JP7534597B2 - Lap fillet welded joints and automotive parts - Google Patents

Description

The present invention relates to lap fillet welded joints and automotive parts.

Automobile suspension parts are usually manufactured by overlapping and welding multiple steel materials together using gas-shielded arc welding or similar techniques. In recent years, there has been a strong demand for improving the fatigue strength of welds in order to increase the strength of vehicle bodies. In conventional technology, efforts have been made to suppress the generation of cracks from the toes of the weld metal in order to improve the fatigue strength of welded joints.

For example, Patent Document 1 discloses a welding material that is characterized by being made of an iron alloy having a martensitic transformation start temperature of 400 to 150°C and containing, by mass%, C: 0.001 to 0.2%, Mn: 3 to 10%, and Si: 0.1 to 1.0%.

Patent Document 2 discloses a solid wire for pulse MAG welding that contains, in mass % relative to the total mass of the wire, less than 0.10% Si, 0.01-0.15% C, 1.80-2.50% Mn, 0.001-0.070% S, 0.030% or less P, 0.010% or less O, and the remainder consisting of Fe and unavoidable impurities. It also discloses a solid wire for pulse MAG welding that contains a total of 0.20% or less of one or both of Ti and Al.

Patent Document 3 discloses a fillet welded joint with excellent fatigue crack initiation resistance, characterized in that in a fillet welded joint of steel plates, the ratio of the hardness Hv (FL-0.5) of the weld metal at a position 0.5 mm away from the fusion boundary FL at the weld toe on the weld metal side to the hardness Hv (FL+0.5) of the heat-affected zone at a position 0.5 mm away from the fusion boundary FL on the heat-affected zone side is 0.3 or more and 0.9 or less.

Patent Document 4 discloses a method for fillet arc welding of thin steel plates, characterized in that when fillet arc welding of steel plates 1.6 to 6 mm thick is performed using a solid wire and gas-shielded arc welding at a welding speed of more than 80 cm/min and not more than 150 cm/min, the steel plate is a TS280 to 600 MPa class steel plate containing C = 0.001 to 0.15%, Si = 0.2 to 2.0%, Mn = 0.5 to 2.5%, with the balance being iron and unavoidable impurities, the solid wire is a solid wire containing C = 0.03 to 0.15%, Si = 0.2 to 2.0%, Mn = 0.5 to 2.5%, Cu ≦ 0.5%, with the balance being iron and unavoidable impurities, and the steel plate and arc welding solid wire are combined so that {Si (steel plate) + 0.1 × Si (wire)} ≧ 0.32.

JP 2004-98108 A JP 2009-166066 A JP 2008-178910 A JP 2009-226476 A

All of the techniques in Patent Documents 1 to 4 are intended to suppress crack initiation from the toe of the weld metal. However, fatigue cracks in welded joints occur at the weld toe and root. The inventors considered that in order to suppress fatigue cracks, it was also necessary to prevent stress concentration at the root. However, none of the above-mentioned prior art techniques improves both the toe shape and the root shape. Therefore, even with these prior art techniques, it is difficult to meet the demand for further improvement in fatigue strength.

In view of the above, the present invention aims to provide a lap fillet welded joint and an automobile part that have high fatigue strength.

The gist of the present invention is as follows.
(1) A lap fillet welded joint according to one aspect of the present invention is a lap fillet welded joint comprising an upper plate having a plate thickness t1, a lower plate having a plate thickness t2, and a weld metal joining the upper plate and the lower plate, wherein components of the weld metal contain, in mass%, C: 0.05 to 0.20%, Mn: 0.6 to 2.7%, P: 0.020% or less, S: 0.020% or less, and Si, with the balance being Fe and impurities, the amount of Si in mass% of the weld metal is not more than the largest value among 0.15%, a value obtained by multiplying the amount of Si in mass% of the upper plate by 0.62, and a value obtained by multiplying the amount of Si in mass% of the lower plate by 0.62, a radius of curvature of a toe of the weld metal is 0.14×t2 or more, and a throat thickness of the weld metal is 0.9×t1 or more.
(2) In the lap fillet welded joint described in (1) above, a component of the weld metal may include one or two elements selected from the group consisting of Al and Ti in place of a portion of the Fe, a total value of the Al amount and the Ti amount, by mass%, of the weld metal may be 0.05% or more, and the Si amount, the Mn amount, the Ti amount, and the Al amount, by mass%, of the weld metal may satisfy Formula 1.
7×Si+7×Mn-112×Ti-30×Al≦12 <Formula 1>
In addition, when an element with a content of 0% is present, zero is substituted for the element with a content of 0% in formula 1.
(3) In the lap fillet welded joint described in (1) or (2) above, a component of the weld metal may include, in mass%, one or more selected from the group consisting of Al: 0.30% or less, Ti: 0.30% or less, Zr: 0.30% or less, B: 0.0040% or less, Sn: 0.4% or less, Cu: 0.50% or less, Cr: 1.5% or less, Nb: 0.3% or less, V: 0.3% or less, Mo: 1.0% or less, and Ni: 3.0% or less, in place of a portion of the Fe, and the Si content may be 0 to 0.8%.
(4) An automobile component according to another aspect of the present invention includes the lap fillet welded joint according to any one of (1) to (3) above.

