CN1140648C - Hot-dip galvanized and zinc alloy steel sheet with reduced bare spot and excellent coating adhesion and method for manufacturing same - Google Patents

Abstract

A hot-dip galvanized and zinc-alloy steel sheet and an alloyed hot-dip galvanized and zinc-alloy steel sheet, and a method for manufacturing the steel sheets, which are used for automobiles. A hot dip galvanized and zinc alloy steel sheet having an oxide formed of an easily oxidizable element directly under a plating layer has a reduced number of bare spots and excellent plating adhesion. According to this method, the coiling temperature during hot rolling is set to not less than 600 ℃ and cooling is performed at a slow cooling rate to retain oxides until after the subsequent steps.

Description

Hot-dip galvanized and zinc-alloyed steel sheet with reduced bare spots and excellent coating adhesion and method of manufacturing the same

technical field

The present invention relates to hot-dip galvanized and zinc-alloyed steel sheets with a reduced number of bare spots and excellent coating adhesion, and a method of manufacturing the same.

Background technique

Because hot-dip galvanized and zinc-alloy steel sheets are mainly used in automobile bodies due to their low price and excellent corrosion resistance, in addition to the corrosion resistance produced by the coating, for steel sheets to be used in automobile bodies, it is necessary to Coating adhesion during processing. When the coating adhesion is lowered, the coating flakes into powder or lumps, and this phenomenon sometimes causes abrasion during press forming or reduces the corrosion resistance of the peeled part of the coating; also, the peeled off fragments are harmful to the steel plate.

As an existing technique for improving coating adhesion, Japanese Patent Laid-Open No. 61-276961 discloses a technique of alloying Fe and Zn at a high temperature range of 700-850° C. after hot-dip plating. However, alloying at high temperature increases not only the cost but also the cost of equipment such as rolls.

Also, in Japanese Patent Laid-Open No. 3-232926, the steel contains at least one element of Zr, La, Ce, Y and Ca, and the cooling rate from recrystallization annealing to plating is set to not less than 50°C/sec. The cost is increased due to the addition of Zr etc. to the steel and the productivity is lowered because the steel sheet charge has to be reduced due to the cooling capacity.

Also, in Japanese Patent Laid-Open No. 2-163356, the contents of O, Al, and N in steel are set to be not more than 0.0045 wt%, (25×Nwt%) to 0.15 wt%, and not more than 0.0030 wt%, respectively. wt%. In addition, according to Japanese Patent Laid-Open No. 6-81101, the content of Ti, Si and P is restricted to satisfy Si(wt%)+P(wt%)≥Ti(wt%). In any case, desired properties of the steel sheet such as strength and elongation cannot always be obtained by such content limitation, possibly due to degraded coating adhesion due to deviation from the predetermined composition range.

In Japanese Patent Laid-Open No. 4-333552, plating adhesion is improved by pre-plating Ni before hot-dip galvanizing. However, in general, a continuous hot-dip galvanizing line (hereinafter referred to as "CGL") does not have such equipment, and a large investment is required for equipment improvement.

At the same time, because the amount of exhaust gas emitted is now limited, the body of the car needs to be lightened. Thinning the steel plate is one way to make the car body lighter. According to this method, it is necessary to increase the strength of the reduced thickness steel plate in order to ensure safety. Therefore, steel sheets with high tensile strength have been developed by increasing the contents of Si, Mn, and P elements in the steel sheets to strengthen the steel sheets. Since steel sheets for automobiles are formed by press, excellent material properties with high r values (high Lankford values) are required, and in particular, addition of these elements is essential for high tensile strength steel sheets.

For hot-dip galvanized steel sheets, recrystallization annealing at a high temperature range of about 700-900°C is necessary to obtain excellent material properties. In CGL, recrystallization annealing is usually carried out under nitrogen atmosphere in the presence of hydrogen (hereinafter referred to as reduction annealing), although this atmosphere is a reducing atmosphere for Fe, it is an oxidizing atmosphere for some elements such as Si, Mn and P . Therefore, elements that are more easily oxidized than Fe such as Si, Mn, and P (referred to as easily oxidizable elements) diffuse to the outside during reduction annealing and bond with oxygen on the surface of the steel sheet to form oxides (referred to as "surface segregation layer") . Since these oxides significantly hinder the wettability between the molten zinc and the steel sheet, so-called bare spots, defects that occur when the zinc is not attached to the steel sheet, are observed.

