JP2011208264A - Vehicle chassis member excellent in corrosion resistance and method of manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a vehicle chassis member excellent in the corrosion resistance in an arc welding part and having high strength.SOLUTION: The vehicle chassis member has the arc welding joint of molten Zn-Al-Mg alloy-plated steel sheet members having a sheet thickness of 1.0-3.0 mm, wherein the steel sheet surface that has had a plated layer before welding is covered with a Zn-Al-Mg alloy layer up to a welding bead end part continuously, an Al-Mg alloy layer exists between the Zn-Al-Mg alloy layer and the steel substrate. In the steel sheet surface within 2.0 mm from the welding bead end part, the Zn-Al-Mg alloy layer has an average Al concentration of 0.2-22.0 mass% and an average Mg concentration of 1.0-10.0 mass%, and Fe-Al alloy layer has an average Fe concentration of at most 70.0 mass%.

Description

  TECHNICAL FIELD The present invention relates to an automobile chassis member such as a frame and a suspension member that are assembled by joining steel plates by an arc welding method, and more particularly to an automobile chassis member excellent in corrosion resistance of an arc welded portion and a manufacturing method thereof.

  Chassis members such as automobile suspension members are assembled by forming hot-rolled steel plates into steel plate members of a predetermined shape mainly by press forming or the like, and joining them by arc welding. Then, after giving a phosphate process in order to provide coating-film adhesiveness, a cationic electrodeposition coating is given and it uses.

  In hot-rolled steel sheets that have been subjected to cationic electrodeposition coating, if the coating film is damaged by chipping with stepping stones, corrosion proceeds from that part. In addition, overlapped fillet welding is often performed in arc welding of automobile chassis members, but in the vicinity of the weld bead portion and the bead toe portion, Fe scale is generated on the surface of the steel sheet due to welding heat input during arc welding, and traveling The Fe scale peels off together with the cationic electrodeposition coating film due to fatigue caused by vibrations therein, resulting in a decrease in corrosion resistance. For this reason, it is necessary to design the strength of an automobile chassis member in anticipation of a reduction in plate thickness due to corrosion. Specifically, conventionally, a high-strength hot-rolled steel sheet having a thickness of 3 to 4 mm made of a steel type having a tensile strength of 340 to 440 MPa has been often used from the viewpoint of collision safety.

  In recent years, further improvement in collision safety and weight reduction have been desired, and the need to use a high-strength steel plate of 590 MPa or higher is increasing as a steel plate for chassis members. There is also a demand for improved rust prevention performance for longer life. Therefore, steel plates obtained by subjecting steel plates having a tensile strength of 590 MPa or more to alloying hot dip galvanizing have begun to be used for automobile chassis members. However, when arc welding is performed on a galvanized steel sheet, the plating layer evaporates and disappears in the vicinity of the weld bead toe exposed to a high temperature, and Fe scale is generated in that portion. For this reason, the fault of the conventional hot-rolled steel sheet that a coating film is easy to peel with Fe scale is not improved even if it uses a zinc plating steel plate. In addition, at the part where the plating layer is melted during welding, a Fe-Al alloy layer with a high Fe concentration is formed at the interface with the steel substrate, as will be described later, and the problem of hindering the original corrosion resistance of the zinc-based plating layer is also manifested. did.

  Patent Document 1 discloses a method of joining a hot-dip Zn-Al alloy-plated steel plate or a hot-dip Zn-Al-Mg alloy-plated steel plate having higher corrosion resistance than a general hot-dip galvanized steel plate by arc welding. According to it, if arc welding is performed under appropriate conditions using a joining material of stainless steel or copper alloy, Zn evaporates in the vicinity of the bead toe during welding, but Al having a higher boiling point than Zn is present. It is said that the corrosion resistance is improved because it remains and coats the steel sheet surface as an Al oxide. However, according to studies by the inventors, the Al oxide is a factor that reduces the adhesion of the cationic electrodeposition coating film because it inhibits the formation of phosphate crystals. For this reason, although the technique of patent document 1 is applicable to the building material in which the coating film peeling by chipping or vibration fatigue does not occur, the automobile chassis member lacks corrosion resistance.

  Moreover, the technique of patent document 1 is applied to the zinc-plated steel plate in which plate | board thickness exceeds 3 mm. This is because, when the plate thickness is as thin as 3 mm or less, the plating layer on the back surface of the steel plate at the joint portion evaporates due to heat input during welding, resulting in plating damage, which may lead to deterioration of the corrosion resistance on the back surface of the steel plate. . In this respect as well, the technology of Patent Document 1 cannot sufficiently meet the needs for reducing the weight of automobile chassis members.

  Furthermore, it is known that molten metal embrittlement cracking is likely to occur when arc welding is performed on a molten Zn—Al—Mg alloy-plated steel sheet (Patent Document 2). In this case, the fatigue strength of the arc welded portion may be reduced, causing a problem. In the technique of Patent Document 1, no special consideration is given to the molten metal embrittlement crack.

