JP7234739B2 - Manufacturing method for high-strength hot-dip plated steel strip - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot dip plating method for metal materials, and more particularly to a hot dip plating method for steel strips.

Conventionally, hot-dip plated steel strip conforming to the standard of ASTM A653 Grade 80 (former name: ASTM A446 Grade E) is sometimes used in the field of construction, for example, where high strength is required. The hot-dip plated steel strip conforming to this standard uses, for example, steel materials to which C (carbon), Si (silicon), Mn (manganese), etc. are added, and is hot-rolled and cold-rolled under normal conditions. After that, in order to prevent the steel material from softening, the continuous annealing temperature is set below the recrystallization temperature and only recovery annealing is performed to ensure the strength of the steel material.In other words, recrystallization is performed after cold rolling. It was manufactured by hot-dip galvanizing without annealing. The recrystallization temperature of the steel material depends on the composition of the steel material and the amount of strain introduced into the steel material by cold rolling.

By the way, a general continuous hot-dip plating facility used for continuously plating a steel strip with a molten metal usually consists of a pretreatment facility, a heating reduction furnace, a hot-dip plating section (a hot-dip metal pot), and a post-treatment. Including equipment. The pretreatment equipment removes the rolling oil and dirt adhering to the steel strip. In the reduction heating furnace, the steel strip is heated in an atmosphere containing H2 to reduce Fe oxide present on the surface of the steel strip. In the hot-dip plating section, the steel strip processed in a reduction heating furnace is immersed or passed through a hot-dip metal plating bath while being held in a reducing atmosphere or in an atmosphere that prevents reoxidation of the steel strip surface. Hot-dip plating is applied to the band. The post-treatment equipment performs various treatments on the hot-dip plated steel strip depending on the application.

In the reduction heating furnace of the continuous hot-dip plating equipment, the material itself of the steel strip is annealed and the Fe oxide film present on the surface of the steel strip is reduced. This annealing treatment is for adjusting the metal structure of the steel strip so that mechanical properties such as tensile strength and elongation of the steel strip are desired. It is done by recrystallization by heating to temperature. Moreover, this reduction treatment is performed by heat-treating the steel strip in a mixed atmosphere of nitrogen gas and hydrogen gas, for example. In this heat treatment, the heating temperature is set according to the intended use of the plated product. The steel strip is heated to a temperature at least equal to or higher than the temperature of the hot dipping bath.

As described above, the heat treatment in the thermal reduction furnace of a general continuous hot-dip plating facility adjusts the mechanical properties of the steel strip and removes the oxide film on the surface of the steel strip. Because the mechanical properties are adjusted and the reactivity between the steel strip and the hot-dip galvanizing bath is good, the hot-dip galvanized steel strip is produced stably without the occurrence of plating defects such as non-plating and pinholes. be able to.

However, as described above, when producing hot-dip metal plated steel strips that require high strength, the temperature of the reduction heating furnace must be lower than the recrystallization temperature of the steel material. In this case, there is concern that the oxide film on the surface of the steel strip is not sufficiently removed, and sufficiently good reactivity between the steel material and the hot-dip plating bath cannot be obtained.

Some prior arts exist as countermeasures for such cases. Patent Document 1 is an invention of a non-recrystallized Zn-Al hot-dip plated steel sheet and its manufacturing method, wherein one or two of Ti (titanium), Nb (niobium), and B (boron) are used in low-carbon steel. By containing the above elements, the recrystallization temperature of the steel strip is increased by 100°C or more. As a result, a non-recrystallized Zn-Al hot dip plated steel sheet is produced which does not cause non-plating even when the steel is annealed at an annealing temperature exceeding 600°C and which provides a tensile strength of 50 kg/mm2 or more.

Patent Document 2 is an invention of a method for manufacturing a non-recrystallized hot-dip galvanized steel sheet in a continuous hot-dip galvanizing line. In the continuous hot-dip galvanizing line that manufactures ordinary general materials, a heating zone, a soaking zone and a cooling zone are arranged in this order in front of the plating bath. Although it is possible to manufacture non-recrystallized hot-dip galvanized steel sheets by lowering the temperature of the heating zone, if the temperature of the heating zone is lowered, the steel sheet exiting the soaking zone will be supercooled in the cooling zone. The temperature of the incoming steel sheet is much lower than that of general materials. As a result, plating defects such as dross drawing and wrinkles tend to occur on the surface of the steel plate after plating, and dross adheres to the bath rolls, which significantly shortens the life of the bath rolls. Since the steel sheet is heat-treated at a low temperature of 100°C, the surface of the steel sheet is not always sufficiently cleaned, and the problem is that poor coating adhesion tends to occur. To solve this problem, the invention of Patent Document 2 measures the temperature of the steel sheet entering the plating tank, and reheats the steel sheet based on the measured value by an induction heating device provided at the rear of the cooling zone. , the problem is solved by controlling the temperature of the steel sheet entering the plating tank so that it reaches the target temperature.
Here, the general material refers to a hot-dip plated steel plate obtained by heating the steel plate to a temperature equal to or higher than the recrystallization temperature, performing softening annealing and cleaning the steel plate surface, and then subjecting the steel plate to a predetermined hot-dip coating. .

