Cooling system of hot stamping of quenchable steel BR1500HS: optimization and manufacturing methods

The innovative hot stamping press of quenchable steel mainly includes two stages: hot forming of blank at elevated temperature and quenching of hot stamped part in closed water-cooled tools. Crucial stage in hot stamping process is quenching of hot formed part in water-cooled tool to achieve final high ultimate tensile strength. In quenching stage, cooling rate of hot stamped part has a significant effect on final mechanical properties of hot stamped part. Hence, in this paper, a new method is proposed to optimize cooling system to improve effectiveness of quenching and thus final mechanical properties of hot stamped part. In this method, cooling channel is introduced in tools, and hot stamping experiments are combined with corresponding finite element numerical simulations to optimize cooling system. According to cooling system optimization, two different sets of tools used to manufacture square-box-shaped part and B pillar of automobile are manufactured using drilling method and pre-embedding method, respectively. Hot stamping experiments and corresponding simulations are performed to verify the effectiveness of optimized cooling system.     

Estimation of die service life for a die cooling method in a hot forging process

Dies may have to be replaced for a number of reasons, such as changes in dimensions due to wear or plastic deformation, deterioration of the surface finish, breakdown of lubrication, and cracking or breakage. In this paper, die cooling methods are suggested to improve die service life with regards to wear and plastic deformation in a hot forging process. The yield strength of die decreases at higher temperatures and is dependent on hardness. Also, to evaluate die life due to wear, a modified Archard’s wear model has been proposed by considering the thermal softening of die expressed in terms of the main tempering curve. This paper describes the effects of die cooling methods such as cooling hole and direct spray cooling on the life of finisher die during the hot forging of an automobile part. It is shown that the cooling hole method during hot forging is necessary for an effective die service life to be obtained.     

Application of hot press forming process to manufacture an automotive part and its finite element analysis considering phase transformation plasticity

An automotive flex plate that is installed in the automotive engine and delivers a torque to a transmission is manufactured by hot press forming technique. By this technique, significant increase of strength through quenching of heated high carbon SK5 steel sheet and higher dimensional stability after forming through the press operation at high temperature can be attainable. The indirect method with a pre-forming step and direct oil quenching to attain uniform and fast cooling rate are employed considering cooling characteristics of the SK5 steel and large thickness of the flex plate. A new tool design is proposed for the hot press forming process, and an optimized heat treatment condition is determined by studying two frequently utilized heat treatments: austempering and quenching with tempering heat treatments. By introducing designed tools and selected heat treatment condition, the target product can be successfully manufactured, which satisfies two main manufacturer's specifications: high hardness and good dimensional accuracy. Moreover, finite element analysis, which considers transformation induced plasticity (TRIP) during phase transformations, is performed to understand the thermo-mechanical behavior of hot press formed sheet. The analysis verifies that phase transformations play significant roles in strengthening by transforming hard martensitic phase and in reducing dimensional change by additional plastic deformation during phase transformations.     

Integrated Modelling of Microstructure Evolution and Mechanical Properties Prediction for Q&P Hot Stamping Process of Ultra-High Strength Steel

High strength steel products with good ductility can be produced via QP hot stamping process, while the phase transformation of the process is more complicated than common hot stamping since two-step quenching and one-step carbon partitioning processes are involved. In this study, an integrated model of microstructure evolution relating to QP hot stamping was presented with a persuasively predicted results of mechanical properties. The transformation of diffusional phase and non-diffusional phase, including original austenite grain size individually, were considered, as well as the carbon partitioning process which affects the secondary martensite transformation temperature and the subsequent phase transformations. Afterwards, the mechanical properties including hardness, strength, and elongation were calculated through a series of theoretical and empirical models in accordance with phase contents. Especially, a modified elongation prediction model was generated ultimately with higher accuracy than the existed Mileiko's model. In the end, the unified model was applied to simulate the QP hot stamping process of a U-cup part based on the finite element software LS-DYNA, where the calculated outputs were coincident with the measured consequences.     

