KR20200075336A - Resistance spot welding method for aluminium based plated steel - Google Patents

Abstract

The present invention relates to a resistance spot welding method of a plated steel material, and more specifically, to a resistance spot welding method of an aluminum-based plated steel material for hot press forming. The present invention includes the steps of: bringing an electrode in contact with the aluminum-based plated steel material; performing a pre-pulse; resting; and performing a main pulse.

Description

RESISTANCE SPOT WELDING METHOD FOR ALUMINIUM BASED PLATED STEEL

The present invention relates to a resistance spot welding method for a plated steel material, and more particularly, to a resistance point welding method for an aluminum-based plated steel material for hot press forming.

Recently, due to exhaustion of energy resources and high interest in the environment, regulations on improving fuel efficiency of automobiles are becoming more and more powerful, and safety of automobiles is also increasing. In order to satisfy this situation, interest in high-strength steel materials having a thin thickness is increasing.

However, while high-strength steel has high strength, there is a problem in that workability is deteriorated, and hot press forming (HPF) has been proposed to solve this problem. The hot press forming method (HPF) is a method of forming a low-temperature structure such as martensite in a steel material by processing the high-strength steel material at a high temperature to be processed and rapidly cooling it to a low temperature, thereby increasing the strength of the final product.

However, in the case of the hot press forming method, since the steel material needs to be heated to a high temperature, the surface of the steel material is oxidized, and there is a problem that a process of removing the oxide on the steel material surface after molding has to be added. As a method for solving this problem, a method of hot press forming a steel material subjected to aluminum plating has been used. Recently, in order to reduce mold contamination during hot press forming, metal heating is possible, and to reduce the melting of the plating layer, alloying of aluminum plating through pre-heating before hot press forming the aluminum plated steel material is followed by hot press forming. Methods are also proposed.

On the other hand, it is necessary to bond one or more steel materials in order to manufacture automobile parts, and in the automobile industry for manufacturing automotive parts, resistance spot welding (RSW) is generally used. The resistance spot welding is a process in which a current is flowed under pressure, and heat generated by a contact resistance generated at a contact surface between metals and an inherent resistance of the metal, and bonding is performed by pressure applied while the metal is heated and melted. Say the public method.

In the case of an aluminum plated steel having an aluminum plated layer alloyed through the pre-heat treatment, there is a problem in that weldability deteriorates as the surface resistance increases due to an increase in the oxide layer on the surface of the material due to the pre-heat treatment.

One aspect of the present invention is to provide a welding method capable of reducing spatter generation during resistance point welding of aluminum-based plated steel and securing an appropriate weld size.

The subject of this invention is not limited to the above-mentioned matter. Additional objects of the present invention are described in the overall contents of the specification, and those skilled in the art to which the present invention pertains will have no difficulty in understanding additional objects of the present invention from the contents described in the specification of the present invention.

An aspect of the present invention is a step of pressing an electrode on an aluminum-based plated steel material, and contacting the electrode with an aluminum-based plated steel material;

After the electrode is in contact with the aluminum-based plated steel, performing pre-pulse to be 75% or less of the initial contact resistance;

Rest after the pre-energization; And

After the pause, for a period of 200 ms or more, the present invention relates to a method for welding a resistance point of an aluminum-based plated steel material, comprising performing main current with 115% or more of the pre-current current.

In the case of welding the aluminum-based plated steel by the welding method according to the present invention, a sufficient size of the welded portion can be secured to meet the customer's needs, and there is an effect of reducing spatter generation.

1 is a graph showing a conventional method of resistance spot welding.
2 is a graph showing a resistance spot welding method of the present invention.

When the aluminum-based plated steel is heat-treated, the contact resistance increases due to the oxide layer present on the surface. As an example, after hot press forming (Hot Press Forming, HPF), when pre-heating an aluminum plated steel material having a tensile strength of 1470 MPa or higher according to Table 1 below, it can be seen that an oxide layer is formed on the surface, thereby increasing contact resistance. have.

