Low specific gravity and high strength steel sheets with excellent ridging resistibility and manufacturing methods thereof

1. A low specific gravity and high strength steel sheet comprising on the basis of weight %: C: 0.2% to 0.8%, Mn: 2% to 10%, P: 0.02% or less, S: 0.015% or less, Al: 5.1% to 15%, and N: 0.01% or less, wherein a ratio of Mn/Al is 0.4 to 1.0, wherein the steel sheet has ridging height of 10 μm or less under conditions of 2.5 mm cutoff after 5% elongation, and wherein retained austenite in a structure is included within a range of 1% or more.

2. The steel sheet of claim 1 , further comprising one or two or more elements selected from the group consisting of on the basis of weight %: Si: 0.1% to 2.0%, Cr: 0.1% to 0.3%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, B: 0.0005% to 0.003%, Ti: 0.01% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, W: 0.1% to 1.0%, Sb: 0.005% to 0.2%, and Ca: 0.001% to 0.2%.

3. The steel sheet of claim 1 , comprising on the basis of weight %: C: 0.6%.

4. The steel sheet of claim 1 , wherein the steel sheet has tensile strength of 600 to 1000 Mpa and specific gravity of 7.2 g/cm 3 or less.

5. The steel sheet of claim 2 , wherein the steel sheet has tensile strength of 600 to 1000 Mpa and specific gravity of 7.2 g/cm 3 or less.

6. A low specific gravity and high strength galvanized steel sheet comprising on the basis of weight %: C: 0.2% to 0.8%, Mn: 2% to 10%, P: 0.02% or less, S: 0.015% or less, Al: 5.1% to 15%, and N: 0.01% or less, wherein a ratio of Mn/Al is 0.4 to 1.0, wherein retained austenite in a structure is included within a range of 1% or more, wherein the steel sheet has ridging height of 10 μm or less under conditions of 2.5 mm cutoff after 5% elongation, and wherein one of coating layers consisting of Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si in thickness of 10 μm to 200 μm per one side thereof is coated on the surface thereof.

7. The steel sheet of claim 6 , further comprising one or two or more elements selected from the group consisting of on the basis of weight %: Si: 0.1% to 2.0%, Cr: 0.1% to 0.3%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, B: 0.0005% to 0.003%, Ti: 0.01% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, W: 0.1% to 1.0%, Sb: 0.005% to 0.2%, and Ca: 0.001% to 0.2%.

8. The steel sheet of claim 6 , wherein the steel sheet has tensile strength of 600 to 1000 Mpa and specific gravity of 7.2 g/cm 3 or less.

9. The steel sheet of claim 7 , wherein the steel sheet has tensile strength of 600 to 1000 Mpa and specific gravity of 7.2 g/cm 3 or less.

10. A manufacturing method of low specific gravity and high strength hot-rolled steel sheet using a steel slab comprising on the basis of weight %: C: 0.2% to 0.8%, Mn: 2% to 10%, P: 0.02% or less, S: 0.015% or less, Al: 5.1% to 15%, and N: 0.01% or less, wherein a ratio of Mn/Al is 0.4 to 1.0, the method comprising: heating the steel slab within a range between 1000° C. and 1200° C.; finishing hot-rolling the heated steel slab within a range between 700° C. and 850° C.; and coiling the hot-rolled steel slab at 600° C. or less.

11. The manufacturing method of claim 10 , wherein the steel slab further comprises one or two or more elements selected from the group consisting of on the basis of weight %: Si: 0.1% to 2.0%, Cr: 0.1% to 0.3%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, B: 0.0005% to 0.003%, Ti: 0.01% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, W: 0.1% to 1.0%, Sb: 0.005% to 0.2%, and Ca: 0.001% to 0.2%.

12. A manufacturing method of low specific gravity and high strength cold rolled steel sheet using a steel slab comprising on the basis of weight %: C: 0.2% to 0.8%, Mn: 2% to 10%, P: 0.02% or less, S: 0.015% or less, Al: 5.1% to 15%, and N: 0.01% or less, wherein a ratio of Mn/Al is 0.4 to 1.0, the method comprising: heating the steel slab within a range between 1000° C. and 1200° C.; finishing hot-rolling the heated steel slab within a range between 700° C. and 850° C.; coiling the hot-rolled steel slab at 600° C. or less; cold-rolling the hot-rolled steel slab at a rolling reduction of 40% to 90%; and annealing the cold-rolled steel slab within a temperature range between recrystallization temperature and 900° C. at an annealing rate of 1 to 20° C./s for 10 to 180 seconds.

13. The manufacturing method of claim 12 , wherein the steel slab further comprises one or two or more elements selected from the group consisting of on the basis of weight %: Si: 0.1% to 2.0%, Cr: 0.1% to 0.3%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, B: 0.0005% to 0.003%, Ti: 0.01% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, W: 0.1% to 1.0%, Sb: 0.005% to 0.2%, and Ca: 0.001% to 0.2%.

14. A manufacturing method of low specific gravity and high strength galvanized steel sheet using a steel slab comprising on the basis of weight %: C: 0.2% to 0.8%, Mn: 2% to 10%, P: 0.02% or less, S: 0.015% or less, Al: 5.1% to 15%, and N: 0.01% or less, wherein a ratio of Mn/Al is 0.4 to 1.0, the method comprising: heating the steel slab within a range between 1000° C. and 1200° C.; finishing hot-rolling the heated steel slab within a range between 700° C. and 850° C.; coiling the hot rolled steel slab at 600° C. or less; cold-rolling the hot-rolled steel slab at a rolling reduction of 40% to 90%; annealing the cold-rolled steel slab within a temperature range between recrystallization temperature and 900° C. at an annealing rate of 1 to 20° C./s for 10 to 180 seconds; and coating the annealed steel slab with one of coating layers consisting of Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Al—Mg, Al—Si, and Al—Mg—Si at a thickness of 10 μm to 200 μm per one side thereof.

15. The manufacturing method of claim 14 , wherein the steel slab further comprises one or two or more elements selected from the group consisting of on the basis of weight %: Si: 0.1% to 2.0%, Cr: 0.1% to 0.3%, Mo: 0.05% to 0.5%, Ni: 0.1% to 2.0%, Cu: 0.1% to 1.0%, B: 0.0005% to 0.003%, Ti: 0.01% to 0.2%, Zr: 0.005% to 0.2%, Nb: 0.005% to 0.2%, W: 0.1% to 1.0%, Sb: 0.005% to 0.2%, and Ca: 0.001% to 0.2%.