JP5555992B2 - Manufacturing method of high-strength hot-dip galvanized steel sheet with excellent surface appearance and plating adhesion - Google Patents

Description

  The present invention relates to a method for producing a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet using a Si-containing high-strength steel sheet as a base material. The present invention relates to a method for producing excellent high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets that give rust prevention to raw steel sheets, especially hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent rust prevention properties. It is used.

  Generally, a hot dip galvanized steel sheet is manufactured by the following method. First, using a thin steel plate obtained by hot-rolling, cold-rolling or heat-treating a slab, the base steel plate surface is degreased and / or pickled and cleaned in the pretreatment step, or the pretreatment step is omitted. After the oil on the surface of the base steel plate is burned and removed, recrystallization annealing is performed by heating to a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or a reducing atmosphere. Thereafter, the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere, and in a molten zinc bath to which a small amount of Al (about 0.1 to 0.2 mass%) is added without being exposed to the air. Immerse in.

  In addition, the alloyed hot-dip galvanized steel sheet is manufactured by subsequently heat-treating the steel sheet in an alloying furnace after hot-dip galvanizing.

  By the way, in recent years, weight reduction has been promoted along with higher performance of raw steel plates, and higher strength of raw steel plates has been demanded, and the amount of high-strength hot-dip galvanized steel plates having rust prevention properties is increasing.

  Addition of solid solution strengthening elements such as Si, Mn, P, and Al is performed to increase the strength of the steel sheet. Among these, Si and Al have an advantage that the strength can be increased without impairing the ductility of the steel, and the Si-containing steel plate is promising as a high-strength steel plate. However, when producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that use a high-strength steel sheet containing a large amount of Si as a base material, there are the following problems.

  As described above, the hot dip galvanized steel sheet is subjected to hot dip galvanizing treatment after heat annealing at a temperature of about 600 to 900 ° C. in a reducing atmosphere. However, Si in steel is an easily oxidizable element and is selectively oxidized even in a generally used reducing atmosphere to concentrate on the surface to form an oxide. Since this oxide reduces the wettability with molten zinc during the plating process and causes non-plating, the wettability rapidly decreases as the Si concentration in the steel increases and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor.

  Further, when Si in the steel is selectively oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying process after hot dip galvanizing. As a result, productivity is significantly inhibited. If an alloying treatment is attempted at an excessively high temperature in order to ensure productivity, there is a problem that the powdering resistance is deteriorated, and it is difficult to achieve both high productivity and good powdering resistance.

For such problems, it has been proposed to improve wettability with molten zinc by heating the steel sheet in an oxidizing atmosphere in advance to form iron oxide on the surface, followed by reduction annealing. . (For example, Patent Document 1)
Prior to hot dipping, sulfur or a sulfur compound is deposited in an amount of 0.1 to 1000 mg / m ² as an S amount, and then a preheating step is performed in a weakly oxidizing atmosphere, followed by annealing in a non-oxidizing atmosphere containing hydrogen. A method is disclosed. (For example, Patent Document 2)
Moreover, the method of removing the surface oxide by performing pickling after annealing a steel plate, and then annealing again and performing hot dip galvanization is proposed. (For example, Patent Document 3)
The technique described in Patent Document 1 intends to suppress Si surface concentration during reduction annealing by heating in an oxidizing atmosphere in advance to form iron oxide on the surface of the steel sheet. However, as is generally known, since the oxidation rate on the steel sheet surface greatly decreases as the Si concentration in the steel increases, the steel sheet having a high Si concentration in the steel can be obtained only by the oxidation means disclosed in Patent Document 1. Sufficient oxidation does not proceed, and it is difficult to obtain an amount of iron oxide necessary for suppressing Si surface concentration.

