JP7433972B2 - Al-plated steel pipes and aluminum-plated steel pipe parts for STAF (registered trademark) construction method, and their manufacturing method - Google Patents

Description

The present invention relates to Al-based plated steel materials for hot forming, Al-based plated steel parts, and methods of manufacturing them.

Steel materials are generally used for vehicle body parts such as automobile bodies and frames. For example, in recent years, steel pipes have been increasingly used to reduce the weight of vehicle body parts. Additionally, a flange portion is sometimes formed on a vehicle body component to join the vehicle body components together.
Methods using cold pressing, hydroforming, hot stamping, etc. are known as methods for manufacturing steel parts having flanges (hereinafter referred to as "flanged steel parts"). In the method using cold pressing and hot stamping, two steel plates are press-formed into a shape that will become the pipe body and flange when combined, and then welded together to produce flanged steel parts. be able to. In addition, in the method using hydroforming, a steel pipe is formed by hydroforming to obtain a pipe body, and then a flange made separately by press forming etc. is welded to the pipe body to produce flanged steel parts. can do. All of these methods require welding, which is time consuming and increases manufacturing costs.

Therefore, a method has been proposed in which a steel pipe is heated and a high-pressure fluid is supplied into the steel pipe to form the pipe (including formation of a flange portion), and then to perform quenching (for example, Patent Document 1). This method is called Steel Tube Air Forming, or STAF (registered trademark) construction method.
As the steel pipe used in the STAF (registered trademark) method, a hardenable steel pipe is used. However, when this steel pipe is heated in the atmosphere, scale (oxide) is likely to be generated on the surface. Surface scale can be removed by shot blasting, pickling, etc., but such treatments lead to increased manufacturing costs. Therefore, the steel pipe used in this manufacturing method is required to have excellent high-temperature oxidation resistance.
Therefore, it has been proposed to use a steel pipe with an Al-based plating layer formed on its surface in the STAF (registered trademark) construction method (for example, Patent Documents 2 and 3).

Japanese Patent Application Publication No. 2018-167312 JP 2019-73778 Publication JP 2019-73779 Publication

However, when a steel material with an Al-based plating layer formed on its surface is hot-formed, the steel material is heated to a temperature higher than the melting point of the Al-based plating layer, so there is a problem that significant plating sag occurs. This plating sag causes the formation of parts where the thickness of the Al-based plating layer on the surface is significantly uneven in steel parts obtained by the various methods described above, especially the STAF (registered trademark) construction method.

The present invention has been made to solve the above-mentioned problems, and provides an Al-based plated steel material for hot forming that can suppress plating sag when manufacturing Al-based plated steel parts by hot forming. The object of the present invention is to provide a method for manufacturing the same.
Another object of the present invention is to provide an Al-based plated steel component manufactured by hot forming in which the thickness of the Al-based plated layer has small variations, and a method for manufacturing the same.

As a result of intensive research on Al-based plated steel materials used for hot forming, the present inventors have pre-alloyed the Al-based plating layer to a specific ratio, and have determined that The inventors have discovered that the above problems can be solved by controlling the ratio of the maximum thickness of the plating layer within a specific range, and have completed the present invention.

That is, the present invention has an Al-based plating layer with an alloying ratio of 35% or more on at least the outer surface of the steel pipe , and the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer is This is an Al-based plated steel pipe for use in the STAF (registered trademark) construction method with a hardness of 1.2 to 2.0 .

The present invention also provides a method for manufacturing the Al-based plated steel pipe, which includes a step of forming an Al-based plating layer on at least one surface of a steel plate, and forming a pipe from the steel plate on which the Al-based plated layer is formed. , manufacturing a steel pipe with the Al -based plating layer formed on at least the outer surface, and heating the steel pipe with the pipe axis direction horizontal so that the alloying rate of the Al-based plating layer is 35% or more. This is a method for manufacturing an Al-based plated steel pipe.

The present invention also provides an Al -based plated steel pipe component including a formed body of the Al-based plated steel pipe formed by the STAF (registered trademark) method , wherein the Al-based plated steel pipe part has at least one surface. The present invention is an Al-based plated steel pipe component having an Al-based plating layer, the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer being 1.0 to 10.0.

Furthermore, the present invention provides an Al -based plated steel pipe in which the Al-based plated steel pipe is heated to a temperature equal to or higher than the A1 transformation point, and after forming by supplying high-pressure fluid into the Al-based plated steel pipe , quenching is performed. This is a method for manufacturing steel pipe parts.

According to the present invention, it is possible to provide an Al-based plated steel material for hot forming, which can suppress plating sagging when producing Al-based plated steel parts by hot forming, and a method for manufacturing the same.
Further, according to the present invention, it is possible to provide an Al-based plated steel component manufactured by hot forming and with small variations in the thickness of the Al-based plated layer, and a method for manufacturing the same.

Embodiments of the present invention will be specifically described below. The present invention is not limited to the following embodiments, and modifications and improvements may be made to the following embodiments as appropriate based on the common knowledge of those skilled in the art without departing from the spirit of the present invention. It is to be understood that such materials also fall within the scope of the present invention.

