Tailored blank material, its manufacturing method, and press-molded product

Abstract

To provide a tailored blank material which has a joint part excellent in moldability and is made of metal materials having different strengths and a method for simply and efficiently manufacturing the tailored blank material, and a press molding using the tailored blank material.SOLUTION: There is provided a method for manufacturing a tailored blank material that includes linear friction bonding one member to the other member, wherein a difference in tensile strengths between the one member and the other member is set at 150 MPa or more. When a bonding pressure applied in linear friction bonding is represented by P, a temperature at which tensile strength of the one member is P is represented by T1(°C) and a temperature at which tensile strength of the other member is P is represented by T2(°C), P is preferably set so that a difference between T1 and T2 is within 100°C.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a tailored blank material, a manufacturing method thereof, and a press-formed product using the tailored blank material. [Background technology]

[0002] In practical use, high-strength steel sheets for automobiles are sometimes fabricated by welding multiple steel sheets together to produce tailored blanks, which are then processed by press forming, etc. Therefore, formability is required not only for the base material but also for the joints.

[0003] However, the joints obtained by conventional welding techniques tend to harden and / or soften, and it is not possible to give the joints the same degree of formability as the base material. In particular, as the strength of the steel plates increases, the difference in mechanical properties between the base material and the joints becomes greater, and the joints become distinct singular points in structural design and processing.

[0004] In response to this, for example, Patent Document 1 (JP 2014-83565 A) discloses a manufacturing method for a tailored blank, which includes a welding process in which a thick plate and a thin plate are butt-welded together so that one surface forms a flat surface, and a remelting process in which the welded portion welded in the welding process is remelted, and which is characterized in that in the remelting process, a remelting scanning speed and heat input density for remelting the welded portion are set to values ​​at which the welded portion is annealed and cooling of the remelted portion progresses.

[0005] In the manufacturing method of tailored blanks described in the above Patent Document 1, it is stated that "by slowing down the remelt scanning speed, the time for heat to be conducted from the remelted portion to the periphery of the remelted portion during remelting is increased, and the heated area is expanded. By maintaining a wide heated area around the weld at a high temperature, the temperature difference between the remelted portion and the heated area can be reduced, the cooling rate of the remelted portion can be slowed, and cooling proceeds at a rate at which the weld is annealed. By annealing and cooling the weld in this way, the hardness of the joint can be reduced. In addition, by remelting the irregularities formed on the surface of the weld during welding, the surface of the joint can be made smooth. These make it possible to alleviate stress concentration at the joint and cracks at the joint."

[0006] Furthermore, Patent Document 2 (JP Patent Publication No. 2015-510453) discloses a method for manufacturing a tailored blank, which is characterized by joining plated steel sheet blanks having different strengths or thicknesses by laser welding using a filler wire.

[0007] The manufacturing method of tailored blanks described in Patent Document 2 above states that "by manufacturing tailored blanks using a filler wire designed with consideration for the penetration of the plating layer, the welded part has a full martensite structure after hot stamping. As a result, the plating layer removal process and re-plating process are not required when manufacturing tailored blanks, which results in reduced costs and improved productivity." [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2014-83565 A [Patent Document 2] Special Publication No. 2015-510453 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the manufacturing methods of tailored blanks disclosed in Patent Documents 1 and 2 form a welded part consisting of a molten solidified structure by fusion welding, and there is a limit to the control of the microstructure and mechanical properties of the welded part. In general carbon steel materials, martensite is generated in the welded part, causing embrittlement and hardening. In addition, since the temperature is inevitably raised to a temperature above the melting point of the welded material during fusion welding, softening of the heat-affected zone becomes a problem, especially for high-strength metal materials.

[0010] In other words, the welds formed by conventional welding methods are singular points that have mechanical properties different from those of the base metal, and it has been difficult to impart good formability to tailored blank materials that include such welds. In particular, in tailored blank materials in which metal materials having different strengths are joined, it is difficult to form a good joint interface that has sufficient strength and has isotropic mechanical properties without anisotropy that do not inhibit the plastic deformation of the joint, and it is also extremely difficult to control the mechanical properties of the joint.

[0011] In view of the problems in the conventional techniques as described above, an object of the present invention is to provide a tailored blank material having a joint with excellent formability and made of metal materials having different strengths, and a method for simply and efficiently manufacturing the tailored blank material. Another object of the present invention is to provide a press-formed product using the tailored blank material of the present invention. [Means for solving the problem]

[0012] In order to achieve the above-mentioned object, the inventors conducted extensive research into the formability of joints in tailored blank materials and suitable joining methods, and discovered that the use of linear friction joining is extremely effective, leading to the present invention.

[0013] That is, the present invention provides: A linear friction joint is formed between one member and another member, The difference in tensile strength between the one member and the other member is 150 MPa or more; The present invention provides a method for manufacturing a tailored blank material, characterized by the above.

[0014] By using linear friction welding, it is possible to obtain a good solid-state weld even between metal components with a tensile strength difference of 150 MPa or more. In addition, compared to friction stir welding (FSW), a typical solid-state welding method, an extremely thin weld area that is homogeneous in the plate thickness direction is formed, so the effect of the weld on the formability of the tailored blank material can be minimized. Here, from the viewpoint of the usefulness and applicability of the tailored blank material, it is more preferable that the difference in tensile strength between one component and the other component is 300 MPa or more.

[0015] Figure 1 shows a schematic diagram of the situation during linear friction welding. Linear friction welding is a solid-state welding method in which the main heat source is frictional heat generated when the materials to be welded are rubbed together in a linear motion. The oxide film formed at the welded interface is removed by expelling the softened material as burrs from the welded interface as a result of the temperature increase, and a weld can be obtained by bringing the newly formed surfaces together.

