JP2964911B2 - Alloying hot-dip galvanizing method for P-added high-strength steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved hot-dip galvanizing method for a P-added high-tensile steel material, and more particularly to a method for producing an alloyed hot-dip galvanized steel sheet suitable as a steel sheet for automobiles.

[0002]

2. Description of the Related Art In recent years, in the industrial fields such as home appliances, building materials, and automobiles, hot-dip galvanized steel sheets that can be produced at relatively low cost have been used in large quantities as rust-preventive steel sheets. In terms of performance after painting, galvannealed steel sheets are widely used.

[0003] A hot-dip galvanized steel sheet is preheated in a weakly oxidizing atmosphere or a reducing atmosphere after a suitable degreasing and washing step or without degreasing, and then a reducing atmosphere of hydrogen and nitrogen (reducing atmosphere). In a continuous hot-dip galvanizing method, the steel sheet is annealed in a furnace, cooled to near the plating temperature, immersed in molten zinc, and the amount of coating is controlled at the exit of the plating bath (eg, with a gas wiping nozzle). Manufactured. The coating weight is 20-15 per side
A range of 0 g / m ² is common. It is difficult to produce a plated layer having an adhesion amount of 20 g / m ² or less by a normal hot-dip galvanizing method.

[0004] In hot-dip galvanizing, it is considered that a thin oxide film of about 80 nm, that is, a film of iron oxide is formed on the steel sheet surface during preheating in terms of wettability with hot-dip zinc. This thickness corresponds to about 0.04 g / m ² in terms of Fe when converted to the amount of iron oxide attached. However, it has been considered that the formation of an oxide film having a thickness larger than that has an adverse effect on dross generation and adhesion of hot-dip plating.

[0005] The hot-dip galvanized layer is formed at the plating / steel interface.
The Fe-Zn alloy layer adheres to the iron substrate by forming, but this alloy layer is hard and brittle because it is an intermetallic compound. Therefore,
To suppress the formation of this alloy layer and prevent the alloy layer from becoming unnecessarily thick, 0.08 to 0.14 wt%
Al is present. As a result, the film processability is maintained, and the powdering resistance of the plating film is secured.
Generation of dross during production is suppressed.

[0006] The alloyed hot-dip galvanized steel sheet is prepared by heating a steel sheet continuously hot-dip galvanized by the above-mentioned method to a material temperature of about 500 to 600 ° C in a heat treatment furnace immediately after leaving the plating bath.
By heating for up to 60 seconds, the entire plating layer is made into an Fe-Zn alloy by mutual diffusion between the zinc plating layer and the steel base. The plating layer becomes an Fe-Zn intermetallic compound, and its average Fe concentration is generally 8 to 12 wt%.

[0007] The coating weight of the galvannealed steel sheet is usually about 25 to 70 g / m ² per side. When the adhesion amount exceeds 70 g / m ² , it is difficult to secure the powdering resistance of the alloyed plating layer, and therefore, it is not generally supplied to the galvannealed steel sheet. In the case of an alloyed hot-dip galvanized steel sheet, Al is also present in the plating bath for the same purpose as above, but since Al exerts an inhibitory effect on the alloying reaction after hot-dip coating,
Usually, the Al concentration is 0.08 to 0.11 wt%, which is generally lower than that of hot-dip galvanizing.

Conventionally, low-carbon Al-killed steel sheets, ultra-low-carbon Ti-added steel sheets, etc. have been mainly used as the base material of these hot-dip coated steel sheets. However, with the demand for higher strength of automotive materials, ductility and toughness have been increased. In order to obtain superior materials, P-added steel is being used.

However, it is known that, in a steel material to which P is added, if the P content exceeds 0.02 wt%, the alloying speed after hot-dip galvanizing is significantly delayed, and the production efficiency is reduced. At the same time, if the P content exceeds 0.02 wt%, the adhesion of the plating film is also a problem, and as a result, the film peels off when the vehicle body undergoes stone splashing in the cold season. This causes rust to occur starting from the peeling point.

[0010] Regarding the promotion of alloying, there is a method of performing pre-Fe plating (JP-A-57-79160). However, since electroplating is added, there are problems such as a large increase in equipment cost and production cost, which is not practical.

