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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.

The increased usage of high-strength, cold-rolled steels has revealed differences in spot welding characteristics between the thin, high strength steels and the plain carbon steels commonly used in the automotive industries. The response of four, 275 MPa yield strength, cold-rolled steels to a variety of spot welding schedules was determined through an evaluation of mechanical property and fracture appearance tests. The results of this study were compared to various weld quality evaluation criteria and suggestions for a revised criteria were proposed.

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.