The Research And Development Of High Performance Offshore Structure Steels

Based on the developing requirement of high quality offshore materials,the issues of high strength offshore steels,such as high yield ratio,poor weldability and unstable low temperature ductility of ultra-thick plate,the current thesis investigates the alloy design,structure and properties relationship of 460MPa offshore steel with low yield ratio and 690MPa low carbon offshore steel with high ductility.The processing route and technical principle of controlled multi-phase structure are studied in 460MPa ultra thick offshore steel plate to reduce the yield ratio,increase the low temperature ductility and improve the ductility of welding heat affected zone(HAZ).The improvement of low temperature ductility and weldability of 690MPa ultra thick offshore steel plate is studied by the application of low carbon micro-alloying and optimized quenching and tempering technique.The different combinations of low carbon high Mn,Ni,Cr and Mo alloying system are adopted in order to investigate the processing technique and structure control,relationship between structure and mechanical properties,and the relationship of the structure and properties in the welding HAZ for the 460MPa offshore steel and 690MPa low carbon offshore steel,which are applied in industrial production.The study of processing technique and structure control of the 460MPa ultra thick offshore steel plate shows that,addition of Cr,Ni,Mo and other alloys improve the austenite stability and influence the thermodynamic and kinetic parameters of ferrite and bainite transformation.Therefore,the proper control of Cr,Ni and Mo alloy is the key factors,which ensures the uniform multi-phase structure through the whole thickness.The research of the phase transformation kinetics illustrates that as for the ferrite phase transformation the peak temperature of the maximum transformation rate is decreased with the increased cooling speed,which is the phase transformation principle to achieve the ferrite structure on different locations of through the thickness.During the accelerated cooling,the reduction of the finishing temperature the transition from granular bainite to lath bainite can be promoted.Meanwhile,the size and amount of the martensite austenite(MA)islands in the core can be reduced and the grain boundary density between bainite laths can be improved.Hence the proper design of the chemical composition and precise control of the ferrite and bainite phase transformation are the key factor to achieve the multi-phase ultra-thick steel plates.The research of structure and mechanical properties for the ultra-thick 460MPa offshore steel plates shows that multi-phase of ferrite and bainite is achieved across the whole thickness,although the phase proportion is varying with different locations of the thickness.The percentage of ferrite continuously decreased from surface to core:60%ferrite on 1/4 thickness and 40%ferrite on 1/2 thickness.And the type of bainite transits from lath to granular bainite,and fine MA islands dispersed in the core.Through the intersected distribution,different ratio and form of soft and hard phases as well as the fine MA island the multi-phase steel possesses on one side low yield ratio,high uniform elongation and homogeneous strength through thickness and high crack initiation energy and extension energy were obtained,resulting in superior low temperature ductility than single phase bainitic steel.The submerged arc welding experiment of the invented steels demonstrates that the low temperature ductility of the subcritical HAZ can be significantly improved by multi-phase structure,comparing with the single phase bainitic steel.This is because the large angle grain boundary distribution between ferrite and bainite,the multi-phase steel provides more nucleation sites for the partial austenization in the subcritical HAZ.Fine recrystallized structure is achieved and coarse chain MA structure is avoided,therefore welding joint with better ductility than that of the single bainitic steel.The study of 690MPa low carbon offshore thick steel plate with in focus on the structure control and process technique.The study shows that during heating of quenching and tempering(QT)process different austenization temperature have significant influence on the original austenite grain size,variant selection and ductile-brittle transition temperature(DBTT).When austenization temperature is above 930 ?,DBTT and original austenite grain size accordance with Cottrell-Petch relationship.The original austenite can represent the effective grains.When quenching temperature is 880?,the crystal structure unit(effective grain),which is influenced by the block and packet interfaces in the original austenite,is in correspondence with DBTT.The critical austenization temperature(880?)influences remarkably the variant selection of the phase transformation structure.The higher the block and packet grain boundary density,the obviously lower the DBTT temperature.In the industrial circumstance by applying low carbon alloying and optimized QT heat treatment technique,the uniform lath bainite is achieved through the 100mm thickness section of the investigated steel.On the 1/4 thickness the BDT temperature is between-80?and-100?.The impact energy at-100?can be 150J.On the 1/2 thickness the DBTT temperature is between-60? and-80?.The impact energy at-80? can be 100J.The welding thermal simulation of 690MPa low carbon offshore steel plate with high ductility shows that,in the coarse grain HAZ of the single pass welding,structure type,MA island size and quantity distribution are the main control factors.The cooling rate is decreased with the increased input energy,the structure transforms from lath martensite to lath bainite,which is beneficial for the enhancement of the ductility.Lots of coarsening MA islands were obtained with the decreased cooling rate,while the coarse MA lead to lower ductility.Under the dual influence,the impact energy of the welding coarse grain HAZ is increased first,then decreased with the increase of input energy,which reaches maximum at 30KJ/cm.The multi pass submerged welding experiment shows that the ductility in the whole welding HAZ shows increase-decrease-increase of impact energy with the distance from the welding joint.The lowest impact energy is 112J without obvious brittle zone.In the coarse grain welding zone coarse MA structure does not appear.The grain size of the original austenite has no obvious coarsening,which has better low temperature ductility than the coarse grain zone.