JP3514837B2 - Hot-dip galvanizing method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dip coating method for forming a plating layer which has no defects such as non-plating and has excellent adhesion on the surface of a hot rolled steel sheet.

[0002]

2. Description of the Related Art In order to impart corrosion resistance to hot-rolled steel sheets, Z
Hot-rolled hot-rolled steel sheets produced by hot-dip-coating n, Al, Zn-Al alloys, etc. are manufactured and used in various fields including building materials. This hot-dip hot-rolled steel sheet is manufactured in a hot-dip galvanizing line of a Zenzimer type, a Cirrus type, etc. after descaling the hot-rolled steel sheet by pickling. However, when the hot rolled steel sheet is descaled by pickling, smut mainly composed of cementite Fe ₃ C remains on the surface of the steel sheet. When a hot-rolled steel sheet with smut adhered thereto is treated in a hot dip coating line, plating repellency (so-called non-plating) occurs due to smut that is not decomposed and removed during reduction heating.

In hot dip plated steel sheet in which non-plating has occurred,
Since the base steel is exposed at the non-plated portion, quality characteristics such as corrosion resistance are deteriorated. Therefore, the hot-dip galvanized steel sheet in which non-plating has occurred is treated as a defective product, which causes a reduction in yield. A method of mechanically descaling the surface of a steel plate with a grinding brush in order to eliminate non-plating (JP-A-49-
13037), a method of grinding the surface of a steel sheet after descaling and pickling (Japanese Patent Application Laid-Open No. 3-61352) and the like. Further, in Japanese Patent Laid-Open No. 3-24255, Ni
The method of pre-plating etc. is introduced.

[0004]

In the method of mechanically descaling with a grinding brush, the method of grinding a steel sheet surface after descaling and pickling, etc., when the steel sheet is curved or the steel sheet surface has microscopic unevenness. If there is, the brush is not evenly contacted, so that oxide scale or smut is likely to remain locally. The method of performing pre-plating with Ni or the like complicates the hot-dip plating line. That is, in the hot-dip plating line lozenges mer type or Cirrus type, the steel sheet oxide formed on the steel sheet surface when heated in the preheating furnace, in a reducing furnace which is maintained at H ₂ -N ₂ atmosphere reduction furnace It is reduced and removed by heating at high temperature for a long time. The gas-reduced steel sheet is cooled to near the bath temperature of the plating metal in a cooling furnace and then introduced into a hot dipping bath. If a pre-plating facility is additionally attached to this hot dip plating line, the manufacturing process and manufacturing conditions are complicated and the manufacturing facility is enlarged. As a result, the manufacturing cost is increased and it cannot be said to be a practical solution.

In recent years, Si and M have been used to increase the strength.
Demand for high-strength hot-rolled steel sheets containing alloy elements such as n and Cr is rapidly increasing, and when such hot-rolled steel sheets are heated at high temperature for a long time, Si, Mn, Cr, etc. are selectively oxidized and the steel sheet surface Concentrate to. This surface concentrated layer also causes non-plating. The present invention has been devised to solve such problems, and by rapidly heating the hot-rolled steel sheet after pickling / electrolytic cleaning to a predetermined temperature in a reducing atmosphere, a smut or a surface concentrated layer It is an object of the present invention to provide a hot-dip hot-dip galvanized steel sheet that suppresses the occurrence of non-plating caused by the above and has excellent adhesion and corrosion resistance.

[0006]

In order to achieve the object, the hot-dip galvanizing method of the present invention is to subject hot-rolled steel sheet to pickling and electrolytic cleaning, and then heat it in an atmosphere of H ₂ -N ₂ to form hot-dip metal. In the hot dipping method of immersing in a bath, the current density is 0.1 to 10 KA / m in a strong alkaline aqueous solution containing a chelating agent.
² and electrolysis for 1 to 60 seconds for electrolytic cleaning and desmutting. P. (℃) when a, M. a steel sheet after desmutting in H ₂ -N ₂ atmosphere P.
~ M. P. It is characterized in that hot-dip plating is performed after heating to a temperature range of + 100 ° C. The hot-rolled steel sheet after desmutting was subjected to M.V. at a heating rate of 50 ° C./sec or more in a H ₂ —N ₂ reducing atmosphere. P. ~ M. P. The temperature is raised to + 100 ° C.
The hot-rolled steel sheet to be hot-dipped is a medium carbon hot-rolled steel sheet or S
i: 0.2 wt% or more, Mn: 1 wt% or more and Cr:
There is a hot-rolled steel sheet containing 0.2% by weight of one kind or two or more kinds.

