KR102507236B1 - Welding method for zinc coated steel - Google Patents

Abstract

A method for welding galvanized steel according to the present invention includes a preparation step of preparing a base steel material containing retained austenite in the structure; A plating step of preparing a coated steel material by forming a galvanized layer on the base steel material; A heat treatment step of locally heating a surface of a surface of the plated steel at a position to be welded to transform the galvanized layer into an α-Fe (Zn) phase; And a welding step of resistance welding the surface of the plated steel by pressing the position where the α-Fe (Zn) phase is formed on the surface of the plated steel material, and the melting point of the α-Fe (Zn) phase formed during the heat treatment step is In the welding step, it is characterized in that the plated steel material and the counter material are higher than the temperature at which resistance welding is performed.

Description

Zinc coated steel welding method {WELDING METHOD FOR ZINC COATED STEEL}

The present invention relates to a method for welding galvanized steel, and more particularly, to a method for welding galvanized steel that is galvanized and contains austenite in its structure.

Recently, in the trend of gradually increasing the strength of steel sheets for automobiles, the application of TRIP (Transformation Induced Plasticity) steel having high strength is gradually expanding. TRIP steel is a steel material in which strength and elongation are increased by inducing retained austenite in the structure, and has an advantage of excellent workability while having high rigidity.

On the other hand, in order to improve the corrosion resistance of steel materials and improve the appearance quality, galvanized steel sheets coated with zinc on the surface of steel materials are widely used, and welding is often used as a method of joining steel materials to steel materials. 1 pressurizes the two base steels 110 with the electrodes 200 and joins the two base steels 110 through resistance spot welding using power supplied from the power supply 300 is schematically shown there is.

The welding heat generated when welding galvanized steel sheet to the other material is 900~1000℃, which is higher than the melting point of pure zinc (about 429℃) or the melting point of zinc alloyed with a small amount of iron (up to about 782℃). , the galvanized layer can be easily melted during the welding process. As shown in FIG. 2, when welding is performed in the LME area of a predetermined composition and temperature range, the galvanized layer is melted.

As shown in FIG. 3, when zinc is melted and this part is pressed in a state in which liquid metal is generated, the liquid metal penetrates along the grain boundary from the surface of the base steel 110 in a solid state, and around the weld area W A liquid metal embrittlement (LME) phenomenon that causes and expands cracks (C) occurs.

In general, among steel structures, ferrite has excellent resistance to this LME phenomenon, but since austenite is easily cracked by LME, steel materials containing retained austenite in the structure such as TRIP steel are welded in a galvanized state. It was difficult to do.

That is, molten zinc penetrates between the grain boundaries of austenite to cause peeling, and cracks occur at the welded part as these cracks expand.

Therefore, there is a demand for a new welding method in which the LME phenomenon does not occur while welding a galvanized steel material containing retained austenite in a structure such as TRIP steel.

The matters described as the background art above are only for improving understanding of the background of the present invention, and should not be taken as an admission that they correspond to prior art already known to those skilled in the art.

KR 10-2012-0017955 A (2012.02.29)

Y. G. Kim, Materials Transactions 55 (1) 2014 pp. 171-175

The present invention has been made to solve these problems, and an object of the present invention is to provide a galvanized steel welding method capable of preventing the LME phenomenon even when welding a steel material that is galvanized and contains retained austenite in its structure. is to do

In order to achieve the above object, a galvanized steel welding method according to an embodiment of the present invention includes a preparation step of preparing a base steel material containing retained austenite in the structure; A plating step of preparing a coated steel material by forming a galvanized layer on the base steel material; A heat treatment step of locally heating a surface of a surface of the plated steel at a position to be welded to transform the galvanized layer into an α-Fe (Zn) phase; And a welding step of resistance welding the surface of the plated steel by pressing the position where the α-Fe (Zn) phase is formed on the surface of the plated steel material, and the melting point of the α-Fe (Zn) phase formed during the heat treatment step is In the welding step, it is characterized in that the plated steel material and the counter material are higher than the temperature at which resistance welding is performed.

In the heat treatment step, the galvanized layer may be heated to 600 to 900°C.

By using the heat generated during the heat treatment step, Fe is diffused from the base steel material to the galvanized layer so that the galvanized layer can be transformed from a δ phase to an α-Fe(Zn) phase.

