Welded joint

Abstract

To provide a welded joint that achieves superior suitability for electrodeposition coating and for chemical conversion in the welded zone through a novel structure.SOLUTION: A welded joint (1) comprises: a steel material; a weld metal (13); a heat-affected zone (14); a scale (15) formed on the surface of the heat-affected zone (14); and a chemical conversion coating (16) formed on the surface of the scale (15) and having a thickness of 0.3-5.0 μm. The chemical composition of the steel material includes predetermined amounts (mass%) of C, Si, Mn, Al, Ti, Cr, Fe, and impurities; the Vickers hardness of the steel material is 250 HV or more; a P ratio, represented as the ratio of the X-ray diffraction intensities of phosphophyllite to hopeite in the chemical conversion coating (16), is 70.0% or more; and the chemical composition index AC of the steel material, the P ratio PR of the chemical conversion coating (16), and the hematite ratio SR of the scale (15) satisfy the following formula (1): AC×PR / SR≥10.0.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a welded joint.

Background Art

[0002] While the issue of climate change is regarded as a socially important issue, in the automotive industry, improvement in fuel consumption and electricity consumption by reducing the weight of the vehicle body is demanded. From the perspective of steel materials, it is possible to reduce the weight compared to conventional vehicle bodies by increasing the strength and reducing the thickness of the steel sheets used.

[0003] On the other hand, the progress of vehicle body weight reduction varies depending on the members. Particularly in the case of chassis members, the increase in strength and reduction in thickness of the steel sheets tend to lag behind those of the body members. This tendency is related to the characteristics required for the members. Compared to body members, chassis members are required to have long-term durability.

[0004] The durability of chassis members often deteriorates due to the welded parts. To ensure durability, it is necessary to improve the fatigue resistance and corrosion resistance of the welded parts. Among them, the corrosion resistance of the welded parts is ensured by chemical conversion treatment or electrocoating to prevent rust. When the chemical conversion processability or electrocoating property becomes inferior, the corrosion resistance significantly decreases.

[0005] To ensure corrosion resistance in chassis members, it is common to apply an electrocoating film with high rust prevention ability, and the electrocoating property of the welded parts becomes an issue. The main factors for the decrease in the electrocoating property of the welded parts, that is, the corrosion resistance of the welded parts, are as follows.

[0006] (1) On the front side of the welded part, the oxidizing gas in the shielding gas reacts with the easily oxidizable elements in the molten metal to generate non-conductive welding slag, leading to poor formation of the electrocoating film. For the problem on the front side of the welded part, a method of removing the slag on the front side of the welded part by post-treatment after welding (for example, shot blasting, etc.) or a method of adopting a welding process or welding material that imparts conductivity to the slag is known to be effective in reducing the poor formation of the electrocoating film due to the slag.

[0007] (2) On one hand, inside the welded part, scale is generated in the heat affected zone (hereinafter sometimes referred to as "HAZ"), which reduces the adhesion of the electrodeposited coating film, and as a result, the corrosion resistance of the welded part decreases. The scale is an iron-based oxide film formed by the reaction of the surface of the steel plate heated by the heat input during welding with oxygen in the air. Furthermore, the inside of the welded part is often located inside a closed cross-sectional structure, and it is difficult to improve the corrosion resistance by post-treatment, so countermeasures by methods other than post-treatment are desired. Regarding the problems on the inside of this welded part, it is known that generating an oxide effective for improving adhesion at the interface between the base metal and the scale is effective in exhibiting good corrosion resistance.

[0008] Regarding the electrodeposition paintability of such welded parts, for example, Patent Document 1 discloses a welded joint including a first steel plate, a second steel plate, a weld metal, a HAZ, and scale generated on the HAZ, wherein the Si content of the first steel plate is 0 to 0.20% by mass, the Si content of the second steel plate is 0.20 to 1.20% by mass, the area ratio of slag adhering to the weld metal is 9% or less, the penetration depth of the weld metal is 15% or more and 85% or less with respect to the thickness of the second steel plate, and the second surface of the second steel plate further includes fayalite detected from the HAZ. According to the welded joint disclosed in this Patent Document 1, it is said that electrodeposition coating defects can be suppressed and scale adhesion can be enhanced.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0010] However, as the high-strength steel plate is advanced (specifically, the high-strength steel plate has a tensile strength of 780 MPa or more or a Vickers hardness of 250 HV or more), the oxidation-prone elements that form the slag increase, and the electrocoating property of the welded part tends to deteriorate.

[0011] Also, even when the scale has good adhesion, it is difficult to completely suppress the intrusion of oxygen and water from the coating surface, and corrosion under the coating proceeds to a certain extent. In order to reduce such corrosion under the coating, a chemical conversion treatment (zinc phosphate treatment) is applied to the welded joint. However, the formation of the chemical conversion film at the welded part is more difficult than that at the base metal part, and the favorable film formation conditions have not been clarified.

[0012] In view of the above circumstances, an object of the present invention is to provide a welded joint having excellent electrocoating property and chemical conversion treatability at the welded part with a novel configuration.

Means for Solving the Problems

[0013] The present invention includes the following aspects.

[0014] (Aspect 1) A welded joint including a steel material, a weld metal, and a heat-affected zone, wherein the welded joint further includes a scale formed on the surface of the heat-affected zone and a chemical conversion film having a thickness of 0.3 to 5.0 μm formed on the surface of the scale, the chemical composition of the steel material is, by mass%, C: 0.03 to 0.30%, Si: 0.01 to 1.10%, Mn: 0.30 to 3.50%, Al: 0.01 to 0.35%, Ti: 0.01 to 0.30%, Cr: 0.02 to 2.00%, B: 0 to 0.0040%, Nb: 0 to 0.050%, Mo: 0 to 0.250%, V: 0 to 0.250%. Ni: 0 to 0.250%, and the balance: Fe and impurities, the Vickers hardness of the above steel material is 250 HV or more, the P ratio represented as the ratio of the X-ray diffraction intensity of phosphophyllite to hopite in the above chemical conversion film is 70.0% or more, the chemical composition index of the above steel material: A C and, the P ratio of the above chemical conversion film: P R and, the hematite ratio of the above scale: S R characterized in that the following formula (1) is satisfied, a welded joint. A C ×P R / S R ≧10.0 ···(1) However, in formula (1), A C , P R and S R are values respectively obtained from the following formulas (i) to (iii). A C =(2.5[Ti] + 5[Mn] + 2[Al] + 6[Cr]) - (10[Si] / [Mn] + 6[C] + 15[S] + 15[P]) ···(i) In formula (i), [Ti], [Mn], [Al], [Cr], [Si], [C], [S] and [P] respectively represent the contents in mass% units of Ti, Mn, Al, Cr, Si, C, S and P contained in the above steel material. P R (%) = {P / (P + H)} × 100 ···(ii) In formula (ii), P represents the X-ray diffraction intensity of phosphophyllite contained in the above chemical conversion film, and H represents the X-ray diffraction intensity of hopite. S R (%) = {S H / (S H +S M +S W +S I )} × 100 ···(iii) In formula (iii), S H represents the X-ray diffraction intensity of hematite contained in the above scale, and S Mrepresents the X-ray diffraction intensity of magnetite contained in the above scale, S W represents the X-ray diffraction intensity of wustite contained in the above scale, S I represents the X-ray diffraction intensity of the ferrite matrix.