JP-A-2-149655 JP-A-10-176253 (Patent Document 3) JP-A-4-41620

In order to produce a high-strength hot-dip plated steel strip, the present invention provides a cold-rolled steel strip with good plateability even if the heat treatment before plating is less than recrystallization annealing. It is an object of the present invention to provide a resulting hot dip plating method. In addition, "good plating property" in the present invention deals with "plating wettability between the molten metal and the steel strip".

In order to solve the above problems, the method for producing a high-strength hot-dip plated steel strip of the present invention comprises a cold-rolling step to obtain cold rolling by cold-rolling a hot-rolled steel strip or an annealed steel strip; A method for producing a hot-dip metal plated steel strip, which includes a plating step in which a cold-rolled steel strip is introduced into a metal plating bath to coat the cold-rolled steel strip with the molten metal, the cold-rolled steel After the cold rolling step, the strip is entered into a plating bath, the temperature of which is maintained below the recrystallization temperature of the cold rolled steel strip, the plating step applying the cold rolled steel to the molten metal. A method for producing a high-strength hot-dip metal plated steel strip characterized by coating the cold rolled steel strip with the molten metal while imparting vibrations in the plating bath while the strips are in contact.

In the present invention, the plating process is a plating process in which the cold-rolled steel strip is coated with the molten metal while applying vibration to the plating bath while the cold-rolled steel strip is in contact with the molten metal. Since the plating wettability between the molten metal to the steel strip can be ensured, even when the temperature of the cold rolled steel strip is maintained below the recrystallization temperature of the cold rolled steel strip and enters the plating bath, Not only can the plating wettability between the molten metal and the cold-rolled steel strip be ensured, but also the cold-rolled steel strip can be molten metal-plated without softening due to recrystallization annealing. Therefore, the plastic working strain introduced into the cold-rolled steel strip is maintained even after the hot-dip metal plating, so that a high-strength hot-dip metal-plated steel strip can be obtained.

(a) is a schematic diagram showing how a steel sheet enters a plating bath in an air atmosphere, and (b) is a schematic enlarged view of the area (A1) in the diagram shown in (a). It is an enlarged view. (Same as Fig. 10 of the basic application) It is an acoustic spectrum observed when vibration is applied to the plating bath using an ultrasonic vibrator with an output of 380 W. (Same as Figure 11 of the basic application) 1 is a schematic diagram showing the configuration of a plating apparatus; FIG. (Same as Figure 9 of the basic application) FIG. 4 is a side view showing the state of the test material after plating; (Same as Figure 6 of the basic application)

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following description is for better understanding of the gist of the invention, and does not limit the invention unless otherwise specified. In addition, in the present application, "A to B" indicates A or more and B or less. The shapes and dimensions of the configurations shown in each drawing in this application do not necessarily reflect the actual shapes and dimensions, and are changed as appropriate for clarity and simplification of the drawings.

(Definition of terms)
In this specification, various molten metals (molten metals) constituting the hot dipping bath are sometimes referred to as "hot dipping bath metals". Further, in this specification, the material and shape of the steel material to be hot-dip plated using the hot-dip plating bath are not particularly limited unless otherwise specified. In addition, "steel plate" may be read as "steel strip" as long as there is no inconvenience.

<Schematic description of findings of the invention> (Fig. 1 is the same as Fig. 10 of the basic application)
Generally, (i) a steel sheet (steel strip) that has not been subjected to reduction treatment is allowed to enter the hot dipping bath, or (ii) the steel sheet is allowed to enter the hot dipping bath in an atmospheric (high oxygen concentration) atmosphere without using a snout. If it is allowed to enter, the reaction between the steel sheet and the hot-dip plating bath metal is hindered, and good plating properties cannot be obtained. The reason for this will be described in detail with reference to FIG. 1 as follows. FIG. 1(a) is a schematic diagram showing how a steel sheet is introduced into a hot-dip plating bath in an air atmosphere. (b) of FIG. 1 is a partially enlarged view schematically showing an enlarged area (A1) of the figure shown in (a).

As shown in FIG. 1(a), a steel sheet 100 that has not undergone a reduction treatment is introduced into a hot-dip plating bath 110 in an air atmosphere. An oxide film is formed on the surface of the steel plate 100 . Further, in the hot-dip plating bath 110, a bath surface oxide 112 exists at the boundary between the hot-dip plating bath metal 111 inside the plating bath and the outside of the plating bath (atmosphere).

