JP2021183342A - Lap fillet welded joint, and automobile component - Google Patents

Abstract

To provide a lap fillet welded joint having high fatigue strength, and an automobile component.SOLUTION: A lap fillet welded joint includes: an upper plate whose plate thickness is t1; a lower plate whose thickness is t2; and weld metal bonding the upper plate and the lower plate. Components of the weld metal contains 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 remainder Fe and impurities, and an amount of the Si of the weld metal in mass% is equal to or below a maximum value of 0.15%, value after multiplying the Si amount in mass % of the upper plate with 0.62, and value after multiplying the Si amount of the lower plate in mass % with 0.62. Curvature radius of a toe of the weld metal is 0.14×t2 or larger, and throat thickness of the weld metal is 0.9×t1 or larger.SELECTED DRAWING: Figure 2

Description

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

Undercarriage parts of automobiles are usually manufactured by laminating and welding a plurality of steel materials by gas shield arc welding or the like. In recent years, in order to improve the strength of the vehicle body, there is a strong demand for improving the fatigue strength of the welded portion. In the prior art, it has been studied to suppress the generation of cracks from the toe of the weld metal in order to improve the fatigue strength of the welded joint.

For example, in Patent Document 1, the martensitic transformation start temperature is 400 to 150 ° C., C: 0.001 to 0.2%, Mn: 3 to 10%, Si: 0.1 to 1.0% in mass%. A welding material characterized by being made of a ferroalloy contained therein is disclosed.

In Patent Document 2, in mass% with respect to the total mass of the wire, Si: less than 0.10%, C: 0.01 to 0.15%, Mn: 1.80 to 2.50%, S: 0.001 to It is characterized by containing 0.070%, P: 0.030% or less, O: 0.010% or less, and the others consisting of Fe and unavoidable impurities. Further, a solid wire for pulse MAG welding, characterized in that the total of one or two of Ti and Al is contained in an amount of 0.20% or less is disclosed.

Patent Document 3 describes the hardness Hv (FL-0) of the weld metal at a position 0.5 mm away from the base point on the weld metal side with the melt boundary FL at the weld toe as the base point in the fillet welded joint of the steel plate. .5) and the hardness Hv (FL + 0.5) of the heat-affected zone at a position 0.5 mm away from the base point on the heat-affected zone side shall be 0.3 or more and 0.9 or less. A fillet welded joint having excellent fatigue crack resistance characteristics is disclosed.

According to Patent Document 4, when fillet arc welding of a steel plate having a thickness of 1.6 to 6 mm is performed by gas shielded arc welding using a solid wire at a welding speed of more than 80 cm / min and 150 cm / min or less, the steel plate is C. A TS280-600MPa class steel plate containing 0.001 to 0.15%, Si = 0.2 to 2.0%, Mn = 0.5 to 2.5%, and composed of the balance iron and unavoidable impurities. The solid wire contains C = 0.03 to 0.15%, Si = 0.2 to 2.0%, Mn = 0.5 to 2.5%, Cu ≦ 0.5%, and the balance. It is characterized by making a solid wire composed of iron and unavoidable impurities, and further combining the steel plate and the solid wire for arc welding so that {Si (steel plate) + 0.1 × Si (wire)} ≧ 0.32. A fillet arc welding method for thin steel plates is disclosed.

Japanese Unexamined Patent Publication No. 2004-98108 Japanese Unexamined Patent Publication No. 2009-166066 Japanese Unexamined Patent Publication No. 2008-178910 Japanese Unexamined Patent Publication No. 2009-226476

All of the techniques of Patent Documents 1 to 4 are intended to suppress the generation of cracks from the toe of the weld metal. However, the places where fatigue cracks occur in the welded joint are the weld toe and the root. The present inventors considered that it is also necessary to prevent stress concentration at the root portion in order to suppress fatigue cracks. However, none of the above prior arts improve both the toe shape and the root shape. Therefore, even with these prior arts, it is difficult to meet the demand for further improvement in fatigue strength.

In view of the above circumstances, it is an object of the present invention to provide a lap fillet welded joint having high fatigue strength and an automobile part.

