JPH08170160A - Production of silicon-containing high tensile strength hot dip galvanized or galvannealed steel sheet - Google Patents

Abstract

PURPOSE: To prevent the occurrence of bare spot by securing wettability of plating and to efficiently produce a plated steel sheet with excellent surface quality by accelerating the velocity of alloying treatment at the time of producing a hot dip galvanized steel sheet and a galvannealed steel sheet in which a high tensile strength steel sheet of >=0.2wt.% Si content is used as a base material. CONSTITUTION: An oil or solution, containing calcium and/or magnesium compound, is applied to a steel sheet before plating or the calcium and/or magnesium compound is incorporated into rolling oil, washing water, etc., by which the calcium and/or magnesium compound in the amount of (0.1-500)mg/m<2> , in total, expressed in terms of metal is allowed to adhere to the surface of the steel sheet. Then, preheating is performed in a nonoxidizing atmosphere or in an oxidizing atmosphere in which the amount of adhering iron oxide after preheating reaches <=5g/m<2> expressed in terms of Fe. Successively, the steel sheet is heated and reduced at 600-900 deg.C and then plated by immersion in a molten zinc bath, followed by alloying treatment, if necessary.

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 an unplated and excellent surface quality, and a method for producing an alloyed hot-dip galvanized steel sheet, which uses an alloy steel, particularly a Si-containing high-tensile steel sheet as a base material. In particular, the present invention relates to a method for producing a rust-preventive steel sheet suitable as a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, in the industrial fields of home appliances, building materials, and automobiles, hot-dip galvanized steel sheets, which can be manufactured at relatively low cost, have been used in large amounts as rust-preventing steel sheets, and in particular, their economic efficiency and their rust-preventing function Alloyed hot-dip galvanized steel sheets are widely used in terms of performance after coating.

Hot-dip galvanized steel sheets are obtained by preheating the base steel sheet in a weakly oxidizing or reducing atmosphere after suitable degreasing and washing, or without degreasing and washing, and then H ₂
A continuous process consisting of heating and reducing the steel sheet in a reducing atmosphere of + N ₂ to anneal it, then cooling the steel sheet to near the plating temperature, immersing it in a molten zinc bath, and controlling the amount of deposit at the plating bath outlet. It is generally manufactured by a hot dip galvanizing process.

The galvannealed steel sheet is obtained by continuously galvanizing a steel sheet hot-dip galvanized as described above in a heat treatment furnace at a material temperature of 500-600 ° C.
By heating for 60 seconds, the entire plated layer was Fe-Zn alloyed by mutual diffusion between the zinc layer and the steel base. The plating layer is an Fe-Zn intermetallic compound, and generally has an average Fe concentration of 8 to 12 wt%.

The base materials of these hot-dip galvanized and alloyed hot-dip galvanized steel sheets have conventionally been low-carbon Al-killed steel sheets, ultra-low carbon Ti-added steel sheets, etc., but there is a demand for higher strength automotive materials. Along with this, Si-containing steel plates containing 0.2% by weight or more of Si are about to be used. Si-added steel has the advantage that it can improve strength while ensuring ductility, and is promising as a high-strength steel material for automobiles.

However, this Si-containing high-strength steel sheet has a fatal drawback in hot dip galvanizing. That is, in the above continuous hot dip galvanizing process, if a Si-containing steel plate is used as the plating base material, a trace amount of water in the atmosphere reacts with Si contained in the steel in the annealing process before plating, and the hot dip zinc A low-wettability Si oxide precipitates on the surface of the steel sheet in an amount that depends on the Si concentration in the steel. Therefore, as the Si concentration in the steel increases, the wettability with molten zinc sharply decreases and non-plating frequently occurs. Further, there is a problem that the Si oxide on the surface of the steel sheet suppresses the alloying reaction in the alloying process and reduces the alloying treatment rate.

It is known that the wettability of a Si-containing steel sheet with molten zinc is improved by forming a Fe oxide film on the surface of the Si-containing steel sheet by performing the preheating step before plating in a weakly oxidizing atmosphere. is there. However, when the Si content is as high as 0.2% by weight or more, preheating in a weak oxidizing atmosphere in the conventional molten zinc process (for example, if the air-fuel ratio of the non-oxidizing furnace is 1 to
1.35), it is difficult to secure wettability and prevent non-plating.Also, especially when using galvannealed steel sheet, the alloying processing speed is remarkably delayed, which greatly hinders production efficiency. There was a drawback.

