JP2023108212A - Steel strip joining method and steel strip joining device - Google Patents

Abstract

To propose a steel strip joining method which prevents a weld zone from breaking in the middle of plate passing, and shortens processing time as much as possible.SOLUTION: Temperature of a weld zone is measured after welding a rear end of a preceding steel strip and a tip of a following steel strip, and after cooling the weld zone to a temperature in a range of (MF-5°C)-(MF-20°C) using a low MF as a reference, out of completion temperatures MF (°C) for forming a low temperature transformation composition of a weld zone, post-heating is immediately applied to the weld zone. Accordingly, in steel plate manufacturing at a steel mill, a weld zone is prevented from breaking in the middle of plate passing despite that steel strips are welded together and continuous processing such as rolling is conducted, and thus, steel strip joining time including post-heating can be considerably shortened.SELECTED DRAWING: Figure 2

Description

TECHNICAL FIELD The present invention relates to a steel strip joining method and a steel strip joining apparatus, and more particularly to prevention of breakage of a welded portion after joining steel strips.

From the viewpoint of improving productivity, it is common to continuously perform processing such as rolling in a steel sheet production line of a steel plant. For this reason, the trailing end of the preceding steel strip and the leading end of the following steel strip are welded to enable continuous processing. However, since the welded part is locally heated and then rapidly cooled, a low-temperature transformation structure such as martensite is generated in the welded part, which increases hardness and decreases toughness, resulting in fracture during continuous processing. may occur.

To address such problems, for example, Patent Document 1 proposes a method for joining steel strips. In the method for joining steel strips described in Patent Document 1, one end of a first steel strip and one end of a second steel strip are butted together and laser-welded, and the weld formed by the laser welding is induction-heated. , the heating temperature by the induction heating is measured at least three points of the heat affected zone of the first steel strip, the heat affected zone of the second steel strip and the welded zone, and the heating in the first steel strip obtained in advance From the relationship between temperature and hardness and the measured heating temperature of the heat affected zone of the first steel strip, the hardness of the heat affected zone of the first steel strip is obtained, and the previously determined heating of the second steel strip is performed. The hardness of the heat affected zone of the second steel strip is obtained from the relationship between temperature and hardness and the measured heating temperature of the heat affected zone of the second steel strip, and the heating temperature of the weld zone and the first It is determined whether or not the difference in hardness between the heat affected zone of the steel strip and the heat affected zone of the second steel strip satisfies a predetermined condition, and the quality of joining of the steel strips is determined.

Japanese Patent Application Laid-Open No. 2020-127947

However, in the steel strip joining method described in Patent Document 1, it is necessary to determine in advance the relationship between the post-heat treatment heating temperature of the weld zone and the heat-affected zone and the hardness for each steel strip to be joined. In addition, even if post-heat treatment is performed on the welded part (joint part) after joining the steel strips, the effect of post-heat treatment cannot be sufficiently obtained because the starting temperature of post-heat treatment is not known. , there were cases where the joint was broken during the threading.

Therefore, the present invention proposes a method for joining steel strips that can reliably exert the effect of the post-heat treatment, prevent breakage of the welded portion (joint portion) during strip threading, and shorten the processing time as much as possible. intended to

In order to achieve the above-described object, the present inventor first made earnest studies on a means for reliably exhibiting the effect of the post-heating treatment. The reason why the effect of the post-heating treatment does not appear reliably is that the post-heating treatment was performed before the transformation of the weld was completed when the steel strips were welded together. Measure the temperature of the welded part (joint part), and confirm that the obtained temperature of the welded part is below the "predetermined temperature" at which the formation of the low-temperature transformation structure is completed, and then start the post-heating process. I came to the realization that I needed to. It should be noted that the term "welded zone" as used herein refers to a "melted zone" and a "heat-affected zone".

