Method for manufacturing tailored blank material and tailored blank material

Abstract

To provide a method for manufacturing a tailored blank material, which improves press formability and stabilizes welding quality, and to provide the tailored blank material.SOLUTION: A method for manufacturing a tailored blank material includes a tailored welding step of irradiating a portion where a side surface of a first steel sheet 1 and a side surface of a second steel sheet 2 having a strength larger than that of the first steel sheet are butted against each other with a laser beam to join the first and second steel sheets, and a heat-affected-portion re-irradiation step of irradiating a HAZ softened portion 4A of a heat-affected portion generated in the first steel sheet among heat-affected portions 4 generated on both sides of a first melted portion 3 with a laser beam to form at least a part of the HAZ softened portion into a second melted portion 5. The tailored blank material includes a first steel sheet and a second steel sheet, a side surface of the first steel sheet and a side surface of the second steel sheet are joined by a first fusion zone, and the first steel sheet has a heat affected zone adjacent to the first fusion zone and a second fusion zone adjacent to the heat affected zone on a side opposite to the first fusion zone.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a tailored blank material and a tailored blank material. [Background technology]

[0002] In recent years, the use of tailored blanks made from high-tensile steel sheets has been increasing with the aim of reducing the weight of automobile bodies and improving collision safety. However, it is known that when high-tensile steel sheets are welded, a heat-affected zone (HAZ) forms around the weld, and a softened zone (hereinafter also referred to as a HAZ softened zone) is formed in the HAZ. Such HAZ softened zones are particularly noticeable in welds made from high-tensile steel sheets with high tensile strength. Furthermore, it has been reported that when such welded products are press-formed, if the tension applied perpendicular to the weld bead is large, strain accumulates in the HAZ softened zone, causing fracture.

[0003] Patent Document 1 discloses a laser-butt-welded thin steel sheet, particularly an ultra-high-tensile strength steel sheet with a tensile strength of 1000 MPa or more, that solves the formability problem caused by fracture in the HAZ softened zone during press forming, thereby enabling the application of the ultra-high-tensile strength steel sheet to tailored blanks. Specifically, Patent Document 1 discloses a laser-butt-welded thin steel sheet, characterized in that the width of the heat-affected softened zone near the weld of the welded steel sheet is 25% or less of the sheet thickness, and that at least one base metal has a tensile strength of 780 MPa or more. Patent Document 1 further discloses a laser-butt-welded thin steel sheet method, characterized in that the welding speed is 8 meters per minute or more. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-218500 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, when laser welding thin steel plates, the welding speed is 8 m per minute or more, which narrows the width of the softened HAZ, but also narrows the width of the weld bead, making it difficult to accurately irradiate the butt joint of the steel plates with the laser beam, which tends to result in misalignment of the welding position and, further, the influence of the end faces of the thin steel plates, which tends to result in gaps. Therefore, there is concern that the welding method in Patent Document 1 will result in a high rate of defective welds during production.

[0006] The present invention has been made in view of the problems inherent in the prior art, and an object of the present invention is to provide a tailored blank manufacturing method and tailored blank material that can improve the press formability of tailor-welded blank material and stabilize the weld quality. [Means for solving the problem]

[0007] A manufacturing method for a tailored blank material according to a first aspect of the present invention comprises a tailored welding process in which a laser beam is irradiated to the area where a side surface of a first steel plate and a side surface of a second steel plate having a strength greater than that of the first steel plate are butted together to join them, and a heat-affected zone re-irradiation process in which a laser beam is irradiated to a HAZ-softened portion of the heat-affected zone formed in the first steel plate, of the heat-affected zones formed on both sides of the first molten zone produced by the tailored welding process, to turn at least a part of the HAZ-softened portion into a second molten zone.

[0008] A tailored blank material according to a second aspect of the present invention comprises a first steel plate and a second steel plate having a greater strength than the first steel plate, wherein a side surface of the first steel plate and a side surface of the second steel plate are joined by a first fusion zone, and heat-affected zones exist on both sides of the first fusion zone, and the first steel plate has the heat-affected zone adjacent to the first fusion zone and a second fusion zone adjacent to the side of the heat-affected zone opposite to the first fusion zone. [Effects of the Invention]

[0009] According to the present invention, it is possible to provide a tailored blank manufacturing method and a tailored blank material that can improve the press formability of a tailored welded blank material and stabilize the welding quality. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic perspective view for explaining a method for manufacturing a tailored blank material according to the present embodiment. [Figure 2] 1 is an optical microscope photograph at 50x magnification showing a cross section of a first steel sheet and a second steel sheet after a tailor welding process. [Figure 3] FIG. 1 is a schematic perspective view for explaining a method for manufacturing a tailored blank material according to the present embodiment. [Figure 4] 1 is an optical microscope photograph at 50x magnification showing the cross sections of the first steel sheet and the second steel sheet after the heat-affected zone re-irradiation step. [Figure 5] FIG. 10 is a schematic diagram showing the configuration of a tailored blank material of Example 2. [Figure 6] FIG. 10 is a contour diagram showing the results of measuring the strain distribution of thermal strain in each tailored blank material of Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] The tailored blank material and the manufacturing method thereof according to the present embodiment will be described in detail below with reference to the drawings. Note that the dimensional proportions in the drawings are exaggerated for the sake of convenience and may differ from the actual proportions.