In the heat treatment step, the α-Fe(Zn) phase generated by transforming the galvanized layer may have a Fe content of 70% or more.

In the welding step, the resistance welding temperature of the plated steel material and the counter material may be 900 to 1000 ° C, and the melting point of the α-Fe (Zn) phase formed in the heat treatment step may be 1000 ° C or more.

The heating position in the heat treatment step may be a region in contact with an electrode that presses the surface of the plated steel material in the welding step.

The heating position in the heat treatment step may be a region in contact with an electrode that presses the surface of the plated steel member in the welding step and a region in contact with the plated steel member and the counter material in the welding step.

In the welding step, the plated steel material and the counter material to be welded may be the same as the plated steel material that has undergone the preparation step, the plating step, and the heat treatment step.

The base steel prepared in the preparation step may be TRansformation-Induced Plasticity (TRIP) steel.

The base steel material prepared in the preparation step may include C: 0.25% or less (excluding 0%), Mn: 2.0% or less (excluding 0%), Al+Si: 2.0% or less (excluding 0%), and the balance Fe. .

In the base steel material prepared in the preparation step, the total volume fraction of martensite and bainite in the structure may be 50 to 90%, and the volume fraction of retained austenite may be 10 to 50%.

The heat treatment step may be performed by irradiating a laser to the surface of the plated steel material.

According to the galvanized steel welding method according to the present invention, the following effects are obtained.

First, even if galvanized steel containing retained austenite is welded, the LME phenomenon can be prevented.

Second, it is easy to apply because the LME phenomenon can be prevented without separate steel component adjustment.

Third, by preventing the LME phenomenon, durability and rigidity can be secured even when galvanized steel is welded.

1 is a configuration diagram showing the state of joining steel materials using a resistance spot welding (RSW) device,
2 is a phase diagram of Fe-Zn,
Figure 3 is a photograph showing the appearance of cracks formed in the welded portion by the LME phenomenon,
4 is a flowchart of a method for welding galvanized steel according to an embodiment of the present invention;
Figure 5 is a schematic diagram showing the appearance of heat treatment and welding of steel materials according to an embodiment of the present invention,
6 is a schematic diagram showing heat treatment according to various embodiments of the present invention.

The terminology used herein is intended only to refer to specific embodiments and is not intended to limit the present invention. As used herein, the singular forms also include the plural forms unless the phrases clearly indicate the opposite. As used herein, the meaning of "comprising" specifies specific characteristics, regions, integers, steps, operations, elements, and/or components, and other specific characteristics, regions, integers, steps, operations, elements, elements, and/or groups. does not exclude the presence or addition of

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs. Terms defined in commonly used dictionaries are additionally interpreted as having meanings consistent with related technical literature and currently disclosed content, and are not interpreted in ideal or very formal meanings unless defined.

Hereinafter, a method for welding galvanized steel according to a preferred embodiment of the present invention will be described with reference to the accompanying drawings.

Figure 4 is a flow chart of a galvanized steel welding method according to an embodiment of the present invention, Figure 5 is a schematic diagram showing the appearance of heat-treating and welding a steel material according to an embodiment of the present invention, Figure 6 is a number of various aspects of the present invention It is a schematic diagram showing the appearance of heat treatment according to the embodiment.

4 to 6, the galvanized steel welding method according to the present invention largely includes a preparation step (S100), a plating step (S200), a heat treatment step (S300), and a welding step (S400). .

The preparation step (S100) is a step for preparing the base steel 110 for performing the welding method according to the present invention, and the plating step (S200) is to form a galvanized layer 120 on the surface of the base steel 110 It is a step for preparing the coated steel material 100, and the heat treatment step (S300) is a step of heating a portion of the galvanized layer 120 to transform it into a deformable portion 121 composed of α-Fe (Zn) phase, and a welding step. (S400) is a step of resistance welding the counter material by pressing the deformable part 121 of the plated steel material 100.

The base steel 110 prepared in the preparation step (S100) is characterized by including retained austenite in the structure. This is because when the galvanized layer 120 is formed on the base steel 110 including the austenite structure and then resistance welded, the galvanized layer 120 melts and penetrates into the base steel 110, causing brittle fracture by the LME phenomenon. because it does

At this time, since the austenite structure is particularly vulnerable to brittle fracture caused by the LME phenomenon, it is preferable to apply the welding method according to the present invention to steel materials containing retained austenite, such as the base steel material 110.

