JP2864338B2 - Flash welding method for high strength steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a tensile strength (hereinafter referred to as TS).
In the flash welding method of high tensile steel of more than 600MPa and 2000MPa or less, particularly in the welding process of steel material such as continuous pickling and rolling of steel material strip, welding of products using steel material such as wheel rim, etc. It relates to welding in which a welded portion undergoes bending or processing after welding. Furthermore, specifically, in a method of performing flash welding by buttping steel plates, etc., the mechanical quality of the welded portion is reduced by suppressing the residual oxide that is a starting point of cracking in the welded portion and improving the structure of the welded portion. It relates to a welding method to be improved.

[0002]

2. Description of the Related Art A flash welding method utilizes a butt end face by utilizing Joule heat generated by a current flowing through a welded portion when a butt end face of a material to be contacted and arc heat generated after the contact portion is melted and scattered. Is heated and then the end faces are pressed together. Regarding the principle of this welding method, also called flash welding method or flash butt welding method,
It is also described in -118282.

Conventional flash welding has been disclosed in
As disclosed in Japanese Unexamined Patent Publication No. -275581, for welding in a manufacturing process such as continuous pickling of a steel material, oil is applied to the vicinity of a butt portion of a material to be welded, and the vicinity of the weld portion is exposed from the atmosphere during welding. Shielding and reduction of residual oxides at the bonding interface have been performed, and a method of applying oil has been developed as disclosed in Japanese Patent Application Laid-Open No. 63-203281. Also, Japanese Unexamined Patent Publication No.
Japanese Patent No. 18282 discloses a welding method in which a material containing zinc or zinc powder, or an alloy or an intermetallic compound containing calcium or magnesium, or a material containing the same is adhered to the vicinity of a butt portion and has the same effect as oil. .

[0004] As disclosed in Japanese Patent Application Laid-Open No. 50-51941, heat treatment of a flash weld is also performed. Usually, a strip-shaped steel material is welded, and after the flash metal is ground, annealing is often performed by high-frequency induction heating outside the welding machine. Also, Japanese Patent Application Laid-Open No. 55-360
No. 17, JP-A-55-36018 discloses a heat treatment method using a welding power source.

[0005]

However, the conventional methods have various problems, especially TS600MPa.
In the welding of steel materials exceeding the above, the reliability of the mechanical quality of the weld was not always sufficient. That is, in the method disclosed in JP-A-59-118282,
There is a problem that the mechanical properties are poor due to too high a weld hardness, and the method disclosed in Japanese Patent Application Laid-Open No. 50-51941 has a problem that inclusions remain at the joint interface. Therefore, when the strip-shaped steel material was butt-butted and flash-welded with the conventional technology, and passed through an pickling line or a cold rolling line, the welded part was broken during the passing of the plate, which significantly increased the production cost of the steel material. .

Hereinafter, the problems of the prior art will be specifically described. Even if oil, zinc, calcium, or magnesium is applied near the butt of the materials to be welded and welded (hereinafter collectively referred to as shield welding), the effect of shielding the area around the weld during welding is not always perfect. Therefore, in a steel material exceeding TS600 MPa containing a relatively large amount of strongly oxidizable elements such as Si and Mn, oxides of strongly oxidizable elements such as Si and Mn remain to some extent at the butt joint interface even after the welding process. Resulting in.

[0007] By the way, TS600MPa more than 2000M
In a steel material of Pa or less, the C content is high, so that the microstructure of the flash weld becomes a martensite or bainite structure. These structures have high notch susceptibility, and if oxides remain at the joint interface, cracks will occur starting from oxide defects at the joint interface when bending stress is applied to the welded part, such as in strip passing. A problem that occurs and propagates to form a large crack, and as a result, a welded portion is broken during threading.

On the other hand, even if ordinary flash welding is performed without performing shield welding and then heat treatment is performed on the welded portion, oxides such as Si, Mn, and Al remaining at the joint interface do not disappear. Therefore, when bending stress is applied to the welded part in strip passing and the like, many small cracks are generated at the joining interface softened by heat treatment starting from the oxide at the joining interface, and these gradually increase during the passing and interconnect with each other. Finally, there is a problem that the weld may be broken.

[0009] For the above reasons, in the steel material manufacturing process, high strength steel materials of TS600MPa or more and 2000MPa or less are not continuously butted against each other, and such high strength materials are connected to mild steel. In many cases, the steel sheet was passed alternately with mild steel.

The present invention has been made in view of the above circumstances, and has as its object to provide a flash welding method capable of realizing a weld having excellent mechanical quality.

[0011]

The present invention solves the above-mentioned problems, and has a tensile strength of more than 600 MPa and 2000 M.
In the flash welding method for steel materials of Pa or less, oil is applied to the vicinity of the butt portion of the material to be welded, flash welding is performed while burning the oil during welding to shield the periphery of the welded portion from the atmosphere, and after welding, the welding is performed. Flash
Ground until it is about the same height as the base metal surface.
Immediately afterward, the welding part is directly
Performing contact energizing a flash welding method of the high strength steel, characterized in that a heat treatment at a temperature below the A ₁ transformation point of 400 ° C. or higher steel material of the weld portion.

