The controlled rolling and cooling test was conducted on V-N-Cr microalloyed low carbon steel. There are three cooling mode

including cooling with furnace

cooling with insulation quilt

and air cooling

and the microstructure

mechanical properties and fracture morphology of three kinds of cooling system were analyzed. The results that show the fine polygonal ferrite and acicular ferrite multiphase structure were obtained in air cooling steel plate. The grain size of ferrite is 5 to 8 pim. Acicular ferrite consists of interwoven laths with width of 1 to 3μm. The tempering occurred in polygonal ferrite and acicular ferrite

and small dispersed carbides were precipitated due to the slow cooling rate in cooling with furnace and cooling with insulation quilt. Under three kinds of cooling conditions

the yield strength ≥585 MPa

tensile strength ≥ 694 MPa

elongation ≥ 27%

and 1/2 sample impact toughness at -60 °C ≥36 J are obtained. The comprehensive mechanical properties is better than GB requirements of Q550F grade. Fine grain strengthening

precipitation strengthening

and microstructural strengthening were the main ways to strengthen the steel. The impact fracture are composed of dimples

exhibiting toughness fracture mode. The grain refinement and acicular ferrite formation caused by controlled rolling and cooling effectively resist the propagation of cleavage cracks

enhancing the low temperature toughness.