Development And Corrosion Resistance Study Of Q420qNH Medium Plate

With the rapid development of bridge construction in China,the demand for steel plates of bridges will increase largely.Under this background,the use of high-grade,high-demand bridge steel has become an inevitable choice.In order to comply with the development requirements of the bridge industry,this article has designed two kinds of weathering bridge steels with different components by referring to the existing standards,and has carried out trial production on this basis.Its microstructure,mechanical properties and weathering properties were studied.The main results of this paper are as follows:(1)The influence of the phase transformation behavior of the weathering bridge steel during the continuous cooling process was studied,and a reasonable cooling system was provided for the test steel to obtain the desired structure.The experimental results showed:Within the range of measured cooling speeds,there are three types of phase transformation zones in the test steel without Mo,they are pre-eutectoid ferrite precipitation,pearlite transformation and bainite transformation.There are also three types of phase transformation zones in the test steel containing Mo,they are pre-eutectoid ferrite precipitation,bainite transformation and martensite transformation.By comparing the dynamic static CCT curves of test steels without Mo and Mo,the addition of Mo inhibits the transformation of ferrite and pearlite,promoted the formation of martensite and improves the microhardness.(2)After TMCP+tempering,the comprehensive mechanical properties of Mo test steel are significantly better than that of the test steel without Mo,and the yield ratio during tempering continues to increase,the elongation after fracturing did not change significantly,and the low-temperature impact toughness was greatly improved.The content,quantity and size of M-A islands in granular bainite were studied.The results showed that after the tempering of experimental steel,the hard phase M-A islands in the original structure gradually decomposed,the content and the average size gradually decreased.Polygonal ferrite increased gradually in soft phase,resulting in a decrease in strength and a decrease in tensile strength.(3)The effects of fine structure,dislocations,grain boundaries,and precipitated particles on the mechanical properties after tempering were systematically studied.The experimental results showed that with the increase of tempering temperature,the long and sharp M-A islands located in the grain boundary in the original structure were decomposed and converted into granular M-A islands,and the bainite laths merge and coarsen.The tangled dislocation lines showed a sparse state,the number of grain boundaries at a small angle decreased,and reduced the tensile strength,improved the impact toughness,and increased the number of fine precipitated particles,also increased the contribution of precipitation strengthening,and lead to an increase in the yield strength.The joint effect of the two will eventually lead to an increase in the yield ratio.(4)Corrosion weight loss test,electrochemical test,SEM morphology observation and XRD examination were used to compare the corrosion resistance of two different compositions of weathering resistant bridge steel and contrast steel Q345B.The experimental results showed that after 360 hours of corrosion,the weight loss rates of the test steel without Mo and Mo are 0.848 g(m2·h)and 0.793 g(m2·h)respectively.The corrosive particles in the rust layer are fine,the pores are also small,and more protective ?-FeOOH is formed,which promoted the formation of a dense rust layer.However,in the cross section of the rust layer,pitting phenomenon occur in the test steel containing without Mo.Compared with the test steel,the weight loss rate of the comparative steel Q345B is 1.679 g(m2·h),coarse rust particles,large pores,and loose rust layer.It was found that the test steel containing Mo exhibited the best corrosion resistance,the corrosion resistance of the test steel without Mo followed,and the corrosion resistance of the comparative steel Q345B was the worst.