JP7376779B2 - Welded joints and auto parts - Google Patents

Description

The present invention relates to welded joints and automobile parts.

For example, in the field of automobiles, in order to preserve the environment, there is a need to improve fuel efficiency by reducing the weight of vehicle bodies, as well as to improve collision safety. Various technological developments have been carried out to reduce the weight of car bodies and improve collision safety, such as using thinner high-strength steel plates as structural members of car bodies and optimizing car body structures. ing. Note that automobile parts also include welded joints having multiple high-strength steel plates as base materials.
In the automobile field, as a method for manufacturing welded joints, an arc welding method in which arc welding is performed using two steel plates joined together as steel base materials is widely adopted.

Automotive parts are used in environments with vibration or repeated external force loads. Therefore, in addition to normal static tensile strength, automobile parts are required to have sufficient fatigue strength to withstand repeated forces. The fatigue strength (fatigue limit) of the steel base material increases in proportion to its tensile strength, but it is generally known that the fatigue strength of arc welded joints is lower than the fatigue strength of the steel base material. There is.

Therefore, techniques for improving the fatigue strength of arc welded joints have been studied.

For example, Patent Document 1 describes a steel fillet welded joint formed by arc welding, in which the martensitic transformation start temperature (Ms point) of the weld metal is 400°C or more and 550°C or less, and the weld toe is The value (ρ/t) obtained by dividing the end radius ρ by the plate thickness t of the base material is 0.25 or more, and the formula: Ms (°C) ≦ 375 × [ρ/t] + 320 is satisfied, and there are no cracking defects. Fillet weld joint. ' has been disclosed.

Furthermore, Patent Document 2 states that "C: 0.01 to 0.15%, Si: 0.3 to 2.0%, Mn: 1.0 to 3.0%, P: 0.05% or less, S: 0.005% or less, Al: 0.005 to 0.05%, Ti: 0.005 to 0.08%, N: 0.003 to 0.025%, and the Al, Ti and Fatigue steel consisting of a steel material with a composition in which N is contained in a predetermined relationship and the remainder is Fe and unavoidable impurities, and a weld metal with a predetermined chemical composition obtained by gas-shielded arc fillet welding of the steel material. A fillet welded joint with excellent properties.'' is disclosed.

Japanese Patent Application Publication No. 2012-11429 JP2002-224834A

Here, the fatigue strength of an arc welded joint has dependence on the shape of the toe of the weld metal.
When the toe of the weld metal has a steep shape (when the toe angle β of the weld metal satisfies β≧35°), the stress concentration at the toe becomes extremely high, and the arc welded joint is repeatedly pulled. If the load continues to be applied, fatigue cracks will occur at an early stage at the boundary (melt boundary) between the toe of the weld metal and the steel base metal.
On the other hand, when the toe of the weld metal has a "sloping" shape (when the toe angle β of the weld metal satisfies 0<β≦35°), the stress concentration at the toe becomes gentle and Fatigue cracks occur on the surface of the weld metal at the edges.

The technique disclosed in Patent Document 1 involves making the toe of the weld metal a "sloping" shape, making the stress concentration at the toe gentle, and raising the Ms point to cause transformation at a low temperature. This technology improves fatigue strength by alleviating welding residual stress.
The technology of Patent Document 2 improves the fatigue strength of the weld metal by making the toe of the weld metal a "sloping" shape, reducing stress concentration at the toe, and leaving a large amount of solid solution N. It is a technology that allows

However, the technologies disclosed in Patent Documents 1 and 2 are technologies for improving fatigue strength when welding steel base materials with a tensile strength of about 780 MPa, and improve fatigue strength when welding steel base materials with a tensile strength of 980 MPa or more. has not been considered.

Therefore, an object of the present invention is to provide a welded joint in which a pair of steel base metals, at least one of which has a tensile strength of 980 MPa or higher, is welded and has excellent fatigue strength, and an automobile part equipped with the welded joint.

Means for solving the problem includes the following aspects.