The gist of the present invention is as follows.
(1) The lap fillet welded joint according to one aspect of the present invention includes an upper plate having a plate thickness of t1, a lower plate having a plate thickness of t2, and a weld metal that joins the upper plate and the lower plate. A lap fillet welded joint comprising, C: 0.05 to 0.20%, Mn: 0.6 to 2.7%, P: 0.020 in mass%. % Or less, S: 0.020% or less, and Si, the balance Fe and impurities, and the Si amount in mass% of the weld metal is 0.15% and the mass% of the top plate. It is less than or equal to the largest value among the value obtained by multiplying the amount of Si by 0.62 and the value obtained by multiplying the amount of Si in mass% of the lower plate by 0.62, and the radius of curvature of the toe of the weld metal. However, it is 0.14 × t2 or more, and the throat thickness of the weld metal is 0.9 × t1 or more.
(2) In the lap fillet welded joint according to (1) above, the component of the weld metal contains one or two selected from the group consisting of Al and Ti in place of a part of Fe. The total value of Al amount and Ti amount in mass% of the weld metal is 0.05% or more, and the Si amount, Mn amount, Ti amount, and Al of the weld metal in mass%. The quantity may satisfy Equation 1.
7 × Si + 7 × Mn-112 × Ti-30 × Al ≦ 12 <Equation 1>
When an element having a content of 0% is present, zero is substituted for the element having a content of 0% in Equation 1.
(3) In the lap fillet welded joint according to (1) or (2) above, the component of the weld metal is replaced with a part of the Fe in mass%, 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 Contains one or more selected from the group consisting of 3.3% or less, V: 0.3% or less, Mo: 1.0% or less, and Ni: 3.0% or less, and the Si content. May be 0 to 0.8%.
(4) The automobile part according to another aspect of the present invention includes the lap fillet welded joint according to any one of (1) to (3) above.

According to the present invention, it is possible to provide a lap fillet welded joint having high fatigue strength and an automobile part.

It is sectional drawing which shows the normal lap fillet welded joint during welding. It is sectional drawing which shows the lap fillet welded joint which concerns on this embodiment during welding. It is sectional drawing of an example of the lap fillet welded joint which concerns on this embodiment. It is sectional drawing of an example of the lap fillet welded joint which concerns on this embodiment.

In order to improve the fatigue strength of the lap fillet welded joint, it is necessary to smooth the shape near the interface between the weld bead and the plate (upper plate and lower plate) and alleviate the stress concentration. Those thought. Then, the present inventors focused on the surface tension of the molten metal. The weld metal that joins the plates is formed by solidifying the molten plate and welding material. Before the weld metal solidifies, it is derived from the steel plate between the molten metal B derived from the welding material and the steel plate (upper plate 11 or lower plate 12), for example, as shown in FIG. 1 or FIG. It is considered that there is a molten metal A to be used.

Here, it is 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 derived from the plate and the surface tension γ2 of the molten metal B composed of a mixture of the welding material and the steel plate. rice field. Specifically, when γ1 is smaller than γ2, it is considered that the molten metal is raised and the weld metal is also raised (so-called convex bead shape) (see FIG. 1). As a result, the weld metal near the toe on the upper plate 11 side is constricted and has a concave shape, and the weld metal near the toe on the lower plate 12 side tends to have a concave undercut shape than the surface of the lower plate 12. it is conceivable that. On the other hand, when γ1 is larger than γ2, it is considered that the molten metal is attracted toward the plate to form a flat shape, and the weld metal also becomes flat (see FIG. 2). In this case, the shape near the interface between the weld bead and the plate becomes smooth, and the stress concentration is relaxed.