[0016] The conditions for linear friction welding are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known welding conditions can be used, but by setting the welding pressure to a high value, the welding temperature can be reduced and a narrower welding area can be obtained. The frequency, amplitude, and distance can be appropriately adjusted depending on the type, size, shape, etc. of the materials to be welded.

[0017] In the manufacturing method of the tailored blank material of the present invention, it is preferable to set the joining pressure applied during linear friction joining as P, the temperature at which the tensile strength of the one member becomes P as T1 (°C), and the temperature at which the tensile strength of the other member becomes P as T2 (°C), so that the difference between T1 and T2 is within 100°C. If the difference between T1 and T2 is within 100°C, even if the metal members have a tensile strength difference of 300 MPa or more, both materials (one member and the other member) deform near the interface to be joined during linear friction joining, and a sufficient amount of burrs required for joining by contact between newly formed surfaces can be discharged. Here, yield strength can be used instead of tensile strength.

[0018] In the manufacturing method of a tailored blank material of the present invention, it is preferable that the one member and / or the other member is a steel plate having a tensile strength of 980 MPa or more. By using linear friction joining, a good solid-state weld can be formed even with high-tensile steel having a tensile strength of 980 MPa or more.

[0019] In addition, in the manufacturing method of the tailored blank material of the present invention, it is preferable that the one member and / or the other member is a zinc-plated steel sheet. When zinc-plated steel sheets are welded, zinc plating components are inevitably mixed into the welded part, and the mechanical properties of the welded part are deteriorated. In contrast, in linear friction welding, burrs are discharged from the entire circumference of the joint interface to achieve welding, so that even if the zinc plating evaporates or melts during welding, the zinc plating components can be effectively prevented from being mixed into the joint. In particular, when zinc-plated steel sheets are linearly friction-welded, burrs can be quickly discharged from the long side that occupies most of the joint surface by linearly sliding in the direction perpendicular to the plate thickness, and the mixing of zinc plating components into the joint can be extremely effectively prevented.

[0020] In addition, the inventors of the present invention observed linear friction welds of zinc-plated steel sheets in detail and found that the zinc-plated layer formed on the surface of the steel sheet deforms and / or moves following the moderately softened burrs, so that the linear friction weld is covered with the zinc-plated layer up to the base of the burrs. In other words, the use of linear friction welding not only prevents zinc from being mixed into the weld, but also allows the surface of the weld to be sufficiently covered with the zinc-plated layer even after welding.

[0021] When the material to be joined is a galvanized steel sheet, the type, size and shape of the galvanized steel sheet are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known galvanized steel sheets can be used. Examples of galvanized steel sheets include hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), electrogalvanized steel sheets (EG) and two-layer alloyed hot-dip galvanized steel sheets (GAE). The same method can also be applied to galvanized steel sheets of different compositions, such as highly corrosion-resistant hot-dip zinc, aluminum, magnesium alloy-plated steel sheets (ZAM (registered trademark), Superdyma (registered trademark): highly weather-resistant plated steel sheets), zinc-aluminum alloy-plated steel sheets, zinc-nickel alloy-plated steel sheets, and zinc-magnesium-plated steel sheets. In addition, the coating weight (plating thickness) of each galvanized steel sheet is not particularly limited as long as it does not impair the effects of the present invention, and can be various conventionally known values.

[0022] Furthermore, in the manufacturing method of the tailored blank material of the present invention, it is preferable that the joining temperature is the A1 point or lower of the steel plate. By setting the joining temperature to the A1 point or lower of the steel plate, softening and embrittlement of the steel plate can be suppressed. Here, in the present invention, "joining temperature" means the desired maximum temperature reached at the interface to be joined during linear friction joining. In steels with a high carbon content, such as medium and high carbon steels, brittle martensite is formed by phase transformation, making joining difficult and causing the joint to become embrittled. In contrast, by setting the joining temperature to the A1 point or lower, no phase transformation occurs, and therefore the formation of brittle martensite can be completely suppressed. In addition, the reduction in the joining temperature can suppress softening in the heat-affected zone.

[0023] Here, when the applied pressure of linear friction welding is increased, the frictional heat increases, but since the softened material is continuously discharged as burrs, the "welding temperature" is determined by the pressure applied to the softened material (the force that discharges the burrs). In other words, when the applied pressure is set high, the welded materials with higher strength (high yield strength) can be discharged as burrs. Here, "higher yield strength" means "lower temperature", so the "welding temperature" decreases with an increase in applied pressure. Since the relationship between yield strength and temperature is approximately constant depending on the material, the welding temperature can be controlled extremely accurately compared to the case where frictional heat is used.

[0024] The present invention also provides a tailored blank material, characterized in that it has a linear friction welded portion in which one member and another member are integrated via a linear friction welded interface, and the difference in tensile strength between the one member and the other member is 150 MPa or more.

[0025] The tailored blank material of the present invention has a difference in tensile strength between one member and the other member of 150 MPa or more, and can be suitably used as a tailored blank material when the strength and plate thickness required at each portion are significantly different. In addition, since one member and the other member are firmly joined via a linear friction joint with a narrow joining area compared to other solid-state joining methods, the influence of the joint on the formability of the tailored blank material is extremely small. The difference in tensile strength between one member and the other member is preferably 300 MPa or more.

