Three experimental advanced high strength steels were examined to assess their response to copper brazing heat treatments. A softening treatment consisting of a 2h subcritical anneal at 675 deg C was successful in generating a microstructure consisting of ferrite and spheroidized cementite. The corresponding tensile properties indicated attractive formability, as evidenced by total elongations ranging between 22.7 and 31.4% for the different steels. A variety of microstructures was obtained in the different advanced high strength steels after simulated brazing processes. These differences are attributed to the effects of alloy elements on phase transformation during the slow continuous cooling step after brazing in the austenite phase field. Compared to the tensile properties of the materials after the softening treatment, the brazed steels showed continuous yielding and an increase in tensile strength. A prominent increase in yield strength was observed in the DP-2 steel, along with a substantial increase in tensile strength. The re-brazing procedure coarsened the overall microstructure and lowered the strength somewhat. In comparison to existing technology wherein precipitation strengthened steels can be processed to have yield strengths of approximately 280 MPa prior to forming and brazing, and up to 350MPa after brazing, the present steels have slightly higher strengths after the softening treatment, and either comparable or higher strength after brazing. For DP-2 the strength is considerably higher, and this steel may thus offer a potential opportunity for product improvement. Weldability and steel processing issues are relevant, so further alloy optimization would be helpful, along with process optimization to lower the pre-brazing strength levels, preferentially via the hot-rolling process rather than an off-line softening treatment.
Copper brazing response of some advanced high strength steel grades
Journal of Materials Processing Technology ; 209 , 1 ; 70-76
2009
7 Seiten, 5 Bilder, 3 Tabellen, 14 Quellen
Article (Journal)
English
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