[0015] (Aspect 2) The welded joint according to the above Aspect 1, characterized by having slag adhering to the surface of the above weld metal, an electrodeposited coating film formed on the surfaces of the above weld metal and the above slag, and a topcoat film formed on the surface of the above electrodeposited coating film.

[0016] (Aspect 3) The welded joint according to the above Aspect 1 or 2, characterized in that the Vickers hardness of the above steel material is 300 HV or more. [Advantages of the Invention]

[0017] According to the present invention, it is possible to provide a welded joint excellent in electrodepositing paintability and chemical conversion treatment properties of the welded part. [Brief Description of the Drawings]

[0018]

Figure 1

[0019] Hereinafter, preferred embodiments of the welded joint of the present invention will be described in detail. In this specification, various numerical ranges mean ranges including their upper and lower limit values unless otherwise specified.

[0020] In order to achieve the above object, the present inventors first focused on the influence of the increase in easily oxidizable elements accompanying the high-strengthening of the steel sheet on the electrodepositing paintability, and the influence of the steel sheet components and welding conditions on the chemical conversion treatment properties of the welded part, and searched for the optimum steel sheet components that can obtain a welded joint excellent in corrosion resistance. As a result, by including a certain amount or more of Si in the steel sheet, it is possible to stably increase the strength of the steel sheet while improving the film adhesion and corrosion resistance on the back side (scale) of the welded part. However, it was found that the electrocoating property and corrosion resistance decrease on the front side (slag) of the welded part. Since the characteristics of the front side and the back side of such a welded part are in a trade-off relationship with each other, the inventors of the present invention focused on C, Si, Mn, Al, Ti, and Cr that affect these characteristics, and further studied the steel sheet components that satisfy both the characteristics of the front side and the back side of the welded part. Then, the inventors of the present invention clarified the influence of C, Si, Mn, Al, Ti, and Cr on the scale formation in the heat-affected zone and the formation of conversion crystal grains during the conversion treatment, and found that the corrosion progressing under the coating film can be reduced by adjusting the steel sheet components and optimizing the welding conditions. Specifically, the inventors of the present invention determined the index regarding the chemical composition of the steel material, and the P ratio: P, which is the ratio of phosphophyllite and hopite in the conversion coating R and the hematite ratio in the scale: S R By adjusting so as to satisfy a specific relational expression (1), it was found that a welded joint having good corrosion resistance, excellent electrocoating property and conversion treatability on the front side and the back side of the welded part can be manufactured. The present invention has been completed based on such findings.

[0021] Hereinafter, a preferred embodiment of the welded joint of the present invention will be described in detail with reference to the drawings. Here, FIG. 1 is a diagram schematically showing a cross section centered on the welded part of the welded joint 1 according to an embodiment of the present invention.

[0022] <Welded joint> As shown in Fig. 1, the welded joint 1 according to this embodiment includes two steel plates as steel materials, namely, the first steel plate 11 and the second steel plate 12, a welding metal 13 that joins the end 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12, a heat-affected zone 14 that is formed around the welding metal 13 and exposed to each of the first surface 121 of the first steel plate 11 and the second steel plate 12 and the second surface 122 of the second steel plate 12, a scale 15 formed on each of the surface of the heat-affected zone 14 and the surface of the welding metal 13, and a chemical conversion film 16 having a thickness of 0.3 to 5.0 μm formed on the surface including the scale 15.

[0023] Note that the welded joint 1 shown in Fig. 1 is a fillet weld joint, but the welded joint of the present invention is not limited to such a form. The welded joint of the present invention can be a welded joint in any form according to the type of parts to which the welded joint is applied. Examples of such forms of welded joints include, for example, T-joints, cross-joints, and butt joints by fillet welding.

[0024] In any case, the welded joint 1 according to this embodiment has a structure in which one of at least two steel materials (for example, the first steel plate 11 and the second steel plate 12) is welded to the other steel material.

[0025] As shown in Fig. 1, the welded joint 1 of this embodiment has a welding metal 13 that joins the end 111 of the first steel plate 11 and the first surface 121 of the second steel plate 12. The welding metal 13 is a metal that has melted and solidified during welding. Specific examples and formation conditions (that is, welding conditions) of this welding metal 13 will be described later.

[0026] Furthermore, the welded joint 1 has a heat-affected zone 14 that is formed around the welding metal 13 and exposed to the second surface 122 of the second steel plate 12. The heat-affected zone 14 is a portion that has not melted during welding but whose structure, metallurgical properties, and mechanical properties have changed due to the welding heat. Note that the welding metal 13 and the heat-affected zone 14 may be collectively referred to as the "welded portion".

[0027] In addition, as shown in Fig. 1, on the second surface 122 of the second steel plate 12, a scale 15 is formed on the surface of the heat-affected zone 14 exposed outside the second steel plate 12. The scale 15 is an oxide film composed of wustite (FeO), magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) generated on the steel plate surface by welding heat. The scale 15 can be visually recognized as a discolored portion caused by welding heat when the second steel plate 12 is viewed from the back side of the weld metal 13.

[0028] Note that by reducing the heat input during welding, the heat-affected zone 14 can be reduced, and the amount of scale 15 generated on the second surface 122 of the second steel plate 12 can be decreased. As a result, the adhesion of the scale 15 can be improved, and the electrodeposition coating film can be held more firmly. However, since there is a risk that it may be difficult to ensure the joining strength of the welded joint 1, the welded joint 1 of the present embodiment is manufactured by welding with a heat input such that the heat-affected zone 14 reaches the second surface 122 of the second steel plate 12.

[0029] Furthermore, as shown in Fig. 1, the welded joint 1 is provided with a chemical conversion film 16 formed on the surface of the scale 15 and having a thickness of 0.3 to 5.0 μm. The chemical conversion film 16 is a film formed by a chemical conversion treatment (zinc phosphate treatment) performed in a post-process after welding. The chemical conversion film 16 is composed of phosphophyllite (Zn 2 Fe(PO 4 ) 2 ·4H 2 O), which is zinc iron phosphate, and hopeite (Zn 3 (PO 4 ) 2 ·4H 2 O), which is zinc phosphate. Such a chemical conversion film 16 has the effect of enhancing the adhesion between the electrodeposition coating film and the ferrite base (steel plate surface) (anchor effect) and the effect of reducing corrosion under the coating film. Specific examples, formation conditions, etc. of this chemical conversion film 16 will be described later.