[0026] In the tailored blank material of the present invention, the one member and / or the other member is preferably a steel plate, and the tensile strength of the steel plate is preferably 980 MPa or more. Since the one member and / or the other member is a high-tensile steel plate having a tensile strength of 980 MPa or more, the blank material can be suitably used as a structural member for automobiles, etc.

[0027] In the tailored blank material of the present invention, the one member and / or the other member is preferably a galvanized steel sheet. By using a galvanized steel sheet, the material can be applied to members that require corrosion resistance, and can be suitably used, for example, as structural members for automobiles, etc.

[0028] In the tailored blank material of the present invention, it is preferable that the Vickers hardness (H) of the linear friction weld interface is 1.1 times or less the higher of the Vickers hardnesses of the one member and the other member, and 0.9 times or more the lower of the Vickers hardnesses of the one member and the other member. By setting the Vickers hardness (H) of the linear friction weld interface within such a range, a good weld can be obtained that has sufficient strength and has isotropic mechanical properties without anisotropy that do not inhibit plastic deformation.

[0029] In addition, in the tailored blank material of the present invention, it is preferable that the Vickers hardness (H) of the linear friction welded interface, the Vickers hardness (H1) of the one member, and the Vickers hardness (H2) of the other member satisfy the relationship 0.8[(H1+H2) / 2]≦H≦1.2[(H1+H2) / 2].

[0030] In a tailored blank material in which metal materials with different strengths are joined, from the viewpoint of imparting good formability, it is preferable that the hardness of "one member-joint-other member" smoothly and continuously change (the joint is not a singular point). Here, by having the Vickers hardness (H) of the joint be in the range of 0.8 to 1.2 times the average value of the Vickers hardness (H1) of one member and the Vickers hardness (H2) of the other member, it is possible to effectively prevent the joint from becoming a singular point.

[0031] The tailored blank material of the present invention can be suitably obtained by using the tailored blank material manufacturing method of the present invention.

[0032] Furthermore, the present invention also provides a press-formed product, characterized in that the linear friction welded portion of the tailored blank material of the present invention is plastically deformed.

[0033] The linear friction weld of the tailored blank material of the present invention is not a singular point of the tailored blank material from the viewpoint of mechanical properties, and has good formability. As a result, the press-formed product of the present invention has no cracks or wrinkles at the plastically deformed linear friction weld, and has a good appearance and high reliability. Effect of the Invention

[0034] According to the present invention, it is possible to provide a tailored blank material having a joint with excellent formability and made of metal materials having different strengths, and a method for simply and efficiently manufacturing the tailored blank material. It is also possible to provide a press-formed product using the tailored blank material of the present invention. [Brief description of the drawings]

[0035] [Figure 1] FIG. 2 is a schematic diagram showing the situation during linear friction welding. [Diagram 2] FIG. 2 is a schematic diagram showing a joining process of the linear friction joining of the present invention. [Diagram 3] 1 is a graph showing the flow stress (yield stress) of carbon steel at each temperature. [Figure 4] 1 is a graph showing the tensile strength of various metals at each temperature. [Diagram 5] FIG. 1 is a schematic cross-sectional view showing an example of a tailored blank material of the present invention. [Figure 6] FIG. 1 is a schematic cross-sectional view showing an example of a press-molded product of the present invention. [Figure 7] 1 shows the base material structure of a 590 MPa-class high-strength steel plate used in the examples. [Figure 8] 2 shows the base material structure of a 980 MPa-class high-strength steel plate used in the examples. [Figure 9] 2 shows the base material structure of a 1250 MPa-class high-strength steel plate used in the examples. [Figure 10]4 is a graph showing the temperature dependence of strength of each high-strength steel plate used in the examples. [Figure 11] 1 is a cross-sectional macro photograph of the embodiment 590 / 980 joint. [Figure 12] 1 is a cross-sectional macro photograph of the embodiment 590 / 1250 joint. [Figure 13] 1 is a cross-sectional macro photograph of a comparative 590 / 980 joint. [Figure 14] 1 is a cross-sectional macro photograph of a comparative 590 / 1250 joint. [Figure 15] 1 is a microstructure photograph of the vicinity of the linear friction weld interface of the 590 / 980 joint in Example. [Figure 16] 1 is a microstructure photograph of the vicinity of the linear friction weld interface of the 590 / 1250 joint in Example. [Figure 17] 1 is a microstructure photograph of a weld of a comparative 590 / 980 joint. [Figure 18] 1 is a microstructure photograph of a weld of a comparative 590 / 1250 joint. [Figure 19] 1 shows the Vickers hardness distribution of the experimental 590 / 980 joint and the comparative 590 / 980 joint. [Figure 20] 1 shows the Vickers hardness distribution of the experimental 590 / 1250 joint and the comparative 590 / 1250 joint. [Figure 21] FIG. 1 is a schematic diagram of the Erichsen test. [Figure 22] These are the results of the Erichsen test for the experimental 590 / 980 joint and the comparative 590 / 980 joint. [Figure 23] 4 shows the results of an Erichsen test on the experimental 590 / 1250 joint and the comparative 590 / 1250 joint. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0036] Representative embodiments of the tailored blank material of the present invention, the manufacturing method of the tailored blank material, and the press-formed product using the tailored blank material will be described in detail below with reference to the drawings, but the present invention is not limited to these. In the following description, the same or equivalent parts are given the same reference numerals, and duplicated descriptions may be omitted. In addition, since the drawings are intended to conceptually explain the present invention, the dimensions of each component shown and their ratios may differ from the actual ones.