JP 2006-35294 A JP 2003-3238 A

  As mentioned above, with conventional automobile chassis members assembled by arc welding, the corrosion resistance of the arc welded part is insufficient, making it difficult to design a thin wall and satisfy both long life and high strength and light weight. I could not. An object of the present invention is to solve these problems and to provide a high-strength automobile chassis member excellent in corrosion resistance of an arc welded portion.

  As a result of detailed studies, the inventors have found that even when a molten Zn-Al-Mg based steel sheet is arc welded under specific arc welding conditions, even if the plating layer evaporates near the bead toe, it melts around it. It has been found that it is possible to realize a phenomenon in which the plated metal is rapidly wetted and spreads to the bead toe in the temperature lowering process after welding. And, if the composition of the Zn-Al-Mg alloy layer that has spread and the Fe-Al alloy layer formed under the wet condition is appropriately controlled according to the welding conditions, the corrosion resistance at the welded portion can be stabilized. It was found that it can be improved. The present invention has been completed based on such findings.

That is, in the present invention, a member having a joined portion obtained by joining the molten Zn-Al-Mg-based alloy plated steel plate members having a plate thickness of 1.0 to 3.0 mm by arc welding,
The surface of the steel plate having a plating layer before welding is continuously covered with a Zn-Al-Mg alloy layer up to the weld bead toe, and between the Zn-Al-Mg alloy layer and the steel substrate. Has an Fe-Al alloy layer,
In the steel plate surface layer portion whose distance from the weld bead toe is within 2 mm, the Zn—Al—Mg-based alloy layer has an average Al concentration of 0.2 to 22.0 mass% and an average Mg concentration of 1.0 to 10. An automobile chassis member having 0 mass% and an Fe-Al alloy layer having an average Fe concentration of 70.0 mass% or less is provided.

  A molten Zn-Al-Mg alloy-plated steel sheet member is a member formed from a molten Zn-Al-Mg alloy-plated steel sheet as a raw material, and includes a cross section in the plate thickness direction such as a cut end face, a punched surface, and a punched portion. It has a molten Zn—Al—Mg-based alloy plating layer on the surface excluding the exposed portion. The plate thickness means the plate thickness of the “plating original plate” not including the plating layer. The weld bead toe is a position where the surface of the base material and the surface of the weld bead intersect.

  The composition of the molten Zn—Al—Mg alloy plating is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, B : 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, balance Zn and inevitable impurities are more preferable.

  The plating base plate of the hot-dip Zn—Al—Mg alloy-plated steel sheet is in mass%, C: 0.05 to 0.25%, Si: 0.10 to 1.50%, Mn: 1.02 to 2. 50%, Al: 0.010 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0001 to 0.0100%, and if necessary, Nb, Cr, Mo, P It is more preferable that one or more of these be contained in a total amount of 1.00% or less, and the remaining Fe and unavoidable impurities.

As a manufacturing method of the above-mentioned automobile chassis member, when the molten Zn-Al-Mg alloy-plated steel sheet members are arc-welded to each other, a molten Zn-Al-Mg system having a plating adhesion amount per side of 20 to 250 g / m ² A manufacturing method that uses an alloy-plated steel plate member and controls the welding heat input Q (J / cm) represented by the following formula (1) within the range established by the following formula (2) according to the amount of plating applied can be adopted.
Q = (I × V) / v (1)
13W + 1140 ≦ Q ≦ 15W + 8720 (2)
However, I: Welding current (A), V: Arc voltage (V), v: Welding speed (cm / sec), W: Plating adhesion amount per side (g / m ² )

  ADVANTAGE OF THE INVENTION According to this invention, the corrosion resistance fall in the bead toe part vicinity part of the arc welding part which was a problem with the conventional automobile chassis member is avoided, and it is possible to construct an automobile chassis with high strength and high corrosion resistance. It became. Thereby, the lifetime improvement of a member is implement | achieved and the need for weight reduction by thickness reduction can also be responded.

The figure which showed typically the welding part cross-section of a lap fillet welded joint. The figure which showed typically the welded joint cross-section in case a steel plate member is the conventional hot-rolled steel plate. The figure which showed typically the cross-sectional state of the high temperature welded part immediately after the arc passed at the time of arc welding of a high intensity | strength molten Zn-Al-Mg type plated steel plate. The figure which showed typically the cross-section of the conventional high intensity | strength molten Zn-Al-Mg type plated steel plate welded joint cooled from the state of FIG. The figure which showed typically the cross-section of the high intensity | strength molten Zn-Al-Mg type plated steel plate welded joint of this invention cooled from the state of FIG. The graph which illustrated the influence of the plating adhesion amount W and the welding heat input Q exerted on the coating state in the vicinity of the toe portion and the appearance of the welded portion when arc welding was performed on a molten Zn-Al-Mg plated steel sheet. The figure which showed the welding experiment method for investigating the molten metal embrittlement cracking resistance. The figure which showed typically the shape of the lap fillet weld joint used for the corrosion resistance test. The figure which showed the composite corrosion test (CCT) conditions.

