Effects of transformed ferrite growth on the tensile fracture characteristics of a dual-phase steel
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I.
INTRODUCTION
D u A L - p h a s e steels typically consist of a strong martensite phase finely dispersed in a soft ferrite matrix. ~ Consideration of the fracture behavior in dual-phase steels has been mainly descriptive. It has been reported that void formation arises from martensite-ferrite interface decohesion2'3'4 or by both martensite particle fracture and interface decohesion 5'6 at high levels of strain, with void growth and coalescence leading to subsequent failure. Stevenson 7 demonstrated that the mechanisms of crack initiation and subsequent crack propagation in a particular dual-phase steel are dependent on its thermal history. Kim and Thomas 8 also showed that different mechanisms of fracture are involved in the failure of dual-phase steels possessing various morphologies. Therefore, it is not to be expected that a single fracture criterion would apply to all dual-phase steels. Two different types of ferrite may be identified in most intercritically annealed dual-phase steels: the ferrite which is present at the intercritical annealing temperature, referred to as retained ferrite, and the ferrite which forms from austenite phase upon cooling. 9-12 The ferrite which formed on cooling grows epitaxially from the retained ferrite and is referred to as epitaxial ferrite or transformed ferrite. 9-12 Recently Jeong and Kim" reported that the growth behavior of transformed ferrite could vary remarkably with cooling rate and that this variation in the growth behavior of the transformed ferrite resulted in markedly different microstructures. It is then realistic to expect that such variations in the microstructure caused by different growth behaviors of transformed ferrite also influence the fracture of dualphase steel. However, the effect of transformed ferrite on the fracture behavior of dual-phase steels has never been reported. The aim of this study is to understand the role of transformed ferrite on the crack initiation and propagation and tensile fracture. It is also hoped to gain a clearer insight into the fracture mechanisms of dual-phase steels. W.C. JEONG is Senior Researcher, Research Institute of Industrial Science and Technology, P.O. Box 135, Pohang 680, Korea. C. H. KIM is Professor, Department of Materials Science and Engineering, Korea Advanced institute of Science and Technology, P.O. Box 131, Chongryang, Seoul 131, Korea. Manuscript submitted August 11, 1986. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL PROCEDURE
The chemical composition of the steel used in this study in weight percent was 0.13 pct C, 1.42 pct Mn, 0.63 pct Si, 0.16 pct Mo, 0.0054 pet P, 0.0068 pct S, 0.09 pct A1, and the balance, Fe. The steel was prepared as 50 lb, air induction melted heat. This ingot was hot forged, hot rolled, and finally cold rolled to the sheets of 1.5 mm thickness. Subsize tensile specimens with a reduced gage length of 25.4 mm and a reduced cross section of 6.25 mm by 1.5 mm were machined parallel to the rolling direction. All tensile specimens were sealed in vacuum in a quartz tube and austen
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