JP4476016B2 - Manufacturing method of steel / aluminum joint structure - Google Patents

Description

  The present invention relates to a method for producing a steel / aluminum bonded structure that combines the excellent lightness and corrosion resistance of an aluminum material with the excellent mechanical strength of a steel material.

  Aluminum materials such as aluminum and aluminum alloys are used in various fields by utilizing their light weight and excellent corrosion resistance. However, in applications where strength is required, the required strength is satisfied by increasing the thickness. However, thickening impairs the lightness that is an advantage of the aluminum material, and is not suitable as a structural member corresponding to a compact design. When a steel material with good mechanical strength is laminated with an aluminum material, the required strength can be obtained without the need for thickening.

  For the lamination of the aluminum material and the steel material, mechanical coupling methods such as bolts, nuts, rivets, and fitting have been adopted, but it is difficult to obtain excellent joints by the mechanical coupling method, and the productivity is low. When the aluminum material / steel material can be welded, a steel / aluminum bonded structure having significantly higher productivity and good characteristics can be obtained as compared with the mechanical bonding method. However, when steel materials and aluminum materials are joined by a normal melt joining method, a large amount of very fragile intermetallic compounds are generated at the joining interface, and the joint strength is significantly reduced.

  An intermetallic compound is produced by an interdiffusion reaction of atoms of steel and aluminum materials at the interface. In patent document 1, the production | generation of an intermetallic compound is suppressed by managing appropriately the reaction temperature, time, etc. which regulate diffusion reaction at the time of friction welding. However, since it is a joint by friction welding, it is necessary to devise the joint design, and there is room for improvement in simplifying the joining process. Application of spot welding is also being studied, and Patent Document 2 introduces a method of resistance welding a hot-dip aluminized steel sheet to an aluminum material.

  A hot-dip aluminum-plated steel sheet tends to be considered to exhibit a behavior similar to that of an aluminum material at the time of joining since a hot-dip aluminum plating layer is present on the surface layer. However, the joint interface is heated to a high temperature exceeding the melting point of Al (660 ° C.) during spot welding. Fe, Si, etc. diffuse from the Al—Fe—Si ternary alloy layer at the base steel / plating layer interface into the molten Al produced by high-temperature heating, but Fe reprecipitates during the cooling process during welding, resulting in a large diffusion coefficient. Si is dispersed in molten Al. As a result, when the bonded interface after cooling is observed, a nugget in which a fragile Al—Fe binary alloy layer is formed in the entire bonded interface is detected, and the joint strength is extremely low.

In order to suppress the adverse effect of the Al—Fe binary alloy layer on the joint strength, Patent Document 3 regulates the ratio of the intermetallic compound in the joint interface. In order to suppress the formation of intermetallic compounds, a method of biasing the heat generated during spot welding to the molten aluminum plated steel sheet with the molten aluminum plated steel sheet on the positive electrode side and the aluminum material on the negative electrode side is still used. Large production is inevitable.
JP 2003-33885 A JP-A-6-39558 JP2003-145278

  The present invention has been devised to solve such problems, and as a result of investigating and examining the behavior of Fe and Si that diffuses and reprecipitates in molten Al by high-temperature heating during spot welding, the nugget center The steel / aluminum joint structure with high joint strength by controlling the heat input distribution along the radial direction from the joint to suppress the formation and growth of Al-Fe binary alloy layers harmful to joint strength With the goal.

In the production method of the present invention, Si: 3 to 12% by mass, Fe: 0.5 to 5% by mass, and the balance: a substantially Al plating layer is formed, and at the plating layer / underlying steel interface, 0.5% is formed . Aluminum or aluminum alloy is superimposed on a hot-dip aluminum-plated steel sheet on which an Al-Fe-Si ternary alloy layer having a thickness of 1 μm or more is formed. And spot welding.

  As a hot-dip aluminum-plated steel sheet, a steel material containing 0.002 to 0.020 mass% of N is used as a base steel, and an N-concentrated layer of N: 3.0 atomic% or more is formed at the interface between the base steel and the hot-dip aluminum plating layer. A plated steel sheet is also a suitable material to be joined. The aluminum material of the counterpart material is preferably aluminum whose Fe content is regulated to 0.1% by mass or less and includes Mg: 0.1 to 6.0% by mass, Si: 3.0% by mass or less as required. An alloy is used.

