Hot-Rolled Steel Sheet Phase Control for Strength and Ductility
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Solution Overview
Problem
Existing high-strength steel sheets struggle to achieve both high ductility and stable shearing workability, particularly in vehicle components, with variations in end surface accuracy and sheared section proportion, and existing technologies do not effectively address both properties simultaneously.
Innovation Solution
A hot-rolled steel sheet with a specific chemical composition and metallographic structure, including controlled grain boundaries and phases, such as martensite, ferrite, and tempered martensite, optimized through controlled heating and cooling processes to enhance strength, ductility, and shearing workability.
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 invention changes the metallurgical parameters of the steel sheet by controlling the ratio of martensite to bainite in the metallographic structure. By adjusting this phase composition ratio and controlling grain size, the material achieves both high strength (1320 MPa or more) and adequate ductility, resolving the contradiction between weight reduction and collision resistance
Solution Approach 2:
The invention creates a composite microstructure containing both martensite and bainite phases in specific proportions. This composite metallurgical structure combines the high strength characteristics of martensite with the ductility contributions from bainite, enabling the steel sheet to simultaneously achieve weight reduction and maintain collision resistance
2Stability of the object's composition
If residual austenite is increased to improve ductility through transformation-induced plasticity, then ductility is improved, but hole expansibility is impaired due to full hard martensite formation
Solution Approach 1:
The invention changes the phase composition parameters by controlling the ratio of martensite to bainite and refining grain size. By optimizing these parameters, the steel achieves improved ductility through controlled transformation-induced plasticity while maintaining hole expansibility through appropriate grain refinement, avoiding the formation of excessive full hard martensite
3Strength
If high strength steel sheet with tensile strength of 980 MPa or more is used, then strength is improved, but shearing workability deteriorates with unstable sheared section proportion and varying end surface accuracy
Solution Approach 1:
The invention changes the metallographic structure parameters by controlling the martensite-to-bainite ratio and grain size distribution. This parameter optimization enables the steel sheet to achieve tensile strength of 980 MPa or more while maintaining stable shearing workability and consistent end surface accuracy, resolving the contradiction between strength and shearing performance
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 tensile strength of 980 MPa or more with stable shearing workability, reducing the likelihood of cracking and ensuring consistent end surface accuracy, making it suitable 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
Implementation Method 2
a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite
Data Source
AI summary
This hot-rolled steel sheet has a predetermined chemical composition, in which a metallographic structure contains, by area %, less than 3.0% of residual austenite, 15.0% or more and less than 60.0% of ferrite, and less than 5.0% of pearlite, has a ratio L60/L7 of a length L60 of a grain boundary having a crystal misorientation of 60° to a length L7 of a grain boundary having a crystal misorientation of 7° about a <110> direction of 0.60 or more, has a standard deviation of a Mn concentration of 0.60 mass % or less, and has a tensile strength of 980 MPa or more.