Welded joint and method for producing same

Abstract

Provided is a welded joint having excellent normal-temperature toughness using a relatively thin high-strength steel plate as a base material. The thickness of each of the base steel sheets is 0.8-10 mm, at least one of the base steel sheets has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more, and the total of the oxygen content of the weld metal in the welded portion and the area ratio of martensite and bainite in the weld metal satisfies a predetermined relationship.

Description

Technical Field

[0001] The present invention relates to a welded joint and a method for making the same. Background Art

[0002] In the automotive field, the suppression of CO2 for the purpose of preventing global warming is 2 There is a growing social demand to reduce CO emissions and improve the safety of passengers and pedestrians in the event of a collision (crash safety). 2 In terms of emissions, the effect of reducing the weight of the vehicle body is very significant. For example, reducing the weight of the vehicle body by 100kg can save an average of about 1km / L of fuel consumption, thereby reducing CO 2 Emissions.

[0003] On the other hand, the standards for collision safety are becoming stricter year by year. Therefore, it is necessary to ensure collision safety by improving the strength and rigidity of the vehicle body and optimizing the strength distribution. Generally speaking, if the strength of the vehicle body is improved, the weight of the vehicle body will increase and CO 2 Therefore, it is important to improve the collision safety and reduce the weight of the vehicle body (i.e. reduce CO 2 Among them, steel sheets and other iron and steel materials are the main raw materials accounting for about 70% of the weight of automobiles, and their high strength is being promoted.

[0004] In addition, in order to achieve lightweighting of the vehicle body, that is, lightweighting of automobile parts, not only the steel plate as the base material but also the welded portion of the welded joint obtained by welding the steel plates needs to have good strength characteristics. In particular, it is also important to ensure good strength characteristics in the welded portions of the running parts, frame parts, etc. among automobile parts.

[0005] As a technology related to improving the strength characteristics of the welded portion, for example, Patent Document 1 discloses "a fillet welding method for steel plates having excellent fatigue strength of welded portions, characterized in that in the method for performing fillet welding of steel plates, a steel plate having a plate thickness of 1.0 to 4.0 mm and a tensile strength of 680 MPa or more is used, and a weld metal having a phase transformation starting temperature of 475 to 550° C. and a tensile strength of 680 MPa or more is formed at the welded portion in such a manner that the degree of constraint of the welded portion of the steel plate is 4000 N / mm·mm or less and the penetration of the weld metal at the welded portion is 1 / 3 or less of the plate thickness of the steel plate."

[0006] In addition, Patent Document 2 discloses “an ultra-high strength welded joint having excellent strength and toughness, characterized in that it is an ultra-high strength welded joint obtained by using a weld metal containing a martensitic single-phase structure and using a steel plate having a plate thickness of 4 to 12 mm, wherein in a cross section of the welded joint perpendicular to the welding direction, the width of the weld metal at the surface layer of the steel plate is set to W1, and the width of the weld metal at a distance of 3 / 4 of the plate thickness from the surface layer of the steel plate is set to W2, and the cross-sectional shape of the weld metal is 2.0 mm to 7.0 mm and W2 is 0.5 mm to 2.4 mm,

[0007] Furthermore, the weld metal contains, by mass%, C: greater than 0.09 and less than 0.24%, Si: 0.2-1.0%, Mn: 0.5-2.5%, P: less than 0.02%, S: less than 0.02%, Al: 0.004-0.08%, Ti: 0.005-0.15%, O: 0.005-0.05%, Ni: 1.0-9%, and further, the carbon equivalent (Ceq) defined by the following (Formula 1) is 0.40-1.00%, and Y defined by the following (Formula 2) is 0.07-0.20%, and the remainder is composed of unavoidable impurities and Fe.

[0008] Ceq=[C]+[Si] / 24+[Mn] / 6+[Ni] / 40+[Cr] / 5+[Mo] / 4+[V] / 14···(Formula 1)

[0009] Y=([Si]+[Mn]) / 40+[Al]+[Ti]···(Formula 2)

[0010] In (Formula 1) and (Formula 2), the elements with brackets [] represent the content (mass %) of each element.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Publication No. 2004-001075

[0014] Patent Document 2: International Publication No. 2011 / 155620 Summary of the invention

[0015] However, in the strength characteristics of welded parts, especially welded parts of welded joints obtained by welding thick steel plates (hereinafter also referred to as welded parts of thick steel plates), preventing brittle fracture becomes an important factor. Among them, brittle fracture refers to the phenomenon that the material is directly fractured from a small elastic deformation without almost any plastic deformation.

[0016] In the past, efforts have been made to improve the toughness in low-temperature environments below 0°C (hereinafter also referred to as low-temperature toughness) from the viewpoint of preventing brittle fracture. That is, metal materials including welds are generally regarded as ductile materials at room temperature. However, when metal materials, especially thick-walled metal materials, are placed in a low-temperature environment, they become brittle and brittle fractures occur. Therefore, efforts have been made to improve the low-temperature toughness of welds of thick steel plates. In fact, in Patent Document 2, low-temperature toughness is also evaluated based on the absorbed energy (vE-40) in a 2mm V-notch Charpy impact test at -40°C.

[0017] However, from the research of the inventors described later, it is known that in welded joints, especially lap fillet welds, obtained by using thinner high-strength steel plates used in automobile parts, for example, steel plates with a thickness of 0.8 mm to 10 mm and a tensile strength of 980 MPa or more, as the joined materials, there is a risk that stress concentration occurs at specific locations of the weld, and brittle fracture occurs at temperatures near room temperature (hereinafter also referred to as room-temperature brittle fracture).

