Effects of annealing cooling rates on mechanical properties, microstructure and texture in continuous annealed IF steel

The mechanical properties, microstructure and texture of interstitial-free (IF) steel processed through continuous annealing routes under five cooling rates (5 °C/s, 50 °C/s, 200 °C/s, 500 °C/s and 1000 °C/s) have been studied. The cooling rates of 200 °C/s, 500 °C/s and 1000 °C/s are defined as ultra-fast cooling rates. This paper focuses on the differences of mechanical properties, microstructure and texture evolution between ultra-fast and common cooling rates (5 °C/s, 50 °C/s) for the first time. The overall structural features of specimens were observed by optical microscopy and nano-sized precipitates were observed by transmission electron microscope (TEM) on carbon extraction replicas. Texture measurement was carried out using D8 advance X-Ray Diffraction (XRD) and the (110), (200) and (112) pole figures were determined. The yield strength increases with increasing cooling rates, from 96 MPa under the cooling rate of 5 °C/s to 136 MPa under the cooling rate of 1000 °C/s, while the ultimate tensile strength changes very little (around 280 MPa). The samples annealed under the cooling rates of 5 °C/s and 500 °C/s have low rave values (2.13 and 2.21 respectively). By contrast, the samples in cases of 50 °C/s, 200 °C/s and 1000 °C/s possess higher rave values (2.32, 2.40 and 2.51 respectively). Three morphologies of TiN particles can be observed in all samples. TiN, with CaO and SiO2 acting as heterogeneous nuclei, is prone to have big size. The size of pure TiN precipitates deceases as increasing of cooling rates. FeTiP is found near TiC precipitates under the cooling rate of 5 °C/s. The sample annealed under the cooling rate of 5 °C/s has the weakest {111} <112> and {111} <110> orientations. The intensity of {111} <110> component increases as the cooling rate increases from common cooling rates of 5 °C/s and 50 °C/s to ultra-fast cooling rates of 200 °C/s, 500 °C/s and 1000 °C/s. The intensity of {011} <100> orientation decreases as the cooling rate increases from 5 °C/s to 500 °C/s. The relationship between r value, microstructure and texture components has been studied in this paper.