Hot stamping and quenching of ultra high strength steel (UHSS) is a novel technology to widely produce automotive structural parts with excellent crash performance. It is a complicated multi-field coupled process, and its forming quality is affected by many process parameters. Experiment and numerical simulation were both taken to study the effects of these parameters on the hot stamping of the UHSS square-cup parts. Then the process parameters were optimized to improve the quality of UHSS hot stamping. By analyzing the defects in the hot stamping of the UHSS square-cup parts, the determining methods and criterion of the hot-forming limit were proposed. Through the numerical simulation of the hot stamping process of UHSS square-cup parts, the main influencing factors and mechanism of the hot-forming limit were analyzed. Besides, the influencing law of the main parameters on the hot-forming limit of UHSS square-cup part was gained and verified by experiments, such as blank holder force (BHF), friction coefficient. The experiment and simulation results showed that the numerical simulation method proposed could make a good prediction to the hot-forming limit of the square-cup parts, and it could provide an effective way to predict the forming limit of the non-axisymmetrical parts for UHSS in hot stamping. It could also offer a reliable basis for realizing the optimization of the UHSS hot stamping process.
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