JP4140959B2 - Method for fusion welding of Zn-based plated steel sheet - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a welding method that suppresses the occurrence of molten metal embrittlement cracks when a Zn-based plated steel sheet is fusion welded.
[0002]
[Prior art]
Zn-based plated steel having excellent corrosion resistance is widely used as a steel material used not only for automobiles but also in places where corrosion is a problem, such as building structures and home appliances. And it is rarely used in the form of a plate, and after forming into a required shape by some means, each part is welded to produce a final product. That is, when a desired product is manufactured using a Zn-based plated steel sheet, a welding process is mostly included.
As types of welding, there are resistance welding as typified by spot welding and fusion welding as typified by arc welding. For building structures and undercarriage parts of automobiles, fusion welding may be used in consideration of the fact that relatively high joint strength is required, the plate thickness is relatively large, and the electrode has a low life in resistance welding. Many.
[0003]
Melt welding is a method in which a very high amount of heat is applied to a material to be welded to melt and solidify, and in some cases, a welding wire is supplied for welding. When a Zn-based plated steel sheet is melt welded, the steel base material, which is the plating base plate, is also melted, but the plating layer coated on the surface of the base material also remelts and evaporates.
In the case of Zn-based plated steel sheets, the melting point of the plating layer is considerably lower than the melting point of the steel sheet that is the base material. Meanwhile, the plating layer is present on the steel sheet surface in a molten state. It is known that when a certain tensile stress, which is a state in which Sn, Cu, Zn or the like is present in a molten state, acts on a steel plate, a crack occurs in the steel plate. This is what is called "molten metal embrittlement cracking" (for example, Shuzo Ueda, "Shusho, the flow of steel technology, 1st series, volume 9, welding of structural steel-properties and metallurgy of low alloy steel") 1997.6.1 Jinjinshokan, p.274-276).
[0004]
[Problems to be solved by the invention]
Molten metal embrittlement cracking occurs when a certain level of tensile stress is applied to a solid metal under a specific combination of molten metal and solid metal. It is a phenomenon that breaks. When the solid metal is steel, it is generated by welding or brazing when the parts after welding are immersed in a molten Zn bath and hot-dip Zn plating is applied, when Cu brazing is performed, or when a Zn-based plated steel sheet is fusion welding. In many cases, Zn is infiltrated in the coarse-grained region of the heat-affected zone and cracking occurs.
In particular, recently, Zn-based plated steel sheets having excellent corrosion resistance have been widely used. However, when a plurality of conditions at the time of welding overlap, it is a problem because molten metal embrittlement cracks are likely to occur in the vicinity of the heat affected zone.
[0005]
In order to suppress the occurrence of molten metal embrittlement cracks in the vicinity of the heat-affected zone, the plating layer in the vicinity of the weld zone is also removed prior to welding. However, not only does the plating layer removal process disperse dust, but the working environment is deteriorated, and the welded part from which the plating layer has been removed is exposed compared with the base metal part because the base steel is exposed. Corrosion resistance is poor. Although degradation of corrosion resistance can be prevented by forming a sprayed layer made of the same material as the plating layer in the weld zone, it requires manufacturing by removing the plating layer before welding and forming a sprayed layer after welding and an extra step. This increases the load on the system and is not realistic.
In addition, measures are taken such as relaxing the tensile stress acting by changing the method of restraining the material to be welded and reducing the residual stress of the material to be welded in advance. However, this method also has a problem of lack of certainty because various amounts of stress are not constant. There is also a method to reduce the heat stress generated and remaining by reducing the heat input during welding as much as possible, but if the heat input during welding is small, sufficient penetration may not be obtained, There is a problem that the bonding strength of this becomes unstable.
[0006]
In Japanese Patent Application Laid-Open No. 2003-3238, the present applicant has proposed a technique for suppressing molten metal embrittlement cracking at the time of welding by combining the composition of the base steel and the composition of the hot dip plating layer. However, prior to investigating the technique to prevent molten metal embrittlement cracking that is likely to occur in the heat affected zone during welding when a Zn-plated steel sheet is formed into an appropriate shape and then welded to construct a structure. There are few examples.
The present invention has been devised to solve such a problem. When a Zn-plated steel sheet is fusion welded, it is melt-welded without removing the plating layer in advance and without suppressing the heat input. However, it aims at providing the fusion welding method in which a molten metal embrittlement crack does not generate | occur | produce in a welding heat affected zone.
