Low-density clad steel sheet having excellent formability and fatigue property

1. A clad steel sheet having excellent formability and fatigue properties, the clad steel sheet comprising:

a base material;

a cladding material provided on both side surfaces of the base material; and

a surface hardness of 400 Hv or more, the surface hardness being measured by measuring a size of an indentation after press-fitting for 10 seconds with a load of 300 gf using a Micro Vickers hardness tester,

wherein the base material is a lightweight steel sheet including, by weight, C: 0.3 to 1.0%, Mn: 4.0 to 16.0%, Al: 4.5 to 9.0%, and a remainder of Fe and inevitable impurities, and

the cladding material is a martensitic carbon steel including, by weight, C: 0.1 to 0.45%, Mn: 1.0 to 3.0%, and a remainder of Fe and inevitable impurities.

2. The clad steel sheet of claim 1 , wherein the lightweight steel sheet further comprises, by weight, Si: 0.03 to 2.0%, Ni: 0.1 to 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 martensitic carbon steel further comprises, by weight, Si: 0.03 to 2.0%, Al: 0.02 to 0.3%, N: 0.04% or less (excluding 0%), B: 0.0005 to 0.005%, P: 0.03% or less, and S: 0.03% or less.

4. The clad steel sheet of claim 3 , wherein the martensitic carbon steel further comprises, by weight, one or more of Cr: 0.1 to 1.0%, Ni: 0.1 to 1.0%, Mo: 0.05 to 1.0%, Ti: 0.005 to 0.05%, and Nb: 0.005 to 0.05%.

5. The clad steel sheet of claim 1 , wherein the cladding material has a thickness of 20 μm or more, and the thickness of the cladding material is 30% or less of a thickness of the clad steel sheet.

6. The clad steel sheet of claim 1 , further comprising: a fatigue strength of 500 MPa or more, an elongation of 25% or more, and a density of 7.4 g/cm 3 or less,

wherein the fatigue strength is measured under a stress ratio −1 and a fatigue limit of 2,000,000 using a bending fatigue tester.

7. The clad steel sheet of claim 1 , wherein the lightweight steel sheet comprises 10 area % or more of retained austenite.

8. The clad steel sheet of claim 1 , wherein the martensitic carbon steel comprises: a matrix structure of martensite; and, as second phase, one or more of carbide, ferrite, retained austenite, and bainite.

9. The clad steel sheet of claim 1 , wherein the martensitic carbon steel comprises: a matrix structure of tempered martensite; and, as a second phase, one or more of carbide, ferrite, retained austenite, and bainite.

10. The clad steel sheet of claim 1 , further comprising: a plated layer formed on the cladding material.

11. The clad steel sheet of claim 10 , wherein the plated layer is one selected from the group consisting of a Zn-based plated layer, a Zn—Fe-based plated layer, a Zn—Al-based plated layer, a Zn—Mg-based plated layer, a Zn—Mg—Al-based plated layer, a Zn—Ni-based plated layer, an Al—Si-based plated layer, and an Al—Si—Mg-based plated layer.

12. The clad steel sheet of claim 1 , wherein a solid-phase junction is formed at a bonding interface between the base material and the cladding material, the base material and the cladding material is directly solid-phase bonded through the solid-phase junction.