| The bainite structure has the characteristics of high strength,high toughness and high wear resistance,which makes bainite steel widely used in power,military,sea and other harsh service conditions.With regard to low-carbon bainitic steel,lower carbon content can improve the formability and good weldability of the material.Based on the alloying principle of metal materials,from the perspective of theoretical analysis and practical application,the design of carbide-free bainite as parent phase carrier was considered,and the Cu-rich phase was dispersed in the bainite ferrite matrix as the strengthening phase after precipitation.On the basis of the traditional bainite steel composition,the proportion of Si,Cr,Cu and other alloying elements is increased to inhibit carbide precipitation and shorten the incubation period of bainite transition.Based on the influence of alloying elements on the transformation dynamics,an ideal TTT kinetic curve for separating the bainite transition region from the pearlite transition region is designed to create conditions for the isothermal transformation of bainite structure.In summary,experimental steel（15Cr12Cu Si Mo Mn）was designed and prepared as the research material in this thesis.Thermal expansion method was used to measure and draw the transformation curve of supercooled austenite（CCT,TTT）,and the Arrhenius formula was used to establish a dynamic model of grain growth of primary austenite suitable for this material.The determination of austenitizing temperature is the primary parameter information for formulating a reasonable heat treatment process in the later stage.By making the best heat treatment process with good strength and toughness,the phase transformation product is a complex structure of bainitic ferrite and residual austenite,which is quasi-bainitic structure.In addition,by optimizing the aging heat treatment process,Cu elements precipitate Cu-rich phase from the matrix in the subsequent tempering aging stage,which plays a role of precipitation strengthening.A new steel grade with practical application value was obtained through the combination of bainite microstructure fine grain strengthening and second phase precipitation strengthening.At the same time,the coupling mechanism of bainite structure regulation and Cu-rich phase synergistic strengthening and toughening was discussed.In order to provide theoretical reference for the further study of Cu-containing bainitic precipitation steel and the development of related fields.The influence of isothermal quenching temperature and time on the microstructure properties of bainite can be attributed to the correlation between variant selection and properties,and the overall interface orientation difference between the large Angle boundary HAGBs（45～65°）and the small Angle boundary LAGBs（2～15°）shows a"bimodal"distribution.At present,there is little report on the correlation between variant selection and crystal orientation difference.Based on the K-S relationship betweenα-Fe andγ-Fe,the evolution law of variant selection and the orientation difference distribution law of special crystallographic surface during isothermal transformation are analyzed and characterized.The results show that the{100}polar diagram of experimental steel is consistent with the{100}standard polar diagram,and the orientation relationship between bainite variants and parent austenite is consistent with K-S relationship.When the isothermal quenching temperature increases,the bainite phase transition rate increases,and the variant selection ability decreases.At the same time,the interface content of HAGBs（45～65°）and LAGBs（2～15°）at large Angle grain boundaries decreased.When the isothermal quenching temperature decreases,the number of variants V1/V2,V1/V3 and V1/V5 in the same CP group increases,the bainite variants in the same CP group can increase the large Angle grain boundaries,and increase the overall interface orientation difference,cleavage plane orientation difference（100）and slip plane orientation difference（110）.By controlling the bainite variant type,the distribution range of the effective cleavage plane orientation difference（100）and slip plane orientation difference（110）is affected,and increasing the special crystal plane orientation difference range is conducive to optimizing the bainite structure and finally improving the mechanical properties.The influence of the second phase precipitation enhancement on the microstructure and properties of bainite can be attributed to the interaction between the precipitation habitus and dislocation of Cu-rich phase,that is,the cutting and bypassing mechanism is closely related to the critical size of the precipitated phase.The evolution law and characteristics of Cu-rich phase precipitation during the aging treatment of experimental steel were studied.The orientation relationship between B2 structure,9R（single crystal,twin）structure and bcc matrix is proved:（1（?）1）α-Fe∥（009）9R-Cu,[（?）10]fcc∥[（?）10]9R-Cu;（1（?）1）fcc∥（011）bcc,[（?）10]fcc∥[1（?）1]bcc.A metastable 9R-Cu crystal structure model is established.Based on phase field simulation and first principles,the total free energy,vacancy energy,binding energy and transition energy of the Cu-rich phase in the transition phase from BCC-Cu to 9R-Cu and 9R-Cu to the twin FCC-Cu+9R-Cu are calculated.By constructing a disordered solid solution structure model of Fe-Cu binary system,the transformation energy path is simulated and calculated,and the relationship between the evolution law of precipitated phase and the simulation calculation is analyzed and characterized.The precipitation tendency and sequence of Cu-rich phase are analyzed and explained from the perspective of energy.The quasi-bainitic structure+Cu-rich phase has been successfully obtained in non-carbide bainitic ferrite steel by the combination of isothermal quenching and aging heat treatment.The bainitic ferrite structure with higher dislocation density was obtained by the former through tissue control.The strength plastic volume of the experimental steel reached the maximum value of 21887MPa·%after 60min isothermal quenching at 320℃.Excellent comprehensive mechanical properties.The precipitation strengthening of the Cu-rich phase during the later aging heat treatment significantly improved the yield strength of the experimental steel,and at the same time made up for the strength reduction caused by the tempering and softening of the bainite ferrite matrix.Based on the interaction between the precipitation strengthening mechanism and the tissue tempering and softening mechanism,a model was established and calculated that the strengthening effect reached 378MPa and the softening effect reached 174MPa.Finally,the calculated increment of yield strength is 204MPa,and the difference between the yield strength at the early stage of aging and the peak of aging is 256MPa,which is basically consistent with the calculated value.The experimental steel has good mechanical properties such as yield strength of 1621MPa,elongation of 12.7%and strong plastic volume of 23202MPa·%,reaching the aging peak after 60min at 500℃. |
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