Effect Of Titanium On Mechanical Properties And Initial Corrosion Behavior Of Q420qENH Steel

Weathering resistant steel containing copper,phosphorus,chromium,nickel,silicon,and other alloying elements is an important anticorrosive steel.In recent years,with the development and utilization of Marine resources in the world,weathering steel has excellent corrosion resistance and good mechanical properties,and has become the main material of large structure bridge steel in coastal areas.The study had researched the effect of different Ti contents(0.014%,0.044%,0.084%,0.144%)on the mechanical properties and the initial corrosion behavior of Q420qENH steel.And it explored the mechanism of strengthening and toughening of test steels with different Ti contents.The effect of Ti content on the mechanical properties of Q420 QENH steel was studied by the microstructure characterization,tensile properties and impact properties of the test steel.The mixed microstructure of the four Ti content steels is composed of granular bainitic ferrite(GBF),quadrate ferrite(QF),acicular ferrite(AF)and martensitic-austenite(M-A)components.With the increase of Ti content,the volume of granular bainite increases,the number of polygonal ferrite and M-A constituents decreases,the effective grain size of microstructure decreases,the dislocation density increases,and the strengthening effect of the second phase increases,which makes the yield strength and tensile strength of the test steel increase significantly;When the Ti content increases from 0.014% to 0.144%,the impact energy of the test steel decreases from 212 J to 12 J,and the impact toughness deteriorates significantly;When the content of Ti is 0.044%,0.084% and 0.144%,large TiN particles are found at the grain boundary of microstructure.With the increase of Ti content,the average size of TiN increaseed.The large size of TiN inclusion is the main factor leading to the deterioration of impact toughness.The accelerated corrosion test was carried out in the range of 0.044% ? 0.144% Ti.The morphology and size of the TiN inclusions in the test steel with 0.144% Ti content were studied by means of metallographic microscope,scanning electron microscope and energy dispersive spectrometer.The incipient corrosion behavior of TiN inclusions in weathering steel was investigated by analyzing the corrosion dissolution of inclusions and matrix.When TiN inclusions are contained in the weathering steel,the corrosion behavior was preferred to the inclusions,resulting in uneven corrosion.With the increase of corrosion time,TiN inclusions induce pitting corrosion behavior.A large amount of?-FeOOH and a small amount of ?-FeOOH are accumulated in the TiN inclusions,which can prevent further pitting corrosion and make the weathering steel corroded uniformly.And the pitting potential decreases gradually with the prolongation of corrosion time,the impedance value increased,and the pitting resistance improved as well.According to the experimental results of the initial corrosion,the initial corrosion process can be divided into the induction period of the pitting behavior,the development period of the pitting corrosion and the later period of the pitting corrosion.