[0037] (1) Tailored blank manufacturing method (linear friction joining method) 2 is a schematic diagram showing the joining steps of linear friction joining in the present invention. Linear friction joining includes a first step of bringing one member 2 into contact with the other member 4 to form a joining interface 6, a second step of repeatedly sliding the one member 2 and the other member 4 on the same trajectory while applying pressure substantially perpendicular to the joining interface 6, and discharging burrs 8 from the joining interface substantially parallel and perpendicular to the sliding direction, and a third step of stopping the sliding to form a joining surface. Each step will be described in detail below.

[0038] (1-1) First step The first step is a step of bringing one member 2 into contact with the other member 4 to form the bonded interface 6. The one member 2 and / or the other member 4 are moved to a location where it is desired to form a bond, and the surfaces to be bonded are brought into contact with each other to form the bonded interface 6. The shapes and sizes of the one member 2 and the other member 4 are not particularly limited as long as they do not impair the effects of the present invention, and the shapes and sizes of the one member 2 and the other member 4 may be different. In addition, the shape and size of the end face having the bonded interface 6 may be different between the one member 2 and the other member 4.

[0039] The manufacturing method of the tailored blank material of the present invention is characterized in that the difference in tensile strength between the one member 2 and the other member 4 is 150 MPa or more. Moreover, the difference in tensile strength is preferably 300 MPa or more. When the difference in strength between the one member 2 and the other member 4 is large, it is very difficult to prevent the welded portion from becoming a singular point of mechanical properties. However, by using linear friction welding to make the width of the welded portion extremely narrow, and by controlling the joining temperature (low temperature) to effectively suppress hardening caused by the formation of martensite and softening of the heat-affected zone, it is possible to obtain a good tailored blank material.

[0040] It is also preferable that the one member 2 and / or the other member 4 is a steel plate having a tensile strength of 980 MPa or more. By using linear friction welding, a good solid-phase weld can be formed even with high-tensile steel having a tensile strength of 980 MPa or more. Here, the more preferable tensile strength of the steel plate is 1180 MPa or more.

[0041] In addition, it is preferable that the one member 2 and / or the other member 4 are galvanized steel sheets. When galvanized steel sheets are welded, galvanized components are inevitably mixed into the welded portion, which reduces the mechanical properties of the welded portion. In contrast, in linear friction welding, burrs are discharged from the entire circumference of the joint interface to achieve welding, so that even if the galvanized plating evaporates or melts during welding, the mixing of the galvanized components into the joint can be effectively suppressed. In particular, when linearly friction welding galvanized steel sheets, burrs can be quickly discharged from the long sides that occupy most of the joint surface by linearly sliding them in a direction perpendicular to the plate thickness, and mixing of the galvanized components into the joint can be extremely effectively suppressed.

[0042] In addition, the galvanized layer formed on the surface of the steel sheet deforms and / or moves in accordance with the softened burr, so that the linear friction joint is covered with the galvanized layer up to the base of the burr. In other words, the use of linear friction welding not only prevents zinc from being mixed into the joint, but also ensures that the surface of the joint is sufficiently covered with the galvanized layer even after joining.

[0043] When the material to be joined is a galvanized steel sheet, the type, size and shape of the galvanized steel sheet are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known galvanized steel sheets can be used. Examples of galvanized steel sheets include hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), electrogalvanized steel sheets (EG) and two-layer alloyed hot-dip galvanized steel sheets (GAE). The same method can also be applied to galvanized steel sheets of different compositions, such as highly corrosion-resistant hot-dip zinc, aluminum, magnesium alloy-plated steel sheets (ZAM (registered trademark), Superdyma (registered trademark): highly weather-resistant plated steel sheets), zinc-aluminum alloy-plated steel sheets, zinc-nickel alloy-plated steel sheets, and zinc-magnesium-plated steel sheets. In addition, the coating weight (plating thickness) of each galvanized steel sheet is not particularly limited as long as it does not impair the effects of the present invention, and can be various conventionally known values.

[0044] (1-2) Second process The second step is a step in which one component 2 and the other component 4 are repeatedly slid on the same trajectory while applying pressure P approximately perpendicular to the joined interface 6, and burrs 8 are expelled from the joined interface 6 approximately parallel to and approximately perpendicular to the sliding direction.

[0045] The method of repeatedly sliding one member 2 and the other member 4 on the same trajectory is not particularly limited as long as it does not impair the effects of the present invention, and both members may be vibrated together, or one may be fixed and the other may be vibrated.

[0046] The conditions of linear friction welding are not particularly limited as long as they do not impair the effects of the present invention, and various conventionally known welding conditions can be used. However, by setting the welding pressure to a high value, the welding temperature can be reduced and a narrower welding area can be obtained. The frequency, amplitude, and approach allowance can be appropriately adjusted according to the type, size, shape, and other factors of the materials to be welded. Here, by increasing the frequency and / or amplitude, the heating rate and cooling rate can be increased, and the softening of the heat-affected zone can be effectively suppressed. In addition, as long as sufficient welding strength can be obtained by the contact between the newly formed surfaces, the approach allowance can be reduced to shorten the welding time and reduce the thermal influence on the welded area. In addition, the welding time can also be shortened by increasing the frequency and amplitude.

[0047] In addition, when the joining pressure applied during linear friction joining is P, the temperature at which the tensile strength of one member 2 becomes P is T1 (℃), and the temperature at which the tensile strength of the other member 4 becomes P is T2 (℃), it is preferable to set P so that the difference between T1 and T2 is within 100℃. If the difference between T1 and T2 is within 100℃, even if the metal members have a tensile strength difference of 300MPa or more, both materials (one member 2 and the other member 4) deform near the interface to be joined during linear friction joining, and a sufficient amount of burrs required for joining by abutting the newly formed surfaces can be discharged. It is more preferable to set the difference between T1 and T2 within 50℃, and most preferably within 30℃.

