| With the depletion of shallow-layer oil resources in China,further exploration and exploitation have to turn to those oil and gas fields in seriously corrosive circumstances with sour hydrogen sulfides.Therefore,high-quality steels are required to make casing steel with good resistance to the aggressive environment.However,the sulfide stress corrosion(SSC)resistance ability of microalloyed highstrength casing steel pipe is closely related to its metallurgical defects in the production process.On the one hand,non-metallic inclusions in steel may act as the nucleation sites of hydrogen-induced cracks.On the other hand,varying degrees of banded structures were formed in the subsequent rolling process due to the segregation generated during continuous casting,which will damage the mechanical properties and corrosion resistance of products.It is reported that rare earth(RE)in steel is able to modify the inclusions and improve the solidification microstructure,which is expected to become an effective means to improve the structures and properties of high-strength sulfur-resistant casing steel.The present study focuses on the demand for inclusion modification and solidification refinement of high strength casing steel.On the basis of laboratory melting experiments and thermodynamic calculation,the reasonable addition amounts of rare earth alloy are proposed.Then,the industrial experiments of C110 and P110 grade casing steel were carried out with EAF→LF→VD→CC production process route.The recovering rate of RE and the effects of RE on the inclusion modification in different refining stages after RE addition are compared and analyzed.Besides,the effect of RE inoculation on the as-cast microstructures and banded defects is studied.At last,the nozzle clogging during the industrial test is studied experimentally based on morphological analysis and the related thermodynamic calculations.Firstly,the modification behavior of inclusions in C110 grade casing steel with different Ce content was studied by high temperature experiments in a tube furnace.The results show that the composition of inclusions in molten steel changes significantly at different stages of the holding process after Ce addition.With the prolongation of the reaction time,the modification sequence of the inclusions in the steel is CaO·Al203→CeAlO3→Ce2O3→Ce2O2S,and the final stable product is related to the Ce content.Based on the experimental research and Factsage thermodynamic calculation,the reasonable addition amount of RE for the specific purpose of industrial production is proposed.It is pointed out that to modify largesize calcium aluminate inclusions in steel,the reasonable addition amount of rare earth should be controlled between 20 and 90 ppm.Furthermore,in order to obtain the RE oxides with better homogeneous nucleation effect,the addition amount of RE should not be less than 180ppm.Meanwhile,considering that the formation of RE oxides is concerned with the diffusion kinetics in molten steel.it is suggested that RE alloys should be added into molten steel at the refining stages with argon blowing.Industrial experiments were carried out to modify inclusions in C110 grade petroleum casing steel.The RE ferroalloy was added to molten steel after VD treatment(test A)or after LF treatment(test B)with a RE content of 61 and 76 ppm,respectively.The recovery rate of RE,the content of Ca,O,S in the steel,and the changes in the composition and morphology of inclusions during refining processes,were analyzed.The results show that the Ca content in the molten steel has a significant influence on the occurrence state of RE in the steel.With the increase of Ca content,the content of dissolved[RE]increased,resulting in a decrease in the modification efficiency.For test A,RE was added to the molten steel after the Ca treatment at the end of VD refining.RE partially modified Ca-Al-O inclusions in molten steel,which decreases the size and number of inclusions but there are still unmodified large size Ca-Al-O inclusions residual in the steel.Thermodynamic calculations show that most RE exists in the molten steel in the form of dissolved[RE]under this condition,and the recovery rate of RE in molten steel is as high as 80%.Test B added RE after LF refining and canceled the Ca treatment at this stage.Under this condition,most RE was generated as inclusions by modifying MgAl2O4 in molten steel to form REAlO3.After 5 minutes of argon blowing,the yield of RE in molten steel was 74%,but almost all of them were removed during the subsequent VD degassing process.Therefore,the recovery rate of RE has dropped to 9%at the end of ladle refining.However,it is worth noting that,due to the low oxygen activity in the steel under the condition of low rare earth yield,the CaO in slag decomposed and formed CaS in the molten steel instead of Ca-Al-O inclusion.Because the liquid Ca-Al-O inclusions are easy to collide and grow in the VD degassing stage,the size and number density of inclusions with RE addition decreased,although the types of inclusions in the molten steel remain unchanged.As a consequence,the NACE A method testing results show that both the industrial experiments can significantly improve the anti-SSC performance of casing steel,and the pass rate is increased from less than 70%in the original process to 100%.In order to improve the as-cast microstructure of casing steel,industrial experiments with higher RE content(418 ppm)addition have been conducted.RE was added to the molten steel after the VD refining stag at the last heat of a continuous casting process.Steel samples were taken from round billets at different stages of the casting process,which shows an increasing RE content in the steel from 122 ppm to 174 ppm along with the casting operation.In this case,the original inclusions in the steel are almost entirely modified to RE2O3.In addition,RE(As,P)is generated on the surface of RE2O3 to form RE-O-As-P composite inclusions.However,the size and the number density of inclusions increased with the increase of RE content,and the proportion of RE-O-As-P inclusions increases as well.According to DSC experiments,it was found that RE2O3 can decrease the solidification undercooling and act as heterogeneous nucleation sites,while the heterogeneous nucleation ability of RE-O-As-P composite inclusions is comparatively weak.Therefore,the solidification structure will be refined only by restricting the RE content in the P110 grade casing steel to 122-152 ppm.The solidification microstructure refinement of casing steel specifically refers to the increase of equiaxed grain ratio with an improved CET transformation consistency,the increase of nucleation rates,and the decrease of secondary dendrite spacing.Therefore,the negative pressure generated by shrinkage at the end of the solidification process is weakened,which is beneficial to eliminate the spot segregation with a diameter larger than 1 mm.As a result,the number and maximum width of banded defects in hot-rolled and Q&T steel pipes are reduced,leading to improved hardness uniformity in casing steel products.Meanwhile,with the increase of RE content,the strength of the casing steel continues to improve regardless of whether RE has refined the solidification structure.This could be explained by the fact that RE can restrict the segregation of As and P at the grain boundary.However,due to the increasing number density and size of inclusions in the steel,the impact toughness deteriorates.At last,the submerged entry nozzle clogging of a RE-treating sulfur resistant casing steel is studied.The clogging occurs in the inner-wall and outlet regions of the straight-through nozzle.Among them,the clogging area in the inner-wall reaches 30%while that of the outlet area reaches 77%.The clogging mechanism is studied experimentally based on the constitution of clogging phases analysis and the related thermodynamic calculations.The results indicate that the clogging is caused by inclusions from molten steel and reoxidation products by the reaction between RE,Ca,Al in molten steel and the previously formed cloggings on nozzle refractory.The number density of solid inclusions increased after RE addition,which significantly aggravates the clogging rate.Meanwhile,RE also decreases the oxygen activity in the molten steel,which promotes the reoxidation reactions at the outlet region.Therefore,the clogging is enhanced to resist the scouring of molten steel,which is the main reason for clogging appearing in the outlet region.To prevent nozzle clogging,suggestions are given for continuous casting of the REtreating casing steel with an improved RE treatment strategy and the optimization of nozzle geometry and its refractory materials. |
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