  As a result, the occurrence of non-plating during hot dipping cannot be sufficiently suppressed, and in the case of alloying, the problem of significant delay of alloying, which is a concern in the alloying process, can be sufficiently solved. Can not. If the alloying speed is slow, the CGL with a limited length of the alloying furnace must be manufactured in consideration of the predetermined productivity, and the alloying temperature must be increased. The powdering property is forced to deteriorate.

  The technique described in Patent Document 2 is intended to improve wettability with molten zinc by a sulfide layer formed on the surface of a steel plate. However, when applied to a steel sheet having a high Si concentration in the steel, the surface concentration of Si cannot be sufficiently suppressed only by the effect of the sulfide layer, and thus the problem of the performance of the plating layer cannot be solved as described above. Even if the preheating step is performed in a weakly oxidizing atmosphere, when applied to a steel sheet having a high Si concentration in the steel, the problem of powdering resistance cannot be solved as described above.

  Furthermore, in the technique disclosed in Patent Document 2, sulfur or a sulfur compound is attached to the surface of the steel plate prior to the heat treatment, so that in the subsequent heat treatment process, sulfur components such as sulfur dioxide and hydrogen sulfide are heated in the heating furnace. A large amount of corrosive gas is released. Corrosion damage to the heating furnace and in-furnace equipment becomes severe, requiring frequent repairs and deterioration updates. In addition, if the furnace gas is released into the atmosphere, air pollution is prevented. From this viewpoint, it is necessary to provide a desulfurization apparatus. Therefore, further improvement is necessary to realize the technique described in Patent Document 2.

In the technique described in Patent Document 3, annealing is performed twice, and the surface concentrated product of Si formed on the surface after the first annealing is removed by pickling, thereby suppressing the formation of the surface concentrated product during the second annealing. It is what. However, when the Si concentration is high, pickling cannot completely remove the surface concentrate, so that the problem of the performance of the plating layer cannot be solved as described above. Furthermore, there is another problem that the pickling equipment for removing the surface concentrate of Si is newly required, which is costly.
Japanese Patent No. 2587724 Japanese Patent Laid-Open No. 11-50223 Japanese Patent No. 3957550

  The present invention provides a method for producing a high-strength hot-dip galvanized steel sheet having a beautiful surface appearance free of non-plating and excellent plating adhesion, using a Si-containing high-strength steel sheet as a base material, and is free from non-plating. It is an object of the present invention to provide a method for producing a high-strength galvannealed steel sheet having a beautiful surface appearance and excellent plating adhesion.

  As described above, in the case of a steel sheet having a high Si concentration in the steel, it has been difficult to completely suppress non-plating by either removing the Si surface enrichment or suppressing the surface enrichment by oxidation. Of the conventional technologies, it is considered effective to suppress surface enrichment due to oxidation for steel sheets with high Si concentration in steel, but oxidation does not proceed with the conventional oxidation method alone, and non-plating is improved. Therefore, it was difficult to obtain the necessary amount of iron oxide. Therefore, in the case of a steel plate having a high Si concentration in steel, it is necessary to promote oxidation by some method.

Therefore, the inventors investigated oxides formed on a steel sheet having a high Si concentration, and found that when SiO ₂ was formed at the interface between the steel sheet surface and Fe oxide, the oxidation of the steel sheet was suppressed. . Furthermore was investigating focuses on the interface oxide formed between the steel sheet surface and the Fe oxide, the oxide other than SiO ₂ is formed, if the SiO ₂ is not formed, or by SiO ₂ surface of the steel sheet It was found that when the coverage is small, the oxidation of the steel sheet is not suppressed.

Specifically, it has been found that if the heat treatment is performed in a reducing atmosphere and the generation of SiO ₂ is suppressed before the oxidation treatment, the oxidation of the steel sheet can be promoted by the subsequent oxidation treatment.

  That is, the gist configuration of the present invention is as follows.

