Modeling the ductile-brittle transition behavior in thermomechanically controlled rolled steels
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I. INTRODUCTION
DESPITE many advances made in (sharp crack) fracture toughness testing and assessment procedures over the last 30 years, the vast majority of steel products are released to a specification that is based on energy absorption in the Charpy notched bar impact test (defined in ASTM Standard E 23), carried out at a prescribed low temperature. Body-centered cubic metals, such as mild steels, demonstrate a transition from a ductile fracture mode to a brittle cleavage failure mechanism as the test temperature decreases. The ductile-brittle transition may occur over a temperature range of only a few degrees Celsius, or a wide range of over 100 degrees, depending upon the microstructure and specimen geometry (thereby the applied stressstrain states in the specimen). Extensive studies have been carried out, over the past 40 years, to determine the quantitative structure-property relationships in steels. As a result, many empirical equations have been proposed to relate the strength and impact behavior to the microstructure and composition of the steels.[1–4] It is well established for hot-rolled or normalized ferrite-pearlite microstructures that the yield strength (Y),[5,6] the critical local fracture stress (F),[7] and the Charpy impact transition temperature (ITT)[8] are a linS.J. WU, formerly Research Fellow, Department of Metallurgy and Materials, The University of Birmingham, is Assistant Professor, School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100083, P.R. China. C.L. DAVIS, Senior Lecturer, is with the Department of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, United Kingdom. Contact e-mail: [email protected] A. SHTERENLIKHT, formerly Research Student, Department of Mechanical Engineering, The University of Sheffield, is Research Fellow, Materials Science Centre, University of Manchester, Manchester, M1 7HS, United Kingdom. I.C. HOWARD, Professor, is with the Department of Mechanical Engineering, The University of Sheffield, Sheffield S1 3JD, United Kingdom. Manuscript submitted May 6, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
ear function of (d )1/2, where d is the mean grain size of polygonal ferrite. The empirical equations, however, can only be applied to materials regarded as “uniform, homogeneous,” such as normalized steels, which have a single size distribution of ferrite grains together with small and finely distributed carbides, thereby allowing an average grain size to be used for prediction. The fracture properties of such materials are essentially single-valued functions within random experimental errors and can be expressed using average microstructural parameters such as mean grain size. Whereas for nonhomogeneous materials, such as steels with a duplex ferrite grain structure, seen in many thermomechanically control rolled (TMCR) steels, the average grain size parameter does not properly represent the microstructure,[9] and therefore cannot be used to predict the cleavage fracture str
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