Furthermore, the present inventors have repeatedly investigated factors that affect the surface tension of molten metal. Then, it was found that the Si concentration of the molten metal greatly affects the surface tension of the molten metal. Si easily binds to oxygen. Therefore, it is considered that a large amount of oxygen adheres to the surface of the molten metal containing a small amount of Si _{without becoming SiO 2.} It is presumed that this oxygen reduces the surface tension of the molten metal. On the contrary, it is considered 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 present inventors reduced the Si amount of the welding material to the Si amount of the plate, and made the Si amount of the weld metal 0.62 times the Si amount of the plates (upper plate and lower plate). As a result of the following, it was possible to obtain a lap fillet welded joint in which the shape near the interface between the weld bead and the plate was smooth and the stress concentration was relaxed. It is presumed that this is because the surface tension γ1 of the molten metal derived from the plate having a high Si content is larger than the surface tension γ2 of the molten metal derived from the welding material having 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 generate the dent of the weld metal as shown in FIG. 1, but it seems that the weld metal has a raised shape without spreading on the surface of the steel sheet. Therefore, the rising angle of the weld metal on the lower plate 12 side may be large. Therefore, in addition to controlling the relationship between the Si amount of the plate and the Si amount of the weld metal, it is also possible to devise welding conditions such as the welding posture and the shield gas component to obtain the shape near the interface between the weld bead and the plate. It was also found that it may be necessary to smooth and relieve stress concentration.

The lap fillet welded joint 1 (see FIG. 3) according to one aspect of the present invention obtained by the above studies includes an upper plate 11 having a plate thickness of t1 and a lower plate 12 having a plate thickness of t2. The weld metal 13 for joining the upper plate 11 and the lower plate 12 is provided, and the components of the weld metal 13 are C: 0.05 to 0.20% and Mn: 0.6 to 2.7% in mass%. , P: 0.020% or less, and S: 0.020% or less, consisting of the balance Fe and impurities, the Si amount of the weld metal 13 in% by mass is 0.15%, and the upper plate 11 It is less than or equal to the largest value among the value obtained by multiplying the amount of Si in mass% by 0.62 and the value obtained by multiplying the amount of Si in mass% of the lower plate 12 by 0.62, and is the toe of the weld metal 13. The radius of curvature r of the portion 131 is 0.14 × t2 or more, and the throat thickness t3 of the weld metal 13 is 0.9 × t1 or more. Hereinafter, the lap fillet welded joint 1 according to the present embodiment will be described in detail.

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

In the present embodiment, the term "upper plate" is defined as a fillet welded end face of two or more parts constituting a lap joint. The number of upper plates 11 may be two or more. When the number of upper plates 11 is two or more, the plate thickness t1 of the upper plates 11 is defined as the total value of the plate thicknesses of the upper plates 11. In the present embodiment, the term "lower plate" is defined as a fillet welded surface of two or more parts constituting a lap joint.

The term "welded metal" is defined as "a part of the weld that melts and solidifies during welding" (JIS Z 3001-1: 2018). In the present embodiment, the term "component of weld metal" is used to (1) specify a region of weld metal in advance by visually observing a cross section perpendicular to the longitudinal direction at the central portion in the longitudinal direction of the welded portion, and (2). Welded metal chips are collected by cutting the area with a drill, and (3) defined as the value measured by measuring the chips with high frequency inductively coupled plasma (ICP) emission spectroscopic analysis. Will be done.

In the present embodiment, the term "throat thickness" is slightly different from the definition defined by the JIS standard, and as shown in FIG. 3, the lower surface of the upper plate 11 (the surface facing the lower plate 12). When there are two or more plates 11, the shortest distance from the intersection of the upper plate 11 closest to the lower plate 12 facing the lower plate 12) and the weld metal 13 to the outer surface of the joint of the weld metal 13. Means distance.

In this embodiment, the term "stop" is defined as the toe and its surroundings. The term "toe" is defined as "the point where the surface of the base metal meets the surface of the weld bead" (JIS Z 3001-7: 2018 "Welding Term-Part 7: Arc Welding"). The term "radius of curvature of the toe" is defined by (1) first taking a micrograph of the cross section of the toe where the metallographic structure of the weld metal is exposed, and (2) then taking a micrograph at a magnification of 40 to 50 times. Observe the photograph while enlarging it on a monitor or the like, and set the toe as the lowest point and the diameter of the circle φ1 and the toe that pass through the bead surface at a height of 5.0% of the plate thickness t2 of the lower plate 12. The diameter φ2 of a circle passing through the bead surface 7.5% higher than the plate thickness t2 of the lower plate 12 as the lowest point, and 10.0 of the plate thickness t2 of the lower plate 12 with the toe as the lowest point. The diameter φ3 of the circle passing through the bead surface of% height is measured (3), and the average value of the radius calculated from the radius of the circles having these diameters φ1 to φ3 is defined as the “radius of curvature of the toe”. (See FIG. 4).