[0048] Here, the pressure P during linear friction joining can be set to be equal to or greater than the yield stress and equal to or less than the tensile strength of one member 2 and / or the other member 4 at a desired joining temperature, thereby controlling the joining temperature. For example, the pressure P can be set to be equal to or greater than the yield stress and equal to or less than the tensile strength of a hot-dip galvanized steel sheet at a desired joining temperature, thereby determining the joining temperature based on the hot-dip galvanized steel sheet. By setting the pressure P to be equal to or greater than the yield stress of the hot-dip galvanized steel sheet, the discharge of burrs 8 from the joined interface 6 begins, and by increasing the pressure P until the tensile strength is reached, the discharge of burrs 8 is accelerated. As with the yield stress, the tensile strength at a specific temperature is also approximately constant depending on the joined materials, so that the joining temperature corresponding to the set pressure P can be realized.

[0049] As a specific example, the flow stress (yield stress) of carbon steel at each temperature is shown in Figure 3, and the tensile strength of various metals at each temperature is shown in Figure 4. Note that Figure 3 is a graph published in "Tetsu to Hagane, No. 11, 1981, p. 140," and Figure 4 is a graph published in "Tetsu to Hagane, No. 6, 1986, p. 55." As shown in these figures, the tensile strength and yield stress at a specific temperature are approximately constant depending on the material.

[0050] That is, when the pressure P during joining is set high, the joined materials with higher yield strength and tensile strength can be discharged as burrs, and the joining temperature can be reduced. Also, as shown in Figures 3 and 4, the tensile strength and yield stress at a specific temperature are approximately constant depending on the material, so by setting the joining pressure P based on the temperature dependency of the strength of the joined materials, the joining temperature can be controlled extremely accurately.

[0051] In linear friction welding, it is necessary to set welding parameters other than the pressure P (frequency and amplitude for vibrating the workpieces, welding time, and gap, etc.), but these values ​​are not limited as long as they do not impair the effects of the present invention, and may be set appropriately depending on the material, shape, size, etc. of the workpieces. Here, by increasing the amplitude and frequency for sliding the workpieces, the heating rate and the cooling rate after welding increase, but the maximum temperature (welding temperature) remains unchanged.

[0052] Furthermore, when the one member 2 and / or the other member 4 are made of steel, it is preferable to set the joining temperature to the A1 point or lower of the steel plate. By setting the joining temperature to the A1 point or lower of the steel plate, softening and embrittlement of the steel plate can be suppressed. In steel, brittle martensite is formed by phase transformation, which may make joining difficult or may cause the joint to become embrittled. In contrast, by setting the joining temperature to the A1 point or lower, no phase transformation occurs, and therefore the formation of brittle martensite can be completely suppressed. In addition, the reduction in joining temperature can suppress softening in the heat-affected zone.

[0053] Furthermore, when one member 2 and / or the other member 4 are made of galvanized steel sheets, the joining temperature is preferably set to the boiling point of zinc (907°C) or lower, and more preferably to the melting point of the galvanized steel sheet. In linear friction joining, the joining temperature can be accurately determined by the joining pressure P, and by setting the joining temperature to the boiling point of zinc or lower, changes in the galvanized layer formed on the surface of the steel sheet can be suppressed. Furthermore, by setting the joining temperature to the melting point of the galvanized steel sheet, changes in the galvanized layer can be more reliably suppressed.

[0054] (1-3) Third step The third step is a step of stopping the sliding in the second step and forming a joining surface. In the linear friction joining method of the present invention, a good joined body can be obtained by stopping the sliding after the burrs 8 are discharged from the entire surface of the joined interface 6. Furthermore, by discharging the burrs 8 from the entire surface of the joined interface 6, it is possible to prevent the zinc plating components from mixing into the joint. The pressure P applied to the joined materials in the second step may be maintained as it is, or may be increased to a higher value in order to discharge the burrs 8 and to bring the newly formed surface into stronger contact.

[0055] Here, the timing for stopping the sliding is not limited as long as the burrs 8 are discharged from the entire surface of the joined interface 6, but by observing the joined interface 6 from a direction approximately perpendicular to the sliding direction and stopping the sliding at the moment the burrs 8 are discharged approximately parallel to the sliding direction, it is possible to form a good joint while minimizing the amount of burrs 8 discharged (while minimizing consumption of the joined materials). Note that "a direction approximately perpendicular to the sliding direction" and "a direction approximately parallel to the sliding direction" are both directions approximately perpendicular to the applied pressure.

[0056] (2) Tailored blank materials Fig. 5 is a schematic cross-sectional view showing an example of a tailored blank material of the present invention. The tailored blank material 10 is formed by linearly friction-welding one member 2 and another member 4, has a linear friction-welded part 14 where the one member 2 and the other member 4 are integrated via a linear friction-welded interface 12, and is characterized in that the difference in tensile strength between the one member 2 and the other member 4 is 150 MPa or more. In addition, it is preferable that the difference in tensile strength between the one member 2 and the other member 4 is 300 MPa or more. Fig. 5 shows a case of butt-welding where the ends to be linearly friction-welded have the same size and shape, but the tailored blank material of the present invention is not limited to this, and the one member 2 and the other member 4 may have different sizes and / or shapes, or may be made of different materials.

