JP2021055136A - HOT-DIP Zn-Al-Mg BASED PLATED STEEL SHEET AND METHOD FOR PRODUCING SAME - Google Patents

Abstract

To reduce hydrogen embrittlement while maintaining excellent corrosion resistance and workability.SOLUTION: A hot-dip Zn-Al-Mg plated steel sheet has a Zn-Al-Mg coating layer on a surface of a base steel sheet, and a diffusible hydrogen concentration in the base steel sheet is 0.25 ppm or less.SELECTED DRAWING: Figure 3

Description

The present invention relates to a molten Zn-Al-Mg-based plated steel sheet and a method for producing the same.

In recent years, there has been an increasing need for high-strength, high-rust-preventive steel sheets for the purpose of weight reduction and resource saving in the fields of automobiles and building materials. Since the high-strength, high-rust-preventive steel sheet is subjected to various processes such as press working and bending, it is important to have excellent workability in addition to high strength and high corrosion resistance. As a surface-treated steel sheet having a high rust preventive effect, which is in increasing demand these days, there is a molten Zn-Al-Mg-based plated steel sheet.

However, in this type of plated steel sheet, when high-strength steel is used for the original plating plate, so-called hydrogen embrittlement is likely to occur due to hydrogen that inevitably enters the steel sheet at the plating line, which is a problem depending on the application. Become. In a general hot-dip galvanizing line, the base steel sheet, which is the original plating plate, is heat-treated in a reducing atmosphere containing hydrogen gas immediately before the plating bath. Hydrogen in this heating atmosphere penetrates into the base steel sheet and causes hydrogen embrittlement. In addition, hydrogen may enter in a wet process such as electrolytic degreasing performed before plating, which can also cause hydrogen embrittlement.

It is known that hydrogen embrittlement in plated steel sheets is usually caused by hydrogen storage in the pickling process of electroplating and its pretreatment, and it is a problem especially when high-strength steel of 980 MPa class or higher is used for the plating base plate. It is easy to become. However, in hot-dip Zn-Al-Mg galvanized steel sheets, even if high-strength steel with a relatively low strength level such as 780 MPa class or 590 MPa class is used, brittle fracture occurs when very severe processing such as close contact bending is performed. May occur.

This type of brittle fracture has also been found to be an event caused by hydrogen invading the plating line. Further, it has been found that in the hot-dip Zn-Al-Mg-based plated steel sheet, the plating layer tends to be a "barrier" that prevents hydrogen from being separated from the steel sheet, as compared with other general hot-dip galvanized steel sheets. Therefore, in order to improve the reliability level for processing a high-strength steel sheet subjected to hot-dip Zn-Al-Mg plating, it is desired to establish a technique for suppressing hydrogen embrittlement of the steel sheet.

As a method for preventing hydrogen embrittlement of steel sheets, Patent Document 1 states that cracks are generated in the plating layer of a hot-dip Zn-Al-Mg-based plated steel sheet using high-tensile steel as a galvanized steel sheet, and then baking treatment is performed. A molten Zn-Al-Mg-based plated steel sheet in which the concentration of diffusible hydrogen in the steel sheet is reduced by releasing hydrogen that has penetrated into the steel sheet is disclosed.

Patent No. 6271067

However, the molten Zn-Al-Mg-based plated steel sheet disclosed in Patent Document 1 is not always sufficient from the viewpoint of reducing hydrogen embrittlement while maintaining excellent corrosion resistance and workability.

One aspect of the present invention has been made in view of the above problems, and an object thereof is molten Zn-Al-, which can reduce hydrogen embrittlement while maintaining excellent corrosion resistance and processability. The purpose is to realize a Mg-based plated steel sheet.

In order to solve the above-mentioned problems, the method for producing a molten Zn-Al-Mg-based plated steel sheet according to one aspect of the present invention is, in terms of mass%, C: 0.01 to 0.20%, Si: 0.01. ~ 0.50%, Mn: 0.01 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, the balance of the base steel sheet having a steel composition of Fe and unavoidable impurities, the temperature of the base steel sheet at the end of finish rolling becomes 800 ° C. or higher and 950 ° C. or lower. After hot-rolling as described above, the hydrogen concentration is set to 2 for the hot-rolled step of winding the base steel sheet so that the temperature of the base steel sheet becomes 650 ° C. or lower and the hot-rolled steel sheet obtained in the hot-rolled step. A fused Zn-Al-Mg system for a ablation step in which the following treatments (i) to (iii) are performed in a furnace having a volume% or more and less than 20% by volume, and a hot-rolled steel sheet annealed in the ablation step. It is a method including a hot-dip plating step of performing plating;
(I) The reduction reaction is promoted by heating the hot-rolled steel sheet to a temperature of 400 ° C. or higher and 750 ° C. or lower.
(Ii) The hot-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 580 ° C. or higher and 750 ° C. or lower.
(Iii) The hot-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more.

