Effect of manufacturing conditions on toughness in the middle carbon martensitic steels produced by the reheat-quenching process

Effect of processing conditions on toughness in the martensitic steels containing 0.22-0.24mass% carbon and single or multiple microalloying elements, such as Nb, V, and Ti, was studied. Ingots of these steels, prepared by a lab scale induction melting furnace, were reheated at various temperatures ranging from 1323 to 1473K, and hot rolled under the various conditions. Then, the obtained plates were heat treated at 1173K for 600s, followed by water quenching. The final microstructure was fully martensitic, with typical surface hardness of HB465 , and the tensile and yield strengths of 1450 and 1200MPa, respectively. Optical and electron microscopic observations, measurement of the prior austenite grain size and Charpy impact tests were carried out to understand effects of alloy compositions and processing conditions on microstructure and toughness. In the steels with Ti single addition, the prior austenite grain size was getting coarser with the higher reheating temperature and/or higher finishing temperature, due to the coarse microstructure before the final heat treatment. However, in the steels with Nb-Ti multiple additions, the prior austenite grain size was getting finer with the higher reheating temperature, since much amount of dissolved microalloying elements was precipitated as carbides or carbo-nitrides during the final heat treatment at 1173K This clearly shows the importance of precipitation control by selecting the optimum hot rolling condition before the final heat treatment Relationships between processing conditions, microstructural characteristics and toughness will be discussed in detail in terms of microstructural control through dissolution and precipitation of the microalloying elements during reheating and hot rolling.