JP2010196083A - Method for manufacturing high-strength hot-dip galvanized steel sheet and high-strength hot-dip galvannealed steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvannealed steel sheet, which have beautiful surface appearances free from plating deficiency and are superior in plating adhesiveness, while using an Si-containing high-strength steel sheet as a base material. <P>SOLUTION: This manufacturing method includes: heating a steel sheet containing, by mass%, 0.05-0.30% C, 0.1-3.0% Si, 0.5-3.0% Mn, 0.01-3.0% Al, 0.001-0.01% S and 0.001-0.1% P to a temperature in a range of 800-900&deg;C in an atmosphere containing 1-20 vol.% O<SB>2</SB>and 1-50 vol.% H<SB>2</SB>O; subsequently heating the steel sheet to a temperature in the range of 800-900&deg;C in an atmosphere containing 0.01 or more but less than 0.1 vol.% O<SB>2</SB>and 1-20 vol.% H<SB>2</SB>O; subsequently heating the steel sheet to a temperature in the range of 800-900&deg;C in a reducing atmosphere containing 1-50 vol.% H<SB>2</SB>and having a dew point of 0&deg;C or less; and then hot-dip galvanizing the steel sheet. <P>COPYRIGHT: (C)2010,JPO&amp;INPIT

Description

  The present invention relates to a method for producing a hot-dip galvanized steel sheet using a Si-containing high-strength steel sheet as a base material, and in particular, a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel that have a beautiful surface appearance without unplating and excellent plating adhesion. The present invention relates to a method for producing a plated steel sheet.

  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. in use.

  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 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 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 atmosphere. Dip plating.

  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 galvanization after being heat-annealed at a temperature of about 600 to 900 ° C. in a non-oxidizing atmosphere or 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 wettability with molten zinc during plating and causes non-plating, the wettability decreases rapidly as the Si concentration in the steel increases, resulting in frequent non-plating. 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.

In order to solve such problems, a method has been proposed in which surface oxides are removed by pickling after annealing a steel sheet, and then annealing and hot dip galvanizing are performed again. (For example, Patent Document 1)
Further, it has been proposed to improve wettability with molten zinc by heating steel sheets in an oxidizing atmosphere in advance to form iron oxide on the surface, followed by reduction annealing. (For example, Patent Document 2)
The technique described in Patent Document 1 performs annealing twice and suppresses the formation of surface concentrate during the second annealing by pickling and removing the surface concentrate of Si generated on the surface after the first 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.

  Furthermore, the technique described in Patent Document 2 attempts to suppress Si surface concentration during reduction annealing by heating in an oxidizing atmosphere in advance to form iron oxide on the steel sheet surface. However, as is generally known, since the oxidation rate on the steel sheet surface greatly decreases with an increase in the Si concentration in the steel, it is necessary to suppress the surface concentration of Si only by the description in Patent Document 2. It is difficult to obtain an adequate amount of iron oxide.

Japanese Patent No. 3957550 Japanese Patent No. 2587724

  The present invention provides a method for producing a 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 beautiful without non-plating. It is an object of the present invention to provide a method for producing an galvannealed steel sheet having a surface appearance and excellent plating adhesion.

Means of the present invention for solving the above-mentioned problems are 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 In performing hot-dip galvanizing on a steel sheet containing 3.0%, S: 0.001 to 0.01%, P: 0.001 to 0.1%, and the balance Fe and inevitable impurities, O ₂ : _1~20vol%, H 2 O: containing 1~50vol%, the balance of _N 2, CO, steel plates 800 to 900 ° C. in an atmosphere composed of one or more and incidental impurities CO ₂ range a heating step of heating so that a temperature of the inner, _then, O 2: less than _{0.01vol% ~0.1vol%, H 2 O} : containing less 1～20Vol%, the balance being _N 2, CO, CO ₂ and 800 steel plates in an atmosphere consisting of one or more of H ₂ and inevitable impurities B heating step for heating to a temperature in the range of ˜900 ° C., then H ₂ : 1-50 vol%, and the steel sheet is in the range of 800-900 ° C. in a reducing atmosphere with a dew point of 0 ° C. or less. A method for producing a high-strength hot-dip galvanized steel sheet excellent in surface appearance and plating adhesion, characterized by performing a hot-dip galvanizing treatment after performing a C heating step of heating to a temperature.

[2] In the heating step A, an oxide film having a structure in which 50 mass% or more is hematite (Fe ₂ O ₃ ) and Mn is concentrated on the outermost surface of hematite 5 to 100 nm is formed on the steel sheet surface. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance and plating adhesion as described in [1].

