The steel sheet null C which is superior in fatigue crack development control quality

摘要

PROBLEM TO BE SOLVED: To provide a steel sheet comprising mainly ferrite and is excellent in both fatigue crack propagation suppressing characteristic and toughness.;SOLUTION: The steel sheet has a suitably controlled chemical composition and a structure in which a ferrite phase is contained in an amount of 90% or more by area, wherein the average hardness of the ferrite phase is 150 Hv or less; when a region surrounded by high angle grain boundaries with an orientation difference of two adjacent ferrite crystals of 15° or more is defined as a crystal grain, the area rate of the crystal grain in the whole ferrite phase is 80% or more, and the average circle-equivalent diameter of the crystal grain is 20 to 150 μm; and the relationship of the following formula (1) is satisfied: {(α/5)+[(β×50)/(2α+40)]}×(γ/3)≥1,200 (1), wherein α denotes the average circle-equivalent diameter of the crystal grain (μm); β denotes the area rate of the crystal grain in the whole ferrite phase (by area%); and γ denotes the average hardness (Hv) of the ferrite phase.;COPYRIGHT: (C)2012,JPO&INPIT