JPS59212767A - Method for discriminating weld zone of steel plate for continuous heat treatment - Google Patents

Abstract

PURPOSE:To discriminate exactly whether a seam weld zone is defective or non-defective by cutting a part of the seam weld zone in the direction orthogonal with a weld line, sampling a test specimen from the section thereof, etching the section of the specimen and evaluating the metal flow of the weld zone. CONSTITUTION:A sectional test specimen in the direction A orthogonal with the weld line L of a welded material 1 obtd. by welding the same materials under the same condition as in the actual operation and the section of said specimen is etched then the metal flow of the weld zone is evaluated. Whether the weld zone is defective or non-defective is thus exactly discriminated.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for determining the weld seam of a steel plate (steel strip) fed into a continuous heat treatment furnace.

In recent years, continuous annealing methods have been adopted for heat treatment (annealing) of cold-rolled steel strips used for automobile bodies, etc.
It has had great effects. This continuous annealing equipment is roughly divided into inlet equipment such as a payoff reel, welding machine, and cleaning equipment; a furnace body consisting of a heating zone, a soaking zone, a primary cooling zone, an overaging zone, and a secondary cooling zone; It consists of rolling mill, shear, take-up reel and other exit equipment.

However, the steel strip supplied to the above-mentioned continuous annealing equipment is attached to a payoff reel in the form of a coil of a certain length, but since annealing is performed continuously, once one coil has been unwound, the rear end of the steel strip is attached to the payoff reel. It is connected to the tip of the next steel strip coil by welding. As shown in Fig. 1, this welding machine usually consists of a pair of disk-shaped electrode rollers 2A and 2B that pinch and rotate the stacked materials 1 to be welded, and a current-carrying part for passing current through them. machine is used.

When seam welding steel plates for continuous annealing, extremely severe conditions are imposed on the seam welding, considering the severe situation in which the seam weld is repeatedly bent by upper and lower hearth rolls in the high-temperature atmosphere in the furnace.

For example, in narrow lap seam welding (the lap thickness of the steel strip is preferably as narrow as possible), the welding conditions are (1) welding current (kAmp), (2) sheet lap width (m
m), (3) welding speed (mpm), (4) electrode pressing force (
kg), (5) Swaging force (kg), (6)
The compensate amount (mm) is mentioned, and among these, a particularly important condition is the welding current value. If the current is insufficient, there will be insufficient welding and the connected plates will separate, while if the current is too much, welding burrs will occur, and in extreme cases, holes will be drilled in the plates.

Therefore, when performing continuous annealing of a steel strip, it is necessary to seam weld the steel strip under specific welding conditions (especially current value) in advance, and check whether the seam weld is appropriate or not, in other words, whether it can withstand continuous annealing. It is necessary to determine whether or not the welding conditions are met, and to set welding conditions based on the determination result.

In general, mechanical tests such as tensile tests and Erichsen tests are considered to be easy methods for determining the suitability of welded parts, but they are subjected to repeated hot bending by hearth rolls in a furnace such as a continuous annealing furnace. In some cases, these mechanical test methods are inadequate for the following reasons.

First, in a tensile test, there is almost no correlation between tensile strength and hot repeated bending strength, so it is inappropriate as a judgment method. In addition, in the Erichsen test, a steel ball is pressed against the weld line L of the welded steel plate 1 as shown in Fig. 2, and the quality is judged from the cracked part as shown in Fig. 3 (in the figure, (a) ) is peeling from the welded part,
(b) shows a crack in the direction perpendicular to the weld line, and (c) shows a crack from the heat affected zone. ) This method is a simple method, but if the strength of the base metal is low, even if it is not well welded, (b), (c)
occurs. Further, (a) occurs only in the case of extreme current shortage. For the above reasons, this Erichsen test cannot be said to be an accurate evaluation method, and weld peeling is likely to occur under current conditions set based on this test.

The present invention was developed in view of the above circumstances, and its purpose is to be able to accurately judge the quality of seam welds of steel strips, and to provide continuous heat treatment that undergoes repeated bending under high-temperature conditions. An object of the present invention is to provide a seam weld determination method that is optimal for determining welds in steel plates for industrial use.

The details of the present invention will be explained below.

When applying the method of the present invention, for example, before starting continuous annealing work, it is necessary to make sure that the welded part is sufficiently durable in the furnace under the welding conditions (especially welding current) set in the seam welding machine installed on the inlet side of the annealing furnace. Now is the time to decide whether you can do it or not.

