| This paper studied the possible kinetics and mechanism of formation of large-dimension RE inclusions, discussed the effects of refinement, purification, microalloying and modification of rare-earth elements, taking 5CrNiMo steel with addition of RE and other microalloying elements as example. The microstructure and inclusions of the samples under different heat-treatment conditions were observed by metallographic microscope, SEM (scanning electronic microscope) and EPMA (electron probe microanalyzer), the composition and conventional mechanical properties were systematically examined and analyzed. The results showed that:Large-dimension RE inclusions were formed inevitably with the additions of RE elements. The dimension of RE inclusions formed in steels exceeded the value specified in current standard and restricted the comprehensive application. Traditional documents showed that: With RE additions,impurities gathered to form big-dimension RE inclusions was promoted by the high temperature, the fluctuation of energy and structure and high activity. And the formation of large-dimension inclusions may relate to the contents of O, S impurities. This studing peoved that by EPMA (electron probe microanalyzer)and transmission electron microscope(TEM): RE inclusions nucleated on oxides and sulphides with high melting point and grew up independently, which grew up a 0.3-0.5μm in length, In view of high adsorbability for RE inclusions, they could adsorb each other and merge into a large-dimension with a 1-5 μm in length. As the RE inclusions were formatted, they had different dimensions on the order of magaitude, and the shape was spherical or approximately spherical or elliptic, but there were no sharp pointed end on them. The study shows that the RE element promoted the RE inclusion shape variation through modification.Compared with 5CrNiMo steel without RE additions under heat treatment of anneal(790℃×5h), quench(850℃×20min) and temper after quench(520℃×3h) the...
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