Improve bendability of a Cr-alloyed press-hardening steel through an in-line quenching and non-isothermal partitioning process

Abstract

A new hot stamping process consisting of an in-line quenching and simultaneous non-isothermal partitioning was applied to improve the bendability of a Cr-alloyed press-hardening steel (Fe-0.23C-1.1Mn-(Cr + Si) < 5.0, wt%). The quenching temperature was controlled by the dwell time during die quench, and the non-isothermal partitioning was achieved through the subsequent air cooling after die opening. The feasibility of carbon partitioning during the air cooling stage was verified by kinetics calculation. Compared with the conventional hot stamping (HS) sample, around 6 vol% nano-sized metastable austenite was obtained in the sample processed by hot stamping with the in-line quenching and non-isothermal partitioning process (HS&QP). As a result, the average bending angle of the HS&QP samples increased by 10 % from 51.2° to 56.2°, and the total absorbed energy increased by 11 % from 28.7 J to 31.9 J compared with conventional HS samples, respectively. Meanwhile, the average ultimate tensile strength (1590 MPa) and total elongation (7.6 %) of the HS&QP samples are still better than that of the commercial-grade AlSi-coated 22MnB5. The proposed in-line quenching and non-isothermal partitioning process can significantly improve the bending properties of the newly developed Cr-alloyed PHS, increase press utilization, while balancing the requirements for strength, ductility and dimensional accuracy. It is a technically promising alternative to the conventional hot stamping process with economic value suitable for the newly developed Cr-alloyed PHS.