Optimized method for cooling a weld seam of a resistance-butt-welded workpiece

Description

[0001] The present invention relates to a method for cooling a weld seam of a resistance butt-welded steel strip after joining two ends by welding, wherein the ends are ends of the steel strip or ends of two individual parts of the steel strip.

[0002] Methods for cooling a weld seam of a resistance-butt-welded steel strip according to the preamble of claim 1 are known, for example, from the documents US 2010 / 059488 A1, WO 2013 / 134112 A2 and GB 688 244 A. I

[0003] When producing a weld seam of a steel strip using a resistance butt welding process, a particularly high quality of the weld seam is important in some areas of application.

[0004] For example, steel bands are welded to be used as band knives and / or band saws, among other things.

[0005] After the welding process, these are ground and sharpened to a specified thickness. One application of these band knives is, for example, the leather processing industry. Band saws, for example, are used for cutting a wide variety of materials.

[0006] Under these conditions, even the smallest changes in the surface structure lead to the rejection of the welded steel strips. In particular, differences in the hardness of the material within a weld seam cause problems. When used as a band knife or band saw, additional bending loads during bending processes, for example, at deflection pulleys, lead to tension in the material, which can shorten the service life of the weld seam.

[0007] Furthermore, the cooling of the weld seam affects the microstructure. This can lead to inhomogeneities that can cause a strip tear at the weld seam.

[0008] Resistance butt-welded steel strips typically cool down at ambient temperature after welding without any further control of this cooling process.

[0009] Once the welding temperature is reached, the two ends of the steel strip are moved toward each other for welding, and then the current is switched off. The cooling rate is determined by the temperature difference between the steel strip and the surrounding area.

[0010] However, temperature differences within the weld seam occur during cooling, which can be attributed to various causes. Contamination on the strip cannot be ruled out. This results in a different energy input into the steel strip due to the current applied during welding. The geometry of the weld seam across the strip cross-section also influences the natural cooling behavior. The weld seam cools differently at the edges than in the center. This is known from general physical processes.

[0011] The varying temperatures within the weld during cooling can lead to cracks and / or defects forming within the weld. Stresses in the material within the strip cross-section are also possible. This can lead to differences in hardness within the material, which reduces the quality of the weld.

[0012] This is where the present invention comes in.

[0013] The invention was based on the problem of improving a method of the type mentioned at the beginning in such a way that a uniform cooling of the weld seam is achieved, with which the desired quality requirements can be met.

[0014] This object is achieved according to the invention by the method for cooling a weld seam of a resistance butt-welded steel strip according to claim 1 and the device according to claim 4.

[0015] By actively influencing the cooling of the weld, it is possible to reduce or even prevent defects and / or cracks, as well as tensions within the weld. Controlled cooling of the weld makes it possible to avoid tensions in the material caused by temperature differences during cooling.

[0016] According to the invention, a current i 2 is passed through the weld seam during cooling.

[0017] Advantageously, it can be provided that the cooling is controlled in such a way that, by adjusting the current i 2 and by monitoring an existing temperature, a temperature distribution is achieved within the weld seam which limits the temperature differences during cooling within the weld seam to ± 10%.

[0018] According to the invention, the current i 2 is set as follows : the actual temperature distribution within the weld seam is monitored; in the event of temperature deviations, the ohmic resistance value at the points of the weld seam with a temperature difference outside a specified range is determined using a table; the energy required for temperature equalization is determined from the temperature difference; the required energy is used to calculate the required current i 2.

[0019] It can be provided that for producing a weld seam of a resistance butt-welded steel strip by joining two ends by welding, wherein the ends are ends of the steel strip or ends of two individual parts of the steel strip, wherein the two ends are clamped in a clamping device, the two ends are pressed together so that contact is created at the abutting surfaces over the entire cross-sectional area, a current is applied which heats the welding point to welding temperature, whereby when the required temperature is reached, the current is switched off and the two ends are compressed by means of a feed movement and welded to form the steel strip, the weld seam is cooled according to one of the preceding claims.

[0020] Furthermore, it is possible that in a first time period the current is set as current i 1 so that the energy generated is sufficient to heat the steel strip to welding temperature and in a second time period the current i 2 is set so that the energy generated is dimensioned such that a uniform temperature distribution within the weld seam is achieved or maintained during the cooling of the weld seam.

