JP2000309847A - Hot rolled steel sheet, hot-dip metal coated hot rolled steel sheet, and their manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled steel sheet having excellent hot dipping property and chemical conversion treatment property and a hot-dip metal coated hot rolled steel sheet. SOLUTION: In the processes of manufacturing a hot rolled steel sheet and a hot-dip metal coated hot rolled steel sheet, heat treatment is applied, after hot rolling in particular, to the resultant hot rolled steel sheet at 650-950 deg.C in a practically reduction-free atmosphere while keeping the black scale adhering to the steel sheet, by which an internal oxidation layer is formed in the vicinity of the ferrite surface of the steel sheet. A pure iron layer is formed on the outermost surface of the ferrite above the internal oxidation layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-rolled steel sheet and hot-rolled hot-rolled steel sheet used for automobile parts and the like, and a method for producing them. Is what you do.

[0002]

2. Description of the Related Art In recent years, the strength of automobile members has been increased from the viewpoint of reducing the weight of a vehicle body and improving reliability and safety. This tendency is not an exception for hot-dip galvanized steel sheets, such as hot-dip galvanized steel sheets, which are frequently used as automotive steel sheets, and various improvement measures have been proposed for increasing strength.

[0003] For example, Japanese Patent Application Laid-Open No. 59-193221 discloses Si
There has been proposed a method for increasing the strength of a steel sheet by incorporating a relatively large amount of a solid solution strengthening element such as iron or Mn. However, in this method, another problem derived from a large amount of Si or Mn is contained, that is, deterioration of hot-dip plating property due to surface concentration of Si or Mn (generation of non-plated parts, ie, generation of non-plating). And a problem of chemical conversion deterioration (a chemical conversion film such as zinc phosphate applied to a cold-rolled steel sheet as an undercoating treatment is not formed on the cold-rolled steel sheet) occurs.

As a solution to the above problem, a method of forcibly oxidizing a steel sheet under a high oxygen partial pressure, and then reducing and hot-dip coating the steel sheet (Japanese Patent Application Laid-Open No. 55-122865), Although a method of performing pre-plating (Japanese Patent Laid-Open No. 58-104163) has been proposed, these methods do not provide sufficient control of surface oxides during heat treatment, and thus are not always stable depending on steel components and plating conditions. There is also a problem that hot-dipability or chemical conversion property cannot be obtained and that an extra process is added to increase the production cost.

Japanese Patent Application Laid-Open No. Hei 9-310163 discloses a method for improving the deterioration of hot-dip coating described above, by performing high-temperature winding after hot rolling to obtain a crystal grain boundary on the surface layer of a steel sheet. And a method of forming an oxide in a crystal grain, that is, a method of forming an internal oxide layer. However, in the above method, a sufficient internal oxide layer cannot be secured depending on the type of steel and the manufacturing history, so that a problem remains in that not always satisfactory hot-dipability and chemical conversion treatment are necessarily obtained. Was.

In particular, recrystallization annealing before hot-dip plating is performed
This tendency was significant when the radiation type heating method such as a radiant tube was used. In the case of the direct heating type heating method, an oxide layer was formed to some extent during this annealing, which was better than the case of the radiant type heating. It was difficult to form a layer.

[0007] Recently, for some automotive components, hot rolled steel sheets have been used in place of conventional cold rolled steel sheets. This hot-rolled steel sheet does not require recrystallization annealing, unlike the cold-rolled steel sheet.Therefore, the surface enrichment of Si and Mn, which mainly occurs during such recrystallization annealing, and consequently the adverse effects caused by such surface enrichment It is considered that occurrence is small. However, when a hot-rolled steel sheet was actually investigated for hot-dipability and chemical conversion treatment properties, a sufficiently satisfactory result was not obtained.

[0008]

DISCLOSURE OF THE INVENTION The present invention advantageously solves the above-mentioned problems. When used as a hot-rolled steel sheet, the hot-rolled steel sheet can exhibit excellent hot-dipability and chemical conversion properties stably. An object of the present invention is to propose a steel sheet and a hot-dip hot-rolled steel sheet together with their advantageous production methods. The chemical conversion property in the present invention means the ability to form a chemical conversion film such as zinc phosphate when a hot-rolled steel sheet is used as it is as a member for an automobile.

