JP2023127690A - Welding method of metal strip and welding apparatus - Google Patents

Abstract

To provide a welding method of a metal strip and a welding apparatus in which tempering of a weld zone after mash seam weld of the metal strip is surely performed to decrease hardness difference between a base material part and the weld zone to be capable of preventing sheet fracture in a continuous processing line.SOLUTION: A welding method of a metal strip in a metal strip continuous processing line comprises: a welding process where a tail end of a preceding metal strip 1 is superimposed on a fore-end of a succeeding metal strip 2, and mash seam weld of the superimposed portion is performed by rotating a top and bottom pair of electrode rings 4; a rolling process where a weld zone 20 welded in the welding process is rolled by a top and bottom pressure rollers 5; a post-heating process where heating treatment of the weld zone rolled in the rolling process is performed; and a cooling process where the weld zone is cooled before the post-heating process. A method of the cooling process is preferably selected from at least one of: a method where cooling medium is blown out over the weld zone; a method where rolling speed is decreased in the rolling process; and a method where a prescribed standby time is set before the post-heating process.SELECTED DRAWING: Figure 1

Description

The present invention relates to a metal strip welding method and a welding device in a metal strip continuous processing line, and in particular to high-strength steel plates, the tail end of a leading metal strip and the tip end of a trailing metal strip are welded by mash seam welding. The present invention also relates to a metal strip welding method and welding apparatus that temper the welded part by post-heating to reliably reduce the hardness of the welded part, prevent plate breakage, and realize high productivity.

In a continuous processing line for metal strips such as steel strips, a leading metal strip (hereinafter also referred to as "preceding material") and a trailing metal strip (hereinafter also referred to as "following material") are welded at the entry side of the line. By linking them together, continuous processing is possible. There are various welding methods, such as butt resistance welding, overlap resistance welding (referred to as "mash seam welding", hereinafter also referred to as "MSW"), and laser welding, but in general, thick materials with a thickness of 2 to 3 mm or more are used. Butt resistance welding and laser welding are used for thin materials of 2 mm or less, and overlap resistance welding (MSW) is used for thin materials of 2 mm or less.

In recent years, demand for high-strength steel sheets has increased, but since high-strength steel sheets contain a large amount of carbon (C), there is a problem in that when they undergo martensitic transformation due to heat treatment, the hardness of the welded part increases significantly. .

In MSW, during welding, the welded part is heated to a temperature in the austenite region exceeding 1000°C, which is the A3 transformation point, but is then rapidly cooled (quenched) to the martensite starting point (Ms point) during the natural cooling process. , the welded part undergoes martensitic transformation and hardens. Since the base metal part where no welding heat is input is not affected by thermosetting, there is a difference in hardness between the base metal part and the welded part, which may cause the plate to break when subjected to repeated bending on a continuous processing line. There is. If the sheet breaks in the annealing furnace, it will cause serious problems such as damage to the inside of the furnace, which will cause a reduction in productivity due to furnace maintenance and operation stoppages required to remove the broken plate. In order to eliminate the hardness difference between the base metal part and the welded part that causes such troubles, it is necessary to reduce the hardness difference by tempering the welded part that has undergone martensitic transformation by heat treatment.

As a method for welding metal strips in a continuous metal strip processing line, Patent Document 1 describes a method of improving the mechanical properties of the joint formed by mash seam welding (MSW) on the overlapped portion of a plurality of thin steel plates. After welding with a disc electrode (electrode ring) made of the same type of copper alloy, the electrode ring of the MSW device is moved back and forth again to perform MSW and post-heat treatment (MSW) using the same electrode ring. Disclosed is a method for forming a welded joint, in which a welded joint is tempered (tempering treatment) of the welded part that has been hardened by heating, and further, simultaneously with the post-heating treatment, processing (swaging) is performed to reduce the thickness of the overlapped part. There is.

In addition, Patent Document 2 describes a mash seam welding method different from Patent Document 1, in which the ends of two metal plates are overlapped, the overlapped portion is pressurized with a pair of upper and lower electrode rings, and while a welding current is applied, A mash seam welding method is disclosed that includes a rolling step of continuously welding and joining the two metal plates, and then rolling the joined portion of the metal plates with a pair of upper and lower pressure rollers.

Furthermore, Patent Document 3 discloses that in seam welding of steel structures such as automobile bodies, after seam welding, heat treatment is performed to cool the seam weld to below the martensite start temperature (Ms point), and then the seam weld is A method of improving the performance of seam welded joints using post-weld heat treatment is disclosed, in which the weld is tempered, the tempering being heated above the martensitic onset temperature but not above the lower critical temperature. There is.

In addition, Patent Document 4 discloses that in order to save labor in a mash seam welding machine, a cooling medium is applied immediately after welding so that the overlapping margin of the metal plates can be reduced, and this is done to reduce the risk of welding defects. A welding machine is disclosed that prevents changes in overlap.

Japanese Patent Application Publication No. 2015-136725 International Publication WO2012/039060 Publication Special Publication No. 2010-516471 Japanese Patent Application Publication No. 8-71762

The method for joining welded joints described in Patent Document 1 uses the same electrode ring during mash seam welding and post-heat treatment, and pressurizes and energizes stacked steel plates while applying pressure to the upper electrode ring and the upper electrode ring. By setting the direction of movement relative to the lower electrode wheel in opposite directions, the equipment for mash seam welding and the equipment for post heat treatment are made common. However, in the method described in Patent Document 1, it is necessary to reciprocate the upper electrode wheel and the lower electrode wheel with respect to the planned joining location of the stacked steel plates, which leads to a problem in that it leads to a decrease in productivity. there were.