  FIG. 1 schematically illustrates a cross-sectional structure of a welded portion of a lap fillet welded joint. This type of welded joint by arc welding is frequently used in automobile chassis. A base material 1 and a base material 1 ′, which are steel plate members, are arranged so as to overlap each other, a weld bead 2 is formed on the surface of the base material 1 and an end surface of the base material 1 ′, and both members are joined.

2 to 5 schematically show an enlarged cross-sectional structure of a weld joint in a portion corresponding to the vicinity of the toe portion 3 shown in FIG.
FIG. 2 schematically shows a welded joint cross-sectional structure when the steel plate member is a conventional hot-rolled steel plate. An Fe scale 4 is formed on the surface of the base material 1 in the vicinity of the toe portion 3 of the weld bead 2 due to the base material 1 being exposed to high temperature by welding heat input. Since the Fe scale 4 is inferior in adhesion to the base material 1, when coating is performed in such a state, the coating film in the vicinity of the toe portion 3 together with the Fe scale 4 is formed when the automobile is repeatedly vibrated. Easy to peel off from. If the coating film falls off, the corrosion resistance of that part is not ensured, so it is necessary to design a thick wall in anticipation of corrosion. Note that the thickness of the Fe scale 4 in FIG. 2 is exaggerated.

  FIG. 3 schematically shows a cross-sectional state in the vicinity of a high-temperature welded portion immediately after the arc has passed during arc welding of a high-strength molten Zn—Al—Mg-based steel sheet. The surface of the base material 1 was covered with the uniform plating layer 7 before welding, but the molten plating layer-derived metal evaporated and disappeared near the toe portion 3 due to the passage of the arc (plating layer). Evaporation area 9). In the portion where the distance from the toe portion 3 is larger than that, the original plating layer 7 is melted to become the Zn—Al—Mg based molten metal 8, but has not disappeared due to evaporation. When the distance from the toe 3 is further increased, the original plating layer 7 exists without melting. In FIG. 3, the thicknesses of the Zn—Al—Mg-based molten metal 8 and the plating layer 7 are exaggerated.

  FIG. 4 schematically shows a cross-sectional structure of a conventional high-strength molten Zn—Al—Mg-based plated steel plate welded joint cooled from the state of FIG. Fe scale 4 is formed in a region in the vicinity of toe 3 where the plating layer has evaporated during welding (plating layer evaporation region 9 in FIG. 3). Next to this, there is a melt-solidified region 10, in which Zn—Al—Mg-based molten metal (reference numeral 8 in FIG. 3) that remains without being evaporated solidifies. Further, a plating layer unmelted region 11 is present in a region far from the toe 3, and the original plating layer (reference numeral 7 in FIG. 3) is present in this region. Reference numeral 5 denotes a Zn—Al—Mg alloy layer, which is composed of a molten and solidified metal and an original plating layer. An Fe—Al alloy layer 6 is interposed between the Zn—Al—Mg alloy layer 5 and the base material 1. The Fe—Al-based alloy layer 6 is a reaction between Fe as a base material component and Al as a plating component, and is responsible for adhesion between the Zn—Al—Mg-based alloy layer 5 and the base material 1. However, the Fe—Al-based alloy layer 6 in the melt-solidified region 10 grows thicker than that existing under the original plating layer. Conversely, when this layer grows excessively, the plating adhesion deteriorates. Further, when the Fe concentration in the Fe—Al-based alloy layer 6 becomes excessive, the corrosion resistance is impaired. In the cross-sectional structure shown in FIG. 4, the formation of the Fe scale 4 causes peeling of the coating film and deteriorates the corrosion resistance. In FIG. 4, the thicknesses of the Fe scale 4, the Zn—Al—Mg alloy layer 5 and the Fe—Al alloy layer 6 are exaggerated.

  FIG. 5 schematically shows the cross-sectional structure of the high strength molten Zn—Al—Mg based plated steel plate welded joint of the present invention cooled from the state of FIG. In this case, the Zn—Al—Mg-based molten metal 8 in FIG. 3 wets and spreads in the region in the vicinity of the toe portion 3 (plating layer evaporation region 9 in FIG. 3) where the plating layer has evaporated during welding, and the plating layer unmelted region 11. The entire region closer to the toe 3 is a melt-solidified region 10. That is, by optimizing the arc welding conditions as described later, the Zn—Al—Mg based molten metal 8 of FIG. 3 quickly flows to the toe 3 in the cooling process after welding, and the steel of the base material 1. Cover the surface of the substrate. For this reason, the Fe scale 4 shown in FIG. 4 is not formed. In this way, the surface of the base material 1 having the plating layer before welding is continuously covered with the Zn—Al—Mg alloy layer from the plating layer that has not been melted at the time of welding to the weld bead toe. And the chemical composition of the Zn—Al—Mg alloy layer 5 in the vicinity of the toe 3 and the Fe—Al alloy layer 6 existing in the underlayer is adjusted to an appropriate range, Good corrosion resistance in the vicinity is maintained. Such a welded joint is realized by an arc welding condition in an appropriate range shown in FIG. In FIG. 5, the thicknesses of the Zn—Al—Mg alloy layer 5 and the Fe—Al alloy layer 6 are exaggerated.