Further, in order to promote alloying, a Si-added steel having the same problem is pre-oxidized in a non-oxidizing furnace so as to form a thick oxide film on the surface of the steel sheet, and then annealed in a reducing furnace. JP-A-55-122865) has been proposed. This is because reducing the iron oxide generated by pre-oxidation
The generated reduced iron has a large surface area, and utilizes the phenomenon that the Fe-Zn reaction at the time of alloying is promoted by the increase in the reaction area.

Japanese Patent Application Laid-Open No. 5-306448 discloses that a steel sheet having P ≧ 0.03 wt% or more, which does not pass through a non-oxidizing furnace, has an oxide film thickness reduced in a reducing furnace divided into two or more zones having different dew points. A method has been proposed in which reduction is carried out so as to control the temperature in the first zone to 100 to 1000 ° and in the second zone to 200 ° or less.
However, this method is difficult to control.

Also, none of the above methods has an effect of improving the adhesion of the plating film. Attempts have been made to improve the film adhesion by controlling the conditions during the alloying treatment, but it has not been able to achieve a sufficient effect.

[0014]

In the galvannealing of P-added steel, a manufacturing method capable of promoting alloying to make the production economical and efficient and at the same time improving the adhesion of the plating film. Is required. An object of the present invention is to establish a method for galvannealing a P-added high-strength steel material in response to this demand. In particular,
An object of the present invention is to provide a galvannealing method for a P-added high-strength steel material capable of accelerating the alloying speed of a high-strength steel material with P> 0.02 wt% and simultaneously forming a plating film having good adhesion. .

[0015]

Means for Solving the Problems The present inventors have established that P> 0.02
The effect of heat treatment before plating of wt% P-added steel on alloying rate was examined.

As a result, it was confirmed that the alloying rate was accelerated when preoxidation was performed so as to form a thick oxide film exceeding 1 g / m ^{2 in} terms of Fe and then reduced at 750 ° C. or more. . However, formation of such a thick oxide film has a problem of peeling of the oxide film in a reduction furnace. On the other hand, when the amount of iron oxide in the pre-oxidation was reduced to less than 1 g / m ^{2 in} terms of Fe and then reduced at 750 ° C. or higher, the effect of accelerating the alloying rate was small.

As a result of studying a method capable of accelerating the alloying speed while keeping the amount of iron oxide at less than 1 g / m ² ,
It has been found that the alloying speed is enhanced by setting the reduction temperature after pre-oxidation at 550 ° C. or higher and lower than 750 ° C., which is lower than the recrystallization temperature required for annealing. However, in this case, since recrystallization did not occur, material properties such as ductility and toughness were poor. This problem can be solved by using a material that has been annealed in advance. However, a separate annealing step is required, which is not practical because of an increase in cost due to an increase in the number of steps.

Then, as a result of examining the annealing in the hot-dip plating equipment, the pre-oxidation of less than 1 g / m ^{2 in} terms of Fe was performed, followed by heating to the recrystallization temperature in an atmosphere in which iron oxide was not reduced to perform annealing. It has been found that alloying is promoted when the iron oxide is cooled to 750 ° C. or lower in this atmosphere and reduced at 550 ° C. or higher and lower than 750 ° C. Further, it has been found that the adhesion of the plating film can be improved by using a hot-dip galvanizing bath having a relatively high Al content of 0.12 wt% or more.

Here, the present invention relates to a method in which a high-strength steel material of P> 0.020 wt% is heated to 900 ° C. in a non-reducing atmosphere at a temperature not lower than the recrystallization temperature.
After heating to the following temperature range, cool down to 750 ° C or less,
During this time, 0.05 g / m ² or more, 1.0 g / m ²
An alloy of a P-added high-strength steel material characterized by forming the following oxide film, reducing it in a reducing atmosphere of less than 750 ° C and at least 550 ° C, applying galvanizing, and performing an alloying heat treatment. This is a chemical galvanizing method.

According to this method, the alloying time can be reduced as compared with the conventional method. In a preferred embodiment, the hot-dip galvanizing is performed in a plating bath having an Al concentration in the bath of 0.12 to 0.20 wt%. Thereby, the plating adhesion is improved as described above.

[0021]

Hereinafter, the present invention will be described in more detail.
In the following description,% is wt% unless otherwise specified.
%, But the percentage with respect to the gas composition is vol%. Further, in the following description, a case where the plating base material is a steel plate will be described. However, the method of the present invention is not limited in principle to alloyed hot-dip galvanizing of a steel plate.
It goes without saying that other steel materials such as deformed materials can be applied.

