JP3468004B2 - High strength hot-dip galvanized steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-dip galvanized hot-rolled steel sheet, and particularly used for automobile bodies and the like, which is excellent in press workability and plating adhesion which are alloyed as necessary. It is a high strength hot-dip galvanized hot rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, it has become necessary to reduce the weight of automobile bodies from the viewpoint of exhaust gas regulations. One of the effective means of reducing the weight of the vehicle body is to reduce the thickness of the steel sheet used, but in order to ensure safety, it is necessary to improve the strength of the steel sheet by the amount of reduction in thickness. there were. Therefore, at the present time, the steel sheet composition is made to contain a composite of strengthening elements such as Si, Mn, Cr, C, and P to enhance the strength.

Such high-strength steel sheets are usually obtained by pickling an oxide film (commonly called black skin) produced by hot rolling,
In order to impart corrosion resistance, "plating", especially "hot dip galvanizing" is applied to obtain a high-strength hot-dip galvanized hot rolled steel sheet. When applying this "hot dip galvanizing", first,
A pickling treatment is performed to remove the black skin. Then, after continuously annealing the steel sheet, an extremely thin so-called "invisible oxide film" formed on the surface of the steel sheet,
Reduction is performed in a reducing atmosphere of N ₂ —H ₂ .

However, when the temperature in the furnace is low and this reduction is insufficient, the oxide film remains and so-called "non-plating" occurs. The remaining oxide film is usually re-reduced, but if the temperature at the time of the re-reduction is too high, Si and M, which are more easily oxidizable than Fe after the reduction, are reduced.
Elements such as n and Cr are concentrated on the surface of the steel sheet as oxides,
Even in such a case, the "non-plating" occurs. The reason is that if the content of Si, Mn, Cr, etc. is large, there is no suitable reduction temperature range.

As one of the conventional techniques for improving these,
Japanese Examined Patent Publication No. 61-9686 proposes to perform "undercoating" of Ni on the surface of a steel sheet prior to hot dip galvanizing. However, in this method, C: 0.0001-
0.30 wt%, Si: 0.001-3.0 wt%, M
n: 0.1 to 3.0 wt%, Cr: 0.001 to 2.0
When targeting steel sheets containing wt% and P: 0.001 to 0.10 wt% respectively, it is necessary to perform Ni plating with an adhesion amount of 10 g / m ² or more, which is "plating cost".
Cause rise. Further, when such a large amount of base Ni plating is applied, the wettability of the hot dip galvanizing is improved, but there is a problem that defects due to Si and Ni frequently occur on the plating surface during the alloying process.

As this undercoat plating, for example, Japanese Patent Laid-Open No.
As disclosed in 7-70268, there is also a method of using Fe. However, this method was able to prevent “non-plating” of Si-added steel, but for that purpose, Fe plating of 5 g / m ² or more was required, and N
As in the case of i, it was extremely uneconomical. Further, as an improvement technique other than the undercoat plating, JP-A-55-122
865 and Japanese Patent Laid-Open No. 4-254531. It is a method of oxidizing a steel sheet in advance to form an iron oxide film on the surface thereof before applying hot dip galvanizing and then reducing annealing the steel sheet to suppress the formation of an oxide film of an alloy element. . This method is a method of adjusting the thickness of the iron oxide film remaining before plating by reduction annealing to a certain value or less, so if the iron oxide film is reduced too much by reduction annealing, the alloying elements are rather concentrated on the surface. However, there is a problem that the plating property becomes poor, that is, the balance between the oxide film and the reduction amount is lost. In addition, to prevent this reduction too much,
A huge amount of iron oxide is required. However, the iron oxide film is peeled off by a roll or the like during rolling, and selective oxygen of alloying elements occurs during reduction annealing after that, the plating property is hindered, or the peeled iron oxide film is scattered in the reduction annealing furnace. There was also a problem that it adversely affected the operation.

As described above, the high-strength hot-rolled steel sheet, which is attractive as a high-strength material for automobiles, currently lacks an effective means for applying hot-dip galvanizing.

[0008]

In view of such circumstances, the present invention is a high-strength molten zinc which is excellent in press workability and plating adhesion without causing so-called "non-plating" during galvanizing. It is an object of the present invention to provide a galvanized hot rolled steel sheet or an alloyed hot-dip galvanized hot rolled steel sheet.

