JP2012012683A - Method for manufacturing hot dip galvanized steel sheet - Google Patents

Abstract

A Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties is produced by detoxifying Si oxides that inhibit the reaction between the steel sheet and zinc during alloying hot-dip galvanizing.
At least Si: 0.2 to 2.0, Mn: 0.2 to 3.0%, Al: 0.001 to 1.5%, and the ratio of Si, Mn, and Al is respectively The steel sheet satisfying the formulas (1) to (3) is continuously hot dip galvanized in a hot dip galvanizing line having a reducing furnace, and the hydrogen partial pressure and water vapor of the atmospheric gas in the heating furnace or heat insulation furnace The logarithmic ratio of the partial pressure is made to satisfy the following formula (4).
28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
−1.39 ≦ log (P _H2O / P _H2 ) ≦ −0.695 (4)
[Selection] Figure 5

Description

  The present invention relates to a method for producing a hot-dip galvanized steel sheet, and specifically relates to a method for producing a Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties.

  In recent years, high-strength steel sheets have been actively adopted as automobile steel sheets in order to reduce the weight of automobiles. Automotive steel plates are often used after being pressed. For this reason, it is considered that the strengthening of the steel sheet for automobiles is preferably solid solution strengthening with less reduction in ductility than the strengthening using the second phase such as precipitation strengthening and transformation strengthening. In particular, Si is a practically effective element as a solid solution strengthening element because it can be increased in strength without significantly reducing ductility and is inexpensive.

On the other hand, since steel plates for automobiles are also required to have rust prevention properties that can withstand harsh natural environments, alloyed hot-dip galvanized steel plates are frequently used as automotive steel plates.
Accordingly, there is a need for alloyed hot dip galvanizing with high strength and good ductility.

  As is well known, a steel plate containing a relatively large amount of Si is a steel plate that is formed into a film so that Si oxide covers the steel plate surface in an annealing furnace when plating is performed in a continuous galvanizing line. The reaction between zinc and zinc is hindered, and non-plating and a decrease in alloying rate are likely to occur in the plating process. When non-plating or a decrease in alloying rate occurs, not only the appearance quality of the galvannealed steel sheet is deteriorated due to uneven galvanization but also the rust prevention property is lowered. In order to suppress Si oxidation on the steel sheet surface, theoretically, the oxygen potential in the annealing furnace may be lowered, but the oxygen potential for suppressing Si oxidation cannot be industrially realized.

Thus, many methods have been proposed so far for suppressing the formation of Si oxide on the steel sheet surface in the annealing process.
Patent Documents 1 and 2 disclose a method of suppressing Si oxide generated on a steel sheet surface in an annealing process by pre-plating the steel sheet surface with Fe, Ni, Co or the like before hot dip galvanizing.

Patent Document 3 discloses a method of suppressing the surface concentration of Si oxide by oxidizing a steel plate in a weakly oxidizing atmosphere before plating and generating a Fe oxide film on the surface of the steel plate.
In Patent Document 4, the solid solution Si in the surface layer is used as an internal oxide by performing heat treatment in a temperature range of 650 to 950 ° C. in an atmosphere in which the black skin scale is not reduced while the black skin scale is attached after hot rolling. A method of fixing and suppressing surface Si oxidation in the annealing process is disclosed.

However, since the methods disclosed in Patent Documents 1 and 2 need to be pre-plated, an increase in the number of manufacturing steps and an accompanying increase in manufacturing cost are inevitable.
The method disclosed in Patent Document 3 weakly oxidizes the surface of the steel sheet to form a film of Fe oxide, so that this Fe oxide is wound around a transport roll in the furnace and transferred to the surface of the steel sheet. Will occur.

Furthermore, the method disclosed in Patent Document 4 inevitably increases the manufacturing cost because it is necessary to heat-treat the hot-rolled sheet in a temperature range of 650 to 950 ° C. before pickling.
In both Patent Documents 5 and 6, a humidified gas is introduced into the reduction zone of the indirect heating furnace in the continuous annealing furnace of the continuous galvanizing line to control the furnace atmosphere and prevent Si concentration on the steel sheet surface. A method for improving plating properties by producing Si oxide in steel is shown.

  Specifically, Patent Document 5 describes Si: 0.4 to 2.0% (in the present specification, “%” means “mass%” unless otherwise specified) and Mn: 1. In an indirect continuous annealing furnace that performs hydrogen reduction on a steel sheet containing 0 to 3.0%, a humidified gas is introduced into the indirect heating furnace to control a specific hydrogen partial pressure and water vapor partial pressure, A method of generating in steel and preventing Si concentration on the steel sheet surface is disclosed.

