Hot Stamped Components Research Articles

Effect of Contact Pressure during Quenching on Microstructures and Mechanical Properties of Hot-stamping Parts

An experimental apparatus with cooling system and pressure-adjustment assembly for simulating quenching was constructed to investigate the effect of contact pressure on the microstructures and mechanical properties of hot-stamping parts. Qualitative and quantitative analyses of the microstructures of the as-quenched parts were conducted; moreover, hardness and tensile tests were performed to measure their mechanical properties. The results indicated that contact pressure during quenching strongly affected the structures and performances of hot-stamping components. An excessive low contact pressure led to insufficient martensitic transformation. The critical contact pressure for complete martensitic transformation for 4.0 mm 22MnB5 steel was 0. 4 MPa when the temperature of the coolant was 20 °C. However, in consideration of the efficiency of practical production, a contact pressure higher than 1.25 MPa is recommended.

Hot stamping of boron steel sheets with tailored properties: A review

The paper gives a review of the main research activities recently carried out in the framework of hot stamping of boron steel sheets with tailored properties. The focus is on the process variants developed to locally adjust the mechanical properties of the hot stamped component as well as on the testing and modeling techniques needed to calibrate the numerical models of the tailored tempering processes and to evaluate the post-forming properties of the stamped products.

Effect of cooling path on the phase transformation of boron steel 22MnB5 in hot stamping process

Boron steels have phase transformation in hot stamping process, and the mechanical properties of hot stamped parts are determined by the microstructure generated by phase transformation. Cooling path has a significant effect on phase transformation and microstructure of the deformed parts; in order to obtain the tailored-customized mechanical properties, the relationship among cooling path, the phase transformation, and the corresponding microstructure must be investigated. In this paper, the dilatometric experiment with the cooling rate of 2, 10, and 50 °C/s and various dwell temperatures was conducted to explore the influence of cooling path on the start and finish temperatures of phase transformations from austenite to ferrite, bainite, and martensite; lasting times of transformations; and final microstructures in hot stamping process. The controllable factors which influence the mechanical properties of hot stamped boron steel component were also determined. In addition, the metallographic and Vickers hardness experiments were conducted as well to confirm the analysis result of dilatometric experiment. The knowledge about the relationship of cooling path, phase transformation, microstructure, and mechanical property of the hot stamped parts helps the design of hot stamping process of the stamped parts with tailored-customized mechanical properties.

A method to evaluate the formability of high-strength steel in hot stamping

Hot stamping is an innovative operation in metal-forming processes that almost completely prevents the cracking and wrinkling of high-strength steel (HSS) sheets. The examination of HSS defects using traditional forming limit diagrams (FLDs) is challenging. In this paper, we have used a new FLD, termed 3D FLD, which considers phase transformations (PT), to evaluate the formability of HSS in hot stamping. A numerical model of the hot stamping process was developed to predict the major and minor strain distributions in hot-stamped components. The effect of blank geometry on forming quality and other main process parameters, such as stamping velocity and forming temperature, were investigated. The measured thicknesses of typical points near the crack position agree well with those of the simulated points. These results confirm that the simulation strain was relatively accurate. Cracking and wrinkling can therefore be predicted accurately using the proposed 3D FLD. The blank was found easy to avoid rupture with optimized process parameters and die parameters. Traditional wrinkling criteria can be used to predict the wrinkling of formed parts within engineering error.

Sensitivity of the final properties of tailored hot stamping components to the process and material parameters

The final mechanical properties of hot stamped components are affected by many process and material parameters due to the multidisciplinary nature of this thermal–mechanical–metallurgical process. The phase transformation, which depends on the temperature field and history, determines the final microstructure and consequently the final mechanical properties. Tailored hot stamping parts – where the cooling rates are locally chosen to achieve structures with graded properties – has been increasingly adopted in the automotive industry. In this case, the robustness of final part properties is more critical than in the conventional hot stamping parts, where the part is fully quenched. In this study, a wide range of input parameters in a generalized hot stamping model have been investigated, examining the effect on the temperature history and resulting final material properties. A generic thermo-mechanical finite element model of hot stamping was created and a modified phase transformation model, based on Scheil's additive principle, has been applied. The comparison between modeling and experiments shows that the modified phase transformation model coupled with the incubation time provides higher accuracy on the simulation of transformation kinetics history. The robustness of four conditions relevant to tailored hot stamping was investigated: heated tooling (with low and high tool conductance), air cooling, and conventional hot stamping. The results show the high robustness of the conventional hot stamping compared to tailored hot stamping, with respect to the stamped component's final material properties (i.e. phase fraction and hardness). Furthermore, tailored hot stamping showed higher robustness when low conductivity tools are used relative to high conductivity tools.

