HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET HAVING EXCELLENT YIELD STRENGTH AND FORMABILITY, AND MANUFACTURING METHOD THEREFOR

摘要

A hot-dip galvanized steel sheet contains specific amounts of C, Si, Mn, P, S, Ti, Al, and N with the remainder being iron and unavoidable impurities. Bainitic ferrite, martensite, retained γ, and ferrite (α) are present each in a specific area ratio. The remainder γ has a specific C concentration. Sub-grains in the recrystallized α and un-recrystallized α have a specific grain diameter. The surface area ratio of α and worked α having a grain diameter of 5 μm or more is 5% or less. The average particle diameter of TiC particles inside a grains is 10 nm or less.

主权项

1. A high strength hot-dip galvanized steel sheet excellent in yield strength and formability, having a steel sheet with a galvanized layer formed on the surface,
the steel sheet containing a chemical composition by mass% (identical here and hereinafter for chemical composition), C: 0.05 to 0.3%, Si: 1 to 3%, Mn: 1 to 3%, P: 0.1% or less (inclusive 0%), S: 0.01% or less (inclusive 0%). Ti: 0.02 to 0.2%, Al: 0.001 to 0.1%, N: 0.002 to 0.03%, and the balance consisting of iron and unavoidable impurities, and further satisfying both
10[C]+0.4[Si]+[Mn]≦4.5  Formula (1),and
0.8×(723−10.7[Mn]−16.9[Ni]+29.1[Si]+16.9[Cr])+0.2×(910−203√[C]+44.7[Si]+31.5[Mo]−30[Mn]−11[Cr]−20[Cu]+700[P]+400[Al]+400[Ti])≦780  Formula (2)(where [ ] means content (mass %) for each of chemical composition) and containing by area ratio based on the entire microstructure (identical here and hereinafter for the microstructure):
bainitic ferrite: 40 to 65%, martensite+retained austenite: 15% or more the martensite: 15% or less, the retained austenite: 5% or more, and ferrite: 20 to 40%, in which a C concentration of the retained austenite (CγR) is 0.9 mass% or more, both of an average grain diameter of recrystallized ferrite and an average grain diameter of sub-grains in un-recrystallized ferrite particles are 3 μm or less and an area ratio of a ferrite having a grain diameter of 5 μm or more is 5% or less and an area ratio of worked ferrite is 5% or less for the ferrite and, further, an average grain diameter of TiC particles present in the ferrite particles is 10 nm or less.