JP2674429B2 - Hot-dip galvanizing method for silicon-containing steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet containing silicon (Si) as a base material.

[0002]

2. Description of the Related Art In recent years, a large amount of hot-dip galvanized steel sheets have been used in the industrial fields of home appliances, building materials, automobiles, etc. Hot-dip galvanized steel sheets are widely used.

Hot-dip galvanized steel sheets are usually preheated in a protective or reducing atmosphere after a suitable degreasing and cleaning step or without degreasing and cleaning, and then a reduction consisting of a mixed gas of hydrogen and nitrogen. It is manufactured by annealing in a neutral atmosphere, then cooling to near the plating temperature and immersing in a molten zinc bath.

At the time of preheating in the above process, it is preferable that an oxide film having a thickness of about 80 nm is formed on the surface of the steel sheet in terms of wettability with molten zinc, but an oxide film having a thickness larger than that is preferable. Is considered to have an adverse effect of increasing the generation of dross and impairing the adhesion of hot dip coating. 0.08 to 0.18% (hereinafter, unless otherwise specified, "%" means "% by weight") of Al (aluminum) in the hot-dip galvanizing bath, including the manufacturing range of alloyed hot-dip galvanized steel sheet described below.
Is included.

Alloyed hot-dip galvanized steel sheets are usually prepared by continuously hot-dip galvanizing steel sheets in a heat treatment furnace at 500 to 600 ° C.
The Fe-Zn alloy plating layer is formed by heating the material temperature to 3 to 30 seconds. The plating layer is made of an Fe-Zn intermetallic compound and generally has an average Fe concentration of 8 to 12%.
The amount of plating applied is usually 25 to 70 g / m ^{2 on} one side, and those below this range are difficult to produce by ordinary means, and those above this range reduce the powdering resistance of the plating layer. It is generally not supplied because it is difficult to secure.

[0006] Al alloying hot-dip galvanized steel sheets often contain about 0.12 to 0.2% Al.
One of the causes is to suppress the formation of an alloy layer at the interface between the plating film and the base material steel plate in the production of a normal hot-dip galvanized steel plate, and to maintain the film workability, Al is added in the bath.
Therefore, when an alloyed hot-dip galvanized steel sheet is manufactured using the same bath, Al is unavoidably mixed in the plating film. Another cause is that it is rather desirable to contain Al in the plating bath in order to secure the powdering resistance of the coating film of the galvannealed steel sheet and suppress the generation of dross during the production. Is
This is because a molten zinc bath containing Al of about 0.08 to 0.11% is usually used. Since Al tends to be enriched in the plating layer during plating, plating in the above bath will reduce the Al concentration in the film.
The range is 0.12 to 0.2%.

As the base material of the plated steel sheet as described above,
Conventionally, low carbon Al killed steel sheets and ultra low carbon Ti-added steel sheets have been mainly used. However, in recent years, for automotive materials, for example, as one measure for weight reduction, high strength steel sheets have been demanded, and silicon-containing steel sheets containing 0.2% or more of Si are about to be used. Since Si is an element that improves the strength while ensuring the ductility of steel, it can be said that silicon-added steel is a desirable steel material that meets the above requirements.

However, the Si-containing steel sheet has a major drawback as a base material for hot dip galvanizing. When a silicon-containing steel sheet is treated according to the above-mentioned normal process, the trace amount of water in the atmosphere reacts with Si in the steel sheet in the annealing process,
Si-Oxide, which impairs wettability with molten zinc, is generated on the surface of the steel sheet. Therefore, non-plating frequently occurs as the Si concentration in steel increases. It is known that wettability is improved by forming Fe oxide on the surface of a silicon-containing steel sheet by heating it in an oxidizing atmosphere in advance. However, the Si content
If it exceeds 0.2%, the oxidizing atmosphere in the conventional process
It is difficult to recover the wettability only by preheating in an atmosphere where the air-fuel ratio of the non-oxidizing furnace is set to 1-1.35.

Further, in the case of producing a galvannealed steel sheet using a silicon-containing steel sheet as a base material, there is a problem that the alloying treatment speed is remarkably slowed and the production efficiency is lowered.

