KR20140085088A - High specific strength steel sheet with excellent ductility and method of manufacturing the same - Google Patents

Abstract

The present invention relates to a high specific strength steel sheet with excellent ductility including 0.15 to 0.5 wt% of carbon (C), 6.0 to 8.0 wt% of manganese (Mn), 5.0 to 6.0 wt% of aluminum (Al), 0.05 to 0.5 wt% of silicon (Si), less than 0.02 wt% (excluding 0) of sulfur (S), and the remainder consisting of iron (Fe) and inevitable impurities. The yield strength is greater than or equal to 550 MPa. Tensile strength multiplied by elongation is greater than or equal to 28,000 MPa%.

Description

TECHNICAL FIELD [0001] The present invention relates to a high strength steel sheet having excellent ductility and a manufacturing method thereof,

The present invention relates to a high strength steel sheet excellent in ductility and capable of contributing to weight reduction of transportation equipment including automobiles and a method for manufacturing the same.

In recent years, due to rising energy prices, there is a growing demand for light weight for improving the fuel efficiency of transportation equipment including automobiles. At the same time, there is an increasing need for a steel plate having both strength and ductility due to integration of parts and complexity of shapes. Up to now, a method of reducing the thickness or reducing the size of parts has been mainly used instead of applying a steel plate having a higher strength than the conventional one in order to reduce the weight of the transportation equipment parts. However, the rigidity of the parts is lowered, do. Therefore, in order to meet the continuously increasing weight-reduction requirement, it is becoming more important to improve the non-strength (strength / specific gravity) by lowering the specific gravity while the strength is high. As a method of lowering the specific gravity of the steel, it is known to add Al, which is a substitutional alloy element having a small atomic weight, and it has been reported that the specific gravity reduction effect is about 1.5% per 1% Al by weight.

Aluminum-containing lightweight steels are classified into ferritic steels and austenitic steels by phase. Ferritic steels have a tensile strength of 400 to 600 MPa and an elongation of 20 to 30%. The product of tensile strength and elongation is limited to 10,000 to 15,000 MPa%. On the other hand, although a case where the tensile strength is high is also reported, the elongation is low and the workability is insufficient.

On the other hand, the austenitic lightweight steel has been reported to have excellent properties expressed as a product of tensile strength and elongation. However, these austenitic light-weight steels generally have a low yield strength of 300 to 500 MPa, which is the biggest drawback of lowering the industrial applicability. To overcome this, there is known a method of improving the yield strength by applying plastic working, There is a problem that the manufacturing cost is increased. Further, in order to obtain a stable austenite phase at room temperature while containing a large amount of Al, which is a strong ferrite stabilizing element, the content of austenite stabilizing elements such as Mn must be high, which also increases the manufacturing cost and makes industrial use difficult.

Accordingly, the present invention has been made to solve the problems of the conventional ferritic and austenitic lightweight steels, and it has been found that the alloy contains a large amount of Al and the alloy element content such as Mn is lower than that of the austenitic lightweight steel, It is necessary to develop a method for easily obtaining excellent high strength steel.

It is an object of the present invention to provide a high strength steel sheet having a yield strength of 550 MPa or more, a product of a tensile strength and an elongation multiplied by 28,000 MPa% or more, and a method of manufacturing the same. . However, these problems are exemplary and do not limit the scope of the present invention.

A high strength steel sheet excellent in ductility according to one aspect of the present invention is provided. The high ductility steel sheet excellent in ductility contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si, less than 0.02% of S (excluding 0) , And the remainder being Fe and other unavoidable impurities.

The high strength steel sheet having excellent ductility is characterized by containing 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu, 0.01 to 2.0% 0.01 to 1.0%.

The high strength steel sheet having excellent ductility may further contain at least one selected from the group consisting of 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr, .

The high strength steel sheet having excellent ductility may further contain at least one member selected from the group consisting of 0.001 to 0.02% of Ca, 0.001 to 0.2% of Mg, 0.0005 to 0.01% of B, and 0.0005 to 0.05% of N, .

