JP4631379B2 - Hot-dip galvanized steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention provides a hot-dip galvanized steel sheet suitable as a surface-treated steel sheet for automobiles, home appliances, and building materials, obtained by subjecting a steel sheet containing various reinforcing elements to a base material and hot-dip galvanizing to the base material. It relates to a manufacturing method.

  In recent years, from the standpoint of improving automobile crash safety and protecting the global environment, there has been a demand for steel sheets with high-tensile galvanized high-strength steel sheets (steel strips) as steel sheets for automobiles for the purpose of reducing weight for improving fuel efficiency. Is increasing. This high-tensile hot-dip galvanized steel sheet is formed by hot-dip galvanizing the base steel sheet, using a high-strength steel sheet containing a number of easily oxidizable solid solution strengthening elements and precipitation-type strengthening elements as the base steel sheet. It has high strength and high workability.

  However, when the amount of the solid solution strengthening element added to improve the strength of the high-tensile steel sheet increases, the recrystallization annealing process before performing the plating process in a continuous galvanizing line (CGL) In the process, there is a disadvantage that a concentrated layer of easily oxidizable reinforcing elements is generated on the steel sheet surface. This concentrated layer is selectively oxidized because the reducing atmosphere for reducing Fe is an oxidizing atmosphere for the oxidizable reinforcing elements present in the steel sheet, and these oxides accumulate on the steel sheet surface. It is formed by letting it.

  If such a concentrated layer is formed on the surface of the steel sheet, the wettability between the steel sheet and the molten zinc is remarkably lowered, so that the plating property is lowered. In particular, in the case of a high-tensile steel sheet having a high content of easily oxidizable solid solution strengthening element, there is a problem that so-called non-plating occurs in which partial plating is not performed.

  In order to solve such a problem, there has been proposed a means for improving the plating property by forcibly oxidizing the steel plate under a high oxygen partial pressure and then reducing it prior to heating in the plating process (Patent Document 1). reference).

  In addition, by performing shot blasting on the steel plate before heat treatment in a strong oxidizing atmosphere, means for improving plating properties (see Patent Document 2), or by performing preliminary plating before performing the plating treatment, Means (see Patent Document 3) for improving plating properties have been proposed.

  Further, means for improving the plating property by performing recrystallization annealing before the plating treatment, generating a surface oxide in advance, pickling and removing the surface oxide, and then performing hot dip galvanization. Has also been proposed (see Patent Document 3).

JP 55-1212865 JP-A-6-158254 JP 58-104163 A JP-A-6-287684 JP-A-8-085858

  However, in the means proposed in Patent Document 1 described above, the problem that the concentrated layer is not sufficiently suppressed in forced oxidation remains unsolved. In addition, there is a problem that it is difficult to ensure stable plating depending on the in-steel component conditions and the plating treatment conditions in which any of the easily oxidizable solid solution strengthening elements is contained in an amount of 1 mass% or more.

In the means proposed in the above-mentioned Patent Document 2, it is difficult to ensure stable plating with a steel plate having a high content of easily oxidizable solid solution strengthening elements.
On the other hand, the means proposed in Patent Document 3 described above has a problem in that the cost for equipment and operation for performing the plating process increases, resulting in an increase in the manufacturing unit price.

  Furthermore, the means proposed in the above-mentioned Patent Documents 4 and 5 have a drawback that the occurrence of non-plating cannot be completely prevented in a steel type having a high content of easily oxidizable strengthening elements.

  Therefore, the present invention has been made in view of the above circumstances, and in particular, even when a high-tensile steel sheet having a high content of easily oxidizable solid solution or precipitation strengthening element is used as a base material, non-plating is performed. It is an object of the present invention to provide a high-tensile hot-dip galvanized steel sheet and a method for producing the same that can improve plating properties without causing them to occur.