  When a hot-dip aluminized steel sheet or aluminum material is energized and heated using an electrode with a rounded tip, the center of the electrode is heated to the highest temperature, and the temperature decreases toward the periphery of the electrode. According to this temperature gradient, an Al-Fe binary alloy layer is formed in the part corresponding to the center of the electrode, but as the distance from the center of the electrode increases, the generation of the Al-Fe binary alloy layer decreases, and the alloy layer disappearing region is reduced. A nugget is formed. As an electrode that provides a suitable temperature gradient, for example, an electrode having a round tip of 5 to 300 mm is used.

  In spot welding of a hot-dip aluminum plated steel plate / aluminum plate, the hot-dip aluminum plated steel plate 1 and the aluminum material 2 are overlapped and pressed by the electrode 3, for example, under a pressurizing condition of about 3 kN, welding current: 15 to 25 kA, energization time: Energize in 3 to 40 cycles. The aluminum material 2 and the molten aluminum plating layer 4 at the joint are melted by heat generated by energization, and are fused by mutual diffusion reaction.

  Fe and Si dissolve into the molten Al from the Al—Fe—Si ternary alloy layer 6 formed at the interface of the base steel 5 / plating layer 4, and the Al—Fe—Si ternary alloy layer 6 disappears at the joining interface. . There is also Fe that melts into the molten Al from the base steel 5. The melted Fe is reprecipitated during the cooling process during welding, and a fragile Al—Fe binary alloy layer 7 tends to be generated at the joint interface. At this time, as seen in the steel plate / aluminum material 2 joining, when the Al—Fe binary alloy layer 7 is formed in the entire joining interface, a nugget 8 having extremely reduced joint strength is formed (FIG. 1a). On the other hand, when the Al—Fe binary alloy layer 7 has not grown to the entire area of the nugget 8 and there is an alloy layer disappearing region 9 in which the plating layer 4 is in direct contact with the base steel 5, the bonded state is maintained in this portion. (FIG. 1b). As the alloy layer disappearing region 9 increases, the joint strength increases.

As a result of various investigations and examinations on the conditions for obtaining the joint interface shown in FIG. 1b, the present inventors have found that the composition of the plated layer of the hot-dip galvanized steel sheet as the material to be joined and the current-carrying conditions during welding are Al—Fe It has been found that it has a great influence on the formation of the original alloy layer.
When a hot-dip aluminum-plated steel sheet on which a hot-dip aluminum plating layer containing Si: 3 to 12 mass% and Fe: 0.5 to 5 mass% is formed is joined with an aluminum material by spot welding, there is an alloy layer disappearance zone 9 A bonding interface is formed. The influence of the Si and Fe concentration of the molten aluminum plating layer on the formation of the Al—Fe binary alloy layer is presumed as follows.

  The Al—Fe binary alloy layer 7 is a result of reprecipitation of Fe dissolved in molten Al generated by high-temperature heating during spot welding during the cooling process. The penetration amount of Fe into molten Al is affected by the Fe concentration gradient of the base steel / plating layer, and increases as the concentration gradient increases (in other words, as the Fe concentration in the plating layer decreases). The eluted Fe exists in the vicinity of the base steel because of its relatively small diffusion coefficient, and becomes a large amount of the Al—Fe binary alloy layer 7 during the cooling process, and reprecipitates at the joint interface. Therefore, if the Fe concentration of the plating layer 4 is increased in advance, Fe that dissolves into the plating layer 4 from the base steel 5 decreases, and as a result, the formation of the Al—Fe binary alloy layer 7 is suppressed.

