自動車の燃費向上のため使用する鋼材の高強度化が求めらrnれている．その中で足回り部品に用いられる熱延鋼板についrnては引張強度と同時に疲労強度を兼備する必要がある．rnそのため，熱延鋼板の疲労特性の改善を目的に疲労特性に及rnぼす強化機構の影響について検討されてきた．%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.rnAs 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.
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