JP7617430B2 - Friction welded joint, tailored blank material, molded product, and method for manufacturing friction welded joint - Google Patents

Description

The present invention relates to friction welded joints, tailored blanks, molded products, and methods for manufacturing friction welded joints.

Since Al-based plated steel sheets have excellent corrosion resistance, they are used as materials for various machine parts. However, when using Al-based plated steel sheets as materials for machine parts that require high strength, a decrease in joint strength due to Al concentration in the welded parts becomes an issue.

When aluminum-plated steel sheets are welded, the aluminum plating is incorporated into the weld metal, forming soft aluminum-enriched areas. These aluminum-enriched areas act as fracture initiation points when stress is applied to the welded joint, reducing the strength of the joint.

One way to avoid this problem is to remove the Al-based coating from the ends of the Al-based plated steel sheet before welding the Al-based plated steel sheet. For example, Patent Document 1 discloses a welded blank obtained by butt welding at least two sheets, characterized in that a welded joint is provided on the end face adjacent to the area where the alloy layer has been removed. However, the step of removing the Al-based coating increases the manufacturing costs of the machine part. Therefore, it is preferable to join the Al-based plated steel sheet without removing the Al-based coating.

Patent Document 2 discloses a tailored blank for hot stamping formed by butt-welding aluminum-plated steel sheets of different strengths, characterized in that the average concentration of Al in the weld metal formed by the butt-welding laser is 0.3 mass% or more and 1.5 mass% or less, the Ac3 point of the weld metal is 1250°C or less, and the steel sheets to be butt-welded are combined and welded so that the value of the product of the hardness of the weld metal after hot stamping and the thickness of the thinnest part of the weld metal is higher than the value of the product of the hardness and the sheet thickness after hot stamping of the steel sheet on the low-strength side. According to this technology, it is said that a tailored blank with sufficient joint strength can be obtained by butt-welding the plated layer of the welded part as it is without removing it. However, even with this technology, Al cannot be sufficiently discharged from the joint.

Patent No. 5237263 Patent No. 5316664

The objective of the present invention is to provide a friction welded joint, tailored blank material, formed product, and method for manufacturing a friction welded joint in which one or both of the base steel sheets have Al-based plating and have high joint strength.

The gist of the present invention is as follows:

(1) A friction welded joint according to one aspect of the present invention comprises a first steel plate having an Al-based plating, a second steel plate arranged substantially on the same plane as the first steel plate, and a friction welded surface joining an end face of the first steel plate and an end face of the second steel plate, wherein the Al concentration in an entire joint, which is a region within 0.5 mm from the friction welded surface in a direction perpendicular to the friction welded surface, is less than twice the Al concentration of the base metal of either the first steel plate or the second steel plate, which has a higher Al concentration.
(2) A friction welded joint according to another aspect of the present invention comprises a first steel plate having an Al-based plating, a second steel plate arranged substantially in the same plane as the first steel plate, and a friction welded surface joining an end face of the first steel plate and an end face of the second steel plate, wherein the average Al content in a joint which is a region within 0.5 mm from the friction welded surface in a direction perpendicular to the friction welded surface is less than 1.0 mass%.
(3) In the friction welded joint described in (1) or (2) above, the thinner of the first steel plate and the second steel plate may have a thickness of 1.0 mm to 4.0 mm.
(4) In the friction welded joint described in any one of (1) to (3) above, the product of the tensile strength and the plate thickness of the first steel plate after quenching may be smaller than the product of the tensile strength and the plate thickness of the second steel plate after quenching.
(5) In the friction welded joint described in any one of (1) to (4) above, the C content of the first steel plate may be 0.10 mass % or more.
(6) In the friction welded joint according to any one of (1) to (5) above, the first steel plate and the second steel plate may be hot stamping steel plates.
(7) In the friction welded joint described in any one of (1) to (6) above, the friction welded joint may have a burr at the friction welded surface that protrudes perpendicularly to the surfaces of the first steel plate and the second steel plate, and an Al-based plating may be provided continuously from the surface of the first steel plate on a part of the surface of the burr.
(8) A tailored blank material according to another aspect of the present invention includes the friction welded joint described above in any one of (1) to (7).
(9) A molded article according to another aspect of the present invention includes the friction welded joint described in any one of (1) to (7) above.
(10) In the molded product described in (9) above, the first steel plate may have a tensile strength of 1000 MPa or more.
(11) In the molded product described in (9) or (10) above, the product of the tensile strength and the plate thickness of the first steel plate may be smaller than the product of the tensile strength and the plate thickness of the second steel plate.
(12) A manufacturing method for a friction welded joint according to another aspect of the present invention includes a step of butting an end face of a first steel plate having an Al-based plating with an end face of a second steel plate to perform linear friction welding, wherein in the linear friction welding, an applied pressure is 50 MPa or more, a frequency is 10 Hz or more, and an amplitude is ±1 mm or more, and the linear friction welding is continued until burrs are removed from the entire butt faces of the first steel plate and the second steel plate.
(13) In the method for producing a friction welded joint described in (12) above, the Al-based plating may be formed up to the end face of the first steel plate at the start of the linear friction welding.