<1>
a pair of steel base materials, at least one of which has a tensile strength of 980 MPa or more;
The weld metal extends along the boundary formed by the surface of one steel base material and the surface of the other steel base material among the pair of steel base materials, and the toe angle β of the weld metal is A weld metal that satisfies 0°<β≦35° and satisfies (surface hardness of weld metal)/(internal hardness of weld metal)≧0.85,
Welded joints with.
<2>
The welded joint according to <1>, wherein the steel base material has a plate thickness of 0.6 to 4.0 mm.
<3>
The weld metal has C: 0.01 to 0.30% in mass%
Si: 0.02-2.00%
Mn: 1.5-4.0%
Ti: 0.01-2.00%
P: 0.100% or less S: 0.0500% or less Cr: 0 to 2.0%, and Mo: 0 to 1.0%
The welded joint according to <1> or <2>, which is made of steel containing.
<4>
The welded joint according to any one of <1> to <3>, which satisfies the following formula (1).
Mn+3.33Cr+2.61Mo≧1.70...(1)
In formula (1), the element symbol indicates the content (mass%) of the corresponding element.
<5>
An automobile part comprising the welded joint according to any one of <1> to <4>.

According to the present invention, it is possible to provide a welded joint in which a pair of steel base metals, at least one of which has a tensile strength of 980 MPa or more, is welded and has excellent fatigue strength, and an automobile part including the same.

1 is a sectional view showing an example of a welded joint of the present invention. FIG. 1 is a plan view showing an example of a welded joint of the present invention. In the welded joint of the present invention, it is a schematic diagram explaining the measuring method of the surface layer hardness of weld metal, and the internal hardness of weld metal. In the welded joint of the present invention, it is a cross-sectional view (optical micrograph) of the weld metal after measuring the surface hardness of the weld metal and the internal hardness of the weld metal.

The present invention will be explained below.
In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower limit and upper limit.
In numerical ranges described in stages, the upper limit or lower limit described in a certain numerical range may be replaced with the upper limit or lower limit of another numerical range described in stages.
In a numerical range, the upper limit value or lower limit value described in a certain numerical range may be replaced with the value shown in the Examples.
The term "step" is included in the term not only an independent step but also a step that cannot be clearly distinguished from other steps, as long as the intended purpose of the step is achieved.
A "combination of preferred embodiments" is a more preferred embodiment.

The welded joint of the present invention is
a pair of steel base materials, at least one of which has a tensile strength of 980 MPa or more;
A weld metal that extends along the boundary formed by the surface of one steel base material and the surface of the other steel base material among a pair of steel base materials, and the toe angle β of the weld metal is 0. A weld metal that satisfies °<β≦35° and (surface hardness of weld metal)/(internal hardness of weld metal)≧0.85,
has.

Due to the above configuration, the welded joint of the present invention has excellent fatigue strength even when a pair of steel base materials, at least one of which has a tensile strength of 980 MPa or more, are welded together.
The welded joint of the present invention was discovered based on the following findings.

If the stress concentration at the toe of the weld metal is extremely high, the influence of the weld metal structure on fatigue strength will be small and the fatigue strength will be low. Therefore, the stress concentration at the toe needs to be gentle enough to cause fatigue cracks to occur in the weld metal. In other words, the toe angle β of the weld metal needs to satisfy 0°<β≦35°.

On the other hand, a steel base material with a tensile strength of 980 MPa or more contains various alloying elements, and the weld metal formed by melting and solidifying the steel base material and welding wire is the weld metal that is produced when the mild steel base material is welded. Easily has high strength compared to metal. However, when welding a steel base material with a tensile strength of 980 MPa or more, fatigue strength commensurate with the high-strength weld metal may not be obtained.

Therefore, the inventors investigated the cause and found the following findings.
When welding a steel base material of 980 MPa or more, the weld metal has a hardness near the surface of the weld metal that is softer than the inside of the weld metal. Furthermore, this is because decarburization occurs from the surface of the weld metal and carbon is lost from the surface of the weld metal.
Therefore, in order to prevent softening near the surface of the weld metal and suppress the occurrence of cracks from the surface of the weld metal, it is effective to (1) improve the hardenability of the surface layer of the weld metal. (2) It is effective to improve the shielding properties of the shielding gas, that is, to bring the "surface hardness of the weld metal" closer to the "internal hardness of the weld metal."

From the above findings, it has been found that the welded joint of the present invention has a pair of steel base materials welded together, at least one of which has a tensile strength of 980 MPa or more, and has excellent fatigue strength.