[0030] And the welded joint 1 of this embodiment has the following specific characteristic configurations. First, in the welded joint 1, the chemical compositions of the steel materials, that is, the first steel plate 11 and the second steel plate 12, are in mass %, C: 0.03 to 0.30%, Si: 0.01 to 1.10%, Mn: 0.30 to 3.50%, Al: 0.01 to 0.35%, Ti: 0.01 to 0.30%, Cr: 0.02 to 2.00%, B: 0 to 0.0040%, Nb: 0 to 0.050%, Mo: 0 to 0.250%, V: 0 to 0.250%, Ni: 0 to 0.250%, and The balance: Fe and impurities. Furthermore, the Vickers hardness of this steel material is 250 HV or more. Also, the P ratio, represented as the ratio of the X-ray diffraction intensities of phosphophyllite and hopite in the conversion coating 16, is 70.0% or more. And the chemical composition index of the steel material: A C and the P ratio of the above conversion coating: P R and the hematite ratio of the above scale: S R satisfy the following formula (1). A C ×P R / S R ≧10.0 ···(1) However, in formula (1), A C , P R and S R are values obtained from the following formulas (i) to (iii), respectively. A C =(2.5[Ti]+5[Mn]+2[Al]+6[Cr])-(10[Si] / [Mn]+6[C]+15[S]+15[P]) ···(i) In formula (i), [Ti], [Mn], [Al], [Cr], [Si], [C], [S] and [P] respectively represent the contents in mass % units of Ti, Mn, Al, Cr, Si, C, S and P contained in the above steel material. P R (%) = {P / (P + H)} × 100 ··· (ii) In formula (ii), P represents the X-ray diffraction intensity of phosphophyllite contained in the above chemical conversion film, and H represents the X-ray diffraction intensity of hopite. S R (%) = {S H / (S H + S M + S W + S I )} × 100 ··· (iii) In formula (iii), S H represents the X-ray diffraction intensity of hematite contained in the above scale, S M represents the X-ray diffraction intensity of magnetite contained in the above scale, S W represents the X-ray diffraction intensity of wustite contained in the above scale, S I represents the X-ray diffraction intensity of the ferrite matrix.

[0031] By having these specific characteristic configurations, the welded joint 1 of the present embodiment has become a welded joint excellent in electrocoating property and chemical conversion treatment property of the welded part. Hereinafter, these specific characteristic configurations of the welded joint 1 will be described in detail.

[0032] [Steel material] In this embodiment, the steel material consists of two steel plates, i.e., the first steel plate 11 and the second steel plate 12 as shown in FIG. 1, but it is not limited to such a form. Any steel material can be adopted according to the type of the component to which the welded joint 1 is applied, as long as it has the above-specified chemical composition and Vickers hardness. Examples of such steel materials include steel plates and steel bars having the above-specified chemical composition and Vickers hardness. The steel material may be two steel plates, three or more steel plates, two steel bars, or three or more steel bars, as long as it has the above-specified chemical composition and Vickers hardness.

[0033] In addition, when the steel material consists of two or more steel materials such as two or more steel plates or two or more steel bars, all of the two or more steel materials may be of the same type, only some of the steel materials may be of the same type, or all of the steel materials may be of different types. Here, the type of the steel material refers to that distinguished by the chemical composition, microstructure, strength, and plate thickness of the steel material.

[0034] When a steel plate is used as the steel material, the plate thickness of the steel plate is not particularly limited, and any plate thickness can be adopted according to the type of the component to which the welded joint 1 is applied. Examples of such plate thicknesses include plate thicknesses of 1 mm or more, 2 mm or more, or 3 mm or more. Also, the plate thickness may be, for example, 8 mm or less, 6 mm or less, or 4 mm or less.

[0035] [Chemical Composition] And the steel plate has a specific chemical composition as described above. That is, the chemical composition of the steel material is in mass%, C: 0.03 to 0.30%, Si: 0.01 to 1.10%, Mn: 0.30 to 3.50%, Al: 0.01 to 0.35%, Ti: 0.01 to 0.30%, Cr: 0.02 to 2.00%, B: 0 to 0.0040%, Nb: 0 to 0.050%, Mo: 0 to 0.250%, V: 0 to 0.250%, Ni: 0 to 0.250%, and the balance: Fe and impurities.

[0036] In addition, in this embodiment, for the chemical composition of the steel material, P, S, N, and O, which are inevitable impurities, are, in mass%, P: 0.020% or less, S: 0.0060% or less, N: 0.0600% or less, and O: preferably 0.0050% or less.

[0037] Hereinafter, each component constituting this steel material will be described in detail. In the following description, "%" means "mass%" unless otherwise specified, and the content of each component means the mass ratio of each component to the total mass of the steel material.

[0038] [C: 0.03 to 0.30%] C is an element necessary for ensuring the strength of the steel material. In order to sufficiently obtain such an effect, the C content is set to 0.03% or more. The C content may be 0.04% or more, 0.05% or more, or 0.06% or more. On the other hand, if C is contained excessively, blisters due to CO gas are likely to occur, and the scale with the chemical conversion film attached is likely to fall off. Therefore, the C content is set to 0.30% or less. The C content may be 0.25% or less, 0.20% or less, or 0.15% or less.

[0039] [Si: 0.01 to 1.10%] Si is a deoxidizing element of steel and is an element effective for increasing the strength without impairing the ductility of the steel material. In order to sufficiently obtain these effects, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, or 0.15% or more. On the other hand, if Si is contained excessively, Si combines with oxygen during welding to form an oxide (SiO 2 ) at the interface between the ferrite matrix and the scale, and this SiO2 By inhibiting the elution of Fe, it may be difficult to form a chemical conversion film. Furthermore, since amorphous slag is formed on the surface of the welded part, it may cause a decrease in conductivity and poor electrocoating. Therefore, the Si content should be 1.10% or less. The Si content may be 1.00% or less, 0.90% or less, or 0.80% or less.

[0040] [Mn: 0.30 - 3.50%] Mn is an element necessary for ensuring the strength of steel. To sufficiently obtain such an effect, the Mn content should be 0.30% or more. The Mn content may be 0.50% or more, 1.00% or more, or 1.50% or more. On the other hand, if the Mn content is excessively high, microsegregation and macrosegregation are likely to occur, which may deteriorate the workability of the steel and may also lead to a decrease in chemical conversion treatability. Therefore, the Mn content should be 3.50% or less. The Mn content may be 3.00% or less, 2.80% or less, or 2.50% or less.

[0041] [Al: 0.01 - 0.35%] Al is an element that functions as a deoxidizer and is effective in increasing the strength of steel. Also, Al is an element effective in enhancing the adhesion at the interface between the ferrite matrix and the scale and making it difficult for the scale with the chemical conversion film to fall off. To sufficiently obtain these effects, the Al content should be 0.01% or more. The Al content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if the Al content is excessively high, it may combine with N to form nitrides and reduce the fatigue strength of the welded joint. Therefore, the Al content should be 0.35% or less. The Al content may be 0.34% or less, 0.33% or less, 0.32% or less, or 0.31% or less.

[0042] [Ti: 0.01 - 0.30%] Ti is an element that has the effect of reducing the amounts of S, N, and O that generate coarse inclusions acting as fracture initiation points. Furthermore, Ti is also an element effective in enhancing the adhesion at the interface between the ferrite matrix and the scale, making it difficult for the scale with the conversion coating to fall off. Also, Ti is an element effective in refining the microstructure and enhancing the balance between the strength and formability of the steel material. To fully obtain these effects, the Ti content should be 0.01% or more. The Ti content may be 0.02% or more, 0.03% or more, or 0.04% or more. On the other hand, if Ti is contained in excess, coarse Ti sulfides, Ti nitrides, and / or Ti oxides may form, which may reduce the formability of the steel material. Therefore, the Ti content should be 0.30% or less. The Ti content may be 0.25% or less, 0.20% or less, or 0.15% or less.