(Al-based plated steel material for hot forming)
The Al-based plated steel material for hot forming (hereinafter sometimes abbreviated as Al-based plated steel material) according to the embodiment of the present invention has an Al-based plating layer on at least one surface of the steel material.
Here, the type of steel material is not particularly limited, and steel plates, steel pipes, etc. can be used. Moreover, when the steel material is a steel plate, the Al-based plating layer can be formed on one side of the steel plate or on both sides of the steel plate. Moreover, when the steel material is a steel pipe, the Al-based plating layer can be formed on the outer surface of the steel pipe, or on the outer surface and inner surface of the steel pipe.
Moreover, in this specification, "hot forming" means a method of heating and forming, such as hot stamping and STAF (registered trademark).

Al-based plated steel with the above structure has excellent properties such as high-temperature oxidation resistance and high-temperature sliding properties, so it is suitable for manufacturing Al-based plated steel parts using hot forming, especially the STAF (registered trademark) method. suitable for use.
Here, the production of Al-based plated steel parts using the STAF (registered trademark) method involves heating a tubular Al-based plated steel material (hereinafter referred to as "Al-based plated steel pipe") and introducing high-pressure fluid into the Al-based plated steel pipe. This means that after supplying and forming, quenching is performed. In this construction method, the shaping may include not only the shaping of the pipe main body but also the shaping of the flange.

On the other hand, when Al-based plated steel materials having the structure described above are used in hot forming, especially the STAF (registered trademark) method, the Al-based plating layer melts and moves vertically downward of the steel material during heating. Dragging is likely to occur.
Therefore, in the Al-based plated steel material according to the embodiment of the present invention, the Al-based plated layer is alloyed in advance to a predetermined ratio, and the thickness of the Al-based plated layer varies (ratio of maximum thickness to minimum thickness). By controlling the amount within a specific range, plating sag during hot forming is suppressed.

The Al-based plating layer has an alloying ratio of 35% or more, preferably 40% or more, and more preferably 50% or more. By controlling the alloying ratio within the above range, plating sag can be suppressed when producing Al-based plated steel parts using hot forming, especially the STAF (registered trademark) method (especially during heating). I can do it.
Here, the alloying rate of the Al-based plating layer can be determined as follows.
When the Al-based plated steel material is an Al-based plated steel pipe, first, the Al-based plated steel pipe is cut in a direction perpendicular to the pipe axis direction, embedded in epoxy resin, and then polished, and the polished cross section is examined under an optical microscope. Take a photo with From the photograph taken, the ratio of the thickness of the alloyed Al-based plating layer to the total thickness of the Al-based plating layer is defined as the alloying ratio. Note that the thickness of the Al-based plating layer is preferably an average value at five locations in one field of view. In addition, if the Al-based plated steel material is a plate-shaped Al-based plated steel sheet (hereinafter referred to as "Al-based plated steel sheet"), the process can be performed in the same manner as above except that the Al-based plated steel sheet is cut in the sheet width direction. good.

The Al-based plating layer has a ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer (maximum thickness of the Al-based plating layer/minimum thickness of the Al-based plating layer) of 1.0 to 5. .0, preferably 1.1 to 3.0, more preferably 1.2 to 2.0. By controlling the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer within the above range, Al-based plated steel parts can be manufactured using hot forming, especially the STAF (registered trademark) method. (particularly during heating), plating sag can be suppressed.
Here, the thickness of the Al-based plating layer can be measured by photographing a cross section with an optical microscope in the same manner as the alloying rate, and from the photograph taken. In addition, when the Al-based plated steel material is an Al-based plated steel pipe, the "minimum thickness of the Al-based plated layer" means that the Al-based plated layer is It means the thickness of the part where is the smallest. Similarly, "maximum thickness of Al-based plating layer" means the thickness of the part where the Al-based plating layer is the largest in a photograph of a cross section of an Al-based plated steel pipe in a direction perpendicular to the pipe axis direction. do. It is preferable that the minimum thickness and maximum thickness of the Al-based plating layer be the average value of five locations in one field of view. In addition, when the Al-based plated steel material is an Al-based plated steel sheet, the "minimum thickness of the Al-based plated layer" refers to the part where the Al-based plated layer is the smallest in the photograph of the cross section in the sheet width direction of the Al-based plated steel sheet. It means the thickness of Similarly, "the maximum thickness of the Al-based plating layer" means the thickness of the portion where the Al-based plating layer is the largest in a photograph of a cross section in the sheet width direction of an Al-based plated steel sheet.
Note that when the Al-based plated steel material is an Al-based plated steel pipe, the Al-based plated steel pipe is often heated with the pipe axis direction in the horizontal direction. At this time, if the vertically upward position with respect to the center of the tube axis is 0°, the part where the thickness of the Al-based plating layer is maximum is around the part (bottom) located at 180° with respect to the center of the tube axis. Therefore, the portions where the thickness of the Al-based plating layer is the minimum are around the portions (side portions) located at positions of 90° and 270° with respect to the center of the tube axis.

Conventional Al-based plated steel materials, which are prone to plating sag, cause a localized increase in surface pressure between the forming mold and the forming die during hot forming, especially in the STAF method (registered trademark). In addition to reducing the service life, it may also reduce high-temperature slidability during molding. Furthermore, depending on the product shape, the dimensional accuracy of product parts may be degraded. On the other hand, the Al-based plated steel material according to the embodiment of the present invention can suppress plating sag when producing Al-based plated steel parts using hot forming, particularly the STAF (registered trademark) method. Problems such as those mentioned above can be resolved. In addition, when painting Al-based plated steel parts manufactured using hot forming, particularly the STAF method (registered trademark), the Al-based plated steel materials according to the embodiments of the present invention may be used after painting due to the effects of plating dripping. It is also possible to suppress deterioration of the appearance of the product.