The reason why the heat treatment before hot-dip plating of a P-added steel sheet according to the method of the present invention promotes alloying after plating has not been completely elucidated, but is presumed as follows.

When the P-added steel is pre-oxidized, P is simultaneously oxidized, and the resulting oxide film contains a trace amount of P oxide together with iron oxide. Thereafter, when reduction annealing is performed at 750 ° C. or higher as in the conventional case, the iron oxide is reduced to reduced iron, and at the same time, a small amount of coexisting P oxide is also reduced.
The reduced P participates in the Fe-Zn reaction during alloying and significantly delays this reaction. Therefore, in this case, although the activation of the Fe-Zn reaction is obtained by the increase in the effective surface area due to the generation of reduced iron, this activation is offset by the inhibition of the reaction by the reduced P, and the alloying promotion effect is reduced. Become smaller.

On the other hand, when the reduction is performed at a relatively low temperature of less than 750 ° C. as in the method of the present invention, the iron oxide is reduced to reduced iron, but the P oxide is not reduced. Therefore, the composition of reduced iron contains iron and a trace amount of P oxide. Unlike reduced P, P oxide does not participate in the Fe-Zn reaction at the time of alloying, so that the steel sheet surface has a high reactivity close to that of pure iron. Therefore, it is considered that the effect of increasing the effective surface area by the reduced iron is sufficiently exhibited, and the alloying speed is dramatically improved. However, when the amount of iron oxide exceeds 1 g / m ² , there is a problem that the oxide remains because the reduction is performed at a relatively low temperature or the oxide is separated in a furnace.

Although the reason why the adhesion of the plating film is improved by increasing the Al concentration in the bath has not been completely elucidated, it is presumed as follows at present. The surface to be plated made of reduced iron has a high activity, and if the Al concentration in the bath is high, a large amount of an Fe-Al alloy layer is produced at the interface during plating. this
Since the Fe-Al alloy layer suppresses the Fe-Zn reaction during alloying, the Fe-Zn reaction is unlikely to occur when the temperature of the alloying heat treatment is increased, and the Fe-Al alloy layer is destroyed at a high temperature and alloying proceeds. I do. The reaction at a high temperature causes the Fe—Al alloy layer to break unevenly, so that irregularities are formed at the film interface. It is considered that these irregularities have led to an improvement in adhesion.

The plating base material in the plating method of the present invention is mainly a steel sheet (eg, a cold-rolled steel sheet) that requires in-line reduction annealing in a continuous galvanizing apparatus.
As long as the change in mechanical properties during the heat treatment process of the present invention is not particularly problematic, the present invention can be applied to a steel sheet which is subjected to out-of-line annealing, such as a hot rolled steel sheet.

The steel type to be used in the method of the present invention is a P-added high-strength steel. When the P content is more than 0.020%, there is a problem in that the alloying speed is delayed and the plating adhesion is lowered. Therefore, the method of the present invention is applied to steel having a P content of more than 0.020%. The upper limit of the amount of P added is not particularly limited, but the upper limit is about 0.2% as a region where cracks and the like do not enter the steel sheet in hot and cold rolling.

The other components of the base steel sheet are not particularly limited. In addition to Fe and inevitable impurities, C, S, Si, Mn, Ti, M
One or more elements such as g, Cr, Ni, Cu, Nb, Ta, and Al can be contained. In order not to lower the mechanical properties of the high-strength steel sheet, it is preferable that these elements have the following steel concentrations. Si <0.05%, C <0.2
%, S <0.03%, 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%. Other elements are preferably less than 0.01% for each element and up to 2.0% or less in total.

FIG. 1 shows an apparatus for manufacturing an alloyed hot-dip galvanized steel sheet (a continuous hot-dip galvanizing line equipped with an alloying heat treatment furnace) that can be used to continuously carry out the method of the present invention.
Here is an example. The method of the present invention will be described below in the order of steps with reference to FIG.

Degreasing Step The base steel sheet subjected to cold rolling or hot rolling is first degreased as required. Degreasing is performed, for example, by immersing in a 2-3% aqueous sodium hydroxide solution at about 60 ° C. for 10 to 300 seconds. Alternatively, degreasing with an organic solvent such as trichlene or thinner, electrolytic degreasing in an aqueous sodium orthosilicate solution, and the like are also possible.