As shown in FIG. 1B, the steel sheet 100 has (i) a bath surface oxide 112 and (ii) an air entrainment layer 120 formed by the atmospheric gas (air) on the surface of the hot dipping bath 110. It enters the hot-dip plating bath 110 in a rolling manner. As a result, a reaction inhibition portion 130 is formed between the hot dipping bath metal 111 and the oxide film 101 of the steel sheet 100 inside the hot dipping bath 110 . This reaction inhibition portion 130 is formed in a composite manner by the bath surface oxide 112 and the air entrapment layer 120 . Since the reaction between the steel sheet 100 and the hot-dip plating bath metal 111 is inhibited by the oxide film 101 and the reaction inhibition portion 130, the surface of the plated product pulled up from the hot-dip plating bath 110 has plating defects (pinholes or unplated etc.) easily occurs.

Therefore, in the hot dip plating method in the prior art, as described above, the steel sheet in which the oxide film on the surface of the steel sheet has been reduced using a heating furnace is introduced into the plating bath through the snout held in a reducing atmosphere (for example, , see Patent Documents 1 and 2). In this case, when the steel sheet enters the plating bath, the reaction between the steel sheet and the hot dipping bath metal proceeds rapidly.

However, in this conventional hot-dip plating method, the reactivity between the steel sheet and the hot-dip plating bath metal is obtained by reducing the oxide film on the surface of the steel sheet. progress of recrystallization is inevitable. Rather, in the conventional hot-dip plating method, passing the steel sheet through a heating furnace not only reduces the oxide film on the surface of the steel sheet, but also promotes recrystallization of the metal structure of the steel sheet, that is, the fact that annealing can be performed. Then, the temperature of the heating furnace and the residence time of the steel sheet in the heating furnace are controlled so that the metal structure of the steel sheet that has passed through the heating furnace has predetermined mechanical properties (e.g., yield strength, tensile strength, etc.) has become common.

However, when it is desired to manufacture a hot-dip galvanized steel strip for use in applications requiring high strength, the annealing temperature should be limited to below the recrystallization temperature so that recrystallization of the steel strip itself does not proceed, as described above. In other words, it was necessary to manufacture the cold-rolled steel strip by hot-dip galvanizing without performing recrystallization annealing. However, even in that case, for example, because the temperature of the heating furnace is limited to below the recrystallization temperature, depending on the composition of the steel strip, the oxide film on the surface of the steel strip cannot be sufficiently reduced, resulting in poor plating properties. In some cases, high-strength hot-dip galvanized steel strips could not be produced due to the lack of such properties.

In the case of hot-dip plating containing Al (aluminum) as a main component, such as 55% by mass Al--Zn (zinc)-based plating or Al-9% by mass Si (silicon)-based plating, the plating bath temperature is It will be around 660°C. If the temperature of the cold-rolled steel strip is adjusted to this temperature by the hot-dip plating method in the prior art and it is attempted to enter the plating bath, this temperature exceeds the recrystallization temperature of the cold-rolled steel strip. It has not been possible to produce strong hot-dip aluminized steel strips.

The inventors of the present invention have diligently studied a hot dip plating method for obtaining a high strength hot dip plated steel strip by a new method different from the above-described conventional techniques. As a result, when the steel strip enters the hot dipping bath, the reactivity between the steel strip and the hot dipping bath metal can be enhanced by the vibration activation effect generated by applying vibrations under specific conditions to the hot dipping bath. I found a new knowledge that it can be done. According to this finding, even when a steel strip at room temperature is introduced into a hot-dip plating bath in an air atmosphere, the plateability of the steel strip can be improved. By utilizing the vibration activation effect produced by applying vibration under this specific condition, good plating properties between the steel strip and hot-dip metal can be obtained even if the temperature of the heating furnace is limited to below the recrystallization temperature. As a result, a high-strength hot-dip galvanized steel strip can be produced regardless of the composition of the steel strip.

In addition, even if the hot-dip plating is a plating containing Al (aluminum) as a main component, the temperature of the cold-rolled steel strip can be controlled by utilizing the vibration activation effect generated by applying vibration under this specific condition. Good platability can be obtained even if the temperature is adjusted to be much lower than the bath temperature and less than the recrystallization temperature before entering the hot-dip galvanizing bath, making it possible to manufacture high-strength hot-dip aluminized steel strip rice field.