  In Patent Document 6, in a steel sheet containing Si, in an indirect continuous annealing furnace, the steel sheet temperature is set to 550 ° C. or higher and 750 ° C. or lower and the dew point is set to −25 ° C. or lower and the Fe oxidation is suppressed. A method is disclosed in which the latter stage of the heating zone is humidified with a humidified gas and the dew point is set to -30 ° C. or higher and 0 ° C. or lower to promote internal oxidation of Si and suppress Si concentration on the steel sheet surface.

  In the methods disclosed by Patent Documents 5 and 6, both the hydrogen partial pressure and the water vapor partial pressure are set for the purpose of suppressing the generation of Si oxide on the steel sheet surface and generating Si oxide inside the steel sheet. Control to a specific range.

JP 2000-303158 A JP 7-197225 A JP 7-216524 A JP 2000-309824 A JP 2007-191745 A International Publication No. 2007/043273 Pamphlet

  The inventors of the present invention controlled the atmosphere in the furnace to the hydrogen / water vapor partial pressure disclosed in Patent Documents 5 and 6 and annealed a steel sheet containing Si. It was found that the plateability of the steel sheet was improved only when the steel sheet had a specific Si—Mn—Al ratio.

  That is, it is only with respect to the steel plate which has a very limited composition that the plating property of the steel plate containing Si can be improved to a practically satisfactory level by the methods disclosed in Patent Documents 5 and 6. It is necessary to improve the plateability of the steel sheet contained.

  An object of the present invention is to provide a method for producing a Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties by detoxifying the Si oxide that inhibits the reaction between the steel sheet and zinc during galvannealing. is there.

The present invention is as follows.
(I) It contains at least Si: 0.2 to 2.0, Mn: 0.2 to 3.0%, Al: 0.001 to 1.5%, and the ratio of Si, Mn and Al is as follows. A method for producing a hot dip galvanized steel sheet in which hot dip galvanizing treatment is continuously performed on a steel sheet satisfying the formulas (1) to (3) in a hot dip galvanizing line having a reducing furnace, wherein the atmospheric gas in the reducing furnace A method for producing a hot-dip galvanized steel sheet, wherein the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure satisfies the following formula (4):

Note that equation (4) corresponds to the range of dew point -20 to 0 ° C. with 3% _{H 2} atmosphere.
28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
-1.39 ≦ log (P _H2O / P _H2 ) ≦ −0.695 (4)

(II) At least Si: 0.2 to 2.0%, Mn: 0.2 to 3.0%, Al: 0.001 to 1.5%, and the ratio of Si, Mn or Al is as follows. A method for producing a hot dip galvanized steel sheet in which hot dip galvanizing treatment is continuously performed on a steel sheet satisfying the formulas (1) to (3) in a hot dip galvanizing line having a reduction furnace, the steel sheet temperature in the reduction furnace The logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the temperature range of 650 to 750 ° C. satisfies the following formula (5), and the hydrogen partial pressure of the atmospheric gas in the temperature range of 750 ° C. to 950 ° C. And a logarithmic ratio of water vapor partial pressure satisfy the following formula (6):

(5) is to fall within the scope of the dew point of -30 ° C. or less at 10% _{H 2} atmosphere, (6) corresponds to the range of dew point -20 to + 10 ° C. in 10% _{H 2} atmosphere.
28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
log (P _H2O / P _H2 ) ≦ −2.29 (5)
-1.91 ≦ log (P _H2O / P _H2 ) ≦ −0.915 (6)

(III) Logarithmic ratio (P _H2O / _PH2 ) is controlled by humidifying nitrogen gas and introducing it into a reduction furnace. The hot-dip galvanized steel sheet described in (I) or (II) above Manufacturing method.
(IV) The method for producing a hot-dip galvanized steel sheet according to any one of items (I) to (III), wherein at least the hydrogen concentration in the atmosphere gas of the reduction furnace is 10% by volume or more.

(V) The manufacturing method of the hot-dip galvanized steel sheet described in the above item (IV), wherein the reducing furnace is a horizontal type.
(VI) After hot dip galvanizing is applied to the steel sheet, the steel sheet is further heated to a temperature of 460 to 600 ° C. for alloying treatment, and the alloyed hot dip galvanized steel sheet having an Fe content of 7 to 15%. A method for producing a hot-dip galvanized steel sheet according to any one of items (I) to (V).

