Cold-Rolled Steel Sheet Cooling to Prevent End Portion Cracking
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Solution Overview
Problem
High-tensile cold-rolled steel sheets often experience end portion cracking during manufacturing due to insufficient ferrite/pearlite transformation at the width-direction and longitudinal-direction ends, leading to fractures and yield decreases, especially when using steel with high Mn content.
Innovation Solution
A method involving hot-rolling slabs with specific chemical compositions to achieve an outlet temperature of 800° C. to 940° C. from the finishing rolling mill, followed by rapid water-cooling and coiling at 550° C. or higher to promote ferrite/pearlite transformation and soften the end portions, preventing cracking during cold-rolling.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high Mn content steel is used to improve hardenability and achieve high tensile strength, then tensile strength is improved, but end portion cracking occurs during cold-rolling
Solution Approach 1:
The invention applies different cooling intensities to different regions of the steel sheet. The width-direction end portions receive intensified cooling (higher cooling speed) compared to the central portion, creating local microstructure differences. This local quality adjustment ensures that end portions have sufficient ferrite/pearlite transformation even with high Mn content, preventing cracking while maintaining overall high strength
Solution Approach 2:
The invention changes the cooling speed parameter specifically at the width-direction end portions during the cooling process. By increasing the cooling speed at these locations, the microstructure transformation is controlled to achieve adequate ferrite/pearlite content, thereby improving end portion cracking resistance while maintaining the high tensile strength properties of the steel
2Productivity
If conventional cooling is used after hot-rolling, then production efficiency is maintained, but end portions have insufficient ferrite/pearlite transformation and develop hard structures
Solution Approach 1:
The invention implements local quality control by applying different cooling speeds to different regions. The width-direction end portions are cooled faster than the central portion, creating intentional microstructure variations that prevent end portion hardening while maintaining overall production efficiency and microstructure stability
3Reliability
If additional high-temperature heating facilities are introduced to prevent end portion cracking, then end portion cracking resistance is improved, but device complexity and manufacturing cost increase
Solution Approach 1:
The invention performs preliminary action by controlling the cooling process immediately after hot-rolling to establish the desired microstructure before cold-rolling. By adjusting cooling speeds to ensure adequate ferrite/pearlite transformation at end portions, the need for subsequent high-temperature heating facilities is eliminated, reducing device complexity while maintaining end portion cracking resistance
Solution Approach 2:
The invention uses the existing cooling infrastructure to achieve the desired microstructure control. The cooling process itself serves the dual purpose of temperature reduction and microstructure optimization, eliminating the need for additional heating facilities and making the system self-sufficient
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 method effectively prevents end portion cracking during cold-rolling without requiring additional high-temperature heating facilities, maintaining yield and achieving a tensile strength of 980 MPa or more in the steel sheets.
Implementation Method 1
hot-rolling a slab having chemical composition containing: C: 0.15% by mass or more and 0.25% by mass or less, Si: 0.8% by mass or more and 3.0% by mass or less, Mn: 1.8% by mass or more and 3.0% by mass or less
Implementation Method 2
cooling at a water volume density of 100 L/min/m2 or more for 0.1 seconds or more, within 3.0 seconds after passing through a final stand of the finishing rolling mill
Implementation Method 3
ferrite/pearlite transformation does not sufficiently proceed at both the width-direction end portions of the steel sheet
Data Source
AI summary
A method for manufacturing a steel sheet for cold rolling, including hot-rolling a slab so that an outlet temperature of a finishing rolling mill is 800° C. or higher and 940° C. or lower, cooling at least a portion of the hot-rolled steel sheet, at a water volume density of 100 L/min/m2 or more for 0.1 seconds or more, within 3.0 seconds after passing through a final stand of the finishing rolling mill and being sent out on a run-out table, and coiling, at a coiling temperature of 550° C. or higher, the cooled hot-rolled steel sheet.