阅读说明：本技术 汽车用行走部件 (Automobile traveling part ) 是由 大塚研一郎 东昌史 森阳一郎 儿玉真二 松叶正宽 于 2020-03-27 设计创作，主要内容包括：本发明的汽车用行走部件具备下述焊接接头：抗拉强度为780MPa以上的第一钢板与第二钢板重叠、在第一钢板的端面与第二钢板的表面之间形成角焊缝焊接部而成,其中,形成所述焊接接头的焊接金属的化学成分以相对于焊接金属的总质量而言的质量％计含有C：0.02～0.30％、Si：0.10％以上且低于1.0％、Mn：1.2～3.0％、Al：0.002～0.30％、Ti：0.005～0.30％、P：超过0％且为0.015％以下、S：超过0％且为0.030％以下,并且满足下述式(1A)、式(1B)、式(2)及式(3),此外,角焊缝焊接部的缝边部的熔渣满足式(4)。[Al]+[Ti]>0.05式(1A)[Ti]/[Al]>0.9式(1B)7×[Si]+7×[Mn]-112×[Ti]-30×[Al]≤12式(2)2.0<[Si]+[Mn]式(3)[熔渣表面的Ti含量]>[熔渣表面的Si含量]式(4)。(The automotive chassis part of the present invention comprises the following welded joints: and a first steel sheet and a second steel sheet having a tensile strength of 780MPa or more, and a fillet welded portion is formed between an end face of the first steel sheet and a surface of the second steel sheet, wherein a chemical composition of a weld metal forming the welded joint contains, in mass%, C: 0.02 to 0.30%, Si: 0.10% or more and less than 1.0%, Mn: 1.2-3.0%, Al: 0.002-0.30%, Ti: 0.005-0.30%, P: more than 0% and 0.015% or less, S: more than 0% and not more than 0.030%, and satisfies the following formulae (1A), (1B), (2) and (3), and the slag at the hem portion of the fillet weld satisfies formula (4). [ Al ] + [ Ti ] >0.05 formula (1A) [ Ti ]/[ Al ] >0.9 formula (1B)7 x [ Si ] +7 x [ Mn ] -112 x [ Ti ] -30 x [ Al ] < 12 formula (2)2.0 [ Si ] + [ Mn ] formula (3) [ Ti content on slag surface ] > [ Si content on slag surface ] formula (4).)

1. A vehicle chassis member, characterized by comprising the following weld joints: a first steel plate having a tensile strength of 780MPa or more and a second steel plate, and a fillet weld portion is formed between the end face of the first steel plate and the surface of the second steel plate,

wherein the chemical composition of the weld metal forming the weld joint is, in mass% relative to the total mass of the weld metal:

C：0.02～0.30％、

si: more than 0.10% and less than 1.0%,

Mn：1.2～3.0％、

Al：0.002～0.30％、

Ti：0.005～0.30％、

P: more than 0% and not more than 0.015%,

S: more than 0% and not more than 0.030%,

Cu：0～0.50％、

Cr：0～1.5％、

Nb：0～0.3％、

V：0～0.3％、

Mo：0～1.0％、

Ni：0～2.5％、

B：0～0.005％，

The remaining part contains iron and impurities,

satisfying the following formula (1A), formula (1B), formula (2) and formula (3),

the slag at the seam edge portion of the fillet weld portion satisfies formula (4),

[ Al ] + [ Ti ] >0.05 formula (1A)

[ Ti ]/[ Al ] >0.9 formula (1B)

7 x [ Si ] +7 x [ Mn ] -112 x [ Ti ] -30 x [ Al ] ≦ 12 formula (2)

2.0< [ Si ] + [ Mn ] formula (3)

[ Ti content on slag surface ] > [ Si content on slag surface ] formula (4)

Wherein [ Si ], [ Al ], [ Ti ], [ Mn ] means the content of each component in mass% with respect to the total mass of the weld metal.

2. The running part for an automobile according to claim 1, wherein a chemical composition of the weld metal contains, in mass%, with respect to a total mass of the weld metal, Cu: 0.005-0.50%, Cr: 0.05 to 1.5%, Nb: 0.005-0.3%, V: 0.005-0.3%, Mo: 0.005-1.0%, Ni: 0.05-2.5%, B: 0.0005-0.005% of 1 or more than 2.

3. The automobile chassis according to claim 1 or 2, wherein a plate thickness of the first steel plate is 0.8mm to 4.0 mm.

Technical Field

The present invention relates to a vehicle chassis member. The present invention particularly relates to an automobile chassis member having a welded joint with excellent strength and corrosion resistance of the weld metal.

