Research Of The Microstructure And Properties Based On Mg Or Zr Treated Ship-building Steel Plates

Oxide metallurgy,one of the paramount methods to control the microstructure and properties of steel utilizing some.special thermal stable oxide inclusions,is an important direction of non-metallic inclusion to take advantage of its function in steelmaking process.It is the basis of the development and application of oxide metallurgy technology to study the relationship between the properties of non-metallic inclusions,microstructure transformation and properties of steel.In the present work,the FH40 ship-building steel containing different Mg,Zr contents were refined with a vacuum induction furnace and rolled with dbuble-stick reversible rolling mill.The functions of Mg or Zr in the different metallurgical processes,such as solidification,rolling and welding,were systematically investigated.Moreover,the effects of Mg or Zr oxide particles on microstructure transformation behavior of the steel were deeply discussed.The present work holds scientific significance for the enrichment and perfection of the oxide metallurgy theory,and provides theoretical guidance for introducing oxide metallurgy technology into Al-deoxidized low carbon steel in future.The Mg or Zr treated ship-building steel plates were prepared according to the GB chemical composition of FH40,and the characteristics of inclusions including particle size,chemical composition and morphology in steels were declared.On the basis of this,the influence of Mg or Zr treatment and treatment intensity on the solidification structure was analyzed,and the effect of Mg or Zr oxide particles on intragranular acicular ferrite?IAF?formation and its mechanism was further discussed.It shows that the dominated Al₂O₃ inclusions can be modified into spinel with reasonable Mg addition?0.0026°/%?,and changed into partially Al-Zr-O with 0.0022%Zr and 0.0050%Zr content,respectively.Under Mg and Zr composite treatment,the AI2O3 inclusions are modified into Al-Mg-Zr-O with 0.0024%Mg and 0.0054%Zr content.The results indicate that the Al-Mg-O+MnS and Al-Mg-Zr-O+?MnS?multiphase particles with 1.5?3.0?m size are effective nuclei of IAF,whereas the Al₂O₃+MnS and Al-Zr-O+MnS are inert for IAF formation in solidification process.Moreover,the IAF structure are generally originated from the surface of oxides and the wrapped MnS phase on it.The IAF formed on the surface of Mg-containing particles can be explained with low mismatch strain and Mn-depletion zone mechanisms.The obtained steel ingots were then subjected to forging and TMCP experiment,and the characteristics of rolled microstructure and mechanical properties were systematically analyzed.Additionally,the effect of Mg or Zr oxide particles on the IAF formation in the rolling process was also characterized.On the basis of this,the strengthening mechanism of Mg and Zr treatment on rolled steel was studied combined with the experimental results of the precipitation phase composition and microstructure of the experimental steel.It shows that the Al-Mg-O and Al-Mg-Zr-O,which caused by Mg and Mg-Zr treatment,respectively,can induce the IAF formation,while the Al-Zr-O caused by Zr treatment is inert for the IAF formation in the present rolling process.In the present study,the strengthening mechanisms of the Mg and Zr in steel involve the grain refinement strengthening,dislocation strengthening and the precipitation strengthening,of which the grain refinement strengthening should be the main mechanism for Mg.In the case of Zr,the main strengthening mechanism is the precipitation strengthening,which results in a poor plasticity for the Zr-treated steels.Moreover,the toughness of the steel is significantly reduced when the concentration of Zr is as high as 0.0050%.The obtained rolled steel plates were subjected to a combination of welding thermal simulation and submerged arc welding experiments for further investigation.The toughness and microstructure of coarse-grained heat-affected zone?CGHAZ?were studied,and the effect of Mg or Zr oxide particles on IAF formation in the welding process was also characterized.The welding tests indicate that the toughness of CGHAZ can be improved by adding with Mg or Zr into the steel.It is attributed to the suppression of grain boundary and Widmanstatten ferrite and to the promotion of IAF induced by Al-Mg-O+MnS,AI-Zr-O+MnS,and AI-Mg-Zr-O+MnS.It should be mentioned that the quasi-polygonal ferrite?QF?transformation tends to be obvious with the heat input energy increase from 54 kJ/cm to 100 kJ/cm.It confirms that the large amount of fine QF in the Zr-treated steel is also responsible for improving the toughness of CGHAZ except for the IAF structure.The characterization of y-a phase transformation during continuous cooling under different cooling rates for the steels was investigated based on the results of solidification,rooling and welding process,and the individual conditions of IAF formation for the different steels were also clarified.For the Mg-treated steel,since the AI-Mg-O has a strong ability to facilitate the IAF,the IAF structure forms in a relatively wide cooling rate range of 1.0?20 ?/s,which eventually tends to expland the bainite transformation.In the case of the Zr-treated steel,since the Al-Zr-O may have a weaker ability,the cooling rate suitabale for IAF formation is narrow,and only a small amount of IAF forms at 5.0?/s.The addition of Zr could promte the transformation of proeutectoid ferrite,while the bainite transformation is suppressed.As for Mg-Zr-treated steel,the Al-Mg-Zr-O also has a strong property for IAF,and a relatively wide cooling rate of 2.0-10?/s can be obtained,which finally leads to the expansion of bainite transformation and the diminution of the proeutectoid ferrite.It is observed that these associated compounds are followed by Al-Mg-O,Al-Mg-Zr-O,Al-Zr-O,and Al₂O₃ in decreasing order of effectiveness for IAF nucleation considering the inclusion composition and cooling condition synthetically.