[0018] In this regard, the techniques disclosed in Patent Documents 1 and 2 do not take into account the prevention of the above-mentioned room-temperature brittle fracture, and it is difficult to say that the toughness in the room-temperature environment (hereinafter also referred to as room-temperature toughness) as described above can be ensured.

[0019] The present invention has been developed in view of the above-mentioned current situation, and an object of the present invention is to provide a welded joint having excellent room-temperature toughness using a relatively thin high-strength steel plate as a base material, and an advantageous method for manufacturing the welded joint.

[0020] Therefore, the inventors have conducted intensive studies to achieve the above-mentioned object, and as a result, have obtained the following findings.

[0021] (1) Generally speaking, in the welded portion of a thick steel plate having a thickness greater than 10 mm, especially greater than 20 mm, the possibility of brittle fracture at room temperature is small unless the welded portion is excessively brittle and an excessive load greater than the allowable stress is applied.

[0022] (2) However, in welded joints obtained by joining steel plates with a thickness of 0.8 mm to 10 mm and a tensile strength of 980 MPa or more, especially lap fillet welds, even when a tensile stress below the allowable stress is applied to the weld, stress concentration occurs at specific locations of the weld, and the risk of brittle fracture at room temperature increases.

[0023] (3) For example, in Figure 1When tensile stress is applied to the lap fillet weld shown, a large stress concentration occurs at the root. In the figure, symbols 1 and 2 are steel plates (parent steel plates), and 3 is a weld (weld metal). In addition, as the steel plates as the joined materials become higher in strength, it is difficult for the parent steel plates to fracture (ductile fracture). Therefore, cracks are generated from the stress concentration part at the root, and brittle fracture at room temperature occurs inside the weld metal or at the boundary between the weld metal and the heat-affected zone (hereinafter also referred to as HAZ), i.e., the joint.

[0024] (4) Room-temperature brittle fracture cannot be simply explained because it is affected by the complex influence of various factors. However, according to the research of the inventors, it was found that oxides present in the joint and weld metal, such as Ti-based oxides, become the starting point of room-temperature brittle fracture.

[0025] (5) In addition, the amount of oxides present in the joint and the weld metal is related to the amount of oxygen contained in the weld metal. Furthermore, the relationship between the amount of oxygen contained in the weld metal (hereinafter also referred to as the oxygen amount of the weld metal) and the total area ratio of martensite and bainite in the weld metal is appropriately controlled. Thus, even in the welded portion of a high-strength base steel plate, for example, a base steel plate having a tensile strength of 980 MPa or more containing a large amount of martensite and bainite, the occurrence of room temperature brittle fracture can be effectively prevented.

[0026] The present invention has been completed as a result of further studies based on the above findings.

[0027] That is, the gist of the present invention is as follows.

[0028] 1. A welded joint comprising two or more base steel plates and a welded portion joining the base steel plates, wherein the base steel plates are each 0.8 mm to 10 mm thick,

[0029] At least one of the above-mentioned base steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more.

[0030] The oxygen content of the weld metal in the welded portion and the total area ratio of martensite and bainite in the weld metal satisfy the relationship of the following formulas (1) to (3).

[0031] 10≤Ow≤600···(1)

[0032] 10≤Ow≤350

[0033] 10-(Ow-10) / 34≤Mw≤100···(2)

[0034] ·When 350<Ow≤600

[0035] 0≤Mw≤100-(Ow-350)×50 / 250···(3)

[0036] in,

[0037] Ow: oxygen content of weld metal (ppm by mass),

[0038] Mw: The total area ratio of martensite and bainite in the weld metal (%).

[0039] 2. The welded joint according to item 1 above, which is a lap fillet welded joint.

[0040] 3. The welded joint according to 1 or 2 above, wherein the components of the base steel plates are:

[0041] Ti content: 0 to 0.20 mass% and

[0042] B content: 0 to 0.0100 mass %.

[0043] 4. A method for manufacturing a welded joint, comprising welding two or more steel plates as joined materials to obtain a welded joint,

[0044] The thickness of the above steel plates is 0.8mm to 10mm.

[0045] The surface oxide film thickness of the above steel plates is less than 50 μm.

[0046] At least one of the steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more.

[0047] The CO shielding gas used in the above welding 2 and O 2 The total volume fraction of and the Ti content and B content of the steel plate satisfy the relationship of the following formula (4), and:

[0048] The average value of the heat input amount in the welding, the thickness of the material to be joined, and the thickness of the surface oxide film of the steel plate satisfies the relationship of the following formula (5).

[0049] 0≤Ti+100×B≤1.2-Og×0.8 / 100···(4)

[0050] 1200×t all 1 / 3 ×(1+0.1×t o 1 / 5 )≤Q≤4000×t all 1 / 3 ×(1+0.1×t o 1 / 5 )···(5)

[0051] in,

[0052] Og: CO of protective gas 2 and O 2 Total volume fraction (%),

[0053] Ti: Ti content of steel plate (mass %),

[0054] B: B content of steel sheet (mass %),

[0055] Q: heat input (J / cm),

[0056] t all : Thickness of the materials to be joined (mm),

[0057] t o : Average value of the thickness of the surface oxide film of the steel plate (μm).

[0058] 5. The method for manufacturing a welded joint according to 4 above, wherein the filler material used in the welding is a solid welding wire.

[0059] 6. The method for manufacturing a welded joint according to 4 or 5 above, wherein the welding is gas metal arc welding.

