Steel welded components

Abstract

The present invention provides a steel welded member which is a steel welded member in which a plurality of Zn-based plated steels having a Zn-based plated layer on the surface of the steel material containing, in mass %, C: 0.05 to 0.40 %, Si: 0.2 to 3.0 %, and Mn: 0.1 to 5.0 % are joined via at least one spot welding region, and in a region 0.5 mm from the end of the pressure contact portion of the spot welding region, contains 20 to 80 % of a Γ phase (Fe3Zn 10 ) in area ratio, and contains one or more oxides having an aspect ratio of 0.5 μm or more.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel welded member. More particularly, the present application relates to a steel welded member having a high corrosion resistance in a spot welding region. BACKGROUND

[0002] In recent years, steel sheets used in various fields such as automobiles and building materials are being developed to be high-strength. For example, in the field of automobiles, high-strength steel sheets are increasingly used in order to reduce the weight of the vehicle body for the purpose of improving fuel efficiency. Such high-strength steel sheets typically contain elements such as C, Si, and Mn in order to increase the strength of the steel.

[0003] Generally, such high-strength steel sheets are required to have a high corrosion resistance in order to ensure the strength and appearance design property, particularly in the case of outdoor use. As a steel sheet having a high corrosion resistance, a zinc-based plated steel sheet having a zinc-based plating layer (e.g., a Zn-Al plating layer, a Zn-Al-Mg plating layer, etc.) formed on the steel sheet is known.

[0004] For example, an automobile member formed using a zinc-based plated steel sheet is generally shaped by press working or the like, and then assembled by welding (e.g., spot welding). Therefore, in such a member in which a plurality of plated steel sheets are joined via a welding region, not only the corrosion resistance of the plated steel sheet itself but also the corrosion resistance of the welding region (e.g., the spot welding region) is required. Generally, the corrosion resistance of the welding region is known to be inferior to that of the sound portion not subjected to welding.

[0005] In this regard, Patent Literature 1 discloses a vapor-deposited metal material having a SiOx-Zn-based (where 1.3 ≤ x ≤ 2.1) vapor-deposited layer formed thereon, and having excellent corrosion resistance, adhesion, and spot weldability. Further, Patent Literature 2 discloses a method for improving the corrosion resistance, tensile strength, and fatigue strength of a joint obtained by spot welding a high-strength plated steel sheet, characterized in that an ultrasonic impact treatment is applied to a crack occurrence portion of a nugget portion and a heat-affected zone around the nugget portion from one side or both sides of the spot welding region.

[0006] Prior Art Documents

[0007] Patent Literature

[0008] Patent Literature 1: Japanese Patent Application Laid-Open No. 4-218661

[0009] Patent Literature 2: Japanese Patent Application Laid-Open No. 2005-103608 SUMMARY

[0010] Problems to be Solved by the Invention

[0011] In Patent Literature 1, although the corrosion resistance and spot weldability of a vapor-deposited metal material were studied, no study was made on improving the corrosion resistance of a spot weld. Further, the invention described in Patent Literature 2 is to repair a crack occurring in a spot weld or the like by performing ultrasonic impact treatment, thereby preventing moisture from entering the crack, and thus improving the corrosion resistance, and therefore Patent Literature 2 does not necessarily make a sufficient study on improving the corrosion resistance of a spot weld in a welded state.

[0012] The present application was made in view of such circumstances, and an object thereof is to provide a steel welded member in which a spot weld has high corrosion resistance.

[0013] Means for solving the object

[0014] The present inventors found that it is important that a prescribed amount of a Γ phase (Fe3Zn 10 ) be present in the vicinity of the end portion of the press-contact portion of a spot weld, and that a coarse oxide (typically containing Si, Mg, O, and Fe) be present, and further found that the corrosion resistance of a spot weld between Zn-based plated steels is significantly improved by the presence of the Γ phase and the coarse oxide.

[0015] The present application was made based on the above insight, and the gist thereof is as described below.

[0016] (1) A steel welded member in which a plurality of Zn-based plated steels having a Zn-based plated layer on the surface of a steel material are joined via at least one spot weld, wherein

[0017] The steel material has a composition containing, in mass%:

[0018] C: 0.05 to 0.40%,

[0019] Si: 0.2 to 3.0%,

[0020] Mn: 0.1 to 5.0%,

[0021] sol. Al: 0 to less than 0.4000%,

[0022] P: 0.0300% or less,

[0023] S: 0.0300% or less,

[0024] N: 0.0100% or less,

[0025] B: 0 to 0.010%,

[0026] Ti: 0 to 0.150%,

[0027] Nb: 0 to 0.150%,

[0028] V: 0 to 0.150%,

[0029] Cr: 0 to 2.00%,

[0030] Ni: 0 to 2.00%,

[0031] Cu: 0 to 2.00%,

[0032] Mo: 0 to 1.00%,

[0033] W: 0 to 1.00%,

[0034] Ca: 0 to 0.100%,

[0035] Mg: 0 to 0.100%,

[0036] Zr: 0 to 0.100%,

[0037] Hf: 0 to 0.100%, and

[0038] REM: 0 to 0.100%, the remainder (or balance) consisting of Fe and impurities;

[0039] In a region of the spot-welded zone 0.5 mm from the end of the pressure-welded portion, 20 to 80% of the Γ phase (Fe3Zn 10 ) is contained in terms of area ratio, and one or more oxides having a length diameter of 0.5 μm or more are contained.

[0040] (2) The steel welded member according to the above (1), wherein one or more oxides having a length diameter of 1.5 to 10.0 μm are contained in the region.

[0041] (3) The steel welded member according to the above (1) or (2), wherein the α phase is further contained in the region in terms of area ratio at 5 to 50%.

[0042] Effects of the Invention

[0043] According to the present application, a steel welded member can be provided in which, in a steel welded member obtained by spot-welding a plurality of Zn-based plated steels, since 20 to 80% of the Γ phase (Fe3Zn 10 ) is present in terms of area ratio in a region of the spot-welded zone 0.5 mm from the end of the pressure-welded portion, and further, coarse oxides, specifically, oxides having a length diameter of 0.5 μm or more are present in the region, such coarse oxides significantly improve the corrosion resistance of the spot-welded zone due to an increase in the contact area with the Γ phase. As a result, a member excellent in overall corrosion resistance, particularly, a member for an automobile can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0044] Figure 1is a sectional view for illustrating a spot-welded region of an example of a steel welded member to which the present application is applied.

[0045] Figure 2 is a view for illustrating an end portion and a region near the end portion of a press-contact portion of an example of a steel welded member to which the present application is applied, and is Figure 1 is an enlarged view of a dotted line portion of

[0046] Figure 3 is a view for observing a cross section of a press-contact end portion of a spot-welded region of an example of a steel welded member to which the present application is applied, by a backscattered electron image (BSE image) of a scanning electron microscope (SEM). DETAILED DESCRIPTION

[0047] <STEEL WELDED MEMBER>

[0048] The present application relates to a steel welded member which is a steel welded member in which a plurality of Zn-based plated steels having a Zn-based plated layer on a surface of a steel material are joined via at least one spot-welded region, characterized in that:

[0049] The component composition which the steel material has contains, in mass%:

[0050] C: 0.05 to 0.40%,

[0051] Si: 0.2 to 3.0%,

[0052] Mn: 0.1 to 5.0%,

[0053] sol. Al: 0 to less than 0.4000%,

[0054] P: 0.0300% or less,

[0055] S: 0.0300% or less,

[0056] N: 0.0100% or less,

[0057] B: 0 to 0.010%,

[0058] Ti: 0 to 0.150%,

[0059] Nb: 0 to 0.150%,

[0060] V: 0 to 0.150%,

[0061] Cr: 0 to 2.00%,

[0062] Ni: 0 to 2.00%,

[0063] Cu: 0 to 2.00%,

[0064] Mo: 0 to 1.00%,

[0065] W: 0 to 1.00%,

[0066] Ca: 0 to 0.100%,

[0067] Mg: 0 to 0.100%,

[0068] Zr: 0 to 0.100%,

[0069] Hf: 0 to 0.100%, and

[0070] REM: 0 to 0.100%, the remainder consisting of Fe and impurities;

[0071] In a region of the spot-welded zone 0.5 mm from the end of the pressure-welded portion, 20 to 80% of the Γ phase (Fe3Zn 10 ) is contained in terms of area ratio, and one or more oxides having an aspect ratio of 0.5 μm or more are contained.

