Depth-Controlled Hot-Rolled Steel Sheet for Strength and Bendability
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Solution Overview
Problem
Existing high-strength steel sheets with a martensite single phase have poor bendability, as evidenced by the insufficient performance of steel sheets described in Patent Documents 1 and 2.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and controlled microstructure, including a total area ratio of martensite and tempered martensite at 90% or more at ¼ the sheet thickness and 70% to 90% at 200 μm from the surface, along with controlled finish rolling and cooling conditions, to achieve high strength and excellent bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If a martensite single phase microstructure is used to increase steel sheet strength, then collision safety is improved, but bendability deteriorates
Solution Approach 1:
The patent applies local quality by creating different microstructure compositions at different depths from the surface. The surface region (0-200μm) contains 3-15% non-martensitic phases to improve bendability, while the core region maintains 90-100% martensite for high strength. This spatial variation in microstructure composition resolves the contradiction between overall strength and surface bendability.
Solution Approach 2:
The patent segments the steel sheet microstructure into distinct regions based on depth from the surface: a surface region (0-200μm) with modified phase composition for bendability, and a core region (>200μm) with martensite single phase for strength. This segmentation allows each region to optimize for its specific functional requirement.
2Strength
If the volume ratio of martensite phase is increased to 90% or more, then strength is improved, but ductility deteriorates
Solution Approach 1:
The patent introduces local quality variations by controlling the depth-dependent distribution of martensite and non-martensitic phases. The surface region contains 3-15% non-martensitic phases (ferrite, bainite, or pearlite) to provide ductility and bendability, while the core region maintains 90-100% martensite for strength, resolving the contradiction between strength and ductility.
3Reliability
If the average grain size of prior austenite grains is reduced to improve toughness, then low temperature toughness is improved, but manufacturing complexity increases
Solution Approach 1:
The patent applies parameter changes by precisely controlling the average grain size of prior austenite grains to 20μm or less, with the aspect ratio controlled to 18 or less. This parameter control achieves improved low temperature toughness while maintaining manufacturability through defined rolling and cooling process parameters.
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 steel sheet achieves high strength and improved bendability, with a tensile strength of 980 MPa or more and a favorable R/t ratio of 1.5 or less, enhancing its suitability for automotive applications.
Implementation Method 1
a microstructure contains, in area ratio, at a location at 1/4 a sheet thickness, a total amount of 90% or more of one or more of martensite and tempered martensite
Data Source
AI summary
In a hot-rolled steel sheet, a microstructure contains, in area ratio, at a location at ¼ a sheet thickness, a total amount of 90% or more of one or more of martensite and tempered martensite and a total amount of 10% or less of one or more of ferrite, bainite, and pearlite, at a location of 200 μm from the surface, a total amount of 70% or more and less than 90% of one or more of martensite and tempered martensite and a total amount of more than 10% and 30% or less of one or more of ferrite, bainite, and pearlite.