Surface Hot-Shortness of Steels Induced by a Small Amount of Copper and Tin from Scrap Steels and its Suppression Methods

Abstract

The recycling of scrap steels can be difficult due to the tramp elements that they can contain. During the steelmaking process, tramp elements such as Cu and Sn are difficult to be removed; and it is these elements that cause surface cracking of steels during hot rolling process (i.e. Cu and Sn liquid embrittlement).The paper consists of three different experiments into the suppression of surface cracking during the hot rolling process. For the oxidation in air, the surface cracking most severely occurred in the specimens which were oxidized around 1100°C in the tested range of 950-1200°C after a 1250°C heating. For the change in oxidation atmosphere from air to water vapor, the surface cracking occurred more severely although the mass gains were smaller in water vapor than in air oxidation. For the addition of Si and Ni in the water vapor conditions of 0%-30%H2O, the surface cracking was found to be suppressed effectively when the mass gain increased. The Cu and Sn enriched alloys at the scale/steel interface were closely observed by optical microscopy and scanning electron microscopy. The mechanism for suppression of the surface cracking was explained in terms of back diffusion of Cu and Sn into the steel and/or occlusion of Cu and Sn into the scale through the development of a rugged scale/steel interface.