Cold-Rolled Steel Sheet Microstructure for Strength and Formability
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Solution Overview
Problem
Current methods for producing high-strength steel sheets with excellent formability and yield ratio face challenges such as material defects, reduced workability, and increased manufacturing costs, particularly in achieving ultra-high strength of 1180 MPa or more while maintaining ductility and formability for applications like vehicle body members.
Innovation Solution
A cold-rolled steel sheet with a specific alloy composition (C: 0.10-0.20%, Si: 0.05-0.495%, Al: 0.01-0.18%, Mn: 2.4-3.5%, Cr: 0.05-0.8%, Mo: 0.05-0.8%, B: 0.0001-0.003%, Nb: 0.005-0.07%, Ti: 0.005-0.07%) and a microstructure comprising fresh martensite, tempered martensite, bainite, retained austenite, and ferrite, along with precise control of annealing and cooling processes, is developed to achieve the desired strength and formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If water cooling is used during continuous annealing to increase yield strength, then yield ratio is improved, but shape quality of coil deteriorates due to temperature deviation
Solution Approach 1:
The patent changes the cooling parameter from water cooling to air cooling, and adjusts the annealing temperature range to 550-750°C to achieve the desired yield ratio while preventing temperature deviation that causes shape quality deterioration
Solution Approach 2:
The patent creates a dual-phase microstructure with specific proportions (martensite 5-20% and ferrite 80-95%) to achieve local optimization of both strength and shape quality, rather than uniform transformation
2Strength
If strength of steel sheet is increased to achieve ultra-high strength of 1180 MPa or more, then tensile strength is improved, but elongation decreases causing formability deterioration
Solution Approach 1:
The patent creates a composite microstructure consisting of martensite and ferrite phases in specific proportions, combining the high strength of martensite with the ductility of ferrite to achieve both ultra-high tensile strength and acceptable formability
Solution Approach 2:
The patent controls the annealing temperature (550-750°C) and cooling rate to achieve specific phase proportions, where martensite provides strength and ferrite provides elongation, resolving the contradiction between tensile strength and formability
3Ease of operation
If excessive Cu content of 2 to 5% is added to precipitate fine Cu particles for improving processability, then hole expandability is improved, but red heat embrittlement occurs and manufacturing costs increase excessively
Solution Approach 1:
The patent removes copper from the alloy composition entirely, achieving hole expandability through a different mechanism (microstructure control with martensite and ferrite phases) rather than copper precipitation, thereby avoiding red heat embrittlement and excessive cost increase
Solution Approach 2:
The patent uses common, low-cost alloying elements (C, Si, Mn, Al, Cr, Mo, B, Nb, Ti) instead of expensive copper, achieving the desired properties through cost-effective means without the harmful side effects of copper addition
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 results in a steel sheet with yield strength of 800-1200 MPa, tensile strength of 1180-1400 MPa, and high elongation, enabling excellent strength and formability, including improved resistance to Liquid Metal Embrittlement (LME) cracks and weldability, while maintaining economic feasibility.
Implementation Method 1
a steel sheet in which a microstructure thereof is transformed from martensite into tempered martensite may be manufactured by being soaked in an annealing process, and then immersed in water and tempered
Implementation Method 2
immersed in water and tempered
Implementation Method 3
followed by an overaging treatment for 1 to 15 minutes at a temperature within a range of 120 to 300° C.
Data Source
AI summary
The present invention relates to a cold rolled steel sheet and a manufacturing method therefor, and, more specifically, to a cold rolled steel sheet and a manufacturing method therefor, the cold rolled steel sheet having excellent strength and formability, which can be preferably applied to a structure member such as the members, seat rails, and pillars of a vehicle, and the like.