Effect of Mn on Strain Aging Behavior of Ultra Low Carbon Steel Sheets

Abstract

Aging behavior of low Carbon steel depends on chemical compositions. Mn were reported to reduce BH (Bake Hardening) value at low carbon mild steel, but its mechanism are not cleared. In this paper, strain aging behavior of ultra low carbon steel with Mn were examined by measuring Yield Point, Yield Point Elongation and BH.
The following results are obtained:
(1) Aging rate is reduced by 1.0% Mn addition. BH value is also reduced by Mn addition, which means that Mn does not improve the relationship between aging rate and BH value.
(2) The BH values of 300 min aging of the samples with Mn show higher values than the BH with 20 min, and show almost same values of the samples without Mn. It seems that no stable phase is formed by Mn addition.
(3) Atomic level interaction between Mn and Carbon is assumed to exist and its value is smaller than that of Mo-C. The trapping ratio of C by Mn at 25°C is smaller than that of Mo, and the trapping ratio of C by Mn at 170°C is larger than that of Mo, which cause the BH deterioration at Mn added steel.