On the other hand, as a mash seam welding method disclosed in Patent Document 2, a method is shown in which the ends of two metal plates are overlapped and continuously welded, and then rolled with a pressure roller. In addition to this, it is stated that productivity can be improved by arranging a post-heat treatment device behind the pressure roller and performing heat treatment continuously, but the mash disclosed in Patent Document 1 Although productivity can be improved compared to the seam welding method, the distance between the pressure roller and the post-heat treatment device is narrow, and it takes as much time from the end of mash seam welding to the start of heat treatment (post-heating) as in Patent Document 1. There are cases where it is difficult to ensure that the weld zone is tempered reliably, and the difference in hardness between the base metal and the weld zone is not reduced and the hardness of the weld zone remains high. Ta.

Furthermore, Patent Document 3 relates to seam welding of steel structures such as automobile bodies, and relates to a seam welding method for the end of a metal strip in a continuous metal strip processing line, which improves productivity and stabilizes the tempering of the welded part. There is no mention or suggestion of a method to achieve both.

Further, Patent Document 4 only discloses an apparatus that simply performs mash seam welding, and does not describe rolling after welding and subsequent heat treatment. The quenching treatment after welding is also done to ensure overlap, and the problem of increasing the hardness of the welded part without reducing the difference in hardness between the base metal and the welded part remains unresolved.

The present invention has been made in view of the above-mentioned problems, and it is possible to reliably temper the welded part after mash seam welding of a metal strip, reduce the hardness difference between the base metal part and the welded part, and continuously process the welded part. An object of the present invention is to provide a metal strip welding method and a welding device that can prevent plate breakage in a line.

The present inventors have discovered that in a metal strip welding method using MSW in a continuous metal strip processing line, even if the welded part is rolled with a pressure roller after MSW and subsequently tempered with a post-heating device, We conducted a thorough investigation into the reasons why the tempering process was not effective and the hardness difference between the base metal and the weld was not reduced, leaving the weld with a martensitic structure with high hardness.

As a result, it was found that due to variations in the temperature of the weld immediately after MSW, when the temperature is higher than a predetermined temperature, the weld remains a martensitic structure with high hardness.

MSW is made by clamping the tail end of the leading metal band and the tip of the trailing metal band, overlapping the leading metal band and the trailing metal band, and sandwiching the overlapping part between a pair of upper and lower electrode rings. This is a joining method in which a current is applied by applying a certain pressure, and the Joule heat generated by the contact resistance of the overlapped parts is used to melt and bond them together. Determined by current carrying resistance which is affected by pressure.

However, the overlapping margin between the tail end of the leading metal band and the leading end of the trailing metal band may vary by several percent depending on the mechanical accuracy of the clamp frame or the like that clamps each metal band. In addition, variations in the pressing force may occur due to wear and deterioration of the electrode wheel. Further, due to voltage pulsations due to power supply load fluctuations within the factory, actual welding current varies with respect to the welding current setting. It has been found that due to these causes, the temperature of the welded part immediately after MSW varies in the range of 1000 to 1100°C.

Furthermore, if the temperature of the weld zone immediately after MSW is higher than a predetermined temperature, the temperature of the weld zone will not fall below the martensite start temperature (Ms point) before the weld zone is tempered by the post-heating device. No martensitic transformation occurs, the metal structure remains austenite, and even if the welded part is tempered using a post-heating device, it will not be tempered. In addition, in high-strength steel sheets containing a large amount of carbon and having a strength of 1470 MPa or more, during the subsequent standby air cooling, even if the weld is not rapidly cooled to below the Ms point, the welded part will be formed with high hardness martensite during the slow cooling process to room temperature. Transform into an organization. Therefore, it has been found that even after heat treatment using a post-heating device, the welded portion remains in a martensitic structure with high hardness.

The present invention has been made based on the knowledge obtained from the results of the above-mentioned studies, and further studies have been made, and the configuration thereof is as follows.
[1] A metal strip welding method in a metal strip continuous processing line, comprising:
a welding process in which the tail end of the leading metal band and the tip of the trailing metal band are overlapped, and the overlapping part is mash-seam welded by rotating a pair of upper and lower electrode wheels;
a rolling step of rolling the welded part welded in the welding step with a pair of upper and lower pressure rollers;
a post-heating step of performing heat treatment on the welded part rolled in the rolling step;
has
The method for welding metal strips further comprises a cooling step of cooling the welded portion before the post-heating step.
[2] In [1] above, the cooling step includes a method of spraying a cooling medium onto the welded part, a method of reducing the rolling speed in the rolling step, and a method of waiting for a predetermined time before the post-heating step. A method for welding a metal strip, characterized by being at least one of the following methods.
[3] The method for welding a metal strip according to [1] or [2], wherein the cooling step is performed based on the temperature of the welded part immediately after the welding step.
[4] The method for welding a metal strip according to any one of [1] to [3], wherein the metal strip is a high-tensile steel plate having a thickness of 2 mm or less and a strength of 1470 MPa or more.
[5] A metal strip welding device in a metal strip continuous processing line, comprising:
Welding means for overlapping the tail end of the leading metal band and the tip end of the trailing metal band, and performing mash seam welding on the overlapping part by rotating a pair of upper and lower electrode wheels;
a rolling means for rolling the welded part welded by the welding means with a pair of upper and lower pressure rollers;
post-heating means for heat-treating the welded part rolled by the rolling means;
Equipped with
The apparatus for welding metal strips further comprises a cooling means for cooling the welded portion upstream of the post-heating means.
[6] The metal strip welding device according to [5] above, wherein the cooling means is a cooling medium injection device that sprays a cooling medium onto the welded portion.
[7] The metal strip welding device according to [6] above, wherein the cooling medium injection device is a high-pressure air injection device.
[8] The metal strip welding device according to any one of [5] to [7], wherein the post-heating means is an induction heating device.
[9] The metal strip welding device according to any one of [5] to [8], wherein the metal strip is a high-tensile steel plate having a thickness of 2 mm or less and a strength of 1470 MPa or more.

According to the present invention, by providing a cooling process for cooling the welded part before the post-heating process, even if the temperature of the welded part immediately after mash seam welding varies before the post-heating process, the welded part can be The martensitic transformation can occur before the post-heating step by lowering the temperature. This makes it possible to reliably temper the welded part in the post-heating process and reduce the difference in hardness between the base metal part and the welded part. Breakage can be prevented.