In order to solve the above-mentioned problems, the method for producing a molten Zn-Al-Mg-based plated steel sheet according to one aspect of the present invention is, in terms of mass%, C: 0.01 to 0.20%, Si: 0.01. ~ 0.50%, Mn: 0.01 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, the balance of the base steel sheet having a steel composition of Fe and unavoidable impurities, the temperature of the base steel sheet at the end of finish rolling becomes 800 ° C. or higher and 950 ° C. or lower. After hot-rolling as described above, the hot-rolling step of winding the base steel sheet so that the temperature of the base steel sheet becomes 600 ° C. or lower and the hot-rolled steel sheet obtained in the hot-rolling step are pickled. The following (i) to (iii) in a furnace in which the hydrogen concentration was 2% by volume or more and less than 20% by volume with respect to the cold rolling step of cold rolling and the cold rolled steel sheet obtained in the cold rolling step. ), And a hot-dip plating step of applying hot-dip Zn-Al-Mg-based plating to the cold-rolled steel sheet that has been annealed in the bubbling step;
(I) The reduction reaction is promoted by heating the cold-rolled steel sheet to a temperature of 400 ° C. or higher and 900 ° C. or lower.
(Ii) The cold-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 650 ° C. or higher and 900 ° C. or lower.
(Iii) The cold-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more.

In order to solve the above-mentioned problems, the molten Zn-Al-Mg-based galvanized steel sheet according to one aspect of the present invention has a mass% of C: 0.01 to 0.20% and Si: 0.01 to 0.01. 50%, Mn: 0.01 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0 On the surface of a base steel sheet having a steel composition of .01 to 0.10%, the balance of which is Fe and unavoidable impurities, in mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10. 0%, Si: 0 to 2.0%, Ti: 0 to 0.10%, B: 0 to 0.05%, Fe: 2.0% or less, the balance is Zn and Zn-Al, which is an unavoidable impurity. It has a −Mg-based coating layer, and the diffusible hydrogen concentration in the base steel sheet is 0.25 ppm or less.

According to one aspect of the present invention, it is possible to reduce hydrogen embrittlement while maintaining excellent corrosion resistance and processability.

It is a schematic diagram which shows the outline structure of the annealing furnace and the pot for carrying out the annealing process and hot dip galvanizing process which concerns on one Embodiment of this invention. It is a table which shows the chemical composition (mass%) of the base steel sheet with respect to the molten Zn-Al-Mg based galvanized steel sheet which concerns on one Embodiment of this invention. It is a table comparing the hot-dip condition, the cold-dip condition, the hot-dip galvanizing condition, the characteristic of a hot-dip galvanized steel sheet, the diffusible hydrogen concentration in a steel sheet, and the plating quality about the hot-dip galvanized steel sheet of an Example and a comparative example.

[Chemical composition of base steel sheet]
First, the chemical composition of the base steel sheet corresponding to the original plating plate will be described with reference to FIG. In the present specification, "%" regarding the chemical composition of the base steel sheet means "mass%" unless otherwise specified. The steel composition (chemical composition) of the base steel sheet is C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.1 to 2.50%, P in mass%. : 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, the balance is Fe and unavoidable impurities Is. The steel types A to I shown in FIG. 2 show a composition example of the chemical composition of the base steel sheet of the example.

(C)
C is an element necessary for increasing the strength of steel. In order to obtain a strength level of tensile strength of 590 MPa or more, a C content of 0.01% or more is required. When the C content is excessive, the non-uniformity of the structure becomes remarkable and the processability is lowered. The C content is limited to 0.20% or less and may be controlled to 0.16% or less.

(Si)
Si is effective for increasing the strength and also has an effect of reducing the precipitation of cementite, and is effective for reducing the formation of pearlite and the like. In order to fully exert these effects, a Si content of 0.01% or more is secured. If a large amount of Si is contained, a Si-concentrated layer is formed on the surface of the steel sheet, which causes a decrease in plating property. The Si content is limited to 0.50% or less, more preferably 0.25% or less.