  As a result of the inventors conducting various studies on the means for solving the above-mentioned problems, two elements α satisfying a predetermined relationship in a temperature range of 300 to 900 ° C. as a strengthening element other than Fe. In a high-tensile steel sheet containing at least β and β, when recrystallization annealing is performed using a continuous annealing furnace having a direct-fired or non-oxidizing furnace-type heating zone before hot-dip galvanizing, these elements α and Since β is more oxidizable than Fe, it is fixed (internal oxidation) as a complex oxide of α and β in the depth region within 100 μm from the surface of the underlying steel plate, and therefore it can be diffused to the surface of the steel plate. It has been found that if α and β in the molten state are reduced, it is possible to prevent the formation of a concentrated layer on the steel sheet surface.

Furthermore, when the elements α and β are added simultaneously, the internal oxidation of the element α can be promoted as compared with the case where only the element α is added without adding the element β which is more easily oxidizable than the element α. I also found that it was possible.
From the above knowledge, a means for providing a high-tensile hot-dip galvanized steel sheet without occurrence of non-plating has been derived.

That is, the gist of the present invention is as follows.
1. As the strengthening elements, elements α and β satisfying the relationship of the following formulas (1) and (2) in the temperature range of 300 to 900 ° C. (however, combinations of the elements α and β are Mn-Si, Si-Al, Cr) -Si) , a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the steel sheet, and an internal oxide containing Fe and elements α and β in a depth region within 100 μm from the surface of the steel sheet And a total amount of internal oxides (excluding Fe single oxide) in the depth region is 0.01 to 1.0 g / m ² per one surface of the steel sheet in terms of O amount.
ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ---- (1)
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ ) ---- (2)
here,
ΔG ^O _X : Standard free formation of reaction (2m / n) X + O ₂ = (2 / n) XmOn
Energy change ΔG ^O _Fe : Reaction 3/2 Fe + O ₂ = 1/2 under conditions of temperature less than 570 ° C.
Under the conditions of Fe ₃ O ₄ and a temperature of 570 ° C. or higher, the reaction 2Fe + O ₂ =
Standard free energy change of 2FeO

2. As the strengthening elements, elements α and β satisfying the relationship of the following formulas (1) and (2) in the temperature range of 300 to 900 ° C. (however, combinations of the elements α and β are Mn-Si, Si-Al, Cr) When performing hot dip galvanizing treatment on a steel sheet containing at least -Si) ,
Heat treatment is performed in an oxidizing atmosphere at 300 to 900 ° C., and an oxide layer having an Fe and O content of 90 mass% or more is formed on the surface of the steel sheet in an amount of 0.03 to 2.0 g / m in terms of O amount on one side of the steel sheet. ^A method for producing a hot-dip galvanized steel sheet, characterized in that the hot-dip galvanized steel sheet is formed at ² and then subjected to a reduction treatment, followed by a hot-dip galvanizing treatment.
ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ---- (1)
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ ) ---- (2)
here,
ΔG ^O _X : Standard free formation of reaction (2m / n) X + O ₂ = (2 / n) XmOn
Energy change ΔG ^O _Fe : Reaction 3/2 Fe + O ₂ = 1/2 under conditions of temperature less than 570 ° C.
Under the conditions of Fe ₃ O ₄ and a temperature of 570 ° C. or higher, the reaction 2Fe + O ₂ =
Standard free energy change of 2FeO

3. 3. The method for producing a hot-dip galvanized steel sheet according to 2 above, wherein the oxidizing atmosphere of the heat treatment has an O ₂ concentration of 0.01 vol% or more and 20 vol% or less.

4 . The steel plate
C: 0.0005 to 0.20 mass%,
α: 1.0 to 3.0 mass% and β: 0.10 to 2.0 mass%
The method for producing a hot-dip galvanized steel sheet according to 2 or 3 above, wherein the balance has a component composition of Fe and inevitable impurities.

4 . In the reduction treatment, the steel sheet is kept in a temperature range of 300 to 900 ° C. for 5 seconds or more in an H ₂ —N ₂ gas atmosphere having a dew point of −60 ° C. or more and less than 0 ° C. and containing H ₂ : 1 vol% or more. The method for producing a hot dip galvanized steel sheet according to any one of the above 2 or 3 , wherein the method is performed.