In order to solve such problems, Japanese Patent Post-examination Publication No. 61-9386 proposes a method of pre-plating the surface of a steel sheet with Ni before the hot-dip galvanizing process. However, according to this method, when the steel contains at least one element among Si and 0.2-2.0wt% Si, 0.5-2.0wt% Mn and 0.1-20wt% Cr, it is necessary that the amount of Ni plating is not less than 10g/ ^m2 Yes, this increases the cost. In addition, although such a large amount of nickel plating improves wettability between hot-dip galvanizing and steel sheet, it is disadvantageous that defects caused by Si and Ni often occur on the surface of the plating layer during alloying.

Also, for example, Japanese Patent Laid-Open No. 57-70268 proposes a method of pre-plating the surface of a steel sheet with Fe prior to the hot-dip galvanizing process. According to this method, bare spots in Si-containing steel can be prevented by pre-plating, but Fe plating of not less than 5 g/m ² is required, which is rather uneconomical.

In addition, other methods are disclosed in Japanese Patent Laid-Open No. 55-122865 and No. 4-254531. In these methods, the steel sheet is oxidized in advance to form an Fe oxide film on its surface, and then subjected to reduction annealing. However, according to these methods, alloying elements such as Si segregate on the surface to form an oxide film due to excessive reduction during reduction annealing, thereby causing a problem of inferior plating. In order to prevent this excessive reduction, a large amount of Fe oxide is required. However, if the amount of the Fe oxide film is too large, the Fe oxide film is peeled off due to rolling or the like, and thus, conversely, a surface segregation layer is generated and causes poor plating and adverse effects on operation because of the peeling off. The Fe oxide film is scattered in the furnace.

In addition to this, known proposals for steel composition and hot rolling conditions for hot-dip galvanized steel sheets with high tensile strength, Japanese Patent Laid-Open No. 6-158172 discloses a method in which Si ≤ 0.2wt% and Mn≤1.5wt% steel coiled at a temperature of not less than 650°C followed by pickling, cold rolling, annealing, and hot-dip galvanizing; and Japanese Patent Laid-Open No. 6-179943 discloses a method in which Steel containing 0.10-1.5 wt% Si and 1.00-3.5 wt% Mn was coiled at a temperature range of 500-680°C, both included, followed by pickling, cold rolling, annealing, and hot-dip galvanizing.

Although these methods give specific process conditions for a series of production steps, such as steel composition and hot rolling conditions, none of these methods can suppress the formation of surface segregation layers during reduction annealing, or improve bare spot or coating adhesion.

invention disclosure

As a result of extensive experimentation, the inventors of the present invention have found that bare spot and coating adhesion can be significantly improved by providing oxides of readily oxidizable elements just below the coating on hot-dip galvanized and zinc alloy steel sheets.

In other words, the present invention provides a hot-dip galvanized and zinc-alloyed steel sheet having an oxide of an easily oxidizable element directly below the coating.

In addition, in hot-dip galvanized and zinc alloy steel sheets, the oxygen concentration is preferably not less than 1 ppm, more preferably 2-200 ppm, and most preferably 3-100ppm.

In addition, such a hot-dip galvanized steel sheet is preferably thermally alloyed after hot-dip galvanizing, thereby obtaining an excellently alloyed hot-dip galvanized and zinc-alloyed steel sheet. Also in alloyed hot-dip galvanized and zinc alloy steel sheets, the oxygen concentration is preferably not less than 1 ppm, more preferably 2 to 200 ppm in the range from the surface layer of the steel sheet substrate immediately under the coating to a depth of 3 μm in the thickness direction of the steel sheet , most preferably 3-100ppm.