[0012] [Manufacturing method for tailored blank materials] The manufacturing method of the tailored blank material of this embodiment includes a tailored welding process in which a laser beam is irradiated onto the area where the side of the first steel plate and the side of the second steel plate are butted together to join them, and a heat-affected zone re-irradiation process in which a laser beam is irradiated onto the HAZ softened portion of the first steel plate.

[0013] The first and second steel plates used in the tailored blank material are not particularly limited, but may be high-tensile steel plates with a thickness of 0.8 mm to 3.0 mm and a tensile strength of 590 MPa to 1470 MPa. Specifically, high-tensile steel plates with a tensile strength of 780 MPa and ultra-high-tensile steel plates with tensile strengths of 980 MPa, 1180 MPa, or 1470 MPa may be used. The tensile strength may be measured in accordance with Japanese Industrial Standard JIS Z2241:2022 (Method of Tensile Testing for Metallic Materials). However, the second steel plate used should have a higher strength than the first steel plate. In this specification, "strength" refers to the product of the thickness and tensile strength of a steel plate. Therefore, "the strength of the second steel plate is greater than the strength of the first steel plate" means "the product of the thickness and tensile strength of the second steel plate is greater than the product of the thickness and tensile strength of the first steel plate."

[0014] In the tailored welding process, first, as shown in FIG. 1, the side surfaces of a first steel sheet 1 and a second steel sheet 2 are brought into contact with each other. Then, a laser beam is irradiated from above onto the butt surface of the first steel sheet 1 and the second steel sheet 2. Specifically, the laser welder includes a laser oscillator, an optical path (optical fiber), and a welding head that focuses the laser. Then, as shown in FIG. 1, the welding head 10 is moved along the butt surface of the first steel sheet 1 and the second steel sheet 2 while irradiating the butt surface with a laser beam. In this way, the butt surfaces of the first steel sheet 1 and the second steel sheet 2 are laser welded.

[0015] The laser beam irradiated onto the butt surfaces is not particularly limited as long as it is capable of laser welding the first steel plate 1 and the second steel plate 2 together, but for example, a carbon dioxide laser, a YAG laser, or a fiber laser can be used.

[0016] In the tailored welding process, the laser beam irradiated onto the butted surfaces of the first steel sheet 1 and the second steel sheet 2 may have a ring profile. A ring profile is a beam profile in which a ring-shaped top hat beam is arranged around a Gaussian-shaped center (core) beam. By using a ring profile laser beam, spatter in the tailored welding process can be reduced.

[0017] The welding speed of the butt surfaces of the first steel sheet 1 and the second steel sheet 2 is not particularly limited, but is preferably 5 to 20 m / min, and more preferably 10 to 15 m / min. The output of the laser beam irradiated onto the butt surfaces varies depending on the plate thicknesses of the first steel sheet 1 and the second steel sheet 2 and the welding speed, but is preferably 2 to 6 kW. For example, if the plate thicknesses of the first steel sheet 1 and the second steel sheet 2 are both 1.2 mm and the welding speed is 10 m / min, the output of the laser beam is preferably about 3.5 kW. Since an increase in the welding speed reduces the heat input to the first steel sheet 1 and the second steel sheet 2, it is necessary to increase the output of the laser beam. Therefore, the output value of the laser beam depends on the welding speed.

[0018] Here, Fig. 2 is an optical microscope photograph showing the results of observing a cross section when a side surface of a first steel plate 1 and a side surface of a second steel plate 2 are tailor welded together. As shown in Fig. 2, by performing tailor welding, a first fusion zone 3 is formed between the first steel plate 1 and the second steel plate 2, where the metal of the first steel plate 1 and the metal of the second steel plate 2 melt and solidify. The first fusion zone 3 has a structure mainly composed of martensite, and is a region with high hardness. The first fusion zone 3 is formed from the front surface to the back surface of the first steel plate 1 and the second steel plate 2.