[0021] It can be provided that the current i 1 > i 2 .

[0022] By changing the current, the entire process can be completed with a single power source. It is entirely possible for a period between the first and second time periods to occur during which the current is completely or almost completely switched off. This can particularly be the period during which the ends of the steel strip are moved toward each other by a feed motion.

[0023] The temperature difference is used to determine the energy required for temperature equalization. It is also possible to determine the energy required to increase the temperature within the weld seam through a series of tests. Based on the Joule heat of an electric current flowing through the weld seam, the current requirement can be determined from a known resistance value. As a first approximation, the equation W s = I 2 2 R s t s be an approach. W s the required energy, R s the resistance at the weld at the point under consideration, ts the time and I 2 the current in the second time period.

[0024] For this type of cooling control, it is important to know the specific properties of the material used.

[0025] For steel strips, stainless steels, heat-treatable steels, and / or carbon steels are suitable. These have different resistance values for different temperatures.

[0026] The following table shows examples of temperature-dependent resistance values for steel. Steel Temperature [°C] Resistance [Ω] Resistance [µΩ] Change in resistance[%] 20 0,00017361 173,61 32% 100 0,00025139 251,39 46% 200 0,00034861 348,61 64% 300 0,00044583 445,83 82% 400 0,00054306 543,06 100% 500 0,00064028 640,28 118% 600 0,00073750 737,50 136% 700 0,00083472 834,72 154% 800 0,00093194 931,94 172%

[0027] The resistance value at 400°C is set to 100%, as this temperature is assumed to be the welding temperature. This value can be selected differently for other processes. In this case, this value is chosen because this temperature is the welding temperature of some materials.

[0028] Studies have shown that temperature differences of 200° to 300° are usually present within a cooling weld.

[0029] From the table below it can be seen that at a temperature difference of ±200° from the standard temperature of 400°, the resistance drops to 64% (at 200°) or increases to 136% (at 600°) relative to the resistance value at 400°.

[0030] During the second period, a current is applied to even out the temperature within the weld seam. Naturally, the current flows more strongly in the areas with lower resistance than in the areas with higher resistance. In the areas with a higher current flow, the steel strip is heated more strongly. This then leads to an equalization of the temperature within the weld seam.

[0031] By monitoring the temperature of the weld seam, the resistance at each point is determined . This is then used to determine the energy required to achieve a desired temperature within the weld seam.

[0032] This process makes it possible to control the cooling of the weld seam as required to achieve the desired material properties.

[0033] According to claim 4, a device for producing a weld seam of a resistance-butt-welded steel strip by joining two ends by welding, wherein the ends are ends of the steel strip or ends of two individual parts of the steel strip, is provided, which device has a control and / or regulating means, wherein the control and / or regulating means is suitable and configured to carry out a method according to one of claims 1-3.

[0034] Further features and advantages of the present invention will become clear from the following description of preferred embodiments with reference to the accompanying drawings. Fig. 1 a flow diagram of a method according to the claims Fig. 2a, 2b a simplified representation of a steel strip with a weld seam

[0035] Figure 1 shows a flow chart illustrating the cooling of a weld according to the present invention.

[0036] First, all preparatory steps for the welding process are carried out. This includes clamping the ends and other work.

[0037] During a first period, a required current i 1 is applied, which heats the ends to welding temperature. Once this temperature is reached, the current i 1 of the first period is switched off, and the steel strip is moved toward each other by a feed motion of the ends. The actual welding of the steel strip is performed.

[0038] After the current of the first period is removed, the cooling of the steel strip and thus also of the weld seam begins.

[0039] Without further measures, the weld would cool unevenly to ambient temperature. The resulting temperature differences can cause defects in the weld, which would lead to differences in the hardness of the material. These quality differences are generally undesirable and result in the steel strip being rejected. For this reason, a current i2 is applied to the ends in a second time period to reheat the weld.

[0040] By appropriate monitoring of the existing temperature, for example an optical control of the weld seam with a pyrometer, the resulting temperature is monitored .

[0041] If the temperature is within a predetermined tolerance band of the temperature at the time of cooling, the cooling process continues until the desired final temperature is reached.

[0042] If this specified tolerance band is deviated from, the optimal current i 2 for the second time period must be determined.

[0043] Different variants for the current flow are conceivable.

