Hot-Rolled Steel Sheet Microstructure for High Strength and Ductility
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing hot-rolled steel sheets lack a technique for achieving both high strength and excellent ductility while maintaining a smooth shearing surface, leading to increased manufacturing costs due to required post-treatments.
Innovation Solution
A hot-rolled steel sheet with a chemical composition of C: 0.100% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, and a metallographic structure containing 3.0% or more retained austenite, a ratio of grain boundaries with specific crystal misorientations, and a standard deviation of Mn concentration of 0.60 mass % or less, achieving a tensile strength of 1180 MPa or more.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Weight of moving object
If high strength steel sheet is used to reduce vehicle body weight, then weight reduction is achieved, but collision resistance and safety become difficult to secure
Solution Approach 1:
The patent changes the microstructural parameters of the steel sheet by controlling the ratio of martensite (30-70%) and retained austenite (10-40%), and by controlling the average grain size (10 μm or less) to achieve both high strength and good formability simultaneously
Solution Approach 2:
The patent creates a composite microstructure consisting of martensite and retained austenite phases, where martensite provides high strength and retained austenite provides ductility through TRIP effect, achieving both collision resistance and weight reduction
2Strength
If steel sheet strength is increased to 1180 MPa or more, then high strength is achieved, but ductility and hole expansibility deteriorate
Solution Approach 1:
The patent optimizes the microstructural parameters by controlling the martensite ratio (30-70%) and retained austenite ratio (10-40%), and by controlling the average grain size to 10 μm or less, achieving both high tensile strength (1180 MPa or more) and good ductility
Solution Approach 2:
The patent utilizes the phase transition of retained austenite to martensite during deformation (TRIP effect) to maintain ductility and hole expansibility even when the base steel has high strength
3Ease of manufacture
If conventional steel sheet processing is used, then manufacturing simplicity is maintained, but shearing surface quality deteriorates with high burr formation
Solution Approach 1:
The patent changes the microstructural parameters by controlling grain size (10 μm or less) and phase distribution (martensite and retained austenite ratios), which inherently improves shearing surface quality and reduces burr formation without requiring additional post-processing
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 steel sheet achieves excellent strength, ductility, and a smooth shearing surface, reducing manufacturing costs and enhancing its suitability for vehicle and structural applications.
Implementation Method 1
the austenite is transformed into martensite during working and large elongation is exhibited due to transformation-induced plasticity
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, 3.0% or more of retained austenite, has a ratio L52/L7 of a length L52 of a grain boundary having a crystal misorientation of 52° to a length L7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of more than 0.18, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 1180 MPa or more.