TWI592501B - High-strength hot-dip galvanized steel sheet with excellent impact-resistant peelability and machined part corrosion resistance - Google Patents

Description

High-strength hot-dip galvanized steel sheet with excellent impact peel resistance and corrosion resistance of processed parts Field of invention

The present invention relates to a high-strength hot-dip galvanized steel sheet, and more particularly to a plated steel sheet which can be made into a high-strength hot-dip galvanized steel sheet excellent in impact resistance and plating adhesion, and is suitable for various uses. For example, strength members for automobiles.

Background of the invention

Hot-dip galvanized steel sheets are widely used in automobiles, automobiles, and building materials. The steel sheet for automobiles which is rolled into a complicated shape needs to have a very high formability, and in recent years, the demand for the rust-proof performance of automobiles has increased, and the situation in which the hot-dip galvanized steel sheet is applied to the steel sheet for automobiles has also increased.

Further, in recent years, from the viewpoint of weight reduction of the vehicle body, the demand for high-strength steel sheets excellent in strength and ductility has been increasing. For example, Patent Document 1 discloses a steel sheet in which a steel sheet structure is a structure in which a ferrite-grained iron phase, a toughened iron phase, and a three-phase mixture of a Worstian iron phase are mixed. In addition, there is also a literature which discloses a steel sheet which exhibits a high ductility-induced transformation-induced plasticity during the forming process by utilizing the residual Wolster iron phase to be converted into a granulated iron.

Such a steel sheet contains, for example, 0.05 to 0.4% by mass of C, 0.2 to 3.0% by mass of Si, and 0.1 to 2.5% by mass of Mn, and after annealing in the two-phase region, a composite structure is formed by controlling a temperature change pattern of the cooling process. Therefore, it is possible to ensure characteristics required without using expensive alloying elements. In recent years, in order to ensure rust prevention, even if such a high-strength steel sheet is subjected to hot-dip galvanizing on the surface of the steel sheet base material, the demand for high-strength hot-dip galvanized steel sheets is gradually increasing.

Such a high-strength steel plate is used not only for the strength member used for the interior panel, but also because the vehicle may be washed by flying stones or obstacles, so the chance of being applied to the external member is also increased. In addition, it is necessary to have high workability when applied to a member having a complicated shape. In the case of a high-strength hot-dip galvanized steel sheet, it is considered that not only the usual 60°V bending, but also the plating adhesion when it is washed by stones or obstacles that are flying during traveling, or when heavy working (Heavy working). Testing, such as the stringent evaluation of the ball impact test or the drawbead test, must also ensure plating adhesion.

Further, since the plating layer and the iron base material are cracked at the portion where the extremely severe processing is performed at the top of the 180° bending head, it is easy to start corrosion from the portion after the chemical conversion treatment or the plating coating. If a little corrosion occurs in this portion, hydrogen will intrude from the corroded portion. Especially in the case where the base material is a high-strength steel sheet, the possibility of hydrogen embrittlement cracking is high.

When the high-strength steel sheet is galvanized by the continuous hot-dip galvanizing equipment, if the Si amount of the steel sheet exceeds 0.3% by mass, the plating wettability is largely lowered. Therefore, using the usual Sendzimir method with an aluminum-plated bath, plating will occur. Failure to apply, the appearance quality deteriorated. Further, since the plating adhesion is also greatly reduced, it is difficult to ensure the plating adhesion at the time of punching or rework.

This is considered to be because an external oxide film is formed on the surface of the steel sheet during the reduction annealing, and the film contains an oxide of Si or Mn which is inferior in wettability to molten Zn.

Regarding the means for solving this problem, Patent Document 2 proposes a method of heating a steel sheet in a gas atmosphere having an air ratio of 0.9 to 1.2 to produce an oxide of Fe, and then oxidizing by a reduction belt containing H ₂ . The thickness of the object was 500 Å or less, and then plating was performed with a plating bath to which Mn and Al were added. However, in actual production lines, various steel sheets containing various additive elements are subjected to teething, and thus it is difficult to appropriately control the thickness of the oxide, and the range of actual machine manufacturing conditions is narrow. In addition, although the effect of improving the wettability and the plating adhesion during normal processing can be expected, the effect of improving the plating adhesion at the time of punching or rework is small.

Regarding other means for suppressing plating failure, Patent Document 3 discloses a method of performing specific plating on the lower layer to improve plating properties. However, in this method, plating equipment must be additionally provided in the front portion of the annealing furnace of the melt plating line, or pre-plating treatment must be performed in the plating line. In either case, it was found that the manufacturing cost increased significantly. Further, the plating adhesion at the time of punching or rework or the effect of improving the corrosion resistance of the processed portion is small.

On the other hand, Patent Document 4 discloses a means for adjusting the oxygen potential of the annealing gas atmosphere during annealing to prevent the Fe in the steel sheet from being oxidized to produce a hot-dip galvanized steel sheet. In this way, by controlling the oxygen in the gaseous environment In the steel, the oxidizable element such as Si or Mn in the steel is internally oxidized, and the formation of the external oxide film is suppressed, and the plating property is improved. By applying this means, sufficient adhesion can be ensured during normal processing, but the effect of improving the plating adhesion at the time of punching or rework and the corrosion resistance of the processed portion cannot be expected.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 05-59429

[Patent Document 2] Japanese Patent Publication No. 04-276057

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2003-105514

[Patent Document 4] Japanese Patent No. 4177782

Summary of invention

The present invention has been made in view of the above-described circumstances, and an object thereof is to provide a high-strength hot-dip galvanized steel sheet having excellent impact peeling resistance and corrosion resistance of a processed portion.

The inventors of the present invention have conducted research and review in order to solve the above problems. As a result, it has been found that even when a steel plate containing a large amount of Si or Mn is used as the plating substrate, by forming a convex alloy layer in the plating layer of the high-strength hot-dip galvanized steel sheet, it is possible to further enhance the time during the punching or the rework. Plating adhesion. In addition, it has been found that the structure of the base material side of the steel sheet is controlled to be fine. The three-layer structure of the layer, the decarburization layer, and the inner layer can significantly suppress the penetration of the base material into the surface layer of the plating layer, starting from the top of the 180° bending head, in the extremely severe deformation state. The occurrence and deterioration of cracks. Further, by designing the plating layer and the steel plate base material in the above-described configuration, it is possible to maintain a high strength of 590 MPa and exhibit an effect of further improving the corrosion resistance of the processed portion, but the present invention has been completed.

The present invention has been completed based on the above findings, and the gist thereof is as follows.

(1) A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, which has a hot-dip galvanized layer on a steel sheet base material, wherein the steel sheet base material contains: C: 0.05 to 0.4 Mass%, Si: 0.4 to 3.0% by mass, Mn: 1.0 to 4.0% by mass, P: 0.0001 to 0.1% by mass, S: 0.0001 to 0.01% by mass, Al: 0.005 to 0.1% by mass, and N: 0.0005 to 0.01% by mass , O: 0.0001 to 0.01% by mass, the remainder consists of Fe and unavoidable impurities, and the tensile strength is 590 MPa or more; the molten galvanized layer is composed of Fe: 0.01 to 6.9 mass%, and Al: 0.01 to 1.0 mass. %, the remaining part of Zn, and unavoidable impurities; the plating layer has: a convex alloy layer contacting the steel plate base material; the convex alloy layer is observed from the cross-sectional direction to the interface of the steel plate base material and the plating layer per unit length The number density is 4 pieces/mm or more, and the maximum grain size of the convex alloy layer at the interface is 100 μm or less; and the steel plate base material has a micronized layer which is in direct contact with the interface between the steel plate base material and the plating layer, The decarburized layer is in contact with the micronized layer and is present inside the steel plate base material to Inner layers, the fine-based layer and a layer other than the decarburized; the average thickness of the fine layer of 0.1 ~ 5 μ m, the average particle diameter of the fine ferrite phase layer is 0.1 ~ 3 μ m; the removal The average thickness of the carbon layer is 10 to 200 μ m, the average grain size of the ferrite phase in the decarburization layer is 5 to 30 μm , and the average volume fraction of the ferrite phase in the decarburized layer is 70% or more. The remaining part of the structure is composed of Worthite iron, toughened iron, 麻田散铁, or Borne iron; the average Vickers hardness Hv(surf) of the decarburized layer and the average Vickers hardness Hv(bulk) of the inner layer The ratio Hv(surf)/Hv(bulk) is 0.3 to 0.8; and one or two or more kinds of oxides of Si and Mn are contained in the layer of the fine layer, the decarburized layer, and the convex alloy layer.

