Investigation On Microstructure And Property Control Of Advanced High Strnegth Steel For Engineering Machinery And Development Of The Key Production Technology

In the face of the current severe situation in the steel industry,it is urgent to study the micro-alloying composition design and TMCP(Therm-Mechanical Control Process)to achieve microstructure control in steel,so as to prepare low-cost?high-performance and high-strength steel to meet the demand of Materials for engineering machinery with high-strength,high-tenacity,light-weight,low-cost quality.This paper proposes two kinds of Advanced high strength steel for engineering machinery: one is Multiphase 960 mpa grade high strength steel used in the filed of Oversize truck crane,Concrete pump truck arm gently.Another kind is Ferritic series high strength steel(yield strength 500\600\700MPa),applied in Large and medium-sized automobile crane and foundation bed.we use lab study and Large production process test,toghther with Gleeble3800,Optical microscope(OM),Scanning electron microscopy(SEM),transmission electron microscopy(TEM),X-ray diffraction(XRD),Energy Dispersive Spectrometer(EDS),Mechanical properties test to study its Microstructural evolution,Performance characteristics and Strengthening mechanism.The further research was carried out on key production process and industrialization application of ferritic series high strength steel.The main results are listed as follows:1.Experiments were conducted focusing the effect of thermal deforming pressure,deformation rate,deformation temperature on microstructure of high strength steel based on application of Thermal simulation Gleeble3800.The results of the study indicate that the microstructure is composed of polygonal and acicular ferrite including dislocation structure,fine needle shaped and plate ferrite.With the increase of the thermal deformation pressure,the nucleation point and the phase change driving force increase,the dislocation density increases,the size of the polygonal ferrite decreases,especially the formation of about 150 nm thick,1-3?m long in the plate ferrite,and the microstructure is further refined,which is beneficial to the performance improvement.With the increase of the deformation rate,the fine necklace like ferrite grain appears in the microstructure,the orientation of martensitic lath tends to be consistent,the substructure in its interior disappears,the strip ferrite width increases,the internal needle like tissue disappears,the deformation temperature affects the deformation mechanism of the test steel,and the lower temperature is bainite.The microstructure is composed of massive,acicular ferrite and granular bainite,and a martensitic phase transition at a higher temperature.The microstructure is composed of martensitic region and polygonal ferrite formed by the interlaced arrangement of strip ferrite.2.The results of TMCP test showed that the microalloying design combined with TMCP technology was used to produce 960 MPa grade high strength steel.Its microstructure was composed of polygonal ferrite,lath ferrite,retained austenite or M-A Island,high density dislocation and precipitates.The dislocation,precipitation strengthening and strip like structure are the strengthening mechanisms of experimental steels.With the increase of finishing rolling temperature,the strip like structure and the polygon ferrite size decrease,the dislocation density increases,the strength and toughness of steel increase,and the ductility decreases.With the increase of coiling temperature,the formation of bainite ferrite,the appearance of precipitates,the decrease of dislocation density,the decrease of steel strength and the increase of toughness,the impact energy absorbed of the test steel decreases and then increases with the decrease of coiling temperature,but the increment is less than the drop value.The dislocation structure can improve the impact toughness,the dislocation wall has a greater degree of improvement in impact toughness than dislocation networks and dislocation tangles.When the finish rolling temperature and coiling temperature are 860oC and 290oC respectively,the yield strength is 1069 MPa,the elongation is 10%,the impact power is 50 J,and the comprehensive mechanical properties are best.3.The results of the tempering test show that the yield strength and the yield strength ratio of the test steel increase with the increase of tempering temperature,and the tensile strength and impact energy absorbed increase first and then decrease,and the total elongation has little change.The largest increment in yield strength after 220oC tempering was 70 MPa,after 620oC tempering the impact energy absorbed reduced 16.6J.When the tempering temperature rises,the fine precipitates size in the microstructure of the test steel appear first and increase with the tempering temperature,the dislocation density decreases,the strip like tissue is merged and grown,and the retained austenite thin film is decomposed,resulting in the corresponding mechanical properties.4.Comparative test analysis of Nb-Ti composite strengthening and single titanium strengthened ferrite body strengthening mechanism of high strength steel series.The results show that the mass fraction of precipitated phase below 18 nm in niobium and titanium reinforced high strength steel is over 35%,the average diameter of ferrite grain is 3.3?m,and the yield strength can be increased to 700 MPa by the precise matching of precipitation enhanced temperature control and ultrafine crystal technology.From the contrast of the strengthening mechanism of the single titanium microalloyed ST-TQ500 and niobium and titanium composite NT-TQ500,the solid solution strengthening is equivalent to the fine grain strengthening,the difference is less than 2MPa,and the precipitation strengthening effect of ST-TQ500 is about 88 MPa higher than that of NT-TQ500,which is related to the stronger precipitation strengthening with the second phase of the Ti nanometers in the coiling stage;the dislocation strengthening of ST-TQ 500 is more than NT-TQ 500 lower about 80 MPa,which is related to the more obvious inhibition of austenite recrystallization and high temperature softening behavior in the finishing stage of Nb steel.The average size of ferrite grains strengthened by single titanium is 3.8?m,and the average size of nanoscale precipitates in the crystal is 12.5nm,the volume fraction is 6.8%,and the yield strength can be increased to 600 MPa.5.The study of key production technology shows that the fluctuation range of high content Ti(0.1%)is 0.03% through the comprehensive control of oxygen in the whole process.Through precise control of the temperature in different stages,such as casting billet heating,finishing rolling and coiling,the technical difficult problem of the titanium microalloyed steel has been solved,and the intensive cooling technology is used to solve the technical problems.The multiphase 960 MPa grade high strength steel has been manufactured in a small scale and has good mechanical properties.It has accumulated experience for industrial application.