Pre-heating conditions Contact resistance (mΩ) Before pre-heat treatment 0.1365 900℃, 220 seconds 0.2113 900℃, 600 seconds 0.5543 930℃, 220 seconds 0.4007 930℃, 600 seconds 1.4847

In general, the size of the weld is preferably 4*√ (material thickness) or more, based on the material thickness. For example, in the case of welding a steel material having a thickness of 1.4 mmt, it is preferable that the appropriate weld size is 4.73 mm or more.

Table 2 shows the results of resistance spot welding of an aluminum plated steel material having a thickness of 1.4 mmt and a contact resistance of 0.2113 mΩ in the conventional manner as shown in FIG. 1. The welding conditions at this time are as shown in Table 2 below.

number Welding time (ms) Welding current (kA) Spatter occurrence Weld area size(mm) One 400 4.0 × 0 2 400 4.4 × 0 3 400 4.8 × 0 4 400 5.5 × 3.74 5 400 5.6 × 5.45 6 400 5.8 ○ 6.13 7 400 6.0 ○ 5.63 8 400 6.2 ○ 5.23 9 400 6.4 ○ 6.43 10 600 3.2 × 0 11 600 3.6 × 0 12 600 4 × 4.21 13 600 4.4 × 4.56 14 600 4.8 × 5.09 15 600 5 ○ 5.18 16 800 3.2 × 0 17 800 3.6 × 0 18 800 4 × 4.36 19 800 4.4 × 4.85 20 800 4.8 × 5.44 21 800 5 ○ 5.70

Looking at the results of Table 2, it can be seen that in the case of 5, 14, 19, and 20, no spatter occurs and the size of the weld exceeds 4.73 mm. However, in the case of 14, 19, and 20, except for the case of 5, the welding time exceeds 400 ms, which is difficult to commercialize due to poor efficiency or productivity.

Therefore, except for the case of 5, there is a problem that welding conditions for efficiently securing a weld of an appropriate size without spattering are very limited.

As a result of deep research to solve this problem, the present inventors performed pre-pulse to apply a pre-current during resistance point welding, thereby forming a predetermined contact to reduce the contact resistance, and thereafter, Main pulse) was performed to devise a method to efficiently secure an appropriate size without generating spatter, and thus, the present invention has been completed.

Hereinafter, the present invention will be described in detail. The resistance spot welding method for an aluminum-based plated steel of the present invention is a step of pressing an electrode against an aluminum-based plated steel to contact the electrode with an aluminum-based plated steel, and pre-energizing after the electrode is contacted with the aluminum-based plated steel. And performing pre-pulse, stopping after the pre-energization, and performing main pulse after the pause.

First, the electrode is pressed against the aluminum-based plated steel prepared for welding. At this time, the pressing is preferably performed in a state where the electrode is brought into contact with the aluminum-based plated steel. The aluminum-based plated steel is, for example, an aluminum plated steel for hot press forming, and may have an oxide layer formed on its surface by pre-heating.

Next, it is preferable to perform pre-pulse by applying a current to the electrode that pressurizes the aluminum-based plated steel. The preliminary energization has a purpose of lowering the contact resistance during the main pulse by making a predetermined melting portion and a heat-affected portion between materials. This is because, if the contact resistance is low during the current application, a larger current can be made by applying a higher current without spattering. Therefore, the preliminary energization requires a quantity of heat capable of making the contact resistance lower than the initial contact resistance of the material, which is expressed as a heat quantity in the case of the material. The amount of heat input can be calculated as the product of the contact resistance of the material and the square of the current and time. The initial contact resistance means the contact resistance of the material before pre-energization.

It is preferable to perform the preliminary energization such that the contact resistance of the material after preliminary energization is 75% or less of the initial contact resistance. After the pre-energization, the lower the contact resistance of the material, the better, but it is preferable to conduct preliminary energization to be 65% or more of the initial contact resistance in order to form an appropriately sized weld through subsequent energization. .