The present invention provides lap fillet welded joints and automotive parts with high fatigue strength.

FIG. 1 is a cross-sectional schematic diagram showing a typical lap fillet welded joint during welding. FIG. 2 is a cross-sectional schematic diagram showing the lap fillet welded joint according to the present embodiment during welding. FIG. 2 is a cross-sectional schematic diagram of an example of a lap fillet welded joint according to the present embodiment. FIG. 2 is a cross-sectional schematic diagram of an example of a lap fillet welded joint according to the present embodiment.

The inventors considered that in order to improve the fatigue strength of lap fillet welded joints, it was necessary to smooth the shape near the interface between the weld bead and the plates (upper plate and lower plate) and reduce stress concentration. The inventors then focused on the surface tension of molten metal. The weld metal that joins the plates is formed when the molten plate and welding material solidify. Before the weld metal solidifies, it is thought that molten metal A originating from the steel plate exists between molten metal B originating from the welding material and the steel plate (upper plate 11 or lower plate 12), as shown in Figure 1 or Figure 2, for example.

Here, it was presumed that the shape of the weld metal is determined according to the magnitude relationship between the surface tension γ1 of the molten metal A originating from the plate and the surface tension γ2 of the molten metal B consisting of a mixture of the welding material and the steel plate. Specifically, it was presumed that when γ1 is smaller than γ2, the molten metal rises up and the weld metal also has a raised shape (so-called convex bead shape) (see Figure 1). As a result, it is presumed that the weld metal near the toe on the upper plate 11 side has a constricted and concave shape, and the weld metal near the toe on the lower plate 12 side is likely to have an undercut shape that is more concave than the surface of the lower plate 12. On the other hand, when γ1 is larger than γ2, it is presumed that the molten metal is drawn toward the plate and has a flat shape, and the weld metal also becomes flat (see Figure 2). In this case, the shape near the interface between the weld bead and the plate becomes smooth, and stress concentration is alleviated.

Furthermore, the inventors have repeatedly studied factors that affect the surface tension of molten metal. They have found that the Si concentration of molten metal has a large effect on the surface tension of molten metal. Si easily bonds with oxygen. Therefore, it is believed that oxygen adheres in large quantities to the surface of molten metal with a low Si content without becoming _SiO2 . It is presumed that this oxygen reduces the surface tension of the molten metal. Conversely, it is believed that the higher the Si content of the molten metal, the higher the surface tension of the molten metal.

Based on the above findings, the inventors reduced the Si content of the welding material relative to the Si content of the plates, and reduced the Si content of the weld metal to 0.62 times or less the Si content of the plates (upper and lower plates). This resulted in a lap fillet welded joint with a smooth shape near the interface between the weld bead and the plates and with reduced stress concentration. This is presumably because the surface tension γ1 of the molten metal resulting from the plate with a high Si content was greater than the surface tension γ2 of the molten metal resulting from the welding material with a low Si content.

However, the shape of the weld metal may change due to the influence of components other than Si. For example, Al does not cause the weld metal to have a dent as shown in Figure 1, but the weld metal appears to have a raised shape rather than spreading over the steel plate surface. This may result in a large rise angle of the weld metal on the lower plate 12 side. Therefore, in addition to controlling the relationship between the Si content of the plate and the Si content of the weld metal, it has also been found that it may be necessary to devise welding conditions such as the welding position and shielding gas composition in order to smooth the shape near the interface between the weld bead and the plate and to alleviate stress concentration.

The lap fillet welded joint 1 (see FIG. 3) according to one embodiment of the present invention, which was obtained through the above study, comprises an upper plate 11 having a plate thickness of t1, a lower plate 12 having a plate thickness of t2, and a weld metal 13 joining the upper plate 11 and the lower plate 12, and the components of the weld metal 13 include, in mass %, C: 0.05-0.20%, Mn: 0.6-2.7%, P: 0.020% or less, and S: 0.020% or less. The balance is Fe and impurities, the Si content in mass % of the weld metal 13 is equal to or less than the largest value among 0.15%, the Si content in mass % of the upper plate 11 multiplied by 0.62, and the Si content in mass % of the lower plate 12 multiplied by 0.62, the radius of curvature r of the toe 131 of the weld metal 13 is equal to or greater than 0.14 x t2, and the throat thickness t3 of the weld metal 13 is equal to or greater than 0.9 x t1. The lap fillet welded joint 1 according to this embodiment will be described in detail below.

