Effect of Microstructure Parameters on Mechanical Properties of Cold-Rolled, Niobium-Microalloyed Steels after Continuous Annealing

We simulate heat treatment of cold-rolled, niobium-microalloyed sheet steels of four different compositions in units of continuous annealing. To study the effect of the microstructure characteristics on the mechanical properties of the rolled products after annealing, methods of metallography, transmission electron microscopy, and statistical analysis are applied. The relationship between the mechanical properties, grain structure parameters, and characteristics of nanosized precipitates of niobium carbide is discussed. We demonstrate that the key effect on the strength parameters is exerted by the density of the nanosized precipitates. The average particle diameter, when it fluctuates in the range of 1.4–1.8 nm, exerts a weaker effect on the mechanical properties. Among the parameters of grain structure, grain size in the transverse direction, which correlates with the parameters of the particles, exerts the greatest influence on the strength characteristics. Based on the results obtained, cassette technologies for production of high strength, cold-rolled steel can be developed, which enables to obtain rolled products of various strength classes from microalloyed steels of same chemical composition.