Hot dip galvanized high strength steel sheet excellent in plating adhesion and hole expandability and method of production of same
View Patent ↗The present invention provides hot dip galvanized high strength steel sheet excellent in plating adhesion and hole expandability and a method of production of the same, that is, hot dip galvanization steel sheet excellent in plating adhesion and hole expandability containing, by mass %, C: 0.08 to 0.35%, Si: 1.0% or less, Mn: 0.8 to 3.5%, P: 0.03% or less, S: 0.03% or less, Al: 0.25 to 1.8%, Mo: 0.05 to 0.35%, and N: 0.010% or less and having a balance of Fe and unavoidable impurities, said hot dip galvanized high strength steel characterized in that the steel sheet has a metal structure having ferrite, bainite, by area percent, 0.5% to 10% of tempered martensite, and, by volume percent, 5% or more of residual austenite, and a method of production comprising annealing by a continuous annealing process at 680 to 930° C. in temperature, then cooling to the martensite transformation point or less, then hot dip galvanizing the steel during which heating the steel to 250 to 600° C., then hot dip galvanizing it.
1. A production method of a hot dip galvanized high strength steel sheet from a slab consisting of, by mass %,
C: 0.08 to 0.35%,
Si: less than 0.2%,
Mn: 0.8 to 3.5%,
P: 0.03% or less,
S: 0.03% or less,
Al: 0.25 to 1.8%,
Mo: 0.05 to 0.35%,
N: 0.010% or less,
Ti: 0.0002% or less,
optionally one or more of Nb: 0.01 to 0.3%, V: 0.01 to 0.3%, Cu: 1% or less,
Ni: 1% or less, Cr: 1% or less, or B: 0.0001 to 0.0030%, and
a balance of Fe and unavoidable impurities,
the method comprising the steps of:
hot rolling the slab to make a steel sheet, coiling the sheet at a temperature of 400 to 750° C., cold rolling the sheet, annealing the cold rolled sheet by a continuous annealing process at a temperature of 680 to 930° C., overaging the annealed sheet at a temperature of 300 to 500° C. for 60 seconds to 20 minutes, cooling the overaged sheet to the martensite transformation point or less, heating the cooled sheet to 250 to 600° C., then hot dip galvanizing the heated sheet,
wherein the hot dip galvanized sheet has by area percent, 3.5% to 10% of tempered martensite, and by volume percent, 7% or more of residual austenite.
2. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , further comprising cooling the steel sheet to the martensite transformation point or less of said continuous annealing process, then pickling it, or not pickling it, then pre-plating the steel sheet with one or more of Ni, Fe, Co, Sn, and Cu to 0.01 to 2.0 g/m 2 per side.
3. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , further comprising alloying the galvanized sheet obtained by said hot dip galvanization process.
4. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , further comprising post-treating said galvanized sheet or galvannealed sheet with one or more of chromate treatment, inorganic film coating, chemical conversion, and resin film coating.
5. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , wherein the slab contains, by mass %, Si: 0.191% or less.
6. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , wherein the slab contains, by mass %, Si: 0.187% or less.
7. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , wherein the slab contains, by mass %, Si: 0.169% or less.
8. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , wherein the slab contains, by mass %, Si: 0.1% or less.
9. The production method of a hot dip galvanized high strength steel sheet as set forth in claim 1 , wherein the hot dip galvanized sheet has by volume percent, 10% or more of residual austenite.
10. A production method of a hot dip galvanized high strength steel sheet from a slab consisting of, by mass %,
C: 0.08 to 0.35%,
Si: less than 0.2%,
Mn: 0.8 to 3.5%,
P: 0.03% or less,
S: 0.03% or less,
Al: 0.25 to 1.8%,
Mo: 0.05 to 0.35%,
N: 0.010% or less,
Ti: 0.0002% or less,
Nb: 0.01 to 0.3%,
V: 0.01 to 0.3%,
Cu: 1% or less,
Ni: 1% or less,
Cr: 1% or less,
B: 0.0001 to 0.0030%, and
a balance of Fe and unavoidable impurities,
the method comprising the steps of:
hot rolling the slab to make a steel sheet, coiling the sheet at a temperature of 400 to 750° C., cold rolling the sheet, annealing the cold rolled sheet by a continuous annealing process at a temperature of 680 to 930° C., overaging the annealed sheet at a temperature of 300 to 500° C. for 60 seconds to 20 minutes, cooling the overaged sheet to the martensite transformation point or less, heating the cooled sheet to 250 to 600° C., then hot dip galvanizing the heated sheet,
wherein the hot dip galvanized sheet has by area percent, 3.5% to 10% of tempered martensite, and by volume percent, 7% or more of residual austenite.