| In order to refine microstructure and improve the properties of steels bytiny.oxides,some basic key questions, i.e,to control the composition, size, quantity,anddistribution of oxides and to investigate the effect of their characteristics anddiversification of precipitates on microstructure and performance of micro-alloyedstructure steel, should be studied systematically, which is very valuable to thetheoretical developments and application of the oxide metallurgy technology.This paper takes high deformability pipeline steel with high cleanliness, highperformance as an object,the formation conditions of oxides in the steel produced withprocess of oxide metallurgy were discussed through the thermodynamic calculation ofthe Al-Ti-Mg(Ca) composite deoxidization and growth kinetics of the deoxidationproducts. Steels deoxidized with titanium or aluminum were smelted using vacuuminduction furnace,and rolled into steel plates by experimental mill in the process ofthermal mechanical control processing and relaxation.The characteristics of oxides,diversification of precipitates in the steel,and phase transformation mechanism inducedby inclusions were studied with scanning electron microscope(SEM),the effect ofcharacteristics of inclusions on microstructure, mechanical properties, resistance tohydrogen induced crack (HIC),and welding properties iof the steel were investigated.The main contributions to the current understanding of this topic are described below:The thermodynamics of formation of products deoxidized by compositeAl-Ti-Mg(Ca) and their growth kinetics were calculated by Factsage6.1. The resultsshowed that the Als content should be less than0.004%to generate the TiOx oxideswhen titanium content is0.01%.The second phase particles generated at the beginningof the solidification could grown to1.1(0.5) μm in size till the final stage ofsolidification while the cooling rate was100(500) K/min. The stronger deoxidizer of Mg or Ca could reduce fastly the supersaturation of oxygen to control oxides growthby oxygen diffusion, and that their deoxidation products have high-melting-point andare difficult to grow up by accumulation themselves,which could refine inclusions inthe steel complex deoxidized by Ti and stronger deoxidizer.The microstructure and mechanical property of steels were tested by opticalmicroscopy and other devices of performance testing such an SEM, universal tensiletest machine and the low temperature impact tester. The results showed that themicrostructures of the as-rolled steel plates with Al-killed consisted of a lot of granularbainite, compared with a lot of acicular ferrite, polygonal ferrite in the steels withTi-killed.The tensile mechanical properties of steels with different deoxidation processhad not significantly difference,for example, the stress strain curves were like roundhouse, the uniform elongation were over13%, yield ratio were less than0.85, andsection extending rate were between60%and70%. However, the impact ductility ofsteel with Ti-Mg complex deoxidation was obviously higher than that with Al-killedand Mg treatment, which is attributed to the microstructure with a lot of acicular ferriteinduced by a number of fine oxides with TiOx. In addition, different from the steelswith Mg treatment,the steels with Ti-Ca complex deoxidation had almost the sameimpact ductility as that with Al-killed steel and Ca treatment, which was due to thecombined action between refining microstructure and crack source of oxides in steelwith different deoxidation process.By means of SEM and confocal laser scanning high-temperature microscope(CLSM), the two, three-dimensional inclusions characteristics, diversification of MnSprecipitates in the steel,and the mechanism of refining microstructure of steels inducedby inclusion particles were explored. The results showed that with Ti-Mg complexdeoxidation process,(Ti, Mn) Ox and precipitation of MnS on the surfaces of titaniumoxide were found, the complex oxdies with some TiOx in steels with two differentdeoxidation process could induce acicular ferrite.As a result, the cation vacancy mechanism and the poor manganese mechanism incurred by inclusions played themain role in this research.Compared with inclusion characteristics in high aluminumsteel, the core oxides in low aluminum steel containning more TiOx were more in thenumber of inclusions,smaller in size,and less in the deformation aspect ratio (1~2),as aresult,which reduce the harm of MnS and complex oxides by improveing distributionand morphology of MnS precipitated on surface of these oxides.Hence, the process ofcomplex deoxidation by Ti played a modification role on MnS incolusions.The steels with different deoxidation process and Mg or Ca treatment were observedby CLSM in high temperature to evaluate the pinning effect of second-phase particleon grain boundary. The results showed that while at1250℃,1400℃insulationcondition, the size of crystalline grains were betyeen40μm and70μm,betyeen60μmand160μm respectfully,but their fine inclusions restraining grain growth by pinningeffect were different,which were attributed to second-phase particles (not inclusionswith oxides) and inclusions with oxides respectfully,grain growth trend of steel withcalcium treatment were smaller than that with magnesium treatment，which is due tothe difference of the pinning effect due to the different number of particles and lowgrowth rate by competitive growth of fine grains(in1400℃insulation condition).The resistance to HIC of the steels were tested by NACE solution at normaltemperatures and pressures. The study showed that steel materials with differentdeoxidation process in this research were not sensitive to HIC, which were not relevantwith the content of total oxygen, but also the number of inclusions in steels, and therate of deformation of the inclusions. In the other hand, HIC sensitivity and hydrogenbubbling have a strong correlation with the number of inclusions larger than5μm.Thermal simulation experiments of steels were tested by Gleeble2000thermalsimulated test machine,then the Vickers hardness, the optical microstructure,and the-25℃Charpy V impact properties of the welding HAZ ofsteels were testedrespectfully. It was found that the hardness of HAZ decreased with the increase of the heat input energy (20KJ/cm,30KJ/cm,50KJ/cm), which in Ti deoxidized steel,however, decreased slowly than that in Al-killed steel because of smaller grains in Tideoxidized steel with a lot of acicular ferrite induced by oxides with TiOx. Impactproperties of HAZ in Ti-Ca complex deoxidation steel decreased slowly than that inTi-Mg complex deoxidation steel, which were due to smaller microstructure of HAZ inTi-Ca complex deoxy steel with smaller grains, and large-angle intertwined grainwhich improve its toughness. However, HAZ of Ti-Mg complex deoxidation steel hadlarger grains, which were attributed to the dual role of oxide inclusions that can refinemicrostructure by inducing acicular ferrite, but reduce the number of second-phaseparticles, reduced its toughness with the increase of heat input energy.This research revealed the mechanism of fining inclusions by strong deoxidizerMg(Ca) in Ti-Mg(Ca) deoxidized steel through thermodynamic calculation, and thecharacteristics of inclusions in steels, precipitation behavior of manganese sulfide,and their pinning effect at high temperature and effect of inducing acicular ferrite werestudied systematically,it were helpful to reveal the phase transformation mechanismInduced by inclusions, and to put forward a modification role on MnS inclusions withthe process of oxide metallurgy.Furthermore, It was found that Ti-Ca complexdeoxidation process could improve impact toughness of HAZ of Ti-Ca deoxidizedsteel at high input heat energy by CLSM observation and welding simulation. Besides,a large number of finely dispersed oxides had a dual function such as reducing thenumber of single second phase particles, refining microstructure and leading to crack.This study provides the train of thought and the key research direction of theapplication of oxide metallurgy on high cleanliness and high performance steel. |
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