Study On Strengthening And Toughening Of X100 High Steel Grade Seamless Steel Tube For Pile Legs

The development of offshore oil and gas resources directly drives the development of offshore drilling platforms in technology and market.Deep-sea drilling platforms are required to have high safety and reliability when serving in extremely harsh environments.Therefore,strict requirements are imposed on the mechanical properties and service performance of pile-leg support pipes of offshore platforms.It must have higher strength and toughness and a certain degree of plastic deformation,that is,a lower yield ratio.At the same time,manufacturers also demand to reduce construction costs to ensure their economic benefits.This paper takes X100 high steel grade seamless steel tube for pile legs as the research object.With the yield-strength ratio of 0.85 as the research target,experiments were carried out from the composition optimization design and quenching and tempering treatment process.According to the mechanical properties of X100 high steel grade leg seamless steel pipe,five alloy composition systems were designed.In the initial design,the content of C,Mn,Cr and other elements is relatively high,and C and Mn are inexpensive strengthening elements,which can effectively improve the hardenability of steel.Cr and Ni can further improve the hardenability,promote the transformation of phase transformation,form a uniformly distributed alloy cementite,and improve the strength by solid solution strengthening,and Ni can also improve the corrosion resistance.Nb,V and Ti can form strong carbides and disperse after heat treatment to improve strength and low temperature toughness,then carried out laboratory smelting tests to obtain experimental samples.The subject used metallographic microscope,hardness analyzer,scanning electron microscope,transmission electron microscope,tensile and impact performance test to conduct microstructure observation and mechanical properties test of the heat-treated test steel,analyzed the microstructure and adjusted the alloying element content according to the performance index requirements,and finally determined the C-Mn-V composition system applicable to the X100 leg tube.In this paper,low-cost C-Mn-V component system test steel was used for quenching and tempering treatment,and the effects of single quenching + tempering and secondary quenching + tempering heat treatment on microstructure and properties were investigated.The quenching and tempering process of X100 seamless steel pipe for pile leg was obtained by adjusting the yield ratio while the mechanical properties meet the requirements.The microstructure of test steel is homogeneous and fine tempered sorbite after quenching at 910-930? and tempering at 690?.The strength of the tested steel meets the requirements of X100 grade,the yield-strength ratio reaches 0.92 level,and the impact toughness at low temperature is also ideal.This composition and process are finally determined to be the ideal composition and process for the economical X100 seamless steel tube for legs.The secondary quenching and tempering process research found that the change of the secondary quenching temperature had little effect on the strength of the test steel,but the strength gradually decreased with the increase of the tempering temperature.C-Mn-V component system test steel can meet the strength requirements of X90-stage seamless pipe after being treated by "900? quenching + 780? quenching + 580? tempering" process.At the same time,the process significantly reduces the yield strength of the test steel to below 0.90,and the low temperature impact toughness of-50°C reaches about 100 J.The process is an effective way to improve the toughness and plasticity of the material,and provides effective technical support for the production of high-grade low-reinforcement ratio seamless pipe.