JP2009299139A - Steel material to be joined to dissimilar material, joined body of dissimilar materials, and method for joining dissimilar materials - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a joined body of dissimilar materials, which is formed by joining a steel material to an aluminum alloy material and gives few restrictions to an application condition for spot welding or the like, is excellently versatile, and also can obtain a joint part having a high joining strength without hindering the reliability of a joint by producing a weak reactive layer (intermetallic compound layer) in the joint part, and to provide a method for joining the dissimilar materials. <P>SOLUTION: This joined body is a joined body of the dissimilar materials of the steel material and the aluminum alloy material. The steel material to be joined has a particular composition and has an outer oxide layer on the surface and an oxide in the inner part each having a particular composition. On the other hand, the aluminum alloy material to be joined is an Al-Mg-Si-based aluminum alloy having a particular composition. In a joined interface of the aluminum alloy material side of the joined body of the dissimilar materials, an Fe content is regulated. Then, the joined body of the dissimilar materials has a reactive layer of Fe and Al formed in the joined interface thereof, and acquires a high joining strength. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a steel for dissimilar material joining with an aluminum alloy material, a dissimilar material joined body obtained by joining a steel material and an aluminum alloy material, and a dissimilar material joining method, which can obtain high joint strength.

  In recent years, with respect to global environmental problems caused by exhaust gas and the like, improvement in fuel efficiency has been pursued by reducing the weight of the body of a transport aircraft such as an automobile. In addition, it has been pursued to improve safety at the time of automobile body collision without hindering the weight reduction as much as possible. For this reason, in particular, aluminum alloy materials that are lighter in weight and superior in energy absorption are being increasingly used in place of steel materials that have been used in the past for automobile body structures. Here, the aluminum alloy material is a general term for aluminum alloy rolled plate materials, extruded materials, forged materials, and the like.

  For example, panels such as outer panels (outer plates) and inner panels (inner plates) of panel structures such as automobile hoods, fenders, doors, roofs, trunk lids, etc. are made of Al-Mg-Si AA to JIS6000 series. The use of aluminum alloy plates such as AA to JIS5000 (hereinafter simply referred to as 5000), such as Al-Mg based (hereinafter simply referred to as 6000), is being studied.

  In addition, as energy absorbers or reinforcements such as bumper reinforcements (also called bumper reinforcements or bumper armatures) or door reinforcements (also called door guard bars or door beams) to ensure the safety of automobile body collisions Aluminum alloy extruded profiles such as Al-Zn-Mg AA to JIS 7000 (hereinafter simply referred to as 7000) and the 6000 alloy are used. Furthermore, the aluminum alloy forging material of the 6000 series alloy is used for automobile undercarriage parts such as suspension arms.

  These aluminum alloy materials are inevitably used by being joined to steel materials (steel members) such as steel plates and mold steels that are generally used in ordinary automobile bodies unless they are all-aluminum automobile bodies. Therefore, when an aluminum alloy material is used for an automobile body (a member combining a steel material and an aluminum alloy material), this inevitably also involves dissimilar joining of Fe-Al (dissociation between dissimilar metal members of iron-aluminum). ) Is necessary.

  However, as a problem when welding this Fe-Al dissimilar material by welding, the formation of an intermetallic compound layer (hereinafter also referred to as a reaction layer) of high hardness and very brittle Fe and Al at the joint interface. is there. For this reason, even if they are apparently bonded to each other, due to the formation of the present compound layer, sufficient bonding strength cannot often be obtained for the dissimilar material joined body by Fe—Al dissimilar material welding by welding.

  Reflecting this, conventionally, these dissimilar material joined bodies (joints of dissimilar metal members) are joined not only by welding, but also by using bolts, rivets, etc., or using an adhesive, There are problems such as complexity of the joining operation and an increase in joining cost.

  In view of this, conventionally, welding by efficient spot welding, which is widely used for joining automobile bodies, has been studied as a welding method for joining different materials of Fe-Al. For example, a method of inserting an aluminum-steel clad material between an aluminum material and a steel material has been proposed. In addition, methods have been proposed in which a metal having a low melting point is plated or inserted on the steel material side. Furthermore, a method of sandwiching insulator particles between an aluminum material and a steel material, a method of providing unevenness in a member in advance, and the like have been proposed. Further, after removing the non-uniform oxide film of the aluminum material, it is heated in the atmosphere to form a uniform oxide film, and in a state where the contact resistance of the aluminum surface is increased, two layers of aluminum-steel are formed. A method of spot welding using a steel plate as an insert material has also been proposed.

  On the other hand, when elements that easily form oxides such as Si, Mn, and Al are added on the steel material side to increase the strength of the steel sheet, the surface of the base metal contains oxides containing these Si, Mn, and Al. Is known to form. It is also known that these oxides containing Si, Mn, Al, etc. inhibit the adhesion between the surface coating such as galvanization and the steel sheet. On the other hand, if the thickness of the oxide layer containing Si, Mn, Al, etc. is in the range of 0.05 to 1 μm by pickling the steel plate, the adhesion between the surface coating such as galvanization and the steel plate and It is also known that spot weldability between steel plates is improved (see Patent Document 1).

  However, in these conventional technologies, sufficient joint strength is obtained for the dissimilar welded joint of Fe-Al under the efficient spot welding joining conditions that are commonly used for joining automobile bodies. I can't. In other words, the spot welding conditions for obtaining the joint strength must be complicated, which is not realistic.

  On the other hand, in particular, there is also a technique intended for spot welding of a dissimilar joint between a 6000 series aluminum alloy material and the like, which are widely used for automobile bodies, and a high-strength steel plate (high-tensile material) having a tensile strength of 450 MPa or more. Various proposals have been made.

  For example, in Patent Documents 2 and 3, spot welding is performed in such a manner that two or more steel materials are stacked or steel materials with a plate thickness limited to 3 mm or less or steel materials are sandwiched between aluminum alloy materials. Proposed. Patent Document 4 proposes to improve the joint strength by defining the nugget area and the thickness of the interface reaction layer in the spot weld. In Patent Documents 5 and 6, it is proposed that the composition, thickness, area, and the like of each generated compound on the steel material side and the aluminum alloy material side at the welding interface are each finely defined to improve the joint strength. .

Further, in Patent Document 7, in a high-strength steel sheet having a specific composition, an external oxide layer newly formed after once removing an existing oxide layer on the steel sheet surface is oxidized with specific ratios of Mn and Si compositions. In addition, the ratio of the internal oxide containing Mn and Si in a total amount of 1 at% or more in the steel region having a depth of 10 μm or less from the steel dough surface of this steel material is specified, and an appropriate spot It has been proposed to obtain a high joint strength of a dissimilar material joined body under welding conditions. In this patent document 7, diffusion of Fe and Al during spot welding is suppressed by a newly formed outer oxide layer containing Si, Mn, and the like, and an inner oxide layer immediately below the surface of the steel material, thereby joining the interface. This suppresses excessive formation of an Al—Fe-based brittle intermetallic compound layer. Incidentally, in Patent Document 7, the welding technique is not limited, and different material joining is performed by spot welding as Example 1, laser welding as Example 2, and MIG welding as Example 3.
JP 2002-294487 A JP 2007-144473 A JP 2007-283313 A JP 2006-167801 A JP 2006-289552 A JP 2007-260777 A JP 2006-336070 A

  These Patent Documents 2 to 7 are commonly intended for spot welding of a dissimilar joint of a 6000 series aluminum alloy material and a high-strength steel sheet, have few restrictions such as application conditions, and have excellent versatility. The purpose is to improve the bonding strength by suppressing the formation of fragile intermetallic compounds.

  However, even in these Patent Documents 2 to 7, there is still room for improvement in terms of improving the bonding strength and the like, particularly with respect to spot welding of a dissimilar joint of a 6000 series aluminum alloy material and a high-strength steel plate. In particular, Patent Document 7 suppresses diffusion of Fe and Al during spot welding by using an outer oxide layer containing Si, Mn and the like newly formed on the steel surface and an inner oxide layer immediately below the surface of the steel material. Thus, it is effective in that excessive formation of an Al—Fe brittle intermetallic compound layer at the bonding interface is suppressed. However, the peel strength measured by the cross tensile test piece of this dissimilar material joined body is at most less than 2 kN, and there is still room for improvement in order to obtain a joint strength of 2 kN or more.

  The present invention has been made in view of such problems, and its purpose is that there are few restrictions such as spot welding application conditions, excellent versatility, and a brittle intermetallic compound or the like is generated at the joint. An object of the present invention is to provide a steel for dissimilar material joining, a dissimilar material joined body, and a dissimilar material joining method capable of obtaining a joint having high joining strength without impeding the reliability of joining.

