the accompanying drawings, which are incorporated herein, and in the following Detailed Description of the Invention, which together serve to explain certain principles of the present invention.
Reference will now be made in detail to exemplary embodiments of a steel sheet and a method of manufacturing the same according to the present invention.
First, a steel sheet according to an exemplary embodiment of the invention has a composition including, by weight, 0.8 to 1.4% C, 5.0 to 10.0% Mn, 2.0 to 8.0% Cr and the balance Fe, and unavoidable impurities.
The present invention has been devised in order to overcome the problems of twin induced plasticity (TWIP) steel of the related art. In the steel sheet according to this embodiment, suitably adjusted amounts of C, Cr and the like are added, whereas the Mn content is lowered to be, by weight, 10% or less. This can consequently obtain a single austenite phase at room temperature while minimizing the Mn content. Accordingly, the composition of the steel sheet according to the present invention includes these alloy elements, thereby achieving a large elongation and high yield strength and tensile strength than related-art TWIP steel while reducing the contents of expensive alloy elements, such as Mn and Al, below those of the related-art TWIP steel.
Specifically, the steel sheet according to an exemplary embodiment of the invention includes, by weight, 5.0 to 10.0% Mn. Since TWIP steel has mechanical twins formed in an austenite matrix at room temperature during plastic deformation, it is important to expand an austenite region from high temperature to room temperature on the Fe—C phase diagram. In addition, stacking fault energy must be about 50 mmJ/m2 or less in order to form mechanical twins during plastic deformation. According to the present invention, Mn is employed as an austenite stabilizing element.
At a Mn content less than 5% by weight, austenite stability is significantly reduced, allowing ferrite or martensite to form in the austenite region during cooling after hot rolling. In addition, at the Mn content less than 5% by weight, the stacking fault energy of the austenite phase excessively increases, thereby making it difficult to form mechanical twins.
On the other hand, if the Mn content is too great, the stacking fault energy excessively increases, such that plastic deformation may occur in the austenite phase. In addition, since one object of the invention is to minimize the content of expensive Mn, it is preferable to maintain the Mn content within 10% by weight according to this embodiment in order to reduce manufacturing costs as low as possible.
In addition, the composition of the steel sheet according to this embodiment includes, by weight, 0.8 to 1.4% C. Here, Fe—Mn binary alloys have ε or α′ martensite partially formed therein instead of a single austenite phase at room temperature. Therefore, according to this embodiment, C is added as an inexpensive and effective austenite stabilizing element in order to obtain a single austenite phase structure at room temperature.
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 carbide 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 is an element that increases stacking fault energy, a large C content makes it difficult to form mechanical twins during deformation, which is problematic. Accordingly, it is preferable that the C content is limited to the range from 1.0 to 1.4% by weight.
Furthermore, the composition of the steel sheet according to this embodiment includes, by weight, 2.0 to 8.0% Cr. Cr has been mainly used in stainless steel since it improves corrosion resistance. Cr not only functions as a ferrite stabilizing element, but also stabilizes austenite by lowering martensite transformation temperature when added to austenite steel. When Cr is added to the Fe—Mn—C ternary system, Cr can control martensite transformation to promote the formation of mechanical twins in the austenite matrix. In contrast, at a Cr content less than 2.0% by weight, austenite stability is insufficient, and strain induced martensite is formed instead of mechanical twins during plastic deformation, thereby producing TRIP steel.
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.
EXAMPLES3 mm thick steel sheets, the chemical compositions of which are presented in Table 1 below, were formed by heating at a temperature of 1100° C. or higher, followed by hot rolling at a temperature of 900° C. or higher. The steel sheets were subsequently subjected to oil cooling or water cooling, thereby manufacturing steel samples (Inventive Examples 1 to 3 and Comparative Examples 1 to 7). In addition, part of the hot-rolled steel samples were subjected to annealing heat treatment at 800 to 1200° C. for 5 to 10 minutes, followed by oil cooling or water cooling.
