Development of a novel Fast-Warm stamping (FWS) technology for manufacturing high-strength steel components

Abstract

Hot and warm stamping are preferable sheet metal forming technologies used in manufacturing high-strength parts with the twofold objectives of reducing fuel consumption and improving automotive crashworthiness. Great efforts have been made to improve the production rate in these processes and it is difficult to further improve productivity. Therefore, the development of new forming technologies may be an alternative solution to form high-strength steels into complex shapes whilst reducing the cycle time. The present work aims to develop a novel lightweight forming technology, namely fast-warm stamping (FWS) technique, to manufacture high-strength steel components with the desired properties. The concept of this process is to utilise ultra-fast heating of a steel blank to an appropriate temperature, whilst minimising the major negative changes to microstructure which are detrimental to the post-form strength. Mechanical properties such as ductility and post-form strength (PFS) of the MS-W900Y1180T (MS1180) steel were examined via uniaxial tensile tests at various temperatures (25–500°C) and strain rates (0.01–5/s). Special attention has been afforded to the effect of heating rate on thermo-mechanical properties and microstructure of the MS1180 steel with different heating rates. The results suggest that the ductility and post-form hardness of the MS1180 steel were simultaneously improved by 25.7% and 5%, with an increase in heating rate from 1 to 150°C/s. The increased hardness is attributed to the finer precipitated carbides and lower recovery at fast heating rate conditions, which was validated by microstructural observations. The validation of the FWS technology was conducted by forming U-shaped components through a dedicated pilot production line caller Uni-form. The fast-warm stamped components exhibited over 92% mechanical strength of the original as-received material consisting of 1140MPa post-form strength and 370HV hardness. The overall manufacturing cycle time in the FWS process was within 10 seconds. Springback of the formed parts under FWS conditions IV was successfully characterized at various temperatures and forming speeds. Close agreements were achieved between the experimental and simulated results for temperature, thickness distribution and springback prediction of the formed parts which validated the accuracy of the developed finite element (FE) model. FWS technology is a promising solution to manufacture components with desirable mechanical properties and dimensional accuracy. In this work, a feasibility study of the FWS technology was extended from martensitic steels to 60Si2Mn spring steel by producing commercialized disc springs. A separate forming tool set with a replaceable forming surface was developed to reduce manufacturing cost. Experimental results showed that a disc spring was successfully formed using the proposed forming process with the required dimensional precision, post-form strength and surface roughness. This forming technique has shown to enable a tremendous reduction of overall cycle time from 30 minutes to less than 20 seconds and subsequent productivity improvement for a mass-production setting.