Mictrostructure and Mechanical Property Development in the Heat Affected Zone of Ultrafine Grained HSLA Steel

Abstract

The ferrite grain refinement is a powerful mechanism to improve mechanical properties of low carbon steels providing steels with high strength and toughness at low temperatures and good weldability characteristics. The grain size refining is the only mechanism capable of to increase both mechanical strength and toughness. By refining the grain size of low carbon steel from 5 μm to 1 μm, its yield strength can be theoretically increased from 450 MPa to 650 MPa. In this way refining of ferritic grain is a very attractive processing route. This work aimed to investigate the characteristics of the heat affected zone of a microalloyed low carbon-manganese (0.11% C, 1.41% Mn, 0.028%Nb, and 0.012%Ti) steel with ultra-fine ferrite grain structure produced through quenching, warm rolling, followed by sub and intercritical annealing in laboratory. Four intercritical annealing treatments were performed after the same warm rolling processing to obtain different grain sizes with residual work hardening of the base metals. Specimens were TIG welded with 4 different levels of heat input. Cooling conditions during tests were recorded and used to evaluated the microstructure of the heat affected zones and their hardness. Cooling times between 800 and 500°C from 0.6 to 17 s were obtained. Martensite was observed in the heat affected zones for low-heat-input welding conditions. No softened zone was found in the heat affected zone in any of the performed tests.