Hot-Rolled Steel Sheet Cooling for Strength and Formability
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Solution Overview
Problem
Existing hot-rolled steel sheets used in vehicle chassis components face challenges in achieving high strength, excellent fatigue performance, and uniform material distribution while maintaining good formability, which is essential for press forming and to prevent processing cracks.
Innovation Solution
The development of an ultrahigh-strength hot-rolled steel sheet with a specific alloy composition and a controlled cooling process, including reheating, hot rolling, primary cooling, secondary cooling, and tertiary cooling, to achieve a microstructure comprising 75-90% low-temperature bainite and martensite, 10-25% acicular ferrite and bainitic ferrite, and minimal other phases.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If water cooling is used to cool the steel sheet to temperatures below Ms to achieve high strength through martensite formation, then tensile strength is improved to 1180 MPa or more, but shape defects occur due to rapid phase transformation and material deviation increases
Solution Approach 1:
The cooling process is divided into multiple stages: rapid cooling to suppress austenite formation, followed by controlled cooling through the martensite transformation region. This segmented approach allows the steel sheet to achieve high strength through martensite formation while minimizing shape defects by avoiding excessive thermal gradients and rapid phase transformation in a single step.
Solution Approach 2:
The steel sheet is pre-cooled to a temperature below the austenite transformation range before entering the martensite transformation region. This preliminary action suppresses austenite formation that would otherwise cause excessive volume expansion and shape defects during subsequent martensite transformation, thereby achieving high strength with better shape uniformity.
2Strength
If rapid cooling is applied to form low-temperature bainite and martensite for high strength, then tensile strength reaches 1180 MPa or more, but the possibility of shape defects increases due to rapid phase transformation
Solution Approach 1:
The cooling rate is dynamically adjusted during the cooling process. Rapid cooling is applied initially to suppress austenite formation and promote bainite/martensite formation, then the cooling rate is moderated as the steel sheet approaches the martensite transformation region. This dynamic control achieves high strength through rapid phase transformation while minimizing shape defects by reducing thermal gradients and transformation stress.
3Strength
If local temperature deviations occur during cooling to achieve ultrahigh strength microstructure, then tensile strength is improved, but material deviation increases causing non-uniform formability
Solution Approach 1:
The cooling process is designed to create different cooling conditions in different regions of the steel sheet. The center region, which is more prone to temperature deviation, is cooled at a controlled rate to prevent excessive thermal gradients. This local quality control ensures uniform material composition and formability while still achieving the required ultrahigh strength microstructure throughout the sheet.
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 resulting steel sheet exhibits tensile strength of 1180 MPa or more, excellent formability, and uniform material distribution, making it suitable for use in vehicle chassis structure members while ensuring driving stability and productivity.
Implementation Method 1
reheating a steel slab at a temperature range of 1100 to 1350° C.
Implementation Method 2
primarily cooling the hot-rolled steel sheet to a temperature equal to or less than Bs at an average cooling rate of 50° C./s or more
Implementation Method 3
after the secondary cooling, performing tertiary cooling at an average cooling rate of 30° C./s or more to a temperature range of Ms° C. to 500° C.
Data Source
AI summary
The present invention relates to a hot-rolled steel sheet that can be suitably applied to an automobile chassis structure member or the like and, more specifically, to an ultrahigh-strength hot-rolled steel sheet having tensile strength of 1180 MPa or more, excellent formability, and uniform material distribution in the steel sheet, and a manufacturing method therefor.