Determination of the Conditions of the Chromium Nitride Formation in a ULC Steel of Fe–Cr–N Type with 0.7% Cr

Abstract

The nitrogen content in solution in the extra-mild steel sheets used in automotive or packaging industry after the different steps of their processing plays an important role both on the formability of the sheets and on their ability for strain ageing. This is why, the aim of the present work is to determine the conditions of the chromium nitride formation in a cold-rolled ULC steel with 0.7% Cr. In order to simulate the precipitation which may occur during the coiling or during the recrystallization annealing of the sheets, the investigated steel was submitted to different thermal treatments at temperatures ranging from 600 to 850°C. Then, the nitrogen content remaining in solution was evaluated after having defined a specific procedure based on thermoelectric power (TEP) measurements and using the segregation of the nitrogen atoms in solution to the dislocations introduced in the steel by cold-rolling. It was shown that the maximum precipitation speed is obtained at 650°C and that the precipitation is considerably delayed when the steel was recrystallized before the treatment.