That is, in the present invention, the temperature of the welded portion is measured in the cooling process after welding, and after the temperature of the welded portion is cooled to a “predetermined temperature” or lower at which the formation of the low-temperature transformed structure is completed, the post-heating treatment is immediately performed. decided to start. As a result, the inventors have found that the effect of the post-heat treatment can be reliably exhibited while shortening the waiting time for cooling of the weld zone, and furthermore, the welding process of the steel strip can be shortened. Here, the "predetermined temperature" is defined as the formation end temperature MF (°C) of the low-temperature transformation structure of martensite, bainite, or the like, or a temperature based on MF. If the generation of the low-temperature transformed structure has been completed, the post-heating treatment sufficiently softens the generated low-temperature transformed structure and makes it tough, so that the effect of the post-heating treatment can be reliably exhibited. However, if the post-heating treatment is started before the transformation is completed during cooling, the effect of the post-heating treatment will be insufficient.

Next, the experimental results that form the basis of the present invention will be described.
A first steel strip and a second steel strip were prepared. Both the first steel strip and the second steel strip were 1.5 GPa class high-strength cold-rolled steel strips (thickness: 2.0 mm). In addition, these steel strips are obtained by the following formula (1)
Ceq = C + Si/50 + Mn/25 + Cr/50 + P/2 (1)
Where, C, Si, Mn, Cr, P: content of each element (% by mass)
It is a steel strip having a composition with a carbon equivalent Ceq of 0.3% defined by .

The rear end of the first steel strip whose end face was cut with a shear and the front end of the second steel strip were overlapped, welded using a mash seam welder, and then cooled. Cooling was allowed to cool. The surface temperature of the weld during the cooling (cooling) process was measured with a radiation thermometer to determine the maximum temperature of the weld. Then, after cooling was stopped at various temperatures (cooling stop temperature) in the middle of cooling, a post-heating treatment was immediately performed by heating to a predetermined temperature (600° C.) with an induction heating device. After the post-heating treatment, after cooling (standing to cool) to room temperature, a test piece was taken from the welded portion and the hardness distribution of the welded portion was measured. The maximum hardness HV was obtained from the hardness distribution of the obtained weld zone. Fig. 1 shows the relationship between maximum weld hardness HV and cooling stop temperature.

From FIG. 1, by setting the cooling stop temperature to 400° C. or less, the hardness of the weld zone after the post-heat treatment is significantly reduced. When the cooling stop temperature is lower than the above temperature (400°C), the transformation is completed in the cooling process, and the low-temperature transformed structure such as martensite that is generated is sufficiently tempered by the post-heat treatment, and the hardness of the weld is reduced. It is considered that In the present invention, this temperature (cooling stop temperature: 400° C.) is referred to as the "finish temperature MF of formation of low-temperature transformation structure of the weld zone" in the steel strip.

For this reason, in the present invention, after welding (joining) the steel strips together, the temperature (surface temperature) of the welded portion is measured, and the maximum temperature of the welded portion is the above-described temperature MF (or below the temperature MF). After being cooled to 100°C, a post-heating treatment was performed. As a result, the welded portion is sufficiently tempered, the hardness is lowered, and breakage during sheet threading can be prevented.

