Fe-C-Mn-Si钢中奥氏体共格孪晶界对贝氏体铁素体变体选择的影响

将C含量(质量分数)分别为0.05%和0.4%的Fe-C-Mn-Si钢进行等温处理得到贝氏体组织,采用EBSD技术对奥氏体共格孪晶界上形成的贝氏体铁素体变体进行分析.结果表明,2种钢中的贝氏体铁素体与母相奥氏体均成近似K-S取向关系.奥氏体孪晶界两侧形成取向相同的变体对.此变体对形成后,孪晶界基本不再显现.晶体学分析表明,共格孪晶界两侧可能出现的变体对最多不超过3组,且这3组变体对的惯习面均与孪晶界平行,因此,贝氏体铁素体变体都将沿孪晶界生长.含C量为0.05%的Fe-C-Mn-Si钢中奥氏体孪晶界上只观察到一组贝氏体铁素体变体对的形成,这是因为C含量较低,贝氏体铁素体生长速度较快,消除了其它变体对的形核机会,先形核的变体对一旦形核就迅速覆盖整个孪晶面.而在含C量为0.4%的Fe-C-Mn-Si钢中,由于C含量较高,贝氏体铁素体生长速度较慢,3组变体对均有机会形核,因此,在孪晶界上可以观察到这3组变体对同时出现.

INFLUENCE OF COHERENT AUSTENITE TWIN BOUNDARIES ON THE VARIANT SELECTION OF BAINITIC FERRITE IN Fe-C-Mn-Si STEELS

To develop higher performance steels the requirements of the control of the microstructure is increasingly enhanced.The crystallographic orientation characteristics in the microstructure attract more and more attention because the mechanical properties are usually determined by the grain boundary density,especially high angle grain boundary density.So far researchers have conducted comprehensive investigation of the variant selection during the phase transformation.To decrease the nucleation barrier,the new phase usually chooses a specific orientation(variant) to decrease the interfacial energy between the parent and new phases.The variant selection occurs on the grain boundaries has been well studied.The nucleation and variant selection on austenite twin boundaries,however, are seldom reported,especially in the low carbon steels in which austenite can hardly be retained to the room temperature.In this work,the variant selection of bainitic ferrite on coherent austenite twin boundaries was studied using electron back scattering diffraction(EBSD) in Fe-C-Mn-Si steels with 0.05%C or 0.4%C(mass fraction).The orientation relationship between bainitic ferrite and austenite is close to K-S relationship in both steels.It was observed that the variant pairs with the similar crystallographic orientation nucleated on both sides of austenite twin boundaries.The twin grain boundaries were erased after the bainitic ferrite variants grew up.Crystallographic analysis showed that no more than three pairs of variants could be formed on one austenite twin boundary,the habit planes of which were all parallel to the twin boundary.As a result,the bainitic ferrite variant nucleated first would grow and expand along the twin boundary.In 0.05C steel,only one pair of variants was observed on the austenite twin boundary because the first nucleated bainitic ferrite variant pair grew fast due to the low carbon content which covers the twin boundary very soon and leaves no chance for the other variant pairs to nucleate on this twin boundary.In 0.4C steel,all three pairs of variants could be formed on one twin boundary because the higher carbon content slowed down the growth of first nucleated bainitic ferrite and more variant pairs could be nucleated on the twin boundary.