Effect Of Si Content On Mechanical Properties And Weldabilities Of Q420qENH Steel

Large-scale bridge structural steels used in offshore areas have high requirements in terms of corrosion resistance,mechanical property,and welding performance due to the harsh application environments.High performance weathering bridge steel has become widely used in the field of bridge construction in China due to its excellent comprehensive property.With the increasing application demand of high performance weathering bridge steel,the bridge steel is developing in the direction of high strength,high toughness and excellent weldability.It is a hotspot in recent years to improve the mechanical property and weldability of bridge steel by optimizing the content of alloy element.Silicon(Si)is the basic element in bridge steel,which has a great influence on the strength of steel plate and the impact behavior of welding HAZ.This paper was aimed to study the effect of Si content(0.12%,0.30%,0.48%,0.75%)on the weldability and mechanical property of Q420 qENH steel.The strengthening and toughening mechanism of test steels with different Si content were explored,and the optimum range of Si content in Q420 qENH steel was put forward.In order to explore the effect of Si content on the mechanical property of Q420 qENH steel,Test steels with different Si content were induced by the thermo mechanical control process(TMCP).The microstructure of each sample was haracterized and the tensile properties were measured.The results show that a microstructure consisting of granular bainitic ferrite(GBF),quasi-polygonal ferrite(QF),and martensite-austenite(M-A)constituent formed in all of the samples.Moreover,the increasing Si content led to an an increase in the number quasi-polygonal ferrite and a decrease in the number of granular bainitic ferrite relatively;the Si addition also increase in the effective grain size and a decrease in the density of dislocations.However,the enhancement of solid solution strengthening and the second phase strengthening of martensite-austenite constituent increased the yield strength of the steel slightly,but increased the tensile strength significantly,lowered yield-to-tensile strength ratio,and improved the strain-hardening capacity of the steel eventually.In order to make clear the effect of Si content on the weldability of Q420 qENH steel,thermal simulation under the heat input of 20 kJ/cm was thus carried out to reproduce the CGHAZ of this type of steel with differing silicon(Si)content from 0.12% to 0.75%.Charpy V-notch impact tests and microstructure observations were performed to analyze the effects of Si content on the microstructure and the impact behavior of test steels.The results showed that a microstructure composed of granular bainite(GB),acicular ferrite(AF),lath bainite(LB),and martensite-austenite(M-A)constituents formed in each steel.When the Si content increased from 0.12% to 0.75%,the LB decreased and in contrast,the GB increased.In addition,the area fraction of the M-A constituents increased with increasing Si content,while the number fraction of effective grains with high-angle grain boundaries decreased.Accordingly,the Charpy impact energy of the CGHAZ decreased sharply from 208 J to 42 J.The increased large M-A constituents significantly enable the microcrack nucleation,while the reduced HAGBs inefficiently impede the microcrack propagation,finally leading to a deteriorated toughness in the CGHAZ of 0.75%Si steel.Comprehensive analysis shows that the content of Si in Q420 qENH steel should be limited to 0.30% ~ 0.48%.