Economically alloyed steel for the needs of the mining industry

Abstract

It has been established that the downtime of ball mills due to the low durability of cast parts is 70...80 % of the total number of downtime, while material damage is estimated in hundreds of thousands of hryvnia. One of the weakest units of these mills are the discharge grids and scoops of the cochlear feeder, the durability of which reaches 5.5...6 months and 1.0...1.5 months, respectively, with the resource of operation of other parts of the mill 9...12 months. As a result of the analysis of the operating conditions of the gratings and scoops of the MSHR, it was found that the duration of operation and the mechanical properties of these parts depend mainly on the material from which they are made. Steel with a high level of plasticity and toughness (35GFL, 35HNL, 34HNML, etc.) had insufficient wear resistance due to low strength, hardness. Materials with a high level of hardness and strength (60Kh2SML) had a low impact toughness, which was the reason for emergency shutdowns of the mills as a result of breakdowns of parts. The analysis of the mechanical properties of ladles and gratings has established the required level of properties of these parts; σВ ≥ 850 MPa; НВ ≥ 2600 МPa; δ ≥ 8 %; Ψ ≥ 20 %, KCU ≥ 0,20 MJ/m2. The effect of changing the silicon concentration on the mechanical properties and wear resistance of steel containing 1.5% chromium and 0.40% manganese with different carbon contents has been studied. With an increase in the silicon content, the strength and hardness indicators increase. The effect of silicon on the plastic properties of steel, impact strength and wear resistance is nonlinear, with 0.9% manganese in this case, a monotonic decrease is observed not only in plastic properties and toughness, but also in wear resistance. This is explained by the fact that with an increase in the manganese content in steel, the segregation heterogeneity and stability of supercooled austenite increase, leading, after normalization, to the formation of a martensite-like structure. It is shown that in the production of castings operating under conditions of abrasive wear in combination with shock loads, it is advisable to observe the ratio of silicon to carbon 2.0...2.5. Mathematical planning of the experiment made it possible to obtain models that adequately describe the effect of the composition of steel on the level of its mechanical and operational properties. Based on the processing of the data obtained from the «STATISTICA» software package, a wear-resistant economically alloyed steel of the following composition was proposed: 0.45...0.50 % carbon; 1.7...2.0 % chromium; 0.5...0.7 % manganese; 1.2...1.5 % silicon. The use of such a composition of steel for the manufacture of unloading gratings and scoops of snail feeder allowed to increase their service life by 1.5 times by improving performance, and, consequently, reduce the maintenance cycle of ball mills, which increased economic efficiency. Keywords: ball grinding mill, alloy steel, mathematic modeling, structure, mechanical properties.