Study On The Microstructure And Property Of NM550 Grade High Strength Low Alloy Abrasion-Resistant Steel

High strength low alloy abrasion-resistant steel has been widely used in the fields of abrasion-resistant materials due to its high hardness and good toughness.With the development of technology, the working conditions of mechanical parts are increasingly hashed. The poor working conditions of construction equipment and mining machinery requires it not only has a high abrasion resistance, but also be able to withstand strong shock and high stress, so as to to extend the service life of the equipment. At present, the main grades of high strength low alloy abrasion-resistant steel plates are NM400 and NM360 in China, which cannot meet the requirements, so the research of higher grade like NM550 has great significance. In this article, based on low-cost and high-performance principle, high strength low alloy abrasion-resistant steel grade NM550 with high hardness, high strength and good toughness has successfully developed by adding a certain amount of Cr, B, Mo and other alloying elements in the ordinary C-Mn steel, and applying reasonable hot rolling process and off-line heat treatment process. The major studies are as follows:(1) The continuous cooling transformation behavior of the experimental steel was investigated. Results show that the dynamic CCT (continuous cooling transformation) curve of two experimental steels can be roughly divided into three parts:high temperature transition zone, the transition microstructures are ferrite and granular bainite; mid-temperature transition zone, the main transition microstructure is bainite; low temperature transition zone, the main transition microstructure is martensite. And both two kinds of the experimental steels have a lower critical cooling rate, also a better ability to obtain martensitic structure. When the cooling rate is above 10?/s, a full martensite structure can be got, and the vickers hardness is more than 550HV.(2) The 12mm and 35mm thick steel plates had been got by applying reasonable hot rolling process and off-line heat treatment process. When the quenching temperature at 880? to 900? range, and the tempering temperature at 170? to 190? range, two experimental steels have a good mechanical properties. The surface Brinell hardness is at 540HBW to 560HBW range, meeting the requirements of national standards in GB/T24186-2009 for NM550 grade abrasion-resistant steel, and two experimental steels also have high strength, good toughness and elongation.(3) Effects of quenching temperature and tempering temperature on the microstructure and mechanical properties of experimental steels were studied. The morphology and sub-structures of martensite, obtained under different quenching temperature, were analyzed and microstructure changes under different tempering temperatures were observed, like redistribution of carbon atoms, precipitation of carbides, the transition of carbide type.When quenching temperature increased from 880? to 1000?, the mechanical properties of experimental steels decreases. Micrographs show that austenite grain size increased. And with the austenite grain size increased, packet size and block width increased, but the lath width did not show a significant change.When tempering temperature increased from 150? to 230?, Brinell hardness of the experiment steels decreased. And impact energy of-40? shows a slightly increase when tempering temperature increased from to 190?, while tempering temperature increased above 210?, it decreased. Tempering at 170? to 190? resulted in a good combination of hardness and impact toughness due to the lath structure with a high dislocation density, the interstitial solid solution of the C atoms, and the tiny ?-carbide in matrix, which showed in TEM micrographs.(4) MLD-10 abrasion test machine was used to study the wear behavior of tested steel and other grade abrasion-resistant steels. The NM550 grade high strength low alloy abrasion-resistant steel has a minimal abrasive weight loss, means it has an excellent abrasion resistance. Wear mechanisms of experimental steels include micro cutting and plastic deformation, and the latter plays a major role.