车门防撞梁热成形工艺优化仿真与试验

基于DYNAFORM软件热冲压模块,研究了由B1500HS钢制成的某型轿车车门防撞梁热成形过程。通过引入H13模具钢热物性参数,并将相变潜热因素考虑在内,利用多步差分法得到了温度场单元体变分公式;依据车门防撞梁结构特征进行工艺分析,建立了车门防撞梁的两种热力相耦合模型。通过对比分析两种方案下整体及典型M形截面应力场、温度场和厚度分布结果发现：带压料板式方案所得产品热成形及淬火后应力场、温度场及厚度分布均匀,更适合热冲压成形。最后依据压料板式方案进行了成形试制,得到车门防撞梁的显微组织均为马氏体,底部与顶部的马氏体板条更为细小,维氏硬度超过450HV,符合热冲压零件性能标准。     

热冲压过程传热耦合研究*

提出一种新的思路解决热冲压过程的传热耦合模拟。板料和模具之间的界面传热系数借助圆柱形冲压试验数据并通过有限元优化反算的方法获得,将获得的界面传热系数(Interfacial heat transfer coefficient, IHTC)引入到ABAQUS的U形冲压模型中,进行单次热冲压模拟,得到一个冲压周期内模具表面任意节点的热流密度曲线。通过调用SAS程序求得模面任意节点对应的时均热流密度,将每个节点的时均热流密度和笛卡儿坐标系下的空间坐标对应关系引入到STAR-CCM+中作为模具淬火传热模拟的第二类边界条件。接着通过网格数据映射将稳态淬火模拟得到的模具温度场,水道壁表面传热系数(Film heat transfer coefficient, FHTC)和壁面冷却水温度重新导入到ABAQUS的冲压模型中进行新的热冲压模拟,此时的热冲压过程即同时实现板料-模具的界面传热和模具-水流的表面传热的稳态模拟。设计并制造仿真中使用的U形模具进行连续热冲压试验,采集特征点的稳态温度,与仿真结果进行对比。结果表明：耦合后的板料和模具的最大温度和未耦合的单次冲压相比升高近一倍,温度分布沿着水道进口向出口方向发生迁移。耦合后的板料和模具特征点温度与达到稳态的试验采集温度误差均小于10%。     

Geometric compensation for automotive stamping die design integrating structure deflection and blank thinning

Targeting to improve dimensional accuracy of automotive body panels, elastic deformation of stamping die structure and blank thinning are considered for compensation on the die surfaces. The thinning distribution on the deformed blank is extracted from stamping process simulation results and converted into first quantitative compensation. The boundary load required for die structural FEM analysis can be automatically mapped from process simulation results by using an updated load mapping algorithm, and the die deflection is then calculated and the deflection of die surfaces is transferred into second quantitative compensation. These two quantitative contributions are referred together for the compensation on the die surface model. The proposed methodologies have been programmed and can be integrated with LS-Dyna and HyperWorks or with Autoform and CATIA. Additionally, a software toolkit used to calculate the contacting ratio between the formed blank and die face has also been developed to automatically evaluate the effectiveness of die face compensation rather than virtually check the contacting ratio. The proposed methodologies and developed software have been verified by a case study of process and die design for an automotive panel part, showing the benefit in improving the contacting ratio.     

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.     

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.     

高强度钢板热冲压工艺与装备研究综述

高强度钢板热冲压工艺是实现汽车轻量化、同时提高汽车碰撞安全性的重要途径之一，在汽车制造业中得到了广泛应用，是钢铁、模具、设备、零部件和汽车制造商关注和国内外学者研究的重点。从实际生产角度出发，对传统热冲压工艺研究中的应用要点进行综述：① 热冲压材料化学成分对其性能的影响、常用镀层的特点与适用范围；② 热冲压加热、成形、淬火、裁切和表面处理工序及零件连接工艺的应用要点；③ 热冲压数值模拟的重要影响因素和流程；④ 热冲压加热系统和压力机的类型与特点；⑤ 热冲压模具选材、设计与制造的要点及优化设计的流程。并对热冲压相关值得关注的研究方向做出展望。     

密封轴承卷边防尘盖冲模双锥面结构设计

卷边防尘盖冲模结构直接影响防尘盖冲压装配质量,针对单锥面冲模出现的质量问题,进行了防尘盖卷曲变形过程的受力分析计算,提出了双锥面冲模结构,确定了合理的锥面角度,并给出了锥面结构尺寸计算公式。经对多种卷边防尘盖冲压装配试验,防尘盖可靠无松动,并能保持轴承外圆精度。     