The steel sheet which has been degreased as required in the pre-oxidation and annealing steps is sufficiently washed with water, dried with hot air in a drier, sent to a heating furnace, and heated in a non-reducing atmosphere to a temperature not lower than the recrystallization temperature and not higher than 900 ° C. After heating to the temperature range, the pre-oxidation and annealing are performed by cooling to 750 ° C or lower.

The oxide film formed on the steel sheet surface by the pre-oxidation has an iron oxide content of 0.05 g / m ² or more and 1.0 g / m ² or less in terms of Fe after the above-mentioned heating and cooling. To be. If the amount of iron oxide is less than 0.05 g / m ² , the coating of the oxide is small, and even if reduced iron is generated in the next reduction step, the alloying promoting effect cannot be sufficiently obtained. If the amount of iron oxide is more than 1.0 g / m ² , iron oxide is picked up in the furnace, causing surface defects.

The pre-oxidation may be carried out in any temperature range while the steel sheet is heated from room temperature to the above temperature range. However, since the amount of iron oxide is as small as 1.0 g / m ² or less, the speed and control of the oxidation reaction are controlled. In consideration of easiness, it is preferable that the oxidation mainly proceeds in the temperature range of 500 to 750 ° C. at the beginning of the oxidation. At a high temperature of 750 ° C. or higher, the oxidation reaction is so fast that it becomes more difficult to control the amount of iron oxide.

In this case, as shown in the figure, the inside of the heating furnace in the non-reducing atmosphere is divided into a preheating furnace and an annealing furnace, and heating is performed in the temperature range of 500 to 750 ° C. in the first preheating furnace. ,
Perform most of the necessary oxidation. While passing through the next annealing furnace,
The temperature is raised above the recrystallization temperature (about 750 ° C) to 900 ° C,
Recrystallize. The non-reducing atmosphere in the annealing furnace has a lower oxidizing property than the atmosphere in the preheating furnace, and does not cause reduction of the oxide film, but does not cause much oxidation. Then, it is cooled to 750 ° C. or lower in a non-reducing atmosphere in a cooling zone provided before the reduction furnace. Thereby, annealing can be performed without increasing the amount of iron oxide. By performing the oxidation mainly in the temperature range of 500 to 750 ° C, the amount of iron oxide after annealing (after cooling) is reduced to 0.05 to 1.
It can be easily controlled within the range of 0 g / m ² .

In the preheating furnace, the temperature is raised to a temperature range of 500 to 750 ° C. necessary for oxidation by a heating method such as burner heating, electric heating, induction heating, or infrared heating. The atmosphere in the furnace is a weakly oxidizing atmosphere in which necessary pre-oxidation of 1.0 g / m ² or less occurs. The preferred atmosphere is oxygen (O ₂ )
Is an oxidizing atmosphere containing 5 to 20,000 ppm, with the balance being an inert gas (N ₂ is cheap and preferred). Instead of O ₂
H ₂ O (dew point 0 to + 40 ° C.) and a rare gas such as Ar or He may be used instead of N ₂ . In the case of the burner heating method, it is possible to oxidize by setting the mixture ratio (air-fuel ratio) of air and fuel gas to 1.0 or more.

The steel sheet oxidized in the preheating furnace is then heated in an annealing furnace to a temperature within a temperature range of 750 to 900 ° C., kept at this temperature for a short time, and cooled in a cooling zone.
Perform annealing. Annealing is necessary to remove strain generated during rolling, particularly during cold rolling, and to obtain appropriate material properties (such as ductility and toughness). Recrystallization of P-added steel generally requires temperatures above 750 ° C. The upper limit of 900 ° C is that annealing at a higher temperature not only causes the base material crystal grains to become coarse and deteriorates the material properties, but also causes the steel sheet to soften and lose its shape, resulting in a risk of fracture. Because there is also the nature.

This annealing is performed in a non-reducing atmosphere so as not to cause reduction during annealing. When reduction occurs during annealing, a small amount of P oxide contained in the oxide film is reduced as described above, and the effect of accelerating the alloying rate is significantly reduced. Also, since the annealing is performed at a high temperature, if the oxidizing property of the atmosphere is strong, the oxidation proceeds more remarkably, and not only pick-up of the iron oxide occurs in the annealing furnace, but also the iron oxide cannot be completely reduced in the next reduction step. Plating may occur.
For this reason, it is preferable that the annealing atmosphere be a slightly oxidizing or substantially inert atmosphere having significantly lower oxidizing property than the atmosphere of the preheating furnace.