The high-strength steel sheet excellent in ductility contains 25 to 75% by volume of austenite in the structure of the steel sheet, and when plastic deformation is applied, a part of the austenite is transformed into martensite and deformed until the steel sheet is broken due to plastic deformation 20% or more of the total amount of the austenite can be converted into martensite.

A method of manufacturing a high strength hot rolled annealed steel sheet excellent in ductility according to another aspect of the present invention is provided. A method for producing a high strength hot-rolled annealed steel sheet excellent in ductility, comprising: 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si, less than 0.02% of S (Excluding 0), the balance being Fe and other unavoidable impurities; heating the steel slab to 1000 to 1250 캜; A hot rolling step of hot-rolling the heated steel slab at 750 DEG C or higher and cooling the steel slab to form a steel sheet; Annealing the hot-rolled and cooled steel sheet at 800 to 1000 占 폚; And a cooling step of cooling the annealed steel sheet at a temperature of 100 ° C to 700 ° C at a rate of 1 ° C / s or higher.

A method for manufacturing a high strength cold rolled annealed steel sheet excellent in ductility according to another aspect of the present invention is provided. A method for producing a high strength cold rolled annealing steel sheet excellent in ductility comprising 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si, less than 0.02% of S, (Excluding 0), the balance being Fe and other unavoidable impurities; heating the steel slab to 1000 to 1250 캜; A hot rolling step of hot-rolling the heated steel slab at 750 DEG C or higher and cooling the steel slab to form a steel sheet; A cold rolling step of pickling the hot-rolled and cold-rolled steel sheet followed by cold rolling at 200 ° C or lower; Annealing the cold-rolled steel sheet at 800 to 1000 占 폚; And a cooling step of cooling the annealed steel sheet at a temperature of 100 ° C to 700 ° C at a rate of 1 ° C / s or higher.

Wherein the steel slab comprises 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cr, To 2.0%, Cu: 0.01 to 2.0%, and W: 0.01 to 1.0%.

Wherein the steel slab contains 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, 0.005 to 1.0% of Zr, and 0.005 to 1.0% of vanadium, And 0.005% to 1.0%.

Wherein the steel slab comprises 0.001 to 0.02% of Ca, 0.001 to 0.2% of Ca, 0.0005 to 0.01% of B, and 0.0005 to 0.01% of B, And 0.0005 to 0.05%.

According to an embodiment of the present invention as described above, the high strength steel sheet having excellent ductility has a microstructure containing 25 to 75% of austenite in a volume fraction, a density of 7.4 g / cm 3 or less, and a yield strength And the product of tensile strength and elongation is higher than 28,000 MPa% due to the deformation organic transformation phenomenon of austenite. Therefore, it is required to have high non-rigidity and formability for manufacturing parts, . ≪ / RTI >

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. It is provided to let you know. In the drawings, the components may be exaggerated or reduced in size for convenience of explanation.

The present invention provides effective means for obtaining high yield strength, tensile strength and elongation by controlling the composition and production conditions of Al-added lightweight steels and controlling the austenite volume fraction and stable degree in the microstructure.

Hereinafter, the composition range of a high strength steel sheet having excellent ductility according to an embodiment of the present invention will be described in detail. (% By weight)

(1) C: As C is an austenite stabilizing element, the volume fraction of austenite increases as the addition amount thereof increases, and the strength of the solution strengthening is increased, thereby improving the strength of austenite. However, in a high strength steel sheet excellent in ductility according to an embodiment of the present invention, when the C content exceeds 0.5%, the carbide tends to precipitate in the austenite grains or in the grain boundaries in the process of hot rolling or cooling after annealing, In this case, it was confirmed that the volume fraction of austenite was reduced and ductility was remarkably lowered. On the other hand, the inventors have found that when the addition amount of C is less than 0.15%, the volume fraction of austenite becomes low and the yield strength, tensile strength and elongation both decrease. Therefore, its content is limited to 0.15 ~ 0.5%.

