Two-Step Press Forming for High-Strength Steel Spring Back
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Solution Overview
Problem
Press forming methods for high-tensile and ultra-high-tensile strength steel sheets in vehicle body structural components face issues such as spring back, twisting, and dimensional accuracy fluctuations due to stress variations, particularly when forming components with U-shaped or L-shaped cross-sections, leading to increased production costs and complexity.
Innovation Solution
A two-step press forming method using a die set with adjusted vertical wall angles, first forming an intermediate component with a smaller angle followed by a second step with a larger angle to match the target component shape, reducing stress and improving dimensional accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a single-step press forming method is used with a die set matching the target component shape, then the forming process is simple, but spring back and twisting occur due to stress concentration at the bottom dead center
Solution Approach 1:
The press forming process is divided into two distinct steps: a first step using a die set with a vertical wall angle smaller than the target component shape, and a second step using a die set with a vertical wall angle equal to or greater than the target component shape. This segmentation allows stress distribution optimization, reducing spring back and twisting while maintaining process simplicity.
Solution Approach 2:
The first step performs preliminary forming with a die set designed to create an intermediate component with reduced stress concentration. By pre-forming the component with a smaller vertical wall angle, the material is prepared in a way that minimizes spring back in the subsequent second step, improving final dimensional accuracy.
2Productivity
If the vertical wall angle of the die set is made equal to the target component shape from the beginning, then the forming process is efficient, but stress concentration causes increased spring back and twisting
Solution Approach 1:
The forming process is segmented into two steps with different die set configurations. The first step uses a die set with a smaller vertical wall angle to reduce stress concentration, while the second step uses a die set with the target vertical wall angle to achieve the final shape. This segmentation maintains productivity by using a straightforward two-step process while improving vertical wall angle accuracy through optimized stress distribution.
3Strength
If ultra-high-tensile strength steel sheets are used to improve collision safety and weight reduction, then material strength increases, but stress at the bottom dead center increases leading to greater spring back
Solution Approach 1:
The vertical wall angle parameter of the die set is changed between the first step (smaller angle) and second step (target angle). This parameter change allows the process to accommodate ultra-high-tensile strength steel sheets by reducing stress concentration in the first step, thereby controlling spring back despite the high material strength.
4Reliability
If material strength fluctuation occurs in ultra-high-tensile strength steel sheets, then production cost and complexity increase, but dimensional accuracy fluctuates due to changes in spring back amount
Solution Approach 1:
By changing the vertical wall angle parameter of the die set between the first and second steps, the process becomes less sensitive to material strength fluctuations. The first step with a smaller angle reduces stress concentration, making the overall process more robust against variations in ultra-high-tensile strength steel sheet properties, thereby stabilizing dimensional accuracy.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method effectively reduces twisting and camber back, maintains vertical wall angle stability, and enhances dimensional accuracy, even with material strength fluctuations, using a simpler die structure.
Implementation Method 1
a first step of press-forming a metal sheet into an intermediate component and a second step of press-forming the intermediate component
Implementation Method 2
forming defects such as spring back are problems in performing press forming
Data Source
Figure 1A~1C
Figure 2
Figure 3
AI summary
Even in a case where a high-tensile strength steel sheet or the like is used, a dimensional accuracy fluctuation in a case where twisting or camber back, wall opening, and a material strength fluctuation thereof occurs is reduced with a simple die structure. A method for manufacturing a press formed article for forming a metal sheet (3) into a target component shape (1) having a bending portion (2) which is bent along a longitudinal direction in a cross section including a top plate portion (1A) and a vertical wall portion (1B) is provided. An angle between the top plate portion (1A) and the vertical wall portion (1B) is referred to as a vertical wall angle. A first step (10A) of press-forming an intermediate component (4) using a die set having a forming surface of a first vertical wall angle smaller than a vertical wall angle of the target component shape (1), and a second step (10B) of press-forming the intermediate component (4) with a die set having a forming surface of a second vertical wall angle equal to or greater than the vertical wall angle of the target component shape (1) are included. The vertical wall angle of the intermediate component (4) after release is set to the first vertical wall angle having an angle smaller than the vertical wall angle of the target component shape (1).