The difference between the knowledge found by the present inventors and the prior art will be described in more detail below. That is, conventionally, a technology has been proposed in which a high-power (for example, several hundred W class) ultrasonic transducer is used to apply vibration of high sound pressure to a plating bath. A spectrum (a white noise-like spectrum with few characteristic peaks) is observed. FIG. 2 shows an acoustic spectrum observed when a plating bath is vibrated using an ultrasonic vibrator with an output of 380 W. FIG. This type of technology physically destroys the oxide film existing on the surface of the steel sheet (or the oxide film remaining on the surface of the steel sheet after reduction treatment) by using the cavitation effect of irradiating the plating bath with high-power ultrasonic waves. By doing so, the plateability of the steel sheet was improved.

On the other hand, the present inventors found that the vibration activation effect of the present invention is recognized even when a low-output ultrasonic transducer is used, and that the plateability of the steel sheet is effectively improved. Found it. In this case, a characteristic peak is observed in the acoustic spectrum, which will be specifically described later. The inventors of the present invention consider the above-described vibration activation effect, which is different from the prior art and is exhibited even at a low sound pressure, as follows.

Specifically, although it is not clear yet, even when a low sound pressure is applied to the hot dipping bath, the hot dipping metal in the molten state undergoes pressure vibration due to sound waves, and this vibration causes air bubbles in the plating bath. occurs. Then, when the generated bubble is crushed by the pressure vibration, it is considered that a shock wave is generated toward the periphery of the bubble. Moreover, it is considered that pressure vibration causes the bubble to expand and contract repeatedly, and it is also conceivable that this expansion and contraction causes a local flow of hot-dip plated metal around the bubble. The effects of the shock wave and the local flow based on the acoustic energy promote mass transfer at the interface between the steel material and the plating bath, resulting in effects such as reducing the thickness of the boundary layer or increasing the mass transfer speed. A possible mechanism is that this ensures the wettability between the steel material and the plating bath.

In addition, it is considered that the phenomenon of accelerated mass transfer at the interface between the steel material and the plating bath also occurs in the conventional technology (when applying high sound pressure vibrations to the plating bath). However, according to the findings of the present invention, it is not necessary to apply high sound pressure vibration to the plating bath, and the energy of the vibration is just enough to generate a vibration activation effect that can ensure the wettability between the steel material and the plating bath. It turned out to be good. In addition, the conventional technique of applying high sound pressure vibrations to the plating bath has the following disadvantages.

In other words, when a plating bath is subjected to high sound pressure vibrations, the cavitation effect that occurs simultaneously with the shock wave and local flow quickly dissolves the steel material into the plating bath, easily causing a corrosion phenomenon called erosion. Inconvenience will occur. If the steel material is a steel sheet, this means that the thickness of the steel sheet after hot-dip plating is smaller than before entering the hot-dip plating bath, making it difficult to guarantee the product thickness of the hot-dip plated steel sheet. I have concerns. In addition, the reaction in which the steel material dissolves in the plating bath increases the concentration of steel components such as iron (Fe) in the plating bath, and as a result, there is a concern that dross is likely to occur. be. Furthermore, erosion of members immersed in the plating bath is likely to occur in order to apply vibrations of high sound pressure to the plating bath, and maintenance of these members becomes complicated.

A brief description of the hot-dip plating method based on the findings of the present inventors is as follows. That is, for example, by applying ultrasonic vibrations to the hot dipping bath using a vibration plate, low sound pressure vibrations are applied to the hot dipping bath. Then, the acoustic spectrum is measured using an acoustic measuring instrument immersed in the hot dipping bath. In this hot-dip plating method, the ultrasonic vibration is applied to the hot-dip plating bath so that the acoustic spectrum satisfies a predetermined condition. A vibration activation effect is produced in the hot-dip plating bath by ultrasonic vibrations imparted to the steel or diaphragm. The predetermined conditions are defined to indirectly specify the degree of strength of the vibration activation effect using the acoustic spectrum in the hot dipping bath so that a vibration activation effect above a certain level is generated.

In one aspect of the method for producing a high-strength hot-dip plated steel strip of the present invention, the hot-dip plating method includes allowing a steel material to enter a plating bath that is a molten metal, and plating the steel material while the steel material is in contact with the molten metal. It includes a plating step of coating the steel material with the molten metal while imparting vibration to the bath. The frequency of the vibration given to the plating bath is defined as a fundamental frequency. In the plating step, the vibration is applied so that the acoustic spectrum measured in the plating bath satisfies the relationship of formula (1) below.

