JP5811841B2 - Method for producing Si-containing high-strength galvannealed steel sheet - Google Patents

Description

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

  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 increase the strength without significantly reducing the 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.
Therefore, an alloyed hot-dip galvanized steel sheet having high strength and good ductility is strongly demanded as a steel sheet for automobiles.

  As is well known, when a steel sheet containing a relatively large amount of Si is plated in a continuous hot dip galvanizing line, the Si oxide is formed into a film so as to cover the surface of the steel sheet in an annealing furnace. The reaction between the steel sheet and zinc is hindered, and non-plating and alloying speed are likely to decrease. 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 surface of the steel sheet, theoretically, the oxygen potential in the annealing furnace may be lowered, but a low oxygen potential capable of suppressing Si oxidation cannot be industrially realized.

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

  Patent Document 3 discloses a method of suppressing the concentration of Si oxide on the surface of the steel sheet by oxidizing the steel sheet in a weakly oxidizing atmosphere before hot dip galvanizing and generating an Fe oxide film on the surface of the steel sheet. It is disclosed.

  In Patent Document 4, by performing heat treatment at 650 to 950 ° C. in an atmosphere in which the black skin scale is not reduced while the black skin scale is adhered to the surface of the hot rolled sheet, the solid solution Si on the surface layer of the steel plate is internally oxidized. A method of fixing Si as a product and suppressing Si oxidation on the surface of a steel sheet in an 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.
In the method disclosed in Patent Document 3, the surface of the steel sheet is weakly oxidized to form a film of Fe oxide, and this Fe oxide is wound around a transport roll in the furnace and transferred to the surface of the steel sheet, Surface flaws occur.

In the method disclosed in Patent Document 4, since it is necessary to heat-treat the hot-rolled sheet at 650 to 950 ° C. before pickling, an increase in manufacturing cost is inevitable.
Furthermore, in Patent Documents 5 and 6, both the humidification gas is supplied to the reduction zone of the indirect heating furnace in the continuous annealing furnace of the continuous galvanizing line to control the furnace atmosphere, and the Si oxide is on the surface of the steel sheet. A method for improving the plating property by preventing it from being present in steel is disclosed.

  Specifically, in Patent Document 5, Si: 0.4 to 2.0% (in the present specification, “%” for chemical composition means “mass%” unless otherwise specified) and Mn: In an indirect continuous annealing furnace that performs hydrogen reduction on a steel sheet containing 1.0-3.0%, a humidifying gas is introduced into the indirect heating furnace to control a specific hydrogen partial pressure and water vapor partial pressure, and Si oxidation A method is disclosed in which a product is produced in steel and Si concentration on the surface of the steel plate is prevented.

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

  In any of the methods disclosed in Patent Documents 5 and 6, by controlling the hydrogen partial pressure and the water vapor partial pressure within a specific range, the generation of Si oxide on the surface of the steel sheet is suppressed, It is generated inside the steel plate.

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

  When the inventors controlled the hydrogen atmosphere and water vapor partial pressure disclosed in Patent Documents 5 and 6 and annealed the steel sheet containing Si, the present inventors are the case where the steel sheet is a partial steel type. Specifically, it has been found that only when the chemical composition has a specific Si—Mn—Al ratio, an improvement in the plateability of the steel sheet is recognized. That is, it is only with respect to the steel plate which has a very limited chemical composition that the plating property of the steel plate containing Si is improved to a practically no problem by the methods disclosed in Patent Documents 5 and 6. It is necessary to improve the plateability of the steel sheet containing Si.

Therefore, the present inventors previously described at least Si: 0.2 to 2.0%, Mn: 0.2 to 3.0%, Al, according to Japanese Patent Application No. 2010-151855 (hereinafter referred to as the prior application). : 0.001 to 1.5%, and the ratio of Si, Mn, and Al is 28 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54, 30 ≦ (Mn / (Si + Mn + Al)) × 100, respectively. ≦ 70, 0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 A hot-dip galvanized steel sheet is obtained by continuously subjecting a steel sheet having a chemical composition to a hot-dip galvanizing line having a reduction furnace. When the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the reduction furnace is set to the range of -1.39 ≦ log (P _H2O / P _H2 ) ≦ −0.695, Steel sheet The invention has been proposed in which the film-like Si oxide on the surface can be granulated to reduce the coverage of the Si oxide on the surface of the steel sheet and improve the plating properties.

