Effects of auto-tempering on microstructure and mechanical properties in hot rolled plain C-Mn dual phase steels

In this paper, the behavior of auto-tempering in hot rolled plain C-Mn dual phase steels was investigated. The steels with thickness of 11 mm were fabricated on the compact strip production line and coiled at 150 °C and 260 °C, respectively. The auto-tempering in the steel with coiling temperature of 260 °C caused the softening of ferrite and martensite and the reduction in hardness difference between these two phases, ultimately led to the decrease in tensile strength and strain hardening ability and the increase in post-necking elongation. The Charpy impact test results indicated that the toughness of auto-tempered steel was superior to the non-tempered steel. When the tensile test or impact test was conducted on the auto-tempered steel, both ferrite grains and tempered martensite islands were readily elongated as fibrous structure. The acquired fibrous structure could inhibit the formation of microvoids and propagation of cracks in tensile test, and delayed the tensile fracture. In addition, the fibrous structure could inhibit the propagation of main crack during impact test, whereas induced the branching cracks, leading to high impact energy.