(Definition of terms)
In this embodiment, the term "lap fillet welded joint" is defined as a lap joint manufactured by fillet welding. Here, the term "lap joint" is defined as "a joint in which parts are placed parallel to each other at an angle of 0°≦α≦5° and overlap each other" (JIS Z 3001-1:2018). Here, the fact that "α" is the included angle between the upper plate and the lower plate is illustrated in the above standard. The term "fillet weld" is defined as "a weld having a triangular cross section between members without providing a groove in a lap joint, T-joint, corner joint, etc." (JIS Z 3001-1:2018).

In this embodiment, the term "upper plate" is defined as one of the two or more components that make up the lap joint, the end faces of which are fillet welded. There may be two or more upper plates 11. When there are two or more upper plates 11, the plate thickness t1 of the upper plate 11 is defined as the total plate thickness of the upper plates 11. In this embodiment, the term "lower plate" is defined as one of the two or more components that make up the lap joint, the surface of which is fillet welded.

The term "weld metal" is defined as "a part of the weld that melts and solidifies during welding" (JIS Z 3001-1:2018). In this embodiment, the term "composition of the weld metal" is defined as a value measured by (1) identifying the weld metal area in advance by visually observing a cross section perpendicular to the longitudinal direction at the longitudinal center of the weld, (2) cutting the area with a drill to obtain chips of the weld metal, and (3) measuring the chips as a sample by optical emission spectroscopy using high-frequency inductively coupled plasma (ICP).

In this embodiment, the term "throat thickness" is slightly different from the definition set out in the JIS standard, and as shown in Figure 3, it means the shortest distance from the intersection of the lower surface of the upper plate 11 (the surface facing the lower plate 12. If there are two or more upper plates 11, it means the surface of the upper plate 11 closest to the lower plate 12 that faces the lower plate 12) and the weld metal 13 to the outer surface of the joint of the weld metal 13.

In this embodiment, the term "toe" is defined as the toe and its surroundings. The term "toe" is defined as "the point where the surface of the base material and the surface of the weld bead intersect" (JIS Z 3001-7:2018 "Welding terminology - Part 7: Arc welding"). The term "toe curvature radius" is defined as follows: (1) First, a micrograph of the toe cross section revealing the metallurgical structure of the weld metal is taken at a magnification of 40 to 50 times; (2) Next, the photograph is observed while being enlarged and displayed on a monitor or the like; and the diameter φ1 of a circle whose lowest point is the toe and passes through the bead surface at a height of 5.0% of the thickness t2 of the lower plate 12, the diameter φ2 of a circle whose lowest point is the toe and passes through the bead surface at a height of 7.5% of the thickness t2 of the lower plate 12, and the diameter φ3 of a circle whose lowest point is the toe and passes through the bead surface at a height of 10.0% of the thickness t2 of the lower plate 12; (3) The average radius calculated from the radii of the circles with diameters φ1 to φ3 is defined as the "toe curvature radius" (see Figure 4).

(Upper and lower plates)
The lap fillet welded joint according to the present embodiment includes an upper plate 11 and a lower plate 12. The chemical components of the upper plate 11 and the lower plate 12 are not particularly limited, and can be appropriately selected within a range that satisfies the components of the weld metal described below. Preferred components of the upper plate 11 and the lower plate 12 are, for example, C: 0.05 to 0.35 mass%, Si: more than 0 to 1.5 mass%, Mn: 0.5 to 3 mass%, P: 0.03 mass% or less, S: 0.02 mass% or less, Al: 0.01 to 0.5 mass%, and the balance: iron and impurities. In order to further increase the strength, Ti, Nb, V, Mo, Cr, Ni, and B may be contained in the upper plate 11 and/or the lower plate 12 as necessary. The tensile strength of the upper plate 11 and the lower plate 12 is not particularly limited, but may be, for example, 780 MPa or more. This is because there is a particularly high need for improved fatigue strength in steel plates with a tensile strength of 780 MPa or more. The components and tensile strengths of the two or more upper plates 11 and lower plates 12 may be the same or different.

The structure and strength of the upper plate 11 and the lower plate 12 are not particularly limited. Since the part of the lap fillet welded joint that has the lowest fatigue strength is the weld metal, the structure and strength of the upper plate 11 and the lower plate 12 do not directly affect the fatigue strength of the lap fillet welded joint. The thickness t1 of the upper plate 11 and the thickness t2 of the lower plate 12 are also not particularly limited. For example, the thickness t1 of the upper plate 11 may be 0.8 to 4.5 mm. The thickness t2 of the lower plate 12 may be 0.8 to 4.5 mm. t1 and t2 may be the same or different.

