Hot-rolled steel sheet, method for making the same, and worked body of hot-rolled steel sheet

摘要

A steel sheet contains, in terms of percent by mass, C: 0.01 to 0.2%, Si: 2.0% or less, and Mn: 3.0% or less and has a martensite phase as dominant phase and ferrite with a grain size of 20 μm or less as a second phase. The ferrite is contained in area ratio of 1% to 30% and the amount of solute carbon being 0.01 percent by mass of more. The steel sheet can provide a hot-rolled steel sheet suitable for automobile steel sheet, i.e., has excellent press workability and excellent strain aging property whereby the tensile strength significantly increases by heat treatment at about the same temperature as typical baking process after the press-working. Moreover, hardening of the ferrite phase improves the fatigue strength after the strain aging.

主权项

1. A hot-rolled steel sheet comprising, in terms of percent by mass,
C: 0.01 to 0.2%, Si: 2.0% or less Mn: 1.5 to 3.0%, P: 0.1% or less S: 0.02% or less, Al: 0.1% or less N: 0.02% or less, the balance being Fe and inevitable impurities, wherein a non-tempered martensite phase is dominant phase, a ferrite phase as a second phase is contained in the range of 3% or more and 20% or less in terms of area ratio, the average grain size of the ferrite phase being 2.8 μm to 20 μm, and an amount of solute carbon is 0.01 percent by mass or more; wherein the hot-rolled steel sheet has a strain aging property ΔTS of 100 MPa or more, ΔTS being an increase in tensile strength by strain aging corresponding to the difference between a tensile strength of the steel sheet after strain aging and a tensile strength of the steel sheet before strain aging; and wherein the hot-rolled steel sheet has a fatigue strength ratio FL′/TS of 0.8 or more, FL′/TS being a ratio of a fatigue limit under pulsating tension FL′ to a tensile strength TS of the steel sheet before strain aging.