Study On Mechnism Of Toughening Of Heat Affected Zone In High Strength Low Alloy Ship Hull Steel

To improve the weldability and high heat input welding adaptability of high strength low alloy ship hull steel, effect of ultra-low carbon alloy designing and the second phase particles (including carbonitride and oxide) on the microstructures and properties of Heat Affected Zone (HAZ) in high strength low alloy ship hull steel were studied in this dissertation. The results showed that carbon content and carbon equivalent (Ceq) were greatly decreased when adopting ultra-low carbon alloy designing. Yield strength of the steel significantly increased through grain refinement and Cu precipitation. Weldability of ultra-low carbon content 440MPa and 550MPa ship hull steel significantly increased after adopting the upper alloy designing idea. Different inclusions have different ability to promote the nucleation of ferrite. For all the inclusions focused in this dissertation, Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote the formation of acicular ferrite during welding. Large amounts of Ti oxides gained through Ti treated, not traditional Al treated, and the Ti-bearing oxide inclusions promote the formation of acicular ferrite which suppressed the growth of grain boundary ferrite (GBF) and ferrite side plate (FSP), it refined the austenite grain size indirectly. Microcontent Mg (⁰.0010%Mg) or Zr (⁰.0043%Zr) added to the Ti steel could further refine the size of inclusions and increased the area fraction of inclusions, which increased the ability of Ti-bearing inclusions promoting the formation of acicular ferrite and improve the toughness of HAZ significantly.Ultra-low carbon alloy designing was carried out based on traditional 440MPa and 550MPa high carbon content ship hull steel. Nb microalloying+TMCP technique were used to improve the yield strength of 440MPa steel and Cu age hardening precipitation was employed to improve the yield strength of 550MPa steel. The results showed that the two strength grade steel both gained the high strength and toughness after ultra-low carbon alloy designing, and low temperature toughness and cold cracking resistance were significantly increased, the experimental steel could be welded at 0℃with no preheating. It was feasible to gain high strength, high toughness and good weldability for the ship hull steel through ultra-low carbon content alloy designing with grain refining strengthening or with age hardening precipitation.The role of Ti₂O₃, TiO₂, ZrO₂, TiN, CeO₂, MgO and Ti₂O₃+MgO compound inclusions promoting the nucleation of ferrite were systematic investigated through force bonding experiment. The results showed the mechanism of inclusions promoting the nucleation of ferrite is different for different inclusions. It including the following three types: chemical reaction occurred in the boundary, and Mn/Si depletion region formed which promote the nucleation of ferrite. These inclusions including TiO₂, ZrO₂, CeO₂, MgO; no chemical reaction occurred in the boundary and the inclusions also promote the nucleation of ferrite, the inclusions including Ti₂O₃; no chemical reaction occurred and no ferrite formed in the boundary, the inclusions including TiN. The austenite temperature showed great effect on the ability of Ti₂O₃ promoting the nucleation of ferrite. Mn diffused much faster in the austenite at higher austenite temperature, which was favorable for the inclusions promoting the nucleation of ferrite. Ti₂O₃ was the most stable inclusions at high temperature and showed the strongest ability to promote nucleation of ferrite in the present study.Effect of Ti and Ti compound oxides on the microstructures and properties of HAZ and the phase transformation regularity of coarse-grain HAZ in Ti treated steel were investigated. The results showed that Ti and Ti compound oxides could promote the formation of acicular ferrite, the low temperature toughness of high heat input welding HAZ significantly improved. The phase transformation starting temperature of GBF, FSP and Intragranular ferrite were almost the same, but kinetics of microstructures growth was different. The growing rate of GBF was the fastest at the temperature of 725⁶50℃, but FSP and Intragranular ferrite showed the fasted growing rate at the temperature of 650⁵00℃. Acicular ferrite formed in the austenite suppressed the growth of GBF and FSP. The results also showed the key points to improve the high heat input welding HAZ toughness was how to get large amounts, more fine and dispersed Ti-bearing inclusions in the steel.To get large amounts of dispersed Ti-bearing inclusions in the ship hull steel, effect 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. It was first time to find, Mg content greatly affected the types, size, distribution of Ti-bearing inclusions. The high input welding HAZ showed the highest low temperature toughness and the size of inclusions was much fine when 12ppm Mg added to the Ti treated steel, and the same amounts of Ti₂O₃ and Mg₂TiO₄ formed 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. Zr content also affected the types and size of inclusions in Ti treated steel greatly. The same amounts of Ti₂O₃ and ZrO₂ formed and HAZ toughness was the highest during high input welding when 43ppm 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 of Ti oxides and increase the density of nucleation core of Intragranular acicular ferrite.