Phase and Microstructure Evolution of a Low-Alloyed Steel During Intercritical Annealing and Quenching
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PHASE (DP) steels consist of a soft ferritic (a) and a hard martensitic (a¢) phase.[1–3] This combination of phases with different mechanical properties results in steels with low yield strength (due to ferrite) and high early-stage strain hardening.[4,5] Dual-phase steels have been developed in the 1970s and have been in industrial production since the 1990s.[6] DP steels are the most widely used advanced high-strength steels in the global transportation industry and are for example used in the automotive industry for car body parts, wheels, and bumpers.[7] Due to their combination of high ultimate strength and high ductility, they can absorb
MICHAEL PFUND, MORITZ WENK, and REINER MO¨NIG are with the Institute for Applied Materials, Karlsruhe Institute of Technology, Hermann-von Helmholtz-Platz 1, 76344 EggensteinLeopoldshafen, Germany. Conatct e-mail: [email protected] Manuscript submitted March 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
large amounts of energy during deformation which is for example of importance for energy absorption during car accidents. Ferrite is very ductile and can yield at low stresses while martensite is very strong. Due to this large discrepancy in the mechanical properties, the strength and the deformation behavior of dual-phase steels critically depend on their microstructures.[8,9] In particular, the spatial distribution and size of the martensitic regions are expected to affect the mechanical properties of the steel.[10–16] The dual-phase microstructure formed during the heat treatment of a DP steel is strongly influenced by the annealing temperature, quenching rate, and chemical composition.[17–20] Both, the intercritical annealing process as well as the quenching process control the final microstructure.[21] First, the microstructure that is present in the two-phase region during intercritical annealing depends on temperature, carbon content and alloying elements. Temperature does not only affect the ratio of ferrite and austenite (c), it also affects the carbon content in the austenite phase. Second, the quenching
process strongly affects the microstructure.[22] The fractions of diffusive (ferrite) or displacive (martensite) transformation products formed from austenite are controlled by the quenching rate.[23] The critical cooling rate which is necessary for the formation of martensite again depends on the carbon content and the austenite grain size. This complex interplay of heat treatment, phase transformation, alloy chemistry and crystallography eventually determines the mechanical properties of the steel. To optimize the heat treatment, knowledge of the through-process microstructural development is needed.[24] The microstructural evolution during the austenitization of low-alloyed steels is the first step in this process. The crystallographic orientation relationship (OR) between the two phases is commonly described by the models developed for the martensitic transformation. Several orientation relationships between martensite and its mother-phase austenite were pro
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