(2) A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion as described in the above (1), wherein the fine layer, the decarburized layer, and the layer of the convex alloy layer contain oxidation The material is one or more selected from the group consisting of SiO ₂ , Mn ₂ SiO ₄ , MnSiO ₃ , Fe ₂ SiO ₄ , FeSiO ₃ , and MnO.

(3) The high-strength hot-dip galvanized steel sheet excellent in the punching resistance and the corrosion resistance of the processed portion as described in the above (1) or (2), wherein the maximum particle diameter of the oxide contained in the convex alloy layer is 0.05. ~0.4μm, the number density is 20~100/μm ² .

(4) The high-strength hot-dip galvanized steel sheet excellent in the punching resistance and the corrosion resistance of the processed portion as described in any one of the above (1) to (3), wherein the maximum particle diameter of the oxide contained in the fine layer is It is 0.01 to 0.2 μm and the number density is 20 to 100/mm ² .

(5) The high-strength hot-dip galvanized steel sheet excellent in the punching resistance and the corrosion resistance of the processed portion as described in any one of the above (1) to (4), wherein the outermost surface of the hot-dip galvanized layer does not have a convex shape Alloy layer.

(6) A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion as described in any one of the above (1) to (5), wherein the steel sheet base material further contains Ti: 0.001 to 0.15 mass%; Nb: One type or two types of 0.001 to 0.10% by mass.

(7) The high-strength hot-dip galvanized steel sheet which is excellent in the punching resistance and the corrosion resistance of the processed portion as described in any one of the above (1) to (6), wherein the steel sheet base material further contains Mo: 0.01 to 2.0% by mass, Cr: 0.01 to 2.0% by mass, Ni: 0.01 to 2.0% by mass, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass of one or two or more.

The high-strength alloyed hot-dip galvanized steel sheet according to the present invention is a high-strength steel sheet containing a large amount of Si or Mn as a base plate, but can ensure plating adhesion at the time of punching or rework, and even a bending head such as 180° The extremely harsh processing section of the top can also exhibit excellent corrosion resistance of the processed part, and can provide high-strength alloyed hot-dip galvanized steel sheets, which is extremely effective as an inner and outer panel or a high-strength member of an automobile.

1‧‧‧ plating

2‧‧‧ convex alloy layer

3‧‧‧Measurement direction of the grain size of the convex alloy layer

4‧‧‧Steel base metal

5‧‧‧Microlayer

6‧‧‧Decarburization layer

7‧‧‧Internal layer

8‧‧‧Fat iron phase

9‧‧‧The remaining part of the organization (Worthfield iron phase, toughened iron phase, Ma Tian iron phase, Bora iron phase)

Fig. 1 is a view showing an example of a schematic view showing a cross-sectional structure of a high-strength hot-dip galvanized steel sheet according to the present invention.

Fig. 2 is a cross-sectional photograph of the inner layer of the comparative example and a cross-sectional photograph of the inner layer of the example of the present invention.

Detailed description of the preferred embodiment

Hereinafter, a high-strength hot-dip galvanized steel sheet having excellent impact peeling resistance and corrosion resistance of a processed portion, and a method for producing the same, which are related to one embodiment of the present invention, will be described in detail.

The high-strength hot-dip galvanized steel sheet according to the present invention has a hot-dip galvanized layer on a steel sheet base material, and the steel sheet base material contains C: 0.05 to 0.4% by mass and Si: 0.4 to 3.0% by mass. Mn: 1.0 to 4.0% by mass, P: 0.0001 to 0.1% by mass, S: 0.0001 to 0.01% by mass, Al: 0.005 to 0.1% by mass, N: 0.0005 to 0.01% by mass, O: 0.0001 to 0.01% by mass, and the remainder It is composed of Fe and unavoidable impurities, and has a tensile strength of 590 MPa or more; the molten zinc-plated layer is composed of Fe: 0.01 to 6.9 mass%, Al: 0.01 to 1.0 mass%, and the remaining portion of Zn, and cannot be avoided. The plating layer has a convex alloy layer that contacts the steel sheet base material; and the convex alloy layer has a number density per unit length of the interface between the steel sheet base material and the plating layer as viewed in the cross-sectional direction of 4 pieces/mm or more. The maximum grain size of the convex alloy layer at the interface is 100 μm or less; and the steel plate base material has a micronized layer which is in direct contact with the interface between the steel plate base material and the plating layer, and a decarburization layer which is in contact with the micronized layer and The inner layer of the steel plate base material and the inner layer are the fine layer and Other than the carbon layer; the average thickness of the fine layer is 0.1 ~ 5 μ m, the average particle diameter of the fine ferrite phase layer is 0.1 ~ 3 μ m; the average thickness of the decarburized layer is 10 ~ 200 μ m, the average particle size of the ferrite phase in the decarburization layer is 5~30 μ m, and the average volume fraction of the ferrite phase in the decarburization layer is above 70%; the remaining part of the structure is from Worthite , toughened iron, 麻田散铁, or ferritic; the ratio of the average Vickers hardness Hv(surf) of the decarburized layer to the average Vickers hardness Hv(bulk) of the inner layer Hv(surf)/Hv(bulk In the layer of the fine layer, the decarburized layer, and the convex alloy layer, one or two or more kinds of oxides of Si and Mn are contained.

A cross-sectional schematic view of a plating layer, a micronized layer, a decarburized layer, and an inner layer in the high-strength hot-dip galvanized steel sheet of the present invention is shown in Fig. 1 .

"The convex alloy layer in the coating"

In the high-strength hot-dip galvanized steel sheet according to the present invention, by including the convex alloy layer in the plating layer, the plating adhesion at the time of punching or rework can be ensured. By including the convex alloy layer 2 as shown in FIG. 1 in the plating layer, a large uneven shape can be formed at the interface between the steel plate base material and the plating layer, and even at the interface direction between the steel plate base material and the plating layer, even if it is subjected to punching or reworking. When a high shear stress is generated, a significant plating adhesion improving effect can be expected by the anchoring effect. As for the form of the convex alloy layer 2, a form in which the small convex alloy layer is dispersed can be expected to have a higher anchoring effect than the formation of the loose and coarse convex alloy layer. Therefore, when the maximum particle diameter of the convex alloy layer 2 at the interface between the base material 4 and the plating layer 1 shown in 3 of FIG. 1 exceeds 100 μm, an effective anchoring effect cannot be expected due to an excessively large size. Therefore, the upper limit of the maximum length (maximum particle diameter 3) of the convex alloy layer is set to 100 μm . It should be limited to 40 μm . Further, the lower limit of the maximum length of the convex alloy layer 2 is not particularly limited, and is preferably set to 3 μm or more. In addition, when the interface between the steel plate base material and the plating layer is observed in the cross-sectional direction of the number density of the convex alloy layer, the length of the interface between the steel plate base material and the plating layer is 4 or more per 1 mm, and the adhesion is enhanced. Sexual effect. On the other hand, in the case where the number density of the convex alloy layer exceeds 100/mm, not only the effect is saturated, but also the chipping resistance may be deteriorated. Therefore, it is desirable to set the upper limit of the number density of the convex alloy layers to 100 pieces/mm. It should be set in the range of 10~60/mm. As shown in FIG. 1, the convex alloy layer 2 contacts the interface between the base material 4 and the plating layer 1, and has a structure in which the interface enters the plating layer 1 from the interface. The convex alloy layer 2 may have any shape as long as it enters the plating layer 1 by contacting the interface 3. Since the interface between the convex alloy layer 2 and the base material 3 does not pass through the Fe-Al phase and protrudes into the plating layer 1, it is considered that the anchoring effect is used to improve the plating adhesion.