[0019] A heat-affected zone 4 is formed outside the first fusion zone 3. The heat-affected zone 4 is a region where fine metal crystal grains of the first steel plate 1 and the second steel plate 2 have grown and coarsened due to the influence of heat from laser welding. A low-hardness HAZ-softened zone 4A is formed at the interface between the first steel plate 1 and the base metal 1A in the heat-affected zone 4. Similarly, a low-hardness HAZ-softened zone 4A is formed at the interface between the second steel plate 2 and the base metal 2A in the heat-affected zone 4. Strain accumulates in such a HAZ-softened zone 4A during press forming, which may cause fracture to originate from the HAZ-softened zone 4A. In particular, there is a possibility that fracture may originate from the HAZ-softened zone 4A present in the first steel plate 1, which has lower strength than the second steel plate 2.

[0020] For this reason, in this embodiment, a heat-affected zone re-irradiation process is performed after the tailored welding process. The heat-affected zone re-irradiation process is a process in which a laser beam is irradiated onto a HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel plate 1 by the tailored welding process, thereby changing at least a part of the HAZ-softened zone 4A into a second fusion zone 5. Specifically, as shown in Fig. 3 , the welding head 11 is moved so that the laser beam is irradiated onto the HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel plate 1 from the surface of the first steel plate 1. The HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel plate 1 is located at a position shifted in parallel by approximately 0.5 mm to 2.0 mm from the first fusion zone 3.

[0021] The second fusion zone 5 transformed from the HAZ-softened zone 4A is a region with high hardness and has a structure mainly composed of martensite, similar to the first fusion zone 3. Therefore, by transforming the HAZ-softened zone 4A of the heat-affected zone 4 generated in the first steel plate 1 into the second fusion zone 5, accumulation of strain during press forming can be suppressed, and the possibility of fracture originating from the HAZ-softened zone 4A before it transforms into the second fusion zone 5 can be reduced.

[0022] The laser beam irradiated onto the HAZ softened portion 4A can be a carbon dioxide laser, a YAG laser, or a fiber laser, as in the tailored welding process.

[0023] The laser beam irradiated onto the HAZ-softened portion 4A may be ring-profiled. By using a ring-profiled laser beam, fracture originating from the HAZ-softened portion of the tailored blank material can be suppressed, thereby further improving press formability. In the ring profile, the ratio of the irradiation diameters of the core beam and the ring beam (core / ring ratio) is not particularly limited, but may be, for example, 60 / 40 to 100 / 0. By bringing the irradiation diameter of the core beam closer to 100, the press formability of the tailored blank material can be further improved, and thermal distortion of the tailored blank material can also be reduced, stabilizing quality.

[0024] Here, the irradiation speed of the laser beam irradiated onto the HAZ-softened zone 4A is preferably faster than the welding speed in the tailored welding process. Specifically, the irradiation speed of the laser beam is preferably 5 to 20 m / min, and more preferably 10 to 15 m / min. For example, the welding speed in the tailored welding process can be 3 m / min, and the irradiation speed in the HAZ re-irradiation process can be 10 m / min. The HAZ re-irradiation process only needs to transform at least a portion of the HAZ-softened zone 4A of the first steel plate 1 into the second fusion zone 5, and as will be described later, the cross-sectional area of ​​the second fusion zone 5 can be smaller than the cross-sectional area of ​​the first fusion zone 3. Therefore, it is preferable to set the irradiation speed of the laser beam in the HAZ re-irradiation process faster than the welding speed in the tailored welding process to efficiently transform the HAZ-softened zone 4A.

[0025] Furthermore, the output of the laser beam irradiated onto the HAZ-softened zone 4A of the first steel sheet 1 in the heat-affected zone re-irradiation step is preferably greater than the output of the laser beam irradiated onto the butt surface between the first steel sheet 1 and the second steel sheet 2 in the tailored welding step. As described above, the laser beam irradiation speed in the heat-affected zone re-irradiation step is faster than the welding speed in the tailored welding step, so it is preferable to increase the output of the laser beam to increase the laser beam absorption rate. Specifically, the output of the laser beam irradiated onto the HAZ-softened zone 4A of the first steel sheet 1 in the heat-affected zone re-irradiation step is preferably 2 to 6 kW. For example, the output of the laser beam irradiated onto the butt surface between the first steel sheet 1 and the second steel sheet 2 in the tailored welding step can be 2.8 kW, and the output of the laser beam irradiated onto the HAZ-softened zone 4A of the first steel sheet 1 in the heat-affected zone re-irradiation step can be 3.5 kW.

[0026] Even when the laser beam irradiated onto the HAZ-softened portion 4A has a ring profile, the irradiation speed of the laser beam can be varied as described above, and further, the laser output can be changed.