[0007]
[Means for Solving the Problems]
In order to achieve the object, the method for fusion welding of a Zn-based plated steel sheet according to the present invention, on the outside of the welded portion of the Zn-based plated steel sheet, when the Zn-based plated steel sheet obtained by plating a Zn-based alloy on the steel sheet surface is fusion welded. The present invention is characterized in that a wall for preventing movement of a hot- dip metal having a heat resistance of 900 ° C. or higher is provided.
[0008]
[Action]
The inventors of the present invention have reported that the occurrence of cracks in the vicinity of the weld heat affected zone when a steel sheet plated with a Zn alloy is melt welded is a Zn-Al-Mg based plating that is slightly more sensitive to cracking than a Zn plated steel sheet as a plated steel sheet. Observation was made by performing fillet welding by CO ₂ arc welding using a steel plate.
That is, as shown in FIG. 1, a Zn—Al—Mg-based plated steel sheet 1 and a fillet portion 3 of plain steel 2 that has not been plated were arc welded by a CO ₂ torch 5 using a welding wire 4. When the welded portion 6 is observed in detail, as shown in FIG. 2, the crack 9 is generated in the vicinity of the surface of the plated steel plate of the heat affected zone 7 in which the crystal grains in the vicinity of the weld metal 8 are coarsened. . And when the component analysis was performed about the vicinity of a crack, it turned out that Zn has adhered to the side wall of the crack 9. FIG. Therefore, the crack 9 was found to be a molten metal embrittlement crack due to Zn.
[0009]
Moreover, when the distribution state of the plating layer after welding is seen, similarly as shown in FIG. 2, it is 1000 degreeC or more on the welding metal 8 with which the base material part which is a plating base plate of a Zn-Al-Mg type plated steel plate was fuse | melted. The plated layer 10 was scattered in the coarse grain region of the heat affected zone 7. Furthermore, on the surface of the plated steel sheet in a region other than the heat-affected zone 7, a plating layer 12 having substantially the same thickness was present after the corrugated plating layer 11. When the components of the plating layers 10, 11, and 12 were analyzed, all of them contained Zn, Al, and Mg.
[0010]
By the way, when the steel is arc-welded, it is known that the weld metal 8 is a portion where the steel is melted, so that the temperature is 1500 ° C. or higher, and the coarse region of the heat affected zone 7 is also 1000 ° C. Yes. On the other hand, the melting temperatures of Zn, Al, and Mg are 420 ° C., 660 ° C., and 650 ° C., respectively, and the evaporation temperatures are 930 ° C., 2060 ° C., and 1107 ° C., respectively. That is, at the time of welding, Zn is evaporated at least in the vicinity of the weld metal 8 and in the coarse grain region of the heat affected zone 7. Therefore, the plating layer 10 scattered on the weld metal 8 or in the coarse-grained region of the heat-affected zone 7 at 1000 ° C. or higher may flow from the periphery due to the re-melting of the plating layer outside the weld zone 6. There is.
[0011]
In arc welding, as shown in FIG. 3, an inward electromagnetic force 15 is generated by energizing the molten droplet 14 in which the welding wire 13 is melted. The electromagnetic force 15 is called a pinch force and is generated when a current is passed through the molten metal.
Since the re-melted plated layer on the plated steel sheet is also in a molten metal state, when energized by the arc 16, an inward electromagnetic force acts like the droplet. It is considered that the plated metal melted by the electromagnetic force approaches the welded portion 6 and the plated layer 10 is scattered in the vicinity of the weld metal 8 or in the coarse grain region of the heat affected zone 7.
Molten metal exists in the coarse-grained region of the heat-affected zone 7, and in combination with the effects of thermal stress caused by thermal expansion and contraction due to welding, molten Zn penetrates into the coarsened grain boundary very easily. It is estimated that molten metal embrittlement cracking occurred.
[0012]
Therefore, the present inventors have found that in order to prevent the occurrence of molten metal embrittlement cracking, it is necessary to dam the molten plated metal so as not to approach the welded portion 6 during welding. For example, as shown in FIG. 4, on the surface of a Zn—Al—Mg based plated steel sheet before welding, for preventing the movement of the hot- dip plated metal having heat resistance of 900 ° C. or more outside the position where the weld 6 is present. A wall 17 was provided to dam the plated metal that was remelted into a molten material so that the plated metal did not approach the weld 6.
When welding was actually performed in that state, no molten metal embrittlement cracking occurred, and the plating layer 10 was not scattered in the vicinity of the weld metal 8 or in the coarse grain region of the heat affected zone 7. And a good quality welded joint could be obtained by such a method.
[0013]
In addition, since the evaporating temperature of Zn is 930 ° C., the installation position of the hot dipped metal movement prevention wall 17, which is a feature of the present invention, is the position immediately before the temperature of the plating original plate reaches 930 ° C. during welding. It is preferable to provide it at a position before reaching 900 ° C.