  [3] The steel sheet according to [1] and [2] is further, as a chemical component, in mass%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: One or more selected from 0.01 to 0.1%, Nb: 0.01 to 0.1%, and B: 0.0005 to 0.0050% [1] Or the manufacturing method of the high intensity | strength hot-dip galvanized steel plate which is excellent in the surface external appearance and plating adhesiveness as described in [2].

  [4] The A heating step is performed in a direct fired furnace (DFF) or a non-oxidizing furnace (NOF) under the condition of an air ratio of 1 to 1.35, and the B heating process is performed in a direct fired furnace (DFF) or a non-oxidizing furnace. The method for producing a high-strength hot-dip galvanized steel sheet having excellent surface appearance and plating adhesion as described in any one of [1] to [3], wherein the method is carried out under a condition with an air ratio of less than 1 using (NOF).

  [5] After producing a high-strength hot-dip galvanized steel sheet by the method according to any one of [1] to [4], further alloying treatment is performed, and the surface appearance and plating adhesion are excellent. A method for producing high-strength galvannealed steel sheets.

  According to the production method of the present invention, even when a Si-containing high-strength steel plate is used as a base material, a hot-dip galvanized steel plate having a beautiful surface appearance without unplating and excellent plating adhesion and an unplated steel An alloyed hot-dip galvanized steel sheet having a beautiful surface appearance is obtained.

An example of the GDS profile of Fe, Mn, and O in the depth direction from the surface of the iron oxide of the steel plate formed in the A heating step is shown.

  As described above, in the case of a steel sheet having a high Si concentration in steel, it has been difficult to completely suppress non-plating by either removing the Si surface enrichment or suppressing the surface enrichment by oxidation. For steel sheets with a high Si concentration in steel, it is considered effective to suppress the surface concentration of Si by iron oxide formed on the steel sheet surface, but oxidation does not proceed with conventional oxidation means alone, and it is not possible. It was difficult to obtain the amount of iron oxide necessary for improving plating.

The inventors investigated not only the amount of iron oxide but also the composition of iron oxide and investigated the relationship with non-plating. As a result, with regard to the amount of oxidation, increasing the amount of iron oxide by strengthening the oxidation improves the non-plating to some extent, but when the amount of oxidation is large, iron oxide adheres to the roll in the furnace and the steel sheet is pressed. It has been found that a so-called pickup phenomenon occurs. On the other hand, regarding the composition of iron oxide, it was found that the pick-up phenomenon occurs when a lot of wustite is generated. It is known that wustite has a structure containing more voids than other iron oxide species. If there are many voids in the oxide film, it is considered that when the steel sheet is pressed against the in-furnace roll, the iron oxide peels from the vicinity of the void and the peeled iron oxide adheres to the in-furnace roll. Therefore, the inventors focused on the composition and structure of iron oxide, and as a result of further studies, when high temperature and oxygen concentration are relatively high, an oxide film having a large amount of hematite having a relatively dense structure can be obtained. It was also found that when hematite accounts for 50% or more, the amount of wustite produced is suppressed, and as a result, the pickup phenomenon is suppressed. In addition, when an oxide film having a structure in which Mn is concentrated is formed on the outermost surface of hematite (Fe ₂ O ₃ ), the concentrated Mn moves to the inside of the oxide film, that is, the steel sheet side during reduction. It was found that non-plating was suppressed.

  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 in the present invention 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 promote oxidation even if the production process of the present invention described later is applied, so that plating adhesion is sufficient. However, non-plating also occurs. 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 preferably 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 Fe and unavoidable impurities, but 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 difference between the martensite phase and the ferrite phase, and effectively contributes to improving the 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.
A heating step: steel sheet in an atmosphere containing O ₂ : 1 to 20 vol%, H ₂ O: 1 to 50 vol%, and the balance consisting of one or more of N ₂ , CO and CO ₂ and unavoidable impurities Is heated to a temperature in the range of 800 to 900 ° C. B heating step: O ₂ : 0.01 vol% to less than 0.1 vol%, H ₂ O: 1 to 20 vol% or less, the balance being N ₂ , CO, CO ₂ , H _{2 in} an atmosphere composed of one or more of H ₂ and unavoidable impurities, and heating the steel plate to a temperature in the range of 800 to 900 ° C. Heating process: H ₂ : 1 to 50 vol The steel sheet is heated in a reducing atmosphere with a dew point of 0 ° C. or less to a temperature in the range of 800 to 900 ° C. Hereinafter, the reason for limitation of heating consisting of the above three steps will be described.

