CONTINUOUS GALVANIZING OF MARTENSITIC AND COMPLEX PHASE STEELS FOR AUTOMOTIVE ANTI-INTRUSION APPLICATIONS(129)

摘要

From the perspective of crashworthiness and passenger safety, martensitic and complex phase UltraHigh 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 longtermintegrity of the structures involved for which continuous galvanizing is a cost-effectivesolution. Several challenges have to be overcome in order to process the above steels in thecontinuous galvanizing line (CGL) while achieving the minimum target tensile strength of1250 MPa. Steel chemical compositions should be selected in such a way that maintaining asuitable cooling rate produces martensite or bainite, and also provides a substrate surface withsufficient reactive wetting suitable for galvanizing. In the present study, steel chemistries weredesigned around relatively lean compositions based on carbon, manganese and silicon withadditional hardenability being provided by molybdenum or chromium additions. Annealing cycleswere determined based on the continuous cooling transformation behaviour of the steels. For bothsteel compositions the target tensile strength of 1250 MPa was achieved using austenitic annealingfor 120s followed by cooling to room temperature at -50°C/s. The steels were successfullyreactively wet by the Zn(Al,Fe) bath using a 95%N_2-5%H_2, -30°C dew point process atmosphere.From scanning electron microscopy, X-ray photoelectron spectroscopy and scanning Augermicroscopy it was determined that oxides of manganese, silicon and chromium formed duringannealing. However, these oxides did not have an adverse effect on coatability and both steelsformed high quality, adherent coatings.