JPH06128758A - Method for hot dip galvanizing steel sheet containing silicon - Google Patents

Abstract

(57) [Summary] [Purpose] To improve the plating property of high Si steel. [Constitution] The surface of a steel sheet containing 0.2% by weight or more of Si was electroplated with at least one of Ni, Fe, Cu and Co in advance (adhesion amount: 5 to 70 mg / m ² ), and then in an oxidizing atmosphere.
Iron oxide is formed on the surface of the steel sheet by heating at 00 ° C or lower (0.3 to 1.5 g / m ² in terms of Fe), reducing at 500 to 650 ° C, and then hot dip galvanizing. Further, alloying treatment may be performed. [Effect] It is possible to manufacture a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which have no unplating and have excellent surface quality.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot dip galvanizing method capable of plating a low alloy steel sheet, particularly a silicon-containing steel sheet, without causing non-plating.

[0002]

2. Description of the Related Art In recent years, a large amount of galvanized steel sheets have been used in the industrial fields of home appliances, building materials, automobiles, etc., but especially alloying fusion which is excellent in economic efficiency and its rust prevention function and performance after painting. Galvanized steel sheets are widely used.

Hot-dip galvanized steel sheets are usually subjected to an appropriate degreasing and washing step or without degreasing and washing,
Manufactured by preheating a steel sheet in a weakly oxidizing atmosphere or a reducing atmosphere, annealing it in a reducing atmosphere of hydrogen + nitrogen, then cooling it to a temperature suitable for plating, and then immersing it in molten zinc. It It is considered desirable to form an oxide film with a thickness of about 80 nm on the surface of the steel sheet during preheating in the pre-annealing process from the viewpoint of wettability with molten zinc. Rather, it is thought to increase the generation of dross and adversely affect the adhesiveness of hot dip coating. As the zinc bath, a bath containing 0.08 to 0.18% by weight of Al is used in consideration of the production range of the alloyed hot-dip galvanized steel sheet described below.

The alloyed hot-dip galvanized steel sheet is usually prepared by heating a hot-dip galvanized steel sheet as described above in a heat treatment furnace at a material temperature of 500 to 600 ° C. for 3 to 30 seconds to form a zinc layer. It is manufactured by causing interdiffusion of Fe and Zn with the steel sheet substrate and making the plating layer an Fe-Zn alloy. Therefore, the plated layer of the galvannealed steel sheet is made of an Fe-Zn intermetallic compound, and its average Fe concentration is generally adjusted to 8 to 12% by weight.

[0005] coating weight of the galvannealed steel sheet is per side 25~70g / m ^2, well below the 25 g / m ² is difficult to manufacture by conventional means, those greater than 70 g / m ² Since it is difficult to ensure the powdering resistance of the plating layer, is not generally supplied. Further, the plating film usually contains about 0.12 to 0.2% by weight of Al. This is added to the plating bath in order to suppress the formation of an alloy layer at the interface between the galvannealed steel sheet and the galvanized galvanized steel sheet manufactured with the same equipment as the galvannealed steel sheet and to maintain the workability of the galvanized film. Al may inevitably be mixed in the plating film, but in order to secure the powdering resistance of the galvannealed film and suppress the generation of dross during manufacturing,
It is considered rather suitable to mix Al in the range of 0.08 to 0.11% by weight. Al in the plating bath tends to be enriched in the plating layer, so 0.08 to 0.11% by weight
When plating is performed in a plating bath containing Al, the Al in the plating film
The concentration is in the range of 0.12 to 0.2% by weight.

Conventionally, low carbon Al-killed steel sheets, ultra-low carbon Ti-added steel sheets, etc. have been mainly used as base materials for hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets. May be required,
A silicon-containing steel sheet containing 0.2% by weight or more is about to be used. Si has the advantage of improving the strength without impairing the ductility of the steel, and in that sense, silicon-added steel is promising as a base material for hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets for automobiles.

However, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets having a silicon-containing steel sheet as a base material have problems in terms of quality and production as described below.