(Weld metal composition)
The lap fillet welded joint according to this embodiment includes a weld metal 13 that joins an upper plate 11 and a lower plate 12. Below, a description will be given of the components of the weld metal 13. Unless otherwise specified, the content of an element described below means the content of the element in the components of the weld metal 13, and the unit "%" means mass %.

[C: 0.05-0.20%]
C has the effect of stabilizing the arc and making droplets finer. If the C content is less than 0.05%, the droplets become larger, the arc becomes unstable, and the amount of spatter increases. As a result, the bead shape becomes uneven, which causes variation in fatigue strength of the welded portion. Also, if the C content is less than 0.05%, the tensile strength of the weld metal cannot be ensured, and it is difficult to obtain the desired joint strength. Therefore, the C content is set to 0.05% or more, preferably 0.08% or more, and more preferably 0.15% or more.

On the other hand, if the C content exceeds 0.20%, the weld metal 13 will become excessively hardened, reducing its crack resistance and making the weld metal 13 more susceptible to fracture. Therefore, the C content is set to 0.20%. The C content is preferably 0.18% or less, or 0.16% or less.

[Si: not more than the largest of (1) 0.15%, (2) the value obtained by multiplying the amount of Si in mass% of the upper plate 11 by 0.62, and (3) the value obtained by multiplying the amount of Si in mass% of the lower plate 12 by 0.62]
General welding materials contain about 0.5% of Si for deoxidizing the weld metal 13. Therefore, the weld metal 13 obtained by using general welding materials also contains the same amount of Si. However, it is considered that Si has the function of increasing the surface tension of the molten metal. As shown in Figs. 1 and 2, the greater the surface tension γ1 of the molten metal A derived from the plate relative to the surface tension γ2 of the molten metal B derived from the welding material, the flatter the molten metal becomes, and the smoother the shape of the vicinity of the interface between the weld metal 13 and the plate (upper plate 11 and lower plate 12) becomes. On the other hand, if the Si content of the weld metal 13 becomes excessive, the weld metal 13 becomes convex, the radius of curvature r of the toe 131 decreases, and further, the throat thickness t3 of the root 132 decreases and the weld metal 13 is necked. The toe 131 with a small radius of curvature r and the necked portion of the weld metal 13 are likely to become the starting point of fatigue cracks.

Therefore, the upper limit of the Si content of the weld metal is set to the larger of (2) the amount of Si in mass percent of the upper plate 11 × 0.62 or (3) the amount of Si in mass percent of the lower plate 12 × 0.62.
However, when the Si content of the plates is small, it is considered that the shape of the weld metal 13 can be made preferable by setting the Si content of the weld metal to 0.15% or less. Therefore, when (2) the Si content in mass% of the upper plate 11 × 0.62 or (3) the Si content in mass% of the lower plate 12 × 0.62 is 0.15% or less, the upper limit of the Si content of the weld metal is (1) 0.15%.

In addition, the upper and lower limits of the Si content may be determined separately while satisfying the above requirements. For example, Si may be contained in the welding material or plates (upper plate 11 and lower plate 12) as a deoxidizing element. Si in the welding material improves the tensile strength of the weld metal 13 by promoting deoxidation of the molten pool. Therefore, the Si content may be 0.01% or more, 0.02% or more, or 0.05% or more. However, the fatigue strength of the lap fillet welded joint 1 can be ensured by other elements. Therefore, the lower limit of the Si content may be 0%.

In addition, if the Si content is excessive, the amount of non-conductive Si-based slag increases, and red rust is likely to occur between the slag and the weld metal 13. From this perspective, the Si content may be set to 0.8% or less while satisfying the above-mentioned requirements. This means that if the "Si content of the upper plate 11 or the lower plate 12 × 0.62%" is more than 0.8%, the Si content of the weld metal 13 is set to 0.8% or less, and if the "Si content of the upper plate 11 or the lower plate 12 × 0.62%" is 0.8% or less, the upper limit of the Si content of the weld metal 13 is set to "Si content of the upper plate 11 or the lower plate 12 × 0.62%". While satisfying the above-mentioned requirements, the Si content may be set to 0.7% or less, or 0.6% or less.

[Mn: 0.6-2.7%]
Mn, like Si, is a deoxidizing element that promotes deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal 13. If the weld metal 13 is used, the tensile strength of the weld metal 13 cannot be sufficiently ensured, and the weld metal 13 is likely to break. Therefore, the Mn content is set to 0.6% or more. The Mn content is preferably set to 0.8% or more. 1.0% or more, or 1.2% or more.

On the other hand, if the Mn content is excessive, the viscosity of the molten metal during welding increases. This causes the molten metal to not flow properly into the welded area, especially when the welding speed is high, making it more likely that a humping bead (i.e., poor bead shape) will occur. As a result, red rust will more easily occur. Therefore, the Mn content should be 2.7% or less. The Mn content is preferably 2.5% or less, 2.2% or less, or 2.0% or less.

