Effect of carbide formation on the microstructure and texture of warm rolled and annealed low carbon steels containing Cr and B

The development of the desirable ND fibre, which is responsible for high r-values in warm rolled and annealed steels, has been associated with the presence in the microstructure after rolling of a high volume fraction of grains containing shear bands. These shear bands then become preferential nucleation sites for ND fibre grains during subsequent annealing. In the current work, the effect of chromium and boron addition on the microstructure and on texture formation was studied in a warm rolled and annealed low carbon steel. Optical microscopy, electron backscattering diffraction and scanning and transmission electron microscopy were employed. All the warm rolling microstructures consisted of polygonal ferrite grains. The addition of the chromium and boron led to the formation of coarse Cr23C6 carbides and fine strain-induced Cr23C6, C3C2 and B49C2 precipitates. The latter affected the microband thickness, dislocation density and stored energy. As a result, the rate of work hardening and the number of grains containing shear bands increased in the Cr alloyed steels. This produced a higher fraction of the ND fibre in the annealing texture of the Cr-containing steels compared to the unmodified steel.