JPH10263863A - Laser tack welding method with constant melting depth - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the generation of spiking and burn through and to eliminate dispersion in molten depth by specifying the speed scanning a laser beam along a butting line and the frequency and amplitude oscillating the same in the direction orthogonal to the butting line and stack welding the tail end part of the preceding slab and the succeeding slab in continuos hot rolling. SOLUTION: The laser beam is scanned at the welding speed of 2-10 m/min along the butting line. Also, the laser beam is oscillated at the frequency of 40-80 Hz and the amplitude of 0.4-1.0 mm in the direction orthogonal to the butting line. Since the laser beam is oscillated at high speed, the dot-like heat source A1 formed in the bottom part of a keyhole is enlarged more than the dot-like heat source A0 in the conventional method. Properties of the dot-like heat source are stabilized and the depth of the secondary molten part due to this is stabilized by making the energy density of the dot-like heat source small like this.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for joining steel slabs such as slabs or sheet bars by continuously hot rolling them to produce hot rolled steel sheets or strips.

[0002]

2. Description of the Related Art In a hot rolling line, in order to continuously hot-roll a slab such as a slab or a sheet bar, a rear end of a preceding rolled material being rolled and a front end of a subsequent rolled material are rolled. There is a method of joining and continuously rolling the joined steel slab. For example, WO. No. 94/6838 discloses that after the rear end of a preceding rolled material being rolled and the front end of a succeeding rolled material are butted, the butted portion is tack-welded with a laser beam to continuously perform hot rolling. A method of doing so is disclosed.

Here, the principle of laser welding will be described. When the laser beam is condensed and radiated onto the steel slab so as to have a sufficient power density, a keyhole as an evaporation hole is formed at the irradiation point. Using the keyhole as a heat source, the surrounding steel slab is melted to form a fused portion. Here, if the laser beam is scanned along the welding line, the keyhole will be scanned, and a fused portion is formed along the welding line, and this is solidified to complete the laser welding.

[0004] Laser welding is suitable for tack welding in a hot rolling line in terms of high-speed processing, deep penetration, and the like, but has a drawback that misalignment tends to occur. In order to solve this drawback, Japanese Patent Application Laid-Open No. Hei 8-257
No. 774 discloses that when a butt portion is temporarily welded with a laser beam, the laser beam is scanned at a speed V along a butt line and the laser beam is vibrated at an average speed v perpendicular to the butt line. , 1 ≦ V / v ≦ 2.5 m
There is disclosed a method for preventing disconnection at the time of laser tack welding by setting / min ≦ V ≦ 20 m / min.

[0005]

However, when laser tack welding is performed on a hot-rolled steel slab, there has been a problem that spiking in which the melting depth is partially increased and further burn-through occurs. FIG. 1A is a sectional view of a laser irradiation point in laser tack welding. When the laser beam 3 is irradiated on the butted portion of the hot-rolled steel slab 1, a keyhole 5 is formed at the irradiation point, and the primary melting portion 7 is melted using the keyhole 5 as a heat source. On the other hand, the laser beam 3 is multiple-reflected inside the keyhole 5 and converges at the bottom of the keyhole 5 to generate a point-like heat source 6. This point heat source 6 forms a secondary fusion zone 8 below the primary fusion zone 7.

Here, the properties of the point-like heat source 6 are not constant, but depend on the internal shape of the keyhole 5, the effect of the evaporated metal plasmaized in the keyhole 5, the amount of heat output to the secondary melting portion 8, and the like. Change. For this reason, the depth of the secondary fusion part 8 partially increases, and spiking of the fusion part 4 and further burn-through occur.

In laser tack welding, particularly in a hot rolling line, a metal easily reaches a boiling point by a laser and is turned into plasma. When the plasma absorbs the laser beam, the amount of heat input to the hot-rolled steel slab, which is the workpiece, changes, and the penetration depth fluctuates, which may cause spiking.

FIG. 1B shows the shape of the melted portion in which spiking and burn-through have occurred as described above. FIG.
(B) has shown the shape of the fusion part in an abutting surface. As described above, the depth of the fusion portion 5 is not uniform, and even if a portion having a small fusion depth is intended to obtain a sufficient fusion depth,
It is feared that burn-through 13 occurs and the contact area ratio is extremely reduced.