  According to the invention proposed by the prior application, it is possible to detoxify the Si oxide that hinders the reaction between zinc and a steel sheet having a chemical composition that satisfies the above Si-Mn-Al ratio during alloying hot dip galvanization, Thereby, the Si containing high intensity | strength hot-dip galvanized steel plate excellent in surface property can be manufactured reliably.

  However, as a result of further studies to further improve the surface quality of the hot dip galvanized steel sheet, the present inventors do not satisfy the above Si—Mn—Al ratio in the invention proposed by the prior application. It has been found that the plateability of a steel sheet having a chemical composition, such as a high-strength steel sheet having a composite structure containing about 0.2 to 0.9% of Si cannot be improved.

The present invention has been made in view of the problems of the invention proposed by the prior application, and the Si-Mn-Al ratio defined by the prior application cannot be improved by the invention proposed by the prior application. Steel plate having a chemical composition that does not satisfy the requirements, for example, Si-Mn-Al oxide of a composite structure steel-based high-tensile steel plate containing about 0.2 to 0.9% of Si is granulated, thereby improving plating properties. 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 quality.

The present inventors obtained the knowledge (a) to (g) listed below and completed the present invention.
(A) As a result of observing oxides formed on the surface of the steel sheet at the time of annealing the steel sheet with the changed Si-Mn-Al ratio under various hydrogen partial pressure and water vapor partial pressure, The produced Si—Mn—Al oxide exists in the form of a film or particles.

  (B) If the Si-Mn-Al oxide is granular, Fe is exposed on the surface of the steel sheet, the wettability between the steel sheet and zinc is improved, and the alloying reaction is promoted. Even if it exists, the hot dip galvanized steel plate excellent in surface property can be manufactured.

  (C) The oxide generated on the surface of the steel sheet becomes granular when the oxide melts and fluidizes in the annealing temperature region and the wettability with the steel sheet is poor.

  (D) From the result of component analysis of the granulated oxide, it was found that the oxide becomes granular when the chemical composition of the oxide is within a specific range.

(E) The composition of the SiO ₂ —MnO—Al ₂ O ₃ oxide formed on the steel sheet surface depends on the ratio of Si, Mn, and Al added to the steel sheet and the hydrogen / water vapor partial pressure in the annealing atmosphere.

  (F) Steel sheet having a chemical composition that does not satisfy the Si-Mn-Al ratio specified in the prior application, for example, Si-Mn- of a high-strength steel sheet having a multi-structure structure steel containing about 0.2 to 0.9% of Si. In order to granulate the Al oxide, the chemical components of the steel sheet and the hydrogen partial pressure and water vapor partial pressure in the annealing atmosphere should satisfy a specific relationship.

  (G) Furthermore, when these partial pressures satisfy a specific relationship, the Si—Mn—Al oxide is also granulated for a steel sheet having a chemical composition that does not satisfy the Si—Mn—Al ratio defined in the prior application. .

The present invention is as follows.
(1) 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. Hot dip galvanized steel sheet that continuously galvanizes steel sheets having chemical compositions satisfying the formulas (1), (2), and (3) in a galvanizing line having a radiant tube type reduction furnace. The logarithmic ratio between the hydrogen partial pressure of the atmospheric gas and the water vapor partial pressure in a temperature range of (a) 650 ° C. or more and less than 750 ° C. is the temperature during heating and heating of the steel sheet in the reduction furnace. In addition to satisfying the following equation (4), (b) the logarithmic ratio of the hydrogen partial pressure of the atmospheric gas and the water vapor partial pressure in the temperature range of 750 ° C. to 950 ° C. satisfies the following equation (5): A method for producing a hot-dip galvanized steel sheet.