[0027] In the heat-affected zone re-irradiation step, a laser beam is irradiated from the surface of the first steel sheet 1 to the HAZ-softened zone 4A of the heat-affected zone 4 generated in the first steel sheet 1, so the laser beam in the heat-affected zone re-irradiation step is irradiated along the laser beam trajectory 20 of the tailored welding step. Specifically, as shown in Fig. 3 , the laser beam is irradiated onto the HAZ-softened zone 4A while the welding head 11 is moved along the laser beam trajectory 20 of the tailored welding step. Therefore, the laser beam trajectory 20 of the tailored welding step and the laser beam trajectory 21 of the heat-affected zone re-irradiation step are approximately parallel to each other.

[0028] In the heat-affected zone re-irradiation step, as shown in Fig. 3 , the HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel plate 1 may be continuously irradiated with a laser beam. In other words, the entire HAZ-softened zone 4A of the heat-affected zone 4 of the first steel plate 1 may be transformed into the second molten zone 5. However, in the heat-affected zone re-irradiation step, the laser beam may be intermittently irradiated with a laser beam. In other words, only a part of the HAZ-softened zone 4A of the heat-affected zone 4 of the first steel plate 1 may be transformed into the second molten zone 5.

[0029] FIG. 4 is an optical microscope photograph showing the results of observing the cross sections of the first steel plate 1 and the second steel plate 2 after the heat-affected zone re-irradiation process. As shown in FIGS. 2 and 4, it can be seen that the HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel plate 1 has been transformed into a second fusion zone 5 by performing the heat-affected zone re-irradiation process. Furthermore, new heat-affected zones 6 have been formed on both sides of the second fusion zone 5. A low-hardness HAZ-softened zone 6A has been formed at the interface of the heat-affected zone 6 with the heat-affected zone 4. Furthermore, a low-hardness HAZ-softened zone 6A has also been formed at the interface of the heat-affected zone 6 with the base metal 1A of the first steel plate 1. However, it can be seen from FIGS. 2 and 4 that the width of the HAZ-softened zone 6A of each heat-affected zone 6 is narrower than the width of the HAZ-softened zone 4A of each heat-affected zone 4. That is, the HAZ re-irradiation step reduces the HAZ softened zone present in the first steel plate 1, making it possible to suppress fractures originating from the HAZ softened zone.

[0030] 2 and 4, the total width W1 of the second fusion zone 5 generated by the heat-affected zone re-irradiation step and the heat-affected zones 6, 6A on both sides of the second fusion zone 5 is preferably smaller than the total width W2 of the first fusion zone 3 generated by the tailored welding step and the heat-affected zones 4, 4A on both sides of the first fusion zone 3. By irradiating the laser beam in the heat-affected zone re-irradiation step so as to achieve such a configuration, the HAZ-softened zone present in the first steel plate 1 is reduced, making it possible to suppress fractures originating from the HAZ-softened zone.

[0031] When the cross sections of the first steel plate 1 and the second steel plate 2 after the heat-affected zone re-irradiation step are observed, the area of ​​the second fusion zone 5 can be made smaller than the area of ​​the first fusion zone 3. In this embodiment, if at least a part of the HAZ-softened zone 4A of the heat-affected zone 4 generated in the first steel plate 1 is changed to the second fusion zone 5, it is possible to suppress fracture originating from the HAZ-softened zone 4A. Therefore, even if the area of ​​the second fusion zone 5 is made smaller than the area of ​​the first fusion zone 3, the fracture suppression effect can be obtained.

[0032] The cross-sectional area of ​​the second fusion zone 5 may be larger than that of the first fusion zone 3. However, if the cross-sectional area of ​​the second fusion zone 5 is larger, the cross-sectional area of ​​the heat-affected zone 6 and the cross-sectional area of ​​the HAZ-softened zone 6A will also be larger. Therefore, it is preferable that the cross-sectional area of ​​the second fusion zone 5 is smaller than that of the first fusion zone 3. Furthermore, as shown in FIG. 4 , the second fusion zone 5 does not have to be formed from the front surface to the back surface of the first steel sheet 1. The second fusion zone 5 has a structure mainly composed of martensite and is a region with high hardness. Therefore, if the second fusion zone 5 is not formed from the front surface to the back surface of the first steel sheet 1, the second fusion zone 5 will be easier to bend, thereby improving the formability of the resulting tailored blank material.

[0033] As described above, the manufacturing method of the tailored blank material of this embodiment comprises a tailored welding process in which a laser beam is irradiated to the area where the side of a first steel plate 1 and the side of a second steel plate 2 having greater strength than the first steel plate 1 are joined together, and a heat-affected zone re-irradiation process in which a laser beam is irradiated to the HAZ softened zone 4A of the heat-affected zone 4 generated on the first steel plate 1, of the heat-affected zones 4 generated on both sides of the first molten zone 3 produced by the tailored welding process, to turn at least a part of the HAZ softened zone 4A into a second molten zone 5.