The present invention provides friction welded joints, tailored blanks, formed products, and methods for manufacturing friction welded joints in which one or both of the base steel sheets have Al-based plating and have high joint strength.

FIG. 2 is a cross-sectional view of a friction welded joint. FIG. 2 is an enlarged cross-sectional view of the friction welded surfaces and the bonded portion of the friction welded joint. FIG. 1 is a schematic diagram of a friction welded joint in which an Al-based plating is provided on part of the surface of the burr. 10 is a diagram illustrating the protruding length h when the plate thicknesses of the first steel plate and the second steel plate are different. FIG. 10 is a diagram illustrating the protruding length h when the plate thicknesses of the first steel plate and the second steel plate are the same. FIG.

(Friction welded joint according to the first embodiment)
First, a friction welded joint 1 according to a first embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a cross-sectional view showing the friction welded joint 1 according to the first embodiment, and Fig. 2 is an enlarged cross-sectional view of a friction welded surface 13 of the friction welded joint 1.

As shown in FIG. 1, the friction welded joint 1 according to this embodiment is a joint obtained by linearly friction-welding at least two steel plates 11 and 12 by butting their end faces together. In other words, the friction welded joint 1 is a so-called butt welded joint. Linear friction welding is a type of welding in which the temperature near the joint is increased by using frictional heat generated by linearly oscillating the members while they are in contact with each other, and pressure is applied (see JIS Z 3000-2:2018).

At least one of the two steel sheets 11 and 12 has an Al-based plating 111. The Al-based plating 111 means a plating whose main component is Al and contains elements other than Al as necessary. The elements other than Al contained in the Al-based plating 111 are, for example, 3 to 15 mass % Si. The Al-based plating 111 is, for example, hot-dip Al plating and electric Al plating. Hereinafter, for convenience, the steel sheet having the Al-based plating 111 is referred to as the first steel sheet 11. Also, the steel sheet joined to the first steel sheet 11 is referred to as the second steel sheet 12. However, both the first steel sheet 11 and the second steel sheet 12 may have an Al-based plating. In this case, any one of the two steel sheets 11 and 12 can be regarded as the first steel sheet 11. Also, the second steel sheet 12 may have a plating other than the Al-based plating.

The thickness of the first steel plate 11 and the second steel plate 12 is not particularly limited. As illustrated in FIG. 1, the thickness of the first steel plate 11 and the second steel plate 12 may be the same, or as illustrated in FIG. 2, the thickness of the first steel plate 11 and the second steel plate 12 may be different.

The friction welded joint according to this embodiment is a so-called butt welded joint, so the first steel plate 11 and the second steel plate 12 are arranged substantially on the same plane. However, a slight angle between the first steel plate 11 and the second steel plate 12 is permitted. In this embodiment, the smaller of the angles between the first steel plate 11 and the second steel plate 12 measured in a cross section perpendicular to the extension direction of the joint 14 is, for example, 170° to 180°.

The friction welded joint 1 has a friction welded surface 13 that joins the end face of the first steel plate 11 and the end face of the second steel plate 12. Unlike a welded joint, no molten solidified portion is formed at the joint of the friction welded joint 1, which is formed by solid-state welding. When the cross section of the welded joint is corroded with picric acid and observed, dendrites, which are traces of molten solidification, are clearly visible, but the corroded cross section of the friction welded joint 1 does not contain dendrites, and instead, it can be seen that the friction welded surface 13 exists in a streak-like manner.

The friction welded surface 13 is formed by linear friction welding. In the welded portion 14 of the friction welded joint 1 according to the first embodiment, obtained by appropriate linear friction welding, the Al concentration is less than twice the Al concentration of the base material of either the first steel plate 11 or the second steel plate 12, whichever has the higher Al concentration. Here, the welded portion 14 refers to the region within 0.5 mm from the friction welded surface 13 in the direction perpendicular to the friction welded surface 13. For example, in the cross section of the friction welded joint 1 shown in Figure 2, the region surrounded by dashed lines A to D is the welded portion 14.

