The effect of relaxing on the grain refinement of low carbon high strength microalloyed steel produced by compact strip production

The microstructural characterization of a low carbon high strength microalloyed steel produced by compact strip production in conjunction with relaxation precipitation controlling the transformation technique was investigated. The microstructural observations were analyzed by means of optical microscopy, scanning electron microscopy, transmission electron microscopy and electron backscattering diffraction. The microstructure of the investigated steel consisted of predominantly granular bainite and lots of acicular ferrite and polygonal ferrite. The average crystallographic grain size was approximately 4 mm. Relaxation before fast cooling facilitated the formation of dislocation cells and intragranular acicular ferrite grains. Lath-like or plate-like acicular ferrite partitioned the austenite grains into many smaller parts, and the transformation of granular bainite at lower temperatures was confined to the smaller zones, resulting in smaller grain sizes. The yield strength, elongation and low temperature (−60 °C) impact toughness of the steel plates were 614 MPa, 24.1 %, 116 J, respectively. The excellent combination of mechanical properties was attributed to the formation of fine grains and sub-cellular structures.