高强度钢韧化机制及与亚结构关系的研究

Study on toughening mechanism of high strength steel and its relationship with substructure

以实际生产制备的800 MPa级调质态水电钢为研究对象,结合扫描电子显微镜(SEM)、电子背散射衍射(EBSD)和冲击实验等,利用显微组织晶体学结构可视化与定量化方法,研究了钢板厚度方向不同取样位置低温韧性差异的本质原因。结果表明,高强度中厚板厚度方向由表面向心部过渡,显微组织由板条状贝氏体向粒状贝氏体过渡,低温冲击韧性降低,韧脆转变温度(DBTT)升高。随显微组织由表面向心部过渡(冷速降低),变体选择加强,心部形成了以单一贝恩(Bain)组为主导的相变组织,大角度晶界密度显著降低,且韧脆转变温度的升高与block界面密度降低紧密相关。此外,研究发现奥氏体晶粒内部的block界面和奥氏体晶界可以有效地偏折和阻止裂纹扩展,但由于奥氏体界面密度显著低于block界面,故对冲击实验过程中裂纹扩展阻力的贡献主要来自晶内的block界面。

Taking the 800 MPa grade quenched and tempered hydroelectric steel as the research object, the essential reason for the difference of low temperature toughness in different sampling positions along the thickness direction was studied by using the visualization quantitative method of microstructure, by scanning electron microscope(SEM), electron backscatter diffraction(EBSD) and impact test. The results show that transition from surface to center in the direction of ultra-high strength steel plate thickness, the microstructure changes from lath bainite to granular bainite, the low temperature toughness decreases and the ductile brittle transition temperature(DBTT) increases. With the transition of microstructure from the surface to the center(i.e., cooling rate decreases), the variant selection is strengthened, and the transformed microstructure dominated by single Bain group is formed in the center. The density of high angle grain boundary is significantly reduced, and the increase of DBTT is closely related to the density decrease of block boundaries. In addition, the study found that the block boundaries within austenite grain and prior austenite grain boundary can effectively deflect and prevent crack propagation. However, because the density of austenite grain boundary is significantly lower than that of block boundaries, the contribution to crack propagation resistance in the process of impact test mainly comes from the block boundaries.