(Summary of steel for joining different materials)
The gist of the steel for joining different materials of the present invention for achieving this object is a steel material for joining different materials with a 6000 series aluminum alloy material, and the composition of this steel material is expressed by mass%, C: 0.01-0. 30%, Si: 0.1 to 3.00%, Mn: 0.1 to 3.00%, P: 0.10% or less (including 0%), S: 0.05% Below (including 0%), N: 300 ppm or less (including 0%), respectively, and the oxides present in the steel region up to 20 μm in depth from the steel dough surface are crystal grains. The ratio of the oxide present in the boundary and the oxide present in the crystal grains containing 1 at% or more in total amount of Mn and Si is 5% or more and less than 20% as an average area ratio in this steel region Yes, the total amount of Mn and Si present on the steel surface is 1 at% or less. The ratio of the outer oxide to be included is 0.1% or more and 50% as an average ratio of the total length of the oxide to the length of 1 μm in the substantially horizontal direction of the interface between the steel base and the outer oxide layer. Is less than.

(Summary of dissimilar materials)
In addition, the gist of the present invention, the dissimilar material joined body for achieving the above object, is a dissimilar material joined body of the dissimilar material joining steel material and the aluminum alloy material of the above gist, wherein the aluminum alloy material is in mass%, Mg: Made of 6000 series aluminum alloy containing 0.1 to 3.0%, Si: 0.1 to 2.5%, and Cu: 0.001 to 1.0%, respectively, the aluminum alloy material side of the dissimilar material joined body The Fe content at the bonding interface is 2.0 mass% or less, and a reaction layer of Fe and Al is formed at the bonding interface.

(Summary of different material joining method)
Further, the gist of the dissimilar material joining method of the present invention for achieving the above object is a dissimilar material joining method between a steel material and an aluminum alloy material, and is a dissimilar material joining method between a steel material and an aluminum alloy material that are welded to each other. A steel material for joining different materials of the gist and 6000 series containing Mg: 0.1 to 3.0%, Si: 0.1 to 2.5%, and Cu: 0.001 to 1.0% in mass%. This is spot welding or friction spot joining (friction stir welding) with an aluminum alloy material made of an aluminum alloy.

(Configuration of external oxide layer)
Here, the balance other than the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer in the present invention is an oxide whose content of Mn and Si is less than 1 at% in the total amount. The outer oxide layer in the present invention is composed of an oxide containing Mn and Si in a total amount of 1 at% or more, an oxide containing Mn and Si in a total amount of less than 1 at%, and a void. The

(Preferred embodiment of the present invention)
The dissimilar material joined body is spot-welded, and the nugget depth direction of the intermetallic compound layer of Fe and Al formed at the joint interface between the steel material and the aluminum alloy material as a condition for each spot welding location Is preferably in the range of 0.1 to 3 μm, and the formation range of the intermetallic compound layer of Fe and Al is preferably 70% or more of the spot welding joint area. Moreover, it is preferable that the peel strength measured by the cross tensile test piece of the dissimilar material joined body is 2 kN or more. Moreover, it is preferable that the said dissimilar material joining body is a body structure of a motor vehicle. Furthermore, as a condition for each joint portion between the steel material and the aluminum alloy material, an interelectrode current of 10 to 35 kA is applied at an applied pressure between electrodes of 2.0 to 3.0 kN, and the thickness tmm of the aluminum alloy material portion to be welded. Therefore, it is preferable that the steel material and the aluminum alloy material are spot-welded by energizing for a time of 200 × tmsec or less.

The present invention allows diffusion of Fe and Al during spot welding by both the outer oxide layer containing Mn and Si on the surface of the steel material and the inner oxide layer containing Mn and Si immediately below the surface of the steel material. This is the same as Patent Document 7 in that it suppresses excessive generation of an Al—Fe-based brittle intermetallic compound layer at the bonding interface. However, the main difference from Patent Document 7 is that the proportion of the external oxide (layer) containing Mn and Si in the external oxide layer is less than that of Patent Document 7 as described in the above gist. . At the same time, the inner oxide layer containing Mn and Si is made deeper and more present from the steel material surface.

  In particular, in spot welding of a dissimilar joined body of a 6000 series aluminum alloy material and a steel material, welding is performed under such a condition that only the aluminum alloy material side is melted without melting the steel material side. In this case, the present invention has found that the diffusion of Fe and Al during spot welding is greatly influenced by the quantitative composition balance between the outer oxide layer and the inner oxide layer. In other words, the appropriate composition balance condition between the outer oxide layer and the inner oxide layer differs depending on the alloy composition (type) of the aluminum alloy material to be joined, and the balance between the outer oxide layer and the inner oxide layer is optimized. For the first time, the diffusion of Fe and Al during spot welding can be effectively suppressed. And the effect which suppresses the excessive production | generation of the Al-Fe-type brittle reaction layer (intermetallic compound layer) in a joining interface becomes higher.

  On the other hand, in the said patent document 7, although 6000 series aluminum alloy material is made into the Example, there is no limitation in the combination of the welding method and the material of the aluminum alloy material and steel material to be welded. In other words, the above-mentioned patent document 7 describes the same conditions for the outer oxide layer and the inner oxide layer [FIG. 1 (described later), even if the welding technique and the combination of materials of the aluminum alloy material and the steel material to be welded are different. It is going to join by the condition of b). As a result, under the conditions of the outer oxide layer and the inner oxide layer specified in Patent Document 7, in particular, in the case of a dissimilar material joint in spot welding of a 6000 series aluminum alloy material and a steel material, as described later, The balance between the oxide layer and the internal oxide layer is deteriorated. For this reason, as disclosed in Patent Document 7, in particular, the peel strength measured by a cross tensile test piece of a dissimilar material joint in spot welding of a 6000 series aluminum alloy material and a steel material is at most 2 kN. It must be lower and lower. On the other hand, if the quantitative composition balance between the outer oxide layer and the inner oxide layer is appropriately controlled [to the condition of FIG. 1 (c) described later], in particular, the 6000 series aluminum alloy material and the steel material. With regard to spot welding of the dissimilar material joined body, a high joint strength of 2 kN or more can be obtained.

  In the present invention, similar to Patent Document 7, the existing oxide layer on the steel surface is once removed by pickling or the like, and further annealed in an atmosphere in which the oxygen partial pressure is controlled. The newly formed outer oxide layer existing on the steel material surface of the steel material is a target.

  In this respect, it is also common with Patent Document 1 in which the oxide layer on the steel material surface is once removed by pickling or the like. However, in Patent Document 1, as in the present invention, annealing is performed in an atmosphere in which the oxygen partial pressure is controlled, and the formation ratio of the outer oxide layer and the inner oxide layer depth are not actively controlled. . For this reason, in the outer oxide layer of Patent Document 1, the proportion of the oxide containing 1 at% or more of the total amount of Mn and Si as defined in the present invention is approximately the interface between the steel base and the outer oxide layer. As an average ratio of the total length of the oxide to the horizontal length of 1 μm, it easily exceeds 80%. As a result, in Patent Document 1, a reaction layer (intermetallic compound layer of Fe and Al) is not sufficiently formed particularly in spot welding of a dissimilar joint of a 6000 series aluminum alloy material and a steel material. Metallurgical joining in the joined body is not possible.

  Unlike spot welding between steel materials, in particular, in the case of dissimilar material joining in which 6000 series aluminum alloy material and steel material are spot welded, welding is performed under the condition that only the aluminum alloy material side is melted without melting the steel material side. In welding under such conditions, as described above, a highly hard and very brittle intermetallic compound layer of Fe and Al is formed at the joint interface. For this reason, the welding mechanism is completely different from the spot weldability between steel materials, which is the subject of Patent Document 1, and welding of dissimilar metals becomes extremely difficult.

  More specifically, when joining dissimilar materials of steel and aluminum, the steel has a higher melting point, higher electrical resistance and lower thermal conductivity than the aluminum, so the heat generation on the steel side is increased. Low melting point aluminum melts. In welding under the condition that only the aluminum alloy material side does not melt, such as spot welding of aluminum alloy material and steel material, only the aluminum alloy material side melts, the steel material side does not melt, and Fe from this steel material side does not melt. When diffused, an Al-Fe-based brittle reaction layer is formed at the interface.

  For this reason, in order to obtain high joint strength in welding under conditions where only the aluminum alloy material side is melted without melting the steel material side, the Al-Fe-based reaction layer needs to be minimized. is there. However, the outer oxide layer on the steel material side is not destroyed, and the diffusion of Fe from the steel material side and the generation of the Al—Fe-based reaction layer are suppressed too much, and the formation area of the reaction layer with respect to the total area of the joint is small. Even if it is too high, metallurgical bonding cannot be performed, so that high bonding strength cannot be obtained. Therefore, in order to realize a high bonding strength, it is necessary to form an Al—Fe reaction layer having a minimum thickness necessary for metallurgical bonding in a wide range as much as possible.

  In this way, particularly in the case of dissimilar material joining in which spot welding is performed between a 6000 series aluminum alloy material and a steel material, the welding mechanism is completely different from spot welding between steel materials, and it is remarkable that high joint strength between dissimilar metals is realized. It becomes difficult.