New Process of Hot Stamping in Combination with Q-P-T Treatment for Higher Strength-Ductility Auto-Parts

Hot stamping has been developed into a sophisticated stage with standardized technology and typical product USIBOR1500. Most of the hot stamped components from 22MnB5 blanks possess high strength-ductility with TS⋅El product from 10 to 15 GPa%. To meet the further requirement of automakers on body lightening, energy saving and impact safety, the new HFQPT process is proposed, i.e. Hot Stamping in combination with Quenching-Partitioning-Tempering treatment, for higher strength-ductility auto parts with TS⋅El product 20∼30 GPa%. Based on the key technology of AHSS in retained austenite RA and TRIP effect, the Q-P-T treatment is integrated to hot stamping process to obtain retained austenite on the martensite matrix to improve ductility of hot stamped ultra-high-strength components. A low temperature P-T furnace is added to the hot stamping line followed the hot pressing machine to perform Partitioning-Tempering treatment immediately after Hot Press Forming and Quenching, named as HFQPT process. The blank steels for new hot stamping process HFQPT line can be further developed on the basis of low alloyed TRIP assisted steels, Q-P steels and Q-P-T steels, and produced in traditional hot strip mill HSM or in compact steel production CSP line through thin slab casting & direct rolling to deliver ultra-thin-hot-strip (thickness t = 1.2 ∼2.0 mm).

Robustness of the Tailored Hot Stamping Process

The final mechanical properties of hot stamped components are affected by many process and material parameters due to the multidisciplinary nature of this thermal-mechanical-metallurgical process. The phase transformation, which depends on the temperature field and history, determines the final microstructure and consequently the final mechanical properties. Tailored hot stamping parts – where the cooling rates are locally chosen to achieve structures with graded properties – has been increasingly adopted in the automotive industry. Robustness of the final part properties is more critical than in the conventional hot stamping. In this paper, the robustness of a tailored hot stamping set-up is investigated. The results show that tailored hot stamping is very sensitive to tooling temperature, followed by latent heat radiation emissivity, and convection film coefficient. Traditional hot stamping has higher robustness compared to tailored hot stamping, with respect to the stamped component’s final material properties (i.e. phase fraction, hardness).

Investigation of Hot Stamping Process of 22MnB5 Based on Metallo-Thermo-Mechanical Theory

In this study, based on the metallo-thermo-mechanical coupling theory, a FEM model of hot stamping including forming and quenching is built to investigate the cooling behavior and the microstructure evolution, and to predict the final mechanical properties of hot-stamped components. The results show that, after about 16s, the temperature of the entire component is lower than Mf of 22MnB5 boron steel and with a continuous uniform distribution. Most of austenite in component has transformed into martensite. To satisfy the required mechanical properties, the sufficient holding time of quenching in die is essential and it plays an important role in ensuring the required hardness. The predicted Rockwell hardness of component after hot stamping process is almost 512HV, which shows a good agreement with the experimental results. It implies that the metallo-thermo-mechanical numerical model established in this study is reasonable and reliable, which can provide a theoretical guidance for optimizing the hot stamping procedure.

Thermal Processing History and Resulting Impact Response of a Hot-Formed Component with Tailored Properties – Numerical Study

The impact modeling of a hot-formed component with tailored mechanical properties is studied to understand the influence of the thermal processing history and how the final properties of the component will affect its impact response. This paper presents a numerical study of the forming and quenching process and subsequent impact simulations. The processing simulations serve to predict the final microstructure and hardness distribution within a lab-scale B-pillar component that is processed using a tool with separate heated and cooled regions. A remapping algorithm is used to translate the results of the forming simulation to the impact simulation. A strain-rate sensitive material model is applied to model the response of these tailored microstructures during impact events. A comparison between a component that is fully hardened and a tailored component with regions of lower strength but increased ductility is presented in this work. Simulations that do not consider the onset of fracture predict superior peak impact load and energy absorption of the fully martensitic component due to its higher overall strength. However, the bainitic regions within the tailored component exhibit much higher ductility. Current work is addressing the introduction of failure criteria into simulations of tailored hot stamped components under impact loading for which the tailored component is expected to demonstrate superior resistance to cracking relative to the fully hardened component.