5 . The galvanizing treatment is molten zinc according to any one of 2 to 4 the bath temperature is 440 to 500 ° C. and the bath Al concentration and performing with zinc plating bath 0.10～0.30Mass% Manufacturing method of plated steel sheet.

6 . 6. The method for producing a hot dip galvanized steel sheet according to any one of 2 to 5 above, wherein after the hot dip galvanizing treatment, the galvanizing is further alloyed by heating at 460 to 580 ° C.

  According to the present invention, for example, even when a high-strength steel sheet containing a large amount of a solid solution strengthening element such as Si or Mn is used as a base material, it is possible to suppress the occurrence of non-plating and improve plating properties. . Accordingly, a hot-dip galvanized steel sheet having high strength and high workability can be provided.

  Further, by alloying the high-tensile hot-dip galvanized steel sheet of the present invention, a high-tensile alloyed hot-dip galvanized steel sheet without uneven alloying can be provided.

  In the method of the present invention, before performing hot dip galvanization, for example, by using a continuous annealing furnace having a heating zone of a direct type or a non-oxidizing furnace type, heat treatment is performed, and within 100 μm from the surface of the steel sheet as the base material In the depth region, elements α and β added as reinforcing elements are fixed as oxides, reducing α and β in a solid solution state that can diffuse to the steel sheet surface layer, and creating a concentrated layer on the steel sheet surface. There is a feature in the place to prevent.

  At that time, the selection of the elements α and β requires that the element β is an element that is more easily oxidizable than the element α. This is because a more stable α and β composite oxide than α alone oxide is generated inside the steel plate within 100 μm from the surface of the underlying steel plate. This is because the effect of reducing the increase becomes greater. This makes it possible to provide a high-tensile hot-dip galvanized steel sheet that is free from unplating.

That is, in the heat treatment prior to the reduction treatment of the steel sheet surface layer, in order to fix α and β as oxides in the steel plate within a depth of 100 μm from the surface of the underlying steel plate, first, the following formula (1) It is necessary to add at least elements α and β satisfying the relationship of (2).
ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ---- (1)
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ ) ---- (2)
here,
ΔG ^O _X : Standard free energy of formation of reaction (2 m / n) X + O ₂ = (2 / n) XmOn
-Change ΔG ^O _Fe : Reaction 3 / 2Fe + O ₂ = 1 / 2Fe ₃ O under the temperature of 570 ° C or less
₄ and at a temperature of 570 ° C. or higher, the reaction 2Fe + O ₂ = 2FeO
Quasi-generated free energy change

  Further, by subjecting the steel sheet having the above component composition to a heat treatment before the hot dip galvanizing, and then to a reduction treatment, the steel strengthening elements α and β are used as oxides immediately before the hot dip galvanizing treatment. Fixing inside the steel plate except the steel plate surface within 100μm from the steel plate surface, it is possible to reduce the solid solution α and β that can diffuse to the steel plate surface layer, and prevent the generation of the concentrated layer on the steel plate surface. It is possible to provide a high-tensile hot-dip galvanized steel sheet that does not generate.

Here, it is important that the total amount of internal oxides (excluding Fe single oxide: hereinafter the same) in the depth region is 0.01 to 1.0 g / m ² per one surface of the steel sheet. This is because the total amount of internal oxidation amount including alpha and beta is less than 0.01 g / m ^2, elements in steel alpha, can not be suppressed surface oxidation of the beta, leading to plating degradation. On the other hand, when the amount of internal oxidation exceeds 1.0 g / m ² , plating adhesion, weldability, corrosion resistance after various coatings, and the like deteriorate.
Further, since the internal oxide existing inside the steel plate exceeding 100 μm from the surface of the base steel plate deteriorates the material strength, the internal oxide needs to exist inside the steel plate within 100 μm.

