Development of low-temperature hot stamping technology for medium-mn steel
File(s)
Author(s)
Tong, Chenpeng
Type
Thesis
Abstract
Hot stamping is an advanced technology for forming ultra-high-strength steels (UHSS), giving good formability and negligible springback. Medium-Mn (MMn) steels are one of the classes of third-generation advanced high-strength steels that have attracted much research attention recently thanks to their low austenitisation temperatures, giving cost savings and improvements in productivity; this and their excellent combination of mechanical properties make them a good candidate material for lightweight vehicle components. In this study, in order to facilitate the use of MMn steels in hot stamping applications, research gaps related to understanding the mechanical and microstructural response of a representative MMn (8Mn) steel under different hot stamping conditions have been filled through carrying out a range of investigations. This consisted of the determination of phase transformation behaviour (i.e. austenite and martensite transformation), thermomechanical behaviour (i.e. flow curves at hot deformation), forming limit curves and final mechanical properties, as well as the influence of initial material state on hot stamping performance. The results demonstrate that the low-temperature hot stamping technology for the MMn steel shows great feasibility in hot stamping, as shown in the following main points: 1) The full austenitisation only requires heating to 740-800 °C and hold for 1-2 minutes, saving costs and improving productivity. 2) The critical quenching rate for achieving a completely martensitic microstructure is very low (> 0.1 °C s-1), providing excellent hardenability. 3) The formability, represented by strain hardening capability and forming limit strains, and final mechanical properties are better than those of conventional steels. A yield point phenomenon is observed for the first time in deformation under hot stamping conditions in the MMn steel, which is due to an excessively fine-grained microstructure...
Version
Open Access
Date Issued
2022-08
Date Awarded
2023-01
License URL
Advisor
Shi, Zhusheng
Lin, Jianguo
Sponsor
Shougang Group (Firm)
Grant Number
MESM_P72577
Publisher Department
Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)