[0012]

According to the present invention, (1) by applying oil in advance to the vicinity of the butted portion of the members to be welded and performing flash welding, oxides remaining at the joint interface are suppressed as much as possible. The heat treatment reduces the hardness of the weld and reduces the notch sensitivity. And these two
By combining the two functions and effects, the quality of the weld is significantly improved.

[0013] In the following, the present invention will be described in detail. Since the oil applied near the butt weld is burned during welding during welding to shield the periphery of the weld from the atmosphere, the amount of oxide remaining at the joint interface is reduced, and the number of defects serving as crack initiation points is reduced. Further, the heat treatment is applied to the welded portion to reduce the notch sensitivity of the welded portion, so that the small number of oxide inclusion defects do not induce large cracks. Further, since the amount of oxide at the joint interface is sufficiently reduced by shield welding, even if these individual defects are expanded to some extent, they are not connected to each other and eventually do not lead to a large crack.

As will be described in detail in Examples, the effect of improving the mechanical properties of the welded portion when shield welding and heat treatment are combined is compared with the case where these are not used and the case where one of them is used. Some are remarkable. This is because there is little oxide at the joint interface due to shield welding,
As a result of thinning, these oxides are decomposed by heat treatment,
It is thought that this was due to the disappearance.

As the oil used for the shield welding, any kind of oil may be used as long as it is not highly inflammable such as gasoline and has no danger of explosion. At this time, if the viscosity of the oil is high, such as grease, it is only necessary to apply it near the welding end face before welding. Just spray it.

The flash welding conditions are not particularly limited.
Normal welding conditions can be used. TS600MPa
In the welding of the following steel materials, only one of the shield welding and the heat treatment after the welding need only be adopted, and is not an object of the present invention. In addition, TS2000MPa
However, even if the present invention is applied to steel materials exceeding the above, the effect of improving the mechanical properties of the weld is not so high. this is,
This is considered to be because micro-cracks occurred in the martensite structure generated in the welded portion, and the mechanical properties of the welded portion could not be improved by heat treatment after welding.

[0017] Incidentally heat treatment method can be classified into several, by adopting a tempering at a temperature of ₁ transformation point 400 ° C. or higher A, it is possible to shorten the heat treatment time. Austenite transformation point (A ₃ points, 0.2w
In the annealing process of raising the temperature beyond 845 ° C.) in t% C material, A ₃ constant transformation point or higher time Hojogo, must be cooled to manage the cooling rate, to suppress the hardness of the final structure low Requires a long cooling time. In contrast, the heating temperature in the tempering of the martensitic structure to the following A ₁ transformation point is the martensite is unstable phase decreases weld hardness by decomposing the ferrite and carbides, after heating There is an advantage that heat treatment can be completed in a very short time without having to control the cooling rate. At this time, the tempering treatment itself can be performed at a temperature of 200 ° C. or more and 400 ° C. or less, but a long holding time is required to sufficiently reduce the hardness. Therefore, in the present invention, the heating temperature is set to 400 ° C. or higher in order to shorten the heat treatment time.

Furthermore, if the applying electricity directly using a welding power source, there is no need to install a separate heat treatment apparatus,
Equipment costs can be reduced. Usually, the capacity of the power source for flash welding is too small to perform heat treatment by direct energization, and requires a very long time. However, direct energization and tempering immediately after flash metal grinding can be performed economically because the heat treatment can be performed in a short time without increasing the capacity of the welding power source for the following reasons. is there.

The first reason is the flash immediately after metal grinding high temperature of the weld, unless heated by energization since this
The difference with the heat treatment temperature at which the heat treatment does not occur can be small . In fact, since flash metal is hard to grind when cooled to room temperature, it is ground at a very high temperature. As a result of measuring the temperature with various welding devices, the surface temperature of the welded portion after flash metal grinding was about 300 ° C.

[0020] Second, the electrical resistance of the steel material is dependent on the material temperature, the electric resistance value as the temperature becomes higher tends to increase. Also, the thermal conductivity decreases. As a result, the higher the temperature immediately before energization, the higher the temperature rise in a shorter time.

[0021] Finally, why must grind the flash metal upon direct current is as follows. That is, when there is a flash metal, just the cross-sectional area is increased by the butt welding portion to be heated, it takes longer to warm current density will decrease at the butt portion proximity during direct current supply, butt have the size of the cross-sectional area This is because the heat generated in portions other than the portions increases, and when the butted portions are raised to a predetermined temperature, burn-through occurs in portions other than the butted portions. The flash metal at the weld is usually ground using a cutting tool until it is flush with the base metal surface. A specific grinding method is described in Japanese Patent Application Laid-Open No. 5-138435.

