Hot stamping process of B-pillar high strength steel

1.Hot stamping process of high strength steel
The hot stamping process of high-strength steel, also known as hot forming technology or stamping hardening technology, is a kind of processing process in which high-strength steel plates in red-hot stage are used as raw materials for stamping and finally forming. In the process of hot stamping, it is first necessary to calculate the consumption of raw materials and blank them for processing, then heat the cut high-strength steel in a heating device until the high-strength steel is austenitized, at which time the heating temperature of the heating device is in the range of 880 ~ 950℃, and finally put the austenitized high-strength steel material on a die with cooling function for material forming. In this process, the surface of the die will make the high-strength steel cool and quench, and phase change will occur, and finally martensite will be formed. Relevant experimental research shows that the hot stamping process can improve the strength of steel from 500~600 MPa to 1 500 MPa, which means that the strength of automobile B-pillar processed by hot stamping process is greatly improved, which can be improved to about 250% of the original strength. Therefore, hot stamping technology has been widely respected in the forming process of automobile high strength steel.

Compared with the traditional processing method of common cold stamping, the differences and advantages of hot stamping are mainly manifested in three aspects. First, the high-strength steel material heated by hot stamping process has less deformation resistance, good plasticity, higher forming limit, and is easier to form. The above advantages have a great role in reducing the requirements of materials for forming equipment and molds; Secondly, in view of the very high strength of automobile parts made of ultra-high strength steel, it will play a vital role in improving the impact resistance and fatigue ability of the car body and increasing the safety of the car. Thirdly, because the hot stamping process of high-strength steel can greatly improve the strength and impact resistance of parts, it can effectively reduce the number of reinforcing plates, simplify the design of body structure and parts, and greatly promote the development goal of automobile industry towards lightweight body. Hot stamping parts are widely used in automobile body supporting structure parts such as B-pillar, front and rear bumpers, longitudinal load-bearing beams, etc. Taking automobile B-pillar high-strength steel forming as an example, the hot stamping process is studied and discussed, which is helpful to quickly and accurately study the forming law and heat transfer law of hot stamping process, and provides new ideas and solutions for the analysis and solution of key problems such as die design related to hot stamping process.

2. Hot stamping process of automobile B-pillar
When forming high-strength steel for automobile B-pillar, the structural characteristics of automobile B-pillar, such as high boss at the top, deep steps at both ends and other complex and difficult structural factors, should also be taken into account, such as the problems and difficulties that the weak areas are easy to deform and wrinkle during processing and forming. At present, the technological process for forming high-strength steel for automobile B-pillar includes four processes: drawing, trimming, punching and shaping. When analyzing the hot stamping process of automobile B-pillar high-strength steel, the computer analysis software is used to study it according to three steps: determining the parameter value, numerical simulation pre-processing and numerical result analysis.

2.1 Determine the parameter values
The finite element model of automobile B-pillar is built with the help of three-dimensional modeling software CATIA, which is convenient for subsequent research and analysis. In view of the fact that there are many influencing factors of hot stamping process parts, and the influencing factors are not independent, such as temperature and plasticity, the thermal-mechanical-phase transformation coupling analysis method is often used in engineering practice. Setting material parameters, steel thickness and final temperature after heating when modeling high strength steel. In the early stage of hot stamping process, although the mold temperature will gradually increase due to contact with the sheet metal, the internal cooling channel will also play a cooling role, so the mold temperature in the early stage is approximately unchanged. When the hot stamping process of automobile B-pillar high-strength steel is simulated by finite element method, the whole process is heating, conveying, forming, quenching and cooling in turn.

2.2 Pre-processing of numerical simulation
The pre-processing steps of numerical simulation of hot stamping process for automobile B-pillar high-strength steel are as follows: model introduction, clear stamping, blank surface design, drawbead design and supplementary surface design. Firstly, the finite element model of automobile B-pillar is constructed, and the property of adding B-pillar high-strength steel to the model is simulated by software. The B-pillar finite element model constructed in CATIA is imported into Autoform software, and the bending reinforced membrane element type is given to analyze the B-pillar finite element model. Secondly, the specific direction of stamping is determined, and the stamping direction is determined after the parameter values are clear. The determination principle of this process should ensure that the stamping direction is consistent with that of other processes as much as possible. The purpose of this step is to reduce the turnover times, which is beneficial to assembly line operation, save production time and ensure the quality of model manufacturing. Secondly, determining the stamping direction under this principle is also conducive to relieving the pressure of operators and reducing their labor intensity.