[P: 0.020% or less]
[S: 0.020% or less]
Both P and S are impurities. By making the content of these elements 0.020% or less, various properties (e.g., toughness, etc.) of the weld metal 13 can be ensured. Therefore, the P content The lower the P and S contents, the better. However, taking into consideration refining capacity and refining costs, the respective P and S contents are The lower limit may be 0.001%, 0.002%, or 0.005%.

The remainder of the components of the weld metal 13 of the lap fillet welded joint 1 according to this embodiment is Fe (iron) and impurities. The impurities refer to, for example, components that are mixed in due to raw materials such as ores or scraps, or various factors in the manufacturing process, during the industrial production of the upper plate 11, the lower plate 12, and the welding material, and are permissible within a range that does not adversely affect the characteristics of the lap fillet welded joint 1 according to this embodiment.

In addition, instead of a portion of the iron in the remaining components of the weld metal 13, the weld metal 13 may further contain the optional elements listed below. However, the lap fillet welded joint 1 according to this embodiment can obtain a satisfactory high fatigue strength without including these optional elements. Therefore, it is not necessary to add the optional elements listed below to the weld metal.

[Total content of Al and Ti: preferably 0.05% or more]
The components of the weld metal 13 of the lap fillet welded joint 1 according to this embodiment may further include one or two elements selected from the group consisting of Al and Ti. In this case, the total value of the Al content and the Ti content in mass % is preferably 0.05% or more.
Both Al and Ti can suppress the formation of coarse ferrite, thereby ensuring sufficient strength of the weld metal. This effect can be obtained by making the total content of Al and Ti 0.05% or more. Therefore, the lower limit of the total content of Al and Ti is preferably 0.05% or more. The total content of Al and Ti is preferably 0.10% or more, and more preferably 0.15% or more.

The upper limit of Al+Ti is not particularly limited, and may be 0.60% calculated from the respective preferred upper limits of Al and Ti described below. However, if the upper limit of Al+Ti is 0.30% or less, the generation of Al-based slag and Ti-based slag is suppressed, and the generation of red rust between the Al-based slag and Ti-based slag and the weld metal is suppressed, so this is even more preferable. The upper limit of Al+Ti is even more preferably 0.20%.

[Si, Mn, Ti, Al]
The contents of Si, Mn, Ti, and Al preferably satisfy the following formula 1. When an element with a content of 0% exists, the element with a content of 0% in formula 1 is Zero is substituted.
7×Si+7×Mn-112×Ti-30×Al≦12 <Formula 1>
The inventors investigated whether red rust occurred between the slag and the weld metal for weld metals having various component systems, and found that the index for the occurrence of red rust is 7×Si+7×Mn-112×Ti-30× It has been revealed that when the value of Al is 12 or less, the generation of red rust is suppressed and the corrosion resistance is further improved. Therefore, the upper limit in formula 1 is set to 12. It is preferable that formula 1 is satisfied. .
The lower limit of 7×Si+7×Mn−112×Ti−30×Al is not particularly limited, and may be, for example, −42.25. This value is determined by the lower limits of Si and Mn and the values of Al and Mn described later. This is calculated from the upper limit of Ti.

Although the embodiment in which the Al content and the Ti content are defined by their total content has been described above, the Al content and the Ti content may each be determined individually. The Al content and the Ti content may each be determined individually, and the total content of these may also be determined.

[Al: preferably 0.30% or less]
Al is a strong deoxidizing element and has the effect of promoting deoxidation of molten metal during arc welding and suppressing the occurrence of blowholes. In order to obtain this effect, the Al content may be 0.001% or more. The Al content is more preferably 0.01% or more, or 0.02% or more.

On the other hand, if the Al content is 0.30% or less, the generation of non-conductive Al-based slag is suppressed, and the generation of red rust between the slag and the weld metal can be further prevented. Therefore, the Al content of the weld metal may be 0.30% or less. The Al content is more preferably 0.25% or less, or 0.20% or less.

[Ti: preferably 0.30% or less]
Ti is a deoxidizing element and is therefore effective in suppressing the occurrence of blowholes. In order to obtain this effect, the Ti content may be set to 0.05% or more. The Ti content is more preferably 0.10% or more, or 0.15% or more.

On the other hand, if the Ti content is 0.30% or less, the increase in Ti-based slag can be suppressed and the occurrence of red rust can be suppressed. Therefore, the Ti content may be 0.30% or less. The Ti content is more preferably 0.25% or less, or 0.20% or less.

[Zr: preferably 0.30% or less]
Zr has a deoxidizing effect, and is therefore effective in suppressing the occurrence of blowholes. In order to obtain this effect, the Zr content may be set to 0.05% or more. The Zr content is more preferably 0.10% or more, or 0.15% or more.