[0060] According to the present invention, a welded joint having excellent room temperature toughness and using a relatively thin high-strength steel plate as a base material can be obtained. By using such a welded joint in automobile parts, such as running parts and frame parts, the weight of the automobile can be reduced while further improving the safety performance of the automobile. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 This is a diagram schematically showing an example of a cross section of a lap fillet weld joint.

[0062] Figure 2 This is a diagram showing the results of confirming the fracture morphology in the tensile test on various welded joints.

[0063] Figure 3 This is a diagram schematically showing an example of a tensile test piece in the case of a lap fillet weld joint.

[0064] Figure 4 This is a diagram schematically showing an example of a tensile test piece in a butt welded joint.

[0065] Figure 5 It is a schematic diagram explaining the evaluation method of room-temperature toughness. DETAILED DESCRIPTION

[0066] The present invention will be described based on the following embodiments. First, a welded joint according to one embodiment of the present invention will be described.

[0067] [1] Welded joints

[0068] A welded joint according to one embodiment of the present invention comprises two or more base steel plates and a weld portion that joins the base steel plates to each other.

[0069] The thickness of the above-mentioned base steel plates is 0.8 mm to 10 mm.

[0070] At least one of the above-mentioned base steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more.

[0071] The oxygen content of the weld metal in the welded portion and the total area ratio of martensite and bainite in the weld metal satisfy the relationship of the following formulas (1) to (3).

[0072] 10≤Ow≤600···(1)

[0073] 10≤Ow≤350

[0074] 10-(Ow-10) / 34≤Mw≤100···(2)

[0075] ·When 350<Ow≤600

[0076] 0≤Mw≤100-(Ow-350)×50 / 250···(3)

[0077] in,

[0078] Ow: oxygen content of weld metal (ppm by mass),

[0079] Mw: The total area ratio of martensite and bainite in the weld metal (%).

[0080] Hereinafter, a weld joint according to an embodiment of the present invention will be described in detail.

[0081] [1-1] Base material steel plate

[0082] As described above, the base steel plates constituting the welded joint according to one embodiment of the present invention have a thickness of 0.8 to 10 mm, and at least one of the base steel plates has a steel structure in which the total area ratio of martensite and bainite exceeds 50% and a tensile strength of 980 MPa or more.

[0083] Thickness of base steel plate: 0.8mm~10mm

[0084] The thickness of the parent steel plate is 0.8 mm to 10 mm. In welded joints obtained with steel plates with a thickness of 0.8 mm to 10 mm as joined materials (hereinafter also referred to as welded joints with a thickness of 0.8 mm to 10 mm), especially lap fillet welds, it can be said that stress concentration is prone to occur at specific locations of the weld, and the risk of brittle fracture at room temperature is particularly high. That is, in such welded joints with a thickness of 0.8 mm to 10 mm, improvement in room temperature toughness is particularly required. Therefore, the thickness of the parent steel plates used as the object is 0.8 mm to 10 mm. The thickness of the parent steel plate is preferably 1.0 mm or more. In addition, the thickness of the parent steel plate is preferably 6 mm or less, and more preferably less than 4 mm. It should be noted that the thickness of the parent steel plates can be the same or different as long as they are within the above range.

[0085] Total area ratio of martensite and bainite: more than 50%

[0086] Room temperature fracture brittle fracture of the weld is particularly prone to occur under conditions where it is difficult for the base steel plate to fracture. Here, if the combined area ratio of martensite and bainite of the base steel plate increases, the occurrence of ductile cracking in the base steel plate is suppressed. As a result, it is difficult for the base steel plate to fracture, and the possibility of room temperature fracture brittle fracture of the weld is increased. That is, in a welded joint composed of such a base steel plate, it is particularly necessary to improve room temperature toughness. Therefore, for the steel structure of at least one base steel plate in the base steel plate, the combined area ratio of martensite and bainite is greater than 50%. The combined area ratio of martensite and bainite of the base steel plate is preferably 60% or more. It should be noted that the combined area ratio of martensite and bainite of the base steel plate is not particularly limited and can also be 100%. It should be noted that the area ratio mentioned here is the area ratio relative to the overall steel structure.

[0087] The area ratio of the remaining structure other than martensite and bainite is preferably less than 50%, more preferably 40% or less. The area ratio of the remaining structure may be 0%. As an example of the remaining structure, ferrite can be mentioned.

[0088] The total area ratio of martensite and bainite is calculated as follows. That is, the base steel plate is cut along the plate thickness direction, the cut surface is ground, and then corroded with 3% by mass nitric acid alcohol solution. Next, a scanning electron microscope (SEM) is used to photograph any 5 locations of the area other than the heat-affected zone of the cut surface at a magnification of 1000 times, and the area ratio of each phase is calculated by the point algorithm. Specifically, points where multiple lines arranged at equal intervals and in a straight line intersect, the so-called lattice points, are arranged on the SEM photo. Then, the ratio of the number of lattice points in the area identified as martensite and bainite to the total number of lattice points is set as the total area ratio of martensite and bainite. It should be noted that in the SEM photo, the area of ​​the lath-shaped microstructure is identified as martensite and bainite, and the area other than that is identified as the remaining part of the structure. In addition, the lattice point spacing is 10μm. In addition, the area ratio of the remaining part of the structure is calculated by subtracting the total area ratio of martensite and bainite from 100%.