(2) Mn: Mn is an austenite stabilizing element, which increases the volume fraction and stability of austenite. In a high strength steel sheet excellent in ductility according to an embodiment of the present invention, when the content of manganese is less than 6.0%, the stability of austenite is low and carbides are precipitated in austenite grains or grain boundaries during hot rolling or cooling after annealing Martensitic transformation occurred and the volume fraction of austenite was decreased and ductility was remarkably decreased. The inventors have found that the higher the Mn content is, the higher the austenite volume fraction and the ductility are improved, but the yield strength is lowered and the manufacturing cost is increased. Particularly, when the Mn content exceeds 8.0% Respectively. Therefore, its content is limited to 6.0 ~ 8.0%.

(3) Al: Al reduces the volume fraction of austenite with ferrite stabilizing element. In the high-strength steel sheet excellent in ductility according to an embodiment of the present invention, the inventors have found that it is preferable to add 5.0% or more of the steel as a substitute lightweight element, since it is effective in reducing the specific gravity of the steel as the content is high. It is necessary to add a large amount of elements such as Mn and Ni in order to secure austenite, so that the manufacturing cost is increased. In addition, the stacking fault energy of austenite is increased, so that deformation organic transformation phenomenon is difficult to occur and ductility is lowered. Therefore, its content is limited to 5.0 ~ 6.0%.

(4) Si: According to one embodiment of the present invention, the inventor of the present invention found that Si added in an amount of 0.05% or more in a high ductility steel sheet having excellent ductility is a substitutional lightweight element similar to Al and lowers the specific gravity of steel, It is advantageous to relax the change of austenite stability according to the annealing temperature and thus to lower the quality deviation. On the other hand, when Si is added in an amount exceeding 0.5%, the ductility is largely lowered, and surface oxidation is excessively generated during hot rolling, thereby deteriorating the surface quality. Therefore, the content thereof is limited to 0.05 to 0.5%.

(5) S: In a high strength steel sheet having excellent ductility according to an embodiment of the present invention, S lowers hot workability and toughness to cause high-temperature brittleness, so that the content thereof is preferably less than 0.02% (excluding 0) Respectively.

Ni, Mo, Co, Cu, Ti, V, Nb, and the like to improve the properties such as strength, ductility and toughness, if necessary, in addition to the above elements as well as Fe and other unavoidable impurities. , Zr, W, Sn, Sb, Ca, Mg, B and N may be added.

(6) Cr: The inventor has found that when Cr is added in an amount of 0.01% or more, the strength and toughness are improved. However, when it is added in an amount exceeding 2.0%, the austenite volume fraction is lowered and the ductility is lowered. To 2.0%.

(7) Ni: The inventor of the present invention has the effect of improving ductility and toughness and stabilizing austenite when added with 0.01% or more of Ni. However, when the cost is high and the cost is increased and the excess amount is over 3.0% It is confirmed that the modified organic transformation is suppressed and the ductility is lowered. Therefore, the content thereof is limited to 0.01 to 3.0%.

(8) Mo: The inventor has the effect of improving the strength and toughness by adding Mo at a content of 0.01% or more. However, when the cost is high and the production cost is increased and the excess amount exceeds 2.0%, the inventor lowers the austenite volume fraction and increases the specific gravity. , The content thereof is limited to 0.01 to 2.0%.

(9) Co: The inventor confirmed that the addition of 0.01% or more of Co has an effect of improving ductility and toughness, but it is limited to 0.01 to 2.0% because the cost is high and the manufacturing cost is increased.

(10) Cu: The inventor has found that when Cu is added in an amount of 0.01% or more, it has an effect of improving ductility and toughness, but when it is added in an amount exceeding 2.0%, it is confirmed that it causes high-temperature brittleness. Limit.

(11) W: The inventor has found that the addition of 0.01% or more of W improves the strength and toughness. However, since it is confirmed that it increases the manufacturing cost by increasing the cost and increases the specific gravity when it is added in excess of 1.0% 0.01 to 1.0%.