  According to the present invention, Si oxide that inhibits the reaction between the steel sheet and zinc during alloying hot dip galvanization is rendered harmless, and this makes it possible to produce a Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties. .

FIG. 1 is a surface SEM photograph of a laboratory annealed test material in which Si—Mn—Al-based oxide is granulated. FIG. 2 is an explanatory view showing the shape change of the synthetic Si—Mn—Al-based oxide during heating. FIG. 3 is a graph showing a change in the amount of heat of the Si—Mn—Al-based oxide. FIG. 4 is a surface SEM photograph of a laboratory annealing test material in which a film-like Si—Mn—Al-based oxide was generated. FIG. 5 is an explanatory diagram showing a temperature pattern of the heating test and atmospheric conditions. FIG. 6 is an explanatory diagram showing a temperature pattern of the heating test and atmospheric conditions. FIG. 7 is an explanatory diagram showing a simplified example of a continuous hot dip galvanizing line to which the present invention is applied.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the accompanying drawings.
The Si-containing steel sheet is continuously hot dip galvanized in a hot dip galvanizing line having a reduction furnace. At this time, the steel sheet contains at least Si: 0.2 to 2.0, Mn: 0.2 to 3.0%, Al: 0.001 to 1.5%, and the ratio of Si, Mn and Al. Satisfies the above formulas (1) to (3), and the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the reduction furnace satisfies the above formula (4). The reason for this will be described below. Note that equation (4) corresponds to the range of dew point -20 to 0 ° C. with 3% _{H 2} atmosphere.

1. Steel sheets with varying Si content, Mn content, Al content were annealed under various hydrogen partial pressures and water vapor partial pressures, and as a result of observing the surface oxides generated at that time, The form of the Si-Mn-Al oxide to be produced includes a film form and a granular form. If a film-like Si-Mn-Al oxide is present on the steel sheet surface, the plateability of the steel sheet is unsatisfactory. When the Si-Mn-Al oxide is present on the steel sheet surface, Fe is exposed on the steel sheet surface, and the wettability between the steel sheet and zinc or the alloying reaction is promoted. It was also found that a hot-dip galvanized steel sheet with excellent surface properties can be produced.

  Cold of Si-containing steel containing Si: 1.3%, Mn: 1.6%, Al: 0.3% and the ratio of Si, Mn and Al satisfying the above formulas (1) to (3) An annealing test was performed using a rolled sheet. The atmosphere gas in the furnace at the time of annealing was 10% by volume of hydrogen concentration-nitrogen gas having a dew point adjusted to −20 ° C., held at 850 ° C. for 2 minutes, and then rapidly cooled with nitrogen gas having a dew point of −68 ° C. The result is shown in FIG.

FIG. 1 is a surface SEM photograph of a laboratory annealed test material in which Si—Mn—Al-based oxide is granulated.
As shown in FIG. 1, the granular material parted on the steel plate surface was observed. When the component analysis of this granular material was conducted, it was Si-Mn-Al type oxide. The rest of the region was a region where no Fe-based oxide was present.

  When Si-containing steel is annealed, it has been conventionally thought that a film-like Si oxide is generated and covers the steel sheet surface, but the shape of the oxide on the steel sheet surface of this laboratory annealed test material is not a film, The Si oxide was divided, and Fe which is the main component of the steel sheet was exposed.

Thus, if Fe is exposed to the steel plate surface after annealing, the reactivity with zinc will not be impaired at the time of plating, and it is thought that favorable zinc wettability is ensured.
The mechanism by which Si oxide is divided during annealing is thought to be due to the fact that Si oxide is softened into glass, has fluidity, and is agglomerated and divided.

Water vapor is necessary for the Si oxide to cause glass softening.
FIG. 2 is an explanatory diagram showing the shape change of the synthetic Si—Mn—Al-based oxide during heating, and shows the influence of water vapor on the glass softening behavior of the Si oxide.

  As shown in Fig. 2, Si-Mn-Al-based oxides were synthesized, and glass softening behavior at high temperatures was investigated under two conditions, dry and wet (with different water vapor contents in a hydrogen-nitrogen mixed gas atmosphere). As a result, no change in shape of the Si—Mn—Al oxide was observed even at 1000 ° C. in the dry state, but a change in shape of the Si—Mn—Al oxide was observed near 770 ° C. in the wet state. Granulated.

