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

Abstract

The present invention relates to a laser cutting method of a steel strip, which is a laser cutting method of a steel strip in which a width direction end portion of a steel strip including a joint portion after joining a rear end portion of a preceding steel strip and a front end portion of a following steel strip is cut using a laser, wherein the steel strip is cut in a manner that an interval between dregs having an aspect ratio of 1.0 or more at the width direction end portion after cutting becomes 1.0 mm or more and an interval between a dreg having an aspect ratio of 1.0 or more and a dreg having an aspect ratio of less than 1.0 becomes 1.0 mm or more.

Description

Technical Field

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

[0002] In the cold rolling process of steel strip, the rear end of the preceding material (leading steel strip) and the front end of the following material (following steel strip) are joined together with the aim of improving productivity and yield, and the steel strip is continuously supplied to the cold rolling line. As a result, the steel strip can be rolled under tension throughout its entire length, and the thickness and shape can be controlled with high precision 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 the welding method, a step inevitably forms at the width of the joint (welded section) between the preceding and following materials due to differences in width and thickness, positional misalignment, etc. Therefore, if the steel strip is rolled in this condition, stress concentration occurs at the stepped section, potentially causing the strip to break at the welded section. If the strip breaks at the welded section, the cold rolling line must be stopped, significantly reducing productivity. Furthermore, it necessitates replacing work rolls damaged by broken strips, thus increasing production costs.

[0004] In recent years, in particular, the demand for thinner and higher-strength cold-rolled steel strips has become increasingly stringent, 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 the stepped sections and a higher steel strip fracture rate. Therefore, to suppress steel strip fracture at the welded sections and alleviate stress concentration near the stepped sections, a process is performed where a notch (cut) is formed at the width-direction end of the steel strip before rolling. It should be noted that at the width-direction end of the steel strip, the butt joint accuracy is inconsistent, the weld becomes insufficient, and the strength tends to decrease. Therefore, the purpose of the partial blanking is also to cut off the lower-strength portion (approximately 30mm from the width-direction end).

[0005] As a method for partial blanking, a common approach is to mechanically cut a semi-circular notch without corners, as described in Patent Document 1. However, in a semi-circular notch, the curvature of the outer edge is uniform, and the width of the steel strip becomes 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 partial blanking 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, currently, the local punching 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] The present invention was made in view of the above-mentioned problems, and its object is to provide a laser cutting method and laser cutting apparatus 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. Furthermore, another object of the present 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. Moreover, another object of the present 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 recognized that slag generated in the cross-section of a steel strip after laser cutting, which extends into a sharp shape due to cold rolling, could potentially become the initiation point for cracks originating from the width-direction end of the steel strip. Therefore, the relationship between slag and the crack susceptibility of the steel strip was investigated, and it was found that the crack susceptibility of the steel strip increases when the slag generated during laser cutting is dense. Furthermore, the inventors of this invention recognized that by controlling the aspect ratio (slag height / slag width) and the spacing of the slag in the laser-cut surface of the steel strip, cracks originating from the width-direction end of the steel strip can also be suppressed after cold rolling.

[0016] Furthermore, it has been recognized that even if the steel strip is cut using a laser, if a work-hardened shear surface remains near the weld, this area can become a starting point for stress concentration and a cause of strip fracture near the weld. Specifically, for cold-rolled steel strip near the weld, a circular hole is machined near the center of the width direction using a piercing mill during welding. This hole machining is performed to allow the cold rolling mill to track the weld point position, but since the piercing mill is a punching process, the piercing mill cross-section becomes a shear surface. Therefore, if high-load cold rolling is performed, stress concentration occurs from the piercing mill cross-section, causing strip fracture near the weld.

[0017] This invention is based on the above-mentioned ideas and has the following features.

[0018] The laser cutting method for steel strips disclosed in this invention is a laser cutting method for cutting the width-direction end of a steel strip, which includes a joint portion formed by joining the rear end of a preceding steel strip and the front end of a subsequent steel strip, using a laser. The steel strip is cut in such a manner that the spacing between slag particles with an aspect ratio of 1.0 or higher at the cut width-direction end is 1.0 mm or higher, and the spacing between slag particles with an aspect ratio of 1.0 or higher and slag particles with an aspect ratio less than 1.0 is 1.0 mm or higher.

[0019] Preferably, the cutting process of cutting off the end in the width direction and the hole forming process of forming one or more holes in the steel strip using a laser are performed continuously while the steel strip is stopped.

[0020] The laser cutting equipment for steel strips involved in this invention performs the laser cutting method for steel strips involved in this invention.

