Steel Sheet Boron Gradient for Strength and Bendability
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Solution Overview
Problem
Current technologies face challenges in maintaining the strength of high-strength steel sheets while improving their bendability, particularly for ultra-high strength hot-dip galvanized steel sheets, as existing methods either compromise strength or suffer from reduced fatigue strength due to oxidized layers or difficulties in applying heat treatment on hot-dip galvanization lines.
Innovation Solution
A high-strength cold-rolled steel sheet, hot-dip galvanized steel sheet, and galvannealed steel sheet are developed by controlling the presence of boron (B) as a hardenability element, with specific chemical compositions and microstructures that balance strength and bendability, including a boron ratio in the surface and interior layers, and optimized production processes such as hot rolling, cold rolling, and annealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheet is increased to achieve ultra-high strength (980 MPa or more), then the tensile strength is improved, but the bendability deteriorates and fissures occur during bending
Solution Approach 1:
The invention applies local quality by creating a soft layer (ferrite-rich region) at the surface layer portion (0-30 μm depth) while maintaining a hard layer (martensite-bainite region) at the interior. This gradient structure allows the surface to accommodate bending deformations without cracking while the interior maintains high strength. The specific chemical composition controls this microstructure formation, with B content of 0.0003-0.005% and C content of 0.05-0.40% being critical for achieving the desired surface softness and interior hardness.
Solution Approach 2:
The invention utilizes parameter changes by controlling the chemical composition parameters (C: 0.05-0.40%, B: 0.0003-0.005%, Mn: 1.0-5.0%, Si: 0.01-3.0%) and heat treatment parameters (heating temperature, cooling rate) to transform the microstructure. The specific parameter ranges enable the formation of a dual-zone microstructure with different mechanical properties, achieving both high strength and improved bendability through compositional and thermal parameter optimization.
2Ease of operation
If a soft layer is formed at the surface layer portion to improve bendability, then the bendability is improved, but the strength may be compromised
Solution Approach 1:
The invention segments the steel sheet into two distinct functional zones: a surface layer portion (0-30 μm) with soft ferrite microstructure for bendability, and an interior layer with hard martensite-bainite microstructure for strength. This segmentation is achieved through controlled heat treatment that creates a clear microstructural boundary, allowing each zone to fulfill its specific function without compromising the other.
Solution Approach 2:
The invention creates a composite microstructure within the steel sheet, combining ferrite (soft phase) in the surface layer with martensite and bainite (hard phases) in the interior. This composite structure at the micro-scale enables the material to exhibit both soft surface behavior for bending and hard interior behavior for strength, effectively achieving properties not possible in homogeneous materials.
3Reliability
If hot-dip galvanization is applied to regions requiring rust preventing properties, then the corrosion resistance is improved, but the fatigue strength may be reduced due to oxidized layers
Solution Approach 1:
The invention applies local quality by forming a soft ferrite-rich surface layer that serves as a protective barrier for the hot-dip galvanized coating. This surface layer modifies the local properties at the coating-substrate interface, reducing stress concentration and improving fatigue strength while maintaining the corrosion resistance benefits of the zinc coating.
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 achieves high tensile strength while maintaining excellent bendability, as evidenced by a tensile strength of 980 MPa or more and a favorable bending radius-to-thickness ratio, without compromising weldability or corrosion resistance.
Implementation Method 1
controlling the presence of boron (B) as a hardenability element, with specific chemical compositions and microstructures that balance strength and bendability, including a boron ratio in the surface and interior layers
Implementation Method 2
optimized production processes such as hot rolling, cold rolling, and annealing
Data Source
Figure 1
AI summary
A high-strength cold-rolled steel sheet is provided that has a chemical composition that consists of, by mass%, C: 0.050 to 0.40%, Si: 0.01 to 3.0%, Mn: 1.0 to 5.0%, sol. Al: 0.001 to 1.0%, Ti: 0.005 to 0.20%, B: 0.0005 to 0.010%, P: 0.1% or less, S: 0.01% or less, O: 0.1% or less, N: 0.01% or less, Cr: 0 to 1.0%, Mo: 0 to 1.0%, Ni: 0 to 1.0%, Cu: 0 to 1.0%, Sn: 0 to 0.50%, Nb: 0 to 0.20%, V: 0 to 0.50%, W: 0 to 0.50%, Ca: 0 to 0.01%, Mg: 0 to 0.01%, Bi: 0 to 0.01%, Sb: 0 to 0.10%, Zr: 0 to 0.01%, and REM: 0 to 0.01%, with the balance being Fe and impurities, and that satisfies the formulas [sol. Bs/B ≤ 0.50] and [sol. Bq/B > 0.50] (where, B: B amount in the steel; sol. Bs: soluble B amount in a surface layer portion of the steel; and sol. Bq: soluble B amount in the steel interior).