When hot dip galvanizing a silicon-containing steel plate by the above-mentioned ordinary process, a very small amount of water in the atmosphere reacts with Si in the steel plate in the annealing process before plating to form Si-Oxide on the steel plate surface. Therefore, the wettability with molten zinc sharply decreases as the Si content of steel increases. As a result, non-plating frequently occurs. For such a problem, it is known that the wettability is improved by heating a steel sheet in advance in an oxidizing atmosphere to form iron oxide on the surface thereof. However, in the case of a steel sheet with a Si content exceeding 0.2% by weight, preheating in an oxidizing atmosphere in a normal hot dip galvanizing process, for example, in an oxidizing atmosphere in which the air-fuel ratio of a non-oxidizing furnace is 1-1.35, Not only is sufficient iron oxide not formed until the wettability is improved, but the alloying treatment rate in the case of alloying treatment after plating is extremely slow, and production efficiency is greatly impaired. In particular, in the case of a steel sheet in which Si is added to an ultra-low carbon steel with Ti added to improve the formability of the steel sheet, the annealing temperature for recrystallization is as high as 800 ° C or higher, Precipitation of Si-Oxide on the surface of the steel sheet becomes more remarkable, and it becomes more difficult to secure wettability.

As another method for improving the wettability with hot-dip zinc, a method is known in which the surface of the steel sheet is subjected to Ni undercoating prior to hot-dip plating (for example, Japanese Patent Publication No. 61-9386).
Issue). However, with this method, when targeting steel with a Si content of 0.2 wt% or more, the adhesion amount of 200 mg / m ² or more
Since it is necessary to apply Ni plating, this causes an increase in cost, and when such a large amount of Ni plating is applied,
Although the wettability of hot-dip galvanizing is improved, there is a problem in that defects caused by Si and Ni frequently occur on the plating surface during the alloying process.

Further, it is possible to prevent the non-plating of the Si-added steel by applying Fe plating or the like in advance in addition to Ni plating, but for this purpose, it is necessary to apply Fe plating of 1 g / m ² or more. It is extremely uneconomical.

Under the circumstances, the silicon-containing steel sheet, which is attractive as a high-strength material for automobiles, lacks practical means for hot-dip galvanizing or alloying it.

[0012]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and is intended to cause non-plating by using a low alloy steel sheet, particularly a steel sheet having a Si content of 0.2% by weight or more as a base material. It is an object of the present invention to provide a hot-dip galvanizing method that can perform plating without any problems, does not reduce the alloying treatment rate of the steel sheet after plating, and is highly economical.

[0013]

As a result of repeated studies to solve the above problems, the present inventors have found that (a) base steel sheet (Si
The surface of the steel sheet containing steel) is pre-plated with a very small amount of metal, (b) the plated steel sheet is oxidized in a weakly oxidizing atmosphere, and (c) the steel sheet after oxidation treatment is performed at 700 ° C or less. It has been found that the hot dip galvanizing and the subsequent alloying treatment can be carried out smoothly by incorporating a temperature reducing step. As the trace amount of metal used for the electroplating, those having an oxygen affinity equal to or smaller than Fe constituting the base material are preferable, and Fe, Cu, Ni, Co and the like are preferable.

The present invention has been made based on the above findings, and the gist of the invention lies in the following hot dip galvanizing method.

On the surface of a steel plate containing 0.2% by weight or more of Si, one or more of Ni, Fe, Cu and Co is preliminarily added in an amount of 5-70 mg / m ²
Electroplated with the amount of deposited on the steel sheet surface and then heated in an oxidizing atmosphere at 700 ° C or less to convert it to Fe on the steel plate surface of 0.3 to 1.5 g / m
^A method for hot dip galvanizing a silicon-containing steel sheet, which comprises forming iron oxide ( ² ), reducing it in a temperature range of 500 to 650 ° C., and then performing hot dip galvanizing.

A hot dip galvanizing method for a silicon-containing steel sheet, characterized by performing an alloying treatment after the hot dip galvanizing.

[0017]

The method of the present invention will be described in detail below in the order of steps.

First, the steel sheet targeted by the present invention is made of Si
A steel sheet containing 0.2% by weight or more.

This is because a steel sheet having a Si content of less than 0.2% by weight can be dealt with within the range of the prior art.