(1) As chemical components, in mass%, C: 0.05 to 0.30%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to When hot-dip galvanizing is performed on a steel sheet containing 3.0%, S: 0.001 to 0.01%, P: 0.001 to 0.1%, and the balance Fe and unavoidable impurities, H ₂ : first heating step of the dew point comprises a 1～50Vol% heats the steel plate at 0 ℃ in the following atmosphere so that the temperature in the range of 600 to 850 ° C., then _O 2: _0.01~20vol%, H ₂ O: second heating step of heating the steel sheet in an atmosphere containing 1～50Vol% so that the temperature in the range of 400 to 850 ° C., then _H 2: dew point comprises 1～50Vol% is 0 ℃ 3rd heating which heats a steel plate so that it may become the temperature within the range of 750-900 degreeC in the following atmospheres After extent, the method of producing a high strength galvanized steel sheet excellent in coating adhesion and surface appearance, characterized in that performing molten zinc plating treatment.

  (2) The steel sheet described in the above (1) is further in mass% as a chemical component, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: 0.01 to It contains 1 or 2 or more types chosen from 0.1%, Nb: 0.01-0.1%, and B: 0.0005-0.0050%, Said (1) characterized by the above-mentioned A method for producing high-strength hot-dip galvanized steel sheets with excellent surface appearance and plating adhesion.

(3) the second heating step is previous _{stage, O 2: 0.1~20vol%, H} 2 O: comprising a steel sheet in an atmosphere containing 1～50Vol% to a temperature in the range of 400 to 750 ° C. In the atmosphere containing O ₂ : 0.01 vol% to less than 0.1 vol% and H ₂ O: 1 to 20 vol% or less, the steel sheet is heated to a temperature in the range of 600 to 850 ° C. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance and plating adhesion as described in (1) or (2) above, wherein heating is performed as described above.

  (4) In the second heating step, the first stage is performed with a direct flame furnace or a non-oxidizing furnace under an air ratio of 1 or more and 1.35 or less, and the second stage is performed with a direct flame furnace or a non-oxidizing furnace, and the air ratio is less than 1. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance and plating adhesion as described in (3) above, which is carried out under the above conditions.

(5) The surface appearance and plating adhesion according to any one of (1) to (4) above, wherein the SiO ₂ coverage on the steel sheet surface after the first heating step is 30% or less. A method for producing high-strength hot-dip galvanized steel sheets.

  (6) After producing a high-strength hot-dip galvanized steel sheet by the method according to any one of (1) to (5) above, the alloy is further subjected to an alloying treatment, which is excellent in surface appearance and plating adhesion A method for producing a high-strength galvannealed steel sheet.

  According to the present invention, even when a Si-containing high-strength steel sheet is used as a base material, the high-strength hot-dip galvanized steel sheet having a beautiful surface appearance with no plating and excellent plating adhesion and no plating A high-strength galvannealed steel sheet having a beautiful surface appearance can be obtained.

  Hereinafter, the present invention will be specifically described.

  First, the reason why the component composition of the steel sheet is limited to the above range will be described. In addition, unless otherwise indicated, the "%" display regarding a component shall mean mass%.

C: 0.05-0.30%
C is an element that stabilizes the austenite phase, and is an element necessary for increasing the strength of the steel sheet. If the C content is less than 0.05%, it is difficult to ensure the strength, and if the C content exceeds 0.30%, the weldability decreases. Therefore, the C content is in the range of 0.05 to 0.30%.

Si: 0.1-3.0%
Si has the effect of improving the formability of the steel sheet by concentrating the solid solution C in the ferrite phase in the austenite phase and increasing the temper softening resistance of the steel. In order to obtain the effect, a content of 0.1% or more is necessary. On the other hand, Si has an effect of suppressing oxidation of the steel sheet, and if the content exceeds 3.0%, it is difficult to suppress the formation of an oxide film even if the production process of the present invention described later is applied. Sex is not improved sufficiently. Accordingly, the Si content is within the range of 0.1 to 3.0%.

