High-strength Cold-rolled Steel Sheet Yield Ratio Bendability
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Solution Overview
Problem
Current high-strength steel sheets for automotive components face challenges in achieving a high yield ratio and good bendability, particularly at high-rate deformation, due to the presence of bainite and martensite microstructures which lead to decreased formability and increased risk of cracking during processing.
Innovation Solution
A high-strength cold-rolled steel sheet with a controlled component composition and production conditions to inhibit ferrite formation, featuring a composite microstructure with bainite and martensite, along with controlled martensite-austenite (MA) and cementite amounts, which enhances both yield ratio and bendability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If the strength of steel sheets is increased by using composite microstructure of bainite and martensite, then the yield ratio and crashworthiness are improved, but the bendability and formability are degraded
Solution Approach 1:
The patent applies parameter changes by precisely controlling the composition parameters (C: 0.11-0.20%, Si: 0.01-1.00%, Mn: 2.20-3.50%, Al: 0.01-0.10%, Ti: 0.01-0.10%, V: 0.01-0.10%, B: 0.0005-0.0050%) and microstructure parameters (martensite 40-90%, bainite 10-60%, ferrite 0-10%, retained austenite 0-5%) to achieve a balance between strength and bendability. The controlled composition and microstructure parameters enable the steel sheet to achieve tensile strength ≥980 MPa and yield ratio ≥0.70 while maintaining bendability R/t ≥1.5
Solution Approach 2:
The patent employs composite materials principle by creating a composite microstructure consisting of multiple phases (martensite, bainite, ferrite, and retained austenite) with specific area percentages. This multi-phase composite microstructure combines the high strength of martensite with the ductility contribution from bainite and controlled ferrite, achieving both high yield ratio and acceptable bendability
2Weight of moving object
If the thickness of steel sheets is reduced to decrease automotive body weight, then the fuel economy is improved, but the strength and safety of components are compromised
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters (particularly C: 0.11-0.20%, Mn: 2.20-3.50%, Si: 0.01-1.00%) and microstructure parameters (martensite content 40-90%, bainite content 10-60%) to achieve ultra-high tensile strength ≥980 MPa. This enables the use of thinner steel sheets while maintaining or improving component strength, thereby reducing automotive body weight without compromising safety
3Productivity
If a high-strength steel sheet is processed by bending at high speed, then the productivity is improved, but cracking may occur due to insufficient bendability
Solution Approach 1:
The patent applies parameter changes by controlling the microstructure parameters (bainite content 10-60%, martensite content 40-90%, ferrite 0-10%, retained austenite 0-5%) and composition parameters to achieve high bendability R/t ≥1.5. The controlled bainite content provides ductility that prevents cracking during high-speed forming operations, enabling both high productivity and reliable crack-free forming
Data Source
AI summary
A high-strength cold-rolled steel sheet has a component composition containing, on a percent by mass basis, C: 0.12% or more and 0.25% or less, Si: less than 0.5%, Mn: 2.0% or more and 3.0% or less, P: 0.05% or less, S: 0.005% or less, Al: 0.01% or more and 0.10% or less, and N: 0.010% or less, the balance being Fe and incidental impurities, the total area percentage of martensite and tempered martensite satisfying 20% or more and 90% or less, the area percentage of ferrite satisfying 10% or less, the area percentage of bainite satisfying 10% or more and 80% or less, the area percentage of a martensite-austenite constituent in the bainite being 1% or more and 10% or less, the area percentage of cementite having an average grain size of 1 μm or less in the bainite being 0.1% or more and 5.0% or less.