| In recent years,shipbuilding industry in our country has developed rapidly,leading to the demand increase for high-end ship plate steel day by day.The large-scale and high-speed ship requirements for ship structural materials are also getting higher and higher,requiring ship plate steel with high strength,good low-temperature impact toughness,welding performance and corrosion resistance.In this study,the aim is to improve the impact performance and the stability of other mechanical properties of a domestic enterprise DH36 high-strength ship plate steel which does not meet the classification society standards of shipping in the export quality inspection.On the basis of investigating the internal quality,microscopic and macroscopic defects of steel billets,the whole metallurgical process has been studied by the metallurgical physical and chemical principles and metallurgical methods systematically.Under the national standards,the chemical composition,steelmaking process,thermal rolling process of DH36 ship plate steel has been optimized during the whole metallurgical process,and the DH36 ship plate steel with stable process and excellent performance has been obtained.In the range of low S content,the seawater corrosion protection mechanism and super-hydrophobic zinc-nickel alloy coating of DH36 ship plate steel have been studied,which is of great significance for the mechanical properties and seawater corrosion resistance of our high-end ship plate steels,aircraft carriers,and naval vessel steels.The completed main researches in the thesis are as follows:(1)By combination with the production process,the low impact performance of DH36 high-strength ship plate steel is analyzed through low magnification,metallographic and inclusion analysis,fracture analysis,scanning electron microscopy,which could be attributed to the internal defects caused by inclusions,especially sulfide inclusions.In the wide bainite,martensite,and pearlite banded structures produced in the center of the steel plate,abnormal core structures and strip-shaped MnS,nitrides and other inclusions caused by the enrichment of C and Mn elements and component segregation are found.Their interfaces with the steel substrate become the crack source,which will cause cracks in the steel plate under the action of tensile stress during the cooling or straightening process after rolling.Combined with the previous big data model study of the mechanical properties of S,P,conventional elements and impact toughness of the DH36 plate steel by our research team,it was determined that the high-performance DH36 plate steel must have the extremely low S content in LF refining,and the P controlled at 0.018-0.020%in the whole process,which will possess the maximum impact toughness and greatly reduce the influence of the fluctuation of C,Si,Mn,Al and other elements on the mechanical properties such as impact toughness.Through research on steel billets with lower S content of 30-60 ppm obtained after improving the steelmaking process,it is found that the precipitation temperature of MnS and the size of inclusions such as sulfides and nitrides still have a greater impact on the impact performance.Even if the size of MnS inclusions is relatively small,the continuity of the internal structures can also be affected in the steel plate.The crack source is likely to occur at the location of the inclusions and the expansion of microcracks reduces the impact performance of the steel under the impact by external forces.MnS precipitates in the austenite solid phase zone.The lower the S content,the lower the precipitation temperature of MnS in the austenite zone,and the smaller the size.It is found in this study that the chemical composition optimization principles of high-performance DH36 plate steel are:low C,medium Mn,Nb,V microalloying,the controlled content of A1 and V at the low limit,ultra-low content of S,the controlled P at 0.018-0.020%,the optimized continuous casting parameters with draw speed of 0.95 m/min,specific water volume of 0.5 L/kg and degree of superheat of 25?.After optimization of the whole process parameters,high performance DH36 plate steel was obtained by smelting,with reduce of the center segregation of the cast slab significantly and the disappearance of the cracks generated by the band structure of the steel plate,leading to the improvement of the impact performance significantly,the reduce of fluctuation range greatly,and the improvement of welding performance significantly.(2)The continuous cooling transition curve(CCT curve)of the DH36 high-strength ship plate steel with excellent performance obtained after the improvement of the steelmaking process was tested on the Gleeble-1500 thermal simulation test machine.Combined with controlled rolling and cooling,the percentage of austenite recrystallization after the single-pass rolling under different deformation amounts and deformation temperatures was determined.Under the best final rolling temperature of 800-820?,cooling rate of 5-7?/s,and final cooling temperature of 690-710?,the low-temperature impact toughness is improved significantly,which not only meets the classification society standards of shipping,but also impel the low-temperature toughness at-40?and-60? much higher than the standard value.For the DH36 ship plate steel with a thickness of 30 mm,when the welding heat input is 15 KJ/cm and 50 KJ/cm respectively,the flaw detection results are all level 1,and the tensile,bending impact performance and hardness tests of the weld butt joints have passed the ship material verification requirements,which solves the problem of unstable welding performance.