High-Strength Steel Sheet Bending Workability via Multi-Phase Control
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Solution Overview
Problem
Current high-strength steel sheets with increased strength suffer from decreased bending workability and variability in mechanical properties, making them unsuitable for applications requiring stable bending performance across the product, particularly in automobile manufacturing.
Innovation Solution
A high-strength steel sheet with a tensile strength of 980 MPa or more is achieved by controlling the chemical composition and microstructure to include a multi-phase microstructure of ferrite, martensite, and bainite phases, with specific area ratios and grain diameters, and employing a manufacturing process involving hot rolling, pickling, cold rolling, and continuous annealing to stabilize bending workability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of a steel sheet is increased to reduce weight, then the tensile strength is improved, but the bending workability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 0.70-1.50%, Mn: 1.50-3.00%, etc.) and microstructural parameters (grain size, phase distribution) to achieve a balance between high tensile strength (1320-1570 MPa) and good bending workability, resolving the contradiction through optimized material parameters
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, martensite, bainite) with specific area ratios (ferrite: 30-60%, martensite: 20-50%, bainite: 10-40%). This multi-phase composite structure provides both the strength from hard phases and the ductility from softer phases, simultaneously achieving high strength and bending workability
2Strength
If the strength level of a high-strength steel sheet is increased, then the tensile strength is improved, but the variation in mechanical properties within a product increases
Solution Approach 1:
The patent controls composition parameters within narrow ranges (C: 0.15-0.35%, Si: 0.70-1.50%, Mn: 1.50-3.00%) to minimize variation. This precise parameter control ensures consistent mechanical properties across the product while maintaining high tensile strength, reducing the variation that typically increases with strength level
Solution Approach 2:
The patent promotes microstructural homogeneity by controlling phase distribution and grain size uniformity across the steel sheet. The multi-phase structure is distributed evenly throughout the product, ensuring consistent bending workability and mechanical properties at different positions, thereby reducing variation within the product
3Ease of operation
If long-time annealing in a batch annealing furnace is performed after cold rolling, then the bending workability is improved, but the productivity decreases
Solution Approach 1:
The patent employs continuous annealing instead of batch annealing, where the steel sheet passes continuously through the annealing furnace. This continuous process eliminates the downtime between batches while maintaining the necessary annealing effect for good bending workability, thereby preserving productivity
Solution Approach 2:
The patent performs cold rolling to a predetermined thickness and microstructure state before annealing, preparing the steel sheet in advance with optimal characteristics. This preliminary cold rolling creates a microstructure that responds well to subsequent annealing, achieving good bending workability through the combined effect of controlled cold rolling followed by continuous annealing
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 resulting steel sheet exhibits excellent bending workability stability and strength, enabling weight reduction and increased fuel efficiency in automobiles while maintaining part yield and productivity.
Implementation Method 1
a multi-phase microstructure including, in terms of area ratio, 30% or more of a ferrite phase, 40% to 65% of a bainite phase and/or a martensite phase
Implementation Method 2
a method for manufacturing the steel sheet, comprising a hot rolling process, a pickling process, and a continuous annealing process
Implementation Method 3
continuous annealing to stabilize bending workability
Data Source
AI summary
Provided are a high-strength steel sheet having a tensile strength of 980 MPa or more and excellent bending workability and a method for manufacturing the steel sheet. The high-strength steel sheet excellent in terms of bending workability has a specified chemical composition with the balance being Fe and inevitable impurities, a microstructure including, in terms of area ratio, 30% or more of a ferrite phase, 40% to 65% of a bainite phase and/or a martensite phase, and 5% or less of cementite, in which, in a surface layer that is a region within 50 µm from the surface in the thickness direction, the area ratio of a ferrite phase is 40% to 55% and the total area ratio of a bainite phase having a grain diameter of more than 5 µm and/or a martensite phase having a grain diameter of more than 5 µm is 20% or less, and a tensile strength is 980 MPa or more.