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

摘要

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 TS280 approx 880MPa 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 percent to 27 percent of volume fraction of harder second phases (V_2) 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 (H_2) on FL is also small compared with that on TS. The endurance ratio (FL/TS) has a tendency to decrease as H_2 increases. The changes in TS X El and FL X El values with V_2 variation are small. The change in TS X El values with H2 variation is small. FL X El values, on the other hand, have a tendency to decrease with the increase of H_2. FL/TS and FL/YS (yield stress) increase as the volume fraction of ferrite increases from 73 percent to 100 percent. The following empirical formula are proposed from a least squares analysis to estimate FL. FL (MFa)=10,4 X d~(-1/2)+51 X (percent Si)+16 X (percent Mn)+80.