Hereinafter, the welded joint of the present invention will be described in detail with reference to the drawings.

As shown in FIGS. 1 and 2, the welded joint 10 of the present invention includes, for example, a pair of steel base materials superimposed on each other (in FIG. 1, 1 is a lower first steel base material, 2 is an upper first steel base material, a weld metal (weld bead) 3 extending along a boundary 4 formed by the surface 1a of the first steel base material 1 and the end surface 2a of the second steel base material 2; Equipped with

Here, in FIGS. 1 and 2, the weld metal 3 is connected to the surface 1a of the first steel base material 1 (the surface facing the sheet thickness direction) and the end surface 2a of the second steel base material 2 (in the direction orthogonal to the sheet thickness direction). Although the embodiment is shown in which it extends along the boundary 4 formed by the surface (facing the surface), the embodiment is not limited to this embodiment.
For example, the weld metal 3 is made by arranging the first steel base material 1 and the second steel base material 2 in an L-shape or a T-shape, and the mutual surfaces of the first steel base material 1 and the second steel base material 2 ( It may also be a mode in which it extends along a boundary 4 formed by two surfaces (surfaces facing each other in the plate thickness direction). Alternatively, the first steel base material 1 and the second steel base material 2 are arranged on the same surface with their ends facing each other to form a butt joint, and the weld metal 3 extends along the boundary 4. It's okay.

Note that FIG. 1 is a diagram of the welded joint 10 seen in a cross section perpendicular to the weld line W of the weld metal 3 (see FIG. 2). Further, as shown in FIGS. 1 and 2, the direction parallel to the weld line W is the Z-axis direction, and the direction perpendicular to the Z-axis direction and parallel to the surface 1a of the first steel base material 1 is the X-axis direction. , the direction perpendicular to the X-axis direction and the Z-axis direction and parallel to the thickness direction of the first steel base material 1 is defined as the Y-axis direction.

<Welded metal>
[Toe shape of weld metal 3]
As shown in FIG. 1, if the position of the molten boundary existing on the surface 1a of the first steel base material 1 is set to point A, the weld metal 3 rises from point A with a toe angle β, and further rises from point A to a second It stands up from a position closer to the steel base material 2 with a flank angle θ. The flank angle θ is generally used as a parameter representing the toe shape of the weld metal 3, but in the present invention, the toe angle β is used as a parameter representing the toe shape of the weld metal 3. The toe angle β is defined as follows.

As shown in FIG. 1, point C is a position 0.3 mm away from point A toward weld metal 3 in the X-axis direction. Moreover, the intersection of the straight line passing through point C and extending in the plate thickness direction (that is, the Y-axis direction) of first steel base material 1 and the surface of weld metal 3 is defined as point B. In this way, when points B and C are defined, the angle between the straight line connecting points A and B and the straight line connecting points A and C is defined as the toe angle β of weld metal 3. .

When the toe angle β is defined, the weld metal 3 of the welded joint 10 satisfies the following conditional expression (1). By satisfying conditional expression (1), the shape of the toe of the weld metal 3 becomes a gentle shape, so that concentration of stress on the toe of the weld metal 3 can be suppressed. When the toe angle β is 35° or more, the toe of the weld metal 3 has a steep shape, and stress tends to concentrate on the toe of the weld metal 3.
0°<β≦35°…(1)

In FIG. 1, for convenience of explanation, the surface 1a of the first steel base material 1 and the end surface 2a of the second steel base material 2 included in the weld metal 3 are shown by dotted lines to indicate the position of the boundary 4. ing. However, in reality, the dotted line portion is melted into the weld metal 3, so even if a cross-sectional photograph of the weld metal 3 is obtained using an optical microscope, the dotted line portion cannot be observed. Therefore, by specifying the three points A, B, and C defined as above on the cross-sectional photograph of the weld metal 3, the toe angle β of the weld metal 3 can be determined from the cross-sectional photograph of the weld metal 3. can be obtained easily. Note that, as long as a photograph sufficient to identify the toe angle β of the weld metal 3 can be obtained, not only an optical microscope but also a scanning electron microscope (SEM), a microscope, or the like may be used. The toe angle β of the weld metal 3 is measured at one location near 1/2 the length of the terminal end and the starting end in the direction of the weld line W (z-axis).

