Method for laser cutting of steel strip, laser cutting device, cold rolling method and method for manufacturing cold rolled steel strip

Abstract

The laser cutting method of the steel strip of the present application is a laser cutting method of a steel strip in which the vicinity of a joint portion in which the rear end portion of a preceding steel strip is joined to the front end portion of a following steel strip is cut using a pulsed laser, wherein the output power of the pulsed laser is set to 0.5 kw or more per 1 millisecond, the processing point diameter of the pulsed laser is set to 0.1 mm or more and less than 0.6 mm, and the ratio of the pulse period time to the rest time is set to 0.3 or more and less than 0.8.

Description

Technical Field

[0001] This invention relates to a laser cutting method for steel strip, laser cutting equipment, a cold rolling method, and a method for manufacturing cold-rolled steel strip. Background Technology

[0002] In the cold rolling process of steel strip, to improve productivity and yield, the rear end of the preceding material (preceding steel strip) is generally joined with the front end of the following material (following steel strip), and the steel strip is continuously supplied to the cold rolling production line. This allows the steel strip to be rolled under tension throughout its entire length, and enables high-precision control of thickness and shape at both the front and rear ends of the steel strip.

[0003] With the increasing alloying of cold-rolled steel strips and advancements in laser welding machines, laser welding is becoming the mainstream method for joining preceding and following materials, replacing traditional flash butt welding. However, regardless of welding methods like flash butt welding or laser welding, a height difference inevitably forms at the width end of the joint (welded section) between the preceding and following materials due to differences in width and thickness, as well as positional offsets. Therefore, when rolling steel strips under these conditions, stress concentration occurs at the height difference, potentially causing the steel strip to break at the welded section. When the steel strip breaks at the welded section, the cold rolling production line must be stopped, significantly reducing productivity. Furthermore, production costs increase because work rolls damaged by the broken strips need to be replaced.

[0004] In recent years, in particular, the demand for thinner and higher-strength cold-rolled steel strips has been increasing, aiming to achieve lighter components and improved performance. Furthermore, this has led to higher requirements for cold rolling reduction rates and rolling loads, resulting in increased stress concentration at weld joints and a higher steel strip fracture rate. Therefore, to suppress steel strip fracture at weld joints and mitigate stress concentration near weld joints, a notch (cut) is created at the width-direction end of the weld joint before rolling the steel strip. It should be noted that at the width-direction end of the steel strip, the butt joint precision is inconsistent, leading to insufficient welding and reduced strength. Therefore, the notch also serves to remove the low-strength portion (approximately 30mm from the width-direction end).

[0005] As a method for creating grooves, such as that described in Patent Document 1, a semi-circular notch without corners is generally mechanically cut. However, in a semi-circular notch, the curvature of the outer edge is uniform, and the width of the steel strip is minimal at the weld, thus generating maximum stress at the weld. Therefore, Patent Document 2 describes a method for forming the notch using laser cutting as a way to shorten the grooving time. Furthermore, Patent Document 3 describes a laser cutting method that produces less slag (molten material during laser cutting).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 5-76911

[0009] Patent Document 2: Japanese Patent Application Publication No. 60-115387

[0010] Patent Document 3: Japanese Patent No. 6354793 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] However, the current situation is that the grooving method described above, especially in the cold rolling of brittle materials and high alloy materials such as silicon steel plates and high-tensile steel plates with high Si and Mn content, cannot achieve sufficient effect and cannot adequately suppress the breakage of the steel strip at the weld.

[0013] This invention was made in view of the above-mentioned problems, and its object is to provide a laser cutting method and laser cutting equipment for steel strip that can suppress the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials. Another object of this invention is to provide a cold rolling method for steel strip that can stably perform cold rolling while suppressing the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials. Furthermore, another object of this invention is to provide a method for manufacturing cold-rolled steel strip that can stably manufacture cold-rolled steel strip while suppressing the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials.

[0014] Methods for solving problems

[0015] The inventors of this invention discovered that dross generated on the cross-section of a steel strip after laser cutting can extend into a sharp shape due to cold rolling, thus becoming the starting point for cracking from the width end of the steel strip. Therefore, they investigated laser cutting conditions that can further suppress dross generation and found that by making the ratio of pulse period time to pulse time (duty factor) less than a certain value, the length of dross generation relative to the laser cutting length can be shortened.

