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The purpose of this paper is to show the approach used by automotive and line pipe steel producers to develop a higher strength sheet steel with improved formability, toughness, and welding characteristics. Also covered are the appropriate stages of steelmaking and hot rolling leading to the development of a columbium-bearing steel with a minimum yield strength of 50 ksi. Typical mechanical properties and forming capabilities are presented, together with a discussion of the strengthening mechanisms and ductility factors utilized to achieve this formable 50 ksi product.

Current trends in the automotive industry seem to be to use the minimum panel curvature that is possible, on the theory that a box provides the most space at the least weight. The purpose of this paper is to investigate the premise that increased curvature of body panels results in a net weight reduction, since the contribution of the increase in surface area is less than the reduced thickness resulting from the increased overall stiffness of panels with increased curvature. In assessing this concept, an expression has been developed for calculating the surface area as a function of panel curvature and panel size in two perpendicular directions. Using this relationship and previously published Chrysler work on the effect of these same two curvature and panel size variables on panel stiffness, an expression is developed for percent weight change at constant stiffness.

Mechanical properties and welding characteristics of a commercial, dual-phase, low-carbon, cold-rolled steel are described. The new steel, HI-FORM 80d, exhibits a total elongation of about 24% as produced and develops a yield strength of about 625 MPa (91 ksi) in a formed and paint-baked part. Property uniformity is excellent and the weldability essentially equivalent to an AISI 1006 steel. In addition, the aging response of HI-FORM 80d is such that yield strengths near 550 MPa (80 ksi) can be achieved with strains of less than 2% and lower paint-bake temperatures than are currently in use.

The mechanical properties and microstructures of low carbon alloy-free martensites are discussed as well as engineering aspects such as joining techniques, fatigue, protection from corrosion, and forming practice. With the properties that have been developed, it is shown that the low carbon martensitic steels can compete with high carbon quenched and tempered or austempered steels as well as such high priced materials as aluminum, titanium, and stainless steel. Similarly, the relatively low cost of low carbon martensitic steels, plus their high strength to weight ratio, makes these steels potential substitutes for plastics and fiber glass. Crash bars, welded tubing, fasteners, small spring-type parts, and corrugated panels are discussed as applications and supporting data are presented.

Eleven coated steels and cold rolled steel were painted with three automotive paint systems and subjected to a variety of corrosion test environments. The results of highly accelerated tests correlated poorly with the results of atmospheric and on-vehicle tests. The results of one accelerated atmospheric exposure (“Volvo” test) agreed with those of the longer-term tests. In a comparison of materials, it was found that all zinc-coated steels performed better than cold rolled steel, and that heavier zinc coatings provided longer-term protection than lighter zinc coatings.