  On the other hand, the outer oxide layer and the inner oxide layer containing the newly generated Mn, Si, etc., as in the present invention, are balanced with each other as described above. In the case of dissimilar materials joining spot welding 6000 series aluminum alloy materials and steel materials, the excessive generation of the reaction layer is suppressed, and Al having the minimum thickness necessary for metallurgical joining is present. The effect of forming the -Fe reaction layer over a wide range at the joint is exhibited. As a result, in the case of welding dissimilar material joining under the condition that only the aluminum alloy material side is melted without melting the steel material side, such as spot welding of a 6000 series aluminum alloy material and steel material, a high joint of 2 kN or more Strength can be realized.

(Oxide composition of steel)
Hereinafter, a specific quantitative composition balance between the outer oxide layer and the inner oxide layer, which is characteristic of the present invention, will be described.

  FIGS. 1A to 1C schematically show oxides (steel cross-sectional structures) on steel surfaces containing Mn and Si, which are once pickled and then annealed in atmospheres controlled to different oxygen partial pressures. Indicate. FIG. 1A shows the case where annealing is performed in a low oxygen partial pressure (low dew point) atmosphere. FIG. 1 (b) shows the case where annealing is performed in an atmosphere of medium oxygen partial pressure (relatively high dew point). FIG. 1 (c) shows the case of annealing in a high oxygen partial pressure (high dew point) atmosphere. Among these, FIG.1 (c) shows the specific quantitative composition balance of the external oxide layer and internal oxide layer which are the characteristics of this invention.

FIG. 1 (a):
In the case of the low oxygen partial pressure atmosphere annealing shown in FIG. 1 (a), the steel material containing Mn and Si, which has been once pickled and the existing outer oxide layer is removed, has a thin outer surface with a steel material surface of about 50 nm. Covered by an oxide layer. However, since the oxygen partial pressure is low, oxygen does not enter (diffuse) into the steel material, and internal oxides including grain boundary oxides are not formed in the steel material below from the steel material surface.

  This external oxide layer is the same as that shown in FIG. 1B and FIG. 1C described later, and is an oxide newly generated by the annealing after the existing oxide layer is removed. A layer, Mn and Si are concentrated, an oxide comprising Mn and Si in a total amount of 1 at% or more, an oxide made of Fe oxide having a total amount of Mn and Si of less than 1 at%, and It is composed of voids. The oxide containing 1 at% or more of Mn and Si is typically composed of an oxide made of Mn2SiO4, SiO2, or the like. The oxide having a total content of Mn and Si of less than 1 at% is typically composed of an oxide made of Fe3O4, for example.

  In the case of FIG. 1 (a), since the outer oxide covers the entire surface of the steel material of the steel material, the ratio of the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer Is as high as 80 to 100% as an average ratio of the total length of the oxide to the length of 1 μm in the substantially horizontal direction of the interface between the steel material and the outer oxide layer. Therefore, such an external oxide layer has a larger proportion of oxide containing 1 at% or more of Mn and Si in total than the external oxide layers of FIGS. 1B and 1C described later. More difficult to destroy. In the case of the outer oxide layer shown in FIG. 1A, the inner oxide is inevitably reduced. Therefore, for example, the internal oxide existing in the steel region having a depth of up to 20 μm from the surface of the steel dough is an intragranular oxide containing 1 at% or more in total of oxide existing at the grain boundary and Mn and Si. The average area ratio is 0% or even less than 5%.

FIG. 1 (b):
On the other hand, in the case of atmospheric annealing in FIG. 1 (b) where the oxygen partial pressure is relatively higher than that in FIG. 1 (a) and has an intermediate oxygen partial pressure, oxygen penetrates (diffuses) into the steel material. For this reason, the steel material containing Mn and Si once pickled to remove the existing outer oxide layer is relatively shallow inside the steel material from the surface of the steel material, together with the outer oxide layer described above, for example, An internal oxide is formed in a steel region having a depth of 10 μm or less from the surface of the steel material. Even if the welding method and the combination of materials of the aluminum alloy material and the steel material to be welded are different in Patent Document 7, the conditions for the same outer oxide layer and inner oxide layer are as shown in FIG. It is the condition of b).

  Among these internal oxides, the oxides generated in the grains are commonly composed of oxides containing 1 at% or more of Mn and Si, including SiO 2 and Mn 2 SiO 4, including FIG. It is an oxide such as Fe3O4 in which the total amount of spherical or granular oxide, Mn and Si is less than 1 at%. At this time, including FIG. 1C described later, in common, grain boundary oxides are also formed on the grain boundaries of the steel. These grain boundary oxides generally include Mn and Si in a total amount. It is a granular oxide containing 1 at% or more.

  As the oxygen partial pressure of atmospheric annealing increases, does oxygen penetrate (diffusion) into the steel material more, or more oxygen penetrates (diffusion), and the region where these internal oxides exist will expand? The amount of these internal oxides increases.

  On the other hand, in contrast to these internal oxides, as the oxygen partial pressure of atmospheric annealing increases, the proportion of oxides containing Mn and Si in the external oxide layer decreases. That is, in the outer oxide layer in FIG. 1B, the ratio of the oxide containing 1 at% or more of Mn and Si in a total amount is 1 μm in the length in the substantially horizontal direction of the interface between the steel material and the outer oxide layer. It becomes 50 to 80% as an average ratio of the total length of this oxide to occupy, and becomes lower than the case of FIG.

FIG. 1 (c):
FIG. 1C shows the case of atmospheric annealing with a high oxygen partial pressure, in which the oxygen partial pressure is higher than that in FIG. 1B. The characteristic features of the outer oxide layer and the inner oxide layer according to the present invention are shown in FIG. The specific quantitative composition balance of is shown. In the case of FIG. 1C, oxygen penetrates (diffuses) into the steel material further than in FIG. 1B. For this reason, in steel materials containing Mn and Si that have been pickled once and the existing outer oxide layer has been removed, the inner oxide described above, together with the outer oxide layer, is located inside the steel material below from the steel fabric surface. The relatively deep region of the steel material is formed deeper inside. These internal oxides are mainly formed in a steel region from the surface of the steel material to a depth of 20 μm.

  On the other hand, the ratio of the oxide containing Mn and Si in the outer oxide layer is further reduced in the case of FIG. 1C than in the case of FIG. That is, in the case of FIG. 1C, the ratio of the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer is the length of the substantially horizontal direction of the interface between the steel material and the outer oxide layer. The average ratio of the total length of the oxide to 1 μm is the lowest, 0.1% or more and less than 50%. Such an external oxide layer has the smallest proportion of oxide containing Mn and Si in a total amount of 1 at% or more than the external oxide layers of FIGS. 1 (a) and 1 (c). More easily destroyed.

  Here, the outer oxide layer on the surface of a steel material such as a normal mild steel material is usually composed of oxides such as αFeOOH, γFeOOH, amorphous oxyhydroxide, and Fe3O4. On the other hand, high oxidation containing Mn and Si as in the present invention, once pickled, and then annealed in an atmosphere with controlled oxygen partial pressure as described above, external oxidation on the surface of the steel material The layer is composed of the above oxide containing Mn and Si in a total amount of 1 at% or more, and the balance is composed of an oxide such as Fe3O4 in which the total amount of Mn and Si is less than 1 at%, and voids.

(Operation of external oxide layer)
When welding the steel material and the aluminum alloy material in FIG. 1, the outer oxide layer on the steel material surface is broken, and an Al—Fe reaction layer is formed on the joint surface between the steel material and the aluminum alloy material. In this respect, the outer oxide layer on the steel material surface has an effect of suppressing the diffusion of Fe and Al during bonding and suppressing the formation of an Al—Fe-based brittle intermetallic compound layer (reaction layer).

  However, particularly in the case of dissimilar material joining such as spot welding of a 6000 series aluminum alloy material and a steel material, such an effect is not exhibited if there is an external oxide layer of the above composition on the steel material surface, but a certain percentage. The oxide phase containing Mn and Si is limited to a relatively small amount of a certain amount or less. That is, as in the case of FIG. 1 (c), the proportion of the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer is substantially horizontal at the interface between the steel fabric and the outer oxide layer. Such an effect is exhibited only when the average ratio of the total length of the oxide to the direction length of 1 μm is 50% or less.

  The reason for this is that a normal 6000 series aluminum alloy material does not contain an alloy element that is a strong reducing agent that can break the outer oxide layer on the steel surface by reduction. This is because alloy elements cannot be present. For this reason, in spot welding where only the aluminum alloy material side melts without melting the steel material side, for example, even if the aluminum alloy side melts, an external oxide layer on the steel material surface is formed at the interface with the steel material. It becomes difficult to break down by reduction and promote the diffusion of Fe from the steel material side. As a result, it becomes difficult to form an Al—Fe reaction layer having a minimum thickness necessary for metallurgical bonding as widely as possible in the bonding portion.

  One of the main reasons why the joint strength in the spot welding example of the dissimilar joint of the 6000 series aluminum alloy material and the steel material of Patent Document 7 is less than 2 kN even if it is high and a joint strength of 2 kN or more cannot be obtained. Is for this. In Patent Document 7, an external oxide layer newly formed on the surface of a high-strength steel plate is an oxide having a specific ratio of Mn and Si as in the present invention. However, the ratio is the same as that of the outer oxide layer in FIG. 1B, and the average ratio is too large, 50% or more (50-80%). As a result, for the 6000 series aluminum alloy material that does not contain an alloying element that becomes a strong reducing agent, the outer oxide layer works too much as a barrier that is not easily destroyed, and diffusion of Fe from the steel material side during welding It becomes difficult to encourage.