[0016] Furthermore, it was found that even when laser-cutting the steel strip, the presence of work-hardened shear surfaces near the weld area can become a starting point for stress concentration, thus causing steel strip fracture near the weld area. Specifically, in the cold-rolled steel strip near the weld area, circular holes are punched towards the center of the width direction during welding. This punching is intended to allow the tandem cold rolling mill to track the weld point; however, because the punching is a blanking process, the punched section becomes a shear surface. Therefore, during high-load cold rolling, stress concentration occurs from the punched section, leading to steel strip fracture near the weld area.

[0017] The present invention is based on the above concept and has the following features.

[0018] The laser cutting method for steel strip of the present invention is a method for cutting a steel strip near the joint of the rear end of the preceding steel strip and the front end of the following steel strip using a pulsed laser. The output power of the pulsed laser is set to 0.5 kW or more per millisecond, the processing point diameter of the pulsed laser is set to 0.1 mm or more and less than 0.6 mm, and the ratio of pulse period time to rest time is set to 0.3 or more and less than 0.8.

[0019] The cutting section using the pulsed laser cutting method may include two end faces of the steel strip in the width direction and one or more closed cross-sectional shapes.

[0020] Compressed air at a pressure of 0.5 MPa or higher can be used as the gas for the pulsed laser.

[0021] The laser cutting device for steel strip of the present invention is a laser cutting device for cutting a steel strip near the joint of the rear end of the preceding steel strip and the front end of the following steel strip using a pulsed laser. The output power of the pulsed laser is set to 0.5 kW or more per millisecond, the processing point diameter of the pulsed laser is set to 0.1 mm or more and less than 0.6 mm, and the ratio of pulse period time to pulse time is set to 0.3 or more and less than 0.8.

[0022] The cutting section using the pulsed laser cutting method may include two end faces of the steel strip in the width direction and one or more closed cross-sectional shapes.

[0023] Compressed air at a pressure of 0.5 MPa or higher can be used as the gas for the pulsed laser.

[0024] The cold rolling method of the present invention is used to cold roll a steel strip that has been cut by the laser cutting method of the present invention.

[0025] The method for manufacturing cold-rolled steel strip of the present invention manufactures cold-rolled steel strip through a process including the cold rolling method of the steel strip of the present invention.

[0026] Invention Effects

[0027] According to the laser cutting method and laser cutting equipment for steel strip of the present invention, even in the case of brittle materials and high-alloy materials, fracture of the steel strip at the welded portion can be suppressed. Furthermore, according to the cold rolling method for steel strip of the present invention, even in the case of brittle materials and high-alloy materials, fracture of the steel strip at the welded portion can be suppressed and cold rolling can be performed stably. Moreover, according to the manufacturing method of cold-rolled steel strip of the present invention, even in the case of brittle materials and high-alloy materials, fracture of the steel strip at the welded portion can be suppressed and cold-rolled steel strip can be manufactured stably. Attached Figure Description

[0028] Figure 1 This is a schematic diagram illustrating a laser cutting method for steel strip as one embodiment of the present invention.

[0029] Figure 2 A diagram showing cross-sectional and surface images of the test material.

[0030] Figure 3 This is a diagram used to illustrate the duty cycle. Detailed Implementation

[0031] Hereinafter, with reference to the accompanying drawings, a laser cutting method for steel strip, a laser cutting apparatus, a cold rolling method, and a method for manufacturing cold-rolled steel strip, as embodiments of the present invention, will be described. It should be noted that the embodiments shown below exemplify apparatus and methods for embodying the technical concept of the present invention, and do not define the materials, shapes, structures, and configurations of the constituent components as shown in the embodiments below. Furthermore, the accompanying drawings are schematic. Therefore, attention should be paid to the differences between the relationship between thickness and planar dimensions, ratios, etc., and reality; the drawings also include portions with different dimensional relationships and ratios.

[0032] Figure 1 This is a schematic diagram illustrating a laser cutting method for steel strip as one embodiment of the present invention. Figure 1As shown, in a laser cutting method for steel strip according to one embodiment of the present invention, a circular arc-shaped notch 11 is formed by laser cutting (laser cutting) over a defined range of the steel strip including the rear end of the preceding steel strip 1 and the front end of the following steel strip 2, in the width direction of the weld portion 3. This allows the notch 11 to be formed without work hardening at the width direction end of the weld portion 3. Furthermore, even in the case of brittle materials or high-alloy materials such as silicon steel sheets and high-tensile steel sheets with high Si and Mn content, fracture at the weld portion 3 will not occur, allowing continuous cold rolling of the preceding steel strip 1 and the following steel strip 2. It should be noted that the shape of the notch 11 and the laser cutting trajectory (laser scanning trajectory) are not limited to this embodiment; the shape of the notch 11 can be a semi-circular shape, a roughly isosceles trapezoid, or other shapes without any problem. Furthermore, in this embodiment, laser cutting with a closed cross-section shape is used to perform the punching process near the center portion of the steel strip in the width direction, which is conventionally performed by punching. It should be noted that in... Figure 1 In the example shown, a hole 12 with a closed cross-sectional shape is formed on the preceding steel strip 1, but the hole 12 can also be formed on the following steel strip 2. Furthermore, there are no particular limitations on the closed cross-sectional shape, the number of holes, or the coordinates of the cutting position. Moreover, when using methods for tracking weld points such as magnetic flux leakage or other image-based determination methods, forming the hole is not necessary.

