Analysis of Hot Tensile and Compression Curves to Assess the Hot Ductility of C-Mn Steels

摘要

The unbending operation is a critical stage of steel continuous casting because it is carried out at thermomechanical conditions for which embrittlement mechanisms can appear leading to transverse cracking. The hot tensile test is commonly used to simulate such thermomechanical conditions, at the surface of the slab, and, the reduction in area of the samples tested to fracture is taken as a measure of the susceptibility to cracking of the steel. However, a further metallographic and fractographic evaluation of the samples is required in order to identify the embrittlement mechanisms. These mechanisms are usually related to transformations in the microstructure, such as precipitation or the appearance of deformation induced ferrite, which imply changes in the strength of the material and should therefore be detectable in the flow curves. However, the features of tensile curves are not usually analyzed when evaluating the hot ductility because necking makes the interpretation of the curves complicated. In this work the hot ductility of a C-Mn steel will be discussed by means of hot tensile and compression tests. The embrittlement mechanism identified for this steel is the appearance of a ferrite layer at austenite grain boundaries. The effect of this mechanism on the features of the tensile curves will be discussed. Moreover, these curves will be compared to compression curves obtained under the same testing conditions to see whether transformation induced ferrite can be detected by means of hot compression testing. The possibility of assessing the ductile behavior of different steel grades through hot compression, which requires less material and is easier to control, will be discussed.