Microstructures And Properties Of High Pure Ferritic Stainless Steels Stabilized By Niobium And Titanium

With the increasing price of nickle, the costs of traditional austinetic stainless steels containing nickle have increased year by year; therefore, ferritic stainless steels with none or little nickle have been widely used in many fields. Ferritic stainless steel with the total content of carbon and nitrogen less than 300×10-6 is one of the most important development directions for stainless steel industry. Some stabilizing elements such as niobium and titanium are generally added into those steels to react with carbon and nitrogen to form carbonitrides of niobium and titanium. Mechanical properties and corrosion resistance of the steels are dramatically affected by those precipitates. Effects of niobium and titanium on various properties of the steels were investigated and the contents of niobium and titanium were optimized. Finally, a kind of high pure ferritic stainless steel with low costs was developed and applied to industrial trial production.This research firstly focused on the study of the occurrence of niobium and titanium and precipitation condition of the second phase containing niobium and titanium in high pure ferritic stainless steels. Then the change rules of combination property, including formability, drawability and corrosion resistance, were studied for the steels with different niobium and titanium contents. The results show that dual stabilizing is a proper way to improve the deep drawability and wrinkle resistance simultaneously, while it has little effect on the drawability. Increasing niobium content is of benefit to the wrinkle resistance and decreasing Ar. Increasing titanium content appropriately is beneficial to the improvement of r-values, but its content should be lower than 0.20%. Niobium and titanium have great influences on the formability of annealed cold-rolled sheets of high pure ferritic stainless steels through affecting the recrystallization microstructures of hot-rolled sheets. To complete recrystallization and obtain the recrystallization microstructures with a small grain size for hot-rolled sheets are of benefit to the formability of annealed cold-rolled sheets.Since the significant effect of recrystallization microstructures of hot-rolled sheets, effects of the contents of niobium and titanium on recrystallization were investigated, based on the current annealing process. A model of recrystallization kinetics of hot-rolled sheets was established and proved to be very precise by experimental results. Niobium and titanium have different effects on recrystallization. Fine precipitates rich in niobium suppress recrystallization but TiN precipitates with size bigger than 1?m promote recrystallization instead.On the basis of the results described above, the proper contents of niobium and titanium, showed as the ratio of (Nb+Ti)/(C+N), were determined to be in the range of 13 to 23. According to the optimized contents of niobium and titanium, a new type of ferritic stainless steel with low costs was developed and applied to industrial trial production. The annealing processes of hot-rolled sheets and cold-rolled sheets were optimized. Compared with the unoptimized high pure ferritic stainless steel, the r-value of annealed cold-rolled sheets of the optimized one increased from 1.84 to 2.22, the pitting potential increased from 0.09V to 0.133V and the reactivation rate decreased from 0.0067 to 0.0042. Meanwhile, the production costs were decreased by 561.6 yuan/ton.