Effects of morphology on the mechanical behavior of a dual phase Fe/2Si/0.1C steel
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I. I N T R O D U C T I O N
DUALphase steels whose structures consist of ferrite and martensite have received great attention in the past few years because of their characteristic mechanical properties. It has been shown that at a given tensile strength level, these dual phase steels have superior formability to commercially available high strength low alloy steels. 1-9 Several important points regarding the deformation mechanisms in dual phase steels have already been documented. The continuous yielding has been attributed to the presence of mobile dislocations in the ferrite matrix, l,z3 These dislocations are by-products of the austenite to martensite transformation. Additionally, tensile strength has been shown to vary linearly with martensite volume fraction by the mixture rule ~,2,4with the exception of steels having fine precipitates in ferrite: ,5 Also, the total elongation has been found to depend on the martensite volume fraction. 3,4 From the viewpoint that the mechanical properties, in general, are sensitive to the microstructures of the system, studies have been made to correlate the microstructure with the mechanical properties of dual phase steels. 6,7 However, the role of morphology (size, shape, distribution) of the constituents in dual phase steels is not yet clear. Therefore, the aim of this investigation is to clarify the effects of the morphology on the mechanical behavior of dual phase steels. II. E X P E R I M E N T A L P R O C E D U R E The material used in this investigation was high purity Fe/2Si/0.1C steel. This steel was homogenized under an argon atmosphere at 1100 ~ for 24 h, and then forged and hot rolled to 1.5 cm thick plate. Tensile and transmission electron microscopy specimens were prepared by cutting the sheets into slightly bigger N. J. KIM and G. THOMAS are both with the Materials and Molecular Research Division, LawrenceBerkeleyLaboratory and the Department of Materials Scienceand Mineral Engineering,University of California, Berkeley,CA 94720. Manuscript submitted May 23, 1980. METALLURGICALTRANSACTIONSA
dimensions than the exact tensile specimens, and were heat treated as described later. Transmission electron microscopy specimens made by twin-jet polishing were examined in a Philips EM301 microscope and also in a Philips EM400 microscope. The latter was used for X-ray analysis of the silicon distribution in the ferrite and martensite. Fracture surfaces of tested specimens were examined with an A M R 1000 scanning electron microscope. Also, to clarify where cracks are first nucleated and propagated, tensile specimens at the various stages of deformation were cut along the tensile axis and examined normal to the tensile axis with an A M R 1000 scanning electron microscope. All tensile tests were performed using 1.25 in. gage round specimens at room temperature on an Instron with a cross head speed of 0.05 cm/min. In order to obtain dual phase steels with various morphologies, three kinds of heat treatments were used as shown in Fig. 1 For the intermediate quenching treatmen
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