This steel sheet is annealed cold-rolled steel sheet (however,
It is a hot-rolled steel plate after cold picking (except for full-hard steel) or after pickling. That is, in the method of the present invention, the material is not adjusted by annealing or heat treatment of a steel sheet, which has been conventionally performed in a series of steps using a continuous hot-dip galvanizing facility (CGL), but is preliminarily tempered with predetermined material characteristics. Steel plate is used as a base material. This is a condition taken from the viewpoint of separating the annealing from the hot dip coating and avoiding the direct contact of the oxide of Si concentrated on the surface with the hot dip zinc in the annealing process. Therefore, if the object is a cold-rolled steel sheet, it is annealed in advance in a batch type or continuous type annealing furnace so that predetermined characteristics can be obtained. However, even a cold-rolled steel sheet need not be annealed if it is a full-hard material as cold-rolled.

When hot-rolled steel sheet is used, it may be pickled.

Electroplating, which is the first step of the present invention, is carried out using the above Si-containing steel plate as a plating base material, and it is desirable to carry out ordinary degreasing and pickling as pretreatment. When hot-rolled steel sheet is targeted, it is not necessary in principle because it has already been pickled, but if the storage period after sheet making is long, it may be pickled again if necessary. . Also, when performing annealing in a continuous annealing furnace, there are some that are equipped with pickling equipment on the exit side of the annealing furnace, so degreasing and pickling steps are not necessary for steel sheets annealed in such equipment. Is.

For pickling, a diluting solution of hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, etc. is used. Pickling has the effect of dissolving some of the Si-based oxide film formed on the surface of the steel sheet in the hot rolling step in the case of hot rolled steel sheet and in the annealing step in the case of pre-annealed cold rolled steel sheet. However, there is also an adverse effect such as thickening oxides such as Ti on the surface of the steel sheet. Therefore, it is desirable to avoid excessive pickling. Preferable conditions for pickling are hydrochloric acid of 3 to 15%, liquid temperature: 40 to 95 ° C., pickling time: 2 to 20 seconds.

On the surface of the steel sheet after pickling, Fe, Cu, Ni and
At least one of Co is electroplated. At least one kind may be single layer plating of these metals,
For example, after Fe plating, Ni plating may be applied thereon. Alternatively, Fe-Ni alloy plating may be applied.

When the coating weight is less than 5 mg / m ² , the wettability with molten zinc is insufficiently improved and non-plating is likely to occur. On the other hand, when it exceeds 70 mg / m ² , the effect is saturated and uneconomical. Therefore, the dose should be 5 to 70 mg / m ² . It is unclear why such an extremely small amount of metal plating is effective in improving hot-dip galvanizing properties, but since the annealing treatment is not performed after applying this metal plating, these metals diffuse into the base metal. It is presumed that it partially remains on the surface of the steel sheet without doing so, and when the steel sheet comes into contact with molten zinc, it functions as an active point of the reaction in which zinc adheres to the base material.

Next, the above electroplated steel sheet is heated in an oxidizing atmosphere to form iron oxide on the surface of the steel sheet. When the heating temperature exceeds 700 ° C, Si in the steel diffuses to the surface of the steel sheet and non-plating easily occurs, and the oxide film easily peels off, and this peeled oxide film adheres to the furnace roll. 700 ℃ as it causes surface defects on the steel sheet
Hereafter, the temperature is preferably 400 to 650 ° C. The lower limit is not specified, but the oxidation rate is low at 300 ° C or lower and is not suitable.
Further, as the oxidizing atmosphere at the time of heating, an atmosphere containing an oxidizing gas such as carbon dioxide, oxygen and water may be used.

When the amount of iron oxide formed on the surface of the steel sheet is less than 0.3 g / m ² in terms of Fe, non-plating is apt to occur and 1.5
If it exceeds g / m ² , the reduction of iron oxide in the next step will be insufficient and non-plating will also occur easily. Therefore, it should be 0.3 to 1.5 g / m ² in terms of Fe.

After forming iron oxide on the surface of the steel sheet,
Reduce in the temperature range of 650 ℃. For this purpose, an atmosphere containing 5 vol% or more of hydrogen and the balance consisting of a non-oxidizing gas is preferable, for example, an atmosphere containing hydrogen and nitrogen and having a dew point of -15 ° C or lower. In this case, if the temperature is less than 500 ° C, the reducing power is weak and the reduction of iron oxide is insufficient.
If it exceeds, the diffusion of Si in the steel is likely to occur and
Is thickened and non-plating is likely to occur. Processing time is 15
~ 20 seconds is appropriate.