[0089] Tensile strength: above 980MPa

[0090] The higher the strength of the parent steel plate, especially when the tensile strength is above 980MPa, the less likely the parent steel plate will break. In addition, the solidification of the weld metal is promoted. As a result, the possibility of room temperature brittle fracture of the weld is increased. That is, in a weld joint composed of such a parent steel plate, improvement in room temperature toughness is particularly required. Therefore, the tensile strength of at least one of the parent steel plates is above 980MPa. The tensile strength of the parent steel plate is preferably above 1180MPa. The upper limit of the tensile strength of the parent steel plate is not particularly limited. The tensile strength of the parent steel plate is, for example, preferably below 2500MPa.

[0091] In addition, as described above, at least one of the parent steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980MPa or more. The remaining parent steel plates may or may not have a steel structure in which the total area ratio of martensite and bainite is greater than 50%. In addition, the tensile strength of the remaining parent steel plates may be greater than 980MPa or less than 980MPa. However, as described above, the greater the total area ratio of martensite and bainite in the parent steel plate, the more room temperature toughness improvement is required. Similarly, the higher the strength of the parent steel plate, the more room temperature toughness improvement is required. Therefore, all parent steel plates, including the remaining parent steel plates, preferably have a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980MPa or more.

[0092] In addition, the base steel plate preferably has the following components:

[0093] Ti content: 0 to 0.20 mass% and

[0094] B content: 0 to 0.0100 mass %.

[0095] As described above, the Ti-based oxides present in the joint and the weld metal become the starting point of room temperature brittle fracture. Therefore, if the Ti-based oxides present in the joint and the weld metal increase, it is difficult to prevent room temperature brittle fracture. Therefore, in the component composition of the base steel plate, the Ti content is preferably 0.20 mass% or less. It should be noted that the lower limit of the Ti content is not particularly limited and may be 0 mass%.

[0096] In addition, B is an element that improves the hardenability of the weld metal and helps to ensure the hardness of the weld metal. However, adding more than 0.0100% by mass of B has the risk of reducing the toughness of the weld. In other words, making the B content 0.0100% by mass or less is effective in suppressing room temperature brittle fracture. Therefore, the B content is preferably 0.0100% by mass or less. It should be noted that the lower limit of the B content is not particularly limited and can be 0% by mass. In addition, regardless of other elements contained in the composition of the parent steel plate, in order to ensure the hardenability of the weld metal, the B content is preferably 0.0010% by mass or more.

[0097] The component composition of the base steel plate other than Ti and B is not particularly limited. For example, the following component composition can be exemplified: in mass %:

[0098] C: 0.04~0.40%,

[0099] Si: 0.01-2.50%,

[0100] Mn: 1.00~5.00%,

[0101] P: 0.050% or less,

[0102] S: 0.010% or less,

[0103] Ti: 0~0.20%,

[0104] Al: 0.01~0.30% and

[0105] B: 0~0.0100%,

[0106] The material contains 10% or less of one or more of any additional elements selected from Cr, Ni, Mo, W, V, Nb, Cu, N, and O (more preferably 0.1% or more of the total amount when any additional elements are contained).

[0107] The remainder is Fe and inevitable impurities.

[0108] In addition, the base steel plate may be a surface treated steel plate (hereinafter also referred to as a plated steel plate) having a metal coating on the surface of the base steel plate. The type and composition of the metal coating are not particularly limited. As the metal coating, for example, a Zn-based coating (a coating having a Zn content greater than 50% by mass) and an Al-based coating (a coating having an Al content greater than 50% by mass) may be cited. In the case where corrosion resistance is required, a Zn-based coating is preferred over an Al-based coating. This is because in a Zn-based coating, the corrosion rate of the base steel plate is reduced due to the sacrificial corrosion protection effect of Zn. As a Zn-based coating, examples include a hot-dip galvanized layer (GI), an alloyed hot-dip galvanized layer (GA), an electroplated zinc layer (EG), a Zn-Ni-based coating (for example, a coating containing 10 to 25% by mass of Ni in addition to Zn), a Zn-Al-based coating, a Zn-Mg-based coating, a Zn-Al-Mg-based coating, and the like. In addition, examples of the Al-based plating layer include an Al—Si-based plating layer (for example, a plating layer containing 10 to 20 mass % of Si in addition to Al) and the like.

[0109] It should be noted that the amount of metal plating in the plated steel sheet is not particularly limited. For example, from the viewpoint of weldability, the amount of metal plating is preferably 120 g / m per single side. 2 In addition, for example, from the perspective of ensuring rust resistance, the amount of metal plating is preferably 20 g / m per single side. 2 above.

[0110] In addition, the number of base steel plates constituting the welded joint may be two or more. As a welded joint having three or more base steel plates, an example of a lap fillet welded joint can be exemplified in which three or four base steel plates are overlapped and joined with one weld.

[0111] [1-2] Welding section

[0112] As described above, the welded portion constituting the welded joint according to one embodiment of the present invention is a portion where the above-mentioned base steel plates are joined to each other, and is composed of weld metal. Moreover, in the welded portion, it is important that the total amount of oxygen in the weld metal and the area ratio of martensite and bainite in the weld metal satisfy the relationship of the following equations (1) to (3).

[0113] 10≤Ow≤600···(1)

[0114] 10≤Ow≤350

[0115] 10-(Ow-10) / 34≤Mw≤100···(2)

[0116] ·When 350<Ow≤600

[0117] 0≤Mw≤100-(Ow-350)×50 / 250···(3)

[0118] in,

[0119] Ow: oxygen content of weld metal (ppm by mass),

[0120] Mw: The total area ratio of martensite and bainite in the weld metal (%).

