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－　　－5Published PaperEffect of Strain Rate on Tensile Properties of Dual Phase Sheet SteelTomohiro KAMIURA and Susumu TAKAHASHIHigh tensile steel sheets are widely used to reduce the weight of automobiles. Dual phase steel sheet is a kind of high tensile steel sheet. Tensile tests of dual phase steel sheets at high strain rates were performed to investigate the relationship between strain rate, elongation and temperature. The strain did not change much at uniform elongation at the strain rate up to 5/s, and decreased in local elongation. In quasi-static tensile tests, the n value did not change when the temperature was 373K or less, but increased when over 473K. Softening of the sheet did not occur at slow strain rates. Similarly, at the strain rate of 100/s, strain did not change in uniform elongation, but increased in local elongation. The temperature during uniform elongation was about 300K, but exceeded 473K at the end of local elongation. At the high strain rate, strain increased in both uniform elongation and local elongation. The conclusions obtained for the elongation are as follows. At strain rates less than 5/s, stress concentration due to temperature difference causes local elongation to decrease as the strain rate becomes faster. At strain rates above 100/s, local elongation increased due to softening of the steel sheet caused by plastic heat generation, as well as increased n value.Keywords: Tensile test, Temperature measurement, High tensile steel, Mechanical propertiesDual Phase鋼板の引張特性におよぼすひずみ速度の影響　自動車の軽量化では，高張力鋼板が多く使用されている．Dual Phase鋼板は，高張力鋼板の一種である．本論文では，Dual Phase鋼板の高速変形時の温度上昇と延性の変化に関して検討した結果を述べている．ひずみ速度が5/sまでは，一様伸び部のひずみはあまり変化しなかった．一方，局所伸び部のひずみは減少した．準静的引張試験におけるn値は温度が373K以下では変化しなかったが，473K以上になると増加した．低ひずみ速度では，軟化は発生しなかった．ひずみ速度100/sでも一様伸び部のひずみ量は変化しなかった．局所伸び部のひずみ増加した一様伸び変形時の温度は約300Kだった．局所伸び変形中に温度は473K以上となった．高いひずみ速度の場合，一様伸びの時のひずみも局部伸び部のひずみも増加した．Dual Phase鋼の延性特性について以下の結論を得た．ひずみ速度5/s以下では温度差による応力集中により，速度が速いほど局所伸びが減少する．ひずみ速度100/s以上では，塑性発熱による軟化およびn値の増加により，局所伸びは増加したことが分かった．キーワード：引張試験，温度計測，高張力鋼板，機械的特性Journal（掲載誌）Journal of Japan Society for Experimental Mechanics, Vol.18, No.3, pp.169-175, 2018