Next, the manufacturing method of the hot dip galvanized steel sheet of the present invention will be described in detail for each requirement.
First, the significance of the above-described formulas (1) and (2) relating to the reinforcing elements α and β in the base steel sheet will be described.
[ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ]
If a strengthening element having a standard production free energy change equal to or greater than the standard production free energy change of the reaction 3 / 2Fe + O ₂ = 1 / 2Fe ₃ O ₄ is added, Fe will be oxidized first. At the time of recrystallization annealing immediately before the hot dip galvanizing treatment, it becomes impossible to fix the reinforcing element in the steel as an oxide inside the steel plate in the vicinity of the surface layer portion of the steel plate.
Therefore, Fe and the elements α and β satisfy the relationship ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe .

[ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ )]
By adding β, which is a more oxidizable element than α, a composite oxide of α and β that is more stable than the oxide of α alone is generated inside the steel plate within 100 μm from the underlying steel plate surface. Therefore, the effect of reducing α and β in a solid solution state that can be diffused to the surface layer of the steel sheet is further increased. From this, the elements α and β are
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ )
It is important that Under the condition of ΔG ^O _α −ΔG ^O _β ≦ 20, it becomes difficult to form a composite oxide of α and β. Therefore, it becomes difficult to fix the steel plate within 100 μm.

[Elements α and β are elements that can increase the tensile strength of a steel sheet by 1 MPa or more when 0.1 mass% is added respectively]
Furthermore, regarding the elements α and β, when 0.1 mass% is added to the steel sheet, sufficient mechanical properties may not be obtained unless the tensile strength of the steel sheet can be increased by at least 1 MPa. Such elements are preferably used for α and β. That is, the elements α and β are solid solution strengthening or precipitation strengthening elements, and when 0.1 mass% is added, it is preferable that the tensile strength of the steel sheet can be increased by 1 MPa or more.

[C content of steel sheet as base material: 0.0005 to 0.20 mass%]
Further, the steel plate preferably contains C in an amount of 0.0005 to 0.20 mass%. That, C (carbon) is it possible to improve the strength by precipitating generation of multi Te onsite phase or bainite phase generated at a low temperature, a carbide of Ti (titanium) and Nb (niobium). However, if C is added excessively, spot weldability deteriorates, so the upper limit is preferably 0.20 mass%. On the other hand, considering the manufacturing cost, the lower limit is preferably 0.0005 mass%.

[Α and β contents of steel sheet as base material: α: 1.0 to 3.0 mass%, β: 0.10 to 2.0 mass%]
By adding β, which is an element that is more easily oxidizable than α, a composite oxide of α and β that is more stable than the oxide of α alone is generated inside the steel plate within 100 μm from the surface of the underlying steel plate. The effect of reducing α and β in a solid solution state capable of diffusing up to the steel sheet surface layer becomes greater. When the content of α and β is less than 1.0 mass% and 0.10 mass%, respectively, a stable complex oxide of α and β is difficult to be formed. Moreover, when there are more content of (alpha) and (beta) than 3.0 mass% and 2.0 mass%, respectively, there exists a possibility that (alpha) and (beta) may surface-enrich large at the time of recrystallization annealing, and plating property may be inhibited.

Here, the combination of the elements α and β is selected from Mn—Si, Si—Al and Cr—Si . These elements have a smaller standard generation free energy change (ΔG ^O _X ) than that of Fe (ΔG ^O _Fe ), and can satisfy the above-mentioned formulas (1) and (2). is there.

Next, the steel plate having the above component composition is subjected to a heat treatment at 300 to 900 ° C. to form an oxide inside the steel plate, and then subjected to a reduction treatment.
[Oxide layer with Fe and O content of 90 mass% or more: 0.03 to 2.0 g / m ^{2 in} terms of O amount per one side of the steel sheet]
The reason why the surface oxidation amount of the base steel sheet after the heat treatment is limited to 0.03 g / m ² or more is that if the oxidation amount is less than 0.03 g / m ² , surface concentration of the alloy elements α and β cannot be prevented. Further, the plating property is deteriorated.
On the other hand, if the oxidation amount exceeds 2.0 g / m ² , the surface oxide film remains in the reduction treatment after the heat treatment without being completely reduced. Inviting, the plating adhesion deteriorates.
Therefore, the surface oxidation amount is 0.03 to 2.0 g / m ² .