[0121] 10≤Ow≤600···(1)

[0122] If the oxygen content of the weld metal, i.e., Ow, is greater than 600 mass ppm, a large amount of oxides, such as Ti-based oxides, will exist in the joint and the weld metal, making it difficult to prevent room temperature brittle fracture. On the other hand, in order to suppress Ow to less than 10 mass ppm, in addition to the base steel plate, it is necessary to extremely reduce the oxygen content of the filler (welding wire) and shielding gas used in welding, which is unrealistic. Therefore, Ow satisfies the above formula (1). Ow is preferably less than 400 mass ppm. In addition, Ow is controlled according to the thickness of the base steel plate, and specifically, it is more preferably less than 600-(t-0.8)×1500 / 92. It should be noted that t is the thickness (mm) of the base steel plate with the smallest thickness in the base steel plate.

[0123] 10≤Ow≤350

[0124] 10-(Ow-10) / 34≤Mw≤100···(2)

[0125] ·When 350<Ow≤600

[0126] 0≤Mw≤100-(Ow-350)×50 / 250···(3)

[0127] As described above, in order to prevent room temperature brittle fracture of the weld, it is necessary to appropriately control the relationship between the oxygen content of the weld metal and Mw, which is the sum of the area ratios of martensite and bainite in the weld metal, according to Ow.

[0128] That is, the larger the Mw is, especially when both Ow and Mw become larger, room temperature brittle fracture is more likely to occur. Therefore, when 350<Ow≤600, it is necessary to make Mw 100-(Ow-350)×50 / 250 or less. It should be noted that when 10≤Ow≤350, the upper limit of Mw is not particularly limited, and Mw can be 100 or less.

[0129] On the other hand, when Ow is small, if Mw becomes too small, the weld metal becomes soft and it is difficult to ensure the specified joint strength. Therefore, when 10≤Ow≤350, Mw is set to 10-(Ow-10) / 34 or more. It should be noted that when 350<Ow≤600, the lower limit of Mw is not particularly limited, and Mw can be 0 or more.

[0130] in, Figure 2 The following are the results of tensile tests performed on various welded joints produced using various steel plates under various welding conditions (welding wires, shielding gases, and joint forms) in the same manner as in the examples described below, and of confirming their fracture morphologies. Figure 2 In the figure, the horizontal axis is Ow and the vertical axis is Mw. In addition, 0 indicates that the fracture morphology in the tensile test is ductile fracture, that is, excellent room temperature toughness is obtained. × indicates that the fracture morphology in the tensile test is brittle fracture, that is, sufficient room temperature toughness is not obtained. In addition, the inside of the dotted line is the range that satisfies the above formulas (1) to (3). Figure 2 It can be seen that in the range satisfying the above formulas (1) to (3) (inside the dotted line), excellent room temperature toughness can be obtained, while on the other hand, in the range not satisfying at least one of the above formulas (1) to (3) (outside the dotted line), sufficient room temperature toughness cannot be obtained.

[0131] It is particularly preferable that the following conditions are satisfied.

[0132] That is, it is preferred that Ow be 400 mass ppm or less.

[0133] When 10≤Ow≤350, the above formula (2) is satisfied.

[0134] When 350<Ow≤400, Mw is set to be 100-(Ow-350)×0.4 or less.

[0135] In addition, in lap fillet welding, the larger Ow is, and the thicker the base steel plate is, the more rigid the weld is against rotational deformation. Therefore, stress concentration at the root is likely to increase. Therefore, when Ow is large, that is, 350<Ow≤600, it is preferred to more strictly control Mw according to the thickness of the base steel plate. Specifically, it is preferred to satisfy the following relationship of formula (3)′.

[0136] 0≤Mw≤100-(Ow-350)×{1+(t-0.8) / 46}×50 / 250···(3)′

[0137] In addition, t is the plate thickness (mm) of the base material steel plate having the smallest plate thickness among the base material steel plates.

[0138] Among them, Ow can be obtained by the inert gas melting-infrared absorption method. In addition, Mw can be obtained as follows. That is, the weld constituting the welded joint is cut along the plate thickness direction, the cut surface is polished, and then corroded with 3% by mass nitric acid alcohol solution. Next, a scanning electron microscope (SEM) is used to photograph any 5 places of the cut surface of the weld at a magnification of 1000 times, and the area ratio of each phase is calculated by the point algorithm. Specifically, points where multiple lines arranged at equal intervals and in a straight line intersect, the so-called lattice points, are arranged on the SEM photo. Then, the ratio of the number of lattice points in the area identified as martensite and bainite to the total number of lattice points is set as the total area ratio of martensite and bainite. It should be noted that in the SEM photo, the area of ​​the lath-shaped microstructure is identified as martensite and bainite, and the area other than that is identified as the remaining part of the structure. In addition, the lattice point spacing is 10μm. The area ratio of the remaining structure can be obtained by subtracting the total area ratio of martensite and bainite from 100%.

[0139] The composition of the weld metal is not particularly limited, and for example, the following composition can be exemplified: in terms of mass %:

[0140] C: 0.03~0.30%,

[0141] Si: 0.01-2.50%,

[0142] Mn: 0.80~5.00%,

[0143] P: 0.050% or less,

[0144] S: 0.010% or less,

[0145] Ti: 0-0.2%,

[0146] Al: 0.001~0.30%,

[0147] B: 0~0.0100% and

[0148] O: 0.001~0.06%,

[0149] The material may contain 10% or less of one or more of any additional elements selected from Cr, Ni, Mo, W, V, Nb, Cu, and N (more preferably 0.01% or more of the total amount of such additional elements if contained).

[0150] The remainder is Fe and inevitable impurities.