The base steel 110 prepared in the preparation step (S100) is preferably TRIP (TRansformation-Induced Plasticity) steel having excellent strength and elongation, and its composition is, for example, C: 0.25% or less (excluding 0%), Mn : 2.0% (excluding 0%) or less, Al+Si: 2.0% or less (excluding 0%), the balance may include Fe.

The structure of the base steel 110 may have a total volume fraction of martensite and bainite of 50 to 90% and a volume fraction of retained austenite of 10 to 50%.

When the volume fraction of retained austenite is less than 10%, elongation decreases and processing becomes difficult, and when the volume fraction of retained austenite exceeds 50%, the required stiffness is not met, making it unusable for high-strength automobile bodies.

The plating step (S200) is a step of preparing a coated steel material 100 in which a galvanized layer 120 is formed on the surface of the base steel material 110, preferably, all surfaces of the base steel material 110 are coated with a galvanized layer 120 For example, the galvanized layer 120 may be formed by passing the strip of the base steel material 110 through a galvanizing bath so that the galvanized layer 120 may be formed.

The above-described preparation step (S100) and plating step (S200) are not specifically limited, and various manufacturing methods may be applied to prepare the base steel material 110 and the plated steel material 100.

In the heat treatment step (S300), a portion of the galvanized layer 120 is formed by heating a portion of the surface of the plated steel 100 to be pressed by the electrode 200 in the welding step (S400), which will be described later, to include the α-Fe (Zn) phase. It is deformed by the transforming part 121 that does. At this time, it is preferable to use a method of irradiating a laser (L) capable of locally heating only the surface as the heating means.

In general, immediately after the galvanized layer 120 is formed, it has a δ phase. As shown in FIG. 2, the δ phase has a melting point of about 670 to 782° C., and may be melted by welding heat (900 to 1000° C.) in a welding step (S400) to be described later.

Therefore, by irradiating the laser L to heat the galvanized layer 120 at 600 to 900 ° C. to diffuse the Fe element from the base steel 110 to the galvanized layer 120, the δ-phase galvanized layer 120 is α- The Fe content is increased so that the Fe(Zn) phase can be transformed into the deformation portion 121.

As shown in FIG. 2, when the content of Fe in the galvanized layer 120 is 70% or more, the melting point exceeds 1000° C., and the deformed portion 121 is melted by the welding heat generated in the welding step (S400) to be described later. This will prevent the LME phenomenon.

The reason why the laser (L) is used in the heat treatment step (S300) is that, when performing a general heat treatment, the temperature of the base steel 110 rises and the martensitic structure is annealed, which may cause a problem in that strength is lowered. That is, when the heat treatment is performed using the laser L, deformation of the base steel 110 can be minimized while the surface of the plated steel 100 is locally heated to diffuse Fe into the galvanized layer 120.

In addition, since the pressing force is essential for the LME phenomenon to occur, the object is heated and the galvanized layer 120 is heat treated and deformed by heating the object without additional pressurization through the laser L, so that even if the galvanized layer 120 is melted during the deformation process, the base The LME phenomenon may not occur because it cannot penetrate into the steel 110.

The welding step (S400) is a step of welding the coated steel material 100 transformed into the deformed portion 121 by irradiating a laser (L) to a part of the galvanized layer 120 with the counter material, such a counter material is prepared, for example The plated steel material 100 manufactured through the step (S100), the plating step (S200) and the heat treatment step (S300) may be used. That is, in the welding step (S400), the two plated steel materials 100 may be joined by welding. Of course, the present invention is not limited thereto, and the plated steel material 100 may be welded to another counter material different from the plated steel material 100. Hereinafter, a case in which two sheets of plated steel 100 are welded will be described.

In the welding step (S400), after overlapping the two sheets of plated steel 100 in which the deformed portion 121 is formed, the electrode is brought into contact with the deformed portion 121, so that the two sheets of coated steel 100 are close to each other. , Electricity is applied to the electrode so that the two plated steel materials 100 are welded by resistance heat. At this time, the welding heat generated by the resistance heat is 900 to 1000 ° C, and the two plated steel materials 100 are resistance welded through this heat and the pressing force of the pair of electrodes.

