JP7485953B2 - Steel - Google Patents

Description

The present invention relates to steel materials.

Steel structures are used in various types of infrastructure, and steel structures used outdoors require high corrosion resistance. Therefore, the steel materials used as materials for these structures (such as structural steel and steel pipes) are often required to have high corrosion resistance as well.

In general, it is known that surface treatment of steel materials is an effective method for improving the corrosion resistance of steel materials.
For example, Non-Patent Document 1 discloses a hot-dip Zn-Al-Mg-Si plated steel sheet for highly corrosion-resistant building materials. Non-Patent Document 1 discloses that this plated steel sheet has good bare corrosion resistance in outdoor exposure environments.

However, for example, lightweight welded H-beams used as building structural materials for houses and prefabricated structures are continuously manufactured from hot-rolled coils by high-frequency resistance welding. Steel materials manufactured by such welding have a welded portion and the other portion (base material portion). When a plated steel sheet is used as the base steel sheet, the plated layer may disappear in the portion that is melted by the heat of welding.
For example, a bead portion without a plating layer formed by melting due to the heat of welding and then solidifying has inferior corrosion resistance compared to other parts having a plating layer. Therefore, when the steel is used as a steel structure, a coating is formed on the part without a plating layer (coating formation by partial repair).

For example, Patent Literature 1 discloses a hot-dip metal plated welded H-section steel and a manufacturing method thereof. Patent Literature 1 discloses a technique for forming a sprayed coating of a metal constituting the hot-dip metal plated coating on a peeled portion of the hot-dip metal plated coating caused by welding.
However, because scale also forms on the weld bead due to the high temperatures during welding, this technique has the problem that the adhesion of the scale itself is low, resulting in a decrease in the performance of the formed coating.

To remove scale from such bead parts, physical descaling such as shot blasting is sometimes performed before the coating is formed. However, the shape of the bead part is complex and not uniform. Therefore, the scale cannot be completely removed by shot blasting. Also, with welded H-shaped steel as described above, the weld part is a corner, so it is difficult to concentrate shot blasting on the bead part, which is only a few mm wide.

In response to these issues, Patent Document 2 discloses a welded lightweight H-shaped steel with excellent corrosion resistance (sacrificial corrosion resistance) and a manufacturing method thereof, which is capable of forming a thermal spray coating with good adhesion to the welded portion of the welded lightweight H-shaped steel, even if there is a weld bead and scale on it at the welded portion.

However, the method of Patent Document 2 has the problem that, when shot blasting is not performed, the initial adhesion is good when visually inspected, but the film adhesion evaluated by a tape peel test is somewhat poor.

JP 2002-1530 A JP 2014-208880 A

Yasuhide Morimoto et al., Highly Corrosion-Resistant Hot-Dip Zn-Al-Mg-Si Alloy-Coated Steel Sheet for Construction Materials "Super Dima", Nippon Steel Technical Report, Japan, 2002, No. 377, pp. 22-24

Assuming that shot blasting is not performed and a weld bead and scale are present in the weld, in order to improve the corrosion resistance of the steel material obtained by welding plated steel sheets, it is necessary to ensure the adhesion of the repair coating formed on the weld bead and to ensure the corrosion resistance provided by the repair coating.
Therefore, an object of the present invention is to provide a steel material that is manufactured by welding plated steel sheets and that has excellent adhesion to a repair coating even when a weld bead and scale are present in the welded portion.

The present inventors have investigated the adhesion of the repair coating. To improve the adhesion of the repair coating, it is necessary to improve the adhesion between the steel sheet and the scale, as well as the adhesion between the scale and the repair coating.
As a result of investigations, the present inventors have found that by allowing Al and Mg contained in a zinc plating film to remain in the scale, the adhesion between the scale and the steel sheet is improved.
They also found that by forming a repair coating containing Al on the scale, the Mg and Al in the scale interact with the Al in the repair coating, improving the adhesion between the scale and the repair coating.