5 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 95 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
log (P _H2O / P _H2 ) ≦ −1.55 (4)
− 0 . 91 ≦ log (P _H2O / P _H2 ) ≦ −0.635 (5)

  (2) The method for producing a hot-dip galvanized steel sheet according to (1) above, wherein at least the hydrogen concentration in the atmosphere gas of the reducing furnace is 10% by volume or more.

(3) Melting as described in the above item (1) or (2), wherein the atmosphere gas P _H2O / P _H2 of the reduction furnace is controlled by introducing humidified nitrogen gas into the reduction furnace Manufacturing method of galvanized steel sheet.

  (4) After manufacturing the hot dip galvanized steel sheet by the manufacturing method according to any one of the above items (1) to (3), the hot dip galvanized steel sheet is brought to a temperature range of 460 ° C or higher and 600 ° C or lower. A method for producing an alloyed hot-dip galvanized steel sheet, wherein the alloying treatment is performed by heating, and the Fe content of the plating layer is in the range of 7 to 15%.

  FIG. 1 is a graph showing a region of the Si—Mn—Al ratio defined in the present invention. In FIG. 1, a region surrounded by a solid broken line is a region having a Si—Mn—Al ratio defined in the present invention.

  According to the present invention, the steel sheet having a chemical composition that does not satisfy the Si—Mn—Al ratio, such as Si, cannot be improved with the invention proposed by the prior application, for example, 0.2 to 0.9% of Si. Si-Mn-Al oxide of complex structure steel-based high-strength steel sheet containing about a grade can be granulated to improve plating properties, and Si-containing high-strength hot-dip galvanized steel sheet and Si-containing high-strength alloy with excellent surface quality It becomes possible to manufacture a galvannealed steel sheet.

FIG. 1 is a graph showing a region of the Si—Mn—Al ratio defined in the present invention. FIG. 2 is a surface SEM photograph of a laboratory annealed test material in which Si—Mn—Al-based oxide is granulated. FIG. 3 is a surface SEM photograph of a laboratory annealing test material in which a film-like Si—Mn—Al-based oxide was generated. FIG. 4 is a graph showing the influence of humidification conditions on the oxide surface coverage. FIG. 5 is an explanatory view showing a laboratory annealing pattern. FIG. 6 is an explanatory view schematically showing a horizontal annealing line.

The present invention will be described with reference to the accompanying drawings.
In the present invention, a high strength steel sheet containing Si is continuously hot dip galvanized in a hot dip galvanizing line having a radiant tube type reduction furnace. You may have a direct-fire reduction furnace and a non-oxidation furnace in the line entrance side rather than 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 Si, Mn, and Al. The ratio has a chemical composition satisfying the above formulas (1), (2) and (3).

In addition, the temperature of the steel sheet during heating and heating in the reduction furnace is
While the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the temperature range of 650 ° C. or more and less than 750 ° C. satisfies the above-mentioned formula (4)
The logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the temperature range of 750 ° C. or higher and 950 ° C. or lower satisfies the above formula (5).

Note that equation (4), it corresponds to a range of dew point -30 ° C. or less at 10 _vol% H 2 + _{N 2} atmosphere, further, (5), the dew point -20 to + 20 ° C. with 10 vol% _{H 2} atmosphere Applicable to the range.

1. Chemical composition of steel sheet The chemical composition of the steel sheet contains at least Si: 0.2 to 2.0%, Mn: 0.2 to 3.0%, Al: 0.001 to 1.5%, Si, The reason why the ratio of Mn and Al satisfies the above formulas (1), (2) and (3) will be described.

  When the Si content is less than 0.2%, the problems of unplating and alloying delay, which are the problems to be solved by the present invention, do not occur. On the other hand, when the Si content exceeds 2.0%, the Si—Mn—Al oxide cannot be granulated. Therefore, the Si content is 0.2 to 2.0%.

