Elasto-viscoplastic behavior of 980 MPa nano-precipitation strengthened steel sheet at elevated temperatures and springback in warm bending

Abstract

High-strength steel sheets strengthened by nano-carbide precipitation (nano-HSS) are suitable for warm forming because of their stable microstructure at elevated temperatures. Elasto-viscoplastic behavior of 980 MPa nano-HSS was examined by performing uniaxial tension and stress relaxation experiments at temperatures ranging from room temperature to 973 K. Warm V- and U-bending of the sheet were experimentally investigated, specifically focusing on temperature- and forming-speed-dependent springback. Furthermore, to clarify the effects of stress relaxation and unloading creep, the numerical analysis of warm bending and springback was conducted using the material model of elasto-viscoplasticity that described the material behavior adequately. Consequently, it was determined that stress relaxation and unloading creep played critical roles in some cases of warm bending, although the primary reason for the reduction of springback in warm forming was the low flow stress at elevated temperatures.