Effect of Tempering Temperature on the Bendability of Martensitic Steels

Effect of tempering temperature on bendability of ultra-high-strength steel sheets with fully martensitic structure having tensile strength ranging from 1100 to 1650 MPa was investigated. In this study, cold-rolled steel sheets with carbon content ranging from 0.15 to 0.4% were quenched from austenite region and subsequently tempered at various temperatures below 500°C. Bendability seriously deteriorated at tempering temperature around 300°C when the tempered martensite embrittlement occurs. Under the same tensile strength level, steel sheets tempered below 200°C were superior in bendability to those tempered at even over 400°C when the ductility recovered. Minimum bending radius was well correlated with the elongated cementite particle number density in the tempered martensite matrix. In the cross section of the bent specimens, cracks along the elongated cementite particles were frequently observed. These results suggested that deterioration in bendability in steels tempered at higher temperature over 200°C was caused by the acceleration of crack propagation during bending deformation due to the elongated cementite precipitates. On the other hand, bendability of fully martensitic steels could not be clarified by the uniform elongation obtained by a tensile testing. Uniqueness of the material deformation behavior in bending was also discussed from the viewpoint of plastic working theory.