Continuous-cooling-precipitation kinetics of Nb(CN) in high-strength low-alloy steels
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I.
INTRODUCTION
THE retardation of austenite recrystallization by means of Nb additions to low carbon steels has been discussed by many investigators and has been attributed to (a) solute drag t~-61 and (b) the strain-induced precipitation of fine N b ( C N ) . 12"3"4"7-1~ The recrystallization retardation behavior changes drastically when Nb forms precipitates. Thus, precipitation-time-temperature ( P I T ) diagrams were introduced to show the relationship between the appearance of precipitates and time and temperature. 12.3::~j These diagrams have been widely used to verify that the abrupt retardation of recrystallization occurs after the start of precipitation. However, with the need for increasing precision, such approximations are frequently inadequate. It is therefore of interest to be able to characterize the continuous-cooling-precipitation (CCP) behavior so that the kinetics of precipitation under industrial forming conditions can be accurately predicted. Until now, few attempts have been made, either experimentally or theoretically, to understand and model such precipitation during continuous cooling, t~21 To calculate CCP diagrams, it is first necessary to be able to predict the P, and P: times during isothermal holding. Unfortunately, the two prediction methods currently available are limited to P, times. In 1987, Dutta and Sellars t~31 developed a thermodynamic model (the D-S model) to predict Ps times for niobium carbonitride, and in 1989, Liu and Jonas II41 developed another model (the L-J model) for titanium carbonitride. Since these models are limited to predicting the P, times, a new calS.H. PARK is Senior Re.archer, Steel Prr,ducts Department, RIST, PO Box 135, Pohang 790-600, Republic of Korea. S. YUE, Assistant Professor. and J.J. JONAS, CSIRA-NSERC Professor of Steel Processing. are with the Department of Metallurgical Engineering. McGill University, 3450 University Street, Montreal, PQ, Canada H3A 2A7. Manuscript submitted August 29, 1991. METALLURGICAL TRANSACTIONS A
culation method for the P/time is proposed. Using these models, the Ps and P: times during isothermal holding at any temperature can be calculated if certain thermodynamic factors and isothermal PTT data are available. The precipitation behavior during continuous cooling can then be simulated using the additivity rule proposed by Scheil f~Sj in 1935. This rule has often been employed to predict phase transformation behavior during continuous cooling from isothermal data. t~6-21~ In the present work, the precipitation of Nb(CN) during isothermal holding was studied by means of the stress relaxation method, t22"23'241 From these isothermal data, the P~ times were calculated using the L-J model and the P/times were predicted using the newly developed P/ model. The CCP behavior was then calculated and compared to the Ps and P/times obtained from an analysis of the evolution of particle size during continuous-cooling tests.
II.
MODELING
A. Methods of Predicting Ps Times during Isothermal Holding The L-J model can be rewritten f
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