[0009]

Means for Solving the Problems Now, the present inventors have conducted intensive studies to achieve the above object, and have found that the chemical conversion treatment and hot-dipability of a hot-rolled steel sheet and the alloy of hot-dip galvanized steel sheet The causes of the deterioration of the oxidizing behavior include the conventional so-called surface concentration of Si and Mn during heating before hot-dip plating, and the addition of Si, Mn, P, etc. to the surface of hot-rolled steel sheet after pickling. It has been determined that the essential cause is that the oxide of the compound remains. It is considered that this is because oxides such as Si and P and composite oxides with iron are difficult to dissolve during pickling.

Therefore, in order to solve the above problem,
It is considered effective to make the hot-rolled steel sheet surface after pickling pure iron. However, it is not always easy to turn the surface layer of ground iron into pure iron.

Therefore, the present inventors have conducted intensive research on this point, and as a result, formed an internal oxide layer near the surface of the ground iron, and as the internal oxide layer forming element, Si, Mn,
It has been found that the surface of a hot-rolled steel sheet can be easily turned into pure iron by confining P and the like in the inside. In addition, in order to stably form such an internal oxide layer, it is extremely effective to perform heat treatment after hot rolling in an atmosphere that does not substantially cause reduction while keeping the black scale. I got some knowledge. The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows. 1. After hot rolling the raw steel slab, with the black scale attached, in an atmosphere in which reduction does not substantially occur, 650-95
Heat treatment in a temperature range of 0 ° C, forming an internal oxide layer near the surface of the base iron of the steel sheet and forming a pure iron layer on the outermost surface of the base iron, and then performing pickling according to a conventional method. A hot-rolled steel sheet.

2. In producing a hot-rolled steel sheet by hot-rolling and then pickling a raw steel slab, after hot-rolling, with the black scale attached, in an atmosphere in which reduction does not substantially occur, 650 mm. By performing heat treatment in the temperature range of ~ 950 ° C, an internal oxide layer is formed in the vicinity of the surface layer of the base iron of the steel sheet, and a pure iron layer is formed on the top surface of the base iron above the internal oxide layer. A method for producing a hot-rolled steel sheet.

3. Hot-rolled hot-dip steel sheet, after hot rolling, heat-treated in a temperature range of 650 to 950 ° C in an atmosphere in which reduction does not substantially occur, with the black scale remaining attached. After forming an inner oxide layer near the surface layer of the steel plate and a pure iron layer on the outermost surface layer of the steel plate, it is obtained by subjecting it to pickling, heating and hot-dip plating in accordance with a conventional method. Hot-rolled steel sheet.

4. The raw steel slab is hot-rolled, then pickled, then subjected to heat treatment and hot-dip plating to produce a hot-dip hot-rolled steel sheet.After hot rolling, the black scale is substantially adhered. In the atmosphere where no reduction occurs, heat treatment is performed at a temperature range of 650 to 950 ° C to form an internal oxide layer near the surface layer of the steel, Is a method for producing a hot-dip galvanized steel sheet, wherein a pure iron layer is formed.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The results of an experiment on which the present invention is based will be described below. FIG. 1 shows a so-called hot-rolled steel sheet in which the black scale is removed in advance by pickling.
(a)) and the result of observing the cross-section of the hot-rolled sheet after heat treatment on the hot-rolled steel sheet with the black scales attached (so-called black-scale hot-rolled sheets (Figs. (b) and (c))). Are shown in comparison. In addition, as materials, Si: 0.5 mass%, Mn: 1.5 ma
An Si-Mn steel containing ss% was used, and the heat treatment conditions for the hot-rolled sheet were 750 ° C. and 5 hours.

As shown in the figure, when the hot-rolled sheet heat treatment was performed while the black scale was attached (FIGS. (B) and (c)), the internal oxidation occurred near the surface layer of the steel sheet. Layer formation was observed. When the heat treatment atmosphere is 100 vol% N ₂ (atmosphere that does not substantially cause reduction: FIG.
The formation of reduced iron was hardly observed on the surface of the black scale scale and at the interface with the ground iron, whereas in the case of 5 vol% H ₂ -N ₂ (atmosphere causing slight reduction: (c) in the same figure). Is
The formation of reduced iron was observed at the interface between the partial surface of the black scale and the ground iron. On the other hand, in the case of the hot-rolled bark, the formation of an internal oxide layer was not observed at all. In addition, black scale hot rolled sheet
An investigation was also made of a case where the heat treatment was performed in a 100 vol% H ₂ atmosphere (strong reducing atmosphere). In this case, only the reduction of the scale scale itself proceeded, and almost no internal oxide layer was formed. Then, oxides such as Si, Mn, and P remain in the reduced iron.