[Hardness of weld metal 3]
Weld metal 3 satisfies the formula: (surface hardness of weld metal)/(internal hardness of weld metal)≧0.85. The fatigue strength of the weld metal 3 can be improved by bringing the surface hardness of the weld metal 3 close to the relatively hard internal hardness without being affected by decarburization.
The lower limit of (surface hardness of weld metal)/(internal hardness of weld metal) is preferably 0.90 or more, more preferably 0.95 or more, from the viewpoint of improving the fatigue strength of weld metal 3.

Here, from the viewpoint of improving the fatigue strength of the weld metal 3, the internal hardness of the weld metal 3 is preferably 120 to 450 HV, more preferably 150 to 380 HV.

The surface hardness of the weld metal 3 is measured as follows (see FIGS. 3 and 4).
In the cross section of the weld metal 3 (a cross section cut along the direction perpendicular to the weld line), the melting boundary (AA in FIG. 3) at a depth of 20 μm from the surface of the weld metal 3 is the origin, and from the surface of the weld metal 3 Measurement is performed at a depth of 20 μm at intervals of 36 μm along the outer shape of the weld metal 3 with an indentation load of 25 gf. If the indentation is too large and protrudes from the surface, or if it is adjacent to an adjacent indentation, the load may be further reduced. Furthermore, if the indentation is too small and hardness measurement is difficult, the load may be increased. If the measurement load is small, the variation may be large, so if the difference in hardness between both adjacent points on both sides is 100 HV or more, that point is excluded from the measurement results.
Then, an The surface hardness of the weld metal 3 is calculated as the average value of the measured values at 5 or more points between the X coordinate of -0.03 mm and -0.25 mm.

The internal hardness of the weld metal 3 is measured as follows (see FIGS. 3 and 4).
In the cross section of the weld metal 3 (cross section cut along the direction perpendicular to the weld line), the melting boundary (BB in FIG. 3) at a depth of 100 μm from the surface of the weld metal 3 is the origin, and the first steel base material 1 Measurements are made at 0.2 mm intervals along the direction parallel to the surface of the indenter with an indentation load of 200 gf. If the hardness difference between adjacent points on both sides is 100 HV or more, that point is excluded from the measurement results.
Then, an +, weld metal side as -), calculate the average value of the measured values at three or more points between the X coordinate -0.1 mm and -1.0 mm as the internal hardness of weld metal 3. . Note that the internal hardness is measured at one location near 1/2 of the length of the terminal end and the starting end in the direction of the weld line W (z-axis).

Note that the hardness of the weld metal 3 is "Vickers hardness". The "Vickers hardness" is measured in accordance with JIS Z 2244 (2009). The conditions other than the load were that the indenter was a Vickers square pyramidal diamond indenter with a facing angle of 136°, and the indentation time was 20 seconds.

[Chemical composition of weld metal 3]
The chemical composition of the weld metal 3 is not particularly limited, but C: 0.01 to 0.30% by mass.
Si: 0.02-2.00%
Mn: 1.5-4.0%
Ti: 0.01-2.00%
P: 0.100% or less S: 0.0500% or less Cr: 0 to 2.0%
Mo: 0-1.0%
Preferably, the steel is made of steel containing.
Specifically, for example, the weld metal 3 contains C: 0.01 to 0.30%, Si: 0.02 to 2.0%, Mn: 1.5 to 4.0%, and Ti: 0.01. ~2.0%, P: 0.1% or less, S: 0.05% or less, Cr: 0 to 2.0%, Mo: 0 to 1.0%, and any elements other than Cr and Mo described below. , and the remainder is preferably Fe and impurities.

Note that the chemical composition of the weld metal 3 is the chemical composition inside the weld metal 3 at a distance of 500 μm or more from the surface of the weld metal 3 and the fusion boundary, respectively.

-C:0.01~0.30%-
C is important for ensuring the strength of the weld metal 3, and if it is too small, the strength of the weld metal 3 may not be guaranteed. Therefore, the amount of C is preferably 0.01% or more. On the other hand, the strength of the surface layer of the weld metal 3 tends to decrease due to decarburization. Therefore, the amount of C is preferably 0.30% or less. The preferable amount of C is 0.02 to 0.2%.

