High Strength Steel Plate Stretch Flange Formability
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Solution Overview
Problem
High strength hot rolled steel plates face challenges in achieving both superior stretch flange formability and fatigue characteristics due to the deformation of MnS inclusions, which act as starting points for cracking during hole expansion, leading to compromised ductility and fatigue resistance.
Innovation Solution
The introduction of Ce and La as deoxidizing agents forms fine, hard oxides and oxysulfides that precipitate MnS, preventing its deformation and clustering, thereby reducing the number of stretched coarse MnS inclusions and enhancing the steel's resistance to cracking and fatigue.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high strength steel plate is used to increase strength and reduce weight, then strength and fatigue resistance are improved, but hole expandability and ductility deteriorate
Solution Approach 1:
The invention changes the chemical composition parameters by strictly controlling S content (0.0005% or less) and Mn content (0.5-3.0%), and adding specific elements (Ti: 0.003-0.10%, Nb: 0.003-0.10%, V: 0.003-0.05%) to modify the microstructure and inclusion characteristics, thereby achieving both high strength and good hole expandability
Solution Approach 2:
The invention creates a composite microstructure consisting of bainite phase as the main phase with fine dispersed carbides and controlled inclusions, combining the advantages of different phases to achieve superior mechanical properties and formability simultaneously
2Strength
If MnS inclusions are present to contribute to strength, then strength is improved, but stretched coarse MnS inclusions form cracking starting points that deteriorate fatigue characteristics and stretch flange formability
Solution Approach 1:
The invention extracts and removes the harmful stretched coarse MnS inclusions by严格控制 S content to 0.0005% or less and adding Ti and Nb elements that precipitate as fine, non-stretchable inclusions, thereby eliminating the cracking starting points while maintaining strength
Solution Approach 2:
The invention introduces Ti and Nb as intermediary elements that act as inclusions modifiers, transforming the harmful stretched MnS inclusions into fine, stable precipitates that do not stretch during deformation, thereby mediating between strength requirements and fatigue resistance
3Reliability
If Ce and La are added to form fine hard oxides and oxysulfides that precipitate MnS, then stretched coarse MnS inclusions are reduced and fatigue characteristics are improved, but manufacturing complexity increases
Solution Approach 1:
The invention changes the compositional parameters by specifying precise ranges for Ce (0.0003-0.04%) and La (0.0003-0.04%) addition, along with coordinated control of Ti, Nb, and other elements, achieving fine inclusion precipitation and improved fatigue characteristics through controlled parameter adjustment
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 results in high strength steel plates with improved stretch flange formability and fatigue characteristics by minimizing the formation of deformed MnS inclusions, leading to better durability and resistance to cracking during deformation and hole expansion.
Implementation Method 1
The introduction of Ce and La as deoxidizing agents forms fine, hard oxides and oxysulfides that precipitate MnS
Implementation Method 2
forms fine, hard oxides and oxysulfides that precipitate MnS, preventing its deformation and clustering
Data Source
Figure 1
AI summary
The present invention provides high strength hot rolled steel plate superior in stretch flange formability and fatigue characteristics comprising steel plate containing C: 0.03 to 0.20%, Si: 0.08 to 1.5%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.0005% or more, N: 0.0005 to 0.01%, acid soluble A1: 0.01% or less, acid soluble Ti: less than 0.008%, and a total of one or both of Ce or La: 0.0005 to 0.04%, having a balance of iron and unavoidable impurities and having a number ratio of 20% or less of stretched inclusions present in the steel plate having a circle equivalent diameter of 1 µm or more and a long axis/short axis of 5 or more.