[0009]

Means for Solving the Problems C, Si, Mn, Cr,
When plating a high-strength hot-rolled steel sheet to which a strengthening element such as P is added, these elements exist as surface oxides during annealing or pickling, and impede wettability with molten zinc. "Non-plating" occurs. Therefore, the inventor has made detailed studies on the surface layer structure of the steel sheet that suppresses the surface concentration of the above elements. As a result, the high-strength hot-rolled steel sheet containing a certain specified component, in the steel sheet base iron crystal grain boundaries and / or crystal grains of the surface layer, if the various oxides of the specific component are generated in advance. It was found that plating can be performed without causing "non-plating" during hot dipping, and that press workability and plating adhesion are dramatically improved. The present invention embodies this finding, with C: 0.0001 to 0.30 wt%, Si: 0.
001 to 3.0 wt%, Mn: 0.1 to 3.0 wt%,
Cr: 0.001-2.0 wt%, P: 0.001-
A base plate is a steel plate containing 0.10 wt% and consisting of balance Fe and unavoidable impurities, and having an oxide effective in improving press workability and plating adhesion in crystal grain boundaries and / or crystal grains in the surface layer portion. a high strength galvanized hot-rolled steel sheet characterized by comprising forming a molten zinc plating layer on the surface of it.

Further, in the present invention, the above oxide is SiO ₂ ,
MnO, FeSiO ₃ , Fe ₂ SiO ₄ , MnSiO
₃ , Mn ₂ SiO ₄ , P ₂ O ₅ , Cr ₂ O ₃ , FeCr
O ₄ , FeCr ₂ O ₄ , (Fe, Mn) O, (Fe, C
r) ₂ O ₃ , (Fe, Mn) SiO ₃ and (Fe, M
n) One or more selected from ₂ SiO ₄ is a high-strength hot-dip galvanized hot-rolled steel sheet.

Further, according to the present invention, the above oxide is distributed from the surface layer of the mother plate to a depth of 0.1 to 100 μm, and in the presence of the above oxide, the oxygen content of the whole mother plate is before the formation of the oxide. It is a high-strength hot-dip galvanized hot-rolled steel sheet characterized in that it is higher by 1 ppm or more. In addition,
It is also a high-strength hot-dip galvanized hot-rolled steel sheet characterized by alloying the hot-dip galvanized layer.

In the present invention, since many kinds of oxides of specific components are present in the surface layer portion, "non-plating" during hot dipping
In addition, when the steel sheet is processed, deep drawability, bending, etc. are very good.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, including the background to the invention,
The contents of the present invention will be described in detail. Conventionally, C: 0.000
1 to 0.30 wt%, Si: 0.001 to 3.0 wt
%, Mn: 0.1-3.0 wt%, Cr: 0.001-
The high-strength hot-rolled steel sheet containing 2.0 wt% and P: 0.001 to 0.10 wt% is subjected to pickling treatment before hot dip galvanizing in order to remove the black skin of the surface layer of the steel sheet. . At that time, since an extremely thin so-called "invisible oxide film" is formed on the surface of the steel sheet, in order to reduce it, the steel sheet is passed through a continuous annealing furnace to reduce N ₂ -H ₂ before plating. Reduce in a sexual atmosphere. However, if the temperature inside the furnace is low and the reduction is insufficient, an oxide film remains and "non-plating" occurs during plating.
Occurs. If the temperature at which the remaining oxide film is re-reduced is too high, elements such as Si, Mn, and Cr will be concentrated as oxides on the steel surface after the reduction, and "non-plating" will occur also in this case. .

Therefore, the inventor diligently studied the countermeasure, and if the oxides of the specific components were present in the surface layer of the steel sheet, the surface concentration of these components was suppressed and the oxides were not formed on the surface of the steel sheet. I knew that. The oxides generated in the grain boundaries and / or crystal grains of the steel sheet surface layer can be generated when the steel sheet is hot-rolled, and in particular, it has a high coiling temperature of the steel strip. , And then grows when the cooling rate is slow. The observation result of the oxide formed during this hot rolling is shown in FIG. 1 in a state where it is immediately below the so-called black skin. In addition, in the oxides, oxygen is dissociated from FeO, which is the main component of the black skin, due to the high temperature when the steel strip is wound into a coil and the surface of the steel sheet is shielded from the atmosphere. It is generated as a result of internal oxidation of the surface layer of the steel sheet under rolling.