The present invention has been made in view of the above findings.
(1) A steel material comprising: a base metal portion having a steel plate and a plating layer formed on the steel plate; and a welded portion having a weld bead and a scale formed on the weld bead,
The plating layer has a chemical composition, in mass%, of Al: 2.0 to 19.0%, Mg: 1.0 to 10.0%, Si: 0 to 2.0%, and the balance: Zn and impurities;
The scale contains Al, Mg, Fe, O, and optionally further contains Si and Zn, and the ratio of a total content of Al and Mg to a total content of Al, Mg, Fe, O, Si, and Zn in the scale, in mass%, is 3.0 to 25.0%.
(2) The steel material according to (1), wherein the steel material is an H-shaped steel, the steel plates are a web and a flange, and the weld is present at the joint between the web and the flange.
(3) The steel material according to (1) or (2), further comprising a coating containing 5.0 mass % or more of Al present on the scale of the welded portion.

According to the present invention, it is possible to provide a steel material manufactured by welding plated steel sheets, which has excellent adhesion to the repair coating even when weld beads and scale are present in the welded portion.

FIG. 2 is a diagram showing an example of a portion including a joint of a steel material (when the steel material is an H-shaped steel material) according to this embodiment manufactured by high-frequency resistance welding. FIG. 2 is a diagram showing an example of a portion including a joint of a steel material (when the steel material is a steel pipe) according to this embodiment manufactured by butt resistance welding. FIG. 10 is a diagram showing an example of a case where a coating is further formed on a weld bead in the steel material according to the present embodiment.

Hereinafter, a steel material according to one embodiment of the present invention (steel material according to this embodiment) will be described with reference to the drawings.
1 or 2, the steel material 1 according to this embodiment includes a base material portion and a welded portion. The base material portion includes a steel plate 11 and a plating layer 12 formed on the steel plate 11, and the welded portion includes a weld bead 21 and a scale 22 formed on the weld bead 21.
As shown in FIG. 3, in the steel material 1 according to this embodiment, a coating 101 containing 5.0 mass % or more of Al may be further present on the scale 22 of the weld.

The steel material 1 according to this embodiment may be, for example, an H-shaped steel in which the steel plate 11 is a web and a flange, and the weld is present at the joint between the web and the flange.

<Base material part>
The steel material 1 according to this embodiment is obtained by welding plated steel sheets each having a plated layer formed on its surface. In this embodiment, a portion that is not affected by heat from welding is referred to as a base material portion. The base material portion includes a steel sheet 11 and a plated layer 12 formed on the steel sheet 11.

[Steel plate]
In the steel material 1 according to the present embodiment, the steel sheet to be the base sheet of the plated steel sheet is not particularly limited. It may be determined depending on the product to which it is applied, the required strength, sheet thickness, etc. For example, a hot-rolled steel sheet described in JIS G3193:2008 can be used.

[Plating layer]
In the steel material 1 according to this embodiment, a plating layer 12 is formed on a steel plate 11 in the base material portion.
The plating layer 12 contains the following components. The percentages of the contents are by mass unless otherwise specified.

Al: 2.0 to 19.0%
Al is an element effective for ensuring corrosion resistance in a plating layer containing aluminum (Al), zinc (Zn), and magnesium (Mg). In addition, in the steel material according to the present embodiment, Al improves the adhesion between the plating layer and the base material, and is also an element that, when contained in the scale formed in the weld, improves the adhesion between the scale and a coating containing Al when the coating is formed on the scale.
In order to fully obtain the above effects, the Al content is set to 2.0% or more, preferably 5.0% or more, and more preferably 10.0% or more.
On the other hand, if the Al content exceeds 19.0%, the effect of improving the corrosion resistance may not be observed, so the Al content is set to 19.0% or less, and preferably 13.0% or less.

Mg: 1.0 to 10.0%
Mg is an element that has the effect of enhancing the corrosion resistance of the plating layer. In addition, in the steel material according to this embodiment, Mg is also an element that, when contained in the scale formed at the welded portion and a coating containing Al is formed on the scale, improves the adhesion between the scale and the coating. In order to fully obtain the above effect, the Mg content is set to 1.0% or more, and preferably 2.0% or more.
On the other hand, if the Mg content exceeds 10.0%, the plating layer becomes brittle and may not be able to achieve excellent adhesion to the base material, so the Mg content is set to 10.0% or less, preferably 5.0% or less.

