Studies on the development of high-strength dual-phase steel sheets with high r m values
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I.
INTRODUCTION
THE mechanical
properties of dual-phase steels having 10 to 20 pet martensite in a fine-grained ferrite matrix are characterized by a continuous yield upon plastic deformation, a relatively high initial rate of strain hardening, a low yield-to-tensile-strength ratio, and a comparatively large total elongation. 1,2,3These highly desirable properties of cold-rolled dual-phase steel sheets, and the possible future application of these steels to automobile body panels to achieve weight reductions, have been discussed recently by Goodman and Mould. 4 As is well known, steel sheets for body-panel applications must have deep-drawing capabilities. On the basis of limited available data, the rm values* of dual-phase steel sheets are *rm or F = (ro + 2r45 + r9o)/4, where numerical subscripts denote angles from rolling direction.
invariably poor, around unity. 5 In comparison with the solution-strengthened deep-drawing steels--for example, the silicon-phosphorus steels 6 having rm values around 2.0 and yield strength around 40 ksi (276 MPa) 4 - dual-phase steels are much inferior in deep-drawing properties. Very little research effort seems to have been devoted to the development of dual-phase cold-rolled steel sheets with good deep-drawing properties. The present investigation was undertaken to explore this possibility.
II.
M E T A L L U R G I C A L CONSIDERATIONS
Although a strong (111) texture can be produced successfully in the various deep-drawing low-carbon sheets steels, 7 the development of an equally strong ( I l l ) texture in a ferrite plus martensite aggregate would be more difficult. This is understandable because it is difficult to achieve a sharp texture in the martensite because of the nature of the transformation variants. Even if the ferrite texture could be developed successfully, the undesirably oriented martensite, which constitutes a substantial volume fraction of the material, would greatly reduce the rm value, and hence the deep-drawing capability, of the dual-phase steel sheets. A simplest approach to producing a deep-drawing sheet with tensile properties characteristic of the dual-phase steels is first to develop a predominantly (111) texture, hence an HSUN HU is Senior Research Consultant with United States Steel Corporation, Research Laboratory, Monroeville, PA 15146. Manuscript submitted May 27, 1981. METALLURGICAL TRANSACTIONS A
extraordinarily high rm value, in the cold-rolled and annealed steel sheet. Then the sheet could be briefly annealed to an intercritical temperature so that local regions with a high carbon content (such as areas adjacent to grainboundary carbides and pearlite colonies) would become austenite pools at the intercritical temperature. Upon rapid cooling, these austenitized regions would transform to martensite. The texture of the ferrite matrix should not be affected significantly by these heat treatments if the amounts of martensite is very small. On the basis of these considerations, and for the present initial explorations, it would be most desirab
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