In particular, in order to improve the formability of steel,
In the steel composition in which Si is added to the Ti-added ultra-low carbon steel base, the annealing temperature for recrystallization increases to 800 ° C or higher, so the precipitation of Si oxides on the steel plate surface increases and the wettability It becomes more difficult to secure the alloy, and the alloying processing speed becomes more noticeable.

In order to solve this problem, there are known a method of applying a thin underlayer electroplating of Ni, Fe or the like prior to hot dipping, or a method of oxidizing the steel sheet surface after grinding. However, the former undercoating method is not practical because it requires an electroplating step, which greatly increases equipment and production costs. The latter method of grinding before oxidation has problems that the inside of the heating furnace is heavily contaminated and the yield is reduced by grinding. In addition, in any of the methods, with steel having a high Si content of 0.2% by weight or more, neither sufficient prevention of non-plating by securing wettability nor delay of alloying treatment speed can be obtained sufficiently.

Therefore, even Si-containing high-strength steel sheets, which are attractive in terms of materials, cannot find a rational means for efficiently performing continuous hot-dip galvanizing and alloying treatment thereof.
This has hindered the spread of hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets using such steel sheets as base materials.

[0011]

The object of the present invention is that, in the conventional continuous hot-dip galvanizing process, practically satisfactory hot-dip galvanizing of Si-containing high-strength steel sheets is impossible,
In addition, it is to solve the problem that the productivity is very low when the alloying treatment is performed.

A specific object of the present invention is that the Si content is 0.2.
A high-strength steel sheet with a weight percentage of more than 100% is used as a base material, and hot-dip galvanizing is performed continuously without causing non-plating so that uniform alloying treatment is possible and a sufficient alloying treatment speed is obtained. A method for producing a galvanized steel sheet and a galvannealed steel sheet is provided.

[0013]

[Means for Solving the Problems] As a result of extensive studies by the present inventors regarding the wettability between steel and hot-dip zinc in hot-dip galvanizing of Si-containing steel, as is conventionally known, Si contained in the steel is known. Is concentrated on the surface of the steel sheet as Si oxide during heat reduction, and the surface energy of the Si oxide is small,
It was reconfirmed that the wettability with molten zinc was lowered.

Therefore, by adhering a specific compound on the surface of the steel sheet to suppress the surface concentration of the Si oxide, the wettability of the hot dip coating of the Si-containing steel sheet is improved, and at the same time, the alloying rate is accelerated. Repeated studies to achieve this.

As a result, an alkaline earth metal compound such as a calcium compound or a magnesium compound is obtained before the heat reduction.
(Example, nitrates, acetates, etc.) If adhered to the surface of the steel sheet, it was found that the surface concentration of Si oxide during heat reduction was significantly suppressed, and the above object could be achieved, and the present invention was reached. did.

The gist of the present invention is 0.2% by weight of Si.
When performing hot dip galvanizing on the Si-containing high-strength steel sheet containing the above, at least one compound selected from calcium compounds and magnesium compounds is added in a metal equivalent amount.
Preheat a steel sheet with 0.1 to 50 mg / m ^{2 on the} surface in advance in a non-oxidizing atmosphere, or oxidize it so that the amount of iron oxide after preheating is 5 g / m ² or less in terms of Fe. A method for producing a high-strength hot-dip galvanized steel sheet containing Si, characterized by preheating in an atmosphere, followed by heat reduction in a temperature range of 600 to 900 ° C, and then dipping in a hot dip zinc bath for plating. .

According to the present invention, further, the steps up to the hot dip galvanizing according to the above method are carried out, and then the alloying heat treatment of the plating layer is carried out. Manufacturing methods are also provided.

[0018]

According to the present invention, the surface of the steel sheet is preliminarily converted into metal.
By attaching a small amount of calcium compound and / or magnesium compound of 0.1 to 50 mg / m ² and then performing preheating and heat reduction, the phenomenon that Si oxide is concentrated on the steel plate surface during heat reduction is significantly suppressed. It is possible to avoid the problems caused by the surface concentration of the Si oxide, that is, the problem of non-plating due to the decrease in wettability with molten zinc and the problem of delaying the alloying process speed during the alloying process. can do.