[0043] [Cr: 0.02 - 2.00%] Cr is an element that contributes to the improvement of the strength of the steel material and the fatigue strength of the welded joint. Also, Cr is an element effective in enhancing the adhesion at the interface between the ferrite matrix and the scale, making it difficult for the scale with the conversion coating to fall off. To fully obtain these effects, the Cr content should be 0.02% or more. The Cr content may be 0.03% or more, or 0.04% or more. On the other hand, if Cr is contained in excess, there is a risk of forming coarse Cr carbides that become fracture initiation points, and there is also a risk of reducing the formability. Therefore, the Cr content should be 2.00% or less. The Cr content may be 1.80% or less, 1.60% or less, or 1.40% or less.

[0044] [P: 0.020% or less] P is an element inevitably mixed in during the refining of steel. The P content may be 0%, but reducing the P content to less than 0.001% will lead to an increase in the dephosphorization cost. Therefore, the P content is preferably at least 0.001%. The P content may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, when the P content exceeds 0.020%, significant solidification segregation occurs during casting, reducing internal cracking and workability, and at the same time, it may cause embrittlement of the welded part. Therefore, the P content is preferably 0.020% or less. The P content may be 0.019% or less, 0.018% or less, or 0.016% or less.

[0045] [S: 0.0060% or less] S is an element inevitably mixed in during the refining of steel. The S content may be 0%, but reducing the S content to less than 0.0010% will increase the manufacturing cost. Therefore, the S content is preferably at least 0.0010%. The S content may be 0.0012% or more, 0.0014% or more, or 0.0015% or more. On the other hand, S may combine with Mn and Ti to form precipitates and deteriorate workability. Therefore, the S content is preferably 0.0060% or less. The S content may be 0.0055% or less, 0.0050% or less, or 0.0045% or less.

[0046] [N: 0.0600% or less] N is an element inevitably mixed in during the refining of steel. The N content may be 0%, but reducing the N content to less than 0.0010% will increase the manufacturing cost. Therefore, the N content is preferably at least 0.0010%. The N content may be 0.0012% or more, 0.0014% or more, or 0.0015% or more. On the other hand, N may form nitrides and deteriorate workability. Therefore, the N content is preferably 0.0600% or less. The N content may be 0.0500% or less, 0.0400% or less, or 0.0300% or less.

[0047] [O: 0.0050% or less] O is an element that inevitably mixes in during the refining of steel. The O content may be 0%, but reducing the O content to less than 0.0008% increases the manufacturing cost. Therefore, the O content is preferably at least 0.0008%. The O content may be 0.0010% or more, or 0.0012% or more. On the other hand, since O forms coarse oxides and may deteriorate the workability, the O content is preferably 0.0050% or less. The O content may be 0.0048% or less, 0.0046% or less, or 0.0045% or less.

[0048] The basic chemical composition of the steel material of this embodiment is as described above. Furthermore, in this embodiment, the steel material may contain one or more of the following optional elements, as necessary, in place of a part of the remaining Fe. These optional elements will be described in detail below.

[0049] [B: 0 to 0.0040%] B is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of the steel material. The B content may be 0%, but in order to sufficiently obtain such an effect, the B content is preferably at least 0.0001%. The B content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more. On the other hand, if B is contained excessively, B precipitates may form and the strength of the steel material may decrease. Therefore, the B content is preferably 0.0040% or less. The B content may be 0.0035% or less, 0.0030% or less, or 0.0025% or less.

[0050] [Nb: 0 to 0.050%] Nb contributes to the improvement of the strength of steel due to precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite grains, and / or dislocation strengthening by suppressing recrystallization. The Nb content may be 0%, but in order to fully obtain these effects, the Nb content is preferably 0.001% or more. The Nb content may be 0.003% or more, 0.004% or more, or 0.005% or more. On the other hand, if the Nb content is excessive, the amount of unrecrystallized ferrite may increase and the formability of the steel may decrease. Therefore, the Nb content is preferably 0.050% or less. The Nb content may be 0.040% or less, 0.030% or less, or 0.020% or less.

[0051] [Mo: 0 to 0.250%] Mo suppresses the phase transformation at high temperatures and contributes to the improvement of the strength of steel. The Mo content may be 0%, but in order to fully obtain such an effect, the Mo content is preferably 0.001% or more. The Mo content may be 0.002% or more or 0.004% or more. On the other hand, if the Mo content is excessive, the workability may decrease and the productivity may decrease. Therefore, the Mo content is preferably 0.250% or less. The Mo content may be 0.200% or less, 0.150% or less, or 0.100% or less.

[0052] [V: 0 to 0.250%] V contributes to the improvement of the strength of steel due to precipitation strengthening, grain refinement strengthening by suppressing the growth of ferrite grains, and / or dislocation strengthening by suppressing recrystallization. The V content may be 0%, but in order to fully obtain these effects, the V content is preferably 0.001% or more. The V content may be 0.002% or more or 0.004% or more. On the other hand, if the V content is excessive, the formability of the steel may decrease. Therefore, the V content is preferably 0.250% or less. The V content may be 0.200% or less, 0.150% or less, or 0.100% or less.

[0053] [Ni: 0 to 0.250%] Ni is an element that suppresses phase transformation at high temperatures and contributes to the improvement of the strength of steel materials. The Ni content may be 0%, but in order to sufficiently obtain such an effect, the Ni content is preferably 0.001% or more. The Ni content may be 0.002% or more or 0.004% or more. On the other hand, if Ni is contained excessively, the weldability of the steel material may decrease. Therefore, the Ni content is preferably 0.250% or less. The Ni content may be 0.200% or less, 0.150% or less, or 0.100% or less.

[0054] Regarding the above optional elements, in this embodiment, the chemical composition of the steel material is, in mass%, B: 0 to 0.0040%, Nb: 0 to 0.050%, Mo: 0 to 0.250%, V: 0 to 0.250%, and Ni: 0 to 0.250% may further contain one or more selected from the group consisting of.

[0055] When the steel material contains such optional elements in this way, it is possible to more reliably obtain a welded joint that is excellent in electrodeposition paintability and chemical conversion treatability of the welded part while maintaining high joint strength.

[0056] In the steel material of this embodiment, the balance other than the above elements consists of Fe and impurities. Here, the impurities are components such as those mixed in due to various factors in the manufacturing process starting from raw materials such as ores and scraps when the steel material is industrially manufactured. Examples of the impurities include Cu, Sn, As, Sb, Ca, Mg, Hf, Bi, REM, Zr, Co, Zn, and W. The impurities may be contained in a total amount of 0.100% or less.

[0057] The chemical composition of the steel material may be measured by a general analysis method. For example, the chemical composition of the steel material may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES). Specifically, when the steel material is a steel plate, a test piece with a size of 35 mm square is sampled from a position near the depth of 1 / 2 of the plate thickness of the steel plate, and the chemical composition of the steel plate can be specified by measuring it under the conditions based on the calibration curve prepared in advance using a measuring device such as ICPS-8100 manufactured by Shimadzu Corporation. C and S that cannot be measured by ICP-AES may be measured using the combustion-infrared absorption method, N may be measured using the inert gas fusion-thermal conductivity method, and O may be measured using the inert gas fusion-non-dispersive infrared absorption method.

[0058] [Vickers hardness: 250 HV or more] In this embodiment, the steel material has a Vickers hardness of 250 HV or more. Usually, such high-strength steel materials tend to have an increase in easily oxidizable elements that form the basis of slag and a decrease in the electrocoating property of the welded part with the increase in strength. However, the welded joint 1 of this embodiment has the above-described specific characteristic configuration even when using a high-strength steel material with a Vickers hardness of 250 HV or more, and thus can exhibit excellent electrocoating property and chemical conversion treatment property at the welded part.

