Effects of Strengthening Mechanisms on Fatigue Properties in Hot-Rolled Sheet Steels with Ferrite Matrix

Abstract

This study was carried out with a search for hot-rolled high strength sheet steels having excellent formability and fatigue properties as background. The effects of strengthening mechanisms on fatigue and tensile properties were investigated using hot-rolled TS280880MPa class sheet steels composed of ferrite matrix and harder second phases. The following results were obtained. The improvement of fatigue limit (FL) associated with an increase from 21% to 27% of volume fraction of harder second phases (V₂) is small compared with that of tensile strength (TS). The effect of an increase from 260 points to 480 points of the second phase micro-vickers hardness (H₂) on FL is also small compared with that on TS. The endurance ratio (FL/TS) has a tendency to decrease as H₂ increases. The changes in TS×El and FL×El values with V₂ variation are small. The change in TS×El values with H₂ variation is small. FL×El values, on the other hand, have a tendency to decrease with the increase of H₂. FL/TS and FL/YS (yield stress) increase as the volume fraction of ferrite increases from 73% to 100%. The following empirical formula are proposed from a least squares analysis to estimate FL. FL (MPa)=10, 4×d^-1/2+51×(%Si)+16×(%Mn)+80