齿轮钢铸态点状偏析及其在热轧棒材中的演变

As-Cast Spot Segregation of Gear Steel and Its Evolution in the Rolled Products

对齿轮钢铸坯中的点状偏析和轧材中的带状偏析缺陷进行了实验研究以探究其相关性。利用低倍侵蚀、枝晶侵蚀、电子探针和特征参数统计计算等方法,研究了点状偏析和带状缺陷的形貌特征、位置分布及元素分布规律,分析了2者之间的演变规律和差异,并讨论了带状缺陷的成因及其热处理消除的可能性。实验结果表明,起源于凝固过程的点状偏析主要分布在铸坯中心等轴晶区域,偏析点内合金元素表现为显著的正偏析。带状缺陷表现为单条型带和聚合型带,轧材中心区域的带状缺陷在位置及元素分布规律上都与铸态点状偏析具有对应性。统计与扩散计算表明,枝晶偏析和相互独立的点状偏析在轧制过程中被压延形成单条型的带状缺陷;一种相互连接的复杂形貌点状偏析经轧制塑性变形后将形成聚合型带,从而导致轧材带状缺陷的条数多于点状偏析的数量。对于带宽约40μm以下的带状缺陷,1200℃保温热处理工艺可以减轻其中的偏析程度;但对于约40μm以上的粗大带状缺陷,后续热处理改善效果有限,必须从连铸源头铸态组织控制着手。

As a crucial transmission component and basic component in mechanical equipment,gears have extremely high requirements for homogeneity of elements.However,the banded segregation defects in a rolled bar will reduce the hardenability of the finished gear and cause uneven deformation in heat treatment.Generally,the formation of banded defects is believed to inseparable from semimacro segregation in the bloom castings.Spot segregation in gear steel bloom and banded segregation defects in a rolled bar were experimentally studied to explore its evolution.The correlation between the spot segregation and banded defects was clarified from the aspects of morphology characteristics,position distribution,element segregation,and spacing statistics using hot acid etching,dendritic etching,electron probe microanalysis(EPMA),and statistical calculations.The evolution law of spot segregation and the possibility of elimination of banded defects in the heat treatment stage were also discussed.Experimental results show that spot segregation mainly exists in the central equiaxed crystal area of the castings,and solute elements such as C,Mn,and Cr in the segregated spot show positive segregation.The wide banded defects are consistent with the spot segregation in their geometrical location and element distribution pattern.Morphologically,there are two types of banded defects in which the dendrite microsegregation and the independently existing spot segregation are changed into separate band structures during the hot rolling process,while the interconnected spot segregations convert into a type of converged band defect through the hot deformation.Thus,more fine and coarse banded defects have been observed in the final rolled bar products than the numbers of as-cast spot segregation.Heat treatment through heating and insulation at 1200℃ can reduce the degree of solute segregation in the banded defects with a width less than 40μm,but it shows little beneficial effect on the more coarse-sized banded defects,which suggests that they should be controlled from the original as-cast dendrite morphology during the casting process.