Si contained in the plating layer 4 has a diffusion coefficient larger than that of Fe, and easily shifts from the Al—Fe—Si ternary alloy layer 6 to molten Al by high-temperature heating during spot welding and is dispersed in the nugget 8. The Therefore, Si diffusion from the Al—Fe—Si ternary alloy layer 6 to the molten Al is delayed by setting the Si concentration of the plating layer 4 as high as 3 to 12 mass%.
In general, diffusion of Fe and Si into molten Al progresses more actively as the temperature becomes higher. Actually, the Al—Fe binary alloy layer 7 is formed and grown thickly in the entire joining interface in the welded portion where excess power is applied, but the Al—Fe binary alloy layer 7 becomes thinner as the input power is reduced. Therefore, if a temperature gradient at which the maximum temperature during welding is high at the center of the electrode and decreases toward the periphery of the electrode can be formed, the Al—Fe binary alloy layer 7 is generated at the center of the joint interface, but at the periphery, It is expected that the Al—Fe binary alloy layer 7 is thin and further becomes a bonding interface having an alloy layer disappearing region 9.

  In the present invention, a temperature gradient effective for suppressing the formation of the Al—Fe binary alloy layer 7 is realized by using an electrode with a rounded tip. When the rounded electrode 3 is pressed against the aluminum material 2, the center portion of the electrode 3 is in close contact with the aluminum material 2, but the contact pressure of the aluminum material 2 / electrode 3 decreases toward the periphery (FIG. 2). Depending on the contact state of the electrode 3 with the aluminum material 2, the current during spot welding also flows more in the central part of the electrode 3 and less in the peripheral part. As a result, the aluminum material 2 has the highest temperature at the portion corresponding to the center of the electrode 3, and a temperature distribution having a lower maximum temperature toward the periphery is obtained.

  Since the maximum temperature reached at the joint that hits the periphery of the electrode 3 is lowered, the generation and growth of the Al—Fe binary alloy layer 7 is suppressed, and the alloy layer disappearing region 9 is generated. In the peripheral portion where the maximum temperature is low, Fe and Si diffusion from the Al—Fe—Si ternary alloy layer 6 at the plating layer / underlying steel interface to the molten Al is reduced, so that Al—Fe—Si is also present after welding. A part of the ternary alloy layer 6 remains at the bonding interface. This also improves the joint strength.

Fe diffusion from the base steel 5 to the molten Al can also be suppressed by forming an Fe diffusion prevention layer at the base steel / plated layer interface. As the Fe diffusion preventing layer, an N enriched layer developed by the present applicant as an aluminized steel sheet for brazing (Patent Document 4) developed by the present applicant is effective. Since the Fe enriched in the molten Al from the base steel 5 is reduced by the N enriched layer, the fragile Al—Fe binary alloy layer 7 generated at the joint interface is further reduced, and a joint structure with high joint strength is obtained.
Japanese Patent Laid-Open No. 9-228018

There are low carbon steel, medium carbon steel, low alloy steel, stainless steel and the like as the plating original plate, and steel types to which Si, Mn, Cr, Ni or the like is added are used depending on the application. Of these, a plating original plate to which 0.002 to 0.020 mass% of N that suppresses Al—Fe interdiffusion is added is preferable.
When the plating original plate is dipped in the molten aluminum plating bath and pulled up, the molten plated metal lifted from the molten plating bath accompanying the plating original plate is solidified to form a molten aluminum plating layer. The thickness of the molten aluminum plating layer is adjusted by controlling the amount of adhesion such as wiping gas spraying on the steel strip immediately after pulling, but the thickness of the Al—Fe binary alloy layer is delayed as the thickness increases, so that the thickness of the molten aluminum plating layer exceeds 5 μm. It is preferable to do.

  In order to obtain a steel / aluminum bonded structure with high bonding strength, the Si and Fe concentrations contained in the hot-dip aluminum plating layer were set to values not including the alloy layer formed at the interface of the base steel / hot-dip aluminum plating layer, respectively. : 3 to 12% by mass, Fe: 0.5 to 5% by mass. The plating layer containing an excessive amount of Si impairs the workability of the hot-dip aluminized steel sheet, so the upper limit of the Si concentration is restricted to 12% by mass. When it is necessary to improve properties other than welding, elements such as Ti, Sr, B, Cr, Mn, and Zn that do not greatly affect the Al—Fe interdiffusion reaction can be appropriately included in the molten aluminum plating layer.