Conventionally, continuous galvanizing line (CGL)
In the reduction annealing at, Si, Mn, Cr and the like were selectively oxidized and concentrated on the surface of the steel sheet. However, when the high-strength hot-rolled steel sheet according to the present invention is used, the surface layer portion is internally oxidized as described above before plating, and the above elements become widely distributed in the steel as oxides. Therefore, even when the steel sheet is subjected to reduction annealing at a high temperature, elements such as Si, Mn, and Cr do not diffuse inward in the steel and do not move to the surface, but are concentrated / precipitated on the surface of the steel sheet as oxides. I can't come. Then, during plating, the wettability between the molten zinc and the steel sheet is not hindered, and "non-plating" does not occur.

During such internal oxidation, elements such as P, Cr, Si, and Mn, which are more easily oxidizable than Fe, diffuse into the black scale inside and outside the steel sheet, and as a result, black by pickling. The amount of solid solution of these elements in the vicinity of the surface layer of the ground iron after the peeling is reduced. GDS (glow discharge emission spectrometry)
Is used to analyze the high-strength hot-rolled steel sheet in which the oxide is formed and those not, and the distribution of these easily oxidizable elements in the depth direction is investigated, and the results are shown in FIG. From FIG. 2, it can be seen that in the high-strength hot-rolled steel sheet in which the oxide is generated, the solid solution amount of these elements in the surface layer portion is reduced.

By the way, at the time of pickling treatment performed before hot dip galvanizing, an element dissolved in steel is oxidized with the dissolution of base iron, and an oxo acid ion in a form of being combined with oxygen, for example, PO ₄ ^3- , SiO ₃ ^2-, etc., and Fe
It forms a complex with cations such as ions and adheres / precipitates on the surface due to its low pH. Particularly, the phosphate complex is easily attached. Therefore, in the so-called "invisible oxide film" that is formed on the surface layer of the steel sheet after pickling, Fe, Si,
A complex oxide composed of Mn, Cr, P, etc. comes to be contained. Since these composite oxides are generally much less likely to be reduced than Fe oxides, Si, Mn, Cr, etc. are so-called surface-concentrated on the surface of the steel sheet, and "non-plating" occurs. Therefore, conventionally, reduction could not be completed completely unless the temperature was raised to a high temperature at which hot dip galvanizing was impossible. On the other hand, a low temperature range (eg 5
(00 to 600 ° C.), the oxide film could not be sufficiently reduced, so that “non-plating” was still generated. However, as described above, during hot rolling, Si, Mn, and
When solid solution elements such as Cr and P are fixed as oxides in the ground iron grain boundaries and in the grains, complex oxides such as Si, Mn, Cr and P will not be contained in the invisible oxide film. . In addition, since those that have been previously formed as oxides are easily removed during pickling, Si, Mn, and C derived from these are removed.
Oxides such as r and P will not be contained in the “invisible oxide film” at the same time.

FIG. 3 shows the result of observing the surface of the hot-rolled steel sheet after pickling and removing the black leather by ESCA (X-ray photoelectron spectroscopy). The presence of P—O bonds could not be confirmed in the steel sheet in which oxides were formed on the grain boundaries and / or inside the grains, but the presence of P—O bonds was observed in the steel sheet in which no oxides were formed. This suggests that a hardly-reducing phosphoric acid compound is produced during pickling of hot rolled steel sheet with black leather.

As described above, most of the invisible oxides formed on the surface layer of the hot rolled steel sheet on which the oxides have been formed are mainly Fe-based oxides, which are easily reducible. Therefore, the plating property is improved in the low temperature reduction temperature range (500 to 600 ° C.) where the surface is not concentrated and the Fe oxide can be reduced, and “non-plating” does not occur. The oxide forming technology in the steel sheet surface layer portion used in the present invention is not limited to hot-dip galvanized steel sheets, hot-dip aluminum plating, hot-dip aluminum-zinc plating 5% aluminum-zinc plating, or so-called galvalume plating, etc. It can also be applied to other hot-dip galvanized steel sheets. This is because the concentration of oxides of Si, Mn, Cr, etc. on the surface is suppressed, wettability with not only zinc but other molten metals such as aluminum is improved, and "non-plating" is also suppressed. Because. So, after all,
By forming an oxide in advance on the surface layer of the high-strength hot-rolled steel sheet, concentration of oxides such as Si, Mn, and Cr on the surface is suppressed, and the addition amount of Si, Mn, and Cr is high. Even with a high-strength hot-rolled steel sheet, the hot-dip galvanizing property becomes good regardless of the metal type.

