Study On High Strength And High Elongation Ductile Iron (QT700-10)

This project is the development of high strength and high elongation ductile iron (QT700-10), and involves some relevant fundamental researches. The main purpose is to achieve QT700-10 in as-cast condition steadily. The fundamental researches include the mechanisms of spheroidizing and inoculation, the discussion of their internal relation, and the analysis of the morphology characteristic of the original austenite.Using the technology solution of high silicon ferritic ductile iron for reference, compared with the traditional chemical composition, this paper increased the ratio of silicon and carbon and acquired the refined duplex matrix which is composed by the solid solution strengthened ferrite and the refinement pearlite. Through composition optimization and process improvement, this new brand of ductile iron was finally realized. Firstly, through microstructure analysis, the ductile iron which can meet the requirements of high strength and high elongation should have the following characteristics:small and round graphite nodules, coherent austenite matrix, and sufficient strengthened ferrite with suitable content. Subsequently, the possible solutions were designed and chose. The synthetic way through the recarburization of scrap steel to produce ductile iron and the increased ratio of Si and C should be beneficial to obtain strong austenite skeleton. The processes of melting, spheroidizing and inoculation needed to be optimized and stabilized to ensure good graphite characteristics. Earning small and round graphite nodules, destructive effects of segregation could also be weakened since the eutectic cells were refined. The difficulty of the matrix microstructure control was to obtain enough amounts of ferrite and ensure the ferrite adequate reinforced at the same time. Si, Mn and Cu were selected as the main alloying elements because they all had the ability to strengthen the ferrite. Si played a major role in the solid solution strengthening of the ferrite, Cu and Mn also had the function of refining the pearlite. Adjusting the contents of those elements, the suitable volume proportion of ferrite could be earned. Through above work, the specific solutions that could achieve QT700-10 were acquired. Thereafter, those solutions should be repeated and their stability and reproducibility were checked.The results showed that under the melting process of which the absorption rate of recarburizer was 75%, using 1.7wt% of nodulizer,1.0wt% of inoculant and 0.2wt% of strontium-bearing stream inoculant, good spheroidizing and inoculation effect could be ensured. Settling the contents of main elements in C 3.2wt%, Si 3.2wt%, Mn 0.8wt%, Cu 0.4-0.5wt% could achieve QT700-10 in as-cast condition. This technical scheme had a good reproducibility. The appropriate microstructure was a matrix of about 40% of solid solution strengthening ferrite plus fine pearlite with small and round graphite nodules dispersed uniformly in it and the nodularity remained at around 90%.It was found that the spheroidizing process intensified the fading of inoculation. The more amounts of nodulizer was used, the more sever and easier the recession would be. It was difficult to obtain spheroidal or spherical graphite nodules and eliminate the degeneration of graphite by just changing the amounts of nodulizer. Stream inoculation was necessary to acquire better graphite characteristic and improve the ability of anti-degeneration significantly. The strontium-bearing inoculant had a better effect than common ferrosilicon on the inoculation and the reason should be that the activity of Sr was stronger than Mg and therefore the hexagonal silicate phases, such as SrOSiO2, could be formed even though the dosage of nodulizer was not just appropriate.This paper argued that spheroidizing and inoculation were inseparable. They regulated the content and the existence form of O in the melt together, and then controlled the graphite characteristic and promoted the formation of spheroidal graphite. The main function of spheroidizing elements was purifying the melt, however excessive deoxygenation should be avoided. The inoculation elements enhanced the roundness of the graphite nodules mainly by promoting the nucleation of graphite and refining those nodules. They also had the ability of deoxygenation, but they were auxiliary. Exploded and vermicular graphite could be eliminated through appropriate spheroidizing process, but for the formation of nodular or spheroidal graphite, well inoculation was absolutely necessary. The melting process influenced the O content of the melt and then influenced the spheroidizing and inoculation.In addition, a method for remaining and revealing the characteristic of original austenite grain was presented in this paper. The results suggested the presence of the large scale austenite grain or dendrite.