The present application claims priority based on Japanese application No. 2019-061002, filed on 3/27/2019, the contents of which are incorporated herein by reference.

Background

Automotive chassis parts are generally manufactured by welding a plurality of steel materials in a superimposed manner by gas shielded arc welding or the like.

The standard for corrosion resistance of automobiles has become more stringent year by year, and particularly, there is an increasing demand for suppressing red rust at joints (arc-welded parts) of traveling members such as arms, sub-frames, and beams. The components include various lower arms, various upper arms, a toe control arm, a trailing arm, a torsion beam, a bracket, a sub-frame, a side member, a cab, a lower guard, a wheel, and a floor cross member.

These automotive underbody parts are manufactured by arc welding a plurality of steel members using a welding wire, and then coated. In this coating, if a coating defect occurs on the surface of the weld metal, not only the appearance is deteriorated, but also the corrosion resistance may be lowered. Even if the coating is applied to the weld metal in good appearance, red rust may be generated between the oxidized slag of the weld metal and the coating film if a coating defect such as floating or peeling of the coating occurs.

Among them, patent document 1 discloses a gas metal arc welding method for improving post-coating corrosion resistance of a welded portion and its vicinity, which is characterized in that it is a method for performing gas metal arc welding using a steel wire for a carbon steel base material which is electro-deposit-coated after welding, and uses a wire having a composition in which the total Si content of the base material and the wire is 0.04 to 0.2% by weight and the total Mn content of the base material and the wire is 0.5% or more.

According to the technique of patent document 1, the corrosion resistance of the welded portion and the vicinity thereof by gas metal arc welding after coating can be improved by suppressing the generation of insulating Si slag.

Documents of the prior art

Patent document

Patent document 1: japanese laid-open patent publication No. 8-33997

Disclosure of Invention

Problems to be solved by the invention

However, in the case of only reducing Si as in the technique of patent document 1, no study has been made on securing the strength of the weld metal. Therefore, it is difficult to apply the composition to a vehicle chassis member requiring high strength of 780MPa or more.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a vehicle chassis member having a welded joint formed of a base steel sheet having 780MPa or more, which is excellent in strength and corrosion resistance of a weld metal.

Means for solving the problems

The specific method of the present invention is as follows.

[1] A first aspect of the present invention is a vehicle running part including the following weld joints: a first steel plate having a tensile strength of 780MPa or more is overlapped with a second steel plate, and a fillet weld is formed between an end face of the first steel plate and a surface of the second steel plate. The chemical composition of the weld metal forming the weld joint is C: 0.02 to 0.30%, Si: 0.10% or more and less than 1.0%, Mn: 1.2-3.0%, Al: 0.002-0.30%, Ti: 0.005-0.30%, P: more than 0% and 0.015% or less, S: more than 0% and 0.030% or less, Cu: 0-0.50%, Cr: 0-1.5%, Nb: 0-0.3%, V: 0-0.3%, Mo: 0-1.0%, Ni: 0-2.5%, B: 0 to 0.005%, and the balance of iron and impurities, and satisfies the following formulae (1A), (1B), (2), and (3), and the slag at the edge of the fillet weld satisfies formula (4).

[ Al ] + [ Ti ] >0.05 formula (1A)

[ Ti ]/[ Al ] >0.9 formula (1B)

7 x [ Si ] +7 x [ Mn ] -112 x [ Ti ] -30 x [ Al ] ≦ 12 formula (2)

2.0< [ Si ] + [ Mn ] formula (3)

[ Ti content on slag surface ] > [ Si content on slag surface ] formula (4)

Wherein [ Si ], [ Al ], [ Ti ], [ Mn ] are contents in mass% of each component with respect to the total mass of the weld metal.

[2] The automobile underbody part according to [1], wherein the chemical component of the weld metal may contain Cu: 0.005-0.50%, Cr: 0.05 to 1.5%, Nb: 0.005-0.3%, V: 0.005-0.3%, Mo: 0.005-1.0%, Ni: 0.05-2.5%, B: 0.0005-0.005% of 1 or more than 2.

[3] The automotive underbody part according to any one of [1] and [2], wherein a thickness of the first steel sheet may be 0.8mm to 4.0 mm.

Effects of the invention

According to the traveling member of the present invention, since the composition of the weld metal is appropriately controlled, the weld metal in the welded joint formed of the base steel sheet of 780MPa or more can exhibit excellent strength and corrosion resistance.

Drawings

Fig. 1 is a perspective view of the automotive underbody part of the present embodiment.

Fig. 2 is a schematic cross-sectional view showing a welded joint of the automotive underbody part according to the present embodiment.

