Effect of Coiling Temperature on Formability and Mechanical Properties of Mild Low Carbon and HSLA Steels Processed by Thin Slab Casting and Direct Rolling

Production of hot rolled HSLA steel strip is nowadays a well known practice; however information about the possibility to set their mechanical and formability performances through the control of the coiling temperature is quite rare, although the knowledge of this aspect can really improve the efficiency of the production process avoiding the application of other successive thermal treatment to condition the steel. In the present investigation a Nb-alloyed grade, a Nb-V grade and a V-grade have been rolled and coiled at different temperature and their mechanical, formability properties and attitude to the strain aging has been measured as a function of two different coiling temperatures. The results obtained on a mild low carbon steels which have undergone the same technological route have been used as a term of comparison. In the mild steel and in the Nb-alloyed ones the lowering of the coiling temperature improves the strength properties and the attitude to the strain aging, while decreases the formability of the steels while in the V-grade the coiling temperature performs a lower influence on these properties. This situation is confirmed also by the textural measurements and is probably related to the different removing rate of the interstitial elements, i.e. C and N. Actually, in the vanadium grades the precipitation temperature of the compounds formed by the vanadium and interstitial elements takes place about at the same thermal range of the coiling temperature producing a significant slowing of the kinetics of the nucleation and growth of those phases. This statement (concerning the fundamental role played by the concentration of the interstitial elements kept in solution in the metal matrix) seems to be supported by the strain aging measurements, which show a clear strain aging of the V-grade only after the artificial heating up which promotes their precipitation of the compounds formed by the interstitial elements