The strength and elongation were measured from the samples manufactured by the above-described process, and the results are presented in Table 2. As presented in Table 2, tensile properties were significantly different according to the annealing temperatures even at the same composition. This relates to chromium carbides formed by the addition of Cr. As illustrated in
Comparative Examples 4, 5 and 6 have a greater amount of precipitations within the austenite matrix than the examples annealed at 1000° C., since Comparative Examples 4, 5 and 6 were annealed at 800° C. where the precipitation of chromium carbides is most active. Since the stability within the austenite matrix is significantly lowered, stress induced martensite is formed during plastic deformation. It is appreciated that the maximum tensile stress is very high but the elongation is very short.
Even in the case where Mn or C is within the range of the inventive examples, if Cr is added at an amount smaller than the reference content, no superior tensile properties are obtained, as apparent from Comparative Example 7. The microstructure of Inventive Example 3 during plastic deformation was observed using a scanning electron microscope (SEM) in order to examine the formation of mechanical twins during plastic deformation. As illustrated in
As set forth above, the exemplary embodiments of the high-strength and high-ductility steel sheet and the method of manufacturing the same according to the present invention have been described in detail. However, a person skilled in the art can make various alternatives and modifications of the exemplary embodiments. It should be therefore understood that the scope of the present invention shall be defined by the Claims appended hereto and their equivalents.
Claims
1. A high-strength and high-ductility steel sheet having a composition comprising, by weight, 0.8 to 1.4% carbon, 5.0 to 10.0% manganese, 2.0 to 8.0% chromium and the balance iron, and unavoidable impurities.
2. The steel sheet according to claim 1, comprising twinning-induced plasticity steel.
3. The steel sheet according to claim 1, excluding chromium carbides.
4. The steel sheet according to claim 1, comprising a hot-rolled steel sheet heat-treated at a temperature of 1100° C. or higher.
5. The steel sheet according to claim 4, wherein the hot-rolled steel is manufactured by heating the steel sheet at a temperature of 1100° C. or higher, hot-rolling the heated steel sheet at a temperature of 900° C. or higher, and subsequently cooling the hot-rolled steel sheet by air cooling or forced cooling.
6. The steel sheet according to claim 4, comprising a cold-rolled annealed steel manufactured by cold-rolling the hot-rolled steel.
7. The steel sheet according to claim 6, wherein the cold-rolled annealed steel is manufactured by cold-rolling the cooled steel sheet with a thickness reduction ratio of 30% or greater at room temperature, annealing the cold-rolled steel sheet at a temperature of 800° C. or higher, and subsequently cooling the annealed steel sheet by air cooling or forced cooling.
8. The steel sheet according to claim 1, wherein a value obtained by multiplying a tensile strength with a total elongation is 30,000 MPa % or greater.
9. The steel sheet according to claim 1, wherein an amount of the chromium ranges from 4.0 to 7.0% by weight.
10. The steel sheet according to claim 1, wherein the composition further comprises, by weight, 0.1 to 2.0% aluminum.
11. A method of manufacturing a high-strength and high-ductility steel sheet, the method comprising:
- preparing a steel sheet having a composition comprising, by weight, 0.8 to 1.4% carbon, 5.0 to 10.0% manganese, 2.0 to 8.0% chromium and the balance iron, and unavoidable impurities;
- heating the steel sheet at a temperature of 1100° C. or higher;
- hot-rolling the heated steel sheet at a temperature of 900° C. or higher; and
- cooling the hot-rolled steel sheet by air cooling or forced cooling.
12. The method according to claim 11, further comprising:
- cold-rolling the cooled steel sheet with a thickness reduction ratio of 30% or greater at room temperature;
- annealing the cold-rolled steel sheet at a temperature of 800° C. or higher; and
- cooling the annealed steel sheet by air cooling or forced cooling.
13. The method according to claim 11, wherein an amount of the chromium ranges, by weight, 4.0 to 7.0%.
14. The method according to claim 11, wherein the composition further comprises, by weight, 0.1 to 2.0% aluminum.
Type: Application
Filed: Oct 21, 2014
Publication Date: Apr 23, 2015
Inventors: Young-Kook LEE (Seoul), Yeon-Seung JUNG (Seoul), Singon KANG (Seoul), Jeogho HAN (Inchon-si), Dongjoon MIN (Seoul)
Application Number: 14/519,287
International Classification: C21D 8/02 (20060101); C22C 38/38 (20060101); C21D 6/00 (20060101);
