High-Strength Cold-Rolled Steel Sheet With Controlled Flatness
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Solution Overview
Problem
Existing high-strength steel plates face challenges in achieving a balance of strength and elongation while maintaining excellent flatness, particularly due to instability of retained austenite and deformation during quenching processes.
Innovation Solution
A high-strength cold-rolled steel plate with a specific microstructure and controlled cooling rates, including multi-stage cooling and partitioning, to stabilize retained austenite and suppress deformation, resulting in a composition of 25-35% ferrite, 10-18% retained austenite, and 5% or less M-A phase, with controlled flatness of 3.0 mm or less.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If quenching process is used to form martensite matrix for high strength, then strength is improved, but deformation and flatness deteriorate
Solution Approach 1:
The quenching process is divided into multiple stages with different cooling rates. The first cooling stage uses a relatively slow cooling rate to minimize deformation, while the second cooling stage uses a faster cooling rate to ensure martensite formation and achieve high strength. This segmented approach to cooling allows simultaneous achievement of high strength and good flatness.
2Stability of the object's composition
If transformation-induced plasticity is used to secure elongation through retained austenite, then elongation is improved, but stability of retained austenite deteriorates
Solution Approach 1:
The patent optimizes the composition parameters, specifically controlling the carbon content at 0.1-0.3% and silicon content at 1.0-2.0%, to achieve the right balance between retained austenite stability and elongation. By precisely controlling these compositional parameters, the retained austenite maintains sufficient stability while still providing the transformation-induced plasticity effect for high elongation.
3Strength
If high carbon content is used to increase strength, then strength is improved, but elongation and moldability deteriorate
Solution Approach 1:
The patent optimizes the carbon content parameter to a specific range of 0.1-0.3%, which is lower than conventional high-strength steels. This controlled carbon level, combined with 1.0-2.0% silicon and 1.5-3.0% manganese, achieves high strength through the martensite matrix and transformation-induced plasticity while maintaining good elongation and moldability.
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 yield strength of 550 MPa or more, tensile strength of 980 MPa or more, elongation index of 20% or more, and tensile strength*elongation index of 20,000 MPa % or more, while maintaining excellent flatness and operational stability.
Implementation Method 1
transformation-induced plasticity steels that simultaneously secure strength and elongation through phase transformation of residual austenite remaining in the structure during plastic deformation
Implementation Method 2
quenching and partitioning as methods of making high-strength transformation-induced plasticity steel based on martensite
Implementation Method 3
quenching and partitioning as methods of making high-strength transformation-induced plasticity steel
Data Source
AI summary
Provided is a high-strength cold-rolled steel plate. According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate includes: in % by weight, carbon (C): 0.1% to 0.3%, silicon (Si): 1.0% to 2.0%, manganese (Mn): 1.5% to 3.0%, aluminum (Al): 0.01% to 0.05% or less, phosphorus (P): 0.02% or less, sulfur(S): 0.005% or less, a remainder being iron (Fe) and other inevitable impurities. According to an embodiment of the present disclosure, the high-strength cold-rolled steel plate has a microstructure including, by area ratio, 25 to 35% ferrite, 10 to 18% retained austenite, 5% or less M-A (martensite-austenite composite phase) and the remainder being martensite.