In the production of stainless steel bodies for rail vehicles, many beam and column components, such as the pillars of side walls and the curved beams of the roof, require stamping forming. Due to their complex shapes, high precision requirements, and large dimensions, it is difficult to achieve the required shape and accuracy using conventional stamping methods, which can lead to quality issues such as wrinkling and springback. The bending process of CNC stretch forming technology, by changing the internal stress distribution of the section, reducing springback, eliminating wrinkling defects on the compressed edges of the section, and significantly improving forming accuracy, has become a core technology in the manufacturing of stainless steel vehicle bodies. It is of great significance in improving the quality and production capacity of rail vehicles and reducing production costs.
Principle of Stretch Forming Process
Bending refers to the stretch forming of the material while applying tangential tension, which changes the internal stress of the section to tensile stress. It is a profile bending process for deforming materials.
Applicable Range of Stretch Forming Process
The stretch forming process is mainly used for stretch forming parts with large length and curvature radius. When using conventional methods to bend such parts, most of the deformation zone is in the elastic deformation state, resulting in significant springback and difficult forming. By applying the stretch forming process, the internal stress distribution of the section can be changed, reducing springback and improving forming accuracy. It can also eliminate wrinkling defects on the compressed edges of the section and improve forming quality.
Factors Influencing the Bending Process
The stretch forming process is a highly complex bending technique, and the forming process is influenced by many factors, such as friction and contact, geometric structure of the bending die, material properties, profile shape parameters, and stretch forming process parameters. This is mainly reflected in how to overcome the influence of springback.
- Springback: Springback is an essential part of metal deformation as long as there is elastic deformation. After the surface of the bent part enters the plastic zone, the center remains in the elastic state. Therefore, springback occurs after the external force is removed. It also involves the adjustment of residual stresses within the workpiece after forming.
- Factors affecting springback: Material mechanical properties, bending radius and wall thickness, bending angle, and the shape of the bent part.
- Methods to reduce springback: Improving the internal stress distribution of the section, making the section stress uniform, reducing springback, and improving forming accuracy.
Distribution of Section Strain in Bending Forming
The section strain distribution of the bending-formed workpiece is uniform, with both upper and lower layers experiencing tensile stress and the opposite direction of springback, effectively reducing springback.
Main Features of CNC Bending for Stainless Steel Profiles
- Difficult deformation material, large springback, and challenging control of profile accuracy.
- Thin wall thickness, poor stiffness, significant variation in section dimensions, and difficult control of section size.
Analysis of Main Defects and Solutions for Stainless Steel Profile Bending Process and Components in Urban Rail Vehicles
The materials used for the curved beams of the stainless steel car roof in urban rail vehicles are all SUS301L stainless steel. The following discusses the main defects in stainless steel profile bending and the process methods to overcome these defects, using the example of the first type of curved beam section.
Solutions for main defects:
- Wrinkling and fracture control: Overcome wrinkling defects by adjusting the tension parameters to ensure that the minimum strain in the section is a positive value. Also, pay attention to controlling the maximum strain value of the curve to be smaller than the material’s forming limit to avoid fracture.
- Control of section distortion defects: By analyzing and predicting the contraction dimensions of various parts of the bent section and compensating for the section size accordingly during blanking and bending, the method of eliminating section distortion is employed.
Stretch Forming Mchine
The main equipment for stretch forming is the profile bending machine, which can be divided into two types based on the operating system: CNC and non-CNC. Based on the position of the stretch forming arm, it can be classified into vertical and horizontal types. Based on the working table’s action mode, it can be categorized as rotary table and swing arm types. The selection of the bending machine for bending forming depends mainly on the product requirements.
This article proposes a lightweight stainless steel urban rail vehicle component stretch forming process, which combines the situation in the bending industry at home and abroad with the actual production of urban rail vehicles in the country. It has been widely applied in practical production and has significant technological research space and practical economic value. The article discusses the main defects and quality issues that may occur during the stretch forming process of stainless steel components and presents feasible solutions, which have extensive guiding significance for improving the industry’s technological level and the manufacturing level of lightweight stainless steel urban rail vehicles.