Hot-Rolled Steel Sheet Microstructure for Stable Shearing and Ductility
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Solution Overview
Problem
Current techniques fail to simultaneously achieve high strength, ductility, and stable shearing properties in hot-rolled steel sheets, particularly for applications requiring strength above 980 MPa, and often result in poor end surface accuracy and inside bend cracking.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and microstructure, including a full hard structure with martensite or bainite, combined with precipitation-hardened ferrite, controlled Mn segregation, and optimized hot rolling and cooling processes to achieve a stable sheared surface proportion and reduced microstructural periodicity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high-strength steel sheet with tensile strength 980 MPa or more is used to reduce vehicle body weight, then weight reduction is achieved, but collision resistance and formability become difficult to maintain
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.15-0.35%, Si: 2.0-3.5%, Mn: 1.0-3.0%, P: 0.01-0.10%, S: 0.001-0.05%, Al: 0.01-2.00%) and microstructural parameters (ferrite grain size 5-15 μm, residual austenite 1-10%, martensite 80-95%) to achieve tensile strength of 980 MPa or more while maintaining formability and collision resistance
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, residual austenite, and martensite) within the steel sheet. This composite structure combines the ductility of ferrite, the transformation-induced plasticity of residual austenite, and the high strength of martensite, achieving both weight reduction and collision resistance
2Stability of the object's composition
If residual austenite is dispersed in ferrite to improve ductility through transformation-induced plasticity, then ductility is improved, but hole expansibility deteriorates due to full hard martensite formation
Solution Approach 1:
The patent controls the ratio and distribution of microstructural phases by adjusting chemical composition parameters (particularly C, Si, Mn content) and heat treatment parameters, maintaining residual austenite at 1-10% to provide transformation-induced plasticity while limiting full hard martensite formation to preserve hole expansibility
Solution Approach 2:
The patent creates local quality differences by dispersing residual austenite and martensite phases within the ferrite matrix. The residual austenite provides local transformation-induced plasticity during deformation, while the ferrite matrix maintains overall ductility and hole expansibility
3Productivity
If shearing working is used to manufacture blank sheets for high productivity, then productivity is improved, but end surface accuracy deteriorates when sheared surface proportion is unstable
Solution Approach 1:
The patent controls microstructural parameters (ferrite grain size 5-15 μm, phase distribution) to stabilize the sheared surface proportion during shearing working, ensuring end surface accuracy is maintained while using high-productivity shearing processes
4Shape
If press forming is used to form vehicle members, then various shapes can be achieved, but formability is limited by insufficient ductility
Solution Approach 1:
The patent utilizes phase transitions by incorporating 1-10% residual austenite that transforms into martensite during press forming deformation. This transformation-induced plasticity enhances ductility and formability, enabling complex vehicle member shapes to be achieved while maintaining material integrity
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 excellent strength, ductility, and shearing properties, while suppressing inside bend cracking, making it suitable for vehicle and mechanical components.
Implementation Method 1
the austenite is transformed into martensite during working and large elongation is exhibited due to transformation-induced plasticity
Implementation Method 2
it is effective to perform intermediate air cooling in the hot rolling process to obtain an appropriate amount of precipitation-hardened ferrite
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in a microstructure, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is less than 1.020, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.