| Recent years,the microalloying technology has been rapidly developed and widely used,and the requirements for the properties of low alloy steel are getting higher and higher.At present,improving the performance of the steel depends on not only the matrix refinement,but as far as possible the comprehensive use of various means of strengthening control microstructure to achieve transformation.Through the establishment of a rapid tempering process of second phase precipitation kinetics model,the article quantitativeiy calculated nucleation rate varied with temperature and precipitation of PTT curves of different precipitation temperature,got the fastest precipitation temperature,and provided a theoretical basis for establishing reasonable tempering process.The article used low alloy steel Q345 D iron based on the actual steel production,then used Gleeble-3800 thermal simulation machine for rapid heating to finish rapid tempering test of hardened steel which is compared with the traditional tempering process,further more,focused on the rapid tempering process of microstructure evolution and precipitation behavior of carbonitride,controlled the influence of the precipitates morphology on the low temperature toughness of materials,so as to verify the correctness of the precipitation model.The first test of Q345 D steel is solution treatment with 1200oC high temperature,quenching in cold saline water,then material is divided into two groups to conduct the traditional tempering and rapid tempering test.The traditional tempering was carried out in the high temperature resistance furnace,respectively,at 620oC,650oC and 680oC,air cooling treatment after 60 min insulation.Rapid tempering specimens were heated at a heating rate of 20oC/s,and all samples of these three tempering temperatures are maintained 40 s,80s and 120 s,and then air cooling;finally the impact test was carried out in-40oC low temperature environment,measured the variation of the low temperature toughness of materials under different temperature value;the article comprehensively used the optical microscopy,scanning electron and transmission electron microscopy to observe the microstructure and morphology of precipitates,and determinate the chemicalcomposition of precipitates.The results showed that the rapid tempering microstructure is mainly lath bainite ferrite and a small amount of equiaxed ferrite,also a small number of micro M/A islands,When temper growing,partial strip film austenite between lath,got higher stability and improved the material toughness.By using scanning electron and transmission electron microscopy,we can see carbonitride particle dispersed and distributed in matrix,compared with the traditional tempering process,ferrite lath finer,precipitates size was obviously smaller and uniformly distributed.Especially,nanocarbonitride particle size at 620oC is smaller,with the maximum volume fraction.With the increasing of temperature,the amount of precipitates was decreased,and the growthing and coarsening.For the low temperature impact toughness value of-40oC,the tempering temperature is the same,low temperature impact toughness with holding time decreased and then increased;when the holding time is the same,the impact value with the tempering temperature increased at decreased;rapid tempering 120 s,the impact toughness value reached the minimum,which is lower than the traditional tempering impact,especially in 620oC,40 s insulation,we can got the optimal performance,up to 41.8J.To sum up,we can get that the tempering temperature 620oC insulation 40 s is the best of rapid tempering process. |
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