Mn: 0.5 to 3.0%
Mn is an element useful for increasing the hardenability and increasing the strength of the steel sheet. The effect cannot be obtained at less than 0.5%. On the other hand, if the content exceeds 3.0%, segregation of Mn occurs and the workability decreases. Therefore, the amount of Mn is in the range of 0.5 to 3.0%.

Al: 0.01 to 3.0%
Al is an element added in a complementary manner to Si, and is preferably contained in an amount of 0.01% or more. However, if the Al content exceeds 3.0%, the weldability and strength ductility balance are adversely affected. Therefore, the Al content is set in the range of 0.01 to 3.0%.

S: 0.001 to 0.01%
S is an element inevitably contained in steel, and lowers formability by producing plate-like inclusions MnS after cold rolling. MnS is not produced until the S content is 0.01%, but excessive reduction is accompanied by an increase in desulfurization cost in the steelmaking process. Therefore, the S content is within the range of 0.001 to 0.01%.

P: 0.001 to 0.1%
P is an element inevitably contained in steel, and is an element contributing to strength improvement. On the other hand, it is also an element that deteriorates weldability, and when the amount of P exceeds 0.1%, the influence appears remarkably. On the other hand, excessive P reduction is accompanied by an increase in manufacturing cost in the steelmaking process. Therefore, the P content is within the range of 0.001 to 0.1%.

  In the present invention, the above component composition is an essential component, and the balance is iron and unavoidable impurities. However, if necessary, one or more of the following components can be appropriately contained.

Cr: 0.1 to 1.0%
Cr is an element effective for improving the hardenability of steel, and in order to obtain this effect, addition exceeding 0.1% is required. Cr strengthens the ferrite phase in a solid solution, reduces the hardness of the martensite phase and the ferrite phase, and contributes effectively to the improvement of formability. However, if the Cr content exceeds 1.0%, this effect is saturated, but rather the surface quality is significantly degraded. Therefore, the Cr content is within the range of 0.1 to 1.0%.

Mo: 0.1 to 1.0%
Mo is an element effective for improving the hardenability of steel and is an element that develops tempering secondary hardening. In order to obtain this effect, addition of 0.1% or more is required. However, if the amount of Mo exceeds 1.0%, this effect is saturated and causes an increase in cost. Therefore, the Mo amount is set within a range of 0.1 to 1.0%.

Ti: 0.01 to 0.1%
Ti forms C or N and fine carbides or fine nitrides in steel, and thus effectively acts to refine the structure after annealing and to impart precipitation strengthening. In order to obtain this effect, addition of 0.01% or more is necessary. However, this effect is saturated when the Ti content exceeds 0.1%. Therefore, the Ti amount is within the range of 0.01 to 0.1%.

Nb: 0.01 to 0.1%
Nb is an element that contributes to improvement in strength by solid solution strengthening or precipitation strengthening. In order to obtain this effect, addition of 0.01% or more is required. However, if the content exceeds 0.1%, the ductility of ferrite is lowered, and the workability is lowered. Therefore, the Nb content is in the range of 0.01 to 0.1%.

B: 0.0005 to 0.0050%
B is necessary for improving the hardenability, suppressing the formation of ferrite during annealing cooling, and obtaining a desired amount of martensite. In order to acquire this effect, it is necessary to add B amount 0.0005% or more, but if it exceeds 0.0050%, this effect will be saturated. Therefore, the B amount is within the range of 0.0005 to 0.0050%.

  Next, the manufacturing method of a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet will be described. Unless otherwise specified, “%” in relation to the atmosphere means vol%.