Further, hot-dip galvanized and zinc-based alloy steel sheets and alloyed hot-dip galvanized and zinc-alloy steel sheets each preferably contain at least one element selected from Si, Mn, and P as a steel component, which The content range is as follows:

  0.001≤Si≤3.0wt%

  0.05≤Mn≤2.0wt%

  0.005≤P≤0.2wt%

Besides, the present invention provides a method for manufacturing the above-mentioned hot-dip galvanized and zinc-alloyed steel sheets or alloyed hot-dip galvanized and zinc-alloyed steel sheets, both of which have reduced bare spots and excellent coating adhesion. In other words, the methods provided by the present invention include:

Step A: In the process of coiling the hot-rolled steel strip, by setting the temperature of the steel strip to be not less than 600°C, and setting the average slow cooling rate to be cooled to 540°C to be not greater than (the temperature of the process of coiling the steel strip -540) ^0.9 ÷ 40 (°C/min), forming an oxide directly under the scale, this oxide is formed by an element that is more easily oxidized than iron, and the element is Si, Mn, P or Al; and

Step B: Hot-dip galvanizing and zinc alloying the steel strip. According to this method, step B follows step A, but other steps may also be used between steps A and B. In general, steps of pickling, degreasing, cold rolling, annealing, etc. are suitable as such intermediate steps.

In addition, according to the method of the present invention, the oxide formed in step A is preferably retained until the treatment in the annealing furnace immediately before step B, which is carried out after step A of.

Also, according to these methods, the hot-rolled slab preferably contains at least one element selected from Si, Mn and P as a steel component in the following ranges:

  0.001≤Si≤3.0wt%

  0.05≤Mn≤2.0wt%

  0.005≤P≤0.2wt%

Also, alloyed hot-dip galvanized and zinc-alloyed steel sheets may be produced by performing a heat alloying treatment after Step B according to any of the above methods.

The oxide formed by the easily oxidizable element located directly under the plating layer is explained below.

Oxides of these easily oxidizable elements are formed during hot rolling, and specifically, these oxides grow when the coiling process temperature (hereinafter referred to as "CT") is high and the cooling rate after coiling is low.

It was observed that the oxide formed during hot rolling was located just below the scale, as shown in Figure 6. However, in the conventional hot-rolled steel sheet, oxides were not observed directly under the scale, as shown in FIG. 7 . The oxides formed during hot rolling were analyzed by an electron probe microanalyzer (hereinafter referred to as "EPMA"), and the results are shown in FIG. 1 . Since Mn, P, Al, and O have peaks, it should be considered that oxides of these elements are formed. The steel sheets shown in Figures 6 and 1 contained 0.1 wt% Mn, 0.006 wt% P and 0.03 wt% Al. They do not contain particularly large amounts of Mn, P or Al.

Preparation of oxides according to the invention directly under the coating of hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets, so that oxides formed directly under the scale during hot rolling can be formed even after post-treatments such as pickling and It remains after plating.

The mechanism of oxide formation directly under the scale is as follows: Oxygen in the scale layer consisting essentially of iron oxide also formed during hot rolling diffuses inwardly into the steel during or after coiling and then in the steel Forms oxides of readily oxidizable elements. Therefore, oxides are formed even when the steel contains only trace amounts of easily oxidizable elements.

Although oxides of elements more oxidizable than iron are present directly under the hot-dip galvanized and zinc alloy coatings of the present invention, oxides of elements less oxidizable than iron oxide or iron may also be contained. In addition, such oxides are preferably formed on the grain boundaries of the hot-rolled steel sheet.

As a result of studies and experiments conducted on various steel sheets, the present inventors have found oxides of Si—O, Mn—O, Al—O, P—O, and Fe—Si—O present in steel sheets.

Fig. 2 shows the elemental analysis results of conventional steel sheets, and Fig. 3 shows the elemental analysis results of non-annealed cold-rolled steel sheets in which oxides were observed, both of which were analyzed at about 10 μm from the surface of each steel sheet to the depth direction It was carried out with a glow discharge spectrometer (hereinafter referred to as "GDS") in the range of. The peaks of Mn, Al, P and O observed at a depth of about 0.3-4 μm from the surface layer correspond to oxides.