[0151] In addition, the joint form is not particularly limited, and examples thereof include lap fillet welded joints and butt welded joints. Among them, lap fillet welded joints are preferred.

[0152] [2] Method for manufacturing welded joints

[0153] Method for manufacturing a welded joint according to one embodiment of the present invention

[0154] Welding two or more steel plates as joined materials to obtain a welded joint, a method for manufacturing a welded joint,

[0155] The thickness of the above steel plates is 0.8mm to 10mm.

[0156] The surface oxide film thickness of the above steel plates is less than 50 μm.

[0157] At least one of the steel sheets has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or greater.

[0158] The CO shielding gas used in the above welding 2 and O 2 The total volume fraction of and the Ti content and B content of the steel plate satisfy the relationship of the following formula (4), and:

[0159] The average value of the heat input amount in the welding, the thickness of the material to be joined, and the thickness of the surface oxide film of the steel plate satisfies the relationship of the following formula (5).

[0160] 0≤Ti+100×B≤1.2-Og×0.8 / 100···(4)

[0161] 1200×t all 1 / 3 ×(1+0.1×t o 1 / 5 )≤Q≤4000×t all 1 / 3 ×(1+0.1×t o 1 / 5 )···(5)

[0162] in,

[0163] Og: CO of protective gas 2 and O 2 Total volume fraction (%),

[0164] Ti: Ti content of steel plate (mass %),

[0165] B: B content of steel plate (mass %),

[0166] Q: heat input (J / cm),

[0167] t all : Thickness of the material to be joined (mm),

[0168] t o : Average value of the thickness of the surface oxide film of the steel plate (μm).

[0169] Hereinafter, a method for manufacturing a welded joint according to an embodiment of the present invention will be described in detail. It should be noted that the description of the plate thickness, steel structure, and component composition of the steel plate as the material to be joined is the same as the description of the plate thickness, steel structure, and component composition of the parent steel plate in the above-mentioned [1-1], and therefore is omitted here.

[0170] The thickness of the oxide film on the surface of the steel plate: 50μm or less

[0171] The oxide film on the surface of the steel plate, which is the material to be joined, melts during welding and enters the weld metal. This leads to an increase in the oxygen content of the weld metal, namely Ow. Therefore, the thickness of the surface oxide film of the steel plate is 50 μm or less. The thickness of the surface oxide film of the steel plate is more preferably 30 μm or less, and further preferably 10 μm or less. The thickness of the surface oxide film is not particularly limited. However, even if the steel plate is pickled to remove the oxide film, a natural oxide film of about 0.001 μm is formed. Therefore, the thickness of the surface oxide film of the steel plate is preferably 0.001 μm or more. It should be noted that the thickness of the surface oxide film of the steel plate referred to is the thickness on each single side.

[0172] Among them, the surface oxide film of the steel plate is an oxide film formed on the surface of the steel plate. That is, the surface oxide film of the steel plate is a layer formed by oxidation of iron and / or other metal elements on the surface of the steel plate. In addition, the thickness of the surface oxide film of the steel plate can be measured according to conventional methods. For example, it can be measured by mirror polishing the cross section of the steel plate and then observing the cross section of the steel plate using a field emission scanning electron microscope (FE-SEM). In addition, when the surface oxide film of the steel plate becomes very thin due to pickling, etc., the thickness of the surface oxide film of the steel plate can be measured by observing the surface part of the steel plate using a transmission electron microscope (TEM). The thickness of the surface oxide film of the steel plate can be calculated, for example, as the average value of the thickness of the oxide film on the surface of the steel plate measured at any 5 locations. However, areas where the oxide film is not confirmed due to scratches on the surface of the steel plate are excluded. When the surface oxide film thickness of the front and back surfaces of the steel plate is greatly different, the thickness of the oxide film on the surface of the steel plate is measured at 5 random locations on each of the front and back surfaces, and the average value is taken as the surface oxide film thickness of the steel plate.

[0173] It should be noted that the thickness of the surface oxide film of the steel plate after welding, that is, the base steel plate constituting the welded joint, is not particularly limited. That is, due to the heat effect of welding, the surface oxide film of the base steel plate is sometimes thicker than the steel plate before welding. In addition, depending on the environment in which the welded joint is stored and used after the manufacture of the welded joint, the surface oxide film of the base steel plate may also become thicker with the progress of corrosion. However, the thickness of the surface oxide film of the base steel plate has no particular effect on the oxygen content of the weld metal, that is, Ow. Therefore, the thickness of the surface oxide film of the base steel plate is not particularly limited.

[0174] 0≤Ti+100×B≤1.2-Og×0.8 / 100···(4)

[0175] As described above, in order to prevent room temperature brittle fracture of the weld, the relationship between the oxygen content of the weld metal and the total area ratio of martensite and bainite in the weld metal should be appropriately controlled. Specifically, it is necessary to set it to the range that satisfies the above-mentioned formulas (1) to (3). To this end, the surface oxide film thickness of the steel plate as the joined material is made to be 50 μm or less and satisfies the relationship of the later-described formula (5). Moreover, at the same time, it is necessary to appropriately control the total Ti content and B content of the steel plate as the joined material and the CO content in the shielding gas used for welding. 2 and O 2 The relationship between the total volume fraction of. Specifically, it is necessary to satisfy the above formula (4). It should be noted that for the above formula (4), the upper limit is preferably 0.4+{-0.8+(t-0.8)×3 / 46}×(Og-100) / 100. For the above formula (4), the lower limit is preferably 0.01.