Since the Al-based plated steel material according to the embodiment of the present invention has excellent high-temperature oxidation resistance, it is difficult to generate scale on at least one surface even when it is used in hot forming, particularly in the STAF (registered trademark) method. On the other hand, when a steel material that does not have an Al-based plating layer on at least one surface is used for hot forming, scale is generated on at least one surface. In particular, when used in the STAF (registered trademark) construction method, scale peels off during the process of forming the flange portion or the pipe body. As a result, the peeled scale tends to cause flaws on at least one surface of the steel material, and the scale adheres to the mold, making cleaning the mold labor-intensive. Therefore, according to the Al-based plated steel material according to the embodiment of the present invention, it is possible to solve the problem caused by the occurrence of scale.

Furthermore, since the Al-based plated steel material (Al-based plated steel pipe) according to the embodiment of the present invention has excellent high-temperature sliding properties, it is easy to form a flange portion using the STAF (registered trademark) method. . Specifically, the Al-based plated steel material (Al-based plated steel pipe) according to the embodiment of the present invention has low friction with the mold at high temperatures, so when forming the flange part, the upper mold and the lower mold are It becomes easier to expand a part of the Al-based plated steel pipe in the half-opened part of the mold.
Here, in this specification, "Al-based plating layer" means a plating layer containing Al as a main component, and is a concept that includes an Al-based plating layer consisting only of Al. The Al content in the Al-based plating layer is not particularly limited, but is preferably 50% by mass or more, more preferably 70% by mass or more, and still more preferably 80% by mass or more.

The Al-based plating layer can be formed by a known method such as a hot-dip plating method, an electroplating method, a vacuum evaporation method, or a cladding method. Among these, an Al-based plating layer formed using the hot-dip plating method, which is currently the most widely used in industry, is preferable.

In one embodiment of the present invention, the Al-based plating layer preferably contains Si as well as Al. By containing Si in the Al-based plating layer, formation of an alloy layer can be suppressed during hot-dip plating, for example. From the viewpoint of ensuring such an effect, the Si content in the Al-based plating layer is preferably 1 to 15% by mass. In addition, from the viewpoint of improving corrosion resistance, this Al-based plating layer has Cr: 0.1 to 1% by mass, Mg: 0.5 to 10% by mass, Ti: 0.1 to 1% by mass, and Sn: 1 to 1% by mass. 5% by mass, Zn: 1 to 50% by mass, etc. These elements can be contained alone or in combination of two or more.

The amount of deposited Al-based plating layer is not particularly limited, but is preferably 10 to 150 g/m ² , more preferably 20 to 100 g/m ² , even more preferably 20 to 80 g/m ² , particularly preferably 25 to 60 g/m 2 ^m2 . By setting the adhesion amount of the Al-based plating layer to 10 g/m ² or more, high-temperature oxidation resistance and high-temperature slidability can be ensured, thereby suppressing the formation of scale on the surface when heated in the atmosphere. At the same time, it becomes easier to form the flange portion. Furthermore, by controlling the amount of the Al-based plating layer to be 150 g/m ² or less, pipe formability can be improved when the Al-based plated steel material is an Al-based plated steel pipe. In addition, when an Al-based plating layer is formed on both sides of an Al-based plated steel pipe and an Al-based plated steel plate, the amount of the Al-based plating layer deposited above means the amount of the Al-based plating layer deposited on each surface.

The composition of the steel used for the Al-based plated steel is not particularly limited as long as it can be hardened. In one embodiment of the present invention, the steel material contains C: 0.10 to 0.50 mass%, Si: 0.10 to 2.00 mass%, Mn: 0.10 to 3.00 mass%, and the remainder It is preferable to have a composition in which is Fe and unavoidable impurities. Further, the steel material may be Cr: 0.10 to 5.00 mass%, Mo: 0.01 to 3.00 mass%, Ni: 0.01 to 3.00 mass%, Cu: 0. One or more selected from 01 to 3.00% by mass, Ti: 0.01 to 0.20% by mass, Al: 0.002 to 0.10% by mass, and B: 0.0003 to 0.0050% by mass. It may further include. Note that in this specification, "inevitable impurities" refer to components such as O, N, P, and S that are difficult to remove. Unavoidable impurities are inevitably mixed into raw materials during the melting process.

The reason for limiting the composition of the above steel material is as follows.
C is an element effective in improving the hardenability as well as the strength of martensite generated during cooling. In order to make the tensile strength (TS) of the Al-based plated steel material 1200 N/mm ² or more after quenching, the C content is preferably 0.10% by mass or more, more preferably 0.11% by mass or more, and even more preferably The content shall be 0.12% by mass or more. On the other hand, if the C content is too large, it will be difficult to ensure energy absorption capacity during collision deformation, so the C content is preferably 0.50% by mass or less, more preferably 0.45% by mass or less, and even more preferably shall be 0.40% by mass or less.

Si is an element effective in improving strength. In order to fully obtain this effect, the Si content is preferably 0.10% by mass or more, more preferably 0.11% by mass or more, and even more preferably 0.12% by mass or more. On the other hand, if the Si content exceeds 2.00% by mass, the effect will be saturated. Therefore, the Si content is preferably 2.00% by mass or less, more preferably 1.80% by mass or less, even more preferably 1.50% by mass or less.

