JP2650558B2 - Manufacturing method of high workability welded steel pipe - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a welded steel pipe having good weldability such as workability.

[0002]

2. Description of the Related Art A welded steel pipe is manufactured by forming a steel strip into a tubular shape with a group of forming rolls, and joining opposing ends to each other by appropriate welding means. Welding methods for manufacturing such welded steel pipes are broadly classified into fusion welding methods such as TIG welding, plasma welding, and submerged arc welding, and pressure welding methods represented by ERW (electric resistance welding).

[0003] The fusion welding method is advantageous in that welding defects are less likely to occur and the welding performance is excellent. In particular, TIG and plasma welding are applied to high-grade steel pipes such as stainless steel due to the good cleanliness of the weld metal. Have been. However, on the other hand, it takes a long time for the welding base metal and the deposited metal to penetrate, so that the welding speed is low and productivity is a problem. On the other hand, a pressure welding method such as the ERW method is extremely excellent in efficiency, contrary to the fusion welding method, but has a drawback that a welding defect such as a penetrator is easily generated. In this case, the reliability of the weld becomes a problem.

Therefore, in order to develop a pipe welding method having the same speed as ERW and the same weld performance as arc welding, use of a carbon dioxide laser as a welding heat source has recently been studied. It has been put into practical use for ferritic stainless steel welded steel pipes. JP 56-1689A
No. 88 and JP-A-63-278689.

[0005] Laser welding, like TIG, is fusion welding, so that defects are unlikely to occur, and since the energy density of the heat source is higher than that of ordinary arc welding, penetration is deeper and faster, so high-speed welding is possible. In addition, since the heat energy concentration is high, the total heat input can be reduced, so that the performance of the welded portion is good.

[0006] Today, there is an increasing demand for steel pipes having good secondary workability, mainly as structural steel pipes for automobile production. Carbon steel or ferritic stainless steel is used as the material, and bending, drawing,
Expansion and their combinations. A problem during the secondary processing is a crack in the welded portion, and improvement in ductility and toughness of the welded portion and uniform hardness distribution have been issues.

Conventionally, most of these steel pipes have been manufactured by ERW. However, as described above, ERW is liable to cause welding defects, and cracks are likely to occur at the time of secondary working from the welding defects. On the other hand, the laser welding method has no defects because it is fusion welding, and since the welding heat input can be reduced compared to conventional fusion welding, the crystal grains become finer and the toughness and processing The property becomes good. Further, the laser welding method enables high-speed welding and has good productivity as compared with the conventional arc welding method. Thus, it can be said that the pipe manufacturing method by laser welding is more suitable for manufacturing a high workability steel pipe than the conventional pipe manufacturing method by ERW or arc welding.

[0008]

As a method of further improving the workability of the welded portion, a method of rolling the weld bead portion in the thickness direction has been proposed (Japanese Patent Laid-Open No. Sho 62-13).
No. 524). Although this method was developed for the purpose of improving the corrosion resistance of the welded portion, it has been clarified by the inventors that the method is also effective for improving the workability.

[0009] The reason why the workability is improved by rolling the bead portion in this way is presumably because the anisotropy of the strength characteristics is eliminated due to the loss of directionality of the crystals formed during welding. Therefore, it is important to secure a sufficient processing amount in order to obtain the effect of improving workability by rolling.

However, as described above, the laser welding method is characterized in that welding with a small bead cross-sectional area can be performed with a small heat input, and as a result, it is extremely difficult to remove immediately after welding by cutting or grinding means. Inner bead extra height is usually 0.1 mm or less, more preferably 0.05
mm or less, and even if bead rolling is performed on this, there is a problem that little effect can be obtained due to a small processing amount.

It is an object of the present invention to apply laser welding to the production of welded steel pipes and to solve the problem of difficulty in securing workability due to insufficient machining in bead rolling, which is observed in laser welding. For example, it is an object of the present invention to provide a method for manufacturing a welded steel pipe which achieves a workability of a critical drawing ratio of 35% or more and an expansion of 20% or more.

[0012]

Means for Solving the Problems First, in laser welding, a method of increasing the upset amount is considered as a method of increasing a margin necessary for securing a sufficient processing amount.
When a pipe is made from a thin steel strip, FIG. 1 (a)
As shown in (b), it is not practical because the welded end faces are misaligned or buckled. Therefore, the present inventors have conducted various studies and found that a method of preheating the vicinity of the welded end face before laser irradiation is effective as a method of increasing the margin, and completed the present invention.

