Structural Steel Microstructure for High-Strength Cold Bending
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Solution Overview
Problem
High-strength structural steels with tensile strength of 800 MPa or more face challenges in cold bending due to inferior elongation and surface cracking, as existing methods like cooling after rolling lead to bainite or martensite formation, reducing elongation and increasing crack susceptibility.
Innovation Solution
A high-strength structural steel composition with specific elements (C, Si, Mn, Al, P, S, N, Ni, Cu, Cr, Mo, Ti, Nb, V, B, and Ca) and a manufacturing process involving reheating, rough rolling, first cooling, heat recuperative treatment, and finish rolling to create a microstructure with tempered bainite and bainitic ferrite, enhancing cold bendability and suppressing surface cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If steel is produced by cooling after rolling to achieve high strength (800 MPa or more), then tensile strength is improved, but elongation is significantly lowered due to formation of bainite or martensite structure
Solution Approach 1:
The patent applies different microstructures to different regions of the steel sheet. The surface layer (0-5mm depth) contains equiaxial ferrite grains with 5-15μm average diameter for ductility, while the interior layer (5mm depth to center) contains bainitic ferrite for strength. This local differentiation resolves the contradiction between surface elongation and overall strength.
Solution Approach 2:
The steel sheet microstructure is segmented into two distinct layers: surface layer and interior layer. The surface layer is optimized for cold bendability with equiaxial ferrite, while the interior layer provides high strength with bainitic ferrite. This segmentation allows each layer to fulfill its specific functional requirement independently.
2Reliability
If excessive cooling is applied to improve impact toughness through finer structure, then toughness is improved, but elongation rate is significantly lowered due to bainite or martensite formation
Solution Approach 1:
The patent implements local quality control by applying different cooling rates to different regions. The surface layer experiences slower cooling to form equiaxial ferrite for elongation, while the interior experiences faster cooling for bainitic ferrite formation providing toughness. This resolves the contradiction between toughness and elongation rate.
3Strength
If cold bending is performed on steel with fine structure, then plasticity occurs on the surface, but cracks occur in the surface of processed part in the thickness direction
Solution Approach 1:
The patent optimizes the surface layer microstructure specifically for cold bending performance by creating equiaxial ferrite grains with 5-15μm diameter in the top 0-5mm layer. This local optimization prevents surface cracking during cold bending while maintaining the high-strength bainitic ferrite in the interior, resolving the contradiction between plasticity and surface crack resistance.
4Strength
If heat treatment method such as quenching-tempering is applied to achieve high strength, then tensile strength of 800 MPa or more is obtained, but production costs increase and weldability deteriorates
Solution Approach 1:
The patent changes the microstructural parameters by controlling cooling rates and temperatures during rolling to form equiaxial ferrite and bainitic ferrite structures. This achieves high strength (800 MPa or more) without quenching-tempering heat treatment, thereby reducing production costs and maintaining good weldability while meeting the strength requirement.
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 achieves excellent cold bendability with a critical curvature ratio of 1.0 or less, maintaining high tensile strength and preventing surface cracking during severe bending, while optimizing microstructure and composition for improved weldability and toughness.
Implementation Method 1
a heat recuperative treatment in which a surface layer of the rough-rolled bar is reheated is performed
Implementation Method 2
the central part contains bainitic ferrite as a matrix structure... the surface layer part includes tempered bainite as a matrix structure
Data Source
Figure 1
Figure 2
Figure 3(a)~3(d)
AI summary
A high-strength structural steel having excellent cold bendability, according to one embodiment of the present invention, comprises, by wt%, 0.02-0.1% of C, 0.01-0.6% of Si, 1.7-2.5% of Mn, 0.005-0.5% of Al, 0.02% or less of P, 0.01% or less of S, 0.0015-0.015% of N, and the balance of Fe and other inevitable impurities, wherein an outer surface layer part and an inner central part thereof are microstructurally divided in a thickness direction, the surface layer part can comprise tempered austenite as a matrix structure, and the central part can comprise bainitic ferrite as a matrix structure.