1 is a schematic side view showing an example of the basic device configuration of a mash seam welding device according to the present invention. It is a mash seam welding device concerning the present invention, and is a schematic side view showing an example of device composition provided with a cooling medium injection device. (a) A side view of the entire device, (b) a partially enlarged view, and (c) a front view of the entire device. The cooling step in the mash seam welding method according to the present invention is a schematic explanatory diagram showing an example of a standby method before post-heating. It is a graph showing the difference in temperature change of a welded part when standby cooling is performed and when a cooling medium injection method is combined. It is a schematic diagram explaining the temperature change (heat cycle) of a welding part due to the difference in the cooling process in the welding method according to the present invention. (a) Conventional example, (b) Invention example 1, (c) Invention example 2, (d) Invention example 3. FIG. 3 is a schematic cross-sectional view illustrating the state of a welded portion during welding. 2 is a graph showing measurement results of Vickers hardness distribution around a welded part.

[Mash seam welding equipment]
A specific embodiment of the mash seam welding apparatus according to the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram showing a schematic configuration of an embodiment of a mash seam welding apparatus according to the present invention.
A leading metal band 1 and a trailing metal band 2, a pair of upper and lower electrode wheels 4, a pair of upper and lower pressure rollers (also referred to as "swaging rolls") 5, a post-heating device 6, a post-welding thermometer 7, and a temperature after rolling. A total of 8 post-heating thermometers 9 are provided, and these are mounted on a carriage 3 for transporting the electrode ring 4 and the like together.

The carriage 3 moves in the direction of the arrow with the leading metal band 1 and the trailing metal band 2 superimposed on each other.

The pair of upper and lower electrode wheels 4 can be driven in the vertical direction by, for example, a hydraulic or pneumatic cylinder device (not shown), and pressurizes the overlapping portion of the leading metal band 1 and the trailing metal band 2, Welding is performed continuously over the entire width of the metal strip from one end to the other end in the width direction while applying a welding current.

A pair of upper and lower pressure rollers 5 are disposed on the downstream side of the electrode wheel 4, and can be similarly driven in the vertical direction by a hydraulic or pneumatic cylinder device (not shown), for example, to continuously weld the welded part. Roll and crush the weld to flatten the welded part. Note that the pressure roller 5 has a structure in which an increase in temperature on the roller surface is suppressed by internal water cooling. Further, the rotation surfaces of the upper and lower rolls may cross each other at an angle of 5° or less, for example.

The post-heating device 6 can be heated using a direct heating method using a burner, a heating method using an induction heating device, or a heating method using an electrode wheel as in Patent Document 1. It is preferable to perform heating using an induction heating device that has a fast heating rate and can heat from one side from the viewpoint of installation space.

Further, in order to measure the temperature of the welded part in each of the welding process, rolling process, and post-heating process that constitute the metal strip welding method according to the present invention, which will be described later, it is possible to arrange the following thermometers. preferable. The temperature of the welded part after the welding process is measured by a post-weld thermometer 7 installed immediately after the pair of upper and lower electrode wheels 4. Further, the temperature of the welded portion after the rolling process is measured by a post-rolling thermometer 8 installed immediately after the pair of upper and lower pressure rollers 5. Note that this post-rolling thermometer 8 indicates the temperature when a standby time is provided after the rolling process (=temperature before the post-heating process), as in the case of executing the standby method before post-heating in the cooling process to be described later. It also happens. Further, the temperature of the welded part during the post-heating process is measured by a post-heating thermometer 9 installed at the top of the post-heating device 6. It is preferable to use a non-contact infrared radiation thermometer as each of these thermometers.

Here, as shown in FIG. 6, the welded part is a part where the leading metal band 1 and the trailing metal band 2, which are base materials, are joined by welding, and the initial overlapping part is heated under pressure. A molten zone 21 (also called a "nugget") at the melted center, a solid-phase joint 22 where the end surfaces of both base materials are deformed and diffusion-bonded, and a heat-affected zone 23 ("HAZ") deformed by welding heat. ). Then, a weld step 24 is formed at a point symmetrical position with respect to the center of the weld. The temperature of this welded part is measured by measuring the temperature at a position S on the surface of the metal band directly above the molten part (nugget) 21 at the center of the welded part 20 shown in FIG.

In one embodiment of the mash seam welding device according to the present invention described above, by moving the carriage 3 in the direction of the arrow, the welding process, rolling process, and post-heating that constitute the metal strip welding method according to the present invention described later are performed. Since the process can be performed continuously, productivity can be increased.

[Metal strip welding method]
Next, the metal strip welding method according to the present invention includes a welding process, a rolling process, a post-heating process, and a cooling process.

That is, the feature of the present invention is to provide a cooling process for cooling the welded part in either the welding process or the rolling process before entering the post-heating process, or after the welding process. Even if there are temperature variations in the weld, the temperature of the weld is lowered and martensitic transformation occurs before the entry side of the post-heating device 6, ensuring that the weld is tempered and the base metal part It is possible to reduce the difference in hardness between the weld and the weld.
Each of the above-mentioned steps will be explained in more detail below.

[Welding process]
As described above, the mash seam welding process involves clamping and overlapping the tail end of the leading metal band 1 and the leading end of the trailing metal band 2, respectively, and sandwiching the overlapping portion between a pair of upper and lower electrode rings 4. This is a process of continuously welding the metal strip over its entire width from one end to the other end in the width direction while applying pressure while rotating and applying a welding current.

The target metal strip is a thin material with a thickness of 2 mm or less as described above, but a plate thickness of 5 mm or less is also applicable, and is more preferably in the range of 0.5 to 2.5 mm.

The overlapping margin, which is the length (width) of the overlapping part in the conveyance direction, is preferably in the range of 0.5 to 1.5 mm from one end of the sheet width to the other. It is more preferable to decide the length according to the specifications such as the type, plate thickness, plate width, etc., and it is preferable to increase the overlap margin for thicker and higher strength materials.