When the calcium and / or magnesium compound is adhered to the surface of the steel sheet according to the present invention, the fact that the concentration of Si oxide on the surface of the steel sheet after heat reduction is reduced to less than half is actually shown on the surface of the specimen. It was confirmed by measuring the Si concentration by ESCA. Further, after depositing a calcium and / or magnesium compound on the surface of the steel sheet, the steel sheet surface is oxidized by a preheating step before heat reduction, which is conventionally known as a method for improving the wettability of hot dip galvanizing of Si-containing steel sheet. If the method of forming an iron oxide film is also used for the above, the wettability is further improved.

As described above, calcium and / or
Alternatively, the reason why the plating wettability can be ensured by depositing a magnesium compound in advance is presumed to be as follows. That is, Si, which has a higher affinity for oxygen than Fe
When such alloying elements as above are present in the steel, the easily oxidizable alloying elements are concentrated on the surface of the steel sheet during the preheating and heat reduction processes, and an oxide film that is extremely difficult to reduce is formed. At this time, Si
Since the oxide has a very small surface free energy, the wettability of the plating is reduced. However, when a calcium and / or magnesium compound that is more easily oxidized than Si is present on the surface of the steel sheet as described above, calcium and / or Since the magnesium compound forms an oxide preferentially over Si, the concentration of Si oxide on the surface of the steel sheet is suppressed.

The oxides of calcium and / or magnesium (CaO and / or MgO) formed on the surface of the steel sheet are
As disclosed in Japanese Patent Publication No. 56-9228, the coating amount of a suspension containing CaO after drying is 3 to 20 g / m ² (Ca equivalent 2.1 to 14 g / m ² ).
When it is present in an amount that completely covers the surface of the steel sheet, as in the case of applying in the range of, it acts as a plating inhibitor. However, as in the present invention, when a trace amount of Ca and / or Mg oxide that does not completely cover the steel plate surface is present,
The effect of preventing plating is small, but rather the effect of suppressing the concentration of Si oxide on the surface of the steel sheet due to the presence of Ca and / or Mg oxide is more remarkable. Moreover, since the surface free energy of Ca and / or Mg oxide is not as small as that of Si oxide, it is considered that the wettability with molten zinc during immersion in the plating bath is improved.

Further, after depositing calcium and / or magnesium compound on the surface of the steel sheet, preheating and heat reducing, then hot dip galvanizing, followed by alloying treatment, alloying and melting excellent in surface quality is performed. A galvanized steel sheet can be obtained, and the alloying treatment speed thereof can be improved. This is because the surface concentration of Si oxide is suppressed and Si
It is presumed that this is because the inhibition of the alloying reaction by the oxide is reduced.

FIG. 1 shows an example of a continuous galvannealing line that can be used to continuously carry out the method of the present invention. With reference to this figure, the method of the present invention will be described below in the order of processing steps. In the following description,% is weight% unless otherwise specified, but% related to gas concentration is vol%.

Base Material Steel Plate The steel plate targeted by the present invention is a steel plate containing Si.
The Si-containing steel sheet generally has poor wettability with molten zinc, is likely to cause non-plating, is likely to cause uneven alloying, and has a low alloying reaction rate. This phenomenon is due to the fact that the Si content in steel is 0.03%.
Although observed in the above steel sheets, it becomes extremely remarkable especially when the Si content is 0.2% or more. Therefore, the present invention is directed to a Si-containing steel sheet having a Si content of 0.2% by weight or more. The upper limit is not particularly specified, but the Si content is preferably about 2% or less from the viewpoint of material properties.

The other components of the base steel sheet are not particularly limited, and in addition to Fe and inevitable impurities, C, S, P, Mn, Ti, M
A carbon steel sheet containing one or more elements such as g, Cr, Ni, Cu, Nb, Ta and Al may be used. The content of these elements is not particularly limited, but is preferably within the following range. C <0.2%, S <0.03%, P <0.2%, Mn <2.0
%, Ti <0.1%, Mg <1.0%, Cr <2.0%, Ni <2.0
%, Cu <2.0%, Nb <0.1%, Ta <0.1%, Al <0.1
%. Regarding other elements, it is preferable that the content of each element is less than 0.01% and the total content is 2.0% or less.