[0057] In the tailored blank material 10, the difference in tensile strength between the one member 2 and the other member 4 is 150 MPa or more (preferably 300 MPa or more), and the tailored blank material can be suitably used as a tailored blank material when the strength required at each portion differs greatly. Furthermore, since the one member 2 and the other member 4 are firmly joined at the linear friction weld 14 (linear friction weld interface 12), which has a narrow joining region compared to other solid-state joining methods, the effect of the weld on the formability of the tailored blank material is extremely small.

[0058] In the tailored blank material 10, the one member 2 and / or the other member 4 are steel plates, and the tensile strength of the steel plates is preferably 980 MPa or more. Since the one member 2 and / or the other member 4 are high-tensile steel plates having a tensile strength of 980 MPa or more, the tailored blank material 10 can be suitably used as structural members for automobiles and the like. Here, the tensile strength of the one member 2 and / or the other member 4 is more preferably 1180 MPa or more.

[0059] Moreover, it is preferable that the one member 2 and / or the other member 4 of the tailored blank material 10 is a galvanized steel sheet. By using a galvanized steel sheet, the material can be applied to members that require corrosion resistance, and can be suitably used, for example, as structural members for automobiles and the like.

[0060] When one member 2 and / or the other member 4 are galvanized steel sheets, the linear friction joint 14 is free of components of the galvanized layer formed on the surface of the hot-dip galvanized steel sheets. "Zinc plating components are not mixed into the linear friction joint" can be confirmed by elemental analysis using SEM-EDS on the cross section of the joint, but since the quantitative value of zinc will be subject to errors due to the influence of peaks caused by iron, for example, it can be determined by obtaining elemental mapping of the entire cross section of the joint and checking whether or not there are clear locations of zinc present inside the joint.

[0061] Furthermore, in the tailored blank material 10, after linear friction welding, a burr 8 is formed on the outer edge of the linear friction welding interface 12, but when the one member 2 and / or the other member 4 are zinc-plated steel sheets, the surface of the linear friction welding portion 14 is covered with a zinc plating layer up to the root of the burr 8. By covering the surface of the linear friction welding portion 14 with a zinc plating layer up to the root of the burr 8, a welding portion with excellent corrosion resistance can be realized.

[0062] Furthermore, in the tailored blank material 10, it is preferable that the Vickers hardness (H) of the linear friction weld interface 12 is 1.1 times or less the higher of the Vickers hardnesses of the one member 2 and the other member 4, and is 0.9 times or more the lower of the Vickers hardnesses of the one member 2 and the other member 4. By setting the Vickers hardness (H) of the linear friction weld interface 12 within this range, it is possible to obtain a good weld having sufficient strength and isotropic mechanical properties without anisotropy that do not inhibit plastic deformation.

[0063] In addition, it is preferable that the Vickers hardness (H) of the linear friction weld interface 12, the Vickers hardness (H1) of the one member 2, and the Vickers hardness (H2) of the other member 4 of the tailored blank material 10 satisfy the relationship 0.8[(H1+H2) / 2]≦H≦1.2[(H1+H2) / 2].

[0064] In a tailored blank material 10 in which metal materials with different strengths are joined, from the viewpoint of imparting good formability, it is preferable that the hardness of "one member - joint - other member" changes smoothly and continuously (the joint is not a singular point). Here, by setting the Vickers hardness (H) of the joint to be in the range of 0.8 to 1.2 times (more preferably in the range of 0.9 to 1.1 times) the average value of the Vickers hardness (H1) of one member 2 and the Vickers hardness (H2) of the other member 4, it is possible to effectively prevent the joint from becoming a singular point.

[0065] The width of the linear friction welded interface 12 is extremely thin, about 0.2 to 2.0 mm depending on the linear friction weld conditions, so it may be difficult to measure the hardness of only that region. Therefore, the Vickers hardness (H) of the welded portion can be determined by determining the measurement position so that the indenter of the Vickers hardness tester is approximately at the center of the linear friction welded interface.

[0066] (3) Press-molded products 6 is a schematic cross-sectional view showing an example of a press-formed product of the present invention. The press-formed product 20 is characterized in that the linear friction welded portion 14 of the tailored blank material 10 is plastically deformed.

[0067] From the viewpoint of mechanical properties, the linear friction weld 14 is not a singular point of the tailored blank material 10 and has good formability. As a result, the press-formed product 20 has no cracks or wrinkles in the plastically deformed linear friction weld 14, and has a good appearance and high reliability.

[0068] The type and magnitude of the plastic processing applied to the linear friction joint 14 is not particularly limited as long as it does not impair the effects of the present invention, and may be appropriately determined depending on the desired shape of the press-formed product 20, etc.

[0069] Representative embodiments of the present invention have been described above, but the present invention is not limited to these, and various design modifications are possible, all of which are included in the technical scope of the present invention. EXAMPLES

[0070] Example The materials used for joining were three types of high-strength steel plates with different strength levels: 590MPa-class high-strength steel plate (precipitation hardened steel, 0.07%C-0.02%Si-1.87%Mn-0.015%P-0.003%S), 980MPa-class high-strength steel plate (DP steel, 0.113%C-0.941%Si-2.09%Mn-0.018%P-0.002%S), and 1250MPa-class high-strength steel plate (martensitic steel, 0.15%C-0.18%Si-1.23%Mn-0.016%P-0.004%S).

[0071] The base material structure of the 590MPa-class high-strength steel plate is shown in Fig. 7, the base material structure of the 980MPa-class high-strength steel plate in Fig. 8, and the base material structure of the 1250MPa-class high-strength steel plate in Fig. 9. The 590MPa-class high-strength steel plate has a microstructure consisting of ferrite and cementite, the 980MPa-class high-strength steel plate has a microstructure consisting of ferrite and martensite, and the 1250MPa-class high-strength steel plate has a microstructure consisting of martensite.

