EFFECT OF BORON ADDITION ON THE MICROSTRUCTURE OF HOT-DEFORMED TI-ADDED INTERSTITIAL-FREE STEEL

Effect of boron (B) addition and hot-deformation on microstructure of ferrite was studied using Ti-added interstitial free (IF) steels conta ining different amount of B. It was clarified that bainitic ferrite ha ving typically lath or plate morphology with high dislocation density can be obtained also in plain IF steels by rapid cooling from austenit e region. B addition largely affected the austenite-ferrite transforma tion and the amount of bainitic ferrite increased with increasing B co ntent. Hot-deformation of austenite enhanced the transformation to pol ygonal or quasi-polygonal ferrite, resulting in decrease of bainitic f errite. Interrupting compression tests indicated that enhanced transfo rmation by hot-deformation is mainly due to grain refinement of austen ite by recrystallization after hot-deformation. The cooling rate after hot-deformation produced the reversed effect of B on grain size of po lygonal or quasi-polygonal ferrite. Ferrite grain size slightly decrea sed with increasing B content in the case of relatively targe cooling rate. When the materials were cooled slowly after hot-deformation, on the other hand, the ferrite grain size increased with increasing B con tent. The coarse ferrite in B-bearing steels had fairly irregular shap e of grain boundaries and inhomogeneous grain size, which suggested th e contribution of somewhat discontinuous grain growth. This coarsening of ferrite would be responsible for the previously reported fact that B addition lowers Lankford Value of cold-rolled and annealed IF steel sheets.