The Influence of Phosphorus on the Mechanical Properties of Carbon Steels near the Melting Point at Low Strain Rate

摘要

Microsegregation calculation, tensile testing and metallographic examination yield a uniform view of the influence of P on crack susceptibility of steel in BTRI. 1. The effect of P depends strongly on C-content. Low carbon steels are rather insensitive, although the increasing P-content lowers the critical limits of shell deformation. Straining by 1 % seems uncritical for the tested low carbon. P-(micro-)alloyed (< 0.046% P) steels, making them rather harmless in conventional continuous casting. Current trends towards higher casting speed and a further increase of P-content may change this situation. 2. Exceeding a carbon equivalent of 0.1 %, phosphorus becomes more harmful. For 0.011 % C-steels, P-contents of 0.024 % lower the critical strain to 0.8 %. The microsegregation calculations yield a steep increase of P-enrichment in the last residual liquid above 0.1 % C, which leads to enlarged critical temperature range, and a higher crack susceptibility. The microprobe analysis confirms these results, and indicates steadily increasing segregation ratio P_(max)/P_0 with higher carbon content. 3. The change in strength at solidus temperature with varying P-content also depends on C-content. The low carbon steels of series A point to a negligible influence of phosphorus, whereas an increase of P-content from 0.01 to 0.02 % lowers the strength of higher carbon steels by 1 MPa. 4. The alloying elements Si, Mn, P and S show extremely high concentration within segregated internal cracks. The phosphorus distribution is characterized by point-like peaks. The maximum concentration amounts to more than 1 %, even when the initial concentration is low. The existence of phosphides seems likely, but a clear correlation with the enrichment of other elements can not be found. The SSCT-test method, combined with theoretical and metallographic analysis gives a comprehensive view of the mechanical properties and crack susceptibility of solidifying steel. The results consider the harmful influence of macro- and microscopic inhomogeneity, like uneven shell growth, segregation and precipitation. These unavoidable phenomena lead to uneven stress distribution over the height of the specimen, and thus, the initiation of cracks occurs earlier than in conventional hot tensile testing. This is a reasonable explanation for the comparatively lower strength values. The possibility of subjecting the solidified specimen to metallographic examination enables the detection of defects. The testing parameters like applied strain, strain rate and steel composition are correlated with the extent of damage. This allows the definition of critical limits of shell deformation (critical strain or stress) and steel composition in order to prevent crack formation. The main target of current work is the implementation of FEM-analysis of shell deformation in the SSCT-test and the determination of creep laws as a function of steel composition. Subsequently, the influence of microscopic inhomogeneity on defect formation and mechanical properties will be considered by micro-modeling, based on the results of extensive metallographic work. These new approaches promise a deeper insight into defect formation, and improved process related material data for further modeling work is another expected result.