Gradient Steel Material High-Plasticity Surface Layer Manufacturing
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Solution Overview
Problem
Existing steel materials face a challenge in balancing high strength and plasticity, as these properties are often contradictory, with surface layers requiring high plasticity for deformation but inner layers needing high strength, which is difficult to achieve simultaneously without compromising material integrity or increasing carbon content.
Innovation Solution
A gradient steel material with a high-plasticity ferrite surface layer and a high-strength inner layer of ferrite+bainite microstructure is developed through a single-material processing method involving smelting, casting, rolling, and heat treatment, where the surface layer is preferentially austenitized and rapidly cooled, while the inner layer undergoes slower cooling, resulting in a continuous material with low carbon content and good welding properties.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If steel material pursues high strength, then strength is improved, but plasticity deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct microstructure zones within the steel material: the surface layer contains ferrite and pearlite for high plasticity, while the inner layer contains bainite and martensite for high strength. This spatial differentiation of microstructures allows each region to exhibit properties optimized for its functional requirements, resolving the contradiction between overall strength and surface plasticity.
Solution Approach 2:
The steel material is segmented into different microstructural regions based on cooling rate gradients. The surface region experiences rapid cooling forming plastic ferrite/pearlite, while the interior experiences slower cooling forming strong bainite/martensite. This segmentation of the material into functional zones resolves the contradiction by allowing simultaneous optimization of strength and plasticity in different locations.
2Stability of the object's composition
If compound preparation methods are used to achieve gradient microstructure, then microstructure gradient is achieved, but manufacturing complexity increases
Solution Approach 1:
The patent employs self-service by utilizing the natural heat transfer characteristics during cooling, where the surface cools faster than the interior automatically. By controlling the cooling rate parameters, the material self-organizes into the desired gradient microstructure without requiring complex external intervention, multiple material layers, or sophisticated equipment. This resolves the contradiction by achieving microstructure gradient through inherent physical processes rather than complex manufacturing systems.
Solution Approach 2:
The patent achieves gradient microstructure by changing the cooling rate parameter across different regions of the material. The surface layer is cooled at a higher rate (10-100°C/s) while the interior is cooled at a lower rate (1-10°C/s). This parameter variation in the cooling process directly produces the desired microstructure gradient, simplifying manufacturing compared to methods requiring multiple material combinations or complex processing steps.
3Strength
If high carbon content is used to achieve high strength, then strength is improved, but welding property deteriorates
Solution Approach 1:
The patent applies local quality by creating distinct microstructure zones within the steel material: the surface layer contains ferrite and pearlite for high plasticity, while the inner layer contains bainite and martensite for high strength. This spatial differentiation of microstructures allows each region to exhibit properties optimized for its functional requirements, resolving the contradiction between overall strength and surface plasticity.
Solution Approach 2:
The steel material is segmented into different microstructural regions based on cooling rate gradients. The surface region experiences rapid cooling forming plastic ferrite/pearlite, while the interior experiences slower cooling forming strong bainite/martensite. This segmentation of the material into functional zones resolves the contradiction by allowing simultaneous optimization of strength and plasticity in different locations.
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
This approach allows for a continuous, cost-effective, and simple manufacturing process that achieves a gradient microstructure without the need for compound preparation, maintaining good metallurgical strength at the interface and enhancing the material's ability to withstand large deformations while maintaining low overall carbon content.
Implementation Method 1
a gradient change of the steel microstructure, a microstructure with high strength, high hardness and high wear resistance, such as martensite and bainite, is distributed on the surface; and a microstructure with high toughness and good plasticity, such as ferrite and pearlite, is distributed inside the material
Implementation Method 2
Northeastern University conducted unidirectional cooling on steel plates through controlled cooling, which results in a gradient change of the steel microstructure
Data Source
AI summary
A gradient steel material with a high plastic surface layer and a high strength inner layer, and a manufacturing method are provided. Weight percentages of the components of the gradient steel material are: C≤0.15%, Si≤1%, Mn≤1.5%, and the balance of Fe and inevitable impurities, the surface layer of the steel material being ferrite, and the inner layer being ferrite+bainite. The manufacturing method therefor comprises: smelting, casting, rolling, and a heat treatment, wherein in the heat treatment step, a steel material is heated to an austenite temperature Ac3 or more and kept at said temperature for more than 3 min; thereafter, the material is cooled to a temperature range between Ar3 and Ar1 in a two-phase zone at a cooling rate of less than 0.5° C./s, and is then cooled to room temperature at a cooling rate of greater than 5° C./s. The present steel material does not need to be obtained by means of the compound preparation of different materials as only a single material is processed. At the same time, the composition of the steel material is simple. Although the internal and external microstructures are different, the difference is a gradual process, and the strength at the interface is good.