Continuous galvanizing of martensitic and complex phase steels for automotive anti-intrusion applications

Abstract

From the perspective of crashworthiness and
passenger safety, martensitic and complex
phase Ultra High Strength Steels (UHSS) are
ideal candidates for automotive anti-intrusion
components. However, these steels must be
protected from corrosive environments in order
to maintain the longterm integrity of the
structures involved for which continuous
galvanizing is a cost-effective solution. Several
challenges have to be overcome in order to
process the above steels in the continuous
galvanizing line (CGL) while achieving the
minimum target tensile strength of 1250 MPa.
Steel chemical compositions should be selected
in such a way that maintaining a suitable
cooling rate produces martensite or bainite,
and also provides a substrate surface with
sufficient reactive wetting suitable for
galvanizing. In the present study, steel
chemistries were designed around relatively
lean compositions based on carbon,
manganese and silicon with additional
hardenability being provided by molybdenum
or chromium additions. Annealing cycles were
determined based on the continuous cooling
transformation behaviour of the steels. For both
steel compositions the target tensile strength of
1250 MPa was achieved using austenitic
annealing for 120s followed by cooling to room
temperature at 50°C/s. The steels were
successfully reactively wet by the Zn(Al,Fe) bath
using a 95%N2-5%H2, -30°C dew point
process atmosphere. From scanning electron
microscopy, X-ray photoelectron spectroscopy
and scanning Auger microscopy it was
determined that oxides of manganese, silicon
and chromium formed during annealing.
However, these oxides did not have an adverse
effect on coatability and both steels formed high
quality, adherent coatings.