[0033] In this embodiment, to evaluate the impact of residual slag from laser cutting on cold rolling, a laboratory-scale rolling experiment was conducted as described below. Specifically, a silicon steel sheet containing 3.3% by mass Si, with a thickness of 2 mm after laser cutting at both ends in the width direction, was used as the test material. Then, without applying tension, the test material was cold-rolled using a rolling mill with a work roll diameter of 500 mm at a total reduction rate of 50%.

[0034] Figure 2 (a) shows a cross-sectional image of the test material at a reduction rate of 0%. Figure 2 (b) Shows a cross-sectional image of the test material with a reduction rate of 50%. Figure 2 (c) A surface image of the test material at a reduction rate of 50%. Figure 2 As shown in (a) to (c), when slag remains due to poor laser cutting conditions, it persists after cold rolling and extends into a sharp shape. In this experiment, although no tension was applied to the test material, it is speculated that the slag extending into a sharp shape would become the starting point of stress concentration and lead to strip fracture in tandem rolling with tension applied as in actual production.

[0035] Therefore, in this embodiment, to minimize the adhesion area of ​​the scum as described above, the laser output power is set to 0.5 kW or more per millisecond, and the laser processing point diameter is set to 0.1 mm or more and less than 0.6 mm as laser cutting conditions. Figure 3 The ratio of pulse period time to pulse time (duty factor) is set to 0.3 or higher and less than 0.8. Under laser cutting conditions such as a laser processing point diameter of 0.6 mm or higher or a duty factor greater than 0.8, the heat input increases, making it easier to generate slag. On the other hand, under laser cutting conditions such as a laser processing point diameter less than 0.1 mm or a duty factor less than 0.3, the heat input decreases, making it impossible to cut steel strips.

[0036] The laser processing point diameter is more preferably 0.2 mm to 0.3 mm, and the duty cycle is more preferably 0.5 to 0.75. Furthermore, compressed air with a pressure of 0.5 MPa or higher is preferably used for the pulsed laser. This is because at pressures below 0.5 MPa, the generated slag will not be blown away and will easily remain on the cutting surface.

[0037] Furthermore, when oxygen is used as the gas in laser cutting, cutting can be performed with low output power by utilizing the heat of oxidation. However, under conditions of slow laser scanning speed and high laser output power, spontaneous combustion occurs in the un-illuminated portion, leading to melting and combustion. Nitrogen can achieve the same effect, but under conditions of slow scanning speed and high output power, nitrogen dissolves in the molten portion or slag, forming hard nitrides or nitrided layers, which can sometimes become the starting point for stress concentration. More preferably, in addition to being inexpensive, is compressed air, which can utilize the heat of oxidation by moderately containing oxygen.

[0038] Furthermore, for ordinary low-carbon steel, even with conventional shearing processes, no cracking (edge ​​cracking) will occur at the ends in the width direction. Therefore, this invention is not necessarily required for steel grades where fracture near the weld is almost nonexistent; it should be applicable to brittle materials, high-alloy materials, and other steel grades where weld fracture occurs through shearing. However, tandem cold rolling mills are sometimes dedicated to silicon steel sheets and high-tensile steel sheets, but sometimes they are not dedicated to silicon steel sheets or high-tensile steel sheets, but are also multi-purpose mills that roll low-carbon steel, etc. In this case, there is no problem in using laser cutting to groove the low-carbon steel. Alternatively, both shearing machines and laser cutting machines can be installed simultaneously and used separately depending on the steel grade.