In such a case, as shown in Fig. 2, in weld material 1 obtained by welding the same material as in actual operation under the initial welding conditions, (A
), cut and collect a cross-sectional specimen in the direction perpendicular to the weld line L, and etch the cross-section of the specimen with an etching solution,
The metal flow in the welded part is evaluated, and based on the quality of the metal flow state, it is determined whether the initial welding conditions, especially the welding current value, are appropriate.

This method of etching the cross section of a steel material and observing its metal flow has been used in the past for determining the cross section of thick plates, but as in the present invention, it is possible to determine the cross section of a thin steel plate passed through a vertical continuous annealing furnace. It is not used for target determination. It has been thought that simple tensile tests, Erichsen tests, etc. are sufficient to judge welds on thin steel plates, but as mentioned above, these tests cannot accurately determine the quality of boom welds on continuous annealing steel plates. . The present invention employs the metal flow method to judge the seam welds of thin steel sheets passed under harsh conditions such as continuous annealing furnaces, and uses the judgment results to determine the appropriateness of seam welding conditions, especially welding current values. It is characterized by being connected.

Specifically, the determination method of the present invention involves cutting out a specimen of a seam weld of a desired size, polishing the cross section, and then immersing it in an etching solution to clearly identify the flow of metal and the joint state of both joining members in the cross section. After that, from this cross section, it is determined whether there is insufficient heat input or excessive heat input, for example, by microscopic observation, and if there is, the welding current value is corrected to high or low.
Ensure proper metal flow. Any etching solution containing picric acid and ethyl alcohol as main components may be used.

FIG. 4 is a graph showing the results of comparing the set current values using the Erichsen test and the metal flow determination method of the present invention, plate lap distance: 2 mm, welding speed: 10 mp.
m condition, h_1 on the horizontal axis is the thickness of the preceding plate (mm), h_
2 is the thickness of the trailing plates (mm).

From this figure, Erichsen evaluation shows that the current value is generally low, and there is a tendency for heat input to be insufficient.

Next, examples of the present invention will be given.

Steel plate JIS, SPCC (width 12
0mm×thickness 0.8mm) with a lap width of 2mm, a welding speed of 10mpm, a welding current value of 15kA, and an electrode pressure of 75.
Welding was carried out using the seam welding machine shown in Figure 1 at 0 kg.

When this welded material was evaluated for quality by pressing a steel ball against the weld line as shown in FIG. 2 and subjected to an Erichsen test, it was judged to be good in all directions. When a current value that passed the Erichsen test was set and applied to actual annealing work, peeling of the weld was observed.

Therefore, a sample was cut out from the welded material as shown in Fig. 2, and the sample was etched with an etching solution to observe the cross-sectional metal flow.
As shown in the figure. As shown in the figure, it can be seen that the melting of the welded portion is insufficient and the joint state is poor.

Next, using the same steel type and size as the materials mentioned above,
In addition, specimens were taken from the welded parts obtained by seam welding with only a high current value (18 kA) among the welding conditions, and the cross section was observed using the metal flow method.

The cross-sectional sketch diagram is shown in FIG. 6, and the joint was in good condition. When the weld was put into practical use under the same conditions, no problems occurred at the weld.

As explained above, according to the welded part determination method of the present invention, it is possible to make a determination that accurately reflects the welding situation, so it is useful for determining the quality of welded parts of steel plates that are subjected to repeated bending at high temperatures such as continuous annealing. Optimal.

[Brief explanation of the drawing]

Figures 1 (a) and (b) are front and side views showing an overview of the seam welding machine, Figure 2 is an explanatory diagram of the welded part, Figure 3 is an explanatory diagram of Erichsen evaluation, and Figure 4 is an illustration of the present invention. 5 and 6 are cross-sectional sketch diagrams showing the metal flow at the weld joint.

Claims (1)

[Claims] When determining in advance the seam welds at the ends of the leading and trailing steel strips to be passed through the continuous heat treatment furnace, a section of the seam weld is cut in a direction intersecting the weld line and a cross-sectional specimen is taken. A method for determining a welded part of a steel plate for continuous heat treatment, characterized in that the cross section of the specimen is etched with an etching solution to evaluate metal flow in the welded part.