In addition, the same applies to alloying after plating, and the fact that there is a correlation with the amount of surface enrichment of the above-mentioned elements has a correlation not only with the plating property but also with the alloying rate, and the one with a smaller amount of surface enrichment. It has been confirmed that the plating property is improved and the alloying speed is increased. Therefore, Si, Mn, Cr, P
In order to dramatically improve the hot-dip galvanizability of high-strength hot-rolled steel sheets to which strengthening elements such as are added, after all, during winding of the hot-rolled steel sheets, the base iron crystal grain boundaries and / or crystal grains By forming oxides of solid solution elements such as Si, Mn, Cr, and P, not only the surface concentration at the time of high-temperature reduction annealing is suppressed, but also the steel sheet surface layer portion is formed at the time of pickling. It is most effective and appropriate because elements other than Fe such as Si, Mn, Cr and P are not contained as a complex oxide in the so-called "invisible oxide film".

The lower limits of the amounts of Si, Mn, and Cr are set to 0.001, 0.1, and 0.001 wt /%, respectively. This is because hot dip galvanizing is possible with a CGL line equipped with a tube (RTH) type or non-oxidizing furnace (NOF) type annealing furnace. Also, regarding the alloying reaction, there is no particular decrease in the alloying reaction rate, and the same alloying equipment and alloying temperature as before, alloying time, alloying time, heating rate during heating, cooling rate during cooling, etc. In the present invention, the amount of Si is 0.001% or more, the amount of Mn is 0.1 or more, and the amount of Cr is 0.001% or more.
The amount of P was 0.001% or more. Further, the lower limit of the amount of C, 0.0001 wt%, is an unavoidable content in a normal steelmaking method.

The upper limit of the amount of Si is 3.0 wt% and Mn is
The upper limit of the amount of Cr is 3.0wt%, the upper limit of the amount of Cr is 2.0wt
%, The upper limit of P is set to 0.10 wt%. If these values are exceeded, an oxide film is formed on the steel sheet surface even during pickling or annealing immediately before hot dip coating, and the adhesion of the plating bath is increased. To significantly reduce In addition, P is 0.1
If it exceeds 0 wt%, a significant alloying delay may occur. Furthermore, the upper limit of the amount of C is set to 0.30 wt%, and if it exceeds this, hardness of the steel sheet increases, ductility decreases, and some mechanical property values such as deep drawability decrease, so it is preferable. Because there is no.

B has a great effect on the secondary working brittleness of steel, and is therefore an essential element for high-strength hot-rolled steel sheets.
These do not significantly impair the wettability with molten zinc during annealing or after pickling. Further, even if degreasing and pickling after annealing is not sufficient and remains on the surface layer, it is unlikely to cause "non-plating". Therefore, in the present invention, the content of B is not particularly limited.

The oxide layer has a thickness of 0.1 or more and 100 μm.
If the thickness is less than 0.1 μm, it is impossible to suppress deterioration of wettability when the thickness is less than 0.1 μm, and if the thickness exceeds 100 μm, the oxide is brittle, and thus the steel sheet itself. This is because there is a risk that the mechanical properties of It is needless to say that the above-mentioned effects will be produced to some extent if any of these oxide layers are formed.
When the amount of oxygen in steel is measured by some analysis method, for example, "impulse furnace melting-infrared absorption method", in the steel sheet in which even a slight oxide layer is formed, the oxygen content in the steel sheet is increased by 1 ppm or more as compared with the conventional steel sheet. Was. When the amount of increase in the oxygen content in steel is less than 1 ppm, it is not effective in improving the plating property and the like. Therefore, the fact that the amount of oxygen in the steel sheet is increased by 1 ppm or more due to the presence of the internal oxide was also added to the conditions of the present invention.

Further, the presence or absence of oxide formation was determined by the following oxygen content analysis. Steel of hot-dip galvanized steel, as-hot-dip galvanized steel, and steel of which the surface is ground and the surface plating layer and 0.5 mm or more of the surface of the base steel plate immediately below the plating layer are ground and removed at the same time. The value obtained by subtracting the latter value from the former value was compared with the medium oxygen content, and the increase in oxygen content in the steel due to the oxide was taken as the increase in oxygen content. Further, even when the cross section of the plated steel sheet is polished and the surface layer portion of the steel sheet is observed with an optical microscope, the presence or absence of the oxide immediately below the surface layer of the steel sheet can be confirmed as a black streak pattern of the oxide layer at the grain boundary portion. In addition, after the cross section, 10 to 20 with 1% Nital solution
It can be observed even if it is lightly etched for about a second.

