Research On Microstructure Regulation And Surface Strengthening Mechanism Of Ultra-fine High Strength Bainitic Steels

Ultra-fine high strength bainitic steels with high strength and excellent toughness are the typical representative of a new generation of advanced high strength steels.With the development tendency of ultra-high strength steels,higher requirements are put forward for microstructure refinement and property optimization of bainitic steels.In recent years,in order to obtain ultrafine or nanoscale lath bainite and excellent mechanical properties,effects of various processing methods,i.e.,adding alloying elements,multi-step processing route,deformation and surface coating,on the microstructure and properties of bainitic steels,attract extensive attentions.How to use low alloying composition design and effective processing route to fabricate steels with high strength and good plasticity is of great significance.In the present study,ultra-fine Fe-C-Mn-Si bainitic steels are mainly used as the research objectives.Effects of important alloying element Si and conventional heat treatment processes（austempering time and prior ferrite formation）on the kinetics of bainite transformation and mechanical properties of high strength bainitic steels were investigated;the influences of ausforming and applied stress on microstructure evolution and mechanical behavior of a bainitic steel were revealed;the effect mechanism of the integrated treatment of austempering and Q&P process（AQ&P）on refining microstructure and improving properties of a bainitic steel was discussed;the strengthening mechanism of the integrated processing route of boriding and austempering to improve the surface properties of bainitic steels is clarified by means of thermal simulator,scanning electron microscope,transmission electron microscope and tensile testing machine,etc.The following conclusions are drawn:（1）Different from the previous results,the present work finds that the effect of prior ferrite on bainite transformation depends on the competition between the increase of transformation rate at the interface and the stability of residual austenite.The formation of ferrite accelerates the rate of initial bainite transformation and increases the amount of retained austenite,but reduces the amount of transformed bainite.（2）AQ&P process can effectively refine the microstructure,especially the blocky M/A island in austempering process and the large-sized tempered martensite in Q&P process.Compared with austempering process or Q&P process,AQ&P process can obtain more retained austenite with higher carbon content,so as to optimize the comprehensive properties of ultra-high strength bainite steel.In addition,the mechanical properties of a medium carbon bainite steel can be further improved by adjusting the austempering time in AQ&P process.（3）A new integrated process of boriding and austempering is utilized to significantly improve the surface property of an ultra-fine high-strength bainite steel.After the integrated treatments,the surface hardness of the steel is more than 1500 HV,and the corrosion resistance and wear resistance are significantly enhanced.The bainite matrix and the produced layer are almost not affected by each other,and the bainitic substrate and the as-prepared layer show good adhesion strength.The borided layer is poor in Si and C atoms,and Fe₂B and Fe B phases tend to grow at（100）plane.Compared with bainite matrix,the surface friction coefficient of the steel is reduced by 20%and the sliding wear property is increased by 2.2 times after the integrated treatment.（4）Ausforming at a low temperature can not only accelerate the isothermal bainitic transformation and refine the bainite phase,but also increase the volume fraction of retained austenite and its stability,so as to optimize the comprehensive properties of an ultra-high strength bainite steel,fabricate a 0.42C-2.00Si-2.80Mn bainite steel with the strength of 1733 MPa and the elongation of 15.7%.Ausforming at a high temperature promotes ferrite transformation,inhibits bainite transformation,but refines lath martensite during continuous cooling process.With increasing the strain,the retained austenite decreases first and then increases,while the average dislocation density increases first and then decreases.（5）Effect of Si content on bainitic kinetics and mechanical properties.With increasing Si content,the kinetics parameter b generally decreases while n shows an opposite tendency,indicating that the bainitic transformation amount and transformation rate decrease with increasing Si content.Increasing Si content delays bainitic transformation while it exhibits little influence on transformation finishing time at a lower transformation temperature.Increasing Si content has little influence on incubation time of bainite transformation while it extends the transformation finishing time obviously at a higher transformation temperature.With high Si content,the differences of incubation period and completion time of bainite transformation become smaller due to different transformation temperature.The increase of Si content is beneficial to improve strength and elongation.（6）Prolonging the austempering time promotes the further redistribution of carbon atoms,increases the chemical stability of untransformed austenite,hinders the formation of blocky M/A islands,and ultimately increases the volume fraction of retained austenite and its carbon content in the microstructure,contributing to the improvement of elongation.（7）Effect of compressive stress on martensite start temperature（M_s）of a0.42C-2.00Si-2.80Mn bainitic steel.The maximum mechanical driving force of 60.2J/mol is provided by applying stress of 70 MPa,which improves the M_sabout 10°C,refines the martensitic lath,and increases the tensile strength from 1470 to 2170 MPa.