Effect Of Duplex Microstructure Of Martensite/bainite On Mechanical Properties For Hot Stamped TTP Parts

With the increasing requirements of automotive industry for energy saving,emission reduction,light weighing and safety improving,hot stamping of ultra-high-strength steel has been increasingly applied.However,with the continuously increasing safety performance requires,the hot stamped parts with high strength and low elongation can not meet the requirements for low energy absorbing in a collision.So,the TTP(Tailored Tempering Properties)process is proposed,which allows for different mechanical properties distribution in regions of the same parts.For a TTP part,the microstructure of transition zone consists of multiple phases,and the fraction and morphology of each phase determines the mechanical property of the part.It is necessary to quantitatively study the relationship between microstructure and mechanical properties.Based on the above background,the following research is carried out:Firstly,the CCT(continuous cooling transformation)behavior of the experimental hot stamping steel under different cooling rates was studied.Based on cooling curves and expansion curves,the phase transformation temperatures were obtained.Therefore,the CCT curve of the steel was reviced with the regard of hardness and microstructure analysis.The results showed that at a uniform cooling rate of 10~30 °C/s,the duplex microstructure of martensite / bainite was obtained;wherein the cooling rate increased from air cooling(about 27 °C/s)to the critical cooling rate(about 30 °C/s),the fraction of martensite increased from 50% to 90%,and the hardness value increased significantly from 312 HV to 426 HV.The phase transformation varied when the cooling rate changes.At a cooling rate was below 10 °C/s,ferrite/pearlite appeared;and at a cooling rate was above 30 °C/s,fully martensitic microstructure was obtained.Martensite fraction increased with rapid increased cooling rate,which resulted in hardness increased.Secondly,according to the microstructure evolution,selected the cooling path to obtain TTP properties of parts by heated tools.And the influence of tool temperature and holding time on martensite/bainite transformation and mechanical properties for stamped parts were analyzed.The tool temperature is the primary factor,which enables phase transformation at specific temperature range.For same holding time of 30 s,the bainite fraction increased from 19.6% to 84.3% as the tool temperature increased from 300 to 450 °C.The corresponding tensile strength was continued to decrease from 1350 MPa to 861.1 MPa.However,the elongation was constant of about 6% at tool temperature of less than 400 °C.And the elongation increased to 9.6% at tool temperature reached 450 °C.Then,the RVE(Representative Volume Elements)model was established to analyze the relationship between duplex microstructure of martensite/bainite(which including fraction of each phase,distribution,morphology)and mechanical properties.The gray-scale co-occurrence matrix combined with k-means clustering method is used to identify the phase from SEM images to automatically build the RVE geometric model.It improves the modeling efficiency and reduces the subjective bias of human analysis.For the analyzed microstructures,the appropriate calculation size of the representative volume element is determined.With the SEM magnification of 2000 times(the calculation size of 50×40 μm),and element size of 0.3 μm,calculation accuracy can be guaranteed.It is found that during the deformation,bainite undertakes the most strain while martensite undertakes the most stress.Stress concentration in fine martensite region leads to crack initial,and large fraction of bainite improves elongation.The compressive stress generated by the martensitic transformation on the surrounding bainite is analyzed by the nanoindentation test.It is assume that bainite away from the martensite-bainite interface is free of stress.A compressive stress of-182.8 MPa in the vicinity of martensite is calculated by Suresh equation.It causes work hardening in adjacent bainite.Therefore,the RVE model considering the influence of the martensite-bainite interface is established.It is assumed that deformation starts in bainite areas with low yield stress(away from interface)and spreads with increasing plastic strain into areas with high yield stress(adjacent interface).The simulating accuracy is improved.The full-size TTP part of rear rails are stamped by divided tools with different temperature.The mechanical properties of the soft area,cooling area and transition area are analyzed.The RVE models are established with the identified SEM images of the transition area to analyze the properties.Under the heating zone temperature of 550 °C,and holding for 8 s,the elongation of soft area reaches 18%.The microstructure of soft area consists of ferrite/pearlite and uniformly distributed granular bainite.It is also found that the continuous distribution of martensite has better mechanical property than block distribution.In summary,the duplex microstructure of martensite/bainite is obtained by heating the tool temperature,and the studying the relationship between duplex microstructure and mechanical properties provides a basis for determination of TTP process parameters.