  Therefore, in spot welding in which the 6000 series aluminum alloy material and the steel material are dissimilarly joined by spot welding and only the aluminum alloy material side is melted without melting the steel material side, Mn in the outer oxide layer is reduced. The ratio of the oxide containing 1 at% or more of the total amount of Si is the average ratio of the total length of the oxide to the length of 1 μm in the substantially horizontal direction of the interface between the steel base and the outer oxide layer. Less than 50%.

  On the other hand, if the average ratio of the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer is too small, the outer oxide layer on the surface of the steel material such as a normal mild steel material described above, There will be no big difference. For this reason, even in the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, or in the case of a 6000 series aluminum alloy material that does not contain an alloy element that becomes a strong reducing agent. The outer oxide layer is easily destroyed. As a result, the diffusion of Fe from the steel material side to the molten aluminum alloy side is excessively promoted, and it is impossible to suppress the excessive formation of an Al—Fe-based brittle reaction layer at the bonding interface. Body joint strength is significantly reduced.

  Therefore, first, in the present invention, in the outer oxide layer existing on the surface of the steel material of the steel material, the ratio of the oxide containing Mn and Si in a total amount of 1 at% or more is the ratio between the steel material and the outer oxide layer. The average ratio of the total length of the oxide to the length of 1 μm in the substantially horizontal direction of the interface is 0.1% or more and less than 50%, preferably 0.1% or more and less than 30%, more preferably 0.1% or more and less than 5%.

  In order to obtain such an outer oxide layer, as in the case of FIG. 1 (c), the oxygen partial pressure of atmosphere annealing is increased, and the inner oxide is formed deeper into the steel material. On the other hand, the case of atmospheric annealing with a high oxygen partial pressure in which the proportion of oxides containing Mn and Si in the outer oxide layer is greatly reduced is assumed.

  As a result, in the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, the diffusion of Fe and Al during bonding is suppressed, and an Al-Fe based brittle intermetallic compound layer (reaction Layer) The effect of suppressing generation becomes larger. As a result, the average thickness of the reaction layer at the spot weld joint interface is controlled within the optimum range of 0.1 to 10 μm, as will be described later. As a result, a high joint strength of 2 kN or more can be obtained particularly for a dissimilar material joint obtained by spot welding of a 6000 series aluminum alloy material and a steel material.

(Internal oxide action)
In the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, the inner oxide layer just below the surface of the steel material is the same as the outer oxide layer on the steel material surface. There is an effect of suppressing diffusion of Fe and suppressing formation of an Al—Fe-based brittle intermetallic compound layer (reaction layer).

That is, at the time of welding joining between a steel material and an aluminum alloy material, the inner oxide of the steel material dissolves in the Al-Fe reaction layer formed by breaking the outer oxide layer on the steel material surface, and Fe, Al Suppression of diffusion and generation of an excessive reaction layer are suppressed. The internal oxide having these functions is made of a spherical oxide such as SiO2 or Mn2SiO4, and contains Mn and Si in a total amount of 1 at% or more.

  However, particularly in the case of dissimilar material joining such as spot welding of a 6000 series aluminum alloy material and a steel material, such an effect is not exhibited if there is an internal oxide layer directly under the surface of the steel material, but a certain percentage. This is limited to the case where the oxide phase containing Mn and Si is present in a certain amount or more and at a certain depth or more directly below the steel material surface. That is, as in the case of FIG. 1C, only when the internal oxide layer containing these internal oxides in a predetermined ratio is formed in a depth direction of 20 μm or more from the steel material surface of the steel material, Such an effect is not exhibited.

  The reason for this is that, as described above, a normal 6000 series aluminum alloy material does not contain an alloy element that becomes a strong reducing agent that can break the outer oxide layer on the steel surface by reduction, and the interface with the steel material. In addition, these alloy elements cannot be present. For this reason, in the present invention, as described above, the outer oxide layer is a barrier that can be relatively easily broken against a 6000 series aluminum alloy material that does not contain an alloying element that is a strong reducing agent. However, in this way, when the outer oxide layer is relatively easily destroyed, the barrier effect on the diffusion of Fe and Al in the outer oxide layer is relatively lowered. In order to effectively suppress the diffusion of Al, the function of the internal oxide becomes more important.

  That is, in the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, the inner oxide is formed one after another by breaking the outer oxide layer on the steel material surface. In the Fe reaction layer, it is necessary to continuously dissolve more throughout the welding process to suppress the diffusion of Fe and Al, and to prevent the reaction layer from being excessively formed. For this purpose, as in the case of FIG. 1C, in order to secure the amount of internal oxide, an internal oxide layer containing the internal oxide at a predetermined ratio is secured together with securing the density of the internal oxide. It is necessary to form at least 20 μm or more from the steel material surface of the steel material.

  One of the main reasons why the joint strength in the spot welding example of the dissimilar joint of the 6000 series aluminum alloy material and the steel material of Patent Document 7 is less than 2 kN even if it is high and a joint strength of 2 kN or more cannot be obtained. Is for this. In Patent Document 7, the internal oxide layer is present as an oxide having a specific ratio of Mn and Si as in the present invention. However, the region where the internal oxide layer is present is the same as the internal oxide layer in FIG. 1B, and is formed only in a relatively shallow steel region of 10 μm or less from the steel material surface of the steel material. For this reason, the point that the internal oxide dissolves in the Al-Fe reaction layer formed one after another by breaking the outer oxide layer on the steel surface is the same, but it is continuous throughout the welding. More solid solution is not possible. In other words, the effect of suppressing the diffusion of Fe and Al and suppressing the excessive generation of the reaction layer is less with respect to the formed Al—Fe reaction layer. As a result, in the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, an Al—Fe reaction layer having a minimum thickness necessary for metallurgical joining is formed at the joint. It becomes difficult to form as wide a range as possible.

  However, in a steel material having such a surface structure, if a large amount of oxide containing 1 at% or more of Mn and Si is present deep inside the steel material, depending on the welding conditions, diffusion of Fe and Al at the time of joining is rather reversed. It is suppressed too much, so that the thickness of the reaction layer cannot be sufficiently secured, or it becomes difficult to uniformly generate the reaction layer, and high bonding strength may not be obtained. Therefore, it is not necessary to provide this inner oxide layer deeper than necessary.

Therefore, in spot welding where 6000 series aluminum alloy material and steel material are spot-welded, and the steel material side does not melt, but only the aluminum alloy material side melts, the total amount of Mn and Si is An internal oxide layer containing an internal oxide containing 1 at% or more in a predetermined ratio is present in a steel region having a depth of at least 20 μm from the steel material surface of the steel material. To define this more specifically, it is an intragranular oxide containing a grain boundary oxide, Mn, and Si in a total amount of 1 at% or more present in a steel region having a depth from the steel dough surface of up to 20 μm. The occupying ratio is 5% or more and less than 20% as the average area ratio occupying within the visual field area 10 μm ² of this steel region. Among the internal oxides, the oxides generated in the grains are, as described above, spherical or granular oxides containing Mn and Si in a total amount of 1 at% or more, and Mn and Si in a total amount of less than 1 at%. On the other hand, the oxide formed on the grain boundary of steel is generally a granular oxide containing Mn and Si in a total amount of 1 at% or more. Therefore, in the present invention, in the definition of the internal oxide, the proportion of the oxide existing in the grain boundary and the oxide existing in the crystal grains containing Mn and Si in a total amount of 1 at% or more is 5% or more and 20%. %.

  As a result, in the case of spot welding in which only the aluminum alloy material side is melted without melting the steel material side, the diffusion of Fe and Al during bonding is suppressed, and an Al-Fe based brittle intermetallic compound layer (reaction Layer) The effect of suppressing generation becomes larger. As a result, the average thickness of the reaction layer at the spot weld joint interface is controlled within the optimum range of 0.1 to 10 μm, as will be described later. As a result, a high joint strength of 2 kN or more can be obtained particularly for a dissimilar material joint obtained by spot welding of a 6000 series aluminum alloy material and a steel material.

  If the density of the internal oxide containing 1 at% or more of Mn and Si is less than 5% as the average area ratio, even if the depth region of the steel material in which the internal oxide exists is satisfied, the density of the internal oxide Is too small, and the amount of internal oxide for exhibiting the effect is insufficient. On the other hand, when the density of the internal oxide containing 1 at% or more of Mn and Si is 20% or more as the average area ratio, the reaction layer at the joining interface between the steel material and the aluminum material is locally There is a high possibility that metallurgical joining is impossible even if the welding conditions are appropriate and the welding conditions are appropriate.

(Internal structure of steel)
As described above, in the steel material, it is not necessary to provide the inner oxide layer deeply exceeding 20 μm. Accordingly, the ratio of the grain boundary oxide and the intragranular oxide containing 1 at% or more of the total amount of grain boundary oxide and Mn, Si existing in the steel region where the depth from the steel dough surface exceeds 20 μm and is 30 μm or less. The average area ratio of the steel region in the visual field area of 10 μm ² is preferably 10% or less.