[0059] From the viewpoints of the strength and impact resistance of the welded joint, the Vickers hardness of the steel material may be 255 HV or more, 260 HV or more, 280 HV or more, 300 HV or more, or 320 HV or more. Also, the upper limit of the Vickers hardness of the steel material is not particularly limited. The Vickers hardness of the steel material may be 450 HV or less, 400 HV or less, or 350 HV or less.

[0060] (Method for measuring the Vickers hardness of the steel material) The Vickers hardness of the steel material can be measured in accordance with JIS Z 2244:2009. The Vickers hardness of the steel material can be obtained as the average value of 10 measurements carried out under the conditions of a load of 1 Kgf (about 9.80 N) and a holding time of 20 seconds at a depth of 1 / 4 of the thickness of the steel material (in the case where the steel material is a steel plate, at a depth of 1 / 4 of the plate thickness of the steel plate). At this time, the distance between the measurement positions ensures a distance of at least 3 times the indentation. Note that the position at a depth of 1 / 4 of the thickness of the steel material means the position in the thickness direction within the steel material starting from the surface of the steel material, which is separated by a distance equal to 1 / 4 of the total thickness of the steel material from the surface of the steel material. For example, in the case where the steel material is a steel plate, the position at a depth of 1 / 4 of the plate thickness of the steel plate means the position in the plate thickness direction within the steel plate starting from the surface of the steel plate, which is separated by a distance equal to 1 / 4 of the total plate thickness of the steel plate from the surface of the steel plate.

[0061] (Conversion coating) In the present embodiment, the welded joint 1 is provided with a conversion coating 16 formed on the surface of the scale 15 and having a thickness of 0.3 to 5.0 μm. The conversion coating 16 is a coating formed by a zinc phosphate treatment (i.e., a conversion treatment) performed in a post-process after welding, and is composed of phosphophyllite (Zn 2 Fe(PO 4 ) 2 ·4H 2 O), which is zinc iron phosphate, and hopeite (Zn 3 (PO 4 ) 2 ·4H 2 O), which is zinc phosphate.

[0062] Incidentally, zinc phosphate treatment is a treatment using a chemical solution mainly composed of phosphoric acid and Zn ions, and is a chemical reaction that generates crystals of phosphophyllite with Fe ions eluted from the steel sheet. The technical points of zinc phosphate treatment are to elute Fe ions to promote the reaction and to densely form phosphophyllite crystals on the steel sheet surface. In particular, regarding the former, if oxides resulting from the formation of Si scale remain on the steel sheet surface, the elution of Fe is hindered, resulting in the appearance of scum or the non-elution of Fe, which may form hopite and deteriorate the performance after painting.

[0063] Here, phosphophyllite means zinc iron phosphate in which the composition ratios of the elements Zn, P, and Fe (excluding O) constituting it are within the ranges of Zn: 15 to 40, P: 10 to 35, and Fe: 20 to 70 (where Zn + P + Fe ≤ 100), respectively. Also, hopite means zinc phosphate in which the composition ratios of the elements Zn and P (excluding O) constituting it are within the ranges of Zn: 5 to 60 and P: 5 to 50 (where Zn + P ≤ 100, and it may further contain Fe in the range of 0 to 5).

[0064] Such a chemical conversion coating 16 has the effect of enhancing the adhesion (anchor effect) between the electrodeposition coating film and the ferrous substrate (steel sheet surface) and the effect of reducing corrosion under the coating film. On the one hand, the surface of the weld metal and the heat affected zone has different initial processes before the chemical conversion treatment. Furthermore, since scale is formed on their surfaces, the chemical conversion treatability varies depending on the presence or absence of welding. Generally, it is said that the weld metal and the heat affected zone have poor chemical conversion treatability or that chemical conversion crystal grains are not formed. However, the inventors of the present invention performed X-ray diffraction (XRD) analysis on the steel sheet and the welded part after the chemical conversion treatment and confirmed that chemical conversion crystal grains are also formed in the heat affected zone. Furthermore, the inventors of the present invention found that there is no difference in the peak of the X-ray diffraction intensity of matrix Fe and phosphophyllite in samples obtained by subjecting various non-welded steel sheets to chemical conversion treatment, whereas there is a difference in the ratio of the peaks of the X-ray diffraction intensity of matrix Fe, phosphophyllite, and hopite in samples obtained by subjecting various welded steel sheets to chemical conversion treatment. Specifically, it was confirmed that a better P ratio can be obtained for steel sheets utilizing Al or Cr compared to steel sheets with a high content of Si, Mn, and B.

[0065] [P ratio: 70.0% or more] The film formation state of the chemical conversion film formed by zinc phosphate treatment can be evaluated by the P ratio, which is represented as the ratio of the X-ray diffraction intensities of phosphophyllite and hopite in the chemical conversion film. The P ratio can be measured using an X-ray diffractometer. It is evaluated by taking the ratio of the X-ray diffraction intensity P of the (100) plane of phosphophyllite and the X-ray diffraction intensity H of the (020) plane of hopite, and P ratio (%) = {P / (P + H)} × 100. The higher this P ratio, the more phosphophyllite is contained in the film, meaning that phosphophyllite crystals are densely formed on the surface of the steel sheet. Phosphophyllite has better chemical stability against acids and alkalis than hopite. In the mixed crystal of phosphophyllite and hopite, the chemical stability against acids and alkalis improves as the ratio of phosphophyllite increases. Therefore, a higher P ratio indicates better corrosion resistance.

[0066] In this embodiment, the P ratio of the chemical conversion film 16 is 70.0% or more. The P ratio may be 72.0% or more, 74.0% or more, 76.0% or more, 78.0% or more, or 80.0% or more. The upper limit of the P ratio is not particularly limited and is, for example, 100.0%.

[0067] Note that the deposition amount of the chemical conversion film 16 (i.e., the mass per unit area of the total solid content of the chemical conversion film) is not particularly limited as long as the effects of the present invention are not inhibited. For example, the deposition amount is about 0.1 to 10.0 g / m 2 per side. The deposition amount of the chemical conversion film 16 may be 1.0 to 6.0 g / m 2 or 3.0 to 4.0 g / m 2 .

[0068] [Relationship between the chemical composition index A of the steel material C , the P ratio P of the chemical conversion film R , and the hematite ratio S of the scale R : Equation (1)] And, the welded joint 1 of this embodiment has A C , which is the chemical composition index of the steel material, R , which is the P ratio of the chemical conversion film 16, and R , which is the hematite ratio of the scale 15, satisfying the following equation (1). A C × P R / S R ≧ 10.0 ···(1) However, in Equation (1), A C , P R and S R are obtained from the following equations (i) to (iii), respectively.

[0069] The chemical composition index A of the steel material C is an index related to the chemical composition of the steel material and is a conditional factor for obtaining excellent joint strength and electrodeposition paintability of the welded part. The chemical composition index A of the steel material C is obtained by the following equation (i). A C=(2.5[Ti] + 5[Mn] + 2[Al] + 6[Cr]) - (10[Si] / [Mn] + 6[C] + 15[S] + 15[P]) ···(i) In formula (i), [Ti], [Mn], [Al], [Cr], [Si], [C], [S], and [P] respectively represent the contents in mass % units of Ti, Mn, Al, Cr, Si, C, S, and P contained in the above steel material.