V単独，V-Nb複合添加超高強度ホットスタンピング部材の組織と機械的性質

As a method for producing ultrahigh strength automotive steel components with high gauge accuracy, the hot stamping technology draws attention and the amount of hot stamped automotive components has exponentially increased in the last decade. The concerns about the potential danger of embrittlement and delayed fracture, however, limit the strength of conventional hot stamped components to 1500 MPa. If these concerns are eliminated, automotive steel components with higher strength which is demanded by the automotive industry can be further commercialized.In this study, microstructures and mechanical properties of V and/or Nb bearing ultrahigh strength hot stamped steel components have been investigated. The V addition improves the resistance to delayed fracture by forming VC whose interface with matrix is supposed to act as preferential hydrogen trap site. The Nb addition refines the microstructure, which improves both the toughness and the resistance to delayed fracture. Proper hot stamping conditions for these steels has been discussed to create an ultrafine martensite microstructure and finely dispersed precipitates. The results indicate that 1900 MPa class steel components with good toughness and high resistance to delayed fracture can be produced.

Investigations on the Manufacturability of Thin Press Hardened Steel Components

In the recent years, the automotive industry is focusing on the reduction of the vehicles weight, so as to minimize the CO2 emissions, while improving the crashworthiness levels. In order to achieve a further body-in-white weight reduction and exploit the potential for lightweight construction of the hot-dip aluminized press hardening manganese-boron steel, further research on the design and development of a process chain for the production of thin hot stamped components is carried out. For this purpose the impact of different blank thickness-dependent process parameters on the component properties is determined through both simulation and experimental analysis. The temperature profile of the heating process is determined for different blank thicknesses and the development of the diffusion layer between the AlSi-coating and the base material is examined. With respect to the transfer process of the thin austenitized blanks from the roller hearth furnace into the forming tool, the time slot is determined via the analysis of the corresponding time-temperature curves. In addition, different simulation models are developed, aiming at the validation and optimization of the transfer process. In order to further investigate the manufacturability of thin press hardened components, the simulation of a hot stamping process is carried out. The developed models are verified by an optical 3D forming analysis of the hot stamped components. As a conclusion of the current investigations, thin hot stamped components can be manufactured only under the precondition that the process is optimally designed and the process chain properly adjusted.

Hot Stamping of High Strength Steel with Tailored Properties by Two Methods

Hot stamped components with full martensite exhibit ultrahigh tensile strength and hardness, whereas the poor ductility restricts their application on certain components, such as B pillar, which needs multi strength in different regions. It is essential to partly achieve softer structure with high ductility, to enhance the crashworthiness and energy absorption of the whole component and thus improving the safety performance. This work described the hot stamping of high strength steel with tailored properties by two methods, namely applying different die temperatures and annealing processes. The hardness distribution, tensile strength and ductility in different regions of the hot stamped components were gained under the conditions of above two methods to realize the tailored properties. Regardless of the decrease of tensile strength, the total elongation of annealed zone can reaches 20.6% while the heating zone can only obtain its elongation of 10.8%. The results show that annealing hardened part to get tailored properties brings about better performance.

Cooling channel design of hot stamping tools for uniform high-strength components in hot stamping process

Although high strengths up to 1,500 MPa can be obtained by means of hot stamping, the strength and uniformity of components cannot be guaranteed during mass production if the cooling channel is not properly designed. In this paper, we propose a method for designing the cooling channel by means of the energy balance principle and arrangement methods such as the triangular method, tool split, and the connection rule for 2D sections to improve the strength and uniformity of hot stamped components. Two separate approaches for designing the cooling channel are suggested. One is to reduce facility and maintenance costs, and the other is to reduce the cooling cycle time, which critically affects productivity. These approaches were applied to hot stamping tools for manufacturing a roof side that is used as part of the structure of an automobile. Cooling performance and cooling uniformity of the designed cooling channel were verified with finite element simulation. Finally, hot stamping tools for manufacturing a roof side were designed to reduce cooling cycle time and were then manufactured to verify the proposed design methods. A roof side was manufactured by means of the hot stamping tools with the designed cooling channel, and the strength and the uniformity of the component were evaluated by means of a tensile test, observation of the microstructure, and measurement of the micro-Vickers hardness.

ホットスタンピング部材のマルテンサイト組織の微細化に及ぼす諸因子の影響

ホットスタンピング部材のマルテンサイト組織の微細化に及ぼす諸因子の影響

New Trends in Laser within the Process Chain of Hotforming Production

Abstract The laser is an established tool in the process chain to produce hot stamped components. While the cutting is the most wide spread application, there are a variety of other laser applications which help to be more efficient. Continuous improvements in machine/laser performance and changing part trends, ask for accessory developments to prevent bottlenecks in production. A new trend in hot forming, is the use of tailor welded blanks, to further reduce weight, or to combine different material properties. The AlSi coating on the material, needs to be eliminated in an efficient way, to assure quality standards in the strength of the welding and corrosion prevention in the welding area.