Mn is an element necessary for increasing the strength, and is contained by 0.2% or more. On the other hand, if the Mn content exceeds 3.0%, the ductility is lowered. Therefore, the Mn content is set to 0.2 to 3.0%.
Since Al is difficult to remove completely from steel in an industrial production process, the lower limit of the Al content is 0.001%. On the other hand, if the Al content exceeds 1.5%, ductility is reduced. Therefore, the Al content is set to 0.001 to 1.5%.

Furthermore, unless the ratio of Si, Mn and Al satisfies the above formulas (1), (2) and (3), the Si—Mn—Al oxide cannot be granulated.
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.

You may contain arbitrary elements other than the above. Hereinafter, typical arbitrary elements will be described.
Ti, Nb, and V delay the recrystallization and refine the crystal grains, and may 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 may 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 of each element is 1% or less.

  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.

  This steel plate is a Si-containing high-strength steel plate, for example, a composite structure steel-based high-tensile steel plate containing about 0.2 to 0.9% of Si. The tensile strength is 440 to 980 MPa.

2. Atmosphere during heating and heating of steel sheet in reduction furnace To granulate Si-Mn-Al oxide, oxidation by water vapor is suppressed in a region where the steel sheet temperature during temperature rising is 650 ° C. or higher and lower than 750 ° C. It is necessary to oxidize with water vapor at a temperature higher than or equal to C.

Specifically, the temperature of the steel sheet during heating and heating is
(A) The logarithmic ratio of the hydrogen partial pressure of the atmospheric gas and the water vapor partial pressure in the temperature range of 650 ° C. or higher and lower than 750 ° C. satisfies log (P _H2O / P _H2 ) ≦ −1.55, and (b) When the logarithmic ratio of the hydrogen partial pressure of the atmospheric gas and the water vapor partial pressure in the temperature range of 750 ° C. or more and 950 ° C. or less satisfies −1.91 ≦ log (P _H2O / P _H2 ) ≦ −0.635, Si-Mn-Al oxide is granulated.

Here, in the actual operation, the temperature of the steel plate is 950 ° C. or lower, and therefore the upper limit of the temperature of the steel plate is 950 ° C.
The logarithmic ratio of the hydrogen partial pressure of the atmospheric gas and the water vapor partial pressure in the temperature range where the temperature of the steel sheet during heating and heating is less than 30 to 650 ° C. is not particularly limited. For example, when it is in this temperature range in a radiant tube type reduction furnace, it is in the range of the logarithmic ratio of hydrogen partial pressure and water vapor partial pressure of the actual production line, -4.29 ≦ log (P _H2O / P _H2 ). This corresponds to a range of dew point of −70 ° C. or more in a 10% by volume H ₂ + N ₂ atmosphere.

When a direct flame reduction furnace or a non-oxidation furnace is installed on the entry side of the reduction furnace of the continuous hot dip galvanizing line, the steel sheet may be rapidly heated to about 650 ° C. by these furnaces.
In actual operation, the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure in the above-described ranges can be maintained by introducing a humidified gas into the reduction furnace, as will be described later.
Si, Mn, and Al in the steel sheet react with water vapor in the reduction furnace immediately to form oxides. As a result, the hydrogen partial pressure increases and the water vapor partial pressure decreases. When the hydrogen concentration in the reduction furnace is low, the amount of water vapor must be kept low correspondingly, but in such a state, if the hydrogen concentration increases due to oxidation of Si, Mn, and Al in the steel sheet, the hydrogen partial pressure and It is difficult to maintain an appropriate logarithmic ratio of the water vapor partial pressure.

For example, when the furnace volume is 25 m ³ and the hydrogen concentration in the furnace is 3% by volume, the logarithmic ratio between the hydrogen partial pressure and the water vapor partial pressure is −1.91 ≦ log (P _H2O / P _H2 ) ≦ −0. To obtain 635, the water vapor amount needs to be 0.0374 to 0.7044 kPa. Here, when a steel sheet having a chemical 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 / s, the mother of the front and back surfaces is 1 μm. Assuming that the materials Si, Mn, and Al are oxidized by water vapor, water vapor is consumed at a rate of 6.2 × 10 ⁻⁴ m ³ / s.