  N: When a steel sheet containing 0.002 to 0.020 mass% is subjected to hot-dip aluminum plating as a plating base plate and then heat-treated under specific conditions, an N-concentrated layer is formed at the interface between the alloy layer and the base steel produced during hot-dip plating. . When the N content in the concentrated layer is 3.0 atomic% or more, Al—Fe interdiffusion is remarkably suppressed, and a hot-dip aluminized steel sheet suitable as a steel / aluminum joint structure is obtained. The effect of suppressing the interdiffusion of Al—Fe by the N-enriched layer improves as the N content of the base steel increases when the heat treatment conditions after hot dipping are made constant. However, when an excessive amount of N exceeding 0.02 mass% is included, the manufacturability of the plating original plate itself is lowered.

  The aluminum material of the mating member does not impose any particular restrictions on the material, but most aluminum or aluminum alloy can be used as long as it is a wrought material. Fe contained in the aluminum material also has the effect of suppressing the formation and growth of the Al—Fe binary alloy layer as in the case of the molten aluminum plating layer, but Al—Fe 2, which is an interfacial reaction between the base steel and the molten aluminum plating layer. The influence and generation of the original alloy layer is much smaller than that of Fe in the hot-dip aluminum plating layer. Therefore, it is preferable to limit the Fe concentration of the aluminum material to 1.0 mass% or less in consideration of the corrosion resistance, workability, etc. of the aluminum material itself.

Aluminum alloy is required to add 3.0% by mass or less, especially about 1% by mass of Si and 0.1 to 1.5% by mass of Mg and to precipitate fine Mg ₂ Si by heat treatment such as aging treatment. Strength is given. In order to improve the strength by Mg ₂ Si precipitation, it is preferable to set the lower limit of the Si content to 0.1% by mass. When 1.5 to 6.0% by mass of Mg is added, high strength can be obtained even by solid solution strengthening. Such an effect is seen in 0.1 to 6.0 mass% of Mg and 3.0 mass% or less of Si, and the contents of Mg and Si are determined according to the required strength. However, if an excessive amount of Mg exceeding 6.0% by mass is included, defects are likely to occur during spot welding, and if an excessive amount of Si exceeding 3.0% by mass is included, coarse precipitates in the aluminum matrix or Crystallized matter may be generated and bonding strength may be reduced.

  The joint structure is manufactured by superposing a molten aluminum plated steel sheet and an aluminum material cut into a predetermined size and spot welding at a predetermined pitch. The welding conditions are determined by the combination of the welding current and the energizing time. However, in the case of an appropriate molten aluminum plating layer composition, the bonding strength tends to increase with an increase in the welding current. When spot welding is performed using a copper alloy tip having an electrode tip R of 75 mm under the conditions of welding current: 25 kA and energization time: 12 cycles, a good tensile shear strength exceeding 4 kN is obtained.

  It is difficult to directly measure the temperature at the joint interface during spot welding, but by observing the nugget (a molten part of the aluminum alloy) formed on the aluminum alloy sheet side and the heat-affected area of the molten aluminum-plated steel sheet base material The temperature during welding at the joint interface can be estimated. When the cross section of the nugget is observed, the melted portion is the largest at the center of the nugget and becomes smaller toward the outer periphery. Even on the side of the hot-dip aluminized steel sheet, the region affected by heat is the largest at the center of the nugget and becomes smaller toward the outer periphery.

  The quantitative change along the radial direction of the nugget in the molten part of the aluminum alloy plate and the heat-affected part of the hot-dip aluminum-plated steel sheet means that the center part of the nugget is held at a higher temperature for a longer time. The heating state of the nugget center is greatly affected by the tip radius of the electrode used for spot welding. Therefore, the relationship between the distance from the center of the nugget and the maximum temperature was investigated for nuggets formed by spot welding using electrodes with different tip radiuses.

