Nb(C, N) Precipitation and Austenite Recrystallization
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I.
INTRODUCTION
U L T R A L O W carbon bainitic (ULCB) high-strength low-alloy (HSLA) steels are characterized by the combined addition of boron and transition metals, such as Nb and Ti. tl-4j The presence of the latter elements inhibits grain growth, retards recovery and recrystallization, and also lowers the austenite-to-ferrite transformation temperature. 15-~u These effects were attributed, in the lower temperature ranges of steel processing, to their precipitation as carbonitrides on the dislocation substructure. 112-~SJ As a consequence, the precipitation kinetics of Nb and Ti carbonitrides have been studied extensively and precipitation-time-temperature (PTT) diagrams determined for various steel compositions and experimental conditions. 116-221 While the single addition of boron to a C-Mn steel can substantially improve the hardenability, t23-261 boron alone does not retard the recrystallization of austenite, t27.281By contrast, the combined addition of Nb and boron leads to large retardations, indicating the existence of a synergism between Nb and boron atoms, t29,3~ The refining effect of boron addition on the size distribution of the carbonitrides in austenitic stainless steels has been reported by Thomas and Henry. t3~J Unfortunately, no quantitative data are yet available on the way that boron affects the kinetics of Nb(C, N) precipitation. For this reason, a research program was undertaken M. DJAHAZI, Assistant Professor, is with Tarbiat Modarres University, Tehran, Islamic Republic of Iran. X.L. HE, Associate Professor, is with the Department of Metal Physics, Beijing University of Iron and Steel Technology, Beijing, People's Republic of China. J.J. JONAS, CSIRA-NSERC Professor of Steel Processing, and W.P. SUN, CSIRA Research Associate, are with the Department of Metallurgical Engineering, McGill University, Montreal H3A 2A7, PQ, Canada. Manuscript submitted April 16, 1990. METALLURGICAL TRANSACTIONS A
with the following main objectives: (1) to determine how the presence of boron affects both the precipitation kinetics of Nb(C, N) and the recrystallization of austenite; (2) to study the interaction between recrystallization and precipitation in boron-modified steels; and (3) to study the influence of prior austenite grain size, amount of deformation, and holding time on the softening and strengthening processes. The availability of such information will improve the understanding of the role played by boron in high-temperature austenite. It will also lead to the better control of industrial processes involving the simultaneous deformation and cooling of such steels.
II. EXPERIMENTAL MATERIALS AND METHODS
A. Experimental Materials The experimental materials were prepared at the Metals Technology Laboratories of the Department of Energy, Mines and Resources, CANMET, Ottawa, Canada. Their chemical compositions are shown in Table I. The low carbon level (0.026 wt pct) was chosen so as to be representative of the ULCB steels. Titanium and AI were added to tie up the nitrogen and to prevent the format
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