Thermomechanical modelling strategies for multiphase steels

In multiphase steels, heat treatments such as quenching are usually applied to achieve a desired metallurgical composition to attain the expected mechanical properties. In these processes, residual thermal stresses arise during the cooling of the material which may induce a permanent deformation leading to dimensional instability. This deformation can be increased by the existence of phase transformations in the steel which should not be overlooked.In the current work, the thermomechanical modelling of multiphase materials is discussed. Firstly, a multiphase thermo-elasto-plastic-viscoplastic model is presented and applied to simulate several quenching heat treatments in a high carbon steel. The model uses the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation to describe the diffusion transformation and the Koistinen-Marburger model to characterise the diffusionless martensitic transformation in non-isothermal kinetics. This allows the observation of the evolution of the different steel phases (austenite, ferrite, pearlite, bainite and martinsite) during the cooling process. However, it is not possible to determine the residual stresses that arise in the intersection of the different phases.In the second part, the model that considers generalised multiphase transformation being compared with a multiphase homogenisation model for the case of a dual-phase steel during cooling and subsequent forming. The homogenisation micro-model operates over a periodic Representative Unit-Cell (RUC), detailing the heterogeneous material distribution due to the different metal phases. Therefore, it is possible to determine the residual stress fields in the intersection of the different phases. On the other hand, this model does not allow to reproduce the transformation process from austenite during cooling. Continuous cooling processes are studied in both parts. Following the heat treatment, tensile and shear test curves are presented and compared with experimental results for the second part.     

基于数值模拟和3D打印的热冲压模具随形水道设计制造研究*

针对传统热冲压模具深孔对钻冷却系统冷却不均的现象,基于热冲压过程的多场耦合传热理论,提出热冲压模具随形冷却系统设计方法。利用Star-ccm+软件对热冲压冷却淬火过程进行热流固耦合模拟。以自主研发的彩虹电动汽车B-柱模具镶块为研究对象,对传统深孔对钻和新提出多种随形设计方案进行了冷却模拟分析。根据模拟结果,比较分析了不同冷却方案的冷却效果,并提出B柱模具镶块纵向随形冷却水道的优化方案,分析了优化前、后模面温度场的变化。提出运用覆膜砂3D打印造型结合传统铸造工艺的方法,制造具有随形冷却水道的热冲压模具。通过对比五种方案发现,采用纵向随形设计的热冲压模具,与冷却最差的平行随形设计相比,模面最大温度降低47.4%,模面平均温度降低40.9%,模面温度均匀性提高1.8%。根据纵向随形水道设计流程,将水道形状和位置参数R、H、r作为优化参数,模面温度场作为优化目标,对纵向随形水道进行优化,结果显示,优化后模面最大温度下降了49.8%,模面平均温度下降了46.8%,模面温度标准偏差下降了67.5%,模面温度均匀性提高了1.9%。因此,采用随形冷却的热冲压模具,有效地提高了生产加工效率和使用寿命,改善了热冲压件的平均强度和组织与力学性能的均匀性。     

Research on the influence of hot stamping process parameters on phase field evolution by thermal-mechanical phase coupling finite element

In order to study the effect of austenitizing temperature and packing time on phase evolution more accurate, a coupling element finite model of hot stamping process of BR1500HS is established based on the platform of DEFORM-3D, which takes the interaction of heat, plastic work, and phase transformation into consideration. The simulated results reveal that the final microstructure mainly consists of martensite, and the martensite volume fraction increases at an austenitizing temperature range from 800 to 900 °C and has a little change with the increasing packing time. Subsequently, a series of physical experiments are conducted to evaluate the reliability of the simulated results. The micrographs show that influence of the austenitizing temperature and packing time on martensite volume fraction is consistent with the simulated results. Further, the tested hardness distribution also proved that the present thermal-mechanical phase coupling element finite model is an effective tool to predict the microstructure during the hot stamping process.     