The electrode ring 4 is mounted on a carriage 3 that moves in the width direction of the plate in order to weld the overlapped portion over the entire width from one end to the other end in the width direction of the plate. A pressurizing device (not shown) is provided above the upper electrode ring 4 to press the electrode ring 4 and pressurize the overlapped portion of the metal strips. The lower electrode ring 4 has a structure that supports pressurization from above. Each of the upper and lower electrode wheels 4 is provided with an electrode wheel drive motor (not shown) for rotation.

Other welding conditions for welding overlap include welding current value, electrode ring pressure, welding speed, etc.
The larger the electrode ring pressure, the larger the energized area and therefore the larger the required welding current. If the welding current is too large, the molten part (nugget) at the center of the weld will break through the surface layer of the metal strip, causing dust to be ejected to the outside, while if it is too small, it will cause weld flaking. Therefore, the welding current value and electrode ring pressure are appropriately determined by conducting a welding test and based on the cross-sectional macrostructure photograph of the welded part and the strength of the welded part by an Erichsen test or the like. For example, the welding current value is about 10 to 25 kA, and the electrode wheel pressing force is about 5 to 40 kN.

Furthermore, the welding speed (carriage movement speed) should be set as fast as possible without insufficient heat input to the welding part under the conditions of the welding current value and electrode wheel pressure determined by the welding test. Preferably, the range is 5.0 to 20.0 mpm.

[Rolling process]
The rolling process is a process in which the welded part is continuously rolled by a pair of upper and lower pressure rollers 5.
The pressure roller 5 is mounted on the carriage 3 in the same way as the electrode wheel 4 described above, and is arranged on the downstream side of the electrode wheel 4. A pressure device (not shown) is provided above the pressure roller 5 to press the pressure roller 5 to press the welded portion of the metal strip. The lower pressure roller 5 has a structure that supports pressure from above. The upper and lower pressure rollers 5 are each provided with a pressure roller drive motor (not shown) for rotation.

The pressing force of the pressure roller 5 is the pressure applied to the pressure roller in the same way as the electrode wheel pressing force. This pressure was determined by conducting a rolling test of the weld and based on the results of cross-sectional macrostructure photographs of the weld, etc., to a pressure that would flatten the weld step 24 (Figure 6) of the weld into a smooth shape. do. For example, the pressing force of the pressure roller is about 10 to 30 kN.

[Post-heating process]
The post-heating step is a step in which the rolled welded portion is heat-treated by the post-heating device 6.
As the post-heating device (also referred to as "post-annealer"), as described above, an induction heating device that has a fast temperature increase rate and can heat from one side is preferable.

The post-heating temperature is preferably set within a range where the temperature of the weld does not exceed the austenite starting point (Ac ₁ point), as will be described later, so as to obtain the effect of tempering the martensitic structure of the weld. Note that the Ac ₁ point is determined by the following equation (1).

Ac ₁ =750.8-26.6C+17.6Si-11.6Mn-22.9Cu-23Ni+24.1Cr+22.5Mo-39.7V-5.7Ti+232.4Nb-169.4Al-894.7B... (1)
Here, the element symbol indicates the content (mass%) of the element in the metal band composition. Elements that are not contained are set to 0.

For example, when the material types of the target metal strip are C: 0.1 to 0.2%, Si: 0.5 to 1.5%, and Mn: 1 to 5% as described above, Ac ₁ The range of points is 696-763°C. As an example, in the case of the above-mentioned steel type, C: 0.20% by mass, Si: 1.50% by mass, Mn: 4.60% by mass, balance: Fe and unavoidable impurities, the Ac ₁ point is approximately 720°C. be.

The above-mentioned variation in the temperature of the welded part immediately after MSW is about 100°C (1000-1100°C) in the microwave, so the post-heating temperature is determined by taking into consideration the variation in the post-heating temperature caused by this variation. do. For example, taking into account the temperature variation (approximately 100°C in the microwave) and a margin of 20°C, the post-heating temperature is set at a temperature 120°C lower than the Ac ₁ point. When the content (mass %) of C, Si, and Mn of the material type of the metal strip is within the above-mentioned range, the post-heating temperature is 576 to 643°C. Further, as an example, in the case of the above-mentioned steel type, C: 0.20% by mass, Si: 1.50% by mass, Mn: 4.60% by mass, balance: Fe and unavoidable impurities, the temperature is about 600°C.

[Cooling process]
The cooling process is a process of cooling the welded part before the above-mentioned post-heating process, that is, after the welding process and/or the rolling process before entering the post-heating process.

Specific methods for cooling the welded part include (1) a method of spraying a cooling medium onto the welded part (hereinafter also referred to as "cooling medium injection method"), and (2) a method of reducing the rolling speed in the rolling process. (hereinafter also referred to as "rolling speed reduction method"), and (3) method of waiting the metal strip for a predetermined time before the post-heating step (hereinafter also referred to as "standby method before post-heating"). The methods (1), (2), and (3) above may be performed separately or in combination as appropriate. Which cooling method to adopt may be selected depending on the work situation of the welding process.

Furthermore, cooling methods may be selected and combined based on the temperature of the welded part immediately after the welding process. The reason why this is performed based on the temperature of the welded part immediately after the welding process is as follows. In the mash seam welding apparatus whose schematic configuration of one embodiment is shown in FIG. 1, the welding process, rolling process, and post-heating process are performed continuously by moving the carriage 3 in the direction of the arrow. The time from immediately after the completion to immediately before the post-heating step is as short as 10 seconds or less. Therefore, if the temperature of the welded part immediately after mash seam welding is high, for example, method (1) alone cannot cause martensitic transformation before entering the post-heating device 6, and (1) and ( It may be appropriate to select a combination of 2). In other words, by selecting and combining appropriate cooling methods according to the temperature variations in the weld zone immediately after mash seam welding, the post-heating device can be turned on even in a short period of time from immediately after the welding process to just before the post-heating process. It becomes possible to reliably cause martensitic transformation up to the side.