  The surface oxidation amount can be obtained, for example, by measuring by a method called “impulse furnace melting-infrared grade method”. Further, by preparing a calibration curve in advance, it is possible to easily quantify O with fluorescent X-rays.

Here, when the heating temperature of the heat treatment is less than 300 ° C., the amount of Fe oxide in the steel sheet after the heat treatment is small, surface concentration of the elements α and β cannot be prevented, and the plating property is deteriorated. Become. On the other hand, when the heating temperature of the heat treatment is higher than 900 ° C., the amount of Fe oxide in the steel sheet is too large, and unreduced oxide that has not been reduced during the subsequent recrystallization annealing remains, which deteriorates the plating properties. Become.
Therefore, heat treatment is performed in the temperature range of 300 to 900 ° C.

By passing through the above steps, the base steel plate after the heat treatment is 0.01 to 1.0 g / m per side in terms of O amount in the region excluding the steel plate surface 100 μm deep from the surface as described above. ^Two internal oxides are formed.

The internal oxidation amount of this steel sheet is also measured by the aforementioned “impulse furnace melting-infrared grade method”. However, it is necessary to subtract the amount of oxygen contained in the material (that is, the high-tensile steel plate before annealing). Therefore, in the present invention, the surface layer portion of the high-tensile steel both surfaces after continuous annealing by polishing or 100μm measured oxygen concentration in the steel, the amount of oxygen O _H contained the measured value to the material, also after continuous annealing The oxygen concentration in the steel in the entire thickness direction of the high-tensile steel plate is measured, and the measured value is defined as the oxygen amount O _I after internal oxidation.
And oxygen O _I after internal oxidation of the steel sheet thus obtained, by using the amount of oxygen O _H contained in the material, calculating the difference between the O _I and _{O H (= O I -O H} ), further one-sided unit surface top (i.e. 1 m ²⁾ value converted into the amount per (g / ^{m 2)} is an internal oxide amount.

[Oxidizing atmosphere for heat treatment: O ₂ concentration is 0.01 vol% or more and 20 vol% or less]
In the heating process, it was limiting the O ₂ content of the oxidizing atmosphere with 0.01 vol% or more, in the O ₂ is less than 0.01 vol%, surface segregation of the alloying elements α and β for Fe oxidation of the steel sheet is small This is because it is impossible to prevent the deterioration of the tightness.
On the other hand, in view of manufacturing costs, the upper limit of the O ₂ content in the heat treatment is desirably 20 vol%.

In general, the heating zone of CGL that can be used for both heat treatment and reduction treatment is generally an atmosphere containing N ₂ and / or H ₂ O.

[Reduction treatment: steel sheet is kept in a temperature range of 300 to 900 ° C. for 5 seconds or more in an H ₂ —N ₂ gas atmosphere having a dew point of −60 ° C. or more and less than 0 ° C. and containing H ₂ : 1 vol% or more]
In the reduction treatment performed after the heat treatment, if the dew point is less than −60 ° C. and / or the temperature exceeds 900 ° C., the Fe oxide is excessively reduced and the elements α and β are concentrated on the surface, which may deteriorate the plating properties. There is. On the other hand, when the dew point is 0 ° C. or higher, the H ₂ concentration is less than 1%, the temperature is less than 300 ° C., and the holding time is less than 5 seconds, the reduction amount of Fe oxide decreases and is not reduced. There is a risk that objects will remain and unplating will occur. For the above reasons, the reduction treatment is performed in a temperature range of 300 to 900 ° C. for 5 seconds in a H ₂ —N ₂ gas atmosphere having a dew point of −60 ° C. or higher and lower than 0 ° C. and containing H ₂ : 1 vol% or higher. It is preferable to hold the above.