Thus, it was found that the formation of the internal oxide layer in the hot-rolled sheet is greatly affected by the atmosphere during the heat treatment of the hot-rolled sheet. FIG. 2 schematically shows the influence of the heat treatment atmosphere of the hot-rolled steel sheet on the formation of the internal oxide layer. As shown in FIG. 2 (a), when heat treatment is performed in a non-reducing (substantially no reduction) atmosphere (for example, 100 vol% N ₂ atmosphere), oxygen in the black scale is mainly crystallized. penetrates along the grain boundary, FeSiO ₃ and Mn _x Fe _y O _z is formed. That is, it is considered that oxygen in the scale is used only for forming the internal oxide layer.

On the other hand, as shown in FIG. 2B, a reducing (substantially reducing) atmosphere (for example, 100 vol% H _{2) is used.}
Or 5vol% H ₂ -N ₂ atmosphere), oxygen in the black scale is used not only for the formation of an internal oxide layer but also for reduction of the black scale (FeO + H ₂ → Fe + H ₂ O) Therefore, the formation of the internal oxide layer becomes insufficient, and the disadvantage that the black scale layer is reduced to form reduced iron mixed with oxides such as Si and Mn also occurs.

Next, FIGS. 3A and 3B respectively show 0.08 mass
% C-1.0 mass% Si-1.5 mass% Mn-0.07mass% P The hot-rolled black scale sheet having a composition of P was heat-treated in nitrogen or a non-heat-treated comparative material.
The results obtained by examining the element distribution in the depth direction by DS (Grim-glow emission spectroscopy) are shown in comparison. Comparative material S
i, Mn, etc. are metallic and homogeneous inside,
In the surface layer, the concentration of Si, which is a residue of the oxide, is increasing.

On the other hand, in the heat-treated material of black scaled hot rolled sheet in nitrogen, peaks due to oxides such as Si and Mn are observed inside the surface layer of the base iron, and the metal element is confined inside as an oxide. You can see that. These in the internal oxide layer are oxides, and these elements as metal elements are significantly reduced. Then, the metal elements such as Si and Mn in the outermost layer are significantly reduced as compared with the inside of the base iron and the comparative material, and it can be seen that the outermost layer is almost pure iron.

As for the oxidation behavior, both internal oxidation and surface layer oxidation can occur. The mechanism by which Si, Mn, etc. decrease on the outermost layer side from the inner side has not been clearly elucidated. It is conceivable that the oxide of the metal moves inside due to internal oxidation, moves into the scale, and is easily removed together with the scale during pickling. Then, it is considered that the outermost layer becomes pure iron by such a mechanism.

Next, the hot-rolled sheet obtained as described above is pickled, and then subjected to heating → hot-dip galvanizing → heat alloying treatment using a salt bath using a vertical type hot-dip plating simulation apparatus manufactured by Resca. And hot-rolled galvannealed steel sheets. FIG. 4 shows the results of examining the occurrence of non-plating during hot-dip plating. The evaluation of non-plating was performed by obtaining the area of the non-plated portion by image processing. As is clear from the figure, it was confirmed that when the heat treatment atmosphere of the hot rolled sheet was substantially non-reducing (A) while the black scale was still attached, no non-plating occurred. Was.

Therefore, in the present invention, in order to stably form a pure iron layer on the outermost layer, the heat treatment of the hot-rolled sheet is performed in a substantially non-reducing atmosphere with the black scale remaining. I decided.

[0025]

The composition of the raw steel slab of the present invention is not particularly limited, and so-called low-carbon steel sheets, ultra-low-carbon steel sheets, Mn-added high-tensile steel sheets and Si-Mn-added high-tensile steel sheets are conventionally known. Any one fits. In particular, Mn-based high-strength steel sheets with a relatively large amount of Mn added to improve strength
It is suitable for high Si-Mn high strength steel sheets to which Si or Mn is added.
If necessary, Ti, Nb, B, Mo, Sb, P,
S, C, N, Cu, Ni, Cr, V, Zr and the like can be contained.

