Analysis Of Precipitation Strengthening Mechanism Of Microalloyed Steel

With the development of economy,we need more natural gas,which requires the development of pipeline steel.In some harsh environment,the development of marine steel is needed.Precipitation strengthening is an important strengthening method and has been one of the development directions of microalloyed steel.In this paper,the effects of isothermal temperature,isothermal time and multi-pass hot rolling on the microstructure,properties and precipitation phase strength of low carbon X90 pipeline steel and three kind marine steels with different Ni content were systematically studied by means of thermal simulation,heat treatment and several mechanical tests.The main contents are as follows:(1)The effects of different isothermal time on the structure and properties of 50%deformed pipeline steel and the precipitation on the structure strength were studied by thermal simulation test.The results show that with the increase of isothermal time,the hardness of the samples decreases firstly,then increases and decreases.The main reason is attributed to the comprehensive influence of nucleation and growth of precipitated phase particles and microstructure changes.However,martensite appears in the microstructure at 700 ?isothermal temperature.(2)The thermal simulation results of marine steel under different technological parameters show that different holding time affects microstructure and properties.Generally,with the increase of holding time,the hardness presents a trend that increases firstly and then decreases.Deformation has an effect on the microstructure and properties.Deformation of tested steel will result in distortion of microstructure,increase of dislocation density and change of phase composition.(3)After multi-pass hot rolling,the tested steel is composed of bainite,martensite,ferrite and M-A island phases.The higher the content of Ni is,the higher the content of hard phase M-A island is,and the higher the hardness is.The Ni element has a certain effect on the tensile properties of the tested steel.The test steel still has good impact toughness at low temperature of-80?.With the increase of Ni content,the difference of impact absorption energy caused by hot rolling in different directions is reduced.