(Mn)
Mn is effective for increasing the strength. In order to stably obtain a strength level of tensile strength of 590 MPa or more, a Mn content of 0.10% or more is secured. It is more effective to set it to 0.50% or more. If the Mn content is excessive, segregation is likely to occur and workability is reduced. The Mn content shall be 2.50% or less.

(P)
P is effective for solid solution strengthening. Here, the content of P of 0.005% or more is secured. It may be managed to 0.010% or more. If the P content is excessive, segregation is likely to occur and workability is reduced. The content of P is limited to 0.050% or less.

(B)
B reduces the austenite-ferrite transformation of steel and contributes to the strengthening of the transformed structure. Further, when Ti or Nb is added, it has the effect of lowering the precipitation temperature of Ti-based carbides and Nb-based carbides by reducing the austenite-ferrite transformation and refining those carbides. In order to obtain the above effect sufficiently, the content of B of 0.0005% or more is secured. It is more effective to set it to 0.001% or more. A large amount of B is a factor that causes a decrease in workability due to the formation of boride. When B is added, it is necessary to carry out in the range of 0.010% or less, and it may be controlled to 0.005% or less.

(Ti)
Ti combines with C to form fine Ti-based carbides, which contributes to high strength. In order to fully exert its action, the Ti content of 0.01% or more is secured. Excessive Ti content causes a decrease in workability. The Ti content may be 0.20% or less and may be controlled to 0.15% or less.

(Al)
Al has a deoxidizing effect. In order to fully exert its action, it is desirable to add Al so that the Al content in the steel sheet is 0.01% or more. Excessive Al content causes a decrease in workability. The Al content is limited to 0.10% or less and may be controlled to 0.05% or less.

(S)
S mixed as an impurity is allowed up to 0.010%, but more preferably 0.005% or less. Since excessive reduction in S causes an increase in the steelmaking load, the S content may usually be 0.0005% or more.

(Nb)
Nb combines with C to form fine Nb-based carbides, which contributes to high strength. It is also effective for making the structure finer and more uniform. Therefore, Nb can be contained if necessary. In order to obtain the above effect sufficiently, it is more effective to secure the content of Nb of 0.005% or more. A large amount of Nb content causes a decrease in workability. When Nb is added, it is carried out in the range of 0.10% or less.

(Mo, Cr)
Since both Mo and Cr have an action of improving the strength by strengthening the solid solution, one or two kinds of Mo and Cr can be added as needed. In order to fully exert the above action, it is more effective to secure a content of 0.01% or more for Mo and 0.01% or more for Cr. A large amount of these elements causes a decrease in ductility. When one or two of these are added, the Mo content is in the range of 0.50% or less, and the Cr content is also in the range of 0.50% or less.

[Zn-Al-Mg-based coating layer]
A Zn-Al-Mg-based coating layer is formed on the surface of the base steel sheet having the above-mentioned chemical composition of the present embodiment. The coating layer is derived from a plating layer formed by molten Zn-Al-Mg-based plating, and is referred to as "Zn-Al-Mg-based coating layer" in the present specification. The Zn-Al-Mg-based coating layer contains Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.0% by mass. 10%, B: 0 to 0.05%, Fe: 2.0% or less, the balance is Zn and unavoidable impurities.

In order to maintain the excellent rust preventive effect of the Zn-Al-Mg-based coating layer for a long period of time, the average thickness of the Zn-Al-Mg-based coating layer is preferably 3 μm or more. It is uneconomical to form an excessively thick layer, and it also leads to a decrease in workability of the coating layer itself. Usually, the average thickness of the Zn-Al-Mg-based coating layer may be in the range of 100 μm or less. Here, the average thickness of the coating layer can be obtained by observing a cross section parallel to the plate thickness direction. Further, it is preferable that the Zn-Al-Mg-based coating layer of the present embodiment has no cracks. As a result, the workability of the molten Zn-Al-Mg-based plated steel sheet can be further improved.