  For this reason, in order to maintain the logarithmic ratio between the hydrogen partial pressure and the steam partial pressure, it is necessary to replenish the same amount. And steel plate oxidation occurs. In order to prevent this, a large number of nozzle holes for introducing water vapor into the reduction furnace may be installed, but this is not practical.

On the other hand, if the hydrogen concentration in the reduction furnace is set 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 that the water vapor is 6.2 × 10 ⁻⁴ m ^3. Even if / s is input, the problem of density unevenness does not occur, and the atmospheric gas can be easily controlled to a desired state. Therefore, it is desirable that at least the hydrogen concentration in the atmosphere gas of the reduction furnace is 10% by volume or more.

3. _P H2O _{/ P H2} of atmospheric gas control reduction furnace of _P H2O _{/ P H2} of the atmospheric gas in the reducing furnace by introducing the humidified nitrogen gas to the reduction furnace is controlled.

  It contains at least Si: 0.4%, Mn: 2.5%, Al: 0.4%, and the ratio of Si, Mn and Al satisfies the above formulas (1), (2) and (3). An example in which an annealing test was performed using a cold-rolled sheet made of Si-containing steel having a satisfactory chemical composition is shown below.

  First, the atmosphere gas in the furnace at the time of annealing was 10% by volume of hydrogen, the remainder was nitrogen gas, held at 850 ° C. for 2 minutes, and then rapidly cooled with nitrogen gas having a dew point of −68 ° C. At this time, nitrogen gas adjusted to a dew point of −20 ° C. was introduced into the furnace when reaching 750 ° C. in the temperature raising process up to 850 ° C.

FIG. 2 is a surface SEM photograph of a laboratory annealed test material in which Si—Mn—Al-based oxide is granulated.
As shown in FIG. 2, the granular material parted on the surface of the steel plate was observed. When the component analysis of these granular materials was conducted, it was Si-Mn-Al type oxide. The rest of the region was a region where no Fe-based oxide was present.

  Although it was conventionally considered that when Si-containing steel is annealed, a film-like Si oxide is formed and covers the surface of the steel sheet, the surface oxide shape of this laboratory annealed test material is not film-like, but Si The oxide was divided and Fe which is the main component of the steel sheet was exposed.

Thus, if Fe is exposed on the surface of the steel sheet after annealing, it is considered that the reactivity with zinc is not impaired during plating, and good zinc wettability is ensured.
On the other hand, FIG. 3 uses the same steel plate as that shown in FIG. 2 and was produced under the same conditions as those in FIG. 2 except that the dew point was kept from room temperature to −20 ° C. during annealing. It is the surface SEM photograph of the laboratory annealing test material which Al system oxide produced | generated.

As shown in FIG. 3, when compared with FIG. 2 humidified from the time of reaching 750 ° C., it can be seen that a film-like Si—Mn—Al-based oxide is formed on the surface when humidified from room temperature.
Next, the test which investigated the influence which the timing of the oxidation by humidified gas has on the surface oxide shape using the cold rolled sheet of Si-containing steel will be described. In this test, the atmosphere gas at the time of annealing was 10% by volume of hydrogen, and the remainder was nitrogen gas. After holding at 900 ° C. for 2 minutes, it was quenched with nitrogen gas having a dew point of −68 ° C. At this time, there were two conditions: a case where nitrogen gas having a dew point adjusted to −40, −30, −20, −10, and 0 ° C. was introduced from room temperature, and a case where the nitrogen gas was introduced only when maintained at 900 ° C.

FIG. 4 is a graph showing the influence of humidification conditions on the oxide surface coverage.
As shown in the graph of FIG. 4, when humidifying from room temperature, the oxide became a film regardless of the dew point, and the surface coverage increased. On the other hand, when humidified only at the time of holding at 900 ° C., the oxide became a film at dew points of −30 ° C. and −40 ° C., but at other dew points, the oxide was granulated and the surface coverage decreased. did.