Hot stamping: manufacturing functional optimized components

Due to rising requirements regarding passenger safety and weight reduction hot stamped components get more and more important regarding security relevant parts because of their high strength. Currently, the new challenge is manufacturing components with locally adjusted mechanical properties in order to simultaneously guarantee structural integrity and energy absorption in case of a crash. Therefore, the so called Tailored Tempering process is a promising strategy controlling the cooling rate of the austenitized blank by local heating of the tool and consequently adjusting microstructure development and resulting mechanical properties. As the adjustment of mechanical properties is a temperature dependent process which is also influenced by process parameters and contact conditions, accurate process knowledge is essential. Within this paper the influence of tool temperature and applied contact pressure on the resulting heat transfer coefficient as well as contact condition between the surfaces of former investigations will be shown in a short overview. Based on the results of these fundamental investigations, design principles for Tailored Tempering process will be derived. Furthermore, a hat-profile geometry will be investigated experimentally and characterized by mechanical tests as well as geometrical aspects like sheet thickness and distortion. Additionally, an FE-Modell based on the results of the thermo-mechanical characterization is build up and validated by comparison with the results of experimental work.

Predictions of the Mechanical Properties and Microstructure Evolution of High Strength Steel in Hot Stamping

Hot stamping is an innovative operation in metal-forming processes which virtually avoids the cracking and wrinkling of high strength steel (HSS) sheets. Examining the phase transformation and mechanical properties of HSS by means of experiments is challenging. In this article, a numerical model of the hot stamping process including forming, quenching, and air cooling was developed to reveal the microstructure evolution and to predict the final mechanical properties of hot-stamped components after multi-process cycles. The effects of the number of process cycles and the holding times on the temperature of HSS were examined using the model. The microstructure evolution of HSS under variable holding times is illustrated. The mechanical properties, particularly hardness and tensile strength, were predicted. It was found that the martensitic content increased with increasing holding time, and the martensitic content of the formed component at the flange and end was higher than for the sidewall, and lowest for the bottom. The hardness trend was consistent with the martensitic content. After six process cycles, the predictive errors of the model for hardness and tensile strength were acceptable for practical applications in engineering. Comparison between the predicted results and the experiment results showed that the developed model was reliable.

핫스템핑 부품의 전단가공에서 전단 하중의 감소 및 트리밍 금형 수명 향상을 위한 국부 연화 방법

In general, hot stamped component is trimmed by costly and time consuming laser cutting when the material strength is over 1,500MPa. The aim of this work was to demonstrate that the trimming die life is improved and the trimming load is decreased by lowering the strength of the region to be trimmed. The model employed in this study was a hat shape, similar to the cross section of many hot stamped products. FE-analysis of hot stamping process was performed to evaluate the effect of tool shape on cooling rate at the area to be trimmed. The best tool shape was thus identified, which created slower cooling and lower hardness at the region to be trimmed. The wear at the cutting tool edge was also reduced.

Indirect Hot Stamping of Boron Steel 22MnB5 for an Upper B-Pillar

During the indirect hot stamping process of boron steel, the pre-deformed component undergoes air cooling, one-side-contact cooling and both-side-contact cooling phases successively. The effects of pre-deformation and cooling rate on the phase transformation should be understood before conducting indirect hot stamping experiments of vehicle components. Uniaxial tensile tests of boron steel at RT were carried out to obtain specimens with different pre-strain levels. Then they were heated to 900°C according to the indirect hot stamping process and quenching tests were performed on them at different cooling rates. Metallographic observations were performed on the quenched specimens and their hardness was measured. The results show that the pre-strain at RT has little influence on the phase transformation of boron steel. This is due to the dislocation structure introduced by deformation at RT recovered during the heating process and it is good for the indirect hot stamping. Upper B-pillar parts were first cold pre-formed, and then were heated and hot stamped. The microstructure and hardness results at different locations on the indirect hot stamped components are demonstrated qualified.

Comparative study of the prediction of microstructure and mechanical properties for a hot-stamped B-pillar reinforcing part

Automobile manufacturers have been increasingly adopting hot-stamped parts for use in newly designed vehicles to improve crash worthiness and fuel efficiency. However, the hot-stamped parts require extreme mechanical properties with ultimate tensile strengths as high as 1500 MPa (∼450 Vickers hardness) while still maintaining adequate formability during the stamping operation. The ultra high strength of hot-stamped components is attributed to the martensitic phase transformation that occurs after the part has been formed at temperatures corresponding to the austenite phase field where formability is enhanced. In the present study, a computer-aided design method incorporating Kirkaldy and Venugopalan type phase transformation models has been implemented following a thermo-mechanical coupled finite element analysis to predict the mechanical properties of hot-stamped parts made with a boron-modified steel. Three empirical models which are typically used for hot stamping analysis are employed and the prediction capability of the models is compared using continuous cooling dilatometry and forming experiments of a modified B-pillar part.