Steel Sheet Bake-Hardenability via Microsegregation Control
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Solution Overview
Problem
Current steel sheets for vehicles face challenges in achieving high bake-hardenability while maintaining sufficient ductility and workability, particularly when pre-strain is low, which limits their application in forming processes.
Innovation Solution
A steel sheet with a chemical composition of C: 0.05% to 0.30%, Si: 0.2% to 2.0%, Mn: 2.0% to 4.0%, and controlled microsegregation to achieve a hard structure of 95% or more, along with optimized hot rolling and temper rolling processes to increase dislocation density, enhancing bake-hardenability and press formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening is applied to improve fuel efficiency, then vehicle weight reduction is achieved, but ductility decreases and cold press forming becomes difficult
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area ratio of hard structure to 95% or more and adjusting chemical composition parameters (C: 0.05-0.30%, Si: 0.2-2.0%, Mn: 2.0-4.0%) to achieve both high strength and improved ductility through optimized phase distribution
2Strength
If high bake-hardenability is achieved through increased pre-strain, then bake-hardening amount increases, but ductility decreases and application to members requiring excellent ductility becomes difficult
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area ratio of hard structure to 95% or more and adjusting chemical composition parameters to achieve high bake-hardenability without requiring increased pre-strain, thereby maintaining ductility
Solution Approach 2:
The invention creates a composite microstructure consisting of hard structure (martensite/bainite) and residual austenite, where the hard structure provides strength and the controlled microsegregation enhances bake-hardenability, achieving both high strength and ductility
3Strength
If microsegregation is present in DP steel, then hard phase becomes continuous in band shape, but hole expansibility and bendability significantly deteriorate
Solution Approach 1:
The invention changes the microstructural parameters by controlling the area ratio of hard structure to 95% or more while managing the distribution characteristics through chemical composition control, achieving strength without the harmful continuous band structure that deteriorates hole expansibility
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 steel sheet exhibits excellent bake-hardenability with a bake-hardening amount of 150 MPa or more at 0.5% pre-strain, improving formability and strength, making it suitable for vehicle structural components.
Implementation Method 1
Mn is diffused by being held in a temperature range of 1200°C or higher and 1300°C or lower for 0.5 h or longer and 5 h or shorter before a hot rolling step
Implementation Method 2
Bake-hardening is a strain aging phenomenon that occurs when interstitial elements (carbon and nitrogen) are locked to dislocations that are introduced by press forming
Data Source
AI summary
A steel sheet includes, as a chemical composition, by mass%: C: 0.05% to 0.30%; Si: 0.2% to 2.0%; Mn: 2.0% to 4.0%; Al: 0.001% to 2.000%; P: 0.100% or less; S: 0.010% or less; N: 0.010% or less; Ti: 0% to 0.100%; Nb: 0% to 0.100%; V: 0% to 0.100%; Cu : 0% to 1.00%; Ni: 0% to 1.00%; Mo: 0% to 1.00%; Cr: 0% to 1.00%; W: 0% to 0.005%; Ca: 0% to 0.005%; Mg: 0% to 0.005%; a rare earth element (REM): 0% to 0.010%; B: 0% to 0.0030%; and a remainder of Fe and impurities, in which a metallographic structure contains, by area ratio, 95% or more of a hard structure and 0% to 5% of residual austenite, by mass% in a cross section in a thickness direction, C1/C2 which is a ratio of an upper limit C1 of a Mn content to a lower limit C2 of the Mn content is 1.5 or less, and a bake-hardening amount BH is 150 MPa or less.