FIG. 3 is a graph showing the calorie change of the Si—Mn—Al-based oxide, and shows the measurement result of differential thermal analysis (DSC).
As shown in FIG. 3, the shape change of the Si—Mn—Al oxide is measured as an endothermic reaction (glass softening) and an exothermic reaction (crystallization), and the Si—Mn—Al oxide is added with water vapor. Glass softens and granulates.

By analyzing the component of the granulated oxide, it was found that the Si-Mn-Al oxide becomes granular when the composition of the Si-Mn-Al oxide is in a specific component range.
The composition of the SiO ₂ —MnO—Al ₂ O ₃ oxide generated on the steel sheet surface depends on the ratio of Si, Mn, and Al added to the steel sheet and the hydrogen partial pressure / water vapor partial pressure in the annealing atmosphere. .

  Specifically, in order for the Si—Mn—Al oxide to be granulated, the steel sheet is at least Si: 0.2 to 2.0%, Mn: 0.2 to 3.0%, Al: 0.00. 001 to 1.5%, and the ratios of Si, Mn, and Al are 28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54, 30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70, 0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 should be satisfied.

Hereinafter, the reasons for limiting the composition of the steel sheet will be described.
If the Si content is less than 0.2%, the problem of non-plating and alloying delay due to the Si oxide does not occur in the first place. On the other hand, if the Si content exceeds 2.0%, the present invention However, the effect of improving the plating property cannot be obtained. Therefore, the Si content is 0.2% or more and 2.0% or less. From the viewpoint of increasing the strength of steel, Si is often contained in an amount of 1.0% or more, but the present invention is particularly useful in such a component range.

  Mn is contained in an amount of 0.2% or more for increasing the strength. On the other hand, when the Mn content exceeds 3.0%, ductility is reduced. Therefore, the Mn content is 0.2% or more and 3.0% or less.

  Since Al is difficult to remove completely from the steel, the Al content is 0.001% or more. On the other hand, if the Al content exceeds 1.5%, ductility is reduced. Therefore, the Al content is set to be 0.001% or more and 1.5% or less.

The reason why the respective ratios of Si, Mn, and Al in the steel sheet are set in the above-described range is that the oxide does not become granulated if the ratio deviates from this range.
Components other than the above will be described.

C is effective for obtaining a high tension, but on the other hand, if contained in excess, the toughness and weldability are lowered, so the C content is preferably 0.03 to 0.20%.
When P is contained excessively, the toughness is deteriorated, so the P content is preferably 0.1% or less.

Since S becomes MnS in steel and degrades the bendability, the S content is preferably 0.01% or less.
Since N forms a nitride during continuous casting and causes cracks in the slab, it is preferable that the N content is low. Therefore, the N content is 0.01% or less.

In addition to the above, an optional additive element may be contained. Hereinafter, typical optional added elements will be described.
Since Ti, Nb, and V delay the recrystallization and refine the crystal grains, they can be contained as necessary. For example, in order to more stably secure a tensile strength of 980 MPa or more, the content of any element of Ti, Nb, and V is preferably 0.003% or more. However, for each element, if it exceeds 0.25%, it becomes saturated and disadvantageous in cost.

  Both Cr and Mo have the function of promoting transformation strengthening by stabilizing austenite in the same manner as Mn, and are effective in increasing the strength of the steel sheet, so they can be contained as necessary. However, since Cr and Mo are easily oxidizable elements, a large amount can adversely affect the plating property. Therefore, the content is set to 1% or less for each element.

  Cu and Ni have a corrosion-inhibiting effect and have a function of concentrating on the surface to suppress the intrusion of hydrogen and suppress delayed fracture, so that they can be contained as necessary. However, in any case, when the content exceeds 1%, this effect is saturated and disadvantageous in cost.

  Ca, Mg, REM, and Zr all contribute to inclusion control, in particular, fine dispersion of inclusions, and further improve bendability, so that they can be contained as required. In order to obtain the above effect more reliably, the content of any element is preferably 0.001% or more. However, if the content is excessive, the surface properties are deteriorated, so each content is set to 0.01% or less.

  B suppresses the nucleation from the grain boundary, enhances the hardenability and contributes to the increase in strength, and can be contained as necessary. In order to obtain this effect more reliably, the B content is preferably 0.0005% or more. If the B content exceeds 0.01%, the effect is saturated, so the B content is 0.01% or less.