Fig. 3 is a schematic view showing a distance between the nozzle and the base material.

Detailed Description

The present inventors have intensively studied a solution to the above problems, and as a result, they have obtained the following findings.

(A) If the Si content of the weld metal is increased in order to improve the strength, Si-based slag is generated, and a coating defect occurs at the generation position of the Si-based slag, which leads to easy generation of red rust.

(B) When Al and Ti coexist in the weld metal in an appropriate balance, the effect of suppressing red rust can be obtained.

(C) In the case where Ti and Si on the slag surface coexist in an appropriate balance at the seam edge portion of the fillet weld portion where slag is likely to accumulate, the effect of suppressing red rust can be obtained.

(D) When the welding conditions for forming the fillet weld portion are set such that the nozzle inner diameter is 14 to 20mm, the gas flow rate is 20 to 30L/min, and the distance between the nozzle and the base metal is 20mm or less, Ti and Si on the slag surface can coexist in an appropriate balance.

The present invention has been made based on the above-described knowledge. Hereinafter, the automobile underbody part according to the embodiment of the present invention will be described in detail.

The vehicle traveling member according to the present embodiment is, for example, a lower arm, an upper arm, a toe control arm, a trailing arm, a torsion beam, a bracket, a sub-frame, a side member, a cab, a lower guard, a wheel, and a floor cross member. Fig. 1 is a perspective view of a lower arm as an automobile chassis component, and as shown in fig. 1, the automobile chassis component according to the present embodiment includes a welded joint 1 in which two steel plates are welded in an overlapping manner.

Fig. 2 is a schematic cross-sectional view showing a welded joint 1 of the automobile chassis member shown in fig. 1. As shown in fig. 2, the welded joint 1 is configured by: the two steel plates 2 and 3 (first steel plate, second steel plate) are overlapped and a fillet weld 4 is formed between the end face of one steel plate 2 and the surface of the other steel plate 3.

The fillet weld 4 is formed by arc welding the two steel plates 2 and 3 with a welding wire.

The two steel plates 2 and 3 may be the same type of steel plate or different types of steel plates. The thickness of the steel sheet 2 having the fillet weld 4 formed on the end face is preferably 0.8mm to 4.0 mm.

If the thickness of the steel plate 2 is 0.8mm or more, the occurrence of welding defects during arc welding can be suppressed, and if the thickness of the steel plate 2 is 4.0mm or less, the increase in weight can be suppressed. The thickness of the steel sheet 2 is preferably 1.4mm or more, more preferably 2.0mm or more. The thickness of the steel sheet 2 is preferably 3.5mm or less, more preferably 3.0mm or less.

Further, the plate thicknesses of the steel plates 2 and 3 are more preferably 0.8mm to 4.0 mm.

If the plate thicknesses of the steel plates 2 and 3 are 0.8mm or more, the occurrence of welding defects during arc welding can be suppressed, and if the plate thicknesses of the steel plates 2 and 3 are 4.0mm or less, the increase in weight can be suppressed. The thickness of the steel sheets 2 and 3 is preferably 1.4mm or more, more preferably 2.0mm or more. The thickness of the steel sheets 2 and 3 is preferably 3.5mm or less, more preferably 3.0mm or less.

The composition of the weld metal 4 of the welded joint 1 can be adjusted by the steel sheet composition and the wire composition. The respective component compositions of the weld metal 4 will be described below.

In the following description, the term "satisfying corrosion resistance" means that red rust does not occur when a composite cycle test (CCT, 5% NaCl, humidity of 50%) defined in JASO method M610 is evaluated at 75 cycles.

The satisfactory strength means that the welded metal does not break and the base metal does not break when the welded test piece is subjected to a tensile test.

The "weld metal" is a metal obtained by melting and mixing a steel plate base material and a welding wire.

The chemical composition of the weld metal is defined as a chemical composition expressed in mass% relative to the total mass of the weld metal, and the description of "mass%" thereof will be abbreviated as "%".

The chemical composition of the weld metal can be measured by a luminescence spectroscopy method using high-frequency Inductively Coupled Plasma (ICP). Specifically, (1) a region of the weld metal is determined in advance by visually observing a cross section perpendicular to the longitudinal direction at the central portion in the longitudinal direction of the weld part, (2) a cutting powder of the weld metal is collected by cutting the region with a drill, and (3) measurement is performed by a spectroradiometric method using a high-frequency Inductively Coupled Plasma (ICP) using the cutting powder as a sample.