(Measurement method of oxide)
The oxide in the present invention is measured with a TEM (transmission electron microscope) at a magnification of 10,000 to 30,000 times in combination with EDX (energy dispersive X-ray spectroscopy). That is, the external oxide is analyzed by EDX by analyzing the interface between the steel material and the external oxide layer in a substantially horizontal direction in the cross section in the thickness direction of the steel material, so that Mn and Si in the external oxide layer near the interface are analyzed. The total amount is obtained, and the oxide phase (plurality of oxides) in the vicinity of the interface containing 1 at% or more of Mn and Si is identified and distinguished from the other phases. Next, the length in the substantially horizontal direction at the interface of the oxide phase containing 1 at% or more of the total amount of Mn and Si in the same interface region as in the EDX analysis is determined by TEM. And the ratio of the total length of this oxide phase which occupies with respect to 1 micrometer of the length of the substantially horizontal direction of an interface is calculated | required. This is performed at a plurality of locations and averaged.

The internal oxide is an oxide containing Mn and Si in a total amount of 1 at% or more at a plurality of locations in a predetermined steel region up to 20 μm in depth from the steel material surface of the steel material, according to the EDX described above. It is specified separately from the other phases. Then, each area ratio of the oxide phase containing 1 M% or more of Mn and Si in the total amount in the same interface region as that of EDX in the visual field area of 10 μm ² is determined by TEM. Here, the area occupied by the grain boundary oxide in the steel region is also added to the area ratio occupied by the internal oxide as an internal oxide containing 1 at% or more of Mn and Si in the total amount as described above. This is performed at a plurality of locations and averaged. In addition, the depth from the steel dough surface of steel materials exceeds 20 micrometers, and the grain boundary oxide which exists in the steel area | region of 30 micrometers or less, and the intragranular oxide which contains 1 at% or more of Mn and Si in total amount occupy The ratio is measured by the same method.

(Oxide layer control)
Control of the external oxide and internal oxide of these steel materials can be performed by controlling the annealing conditions (oxygen partial pressure) of the steel materials as described above. More specifically, the oxygen partial pressure (dew point) in the annealing atmosphere of the steel material can be changed and controlled. In any steel type, when the oxygen partial pressure (dew point) is high, the amount of oxide enriched in Mn and Si in the outer oxide layer on the steel material surface decreases. In addition, the steel is oxidized to the inside, and internal oxidation and grain boundary oxidation progress. As a result, SiO2, Mn2SiO4, etc. are formed in the steel, and the area ratio of oxides containing Mn and Si in the steel increases.

  On the other hand, in any steel type of high strength steel, when the oxygen partial pressure (dew point) is low, oxides enriched in Mn and Si such as Mn2SiO4 and SiO2 in the outer oxide layer on the steel surface are formed. However, the amount or area ratio increases. On the other hand, the oxidation inside the steel is difficult to proceed, the amount of SiO2 and Mn2SiO4 in the steel is reduced, and the area ratio of the oxide containing Mn and Si in the steel is reduced.

(Reaction layer at the bonding interface of dissimilar materials)
In the dissimilar material joined body in which the steel material with the controlled oxide layer on the surface and the aluminum material are joined by welding as described above, high joining strength can be obtained by setting appropriate welding conditions. However, even if the conditions on the welding material side are adjusted, depending on the welding conditions (welding conditions), high joint strength may not be realized.

  For this reason, it is necessary to define conditions for obtaining a high joint strength when viewed from the dissimilar material joint side, and to control and optimize the welding conditions so as to meet the dissimilar material joint side conditions. Therefore, in the present invention, preferably, spot welding conditions for obtaining high joint strength as a dissimilar material joined body are defined.

  As described above, when viewed from the dissimilar material joined body side, it is necessary to form a reaction layer of Fe and Al having a minimum thickness necessary for metallurgical joining as widely as possible in the joint. That is, first, as the necessary and minimum thickness for metallurgical joining, the average thickness in the nugget depth direction (steel thickness direction) of the reaction layer at the joining interface with the aluminum material is controlled within the range of 0.1 to 10 μm. It is necessary to.

  At the weld joint interface between steel and aluminum, the reaction layer is a layered Al5Fe2 compound layer on the steel side, and a layer of granular or acicular Al3Fe compound and Al19Fe4Si2Mn compound on the aluminum side. Have each.

  When the thickness of these fragile reaction layers in the nugget depth direction exceeds 10 μm, the bonding strength is significantly reduced. On the other hand, when the thickness of the reaction layer in the nugget depth direction is less than 0.1 μm, metallurgical bonding becomes insufficient, and sufficient bonding strength cannot be obtained. Therefore, the average thickness of the reaction layer at the bonding interface between the steel material and the aluminum material, in which the oxide layer on the surface is controlled, is in the range of 0.1 to 10 μm.

(Reaction layer formation range)
Next, it is necessary to form the reaction layer of Fe and Al in the dissimilar material joined body in as wide a range as possible. In other words, the formation range of the reaction layer after joining is 70% or more of the joining area (substantially horizontal direction of the steel material, direction perpendicular to the nugget depth direction) in spot welding such as spot welding and FSW (friction stir welding). It is preferable that it is an area.

  If the appropriate thickness range is not formed as uniformly as possible over the above appropriate thickness range, the reaction layer may not be able to reliably achieve metallurgical bonding. On the other hand, if the reaction layer having the appropriate thickness range is formed to be 70% or more, sufficient bonding strength can be obtained with certainty.

(Measurement of reaction layer at joint interface of dissimilar materials)
The measurement of the reaction layer in the present invention is performed by cutting the steel-aluminum joint as in the examples described later and observing the joint interface from the cross section with an SEM (scanning electron microscope). Perform the above measurements.

(Chemical composition of steel)
First, the component composition of the steel material targeted by the present invention will be described below. In the present invention, a high strength steel material (high tensile) having a tensile strength of 450 MPa or more including Si, Mn and the like is mainly targeted. Furthermore, when the existing oxide layer on the surface is once removed by pickling or the like and then annealed in an atmosphere in which the oxygen partial pressure is controlled, an external oxide layer containing a predetermined amount of Si, Mn, etc. The target is steel materials that can be newly generated.

  For this reason, about the component composition of steel materials, C: 0.01-0.30%, Si: 0.1-3.00%, Mn in mass% on the assumption that Si, Mn, etc. contain predetermined amount. : 0.1 to 3.00% each, preferably with the balance being Fe and inevitable impurities. In addition to this, a composition containing Al: 0.002 to 0.1%, with the balance being Fe and inevitable impurities may be used. Further, in addition to or instead of Al, Nb: 0.005 to 0.10%, Ti: 0.005 to 0.10%, Zr: 0.005 to 0.10%, Cr : 0.05 to 3.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Ni: 0.01 to 3.00%, one or more The balance may be composed of Fe and inevitable impurities.

  Here, P, S, N, and the like as impurities of the steel material deteriorate various properties such as toughness, ductility, and bonding strength of the steel material, so P: 0.10% or less (including 0%), S: It regulates to 0.05% or less (including 0%) and N: 300 ppm or less (including 0%), respectively. In addition, the unit (content of each element) of the chemical component in this invention is mass% altogether including an aluminum alloy.

The reasons for limiting the constituent elements of the steel are as follows.
C:
C is an element necessary for increasing the strength, but if the content is less than 0.01%, the strength of the steel cannot be secured, and if it exceeds 0.30%, the cold workability decreases. Therefore, the C content is in the range of 0.01 to 0.30%.

Si, Mn:
Si and Mn form the outer oxide layer containing a predetermined amount of Si or Mn on the surface of the steel material. In the case of dissimilar joining of Fe and Al, this external oxide layer can prevent the diffusion of Fe and Al from the base material side of each other, and can minimize the formation of brittle intermetallic compounds. It also helps to improve the brittleness of intermetallic compounds.

  Further, Si and Mn form the internal oxide layer containing a predetermined amount of Si or Mn inside the steel material. This inner oxide layer is dissolved in the Al-Fe reaction layer formed by breaking the outer oxide layer on the steel surface, preventing the diffusion of Fe and Al from the base material side of each other, and the reaction layer Is suppressed from being excessively generated.

  Therefore, if there is too little content of Si and Mn in the steel material, the outer oxide layer and the inner oxide layer are insufficient, and as described later, the bonding strength of the dissimilar material joined body cannot be improved. On the other hand, when there is too much content of Si and Mn in steel materials, the joining strength of a dissimilar material joined body will be reduced on the contrary as it mentions later. For this reason, in order to form the said suitable outer oxide layer and internal oxide layer, it is necessary for Si and Mn in steel materials to be within the range of the contents defined in the present invention.

Si:
Si is important as an element that can ensure the necessary strength without degrading the ductility of the steel material, and for that purpose, a content of 0.1% or more is necessary. On the other hand, when it contains exceeding 3.00%, ductility will deteriorate. Therefore, the Si content is in the range of 0.1 to 3.00% for this reason.

