| In this paper,for the reason get better product cost,mechanical performance and applicability.a 1000 MPa strength grade Q&P steel with composition of 0.2% c-1.25%si-2.0% Mn was designed for the production process of "two-step"(fast cooling termination temperature is lower than partition temperature)based on continuous annealing production line,Different Q&P processes were simulated by continuous annealing simulator,and the tensile properties of the samples under different processes were tested at room temperature.The changes of microstructure and composition of the experimental steel under different process conditions were studied by optical microscope,SEM,TEM and XRD.The transformation law of austenite(obtained by critical zone treatment)to martensite during rapid cooling process of experimental steel was studied by means of thermal dilatometer and TEM.Based on this,a modified CCE model suitable for the experimental steel was proposed.According to the partitioning process,the diffusion law of carbon between martensite and austenite in the partitioning process is studied,and verified by experiments.On the basis of the above experimental research,the industrial trial production of Q&P980 was carried out,and the industrial trial production steel plate was evaluated.In order to determine the continuous annealing process window of 1000 MPa grade Q&P steel produced by this experimental steel,three different Q&P treatment processes were designed.The difference between them is that the "Q" process is different,which are: direct quenching after soaking in single phase zone,slow cooling and quenching after soaking in single phase zone,and quenching in critical zone.Then the same "P" process is carried out: 420℃ for 370 s.The results show that the microstructure of the steel is lath martensite + thin film austenite.The tensile strength ranges from 1182.4mpa to 1249.5MPa;it has a certain influence on the elongation,ranging from 3.75% to 7.5%.The maximum thickness of retained austenite is 0.085μm at 270 ℃.At this time,the highest elongation of the experimental steel is 7.5%,and the strength plastic product reaches the maximum of 8.87 GPa%.The microstructure of the steel is lath martensite + thin film austenite,proeutectoid ferrite,massive retained austenite and M-A island.When the final temperature of slow cooling is 680 ℃,the volume fraction of ferrite is about 25%,and the tensile strength reaches 994.3MPa.Under this condition,when the fast cooling temperature is 250℃,the elongation reaches the maximum value of 22.8%,the strength plastic product is22.6GPa%,and the final microstructure is 68% lath martensite + thin film austenite +23% ferrite + 8% M-A Island + massive retained austenite.In the case of critical zone quenching,when the soaking temperature is 790℃,the volume fraction of ferrite is about 22%,and the tensile strength reaches 1046.9 MPa.Under these conditions,when the fast cooling temperature reaches 220 ℃,the elongation reaches the maximum value of 14.5%,the strength plastic product is 15.2GPa%,and the final microstructure is 43% lath martensite + thin film austenite + 22% recrystallized ferrite+ 33% M-A Island + massive retained austenite.It can be seen from the comparison that according to the performance requirements of 1000 MPa grade Q &P steel,the suitable process is to adopt the Q&P process of soaking in single phase zone,then slow cooling and quenching.In order to accurately predict the best final temperature of rapid cooling for obtaining the maximum volume fraction of retained austenite after Q&P process treatment,austenite with different carbon content was obtained by heat preservation treatment in critical zone(760 ～ 850 ℃)The relationship between the kinetic constant parameter ‘a’ of martensitic transformation and the carbon content of austenite is obtained;It is confirmed that the isoactivity relationship of Fe-C binary system can still be used after adding Si and Mn alloys.On the basis of the above,a modified CCE model suitable for the experimental steel was obtained.It has higher accuracy than the classical CCE model in predicting the relationship between the volume fraction of retained austenite and the fast cooling temperature under three different Q&P process conditions.In the early stage of partitioning,the carbon in martensite diffuses rapidly into austenite.After about 1s partitioning time,the carbon content in martensite is close to equilibrium composition,which greatly prolongs the incubation period of cementite.No cementite precipitation is observed in martensite within 400 s partitioning time.At the initial stage of partitioning,the distribution of carbon in austenite is uneven and the austenite is unstable,which shows that the carbon content detected by XRD fluctuates greatly,the volume fraction of retained austenite is low and the dislocation density of Q&P steel is high after secondary quenching.When the partitioning time is longer than 20 s,the distribution of carbon in austenite tends to be uniform,and the purpose of stabilizing retained austenite is achieved.When the partition time is 100 s,the measured carbon concentration is close to the theoretical value.According to the related technical achievements of Q&P steel,the industrial trial production of Q&P980 steel plate was carried out in the continuous annealing production line of Handan Iron and Steel Co.,Ltd.the overall index of the steel plate met the technical requirements of CR550/980QP: tensile strength 1006-1020 MPa,yield strength 632-635 MPa,elongation 22.2%-22.6%,strength plastic product22.6-22.7GPa%.Compared with DP980 with the same strength grade,the elongation of Q&P980 increases greatly from 13.5% to more than 22%,the strength plastic product increases from 13.9GPa% to more than 22GPa%,and the welding effect of steel plate are good.The formability of Q&P980 is about the same as that of DP780,which has the potential of material substitution and lightweight optimization on DP780 parts. |
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