Si: 0 to 2.0%
Silicon is an element that contributes to improving the adhesion between the plating layer and the base material. Silicon does not necessarily have to be contained in the plating layer, and the content may be 0%, but in order to obtain the above effects, the Si content is preferably 0.01% or more, and more preferably 0.1% or more.
On the other hand, if the Si content exceeds 2.0%, the above-mentioned effect of improving adhesion may not be observed, so if Si is contained, the Si content is set to 2.0% or less.

The plating layer of the base metal part of the steel material according to this embodiment basically has the above-mentioned chemical composition, with the balance being Zn and impurities. The total content of impurities is preferably 5.0% or less, and more preferably 3.0% or less.
However, the plating layer of the base material of the steel material according to this embodiment may further contain, in place of a portion of Zn, one or more of La, Ce, In, Bi, Sn, Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, Fe, and Sr in the ranges described below. These elements are optional elements that do not necessarily need to be contained, so the lower limit is 0%. Furthermore, even if these elements are contained at the impurity level (for example, a content less than the lower limit of the preferred range described below), they do not substantially affect the properties of the plating layer.

La: 0.001 to 0.50%
Ce: 0.001 to 0.50%
La and Ce are elements that strengthen both the protective action due to aluminum in the plating and the sacrificial corrosion protection action due to zinc. In order to further improve the corrosion resistance, it is preferable to contain at least 0.001% of one or both of La and Ce.
On the other hand, even if La and Ce are contained in an amount exceeding 0.50%, the effect is saturated. Therefore, when La and Ce are contained, the content of each is preferably 0.50% or less.

In: 0.01 to 1.00%
Bi: 0.01 to 1.00%
Sn: 1.0 to 10.0%
In, Bi, and Sn are elements that contribute to improving corrosion resistance. In order to obtain this effect, it is preferable that the In content be 0.01% or more, the Bi content be 0.01% or more, and/or the Sn content be 1.0% or more.
On the other hand, if the contents of these elements are excessive, the appearance after plating may become rough, so when these elements are contained, the In content is preferably 1.00% or less, the Bi content is preferably 1.00% or less, and the Sn content is preferably 10.0% or less.

Ca: 0.01 to 0.50%
Be: 0.01 to 0.20%
Ti: 0.01 to 0.50%
Cu: 0.1 to 1.0%
Ni: 0.01 to 1.00%
Co: 0.01 to 0.30%
Cr: 0.01 to 0.20%
Mn: 0.01 to 0.50%
Fe: 0.01 to 3.00%
Sr: 0.01 to 0.50%
Ca, Be, Ti, Cu, Ni, Co, Cr, Mn, Fe, and Sr are elements that contribute to improving corrosion resistance after painting. To obtain this effect, it is preferable to contain one or more of the following: Ca: 0.01% or more, Be: 0.01% or more, Ti: 0.01% or more, Cu: 0.1% or more, Ni: 0.01% or more, Co: 0.01% or more, Cr: 0.01% or more, Mn: 0.01% or more, Fe: 0.01% or more, and Sr: 0.01% or more.
On the other hand, if the contents of these elements are excessive, the appearance after plating may become rough, and therefore, when these elements are contained, it is preferable that the Ca content is 0.50% or less, the Be content is 0.20% or less, the Ti content is 0.50% or less, the Cu content is 1.0% or less, the Ni content is 1.00% or less, the Co content is 0.30% or less, the Cr content is 0.20% or less, the Mn content is 0.50% or less, the Fe content is 3.00% or less, and the Sr content is 0.50% or less.

The chemical composition of the plating layer is measured by the following method.
First, the plating layer is stripped and dissolved with an acid containing an inhibitor that suppresses corrosion of the base steel (steel material) to obtain an acid solution. The obtained acid solution is then measured by ICP analysis to obtain the chemical composition of the plating layer. There are no particular limitations on the type of acid as long as it is an acid that can dissolve the plating layer. The chemical composition is measured as an average chemical composition.

<Welded parts>
[Weld bead]
As described above, the steel material 1 according to the present embodiment is obtained by welding plated steel sheets having a plating layer formed on the surface. Although there is no limitation on the welding method, examples include welding by high-frequency resistance welding or the like in a state where the plated steel sheets are in contact with each other (for example, in the case of a welded H-shaped steel, a surface corresponding to the plate thickness direction of the web is in contact with a surface of a flange, and in the case of a steel pipe, ends of steel sheets processed into a cylindrical shape are in contact with each other).
At the welded portion, the abutting materials melt together and the steel material is pushed out by the contact pressure, forming a weld bead 21 on the steel sheet 11 where no plating layer is present.

