Abstract
The present study concerns the deformation and fracture behavior of two ferrite–martensite dual phase steels (FMDP660 and FMDP780) with different phase fractions subjected to different strain rate (0.001 s−1 to 1000 s−1) tensile testing. For both steels, the yield strength (YS) monotonically increased with strain rates, whereas the values of ultimate tensile strength (UTS), uniform elongation (UE) and post-uniform elongation (PUE) were maintained stable at the low strain rate range (0.001–0.1 s−1), followed by a significant increase with strain rate at high strain rate levels (0.1–1000 s−1). The FMDP780 steel with a higher fraction of martensite possessed a stronger strain rate sensitivity of tensile strength and elongation (UE and PUE) values at the high strain rate stage, compared with the FMDP660 sample. The change of UTS and UE with different strain rates and phase fractions was highly related to the strain hardening behavior, which was controlled by the dislocation multiplication in ferrite, as validated by transmission electron microscopy (TEM). The fracture surface of the two steels was characterized by dimpled-type fracture associated with microvoid formation at the ferrite–martensite interfaces, regardless of the strain rates. The change of the dimple size and PUE value of the two steels with strain rates was attributed to the effect of adiabatic heating during tensile testing.
Highlights
The automotive industry is facing stringent regulations and significant challenges on weight reduction of road transportation vehicles for fuel economy, as well as improvement of crashworthiness performance to ensure vehicle safety
It is obvious that white martensite islands are imbedded in dark ferrite matrix and mainly distributed on the ferrite grain boundaries; the volume fraction of martensite is around 20% and 35% for the FMDP660 and FMDP780 steel, respectively
It is shown that both steels yield without an obvious yielding platform, namely, the two steels undergo continuous yielding, which is usually associated with the existing high mobile dislocation density in the microstructure [16,17,18]
Summary
The automotive industry is facing stringent regulations and significant challenges on weight reduction of road transportation vehicles for fuel economy, as well as improvement of crashworthiness performance to ensure vehicle safety The response to these challenges requires the intensive use of advanced high strength steels (AHSS) in body-in-white assemblies. Despite the large amounts of the study on the dynamic deformation behavior of DP steels [8,9,10,11], some inconsistencies still remain with respect to the effect of strain rate on the change of mechanical property values. It is necessary to carry out systematic studies on the influence of the microstructure with respect to different phase fractions on the dynamic deformation behavior of FMDP steels. The effects of the phase fraction on the dynamic mechanical properties and energy absorption, the strain hardening behavior associated with dislocation motion, fracture mechanism, the adiabatic heating and fracture behavior were addressed in detail
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