The present invention has been completed based on these findings and further studies. That is, the gist of the present invention is as follows.
[1] In joining the leading steel strip and the trailing steel strip by welding the trailing end portion of the leading steel strip and the leading end portion of the trailing steel strip, after completing the welding, A method for joining steel strips, characterized by measuring the temperature of a welded portion formed by welding, and subjecting the welded portion to a post-heating treatment immediately after the welded portion is cooled to a predetermined temperature T (°C). .
[2] The predetermined temperature T (°C) is such that, of the low-temperature transformation structure generation finish temperature MF (°C) of the weld zone of the preceding steel strip and the following steel strip, the lower MF is used as a standard, (MF The method for joining steel strips according to [1], wherein the temperature is in the range of -5°C) to (MF -20°C).
[3] The method for joining steel strips according to [1] or [2], wherein the welding is mash seam welding.
[4] The method for joining steel strips according to any one of [1] to [3], wherein the temperature of the weld is measured using a radiation thermometer.
[5] The method for joining steel strips according to any one of [1] to [4], wherein the post-heat treatment is a heat treatment using induction heating.
[6] Either the first steel strip or the second steel strip has the following formula (1)
Ceq = C + Si/50 + Mn/25 + Cr/50 + P/2 (1)
Where, C, Si, Mn, Cr, P: content of each element (% by mass)
The method for joining steel strips according to any one of [1] to [5], wherein the steel strip has a composition having a carbon equivalent Ceq of 0.1% or more.
[7] A steel strip joining apparatus comprising: a cutting means for cutting a trailing end portion of a preceding steel strip and a leading end portion of a following steel strip; Welding means for welding the rear end portion of the steel strip and the leading end portion of the following steel strip, temperature measuring means for measuring the temperature of the welded portion formed by the welding, and post-heating the welded portion for a predetermined amount. a post-heating means for heating to a treatment temperature;
and a control means for instructing the post-heating means to perform the heating based on the temperature of the weld zone during cooling measured by the temperature measuring means.
[8] The steel strip joining apparatus according to [7], wherein the welding means is a mash seam welder.
[9] The steel strip joining apparatus according to [7] or [8], wherein the temperature measuring means is a radiation thermometer.
[10] The steel strip joining apparatus according to any one of [7] to [9], wherein the post-heat treatment means is an induction heating device.
[11] Judging means for judging that the measured temperature of the weld zone is a predetermined temperature, and instructing means for instructing the post-heat treatment means to perform the heating based on the result of the judgment. The steel strip joining apparatus according to [7], characterized by having

According to the present invention, even when steel strips are welded to each other and subjected to continuous processing such as rolling in steel sheet production at a steel mill, the effect of the post-heating treatment after welding and joining is reliably exhibited, and continuous strip threading is performed. Breakage of the welded portion (joint portion) in the middle is prevented, productivity is improved, and there is a marked industrial effect. Moreover, according to the present invention, there is also the effect that the processing time including welding and subsequent post-heating can be greatly shortened.

4 is a graph showing the relationship between the hardness of the weld zone after post-heat treatment and the cooling stop temperature. BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows typically the joining apparatus of the steel strip suitable for implementation of this invention. BRIEF DESCRIPTION OF THE DRAWINGS It is explanatory drawing which shows typically the outline|summary of (a) mash seam welding and (b) laser welding. FIG. 4 is an explanatory diagram schematically showing a heating and cooling pattern of a welded portion in an example;

The present invention is a steel strip in which the rear end portion of the preceding steel strip and the leading end portion of the following steel strip are welded to form a welded portion, the steel strips are joined together, and then the welded portion is post-heated. This joining method enables continuous processing such as rolling. The steel strip targeted by the present invention is a steel strip in which a low-temperature transformation structure such as martensite or bainite is likely to be formed in the weld zone during welding. As such a steel strip, for example, the following formula (1)
Ceq = C + Si/50 + Mn/25 + Cr/50 + P/2 (1)
Where, C, Si, Mn, Cr, P: content of each element (% by mass)
A material having a carbon equivalent Ceq of 0.1% or more can be exemplified. In addition, the calculation in the above formula (1) is assumed to be zero when the element is not contained.

Examples of such materials (steel strips) include carbon steel, TRIP steel (transformation-induced plasticity steel), DP steel (Dual Phase steel), and the like. Classified by strength level, various steel sheets such as TS 1.0 GPa to 1.5 GPa grade high-strength steel sheets (steel strips) for automobile bodies and various high-strength surface-treated steel sheets can be exemplified.

In the present invention, when joining the steel strips, the rear end of the first steel strip and the front end of the second steel strip are welded. The first steel strip and the second steel strip may be steel strips of the same type or steel strips of different types.

The method of welding the steel strips is not particularly limited, but mash seam welding or laser welding is preferred.

In mash seam welding, the rear end of the first steel strip and the front end of the second steel strip are cut with a shear, overlapped in parallel, and the overlapped portion is welded with an electrode wheel of a mash seam welder. By pressing and energizing, the welded portion is formed and crushed, and while being flattened by the swaging roll SWR, it is run in the width direction and joined.