Next, the manufacturing process of the present invention will be described. First, as a method for manufacturing a billet, a continuous casting method is advantageously applied, but it goes without saying that an ingot-bulking method may be used. The hot rolling is not particularly limited, and may be performed according to a conventionally known method. Typical hot rolling conditions are a reduction ratio of 80 to 99%, a hot rolling end temperature of 600 to 950 ° C, and a winding temperature of 300 to 750 ° C. For hot-rolled steel sheets, the thickness is usually 1.6 to 6.
Although it is about 0 mm, it can be applied to thin materials of about 0.8 mm obtained by recent advances in high-pressure technology in hot rolling.

The hot-rolled steel sheet obtained as described above is
Normally, after removing the black scale by pickling as it is, it is used as a product, or hot-dip galvanized by hot-dip coating, but in the present invention, after the above hot rolling,
The hot-rolled steel sheet with the black scale attached is heat-treated in an atmosphere in which reduction does not substantially occur to form an internal oxide layer near the surface layer of the steel sheet, By using a pure iron layer, stable hot-dip plating properties and chemical conversion treatment properties are improved.

In the present invention, the pure iron layer does not mean that 100% iron does not contain any other element, and that the easily oxidizable metal elements such as Si and Mn are remarkably reduced from the inside of the base iron. It means that it is pure iron. Note that elemental analysis cannot distinguish between a metal state and an oxide state. However, in a typical case, a pure iron layer exists on the surface side of the internal oxide by GDS as shown in FIG. You can confirm that you are doing. Since it is sometimes difficult to directly confirm the pure iron layer, it is easy to confirm that the pure iron layer exists on the surface layer by simply confirming the internal oxide layer with an optical microscope. can do. This is because the outermost layer is made pure iron by the formation of the internal oxide layer.

Here, in order to stably obtain excellent hot-dipability, the thickness of the internal oxide layer is set to about 5 to 40 μm and the area ratio of the internal oxide layer in the surface layer is set to about 1 to 20%. It is desirable. Note that this value can be easily determined as the area ratio of a portion that looks black at the time of cross-sectional observation (1000 times) in no etching.

In the heat treatment step of the hot-rolled steel sheet,
The processing temperature must be 650-950 ° C. This is because if the heat treatment temperature exceeds 950 ° C, the crystal grain size becomes coarse and the skin becomes rough.
If the temperature is lower than ℃, a sufficient pure iron layer cannot be formed. The heat treatment time is not particularly limited, but is preferably about 4 to 40 hours.

In the present invention, as an atmosphere which does not substantially cause reduction, an atmosphere of 100 vol% N ₂ is the best, and an H ₂ -N ₂ mixed atmosphere having an H ₂ content of less than 5 vol% is advantageously suited. I do. In this regard, when the H ₂ content is 5 vol% or more, the formation of an internal oxide layer is significantly reduced, and not only is it difficult to form a pure iron layer on the surface, but also a metal oxide is contained on the surface of the black scale. Undesired reduced iron is generated, which hinders the removal of residual scale in the pickling step, which is not preferable. In an oxidizing atmosphere containing a large amount of oxygen, such as in the atmosphere, oxidation of easily oxidizable metal elements in steel or iron itself proceeds only on the surface of the base iron, and the formation of an internal oxide layer is extremely small, and the surface layer This is not suitable because a pure iron layer is not formed on the surface. However, 100vol% N ₂ atmosphere or H ₂ amount is 5v
If the O ₂ content in a H ₂ —N ₂ mixed atmosphere of less than 1 vol% is 1 vol% or less, the oxidation of iron is so small that there is no problem, an internal oxide layer is generated, and the surface layer becomes pure iron. As we go,
If so, it may be contained. Eliminating O ₂ completely has a significant economic disadvantage.

Next, pickling is performed. The pickling conditions are also not particularly limited, and may be performed by adding a pickling accelerator and a pickling inhibitor as needed with hydrochloric acid or sulfuric acid according to a conventional method. It is better not to perform excessively excessive pickling to remove more than μm.

Thereafter, in the case of hot-dip plating, heating is performed to reduce the oxide (invisible oxide) covering the surface or to promote the activation of the surface. However, the heating conditions are not particularly limited. According to the usual law, for example,
H ₂ : 2 to 20 vol%, balance: in an atmosphere of N ₂ , dew point: −
50 to + 10 ° C, temperature: 500 to 950 ° C, time: 10 seconds to 10 min
What is necessary is just to perform on conditions of about. By performing such heating, oxides such as Si, Mn, and P on the surface of the ground iron, and composite oxides with iron are wiped off from the surface, so that excellent hot-dipability and alloying characteristics can be obtained. It is.

