Abstract: Thermal-mechanical-transformation coupled relations ofboron steel are investigated in the hot forming (HFS, hot stamping) processby tensile and quenching experiments at high temperature. In theexperiments, plate specimens of boron steel are austenitized for fiveminutes at 950℃, then formed in tension and quenched. The force,displacement, expansion and temperature in the experimental process aremeasured. Based on the analysis of the above physical quantities variationand the specimen's microstructure, thermal-mechanical-transformation coupledrelations of boron steel are researched and thethermal-mechanical-transformation coupled constitutive models are developed.The multi-phase mixed relationship is introduced to analyze the effectivethermo-mechanical parameters and mechanical properties of multi phasesduring the hot forming. The components of strain and their evolved mechanismfor hot forming are investigated. The phase-transformation volume stress andphase-transformation plastic stress are defined and expressed to explain themechanism of thermal-mechanical-transformation coupled relations. Themodified Norton-Hoff flow stress is applied to describe the flow propertiesof boron steel at high temperatures and the material parameters is obtainedby analyzing the measured data of high-temperature tensile tests. Based onthe above research, the thermal-mechanical-transformation coupled models areintroduced into the constitutive equations of hot forming, and theintegrated and incremental constitutive equations are developed,respectively. Numerical simulation of U-shaped hot forming process isimplemented and compared with the experimental results. The results provethe validity of the developed constitutive equations.