[0070] P, which is the P ratio of the chemical conversion film R is, as described above, an evaluation index for the film formation state of the chemical conversion film formed by zinc phosphate treatment. The P ratio P of the chemical conversion film R is obtained by the following formula (ii) and has a value of 70.0% or more in this embodiment as described above. P R (%) = {P / (P + H)} × 100 ···(ii) In formula (ii), P represents the X-ray diffraction intensity of phosphophyllite contained in the above chemical conversion film, and H represents the X-ray diffraction intensity of hopite.

[0071] S, which is the hematite ratio of scale 15 R is an evaluation index for the formation state of scale 15. As described above, scale 15 is composed of iron oxides of magnetite (Fe 3 O 4 ), hematite (Fe 2 O 3 ), and wustite (FeO). These iron oxides are generated by the reaction of iron (Fe) diffusing from the base iron side with oxygen (O 2 ) in the atmosphere. Among these iron oxides, hematite particularly affects the adhesion between the base iron and the scale. Therefore, the hematite ratio in scale 15 becomes an evaluation index for the formation state of scale 15. The hematite ratio S of scale 15 R is obtained by the following formula (iii). S R (%) = {S H / (S H + S M + S W + S I )} × 100 ···(iii) In formula (iii), S H represents the X-ray diffraction intensity of the (104) plane of hematite contained in the scale, and S M represents the X-ray diffraction intensity of the (311) plane of magnetite contained in the scale, and S W represents the X-ray diffraction intensity of the (200) plane of wustite contained in the scale, and S I represents the X-ray diffraction intensity of the (110) plane of the ferrite matrix. Note that the hematite ratio of the scale can be obtained according to the following measurement method.

[0072] (Measurement method of hematite ratio) Collect the scale from the welded joint to be measured. The scale is collected at any plurality of locations, and at least 1.0 g of scale is collected at each location. For the collected scale, the X-ray diffraction intensity of hematite, the X-ray diffraction intensity of magnetite, the X-ray diffraction intensity of wustite, and the X-ray diffraction intensity of the ferrite matrix are obtained by X-ray diffraction (XRD) analysis. Then, the hematite ratio is calculated according to the above formula (iii). The average value of the hematite ratios of the scales at the above-mentioned plurality of locations is adopted as the hematite ratio S R used in the above formula (1).

[0073] The welded joint 1 of the present embodiment has the chemical composition index A of the steel material C and the P ratio P of the chemical conversion film 16 R and the hematite ratio S of the scale 15 R satisfy the above formula (1), so that the electrodeposition paintability and chemical conversion treatability on the front and back sides of the welded part are excellent, and good corrosion resistance can be exhibited.

[0074] In the present embodiment, the left side A of formula (1) C ×P R / S R may be 15.0 or more, 20.0 or more, or 25.0 or more. Note that the upper limit of the left side of formula (1) is not particularly limited, but for example, it is 90.0 or less or 80.0 or less.

[0075] In addition, in this embodiment, the thickness of the chemical conversion film 16 is 0.3 to 5.0 μm as described above. The thickness of the chemical conversion film 16 may be 0.4 μm or more, 0.5 μm or more, or 0.6 μm or more. Further, the thickness of the chemical conversion film 16 may be 4.8 μm or less, 4.6 μm or less, or 4.5 μm or less.

[0076] The welded joint 1 of this embodiment may have any additional configuration as long as it does not inhibit the effects of the present invention. For example, the welded joint 1 may have slag (not shown) adhering to the surface of the weld metal 13, an electrodeposition coating film (not shown) formed on the surfaces of the weld metal 13 and the slag, and a topcoat film (not shown) formed on the surface of the electrodeposition coating film. A welded joint with slag adhering to the surface of the weld metal generally tends to have a reduced electrodeposition coating property at the welded part. However, the welded joint 1 of this embodiment can exhibit excellent corrosion resistance by having an electrodeposition coating film and a topcoat film. Note that the electrodeposition coating film and the topcoat film may be formed so as to cover not only the weld metal 13 and the slag, but also the heat affected zone 14, the scale 15, and the chemical conversion film 16.

[0077] (Application Example) Since the welded joint of the present invention is excellent in the electrodeposition coating property and chemical conversion treatment property of the welded part and has good corrosion resistance, in addition to the above-described automobile chassis members, various parts of transportation machines such as automobiles, various parts of industrial machines, and various structural members of buildings, etc. It can be suitably used for parts in various fields. In particular, when the welded joint of the present invention is applied to an automobile chassis member, it becomes possible to further extend the life and reduce the thickness of the chassis member, which has been difficult in the past.

[0078] Next, a preferred manufacturing method of the welded joint 1 according to another aspect of the present invention will be described. The following description is intended to exemplify a characteristic method for manufacturing the welded joint 1 according to an embodiment of the present invention, and is not intended to limit the welded joint to be manufactured by the manufacturing method as described below.

[0079] <Manufacturing Method of Welded Joint> The manufacturing method of the welded joint 1 of the present embodiment includes at least a steel material manufacturing process for manufacturing the first steel plate 11 and the second steel plate 12 as steel materials, a welding process for welding the first steel plate 11 and the second steel plate 12, and a chemical conversion treatment process for subjecting the welded joint to zinc phosphate treatment (i.e., chemical conversion treatment) to form a chemical conversion film 16. Hereinafter, the preferred conditions and the like of these processes will be described.

[0080] (Steel material manufacturing process) The means and conditions of the steel material manufacturing process are not particularly limited as long as a steel material having the above specific chemical composition and Vickers hardness can be obtained. For example, when manufacturing the first steel plate 11 and the second steel plate 12 of the present embodiment, the steel material manufacturing process can be performed by a method including a casting process for casting a slab having the above specific chemical composition and a hot rolling process for hot rolling the cast slab.

[0081] Here, the casting process is a process for casting a slab having the above specific chemical composition. The casting process uses a continuous casting machine equipped with a plurality of reduction rolls adjacent to each other in the conveyance direction of the slab.

[0082] Also, the hot rolling process is a process for hot rolling the cast slab. In the hot rolling process, it is preferable to heat the slab to a predetermined temperature (for example, 1200°C) or higher prior to hot rolling. By heating in this way, the rolling reaction force does not become excessively large during hot rolling, and the target thickness is easily obtained.

[0083] Also, in the hot rolling process, rough rolling and finish rolling are performed on the heated slab. Here, by appropriately adjusting various conditions of rough rolling and finish rolling, it is possible to easily control the Vickers hardness of the steel plate within the above specific range. Examples of the conditions to be adjusted in rough rolling and finish rolling include the starting temperature of rough rolling, the finishing temperature of finish rolling, the diameter of the rolling roll, and the coiling temperature.

[0084] The steel sheet after the hot rolling process may be subjected to skin pass rolling for the purpose of shape correction. Further, the steel sheet after the hot rolling process or the steel sheet after the skin pass rolling may be subjected to an optional processing step such as plating, if necessary.

[0085] Then, the steel materials obtained by the above manufacturing method, that is, the first steel sheet 11 and the second steel sheet 12, are subjected to the following welding process.

[0086] (Welding process) The welding process can weld the first steel sheet 11 and the second steel sheet 12, and the hematite ratio S of the scale 15 formed R is such that, as long as conditions can be adopted that satisfy the above formula (1) together with the chemical composition index A of the above steel material C and the P ratio P of the chemical conversion coating R the means, conditions, etc. are not particularly limited.