As the substantially inert or slightly oxidizing atmosphere gas gas suitable for annealing, (a) an N ₂ -O ₂ gas containing ₁ to 500 ppm of O ₂ and the balance consisting of N ₂ , and (b) ) 0.4 ≦ P (H
H ₂ OH ₂ —N ₂ gas of about ₂ O) / P (H ₂ ) ≦ 1.0.
As in the pre-oxidation atmosphere, a rare gas such as Ar or He may be used instead of N ₂ . The lower limit of the O ₂ or H ₂ O concentration of these gases is a minimum concentration that does not cause reduction of Fe, and the upper limit is a concentration that does not significantly promote iron oxidation in a high-temperature annealing furnace.

In order to maintain the gas atmosphere in the annealing furnace as described above, it is preferable to shut off the atmosphere between the preheating furnace and the annealing furnace. This interruption of the atmosphere is achieved by providing a seal roll or throat between the preheating furnace and the annealing furnace, or by using a gas shield such as an air curtain. If the more oxidizing gas in the preheating furnace enters the higher temperature annealing furnace, the oxidation may be promoted too much in the annealing furnace, and the above-mentioned problem may occur.

The heating of the steel sheet in the annealing furnace can be achieved by a heating method such as induction heating, electric heating, radiant tube, infrared heating, and the like. Since the purpose is annealing, the rate of temperature rise in the annealing furnace is not particularly limited, but rapid heating is preferable in terms of production efficiency. Actually, in the above gas atmosphere, a temperature rising rate of 10 ° C./s or more, particularly 10 to 100 ° C./s, is sufficient. The holding time at the annealing temperature (attained temperature) is the time required for recrystallization.
Seconds are sufficient. The actual holding time will be about 10 to 100 seconds, depending on the heating method and its control method.

When the steel sheet at the annealing temperature is immediately sent to the next reduction furnace, the iron oxide film on the steel sheet surface is reduced at a high temperature of 750 ° C. or more, and the P oxide is reduced. The steel sheet is cooled in an atmosphere similar to that of the annealing in the annealing furnace or in a cooling zone during transfer to the reduction furnace so that the temperature falls to 750 ° C. or less. The cooling rate at this time is not particularly limited, but is preferably 5 to 20 ° C / s.

It should be noted that it is preferable to perform heating separately in the preheating furnace and the annealing furnace as described above because the amount of iron oxide can be easily controlled, but this is not always essential. Although it becomes more difficult to control the amount of iron oxide, the oxidizing property of the annealing furnace is slightly increased,
Only heating to 750-900 ° C in an annealing furnace can achieve both pre-oxidation and annealing purposes. In short, the steel sheet that has been annealed and cooled to 750 ° C. or less has not been substantially reduced after oxidation and has an iron oxide content of 0.05 to 1.0 g / m ^2.
It should just be in the range of.

Reduction Step As described above, the steel sheet annealed without reduction is then sent into a reduction furnace. While passing through the reduction furnace, 750
Reduces iron oxide on the steel sheet surface to reduced iron at a temperature lower than ℃ and higher than 550 ℃. However, since the reduction temperature is lower than 750 ° C., a small amount of the P oxide present in the iron oxide is not reduced and remains in the oxide state. As described above, this P oxide is inactive in the alloying step after plating, and does not cause a delay in the Fe-Zn reaction. The conversion speed is significantly increased.

If the reduction temperature is lower than 550 ° C., the reduction rate is low, and iron oxide may remain on the steel sheet surface, which may cause non-plating. When the reduction temperature exceeds 750 ° C., the P oxide in the iron oxide is reduced together, and the reduced iron contains reduced P at a concentration equivalent to that of the base material, which participates in the Fe-Zn reaction, Since the reaction is delayed, the effect of promoting alloying is reduced.

The reduction is performed until the iron oxide on the steel sheet surface is reduced to reduced iron. Preferred reduction times are in the range of 30 to 120 seconds. If the time is less than 30 seconds, the iron oxide cannot be completely reduced, and non-plating occurs. Also, over 120 seconds,
Measures such as lengthening the line length or decreasing the line speed are required, and cost for equipment or production is required. In order to maintain the temperature, heating equipment is generally required even in the reduction furnace. Heating method is induction heating, electric heating,
Radiant tube method, infrared heating method, etc. are possible.