  Bi has the effect of concentrating on the solidification interface of the molten steel to narrow the dendrite interval and reduce the solidification segregation. As a result, there is also an effect of preventing a bending crack at the segregation part. In order to expect this effect, the Bi content is preferably 0.0002% or more. Even if it contains more than 0.05%, the effect is saturated.

2. Logarithmic ratio of hydrogen partial pressure and water vapor partial pressure of the atmospheric gas in the reduction furnace In order to granulate the oxide, the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas is set to -1.39 ≦ log (P _H2O / P _H2 ) ≦ −0.695. Beyond this range, the oxide does not granulate.

  In actual operation, nitrogen gas is humidified and introduced into the reduction furnace to control the logarithmic ratio of the hydrogen partial pressure and water vapor partial pressure in the above-described range. Si, Mn, and Al in the steel sheet immediately react with water vapor in the reduction furnace to form oxides. As a result, the hydrogen partial pressure increases and the water vapor partial pressure decreases.

  Here, when the hydrogen concentration in the reduction furnace is low, the corresponding water vapor amount is also low. In such a state, if the hydrogen concentration increases due to oxidation of Si, Mn, Al in the steel sheet, the hydrogen partial pressure and water vapor It becomes difficult to control the logarithmic ratio of the partial pressures within the appropriate range described above.

For example, when the furnace volume of the reduction furnace is 25 m ³ and the hydrogen concentration in the furnace is 3% by volume, the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure is set to −1.39 ≦ log (P _H2O / P _H2 ) ≦ −. In order to make 0.695, it is necessary to make the amount of water vapor 0.124-0.614 kPa. Here, a case where a steel sheet having a composition of 1.5Si-1.6Mn-0.2Al and a width of 1.2 m is passed through a continuous hot-dip galvanizing line at a speed of 1 m / sec will be considered as an example. . Assuming that Si, Mn, and Al in the region of 1 μm depth on the front and back surfaces of the steel plate are oxidized by water vapor, water vapor is consumed at a rate of 6.2 × 10 ⁻⁴ m ³ / sec. In order to maintain the logarithmic ratio of the pressure within the above-described proper range, it is necessary to replenish the same amount of water vapor.

  However, since the water vapor concentration is excessively high in the portion supplemented with water vapor, decarburization and oxidation occur in the steel sheet present in this portion. In order to prevent this, it is only necessary to install a large number of nozzle holes for introducing steam into the furnace and supply the steam to the steel plate as uniformly as possible. However, this is not realistic from the viewpoint of equipment costs.

On the other hand, when the hydrogen concentration in the furnace is high, the amount of water vapor that makes the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure within an appropriate range increases, so the water vapor is 6.2 × 10 ⁻⁴ m ³ / s. Even if it is added, the problem of density unevenness does not occur, and the atmosphere in the furnace can be easily controlled to a desired atmosphere. Therefore, the hydrogen concentration in the atmosphere gas of the heating furnace or heat insulation furnace is preferably 10% by volume or more.

  In addition, it is preferable that the reduction furnace in this invention is not a saddle type but a horizontal type. When the humidified gas is introduced into the furnace, the horizontal furnace volume is smaller than the vertical furnace volume, so using the horizontal type makes it easier to throttle the zone where the steel sheet and water vapor react. Since oxidation can be suppressed as compared with the case of using a saddle type, it is advantageous for more reliably producing a Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties.

  In addition, when the temperature of the steel sheet during the temperature increase is in the range of 650 ° C. or higher and lower than 750 ° C., oxidation by water vapor is suppressed, and in the region of 750 ° C. or higher, the hydrogen partial pressure and water vapor at which the oxide is granulated are obtained. The condition of the logarithmic ratio of the partial pressure becomes wider.

Specifically, the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure in a region where the temperature of the steel sheet during temperature rise is less than 750 ° C. is log (P _H2O / P _H2 ) ≦ −2.29, and is 750 ° C. or higher. It was confirmed that the oxide was granulated when the temperature region was -1.91 ≦ log (P _H2O / P _H2 ) ≦ −0.915.

  The logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure in the atmosphere is defined with a steel plate temperature of 750 ° C. as a boundary. When oxidation starts at less than 750 ° C., a film-like Si—Mn—Al-based oxide is formed on the surface. In order to produce | generate, it was 750 degreeC or more to make high dew point atmosphere. When the steel plate temperature is less than 750 ° C., it is necessary to perform annealing in a low dew point atmosphere to suppress oxidation as much as possible.

