The present study examined the effect of microstructural characteristics on the toughness properties of a hot stamping martensitic stainless steel. Moderately slow cooling during the martensitic transformation leads to the auto-tempering of the martensite laths and the stabilization of thin austenite films. The amounts of retained austenite and cementite precipitates were quantified for various cooling conditions. Charpy impact toughness tests were performed over a large range of temperatures to characterize the ductile-to-brittle transition. Decreasing the cooling rate from 300 °C/s down to 3 °C/s increased the retained austenite fraction from 0.6% up to 2.6% and decreased the ductile-to-brittle transition temperature by 140 °C. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the cooling rate, by applying the local approach to fracture. However, it has been demonstrated that a higher retained austenite fraction modifies incipient plasticity and decreases the yield stress by 60 MPa. As a result, retained austenite delays cleavage fracture by increasing the strain necessary to reach the critical cleavage fracture stress required to trigger cleavage initiation in the ductile-to-brittle transition domain. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature. It is thus beneficial to design cooling rates in order to increase the retained austenite fraction and to improve impact toughness at low temperatures.

本研究研究了微观结构特征对热冲压马氏体不锈钢韧性的影响。马氏体相变期间的适度缓慢冷却导致马氏体板条的自动回火和薄奥氏体薄膜的稳定化。对于各种冷却条件，对残留的奥氏体和渗碳体沉淀物的量进行了定量。在很大的温度范围内进行了夏比冲击韧性测试，以表征韧性到脆性的转变。将冷却速率从300°C / s降低到3°C / s，可将残余奥氏体分数从0.6％提高到2.6％，并使韧性至脆性转变温度降低140°C。无论冷却速率如何，断裂解理的临界应力均约为2400 MPa，通过对骨折采用局部方法。但是，已经证明较高的残余奥氏体分数会改变初期的可塑性，并使屈服应力降低60 MPa。结果，残余奥氏体通过增加达到延性-脆性转变域中触发解理起始所需的临界解理断裂应力所需的应变来延迟解理断裂。这样，残留的奥氏体起着降低延性至脆性转变温度的决定性作用。因此，有利的是设计冷却速率，以增加残余奥氏体分数并改善低温下的冲击韧性。结果，残余奥氏体通过增加达到延性-脆性转变域中触发解理起始所需的临界解理断裂应力所需的应变来延迟解理断裂。这样，残留的奥氏体起着降低延性至脆性转变温度的决定性作用。因此，有利的是设计冷却速率，以增加残余奥氏体分数并改善低温下的冲击韧性。结果，残余奥氏体通过增加达到延性-脆性转变域中触发解理起始所需的临界解理断裂应力所需的应变来延迟解理断裂。这样，残留的奥氏体起着降低延性至脆性转变温度的决定性作用。因此，有利的是设计冷却速率，以增加残余奥氏体分数并改善低温下的冲击韧性。