Development Of Low Alloy Martensite Abrasion Resistant Steel With High Strength And Toughness

Based on the heavy plate production line with high level modernization process equipments, a new abrasion resistant steel product of NM500 that contains mainly low alloy chemical composition of C-Mn-Cr-Mo-Ni was designed and developed in this paper.The problems on chemical composition design, continuous cooling transform rules, controlled rolling and controlled cooling process, and heat treatment etc. were analyzed and researched carefully.The low cost manufacturing technique for making low alloy wear resistant steel with high strength and toughness was proposed in the paper after discussing the effects of the different original microstructure on re-heat austenite microstructure and quenching performance. The main study contents and results are as follows:(1) The design ideas on low cost composition of NM500 were:the 0.25%C, 0.56%Mn and 0.67%Cr were designed to ensure the fundamental strength and hardness of lath martensite; the 0.23%Mo and 0.21%Ni were designed to improve toughness of the microstructure; the 0.025%Ti element combined with Mo was added to strengthen the effects of controlled rolling and controlled cooling;and the appropriate amount of 0.0014%B was added to improve hardness of the steel. In order to obtain good welding performance while maintaining fine metal properties, the lower carbon equivalent (CEQ? 0.55%) and weld cold cracking sensitivity (Pcm? 0.37%) should be considered.(2) The transformation rules of the test steel's microstructure state and hardness varied with different cooling rate were studied under the condition of continuous cooling. When cooling speed was between 5?/s?10?/s, phase transition temperature range was between 651??336?, the mixed microstructure of lower bainite and martensite with 400-477HBW Brinell hardness was got. When cooling speed was above 12.5?/s, phase transition temperature range was between 494??194?, the whole single martensite microstructure with 490-538HBW Brinell hardness can be got.(3) In the light of different rolling and cooling process, the effects of different original microstructure on the re-heat austenite microstructure and quenching properties were studied. Experiment results showed that, low-temperature controlled rolling plus online quenching process can obtain fine martensite microstructure which has high dislocation density and Nano-precipitates. Compared with pearlite microstructure gained using traditional rolling process in higher temperature, the martensite microstructure can keep the grain finer for the re-heat austenite. It can make the average grain size reduced from 14 ?m to 6.5 ?m at 900? heating temperature.Due to the refinement effect to the re-heat austenite grain size, the number of large-angle grain boundaries tested by the EBSD was increased obviously after quenching, and average effective grain size (EGS) was changed from 5.6?m down to 2.1 ?m. Using low temperature controlled rolling+online quenching technique, the toughness of steel can be improved by increased number of large-angle grain boundaries. And the impact energy in transition temperature region was increased, and ductile-brittle transition temperature range expanded about 40?.Within the temperature range of 0??-60?, the impact energy was increased about 36J, and the slope of the curve of the impact energy decreased with temperature decreasing was declined markedly. With decreasing of the size of lath block and lath packet, the interface were increased and the martensite strength was improved significantly.Experiment results showed that, compared with conventional hot-rolled process, the NM500 steel quenched directly after low temperature controlled rolling process has distinct improvement in strength, toughness, hardness and bending properties. The steel has nice synthetic mechanical properties without tempering, so this process innovation has positive significance and great value in actual industrial production.(4) The effects of gas shielded metal arc welding (GMAW) on toughness of Coarse-grained heat affected zone (CGHAZ) were studied, and the type y-groove weld cracking test, the tests on change of hardness and toughness of welded joints were carried out.The welding thermal simulation experiments of NM500 steel showed that when T8/5 is in 5-15 seconds with line energy of 10-25KJ/cm, the toughness of CGHAZ is ideal. When T8/5 is more than the 20 seconds and line energy is greater than 35KJ/cm, the toughness of CGHAZ is in deterioration.Oblique y-groove weld cracking test showed that using ER80-G flux-cored wire with 100? temperature preheated, section crack rates and surface crack rates are all zero; using ER50-G wire with 80? temperature preheated, section and surface crack rates are all zero. Therefore the base metal has good cold crack resistance capacity.Using ER80-G wire with 80%Ar+20%CO2 protection, and using ER50-G wire with 100% CO2 protection, the impact energy of welded joints and HAZ coarse grain zone were all close to that of base metal. It indicated that the two welded joints have good toughness for the two welding process suggested above. Metallographic examination showed that the welded joints have no crack and any abnormal microstructure.