Experimental and theoretical study on the hot forming limit of 22MnB5 steel

To reveal the effects of forming temperature and blank thickness on the formability of boron steel 22MnB5, which is represented by forming limit diagram, the hot forming limit experiments and B-pillar hot stamping simulations are conducted. Combined with air cooling test, the forming limit diagrams of boron steels with different blank thicknesses of 1.8, 1.6, and 1.4 mm and forming temperatures of 800, 700, and 600 °C are established. The relationships between the formability and the crystal structure of steel, the forming limit diagram, and the effect of blank thickness and forming temperature on the formability of boron steel are extensively investigated. A model for prediction of hot forming limit of 22MnB5 steel before the occurrence of phase transformation induced by cooling based on Oh's ductile fracture criterion and Logan–Hosford yield criterion is derived and verified by experiments. The research thus provides an in-depth understanding of the formability of 22MnB5 steel for its process determination and process parameter configuration in industries.     

精密轴承套圈淬火油的选择

减少轴承套圈淬火变形是热处理工作者的永恒课题,而淬火油的选择是解决淬火变形的重要环节.选择冷却速度介于快速淬火油(冷油)和等温淬火油(热油)之间的温油作为淬火油,解决壁厚小于16mm套圈的淬火变形问题.     

22MnB5超高强钢热冲压成形工艺及试验

考虑材料的热物理性能参数、力学性能与温度的关系,利用ABAQUS软件建立了22MnB5超高强钢热冲压过程的热力耦合有限元模型,选用合适的热冲压工艺参数进行数值分析,得到了坯料热冲压成形的应力应变分布,并以板料初始温度为变量,研究不同初始温度对零件厚度分布、回弹及冷却速率的影响。进行了板料初始温度为900℃的22MnB5超高强钢热冲压试验,零件厚度分布及回弹量与模拟结果基本吻合,各区域淬火组织均为板条状马氏体,由于零件底部的淬火冷却速率较大,马氏体组织更加均匀细小。     

20Cr1Mo1VTiB钢的连续冷却转变行为

采用热膨胀仪测定20Cr1MolVTiB钢在不同淬火温度(9501100℃)下的贝氏体转变点和较佳淬火温度下的连续冷却相变点,结合组织观察和硬度测试绘制连续冷却转变曲线;利用经验公式建立相变点和相转变量与冷却速率的关系,并计算相变激活能。结果表明:随着淬火温度升高,试验钢中贝氏体转变温度降低,较佳淬火温度为1050℃;冷却速率不高于0.5℃·s-1时,过冷奥氏体转变产物为先共析铁素体、珠光体和贝氏体,冷却速率超过0.5℃·s^-1时则为单一贝氏体;相变点-冷却速率和相转变量-冷却速率拟合曲线与试验结果吻合较好,先共析铁素体和贝氏体相变激活能分别为744.8,274.9 kJ·mol^-1。     

面向设计的板料成形快速仿真系统FASTAMP

FASTAMP是基于改进的有限元逆算法和动力显式算法的板料成形快速仿真软件。改进的逆算法求解器采用了考虑弯曲效应的DKQ四边形单元及方程组快速求解算法,真实考虑了摩擦、压边力、背压力和曲压料面等实际工艺条件,在模拟精度和速度上均有较大的提高。系统结合了两种算法的优势,将产品设计、选材和工艺设计三个独立的过程紧密结合起来,可快速分析产品设计中的潜在缺陷,为工艺设计人员提供有效的工艺设计参考和强有力的设计辅助分析工具。     

One step positive approach for sheet metal forming simulation based on quasi-conjugate-gradient method

In one step inverse finite element approach, an initial blank shape is normally predicted from the final deformed shape. The final deformed shape needs to be trimmed into a final part after stamping, the trimmed area, therefore, needs to be compensated manually before using one step inverse approach, which causes low efficiency and in consistency with the real situation. To solve this problem, one step positive approach is proposed to simulate the sheet metal stamping process. Firstly the spatial initial solution of one step positive method is preliminarily obtained by using the mapping relationship and area coordinates, then based on the deformation theory the iterative solving is carried out in three-dimensional coordinate system by using quasi-conjugate-gradient method. During iterative process the contact judgment method is introduced to ensure that the nodes on the spatial initial solution are not separated from die surface. The predicted results of sheet metal forming process that include the shape and thickness of the stamped part can be obtained after the iterative solving process. The validity of the proposed approach is verified by comparing the predicted results obtained through the proposed approach with those obtained through the module of one step inverse approach in Autoform and the real stamped part. In one step positive method, the stamped shape of regular sheet can be calculated fast and effectively. During the iterative solution, the quasi-conjugate-gradient method is proposed to take the place of solving system of equations, and it can improve the stability and precision of the algorithm.