On the other hand, if the Zr content is 0.30% or less, the occurrence of red rust can be suppressed. Therefore, the Zr content may be 0.30% or less. The Zr content is more preferably 0.25% or less, or 0.20% or less.

[B: preferably 0.0040% or less]
B may be contained in an amount of 0.0005% or more in order to improve the hardenability of the welded portion and thereby improve the tensile strength. Furthermore, if the B content is 0.0040% or less, the decrease in elongation of the welded portion can be suppressed. Therefore, the B content may be 0.0040% or less. More preferably, the B content is 0.0030% or less.

[Sn: preferably 0.4% or less]
Sn is an element that improves the corrosion resistance of the weld metal. Although the inclusion of Sn is not essential, increasing the corrosion resistance of the weld metal is also very advantageous for mechanical parts. In order to obtain the effect of improving the corrosion resistance of Sn, Sn may be set to 0.05% or more. On the other hand, if Sn exceeds 0.4%, the cracking sensitivity of the weld metal increases and hot cracking is more likely to occur. Furthermore, if Sn exceeds 0.4%, Sn segregates at the grain boundaries of the weld metal, reducing the toughness. Therefore, Sn may be set to 0.4% or less.

[Cu: preferably 0.50% or less]
By copper plating the surface of the welding material with Cu, the efficiency of the welding work can be improved. Therefore, the Cu content may be set to 0.05% or more. Furthermore, by setting the Cu content to 0.50% or less, the occurrence of weld cracks can be suppressed. Therefore, the Cu content may be set to 0.50% or less.

[Cr: preferably 1.5% or less]
Cr enhances the hardenability of the welded portion and improves the tensile strength. Therefore, the Cr content may be set to 0.05% or more. Furthermore, when the Cr content is set to 1.5% or less, the decrease in elongation of the welded portion can be suppressed. Therefore, the Cr content may be set to 1.5% or less.

[Nb: preferably 0.3% or less]
Nb enhances the hardenability of the welded portion and improves the tensile strength. Therefore, the Nb content may be 0.005% or more. Furthermore, when the Nb content is 0.3% or less, the decrease in elongation of the welded portion can be suppressed. Therefore, the Nb content may be 0.3% or less.

[V: preferably 0.3% or less]
V enhances the hardenability of the welded portion and improves the tensile strength. Therefore, the V content may be set to 0.005% or more. Furthermore, when the V content is set to 0.3% or less, the decrease in elongation of the welded portion can be suppressed. Therefore, the V content may be set to 0.3% or less.

[Mo: preferably 1.0% or less]
Mo enhances the hardenability of the welded portion and improves the tensile strength. Therefore, the Mo content may be 0.05% or more. When the Mo content is 1.0% or less, the elongation decrease of the welded portion can be suppressed. Therefore, the Mo content may be 1.0% or less.

[Ni: preferably 3.0% or less]
Ni improves the tensile strength and elongation of the welded portion. Therefore, the Ni content may be 0.05% or more. Furthermore, when the Ni content is 3.0% or less, the occurrence of weld cracks can be suppressed. Therefore, the Ni content may be 3.0% or less. The Ni content is more preferably 2.0% or less.

(Shape of the toe of the weld metal)
In the lap fillet welded joint 1 according to this embodiment, the radius of curvature r of the toe 131 of the weld metal 13 is set to be 0.14×t2 or more. This makes it possible to suppress stress concentration at the toe 131 and improve the fatigue strength of the lap fillet welded joint 1. The radius of curvature r of the toe 131 may be set to be 0.16×t2 or more, 0.19×t2 or more, or 0.20×t2 or more.

(throat thickness of weld metal)
In the lap fillet welded joint 1 according to this embodiment, the throat thickness t3 of the weld metal 13 is set to 0.9×t1 or more. This means that the necking near the interface between the weld metal 13 and the upper plate 11 is small. As a result, in the lap fillet welded joint 1 according to this embodiment, it is possible to suppress stress concentration near the interface between the weld metal 13 and the upper plate 11 and improve the fatigue strength of the lap fillet welded joint 1. The throat thickness t3 of the weld metal 13 may be set to 0.95×t1 or more, 1.0×t1 or more, or 1.1×t1 or more.

The manufacturing method for weld metal 13 having the above-mentioned composition and shape is described below.

The composition of the weld metal 13 is determined according to the composition of the upper plate 11, the composition of the lower plate 12, the composition of the welding material, and the migration rate of the welding material. Therefore, by appropriately controlling the composition and migration rate of the welding material according to the composition of the upper plate 11 and the composition of the lower plate 12, the composition of the weld metal 13 can be kept within the above-mentioned range.