The convex alloy layer in the present invention can be formed by performing a mild alloying heat treatment after being immersed in a plating bath as will be described later. In the plating bath, fine columnar crystals (hereinafter referred to as precipitates in the bath) which are formed at the interface between the steel sheet base material and the molten zinc and which are fine and are columnar ζ phase (FeZn ₁₃ ) or δ 1 phase (FeZn ₇ ) are directly precipitated. Even if it coexists with a convex alloy layer, it does not have any adverse effect of the effect of this invention, However, the effect of improving the adhesiveness at the time of a punching or a rework is not expectable. Therefore, in order to distinguish the convex alloy layer from the precipitated phase in the bath, the convex alloy layer is defined to have a thickness of 2 μm or more, and a layer of the Fe-Al phase is not formed at the interface between the convex alloy layer and the steel plate base material. The upper limit of the thickness of the convex alloy layer is not particularly limited, and is preferably set to be 90% or less of the total thickness of the plating layer. The convex alloy layer is directly deposited at the interface with the steel base material, and there is no Fe-Al phase between the interfaces. Since the convex alloy layer and the base material are not in direct contact with each other via the Fe-Al phase, it is considered that the adhesion can be effectively improved.

The type of the phase constituting the convex alloy layer is not particularly limited, and is selected from the ζ phase (FeZn ₁₃ ), the δ 1 phase (FeZn ₇ ), and the Γ 1 phase (Fe ₅ Zn ₂₁ ) selected from the Fe—Zn-based intermetallic compound phase. A single phase structure or a multiphase structure of the Γ phase (Fe ₃ Zn ₁₀ ) is preferred.

"Method for Measuring Convex Alloy Layer"

The maximum length and the number density of the convex alloy layer are determined by potting and grinding the profile, etching with a 0.5% nitric oxide, and taking a 200-times photograph with an optical microscope to obtain a unit length per unit length. The number density. In addition, the maximum length of the convex alloy layer among the same photographs was also measured. Five photographs were taken at 200 times for one sample, and the length of the convex alloy layer was measured, and the maximum value was determined as the maximum length of the convex alloy layer in the sample.

Further, the convex alloy layer is produced by alloying reaction between the plating layer and the steel sheet base material. When the convex alloy layer reaches the outermost surface of the plating layer, the surface gloss is lowered, and the appearance uniformity is lowered. Therefore, among the high-strength hot-dip galvanized steel sheets of the present invention, it is preferable that the outermost surface of the hot-dip galvanized layer does not have a convex alloy layer.

"Fe concentration of the coating"

As described above, it is important to control the form of the convex alloy layer for the high-strength hot-dip galvanized layer of the present invention. By setting the Fe concentration to 0.01% by mass or more, a convex alloy layer can be contained in the plating layer. In addition, when the Fe concentration is more than 6.9 mass%, a part of the alloying reaction progresses to the surface of the plating layer, and the effect of improving the plating adhesion becomes small. Therefore, the Fe concentration in the plating layer is limited to the range of 0.01 to 6.9 mass%. It should be set at 2.0 to 6.9 mass%.

"Al concentration of plating"

When the Al concentration in the plating layer is less than 0.01% by mass, the plating cannot be controlled. Excessive Fe-Zn reaction in the bath makes it difficult to control the structure of the coating. In addition, when the Al concentration is 1.0% by mass, a dense Al 2 O 3 film is formed on the surface of the plating layer, and thus there is a concern that the welding property is inhibited when the spot welding is performed. From the viewpoint of controlling the plating structure, it is preferred to set the Al concentration in the plating layer to 0.03 mass% to 0.8 mass%. More preferably, it is set in the range of 0.1% by mass to 0.5% by mass.

"Other unavoidable impurities"

In the embodiment of the present invention, the hot-dip galvanizing layer may contain or mix Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sr, In. One or two or more of Cs and REM. Even if one or two or more types of the above-mentioned elements are contained or mixed in the hot-dip galvanized layer, the effects of the present invention are not impaired, and depending on the content, corrosion resistance and workability may be improved.

"Ply composition measurement method"

In order to measure the Fe concentration and the Al concentration in the plating layer, the plating layer may be dissolved in a 5% aqueous solution of HCl added with an inhibitor, and the solution may be quantified by ICP analysis.

"Structure of the base material side of the steel plate"

In the high-strength hot-dip galvanized steel sheet according to the present invention, the structure of the steel sheet base material side will be described in detail below.

"Microlayer"

As shown in Fig. 1, in the high-strength hot-dip galvanized steel sheet according to the present invention, the steel sheet base material side has a fine layer 5 which directly contacts the interface between the steel sheet base material and the plating layer. In the micronized layer 5, extremely fine particles mainly composed of ferrite grains and iron phases are formed. Even if it is a part of the layer which is extremely severely deformed like the top of the 180° bending head, it is possible to suppress cracks occurring from the inside of the steel sheet base material or deterioration of subsequent cracks.

By setting the average thickness of the fine layer to 0.1 μm or more, it is possible to exhibit an effect of suppressing occurrence or deterioration of cracks during processing. Further, when the average thickness of the fine layer is set to more than 5 μm , excessive alloying occurs in the plating bath, and the plating structure of the present invention cannot be obtained. Therefore, the average thickness of the micronized layer is limited to the range of 0.1 to 5 μm . The average thickness of the micronized layer should be set in the range of 0.1 to 3 μm . In addition, by setting the average particle diameter of the ferrite-particle iron phase in the fine layer to 0.1 μm or more, it is possible to suppress the occurrence or deterioration of cracks during processing, and if it is more than 3 μm , The effect is limited. Therefore, the average particle diameter of the ferrite iron phase in the fine layer is limited to the range of 0.1 to 3 μm . It should be set in the range of 0.1~2 μ m.

In the present invention, since the fine layer and the decarburized layer are produced as described later, annealing is performed under the conditions of the specific temperature region and the specific gas atmosphere during the annealing step. As a result, a decarburization reaction occurs in the surface layer of the steel sheet base material in a specific temperature region. Since the steel base material is decarburized in the fine layer, the constituent phase in the fine layer is a structure mainly composed of a ferrite-rich iron phase in addition to oxide or inclusion particles.

In the present invention, as described above, the effect of the fine layer on the side of the steel sheet base material can suppress the occurrence or deterioration of cracks during rework. At the same time, the grain size of the ferrite iron in the surface layer of the steel plate base material is made fine, and in the process of the heating alloying treatment after the hot-dip galvanizing to form the convex alloy layer, the Fe having the steel base material and the hot-dip galvanized layer is promoted. - The effect of the Zn alloying reaction. Thus, in In the state of having a fine layer, the amount of heat required to form the convex alloy layer can be reduced, and the heating temperature during the alloying process can be lowered. If the heating temperature of the alloying treatment process is lowered, the Fe-Zn reaction rate is lowered. Therefore, before the convex alloying layer covers the entire plating layer, the reaction is easily slowed down, and the range of conditions under which the production can be performed can be amplified.