As described above, in a friction welded joint having a joint 14 with a suppressed Al concentration, substances present on the end faces of the first steel plate 11 and the second steel plate 12 are expelled to the outside of the friction welded surface 13. The substances present on the end faces are, for example, oxides and sagging of the Al-based plating 111. In other words, in the friction welded joint 1 according to the first embodiment, the Al-based plating 111 is expelled from the joint 14. In a normal welded joint, the Al-based plating is taken up by the weld metal, forming a soft Al-enriched portion that reduces the joint strength. On the other hand, when the Al-based plating 111 is expelled from the joint 14, no softened portion is formed at the joint, and high joint strength is ensured.

In addition, burrs 15 protruding vertically from the surfaces of the first steel plate 11 and the second steel plate 12 may be formed near the friction welded surface 13. The burrs 15 are formed when material present on the end surfaces of the first steel plate 11 and the second steel plate 12 is expelled to the outside of the friction welded surface 13. The burrs 15 may contain Al-based plating 111. This is because the burrs 15 do not affect the joining strength of the friction welded joint 1. On the other hand, the burrs 15 may be removed from the friction welded joint 1 for the purpose of maintaining the aesthetic appearance of the friction welded joint 1, etc.

The burr 15 may have an Al-based plating formed continuously from the surface of the first steel sheet 11 on a part of its surface. As shown in FIG. 3, the burr 15 is generated by the rise of metal softened by the temperature rise near the friction welded surface 13. When a steel sheet with an Al-based plating arranged up to the end surface is used for linear friction welding, the Al-based plating arranged near the end surface is turned up following the rise of the burr 15. Therefore, when a steel sheet with an Al-based plating arranged up to the end surface is used for linear friction welding, the Al-based plating is arranged continuously from the surface of the first steel sheet 11 on a part of the surface of the burr 15. It is presumed that the process of removing the Al-based plating from the end surface during linear friction welding has been omitted in the friction welded joint having such a burr 15. Therefore, the friction welded joint having such a burr 15 is superior in terms of manufacturing costs.

Whether or not the Al-based plating 111 is expelled from the joint 14 can be determined by measuring the element distribution in the cross section of the friction welded joint 1 using EPMA. FIG. 2 shows an enlarged cross section of the friction welded joint 1. The dashed lines A and C in FIG. 2 are lines extending from the surface of the thinner of the first steel plate 11 and the second steel plate 12 (the first steel plate 11 in FIG. 2) that has not been subjected to plastic deformation. The interface between the first steel plate 11 and the second steel plate 12, and the region located inside the dashed lines A and C in the plate thickness direction, is regarded as the friction welded surface 13. The dashed lines B and D in FIG. 2 are lines parallel to the friction welded surface 13 and 0.5 mm away from the friction welded surface 13. The region surrounded by the dashed lines A to D is the region within 0.5 mm from the friction welded surface 13 in the direction perpendicular to the friction welded surface 13, that is, the joint 14. The Al concentration in the region surrounded by dashed lines A to D is measured at 100 μm intervals along the direction along the friction welded surface 13 and along the direction perpendicular to the friction welded surface 13. As a result, the measurement points of the Al concentration of the joint 14 are arranged in a lattice pattern with 100 μm intervals. A friction welded joint that does not have a measurement point with an Al concentration twice or more higher than the Al concentration of the steel plate in the region surrounded by dashed lines A to D is considered to be a joint in which the Al-based plating 111 has been expelled from the joint 14. If the Al concentration of the first steel plate 11 differs from the Al concentration of the second steel plate 12, the Al concentration of the steel plate with the higher Al concentration is used as the above-mentioned "Al concentration of the steel plate."

Note that areas (e.g., burrs 15) that are outside dashed lines A and C in the plate thickness direction are not evaluated. In addition, in FIG. 2, where the first steel plate 11 and the second steel plate 12 are on the same plane, the cross section of the joint 14 is rectangular, but the first steel plate 11 and the second steel plate 12 may form a slight angle. In this case, the angle between dashed lines A and C and dashed lines B and D is not 90°, but may vary in the range of approximately 80° to 100°. However, even in this case, it is sufficient to arrange the measurement points in a grid pattern with 100 μm intervals as described above.

