WO2024053669A1

WO2024053669A1 - Welded joint

Info

Publication number: WO2024053669A1
Application number: PCT/JP2023/032519
Authority: WO
Inventors: 卓哉光延; 卓史横山; 浩史竹林
Original assignee: 日本製鉄株式会社
Priority date: 2022-09-06
Filing date: 2023-09-06
Publication date: 2024-03-14

Abstract

The present invention addresses the problem of providing a welded joint in which LME cracking during manufacturing is suppressed. A welded joint according to the present invention is characterized in that: a plated steel sheet forming the welded joint has a prescribed chemical composition; when the C-concentration is measured by GDS in a thermally non-affected portion of the welded joint in the depth direction of a base steel sheet starting from the interface between the base steel sheet and a plating layer of the plated steel sheet, the depth at which the C-concentration is 0.05% or less is 10 μm or more; the roughness expressed as Ra of the interface between the base steel sheet and the plating layer of the plated steel sheet in the thermally non-affected portion is 3.0 μm or less; and, at a position 500 μm apart from an end of a pressure welded portion of the welded joint, the thickness of a layer where the area percentage of a ferrite phase is 90% or more is at least 15 μm in the depth direction of the base steel sheet starting from the interface between the base steel sheet and the plating layer of the plated steel sheet.

Description

welded joints

The present invention relates to a welded joint. More specifically, the present invention relates to a welded joint that suppresses LME cracking during manufacturing.

In recent years, efforts have been made to increase the strength of steel plates used in various fields such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of welded joints made by spot welding high-strength steel plates is increasing in order to reduce the weight of vehicle bodies in order to improve fuel efficiency.

When welding zinc-plated steel sheets, especially high-strength steel sheets, there may be a problem of reduced weldability due to liquid metal embrittlement (LME) cracking, as described in Patent Document 1, for example. LME cracking is thought to occur when the surface layer of the steel plate transforms into austenite during welding, molten zinc intrudes into the grain boundaries and embrittles the steel plate, and furthermore, tensile stress is applied to the steel plate during welding.

Furthermore, as disclosed in Non-Patent Document 1, it is known that ferrite phase grain boundaries have lower LME susceptibility than austenite grain boundaries with respect to LME cracking.

Note that Patent Document 2 discloses that, as a steel sheet that suppresses LME cracking and improves weldability, Si oxide particles with a particle size of 20 nm or more are contained in a number density of 3000 to 6000 pieces/mm ² in the surface layer of the steel sheet. A steel sheet having a grain size distribution is disclosed.

International Publication No. 2019/116531 International Publication No. 2020/218575

In order to prevent LME cracking during the production of welded joints, it is effective to prevent Zn, etc. contained in the plating layer from penetrating into the austenite-transformed steel plate. In this respect, there is room for improvement.

In view of these circumstances, it is an object of the present invention to provide a welded joint that suppresses LME cracking during manufacturing.

The present inventors have diligently studied means for solving the above problems. As a result, by containing a large amount of Si and Al in the steel plate to be spot welded and subjecting the steel plate to an appropriate surface condition and subjecting it to high dew point annealing, the area around the weld is decarburized and the ferrite (α) phase is stabilized. It was discovered that LME can be suppressed by covering the surface layer of a steel plate with a ferrite phase with a low solid solution amount of C, and by maintaining the ferrite phase in a stable state even during the production of welded joints.

The present invention has been made based on the above findings and further studies, and the gist thereof is as follows.

(1) A plurality of superimposed steel plates, a nugget that joins the plurality of steel plates, a spot weld having a pressure weld and a heat affected zone formed around the nugget, and a separation formed around the pressure weld. one or more of the plurality of steel plates is a plated steel plate including a base steel plate and a plating layer, and the plating layer is at least on the overlapping surface of the plurality of steel plates. The plated steel sheet is formed on the corresponding surface and contains Zn, the tensile strength of the plated steel sheet is 780 MPa or more, and the chemical composition of the base steel sheet is C: 0.05 to 0.40% in mass%. , Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.7-2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.010%, Ti: 0-0. 150%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100 %, REM: 0 to 0.100%, the remainder being Fe and impurities, Si and sol. The total content of Al is 1.8% or more, and in the non-heat-affected zone of the welded joint, the base steel plate starts from the interface between the plating layer of the plated steel plate and the base steel plate. In the depth direction, the depth at which the C concentration measured by GDS is 0.05% or less is 10 μm or more, and the roughness of the interface between the plating layer of the plated steel sheet and the base steel sheet is Ra 3. 0 μm or less, and at a position 500 μm outward from the end of the press-welded portion, a ferrite phase is formed in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate. A welded joint characterized in that the layer having an area ratio of 90% or more has a thickness of 15 μm or more.

(2) In the non-heat affected zone of the welded joint, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0. The welded joint according to (1) above, wherein the depth of .05% or less is 15 μm or more.

(3) At a position 500 μm outward from the end of the press-welded part, the area of the ferrite phase in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate. The welded joint according to (1) above, characterized in that the thickness of the layer having a ratio of 90% or more is 30 μm or more.

(4) In the non-heat affected zone of the welded joint, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0. .05% or less, the base material has a depth of 15 μm or more, and is located at a position 500 μm outward from the end of the press-welded portion, starting from the interface between the plating layer of the plated steel sheet and the base steel sheet. The welded joint according to (1) above, wherein the thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 30 μm or more in the depth direction of the steel plate.

(5) The roughness of the interface between the plating layer of the plated steel sheet and the base steel sheet in the non-heat affected zone is 2.0 μm or less in terms of Ra. Any welded fittings.

According to the present invention, it is possible to obtain a welded joint that suppresses LME cracking during manufacturing.

1 is a diagram schematically showing an example of a welded joint of the present invention. FIG. 3 is a diagram showing a layered ferrite (α) phase formed around a welded portion of a welded joint of the present invention. It is a figure explaining LME resistance evaluation in an example.

The present invention will be explained below. The present invention is not limited to the following embodiments. First, an outline of improving the LME resistance during manufacturing in the welded joint of the present invention will be explained.