[0022]

EXAMPLE 0.14% by weight of carbon, 0.544% of silicon by weight.
%, Manganese 2.51%, and other trace addition elements, plate thickness 3.0mm, plate width 1500mm and tensile strength 10
A hot-rolled carbon steel strip of 00 MPa was butted, grease was applied near the butted portion, and flash welding was performed. At this time, the welding conditions were a flash time of 8 seconds, a flash length of 14 mm, and an upset allowance of 3.0 mm.

[0023] After the flash welding, the flash metal of the welded portion was ground until it is flush with the surface of the base material by the bytes of the welding machine accessory. The weld temperature after flash metal grinding was 320 ° C. As a sample for evaluating the quality of the welded part, after the flash metal grinding, the welded specimen was taken out of the welding machine, cooled to about room temperature, and heated by high-frequency induction heating until the sheet surface temperature reached 600 ° C in about 30 seconds. It was heated and then allowed to cool (Example 1 of the present invention).

Furthermore, as Example 2 of the present invention, after the flash metal grinding it was subjected to tempering the weld with the welding power source. The temperature of the weld at the start of energization was 280 ° C., and the tempering target temperature (600 ° C.) was reached in about 5 seconds. Immediately after that, energization was stopped and the sample was allowed to cool.

For comparison, only flash welding was performed under the above conditions, the flash metal was ground with a bite provided with a welding machine (Comparative Example 1), grease was applied and welding was performed under the above conditions. Flash metal was only ground with a bite and tempering was not performed (Comparative Example 2). Flash welding was performed under the above conditions without applying grease, and flash metal was ground with a bite attached to the welding machine, and then a welding power source was used. (Comparative Example 3) in which only the tempering treatment was performed by direct energization by the method described above.

In order to evaluate the mechanical quality of the weld, the hardness of the weld was measured, and 20 50 mm test pieces were cut out from the 1500 mm weld and subjected to a 180 ° bending test with a 6R punch. In the bending test, the lengths of individual cracks generated in the weld were measured, and the total of these was divided by the length of the specimen (50 mm) to represent the weld quality by percentage. At this time, in a normal pickling line or a cold rolling line, if the crack rate in a bending test using a 6R punch is 20% or less, the sheet can be passed.

As shown in Table 1, the hardness of the weld was sufficiently softened by tempering after welding.

[0028]

[Table 1]

Further, results of the bending test are as shown in FIG. 1, all materials in Comparative Example 1 undergoes a total cracking, in Comparative Example 2 was subjected to oil-shielded welding, although there is also a low cracking rate specimen, Cracks propagated from the oxide remaining at the joint interface as a starting point, and on average, only a slight improvement in mechanical quality was observed as compared with Comparative Example 1 in which normal welding was performed.

In Comparative Example 3 in which only tempering treatment was performed after welding, there was no test piece having a low cracking rate, but since the welded portion was softened, cracks generated at the joint interface were not largely propagated. On average, the mechanical quality was judged to be better than Comparative Example 2.

On the other hand, with oil-shielded welding, the present invention Examples 1 and 2 was subjected to post-weld tempering, not only less occurrence itself of cracking, never generated cracks is large propagation, very good mechanical Indicated quality.

[0032]

As described above in detail, according to the present invention, the bending strength or the tensile strength at which the working is applied to the weld after welding is 600M.
In flash welding of a steel material having a pressure of more than Pa and not more than 2000 MPa, the quality of the welded portion can be greatly improved in a simple manner , which is industrially advantageous, for example, in the manufacture of a steel material or a steel wheel rim.

[Brief description of the drawings]

FIG. 1 is a graph showing an example of mechanical quality when the present invention is used.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Yoshifumi Kobayashi 5-3 Tokaicho, Tokai City, Aichi Prefecture Nippon Steel Corporation Nagoya Works (72) Inventor Nobuyuki Kino 5-3 Tokaicho, Tokai City, Aichi Prefecture Nippon Steel Corporation Nagoya Works (56) References JP-A-1-186282 (JP, A) JP-A-51-142446 (JP, A) JP-A 2-93085 (JP, U) Hei 1-92320 (JP, U) (58) Field surveyed (Int. Cl. ⁶ , DB name) B23K 11/04

Claims (1)

(57) [Claims] 1. A tensile strength of more than 600 MPa and 2000 MPa
In the following flash welding method for steel material, oil is applied to the vicinity of a butt portion of a material to be welded, flash welding is performed while burning the oil during welding to shield the periphery of the welded portion from the atmosphere, and the welded portion is welded after welding. Flash metal
Is ground until it is substantially the same height as the surface of the base material, and immediately thereafter , current is directly applied to the weld using the power source for flash welding, and the weld is heated to 400 ° C. or higher. flash welding method of the high strength steel, characterized in that a heat treatment at a temperature below the a ₁ transformation point.