| Q345 structural steel has the advantages of high strength,good seismic resistance,short production cycle,and is widely used in construction,machinery,rail transportation and other fields.One of the biggest problems in steel production is energy waste and environmental pollution,to realize the device lightweight and achieve the purpose of energy saving and emission reduction,the development of high strength steel plate is imminent.At present,the research on structural steels mainly focuses on the study of the hardenability of structural steels and the analysis of the mechanical properties of complete quenching,but less research on the changes of microstructure and mechanical properties of structural steel after intercritical quenching and tempering treatment have been done by scholars home and abroad.Q345 hot rolled plate produced by Laiwu steel,is taken as the research object in this paper.Different intercritical heating temperature,intercritical holding time and tempering temperature are set for heat treatment of Q345 structural steel.By means of metallographic experiment and SEM,the effect of heat treatment process on the microstructure of materials was studied,and the effect of heat treatment process on the mechanical properties of materials was studied by means of hardness test and tensile test.Combined with the microstructure and mechanical properties,the effect mechanism of critical heating treatment on the strength and toughness of materials was analyzed.Q345 hot-rolled steel undergoes step quenching process and intercritical quenching process,according to the result of experiment shows that A1 and A3 are 700? and 820? respectively.First of all,study the effect of intercritical quenching temperature,analysis the result of the experiment shows that below 700?,materials doesn't enter critical area,quenched organization for ferrite and pearlite;higher than 700 ?,organization of the water quenched samples is martensite and ferrite.When the temperature increases in the critical zone,the hardness and strength increase obviously and the plasticity decreases.Quenched at 760?,the strength-elongation balance of step quenched specimens and intercritical quenched specimens reaches the maximum,they are19701.3Mpa ·%and 17630.4Mpa·%,9.6%and 19.1%higher than that of the original hot-rolled plate.They show good comprehensive performance.760? is the best critical quenching temperature,the proportion of ferrite and martensite is 1:1.Then,the influence of tempering temperature is studied,and the results showed that the process of martensite softening and carbide precipitation occurred during tempering.The organization is ferrite and troostite.With the increase of tempering temperature,the hardness and strength of samples decreased gradually,and the plasticity improved.Recrystallization temperature of Intercritical quenched samples is lower than step quenched samples,the partial recrystallization occurs when the tempering temperature is 500?,the original martensite structure of dark area disappears,ferrite grain size reduces.Last,the influence of critical holding time is studied.The results showed that the holding time of step quenched samples is short,the martensite content was high,with the extension of the time,the martensite decreases until it becomes constant;the holding time of intercritical quenched samples is short,the content of martensite is low,with the increase of the time,the martensite increases to constant.When the holding time is 20 minutes,the strength-elongation balance of step quenched samples and intercritical quenched samples reaches the maximum value of 17866.2Mpa·%and 19945.3Mpa·%,respectively.The best critical holding time is 20 minutes.After 450? tempering,hardness and strength are slowly down,the plastic improves.In conclusion,the dual phase steel plates with good strength and plasticity can be obtained by adjusting the heat treatment parameters of the steel plates after the two processes.Q345 structural steel quenched at 760? for 20 minutes and then tempered at 450? for 60 minutes has the best microstructure and mechanical properties. |
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