Effects of microstructure and strain rate on deformation behavior in advanced high strength steels

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Mishra, Kishlay
Gokhale, Arun M.
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With an increasing demand for better fuel efficiency and better crashworthiness, automotive and steel industries in the recent past have seen enormous interest in developing thinner and stronger Advanced High Strength Sheet (AHSS) steels. During forming processes, AHSS steels can undergo deformation at 10-100/sec strain rate, while in a crash they can experience deformation at around 1000/sec. Therefore, it is important to understand their deformation at slow as well as high strain rates while designing automotive body parts. Most studies in this area have been in understanding the mechanical response of AHSS under different loading conditions. Some studies have also been done to qualitatively study the fracture surfaces, to understand the mechanisms of failure in these steels. However, very few quantitative studies have been done to understand these mechanisms of failure. Therefore, the objective of this study was to quantitatively understand the effects of microstructure and strain rate on the deformation behavior of three different grades of AHSS steels; HSLA 590, Ductibor® 500 and Usibor® 1500. Differences in microstructures were achieved using different grades of steel, as well as by changing the austenitization conditions of hot-stamping process. Microstructures were quantified for surface area per unit volume of different interfaces, volume fraction of different microstructural constituents, and length per unit volume of grain edges (for HSLA 590). HSLA 590 specimens were tested at various strain rates ranging from 10-4/sec to 3200/sec and their mechanical response were studied. Similarly, mechanical response of Ductibor 500 and Usibor 1500 were studied for strain rates ranging from 10-4/sec to 1000/sec. Various micro-mechanisms of failure were quantified using quantitative fractography and digital image analysis. Mechanical testing data shows that tensile properties of HSLA 590 are most strain rate sensitive. Tensile properties of Usibor 1500 are least sensitive to changing strain rates. Tensile properties of Ductibor 500 are more sensitive to hot-forming process parameters compared to Usibor 1500. Also, some fracture micro-mechanisms have been seen to vary significantly with processing conditions, even though their effects on mechanical behavior is minimal.
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