Effects Of Cooling Method And Section Size On Microstructure And Properties Of 20CrSiMn2MoV Bainitic Steel

With the rapid development of social economy,the requirements for mechanical properties of metal materials such as steel are more urgent.For large units,how to maintain the microstructure uniformity and performance stability is particularly important.Therefore,it is significance for the application of large-size actual steel parts to study the influence of size factors on the properties of solid parts by heat treatment and sampling.At the same time,the heat treatment process in the iron and steel industry is considered to be one of the important processes to change the microstructural and mechanical properties.In this paper,the high-strength bainitic steel 20Cr Si Mn2Mo V is regarded as the research object,the phase transition temperature is measured by hardness-temperature method,Meanwhile,the effects of cooling methods and cross-section sizes on the microstructure and properties of large-diameter bars are studied through different cooling experiments,Tensile tests,hardness,impact tests,microstructure observation of metallography and scanning,XRD phase analysis,etc.The results show that the hardness-temperature method is used to measure the phase transition temperature,which is 710?and 880?respectively.?70mm bainite steel rod is heated up to 920?,then air cooling,and tempered at 300?,heated up to 920?,then water cooling for 30s before air cooling and tempered at 300?,The microstructure of different parts of the rod is bainite ferrite and retained austenite.After heating up to 920?and water cooling and tempering at 200?,the microstructure of different parts of the bar is made up of tempered lath martensite and retained austenite,and the experimental material has good bainite and martensite hardenability.The mechanical properties of?70mm bar obtained by sampling at half radius(R/2)of air cooling after heat treatment are tensile strength Rm of 1226MPa,elongation A of 17%,area shrinkage Z of 52.7%and impact energy KV₂of 75.5 J;The mechanical properties Rm,elongation A,area shrinkage Z,and KV₂obtained by water cooling for 30s and air cooling R/2 sampling are 1254MPa,18%,58.1%and 76J,respectively.The mechanical properties of bar R/2 after heating at 920?,water cooling at 200?and tempering at 200?are tensile strength Rm is 1513MPa,elongation A is 17%,area shrinkage Z is 47.4%,and impact energy KV₂is 73.2J.The impact performance of?70mm bainite steel bar is improved by water cooling for a certain time before air cooling.In order to obtain high strength and good toughness of bainitic steel bars,air cooling or water cooling-air cooling can be used for heat treatment,and in order to obtain ultra-high strength and high toughness,water cooling and low temperature tempering can be used for heat treatment of bainitic steel bars.The bars of different sizes are heated up to 920?for air cooling.With the increase of sample size,the cooling rate decreases and the bainite transformation temperature Bs increases.The bars with 10-70mm diameter air-cooled at 920?and tempered at 300?consists of bainite ferrite and retained austenite.With the increase of bar diameter,the length of bainite lath and the proportion of granular bainite increases.With the increase of bar size,its strength and hardness tend to decrease.The tensile strength of?10mm bar is the highest,reaching up to1571MPa,and the tensile strength of?70mm bar R/2 is the lowest,1226MPa,which is not much different from the tensile strength 1254MPa sampled at the edge.With the increase of bar diameter,the impact energy tends to decrease.The impact energy KV₂of?70 mm bar edge sampling is 80.5J,the impact energy KV₂of R/2 sampling is 75.5J,and the impact energy KV₂of the center is 61J,which keeps a high level.The hardness values of the edge and the center of the bar with the same diameter have little difference.The experimental material has good bainite quenching hardenability,and the bar with the diameter of 70mm bainite steel has good strength and toughness.