Since the pre-process is a non-reducing atmosphere in which iron oxidation can occur, it is necessary to shut off the atmosphere between the pre-process and the reduction process. For this blocking, a seal roll, a throat, an air shield, or the like can be used.

The reducing atmosphere is set so that iron oxide is reduced in the above temperature range, but P oxide is not reduced. Specifically, the hydrogen concentration is 2 to 25% and the dew point is -60 ° C.
If it is 0 ° C., such a condition is satisfied. Low dew point,
The higher the hydrogen concentration, the easier the iron oxide is to reduce, so that the line speed can be increased and the productivity is improved. But,
When the reduction temperature is relatively high within the range of the present invention, that is, 700 ° C. or higher and lower than 750 ° C., when the reduction is performed in a reducing atmosphere having a low O ₂ (H ₂ O) potential, the above-described P oxide Since reduction may occur and the improvement effect may be reduced, it is preferable to set the dew point to -30 ° C or higher and the reduction time to 30 to 90 seconds.

The oxidation annealing step under the conditions described above molten zinc plating step, and after a reduction step, the steel sheet was cooled usually to a plating bath temperature, immersed in a galvanizing in the molten zinc plating bath Do. This plating step itself may be performed under the same conditions as in the past, and no particular conditions are set in the present invention.

The plating bath is mainly composed of Zn and Al. The Al concentration is preferably in the range of 0.03 to 0.2%, and if the Al concentration is within this range, the alloying promoting effect of the present invention can be sufficiently expected. That is, in the present invention, the range of the Al concentration can be made wider than the Al concentration of 0.08 to 0.11% conventionally used for galvannealing. This is because Al conventionally shows an effect of suppressing alloying, so that the concentration has been limited, whereas in the present invention, since the alloying promoting effect is exerted as described above, even if the Al concentration is high, This is because there is no delay in alloying.

The lower limit of the Al concentration is due to dross formation.
If it is less than 0.03%, dross is generated frequently and operation is difficult.
The upper limit of the Al concentration is because the target product of the present invention is mainly a steel plate for an automobile, and a Zn-Al alloy plated steel plate is not a target. Therefore, there is almost no plating at an Al concentration of 0.2% or more.

As described above, the concentration of Al in the plating bath is set to 0.1.
If it is 12 to 0.2%, which is higher than the above range, the adhesion of plating after alloying is remarkably improved. Therefore, among alloyed hot-dip galvanized products that require special adhesion, such as those used for outer plates, 0.12-0.2%
It is preferable to perform plating in a plating bath containing Al.

Other components of the plating bath include the inclusion of Fe due to elution of the steel sheet, but there is no effect if the Fe concentration is 0.05% or less (does not include dross). In addition, Ni, Co, Cr, Cu, Si, Ti, Li, Nb, Mo, Ta,
Even if one or more metals such as Ca, Mg, Mn, K, Na, Pb, Sn, and W are mixed, if the concentration per each element is 0.02% or less and the total concentration is 0.5% or less, the influence is obtained. Almost no.

[0053] The temperature of the plating bath is usually in the range of 420 to 520 ° C. If the temperature is lower than 420 ° C., the bath may be solidified because the temperature is near the freezing point, making the operation difficult. If the temperature is higher than 520 ° C., the amount of Fe eluted increases and dross generation becomes remarkable. In order to avoid a rise in the temperature of the plating bath, the temperature of the steel sheet when entering the plating bath is also set as close as possible to a plating bath temperature in the range of 420 to 520 ° C.

The plating adhesion amount is the same as the conventional one
About 25-70 g / m ² is common. This plating weight is
This is performed by means for controlling the amount of deposition provided on the plating bath (eg, a gas wiping nozzle).

Alloying heat treatment step The hot-dip galvanized steel sheet that has exited the plating bath is heated in a heat treatment furnace provided immediately downstream of the plating bath to form a Zn-plated layer of Zn-Fe.
Change to alloy layer. This alloying heat treatment may be performed in the same manner as in the prior art, and no particular conditions are set in the present invention.

Usually, by heating at a temperature of about 480 to 600 ° C. for about 3 to 60 seconds, a Zn—Fe alloy layer in which the Fe concentration in the film is adjusted to about 8 to 12% is formed. As for the heating method, induction heating, direct energization, burner, heating by infrared rays, etc. are possible. The heating atmosphere is usually in the air. The galvannealed steel sheet exiting the heat treatment furnace is cooled by a suitable cooling means (eg, water cooling and / or air cooling),
Usually, the sheet is rolled after being subjected to skin pass rolling to remove distortion.

