Based on the combination of materials science and mechanical engineering,hot press forming process of the vehicle high strength steels was analyzed. The hot forming process included:heating alloys rapidly to austenite microstructures,stamping and cooling timely,maintaining pressure and quenching. The results showed that most of austenite microstructure was changed into uniform martensite by the hot press forming while the samples were heated at 900 ℃ and quenched. The optimal tensile strength and yield strength were up to 1530 MPa and 1000 MPa,respectively,and the shape deformation reached about 23%. And springback defect did not happen in the samples.
CHANG Ying MENG Zhao-huan YING Liang LI Xiao-dong MA Ning HU Ping
Anisotropic Sm2Co17 flakes with high aspect ratio were prepared by magnetic field assisted high energy ball milling in the presence of heptane and oleic acid(OA).The thickness of flakes was only tens of nanometers.Coercivity of 3 kOe was achieved in the nano-flakes.Most interestingly,the magnetic crystalline anisotropy of Sm2Co17 flakes was improved compared to that of particles made by traditional ball milling.These anisotropic Sm2Co17 nano-flakes could be the building blocks for the future high-performance nano-composite permanent magnets with an enhanced energy product.
A new preparation method of Sm2Co17nanoflakes was investigated.Hard magnetic Sm2Co17nanoflakes with thickness of 20-100nm were obtained by milling in heptane and oleic acid under a magnetic field of 1.5Tfor0.5-20h.It was shown that higher anisotropic magnetic properties would be induced by the flake-shape anisotropy when the prepared Sm2Co17particles are milled with a magnetic field.The magnetic anisotropy of flakes after being aligned under the magnetic field of 1.5Tcould be further enhanced,and the value of(BH)m was 128kJ·m-3.Both anisotropyand properties are better than those of the nano-particles milled without a magnetic field.
A series of magnetic nanocomposites based on poly(ε-caprolactone) (PCL) and Fe3O4 nanoparticles were prepared using a facile in situ polymerization method. The chemical structures of the PCL/Fe3O4 nanocomposites were characterized by Fourier transform infrared (FTIR) spectroscopy. Results of wide-angle X-ray diffraction (WAXD) showed that the incorporation of the Fe3O4 nanoparticles did not affect the crystallization structure of the PCL. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and dispersion of the Fe3O4 nanoparticles within the as-synthesized nanocomposites. Results of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM) showed that the crystallization temperature was raised and the spherulites size decreased by the presence of Fe3O4 nanoparticles in the nanocomposites due to the heterogeneous nucleation effect. The thermal stability of the PCL was depressed by incorporation of Fe3O4 nanoparticles from thermogravimetric analysis (TGA). The superparamagnetic behavior of the PCL/Fe3O4 nanocomposites was testified by the superconducting quantum interference device (SQUID) magnetometer analysis. The obtained biodegradable nanocomposites will have a great potential in magnetic resonance imaging contrast and targeted drug delivery.
Use of hot-stamped high strength steels(HSHSS)not only reduces the vehicle weight,but also improves the crash safety,therefore more and more mentioned steels are used to produce automobile parts.However,there are several problems especially the low ductility and toughness,which have restricted the application of HSHSS in automobile body.Suitable process parameters are very crucial to improve strength and toughness.In order to study the effect of austenization temperature,soaking time and start deformation temperature on strength and toughness of boron steel 22MnB5,an L9(34)orthogonal experiment which was analyzed by means of comprehensive evaluation was carried out based on Kahn tear method to obtain the value of fracture toughness.The results indicate that the excellent formability,high strength and toughness of boron steel 22MnB5with 1.6mm in thickness are obtained when the austenization temperature is in the range of 920-950℃,the soaking time is 1min and the start deformation temperature is in the range of 650-700℃.The optimal parameters were used for typical hot stamping structural parts tests.Properties of samples such as tear strength,unit initiation energy and ratio of strength to toughness(RST)were improved by 10.91%,20.32%and 22.17%,respectively.Toughness was increased substantially on the basis of a small decrease of strength.
