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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.

Usage of AHSS may reduce the weight of automobiles and improve the crash safety, consequently, AHSS have been used on automobile increasingly. As the typical representative of AHSS, boron alloyed steels have been the point of focus for the materials choice in hot stamping. In this paper, hot stamped steel produced by Ma Steel is taken as the study object. The CCT curve is measured by Gleeble 3500. In addition, parts are trial produced by hot stamping steel sheet, microstructure observation and micro-hardness test are carried out as well. The parameters of hot stamping are determined and the mechanical properties of the parts made by tested steel meet requirements as well.

As more and more structural and safety automobile components are manufactured using low alloy steels by hot stamping and subsequent quenching technology, attempts for improved ductility of the quenched martensitic microstructure have been made by developing new steels and according processing procedures. In present research, a cold rolled sheet of 40Cr13 martensitic stainless steel was fabricated after hot rolling and annealing. Mixed microstructures of martensite and austenite (about 10vol.%) were obtained through appropriated austenitizing, quenching and partitioning treatment, which exhibits not only enhanced strengths of Rel=1250MPa, Rm=1650MPa, but also significantly improved ductility of A=17%.

High temperature constitutive model of high strength boron steel is an indispensable part of the numerical simulation in hot stamping. In order to investigate the flow behavior of high strength boron steel for hot stamping at high temperature, a series of isothermal uniaxial tensile tests were performed within the temperature range of 873-1173K and strain rate range of 0.01-10 s^–1 by using Gleeble 3500 thermo-mechanical testing system. The modified Zerilli-Armstrong model was then proposed to characterize the hot plastic flow behavior of the investigated steel. Analysis results indicate that the flow stress values of B1500HS predicted by the modified Zerilli-Armstrong (ZA) model agree well with the experimental ones in most test conditions. Moreover, the modified ZA model can be used for prediction of the plastic flow behavior of high strength boron steel in the process of hot stamping.

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.

AA7075-T6 alloy shows poor formability at room temperature. In this paper, thermal experiments were first conducted to investigate the effect of different heat treatment conditions with AA7075-T6 alloy. Then thermal-mechanical uniaxial tensile experiment of AA7075-T6 were first carried out under different temperatures (100-475°C) in order to find the optimum forming conditions of AA7075-T6. And then the specimens were artificially aged at 120°C for 24 hours to regain the T6-temper properties after stretching. Results showed that AA7075-T6 formed at 475°C, 0.1s-1 had a significant improvement in elongation. The following artificial aging treatment produced equivalent material properties to the as-received T6 sheet. A modified Fields-Backofen constitutive equation of AA7075-T6 was obtained to describe the thermo-mechanical flow behavior of AA7075-T6 and then applied in FEM simulation of deep drawing box. Taking displacement as the criteria, results of numerical simulation proved hot stamping technology with HFQ process can improve the formability of AA7075-T6 high strength aluminum alloy.

In this study, the crack propagation process in Al-Si coating on boron steel was investigated by performing isothermal hot tensile tests on Gleeble 3500. A scanning electro microscope was used to observe the crack morphologies under different strain conditions. Furthermore, surface morphologies of the segment coating under different deformation temperatures or strain rates were observed with a 3-D ultra-depth microscope. The results showed that coating cracks could be classified into two patterns with the increasing applied tensile strain. Under lower strain conditions, only cracks which were almost perpendicular to the coating/substrate interface were observed. As the tensile strain was large enough, interfacial cracks occurred and resulted in crack debonding. By comparing the surface morphologies and crack densities under different deformation conditions, it suggested that a higher deformation temperature or a lower strain rate could reduce the crack number.

To evaluate the influence of key process parameters on the quality of hot stamping part, a metallo-thermo-mechanical coupling FE model of hot stamping process is built to investigate the influence of initial temperature and stamping speed on the microstructure and tensile strength distribution of formed component. The results show that, during the forming stage, the formability of component is closely related to the stamping speed. With the increase of stamping speed, the lowest temperature of component greatly increases, while the max principal stress of component evidently decreases. During the quenching stage, the initial temperature plays a more important role in ensuring the required tensile strength. When the initial temperature of component is at the range from 750°C to 800°C, the martensite volume fraction of component after quenching is exceed 85% and with a uniform distribution, and its tensile strength is more than 1500 MPa. However, with the further improving initial temperature, the microstructure of component exhibits a non-uniform distribution and its tensile strength cannot meet the requirement of mechanical properties of formed component.

The hot stamping technology is widely applied to ultra-high strength steel (UHSS) automobile parts for weight reduction and safety improvement. However, hot-stamped components may have defects such as wrinkling, thinning and folding. In this paper, the function relationship between structural characteristics of parts and wrinkling in hot stamping process was built by analyzing the wrinkling behavior of automotive beams with various structural characteristics. According to the geometries, the structural characteristics of parts were classified into five types and defined as structural elements with non-verbosity and high recognition. Finite element software AUTOFORM was used to simulate the hot stamping of parts. By comparing the simulated results of parts with various structural characteristics, function relationship between structural elements and wrinkling trends was established. Results show that the more complex shapes induce more serious tendencies to wrinkling. Side wall has a positive effect on wrinkling while boss plays an opposite role. The influences of top face on wrinkling are indistinctive.

To improve the automotive safety and achieve light weighting, high strength steel was used in automobile pillar. The numerical simulation by Dynaform was finished for hot stamping of automobile pillar with thickness of 1mm and material of high strength steel 22MnB5. The simulation results analyzed and predicted defects such as cracking, wrinkling in hot stamping of high strength steel. The research can be used to guide the optimization of stamping process and die design for the automotive pillar to obtain the qualified products.

