In a world where fuel economy and crash safety is becoming an important factor in the automotive industry, the need for materials with very high strength-to-weight ratio is growing rapidly. One of the materials used for this purpose is the boron steel 22MnB5. Since the material has very high mechanical yield limit it is hard to produce parts using cold forming, which is the standard procedure for sheet metal forming. Therefore, the use of hot stamping is increasing. Hot stamping enables manufacturing of boron steel parts with good dimensional accuracy and low spring back. However, some amount of spring back is still present during the process. In this thesis, spring back of a hot formed 22MnB5 A-pillar is investigated using computer simulations in the software LS-DYNA. The main focus was to develop a process for simulating spring back in hot stamping. The work started with simulations of the forming and quenching stages of the hot stamping process, both on a full size and on a smaller section of the blank. Simultaneously as the simulations, a literature study was also conducted. The literature study was aimed at finding hints and information on how to build the simulations. Furthermore, interviews were made with experts on both LS-DYNA and hot stamping. A process for spring back evaluation was then created and written as an LS-DYNA keywordfile. In the developed spring back simulation, the part is taken out of the press right after the quenching is finished and placed in a space where it can cool and move freely. The simulation is conducted until the part reaches room temperature. After the quenching is done, data containing temperatures, stresses and strains of the part is exported. This data is then included in the spring back simulation where the part is cooled to room temperature. During the cooling, the stresses, strains and temperatures are equalized until the spring back reaches a steady state. The results indicate that the developed method for spring back evaluation can be used to foresee shape deviations for the intended part and process.

No description available.

Hot stamping and die quenching (HS/DQ) process of AA7075 aluminum alloy is one of attractive forming techniques for producing high strength automotive structural components to encounter their poor formability at room temperature. In this technique, quenching rate of this alloy is very crucial as it affects precipitation kinetics after artificial ageing of part formed, which in turn determines the final in-service mechanical properties and corrosion performance of part. Thermal contact resistance (TCR) between two solid surfaces is the main parameter that controls heat transfer between hot AA7075 sheet and cold steel dies, and thus affects quenching rate of part formed. Therefore, the final properties of automotive parts produced by hot stamping is indirectly influenced by TCR. The common methods of determining TCR in HS/DQ are often impracticable as they require thermocouples to be inserted into complex-shaped stamping dies, punches and thin aluminum sheet (blank) to be formed. A potential mechanistic approach for determining TCR could be an attractive alternative due to its avoidance of embedded thermocouples into the tooling and blank. The mechanistic method emphasizes on physical mechanisms (roughness etc.) governing interfacial heat transfer between cold forming tools and hot blank. The proposed work focuses on utilizing the mechanistic method to predict TCR between multiple cylindrical asperities on a nominally flat (and heated) AA7075 blank surface and a rigid, flat, asperity-free (and cold) steel die surface. The asperities were considered to deform elastoplastically, increasing contact area. Subsequently, TCR correlation as a function of temperature, contact load, and contact area was formulated. To validate the mechanistic model, a series of surface asperity flattening experiments using thermocouple-embedded AA7075 blank and polished stainless steel planar dies were carried out. Good agreement between mechanistic model predictions and experimental results in term of contact area and TCR as a function of contact load were observed. / Thesis / Doctor of Philosophy (PhD)

O processo de estampagem à quente é um dos métodos possíveis para a conformação de aços de alta resistência. Para isso é necessário o estudo das propriedades termomecânicas para obtenção de parâmetros ótimos para a construção de uma linha de estampagem eficiente. Neste trabalho, foram realizados ensaios para determinar os parâmetros para a estampagem a quente do aço 22MnB5, e posteriormente a construção de uma linha de estampagem a quente completa. Obteve-se ao fim do processo, uma microestrutura martensitica com dureza de 430 HV com resistência à tração de aproximadamente 1365 MPa. / The hot stamping process is one of the possible ways to high strenght steel conformation. In this work, study of 22MnB5 steel thermomechanical properties was performed in order to obtain optimum parameters to hot stamping process. After, a complete hot stamping line was built. At the end of hot stamping process it was possible to obtain a martensitic microstructure with 1365 MPa strenght resistance and 430 HV.