[(1) Coolant injection method]
One embodiment of the coolant injection method is shown in FIG. As shown in the enlarged view of FIG. 2, for example, a compressor or the like (not shown) is used as a cooling medium between the electrode wheel 4 and the pressure roller 5, and between the pressure roller 5 and the post-heating device 6. Headers 10 and 11 are provided to inject high-pressure air at a pressure of about 4 kg/cm ² , and the high-pressure air is discharged toward the welded area after mash seam welding and the welded area after rolling by the pressure roller. Since the higher the temperature of the welding part, the higher the heat removal effect by high-pressure air, it is particularly preferable to provide a header 10 between the electrode ring 4 and the pressure roller 5. Alternatively, air that is -40 to -80° C. lower than room temperature may be used as the cooling medium via a vortex tube that utilizes the vortex effect that separates high-pressure air (compressed air) into hot air and cold air. For example, in the case of a 1.6 mm steel plate, the cooling rate by the cooling medium injection method is 0.89 °C/s in the case of natural cooling using radiant heat at a room temperature of 20 °C, but the cooling rate by a compressor etc. is 3 to 4 kg/cm. In the case of injection cooling using high-pressure air at a pressure of about ² , the rate is 2.63℃/s, which is 2.3 times that of natural cooling, and in the case of injection cooling using low-temperature air at -30℃ via a vortex tube. is 2.93°C/s, and the cooling effect can be increased to 2.9 times that of natural cooling.

Furthermore, in addition to the above-mentioned air, compressed inert gas such as argon gas or helium gas can also be sprayed as a cooling medium, although this is not practical (in terms of cost).

[(2) Rolling speed reduction method]
In contrast to the cooling medium injection method described above, this rolling speed reduction method does not require any special cooling means. This method reduces the rolling speed of the pressure roller in the rolling process to increase the contact time between the pressure roller, whose surface is kept low by internal water cooling, and the welding area, thereby removing contact heat. This is the way to do it.

Here, the rolling speed in the rolling process is the speed at which the welded portion of the leading metal band 1 and the trailing metal band 2 is rolled by the pressure roller 5 in the welding apparatus shown in FIG. It refers to the speed at which the metal strip is moved relative to the metal strip in the direction of the arrow, and is also called the carriage movement speed, and is equivalent to the welding speed described above. The rolling speed is preferably in the range of 5 to 20 mpm.

During the rolling process, the average cooling rate during the period from the pressure roller 5 to the subsequent post-heating device 6 (distance of about 60 mm) is extremely high at 140 to 160°C/s for a 1.6 mm steel plate, so the carriage movement By lowering the speed and lengthening this period, the amount of heat removed by contact with the welded part during pressurization by the pressure roller 5 and by cooling (natural cooling) during movement before and after pressurization will be increased. be able to. This narrows the temperature variation range on the entry side of the post-heating device 6 to the low temperature side, so that it can be reliably kept below the Ms point. For example, when the distance between the electrode wheel 4 and the post-heating device 6 is 500 mm, the rolling speed is 10 mpm, and the average cooling rate from after welding to after rolling is 140°C/s, the temperature is adjusted at the inlet side of the post-heating device 6. When lowering the temperature by -20°C, the rolling speed should be set to 9.54 mpm (minus 5%), and the residence time should be increased by 0.14 s compared to 3 s.

In addition, by implementing the cooling medium injection method that injects a cooling medium into the welded part to cool it in conjunction with the rolling speed reduction method in this rolling process, it is possible to cool the welded part by allowing it to cool during movement before and after pressurization (natural cooling). ), it is possible to increase the amount of heat removed by forced convection, and the welded part can be cooled more efficiently and in a shorter time, which is preferable.

[(3) Post-heating standby method]
Another cooling method is a method in which the metal strip is allowed to stand by for a predetermined period of time before the post-heating step and allowed to cool.

One aspect of the post-heating standby method will be described with reference to FIG. 3.
FIG. 3(a) shows a state in the middle of welding in the mash seam welding method. While the carriage 3 moves from right to left, the overlapping portions of the leading metal band 1 and the trailing metal band 2 are welded by a pair of upper and lower electrode wheels 4, and then the welded metal bands are welded by a pressure roller 5. is being rolled. In this state, the post-heating device 6 is in an OFF state in which it is not in operation. Normally, post-heating treatment is performed with the post-heating device 6 in operation, that is, in the ON state, but in the case of the standby method before post-heating, the post-heating device 6 is left in the OFF state. , the state shown in FIG. 3(b) is reached. That is, welding and rolling have been completed and the carriage 3 has reached the left end. Here, only the upper roll of the electrode wheel 4 and pressure roller 5 is raised to open a gap between the upper and lower sides. Thereafter, the carriage 3 is moved to the right as shown in FIG. 3(c). At this time, the gap remains open. Then, when the carriage 3 returns to the initial position (the state where the carriage 3 is at the right end), as shown in FIG. 3(d), the post-heating device 6 is turned on and the carriage 3 is moved to the left. , the welded metal strip is passed through to carry out post-heat treatment of the metal strip.

By this method, the welded metal strip can wait for the carriage 3 to reciprocate from the rolling process to the post-heating process, and the temperature of the welded part is reduced by natural cooling during that time. The waiting time in this case is preferably 40 to 80 seconds.

Furthermore, in addition to this post-heating standby method, the above-mentioned cooling medium injection method may be used in combination. At this time, it may be allowed to wait until the temperature of the welded part falls below a predetermined temperature (for example, the Ms point), or it may be made to wait for a predetermined time depending on the temperature of the welded part immediately after the welding process. .