(IB-NB)/(IA-NA)>0.2 (1)
(here,
IA: Average sound pressure over the entire frequency band measured IB: (i) between the sound pressure peak at the fundamental frequency and the sound pressure peak at the second harmonic frequency, and (ii) the sound pressure at a plurality of harmonic frequencies Average value of sound pressure in a specific frequency band between adjacent peaks NA: Average value of sound pressure in the entire measurement frequency band when the vibration is not applied NB: The above specified for the IB It is the average value of sound pressure in a specific frequency band when the above vibration is not applied)

(advantageous effect)
As described above, according to the hot-dip plating method in one aspect of the method for producing a high-strength hot-dip plated steel strip of the present invention, while the steel plate 2 and the plating bath are in contact with each other, the steel plate is vibrated so as to satisfy the predetermined conditions. 2. As a result, the bath surface oxide 22 and air entrained in the plating bath are dispersed in the bath. That is, the reaction inhibitor is dispersed in the bath. In addition, mass transfer is promoted at the interface between the steel material and the plating bath, resulting in effects such as reducing the thickness of the boundary layer or increasing the mass transfer rate. This ensures wettability between the steel material and the plating bath. Therefore, the hot-dip plating bath metal 21 and the steel plate 2 react smoothly. As a result, even when the steel sheet 2 that has not been subjected to heat treatment (reduction treatment) in advance is used, the plateability of the steel sheet 2 can be improved. Alternatively, the steel plate 2 may be preheated to a temperature lower than the recrystallization temperature of the steel plate. While the steel sheet 2 and the plating bath are in contact with each other, the steel sheet 2 is subjected to vibrations that satisfy predetermined conditions, so that the wettability between the hot-dip plating bath metal 21 and the steel sheet 2 is good, and the steel sheet is regenerated. It is possible to provide a method for producing a high-strength hot-dip metal plated steel strip that is hot-dip metal plated without crystal annealing.

(Fig. 9 of the basic patent, paragraphs 0098-0104, paragraphs 0070-0073)
(Hot-dip plating equipment)
A hot dip plating apparatus 60 for carrying out the hot dip plating method in this embodiment will be described with reference to FIG. The hot-dip plating apparatus 60 is an example, and the apparatus for carrying out this hot-dip plating method is not particularly limited. FIG. 9 is a schematic diagram showing a hot dip plating apparatus 60 for carrying out the hot dip plating method according to this embodiment.

As shown in FIG. 3, the hot-dip plating apparatus 60 includes a gas reduction heating zone 61, a hot-dip plating section 62, an ultrasonic horn 10, and a measuring device 50 for measuring an acoustic spectrum. The gas reduction heating zone 61 includes an atmospheric gas introducing portion 61a and a heating portion 61b, and is capable of performing heat treatment on the steel plate 2 in a desired atmosphere.

(Structure of hot-dip plated part)
As shown in FIG. 3 , the hot-dip plating section 62 of the hot-dip plating apparatus 60 includes a crucible furnace 41 containing a carbon crucible 42 , and heats the carbon crucible 42 by generating resistance heating in a heating zone 43 . do. Hot-dip plating bath metal 21 is stored in carbon crucible 42 , and bath surface oxide 22 may be formed on the surface of hot-dip plating bath metal 21 .

(Measuring device for measuring acoustic spectrum)
The hot-dip plating section 62 also includes a waveguide rod 51, an acoustic emission sensor (hereinafter sometimes referred to as an AE sensor) 52, and a measuring section 53 as a measuring device 50 for measuring the acoustic spectrum. Measurement unit 53 includes a spectrum analyzer and an amplifier. One end of a waveguide rod 51 is immersed in the molten plating bath metal 21 and the other end is connected to an AE sensor 52 .

Various devices used in the measurement unit 53 of the hot dip plating apparatus 60 in this embodiment are specifically as follows.
・Waveguide rod 51: Made of SUS430, φ6mm×300mm
・ AE sensor 52: NF circuit design block, AE-900M
・Amplifier: NF Circuit Design Block Co., Ltd., AE9922
• Spectrum Analyzer: Agilent, E4408B.

(Gas reduction heating zone)
A gate valve 63 is provided between the gas reduction heating zone 61 and the hot-dip plating section 62 . The steel sheet 2 treated in the gas reduction heating zone 61 opens the gate valve 63 and is transferred to the hot-dip plating section 62 without being exposed to the atmosphere. The steel sheet 2 is subjected to pretreatment such as atmosphere control and heating in the gas reduction heating zone 61 above the gate valve 63 , and then enters the hot dipping bath metal 21 .

In the hot-dip plating section 62 , the space above the crucible furnace 41 is shielded from the atmosphere by a port flange 64 and an O-ring 65 . An atmosphere gas introducing portion 66 is provided in a part of the port flange 64 so that the atmosphere in the hot dipping portion 62 can be controlled.

(diaphragm)
Further, in the hot-dip plating apparatus 60 of the present embodiment, a vibration plate 70 is fixed to the tip of the ultrasonic horn 10 , and this vibration plate 70 imparts vibration to the hot-dip plating bath metal 21 . This gives vibration to the steel plate 2 . That is, the hot-dip plating apparatus 60 indirectly applies vibrations to the steel sheet 2 . Note that the diaphragm 70 is not limited to the above materials. The vibrating plate 70 preferably has high corrosion resistance when immersed in the hot dipping bath and poor wettability with respect to the hot dipping bath. For example, ceramics can be used.