The steel sheet subjected to the above reduction treatment is subjected to a hot dip galvanizing treatment using a galvanizing bath having a bath temperature of 440 to 500 ° C. and an Al concentration in the bath of 0.10 to 0.30 mass%.
[Bath temperature: 440-500 ° C]
The bath temperature affects the reactivity between the steel sheet and the bath. When the bath temperature is less than 440 ° C., the viscosity of the bath increases and the appearance after plating or plating is deteriorated. Moreover, the reactivity with a steel plate also falls.
On the other hand, when the bath temperature exceeds 500 ° C., the reactivity between the bath and the steel plate increases, the Fe—Al—Zn alloy layer decreases, the Fe—Zn alloy layer increases, and the powdering resistance decreases. Therefore, the bath temperature is preferably 440 to 500 ° C.

[Al concentration in bath: galvanizing bath with 0.10 to 0.30 mass%]
When the Al concentration in the bath is less than 0.10 mass%, the growth of the alloy layer in the plating bath becomes remarkable and it becomes difficult to control the amount of plating adhesion, and the powdering resistance is also deteriorated.
On the other hand, when the Al concentration in the bath is increased to exceed 0.30%, Fe-A1-Zn alloy that causes deterioration of weldability in hot dip galvanized steel sheets and suppresses alloying in galvannealed steel sheets. The amount of the layer increases, and when the alloying is performed in the alloying furnace, the treatment is required for a long time at a higher temperature, thereby hindering the productivity. Therefore, the Al concentration in the bath is preferably 0.10 to 0.30 mass%.

After the above hot dip galvanizing treatment, the galvanization can be alloyed by further heating at 460 to 580 ° C.
[Alloying at 460-580 ° C]
When the alloying temperature is less than 460 ° C., it takes time to diffuse and contain Fe of the base steel sheet in the plating layer, and it becomes difficult to further improve the adhesion between the base steel sheet and the plating layer by Fe diffusion.
On the other hand, when the alloying temperature is higher than 580 ° C., the reaction between the base steel plate and the plating layer is sufficiently promoted, but a thick Fe—Zn alloy layer harmful to powdering resistance is generated. . Therefore, the alloying temperature is preferably 460 to 580 ° C.
In addition, the ratio (alloying degree [Femass%]) which occupies in the plating layer for the quantity of Fe diffused from the base steel sheet into the plating layer is 7 to 15 mass%, preferably 9 to 11 mass%.

  The present invention can cope with almost all steel types only by changing the heating temperature to change the oxidation amount, and a stable effect can be obtained by controlling the heating temperature.

  The high-tensile steel plate having the component composition shown in Table 1 was subjected to the heat treatment shown in Table 2, followed by reduction treatment, and then hot dip galvanized and further partially alloyed. The thus obtained hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet were evaluated for mechanical properties, plating properties, appearance after alloying, and plating layer adhesion. The results are also shown in Table 2. Each evaluation method is as follows.

"Mechanical properties"
The mechanical properties were evaluated by taking a JIS No. 5 tensile specimen from each plated steel sheet and measuring the tensile strength TS (MPa) and elongation EI (%) measured from the tensile test. % Indicates a good balance of strength and ductility, the mechanical properties are good (◯), and less than 20000 MPa ·% is bad (×).

"Plating property"
The plating property was evaluated visually as follows.
○ = Non-plating △ = Slightly non-plating (can be improved by infiltration plate temperature, bath temperature, etc.)
× = Non-plating (impossible to improve under plating conditions)

"Appearance after alloying"
The appearance after alloying was evaluated visually as follows.
○ = No alloying irregularity △ = Slightly uneven alloying, etc. (can be improved by alloying temperature, etc.)
× = There is uneven alloying (impossible improvement)

"Plating layer adhesion"
Ball impact test is performed on ungalvanized hot-dip galvanized steel sheet (GI), and after peeling off a commercially available cellophane tape on the processed part, the presence or absence of peeling of the plating layer is evaluated according to the following criteria did.
○ = No peeling of plating layer △ = A little peeling of plating layer × = Remarkably peeling of plating layer After the tape surface was bent 90 ° and bent back, the tape was peeled off, and the amount of plating peeled per unit length was measured by measuring the Zn count with fluorescent X-rays and evaluated according to the following criteria.
○ = X-ray Zn count less than 1000 × = X-ray Zn count greater than 1000

