Abstract
The effects of the accelerated cooling after the controlled rolling (CR) on mechanical properties were investigated of the various microalloyed steels. The controlled rolling conditions were a reheating temperature of 1 100°C or 1 200°C, cumulative rolling reduction of 70% below 900°C, and a finishing rolling temperature of 800°C with a plate thickness of 20mm. The interrupted accelerated cooling (IAC) was carried out in the temperature range from 780 to 600°C, followed by air cooling, and the maximum cooling rate was 12°C/s. The increase in cooling rate by IAG caused microstructural changes such as the refinement of ferrite grain size, the disappearance of pearlite banding and the formation of refined bainite in place of pearlite. The increment of strength due to IAC with cooling rate of around 10°C/s that gave rise to the very refined duplex structure of ferrite and bainite was 5 to 10kg f/mm2, depending on the content of the microalloying elements such as Nb, V, or Ti. The Charpy V-notch transition temperature was improved by IAC when the reheating temperature was 1 100°C, and the superior low temperature toughness (such as a transition temperature below -100°C) was attained with occurrence of very few splittings on the fracture surface of the Charpy V-notch impact specimens. The optimum cooling conditions and the transformation behavior resulting from IAC are discussed. The increment of strength due to IAC was analyzed based on quantitative investigation of the transformed microstructure.