(Friction welded joint according to the second embodiment)
Next, a friction welded joint 1 according to a second embodiment of the present invention will be described. The friction welded joint 1 according to the second embodiment has a first steel plate 11 having an Al-based plating 111, a second steel plate 12 arranged substantially on the same plane as the first steel plate 11, and a friction welded surface 13 joining the end surface of the first steel plate 11 and the end surface of the second steel plate 12, as in the first embodiment. The configurations of the first steel plate 11, the second steel plate 12, and the Al-based plating 111 can be the same as in the first embodiment. In addition, the friction welded joint 1 according to the second embodiment may have a burr 15 as in the first embodiment.

The friction welded joint 1 according to the second embodiment has a friction welded surface 13 that joins the end face of the first steel plate 11 and the end face of the second steel plate 12, and the average Al content in the joint 14, which is a region within 0.5 mm from the friction welded surface 13 in a direction perpendicular to the friction welded surface 13, is less than 1.0 mass%. The joint 14 is the region surrounded by dashed lines A to D in FIG. 2, as in the first embodiment. When the average Al content in the joint 14 is less than 1.0 mass%, no softening due to Al occurs in the joint of the friction welded joint 1, and high joint strength is ensured.

The average Al content in the joint 14 can be determined by a method similar to that for determining whether the Al-based plating 111 is discharged from the joint 14. Specifically, the average Al content in the joint 14 is determined by measuring the element distribution in the cross section of the friction welded joint 1 using an EPMA. That is, the Al concentration in the region surrounded by dashed lines A to D in FIG. 2 is measured at 100 μm intervals along the direction along the friction welded surface 13 and along the direction perpendicular to the friction welded surface 13. As a result, the measurement points for the Al concentration in the joint 14 are arranged in a lattice pattern with 100 μm intervals. The average value of the Al concentration at these measurement points is then calculated, and this is regarded as the average Al content in the joint 14.

In both the determination of whether the Al-based plating 111 is discharged from the joint 14 and the measurement of the average Al amount in the joint 14, the EPMA measurement conditions are as follows.
Acceleration voltage: 15 kV
・Beam diameter 100 nm
- Irradiation time per point: 1000 ms

The friction welded joint 1 may have both the configuration according to the first embodiment and the configuration according to the second embodiment. In addition, as long as the above-mentioned requirements are met, the configurations of the friction welded joint 1 according to the first embodiment and the second embodiment are not particularly limited. A more suitable form of the friction welded joint 1 according to these embodiments will be described below as an example. Unless otherwise specified, the configuration described below is applicable to both the friction welded joint 1 according to the first embodiment and the friction welded joint 1 according to the second embodiment.

(Thickness)
The thickness of at least the thinner steel plate of the first steel plate 11 and the second steel plate 12 may be 4.0 mm or less. The thicknesses of both steel plates may be 4.0 mm or less. By making the thickness of the steel plate 4.0 mm or less, it becomes possible to use the friction welded joint 1 as a material for press-molded parts. This makes it possible to apply the friction welded joint 1 according to this embodiment to a wide range of thin plate structural members such as automobile parts. The thickness of one or both of the first steel plate 11 and the second steel plate 12 may be 3.5 mm or less, 3.0 mm or less, or 2.8 mm or less.

On the other hand, by making the thickness of at least the thinner steel plate 1.0 mm or more, it is possible to effectively prevent buckling from occurring during friction welding of the first steel plate 11 and the second steel plate 12. The thickness of the thinner steel plate may be 1.2 mm or more, 1.5 mm or more, or 2.0 mm or more. There is no prohibition on applying the above-mentioned values to the lower limit of the thickness of the thicker steel plate.

(Tensile strength and thickness of steel plate)
By specifying the quench hardening ability (a property of an alloy that indicates the ease of quench hardening by heat treatment, and a property that indicates how deep and hard the structure can be obtained when quenched) and thickness of the steel plate, the superiority of the friction welded joint 1 according to this embodiment over the prior art becomes even more prominent. One example of an evaluation index of quench hardening ability that is commonly used is carbon equivalent (a value obtained by converting the content of alloy elements in a steel material into the amount of carbon), but in the friction welded joint 1 according to this embodiment, "tensile strength after quenching" is used as an evaluation index of hardenability. The tensile strength of a steel plate after quenching means the tensile strength of the steel plate measured after the steel plate is heated to 800 to 1000 ° C and then quenched by mold cooling or water cooling. During quenching, the time required for the temperature of the steel plate to decrease from 800 ° C to 500 ° C is set to be smaller than the value t _M calculated by the following formulas 1 to 4.
log ₁₀ t _M =4.6×CEI-2.08 (Equation 1)
The CEI is a value calculated by the following formula.
CEI=Cp+Si/24+Mn/6+Cu/15+Ni/12+Cr×(1-0.16√Cr)/8+Mo/4+0.09 (Formula 2)
Cp=C (C≦0.3%) (Formula 3)
Cp=C/6+0.25 (C>0.3%) (Formula 4)
The element symbols in Formulas 2 to 4 represent the content, in mass %, of the alloy element corresponding to the symbol in the steel sheet.

