Effect of Cold Rolling before Hydrogen Reduction on Reduction Behavior and Morphologies of Oxide Scale on Hot-rolled Low-carbon Steel

Abstract

A new ‘acid-free picking’ method that adding cold rolling prior to hydrogen-reduction descaling was put forward in this study. The purpose of cold rolling was to break the completeness of oxide scale and increase reduction efficiency. Subsequently, the high-temperature reduction behavior of oxide scale on the surface of hot-rolled low carbon steel, which reacted with hydrogen after 10% deformation of cold rolling in 20%H₂-Ar at the temperature range of 400–850°C, was investigated to validate the feasibility. The mass loss of specimens in the non-isothermal and isothermal reduction experiment was clearly measured by thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) was used to observe surface morphology and cross-section microstructure of reduction products. The phase composition of reduction products was identified by X-ray diffraction (XRD). The experimental results indicated that the reduction reaction degree can be controlled by three main factors: reduction temperature, time and cold rolling. After 10% deformation of cold rolling, the completeness of original oxide scale was broken and then a large amount of micro-cracking appeared in oxide scale and the gap formed at oxide/substrate interface. The reduction rate of broken oxide scale was significantly faster than that of original oxide scale when the reduction temperature achieved more than 700°C. The effect of micro-cracking and gap on the mechanism of reduction process were clarified based on reaction of gas (H₂) – solid (iron oxides) interface.