[0013] Conventionally, preheating has the effect of increasing the penetration of laser welding and increasing the limit welding speed.
Although the effect has been found in the improvement of production rather than the improvement of the performance of the welded portion, it has been found that in the present invention, the combination with bead rolling is also effective in the performance of the welded portion. That is, conventionally, preheating itself during laser welding was known, but not in combination with bead rolling performed after welding.
Rather, as described above, in the past, efforts have been made to either not actively form a weld bead or, if formed, to eliminate it by cutting or the like.

Therefore, the gist of the present invention is to supply a steel strip to a group of forming rolls, form the steel strip continuously into an open pipe shape, and press the opposite side squeeze rolls to bring both end faces into contact with each other. In the pipe welding method of irradiating a laser beam to the abutting portion to obtain a welded pipe, the vicinity of the end face to be welded is preheated to 250 ° C. or more before laser irradiation to form at least an inner surface extra height 0.15 mm or more, A method for manufacturing a welded steel pipe having high workability, characterized in that after welding is completed, a weld bead portion is lowered in the thickness direction to eliminate the extra height of the inner surface.

The elimination of the extra height of the inner surface may be performed in-line at the same speed as the welding. Further, after elimination of the extra height of the inner surface, at least the vicinity of the welded portion may be heat-treated to ensure the workability.

[0016]

Next, the operation of the present invention will be described in more detail with reference to the accompanying drawings. FIG. 1 is a diagram for explaining a bead shape in the conventional method at the time of high upset. In laser welding, since the heat input is small and the heating area is small, even if the upset is set large by the squeeze roll, FIG. Abnormal deformation (misalignment, buckling) as shown in (b) occurs, and the margin cannot be increased. In the drawing, reference numeral 10 denotes both end portions between welds, 12 denotes a weld bead, FIG. 1 (a) shows a state where both end surfaces are misaligned, and FIG. 1 (b) shows a state where a steel plate buckles at both ends. Shown respectively.

On the other hand, when the preheating is performed in accordance with the present invention, as shown in FIG. 2 (showing the state after the removal of the extra surface protruding portion), the region near the preheated end face is easily deformed. It is possible to increase the margin without causing the problem. In the figure, the same parts as those in FIG. 1 are denoted by the same reference numerals. Numeral 14 denotes an extra inner surface portion. In addition, the inner surface extra height refers to the height H between the virtual inner surface and the inner surface extra portion indicated by a dotted line in the figure.

FIG. 3 is a graph showing the relationship between preheating temperature, upset amount and inner bead height when ferritic stainless steel is carbon dioxide laser-welded according to a conventional method. In order to secure the height (H) of the excess metal part, the preheating temperature must be 250 ° C or higher, and if it is less than that, the strength of the preheating area is insufficiently reduced and no increase in the excess metal can be expected.

FIG. 4 is a graph showing the relationship between the roll-erased inner surface fill height of the welded steel pipe having an internal fill plate obtained in the above example and the weldability (evaluated by the limit drawing ratio).
It can be seen that performing the rolling elimination of the previously formed inner surface margin in accordance with the method of the present invention is extremely effective for improving the workability.

As shown in FIG. 5 (a), the bead rolling, particularly the inner bead rolling, improves the workability of the welded portion, as schematically shown in FIG. 5 (a). (Grain boundary at the center of the bead) It is considered that the direct cause is that the 20 is broken by the bead rolling as shown in FIG. 5 (b).

According to the present invention, a welded steel pipe is manufactured through the steps of upsetting preheating of a steel strip with a forming squeeze roll, laser welding, and bead rolling, and each of these unit operations is already performed by welding. It is known as a method for manufacturing a steel pipe, and it may be used in the present invention.

For example, in the present invention, preheating performed prior to laser welding may be performed by a high frequency induction method or the like.
At that time, preferably 10KH is used to preheat only the vicinity of the end face.
The frequency should be higher than z. The laser welding used in the present invention may be one already used as a pipe manufacturing method such as a carbon dioxide laser method, and there is no particular limitation including the welding conditions. In general, it may be performed under the following conditions.

