Abstract
The precipitation behavior of Nb in deformed austenite and the effect of precipitation on rolling load and mechanical properties were studied in two Nb treated steels and a Nb free steel. The steel slabs were solution treated at 1250°C, hot rolled in a five pass schedule and cooled at 40°C/s. Precipitation treatment of Nb was performed between third and fourth passes at 950°C for 020 min. Nb (CN) was quantitatively analysed as Insol. Nb by using electrolysis method.
Sol. Nb( Total Nb-Insol. Nb), rolling load and tensile strength(TS) decrease with holding time at 950°C. The decreasing rate of these values measured for 0.03%Nb steel are larger than that for 0.02%Nb steel. This result shows that the amount of Nb disolved in austenite accelerates the rate of precipitation. TS increase with increasing Sol. Nb and in case of 0.03%Nb steel the increment of TS depending on an increase in precipitation hardening in ferrite is estimated at 12 kgf/mm2. These results show that Sol. Nb is able to be used for the prediction of the mechanical properties of Nb treated hot rolled steel sheet.