[0057]

EXAMPLES Example 1 As a plating base material, unannealed materials (sheet thickness 0.8 mm) of the following six types of carbon steel cold-rolled steel sheets A to F having the compositions shown in Table 1 and differing in the amount of P added were used. , 250 x 100 mm.

[0058]

[Table 1]

Each of the above steel sheets is degreased with a 10% NaOH aqueous solution in advance, and then can be subjected to a heat treatment in a predetermined atmosphere and can be directly hot-dipped from a reducing atmosphere. Using the following steps: (1) pre-oxidation, (2) annealing, and (3) reduction
(4) Hot-dip plating was performed. The absolute pressure at the time of each heating in the apparatus was 1 atm.

(1) Pre-oxidation The degreased steel sheet was placed in the heat treatment furnace of the above plating apparatus, as shown in Table 2.
A to e, in an O ₂ -N ₂ mixed gas having an O ₂ concentration of 5 to 5000 ppm or in the air under the conditions shown in Table 2 at 500 to 700 ° C.
, And the surface of the steel sheet was oxidized to form an oxide film. The amount of iron oxide on the steel sheet surface was measured by a solution analysis method. Table 4 shows the measurement results as Fe conversion amounts.

[0061]

[Table 2]

(2) Annealing After preheating and pre-oxidation as described above, the steel sheet is once cooled to 200 ° C. in the above atmosphere, and then the atmosphere gas is subjected to any of the non-reducing (Slightly oxidizing or substantially inert) gas, and in this gas atmosphere,
At a heating rate of 20 ° C / s, the temperature is raised to a temperature not lower than 750 ° C and not higher than 900 ° C, maintained at this temperature for 10 to 90 seconds, and then cooled at a cooling rate of 10 ° C / s to a reduction temperature of less than 750 ° C. By doing so, annealing was performed. The amount of iron oxide after this annealing was also measured in the same manner as above. Table 4 shows the attained temperature, the holding time, the reduction temperature, and the amount of iron oxide after annealing in the annealing step.

[0063]

[Table 3]

(3) Reduction After cooling to a reduction temperature in an annealing atmosphere, the atmosphere gas is replaced with an N ₂ -H ₂ mixed gas having a dew point of −60 ° C. to 0 ° C. and an H ₂ concentration of 5 to 30%, 550 ° C or more, 750 in this iron reducing atmosphere
The reduction was carried out at a reduction temperature shown in Table 4 below 20C for 20 to 90 seconds. Table 4 shows the reduction temperature and the retention time. For comparison, reduction temperatures outside the range of the present invention (less than 550 ° C or 750 ° C)
) Or the atmosphere gas is not a reducing atmosphere (ie, the dew point is more than 0 ° C.), and the same reduction was performed. Table 4 also shows the conditions at this time.

(4) Hot-dip galvanizing After the temperature holding in the reduction step was completed, the steel sheet was cooled in this reducing atmosphere until the temperature of the steel sheet reached about 460 ° C. The cooled steel sheet was then immersed in a hot-dip zinc bath having an Al concentration of 0.03% or more and 0.2% or less and the balance being Zn and having a bath temperature of 460 ° C. to perform hot-dip coating on both sides. The plating bath immersion time was 2 seconds, and the amount of Zn deposited was adjusted to about 60 g / m ² (per one side) with a gas wiper.

(5) Heat treatment for alloying After the plating, the alloy was heat-treated for alloying in a salt bath at 500 ° C., and the time when the Fe concentration in the film became 9 to 11% was measured as the alloying completion time. The alloying time was determined to be を for 20 seconds or less, Δ for 20 to 40 seconds, and × for 40 seconds or more. The evaluation of this alloying is also shown in Table 4.

[0067]

[Table 4]

As can be seen from Table 4, according to the present invention,
After oxidizing the steel sheet surface by pre-oxidation so as to be 0.05 to 1.0 g / m ^{2 in} terms of Fe, annealing in a non-reducing atmosphere so that the iron oxide amount does not exceed 1.0 g / m ^2. And then 75
By performing the plating pretreatment in the order of reduction at a temperature lower than 0 ° C., alloying of the P-added steel after hot-dip galvanizing was promoted, and the alloying treatment could be completed in a short time.
On the other hand, when the reducing conditions were out of the range of the present invention as in the comparative example, no alloying promoting effect was obtained.

