Cut steel plates, tailored blanks, methods for manufacturing tailored blanks, hot-pressed products, and methods for manufacturing hot-pressed products.

Abstract

To provide a cut steel plate that eliminates or reduces the amount of trimming required on the laser-cut surface, and a tailored blank, a method for manufacturing a tailored blank, a hot-pressed product, and a method for manufacturing a hot-pressed product using the same. [Solution] A cut steel sheet having a base steel sheet, an Al plating layer, an intermetallic compound layer, and a surface coating provided on at least one of the Al plating layers provided on one side of the base steel sheet or on both sides of the base steel sheet, the coating containing a binder resin and carbon black, and having a carbon black content of 1 to 20% by mass relative to the surface coating, wherein at least a portion of the end face of the cut steel sheet is a laser cut surface, and the height of the dross at the laser cut surface is 0.50 mm or less, and a tailored blank, a method for manufacturing a tailored blank, a hot press-formed product, and a method for manufacturing a hot press-formed product using the same.

Description

[Technical Field]

[0001] This disclosure relates to cut steel plates, tailored blanks, methods for manufacturing tailored blanks, hot press-formed products, and methods for manufacturing hot press-formed products. [Background technology]

[0002] In recent years, from the perspective of protecting the global environment, reducing CO2 gas emissions has become an urgent issue in the automotive sector, and therefore, the lightweighting of automobile bodies is being actively explored. In response, the application of high-strength steel sheets is being actively considered. The strength of steel sheets (plated steel sheets) is also increasing.

[0003] On the other hand, when forming high-strength steel sheets, the formability decreases as the strength of the steel sheet increases. In particular, high-strength steel sheets with a tensile strength exceeding 1500 MPa have poor ductility, making it difficult to cold press form them into the complex shapes of automotive structural components.

[0004] Hot press forming (also known as "hot stamping") is attracting attention as one of the technologies for forming automotive components. In hot press forming, a steel sheet for hot pressing is heated to a high temperature and pressed in a temperature range above the Ar3 transformation temperature. Furthermore, in hot press forming, the pressed steel sheet is rapidly cooled by heat removal using a die, causing a transformation to occur simultaneously with the forming process while press pressure is applied. As a result, hot press forming is a technology that can manufacture press-formed products with high strength and excellent shape retention.

[0005] As for technologies related to hot press forming, for example, Patent Document 1 states that "an Al plating layer is formed on one or both sides of a steel sheet, the Al plating layer contains Si: 3 to 15% by mass, the remainder being Al and unavoidable impurities, the surface roughness is Ra of 1.0 to 4.0 μm, and on the Al plating layer, L * An Al-plated steel sheet for hot pressing is disclosed, characterized in that a surface coating layer having a density of 10 to 50 is laminated therein.

[0006] On the other hand, in order to improve the yield and functionality of press-formed automotive components, tailored blanks, which are made by joining the ends of at least two steel plates by laser welding, plasma welding, etc., are being used as press-formed materials. Tailored blanks allow for the joining of multiple cut steel plates according to the purpose, making it possible to freely change the plate thickness and strength within a single component. As a result, using tailored blanks makes it possible to improve the functionality of automotive components and reduce the number of parts in an automotive component. Furthermore, by hot press forming using tailored blanks, it is possible to manufacture high-strength press-formed products with freely adjustable plate thickness, strength, etc.

[0007] Furthermore, it is known that steel sheets for producing tailored blanks can be obtained, for example, by laser cutting an Al-plated steel sheet (for example, a coil of that steel sheet) into an arbitrary shape (Patent Documents 2-6). [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Patent No. 5573195 [Patent Document 2] Special Publication No. 2014-512457 [Patent Document 3] Patent No. 6961815 [Patent Document 4] Patent No. 6961816 [Patent Document 5] Patent No. 7440637 [Patent Document 6] Patent No. 6826999 [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] Laser cutting is a highly flexible cutting method that does not require molds, unlike mold blanking. On the other hand, because the surface of the Al plating layer on an Al-plated steel sheet has a high reflectivity to the laser, the surface of the Al-plated layer requires more laser irradiation energy when laser cutting compared to the surface of the base steel sheet (i.e., the base metal). When laser cutting, increasing the laser output and applying a large amount of laser irradiation energy causes the surface of the Al plating layer to melt upon laser irradiation. On the other hand, after the Al plating layer has melted, when the laser reaches the base steel sheet, a large amount of laser irradiation energy is applied to the base steel sheet (i.e., the base metal). When a large amount of laser irradiation energy is applied to the base steel plate, a large amount of molten metal is generated, and it becomes difficult to remove the molten metal. Any molten metal that is not completely removed remains as dross on the laser-cut surface. Dross causes cosmetic defects or welding defects, requiring trimming. Furthermore, a larger amount of dross increases the amount of trimming required. Therefore, it is desirable to have as little dross as possible remaining on the laser-cut surface. However, including Patent Documents 2 to 6 (particularly Patent Document 2), there is room for improvement in miniaturizing the dross remaining on the laser-cut surface of Al-plated steel sheets by laser cutting.

[0010] Therefore, the object of this disclosure is to provide a cut steel sheet that allows for the omission or reduction of trimming of the laser-cut surface, as well as a tailored blank, a method for manufacturing a tailored blank, a hot press-formed product, and a method for manufacturing a hot press-formed product, all utilizing the same. [Means for solving the problem]

[0011] The means for solving the problem include the following: <1> Base material steel plate, An Al plating layer provided on one or both sides of the base steel sheet, A layer of intermetallic compound containing iron and aluminum is provided between the base steel sheet and the Al plating layer, Provided on at least one of the Al plating layer provided on one side of the base metal steel plate or the Al plating layers provided on both sides of the base metal steel plate, and containing a binder resin and carbon black, a surface film having a carbon black content of 1 to 20% by mass with respect to the surface film, having, at least a part of the end face of the cut steel plate is a laser cut surface, a cut steel plate, wherein the height of dross on the laser cut surface is 0.50 mm or less. <2> The cut steel plate according to <1>, wherein the surface roughness Ra of the surface film is 0.5 to 2.0 μm. <3> The cut steel plate according to <1> or <2>, wherein the total thickness of the intermetallic compound layer and the Al plating layer is 9 to 60 μm in average thickness. <4> The cut steel plate according to any one of <1> to <3>, wherein the plate thickness of the base metal steel plate is 0.7 to 3.2 mm. <5> Two or more cut steel plates having the cut steel plate according to any one of <1> to <4>, a welding metal connecting the two or more cut steel plates, a tailored blank having. <6> A method for manufacturing a tailored blank, comprising welding two or more cut steel plates having the cut steel plate according to any one of <1> to <4> and connecting them with a welding metal. <7> A hot press molded product of the tailored blank according to <5>. <8> A method for manufacturing a hot press molded product, comprising hot press molding the tailored blank according to <5>.

