JPH05106001A - Method for plating molten zinc on steel sheet containing silicon - Google Patents

Abstract

(57) [Summary] [Structure] A steel sheet containing 0.2% by weight or more of Si is annealed in a non-oxidizing atmosphere, and then subjected to carburizing treatment before dipping in hot dip galvanizing bath. .. After carburizing, 0.2 g / m ² or more of iron oxide is formed as Fe on the surface of the steel sheet, and then 500 to 900 ° C in a non-oxidizing atmosphere.
It is even more effective if the method of annealing in the temperature range is adopted. [Effect] A hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet which has no surface and is excellent in surface quality can be efficiently manufactured. By adopting a method of forming iron oxide on the surface of the steel sheet after the carburizing treatment and annealing, good plating property can be secured for the steel sheet having a Si content of more than 1%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having a silicon-containing steel sheet as a base material and having an excellent non-plating surface quality and an alloyed hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art In recent years, hot-dip galvanized steel sheets have been used in large quantities in industrial fields such as home appliances, building materials, and automobiles. However, alloyed hot-dip zinc is particularly excellent in economic efficiency and its rust-preventive function and performance after painting. Plated 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 said that an oxide film with a thickness of about 80 nm should be formed 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 adversely affect the generation of dross and 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 alloyed hot-dip galvanized steel sheet described later.

The alloyed hot-dip galvanized steel sheet is obtained by heating the hot-dip galvanized steel sheet as described above in a heat treatment furnace to a material temperature of 500 to 600 ° C. for 3 to 30 seconds to obtain a zinc layer and a steel sheet base material. The inter-diffusion between the
It is manufactured by using an alloy. Therefore, the plated layer of the galvannealed steel sheet is made of an Fe-Zn intermetallic compound, and the average Fe concentration is generally adjusted to 8 to 12% by weight.

[0005] coating weight of the galvannealed steel sheet is usually per side 25~70g / m ^2, well below the 25 g / m ² is difficult to manufacture in conventional means, 70 g / m ²
However, it is difficult to ensure the powdering resistance of the plating layer, so that it is not generally supplied.
In addition, the plating film usually contains about 0.12 to 0.2% by weight of Al.
Is often contained. This suppresses the formation of an alloy layer at the interface between the galvanized steel sheet and the galvanized hot-dip galvanized steel sheet, which is manufactured in the same equipment as the galvannealed steel sheet, and is used in the plating bath to maintain the workability of the galvanized film. Although the added Al may inevitably be mixed into the plating film, the plating is usually performed in order to secure the powdering resistance of the galvannealed film and suppress the generation of dross during manufacturing. It is considered appropriate to mix 0.08 to 0.11% by weight of Al in the bath. Since Al in the plating bath tends to be enriched in the plating layer,
When plating is performed in a plating bath containing about 0.08 to 0.11% by weight of Al, the Al concentration in the plating film is in the range of 0.12 to 0.2% by weight.

[0006] By the way, 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. Since higher strength is required, a silicon-containing steel sheet containing 0.2% by weight or more of Si is about to be used. Si has the advantage of improving the strength while ensuring the ductility of steel, and in that sense silicon-containing steel plates are promising as high-strength materials for automobiles.

[0007]

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 concentration in the steel increases. 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. Therefore, precipitation of Si-Oxide on the surface of the steel sheet becomes more remarkable, and it becomes more difficult to secure wettability. As a result, non-plating frequently occurs.

In order to solve such problems, it is known that the wettability is improved by heating the steel sheet in an oxidizing atmosphere in advance and forming iron oxide on the surface thereof. But,
In the case of a steel sheet with a Si content of 0.2% by weight or more, wettability is improved by 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. In addition to not forming sufficient iron oxide, the alloying rate in the case of alloying treatment after plating is remarkably slow, and production efficiency is greatly impaired.

As another method for improving the wettability with hot-dip zinc, prior to hot-dip galvanizing, Ni or Fe is applied to the surface of the steel sheet.
There is also known a method of underplating a metal such as, but this method causes an increase in cost, and a steel sheet having a high Si content does not have a sufficient effect, and in the case of a steel sheet having a high Si content, Has a problem that uniform alloying is very difficult.

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

The present invention has been made in view of the above circumstances, and provides a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which have a silicon-containing steel sheet as a base material and are excellent in surface quality without unplating. It is an object of the present invention to provide a hot dip galvanizing method that can be efficiently manufactured.

[0013]

Means for Solving the Problems As a result of studies on the wettability of a steel sheet and molten zinc in hot dip galvanizing of a silicon-containing steel sheet, the present inventors have found that even if the Si content in the steel is the same, the coexistence The wettability changes drastically depending on the C concentration, and by increasing the C concentration on the steel sheet surface, the wettability with molten zinc is greatly improved and the plating property is improved, and for that purpose, carburizing treatment is performed. Has found that is extremely effective.

The present invention has been made on the basis of such findings, and its gist resides in the following hot dip galvanizing method.