Further, the plating base material is a steel sheet (eg, cold rolled steel sheet) which mainly requires in-line heat reduction, and changes in mechanical properties in the heating step before plating in the method of the present invention are particularly If it is not a problem, the method of the present invention can be applied to a steel sheet such as a hot-rolled steel sheet that is subjected to off-line annealing.

Calcium and / or magnesium compound
If necessary, the base steel sheet that has been subjected to the adhesion cold rolling or hot rolling is subjected to surface cleaning treatment such as pickling and degreasing, if necessary. These may be carried out by a conventional method. Generally, hot-rolled steel sheets are pickled and degreased, and cold-rolled steel sheets are only degreased. Degreasing can be either alkaline degreasing, organic solvent degreasing (eg thinner, degreasing with trichlene, etc.) or electrolytic degreasing, but in a continuous hot dip coating line 60 to 95 ° C 2-10% NaOH
Alkaline degreasing, which is performed by dipping in an aqueous solution for several seconds to several minutes, is common. The degreased steel plate is washed with water and dried if necessary.

In a normal hot dip galvanizing line, after degreasing, preheating and heat reduction are carried out in one or more heating furnaces, but in the present invention, calcium and / or magnesium compounds are attached to the surface of the plated steel sheet before that. . Calcium compounds include calcium oxide (CaO) and Ca during the heating process (preheating and heat reduction process) before plating.
Any calcium compound that changes to O 2 can be used. Similarly, as the magnesium compound, magnesium oxide (MgO), and any magnesium compound that changes into MgO during the heating step (preheating and heat reduction step) before plating can be used. Therefore, the calcium and / or magnesium compound previously attached to the surface of the steel sheet exists as an oxide on the surface of the steel sheet when immersed in the hot dip galvanizing bath.

Examples of calcium compounds that can be used include calcium oxide [CaO], calcium nitrate [Ca (N
O ₃ ) ₂ ], calcium chloride [CaCl ₂ ], calcium perchlorate
[Ca (ClO ₄ ) ₂ ], calcium hydroxide [Ca (OH) ₂ ], calcium carbonate [CaC ₂ ] and other inorganic calcium compounds, as well as organic acid calcium salts {eg, calcium acetate [(CH ₃ CO ₂ ) ₂ Ca
H ₂ O], calcium benzoate [(C ₆ H ₅ CO ₂ ) ₂ Ca ・ 3H ₂ O]
Etc.}, calcium alkoxide, calcium acetylacetone complex and other organic calcium compounds.

Examples of magnesium compounds that can be used include magnesium oxide [MgO], magnesium hydroxide [Mg
(OH) ₂ ], magnesium chloride [MgCl ₂ ], magnesium perchlorate [Mg (ClO ₄ ) ₂ ], and other inorganic magnesium compounds. In the present invention, one or more compounds selected from such calcium compounds and magnesium compounds can be used.

The timing of adhesion of the calcium and / or magnesium compound to the steel plate surface is not particularly limited as long as it is before the preheating step, and the adhesion method can also physically adhere the calcium and / or magnesium compound to the steel plate surface. Any method can be adopted. In short, in order to prevent the Si oxide from concentrating on the surface of the steel sheet during heat reduction,
It suffices that the calcium and / or magnesium compound is attached to the surface of the steel sheet by the heat reduction.

The calcium and / or magnesium compound may be adhered by, for example, applying a liquid containing a calcium and / or magnesium compound to a clean degreased steel plate surface to adhere the calcium and / or magnesium compound, and calcium. It is possible to use a rolling oil containing a magnesium compound and / or a magnesium compound to deposit a calcium and / or magnesium compound during rolling. The application of the calcium and / or magnesium compound is usually applied or sprayed in the form of a solution or a liquid dispersion, but it can also be applied directly to the surface of the steel sheet.

In the method of depositing the calcium and / or magnesium compound after degreasing, foreign matter such as Fe powder attached to the surface of the steel sheet during rolling can be removed, so that a product with a clean surface can be obtained. In this case, it is efficient to perform the step of depositing the calcium and / or magnesium compound in the plating line, but it is also possible to perform it in another line.

The adhesion of the calcium and / or magnesium compound after degreasing is carried out, for example, by adding a calcium and / or magnesium compound to an oil containing one or more kinds of animal oil, vegetable oil and synthetic oil as a main component ( It can be carried out by dissolving or dispersing in fine powder form) and applying it to the degreased steel plate surface. In this case, the oil adhering to the steel plate surface volatilizes due to the high temperature in the furnace,
When burner is heated, it burns and does not affect plating.

