Effective grain size of dual-phase steel

doi:10.1016/0025-5416(86)90181-3

Materials Science and Engineering

Volume 83, Issue 1, October 1986, Pages 145-149

https://doi.org/10.1016/0025-5416(86)90181-3 Get rights and content

Abstract

The effect of second-phase martensite on cleavage crack propagation was studied to define the effective grain size of dual-phase steels which controls the ductile-to-brittle transition temperature. It has been shown that the introduction of second-phase martensite into the ferrite matrix has no effect on changing the direction of cleavage crack propagation; therefore the effective grain size of dual-phase steels is the same as that of the initial structure prior to two-phase annealing, regardless of changes in the ferrite mean free path.

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As illustrated in Table 1, the UTS × UE values of the developed DP steels owing to the high strength and sufficient elongation is higher than that of the as-received sample. The comparison of the present DP steels with the previous works and commercial grades of DP steels [6,9,11,23,47,57–62], has been showed in Fig. 9. The presented steels in this plot are containing less than 0.20 wt % C, 2 wt % Mn and 1.5 wt % of other alloying elements.
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Effect of holding time at an intercritical temperature on the microstructure and tensile properties of a ferrite-martensite dual phase steel
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Dual phase (DP) steels consist of hard brittle martensite phase and soft ductile ferrite phase. With a novel bending under tension test system, in-situ symmetrical bending under tension experiments were carried out and photomicrographs of bending surface were recorded. The microstructure “earthquake” of DP780 dual phase steels was observed in the bending under tension process. By analyzing the in-situ images serious, the initiation, coalescence of cavities and propagation of micro-cracks until final fracture were analyzed. The micro-cracks form only in the outside surface of bending radius, and mainly appear near the phase boundary of ferrite and martensite. Micro-cracks coalesce and propagate in the direction perpendicular to the stretching direction approximately, and at the phase boundary of martensite and ferrite. Furthermore, digital image correlation technology was used in this study to analysis the strain distribution between ferrite and martensite during the bending under tension deformation and fracture.
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Citation Excerpt :
As a result, the automotive industry is continuously exploring methods of enhancing their ductility and strength simultaneously to develop superior grades of such steels. The overall behavior is governed by the hard martensite island-like particles embedded into a ductile ferrite matrix (Rashid, 1981; Speich and Miller, 1979) along with their volume fraction and morphology (i.e., shape, size, and distribution), carbon content, and grain size (Davies, 1978; Erdogan, 2002; Erdogan and Priestner, 2002; Kim et al., 1986; Kim and Thomas, 1981; Park et al., 2014; Pierman et al., 2014). In the last decade, there have been numerous attempts in relating microstructural features of DP steels to their overall performance through microstructural modeling of representative volume element (RVE).
Strong and ductile dual-phase (DP) steels are an important class of advanced high strength steels that are commonly used by the automotive industry. Micromechanical computational modeling is imperative for guiding the material design of DP steels with simultaneous enhancements in strength and ductility. In this study, the effect of the morphology of the martensite hard phase on the overall stress-strain response of DP steels is studied. Through generating realistic virtual representative volume elements (RVEs) and adapting a microstructural-based approach using a finite deformation elastic-viscoplastic constitutive model, it was possible to conduct various parametric studies of the effects of martensite phase on the strength and ductility of DP steel. Morphological parameters of the martensite phase that are studied include (1) effect of aspect ratio (equiaxed versus elongated), (2) effects of the combination of various aspect ratio within an RVE, and (3) effect of interconnectivity between martensite particles and its thickness. It is shown that elongated martensite particles lead to a better overall performance than that of equiaxed martensite particles. It is found that by having the martensite phase interconnected, both strength and ductility increase. Furthermore, it is shown that the increase in the volume fraction of equiaxed martensite particles within microstructures of elongated particles reduces the strength and ductility simultaneously. Overall, it is concluded that the morphology of the martensite phase in DP steels impacts more the ductility than strength.

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^∗: Present address: Allied Corporation, Metals and Ceramics Laboratory, Morristown, NJ 07960, U.S.A.

^†: Present address: Westinghouse Electric Corporation, Marine Division, Sunnyvale, CA 94088, U.S.A.

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Effective grain size of dual-phase steel

Abstract

Scr. Metall.

Metall. Trans. A