(3)According to the theory of ion-molecule coexistence(IMCT),a prediction model of the phosphorus distribution ratio between the CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5-TiO2 nine-element slag system of converter smelting DH36 ship plate steel and the molten steel was established.The actual production data,which was obtained using the slag-steel composition at the end of the converter smelting DH36 ship plate steel in company,verified the accuracy of the phosphorus distribution ratio prediction model used in the DH36 smelting to control the phosphorus content.The thermodynamic theory proved that the characterization equation of the key parameters NFetO in the dephosphorization model must use the "full oxygen method",and the data obtained at the production site also confirmed the accuracy of the theoretical characterization equation,which strongly supported the implementation of the oxidative dephosphorization model.The relationship between[%P]and lg LP,measured is obtained by the thermodynamic model.The lg LP of DH36 at the end of converter smelting corresponding to the[%P]concentration range of 0.018-0.020 is 3.86-4.07,and the smelting temperature is T=1617-1634?.The characteristics and composition range of the corresponding end slag are:binary basicity R2=2.5-3.5,(%MgO)=8-11.6,(%FeO)=11.9-13.8.The composition of(%Fe2O3),(%MnO),(%Al2O3)has little effect on the distribution ratio of P.It was also found that when the(%TiO2)content of slag was less than 1.0%,the(%TiO2)content has little effect on lg LP,but when the(%TiO2)content was in the range of 1.0-1.3%,lgLP fluctuated greatly,which needed to be further studied.Based on the IMCT theory,a thermodynamic model of SiO2-Al2O3-CaO-MgO-MnO-TiO2-FeO seven-element slag refining and desulfurization of DH36 ship plate steel LF furnace was established.The 30 sets of industrial data showed that the theoretical prediction results were in good agreement with the measured data.It was found that the contribution of LS,MgS to the total sulfur distribution ratio was very little and could be ignored;the contents of MnO and TiO2 in the slag and the refining temperature have little effect on the sulfur distribution ratio.The greatest impact on the distribution ratio of sulfur is the slag basicity and the oxygen content in the molten steel[%O](or the content of(%FeO)in the slag).When the slag basicity increases from 2 to 6,the distribution ratio of sulfur increases by 10.When the oxygen content in the molten steel is less than 50 ppm or the(%FeO)content in the refining slag is less than 1,the sulfur distribution ratio increases sharply.(4)The corrosion behavior of the smelted low-S,controlled-P DH36 ship plate steel was studied in the simulated seawater.The electrochemical polarization curve and impedance spectroscopy(EIS)showed that among the steels with controlled P content at 0.018-0.020%and the S content of 30 ppm,50 ppm and 60 ppm,the lower 30 ppm S DH36 steel exhibited the best corrosion resistance.The corrosion morphology of the sample analyzed by scanning electron microscope confirmed that the steel was corroded uniformly,which could be attributed to the formation of corrosion micro-batteries between the small amount of MnS inclusions and the surrounding iron matrix in the low-sulfur state steel.The result indicated that the low-S ship plate steel still couldn't prevent the corrosion of seawater,which required further study on other corrosion protection technique for the ship plate steel.(5)To explore the corrosion protection mechanism of DH36 ship plate steel,the zinc-nickel alloy coating was prepared by electrochemical deposition.It was found that when deposited at lower potentials of-0.8 V and-1.0 V,nickel ions with positive precipitation potentials were precipitated preferentially.The zinc-nickel deposition process belonged to normal co-electrodeposition,and the deposition speed was slow,leading to the uncovered entire surface.When deposited at-1.2 V,zinc with lower standard electrode potential was precipitated quickly and the deposition of nickel was inhibited,forming a Zn(OH)2 colloidal film with a faster abnormal co-deposition,and finally leading to the formation of a dense zinc-nickel alloy,which improved greatly the corrosion resistance of DH36 ship plate steel.However,when deposited at-1.4 V,larger particles and holes were formed under the abnormal co-deposition,which reduced the corrosion resistance of the zinc-nickel alloy coating.(6)Finally,in order to obtain super corrosion resistant ship plate steel,a micro-nano structure superhydrophobic zinc-nickel alloy coating was prepared on the surface of DH36 ship plate steel by electrochemical deposition,and the influence of deposition time on the coating morphology,chemical composition,crystal structure and wettability was studied.After the PFTEOS treatment,the wetting behavior of a micro-nano structured zinc-nickel alloy coating formed on DH36 surface after deposition time for 3000 s changed from super-hydrophilic to super-hydrophobic and its static water contact angle exceeded 160°.The polarization curves in 3.5%NaCl solution confirmed that the corrosion resistance of the as-prepared superhydrophobic zinc-nickel alloy coating was about 32 times higher than that of the uncoated 30ppm low-sulfur DH36 ship plate steel.This research brings new hope to solve systematically the corrosion problem of high-end ship plate steels in seawater in the near future. |
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