The steel sheet having the above composition is subjected to the following three heat treatments and then plated. This heat treatment is an important requirement in the present invention, and in particular, the first heating step is the most important requirement. By performing the first heating step before the second heating step under the following conditions, the generation of SiO ₂ at the interface between the steel plate and the Fe oxide during the second heating step is suppressed, and a steel plate containing a large amount of Si. It becomes possible to promote the oxidation.
The first heating step: _H 2: a dew point comprises 1-50% of 0 ℃ in the following atmosphere, the steel sheet heated so that the temperature of 600 to 850 ° C. The second heating step: _O 2: 0.01 to 20 %, H ₂ O: In an atmosphere containing 1-50%, the steel sheet is heated to a temperature of 400-850 ° C. Third heating step: H ₂ : 1-50% inclusive and dew point of 0 ° C. or less Heat the steel plate to a temperature of 750-900 in the atmosphere

First heating step:
The first heating step is carried out for the formation of excessive suppression and Si-Mn oxide formation of SiO ₂ in the surface of the steel sheet. The atmosphere is such that H ₂ is 1% to 50% and the dew point is less than 0 ° C. When H ₂ is less than 1%, Si does not generate an oxide, and Al forms a surface oxide. Since Si that has not formed an oxide forms SiO ₂ in the subsequent second heating step, oxidation of Fe is suppressed, and as a result, non-plating occurs. On the other hand, if H ₂ is 50% or more, excessive SiO ₂ is generated on the surface in the first heating step, so that oxidation of Fe is suppressed in the subsequent second heating step, and non-plating occurs. Further, the dew point is preferably 0 ° C. or less from the humidification cost.

In the first heating step, the steel sheet temperature is heated from room temperature to a temperature of 600 to 850 ° C. When the temperature of the steel sheet heated in the first heating step is less than 600 ° C., Si does not form an oxide, and when it exceeds 850 ° C., a large amount of SiO ₂ is formed. Therefore, in the first heating step, the steel plate temperature is 600 ° C. or higher. The steel sheet is heated to a temperature in the range of 850 ° C. or lower.

In the first heating step, _SiO 2, Si-Mn oxide is formed on the surface of the steel sheet. SiO ₂ and / or Si—Mn oxide is generated on the surface of the steel sheet after the first heating step. If the steel plate surface is covered with SiO ₂ , the oxidation of Fe is hindered in the next second heating step, and the surface of Si and Mn is concentrated in the third heating step. Occur. In order to ensure a sufficient amount of Fe oxidation for suppressing non-plating in the second heating step, the SiO ₂ coverage on the steel sheet surface after the first heating step is desirably 30% or less. Incidentally, the coverage of the SiO ₂ performs mapping by AES, the portion where Si and O are concentrated as SiO ₂ coating unit, can be determined by image processing.

Second heating step:
A 2nd heating process is performed in order to oxidize a steel plate actively and to produce Fe oxide on the steel plate surface. Therefore, O ₂ must have a sufficient amount for oxidation, and is 0.01% or more. For economic reasons, it is preferably 20% or less of the atmospheric level. H ₂ O is made 1% or more in order to promote oxidation. Further, considering the humidification cost, 50% or less is preferable. If the temperature of the steel sheet heated in the second heating step is less than 400 ° C., it is difficult to oxidize, and if it exceeds 850 ° C., it will oxidize too much and the picking with the in-furnace roll will generate in the third heating step. The steel plate is heated so that the steel plate temperature is in the range of 400 ° C. or higher and 850 ° C. or lower.

The second heating step may be performed subsequent to the first heating step, or a cooling step may be provided between the first heating step and the second heating step. The atmosphere of the cooling process is preferably an N ₂ atmosphere. The first heating step and the second heating step may be performed by different steel plate manufacturing facilities.

Further, in order to more effectively suppress the press flaws described above divides the second heating step in two stages, the front _{O 2: 0.1~20vol%, H 2} O: containing 1～50Vol% temperature of the steel strip in an atmosphere is heated so that the temperature in the range of 400 to 750 ° C., the subsequent _O 2: less than _{0.01vol% ~0.1vol%, H 2 O} : containing the following 1～20Vol% It is desirable to heat the steel sheet so that the steel sheet temperature is in the range of 600 to 850 ° C. in the atmosphere (however, the latter steel sheet temperature is higher than the preceding steel sheet temperature).