  The results of the investigation show that the maximum temperature reached abruptly decreases with increasing distance from the center of the nugget as the electrode with a smaller tip radius is used (upper part of FIG. 3). The generation amount of the Al—Fe binary alloy layer also corresponds to the temperature distribution in the radial direction of the nugget, and the generation amount of the Al—Fe binary alloy layer increases along the radial direction of the nugget as an electrode with a smaller tip radius is used. A tendency to decrease rapidly was shown (bottom of FIG. 3). From the influence of the nugget tip radius on the amount of Al-Fe binary alloy layer produced, spot welding using an electrode with an appropriate tip radius forms an alloy layer disappearance zone from the nugget center to the nugget outer periphery. It can be seen that

  C: 0.04 mass%, Si: 0.01 mass%, Mn: 0.20 mass%, P: 0.01 mass%, S: 0.007 mass%, Al: 0.010 mass%, N: After forming a hot-dip aluminum plating layer of Si: 9.2 mass% and Fe: 1.8 mass% on a cold rolled steel sheet with a thickness of 1.0 mm containing 120 ppm: 20 μm, 450 ° C. × 15 hours A hot-dip aluminized steel sheet in which 5 atomic% of N was concentrated at the base steel / plated layer interface by post heating was used as one material to be joined. In the hot dip galvanized steel sheet, an Al—Fe—Si ternary alloy layer of Al-10.9% by mass Si-35.8% by mass Fe was formed at the base steel / plated layer interface.

The counterpart materials were Si: 0.11 mass%, Fe: 0.25 mass%, Mg: 5.52 mass%, Cu: 0.35 mass%, Cr: 0.02 mass%, Zn: 0.01 An aluminum alloy plate having a thickness of 1.0 mm with a mass% and the balance Al was used.
A test piece cut out from a hot-dip aluminum-plated steel plate or aluminum alloy plate was degreased and washed, and then overlapped and sandwiched between electrodes for spot welding, and a pressure of 3 kN was applied. Four types of copper alloy chips (diameter: 16 mm) having different tip radiuses of 4, 10, 75, and 200 mm were used for the electrode 3, and spot welding was performed at a maximum welding current of 25 kA, a frequency of 60 Hz, and a cycle number of 12.

The bonding interface formed by spot welding was observed, and the formation state of the Al—Fe binary alloy layer and the Al—Fe—Si ternary alloy layer was investigated. The observation result of the joint interface was image-processed, and the line segment ratio of the Al—Fe binary alloy layer was calculated from the Al—Fe binary alloy layer crossing a concentric circle every 1 mm from the nugget center. The production amount of the Al—Fe binary alloy layer was evaluated based on the calculated line segment ratio.
As shown in FIG. 3, the outer periphery of the nugget with the lowest maximum temperature becomes wider as the radius of the electrode tip becomes smaller, and accordingly, the generation region of the Al—Fe binary alloy layer is limited to the center of the nugget. When spot welding is performed using an electrode with a tip radius as small as 4 mm, almost no Al—Fe binary alloy layer is formed on the outer side of the nugget 2 mm or more from the center of the nugget, and part of the Al—Fe—Si ternary is formed. The alloy layer was detected and disappeared.

On the other hand, when an electrode having a large tip radius of 200 mm was used, the concentration of the welding current at the electrode center was small, and the line segment ratio of the Al—Fe binary alloy layer was as high as 98%.
Subsequently, a test piece was cut out from each joint structure, and joint strength was measured by a shear test. Tip R: In a joint structure obtained by spot welding using an electrode of 200 mm, the aluminum material is separated from the molten aluminum plated steel plate with a shearing force of 2.2 kN, and the trace of the aluminum material is present at the joint of the molten aluminum plated steel plate. Was not detected. On the other hand, some joint structures obtained by spot welding using an electrode having a small tip radius exhibit a high shear strength of 2.6 kN or more and break the base material. (Table 1)

C: 0.05% by mass, Si: 0.1% by mass, Mn: 0.25% by mass, P: 0.012% by mass, S: 0.006% by mass, Al: 0.006% by mass The rolled steel sheet was hot dip aluminum plated. In hot dip aluminum plating, the Si content of the hot dip aluminum plating layer is three levels of 1.8 mass%, 5.5 mass%, and 9.5 mass%, and the Fe content is 0.2 to 0.3 mass%, 0. Five levels of 0.8-0.9% by mass, 1.8-2.2% by mass, 4.2-4.6% by mass, 5.7-6.1% by mass, and a thickness of 25-30 μm. The composition of the molten aluminum bath and the conditions of the hot dipping were adjusted.