[0007]

Function: Electrolytic cleaning is a method known as a desmutting treatment method for metallic materials, and is widely adopted as a pretreatment for batch-type electroplating especially for steel materials. However, there is no example of applying this electrolytic cleaning method to the pretreatment of continuous hot dip plating. The present inventors have variously investigated and studied the applicability of electrolytic cleaning to a continuous hot-dip galvanizing line. As a result, it was found that when the hot-rolled steel sheet is subjected to descaling pickling and then electrolytic cleaning, the smut remaining on the surface of the steel sheet is uniformly removed and an activated surface suitable for hot dip coating is obtained. Further, the activation state of the steel sheet surface, when heated rapidly following the electrolysis washed with H ₂ -N ₂ atmosphere, was found to be maintained until a state suitable for hot dipping without being oxidized.

The present invention will be described in detail below. When the hot-rolled steel sheet is descaled and pickled, smut containing cementite as a main component remains on the surface of the steel sheet. In addition, Si, Mn, C
In the high-strength hot-rolled steel sheet to which r or the like is added, there are smuts such as Si-based oxides, Mn-based oxides, and Cr-based oxides in addition to residual smut. These smut are not well resolved or reduced even if a high temperature heating in H ₂ -N ₂ atmosphere. Therefore, in the portion where the smut remains, the reaction between the plated metal and the surface of the steel sheet is hindered, and non-plating is likely to occur. The smut remaining on the surface of the steel sheet is easily removed from the surface of the steel sheet by electrolytic cleaning. For electrolytic cleaning at this time, NaO
A strong alkaline aqueous solution containing H, KOH or the like as a main component is used. When a chelating agent containing an oxycarboxylic acid salt, an aminopolycarboxylic acid salt or the like as a main component is added to the strongly alkaline aqueous solution, the remaining smut is chelated and the desmutting effect is promoted. In this respect, it is preferable to use a strong alkaline aqueous solution containing a chelating agent as an electrolytic cleaning solution.

For electrolytic cleaning, an anodic electrolysis method, a cathodic electrolysis method,
A PR electrolysis method or the like in which the anode and the cathode are alternately switched is adopted. Oxygen gas is generated at the anode and hydrogen gas is generated at the cathode. The amount of gas generated at the anode is half that of the cathode. However, the desmutting effect is greater in the anodic electrolysis because it has a physical removing action by gas generation and a removing action by melting the steel sheet substrate. In addition, the PR electrolysis method in which the physical removal action due to the generation of gas and the removal action due to the dissolution of the steel sheet substrate act synergistically is more effective for desmutting. From this point, the anodic electrolysis method or the PR electrolysis method is suitable for desmutting. Current density of 0.1 to 10 for electrolytic cleaning
Electrolysis conditions of KA / m ² and electrolysis time of 1 to 60 seconds are adopted. If the current density is less than 0.1 KA / m ² or the electrolysis time is less than 1 second, the smut is not completely removed. Conversely, 10
At current densities above KA / m ² or electrolysis times above 60 seconds, unnecessarily large-scale electrolytic cleaning equipment is required,
Manufacturing cost is high.

The electrolytically cleaned steel sheet is washed with water and dried, and subsequently heated in an H ₂ —N ₂ atmosphere. Here, the melting point of the plating metal is M. P. (℃),
M. P. ~ M. P. It is rapidly heated to a temperature range of +100 (° C). The heating temperature is M. P. When it is below, the hot-rolled steel plate having a melting point of the plating metal or less is introduced into the plating bath, the plating metal in contact with the steel plate is solidified before reacting with the steel plate, and a necessary plating layer is not formed. Conversely, M. P. +100
Heating to a temperature above (° C.) not only wastes thermal energy consumed for heating, but also Si, Mn, C
r and the like are concentrated on the surface of the steel sheet, and non-plating is likely to occur.
Usually, in the hot dip plating, in order to suppress the growth of the alloy layer generated at the interface between the plating layer and the steel sheet, 0.14 to 0.18 wt% of Al in the hot dip Zn plating and approximately 9 wt% of the hot dip Al plating. Si is added to the plating metal bath.
Therefore, the melting point of the plated metal is about 420 ° C. for the hot-dip Zn plating and about 600 ° C. for the hot-dip Al plating. Therefore, the heating temperature range is about 420 to 520 ° C. for hot dip Zn plating and about 600 to 7 for hot dip Al plating.
Set to 00 ° C.

