Cold-Rolled Steel Sheet Composition for Weldability and Formability
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Solution Overview
Problem
Existing high-strength steel sheets face challenges in achieving excellent weldability, strength, and formability due to decreased ductility and formability when strengthening, and issues such as Liquid Metal Embrittlement (LME) and hydrogen embrittlement.
Innovation Solution
A cold-rolled steel sheet with a specific alloy composition (C: 0.14-0.16%, Si: 0.3-0.6%, Al: 0.01-0.3%, Mn: 2.6-3.0%, Cr: 0.01-0.25%, Mo: 0.15-0.4%, B: 0.0001-0.005%, Nb: 0.001-0.05%, Ti: 0.001-0.05%, P: 0.04% or less, S: 0.01% or less, N: 0.01% or less) and a microstructure consisting of retained austenite, fresh martensite, bainite, tempered martensite, and ferrite, satisfying certain relational expressions to enhance weldability, strength, and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of the steel sheet is increased to achieve high tensile strength of 980 MPa or higher, then the strength is improved, but the ductility and formability decrease
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters (C: 0.14-0.16%, Si: 0.3-0.6%, Mn: 2.6-3.0%, Cr: 0.01-0.25%, Mo: 0.15-0.4%, B: 0.0001-0.005%, Nb: 0.001-0.05%, Ti: 0.001-0.05%) and processing parameters (finishing hot rolling temperature: 830-980°C, coiling temperature: 450-700°C, continuous annealing temperature: 800-840°C, primary cooling rate: less than 10°C/s, secondary cooling rate: 10°C/s or more, reheating temperature: 380-480°C) to achieve a balanced microstructure that simultaneously provides high strength and good formability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (tempered martensite as the primary phase, retained austenite, fresh martensite, bainite, and ferrite) that work together to provide both high strength and excellent formability, overcoming the trade-off between these properties
2Ease of operation
If a large amount of Si and Al is added to introduce retained austenite for improving formability, then the formability is improved, but Liquid Metal Embrittlement (LME) occurs during spot welding
Solution Approach 1:
The patent optimizes the parameters of Si and Al content within specific ranges (Si: 0.3-0.6%, Al: 0.01-0.3%) rather than using large amounts, and combines them with other alloying elements to achieve the desired microstructure while preventing LME during welding
Solution Approach 2:
The patent creates a composite microstructure with controlled retained austenite content (more than 1% and 5% or less) combined with tempered martensite as the primary phase, which provides formability through the TRIP effect while the tempered martensite matrix prevents LME during welding
3Strength
If the strength of the steel sheet is increased, then the strength is improved, but hydrogen embrittlement and severe corrosion may occur
Solution Approach 1:
The patent optimizes the chemical composition parameters, particularly controlling C content (0.14-0.16%) to an appropriate level and adding specific amounts of Cr (0.01-0.25%) and Mo (0.15-0.4%) which improve both strength and corrosion resistance, while limiting P (0.04% or less) and S (0.01% or less) to prevent hydrogen embrittlement and severe corrosion
Solution Approach 2:
The patent converts the potential harm of high carbon content (which would cause hydrogen embrittlement) into a benefit by precisely controlling C content within a narrow range and using it in combination with other alloying elements to achieve high strength through the microstructure rather than through excessive carbon content
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 proposed cold-rolled steel sheet achieves excellent weldability, strength, and formability with a tensile strength of 1180 to 1350 MPa, yield strength of 740 to 980 MPa, elongation of 8% or more, hole expandability of 20% or more, and improved resistance to Liquid Metal Embrittlement.
Implementation Method 1
a method of using a Transformation Induced Plasticity (TRIP) phenomenon by introducing retained austenite as in Patent Document 1 is widely used as a method to increase elongation
Implementation Method 2
a microstructure consists of retained austenite: more than 1% and 5% or less, fresh martensite: 10% or more and less than 25%, bainite: less than 20% (excluding 0%), tempered martensite: 55% or more and less than 80%, and ferrite: 5% or less (including 0%)
Data Source
AI summary
The present disclosure relates to a cold rolled steel sheet having excellent weldability, strength and formability and a method of manufacturing the same.An aspect of the present disclosure is to provide a cold-rolled steel sheet having excellent weldability, strength, and formability, and a method of manufacturing the same.As set forth above, according to an aspect of the present disclosure, a cold-rolled steel sheet having excellent weldability, strength, and formability and a method of manufacturing the same may be provided.