The other material is Si: 1.21% by mass, Fe: 0.37% by mass, Mg: 0.7% by mass, Cu: 0.02% by mass, Zn: 0.01% by mass, Al: remaining plate An aluminum alloy plate having a thickness of 1.0 mm was used.
A test piece cut out from a hot-dip aluminum-plated steel plate or aluminum alloy plate was degreased and washed, and then spot welded with an AC spot welder (60 Hz). As welding conditions, a tip end radius: a 75 mm copper alloy tip was used as an electrode, the welding current was set to 21 kA, and the energization time was set to 12 cycles.

The joint strength of the produced joint structure was measured by a tensile shear test and a cross tension test.
As can be seen from the test results in Table 2, when the Si and Fe concentrations in the molten aluminum plating layer are maintained within the appropriate ranges (Si: 3 to 12% by mass, Fe: 0.5 to 5% by mass), tensile shear A bonded structure having a high bonding strength of strength: 2.4 kN or more and cross tensile strength: 1.3 kN or more was obtained.

  On the other hand, when the Si and Fe concentrations in the hot-dip aluminum plating layer are low, the line segment ratio of the Al—Fe binary alloy layer is high, and the tensile shear strength is 2.4 kN or less, and the cross tensile strength is 0.9 kN. did. When observing the fractured interface in the tensile test, it was confirmed that continuous cracking occurred between the alloy layers up to the center, and the bonding strength was reduced by the Al-Fe binary alloy layer in the entire area of the interface. .

Also, in order to investigate the effect of the thickness of the Al-Fe-Si ternary alloy layer at the interface between the molten aluminum plating layer and the base steel, the melting of Si: 9.5 mass%, Fe: 2.2 mass% Adjusting the thickness of Al-Fe-Si ternary alloy layer to 0.08μm by increasing the cooling rate after hot dipping to -40 ℃ / sec or more for hot dipped aluminum plated steel sheet manufactured using aluminum plating bath did. In this case, since the thickness of the Al—Fe—Si ternary alloy layer does not reach the target value of 0.1 μm, an Al—Fe binary alloy layer is likely to be formed during spot welding, resulting in a tensile shear strength of 2.6 kN. The cross tensile strength was 1.1 kN, and the joint strength tended to decrease.

  As described above, when spot welding a hot-dip aluminized steel sheet and aluminum material using an electrode with a rounded tip, the temperature distribution of the joint from the center of the electrode toward the periphery is an Al-Fe binary alloy layer. A bonding interface having a temperature gradient effective for suppressing the formation of the alloy layer and having an alloy layer disappearing region without the Al—Fe binary alloy layer is formed. An Al—Fe—Si ternary alloy layer that is effective in improving joint strength may remain and be generated in a part of the alloy layer disappearance region. For this reason, steel materials and aluminum materials are firmly joined and used as various structural members such as vehicle structures and heat exchangers as joint structures utilizing the advantages of aluminum materials and steel materials.

Schematic diagram comparing the Al-Fe binary alloy layer formed at the joint interface between spot-welded plain steel plate / aluminum alloy plate (a) and hot-dip aluminized steel plate / aluminum alloy plate (b) Conceptual diagram of spot welding of aluminum sheet and aluminum material using electrode with tip radius and graph showing temperature distribution of joint A graph showing that the maximum temperature reached at the joint and the amount of Al-Fe binary alloy layer produced differ depending on the tip radius of the electrode

Explanation of symbols

1: Hot dip galvanized steel sheet 2: Aluminum material 3: Electrode 4: Hot dip aluminum plating layer 5: Base steel 6: Al—Fe—Si ternary alloy layer 7: Al—Fe binary alloy layer 8: Nugget 9: Alloy layer Vanishing zone

Claims (1)

Si: 3 to 12% by mass, Fe: 0.5 to 5% by mass, balance: a substantially Al plating layer is formed, and has a thickness of 0.1 μm or more at the plating layer / underlying steel interface. Aluminum or aluminum alloy is superimposed on a hot-dip aluminum-plated steel sheet on which an Al-Fe-Si ternary alloy layer is formed, and the hot-dip aluminum-plated steel sheet and aluminum or aluminum alloy are pressed with an electrode with a rounded tip, and spot welding is performed. A method for producing a steel / aluminum bonded structure.

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