-Si:0.02~2.00%-
Si has the role of a deoxidizing element. Therefore, the amount of Si is preferably 0.02% or more. On the other hand, when the amount of Si is large, scale and slag tend to be excessive. Therefore, the amount of Si is preferably 2.00% or less. The preferred amount of Si is 0.02 to 1.5%.

-Mn:1.5~4.0%-
Mn is important for increasing the hardenability of the weld metal 3. Therefore, the Mn content is preferably 1.5% or more. If the amount of Mn is large, the weld metal 3 becomes too hard, and there is a concern that hydrogen embrittlement cracking of the weld metal 3 may occur. Therefore, the Mn amount is preferably 4.0% or less. The preferable amount of Mn is 1.5 to 3.0%.

-Ti:0.01~2.00%-
Ti has the function of refining the structure of the weld metal 3 and preventing the surface hardness of the weld metal 3 from decreasing. Therefore, the Ti amount is preferably 0.01% or more. An excessive amount of Ti may form excessive precipitates and cause a decrease in the toughness of the weld metal 3. Therefore, the Ti amount is preferably 2.0% or less. A preferable amount of Ti is 0.05 to 0.15%.

-P: 0.100% or less-
P has the function of strengthening the weld metal 3, but causes a significant deterioration in the toughness of the weld metal 3. Therefore, the amount of P is preferably 0.100% or less. On the other hand, although the amount of P may be 0%, it is economically disadvantageous to completely remove P mixed in from raw materials of the steel base material. Therefore, the amount of P is preferably 0.0001% or more.

-S: 0.0500% or less -
S, like P, is an element that deteriorates the toughness of the weld metal 3, and if contained in a large amount, causes solidification cracking of the weld metal 3. Therefore, the amount of S is preferably 0.0500% or less, but on the other hand, it is economically disadvantageous to completely remove S mixed in from raw materials of the steel base material. Therefore, the amount of S is preferably 0.0001% or more.

-Cr: 0 to 2.0%-
Cr is an optional element, but like Mn, it is an element that increases the hardenability of the weld metal 3. Moreover, an excessive amount of Cr is uneconomical. Therefore, the Cr content is preferably 2.0% or less.

-Mo: 0 to 1.0%-
Mo is an optional element, but like Mn, it is an element that increases the hardenability of the weld metal 3. Moreover, an excessive amount of Mo is uneconomical. Therefore, the amount of Mo is preferably 1.0% or less.

-Formula (1)-
The chemical composition of the weld metal 3 preferably satisfies formula (1) from the viewpoint of improving the hardenability of the surface layer of the weld metal 3 and bringing the surface hardness of the weld metal 3 close to the internal hardness of the weld metal 3, More preferably, formula (1-2), formula (1-3) or formula (1-4) is satisfied.
However, the upper limit of "Mn+3.33Cr+2.61Mo" is preferably 13.26 or less, preferably 10 or less, and more preferably 5 or less.
Mn+3.33Cr+2.61Mo≧1.70...(1)
Mn+3.33Cr+2.61Mo≧1.70...(1-2)
Mn+3.33Cr+2.61Mo≧1.75...(1-3)
Mn+3.33Cr+2.61Mo≧1.80...(1-4)
In formula (1) and formula (1-2), the element symbol indicates the content (mass%) of the corresponding element. Note that in formula (1) and formula (1-2), "Mn+3.33Cr+2.61Mo" is calculated assuming that the elements not contained are 0%.

The chemical composition of the weld metal 3 is, as arbitrary elements, in mass%, Al: 0 to 1.0%, N: 0 to 0.1000%, B: 0 to 0.01%, Nb: 0 to 0.5 %, Ni: 0 to 5.0%, and Cu: 0 to 5.0%.

-Al: 0 to 1.0%-
Al acts as a deoxidizing element, and most of it is discharged outside the weld metal 3 as slag. If there is too much Al, the toughness of the weld metal 3 tends to deteriorate. Therefore, the amount of Al is preferably 1.0% or less.

-N: 0~0.1000%-
When N is contained in a large amount, it causes blowholes and the like. Therefore, the upper limit of the amount of N is 0.1% or less.
On the other hand, like C, N strengthens the strength of the weld metal 3. Therefore, the amount of N is preferably 0.0001% or more.

