Study On Mechanical And Corrosion Behaviour Of Zr Microalloyed Steel Used In Typical Ocean Engineering

With the depletion of fossil energy on land,countries around the world have turned their attention to the development of marine resources.The development of marine resources in China started late,but after many years of development,great progress has been made.The report of the 20th Party Congress proposed the strategy of developing the marine economy,protecting the marine ecological environment and strengthening the construction of a strong marine state,which has put forward higher requirements for energy saving and emission reduction in marine development.Steel is used in a large number of applications in the development of marine resources,especially in marine platforms and subsea oil and gas transportation pipelines.The development of steel materials with excellent mechanical and corrosion resistance properties is an important way to promote energy and material conservation in the industry and to help reach the peak of carbon,carbon neutral.To this end,this project uses the current emerging Zr micro-alloying technology for high-strength low-alloy martensitic steel（HSLA）and 304 austenitic stainless steel used in offshore platforms and oil and gas pipelines,to improve the organisation,mechanical properties and corrosion resistance to improve their serviceability.The main findings are summarised below.The optimization of the organization of the two offshore steels by Zr microalloying is mainly reflected in the precipitation phase,grain size,inclusions morphology,size and distribution.On the one hand,the addition of Zr elements has led to a reduction of grain size by 1/2 and 1/3,and inclusions by nearly 3/2 and 1/3,respectively,with a more uniform tissue distribution for the two alloyed steels.On the other hand,the inclusions morphology also changed from Mn-rich long spindle type to Zr-containing spherical composite inclusions,which had an impact on the organization,mechanical properties and corrosion resistance of the alloy steels.The addition of Zr elements results in an improvement in the hardness of HSLA martensitic alloy steels and the impact toughness of 304 austenitic stainless steels.When the Zr content is0.2 wt%,compared to HSLA steel without Zr,its hardness increases from 227 HV to 247 HV,which is related to the diffuse distribution of hard Zr C in the matrix;while the impact work of304 austenitic stainless steel is 241 J,an increase of 43 J.This is mainly due to the generation of Zr O₂ to strengthen the solid solution strengthening effect,the refinement of grain size to play a fine grain The impact toughness of Zr-304 austenitic stainless steel was enhanced by the combined effect.In terms of tensile properties,the addition of moderate amounts of Zr elements is beneficial to the HSLA martensitic alloy steel tensile strength and plasticity.With the increase of Zr content alloy steel tensile strength and plasticity show the law of first increase and then decrease,when the Zr content of 0.04 wt%,the alloy steel tensile strength and elongation of the highest,respectively 780 MPa and 26%,which is mainly related to the refinement of inclusions.The Zr microalloying facilitates the improvement of the corrosion resistance of the two offshore steels,with the overall best corrosion resistance of HSLA martensitic alloyed steel and304 austenitic stainless steel at 0.04 wt%and 0.2 wt%Zr content respectively.In the pre-corrosion stage,the corrosion resistance of the two marine engineering steels is better because the addition of Zr elements refines the size of the inclusions,resulting in less intrusion of Cl^-into the matrix.In the middle and late stages of corrosion for HSLA martensitic alloy steels,the addition of Zr elements promotes the formation of Fe-Zr-O spinel oxides,which protect the substrate by covering the film and the substrate.For 304 austenitic stainless steel,on the one hand,Zr elements solid solution in the matrix to promote the generation of Zr O₂,combined with Mn S-rich inclusions and precipitation,consuming inclusions at the content of Mn elements,delaying the occurrence of pitting corrosion.On the other hand,thickening the thickness of the passivation film layer,generating high-valent Cr oxides,improving the stability of the film layer,the combined effect of improving the pitting resistance of 304 stainless steel.In summary,the addition of the right amount of Zr elements will help to improve the structure of the two types of marine engineering steel,improve its mechanical properties and corrosion resistance.This will have the effect of extending the service time of marine engineering steels and promoting the development of marine resources.