(Upper plate and lower plate)
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 can satisfy the components of the weld metal described later. Preferred components of the upper plate 11 and the lower plate 12 are, for example, C: 0.05 to 0.35% by mass, Si: more than 0 to 1.5% by mass, Mn: 0.5 to 3% by mass, P: 0. .03 or less mass%, S: 0.02 or less mass%, Al: 0.01 to 0.5% by mass, and the balance: iron and impurities. Ti, Nb, V, Mo, Cr, Ni, and B may be contained in the upper plate 11 and / or the lower plate 12 as necessary in order to further increase the strength. 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 strong need for improving fatigue strength in steel sheets of 780 MPa or more. The components and tensile strengths of the two or more upper plates 11 and the lower plates 12 may be the same or different.

The structure and strength of the upper plate 11 and the lower plate 12 are also not particularly limited. Since the portion of the lap fillet welded joint having 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.

(Component of weld metal)
The lap fillet welded joint according to the present embodiment includes a weld metal 13 for joining the upper plate 11 and the lower plate 12. Hereinafter, the components of the weld metal 13 will be described. Unless otherwise specified, the content of the elements described below means the content of the elements in the components of the weld metal 13, and the unit "%" means mass%.

[C: 0.05 to 0.20%]
C has the effect of stabilizing the arc and atomizing the droplets. When the C content is less than 0.05%, the droplets become large, the arc becomes unstable, and the amount of spatter generated increases. As a result, the bead shape becomes uneven, which causes variation in the fatigue strength of the welded portion. Further, if the C content is less than 0.05%, the tensile strength of the weld metal cannot be secured and the desired joint strength cannot be obtained. Therefore, the C content is set to 0.05% or more. The C content is preferably 0.08% or more, more preferably 0.15% or more.

On the other hand, when the C content exceeds 0.20%, the weld metal 13 is excessively hardened, so that the crack resistance is lowered and the weld metal 13 is easily broken. Therefore, the C content is 0.20%. The C content is preferably 0.18% or less, or 0.16% or less.

[Si: (1) 0.15%, (2) the value obtained by multiplying the Si amount in mass% of the upper plate 11 by 0.62, and (3) the Si amount in mass% of the lower plate 12 is 0. The value multiplied by 62, less than or equal to the largest value]
Si is contained in a general welding material in an amount of about 0.5% for deoxidizing the welding metal 13. Therefore, the weld metal 13 obtained from a general welding material also contains the same amount of Si. However, Si is considered to have a function of increasing the surface tension of the molten metal. As shown in FIGS. 1 and 2, the larger the surface tension γ1 of the molten metal A derived from the plate with respect to the surface tension γ2 of the molten metal B derived from the welding material, the flatter the molten metal becomes and the more the weld metal becomes. The shape near the interface between the plate 13 and the plate (upper plate 11 and lower plate 12) becomes smooth. On the other hand, when the Si content of the weld metal 13 becomes excessive, the weld metal 13 becomes convex, the radius of curvature r of the toe portion 131 is reduced, the throat thickness t3 of the root portion 132 is reduced, and the weld metal 13 is constricted. Invite. The constriction of the toe portion 131 having a small radius of curvature r and the weld metal 13 tends to be a starting point for the occurrence of fatigue cracks.

Therefore, the upper limit of the Si content of the weld metal is (2) Si amount in mass% of the upper plate 11 × 0.62, or (3) Si amount in mass% of the lower plate 12 × 0.62. , The larger one.
However, when the Si content of the plate 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, if (2) the amount of Si in the mass% of the upper plate 11 x 0.62 or (3) the amount of Si in the mass% of the lower plate 12 x 0.62 is 0.15% or less, welding is performed. The upper limit of the Si content of the metal is (1) 0.15%.

After satisfying the above requirements, the upper and lower limits of the Si content may be separately determined. For example, Si may be contained in a welding material or a plate (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 set to 0%.