〔C：0.02～0.30％〕

C has an effect of stabilizing an arc and making droplets fine, and when the C content is less than 0.02%, the droplets become large and the arc becomes unstable, resulting in an increase in the amount of spatter generated. As a result, the bead shape becomes uneven and defective, and red rust occurs. The reason why red rust is generated due to the poor bead shape is that: weld slag is likely to be generated in the recesses due to the defects, and water, mud containing moisture, and the like, which cause red rust, are likely to accumulate. If the C content is less than 0.02%, the tensile strength of the weld metal cannot be obtained, and the desired tensile strength cannot be obtained. Therefore, the lower limit of C is 0.02% or more, preferably 0.04% or more, and more preferably 0.06% or more.

On the other hand, if the C content exceeds 0.30%, the weld metal is hardened to lower the cracking resistance, and the weld metal is likely to be broken. Therefore, the upper limit of C is 0.30%, preferably 0.25%.

[ Si: 0.10% or more and less than 1.0% ]

Si is contained as a deoxidizing element in the wire or the base material. In particular, Si in the wire increases the tensile strength of the weld metal by promoting deoxidation of the weld pool. Therefore, the lower limit of Si is 0.10%, preferably 0.20%, and more preferably 0.30%. On the other hand, if Si is excessively contained, the amount of non-conductive Si-based slag increases, and red rust tends to be generated between the slag and the weld metal. Therefore, the upper limit of Si is less than 1.0%, preferably less than 0.9%, and more preferably less than 0.8%. More preferably 0.7% or less. Still more preferably less than 0.62%.

〔Mn：1.2～3.0％〕

Mn is also a deoxidizing element, similarly to Si, and is an element that promotes deoxidation of the molten pool during arc welding and also improves the tensile strength of the weld metal. If the Mn content is small, the tensile strength of the weld metal cannot be sufficiently ensured, and the weld metal is likely to break. Therefore, the lower limit of Mn is 1.2% or more, preferably 1.5% or more.

On the other hand, if Mn is contained excessively, the viscosity of the molten metal becomes high, and when the welding speed is high, the molten metal cannot flow into the welding portion properly, and a humped bead is formed, and a bead shape defect is likely to occur. As a result, the bead shape becomes uneven and defective, and red rust occurs. Therefore, the upper limit of Mn is 3.0% or less, preferably 2.5% or less.

〔Al：0.002～0.30％〕

Al is a strong deoxidizing element, and has an effect of promoting deoxidation of molten metal during arc welding to suppress generation of blowholes. Therefore, the lower limit of the Al content is 0.002%, preferably 0.01%, and more preferably 0.02%.

On the other hand, if the Al content is excessive, the non-conductive Al-based slag increases, and red rust tends to occur between the slag and the weld metal. Therefore, the upper limit of the Al content of the weld metal is 0.30%, preferably 0.25%, and more preferably 0.20%.

〔Ti：0.005～0.30％〕

Ti is a deoxidizing element, and therefore has an effect of suppressing the generation of pores. Therefore, the lower limit of the Ti content is 0.005%, preferably 0.01%, more preferably 0.05%.

On the other hand, if Ti is excessively contained, Ti-based slag increases, adhesion between the Ti-based slag and the weld metal decreases, and peeling becomes easy. Therefore, red rust is likely to be generated from the peeled portion. Therefore, the upper limit of the Ti content is 0.30%, preferably 0.25%, and more preferably 0.20%.

[ synergistic Effect due to the coexistence of Al and Ti ]

Both Al and Ti are elements that suppress Si-based slag and generate Al-based slag and Ti-based slag, and contribute to suppression of poor coating. However, when Al or Ti is contained at the lower limit or more, the following tendency is exhibited: only Al-based slag or only Ti-based slag is agglomerated on the welding bead. When these slags agglomerate, even if there is no coating defect on the welding bead, a gap is easily formed between the weld metal and the slags, and red rust is generated from the gap. That is, by forming both Al-based slag and Ti-based slag, the aggregation of the same slag can be suppressed, and as a result, red rust can be suppressed. Therefore, in the present invention, by setting the weld metal to a composition system containing Al and Ti at the same time, excellent corrosion resistance can be obtained.

〔Al、Ti〕

The contents of Al and Ti satisfy the following formulas (1A) and (1B).

[ Al ] + [ Ti ] >0.05 formula (1A)

[ Ti ]/[ Al ] >0.9 formula (1B)

Both Al and Ti can sufficiently secure the strength of the weld metal by suppressing the generation of coarsened ferrite. When the total content of Al and Ti is 0.05% or less, ferrite of the weld metal is easily coarsened even when no blowholes are generated, the strength of the weld metal is not sufficiently obtained, and fracture is easily generated in the weld metal. Therefore, the lower limit of the total content of Al and Ti exceeds 0.05%, preferably 0.10%, and more preferably 0.15%.