[0087] For example, when welding the first steel sheet 11 and the second steel sheet 12 of the present embodiment, the welding process can be performed by gas shielded arc welding using a welding wire as the welding metal and a shielding gas. That is, the welding process in the present embodiment is a process of forming a welding metal 13 by gas shielded arc welding the end portion 111 of the first steel sheet 11 and the first surface 121 of the second steel sheet 12.

[0088] The shielding gas used for this gas shielded arc welding is not particularly limited. For example, 100% by volume of Ar gas (pure Ar gas), 100% by volume of carbon dioxide gas (pure carbon dioxide gas), a mixed gas of Ar and 3 - 30% by volume of CO 2 and the like can be used.

[0089] Regarding other various welding conditions such as welding current, welding voltage, shielding gas flow rate, welding speed, etc., the first steel sheet 11 and the second steel sheet 12 can be welded as described above, and the hematite ratio S of the scale 15 formed R is such that C together with the chemical composition index A of the above steel material RThere are no particular limitations as long as the conditions satisfy the above formula (1), and normal welding conditions corresponding to the desired product form, type of welding method, etc. can be adopted. Examples of such welding conditions include welding current: 180 A to 320 A, welding voltage: 12 V to 30 V, shielding gas flow rate: 15 L / min to 30 L / min, welding speed: 50 cm / min to 180 cm / min, etc.

[0090] Also, the welding wire is not particularly limited, but for example, solid wires for mild steel or high-tensile steel can be used. Further, when using a solid wire as the welding wire, the chemical composition, by mass%, is C: 0.05 to 0.20%, Si: 0.15% or less, Mn: 0.3 to 2.5%, P: 0.02% or less, S: 0.04% or less, Ti: 0.02 to 0.20%, B: 0.012% or less, Al: 0.22% or less, Cr: 0.5% or less, Nb: 0.3% or less, V: 0.3% or less, Mo: 1.0% or less, Ni: 3.0% or less, Zr: 0.200% or less, Cu: 0.5% or less, and the balance: iron and impurities, and a solid wire may be used.

[0091] Regarding the welding conditions, when it is difficult to perform welding in one pass due to reasons such as a large thickness of the steel material, welding may be performed in multiple passes.

[0092] Furthermore, the type of welding is not limited to the above-mentioned gas shielded arc welding, and any arc welding method using a cored flux wire or the like or a laser welding method may be adopted.

[0093] And the welded joint obtained by the above welding process, that is, the welded joint in which the first steel plate 11 and the second steel plate 12 are welded by the welding metal 13, is subjected to the following chemical conversion treatment process.

[0094] (Chemical conversion treatment process) The chemical conversion treatment process (i.e., the zinc phosphate treatment process) is such that the P ratio of the formed chemical conversion film is 70.0% or more, and the P ratio (P R ) of the chemical conversion film is the chemical composition index A of the above-mentioned steel material Cand the hematite ratio S of the scale 15 R As long as conditions satisfying the above formula (1) can be adopted, the means, conditions, etc. are not particularly limited.

[0095] For example, in the case of this embodiment, the chemical conversion treatment step can be performed as follows. First, known degreasing treatment and cleaning treatment are performed on the surfaces of the steel plates that will be the formation surfaces of the chemical conversion film, that is, the surfaces of the first steel plate 11, the second steel plate 12, the weld metal 13, the heat-affected zone 14, and the scale 15, in order to remove impurities such as attached oil. Next, a zinc phosphate treatment agent is adhered to the surfaces after these treatments by any means such as coating or dipping to precipitate chemical conversion crystals. Then, a chemical conversion film is formed by drying under arbitrary drying conditions.

[0096] As described above, the welded joint 1 of this embodiment can be manufactured. Further, an arbitrary step such as an electrodeposition coating step may be performed on the welded joint 1 after the chemical conversion treatment step.

[0097] When the electrodeposition coating step is performed after the chemical conversion treatment step, as long as the effects of the present invention are not inhibited, the means, conditions, etc. are not particularly limited. In the case of this embodiment, the electrodeposition coating step is a step of performing electrodeposition coating on the surface that will be the formation surface of the electrodeposition coating film and that at least partially includes the chemical conversion film 16. The type of electrodeposition coating, etc. is not particularly limited. For example, cationic electrodeposition coating or the like can be adopted. Also, the film thickness of the electrodeposition coating film formed by electrodeposition coating can be an arbitrary film thickness according to the use of the welded joint, etc. For example, in the case of a dried coating film, the film thickness can be 8 μm or more, 10 μm or more, or 12 μm or more. Also, the film thickness of the electrodeposition coating film may be 50 μm or less, 40 μm or less, or 30 μm or less.

[0098] Note that the welded joint after electrodeposition coating may be further subjected to a topcoat step according to its use, etc.

[0099] The welded joint of the present invention is not limited to the above-described embodiments, examples described later, etc., and can be appropriately combined, substituted, changed, etc. within the range not departing from the object and gist of the present invention. In this specification, ordinal numbers such as "first", "second", etc. are for distinguishing the matters to which the ordinal numbers are attached, and do not mean the order, priority, importance, etc. of each matter.

Example

[0100] Hereinafter, the present invention will be described more specifically by way of examples, but the present invention is not limited to such examples only.

[0101] (Fabrication of welded joint) In order to verify the effects of the present invention, first, two sheets of each of steel plates A to S having the chemical compositions shown in Table 1 below were prepared as a set. Note that these steel plates can be welded even with different combinations of chemical compositions. Then, one end of one of the two sheets of steel plate and one surface of the other steel plate were gas shielded arc welded under the welding conditions shown in Table 2 below to obtain various fillet welded joints. Note that during welding, a jig in which the back side of the welded portion is hollow was used, and a solid wire (wire diameter: 1.2 mm) was used as the welding wire. Note that the underlines attached to the various numerical values in Table 1 indicate that they are outside the scope of the present invention or are unfavorable conditions.

[0102] Next, while taking care not to contact the front and back sides of the welded portion, which are the surfaces to be evaluated for electrodeposition paintability and chemical conversion treatability, of these various fillet welded joints, a degreasing agent (trade name: Fine Cleaner E2001, manufactured by Nippon Parkerizing Co., Ltd.) was used for degreasing treatment at 40 ° C. and for 120 seconds. After that, surface conditioning was performed at 25 ° C. and for 15 seconds using a surface conditioner (trade name: Surf Fine GL, manufactured by Nippon Paint Surf Chemicals Co., Ltd.). Then, on the surface after the surface conditioning, a zinc phosphate treatment agent (Surf Dyn SD5350, manufactured by Nippon Paint Surf Chemicals Co., Ltd.) was used, and the application amount was 2.5 to 3.5 g / m 2, by performing chemical conversion treatment under the conditions of 35°C and 120 seconds and then drying, a chemical conversion film was formed on the surface of the welded joint. Further, on the surface where this chemical conversion film was formed, cationic paint (PN1010E, gray) was used to perform electrodeposition coating under the conditions of a film thickness of 20 ± 1 μm, a voltage of 160 V, 30°C, and 120 seconds, and then baking was performed at 170°C for 20 minutes to obtain the welded joints of Examples 1 to 6 and Comparative Examples 1 to 13.