[Arc welding conditions]
FIG. 6 shows a lap fillet weld of the type shown in FIG. 1 by arc welding using a molten Zn-6 mass% Al-3 mass% Mg plated steel sheet having a thickness of 2 mm and a coating weight of 20 to 250 g / m ² . The result of investigating the influence on the covering state in the vicinity of the toe portion 3 and the appearance of the arc welded portion when the joint is produced and the plating adhesion amount W and the welding heat input Q are variously changed will be exemplified. Welding gas was Ar + 20vol.% CO _2. The welding heat input Q was adjusted by changing the welding current I and the welding speed v. The value of welding heat input Q (J / cm) is determined by the equation (1). A cross section in the vicinity of the weld corresponding to FIG. 5 was observed with a scanning electron microscope. A portion where the Zn-Al-Mg-based alloy layer 5 is formed up to the bead toe 3 and no spatter is generated or is slight, and the plating layer in the vicinity of the bead toe 3 remains disappeared by evaporation ( The case where a Zn—Al—Mg-based alloy layer non-existing portion) was observed was evaluated as x. Further, the appearance of the arc welded portion was visually observed, and a case where it was judged that there was much spatter generation and it did not have a good appearance as a product was marked with Δ. And the thing of (circle) evaluation was determined to be a pass.

  As can be seen from FIG. 6, the Zn—Al—Mg based alloy layer 5 was formed up to the bead toe 3 when the welding heat input Q was within the range of the formula (2). Further, in these, the thickness and chemical composition of the Zn—Al—Mg alloy layer 5 in the vicinity of the toe portion 3 and the chemical composition of the Fe—Al alloy layer 6 existing in the underlayer are within an appropriate range described later. It has been separately confirmed that it has been adjusted and has excellent corrosion resistance in the vicinity of the weld.

On the other hand, in the region where the welding heat input Q is Q <13W + 1140 in FIG. 6, the molten metal derived from the plating layer does not reach the bead toe 3.
On the other hand, in the region where the welding heat input Q is Q> 15W + 8720 in FIG. 6, the bead toe 3 was covered with the Zn—Al—Mg alloy layer 5, but spatter was significantly generated due to excessive heat input. If the spatter is significant, not only the appearance of the coating is impaired, but also the starting point of corrosion and the corrosion resistance decreases. Further, when the welding heat input Q is excessive, the Fe—Al-based alloy layer 6 grows thick and the Fe concentration increases as will be described later, and the Al concentration in the Zn—Al—Mg-based alloy layer 5 decreases or adheres. Incurs a decline.

  As a result of conducting a similar investigation using materials having a plate thickness of 1 mm and 3 mm, it was confirmed that the same result as in FIG. 6 was obtained if the welding heat input Q was within the range of the above equation (2). Although the reason why the plating layer remains lost when the welding heat input Q is smaller than the range of the present invention is not clear, the welding heat input Q is the melting amount of the original plating layer 7 or Zn—Al—. It is presumed that the fluidity of the Mg-based molten metal 8 (FIG. 3) is affected.

In arc welding assuming an automobile chassis member, the plating layer in the vicinity of the toe portion 3 evaporates and disappears usually in a range from the toe portion up to 2 mm. Therefore, the effect of improving the corrosion resistance of the welded portion can be evaluated by the characteristics of the coating layer formed in a region within 2.0 mm from the toe portion 3 (hereinafter, simply referred to as “the 2 mm region in the vicinity of the toe portion”). . In FIG. 5, a 2 mm region in the vicinity of the toe portion is indicated by reference numeral 12.
Hereinafter, “%” in the chemical composition of each alloy layer, plating bath, and original plating plate means “% by mass” unless otherwise specified.

[Zn-Al-Mg-based alloy layer in the 2 mm region near the toe part]
As a result of various studies, the average Al concentration of the Zn-Al-Mg alloy layer 5 is 0.2 to 22.0% and the average Mg concentration is 1.0 to 10.0% in the 2 mm region near the toe portion. There is a need. If the average Al concentration in this region is less than 0.2%, the corrosion resistance improving effect is insufficient, and if it exceeds 22.0%, the corrosion resistance improving effect is saturated. Further, if the average Mg concentration in this region is less than 1.0%, the corrosion resistance improving effect is insufficient, and even if it exceeds 10.0%, the corrosion resistance improving effect is saturated. The composition of the Zn—Al—Mg alloy layer 5 in this region can be controlled by the composition of the plating layer 7 and the welding conditions (particularly, the growth method of the Fe—Al alloy layer 6). In order to optimize the composition of the Zn—Al—Mg-based alloy layer 5, it is advantageous to set the composition of the hot-dip plating layer 7 in the range described later.