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

Further, in a component system having a high content of Si and Mn, the balance of Al and Ti is important. Therefore, the content of Al and Ti is adjusted so as to satisfy the above formula (1B). If this formula (1B) is not satisfied, that is, if Ti/Al is 0.9 or less, Al-based slag is aggregated on the weld bead to be thick, and a gap is easily formed between the weld metal and the slag, and red rust may be generated from the gap. The upper limit of the Ti/Al value is not particularly limited.

〔Si、Mn、Ti、Al〕

Further, the contents of Si, Mn, Ti and Al satisfy the following formula (2).

7 x [ Si ] +7 x [ Mn ] -112 x [ Ti ] -30 x [ Al ] ≦ 12 formula (2)

The inventors of the present invention examined the presence or absence of red rust between slag and weld metal for weld metals having various component systems, and found as a result that: if the value of 7X [ Si ] + 7X [ Mn ] -112X [ Ti ] -30X [ Al, which is an index for the occurrence of red rust, exceeds 12, red rust occurs at an early stage and corrosion resistance is poor. Therefore, in the above formula, the upper limit is set to 12.

The lower limit is not particularly limited, and is-33.5 calculated from the lower limits of Si and Mn and the upper limits of Al and Ti.

〔Si、Mn〕

The contents of Si and Mn satisfy the following formula (3).

2.0< [ Si ] + [ Mn ] formula (3)

When the total amount of Si and Mn exceeds 2.0%, the strength of the weld metal can be ensured, and the weld metal can be prevented from breaking when subjected to a tensile load.

The upper limit of the content of Si and Mn is not particularly limited, but is less than 4.0% as calculated from the upper limits of Si and Mn.

In the above formula, [ Si ], [ Al ], [ Ti ], [ Mn ] means the content of each component in mass% with respect to the total mass of the weld metal.

Slag at the edge of the seam: [ Ti content on slag surface ] > [ Si content on slag surface ]

Since the hem portion of the fillet weld portion is a place where slag is likely to accumulate, red rust tends to occur more easily than portions other than the hem portion. Therefore, in the fillet weld, the slag at the hem portion needs to satisfy the formula (4).

[ Ti content on slag surface ] > [ Si content on slag surface ] formula (4)

When the slag at the seam edge portion of the fillet weld portion does not satisfy formula (4), the insulating Si-based slag is condensed on the weld bead to increase the thickness, and a gap is likely to be formed between the weld metal and the slag, which may cause red rust to form from the gap.

The Ti content and the Si content of the slag surface can be measured by SEM (scanning electron microscope) and EDS (energy dispersive X-ray spectrometer).

More specifically, the diagonal weld is collected by cutting and embedded in resin. The resin-embedded sample was then ground to a location where no slag was removed. The slag adhering portion of the cross section of the welded portion was observed by SEM in an enlarged manner, and the average Si concentration and the average Ti concentration in the 100 μm × 100 μm region of the slag were obtained by EDS. In order to prevent unevenness due to different observation sites, the average Si concentration and the average Ti concentration were set to be the average of the observation results of 3 cross sections.

[ P: more than 0% and 0.015% or less ]

P is an element generally mixed as an impurity in steel, and is also included in weld metal because P is also generally included as an impurity in steel sheets and welding wires. Here, P is one of the main elements that cause high-temperature cracking of the weld metal, and therefore is preferably suppressed as much as possible. If the P content exceeds 0.015%, high-temperature cracking of the weld metal becomes significant, so the upper limit of the P content of the weld metal is 0.015% or less.

The lower limit of the P content is not particularly limited, and therefore, is more than 0%, but may be 0.001% from the viewpoint of the cost and productivity of dep.

[ S: more than 0% and 0.030% or less

S is also an element that is generally mixed into steel as an impurity, as in P, and is also generally included in a welding wire as an impurity, and therefore is also included in a weld metal. Here, S is an element that inhibits cracking resistance of the weld metal, and is preferably suppressed as much as possible. If the S content exceeds 0.030%, the resistance to cracking of the weld metal deteriorates, so the S content of the weld metal is 0.030% or less.

The lower limit of the S content is not particularly limited, and therefore is more than 0%, but may be 0.001% from the viewpoint of the cost and productivity of S removal.

Cu, Cr, Nb, V, Mo, Ni and B are not essential elements, but may contain 1 or 2 or more kinds in combination as required. Effects and upper limit values obtained by including each element will be described. The lower limit of the case where these elements are not contained is 0%.