LME cracking occurs when the surface layer of a steel plate is heated during spot welding, the steel plate structure in the surface layer transforms into austenite, and the grain boundaries become brittle as hot-dip plating enters the steel plate structure along the austenite grain boundaries. It is caused by doing. It is thought that LME cracking occurs because tensile stress is applied to the steel plate during welding. The welded joint of the present invention improves LME resistance during manufacture of the welded joint due to the structure formed on the surface layer of the steel plate. Note that in this specification, the surface layer of a steel plate refers to the range from the outermost surface of the steel plate to a depth of 100 μm.

If the C element is contained in the surface layer of a steel sheet, LME cracking is likely to occur, so keeping the C concentration in the surface layer of the steel sheet low is effective in preventing LME cracking. Normally, when a steel plate is heated, such as during annealing, external oxidation occurs and oxides (scale) are formed on the surface of the steel plate, making it difficult for decarburization to proceed. Therefore, the C concentration in the surface layer of the steel sheet is difficult to decrease. On the other hand, in the welded joint of the present invention, in the non-heat affected zone, the C concentration measured by GDS is measured in the depth direction of the base metal starting from the interface between the plating layer of the plated steel plate constituting the welded joint and the base steel plate. is 0.05% or less at a depth of 10 μm or more. This means that the concentration of C, which is an element that tends to cause LME, is low in the surface layer of the plated steel sheets that make up the welded joint, and this state is maintained even after the welded joint is manufactured.

In the present invention, when manufacturing a plated steel plate constituting a welded joint, a strong strain is applied to the surface layer of the base steel plate without increasing the surface roughness, and the steel plate is annealed at a high dew point. This allows oxygen to diffuse into the steel sheet and form internal oxides, making it possible to suppress the formation of external oxides. This reduces the C concentration in the surface layer of the base steel plate.

Furthermore, in the welded joint of the present invention, at a position 500 μm outward from the end of the pressure welded part, the area of the ferrite phase is The thickness of the layer with a ratio of 90% or more is 15 μm or more. That is, the ferrite phase stably exists even around the heat affected zone. Here, the term "heat affected zone" refers to an unmolten part of a steel plate whose structure, metallurgical properties, mechanical properties, etc. have changed due to welding heat. The heat affected zone can be confirmed by observing a cross section in the plate thickness direction using an SEM. Note that the "non-heat-affected zone" refers to a portion other than the heat-affected zone. Furthermore, Si and sol. It is thought that the effect of adding Al in combination can stabilize ferrite.

That is, the welded joint of the present invention has Si and sol. Due to the combined effects of high Al content, applying strain to the surface layer before annealing, and controlling the dew point during annealing, the surface layer of the plated steel sheet has a low C concentration even when affected by the heat from welding, and furthermore, ferrite By forming a layer with a high area ratio of , it is possible to improve the LME resistance during the manufacture of welded joints.

Hereinafter, the present invention will be explained in detail.

《Welded joint》
First, the welded joint of the present invention will be explained with reference to FIG. The welded joint of the present invention is manufactured by spot-welding a plurality (two in FIG. 1) of steel plates 1 using a welding electrode A, as shown in FIG. 1(a). An overlapping surface 6 is formed on the surface where the plurality of steel plates 1 are in contact with each other.

Figure 1(b) shows a welded joint in which two steel plates are welded together by spot welding. The welded joint of the present invention includes a plurality of superimposed steel plates 1 and a nugget 2 that joins the plurality of steel plates. The nugget 2 is a portion where steel components and plating layer components are melted and solidified by spot welding. A pressure welding portion 3 is formed around the nugget 2, in which two steel plates 1 are pressed together. A separation portion 4 exists around the pressure contact portion 3 . The separation portion 4 is a portion where welding or pressure welding by spot welding has not occurred, and is a portion where the two steel plates 1 are not in direct contact. The term “end of the press-contact portion” refers to the position of the press-contact portion 3 that is closest to the separation portion 4 . The term “outside” from the end of the pressure contact portion refers to the direction from the end of the pressure contact portion 3 toward the open portion of the separation portion 4 . Further, around the nugget 2, a heat affected zone 5 is formed in which the structure, metallurgical properties, mechanical properties, etc. have changed due to the welding heat.

The plurality of steel plates constituting the welded joint of the present invention may include both a plated steel plate having a base steel plate and a plating layer, and a non-plated steel plate that is not plated, but one or more of the plurality of steel plates is a plated steel plate. The plating layer is formed at least on a surface corresponding to the overlapping surface 6 of the steel plates. Moreover, the tensile strength of the plated steel sheet is 780 MPa or more.

As mentioned above, LME cracking occurs when molten zinc plating is present on the surface of a high-strength (tensile strength of 780 MPa or more) steel plate during welding. Therefore, for example, when considering a welded joint made up of two steel plates, if one of the two steel plates is a high-strength plated steel plate, LME cracking may occur. The welded joint of the present invention includes a high-strength plated steel plate in the steel plates constituting the welded joint, and can suppress LME cracking during manufacture of the welded joint.

<Chemical composition of steel plate>
Next, the chemical composition of the base steel plate of the plated steel plate constituting the welded joint will be explained. Hereinafter, "%" regarding chemical components means "% by mass". Furthermore, in numerical ranges for chemical components, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.

(C: 0.05-0.40%)
C (carbon) is an element that ensures the strength of steel. In order to obtain a tensile strength of 780 MPa or more, which is the target of the present invention, the C content was reduced to 0, taking into account the balance with weldability, and to prevent the C concentration in the surface layer of the base steel plate from becoming too high. .05 to 0.40%. If the C content is too large, the C concentration in the surface layer of the base steel sheet will not be reduced even by high dew point annealing, which will be described later. The content of C may be 0.07% or more, 0.10% or more, or 0.12% or more. The content of C may be 0.35% or less, 0.30% or less, or 0.25% or less.