(vibration frequency)
The ultrasonic horn 10 may apply vibrations with a frequency of 1 kHz to 150 kHz to the diaphragm 70, for example. As an example of a specific vibration frequency, it can be 20 kHz. The intensity of vibration applied to diaphragm 70 by ultrasonic horn 10 (output of ultrasonic transducer 11) may be set so as to satisfy the above conditions. For example, the level of output of the ultrasonic transducer 11 that satisfies the above conditions may be investigated in advance for each condition of the steel plate 2 and the hot dipping bath 21 .

(Hot-dip plating bath)
As the hot-dip plating bath 21, various known hot-dip plating baths can be used. Examples of hot-dip plating baths include zinc (Zn)-based plating baths, Zn-aluminum (Al)-based plating baths, Zn-Al-magnesium (Mg)-based plating baths, and Zn-Al-Mg-silicon (Si)-based plating baths. bath, Al-based plating bath, Al--Si-based plating bath, Zn--Al--Si-based plating bath, Zn--Al--Si--Mg-based plating bath, tin (Sn)--Zn-based plating bath, and the like.

The hot dip plating apparatus 60 as described above can be applied to a continuous hot dip plating method. That is, although it is difficult to apply vibration directly to the steel sheet in the continuous hot dipping method, it is possible to apply vibration indirectly to the steel sheet 2 as in the hot dip plating apparatus 60 described above. Therefore, the results demonstrated using the hot dip plating apparatus 60 as described above can be applied to the continuous hot dip plating method.

(Plating base material)
Plating base materials A and B shown in Table 1 below and plating base material G shown in Table 2 were used.

(cold rolling)
The annealed materials of the plated base materials A, B, and G are cold-rolled, and the plated base material A is 0.8 mm thick, the plated base material B is 1.4 mm thick, and the plated base material C is 1.4 mm thick. It was finished to 1.0 mm and used as a plating base plate.
As mentioned above, the general continuous hot-dip plating equipment used for continuously plating a steel strip with molten metal usually includes pretreatment equipment, a heating reduction furnace, a hot-dip plating section (a hot-dip metal pot ), and post-treatment equipment. The recrystallization temperature of the steel strip when passing through the heating reduction furnace using such equipment is considered to be about 550°C for the plated base sheets A and B, and about 630°C for the plated base sheet G.

(Hardness evaluation of plated base plate)
A part of the original plate was cut out and embedded in resin. Then, the resin was cut and the surface was polished, and the hardness of the plated base plate was measured with a microhardness tester on the cross section of the plated base material. The test force was 0.2 gf.
The hardness of the plating base sheet finished to a predetermined thickness was as follows.
・Plating original plate A: 257
・Plating original plate B: 273
・Plating original plate C: 287

(Preprocessing)
The plated blank was subjected to alkaline electrolytic degreasing as a pretreatment immediately before being charged into the gas reduction heating zone 61 of the hot-dip plating apparatus 60 .

(Plating bath, plating bath temperature)
In this embodiment, the plating bath and the plating bath temperature of the hot-dip plating portion 62 were set as follows.
・Zn-6% by mass Al-3% by mass Mg plating bath, 460°C
・Al-9% by mass Si plating bath, 580°C

In this example, three types of heat treatment temperatures and four types of heating atmospheres were used for the plating stock sheet in the gas reduction heating zone 61 .
(Heating atmosphere of plated base plate)
・Atmosphere ・N2
・ 3 vol% H2-N2
・30 vol% H2-N2

(Heat treatment of plating base plate)
・No heating (normal temperature)
・400℃
・600℃
The above 400° C. and 600° C. are the temperatures reached by the heating portion 61b of the gas reduction heating zone 61, and the required time from the start of heating to the reached temperature is about 10 seconds.

(Diaphragm, vibration application conditions, acoustic spectrum)
As the vibration plate 70 in the hot-dip plated portion 62, a plate made of ordinary steel (the same steel type as the steel plate A in Table 1) and having a length of 150 mm, a width of 50 mm, and a thickness of 0.8 mm was used. The vibration frequency of the ultrasonic horn was 15 kHz, and the output was 30W. Further, when the steel plate 2 is immersed in the plating bath 21, the distance between the steel plate 2 and the diaphragm 70 is 5 mm. (IB-NB) and the ratio of the average intensity and the acoustic intensity between positive overtones (IB-NB)/(IA-NA) were measured.

