Tempering of Mn and Mn-Si-V Dual-Phase Steels
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I,
INTRODUCTION
ALTHOUGH dual-phase (ferrite-martensite) steels offer an improved combination of strength and ductility compared to existing high-strength low-alloy steels, the factors controlling the strength and ductility of these steels are still only partially understood. In particular, the improved ductility of these steels has been attributed to many factors including a lower carbon content of the ferrite, formation of epitaxial ferrite, higher plasticity of the martensite phase compared to cementite, and the presence of retained austenite (transformation-induced plasticity). 1-6 Similarly, although the strength of these materials is obviously related to the volume fraction of martensite, the effect of the carbon content or strength of the martensite phase on the strength of the ferrite-martensite mixture characteristic of dual-phase steels is still being debated. 1,7-9,12 In the present work we have attempted to clarify some of these questions by studying the tempering behavior of two dual-phase steels, a simple 1.5 Mn water-quenched dualphase steel with only a small amount of retained austenite (3 pet), and a 1.7 Mn-Si-V continuously-annealed dualphase steel with appreciable amounts of retained austenite (9 pet). The tempering process provides a means of measuring both the effect of the hardness of the martensite phase on the tensile strength of dual-phase steels, and the effect of retained austenite on ductility because the hardness of the martensite phase varies with the tempering temperature and because the retained austenite decomposes into a ferritecarbide aggregate upon tempering at 200 to 400 ~
G.R. SPEICH, Research Consultant, A.J. SCHWOEBLE, Research Technician, and G. P. HUFFMAN, Associate Research Consultant, are all with U. S. Steel Research Laboratory, Monroeville, PA 15146. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees.
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II.
MATERIALS AND EXPERIMENTAL PROCEDURE
The chemical compositions of the two steels are given in Table I. The Mn steel was a simple 1.5 Mn sheet steel containing 0.12 pct carbon that had been intercritically annealed one hour at 760 ~ and water-quenched (115 ~ per second). The Mn-Si-V steel was a 1.7 Mn-0.58Si-0.04V sheet steel (rare-earth-treated; columbium was a minor addition) containing 0.12 pct carbon that had been intercritically annealed four minutes at 800 ~ and cooled at 5 ~ per second. Because of the different initial processing, the Mn-Si-V steel had much larger amounts of retained austenite (9 pct) compared to the Mn steel (3 pct), and also contained small amounts of pearlite or bainite that were not present in the Mn steel. Both steels had been hot-rolled to 2 mm (0.080 inch) thick sheets prior to intercritical annealing. The as-received (as-quenched) sheet steels were sheared into 100 x 19 x 2 mm (4 x
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