| Titanium element plays an important role to the improvement of the performanceof steels. The refinement of grain by TiN in low titanium steel can not meet therequirement of large heat input welding. Furthermore, the idea to reduce niobium andincrease titanium could make the advantage of titanium’ low price and the precipitatingstrengthening of TiC more effectively. Thus, the cost of the production of high strengthstructural steels could be reduced. But the chemical property of titanium is active, theprecipitation behavior of titanium-containing inclusions will be affected by manyelements such as aluminum, oxygen, nitrogen, sulfur and carbon content and so on.Therefore, the inclusions in titanium-containing steel are more difficult to control andtheir strengthening effects will be not stable with the increasing of titanium in steel andthus the performance of steel will be not stable. Therefore, the large scale production ofstable titanium-containing steels will be hampered to some extent. So it is important andmeaningful to study the precipitation behavior of inclusions and their effects on themicrostructure in titanium-containing steels.In this study, the thermodynamics and kinetics of inclusion’ precipitation in theexperiment steels with different aluminum, titanium, nitrogen, manganese and sulfurcontent were calculated. The findings are listed as followed:The calcium content and its range to treat steel in low aluminum steel were small.The radius of TiN increases from2μm to9μm while the titanium content increases from0.024%to0.08%(mass percent), and the radius of TiN increases rapidly with theincrease of Ti content. The radius of TiN increases from10μm to11μm while thetitanium content increases from0.08%to0.15%, and the radius of TiN increases slowlywith the increase of Ti content. The closer the solidification fraction to1, the faseter theTiN grows.Secondly, X80pipeline steels with a titanium contant of0.02%were smelted inthis study. The steels with different aluminum content (X1,[Al mass%]=0.0056%, X2,[Al mass%]=0.034%) were deoxidized by Al-Ti and treated by magnesium. Thebehavior of inclusions in these steels and its effect on the microstructure were studied.The findings are listed as followed:The inclusions X1steel were composed of an Al2O3-MgO-TiOxcore with structure of multilayer or mosaic and MnS precipitated wrapped, semi-wrapped or in multiplelocal zones on their surface, compared with Al2O3-MgO core with structure of bundleand MnS precipitated wrapped or semi-wrapped on their surface in X2steel. Thenumber of inclusions was larger, while the thickness of MnS layer on the surface ofoxides and the deformation of inclusions in X1steel was smaller than that in X2steel.The deformation of inclusions was related to the composition of core oxides and thethickness of MnS. The harmful of MnS will be reduced while the steel was complexdeoxidized by Al-Ti and treated by magnesium.The inclusions X1steel were composed of an Al2O3-MgO-TiOxcore with structureof multilayer or mosaic and MnS precipitated wrapped, semi-wrapped or in multiplelocal zones on their surface, compared with Al2O3-MgO core with structure of bundleand MnS precipitated wrapped or semi-wrapped on their surface in X2steel. Thenumber of inclusions was larger, while the thickness of MnS layer on the surface ofoxides and the deformation of inclusions in X1steel was smaller than that in X2steel.The deformation of inclusions was related to the composition of core oxides and thethickness of MnS. The harmful of MnS will be reduced while the steel was complexdeoxidized by Al-Ti and treated by magnesium.The inclusions in Al-Ti-Mg steel could induce acicular ferrites and refine graineffecrively. The ability of inclusion to induce acicular ferrite was effected by itscomposition and size. The inclusion generated in the form of collision andheterogeneous nucleation was small. The inclusion generated in the form of chemicalreaction was large. Furthermore, the structure of the complex oxides core and theprecipitation behavior of MnS (or TiN) on their surface in steel were also important.The acicular ferrites were easly to nucleation between the complex oxides or on thejunction of complex oxides and MnS. The dual effects of Ti and Mg deoxidation arebeneficial for the composite Al2O3-MgO-TiOxoxides to form complex structure and theprecipitation of MnS in multiple local zones on the surface of complex oxides, and thuswould be beneficial to the effective nucleation of interlocking acicular ferrites on oneinclusion and increase the volume fraction of acicular ferrites.The steel samples were reheated and in-situ observed by confocal laser scanningmicroscopy (CLSM). The grain size in X1steel was more uniform than that in X2steelduring the isothermal process at1250°C due to a larger number of finely inclusions with high melting point in X1steel to pin austenite grain boundaries. Furthermore, themicrostructure was dominant AFs in X1steel due to the effective nucleation ofinterlocking acicular ferrites on fine inclusions, compared dominant GBs in X2steelwhile simulate the welding energy of50kJ/cm. The low aluminum steel with Al-Ticomplex deoxidation steel with magnesium treatment will adapt the process of largeheat input welding well.Meanwhile, the HG785steels with high titanium content were melted. Differentaluminum contents, different titanium contents and different time interval between theaddition of aluminum and titanium were designed in these steels. Then, the precipitationbebavior of inclusions and its effects on mircrostructure were studied，the finding arelisted as followed:With the increasing of aluminum content from0.008%,0.013%to0.026%, theaverage size of inclusions was increasing, the number of inclusions was descreasing andthe TiOxcontent in conmplex inclusions was descreasing. With the increasing oftitanium content from0.106%,0.125%to0.153%, the average size as well as thenumber of of inclusions was increasing, the TiOxcontent and CaO content in complexinclusions were increasing, the MgO content in complex inclusions was descreasing.With the increasing of time interval between aluminum and titanium from5min,10minto15min, the number of inclusions increased first and then descreased, the average sizeof inclusions was decreasing, the magnesium content was increasing, the titaniumcontent descreased first and then was stable, the calcium content inscreased first andthen was stable. There was an optimal range of time interval between aluminum andtitanium for the steels to obtain finely inclusions with an appropriate content of titanium,magnesium and calcium, the time in this study was between10-15min.The further study showed that The complex MgO-TiOx-CaS-MnS (little),Al2O3-MgO-TiOx-CaS (little MnS) or Al2O3-MgO-CaS (little MnS) inclusions with astructure of multilayered or bundle, which contain high TiOxor MgO content, werebeneficial to the heterogeneous nucleation of TiN. TiN was not inclined to nucleate onthe mosaic complex inclusions of CaS-MnS(less) with Al2O3-MgO-TiOx. While the sizeof the original complex inclusions was small, TiN would precipitate wrapped on thesurface of complex inclusions with a large thickness. While the size of the originalcomplex inclusions was large, TiN will precipitate semi-wrapped or in local zone on the surface of complex inclusions with a small thickness.The heterogeneous nucleation ofTiN on complex inclusions will be effective while the composition, structure andnumber of inclusions are appropriate controlled. Meanwhile, the homogeneous of TiNwill be delayed and the size of TiN will decrease.While the steel was deoxidized by Al-Ti and treated by magnesium in MgOcrucible, the complex Al2O3-MgO-CaO-TiOx-TiN inclusions will appear in steels andinduce acicular ferrite effectively. Then the grain will be refined and the strength as willas the toughness of steels will be improved. |
PDF Full Text Request