The heating in the previous stage is performed to oxidize the steel sheet, and O ₂ needs a sufficient amount to oxidize and is 0.1% or more. For economic reasons, it is preferably 20% or less of the atmospheric level. H ₂ O is made 1% or more in order to promote oxidation. Further, considering the humidification cost, 50% or less is preferable. If the temperature after heating in the former stage is less than 400 ° C., it is difficult to oxidize, and if it exceeds 750 ° C., it will oxidize too much and pick up will occur in the roll in the third heating step. Heat until

The latter heating is performed to reduce the surface of the once oxidized steel sheet and suppress the pressing. Therefore, it is possible to reduce the surface of the steel sheet by heating in the latter stage, and heating is performed under conditions where pick-up does not occur, that is, low-temperature reducing heating conditions in a low O ₂ concentration atmosphere. In the next third heating step, reduction treatment is performed to such an extent that it reacts with the in-furnace roll and pick-up does not occur. Since O ₂ cannot be reduced when it is 0.1% or more, O ₂ is made less than 0.1% (however, 0.01 vol% or more). If H ₂ O is contained in a large amount, the steel sheet is oxidized, so the content is set to 20% or less (however, 1 vol% or more). When the steel plate temperature is less than 600 ° C., reduction is difficult, and when it exceeds 850 ° C., heating costs are required. Therefore, in the latter stage, heating is performed so that the steel plate temperature is in the range of 600 ° C. to 850 ° C.

  When the pre-stage heating is performed in a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF), it is preferable that the combustion gas is C gas generated in a coke oven and the air ratio is 1 to 1.35. This is because when the air ratio is less than 1, the steel sheet is not oxidized, and when it exceeds 1.35, pickup occurs due to overoxidation. In addition, when post-stage heating is performed in a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF), it is preferable that the combustion gas is C gas generated in a coke oven and the air ratio is 0.6 to less than 1. . This is because if the air ratio exceeds 1, iron oxide on the surface of the steel sheet cannot be reduced, and if the air ratio is less than 0.6, the combustion efficiency is deteriorated.

Third heating step:
A 3rd heating process is performed following a 2nd heating process, and performs a reduction process. Therefore, the atmosphere of the third heating step is such that H ₂ is 1% or more and 50% or less, and the dew point is 0 ° C. or less. When H ₂ is less than 1% and the dew point exceeds 0 ° C., the iron oxide generated in the second heating step is difficult to be reduced. Therefore, sufficient iron oxide and internal oxide to ensure plating adhesion in the second heating step Even if nitride is formed, the plating property is deteriorated. Moreover, _{H 2} leads to cost more than 50%. Since it is difficult to implement industrially when the dew point is less than −60 ° C., the dew point is preferably −60 ° C. or higher.

  In a 3rd heating process, it heats so that steel plate temperature may become the temperature in the range of 750-900 degreeC. When the temperature is lower than 750 ° C., unrecovered amorphous ferrite introduced during cold rolling remains, and workability deteriorates. Moreover, when it exceeds 900 degreeC, a heating cost will start.

  After performing the above three steps, it is cooled and immersed in a hot dip galvanizing bath to perform hot dip galvanizing. For the production of hot dip galvanized steel sheet, a galvanizing bath having a bath temperature of 440 to 550 ° C. and an Al concentration in the bath of 0.14 to 0.24% is used. A galvanizing bath having an Al concentration of 0.10 to 0.20% in the bath is used.

  If the bath temperature is less than 440 ° C, there is a possibility that the solidification of Zn may occur in places where the temperature variation in the bath is large. If the bath temperature exceeds 550 ° C, the evaporation of the bath is severe and the operation cost and vaporized Zn will enter the furnace. There are operational problems due to adhesion. Furthermore, since alloying proceeds during plating, it tends to be overalloyed.