〔Cu：0～0.50％〕

Cu may be contained in the weld metal by copper plating of the wire, and therefore, it may be contained in an amount of 0.005% or more. On the other hand, if the Cu content becomes excessive, weld cracking tends to occur, so the upper limit of Cu is 0.50% or less.

〔Cr：0～1.5％〕

Cr may be contained in an amount of 0.05% or more because it improves hardenability of the welded portion and thereby improves tensile strength. On the other hand, if Cr is excessively contained, the elongation of the welded portion decreases. Therefore, the upper limit of Cr is 1.5% or less.

〔Nb：0～0.3％〕

Nb may be contained in an amount of 0.005% or more because it improves hardenability of the welded portion and improves tensile strength. On the other hand, if Nb is excessively contained, the elongation of the welded portion decreases. Therefore, the upper limit of Nb is 0.3% or less.

〔V：0～0.3％〕

V may be contained in an amount of 0.005% or more because it improves hardenability of the welded portion and improves tensile strength. On the other hand, if V is excessively contained, the elongation of the welded portion decreases. Therefore, the upper limit of V is 0.3% or less.

〔Mo：0～1.0％〕

Mo may be contained in an amount of 0.005% or more because it improves hardenability of the welded portion to improve tensile strength. On the other hand, if Mo is excessively contained, the elongation of the welded portion decreases. Therefore, the upper limit of Mo is 1.0% or less.

〔Ni：0～2.5％〕

Ni may be contained by 0.05% or more because it improves the tensile strength and elongation of the welded portion. On the other hand, if Ni is excessively contained, weld cracking is likely to occur. Therefore, the upper limit of Ni is 2.5% or less. Preferably 2.0% or less.

〔B：0～0.005％〕

B may be contained in an amount of 0.0005% or more because it improves hardenability of the weld portion to improve tensile strength. On the other hand, if B is excessively contained, the elongation of the welded portion decreases. Therefore, the upper limit of B is 0.005%. Preferably 0.003% or less.

The remainder of the above-described components includes Fe and impurities. The impurities are components contained in the raw material, components mixed in during the production process, and components not intentionally contained in the weld metal, or components contained within a range not adversely affecting the automobile underbody part of the present embodiment.

The automotive underbody part of the present embodiment is explained above. The type of the steel sheet forming the base material of the automotive chassis part according to the present embodiment is not particularly limited as long as the tensile strength is 780MPa or more, but the steel sheet preferably contains the following components: c: 0.030 to 0.40%, Si: 0.4-1.8%, Mn: 1.80-3.20%, P: less than 0.05% and S: less than 0.010%. Further, the steel sheet may contain optional components such as Al and Ti in addition to the above components.

Next, a method for manufacturing the automobile underbody part of the present embodiment will be described. The following cases are described as examples: the fillet weld is formed by gas shielded arc welding.

The nozzle has an inner diameter of 14 to 20mm, a gas flow rate of 20 to 30L/min, and a distance between the nozzle and the base material of 20mm or less

In manufacturing the automotive underbody part of the present embodiment, welding conditions when forming the fillet welded portion by gas metal arc welding are set as follows: the nozzle has an inner diameter of 14 to 20mm, a gas flow rate of 20 to 30L/min, and a distance between the nozzle and the base material of 20mm or less. As a result, the weld metal is less likely to be oxidized, and [ Ti content on the slag surface ] is formed in the seam edge portion of the fillet weld portion [ Si content on the slag surface ].

As shown in fig. 3, the nozzle-base material distance 6 is the shortest distance between the tip of the outlet of the shielding gas 52 in the welding nozzle 51 and the target position 7 of the welding wire 53. Fig. 3 is a diagram showing a case where the target position 7 of the wire 53 is an intersection 54, and the intersection 54 is an intersection between a plane passing through the end face of the steel plate 2 and the surface of the steel plate 3.

The method for manufacturing an automobile underbody part according to the present embodiment more preferably satisfies formula (5).

(10/6X [ nozzle inner diameter ] -20/6). times.0.8. ltoreq.gas flow (10/6X [ nozzle inner diameter ] -20/6). times.1.2 formula (5)

By satisfying the formula (5), the protection state of the weld metal surface becomes further favorable, and the weld metal becomes further less likely to be oxidized. Therefore, it becomes easy to control the Ti and Si contents on the slag surface at the hem portion of the fillet weld portion to the relationship of [ Ti content on the slag surface ] > [ Si content on the slag surface ].