-B: 0 to 0.01%-
When B is contained in a large amount, it causes deterioration of the toughness of the weld metal 3. Therefore, the upper limit of the amount of B is preferably 0.01% or less.

-Nb: 0 to 0.5%-
If Nb is contained in a large amount, it may cause deterioration in the toughness of the weld metal 3. Therefore, the amount of Nb is preferably 0.5% or less.

-Ni: 0 to 5.0%-
If Ni is contained in a large amount, it may cause deterioration in the toughness of the weld metal 3. Therefore, the Ni amount is preferably 5.0% or less.

-Cu: 0 to 5.0%-
When Cu is contained in a large amount, the toughness of the weld metal 3 deteriorates. Therefore, the amount of Cu is preferably 5.0% or less.

The chemical composition of the weld metal 3 includes, as arbitrary elements other than the above-mentioned arbitrary elements, in mass % within a range that does not affect the fatigue hardness of the weld metal 3.
O: 0 to 0.04%,
Ca: 0-0.001%,
It may contain one or more types.

<Steel base material>
At least one of the pair of steel base materials has a tensile strength of 980 MPa or more. A steel base material having high tensile strength is particularly suitable as a steel base material for the lap weld joint 10 for automobiles, which is strongly required to reduce weight and improve collision safety. From this point of view, it is preferable that both of the pair of steel base materials have a tensile strength of 980 MPa or more.
Note that the tensile strength is measured according to JIS Z2241 (2011).

The chemical composition of the pair of steel base materials is preferably a chemical composition that provides mechanical properties with a tensile strength of 980 MPa or more.
As an example of the chemical composition of a pair of steel base materials, C: 0.002 to 0.4% in mass%
Si: 0.002-2.0%
Mn: 1.5-5.0%
P: 0.1% or less S: 0.05% or less N: 0 to 0.01%,
Cr: 0-2.0%,
Mo: 0-1.0%,
Ni: 0 to 5.0%,
Ti: 0 to 0.3%,
Al: 0-0.5%,
Balance: A chemical composition consisting of Fe and impurities.

The plate thickness of the pair of steel base materials is preferably 0.6 to 4.0 mm, more preferably 0.8 to 3.2 mm.
This is because, in a thick steel base material, welding residual stress becomes dominant on the fatigue strength, and the influence of the structure of the weld metal 3 becomes small. In other words, the influence of the fatigue strength of the weld metal 3 on the fatigue strength of the welded joint increases. By using thin plates with a thickness of 0.6 to 4.0 mm as the pair of steel base materials, weld metal 3 that satisfies (surface hardness of weld metal) / (internal hardness of weld metal) ≧ 0.85 Contributes to improving the fatigue strength of welded joints with effective microstructure.

<Manufacturing method of welded joint>
An example of the method for manufacturing a welded joint of the present invention is a method for manufacturing a welded joint by arc welding that satisfies the following conditions.
(1) Welding speed: 50 cm/min or more, less than 110 cm/min (2) Shielding gas composition: CO ₂ concentration: 20% or less, balance: Ar
(3) Welding current: 150-250A

First, in the method for manufacturing a welded joint of the present invention, the welding wire is not particularly limited as long as the weld metal 2 satisfies the above structure.
An example of the chemical composition of the welding wire is C: 0.002 to 0.4% by mass.
Si: 0.002-2.0%
Mn: 1.5-5.0%
P: 0.1% or less S: 0.05% or less N: 0 to 0.01%,
Cr: 0-2.0%,
Mo: 0-1.0%,
Ni: 0 to 5.0%,
Ti: 0 to 0.3%,
Al: 0-0.5%,
Cu: 0-3.0%
Nb: 0-1.0%
B: 0-0.02%
Balance: A chemical composition consisting of Fe and impurities.

The welding speed is preferably less than 110 cm/min, more preferably 100 cm/min or less. This is because if the welding speed is too fast, the weld metal 3 will come out of the shielding range of the shielding gas, promoting decarburization of the surface layer of the weld metal 3, reducing the surface hardness and increasing the difference with the internal hardness. It is. As a result, the fatigue strength of the weld metal 3 decreases. Further, from the viewpoint of welding work efficiency, the welding speed is preferably 50 cm/min or more.

