| In our country,it is a promising route to exploit the oil and gas in ocean because ocean owns rich resource and the oil and gas in land become shorter.The existing 690 MPa class offshore platform steel has many disadvantages such as poor plasticity,high alloy cost and so on.Moreover,high cost of the production during the cycle quenching and tempering for a long time also brings many difficulties to preparation of steel products for offshore platforms in our country.As the core component of the ocean platform,the development of new high strength and toughness steel has become the key factor to achieve our goal of building a strong marine country.Based on the knowledge of physical metallurgy,material science and corrosion electrochemistry,this paper developed a new generation of medium manganese steel for offshore platforms using reversed austenite to regulate and control strength and toughness.It is expected that this steel is beneficial for independently fabricated oceanic steels by our country.The demand of mechanical properties for medium manganese steel with the strength and high toughness of offshore platform steel is that yield strength greater than 690 MPa,yield ratio less than 0.88,elongation greater than 20%and the Charpy impact energy at-40?greater than 140 J.Based on the principal that 'Mn/C alloying','Mn instead of Ni','ensuring the weldability and corrosion resistance',the tested steel meeting the demand of mechanical properties was fabricated by melting,hot rolling and intercritical tempering.The regulation mechanism of intercritical tempering process on the microstructure and mechanical properties of tested steel was studied.In addition,corrosion behaviors of medium manganese steel in simulated marine splash zone and marine atmospheric zone are deeply analyzed.The main content in this paper and the innovative results are listed below:(1)In the alloy system of medium manganese steel,the critical cooling rate of martensitic transformation can be reduced significantly.In addition,the hardenability of steel,the stability of microstructure and properties at the thickness dection in a medium heavy plate are also improved.The content of Mn in the tested medium manganese steel is set from 5%to 5.6%.Through the measurments of the critical phase transformation point and austenite continuous cooling curve of medium manganese steel with different chromium(Cr)content,the results show that with the increase of Cr content,the temperature range of two phase region(austenite and ferrite)is expanded,and the temperature of Acl and Ac3 moves up obviously.The high Mn content ensures the hardenability of the experimental steel.Under the conditions of undeformed and 60%deformation,the microstructure of the experimental steels is martensite at different cooling rate.(2)The Cr alloyed medium manganese steel which is mainly composed of tempered martensite and reversed austenite,is obtained by the combination of direct quenching after hot rolling and intercritical tempering process.With the increase of the intercritical tempering temperature,the content of the reversed austenite in the steel increases first and then decreases,which leads to the first decrease of the strength and then rise.In addition,impact energy(-40?),total elongation and product of strength and elongation all show the trend of steady decline after rapid rise.The optimum heat treatment scheme for 0.4Cr mass%medium manganese steel was tempering at 650? for 30 min.The yield strength is 737 MPa,tensile strength is 858 MPa,elongation and impact energy at-40? are 21.7%and 226 J,respectively.The mechanical properties of manganese steel with Cr content of 0.8%after intercritical tempered at 650? for 50 min reached the best,the yield strength and tensile strength are 708 MPa and 840 MPa,and the total elongation and impact energy at-40? is 23.7%and 143 J,respectively.Through laboratory research,medium manganese steel for offshore platform is successfully prepared to meet the requirements of high strength and toughness.(3)Medium manganese steel with 0.4Cr mass%exhibited the excellent low-temperature toughness.The impact toughness of the experimental steel was tested at different impact temperature and the fracture microstructure was analyzed.The results show that all fractographs from 0? to-60? showed a ductile-fractured surface.The size of the fine dimples decreased as the impact temperature decreased because of increased stress concentrations.The deformation induced degree of the metastable austenite transformed to martensite decreased as the impact temperature decreased.The impact toughness of the tested steel exhibited a slight downward trend as the impact temperature decreased.In addition,the stability of austenite and the impact toughness,as functions of impact temperature,were revealed.