(Si: 0.7-3.0%, sol.Al: 0.7-2.0%, Si+sol.Al≧1.8%)
Si (silicon) is an element that promotes ferrite stabilization and decarburization when added in combination with Al (aluminum). In order to obtain the effect of suppressing LME, Si: 0.7 to 3.0%, sol. Al: 0.7 to 2.0% is contained, and Si and sol. The total value of Al content is 1.8% or more. sol. Al refers to acid-soluble Al that is not converted into oxides such as Al ₂ O ₃ and is soluble in acids, and was measured by subtracting the insoluble residue on the filter paper that is generated during the Al analysis process. Required as Al. The content of Si may be 0.8% or more, 0.9% or more, or 1.0% or more. The content of Si may be 2.8% or less, 2.5% or less, or 2.0% or less. sol. The Al content may be 0.8% or more, 0.9% or more, or 1.0% or more. sol. The Al content may be 1.8% or less, 1.6% or less, or 1.5% or less. Si and sol. The total content of Al may be 1.9% or more, 2.0% or more, or 2.2% or more.

(Mn: 0.1-5.0%)
Mn (manganese) is an effective element for improving the strength of steel by creating a hard structure. Considering the balance between the strength of the steel and the deterioration of workability due to Mn segregation, the Mn content is set to 0.1 to 5.0%. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

(P: 0.0300% or less)
P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, weldability may deteriorate. Therefore, the content of P is set to 0.0300% or less. The content of P may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that P is not contained, and the lower limit of the P content is 0%. From the viewpoint of dephosphorization cost, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.

(S: 0.0300% or less)
S (sulfur) is an impurity generally contained in steel. If the S content exceeds 0.0300%, weldability will decrease, and furthermore, the amount of MnS precipitated may increase, leading to a possibility that workability such as bendability will decrease. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0100% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. It is preferable that S is not contained, and the lower limit of the S content is 0%. From the viewpoint of desulfurization cost, the S content may be more than 0%, 0.0001% or more, or 0.0005% or more.

(N: 0.0100% or less)
N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may deteriorate. Therefore, the N content is set to 0.0100% or less. The content of N may be 0.0080% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. It is preferable that N is not contained, and the lower limit of the N content is 0%. From the viewpoint of manufacturing cost, the N content may be more than 0%, 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0005% or more.

(B: 0-0.010%)
B (boron) is an element that increases hardenability and contributes to improvement of strength, and also segregates at grain boundaries to strengthen grain boundaries and improve toughness, so it may be included as necessary. . Since B is not an essential element, the lower limit of the content of B is 0%. Although this effect can be obtained even when B is contained in a trace amount, it is preferable that the content of B is 0.0001% or more. The content of B may be 0.0002% or more, 0.0003% or more, 0.0005% or more, 0.0007% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.010% or less. The content of B may be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less.

(Ti: 0-0.150%)
Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to improving the strength, so it may be included as necessary. Since Ti is not an essential element, the lower limit of the content of Ti is 0%. Although this effect can be obtained even with a trace amount of Ti, the content of Ti is preferably 0.0001% or more. The content of Ti may be 0.0002% or more, 0.0003% or more, 0.0005% or more, 0.0007% or more, or 0.0010% or more. On the other hand, if excessively contained, coarse TiN may be generated and toughness may be impaired, so the content of Ti is set to 0.150% or less. The Ti content is 0.1000% or less, 0.0500% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less. good.

(Nb: 0-0.150%)
Since Nb (niobium) is an element that contributes to improving strength through improving hardenability, it may be included as necessary. Since Nb is not an essential element, the lower limit of the content of Nb is 0%. Although this effect can be obtained even with a trace amount of Nb, the content of Nb is preferably 0.001% or more. The Nb content may be 0.002% or more, 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.150% or less. The Nb content may be 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(V: 0-0.150%)
Since V (vanadium) is an element that contributes to improving strength through improving hardenability, it may be contained as necessary. Since V is not an essential element, the lower limit of the content of V is 0%. Although this effect can be obtained even when V is contained in a trace amount, it is preferable that the content of V is 0.001% or more. The content of V may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. The V content may be 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

(Cr: 0-2.00%)
Cr (chromium) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Cr is not an essential element, the lower limit of the content of Cr is 0%. Although this effect can be obtained even with a trace amount of Cr, the content of Cr is preferably 0.001% or more. The content of Cr may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.08% or more. On the other hand, if it is contained excessively, a large amount of Cr carbide will be formed, and the hardenability may be adversely affected, so the content of Cr is set to 2.00% or less. The Cr content is 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.70% or less, 0.50% or less, or 0.30% or less. good.

(Ni: 0-2.00%)
Since Ni (nickel) is effective in improving the hardenability of steel and increasing the strength of steel, it may be contained as necessary. Since Ni is not an essential element, the lower limit of the Ni content is 0%. Although this effect can be obtained even with a trace amount of Ni, it is preferable that the Ni content is 0.001% or more. The Ni content may be 0.01% or more, or 0.02% or more. On the other hand, since excessive addition of Ni increases cost, the Ni content is set to 2.00% or less. The Ni content is 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.50% or less, 0.30% or less, 0.20 % or less, 0.10% or less, or 0.05% or less.

(Cu: 0-2.00%)
Cu (copper) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Cu is not an essential element, the lower limit of the content of Cu is 0%. Although this effect can be obtained even with a trace amount of Cu, the content of Cu is preferably 0.0001% or more. The content of Cu may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and cracking of the slab after casting, the content of Cu is set to 2.00% or less. The Cu content is 1.8000% or less, 1.5000% or less, 1.2000% or less, 1.0000% or less, 0.5000% or less, 0.1000% or less, 0.0500% or less, 0.0100 % or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

(Mo: 0-1.00%)
Mo (molybdenum) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Mo is not an essential element, the lower limit of the content of Mo is 0%. Although this effect can be obtained even with a trace amount of Mo, the content of Mo is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.08% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.90% or less, 0.70% or less, 0.50% or less, or 0.30% or less.

(W: 0-1.00%)
W (tungsten) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be included as necessary. Since W is not an essential element, the lower limit of the content of W is 0%. Although this effect can be obtained even when a small amount of W is included, it is preferable that the content of W is 0.001% or more. The content of W may be 0.002% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the content of W is set to 1.00% or less. The content of W is 0.90% or less, 0.70% or less, 0.50% or less, 0.30% or less, 0.10% or less, 0.05% or less, or 0.03% or less. good.

(Ca: 0-0.100%)
Ca (calcium) is an element that contributes to control of inclusions, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Ca is not an essential element, the lower limit of the content of Ca is 0%. Although this effect can be obtained even with a trace amount of Ca, the content of Ca is preferably 0.0001% or more. The content of Ca may be 0.0002% or more. On the other hand, since excessive Ca content may cause deterioration of surface properties, the Ca content is set to 0.100% or less. The content of Ca is 0.0800% or less, 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, 0.0010% or less, 0.0008 % or less, or 0.0005% or less.

