Cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property and method of producing the same

摘要

The invention provides a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property comprising, in mass %, C: 0.0005-0.0040%, Si: 0.8% or less, Mn: 2.2% or less, S: 0.0005-0.009%, Cr: 0.4-1.3%, O: 0.003-0.020%, P: 0.045-0.12%, B: 0.0002-0.0010%, Al: 0.008% or less, N: 0.001-0.007%, and a balance of Fe and unavoidable impurities. Ultra-low-carbon steel retaining solute N and containing added Cr, P, B and O is used to produce hot-rolled and cold-rolled steel sheet and hot-dip galvanized cold-rolled steel sheet that exhibit both high paint bake hardenability and ordinary-temperature non-aging property.

主权项

1. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property comprising:
a slab consisting of in mass %,
C: 0.0005-0.0040%,Si: 0.8% or less,Mn: 2.2% or less,S: 0.0005-0.009%,Cr: 0.4-1.3%,O: 0.003-0.020%,P: 0.045-0.12%,B: 0.0002-0.0010%,Al: 0.008% or less,N: 0.001-0.007%,optionally at least one of V, Zr, Ce, Ti, Nb, Mg: 0.001-0.02% in total, and a balance of Fe and unavoidable impurities; hot rolling the slab at a temperature of (Ar3 point −100)° C. or greater; cold rolling the hot-rolled slab at a reduction ratio of 90% or less; annealing the cold-rolled product to reach a maximum temperature of 750-920° C.; and during a post-annealing cooling process, holding the annealed product for 20 to 50 seconds at a temperature range of 550-750° C., wherein the steel satisfies the relationship N−0.52 Al>0%, 50% or more of solute N form pairs with Cr or segregate around oxides or precipitates, and the steel sheet has a yield point elongation equal to or less than 0.08% and a bake hardenability of at least 82 MPa, wherein the cold-rolled steel sheet comprises a structure that consists of ferrite and/or bainite as the main phase, and optionally contains martensite.