[0072] In recent years, for example, as an automobile member, in order to improve fuel efficiency and to achieve light weight, a so-called high-strength steel sheet (for example, a tensile strength of 440 MPa or more) is used. Such a high-strength steel sheet, particularly a high-strength steel sheet which can be used outdoors, requires high corrosion resistance from the viewpoint of ensuring strength and appearance design, and in recent years, as a high-strength steel sheet having excellent corrosion resistance, a Zn-based plated steel sheet in which a Zn-based plated layer is formed on a steel sheet is used in large quantities. On the other hand, an automobile member is generally assembled into a desired member shape by welding (for example, spot welding) after the above-described plated steel sheet is formed by press working or the like. Therefore, the automobile member requires high corrosion resistance not only in the plated steel sheet portion but also in the spot-welded zone. On the other hand, the corrosion resistance of the spot-welded zone is poorer than that of a healthy portion which has not been subjected to welding. Therefore, corrosion starts from the spot-welded zone, the strength of the welded zone decreases, and thus the desired characteristics (particularly, characteristics related to strength) cannot be ensured as an automobile member.

[0073] Therefore, the present inventors have intensively studied a method for improving the corrosion resistance of a spot-welded zone, and as a result, it has been found that by subjecting a steel material having a prescribed chemical composition to a specific grinding treatment as a pre-annealing treatment and an annealing treatment under prescribed conditions, obtaining a Zn-based plated steel material by forming a Zn-based plated layer on the obtained steel material, and spot-welding the Zn-based plated steel material to produce a steel welded member, the corrosion resistance of the spot-welded zone can be significantly improved compared to the case where a conventional plated steel material is used. The end portion of the pressure-welded portion of the spot-welded zone of the thus produced steel welded member was analyzed in detail, and as a result, it has been found that in a region 0.5 mm from the end portion, 20 to 80% of the Γ phase (Fe3Zn 10), and thus, coarse oxides (Si oxides or Mn oxides, etc.) exist in the region. Therefore, it was found that, by the presence of both the Γ phase and the coarse oxides near the end portion of the caul portion, the corrosion resistance of the spot-welded region was greatly improved compared to a steel welded member made of a conventional plated steel material. It is considered that the reason for the improvement in the corrosion resistance of the spot-welded region is that the coarse Si, Mn oxides insulate the Zn-based corrosion product, and thus, the corrosion of the Zn-based plated layer can be suppressed. Further, the end portion of the caul portion was analyzed, and as a result, it was ascertained that, if the contact area of the coarse oxides with the Γ phase increases, the corrosion resistance of the spot-welded region greatly improves. Therefore, it has been ascertained that, by increasing the diameter and / or the number of the coarse oxides on the basis of ensuring a sufficient Γ phase area ratio, the corrosion resistance of the welded region can be greatly improved. Therefore, the present inventors have developed a steel welded member in which the spot-welded region has a high corrosion resistance, which is particularly advantageous for a member for a vehicle.

[0074] Hereinafter, a steel welded member according to the present application will be described in detail. The steel welded member according to the present application is a steel welded member in which a plurality of Zn-based plated steels having a Zn-based plated layer on the surface of a steel material (e.g., a steel sheet) are joined via at least one spot-welded region. Therefore, the steel welded member is constituted by spot-welding a plurality of (i.e., two or more) Zn-based plated steels, and the Zn-based plated steel has a steel material and a Zn-based plated layer formed on the steel material. Other layers (e.g., a Ni plated layer, etc.) can be included between the steel material and the plated layer. The steel welded member according to the present application includes at least one spot-welded region between the Zn-based plated steels, and can include two or more spot-welded regions. The Zn-based plated layer can be formed on one face of the steel material, or can be formed on both faces thereof. However, in order to obtain the steel welded member according to the present application, at least one of the two Zn-based plated steels to be spot-welded has a face having the Zn-based plated layer as a spot-welding joining face. Figure 1 A cross section of a spot-welded region of an exemplary steel welded member 1 according to the present application is shown in FIG. 1. The steel welded member 1 joins two Zn-based plated steels 11 via a spot-welded region 21. The spot-welded region 21 is typically constituted by a nugget portion 23 and a caul portion 25.

[0075] [Steel material]

[0076] Hereinafter, the steel material in the present application will be described in detail. The shape of the steel material is not particularly limited, but a steel sheet is preferred. When the steel material in the present application is a steel sheet, the sheet thickness thereof is not particularly limited, and can be, for example, 0.1 to 3.2 mm.

[0077] (Composition of the steel material)

[0078] The composition of the steel material of the present application will be described. The "%" of the content of the elements means "mass %" unless otherwise specified. In the numerical range of the composition, the numerical range indicated by "~" means the range including the lower limit value and the upper limit value indicated by the numerical values before and after the "~" unless otherwise specified.

[0079] (C: 0.05 to 0.40%)

[0080] C (carbon) is an important element for securing the strength of the steel. In order to secure sufficient strength, the C content is specified to be 0.05% or more. The C content is preferably 0.07% or more, more preferably 0.10% or more, and further preferably 0.12% or more. On the other hand, if the C content is excessive, there is a concern that the weldability will be reduced. Therefore, the C content is specified to be 0.40% or less. The C content can be 0.38% or less, 0.35% or less, 0.32% or less, or 0.30% or less.

[0081] (Si: 0.2 to 3.0%)

[0082] Si (silicon) is an effective element for increasing the strength of the steel. In order to secure sufficient strength and further generate a desired coarse oxide in the vicinity of the end portion of the spot-welded area, the Si content is specified to be 0.2% or more. The Si content is preferably 0.3% or more, more preferably 0.5% or more, and further preferably 1.0% or more. On the other hand, if the Si content is excessive, there is a concern that surface properties will be deteriorated, and further, that a plated layer cannot be properly formed due to the generation of an oxide on the outside of the steel material at the time of annealing, and it is difficult to form a Γ phase in the vicinity of the end portion of the spot-welded area. Therefore, the Si content is specified to be 3.0% or less. The Si content can be 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

[0083] (Mn: 0.1 to 5.0%)

[0084] Mn (manganese) is an effective element for increasing the strength of the steel by obtaining a hard structure. In order to secure sufficient strength and further generate a desired coarse oxide in the vicinity of the end portion of the spot-welded area, the Mn content is specified to be 0.1% or more. The Mn content is preferably 0.5% or more, more preferably 1.0% or more, and further preferably 1.5% or more. On the other hand, if the Mn content is excessive, the metal structure becomes non-uniform due to Mn segregation, and there is a concern that the workability will be reduced. Further, there is a concern that a plated layer cannot be properly formed due to the generation of an oxide on the outside of the steel material at the time of annealing, and it is difficult to form a Γ phase in the vicinity of the end portion of the spot-welded area. Therefore, the Mn content is specified to be 5.0% or less. The Mn content can be 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less.

[0085] (sol. Al: 0 to less than 0.4000%)

[0086] Al (aluminum) is an element that functions as a deoxidizing element. The Al content can also be 0%, but in order to obtain a sufficient deoxidizing effect, the Al content is preferably 0.0010% or greater. The Al content is more preferably 0.0050% or greater, further preferably 0.0100% or greater, and still further preferably 0.0150% or greater. On the other hand, if the Al content is excessive, there are concerns that the workability and surface properties will deteriorate. Therefore, the Al content is specified to be less than 0.4000%. The Al content can also be 0.3900% or less, 0.3800% or less, 0.3700% or less, 0.3500% or less, 0.3400% or less, 0.3300% or less, 0.3000% or less, or 0.2000% or less. The Al content refers to the content of so-called acid-soluble Al (sol. Al).