  The average thickness of the Zn—Al—Mg alloy layer 5 in the 2 mm region near the toe portion is more preferably 1.0 μm or more. Even if it is thinner than that, the effect of improving the corrosion resistance can be obtained, but considering the life in which the high corrosion resistance is obtained, it is advantageous to be 1.0 μm or more. The upper limit is limited by the original plating layer, but is usually in the range of 40 μm or less. The Zn—Al—Mg alloy layer 5 generated in this region inevitably contains elements such as Ti, B, and Si derived from the original plating layer 7 and component elements (particularly Fe) of the base material 1. It may be included.

[Fe-Al alloy layer in the 2 mm region near the toe part]
The Fe—Al-based alloy layer 6 is generated by an alloying reaction between the Zn—Al—Mg-based molten metal 8 and the base material 1 during cooling after arc welding, and the base material 1 and the Zn—Al—Mg-based alloy layer 5 are formed. Contributes to improved adhesion. However, it is important that the average Fe concentration in the Fe—Al-based alloy layer 6 in the 2 mm region in the vicinity of the toe portion is 70.0% or less. When it exceeded that, it turned out that the Fe-Al type alloy layer 6 becomes weak and adhesiveness falls.

As the thickness of the Fe—Al alloy layer 6 increases, Al necessary for forming the Fe—Al alloy layer 6 is consumed from the Zn—Al—Mg alloy layer 5, and the Zn—Al—Mg alloy is consumed. A state where the thickness of the alloy layer 5 and the Al concentration are insufficient is likely to occur. Further, since the Fe—Al-based alloy layer 6 is essentially brittle, if the thickness is excessively increased, the adhesion of the Zn—Al—Mg-based alloy layer 5 tends to be lowered. As a result of various studies, it is more effective for improving the adhesion that the relationship of the following formula (3) is established in the thickness of each alloy layer in the 2 mm region in the vicinity of the toe portion.
[Average thickness of Fe—Al-based alloy layer 6] / [Average thickness of entire alloy layer composed of Fe—Al-based alloy layer 6 and Zn—Al—Mg-based alloy layer 5] ≦ 0.5 (3)
Note that the Fe—Al-based alloy layer 6 generated in this region inevitably contains elements such as Ti, B, and Si derived from the original plating layer 7 and component elements other than Fe of the base material. I do not care.

[Original molten Zn-Al-Mg alloy plating layer]
In order to adjust the composition of the Zn—Al—Mg-based alloy layer 5 in the 2 mm region in the vicinity of the toe portion to the appropriate range described above, the Al concentration in the original plating layer 7 is 3.0% or more and the Mg concentration is 1. It is advantageous to keep it at 0% or more. In particular, Al tends to be consumed for the growth of the Fe—Al-based alloy layer 6 when the welding input Q is relatively large or the amount of plating adhesion is relatively small. In this case, a significant concentration drop occurs. Sometimes. Specifically, the composition range of the molten Zn—Al—Mg-based alloy plating layer 7 is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0. A suitable target is 0.10%, B: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and inevitable impurities. The chemical composition of the hot dipping layer substantially reflects the composition of the hot dipping bath.

  Al is effective in improving the corrosion resistance of the plated steel sheet, and suppresses the generation of Mg oxide dross in the plating bath. If the Al content of the hot dipping bath is 3.0% or more, these effects can be exhibited, but it is more preferably 4.0% or more. On the other hand, if the Al content exceeds 22.0%, the brittle Fe—Al alloy layer 6 grows excessively on the base of the plating layer 7, which causes a decrease in plating adhesion. In order to ensure excellent plating adhesion, the Al content is preferably 15.0% or less, and may be controlled to 10.0% or less.

  Mg exhibits the effect | action which produces | generates a uniform corrosion product on the surface of a plating layer, and raises the corrosion resistance of a plated steel plate remarkably. The effect is manifested when the Mg content in the hot dipping bath is 0.05% or more. In the present invention, the average Mg concentration of the Zn—Al—Mg alloy layer 5 in the 2 mm region near the toe is 1.0. Therefore, it is advantageous that the Mg concentration in the plating layer 7 is also 1.0% or more. On the other hand, if the Mg content exceeds 10.0%, Mg oxide-based dross tends to occur, and it becomes difficult to obtain a high-quality plating layer.

  When Ti and B are contained in the hot dipping bath, there are advantages such as an increase in the degree of freedom of manufacturing conditions during hot dipping. For this reason, 1 type or 2 types of Ti and B can be added as needed. It is more effective to add 0.0005% or more in the case of Ti and 0.0001% or more in the case of B. However, when the content of Ti or B in the plating layer becomes excessive, it causes a poor appearance of the plating layer surface due to the formation of precipitates. When Ti is added to the plating bath, the range is 0.10% or less, and when B is added, the range is 0.05% or less.