[0021] The cold rolling method for steel strips involved in this invention involves cold rolling steel strips that have been cut by the laser cutting method of the steel strips involved in this invention.

[0022] The present invention relates to a method for cutting cold-rolled steel strip, which utilizes a process including the cold rolling method of the steel strip of the present invention to manufacture cold-rolled steel strip.

[0023] Invention Effects

[0024] 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, it is possible to suppress the breakage of steel strip at the weld portion. 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, it is possible to suppress the breakage of steel strip at the weld portion and perform cold rolling 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, it is possible to suppress the breakage of steel strip at the weld portion and produce cold-rolled steel strip stably. Attached Figure Description

[0025] Figure 1 This is a schematic diagram illustrating a laser cutting method for steel strip as an embodiment of the present invention.

[0026] Figure 2 It is a diagram showing the cross-sectional and surface images of the cold-rolled test material in the area with the highest slag production.

[0027] Figure 3 It is a diagram showing the cross-sectional and surface images of the cold-rolled test material in a region with low slag production.

[0028] Figure 4 It is a diagram used to illustrate the duty cycle. Detailed Implementation

[0029] Hereinafter, with reference to the accompanying drawings, a laser cutting method, laser cutting equipment, cold rolling method, and 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 used to concretize the technical concept of the present invention, and do not limit the materials, shapes, structures, and arrangements of the constituent components to the embodiments shown below. Furthermore, the accompanying drawings are schematic. Therefore, it should be noted that the relationship between thickness and planar dimensions, ratios, etc., differ from reality, and the drawings also include portions where the dimensional relationships and ratios differ from each other.

[0030] 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, the laser cutting method for steel strip according to one embodiment of the present invention forms an arc-shaped notch 11 by laser cutting (laser cutting) on ​​a predetermined 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 at the width direction end of the weld portion 3. Therefore, the notch 11 can 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, the preceding steel strip 1 and the following steel strip 2 can be continuously cold-rolled without causing fracture at the weld portion 3. 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; other shapes such as a semi-circular shape or a roughly isosceles trapezoid are also acceptable.

[0031] In this embodiment, to evaluate the effect of slag remaining on the cut surface of the steel strip during laser cutting on cold rolling, a laboratory-scale rolling experiment was conducted as described below. Specifically, a 2mm thick silicon steel sheet containing 3.3% by mass Si and laser-cut at both ends in the width direction was used as the test material. Furthermore, the test material was cold-rolled with a total reduction of 50% using a rolling mill with a work roll diameter of 500mm without applying tension.

[0032] Figure 2 (a) and (b) show cross-sectional and surface images of the cold-rolled test material in areas with high slag production. Additionally, Figure 3 (a) and (b) show cross-sectional and surface images of the cold-rolled test material in areas with low slag production. Figure 2 and Figure 3 The comparison clearly shows that when slag remains due to poor laser cutting conditions, it persists after cold rolling, and the slag extends into a sharp shape. In this experiment, no tension was applied to the test material, but in continuous rolling with tension applied as in actual production, it is presumed that the slag extending into a sharp shape would become the starting point of stress concentration, leading to the breakage of the steel strip at the weld.

[0033] In summary, in this embodiment, the spacing between slag particles with an aspect ratio (slag height / slag width) of 1.0 or higher generated during laser cutting is 1.0 mm or higher, and the spacing between slag particles with an aspect ratio of 1.0 or higher and slag particles with an aspect ratio less than 1.0 is 1.0 mm or higher. Based on this slag state, even in the case of brittle materials and high-alloy materials such as silicon steel sheets and high-tensile steel sheets with high Si and Mn content, the slag shape can be prevented from extending into a sharp shape during cold rolling, thus preventing the steel strip at the weld joint from breaking during cold rolling.

[0034] It should be noted that, in this embodiment, the laser cutting processing conditions are not particularly limited, except that the interval between slag particles with an aspect ratio of 1.0 or higher generated during laser cutting is 1.0 mm or higher, and the interval between slag particles with an aspect ratio of 1.0 or higher and slag particles with an aspect ratio of less than 1.0 is 1.0 mm or higher. These conditions can be appropriately set according to the thickness of the steel strip, etc. For example, the laser output can be 0.5 kW or higher per ms, and the laser processing spot diameter can be 0.1 mm or higher and less than 1.0 mm. Figure 4 The ratio of pulse period time to pulse time (duty factor) shown is 0.3 or more and less than 0.8. Furthermore, compressed air, nitrogen, and oxygen can be used as the gas in the pulsed laser. It is also preferable that the gas pressure is 0.5 MPa or more. In addition, in this embodiment, the punching process near the center of the steel strip in the width direction, which is conventionally performed using a piercing machine, is performed by laser cutting with a closed cross-sectional shape. It should be noted that... Figure 1 In the example shown, a hole 12 with a closed cross-sectional shape is formed in the preceding steel strip 1, but the hole 12 can also be formed in the following steel strip 2. Furthermore, the closed cross-sectional shape, the number of cut holes, and the coordinates of the cut position are not particularly limited. Additionally, if the welding point tracking method is implemented using the magnetic flux leakage method or other image determination methods, it is not necessary to form a hole.