(Mg: 0-0.100%)
Mg (magnesium) is an element that contributes to control of inclusions, particularly to fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Mg is not an essential element, the lower limit of the Mg content is 0%. Although this effect can be obtained even with a trace amount of Mg, it is preferable that the Mg content is 0.0001% or more. The content of Mg may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0008% or more. On the other hand, since excessive Mg content may cause deterioration of surface properties, the content of Mg is set to 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

(Zr: 0-0.100%)
Zr (zirconium) is an element that contributes to control of inclusions, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Zr is not an essential element, the lower limit of the content of Zr is 0%. Although this effect can be obtained even when a small amount of Zr is contained, it is preferable that the content of Zr is 0.001% or more. The content of Zr may be 0.002% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, since excessive Zr content may cause deterioration of surface properties, the Zr content is set to 0.100% or less. The content of Zr may be 0.080% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(Hf: 0-0.100%)
Hf (hafnium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since it is not an essential element, the lower limit of the content of Hf is 0%. Although this effect can be obtained even with a trace amount of Hf, it is preferable that the Hf content is 0.0001% or more. The Hf content may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0008% or more. On the other hand, since excessive Hf content may cause deterioration of surface properties, the content of Hf is set to 0.100% or less. The Hf content may be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

(REM: 0-0.100%)
REM (rare earth element) is an element that contributes to control of inclusions, particularly to fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be included as necessary. Since it is not an essential element, the lower limit of the content of REM is 0%. Although this effect can be obtained even with a trace amount of REM, the content of REM is preferably 0.0001% or more. The content of REM may be 0.0003% or more, or 0.0005% or more. On the other hand, if excessively contained, deterioration of surface properties may become apparent, so the content of REM is set to 0.100% or less. The content of REM may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less. Note that REM is an abbreviation for Rare Earth Metal, and refers to an element belonging to the lanthanide series. REM is usually added as a misch metal.

In the base steel plate of the plated steel plate constituting the welded joint according to the present invention, the remainder other than the above chemical components consists of Fe and impurities. Here, impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap when manufacturing steel sheets industrially, and are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap. It means a substance that does not have an adverse effect on LME resistance.

The chemical components of the base steel plate may be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas melting-thermal conductivity method. These analyzes may be performed using samples taken from the base steel plate using a method compliant with JIS G0417:1999.

One or more of the plurality of steel plates constituting the welded joint of the present invention is a plated steel plate comprising a base steel plate and a plating layer. The plating layer contains Zn. Further, the plating layer is formed on at least a surface corresponding to the overlapping surface of the plurality of steel plates. The plating layer is not particularly limited as long as it contains Zn. As an example, Zn-0.2%Al, Zn-0.5%Al, Zn-1.5%Al-1.5%Mg, Zn-20%Al-7%Mg, Zn-30%Al-10 %Mg. The plating layer may be formed on surfaces other than those corresponding to the overlapping surfaces of the plurality of steel plates.

The chemical composition of the plating layer is determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses the corrosion of the base steel sheet, and measuring the resulting solution using ICP (inductively coupled plasma) emission spectroscopy. be able to.

The thickness of the plating layer may be, for example, 3 to 50 μm. Further, the amount of the plating layer deposited is not particularly limited, but may be, for example, 10 to 170 g/m ² per side. In the present invention, the amount of the plating layer deposited is determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the base steel plate, and from the change in weight of the plating layer before and after pickling and peeling. The thickness of the plating layer may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the plating layer may be 40 μm or less, or 30 μm or less. The amount of the plating layer deposited on one side may be 20 g/m ² or more, 30 g/m ² or more, 40 g/m ² or more, or 50 g/m ² or more. The amount of the plating layer deposited per side may be 150 g/m ² or less, 130 g/m ² or less, 120 g/m ² or less, or 100 g/m ² or less.

The thickness of the plated steel plate constituting the welded joint of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The plate thickness may be 0.8 mm or more, or 1.0 mm or more. The plate thickness may be 3.0 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

Next, the structure of the plated steel plate that makes up the welded joint will be explained.

[C concentration in non-heat affected zone]
In the welded joint of the present invention, in the non-heat affected zone, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0.05%. The depth is 10 μm or more. The heat-affected zone is a part where the material properties have changed due to heat during the spot welding process, and can be confirmed by SEM observation. The non-heat affected zone is a portion other than the heat affected zone. Any position 5 mm or more away from the outer edge of the spot weld may be considered a non-heat-affected zone, so measurements are performed at a position 5 mm or more away from the outer edge of the spot weld.

Since LME sensitivity decreases as the C concentration decreases, lowering the C concentration in the non-heat affected zone improves the LME suppression effect during welded joint manufacturing. Further, since C is an austenite stabilizing element, a small amount of C stabilizes the ferrite phase with low LME sensitivity, which will be described later.

Such a structure can be obtained by changing the chemical composition of the base steel plate of the plated steel plate to one containing a large amount of Si and Al as described above, subjecting it to the heat treatment described below, and spot welding.

If the depth at which the C concentration is 0.05% or less is 10 μm or more, the effect of suppressing LME during manufacturing can be obtained, so the upper limit of the depth is not particularly limited, and the depth is, for example, 50 μm or less. , 40 μm or less, or 30 μm or less. The depth at which the C concentration is 0.05% or less is preferably 25 μm or more.

The GDS measurement is performed five times in the thickness direction, and the average value of these measurements is taken as the C concentration. The measurement conditions are as follows. The starting point of "depth" is the interface between the base steel sheet and the plating layer. The interface between the base steel plate and the plating layer is located at a position where the Fe concentration measured by GDS measurement is 93% of the Fe concentration at a depth of 150 μm.