In laser welding, the rear end of the first steel strip and the front end of the second steel strip are cut with a shear and butted together, and the laser welding head of the laser welding machine is scanned in the width direction at the butted portion. to form a continuous weld.

In the present invention, the temperature of the formed weld (surface temperature) is measured. It is preferable to measure the temperature (surface temperature) of the weld zone continuously or intermittently following the cooling immediately after the completion of welding. Then, in the present invention, the post-heating process is started immediately after the temperature of the weld reaches a predetermined temperature T (°C). The "predetermined temperature T (°C)" referred to here is the final temperature MF (°C) of formation of the low-temperature transformation structure of the weld zone, or a temperature in the range of (MF - 5°C) to (MF - 20°C). preferably.

The temperature MF at which the formation of the low-temperature transformation structure of the weld zone ends is obtained in advance by experiments for a steel strip having the same composition as that of the steel strip. After the steel strip is heated to the welding temperature, it is cooled, and the heated welded portion (the portion corresponding to welding) stops cooling at each temperature (cooling stop temperature) during cooling, and is immediately heated to the predetermined temperature. After post-heating treatment and cooling to room temperature, the hardness of the welded portion (the portion corresponding to welding) is measured, the relationship between the cooling stop temperature and the hardness after post-heating treatment is obtained, and the cooling-stop temperature and post-heating treatment are obtained. It shall be determined from the relationship with hardness later. In the present invention, the highest cooling stop temperature at which the hardness after the post-heat treatment is about the same as the hardness of the base material is defined as the completion temperature MF (°C) of formation of the low-temperature transformation structure of the weld zone. When steel strips having different compositions are to be joined, the lower MF is used among the temperatures MF at which the formation of low-temperature transformation structures of the welded portions of the respective steel strips is completed in advance.

The temperature of the weld (surface temperature) is measured by temperature measuring means. The temperature measuring means is not particularly limited, but it is preferable to use, for example, a non-contact temperature measuring device such as a radiation thermometer. The temperature measurement device may be a device that can measure a specific point such as a fiber-type radiation thermometer, or a device that can measure temperature distribution two-dimensionally such as a thermography camera or an array-type radiation thermometer. .

In the present invention, when the temperature of the weld zone reaches the low-temperature transformation structure generation finish temperature MF of the weld zone during cooling after the completion of welding, the post-heat treatment is immediately performed. In addition, from the viewpoint of ensuring the effect of the post-heating treatment, the temperature at which the post-heating treatment is started should be between (MF-5°C) and (MF -20°C) is preferable.

The post-heating treatment heats and tempers the welded portion to reduce hardness and prevent breakage during threading. The heating temperature of the post-heating treatment is appropriately determined according to the composition of the steel strip, but the temperature in the range of 450 ° C. or higher and lower than the Ac ₁ transformation point of the steel strip is preferably 450 to 650 ° C., more preferably 500 ° C. The temperature is in the range of -600°C, more preferably 550-590°C.

The post-heating means is not particularly limited, but is preferably a burner heating device or an induction heating device. In particular, if induction heating is used, the heating rate is high and the processing time can be shortened. In addition, after the post-heating treatment, it is allowed to cool to room temperature.

As shown in FIG. 2, the steel strip joining apparatus 1 suitable for joining the steel strips described above includes a cutting means 2 for cutting the ends of the steel strips, a welding means 3, a temperature measuring means 4, a rear It has heat treatment means 5 and control means 6 (not shown). In FIG. 2, the cutting means 2, the welding means 3, the temperature measuring means 4, and the post-heating means 5 are mounted on a carriage 10 so that they can be moved integrally in the width direction of the steel strip.

An example of the cutting means 2 is a shear 21 that cuts the trailing edge of the preceding steel strip 11 and the leading edge of the trailing steel strip 12 . As the welding means 3, a mash seam welder and a laser welder can be exemplified. to weld. Although FIG. 2 shows a mash seam welder as the welding means 3, the present invention is not limited to this.

