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Jia Z.,University of Louisiana at Lafayette | Misra R.D.K.,University of Louisiana at Lafayette | O'Malley R.,Nucor Steel Decatur LLC | Jansto S.J.,CBMM Co.
Materials Science and Engineering A | Year: 2011

We describe here the precipitation behavior and mechanical properties of 560MPa Ti-Nb and 770MPa Ti-Nb-Mo-V steels. The precipitation characteristics were analyzed in terms of chemistry and size distribution of precipitates, with particular focus on the crystallography of precipitates through an analysis of electron diffraction patterns. In addition to pure carbides (NbC, TiC, Mo2C, and VC), Nb containing titanium-rich carbides were also observed. These precipitates were of a size range of 4-20nm. The mechanism of formation of these Ti-rich niobium containing carbides is postulated to involve epitaxial nucleation of NbC on previously precipitated TiC. Interface precipitation of NbC was an interesting observation in compact strip processing which is characterized by an orientation relationship of [001]NbC//[001]α-Fe, implying that the precipitation of NbC occurred during austenite-ferrite transformation. © 2011 Elsevier B.V.

Xu K.,University of Illinois at Urbana - Champaign | Thomas B.G.,University of Illinois at Urbana - Champaign | OMalley R.,Nucor Steel Decatur LLC
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2011

The formation of precipitates during thermal processing of microalloyed steels greatly influences their mechanical properties. Precipitation behavior varies with steel composition and temperature history and can lead to beneficial grain refinement or detrimental transverse surface cracks. This work presents an efficient computational model of equilibrium precipitation of oxides, sulfides, nitrides, and carbides in steels, based on satisfying solubility limits including Wagner interaction between elements, mutual solubility between precipitates, and mass conservation of alloying elements. The model predicts the compositions and amounts of stable precipitates for multicomponent microalloyed steels in liquid, ferrite, and austenite phases at any temperature. The model is first validated by comparing with analytical solutions of simple cases, predictions using the commercial package JMat-PRO, and previous experimental observations. Then it is applied to track the evolution of precipitate amounts during continuous casting of two commercial steels (1004 LCAK and 1006Nb HSLA) at two different casting speeds. This model is easy to modify to incorporate other precipitates, or new thermodynamic data, and is a useful tool for equilibrium precipitation analysis. © 2010 The Minerals, Metals & Materials Society and ASM International.

Zhou T.,McMaster University | O'Malley R.J.,Nucor Steel Decatur LLC | Zurob H.S.,McMaster University
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2010

The high-temperature grain-growth kinetics in delta-ferrite and austenite is investigated. The delta-ferrite growth kinetics was observed directly on a model alloy that contained 2.5 wt pct aluminum in order to stabilize delta-ferrite down to room temperature. The gamma graingrowth kinetics was by etching the former austenite grain boundaries in a precipitate-free variant of APIX60 steel. At high temperatures and in the absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to follow a simple parabolic growth law. The findings are applied to the problem of grain-size control during the process of thin-slab casting direct rolling (TSCDR). © The Minerals, Metals & Materials Society and ASM International 2010.

Hibbeler L.C.,University of Illinois at Urbana - Champaign | Hwang I.,University of Illinois at Urbana - Champaign | O'Malley R.J.,Nucor Steel Decatur LLC | Thomas B.G.,University of Illinois at Urbana - Champaign
Iron and Steel Technology | Year: 2013

A new methodology is presented to calibrate the 1D CON1D model with a full 3D finite- element model of the mold. Coupled with models of solidification and interfacial phenomena, this modeling tool is applied to gain insights into many aspects of heat transfer in the process. The CON1D model is based on a 1D finite-difference solidification model of the shell, taking advantage of the large Péclet number that makes axial heat conduction negligible relative to heat transported by the moving steel. It includes conduction and radiation across the interfacial layers, aided by mass, momentum and force balances on the slag, which are all solved analytically. Simplifying the mold geometry for the 1D model requires careful definition of the dimensions in order to retain the thermal characteristics of the system. This calibration of the thermocouple location should be performed after the water channel geometry has been calibrated.

Kapoor M.,University of Alabama | Kapoor M.,U.S. National Energy Technology Laboratory | O'Malley R.,Nucor Steel Decatur LLC | Thompson G.B.,University of Alabama
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2016

Composition analysis of carbide and carbo-nitride precipitates was performed for two Nb-Ti microalloyed steels with yield strengths of 750 and 580 MPa using an atom probe study. In the high-Ti 750 MPa steel, Ti-rich (Ti,Nb)(C,N) and Ti-rich (Ti,Nb)(C) precipitates were observed. In the high-Nb 580 MPa steel, a Ti-rich (Ti,Nb)(C,N) precipitate and (Ti,Nb)(C) clusters were noted. These (Ti,Nb)(C) clusters in the high-Nb 580 MPa steel were smaller than the (Ti,Nb)(C) precipitates in high-Ti 750 MPa steel. In general, a larger number of precipitates were found in the high-Ti 750 MPa steel. This difference in the number density of the precipitates between the two steels is attributed to the difference in Ti content. Combining the atom probe tomography results and thermodynamic calculations, the precipitation sequence in these alloys was inferred to be the following: as the temperature decreases, TiN precipitates out of the solution with successive (Ti,Nb)(C,N) layers of varying composition forming on these Ti-rich precipitates. Once N is depleted from the solution, a second set of (Ti,Nb)(C) precipitates in a similar manner in the matrix and also onto the carbo-nitride phase. This observation is consistent with previous observations in high-strength low-alloy steels containing comparable amounts of only Nb. It was noted that the amount of Nb, Nb/(Nb + Ti), in the precipitates decreased from 0.20 to 0.04 with the size of the precipitate. We believe that this is due to the Nb supersaturation in the matrix when these precipitates nucleate. © 2016 The Minerals, Metals & Materials Society and ASM International

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