[Diffusible hydrogen concentration in base steel sheet]
The hydrogen concentration in the base steel plate, which causes hydrogen embrittlement, can be evaluated by measuring the amount of diffusible hydrogen per unit volume of the base steel plate. The amount of hydrogen released when heated from room temperature to 300 ° C at a heating rate of 5 ° C / min is measured with an atmospheric pressure ionized mass spectrometer, and the total amount of hydrogen released from room temperature to 200 ° C is diffusible. Calculated as the amount of hydrogen. As the measurement sample, a sample consisting of only a base steel sheet from which the Zn-Al-Mg-based coating layer has been removed with abrasive paper can be used.

By using the high-strength steel in the composition range described above as the plating base plate and setting the hydrogen concentration to 2% by volume or more and less than 20% by volume in the reduction heating area 30, the soaking area 40, and the cooling area 50 of the annealing furnace 100 described later. , The diffusible hydrogen concentration in the base steel sheet can be set to 0.25 ppm or less. When the diffusible hydrogen concentration in the base steel sheet is reduced to 0.25 ppm or less, only the hydrogen embrittlement phenomenon, which tends to be a problem in hot-dip Zn-Al-Mg-based plated steel sheets using high-tensile steel of 980 MPa class or higher as the base steel sheet. In addition, the phenomenon of hydrogen embrittlement is remarkably reduced even in a molten Zn-Al-Mg-based galvanized steel sheet using a high-tensile steel having a relatively low strength level of 780 MPa class or 590 MPa class as a base steel sheet. Therefore, in the molten Zn-Al-Mg-based plated steel sheet according to the embodiment of the present invention, the diffusible hydrogen concentration in the base steel sheet is specified to be 0.25 ppm or less. The diffusible hydrogen concentration in the base steel sheet is more preferably 0.20 ppm or less. According to the above configuration, it is possible to realize a molten Zn-Al-Mg-based plated steel sheet capable of reducing hydrogen embrittlement while maintaining excellent corrosion resistance and workability.

[Metal structure of base steel sheet]
It is desirable that the matrix (steel base) of the base steel sheet has a structure composed of a bainitic ferrite phase or a mixed structure of a ferrite phase and a martensite phase. In the latter structure, the amount of martensite is preferably 10 to 50% by volume.

[Mechanical characteristics]
The mechanical properties of the hot-dip galvanized steel sheet on which the Zn-Al-Mg-based coating layer was formed were such that in a tensile test in the direction perpendicular to rolling (JIS Z2241: 2011), the tensile strength was 590 to 1180 MPa and the total elongation at break was 10. It is desirable that it is% or more.

[Plating quality]
The plating quality is confirmed visually and by observing with an optical microscope at a magnification of 200 times. The one with plating uniformly attached to the surface of the steel sheet is regarded as good plating quality (○), and the one with a part without plating is regarded as good. The plating quality was poor (x).

〔Production method〕
Next, a method for producing a molten Zn-Al-Mg-based plated steel sheet according to an embodiment of the present invention will be described with reference to FIG. FIG. 1 is an apparatus for performing an annealing step and a hot-dip galvanizing step, which includes an annealing furnace 100 and a pot 60. Further, the annealing furnace 100 is divided into each area of a preheating area 10, a direct fire area 20, a reduction heating area 30, a soaking area 40, and a cooling area 50. The steel strip (base steel plate) 1 enters from the preheating area 10 of the annealing furnace 100, exits from the cooling area 50, and passes through the pot 60.

First, a hot rolling step is carried out before the annealing step and the hot dip galvanizing step. In the hot rolling step, the steel strip 1 is hot-rolled so that the temperature at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then wound at a temperature of 650 ° C. or lower.

A cold rolling step may be added between the hot rolling step and the annealing step. In this case, the steel strip 1 is hot-rolled in the hot-rolling step so that the temperature at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then wound at a temperature of 600 ° C. or lower and cooled. In the hot-rolling step, the hot-rolled steel sheet obtained in the hot-rolling step is pickled and cold-rolled.

Next, the steps carried out using the annealing furnace 100 and the pot 60 include at least an annealing step and a hot dip galvanizing step. The annealing step is carried out in the annealing furnace 100. Further, the hot-dip galvanizing step is carried out in the pot 60.

In the preheating area 10, preheating is performed on the steel strip 1. In the direct fire area 20, by adopting a direct fire reduction burner (MID burner), it is possible to prevent peroxidation of the steel strip 1 in the direct fire area 20. Here, by direct heating with a burner, the steel strip 1 is heated to 530 ° C. to remove rolling oil and dirt on the surface of the steel strip 1.