In the mash seam welder, the rear end of the first steel strip 11 and the front end of the second steel strip 12, which are cut off by the shear 21, are held by clamps or the like and overlapped in parallel. , the overlapped portion is pressurized and energized by the electrode wheels 31, 31 to form and crush the welded portion, and while further flattened by the swaging rolls SWR 32, 32, it travels in the width direction and continuously The steel strips are joined together by welding (see FIG. 3(a)). Further, in the laser welding machine, the rear end portion of the first steel strip 11 and the front end portion of the second steel strip 12, the ends of which have been cut by the shear 21, are butted against each other, and a laser beam is welded along the butted portion. The welding head 33 is scanned to continuously irradiate and weld the steel strips to form a weld zone and join the steel strips together (see FIG. 3(b)).

As the temperature measuring means 4, for example, a non-contact temperature measuring device such as a radiation thermometer that can measure the temperature (surface temperature) of the weld zone can be exemplified. Although the temperature measuring means 4 is arranged on the lower side (rear side) of the steel strip in FIGS. 2 and 3, it goes without saying that the present invention is not limited to this. The temperature measuring means 4 is provided immediately in front of the post-heating means 5 so as to measure the temperature of the weld zone immediately before the post-heating process, and is mounted on a carriage or the like so as to be movable along the weld line. preferably. The temperature of the weld measured by the temperature measuring means 4 is
It is sent to the control means 6 .

The post-heating means 5 is not particularly limited as long as it can stably heat the welded portion to a predetermined post-heating temperature, such as burner heating or induction heating. From the viewpoint of convenience, etc., it is preferable to use an induction heating device. Although the post-heating means 5 is arranged on the lower side (back side) of the steel strip in FIG. 2, it goes without saying that the present invention is not limited to this. The post-heating means 5 is preferably arranged so as to be movable along the weld line.

The control means 6 compares the temperature of the weld zone measured by the temperature measuring means 4 with a predetermined temperature obtained in advance (low-temperature transformation structure generation end temperature MF of the weld zone), and performs post-heating. and an instruction means for instructing the post-heating treatment means 5 to perform heating (post-heating treatment).

The present invention will be further described below based on examples.

A first steel strip and a second steel strip were prepared. Both the first steel strip and the second steel strip are 1.5 GPa class high-strength steel strips (steel plates) for automobile bodies with a thickness of 2.0 mm, and the carbon equivalent Ceq defined by the formula (1) is 0.3%. Met. In addition, the temperature MF at which the low-temperature transformation structure formation end temperature MF of the welded portion (the portion corresponding to the weld) of the steel strip was obtained in advance by experiment. MF was 400°C.

The rear end portion of the first steel strip 11 and the front end portion of the second steel strip 12 were welded using the steel strip joining apparatus shown in FIG. During welding, the end of the trailing end of the preceding first steel strip 11 and the end of the leading end of the following second steel strip 12 are cut by the cutting means 2 (shear 21), and the first The rear end of the steel strip 11 and the front end of the second steel strip 12 were overlapped. Then, the carriage 10 is caused to travel in the width direction of the steel strip at the overlapped portion, and the electrode wheels 31, 31 of the mash seam welder pressurize and energize the welded portion, forming and crushing the welded portion, and further swaging. Flattening with roll SWRs 32, 32 and mash seam welding were performed to form the steel strip 11 and the steel strip 12 into a continuous steel strip. Further, the temperature (surface temperature) of the obtained welded portion was measured with a radiation thermometer (temperature measuring means 4) arranged on the weld line to determine the maximum temperature at the welded portion. The obtained welded portion was subjected to a post-heating treatment, and then continuously threaded.
(Comparative example 1)
After the welding is finished, the temperature of the weld is allowed to cool, and the temperature of the obtained weld is measured by a radiation thermometer (temperature measurement means 4) placed on the weld line. ° C.), a post-heating treatment was performed by heating to a predetermined temperature (600° C.) with an induction heating device (post-heating means 5), and the mixture was allowed to cool to room temperature. No breakage occurred at the welded portion during continuous sheet threading.
(Comparative example 2)
After the welding is completed, the temperature of the obtained weld zone is measured planarly by a radiation thermometer (temperature measuring means 4) placed on the weld line, and is immediately heated to a predetermined temperature by an induction heating device (post-heating means 5). (600°C), followed by heat treatment and cooling to room temperature. The temperature of the weld zone (maximum temperature) at the start of the post-heat treatment was 550°C. The treatment time from the start of welding to the end of cooling in the post-heat treatment was 50% of the treatment time in Comparative Example 1, which was 100%. It should be noted that the weld was broken during cold rolling by continuous sheet threading.
(Invention Example 1)
After the welding is finished, the temperature of the weld is allowed to cool, and the obtained temperature of the weld (maximum temperature) is measured planarly with a radiation thermometer (temperature measuring means 4) placed on the weld line, and the temperature of the weld (maximum temperature) However, after decreasing to (MF-5°C) based on the pre-obtained low-temperature transformation structure formation end temperature MF (°C) of the weld, the induction heating device (post-heating means 5) is used to heat the material to a predetermined temperature. A post-heating treatment was performed by heating to (600° C.), and after the post-heating treatment was completed, it was allowed to cool to room temperature. The processing time from the start of welding to the end of cooling in the post-heating treatment was 70% of that of Comparative Example 1, and the processing time was shortened. No breakage occurred at the weld zone during cold rolling by continuous sheet threading.

