Low-density clad steel sheet having excellent strength and plateability, and manufacturing method therefor

1. A clad steel sheet, comprising:

a base material; and

a clad material provided on both side surfaces of the base material,

wherein the base material is a ferrite-austenitic duplex steel sheet including, by weight %, C: 0.3-0.7%, Mn: 2.0-9.0%, Al: 4.5-8.0%, and a balance of Fe and inevitable impurities, and

wherein the clad material is ferrite carbon steel including, by weight %, C: 0.0005-0.2%, Mn: 0.05-2.5%, and a balance of Fe and inevitable impurities.

2. The clad steel sheet of claim 1 , wherein the ferrite-austenitic duplex steel sheet further includes, by weight %, Si: 0.03-2.0%, Ni: 0.1-4.0%, N: 0.04% or less excluding 0%, P: 0.03% or less, and S: 0.03% or less.

3. The clad steel sheet of claim 1 , wherein the ferrite carbon steel further includes, by weight %, Al: 0.01-0.1%, N: 0.04% or less excluding 0%, P: 0.03% or less, and S: 0.03% or less.

4. The clad steel sheet of claim 3 , wherein the ferrite carbon steel further includes, by weight %, one or more of Si: 0.03-2.0%, Ti: 0.005-0.05%, and Nb: 0.005-0.05%.

5. The clad steel sheet of claim 1 , wherein a thickness of the clad material on one side surface of the base material is 25 μm or more, and a sum of thicknesses of the clad material on the both side surfaces of the base material is 30% or less of a total thickness of the clad steel sheet.

6. The clad steel sheet of claim 1 , wherein the clad steel sheet has a tensile strength of 780 MPa or more and a density of 7.4 g/cm 3 or less.

7. The clad steel sheet of claim 1 , wherein a microstructure of the ferrite-austenitic duplex steel sheet includes 10-50% of retained austenite in a ferrite matrix by area fraction.

8. The clad steel sheet of claim 1 , wherein a microstructure of the ferrite carbon steel has ferrite as a matrix structure and includes one or more of carbide, pearlite, martensite, and bainite as a second phase.

9. The clad steel sheet of claim 1 , wherein the clad steel sheet further includes a plating layer formed on a surface of the clad material.

10. The clad steel sheet of claim 9 , wherein the plating layer is one selected from the group consisting of Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Mg—Al, Zn—Ni, Al—Si, and Al—Si—Mg.

11. A method of manufacturing a steel sheet, the method comprising:

preparing a base material, a ferrite-austenitic duplex steel sheet including, by weight %, C: 0.3-0.7%, Mn: 2.0-9.0%, Al: 4.5-8.0%, and a balance of Fe and inevitable impurities;

preparing a clad material, ferrite carbon steel including, by weight %, C: 0.0005-0.2%, Mn: 0.05-2.5%, and a balance of Fe and inevitable impurities;

obtaining a laminate by disposing the base material between the two clad materials;

welding an edge of the laminate and heating the laminate in a temperature range of 1050-1350° C.;

obtaining a hot rolled steel sheet by finish-rolling the heated laminate in a temperature range of 750-1050° C.;

coiling the hot rolled steel sheet at 350-700° C.;

obtaining a cold rolled steel sheet by pickling the coiled hot rolled steel sheet and performing cold-rolling by applying a cold rolling reduction ratio of 35-90%; and

annealing the cold rolled steel sheet in a temperature range of 550° C. or higher and A3+50° C. or lower of the clad material.

12. The method of claim 11 , wherein the method further includes forming a plating layer by performing plating by a hot dip plating method after the annealing.

13. The method of claim 12 , wherein the plating layer is one selected from the group consisting of Zn, Zn—Fe, Zn—Al, Zn—Mg, Zn—Mg—Al, Zn—Ni, Al—Si, and Al—Si—Mg.