In the reduction heating area 30, in the case where the cold rolling step is not added between the hot rolling step and the annealing step, the reduction reaction is promoted by heating the hot-rolled steel sheet at a temperature of 400 ° C. or higher and 750 ° C. or lower. .. On the other hand, in the case where the cold rolling step is added between the hot rolling step and the annealing step, the reduction reaction is promoted by heating the cold-rolled steel sheet at a temperature of 400 ° C. or higher and 900 ° C. or lower. Specifically, by heating the steel sheet in H ₂ -N ₂ gas mixture performs reduction of very thin oxide film of the following (1) reaction at the steel sheet surface shown in - (3), to activate the surface ..
(1) _Fe 3 _{0 4} tens 4H ₂ → 3Fe tens _4H 2 0
(2) FeO 10H ₂ → Fe10 H ₂₀
(3) Fe ₂ O ₃ + 3H ₂ → 2 Fe + 3H ₂ O
By such a reduction reaction, the surface of the steel sheet is cleaned and activated, and the plating adhesion is improved. Note that FIG. 1 shows how the steel strip 1 is heated to 685 ° C. in the reduction heating area 30.

In the soaking area 40, in the case where the cold rolling step is not added between the hot rolling step and the annealing step, the temperature of the hot-rolled steel sheet after the treatment is 580 ° C. or higher and 750 ° C. or lower for 10 seconds. The heat is equalized. On the other hand, in the case where the cold rolling step is added between the hot rolling step and the annealing step, the heat is equalized for 10 seconds or more so that the cold-rolled steel sheet after the treatment has a temperature of 650 ° C. or higher and 900 ° C. or higher.

In the cooling area 50, in the case where the cold rolling process is not added between the hot rolling process and the annealing process, the processed hot-rolled steel sheet is heated at an average cooling rate of 5 ° C./sec or more and 500 ° C. or less. Cool to. On the other hand, in the case where the cold rolling step is added between the hot rolling step and the annealing step, the processed cold-rolled steel sheet is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more. .. Note that FIG. 1 shows how the steel strip 1 is cooled to 420 ° C. in the cooling area 50.

In the hot-dip plating step, the pot 60 is used, and in the case where the cold rolling step is not added between the hot rolling step and the annealing step, the molten Zn-Al is applied to the hot-rolled steel sheet annealed in the annealing step. -Mg-based plating is applied. On the other hand, in the case where a cold rolling step is added between the hot rolling step and the annealing step, molten Zn-Al-Mg-based plating is applied to the cold-rolled steel sheet that has been annealed in the annealing step. .. Note that FIG. 1 shows how the steel strip 1 is cooled to 410 ° C. during the hot-dip galvanizing step.

In the method for producing a molten Zn-Al-Mg-based plated steel sheet described above, a furnace in which the hydrogen concentrations in the reducing heating area 30, the soaking area 40 and the cooling area 50 of the annealing furnace 100 are 2% by volume or more and less than 20% by volume. By annealing inside, hydrogen intrusion into the steel sheet is reduced, and the diffusible hydrogen concentration in the steel sheet after plating is reduced. Further, the workability of the plating material is improved by reducing the concentration of diffusible hydrogen in the steel sheet. Further, according to the above method, it is possible to reduce the cost by reducing the amount of hydrogen gas used in the annealing step and the hot dip galvanizing step.

As described above, the manufacturing method of the molten Zn-Al-Mg-based plated steel sheet according to the embodiment of the present invention can be summarized in the following two cases.

(Case 1)
(1) The base steel sheet having the chemical composition is hot-rolled so that the temperature of the base steel sheet at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then the temperature of the base steel sheet is 650. A hot rolling process that winds up to a temperature below ° C,
(2) An annealing step of performing the following treatments (i) to (iii) in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume on the hot-rolled steel sheet obtained in the hot rolling step. When,
(3) The hot-dip galvanizing step of performing hot-dip Zn-Al-Mg-based plating on the hot-rolled steel sheet annealed in the annealing step is included.

(I) The reduction reaction is promoted by heating the hot-rolled steel sheet to a temperature of 400 ° C. or higher and 750 ° C. or lower.
(Ii) The hot-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 580 ° C. or higher and 750 ° C. or lower.
(Iii) The hot-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more.