Equipment: High frequency glow discharge emission spectrometer (manufactured by LECO Japan LLC, model number “GDS850A”)
Ar gas pressure: 0.3MPa
Anode diameter: 4mmφ
RF output: 30W
Measurement time: 200-1500 seconds

[Interface roughness in non-heat affected zone]
In the welded joint of the present invention, the roughness of the interface between the plating layer of the plated steel plate and the base steel plate in the non-heat affected zone is 3.0 μm or less as an arithmetic mean height Ra defined by JIS B0601:2013. Here, "the interface between the plating layer and the base steel sheet" means the actual interface between the plating layer and the base steel sheet. As the roughness increases, cracks are more likely to occur due to stress concentration. The roughness of the interface between the plating layer and the base steel plate may be 2.5 μm or less, or 2.0 μm or less in Ra. Regarding the roughness of the interface between the plating layer and the base steel plate, the surface roughness of the base steel plate measured after removing the plating may be regarded as the roughness of the interface between the plating layer and the base steel plate. The plating is removed by dissolving the plating layer in an acid solution containing an inhibitor that inhibits corrosion of the steel plate.

[Ferrite phase outside the pressure welding part]
In the welded joint of the present invention, at a position 500 μm in an arbitrary direction outward from the end of the pressure welded part, in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate, The thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 15 μm or more. FIG. 2 shows an example of a microstructure photograph taken by SEM around the welded part of the welded joint of the present invention. FIG. 2 shows the vicinity of the pressure welding part and the separation part of the welded joint, and the black part in the upper right corner is the separation part. A layer in which the area ratio of ferrite (α) phase is 90% or more is formed on the surface layer of the lower steel plate. The interior of the steel plate is martensite (M), and some ferrite (α) may also be present. Further, a heat affected zone (HAZ) whose structure has changed due to welding heat is formed near the pressure welded portion.

It is known that ferrite phase grain boundaries have lower LME susceptibility than γ (austenite) grain boundaries (for example, Non-Patent Document 1). Therefore, by forming the periphery of the welded joint of the welded joint into a structure in which the ferrite phase is stable even during welding, it is possible to improve the LME suppression effect during the production of the welded joint. Such a surface structure can be obtained by changing the chemical composition of the base steel plate to one containing a large amount of Si and Al as described above, subjecting it to the heat treatment described below, and then spot welding.

If the thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 15 μm or more, the effect of improving LME resistance can be obtained, so the upper limit of the thickness is not particularly limited, and the thickness is, for example, 100 μm. The thickness may be less than or equal to 80 μm or less than or equal to 60 μm. The thickness of the layer in which the area ratio of the ferrite phase is 90% or more may be 20 μm or more, or 30 μm or more.

The thickness of the layer in which the area ratio of the ferrite phase is 90% or more can be determined by nital-etching a cross section of the welded joint perpendicular to the steel plate surface and observing it with SEM. etc., and find them. Here, the area ratio of the ferrite phase refers to the area ratio determined by observing a cross section cut perpendicular to the surface of the steel plate. Even if there is a local part in the thickness direction where the area ratio of ferrite phase is less than 90% when observing a cross section cut parallel to the steel sheet surface, vertical There is no problem as long as the area ratio of the ferrite phase in the cross section cut is 90% or more. A more specific measurement method is as follows.

The ferrite area ratio is determined by cutting a cross section of the steel plate in the thickness direction perpendicular to the rolling direction, mirror polishing, revealing the steel structure with nital liquid, and taking a secondary electron image using a field emission scanning electron microscope. ,demand. The observation position is 500 μm outward from the end of the press-welded part, and five observation positions are set at equal intervals at arbitrary positions in the rolling direction (direction perpendicular to the plane of the paper in FIG. 1(b)) for observation. The ferrite area ratio is the average value of 5 fields of view. For the obtained tissue photographs, the fraction of each tissue is calculated by the point counting method. More specifically, first, a grid of equal intervals is drawn on the tissue photograph. Next, it is determined whether the structure at each lattice point corresponds to tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite. By finding the number of lattice points corresponding to each tissue and dividing by the total number of lattice points, the fraction of each tissue can be measured. The larger the total number of grid points, the more accurately the volume fraction can be determined. In this embodiment, the grid spacing is 2 μm×2 μm, and the total number of grid points is 1500 points.

The criteria for determining tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite are shown below. A region that has a substructure (lath boundary, block boundary) within the grain and in which carbides are precipitated in a plurality of variants is determined to be tempered martensite. In addition, a region where cementite is precipitated in a lamellar shape is determined to be pearlite. A region with low brightness and no underlying structure is determined to be ferrite. A region where the brightness is high and the underlying structure is not exposed by etching is determined to be fresh martensite or retained austenite. Areas that do not fall under any of the above are determined to be bainite. Simply speaking, the area ratio of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[Tensile strength of plated steel sheet]
Since the present invention suppresses LME occurring in high-strength steel plates, the plated steel plates constituting the welded joints have high strength, and specifically have a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, the tensile strength may be, for example, 2000 MPa or less. The tensile strength may be measured in accordance with JIS Z 2241:2011 by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction. The tensile strength may be 880 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The tensile strength may be 1900 MPa or less, or 1800 MPa or less.

If it is not possible to collect a test piece for tensile strength measurement from the plated steel plate constituting the welded joint, as an alternative, the hardness of the plated steel plate in the non-heat-affected zone located at a distance of 5 mm or more from the outer edge of the spot weld may be used. (Vickers hardness) and estimate the value of tensile strength from the correlation formula below (Correlation between static strength parameters, Norihiko Hasegawa, Junichi Arai, Michishichi Tanaka, "Materials" Vol. 39, No. 442, P859-863).

Hv=0.301×TS+5.701
(However, Hv is Vickers hardness and TS is tensile strength (unit: MPa))

In other words, if the hardness is about 240 Hv or more, the tensile strength can be considered to be 780 MPa or more.

The hardness of the plated steel plate is measured at 1/2 depth at a position that is the non-heat affected zone of the plated steel plate that constitutes the welded joint. Hardness measurement is performed in accordance with JIS Z 2244:2009. The measurement load is 200gf. The hardness of the plated steel plate in the non-heat affected zone located at a distance of 5 mm or more from the outer end of the spot weld may be 245 Hv or more, 250 Hv or more, 260 Hv or more, 270 Hv or more, 300 Hv or more, or 340 Hv or more.

Next, a method for manufacturing a welded joint according to the present invention will be explained. First, a method for manufacturing a base steel plate of a plated steel plate constituting a welded joint will be explained.

