Influence of Ni impurity in steel on the removability of primary scale in hydraulic descaling

Influences of Ni impurity on the removability of primary scale in hydraulic descaling were investigated in 0.02 and 0.1 mass% Si mild steels. And the thickness distribution of residual scale was discussed in relation to the unevenness of the scale/steel interface. The removability of primary scale reduced due to the existence of Ni impurity ranging to 0.05 mass% in each steel. With an increase in Si content up to 0.1 mass%, the removability of Ni added steels showed heating temperature dependence between 1 150 and 1 250 degrees C. The existence of Ni impurity made the scale/steel interface of steel uneven, Estimating the unevenness of the interface by measuring the interface length, it was found that the austenite transgranular unevenness governs the change in the total interface length rather than the oxide invasion into austenite grain boundaries. A reduction of scale removability caused by Ni impurity was explained by the increased interface length. In spite of the large difference in scale removability between 0.02 and 0.1 mass% Si steels containing more than 0.05 mass% Ni, the unevenness of the scale/steel interface was the identical with each other. Therefore, the temperature dependence of the scale removability in Ni added 0.1 mass% Si steel was not explained by this factor. Other mechanisms concerning fayalite formation and its morphology should be taken into account for this behavior.