[scale]
Since the weld bead 21 reaches a high temperature, a scale 22 (oxide scale containing Fe and O) is formed on the weld bead 21. The scale 22 may further contain Zn and Si derived from the plating layer.
Conventionally, such scale has a problem that the adhesion of the scale itself is low, and the performance of the formed coating is deteriorated. However, in the steel material 1 according to the present embodiment, the chemical composition of the plating layer of the plated steel sheet to be welded is controlled within a predetermined range, and the welding conditions are controlled as described below, so that the scale 22 contains Al and Mg in addition to Fe and O, and the ratio of the total content of Al and Mg to the total content of Al, Mg, Fe, O, Si, and Zn in the scale 22 in mass % is set to 3.0% or more. As a result, the adhesion between the weld bead 21 and the scale 22 is improved. In addition, when the scale 22 contains 3.0% or more of Mg and Al, when a coating (repair coating) 101 containing Al is formed on the scale 22, the adhesion between the scale 22 and the coating 101 is improved due to the interaction between the Al and Mg in the scale 22 and the Al in the coating 101.
If the ratio of the total content of Al and Mg to the total content of Al, Mg, Fe, O, Si, and Zn in the scale 22 is less than 3.0%, the coating adhesion is poor. If the ratio of the total content of Al and Mg in the scale 22 exceeds 25.0%, the amount of Fe is relatively reduced, and the strength of the scale 22 decreases or the scale becomes brittle, making it more susceptible to partial cracking.
The scale 22 may further contain about 0.1 to 10% of elements derived from the base material, such as Mn, Ti, and Cr.

[Method for measuring the proportion of Al and Mg content in the total content of Al, Mg, Fe, O, Si and Zn in scale]
The proportions of Al and Mg in the scale are measured by the following method.
A test piece including a weld bead is cut out from the steel material. The test piece is embedded in resin and then polished to prepare a cross-sectional observation sample. The cross-section of the weld bead is observed using a scanning electron microscope, and elemental quantification is performed using an energy dispersive X-ray detector to measure the content of Al and Mg. In this embodiment, the total content of the six elements O, Mg, Al, Si, Fe, and Zn is taken as 100 mass%, and the Al content and Mg content (mass%) are calculated, and the ratio of the total content of Al and Mg to the content of Fe, O, Al, Mg, Si, and Zn in the scale in mass% is taken as the ratio.

As described above, the weld bead is formed on the steel plate. In this embodiment, the weld may also include the portion of the steel plate that is thermally affected by the welding.

<Coating>
Since the plating layer of the weld bead 21 disappears due to welding, the weld bead 21 has inferior corrosion resistance compared to other portions having the plating layer 12. Therefore, when the weld bead 21 is used as a steel structure, a coating is formed on the portion without the plating layer (coating formation by partial repair).
In the steel material 1 according to this embodiment, a coating 101 containing, by mass %, 5.0% or more of Al may be formed as a repair coating on the scale 22 of the weld.
By setting the Al content of the coating 101 to 5.0 mass% or more, adhesion to the scale 22 is excellent, and therefore the corrosion resistance of the steel material is improved. The coating 101 may contain 10 mass% or less of Mg in addition to Al. Furthermore, the balance other than Al and Mg may be Zn and impurities.
The method for forming the coating 101 is not limited, and it may be a metal sprayed layer formed by spraying a metal containing Al.

The Al content of the coating is measured using a cross-sectional observation sample prepared in the same manner as the method for measuring the proportions of Al and Mg content in the scale described above. The cross section of the weld bead is observed using a scanning electron microscope, and elemental quantification is performed using an energy dispersive X-ray detector to measure the Al content in the coating. In this embodiment, the Al content is calculated by taking the total content of the three elements Mg, Al, and Zn as 100 mass% as the proportion of the metal components.

In the steel material according to the present embodiment described above, when thermal spraying is performed without shot blasting, the adhesion of the thermal spray coating is improved, particularly the coating adhesion evaluated by a tape peel test.