[0176] In addition, when the chemical compositions of two or more steel plates as the materials to be joined are different from each other, the chemical compositions of the respective steel plates need to satisfy the above-mentioned formula (4).

[0177] 1200×t all 1 / 3 ×(1+0.1×t o 1 / 5 )≤Q≤4000×t all 1 / 3 ×(1+0.1×t o 1 / 5 )···(5)

[0178] In order to prevent room temperature brittle fracture of the weld, it is necessary to appropriately control the relationship between the oxygen content of the weld metal and the total area ratio of martensite and bainite in the weld metal, specifically, to satisfy the range of the above formulas (1) to (3). To this end, the thickness of the surface oxide film of the steel plate as the joined material is 50 μm or less, and the relationship of the above formula (4) is satisfied. Moreover, at the same time, it is necessary to optimize the heat input Q (J / cm) according to the thickness of the joined material and the thickness of the surface oxide film of the steel plate as the joined material as shown in the above formula (5).

[0179] That is, when the heat input Q is too small, not only will poor melting occur, but the cooling rate after welding will increase, and the combined area ratio of martensite and bainite in the weld metal will increase excessively. As a result, there is a risk of reducing the room temperature toughness of the weld metal. In addition, the volume of the weld metal decreases, and the dilution of oxygen entering the weld metal becomes insufficient. As a result, there is still a risk of reducing the room temperature toughness of the weld metal. On the other hand, when the heat input Q is too large, melt-through occurs and a sound weld joint cannot be obtained. In addition, due to the generation of a large amount of slag, there is a risk of causing corrosion resistance to deteriorate. Therefore, the heat input Q needs to be controlled to satisfy the above formula (5). For the above formula (5), the upper limit is preferably 3000×t all 1 / 3 ×(1+0.1×t o 1 / 5 ). In addition, for the above formula (5), the lower limit is preferably 1300×t all 1 / 3 ×(1+0.1×t o 1 / 5 ).

[0180] It should be noted that in formula (5),

[0181] t all : Thickness of the material to be joined (mm),

[0182] t o : Average value of the thickness of the surface oxide film of the steel plate (μm).

[0183] That is, in the case of a lap fillet weld, t all It is the total thickness of the steel plates to be joined. In the case of butt welded joints and flat plate cladding joints, t all It is the thickness of the steel plate having the largest thickness among the steel plates to be joined.

[0184] In addition, o It is the average value of the thickness of the surface oxide film (per single surface) of the steel plates to be joined.

[0185] In addition, the heat input amount Q is obtained by the following formula.

[0186] Q=I×E×60 / c

[0187] in,

[0188] I: welding current (A),

[0189] E: Arc voltage (V),

[0190] c: welding speed (cm / min).

[0191] When the welding current, arc voltage, and welding speed change during welding, the heat input amount Q is calculated using the average values ​​(average value over time during welding (time integrated value÷welding time)).

[0192] The welding conditions other than those described above are not particularly limited and may be carried out according to a conventional method.

[0193] For example, the type of shielding gas used in welding is not particularly limited as long as it satisfies the above formula (4), and examples thereof include Ar, He, CO 2 , O 2 、N 2 and their mixed gases.

[0194] In addition, as the filler material, for example, solid welding wire, flux-cored welding wire, and metal-cored welding wire can be used. Among them, from the viewpoint of controlling the oxygen content of the weld metal and the sum of the area ratios of martensite and bainite in the weld metal to satisfy the above-mentioned formulas (1) to (3), it is preferable to use a solid welding wire. As the component composition of the filler material, for example, the following component composition can be exemplified: in terms of mass %:

[0195] C: 0.03~0.2%,

[0196] Si: 0.005-2.00%,

[0197] Mn: 0.05~5.00%,

[0198] P: 0.050% or less,

[0199] S: 0.010% or less,

[0200] Ti: 0~0.20%,

[0201] Al: 0~0.30% and

[0202] O: 0~0.01%,

[0203] The material contains 10% or less of one or more of any additional elements selected from Cr, Ni, Mo, W, V, B, Nb, Cu, and N (more preferably 0.01% or more of the total amount when any additional elements are contained).

[0204] The remainder is Fe and inevitable impurities.

[0205] Examples of the welding method include gas metal arc welding and laser arc hybrid welding. As the welding method, gas metal arc welding, which is a wire welding method most commonly used in automobile assembly processes, is particularly preferred.

[0206] There is no particular limitation on the current control method in gas metal arc welding. For example, DC pulse welding can be applied, in which the current is controlled to be pulsed at a certain period. In addition, push-pull welding, which is represented by CMT welding, can also be applied, in which the welding wire is fed forward and reversely at a certain period during welding. In addition, the power supply characteristics of the welding machine are not particularly limited. For example, the average value of the welding current is preferably 100 to 300 A, the average value of the arc voltage is preferably 10 to 30 V, and the welding speed is preferably 40 to 200 cm / min.

[0207] Example

[0208] The steel plates 1 and 2 shown in Table 1 are used as the materials to be joined and welded under the conditions shown in Table 2 to produce a welded joint. The filler material (welding wire) uses a solid welding wire (recorded as SW in Table 2) or a flux-cored welding wire (recorded as FCW in Table 2) having the composition shown above. The joint form is a lap fillet weld or a butt weld. The positional relationship between the steel plates 1 and 2 is as follows: Figure 3 and 4 It should be noted that the conditions other than those explicitly stated were in accordance with conventional methods. In addition, both Steel Plates 1 and 2 had the above-exemplified component compositions, and the area ratios of the steel structures shown in Table 1 were measured in accordance with the above-mentioned procedure.

