Investigation Of High Strength Low Alloy Cr-B Series NM400Wear Resistant Steel With Micro-Nb Addition

Low-alloyed wear resistant steel has characters of simple production technique and excellent mechanical properties. Consequently, it is widely applied in engineering machinery and gradually replaces traditional wear resistant materials and becomes the main type of steel applied. At present, conventional reheat quenching is still main technique adopted by domestic manufactures which can only steadily produce wear resistant steels below the level of NM400. What’s more, domestic wear resistant steels have defects of high alloy content and poor toughness. NM400level and above wear resistant steels of high quality still rely on import.This thesis studied the continuous cooling phase transformation behavior of tested steel and prepared steel plates of superior properties with two different processes. One is the process of austenite recrystallization zone controlled rolling, direct quenching and tempering. And, the other is austenite recrystallization zone controlled rolling, reheat quenching and tempering. The influence of processes on microstructure and mechanical properties was analyzed. Besides, the abrasion test was conducted and the wear mechanism of tested steel was analyzed. The main research results carried out in this thesis are as follows:(1)The hardenability of tested steel is excellent. The microstructure is polygonal ferrite, bainite and little pearlite with cooling rate0.5℃/s. When cooling rate increases to5℃/s, martensite transformation can be observed and the microstructure is complex constituents of martensite and bainite. With cooling rate20℃/s, the microstructure is fully martensite and the hardness is above388HB.(2)After quenching and low temperature tempering, main microstructure of tested steels is tempered martensite of which the width of martensite lath is about0.2μm. Fine carbides and precipitate containing Nb dispersively distribute in the laths. The tesile strength of tested steel is between1277MPa and1465MPa, the elongation is between11.0%and13.5%, the impact energy at-20℃is between25J and69J and the hardness is between421HB between467HB. All the mechanical properties meet requirements of national standard constituted for NM400.(3)The mechanical properties of steels which were quenched directly are superior compared with those of steels with conventional reheat quenching. The analysis shows that there are mainly three reasons as follows. Firstly, direct quenching process reduces the martensite packet size. Therefore, the proportion of high grain boundary is increased. Secondly, a small amount of lower bainite appears along prior austenite grain boundaries. This complex constituent of martensite and lower bainite improves the strength and toughness. At last, direct quenching process preserves lots of defects in deformed austenite which can provide precipitation sites for carbides in tempering process. This improves mechanical properties of tested steel because the carbides are finer and distribute dispersively.(4)Tested steel prepared in this paper has superior abrasive resistance. The relative wear resistances of tested steels were all above2.5with Q345steel as the standard steel. The relative wear resistance of tested steel after950℃direct quenching and300℃tempering is3.33and its wear resistance is the best. The study of wear mechanism shows that the wear is mainly caused by multiple plastic deformation mechanism and wear created by micro-cutting mechanism is little.