An object of the present invention is to provide a method of laser tack welding of a hot-rolled steel slab which does not cause spiking or burn-through and has a uniform melting depth.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a method for producing a hot-rolled slab or strip by joining a plurality of hot-rolled slabs and continuously hot rolling the slabs. In the method of butting the butt portion with the tip of the subsequent billet and temporarily welding the butt portion with a laser beam, the laser beam is scanned along the butt line at 2 to 10 m / min during the laser welding, and the laser beam is Is vibrated in a direction perpendicular to the butt line at a frequency of 40 to 80 Hz and an amplitude of 0.4 to 1.0 mm.

As shown in FIG. 2, the present inventors define the variation Bd (mm) of the fusion zone and the average fusion depth d (mm), and use the variation ratio (Bd) as an index of the variation of the fusion depth.
/ D × 100), a method for reducing the variation in the melt depth was studied. First, in order to examine the relationship between the welding speed and the variation rate, an experiment was conducted in which a laser welding of a butt portion of a billet at 1000 ° C. was performed using a CO ₂ laser having an output of 14 kW. The results are shown in FIG. 3, but the variation rate could not be reduced even when the welding speed was changed. On the other hand, an experiment was conducted in which laser welding was performed at a welding speed of 3 m / min while oscillating the laser beam in a direction perpendicular to the butt line at 50 Hz. The results are shown in FIG. 4. The variation rate is reduced by vibrating the laser beam in the direction perpendicular to the butting line. In this experiment, in particular, amplitudes 0.4 to 1..
Within the range of 0 mm, the effect of reducing the variation rate is high, and the amplitude 0.7
The effect of reducing the variation rate is maximum at mm.

Therefore, the present invention provides a method for scanning a laser beam along a butt line at a rate of 2 to 10 m / min, and for applying a laser beam in a direction perpendicular to the butt line at a frequency of 40 to 8 m / min.
Vibration was performed at 0 Hz and an amplitude of 0.4 to 1.0 mm. Hereinafter, a description will be given of a mechanism of reducing the variation rate of the melt depth by vibrating the laser beam in a direction perpendicular to the butt line.

FIG. 5A is a sectional view of a laser irradiation point according to the present invention. FIG. 5B is a sectional view of a laser irradiation point by a conventional laser tack welding performed under the same conditions except that the laser beam is not vibrated. FIG. 5 (a)
And FIG. 5 (b) and as is apparent from a comparison of the melt width W ₁ of the present invention is wider than the melting width W ₀ of the conventional method. On the other hand, compared with the conventional method having the same welding speed, the fusion depth of the present invention is smaller.

FIG. 5 also shows the behavior of the laser beam multiple-reflected inside the keyhole. Since the vibrating the laser beam at high speed in the present invention, the point-like heat source A ₁ which is formed in the key hole bottom portion is enlarged than the point-like heat source A ₀ in the conventional method, the energy density becomes smaller. The present invention stabilizes the properties of the point-like heat source by reducing the energy density of the point-like heat source in this way, and stabilizes the depth of the secondary fusion zone derived therefrom.

In the present invention, the laser beam scans along the butt line at 2 to 10 m / min. The reason why the welding speed was set to such a range was that if the welding speed was less than 2 m / min, the heat input to the steel slab was too large and the burn-through occurred in the welded portion. Conversely, the welding speed was 10 m / min. This is because a sufficient welding area cannot be secured if the value is excessive.

The laser beam has a frequency of 40-80 Hz and an amplitude of 0.4-1.
Vibrate at 0mm. Frequency 40-80Hz
The reason is that if the frequency is less than 40 Hz, the effect of the present invention cannot be obtained simply because the melted portion meanders.
This is because the effect is saturated even when the value exceeds z. The reason why the amplitude is set to 0.4 to 1.0 mm is that if the amplitude is less than 0.4 mm, the effect of the present invention cannot be obtained because the expansion of the point heat source is insufficient.
If the thickness exceeds 0 mm, the melting depth becomes insufficient.

[0017]

Embodiments of the present invention will be described below. The target of the laser tack welding method of the present invention is a steel slab having a thickness of 25 to 50 mm, and is mainly a sheet bar after rough rolling. The leading end and the trailing end of such a billet are cut by, for example, a running shear to adjust the shape, and the trailing end of the preceding billet traveling on the line and the leading end of the succeeding billet are abutted. Laser tack welding is performed, and it is subjected to continuous processing in a subsequent step. In this case, the temperature of the sheet bar as the workpiece is about 900 to 1150 ° C.