Laser output: 1-6 Kw, welding speed: 1-20
m / min, and if necessary, Ar gas or
Shield welding in which He gas is injected at 10 to 50 l / min may be performed. Rolling of the inner bead may be performed by roll processing, sliding pressure, continuous forging, or the like.

The above method is a method in which the rolling of the inner surface bead is performed in-line at the same speed as the welding, but the effect is the same even if the method is performed off-line. Further, the vicinity of the welded portion after bead rolling or the entire pipe is, for example, 910-950 ° C. × 1.
The heat treatment for up to 30 minutes makes the hardness distribution uniform, so that the workability can be further improved.

The material of the present invention may be any one of stainless steel, general carbon steel, alloy steel, and other high alloy steels as long as it can be a welded steel pipe or a welded steel pipe to which the present invention is applied. Next, the operation of the present invention will be described more specifically with reference to examples.

[0026]

EXAMPLES Table 1 shows the chemical compositions of the raw materials used in this example, and Table 2 shows the welding conditions. The material is ferritic stainless steel
(A), 50 kg class carbon steel (B) and low strength carbon steel (C)
Type, size is steel type A, C is outer diameter 50.8mm, wall thickness 2
mm, steel type B has an outer diameter of 50.8 mm and a wall thickness of 6 mm.

There are three types of welding methods: ERW (Electric resistance welding), Ar gas plasma welding, and carbon dioxide laser welding. In some conditions, heat treatment is performed in the final step, and the condition is that 930 ° C. is allowed to cool. The amount of bead rolling was represented by the extra height of the formed inner bead before the elimination of rolling (the outer bead is ground immediately after welding, so it is irrelevant in this embodiment). Preheating was performed by a high-frequency induction method (frequency: 450 kHz).

As for the performance of the welded portion, the workability is evaluated by drawing and expanding the tube, and the occurrence of welding defects is evaluated by an adhesion flattening test. For the drawing and expansion, the drawing ratio and the expansion ratio at the limit of the occurrence of cracking were indicated by the outer diameter ratio (%), and the welding defects were indicated by the defect length ratio to the specimen length.

In Table 2, Nos. 1 to 7, 13 to 18, 25 to 28
Are conventional examples, Nos. 8, 9, 19, 20, 29 and 30 are comparative examples,
10 to 12, 21 to 24, and 31 to 33 are examples of the present invention. As is clear from the results shown in Table 2, the welded steel pipe manufactured by the method of the present invention has a drawing of 35% or more and an expanded pipe of 20% or more. There is nothing.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

According to the present invention, the processing limit of the laser welded portion is improved, and it is possible to manufacture a welded pipe having extremely good workability. In particular, when the drawing is 40% or more and the expansion is 50% or more, its practical significance is that it can be applied, for example, to steel pipes for automobile exhaust system where the pipe ends are reduced and expanded. large.

[Brief description of the drawings]

FIGS. 1 (a) and 1 (b) show a bead shape according to a conventional method at a high upset, showing cases of misalignment and buckling, respectively.

FIG. 2 is an explanatory view of a bead shape according to the present invention.

FIG. 3 is a graph showing a relationship between a preheating temperature and a buckling limit inner surface extra height.

FIG. 4 is a graph showing a relationship between a height of a formed inner surface margin before rolling erasure and a limit drawability;

FIGS. 5 (a) and 5 (b) are schematic views of cross sections of a rolled bead in a case without bead rolling and in a case with bead rolling, respectively.

Claims (3)

(57) [Claims] 1. A steel strip is supplied to a group of forming rolls, continuously formed into an open pipe shape, and pressed by a squeeze roll provided so that both end surfaces are brought into contact with each other. In the pipe welding method of obtaining a welded pipe, the vicinity of the end face to be welded is preheated to 250 ° C. or more before laser irradiation to form at least an inner surface extra height 0.15 mm or more,
A method of manufacturing a welded steel pipe having high workability, characterized in that after welding is completed, a weld bead portion is lowered in a thickness direction to eliminate the extra height of the inner surface. 2. The method according to claim 1, wherein the elimination of the extra height of the inner surface is performed in-line at the same speed as the welding. 3. The method for producing a weldable steel pipe with high workability according to claim 1, wherein at least the vicinity of the welded portion is heat-treated after elimination of the extra height of the inner surface.