Este trabalho avalia as propriedades mecânicas, microestrutura resultante e força de estampagem da liga DIN 27MnCrB5 para o processo de estampagem a quente nas temperaturas de aquecimento de 750°C, 850°C e 950°C. Para isso foram realizados ensaios nas três temperaturas e medidos os valores da força necessária para a estampagem da chapa, que foi aplicada em uma área de 30mm x 1,50mm. A partir da peça estampada foram realizadas análises metalográficas da microestrutura resultante e medições de dureza para a correlação com as propriedades mecânicas. Para as temperaturas de aquecimento de 750°C e 850°C não foi verificada microestrutura martensítica nas peças estampadas. Para o ensaio realizado com uma temperatura de aquecimento de 950°C ocorreu transformação martensítica na peça na região que esteve em contato com o prensa chapa e a matriz. Esta transformação metalúrgica resultou em valores de dureza de 715±17HV0,2 em comparação com a condição inicial da chapa, cuja dureza era de 187±5HV0,2. As medições de força de estampagem para as três temperaturas de ensaio indicam uma redução da força com o aumento da temperatura de aquecimento da chapa. Para a temperatura de aquecimento da chapa de 750°C o valor médio da força de estampagem foi de 14kN, para a temperatura de 850°C o valor médio da força de estampagem foi de 9kN, enquanto para a temperatura de 950°C foi encontrado o valor médio de 5kN. / This work evaluates the mechanical properties, resulting microstructure and stamping force of the DIN 27MnCrB5 alloy for the hot stamping process at the heating temperatures of 750°C, 850°C and 950°C. For this, tests were carried out at the three temperatures and the values of the force required for the stamping of the sheet were measured, which was applied in an area of 30mm x 1,50mm. From the stamped part metallographic analyzes of the resulting microstructure and hardness measurements were performed for the correlation with the mechanical properties. For the heating temperatures of 750°C and 850°C, no martensitic microstructure was observed in the stamped parts. For the test performed with a heating temperature of 950°C martensitic transformation occurred in the part in the region that was in contact with the blank holder and the die. This metallurgical transformation resulted in hardness values of 715±17HV0,2 compared to the initial condition of the sheet, whose hardness was 187±5HV0,2. Stamping force measurements for the three test temperatures indicate a reduction in force with the increase of the sheet heating temperature. For the sheet heating temperature of 750°C the average value of the stamping force was 14kN, at the temperature of 850°C the average value of the stamping force was 9kN, while at the temperature of 950°C it was found The average value of 5kN.

Este trabalho avalia as propriedades mecânicas, microestrutura resultante e força de estampagem da liga DIN 27MnCrB5 para o processo de estampagem a quente nas temperaturas de aquecimento de 750°C, 850°C e 950°C. Para isso foram realizados ensaios nas três temperaturas e medidos os valores da força necessária para a estampagem da chapa, que foi aplicada em uma área de 30mm x 1,50mm. A partir da peça estampada foram realizadas análises metalográficas da microestrutura resultante e medições de dureza para a correlação com as propriedades mecânicas. Para as temperaturas de aquecimento de 750°C e 850°C não foi verificada microestrutura martensítica nas peças estampadas. Para o ensaio realizado com uma temperatura de aquecimento de 950°C ocorreu transformação martensítica na peça na região que esteve em contato com o prensa chapa e a matriz. Esta transformação metalúrgica resultou em valores de dureza de 715±17HV0,2 em comparação com a condição inicial da chapa, cuja dureza era de 187±5HV0,2. As medições de força de estampagem para as três temperaturas de ensaio indicam uma redução da força com o aumento da temperatura de aquecimento da chapa. Para a temperatura de aquecimento da chapa de 750°C o valor médio da força de estampagem foi de 14kN, para a temperatura de 850°C o valor médio da força de estampagem foi de 9kN, enquanto para a temperatura de 950°C foi encontrado o valor médio de 5kN. / This work evaluates the mechanical properties, resulting microstructure and stamping force of the DIN 27MnCrB5 alloy for the hot stamping process at the heating temperatures of 750°C, 850°C and 950°C. For this, tests were carried out at the three temperatures and the values of the force required for the stamping of the sheet were measured, which was applied in an area of 30mm x 1,50mm. From the stamped part metallographic analyzes of the resulting microstructure and hardness measurements were performed for the correlation with the mechanical properties. For the heating temperatures of 750°C and 850°C, no martensitic microstructure was observed in the stamped parts. For the test performed with a heating temperature of 950°C martensitic transformation occurred in the part in the region that was in contact with the blank holder and the die. This metallurgical transformation resulted in hardness values of 715±17HV0,2 compared to the initial condition of the sheet, whose hardness was 187±5HV0,2. Stamping force measurements for the three test temperatures indicate a reduction in force with the increase of the sheet heating temperature. For the sheet heating temperature of 750°C the average value of the stamping force was 14kN, at the temperature of 850°C the average value of the stamping force was 9kN, while at the temperature of 950°C it was found The average value of 5kN.

