Origin and Propagation of Splits in High-Strength Low-Alloy Strip Steel
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HIGH-STRENGTH strip steels, with a yield stress of approximately 700 MPa, can be used in applications such as earth movers and cranes (known as yellow goods). The high strength and good toughness required for this application are achieved by composition control (low carbon, niobium, titanium, vanadium and manganese additions) and by a fine grain size.[1,2] Strip thicknesses between 8 and 17 mm are used, although increasing thickness is generally associated with reduced toughness[3] along with increased scatter for Charpy impact energies in the ductile-to- brittle transition region. The broad transition region has also been associated with the presence of splits (fissures) on the fracture surface, which is a characteristic of controlled rolled steels.[4] The stress state at the crack tip influences the fracture energy and toughness of the material, which tend to increase as the tensile triaxiality decreases.[3,5–8] Tensile triaxiality can be reduced by relaxing the rzz stress component at the crack tip by the formation of splits that are perpendicular to the thickness direction (parallel with the rolling direction)—the orientation of the splits with RACHEL PUNCH, formerly Ph.D. Student, with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, U.K. is now Graduate Engineer, with Jaguar Land Rover, Whitley, Coventry, Warwickshire, U.K. MARTIN STRANGWOOD, Senior Lecturer, and CLAIRE DAVIS, Professor, are with the School of Metallurgy and Materials, University of Birmingham. Contact e-mail: [email protected] Manuscript submitted December 9, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS A
respect to the Charpy sample, and hence, stress, is shown in Figure 1. As splits occur, the thickness of the sample is effectively reduced, which decreases the rzz value to 0 at the surface of each split. The sample will then act as a group of thinner samples, producing a lower ductile-brittle transition temperature (DBTT)—the closer the stress state approaches plane stress, the lower the DBTT.[3,9] Many authors have investigated the origin of splits, with inclusions[9,10] (such as flattened MnS[11]), segregation of impurity atoms,[12] microstructural banding,[4,12] carbides on the grain boundaries of elongated grains,[4,13,14] and texture[13,15] all being reported to play a role. Song et al.[3] stated that the more cementite particles that are aligned in the material, the greater the number of weak planes, which leads to an increased tendency to form splits. When cohesion between the cementite particles and the matrix is weak, these can act as an initiation site for splits. Work has also been carried out on mild steel containing discrete carbides, which have been found to cause microcracks leading to cleavage fracture.[16] Echeverrı´ a and RodriguezIdabe[17] stated that a brittle particle does not govern brittle failure alone, as any microcracks formed at inclusions need to propagate through the surrounding matrix so that a sufficiently coarse grain size is needed as well. Crystallogra
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