Claims (7)

As the strengthening elements, elements α and β satisfying the relationship of the following formulas (1) and (2) in the temperature range of 300 to 900 ° C. (however, combinations of the elements α and β are Mn-Si, Si-Al, Cr) -Si) , a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the steel sheet, and an internal oxide containing Fe and elements α and β in a depth region within 100 μm from the surface of the steel sheet And a total amount of internal oxides (excluding Fe single oxide) in the depth region is 0.01 to 1.0 g / m ² per one surface of the steel sheet in terms of O amount.
ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ---- (1)
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ ) ---- (2)
here,
ΔG ^O _X : Standard free formation of reaction (2m / n) X + O ₂ = (2 / n) XmOn
Energy change ΔG ^O _Fe : Reaction 3/2 Fe + O ₂ = 1/2 under conditions of temperature less than 570 ° C.
Under the conditions of Fe ₃ O ₄ and a temperature of 570 ° C. or higher, the reaction 2Fe + O ₂ =
Standard free energy change of 2FeO As the strengthening elements, elements α and β satisfying the relationship of the following formulas (1) and (2) in the temperature range of 300 to 900 ° C. (however, combinations of the elements α and β are Mn-Si, Si-Al, Cr) When performing hot dip galvanizing treatment on a steel sheet containing at least -Si) ,
Heat treatment is performed in an oxidizing atmosphere at 300 to 900 ° C., and an oxide layer having an Fe and O content of 90 mass% or more is formed on the surface of the steel sheet in an amount of 0.03 to 2.0 g / m in terms of O amount on one side of the steel sheet. ^A method for producing a hot-dip galvanized steel sheet, characterized in that the hot-dip galvanized steel sheet is formed at ² and then subjected to a reduction treatment, followed by a hot-dip galvanizing treatment.
ΔG ^O _β <ΔG ^O _α <ΔG ^O _Fe ---- (1)
ΔG ^O _α −ΔG ^O _β > 20 (kJ · mol ⁻¹ ) ---- (2)
here,
ΔG ^O _X : Standard free formation of reaction (2m / n) X + O ₂ = (2 / n) XmOn
Energy change ΔG ^O _Fe : Reaction 3/2 Fe + O ₂ = 1/2 under conditions of temperature less than 570 ° C.
Under the conditions of Fe ₃ O ₄ and a temperature of 570 ° C. or higher, the reaction 2Fe + O ₂ =
Standard free energy change of 2FeO The oxidizing atmosphere of the heat treatment method for producing a hot-dip galvanized steel sheet according to claim 2, O ₂ concentration, characterized in that it is 20 vol% or less than 0.01 vol%. The steel plate
C: 0.0005 to 0.20 mass%,
α: 1.0 to 3.0 mass% and
β: 0.10 to 2.0 mass%
The method for producing a hot-dip galvanized steel sheet according to claim 2 or 3 , wherein the balance has a component composition of Fe and inevitable impurities . In the reduction treatment , the steel sheet is kept in a temperature range of 300 to 900 ° C. for 5 seconds or more in an H ₂ —N ₂ gas atmosphere having a dew point of −60 ° C. or more and less than 0 ° C. and containing H ₂ : 1 vol% or more. The method for producing a hot dip galvanized steel sheet according to any one of claims 2 to 4, wherein the method is performed. The said hot dip galvanization process is performed using the galvanization bath whose bath temperature is 440-500 degreeC and Al concentration in a bath is 0.10-0.30 mass%, The fusion | melting in any one of Claim 2 thru | or 5 characterized by the above-mentioned. Manufacturing method of galvanized steel sheet. The method for producing a hot dip galvanized steel sheet according to any one of claims 2 to 6, wherein after the hot dip galvanizing treatment, the galvanizing is further alloyed by heating at 460 to 580 ° C.