  When Si is contained in the hot dip plating bath, excessive growth of the Fe—Al alloy layer 6 formed at the interface between the base material 1 and the plating layer 7 is suppressed, and the workability of the hot dip Zn—Al—Mg plated steel sheet is reduced. This is advantageous for improvement. Therefore, Si can be contained as necessary. In that case, it is more effective to set the Si content of the hot dipping bath to 0.005% or more. However, since excessive Si content increases the amount of dross in the hot dipping bath, the Si content in the plating bath is limited to 2.0% or less.

  In general, it is unavoidable that Fe is mixed in the hot dipping bath because the steel sheet is immersed and passed. The Fe content in the Zn-Al-Mg plating bath is generally allowed to be up to about 2.0%.

It is preferable that the coating amount (per one side) of the molten Zn—Al—Mg-based plated steel sheet is 20 to 250 g / m ² . By setting the plating adhesion amount to 20 g / m ² or more per side, a welding condition is set to ensure the amount of Zn—Al—Mg based molten metal 8 (FIG. 3) sufficient to wet and spread to the bead toe during arc welding. It's easy to do. However, if the plating adhesion amount exceeds 250 g / m ² , blow holes are likely to occur during welding. When blow holes occur, the welding strength decreases.

[Plating plate]
As a hot-dip Zn—Al—Mg-based plated steel sheet for automobile chassis, it is particularly suitable that it has high strength and excellent resistance to molten metal embrittlement cracking. As such a steel plate, in mass%, C: 0.05-0.25%, Si: 0.10-1.50%, Mn: 1.00-2.50%, Al: 0.010-0 .100%, Ti: 0.000 to 0.100%, B: 0.0001 to 0.0100%, and if necessary, one or more of Nb, Cr, Mo and P are further added to a total of 1.00%. It is preferable to use a steel type that is contained in the following range and includes the remaining Fe and inevitable impurities for the plating base plate. Each component element will be briefly described.

  C is an element necessary for increasing the strength. If it is less than 0.05%, it becomes difficult to obtain a tensile strength of 590 MPa or more. If it exceeds 0.25%, the ductility is lowered and the press formability is deteriorated.

  Si is an element effective for increasing the strength without reducing ductility. If it is less than 0.10%, the strength improving effect is small, and if it exceeds 1.50%, Si may be concentrated on the steel sheet surface during reduction heating in the hot dipping line to become an oxide, which may impair plating properties.

  Mn is effective for increasing the strength, similar to Si. If it is less than 1.00%, the effect of improving the strength is small, and if it exceeds 2.50%, Mn concentrates on the surface of the steel sheet during reduction heating in the hot dipping line and becomes an oxide, which tends to hinder plating properties.

  Al is added as a deoxidizer during steelmaking. If the content is less than 0.010%, the deoxidation effect is small, and if it exceeds 0.100%, it may be concentrated on the surface of the steel sheet during reductive heating in the same manner as Si and Mn and become an oxide, which may impair the plating properties.

  Ti is an element that contributes to strength improvement by precipitation strengthening of carbides. If it is less than 0.010%, the effect of improving the strength is small, and if it exceeds 0.100%, the recrystallization temperature becomes remarkably high.

  B segregates at the austenite grain boundary in the heat-affected zone during arc welding, and exhibits an effect of suppressing molten metal embrittlement cracking due to the molten Zn—Al—Mg-based plating layer. If it is less than 0.0001%, the effect of suppressing molten metal embrittlement cracking is small, and even if added over 0.0100%, the effect is saturated and the cost is increased.

  Nb, Cr, Mo, and P, like B, segregate at the austenite grain boundary in the heat-affected zone during arc welding and exhibit an effect of suppressing molten metal embrittlement cracking. For this reason, what is necessary is just to employ | adopt the steel type containing 1 or more types of these elements as needed. It is more effective that the total content of these elements is 0.01% or more. However, even if these total contents exceed 1.0%, the effect is saturated and the cost is increased. For individual elements, Nb is more preferably in the range of 0.10% or less, and more preferably in the range of 0.01 to 0.10%. Cr is more preferably in the range of 0.70% or less, and more preferably in the range of 0.10 to 0.70%. Mo is more preferably in the range of 0.50% or less, and more preferably in the range of 0.10 to 0.50%. P is more preferably in the range of 0.50% or less, and more preferably 0.05 to 0.50%.

[Thickness of plated steel sheet]
In consideration of automotive chassis applications, when a plating original plate having the above chemical composition is used, if the plate thickness is less than 1.0 mm, the fatigue strength of the arc welded portion tends to be insufficient. On the other hand, if the plate thickness exceeds 3.0 mm, it is necessary to increase the welding heat input Q, and sputtering is likely to occur. If it is 3.0 mm or less, it will become advantageous with respect to the conventional hot-rolled steel plate material also from a viewpoint of weight reduction.

[Arc welding]
The method of arc welding is not particularly limited except for the conditions defined in the present invention. In the arc welding method, a mixed gas of Ar, CO ₂ or Ar + CO ₂ is generally used as a shielding gas, but in the present invention, the kind and composition of the shielding gas are not particularly limited. Also, the welding wire is not particularly limited, and YGW12 for mild steel, YGW14 for Zn plating, YGW21 for 590-780 MPa class high-tensile steel, or the like may be used.