As another method, a solution prepared by dissolving a calcium and / or magnesium compound in water or an organic solvent such as alcohol is applied to the surface of the degreased steel sheet, and then heated if necessary to remove the water or organic solvent. Calcium and / or magnesium compounds can also be attached by evaporation.

If the surface of the steel sheet during rolling is relatively clean and it is not necessary to remove foreign matter after rolling, calcium and / or
Alternatively, the magnesium compound can be applied to the surface of the steel sheet by being contained in rolling oil during cold rolling. That is, animal oil,
Calcium and / or magnesium compounds are added to oils containing one or more of vegetable oils and synthetic oils as main components, and used as cold rolling oils to adhere calcium and / or magnesium compounds to the steel sheet surface. Can be made.

The amount of the calcium and / or magnesium compound attached to the surface of the steel sheet is the sum of the metal equivalents (Ca + Mg
If the amount is 0.1 mg / m ² or more, there is an effect of suppressing the concentration of Si oxide, and the effect increases as the amount of adhesion increases. However, the wettability improving effect becomes saturated when the amount of adhesion becomes about 50 mg / m ² , so the upper limit is set to 50 mg / m ² or less. When the surface of the steel sheet is completely covered with a calcium and / or magnesium compound, the calcium and / or magnesium compound acts as a plating inhibitor, which causes a decrease in wettability and non-plating. A coating weight of 1 g / m ² or more is required. Therefore, if the amount is 50 mg / m ² or less, such a decrease in wettability due to coating with a calcium and / or magnesium compound is not observed, and
On the contrary, since the surface concentration of the oxide is suppressed, the wettability is remarkably improved.

When the rolling oil contains a calcium and / or magnesium compound, the amount of the oil remaining after rolling is about 10 to 100 mg / m ² , and therefore the calcium and / or magnesium compound in the rolling oil is used. The content of may be 1% or more in terms of metal amount. The same applies to the case of applying in oil after rolling.

The above-mentioned calcium and / or magnesium compound adhesion treatment is carried out in the rolling step or in the subsequent pretreatment step of hot dip galvanization, but the calcium and / or magnesium compound adhesion in the present invention is performed in these lines. The adhesion process is not limited to the internal adhesion process and may be performed as a single process outside the line. In addition, calcium and / or magnesium compounds may be attached to the surface of the steel sheet by adding it to the pickling solution or the washing solution in the cleaning line.

Preheating and Annealing Step The steel sheet having the calcium and / or magnesium compound attached to the surface as described above is heated (preheating and annealing) before hot dipping . This may be performed according to a method that has been conventionally performed, and iron oxide on the surface of the steel sheet may be reduced to reduced iron after annealing.

For preheating, the steel sheet is heated in a non-oxidizing atmosphere or an oxidizing atmosphere. As described above, if the preheating is performed in an oxidizing atmosphere to oxidize the steel sheet in the preheating stage to form the iron oxide film on the steel sheet surface, the wettability is further improved. Therefore, an oxidizing atmosphere is preferable as the atmosphere. When the steel sheet is oxidized in the preheating stage, this oxidation can be performed in the preheating stage of the preheating continuous furnace. In this case, the amount of iron oxide on the surface of the steel sheet after preheating is 5 g in terms of Fe in consideration of the fact that the effect of improving the wettability with molten zinc is saturated and that the iron oxide is likely to peel off during stripping. / m ² or less, preferably 2 g / m ² or less.

As a specific atmosphere gas, in the case of a non-oxidizing atmosphere, an inert gas (eg, nitrogen and argon,
One or more rare gases such as neon) or a single gas, and an inert gas containing oxygen at a partial pressure of less than 0.05 of the total pressure can be used. In an oxidizing atmosphere, the total pressure
Oxygen-containing inert gas containing oxygen at a partial pressure of 0.05 to 0.4
(Including air) can be used. The preheating temperature is preferably
The temperature is 550 ° C or higher and the annealing temperature or lower.

