Abnormal Strain Rate‐Dependent Work‐Hardening Behavior in Quenching and Partitioning Steel

Due to their high levels of formability and crashworthiness, the new quenching and partitioning (QP) steels are increasingly being used in car‐body structures. Using a unique experimental technique for tensile testing in a large strain‐rate range of 10 −4 –1000 s −1 , an anomalous work‐hardening behavior is found in a QP1180 steel that may affect its formability and crashworthiness. At the lowest strain rate of 10 −4 s −1 , the work‐hardening rate θ decreases linearly with increasing flow stress. With an increasing strain rate, however, an increasingly severe nonlinearity in the work‐hardening rate is evident, as has been observed in metastable austenitic steels when the deformation‐induced martensitic transformation occurs at lower temperatures. This work‐hardening‐rate peak reaches its maximum at 10 s −1 . Above this value, the peak decreases again, and at more than 500 s −1 , it behaves like the quasi‐static test. The corresponding material characteristics, such as elongation and strength, change similarly.