[0087] (P: 0.0300% or less)

[0088] P (phosphorus) is generally an impurity contained in steel. If P is excessively contained, there are concerns that the weldability will deteriorate. Therefore, the P content is specified to be 0.0300% or less. The P content is preferably 0.0200% or less, more preferably 0.0100% or less, and further preferably 0.0050% or less. The lower limit of the P content is 0%, but from the viewpoint of manufacturing cost, the P content can be more than 0% or 0.0001% or greater.

[0089] (S: 0.0300% or less)

[0090] S (sulfur) is generally an impurity contained in steel. If S is excessively contained, there are concerns that the weldability will deteriorate, and further that the workability such as bendability will deteriorate due to an increase in the amount of MnS precipitated. Therefore, the S content is specified to be 0.0300% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and further preferably 0.0020% or less. The lower limit of the S content is 0%, but from the viewpoint of desulfurization cost, the S content can be more than 0% or 0.0001% or greater.

[0091] (N: 0.0100% or less)

[0092] N (nitrogen) is generally an impurity contained in steel. If N is excessively contained, there are concerns that the weldability will deteriorate. Therefore, the N content is specified to be 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less, and further preferably 0.0030% or less. The lower limit of the N content is 0%, but from the viewpoint of manufacturing cost, the N content can be more than 0% or 0.0010% or greater.

[0093] The basic composition of the steel plate involved in this invention is as described above. Additionally, the steel plate may contain any of the following elements as needed. The presence of these elements is not mandatory, and the lower limit of their content is 0%.

[0094] (B: 0~0.010%)

[0095] Boron (B) is an element that contributes to increased strength by improving hardenability, and also enhances toughness by strengthening grain boundaries through segregation towards them. The B content can be 0%, but it may be included as needed to achieve the aforementioned effects. The B content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is preferably 0.010% or less, and may also be 0.008% or less, or 0.006% or less.

[0096] (Ti: 0~0.150%)

[0097] Titanium (Ti) is an element that precipitates as TiC during steel cooling, contributing to increased strength. The Ti content can be 0%, but it can be included as needed to achieve the aforementioned effect. The Ti content can also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ti is present in excess, there is a concern that coarse TiN will form, impairing toughness. Therefore, the Ti content is preferably 0.150% or less, but can also be 0.100% or less, or 0.050% or less.

[0098] (Nb: 0~0.150%)

[0099] Niobium (Nb) is an element that contributes to increased strength by improving hardenability. The Nb content can be 0%, but it may be included as needed to achieve the aforementioned effect. The Nb content can also be 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is preferably 0.150% or less, but can also be 0.120% or less, 0.100% or less, or 0.060% or less.

[0100] (V: 0~0.150%)

[0101] V (Vanadium) is an element that contributes to an increase in strength by improving hardenability. The V content can also be 0%, but it can also be contained as needed in order to obtain the above effect. The V content can also be 0.001% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is preferably 0.150% or less, and can be 0.100% or less or 0.060% or less.

[0102] (Cr: 0 to 2.00%)

[0103] Cr (Chromium) is effective for increasing the strength of steel by improving the hardenability of the steel. The Cr content can also be 0%, but it can also be contained as needed in order to obtain the above effect. The Cr content can also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, if Cr is contained in excess, there is a concern that a large amount of Cr carbide will be formed, which will instead impair the hardenability. Therefore, the Cr content is preferably 2.00% or less, and can be 1.80% or less or 1.50% or less.

[0104] (Ni: 0 to 2.00%)

[0105] Ni (Nickel) is an element that is effective for increasing the strength of steel by improving the hardenability of the steel. The Ni content can also be 0%, but it can also be contained as needed in order to obtain the above effect. The Ni content can also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, the addition of Ni in excess leads to an increase in cost. Therefore, the Ni content is preferably 2.00% or less, and can be 1.80% or less or 1.50% or less.

[0106] (Cu: 0 to 2.00%)

[0107] Cu (Copper) is an element that is effective for increasing the strength of steel by improving the hardenability of the steel. The Cu content can also be 0%, but it can also be contained as needed in order to obtain the above effect. The Cu content can also be 0.001% or more, 0.01% or more, 0.10% or more, or 0.50% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, a slab crack after casting, and a decrease in weldability, the Cu content is preferably 2.00% or less, and can be 1.80% or less, 1.50% or less, or 1.00% or less.

[0108] (Mo: 0 to 1.00%)

[0109] Mo (molybdenum) is an effective element for increasing the strength of steel by increasing the hardenability of the steel. The Mo content can also be 0%, but can be contained as needed in order to obtain the above-mentioned effect. The Mo content can also be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and weldability, the Mo content is preferably 1.00% or less, and can be 0.90% or less, or 0.80% or less.

[0110] (W: 0 to 1.00%)

[0111] W (tungsten) is an effective element for increasing the strength of steel by increasing the hardenability of the steel. The W content can also be 0%, but can be contained as needed in order to obtain the above-mentioned effect. The W content can also be 0.001% or more, 0.01% or more, 0.10% or more, or 0.30% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and weldability, the W content is preferably 1.00% or less, and can be 0.90% or less, 0.80% or less, or 0.60% or less.

[0112] (Ca: 0 to 0.100%)

[0113] Ca (calcium) is an element that contributes to the control of inclusions, particularly contributes to the fine dispersion of inclusions, and has the effect of increasing toughness. The Ca content can also be 0%, but can be contained as needed in order to obtain the above-mentioned effect. The Ca content can also be 0.0001% or more, 0.001% or more, 0.010% or more, or 0.020% or more. On the other hand, if Ca is contained in excess, the deterioration of surface properties is sometimes noticeable. Therefore, the Ca content is preferably 0.100% or less, and can be 0.080% or less, 0.050% or less, or 0.030% or less.

[0114] (Mg: 0 to 0.100%)

[0115] Mg (magnesium) is an element that contributes to the control of inclusions, particularly contributes to the fine dispersion of inclusions, and has the effect of increasing toughness. The Mg content can also be 0%, but can be contained as needed in order to obtain the above-mentioned effect. The Mg content can also be 0.0001% or more, 0.001% or more, 0.010% or more, or 0.020% or more. On the other hand, if Mg is contained in excess, the deterioration of surface properties is sometimes noticeable. Therefore, the Mg content is preferably 0.100% or less, and can be 0.090% or less, 0.080% or less, or 0.050% or less.

[0116] (Zr: 0 to 0.100%)

[0117] Zr (zirconium) is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and has the effect of improving toughness. The Zr content can also be 0%, but can be contained as needed in order to obtain the above effects. The Zr content can also be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if Zr is contained in excess, the deterioration of surface properties is sometimes noticeable. Therefore, the Zr content is preferably 0.100% or less, and can be 0.050% or less, 0.040% or less, or 0.030% or less.

[0118] (Hf: 0 to 0.100%)

[0119] Hf (hafnium) is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and has the effect of improving toughness. The Hf content can also be 0%, but can be contained as needed in order to obtain the above effects. The Hf content can also be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, if Hf is contained in excess, the deterioration of surface properties is sometimes noticeable. Therefore, the Hf content is preferably 0.100% or less, and can be 0.080% or less, 0.050% or less, or 0.030% or less.

[0120] (REM: 0 to 0.100%)

[0121] REM (rare earth element) is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and has the effect of improving toughness. The REM content can also be 0%, but can be contained as needed in order to obtain the above effects. The REM content can also be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.010% or more. On the other hand, if REM is contained in excess, the deterioration of surface properties is sometimes noticeable. Therefore, the REM content is preferably 0.100% or less, and can be 0.050% or less, 0.040% or less, or 0.030% or less. Furthermore, REM is an abbreviation for Rare Earth Metal, and refers to an element belonging to the lanthanide series. REM is usually added in the form of a mixed rare earth.

