Cold Rolled Steel Sheet Low Yield Ratio Formability
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Solution Overview
Problem
Conventional high strength steel sheets fail to simultaneously achieve low yield ratio, high tensile strength, excellent elongation, and good stretch-flange-formability, which are essential for complex-shaped automotive parts, leading to issues like cracking and spring-back during press forming.
Innovation Solution
A high strength cold rolled steel sheet with a specific chemical composition and microstructure, including controlled volume fractions and grain sizes of ferrite, bainite, retained austenite, and martensite, along with optimized annealing processes, to achieve a yield ratio of 64% or less and tensile strength of 590 MPa or more, while maintaining excellent elongation and stretch-flange-formability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If DP steel with ferrite-martensite multi-phase is used to achieve high tensile strength and low yield ratio, then formability and strength are improved, but stretch-flange-formability deteriorates due to cracking at the interface between ferrite and martensite
Solution Approach 1:
The patent applies local quality by creating distinct regions with different microstructures: a first region near the surface with fine-grained ferrite (3-10 μm) for ductility and crack resistance, and a second region in the core with coarse-grained ferrite (10-20 μm) for strength. This spatial differentiation of grain sizes and phase distributions optimizes both stretch-flange-formability and tensile strength by placing appropriate microstructural characteristics in specific locations within the steel sheet thickness direction.
Solution Approach 2:
The patent employs composite materials principles by creating a multi-phase steel sheet with ferrite, bainite, retained austenite, and martensite phases distributed in specific volume ratios. The composite microstructure combines the ductility of ferrite, the strength of martensite, and the transformation-induced plasticity of retained austenite, achieving superior mechanical properties and formability that single-phase or simple two-phase steels cannot provide.
2Stability of the object's composition
If TRIP steel sheet is subjected to forming at temperature higher than or equal to martensite transformation start temperature to obtain large elongation, then ductility is improved, but retained austenite transforms into martensite during blanking causing cracking at the interface with ferrite
Solution Approach 1:
The patent applies parameter changes by precisely controlling the volume ratio of retained austenite (5-15%) and its carbon concentration (0.5-1.5%), along with controlling the grain size of ferrite in different regions. These parameter optimizations ensure that retained austenite provides sufficient elongation through stress-induced transformation while minimizing harmful cracking during blanking operations.
Solution Approach 2:
The patent applies preliminary action by pre-distributing fine-grained ferrite in the first region before forming operations. This fine-grained structure acts as a buffer zone that prevents crack propagation from the surface inward during blanking, preparing the material structure in advance to resist the cracking that would otherwise occur during subsequent forming when retained austenite transforms.
3Strength
If high strength steel sheet is used for press forming to achieve high tensile strength, then strength is improved, but dimensional accuracy deteriorates due to spring-back after forming
Solution Approach 1:
The patent applies local quality by creating a gradient microstructure with fine-grained ferrite (3-10 μm) in the first region near the surface and coarse-grained ferrite (10-20 μm) in the second region in the core. The fine-grained surface region provides lower yield strength and higher ductility, reducing spring-back during press forming, while the coarse-grained core maintains high tensile strength, thereby improving dimensional accuracy without sacrificing strength.
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 a steel sheet with improved elongation, stretch-flange-formability, and resistance to degradation over time, ensuring stable performance in complex-shaped automotive parts with reduced yield ratio and enhanced strength-elongation balance.
Implementation Method 1
When the TRIP steel sheet is subjected to forming at a temperature higher than or equal to the martensite transformation start temperature, large elongation is obtained through stress induced transformation of retained austenite into martensite
Implementation Method 2
performing annealing under the conditions of performing heating to an annealing temperature in a temperature range of 780° C. to 900° C. at an average heating rate of 3° C./s to 30° C./s, performing holding at the annealing temperature for 30 to 500 s
Data Source
AI summary
A high strength cold rolled steel sheet with a low yield ratio has a chemical composition containing C: 0.05% to 0.10%, Si: 0.6% to 1.3%, Mn: 1.4% to 2.2%, P: 0.08% or less, S: 0.010% or less, Al: 0.01% to 0.08%, N: 0.010% or less, and the remainder being Fe and incidental impurities, on a percent by mass basis, and a microstructure in which the average grain size of ferrite is 15 μm or less, the volume fraction of ferrite is 70% or more, the volume fraction of bainite is 3% or more, the volume fraction of retained austenite is 4% to 7%, the average grain size of martensite is 5 μm or less, and the volume fraction of martensite is 1% to 6%, wherein the average C concentration (percent by mass) in the retained austenite is 0.30% to 0.70%, yield ratio is 64% or less, and the tensile strength is 590 MPa or more.