"Method for measuring micronized layer"

In order to measure the micronized layer, the cross-section was processed by a CP (Cross section polisher) device, and the reflected electron image of FE-SEM (Field Emission Scanning Electron Microscopy) was observed at 5000 times, and the average thickness and fineness of the micronized layer were measured. The average crystal grain size of the ferrite iron phase in the layer may be. The definition of the micronized layer means that the average particle diameter of the ferrite-grain iron phase in the outermost layer of the steel sheet base material is 1/2 or less of the average particle diameter of the ferrite-grain iron phase in the decarburized layer. Micro layer. Further, the boundary between the average particle diameter of the ferrite-grained iron phase in the micronized layer and the average particle diameter of the ferrite-grained iron phase in the decarburized layer is defined as the layer boundary between the micronized layer and the decarburized layer.

"decarburization layer"

Among the high-strength hot-dip galvanized steel sheets of the present invention, there is a decarburized layer 6 as shown in Fig. 1 . Compared with the inner layer 7, the decarburized layer 6 has a lower volume fraction and a lower strength of the hard phase (the remaining portion of the structure 9), so that the top of the 180° bending head is less likely to become a severely deformed state. The starting point of the crack can suppress the occurrence of cracks at the top of the 180° bending head. By setting the average thickness of the decarburized layer to 10 μm or more, the top of the 180° bending head can also exhibit the effect of suppressing the occurrence of cracks. If it is set to exceed 200 μm , the properties of the decarburized layer are lowered. The tensile strength of the entire steel base material. Therefore, it is limited to the range of 10 to 200 μm . It should be set in the range of 30~150 μm .

"Steel sheet structure in the decarburized layer"

As shown in Fig. 1, the decarburization layer 6 is mainly composed of the ferrite-grained iron phase 8, and the remaining part of the structure 9 is 1 of the Vostian iron phase, the toughened iron phase, the Matian iron phase, and the Borne iron phase. Kind or more than two kinds of mixed tissues. By setting the volume fraction of the ferrite-rich iron phase in the decarburized layer 6 to 70% or more, the average hardness of the decarburized layer 6 is sufficiently lowered with respect to the inner layer 7, and exhibits a 180° bending processing head. The effect of cracking at the top. In the case where the average particle diameter of the ferrite iron phase in the decarburized layer is less than 5 μm , the effect of softening the decarburized layer is lacking. Further, when the average particle diameter of the ferrite-rich iron phase in the decarburized layer is set to more than 30 μm , there is a possibility that the low-temperature toughness is not good. Therefore, the average particle diameter of the ferrite iron phase in the decarburized layer is limited to the range of 5 to 30 μm . In addition, by setting the decarburization layer to the configuration according to the present invention, the ratio of the average Vickers hardness Hv (surf) of the decarburized layer to the average Vickers hardness Hv (bulk) of the inner layer can be Hv (surf). /Hv(bulk) is set in the range of 0.3 to 0.8. Among the tops of the 180° bending head, in order to suppress cracking in the vicinity of the interface between the steel base material and the plating surface layer, it is necessary to lower the hardness of the decarburizing layer with respect to the overall hardness. When Hv(surf)/Hv(bulk) is less than 0.3, the hardness of the decarburized layer is too low, which may adversely affect the strength of the entire steel base material. Further, in the case where Hv(surf)/Hv(bulk) exceeds 0.8, the decarburized layer is not soft enough for the inner layer, and therefore cracks occur at the top of the 180° bending head. Therefore, in the present invention, Hv(surf)/Hv(bulk) is limited to the range of 0.3 to 0.8. Hv(surf)/Hv(bulk) should be set in the range of 0.3 to 0.6.

"Determination of decarburization layer"

In order to measure the thickness of the decarburized layer, the profile of the steel sheet is first potted and ground, and the hardness curve is measured by the micro Vickers hardness test from the interface between the base material of the steel plate and the plating layer to the side of the base material of the steel plate, and the hardness curve is determined relative to the inner layer. The thickness of the layer in which the hardness is lowered in terms of hardness. The obtained layer thickness includes both the thickness of the decarburized layer and the thickness of the micronized layer, and the layer thickness obtained by the micro Vickers hardness test minus the value of the thickness of the micronized layer obtained by the foregoing method is decarburization. Layer thickness. In addition, the average value of the measured hardness in the decarburized layer is defined as Hv (surf), and the average value of the measured hardness in the inner layer is set to Hv (bulk).

Further, in order to obtain the volume fraction of the ferrite-grain iron phase in the decarburized layer, a sample having a thickness of the plate parallel to the rolling direction of the steel base material was taken as the observation surface, and the observation surface was polished to use a nitrate The etchant is etched, the decarburized layer is observed by FE-SEM, and the area fraction of the ferrite grain iron phase is measured, which can be regarded as a volume fraction. In addition, the particle size of the ferrite iron phase can also be measured.

"Internal organization"

The structure of the inner layer in the present invention is not limited to a specific range as long as the tensile strength of the steel sheet is 590 MPa or more and the Hv (surf)/Hv (bulk) is in the range of 0.3 to 0.8, and the strength is ensured. From the point of view of the balance of ductility, it is preferable that the iron phase of the ferrite is more than 50%, and the remaining part of the structure 9 is composed of the granulated iron, the volcanic iron, the toughened iron, and the ferritic iron.

"Improvement of corrosion resistance of processing parts"

In the high-strength hot-dip galvanized steel sheet according to the present invention, the plating layer has a convex alloy layer, and the steel sheet base material side has a fine layer and a decarburized layer. The individual layers have the same effect as described above, and by having all of these layers present as described in the present invention, In the processed portion which is extremely severely deformed as in the top of the 180° bending head, it is possible to obtain a remarkable effect of improving the corrosion resistance which has not been expected in the past. If only the convex alloy layer is present, and the surface layer of the steel base material does not have a fine layer and a decarburized layer, the surface layer of the steel base material at the top of the 180° bending head is severely deformed, so that cracks may occur, and the crack may pass through. To the surface of the coating, reducing the corrosion resistance of the processed portion. In addition, if the steel base material side has a fine layer and a decarburized layer, and there is no convex alloy layer, the top of the 180° bending head can suppress cracks in the surface layer of the steel base material, but the base material is severely deformed. Therefore, the plating layer is deformed accordingly, and in the vicinity of the interface between the steel base material and the plating layer, the adhesion is remarkably lowered, the plating layer is peeled off, and the corrosion resistance of the processed portion is lowered.

In the present invention, only in the state in which all of the convex alloy layer, the micronized layer, and the decarburized layer are present, the top portion of the 180° bending head is not cracked by the surface layer portion of the steel sheet base material, and The anchoring effect by the presence of the convex alloy layer causes the adhesion of the steel plate base material to the vicinity of the plating layer to be not deteriorated even if the steel base material is deformed to deform the plating layer, and the plating layer does not peel off. Therefore, the corrosion resistance of the processed portion can be remarkably improved.

"Oxide"

In the fine layer and the decarburized layer of the high-strength hot-dip galvanized steel sheet of the present invention, the layer of the convex alloy layer contains an oxide containing one or more kinds of Si and Mn. In addition, the type of the oxide contained in the fine layer, the decarburized layer, and the convex alloy layer is one selected from the group consisting of SiO ₂ , Mn ₂ SiO ₄ , MnSiO ₃ , Fe ₂ SiO ₄ , FeSiO ₃ , and MnO. Two or more types are preferable.