Further, when Si is excessively contained, non-conductive Si-based slag increases, and red rust is likely to occur between the slag and the weld metal 13. From this point of view, the Si content may be 0.8% or less after satisfying the above requirements. This means that when the "Si content of the upper plate 11 or the lower plate 12 x 0.62%" is more than 0.8%, the Si content of the weld metal 13 is 0.8% or less, and the "upper plate 11" is set. Or, when the "Si content of the lower plate 12 x 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 x 0.62". It means "%". After satisfying the above requirements, the Si content may be 0.7% or less, or 0.6% or less.

[Mn: 0.6 to 2.7%]
Mn is also a deoxidizing element like Si, and is an element that promotes deoxidation of the molten pool during arc welding and improves the tensile strength of the weld metal 13. If the Mn content is low, the tensile strength of the weld metal 13 cannot be sufficiently secured, 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 0.8% or more, 1.0% or more, or 1.2% or more.

On the other hand, when the Mn content is excessive, the viscosity of the molten metal at the time of welding becomes high. As a result, especially when the welding speed is high, the molten metal does not properly flow into the welded portion, and a humping bead (that is, a defective bead shape) is likely to occur. As a result, red rust is likely to occur. Therefore, the Mn content is 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 setting the content of these elements to 0.020% or less, various properties (for example, toughness) of the weld metal 13 can be ensured. Therefore, the P content is 0.020% or less, and the S content is 0.020% or less. The smaller the content of P and S, the more preferable, but considering the refining capacity and refining cost, the lower limit of the content of P and S may be 0.001%, 0.002%, or 0.005%, respectively. good.

The balance of the components of the weld metal 13 of the lap fillet welded joint 1 according to the present embodiment is Fe (iron) and impurities. The impurities are, for example, the upper plate 11, the lower plate 12, and the raw materials such as ore or scrap when industrially manufacturing the welding material, or the components mixed by various factors in the manufacturing process. It means that it is permissible within a range that does not adversely affect the characteristics of the lap fillet welded joint 1 according to the present embodiment.

Further, the weld metal 13 may further contain any of the following optional elements instead of a part of iron in the balance of the components of the weld metal 13. However, the lap fillet welded joint 1 according to the present embodiment can obtain good high fatigue strength without containing these optional elements. Therefore, it is not necessary to add any of the following elements to the weld metal.

[Total content of Al and Ti: preferably 0.05% or more]
The component of the weld metal 13 of the lap fillet welded joint 1 according to the present embodiment may further include one or two selected from the group consisting of Al and Ti. In this case, the total value of the amount of Al and the amount of Ti in mass% is preferably 0.05% or more.
Both Al and Ti can sufficiently secure the strength of the weld metal by suppressing the formation of coarsened ferrite. This effect can be obtained by setting the total content of Al and Ti to 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, more preferably 0.15% or more.

The upper limit value of Al + Ti is not particularly limited, and may be 0.60% calculated from the respective preferable upper limit values of Al and Ti, which will be described later. However, when 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, Ti-based slag and the weld metal is suppressed. Therefore, it is more preferable. The upper limit of Al + Ti is more preferably 0.20%.

[Si, Mn, Ti, Al]
Further, the contents of Si, Mn, Ti, and Al preferably satisfy the following formula 1. When an element having a content of 0% is present, zero is substituted for the element having a content of 0% in Equation 1.
7 × Si + 7 × Mn-112 × Ti-30 × Al ≦ 12 <Equation 1>
As a result of investigating the presence or absence of red rust between the slag and the weld metal in the weld metal having various component systems, the inventors have found that it is an index related to the occurrence of red rust, which is 7 × Si + 7 × Mn-112 × Ti-30 ×. It was clarified that when the value of Al was 12 or less, the generation of red rust was suppressed and the corrosion resistance was further enhanced. Therefore, in Equation 1, the upper limit is set to 12. It is preferable that this 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 calculated from the lower limit values of Si and Mn and the upper limit values of Al and Ti described later.

Although the embodiment in which the Al content and the Ti content are defined by the total content thereof is described above, the Al content and the Ti content may be individually determined. The Al content and the Ti content may be determined individually, and the total content thereof may be determined.