Mn:
Mn is also indispensable as an element for ensuring the strength and toughness of the steel material. If the content is less than 0.1%, the effect cannot be obtained. On the other hand, when the content exceeds 3.00%, the strength is remarkably increased and cold working becomes difficult. Therefore, the Mn content is in the range of 0.1 to 3.00% for this reason.

Al:
Al is an element effective for improving the toughness of steel by capturing solid solution oxygen as a deoxidizing element of molten steel and preventing the occurrence of blowholes. When the Al content is less than 0.002%, these sufficient effects cannot be obtained. On the other hand, when the Al content exceeds 0.1%, the weldability is adversely affected, or the toughness of the steel increases due to an increase in alumina inclusions. Deteriorate. Therefore, the Al content is in the range of 0.002 to 0.1%.

One or more of Nb, Ti, Zr, Cr, Mo, Cu, Ni:
Inclusion of one or more of Nb, Ti, Zr, Cr, Mo, Cu, and Ni contributes to increasing the strength and toughness of the steel in common.

  Among these, Ti, Nb, and Zr are precipitated as carbonitrides in the steel to increase the strength, and the microstructure of the steel is refined to improve the strength, toughness and the like. However, when it is contained in a large amount, the toughness is greatly deteriorated. Therefore, when it contains, it is set as each range of Nb: 0.005-0.10%, Ti: 0.005-0.10%, Zr: 0.005-0.10%.

  Of these, Cr, Mo, Cu, and Ni improve the hardenability of the steel and improve the strength. However, if contained in a large amount, the toughness of the steel is greatly deteriorated. Therefore, when contained, Cr: 0.05 to 3.00%, Mo: 0.01 to 3.00%, Cu: 0.01 to 3.00%, Ni: 0.01 to 3.00% The range.

(Strength of steel)
In the present invention, a high strength steel material (high tensile) having a tensile strength of 450 MPa or more is mainly used for applications such as automobile members. Low-strength steels are generally low-alloy steels, and the oxide film is almost iron oxide. Therefore, diffusion of Fe and Al is facilitated, and a brittle reaction layer is easily formed. In addition, since the amount of Si and Mn is small, the oxide containing Si and Mn necessary for suppressing diffusion of Fe and Al of the base material in the present invention is hard to be formed on the surface and inside of the steel material. It is difficult to control the composition and thickness of the oxides (layers) including and outside, and it becomes difficult to control the reaction layer during welding. Furthermore, since the strength of the steel material is insufficient, the pressure of the electrode tip during spot welding increases the deformation of the steel material, and the oxide film is easily destroyed, so the reaction with aluminum is promoted abnormally, Brittle intermetallic compounds are easily formed.

(Aluminum alloy material)
The aluminum alloy material used in the present invention is, by mass, Al containing Mg: 0.1 to 3.0%, Si: 0.1 to 2.5%, and Cu: 0.001 to 1.0%. -It is set as 6000 series aluminum alloy of AA thru | or JIS specification of Mg-Si type. The shape of the alloy material is not particularly limited depending on the use of each part of the automobile body, and the above-described widely used plate material, shape material, forging material, casting material, and the like are appropriately selected. However, the strength of the aluminum material is desirably higher in order to suppress deformation due to pressurization during spot welding, as in the case of the steel material.

  For automobile body panels and the like, various properties such as excellent press formability, BH property (bake hard property), strength, weldability, and corrosion resistance are required. In order to satisfy such requirements, the composition of the 6000 series aluminum alloy plate is, by mass, Mg: 0.1 to 1.0%, Si: 0.1 to 1.5%, Mn: 0.00. An Al—Mg—Si-based aluminum alloy containing 01 to 1.0%, Cu: 0.001 to 1.0%, the balance being Al and inevitable impurities is preferable. In order to further improve the BH property, it is preferable that the Si-Mg mass ratio Si / Mg is an excess Si type 6000 series aluminum alloy plate having a mass ratio of 1 or more.

  Further, as the extruded material for the automobile body reinforcing material, various properties such as excellent bending crushability and corrosion resistance are required. In order to satisfy such a requirement, the composition of the extruded material of 6000 series aluminum alloy is, by mass, Mg: 0.30 to 1.0%, Si: 0.30 to 1.0%, Fe: 0.00. Al-Mg-Si-based aluminum alloy containing 01 to 0.40%, Mn: 0.001 to 0.30%, Cu: 0.001 to 0.65%, and the balance consisting of Al and inevitable impurities It is preferable to do. Furthermore, in addition to each of the preferred compositions described above, one or two of Cr: 0.001 to 0.2% and Zr: 0.001 to 0.2% in total amount of 0.30% or less, or Zn: One or two of 0.001 to 0.25% and Ti: 0.001 to 0.10% may be selectively included.

  Other elements other than these are basically impurities, and the content (allowable amount) of each impurity level is in accordance with AA to JIS standards. However, from the viewpoint of recycling, not only high-purity Al bullion but also 6000 series alloys and other aluminum alloy scrap materials, low-purity Al bullion, etc. are used as melting materials. There is a high possibility that elements will be mixed. Then, reducing these impurity elements to, for example, below the detection limit itself increases the cost, and a certain amount of allowance is required. Accordingly, the other elements are allowed to be contained within a permissible range in accordance with AA to JIS standards.

The significance of each element in the 6000 series aluminum alloy is as follows.
Si: Si, together with Mg, forms an aging precipitate that contributes to strength improvement at the time of artificial aging treatment at low temperatures such as solid solution strengthening and paint baking treatment, and exhibits age hardening ability, for example, 180 MPa or more It is an essential element for obtaining the required strength (proof strength). If the content is insufficient, such an effect cannot be obtained. If the content is too large, the formability such as press formability and bending workability is remarkably deteriorated, and the weldability is greatly hindered.

Mg: Mg also forms an aging precipitate that contributes to strength improvement together with Si during the above-mentioned artificial aging treatment such as solid solution strengthening and paint baking treatment, exhibits age hardening ability, and as a panel, the required proof stress It is an essential element for obtaining. If the content is insufficient, such an effect cannot be obtained. If the content is too large, moldability such as press formability and bending workability is remarkably lowered.

Cu: Cu has the effect of promoting the formation of aging precipitates that contribute to improving the strength of the aluminum alloy material structure in the crystal grains under conditions of artificial aging treatment at a relatively low temperature for a short time. Moreover, solid solution Cu also has the effect of improving moldability. If the content is insufficient, such an effect cannot be obtained, and if the content is too large, the corrosion resistance and weldability are remarkably deteriorated.

Mn: Mn produces dispersed particles (dispersed phase) during the homogenization heat treatment, and these dispersed particles have the effect of hindering the grain boundary movement after recrystallization, so that it is possible to obtain fine crystal grains. is there. The press formability and hemmability are improved as the crystal grains of the aluminum alloy structure are finer. If the content is insufficient, such an effect cannot be obtained, and if the content is too large, the mechanical properties are lowered. In addition, formability such as bending workability is lowered.

Fe: Fe has the same function as Mn, Cr, Zr, etc., generates dispersed particles (dispersed phase), prevents grain boundary movement after recrystallization, prevents crystal grains from becoming coarse, Has the effect of miniaturizing. If the content is insufficient, such an effect cannot be obtained. If the content is too large, coarse crystallized products are likely to be generated, and the fracture toughness and fatigue characteristics are deteriorated.

Zn: Zn contributes to improvement of strength by solid solution strengthening, and also has an effect of remarkably accelerating age hardening of the final product during aging treatment. If the content is insufficient, such an effect cannot be obtained. If the content is too large, the sensitivity to stress corrosion cracking and intergranular corrosion is remarkably increased, and the corrosion resistance and durability are lowered.

Ti: Ti has the effect of refining the crystal grains of the ingot to make the extruded material structure fine crystal grains. If the content is insufficient, such an effect cannot be obtained, and if the content is too large, coarse crystal precipitates are formed, and the required properties such as the bending crushability and corrosion resistance as a reinforcing material, and the bending of the extruded material This may cause deterioration of workability.

Cr, Zr: Transition elements of Cr and Zr, like Mn, generate dispersed particles (dispersed phase) composed of intermetallic compounds such as Al-Cr and Al-Zr to prevent coarsening of crystal grains It is effective to do. If the content is insufficient, such an effect cannot be obtained, and if the content is too large, coarse crystal precipitates are formed, and if the content is too large, the above-described bending crushability and corrosion resistance as a reinforcing material are required. Degrading properties and mechanical properties. In addition, formability such as bending workability is lowered.

(Thickness of steel and aluminum alloy materials)
Moreover, the thickness (plate thickness etc.) of the welded portion of the steel material or aluminum alloy material is not particularly limited, and is appropriately selected or determined from design conditions such as required strength and rigidity of an applicable member such as an automobile member.

  However, assuming an automobile member or the like, the thickness t (of the welded portion) of the steel material is practically selected from 0.3 to 3.0 mm. If the thickness of the steel material is too thin, the necessary strength and rigidity as an automobile member cannot be ensured, which is inappropriate. In addition, in the case of spot welding, for example, due to the pressurization by the electrode tip, the steel material is greatly deformed and the oxide film is easily destroyed, and thus the reaction with aluminum is promoted. As a result, an intermetallic compound is easily formed. On the other hand, when the thickness of the steel material is too thick, spot welding joining itself becomes difficult.

