Hot-Pressed Steel Sheet Decarburization Control
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Solution Overview
Problem
Conventional high-strength hot-pressed steel sheet members face challenges in achieving both high ductility and toughness, with decarburization during the hot pressing process leading to embrittlement and reduced dimensional accuracy, particularly when attempting to form complex shapes for automotive and machine structural components.
Innovation Solution
A hot-pressed steel sheet with a multi-phase structure containing ferrite and martensite, achieved by controlling the chemical composition and processing conditions, including specific ranges of C, Si, Mn, and Si content, and suppressing decarburization through controlled heating and cooling rates, results in a steel sheet with enhanced tensile strength, ductility, and toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a multi-phase structure containing ferrite and martensite is created to improve ductility, then ductility is improved, but decarburization progresses during heating and air cooling, causing a decrease in toughness
Solution Approach 1:
The patent applies inert atmosphere by conducting hot pressing in a controlled atmosphere (vacuum or inert gas) to prevent decarburization during heating and cooling. This resolves the contradiction by maintaining carbon content in the steel, preserving toughness while allowing the multi-phase structure to form for improved ductility.
Solution Approach 2:
The patent changes the atmospheric parameters during hot pressing from air to vacuum or inert gas environment. This parameter change prevents carbon loss through decarburization, thereby maintaining toughness while still enabling the formation of the desired multi-phase structure with ferrite and martensite for improved ductility.
2Weight of moving object
If high-strength steel sheet is used to reduce weight, then weight is reduced, but press formability decreases making it difficult to manufacture components with complicated shapes
Solution Approach 1:
The patent changes the temperature parameter by performing hot pressing at elevated temperatures (Ac3 point or higher). This temporary parameter change restores formability to high-strength steel, enabling complex shapes to be formed while maintaining the weight reduction benefit of using high-strength material.
Solution Approach 2:
The patent applies periodic action by temporarily heating the steel sheet to high temperature during the forming process, then cooling it afterward. This periodic temperature change enables formability during the heating phase for complex shaping, while the cooling phase restores high strength in the final product, resolving the contradiction between weight reduction and manufacturability.
3Weight of moving object
If high-strength steel sheet is used to reduce weight, then weight is reduced, but dimensional accuracy deteriorates due to prominent springback
Solution Approach 1:
The patent changes the temperature parameter by performing hot pressing at elevated temperatures. This reduces springback during forming due to increased ductility at high temperature, thereby improving dimensional accuracy while maintaining the weight reduction benefit of high-strength steel.
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 the production of steel sheets with tensile strengths of 980 MPa or more, combined with excellent ductility and toughness, effectively addressing the limitations of previous methods by minimizing decarburization and maintaining structural integrity.
Implementation Method 1
a steel structure containing ferrite and cementite... a total area ratio of bainite and martensite: 0% to 10%
Implementation Method 2
decarburization is likely to progress and a decrease in toughness by the decarburization is caused during heating and air cooling in hot pressing
Data Source
AI summary
A steel sheet for hot pressing includes: a specific chemical composition; and a steel structure comprising ferrite and cementite and represented, in area %: a total area ratio of bainite and martensite: 0% to 10%; and an area ratio of cementite: 1% or more; and a concentration of Mn in the cementite is 5% or more.