Advantages of the Invention

[0012] According to the present disclosure, there are provided a cut steel plate in which trimming of the laser cut surface can be omitted or the trimming amount can be reduced, and a tailored blank, a method for manufacturing a tailored blank, a hot press molded product, and a method for manufacturing a hot press molded product using the same. [Brief explanation of the drawing]

[0013] [Figure 1] Figure 1 is an enlarged schematic cross-sectional view showing an example of a cut steel plate according to this disclosure. [Figure 2] Figure 2 is a schematic diagram showing an example of a cross-section of a weld in a tailored blank according to the present disclosure. [Modes for carrying out the invention]

[0014] The following describes in detail an example of a preferred embodiment of this disclosure. In this disclosure, a numerical range represented using "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. Regarding the content of components (elements), for example, the content of C (carbon) may be expressed as "C amount." The content of other elements may also be expressed similarly. The term "cross-section" in the context of cut steel plates refers to the cross-section obtained by cutting along the thickness direction of the plate. The term "edge" of a cut steel plate refers to the area located around the cut steel plate, and in its widest case, it represents the area within 5 mm from the edge of the cut steel plate. The "edge" of a cut steel plate refers to the point where the end face meets the opposing surfaces in the thickness direction of the cut steel plate (i.e., the surface of the cut steel plate). The term "end face" of a cut steel plate refers to the face perpendicular to the thickness direction between opposing surfaces in a direction perpendicular to the thickness direction.

[0015] <Cutting steel plate> The cut steel plate of this disclosure is Base material steel plate, An Al plating layer provided on one or both sides of the base steel plate, A layer of intermetallic compound containing iron and aluminum is provided between the base steel sheet and the Al plating layer. A surface coating is provided on at least one of the Al plating layers provided on one side of the base steel sheet or on both sides of the base steel sheet, and contains a binder resin and carbon black, with a carbon black content of 1 to 20% by mass relative to the surface coating. It has. Furthermore, at least a portion of the end face of the cut steel plate is a laser-cut surface, and the height of the dross at the laser-cut surface is 0.50 mm or less.

[0016] The cut steel sheet of this disclosure, with the above configuration, eliminates or reduces the amount of trimming required on the laser-cut surface. The cut steel sheet of this disclosure was discovered through the following findings. Traditionally, although the Al plating layer has a higher reflectivity at room temperature than the base steel sheet (i.e., the base metal), it was thought that laser cutting would have little effect on the cutting performance of the Al plating layer (i.e., dross formation) because it forms a keyhole-shaped molten pool. However, the inventors conducted further research on miniaturizing the dross generated during laser cutting of aluminum-plated steel sheets and obtained the following findings. By applying a black surface film containing 1-20% by mass of carbon black to the surface of the Al-plated steel sheet's Al plating layer, the reflectivity is reduced. This allows for a reduction in the laser irradiation energy during laser cutting. In addition, when an intermetallic compound layer is interposed between the base steel sheet and the Al plating layer, the penetration force during laser irradiation does not change abruptly from the Al plating layer to the base steel sheet. Instead, the penetration force is ensured to be continuous by crossing the intermetallic compound layer containing iron and aluminum, resulting in stable cutting performance. As a result, the dross generated during laser cutting is minimized. Furthermore, by reducing the laser irradiation energy during laser cutting, and by optimizing laser cutting conditions such as applying solid-state lasers (fiber lasers, YAG (Yttrium Aluminum Garnet) lasers, or gas lasers, and increasing the cutting speed, the amount of base metal melted when the laser reaches the base steel plate (i.e., the base metal) was reduced. As a result, the height of the dross at the laser-cut surface was reduced to 0.50 mm or less.

[0017] Based on the above findings, a cut steel sheet according to this disclosure has been found in which trimming of the laser-cut surface can be omitted or the amount of trimming can be reduced. Furthermore, the cut steel plate of this disclosure has a reduced dross height of 0.50 mm or less at the laser-cut surface, resulting in a good appearance. In addition, even when welding is performed on the edges of the cut steel plate of this disclosure, including the laser-cut surface, welding defects are suppressed.

[0018] The details of the cut steel plate of this disclosure are described below.

[0019] The cut steel sheet of this disclosure is a cut steel sheet obtained by laser cutting a coated Al-plated steel sheet having a base steel sheet, an Al plating layer provided on one or both sides of the base steel sheet, an intermetallic compound layer provided between the base steel sheet and the Al plating layer, and a surface coating provided on at least one of the Al plating layer provided on one side of the base steel sheet or the Al plating layer provided on both sides of the base steel sheet (see Figure 1). Furthermore, the laser-cut surface of the cut steel sheet is free of dross, or if dross is present, it has a height of 0.50 mm or less. However, the cut steel sheet of this disclosure may be any cut steel sheet obtained by laser cutting at least a portion of a coated Al-plated steel sheet. In other words, the cut steel sheet of this disclosure may have all its end faces as laser-cut surfaces, or only a portion of its end faces as laser-cut surfaces. Furthermore, if Al plating layers are provided on both sides of the base steel sheet, it is preferable that the surface coating be provided on at least the Al plating layer on the side that is the laser irradiation surface during laser cutting. Furthermore, an embodiment in which some of the end faces are laser-cut surfaces is, for example, when the cut steel plate of this disclosure is a quadrilateral steel plate having four end faces, an embodiment in which at least one of the four end faces is a laser-cut surface.

