| With the increase of energy demand and development of petroleum chemical industry in China,the requirement of Ni-containing cryogenic steels which is used as liquefied gas storage tank and liquefied gas ships structural material is rapidly increased.The liquefied gases are highly flammable and their temperatures are very low.Therefore Ni-containing cryogenic steels are required to possess excellent toughness,high strength,and good welding ability so as to ensure the safty.Along with the continuous increase in the size of liquefied gas storage tanks,higher cryogenic toughness and strength has been required for Ni-containing cryogenic steels.The Ni-containing cryogenic steels have developed relatively late in China and there are still many problems in regard of industrial production need to be resolved.In this paper,high temperature deformation behavior,phase transformation law,influence of QT(Quenching and tempering)heat treatment on microstructure evolution and mechanical properties of Ni-containing cryogenic steels are systematic investagated.The effect of Ni addition on the microstructure and mechanical properties of Ni-containing cryogenic steels is also analyzed.Based on these work,a new process(TMCP-UFC-LT),which could increase the volume fraction of reversed austenite lower Ni content steels,is developed to reduce the Ni addition.The main works of the present paper are as follows:(1)The high temperature deformation behaviors of Ni-containing cryogenic steels were investigated by single hit compression experiment,and the effect of Ni content on dynamic recrystallization behavior were also analyzed.The dynamic recrystallization constitutive equation was obtained through regression analysis on the experimental data.The influence factors of deformation resistance for Ni-containing cryogenic steels were analyzed,and deformation resistance models were established.The static recrystallization behaviors of Ni-containing cryogenic steels were investigated by double hit compression tests,and the results show that static recrystallization softening rate increased with the increase of deformation temperature and interval time.The static recrystallization softening rate of 3.5%Ni steel was larger than that of 5%Ni steel under the same deformation temperature and interval time condition,indicating that the addition of Ni could decrease the static recrystallization softening rate and retard the occurrence of static recrystallization.(2)The austenite phase transformation rule during continuous cooling process of four Ni-containing cryogenic steels and the effect of Ni content on phase transformation behavior were investigated.The results show that the microstructures gradually transform from polygon ferrite structure into lath martensite structure with increasing cooling rate.The phase transformation starting temperature and the average grain size of polygon ferrite decreased with the increase of Ni content.The addition of Ni can significant reduce polygon ferrite transformation field,expand martensite transformation field,and decrease the critical cooling rate for martensite formation.(3)The evolutions of microstructure and mechanical properties after quenching and tempering for Ni-containing cryogenic steels with different Ni contents were investigated.The results showed that after QT treatment,the Ni-containing cryogenic steels were composed of tempered martensite and a certain conent of reversed austenite.The volume fraction of reversed austenite has been increased from 0%up to 6.3%when the Ni content is increased from 3.5%to 9%.The Charpy impact tests indicated that the cryogenic toughness was markedly improved with the increase of Ni content,which can be correlated to the increase of reversed austenite amount.The relationship between the reversed austenite and the impact toughness at-196? follows:EV=10+11.1V?+3V?'2.The main contribution of reversed austenite on toughness lies in(1)the elimination of cementite precipitations by reversed austenite to improve the plastic deformation capacity of matrix,and(2)the hindering of crack propagation by reversed austenite through its plastic deformation.(4)This paper proposes a new process(TMCP-UFC-LT)in order to increase the cryogenic toughness of lower Ni content steels by controlling the volume fraction and morphology of reversed austenite.The microstructure of three Ni-containing cryogenic steels treated by TMCP-UFC-LT treatment consisted of tempered martensite,intercritical ferrite and a certain amount of reversed austenite.The reversed austenite has two types of morphologies:one type is acicular reversed austenite which forms along the lath boundaries;another type is block reversed austenite which mainly forms at prior austenite gain boundaries and martensite packet boundaries.With the increasing tempering temperature,the volume fraction of reversed austenite increased,the width of acicular reversed austenite increased and its morphology gradually transformed into short rod-like or irregular blocky.The stability of reversed austenite decreased with the increase of tempering temperature because of the larger size of reversed austenite and lower content of C,Mn,and Ni elements in reversed austenite.The unstable reversed austenite would retransform to fresh martensite after cooling to cryogenic temperature,resulting in the formation of cracks due to stress concentration in the phase interface,which led to the deterioration of cryogenic toughness.(5)The cryogenic toughness of TMCP-UFC-LT samples was significantly higher than that of QT samples.In 3.5%Ni steel,the TMCP-UFC-LT sample's impact energy at-135 ?was 237 J,which was 115 J more than QT sample.In 5%Ni steel,the TMCP-UFC-LT sample's impact energy at-196 ? was 185 J,which was 157 J more than QT sample.In 7%Ni steel,the TMCP-UFC-LT sample's impact energy at-196 ? was 222 J,which was 113 J more than QT sample.Compared to QLT sample,TMCP-UFC-LT sample possess higher strength.Furthermore,TMCP-UFC-LT process reduced the production cost by deleting conventional reheating and quenching(RQ)stages.It can be seen that the TMCP-UFC-LT process is an effective method to improve the strength and toughness properties of Ni-containing cryogenic steels.(6)Oscillographic impact results show that the crack propagation energy of TMCP-UFC-LT steel significantly increased compared to conventional QT steel,indicating that the microstructure of TMCP-UFC-LT steel exhibited high resistance to the crack propagation.The superior cryogenic toughness of TMCP-UFC-LT is mainly attributed to the refinement of effective grain size and the formation of fine and stable acicular reversed austenite.Compared to blocky reversed austenite,the distribution of acicular reversed austenite was more dispersed,which can hinder the crack propagation more efficiently,leading to improvement of crack propagation energy,resulting in excellent cryogenic toughness.Based on the fundamental research results,Ni-containing cryogenic steels plates have been successfully manufactured in NISCO with its 3500 mm steckel mill,which possess excellent cryogenic temperature toughness and high strength. |
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