After the pre-energization, it is preferable to rest. Spatter may be prevented through the tissue. At this time, the rest time is necessary because the spatter is easily generated due to the rapid increase in the internal pressure of the molten portion when the resistance heat is continuously generated in the welding portion. However, one cycle or more than 16ms is required depending on the equipment and control performance of the welding machine. The upper limit of the idle time is not particularly limited, but since it affects productivity, it is preferable to perform the entire welding time including 400% or less of the preliminary energization, the pause, and this energization.

After the pause, it is preferable to perform main energization. The present energization is to form a weld of a sufficient size. At this time, the size of the welding portion is preferably 4*√ (material thickness) or more. A part of the molten portion was formed by the pre-pulse, but this current is required to secure the required diameter of the weld. When applying the above-mentioned preliminary energization, the welding time of the current energization is preferably at least 200 ms, and the welding current is preferably performed at 115% or more of the preliminary energization.

When the welding time of the current supply is shorter than 200 ms, the welding current must be high to secure the size of the welding part, but in this case, spatters may occur and welding defects may be caused. The upper limit of the welding time of the current energizing is not particularly limited, but as described above, when considering productivity, it is preferable that the total welding time including pre-energizing, resting, and energizing is 400 ms or less.

When the welding current of the current supply is less than 115% of the pre-current supply, it is not preferable because the welding time must be excessively increased to secure the size of the welding section. However, the upper limit of the welding current of the current supply is not particularly limited, but when it exceeds 135% of the pre-current supply, spatter may occur, which is not preferable.

Hereinafter, embodiments of the present invention will be described in detail. The following examples are only for understanding the present invention and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the items described in the claims and the items reasonably inferred therefrom.

(Example)

A pre-heated aluminum plated steel having an oxide layer on the surface was prepared. The contact resistance of this aluminum plated steel was 0.2113 mΩ. The aluminum-plated steel material was subjected to pre-energization, pause, and main energization as shown in FIG. 2 to perform welding under the pre-energization, pause, and main energization conditions under Table 3.

At this time, whether spatter occurred during welding and the size of the weld after welding were measured, and the results are shown in Table 3 below.

number Preliminary energization downtime
(ms) Saw current Spatter occurrence Weld
Size (mm) Remark
(Judgment) Energization time
(ms) Current magnitude
(kA) Contact resistance after pre-energization (mΩ) Energization time
(ms) Current magnitude
(kA) One 180 4.6 0.1538 20 200 4.6 × 0 × 2 180 4.6 0.1538 20 200 4.8 × 3.8 × 3 180 4.6 0.1538 20 200 5 × 4.03 × 4 180 4.6 0.1538 20 200 5.2 × 4.16 × 5 180 4.6 0.1538 20 200 5.4 × 4.98 ○ 6 180 4.6 0.1538 20 200 5.6 × 5.29 ○ 7 180 4.6 0.1538 20 200 5.8 × 6.15 ○ 8 180 4.6 0.1538 20 200 6 × 5.92 ○ 9 180 4.6 0.1538 20 200 6.2 × 6.80 ○ 10 180 4.6 0.1538 20 200 6.4 Occur 6.29 × 11 180 4.0 0.1832 20 200 4.6 × 0 × 12 180 4.0 0.1832 20 200 4.8 × 0 × 13 180 4.0 0.1832 20 200 5 × 2.76 × 14 180 4.0 0.1832 20 200 5.2 × 3.12 × 15 180 4.0 0.1832 20 200 5.4 × 3.67 × 16 180 4.0 0.1832 20 200 5.6 × 4.54 × 17 180 4.0 0.1832 20 200 5.8 × 4.98 ○ 18 180 4.0 0.1832 20 200 6 Occur 5.92 × 19 180 4.0 0.1832 20 200 6.2 Occur 6.80 × 20 180 5.0 Spatter 20 200 4.0 - 0 × 21 180 5.0 Spatter 20 200 3.5 - 0 × 22 150 5.0 Spatter 20 230 4.0 - 0 × 23 150 5.0 Spatter 20 230 3.5 - 0 × 24 280 4.6 0.1365 20 300 4.6 × 2.82 × 25 280 4.6 0.1365 20 300 4.8 × 4.02 × 26 280 4.6 0.1365 20 300 5.0 × 4.34 × 27 280 4.6 0.1365 20 300 5.2 × 4.65 × 28 280 4.6 0.1365 20 300 5.4 × 5.12 ○ 29 280 4.6 0.1365 20 300 5.6 × 5.54 ○ 30 280 4.6 0.1365 20 300 5.8 × 6.43 ○ 31 280 4.6 0.1365 20 300 6.0 × 6.75 ○ 32 280 4.6 0.1365 20 300 6.2 × 6.88 ○ 33 280 4.6 0.1365 20 300 6.4 × 6.74 ○ 34 280 4.6 0.1365 20 300 6.6 × 6.77 ○ 35 280 4.6 0.1365 20 300 6.8 Occur 5.81 ×

