Abstract
Stable precipitation-strength of hot-rolled sheet steel can be realized with precipitates remaining fine even at high temperature coiling. This study deals with a relationship between the strength of ferritic steels strengthened by nanometer-sized carbides and coiling temperature in order to evaluate the stability of the strength against a deviation of coiling temperature. Ti-Mo-bearing and Ti-bearing steels were heat-treated for the precipitation of (Ti, Mo)C and TiC in ferrite matrix, respectively. These steels were hot-rolled and coiled in the temperature range from 550°C to 700°C. Matrix of the steels coiled at temperatures more than 600°C was only ferrite and nanometer-sized carbides in law were generated in the matrix. Ti-Mo-bearing steel exhibited high strength even at the high coiling temperature while the strength of Ti-bearing steel significantly decreased. Hardness of Ti-bearing steel just transformed at 650°C was the same as that at 625°C. In addition, while hardness of Ti-bearing steel coiled at 625°C significantly decreased during holding at 650°C for 86.4 ks, Ti-Mo-bearing steel did not show a large change in hardness. These results confirm that (Ti, Mo)C is not coarsened easily by Ostwald ripening at the high coiling temperature unlike TiC. Interfacial energy between (Ti, Mo)C and ferrite matrix can be the same as that in case of TiC since both carbides have the same structure and the equivalent lattice parameter. The retardation of Ostwald ripening of (Ti, Mo)C is attributed to a small amount of titanium in solution in Ti-Mo-bearing steel.