The purpose of designing the blank pressing surface is to facilitate the sheet metal forming, and to ensure that the high-strength steel sheet will not be damaged and cracked during the hot working forming process. In this process, the shape of B-pillar high-strength steel parts and the design concept of blanking surface need to be considered to ensure that the blanking surface is smooth and convenient for the sheet to move to the die. Drawing is the key point and difficulty of hot stamping process. In order to prevent cracking, necking and wrinkling in the production process, the flow state of sheet metal is improved by designing drawing ribs to improve the flow uniformity of parts and further ensure the forming quality of B-pillar high-strength steel parts. For the supplementary surface, the design process includes two parts: inside and outside. Wherein the internal process supplementary surface is used for filling the internal holes and chamfering of the parts; The function of external process supplementary surface is to connect the blanking surface with the outer contour line of the part according to the designed specific draft angle. All the above processes can be simulated in Autoform software, and the hot stamping process of automobile B-pillar high-strength steel can be pre-processed by numerical simulation.

2.3 Analysis of numerical results
The surface hardness, strength standard and other factors of automobile B-pillar high-strength steel materials are directly related to the application of hot stamping process. The function of finite element simulation design is to make clear the strength, hardness, compressive properties of sheet metal and so on through the analysis of numerical simulation results of automobile B-pillar. Both numerical analysis and engineering practice show that the overall thickness of automobile B-pillar high-strength steel is uniform after hot stamping forming process, and there are no common problems such as damage, cracking and wrinkling in traditional processing. In addition, the hardness and strength of B-pillar are higher than the design standard requirements, and the product forming effect is good.

3. Matters needing attention in selecting hot stamping process die
For the traditional cold stamping die, only the function of the die is needed to be considered when selecting. However, for the hot stamping process of automobile B-pillar high-strength steel, because the function of the die serves the part forming on the one hand and plays the role of cooling and quenching on the other hand, the selection of the die for the hot stamping process is usually more complicated. Therefore, more attention should be paid to the selection of hot stamping process die for automobile B-pillar high-strength steel. Not only the material selection of the die, but also the die design should be controlled more strictly. Only in this way can the die meet the requirements of both component forming and cooling quenching.

First of all, the thermal conductivity of the die material is good, and only in this way can the steel plate and the die surface be promoted to conduct heat efficiently and play a cooling function. Secondly, the mold material has high thermal conductivity, high hardness, good wear resistance and excellent thermal fatigue performance, which can always maintain stable size and good hardness during the forming process and bear the inevitable wear during the processing; In addition, good performance should be maintained under the working condition of alternating hot and cold, which will not affect the service life. Finally, the mold material has good corrosion resistance, in order to avoid internal cooling pipes from being corroded. At present, the choice of die materials for hot stamping in the world is usually given priority to materials containing high Ni and Cr elements. The choice of die materials should meet the actual needs, and the die materials should be reasonably selected by comprehensively considering various factors such as thermal conductivity, thermal conductivity, hardness, wear resistance, thermal fatigue performance, corrosion resistance and service life.

Although the hot stamping process has many advantages, its resilience is poor, which puts forward new requirements for designing the die for the hot stamping process of B-pillar high-strength steel-based on the principle of thermal expansion and cold contraction, the cooling system can be cooled more quickly and evenly. Therefore, in addition to a series of requirements for selecting die materials, external factors such as system sealing should also be considered in the die design of hot stamping process for B-pillar high-strength steel. Combined with numerical result analysis and numerical design, the die for hot stamping process of B-pillar high-strength steel is designed to ensure the pipeline effect. In addition, the hot stamping process has unique and distinct characteristics in the design of forming parts and process debugging, which may have an impact on the forming of B-pillar, so it is still necessary to analyze these characteristics in detail.

High-strength steel materials are widely used in many structural parts of automobiles, especially automobile B-pillar, because they can reduce the weight of automobile body, meet the strength requirements of collision, and save energy and protect the environment. Although high-strength steel has excellent strength, the traditional stamping process can not process high-strength steel because of its poor formability, large forming force, difficult control of springback deformation and tensile crack defects. At present, only hot stamping process can solve a series of problems of traditional stamping process in the field of high-strength steel processing. Through the related research and development of hot stamping process for automobile B-pillar high-strength steel, the difficulties and key points of applying hot stamping process are analyzed and studied from the aspects of hot stamping process flow, numerical simulation analysis and die selection, and the advantages, characteristics, rationality and scientificity of applying hot stamping process are discussed. The hot stamping process of automobile B-pillar high-strength steel can not only meet the performance and processing requirements of hot-formed parts in automobile industry, but also support the development and innovation of domestic automobile lightweight technology and meet the requirements of safety collision. It is of great significance to study the hot stamping process of automobile B-pillar high-strength steel.

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