"Oxide in the convex alloy layer"

The effect of forming the convex alloy layer 2 in the plating layer 1 is as described above, and the plating adhesion at the time of punching and rework is improved. As will be described later, when the steel sheet base material is annealed, an internal oxide is formed on the surface of the steel sheet base material in a specific temperature region, and a mild alloying heat treatment is performed after the hot-dip galvanizing to form a convex alloy layer. By the above reaction, the convex alloy layer 2 as shown in Fig. 1 can be formed, so that the convex alloy layer inevitably contains an oxide. The oxide contained in the convex alloy layer preferably has a maximum particle diameter of 0.05 to 0.4 μm and a number density of 20 to 100 particles/μm ² .

"Oxide in the microlayer"

In the present invention, as described later, in the specific temperature region, an internal oxide is formed in the steel base material in the specific temperature region, and the growth of the iron phase crystal of the surface layer of the steel sheet base material is suppressed by the internal oxide particles. The fine layer 5 of the structure shown in Fig. 1 is used. Therefore, the oxide layer is inevitably contained in the micronized layer. The oxide contained in the fine layer has a maximum particle diameter of 0.01 μm to 0.2 μm and a number density of 20 to 100 particles / μm ² .

"Measurement of oxides"

In order to determine whether or not the oxide layer is present, the type of identification, the maximum particle size, and the number density, the cross-section of the plated steel sheet is processed by FIB (Focused Ion Beam) to prepare a film sample, and then in FE- Among the TEM (Field Emission Transmission Electron Microscopy), observation was performed at 30,000 times. Five fields of view were taken for one sample, and the average value of the oxide number density was taken from all the fields of view, and the number density of the sample was determined. Further, the maximum value of the oxide particle diameter measured in all the fields of view was defined as the maximum oxide particle diameter of the sample.

"Chemical composition of steel base metal"

The chemical composition of the base material steel sheet constituting the high-strength hot-dip galvanized steel sheet according to the embodiment of the present invention will be described.

C: C is an element which increases the strength of the steel, and is contained in an amount of 0.05% by mass or more. However, if it is excessively contained, the strength is excessively increased and the workability is lowered. Therefore, the upper limit is 0.4% by mass. From the viewpoint of workability and weldability, it is preferably set in the range of 0.07 to 0.3% by mass.

Si: Si is an effective element for improving the strength without lowering the ductility, and it is effective to add 0.4% by mass or more. On the other hand, when the addition exceeds 3.0% by mass, the strength increasing effect is saturated, and the ductility is lowered. In addition, the deterioration of the wettability of the plating is remarkable, and the appearance is greatly deteriorated. Therefore, the upper limit is limited to 3.0% by mass. It should be set in the range of 0.5 to 2.5% by mass.

Mn: Mn is an important element required for high strength, and is added in an amount of 1.0% by mass or more. However, when it exceeds 4.0% by mass, the steel blank is easily broken, and the weldability at the time of spot welding is also deteriorated. Therefore, 4.0% by mass is set as the upper limit. From the viewpoint of strength and workability, it is preferably set in the range of 1.5 to 3.5% by mass.

P: P is also an element which increases the strength of steel and on the other hand reduces workability, so it is limited to 0.1% by mass. The refining cost required to lower P to less than 0.0001% by mass is expensive, so the lower limit is 0.0001% by mass. From the balance of strength, workability and cost, it should be set at 0.005 to 0.02% by mass.

S: S is an element which lowers the hot workability and corrosion resistance of steel. When it exceeds 0.01% by mass, hot workability and corrosion resistance are deteriorated, so It is limited to 0.01% by mass. Further, setting at less than 0.0001% by mass is disadvantageous in terms of cost, so the lower limit is made 0.0001% by mass. However, if S is excessively reduced, surface defects are likely to occur, and therefore it is preferably set to 0.001% by mass or more.

Al: Al is a deoxidizing element of steel, and it is also possible to suppress the fine granulation of the hot rolled material by AlN and the coarsening of crystal grains in a series of heat treatment steps, and it is necessary to add 0.005% by mass or more in order to improve the material. However, when it exceeds 0.1% by mass, the weldability may be deteriorated, and therefore it is set to 0.1% by mass or less. Further, from the viewpoint of reducing surface defects by using alumina clusters, it is preferably set to 0.08 mass% or less.

N: N can increase the strength of the steel, and on the other hand, the workability is lowered, so the upper limit is set at 0.01% by mass. In particular, in the case where high workability is required, it is preferably set to 0.005 mass% or less. The smaller the N, the better, but the reduction to less than 0.0005 mass% requires an excessive cost, so the lower limit is set at 0.0005 mass%.

O: O causes the formation of an oxide, and the ductility and the edge formability are deteriorated, so that the content must be suppressed. When the O content is more than 0.010%, the edge formability is remarkably deteriorated, so the upper limit of the O content is set to 0.010%. Further, the O content is preferably 0.007% or less, and preferably 0.005% or less. Even if the lower limit of the O content is not particularly set, the effect of the present invention can be exhibited. However, if the O content is set to less than 0.0001%, the production cost is greatly increased. Therefore, 0.0001% is set as the lower limit. The O content is preferably 0.0003% or more, and 0.0005% or more is preferred.

Further, the molten galvanized steel associated with the embodiment of the present invention In the steel plate base material of the plate, the following elements may be added as necessary.

Ti: Ti is an element which contributes to the strength of the steel sheet by strengthening the precipitates, suppressing the coarse grain strengthening by the growth of the ferrite iron crystal grains, and the dislocation strengthening obtained by suppressing the recrystallization. However, when the Ti content is more than 0.150%, the precipitation of carbonitrides increases, and the formability deteriorates. Therefore, the Ti content is preferably 0.150% or less. From the viewpoint of moldability, the Ti content is preferably 0.080% or less. Even if the lower limit of the Ti content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Ti, the Ti content is preferably 0.001% or more. In order to further increase the strength of the steel sheet, the Ti content is preferably 0.010% or more.

Nb: Nb is an element which contributes to the improvement of the strength of the steel sheet by strengthening the precipitates, suppressing the coarse grain strengthening by the growth of the ferrite iron crystal grains, and dislocation strengthening by suppression of recrystallization. However, when the Nb content is more than 0.100%, precipitation of carbonitrides increases and moldability deteriorates. Therefore, the Nb content is preferably 0.100% or less. From the viewpoint of moldability, the Nb content is preferably 0.050% or less. Even if the lower limit of the Nb content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Nb, the Nb content is preferably 0.001% or more. In order to further increase the strength of the steel sheet, the Nb content is preferably 0.010% or more.

Mo: Mo suppresses phase change at high temperatures and is an element which contributes to high strength and can be added to replace a part of C and/or Mn. When the Mo content exceeds 2.00%, the hot workability is impaired and the productivity is lowered. Therefore, the Mo content is preferably set to 2.00% or less, and preferably 1.40% or less. Even Mo content The lower limit of the present invention is not particularly limited, and the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Mo, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more.

Cr:Cr suppresses phase change at high temperatures and is an element that contributes to high strength, and can be added to replace a part of C and/or Mn. When the Cr content exceeds 2.00%, the hot workability is impaired and the productivity is lowered. Therefore, the Cr content is preferably set to 2.00% or less, and more preferably 1.40% or less. Even if the lower limit of the Cr content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Cr, the Cr content is preferably 0.01% or more, and more preferably 0.10% or more.

Ni: Ni suppresses phase change at high temperatures and is an element that contributes to high strength, and can be added to replace a part of C and/or Mn. When the Ni content exceeds 2.00%, the weldability is impaired. Therefore, the Ni content is preferably set to 2.00% or less, and more preferably 1.40% or less. Even if the lower limit of the Ni content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Ni, the Ni content is preferably 0.01% or more, and more preferably 0.10% or more.