[0034] In the manufacturing method of this embodiment, at least a portion of the HAZ-softened zone 4A generated in the tailored welding process is transformed into a second molten zone 5 in the heat-affected zone re-irradiation process, thereby reducing the amount of HAZ-softened zone present in the first steel plate 1. Therefore, the resulting tailored blank material is less susceptible to fracture originating from the HAZ-softened zone, thereby improving press formability. Furthermore, in the manufacturing method of this embodiment, unlike Patent Document 1, there is no need to increase the welding speed in the tailored welding process, so the width of the weld bead is less likely to become narrow. Therefore, the welding target position is less likely to shift, and the welding is less susceptible to the effects of the end face of the thin steel plate, resulting in stable welding quality.

[0035] Furthermore, in the tailored welding process, a laser beam is irradiated onto the area where the side surface of the first steel plate 1 and the side surface of the second steel plate 2 are butted together to join them. In other words, the tailored welding process does not involve stacking the first steel plate 1 and the second steel plate 2 in the thickness direction and then irradiating the stacked area with a laser beam to join them. Therefore, the resulting tailored blank material does not have an excessively thick welded area, allowing for a good appearance.

[0036] In the manufacturing method of this embodiment, the laser beam trajectory 20 in the tailored welding step and the laser beam trajectory 21 in the heat-affected zone re-irradiation step may be approximately parallel. In this case, the entire HAZ-softened zone 4A of the first steel sheet 1 generated in the tailored welding step can be changed into the second molten zone 5, and fracture originating from the HAZ-softened zone can be further suppressed, thereby improving press formability.

[0037] In the manufacturing method of this embodiment, when a cross section perpendicular to the laser beam trajectory 20 in the tailored welding step is observed, the total width W1 of the second fusion zone 5 generated in the heat-affected zone re-irradiation step and the heat-affected zones 6, 6A on both sides of the second fusion zone 5 may be smaller than the total width of the first fusion zone 3 generated in the tailored welding step and the heat-affected zones 4, 4A on both sides of the first fusion zone 3. With this configuration, the width of the HAZ-softened zone 6A in each heat-affected zone 6 is narrower than the width of the HAZ-softened zone 4A in each heat-affected zone 4, and the number of HAZ-softened zones present in the first steel plate 1 is reduced, making it possible to suppress fractures originating from the HAZ-softened zones.

[0038] In the manufacturing method of this embodiment, the irradiation speed of the laser beam in the heat-affected zone re-irradiation step may be faster than the welding speed in the tailored welding step. This makes the cross-sectional area of ​​the second fusion zone 5 smaller than the cross-sectional area of ​​the first fusion zone 3, and further reduces the HAZ-softened zone present in the first steel plate 1, making it possible to suppress fracture originating from the HAZ-softened zone.

[0039] In the manufacturing method of this embodiment, when a cross section perpendicular to the laser beam trajectory 20 in the tailored welding step is observed, the area of ​​the second fusion zone 5 formed by the heat-affected zone re-irradiation step may be smaller than the area of ​​the first fusion zone 3 formed by the tailored welding step. This reduces the HAZ-softened zone present in the first steel plate 1, making it possible to suppress fractures originating from the HAZ-softened zone.

[0040] In the manufacturing method of this embodiment, the laser beam irradiated to the HAZ-softened zone 4A of the heat-affected zone 4 generated in the first steel sheet 1 in the heat-affected zone re-irradiation step may be a ring profile. This makes it possible to suppress fracture originating from the HAZ-softened zone of the tailored blank material, and further improve the press formability (stretch formability) of the tailored blank material.

[0041] [Tailored blank material] Next, a tailored blank material according to this embodiment will be described. The tailored blank material according to this embodiment is formed by welding a first steel plate 1 to a second steel plate 2 having a strength greater than that of the first steel plate 1. Specifically, in the tailored blank material, a side surface of the first steel plate 1 and a side surface of the second steel plate 2 are joined by a first fusion zone 3.

[0042] In a tailored blank material, as shown in Fig. 4, in the cross section of the first steel plate 1 and the second steel plate 2, heat-affected zones 4 exist on both sides of the first fusion zone 3. The heat-affected zones 4 are regions where fine metal crystal grains in the first steel plate 1 and the second steel plate 2 have grown and coarsened due to the influence of heat from laser welding. In addition, a low-hardness HAZ softened zone 4A is formed at the interface between the second steel plate 2 and the base material 2A in the heat-affected zone 4.

