Study On Microstructure Evolution And Mechanical Properties Of Low Alloy Steel With Yield Strength Of 390MPa

In recent years,with the continuous development of TMCP technology and microalloying technology,the technology of HSLA steel has developed rapidly in the steel industry and obtained many research results.The HSLA steel is mainly used for building structures,industrial plants,lifting transportation and other high-load welded structure.And the most representative HSLA steel is Q345.However,with the development of construction,forming and other industries,the consumer's requirements on the strength,welding,molding and other properties of materials have become higher and higher.In some degree,the mechanical properties of Q345 HSLA steel can't meet consumer's requirements.It's an inevitable trend to develop and produce HSLA steel with higher strength.The Q390 is widely used in buildings,bridges,boilers,oil storage tanks and other welded structures of high load and so on for better mechanical properties.Traditional chemical composition of Q390 needs add more alloying elements Mn and microalloying elements Nb,V,Ti.The(micro)alloying cost is high.Therefore,it has important significance to develop low cost 390MPa grade low alloying steel.Based on this,in this paper,the chemical compositions of Q390 were redesigned.The Mn content was lower than 0.9%,canceled the precious micro alloying elements Nb and V and added cheap micro alloying element Ti.Low cost,low load and high efficient production of Q390 low alloying steel can be realized by fine-grain strengthening and precipitation strengthening from Ti and TMCP process.In this paper,by using the thermal simulation experiments and hot rolling experiments combined with the mechanical properties testing and microstructure analysis technology to study systematically on TMCP process,microstructure evolution and mechanical properties of experimental steel,in order to provide experimental data and control ideals for industrial production.In this paper,the main research work and results are as follows:(1)By single-pass compression experiments,this paper had studied the high temperature deformation behavior of austenite and established the deformation resistance model and dynamic recrystallization kinetics model.The results showed that,in the work hardening phase,the deformation resistance increased with the increase of deformation rate and deformation degree and the decrease of deformation temperature and low strain rate,deformation of high temperature and large deformation promoted the occurrence of recrystallization.According to the experimental data,the activation energy of dynamic recrystallization was got by regression method,which was 257.142kJ/mol.Lastly,The deformation resistance model and dynamic recrystallization kinetics model had high precision.(2)Through thermal simulation experiments,the phase transformation during continuous cooling of experimental steel was studied and the CCT curve was drawn.The results show that,with the increase of cooling rate,the Ar3 temperature of the experimental steel declined continuously,the content of ferrite and pearlite reduced gradually,the content of bainite increased continuously,the organization grain got smaller and the macro hardness value also increased gradually.(3)By using TMCP thermal simulation experiments,this paper had researched and analysised the effects of the cooling rates and finish cooling temperatures on the microstructures and mechanical properties of the experimental steel after deformation.The results show that,with the increase of cooling rate and the decrease of the finish cooling temperature,the content of ferrite and pearlite reduced gradually,the content of bainite increased continuously,the organization grain got smaller and the macro hardness value also increased gradually.With the decrease of the finish cooling temperature,the amount of precipitation of second phase particles TiC reduced gradually,the average particle size decreased,at the same time,the lamellar spacing of pearlite and the width of ferrite lath decreased gradually.(4)Through hot rolling experiments,this paper had researched and analysised the effects of TMCP process on the microstructures and mechanical properties of the experimental steel.The results showed that,with the increase of cooling rate,the decrease of the coiling temperature and finish rolling temperature,the microstructures became finer and the strength increased,however,the elongation decreased gradually.When coiling temperature controlled between 480 and 630?,the cooling rate controlled within the scope of the 26 to 70?/s,and the finish rolling temperature being about 860 ?,could obtain good comprehensive mechanical properties and fulfill the requirements of the mechanical properties of the Q390 low alloy steel.In a certain steel factory of China,this paper had carried on industrial production of the low alloy steel with yield strength of 390MPa and achieved the production with low cost,low load and high efficiency of the low alloy steel with yield strength of 390 MPa.