[0209] Next, 1 to 10 g of sample was collected from the produced weld joint, and the oxygen content Ow of the weld metal was measured by the inert gas melting-infrared absorption method. In addition, the total area ratio Mw of martensite and bainite in the weld metal was measured according to the above method. The results are shown in Table 2. It should be noted that in the column of formula (4) in Table 2, the case where both steel plates 1 and 2 as the joined materials satisfy formula (4) is recorded as "satisfied", and the case where at least one of the steel plates 1 and 2 as the joined materials does not satisfy formula (4) is recorded as "not satisfied".

[0210] In addition, the samples were collected from the welded joints with the longitudinal direction being perpendicular to the welding direction (the direction perpendicular to the welding direction (weld direction) and the thickness direction of the welded joint (the thickness direction of the parent steel plate)). Figure 3 and Figure 4 It should be noted that in the case of lap fillet welds, if Figure 3 As shown in FIG. 1 , the backing plate is joined to the base steel plate by resistance spot welding. Then, the tensile test piece collected is used to perform a tensile test in accordance with JIS Z 2241 (2011). The tensile direction in the tensile test is as follows: Figure 3 and Figure 4 As shown in FIG. 1 , the test speed is 10 mm / s. After the tensile test, the fracture surface of the tensile test piece is observed. Specifically, according to Figure 5 The method shown is to observe a field of view size of 200 μm square at the center of the width direction of the fracture surface of the parallel part of the tensile test piece while moving in the thickness direction. It should be noted that the observation was performed using a scanning electron microscope (SEM) at a magnification of 500 times. Then, in each observation field of view, when the area of ​​the brittle fracture region is more than 50% of the area of ​​the observation field of view, the observation field of view is judged as a brittle fracture surface. It should be noted that the brittle fracture region refers to a region where the fracture surface morphology is a cleavage fracture or a quasi-cleavage fracture. In addition, an area where traces of dimples or shear fractures are confirmed is regarded as a ductile fracture region rather than a brittle fracture region. Then, the total number of the above-mentioned observation fields is set to Na, the number of observation fields judged as brittle fracture surfaces is set to Nb, and the room-temperature toughness is evaluated according to the following criteria using Nb / Na.

[0211] Evaluation A (very good toughness at room temperature): 0≤Nb / Na≤0.2

[0212] Evaluation B (excellent room temperature toughness): 0.2<Nb / Na≤0.5

[0213] Evaluation F (inadequate room temperature toughness): 0.5<Nb / Na≤1.0

[0214] Table 2 shows the evaluation results.

[0215] It should be noted that since the plate thickness, steel structure and component composition of the base steel plate constituting the obtained welded joint are the same as the plate thickness, steel structure and component composition of the steel plate as the joined material, the description thereof is omitted.

[0216]

[0217]

[0218]

[0219] According to Table 2, the welded joints of the inventive examples all achieved excellent room temperature toughness. On the other hand, the welded joints of the comparative examples did not achieve sufficient room temperature toughness.

[0220] Explanation of symbols

[0221] 1 Steel plate (base steel plate)

[0222] 2 Steel plate (base steel plate)

[0223] 3. Welding part (weld metal)

Claims

1. A welded joint comprising two or more base steel plates and a welded portion where the base steel plates are joined together, wherein: The thickness of the base steel plate is 0.8 mm to 10 mm. At least one of the base steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more. The oxygen content of the weld metal of the welded portion and the total area ratio of martensite and bainite in the weld metal satisfy the relationship of the following equations (1) to (3): 10≤Ow≤600···(1) 10≤Ow≤350 10-(Ow-10) / 34≤Mw≤100···(2) ·When 350<Ow≤600 0≤Mw≤100-(Ow-350)×50 / 250···(3) in, Ow: oxygen content of weld metal, expressed in ppm by mass, Mw: The total area ratio of martensite and bainite in the weld metal in %.

2. The weld joint according to claim 1, which is a lap fillet weld joint.

3. The weld joint according to claim 1 or 2, wherein: The components of the parent steel plates are: Ti content: 0 to 0.20 mass% and B content: 0 to 0.0100 mass %.

4. A method for manufacturing a welded joint, comprising welding two or more steel plates as joined materials to obtain a welded joint, wherein: The thickness of the steel plates is 0.8 mm to 10 mm. The thickness of the surface oxide film of the steel plate is less than 50 μm. At least one of the steel plates has a steel structure in which the total area ratio of martensite and bainite is greater than 50% and a tensile strength of 980 MPa or more. The total volume fraction of CO2 and O2 in the shielding gas used in the welding and the Ti content and B content of the steel plate all satisfy the relationship of the following formula (4), and: The average value of the heat input during welding, the thickness of the material to be joined, and the thickness of the surface oxide film of the steel plate satisfies the relationship of the following formula (5): 0≤Ti+100×B≤1.2-Og×0.8 / 100···(4) 1200×t all 1 / 3 ×(1+0.1×t o 1 / 5 )≤Q≤4000×t all 1 / 3 ×(1+0.1×t o 1 / 5 )···(5) in, Og: Total volume fraction of CO2 and O2 in the protective gas in %, Ti: Ti content of the steel plate in mass %, B: B content of the steel plate in mass %, Q: heat input, in J / cm2, t all : The thickness of the material being joined, in mm, t o : The average value of the surface oxide film thickness of the steel plate, in μm.

5. The method for manufacturing a welded joint according to claim 4, wherein: The filler material used in the welding is solid welding wire.

6. The method for manufacturing a welded joint according to claim 4 or 5, wherein: The welding is gas metal arc welding.

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