Electric heating – Metal heating – By arc
Reexamination Certificate
2002-09-25
2004-08-03
Elve, M. Alexandra (Department: 1725)
Electric heating
Metal heating
By arc
C219S121850
Reexamination Certificate
active
06770840
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method of butt-welding hot-rolled steel materials by a laser beam, during the production of a hot-rolled steel plate or a hot-rolled steel strip by continuously hot-rolling a steel material such as a slab or a sheet bar, and a welding nozzle, a filler wire supply nozzle and a welding apparatus.
BACKGROUND ART
In order to continuously hot-roll a steel material such as a slab or a sheet bar in a hot rolling line, the tail end of the preceding steel material has been butt-welded (tack welding) to the front end of the following steel material by a laser beam.
As shown in
FIG. 1
, the principle of a method of welding by a laser beam LB, which is a high energy beam, is to irradiate a workpiece S with a focused laser beam LB while a filler wire W is being supplied from a wire supply nozzle NW, thereby forming a keyhole K as a heat source which is a region having the highest energy density, and to scan the keyhole.
When the keyhole K which is a region where the metal is cylindrically evaporated is scanned, the peripheral molten region gradually solidifies as the laser beam LB passes to form a weld bead, and the workpiece S is butt-welded. The keyhole K is formed during laser welding by a balance between the vapor pressure and the force of gravity of the evaporated metal within the keyhole K. The evaporated metal generated on the keyhole K and the welding gas become a laser-induced plasma P. The mutual action of the laser-induced plasma P and the laser beam LB determines the energy incident on the workpiece S. The state of the mutual action of the laser beam LB and the plasma P changes momentarily, and the depth of penetration d increases and decreases in accordance with the change as shown in FIG.
2
. When the amount of the plasma is increased, the laser beam LB is absorbed by the plasma, and the laser beam LB cannot reach the workpiece S, thereby decreasing the depth of penetration d. On the other hand, when the amount of the plasma is decreased, the laser beam LB easily reaches the workpiece S to cause spiking, thereby increasing the depth of penetration d temporarily. In particular, when welding is conducted by laser with an output as high as at least 25 kW, the size of the plasma becomes extremely large, and the variation ratio of the depth of penetration d becomes significant and as large as from 25 to 30%.
In laser welding hot-rolled steel materials in a hot rolling line, the hot-rolled steel materials have a high temperature, of at least 900° C., at the beginning. The metal is therefore easily heated to its boiling point by laser irradiation, and becomes a plasma; the mutual action of the laser beam and the laser-induced plasma is activated to repeat expansion and contraction of the plasma, thereby destabilizing a plasma generation region. As a result, spiking frequently occurs, and a variation in the depth of penetration increases. When the increase and decrease in the depth of penetration are significant, ensuring a bonding area ratio which is aimed at non-fracture of the joint during rolling becomes very difficult in cases where the steel materials are butt welded after shearing in continuous hot rolling. Moreover, for groove butt welding, when the depth of penetration d becomes excessive due to spiking, the molten metal flows down as shown in
FIG. 2
, that is, so-called meltdown MD takes place. As a result, there arises the possibility that the bonding area ratio falls (the portion R in FIG.
2
).
Furthermore, WO. No. 94/6838 discloses a method of continuous hot rolling, which comprises butting the tail end of the preceding rolled material and the front end of the following rolled material against each other, and tack-welding the butted portions by a laser beam.
Although laser welding is suitable as tack welding in a hot rolling line in view of its high speed and its deep penetrating ability, it has the disadvantage of easily forming an offset. In order to overcome the disadvantage, Japanese Unexamined Patent Publication (Kokai) No. 8-257774 discloses a method of preventing an offset during tack-welding the butt portion with a laser beam, which comprises scanning the butt line with a laser beam at a speed of V while the laser beam is being oscillated in the direction vertical to the butt line at an average speed of v, wherein 1≦V/v≦2, 5 m/min≦V≦20 m/min.
However, even the method mentioned above cannot solve the problems of spiking and generation of meltdown.
FIG. 3
shows a more detailed cross-sectional view of the laser irradiation point in tack welding with laser. When the butt portion of hot-rolled steel materials S is irradiated with a laser beam LB, a keyhole K is formed at the irradiation point. The keyhole K acts as a heat source, and a primary molten portion M
1
is formed. On the other hand, the laser beam LB is multiply reflected in the interior of the keyhole K to converge to the lowest portion thereof and produce a spot-like heat source Q. A secondary molten portion M
2
is formed, under the primary molten portion M
1
, by the spot-like heat source.
The properties of the spot-like heat source Q are not constant, but change in accordance with the internal shape of the keyhole K, the influence of evaporated metal having become plasma within the keyhole K, the heat output to the secondary molten portion M
2
, and the like. As a result, the depth of the secondary molten portion M
2
partially increases so that spiking of the molten portion M or meltdown MD (
FIG. 2
) takes place.
Furthermore, in laser welding steel materials, when the base material S (
FIG. 1
) is a steel that contains metal elements such as Al, Si and Ti having deoxidizing effects in at least a given content, blow holes are hardly generated. However, when the content of a deoxidizing agent is low, blow holes are sometimes included in the weld zone. In particular, when high temperature materials are to be welded, for example, in a hot rolling line, scales (iron oxide) adhere to groove faces. When welding is conducted while the scales are being involved, blow holes are formed in the weld bead. The mechanism of the blow hole formation is as explained below: oxygen contained in the steel materials or the scales adhering to the steel material surface combines with carbon in the steel materials to form carbon monoxide in the melting step during welding, and the carbon monoxide is taken up in a bead in the solidifying step. The blow holes B in the weld bead WB not only lower the bonding strength of the hot-rolled steel materials S, but also relate to an increase and a decrease of a depth of penetration d as shown in
FIG. 4
; any of the blow holes can push a molten portion into the depth direction at the time of forming a carbon monoxide gas to temporarily increase the depth of penetration d by &Dgr;d. Spiking frequently takes place in accordance with the phenomena, and the variation in the depth of penetration Bd increases.
The ratio of the variation in the depth of penetration Bd to the average depth of penetration d as shown in
FIG. 2
is defined as a variation ratio Bd/d. In the conventional laser welding method, the variation ratio Bd/d is about 20%.
As shown in
FIG. 2
, a margin A has heretofore been left as a countermeasure against the variation in the depth of penetration explained above and, particularly, against meltdown MD. For example, when the plate thickness of the hot-rolled steel materials is 35 mm, the effective thickness of the butt portion is 25 mm, and the margin A left becomes 5 mm when the depth of penetration is 20 mm.
A method has heretofore been adopted which is not aimed at welding the substantial effective butt thickness of a workpiece to secure a bonded area ratio but which is aimed at decreasing a variation in the depth of penetration by suppressing a total input energy for the hot-rolled steel materials by, for example, lowering the laser output, decreasing the welding speed, and the like procedure.
However, all these countermeasures are taken at the cost of the ability of deep penetration and the ability of
Kido Motoi
Minamida Katsuhiro
Yamamoto Hiroyuki
Elve M. Alexandra
Nippon Steel Corporation
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