[0122] In the steel material of the present application, the remainder other than the above component composition is composed of Fe and impurities. Here, the so-called impurities are components and the like that are mixed by various causes in the manufacturing process, represented by raw materials such as ores and scrap, and the like, when the steel material is manufactured industrially.

[0123] In the present application, the analysis of the composition of the steel material can be performed by an elemental analysis method known to those skilled in the art, for example, by an inductively coupled plasma mass spectrometry (ICP-MS method). However, as for C and S, a combustion infrared absorption method can be used for the measurement, and as for N, an inert gas melting-thermal conductivity method can be used for the measurement. These analyses can be performed using a sample collected from the steel material according to a method based on JIS G0417: 1999.

[0124] [Zn-based plated steel material]

[0125] The Zn-based plated steel material in the present application is a steel material on the surface of which a Zn-based plated layer is formed. The Zn-based plated layer can be formed on one side of the steel material or on both sides thereof. As the Zn-based plated layer, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electroplated zinc layer, an electroplated alloyed zinc layer, and the like can be exemplified. More specifically, as the plating species, for example, Zn-0.2% Al (GI), Zn-0.09% Al (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si, and the like can be used. In the present application, the Zn-based plated layer can contain Zn as long as it contains Zn, and also includes a plated layer in which the largest component is not Zn. Further, another layer can be included between the steel material and the Zn-based plated layer.

[0126] (Composition of the Zn-based plated layer)

[0127] The composition contained in the Zn-based plated layer in the present application will be described. The "%" of the content of the element means "mass%" unless otherwise specified. In the numerical range of the composition of the plated layer, the numerical range indicated by "~" means the range including the lower limit value and the upper limit value indicated by the numerical values before and after the "~" unless otherwise specified.

[0128] (Al: 0 to 60.0%)

[0129] Al is an element that improves the corrosion resistance of the plated layer by being contained together with Zn or alloyed, and thus can be contained as needed. Therefore, the content of Al can also be 0%. In order to form a plated layer containing Zn and Al, the content of Al is preferably 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, even if Al is excessively contained, the effect of improving the corrosion resistance is saturated, and thus the content of Al is preferably 60.0% or less, for example, 55.0% or less, 50.0% or less, 40.0% or less, 30.0% or less, 20.0% or less, 10.0% or less, or 5.0% or less.

[0130] (Mg: 0 to 15.0%)

[0131] Mg is an element that improves corrosion resistance of the plated layer by being contained or alloyed with Zn and Al, and thus can be contained as needed. Therefore, the Mg content can also be 0%. In order to form a plated layer containing Zn, Al, and Mg, the Mg content is preferably 0.01% or more, and can be, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, if Mg is excessively contained, Mg cannot be completely dissolved in the plating bath and is suspended as an oxide, and if galvanizing is performed with such a plating bath, there is a concern that the oxide will adhere to the surface layer of the plated layer, resulting in poor appearance, or that a plating defect will occur. Therefore, the Mg content is preferably 15.0% or less, and can be, for example, 10.0% or less, 5.0% or less.

[0132] (Fe: 0 to 15.0%)

[0133] Fe can be contained in the plated layer by diffusing from the steel material when the plated steel material is subjected to heat treatment after the Zn-containing plated layer is formed on the steel material. Therefore, Fe can not be contained in the plated layer in a state where heat treatment is not performed, and thus the Fe content can be 0%. Furthermore, the Fe content can be 1.0% or more, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more. On the other hand, the Fe content is preferably 15.0% or less, and can be, for example, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0134] (Si: 0 to 3.0%)

[0135] Si is an element that further improves corrosion resistance if contained in a Zn-containing plated layer, particularly a Zn-Al-Mg plated layer, and thus can be contained as needed. Therefore, the Si content can also be 0%. From the viewpoint of improving corrosion resistance, the Si content can be, for example, 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more. Furthermore, the Si content can be 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.2% or less.

[0136] The basic component composition of the Zn-based plated layer is as described above. In addition, the Zn-based plated layer can optionally contain one or two or more of Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, Ti: 0 to 1.00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%. Although there is no particular limitation, from the viewpoint of sufficiently exerting the effects and functions of the above-described basic components constituting the Zn-based plated layer, the total content of these optional additive elements is preferably 5.00% or less, and more preferably 2.00% or less.

[0137] The remaining portion of the Zn-based plated layer other than the above components is composed of zinc and impurities. The so-called impurities in the Zn-based plated layer are components and the like that are mixed in as raw materials and the like due to various reasons in the manufacturing process at the time of manufacturing the plated layer. In the plated layer, as impurities, elements other than the above-described essential components and optional components can be contained in a trace amount within a range that does not interfere with the effects of the present application.

[0138] The composition of the Zn-based plated layer can be determined by dissolving the plated layer in an acid solution to which a corrosion inhibitor for suppressing corrosion of the steel material is added, and measuring by ICP (high-frequency inductively coupled plasma) emission spectroscopy.

[0139] The thickness of the Zn-based plated layer can be, for example, 3 to 50 μm. In addition, the attached amount of the plated layer is not particularly limited, and can be, for example, 10 to 170 g / m 2 In the present application, the attached amount of the plated layer can be determined from the weight change before and after pickling by dissolving the plated layer in an acid solution to which a corrosion inhibitor for suppressing corrosion of the base metal is added.

[0140] [Spot-welded area]

[0141] The steel welded member according to the present application includes at least one spot-welded area among the above-described Zn-based plated steels. Therefore, a plurality of (two or more) Zn-based plated steels are joined together by spot welding. Figure 1 is a cross-sectional view for illustrating a spot-welded area of an example of the steel welded member according to the present application. Figure 1 In the present application, two Zn-based plated steels 11 are joined together via a spot-welded area 21. Generally, if the two Zn-based plated steels 11 are spot-welded, as shown in Figure 1 the portion pressed by the electrode, a portion formed of a molten solidified portion 23 of the steel component and / or the plated layer component, which is called a nugget portion 23, is formed, and a crimped portion 25 joined in a manner that the above-described components do not melt outside the nugget portion 23 is formed. Therefore, the spot-welded area 21 includes the nugget portion 23 and the crimped portion 25, and is typically composed only of the nugget portion 23 and the crimped portion 25. The nugget portion 23 and the crimped portion 25 are easily distinguished by, for example, a backscattered electron image (BSE image) of a scanning electron microscope (SEM) because the composition is different. In the present application, the shape and the composition of the nugget portion 23 are not particularly limited.

[0142] (Crimped portion)

[0143] The steel welded component of the present invention contains 20-80% by area of ​​a Γ phase and one or more oxides with a major diameter of 0.5 μm or more in the area of ​​the spot weld zone at the end of the crimped portion. Preferably, the above-mentioned area contains a Γ phase, an α phase, and an oxide with a major diameter of 0.5 μm or more. Here, in the present invention, the term "end of the crimped portion" refers to the end of the spot weld zone of multiple Zn-based coated steels, and the boundary between the portion (crimped portion) joined by welding of the multiple Zn-based coated steels and the unjoined portion. More specifically, the "end of the crimped portion" exists in... Figure 1 Within the dashed lines, Figure 2 It is designated by number 27. Therefore, "the area 0.5 mm from the end of the crimped part" refers to the boundary between the joint and non-joint parts of the two Zn-based coated steels. Figure 2 (Number 27) in the opposite direction to the direction of the melting core 23 ( Figure 2 The area from the left side to 0.5mm, Figure 2 The region 29, denoted by number 29 (mesh pattern), is located 0.5 mm from the end of the crimping portion. This region typically has a fan-shaped form, but can vary depending on the spot welding conditions. Therefore, in this invention, the region 29, 0.5 mm from the end of the crimping portion, refers to the area sandwiched between two steel pieces when a circle is drawn with the end 27 of the crimping portion as the center. Hereinafter, the region 0.5 mm from the end of the crimping portion in the spot welding area will be simply referred to as the "end-near region". Figure 3 This invention relates to a scanning electron microscope (SEM) backscattered electron image (BSE image) of a cross-section of the region near the end of the crimped portion of the spot weld area of ​​an illustrative steel welded component. The BSE image shows that the region near the end of the spot weld area between the two Zn-based coated steels 11 contains oxide 13, Γ phase 15, and α phase 17. Furthermore, Figure 3 The black areas visible at the interface between the Zn-plated steel 11 and the spot welding area can be considered not to be oxides, but rather cracks, etc., which can be identified through elemental analysis.