The composition (volume %) of the shielding gas is preferably such that the CO ₂ concentration is 20% or less and the balance is Ar. This is because when the CO ₂ concentration is high, decarburization of the surface layer of the weld metal 3 is promoted, the surface layer hardness decreases, and the difference from the internal hardness increases. As a result, the fatigue strength of the weld metal 3 may decrease.

The welding current is preferably 150 to 250A. While the welding speed is within the above range, the toe shape of the weld metal 3 can be made "sloping". That is, the toe angle β of the weld metal 3 can be set to 0°<β≦35°.

<Automotive parts>
The automobile part of the present invention includes the welded joint of the present invention.
For example, the automobile part of the present invention includes a welded joint shown in FIGS. 1-2.
Specifically, the automobile parts of the present invention are exemplified by car body frame parts, panel parts, and suspension parts, and specifically, suspension arms, suspension frames, chassis frames, etc. that require high strength are preferably used. Can be mentioned.

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples.

(Example)
Lap fillet welding was performed using steel plates having the chemical composition and tensile strength shown in Table 1 as a pair of steel base materials.
The welding conditions were a welding speed of 80 cm/min, a welding current of 235 [A], and a shielding gas composition of Ar+20% _CO2 .
The welding wire used was a wire equivalent to YGW16 of JIS Z 3312 (2009) (see Table 2).

A plane bending fatigue test piece was prepared from the welded specimen, and a plane bending fatigue test was conducted under double-sided loading with a stress ratio R=-1.
The fatigue limit was set as the stress amplitude at which no fracture occurred after 10 million cycles.
The bending stress was based on the maximum bending stress on the surface of the smallest cross section of the test piece (width 20 mm, plate thickness 2.9 mm).

The results obtained are shown in Table 3. The chemical composition of the weld metal was determined by taking an analysis sample from the weld metal of the same test piece as the one from which the fatigue test piece was taken and conducting chemical analysis.

From the above results, it can be seen that Test Examples 1 to 9 and 11 have higher fatigue strength than Test Example 10, with a fatigue limit of 200 MPa or more.
In particular, it can be seen that in Test Examples 1, 3 to 4, 6 to 9, and 11, high fatigue strength with a fatigue limit of 210 MPa or more can be obtained.

1, 2 Steel base material 3 Weld metal

Claims (5)

a pair of steel base materials, at least one of which has a tensile strength of 980 MPa or more;
The weld metal extends along the boundary formed by the surface of one steel base material and the surface of the other steel base material among the pair of steel base materials, and the toe angle β of the weld metal is 0°<β≦35°, and (surface hardness of weld metal defined below )/(internal hardness of weld metal defined below )≧0.85 , and remelted. Welded metal that has not been
Welded joints with.
-Surface hardness of weld metal-
In the cross section along the orthogonal direction of the weld line, the X axis passes through the fusion boundary at a depth of 20 μm from the surface of the weld metal and is parallel to the surface of one steel base metal (with the steel base metal side being +, The average value of the Vickers hardness measured at 5 or more points between the X coordinate of -0.03 mm and -0.25 mm, when the weld metal side is -0.03 mm and -0.25 mm.
-Internal hardness of weld metal-
In the cross section along the orthogonal direction of the weld line, the X-axis passes through the fusion boundary at a depth of 100 μm from the surface of one steel base material and is parallel to the surface of one steel base material (however, the The average value of the Vickers hardness measured at three or more points between the X coordinate of -0.1 mm and -1.0 mm. The welded joint according to claim 1, wherein the steel base material has a plate thickness of 0.6 to 4.0 mm. The weld metal has C: 0.01 to 0.30% in mass%
Si: 0.02-2.00%
Mn: 1.5-4.0%
Ti: 0.01-2.00%
P: 0.100% or less S: 0.0500% or less Cr: 0 to 2.0%, and Mo: 0 to 1.0%
The welded joint according to claim 1 or 2, which is made of steel containing. The welded joint according to any one of claims 1 to 3, which satisfies the following formula (1).
Mn+3.33Cr+2.61Mo≧1.70...(1)
In formula (1), the element symbol indicates the content (mass%) of the corresponding element. An automobile part comprising the welded joint according to any one of claims 1 to 4.