1、Effect of microstructure and chemical composition on dynamic factor of high strength steelsJ. Qu*, W. Dabboussi, J. Nemes and S. YueThe high strain rate properties of high strength steels with various microstructures and static strength levels were studied by means of split Hopkinson bar apparatus i
2、n shear punch mode. The as received cold rolled sheet steels were subjected to a variety of heat treatment conditions to produce several different microstructures, namely ferrite plus pearlite (FzP), ferrite plus bainite(FzB), ferrite plus martensite (FzM) and ferrite plus bainite and retained auste
3、nite (FzBzRA). According to the variation of dynamic factor (ratio of dynamic to static strength) with static strength, two groups of microstructures with two distinct behaviours were identified, i.e. classic dual phase (ferrite plus martensite) and multiphase (including ferritepearlite, ferritebain
4、ite, etc.). It was also observed that the general dependence of microstructure on the dynamic factor was strongly influenced by chemical composition in the case of ferrite plus martensite microstructures.Keywords: DP steel; TRIP steel; Hopkinson bar; Shear punch test; Dynamic factor; IntroductionHig
5、h strength steels are being extensively applied to automobile body structures to improve crashworthiness without increasing weight. In particular, dual phase (DP) and transformation induced plasticity (TRIP) steels have received particular interest, as they exhibit an excellent combination of cold f
6、ormability and strength, compared to conventional high strength low alloy (HSLA) steels.1,2 Dual phase steel is characterised by a matrix of ferrite with small islands of martensite. The hard martensite particles provide substantial strengthening while the ductile ferrite matrix gives good ductility
7、. Therefore, the mixture acts somewhat like a particle reinforced composite and exhibits continuous yielding behaviour and a high work hardening rate. Transformation induced plasticity steel contains ferrite, bainite and retained austenite, and the relatively high ductility of this type of steel res
8、ults from the transformation of metastable retained austenite to martensite under straining. High strength low alloy steel grades, which are low carbon grades with microalloying additions of Nb, Ti, and/or V, are also key materials fulfilling ultra light weight design requirement. The combination of
9、 microalloying and thermomechanical processing allows the exploitation of different strengthening mechanisms.35 . To obtain an indication of crashworthiness of a material, high strain rate deformation performance, quantified by parameters such as dynamic strength and absorbed energy, is usually exam
10、ined by Hopkinson bar testing.6,7 or drop weight crush testing.8,9 Strain rate sensitivity can be defined by the strain rate sensitivity coefficient (e.g. the m-value of the extended Hollomon equation10 or the C-value of the Johnson-Cook equation11), or by the dynamic factor, which is the ratio of t
11、he material dynamic strength to static strength. Although there are some contradictory results in the literature, it is generally found that the dynamic factor decreases with increasing static strength level12,13. However, it seems that the dependence of microstructure on the general behaviour of dy
12、namic factor with respect to static strength has not been clearly understood, and the effect of chemical composition has not been studied adequately. This investigation was undertaken to determine the effect of microstructure on the strain rate sensitivity of three steel compositions, which were ori
13、ginally designed to be high strength DP, TRIP and HSLA steels respectively. Different types of microstructures with a wide range of static strength levels were obtained by various heat treatments on the as received cold rolled steels. The dynamic properties of these microstructures were then measure
14、d by means of shear punch testing, using a split Hopkinson bar apparatus. The effect of chemistry was also studied by comparing the dynamic factor of certain microstructures generated from different steels.ExperimentalMaterialsThe materials used in this study were three cold rolled sheet steels with
15、 a thickness of 1-7 mm, the compositions1 Schematic diagrams of various heat treatment schedulesof which were originally designed to be high strength DP, TRIP and HSLA steels respectively, as listed in Table 1. Steel A has a relatively high level of manganese plus some molybdenum to facilitate the f
16、ormation of the dual phase microstructure. Steel B has a very high level of aluminium to enable the formation of retained austenite. Steel C is mainly strengthened by grain refining and precipitation hardening by a small addition of niobium. Each of these steels was heat treated to generate different types of microstructures, namely ferritezmartensite (FzM), ferritezpearlite (FzP), ferritezbainite (FzB), and ferritezbainitezretained austenite (FzBzRA), as listed in T
