Press-Molded Hat Sections With Two-Step Forming to Prevent Cracks
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Solution Overview
Problem
Existing methods for producing press formed products from high-strength steel sheets, such as those with hat-shaped cross-sections, face challenges in reducing cracks in shape-changing parts and warping of vertical wall parts, particularly due to tensile and bending stresses during pad bending forming.
Innovation Solution
A method involving a two-step press working process using a first and second press forming apparatus, where the first step forms the top panel and ridge line parts with alleviated restraint, and the second step forms the vertical wall and flange parts, employing a pad adjusting mechanism to control the distance between the pad and punch, and a sub punch mechanism to delay the formation of flange parts, ensuring the predetermined height condition is met.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If pad bending forming is used to produce press formed products from high-strength steel sheets, then the shape fixability and material rupture resistance are improved, but cracks occur in shape-changing parts and warping occurs in vertical wall parts due to tensile and bending stresses
Solution Approach 1:
The press forming process is divided into two distinct stages: a first press working step that forms the top panel and ridge line parts, and a second press working step that forms the vertical wall and flange parts. This segmentation allows each stage to optimize for its specific forming requirements, reducing excessive tensile and bending stresses that cause cracks in shape-changing parts.
Solution Approach 2:
The first press working step performs preliminary forming of the top panel and ridge line parts before the second step forms the vertical walls and flanges. This preliminary action prepares the workpiece in an intermediate state that reduces stress concentration during the final forming operation, preventing cracks in shape-changing parts while maintaining shape fixability.
2Strength
If high-strength steel sheets are used to improve collision safety and reduce car body weight, then the tensile strength and fuel economy are improved, but the formability and resistance to cracking during press working deteriorate
Solution Approach 1:
The manufacturing process is segmented into two press working steps, allowing the formation of different product features at optimal times. This segmentation enables high-strength steel sheets to be formed successfully by reducing the complexity of single-step forming, thereby improving ease of manufacture while maintaining the high tensile strength properties.
Solution Approach 2:
The first press working step performs partial forming of the product, creating an intermediate state with formed top panel and ridge line parts. This partial action reduces the overall forming complexity, making it easier to manufacture press formed products from high-strength steel sheets that would otherwise be difficult to form in a single step.
3Object-affected harmful factors
If the pad is held at a position distant from the punch during pushing-in, then the tensile stress in shape-changing parts is reduced, but the forming time and process complexity increase
Solution Approach 1:
The forming process is segmented into two time-efficient steps with the pad positioned optimally in each. This segmentation reduces tensile stress in shape-changing parts by controlling pad position during the first step, while maintaining overall forming efficiency through the structured two-step approach rather than prolonged single-step forming.
Solution Approach 2:
The first press working step performs preliminary forming with the pad held at an optimized distance from the punch, reducing tensile stress in shape-changing parts before the second step completes the forming. This preliminary action with controlled pad positioning reduces harmful stresses while maintaining reasonable forming time through efficient sequential operations.
Data Source
Figure 1~2A
Figure 2B~3
Figure 4~5
AI summary
A press formed product includes a top panel part, vertical wall parts, and ridge line parts between the vertical wall parts and the top panel part, and includes a shape-changing part in a part of the ridge line parts. A producing method of the press formed product includes a preparation step of preparing a metal sheet as a starting material; and a press working step of performing press working on the starting material by using a punch, a pad arranged opposed to a front end surface of the punch, and a die capable of receiving the pad. The press working step includes: during a time period from when pushing-in of the starting material into the die by the punch is started or immediately thereafter until the pushing-in of the punch with respect to the die reaches a predetermined distance short of a bottom dead point for the pushing-in, holding the pad at a position distant from the punch by the predetermined distance; when the pushing-in of the punch with respect to the die reaches the predetermined distance short of the bottom dead point, receiving the pad in the die; and continuing the pushing-in of the punch with respect to the die and the pad to the bottom dead point, so as to form the top panel part, the ridge line parts, and the vertical wall parts.