Annealing Process And Microstructure-Mechanical Properties Control Of High Quality Automobile Steels

To meet the needs of new energy,lightweight,long life cycle of automobile products,and to enhance the competitiveness of steel products,the development of high quality automobile steels should be accelerated.Cold-rolled steels after annealing are widely used in automobiles,especially the cold-rolled dual-phase steels and the hot-dipped galvanizing dual-phase steels.However,how to optimize the annealing processes of the dual-phase steels to improve the surface qualities and to promote the matching between the strength and plasticity of steel plates needs a further study.Additionally,to control the coating quality,such as the peeling mechanism of coating and anti-chalking performance,some other work is also needed.In this dissertation,the continuous cooling transformation of dual-phase steel was studied by thermo-mechanical simulation experiment,and the continuous cooling transformation curve was also established based on the microstructural analysis.The relationships among the continuous annealing process,microstructures and properties of the cold-rolled dual-phase steels and the hot-dip galvanizing dual-phase steels were studied with the continuous annealing simulator.The effects of the continuous annealing process on the structures and properties of the experimental steels and the corresponding mechanisms were discussed.The peeling mechanism of coating,anti-chalking performance and the phase structure of the automobile steel coating were systematically studied.The main work and results of this dissertation are as follows:1.The continuous cooling curve of the Fe-0.15C-1.78Mn-0.1Si-0.08Ti steel was measured,and the microstructural evolution laws under different continuous cooling conditions were specified.Furthermore,the critical cooling transformation temperatures Ac1(720?)and Ac3(870?),and the minimum critical cooling rate of martensite(>5?/s)were obtained.These results can provide theoretical basis for the continuous annealing processes of cold-rolled dual-phase steels and the hot-dip galvanizing dual-phase steels.2.The effects of annealing temperature,holding time and cooling rate on the microstructures and properties of hot-dip galvanizing dual-phase steel were studied.(i)When the annealing temperature is low(e.g.790?C or 820?),the martensite is mainly distributed in the grains,and the martensite islands mainly reveal banded microstructure distribution.A further increase of the annealing temperature to 850? facilitates the formation of twin martensite,and the banded microstructure distribution becomes less appreciable.Meanwhile,the volume fraction of the martensite is increased(about 19.6%).With a comprehensive analysis,the experimental steel has favorable combination properties when the annealing temperature is 850?:Rm=840 MPa;A=18%;Rp0.2/Rm=0.55.(?)At a certain annealing temperature(850?),the average grain size of the ferrite becomes larger and the volume fraction of the martensite is firstly increased and then decreased with extending the annealing time.When annealing time is 100 s,the martensite content is higher with a uniform distribution.With the extension of the annealing time,both the tensile strength and the yield strength increase first and then decrease,but the elongation shows little modification.(?)By changing the cooling rate,the volume fractions of the ferrite and martensite can be effectively controlled to obtain low yield strength and high tensile strength dual-phase steel.After annealing at 850?,the tensile strength increases slightly with the change of the cooling rate,which is attributed to the ferrite,martensite and their interaction.With the increase of the cooling rate,the yield strength of the experimental steel increases firstly and then decreases.3.The effects of heating rate,annealing temperature and slow cooling temperature on the microstructural evolution of cold-rolled dual-phase steel were clarified.(i)At a certain annealing temperature(810?),with increasing the heating rate,the volume fraction of the martensite islands is increased with a morphological change from long bar-like to equiaxed form.Meanwhile,their average size is decreased.When the heating rate is 70?/s,the martensite islands become finer and more scattered,the size of which is about 1?m.Furthermore,with increasing the heating rate,the yield strength and tensile strength of the experimental steel are improved to different degrees.However,the drawing rate decreases slightly.When the heating rate is relatively high(e.g.70?/s),the tensile strength of the experimental steel reaches 920 MPa,and the combination property is favorable,e.g.the product of strength and elongation reaches 16.4 GPa-%.(?)when the heating temperature increases from 780? to 810?,the volume fraction of the martensite increases from 29.5%to 37%.Furthermore,the size of the martensite laths also increases,and the grain boundaries are transformed from clear to vague.With a further increase of the heating temperature,the variation tendency of the martensite phase fraction is attenuated.In addition,both the tensile strength and elongation increase firstly and then decrease,but the elongation shows little modification.(?)With the decrease of slow cooling temperature,the volume fraction of orientated ferrite increases,while the volume fraction of martensite decreases.Furthermore,the orientated ferrite has a low solid solution of carbon,which reduces the yield strength and promotes the ductility of ferrite.4.The effects of the substrate surface roughness,crystallographic texture and coating phase structure on the properties of alloyed coatings were specified.(?)A crystallographic texture analysis on the DC51D+ZF substrate surface indicates that the {001}-{101} orientation is more favorable for the formation of 8 phase,while the{11}-{113}-{313} orientation for the formation of ? phase in the alloyed coating.The anti-chalking performance of the alloyed coating decreases with the increase of the Fe content and the thickness of the ? phase layer,but the stripping performance is improved with the increase of the thickness of the ? phase layer.The texture of the zinc layer itself and the forming ability of the substrate are also the reasons for the resistance to the powder.(?)The different anti-chalking properties of two SCG270 galvanized steel sheets with thicknesses of 0.7 mm and 1.6 mm were analyzed.Results show that:different Fe contents of the coating results in different phase structures,and different substrate roughness leads to different degrees of alloying.Roughness of the thick substrate is greater than that of the thin substrate.A higher roughness accelerates the degrees of alloying,and then gives rise to different Fe contents in the coating.The thickness of the alloyed coating increases as the substrate thickness increases.However,the thicker coating has a worse resistance to chalking.The unevenness of the coating thickness is also a factor that causes the coating to be different in chalking.