<Manufacturing method of base material steel plate>
The base steel plate of the plated steel plate constituting the welded joint according to the present invention can be obtained, for example, by a casting process in which molten steel with adjusted chemical composition is cast to form a steel billet, and a hot rolled steel plate is obtained by hot rolling the steel billet. A hot rolling process, a winding process of winding a hot rolled steel plate, a cold rolling process of cold rolling the wound hot rolled steel plate to obtain a cold rolled steel plate, a pretreatment process of brush grinding the cold rolled steel plate, and It can be obtained by a manufacturing method including an annealing step of annealing a pretreated cold rolled steel sheet. Alternatively, the material may be pickled and then cold-rolled without being wound up after the hot-rolling process.

[Casting process]
The conditions of the casting process are not particularly limited. For example, following melting in a blast furnace, electric furnace, etc., various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting or ingot casting.

[Hot rolling process]
A hot rolled steel plate can be obtained by hot rolling a steel piece obtained by casting. The hot rolling process is performed by directly or once cooling the cast steel billet, then reheating and hot rolling. When reheating is performed, the heating temperature of the steel piece may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction rate of each rolling may be changed as appropriate depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[Winding process]
Hot-rolled steel sheets can be rolled up at a predetermined temperature. The winding temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by unwinding the hot-rolled steel sheet before or after winding. Alternatively, it is also possible to perform cold rolling, which will be described later, by pickling after the hot rolling process without winding.

[Cold rolling process]
After pickling or the like is performed on the hot rolled steel sheet, the hot rolled steel sheet can be cold rolled to obtain a cold rolled steel sheet. The rolling reduction ratio of cold rolling may be changed as appropriate depending on the desired metallographic structure and plate thickness, and may be, for example, 20 to 80%. After the cold rolling process, the material may be cooled to room temperature by, for example, air cooling.

[Pre-treatment process]
As mentioned above, in a welded joint, in the non-heat-affected zone, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0.05%. At a position 500 μm outward from the end of the pressure welding part, the ferrite phase is In order to make the thickness of the layer with an area ratio of 90% or more 15 μm or more, it is necessary to perform a predetermined pretreatment before annealing the steel plate, and then perform the annealing.

The pretreatment includes grinding the surface of the cold rolled steel plate with a grinding brush (brush grinding process). An example of a grinding brush that can be used is M-33 manufactured by Hotani Corporation. Thereby, strain can be introduced without increasing the surface roughness. When grinding, it is recommended to apply a 1.0 to 5.0% NaOH aqueous solution to the surface of the steel plate. It is preferable that the brush reduction amount is 0.5 to 10.0 mm and the rotation speed is 100 to 1000 rpm. By controlling the coating liquid conditions, brush reduction amount, and rotation speed to perform brush grinding, decarburization is promoted in the annealing process described later, and ferrite is stabilized even around the weld during the production of welded joints. The structure can be efficiently formed on the surface layer of the steel sheet.

[Annealing process]
After the pretreatment step, the cold rolled steel sheet is annealed. Annealing is performed under a tension of 1 to 20 MPa. Applying tension during annealing makes it possible to more effectively introduce strain into the steel sheet, promoting decarburization of the surface layer.

The holding temperature in the annealing step is 750 to 900°C. The holding temperature may be 770-870°C. By setting it within such a range, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase can be stabilized. The heating rate up to the holding temperature is not particularly limited, but may be 1 to 10° C./sec.

The holding time at the holding temperature in the annealing step is 20 to 300 seconds. The holding time may be between 30 and 250 seconds. By setting it within such a range, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase can be stabilized.

The atmosphere in the annealing step has a dew point of -30 to 20°C. The dew point may be -10 to 5°C. The atmosphere may be, for example, N ₂ -1 to 10 vol% H ₂ or N ₂ -2 to 4 vol% H ₂ . If the dew point is too high or too low, a phase containing oxides such as Si, Mn, and Al is formed outside the steel sheet, which inhibits the mutual diffusion of the plating components and steel components, resulting in insufficient plating properties. There is.

By the manufacturing method including each of the steps described above, decarburization is promoted in the surface layer of the steel sheet, and a steel sheet in which the ferrite phase is stabilized even during the production of welded joints can be obtained.

<Manufacturing method of plated steel sheet>
The plated steel plate constituting the welded joint according to the present invention can be obtained by performing a plating process to form a plating layer containing Zn on the base steel plate manufactured as described above.

The plating treatment may be performed according to methods known to those skilled in the art. The plating treatment may be performed, for example, by hot-dip plating or electroplating. Preferably, the plating process is performed by hot-dip plating. The conditions for the plating treatment may be appropriately set in consideration of the chemical composition, thickness, amount of adhesion, etc. of the desired plating layer. For example, it may be immersed in a hot-dip galvanizing bath at 420 to 480° C. with adjusted chemical components for 1 to 10 seconds, and then pulled out at 20 to 200 mm/sec after immersion, and the amount of plating deposited may be controlled by N ₂ wiping gas. After the plating process, an alloying process may be performed. The alloying treatment may be performed, for example, at 500 to 550° C. for 10 to 60 seconds.

<Spot welding process>
A plurality of steel plates including the above-mentioned plated steel plates are overlapped and spot welded to obtain a welded joint. The conditions for spot welding are not particularly limited. For example, spot welding can be performed using a dome radius type welding electrode with a tip diameter of 6 to 8 mm, with a pressure of 1.5 to 6.0 kN, a current application time of 0.1 to 1.0 seconds, and a current of 4 to 15 kA. Can be done.

Since the welded joint according to the present invention suppresses LME cracking during manufacturing, it can be suitably used in a wide range of fields such as automobiles, home appliances, and building materials. It is particularly preferred to be used in the automotive field.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The present invention is not limited to these examples.

<Example 1>
(Preparation of plated steel sheet sample)
No. of Table 1 Molten steel adjusted to have the chemical composition described in 1 was melted in a blast furnace and cast by continuous casting to obtain a steel billet. The obtained steel piece was heated to 1200°C and hot rolled at a finish rolling end temperature of 950°C and a finish rolling reduction of 30% to obtain a hot rolled steel plate. The obtained hot-rolled steel sheet was wound up at a winding temperature of 650° C., pickled, and then cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel plate was 1.6 mm.

