Continuous cooling transformation temperatures determined by compression tests in low carbon bainitic grades
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I.
INTRODUCTION
IN modern low carbon and ultralow carbon bainitic steels, boron is an important microalloying element that is added to improve the hardenability.[1–8] By optimizing the boron concentration and processing conditions, this element can be made to segregate on the grain boundaries in the austenite range, and the g-to-a transformation is then suppressed, allowing low carbon bainite to form. Boron can also form coarse precipitates, which promote the formation of polygonal ferrite by acting as nucleation sites. Thus, the form in which boron is present is an important factor affecting the transformation behavior and the resulting microstructure. In more recent developments, strong carbide and nitride formers, such as Nb and Ti, are added to low carbon B steels to tie up the C and N, thereby making more boron available for segregation. The combined effects of Mo and B on the transformation of austenite have been investigated in a series of alloyed low carbon steels.[9,10] The influence of boron on the kinetics of Nb(C,N) precipitation has also been investigated in ultralow carbon bainitic steels.[11,12] It was found that the presence of boron accelerates Nb(C,N) precipitation; recrystallization is therefore retarded more strongly in Nb-B grades than in steels containing only Nb. This was further demonstrated by Bai et al.[13] recently in an investigation of the no-recrystallization temperature (Tnr) in a series of NbB treated steels. Although the effects of various processing parameters on the transformation temperature in a series of D.Q. BAI, Postdoctoral Fellow, S. YUE, Associate Professor, and J.J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. T.M. MACCAGNO, formerly with the Department of Metallurgical Engineering, McGill University, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted August 14, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
ultralow carbon Nb-B steels (boron contents up to 16 ppm) have been studied by Tamehiro et al. by means of metallography,[14,15] that of boron in solution, especially when Nb is present, is not very clear. In the present study, the continuous cooling compression (CCC) technique[16] was employed to investigate the influence of microalloying elements (Nb, Mo, and B) and processing parameters (reheat temperature, prestrain, and holding time) on the transformation temperatures. A detailed study of the associated microstructures is presented in a separate article.[17] II.
EXPERIMENTAL MATERIALS AND PROCEDURE
A. Experimental Materials Four steels containing binary combinations of Nb and B or Mo and B were investigated so as to determine the influence of Nb and Mo addition on the transformation temperature. A carbon steel was included in the study as a baseline for comparison. A ternary Mo-Nb-B steel and a single B-modified grade were also included: the former to examine the combined effects of Nb and Mo, and
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