Non-equilibrium phosphorus grain boundary segregation and its effect on embrittlement in a niobium-stabilized interstitial-free steel

Abstract

Grain boundary segregation of phosphorus in an Nb-stabilized and P-strengthened interstitial-free steel was investigated using Auger electron spectroscopy after the steel specimens were quenched from 880°C and aged at 680°C for different times. The segregation initially increased with increasing aging time until reaching a maximum value and then decreased gradually to its equilibrium value with further increasing aging time, demonstrating a non-equilibrium segregation kinetic characteristic with a critical time of around 30min. The ductile-to-brittle transition temperatures (DBTTs) of the aged specimens were determined through fracture appearance measurements. There was a linear relationship between DBTT and phosphorus boundary concentration (Cp, at%): DBTT(°C)=4.39Cp−64.06, suggesting that a P-strengthened IF steel should avoid slow cooling or staying in the intermediate temperature range so as to avoid apparent phosphorus grain boundary segregation and in turn to avoid embrittling.