[Al: preferably 0.30% or less]
Al is a strong deoxidizing element and has an effect of promoting deoxidation of molten metal during arc welding and suppressing the generation of blow holes. 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, when the Al content is 0.30% or less, the generation of non-conductive Al-based slag can be 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]
Since Ti is a deoxidizing element, it is effective in suppressing the generation of blow holes. In order to obtain this effect, the Ti content may be 0.05% or more. The Ti content is more preferably 0.10% or more, or 0.15% or more.

On the other hand, when 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]
Since Zr has a deoxidizing effect, it is effective in suppressing the occurrence of blowholes. In order to obtain this effect, the Zr content may be 0.05% or more. The Zr content is more preferably 0.10% or more, or 0.15% or more.

On the other hand, when 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 enhance the hardenability of the welded portion and improve the tensile strength. Further, when the B content is 0.0040% or less, it is possible to suppress the decrease in elongation of the welded portion. 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, enhancing the corrosion resistance of the weld metal is also very advantageous for machine parts. In order to obtain the effect of improving the corrosion resistance by Sn, Sn may be set to 0.05% or more. On the other hand, when Sn exceeds 0.4%, the crack sensitivity of the weld metal becomes high and high temperature cracking is likely to occur. Further, when Sn exceeds 0.4%, Sn segregates at the grain boundaries of the weld metal and the toughness decreases. Therefore, Sn may be 0.4% or less.

[Cu: preferably 0.50% or less]
By copper-plating the surface of the welding material with Cu, the welding work can be made more efficient. Therefore, the Cu content may be 0.05% or more. Further, when the Cu content is 0.50% or less, the occurrence of welding cracks can be suppressed. Therefore, the Cu content may be 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 0.05% or more. Further, when the Cr content is 1.5% or less, it is possible to suppress the decrease in elongation of the welded portion. Therefore, the Cr content may be 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. Further, when the Nb content is 0.3% or less, it is possible to suppress the decrease in elongation of the welded portion. 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 0.005% or more. Further, when the V content is 0.3% or less, it is possible to suppress the decrease in elongation of the welded portion. Therefore, the V content may be 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. Further, when the Mo content is 1.0% or less, it is possible to suppress the decrease in elongation of the welded portion. 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 weld. Therefore, the Ni content may be 0.05% or more. Further, when the Ni content is 3.0% or less, the occurrence of welding 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 toe of weld metal)
In the lap fillet welded joint 1 according to the present embodiment, the radius of curvature r of the toe portion 131 of the weld metal 13 is 0.14 × t2 or more. As a result, stress concentration at the toe portion 131 can be suppressed, and the fatigue strength of the lap fillet welded joint 1 can be improved. The radius of curvature r of the toe portion 131 may 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 the present embodiment, the throat thickness t3 of the weld metal 13 is 0.9 × t1 or more. This means that the constriction 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 the present embodiment, stress concentration in the vicinity of the interface between the weld metal 13 and the upper plate 11 can be suppressed, and the fatigue strength of the lap fillet welded joint 1 can be improved. The throat thickness t3 of the weld metal 13 may be 0.95 × t1 or more, 1.0 × t1 or more, or 1.1 × t1 or more.

A method for manufacturing the weld metal 13 having the above-mentioned components and shapes will be described below.

The component of the weld metal 13 is determined according to the component of the upper plate 11, the component of the lower plate 12, the component of the welding material, and the transfer rate of the welding material. Therefore, if the components of the welding material and the migration rate are appropriately controlled according to the components of the upper plate 11 and the components of the lower plate 12, the components of the weld metal 13 can be within the above range.

The radius of curvature r and the throat thickness t3 of the toe portion 131 of the weld metal 13 can be preferably controlled by keeping the Si content of the weld metal 13 within the above range. However, when the welding metal 13 contains a large amount of an element having the property of raising the welding metal 13 (for example, Al), it is desirable to perform welding while tilting the joint in addition to controlling the Si content. For example, if the joint is tilted so that the toe end 131 is lifted and the upper plate 11 is lowered with the direction in which the weld bead is stretched (welding direction) as the rotation axis, the molten metal is tilted toward the upper plate due to the influence of gravity. A gentle shape with a large toe radius r can be obtained. In this case, the throat thickness t3 also increases.

