Research And Application Of Medium Carbon Low-alloyed Wear Resistant Steels

In order to improve production efficiency and decrease manufacturing cost, large-scale equipment and components for the iron ore powder production line are developed. As key components of the grinding machine, the service life of the wear resistant liners directly determine the efficiency and the cost for the production of iron ore powder. However, the material specification for ultra-large grinding machine has not been systematically developed and most of the key equipment for iron ore powder production line are mainly dependent on importation. As a result, the construction cost and schedule of iron ore powder production line are significantly controlled by others. In order to meet the service life requirement of large wear resistant parts, these steels should be with high hardness and strength, excellent toughness and ductility, and good hardenability. Therefore, how to obtain a good combination with high strength and high toughness, and further improve the wear resistance of wear-resistant steel, has been a topic of great concern to researchers. In the present work, a series of novel medium carbon low-alloyed wear resistant steels were designed by microalloying Ti, B and RE. The effects of Ti, B and RE on the microstructure evolution were analyzed. The types, sizes and distributions of Ti-containing precipitates and RE inclusions during solidification and heat treatment and their effects on microstructure and mechanical properties were systematically investigated. Wear mechanism of these wear resistant steels and the relationship between Ti and RE contents, mechanical properties and wear resistance were discussed. Based on the research above, a series of wear resistant liner castings for the mills of iron ore powders were successfully produced, which have got precious experiences and provided referential cases for the domestically producing large wear resistant liner castings. The main research contents are summarized as follows:(1) Ti, B and RE microalloyed alloy design ideas were put forward on the basis of thermodynamic calculations and experimental investigations. The hardenability could be improved by Trace B, and Ti is add to combine with the available N for the desired B-hardenability effect. Meanwhile, the addition of RE (La, Ce) element could purify liquid steel, and improve the shape and distribution of inclusions, and refine microstructure together with Ti. Consequently, high strength, toughness and wear resistance could be obtained. Therefore, novel medium carbon low-alloyed wear resistant steels were designed.(2) The continuous cooling transformation (CCT) curves were determined by using thermal dilatometer, and the microstructure evolution of experimental steels was analyzed. The results showed that only B addition is mainly combined with N dissolved in austenite to form BN precipitate. And also, the RE addition forming inclusions of RE2O3 and RE2O2S in steel melting have little effect on hardenability. While, Ti addition effectively suppresses the formation of BN precipitates, which greatly delays the pearlite transformation and significantly enhances the hardenability of experimental steels. However, superfluous B content could promote the precipitation of M23(C, B)6 carbides in large size, which deteriorate the hardenability of experimental steels.(3) The precipitation behavior of TiN precipitates and RE inclusions in the solidification process and their effects on the solidificational microstructure are investigated by thermodynamic calculations, constant temperature solidification experiments within solid-liquid two phase region and continuous cooling solidification experiments. A large number of TiN precipitates and RE inclusions at the interfaces and frontiers of coarse dendrites and equiaxed grain boundaries in the residual liquid area, and the rest of TiN precipitates and RE inclusions are present within the coarse dendrite and equiaxed grain when holding at different temperatures of solid-liquid two phase region. The comparison of solidification macrostructures with low magnification showed that adding 0.090% Ti and 0.016% RE separately in experimental steels could significantly increase equiaxed grain ratio of solidification macrostructures, and refine the solidified structure.(4) The precipitation behavior of Ti-containing precipitates and RE inclusions in experimental steels after normalizing at 920 ℃ and tempering at 250 ℃ and the effects of Ti and RE contents on grain size and mechanical properties were investigated by OM, SEM, TEM, and EPMA. The results showed that the Ti-containing precipitates dependent on the Ti content were observed to be different types in various size ranges. When Ti content is 0.021%, the Ti-containing precipitates are considered to be TiN distributing in microscale and nanoscale sizes. When Ti content is 0.041% and 0.090%, two compound types of the Ti-containing precipitates are determined to be Ti(C, N) in microscale size and (Ti, Mo)(C, N) in nanoscale size, respectively. The size of the Ti-containing precipitates gradually becomes larger with the increase in Ti content, and the amount of those particles significantly rises up as the Ti content ranges from 0 to 0.090%. Meanwhile, the prior austenite grain size is obviously refined by these precipitates. The strength and toughness increase with the increase in Ti content and reach a peak value when Ti content is 0.021%. Subsequently, the strength decline gradually to a stable value, however, the toughness is dramatically degraded when Ti content is over 0.041%. In addition, most of RE inclusions formed in the smelting process are less than 1 μm. Only RE addition could improve the impact toughness. However, after addition of Ti and RE together, the RE inclusions promote to form the composite precipitates of TiN and RE inclusions, which increase the microscale size of coarse TiN particles and significantly reduce the impact toughness of the experimental steel.(5) The wear mechanism of these wear-resistant steels and the relationship of wear resistance with mechanical properties and Ti and RE elements were studied using MLD-10 type dynamic load abrasive wear testing machine. The results showed that under the condition of the present abrasive wear experiment, the wear mechanism of all experimental steels is mainly based on the fatigue and spalling caused by plastic deformation, and a small amount of micro-cutting wear. Therefore, the wear resistance of experimental steels mainly depends on their comprehensive mechanical properties. Adding Ti and RE separately in experimental steels could inhance the wear resistance by improving comprehensive mechanical properties. However, after addition of Ti and RE together, TiN-RE inclusions promote the initiation and propagation of cracks in the impact wear process, resulting the decrease in the wear resistance.(6) Overall manufacturing processes of component specification, mechanical property requirements, and hot working procedures for large wear resistant liner castings were developed on the basis of combining materials research and manufactory practice. A large number of wear-resistant liner castings made by the novel medium carbon low-alloyed steel were produced in the procedures from electric-arc furnace (EAF) melting-ladle furnace (LF) refining-vacuum oxygen decarbonizing (VOD) and controlling the order of Ti, B and RE additions. Finally,the service life of those liners was up to 75 days which was increased by 15.4% compared with that of the imported products. Up to date, so that the imported products have been almost completed replaced.