In order to improve the formability of steel
When Si-added steel, which is based on additive ultra-low carbon steel, is used as the base material, the annealing temperature for recrystallization is 800 ° C or higher, so precipitation of Si-Oxide on the steel sheet surface becomes more pronounced and wets. It becomes more difficult to secure the sex.

As a method for solving the above-mentioned problems, it is conventionally known to apply an undercoat of Ni, Fe or the like prior to hot dipping, but an extra plating step is added, so that the number of steps is increased and the manufacturing is performed. Despite the cost increase,
Steel having a high Si content has a problem that a sufficient effect for improving wettability with molten zinc cannot be obtained, and a uniform alloying treatment is extremely difficult.

As described above, even in the case of silicon-containing steel, which is attractive in terms of material, it is the current situation that there is no practical method for applying suitable hot-dip galvanizing or alloying hot-dip galvanizing.

[0013]

DISCLOSURE OF THE INVENTION According to the present invention, it is impossible to form a satisfactory hot-dip galvanized coating in the conventional hot-dip galvanizing process, and the productivity is very low when alloying treatment is applied. The task was to develop a practical method for producing a plated steel sheet using the contained steel as a base material.

An object of the present invention is to use a steel sheet containing Si in an amount of 0.2% or more as a base material, perform plating without the occurrence of non-plating points, and sufficiently increase the alloying treatment rate, thereby making it possible to economically melt molten zinc. It is to provide a method for producing a galvanized steel sheet and a galvannealed steel sheet.

[0015]

The gist of the present invention is the following hot dip galvanizing method or a method for producing an alloyed hot dip galvanized steel sheet.

(1) A method for producing a hot-dip galvanized steel sheet using a steel sheet containing 0.2% or more of Si as a base material, wherein the steel sheet whose surface has been removed by grinding at 0.4 to 5 g / m ² has a temperature of 650 ° C or less. Treated in an oxidizing atmosphere and converted to Fe on the steel plate surface to 0.3
~ 1.5 g / m ² of iron oxide layer formed, then 500-650 ° C
A hot dip galvanizing method for a silicon-containing steel sheet, which comprises performing hot dip galvanizing after reduction at the temperature of.

(2) A method for producing an alloyed hot-dip galvanized steel sheet in which hot-dip galvanizing is performed in the step described in (1) above, and then alloying heat treatment is further performed.

The base steel sheet used in the method of the present invention contains Si
A hot-rolled steel sheet or a cold-rolled steel sheet containing 0.2% or more. In the case of hot-rolled steel sheet, since it has not been subjected to cold plastic working, in many cases the workability can be maintained as it is,
The coil wound in the hot rolling process can be used as a base material in a state of being pickled and descaled. However, it may also be subjected to a heat treatment in order to impart certain mechanical properties before it is subjected to the process of the invention.

In the case of a cold rolled steel sheet, it may be used as a base material in an unannealed state, but it is difficult to adjust the mechanical properties (especially workability) of the base material only by the oxidation and reduction steps of the method of the present invention. Therefore, it is desirable to perform annealing in advance so that the predetermined characteristics can be obtained. This annealing may be batch type annealing,
It may be continuous annealing.

The Si-containing steel sheet, whether it is a hot-rolled steel sheet or a cold-rolled steel sheet, has an oxide layer mainly containing Si on its surface. Further, when heated in a continuous hot dip galvanizing furnace in a hydrogen / nitrogen mixed atmosphere at 700 ° C. or higher, moisture in the atmosphere causes selective oxidation of Si, and the oxide layer mainly containing Si rapidly increases.

The basic idea of the present invention is that the Si-enriched oxide on the surface of the base material is not brought into direct contact with molten zinc. For that purpose, grinding of the surface of the base material and subsequent treatment of oxidation and reduction are performed.

