Effect of Zr addition on phase transformation and precipitation in B-added hot stamping steel

• DOI: 10.1007/s12540-013-4001-y
• Publication Date: 2013-07-01T04:08:07Z
• Authors: Moon-Jo Kim; Hoon-Hwe Cho; Sung-Hwan Kim; Seung Man Nam; Seung-Ha Lee; Man-been Moon; Heung Nam Han
• Keywords: Hardenability; Hot stamping; Metallography

The effect of Zr addition on phase transformation and precipitation in B-added hot stamping steels was investigated. First, a thermodynamic calculation was conducted to compare the thermodynamic stability of nitride precipitates for various nitride-forming elements. The equilibrium temperatures for phase transformation and nitride precipitation in B-added hot stamping steels containing Zr were also calculated. The phase transformation kinetics of B-added hot stamping steels with various Zr contents were evaluated by both dilatometry and metallography. It was verified that Zr addition can provide protection of the B-hardenability effect, which produce a fully martensitic microstructure after hot stamping. To confirm the existence of Zr precipitation in the hot stamping steels, transmission electron microscopy and energy dispersive spectroscopy were used. A large number of precipitates were observed in the form of (Ti, Zr)N. The effect of Zr addition in B-added hot stamping steel to provide effective B-protection resulting in an increase of hardenability was identified and discussed.