High-Strength Steel Sheet Microstructure for Bendability
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Solution Overview
Problem
High strength steel sheets with a tensile strength of not less than 1,180 MPa face issues with delayed fracture due to residual stress and poor bendability, particularly at shear end surfaces, necessitating improved formability and resistance to cracking during bending.
Innovation Solution
A high strength steel sheet with specific chemical composition and microstructure, including 70% martensite, 3-20% retained austenite, and limited ferrite and bainitic ferrite, along with controlled cooling and heat treatments, to enhance bendability and delayed fracture resistance, and a wide optimal clearance range for shear end surfaces.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel sheets with tensile strength of not less than 1,180 MPa are used, then strength is improved, but delayed fracture resistance deteriorates due to increase in residual stress
Solution Approach 1:
The invention changes the chemical composition parameters (C: 0.23-0.38%, Si: 1.00-2.00%, Mn: 2.00-4.00%, Al: 0.010-1.000%, Nb: 0.005-0.100%) and microstructural parameters (martensite amount: 70-90%, retained austenite amount: 3-20%, instability index k: 5.0-6.5, instability index d: 4.5-5.7) to achieve a balance between high tensile strength and delayed fracture resistance. The specific composition ranges and microstructural controls modify the material properties to reduce residual stress while maintaining strength.
Solution Approach 2:
The invention creates a composite microstructure consisting of martensite (70-90%), retained austenite (3-20%), and controlled amounts of ferrite and bainitic ferrite. This multi-phase composite structure combines the high strength of martensite with the ductility and stress-absorbing capabilities of retained austenite, thereby improving delayed fracture resistance while maintaining high tensile strength.
2Strength
If high strength steel sheets with tensile strength of not less than 1,180 MPa are used, then strength is improved, but bendability deteriorates due to cracking at bent ridge portion
Solution Approach 1:
The invention adjusts the chemical composition parameters (particularly Si: 1.00-2.00%, Mn: 2.00-4.00%, and Al: 0.010-1.000%) and microstructural parameters (retained austenite amount: 3-20%, instability index d: 4.5-5.7) to improve bendability. The controlled instability indices and retained austenite content allow the material to undergo bending without cracking at the bent ridge portion while maintaining high tensile strength.
3Strength
If high strength steel sheets are used, then strength is improved, but optimal clearance range for shear end surface bending becomes narrow
Solution Approach 1:
The invention modifies the chemical composition parameters (C: 0.23-0.38%, Si: 1.00-2.00%, Mn: 2.00-4.00%, Nb: 0.005-0.100%) and microstructural parameters (instability index k: 5.0-6.5, martensite amount: 70-90%) to expand the optimal clearance range for shear end surface bending. The specific composition and microstructure provide a wider processing window that accommodates variations in shear clearance while maintaining formability and preventing cracking.
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 solution provides a steel sheet with enhanced tensile strength, improved bendability, and resistance to delayed fracture, suitable for automotive applications, contributing to lighter vehicle bodies and better fuel efficiency.
Implementation Method 1
the steel sheet has a microstructure in which an amount of martensite is not less than 70%, an amount of retained austenite is not less than 3% and not more than 20%, and a total amount of ferrite and bainitic ferrite is not more than 10%
Implementation Method 2
a heat treatment A in which the cold rolled steel sheet is retained at a temperature T1 of not lower than 800°C for 10 seconds or more and then cooled
Data Source
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Figure 4~5
AI summary
There is provided a high strength steel sheet having a tensile strength of not less than 1,180 MPa, having excellent bendability and excellent delayed fracture resistance, and having a wide optimal clearance range with respect to bending forming of a shear end surface. A steel sheet included in the high strength steel sheet contains, by mass, C: 0.030%-0.500%, Si: 0.50%-2.50%, Mn: 1.50%-5.00%, P: not more than 0.100%, S: not more than 0.0200%, Al: not more than 1.000%, N: not more than 0.0100%, O: not more than 0.0100%, and Nb: 0.005%-0.100%, with a balance consisting of Fe and inevitable impurities, an amount of martensite is not less than 70%, an amount of retained austenite is 3%-20%, a total amount of ferrite and bainitic ferrite is not more than 10%, an instability index k of retained austenite is less than 6.1, and an instability index d of retained austenite in an initial stage of working is less than 5.7.