[0020] In Figure 1, 100 represents the cut steel plate, 10 represents the base steel plate, 12 represents the Al plating layer, 14 represents the intermetallic compound layer, 16 represents the surface film, 18 represents the dross, 100A represents the laser cut surface, and T represents the height of the dross. Figure 1 shows an example of a cut steel sheet having a surface coating on an Al plating layer provided on both sides of the base steel sheet, and a cut end surface with dross of 0.50 mm or less in height.

[0021] [Dros] Dross is defined in JIS Z 3001:1:2018 No. 17309 as "molten metal or metal oxide generated by thermal cutting (laser cutting in this disclosure) and adhering to and solidifying below the cut surface." Specifically, in the cut steel sheet of this disclosure, the dross is molten metal or metal oxide that protrudes in the thickness direction from the edge of the laser cut surface to the side opposite to the laser irradiation surface in the thickness direction (see Figure 1).

[0022] The height of the dross at the laser cutting surface is 0.50 mm or less, preferably 0.30 mm or less, more preferably 0.10 mm or less, and ideally 0 mm. If the dross height at the laser-cut surface exceeds 0.50 mm, trimming becomes necessary, and the amount of trimming required increases.

[0023] The height of the dross in the laser-cut surface is the length from the edge of the laser-cut surface along the thickness direction of the plate, when the laser-cut surface is observed from a direction perpendicular to the laser-cut surface (see T in Figure 1). The height of the dross at the laser-cut surface is then measured as follows: The end of the cut steel plate to be measured, including the laser-cut surface with dross, is cut along the thickness direction to obtain a steel billet. A steel slab is embedded in epoxy resin, and the surface facing the laser-cut edge is polished to obtain a sample. The laser-cut surface of the obtained sample is observed at 25x magnification using an optical microscope. Then, the height of each observed dross is measured. Perform the above operation once and calculate the arithmetic mean of the multiple dross heights measured.

[0024] Furthermore, confirmation that the surface is laser-cut is performed by checking for the presence or absence of a heat-affected zone caused by the laser at the edge of the cut steel plate.

[0025] [Base material steel plate] The base steel sheet is the steel sheet to which the intermetallic compound layer and the Al plating layer are to be applied (i.e., the steel sheet to which Al plating is to be applied). The base steel sheet can be any sheet obtained by conventional methods and is not particularly limited. The base steel sheet may be either a hot-rolled steel sheet or a cold-rolled steel sheet.

[0026] The thickness of the base steel plate is preferably 0.7 to 3.2 mm, and more preferably 1.0 to 2.6 mm. From the viewpoint of ensuring the tensile strength of the base steel plate, it is preferable that the thickness of the base steel plate be 0.7 mm or more. On the other hand, if the thickness of the base steel plate is 3.2 mm or less, the cutting speed during laser cutting can be increased, reducing the amount of melted base metal. As a result, the dross height tends to be smaller. Therefore, it is preferable that the thickness of the base steel plate be within the above range. The thickness of the base steel plate is determined by measuring the thickness at five arbitrary points on the cut steel plate, excluding the edges, using a micrometer (Mitutoyo Corporation CPM-MX), and calculating the average of these measurements.

[0027] The base steel sheet should preferably be a steel sheet formed to have high mechanical strength (meaning various properties related to mechanical deformation and fracture, such as tensile strength, yield point, elongation, reduction of area, hardness, impact value, fatigue strength, etc.). Specifically, a steel sheet with a tensile strength of 400 to 2700 MPa may be used. Furthermore, steel plates with low mechanical strength may be used as the base material. Specifically, these include 1300 MPa, 1200 MPa, 1000 MPa, 600 MPa, and 500 MPa classes. For example, in the case of a B-pillar of an automobile, it is desirable to use a steel plate with a tensile strength of 1500 to 2000 MPa class for the upper and central parts where deformation is to be prevented, and a steel plate with a tensile strength of 500 MPa to 1500 MPa class for the lower part, which is the energy absorption part. More preferably, the lower part should be a steel plate with a tensile strength of 600 MPa to 1300 MPa class.

[0028] Examples of preferred chemical compositions for the base steel sheet include the following: In other words, a preferred base steel sheet is, in terms of mass%, C: 0.02%~0.58%, Mn: 0.20%~3.00% Al: 0.005%~0.060%, Ti: 0%~0.20%, Nb: 0%~0.20%, V: 0%~1.0%, W: 0%~1.0%, Cr: 0%~1.0%, Mo: 0%~1.0%, Cu: 0%~1.0%, Ni: 0%~1.0%, B: 0%~0.0100%, Mg: 0%~0.05% Ca: 0%~0.05%, REM: 0%~0.05%, Bi: 0%~0.05%, Si: 0%~2.00% Sn: 0%~0.5% P: 0.03% or less, S: 0.010% or less, N: 0.010% or less, and, The remainder includes a base steel sheet having a chemical composition consisting of Fe and impurities. Note that the "%" indicated below to show the content of components (elements) means "mass percent".

[0029] (C: 0.02%~0.58%) Carbon (C) is an important element that enhances the hardenability of the base steel sheet and primarily determines the strength after quenching. Furthermore, it lowers the A3 point and promotes lower quenching temperatures. If the C content is less than 0.02%, its effect may not be sufficient. Therefore, it is preferable to have a C content of 0.02% or more. On the other hand, if the C content exceeds 0.58%, the toughness of the quenched area deteriorates significantly. Therefore, it is preferable to have a C content of 0.58% or less. Preferably, it is 0.45% or less.

[0030] (Mn: 0.20%~3.00%) Mn is a very effective element for improving the hardenability of the base steel sheet and ensuring stable strength after quenching. If the Mn content is less than 0.20%, its effect may not be sufficient. Therefore, it is preferable to have an Mn content of 0.20% or more, preferably 0.80% or more. On the other hand, if the Mn content exceeds 3.00%, not only does its effect saturate, but it may also become difficult to ensure stable strength after quenching. Therefore, it is preferable to have an Mn content of 3.00% or less, preferably 2.40% or less.