Fig. 4 shows the elemental analysis results of a conventional steel sheet, and Fig. 5 shows the elemental analysis results of an annealed cold-rolled steel sheet in which oxides were observed, both of which are at about 10 μm in the depth direction from the surface of each steel sheet within the scope of the GDS. A large amount of surface segregation products generated by reduction annealing was observed in the conventional steel sheet of FIG. 4 , whereas generation of surface segregation products was suppressed and hardly observed in the steel sheet having oxides generated during hot rolling.

Next, by eroding the steel sheet with 1% nitric acid solution for several seconds to tens of seconds, the oxide of the present invention present in the surface layer of the steel sheet (steel base surface layer) directly under the coating can be observed with an optical microscope.

Figure 8 (photograph) and Figure 9 (photograph) show conventionally alloyed hot-dip galvanized steel sheets without oxides and alloyed hot-dip galvanized steel sheets containing the oxides of the present invention, respectively. Both Fig. 8 and Fig. 9 are cross-sectional optical micrographs of alloyed hot-dip galvanized steel sheets magnified 1000 times. The black bands observed just below the plating are oxides (shown by arrows).

In addition, the formation of oxides can also be demonstrated by analyzing the oxygen contained in the steel. As a related art, the oxygen in the steel is analyzed in the thickness direction of the steel sheet using the following steel sheets, which are hot-rolled steel sheets pickled to remove scale after coiling, obtained by dissolving only the coating layer of hot-dip galvanized and zinc alloy steel sheets steel sheets, non-annealed cold-rolled steel sheets, or annealed steel sheets, and the obtained values were compared with those of steel sheets obtained by grinding to form a surface layer of oxides. The steel sheet in which oxides are formed has a greater oxygen value than the ground steel sheet, the oxygen value being analyzed over the entire thickness.

Next, the mechanism for improving bare spot and plating adhesion by forming an oxide directly under the plating is investigated.

First, concerning the improvement of the bare spot, as mentioned above, when the oxide is formed directly under the scale by oxygen diffusion in the interior during or after the coiling process, it is found that the easily oxidizable elements are suppressed during the reductive annealing of the CGL Segregated on the surface.

It is hypothesized that this phenomenon is due to the following reasons: the content of easily oxidizable elements in the surface layer is reduced because the easily oxidizable elements have been precipitated as oxides during or after the coiling process; the formed oxides prevent the easily oxidizable elements from migration from the steel matrix to the surface of the steel sheet (i.e. outward diffusion); and oxidation-reduction reactions occur inside the steel sheet, in other words, during reduction annealing, the Fe-containing oxides formed during or after coiling are transformed into easily Oxides of oxidizing elements.

As a result, surface segregation of readily oxidizable elemental species, which impedes wettability between molten zinc and the steel sheet, is greatly reduced, thereby significantly improving bare spots.

Next, the plating adhesion is explained.

It is known that the spalling of the coating is mainly due to the compressive stress during the press-forming process.

Since the steel sheet has oxides just below the coating, the steel sheet of the present invention has spaces between oxide crystals, zinc penetrates into the steel sheet more easily than conventional oxide-free steel. As a result, the interface between the plating layer and the steel sheet is significantly rough, so that the plating layer can be firmly adhered to the steel sheet. As a result, both the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet included in the present invention obtain excellent coating adhesion during press-forming.

Figures 10 and 11 show the results obtained by observing the steel plate with SEM. The coating on this steel plate has been subjected to constant current method (4% methyl salicylate, 1% salicylic acid, and 10% potassium iodide/methanol solution; 5mA/ cm ² ) was forced to dissolve to the iron potential to expose the steel plate. It will be appreciated that the interface between the coating and the steel sheet is significantly rougher than conventional oxide-free steel sheets.

In addition, when the steel plate contains at least one element selected from Si, Mn and P as a steel component, the technology disclosed in the present invention exhibits a more excellent effect, and the content range of the component is as follows:

  0.001≤Si≤3.0wt%

  0.05≤Mn≤2.0wt%

0.005≤P≤0.2wt% Problems such as bare spots and reduced adhesion hardly occur in steels not containing the above elements, so the lower limits of these elements are preferably 0.001wt% Si, 0.05wt% Mn and 0.005wt% P . Meanwhile, the determination of the upper limit of each element is a preferable range in consideration of both the maximum strengthening effect and the cost.

