Influence of Composition and Hot Rolling on the Subsurface Microstructure and Bendability of Ultrahigh-Strength Strip
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INTRODUCTION
IN recent years, ultrahigh-strength strip steels (UHSS) produced using thermomechanical rolling and direct quenching (TM-DQ) have become interesting materials for structural applications because they can exhibit the good combinations of mechanical properties and weldability.[1–3] The microstructures of these steels typically consist of lower bainite[1] and/or auto-tempered lath martensite.[3,4] In the case of TM-DQ strip steels, cold bending is the most important method of forming in applications such as containers and crane booms. The bendability is generally measured as the minimum ratio of the punch radius (r) to the strip thickness (t) that the steel can tolerate without the appearance of surface defects during bending to an angle of 90º in three-point bending.[5] Failure in bending typically results either from strain localization, i.e., the formation of shear bands, or development of micro-voids.[6–8] Earlier studies[4,9,10] have shown that strain localization is the precursor to failure in the air bending of UHSS and shear bands have ANTTI JUHANI KAIJALAINEN, Ph.D. Candidate, MIA LIIMATAINEN, Ph.D. Student, JOUKO HEIKKALA, Laboratory Manager, and DAVID A. PORTER, Professor, are with the Centre for Advanced Steels Research, Materials Engineering, University of Oulu, P.O. Box 4200, 90014 Oulu, Finland. Contact email: antti.kaijalainen@oulu.fi VILI KESTI, Specialist, Forming Technology, and TOMMI LIIMATAINEN, Technology Expert, are with SSAB Europe, P.O. Box 93, 92101 Raahe, Finland. Manuscript submitted January 1, 2016. Article published online June 3, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
been observed to initiate at depths in the range of 1 to 6 pct from the surface at angles of ~45 deg to the surface, i.e., in the maximum shear stress directions. Therefore, it is reasonable to suppose that the near-surface properties of the strip govern bendability. Kesti et al.[11] have shown that UHSS with a through-thickness and Kaijalainen et al.[10] with a subsurface microstructure comprising a mixture of granular bainite and ferrite has good bendability with a bulk yield strength of 700 and 960 MPa, respectively. The idea behind the present research was to explore whether higher strength steels, with a yield strength above 900 MPa, could be made by limiting the ductile ferritic–granular bainitic (F/GB) layer to the surface while having a stronger bainitic–martensitic (B/M) microstructure in the bulk of the thickness. Additions of niobium (Nb) are known to affect the recrystallization kinetic[12–14] by increasing the plastic deformation of the austenite below its recrystallization stop temperature thereby enhancing the formation of ferrite and granular bainite during cooling. Also carbon (C), manganese (Mn), and molybdenum (Mo) contents affect recrystallization kinetics.[13] Consequently, three chemical composition variants were chosen to explore the possibilities of producing an optimal softer F/GB near-surface layer together with a harder B/M bulk microstructure giving a tough, weldable UHSS strip
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