High-Strength Steel Sheet Warm Working via Low-Temperature Annealing
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Solution Overview
Problem
Current high-strength steel sheets for warm working lack excellent warm workability and yield ratio, particularly at low heating temperatures, due to issues such as increased carbon content in retained austenite, grain size, and equipment costs associated with high-temperature processing.
Innovation Solution
A high-strength steel sheet with a chemical composition of C: 0.05% to 0.20%, Si: 3.0% or less, Mn: 3.5% to 8.0%, and specific microstructure, including 10% to 60% retained austenite, 10% to 80% ferrite, and 10% to 50% martensite, with carbides at grain boundaries, processed using a method involving hot rolling, pickling, annealing, and cooling to achieve a low carbon content in retained austenite and fine grain structure.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-temperature heating is performed to achieve high strength and good workability, then strength and workability are improved, but equipment complexity and running costs increase
Solution Approach 1:
The patent changes the temperature parameter from conventional high-temperature heating (900-1200°C) to low-temperature heating (50-200°C). This parameter change allows achieving high strength (tensile strength ≥1180 MPa) and good workability without requiring complex high-temperature heating equipment, thereby resolving the contradiction between strength improvement and equipment complexity reduction
2Strength
If high-temperature heating is performed to achieve high strength and good workability, then strength and workability are improved, but running costs increase
Solution Approach 1:
The patent reduces the heating temperature parameter from 900-1200°C to 50-200°C, which directly reduces energy consumption and running costs while still achieving the required strength (tensile strength ≥1180 MPa) and workability. This resolves the contradiction between strength improvement and running cost reduction
3Ease of operation
If carbon content in retained austenite is increased to form stable retained austenite, then workability is improved, but yield ratio decreases due to high carbon martensite formation
Solution Approach 1:
The patent optimizes the carbon content parameter in retained austenite to be less than 0.40 mass%, which is lower than conventional levels. This controlled carbon content allows the retained austenite to provide good workability (EL ≥27%) while preventing excessive carbon from transferring to martensite, thereby maintaining a high yield ratio (≥60%) and resolving the contradiction between workability improvement and yield ratio preservation
4Use of energy by stationary object
If heating temperature is decreased to reduce equipment costs, then running costs decrease, but warm workability and yield ratio become insufficient
Solution Approach 1:
The patent identifies and applies an optimal temperature parameter range (50-200°C) for low-temperature heating that maintains excellent warm workability (EL ≥27%) and high yield ratio (≥60%). This optimized parameter range achieves cost reduction through lower heating temperatures while preventing the deterioration of workability and yield ratio, resolving the contradiction between running cost reduction and workability maintenance
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 achieves a high strength with tensile strength of 1180 MPa or more, Vickers hardness of 400 or more, and yield ratio of 60% or more at room temperature, while maintaining excellent warm workability and reducing processing costs through controlled annealing conditions.
Implementation Method 1
a heating process for annealing, a holding process for annealing, and a cooling process for annealing
Implementation Method 2
a microstructure including, in terms of area ratio, 10% to 60% of retained austenite, 10% to 80% of ferrite, 10% to 50% of martensite
Data Source
AI summary
A high-strength steel sheet for warm working having excellent warm workability, and a method for manufacturing the steel sheet. The steel sheet has a chemical composition including, by mass %, C: 0.05% to 0.20%, Si: 3.0% or less, Mn: 3.5% to 8.0%, P: 0.100% or less, S: 0.02% or less, Al: 0.01% to 3.0%, N: 0.010% or less, one or more selected from Nb: 0.005% to 0.20%, Ti: 0.005% to 0.20%, Mo: 0.005% to 1.0%, and V: 0.005% to 1.0%. The steel sheet has a microstructure including, in terms of area ratio, 10% to 60% of retained austenite, 10% to 80% of ferrite, 10% to 50% of martensite, and 0% to 5% of bainite, in which a C content in the retained austenite is less than 0.40 mass % and the average crystal grain diameter of the retained austenite, the martensite, and the ferrite is 2.0 μm or less.