Figure 4 shows the difference in temperature change in the weld when the plate thickness is 1.6 mm and the weld temperature is 335°C, when the weld is allowed to cool naturally and when the cooling medium injection method is combined. show. According to this, when the Ms point is 305°C, it takes about 40 seconds to reduce the temperature of the weld to the Ms point by simply waiting, and when the cooling medium injection method is used in combination. It can be seen that the time is shortened to about 10 seconds.

[Temperature change in welding zone (heat cycle)]
Temperature changes (heat cycles) in the welded portion due to differences in the cooling process in the welding method according to the present invention will be explained using FIGS. 5(a) to 5(d). Here, FIG. 5(a) shows a conventional example without a cooling process. FIG. 5(b) is an example of cooling by the cooling medium injection method (1) of the cooling method described above in Invention Example 1, and FIG. 5(c) is an example of cooling by the cooling method (2) of Invention Example 2. FIG. 5(d) is an example of cooling using the rolling speed reduction method of (3) of the cooling method in Invention Example 3.

Note that symbols [A] to [F] in FIGS. 5(a) to (d) indicate periods, and their contents are as follows.
[A] Period of welding process (period from start to end of welding)
[B] Period from after welding to before rolling (interval period from after welding to before rolling by pressure roller)
[C] Period of rolling process (period of rolling with pressure roller 5)
[D] Period from after rolling to before post-heating (period from after rolling to before post-heating treatment)
[E] Period of post-heating process (period of heat treatment by post-heating device)
[F] Period from post-heating to cooling

First, the transition of temperature change in the conventional example shown in FIG. 5(a) will be explained.
The period [A] in FIG. 5(a) is extremely short and is within 0.5 seconds. During this period, the welded part is heated to 1000° C. or higher, for example, 1033° C., which exceeds the austenite transformation point (hereinafter also referred to as "Ac ₃ point"), and undergoes austenite transformation (hereinafter also referred to as "γ transformation"). ). This _Ac3 point is determined by the following equation (2).

Ac ₃ points = 937.2-436.5C+56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr+38.1Mo+124.8V+136.3Ti-19.1Nb+198.4Al+3315B... (2)
Here, the element symbol indicates the content (mass%) of the element in the metal band composition. Elements that are not contained are set to 0.

For example, when the composition of the steel type is C: 0.1 to 0.2 mass%, Si: 0.5 to 1.5 mass%, and Mn: 1 to 5 mass% among the target metal strips, The range of Ac ₃ points is 779-958°C. As an example, when unavoidable impurities include C: 0.20% by mass, Si: 1.50% by mass, Mn: 4.60% by mass, and the balance is Fe, the Ac ₃ point is about 850°C.

Next, the period [B] in FIG. 5(a) is within 3 seconds, and the welded part is allowed to cool naturally during this period. The cooling rate of natural cooling is about 0.7 to 3.0°C/s when the plate thickness is in the range of 0.5 to 2.0 mm.

The period [C] in FIG. 5A is within 0.5 seconds, similar to the above [A]. Here, a large amount of heat is removed from the welded portion due to contact with the pressure roller 5.
In the next period [D] in FIG. 5(a), natural cooling is further performed.

Here, when the temperature is lowered to below the martensitic transformation starting point (hereinafter also referred to as "Ms point") and martensitic transformation (hereinafter also referred to as "M transformation") occurs, a hard martensitic structure is generated. , If there is no cooling and the temperature drop is small, there is no martensitic transformation and no hard martensitic structure is generated.

This Ms point is determined by the following equation (3).
Ms point=521-353C-22Si-24.3Mn-7.7Cu-17.3Ni-17.7Cr-25.8Mo... (3)
Here, the element symbol indicates the content (mass%) of the element in the metal band composition. Elements that are not contained are set to 0.

For example, when the material types of the target metal strip are C: 0.1 to 0.2%, Si: 0.5 to 1.5%, and Mn: 1 to 5% as described above, the Ms point The range is 296-450°C. As an example, in the case of the above-mentioned steel type, C: 0.20% by mass, Si: 1.50% by mass, Mn: 4.60% by mass, balance: Fe and unavoidable impurities, the Ms point is about 305°C. . When the temperature of the weld zone is lowered to below this Ms point, martensitic transformation occurs and a hard martensitic structure is generated.

Continuing, the period [E] in FIG. 5(a) is within 2 seconds. If the temperature of the weld exceeds the Ms point before entering the post-heating process, that is, if martensitic transformation has not occurred before entering the post-heating process, the weld will remain intact even after heat treatment. Since the weld is not tempered, the difference in hardness between the base metal (leading metal band and trailing metal band) and the weld is not reduced, and the hardness of the weld remains high.

Thereafter, in the period [F] in FIG. 5(a), the welded part is allowed to cool naturally. In this conventional example, even after the post-heating step [E], the welded portion was not tempered and the hardness of the welded portion remained high.

Next, invention example 1 shown in FIG. 5(b) will be described.
This invention example 1 is an example in which cooling was performed by injecting a cooling medium during periods [B] and [D] in FIG. 5(b). In this case, the temperature of the weld zone decreases significantly during the period [B], and after the subsequent rolling of [C], the temperature of the weld zone drops below the Ms point, martensitic transformation occurs, and a hard martensitic structure is formed. generate. Therefore, even if tempered in the subsequent post-heating step [E], the temperature will not reach the Ac ₁ point, so tempered martensite will be generated, reducing the hardness difference between the base metal and the welded part. .

Next, invention example 2 shown in FIG. 5(c) will be described.
Inventive Example 2 is an example in which a rolling speed reduction method of reducing the rolling speed (carriage movement speed) of the rolling process is used in the period [C] in FIG. 5(c). In this case, by reducing the carriage movement speed, the rolling time of [C] and the time for cooling (natural cooling) of [B] and [C] can be gained, so the temperature of the weld zone can be increased. This results in a drop below the Ms point. As a result, as in Invention Example 1 above, the temperature of the welded part does not reach the Ac ₁ point even in the subsequent post-heating process, so tempered martensite is generated and the hardness difference between the base metal and the welded part is will be reduced.

