Effect Of Nb On Mechanical Properties And Corrosion Resistance Of 420 MPa Weathering Steel

Weathering steel can be used outdoors without coating because of its excellent atmospheric corrosion resistance.It has the unique function of"stop rust with rust",which can reduce the environmental pollution caused by the use of paint and greatly reduce the maintenance cost in the later stage.Weathering steel is in line with the guiding concept of green economy,mainly used in bridges,buildings and so on.Due to the high content of Cu,Cr,Si and other weather resistant alloy elements in the steel,the low temperature toughness of the steel plate decreases,especially the low temperature toughness of the coarse grain heat affected zone in the welded joint is seriously damaged.Therefore,how to control the service performance of weathering steel comprehensively and solve the matching problem between strength and toughness,weldability and corrosion resistance is an important research direction.As an effective means to improve the strength and toughness of materials,microalloying is often used in structural steel.Nb,as one of the most important microalloying elements,is often added to steel to improve its mechanical properties.Nb can effectively refine the original austenite grain,refine the transformation structure and improve the strength and toughness of low alloy steel during rolling.However,during the welding thermal cycle,the addition of Nb will increase the hardenability of the steel,which may increase the size and number of M-A component of brittle phase and damage the low temperature toughness.At the same time,the influence mechanism of Nb on the atmospheric corrosion resistance of weathering steel is not clear,so the systematic study of the influence of Nb on the comprehensive performance of weathering steel has a certain reference significance for the system design of high performance weathering steel.In this paper,four kinds of Nb content 420 MPa weathering steels were selected as the research objects,and the influence mechanism of Nb content on strength and toughness,weathering resistance and low temperature toughness of CGHAZ was systematically studied.The main contents and conclusions of this paper are as followsThe effects of Nb content on microstructure,tensile properties at room temperature,impact properties at low temperature and corrosion resistance in industrial atmosphere were analyzed.It is found that the fine Nb precipitates in high temperature region restrict the growth of original austenite and refine the structure by pinning grain boundaries.At the same time,the addition of Nb leads to the decrease of Ar₃,the increase of phase transformation undercooling and the limitation of grain growth.Both of them lead to grain refinement and improve the strength and toughness.In the simulated atmospheric environment corrosion experiment,the addition of Nb promoted the increase of stable phaseα-Fe OOH,and the rust layer became denser.At the same time,the addition of Nb promoted the positive shift of electrochemical self corrosion potential.Both of them improved the atmospheric corrosion resistance,but with the further increase of Nb content,the weatherability changed little.Using Gleeble-3500 thermal simulator,the welding thermal cycle physical simulation of four kinds of Nb content test steels with heat input of 20 k J/cm was carried out,and the effect of different Nb content on the low temperature impact toughness of CGHAZ under this typical heat input was analyzed.It is found that the low temperature toughness increases with the increase of Nb content,which is mainly related to the refinement of microstructure.During the high temperature process,the pinning effect of precipitates restricts the growth of austenite grains,while the addition of Nb reduces Ar₃ and increases the undercooling degree of transformation.Both of them lead to grain refinement,increase of density of large angle grain boundary and increase of resistance to cracks,thus effectively improving the low temperature toughness of CGHAZ.