[0072] The dimensions of all steel plates were 2 mm x 80 mm x 62.5 mm, and the 2 mm x 80 mm end faces were butted together to perform linear friction welding. The combinations of materials to be welded were dissimilar material welding of 590 MPa class high strength steel plate and 980 MPa class high strength steel plate (590 / 980 joint), and dissimilar material welding of 590 MPa class high strength steel plate and 1250 MPa class high strength steel plate (590 / 1250 joint).

[0073] In addition, the temperature dependence of the strength of three types of high-strength steel plates was investigated to determine the joining pressure for linear friction welding. The results of measuring the tensile strength at temperatures of 400 to 800°C with a strain rate of 1.0 / s are shown in Figure 10. In addition to keeping the joining temperature as low as possible, the joining pressure was set to 250 MPa in order to sufficiently remove both joined materials as burrs from the vicinity of the joined interface. The temperatures at which burrs are formed in linear friction welding with a joining pressure of 250 MPa are approximately 650°C for 590 MPa-class high-strength steel plates, approximately 670°C for 980 MPa-class high-strength steel plates, and approximately 710°C for 1250 MPa-class high-strength steel plates.

[0074] The linear friction joining conditions other than the joining pressure were constant at a frequency of 30 Hz, an amplitude of 1 mm, and a distance of 2.7 mm, and a 590 / 980 joint and a 590 / 1250 joint were obtained.

[0075] Comparative Example A comparative 590 / 980 joint and a comparative 590 / 1250 joint were obtained in the same manner as in the examples, except that the joints were formed using laser welding instead of linear friction welding.

[0076] Laser welding was performed in the atmosphere using a disk laser with a laser output of 4 kW, a welding speed of 6 m / s, and a laser spot diameter of 340 μm.

[0077] [evaluation] (1) Cross-sectional macro observation of joints Cross-sectional samples of each of the resulting joints were prepared, mirror-polished, and then observed under an optical microscope.

[0078] A cross-sectional macrophotograph of the Example 590 / 980 joint is shown in Figure 11, and a cross-sectional macrophotograph of the Example 590 / 1250 joint is shown in Figure 12. In both joints, sufficient flash was removed from both of the joined materials, and it was found that a good solid-state joint without defects was formed.

[0079] A cross-sectional macrophotograph of the comparative 590 / 980 joint is shown in Figure 13, and a cross-sectional macrophotograph of the comparative 590 / 1250 joint is shown in Figure 14. In both joints, good laser welds were formed, and no weld defects were observed.

[0080] (2) Microstructural observation of joint To confirm the state of the joint interface, a cross-sectional sample of the joint was observed using an SEM. After polishing and etching the cross-section (4% nital), the structure was observed using a scanning electron microscope (FE-SEM, JEOL Ltd. JSM-7001FA). Emery paper (#600 to #3000) and diamond paste (grain sizes 3 μm and 1 μm) were used for polishing. Samples for observing the base material were also prepared in the same way.

[0081] Figure 15 shows a microstructure photograph near the linear friction weld interface of the 590 / 980 joint. The microstructure on the 590MPa-class high-strength steel plate side is composed of ferrite and a small amount of martensite, while the microstructure on the 980MPa-class high-strength steel plate side is composed of ferrite and martensite. These microstructures indicate that the linear friction weld temperature was near point A1. It can also be seen that there are no microscopic defects at the linear friction weld interface.

[0082] Figure 16 shows a microstructure photograph near the linear friction weld interface of the practical 590 / 1250 joint. The microstructures on both the 590MPa-class high-strength steel plate side and the 1250MPa-class high-strength steel plate side are composed of ferrite and fine cementite, which indicates that the linear friction weld temperature was below the A1 point. In addition, there are no microscopic defects at the linear friction weld interface. Here, the A1 points are 660°C for the 590MPa-class high-strength steel plate, 668°C for the 980MPa-class high-strength steel plate, and 693°C for the 1250MPa-class high-strength steel plate. Based on the temperature dependence of the strength of each steel plate shown in Figure 10, the actual weld temperature and the A1 point are close to each other, and it is considered that in the practical 590 / 1250 joint, both materials were below the A1 point.

[0083] Photographs of the microstructures of the welds of the comparative 590 / 980 joint and the comparative 590 / 1250 joint are shown in Figures 17 and 18, respectively. When laser welding was performed, a microstructure consisting only of martensite was formed in both welds.

[0084] (3) Vickers hardness measurement The Vickers hardness of the cross section of each joint obtained in the above examples and comparative examples was measured. The measurement was performed using a FUTURE-TECH ARS 10K measuring device under the conditions of 1 kg and 10 s.

[0085] FIG. 19 shows the Vickers hardness distribution perpendicular to the joint interface in the cross section of the 590 / 980 joint and the comparative 590 / 980 joint. The Vickers hardness of the 590MPa high-strength steel plate is about 200Hv, and the Vickers hardness of the 980MPa high-strength steel plate is about 350Hv. The Vickers hardness of the linear friction joint is between the values ​​of both steel plates, and the hardness of the 590MPa high-strength steel plate, the linear friction joint, and the 980MPa high-strength steel plate changes gradually. The Vickers hardness of the linear friction joint interface is 250Hv, which is a value between the Vickers hardness of the 590MPa high-strength steel plate and the Vickers hardness of the 980MPa high-strength steel plate, and is in the range of 0.8 to 1.2 times the average value of the Vickers hardness of the 590MPa high-strength steel plate and the 980MPa high-strength steel plate.

