| At present,the high strength low alloy(HSLA)steel has been widely application prospect,but its high yield ratio has always been a problem.Especially in the strength level of 690 MPa above cut deal that the yield ratio is basically more than 0.9,even more than 0.95.The traditional technology is difficult to solve this problem,and often appear the strength or toughness of the problem.This paper takes the control of HSLA steel as research object,adopt the method of relaxation-precipitation+tempering,and studies the influence of process parameters on microstructure and properties during rolling and tempering,and then realizes the research object of yield ratio less than0.88,thus solving the deficiencies of the above problems.Applying the results of this research to the industrial production line,a high-strength steel with low yield ratio of Q690 E has been successfully realized,and the yield ratio reach 0.84.The main research results are as follows:(1)The traditional online quenching in austenitic area+direct tempering obtained single-phase microstructure,although it has high strength and toughness,but the problem of high yield ratio is difficult to solve.The strength are sharp fall and toughness increased by intercritical quenching and tempering of test steel.Although,the yield ratio is reduced compared with the direct tempering steel,but the decrease is not obvious,which cannot meet our requirement of yield ratio.(2)The yield strength of tested steel after rolled can more than 630 MPa and the yield ratio is low than 0.85 at different cooling temperature.The microstructure of the steel is bainitic ferrite and granular bainite.With the decrease of the cooling temperature,the volume fraction of ferrite gradually increases,the lath become wider and the number of M-A island increases,which deteriorates the low temperature impact property of the test steel.In addition,with the decrease of cooling temperature the he volume fraction of precipitate gradually increases,and the size distribution gradually become bipolar,that is,both small and large precipitate increase,but small precipitate predominate.Therefore,the average size of precipitate decreases,and the precipitate strengthening effect was enhanced.Effective pinning of the dislocation due to these fine precipitate is precipitated on the dislocation line.(3)The softening effect of tempering increase significantly the toughness of the test steel after tempering at different temperatures.But the yield strength does not increase significantly,and the tensile strength decreases significantly,lead to an increase in the yield ratio.In this process,the bainite ferrite lath coarsened with tempering temperature increasing.The M-A island was gradually decomposed a large amount of carbides which strengthened the microstructure.But its also reduce the effect of obstruct dislocation movement and prevent crack growth.After tempering at 350℃,the microstructure of the steel has good tempering stability.The many fine precipitates pinning the high-density entangled dislocation by deformation is retained,and some new phases are nucleated out.But due to the low tempering temperature the precipitates precipitated out less and the growth rate of precipitates is slow.At this time,the precipitated speed more than the growth rate of precipitates.so the average size of precipitates decreases compared with the rolled state,which further strengthening the precipitation reinforcement.(4)Applying the results of this research to actual industrial production,the low yield ratio Q690 E grade steel plate was developed.The yield strength of the trial steel reached 713 MPa,the tensile strength reached 850 MPa,the elongation after fracture reached 15%,and the impact energy value at-40°C reached 232 J.The yield ratio drops below 0.85,which has good comprehensive performance and use safety. |
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