Evolution And Control Of Non-metallic Inclusions In CrMo Structural Alloy Steel Produced Through Mini-mill Steelmaking Processes

CrMo structural alloy steel possesses satisfactory tensile strength, corrosion resistance and high-temperature strength which intends for high temperature and high pressure services and is widely applied to the power generation industry and the petro-chemical industry. There are strict requirements for its cleanliness and within non-metallic inclusions. The refining slag used in CrMo structural alloy steel practical production at some steel mill is easy to crust and there are internal defects and cracks in some CrMo structural alloy product caused by MgO-Al2O3-CaO inclusions which contain a few CaO. The34CrMo4gas cylinder steel and26CrMo4s/2,28CrMo47oil drill pipe steel produced through EAF-LF-VD-CC process is investigated in this thesis. Inclusion composition and content is predicted by FactSage thermo software, and the calculation results are verified. The evolution mechanism of inclusions prepared under different steelmaking processes is studied. Slag optimization was proposed according to the research results of the relationship between the low viscosity slags and inclusions investigated by slag-steel equilibrium experiments.The result of thermodynamic FactSage calculation based on liquid steel composition in tundish indicates that non-metallic inclusions in34CrMo4gas cylinder steel consist of Al2O3-CaO oxide, MgAl2O, CaMg2Al16O27, CaS and MnS, which is perfectly agreement with the result of inclusions analysis in continuous casting blank. Thermodynamic calculation predicts that Al inclusions content in steel is43ppm and108ppm for MnS, while in fact the inclusion original position analysis reveals that Al and MnS inclusions content are separately22ppm and53ppm. The inclusions content predicted is double of practical measurement which is resulted from the inclusion flotation in tundish and then reduced the inclusion content in the final casting blank.Experiment results indicate that oxygen content in steel is decreased from180ppm before LF refining to lOppm in tundish during34CrMo4gas cylinder smelting process. SiO2-MnO complex inclusion exists in the steel at LF-VD stage and Al2O3-CaO complex exists during the whole refining process. Al2O3-CaO inclusions composition from each smelting process shows obvious different, namely,12CaO·7Al2O3is the dominant inclusion after VD refining and transfers to high melting point compound because of Al2O3content of inclusion is increased in tundish. Besides Al2O3-CaO complex coated with CaS layer, MgAl2O4and MnS inclusions also appeare in casting round at same time. The content of macro inclusions mainly caused by slag entrapment in CrMo gas cylinder steel is as large as150mg/10kg steel.Industry experiments based on CrMo oil drill pipe steel production process with different deoxidization and calcium treatment are designed and carried out. Inclusions in tundish liquid steel and casting round of CrMo drill pipe steel deoxidized with SiCaBa alloy and treated with calcium wire in VD refining mainly are Al2O3-CaO-CaS, while they are MgO-Al2O3-CaO, SiO2-Al2O3-MnO and clustered AI2O3without calcium treatment in VD refining. On the other side, inclusions after VD refining, in tundish liquid steel and casting round of CrMo drill pipe steel deoxidized with both SiCaBa alloy and aluminum block mainly are MgO-Al2O3-CaO and CaS with calcium treatment in VD refining and mainly are MgO-Al2O3-CaO and MnS without calcium treatment. Although oxygen content in steel is decreased through different processes, they are all below20ppm in final casting round. Macro inclusion content in final casting round is below10mg/10kg steel and it is slightly higher with calcium treatment in VD refining. Macro inclusions mainly consist of spherical aluminate and MgO-CaO oxide with irregular shape. Clustered Al2O3, low melting point SiO2-Al2O3-MnO complex, elongated MnS and MgO-Al2O3-CaO complex with a few CaO in drill pipe steel round have adverse effect on product quality. It is found that deoxidization with SiCaBa alloy and calcium treatment in VD refining is the best process route for CrMo structural alloy steel production according to the comparison of cleanliness and inclusion compositions in steel produced through different processes.The viscosity at1500℃of refining slag after VD refining process is2.5Pa-s when deoxidization with SiCaBa alloy, compared with that of1.5Pas when deoxidization with SiCaBa alloy and Al block simultaneously. The viscosity increases rapidly with temperature decreasing which could result in slag crusting. In order to decrease slag viscosity, synthetic slag E (mass%, CaO,55; SiO2,21; Al2O3,12; MgO,7; CaF2,5) and synthetic slag F (mass%, CaO,51; SiO2,17; Al2O3,25; MgO,7) are designed by reference to the composition of slag used in28CrMo47drill pipe steel production. It is shown that the viscosity of both synthetic slags is about0.5Pas at1500℃. Slag-steel equilibrium experiments indicates that synthetic slag tend to form inclusions at high melting point region close to CaO region in MgO-Al2O3-CaO ternary phase diagram, however, synthetic slag F drives inclusion composition to low melting point region. Hence, slag F is better than slag E considering the decreasing of inclusion content.The effect of Al2O3content in synthetic slag F on slag viscosity and inclusions in steel is investigated. Slag viscosity is between0.5Pa-s to Pa·s during1400℃-1500℃when A12O3content is in range of20%-35%mass fraction. The results of laser Raman measurements of synthetic slags indicate that the non-bridging oxygens content of synthetic slag with25%Al2O3is the least while whose viscosity is the largest at1500℃. Inclusions in steel treated with F series synthetic slag (Al2O3content20%-40%mass fraction) are mainly MgO-Al2O3-CaO complex whose composition distribute scattered. Al2O3content in inclusions is increased with Al2O3content,25%-40%mass fraction in slag increasing. Inclusions composition is relatively concentrated and close to Al2O3-CaO low melting point region when Al2O3content is25%mass fraction in slag. The quantity of1-3μm inclusions in steel treated for F series synthetic slag is similar. There is a strong negative correlation between the quantity of3-5μm inclusions and the ratio of MgO/CaO and Al2O3/(Al2O3+CaO) of inclusion composition. It is indicated that the inclusion quantity is less when inclusion melting point is higher in slag-steel equilibrium experiments, and vice versa. The best synthetic slag composition for CrMo alloy steel refining is CaO,51%; SiO2,17%; A12O3,25%; MgO,7%mass fraction.