[0044] For example, the invention does not allow the current to be adjusted so that it decreases linearly over a period of time or pulses over a period of time. If temperature deviations are detected, the current profile can be adjusted.

[0045] Due to the resistance differences shown, areas with a lower temperature heat up more than areas with a higher temperature. This is also used to determine the optimal current.

[0046] The measured temperature is assigned to a specific resistance value. This, in turn, determines the power required to heat the weld to the desired temperature.

[0047] Once the optimal current has been determined, it is applied in such a way that the weld seam is reheated.

[0048] The process is continued until the weld seam has reached the final temperature under the desired cooling.

[0049] In Figure 2a and Figure 2b Simplified representations of a steel strip 2 with a weld seam 1 are shown. The two ends 3, 4 of the steel strip 2 are clamped in a clamping device 5, with each end 3, 4 being held by its own clamp 5.

[0050] The required current is supplied to the steel strip 2 via terminals 5. After reaching the required welding temperature, the two ends 3, 4 are moved toward each other by a feed movement 7. In this case, the feed movement 7 is performed by the terminals 5. However, it is entirely possible that another device is present that performs the feed movement 7.

[0051] Figure 2a shows a temperature distribution 6 in the weld 1, which occurs when the cooling of the weld 1 occurs without further monitoring at ambient temperature. Different areas with different temperatures form. The Figure 2a shows these areas indicated by different hatchings.

[0052] If the natural cooling behavior is controlled by a targeted use of a reheating current, a temperature distribution 6 can advantageously form in the weld seam 1, as shown in Figure 2b is shown.

[0053] The entire weld seam 1 shows a uniform temperature distribution 6. The exact tolerance band for a uniform temperature distribution 6 depends on the material used for the steel strip 2 and the quality requirements for the hardness differences in the material.

[0054] The temperature tolerance should be determined in advance for each process. With an initial welding temperature of 400°C, cooling can conceivably occur within a tolerance range of ± 10%. Other values are possible depending on the welding requirements.

Claims

1. A method of cooling a weld seam(1) of a resistance butt-welded steel strip (2) after joining two ends (3, 4) by welding, wherein the ends (3, 4) are ends (3, 4) of the steel strip (2) or ends of two individual parts of the steel strip (2), wherein the cooling is controlled and differs from a natural cooling behaviour of the weld seam (1) at ambient temperature , wherein a current i2 is passed through the weld seam (1) during the cooling, characterised in that the current i2 is set as follows: - an actual temperature distribution within the weld seam (1) is monitored, - in the event of temperature deviations, the ohmic resistance value at the points of the weld seam with a temperature difference outside a defined range is determined by means of a table, - the temperature difference is used to determine the energy required to equalise the temperature, - the required energy is used to calculate the required current i2.

2. Method according to claim 1, characterised in that the cooling is controlled in such a way that by setting the current i2 and by monitoring the existing temperature, the temperature distribution (6) within the weld seam (1) is achieved, which limits the temperature differences during cooling within the weld seam to± 10% .

3. Method for producing a weld seam (1) of a resistance butt-welded steel strip (2) by joining two ends (3, 4) by welding, wherein the ends (3, 4) are ends of the steel strip (2) or ends of two individual parts of the steel strip (2), wherein - the two ends (3, 4) are clamped in a clamping device (5), - the two ends (3, 4) are pressed together in such a way that contact is made at the joint surfaces over the entire cross-sectional area, - a current is applied which heats the welding point up to welding temperature, whereby when the required temperature is reached the current is switched off and the two ends (3, 4) are pressed together by means of a feed movement (7) and welded to form the steel strip (2), wherein the weld seam (1) is cooled according to one of claims 1 to 2.

4. Apparatus for producing a weld seam (1) of a resistance butt-welded steel strip (2) by joining two ends (3, 4) by welding, wherein the ends (3, 4) are ends (3, 4) of the steel strip (2) or ends of two individual parts of the steel strip (2), comprising - a power source, - means for monitoring the temperature distribution within the weld seam (1), - a clamping device (5), wherein the two ends (3, 4) of the steel strip (2) are clamped in the clamping device (5) and the required current is supplied to the workpiece via the clamping device (5), - a feed device, wherein the feed device moves the two ends (3, 4) of the steel strip (2) towards each other by a feed movement (7), and - a control means, wherein the control means is suitable and set up to carry out a method according to one of claims 1 to 3.