When the hot rolled steel sheet is heated to this temperature range,
It is necessary to set the temperature rising rate to 50 ° C./second or more. This is not a problem for hot-rolled steel sheets with normal levels of Si, Mn, or Cr, but for high-strength hot-rolled steel sheets with Si: 0.2 wt% or more, Mn: 1 wt% or more, or Cr: 0.2 wt% or more. , If the heating rate is low, the heating time becomes longer, and Si, M
N, Cr, etc. are concentrated on the surface of the steel sheet, and non-plating is likely to occur. The upper limit of the temperature rising rate is not particularly specified, but when considering a normal heating furnace, 200 ° C./second is preferable. A heating rate of more than 200 ° C./sec requires a large-scale heating furnace and causes an increase in manufacturing cost. Such a heating rate is achieved by, for example, high frequency induction heating, direct current heating, or the like.

By heating in an H ₂ -N ₂ atmosphere, the surface of the steel sheet desmutted by electrolytic cleaning is heated to a temperature suitable for hot dip coating in an active state without being reoxidized. It is not necessary to add H ₂ in an N ₂ atmosphere containing neither O ₂ nor H ₂ O at all, but it is practically difficult to obtain such an N ₂ atmosphere. Therefore, H ₂ is added to the N ₂ atmosphere at a ratio of 1 to 75% by volume. The antioxidant effect by the addition of H ₂ is exhibited at 1% by volume or more. But,
Even if H ₂ is added more than necessary, the effect corresponding to the increase of H ₂ is not exerted, and the production cost is rather increased. Further, since an ammonia decomposition gas is used industrially, the upper limit of the amount of H ₂ added is regulated to 75% by volume.

In the hot dipping method according to the present invention, equipment as shown in FIG. 1, for example, is used. Hot rolled steel plate 1
Is introduced into the pickling tank 2 and subjected to descaling pickling. For pickling, a normal method of descaling a hot-rolled steel sheet is adopted, but hydrochloric acid pickling is preferable because it has a high pickling rate and smut hardly remains. The hot-rolled steel sheet 1 that has been pickled is washed in a washing tank 3 and introduced into an electrolytic cleaning tank 4. In the electrolytic cleaning tank 4, the hot-rolled steel sheet 1 is immersed in a strong alkaline aqueous solution containing a chelating agent.
Is subjected to anodic electrolysis or PR electrolysis and desmutted. Next, the hot-rolled steel sheet 1 is washed again with water in the washing tank 5 and dried in the hot air drying furnace 6. After that, the hot-rolled steel sheet 1 is introduced into the heating furnace 7, and M. P. ~ M. P. Rapidly heated to a temperature of + 100 ° C. A seal roll 8 is provided on the inlet side of the heating furnace 7 to prevent atmospheric air from entering the furnace. The heating furnace 7 is connected a gas introduction pipe 9, a predetermined concentration of H ₂ in H ₂
-N ₂ atmosphere gas is introduced into the furnace. The heating means for the hot-rolled steel sheet 1 in the heating furnace 7 is not particularly limited, but high frequency induction heating, which has a high temperature rising rate and excellent controllability of the sheet temperature, and direct current heating are suitable.