  If the Al concentration in the bath is less than 0.14% when producing a hot dip galvanized steel sheet, Fe—Zn alloying progresses and plating adhesion deteriorates, and if it exceeds 0.24%, defects due to Al oxide occur. When the alloyed hot-dip galvanized steel sheet is produced, if the Al concentration in the bath is less than 0.10%, a large amount of ζ phase is generated and powdering properties deteriorate, and if it exceeds 0.20%, Fe-Zn alloying progresses. Absent.

The alloying treatment is optimally performed at a temperature higher than 460 ° C. and lower than 570 ° C. At 460 ° C or lower, alloying progresses slowly, and at 570 ° C or higher, a hard and brittle Zn-Fe alloy layer formed at the iron-iron interface is excessively formed by overalloy and not only the plating adhesion deteriorates, but also the residual austenite phase Since it decomposes, the strength ductility balance also deteriorates. The plating adhesion amount is not particularly defined, but is preferably 10 g / m ² or more (adhesion amount per one surface) for corrosion resistance and plating adhesion amount control. Moreover, since adhesion will fall when there is much adhesion amount, 120 g / m < ² > or less (attachment amount per one side) is desirable.

  Hereinafter, the present invention will be specifically described based on examples.

A slab having a steel composition shown in Table 1 was heated at 1260 ° C. for 60 minutes in a heating furnace, subsequently hot-rolled to 2.8 mm, and wound at 540 ° C. Next, the black scale was removed by pickling and cold rolled to 1.6 mm. Next, as a heating furnace, RTF (radiant tube furnace, the same shall apply hereinafter) -DFF (direct fire furnace) -RTF-cooling zone DFF type CGL or RTF-NOF (non-oxidizing furnace) -RTF-cooling zone Using the NOF type CGL, the first heating process to the third heating process were performed under the heat treatment conditions shown in Tables 2 to 5. DFF and NOF are each divided into four zones, the first zone to the fourth zone. In the second heating step, the first stage heating is performed in the first to third zones of DFF or NOF, and the second stage heating is performed in the fourth zone. It was. C gas generated in a coke oven was used as the fuel gas. In some examples, the first heating step was performed and then the cooling was performed, and the second heating step and thereafter were performed. The cooling process may be performed in the same facility or in a separate facility. The heating temperature is the steel plate temperature at the process outlet. Subsequently, hot dip galvanizing treatment was performed in an Al-containing Zn bath at 460 ° C. to obtain a hot dip galvanized steel sheet. The Al concentration in the bath was 0.14 to 0.20% Al, and the adhesion amount was adjusted to 40 g / m ² per side by gas wiping. Moreover, after performing hot dip galvanization, the alloying hot dip galvanized steel plate was obtained by performing an alloying process at 500-580 degreeC. Moreover, the 2nd heating process-the 3rd heating process were not performed, but the steel plate after the 1st heating process was obtained by letting a plating bath pass. The surface of this steel sheet was mapped by AES, and the surface coverage of SiO ₂ was determined.

  With respect to the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) obtained as described above, the plating appearance was investigated by the method described below. The obtained results are shown in Tables 2 to 5 together with the conditions.

<Surface appearance>
Judging by visual inspection for appearance defects such as non-plating and pushing rods, good (○) when there is no appearance defect, good (△) when there is a slight but poor appearance. When there was an appearance defect, it was determined as (×).