(Case 2)
(1) The base steel sheet having the chemical composition is hot-rolled so that the temperature of the base steel sheet at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then the temperature of the base steel sheet is 600. Hot rolling process that winds up to ℃ or less,
(2) A cold rolling step in which the hot-rolled steel sheet obtained in the hot rolling step is pickled and cold-rolled.
(3) An annealing step of performing the following treatments (i) to (iii) in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume on the cold-rolled steel sheet obtained in the cold rolling step. When,
(4) Includes a hot-dip galvanizing step of performing hot-dip Zn-Al-Mg-based plating on a cold-rolled steel sheet that has been annealed in the annealing step.

(I) The reduction reaction is promoted by heating the cold-rolled steel sheet to a temperature of 400 ° C. or higher and 900 ° C. or lower.
(Ii) The cold-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 650 ° C. or higher and 900 ° C. or lower.
(Iii) The cold-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more.

〔Example〕
Next, based on FIG. 3, regarding the hot-dip galvanized steel sheets of Examples and Comparative Examples, the hot-dip condition, the cold-dip condition, the hot-dip galvanizing condition, the characteristics of the hot-dip galvanized steel sheet, the diffusible hydrogen concentration in the steel sheet, and the plating quality The result of comparison is shown.

The base steel sheet having the chemical composition shown in FIG. 2 was heated to 1250 ° C. and then hot-rolled to obtain a hot-rolled steel sheet for a hot-rolled base plate or a cold-rolled base steel plate. As for the hot-rolling conditions, the temperature of the steel sheet at the time of finish rolling was set to 880 ° C., the temperature of the steel sheet at the time of winding was set to 600 ° C., and the plate thickness was set to 3.2 mm for the hot-rolled plating original plate. On the other hand, for the cold-rolled original plate, the temperature of the steel sheet during finish rolling was set to 880 ° C, the temperature of the steel sheet when winding was set to 460 ° C, and the plate thickness was set to 2 mm. The hot-rolled steel sheet for the hot-rolled plating base plate was pickled and used as it was as the hot-rolled plating base plate. The hot-rolled steel sheet for the cold-rolled plating base plate was pickled and then cold-rolled at the cold rolling ratio (cold rolling ratio) shown in FIG. 3 to obtain a cold-rolled base plate. Using each of the original plating plates, heat treatment was performed on a continuous hot-dip galvanizing line under the conditions shown in FIG. 3 to produce a hot-dip Zn-Al-Mg-based plated steel sheet (Zn-Al-Mg).

Here, the composition of the plating bath of the pot 60 is, in mass%, Al: 6.0%, Mg: 3.0%, Si: 0.01%, Ti: 0.002%, B: 0.0005%. , Fe: 0.1%, and the balance was Zn. The amount of plating adhered was adjusted so that the thickness of the plating layer per one side of the steel sheet was 15 μm. In addition, alloyed hot-dip galvanized steel sheets (GA) were manufactured using some of the steel sheets. The alloying temperature was 550 ° C.

“TS / MPa” in the characteristic item of the hot-dip galvanized steel sheet in FIG. 3 indicates the tensile strength of the test piece according to JIS Z2241, and “TEl /%” indicates the total tensile strength of the test piece according to JIS Z2241. Shows growth. Further, the metal composition "BF" shows a structure composed of a bainitic ferrite phase, and the metal composition "F + M" shows a mixed structure of a ferrite phase and a martensite phase.

Here, when the diffusion hydrogen concentration in the steel sheet is compared, it is 0.14 to 0.19 (wtppm) in the steel type of the example, which is lower than 0.27 to 0.42 (wtppm) of the comparative example. You can see that. From the above, it was found that hydrogen embrittlement was reduced according to the molten Zn-Al-Mg-based plated steel sheet of the example.

In addition, the steel type No. of the comparative example. 11 to 15 are examples of alloyed hot-dip galvanized steel sheets, but the diffusion hydrogen concentration in the steel sheets is relatively low at 0.12 to 0.19 (wtppm). This indicates that the plating coating layer of the alloyed hot-dip galvanized steel sheet is more likely to release hydrogen in the steel sheet than the Zn-Al-Mg-based coating layer.