In numbers 1 to 10, the heat input of the pre-energization is about 805 W·s (= contact resistance 0.0002113 * current 4600A * current 4600A * time 0.180s), and for numbers 11 to 19, about 608 W·s (= 0.0002113* 4000*4000*0.180), and for numbers 20 to 21, about 951 W·s (= 0.0002113*5000*5000*0.180), and for numbers 22 to 23, about 792 W·s (= 0.0002113*5000*) 5000*0.150), and for numbers 24 to 35, about 1152 W·s (= 0.0002113*4600*4600*0.280)

In Table 3, the numbers 1 to 10 were pre-energized to be about 72.8% of the initial contact resistance, but the numbers 1 to 4 did not form a weld having an appropriate size because the current size of the current was small. Spatters were caused by excessive current.

Nos. 11 to 19 indicate that the contact resistance after pre-energization was about 86.7% of the initial contact resistance, and sufficient pre-energization was not achieved. However, in the case of No. 17, the welding portion of an appropriate size was formed without spattering through this energization, but the welding conditions are very narrow, and thus, it is not easy to apply. That is, it can be seen that if the welding current is slightly increased, spatters are generated, as shown in numbers 18 and 19.

Nos. 20 to 23 indicate that the welding current of the pre-energization is too high, so it can be confirmed that spatters are generated during pre-energization.

Nos. 24 to 35 have a contact resistance after pre-energization, about 64.6% of the initial contact resistance, and sufficient pre-energization was performed, but this energization time was excessive to form a weld of an appropriate size without spattering, and the total welding time. There is a problem that this 600ms is required.

Claims (5)

Pressing an electrode against an aluminum-based plated steel material, thereby contacting the electrode with an aluminum-based plated steel material;
After the electrode is in contact with the aluminum-based plated steel, performing pre-pulse to be 75% or less of the initial contact resistance;
Rest after the pre-energization; And
After the pause, for a period of 200 ms or more, a method of welding a resistance point of an aluminum-based plated steel material comprising performing main current with 115% or more of the pre-current current.
The method according to claim 1,
The method of welding a resistance point of an aluminum-based plated steel having a preliminary energization, a pause, and a total time of this energization of 400 ms or less.
The method according to claim 1,
The pause is a method of welding a resistance point of an aluminum-based plated steel material for 16 ms or more.
The method according to claim 1,
The aluminum-based plated steel material is a hot-pressed molding (Hot Press Forming, HPF) after 1470MPa grade aluminum plated steel having a tensile strength of pre-heat treatment method of resistance spot welding of aluminum-based plated steel.
The method according to claim 1,
The present energization is a method of welding a resistance point of an aluminum-based plated steel material performed at 135% or less of the pre-current current.

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