A steel sheet containing 0.2% by weight or more of Si is annealed in a non-oxidizing atmosphere, and then immersed in a hot dip zinc bath for hot dip galvanizing, and the steel sheet is previously carburized. Method for hot dip galvanizing steel sheet containing.

After performing carburizing treatment on a steel sheet containing 0.2 wt% or more of Si, the surface of the steel sheet is converted to Fe.
A silicon-containing steel plate characterized by forming iron oxide of 0.2 g / m ² or more, then annealing in a temperature range of 500 to 900 ° C in a non-oxidizing atmosphere, and then immersing it in a molten zinc bath for plating. Hot dip galvanizing method.

[0017]

The present invention will be described in more detail below.

The hot-dip galvanizing method of the present invention uses 0.2% Si.
The target is steel plates containing more than wt%. This means that for steel sheets with a Si content of less than 0.2% by weight, the usual hot dip galvanizing method described above, that is, after preheating the steel sheet in a weak oxidizing atmosphere annealing furnace or a reducing atmosphere annealing furnace, hydrogen + nitrogen Although it is annealed in a reducing atmosphere, cooled to a temperature suitable for plating, and then dipped in a hot dip galvanizing bath to obtain hot dip galvanized products, the Si content is
This is because in the case of a steel sheet containing 0.2% by weight or more, non-plating occurs in this hot dip galvanizing method, and a product of excellent quality cannot be obtained.

In the present invention, when the above-mentioned silicon-containing steel plate is charged into the continuous furnace of the continuous galvanizing line, carburizing treatment is performed in advance. The mechanism of improving the wettability with molten zinc by increasing the C concentration on the surface of the steel sheet by carrying out carburizing treatment is not clear at present, but
At least the Si on the steel plate surface after the reduction process
This is probably because the formation of -oxide is suppressed and the rate of iron oxide formation in the oxidation process before reduction is increased.

The carburizing treatment may be performed either after annealing the steel sheet or before annealing, but the heat treatment can be performed in the subsequent heating step in a non-oxidizing atmosphere also as the annealing treatment. Therefore, it is economical to perform it on the unannealed material.

If the C content of the target steel sheet is 0.5% by weight or more, relatively good wettability can be recognized without carburizing treatment. Therefore, the method of the present invention uses the C content of 0.5% by weight. Less than 0.1% by weight, particularly, the effect is great for a silicon-containing steel plate, and it is effective for improving the plating property of an ultra-low carbon silicon-containing steel plate containing Ti, Nb and the like. It is desirable that the carbon concentration on the surface of the steel sheet is set to about 0.1 to 0.5% by weight by carburizing.

Any method may be used in the carburizing treatment carried out in the present invention. Taking liquid carburization as an example, molten salt containing NaCN as the main component (commercial products can be used)
Carburizing can be carried out by immersing in. Further, electrolytic carburization in which cathodic electrolysis is performed in a molten carbonate of Na or K is also one of applicable methods.

Gas carburization can also be utilized as a means to carry out the present invention. In this case, a mixed gas of CO and an inert gas is used as the atmospheric gas. The carburization reaction can be promoted by lowering the dew point of the atmospheric gas. In the case of gas carburization, the treatment temperature is preferably 400-750 ° C. If it is less than 400 ° C, the solubility of C in α-Fe is too small to make it difficult to carburize. If it exceeds 750 ° C, carburization becomes excessive depending on the steel type, and problems such as deterioration of mechanical properties due to subsequent heat treatment occur. ..

When the method of the present invention is carried out in a continuous galvanizing line (CGL) having an all-radiant tube type continuous furnace, the atmosphere of the continuous furnace is changed to, for example, normal AX gas by CO 2 at the preheating stage of the steel sheet. The carburizing treatment can be easily performed by setting a carburizing atmosphere composed of nitrogen, hydrogen and CO gas in which 0.1 to 2% is mixed.

As described above, the invention described above is that the carburizing treatment is performed in advance and then the plating is performed by the usual hot dip galvanizing method.

In the invention of (1), after the carburizing treatment, the oxidizing treatment is performed to form iron oxide on the surface of the steel sheet. The amount of iron oxide is represented as Fe, that is, the amount of Fe contained in iron oxide. If it is less than 0.2 g / m ² , wettability with molten zinc is insufficient, so the amount is set to 0.2 g / m ² or more.

From the viewpoint of improving wettability, it is desirable that the amount of iron oxide is large, but if it exceeds 5 g / m ² , reduction failure is likely to occur in the next step, and iron oxide is likely to be peeled off in the strip. Problems such as occur.

Then, the steel sheet having iron oxide formed on the surface thereof is heated in a non-reducing atmosphere containing hydrogen at 500 to 900 ° C. This may be carried out according to a conventional method. As the atmosphere gas, a mixed gas of hydrogen and nitrogen containing 2% or more of hydrogen is usually used. The heating temperature is the annealing temperature when the steel sheet is not annealed and carburized, and when it is already annealed.
The temperature range is 500 to 900 ° C. If the temperature is less than 500 ° C, the reduction is insufficient, and if it exceeds 900 ° C, the ferrite crystal grains grow abnormally and the mechanical properties are degraded. It has a great adverse effect on wettability.