The reduction after preheating is carried out in the temperature range of 600 to 900 ° C. in a reducing atmosphere containing hydrogen according to a conventional method. As the reducing atmosphere gas, hydrogen is contained in an amount of 2 to 25%, the rest is an inert gas, particularly nitrogen, and the dew point is −80 ° C. or higher,
Gases below ℃ are preferred. The holding time of the annealing temperature may be a time sufficient to reduce the iron oxide film on the steel sheet surface to reduced iron, but it is usually in the range of 10 to 60 seconds.

When the plating base material is a Si-containing steel plate, Si in the steel is concentrated as Si oxide on the surface of the steel plate during heat reduction at this high temperature, which lowers the plating wettability. According to this, the calcium and / or magnesium compound previously attached to the surface of the steel sheet is not oxidized (Ca
O, MgO), and this oxide hinders the surface concentration of the Si oxide, thus preventing deterioration of the plating wettability.

Plating Step After the heat-reduced steel sheet is cooled to the plating temperature, it is immersed in a hot dip galvanizing bath for plating. The plating process may be performed in the same manner as the conventional one. As the conditions for hot dip galvanizing, a plating bath with a bath temperature of 460 ± 30 ° C and an Al concentration of 0.03 to 0.2% is often used. If the Al concentration is less than 0.03%, Fe
-Zn alloying reaction is too fast and the film composition is not stable.
Dross is often generated, which is not preferable. Further, even if the Al concentration exceeds 0.2%, operational problems may occur due to the generation of dross. When alloying after plating, it is desirable that the Al concentration be 0.15% or less from the viewpoint of alloying reaction time.

The coating weight is not particularly limited, but is 50 to 250 g / m ^{2 on} both sides in the case of hot dip galvanized steel sheet and 25 to 75 g / m ² per one side in the case of alloyed hot dip galvanized steel sheet.
The degree is normal. It is difficult to obtain a thin coating with an adhesion amount of 25 g / m ² or less per one side by hot dip galvanizing. The coating weight is controlled by the coating weight control means (eg, gas wiper) provided directly above the plating bath. After the plating process, the hot-dip galvanized steel sheet shipped is cooled to room temperature by an appropriate cooling means (eg, water cooling or air cooling) and then wound.

Alloying Heat Treatment Step In the case of a galvannealed steel sheet, the alloying heat treatment after plating may be carried out in the same manner as in the conventional case. The heat treatment is performed under conditions sufficient to change the plated layer into the Fe-Zn alloy layer. Usually, the Fe-Zn alloy layer with the Fe concentration adjusted to 9 to 12% is formed by heating at a temperature of 420 to 600 ° C for about 3 to 60 seconds. As the heating method, direct flame heating by combustion gas or radiant heating, infrared heating, direct current heating,
Any conventionally used heating method such as induction heating can be used. After the alloying heat treatment process is finished, cool to room temperature,
Being rolled up.

In the above, the case where the method of the present invention is carried out in an ordinary Zenzimer type continuous hot-dip galvanizing facility or a facility with a slight improvement has been mainly described.
The method of the present invention is not limited to implementation in these facilities, for example, as in the case of performing only annealing in a separate line, after preliminarily depositing calcium and / or magnesium compound on the steel plate surface, Si to its surface
A process of performing hot dip galvanizing while suppressing the generation of oxides and annealing and preventing oxidation by a flux treatment or the like may be performed.

[0049]

[Examples] Five types of ultra low carbon steel hot rolled steel sheets shown in Table 1
(Unannealed material) as a test material, after cold rolling, or by degreasing the hot-rolled steel sheet as it is, the following method A for adhering calcium and / or magnesium compounds, or calcium and / or calcium in rolling oil for cold rolling. Alternatively, the method B described below in which a magnesium compound is included causes calcium and / or magnesium compound to adhere to the surface of the steel sheet before plating, and then hot dip galvanization and alloying treatment are performed by the method described below to perform non-plating after plating. The state of occurrence of alloying, the occurrence of uneven alloying, and the time required for alloying treatment were investigated.

[0050]

[Table 1]

[Calcium and / or Magnesium Compound Adhesion Method A] The above hot-rolled steel sheet test material was cold-rolled to a thickness of 0.8 mm by using ordinary rolling oil, or the hot-rolled steel sheet (sheet thickness 2.
3 mm) as it is, cut into 220 mm (length) × 80 mm (width), degreased with 10% NaOH aqueous solution, and then (a) or
A calcium or magnesium compound was attached to the surface of the steel sheet by the method shown in (b). In any attachment method,
The amount of calcium or magnesium compound (calculated as Ca or Mg metal) adhering to the steel sheet after the adhering treatment was measured by the fluorescent X-ray method.