Next, a 2.0% NaOH aqueous solution was applied to the cold-rolled steel sheet, and a pretreatment of brush grinding was performed. Brush grinding was performed using Hotani M-33 as a grinding brush at a brush reduction amount of 2.0 mm and a rotation speed of 600 rpm (condition A in Table 2).

Before the annealing process, the surface roughness of the steel plate was measured in accordance with JIS B 0601:2013. That is, 10 locations are randomly selected on the surface of the surface layer side, the surface profile at each location is measured using a contact type surface roughness meter, and the arithmetic mean roughness Ra is obtained by arithmetic averaging of the surface roughness at those locations. , was evaluated as follows.

Evaluation AA: 2.0 μm or less Evaluation A: More than 2.0 μm, 3.0 μm or less Evaluation B: More than 3.0 μm

Thereafter, annealing was performed in a N ₂ -4% H ₂ gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less at a dew point of 0° C., a holding temperature of 800° C., and a holding time of 100 seconds. The temperature increase rate during annealing was 6.0°C/sec up to 500°C, and 2.0°C/sec from 500°C to the holding temperature. The annealing treatment was performed under a tension of 15 MPa.

Following the annealing treatment, the steel plate was subjected to a plating treatment to obtain a hot-dip galvanized steel plate. The plating treatment was performed by immersing the sample in a hot-dip galvanizing bath (Zn-0.2% Al) at 460° C. for 3 seconds. After dipping, it was pulled out at 100 mm/sec, and the coating weight was controlled to 50 g/m ² using N ₂ wiping gas, to obtain a plated steel sheet.

<Examples 2 to 28, Comparative Examples 29 to 41>
A plated steel sheet was prepared in the same manner as in Example 1, except that the chemical components were as shown in Table 1, the conditions for the pretreatment process and annealing process were as shown in Table 2, and the plating type was as shown in Table 3. was manufactured. In addition, No. In No. 40, the pretreatment of brush grinding was omitted. Also, No. In No. 41, a grinding brush D-100 manufactured by Hotani Co., Ltd. was used in the pretreatment (condition B in Table 2). D-100 is a brush with approximately twice the amount of grinding as M-33.

The composition and bath temperature of the plating species shown in Table 3 are as follows. After the plating treatment, F was subjected to alloying treatment at 530° C. for 20 seconds to obtain alloyed hot-dip galvanizing.

A: Zn-0.2%Al (460°C)
B: Zn-0.5%Al (440°C)
C: Zn-1.5%Al-1.5%Mg (500°C)
D: Zn-20%Al-7%Mg (530°C)
E: Zn-30%Al-10%Mg (530°C)
F: Zn-0.14%Al (450°C) alloyed hot-dip galvanizing

(Tensile strength evaluation)
For each plated steel plate, a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction was taken, a tensile test was performed in accordance with JIS Z 2241:2011, the tensile strength was determined, and the evaluation was performed as follows. did.

Evaluation AAA: 1180MPa or more Evaluation AA: 980MPa or more, less than 1180MPa Evaluation A: 780MPa or more, less than 980MPa

(Production of welded joints)
Two plated steel plates were stacked together and spot welded using a dome radius type welding electrode with a tip diameter of 8 mm at a welding angle of 2°, a pressing force of 4.0 kN, a current application time of 0.5 seconds, and a current of 12 kA. A welded joint was produced, and the LME suppression effect (LME resistance) during production was evaluated.

The LME resistance was evaluated as follows using the length of the LME crack that occurred just outside the pressure welded part. Evaluation of LME resistance will be described with reference to FIG. 3. LME resistance is the outer part of the pressure welded part 3 formed by overlapping two steel plates 1 and spot welding, and the LME that occurs immediately outside the pressure welded part, which is a position near the pressure welded part. Evaluation was made by measuring the crack length. The evaluation was made as follows based on the length of the crack 11 just outside the pressure welding part. In this example, if the evaluation was A or higher, it was determined that the LME resistance was excellent. If the evaluation was "A" or higher, it was determined that the LME resistance was excellent and the problem to be solved by the present invention was solved.

Evaluation AAA: 0μm
Evaluation AA: More than 0 μm and less than 60 μm Evaluation A: More than 60 μm and less than 120 μm Evaluation B: More than 120 μm

(Evaluation of the structure around the weld)
A sample was taken by cutting the welded joint, and GDS measurements were performed five times in the thickness direction under the conditions described above, starting from the interface between the plating layer and the base steel plate at the position that would be the non-heat affected zone. The depth at which the C concentration was 0.05% or more was determined and shown in "C≦0.05% depth" in Table 3.

In addition, the plating was removed using a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (manufactured by Asahi Chemical Co., Ltd., Ivit) at the position that would become the non-heat affected zone, and the surface of the exposed base steel sheet was removed. The roughness was measured in the same manner as before annealing, and is shown in "Base material steel plate/plating interface roughness" in Table 3.

In addition, at a position 500 μm outward from the end of the press-welded part, starting from the interface between the plating layer and the base steel plate, the thickness of the layer in which the area ratio of the ferrite phase is 90% or more was measured using the method described above. It is shown in "α phase thickness" in Table 3.

The results of the evaluation are shown in Table 3.