In this step, iron oxide on the surface of the steel sheet is reduced to metallic Fe.

According to the invention of carrying out further oxidation treatment after carburizing treatment, the Si content is 1 as shown in Example 2.
It is possible to carry out hot dip galvanizing of a steel sheet exceeding 100% without causing non-plating.

When the present invention is carried out in a continuous galvanizing line having a continuous furnace, after carrying out carburizing treatment,
By forming iron oxide on the surface of the steel sheet in the preheating stage and further annealing in the reduction zone, it is possible to perform plating online efficiently. Since the existing plating line is used, this method is also economically superior.

The following steps are common to the inventions 1 and 2, but the steel sheet subjected to the above treatment, that is,
In the invention of 1), the steel sheet that has been carburized, and in the invention of 3
After cooling to ~ 600 ° C, it is immersed in molten zinc containing 0.03 to 0.2% by weight of Al, preferably 0.08 to 0.12% by weight,
Perform hot dip plating.

When alloying plating is performed, heat treatment may be performed according to a conventional method after the above steps.

[0034]

Example 1 A cold-rolled steel plate (unannealed material, plate thickness 0.8 mm) of ultra low carbon steel shown in A of Table 1 was cut into 250 mm × 100 mm test samples, which were carburized. After performing the hot-dip galvanizing using a vertical hot dip galvanizing device that can be heat-treated in a predetermined atmosphere and that can directly charge the test material from the reducing atmosphere into the hot-dip galvanizing bath. .. That is, after the test material was degreased with a 10% NaOH solution in advance, it was carburized by various methods and conditions shown in Table 2, and then a mixed gas of N ₂ + 40% H ₂ with an oxygen concentration of 2 ppm (with a dew point of −30 ℃) 750
After annealing at 850 ° C for 60 seconds and cooling to 460 ° C, plating was performed by immersing in a molten zinc bath having a total Al concentration of 0.127% and a total Fe concentration of 0.020%. The plating time was 1 second, and the Zn adhesion amount was adjusted to about 50 g / m ² (per one side) with a gas wiper.

After plating, the occurrence of non-plating was investigated.

The results are shown in Table 2. As is clear from this table, in Nos. 2 to 7 (invention examples), no occurrence of non-plating is observed, and good plating properties can be secured for steel containing 0.2% or more of Si. It was

[0037]

[Table 1]

[0038]

[Table 2]

[0039]

Example 2 A cold rolled steel sheet of ultra low carbon steel (unannealed material, sheet thickness 0.8 mm) shown in B of Table 1 was cut into 250 mm × 100 mm to make a test material, and this test material was carburized. And further subjected to oxidation treatment in a mixed gas of oxygen and nitrogen, and then heat treatment in a predetermined atmosphere is possible, and the test material can be directly charged into the hot dip plating bath from the reducing atmosphere. Hot-dip plating was performed using a vertical hot-dip galvanizing apparatus. That is, after the test material was washed in the same manner as in Example 1, the carburizing treatment was performed under the various methods and conditions shown in Table 3, and then the oxidation treatment was performed under the various conditions shown in Table 3 as well. After annealing at 850 ℃ for 60 seconds in N ₂ + 20% H ₂ mixed gas with oxygen concentration less than 2ppm and cooling to 460 ℃, total Al concentration is 0.115%, total Fe
Plating was performed by immersing in a molten zinc bath with a concentration of 0.016%. The plating time was 1 second, and the Zn adhesion amount was adjusted to about 50 g / m ² (per one side) with a gas wiper.

After plating, the occurrence of non-plating was investigated. Further, the test piece after hot dipping was alloyed in a salt bath at 500 ° C., and the time required for alloying was measured.

The results are shown in Table 3. As is clear from this table, in the invention examples (No. 4, 6, and 8 to 10),
No occurrence of non-plating was observed, and good plating properties were exhibited for steel sheets with a Si content exceeding 1%. The alloying reaction proceeded rapidly. On the other hand, in the invention examples (Nos. 3, 5 and 7) in which the oxidation treatment was not performed, slight non-plating occurred, and alloying took a longer time than the invention.

[0042]

[Table 3]

[0043]

As described above, when a silicon-containing steel sheet is plated by the hot-dip galvanizing method of the present invention, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet which is excellent in surface quality without unplating is produced. can do. Moreover, it can be carried out online by using the existing continuous galvanizing line, which is an excellent method in terms of economy.

[0044]

Claims (2)

[Claims] 1. A steel sheet containing Si in an amount of 0.2% by weight or more is annealed in a non-oxidizing atmosphere, and then immersed in a hot dip zinc bath for hot dip galvanizing. A hot dip galvanizing method for a silicon-containing steel sheet. 2. A steel sheet containing 0.2% by weight or more of Si is carburized, and then 0.2 g / Fe as Fe on the surface of the steel sheet.
A molten zinc for a silicon-containing steel sheet, characterized in that iron oxide of m ² or more is formed, then annealed in a temperature range of 500 to 900 ° C in a non-oxidizing atmosphere, and then immersed in a molten zinc bath for plating. Plating method.