Mn is an element effective in improving strength and hardenability. In order to sufficiently obtain these effects, the Mn content is preferably 0.10% by mass or more, more preferably 0.20% by mass or more, and still more preferably 0.30% by mass or more. On the other hand, if the Mn content exceeds 3.00% by mass, the effect will be saturated. Therefore, the Mn content is preferably 3.00% by mass or less, more preferably 2.80% by mass or less, even more preferably 2.50% by mass or less.

Cr is an element effective in improving hardenability. In order to fully obtain this effect, the Cr content is preferably 0.10% by mass or more, more preferably 0.15% by mass or more, and still more preferably 0.20% by mass or more. On the other hand, if the Cr content exceeds 5.00% by mass, the effect will be saturated. Therefore, the Cr content is preferably 5.00% by mass or less, more preferably 4.50% by mass or less, even more preferably 3.00% by mass or less.

Mo is an element effective in improving hardenability. In order to fully obtain this effect, the Mo content is preferably 0.01% by mass or more, more preferably 0.02% by mass or more. On the other hand, if the Mo content exceeds 3.00% by mass, the effect will be saturated. Therefore, the Mo content is preferably 3.00% by mass or less, more preferably 2.50% by mass or less, even more preferably 2.00% by mass or less.

Ni is an element that is effective in improving hardenability and impact resistance at the time of collision. In order to fully obtain these effects, the Ni content is preferably 0.01% by mass or more, more preferably 0.02% by mass or more. On the other hand, if the Ni content exceeds 3.00% by mass, the effect will be saturated and the cost will also increase. Therefore, the Ni content is preferably 3.00% by mass or less, more preferably 2.50% by mass or less, even more preferably 2.00% by mass or less.

Cu is an element effective in improving hardenability and toughness. In order to sufficiently obtain these effects, the Cu content is preferably 0.01% by mass or more, more preferably 0.03% by mass or more, and even more preferably 0.05% by mass or more. On the other hand, if the Cu content exceeds 3.00% by mass, the effect will be saturated and the cost will also increase. Therefore, the Cu content is preferably 3.00% by mass or less, more preferably 2.50% by mass or less, even more preferably 2.00% by mass or less.

Ti is a component added to adjust the deoxidization of molten steel, but it also has a denitrification effect. In addition, since N dissolved in solid solution is fixed as nitride, the effective amount of B for improving hardenability is increased. Furthermore, Ti forms carbonitrides and has the effect of suppressing coarsening of crystal grains during quenching and heating. In order to stably obtain these effects, the Ti content is preferably 0.01% by mass or more, more preferably 0.02% by mass or more. On the other hand, when the Ti content increases, not only does the cost increase, but also the processability decreases. Therefore, the Ti content is preferably 0.20% by mass or less, more preferably 0.10% by mass or less.

Al is a component used as a deoxidizing agent for molten steel, and also has the effect of fixing N. In order to sufficiently obtain such effects, the Al content is preferably 0.002% by mass or more, more preferably 0.005% by mass or more, and still more preferably 0.010% by mass or more. On the other hand, when the Al content increases, cleanliness is impaired, surface flaws are more likely to occur, and this becomes a factor in deteriorating surface quality. Therefore, the Al content is preferably 0.10% by mass or less, more preferably 0.080% by mass or less, even more preferably 0.060% by mass or less.

B is an element effective in improving hardenability. In order to fully obtain this effect, the B content is preferably 0.0003% by mass or more, more preferably 0.0005% by mass or more, and still more preferably 0.0010% by mass or more. On the other hand, if the B content exceeds 0.0050% by mass, the effect is saturated and there is a risk that cracks may occur during formation of the flange portion or molding of the pipe body. Therefore, the B content is preferably 0.0050% by mass or less, more preferably 0.0045% by mass or less, even more preferably 0.0040% by mass or less.

The Al-based plated steel material according to the embodiment of the present invention may further have a known surface treatment layer such as a chemical conversion treatment layer on the surface of the Al-based plated layer, within a range that does not impede the effects of the present invention.

Although not particularly limited, the Al-based plated steel material according to the embodiment of the present invention includes a step of forming an Al-based plating layer on at least one surface of the steel material, and a process in which the alloying rate of the Al-based plating layer is 35% or more. It can be manufactured by a method including a step of heating to .
In addition, when the Al-based plated steel material is an Al-based plated steel pipe, the process includes a step of forming an Al-based plating layer on at least one surface of the steel plate, a step of forming the steel plate on which the Al-based plating layer is formed, and a step of forming the steel plate on which the Al-based plated layer is formed. It can also be manufactured by a method including a step of heating so that the alloying rate of the plating layer becomes 35% or more.
The pipe-making method is not particularly limited, and methods known in the technical field can be used. For example, after forming a steel plate on which an Al-based plating layer has been formed into a cylindrical shape, both ends in the width direction of the plate may be butted together and electrical resistance welded.

The heating method is not particularly limited, and can be performed using a heating device known in the technical field. Further, when the Al-based plated steel material is immediately hot formed, particularly by the STAF (registered trademark) method, heating may be performed using a forming apparatus capable of performing the forming process.
The heating conditions such that the alloying rate of the Al-based plating layer becomes 35% or more are not particularly limited and may be set appropriately depending on the type and amount (thickness) of the Al-based plating layer.

