Strain rate sensitivity of ultra-high strength hot stamping steel
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摘要: 利用CMT5105電子萬能試驗機和HTM 16020電液伺服高速試驗機對超高強熱成形鋼進行拉伸試驗,應變速率范圍為10-3~103 s-1,模擬熱成形零件在不同應變速率下的碰撞情況.結果表明:在低應變速率階段(10-3~10-1 s-1)實驗鋼的應變速率敏感性不高,隨應變速率的升高,實驗鋼的強度和延伸率變化不大;在高應變速率階段(100~103 s-1)實驗鋼具有高的應變速率敏感性,隨應變速率的升高,實驗鋼的強度和延伸率都呈增大的趨勢,并且抗拉強度的應變速率敏感性要大于屈服強度.這主要是由于在高應變速率階段拉伸時產生的絕熱溫升現象和應變硬化現象共同作用造成的.實驗鋼頸縮后的延伸率隨應變速率的增大而減小,主要是由于高應變速率下馬氏體局部變形不均勻造成的.實驗鋼吸收沖擊功的能力隨應變速率的升高而增大,實驗鋼達到均勻延伸率時吸收沖擊功的大小對應變速率更敏感.與低應變速率階段相比,實驗鋼在高應變速率階段的斷口韌窩的平均直徑更小,韌窩的深度更深,這與高應變速率階段部分馬氏體晶粒的碎化有關.通過掃描電鏡和透射電鏡觀察發現,在高應變速率拉伸時晶粒有明顯的拉長趨勢,并且在應力集中的地方有一些微空洞的存在,應變速率為103 s-1時部分區域有碎化的現象.Abstract: The tensile test of an ultra-high strength hot stamping steel was tested using the CMT5105 electronic universal testing machine and HTM 16020 electro-hydraulic servo high-speed material testing machine. The impacts of the hot stamping parts were simulated at strain rates range of 10-3-103 s-1. The results show that in the low strain rate (10-3-10-1 s-1), the strain rate sensitivity of the tested steel is not very high, and the steel strength and elongation change little with an increase of strain rate. In the high strain rate stage (100-103 s-1), the strain rate sensitivity of the steel is very high, and the steel strength and elongation increase with strain rate. The strain rate sensitivity of the tensile strength is higher than the yield strength mainly because of the adiabatic temperature rise phenomenon and the strain working phenomenon that simultaneously occur during the high strain rate stage. The elongation after necking decreases with an increase of strain rate, mainly because of the local inhomogeneous deformation of the martensite at the high strain rate. The impact energy absorption capacity of the experimental steel increases with strain rate, and is more sensitive at the uniform elongation. Compared with the low strain rate stage, the average fracture diameter of the dimple in the high strain rate stage is smaller, and its depth is deeper; this is related to the fragmentation of the martensite grains region in the high strain rate stage. Scanning elec-tron microscope and transmission electron microscope images reveal that the grains are elongated at high strain rate stage and some microvoids are present in the stress-concentrated regions. Moreover, the fragmentation phenomenon can be found in part of region at the 103 s-1 strain rate.
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參考文獻
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