Precipitation of Nb in Ferrite After Austenite Conditioning. Part I: Microstructural Characterization
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NTRODUCTION
THE metallurgical basis for thermomechanical processing of high-strength low-alloy (HSLA) steels is the use of microalloying elements.[1–4] Strain-induced precipitation of niobium carbonitrides is able to delay austenite recrystallization during interpass times,[5–7] leading to the strain accumulation that enhances nucleation during c fi a transformation and final grain refinement.[8– 11] Depending on the steel composition and the thermomechanical sequences applied to condition the austenite, some Nb can remain in solution at finishing temperature. In particular, it is expected that for sheet rolling, which involves a fast process, short interpass times at the finishing mill (~10 seconds) will not allow full precipitation of Nb. Free Nb remaining in solution after hot working can play different roles, depending on cooling conditions. Free Nb increases hardenability, which enhances the formation of nonpolygonal ferrite grains with irregular shapes or low-temperature transformation products (bainite, acicular ferrite), particularly at high cooling rates.[12] Bainitic microstructures form when sufficiently high cooling rates and low coiling temperatures combine and contain a dislocation density that increases with decreasing transformation temperature.[13,14] However, in the field of Nb containing steels, both low and high dislocation densities were reported at the same time by different authors for the same coiling temperatures. For example, industrial coiling A. IZA-MENDIA and B. PEREDA, Researchers, and I. GUTIE´RREZ, Senior Researcher, are with the Materials Department, CEIT and Tecnun (University of Navarra), 20018 Donostia-San Sebastia´n, Basque, Spain. Contact e-mail: [email protected] M.A. ALTUNA, formerly Researcher, with the Materials Department, CEIT and Tecnun (University of Navarra), is now Metallurgy Engineer, with Grupo WEC, Polı´ gono Industrial no 38, 20829 Itziar Deba, Basque, Spain. Manuscript submitted October 10, 2011. Article published online September 29, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
at 923 K (650 C) of a 0.06C-0.06Nb steel produced an average dislocation density of ~2.5 9 1014 m2,[15] while in another work, a dislocation density of roughly 1012 m2 was reported for a 0.07C-0.043Nb steel also coiled at 923 K (650 C).[16] Surprisingly, when comparing the optical micrographs provided in the original references,[15,16] in spite of the two orders of magnitude difference in measured dislocation densities, the microstructures have a polygonal aspect and do not appear essentially different. Precipitation of Nb remaining in solution after finishing mill can take place in austenite. The MC precipitates exhibiting a ClNa type of lattice adopt a cube-cube orientation relationship with austenite. After transformation to ferrite, these precipitates become incoherent or follow a Kurdjumov–Sach orientation relationship with the matrix. The hot processing conditions of the austenite and the applied cooling rate influence the ability of Nb to precipitate in austenite. In particular, a low deformat
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