[0043] In the tailored blank material, the first steel plate 1 has heat-affected zones 4, 6, 6A adjacent to the first fusion zone 3, and a second fusion zone 5 adjacent to the heat-affected zones 4, 6, 6A on the side opposite to the first fusion zone 3. In addition, heat-affected zones 6, 6A are formed between the second fusion zone 5 and the base metal 1A of the first steel plate 1. In the heat-affected zone 6 of the first steel plate 1, a HAZ-softened zone 6A is formed at the interface with the heat-affected zone 4, and a HAZ-softened zone 6A is also formed at the interface with the base metal 1A of the first steel plate 1.

[0044] The first fusion zone 3 and the second fusion zone 5 are structures mainly composed of martensite and are regions with high hardness. However, the HAZ-softened zone 4A is a region with lower hardness than the first fusion zone 3 and the base material 2A of the second steel plate 2. In addition, the HAZ-softened zone 6A is a region with lower hardness than the second fusion zone 5 and the base material 1A of the first steel plate 1.

[0045] 4, in the tailored blank material of this embodiment, the width of the HAZ-softened zone 6A in each heat-affected zone 6 is narrower than the width of the HAZ-softened zone 4A in each heat-affected zone 4. Furthermore, in the tailored blank material, the cross-sectional area of ​​the HAZ-softened zone 6A in each heat-affected zone 6 is smaller than the cross-sectional area of ​​the HAZ-softened zone 4A in each heat-affected zone 4. Therefore, since there are fewer HAZ-softened zones 6A in the first steel plate 1, it is possible to suppress fractures originating from the HAZ-softened zones 6A.

[0046] In the case of a tailored blank material, when a cross section is observed, the area of ​​the second fusion zone 5 is smaller than the area of ​​the first fusion zone 3. This reduces the HAZ-softened zone 6A present in the first steel plate 1, making it possible to suppress fracture originating from the HAZ-softened zone 6A.

[0047] In the tailored blank material, the first fusion zone 3 is generated from the front surface to the back surface of the first steel plate 1 and the second steel plate 2, and therefore the first steel plate 1 and the second steel plate 2 can be firmly joined. In addition, the heat-affected zone 4 and the HAZ-softened zone 4A are also generated from the front surface to the back surface of the first steel plate 1 and the second steel plate 2. In addition, the heat-affected zone 6 and the HAZ-softened zone 6A are also generated from the front surface to the back surface of the first steel plate 1.

[0048] The second fusion zone 5 may be generated from the front surface to the back surface of the first steel plate 1. However, the second fusion zone 5 does not have to be generated from the front surface to the back surface of the first steel plate 1. This reduces the HAZ-softened zone 6A present in the first steel plate 1, making it possible to suppress fracture originating from the HAZ-softened zone 6A.

[0049] As described above, the tailored blank material of this embodiment comprises a first steel plate 1 and a second steel plate 2 having greater strength than the first steel plate 1. A side surface of the first steel plate 1 and a side surface of the second steel plate 2 are joined by a first fusion zone 3, and heat-affected zones 4 exist on both sides of the first fusion zone 3. The first steel plate 1 has heat-affected zones 4, 6, 6A adjacent to the first fusion zone 3, and a second fusion zone 5 adjacent to the side of the heat-affected zones 4, 6, 6A opposite to the first fusion zone 3.

[0050] As described above, the tailored blank material is obtained by irradiating a laser beam onto the HAZ-softened zone 4A of the heat-affected zone 4 generated in the first steel plate 1, thereby converting at least a part of the HAZ-softened zone 4A into a second molten zone 5. As a result, the HAZ-softened zone present in the first steel plate 1 is reduced, and therefore, in the tailored blank material, fracture originating from the HAZ-softened zone is suppressed, thereby improving press formability.

[0051] Hereinafter, this embodiment will be described in more detail with reference to Examples 1 and 2, but this embodiment is not limited to these Examples. [Example]

[0052] [Making tailored blanks] First, a high-tensile steel plate 1 having a thickness of 1.2 mm and a tensile strength of 980 MPa was prepared as the first steel plate, and a high-tensile steel plate 2 having a thickness of 1.2 mm and a tensile strength of 1180 MPa was prepared as the second steel plate. Therefore, the strength of the second steel plate is greater than the strength of the first steel plate.

[0053] Then, the side of high-tensile steel plate 1 and the side of high-tensile steel plate 2 were butted together, and then a tailored welding process was performed in which a laser beam was irradiated onto the butted area to form a weld bead. The welding power for the tailored welding was 2.8 kW, and the welding speed was 3 m per minute.

[0054] Next, a heat-affected zone re-irradiation process was performed in which a laser beam was irradiated to the HAZ softened portion of the heat-affected zone of the first steel plate, which had low strength, among the heat-affected zones present at a position 1.6 mm away from the first fusion zone generated in the tailored welding process, to form a second fusion zone. Note that the irradiation power of the laser beam in the heat-affected zone re-irradiation process was 3.5 kW, and the irradiation speed was 10 m per minute.