Thus, by introducing nitrogen gas adjusted to a dew point of -30 ° C. or higher and 0 ° C. or lower into the reduction furnace during heating and holding, it is possible to control the P _{2 H 2} / P _{H 2} of the reduction furnace atmosphere gas.

4). Production of alloyed hot-dip galvanized steel sheet After producing a hot-dip galvanized steel sheet by the production method according to the present invention described above, this hot-dip galvanized steel sheet is heated to a temperature range of 460 ° C or higher and 600 ° C or lower and subjected to alloying treatment. An alloyed hot-dip galvanized steel sheet is produced by setting the Fe content of the plating layer to a range of 7 to 15%.

  When the alloying treatment temperature is lower than 460 ° C., alloying failure occurs, and when it exceeds 600 ° C., powdering properties are deteriorated. Further, when the Fe content of the plating layer is less than 7%, alloying is similarly poor, and when it exceeds 15%, the powdering properties are deteriorated.

  In this way, a Si-containing high-tensile alloyed hot-dip galvanized steel sheet is provided. This Si-containing high-tensile alloyed hot-dip galvanized steel sheet has a high tensile strength of 440 to 980 MPa, excellent press workability, and good rust resistance, and can be used very suitably as a steel sheet for automobiles. it can.

The present invention will be described with reference to examples.
A total of 12 steel types were used as test materials: actual equipment AG having chemical compositions shown in Table 1 (Fe and impurities other than those shown in Table 1) and laboratory materials HL.

  For actual equipment A to G, a cold-rolled base material (before annealing) that has been pickled and cold-rolled after hot rolling is obtained, cut into 20 mm squares, immersed in an organic solvent, and 15 with an ultrasonic cleaner. The sample was washed and degreased for a sample material.

  The laboratory materials H to L were cast in a vacuum melting furnace and then forged, and were hot-rolled sheets having a thickness of 3 mm using 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. Furthermore, after mechanical grinding, a cold-rolled sheet having a thickness of 2 to 0.8 mm was produced with a cold rolling mill. 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 to obtain a test material.

Thereafter, the effects of the base material Si—Mn—Al ratio, the hydrogen partial pressure of the atmospheric gas, and the water vapor partial pressure on the shape of the oxide formed on the surface of the steel sheet during annealing were investigated using these test materials.
An annealing test was performed using an infrared image furnace. After setting 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 10% volume H ₂ + N _2. After changing to a gas atmosphere (dew point −60 ° C., 5 L / min) for 3 minutes, the temperature was raised to 850 ° C. at a rate of 15 ° C./sec and held for 2 minutes.

At this time, N ₂ gas passed through ice water or warm water was mixed with atmospheric gas and flowed into the furnace. The flow rate of N ₂ gas was adjusted so that the dew point meter installed on the entrance side of the heating furnace would be −35 ° C., −10 ° C., and + 10 ° C.

FIG. 5 is a graph showing a laboratory annealing pattern.
As shown in the figure, humidification was performed under two conditions: when the temperature of the steel sheet is from room temperature to the end of holding at 850 ° C., and when the temperature is reached from 750 ° C. until the end of holding at 850 ° C. In the latter case, the dew point was -35 ° C up to 750 ° C.

After holding, after cooling to 50 ° C. with N ₂ gas (dew point −68 ° C., 25 L / min), 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.

Below, evaluation of the oxidation state of the surface of the steel plate in a present Example is demonstrated.
In the backscattered electron image mode, a difference in contrast appears as shown in FIGS. When the atomic weights of Fe, which is the main component of the base material, and the oxides generated on the surface are compared, 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. The coverage was 30% or less as ◯, 30 to 70% as Δ, 70% or more as x, and Δ or more as acceptable. The reason for this is that when the coverage is 70% or more, the wettability is remarkably deteriorated in the subsequent wettability evaluation with molten zinc, so that the coverage is less than 70%.

  The results are summarized in Tables 2-4. In addition, in each column of Tables 1 to 4, those that deviate from the conditions defined in the present invention are underlined.