The steel sheet after the reduction treatment is cooled to about 380 to 550 ° C. and hot dip plated in a zinc bath containing about 0.05 to 0.15% Al. If necessary, use an alloying treatment furnace.
An alloying treatment is performed by heating at 450 to 600 ° C for 1 to 120 seconds.

According to the above hot dip galvanizing method of the present invention,
It is possible to obtain a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet which is free from unplating and has excellent surface characteristics, using a silicon-containing steel sheet as a base material. According to this method, hot-dip galvanizing is possible even for steel sheets having a Si content exceeding 1.5% by weight.

[0031]

[Examples] Hot-rolled steel sheets of high Si steel having the chemical composition shown in Table 1 (steel type A, sheet thickness 2.3 mm, unpickled material) and cold-rolled steel sheets (steel types B to D, sheet thickness 0.80 mm, unannealed) Material) is cut into 250 mm x 100 mm to make a test material. For B to D, hydrogen is 15 vol%
In an atmosphere with a dew point of -30 ° C, the balance of which is nitrogen,
Annealing was performed at a temperature rising rate of 15 ° C / sec and 850 ° C x 120 sec.

These steel sheets were pickled in a 5% HCl aqueous solution at 80 ° C. for 5 seconds (some of the steel sheets were not pickled), and then, on the steel sheet surface under the conditions shown in Table 2, 2 ~ 70
Ni plating mg / m ^2, Fe plating ^{2~70mg / m 2, 5mg / m} 2
Cu plating or 10 to 15 mg / m ² Co plating was performed, and then oxidation (preheating) treatment, reduction treatment, and hot dip galvanizing were performed using a vertical hot dipping apparatus. According to this apparatus, the test material can be heat-treated in a predetermined atmosphere, and can be directly immersed in the hot dip bath from the reducing atmosphere. Oxidation treatment is performed under the conditions shown in a to e of Table 3 (however, a is not oxidized), and reduction treatment is N ₂ +25 vol%
After performing the reduction treatment in an atmosphere of H ₂ at 600 ° C. for 60 seconds, the steel sheet was cooled to 460 ° C. and hot dip galvanizing was performed.
Total Al concentration of plating bath is 0.112%, total Fe concentration is 0.020%
Then, the plating time was set to 1 second, and the amount of Zn deposited was adjusted to about 50 g / m ² (per surface) with a gas wiper. The alloys that did not cause non-plating were further alloyed at 500 ° C.

After hot-dip galvanizing, the occurrence of non-plating was investigated. In addition, the non-plated product was subjected to alloying treatment and the time required for alloying was measured.

The survey results are shown in Table 4. In addition, in the table,
The presence or absence of pickling as a pretreatment for electroplating, the basis weight of electroplating, the method of oxidation (preheating), the amount of iron oxide and the reducing conditions are also shown.

As is clear from the results shown in Table 2, when electroplating, oxidation (preheating) treatment and reduction treatment are performed under the conditions specified in the present invention, hot dip galvanization is performed without causing non-plating. I was able to. Also, alloying was easy.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

[Table 4 (1)]

[0040]

[Table 4 (2)]

[0041]

As described above, when the method of the present invention is applied to a steel sheet containing Si, a galvanized steel sheet and an alloyed hot dip galvanized steel sheet having excellent surface quality without unplating are produced. can do.

[0042]

Claims (2)

[Claims] 1. The surface of a steel sheet containing 0.2 wt% or more of Si is electroplated with at least one of Ni, Fe, Cu and Co in an amount of 5 to 70 mg / m ² and then oxidized at 700 ° C. or less. After heating in a strong atmosphere to form 0.3 to 1.5 g / m ² of iron oxide converted to Fe on the surface of the steel sheet, reduce it in the temperature range of 500 to 650 ℃, and then perform hot dip galvanizing. A method for hot dip galvanizing a silicon-containing steel sheet, comprising: 2. A surface of a steel sheet containing 0.2% by weight or more of Si is electroplated with at least one of Ni, Fe, Cu and Co in an amount of 5 to 70 mg / m ² and then oxidized at 700 ° C. or less. After heating in a strong atmosphere to convert 0.3 to 1.5 g / m ² of iron oxide on the surface of the steel sheet to form iron oxide, it was reduced in the temperature range of 500 to 650 ℃, and subsequently hot-dip galvanized. A method of hot dip galvanizing a silicon-containing steel sheet, characterized by performing an alloying treatment thereafter.