Narrow Results

Hot stamped products with ultra-high strength have been wildly used in the automotive industry. As the requirements on lightweight and safety increase, automotive components with tailored mechanical properties have been manufactured by newly developed hot stamping processes. In this work, the most recent developments and challenges in hot stamping of high strength steels have been reviewed. These issues are separated into two aspects: theories and applications. The theoretical aspect includes friction and heat transfer between sheet and tools, mechanism of springback, hydrogen induced delayed fracture, constitutive models integrating thermomechanics and phase transformations, and “performance control” of the hot stamped products, etc. The application aspect includes die wear, thermal fatigue, second development of numerical simulation software with the consideration of phase transformations, and springback control, etc. This paper analyzed the recent developments and challenges in the hot stamping technology, and some challenges can be further discussed according to the research progress of Tongji University.

The emphasis on passenger safety and fuel efficiency has led to a significant increase in the usage of hot stamped boron steels in automobile applications due to their high strength-to-weight ratio. The most common applications of boron steels in vehicles are as structural reinforcements used to resist the impact deformation. The performance of the parts to resist side impact deformation mainly depends on the cold bending property of the material. This paper presents the approach by optimizing the amount of Nb micro alloying in the boron steel to improve its toughness. The results show that the bendability of boron steel can be significantly improved due to the original austenite grain size of the material is refined by adding an appropriate amount of Nb micro alloying.

The hot stamping process has been widely applied to the production of automotive body parts because the stamped panels with both high strength and good shape-accuracy are easily obtained. Microstructure especially the austenite grain size and homogeneity played a great role on mechanical properties of hot stamping part. In this study, multiple heat treatment test (900~970°C) were carried out to estimate the austenite grain growth behavior of boron steel based on Beck Models. The result showed that the austenite grain size grew gradually under high temperature and long soaking times. The mixed grain structure easily appeared during 900~930°C holding 5~10 minutes which would be bad for part properties. The austenite grain growth equation of boron steel at 930°C based Beck Model was proposed as follow [4]: D=D0+0.3t^0.7893, where, D0-original grain size (um), D-grain size (um), t-holding time (s).

Hot stamping of tailor welded blanks (TWBs) is proposed to meet the safety requirements by providing auto structural body parts with tailored mechanical properties. In order to obtain the expected tailored properties, it is essential to select the appropriate materials as TWBs. In this paper, four different kinds of the high strength low alloy steels and dual phase steels have been investigated through hot stamping. The mechanical properties and the microstructures of the samples were analyzed. The results show that these steels, achieving ultimate tensile strength values of 500 - 700MPa as well as the ductility values of 15% - 27%, are suitable as base materials for hot stamped parts with tailored properties.

The paper deals with the latest advances in the field of tribology in case of hot stamping of boron steel sheets. The investigation of the boundary conditions becomes mandatory not only for the calibration of the numerical models of the hot stamping processes, but also for the evaluation of new classes of lubricants, coatings and die steels characterized by increased life performances. Testing equipment and procedures as well as measurements analyses to evaluate the friction coefficient and the wear phenomena are presented. The emphasis is put on physical simulation techniques, capable to reproduce the thermal and mechanical cycles the metal sheets undergo during the real industrial practice. The tribological characteristics of boron steel sheets coated with Al-Si and Zn coatings are presented and discussed.

In hot stamping, the material formability, thermal interaction, and metallurgical microstructure are coupled. Because of the complex forming mechanism, forming defects and product strength are difficult to predict. Therefore, accomplishing the process design and performing strength analysis by using finite element simulations is more efficient. In this study, an experimental platform was developed to measure and derive the coefficients of friction at the interface between the sheet metal and the die surface. To ensure that the designed platform was sufficiently strong to sustain both normal and pulling forces in friction tests, finite element simulations were performed. Friction experiments at various increased temperatures and compression forces were conducted using three different types of boron steel sheets as test specimens, including non-coated 15B22 steel, high temperature oxidation resistance coated 15B22 steel, and aluminum alloy coated 22MnB5 steel. Finite element simulation was then constructed using the experimental data and material properties of boron steel, which were calculated using the JMatPro software. The finite element simulations for hot stamping an automotive B-pillar, including transportation, hot forming, and die quenching analyses, were then performed. The results of the finite element simulation showed that different coefficients of friction according to boron steels with different surface coatings have significant effects on the final hot stamped production part.

The yearly production of hot stamped automotive parts is increasing due to several advantages over the conventional cold forming processes, such as allowing the manufacture of components of much higher strength and of more complex geometries. Hot stamping of uncoated boron steel results in the formation of a loose oxide on the steel surface. This oxide has to be subsequently removed by shot blasting for satisfactory painting. In this paper, a method to avoid shot blasting is presented. The method is based on a chemical pre-treatment allowing anchoring of the formed oxide to the steel surface during hot stamping. Characterization by Scanning Electron Microscopy (SEM) as well as results from adhesion tests and a cyclic corrosion test of the produced material indicate that the scale adhesion is unaffected by subsequent process steps (cleaning, phosphating, and painting). The results also indicate that the corrosion properties of the hot stamped material are directly affected by scale adhesion; improved adhesion results in improved corrosion resistance. Furthermore, the chemical pre-treatment allows more rapid heating than non-modified material.

In order to explore the crack propagation of aluminum-silicon coated boron steel, biaxial tensile tests under complex loads were performed. The biaxial tensile device is installed in the AG-100kN machine at high-temperature, which provides plane-stress conditions. Strain rates of 0.1, 1s^-1 were selected as the forming process parameters. Real data obtained from the biaxial tensile tests produced stress-strain curves of the coated boron steels, which could then be used to simulate the crack propagation path more accurately. When the fatigue crack kept growing in tensile tests, the path could be observed by Photron FASTCAM Viewer, which is in good correspondence with the result of the tests. Fracture surfaces of the coated steels were inspected under SEM. This paper reports the significance to study the microscopic mechanism of failure and the great importance in practical application of aluminum-silicon coated boron steel in the forming process.

Hot stamping is a new manufacturing process that benefits in meeting the most important requirements and regulations like weight reduction, fuel efficiency and safety performance of automobiles. The finite element analysis acts as a tool to investigate the feasibility of manufacturing a part with hot stamping process. The accuracy of finite element analysis depends on the precision of the material model. Visco-plastic damage constitutive equations require close-fitted material constants, which are obtained from thermo-mechanical tests using a Gleeble simulator for various strain rates and temperatures. Due to resistance heating principle, a normal sample is not homogeneously heated along the gauge length, which can result in different phases along the gauge length while attempting to heat the steel sample to above austenite temperature. The stress-strain data obtained at this condition is not accurate enough to precisely simulate the hot stamping process. In order to obtain uniform thermo-mechanical properties of full austenite, a new self-resistance heating test rig and technique to achieve accurate stress-strain data at hot stamping conditions is proposed in this study.