[0144] (Γ phase)

[0145] The steel welded components involved in this invention contain 20-80% Γ phase in the region near the ends, based on area ratio. Here, the "Γ phase" refers to Fe-Zn intermetallic compounds with a body-centered cubic (bcc) lattice structure: Fe3Zn. 10 . Reference Figure 3 The region near the ends of the coated steel 11 contains a Γ phase 15 (the phase seen as light gray). This Γ phase is, for example, determined by SEM-EDS. Figure 3The steel welded member according to the present application can also contain the α phase in the vicinity of the end portion. The α phase is referred to as a zinc solid-solution ferrite phase. Reference is made to JIS G 0555-2: 2005, "Steel - Microstructure - Part 2: Determination of phase fractions".

[0146] (α phase)

[0147] The steel welded member according to the present application can also contain the α phase in the vicinity of the end portion. The α phase is referred to as a zinc solid-solution ferrite phase. Reference is made to JIS G 0555-2: 2005, "Steel - Microstructure - Part 2: Determination of phase fractions". Figure 3 The α phase 17 (phase seen in dark gray) is contained in the vicinity of the end portion between the plated steels 11. This α phase is formed, for example, by performing element analysis on the cross-sectional structure of the vicinity of the end portion by SEM-EDS. Figure 3 The steel welded member according to the present application can also contain the α phase in the vicinity of the end portion. The α phase is referred to as a zinc solid-solution ferrite phase. Reference is made to JIS G 0555-2: 2005, "Steel - Microstructure - Part 2: Determination of phase fractions". The α phase area ratio is not particularly limited, but if a Zn-based plated steel is spot-welded under ordinary conditions, it is formed in a prescribed area ratio in most cases. The α phase area ratio is usually 5% or more in the vicinity of the end portion, and can be, for example, 10% or more, 15% or more, or 20% or more. The upper limit of the α phase area ratio is not particularly limited, but from the viewpoint of forming the Γ phase and the oxide described later in a sufficient area ratio and having a sufficient size, it is preferably 50% or less, and can be, for example, 45% or less, 40% or less, or 35% or less. The steel welded member according to the present application can also not contain the α phase in the vicinity of the end portion, that is, the α phase area ratio can also be 0%. Alternatively, the vicinity of the end portion can also be composed only of the oxide, the Γ phase, and the α phase.

[0148] (oxide having a length of 0.5 μm or more)

[0149] Reference is made to JIS G 0555-2: 2005, "Steel - Microstructure - Part 2: Determination of phase fractions". Figure 3The steel welded member according to the present application contains coarse oxides 13 in the vicinity of the end portion. Such oxides are aggregates of internal oxides formed in the interior of the steel material, which can be obtained by subjecting the steel material to a prescribed annealing treatment (including a pre-treatment for annealing). Further, the coarse oxides 13 are oxides containing Si and / or Mn. The Si and / or Mn greatly contribute to improvement of the corrosion resistance of the steel welded member by promoting the formation of an insulating film of a Zn-based corrosion product in a corrosive environment. The steel welded member according to the present application contains one or more oxides having a length of 0.5 μm or more in the vicinity of the end portion. Here, the "length" of the oxide refers to the length of the largest line segment that cuts the oxide. The shape of the oxide is not particularly limited, and can be round, substantially round, elliptical, polygonal, or the like. Regardless of the shape, the length refers to the length of the largest line segment that cuts the oxide. If the length of the oxide is 0.5 μm or more, the contact area with the Γ phase increases, and the corrosion resistance of the welded portion of the steel welded member is significantly improved. The larger the oxide, and more specifically the longer the circumference of the oxide, the larger the contact area with the Γ phase, and thus the effect of improving the corrosion resistance can be exerted. Therefore, the length of the oxide present in the vicinity of the end portion is preferably 0.7 μm or more, more preferably 1.0 μm or more, and further preferably 1.5 μm or more. The upper limit of the length of the oxide is not particularly limited, but the length of the oxide can also be 10.0 μm or less.

[0150] In the steel welded member according to the present application, as long as one or more oxides having a length of 0.5 μm or more are contained in the vicinity of the end portion, but from the viewpoint of increasing the contact area with the Γ phase, the number of oxides having a length of 0.5 μm or more can also be two or more, three or more, four or more, five or more, ten or more, or twenty or more. Further, the length of the largest oxide (i.e., the largest length of the oxide) among the oxides present in the vicinity of the end portion can also be 0.7 μm or more, 1.0 μm or more, 1.5 μm or more, 2.0 μm or more, or 3.0 μm or more. The upper limit of the largest length of the oxide is not particularly limited, and the largest length of the oxide can also be, for example, 10.0 μm or less.

[0151] The area ratio of the oxides contained in the spot-welded portion is preferably 5 to 45%, and more preferably 10 to 40%.

[0152] (Composition of the Oxide)

[0153] In this invention, the oxide, in addition to oxygen, contains one or more of the elements contained in the aforementioned steel. Typically, its composition contains Si, O, and Fe, and may further contain Mn, depending on the specific circumstances. More specifically, the oxide typically contains Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%. Besides the aforementioned elements, the oxide may also contain elements that may be present in the aforementioned steel (e.g., Cr).

[0154] [Methods for determining the area fraction of the Γ phase and α phase, as well as the major diameter and number of oxides]

[0155] The area fraction of the Γ phase and α phase can be determined by using... Figure 3 Backscattered electron images (BSE images) of such cross-sections are obtained using scanning electron microscopy (SEM). Specifically, first, a BSE image of the end containing the crimped portion is obtained using SEM. Then, from the BSE image, a specific image of the end of the crimped portion is obtained (…). Figure 2 (Number 27), then the specific spot weld area is 0.5mm from the end of the crimped part ( Figure 2 The region near the end of the oxide (29). In the BSE image, since oxides are composed of relatively lighter elements compared to the Γ phase and α phase, they are shown as the darkest (typically black) in the BSE image. Therefore, in order to distinguish between the "Γ phase and α phase" and "oxides", the BSE image is binarized, thereby determining the outline of a specific oxide and the major axis and number of oxides. In addition, the area ratio of oxides is calculated based on this binarized image. Furthermore, cracks and gaps (areas where none exist) in the BSE image are shown as black, but they can be distinguished from oxides using elemental analysis (e.g., EDS) accompanying SEM. Next, since the Γ phase is composed of heavier elements compared to the α phase, it is shown as the brightest in the BSE image. Therefore, in order to distinguish between the "Γ phase" and "α phase and oxides", the BSE image is binarized, thereby calculating the area ratio of the Γ phase for a specific Γ phase region. Then, the area ratio of the α phase is obtained by subtracting the area ratios of oxides, the Γ phase, cracks, and interstitials from 100%. Furthermore, for each phase in the BSE image, ordinary elemental analysis can be performed to determine whether each phase is the Γ phase, the α phase, oxide, or any other phase.

[0156] <Manufacturing Methods for Welded Steel Components>

[0157] The preferred manufacturing method of the steel welded component according to the present invention will be described below. The following description is intended to illustrate a characteristic method for manufacturing the steel welded component according to the present invention, but is not intended to limit the manufacturing of the steel welded component to the manufacturing method described below.

