Research On The Mechanisms Of Microstructure And Properties Control Of 420 MPa Grade Offshore Steel Based On Oxide Metallurgy

China is a major maritime country,relevant marine engineering equipment and advanced marine materials must be developed to improve our marine resource development capabilities and national maritime rights.In the construction of offshore engineering equipment,welding is an important connection process,which can directly affect the quality.Advanced automated high heat input welding technology has been widely used in the field of ship and marine engineering,which can greatly improve the welding efficiency,ensuring the welding quality and performance.However,high peak temperature and slow cooling rate during welding thermal cycle can form undesirable structures,such as ferrite side plate in the heat affected zone（HAZ）of conventional steel,which induce cracking of components and unable to meet the requirements of low temperature impact absorbed energy above E level steel plate.For this reason,EH420 grade marine engineering steels was choosing as the experimental material,related mechanisms about microstructure and mechanical properties were carried out.The main contents of the paper are as follows:（1）Based on the Gibbs free energy,interaction coefficient of different deoxidation elements and experimental characterization methods,the dominant nucleation interval and precipitation mechanism of inclusions during smelting and solidification were explored,and nucleation,growth,floating behavior,distribution state and dynamic interaction mechanism of composite inclusions during metallurgical process was also mastered.The results show that Ti₂O₃ particles have the highest stability among various Ti-containing deoxidation products.When the chemical composition of Ti is 0.015%,the dissolved Al content cannot exceed 0.006%to avoid the formation of Al₂O₃;Mg has a strong deoxidation ability,which can form Mg O at the mount of 0.00033%.Control the dissolved oxygen content before Ti-Fe alloy addition is0.005%,ensuring the growth of titanium oxide nucleation and shortening the argon blowing time are beneficial to obtain a better inclusion distribution and HAZ microstructure.The inclusion will grow up through turbulent and Stokes collision after nucleation,finally will be removed through the molten steel flow.Ti N and Mn S are easy to precipitate at the end of solidification.（2）The new generation TMCP process is used to make oxide particles improve strength and toughness of the matrix,revealing the phase transformation behavior of base metal under the"recrystallization zone rolling+water cooling"process,which can make the comprehensive improvement of mechanical properties and high heat input welding performance.The results show that the microstructure dominated by intragranular acicular ferrite can be formed in the rolled state,which by appropriately increasing compress ratio in recrystallization zone and cooling rates.Acicular ferrite plate was first grown by inclusion induction.As temperature decrease,the supersaturated carbon in plates will discharge to the surrounding austenite,which will cause irregular coarsening of its shape,and the unconverted austenite will be further transformation by diffusion mechanism.Compared with the conventional two-stage rolling,the"recrystallization zone rolling+water cooling"process can improve the strength of experimental steel about 30～50 MPa,and the base metal still has good low-temperature impact toughness at-60°C.（3）Experimental researches about Nb,V,Ni,Cu in high-strength marine engineering steel was carried out,and the influence of micro-alloy elements on the HAZ microstructure and properties was also discussed,which can provide theoretical guidance for the design of low-cost high-performance steel.The results show that an appropriate amount of Nb is beneficial to optimize the structure and properties of HAZ,but excessive Nb will inhibit the transformation of acicular ferrite,which is not conducive to the refinement of the intragranular structure.A proper amount of V can improve the mechanical properties of the experimental steel through fine grain strengthening and precipitation strengthening,and can reduce the energy required for the nucleation of intragranular acicular ferrite,but excessive V will reduce the low temperature toughness of HAZ due to precipitation hardening.Appropriate addition of Ni and Cu can effectively increase the strength of the matrix and inhibit the formation of side slat ferrite.（4）Experimental researches and theoretical calculations were carried out about the pinning ability of different inclusion sizes and contents on HAZ austenite grain boundary and effect of inducing intragranular acicular ferrite.The results show that the experimentally measured intragranular acicular ferrite lath growth speed is between the theoretical value of NPLE and Para model.Carbon diffusion is the main factor to control intragranular acicular ferrite length.By V-microalloying in Ti-Mg oxide metallurgical steel,a new HAZ toughening route was established.On one hand,pinning the grain boundaries by high melting point oxide inclusions.On the other hand,VN composite precipitation on the oxides surface to form a solute depleted zone and a low mismatch degree interface,which achieve synergistic control of grain size and microstructure refinement.（5）Aiming at the fracture behavior of high-strength marine engineering steel HAZ under high heat input welding conditions,main factors control the HAZ fracture and mechanisms of inclusions promoting the formation of cleavage cracks were revealed using oscillographic impact and scanning electron microscope,and a quantitative model between the fracture toughness of oxide metallurgy steel and fracture morphology was established.The results show that under cleavage fracture conditions,crack formation energy is the main factor controlling the fracture behavior of materials.Under ductile fracture conditions,crack propagation energy is the main factor controlling the fracture behavior of materials.The average size of inclusion particles can be used to characterize the critical microcrack size,and the inclusion will induce HAZ fracture through the interface separation mechanism or its own fracture mechanism under different stress conditions.Different fracture morphologies are the result of competition between cleavage fracture and microporous polymerization growth mechanism.Based on this,a quantitative model between fracture toughness and fracture morphology of oxide metallurgical EH420 grade offshore steel was established.The continuous transition from toughness to brittleness was explained.（6）Two oxide metallurgical processes based on the double-station LF furnace and the top-blown oxygen RH furnace were proposed,which realized the industrialized stable production of 60 mm thick EH420 grade high heat input welding offshore steel,and the evolution of inclusions under smelting condition was established.Experimental steel was welded by single-sided V-groove EGW with 361 k J/cm,the average impact energy of HAZ at-40℃was above134 J.The core of inclusions is gradually transformed from Mn S,Si O₂ and Mn O_x-Si O_y into composite oxide,and the microstructures also transformed from ferrite side plate+polygonal ferrite to intragranular acicular ferrite+bainite+polygonal ferrite.Strictly controlling the oxygen concentration of molten steel and appropriately shortening blowing time of RH furnace are beneficial to promote the fine dispersion of inclusions.