In the present invention, even when radiant heating such as a radiant tube is used for heating before hot-dip plating, excellent hot-dip plating properties and alloying characteristics are ensured because the surface layer is close to pure iron. There are advantages that can be done. Further, in the present invention, a temper rolling of 10% or less can be added to the steel strip after the hot-dip plating process described later for shape correction, adjustment of surface roughness, and the like.

The hot-rolled steel sheet obtained as described above may be subjected to hot-dip plating according to a conventionally known method. For example, in the case of hot-dip galvanizing, hot-dip steel sheets are immersed in a hot-dip zinc bath at a bath temperature of about 460 to 490 ° C. to perform hot-dip galvanizing. At that time, the temperature of the sheet when it is immersed in the bath is preferably about 460 to 500 ° C. Further, the amount of Al in the molten zinc bath is preferably about 0.13 to 0.5 wt%. The hot-rolled steel sheet immersed in the hot-dip galvanized bath in this way is pulled up from the bath, and then the amount of coating is adjusted by gas wiping treatment or the like, and becomes a hot-dip galvanized steel sheet. Furthermore, such a hot-dip galvanized steel sheet can be made into an alloyed hot-dip galvanized steel sheet by performing an alloying process thereafter.

Other hot-dip plating treatments include:
There are hot-dip aluminum plating, hot-dip zinc-aluminum plating, hot-dip zinc-magnesium-aluminum plating and the like, and these may be subjected to hot-dip plating according to a conventionally known method. Also, Pb, Sb, B
A small amount of i, REM, Ti, etc. may be added. further,
Regarding the coating weight of hot-dip plating, 20 to 100 g / side
preferably with m ² approximately.

[0037]

EXAMPLE A steel slab adjusted to the composition shown in Table 1 was used.
After heating to 1100 to 1250 ° C, a hot-rolled sheet having a thickness of 2.0 mm was formed by hot rolling, and then subjected to a heat treatment of the hot-rolled sheet under the conditions shown in Tables 2 and 3, followed by pickling. The hot-rolled steel sheet thus obtained is subjected to heat treatment at 700 ° C for 1 min. ・ Bath temperature: 470 ° C ・ Infiltration plate temperature: 470 ° C ・ Al content: 0.14wt% ・ Coating weight: 60 g / m ² (one side)-Hot-dip galvanized steel sheet was manufactured by performing hot-dip galvanizing treatment under the condition of plating time: 1 sec. Further, a part of the steel sheet was then subjected to an alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.
Further, a part thereof was subjected to a hot-dip aluminum plating treatment and a hot-dip zinc-aluminum plating treatment after the above heat treatment. Further, a part of the hot-rolled steel sheet was subjected to a chemical conversion treatment. For comparison, a hot-rolled steel sheet, a hot-dip coated hot-rolled steel sheet, and a hot-rolled hot-rolled alloyed steel sheet were manufactured according to a conventional method. The results obtained by examining the chemical conversion property of the hot-rolled steel sheet thus obtained, the hot-dipability and plating adhesion of various hot-dip hot-rolled steel sheets, and the alloying speed and alloying unevenness of the hot-rolled hot-dip galvanized steel sheet, Tables 4 and 5 show the results.

The evaluation method of each characteristic is as follows. <Chemical conversion treatment property> A hot rolled steel sheet was subjected to a chemical conversion treatment consisting of degreasing → water washing → surface conditioning → chemical conversion shown in Table 6 to form a zinc phosphate film, which was evaluated according to the following criteria. ：: Zinc phosphate coating was uniformly formed on the entire surface ×: Part where no zinc phosphate coating was formed

<Hot Dipability> The appearance after hot-dip plating was image-processed to determine the non-plating area ratio, which was evaluated according to the following criteria. 5: Non-plated area ratio 0% 4: Non-plated area ratio 0 to 0.1% 3: Non-plated area ratio 0.1 to 0.3% 2: Non-plated area ratio 03 to 0.5% 1: Non-plated area ratio 0.5% or more

<Plating adhesion> DuPont impact test (diameter
A 1/4 inch weight of 1 kg was dropped on a steel plate from a height of 50 cm) to evaluate the plating adhesion. The criteria are as follows. ○: No plating peeling ×: With plating peeling

