Effects Of Hot Rolling Process And Niobium/Titanium Content On The Microstructures And Mechanical Properties Of Low-Carbon Microalloyed Steels

Because of the special service condition,the yield strength of the low-carbon microalloyed steels used for manufacturing water heater tanks needs to reach a certain level.For the low-carbon microalloyed steels,the enamel fire annealing process(EFA)is equivalent to the heat treatment process.The microstructure changes,such as ferrite-austenite transformation and grain growth will occur during the EFA process.In this thesis,the effects of dynamic strain induced transformation process(DSIT)and Nb/Ti content on the microstructures and mechanical properties of low-carbon microalloyed steels before and after heat treatment are studied by experiments,thermodynamic calculations and finite element simulations.The main results are as follows.The results of thermodynamic calculations show that the A_e3 temperature of the steels containing 0.01-0.06 wt.%Nb is between 871? and 873?,and the all solution temperature(T_AS)range of niobium carbonitride is between 1009? and 1179?.The A_e3 temperature of the steel containing 0.02 wt.%Ti is 874?,and the T_AS temperature of titanium carbonitride is higher than 920?.The results of quenching experiments show that the A_r3 temperature of the steel with 0.02 wt.%Nb is between 840? and 860?.The effects of different DSIT rolling processes on the microstructures and mechanical properties of the low-carbon microalloyed steels are investigated.The results show that the yield strengths(?_0.2)of the steels with different start rolling temperatures(T₃)during 4^th-6^thhot rolling passes vary by about 10-50 MPa.The average grain size of the steels with different T₃ temperatures is about 3-4?m.When the T₃ temperature range is between 820? and 840?,the properties of the as-rolled steels are better.The finite element simulation results show that the higher T₃ temperature leads to the lower the plastic deformation resistance and rolling force.The yield strength(?_0.2)of the steel rolled with a larger reduction(II#process)is higher than that of the steel with a smaller reduction rate(I#process).The results of finite element simulation show that with a larger reduction rate,the temperature of the steel in 4^th-6^th rolling passes rises up obviously,and the maximum rolling force is also increased;Different hot rolling processes have little effect on the microstructures and mechanical properties of the steels after heat treatment.The effects of different Nb/Ti contents on the microstructures and mechanical properties of low-carbon microalloyed steels are studied.The results show that the yield strength(?426-459 MPa)of the as-rolled steels increases with the increase of Nb content,and the yield platform becomes more obvious.The average grain size of the as-rolled steels containing Nb element is?3-4?m.Comparing the steels with the same level of Nb/Ti content,it can be found that the yield strength of the steel with Nb element(?430 MPa)is about 60MPa higher than that of the steel with Ti element(?371 MPa).The grain size of the steel with Nb element is about 3?m smaller than that of the sample with Ti element(?6?m),and there is no typical yield phenomenon in the tensile curve of the steel with Ti element.The yield strength of the steels containing Nb element after sand cooling(SC)or air cooling(AC)heat treatment(SC:?412-457 MPa;AC:?415-440 MPa)are slightly lower than that of the as-rolled steels,and the grain size after heat treatment is slightly increased(?3-4.5?m).This is mainly due to the release of strain energy caused by rolling during heat treatment,which promotes grain growth.The yield strength(?389-397 MPa)of the steels containing Ti element after heat treatment by SC is slightly higher than that of the as-rolled steels,whereas the yield strength(?353-389 MPa)of steels after heat treatment by AC is slightly lower than that of the as-rolled steels.The strengthening mechanisms of the steels containing 0.02-0.06wt.%Nb samples after heat treatment at 871? by SC are analyzed.It is found that the contributions of various strengthening mechanisms to the yield strength of those samples are as following:fine grain strengthening(?45%),solution strengthening(?25%),dislocation strengthening(?15%)and precipitation strengthening(?10%).