The quench hardening ability of the steel plate can be evaluated by measuring the tensile strength after quenching the steel plate under the above conditions. However, even if the quench hardening ability of the steel plate is specified using the "tensile strength after quenching," the steel plate of the friction welded joint 1 does not need to be quenched. The friction welded joint 1 before quenching is also considered to be the friction welded joint 1 according to this embodiment as long as it meets the specified requirements.

In the friction welded joint 1 according to this embodiment, the product of the tensile strength and thickness after quenching of the first steel plate 11 having the Al-based plating 111 may be smaller than the product of the tensile strength and thickness after quenching of the second steel plate 12. In this case, the friction welded joint 1 has an even greater advantage over a normal welded joint.

In a normal welded joint, the above-mentioned base material conditions are extremely disadvantageous from the viewpoint of ensuring joint strength. The reasons for this include: (1) the Al-based coating 111 incorporated into the weld metal is concentrated near the fusion boundary, which is the boundary between the base material and the weld metal; (2) stress concentration is likely to occur at the fusion boundary of a thin steel plate; and (3) if a soft Al-concentrated area is present at the fusion boundary of a high-strength steel plate, the difference in hardness between the Al-concentrated area and other areas becomes significant.

However, in the friction welded joint 1 according to this embodiment, the Al-based plating 111 is expelled from the friction welded surface and its vicinity, or the average Al concentration in this area is kept extremely low. Therefore, even if the friction welded joint 1 according to this embodiment has the above-mentioned tensile strength and plate thickness after quenching, it is possible to ensure high joint strength.

(C content of steel plate)
The C content of the first steel sheet 11 having the Al-based plating 111 may be, for example, 0.10 mass% or more. It is known that a butt joint obtained by welding an Al-based plated steel sheet having a high C content and therefore high strength is very likely to have a decrease in joint strength. The reason for this is that the hardness difference between the Al-enriched portion and the other portion is significant. However, in the friction welded joint 1 according to this embodiment, the Al-based plating 111 is discharged from the friction welded surface and its vicinity, or the average Al concentration in this region is kept extremely low. Therefore, even if the C content of the first steel sheet 11 having the Al-based plating 111 is 0.10 mass% or more, the friction welded joint 1 according to this embodiment can ensure high joint strength.

(Type of steel plate)
The use of the friction welded joint 1 according to this embodiment is not particularly limited, and the type of steel plate constituting the joint can be appropriately selected according to the use. For example, when the friction welded joint 1 is used as an automobile part, it is preferable that the first steel plate 11 and the second steel plate 12 are hot stamping steel plates. The hot stamping steel plate is a steel plate having a plate thickness and hardenability suitable for hot stamping. The components of the hot stamping steel plate include, for example, in mass%, C: 0.05 to 0.55%, Si: 0.01 to 2.0%, Mn: 0.2 to 3.0%, Cr: 0.01 to 0.5%, B: 0.1% or less (including 0%), and Al: 0.005 to 0.1%, with the balance being Fe and impurities. The impurities refer to, for example, components that are mixed in due to raw materials such as ores or scraps, or various factors in the manufacturing process when industrially manufacturing steel, and are acceptable within a range that does not adversely affect the friction welded joint according to this embodiment. The steel plate may further contain one or more of Ti, Mo, and Nb. Only a part of the numerical ranges of the contents of each alloying element listed above may be applied to the first steel plate 11 and/or the second steel plate 12. For example, the first steel plate 11 and the second steel plate 12 may be steel plates containing Al: 0.005 to 0.1%, with the remainder being iron and any alloying element.

So far, we have described a friction welded joint 1 in which two steel sheets 11 and 12 are butted together at their end faces. However, the friction welded joint 1 may have three or more steel sheets. All of the steel sheets may be joined by the friction welded surfaces 13, or only some of the steel sheets may be joined by the friction welded surfaces 13, with the remaining steel sheets being joined by another means such as welding. For example, when manufacturing a joint by joining one Al-based plated steel sheet and two unplated steel sheets, friction welding may be applied only to the joining of the Al-based plated steel sheet and the unplated steel sheet, and another joining means (e.g. laser welding, etc.) may be applied to the joining of the unplated steel sheets.

