Continuous Steel Casting Soft Reduction for Center Segregation
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Solution Overview
Problem
Current continuous casting methods fail to adequately reduce center segregation in steel slabs, particularly in the slab width direction, leading to hydrogen-induced cracking issues in line pipe steel materials, due to insufficient control over solidification completion position and bulging between rollers, and neglect the influence of reformation zones in curved and vertical-bending machines.
Innovation Solution
A continuous casting method that involves intentional bulging of the slab's wide side surfaces by increasing the roller gap downstream in curved or vertical-bending machines, followed by rolling reduction in a soft reduction zone with a reduced roller gap, ensuring a solid phase fraction less than 0.2 in the reformation zone where the slab shape transforms from circular arc to linear, thereby minimizing tensile forces and segregation variation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If soft reduction is performed with a predetermined rolling reduction amount by adjusting roller gap in segment-type continuous casting machines, then center segregation is reduced, but segmentation of the continuous casting machine causes non-uniform rolling reduction amount across the slab width
Solution Approach 1:
The continuous casting machine is divided into multiple segments, each with independently adjustable roller gaps. This allows the rolling reduction amount to be optimized for different regions of the slab width, compensating for the segmentation effect and achieving uniform rolling reduction across the entire slab width while effectively reducing center segregation.
Solution Approach 2:
Different roller gaps are set for different segments along the width direction of the slab. By locally adjusting the rolling reduction amount in each segment, the patent achieves uniform rolling reduction across the slab width, addressing the non-uniformity caused by machine segmentation while maintaining effective center segregation control.
2Manufacturing precision
If rolling reduction is applied to prevent void formation and molten steel flow, then center segregation is reduced, but tensile forces in reformation zones cause solidification interface separation and internal cracking
Solution Approach 1:
The patent identifies and avoids performing soft reduction in reformation zones where the slab shape is transformed from circular arc to linear. By preliminarily determining the reformation zone locations and excluding them from soft reduction operations, the method prevents tensile forces from causing solidification interface separation and internal cracking, while still achieving center segregation control in safe zones.
Solution Approach 2:
The patent recognizes that reformation zones create harmful tensile forces that lead to internal cracking. By strategically avoiding soft reduction in these zones and concentrating rolling reduction in stable zones, the method converts the potential harm of reformation zone stresses into a benefit by preventing cracking while maintaining segregation control effectiveness.
3Object-affected harmful factors
If solid phase fraction is controlled to be small in reformation zone, then internal cracking is prevented, but center segregation control effectiveness is reduced
Solution Approach 1:
The casting process is segmented into reformation zones (where shape transformation occurs) and non-reformation zones (stable zones). Soft reduction is applied only in stable zones where solid phase fraction can be effectively controlled, while reformation zones are excluded. This segmentation allows center segregation control to be maintained in safe zones without causing internal cracking in reformation zones.
Solution Approach 2:
Different solid phase fraction control strategies are applied to different zones: in reformation zones, the focus is on preventing internal cracking by avoiding excessive rolling reduction, while in stable zones, the focus is on achieving center segregation control through appropriate solid phase fraction management. This local differentiation optimizes both cracking prevention and segregation control.
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 effectively reduces the segregation degree variation across the slab width, enhances hydrogen-induced cracking resistance, and prevents internal cracking, meeting stringent quality requirements for line pipe steel.
Implementation Method 1
performing rolling reduction on the wide side surfaces of the slab, performed after the bulging of the wide side surfaces of the slab, in a soft reduction zone in which the roller gap of a plurality of pairs of slab support rollers is reduced stepwise toward the downstream side in the casting direction
Implementation Method 2
a reformation zone in which a shape of the slab in the casting direction is reformed from a circular arc shape into a linear shape
Implementation Method 3
In a solidification process of steel, solute elements such as carbon (C), phosphorus (P), sulfur (S), and manganese (Mn) are concentrated in the unsolidified liquid side through redistribution during the solidification
Data Source
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AI summary
To reduce an overall segregation level of center segregation of a continuously cast slab in a slab width direction and also reduce variation in a segregation degree in the slab width direction. A continuous casting method of steel according to the present invention includes the step of bulging wide side surfaces of a slab having thereinside an unsolidified layer with a total intentional bulging amount of 3 to 10 mm by increasing stepwise toward a downstream side in a casting direction a roller gap of a plurality of pairs of slab support rollers disposed in a continuous casting machine. The method also includes the step of performing rolling reduction on the wide side surfaces of the slab, performed after the bulging of the wide side surfaces of the slab, in a soft reduction zone in which the roller gap of a plurality of pairs of slab support rollers is reduced stepwise toward the downstream side in the casting direction. The wide side surfaces of the slab undergo rolling reduction at a rolling reduction speed of 0.3 to 2.0 mm/min with a total rolling reduction amount smaller than or equal to the total intentional bulging amount in the soft reduction zone. A solid phase fraction at a center of a thickness of the slab is smaller than 0.2, or is greater than or equal to a flow limit solid phase fraction and not greater than 1.0 in a reformation zone in which a shape of the slab in the casting direction is reformed from a circular arc shape into a linear shape.