[0035] To further clarify, for ordinary low-carbon steel, even conventional shearing processes will not produce cracks (edge ​​cracks) in the width direction. Therefore, this invention may not be necessary for steel grades that rarely experience fractures near the weld; it should be applied to brittle materials and high-alloy steel grades where weld fractures occur during shearing. However, cold rolling mills can be dedicated to silicon steel plates or high-tensile steel plates, or they can be used for rolling low-carbon steel as well. In such cases, there is no problem in performing partial blanking of low-carbon steel using laser cutting. Alternatively, both shearing and laser cutting machines can be installed simultaneously and used separately depending on the steel grade.

[0036] Example

[0037] The present invention will now be described based on embodiments. Using a cold continuous rolling mill consisting of a total of five rolling mills, an experiment was conducted to cold roll a raw material steel sheet containing 2.8–3.3 mass% Si (2.0 mm thickness, 1000 mm width) for electromagnetic steel sheets to a finished thickness of 0.300 mm. In the embodiments, laser cutting was performed based on the implementation of the present invention. Specifically, near the weld between the preceding and following strips, laser cutting was performed in a semi-circular shape relative to both ends of the strip and in a circular closed-section shape relative to the center of the width of the preceding strip. It should be noted that the cutting conditions were set as follows: compressed air, pressure 1.0 MPa, laser output 0.8 kW, duty cycle 0.5, and scanning speed 3.0 m / min. Furthermore, in the comparative example, rolling was performed in the same manner as in the embodiments, except that laser cutting conditions that did not meet the conditions of the present invention resulted in more sharp-shaped slag. The state of slag and the rate of broken steel coils after 100 steel coil rolling in the above-described laser cutting examples and comparative examples are shown in Table 1.

[0038] [Table 1]

[0039]

[0040] As shown in Table 1, in the comparative examples, the weld fracture rate was 8.0% to 12.0%, while in the embodiments, the weld fracture rate 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 sharp-shaped slag can be reduced, and the work-hardened portion near the weld can be eliminated. Furthermore, this suppresses weld fracture, resulting in increased productivity and yield.

[0041] The foregoing has described embodiments using the invention made by the inventors, but the present invention is not limited to the description and drawings that constitute a part of the disclosure of this invention. That is, all other embodiments, examples, and applications made by those skilled in the art based on this embodiment are included within the scope of this invention.

[0042] Industrial availability

[0043] According to the present invention, a laser cutting method and laser cutting apparatus for steel strip 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 strip 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 strip can be provided 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.

[0044] Explanation of reference numerals in the attached figures

[0045] 1. Leading steel strip

[0046] 2. Rear steel belt

[0047] 11 gaps

[0048] 12 holes.

Claims

1. A laser cutting method for steel strip, comprising using a laser to cut the width-direction end of a steel strip including a joint portion formed by joining the rear end of a preceding steel strip and the front end of a subsequent steel strip, wherein, The steel strip is cut in such a manner that the spacing between slag particles with an aspect ratio of 1.0 or higher at the cut end in the width direction is 1.0 mm or higher, and the spacing between slag particles with an aspect ratio of 1.0 or higher and slag particles with an aspect ratio of less than 1.0 is 1.0 mm or higher. The aspect ratio is the height of the slag divided by the width of the slag.

2. The laser cutting method for steel strip according to claim 1, While the steel strip is stopped, a cutting process to cut off the end in the width direction and a hole forming process to form one or more holes in the steel strip using a laser are performed continuously.

3. A laser cutting device for steel strip, performing the laser cutting method for steel strip as described in claim 1 or 2.

4. A cold rolling method for steel strip, wherein the steel strip cut by the laser cutting method of claim 1 or 2 is cold rolled.

5. A method for manufacturing cold-rolled steel strip, comprising the steps of the cold rolling method for steel strip as described in claim 4.

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