Cu: Cu is an element which is increased in strength by being present in the form of fine particles in steel, and may be added in place of a part of C and/or Mn. When the Cu content exceeds 2.00%, the weldability is impaired. Therefore, the Cu content is preferably set to 2.00% or less, and more preferably 1.40% or less. Even if the lower limit of the Cu content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of Cu, the Cu content is preferably 0.01% or more, and more preferably 0.10% or more.

B: B suppresses phase change at high temperatures and is an element which contributes to high strength and can be added to replace a part of C and/or Mn. When the B content exceeds 0.010%, the hot workability is impaired and the productivity is lowered. Therefore, the B content is preferably set to 0.010% or less. From the viewpoint of productivity, the B content is preferably 0.006% or less. Even if the lower limit of the B content is not particularly set, the effect of the present invention can be exhibited. However, in order to sufficiently obtain the effect of increasing the strength of B, it is preferable to set the B content to 0.0001% or more. In order to achieve further high strength, the B content is preferably 0.0005% or more.

"Production method"

Next, a method of producing the high-strength hot-dip galvanized steel sheet of the present invention will be described. In the method for producing a high-strength hot-dip galvanized steel sheet according to the present invention, the steel preform having the composition described above is used as a bottom plate, which is hot rolled, cooled, entangled, pickled, cold rolled, and then annealed by CGL. It is immersed in a molten zinc plating bath to form a high-strength hot-dip galvanized steel sheet.

The steel product to be heated and rolled is not particularly limited as long as it is a product produced by continuous casting of steel or thin steel continuous casting. Further, it is also suitable for a process such as continuous casting-direct hot rolling (CC-DR) which performs hot rolling immediately after casting.

The finishing temperature of the hot rolling is not particularly limited, and from the viewpoint of ensuring the calendering formability of the steel sheet, it is preferably set at 850 to 970 °C. Further, the cooling condition or the winding temperature after the hot rolling is not particularly limited, and the winding temperature is preferably set to 750 ° C or less in order to prevent the difference in the material between the end portions of the coil and the thickness of the rust sheet from being deteriorated. In addition, if the winding temperature is too low, the cold pressing delay is prone to ear rupture, in extreme In the case of a plate rupture, it is preferable to set it at 550 ° C or higher. After that, in order to remove the black rust sheet, the usual pickling is performed, and then the cold pressing delay is performed, and the pressing ratio can be adjusted according to the usual conditions. In order to maximize the workability, the rolling ratio is set at More than 50% is preferred. On the other hand, cold rolling is performed at a rolling ratio of more than 85%, and a large cooling load must be applied. Therefore, it is preferably 85% or less.

As described above, after cold rolling is performed, hot-dip galvanizing is performed. In an example of the method for producing a high-strength hot-dip galvanized steel sheet according to the present invention, it is preferable that the gas atmosphere in the case where the steel sheet having the component composition described above is subjected to hot-dip galvanizing is set to contain 0.1 to 20% by volume of H ₂ , and the remainder The gas environment consisting of N ₂ , H ₂ O, O ₂ and unavoidable impurities sets the gas environment between 650 and the highest heating temperature to satisfy -1.7≦log(P _H2O /P _H2 )≦-0.6 The gas atmosphere is heated at an average temperature increase rate of 0.5 to 5 ° C / s, and then continuously annealed, then cooled to 650 ° C at an average cooling rate of 0.1 to 200 ° C / s, and cooled to an average cooling rate of 3 to 200 ° C / s. Between 650 ° C and 500 ° C, in the zinc plating bath temperature: 450 ~ 470 ° C, the temperature of the steel plate immersed in the plating bath: 430 ~ 500 ° C conditions immersed in the zinc plating bath, followed by heating at 400 ~ 440 ° C Alloying treatment for 1 to 50 s, then cooling to room temperature.

The above-mentioned hot-dip galvanizing is preferably carried out in a full reduction furnace of a continuous type molten plating apparatus. The gas atmosphere during annealing is set to a gas atmosphere containing 0.1 to 20% by volume of H2, and the balance being composed of N ₂ , H ₂ O, O ₂ and unavoidable impurities. When hydrogen is less than 0.1% by volume, the oxide film existing on the surface layer of the steel sheet cannot be sufficiently reduced, and the plating wettability cannot be ensured. Therefore, the amount of hydrogen in the reducing annealing gas atmosphere is set to 0.1% by volume or more. If the hydrogen in the reducing annealing gas atmosphere exceeds 20% by volume, the dew point (corresponding to the water vapor partial pressure P _H2O ) excessively rises, and equipment for preventing condensation must be employed. The use of new equipment leads to an increase in production cost, so the amount of hydrogen in the reducing annealing gas atmosphere is set to be 20% by volume or less. It is preferably set at 0.5% by volume or more and 15% by volume or less.

By setting the gas environment between the temperature 650 and the highest heating temperature to a gas environment satisfying: -1.7≦log(P _H2O /PH ₂ )≦-0.6, the heating is performed at an average heating rate of 0.5 to 5 ° C / s. The fine layer 5 or the decarburized layer 6 as shown in Fig. 1 of the present invention is formed. In the temperature region of less than 650 ° C, the steel sheet structure is in a state where recrystallization does not start to occur. Recrystallization occurs in a temperature region of 650 ° C or higher, and nucleated recrystallized particles slowly grow. In such a temperature range, by increasing the log (P _H2O / P _H2 ) of the gas atmosphere during annealing to a gas atmosphere on the oxidizable side, Si and Mn in the steel sheet base material can be made inside the surface layer of the steel sheet base material. Oxidation, since the internal oxide particles suppress the grain growth of the recrystallized grains of the steel sheet base material, fine recrystallized grains are formed on the surface layer of the steel sheet base material, and the fine refining layer 5 can be formed. Further, at the same time as internal oxidation, a decarburization reaction occurs in the surface layer of the steel sheet base material, and the volume fraction of the ferrite-grain iron phase in the surface layer of the steel sheet base material rises, and the decarburization layer 6 can be formed. When the log (P _H2O /P _H2 ) of the gas environment between 650 ° C and the highest heating temperature is less than -1.7, in the surface layer of the steel sheet, Si or Mn hardly undergoes internal oxidation, and the decarburization reaction also occurs. No progress was made, so that a micronized layer or a decarburized layer could not be formed. In addition, when the log (P _H2O / P _H2 ) is set to exceed -0.6, the thickness of the decarburized layer becomes excessively large, which adversely affects the strength of the entire steel base material. Therefore, it is preferably set in the range of -1.7 ≦ log (P _H2O / P _H2 ) ≦ -0.6. It is preferably set at -1.3 ≦ log (P _H2O / P _H2 ) ≦ -0.7. Further, when the average temperature increase rate in the temperature region exceeds 5 ° C / s, the surface layer of the steel sheet base material is recrystallized before the formation of the internal oxide particles, and the fine layer cannot be obtained. Further, the time for performing the decarburization reaction cannot be sufficiently ensured, and the decarburized layer cannot be formed. On the other hand, when the average temperature increase rate in this temperature range is less than 0.5 ° C / s, the decarburization reaction proceeds excessively, and the strength of the entire steel base material is lowered. Therefore, it is preferable to set the average temperature increase rate between 650 ° C and the maximum heating temperature in the range of 0.5 to 5 ° C / s. Preferably, the average temperature increase rate is set in the range of 0.5 to 3 ° C / s.