As an example, FIG. 4 shows the results when the steel plate used in FIG. 1 is kept from room temperature to a dew point of −20 ° C. during annealing. Other annealing conditions are the same as those in FIG.
FIG. 4 is a surface SEM photograph of a laboratory annealing test material in which a film-like Si—Mn—Al-based oxide was generated.

As shown in FIG. 4, when compared with the result of FIG. 1 humidified only at 850 ° C., film-like Si—Mn—Al-based oxides are formed on the steel sheet surface when humidification is performed from room temperature.
In actual operation, the annealing temperature of the steel sheet is 950 ° C. or lower, so the upper limit of the steel sheet temperature is 950 ° C.

The reason for this will be described below. The steel sheet is rapidly heated up to about 650 ° C. in the preheating zone on the inlet side of the continuous hot dip galvanizing line (a non-oxidizing furnace or a direct-fired furnace is exemplified, but may be integrated with a reducing furnace). At this time, Fe oxide is generated on the surface of the steel sheet. The produced Fe oxide is immediately reduced by hydrogen gas in the furnace, and H ₂ O is generated. The generated H ₂ O reacts with Si, Mn, and Al in the steel to cause oxides generated on the steel sheet surface.

  However, in practice, in the temperature range below 650 ° C., the diffusion rate of Si, Mn, and Al is small, and there is no problem of concentration as an oxide on the surface. Therefore, the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure in the temperature range of less than 650 ° C. does not matter. On the other hand, in the case of a radiant tube type reduction furnace, since no Fe oxide is generated, it is not necessary to consider the above.

The present invention will be described more specifically with reference to examples.
The actual equipment and lab materials (total 26 steel types) having the composition shown in Table 1 were used as test materials.
The actual equipment is obtained after hot rolling, pickling and cold rolling cold rolled base material (before annealing), cut into 20 mm square, immersed in organic solvent, and cleaned with an ultrasonic cleaner for 15 minutes. Degreased. On the other hand, the laboratory material was cast in a vacuum melting furnace and then forged, and a hot-rolled sheet having a thickness of 3 mm was formed by a hot rolling mill.

  The front and back surfaces of the hot-rolled sheet were each mechanically ground by 500 μm to remove the influence of element concentration on the surface layer of the hot-rolled sheet. Further, after mechanical grinding, a cold-rolled sheet was produced with a cold rolling mill (thickness 2 mm → 0.8 mm). Thereafter, the cold-rolled plate was cut into 20 mm square, immersed in an organic solvent, washed with an ultrasonic cleaner for 15 minutes, and degreased.

  Then, using these specimens, the influence of the base material Si—Mn—Al ratio and the hydrogen / water vapor partial pressure of the atmospheric gas on the shape of the oxide formed on the steel sheet surface during annealing was investigated. An annealing test was performed using a table lamp heating device under the temperature pattern and atmospheric gas conditions shown in FIG.

After placing the test material in the furnace using a table lamp heating device, the gas was replaced with N ₂ gas (dew point -68 ° C., 25 L / min) for 3 minutes, and then 3% H ₂ + N ₂ gas. A gas in which (2 L / min) and N ₂ gas passed through ice water or warm water were mixed was allowed to flow into the furnace.

At this time, the flow rate of N ₂ gas was adjusted so that the dew point meter installed on the furnace entrance side would be −30 ° C., −10 ° C., and + 10 ° C. After changing to this mixed gas atmosphere and replacing for 3 minutes, the temperature was raised to 850 ° C. at 15 ° C./second and held for another 2 minutes, then the furnace was stopped and N ₂ gas (dew point −68 ° C., 25 L / min) It was cooled quickly. After cooling to 50 ° C., the sample was taken out into the atmosphere, and the surface was observed in the reflected electron image mode of SEM to evaluate the surface oxidation state.

The evaluation of the oxidation state of the steel sheet surface in this example will be described.
In the backscattered electron image mode, a difference appears in contrast as shown in FIGS. 1 and 4 depending on the constituent atomic species. When comparing the atomic weight of Fe, which is the main component of the base material, and the oxide generated on the surface, the heavy atomic weight Fe is white and the light oxide is black. Therefore, the obtained surface SEM image was binarized with image conversion software, and the black area ratio was defined as the oxide surface coverage. A coverage of 30% or less was evaluated as ◯, 30 to 70% as Δ, 70% or more as x, and Δ or more as a pass.

