Physical Metallurgy of Interstitial-Free Steels: Precipitates and Solutes

摘要

Typical applications of interstitial-free(IF) or ultra-low carbon(ULC) steels are represented by both body structure and inner and outer panels. Depending upon the precise application, these steels must exhibit adequate levels of strength, formability, and weldability. Furthermore, there are applications where the yield point phenomenon must be avoided and where strain aging may or may not be attractive. There are at present two families of IF steels delineated by yield strength. The first is the soft, high formability version where the yield strength is near 150 MPa. The second is the high strength version, with a yield strength around 250 MPa. There are two approaches to achieving the higher strength. The first is based upon solid solution strengthening usually by phosphorus~(1), while the second relies on paint bake hardening of unstabilized steel.~(2) It is now accepted that the success of these steels is critically dependent upon achieving the proper solute carbon content. For example, the solute carbon content should be at or near zero in the low strength, high formability version since solute carbon is very deleterious to texture formation during cold rolling and annealing. This requirement obviously demands complete stabilization of the bulk carbon content. Paint bake hardening, on the other hand, relies on obtaining a certain solute carbon content after annealing to achieve the final strength after baking. Hence, whether one considers the low strength or the paint bake hardening versions, one thing is certainly true: the production of steel components of proper properties and uniformity relies on the precise disposition of carbon between particles and matrix. This disposition must not only be known with some certainty, but also must be obtainable in commercial practice. This paper will review some highlights of our work on the stabilization of carbon, thermomechanical processing, cold work embrittlement, and texture control of IF steels.