In the method for manufacturing the automotive underbody part according to the present embodiment, the inclination angle (standing angle) of the welding torch is more preferably 50 to 70 °. When the inclination angle (rising angle) of the welding torch is 50 ° or more, the protection state of the weld metal surface becomes further favorable, and the weld metal becomes further less likely to be oxidized. Therefore, it becomes easy to control the Ti and Si contents on the slag surface at the hem portion of the fillet weld portion to the relationship of [ Ti content on the slag surface ] > [ Si content on the slag surface ]. On the other hand, the welding metal penetration shape can be favorably maintained by setting the inclination angle (rising angle) of the welding torch to 70 ° or less.

The inclination angle (rising angle) of the welding torch is an acute angle formed by the longitudinal direction of the welding torch and the surface of the steel plate 3.

Examples

Hereinafter, the effects of the present invention will be described more specifically by examples.

A welded joint was produced by overlap fillet arc welding using various welding wires for a steel sheet combination having a tensile strength of 980MPa or a steel sheet combination having a tensile strength of 780MPa, and the weld metal was evaluated. The overlapping amount of one and the other steel plates was set to 15mm and the plates were closely adhered, and the plate thickness of the steel plates was set to 2.6 mm. Regarding the welding posture, the welding line is set to be horizontal, and the inclination angle α of the other steel plate is set to 0 °. The welding method was a pulsed MAG arc welding method in which the angle of inclination (angle of erection) of the welding torch was set to 60 DEG and a shielding gas mainly containing 20 vol% CO was used₂Ar gas of (2). Furthermore, as protective gas, a gas containing 3% of O was also used₂Ar gas of (2) and 20% CO₂And 2% of O₂Ar gas of (2). The wire tip positioning target is set to a corner portion constituted by an end face of one steel plate and a surface of the other steel plate.

In addition, the welding conditions of experiments Nos. 1 to 28 and Nos. 29 to 42 were: the nozzle has an inner diameter of 14 to 20mm, a gas flow rate of 20 to 30L/min, and a distance between the nozzle and the base material of 20mm or less. Further, the relationship between the nozzle inner diameter and the gas flow rate satisfies the above expression (5), and the inclination angle (standing angle) of the welding torch is 50 to 70 °.

As the welding wire, the composition of the weld metal is adjusted by using solid wires of various composition systems.

The tensile strength, sheet thickness and main components of each steel sheet are as follows.

(980MPa Steel plate)

Plate thickness: 2.6mm

Ingredients: 0.060% for C, 1.2% for Si, 2.50% for Mn, 0.01% for P, 0.005% for S, 0.03% for Al, 0.12% for Ti

(780MPa Steel plate)

Plate thickness: 2.6mm

Ingredients: 0.045% of C, 0.02% of Si, 1.55% of Mn, 0.01% of P, 0.005% of S, 0.3% of Al, and 0.13% of Ti

The chemical composition of the weld metal was measured for the welded joint obtained in this manner.

Specifically, (1) a region of the weld metal was previously determined by visually observing a cross section perpendicular to the longitudinal direction at the central portion in the longitudinal direction of the weld part, (2) a cutting powder of the weld metal was collected by cutting the region with a drill, and (3) the chemical composition of the weld metal was measured by a spectroradiometric method using a high-frequency Inductively Coupled Plasma (ICP) using the cutting powder as a sample.

Table 1, table 2, and table 3 show the contents of the respective components and the values of formula (1A), formula (1B), formula (2), and formula (3). In addition, numerical values outside the scope of the present invention are underlined. In addition, the non-added components are set as blanks in the table.

In addition, the Ti content and the Si content of the slag surface at the seam edge portion of the fillet weld were measured by the SEM EDS method by the above-described method. The Ti content and the Si content of the slag surface at the fillet portion of the fillet weld zone are set to OK (good) if the formula (4) is satisfied, and NG (bad) if the formula (4) is not satisfied. Table 4 also shows the strength (fracture position), the determination result obtained by the formula (4), and the evaluation result of red rust for each experimental example.

[ Table 1]

[ Table 2]

[ Table 3]

[ Table 4]

(evaluation of Strength)

The strength was evaluated by the position of the break in the joint tensile test. In the tensile test, the longitudinal ends of two steel sheets of 25mm × 100mm were overlapped by 15mm to weld the overlapped fillet, and the weld was performed at a tensile rate of 10 mm/min in the longitudinal direction. The fracture position is set to OK when the base metal is used, and NG when the weld metal is used.

(evaluation of Red Rust)

The presence or absence of red rust was evaluated by performing 50 cycles, 75 cycles, and 100 cycles of the combined cycle test (CCT, 5% NaCl, and 50% humidity) specified in JASO method M610. The setting is OK when no red rust is generated, and NG when red rust is generated.