In addition, even when a small amount of oxides are observed with an optical microscope in the sections of hot-dip galvanized and zinc-alloy steel sheets corroded with 1% nital solution, the technology disclosed in the present invention can also show sufficient sensitivity to bare spots and coating adhesion. Focus on both effects.

In addition, according to the oxygen analysis in steel, when the value of the following formula is not less than 1ppm, it shows a sufficient effect, especially for bare spots and coating adhesion:

(Oxygen in a steel plate that has been decoated by the hydrochloric acid-antimony method) - (Oxygen in a steel plate that has been decoated by the hydrochloric acid-antimony method and the surface layer has been ground to remove 3 μm)

Next, a method for producing the above-mentioned coated steel sheet will be described. The coiling temperature after hot rolling should be high and the cooling after coiling should be slow, as detailed below.

The coiling temperature after hot rolling must be 600°C or higher to produce oxides, and the cooling rate after coiling to 540°C must not be greater than: (CT-540) ^0.9 ÷ 40 (°C/min) at less than 540°C Even further slow cooling does not form oxides.

In addition, although the scale is removed by pickling and/or grinding before coating, and sometimes electrolytic degreasing or pickling equipment is also installed on the inlet side of the CGL, the surface of the steel sheet during or after coiling in the hot rolling process The oxide formed in the layer must remain after these treatments described above.

Hot-dip galvanizing and zinc alloy of the present invention are general concepts of molten zinc containing zinc, and may not only contain hot-dip galvanizing but also Galfan (aluminum-zinc alloy) and Galvalume (aluminum-zinc alloy), in both Si may be contained in zinc in any case. In addition, Pb, Mg, Mn, etc. may also be contained. Therefore, there are no particular limitations on the conditions of the zinc bath.

There are no special restrictions on the other conditions of the coating, however, in consideration of corrosion resistance, etc., the preferred amount of zinc and zinc alloy coating is about 25-90g/ ^m2 , in the coating of alloyed hot-dip galvanized steel The iron content is preferably 8-13 wt%.

Also, both hot-rolled steel sheets and cold-rolled steel sheets can be used as the substrate for plating.

Brief description of the drawings

[Fig. 1] EPMA analysis graph of oxides observed directly under scale during hot rolling.

[Fig. 2] A graph showing the results of elemental analysis of a conventional non-annealed cold-rolled steel sheet, which was carried out by GDS within about 10 µm from the surface to the thickness direction.

[Fig. 3] A graph showing the results of elemental analysis of the unannealed cold-rolled steel sheet of the present invention, which was carried out by GDS within about 10 µm from the surface to the thickness direction.

[Fig. 4] A graph showing the results of elemental analysis of a conventionally annealed cold-rolled steel sheet, which was carried out by GDS within about 10 µm from the surface to the thickness direction.

[Fig. 5] A graph showing the results of elemental analysis of the annealed cold-rolled steel sheet of the present invention, which was carried out by GDS within about 10 µm from the surface to the thickness direction.

[FIG. 6] A cross-sectional optical micrograph, magnified 1000 times, showing oxides at the position directly under the scale of the hot-rolled steel sheet of one example.

[Fig. 7] Cross-sectional optical micrograph, magnified 1000 times, showing the oxide at the position just below the scale of the conventional hot-rolled steel sheet.

[FIG. 8] Cross-sectional optical micrograph, magnified 1000 times, showing that the alloyed hot-dip galvanized steel sheet of Example contains oxides.

[Fig. 9] Cross-sectional optical micrograph, magnified 1000 times, showing that a conventional alloyed hot-dip-coated steel sheet does not contain oxides.

[FIG. 10] SEM photograph, magnified 1500 times, showing the steel plate of the example in which the plating layer was dissolved.

[Fig. 11] SEM photograph, magnified 1500 times, showing a conventional steel plate in which the coating has been dissolved.

〔Symbol Description〕

1-untreated steel plate part, 2-scale, 3-oxide, 4-coating, 5-oxide.