(12) Ti: When Ti is added in an amount of 0.005% or more, the inventors formed nitrides or carbides to refine the crystal grains and improve the strength and toughness. However, when the cost is high and the manufacturing cost is increased and the excess is added in excess of 1.0% Since it has been confirmed that the ductility is lowered, its content is limited to 0.005 to 1.0%.

(13) V: The inventors have found that when V is added in an amount of 0.005% or more, nitrides or carbides are formed to refine the crystal grains and improve strength and toughness. However, when the cost is high and the production cost is increased and the excess is added in excess of 1.0% Since it has been confirmed that the ductility is lowered, its content is limited to 0.005 to 1.0%.

(14) Nb: When Nb is added in an amount of 0.005% or more, the inventors have found that nitrides or carbides are formed to refine the crystal grains and improve the strength and toughness. However, when the cost is high and the manufacturing cost is increased, It is confirmed that the ductility is lowered and the specific gravity is increased, so the content thereof is limited to 0.005 to 1.0%.

(15) Zr: The inventors have found that when Zr is added in an amount of 0.005% or more, nitrides or carbides are formed to miniaturize grains and improve strength and toughness. However, when the cost is high and the production cost is increased, It is confirmed that the ductility is lowered and the specific gravity is increased, so the content thereof is limited to 0.005 to 1.0%.

(16) Ca: The inventor of the present invention has the effect of lowering the specific gravity when Ca is added in an amount of 0.001% or more and improving the hot workability by forming an emulsion by combining with S causing high temperature brittleness. However, It is confirmed that the ductility is lowered, and its content is limited to 0.001 ~ 0.02%.

(16) Mg: When Mg is added in an amount of 0.001% or more, the inventor has the effect of lowering the specific gravity and improving the hot workability by forming an emulsion by binding with S causing high-temperature brittleness. However, Since it has been confirmed that the ductility is lowered, the content thereof is limited to 0.001 to 0.2%.

(17) B: The inventors found that when B is added in an amount of 0.0005% or more, it is segregated at the grain boundaries to refine the grain and improve the strength. However, it has been confirmed that when the B content is over 0.01% To 0.0005 to 0.01%.

(18) N: The inventors found that when N is added in an amount of 0.0005% or more, nitrides are formed to inhibit grain coarsening and improve strength and stabilize austenite. However, when N is added in excess of 0.05% The content is limited to 0.0005 ~ 0.05%.

The high strength steel sheet according to embodiments of the present invention may have various combinations of the above-mentioned compositions, and the high strength steel sheet according to embodiments of the present invention having a specific composition range may be exemplarily listed as follows .

First steel plate

The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.15 to 0.5%, Mn is 6.0 to 8.0%, Al is 5.0 to 6.0%, Si is 0.05 to 0.5%, S is less than 0.02% (excluding 0) Steel plate containing impurities. In the first steel sheet, Fe in the balance may be 85% to 88.8% by weight.

Second steel plate

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. At least one selected from the group consisting of 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu and 0.01 to 1.0% of W Further comprising: And the remainder being Fe and other unavoidable impurities.

The third steel sheet

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr; And the remainder being Fe and other unavoidable impurities.

4th Steel

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. At least one selected from the group consisting of 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu and 0.01 to 1.0% of W Further comprising: 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr; And the remainder being Fe and other unavoidable impurities.

The fifth steel sheet

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. Further comprising at least one member selected from the group consisting of Ca: 0.001 to 0.02%, Mg: 0.001 to 0.2%, B: 0.0005 to 0.01%, and N: 0.0005 to 0.05% And the remainder being Fe and other unavoidable impurities.

The sixth steel sheet

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. At least one selected from the group consisting of 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu and 0.01 to 1.0% of W Further comprising: Further comprising at least one member selected from the group consisting of Ca: 0.001 to 0.02%, Mg: 0.001 to 0.2%, B: 0.0005 to 0.01%, and N: 0.0005 to 0.05% And the remainder being Fe and other unavoidable impurities.

