Steel sheet having excellent resistance to liquid metal embrittlement cracks and method for manufacturing the same
Provided is a steel sheet having excellent resistance to liquid metal embrittlement cracks and a method for manufacturing the same. The steel sheet includes, by weight %, 0.04% to 0.35% of C, 0.99% or less of Al+Si, 3.5% to 10% of Mn, 0.05% or less of P (excluding 0%), 0.02% or less of S (excluding 0%), and 0.02% or less of N (excluding 0%) with a remainder of Fe and other inevitable impurities, wherein the steel sheet comprises a microstructure comprising at least 10% of residual austenite, at least 60% of annealed martensite and 20% or less of alpha martensite and epsilon martensite, by volume fraction, and an average thickness of a Mn-depleted layer is at least 0.5 μm from a product surface.
1. A steel sheet having excellent resistance to liquid metal embrittlement cracks, the steel sheet comprising:
by weight %, 0.04% to 0.35% of C, 0.99% or less of Al+Si, 3.5% to 10% of Mn, 0.05% or less of P (excluding 0%), 0.02% or less of S (excluding 0%), and 0.02% or less of N (excluding 0%) with a remainder of Fe and other inevitable impurities;
a microstructure comprising: at least 10% by volume fraction of residual austenite, at least 60% by volume fraction of annealed martensite, and 20% or less by volume fraction of alpha martensite and epsilon martensite; and
a Mn-depleted layer having an average thickness of at least 0.5 μm from a surface of the steel sheet.
2. The steel sheet of claim 1 , further comprising: at least one selected from the group consisting of 0.15% or less of Ti, 0.5 or less of Nb, 0.5 or less of V and 0.5 or less of Mo.
3. The steel sheet of claim 1 , further comprising: at least one of 0.001% to 0.1% of Zr and 0.001% to 0.5% of W.
4. The steel sheet of claim 1 , further comprising: at least one selected from the group consisting of 1% or less of Ni, 1.5% or less of Cr and 0.5% or less of Cu.
5. The steel sheet of claim 1 , further comprising: at least one selected from the group consisting of 0.1% or less of Sb, 0.01% or less of Ca and 0.01% or less of B.
6. The steel sheet of claim 1 , wherein the steel sheet is one of a cold-rolled steel sheet, a hot-dip galvanized steel sheet, a hot-dip galvannealed steel sheet, an electrolytic galvanized steel sheet, a PVD galvanized steel sheet and a hot-dip aluminum-plated steel sheet.
7. A method of manufacturing a steel sheet having excellent resistance to liquid metal embrittlement cracks, the method comprising:
reheating a steel slab comprising: 0.04% to 0.35% of C, 0.99% or less of Al+Si, 3.5% to 10% of Mn, 1.5% or less of Cr (excluding 0%), 0.05% or less of P (excluding 0%), 0.02% or less of S (excluding 0%), and 0.02% or less of N (excluding 0%) with a remainder of Fe and other inevitable impurities, by weight %, at a temperature of 1100° C. to 1300° C.;
finish-hot-rolling the reheated steel slab at a temperature of Ar3 to 1000° C. to obtain a hot-rolled steel sheet;
coiling the hot-rolled steel sheet at a temperature of 720° C. or less;
heat-treating the coiled hot-rolled steel sheet at a temperature in a range of Ac1 to Ac1+(Ac3−Ac1)/2 for at least 30 minutes;
pickling the heat-treated hot-rolled steel sheet for at least 5 seconds;
cold-rolling the pickled hot-rolled steel sheet to obtain a cold-rolled steel sheet; and
two-phase annealing the cold-rolled steel sheet a temperature in the range of 580° C. to 900° C. for 10 seconds to 5 minutes.
8. The method of claim 7 , wherein the steel slab further comprises: at least one selected from the group consisting of 0.15% or less of Ti, 0.5% or less of Nb, 0.5% or less of V and 0.5% or less of Mo.
9. The method of claim 7 , wherein the steel slab further comprises: at least one of 0.001% to 0.1% of Zr and 0.001% to 0.5% of W.
10. The method of claim 7 , wherein the steel slab further comprises: at least one selected from the group consisting of 1% or less of Ni, 1.5% or less of Cr and 0.5% or less of Cu.
11. The method of claim 7 , wherein the steel slab further comprises: at least one selected from the group consisting of 0.1% or less of Sb, 0.01% or less of Ca and 0.01% or less of B.
12. The method of claim 7 , further comprising: annealing the cold-rolled steel sheet at a temperature of at least Ac3 for at least 30 seconds and cooling the annealed cold-rolled steel sheet, before the two-phase annealing.
13. The method of claim 7 , further comprising: plating the two-phase-annealed cold-rolled steel sheet.
14. The method of claim 13 , wherein the plating is at least one of hot-dip galvanization, hot-dip galva-annealing, electrolytic galvanization, PVD galvanization and hot-dip aluminum plating.
15. The method of claim 14 , wherein the hot-dip galvanization is achieved by immersing the two-phase-annealed cold-rolled steel sheet in at least one of a Zn plating bath, a Zn—Al plating bath, an Al—Si plating bath, an Al—Si—Mg plating bath and a Zn—Mg—Al plating bath.
16. The method of claim 14 , wherein the PVD galvanization is achieved using an electromagnetic levitation physical vapor deposition (EML-PVD) method.