A heating process:
A heating process is performed in order to form the Fe oxide film of a desired composition and structure | tissue on the steel plate surface. Therefore, the heating atmosphere is an atmosphere containing O ₂ : 1 to 20 vol% and H ₂ O: 1 to 50 vol%. The atmosphere consists of one or more of N ₂ , CO, CO ₂ and unavoidable impurities. When the O _{2 in the} heating atmosphere is less than 1 vol%, the hematite fraction in the oxide film decreases. For economic reasons, O ₂ 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. Further, if the heating temperature is less than 800 ° C., iron oxide having a desired form cannot be obtained. If the heating temperature exceeds 900 ° C., the heating cost is increased.

The steel plate surface after the A heating step is mainly composed of iron oxide. By controlling the production ratio of hematite (Fe ₂ O ₃ ) high on this surface, it is possible to control the production ratio of wustite (FeO) low as a result, effectively preventing pickup in the C heating step. it can. Iron oxide is produced in the order of wustite (FeO), magnetite (Fe ₃ O ₄ ), and hematite (Fe ₂ O ₃ ) in this order from the steel sheet side. Of these, wustite (FeO) is a void inside than other iron oxides. Since the structure has a large amount, the strength is low, so that it peels off from the vicinity of the void and causes pickup. In contrast, hematite has a relatively dense structure and is difficult to peel off. By controlling the production ratio of hematite (Fe ₂ O ₃ ) in the oxide to a high ratio of 50 mass% or more, it is possible to form iron oxide that hardly peels on the steel sheet surface. Further, a concentrated layer of about 5 to 100 nm of Mn is formed on the outermost surface of hematite (Fe ₂ O ₃ ), and this Mn is formed inside the oxide film during the reduction in the B heating step and the C heating step. Since it moves to the side, surface concentration does not occur, non-plating is suppressed, and the mechanism is unknown, but if a Mn concentrated layer is present, the surface quality tends to be improved.

Examples of the method for analyzing the iron oxide composition include a method in which the B heating step, the C heating step, and the plating bath are evacuated and the steel sheet that has been left in the A heating step is collected and quantified by the X-ray diffraction method. The compositional composition ratio of each oxide can be obtained based on the measurement result of a standard sample in which the mixing ratio of each iron oxide species is changed for determination. Further, the thickness of the Mn-enriched layer on the surface of hematite (Fe ₂ O ₃ ) is measured by measuring the depth profile (Depth Profile) of each component in the depth direction from the surface of iron oxide using GDS, AES, etc. A portion where the Mn strength of the oxide surface is twice or more the minimum strength of Mn inside the Fe oxide can be obtained as the Mn concentrated layer. An example of the GDS profile of Fe, Mn, and O in the depth direction from the iron oxide surface of a steel sheet treated (oxidized) in a 5% O ₂ -15% H ₂ O atmosphere at 850 ° C. in the A heating step is shown in FIG. Shown in In FIG. 1, the hatched portion is a Mn concentrated layer.

B heating process:
The heating in the B heating step is performed in order to suppress pickup more effectively by reducing the surface of the steel plate once oxidized in the A heating step. If O ₂ is 0.1% or more, iron oxide is not reduced, and it takes a cost to make it less than 0.01%, so it is made 0.01% or more and less than 0.1%. Further, H ₂ O is made 1% or more so as not to reduce too much, and if it exceeds 20%, the humidification cost is required, so 1% or more and 20% or less. The remainder of the atmosphere consists of one or more of N ₂ , CO, CO ₂ , H ₂ and unavoidable impurities.

  In the B heating step, the steel sheet is heated to a temperature in the range of 800 to 900 ° C. If the heating temperature is less than 800 ° C, the iron oxide is not reduced. If the heating temperature exceeds 900 ° C, the heating cost is increased. When the B heating step is performed following the A heating step, the B heating step is performed such that the steel plate temperature is equal to or higher than the steel plate temperature in the A heating step.

  The A heating step and the B heating step can be performed in different heating furnaces, but considering the industrial productivity and the improvement of the current production line, the same heating furnace is used. It is desirable to perform the A heating process and the B heating process by changing the condition setting and dividing into two or more zones.

  When the A heating step is performed in a direct fired furnace (DFF) or a non-oxidizing furnace (NOF), the air ratio is 1 to 1.35, and the B heating process is performed in a direct fired furnace (DFF) or a non-oxidizing furnace ( NOF) is preferably performed under conditions with an air ratio of less than 1.

