Steel Sheet Martensite Transformation Shape Uniformity
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Solution Overview
Problem
Steel sheets used for automotive parts face challenges in achieving high strength, excellent shape uniformity, and excellent delayed fracture resistance, particularly over the full width of the steel sheet.
Innovation Solution
The production of steel sheets with a microstructure comprising 20% or more martensite and 0% to 80% ferrite, along with controlled residual stress and binding conditions during cooling, to limit transformation strain and enhance shape uniformity and delayed fracture resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If martensite is formed to increase the strength of the steel sheet, then the strength is improved, but the transformation strain degrades the uniformity of the shape of the steel sheet
Solution Approach 1:
The steel sheet is bound with rollers before martensite transformation occurs during water quenching. This preliminary binding action prevents the transformation strain from causing shape degradation, thereby maintaining shape uniformity while still allowing martensite formation for high strength
Solution Approach 2:
The steel sheet is bound with rollers at the appropriate temperature range (Ac3 transformation point or higher) before martensite transformation occurs. This preliminary binding action ensures that when martensite forms during subsequent water quenching, the shape uniformity is maintained while strength is improved
2Shape
If levelling or skin pass rolling is performed to correct the shape of the steel sheet, then the uniformity of the shape is improved, but the dimensional accuracy achieved in forming is degraded
Solution Approach 1:
The steel sheet is bound with rollers before martensite transformation occurs, preventing shape degradation at the source. This eliminates the need for subsequent levelling or skin pass rolling operations, thereby preserving the dimensional accuracy achieved in forming
Solution Approach 2:
The invention removes the need for levelling or skin pass rolling operations by preventing shape degradation through roller binding during martensite transformation. This extraction of the correction step avoids the introduction of additional strain that would degrade dimensional accuracy
3Shape
If the steel sheet is bound with rollers during water quenching to limit shape degradation, then the uniformity of the shape is improved, but the delayed fracture resistance is not enhanced to an excellent level
Solution Approach 1:
The invention changes the parameters of the binding process, specifically applying binding at the Ac3 transformation point or higher temperature and maintaining binding during water quenching. This parameter change ensures both shape uniformity is improved and delayed fracture resistance is enhanced to an excellent level by controlling transformation strain distribution
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 approach results in steel sheets with high strength, excellent shape uniformity, and enhanced delayed fracture resistance over the full width of the steel sheet, improving the performance of automotive structural members.
Implementation Method 1
when martensite is formed, transformation strain degrades the uniformity of the shape of a steel sheet
Implementation Method 2
martensite transformation occurs during water quenching at a high speed
Data Source
AI summary
A steel sheet having a steel microstructure including, by area %, martensite in a range of 20% or more and 100% or less, ferrite in a range of 0% or more and 80% or less, and the balance being 5% or less. A residual stress generated at the transverse center of the steel sheet when the steel sheet is subjected to a V-bending process is 800 MPa or less. A residual stress generated at the transverse end of the steel sheet when the steel sheet is subjected to a V-bending process is 90% or more and 110% or less of the residual stress generated at the transverse center of the steel sheet. The maximum amount of warpage of the steel sheet sheared to a length of 1 m in the longitudinal direction is 15 mm or less.