| Duplex stainless steel has the characteristics of both austenitic stainless steel and ferritic stainless steel.It is an important structural material with high strength,good corrosion resistance and excellent welding performance.Compared with super austenitic stainless steel and nickel based alloy materials with similar performance,it is a resource-saving stainless steel due to the cost advantage of substituting nickel with nitrogen.Meanwhile,duplex stainless steel is designed with long service life and low service cycle cost.Therefore,this type of alloy has great market potential,which has been widely used in oil and gas fields,flue gas desulfurization,seawater desalination and other industrial fields.Super duplex stainless steel（SDSS）S32750is a typical representative of the third generation duplex stainless steel.Due to the high content of chromium,molybdenum and nitrogen,the pitting resistance equivalent number（PREN）of S32750 can reach 40,which can be applied to extreme environments such as seawater heat exchangers and offshore oil and gas extraction.However,the difference in the mechanical responses of austenite and ferrite under hot working conditions,can lead to the formation of edge cracks or an inappropriated surface finish.It is thus difficult to establish the hotworking regime,which makes the production of hot rolled coil face severe challenges.In addition,S32750 suffers from deterioration of mechanical properties and corrosion resistance due to the second phase precipitated during aging treatment,which would bring production accidents and product failure.In order to solve the above problems,the hot deformation behavior,microstructure and texture evolution,dynamic softening mechanism of two phases during hot deformation,precipitation behavior and nucleation mechanism of the second phases during aging treatment,and the influence on properties of S32750 were systematically studied by means of optical microscopy（OM）,scanning electron microscopy（SEM）,electron backscatter diffraction（EBSD）and transmission electron microscopy（TEM）.The main results are as follows:1、The hot deformation behavior of as-cast S32750 was systematically studied,and the microstructure,texture evolution and dynamic softening mechanism of the two phases during hot deformation were clarified.The results show that the hot deformation behavior of as-cast S32750 was closely related to deformation temperature and strain rate.The coupling mechanism of the two phases under different deformation conditions determined two types of flow curve characteristics:when the deformation temperature was low and the strain rate was low,the flow curves exhibited“yield-point-like”characteristic;when the strain rate was high or the deformation temperature was high and the strain rate was low,the flow curves shown typical dynamic recrystallization characteristic.The activation energy of as-cast S32750 was393.75 k J/mol.The optimum hot deformation domains for S32750 were determined as（950～1150）℃/（0.1～1）s-1 and（1150～1200）℃/（0.1～25）s-1.2、When the deformation temperature was low,the softening mechanism of austenite phase was dynamic recovery;As the deformation temperature increased,the softening mechanism changed to discontinuous dynamic recrystallization.The DRX fraction of austenite increased with deformation temperature.The typical recrystallization texture of austenite phase was cubic texture,but its strength was weak.The softening mechanism of ferrite phase was continuous dynamic recrystallization.The Xv of ferrite kept unchanged of about 60%under all deformation conditions.The recrystallization textures of ferrite phase were mainly cubic texture and rotating cubic texture.With the increase of temperature and the decrease of strain rate,the strength of texture increased and concentrated to rotating cubic texture.According to processing maps and microstructure evolution,it was beneficial to the hot ductility of as-cast S32750 by increasing the deformation temperature and increasing the strain rate and strain at high temperature.3、The precipitation behavior and precipitation mechanism of the second phases in S32750hot rolled plate at 650～1000℃was studied.And the influence of different precipitates on the mechanical properties of S32750 was clarified.The results showed that the lamellar Cr2N precipitated at the ferrite grain boundaries after aging at 650～750℃for 60 min.σphase precipitated at 800～1000℃,whileχphase precipitated at 650～850℃.The precipitation mechanism of intragranularχphase in S32750 was strongly temperature dependent.A large amount of lenticularχphase homogenously nucleated within ferrite grains after aging at 650℃.Theχphase/ferrite interface was incoherent,and the OR between two phases was（200）χ∥（101）ferrite,[013]χ∥[111]ferrite.When aging at 800℃,chromium nitrides acted as favorable sites for the heterogeneous nucleation ofχphases,which reduced the energy barrier.The granularχphase and ferrite matrix formed a semi-coherent interface.The OR between three phases was（1010）Cr2N∥（110）χ∥（110）ferrite,[1211]Cr2N∥[111]χ∥[111]ferrite.The driving force was an important factor that influences precipitation mechanisms of intragranularχphase at different aging temperatures.The second phases deteriorated the mechanical properties of S32750 to different degrees,and the degree of influence is closely related to the quantity,morphology and distribution of the second phases.The Cr2N at grain boundaries reduced the plasticity of S32750,andχphase made the material less strong and plastic,whileσphase significantly increased the yield strength but greatly reduced the plasticity.When the amount ofσphase was less than 5%,the5%,the lamellar Cr2N at grain boundary was the main factor affecting the toughness of S32750.When the amount ofσphase was more than 5%,the toughness decreased sharply,andχphase became the key factor affecting the impact properties.4、There were two types of quenched-in nitrides,Cr2N and Cr N in SDSS S32750 after being held at 1100～1250℃ and cooled in different ways.Higher temperature and faster cooling rate would promote the precipitation of nitrides.Due to redistribution of alloying elements at high temperatures,the nitrogen element was supersaturated within the ferrite phase.So the driving force of nitrides created at higher temperatures than the equilibrium precipitation temperature,causing them to precipitate during cooling.It was showed that quenched-in nitrides had essentially no effect on the tensile properties of S32750,and the strength and plasticity were similar to those of solid solution state.The influence of nitrides on impact property was small.The cooling mode had little effect on the impact toughness for samples holding at 1100℃ and 1150℃,and the impact energy can reach 250 J.With increasing of holding temperature,the toughness decreased gradually.The toughness reduction of air-cooling sample was less than that of water-quenching sample.And the impact energy was 173 J after holding at 1250℃ and then water quenching.The pitting resistance of S32750 were related to the amount of nitrides.In the samples without nitride precipitation,the critical pitting temperature（CPT）reached about 90℃.After the precipitation of quenched-in nitrides in the ferrite ferrite,the CPT started to decrease to only 46℃ at 1250℃ with water quenching.The hot continuous rolling process of S32750 was optimized based on the results in this thesis,and the problems of surface and edge cracks were solved.The product yield and production efficiency were improved and the large-scale production was achieved.The specific and effective theoretical data were provided for users to control the heating history of the steel. |
