The effect of thermo-mechanical treatments on the micro-structure and mechanical properties of high strength low alloy steels micro-alloyed with vanadium

A series of vanadium micro-alloyed high strength low alloy steels were tested to evaluate the effect thermo-mechanical treatments had on their mechanical properties. The properties under consideration were the mechanical strengths and strain ageing propensity, as defined by the standard tensile test, and the fracture mode transition temperature, as defined by the Charpy impact test. Ferrite grain size was also evaluated, as was the portion of the interstitial nitrogen in the steel which was present in the form of a micro-alloy precipitate. Cooling rate was found to be the most significant factor in determining the ferrite grain size of the steel. Steel chemistry was found to be the primary factor in determining precipitation strengthening, this being independent of thermo-mechanical history. Carbon rich vanadium carbo-nitride precipitates in the steel were found to coarsen more rapidly than nitrogen rich precipitates, and a vanadium to nitrogen atomic ratio of less than 2:1 was, therefore, required to avoid loss of precipitation strengthening as a result of this effect. The effect of vanadium carbo-nitride precipitates on the fracture mode transition temperature of the steel was found to be essentially independent of cooling rate. It was dependent on the vanadium concentration of the steel, and on whether the carbo-nitride precipitates were coherent or non-coherent with the ferrite lattice. Strain ageing was found to be significantly reduced (less than 15 MPa) if the atomic ratio of vanadium to nitrogen was greater than 1.8:1. The fine precipitate present in the samples were found to be dissolved by standard acid dissolution nitrogen analysis techniques. As a result of this an alternate analysis technique was developed and shown to be more accurate in determining the concentration of nitrogen rich precipitates in the steel samples