Cold-Rolled Steel Sheet Composition for Surface Quality and Low Deviation
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Solution Overview
Problem
High-strength cold-rolled steel sheets with excellent surface quality and low material deviation are needed for structural vehicle components, but existing technologies face challenges such as poor weldability and surface defects due to high Si, Al, Mn, or hardenable element additions, which affect thickness quality and formability.
Innovation Solution
A high-strength cold-rolled steel sheet with specific composition (C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%, N: 0.001-0.01%) and controlled microstructure, manufactured through reheating, hot rolling, cooling, coiling, and continuous annealing, with temperature control in the coiling process to minimize surface defects and material deviation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a large amount of Si, Al, Mn is added to high-strength steel, then strength is improved, but weldability deteriorates and surface defects occur due to dents in furnace during annealing
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.05-0.3%, Si: 0.01-2.0%, Mn: 1.5-3.0%, Al: 0.01-0.1%, P: 0.001-0.015%, S: 0.001-0.01%) to achieve the optimal balance between strength and surface quality. This compositional parameter optimization prevents excessive oxide formation while maintaining high strength, thereby reducing surface defects caused by roll dents during annealing.
Solution Approach 2:
The patent applies local quality by controlling the distribution and concentration of alloying elements to specific ranges that locally optimize both strength properties and surface characteristics. The precise compositional control ensures that elements like Si and Mn are present in amounts sufficient for strength enhancement but limited enough to prevent harmful oxide accumulation on the surface.
2Strength
If a large amount of hardenable elements such as Mn, Cr, Mo is added, then strength is improved, but material deviation of hot-rolled coil occurs, so thickness quality deteriorates during cold rolling
Solution Approach 1:
The patent applies parameter changes by optimizing the content of hardenable elements (Mn: 1.5-3.0%, Cr: 0.01-1.0%, Mo: 0.01-0.5%) to achieve sufficient strength while minimizing material deviation during hot rolling. This controlled composition prevents excessive hardenability that would cause thickness variation, thereby maintaining good thickness quality during subsequent cold rolling operations.
3Strength
If high strength is achieved through high carbon content, then tensile strength is improved, but weldability deteriorates
Solution Approach 1:
The patent applies parameter changes by precisely controlling the carbon content parameter (C: 0.05-0.3%) to achieve the optimal balance between strength and weldability. This controlled carbon level provides sufficient tensile strength (1180 MPa or more) while preventing excessive carbon that would cause poor weldability and carbon equivalent issues.
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 method produces a steel sheet with a tensile strength of 1180 MPa or more, a product of tensile strength and elongation of 8,000 MPa % or more, and less than 15,000 MPa %, suitable for vehicle body parts requiring strength-elongation balance and impact resistance, while reducing surface defects and material deviation.
Implementation Method 1
a microstructure, by area: a sum of bainite and martensite: 80% or more
Implementation Method 2
continuously annealing the cold-rolled steel sheet at a temperature within a range of 740 to 900° C.
Implementation Method 3
cooling the hot-rolled steel sheet to a temperature within a range of 450 to 700° C. at an average cooling rate of 10 to 70° C./s
Data Source
AI summary
The present invention relates to: a high-strength cold-rolled steel sheet having excellent quality and low mechanical property deviation; and a manufacturing method therefor. More particularly, the present invention relates to: a high-strength cold-rolled steel sheet that can be suitably used for automotive parts by ensuring high strength and elongation with little surface defects and low mechanical property deviation; and a manufacturing method therefor.