<Production Method>
Next, a preferred method for manufacturing the steel material according to this embodiment will be described. The steel material according to this embodiment can obtain the effects as long as it has the above-mentioned characteristics regardless of the manufacturing method. However, the following method is preferable because it can be manufactured stably.

Specifically, the steel material according to this embodiment can be manufactured by a manufacturing method including the following steps (I) to (II).
(I) A plating step in which a steel sheet is plated to obtain a plated steel sheet; and (II) A welding step in which the plated steel sheet is welded to obtain a steel material of a predetermined shape including a base material portion and a weld portion. In addition, when a coating is further formed on the weld bead, it is preferable to include the following steps after the welding step.
(III) Coating formation step of forming a coating on the weld bead Preferred conditions for each step are described below. For conditions not described, known conditions can be applied.

[Plating process]
In the plating step, a steel sheet is immersed in a plating bath to obtain a plated steel sheet. The plating method is not limited, and a known hot-dip plating method may be adopted. As described above, the steel sheet is not limited.
The composition of the plating bath may be adjusted so as to obtain a plating layer having the above-mentioned chemical composition.

[Welding process]
The steel material according to this embodiment is produced by continuously welding a plurality of plated steel sheets or a plurality of end faces of plated steel sheets in contact with each other by high-frequency resistance welding, butt resistance welding, or the like.
During welding, as described above, welding is performed on a plated steel sheet having a plating layer containing Al, Mg, and Zn, and the formed weld bead is formed (crushed) by a crushing roller, and then water-cooled. In this case, in the manufacturing method of a steel material according to this embodiment, in order to cause a predetermined amount of Al and Mg to exist in the scale, it is preferable to perform welding with a heating temperature (maximum temperature reached) of 1500°C or higher, and then set the temperature during crushing to 800 to 1100°C and the water-cooling start temperature to 800°C or higher.
If the temperature during crushing exceeds 1100°C, the heat stored in the crushing roller increases, the crushing roller softens, and the formability of the weld bead is poor. If the temperature during crushing is less than 800°C, the weld bead becomes hard, and similarly the formability of the weld bead is poor. If the water cooling start temperature is less than 800°C, the diffusion of plating components into the generated scale is reduced, resulting in poor coating adhesion.
The conditions other than those mentioned above are not particularly limited, and may be set depending on the shape, size, application, etc. of the steel material to be produced.
For example, when a lightweight H-section steel is manufactured by high-frequency resistance welding as the steel material according to this embodiment, the following welding conditions are exemplified.
That is, a steel sheet for the web and a steel sheet for the flange, both cut from a plated steel strip with a thickness of 1.6 to 9.0 mm, are brought into contact with each other to form the shape of an H-beam, and high-frequency resistance welding is performed so that the heating temperature of the contact portion is 1500°C or higher. High-frequency resistance welding is performed by using a copper power supply, setting the frequency of the high-frequency power source to 200 to 400 kHz, and bringing the power supply and the steel strip into contact. After high-frequency resistance welding, the weld bead is formed by a crushing roller so that the crushing temperature (Tf) is 800 to 1100°C, and then water cooling may be performed from a temperature range of 800°C or higher.

The steel material according to the present embodiment is based on the premise that shot blasting is not performed. If shot blasting is performed on the steel material welded under the above conditions, the scale will be removed, or even if the scale partially remains, the Mg and Al contents in the scale will not be absorbed.
When shot blasting is performed on steel materials welded under the above conditions and the scale is completely removed, the coating adhesion is excellent in tape peeling tests, etc., but complete removal is not easy, and if complete removal is performed, productivity will drop significantly. Also, since there is no scale, this is different from the present embodiment, which is based on the premise that shot blasting is not performed.

