Improving the Performance of Cast Iron Rolls by Modifying Melts with Rare Earth Metals

Problem Statement and Current Status

Increasing the output and improving the production of ductile iron castings is one of the most important areas of the comprehensive long-term scientific and technological progress program for foundries in Ukraine. Significant potential for improving casting quality lies in the ability to actively and purposefully shape their structure and properties by treating melts in a liquid state with carefully selected additives.

A review of domestic and international scientific, technical, and patent literature revealed that a universal modifier and modification method suitable for the production of cast mill rolls has not yet been developed. The crystallization and structure formation of modified cast irons remains controversial and requires further research to refine modifier compositions and modification technologies for specific industrial conditions. For over sixty years, the NMAU (National Metallurgical Academy of Ukraine, formerly DMetI – Dnipropetrovsk Metallurgical Institute) has been conducting research on the influence of individual rare earth metals and complex modifiers based on them on the structure and properties of sheet rolling rolls [1-3]. However, the characteristics and patterns of structure formation in the working layer and core of chilled sheet rolling rolls under the influence of complex modifiers have not been adequately covered in the technical literature.

Research Problem Statement

To produce rolling rolls with improved performance properties, further research is needed into the structure formation processes of white cast iron modified with complex modifiers based on rare earth metals.

Key Research Results

The effect of cerium, yttrium, lanthanum, samarium, gadolinium, dysprosium, and complex modifiers based on these elements on the structure of white cast iron was studied. Metallographic analysis was performed using a Neophot-21 optical microscope at magnifications of 100-500. Sections were examined before and after nital etching.

The results of comparative studies of the structure of rolls made of cast iron modified with magnesium and a complex modifier based on rare earth metals showed that, despite similar chemical compositions, the working layer structure of the experimental rolls was free of graphite, while the amount of graphite in the rolls made of magnesium-modified cast iron was 0.5-2.5% at depths of 10 and 20 mm from the working surface. Furthermore, the structure of the metal matrix of complex-modified cast irons was characterized by a greater dispersion of its components.

The modifying effect of increasing amounts of modifiers in white cast irons is manifested in the suppression of honeycomb ledeburite crystallization, an increase in the microhardness of the matrix and cementite, and the transformation of some austenite into martensite. Residual modifier contents optimal for modifying roller cast irons were determined. The effectiveness of using gadolinium and dysprosium as components of complex modifiers for modifying roller cast irons was demonstrated, and the composition of the complex modifier was optimized.

Modification of cast iron melts with a complex modifier allowed for a 2.5-6-fold reduction, depending on the cooling rate, in the number of non-metallic inclusions in the working layer of the roll, while simultaneously reducing their average size. Comparative tests of the mechanical properties of cast iron rolls modified with a complex modifier and those modified with magnesium showed that the strength properties of the material at a depth of 10-160 mm along the roll barrel were 15-20% higher for cast iron modified with a complex modifier than for magnesium-modified cast iron rolls.

The effect of cooling rates in the range of 0.04-2.5% of 950°C on the structure formation processes of complex-modified white roller cast iron was also studied. Over the entire cooling rate range studied, the cast iron austenite underwent a eutectoid transformation, resulting in the formation of pearlite of varying particle size. Studies of the effect of cooling rate on cast iron properties (hardness, elastic modulus, and wear resistance) revealed a 5-23% improvement in properties with increasing cooling rate in the critical temperature range. Heat treatment regimes for mill rolls made of complex-modified cast irons were developed.

A pilot batch of rolls modified with a complex modifier was cast under industrial conditions. A 20-35% increase in service life was achieved in the rolls compared to mass-produced rolls cast from spheroidal graphite iron.

Conclusions

1. A technology for treating roll melts with complex modifiers based on rare earth metals has been developed.
2. By changing the microstructure and mechanical properties of the cast irons, the melt treatment improved the service properties of the rolling rolls.

References:

1. Belay, G.E. "A Study of the Effect of Modification on Cast Iron Crystallization and the Structure and Properties of Sheet Rolls": Abstract of Cand. Sci. (Eng.): Specialist. 05.16.04 "Foundry Production" / G.E. Belay. - Dnepropetrovsk, 1967. 23 p.
2. Kolotilo, E.V. Research and Improvement of Production Processes for Sheet Rolls from Modified Cast Irons: Abstract of Cand. Sci. (Eng.): Specialist 05.16.04 "Foundry Production" / E.V. Kolotilo. - Kyiv, 1977.-21 p.
3. Ivanova L.Kh. Theoretical Foundations and Practical Methods for Maintaining Cast Rolls from Complexly Modified Cast Irons: Abstract of Doctor of Engineering (Eng.): Specialist 05.16.04 "Foundry Production" / L.Kh. Ivanova. - Dnipropetrovsk, 2008. - 35 p.

UDC 621.74.002.6:669.131.7
Ivanova L.Kh. /Doctor of Engineering Sciences/,
Kolotilo E.V. /PhD/,
Maimur Ya.S.,
Mukha D.V.
NMetAU

Received October 6, 2011