Heat-Treated Cold-Rolled Steel Sheet for Strength-Formability Balance
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Solution Overview
Problem
Existing high strength steel sheets struggle to balance high formability with high strength, which is necessary for automotive parts to meet the demands of intricate assembly, crashworthiness, and reduced weight for improved fuel efficiency.
Innovation Solution
A cold-rolled steel sheet with a specific elemental composition and microstructure, including carbon, manganese, silicon, aluminum, chromium, phosphorus, sulfur, nitrogen, and optional elements like niobium, titanium, vanadium, molybdenum, nickel, and calcium, achieving an ultimate tensile strength of 950 MPa or more and total elongation of 14.0% or more, with a yield strength of 600 MPa or more, and a YS/TS ratio greater than 0.55, along with good forming, weldability, and coatability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased to reduce vehicle weight and improve fuel efficiency, then the strength increases, but the formability decreases
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.25%, Si: 0.01-0.70%, Mn: 1.50-3.00%, Cr: 0.05-0.50%, Al: 0.01-1.00%) and processing parameters (cooling rates, heating temperatures) to achieve a microstructure that simultaneously provides high strength and high formability. This resolves the contradiction by finding optimal parameter ranges that balance both properties.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (martensite, bainite, ferrite, and retained austenite) with specific volume percentages. This composite microstructure combines the high strength of martensite with the high formability contribution from retained austenite (8-20%), thereby resolving the strength-formability contradiction through microstructural composition.
2Strength
If the tensile strength is increased beyond 900 MPa, then the strength increases, but the total elongation and formability are compromised
Solution Approach 1:
The patent utilizes phase transitions by controlling the transformation of austenite during cooling and heat treatment processes. By maintaining 8-20% retained austenite after cooling and performing heat treatment at temperatures below Ac3, the patent achieves a microstructure with martensite, bainite, and retained austenite that provides both high tensile strength (950-1200 MPa) and high total elongation (14-18%), resolving the contradiction between strength and elongation.
Solution Approach 2:
The patent employs parameter changes through controlled cooling rates (5-50°C/s), specific heating temperatures (Ac1+50°C to Ac3-50°C), and holding times to achieve the desired microstructure. These parameter adjustments enable the steel to attain tensile strength above 950 MPa while maintaining total elongation of 14% or more, overcoming the limitation of previous methods that could only achieve up to 900 MPa.
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 provides a steel sheet with the desired mechanical properties and manufacturing compatibility, ensuring high strength, formability, and suitability for conventional industrial applications.
Implementation Method 1
heat treated cold rolled steel sheet
Implementation Method 2
heat treated cold rolled steel sheet and a method of manufacturing thereof
Data Source
AI summary
A method for producing a heat treated cold rolled steel sheet having a composition comprising of the following elements, 0.1%≤Carbon≤0.25%, 2.15%≤Manganese≤3.0%, 1%≤Silicon≤0.8%, 0.1%≤Aluminum≤0.9%, 0.05%≤Chromium≤0.5%, 0%≤Phosphorus≤0.09%, 0%≤Sulfur≤0.09%, 0%≤Nitrogen≤0.09%, 2.4%≤C+Mn≤3%, 0%≤Niobium≤0.1%, 0%≤Titanium≤0.1%, 0%≤Vanadium≤0.1%, 0%≤Molybdenum≤1%, 0%≤Nickel≤1%, 0%≤Calcium≤0.005%, 0%≤Boron≤0.01%, 0%≤Cerium≤0.1%, 0%≤Magnesium≤0.05%, 0%≤Zirconium≤0.05% the remainder being composed of iron and unavoidable impurities, the method including hot rolling, cooling the hot rolled steel at a cooling rate of at least 30° C./s to a coiling temperature less than 600° C.; and coiling, cooling to room temperature, cold rolling, annealing the cold rolled steel sheet using a two step heating process, cooling, overaging, and cooling to room temperature.