[Summary]
The method for producing a molten Zn-Al-Mg-based plated steel sheet according to aspect 1 of the present invention is, in terms of mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0. .10 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10. %, The base steel sheet having a steel composition in which the balance is Fe and unavoidable impurities is hot-rolled so that the temperature of the base steel sheet at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then the above. The hydrogen concentration was set to 2% by volume or more and less than 20% by volume with respect to the hot-rolling step of winding the base steel sheet so that the temperature of the base steel sheet was 650 ° C. or lower and the hot-rolled steel sheet obtained in the hot-rolling step. It includes an annealing step of performing the following treatments (i) to (iii) in the furnace and a hot-dip plating step of applying molten Zn-Al-Mg-based plating to the hot-rolled steel sheet annealed in the annealing step. Is the method;
(I) The reduction reaction is promoted by heating the hot-rolled steel sheet to a temperature of 400 ° C. or higher and 750 ° C. or lower.
(Ii) The hot-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 580 ° C. or higher and 750 ° C. or lower.
(Iii) The hot-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more.

According to the method, in the annealing step, the treatments (i) to (iii) are carried out in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume. Therefore, the concentration of diffusible hydrogen in the base steel sheet of the molten Zn-Al-Mg-based plated steel sheet can be reduced, and hydrogen embrittlement can be reduced. According to the above method, a molten Zn-Al-Mg-based plated steel sheet can be produced. Therefore, excellent corrosion resistance and workability can be maintained.

The method for producing a molten Zn-Al-Mg-based plated steel sheet according to aspect 2 of the present invention is, in terms of mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0. .10 to 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10. %, The base steel sheet having a steel composition in which the balance is Fe and unavoidable impurities is hot-rolled so that the temperature of the base steel sheet at the end of finish rolling is 800 ° C. or higher and 950 ° C. or lower, and then the above. A hot rolling step of winding the base steel sheet so that the temperature of the base steel sheet becomes 600 ° C. or lower, and a cold rolling step of pickling and cold rolling the hot-rolled steel sheet obtained in the hot rolling step. The baking step of performing the following treatments (i) to (iii) in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume with respect to the cold-rolled steel sheet obtained in the cold rolling step. This method includes a hot-dip plating step of performing hot-dip Zn-Al-Mg-based plating on a cold-rolled steel sheet that has been annealed in the baking step;
(I) The reduction reaction is promoted by heating the cold-rolled steel sheet to a temperature of 400 ° C. or higher and 900 ° C. or lower.
(Ii) The cold-rolled steel sheet after the treatment of (i) is heated for 10 seconds or more so that the temperature becomes 650 ° C. or higher and 900 ° C. or lower.
(Iii) The cold-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more. According to the method, the same effect as that of the first aspect can be obtained.

In the method for producing a molten Zn-Al-Mg-based plated steel sheet according to the third aspect of the present invention, in the first or second aspect, the base steel sheet is further mass%, Nb: 0 to 0.10%, Mo: It may contain 1 type or 2 or more types of 0 to 0.50% and Cr: 0 to 0.50%. According to the above method, the strength of the molten Zn-Al-Mg-based plated steel sheet can be further improved.

The molten Zn-Al-Mg-based galvanized steel sheet according to the fourth aspect of the present invention has a mass% of C: 0.01 to 0.20%, Si: 0.01 to 0.50%, and Mn: 0.1 to 10. 2.50%, P: 0.005 to 0.050%, B: 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, balance On the surface of the base steel sheet having a steel composition in which Fe and unavoidable impurities are present, in mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2. 0%, Ti: 0 to 0.10%, B: 0 to 0.05%, Fe: 2.0% or less, the balance has Zn and a Zn-Al-Mg-based coating layer which is an unavoidable impurity. The diffusible hydrogen concentration in the base steel sheet is 0.25 ppm or less. According to the above configuration, it is possible to realize a molten Zn-Al-Mg-based plated steel sheet capable of reducing hydrogen embrittlement while maintaining excellent corrosion resistance and workability.

In the molten Zn-Al-Mg-based plated steel sheet according to the fifth aspect of the present invention, it is preferable that there are no cracks in the Zn-Al-Mg-based coating layer in the fourth aspect. According to the above configuration, the workability of the molten Zn-Al-Mg-based plated steel sheet can be further improved.

In the molten Zn-Al-Mg-based plated steel sheet according to the sixth aspect of the present invention, in the fourth or fifth aspect, the base steel sheet is further mass%, Nb: 0 to 0.10%, Mo: 0 to 0. It may contain one or more of .50% and Cr: 0 to 0.50%. According to the above configuration, the strength of the molten Zn-Al-Mg-based plated steel sheet can be further improved.