As the above reason, when the coverage is 70% or more, the wettability is remarkably deteriorated in the subsequent wettability evaluation with molten zinc. Therefore, the coverage is less than 70%.
The results are shown in Tables 2 and 3.

As shown in Table 2 and Table 3, only when both of the formulas (1) to (3) and the formula (4) are satisfied, the generated oxide is softened to a glass, and the coated area ratio on the steel sheet surface is low. I understand. Furthermore, when the test which changed the dew point condition finely was performed and the surface coverage of the oxide was investigated, if Formula (1)-(3) is satisfy | filled, hydrogen / water vapor partial pressure of atmospheric gas will be -1. It has been found that the coverage is less than 70% when controlled within the range of 39 ≦ log (P _H2O / P _H2 ) ≦ −0.695.

Next, using the test materials A, B, and V that were evaluated as ◯ and Δ in Tables 2 and 3, the relationship between the oxide shape generated on the steel sheet surface and the steel sheet temperature when the humidified gas was introduced was investigated. .
Similarly, an annealing test was performed using the table lamp heating device under the temperature pattern and atmospheric gas conditions shown in FIG.

After placing the test material in the furnace, the gas was replaced with N ₂ gas (dew point −68 ° C., 25 L / min) for 3 minutes, and then 10% H ₂ + N ₂ gas (dew point −58 ° C., 5 L / min). Min) for 3 minutes. Thereafter, the temperature was raised at 10 ° C./second while maintaining 10% H ₂ + N ₂ gas (dew point −58 ° C., 5 L / min), the holding temperatures at high temperatures were 700, 750, 850 ° C. When the temperature was reached, N ₂ gas passed through ice water or warm water was flowed into the furnace so that the dew point of the atmospheric gas was −25, −20, −10, 0, + 20 ° C.

After holding in this atmospheric gas for 2 minutes, the furnace was stopped and quenched with N ₂ gas (dew point -68 ° C., 25 L / min). After cooling to 50 ° C., the sample was taken out into the atmosphere, and the surface was observed in the reflected electron image mode of SEM to evaluate the surface oxidation state.

  The evaluation of the oxidation state of the steel sheet surface in the examples is evaluated by the oxide coverage on the steel sheet surface in the same manner as in the evaluation method of Example 1. The coverage is 30% or less ◯, 30 to 70% is △, 70% or more was evaluated as x, and Δ or more was determined as pass. The results are shown in Tables 4 and 5.

Compared with the results shown in Tables 2 and 3, the range in which the evaluation becomes ◯ expands by performing humidification at 750 ° C. or higher than from room temperature. Furthermore, when the test which changed the dew point condition finely was performed and the surface coverage of the oxide was investigated, log (P _H2O / P _H2 ) ≦ −2.29 was satisfied in the steel plate temperature range below 750 ° C., and 750 ° C. In the above case, the covering ratio is less than 70% if -1.91 ≦ log (P _H2O / P _H2 ) ≦ −0.915 is satisfied.

Next, the effect of the present invention was investigated on the horizontal line of the actual machine using the test materials A, H, and D.
FIG. 7 is an explanatory view showing a simplified example of a continuous hot-dip galvanizing line (hereinafter referred to as CGL) to which the present invention is applied, and shows only a part of the CGL.

  This CGL includes a pre-tropical zone 2, a non-oxidizing furnace 3, a reducing furnace 4, a snout 5, a molten zinc pot 6, and an alloying furnace 7. The reduction furnace 4 is divided into a heating zone 4a, a heating zone 4b, a soaking zone 4c, and a cooling zone 4d according to the heating temperature range, and the inside of the furnace is maintained in a reducing atmosphere. The arrow in the figure is the moving direction of the steel plate 1.

  The steel sheet 1 passed through the CGL is heated in the pre-tropical zone 2 and further heated to a recrystallization temperature or lower (up to about 650 ° C.) in the non-oxidizing furnace 3 and then heated to about 700 ° C. in the heating zone 4a. It is heated to the recrystallization temperature or higher (for example, about 850 ° C.) in the heating zone 4b. Thereafter, it is further heated in the soaking zone 4c and completely recrystallized, and then cooled to about 500 ° C. in the cooling zone 4d.

