| Direct quenching was a short process to achieve superior performance of steel plate,which could take place of off-line quenching.But it’s a difficult problem to control the microstructure,properties and shape of high strength steel plate to match the performance of steel in hot working process with the production technology and equipment capacity.The effect of hot processing on the microstructure、properties and the flatness of 960 MPa high strength medium thickness steel plate has been studied in this paper,and it was successfully trial-produced in a 3500 mm production line.The change of microstructure and properties of high strength steel has been studied by the deformation of Gleeble 3800 thermal simulation,hot rolling and tempering tests,scanning electron microscopy(SEM),electron backscattering diffraction(EBSD),transmission electron microscopy(TEM)and mechanical properties tests.The effects of cooling capacity of direct quenching equipment and the effects of air cooling time in laminar flow zone on temperature uniformity in thickness direction of steel plate and the changes of force-energy parameters and flatness of high strength steel with different yield strength during the process of straightening with small amount of bending have been studied by finite element methods.The results are as follows.(1)During the continuous cooling process,the microstructure of the high strength steel consists of bainite and martensite when cooled at 5℃/s,and the hardness is 383 HV;When the cooling rate is ≥10℃/s,the microstructure is martensite and the hardness is ≥403 HV.After austenite deformation,the hardness is≥412.5 HV when the cooling rate is greater than or equal to 10℃/s.In order to obtain the full martensitic structure,the cooling rate during quenching should be≥10℃/s.(2)The heat transfer process of the cooling equipment has been simulated and the results show that the cooling capacity of the on-line quenching equipment has a linear relationship with the water flow density of the header.The cooling capacity of DQ and ACC zones is 5904-7255 W/(m2·℃)and 3012-3369 W/(m2·℃),respectively,within the range of common water flow density of header,which can meet the cooling requirements of 960 MPa grade high strength steel for direct quenching with a maximum thickness of 40 mm.Air cooling in ACC can effectively improve the temperature and hardness uniformity in the thickness direction of high strength steel.After interval air cooling for 3.2 s in ACC,the temperature difference between surface and core of steel plate can be reduced by 88.94%and the cooling rate of core can be reduced by 80.81%.The hardness difference in thickness direction can be reduced from 23 HV to 5 HV.(3)The effects of air cooling relaxation time and quenching termination temperature on microstructure and properties of high strength steel were investigated by rolling and direct quenching after air cooling relaxation.When the relaxation time increases from 15 s to 135 s,the tensile strength decreases from 1450 MPa to 1240 MPa,the elongation increases from 9%to 13%,and the impact energy at-20℃ increases from 33 J to 148 J.With the increase of relaxation time the width of the lath increased and the morphology of dislocation changed gradually from entanglement to discrete and cellular,and the dislocation density decreased,the proportion and the size of the precipitates and the proportion of big Angle grain boundary increased,the content of retained austenite changed were the reasons that the change of the mechanical properties.The dislocation strengthening and the fine grain strengthening were the main strengthening mechanism of the high strength steel,and the reduction of dislocation density caused by relaxation prevents crack nucleation,the increase of large Angle grain boundaries,and the TRIP effect of retained austenite changes the direction of crack propagation were the main reason of toughening.In order to improve the toughness,the relaxation time could be controlled between 65 s and 135 s.When the quenching termination temperature is between 100℃ and 300℃,the microstructure is martensite lath.The tensile strength is between 1390 MPa and 1450 MPa.In order to ensure that the mechanical properties of high strength steel meet the requirements of 960 MPa grade high strength steel,the quenching termination temperature should be controlled below 300℃.(4)For the high strength steel with air cooling relaxation time of 15 s,when tempered between 500℃ and 720℃,the yield strength was 1190-780 MPa and the tensile strength was 1220-840 MPa,the elongation was 10%-12.5%,and the iMPact toughness at-20℃ was 84-108 J.when the relaxation time extended to 135 s,and the steel was tempered between 590-650℃,the yield strength was 1051－1090 MPa and the tensile strength was 1100-1109 MPa,the elongation was 14.2%17.8%,and the impact toughness at-20℃ was 165-240 J.The tempering window of 960 MPa grade high strength steel produced by relaxation+direct quenching+tempering process is 590-650℃.With the increase of tempering temperature,the width of martensitic lath,precipitate proportion and size increased,while the dislocation density was the opposite.Precipitation strengthening made up for the loss of dislocation strengthening and stabilized the strength;The decrease of dislocation density prevents the nucleation of crack,and the increase of large Angle grain boundary changes the direction of crack propagation and improves toughness.(5)During the process of straightening with small amount of bending,the stress and total straightening force of directly quenched steel plate with different yield strength decreased gradually with the increase of straightening pass,and the flatness decreased gradually with the increase of straightening pass.The strain in thickness direction increased with the accumulation of straightening passes.When the accumulated strain at 40%of the thickness from the steel plate surface exceeded the elastic limit strain,the steel plate can obtain good flatness.For the direct quenched steel plate with original flatness of 50 mm/2m and thickness of 20 mm,the flatness of the steel plate can be less than 3.5 mm/2m under the yield strength of less than 1100 MPa. |
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