7th Steel

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr; Further comprising at least one member selected from the group consisting of Ca: 0.001 to 0.02%, Mg: 0.001 to 0.2%, B: 0.0005 to 0.01%, and N: 0.0005 to 0.05% And the remainder being Fe and other unavoidable impurities.

8th steel sheet

The steel sheet contains 0.15 to 0.5% of C, 6.0 to 8.0% of Mn, 5.0 to 6.0% of Al, 0.05 to 0.5% of Si and less than 0.02% of S (excluding 0) in weight%. At least one selected from the group consisting of 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu and 0.01 to 1.0% of W Further comprising: 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr; Further comprising at least one member selected from the group consisting of Ca: 0.001 to 0.02%, Mg: 0.001 to 0.2%, B: 0.0005 to 0.01%, and N: 0.0005 to 0.05% And the remainder being Fe and other unavoidable impurities.

Hereinafter, a method for manufacturing a high strength steel sheet having excellent ductility according to an embodiment of the present invention will be described in detail.

[Slab heating]

First, a steel slab satisfying the above composition is prepared and then heated to 1000 to 1250 캜. The inventors have found that when the heating temperature is less than 1000 캜, the hot-rolled steel sheet is difficult to produce a thin steel sheet because of the low hot-rolling temperature, and the surface oxide film can not be removed due to high-pressure water spraying during rolling. In addition, the inventors confirmed that when the heating temperature exceeds 1250 ° C, there is a problem that uneven deformation and hot-rolled property appear due to formation of a coarseness in a steel slab. Therefore, the slab heating temperature is limited to the range of 1000 to 1250 占 폚.

[Hot Rolling]

The heated steel slab is hot-rolled at 750 ° C or higher and cooled to room temperature. The inventors have found that if the hot rolling is performed at a temperature lower than 750 캜, the strength of the steel increases and the rolling load becomes high, which makes rolling difficult. Therefore, the temperature of the hot rolling is limited to 750 DEG C or higher.

[Cold Rolling]

When the cold-rolled annealed steel sheet is manufactured, the hot-rolled steel sheet is pickled to remove the oxide layer on the surface, followed by cold rolling. The reduction rate is preferably 40% or more so that recrystallization can sufficiently occur during annealing, but it is not necessary to limit the reduction. If necessary, the hot-rolled steel sheet can be heated and rolled for more smooth rolling. In this case, if the temperature of the steel sheet is excessively increased, an oxidation layer is formed on the surface, so that the heating temperature is limited to 200 ° C or less.

[Annealing]

The hot-rolled steel sheet or the cold-rolled steel sheet is annealed in a temperature range of 800 to 1000 ° C. When the annealing temperature is less than 800 ° C., the strain is not so high that the austenite volume fraction is less than 25% or the volume fraction is more than 25%, and therefore the strain is not induced. Thus, the product of the tensile strength and elongation is obtained to be 28,000 MPa% or more it's difficult. On the other hand, when the annealing temperature exceeds 1000 캜, the stability of the austenite is lowered, and a large amount of austenite is transformed into martensite even in a small deformation, so that the tensile strength is increased but the elongation is greatly reduced. When the annealing temperature exceeds 1000 캜, the yield strength is often as low as less than 550 MPa, so the annealing temperature is limited to 800 to 1000 캜. The holding time at the annealing temperature is preferably 10 to 600 seconds in consideration of the time and productivity required for recrystallization and austenite formation.

[Cooling]

The annealed steel sheet is cooled to room temperature, and the cooling rate is 1 ° C / s or more in a temperature range of 100 to 700 ° C. The longer the holding time in the temperature range of 100 to 700 占 폚, the more easily the austenite grains are formed or the carbide is formed in the grain boundaries. The formation of such carbides lowers the austenite volume fraction and stability, .

Hereinafter, the microstructure of a high strength steel sheet having excellent ductility according to an embodiment of the present invention will be described in detail.