It is known that the plating is peeled off mainly by receiving compressive stress during press working.
The hot-rolled steel sheet having an oxide immediately below the coating layer of the hot-dip galvanized hot-rolled steel sheet according to the present invention has a steel sheet surface layer portion which is kept extremely clean, as compared with a conventional hot-rolled steel sheet in which no oxide exists. As a result, since there are many active sites for the reaction between molten zinc and iron, the occurrence of non-plating is suppressed as a result. Therefore, the alloy layer for alloying becomes extremely dense, and as a result, the plating adhesion during press working becomes good. The improvement of the plating adhesion during the press working was confirmed by observing the cross section with an optical microscope and observing even a small amount of the oxide immediately below the plating layer, the effect was confirmed.

In the present invention, the coating amount is not particularly limited, but from the viewpoint of corrosion resistance and the like, as a steel sheet for automobiles, normal Zn-F after alloying a hot dip galvanized steel sheet is used.
An appropriate amount of the e alloy deposited is usually 25 to 90 g / m ² , and an appropriate Fe content in the plating layer is 8 to 13 wt%.
Similarly, the conditions of the hot dip galvanizing bath are not particularly limited, but the aluminum concentration in the galvanizing bath is 0.1.
It is suitable that the Fe concentration be 0.01 to 3 wt% and the Fe concentration be 0.01 wt% to be saturated. Further, Pb, Mg, Mn and the like may be contained in the bath.

In the present invention, the steel sheet after plating is immediately heat-alloyed, if necessary, to produce a galvannealed steel sheet. During this alloying, the temperature was 4
When the temperature is lower than 60 ° C., productivity is lowered unless heating is performed for a long time. Therefore, it is necessary to heat it at a temperature of 460 ° C. or higher, but it must be 600 ° C. or lower in order to secure plating adhesion during press working.

Next, a method for manufacturing a hot-dip galvanized hot-rolled steel sheet mother board according to the present invention will be described. The crystal grain boundaries and / or oxides in the crystal grains in the surface layer of the steel sheet are generated, for example, by coiling at a coiling temperature of 650 ° C. during hot rolling and then cooling at 50 ° C./hour. In other words, this oxide is generated by the black skin (iron oxide) generated by hot rolling dissociating oxygen at high temperature and permeating into the steel sheet, and the surface layer of the base iron is internally oxidized under the oxygen partial pressure. Is. The rate of internal oxidation is a function of time and temperature. The higher the temperature or the longer the time, the more the oxidation reaction proceeds, and the grain boundaries and / or
Alternatively, the amount of oxide in the crystal grains increases.

[0030]

Example A high-strength steel sheet having the composition shown in Table 1 was subjected to pickling treatment after hot rolling and subjected to various surface treatments to produce a large number of surface-treated steel sheets as listed in Table 1. First, 12
After hot rolling the slab heated at 00 to 1250 ° C., 96
Finish rolling at 0-910 ° C to make steel strip, 440-76
It was wound into a coil at 0 ° C. Next, the steel strip was pickled with a CGL line to remove the black skin, and thereafter, cold rolling, reduction annealing, and various surface treatments were performed. The reduction annealing is performed according to the steel type No. 1
Of 550 to 850 ° C., steel type No. 2 is 700 ° C, steel type N
o. 3 is 670 ° C., steel type No. 4 is 650 ℃, steel type N
o. 5 is 680 ° C., steel type No. 6 is 630 ℃, steel type N
o. 7 was performed at 640 ° C. In addition, "thickness" in Table 1
Represents the thickness of the distribution range of the oxide layer from the surface of the steel sheet.