(Tailored blank material according to the third embodiment)
Next, a tailored blank material according to a third embodiment of the present invention will be described. A tailored blank material is a material for press-molded parts, and is manufactured by joining a plurality of steel plates by means of welding or the like. Tailored blank materials are extremely suitable as materials for machine parts manufactured by joining together steel plates of various materials and thicknesses, such as automobile side panels and floor panels.

The tailored blank material according to the third embodiment of the present invention includes the friction welded joint 1 according to the first or second embodiment described above. Therefore, the tailored blank material according to the third embodiment can ensure high joint strength, even though it has an Al-based plated steel sheet as a component.

(Molded product according to the fourth embodiment)
Next, a formed product according to a fourth embodiment of the present invention will be described. The formed product is a part obtained by pressing a tailored blank material, preferably by hot stamping.

The formed product according to the fourth embodiment includes the friction welded joint 1 according to the first or second embodiment described above, as well as the tailored blank material according to the third embodiment. The formed product according to the fourth embodiment can ensure high joint strength despite having an Al-based plated steel sheet as a component. The difference between the tailored blank material and the formed product is whether it is in a state before press working or in a state after press working. The formed product according to the fourth embodiment can be manufactured, for example, by hot stamping the tailored blank material according to the third embodiment. The use of the formed product is not particularly limited, but it can be, for example, an automobile part.

In order to facilitate processing, the tailored blank material before press processing is usually not quenched. On the other hand, the molded product according to the fourth embodiment is preferably quenched. For example, it is preferable that the tensile strength of the first steel sheet 11 is 1000 MPa or more. This allows the strength of the molded product to be dramatically increased. When an Al-based plated steel sheet with a tensile strength of 1000 MPa or more is welded, the joint strength may decrease due to Al concentration in the welded part. On the other hand, in the molded product according to the fourth embodiment, the Al-based plating 111 is discharged from the region within 0.5 mm from the friction welded surface 13, or the average Al amount in the region within 0.5 mm from the friction welded surface 13 is less than 1.0 mass%, so that the joint strength can be ensured even if the tensile strength of the Al-based plated steel sheet is 1000 MPa or more.

In the molded product according to the fourth embodiment, the product of the tensile strength and the thickness of the first steel sheet 11, which is an Al-plated steel sheet, may be smaller than the product of the tensile strength and the thickness of the second steel sheet 12. It is known that molded products made of such Al-plated steel sheets are very susceptible to a decrease in joint strength. The reason for this is that Al concentration is very likely to occur, and furthermore, there is a significant difference in hardness between the Al-concentrated portion and the other portions. However, in the molded product according to the fourth embodiment, the Al-plating 111 is discharged from the joint 14, which is a region within 0.5 mm from the friction-welded surface 13 in a direction perpendicular to the friction-welded surface 13, or the average Al content in the joint 14 is less than 1.0 mass%. Therefore, even if the Al-plated steel sheet has the above-mentioned tensile strength and thickness, the molded product according to the fourth embodiment can ensure high joint strength.

(Method of manufacturing a friction welded joint according to the fifth embodiment)
Next, a method for manufacturing a friction welded joint according to a fifth embodiment of the present invention will be described. The method for manufacturing a friction welded joint according to the fifth embodiment of the present invention includes a process of butting an end face of a first steel plate having an Al-based coating with an end face of a second steel plate to perform linear friction welding. Linear friction welding is a type of welding in which the temperature in the vicinity of the joint is increased by utilizing frictional heat generated by linearly oscillating only one side while the members are in contact with each other, and pressure is applied (see JIS Z 3000-2:2018).

In linear friction welding, the applied pressure is 50 MPa or more, the frequency is 10 Hz or more, and the amplitude is ±1 mm or more. The applied pressure refers to the pressure applied between the first steel plate and the second steel plate. The frequency refers to the number of times the first steel plate and the second steel plate are linearly oscillated per second. The amplitude refers to the maximum amount of displacement when the first steel plate and the second steel plate are linearly oscillated. This allows the first steel plate 11 and the second steel plate 12 to be sufficiently heated and the friction welding surface 13 to be formed.

Furthermore, the linear friction welding needs to be continued until burrs are discharged from the entire butt surface of the first steel plate 11 and the second steel plate 12. "Burrs are discharged from the entire butt surface" means that the material softened by the temperature rise is discharged as burrs from the entire butt interface. It is difficult to confirm whether this state has been achieved without destroying the friction welded joint 1. Therefore, it is sufficient to perform a friction welding test in advance using the same material as the friction welded joint 1 to be manufactured, and to determine in advance the joining time that can achieve the above-mentioned state. By performing friction welding for a time longer than this joining time, burrs can be discharged from the entire butt surface of the first steel plate 11 and the second steel plate 12.

