Study On The Mechanism Of Titanium On The Yield Strength Of Low Carbon Steel

Titanium microalloyed high strength steels have attracted much attention because of their relatively low production cost and excellent comprehensive properties.At present,their physical metallurgical characteristics have been systematically studied,and it is generally believed that the grain refinement strengthening and precipitation strengthening are the main strengthening mechanism of them.However,due to the lack of comparative studies,it's still difficult to clarify the strengthening mechanism of titanium in low carbon steel.Therefore,based on the composition of low carbon steel,this paper added 0.10%titanium element and used Gleeble 3800 thermal simulator to study the microstructure evolution and strengthening mechanism of the two materials during deformation and cooling process.The main contents and results of this paper are as follows:(1)The yield strength of the experimental steel was studied by isothermal compression method after being held at 900 ? for different time and then cooled at20 ?/s to 600 ? for different time.It was found that the yield strength of low carbon steel was significantly increased by adding titanium element under the same process conditions.At the same isothermal time,the yield strength of low carbon steel decreases slowly with the extension of holding time after deformation,while the yield strength of titanium microalloyed steel increases first and then decreases.The isothermal time has little effect on the yield strength of low carbon steel with the same holding time after deformation.For the titanium microalloyed steel with isothermal time of 600 s,most of the yield strength test results are higher than that of isothermal time of 300 s,and it reaches the maximum when the holding time is 100 s after deformation.When the holding time is extended to300 s,the yield strength of the titanium microalloyed steel with isothermal time of 600 s decreases rapidly,which is significantly lower than that at 600? isothermal time of 300 s.(2)Stable Ti N particles can be formed when titanium is added to low carbon steel,which can avoid excessive growth of low carbon steel grains during the austenitization process at 1200 ?.After recrystallization rolling at 1050 ?,the austenite grains of the two experimental steels were significantly refined.In the isothermal process after deformation at 900 ?,the structure of low carbon steel recrystallized,the grains grew slightly with the extension of the holding time;the recrystallization of titanium microalloyed steel was steel elongated state due to solute drag and strain induced precipitation.Under the same process,the difference between grain size of the two experimental steels after isothermal at 600 ? is only about 2 ?m,the microstructure of the two kind of steels at room temperature is mainly quasi-polygonal ferrite;A large number of titanium-containing precipitates were formed in the low carbon steel with the addition of titanium,which led to the troostite in low carbon steel at room temperature is replaced by granular bainite.The effect of titanium on grain refinement in low carbon steel is not obvious,grain refinement strengthening is not the main reason for the difference in yield strength of the titanium microalloyed steels.(3)The intragranular hardness of the experimental steel at 600 ? was measured by Vickers hardness tester.The results show that the intragranular hardness of low carbon steel under different processes is basically at the same level.In the initial stage of holding after deformation at 900 ?,the intragranular hardness of the titanium microalloyed steel isothermal at 600 ? of 600 s is slightly higher than that of 300 s.With the extension of the holding time after deformation,the hardness of the experimental steel begins to decrease gradually.When the holding time after deformation reaches 300 s,the hardness of titanium microalloyed steel which isothermal at 600 ? for 300 s is higher than that of 600 s.The trend of precipitation strengthening effect of titanium microalloyed steel under different processes is roughly the same as that of yield strength.Under the same process,the intragranular hardness of the titanium microalloyed steel is greatly improved compared with the low carbon steel.TEM images show that there are a large number of precipitated particles of about 5 nm in the titanium microalloyed steel,which leads to a strong precipitation strengthening effect.When titanium is added to low carbon steel,a large number of nano-carbides will be precipitated in isothermal process,precipitation strengthening is the main factor to increase the yield strength of low carbon steel.(4)The strengthening effect of titanium on low carbon steel is mainly the result of the coupling of grain refinement strengthening and precipitation strengthening.Compared with the high precipitation strengthening effect,the grain refinement strengthening effect caused by strain induced precipitation is not obvious.On the other hand,grain refinement strengthening is the only way to improve the strength and toughness of materials at the same time,the influence of strain induced precipitation on grain refinement strengthening still needs to be further studied.