(hot dip plating)
When the heat treatment is not performed in the gas reduction heating zone 61, the steel plate is immersed in the hot dipping bath 21 for 12 seconds, of which vibration is applied by the ultrasonic horn 10 for the last 2 seconds, after which the steel plate 21 is melted. It was pulled up from the plating bath.
When the heat treatment at 400° C. or 600° C. is performed in the gas reduction heating zone 61, first, the steel plate 21 is immersed in the hot dipping bath 21 after starting the vibration of the hot dipping bath 21 by the ultrasonic horn 10, and is held for 2 seconds. The steel plate 21 was pulled up from the hot-dip plating bath 21 later.

(Plating wettability evaluation)
Using a sample after hot-dip plating as a test material, the plating property was evaluated as follows. FIG. 4 is a side view showing the state of the test material 3 after plating. As shown in FIG. 4, a plated region 3a is formed on the specimen 3 after plating by hot-dip plating. In addition, a non-plated portion 4 that is not hot-dip plated may exist in a part of the plated region 3a.

For example, the depth of the portion of the test material 3 immersed in the hot-dip plating bath is L11, and the width of the test material 3 is L12. In this case, on the plate surface shown in FIG. 6, L11×L12 is the ideal area α of the plating region. Also, the area β of the unplated portion 4 is measured using a known area measuring means. Then, the non-plating rate was calculated by calculating (β/α)×100. The plating properties of test material 3 were evaluated according to the following criteria.

◎: Non-plating rate is 0%
◯: Non-plating rate is greater than 0% and less than 1% △: Non-plating rate is 1% or more and less than 10% ×: Non-plating rate is 10% or more and less than 80% XX: Non-plating rate is 80% or more.

(Evaluation of hardness after hot-dip plating)
A part of the test material 3 for which the non-plating rate was evaluated was cut out and embedded in resin. Then, the resin was cut, the surface was polished, and the hardness of the plated steel sheet was measured with a microhardness tester on the cross section after the hot dip plating. Also in this case, the test force was 0.2 gf.
The hardness after hot-dip plating was evaluated by comparing the hardness after hot-dip plating with respect to the original sheet before hot-dip plating, and the ratio was evaluated as follows. Those with an evaluation of ◯ were regarded as acceptable.
○: hardness after hot-dip plating is 90% or more compared to before hot-dip plating ×: hardness after hot-dip plating is less than 90% compared to before hot-dip plating

(comprehensive evaluation)
○: Evaluation of wettability is △ or more, and hardness evaluation after hot dipping is also ○ ×: Evaluation of wettability is × or less, or hardness evaluation after hot dipping is × thing

(Results of Zn-6% by mass Al-3% by mass Mg plating)
Table 3 summarizes the evaluation results when the hot-dip plating bath is a Zn-6% by mass Al-3% by mass Mg plating bath. No. In 101 to 112, the plating base plate was not heated, but the plating was performed by applying vibration to the hot dip plating bath under conditions such that the acoustic spectrum within the scope of the present invention was measured in the hot dip plating bath. The non-plating rate of the product was 0%, and no decrease in hardness of the plated product was observed.

No. In 201 to 212, the plated base plate was heated at 400°C. The heating atmosphere was changed to air, nitrogen, 3% by volume hydrogen-nitrogen, and 30% by volume hydrogen-nitrogen. Of these, the plated products heated in the air (Nos. 201 to 203) had a non-plating rate of less than 10%. The plated products (Nos. 204 to 212) heated in nitrogen, 3% by volume hydrogen-nitrogen, and 30% by volume hydrogen-nitrogen atmospheres had a non-plating rate of 0%. Further, no decrease in the hardness of the plated products was observed in any of the plated products. This is presumably because the heating temperature of 400° C. is below the recrystallization temperature for all of the plating blanks A, B, and G, so that they did not soften.

No. In 301 to 312, the plated base plate was heated at 600°C. Among these, No. 1 using plating original plate A. 301 and 310 were greatly softened. It is considered that this is because recrystallization progressed by heating at 600°C. On the other hand, No. For 303 and 312, no decrease in the hardness of the plated products was observed. This is presumably because recrystallization did not proceed by heating at 600°C because the original plating sheet G was used and the recrystallization temperature was considered to be 630°C.

(Results of Al-9 mass% Si plating)
Table 4 summarizes the evaluation results when the hot-dip plating bath is an Al-9% by mass Si plating bath. No. In 401 to 412, the plating base plate was not heated, but the plating was performed by applying vibration to the hot dipping bath under conditions that allow the acoustic spectrum within the scope of the present invention to be measured in the hot dipping bath. The non-plating rate of the product was 0%, and no decrease in hardness of the plated product was observed.