[Film formation process]
The method for forming the coating is not limited, but it is preferably performed by thermal spraying. The thermal spraying method may be any method such as arc spraying, flame spraying, plasma spraying, etc. As an example, in the case of arc spraying, it is preferable to perform it under the following conditions.
That is, a wire containing Al and, if necessary, Mg and Zn is prepared as the spray metal wire. The distance between the spray nozzle of the spray gun and the weld bead of the plated and welded H-shaped steel, which is the part to be sprayed, is set to 50 to 100 mm, the advance angle is set to 0 to -20 degrees, and the moving speed of the plated and welded H-shaped steel is set to 30 to 200 m per minute. In this case, it is preferable to select the spray metal wire so that the Al content of the coating is 5.0 mass% or more.
The coating may be formed before the temperature of the steel material is lowered to room temperature after the welding process (continuous), or may be formed separately after the temperature of the steel material is lowered to room temperature (separate process). Also, the coating may be formed after the temperature of the steel material is lowered to room temperature after the welding process and the steel material is heated to a predetermined temperature (separate process/preheating).
From the viewpoint of process omission, continuous (forming the coating before the temperature of the steel material drops to room temperature) is preferable. The temperature of the steel material during coating formation is not particularly specified, but from the viewpoint of adhesion, 100°C or higher is preferable. The upper limit temperature is not particularly specified, but from the viewpoint of thermal efficiency, 400°C or lower is preferable.

Example 1
A plated steel strip having a thickness of 4.5 mm was prepared, in which a plated layer having the chemical composition shown in Table 2 was formed on a steel sheet corresponding to JIS G3193:2008. This plated steel strip was welded to form a web and a flange, and a welded H-shaped steel having a joint as shown in FIG. 1 was manufactured. During welding, high-frequency resistance welding was performed using a copper power supply, and the frequency of the high-frequency power source was set to 360 kHz so that the heating temperature (welding temperature) of the joint was 1500°C or higher. After high-frequency resistance welding, as shown in Tables 1 and 2, the weld bead was formed by a crushing roller at a crushing temperature Tf (°C), and then water-cooled from Tc (°C). In the chemical composition of the plated layer in Table 2, "-" indicates that no additive was added.
Thereafter, arc spraying was performed at the temperature of the steel material shown in Table 2 to form a coating as shown in FIG. 3. Pure Al, Al-5% Mg, Zn-15% Al, and pure Zn, each having a diameter of 1.6 mm, were prepared as the spray metal wires used in the arc spraying, and these were used in the combinations shown in Table 2. The spraying device used was a BP-400 model with push-type wire supply manufactured by U-Tech Japan Co., Ltd. The distance between the spray nozzle of the spray gun and the weld bead of the plated welded H-shaped steel, which was the part to be sprayed, was 50 mm, the forward angle was set to minus 10 degrees, and the moving speed of the plated welded H-shaped steel was 45 m per minute. At this time, the water cooling conditions described above were changed, and the material temperature after high-frequency resistance welding was adjusted to perform warm spraying.

For the obtained steel material, the cross section of the weld bead was observed using a scanning electron microscope using the method described above, and elemental quantification was performed using an energy dispersive X-ray detector to measure the proportion of each element in the total content of Fe, O, Al, Mg, Si, and Zn in the scale in mass %, and the presence or absence of other elements contained.
The Al content in the coating was also determined by the method described above.
The results are shown in Tables 2 and 3.

The resulting steel materials were also evaluated for film adhesion, in terms of initial adhesion and tape peelability, using the following methods.

[Method for evaluating initial adhesion]
The obtained test materials were visually inspected for peeling of the coating formed on the welded parts while still welded, and were rated as "○" for no peeling, "△" for peeling only on the periphery, and "×" for peeling over the entire surface.

[Method for evaluating tape peelability]
The obtained test materials were evaluated by peeling using polyester tape. Specifically, polyester tape was applied to the coating formed on the welded part in accordance with JIS K5600 5-6 (but without cross-cutting) and peeled off. If no peeling of the coating occurred at all, it was evaluated as "◎", and if the peeling area rate was 5% or less, it was evaluated as "◯". For those that were evaluated as "×" in the evaluation by visual observation (initial adhesion), the evaluation of "tape peeling" was not performed.
If the initial adhesion was ◯ and the tape peelability was ⊚, it was determined that the film adhesion was excellent.