  A cold-rolled steel strip having a thickness of 2 mm and a width of 1000 mm having the composition shown in Table 1 is passed through a hot dipping line to produce a hot-dip Zn-Al-Mg plated steel plate under the plating conditions shown in Table 2, and mechanical properties And the molten metal embrittlement cracking property was investigated.

[Test method for mechanical properties]
A JIS No. 5 tensile test piece was taken in parallel with the rolling direction and subjected to a tensile test at room temperature in accordance with JIS Z2241, and 0.2% yield strength (YS), tensile strength (TS), and total elongation (T.El) were measured. .

[Test method for molten metal embrittlement crack resistance]
As shown in FIG. 7, a steel bar boss (protrusion) 15 having a diameter of 20 mm and a length of 25 mm was vertically set at the center of a 100 mm × 75 mm test piece 14 and arc-welded. One round of the circumference of the boss 15 was made from the welding start point, and after the welding start point was passed, the bead was further piled up and the welding proceeded. During welding, the test piece 14 was held on a flat plate. This test was conducted in a situation where welding cracks are likely to occur experimentally. The welding conditions are as follows.
(Arc welding conditions)
・ Welding current: 160A
・ Arc voltage: 20V
-Welding speed: 0.4 m / min
・ Welding wire: YGW14
Shield _{gas:. Ar-20vol% CO 2} , flow rate 20L / min

After welding, the cut surface 20 passing through the central axis of the boss 15 and passing through the bead overlap portion 17 is observed most in the test piece 14 by observing the test piece 14 near the bead overlap portion 17 with an optical microscope. The depth of the deep crack (maximum crack depth) was measured. This crack is judged to be a “molten metal embrittlement crack”.
These test results are shown in Table 3.

  As can be seen from Table 3, the hot-dip Zn-Al-Mg alloy-plated steel sheet using steels A to L as the plating base plate has a tensile strength of 590 MPa or more and exhibits excellent molten metal embrittlement cracking resistance. Steels A to L have the aforementioned chemical composition. Therefore, when emphasizing the resistance to molten metal embrittlement cracking, it is advantageous to employ a plating base plate having the above-described chemical composition.

  The cold-rolled steel strips (plate thickness 2 mm, plate width 1000 mm) of steels A, C, E, F, H, I, K, and L used in Example 1 were passed through a hot dipping line and shown in Table 4. As shown in FIG. 8, a hot-dip Zn—Al—Mg-based plated steel sheet is produced under various plating conditions, a 100 mm × 100 mm sample is taken from the obtained plated steel sheet, and two samples of the same kind of plated steel sheet are schematically shown in FIG. Joined by lap fillet arc welding. The welding conditions were adjusted to various welding heat inputs Q by changing the welding current and arc voltage within the range shown in Table 5. For the Zn—Al—Mg-based alloy layer 5 and the Fe—Al-based alloy layer 6 in the 2 mm region near the toe portion of the obtained welded joint sample, average thickness measurement and elemental analysis were performed. In addition, the spatter generation state of the welded part and the corrosion resistance of the welded part were examined.

[Method of assuming the average thickness of each alloy layer in the 2 mm area near the toe portion]
About the cross section perpendicular to the weld bead of the welded joint sample, a portion corresponding to the cross section schematically shown in FIG. 5 was observed with an SEM, and the distance from the bead toe 3 was approximately 0.1 mm to 1.9 mm. Cross-sectional photographs (magnification 2000 to 5000 times) of 10 fields are taken at equal intervals, the thicknesses of the Zn—Al—Mg alloy layer 5 and the Fe—Al alloy layer 6 at each position are measured, and the average value thereof is measured. Is the average thickness of the Zn—Al—Mg alloy layer and the Fe—Al alloy layer in the 2 mm region near the toe portion.

[Elemental analysis method for each alloy layer in the 2 mm area near the toe]
At each position where the above thickness measurement (cross-section photography) was performed, using an energy dispersive X-ray fluorescence spectrometer attached to the SEM, the Al concentration and Mg concentration in the Zn—Al—Mg alloy layer 5, and The Fe concentration in the Fe—Al-based alloy layer 6 was measured, and the average value of each field of view was defined as the concentration of each element in the 2 mm region near the toe portion.

[Investigation method for spatter generation in welds]
The appearance of the arc welded part is visually observed, and the occurrence of spatter is not observed or is slight, so that it is judged as having a sufficiently good appearance as a product (good), otherwise x Judged (bad).

[Method of testing corrosion resistance of welds]
The above welded joint sample was subjected to surface adjustment and phosphate treatment under the conditions shown in Table 6, and was subjected to cationic electrodeposition coating under the conditions shown in Table 7. A sample subjected to cationic electrodeposition coating was subjected to a fatigue test under the test conditions of a stress of 50 N / mm ^{2 in} the direction perpendicular to the welding direction and a test number of 1 × 10 ⁵ times in order to simulate fatigue due to vibration. It used for the composite corrosion test (CCT) of the conditions to show, and investigated the presence or absence of red rust generation | occurrence | production after CCT250 cycle. Those in which the occurrence of red rust was not observed in the welded portion were judged as ◯ (good), and the others were judged as x (bad).
The results are shown in Table 8.

