Four high-strength low-alloy (HSLA) steels with varying chemical compositions were forged in two different temperature ranges followed by cooling in various media. Microstructures and mechanical properties of the steels were evaluated. The microstructures obtained in water-quenched low-carbon HSLA steels were lath martensite packet within the pancaked grains. On air or sand cooling predominantly bainitic ferrite or granular bainite structure forms. The strength properties of these steels decreased with decrease in cooling rate and is accompanied by an increase in elongation and impact toughness values. The ductile-to-brittle transition temperature of HSLA-100 grade steel was found to be - 40°C. The impact fracture surface of air cooled HSLA-100 steel showed ductile failure with formation of dimples at 20°C and at - 20°C. The fracture mode changed to brittle failure with formation of cleavage and river pattern at - 40°C and at - 60°C. The microstructures of the ultra-low carbon HSLA steel show lath ferrite or granular ferrite in water-quenched condition. With slower cooling rate, the volume fraction of lath ferrite decreased with an increase in formation of polygonal ferrite. The maximum strength value obtained in air-cooled condition is achieved due to precipitation of fine microalloying carbides and carbonitrides. Slower cooling rate increases the volume fraction of polygonal ferrite which increases the toughness value.
HSLA steel, bainitic ferrite, polygonal ferrite, toughness, naval applications, microstructure
