Key Metallurgical Technology Of High Heat Input Welding Shipbuilding Steel

To improve the weldability and high heat input welding adaptability of high strength low alloy ship hull steel, analysis of formation thermodynamics on the second phase oxide particles in high strength ship steel, microstructures and properties of Heat Affected Zone (HAZ) in Ti-treated high strength ship hull steel, and mechanism of Ti oxide promoting the formation of acicular ferrite in HAZ were studied in this dissertation. Then key alloy design, metallurgical technology and weldability of high heat input welding ship steel were investigated, proposing the technology range of approvingly high heat input welding.Firstly analysis of formation thermodynamics on the oxide particles in high heat input ship steel was studied in this paper, proposing the basic technical thought of Si-Mn complex deoxidization. The result shows that, oxygen content of melt equilibration decrease by using Si-Mn complex deoxidization technique. When Si content was0.3%, oxygen content of melt equilibration was0.0084%by using Si deoxidization, while it lowed to0.0062%by using Si-Mn complex deoxidization. Within the convention scope of Ti (0.01～0.02%) and O(0.0006%-0.0030%) content in ship steel, titanium and oxygen in the liquid steel forms the reaction of Ti2O3pirorly. Also the formation of Ti2O3was effect by aluminium, they can cause oxidation-reduction reaction. So, to promote the formation of Ti2O3, oxygen content of liquid must less than30ppm and aluminium content less than94ppm before Si-Mn complex deoxidization.Nucleation mechanism of acicular ferrite promoted by Ti oxides was investigated. The continuous quenching test of Ti deoxidized steel was performed using Gleeble3500D. With a decrease in temperature to725℃, 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 Ti2O3within austenite grains.The phase transformation of GBF basically completed and the acicular ferrite intragranularly growed rapidly which suppressed the growth of FSB at650℃,the interlocking microstructure of acicular ferrite formed in austenite grains when temperature continued to575℃.Bonding diffusion experiment had been performed to analyze the mechanism of MDZ(Mn-depleted zone) around Ti2O3from macro perspective and the effect of high temperature holding time on ferrite nucleation ability had also been discussed.The results showed that the ferrite layers formed along the interfaces between the steels and the Ti2O3powders and the ferrite layers hadn’t been found along the interfaces between the steels and the Al2O3powders.It confirmed that Ti2O3particle had 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 at950℃.The lOμm width of MDZ formed near steel-Ti2O3interfaces in the bonded apecimens was analyzed by electron probe microanalysis(EPMA).The MDZ developed in the vicility of steel-Ti2O3powder interfaces because Ti2O3itself absorb neighboring Mn within an austenite matrix, then ferrite formed remarkably because of phase equilibrium temperature increased. The nucleation ability decreased at Ti2O3with increasing of high temperature holding time. It was becaused that with increasing of high temperature holding time, manganese diffused into Ti2O3easily and formed into relative saturation, which reduced the MDZ and nucleation ability of intragranular ferrite.To gain abundant, fine, dispersive Ti oxides which promote acicular ferrite nucleation notably and enhance toughness of HAZ, effct of small amounts of Mg, Zr on the inclusions and microstructure and HAZ toughness during large heat input welding in Ti treated steel were systematic studied. The result showed that the high input welding HAZ showed the highest low temperature toughness and the size of inclusions was much fine when12ppm Mg added to the Ti treated steel, and the same amounts of Ti2O3and Mg2TiO4formed at this time. Microcontent Mg added to the Ti treated steel decreased the attractive force of inclusions significantly, effectively refined the size of Ti-bearing oxides. The same amounts of Ti2O3and ZrO2formed and HAZ toughness was the highest during high input welding when43ppm Zr added to the Ti treated steel. The density of Zr oxide was relatively higher, and it was not easy for the oxides rising in the liquid steel. Large amounts of fine Zr oxide could provide the nucleation core for the Ti oxide formed later, which avoided the coarsening o f Ti oxides and increase the density of nucleation core of intragranular acicular ferrite. Key metallurgical technology of high heat input welding ship steel were empoldered firstly. Large amounts of dispersed Ti-bearing inclusions which average granularity was1～3μm and mainly were Mg-Al-Ti-O complex oxides formed in steel by using Ti wire feeding treatment. Deoxidation time affected the formation of inclusions in Ti treated steel greatly, size of inclusions increased and quantity decreased with increasing deoxidation time. High heat input welding ship steel for E36can adapt240KJ/cm welding energy by using Mg-Ti complex treatment.