  As shown in Table 8, test Nos. 1 to 15 according to the present invention are continuously covered with a Zn—Al—Mg alloy layer up to the weld bead toe, and the occurrence of spatter and the corrosion resistance of the weld Was good.

  On the other hand, Comparative Examples No. 21 and 22 were continuously covered with the Zn—Al—Mg alloy layer up to the weld bead toe, but the Zn—Al—Mg alloy layer in the 2 mm region near the toe. Since the Al concentration and Mg concentration of 5 are low, and the Fe concentration of the Fe—Al based alloy layer 6 is high, it peels locally at the interface between the Fe—Al based alloy layer 6 and the base material 1 in the fatigue test. It was inferior to the corrosion resistance of the weld. Nos. 23, 25, 27, 29, and 31 have insufficient welding heat input Q, so that wetting and spreading of the plating layer-derived metal melted at the time of welding becomes insufficient, and the Zn-Al-Mg alloy layer absent portion And Fe scale 4 was formed in that portion. As a result, in the fatigue test, it peeled off at the interface between the Fe scale 4 and the base material 1 and was inferior in the welded portion corrosion resistance. In Nos. 24, 26, and 28, a large amount of spatter was generated because the welding heat input Q was excessive. Further, the Fe concentration of the Fe—Al based alloy layer 6 was increased, and it was locally peeled at the interface between the Fe—Al based alloy layer 6 and the base material 1 in the fatigue test, resulting in poor corrosion resistance of the welded portion. In Nos. 30 and 32, since the welding heat input Q was considerably high, the occurrence of spatter was remarkably increased, and although no red rust was generated in the welded portion, white rust was generated starting from the portion where the spatter adhered.

DESCRIPTION OF SYMBOLS 1, 1 'Base material 2 Weld bead 3 Toe part 4 Fe scale 5 Zn-Al-Mg type alloy layer 6 Fe-Al type alloy layer 7 Plating layer 8 Zn-Al-Mg type molten metal 9 Plating layer evaporation area 10 Melt-solidified region 11 Plating layer unmelted region 12 Toe vicinity 2 mm region 14 Test piece 15 Boss 16 Weld bead 17 Bead overlap portion 18 Molten Zn-Al-Mg alloy-plated steel sheet 19 Weld bead 20 Cut surface

Claims (5)

A member having a joined portion obtained by joining molten Zn—Al—Mg alloy-plated steel plate members having a plate thickness of 1.0 to 3.0 mm by arc welding,
The surface of the steel plate having a plating layer before welding is continuously covered with a Zn-Al-Mg alloy layer up to the weld bead toe, and between the Zn-Al-Mg alloy layer and the steel substrate. Has an Fe-Al alloy layer,
In the steel plate surface layer portion whose distance from the weld bead toe is within 2 mm, the Zn—Al—Mg-based alloy layer has an average Al concentration of 0.2 to 22.0 mass% and an average Mg concentration of 1.0 to 10. An automobile chassis member having 0 mass% and an Fe-Al alloy layer having an average Fe concentration of 70.0 mass% or less.   The composition of the molten Zn—Al—Mg alloy plating is mass%, Al: 3.0 to 22.0%, Mg: 0.05 to 10.0%, Ti: 0 to 0.10%, B The automobile chassis member according to claim 1, comprising: 0 to 0.05%, Si: 0 to 2.0%, Fe: 0 to 2.0%, the balance Zn and unavoidable impurities.   The plating base plate of the hot-dip Zn—Al—Mg alloy-plated steel sheet is in mass%, C: 0.05 to 0.25%, Si: 0.10 to 1.50%, Mn: 1.02 to 2. 50%, Al: 0.010 to 0.100%, Ti: 0.010 to 0.100%, B: 0.0001 to 0.0100%, the balance Fe and unavoidable impurities. Or an automobile chassis member according to 2;   The automobile chassis member according to claim 3, wherein the plating base plate further contains at least one of Nb, Cr, Mo, and P in a total range of 1.00% or less. When arc welding the molten Zn-Al-Mg alloy-plated steel sheet members, using a molten Zn-Al-Mg alloy-plated steel sheet member having a plating adhesion amount of 20 to 250 g / m ² per side, The manufacture of an automobile chassis member according to any one of claims 1 to 4, wherein the welding heat input Q (J / cm) represented by the formula (1) is controlled within a range of the following formula (2) according to the amount of plating adhesion. Law.
Q = (I × V) / v (1)
13W + 1140 ≦ Q ≦ 15W + 8720 (2)
However, I: Welding current (A), V: Arc voltage (V), v: Welding speed (cm / sec), W: Plating adhesion amount per side (g / m ² )

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