Effect of Boron on the Isothermal Bainite Transformation
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AS passenger safety issues are being increasingly emphasized in automotive body-in-white designs, higher strengths are required for steel sheet grades which are pressformed into anti-intrusion structural parts, such as B-pillars and cross members. The required strength levels, typically ranging from 1.0 to 1.5 GPa can be obtained by alloying the steel with suitable hardenability elements. The alloying levels, however, must be kept as low as possible to achieve lean chemical compositions which insure adequate weldability and low production costs. The highest strength levels are usually achieved with a fully martensitic microstructure as the presence of hightemperature transformation products generally reduces the strength. When industrial cooling rate ranges are applied, the high-temperature diffusional transformations, i.e., the pro-eutectoid ferrite and pearlite transformations, can easily be inhibited by the addition of trace amounts of B which has a pronounced nucleation site poisoning effect.[1,2] It was also reported that the strength of a mixed microstructure of upper bainite and martensite decreased with an increase in the volume fraction of upper bainite.[3] Hence, the transformation of part of the microstructure to upper bainite should also be carefully controlled to maximize the strength, in particular when martensitic grades are produced in a TAEJIN SONG, Researcher, is with the POSCO Technical Laboratories, Gwangyang, South Korea. BRUNO CHARLES DE COOMAN, Professor, is with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang, South Korea. Contact e-mail: [email protected] Manuscript submitted June 4, 2012. Article published online November 18, 2012 1686—VOLUME 44A, APRIL 2013
continuous galvanizing line (CGL) to produce an ultrahigh strength corrosion-resistant sheet steel. In a CGL, the strip temperature must be kept at a temperature slightly above the liquid Zn bath temperature. This temperature is typically within the temperature range for the transformation to upper bainite in many low-C, lean-alloyed steel grades suitable for automotive applications. A clear understanding of the transformation behavior and the resulting bainitic microstructure formed during CGL processing is required to achieve control of the mechanical properties of bainite-martensitic steel. In a recent study of the continuous cooling transformation (CCT) behavior of low-C, lean-alloyed steel, it was shown that the bainitic transformation could be suppressed by B-addition.[4] The mechanism by which B additions lead to a suppression of the bainitic transformation is, however, by no means unanimously agreed upon. Some studies concluded that B had a negligible influence on the bainite transformation,[5,6] but the opposite conclusion has also been reported.[7–9] These conflicting reports can be attributed to difference in experimental conditions, such as the austenitizing conditions, the C content of the steel, and the additional alloying with Mo or Nb.[1,7,9–12] It is well known
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