In the linear friction welding, other joining conditions are not particularly limited, and various conditions can be adopted within a range in which the linear friction welding can be properly completed and a friction welded joint without joining defects can be obtained. As a suitable manufacturing condition, for example, it is preferable to set the projection length according to the plate thickness, etc. Specifically, when the thinner plate thickness t (mm) of the steel plates 11 and 12 constituting the friction welded joint 1 is 1.0 mm to 4.0 mm, it is preferable that the plate thickness t, the projection length h (mm) in the linear friction welding, and the applied pressure σ _c (MPa) in the linear friction welding satisfy the relationship of the following formula 5. That is, it is preferable that the applied pressure σ _c is equal to or less than the upper limit value (100×t/h) ² (MPa) defined in the following formula 5.
σ _c ≦(100×t/h) ² (Formula 5)
Here, the protrusion length refers to the length of the part of the thinner of the first steel plate 11 and the second steel plate 12 protruding from the chuck C toward the end face of the steel plate when the plate thicknesses of the first steel plate 11 and the second steel plate 12 are different as in Fig. 4A, and refers to the length of the part of the first steel plate 11 and the second steel plate 12 protruding from the chuck C toward the end face of the steel plate when the plate thicknesses of the first steel plate 11 and the second steel plate 12 are the same as in Fig. 4B. When the thicknesses and protrusion amounts of the first steel plate 11 and the second steel plate 12 are the same, the same results can be obtained regardless of which protrusion amount is used as the protrusion length h.

In friction welding of steel plates, unlike welding, it is necessary to apply pressure to the steel plate perpendicular to the thickness direction of the steel plate. Therefore, if the applied pressure is too large relative to the thickness of the steel plate, the steel plate may buckle. On the other hand, if the applied pressure is too small, the time required for pressure welding may be long and productivity may decrease. In addition, if the time required for pressure welding is long, the temperature of the part away from the joining interface may increase due to frictional heat, soften, and buckling may occur easily. According to the experimental results of the present inventors, as described above, by setting the lower limit of the applied pressure to 50 MPa and the upper limit of the applied pressure σ _c according to the plate thickness of the steel plate, it is possible to more effectively suppress buckling of the steel plate. Note that the smaller the protruding length h, the more preferable it is from the viewpoint of suppressing buckling, but it is necessary to note that if the protruding length h is too small, it becomes difficult to ensure sufficient joining time.

When welding Al-based plated steel sheets, it is common to remove the Al-based plating on the surface of the steel sheet to avoid embrittlement of the joint due to Al. On the other hand, in the manufacturing method of the friction welded joint according to this embodiment, the Al-based plating is expelled from the joint by friction welding, so there is no need to remove the Al-based plating 111 from the steel sheet when friction welding the steel sheets. In other words, in the manufacturing method of the friction welded joint according to this embodiment, the Al-based plating 111 may be formed up to the end face of the first steel sheet 11 at the start of linear friction welding. This allows the process of removing the Al-based plating 111 from the first steel sheet 11 to be omitted, further reducing the manufacturing cost of the friction welded joint.

The effect of one aspect of the present invention will be explained in more detail using an example. However, the conditions in the example are merely one example of conditions adopted to confirm the feasibility and effect of the present invention. The present invention is not limited to this one example of conditions. Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

Various butt joints were manufactured by butting the end faces of hot stamping steel sheets A and B shown in Table 1 together and joining them using the joining method shown in Table 2. Of the two steel sheets, the one with the smaller product of tensile strength and sheet thickness was designated steel sheet a. The specific joining conditions were as follows. In all of the invention examples and comparative examples, steel sheets a and b were joined without removing the Al plating.

(Laser welding conditions)
Laser type: Fiber laser with focal diameter of φ0.6 mm Laser output: 4 to 6 kW (adjusted according to board assembly)
Welding speed: 3 m/min
Shielding gas type: Ar
Shielding gas flow rate: 20 l/min

(Linear friction welding conditions)
Applied pressure: 100 MPa
Frequency: 15Hz
Amplitude: ±2mm
・Protrusion length: For both steel plate A and steel plate B, 7 mm (invention example) or 30 mm (reference example)

The various butt joints obtained were quenched using a flat die. The quenching conditions were a heating temperature of 900°C, a heating time of 4 minutes, and a bottom dead center holding time of 1 minute. JIS No. 5 tensile test pieces were cut out from the quenched test pieces. When cutting, the longitudinal direction of the tensile test pieces was perpendicular to the joint interface. The tensile test pieces were then subjected to a static tensile test to confirm the fracture mode. The tensile test was performed at a tensile speed of 10 mm/min. Joints in which fracture occurred in the base material were judged to have good joint strength, and were marked with "O" in the "Fracture Mode" column in Table 2.

