| 摘要 |
Provided are a high-strength galvanized steel sheet which can preferably be used as a material for automobile parts and a method for manufacturing the steel sheet. The steel sheet has a C content of 0.15% or less, in which an area ratio of ferrite is 10% or less, an area ratio of bainitic ferrite is 2% or more and 30% or less, an area ratio of martensite is 60% or more and 98% or less, an area ratio of retained austenite is less than 2%, an average grain diameter of martensite adjacent to bainite is 15 μm or less, a proportion of massive martensite adjacent only to bainite to the whole metallographic structure is 10% or less, and a value (ΔHv) calculated by subtracting the Vickers hardness at a position located at 20 μm from the surface of the steel sheet from the Vickers hardness at a position located at 100 μm from the surface of the steel sheet is 30 or more. |
| 主权项 |
1. A high-strength galvanized steel sheet having
a chemical composition containing, by mass %, C: 0.05% or more and 0.15% or less, Si: 0.01% or more and 1.00% or less, Mn: 1.5% or more and 4.0% or less, P: 0.100% or less, S: 0.02% or less, Al: 0.01% or more and 0.50% or less, Cr: 0.010% or more and 2.000% or less, Nb: 0.005% or more and 0.100% or less, Ti: 0.005% or more and 0.100% or less, B: 0.0005% or more and 0.0050% or less, and the balance being Fe and inevitable impurities, in which K, which is expressed by equation (1) below, is 3.0 or more and a metallographic structure including, in terms of area ratio determined by performing structure observation at a position located at ¼ of the thickness in a cross section at a right angle to the surface of the steel sheet, ferrite: 10% or less, bainitic ferrite: 2% or more and 30% or less, and martensite: 60% or more and 98% or less, wherein the proportion of retained austenite determined by using an X-ray diffraction method is less than 2%, an average grain diameter of martensite adjacent to bainite is 15 μm or less, the proportion of massive martensite adjacent only to bainite to the whole metallographic structure is 10% or less, and a value (ΔHv) calculated by subtracting the Vickers hardness at a position located at 20 μm from the surface of the steel sheet from the Vickers hardness at a position located at 100 μm from the surface of the steel sheet is 30 or more:
K=−0.4×[Si]+1.0×[Mn]+1.3×[Cr]+200×[B] (1), where, in equation (1) above, [Si] denotes the content [mass %] of Si, [Mn] denotes the content [mass %] of Mn, [Cr] denotes the content [mass %] of Cr, and [B] denotes the content [mass %] of B. |
