Study on the cleanliness of crank steel S34MnV produced by the EAF-LF(VD)-VT

The experimental samples were the marine crank steel S34MnV produced by the EAF-LF(VD)-VC production process, including the electric period, LF-VD refining period, casting period, and forging ingots from a heavy machinery factory in north China to solve the difficulty of being rejected by ultrasonic flaw detection for crank shaft. These samples were analyzed by the methods of metallographic observation, energy spectrum, and the electrolysis. The obtained results indicated that there was a large number of inclusions during the refining stage, and the proportion of inclusions with the size bigger than 15 μm accounting to total inclusions were within 1.65%-10.34% after VD process. Furthermore, the probability of inclusions captured by columnar crystal in the steel ingot cross-section was found changed obviously in different locations of ingot cap. The content of large inclusions in the center of the beginning of the ingot cap was 5.81 mg/10 kg, with increasing the distance from the center to the edge, whereas this value decreased to only 25.9 mg/10 kg in the rear center of ingot cap, with decreasing the distance form the center to the edge. In addition, large numbers of inclusions was found to exist in the forging ingot with the biggest inclusion of a 140 μm DS category, which were difficult to be cut off completely and would be the main reason of being rejected by ultrasonic flaw detection for crank shaft.