Effect of Strain-Induced Precipitation on the Austenite Non-recrystallization ( T nr ) Behavior of a High Niobium Microa
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INTRODUCTION
THE development of high-strength linepipe grade steels of lower carbon content (i.e., C < 0.1 wt pct) with adequate toughness depends on the level of highest grain refinement that can be achieved from pancaked austenite as well as higher finish rolling temperature (FRT).[1–4] The role of niobium as a microalloying addition is well known to delay the recrystallization of austenite (c) by either solute drag effect or by precipitation of carbides/carbonitrides that can pin the dislocations/grain boundaries and thereby increase the non-recrystallization temperature of the steel, i.e., Tnr.[5–9] The Tnr is V. RAJINIKANTH is with the CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India and also with the Department of Materials Science Engineering, IIT Kanpur, Kanpur 208016, India. TIPU KUMAR, B. MAHATO, and SANDIP GHOSH CHOWDHURY are with the CSIR-National Metallurgical Laboratory. Contact e-mail: [email protected] SANDEEP SANGAL is with the Department of Materials Science Engineering, IIT Kanpur. Manuscript submitted April 29, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
defined as that temperature below which no complete static recrystallization of austenite takes place during the interpass time between two successive rolling passes or deformations. The determination of this temperature is very critical in designing the rolling schedule of the microalloyed steels to achieve the highest refinement of ferrite (a) grain size. Higher the Tnr, larger is the window for pancaking of austenite before it touches the Ar3 phase transformation (c + a) line during the hot rolling. This implies that a larger reduction of austenite is possible under lower rolling loads apart from improved grain refinement of ferrite due to phase transformation from the pancaked austenite. The determination of Tnr can be carried out by performing either multihit/multipass deformation method or double-hit deformation method as reported in the literature.[8–17] The multihit or multipass deformation process involves successive hot deformation with decreasing deformation temperatures at a specified strain per pass, strain rate, and interpass time. The deviation in the slope of mean flow stress (MFS) against the inverse of absolute temperature (T, K) from the above experiment denotes the Tnr.[18,19] The multihit or
multipass deformation method is very popular because it is simpler to conduct and generally requires few tests and samples to arrive at Tnr. Despite the capability of multihit methods to predict sufficiently accurate Tnr, it cannot provide accompanying microstructural information, i.e., recrystallization–precipitation interaction which is often required in recrystallization models to be applied during industrial hot rolling. The Tnr temperature can also be obtained by performing double-hit experiments at different test temperatures.[14,15] At each test temperature, the sample is isothermally deformed by two successive passes with a specified interpass time. For each pass, the specimen is deformed up to a specific strain at a
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