[0031] (Al: 0.005%~0.060%) Al functions as a deoxidizing element. Furthermore, Al reduces oxide inclusions that adversely affect the mechanical strength of the base steel sheet. When the Al content is less than 0.005%, it may be difficult to obtain the above effects. Therefore, it is preferable to have an Al content of 0.005% or more. On the other hand, when the Al content exceeds 0.060%, the above effects saturate, making it cost-inefficient. Therefore, it is preferable to have an Al content of 0.060% or less.

[0032] (Ti:0%~0.20%, Nb:0%~0.20%, V:0%~1.0%, W:0%~1.0%) Ti, Nb, V, and W are elements that promote the interdiffusion of Fe and Al in the Al plating layer and the base steel sheet. Therefore, at least one of Ti, Nb, V, and W may be included in the base steel sheet. However, if 1) the amount of Ti and Nb exceeds 0.20%, or 2) the amount of V and W exceeds 1.0%, the effect of the above action saturates, making it cost-inefficient. Therefore, it is preferable to keep the amount of Ti and Nb at 0.20% or less, and the amount of V and W at 1.0% or less. Preferably, the amount of Ti and Nb is 0.15% or less, and the amount of V and W is 0.5% or less. To more reliably obtain the effect of the above action, it is preferable to set the lower limit of the amount of Ti and Nb to 0.01% or more, and the lower limit of the amount of V and W to 0.1% or more.

[0033] (Cr:0%~1.0%, Mo:0%~1.0%, Cu:0%~1.0%, Ni:0%~1.0%, B:0%~0.0100%) Cr, Mo, Cu, Ni, and B are effective elements for improving the hardenability of the base steel sheet and ensuring stable strength after quenching. Therefore, one or more of these elements may be included. However, if the content of Cr, Mo, Cu, and Ni exceeds 1.0% or the amount of B exceeds 0.0100%, the above effects saturate, making it cost-inefficient. Therefore, it is preferable to keep the content of Cr, Mo, Cu, and Ni at 1.0% or less. Also, the amount of B is preferably 0.0100% or less, and more preferably 0.0080% or less. To more reliably obtain the above effects, it is preferable to satisfy either the content of Cr, Mo, Cu, and Ni at 0.1% or more, or the content of B at 0.0010% or more.

[0034] (Ca:0%~0.05%, Mg:0%~0.05%, REM:0%~0.05%) Ca, Mg, and REM have the effect of refining the morphology of inclusions in steel and preventing the occurrence of cracks during hot press forming caused by inclusions. Therefore, one or more of these elements may be included. However, if added in excess, the effect of refining the morphology of inclusions in steel will saturate, and only increase costs. Therefore, it is preferable to keep the amount of Ca at 0.05% or less, the amount of Mg at 0.05% or less, and the amount of REM at 0.05% or less. To more reliably obtain the effects of the above actions, it is preferable to satisfy at least one of the following: the amount of Ca at 0.0005% or more, the amount of Mg at 0.0005% or more, and the amount of REM at 0.0005% or more.

[0035] Here, REM refers to the total of 17 elements including Sc, Y, and lanthanides, and the REM content mentioned above refers to the total content of these elements. In the case of lanthanides, they are added industrially in the form of mischmetal.

[0036] (Bi: 0%~0.05%) Bi acts as a solidification nucleus during the solidification process of molten steel and is an element that suppresses segregation of Mn and other elements. Therefore, Bi may be included. In particular, Bi has the effect of suppressing the deterioration of toughness caused by Mn segregation. Therefore, it is preferable to include Bi in such steel types. However, if the amount of Bi exceeds 0.05%, the effect of the above action will saturate, leading to an increase in cost. Therefore, the amount of Bi should be 0.05% or less. Preferably, it is 0.02% or less. Furthermore, in order to more reliably obtain the effect of the above action, it is preferable to have a Bi amount of 0.0002% or more. Even more preferably, it is 0.0005% or more.

[0037] (Si: 0%~2.00%) Si is a solid solution strengthening element and can be effectively utilized up to 2.00%. However, if the Si content exceeds 2.00%, there is a concern that problems may occur in the plating properties. Therefore, when Si is included, it is preferable to keep the Si content to 2.00% or less. The preferred upper limit is 1.40% or less, and more preferably 1.00% or less. The lower limit is not particularly limited, but to more reliably obtain the effects of the above action, 0.01% or more is preferred.

[0038] (Sn: 0%~0.5%) Sn is an element that improves the corrosion resistance of exposed steel. Therefore, Sn may be included in the base steel sheet. However, including more than 0.5% Sn in the base steel sheet will lead to embrittlement of the base steel sheet. Therefore, the amount of Sn should be 0.5% or less. Preferably, the amount of Sn is 0.3% or less. Furthermore, to more reliably obtain the effects resulting from the above action, it is preferable to have a Sn content of 0.02% or more. More preferably, the Sn content is 0.04% or more.

[0039] (P: 0%~0.03%) P is an element present as an impurity. Excessive P content can easily reduce the toughness of the base steel sheet. Therefore, the P content should be 0.03% or less, preferably 0.01% or less. There is no need to specify a lower limit for the P content; it may be 0% or greater. From a cost perspective, it is preferable to have a P content of 0.0002% or more.

[0040] (S: 0%~0.010%) S is an element present as an impurity, forming MnS and causing the base steel sheet to become brittle. Therefore, the amount of S should ideally be 0.010% or less. A more desirable amount of S is 0.004% or less. There is no need to specify a lower limit for the amount of S. The amount of S may be 0% or greater. From a cost standpoint, it is preferable to have an amount of 0.0002% or more.

[0041] (N: 0%~0.010%) N is an element present as an impurity that forms inclusions in steel and degrades toughness after hot press forming. Therefore, the amount of N should preferably be 0.010% or less. Preferably, it should be 0.008% or less, and more preferably 0.005% or less. There is no need to specify a lower limit for the amount of N. The amount of N may be 0% or greater. From a cost standpoint, it is preferable to have 0.0002% or more.

[0042] (Remainder) The remainder consists of Fe and impurities. Here, impurities refer to components contained in raw materials such as ore and scrap, or components that are mixed in during the manufacturing process, and not components that were intentionally included in the base steel sheet.