No. No. 29 is a comparative example in which the base steel plate of the plated steel plate has a high C content. It is thought that because the base steel plate had a high C content, decarburization in the surface layer did not proceed even with high dew point annealing. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. No. 30 has a low Si content in the base steel plate of the plated steel plate, and Si and sol. This is a comparative example in which the sum of the Al contents is small. It is thought that because the Si content of the base steel sheet was low, decarburization did not proceed in the surface layer even when high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. No. 31 is a comparative example in which the base steel plate of the plated steel plate has a high Si content. Because the base steel sheet had a high Si content, it is thought that even after high dew point annealing, external oxidation progressed and oxides (scale) were formed on the surface layer of the steel sheet, suppressing decarburization at the outermost surface. It will be done. Therefore, the depth at which the C concentration in the non-heat-affected zone is 0.05% or less has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. 32 is the sol. of the base steel plate of the plated steel plate. This is a comparative example with a low Al content. Base material steel plate sol. It is thought that because the Al content was low, decarburization did not proceed in the surface layer even though high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. 33 is the sol. of the base steel plate of the plated steel plate. The content of Al is low, and the content of Si and sol. This is a comparative example in which the sum of the Al contents is small. Base material steel plate sol. The content of Al is low, and the content of Si and sol. It is considered that because the sum of the Al contents was small, decarburization did not progress in the surface layer even if high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. 34 is the sol. of the base steel plate of the plated steel plate. This is a comparative example with a high content of Al. Base material steel plate sol. It is thought that because the Al content was high, even if high dew point annealing was performed, external oxidation progressed and oxides (scale) were formed on the surface layer of the steel sheet, suppressing decarburization at the outermost surface. Therefore, the depth at which the C concentration in the non-heat-affected zone is 0.05% or less has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. No. 35 is a comparative example in which the total content of Si and Al in the base steel plate of the plated steel plate is small. Si and sol. of base material steel plate. It is considered that because the sum of the Al contents was small, decarburization did not progress in the surface layer even if high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. In No. 36, the dew point during annealing during manufacturing of the plated steel sheet was low, so a layer containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and during the plating process, the interaction between the plating components and the steel components occurred. It is thought that diffusion was inhibited. As a result, welded joints were not evaluated because appropriate plating could not be obtained.

No. No. 37 had a high dew point during annealing during the manufacturing of the plated steel sheet, so a layer containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and during the plating process, the interaction between the plating components and the steel components occurred. It is thought that diffusion was inhibited. As a result, welded joints were not evaluated because appropriate plating could not be obtained.

No. It is believed that in No. 38, decarburization was not sufficiently promoted during annealing because the holding temperature during annealing during production of the plated steel sheet was low. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. No. 39 is a comparative example in which the annealing temperature during production of the plated steel sheet was high. It is thought that decarburization was not sufficiently promoted during annealing because the holding temperature during annealing was high. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. No. 40 is a comparative example in which the annealing time during production of the plated steel sheet is short. It is thought that decarburization was not sufficiently promoted during annealing because the holding time during annealing was short. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. In No. 41, brush grinding was not performed in the pretreatment process during production of the plated steel sheet, so strain was not introduced to the surface of the steel sheet, and it is thought that decarburization did not proceed during annealing. Therefore, the depth at which the C concentration is 0.05% or less in the non-heat-affected zone becomes small, and the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part. thickness has become smaller. As a result, the LME resistance during manufacture of welded joints was inferior.

No. In No. 42, a brush with a large amount of grinding was used in the pretreatment process during the production of the plated steel sheet, so the surface roughness of the base steel sheet increased and it is thought that the ferrite phase was not stabilized. Therefore, the thickness of the layer in which the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the press-contact portion was reduced. As a result, the LME resistance during manufacture of welded joints was inferior.

No. Examples 1 to 28 are examples of the present invention, in which LME was suppressed during the production of welded joints. The depth where the C concentration in the non-heat affected zone is 0.05% or less is large, and the thickness of the layer where the area ratio of the ferrite phase is 90% or more at a position 500 μm outward from the end of the pressure welding part is large. In the examples, it was confirmed that the material had particularly excellent LME resistance.

According to the present invention, it is possible to provide a welded joint in which LME cracking during manufacturing is suppressed, and the welded joint can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles. Therefore, the present invention has extremely high industrial applicability.

1 Steel plate 2 Nugget 3 Pressure welding part 4 Separation part 5 Heat affected zone 6 Overlapping surface 11 Cracks just outside the pressure welding part

Claims

multiple overlapping steel plates,
a nugget for joining the plurality of steel plates, and a spot welding part having a pressure welding part and a heat affected zone formed around the nugget;
A welded joint comprising a separation part formed around the pressure welding part,
One or more of the plurality of steel plates is a plated steel plate comprising a base steel plate and a plating layer, and the plating layer is formed at least on a surface corresponding to the overlapping surface of the plurality of steel plates, and Contains Zn,
The tensile strength of the plated steel sheet is 780 MPa or more,
The chemical composition of the base steel plate is in mass%,
C: 0.05-0.40%,
Si: 0.7-3.0%,
Mn: 0.1 to 5.0%,
sol. Al: 0.7-2.0%,
P: 0.0300% or less,
S: 0.0300% or less,
N: 0.0100% or less,
B: 0 to 0.010%,
Ti: 0 to 0.150%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Cr: 0-2.00%,
Ni: 0-2.00%,
Cu: 0-2.00%,
Mo: 0-1.00%,
W: 0-1.00%,
Ca: 0-0.100%,
Mg: 0-0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
REM: 0~0.100%
, the remainder being Fe and impurities,
Si and sol. The total value of Al content is 1.8% or more,
In the non-heat affected zone of the welded joint,
In the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate, the depth at which the C concentration measured by GDS is 0.05% or less is 10 μm or more. ,
The roughness of the interface between the plating layer of the plated steel plate and the base steel plate is 3.0 μm or less in Ra,
At a position 500 μm outward from the end of the pressure contact part,
The thickness of a layer in which the area ratio of the ferrite phase is 90% or more is 15 μm or more in the depth direction of the base steel sheet starting from the interface between the plating layer of the plated steel sheet and the base steel sheet. A welded joint featuring:
In the non-heat affected zone of the welded joint, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0.05%. The welded joint according to claim 1, wherein the depth is 15 μm or more.
At a position 500 μm outward from the end of the press-welded portion, the area ratio of the ferrite phase is 90 μm in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate. The welded joint according to claim 1, characterized in that the thickness of the layer that is 30 μm or more is 30 μm or more.
In the non-heat affected zone of the welded joint, the C concentration measured by GDS in the depth direction of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate is 0.05%. The depth of the base steel plate is 15 μm or more, and at a position 500 μm outward from the end of the pressure welding part, the depth of the base steel plate starting from the interface between the plating layer of the plated steel plate and the base steel plate. 2. The welded joint according to claim 1, wherein the layer having a ferrite phase area ratio of 90% or more has a thickness of 30 μm or more in the transverse direction.
According to any one of claims 1 to 4, the roughness of the interface between the plating layer of the plated steel plate and the base steel plate in the non-heat affected zone is 2.0 μm or less in terms of Ra. welded joints.