Effect Of Spheroidization Treatment On Corrosion Behavior And Mechanical Properties Of Ferrite-Pearlite Steel

The ferrite-pearlite steel is widely used in buildings,bridges,railways,vehicles,ships and various mechanical manufacturing industries because of its excellent mechanical properties,good welding and processing properties along with low cost.However,its poor corrosion resistance poses a great threat to the safety of crude oil transportation when it is used as the bottom plate in cargo oil tank(COT).The International Maritime Organization(IMO)stipulates that corrosion-resistant steel is the alternative of protective coating.Therefore,the research and development of corrosion-resistant high-strength ship plate steel has important economic value and practical significance,and meets the significant strategy needs of China.In the simulated bottom plate environment of COT,the ferrite was preferentially dissolved as anode phase,and cementite was gradually accumulated on the surface of steel as cathode phase,leading to the continuous increase of the cathode-anode area ratio,enhancing the micro-galvanic effect,and then accelerating the corrosion of ferrite.Therefore,the corrosion resistance of ferrite-pearlite steel became worse during long-term immersion.Obviously,the accumulation of cathode cementite was the main reason for accelerated micro-galvanic corrosion.According to the galvanic corrosion theory,the spheroidization process was employed to transform large lamellar cementite into fine granular cementite which was easy to fall off,thus reducing the accumulation of cementite,inhibiting the micro-galvanic effect and improving the corrosion resistance of steel during long-term immersion.However,the spheroidization process will inevitably lead to the change in the mechanical properties of steel,which were the main design index of steel as structural material.Therefore,how to improve the corrosion resistance of steel on the premise of ensuring its mechanical properties is an unavoidable problem in the research and development of corrosion-resistant high-strength steel.In this paper,three ferrite-pearlite steels(CS1 steel,CS2 steel and CS3 steel)with different carbon content(0.79 wt.%,0.44 wt.%and 0.15 wt.%)were selected as the research objects.The effect of spheroidization treatment and Cu microalloying on the corrosion behavior and mechanical properties of ferrite-pearlite steel in the simulated bottom plate environment of COT was studied by means of mechanical properties characterization,corrosion morphology characterization,weight loss experiment and electrochemical tests,and its action mechanism was clarified,providing ideas for microstructure design of corrosion-resistant ship plate steel.The results showed that the pearlite was an important strengthening structure in carbon steel.With the decrease of carbon content,the pearlite content decreased,and the strength of ferrite-pearlite steel decreased while the ductility increased.After spheroidization treatment,ferrite-pearlite microstructure transformed to tempered martensite microstructure,and both ferrite matrix phase and cementite secondary phase were changed.The strength of steel was the result of the interaction of ferrite phase and cementite phase.Because the effect of fine grain strengthening and dislocation strengthening of ferrite phase couldn't make up for the strength loss caused by the morphology transformation of cementite phase,the spheroidization treatment reduced the strength and increased the ductility of CS1 steel and CS2 steel with high carbon content.The cementite phase was the main factor affecting the mechanical properties of steel.For CS3 steel with low carbon content,the strength loss of steel caused by the morphology transformation of cementite phase was limited.Meanwhile,the strength of steel was improved by the fine grain strengthening and dislocation strengthening of ferrite phase.The ferrite phase was the main factor affecting the strength of the steel.Therefore,both the strength and ductility of CS3 steel after spheroidization treatment increased.At the initial stage of immersion,the corrosion behavior of steel mainly depended on its microstructure.The exposed surface area of granular cementite is larger than that of lamellar cementite with same carbon content,and its distribution is more uniform.Therefore,the cathode-anode area ratio in spheroidized steel and the amount of galvanic couples increased,thus enhancing the micro-galvanic effect and accelerating the corrosion of ferrite.In addition,the corrosion tendency of tempered ferrite grains with high dislocation density is high.Therefore,the corrosion rate of three spheroidized steel was accelerated to some extent compared with its as-received steel.During long-term immersion,macroscopic uniform corrosion happened on CS1 steel and CS3 steel,while CS2 steel suffered severe localized corrosion.With the extending of immersion time,the cementite accumulation on the surface of CS1 steel and CS2 steel with high carbon content increased,and the corrosion rate of both steels continuously increased.Due to the localized corrosion on CS2 steel,more cementite remained in corrosion pits,the corrosion rate of CS2 steel was faster.The cementite accumulation on the surface of CS3 steel with low carbon content was few,so its corrosion rate increased slightly with time.The spheroidization treatment mainly relieved the acceleration effect of micro-galvanic corrosion by reducing the cementite accumulation on steel surface,and then improved the corrosion resistance of steel during long-term immersion.Therefore,compared with as-received steel,the corrosion rate of CS1 spheroidized steel and CS2 spheroidized steel during long-term immersion was significantly reduced,while that of CS3 spheroidized steel during long-term immersion is higher.On this basis,the influence of spheroidization treatment and Cu microalloying on the corrosion resistance and mechanical properties of CS2 steel suffered the most serious corrosion was further studied.The results showed that the single factor of spheroidization treatment or Cu microalloying and their cooperation couldn't improve the mechanical properties of CS2 steel,and the morphology transformation of cementite phase was the main reason for the strength loss of steel.In the simulated bottom plate environment of COT,both spheroidization treatment and Cu microalloying could significantly inhibit the development of localized corrosion originating from pearlite region.During long-term immersion,spheroidization treatment and Cu microalloying notably reduced the accumulation of cementite on steel surface,and their cooperation was stronger than any single factor,which further weakened the micro-galvanic effect and improved the corrosion resistance of CS2 steel.Because Cu microalloying played a positive role in improving the corrosion resistance of ferrite-pearlite steel,we further discussed the influence of spheroidization treatment on the corrosion resistance and mechanical properties of Cu containing CS3 steel(AS3 steel).The results showed that the spheroidization treatment improved the strength of AS3 steel.The ferrite phase was the main factor affecting the mechanical properties of AS3 steel,and the fine grain strengthening and dislocation strengthening of ferrite grains were the main strengthening mechanism.Compared with AS3 steel,the initial corrosion rate of AS3 spheroidized steel was slightly faster.Due to the spheroidization treatment reduced the accumulation of cathode cementite,the micro-galvanic effect on AS3 spheroidized steel was weaker,and the corrosion rate was lower during long-term immersion.In summary,spheroidization treatment has achieved comprehensive optimization of mechanical properties and long-term corrosion resistance of AS3 steel.