Study on effect of strain rate on elongation in advanced high strength steel

Abstract

Abstract Automotive parts made of Advanced High Strength Steel (AHSS) deform at various strain rates during the press forming process, car crash, etc. It is known that conventional steels are affected by strain rate. AHSS is increasingly being used for reducing the weight of automobiles. Since such high tensile steels are also used more and more in other industries, it is important to investigate the relationship between stress and strain with changing strain rate for AHSS in order to improve the accuracy of deformation simulation. In this paper, a high speed tensile test method is proposed and results of investigations on the stress-strain curves with changing strain rate for AHSS are reported. High speed tensile tests were conducted using a servo-hydraulic controlled testing machine. Quasi-static tests were carried out by conventional testing apparatus. The test material was dual phase cold-rolled steel sheet with tensile strength of 980MPa. By using specimens with newly designed shapes, they could be broken at the center stably at various tensile velocities. Experimental results showed that the ductility of AHSS presented nonlinear behaviors dependent on the strain rate. Elongation was 23% at strain rate 0.1 s-1 and 29% at 1000 s-1. Uniform and necking elongations were measured at various strain rates. As a result, it was found that the necking elongation was larger than the uniform elongation with respect to strain rate. In order to investigate the reasons for this, the temperature was measured during the tests. The results revealed that at high strain rates, the temperature exceeded 520K, confirming that the tensile stress of AHSS increases due to the strain rate and ductility is changed by heat generated during plastic deformation.