Finally, invention example 3 shown in FIG. 5(d) will be described.
Inventive Example 3 is an example in which a method is used in which a standby time is provided before post-heating to lower the temperature of the welded part during the period [D] in FIG. 5(d). In this case, by lengthening the period [D], the temperature of the welded part will drop below the Ms point, so as in Invention Examples 1 and 2, the subsequent post-heating step will also Since the temperature of the weld zone does not reach the Ac ₁ point, tempered martensite is generated and the difference in hardness between the base metal and the weld zone is reduced.

[Cooling control by temperature measurement]
Furthermore, this cooling process requires measuring the temperature of the weld immediately after the welding process, and if the temperature of the weld exceeds a predetermined value, then the subsequent cooling process can be carried out (from an economic point of view). ) preferred.

Specifically, for example, in the case of the cooling medium injection method shown in FIG. , a header 10 provided between the electrode wheel 4 and the pressure roller 5 that injects high-pressure air, and a header 11 provided after the pressure roller 5 that injects high-pressure air. It is preferable.

This predetermined value is based on the measurement data of the post-welding thermometer 7 and the post-rolling thermometer 8, and is determined in advance as a temperature value that ensures that the temperature of the welded part falls below the Ms point on the entry side of the post-heating device 6. Just leave it there. For example, if the temperature of the weld immediately after MSW varies in the range of 1000 to 1100 °C, the temperature of the weld on the entry side of the post-heating device 6 is in the range of 281 to 309 °C, and the Ms point is 305 °C, If the predetermined value is set to 1070°C, the variation range on the entry side of the post-heating device 6 can be narrowed to 281 to 300°C, and the temperature can be reliably kept below the Ms point. Therefore, when the temperature of the welded part immediately after welding is 1070° C. or lower, by controlling the high pressure air and stopping its injection, unnecessary discharge of high pressure air can be prevented, and running costs can be reduced.

In addition, even when the cooling method is the rolling speed reduction method or the standby method before post-heating, the temperature of the welded part is measured immediately after the welding process, and if the temperature of the welded part exceeds a predetermined value, the cooling The method may be implemented.

In this example, a mash seam welding apparatus shown in FIG. 1 or 2 was used to weld a 1470 MPa class high-strength steel plate (the composition of the steel type was the same as the above steel example, C: 0.20% by mass, Si: 1 .50 mass %, Mn: 4.60 mass %, balance: Fe and unavoidable impurities), and the plate thickness was 1.6 mm. The metal strips 2 are overlapped with an overlap margin of 1.2 mm, continuously welded using an electrode ring 4, successively rolled using a pressure roller 5, and then heat treated using a post-heating device 6. After welding, The temperature of the welded part after the welding process and the temperature of the welded part after the rolling process were measured using the thermometer 7 and the post-rolling thermometer 8, respectively.

Regarding the temperature of the post-rolling thermometer 8, a case where the temperature did not exceed the Ms point (305° C. in the above steel plate) was judged as good (〇), and a case where it was exceeded was judged as bad (×).

Then, it was confirmed whether the hardness of the welded part had been softened by tempering to a level equivalent to the hardness of the base metal strip (400 HV or less). The hardness is determined by measuring the Vickers hardness (HV) in accordance with JIS Z 2244 "Vickers hardness test test method" at a pitch of 0.2 mm at measurement intervals in the transport direction of the welded part with a measurement load (test force) of 200g. When the average value of hardness (HV) was 400 or less (HV≦400), it was determined that the material had been softened by tempering (good: ◯).

In the post-heating step using the post-heating device 6, the welded portion was heated to a target temperature of 600° C., and then allowed to cool naturally.

Table 1 lists the results of Inventive Examples 1 to 14 and Comparative Examples 1 to 3.

Among Examples 1 to 14 of the present invention, Examples 1 to 7 of the present invention are examples in which the cooling step was performed without any particular temperature control, and the post-heating step was performed.

Examples 8 to 14 of the present invention are examples of the invention using a cooling control method based on temperature measurement. This is an example in which temperature is measured and control is performed such that a cooling step is performed when the measured value exceeds the predetermined value.

First, in Example 1 of the present invention, as shown in the welding apparatus shown in FIG. This is a case where headers 10 and 11 are provided, and high-pressure air is injected toward the welded portion after mash seam welding and after rolling by the pressure roller 5. The pressure of the high-pressure air was set at 4 kg/cm ² , and the high-pressure air was sprayed using a flat nozzle so that the overlapping portions of the metal strips were evenly hit. The temperature after rolling was 255°C, which was below the Ms point (305°C), and the average Vickers hardness was 328HV, which was below the target of 400HV.

Inventive Example 2 is a case where the rolling speed in the rolling process is lowered and cooling is performed. In this case, the rolling speed was lowered by 0.5 mpm from the usual 9.5 mpm to 9.0 mpm. As a result, the temperature after rolling was 273° C., which was below the Ms point, and the average value of Vickers hardness was also 344 HV.

In Example 3 of the present invention, after the rolling process is completed and before the post-heating process, a gap is opened between the electrode ring 4 and the pressure roller 5, and the pressure roller 5 is kept on standby for a predetermined period of time (40 s). This is a case where the post-heating treatment is performed after being left to cool naturally. As a result, the temperature after rolling (=temperature before post-heating) was 302° C., which was below the Ms point, and the average value of Vickers hardness was also 358 HV.

Inventive Example 4 is a case in which the cooling medium injection method of Inventive Example 1 and the rolling speed reduction method of Inventive Example 2 are combined for cooling. As a result, the temperature after rolling was 241° C., which was below the Ms point, and the average value of Vickers hardness was also 320 HV.

Inventive Example 5 is a case in which cooling is performed by combining the cooling medium injection method of Inventive Example 1 and the post-heating standby method of Inventive Example 3. As a result, the temperature after rolling was 247° C., which was below the Ms point, and the average value of Vickers hardness was also 336 HV.