Test Nos. 1 to 5, 12 to 15, 19 to 22, and 25 to 32, which are examples of the present invention, were produced under welding conditions (Method A) that performed favorable scale control on plated steel sheets having a plating layer with a predetermined chemical composition, and steel materials having scale containing predetermined Al and Mg were obtained. As a result, when a thermal spray coating was formed after welding without shot blasting, the adhesion was acceptable both visually and by tape peeling.
On the other hand, in Test Nos. 6 to 9, 16 to 18, 23, and 24, in which the welding conditions were not favorable for scale control, the proportions of Al and Mg in the scale were insufficient. Also, in these examples, the tape peelability in terms of adhesion to the coating was insufficient.
In addition, in Test Nos. 10 and 11, in which the chemical composition of the plating layer was outside the range of the present invention, the proportions of Al and Mg in the scale were insufficient. In addition, in these examples, the tape peelability in terms of adhesion to the coating was insufficient.

Example 2
A plated steel strip having a thickness of 4.5 mm was prepared by forming a plating layer having the chemical composition shown in Table 4 on a steel plate corresponding to JIS G3193:2008. The plated steel strip was roll-formed into a tube shape, and high-frequency resistance welding was performed on the butt joint to manufacture a welded round tube. High-frequency resistance welding was performed using a copper power supply and the frequency of the high-frequency power source was set to 320 kHz so that the heating temperature of the joint was 1500°C or higher, and the crushing temperature and water cooling start temperature were as shown in Tables 1 and 4. After high-frequency resistance welding, the weld bead on the outer surface of the tube was ground and removed by a bead cutter as shown in Figure 2, but the bead was not cut on the inner surface.
Thereafter, arc spraying was performed to form a coating at the temperature of the steel material shown in Table 5. A pure Al wire with a diameter of 1.6 mm was used as the spray metal wire used in the arc spraying. A BP-400 model with push-type wire supply manufactured by U-Tech Japan Co., Ltd. was used as the spraying device. The distance between the spray nozzle of the spray gun and the weld bead of the plated-welded H-shaped steel, which was the part to be sprayed, was 40 mm, the forward angle was set to minus 10 degrees, and the moving speed of the plated-welded H-shaped steel was 20 m per minute. At this time, the water cooling conditions described above were changed and the material temperature after high-frequency resistance welding was adjusted to perform warm spraying.

For the obtained steel material, in the same manner as in Example 1, the proportion of each element in the total content of Fe, O, Al, Mg, Si, and Zn in the scale in mass % was measured by the method described above.
In addition, the amounts of elements contained in the coating other than the above elements were also measured. The Al content of the coating was also measured.
The results are shown in Tables 4 and 5.

The obtained steel material was also evaluated for the initial adhesion and tape peelability on the inner surface of the steel pipe in the same manner as in Example 1 to determine the coating adhesion. The results are shown in Table 5.

Test Nos. 101, 104, and 105, which are examples of the present invention, were produced by welding steel sheets having a plating layer with a predetermined chemical composition under welding conditions (Method A) that performed preferable scale control, and thus steel materials having scales containing predetermined Al and Mg were obtained. As a result, when a thermal spray coating was formed after welding without shot blasting, the adhesion was acceptable both visually and by tape peeling.
On the other hand, in Test No. 102, in which the welding conditions were not favorable for scale control, and Test No. 103, in which the chemical composition of the plating layer was outside the range of the present invention, the proportions of Al and Mg in the scale were insufficient. Also, in these examples, the tape peelability in terms of adhesion to the coating was insufficient.

REFERENCE SIGNS LIST 1 Steel material 11 Steel plate 12 Plating layer 21 Weld bead 22 Scale 101 Coating (repair coating)

Claims (3)

A base material portion having a steel plate and a plating layer formed on the steel plate;
a weld having a weld bead and a scale formed on the weld bead;
A steel material comprising:
The plating layer has a chemical composition, in mass%,
Al: 2.0 to 19.0%,
Mg: 1.0 to 10.0%,
Si: 0 to 2.0%, and the balance: Zn and impurities.
Including,
The scale contains Al, Mg, Fe, O, and optionally further contains Si and Zn, and the ratio of the total content of Al and Mg to the total content of Al, Mg, Fe, O, Si, and Zn in the scale, in mass%, is 3.0 to 25.0%. The steel material is an H-shaped steel,
The steel plate is a web and a flange,
the weld is at the joint between the web and the flange;
The steel material according to claim 1. The steel material according to claim 1 or 2, further comprising a coating containing 5.0 mass % or more of Al present on the scale of the welded portion.

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