Microstructure and dynamic material performance of high strength and ultra high strength strip steels

Abstract

The combined need for weight reduction and increased safety in modern vehicles has led automobile manufacturers to utilise the unique properties of high strength steels in their vehicles. Standard quasi-static tensile tests allow the static mechanical properties to be evaluated for any given steel grade. However, automobile designers require more advanced data, including high strain rate tensile data to fully understand the performance of a steel grade during deformation. Knowledge of the role of strain rate on a given strip steel grades deformation characteristics is essential to fully predict the behaviour of the steel during a crash event to help ensure the safety of the vehicle passengers. In the present study, three commercial strip steel grades were subjected to room temperature deformation at variable strain rates, ranging from quasi-static to dynamic loading conditions, where their obtained mechanical properties were measured. The steel grades studied were a HSLA grade (XF450), a carbon manganese grade (CMn800) and an advanced high strength steel grade (DP1400), each with very different microstructures. The present study showed that the XF450 with a ferrite (97%)-pearlite microstructure exhibited superior elongation and was the most strain rate sensitive grade. This was the only grade where energy absorption until 10% elongation could be measured. The DP1400 with a mainly martensitic microstructure showed very little change in strength or ductility properties over the tested strain rates.

Dual phase versus TRIP strip steels: Comparison of dynamic properties for automotive crash performance

“…This was confirmed in the work conducted by Oliver et al 1 However, the true stress-strain curves presented in Figs. 2b, 3b, 4a and 4b for the TRIP600, TRIP800, TRIP950 and TRIP1000 grades respectively imply that in the early stages of deformation (,0 .…”

“…1 In addition to martensite and bainite, the TRIP grades studied have also been shown to contain varying amounts of retained austenite. Previous work 14,18 has shown the beneficial effects of retained austenite on the TRIP strip steel mechanical properties.…”

“…Both have been shown to comprise different microstructures contributing to different properties. 1 This work has shown that both DP and TRIP type grades offer the kind of properties automotive manufactures require for today's vehicles. The following conclusions can be drawn from this study.…”

“…1 The TRIP grades examined in this study generally contained higher levels of C, Si and Al compared to the DP grades, which is required to retain austenite in the structure at room temperature. By comparing the measured volume fractions (Table 2), it can be seen that higher levels of second phase constituents contribute to an increase in the observed strength for all the studied grades.…”

Position-dependent shear-induced austenite–martensite transformation in double-notched TRIP and dual-phase steel samples

“…Low-alloyed transformation-induced-plasticity (TRIP) steels were developed in the late 1980s (Matsumura et al, 1987a,b;Chen et al, 1989) and have ever since attracted significant interest for automotive applications owing to their outstanding combination of high strength and formability (Srivastava et al, 2006;Ská lová et al, 2006;Oliver et al, 2007;Kammouni et al, 2009;Chiang et al, 2011). At room temperature a typical TRIP steel microstructure contains three phases: ferrite, bainite and metastable austenite (Timokhina et al, 2004;Zaefferer et al, 2004).…”