[0043] [Al plating layer] The Al plating layer is a layer formed after applying Al plating to a base steel sheet, excluding the intermetallic compound layer. The method for forming the Al plating layer is not particularly limited. For example, the Al plating layer can be formed on one or both sides of the base steel sheet by a hot-dip galvanizing method. The hot-dip galvanizing method involves immersing the base steel sheet in a molten metal bath mainly containing aluminum, thereby applying Al plating to both sides of the base steel sheet.

[0044] Here, the Al plating layer is a plating layer mainly containing aluminum, and it is sufficient if it contains 50% by mass or more of aluminum. Depending on the purpose, it may also contain elements other than aluminum (for example, silicon), and may contain impurities that are introduced during the manufacturing process. Specifically, examples of Al plating layers include an Al plating layer having a chemical composition in which, by mass%, 5% to 12% Si (silicon) is present and the remainder consists of aluminum and impurities, or an Al plating layer having a chemical composition in which, by mass%, 5% to 12% Si (silicon), 2% to 4% Fe (iron), and the remainder consists of aluminum and impurities. In particular, incorporating Si within the above range into the Al plating layer can suppress the deterioration of workability and corrosion resistance. Furthermore, the thickness of the intermetallic compound layer can be reduced. Furthermore, impurities refer to components that are introduced during the manufacturing process and are not components that were intentionally included in the Al plating layer.

[0045] The thickness of the Al plating layer is not particularly limited, but for example, it is often 8 μm or more in average thickness, and preferably 15 μm or more. Furthermore, the thickness of the Al plating layer is often 50 μm or less in average thickness, preferably 40 μm or less, more preferably 35 μm or less, and even more preferably 30 μm or less.

[0046] [Intermetallic compound layer] The intermetallic compound layer is formed between the base steel sheet and the Al plating layer (i.e., at the boundary) when an Al plating layer is formed on the base steel sheet. Specifically, the intermetallic compound layer is formed by the reaction of iron (Fe) and aluminum (Al) in the base steel sheet with a metal containing aluminum in a molten metal bath mainly containing aluminum. The intermetallic compound layer is mainly Fe x Al y It is formed of multiple types of compounds represented by (x, y represent 1 or more). If the Al plating layer contains Si (silicon), Fe x Al y and Fe x Al y Si z It is formed from multiple types of compounds represented by (x, y, and z represent values ​​greater than or equal to 1).

[0047] The thickness of the intermetallic compound layer is not particularly limited, but for example, it is often 1 μm or more in average thickness, preferably 3 μm or more, and more preferably 4 μm or more. Also, the thickness of the intermetallic compound layer is often 10 μm or less in average thickness, and preferably 8 μm or less. The thickness of the intermetallic compound layer represents the average thickness in the cut steel sheet. The thickness of the intermetallic compound layer can be controlled by the temperature and immersion time of the molten metal bath, which mainly contains aluminum.

[0048] Here, the combined thickness of the intermetallic compound layer and the Al plating layer is preferably 9 to 60 μm on average, and more preferably 15 to 45 μm. When the combined thickness of the intermetallic compound layer and the Al plating layer is within the above range, the amount of laser irradiation energy input is reduced by increasing the thickness of the surface film, while the reflectivity of the Al plating layer surface due to the surface film is reduced, resulting in miniaturization of dross and easier reduction of dross height.

[0049] "Surface coating" The surface coating contains a binder resin and carbon black. The surface coating may also contain other components.

[0050] As the binder resin, either a water-soluble or water-dispersible water-based resin that dissolves or disperses in water, or a solvent-based resin that dissolves or disperses in an organic solvent, may be used. However, from the standpoint of manufacturing cost and environmental suitability, water-based resins are preferred.

[0051] Examples of water-based resins include water-soluble or water-dispersible resins such as polyester resins, urethane resins, polyolefin resins, acrylic resins, epoxy resins, phenolic resins, and mixed resins of two or more of these resins. Examples of solvent-based resins include polyester resins, urethane resins, epoxy resins, acrylic resins, and mixed resins of two or more of these resins.

[0052] Here, the binder resin may be a crosslinked resin having a crosslinked structure or a non-crosslinked resin without a crosslinked structure, but a non-crosslinked resin is preferable from the viewpoint of low-temperature film formation of the coating film. As the crosslinking agent (curing agent) that imparts a crosslinked structure to the binder resin, a water-soluble crosslinking agent is preferred. Specifically, melamine, isocyanates, silane compounds, zirconium compounds, titanium compounds, etc. are preferred as crosslinking agents.

[0053] The binder resin content is preferably 20 to 40% by mass, and more preferably 25 to 35% by mass, relative to the surface film.

[0054] (Carbon Black) Examples of carbon black include well-known types such as Ketjen black, oil furnace black, channel black (i.e., gas black), and acetylene black.

[0055] The carbon black content is 1 to 20% by mass, preferably 5 to 20% by mass, and more preferably 10 to 15% by mass. If the carbon black content is less than 1% by mass, high laser irradiation energy is required during laser cutting, which increases the height of the dross. If the carbon black content exceeds 20% by mass, the cost of surface coating formation increases. Therefore, the carbon black content shall be within the above range. The lower limit of the carbon black content is preferably 3% by mass, 5% by mass, 7% by mass, or 10% by mass.

[0056] (Other ingredients) Other components include one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide.

[0057] In particular, the surface film preferably contains one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide. These oxides tend to improve lubricity during hot pressing and also enhance corrosion resistance after painting. Furthermore, the total content of one or more oxides selected from the group consisting of Zr oxide, Zn oxide, and Ti oxide is 0 to 70% by mass, preferably 0 to 60% by mass, more preferably 1 to 60% by mass, and even more preferably 40 to 60% by mass, relative to the surface film.

[0058] (Thickness of the surface coating) The surface film thickness is preferably 0.5 to 5.0 μm on average, and more preferably 1.0 to 3.0 μm. When the surface film thickness is within the above range, it is most suitable for rapid heating using radiation such as near-infrared rays during hot stamping.

