Multi-Phase Steel Sheet Annealing for Strength and Workability
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Solution Overview
Problem
Existing steel sheets for automotive bodies face challenges in achieving both excellent strength and workability, as previous methods either prioritize one over the other or result in increased yield ratios due to excessive formation of ferrite or bainite.
Innovation Solution
A steel sheet with a specific chemical composition and microstructure is developed, optimizing annealing conditions to control Mn concentration, ensuring a balanced distribution of ferrite, martensite, bainite, and retained austenite, with controlled Mn concentrations in martensite and ferrite, and rapid annealing processes to enhance elongation and strength.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If conventional annealing methods are used to increase elongation, then workability is improved, but yield ratio increases due to excessive ferrite or bainite formation
Solution Approach 1:
The patent applies parameter changes by precisely controlling annealing temperature (Ac1+10℃ to Ac3-10℃), heating rate (3-7℃/sec), and holding time (5-30 sec) to achieve optimal microstructure. This resolves the contradiction by finding the precise parameter window that produces sufficient ferrite for workability while limiting bainite formation to maintain low yield ratio
Solution Approach 2:
The patent employs dynamic control of the annealing process with specific heating rates and holding times that adapt to the steel's transformation behavior. The process dynamically balances ferrite and bainite formation during cooling, ensuring adequate elongation without excessive yield ratio increase
2Ease of operation
If Mn concentration is increased to form retained austenite, then elongation is improved, but Mn concentrates into martensite increasing yield ratio
Solution Approach 1:
The patent applies local quality by creating non-uniform Mn distribution where Mn concentrates in ferrite and retained austenite regions but is suppressed in martensite regions. This is achieved through controlled annealing that promotes Mn diffusion to specific phases, providing local Mn enrichment where it benefits elongation while avoiding martensite hardening
Solution Approach 2:
The patent uses parameter changes in annealing temperature and holding time to control Mn partitioning behavior. By optimizing these parameters, Mn is directed to ferrite and retained austenite phases rather than martensite, achieving high elongation without excessive yield ratio
3Reliability
If annealing holding time is extended to concentrate carbon into austenite, then retained austenite is stabilized, but productivity decreases
Solution Approach 1:
The patent applies partial action by using moderate annealing holding times (5-30 sec) rather than extended times. This partial treatment is sufficient to achieve the required carbon concentration in austenite and form adequate retained austenite (3-20%) while avoiding excessive processing time that would reduce productivity
Solution Approach 2:
The patent optimizes the annealing holding time parameter within a specific range (5-30 sec) to achieve the balance between retained austenite formation and production efficiency. This parameter optimization ensures sufficient carbon concentration in austenite without excessive holding time
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 results in a steel sheet with improved workability and strength, allowing complex automotive shapes without cracking, while maintaining a low yield ratio, suitable for automotive members.
Implementation Method 1
the microstructure that is ferrite-austenite dual phase or austenite single phase during annealing holding transforms from austenite into ferrite, martensite, bainite, or other metallic phase during cooling after the annealing or during holding after the cooling
Implementation Method 2
during the cooling or during the holding after the cooling, Mn migrates from ferrite present since the annealing holding and ferrite formed as a result of the transformation to austenite, and eventually Mn concentrates into martensite
Implementation Method 3
performing annealing more rapidly than in conventional techniques, specifically, increasing the heating rate in a high temperature range up to the annealing temperature
Data Source
AI summary
Provided is a steel sheet having excellent workability while ensuring excellent strength. The steel sheet comprises: a predetermined chemical composition; and a steel microstructure that contains ferrite: 45% to 90%, martensite: 5% to 30%, bainite: 1% to 25%, and retained austenite: 3% or more and in which [Mn]M/[Mn] is 1.00 to 1.15 and [Mn]M/[Mn]F is 1.00 to 1.30, wherein TS×El is 16000 MPa·% or more.