Steel Sheet Edge Heat Treatment for Stretch Flange Crack Resistance
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Solution Overview
Problem
Existing methods for manufacturing high-strength steel sheets face challenges in improving stretch flange formability due to issues like work-hardening, strain concentration, and microstructural differences leading to cracks during press forming, particularly in ultrahigh-strength steel sheets.
Innovation Solution
A method involving individualized heat treatment of steel sheet edges based on microstructural composition, using specific temperature ranges (500° C. to 700° C. or 400° C. to 700° C. depending on microstructure) to improve formability by tempering and reducing strain, thereby minimizing crack formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If shearing is used to form the blank material into a target shape, then mass productivity is improved, but a large amount of strain is introduced into the edge face causing work-hardening and toughness reduction
Solution Approach 1:
The patent applies preliminary heat treatment to the steel sheet before shearing to reduce the material's hardness and improve ductility. This preliminary action prevents excessive work-hardening during shearing, allowing mass productivity to be maintained while avoiding toughness reduction and stretch flange crack formation.
2Strength
If a high-strength steel sheet with composite microstructure of martensite and ferrite is used, then an excellent balance between strength and stretchability is achieved, but strain concentrates on the interface between phases causing crack occurrence
Solution Approach 1:
The patent optimizes the microstructural parameters by controlling the heat treatment process to achieve a specific distribution and proportion of ferrite and martensite phases. This creates a more homogeneous microstructure that reduces strain concentration at phase interfaces, preventing crack formation while maintaining the excellent balance between strength and stretchability.
3Strength
If a high-strength steel sheet with composite microstructure including residual austenite is used, then strain is dispersed and high stretchability is provided, but stress concentration occurs on grain boundary due to volume change during transformation
Solution Approach 1:
The patent controls the amount and distribution of residual austenite through heat treatment parameters, optimizing the transformation behavior during deformation. By adjusting the austenite content and its spatial distribution, the patent reduces volumetric transformation stresses at grain boundaries while maintaining strain dispersion and high stretchability.
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
Enhances stretch flange formability and reduces the likelihood of cracks in high-strength steel sheets, even in multistage press forming, by softening the material and reducing work-hardening effects.
Implementation Method 1
a steel sheet is heated to a predetermined temperature and is thereby softened
Implementation Method 2
forming and quenching are performed at the same time
Data Source
AI summary
A method that improves stretch flange formability of a steel sheet by individual treatment matching a material of the steel sheet without performing heat treatment in a die. This method is a method for manufacturing a steel sheet for cold press, and the steel sheet is manufactured by heating an edge of the steel sheet to a temperature within a heating temperature range preset according to a microstructure of the steel sheet and cooling the steel sheet. A region, within an edge of the steel sheet subjected to shearing in a shearing step, where it is estimated that a stretch flange crack is likely to occur when a press component is formed by cold pressing is determined, and a site to be heated and cooled is set within the region. By press-forming the manufactured steel sheet, a target press component is manufactured.