  Further, the thickness t (of the portion to be welded) of the aluminum alloy material is selected from the range of 0.3 to 5.0 mm when similarly assuming an automobile member or the like. If the thickness of the aluminum alloy material is too thin, the strength as an automobile component is insufficient and inappropriate. In addition, the nugget diameter is not obtained, and the surface of the aluminum material is easily melted, so that it is easy to cause dust and high bonding. There is a possibility that strength cannot be obtained. On the other hand, when the thickness of the aluminum alloy material is too thick, the welding joint itself becomes difficult as in the case of the steel plate thickness described above.

(Joining method)
In the present invention, as a precondition, the welding method is selected so that only the aluminum alloy material side is melted without melting the steel material side. In this respect, the joining method is limited to spot welding or friction spot joining (also referred to as friction stir welding, FSW, spot FSW). In other words, MIG welding and laser welding in which both the steel material side and the aluminum alloy material side melt are excluded, and welding methods such as ultrasonic welding, diffusion welding, friction welding, brazing and the like that do not melt both are also excluded. It is. In addition, joining by spot welding is more preferable than friction spot joining because of productivity and ease of adopting appropriate conditions.

  In addition, as a preferable condition for each spot welding joint location so that only the aluminum alloy material side is melted without melting the steel material side, an electrode pressure of 2.0 to 3.0 kN is 10 A current between .about.35 kA is energized for a time of 200.times.tmsec or less in relation to the thickness tmm of the aluminum alloy material part to be joined. Under these inappropriate spot welding conditions such as a to d shown in Table 4 to be described later, a high joint strength of the dissimilar material joined body cannot be obtained.

  Hereinafter, as an example, different materials were joined by spot welding to produce different materials joined bodies. And the joint strength of each dissimilar material joined body was measured and evaluated.

  Specifically, after steel plates melted and rolled to 1.2 mm thickness with each component composition shown in Table 1 are pickled once to remove the existing surface oxide layer, A and B shown in Table 3 , C, D, E, F, and G, the oxygen partial pressure (dew point) in the annealing atmosphere is variously changed, except that the annealing temperature is 880 ° C. and the annealing time is 200 sec. Steel plates having different surface layer oxidation structures were prepared. Here, the steel sheets having the respective component compositions shown in Table 1 are all high-strength steel sheets targeted by the present invention, and the tensile strengths of the respective steel sheets are all in the range of 780 to 1280 MPa of 450 MPa or more.

  Table 3 also shows oxidation structures such as the outer oxide layer composition and the inner oxide layer composition of each steel plate after annealing. Among the annealing conditions shown in Table 3, D, E, F, and G, in which the oxygen partial pressure (dew point) increases sequentially, are the annealing conditions in which the oxygen partial pressure (dew point) is suitable. For this reason, as shown in Table 3, the annealing conditions D, E, F, and G satisfy the conditions of the present invention for the outer oxide layer and the inner oxide of the steel sheet after annealing. That is, the ratio of the oxide containing 1 at% or more of Mn and Si in the total amount in the outer oxide layer occupies the length of 1 μm in the substantially horizontal direction of the interface between the steel base and the outer oxide layer. The average ratio of the total length of objects is in the range of 0.1% or more and less than 50%.

Moreover, annealing conditions D, E, F, and G are internal oxide 1 (the total amount of grain boundary oxide, Mn, and Si existing in the steel region where the depth from the steel dough surface of the steel sheet is 20 μm). % Of the intragranular oxide) is in the range of 5% or more and less than 20% as an average area ratio in the visual field area 10 μm ² of this steel region. Furthermore, internal oxide 2 (intragranular oxide containing a grain boundary oxide, Mn, and Si in a total amount of 1 at% or more present in a steel region having a depth from the steel dough surface of the steel sheet exceeding 20 μm to 30 μm or less. ) Is 10% or less as an average area ratio in the visual field area 10 μm ^{2 of the} steel region.

  However, among these annealing conditions, G is an example in which the oxygen partial pressure (dew point) is as high as the limit. For this reason, the average ratio of the total length of the oxides in the outer oxide layer of the steel sheet after annealing is within the range, but is close to the lower limit, and is significantly reduced to about 0.5%. On the other hand, the ratio of the internal oxides 1 and 2 is within the range but is close to the upper limit and is significantly high.

  On the other hand, among the annealing conditions shown in Table 3, A, B, and C are comparative examples in which the oxygen partial pressure (dew point) is too low as compared with the annealing conditions D, E, F, and G. For this reason, as shown in Table 3, the average ratio of the total length of the oxide in the outer oxide layer of the steel plate after annealing exceeds 50%. Therefore, among the annealing conditions shown in Table 3, the annealing conditions for A, B, and C are such that the respective oxide structures such as the outer oxide layer composition and the inner oxide layer composition of each steel sheet deviate from the optimum conditions, and the dissimilar material joined body. It is clear that the bonding strength of the material decreases. For this reason, the dissimilar-material joined body by spot welding was not manufactured for each steel plate annealed on these A, B, and C annealing conditions.

In addition, each oxidation structure in the joining equivalent part of each steel plate was measured with the following measuring method, respectively.
(External oxide formation range)
The external oxide is manufactured by a focused ion beam processing device (FIB: Focused Ion Beam Process, manufactured by Hitachi, Ltd .: FB-2000A), and the external oxide in the cross section in the thickness direction of the steel sheet by the EDX (model: NORAN-VANTAGE). The total amount of Mn and Si in the outer oxide layer near the interface is obtained by analyzing the interface between the steel fabric and the outer oxide layer in a substantially horizontal direction, and the total amount of Mn and Si is 1 at% or more. The oxide phase (plurality of oxides) in the vicinity of the interface was identified separately from the other phases. Next, cross-sectional observation was performed with a TEM (JEOL field emission transmission electron microscope: JEM-2010F, acceleration voltage 200 kv) at a magnification of 100,000 times, and the total amount of Mn and Si in the same interface region as the EDX was 1 at%. The length of the oxide phase including the above in the substantially horizontal direction at the interface is obtained. Then, the ratio of the total length of the oxide phase to the length of 1 μm in the substantially horizontal direction of the interface was determined. This was performed for each of three fields of view, and the average value was obtained.

(Internal oxide occupation area ratio)
As shown in FIG. 1 (c), the internal oxide is an internal oxide in the steel region having a depth of 20 μm from the steel dough surface. 1 and the internal oxide in the steel area | region where the depth from the steel dough surface of a steel plate exceeds 20 micrometers and is 30 micrometers or less was made into the internal oxide 2, and these compositions were analyzed. The compositional analysis is performed with the average area ratio of oxides containing 1 at% or more of Mn and Si in the total amount in each steel region. First, an oxide containing Mn and Si in a total amount of 1 at% or more in each steel region is identified and distinguished from the other phases by the EDX. Then, a cross-section was observed with a TEM (JEOL field emission transmission electron microscope: JEM-2010F, acceleration voltage 200 kV) at a magnification of 30,000 times, and the total amount of Mn and Si in the same interface region as the EDX was 1 at%. The ratio of the area of the oxide phase contained above in the visual field area (base metal area) per 10 μm ² was determined. Here, the area occupied by the grain boundary oxide is also added to the oxide containing 1 at% or more of Mn and Si in total. This was performed for each of three fields of view, and the average value was obtained.

  Each of these steel plates having different oxidation structures and a 6000 series aluminum alloy plate having a thickness of 1 to 1.6 mm having the composition shown in Table 2 in common with each example are processed into the shape of a cross tensile test piece described in JIS A 3137. Then, spot welding was performed under the conditions a, b, c, d, e, and f shown in Table 4 to join different materials. Here, as evaluated from peel strength shown in Table 5 described later, a to d shown in Table 4 are inappropriate spot welding conditions, and e and f are appropriate spot welding conditions.

  In addition, the spot welding shown in Table 4 commonly performed spot welding per point with the applied pressure, welding current, and welding time shown in Table 4 using a DC resistance welding tester. In common, a dome-shaped electrode made of a Cu—Cr alloy was used, and the positive electrode was made of an aluminum material and the negative electrode was made of a steel material.

(Interfacial reaction layer thickness and formation range)
The thickness and formation range of the interface reaction layer of each of the dissimilar material assemblies manufactured as described above were measured. These results are shown in Table 5. The thickness of the interface reaction layer was measured by cutting at the center of each spot weld, embedding in resin, polishing, and performing SEM observation at 0.5 mm intervals over the entire joint. When the thickness of the reaction layer is 1 μm or more, it is measured with a field of view of 2000 times, and when it is less than 1 μm, it is measured with a field of view of 10,000 times, and an average value is obtained for each spot weld. The average value was defined as the average thickness of the interface reaction layer. Moreover, the formation range of the interface reaction layer was determined by determining the ratio of the reaction layer formation area to the total spot area in each spot welded portion, and obtaining the average value of 30 spot welded portions.