The material of the hot dipped metal movement prevention wall is not particularly limited as long as it does not react with the plated metal of the Zn-based plated steel sheet and has heat resistance of 900 ° C. or higher. In consideration of ease of use, it is preferable to use ceramics. The width of the wall may be any size as long as it can be stably installed on the Zn-based plated steel sheet. However, the height of the wall varies depending on the thickness of the plating layer. In other words, when the plating layer is thick, the amount of plating metal that needs to be prevented from moving also increases, so it is necessary to increase the amount accordingly. For example, when the thickness of the plating layer was 20 μm, cracking could be prevented by setting the wall height to 1 mm.
[0014]
【Example】
As a Zn-based plated steel sheet, a plated steel sheet with a thickness of 20 μm, a thickness of 4 mm, a width of 100 mm, and a total length of 100 mm is obtained by immersing in a Zn-6% Al-3% Mg alloy plating bath and plating with Zn—Al—Mg alloy. It was used. A plain steel strip 2 having a plate thickness of 10 mm, a width of 30 mm, and a height of 40 mm as shown in FIG.
The arc welding was CO ₂ welding, the welding current was 220 A, the welding voltage was 26 V, the welding speed was 0.3 m / min, the torch angle was 45 degrees, and the flow rate of the CO ₂ shielding gas was 20 l / min. As the welding wire 4, YGW 12 having a diameter of 1.2 mm was used.
The area to be the welded portion 6 was confirmed in advance and was within a range of 18 mm from the center of the ordinary steel strip 2, so that the Al having a width of 10 mm and a height of 5 mm was located 18 mm away from the center of the ordinary steel strip 2. _{A 2} O ₃ ceramic wall was provided.
As a result of arc welding in that state, no molten metal embrittlement cracking occurred, and a good weld joint could be obtained.
[0015]
[Comparative example]
Arc welding was performed under the same conditions as in the above example except that no wall was provided.
As a result, cracks occurred in the coarse grain region of the heat-affected zone, and Zn adhered to the side walls of the cracks. From this, it was found that when a wall was not provided, molten metal embrittlement cracking due to Zn occurred. In addition, plating layers containing Zn, Al, and Mg are scattered in the vicinity of the weld metal and in the coarse grain region of the heat-affected zone.
[0016]
【The invention's effect】
As described above, according to the method of the present invention, when a Zn-based plated steel sheet obtained by plating a Zn-based alloy on the steel sheet surface is melt welded , the heat resistance of 900 ° C. or more is provided on the outer side of the welded portion of the Zn-based plated steel sheet . by providing the molten plating metal movement preventing wall having, during welding, dammed is molten plating metal, does not approach the weld 6, melting in the grain boundary of the coarse crystal grains of the weld heat affected zone Zn It was possible to suppress the occurrence of molten metal embrittlement cracks.
For this reason, even if a Zn-based plated steel sheet, especially a Zn-Al-Mg-based steel sheet with excellent corrosion resistance, is melt welded, a welded product having a sound welded portion is generated without causing cracks due to molten metal embrittlement. Manufactured. The obtained welded product is used as a structural member or the like in various fields by utilizing the original high corrosion resistance of a Zn-based plated steel sheet, particularly a Zn-Al-Mg-plated steel sheet.
[Brief description of the drawings]
[Fig. 1] A diagram for explaining the aspect of fillet welding of strip material on a plated steel plate by arc welding. [Fig. 2] Melt generated in the heat-affected zone when fillet welding of the strip material on a plated steel plate is performed. Diagram explaining metal embrittlement crack [Fig. 3] Diagram explaining electromagnetic force during arc welding [Fig. 4] Provided around the heat-affected zone when fillet-welding strip material on plated steel sheet Diagram explaining the action of the hot- dip plated metal movement prevention wall [Explanation of symbols]
1: Zn-based plated steel sheet 2: Strip material to be welded to fillet 3: Fillet welded portion 4: Welding wire 5: Torch 6: Welded portion 7: Weld heat affected zone 8: Weld metal 9: Molten metal embrittlement crack 10: plated layer 11: plating layer 12: plating layer 13: welding wire 14: droplet 15: electromagnetic force 16: arc 17: molten plating metal movement blocking wall

Claims (1)

When a Zn-based plated steel sheet obtained by plating a Zn-based alloy on the steel sheet surface is melt welded , a hot- dip plated metal movement prevention wall having heat resistance of 900 ° C. or more is provided outside the welded portion of the Zn-based plated steel sheet. A method for fusion welding of a Zn-based plated steel sheet.

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