[0158] The steel welded member according to the present application can be obtained by performing a steel material manufacturing process of manufacturing a steel material, a plating process of forming a Zn-based plated layer on each of the steel material surfaces to manufacture a Zn-based plated steel material, and a welding process of joining two plated steel materials by spot welding. The steel welded member according to the present application is more specifically a steel welded member containing 20 to 80% of a gamma phase in terms of area ratio in the vicinity of the end portion, and containing one or more oxides having a length of 0.5 μm or more. An effective method for obtaining the steel welded member is to form internal oxides in advance in the surface layer of the steel material (for example, a region from the surface of the steel material to a depth of 50 μm, that is, the inside of the steel material) in the steel material manufacturing process. As such internal oxides, there are, for example, granular oxides dispersed in the form of particles in the steel grains or on the grain boundaries, grain boundary oxides present along the grain boundaries of the steel, and / or dendritic oxides present in the form of dendrites in the grains. If these internal oxides are formed in the inside of the steel material, and spot welding is performed after the Zn-based plated layer is formed, the internal oxides flow together with the molten portions of the steel components and the plated layer components to the vicinity of the end portion of the press-contact portion, that is, the vicinity of the end portion, and the internal oxides form aggregates, so that oxides having a length of 0.5 μm or more are easily formed in the vicinity of the end portion of the steel welded member. In contrast, when oxides are formed in the form of a film on the surface of the steel material (external oxides), that is, when an external oxide layer is formed, there is a concern that the formation of oxides having a length of 0.5 μm or more is insufficient, and the corrosion resistance of the spot-welded portion of the steel welded member is insufficient. Furthermore, when the plated layer cannot be properly formed due to the presence of the external oxide layer, as a result, there is a concern that the gamma phase and the oxides cannot be formed. In order to form internal oxides in the surface layer of the steel material, an effective method is to perform an annealing process after a prescribed annealing pre-treatment process (grinding process) is performed after rolling, under prescribed conditions. In the following, a steel material manufacturing process, a plating process, and a welding process will be described with the case where a steel sheet is used as the steel material. Furthermore, the steel material can be in any shape, and the manufacturing method of the steel welded member when a steel material other than a steel sheet is used can be appropriately changed according to methods known in the technical field.

[0159] [Steel material manufacturing process]

[0160] In the following, the case where a steel sheet is used as the steel material will be described. The steel sheet can be obtained, for example, by performing a casting process of casting molten steel having a prescribed composition to form a billet, a hot-rolling process of obtaining a hot-rolled steel sheet by hot-rolling the billet, a coiling process of coiling the hot-rolled steel sheet, a cold-rolling process of obtaining a cold-rolled steel sheet by cold-rolling the coiled hot-rolled steel sheet, a grinding process of introducing dislocations into the surface of the cold-rolled steel sheet, and an annealing process of annealing the ground cold-rolled steel sheet. Alternatively, the coiling process can be omitted after the hot-rolling process, and the cold-rolling process can be performed directly after pickling.

[0161] [Casting process]

[0162] The conditions of the casting step are not particularly limited. For example, it is sufficient to successively perform melting using a blast furnace and an electric furnace or the like, perform various secondary refining, and then cast using a general continuous casting method, an ingot casting method, or the like.

[0163] (Hot rolling step)

[0164] A hot-rolled steel sheet can be obtained by hot-rolling a steel slab cast as described above. The hot rolling step can be performed by directly hot-rolling the cast steel slab or by hot-rolling after temporary cooling and reheating. In the case of reheating, the heating temperature of the steel slab can be, for example, 1100°C to 1250°C. In the hot rolling step, rough rolling and finish rolling are generally performed. The temperature and reduction rate of each rolling can be appropriately changed according to the desired metal structure and sheet thickness. For example, the finish rolling can be performed at a finish rolling end temperature of 900 to 1050°C and a finish rolling reduction rate of 10 to 50%.

[0165] (Winding step)

[0166] The hot-rolled steel sheet can be wound at a prescribed temperature. The winding temperature can be appropriately changed according to the desired metal structure or the like, and can be, for example, 500 to 800°C. The hot-rolled steel sheet can be subjected to a prescribed heat treatment by uncoiling before winding or after winding. Alternatively, the winding step can not be performed, and after the hot rolling step, pickling can be performed, followed by the cold rolling step described later.

[0167] (Cold rolling step)

[0168] After the hot-rolled steel sheet has been subjected to pickling or the like, a cold-rolled steel sheet can be obtained by cold-rolling the hot-rolled steel sheet. The reduction rate of cold rolling can be appropriately changed according to the desired metal structure and sheet thickness, and can be, for example, 20 to 80%. After the cold rolling step, the cold-rolled steel sheet can be cooled to room temperature, for example, by air cooling.

[0169] (Grounding step)

[0170] To produce the steel welded member according to the present application, the internal oxides are sufficiently obtained in the surface layer of the steel material by performing a grinding process before annealing the cold-rolled steel sheet. By this grinding process, a large number of dislocations can be introduced into the surface of the cold-rolled steel sheet. Diffusion of oxygen and the like is faster at the grain boundaries than within the grains, so by introducing a large number of dislocations into the surface of the cold-rolled steel sheet, a large number of channels can be formed at the grain boundaries. Therefore, oxygen easily diffuses (intrudes) into the interior of the steel along these dislocations at the time of annealing, and in addition, the diffusion speed of Si and Mn is increased, and as a result, by the combination of oxygen with Si and / or Mn in the interior of the steel, the internal oxides such as the granular-type oxides, the grain boundary-type oxides, and / or the dendrite-type oxides described above can be promoted to form. Along with the promotion of the formation of such internal oxides due to the grinding process, the Si and Mn concentrations in the steel parent phase around the internal oxides decrease, and a deficiency layer of Si and Mn can be formed in the surface layer of the steel sheet. The grinding process is not particularly limited, and for example, it can be performed by grinding the surface of the cold-rolled steel sheet with a heavy-duty grinding brush under conditions in which the grinding amount is 10 to 200 g / m 2 The grinding amount of the heavy-duty grinding brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited, and for example, it can be adjusted by appropriately selecting the number of heavy-duty grinding brushes, the rotational speed, the brush pressure, the coating liquid used, and the like. By performing such a grinding process, the internal oxides can be efficiently formed in the surface layer of the steel sheet in the annealing process described later. It is considered that the internal oxides and the deficiency layer of Si and Mn thus formed contribute to the formation of the desired end-portion vicinity region. In particular, the internal oxides are aggregated in the end-portion vicinity region at the time of alloying the plating layer and the base metal by spot welding, and form oxides having an aspect ratio of 0.5 μm or more in a manner of coexisting with the Γ phase described above.

[0171] (annealing process)

[0172] It is preferable to anneal the cold-rolled steel sheet on which the grinding process described above has been performed. It is preferable to perform annealing in a state in which tension is applied to the cold-rolled steel sheet in the rolling direction. In particular, in a region in which the annealing temperature is 500°C or higher, it is preferable to perform annealing with increased tension compared to other regions, and specifically, it is preferable to perform annealing in a state in which tension of 3 to 150 MPa, particularly 15 to 150 MPa, is applied to the cold-rolled steel sheet in the rolling direction in a region in which the annealing temperature is 500°C or higher. If tension is applied at the time of annealing, a large number of dislocations can be more efficiently introduced into the surface of the cold-rolled steel sheet. Therefore, oxygen easily diffuses (intrudes) into the interior of the steel along these dislocations at the time of annealing, and in addition, the diffusion speed of Si and Mn is increased, so oxides are easily generated in the interior of the steel sheet.

[0173] To properly form the internal oxides, the holding temperature of the annealing step is preferably 700 to 900°C, more preferably 720 to 870°C. By setting to such a temperature range, formation of the external oxide layer is suppressed and oxides are formed inside the steel sheet. If the above holding temperature is less than 700°C, the desired internal oxides are not sufficiently formed at the time of annealing, and there is a concern that coarse oxides cannot be obtained in the vicinity of the end portion. If the above holding temperature exceeds 900°C, there is a concern that an external oxide layer is formed at the time of annealing and it is difficult to form the Γ phase in the vicinity of the end portion. The rate of temperature increase to the above holding temperature is not particularly limited, and the temperature can be increased at a rate of 1 to 10°C / sec. In addition, the temperature increase can be performed in two stages by a first temperature increase rate of 1 to 10°C / sec and a second temperature increase rate of 1 to 10°C / sec that is different from the first temperature increase rate.

