ABSTRACT

As-cast inherited defects are usually observed especially in quenched and tempered (QT) steels, such as 42CrMo due to macro-segregation. A three-dimensional numerical model has been presented to reveal the heat mass transportation and solidification behaviour of the molten steel in a 250 mm × 280 mm continuously cast bloom with and without M-EMS application. The reliability of the coupled model was proved by comparing the measured data of magnetic flux density with the calculated one along the centreline. The research indicates that the phenomena of a decrease in the solidification rate and an increase in molten steel washing velocity are observed as the current intensity increases from 0 to 450 A, which leads to a decrease of the minimum segregation degree by 0.039 and an increase of the width of the harmful negative segregation band by 3.6 mm. The washing effect that increases with the current intensity can reinforce the heat transfer between molten steel and solidification front and stimulate the formation of the equiaxed crystal. Accordingly, the average, local temperature gradient, and the superheat degree at the centre of mould exit decrease by 0.12, 2.62 K·mm⁻¹, and 2.9 K, respectively, together with an increase of the equiaxed crystal ratio by about 9.44%. It suggests that the M-EMS may lead to the more serious subsurface negative segregation but can improve the equiaxed crystal ratio of bloom castings, which influences the heat treatment performance and flaw detection pass rate of subsequent rolled products.

摘要

通常观察到铸态遗传缺陷，特别是在淬火和回火（QT）钢中，由于宏观偏析，例如42CrMo。提出了一个三维数值模型，以揭示在使用和不使用M-EMS的情况下，在250 mm×280 mm连铸坯中钢水的热质传递和凝固行为。通过将磁通密度的实测数据与沿中心线计算出的数据进行比较，证明了耦合模型的可靠性。研究表明，当电流强度从0增加到450 A时，观察到凝固速率降低和钢水洗涤速度提高的现象，这导致最小偏析度降低0.039并增加最小偏析度。有害的负隔离带的宽度增加了3.6毫米。随着电流强度的增加，清洗效果会增强钢水和凝固前沿之间的传热，并促进等轴晶的形成。因此，平均，局部温度梯度和铸型出口中心的过热度降低了0.12、2.62 K·mm^-1和2.9 K，以及等轴晶率增加了约9.44％。这表明，M-EMS可能导致更严重的表面负偏析，但可以改善大方坯铸件的等轴晶比，从而影响后续轧制产品的热处理性能和探伤合格率。