[0055] In this manner, a tailored blank material according to the example was produced.

[0056] [Evaluation of tailored blank materials] Figure 2 is a photograph showing the results of observing a cross section perpendicular to the laser beam trajectory in a blank material of a first steel plate 1 and a second steel plate 2 obtained by the tailored welding process. As shown in Figure 2, as a result of performing tailored welding, a first fusion zone 3 has been formed between the first steel plate 1 and the second steel plate 2, where the metal of the first steel plate 1 and the metal of the second steel plate 2 have melted and solidified. It can also be seen that a heat-affected zone 4 has been formed outside the first fusion zone 3.

[0057] FIG. 4 shows a photograph of a cross section perpendicular to the laser beam trajectory of a tailored blank material obtained by the heat-affected zone re-irradiation process. As shown in FIG. 4, the heat-affected zone re-irradiation process transformed the HAZ-softened zone 4A of the heat-affected zone 4 formed in the first steel sheet 1 into a second fusion zone 5. New heat-affected zones 6 were formed on both sides of the second fusion zone 5. A low-hardness HAZ-softened zone 6A was formed at the interface of the heat-affected zone 6 with the heat-affected zone 4. Furthermore, a low-hardness HAZ-softened zone 6A was also formed at the interface of the heat-affected zone 6 with the base metal 1A of the first steel sheet 1. However, as shown in FIGS. 2 and 4, the width of the HAZ-softened zone 6A in each heat-affected zone 6 is narrower than the width of the HAZ-softened zone 4A in each heat-affected zone 4, resulting in a reduction in the HAZ-softened zone, making it possible to suppress fractures originating from the HAZ-softened zone. [Example]

[0058] [Making tailored blanks] As in Example 1, first, a high-tensile steel plate 1 having a thickness of 1.2 mm and a tensile strength of 980 MPa was prepared as the first steel plate, and a high-tensile steel plate 2 having a thickness of 1.2 mm and a tensile strength of 1180 MPa was prepared as the second steel plate.

[0059] Then, the side of high-tensile steel plate 1 and the side of high-tensile steel plate 2 were butted together, and then a tailored welding process was performed in which a laser beam was irradiated onto the butted area to form a weld bead. The welding power for the tailored welding was 3.0 kW, and the welding speed was 4 m per minute. A ring profile was used for the laser beam, and the core / ring ratio was set to 30 / 70.

[0060] Next, a heat-affected zone re-irradiation process was performed in which a laser beam was irradiated to the HAZ softened portion of the heat-affected zone of the first steel plate, which had lower strength, among the heat-affected zones present at a position 1.4 mm away from the first fusion zone created in the tailored welding process, to generate a second fusion zone. A ring profile laser beam was used for the laser beam. Furthermore, the core / ring ratio of the ring profile was set to 30 / 70, 60 / 40, or 100 / 0. The laser beam output was 3.9 kW when the core / ring ratio was 30 / 70, 2.6 kW when it was 60 / 40, and 1.8 kW when it was 100 / 0, and the irradiation speed was 10 m / min.

[0061] In this way, tailored blank materials were produced in which the heat-affected zone was re-irradiated with laser light at various core / ring ratios, as shown in Figure 5. Furthermore, tailored blank materials were also produced in which the first steel plate and the second steel plate were tailor welded using the tailored welding process, but the heat-affected zone re-irradiation process was not performed and no second molten zone was not generated.

[0062] [Evaluation of tailored blank materials] (Extension test) Stretching tests were conducted in accordance with ISO 12004-2:2021 for tailored blanks without a second fusion zone and tailored blanks with a second fusion zone at each core / ring ratio. The ball-nosed punch used in the stretching tests had a diameter of 100 mm. The stretching test is a method for evaluating the plastic deformation characteristics during stretching. Specifically, a ball-nosed punch was inserted into the joint of each tailored blank to generate a through-hole crack. The maximum load of the ball-nosed punch on the tailored blank and the travel distance (stretching height) of the ball-nosed punch when a through-hole crack occurred were then measured. The maximum load and stretching height of each tailored blank are shown in Table 1. Furthermore, the percentage increase in the maximum load of each tailored blank, relative to the maximum load of the tailored blank without a second fusion zone (100%), is also shown in Table 1.

[0063] [Table 1]

[0064] As shown in Table 1, it can be seen that the generation of the second fusion zone improves the maximum load and the projection height. In this way, by changing the HAZ-softened zone 4A generated in the tailored welding process into the second fusion zone 5 in the heat-affected zone re-irradiation process, fracture originating from the HAZ-softened zone is suppressed, thereby improving the press formability of the tailored blank material.

