Research On Rolling Process And HAZ Microstructure And Properties Of V-microalloyed Steels For High Heat Input Welding

High heat input wielding steel is one of the important steel for offshore platform construction in 13th Five-Year national planning with large size,high strength and easy welding characteristics.At present,there are many researches on high heat input welding steel in China.Because of the complex process of smelting and rolling,the almost property of the products are not stable.This thesis adapted the smelting process of comprehensive treatment of Al-Ti-V-N as the starting point using of the precipitation strengthening effect of alloy elements and TMCP rolling process to ensure good mechanical properties of steel bar.Also the HAZ toughness was improved by oxide metallurgy,precipitation of sulfide and refinement of grain size.The present researches have indicated that the formation of a large number of AF in the steel was one important way to enhance the toughness of HAZ.So the AF nucleation mechanism became an important research direction.Depending on the theory that the second phase particles including the Al,Ti,Mn and carbon nitride oxide would induced the nucleation,this thermodynamic calculation,rolling process simulation,mechanical testing and physical simulation to analyze the effect of alloy elements in HAZ.Studies showed that improving the AF could increase the toughness of HAZ.Furthermore through the elements of control and optimization of rolling process,HAZ will completely crystalline organization forward and replace the overheated zone of HAZ to improve the toughness of HAZ is also an important means of improving the toughness of HAZ.This improved the toughness obviously.Even beyond the rolled steel toughness,perhaps in the future large line energy or long t8/5 welding has been widely used in the development of research on steel.In view of the fact that the AF nucleation mechanism is not uniform,this paper also studies the nucleation mechanism of AF in HAZ.It is hard to induce the nucleation of AF with Ti alone.When added the Mn element,some MDZ around the inclusions formed and the relationship between inclusions and ferrite is good.Those were the key factors to enduce the nucleation mechanism of AF.Particles of Ti(CN),CuS,V(CN)in sulfide oxidation or complex inclusions precipitated and ferrite and B-N orientation is the key to good AF induced nucleation,because the B-N orientation relationship of AF induced nucleation driving force far less than MDZ nucleation driving force,no Mn steel AF nuclear driving force is often inadequate.However,the study also found that the MDZ shaped nuclear drive is not the decisive driving force of the AF core,Al2O3+V(VN)composite particles can also be formed around the inclusion of large AF shaped nuclear driving force.This thesis is a small part of the development of high heat input welding steel,the optimal steel mechanical parameters:ReL=456MPa,Rm=562MPa,rolled steel AKV(-40?)=310J,100kJ/cm HAZ(-20?)AKV=181J,300 kJ/cm HAZ AKV(-20?)=240J,477 kJ/cm HAZ AKV(-20?)=333J.