As a laser oscillator used in the laser tack welding method of the present invention, for example, a CO having an output of 14 to 45 kW is used.
_Two laser oscillators are suitable. The beam diameter on the laser emission side of such a laser oscillator is 60 to 100 mm, and this is condensed and irradiated on the surface of the workpiece to a diameter of 0.4 to 0.8 mm, so that the laser energy on the surface of the workpiece is The density becomes 10 to 40 MW / cm ² . By irradiating the laser with such an energy density, a keyhole is generated at the laser irradiation point, and the laser irradiation point can be moved along the butt line to perform deep penetration laser welding.

Here, in the present invention, the laser beam is vibrated in a direction perpendicular to the butting line. That is, the laser irradiation point on the steel plate surface is vibrated perpendicularly to the butting line. FIG. 6 shows an example of a method of vibrating the laser irradiation point. FIG. 6 is a schematic diagram of a welding head portion that vibrates the laser irradiation point perpendicularly to the butting line L of the steel slab 1. The welding head includes a condenser mirror 21, a mirror 23, a mirror oscillation shaft 28, and a mirror oscillation mechanism 25. The laser beam 3 incident on the welding head is given a convergence angle by a condenser mirror 21 fixed on the welding head, and is reflected and transmitted to the mirror 23. Mirror 23
Is supported by a mirror vibration shaft 28 and vibrates by a mirror vibration mechanism 25. By oscillating the mirror 23 in this manner, the laser irradiation point can be oscillated perpendicularly to the butting line L.

Although FIG. 6 shows an example in which the mirror 23 is vibrated, the mirror 23 may be fixed and the light-collecting mirror 21 may be vibrated. Also, when condensing and irradiating a laser beam using a transmission optical system, for example, a transmission optical system such as a condenser lens is shaken in a circular shape or perpendicular to the optical axis of the condenser lens. What is necessary is just to support a lens by a suitable axis and to vibrate about an axis.

[0021]

EXAMPLE A laser tack welding of SS41 steel at about 1000 ° C. was carried out in accordance with the method of the present invention using a CO ₂ laser having an output of 45 kW. At this time, welding was performed while spraying helium as a shielding gas to shield from the atmosphere. The welding speed was 3 m / min, the laser beam vibration frequency was 50 Hz, and the laser beam vibration amplitude was 0.4.
mm. As a result of performing the laser tack welding as described above, the variation rate (Bd / d) was about 7%. On the other hand, the variation rate (Bd
/ D) was 15%.

Further, even in the butt welding of hot rolled steel slabs, no melt-through of the molten metal occurred, and the joint area ratio did not decrease.

[Brief description of the drawings]

FIG. 1A is a sectional view of a laser irradiation point in laser tack welding. (B) Drawing which shows the shape of the fusion zone where spiking and burn-through occurred.

FIG. 2 is an explanatory diagram of a variation Bd (mm) of a fusion zone and an average fusion depth d (mm).

FIG. 3 is a table showing a correlation between a welding speed and a variation rate.

FIG. 4 is a table showing a correlation between a laser beam vibration amplitude and a variation rate.

FIG. 5A is a sectional view of a laser irradiation point according to the present invention. (B) It is sectional drawing of the laser irradiation point by a conventional method.

FIG. 6 is a drawing showing an example of a method of vibrating a laser irradiation point.

[Explanation of symbols]

REFERENCE SIGNS LIST 1 billet 3 laser beam 4 fusion part 5 keyhole 6 point heat source 7 primary fusion part 8 secondary fusion part 11 unwelded part 13 burn-off 15 surface dent W ₁ fusion width of present invention W ₀ fusion width of conventional method A ₁ Point heat source of the present invention A ₀ Point heat source of conventional method

Claims (1)

[Claims] When a plurality of hot-rolled billets are joined and continuously hot-rolled to produce a hot-rolled billet or a strip, a rear end portion of a preceding billet and a front end of a succeeding billet are produced. In the method of butting and joining the butted portions with a laser beam, the laser beam is laid along the butt line by 2 to 1 at the time of the laser welding.
Scanning was performed at 0 m / min, and the laser beam was applied in a direction perpendicular to the butt line at a frequency of 40 to 80 Hz and an amplitude of 0.1 Hz.
A laser tack welding method characterized by vibrating at 4 to 1.0 mm.