The corrosion resistance was evaluated in four stages of A, B, C and D below. A and B are set to satisfy corrosion resistance, and C and D are set to not satisfy corrosion resistance.

A: all of 50 cycles, 75 cycles, and 100 cycles are OK.

B: 50 cycles and 75 cycles are OK and 100 cycles are NG.

C: 50 cycles are OK, 75 cycles and 100 cycles are NG.

D: all of 50 cycles, 75 cycles, and 100 cycles were NG.

In the experiments nos. 1 to 28 of the present invention examples, the composition of the weld metal and the composition of the slag surface at the seam edge portion of the fillet weld portion were appropriately controlled by appropriately adjusting the composition of the weld metal and the welding conditions, and therefore, excellent strength and corrosion resistance were obtained at the weld metal. However, in the test nos. 8, 13, 18, 26 and 28 in which the Si content exceeded 0.7%, red rust was generated at 100 cycles although red rust was not generated at 50 cycles and 75 cycles in the combined cycle test.

In the experiment No.29 of the comparative example, since the Si content of the weld metal was excessive and the formula (4) was not satisfied, non-conductive slag was formed and red rust was generated.

In experiment No.30 of the comparative example, the chemical composition of the weld metal did not satisfy formula (2) and did not satisfy formula (4), and therefore the generation of red rust could not be suppressed.

In experiment No.31 of the comparative example, the weld metal was hardened due to the excessive C content of the weld metal, and the desired strength could not be obtained.

In experiment No.32 of the comparative example, the Si content of the weld metal was too small, and the chemical composition of the weld metal did not satisfy formula (1), so the effect of suppressing the generation of coarsened ferrite could not be sufficiently enjoyed, and the strength of the weld metal could not be ensured. In addition, red rust was generated in 75 cycles and 100 cycles of the composite cycle test due to the failure to satisfy formula (4).

In experiment No.33 of the comparative example, the C content of the weld metal was too small, and therefore the tensile strength of the weld metal could not be obtained, and the desired tensile strength could not be obtained. In addition, red rust was generated due to a defective bead shape.

In experiment No.34 of the comparative example, red rust was generated due to a poor bead shape because the Mn content of the weld metal was excessive.

In experiment No.35 of the comparative example, since the Al content of the weld metal was excessive, the Al-based slag increased, and red rust was generated between the Al-based slag and the weld metal.

In experiment No.36 of the comparative example, since the Ti content of the weld metal was 0% and Al and Ti were not present, only Al-based slag aggregated on the weld bead, and a gap was formed between the weld metal and the Al-based slag, and red rust was generated from the gap. In addition, red rust is generated due to the failure to satisfy formula (4).

In experiment No.37 of the comparative example, the Mn content of the weld metal was too small, and therefore the strength of the weld metal could not be ensured, and fracture occurred in the weld metal.

In experiment No.38 of the comparative example, since the Ti content of the weld metal was excessive, the Ti-based slag increased, and the adhesion of the Ti-based slag was reduced, thereby peeling off. Therefore, red rust is generated at the peeled portion.

In experiment No.39 of the comparative example, since the Al content of the weld metal was 0% and Al and Ti were not present, only Ti-based slag aggregated on the weld bead, and a gap was formed between the weld metal and the Ti-based slag, and red rust was generated from the gap. In addition, red rust is generated due to the failure to satisfy formula (4).

In experiment No.40 of the comparative example, since the chemical composition of the weld metal does not satisfy formula (3), the strength of the weld metal cannot be secured, and fracture occurs from the weld metal when a tensile load is applied.

In experiment No.41 of the comparative example, red rust was generated due to the coagulation of Al-based slag and the failure to satisfy formula (4) because the chemical composition of the weld metal did not satisfy formula (1B).

In the experiment No.42 of the comparative example, since the Si content of the weld metal was excessive and the formula (4) was not satisfied, non-conductive slag was formed and red rust was generated.

For experiment No.43 of the comparative example, the welding conditions were set to: the nozzle inner diameter is less than 14mm, the gas flow rate is less than 20L/min, and the distance between the nozzle and the base material exceeds 20 mm. As a result, red rust was generated in 75 cycles and 100 cycles of the combined cycle test because formula (4) was not satisfied.

Industrial applicability

According to the present invention, it is possible to provide a vehicle chassis component having a welded joint formed of a base steel sheet having 780MPa or more, which is excellent in the strength and corrosion resistance of weld metal, and which has a high industrial utility value.

Description of the symbols

1 welded joint

2 first steel plate

3 second steel plate

4 fillet weld welded part, weld metal

51 nozzle (welding torch)

52 protective gas

53 welding wire

54 intersection part

6 distance between nozzle and base material

7 target position