[0065] Furthermore, it was found that the maximum load further increased by forming a ring profile on the laser beam and increasing the core / ring ratio to reduce the beam irradiation diameter during the heat-affected zone re-irradiation process. In other words, by setting the core / ring ratio to 60 / 40 to 100 / 0, the maximum load was more than doubled compared to when the second molten zone was not generated. Therefore, by forming a ring profile on the laser beam and increasing the core / ring ratio to reduce the beam irradiation diameter, the press formability of the tailored blank material could be further improved.

[0066] (thermal strain measurement) We evaluated whether the tailored blanks were subject to thermal strain due to the heat input during the tailored welding process and the heat-affected zone re-irradiation process. Specifically, we first placed tailored blanks without a second fusion zone and tailored blanks with a second fusion zone at each core / ring ratio on a rotating table with their main surfaces aligned vertically. The periphery of each tailored blank was then 3D scanned using a non-contact 3D strain measurement device to measure the strain distribution of each tailored blank. Figure 6 shows the strain distribution of each tailored blank, and Table 2 lists the maximum strain of each tailored blank. Furthermore, Table 2 also lists the percentage increase in maximum strain of each tailored blank, relative to the maximum strain of the tailored blank without a second fusion zone, which is set to 100%.

[0067] [Table 2]

[0068] As shown in Figure 6 and Table 2, it can be seen that the blank material in which the second fusion zone was not generated exhibited less thermal strain. In contrast, the tailored blank material with a core / ring ratio of 30 / 70 exhibited more thermal strain than the blank material in which the second fusion zone was not generated. It can also be seen that the thermal strain of the tailored blank material can be reduced by setting the core / ring ratio to 60 / 40 to 100 / 0. Therefore, it can be seen that the thermal strain of the tailored blank material can be reduced by performing the heat-affected zone re-irradiation process with a ring profile and further increasing the core / ring ratio to reduce the beam irradiation diameter.

[0069] Although the present embodiment has been described above, the present embodiment is not limited to this, and various modifications are possible within the scope of the gist of the present embodiment. [Explanation of symbols]

[0070] 1. First Steel Plate 2. Second Steel Plate 3 First fusion zone 4 Heat-affected zone 4A HAZ softened area in the heat affected zone 5 Second fusion zone 6 Heat-affected zone 6A HAZ softened area in the heat affected zone 20 Laser beam trajectory in tailored welding process 21 Laser beam trajectory during heat-affected zone re-irradiation process

Claims

1. a tailored welding process in which a side surface of a first steel plate and a side surface of a second steel plate having a strength greater than that of the first steel plate are butted together by irradiating a laser beam onto the butted portion; a heat-affected zone re-irradiation process in which a laser beam is irradiated onto a HAZ-softened portion of the heat-affected zone generated in the first steel plate, among the heat-affected zones generated on both sides of the first molten zone generated by the tailored welding process, thereby turning at least a part of the HAZ-softened portion into a second molten zone; A method for manufacturing a tailored blank material, comprising:

2. 2. The method for manufacturing a tailored blank material according to claim 1, wherein a laser beam trajectory in the tailored welding step and a laser beam trajectory in the heat-affected zone re-irradiation step are substantially parallel to each other.

3. 3. The method for manufacturing a tailored blank material according to claim 1 or 2, wherein, when a cross section perpendicular to a laser beam trajectory in the tailored welding process is observed, a total width of the second fusion zone generated by the heat-affected zone re-irradiation process and the heat-affected zones on both sides of the second fusion zone is smaller than a total width of the first fusion zone generated by the tailored welding process and the heat-affected zones on both sides of the first fusion zone.

4. 3. The method for manufacturing a tailored blank material according to claim 1, wherein an irradiation speed of the laser beam in the heat-affected zone re-irradiation step is faster than a welding speed in the tailored welding step.

5. 3. The method for manufacturing a tailored blank material according to claim 1, wherein, when a cross section perpendicular to the laser beam trajectory in the tailored welding process is observed, the area of ​​the second molten zone generated by the heat-affected zone re-irradiation process is smaller than the area of ​​the first molten zone generated by the tailored welding process.

6. 3. The method for manufacturing a tailored blank material according to claim 1, wherein the laser beam irradiated to the HAZ softened portion of the heat affected zone generated in the first steel plate in the heat affected zone re-irradiation step has a ring profile.

7. a first steel plate; a second steel plate having a strength greater than that of the first steel plate, a side surface of the first steel plate and a side surface of the second steel plate are joined by a first fusion zone, a heat-affected zone is present on both sides of the first fusion zone; A tailored blank material, wherein the first steel plate has a heat-affected zone adjacent to the first fusion zone and a second fusion zone adjacent to the heat-affected zone on the opposite side from the first fusion zone.

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