[Additional notes]
The present invention is not limited to the above-described embodiments, and various modifications can be made within the scope of the claims, and the embodiments obtained by appropriately combining the technical means disclosed in the different embodiments. Is also included in the technical scope of the present invention. Furthermore, new technical features can be formed by combining the technical means disclosed in each embodiment.

1 Steel strip (base steel plate)
10 Preheating area 20 Direct fire area 30 Reduction heating area 40 Soaking area 50 Cooling area 60 Pot 100 Annealing furnace

Claims (6)

By mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.1 to 2.50%, P: 0.005 to 0.050%, B: Finish rolling of a base steel sheet having a steel composition of 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, and the balance being Fe and unavoidable impurities. A hot rolling step of hot rolling so that the temperature of the base steel sheet at the end of the process is 800 ° C. or higher and 950 ° C. or lower, and then winding up the base steel sheet so that the temperature of the base steel sheet is 650 ° C. or lower.
The annealing step of performing the following treatments (i) to (iii) in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume with respect to the hot-rolled steel sheet obtained in the hot rolling step.
A method for producing a molten Zn-Al-Mg-based plated steel sheet, which comprises a hot-dip plating step of applying molten Zn-Al-Mg-based plating to a hot-rolled steel sheet that has been annealed in the annealing step;
(I) The reduction reaction is promoted by heating the hot-rolled steel sheet to a temperature of 400 ° C. or higher and 750 ° C. or lower.
(Ii) The hot-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 580 ° C. or higher and 750 ° C. or lower.
(Iii) The hot-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more. By mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.1 to 2.50%, P: 0.005 to 0.050%, B: Finish rolling of a base steel sheet having a steel composition of 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, and the balance being Fe and unavoidable impurities. Hot rolling step of hot rolling so that the temperature of the base steel sheet at the end of is 800 ° C. or higher and 950 ° C. or lower, and then winding so that the temperature of the base steel sheet is 600 ° C. or lower. When,
A cold rolling step of pickling and cold rolling the hot-rolled steel sheet obtained in the hot rolling step, and a cold rolling step.
The annealing step of performing the following treatments (i) to (iii) in a furnace having a hydrogen concentration of 2% by volume or more and less than 20% by volume with respect to the cold-rolled steel sheet obtained in the cold rolling step.
A method for producing a hot-dip Zn-Al-Mg-based plated steel sheet, which comprises a hot-dip plating step of applying hot-dip Zn-Al-Mg-based plating to a cold-rolled steel sheet that has been annealed in the annealing step;
(I) The reduction reaction is promoted by heating the cold-rolled steel sheet to a temperature of 400 ° C. or higher and 900 ° C. or lower.
(Ii) The cold-rolled steel sheet after the treatment of (i) is soaked for 10 seconds or more so that the temperature becomes 650 ° C. or higher and 900 ° C. or lower.
(Iii) The cold-rolled steel sheet after the treatment of (ii) is cooled to a temperature of 500 ° C. or lower at an average cooling rate of 5 ° C./sec or more. The claim that the base steel sheet further contains one or more of Nb: 0 to 0.10%, Mo: 0 to 0.50%, and Cr: 0 to 0.50% in mass%. The method for producing a molten Zn-Al-Mg-based plated steel sheet according to 1 or 2. By mass%, C: 0.01 to 0.20%, Si: 0.01 to 0.50%, Mn: 0.1 to 2.50%, P: 0.005 to 0.050%, B: On the surface of a base steel sheet having a steel composition of 0.0005 to 0.010%, Ti: 0.01 to 0.20%, Al: 0.01 to 0.10%, and the balance being Fe and unavoidable impurities.
By mass%, Al: 1.0 to 22.0%, Mg: 1.3 to 10.0%, Si: 0 to 2.0%, Ti: 0 to 0.10%, B: 0 to 0. 05%, Fe: 2.0% or less, the balance has Zn and a Zn-Al-Mg-based coating layer which is an unavoidable impurity.
A molten Zn-Al-Mg-based galvanized steel sheet having a diffusible hydrogen concentration of 0.25 ppm or less in the base steel sheet. The molten Zn-Al-Mg-based galvanized steel sheet according to claim 4, wherein there are no cracks in the Zn-Al-Mg-based coating layer. The claim that the base steel sheet further contains one or more of Nb: 0 to 0.10%, Mo: 0 to 0.50%, and Cr: 0 to 0.50% in mass%. The hot-dip Zn-Al-Mg-based galvanized steel sheet according to 4 or 5.

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