Typical heating conditions include heating to a temperature of 570 to 800°C at a heating rate of 3 to 200°C/second and holding for 10 minutes or less.
If the temperature increase rate is less than 3° C./sec, the processing time becomes longer and productivity decreases. On the other hand, if the temperature increase rate exceeds 200° C./sec, the diffusion of Al may not be able to keep up with the heating rate and the surface may become rough.
If the heating temperature is less than 570°C, alloying of the Al-based plating layer may become insufficient. On the other hand, if the heating temperature exceeds 800° C., plating sag tends to occur, and the surface of the Al-based plating layer may become rough or oxide scale may occur. The heating temperature is preferably 580°C to 700°C, more preferably 600 to 690°C.
If the holding time exceeds 10 minutes, the processing time becomes longer and productivity decreases. The holding time is preferably 15 seconds to 9 minutes, more preferably 20 seconds to 8 minutes, taking into account the workability of the Al-based plated steel material and the surface condition of the Al-based plated steel parts.

The tensile strength (TS) after quenching of the Al-based plated steel according to the embodiment of the present invention is not particularly limited, but is preferably 1200 N/mm ² or more, more preferably 1300 N/mm ² or more, and even more preferably 1400 N/mm. ² or more. If the tensile strength after quenching is within the above range, the strength required for automobile body parts can be ensured.

(Al-based plated steel parts)
The Al-based plated steel component according to the embodiment of the present invention includes a hot-formed body of the above-mentioned Al-based plated steel for hot forming. In addition, this Al-based plated steel component has an Al-based plated layer on at least one surface, and the ratio of the maximum thickness of the Al-based plated layer to the minimum thickness of the Al-based plated layer (maximum thickness of the Al-based plated layer) (minimum thickness of Al-based plating layer) is 1.0 to 10.0. If the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer is within the above range, it can be said that the variation in the thickness of the Al-based plating layer is small.

The ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer in the Al-based plated steel component is the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer in the Al-based plated steel material. It can be found in the same way as the ratio.
For example, when the Al-based plated steel component is tubular, the thickness of the Al-based plating layer can be measured by photographing a cross section in a direction perpendicular to the tube axis using an optical microscope. In addition, "the minimum thickness of the Al-based plating layer" refers to the thickness of the part where the Al-based plating layer is the smallest in a photograph of a cross section of a tubular Al-based plated steel part in a direction perpendicular to the tube axis direction. means. Similarly, the "maximum thickness of the Al-based plating layer" refers to the thickness of the portion where the Al-based plating layer is the largest in a photograph of a cross section of a tubular Al-based plated steel part in a direction perpendicular to the tube axis direction. It means that. Note that it is preferable that the minimum thickness and maximum thickness of the Al-based plating layer be an average value at five locations in one field of view.
Note that when the Al-based plated steel part is tubular, the Al-based plated steel part is often heated with the tube axis direction in the horizontal direction. At this time, if the vertically upward position with respect to the center of the tube axis is 0°, the part where the thickness of the Al-based plating layer is maximum is around the part (bottom) located at 180° with respect to the center of the tube axis. Therefore, the portions where the thickness of the Al-based plating layer is the minimum are around the portions (side portions) located at positions of 90° and 270° with respect to the center of the tube axis.

The Al-based plated steel parts according to the embodiments of the present invention are manufactured by heating the hot-forming Al-based plated steel material to a temperature equal to or higher than the A1 transformation point, forming it, and then quenching it. In particular, when Al-based plated steel parts are manufactured using the STAF (registered trademark) method, the above-mentioned Al-based plated steel pipe is heated to a temperature equal to or higher than the A1 transformation point, and high-pressure fluid is supplied into the Al-based plated steel pipe. It is manufactured by molding and then quenching. Forming in the STAF (registered trademark) method can include not only forming the pipe main body but also forming the flange.

The Al-based plated steel material is heated to a temperature of A1 transformation point or higher, preferably 850° C. or higher, in order to austenitize the steel structure of the Al-based plated steel material. This heating softens the Al-based plated steel material, allowing it to be formed into a desired shape. In particular, in manufacturing using the STAF (registered trademark) method, the Al-plated steel pipe thermally expands by supplying high-pressure fluid into the pipe, making it possible to form the pipe body and the flange.
The temperature increase rate during heating is not particularly limited, but is preferably 5 to 200°C/sec.

Hardening can be performed by forming the Al-based plated steel material and then rapidly cooling it. By quenching, martensitic transformation occurs, in which austenite transforms into martensite, and the strength of the Al-based plated steel parts is increased.

The method for manufacturing Al-based plated steel parts according to the embodiments of the present invention can be carried out using a forming apparatus known in the technical field. In particular, when manufacturing the Al-based plated steel parts according to the embodiments of the present invention by the STAF (registered trademark) method, a forming apparatus capable of implementing the STAF (registered trademark) method, for example, Japanese Patent Application Publication No. 2018-167312 This can be carried out using the molding apparatus described in the above publication. Specifically, the method for manufacturing Al-based plated steel parts according to the embodiment of the present invention using the STAF (registered trademark) method can be performed as follows.

First, an Al-based plated steel pipe is placed between an upper molding mold (hereinafter referred to as "upper mold") and a lower molding mold (hereinafter referred to as "lower mold"), Al-based plated steel pipe is heated by electricity.
Next, the mold clamping position is adjusted so that the upper mold and the lower mold are in a half-open state, and high-pressure fluid (for example, high-pressure air) is supplied to the inside of the Al-based plated steel pipe. As a result, the Al-based plated steel pipe expands into a shape that follows the inner surface shapes of the upper and lower molds, and a portion of the Al-based plated steel pipe protrudes from the half-open portion of the upper and lower molds. Expand. By clamping the mold in this state, a part of the Al-based plated steel pipe protruding from the half-opened part of the upper mold and lower mold is caught between the upper mold and the lower mold, and the flange part is It is formed.
After the flange portion is formed, by further supplying high-pressure fluid to the inside of the Al-based plated steel pipe, the degree of close contact (degree of conformity) to the inner surface shape of the upper mold and the lower mold is increased. In this way, the Al-based plated steel pipe is formed into a predetermined shape to form the pipe main body.
After the pipe body is formed, it is quenched by being rapidly cooled in a mold, resulting in a flanged Al-plated steel part with increased strength due to martensitic transformation.