  As shown in Tables 2 to 4, it can be seen that only when all of the formulas (1) to (6) are satisfied, the generated oxide is granulated, and the surface area coverage of the steel sheet is reduced to less than 70%. .

The effects of the present invention were investigated in the horizontal continuous hot dip galvanizing line of the actual machine using the test materials F and G evaluated as ◯ in Tables 2 to 4.
FIG. 6 is an explanatory view showing a part of an embodiment of a continuous hot dip galvanizing line (hereinafter abbreviated as “CGL”) to which the present invention is applied.

  The CGL shown in FIG. 6 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 FIG. 7 is the moving direction of the steel plate 1. The steel plate 1 passed through the CGL is heated in the pre-tropical zone 2 and further heated to a recrystallization temperature or lower, for example, about 650 ° C. in the non-oxidizing furnace 3, and then heated to about 750 ° C. in the heating zone 4a. It is heated to the recrystallization temperature or higher, for example, about 850 ° C. in the tropics 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. Further, it is heated to about 600 ° C. in the alloying furnace 7 to be alloyed. 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, humidifiers were installed in the soaking zones 4a, 4b and 4c, 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 dew point meters installed in the heating zone 4a, heating zone 4b, soaking zone 4c, and cooling zone 4d of the reduction furnace 4, respectively.

The test was performed by changing the flow rate of the humidified N ₂ gas and adjusting the dew point in the furnace. The hydrogen concentration in the furnace is 10% by volume, and the temperature of the steel sheet 1 is 640 ° C in the non-oxidizing furnace 3, 740 ° C in the heating zone 4a, 800 ° C in the soaking zone 4b, 880 ° C in the soaking zone 4c, and 500 ° C in the cooling zone 4d. It was.

  The plating evaluation method was performed visually. A steel plate in which no plating was not generated was rated as “◯”, a steel plate in which non-plating was generated was evaluated as “x”, and “◯” was determined as acceptable. The results are shown in Table 5. In each column of Table 5, those that deviate from the conditions defined in the present invention are underlined.

  As shown in Table 5, with respect to the steel sheet satisfying the chemical components defined by the formulas (1) to (3), the steel sheet 1 is heated and heated, which is defined by the formulas (4) and (5). It can be seen that non-plating does not occur when the atmospheric gas conditions are satisfied.

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 (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 and Al is Hot dip galvanization in which a steel sheet having a chemical composition satisfying the following formulas (1), (2) and (3) is continuously galvanized in a galvanizing line having a radiant tube type reduction furnace. A method for producing a steel sheet, wherein the temperature during heating and heating in the reduction furnace of the steel sheet is
While the logarithmic ratio of the hydrogen partial pressure and the water vapor partial pressure of the atmospheric gas in the temperature range of 650 ° C. or more and less than 750 ° C. satisfies the following formula (4),
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 of the atmospheric gas in a temperature range of 750 ° C. or higher and 950 ° C. or lower satisfies the following formula (5):
5 ≦ (Si / (Si + Mn + Al)) × 100 ≦ 54 (1)
30 ≦ (Mn / (Si + Mn + Al)) × 100 ≦ 95 (2)
0 ≦ (Al / (Si + Mn + Al)) × 100 ≦ 30 (3)
log (P _H2O / P _H2 ) ≦ −1.55 (4)
− 0 . 91 ≦ log (P _H2O / P _H2 ) ≦ −0.635 (5)   2. The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein at least a hydrogen concentration in an atmosphere gas of the reduction furnace is 10% by volume or more. The melting according to claim 1 or 2, wherein the atmosphere gas P _H2O / P _H2 of the reduction furnace is controlled by introducing humidified nitrogen gas into the heating furnace or heat insulation furnace. Manufacturing method of galvanized steel sheet.   After manufacturing the hot dip galvanized steel sheet by the manufacturing method according to any one of claims 1 to 3, the hot dip galvanized steel sheet is heated to a temperature range of 460 ° C or higher and 600 ° C or lower and alloyed. And producing an alloyed hot-dip galvanized steel sheet, wherein the Fe content of the plating layer is in the range of 7 to 15% by mass.