C heating process:
The C heating process is installed immediately after the B heating process, and simultaneously reduces the iron oxide while annealing the steel sheet. C heating step is performed in a conventional RTF, heating atmosphere _{is H} 2: a reducing atmosphere containing 50% or more and 1% or less. The balance is N ₂ and inevitable impurities. If H ₂ is less than 1%, sufficient reduction does not occur, and if it exceeds 50%, the cost increases. The dew point is 0 ° C or less. When the dew point exceeds 0 ° C., iron oxide is difficult to reduce. Moreover, since it is difficult to implement industrially when the dew point is less than -60 ° C, the dew point is desirably -60 ° C or higher.

  A C heating process heats a steel plate so that it may become the temperature within the range of 800-900 degreeC. When the heating temperature is less than 800 ° C., the reduction rate is slow, and when it exceeds 900 ° C., the heating cost is increased. When the C heating step is performed following the B heating step, the C heating step is performed such that the steel plate temperature is equal to or higher than the steel plate temperature in the B heating step.

  The iron oxide formed in the A heating process is reduced in the C heating process, so that the steel sheet surface is covered with reduced iron. The occurrence of defects is prevented. The iron oxide that does not easily peel off is formed in the A heating process, and the oxide is reduced in the B heating process, so that the pickup phenomenon in the C heating process is suppressed, so that it is possible to prevent the occurrence of push rods due to the pickup phenomenon. .

  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. For the production of galvannealed steel sheet, the bath temperature is 440 to 550. 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 due to overalloy is generated too much, resulting in deterioration of plating adhesion and 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 in a heating furnace at 1260 ° C. for 60 minutes, 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, DFF type CGL provided with DFF (direct furnace) -RTF-cooling zone, or NOF type CGL provided with NOF (non-oxidizing furnace) -RTF-cooling zone, Tables 2 to 5 are used. The A heating process to the C heating process were performed under the heat treatment conditions shown in FIG. DFF and NOF were each divided into 4 zones of 1st zone-4th zone, A heating process was performed in the 1st-3rd zone of DFF or NOF, and B heating process was performed in the 4th zone. C gas generated in a coke oven was used as the fuel gas. C heating step is carried out in RTF, atmospheric gas used was _H 2 -N ₂ gas.

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 galvannealed steel plate was obtained by performing an alloying process at 500-580 degreeC.

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

<Surface appearance>
Judgment is made by visual inspection for appearance defects such as push rods caused by non-plating or pick-up phenomenon. Good (○) if there is no appearance defect, and if there is a slight appearance defect, it is generally good. When there was good (Δ) and poor appearance, 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. According to the following criteria, those with ranks 1 and 2 were evaluated as particularly good (◯), good (Δ), and those with 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 (5)

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, O ₂ : 1 to 20 vol. %, H ₂ O: 1 to 50 vol%, and the temperature within the range of 800 to 900 ° C. in the atmosphere where the balance is one or more of N ₂ , CO, CO ₂ and unavoidable impurities. A heating step of heating to become, then O ₂ : 0.01 vol% to less than 0.1 vol%, H ₂ O: 1 to 20 vol% or less, the balance being N ₂ , CO, CO ₂ , H _The steel plate is 800 to 90 in an atmosphere composed of one or more of 2 and inevitable impurities. B heating step for heating to a temperature in the range of 0 ° C., and then a temperature in the range of 800 to 900 ° C. in a reducing atmosphere containing H ₂ : 1 to 50 vol% and a dew point of 0 ° C. or less. A method for producing a high-strength hot-dip galvanized steel sheet excellent in surface appearance and plating adhesion, characterized by performing a hot-dip galvanizing treatment after performing a C heating step of heating so as to become. The A heating step is characterized in that an oxide film having a structure in which 50 mass% or more is hematite (Fe ₂ O ₃ ) and Mn is concentrated on the outermost surface of hematite is 5 to 100 nm on the steel sheet surface. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate excellent in the surface external appearance and plating adhesiveness of Claim 1. The steel sheet according to claim 1 or 2 is further, as a chemical component, in mass%, Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%, Ti: 0.01 to 0. 1 or 2 or more types chosen from 1%, Nb: 0.01-0.1% and B: 0.0005-0.0050% of Claim 1 or 2 characterized by the above-mentioned A method for producing high-strength hot-dip galvanized steel sheets with excellent surface appearance and plating adhesion. The A heating step is performed in a direct fired furnace (DFF) or a non-oxidizing furnace (NOF) under the condition of an air ratio of 1 to 1.35, and the B heating process is performed in a direct fired furnace (DFF) or a non-oxidizing furnace (NOF). The method for producing a high-strength hot-dip galvanized steel sheet excellent in surface appearance and plating adhesion according to any one of claims 1 to 3, wherein the air ratio is less than 1. The high strength galvanized steel sheet manufactured by the method according to any one of claims 1 to 4, and further subjected to an alloying treatment. Manufacturing method of galvanized steel sheet.

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