Usually, when performing hot dip galvanizing only by simply performing surface grinding, the required grinding amount is 15 g / m ² or more per side, and when such grinding is performed, a continuous furnace of a hot dip coating line (CGL) is used. If the internal heating temperature is around 600 ° C, hot dipping can be performed without causing non-plating. However, a large amount of grinding as described above cannot be performed with a brush roll installed on the inlet side of a normal CGL, and a special grinder is required, and such a brush for heavy grinding has a short life and requires a low equipment cost. However, operating costs are high.

Grinding has a function of mechanically forming a new surface on the surface of the base material steel plate, refining the crystal grains in the surface layer, and introducing a large amount of dislocations to activate the surface. However, excessive activation due to a large amount of grinding excessively increases the reactivity between the steel sheet and the molten zinc and causes a large amount of dross.

However, the present inventor can significantly reduce the required grinding amount by introducing a process of grinding the surface of the base material and then heating it in an oxidizing atmosphere to form an appropriate iron oxide layer on the surface of the steel sheet. I found it.

[0025]

The function and effect of the conditions defined in the present invention and the reason for limiting the conditions will be described below.

The steel sheet targeted by the present invention has a Si content of 0.2.
% Or more of steel sheet (hot rolled steel sheet or cold rolled steel sheet). If the Si content is less than 0.2%, it is possible to deal with it by devising a conventional technique, and it is not necessary to dare to apply the method of the present invention.
Although there is no particular limitation on the components other than Si, considering that the plated steel sheet produced by the method of the present invention is particularly suitable for automobiles, it has both strength and workability, particularly press formability. It is desirable to choose the composition. For that purpose, C: 0.001-0.15%, Mn: 0.1-1.5%, P: 0.01-
A composition containing 0.1% and Ti: 0.01 to 0.1% is recommended.

As described above, the surface of the steel sheet that has been heat-treated as required and tempered to have predetermined mechanical properties is then ground. There are no restrictions on this grinding method. Since the amount of grinding is small, an ordinary brush roll for grinding is sufficient. Grinding amount is limited to 0.4g / m ² to 5g / m ² . 0.
If it is less than 4 g / m ² , the removal of the Si-based oxide layer is insufficient, and the oxidation rate of Fe is low, resulting in insufficient oxidation in the next step. When the grinding amount exceeds 5 g / m ² , the removal of the Si-based oxide is completed, but it is difficult to adjust the oxidation in the next step to an appropriate range. Further, the reaction with molten zinc is excessively activated and dross generation increases, which causes an increase in operating cost.

The steel plate subjected to the grinding is then heated in an oxidizing atmosphere to be oxidized, and its surface is converted into Fe at 0.3 g / m ²
Up to 1.5 g / m ² of iron oxide is formed.
As the atmosphere, an atmosphere containing an oxidizing gas such as CO ₂ , O ₂ and H ₂ O is used. If the amount of oxidation in terms of Fe is less than 0.3 g / m ² , non-plating is likely to occur, and if it exceeds 1.5 g / m ² , reduction of iron oxide in the next step is likely to be insufficient and non-plating is likely to occur.

The temperature of the oxidation treatment should be 650 ° C. or lower at the material temperature. If this temperature is exceeded, the strain introduced into the surface by grinding will decrease and the oxidation reaction will be suppressed.
The generation of ide becomes insufficient, and as a result, reduced iron is not sufficiently generated in the subsequent reduction step.

Further, diffusion of Si to the surface is likely to occur and non-plating is likely to occur. There is no particular restriction on the lower limit of the heating temperature, but if the temperature is 300 ° C or lower, the oxidation rate is low and the treatment requires a long time, which is not preferable.

The steel sheet having the iron oxide layer formed on the surface is subjected to reduction treatment at a temperature of 500 to 650 ° C. in the next step. For this purpose, it is preferable to use an atmosphere having a dew point of −15 ° C. or lower, which contains 5% or more of hydrogen and the balance is a non-oxidizing gas (usually nitrogen). If the treatment temperature is lower than 500 ° C, the reducing power is weak and the reduction becomes insufficient, and if it exceeds 650 ° C, Si is likely to be diffused, and Si is concentrated again on the surface of the steel sheet, and unplating is likely to occur. The processing time may be 15 to 250 seconds.