[0174] The holding time at the holding temperature of the above annealing step is preferably 10 to 300 seconds, more preferably 30 to 250 seconds. By setting to such a range, formation of the external oxide layer is suppressed and oxides are formed inside the steel sheet. If the above holding time is less than 10 seconds, the desired internal oxides are not sufficiently formed at the time of annealing, and there is a concern that coarse oxides cannot be obtained in the vicinity of the end portion. If the above holding time exceeds 300 seconds, there is a concern that an external oxide layer is formed at the time of annealing and it is difficult to form the Γ phase in the vicinity of the end portion.

[0175] The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C, from the viewpoint of sufficiently forming the internal oxides. If the dew point is too low, there is a concern that an external oxide layer is formed on the surface of the steel sheet and the internal oxides are not sufficiently formed, and thus there is a concern that the Zn-based plating layer cannot be properly formed and the desired Γ phase and coarse oxides cannot be formed. On the other hand, if the dew point is too high, there is a concern that the desired internal oxides are not sufficiently formed at the time of annealing and the desired Γ phase cannot be obtained in the vicinity of the end portion, and in addition, it is not preferable in terms of manufacturing because water droplets are formed more. In addition, the atmosphere in the annealing step can be a reducing atmosphere, and more specifically, it can be a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere in which hydrogen is 1 to 10% (for example, hydrogen 4% and nitrogen in the balance).

[0176] In addition, it is effective to remove the internal oxide layer of the steel sheet at the time of the annealing step in advance. An internal oxide layer is sometimes formed in the surface layer of the steel sheet between the rolling steps, particularly between the hot rolling steps. The internal oxide layer formed by such a rolling step is concerned to hinder the formation of internal oxides in the annealing step, and thus it is preferable to remove the internal oxide layer in advance before the annealing by pickling treatment or the like. More specifically, it is desirable to make the depth of the internal oxide layer of the cold-rolled steel sheet at the time of the annealing step 0.5 μm or less, preferably 0.3 μm or less, more preferably 0.2 μm or less, and further preferably 0.1 μm or less in advance.

[0177] By performing the above-described respective steps, a steel sheet in which oxides are sufficiently contained in the inside of the steel sheet can be obtained.

[0178] [Plating Step]

[0179] The Zn-based plated steel sheet can be obtained by a plating step of forming a Zn-based plated layer on the steel sheet manufactured as described above. The plating step can be performed as long as it is performed according to a method known to those skilled in the art. The plating step can be performed, for example, by hot-dip plating or by electroplating. It is preferable that the plating step be performed by hot-dip plating. The conditions of the plating step can be appropriately set as long as the composition, the thickness, and the adhesion amount of the desired plated layer are taken into consideration. An alloying treatment can be performed after the plating treatment. Typically, the conditions of the plating step are preferably set so as to form a plated layer containing Al: 0 to 60.0%, Mg: 0 to 15.0%, Fe: 0 to 15%, and Si: 0 to 3%, with the remainder being Zn and impurities. More specifically, the conditions of the plating step can be appropriately set, for example, so as to form Zn-0.2% Al (GI), Zn-0.09% Al (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si.

[0180] [Welding Step]

[0181] In the welding step, two or more of the Zn-based plated steel sheets obtained by the above-described plating step are prepared, and spot welding is performed at least at one site. Therefore, a spot-welded region is formed between the two steel sheets by the welding step, and as a result, a steel welded member in which a plurality of Zn-based plated steel materials having a Zn-based plated layer on the surface of the steel sheet are joined via at least one spot-welded region can be obtained. The conditions at the time of spot welding can be set according to conditions known to those skilled in the art. For example, when a welding electrode having a tip diameter of 6 to 8 mm in a dome-shaped radiation type is used, it can be set to a pressing force of 1.5 to 6.0 kN, an electric current time of 0.1 to 1.0 s (5 to 50 cycles, power frequency: 50 Hz), and an electric current of 4 to 15 kA.

[0182] As described above, in the production of a steel welded member, by producing a steel material having internal oxides through prescribed steel material production processes (particularly, a grinding process and an annealing process), and using a Zn-based plated steel material on which a Zn-based plating layer is formed, it is possible to produce a steel welded member that contains 20 to 80% of Γ phase (Fe3Zn 10 ) in terms of area ratio in a region near the end portion of the press-contact portion of the spot-welded region, and contains one or more oxides having a length of 0.5 μm or more.

[0183] Examples

[0184] Hereinafter, the present application will be described in more detail by way of examples, but the present application is not limited by these examples in any way.

[0185] (Production of Steel Material)

[0186] A steel slab was formed by casting molten steel whose composition was adjusted, and a cold-rolled steel sheet was obtained by hot-rolling the steel slab, and cold-rolling after pickling. Next, after air-cooling to room temperature, the cold-rolled steel sheet was subjected to pickling treatment, and the internal oxide layer formed by rolling was removed to the internal oxide layer depth (μm) before annealing shown in Table 1. Next, a sample was collected from each cold-rolled steel sheet in accordance with the method based on JIS G0417: 1999, and the composition of the steel sheet was analyzed by ICP-MS method or the like. The composition of the steel sheet determined is shown in Table 1. The thickness of the steel sheet used was 1.6 mm in all.

[0187] Next, with respect to the cold-rolled steel sheet, after applying an aqueous NaOH solution, a heavy grinding brush was used to perform grinding at 10 to 200 g / m 2The amount of grinding by the grinding brush was 0.5 μm. The surface of the cold-rolled steel sheet was ground (sample No. 1 was not ground). Then, each steel sheet sample was annealed according to the dew point, holding temperature, and holding time shown in Table 1 (annealing atmosphere: hydrogen 4% and nitrogen balance), thereby producing each steel sheet sample. In all of the steel sheet samples, the temperature increase rate to 500°C at the time of annealing was set to 6.0°C / sec, and the temperature increase rate from 500°C to the holding temperature was set to 2.0°C / sec. In the above annealing, the annealing was performed in a state in which a tension of 0.5 MPa or more was applied to the cold-rolled steel sheet in the rolling direction, and a higher tension was applied in the rolling direction in a region at an annealing temperature of 500°C or more than in other regions, specifically, a tension of 3 to 150 MPa was applied (sample No. 116 did not apply such a tension). In Table 1, the presence or absence of grinding based on the grinding brush and the conditions of the annealing (presence or absence of application of a tension of 3 to 150 MPa in a region at an annealing temperature of 500°C or more, dew point (°C), holding temperature (°C), and holding time (sec)) are shown. Further, with respect to each steel sheet sample, a JIS No. 5 tensile test piece having a direction at a right angle to the rolling direction as a length direction was collected, and a tensile test was performed in accordance with JIS Z2241 (2011), and the tensile strength of all of the examples was 440 MPa or more.

[0188] (Zn-based plated steel material sample production)

[0189] After each steel material sample was cut to a size of 100 mm x 200 mm, hot dip galvanizing was performed on each steel sheet sample, followed by alloying treatment. In the hot dip galvanizing step, the cut sample was immersed in a hot dip galvanizing bath at 440°C for 3 seconds. After the immersion, drawing was performed at 100 mm / sec, and the plating adhesion amount was controlled to 50 g / m 2 . Then, alloying treatment was performed at 460°C, thereby obtaining a Zn-based plated steel material sample.

[0190] (Steel welded member sample production)

[0191] Each of the Zn-based plated steel samples was cut into a size of 50 mm x 100 mm and two pieces were prepared, and the two pieces of the Zn-based plated steel samples were spot-welded to obtain a steel welded member sample. The conditions of the spot-welding were set to a pressing force of 3.0 kN, an electric current of 7 kA, and an electric current time of 0.5 seconds (20 cycles, power frequency of 50 Hz) using a welding electrode of a dome-shaped radiation type with a tip diameter of 8 mm. In addition, the obtained steel welded member sample was subjected to zinc phosphate treatment (SD5350 system: a standard prepared by Nippon Paint Industrial Coatings, Inc.), and further subjected to electrophoretic coating (PN110 Power Nix Grey: a standard prepared by Nippon Paint Industrial Coatings, Inc.) to a thickness of 20 μm, and baked at a baking temperature of 150°C for 20 minutes to obtain an evaluation sample.

