Thermo-mechanical process and continuous cooling process were carried out on 20CrNi2MoV steel. Three cooling rates were implemented to optimize the mechanical properties. The microstructure evolution, precipitation behavior, and strengthening mechanisms were systematically investigated, and the fracture mechanisms were analyzed via combination of impact fracture morphologies and deflection-load curves. The experimental results indicate that the transformed microstructure of experimental steel is all complex consisting of granular bainitic ferrite and bainitic ferrite with dispersed martensite/austenite （M/A） constituents in the matrix at cooling rates of 13, 21, and 29 ℃/s. When the cooling rate increases, the grain of the steel is obviously refined. The sizes of the bainitic ferrite are 5.8, 4.7, and 3.1 μm under cooling rates of 13, 21, and 29 ℃/s, respectively. The refinement of the bainitic ferrite plays a dominant role in strength increasing and also contributes to high crack propagation energy. However, the morphologies of M/A constituents obtained under different cooling rates contribute to different crack initiation energies and then affect the impact property.
Jian ZhangChang-Sheng LiBin-Zhou LiZhen-Xing LiXue-Dong Pang
In order to optimize the production process, improve the production efficiency and accelerate the development and application of the domestic dual-phase steel, the effects of heat treatment process parameters on microstructure and mechanical properties of DP440 cold rolled strip were studied by the CAS-300 simulated continuous annealing equipment. When the heating rate increased from 5 to 100 ℃/s, both the tensile strength and yield strength increased because of the decreased grain size. When the intercritical annealing temperature increased from 780 to 850 ℃, the martensite content decreased so that the tensile strength decreased first, then increased, and the yield strength increased. When the rapid cooling rate increased from 25 to 100 ℃/s, because the martensite content increased, the tensile strength increased, while the yield strength decreased. When the overaging temperature increased from 260 to 400 ℃, the tensile strength decreased, while the yield strength increased. When the overaging time increased from 240 to 480 s, the tensile strength decreased a little, while the yield strength increased a little.
Gang HUOYi-ming CENZhen-xing LIGuo-dong LIChang-sheng LI
The thermal scratch seriously affects the surface quality of the cold rolled stainless steel strip. Some researchers have carried out qualitative and theoretical studies in this field. However, there is currently a lack of research on effective forecast and control of thermal scratch defects in practical production, especially in tandem cold rolling. In order to establish precise mathematical model of oil film thickness in deformation zone, the lubrication in cold rolling process of SUS410L stainless steel strip is studied, and major factors affecting oil film thickness are also analyzed. According to the principle of statistics, mathematical model of critical oil film thickness in deformation zone for thermal scratch is built, with fitting and regression analytical method, and then based on temperature comparison method, the criterion for deciding thermal scratch defects is put forward. Storing and calling data through SQL Server 2010, a software on thermal scratch defects control is developed through Microsoft Visual Studio 2008 by MFC technique for stainless steel in tandem cold rolling, and then it is put into practical production. Statistics indicate that the hit rate of thermal scratch is as high as 92.38%, and the occurrence rate of thermal scratch is decreased by 89.13%. Owing to the application of the software, the rolling speed is increased by approximately 9.3%. The software developed provides an effective solution to the problem of thermal scratch defects in tandem cold rolling, and helps to promote products surface quality of stainless steel strips in practical production.
The thermal scratch significantly influences the surface quality of the cold rolled stainless steel strip. In or der to establish a precise mathematical model of oil film thickness in deformation zone, the lubrication in cold rolling process of B443NT stainless steel strip was studied in the laboratory. According to the principle of statistics, a math- ematical model of critical oil film thickness in deformation zone for thermal scratch was built, with fitting and regres- sion analytical method, and then based on temperature comparison method, the criterion for deciding thermal scratch defect was put forward. Storing and calling data through SQI. Server 2005, a software on thermal scratch control was developed in the Microsoft Visual Studio 2008 environment by MFC technique for stainless steel in tandem cold roll- ing, and then it was used into the practical production. Statistical results demonstrated that the hit rate of thermal scratch arrives at 91.47%, the occurrence rate of thermal scratch is decreased by 87.81%, and rolling speed is in- creased by 7.3%. The developed software is of significance to the control of products quality of stainless steel strips, and the analysis and solution to the problem of thermal scratch defects in tandem cold rolling.
Microstructural evolution and mechanical properties of cryogenic rolled Fe-36Ni steel were investigated. The annealed Fe-36Ni steel was rolled at cryogenic temperature（ 123-173 K） with 20%- 90% rolling reduction in thickness.The deformation process was accompanied by twinning at cryogenic temperature,and the mean thickness of deformation twins was about 200 nm with 20% rolling reduction. When the rolling reduction was above 40%,twinning was suppressed due to the stress concentration in the tested steel. Deformation microstructure of Fe-36Ni steel consisted of both twin boundaries and dislocations by cryogenic rolling（ CR）,while it only contained dislocations after rolling at room temperature（ RT）. The tensile strength of Fe-36Ni steel was improved to 930 MPa after 90% reduction at cryogenic temperature,while the tensile strength after 90% reduction at RT was only 760 MPa. More dislocations could be produced as the nucleation sites of recrystallization during CR process.
Jian-jun ZHENGChang-sheng LIShuai HEBan CAIYan-lei SONG
Based on the rigid-plastic finite element method（FEM）, the shear stress field of deformation region for high manganese austenite steel during hot asymmetrical rolling process was analyzed. The influences of rolling parameters, such as the velocity ratio of upper to lower rolls, the initial temperature of workpiece and the reduction rate, on the shear deformation of three nodes in the upper, center and lower layers were discussed. As the rolling parameters change, distinct shear deformation appears in the upper and lower layers, but the shear deformation in the center layer appears only when the velocity ratio is more than 1.00, and the absolute value of the shear stress in this layer is changed with rolling parameters. A mathematical model which reflected the change of the maximal absolute shear stress for the center layer was established, by which the maximal absolute shear stress for the center layer can be easily calculated and the appropriate rolling technology can be designed.
Efect of ball scribing on relative permeability of conventional grain-oriented（CGO） and high permeability grain-oriented（HGO） electrical steel was investigated. The samples were scribed with spacing of 2 mm, 4 mm, 8 mm and 16 mm. The results show that after ball scribing with 16 mm width at 1.0 T, relative permeability of both℃GO and HGO steels was increased by 109% and 80%, respectively. Relative permeability rises as the scribing space increases, with the movement of the peak value of relative permeability to a higher flux density. Relational models describing relative permeability and flux density were constructed with high accuracy based on experimental data. The experimental data curves were analyzed during the magnetizing process.
Hao WANGChangsheng LITao ZHUNkwachukwu ChukwuchekwaBan CAIGang HUO