The radius of curvature r and throat thickness t3 of the toe 131 of the weld metal 13 can be preferably controlled by setting the Si content of the weld metal 13 within the above-mentioned range. However, if the weld metal 13 contains a large amount of an element (such as Al) that has the property of making the weld metal 13 bulge, in addition to controlling the Si content, it is desirable to weld while tilting the joint. For example, if the joint is tilted so that the toe 131 is raised and the upper plate 11 is lowered around the direction in which the weld bead extends (welding direction) as the axis of rotation, the molten metal will tilt toward the upper plate due to the effect of gravity, and a smooth shape with a large toe radius r will be obtained. In this case, the throat thickness t3 will also increase.

The welding method is not particularly limited, but it is preferable to use, for example, gas-shielded arc welding. A suitable gas type is, for example, an inert gas containing 5 to 50 volume % _CO2 , with the remainder being mainly Ar. It is more preferable that the shielding gas contains 20 volume % or less _CO2 .

The welding position in gas-shielded arc welding is not particularly limited. For example, the welding position may be a downward position (a position in which the lower plate is horizontal). On the other hand, the welding position may be changed so that the lower plate is tilted from the horizontal by more than 0° and not more than 60°. Here, the inclination of the lower plate refers to the angle between the horizontal plane and the upper plate (or lower plate) when the lower plate is tilted around the direction in which the weld bead extends (welding direction) as the axis of rotation. Through the inclination of the lower plate, the radius of curvature of the toe of the weld metal and the throat thickness of the weld metal can be more suitably controlled.

An automobile part according to another aspect of the present invention includes the lap fillet welded joint 1 according to this embodiment. As a result, the automobile part according to this embodiment has high fatigue strength. The type of automobile part is not particularly limited. For example, automobile parts according to this embodiment include various lower arms, various upper arms, toe control arms, trailing arms, torsion beams, carriers, subframes, side rails, cabs, underrun protectors, wheels, and floor crosses.

Steel plates S1 to S7 having the components shown in Table 1 were gas-shielded arc-welded using wires W1 to W9 having the components shown in Table 2. The remainder of the components of the steel plates and wires were iron and impurities. The steel plates of the upper and lower plates were of the same type. The content of elements that were not intentionally added to the components of the steel plates and wires is indicated as "-". The shielding gas type and welding position during welding were as shown in Table 3. In the example of the "downward" welding position, the lower and upper plates were horizontally welded. In the example of the "45°" welding position, the plates were welded by tilting the joint by 45° so that the toe 131 was raised and the upper plate 11 was lowered, with the direction in which the weld bead extends (welding direction) as the axis of rotation. In the example of the "-45°" welding position, the plates were welded by tilting the joint by 45° so that the toe 131 was raised and the upper plate 11 was lowered, with the direction in which the weld bead extends (welding direction) as the axis of rotation.

The various lap fillet welded joints obtained in this manner were evaluated using the following methods and are listed in Tables 4 and 5.

(Weld metal composition)
In the longitudinal center of the weld, the area of the weld metal was identified in advance by visually observing a cross section perpendicular to the longitudinal direction, and the area was cut with a drill to collect chips of the weld metal. The chips were used as samples and measured by an emission spectrometry method using high-frequency inductively coupled plasma (ICP). The measurement results are shown in Table 4. The red rust formation index (7×Si+7×Mn-112×Ti-30×Al) calculated from these chemical components is shown in Table 5. In calculating the red rust formation index, the content of elements that were not intentionally added to the components of the steel sheet and wire was considered to be "0". In Table 4, values outside the range of the invention are underlined.

(Toe radius and throat depth)
The lap fillet welded joint was cut perpendicular to the extending direction of the weld metal (weld bead). The cut surface was then polished and etched to reveal the weld metal at the cut surface. The shape of this weld metal was measured to measure the toe radius r and throat thickness t3. The toe radius r and throat thickness t3 were defined as above. The measurement results are shown in Table 5. In Table 5, values outside the range of the invention are underlined.

(Fatigue strength)
A Schenk fatigue tester was used to perform bi-directional bending fatigue tests on the lap fillet welded joints. At 2 million cycles, the lap fillet welded joints were deemed to have no cracks if the torque drop from the torque at the start of the test was less than 40%. The stress amplitude at 2 million cycles is shown in Table 5 as the fatigue strength.

Comparative examples 15 to 20 are fatigue test results when a standard ordinary welding wire was applied to each steel plate, and the fatigue strength was approximately 150 MPa on average. Therefore, lap fillet welded joints that showed a fatigue strength of more than 180 MPa, which is 1.2 times this value, were judged to have excellent fatigue strength.