Inventive Example 6 is a case in which the method of reducing the rolling speed of Inventive Example 2 and the standby method before post-heating of Inventive Example 3 are combined for cooling. As a result, the temperature after rolling was 262° C., which was below the Ms point, and the average value of Vickers hardness was also 350 HV.

Inventive Example 7 is a case in which cooling was performed by combining all three methods: the cooling medium injection method of Inventive Example 1, the rolling speed reduction method of Inventive Example 2, and the standby before post-heating method of Inventive Example 3. As a result, the temperature after rolling was 238° C., which was below the Ms point, and the average value of Vickers hardness was also 312 HV.

Next, Examples 8 to 14 of the present invention are examples in which a cooling process was performed when the temperature after welding exceeded a predetermined value (1070°C), and the contents of the cooling process were as follows. It was carried out corresponding to each method.

As a result, the present invention example 8 was an example using the cooling medium injection method, and the temperature after rolling was 285° C., which was lower than the Ms point, and the average value of the Vickers hardness was also 320 HV.

Inventive Example 9 is an example using the rolling speed reduction method, and the temperature after rolling was 282° C., which was below the Ms point, and the average Vickers hardness was 333 HV.

Invention Example 10 is an example using the standby method before post-heating, and the temperature after rolling was 303° C., which was below the Ms point, and the average value of Vickers hardness was also 350 HV.

Inventive Example 11 is an example in which the cooling medium injection method and the rolling speed reduction method were combined for cooling. As a result, the temperature after rolling was 271° C., which was below the Ms point, and the average Vickers hardness was also 318 HV.

Inventive Example 12 is an example in which cooling was performed by combining the cooling medium injection method and the standby method before post-heating. As a result, the temperature after rolling was 280° C., which was below the Ms point, and the average value of Vickers hardness was also 330 HV.

Inventive Example 13 is an example in which cooling was performed by combining the rolling speed reduction method and the standby method before post-heating. As a result, the temperature after rolling was 303° C., which was below the Ms point, and the average value of Vickers hardness was also 345 HV.

Inventive Example 14 is an example in which the cooling medium injection method, the rolling speed reduction method, and the standby method before post-heating were all combined for cooling. As a result, the temperature after rolling was 270° C., which was below the Ms point, and the average Vickers hardness was also 310 HV.

Comparative Examples 1 and 2 are cases in which the post-heating step using the post-heating device 6 shown in FIG. Since no heat treatment was performed, the average value of Vickers hardness was 501 HV, which significantly exceeded the target value of 400 HV. In Comparative Example 2, the temperature after rolling was 315° C., which was higher than the Ms point, there was no post-heating treatment, and the average value of Vickers hardness was 520 HV, which significantly exceeded the target value of 400 HV.

Comparative Example 3 is an example in which post-heat treatment was performed, but since the temperature after rolling before that was as high as 325°C and there was no cooling process, even if the subsequent post-heat treatment was performed, the average Vickers hardness The value was 482HV, significantly exceeding the target of 400HV.

Furthermore, the Vickers hardness distribution around the welded parts in the inventive example and the comparative example was measured. The measurement results are shown in FIG. Comparative Example 1 with "no post-heating process" and Comparative Example 3 with "no cooling process but with post-heating process" showed no effect of reducing Vickers hardness, but inventive example 4 with "with cooling process and post-heating process". With heating process, the effect of reducing Vickers hardness was confirmed.

From the above, the effectiveness of the present invention was demonstrated.

1 Preceding metal band (preceding material)
2 Trailing metal band (trailing material)
3 Carriage 4 Electrode wheel 5 Pressure roller (swaging roll)
6 Post-heating device 7 Post-welding thermometer 8 Post-rolling thermometer 9 Post-heating thermometer 10, 11 Header (cooling device)
12 Entry side clamp 13 Output side clamp 20 Welded part 21 Melted part (nugget)
22 Solid phase joint 23 HAZ (heat affected zone)
24 Welding step

Claims (9)

A metal strip welding method in a metal strip continuous processing line, the method comprising:
a welding process in which the tail end of the leading metal band and the tip of the trailing metal band are overlapped, and the overlapping part is mash-seam welded by rotating a pair of upper and lower electrode rings;
a rolling step of rolling the welded part welded in the welding step with a pair of upper and lower pressure rollers;
a post-heating step of performing heat treatment on the welded part rolled in the rolling step;
has
The method for welding metal strips further comprises a cooling step of cooling the welded portion before the post-heating step. The cooling step is at least one of the following methods: spraying a cooling medium onto the welded portion, reducing the rolling speed in the rolling step, and waiting a predetermined time before the post-heating step. The method of welding a metal strip according to claim 1, characterized in that: 3. The metal strip welding method according to claim 1, wherein the cooling step is performed based on the temperature of the welded portion immediately after the welding step. 4. The method for welding a metal strip according to claim 1, wherein the metal strip is a high-tensile steel plate having a thickness of 2 mm or less and a strength of 1470 MPa or more. A metal strip welding device in a metal strip continuous processing line,
Welding means for overlapping the tail end of the leading metal band and the tip end of the trailing metal band, and performing mash seam welding on the overlapping part by rotating a pair of upper and lower electrode wheels;
a rolling means for rolling the welded part welded by the welding means with a pair of upper and lower pressure rollers;
post-heating means for heat-treating the welded part rolled by the rolling means;
Equipped with
The apparatus for welding metal strips further comprises a cooling means for cooling the welded portion upstream of the post-heating means. 6. The metal strip welding device according to claim 5, wherein the cooling means is a cooling medium injection device that sprays a cooling medium onto the welded portion. 7. The metal strip welding apparatus according to claim 6, wherein the cooling medium injection device is a high pressure air injection device. The metal strip welding device according to any one of claims 5 to 7, wherein the post-heating means is an induction heating device. The metal strip welding apparatus according to any one of claims 5 to 8, wherein the metal strip is a high-tensile steel plate having a thickness of 2 mm or less and a strength of 1470 MPa or more.

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