[0059] The method for measuring the average thickness of the Al plating layer, intermetallic compound layer, and surface film is as follows. In the cut steel plate, a steel slab is obtained by cutting along the thickness direction, excluding the edges. A steel billet is embedded in epoxy resin, and the cut surface is polished to obtain a sample. The cross-section of the obtained sample is subjected to linear analysis using an electron beam microanalyzer (FE-EPMA), from the surface of the cut steel plate inward, along the thickness direction. The elements to be analyzed in the linear analysis will be Fe and Al. The measurement conditions will be an acceleration voltage of 15kV, a beam diameter of approximately 100nm, and an irradiation time of 1000ms. The measurement pitch will be a grid pattern with a 5μm pitch. Based on the resulting line analysis, by focusing on the concentration changes of specific elements from the base steel sheet towards the surface film, the interfaces between the Al plating layer and the intermetallic compound layer, the intermetallic compound layer and the base steel sheet, and the Al plating layer and the surface film are identified. Specifically, based on the line analysis results from the base steel sheet side, the region with an aluminum concentration of less than 0.06 mass% is considered the base steel sheet, the region with an aluminum concentration of 0.06 mass% or more is considered the intermetallic compound layer, the region with an aluminum concentration of 0.06 mass% or more and an iron concentration of less than 4 mass% is considered the Al plating layer, and the region with an aluminum concentration of less than 0.06 mass% is considered the surface coating layer, and each interface is determined accordingly. Furthermore, the determination of the boundary between the surface film and the plating layer can be made more accurate by monitoring not only the aluminum concentration but also the concentrations of the binder resin components contained in the surface film and the elements specific to carbon black. The length along the thickness direction of the interface between the intermetallic compound layer and the base steel sheet, and the interface between the Al plating layer and the intermetallic compound layer, is defined as the thickness of the intermetallic compound layer. The length along the thickness direction of the Al plating layer is defined as the interface between the Al plating layer and the intermetallic compound layer and the interface between the Al plating layer and the surface film. The length along the thickness direction between the identified interface between the Al plating layer and the surface film and the surface region considered to be the surface film is defined as the thickness of the surface film. Then, the above procedure is performed five times to determine the arithmetic mean of the thicknesses of the Al plating layer, the intermetallic compound layer, and the surface film.

[0060] (Surface roughness Ra of the surface coating) The surface roughness Ra of the surface film is preferably 0.5 to 2.0 μm, more preferably 0.7 to 1.4 μm, and even more preferably 0.7 to 1.4 μm. When the surface roughness Ra of the surface coating is 0.5 μm or higher, the reflectivity of the surface coating decreases, and the laser irradiation energy during laser cutting is reduced. As a result, dross formation is suppressed, and the dross height is reduced. When the surface roughness Ra of the surface film is 2.0 μm or higher, the absorption rate of the surface film decreases, and the laser irradiation energy during laser cutting is reduced. As a result, dross formation is suppressed, and the dross height is reduced. Therefore, the surface roughness Ra of the surface film is preferably within the above range.

[0061] The surface roughness Ra of the surface coating can be controlled, for example, by spraying water in a spray form and cooling it during the solidification stage after molten plating.

[0062] The surface roughness Ra of the surface coating is the arithmetic mean roughness as defined in JIS B 0601:1994. The method for measuring the surface roughness Ra of the surface film is as follows: A sample is taken from the area of ​​the cut steel plate, excluding the edges. The surface roughness Ra of the surface coating of the sample is measured in accordance with JIS B 0601:1994. The measurement conditions are as follows: evaluation length of 4 mm, measurement speed of 0.6 mm / s, and cutoff value of 0.8 mm. Then, the above procedure is performed on five samples, and the arithmetic mean of the surface roughness Ra of the surface film is calculated.

[0063] (Brightness L of the surface film * ) Brightness L of the surface film * is preferably 10 to 50. When the brightness L of the surface film * is 10 to 50, the reflectance of the surface film decreases, and the laser irradiation energy during laser cutting is reduced. As a result, dross generation is suppressed and the dross height is reduced. Therefore, the brightness L of the surface film * is preferably in the above range.

[0064] The brightness L of the surface film * is controlled, for example, by the carbon black content.

[0065] The brightness L of the surface film * is the brightness L in the CIE1976L * a * b * color system shown in * JIS K 5600-4-6:1999. And the method for measuring the brightness L of the surface film * is as follows. A sample is taken from a region excluding the edge in the cut steel plate. The brightness L of the surface film * is measured for the surface of the surface film of the sample using a reflection spectrophotometer. Five samples are measured with a colorimeter, and the arithmetic mean value is obtained.

[0066] [Manufacturing method of cut steel plate] The manufacturing method of the cut steel plate of the present disclosure is obtained by laser cutting a coated Al-plated steel plate having the same layer structure as the cut steel plate of the present disclosure. For example, the manufacturing method of the cut steel plate of the present disclosure can be obtained by pulling out a coated Al-plated steel plate from a steel coil and laser cutting the pulled out coated Al-plated steel plate.

[0067] In particular, for example, by setting the laser cutting conditions as follows, a cut steel plate can be obtained in which the dross height at the laser-cut surface satisfies the above range. Laser source (laser oscillator): Solid-state laser (fiber laser, YAG laser, etc.) with a laser wavelength of 1070 nm and a maximum frequency of 50 kHz. • Light-gathering element: 50 μm • Laser output: Average output 250W ·Cutting speed: 800~1600mm / min • Assist gas: Compressed air

[0068] In recent years, the type of laser source (laser oscillator) used in laser cutting has shifted from carbon dioxide lasers with a wavelength of 10.8 μm to solid-state lasers (fiber lasers, YAG lasers, etc.) with a wavelength of approximately 1 μm. Optical transmission from the laser source (laser oscillator) to the processing head is via mirrors for carbon dioxide lasers, while it is via fiber optic cables for solid-state lasers. Mirror transmission requires the cumbersome maintenance of mirror angles, whereas fiber optic cable transmission is easier to use as it does not require mirrors. Therefore, it is appropriate to use a solid-state laser as the laser source (laser oscillator). On the other hand, for lasers irradiated by a solid-state laser, the Al plating layer has a higher reflectivity at room temperature than the base steel plate (i.e., the base metal), requiring an increase in laser output when laser cutting, and tends to easily generate dross. However, even if a solid-state laser is used as the laser source (laser oscillator), by applying the above surface coating to the Al-plated steel sheet and setting the laser cutting conditions to the above conditions, a cut steel sheet with a dross height satisfying the above range can be obtained. However, a gas laser (such as a carbon dioxide laser) may be used as the laser source. Fiber lasers are a type of solid-state laser that uses an optical fiber as an amplification medium to emit laser light. Examples include continuous wave (CW) lasers and pulsed lasers.

