The Influences Of Ti-Mg-Ca Oxide On Microstructure And Properties Of HAZ In High Heat Input Welding Steel

In recent years,with the increasing pace of marine development in China and the rapid growth of the shipbuilding industry,the demand for steel for marine engineering has increased year by year,and the standard is also increasing.In order to improve welding efficiency,short engineering and manufacturing cycle,and reduce cost,high heat input welding methods are adopted in the field of ship and marine engineering.However,the use of high heat input welding often leads to serious deterioration of the strength and toughness of the heat affected zone of the weld,and is prone to defects such as weld cracks,which affect the safe use performance of the overall structural member.The development of steel for resistance to high heat input welding involves research in the fields of smelting,continuous casting,rolling and welding.The "oxide metallurgy"proposed by Nippon Railway's Takamura and Kouchi in the 1990s had led to the discovery that the introduction of dispersed beneficial oxides in steel can promote the nucleation of intragranular ferrite and improve the HAZ structure.In order to better understand the impact of different oxide metallurgy processes on the promotion of base metal and welding heat affected zone,this paper relies on the national "13th Five-Year",key research and development plan "high strength,large size,easy to weld" marine engineering steel and application topics,the preparation process of EH36 steel plate for large-line energy welding and the structural properties of oxidized inclusions for HAZ welding of steel plates were studied.(1)In this paper,the composition of EH36 steel mainly composed of C and Mn alloys was designed,and three different deoxidation processes of Ti-Ca,Ti-Ca-N and Ti-Mg were carried out.The TMCP rolling process was determined based on the static CCT curves of the three experimental steels,and the microstructure observation and impact performance of the rolled steel sheets were examined.Metallographic observation shows that the experimental steels with different deoxidation processes are subjected to TMCP rolling process,and the microstructure after rolling is a mixed structure of small-sized ferrite and pearlite.The grain boundaries are clear between the microstructures,and the microstructure is obtained effectively refinement.The impact energy of the three experimental steels at 0?,-20? and-40? are higher than 300J,indicating good impact toughness.It is proved that the oxide base metal can be obtained by the oxide metallurgy+TMCP technology.(2)Welding heat simulation experiments with different heat input were carried out for three different deoxidation process experimental steels.The analysis of the low temperature toughness of-40 ? after the welding thermal simulation experiment shows that the simulated CGHAZ of the three experimental steels has excellent toughness,the toughness of Ti-Mg treated steel is particularly prominent.The deoxidized inclusions in the steel can promote the formation of large dimples,thus effectively improving the toughness of the welded HAZ.Metallographic observations show that the formation of ferrite in HAZ can be promoted by oxide metallurgy.The deoxidation product can effectively induce the nucleation of intragranular acicular ferrite.The EBSD technique is used to improve the welding of HAZ by oxide metallurgy.The ratio of the large-angle grain boundaries between the inner grains is effective for the propagation of micro-cracks between the grains.(3)Inclusions are important for improving and promoting the microstructure and properties of welded HAZ.Analysis of inclusions in experimental steel shows that composite inclusions are inclusions that can effectively promote acicular ferrite nucleation,which including Ti,Ca and Mg oxide and Ca,Mg sulfide,and MnS and other materials,the use of Ti-Ca and Ti-Mg in the experimental steel can promote the acicular ferrite nucleation of the number of effective inclusions,have 9104/mm2 and 8584/mm2 respectively,that are about 4 times which of Ti-Ca-N steel.(4)Using CSLM technology finds that the peak temperature and the length of the holding time have a profound effect on the growth of the austenite grains of HAZ;if the holding time is too short,the original austenite cannot be completely converted into acicular ferrite;If the holding time is too long,the austenite grains will be transformed into coarse lath bainite or martensite structure.The Ti-Mg treatment process can suppress the coarsening of high-temperature austenite grains and improve the ability of austenite to form acicular ferrite.