In this reduction step, Fe-ox produced in the previous step
The ide is reduced and a reduced iron layer is formed. This reduced iron layer acts as a barrier for Si diffusion and, if it has a sufficient thickness, suppresses diffusion of Si to the surface during the reduction process. The steel sheet processed in the above process is cooled to about 380 to 550 ° C and Al is removed.
It is dipped in a hot dip zinc bath containing about 0.05 to 0.15% and plated, and the amount of adhesion is adjusted to a predetermined range by gas wiping to obtain a product as a hot dip galvanized steel sheet.

In the case of producing an alloyed hot-dip galvanized steel sheet, the steel sheet plated in the above step is passed through an alloying treatment furnace and heated at 450 to 600 ° C. for 1 to 120 seconds to alloy the plating film.

Thus, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet which is free from unplating and has excellent surface characteristics can be obtained. According to the method of the present invention, sound hot-dip galvanizing can be performed even on a steel sheet having a Si content of more than 1.5%.

[0035]

[Examples] Hot-rolled steel sheets (steel type A, sheet thickness 2.3 mm) and cold-rolled steel sheets (steel types B to D, sheet thickness 0.80) of high Si steel having the chemical composition shown in Table 1
mm, unannealed material) was cut into 250 × 100 mm to make a test material.

For steel types B to D, annealing was performed in advance in an atmosphere of N ₂ + 15% H _{2 with} a dew point of −30 ° C. and a temperature rising rate of 15 ° C./sec to 850 ° C. and holding for 120 sec. However, in Table 3, B'and D'show the steel types B and D that have not been annealed.

The steel plate thus prepared was ground by a brush roll in the range of 20 g / m ² or less per one surface. The steel plate after grinding was subjected to hot dip galvanizing through a vertical hot dip galvanizing apparatus capable of heat treatment in a predetermined atmosphere and capable of hot dip coating directly from a reducing atmosphere. That is, if necessary, the steel sheet is preheated and oxidized in various atmospheres of a to e shown in Table 2 in this apparatus, and then reduced in an atmosphere of N ₂ + 25% H ₂ for 600 ° C. × 60 seconds to 460 ° C. After cooling to the steel plate temperature, the total Al concentration is 0.112
%, Total Fe concentration 0.020%, hot-dip galvanizing was performed in the bath. The plating time was set to 1 second, and the amount of zinc deposited was adjusted to about 50 g / m ² per side with a gas wiper. The occurrence of non-plating was investigated after plating. Furthermore, some of the samples were alloyed at a temperature of 500 ° C. and the time required for alloying was measured. Table 3 shows the time required for alloying for those subjected to plating evaluation and alloying treatment.

[0038]

[Table 1]

[0039]

[Table 2]

[0040]

[Table 3 (1)]

[0041]

[Table 3 (2)]

As can be seen from the results shown in Table 3, there is no non-plating in the examples of the present invention, and the galvannealed steel sheet can be alloyed in a relatively short time. Of the comparative examples, only No. 6 which grinded a large amount close to 20 g / m ² showed good results, but it is difficult to carry out such large-scale grinding on an actual production line. Even if it can be implemented, soaring equipment costs and operating costs are inevitable.

[0043]

According to the method of the present invention, a hot dip galvanized steel sheet or an alloyed hot dip galvanized steel sheet having a Si-containing steel as a base material can be produced comparatively economically without causing non-plating.

Claims (2)

(57) [Claims] 1. A method for producing a hot-dip galvanized steel sheet using a steel sheet containing 0.2% or more of Si as a base material.
0.4 to 5 g / m ^{2 The} steel plate removed by grinding is treated in an oxidizing atmosphere at 650 ° C or less, and converted to Fe on the surface of the steel plate to 0.3 to 1.
A method for hot dip galvanizing a silicon-containing steel sheet, which comprises forming an iron oxide layer of 5 g / m ² and then reducing it at a temperature of 500 to 650 ° C. and then performing hot dip galvanizing. 2. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises hot-dip galvanizing in the step of claim 1 and then further subjected to alloying heat treatment.