Este trabalho avalia as propriedades mecânicas, microestrutura resultante e força de estampagem da liga DIN 27MnCrB5 para o processo de estampagem a quente nas temperaturas de aquecimento de 750°C, 850°C e 950°C. Para isso foram realizados ensaios nas três temperaturas e medidos os valores da força necessária para a estampagem da chapa, que foi aplicada em uma área de 30mm x 1,50mm. A partir da peça estampada foram realizadas análises metalográficas da microestrutura resultante e medições de dureza para a correlação com as propriedades mecânicas. Para as temperaturas de aquecimento de 750°C e 850°C não foi verificada microestrutura martensítica nas peças estampadas. Para o ensaio realizado com uma temperatura de aquecimento de 950°C ocorreu transformação martensítica na peça na região que esteve em contato com o prensa chapa e a matriz. Esta transformação metalúrgica resultou em valores de dureza de 715±17HV0,2 em comparação com a condição inicial da chapa, cuja dureza era de 187±5HV0,2. As medições de força de estampagem para as três temperaturas de ensaio indicam uma redução da força com o aumento da temperatura de aquecimento da chapa. Para a temperatura de aquecimento da chapa de 750°C o valor médio da força de estampagem foi de 14kN, para a temperatura de 850°C o valor médio da força de estampagem foi de 9kN, enquanto para a temperatura de 950°C foi encontrado o valor médio de 5kN. / This work evaluates the mechanical properties, resulting microstructure and stamping force of the DIN 27MnCrB5 alloy for the hot stamping process at the heating temperatures of 750°C, 850°C and 950°C. For this, tests were carried out at the three temperatures and the values of the force required for the stamping of the sheet were measured, which was applied in an area of 30mm x 1,50mm. From the stamped part metallographic analyzes of the resulting microstructure and hardness measurements were performed for the correlation with the mechanical properties. For the heating temperatures of 750°C and 850°C, no martensitic microstructure was observed in the stamped parts. For the test performed with a heating temperature of 950°C martensitic transformation occurred in the part in the region that was in contact with the blank holder and the die. This metallurgical transformation resulted in hardness values of 715±17HV0,2 compared to the initial condition of the sheet, whose hardness was 187±5HV0,2. Stamping force measurements for the three test temperatures indicate a reduction in force with the increase of the sheet heating temperature. For the sheet heating temperature of 750°C the average value of the stamping force was 14kN, at the temperature of 850°C the average value of the stamping force was 9kN, while at the temperature of 950°C it was found The average value of 5kN.

O processo de estampagem à quente é um dos métodos possíveis para a conformação de aços de alta resistência. Para isso é necessário o estudo das propriedades termomecânicas para obtenção de parâmetros ótimos para a construção de uma linha de estampagem eficiente. Neste trabalho, foram realizados ensaios para determinar os parâmetros para a estampagem a quente do aço 22MnB5, e posteriormente a construção de uma linha de estampagem a quente completa. Obteve-se ao fim do processo, uma microestrutura martensitica com dureza de 430 HV com resistência à tração de aproximadamente 1365 MPa. / The hot stamping process is one of the possible ways to high strenght steel conformation. In this work, study of 22MnB5 steel thermomechanical properties was performed in order to obtain optimum parameters to hot stamping process. After, a complete hot stamping line was built. At the end of hot stamping process it was possible to obtain a martensitic microstructure with 1365 MPa strenght resistance and 430 HV.

This rapport discusses a Bachelor's thesis conducted at Gestamp Hardtech in Luleå, a company that invented the press hardening technique and still today is one of the leading companies utilising this type of process. A method used in the manufacture of ultra-high strength steel components. The main use of press hardening is when forming sheet metal for the automotive industry, because of the very high resistance to deformation and in turn low weight parts made from this process can offer. The number of of body parts for cars made with this process is high but yet rising as the method is being advanced, the technique is highly advanced and requires both knowledge and process control to manage. The creation of soft zones is a big part of hot stamping. A soft zone is a part of a material with lower strength and hardness, which is achieved by lowering the cooling rate at a specific area of the piece, resulting in a product that is both hardened and soften. For this project, only the soft zones were focused on and not the relation between hardened zones, this was for the interest in having the same mechanical properties over the whole metal sheet used. The questions that this project will try to answer is the possibilities of introducing new materials that can be used in hot stamping with combination of building in soft zones in them. It will also deep dive in to each of the materials materials mechanical properties achieved when process and give data that in the future can be used to build other projects on. While the project is built on the standard used today on softer materials process parameters, a recipe more based on production experience and default setting in the manufacturing line, it will answer if these settings might still hold true for these materials and if not what kind of parameters are more preferred. The reason why this project is of interest is because the automotive industry today has a great desire in lowering weight of the vehicle without reducing the quality. This project is a big step in the direction of finding a material that can provide the same mechanical properties while reducing the volume of the material. Soft zone plane sheets were made by direct hot stamping in the research line in Luleå. The main parameters changed in the different trials were: material, die temperature and cooling time.

