Cold-rolled steel plate for hot forming, having excellent corrosion-resistance and spot-weldability, hot-formed member, and method for manufacturing same
An aspect of the present invention relates to a cold-rolled steel plate for hot forming, which is excellent in corrosion-resistance and spot-weldability, contains, by weight %, C: 0.1-0.4%, Si: 0.5-2.0%, Mn: 0.01-4.0%, Al: 0.001-0.4%, P: 0.001-0.05%, S: 0.0001-0.02%, Cr: 0.5% to less than 3.0%, N: 0.001-0.02%, and a balance of Fe and inevitable impurities, satisfying formula (1) below, and includes an Si amorphous oxidation layer continuously or discontinuously formed at a thickness of 1 nm-100 nm on the surface thereof. Formula (1): 1.4≤0.4*Cr+Si≤3.2 (wherein element symbols denote measurements of respective element contents by weight %).
1. A method of manufacturing a cold-rolled steel plate for hot-forming, comprising:
heating a slab to 1000˜1300° C., the slab comprising C: 0.1 to 0.4%, Si: 0.5 to 2.0%, Mn: 0.01 to 4.0%, Al: 0.001 to 0.4%, P: 0.001 to 0.05%, S: 0.0001 to 0.02%, Cr: 0.5% or higher and less than 3.0%, N: 0.001 to 0.02%, and a balance of Fe and inevitable impurities by weight %;
obtaining a hot-rolled steel plate by hot-rolling the heated slab at finishing rolling temperature of Ar3 to 1000° C.;
coiling the hot-rolled steel plate within a temperature range of higher than Ms to 750° C. or less;
obtaining a cold-rolled steel plate by cold-rolling the coiled hot-rolled steel plate; and
continuously annealing the cold-rolled steel plate to satisfy equation (2) and equation (3) below:
1≤exp[0.07*DP(I)+(0.6*Cr+3*Si)]≤100 equation (2)
50*exp[0.05*DP(I)−(1.2*Cr+6*Si)]≤2.5 equation (3)
where each element symbol is a value of a content of each element measured by weight %, and DP(I) is a dew point temperature (° C.) when the continuous annealing is performed,
wherein the slab satisfies equation (1) below:
1.4≤0.4*Cr+Si≤3.2 equation (1)
where each element symbol is a value of a content of each element measured by weight %.
2. The method of claim 1 , wherein the slab comprises one or more elements selected from between a) and b) below by weight %:
a) one or more elements selected from among Ti, Nb, Zr, and V: 0.001 to 0.4%
b) B: 0.0001 to 0.01%.
3. The method of claim 1 , wherein the slab comprises one or more elements selected from among c) to e) below by weight %:
c) one or more elements selected from between Mo and W: 0.001 to 1.0%
d) a sum of contents of Cu and Ni: 0.005 to 2.0%
e) one or more elements selected from between Sb and Sn: 0.001 to 1.0%.
4. The method of claim 1 , wherein the cold-rolling is performed at a reduction ratio of 30 to 80%.
5. The method of claim 1 , wherein the continuous annealing is performed within a temperature range of 700 to 900° C.