High strength hot rolled steel sheet, which has an acid pickling property, a chemical conversion treatment capacity, a fatigue property, a stretch beading ability and excellent surface deterioration resistance during molding, and having an isotropic resistance and ductility, and method for producing said high strength hot rolled steel sheet

摘要

A high strength hot rolled steel sheet, which is superior in an acid pickling property, a chemical conversion treatment capacity, a fatigue property, an expandability of holes and a resistance to surface deterioration during shaping, and which has an isotropic resistance and an isotropic ductility, the steel plate consisting of: from the point of view of mass percentage, C: from 0.05 to 0.12%; Yes: from 0.8 to 1.2%; Mn: from 1.6 to 2.2%; Al: from 0.30 to 0.6%; P: 0.05% or less; S: 0.005% or less; N: 0.01% or less, and optionally one or more selected from a group consisting of Cu: 0.002 to 2.0%, Ni: 0.002 to 1.0%, Ti: 0.001 to 0 , 5%, Nb: 0.001 to 0.5%, Mo: 0.002 to 1.0%, V: 0.002 to 0.2%, Cr: 0.002 to 1.0%, Zr: 0.002 to 0 , 2%, Ca: from 0.0005 to 0.0050%, REM: from 0.0005 to 0.0200%, and B: from 0.0002 to 0.0030%, the rest being inevitable impurities and Faith, in the one that a microstructure comprises: 60% in area or more of ferrite phases; more than 10% in martensite phase area; from 0 to less than 5% in the area of bainite phases; and from 0 to less than 1% phases of residual austenite, the maximum concentration of Al detected by a spectroscopic analysis of luminescent discharge emission is in a range of 0.75% by mass or less in an area from a surface of the steel sheet up to 500 nm thick after being etched with acid, in which an average length of the crystalline ferrite grains in a rolling direction is in a range of 20 μm or less in an area from the surface of the steel sheet up to a thickness of 20 μm, and in which a ratio of the fatigue limit is 0.45 or more.