The welding method is not particularly limited, but for example, gas shielded arc welding is preferable. Suitable gas species include an inert gas containing 5-50% by volume CO ₂ and the balance being predominantly Ar. In the shield gas, _{it is more preferable that CO 2} is 20% by volume or less.

The welding posture in gas shielded arc welding is also not particularly limited. For example, the welding posture may be a downward posture (a posture in which the lower plate is horizontal). On the other hand, the welding posture may be changed so that the lower plate is tilted from the horizontal by more than 0 ° and 60 ° or less. Here, the inclination of the lower plate is an angle formed by the horizontal plane and the upper plate (or the lower plate) when the lower plate is tilted with the direction in which the weld bead is stretched (welding direction) as the rotation axis. .. The radius of curvature of the toe of the weld metal and the throat thickness of the weld metal can be more preferably controlled through the inclination of the lower plate.

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

Steel sheets 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 rest of the components of the steel sheet and the wire were iron and impurities. The steel plates of the upper plate and the lower plate are of the same type. The content of elements not intentionally added to the components of the steel plate and wire is indicated by "-". The shield gas type and welding posture at the time of welding are as shown in Table 3. In the example of the welding posture "downward", the lower plate and the upper plate were placed horizontally and the plates were welded. In the example of the welding posture "45 °", the plate is welded by tilting the joint by 45 ° so as to lift the upper plate 11 and lower the toe 131 with the direction in which the welding bead extends (welding direction) as the rotation axis. .. In the example of the welding posture "-45 °", the plate is welded by tilting the joint by 45 ° so as to lift the toe end 131 and lower the upper plate 11 with the direction in which the welding bead extends (welding direction) as the rotation axis. bottom.

The various lap fillet welded joints thus obtained were evaluated by the following methods and are shown in Tables 4 and 5.

(Component of weld metal)
In the central part of the weld in the longitudinal direction, the area of the weld metal is identified in advance by visually observing the cross section perpendicular to the longitudinal direction, and the area is cut with a drill to collect the chips of the weld metal and cut the area. The powder was measured by luminescence spectroscopic analysis using high frequency inductively coupled plasma (ICP) as a sample. The measurement results are shown in Table 4. In addition, the red rust generation index (7 × Si + 7 × Mn-112 × Ti-30 × Al) calculated from these chemical components is shown in Table 5. In calculating the red rust generation index, the content of elements not intentionally added to the components of the steel sheet and wire was regarded as "0". In Table 4, values outside the scope of the invention are underlined.

(Tip radius and throat thickness)
The lap fillet welded joint was cut perpendicular to the extending direction of the weld metal (welded bead). The cut surface was then polished and etched to reveal the weld metal on the cut surface. By measuring the shape of this weld metal, the toe radius r and the throat thickness t3 were measured. The definitions of the toe radius r and the throat thickness t3 are as described above. The measurement results are shown in Table 5. In Table 5, values outside the scope of the invention are underlined.

(Fatigue strength)
A bending fatigue test was conducted on the lap fillet welded joint using a Schenck fatigue tester. Laminated fillet welded joints with a torque reduction of less than 40% relative to the torque at the start of the test at 2 million iterations were considered to have no cracks. The stress amplitude at the time of 2 million repetitions is shown in Table 5 as fatigue strength.

Comparative Examples 15 to 20 are fatigue test results when a standard ordinary welding wire was applied to each steel sheet, and their fatigue strength was about 150 MPa on average. Therefore, the lap fillet welded joint showing a fatigue strength exceeding 180 MPa, which is 1.2 times this value, was determined to have excellent fatigue strength.

(Paintability)
After degreasing and chemical conversion treatment of the lap fillet welded joint, electrodeposition coating was applied so that the film thickness was 20 μm. Then, the electrodeposition-painted part of the weld bead was photographed. The photograph was image-analyzed to measure the ratio of the area of poor electrodeposition coating to the area of the weld bead. Here, the bead length of the welding test piece was set to 120 mm, and the defect rate of electrodeposition coating was obtained from the weld bead having a length of 90 mm excluding the welding start portion (length 15 mm) and the end portion (length 15 mm). .. Gray paint was used for electrodeposition coating. This makes it possible to easily identify defective electrodeposition coating where reddish brown or black slag is exposed. When the area ratio of defective coating was 5% or less, it was judged that the electrodeposition coating ratio was good.