No. In 501 to 512, the plated base plate was heated at 400°C. The heating atmosphere was changed to air, nitrogen, 3% by volume hydrogen-nitrogen, and 30% by volume hydrogen-nitrogen. Of these, the non-plating rate was less than 10% for the plated products heated in the atmosphere (Nos. 501 to 503). Plated products (Nos. 504 to 512) heated in nitrogen, 3% by volume hydrogen-nitrogen, and 30% by volume hydrogen-nitrogen atmospheres had a non-plating rate of 0%. Further, no decrease in the hardness of the plated products was observed in any of the plated products. This is presumably because the heating temperature of 400° C. is below the recrystallization temperature for all of the plating blanks A, B, and G, so that they did not soften.

No. In 601 to 612, the plating base plate was heated at 600°C. Among these, No. 1 using plating original plate A. 601, 604, 607, 610 and No. 602, 605, 608 and 611 were greatly softened. It is considered that this is because recrystallization progressed by heating at 600°C. On the other hand, No. 603, 606, 609 and 612 showed no decrease in hardness of the plated products. This is presumably because recrystallization did not proceed by heating at 600°C because the original plating sheet G was used and the recrystallization temperature was considered to be 630°C.

In this way, when the steel strip enters the hot dipping bath, the reactivity between the steel strip and the hot dipping bath metal is enhanced by the vibration activation effect generated by applying vibrations under specific conditions to the hot dipping bath. Even if the temperature of the heating furnace is limited to less than the recrystallization temperature, good platability can be obtained between the steel strip and the hot-dip plated metal. It has become possible to manufacture strong hot-dip galvanized steel strips.

In addition, even if the hot-dip plating is a plating containing Al (aluminum) as a main component, the temperature of the steel strip can be lowered from the plating bath temperature by utilizing the vibration activation effect generated by applying vibration under this specific condition. Even if the temperature is adjusted to be lower than the recrystallization temperature and entered into the hot-dip plating bath, good plating properties can be obtained.

Claims (5)

a cold-rolling step of cold-rolling a hot-rolled steel strip or an annealed steel strip to obtain a cold-rolled steel strip ;
A method for producing a high-strength hot- dip plated steel strip, comprising a plating step of immersing the cold-rolled steel strip in a plating bath of molten metal to coat the cold-rolled steel strip with the molten metal. hand,
After the cold-rolling step, the cold-rolled steel strip is maintained at a temperature below the recrystallization temperature of the cold-rolled steel strip, and (i) has an oxidized surface without being subjected to a reduction treatment. (ii) entered into the plating bath in a state of having a film, or (ii) in a state of being subjected to a reduction treatment at a temperature of 600 ° C. or less and less than the recrystallization temperature;
In the plating step, the cross-sectional hardness of the high-strength hot-dip plated steel strip obtained by the plating step is 90% or more of the cross-sectional hardness of the cold-rolled steel strip, and the high-strength hot-dip plated steel strip is The molten metal is applied to the cold-rolled steel strip while imparting vibrations in the plating bath while the cold-rolled steel strip is in contact with the molten metal such that the non-plating rate of the cold-rolled steel strip is less than 10%. A method for producing a high-strength hot-dip plated steel strip, characterized by coating with With the frequency of the vibration applied to the plating bath as the fundamental frequency,
The high-strength hot dip plated steel strip according to claim 1, wherein the vibration is applied so that the acoustic spectrum measured in the plating bath satisfies the relationship of the following formula (1): Production method.
(IB-NB)/(IA-NA)>0.2 (1)
(here,
IA: Average value of sound pressure over the entire measurement frequency band IB: (i) between the sound pressure peak at the fundamental frequency and the sound pressure peak at the second harmonic frequency, and (ii) the sound pressure at a plurality of harmonic frequencies Between the adjacent peaks, the average value of the sound pressure in a specific frequency band NA: The average value of the sound pressure in the entire measurement frequency band when the vibration is not applied NB: Defined for the IB It is the average value of the sound pressure in the above specific frequency band when the above vibration is not applied) The plating bath is a Zn-6% by mass Al-3% by mass Mg plating bath,
The cold-rolled steel strip is subjected to (i) a state of having an oxide film on the surface without being subjected to a reduction treatment, or (ii) a state of being subjected to a reduction treatment at a temperature of 400 ° C. or less, and then subjected to the plating. 3. The method for producing a high-strength hot-dip galvanized steel strip according to claim 1 or 2, wherein the step is carried out in a bath. 3. The method for producing a high-strength hot-dip galvanized steel strip according to claim 2, wherein the plating bath is a 55% by mass Al--Zn-based plating bath or an Al-9% by mass Si-based plating bath. The method for producing a high-strength hot-dip plated steel strip according to any one of claims 1 to 4, wherein the non-plating rate of the high-strength hot-dip plated steel strip is less than 1%.

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