[0103]

Table 1

[0104]

Table 2

[0105] Using the welded joints of Examples 1 to 6 and Comparative Examples 1 to 13 obtained as described above, by X-ray diffraction analysis, the X-ray diffraction intensity P of phosphophyllite and the X-ray diffraction intensity H of hopite in the chemical conversion film were measured. Furthermore, the X-ray diffraction intensity of the base steel plate and the X-ray diffraction intensities of wustite (FeO), magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) in the scale were measured. Also, using these measurement results, the P ratio of the chemical conversion film and the value of the left side of the above formula (1) (that is, A C ×P R / S R ) were calculated. And when the P ratio of the chemical conversion film was 70.0% or more, it was judged that the chemical conversion processability was good.

[0106] Furthermore, for the welded joints of Examples 1 to 6 and Comparative Examples 1 to 13, the electrodeposition paint defect area ratio (%) was calculated from the ratio of the paint defect area to the projected area of the weld bead (weld metal) when viewing the surface of the weld bead vertically, and the electrodeposition paintability was evaluated. The weld bead length of the welded joint was 150 mm, and the paint defect area ratio was calculated from the 100 mm long weld bead excluding 50 mm at both ends (25 mm at the starting end and 25 mm at the ending end) of the welded part. When the paint defect area ratio was 8% or less, it was determined that the electrodeposition paintability was good.

[0107] The above measurement results and calculation results are shown in Table 3 below. Note that the underlines attached to various numerical values in Table 3 indicate that they are outside the scope of the present invention, are unfavorable conditions, or that the electrodeposition paintability and chemical conversion treatment property do not meet the above criteria.

[0108]

Table 3

[0109] It was found that the welded joints of Examples 1 to 6 were all excellent in the electrodeposition paintability and chemical conversion treatment property of the welded part.

[0110] In the welded joint of Comparative Example 1, since the Si content of the steel sheet was too high, the elution of iron was suppressed by the formation of fayalite (Fe 2 SiO 4 ), and the phosphophyllite decreased, resulting in poor chemical conversion treatment property.

[0111] In the welded joint of Comparative Example 2, since the Cr content of the steel sheet was too low, the adhesion at the interface between the base iron and the scale decreased, and the scale peeled off together with the chemical conversion film.

[0112] In the welded joint of Comparative Example 3, since the Al content of the steel sheet was too high, alumina (Al 2 O 3 ) was generated, resulting in a decrease in the chemical conversion treatment property.

[0113] In the welded joint of Comparative Example 4, since the Al content of the steel plate was too low, the adhesion at the interface between the substrate and the scale decreased, resulting in the scale peeling off together with the chemical conversion film.

[0114] In the welded joint of Comparative Example 5, since the Mn content of the steel plate was too high, the influence of Si became excessive, resulting in the formation of fayalite and a decrease in chemical conversion treatability.

[0115] In the welded joint of Comparative Example 6, oxides of Fe and Cr with a spinel structure (FeCr 2 O 4 ) were formed at the interface between the substrate and the scale, resulting in a decrease in the adhesion between the substrate and the scale and the scale peeling off together with the chemical conversion film.

[0116] In the welded joint of Comparative Example 7, in the chemical composition of the steel plate, since the Mn content as a hardening element was too low, the strength of the welded joint decreased.

[0117] In the welded joint of Comparative Example 8, since the C content of the steel plate was too high, the pressure of CO gas generated at the interface between the substrate and the scale increased, resulting in the scale peeling off together with the chemical conversion film.

[0118] In the welded joint of Comparative Example 9, in the chemical composition of the steel plate, since the C content as a hardening element was too low, the strength of the welded joint decreased.

[0119] In the welded joint of Comparative Example 10, it concentrated at the grain boundaries in the scale, and due to the decrease in the adhesion of the scale, the scale peeled off together with the chemical conversion film.

[0120] In the welded joint of Comparative Example 11, it concentrated at the grain boundaries in the scale, and due to the decrease in the adhesion of the scale, the scale peeled off together with the chemical conversion film.

[0121] In the welded joint of Comparative Example 12, since the Ti content of the steel plate was too low, alumina (Al 2 O 3) occurred, resulting in a decrease in formation processability.

[0122] In the welded joint of Comparative Example 13, due to the excessive Ti content in the steel plate, the scale was easily peeled off, and as a result, the scale fell off together with the conversion film.

Explanation of Signs

[0123] 1 Welded joint 11 First steel plate 111 End 12 Second steel plate 121 First surface 122 Second surface 13 Weld metal 14 Heat affected zone 15 Scale 16 Conversion film

Claims

1. A welded joint comprising a steel material, a weld metal, and a heat-affected zone, wherein the welded joint further comprises a scale formed on the surface of the heat-affected zone and a chemical conversion coating having a thickness of 0.3 to 5.0 μm formed on the surface of the scale, the chemical composition of the steel material being, in mass %, C: 0.03 to 0.30%, Si: 0.01 to 1.10%, Mn: 0.30 to 3.50%, Al: 0.01 to 0.35%, Ti: 0.01 to 0.30%, Cr: 0.02 to 2.00%, B: 0 to 0.0040%, Nb: 0 to 0.050%, Mo: 0 to 0.250%, V: 0 to 0.250%, Ni: 0 to 0.250%, and the balance: Fe and impurities, the Vickers hardness of the steel material being 250 HV or more, P ratio, represented as the ratio of the X-ray diffraction intensity of phosphophyllite to hopite in the formation film: P Ｒ is 70.0% or more, Chemical composition index of the steel material: A Ｃ and P ratio of the chemical conversion film: P Ｒ and hematite ratio of the scale: S Ｒ A welded joint characterized in that the following formula (1) is satisfied. A Ｃ ×P Ｒ / S Ｒ ≥10.0...(1) However, in Formula (1), A Ｃ , P Ｒ and S Ｒ are values obtained from the following Formulas (i) to (iii), respectively. A Ｃ = (2.5[Ti] + 5[Mn] + 2[Al] + 6[Cr]) - (10[Si] / [Mn] + 6[C] + 15[S] + 15[P])... (i) in formula (i), [Ti], [Mn], [Al], [Cr], [Si], [C], [S], and [P] respectively represent the contents in mass % units of Ti, Mn, Al, Cr, Si, C, S, and P contained in the steel material. P Ｒ P(%) = {P / (P + H)} × 100... (ii) In formula (ii), P represents the X-ray diffraction intensity of phosphophyllite contained in the chemical conversion coating, and H represents the X-ray diffraction intensity of hopite. S Ｒ (%) = {S Ｈ / (S Ｈ + S Ｍ + S Ｗ + S Ｉ )} × 100... (iii) In formula (iii), S Ｈ represents the X-ray diffraction intensity of hematite contained in the scale, and S Ｍ represents the X-ray diffraction intensity of magnetite contained in the scale, and S Ｗ represents the X-ray diffraction intensity of wustite contained in the scale, and S Ｉ represents the X-ray diffraction intensity of the ferrite matrix.

2. The welded joint according to claim 1, further comprising slag adhering to the surface of the weld metal, an electrodeposition coating film formed on the surfaces of the weld metal and the slag, and a topcoat film formed on the surface of the electrodeposition coating film.

3. The welded joint according to claim 1 or 2, wherein the Vickers hardness of the steel material is 300 HV or more.

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