In Comparative Examples 15 and 17, the fatigue strength was insufficient. This exceeds the largest value among the values obtained by multiplying the amount of Si in the weld metal by 0.15% or the amount of Si in mass% of the steel sheet (“steel sheet Si” shown in Table 3) by 0.62. However, it is presumed that this is because the radius of curvature of the toe (the "toe radius" shown in Table 5) is insufficient.
In Comparative Example 16, the fatigue strength was insufficient. It is presumed that this is because the amount of Si in the weld metal exceeds 0.15%, or the largest value among the values obtained by multiplying the steel plate Si by 0.62, and the throat thickness is insufficient.
In Comparative Examples 18, 20, 21, 22, and 24, the fatigue strength was insufficient. It is presumed that this is because the toe radius is insufficient.
In Comparative Example 19, the fatigue strength was insufficient. It is estimated that this is because the amount of Si in the weld metal exceeds 0.15%, or the largest value among the values obtained by multiplying the steel plate Si by 0.62, and the toe radius and throat thickness are insufficient. Will be done.
In Comparative Example 23, the fatigue strength was insufficient. It is presumed that this is because the throat thickness of the weld metal was insufficient.
In Comparative Example 25, the fatigue strength was insufficient. It is presumed that this is because the amount of C in the weld metal is insufficient.
In Comparative Example 26, a crack was generated in the root portion due to hydrogen embrittlement at the completion of welding. Therefore, the fatigue strength of Comparative Example 26 has not been measured. It is presumed that this is because the amount of C in the weld metal was excessive.
In Comparative Example 27, the fatigue strength was insufficient. It is presumed that this is because the amount of Mn in the weld metal is insufficient.
In Comparative Example 28, the fatigue strength was insufficient. It is presumed that this is because the amount of Mn in the weld metal was excessive.

On the other hand, the lap fillet welded joint according to Invention Examples 1 to 14 in which the chemical composition and shape of the weld metal are within a predetermined range has high fatigue strength.

According to the present invention, it is possible to provide a lap fillet welded joint having high fatigue strength and an automobile part. Therefore, the present invention has high industrial applicability.

1 Laminated fillet welded joint 11 Upper plate 12 Lower plate 13 Welded metal 131 Toe end 132 Root part A Molten metal derived from plate B Molten metal derived from welding material γ1 Surface tension of molten metal derived from plate γ2 Welding material Surface tension of molten metal derived from t1 Plate thickness of upper plate t2 Plate thickness of lower plate t3 Throat thickness r Radius of curvature of 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 for joining the upper plate and the lower plate.
The component of the weld metal is mass%,
C: 0.05 to 0.20%,
Mn: 0.6-2.7%,
P: 0.020% or less,
S: 0.020% or less,
And Si
Containing, the balance consists of Fe and impurities,
The Si amount in mass% of the weld metal is 0.15%, the value obtained by multiplying the Si amount in mass% of the upper plate by 0.62, and the Si amount in mass% of the lower plate. It is less than or equal to the largest value multiplied 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 having a throat thickness of 0.9 × t1 or more of the weld metal. The component of the weld metal contains one or two selected from the group consisting of Al and Ti in place of a part of the Fe.
The total value of Al amount and Ti amount in mass% of the weld metal is 0.05% or more.
The lap fillet welded joint according to claim 1, wherein the Si amount, the Mn amount, the Ti amount, and the Al amount of the weld metal satisfy the formula 1.
7 × Si + 7 × Mn-112 × Ti-30 × Al ≦ 12 <Equation 1>
When an element having a content of 0% is present, zero is substituted for the element having a content of 0% in Equation 1. The component of the weld metal is, in mass%, instead of a part of the Fe.
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, Ni: 3.0% or less,
Includes one or more selected from the group consisting of
The lap fillet welded joint according to claim 1 or 2, wherein the Si content is 0 to 0.8%. An automobile part comprising the lap fillet welded joint according to any one of claims 1 to 3.

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