The high strength steel sheet having excellent ductility according to an embodiment of the present invention includes 25 to 75% of austenite in a volume fraction and the remainder may vary depending on the composition and manufacturing conditions and includes ferrite, martensite, carbide and the like . The volume fraction and stable degree of austenite contained in the microstructure of the steel sheet of the present invention play a key role in obtaining excellent strength and ductility. When the austenite is adequately stable, a plastic deformation phenomenon occurs in which a part of the austenite is transformed into martensite by plastic deformation, thereby improving the elongation and improving the tensile strength when the transformation organic transformation is active.

When the volume fraction of austenite is as low as less than 25%, it is difficult to obtain a product of tensile strength and elongation of 28,000 MPa% or more because the effect of improving the tensile strength and elongation due to the transformation organic transformation phenomenon is small. On the other hand, the higher the volume fraction of austenite is, the larger the effect of strain organic transformation becomes, but the yield strength tends to be lowered. Therefore, it is preferable to have a volume fraction of 25 to 75%.

On the other hand, the stable degree of austenite affects both the volume fraction and the deformation organic transformation phenomena. When the austenite is less stable, the austenite formed during annealing is transformed into martensite, ferrite or carbide during cooling, and it is difficult to obtain a volume fraction of 25% or more at room temperature. In such a case, the elongation is largely lowered. On the other hand, when the austenite is excessively stable, strain-induced transformation does not occur or is weak, and it is difficult to obtain a product of tensile strength and elongation at 28,000 MPa% or more. Therefore, austenite is stable enough to have a volume fraction of 25 to 75% at room temperature, and its stable degree is not excessively high, so that at least 20% of the austenite before deformation until fracture of the steel sheet due to plastic deformation becomes martensite It is preferable that it is a degree of phase transformation.

Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the technical idea of the present invention is not limited to the following specific embodiments.

<Examples>

Ingots having the composition shown in Table 1 below were prepared, heated to 1200 ° C. and held for 1 hour, hot-rolled at a temperature of 900 ° C. or higher to 3.0 to 3.5 mm, maintained at 650 ° C. for 1 hour and cooled to room temperature . Some of the hot - rolled steel sheets were pickled to remove the oxide layer and then cold rolled at a reduction ratio of 20 to 70%. In this process, most of the comparative steels were broken. The hot-rolled steel sheet and the cold-rolled steel sheet were annealed under the conditions shown in Table 2 and cooled to room temperature. The holding time at the annealing temperature was 120 seconds. The yield strength, tensile strength and elongation of the annealed specimens were measured by tensile test. The volume fraction of austenite was measured by an X-ray diffraction test and a direct comparison method, and the tensile test piece and the fractured test piece were measured for uniformly stretched portions, Respectively.

As can be seen from the following Table 2, Inventive Examples 1 to 16 according to the embodiments of the present invention exhibited excellent yield strengths of 550 MPa or more and tensile strength and elongation products of 28,000 MPa% or more. Specifically, the high strength steel sheet according to the embodiments of the present invention has a yield strength of 565 MPa to 797 MP, and a product of tensile strength and elongation is 30,053 MPa% to 51,000 MPa%. That is, the high-strength hot-rolled annealed steel sheet according to the embodiments of the present invention has a yield strength of 575 MPa to 797 MP, a product of tensile strength and elongation of 30,053 MPa to 49,476 MPa% The cold-rolled annealed steel sheet according to the present invention has a yield strength of 565 MPa to 781 MP and a product of tensile strength and elongation of 34,960 MPa% to 51,000 MPa%.

On the other hand, in Comparative Examples 1 to 3 in which the inventive steel was used but the annealing temperature or cooling rate did not match the range of the present invention and Comparative Examples 4 to 8 in which the composition did not meet the range of the present invention, the yield strength was 550 MPa or less Or the product of tensile strength and elongation is 28,000 MPa% or less. In the case of the comparative example, the elongation rate is particularly low as compared with the case of the present invention. In this case, the austenite is too low in the volume fraction of less than 25% or the austenite is excessively stable even if it is 25% Is less than 20%.