(4)In a 0.8Cr mass%medium manganese steel intercritically temerped at 700? for 50 min,a new hexagonal closed-packed phase(?-martensite)appear on former reversed austenite by an air-cooled process.The tensile strength of the tested steel was improved to 1 GPa,and the product of strength and elongation was improved to 21.6 GPa·%,and the yield ratio was reduced to 0.50.This result was attributed to the decreasing of austenite volume fraction in existence of ?-martensite.Partitioning behavior of the alloy elements at different intercritical tempering conditions was simulated by DICTRA.With the exception of Mn(enriched at the interface),the elements were enriched at the center of the reversed austenite,synergistically increasing the stability of austenite.According to the partitioning result,stacking faults energies at different tempering conditions were calculated.After intercritical tempering at 700? for 50 min,the ?-martensite was formed at the center of the reversed austenite at the cooling temperatures between 50? and 100?.Moreover,the ?-martensite formation temperature after tempering at 650? for 50 min was between 0? and 50?.In addition,The fracture mode was changed obviously with increasing intercritical tempering temperature:quasi-cleavage-fractured? brittle-fractured? intergranular-fractured was observed.The decline of stability for reversed austenite and the earlier deformation-induced behavior of?-martensite could reduce the toughness of the tested steel due to the change in the intercritical tempering conditions.During the low-temperature impact process,?-martensite exhibited good stability.(5)A cyclic immersion corrosion test was used to study the corrosion behavior of medium manganese steel in simulated marine splash zone.The results show that the main products in the early stage of corrosion are ?-FeOOH with flaky structures.The corrosion product presented loose and porous state,which leads to an increasing corrosion rate.With the increase of corrosion time,?-FeOOH appeared,presented cotton ball-shaped and whisker shaped.In combination with the elemental distribution in the corrosion products,and by electrochemical measurement and establishment of the Eh-pH diagram,it is known that the elemental Mn accumulate in the corrosion products and existed in many forms,such as MnFe2O4 and Mn3O4.The presence of this product will increase the precipitation rate of the cations of the corrosion products.The addition of Mo elements can effectively control the selective permeability of the cation in the form of oxide in the corrosion products.The addition of Ni element increases the ratio of ?-FeOOH in the corrosion products and increases the densification of the rust layer.The effect of Cr element is the most obvious.Through the Cr alloying of medium manganese steel,the corrosion rate in the simulated marine splash zone is obviously reduced.As the content of Cr in the medium manganese steel increased from 0.4%to 0.8%,which can reduce the annual corrosion rate by about 12%in the simulated marine splash zone,so as to effectively reduce the effect of substrate corrosion.(6)The corrosion behavior of the Cr alloying medium manganese steel in a simulated marine atmospheric zone was elaborated under a salt spray environment.The results show that the corrosion behavior of medium manganese in simulated marine atmosphere environment was different from that in simulated marine splash zone.The corrosion rate could be divided into three parts,containing rapid decline stage,slow decline stage and stable stage.The corrosion products mainly consisted of ?-FeOOH,?-FeOOH,FexOy,MnxOy and MnFe2O4.Based on the XPS results of Fe in corrosion product,Fe2O3 acted as the main corrosion products in the early stage of corrosion.The higher intensity of XPS shows strong oxidation reaction emerging.In the second stage of corrosion,MnFe2O4 on the corrosion process acted as an anode material because it has higher activity for the dissolution rate of H+with the ongoing corrosion oxidation.In the stable stage of corrosion rate,the main corrosion product was Fe3O4 that had higher stability.The corrosion behavior was analyzed byinvestigating elemental distribution and valence state in the corrosion products.With increasing the Cr content in medium manganese steel,Cr(as one of the anti-corrosion elements)may form Cr-rich oxides/hydoxides to stabilize the corrosion rate.In addition,the reversed transformation austenite plays a positive role in regulating the corrosion process.The corrosion rate can be weaken by optimizing heat treatment process. |
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