The maximum heating temperature is not particularly limited, and when it exceeds 900 ° C, there is a concern that the shape of the sheet at the time of high temperature toothing is bad, and therefore it is preferably set in the range of 800 to 900 °C.

In the present invention, after the above-described heating and heating, the annealing is continuously performed. The annealing time is not particularly limited, and it is only necessary to set the conditions as necessary, and it is preferably set in the range of 1 s to 300 s from the viewpoint of economy and surface properties of the steel sheet. It is preferably set in the range of 30 s to 150 s.

After the above annealing is completed, it is cooled to the plating bath immersion temperature. The average cooling rate of the highest heating temperature to 650 ° C is desirably set at 0.1 to 200 ° C / s. Setting the cooling rate to less than 0.1 ° C / s will greatly impair the productivity, so it is not suitable. In addition, excessively increasing the cooling rate leads to high manufacturing costs, so it is preferable to set the upper limit at 200 ° C / s. The cooling rate at 650~500°C should be set at 3~200°C/s. If the cooling rate is too small, the Worthite iron phase will be converted into a Borne iron structure during the cooling process, and it is difficult to ensure more than 3% of the Worthite iron. Volume rate. Therefore, it is preferable to limit the lower limit to 3 ° C / s. On the other hand, even if the cooling rate is increased, there is no problem in the material aspect, but an excessively high cooling rate leads to high manufacturing cost. Therefore, the upper limit should be set at 200 ° C / s. The cooling method may be roll cooling, air cooling, water cooling, and any of these methods.

In the hot-dip galvanizing step, the plating bath temperature should be set at 450 to 470 °C. When the plating bath temperature is less than 450 ° C, the bath temperature control is unstable, and there is a concern that a part of the plating bath is solidified. Further, if the bath temperature exceeds 470 ° C, the life of the equipment such as the immersion roll or the zinc tank becomes short. Therefore, the bath temperature of the zinc plating bath should be set at 450 to 470 °C.

The plate temperature of the steel plate entering the plating bath should be set at 430~500 °C. If the inlet plate temperature is set to less than 430 ° C, the plating bath temperature is significantly lowered, and in order to stabilize the bath temperature, a large amount of heat must be supplied to the plating bath. Therefore, the lower limit should be set at 430 °C. Further, when the entering sheet temperature exceeds 500 ° C, the alloying reaction of Fe and Zn in the plating bath cannot be controlled, and it is difficult to control the amount of adhesion. Therefore, the upper limit should be set at 500 °C.

The concentration of Al in the plating bath is not particularly limited. In order to form a convex alloy phase in the plating layer, it is preferable to set the effective Al concentration (the total Al concentration in the bath minus the total Fe concentration in the bath) to be in the range of 0.03 to 0.8% by mass. . It is preferably set in the range of 0.08 to 0.3% by mass.

After immersion in the plating bath, it should be heated and alloyed at 400~440 °C for 1 s to 50 s, and then cooled to room temperature. In the heating alloying step, a local alloying reaction is carried out by alloying at a low temperature to form a convex alloy layer in the high-strength hot-dip galvanized steel sheet of the present invention. When the alloying temperature is less than 400 ° C, a local alloying reaction hardly occurs, so that the convex alloying layer is not formed, and the lower limit is preferably set to 400 ° C. Further, when the temperature exceeds 440 ° C, the alloying reaction is not local, but is enlarged to the whole, and it is difficult to obtain the form of the convex alloy layer. Therefore, the upper limit should be set to 440 ° C. Further, when the heating time is less than 1 s, the convex alloy layer is not formed, and when the heating time exceeds 50 s, the production line of the alloying furnace becomes too long. Therefore, it is preferable to set the heating time in the range of 1 s to 50 s.

Further, in the present invention, it is preferred to stop the heating alloying treatment before the convex alloy layer generated at the interface between the plating layer and the steel sheet base material reaches the surface of the plating layer. Further, it is desirable to carry out the toothing of the steel material having the same composition as the steel to be produced in advance, and to obtain a heating time required for the reaction to be completely alloyed on the surface in a temperature range of 400 to 440 °C. By maintaining the heating for 10 to 80% of the heating time (alloying end time) required for complete alloying in advance, the convex alloy layer does not reach the surface of the plating layer, and can be manufactured with high precision.

[Example 1]

Next, an embodiment of the present invention will be described. The conditions of the examples are examples of conditions employed to confirm the implementation possibilities and effects of the present invention, and the present invention is not limited by the examples of the conditions. The present invention can adopt various conditions without departing from the gist of the present invention and achieving the object of the present invention.

The steel slabs composed of the compositions shown in Table 1 were heated to 1150 to 1250 ° C, and the completion temperature was set to 850 to 970 ° C for hot rolling to obtain a hot rolled steel strip having a thickness of 2.4 mm. After pickling, cold rolling was carried out to form a cold-rolled steel strip having a thickness of 1.0 mm, which was melted according to the conditions shown in Table 2. The molten galvanized steel sheets of Experimental Examples 1 to 34 were produced by the galvanizing line.

The following evaluation tests were carried out for each experimental steel sheet produced by the above method, and the results are disclosed in Tables 3-1 and 3-2.

The amount of adhesion of the plating layer was obtained by dissolving the plating layer of the evaluation surface with hydrochloric acid to which an inhibitor was added, and was obtained by a gravimetric method. At the same time, the Fe concentration and the Al concentration in the plating layer can be measured by quantifying Fe and Al in the solution by ICP.

The maximum length and the number density of the convex alloy layer in the plating layer are as described above, and the cross section is potted and mirror-polished, and then immersed in a 0.5 mass% nitrile etching solution for 1 to 3 seconds for etching. It was obtained by observing at 200 times by an optical microscope.

The average thickness of the fine layer of the steel plate base material and the average grain size of the iron phase of the fat grain in the fine layer were measured by observing the electron image of the FE-SEM at 5000 times by processing the profile CP.

As described above, the average thickness of the decarburized layer of the steel base material is mirror-polished after the section is potted, and the hardness curve is measured by the micro Vickers hardness test from the interface between the steel base material and the plating layer toward the base material side of the steel sheet. The thickness of the layer in which the hardness is lowered in terms of the hardness of the inner layer is obtained by subtracting the thickness of the previously obtained fine layer.

The average particle size of the ferrite iron phase in the decarburization layer and the average volume fraction of the ferrite iron phase in the decarburization layer can be as long as 3% by weight after potting and grinding the profile The etchant was etched and was obtained by observing the FE-SEM secondary electron image of the decarburized layer 2000 times.

Hv(surf)/Hv(bulk) As described above, the average value Hv(surf) of the micro-Vickers hardness of the decarburized layer and the microscopic Vickers of the inner layer can be determined by potting and grinding the profile. The average value of hardness Hv (bulk) is obtained by calculating the ratio.

In order to obtain oxygen in the convex alloy layer, the micronized layer, and the decarburized layer The presence or absence, type, maximum particle diameter, and number density of the compound were measured by FIB processing on the cross section of the plated steel sheet as described above, and the measurement was carried out by observing at 30,000 times using FE-TEM.

In the tensile test, the tensile strength was measured by the steel sheet of each experimental example into the No. 5 test piece described in JIS Z 2201, and the tensile strength (MPa) was measured in accordance with the test method described in JIS Z 2241.

It is assumed that the plating adhesion of the standard processing is evaluated by the V bending test. The V-bend test is a 60°V curved metal model. The curved inner side was used as the evaluation surface, and a metal mold having a front end radius of curvature of 1 mm was used for 60° bending, and a tape was attached to the inside of the bent portion, and the tape was torn. The pulverization characteristics were evaluated by the peeling condition of the plating layer peeled off together with the tape. The evaluation criteria are as follows, and ○ is determined to be qualified.