The foregoing description of specific embodiments of the invention has been presented for purposes of illustration and description. It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention as defined by the appended claims. Do.

Claims (10)

The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.15 to 0.5%, Mn is 6.0 to 8.0%, Al is 5.0 to 6.0%, Si is 0.05 to 0.5%, S is less than 0.02% (excluding 0) Wherein the steel has a yield strength of 550 MPa or more and a product of tensile strength and elongation of 28,000 MPa% or more. The steel sheet according to claim 1, wherein the steel sheet comprises 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu, 0.01 to 2.0% of Cu, : 0.01 to 1.0%. &Lt; / RTI &gt; The steel sheet according to claim 1, wherein the steel sheet contains at least one selected from the group consisting of 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr, Further comprising a high strength steel sheet having excellent ductility. The steel sheet according to claim 1, wherein the steel sheet contains at least one selected from the group consisting of 0.001 to 0.02% of Ca, 0.001 to 0.2% of Mg, 0.0005 to 0.01% of B, and 0.0005 to 0.05% of N, Further comprising a high strength steel sheet having excellent ductility. The steel sheet according to any one of claims 1 to 4, wherein the steel sheet contains 25 to 75% of austenite in a volume fraction, and when plastic deformation is applied, a part of the austenite is transformed into martensite, Wherein the steel sheet is phase-transformed into martensite by 20% or more of the austenite before deformation until the steel sheet is broken. The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.15 to 0.5%, Mn is 6.0 to 8.0%, Al is 5.0 to 6.0%, Si is 0.05 to 0.5%, S is less than 0.02% (excluding 0) A heating step of heating the steel slab to 1000 to 1250 占 폚;
A hot rolling step of hot-rolling the heated steel slab at 750 DEG C or higher and cooling the steel slab to form a steel sheet;
Annealing the hot-rolled and cooled steel sheet at 800 to 1000 占 폚; And
A cooling step of cooling the annealed steel sheet at a rate of 1 DEG C / s or more in the range of 100 to 700 DEG C;
Wherein the high strength steel sheet has excellent ductility. The steel sheet according to any one of claims 1 to 3, wherein the content of C is 0.15 to 0.5%, Mn is 6.0 to 8.0%, Al is 5.0 to 6.0%, Si is 0.05 to 0.5%, S is less than 0.02% (excluding 0) A heating step of heating the steel slab to 1000 to 1250 占 폚;
A hot rolling step of hot-rolling the heated steel slab at 750 DEG C or higher and cooling the steel slab to form a steel sheet;
A cold rolling step of pickling the hot-rolled and cold-rolled steel sheet followed by cold rolling at 200 ° C or lower;
Annealing the cold-rolled steel sheet at 800 to 1000 占 폚; And
A cooling step of cooling the annealed steel sheet at a rate of 1 DEG C / s or more in the range of 100 to 700 DEG C;
Wherein the high strength steel sheet has excellent ductility. 8. The method according to any one of claims 6 to 7,
Wherein the steel slab comprises 0.01 to 2.0% of Cr, 0.01 to 3.0% of Ni, 0.01 to 2.0% of Mo, 0.01 to 2.0% of Co, 0.01 to 2.0% of Cu, 0.01 to 2.0% Wherein the steel sheet further comprises at least one member selected from the group consisting of iron and iron. 8. The method according to any one of claims 6 to 7,
Wherein the steel slab further comprises at least one selected from the group consisting of 0.005 to 1.0% of Ti, 0.005 to 1.0% of V, 0.005 to 1.0% of Nb, and 0.005 to 1.0% of Zr, A method for manufacturing the excellent high strength steel sheet. 8. The method according to any one of claims 6 to 7,
Wherein the steel slab further comprises at least one member selected from the group consisting of 0.001 to 0.02% of Ca, 0.001 to 0.2% of Mg, 0.0005 to 0.01% of B, and 0.0005 to 0.05% of N, A method for manufacturing the excellent high strength steel sheet.