<Plating adhesion>
The plating adhesion of the galvannealed steel sheet was evaluated for powdering resistance. Specifically, the amount of delamination per unit length when cellotape (registered trademark) is applied to an alloyed hot-dip galvanized steel sheet, the tape surface is bent 90 degrees, and bent back, is taken as the Zn count by fluorescent X-rays. Based on the following criteria, those of ranks 1 and 2 were evaluated as particularly good (◯), good (Δ), and those of 3 or more as bad (x).
X-ray fluorescence count Rank 0 to less than 500: 1 (good)
500 to less than 1000: 2
1000 to less than 2000: 3
2000 to less than 3000: 4
3000 or more: 5 (poor)
About the hot-dip galvanized steel sheet which is not alloyed, the ball impact test was performed, the processed part was peeled off with cello tape (registered trademark), and the plating adhesion was evaluated by visually judging the presence or absence of peeling of the plating layer.
○: Plating layer is not peeled ×: Plating layer is peeled

  As can be seen from Tables 2 to 5, the hot-dip galvanized steel sheet and the galvannealed steel sheet of the present invention have a beautiful surface appearance with no unplating or squeezing despite containing Si. The plating adhesion is also good. On the other hand, the hot-dip galvanized steel sheet and the galvannealed steel sheet of the comparative example are inferior in surface appearance and plating adhesion.

  High-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets manufactured by the method of the present invention have a beautiful surface appearance and excellent plating adhesion, so they are widely used mainly in the fields of automobiles, home appliances and building materials Use in applications is expected.

Claims (6)

As chemical components, in mass%, C: 0.05 to 0.30%, Si: 0.1 to 3.0%, Mn: 0.5 to 3.0%, Al: 0.01 to 3.0 %, S: 0.001 to 0.01%, P: 0.001 to 0.1%, and when hot dip galvanizing is performed on a steel sheet composed of the remaining Fe and inevitable impurities, H ₂ : 1 to 50 vol. first heating step of the dew point includes percent is heated so that the steel sheet to a temperature in the range of 600 to 700 ° C. at 0 ℃ in the following atmosphere, then _{O 2: 0.01~20vol%, H 2} O: the second heating step of heating the steel sheet in an atmosphere containing 1～50Vol% so that the temperature in the range of 400 to 850 ° C., then _H 2: dew point comprises 1～50Vol% is 0 ℃ less atmosphere A third heating step of heating the steel plate to a temperature in the range of 750 to 900 ° C. After Tsu, the method of producing a high strength galvanized steel sheet excellent in coating adhesion and surface appearance, characterized in that performing molten zinc plating treatment.   The steel sheet according to claim 1 is, in addition, as a chemical component, in mass%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: 0.01 to 0.1%. The surface appearance and plating adhesion according to claim 1, comprising one or more selected from Nb: 0.01 to 0.1% and B: 0.0005 to 0.0050% Of producing high-strength hot-dip galvanized steel sheet with excellent properties. In the second heating step, the former stage is such that the steel sheet is heated to a temperature in the range of 400 to 750 ° C. in an atmosphere containing O ₂ : 0.1 to ₂₀ vol% and H ₂ O: 1 to 50 vol%. In the subsequent stage, the steel sheet is heated to a temperature in the range of 600 to 850 ° C. in an atmosphere containing O ₂ : 0.01 vol% to less than 0.1 vol% and H ₂ O: 1 to 20 vol% or less. The method for producing a high-strength hot-dip galvanized steel sheet excellent in surface appearance and plating adhesion according to claim 1 or 2, wherein the temperature at the latter stage is higher than the temperature at the former stage.   In the second heating step, the first stage is performed by a direct flame furnace or a non-oxidation furnace under the condition of an air ratio of 1 or more and 1.35 or less, and the second stage is performed by a direct flame furnace or a non-oxidation furnace under a condition where the air ratio is less than 1. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the surface external appearance and plating adhesiveness of Claim 3 characterized by performing. High strength molten zinc excellent in surface appearance and the coating adhesion according to any one of claims 1 to 4, SiO ₂ coverage of the steel sheet surface after the first heating step is equal to or less than 30% Manufacturing method of plated steel sheet.   High strength galvanized steel sheet manufactured by the method according to any one of claims 1 to 5, and further alloying treatment is performed. Manufacturing method of galvanized steel sheet.