Processing To Improve The Hull Steel Welded Haz Microstructure And Properties. Of Ti

The low temperature toughness declined and cold cracking susceptibility increased in traditional Al treated ship hull steel when the high welding heat input was adopt.The disperse distribution of Ti oxides inclusions promoted nucleation of intragranular accicular ferrite in steel making process and it was effective to improve the microstructures and properities of HAZ(Heat Affected Zone)in high strength low alloy ship hull steel.Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote the formation of acicular ferrite during welding among the second phase inclusions.In this paper,first the mechanism about how the intragranular ferrite nucleation facilitated on the Ti oxides was studied.The continuous quenching test of Ti deoxidized steel was performed using Gleeble3500D without Nb and other strengthing elements.With a decrease in temperature to 725℃,the GBF(grain boundary ferrite)and FSP(ferrite side plate)begin to nucleate along the austenite grain boundaries,at the same time the acicular ferrite mucleate at Ti₂O₃ within austenite grains.The phase transformation of GBF basically completed and the acicular ferrite intragranularly growed rapidly which suppressed the growth of FSB at 650℃,the interlocking microstructure of acicular ferrite formed in austenite grains when temperature continued to 575℃.Bonding diffusion experiment has been performed to analyze the mechanism of MDZ(Mn-depleted zone)around Ti₂O₃ from macro perspective and the effect of high temperature holding time on ferrite nucleation ability has also been discussed.The results showed that the ferrite layers formed along the interfaces between the steels and the Ti₂O₃ powders and the ferrite layers hasn't been found along the interfaces between the steels and the Al₂O₃ powders.It confirmed that Ti₂O₃ particle has high ability to promote acicular ferrite nucleation.The width of the MDZ decreased with decreasing austenitizing temperature and the ferrite layers disappeared the austenitizing temperature was at 950℃.The 10μm width of MDZ formed near steel- Ti₂O₃ interfaces in the bonded apecimens was analyzed by electron probe microanalysis(EPMA).The MDZ developed in the vicility of steel- Ti₂O₃ powder interfaces because Ti₂O₃ itself absorb neighboring Mn within an austenite matrix.The nucleation ability decreased at Ti₂O₃ with increasing of high temperature holding time. The thermal cycle of CGHAZ for the welding heat input with different t_8/5was simulated using Gleeble1500D.The charpy v-notch impact toughness at -20℃showed that the toughness of CGHAZ in Ti treated steel was higher than Al treated steel when the welding heat input was higher than 50KJ/cm,and the toughness of Ti treated hull ship steel attained 60J when the welding heat input was 75KJ/cm.It is concluded from Gas Metal Arc welding experiment at -20℃that the toughness of CGHAZ in Al treated steel was lower than 40J with the welding heat input of 50KJ/cm,but the CGHAZ in Ti treated steel was higher than 140J with the welding heat input of 50KJ/cm and 20KJ/cm.So the Ti treated steel improved low temperature toughness of CGHAZ greatly in ship hull steel.It is found that most spherical inclusions in Ti treated steel had Ti rich oxide core with a little MnS layer on the surface of particles,ranging from 2 to 3.2μn,the morphology,size and chemical composition hadn't been affected after thermal cycle with welding heat input of 50KJ/cm.It is also found that most complex irregular inclusions in Al treated steel had Al rich oxides core with outer layer of TiN+Mn-Ti-O and a little MnS on the edge of particles,ranging from 2.5 to 3.5μm.The precipitates of outer layer in complex inclusions dissolved and the size of particles decreased after thermal cycle with welding heat input of 75KJ/cm.The complex inclusions had mostly Al₂O₃ core with outer layer of (Ti,Nb)(C,N).The thermal cycle for the welding heat input of 50KJ/cm was simulated,once the samples continued to 600℃and 650℃,they were quenched in water and a number of acicular ferrites nucleated at Ti oxides were got within austenite grain.The microstructures of acicular ferrites were observed with welding heat input of t_8/5=100s and t_8/5=60s using Metallurgical Microscope and SEM,The acicular ferrite decreased the amount of granular bainite and it split austenite grain and the the microstructures were refined.The austenite grain size growed more quickly in Ti treated hull ship steel than that in Al treated hull ship steel when austenite temperature range from 900℃to 1100℃,The Al treated steel started abnormal grain growth when the austenite temperature above 1300℃,but the Ti treated steel still continued to normal growth.The austenite grain size(180μm)was much more larger in Al treated steel than that in Ti treated steel when the temperature reached 1300℃.As the thermal cycle peak temperature was at 1350℃,The austenite grain size growed more quickly in Ti treated hull ship steel than that in Al treated hull ship steel when t_8/5＜40s,the growth of anstenite was much more quickly in Al treated steel than that in Ti treated steel when t_8/5＞40s,and the size of austenite grain was 160μm in Al treated steel which was much more larger than Ti treated steel.