Ultra-High Strength Steel Sheet With Decarburized Layer Bendability
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Solution Overview
Problem
Existing ultra-high strength steels face issues with shape quality deterioration due to temperature deviations, poor workability during component processing, and decreased elongation, which affect their suitability for cold stamping and bendability, while also requiring high facility investment costs.
Innovation Solution
A steel sheet composition with specific alloy elements (C, Mn, Si, P, S, Al, Cr, Mo, B, Ti, Nb) and a manufacturing process involving heating, hot rolling, coiling, cold rolling, continuous annealing, stepwise cooling, and reheating to achieve a microstructure of 99% martensite and/or tempered martensite, with a decarburization layer for improved bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If water cooling during continuous annealing is used to increase yield strength, then yield ratio increases, but shape quality deteriorates due to temperature deviation and workability decreases
Solution Approach 1:
The invention changes the chemical composition parameters of the steel sheet by precisely controlling the content ranges of alloying elements (C: 0.15-0.30%, Si: 0.05-1.50%, Mn: 1.00-2.50%, Cr: 0.01-0.20%, Mo: 0.01-0.20%, B: 0.0005-0.0050%, Ti: 0.005-0.100%, Nb: 0.005-0.100%, V: 0.005-0.100%). This compositional parameter optimization enables the steel to achieve high yield ratio (70% or more) while maintaining excellent shape quality and bendability, resolving the contradiction between strength improvement and manufacturing precision
Solution Approach 2:
The invention creates local quality differences through the formation of a decarburized layer (5-50 μm thick) on the surface of the steel sheet. This decarburized layer has lower carbon content compared to the interior, providing a soft surface layer that improves bendability and shape quality, while the interior maintains high strength through martensitic microstructure. This local differentiation resolves the contradiction between overall strength and surface quality
2Strength
If steel strength increases to meet safety requirements, then collision performance improves, but elongation decreases and molding processability deteriorates
Solution Approach 1:
The invention optimizes chemical composition parameters to achieve tensile strength of 1200 MPa or more while maintaining elongation of 6% or more. The specific composition ranges (particularly C: 0.15-0.30%, Si: 0.05-1.50%, Mn: 1.00-2.50%) enable this dual achievement, resolving the contradiction between strength and formability
Solution Approach 2:
The decarburized layer formed on the surface (5-50 μm thick) provides local softness that improves molding processability and bendability, while the interior maintains high strength. This local quality differentiation allows the material to be easily formed while achieving ultra-high strength, resolving the contradiction between strength and ease of manufacture
3Strength
If hot press forming method is used to secure high strength, then strength increases, but facility investment costs and process costs increase
Solution Approach 1:
The invention uses conventional cold-rolled steel sheet as a disposable starting material that can be processed using existing, cost-effective equipment. By optimizing the chemical composition and using standard cold rolling and annealing processes, the invention achieves ultra-high strength without requiring expensive hot press forming facilities, thus resolving the contradiction between strength and device complexity
Solution Approach 2:
The invention replaces the mechanical/thermal system of hot press forming with a chemically optimized cold-rolled system. By carefully selecting alloying element compositions, the steel achieves high strength through controlled microstructure formation during conventional annealing, eliminating the need for complex high-temperature press forming equipment
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 ultra-high strength, high yield ratio, and excellent bendability, suitable for cold stamping, while reducing facility costs and improving processability.
Implementation Method 1
continuously annealing the cold-rolled steel sheet for 30 seconds or more at Ac3 or higher
Implementation Method 2
performing primary cooling at an average cooling rate of 1 to 10° C./s to a temperature range of 550 to 750° C. after the continuous annealing; performing secondary cooling at an average cooling rate of 20 to 80° C./s to a temperature of Ms−190° C. or less
Implementation Method 3
performing reheating after the secondary cooling and then performing an over-aging treatment
Data Source
AI summary
The present invention relates to a steel sheet suitable for automobile chassis members, etc., and, more particularly, to an ultra-high strength steel sheet having excellent bendability, and a manufacturing method therefor.