○: no peeling; Δ: peeling occurred; ×: significant peeling.

It is assumed that the plating adhesion at the time of punching and rework is evaluated by the falling ball test. The ball drop test was performed by dropping a metal mold having a tip end portion of a hemisphere having a diameter of 25 mm and a weight of 3.2 kg from a height of 60 cm to a molten plated steel sheet, and observing the convex portion of the deformed molten plated steel sheet by a magnifying glass and Tape peel test to evaluate. The evaluation criteria were as follows, and ◎, ◎, and ○ were determined to be acceptable.

◎ ◎: no peeling or cracking; ◎: there is a slight crack in the local, but there is no problem; ○: there is a slight crack and peeling in the local, but there is no problem; △: peeling was apparently present, and there was a problem; ×: peeling was remarkable, and there was a problem.

In addition, the corrosion resistance of the processed portion of the portion subjected to extremely severe processing was investigated by the 0T bending (180° adhesion bending) test sample. The top of the curved outer head bent at 0 T was used as an evaluation site, and the sample after 0T bending was subjected to chemical conversion treatment and plating coating in accordance with the following conditions.

Chemical treatment: zinc phosphate treatment adhesion amount 2.5g / m ²

Electroplating: Lead-free epoxy-based plating coating film thickness 20 μm

The corrosion-promoting test disclosed in JASO-M609-91 was then performed to evaluate the number of cycles of red rust generated from the top of the 0T bend head. As a result, it was scored according to the following criteria, and ◎, ◎, and ○ were determined to be acceptable.

◎◎: After 150 cycles, there was still no red rust and white rust; ◎: no red rust occurred after 150 cycles, but slight white rust occurred; ○: no red rust occurred after 120 cycles, but slightly White rust; △: red rust after 60 cycles; ×: red rust after 30 cycles.

As is clear from Table 3, in all of the examples of the present invention, the plating adhesion at the time of rework and the corrosion resistance of the processed portion at the time of rework were all at acceptable levels. In the comparative examples which did not satisfy the range of the present invention, either of the plating adhesion at the time of rework or the corrosion resistance of the processed portion at the time of rework was poor. Fig. 2 is a cross-sectional photograph of the inner layer of the comparative example which is presumed to correspond to the experiment number 8, and a cross-sectional photograph of the inner layer of the example of the invention which is presumed to correspond to the experiment number 13.

Claims (15)

A high-strength hot-dip galvanized steel sheet which is excellent in corrosion resistance and corrosion resistance in a processed portion, and has a hot-dip galvanized layer on a steel sheet base material, wherein the steel sheet base material contains C: 0.05 to 0.4% by mass, Si: 0.4 to 3.0% by mass, Mn: 1.0 to 4.0% by mass, P: 0.0001 to 0.1% by mass, S: 0.0001 to 0.01% by mass, Al: 0.005 to 0.1% by mass, N: 0.0005 to 0.01% by mass, O: 0.0001 to 0.01% by mass, the remainder is composed of Fe and unavoidable impurities, and the tensile strength is 590 MPa or more; the molten galvanized layer is composed of Fe: 0.01 to 6.9 mass%, and Al: 0.01 to 1.0 mass%, The remaining portion of Zn and the unavoidable impurities are formed; the plating layer has a convex alloy layer contacting the steel plate base material; and the number of each unit length of the interface between the steel plate base material and the plating layer is observed from the cross-sectional direction of the convex alloy layer. The density is 4 pieces/mm or more, and the maximum grain size of the convex alloy layer at the interface is 100 μm or less; and the steel plate base material has a micronized layer which is in direct contact with the boundary of the steel plate base material and the plating layer. The surface and the decarburized layer are in contact with the fine layer and are present inside the steel base material and the inner layer, and are outside the fine layer and the decarburized layer; the fine layer has an average thickness of 0.1 to 5 μm, and the fine layer The average grain size of the ferrite phase is 0.1 to 3 μm; the average thickness of the decarburization layer is 10 to 200 μm, and the average grain size of the ferrite phase in the decarburization layer is 5 to 30 μm, in the decarburized layer. The average volume fraction of the ferrogranular iron phase is above 70%, and the remaining part of the structure is composed of Worthite iron, toughened iron, 麻田散铁, or Borne iron; the average Vickers hardness of the decarburized layer is Hv (surf The ratio of the average Vickers hardness Hv (bulk) to the inner layer is Hv(surf)/Hv(bulk) is 0.3 to 0.8; in the layer of the fine layer, the decarburized layer, and the convex alloy layer, Si is contained. And one or more oxides of Mn. The high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance of the processed portion, wherein the oxide contained in the layer of the fine layer, the decarburized layer, and the convex alloy layer is SiO ₂ , One type or two or more types of Mn ₂ SiO ₄ , MnSiO ₃ , Fe ₂ SiO ₄ , FeSiO ₃ , and MnO. A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the maximum particle diameter of the oxide contained in the convex alloy layer is 0.05 to 0.4 μm and the number density is 20 100 / μm ² . A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the maximum particle diameter of the oxide contained in the convex alloy layer is 0.05 to 0.4 μm, and the number density is 20 100 / μm ² . A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the oxide contained in the fine layer has a maximum particle diameter of 0.01 to 0.2 μm and a number density of 20 to 100. /mm ² . A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the oxide contained in the fine layer has a maximum particle diameter of 0.01 to 0.2 μm and a number density of 20 to 100. /mm ² . A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the oxide contained in the fine layer has a maximum particle diameter of 0.01 to 0.2 μm and a number density of 20 to 100. /mm ² . The high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance of the processed portion, wherein the oxide contained in the fine layer has a maximum particle diameter of 0.01 to 0.2 μm and a number density of 20 to 100. /mm ² . The high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance of the processed portion according to any one of claims 1 to 8, wherein the outermost surface of the hot-dip galvanized layer does not have a convex alloy layer. The high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance of the processed portion according to any one of claims 1 to 8, wherein the steel base material further contains: Ti: 0.001 to 0.15 mass%, and Nb: 0.001 to 0.10 mass% One or two. A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the steel sheet base material further contains one or two kinds of Ti: 0.001 to 0.15 mass%, and Nb: 0.001 to 0.10 mass%. . Excellent impact resistance and corrosion resistance of the processing part according to any one of claims 1 to 8. The high-strength hot-dip galvanized steel sheet further includes: Mo: 0.01 to 2.0% by mass, Cr: 0.01 to 2.0% by mass, Ni: 0.01 to 2.0% by mass, Cu: 0.01 to 2.0% by mass, and B: One type or two or more types of 0.0001 to 0.01% by mass. The high-strength hot-dip galvanized steel sheet having excellent punching resistance and corrosion resistance of the processed portion, wherein the steel base material further contains: Mo: 0.01 to 2.0% by mass, Cr: 0.01 to 2.0% by mass, and Ni: 0.01 to 2.0. One or two or more kinds of mass %, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass. The high-strength hot-dip galvanized steel sheet having excellent punching resistance and corrosion resistance of the processed portion, wherein the steel sheet base material further contains: Mo: 0.01 to 2.0% by mass, Cr: 0.01 to 2.0% by mass, and Ni: 0.01 to 2.0. One or two or more kinds of mass %, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass. A high-strength hot-dip galvanized steel sheet having excellent impact resistance and corrosion resistance in a processed portion, wherein the steel sheet base material further contains: Mo: 0.01 to 2.0% by mass, Cr: 0.01 to 2.0% by mass, Ni: 0.01 to 2.0% by mass, Cu: 0.01 to 2.0% by mass, and B: 0.0001 to 0.01% by mass of one type or two or more types.