| This article aims at investigating the effects of martensite morphology on the dynamic deformation behavior of dual phase HSLA steels. In the intermediate quenched (IQ) steel sample, very fine fibrous martensites were well distributed in the ferrite matrix, whereas bulk martensites were mixed with ferrite grains in the step quenched (SQ) sample. Dynamic torsional tests were conducted using torsional Kolsky bar for two steel specimens, and then the test data were compared via microstructures, tensile properties, and fracture mode. After the heat-treatment, IQ-sample showed higher tensile strengths with relatively good ductility than those of the as-received sample, but SQ-sample had very poor ductility. The dynamic torsional test results indicated that in IQ-sample, fracture occurred in a ductile mode, irrespective of the loading type and rate. On the other hand, ductile fracture was predominated in SQ-sample under dynamic torsional loading, although cleavage fracture occurred with little plastic deformation under quasi-static tensile loading. This was because the constraint of the ferrite deformation was released by the deformation of martensites themselves, resulting in considerably high fracture shear strain in the dynamic torsional test. The deformation and fracture behavior of SQ-sample also correlated well with the dynamic shear stress-strain curve and observation of the central area of the gage section, i.e., void initiation at martensite ends and propagation through ferrite grains. |
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