(A) <Oil> method: powdered calcium oxide Ca
O ₂ , calcium nitrate Ca (NO ₃ ) ₂ , magnesium oxide MgO ₂ ,
One or two kinds of magnesium carbonate MgCO ₃ are added to an oil containing animal oil (beef tallow) as a main component in a predetermined amount and dispersed, and this oil is applied to the surface of the steel sheet.

(B) <Water + Alcohol> Method: 1 of calcium acetate (CH ₃ CO ₂ ) ₂ Ca · H ₂ O and calcium chloride CaCl ₂
The seeds or two are added in a predetermined amount to water containing 5% by weight of isopropyl alcohol and dissolved, and the resulting solution is applied to the surface of the steel sheet by a sprayer, and alcohol and water are evaporated at 100 ° C.

[Calcium and / or Magnesium Compound Adhesion Method B] <Oil> Method: Using the above hot rolled material with a rolling oil containing a calcium and / or magnesium compound, a plate thickness of 0.8 mm
Cold rolled. As the rolling oil used, one or two kinds of powdered calcium nitrate Ca (NO ₃ ) ₂ , calcium oxide CaO, and magnesium carbonate MgCO ₃ were added in a predetermined amount to an oil containing animal oil (beef tallow) as a main component. It was dispersed. Cold rolled steel sheet with calcium and / or magnesium compound adhered in this way is 220 mm (length) × 80 mm (width)
After cutting into pieces, the pieces were subjected to a plating step without degreasing.
Also in this case, the amount of calcium and / or magnesium compound (metal conversion amount) adhering to the cold-rolled steel sheet is measured by fluorescence X.
It was measured by the line method.

[Plating Method] The test material having the calcium and / or magnesium compound adhered to its surface by the above method A (a), A (b) or B is directly hot-dipped from a reducing atmosphere. Preheating, heat reduction, and hot dip coating were performed using an experimental vertical hot dip galvanizing apparatus that can be charged into the bath. The sample for which surface analysis was performed after preheating was immediately cooled in nitrogen gas after preheating.

Preheating before plating was carried out by any of the following methods (a) to (c) in the vertical hot dipping apparatus. In all cases, the heating rate was 15 ° C./s, and the total pressure of the atmosphere gas was 1 atm. Of these, the method (a) was preheating in a non-oxidizing atmosphere in which substantially no oxidation occurred.
On the other hand, the methods (b) and (c) were preheating in an oxidizing atmosphere, and an iron oxide film was formed on the surface of the steel sheet, so immediately after preheating, a part of the test material was replaced with nitrogen gas as described above. After cooling in, the amount of iron oxide deposited on the surface was measured by a fluorescent X-ray method.

(A) Heat to 550 ° C. in nitrogen gas containing 10 ppm of oxygen. (b) Heat to 600 ° C in nitrogen gas containing 500 ppm of oxygen. (c) Heat to 600 ° C in air.

A preheated steel sheet was then applied, and then an oxygen concentration of 2 ppm
At 26% H ₂ -N ₂ mixed gas (dew point -30 ° C.) in, 600-850
After heat reduction at 60 ° C for 60 seconds, in the same atmosphere at 460 ° C
It was cooled to 0.12% Al—Zn bath (bath temperature 460 ° C.) for 1 second to perform hot dip galvanizing. After plating, the amount of deposited Zn was adjusted to about 50 g / m ² (per surface) with a gas wiper.

After plating, the occurrence of non-plating was evaluated by visual observation of the plating surface according to the following criteria. ◎: When there is no non-plating point after 10 or more observations, ○: Non-plating point less than 1 mm ² out of 10 times (pinhole)
Is observed even once, △: When 1 mm ² or more of non-plating spots is observed 1 to 4 out of 10 times, ×: 1 mm ² or more of non-plating spots is observed 5 times or more of 10 times If done.

The hot-dipped test material was then heated in a salt bath at 500 ° C. for alloying treatment. The alloying process is
The treatment was performed until the Fe concentration in the coating became 9 to 11%, and the treatment time required until the completion of alloying was measured as the alloying time. After alloying, the test materials were visually observed to investigate the occurrence of uneven alloying. The case where no alloying unevenness occurred was evaluated as ◯, and the case where it occurred was evaluated as x.