<Alloying rate> Alloying conditions Heating rate: 20 ° C / s Cooling rate: 15 ° C / s Alloying temperature: 490 ° C Alloying time: 20 seconds Surface of alloyed material treated under the above conditions The alloying speed was evaluated based on whether or not the zinc η phase remained. ○: without zinc η phase ×: with zinc η phase

<Metalization unevenness> Using a salt bath, 10 ×
A 20-cm hot-dip coated plate was alloyed at 490 ° C for 30 seconds, and it was evaluated whether there was uneven alloying by observing the plating appearance after alloying. ○: No burn unevenness (uniform) ×: Burn unevenness

[0043]

[Table 1]

[0044]

[Table 2]

[0045]

[Table 3]

[0046]

[Table 4]

[0047]

[Table 5]

[0048]

[Table 6]

As is clear from Tables 4 and 5, each of the hot-rolled steel sheets obtained according to the present invention has a pure iron layer on the outermost layer, and as a result, the hot-rolled steel sheet obtained by the conventional method is obtained. In comparison, it exhibits excellent chemical conversion treatment properties, hot-dip plating properties, and alloyed hot-dip plating properties.

[0050]

Thus, according to the present invention, after hot rolling,
By performing hot-rolled sheet heat treatment in an atmosphere in which reduction does not substantially occur with the black scale attached, an internal oxide layer is formed near the surface layer of the steel sheet and the upper layer of the internal oxide layer is formed. A pure iron layer can be formed on the outermost surface of the base iron, and as a result, the chemical conversion treatment property and the hot-dip plating property can be remarkably improved.

[Brief description of the drawings]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an optical microscope micrograph showing a cross section of a hot-rolled strip (FIG. 1 (a)) and a hot-rolled strip of black (FIGS. 2 (b) and 2 (c)) after heat treatment.

FIG. 2 is a view showing the influence of a heat treatment atmosphere of a hot-rolled steel sheet on the formation of an internal oxide layer.

Fig. 3 Black-scaled hot-rolled sheet subjected to hot-rolled sheet heat treatment (a)
FIG. 4 is a graph showing a comparison of element distribution in the depth direction after pickling for the case (b) not subjected to such heat treatment.

FIG. 4 is a diagram showing a state of occurrence of non-plating during hot-dip plating.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA02 EA04 EA05 EA11 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA23 EA25 EA26 EA27 EA28 EA31 FA02 FA03 FF02 FF03 GA02 GA05 GA08

Claims (4)

[Claims] After the hot rolling of a raw steel slab, a heat treatment is performed in a temperature range of 650 to 950 ° C. in an atmosphere in which reduction does not substantially occur, with a black scale attached thereto, and A hot-rolled steel sheet obtained by forming an internal oxide layer near the iron surface layer and forming a pure iron layer on the outermost surface layer of the base iron, and then subjecting the layer to pickling according to a conventional method. 2. In producing a hot-rolled steel sheet by subjecting a raw steel slab to hot rolling and then pickling, after hot rolling, substantially no reduction occurs while the black scale is adhered. By performing heat treatment in the atmosphere at a temperature of 650 to 950 ° C, an internal oxide layer is formed in the vicinity of the surface layer of the steel sheet, and a pure iron layer is formed on the outermost layer of the steel sheet above the internal oxide layer. Forming a hot rolled steel sheet. 3. A hot-rolled hot-dip steel sheet, which is obtained by hot rolling a raw steel slab and then keeping a scale of 650 to 950 ° C. in an atmosphere in which reduction does not substantially occur, with a black scale attached. After performing heat treatment in the range, forming an internal oxide layer near the surface layer of the steel sheet and forming a pure iron layer on the outermost layer of the steel sheet,
A hot-dip hot-dipped steel sheet obtained by performing pickling, heating and hot-dip plating in a conventional manner. 4. A raw steel slab is hot-rolled, then pickled, subjected to a heat treatment and a hot-dip plating treatment to produce a hot-dip hot-rolled steel sheet. As it is, heat treatment is performed in a temperature range of 650 to 950 ° C. in an atmosphere in which reduction does not substantially occur to form an internal oxide layer near the surface layer of the steel sheet and to form an internal oxide layer on the internal oxide layer. A method for manufacturing a hot-dip galvanized steel sheet, comprising forming a pure iron layer on the outermost surface of the base iron.