In the above manufacturing method, it is possible to freely form the shape of the Al-plated steel pipe by following the inner surface shape of the upper and lower molds, so it is possible to form the Al-plated steel pipe into any shape that is uneven in the rolling direction. Steel parts can be integrally molded.

EXAMPLES The present invention will be explained in detail below with reference to examples, but the present invention is not to be construed as being limited to these examples.

(Example 1)
<Manufacture of Al-based plated steel>
An Al-based plated steel pipe was manufactured as an Al-based plated steel material. First, steel having the composition shown in Table 1 (the remainder is Fe and unavoidable impurities) is melted, and the slab obtained by continuous casting is hot rolled to form a hot rolled steel sheet with a thickness of 3.2 mm. And so. Next, this hot-rolled steel plate was pickled and then cold-rolled to obtain a cold-rolled steel plate with a thickness of 1.6 mm. Next, this cold-rolled steel sheet was heat-annealed at 720° C. for 10 seconds in a continuous hot-dip Al-based plating line, and then immersed in an Al-based plating bath to form an Al-based plating layer on both sides of the cold-rolled steel sheet. The Al-based plating bath had a composition of 91% by mass Al and 9% by mass Si. The amount of the Al-based plating layer formed on each surface of the cold-rolled steel sheet was 35 g/m ² (the thickness of the Al-based plating layer was 17 μm). Note that the amount of the Al-based plating layer adhered was measured by fluorescent X-ray analysis.
Next, the cold-rolled steel plate on which the Al-based plating layer obtained above was formed was formed into a cylindrical shape, and then both ends in the width direction of the plate were butted together and electrical resistance welded. After welding, the weld bead was cut and Al was sprayed.
Next, the steel pipe obtained above was heated to a temperature of 400 to 800 °C at a heating rate of 7 °C/sec and held for 10 seconds to 15 minutes (specific conditions are shown in Table 2). By doing so, an Al-based plated steel pipe was obtained.

<Manufacture of Al-based plated steel parts>
An Al-based plated steel pipe was placed between the upper mold and the lower mold, and the Al-based plated steel pipe was electrically heated to 950°C at a heating rate of 150°C/sec. Next, after adjusting the mold clamping position so that the upper and lower molds are in a half-open state, high-pressure air is supplied to the inside of the Al-plated steel pipe to keep the upper and lower molds in a half-open state. A flange portion was formed by clamping the mold with a portion of the Al-based plated steel pipe protruding from the portion. Subsequently, high-pressure fluid was supplied to the inside of the Al-based plated steel pipe to form a pipe main body. Next, by rapidly cooling and hardening in a mold, an Al-based plated steel part was obtained.

<Evaluation of surface condition of Al-based plated steel parts>
The surface condition of the Al-based plated steel parts was evaluated using an electron beam microanalyzer (EPMA). The results are shown in Table 2. The results in Table 2 are common to Al-plated steel parts manufactured from Al-plated steel pipes of all steel types. In addition, in Table 2, 〇 indicates that the surface of the Al-based plated steel parts is smooth and no oxide scale is confirmed, and 〇 indicates that the surface of the Al-based plated steel parts is rough or the occurrence of oxide scale is confirmed. Those that have been applied are indicated as ×.

As shown in Table 2, in the production of Al-plated steel pipes of all steel types, 30 seconds to 10 minutes at a temperature of 500°C, 2 seconds to 10 minutes at a temperature of 600°C, and 2 seconds to 10 minutes at a temperature of 700°C. When heated for 10 minutes and held at a temperature of 800° C. for 2 seconds, it was confirmed that the surface of the Al-based plated steel parts was smooth and no oxide scale was generated.
For comparison, a comparison steel pipe was prepared in the same manner as above except that a steel having the composition of steel type 1 shown in Table 1 was not formed, and an Al-plated steel pipe was prepared in the same manner as above. As a result of manufacturing the parts and evaluating the surface condition, it was confirmed that oxide scale was generated under all heating conditions.
In addition, for comparison steel pipes made in the same manner as above except that no heat treatment was performed, Al-based plated steel parts were made in the same manner as above and the surface condition was evaluated. As a result, the surface was rough. This was confirmed. This result was common to all steel types.

<Evaluation of workability of Al-based plated steel>
In the production of the above-mentioned Al-based plated steel material (Al-based plated steel pipe), 10 Al-based plated steel pipes were produced under a heating condition of 600°C for 5 minutes, and 10 Al-based plated steel parts were produced in the same manner as above. Manufactured in pieces. Regarding the obtained 10 Al-based plated steel pipe products, the formation state of the flange portion and the pipe body portion was evaluated by visual observation. As a result, the flange portions and pipe body portions of all ten Al-based plated steel parts were in good condition for all steel types.
For comparison, a comparison steel pipe was prepared in the same manner as above except that a steel having the composition of steel type 1 shown in Table 1 was not formed, and an Al-plated steel pipe was prepared in the same manner as above. As a result of manufacturing the parts and evaluating the workability of the Al-based plated steel pipes, it was found that the formation state of the flange portion was non-uniform in the two Al-based plated steel parts.