The hot dip galvanizing bath has an aluminum concentration of 0.1.
5 wt% was added, and the temperature was 490 ° C.
The appearance of the plating was visually observed, and it was judged whether or not it was good and whether or not non-plating occurred. The alloying treatment was performed at a temperature of 470 ° C to 570 ° C. The alloying state was evaluated after visually confirming whether "alloying unevenness", "alloying delay", etc. did not occur after alloying.
The presence or absence of oxides immediately below the surface layer of the steel sheet was observed by polishing the cross section of the sample and then immersing it in a 1% nital solution for etching. In the press workability evaluation test, an alloyed hot-dip galvanized hot-rolled steel sheet was bent and stretched by 90 degrees, and the pressure-bonded side was tape-peeled, and the amount of zinc peeled was measured by fluorescent X-ray.

[0032]

[Table 1]

Table 2 collectively shows the treatment results of the various hot-dip galvanized hot-rolled steel sheets produced as described above. From Table 2, it is apparent that the plating state is good when the surface oxide having an appropriate thickness is present in the crystal grain boundaries, in the crystal grains, or in both of them. Further, it was generally good when the surface oxide was very thin, but when the oxide layer was not present, "non-plating" occurred. On the other hand, in the case of a steel plate that does not correspond to the present invention, "non-plating" or "uneven amount of adhesion"
However, when the oxide layer was as thick as 150 μm, various problems such as deterioration of mechanical properties of the steel sheet occurred. However, in the steel sheet according to the present invention, let alone "non-plating", no problems such as deterioration of mechanical properties of the steel sheet itself occurred.

Further, the hot-dip galvanized hot-rolled steel sheet produced in the same manner was heat-alloyed at 460 to 560 ° C. to produce an alloyed hot-dip galvanized hot-rolled steel sheet. The result is
The results are shown in Tables 2 and 3. At that time, the press workability was ranked and evaluated as follows. Fluorescent X-ray count, press workability evaluation (rank) 0-500 rank 1 (good) 500-1000 2 1000-2000 3 2000-3000 4 3000 or more 5 (poor)

As is clear from Tables 2 to 4, Comparative Examples 1 to 1
The steel plate of No. 9 had "non-plating" and was poor in press workability and adhesion. However, the steel sheets according to the present invention (Examples 1 to 11) were good in surface appearance, press workability and adhesion.

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

As described above, according to the present invention, S
Even if it contains i, Mn, Cr, P, etc., it is possible to provide a high-strength hot-dip galvanized hot-rolled steel sheet that is as efficient as ordinary steel and is excellent in press workability and plating adhesion. .

[Brief description of drawings]

FIG. 1 is a diagram showing a state in which an oxide is generated on a surface layer of a steel sheet.

FIG. 2 is a diagram showing the distribution of various elements in the steel sheet thickness direction after pickling and removing the black skin, where (a) shows the case where an oxide is formed at the grain boundary / inside of the steel sheet surface layer portion (b). ) Is a case where it is not generated.

FIG. 3 is a diagram showing the results of ESCA analysis of the surface of a steel sheet from which the black skin has been pickled and removed.
In the case where the oxide is generated in the grains, (b) is the case where it is not generated.

─────────────────────────────────────────────────── ─── Continuation of front page (56) References Patent 3312103 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38/60 C23C 2/02 C23C 2 / 06 C23C 2/28

Claims (4)

(57) [Claims] 1. C: 0.0001 to 0.30 wt%, Si: 0.001 to 3.0 wt%, Mn: 0.1 to 3.0 wt%, Cr: 0.001 to 2.0 wt%, P : 0.001 to 0.10 wt%, balance Fe and unavoidable impurities, and oxides effective in improving press workability and plating adhesion in the crystal grain boundaries and / or crystal grains in the surface layer Steel plate as mother board
And, high-strength galvanized hot-rolled steel sheet characterized by comprising forming a molten zinc plating layer on the surface of it. 2. The oxide is SiO ₂ , MnO or FeS.
iO ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ , P
₂ O ₅ , Cr ₂ O ₃ , FeCrO ₄ , FeCr ₂ O ₄ , (F
e, Mn) O, (Fe, Cr) ₂ O ₃ , (Fe, Mn) S
The high-strength hot-dip galvanized hot-rolled steel sheet according to claim 1, which is one or more selected from iO ₃ and (Fe, Mn) ₂ SiO ₄ . 3. The oxide is added in an amount of 0.1 to 10 from the surface of the mother plate.
Distributed to 0 μm depth and in the presence of the above oxides,
The high-strength hot-dip galvanized hot-rolled steel sheet according to claim 1 or 2, wherein the oxygen content of the entire mother plate is 1 ppm or more higher than that before the oxide is generated. 4. The high-strength hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized layer is alloyed.