[0192] (Analysis of metal structure in a region 0.5 mm from the end of the pressure-welded portion)

[0193] For each of the evaluation samples, area ratio measurement of the Γ phase and the α phase was performed using a BSE image of a cross section of the welded zone by SEM. Specifically, first, after a cross section sample of the welded zone was prepared, a BSE image including the end of the pressure-welded portion was obtained by SEM, the end of the pressure-welded portion was specified from the BSE image, and then a region 0.5 mm from the end of the pressure-welded portion (end vicinity region) of the spot-welded zone was specified. In the specified end vicinity region, the BSE image was subjected to binaryzation processing in order to discriminate "Γ phase and α phase" and "oxide", and thereby the outline of the oxide was specified, and the length diameter of each of the observed oxides and the number of the oxides were measured. Further, the area ratio of the oxides was calculated based on the binaryzation image. As for the cracks and gaps and the like in the BSE image, identification from the oxides was performed using SEM-EDS attached to the SEM. Then, in the end vicinity region, the BSE image was subjected to binaryzation processing in order to discriminate "Γ phase" and "α phase and oxide", and thereby the region of the Γ phase was specified, and the Γ phase area ratio was calculated. Then, the α phase area ratio was calculated by subtracting the area ratios of the oxides and the Γ phase and the area ratios of the cracks and gaps and the like from 100%. The area ratios of the Γ phase and the α phase, the number of oxides having a length diameter of 0.5 μm or more (coarse oxide number), and the maximum value of the length diameter of each of the oxides (oxide maximum diameter) are shown in Table 1.

[0194] (Evaluation of corrosion resistance of the spot-welded zone)

[0195] To each steel welded member sample, a cyclic corrosion test was performed in accordance with JASO (M609-91), and the corrosion resistance of the spot-welded area was evaluated based on the corrosion state of the steel after 120 cycles. Regarding each evaluation sample, after the above corrosion test was completed, a cross section (for example, a portion including the spot-welded area (the nugget portion and the caulking portion) and the steel) was observed by SEM. From the observation image, the maximum corrosion depth of the steel portion in a 1 mm range was measured from the vicinity of the end portion of the caulking portion toward the steel (for example, upward in the middle). When the maximum corrosion depth was 0.1 mm or less, it was evaluated as O, and when it exceeded 0.1 mm, it was evaluated as X. The evaluation of each sample is shown in Table 1. Figure 1 Figure 1

[0196]

[0197] ​​The test pieces No. 2 to 8 and 18 to 31 have high corrosion resistance of the spot-welded zone because they all satisfy the steel composition, the area ratio of the Γ phase and the main condition of the coarse oxide. On the other hand, the test piece No. 1 does not have sufficient corrosion resistance of the spot-welded zone because it does not have the grinding treatment before annealing, and thus the internal oxide is not sufficiently formed at the time of annealing, and the Γ phase and the coarse oxide are not formed in the vicinity of the end portion. The test piece No. 9 does not have sufficient corrosion resistance of the spot-welded zone because the dew point at the time of annealing is low, and thus the plating layer is not properly formed, and the Γ phase and the coarse oxide are not formed in the vicinity of the end portion. The test piece No. 10 does not have sufficient corrosion resistance of the spot-welded zone because the dew point at the time of annealing is high, and thus the internal oxide is not sufficiently formed at the time of annealing, and the Γ phase is not formed in the vicinity of the end portion. The test piece No. 11 does not have sufficient corrosion resistance of the spot-welded zone because the holding temperature at the time of annealing is low, and thus the internal oxide is not sufficiently formed at the time of annealing, and the coarse oxide is not formed in the vicinity of the end portion. The test piece No. 12 does not have sufficient corrosion resistance of the spot-welded zone because the holding time at the time of annealing is long, and thus the external oxide layer is formed at the time of annealing, and the Γ phase is not formed in the vicinity of the end portion. The test piece No. 13 does not have sufficient corrosion resistance of the spot-welded zone because the holding time at the time of annealing is short, and thus the internal oxide is not sufficiently formed at the time of annealing, and the oxide is not formed in the vicinity of the end portion. The test piece No. 14 does not have sufficient corrosion resistance of the spot-welded zone because the holding temperature at the time of annealing is high, and thus the external oxide layer is formed at the time of annealing, and the Γ phase is not formed in the vicinity of the end portion. The test piece No. 15 does not have sufficient corrosion resistance of the spot-welded zone because the amount of Si is excessive, and thus the external oxide layer is formed at the time of annealing, the plating layer is not properly formed, and the Γ phase is not formed in the vicinity of the end portion. The test piece No. 16 does not have sufficient corrosion resistance of the spot-welded zone because the amount of Mn is excessive, and thus the external oxide layer is formed at the time of annealing, the plating layer is not properly formed, and the Γ phase is not formed in the vicinity of the end portion. The test piece No. 17 does not have sufficient corrosion resistance of the spot-welded zone because the internal oxide layer before annealing is thick, and thus the plating layer is not properly formed, and the Γ phase and the oxide are not formed in the vicinity of the end portion. The test piece No. 32 does not have sufficient corrosion resistance of the spot-welded zone because the prescribed tension is not applied at the time of annealing, and thus the internal oxide is not sufficiently formed at the time of annealing, and the Γ phase and the coarse oxide are not formed in the vicinity of the end portion.

[0198] Industrial applicability

[0199] According to the present application, a steel welded member having a spot weld zone with high corrosion resistance can be provided, which is suitable for use in applications such as automobiles, building materials, etc., particularly in automobile applications, and can exhibit high corrosion resistance as an automobile steel welded member, and long life can be expected. Therefore, it can be said that the present application is an extremely valuable invention in industry.

[0200] Explanation of symbols:

[0201] 1 Steel welded member

[0202] 11 Zn-based plated steel material

[0203] 13 Oxide

[0204] 15 Gamma phase

[0205] 17 Alpha phase

[0206] 21 Spot weld zone

[0207] 23 Fusion nugget portion

[0208] 25 Press contact portion

[0209] 27 End portion of press contact portion

[0210] 29 Area near end portion (area 0.5 mm from end portion of press contact portion)

Claims

1. A steel welded member which is a steel welded member in which a plurality of Zn-based plated steels having a Zn-based plated layer on the surface of a steel material are joined via at least one spot welding region, wherein the steel material has a composition consisting of, in mass%: Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to less than 0.4000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Hf: 0 to 0.100%, and REM: 0 to 0.100%, with the remainder consisting of Fe and impurities; the region 0.5 mm from the end of the pressure bonding portion of the spot welding region" means a region (29) 0.5 mm from the boundary (27) between the joined portion and the unjoined portion of the two Zn-based plated steels in a direction opposite to the direction of the nugget portion (23). One or more oxides having an aspect ratio of 1.5 to 10.0 μm are contained in the region. C：0.05～0.40％、 Further, a zinc solid-solution ferrite phase, i.e., an α phase, is contained in the region at an area ratio of 5 to 50%. In the region, one or more oxides having an aspect ratio of 1.5 to 10.0 μm are contained. Further, a zinc solid-solution ferrite phase, i.e., an α phase, is contained in the region at an area ratio of 5 to 50%. ​ ​ ​ B：0～0.010％、 ​ ​ V：0～0.150％、 Cr：0～2.00％、 ​ ​ ​ W：0～1.00％、 ​ ​ Zr：0～0.100％、 ​ ​ In a region of the spot welding zone 0.5 mm from the end of the pressure bonding portion, 20 to 80% of the γ phase, i.e., Fe3Zn, is contained in terms of area ratio 10 , and one or more oxides having an aspect ratio of 0.5 μm or more are contained. ​ 2. The steel welded component according to claim 1, wherein, ​ 3. Steel welded construction element according to claim 1 or 2, wherein ​

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