<Evaluation of tensile strength after quenching>
In the production of the above-mentioned Al-based plated steel material (Al-based plated steel pipe), the heating condition was set to 600° C. for 5 minutes to produce the Al-based plated steel pipe, and the Al-based plated steel parts were manufactured in the same manner as above. A tensile test was conducted on the obtained Al-based plated steel parts. In the tensile test, tensile strength was measured in accordance with JIS Z2241:2011 using a JIS No. 11 test piece taken from an Al-based plated steel component and having a mandrel inserted in the grip. The results are shown in Table 3.

As shown in Table 3, the Al-based plated steel parts had a tensile strength of 1200 N/mm ² or more after quenching in all steel types.

(Example 2)
<Manufacture of Al-based plated steel>
Example except that steel type 1 was selected, the amount of Al-based plating layer was set to 24 g/m ² (Al-based plating layer thickness: 10 μm), and the heat treatment conditions were changed to those shown in Table 4. An Al-based plated steel pipe was obtained in the same manner as in Example 1.

<Evaluation of alloying rate of Al-based plating layer in Al-based plated steel material>
After cutting an Al-based plated steel pipe in a direction perpendicular to the pipe axis and embedding it in an epoxy resin, a polishing treatment was performed, and the polished cross section was photographed using an optical microscope. The photographic locations were taken at positions of 90°, 180°, and 270° with respect to the center of the tube axis, assuming that the position of the weld bead portion with respect to the center of the tube axis was 0°. From the photograph taken, the ratio of the thickness of the alloyed Al-based plating layer to the thickness of the entire Al-based plating layer was defined as the alloying ratio. The thickness of the entire Al-based plating layer and the alloyed Al-based plating layer was the average value of five locations in one field of view. Further, the alloying ratio was an average value of positions at 90°, 180°, and 270° with respect to the center of the tube axis, when the position of the weld bead portion with respect to the center of the tube axis was 0°.

<Evaluation of minimum and maximum thickness of Al-based plating layer in Al-based plated steel material>
After cutting an Al-based plated steel pipe in a direction perpendicular to the pipe axis and embedding it in an epoxy resin, a polishing treatment was performed, and the polished cross section was photographed using an optical microscope. The photographic locations were set at 90°, 180°, and 270° with respect to the center of the tube axis, where the position of the weld bead (the position vertically upward during heating) with respect to the center of the tube axis was 0°. Here, the position at 180° with respect to the center of the tube axis is the position where the Al-based plating layer has the maximum thickness, and the positions at 90° and 270° with respect to the center of the tube axis are the positions where the Al-based plating layer has the minimum thickness. And so. From the photographs taken, the thickness of the Al-based plating layer at each position was measured. The thickness of the Al-based plating layer at each position was the average value of five locations in one field of view. The minimum thickness of the Al-based plating layer was the average value of the thicknesses at positions 90° and 270° with respect to the center of the tube axis.
Next, based on the minimum thickness and maximum thickness of the Al-based plating layer obtained as described above, the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer was calculated.

<Manufacture of Al-based plated steel parts>
Al-based plated steel parts were manufactured using the STAF (registered trademark) method. Specifically, an Al-based plated steel pipe was placed between an upper mold and a lower mold with the weld bead facing vertically upward, and the Al-based plated steel pipe was electrically heated under the conditions shown in Table 4. Next, after adjusting the mold clamping position so that the upper and lower molds are in a half-open state, high-pressure air is supplied to the inside of the Al-plated steel pipe to keep the upper and lower molds in a half-open state. A flange portion was formed by clamping the mold with a portion of the Al-based plated steel pipe protruding from the portion. Subsequently, high-pressure fluid was supplied to the inside of the Al-based plated steel pipe to form a pipe main body. Next, by rapidly cooling and hardening in a mold, an Al-based plated steel part was obtained.

<Evaluation of minimum and maximum thickness of Al-based plating layer in Al-based plated steel parts>
After cutting an Al-based plated steel component in a direction perpendicular to the tube axis direction and embedding it in an epoxy resin, a polishing treatment was performed, and the polished cross section was photographed using an optical microscope. The photographic locations were taken at positions of 90°, 180°, and 270° with respect to the center of the tube axis, assuming that the position of the weld bead portion with respect to the center of the tube axis was 0°. Here, the position at 180° with respect to the center of the tube axis is the position where the Al-based plating layer has the maximum thickness, and the positions at 90° and 270° with respect to the center of the tube axis are the positions where the Al-based plating layer has the minimum thickness. And so. From the photographs taken, the thickness of the Al-based plating layer at each position was measured. The thickness of the Al-based plating layer at each position was the average value of five locations in one field of view. The minimum thickness of the Al-based plating layer was the average value of the thicknesses at positions 90° and 270° with respect to the center of the tube axis.
Next, based on the minimum thickness and maximum thickness of the Al-based plating layer obtained as described above, the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer was calculated.
The heating conditions and evaluation results are shown in Table 4.