10 Carriage 11 First Steel Strip 12 Second Steel Strip 2 Cutting Means 21 Shear 3 Welding Means 31 Electrode Wheel 32 Swaging Roll 33 Laser Welding Head 4 Temperature Measuring Means (Radiation Thermometer)
5 post-heating means (induction heating device)

Claims (11)

In joining the leading steel strip and the trailing steel strip by welding the trailing end portion of the leading steel strip and the leading end portion of the trailing steel strip, after the welding is completed, the welding is performed to form A method for joining steel strips, characterized in that the temperature of the welded portion is measured, and after the welded portion is cooled to a predetermined temperature T (°C), the welded portion is immediately subjected to a post-heating treatment. Of the temperatures MF (°C) at which the formation of the low-temperature transformation structure ends in the weld zone of the preceding steel strip and the following steel strip, the predetermined temperature T (°C) is determined by taking the lower MF as a reference (MF-5°C ) to (MF-20°C). The method for joining steel strips according to claim 1 or 2, wherein the welding is mash seam welding. The method for joining steel strips according to any one of claims 1 to 3, wherein the temperature of the welded portion is measured using a radiation thermometer. The method for joining steel strips according to any one of claims 1 to 4, wherein the post-heat treatment is a heat treatment using induction heating. Either the first steel strip or the second steel strip is a steel strip having a composition in which the carbon equivalent Ceq defined by the following formula (1) is 0.1% or more. 6. A method for joining steel strips according to any one of 1 to 5.
Record
Ceq = C + Si/50 + Mn/25 + Cr/50 + P/2 (1)
Where, C, Si, Mn, Cr, P: content of each element (% by mass) A steel strip joining apparatus comprising: a cutting means for cutting a trailing end portion of a preceding steel strip and a leading end portion of a following steel strip; Welding means for welding the rear end portion and the leading end portion of the trailing steel strip, temperature measuring means for measuring the temperature of the welded portion formed by the welding, and heating the welded portion to a predetermined post-heating temperature. post-heating means for heating;
and a control means for instructing the post-heating means to perform the heating based on the temperature of the weld zone during cooling measured by the temperature measuring means. 8. The apparatus for joining steel strips according to claim 7, wherein said welding means is a mash seam welder. 9. The apparatus for joining steel strips according to claim 7, wherein said temperature measuring means is a radiation thermometer. 10. The apparatus for joining steel strips according to claim 7, wherein said post-heat treatment means is an induction heating device. The control means has a judgment means for judging that the measured temperature of the weld zone is a predetermined temperature, and an instruction means for instructing the post-heat treatment means to perform the heating based on the result of the judgment. The steel strip joining apparatus according to claim 7, characterized by:

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