(Amount of elements at the bonding interface on the aluminum alloy material side)
Similarly, the Fe content (mass%: indicated as Fe concentration in Al at the interface in Table 5) at the bonding interface on the aluminum alloy material side of each manufactured dissimilar material joined body was measured. These results are shown in Table 5.

  For the analysis, EPMA: X-ray microanalyzer (JXA-8800RL) manufactured by JEOL Ltd. was used, and measurement was carried out with a constant acceleration voltage of 15 kV and an irradiation current of 0.3 μA. The object of analysis was a cross section cut at the center of each spot welded portion, and the analysis was performed up to the inside of 0.5 mm each on the aluminum alloy material side and the steel material side, centering on the joint interface between the aluminum alloy material and the steel material. . Then, the Fe content originally contained in the aluminum alloy material inside the aluminum alloy material is subtracted, and the Fe content (mass%: in Table 5, Fe in Al at the interface in the aluminum alloy material side joining interface) Concentration and display).

  A cross tension test was performed on each of the manufactured dissimilar material joints to determine the peel strength. These results are also shown in Table 5. With reference to the spot weld joint strength between A6022 aluminum materials = 1.0 kN, the peel strength was evaluated as ◯ if it was 2.0 kN or more, and x if it was less than 2.0 kN.

  As is apparent from Table 5, annealing conditions D, E, F, and G with suitable oxygen partial pressures (dew points) shown in Table 3 using steel plates and 6000 series aluminum alloy plates having the proper component compositions shown in Tables 1 and 2 were used. In each of Inventive Examples 1 to 23 processed in Step 1, the outer oxide layer and the inner oxide of the steel plate after annealing satisfy the conditions of the present invention. In particular, the annealing condition F is a preferable range of 0.1% or more and less than 30%, and the annealing condition G is more preferable as the ratio of the oxide containing Mn and Si in the total amount of 1 at% or more in the total amount. The range of 0.1% or more and less than 5% is satisfied. Further, each invention example using a steel plate that satisfies these oxide conditions and having an appropriate spot welding condition of e and f as the welding conditions includes Fe at the bonding interface on the aluminum alloy material side of the dissimilar material bonded interface. The amount is 2.0% by mass or less. Further, the formation area (formation ratio) of the reaction layer of Fe and Al formed at the joining interface between the steel material and the aluminum alloy material is 70% or more of the spot welding joining area, and the thickness of this reaction layer is also Is appropriate. As a result, as is apparent from Table 5, it can be seen that in each invention example, the bonding strength (peeling strength) of the dissimilar bonded body is higher than 2 kN.

  On the other hand, as is apparent from Table 5, D, E, F, and G having suitable oxygen partial pressures (dew points) shown in Table 3 using steel plates having the proper composition shown in Tables 1 and 2 and 6000 series aluminum alloy plates. Of course, in each of Comparative Examples 24-31 treated under the annealing conditions, the outer oxide layer and the inner oxide of the steel plate after annealing are within the conditions of the present invention. However, the comparative examples 24-31, which are inappropriate spot welding conditions a to d in Table 4, are the Fe content in the joining interface on the aluminum alloy material side, and the joining interface between the steel material and the aluminum alloy material. The formation area (formation ratio) of the reaction layer of Fe and Al formed in 1) or the thickness of this reaction layer is also inappropriate. As a result, as is clear from Table 5, it can be seen that the thickness and the formation range of the interface reaction layer of the dissimilar material joined body do not satisfy the conditions of the present invention, and the joining strength of the dissimilar joined body is remarkably lowered.

  In Comparative Examples 32-38, a 6000 series aluminum alloy plate having an appropriate component composition is used, and the oxygen partial pressure (dew point) shown in Table 3 is processed under the annealing conditions of E, and the welding conditions are e and f. As suitable spot welding conditions, the outer oxide layer and the inner oxide of the steel sheet after annealing are generally within the conditions of the present invention. Further, the Fe content at the joining interface on the aluminum alloy material side is also generally within the conditions of the present invention. However, since the steel plate component compositions 19 to 25 shown in Table 1 are out of the scope of the present invention and are inappropriate, as is apparent from Table 5, the bonding strength of the dissimilar joints is remarkably low.

  In Comparative Example 32, C was too high, a supercooled structure was generated in the spot welded portion, and cracks were generated. In Comparative Example 33, Si was too high, and an optimum reaction layer of Fe and Al could not be formed at the bonding interface. In Comparative Example 34, Mn was too high, a supercooled structure was generated in the spot weld, and cracks were generated. In Comparative Example 35, Al was too high, the ductility of the steel material was lowered, it was brittlely broken in the cross tension test, and the peel strength was low. In Comparative Example 36, N was too high, the ductility of the steel material was lowered, the brittle fracture occurred in the cross tension test, and the peel strength was low. In Comparative Example 37, Cr was too high, a supercooled structure was generated in the spot welded portion, fractured brittlely in the cross tension test, and the peel strength was low. In Comparative Example 38, Nb was too high, the ductility of the steel material was lowered, it was brittlely broken in the cross tension test, and the peel strength was low.

  Therefore, these facts support the critical significance of the component composition and oxide conditions on the steel material side of the present invention. Moreover, the significance of the conditions of the present invention regarding the thickness and formation range of the interface reaction layer of the dissimilar material joined body is understood. In addition, the thickness and range of the interface reaction layer of the dissimilar material joined body satisfy the conditions of the present invention, and in order to increase the joining strength of the dissimilar material joined body, not only a steel plate that satisfies the oxide condition is used, but also welding conditions It is understood that it is necessary to make it appropriate.

  According to the present invention, there are few restrictions such as spot welding application conditions, excellent versatility, and a brittle reaction layer (intermetallic compound layer) or the like is generated at the joint, thereby impairing the reliability of the joint. Therefore, it is possible to provide a dissimilar material joined body and a dissimilar material joining method in which a steel material and an aluminum alloy material are welded and joined, which can obtain a joint having high joint strength. Such a dissimilar material joined body and a dissimilar material joining method can be usefully applied as various structural members and their welding methods in the transportation field such as automobiles and railway vehicles, machine parts, building structures, and the like.

It is a schematic diagram which shows the steel plate cross section for different material joining of this invention.

Claims (7)

  It is a steel material for joining different materials to a 6000 series aluminum alloy material, and the composition of this steel material is, in mass%, C: 0.01 to 0.30%, Si: 0.1 to 3.00%, Mn: 0 0.1 to 3.00%, P: 0.10% or less (including 0%), S: 0.05% or less (including 0%), N: 300 ppm or less (including 0%) In the steel region where the depth of the steel material from the steel dough surface is up to 20 μm, the total amount of oxide existing at the grain boundary, Mn, and Si is 1 at%. The ratio of the oxides present in the crystal grains to be contained is 5% or more and less than 20% as an average area ratio in the steel region, and Mn and Si existing on the surface of the steel material are in total amounts. The proportion of the outer oxide containing 1at% or more is steel material and outer oxide layer. The total length of the average ratio of approximately this oxide occupying the horizontal direction length 1μm of the interface, dissimilar materials bonded steel material, which is a 0.1% to less than 50%.   It is a dissimilar material joined body of the steel material and aluminum alloy material of Claim 1, Comprising: The said aluminum alloy material is the mass%, Mg: 0.1-3.0%, Si: 0.1-2.5 %, Cu: 0.001 to 1.0%, each containing 6000 series aluminum alloy, and the content of Fe at the joining interface on the aluminum alloy material side of the dissimilar material joined body is 2.0 mass% or less A dissimilar material joined body, wherein a reaction layer of Fe and Al is formed at the joining interface.   The dissimilar material joined body is spot-welded, and as a condition for each spot welding location, the average nugget depth direction of the reaction layer of Fe and Al formed at the joining interface between the steel material and the aluminum alloy material 3. The dissimilar material joined body according to claim 2, wherein the thickness is in the range of 0.1 to 3 μm, and the formation range of the reaction layer of Fe and Al is 70% or more of the spot welding area.   The dissimilar material joined body according to claim 2 or 3, wherein a peel strength measured by a cross tensile test piece of the dissimilar material joined body is 2 kN or more.   The dissimilar material joined body according to any one of claims 2 to 4, wherein the dissimilar material joined body is for a vehicle body structure of an automobile.   It is a dissimilar-material joining method of steel materials and aluminum alloy materials, Comprising: The steel materials of Claim 1, and mass: Mg: 0.1-3.0%, Si: 0.1-2.5%, Cu : A dissimilar material joining method characterized by spot welding or friction spot joining (friction stir welding) with an aluminum alloy material made of a 6000 series aluminum alloy each containing 0.001 to 1.0%. As a condition for each joint portion between the steel material and the aluminum alloy material, an interelectrode current of 10 to 35 kA is applied at a thickness tmm of the aluminum alloy material portion to be welded at an interelectrode pressure of 2.0 to 3.0 kN. 7. The dissimilar material joining method according to claim 6, wherein the steel material and the aluminum alloy material are spot-welded by energizing for a time of 200 × tmsec or less.

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