However, the melting point of the deformed portion 121 of the α-Fe (Zn) phase in which the galvanized layer 120 is deformed by receiving laser L irradiation should be higher than the welding heat in the welding step (S400). Through this, even if the two sheets of plated steel 100 are welded in the welding step (S400), the deformed portion 121 of the galvanized layer 120 is not melted, and thus the generation of LME caused by penetration of liquid metal is fundamental. will be blocked by

On the other hand, in the heat treatment step (S300), the position where the deformed portion 121 is formed by irradiating the laser (L) may be formed only in the portion directly in contact with the electrode in the welding step (S400), or the electrode in the welding step (S400). It can also be formed on the opposite part in contact with (200).

When the deformable part 121 is formed only at the portion in contact with the electrode, resistance welding can be continuously performed after irradiating the laser (L) and pressing the electrode 200 in a state in which two sheets of plated steel 100 are overlapped, thereby simplifying the process. There is an advantage of simplification, and when the deformable portion 121 is formed on both sides of the plated steel 100, there is an advantage of preventing LME that may rarely occur in a region where two sheets of plated steel 100 come into contact.

Although the embodiments of the present invention have been described with reference to the accompanying drawings, those skilled in the art can realize that the present invention can be implemented in other specific forms without changing the technical spirit or essential features. you will be able to understand

Therefore, the embodiments described above should be understood as illustrative in all respects and not limiting. The scope of the present invention is indicated by the claims to be described later rather than the detailed description above, and all changes or modified forms derived from the meaning and scope of the claims and equivalent concepts thereof should be construed as being included in the scope of the present invention. .

100: plated steel
110: base steel
120: galvanized layer
121: deformation part
200: electrode
300: power supply
C: crack
W: welding area
L: laser

Claims (12)

A preparation step of preparing a base steel material containing retained austenite in the structure;
A plating step of preparing a coated steel material by forming a galvanized layer on the base steel material;
A heat treatment step of locally heating a surface of a surface of the plated steel at a position to be welded to transform the galvanized layer into an α-Fe (Zn) phase; and
A welding step of resistance welding a relative material by pressing the position where the α-Fe (Zn) phase is formed on the surface of the plated steel material by heating;
The melting point of the α-Fe (Zn) phase formed during the heat treatment step is higher than the temperature at which the plated steel material and the counter material are resistance welded in the welding step.
The method of claim 1,
The heat treatment step is a galvanized steel welding method, characterized in that for heating the galvanized layer to 600 ~ 900 ℃.
The method of claim 2,
The galvanized steel welding method characterized in that the zinc plating layer is transformed from a δ phase to an α-Fe (Zn) phase by diffusing Fe from the base steel material into the galvanized layer using the heat generated during the heat treatment step.
The method of claim 1,
The α-Fe (Zn) phase generated by transforming the galvanized layer in the heat treatment step has a Fe content of 70% or more.
The method of claim 1,
In the welding step, the temperature at which the plated steel material and the counter material are resistance welded is 900 to 1000 ° C.
The galvanized steel welding method, characterized in that the melting point of the α-Fe (Zn) phase formed in the heat treatment step is 1000 ℃ or more.
The method of claim 1,
The heating position in the heat treatment step is a galvanized steel welding method, characterized in that the region in contact with the electrode that presses the surface of the plated steel material in the welding step.
The method of claim 1,
The heating position in the heat treatment step is a galvanized steel welding method, characterized in that a part in contact with the electrode for pressing the surface of the plated steel material in the welding step and a part in contact with the plated steel material and the counter material in the welding step.
The method of claim 1,
The welding method of galvanized steel, characterized in that the counter material to be welded with the plated steel material in the welding step is the same as the plated steel material that has passed through the preparation step, the plating step and the heat treatment step.
The method of claim 1,
The galvanized steel welding method, characterized in that the base steel material prepared in the preparation step is TRANSformation-Induced Plasticity (TRIP) steel.
The method of claim 9,
The base steel material prepared in the preparation step includes C: 0.25% or less (excluding 0%), Mn: 2.0% (excluding 0%) or less, Al+Si: 2.0% or less (excluding 0%), and the balance including Fe. Galvanized steel welding method.
The method of claim 1,
The base steel prepared in the preparation step has a total volume fraction of martensite and bainite in the structure of 50 to 90%, and a galvanized steel welding method, characterized in that the volume fraction of retained austenite is 10 to 50%.
The method of claim 1,
The heat treatment step is galvanized steel welding method, characterized in that carried out by irradiating a laser on the surface of the plated steel material.

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