Example 2 An alloyed hot-dip galvanized steel sheet was produced in the same manner as in Example 1. However, the Al concentration in the bath at the time of hot-dip galvanizing was set to 0.12% or more, and the Fe concentration in the coating was changed to 9 to 11% by the next alloying heat treatment.
I went until it became. With respect to the obtained galvannealed steel sheet, the adhesion of the plating film was evaluated by the following two types of test methods.

Film peeling test after cup squeezing: diameter 60 m
A test piece of a galvannealed steel sheet punched out into a disc shape with a diameter of 30 m was subjected to shearing at room temperature by a cylindrical drawing with a punch diameter of 30 mm and a die shoulder radius of 3R. The weight was measured and evaluated according to the following criteria. ：: Less than 25 mg (good), Δ: 25 mg or more, less than 35 mg (normal), ×: 35 mg or more (bad).

Low-temperature impact test after painting: A test piece of an alloyed hot-dip galvanized steel sheet cut out to a size of 150 × 70 mm was
After a chemical conversion treatment with a commercially available immersion phosphating solution, a three-coat coating (total film thickness: 100 μm) of a base coat → intermediate coat → top coat with a cationic electrodeposition paint was applied. The obtained coated steel sheet was cooled and kept at −20 ° C., and 10 gravel stones (basalt) having a diameter of 4 to 6 mm were air-pressed at a pressure of 2.0 kg / cm ² in a Gravelo testing machine.
Collision was performed at a collision speed of 100 to 150 km / hr, and the peeling diameter of the coating at each collision point was measured. The average of the peel diameters (average peel diameter) was evaluated according to the following criteria. ○: 4.0 mm or less (good), ×: more than 4.0 mm (bad).

The results of the adhesion test are shown together in Table 5 together with the steel type of the base material and each processing condition. In Table 5, the meaning of each symbol is the same as in Example 1. For comparison, the results of examples in which the Al concentration in the bath is less than 0.12%, and examples in which other conditions are outside the scope of the present invention are also shown as comparative examples.

[0073]

[Table 5]

As can be seen from Table 5, when the Al concentration in the plating bath is as high as 0.12% or more, the adhesion of the plated film after the alloying treatment is improved, and the peeling after applying shear in the adhesion test is small. Since a plating film having excellent adhesion that can sufficiently withstand the low-temperature impact of the adhesion test can be obtained, it is particularly suitable for applications such as automobile outer panels. However, the Al concentration in the bath was 0.12%
Even above, if the reduction conditions are out of the range of the present invention,
No improvement in plating adhesion can be obtained. That is, the effect of improving the plating adhesion is obtained when the processing conditions before plating are within the range of the present invention and the Al concentration in the bath is 0.12% or more.

[0075]

As described above, according to the present invention,
In alloying hot-dip galvanizing of a P-added high-strength steel sheet, the alloying speed can be significantly increased as compared with the conventional method, and an alloyed hot-dip galvanized steel sheet can be manufactured efficiently and economically. Further, by adjusting the Al concentration in the plating bath, the adhesion of the plated film after alloying can be enhanced. Although the plated steel sheet produced by the method of the present invention exhibits high performance that can be used as a material for automobiles, particularly as an outer plate material,
Because it can be manufactured relatively inexpensively, it is also useful for other uses such as home appliances and building materials.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a hot dip galvanizing line suitable for carrying out the method of the present invention.

Continuation of the front page (56) References JP-A-6-306561 (JP, A) JP-A-4-232241 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C23C 2 / 00-2/40

Claims (2)

(57) [Claims] 1. A high-tensile steel material of P> 0.020 wt% is heated in a non-reducing atmosphere to a temperature range of not lower than a recrystallization temperature and not higher than 900 ° C., and then cooled to 750 ° C. or lower, during which time the steel sheet surface An oxide film of 0.05 g / m ² or more and 1.0 g / m ² or less in terms of Fe is formed, then reduced in a reducing atmosphere of less than 750 ° C and 550 ° C or more, then hot-dip galvanized, and heat treated for alloying. A galvannealing method for alloying a P-added high-tensile steel material. 2. The hot-dip galvanizing method according to claim 1, wherein the Al concentration in the bath is 0.
The method according to claim 1, wherein the plating is performed in a plating bath of 12 to 0.20 wt%.