[0086] On the other hand, a significant increase in hardness was observed in the joint of the comparative 590 / 980 joint due to the formation of martensite. The hardness of the joint is about 100 Hv higher than that of the 980 MPa-class high-strength steel plate, indicating that the joint is a singular point in terms of mechanical properties.

[0087] Figure 20 shows the Vickers hardness distribution perpendicular to the linear friction weld interface in the cross section of the 590 / 1250 joint. The Vickers hardness of the 590 MPa high-strength steel plate is about 200 Hv, and the Vickers hardness of the 1250 MPa high-strength steel plate is about 450 Hv. The Vickers hardness of the linear friction weld falls between the values ​​of both steel plates, and the hardness changes gradually from the 590 MPa high-strength steel plate to the linear friction weld to the 1250 MPa high-strength steel plate. The Vickers hardness of the linear friction weld interface is 270 Hv, which is between the Vickers hardness of the 590 MPa high-strength steel plate and the Vickers hardness of the 1250 MPa high-strength steel plate, and is in the range of 0.8 to 1.2 times the average value of the Vickers hardness of the 590 MPa high-strength steel plate and the 1250 MPa high-strength steel plate.

[0088] On the other hand, in the joint of the comparative 590 / 1250 joint, the hardness of the 590MPa-class high-strength steel plate increases due to the formation of martensite, while the 1250MPa-class high-strength steel plate side has softened in the heat-affected zone. Also, the hardness near the linear friction weld interface is higher than that of the 1250MPa-class high-strength steel plate. Thus, the hardness of the joint of the comparative 590 / 1250 joint changes significantly, and it can be seen that the joint is a singular point of mechanical properties.

[0089] (4) Evaluation of formability (Erichsen test) An Erichsen test was performed to evaluate the formability of each of the resulting joints. A schematic diagram of the Erichsen test performed is shown in Figure 21. The test specimen was a rectangular plate with a thickness of 1.8 mm, width of 80 mm, and length of 120 mm, and the weld line was located at the center of the test specimen. Burrs removed from the joint of the linear friction welded joint were removed with emery paper (#80).

[0090] The Erichsen test was carried out using an automatic universal deep drawing tester (JT TOHSI SAS-200D) under the conditions of a constant punch speed of 5 mm / s and a blank holding force of 30 kN. The shape of the punch was a hemisphere with a diameter of 20 mm, and a 0.1 mm thick Teflon sheet (registered trademark) was used as a lubricant to break the joint near the top of the punch. The dome height until the plastic deformation region broke was measured, and the formability of each joint was evaluated.

[0091] The results of the experimental 590 / 980 joint and the comparative 590 / 980 joint are shown in Figure 22, and the results of the experimental 590 / 1250 joint and the comparative 590 / 1250 joint are shown in Figure 23. In either case, the experimental joint has a higher dome height than the comparative joint, and the experimental joint of the present invention has excellent formability. Note that the comparative joint was obtained by laser welding, which provides the best formability in fusion welding, and it can be seen that the experimental joint of the present invention has excellent formability compared to a tailored blank material obtained by conventional fusion welding. [Explanation of symbols]

[0092] 2...One member, 4...the other member, 6... bonded interface, 8. Bali, 10 Tailored blank material, 12...Linear friction welding interface, 14...linear friction joint, 20 Press-molded products.

Claims

1. A linear friction joint is formed between one member and another member, The difference in tensile strength between the one member and the other member is 150 MPa or more; The joining pressure applied during linear friction joining is P, the temperature at which the tensile strength of one member becomes P is T1 (°C), and the temperature at which the tensile strength of the other member becomes P is T2 (°C), Setting the temperature P so that the difference between the temperature T 1 and the temperature T 2 is within 100° C.; A manufacturing method for tailored blank materials, characterized by the above.

2. The one member and / or the other member is a steel plate, The tensile strength of the steel plate is 980 MPa or more; A method for manufacturing a tailored blank material according to claim 1, characterized in that

3. The one member and / or the other member is a galvanized steel plate; 3. A method for manufacturing a tailored blank material according to claim 1 or 2, characterized in that

4. The joining temperature is A of the steel plate. 1 points or less, The manufacturing method of a tailored blank material according to claim 2 or 3, characterized in that

5. A linear friction weld is provided between one member and another member, the linear friction weld being integrated through a linear friction weld interface. The difference in tensile strength between the one member and the other member is 150 MPa or more, The Vickers hardness (H) of the linear friction weld interface is 1.1 times or less the higher of the Vickers hardnesses of the one member and the other member, and is 0.9 times or more the lower of the Vickers hardnesses of the one member and the other member. Tailored blank material characterized by:

6. The one member and / or the other member is a steel plate, The tensile strength of the steel plate is 980 MPa or more; The tailored blank material according to claim 5 .

7. The one member and / or the other member is a galvanized steel sheet; 7. The tailored blank material according to claim 5 or 6,

8. The Vickers hardness (H) of the linear friction weld interface and the Vickers hardness (H 1 ) and the Vickers hardness (H 2 ) satisfies the relationship of the following formula (1); The tailored blank material according to any one of claims 5 to 7, 0.8[(H 1 +H 2 ) / 2]≦H≦1.2[(H 1 +H 2 ) / 2] (1)

9. The linear friction weld of the tailored blank material according to any one of claims 5 to 8 is plastically deformed. A press-molded product characterized by the above.

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