Research of Low Carbon Nb-Ti-B Microalloyed High Strength Hot Strip Steels with Yield Strength ≥700 MPa

Abstract

The effects of different compositional alternatives of Nb-Ti-Mo, Nb-Ti-Mo-B, and Nb-Ti-B with the basic composition of 0.08C-1.8Mn were studied aiming at optimization of steel composition and hot rolling and cooling technology of a microalloyed high strength hot strip steels with YS ≥700 MPa. In the work, four experimental steels with different combinations of micro-amounts of Nb, Ti, and B are compared. Moreover, two experimental steels, i.e. the steel of Nb-Ti and the steel of Nb-Ti-B were subjected to CCT diagram experiment in order to find the effects of sub micro-amount B in this type of steels. It has been found that sub micro-amount B addition brings about significant influence on the tensile properties, microstructures, and phase transformation characters of this kind of steels. It has been also found that as coiling temperature after hot rolling, 600°C is better than 570°C when strip steel is applied under as hot rolled condition. If strip steel is applied under as annealed condition, 570°C is the preferable coiling temperature leading to larger strength increment and higher elongation after annealing. Boron microalloying is more suitable for as annealed strip steels. The CCT diagram study shows that sub micro-amount B addition strongly lowers the transformation start temperature and transformation stop temperature and obviously increases hardness at fast cooling rates (44–1°C/s). B suppresses ferrite transformation during continuous cooling at comparatively fast cooling rates promoting the granular bainite formation and increasing steel hardness. It is common that for this type of steels that in both B free and B containing steels, at certain cooling rates martensite islands is prone to occur. B shifts the martensite islands phase presence from faster cooling rates to slower cooling rates. In the case of as hot rolled high strength strip steel, martensite islands are to be avoid; while for the as annealed high strength hot strip steel, martensite islands are beneficial leading to further increase of strength level and to the improvement of elongation value after annealing.