Study On The Oxidation Behavior Under Hot Rolling And The Corrosion Performance Of The Marine Steel

Adding copper,phosphorus,nickel,chromium,antimony,and rare-earth elements to steel can improve its corrosion resistance.Among the available methods,nickel alloying is an efficient technique for enhancing the anti-corrosion performance of steel in the marine environment.However,three problems limit the application and development of marine steel:steel corrosion in the marine environment,surface quality defects caused by high-temperature oxidation of Ni-containing steel,and non-uniform performance across the thickness of medium and heavy plates.To solve these three problems,Cr,Ni,Cu,and P were added to marine steel.Additionally,a corrosionresistant steel composition was designed and a rolling test was performed experimentally.The corrosion rules in different ocean environments were investigated,the high-temperature oxidation behavior of Ni-containing steel in different environments at high temperatures was examined,and the deformational behavior of Ni-containing steel under different controlled-rolling conditions was analysed.The main conclusions obtained were as follows:(1)Cr0.3%-Ni0.3%-Cu0.3%and Cr0.3%-Cu0.3%-P0.1%low alloy-corrosionresistant steels were investigated and trial-produced industrially.Compared with plain carbon steel,the abovementioned steels exhibited relative corrosion rates of less than 67.67%and 61.04%after 72-h and 120-h alternate immersion tests,respectively;(2)Cr0.3%-Cu0.3%-P0.1%corrosion-resistant steel bars measuring φ20 mm,φ22 mm,and φ25 mm were trial-produced industrially at the finishing temperature of 880℃-890℃.After a 72-h alternate immersion test,their relative corrosion rates were 87.07%-90.93%those of corrosion-resistant steel bars trial-produced industrially at 960℃-990℃;(3)The oxidation behavior of steel bars with different Ni contents in an atmosphere containing 5%-20%O2 at 900℃-1200℃ was investigated.The oxidation kinetics rules and the evolution of the structure of the Ni-steel oxidation layer were analysed.Oxidation kinetics equations were formulated for Ni-containing steel;(4)During the high-temperature oxidation of Ni-containing steel,Ni-rich oxides were formed on the substrate surface,which hindered the diffusion of oxygen and metal ions,thus improving the oxidation resistance of the steel.Meanwhile,these Ni-rich oxides were dispersedly distributed,which promoted the binding of the oxide layer and matrix,thus rendering it more difficult for the oxide layer to peel off;(5)The gradient-temperature rolling process effectively increased the central strain of the plate.Under 58.3%deformation after five-pass rolling,compared with the result of uniform-temperature rolling,the central equivalent plastic strain of the heavy plate formed by continuous gradient-temperature rolling and first-pass gradienttemperature rolling increased by 5.40%and 1.04%,respectively.Interpass cooling was performed to increase the temperature difference between the surface and center of the plate,which facilitated the penetration of deformation into the center;(6)Compared with uniform-temperature rolling,gradient-temperature rolling resulted in more refined grains on the surface of and inside the plate,a more uniform distribution of the microstructure and mechanical properties in the thickness direction,and a relative corrosion rate of 98.13%.In summary,when exposed to a simulated coastal-industrial atmosphere,a highsalinity/high-humidity environment,and high salinity-dry/wet alternating conditions,the corrosion-resistant steels investigated in this study exhibited relative corrosion rates of 35.29%(5Ni),57.17%(Ni1Cr3),and 15.47%(Ni1Cr7)respectively,as compared to the plain carbon steel.The high-temperature oxidation characteristics and the formation mechanism of oxide scales for marine steels with different Ni contents were revealed.Additionally,an oxidation-prediction model for Ni-containing marine steels was established.The regulation mechanism of deformation penetration and the microstructure evolution law of the continuous gradient-temperature rolling process of marine steels were clarified.Compared with uniform-temperature rolling,continuous gradient-temperature rolling resulted in a higher central equivalent plastic strain by 5.40%,more refined surface and internal grains.