Effect Of Boron On Impact Toughness And Mechanism Of Hot-Rolled Q345E H-Beams

According to different applications,hot rolled H-beams distribute height to width ratio of its section rationally.It has fine mechanical properties and usability.H-beams are used widely because of its light weight,high strength,good shock resistance, quick commercial process and construction speed,energy conservation and environmental protection.Our country has had the condition to develop steel structure; the constructs of steel structure present a thriving sight.The development of the constructs of steel structure creates good market conditions for the popularization of hot rolled H-beams.The hot rolled H-beams,base on the composition of Q345E,have unstable impact toughness at low temperature,especially in -40℃.The purpose of this research is to improve the impact toughness at low temperature and expand the scope of application by microalloying techniques.The process of this research is divided into three steps:firstly,the previous H-beams were analyzed to find the possible reasons which affect the impact toughness at low temperature;secondly,the laboratory research is to find whether the microalloy treatment is feasible or not;thirdly,commercial process was carried out under laboratory research's guidance.In each stage,the impact energy was tested at different temperature,and then the metallurgical structure,leaf metal,replica carbon film and impact fracture were detected by using optical microscope,TEM and SEM. Boron treatment is the highlight in this paper.In the past research,boron is mainly used to improve the hardenability.But in the research,the purpose is to increase the impact property at low temperature by adding boron element.The impact toughness at low temperature was improved in the commercial process under laboratory research's guidance.It confirmed that B element could help to perfect the impact toughness at low temperature considerably.The mechanism of action may be that boron should purify the boundary of the grains.The boron works with niobium.They fine the grain,postpone austenitic transformation and increase the content of bainite.The strength and the toughness were improved by getting mixture structure consisting of ferrite,pearlite and bainite.This would help to expand the range of application of H-beams in the cold environment.