Giving consideration to both strength and energy absorption in side impact, it is of great significance to investigate the variable strength design and forming of B-pillar reinforcement panel. In this work, the finite element (FE) model of side impact was established for a certain car under the environment of Hypermesh and LS-DYNA. Based on the orthogonal experiment and adaptive response surface method, the optimization design of the variable strength of B-pillar reinforcement panel was carried out to make the distribution of the strength reasonable. After that, the B-pillar reinforcement panel with variable strength was further hot stamped in FE methods. The results show that the car body with the optimized B-pillar reinforcement panel can better meet the requirements of the side impact performance and, meanwhile, realize the aim of reducing weight by 12.1%.

Thermal contact resistance between sheet metal and die is the key fact of influencing parameters in thermal and physical properties of the heat transfer in hot stamping process. Based on the cooling curves of blank and flat die obtained by experiment, the inverse heat conduct problem is solved by using sequential function method (SFM) and the thermal contact resistance is calculated.

In this paper, a FEM method based on optimization was used to estimate the interfacial heat transfer coefficient between blank and die during hot stamping process. The temperature data of blank center and the point 1.5 mm under die surface's center at the holding pressure range from 5 MPa to 40 MPa were foremost obtained during hot stamping experiment by a two-piece cylindrical die experiment. The interfacial heat transfer coefficient (IHTC) depended on holding pressure was computed through FEM method based on optimization, and the effect of thermocouple response time and latent heat resulting from martensitic transformation were taken into account. The results show that the approximate relationship of IHTC and holding pressure is linear, and the relative error between experiment data and simulated data can be controlled within 15%. Moreover, the mean absolute error is less than 24°C, whereas different IHTC values and trends will be gained when using different experiment models and inverse methods.

Hot stamping technology is an important method of processing of high strength steel sheet which make the blank formed and quenched after its austenitizing, its stamping making use of the cooling system to produce martensite in order to improve the strength of the blank. The cooling system determines the quality of the hot stamping as a key part of the hot forming technology, In accordance with the principle of fluid dynamics, combined with the theory of conjugate heat transfer, build the finite element simulation model. Under the certain conditions, do the numerical simulation of the B-pillar in the holding pressure quenching stage with different way of water circulation.

The comprehensive mechanical properties of final parts after the conventional hot stamping process are poor for the limitation of elongation. In this paper, recent innovative processes to increase the product of strength and plasticity are reviewed and compared, in which the quenching & partitioning treatment and its integration with hot stamping technology called as HS+Q&P process are discussed in detail. By using boron steel B1500HS, the microstructure characterizations and mechanical properties after HS+Q&P process are investigated by Q&P tests and hot stamping experiments respectively. The results illustrate the feasibility of the presented process and show its noticeable effects on improving the product of strength and plasticity for final parts.

This paper provides an overview of hot stamping of light alloys using a method known as HFQ^® forming. We present a new method of forming complex shaped automotive components through the use of a hot stamping process for high strength AA6082 alloy. In particular, the component selected for study is a complex automotive door inner component that could not be conventionally formed from such a strong alloy. The forming process is described and the post-formed thickness distribution is presented. An FE simulation is given to demonstrate the potential to predict the forming process and to provide guidance as to the final component thicknesses.

Conventional hot stamping usually adopted 22MnB5 blanks to produce ultra high strength autoparts. Due to the effect of quenching martensite microstructure, the elongation of hot-stamped autoparts are rather low and the product of strength and elongation (PSE) are usually around 10 GPa%. To meet the increasing requirement of new-generation-vehicles on body lightening, energy saving and passive safety, especially on crash intrusion resistance and impact energy absorption, the advanced hot forming treatment AHFT is developed for ultra-high-strength-ductility auto-parts with PSE≥15~30GPa%. Based on the key technology of TRIP assisted & Q-P-T steels, the austempering AT and partitioning-tempering P-T treatment are respectively combined with hot stamping to obtain different F-TRIP, B-TRIP or M-TRIP (i.e. Q-P) steels with better strength-ductility balance for different energy-absorbing and intrusion resisting auto-parts. The blank steels for AHFT process are developed from 22MnB5, TRIP assisted steels and Q-P-T steels for different strength-ductility grades. The combination of thin slab casting and direct rolling TSCR line with AHFT process will start a compact route for the production of ultra-high-strength-ductility auto-parts with high-efficiency, energy-saving, low GHG emission, reduced cost and different strength-ductility grades.

This paper introduces the research work of “High Performance Sheet Metal Forming Team” from Huazhong University of Science & Technology, in hot stamping technology and application. Aiming at the basic theory and engineering application of hot stamping, the team has begun comprehensive study in materials, process and equipment for hot stamping of high strength steel since 2008. The representative achievements in many aspects of the team are illustrated, including microstructure control, heating, simulation of phase transition, frictional behavior between materials and die, and simulation and experiments of tailored tempering process. In addition, the CAD/CAE software and non-destructive testing instrument for hot stamping were developed to meet the industrial requirements.

In order to realistically predict the hot stamping process of TWBs, the boron-manganese steel B1500HS, the joint partner B340LA and the welding zone, including the Heat Affected Zone (HAZ), should be sufficiently accurate to describe thermal, mechanical and metallurgical parameters. In the article, thermal-mechanical modeling of hot stamping of tailor welded blanks is investigated. In addition, the results obtained from the numerical simulations are analyzed and critically compared with experimental results. Finally, the obtained results such as Vicker's hardness and microstructure distribution are highlighted and discussed.