Effects of Hot Rolling Conditions on the Fatigue Limit of Ti Added Dual Phase Hot-Rolled Steel Sheets

Abstract

  The effects of hot rolling conditions on the size of TiC precipitates in Ti-added ferrite-martensite hot-rolled steel sheets and precipitated TiC size on fatigue limit were investigated. Steel sheets were hot-rolled at 1153 K or 1073 K and rapidly cooled to 953 K after hot-rolling and air-cooled for several seconds in order to accelerate ferrite transformation and rapid cooled again to coiling temperature. It was found that TiC particles were coarsened with the decline of finish-rolling temperature (FRT) and the increase of air-cooling time after hot rolling. This is presumably because the decrease of FRT raises the ferrite transformation temperature and TiC precipitation temperature; and the increase of air-cooling time from 953 K results in the coarsening of TiC particles due to growth or Ostwald growth of TiC.
  As for the relationship between TiC particle size and fatigue limit, it was found that fatigue limit was maximized when the average size of TiC particle was about 7 nm. This indicates that the fatigue limit of Ti added ferrite-martensite hot-rolled steel sheets depends upon precipitation strengthening of precipitates which are not cut by dislocations.