In addition, whether or not the Al plating was expelled from the joint was confirmed using the method described above, and the results are also shown in Table 2. The average Al concentration of the laser weld was determined from a range of 0.5 mm from the butt interface before welding.

The joints of Comparative Examples 1 to 5, which were manufactured by laser welding, had weld metal at the joint, and aluminum-enriched areas due to aluminum plating were confirmed in the weld metal. When these were subjected to a fracture test, fracture occurred at the joint (weld metal).

In Reference Example 1, linear friction welding was used as the joining method, but the resulting friction welded joint had poor joining. In other words, the linear friction welding process could not be completed properly. This is presumably because the applied pressure was too large compared to the plate thickness.

Invention Examples 1 to 6 were joints formed by linear friction welding with an appropriate applied pressure. The Al plating was expelled from the joints of these invention examples (areas within 0.5 mm of the friction welded surface). Furthermore, when these invention example joints were subjected to a fracture test, fracture occurred in steel plate A, not in the friction welded surface. In other words, the joints of invention examples 1 to 6 had excellent joint strength, even though one or both of the base steel plates had Al-based plating.

Reference Signs List 1 Friction welded joint 11 First steel plate 111 Al-based plating 12 Second steel plate 13 Friction welded surface 14 Joint 15 Burr C Chuck

Claims (13)

A first steel sheet having an Al-based plating;
a second steel plate disposed substantially flush with the first steel plate;
a friction welded surface joining an end surface of the first steel plate and an end surface of the second steel plate;
Equipped with
A friction welded joint, in which the Al concentration is less than twice the Al concentration of the base metal of either the first steel plate or the second steel plate, whichever has a higher Al concentration, over the entire joint, which is a region within 0.5 mm from the friction welded surface in a direction perpendicular to the friction welded surface. A first steel sheet having an Al-based plating;
a second steel plate disposed substantially flush with the first steel plate;
a friction welded surface joining an end surface of the first steel plate and an end surface of the second steel plate;
Equipped with
A friction welded joint, in which the average Al content in a joint region within 0.5 mm from the friction welded surface in a direction perpendicular to the friction welded surface is less than 1.0 mass%. The friction welded joint according to claim 1 or 2, characterized in that the thinner of the first steel plate and the second steel plate has a thickness of 1.0 mm to 4.0 mm. The friction welded joint according to any one of claims 1 to 3, characterized in that the product of the tensile strength and thickness of the first steel plate after quenching is smaller than the product of the tensile strength and thickness of the second steel plate after quenching. The friction welded joint according to any one of claims 1 to 4, characterized in that the C content of the first steel plate is 0.10 mass% or more. The friction welded joint according to any one of claims 1 to 5, characterized in that the first steel plate and the second steel plate are hot stamping steel plates. The friction welded joint has burrs protruding perpendicularly to the surfaces of the first steel plate and the second steel plate at the friction welded surfaces,
7. The friction welded joint according to claim 1, wherein an Al-based plating is provided continuously on a part of a surface of the burr from the surface of the first steel plate. A tailored blank material including a friction welded joint according to any one of claims 1 to 7. A molded article comprising the friction welded joint according to any one of claims 1 to 7. The molded product according to claim 9, characterized in that the tensile strength of the first steel plate is 1000 MPa or more. The molded product according to claim 9 or 10, characterized in that the product of the tensile strength and thickness of the first steel plate is smaller than the product of the tensile strength and thickness of the second steel plate. The method includes a step of butting an end surface of a first steel sheet having an Al-based plating against an end surface of a second steel sheet, and linearly friction joining the end surfaces of the first steel sheet and the second steel sheet together,
In the linear friction joining, the applied pressure is 50 MPa or more, the frequency is 10 Hz or more, and the amplitude is ±1 mm or more;
A manufacturing method for a friction welded joint, comprising continuing the linear friction welding until burrs are removed from the entire butt surfaces of the first steel plate and the second steel plate. The method for manufacturing a friction welded joint according to claim 12, characterized in that the Al-based plating is formed up to the end surface of the first steel plate at the start of the linear friction welding.

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