Furthermore, a conventional hot-dip galvanized steel sheet (Test Nos. 15 and 16) obtained by performing pretreatment and plating under exactly the same conditions except that the adhesion treatment of calcium and / or magnesium compounds was not performed. The alloy materials were similarly alloyed in a salt bath at 500 ° C, and the alloying speeds (time until completion of alloying = alloying time) were compared. By depositing the calcium and / or magnesium compound according to the present invention, the alloying time is shortened by 5 seconds or more as compared with the non-adhered conventional method, ⊚, and the one in which the alloying time is shortened by 1 to 5 is ∘, and almost no difference Those in which no was observed were marked with x.

With respect to the steel types 3, 4 and 5, unless calcium and / or magnesium compounds were adhered according to the present invention, the entire surface was unplated in the hot dip galvanizing process and no plating film was adhered. Could not be done, and thus the alloying rates could not be compared. Therefore, for steel types 3, 4, and 5, the alloying rate evaluation column is displayed as "-". Table 2 (inventive examples) and Table 3 (comparative examples) separately show the deposition method, deposition conditions and deposition amount of the calcium and / or magnesium compound, the amount of iron oxide on the surface of the steel sheet after preheating, and the evaluation results.

[0063]

[Table 2]

[0064]

[Table 3]

As is clear from Table 3, when the calcium and / or magnesium compound is not adhered as in the conventional case, the Si content is 0.2% as shown in Test Nos. 15 and 16.
In the above Si-containing steel sheet, non-plating occurred during hot dip galvanizing, and especially when the Si content was around 1% or more, the whole surface became non-plating. Test No. 16, which oxidizes the surface of the steel sheet in the preheating process, improves the plating result, but the degree of improvement is slight, and even if the Si content is less than 1%, non-plating cannot be prevented and the Si content is When it was 1% or more, the whole surface was not plated.

On the other hand, by depositing calcium and / or magnesium compounds in advance on the surface of the steel sheet according to the present invention, as shown in Table 2, the Si-containing steel sheet is used as the base material without causing non-plating. It was possible to obtain a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent surface quality. Further, the alloying reaction was promoted, and the alloying reaction time could be significantly shortened.

When the Si content becomes a high concentration of about 1% or more, uneven alloying may occur only by the adhesion of the calcium and / or magnesium compound according to the present invention, but even in that case, in the preheating step By oxidizing the surface of the steel sheet, it was possible to completely prevent the occurrence of alloying unevenness when the Si concentration was as high as 1% or more. However, when the total amount of adhered calcium and / or magnesium compounds was less than 0.1 mg / m ^{2 in} terms of metal, as shown in Table 3, there was some improvement effect, but the effect was insufficient.

[0068]

As described above, according to the present invention, non-plating is apt to occur due to deterioration of plating wettability.
Using the Si-containing high-strength steel sheet as the base material, it is possible to produce hot-dip galvanized steel sheet with no unplating and excellent surface quality, and when alloying after plating, there is no uneven alloying. A plated steel sheet can be efficiently manufactured with a faster alloying reaction time than before.

[Brief description of drawings]

FIG. 1 is an illustration of a continuous hot dip galvanizing line that can be used to carry out the method of the present invention.

Claims (2)

[Claims] 1. When performing hot dip galvanizing on a Si-containing high-strength steel sheet containing 0.2% by weight or more of Si, at least one selected from a calcium compound and a magnesium compound.
Pre-heat a steel sheet with a total of 0.1 to 50 mg / m ² of metal compounds deposited on the surface in a non-oxidizing atmosphere, or the amount of iron oxide deposited after pre-heating is 5 g / Fe. preheated in an oxidizing atmosphere so that m ² or less, followed by 600-90
A method for producing a high-strength hot-dip galvanized steel sheet containing Si, which comprises performing heat-reduction in a temperature range of 0 ° C. and then immersing in a hot-dip zinc bath for plating. 2. The production of a Si-containing high-strength hot-dip galvanized steel sheet, which comprises subjecting the steps up to hot dip galvanizing according to the method of claim 1 to an alloying heat treatment of the plating layer. Method.