(Paintability)
After degreasing and chemical conversion treatment of the lap fillet welded joint, electrodeposition coating was applied to a film thickness of 20 μm. Then, the electrodeposition coating part of the weld bead was photographed. The photograph was subjected to image analysis to measure the ratio of the area of defective electrodeposition coating to the area of the weld bead. Here, the bead length of the welded test piece was 120 mm, and the defective rate of the electrodeposition coating was obtained from a 90 mm long weld bead excluding the welding start part (length 15 mm) and the end part (length 15 mm). Gray paint was used for the electrodeposition coating. This made it easy to identify defective electrodeposition coating parts where reddish brown or black slag was exposed. The electrodeposition coating rate was determined to be good when the defective coating area was 5% or less in area ratio.

The fatigue strength was insufficient in Comparative Examples 15 and 17. This is presumably because the Si content of the weld metal exceeded the largest value among 0.15% and the value obtained by multiplying the Si content in mass% of the steel plate ("Steel plate Si" in Table 3) by 0.62, and the curvature radius of the toe ("Toe radius" in Table 5) was insufficient.
In Comparative Example 16, the fatigue strength was insufficient. This is presumably because the Si content of the weld metal exceeded the largest value between 0.15% and the value obtained by multiplying the Si content of the steel plate by 0.62, and the throat depth was insufficient.
In Comparative Examples 18, 20, 21, 22, and 24, the fatigue strength was insufficient. This is presumably because the toe radius was insufficient.
In Comparative Example 19, the fatigue strength was insufficient. This is presumably because the Si content of the weld metal exceeded the largest value between 0.15% and the value obtained by multiplying the Si content of the steel plate by 0.62, and the toe radius and throat depth were insufficient.
In Comparative Example 23, the fatigue strength was insufficient. This is presumably because the throat thickness of the weld metal was insufficient.
In Comparative Example 25, the fatigue strength was insufficient. This is presumably because the C content of the weld metal was insufficient.
In Comparative Example 26, cracks occurred in the root due to hydrogen embrittlement when welding was completed. Therefore, the fatigue strength of Comparative Example 26 was not measured. This is presumably because the C content of the weld metal was excessive.
In Comparative Example 27, the fatigue strength was insufficient. This is presumably because the Mn content of the weld metal was insufficient.
In Comparative Example 28, the fatigue strength was insufficient. This is presumably because the Mn content in the weld metal was excessive.

On the other hand, the lap fillet welded joints according to the examples of the invention, in which the chemical composition and shape of the weld metal were within the specified ranges, had high fatigue strength.

The present invention can provide lap fillet welded joints and automotive parts with high fatigue strength. Therefore, the present invention has high industrial applicability.

1 lap fillet welded joint 11 upper plate 12 lower plate 13 weld metal 131 weld toe 132 root A molten metal derived from plate B molten metal derived from welding material γ1 surface tension of molten metal derived from plate γ2 surface tension of molten metal derived from welding material t1 plate thickness t2 plate thickness t3 throat thickness r radius of curvature of weld toe

Claims (4)

A lap fillet welded joint comprising an upper plate having a plate thickness of t1, a lower plate having a plate thickness of t2, and a weld metal joining the upper plate and the lower plate,
The composition of the weld metal is, in mass%,
C: 0.05-0.20%,
Mn: 0.6 to 2.7%,
P: 0.020% or less,
S: 0.020% or less,
Al: 0.01-0.30%,
and Si
with the remainder being Fe and impurities,
the Si content in mass% of the weld metal is equal to or less than the largest value of 0.15%, the value obtained by multiplying the Si content in mass% of the upper plate by 0.62, and the value obtained by multiplying the Si content in mass% of the lower plate by 0.62;
The radius of curvature of the toe of the weld metal is 0.14 × t2 or more,
A lap fillet welded joint, wherein the throat thickness of the weld metal is 0.9 x t1 or more. The composition of the weld metal optionally contains Ti instead of a portion of the Fe,
The total value of the Al content and the Ti content in the weld metal in mass% is 0.05% or more,
The Si amount, the Mn amount, the Ti amount, and the Al amount in mass% of the weld metal satisfy Formula 1,
The amount of Al in the weld metal in mass % is 0.20% or less.
2. The lap fillet weld joint of claim 1.
7×Si+7×Mn-112×Ti-30×Al≦12 <Formula 1>
In addition, when an element with a content of 0% is present, zero is substituted for the element with a content of 0% in formula 1. The composition of the weld metal is, in mass %, replacing a part of the Fe ,
Ti : 0.30% or less,
Zr: 0.30% or less,
B: 0.0040% or less,
Sn: 0.4% or less,
Cu: 0.50% or less,
Cr: 1.5% or less ,
V : 0.3% or less,
Mo: 1.0% or less, and Ni: 3.0% or less,
Contains one or more selected from the group consisting of
The Si content is 0 to 0.8%,
The Al content is 0.20% or less.
3. The lap fillet welded joint according to claim 1 or 2. An automobile part having a lap fillet welded joint according to any one of claims 1 to 3.

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