[0069] <Tailored Blanks / Manufacturing Method for Tailored Blanks> The tailored blank of this disclosure comprises two or more cut steel plates, each having the cut steel plate of this disclosure, and a weld metal connecting the two or more cut steel plates (see Figure 2). Furthermore, the tailored blank of the present disclosure is obtained by welding two or more cut steel plates, each having the cut steel plate of the present disclosure, together with weld metal (method of manufacturing the tailored blank of the present disclosure). In Figure 2, 200 represents a tailored blank, 100 represents a cut steel plate, and 120 represents weld metal.

[0070] In the tailored blank of this disclosure, two or more cut steel plates may be used in combination as appropriate for the purpose. The two or more cut steel plates used to obtain the tailored blank may, for example, be steel plates of the same strength class, or steel plates of different strength classes. Furthermore, steel plates of the same thickness may be used, or steel plates of different thicknesses may be used. However, the tailored blank of this disclosure applies the above-mentioned cut steel sheet to at least one of two or more cut steel sheets. Furthermore, at least a portion of the Al plating layer and / or intermetallic compound layer in the area of ​​the tailored blank intended for welding may be removed.

[0071] In the tailored blank of this disclosure, the end of the cut steel plate of this disclosure that connects to the weld metal may be an end having a laser-cut surface or an end not having a laser-cut surface. If the end of the cut steel plate of this disclosure having a laser-cut surface is connected with weld metal, welding defects can be suppressed. Furthermore, the tailored blank of this disclosure may be a tailored blank formed by overlapping and welding two or more cut steel plates, or a tailored blank formed by butt welding two or more cut steel plates. When the ends of the cut steel plates having laser-cut surfaces are butt-welded, the butt welding accuracy is improved.

[0072] In the tailored blanks of this disclosure, the welding method is not particularly limited and examples include laser welding (laser beam welding), arc welding, and electron beam welding. Examples of arc welding include plasma welding, TIG (Tungsten Inert Gas) welding, MIG (Metal Inert Gas) welding, and MAG (Metal Active Gas) welding, with plasma welding being a preferred arc welding method. The welding conditions should be selected according to the desired conditions, such as the thickness of the cut steel plate used. Furthermore, welding may be performed while supplying filler wire as needed.

[0073] <Hot press-formed products / Method for manufacturing hot press-formed products> The hot-pressed product of the present disclosure is a hot-pressed product of the tailored blank of the present disclosure. The hot-pressed articles of this disclosure are obtained by hot-pressing the tailored blanks of this disclosure (method of manufacturing the hot-pressed articles of this disclosure).

[0074] Specifically, the hot press-formed articles of this disclosure can be manufactured as follows: First, the tailored blank is heated to a high temperature to soften it. Then, using a mold, the softened tailored blank is formed by hot press forming (i.e., hot stamping), cooled, and quenched to obtain a hot-pressed product of the desired shape. By quenching the hot-pressed product through heating and cooling, a hot-pressed product with a high tensile strength, for example, of approximately 1500 MPa or more, can be obtained.

[0075] In addition to conventional electric furnaces and radiant tube furnaces, heating methods such as infrared heating, electrostatic heating, and induction heating can be used when performing hot press forming.

[0076] The maximum temperature achievable during hot press forming is not particularly limited, but is preferably, for example, 850°C to 1000°C. In hot press forming, since heating must occur in the austenite region, a temperature of around 900°C to 950°C is usually adopted.

[0077] In hot press forming, a tailored blank heated to a high temperature is press-formed in a mold cooled by water cooling or the like, and simultaneously hardened by the cooling in the mold. Alternatively, if necessary, water may be sprayed directly onto the blank material through gaps in the mold for water cooling. This process yields a hot press-formed product of the desired shape.

[0078] The applications of the hot-pressed products of this disclosure are not particularly limited and include, for example, various automotive components such as automobile bodies and various components for industrial machinery. Examples of automotive components include, specifically, various pillars; stabilizers, door beams, roof rails, bumpers and other reinforcements; frames; arms and other various parts.

[0079] <Other> The cut steel plates of this disclosure are not limited to cut steel plates for the manufacture of tailored blanks, but may also be, for example, cut steel plates for steel pipes. A steel pipe to which the cut steel plate of the present disclosure is applied comprises, for example, a weld metal and a cut steel plate of the present disclosure which is formed in an open tubular shape with two circumferential edges facing each other, and to which the two circumferential edges are connected to the weld metal.

[0080] On the other hand, a steel pipe to which the tailored blank of the present disclosure is applied comprises, for example, a weld metal and the tailored blank of the present disclosure, which is formed in an open tubular shape with two circumferential edges facing each other, and to which the two circumferential edges are connected to the weld metal.

[0081] The method for forming the cut steel plates and tailored blanks into a tubular shape is not particularly limited and may be any method, such as the UOE method or the bending roll method. Furthermore, the welding after forming the tubular shape is not particularly limited and includes, for example, laser welding; plasma welding; electric resistance welding; or electric resistance welding using high-frequency induction heating. [Examples]

[0082] The following are examples of embodiments of this disclosure, but this disclosure is not limited to the following embodiments. It is clear to those skilled in the art that various variations or modifications can be conceived within the scope of the ideas described in the claims, and these will naturally fall within the technical scope of this disclosure.