Then, it passes through the snout 5 held in a reducing atmosphere, is immersed in a hot dip zinc pot 6, and is hot dip galvanized.
Furthermore, it is heated to about 600 ° C. in the alloying furnace 7 and alloyed, and for example, an alloyed hot-dip galvanized steel sheet having a Fe content of 7 to 15% is produced. Note that the atmosphere gas flow in the furnace is in the direction from the snout 5 to the pre-tropical zone 2 in reverse to the moving direction of the steel sheet in order to maintain the atmosphere of the reduction furnace.

In order to investigate the effect of the present invention, a humidifier was installed in the soaking zone 4b, and humidified N ₂ gas was introduced into the furnace. Although the humidification method is not particularly limited, it is preferable to humidify N ₂ gas through a humidifier. The dew point in the furnace was recorded with a dew point meter installed in each zone of the reduction furnace 4.

In the test, the dew point in the furnace was adjusted by changing the flow rate of the humidified N ₂ gas. The hydrogen concentration in the furnace was 10%, the steel sheet temperature was 650 ° C. in the non-oxidizing furnace 3, 700 ° C. in the heating zone 4a, 800 ° C. in the soaking zone 4b, 850 ° C. in the soaking zone 4c, and 500 ° C. in the cooling zone 4d.

  The plating evaluation method was performed visually. A steel plate in which no plating was generated at all was indicated as “◯”, a steel plate on which no plating was generated was indicated as “x”, and “◯” was determined as acceptable. The results are shown in Table 6.

As a result of the test, it was confirmed that the conditions of the atmospheric gas that does not cause non-plating need to satisfy the formula (4) with respect to the steel sheet satisfying the compositions of the formulas (1) to (3).
Moreover, although the hydrogen concentration in the furnace was 10%, decarburization and oxidation did not occur on the surface layer of the steel sheet, and a Si-containing high-strength hot-dip galvanized steel sheet having excellent surface properties can be produced.

DESCRIPTION OF SYMBOLS 1 Steel plate 2 Pre-tropical 3 Non-oxidizing furnace 4 Reduction furnace 4a Heating zone 4b Heating zone 4c Soaking zone 4d Cooling zone 5 Snout 6 Molten zinc pot 7 Alloying furnace

Claims (6)

In mass%, at least Si: 0.2-2.0, Mn: 0.2-3.0%, Al: 0.001-1.5%, and the ratio of Si, Mn and Al respectively A steel sheet satisfying the following formulas (1) to (3) is a method for producing a hot-dip galvanized steel sheet which is continuously hot-dip galvanized in a hot-dip galvanizing line having a reduction furnace, and the atmosphere in the reduction furnace A method for producing a hot-dip galvanized steel sheet, wherein the logarithmic ratio of the hydrogen partial pressure of gas and the partial pressure of water vapor satisfies the following formula (4):
28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
−1.39 ≦ log (P _H2O / P _H2 ) ≦ −0.695 (4) In mass%, at least Si: 0.2-2.0%, Mn: 0.2-3.0%, Al: 0.001-1.5%, and the ratio of Si, Mn or Al is A method for producing a hot-dip galvanized steel sheet, in which hot-dip galvanizing treatment is continuously performed on a steel sheet satisfying the following formulas (1) to (3) in a hot-dip galvanizing line having a reduction furnace, wherein the steel sheet in the reduction furnace The logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the temperature range of 650 to 750 ° C. satisfies the following formula (5), and the hydrogen content of the atmospheric gas in the temperature range of 750 ° C. to 950 ° C. A method for producing a hot-dip galvanized steel sheet, wherein the logarithmic ratio of the pressure and the water vapor partial pressure satisfies the following formula (6):
28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 70 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
log (P _H2O / P _H2 ) ≦ −2.29 (5)
-1.91 ≦ log (P _H2O / P _H2 ) ≦ −0.915 (6) The method for producing a hot-dip galvanized steel sheet according to claim 1 or 2, wherein the logarithmic ratio (P _H2O / _PH2 ) is controlled by humidifying nitrogen gas and introducing it into the reduction furnace.   The method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein a hydrogen concentration in an atmosphere gas of the reduction furnace is 10% by volume or more.   The said reduction furnace is a horizontal type, The manufacturing method of the hot dip galvanized steel plate described in Claim 4.   After the hot dip galvanizing is applied to the steel plate, the steel plate is further heated to a temperature of 460 to 600 ° C. for alloying treatment, and the alloyed hot dip galvanized steel plate whose Fe content in the plating layer is 7 to 15% by mass. The manufacturing method of the hot dip galvanized steel plate described in any one of Claim 1- Claim 5 which manufactures.