The hot-rolled steel sheet 1 is introduced into the plating metal tank 10 and is hot-dipped when the sheet temperature reaches a predetermined temperature range. The hot-rolled hot-rolled steel sheet 1 that has been hot-dip plated is lifted vertically upward, the coating amount of the gas is adjusted by the gas wiping nozzle 11, and then cooled. As is clear from the layout shown in FIG. 1, since electrolytic cleaning is adopted for desmutting, smut is uniformly removed from the surface of the steel sheet after descaling and pickling. Moreover, H ₂ -N subsequent to electrolytic cleaning
₂ Immediately after hot-melting in an atmosphere, hot-dip galvanizing is performed, so high-strength hot-rolled steel sheet containing Si, Mn, Cr, etc. does not cause defects such as non-plating Is formed. Further, it is only necessary to arrange the electrolytic cleaning tank 4, the drying furnace 6, the heating furnace 7 and the plating metal tank 10 on the downstream side of the pickling equipment, and it is not necessary to heat the hot-rolled steel sheet 1 to a high temperature for a long time before plating. . Therefore, the equipment is more compact than the conventional Zenzimer type and Cirrus type, and an increase in equipment load can be avoided. as a result,
An excellent hot-dip Zn-plated hot-rolled steel sheet is produced at a high yield at a low production cost.

[0015]

EXAMPLES Using the equipment shown in FIG. 1, hot-dip Zn plating, Zn-4 wt% Al alloy plating, Zn-55 wt%
Al alloy plating and hot dip Al plating were applied to the hot rolled steel sheet. Manufacturing specifications are shown in Tables 1 to 8. The plating base plate is a medium carbon hot rolled steel plate (C: 2 mm thick and 1000 mm wide).
0.1% by weight, Si: 0.01% by weight, Mn: 0.4% by weight) and high-strength hot-rolled steel sheet (C: 0.1% by weight, Si:
1.2 wt%, Mn: 2 wt%, Cr: 0.2 wt%)
It was used. Bath temperature 8 at a plate speed of 100 m / min
It was introduced into a 20% by weight HCl pickling bath at 0 ° C. for 20 seconds to carry out pickling treatment.

The pickled hot-rolled steel sheet was immersed in warm water at 80 ° C for 5 hours.
After a water washing step of dipping for a second, it was introduced into an electrolytic cleaning bath maintained at a bath temperature of 40 ° C., and electrolytic cleaning was performed by anodic electrolysis.
As the electrolytic cleaning liquid, an aqueous solution prepared by adding 50 g / l of an oxycarboxylic acid salt (chelating agent) to 50 g / l of sodium hydroxide, and 50 g / l of an aminopolycarboxylic acid salt (chelating agent) added to 50 g / l of sodium hydroxide. Sodium hydroxide 50g / l without adding aqueous solution and chelating agent
Three types of aqueous solutions of The hot-rolled steel sheet subjected to electrolytic cleaning was introduced into a heating furnace equipped with a high-frequency induction heating device after a water washing step of immersing the hot-rolled steel sheet in hot water at 80 ° C. for 5 seconds. As the atmosphere of the heating furnace, O ₂ concentration: 5 × 10 ⁻³ vol% and dew point:
An H ₂ —N ₂ atmosphere gas at −20 ° C. was used. H of each example
₂ Concentration, heating rate, heating temperature, etc. are shown in Tables 1-8.

Various hot dip coatings were applied to the reduction-heated hot rolled steel sheet under the following conditions. Hot-dip Zn plating conditions: Composition of plating bath: Zn-0.18 wt% Al (MP
≈420 ° C.) Plating bath temperature: 460 ° C. Immersion time: 2 seconds Plating weight: 60 g / m ² per side Molten Zn-4 wt% Al Plating conditions: Plating bath composition: Zn-4 wt% Al (M. P. 38
0 ° C) Plating bath temperature: 460 ° C Immersion time: 2 seconds Plating coverage: 60 g / m ² per side

Molten Zn-55 wt% Al plating conditions: Composition of plating bath: Zn-55 wt% Al-1.5 wt%
Si of (M.P. ≒ 580 ℃) Plating bath temperature: 600 ° C. Immersion time: 2 seconds coating weight: per side 80 g / m ² melt Al plating conditions: Composition of plating bath: Al-9 wt% Si (M .P.≈60
0 ° C) Plating bath temperature: 650 ° C Immersion time: 2 seconds Plating coverage: 60 g / m ² per side