[0083] <Example 1> As the base steel sheet, a base steel sheet was prepared having a chemical composition of C:0.24%, Mn:1.22%, Al:0.020%, Ti:0.02%, Cr:0.2%, B:0.0025%, Si:0.25%, P:0.02%, S:0.002%, N:0.001%, with the remainder being Fe and impurities, and having a strength class of 1500 MPa after hot pressing. Next, both sides of the base steel sheet were plated with Al using the Zenzimer method with an Al plating bath consisting of Si:9%:the remainder:Al and impurities. The resulting Al plating layer had a chemical composition of Si:9%:Fe:3%:the remainder:Al and impurities. In the Al plating process, the amount of Al plating layer after plating was adjusted using the gas wiping method to the values ​​shown in Table 1. Through the above operations, an Al-plated steel sheet was obtained.

[0084] Next, a surface coating treatment solution containing a water-soluble urethane resin and carbon black (CB) as a binder was prepared. Then, the treatment solution was applied to one side of the Al-plated steel sheet using a roll coater, and the coating film was baked at 80°C to form a surface film with a thickness of 1.2 to 1.5 μm. The composition of the treatment solution (i.e., the composition of the surface film) was as shown in Table 1. Through the above operations, a coated Al-plated steel sheet was obtained.

[0085] Next, a coated Al-plated steel sheet was laser-cut using the laser cutting conditions shown in Table 1 to obtain cut steel sheets measuring 50 × 100 mm. However, the laser irradiation surface for laser cutting was the coated surface of the coated Al-plated steel sheet. The fiber laser used as the laser source has a laser wavelength of 1070 nm and a frequency of 0 to 50 kHz.

[0086] <Examples 1-5, Comparative Examples 1-5> As shown in Table 1, cut steel plates were obtained in the same manner as in Example 1, except that various conditions were changed. The surface roughness Ra of the surface coating was controlled by spraying water in a spray form and cooling it at the time of solidification after hot-dip plating, and adjusted to the values ​​shown in Table 1.

[0087] <Rating> (Dross height) The dross height at the laser-cut surface of each example of cut steel plate was measured according to the method described above. If the cutting speed was too fast and the molten metal could not be completely removed, resulting in failure to laser cut, then "Impossible to cut" was indicated in the "Dross Height" column in Table 1.

[0088] (Weldability) Two cut steel plates were used for each example. After butting the laser-cut surfaces of the steel plates together, they were welded under the following conditions to obtain tailored blanks. The laser welding conditions were adjusted to achieve through-welding within the range of laser power 3.0 to 5.0 kW and welding speed 4.0 m / min to 7.0 m / min. The corrosion resistance of the welded area after painting was then evaluated as follows.

[0089] -Corrosion resistance test after painting- Before painting, the obtained tailored blanks were hot-pressed (hot-stamped) to produce hot-pressed parts. Specifically, the obtained tailored blanks were held in a furnace heated to 920°C for 4 minutes, and then the heated tailored blanks were hot-pressed (hot-stamped) in a water-cooled mold, followed by quenching to produce flat hot-pressed parts. Next, the obtained hot-pressed products were chemically treated and then electrodeposited to produce painted hot-pressed products. The chemical treatment was performed using PB-SX35T chemical treatment solution manufactured by Nippon Parkerizing Co., Ltd. Subsequently, Powernics 110 cationic electrodeposition paint manufactured by Nippon Paint Co., Ltd. was used as the electrodeposition paint, and the hot-pressed products were electrodeposited with a target electrodeposition film thickness of approximately 15 μm. After washing the electrodeposited hot-pressed products with water, they were heated at 170°C for 20 minutes to bake, producing painted hot-pressed products.

[0090] From the obtained hot-pressed painted products, rectangular test pieces with welded sections were taken to be used as test specimens for post-painting corrosion resistance testing. The test plates were 65 mm in length and 100 mm in width, and were taken so that a weld line ran along the entire length in the center of the width.

[0091] Post-painted corrosion resistance testing was performed using the JASO M610-92 automotive parts appearance corrosion test, evaluating the corrosion status after 360 cycles (120 days). Measurements were taken with a test sample size of N=10 to confirm corrosion of the weld metal. Furthermore, in the test, any sample in which corrosion was observed in the weld metal at even one location was counted as a sample with corrosion.

[0092] [Table 1]

[0093] From the above results, it can be seen that the dross height at the laser-cut surface of the steel plate cut in this embodiment is smaller than that of the steel plate cut in the comparative example. As a result, it can be seen that trimming of the laser-cut surface after laser cutting can be omitted or the amount of trimming can be reduced. Furthermore, it can be seen that the cut steel plate in this embodiment has good weldability at the laser-cut surface. [Explanation of Symbols]

[0094] 10 Base steel plate 12 Al plating layer 14 Intermetallic compound layer 16 Surface coating 18 Dros 100A laser cut surface T Dross Height 100 cutting steel plate, 120 Weld metal 200 Tailored Blanks

Claims

1. Base material steel plate and An Al plating layer provided on one or both sides of the base steel sheet, A layer of intermetallic compound containing iron and aluminum is provided between the base steel sheet and the Al plating layer, A surface film is provided on at least one of the Al plating layers provided on one side of the base steel sheet or on both sides of the base steel sheet, and contains a binder resin and carbon black, wherein the carbon black content relative to the surface film is 1 to 20% by mass. It has, At least a portion of the end face of the cut steel plate is a laser-cut surface. A cut steel plate in which the height of the dross at the laser-cut surface is 0.50 mm or less.

2. The cut steel plate according to claim 1, wherein the surface roughness Ra of the surface film is 0.5 to 2.0 μm.

3. The cut steel sheet according to claim 1, wherein the sum of the thicknesses of the intermetallic compound layer and the Al plating layer is 9 to 60 μm on average.

4. The cut steel plate according to claim 1, wherein the thickness of the base steel plate is 0.7 to 3.2 mm.

5. Two or more cut steel plates having the cut steel plate described in any one of claims 1 to 4, The welding metal connecting the two or more cut steel plates, A tailored blank with [specific feature].

6. A method for manufacturing a tailored blank, comprising welding two or more cut steel plates, each having a cut steel plate according to any one of claims 1 to 4, and connecting them with weld metal.

7. A hot-pressed product of a tailored blank as described in claim 5.

8. A method for manufacturing a hot press-formed product by hot press-forming a tailored blank as described in claim 5.

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