1180 MPa Steel Sheet Composition for Press Formability
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Solution Overview
Problem
Conventional techniques fail to produce high strength steel sheets with a tensile strength of 1180 MPa or more and uniform elongation of 6% or more, while maintaining excellent press formability, due to limitations in microstructure and production processes.
Innovation Solution
A high strength steel sheet with a microstructure comprising upper bainite as the main phase, combined with appropriate amounts of fresh martensite and retained austenite, achieved through a balanced chemical composition and specific hot rolling and cooling processes, ensuring a tensile strength of 1180 MPa or more and uniform elongation of 6% or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the tensile strength of steel sheet is increased to 1180 MPa or more, then the strength is improved, but the ductility deteriorates and press formability is reduced
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.30%, Si: 0.70-1.50%, Mn: 2.00-4.00%, P: 0.050% or less, S: 0.030% or less, Al: 0.010-2.000%, B: 0.0005-0.0100%) and microstructure parameters (upper bainite area fraction: 70-90%, fresh martensite and retained austenite area fraction: 7-30%) to achieve both high tensile strength (1180 MPa or more) and high uniform elongation (6% or more), thereby resolving the contradiction between strength and press formability
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases: upper bainite as the main phase (70-90% area fraction), fresh martensite and retained austenite (7-30% area fraction). This composite microstructure combines the high strength characteristics of martensite with the good ductility and press formability of upper bainite, achieving both tensile strength of 1180 MPa or more and uniform elongation of 6% or more
2Productivity
If conventional microstructure control methods are used, then production process is simple, but the uniform elongation cannot reach 6% or more while maintaining high strength
Solution Approach 1:
The patent uses parameter changes by controlling the finish temperature of hot rolling and the cooling rate to achieve the desired microstructure. Specifically, hot rolling is performed to a finish temperature of (RC-50°C) or more and (RC+150°C) or less, followed by cooling at a rate of 5°C/s or more to a temperature of Trs or more and (Trs+250°C) or less. This parameter control achieves uniform elongation of 6% or more while maintaining high strength, without requiring complex additional heat treatment processes
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 enables the production of steel sheets with high tensile strength and excellent press formability, reducing the risk of necking and cracking during press forming, and contributing to weight reduction in automotive components while ensuring safety and environmental benefits.
Implementation Method 1
a microstructure including: upper bainite in an area fraction of 70 % or more as a main phase, and fresh martensite and retained austenite in a total area fraction of 7 % to 30 %
Implementation Method 2
subjecting the heated steel material to hot rolling to obtain a hot-rolled steel sheet under a set of conditions including a rolling finish temperature of (RC - 50° C.) or more and (RC + 150° C.) or less
Implementation Method 3
cooling the hot-rolled steel sheet under a set of conditions including a time from the end of the hot rolling to the start of the cooling of 2.0 s or less, an average cooling rate of 5° C./s or more, and a cooling stop temperature of Trs or more and (Trs + 250° C.) or less
Data Source
AI summary
Provided is a high strength steel sheet that has a tensile strength of 1180 MPa or more and a uniform elongation of 6 % or more. The high strength steel sheet has a chemical composition that contains predetermined components with a MSC of 3.0 mass% to 4.2 mass%. The high strength steel sheet has a microstructure including upper bainite in an area fraction of 70 % or more as a main phase, fresh martensite and retained austenite in a total area fraction of 7 % to 30 %, with the retained austenite having an area fraction of 2 % or more. The high strength steel sheet has a mechanical property with a uniform elongation of 6 % or more and a tensile strength of 1180 MPa or more.