Cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property and method of producing the same
View Patent ↗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.
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 (Ar 3 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.
2. 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 (Ar 3 point−100)° C. or greater;
cold rolling the hot-rolled slab at a reduction ratio of 90% or less;
annealing the cold-rolled product on a continuous hot-dip galvanizing line to reach a maximum temperature of 750-920° C.;
during a post-annealing cooling process, holding the annealed product for 20 to 50 seconds at a temperature range of 550-750° C.; and
immersing the product in a galvanizing bath,
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.
3. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 2 , further comprising:
heat treating the product for 1 second or greater at a temperature of 460-550° C. after immersing it in the galvanizing bath.
4. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 1 , wherein said holding the annealed product at a temperature range of 550-750° C. in a post-annealing cooling process is for 30 seconds or greater.
5. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 2 , wherein said holding the annealed product at a temperature range of 550-750° C. in a post-annealing cooling process is for 30 seconds or greater.
6. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 1 , wherein during the post-annealing cooling process, said annealed product is held for 20 seconds or greater at a temperature range of 600-700° C.
7. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 2 , wherein during the post-annealing cooling process, said annealed product is held for 20 seconds or greater at a temperature range of 600-700° C.
8. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 1 , wherein the steel sheet has a bake hardenability of at least 87 MPa and the main phase is ferrite.
9. A method of producing a cold-rolled steel sheet excellent in paint bake hardenability and ordinary-temperature non-aging property according to claim 2 , wherein the steel sheet has a bake hardenability of at least 87 MPa and the main phase is ferrite.