A sample having a length of 1 m was cut out from the hot-dipped coil and the platability was investigated by observing the presence or absence of non-plating with a loupe having a magnification of 10 times. The survey results are shown in Tables 1 to 8 together with the operational specifications. As shown in each table, when the conditions specified in the present invention are satisfied,
A good plating layer was formed with no occurrence of non-plating. On the other hand, test numbers 1, 15, 30, 44, 59, 73, 8 using an electrolytic cleaning solution without adding a chelating agent
In Nos. 8 and 102, smut remained on the surface of the hot rolled steel sheet after electrolytic cleaning, and non-plating was observed on the hot dip plated steel sheet. Moreover, the test numbers 2, 3, 16, 17, 31, 32, 45, 4 whose electrolytic cleaning conditions are out of the range specified in the present invention.
6,60,61,74,75,89,90,103,1
Also in No. 04, smut remained, and non-plating was observed on the hot-dip plated steel sheet.

Test number 4, 18, 33, 47, 62, 7
In Nos. 6, 91 and 105, the H ₂ concentration in the heating and reducing atmosphere was out of the range specified in the present invention, so that oxidation occurred on the surface of the steel sheet. Test No. 19, 48, 77 with slow heating rate,
In 106, Si, Mn, Cr, etc. are concentrated on the steel plate surface,
Non-plating occurred. In addition, test numbers 5, 6, 20, 21, 34, 35, 49, 50, 63, which have a low heating temperature,
64, 78, 79, 92, 93, 107, 108,
Since the steel plate temperature was lower than the melting point of the plated metal, the interfacial reaction between the steel plate surface and the plated metal did not proceed sufficiently, and non-plating occurred.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

[Table 4]

[0025]

[Table 5]

[0026]

[Table 6]

[0027]

[Table 7]

[0028]

[Table 8]

[0029]

As described above, in the present invention, the hot rolled steel sheet desmutted by electrolytic cleaning is treated with H ₂
By rapid heating in -N ₂ atmosphere, Si, M
The hot-rolled steel sheet is heated to a temperature range suitable for hot dipping while maintaining the active surface without the concentration of n, Cr, etc. on the surface. Therefore, the reaction between the surface of the steel sheet and the plated metal proceeds smoothly, and a hot-dip hot-rolled steel sheet without defects such as non-plating can be obtained. In addition, a compact equipment configuration is required compared to the conventional Zenzimer type and Cirrus type, which reduces manufacturing costs.

[Brief description of drawings]

FIG. 1 is an equipment configuration of a hot-dip galvanizing line in which the present invention is carried out.

[Explanation of symbols]

1: Hot rolled steel sheet 2: Pickling tank 3: Washing tank 4: Electrolytic washing tank 5: Washing tank 6: Hot air drying furnace 7: Heating furnace 8: Seal roll
9: Gas introduction pipe 10: Metal plating tank 11: Gas wiping nozzle

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI C25F 1/06 C25F 1/06 A (72) Inventor Toshiharu Tachibana 5 Ishizu Nishimachi, Sakai City, Osaka Prefecture Nisshin Steel Co., Ltd. Steel Research In-house (56) Reference JP-A-2-197520 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C23C 2/00-2/40 C21D 1/74 C22C 38/00 301 C22C 38/18 C22C 38/38 C25F 1/06

Claims (3)

(57) [Claims] 1. A hot rolled steel sheet is pickled and electrolytically washed, and then H ₂
-Heating in N ₂ atmosphere and immersing in hot dip metal bath
In the hot dipping method, a strong alkali containing a chelating agent
Current density 0.1 to 10 KA / m ^{2 in} aqueous solution and electrolysis
Electrolytic cleaning is performed under the condition of 1 to 60 seconds for desmutting, and the melting point of the plated metal is M. P. (℃) when a, M. a steel sheet after desmutting in H ₂ -N ₂ atmosphere P. ~ M. P. +
After heating to warm to the temperature range of 100 ° C., molten plating method of hot-rolled steel sheet, characterized by hot dipping. 2. After desmutting, the steel plate is heated at 50 ° C./sec or more.
M. at the heating rate. P. ~ M. P. Raised to a temperature of + 100 ° C
The hot dipping method according to claim 1, wherein the hot dipping is performed. 3. Si: 0.2 wt% or more, Mn: 1 wt
% Or more and Cr: 0.2% by weight of one or more kinds
The hot dip coating method according to claim 1 or 2, wherein a hot rolled steel sheet is used .