As shown in Table 4, the alloying ratio of the Al-based plating layer is 35% or more, and the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer is 1.0 to 5. 0, the Al-based plated steel pipes (No. 2-5 to 2-14) have a lower Al-based plating layer than the Al-based plated steel pipes (No. 2-1 to 2-4) that do not satisfy both conditions. An Al-based plated steel component was obtained in which the ratio of the maximum thickness of the Al-based plated layer to the minimum thickness of the aluminum plated layer was 1.0 to 10.0 (that is, the plating sag was small).
In particular, in the case of Al-plated steel parts manufactured using the STAF (registered trademark) method using Al-plated steel pipes in which the Al-based plating layer was not alloyed in advance, the plating sag became significantly large (No. 2-1~ 2-2). In addition, even if the Al-based plating layer is alloyed in advance, the alloying ratio may be insufficient or the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer may not be within the specified range. The plating sag became large in Al-based plated steel parts (Nos. 2-3 to 2-4).

(Example 3)
<Manufacture of Al-plated steel pipes>
Example except that steel type 1 was selected, the amount of Al-based plating layer was set to 35 g/m ² (thickness of Al-based plating layer: 17 μm), and the heat treatment conditions were changed to those shown in Table 5. An Al-based plated steel pipe was obtained in the same manner as in Example 1.
<Evaluation of alloying rate of Al-based plating layer in Al-based plated steel pipe>
It was carried out in the same manner as in Example 2.
<Evaluation of minimum and maximum thickness of Al-based plating layer in Al-based plated steel pipe>
It was carried out in the same manner as in Example 2.

<Manufacture of Al-based plated steel parts>
Example 2 except that an Al-plated steel pipe was placed between the upper mold and the lower mold with the weld bead facing vertically upward, and the Al-plated steel pipe was electrically heated under the conditions shown in Table 5. Al-based plated steel parts were obtained in the same manner.
<Evaluation of minimum and maximum thickness of Al-based plating layer in Al-based plated steel parts>
It was carried out in the same manner as in Example 2.
The heating conditions and evaluation results are shown in Table 5.

As shown in Table 5, the alloying rate of the Al-based plating layer is 35% or more, and the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer is 1.0 to 5. 0, the Al-based plated steel pipes (No. 3-5 to 3-20) have a lower Al-based plating layer than the Al-based plated steel pipes (No. 3-1 to 3-4) that do not satisfy both conditions. An Al-based plated steel component was obtained in which the ratio of the maximum thickness of the Al-based plated layer to the minimum thickness of the aluminum plated layer was 1.0 to 10.0 (that is, the plating sag was small).
In particular, in the case of Al-plated steel parts manufactured using the STAF (registered trademark) method using Al-plated steel pipes in which the Al-based plating layer was not alloyed in advance, plating sag became significantly large (Nos. 3-1 to 3). 3-2). In addition, even if the Al-based plating layer is alloyed in advance, the alloying ratio may be insufficient or the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer may not be within the specified range. The plating sag became large for Al-based plated steel parts that were coated with aluminum (Nos. 3-3 to 3-4).

As can be seen from the above results, the present invention provides an Al-based plated steel material for hot forming that can suppress plating sag when manufacturing Al-based plated steel parts by hot forming, and a method for manufacturing the same. be able to.
Further, according to the present invention, it is possible to provide an Al-based plated steel component manufactured by hot forming and with small variations in the thickness of the Al-based plated layer, and a method for manufacturing the same.

Claims (7)

Having an Al-based plating layer with an alloying rate of 35% or more on at least the outer surface of the steel pipe,
An Al-based plated steel pipe for use in the STAF (registered trademark) method, wherein the ratio of the maximum thickness of the Al-based plated layer to the minimum thickness of the Al-based plated layer is 1.2 to 2.0. The steel pipe contains C: 0.10 to 0.50% by mass, Si: 0.10 to 2.00% by mass, Mn: 0.10 to 3.00% by mass, and the balance is Fe and unavoidable impurities. The Al-based plated steel pipe according to claim 1, having a certain composition. The steel pipe contains Cr: 0.10 to 5.00 mass%, Mo: 0.01 to 3.00 mass%, Ni: 0.01 to 3.00 mass%, Cu: 0.01 to 3.00 mass%. %, Ti: 0.01 to 0.20% by mass, Al: 0.002 to 0.10% by mass, and B: 0.0003 to 0.0050% by mass. 2. The Al-based plated steel pipe according to 2. The Al-based plated steel pipe according to any one of claims 1 to 3, wherein the amount of the Al-based plated layer deposited is 20 to 100 g/m ² . A method for manufacturing an Al-based plated steel pipe according to any one of claims 1 to 4, comprising:
forming an Al-based plating layer on at least one surface of the steel plate;
forming a pipe from the steel plate on which the Al-based plating layer is formed, and producing a steel pipe on which the Al -based plating layer is formed on at least the outer surface;
A method for producing an Al-based plated steel pipe, the method comprising: heating the steel pipe with its axis directed horizontally so that the alloying rate of the Al-based plated layer is 35% or more. An Al-based plated steel pipe component comprising a formed body of the Al-based plated steel pipe according to any one of claims 1 to 4 by the STAF (registered trademark) construction method,
The Al-based plated steel pipe component has an Al-based plating layer on at least one surface, and the ratio of the maximum thickness of the Al-based plating layer to the minimum thickness of the Al-based plating layer is 1.0 to 10.0. Al-based plated steel pipe parts. The Al-based plated steel pipe according to any one of claims 1 to 4 is heated to a temperature equal to or higher than the A1 transformation point, and after forming by supplying high-pressure fluid into the Al-based plated steel pipe, quenching is performed. A method for manufacturing Al-based plated steel pipe parts.