In recent years, there has been an increase in the use of press hardened steel (PHS) in the body-in-white of automobiles, namely in parts such as side impact beams, roof rails, engine firewalls, and the floor area. As these parts are expected to possess corrosion resistance, Al-Si coatings are utilized on them. The implementation of Zn coatings is limited, despite possessing improved corrosion resistance attributed to cathodic corrosion protection, due to the detrimental effects of liquid metal embrittlement (LME) and microcracking. LME can be mitigated if stamping occurs at a temperature lower than the Fe-Zn peritectic temperature of 782 °C, as this ensures that the conditions of LME are not met and the cathodically-protective Γ-Fe3Zn10 phase is formed. The objective of this work was to determine a process window for stamping at lower temperatures for a GA80-coated prototype steel (steel K) with the composition of 0.19C-1.92Mn-0.20Si-0.003B-0.03Ti (wt.%). The target mechanical properties in this process window were UTS ≥ 1400 MPa, and YS ≥ 1000 MPa. To achieve robust cathodic corrosion protection, more than 15 vol% of Γ-Fe3Zn10 in the coating was desired. Furthermore, it was aimed to determine the robustness of this process and the industrial feasibility of it through pilot-scale trials for the GA80-coated prototype steel. To this end, a bare prototype steel (steel I) with the composition of 0.20C-1.96Mn-0.25Si-0.003B-0.01Ti was compared to GA-coated steel K. It was found that stamping a direct hot press forming (DHPF) temperature of 650 °C produced similar mechanical properties such as tensile strengths and ductilities. Microstructural analysis of laboratory-scale DHPF for steel K showed that both the coating and the substrate microstructures were not a strong function of the DHPF temperatures of 550 – 700 °C. The substrate predominantly consisted of martensite with small amounts of ferrite and the coating consisted of α-Fe(Zn) and ≥ 15 vol.% Γ-Fe3Zn10. Based on the fraction of Γ-Fe3Zn10 and the coating thickness, robust cathodic corrosion protection is expected from GA80-coated steel K. Furthermore, no evidence of LME or severe microcracking was observed in the microstructure for DHPF temperatures of 550–700 °C for steel K and ductile fracture was observed in tensile coupons. Tensile testing for laboratory-scale DHPF for steel K showed that the mechanical properties such as YS, UE and PUE were not significantly affected by DHPF temperatures of 550-700 °C. Moreover, the targets of UTS ≥ 1400 MPa, and YS ≥ 1000 MPa were met for all DHPF temperatures of 550 – 700 °C. Through pilot-scale tensile testing of steel K, it was determined that the targets for tensile strengths (UTS and YS), were achieved across various conditions involving austenitization at 890 °C for 60 – 240 s and DHPF temperatures ranging from 550 – 700 °C. These findings strongly suggest that industrial feasibility is attainable for DHPF of GA-coated steel K at lower temperatures, enabling the attainment of desirable mechanical properties and robust corrosion protection. Thus, it can be concluded that the process window yielding desirable properties for steel K was determined to be austenitization at 890 °C for 60 – 240 s and DHPF temperatures between 550 – 700 °C. / Thesis / Master of Applied Science (MASc) / The forming process of the steels used in the automotive industry is not suitable for zinc coatings because it requires pressing at high temperatures to ensure obtaining strong steels. Thus, zinc coatings cannot be utilized despite the desirable cathodic corrosion protection properties they can provide. This study aimed to determine a suitable process window for a zinc-coated prototype steel that would have both high strength and desirable corrosion protection. The zinc-coated prototype steel was pressed at lower temperatures. It was observed that the target mechanical properties were met for all the temperatures tested. Moreover, robust cathodic corrosion protection is expected for all the temperatures. Furthermore, pilot-scale tests yielded comparable results to those obtained in the laboratory, indicating that this process can be successfully applied in the industry as it possesses a sufficiently large process window.