[0039] Example

[0040] The present invention will now be described based on embodiments. Using a tandem cold rolling mill consisting of five rolling mills, an electromagnetic steel sheet containing 2.8–3.3% by mass Si with a base material thickness of 2.0 mm and a width of 1000 mm was cold-rolled to a finishing thickness of 0.300 mm using the raw steel sheet as the rolling material. In the embodiments, laser cutting was performed based on an embodiment of the present invention. That is, near the weld between the preceding and following steel strips, a semi-circular laser cut was performed relative to both ends of the steel strip, and a circular closed-section laser cut was performed relative to the center of the width of the preceding steel strip. On the other hand, in the comparative example, laser cutting conditions that did not satisfy the conditions of the present invention were implemented. Otherwise, rolling was performed in the same manner as in the embodiments. The slag formation and the breakage rate of 100 coils rolled in the embodiments and comparative examples where laser cutting was performed as described above are shown in Table 1. It should be noted that the meanings of ◎, ○, △, and × for the scum height and scum generation length recorded in Table 1 are as described in Table 2.

[0041] [Table 1]

[0042]

[0043] [Table 2]

[0044] scum height Length of scum formation × Cannot be cut Compared to laser cutting length, it is more than 80% △ The scum is larger than 0.2mm. Compared to laser cutting length, it is 40% to 70% or more. ○ Scum less than 0.2mm Compared to laser cutting, the length is 20-30% or more. ◎ No scum The length is more than 10% of that of laser cutting.

[0045] As shown in Table 1, in the comparative examples, the incidence of weld fracture was 6.0% to 10.0%, while in the embodiments, the incidence was less than 2%. This confirms the effectiveness of the present invention. Specifically, by applying the present invention to laser-cut the area near the weld between the preceding and following steel strips, the amount of slag generated can be reduced, and work hardening near the weld can be prevented. Furthermore, this suppresses weld fracture, thereby improving productivity and yield.

[0046] The embodiments of the invention completed by the inventors have been described above. However, the invention is not limited to the descriptions and drawings that constitute a part of the disclosure of the invention in this embodiment. That is, all other embodiments, examples, and applications based on this embodiment by those skilled in the art are included within the scope of the invention.

[0047] Industrial availability

[0048] According to the present invention, a laser cutting method and laser cutting apparatus for steel strips can be provided that can suppress the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials. Furthermore, according to the present invention, a cold rolling method for steel strips can be provided that can stably perform cold rolling while suppressing the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials. Moreover, according to the present invention, a method for manufacturing cold-rolled steel strips can be provided that can stably manufacture cold-rolled steel strips while suppressing the breakage of the steel strip at the weld even in the case of brittle materials and high alloy materials.

[0049] Symbol Explanation

[0050] 1. Leading steel strip

[0051] 2. Rear steel belt

[0052] 11 Notch

[0053] 12-hole section

Claims

1. A laser cutting method for steel strip, comprising using a pulsed laser to cut near the joint of a steel strip where the rear end of a preceding steel strip and the front end of a subsequent steel strip are joined, wherein, The output power of the pulsed laser is set to 0.5 kW or more per millisecond, the processing point diameter of the pulsed laser is set to 0.1 mm or more and less than 0.6 mm, and the ratio of pulse cycle time to rest time is set to 0.3 or more and less than 0.

8.

2. The laser cutting method for steel strip according to claim 1, wherein, The cutting section using the pulsed laser cutting method includes two end faces of the steel strip in the width direction and one or more closed cross-sectional shapes.

3. The laser cutting method for steel strip according to claim 1 or 2, wherein, Compressed air at a pressure of 0.5 MPa or higher is used as the gas for the pulsed laser.

4. A laser cutting device for steel strip, which uses a pulsed laser to cut near the joint of a steel strip where the rear end of a preceding steel strip and the front end of a following steel strip are joined, wherein... The output power of the pulsed laser is set to 0.5 kW or more per millisecond, the processing point diameter of the pulsed laser is set to 0.1 mm or more and less than 0.6 mm, and the ratio of pulse period time to pulse time is set to 0.3 or more and less than 0.

8.

5. The laser cutting equipment for steel strip according to claim 4, wherein, The cutting section using the pulsed laser cutting method includes two end faces of the steel strip in the width direction and one or more closed cross-sectional shapes.

6. The laser cutting equipment for steel strip according to claim 4 or 5, wherein, Compressed air at a pressure of 0.5 MPa or higher is used as the gas for the pulsed laser.

7. A cold rolling method for steel strip, wherein, The steel strip cut by the laser cutting method according to any one of claims 1 to 3 is then cold rolled.

8. A method for manufacturing cold-rolled steel strip, wherein, Cold-rolled steel strip is manufactured by a process including the cold rolling method of the steel strip as described in claim 7.

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