BEST MODE FOR CARRYING OUT THE INVENTION

Examples of the present invention are as follows: Each sample listed in Table 1 was melted in a converter and cast into slabs by continuous casting. Each obtained slab was hot-rolled at a slab heating temperature of 1150-1200°C to a thickness of 1.2-3.5mm, and the finish rolling temperature was 900-920°C. The coiling temperature and cooling rate are listed in Table 2. Afterwards, the resulting steel plates were pickled in 5% HCl aqueous solution at 80°C for 5-15 seconds to remove the scale layer, and then each steel plate was divided into two groups: one group was directly sent to CGL for treatment, while the other group was cold-rolled to 0.7mm thick. In addition, if necessary, the following methods are also used in combination on the CGL inlet side as pretreatment to remove the surface layer of the steel plate.

Electrolytic degreasing: electrolysis in 3% NaOH aqueous solution at 60°C for about 10 seconds.

Pickling: Pickling in 5% HCl aqueous solution at 60°C for about 3 seconds.

Brush roller: a brush roller with abrasive grains.

In CGL, both hot-rolled and cold-rolled steel sheets are hot-dip galvanized at 470°C after annealing at 800-850°C. In addition, an alloyed hot-dip galvanized steel sheet is obtained by subsequently subjecting the annealed steel sheet to an alloying treatment at 480-530° C. for 15-30 seconds.

○Evaluation method for oxides

Observation method of oxides in hot-rolled steel sheets

The cross-section of each hot-rolled steel sheet was ground, but not etched, and then observed with an optical microscope to measure the depth occupied by oxides. The preferred optical microscope has a magnification of 1000 times.

Quantitative evaluation of oxides in hot-rolled steel sheets

The following values are obtained;

(The amount of oxygen in the steel analyzed in the thickness direction of the hot-rolled steel plate that has been pickled to remove the scale)-(Oxygen in the steel that was ground to the depth occupied by the oxides in the thickness direction of the steel after descaling) quantity)

Quantitative evaluation of oxides in coated steel sheets

Immerse each coated steel plate in the solution listed below until the dissolution reaction of the coating is over, and then calculate the concentration of oxygen produced by oxides within 3 μm from the surface of the steel plate to the thickness direction of the plate according to the following formula:

(Oxygen in the steel sheet whose coating was peeled off by the hydrochloric acid-antimony method) - (Oxygen in the steel sheet whose coating was peeled off by the hydrochloric acid-antimony method and the surface layer was removed by 3 μm)

1% nitric acid alcohol solution 1 volume% HNO ₃ -ethanol solution

Hydrochloric acid·antimony method: Sb ₂ O ₃ (20g)+35%HCl(11)

○Evaluation method for bare spots

Each plated steel sheet was evaluated by microscopic observation. Bare spots Not observed: Level 1

Very small amount observed: Grade 2

A small amount was observed: Grade 3

Observed: Grade 4

○A test for the evaluation of coating adhesion

Each coated steel panel was subjected to a DuPont impact test using a 1/2 inch punch, and microscopic observation was used to demonstrate the occurrence of flaking.

No peeling observed: ○

Peeling observed: ×

Each alloyed hot-dip zinc alloy steel sheet was bent to 90°, bent to the opposite direction, and the compressed side of the steel sheet was peeled off with tape to measure the amount of zinc spalled by fluorescent X-rays.

Counted values:

Less than 500 Level 1 (Good)

500-1000 Level 2

1000-2000 Level 3

                                                               Level 4

Greater than 3000 Level 5

Table 3 presents the results for hot-dip galvanized steel panels and Table 4 for alloyed hot-dip galvanized steel panels.

Table 1. Composition of sample steel Offset Cwt% Siwt% Mnwt% Pwt% A 0.105 0.010 0.08 0.008 B 0.070 0.10 0.10 0.01 C 0.070 0.50 2.0 0.07 D. 0.010 1.50 0.10 0.05 E. 0.003 0.003 0.05 0.005 f 0.003 0.01 0.20 0.01 G 0.003 0.30 0.50 0.04 h 0.003 0.05 1.95 0.20

           Table 2 Coiling Conditions, Occupancy of Oxides in Hot-Rolled Steel

Depth, and oxide content in hot rolled steel steel sample CT°C Average cooling rate to 540°C °C/min Depth occupied by oxides in hot-rolled steel μm Oxide content in hot rolled steel ppm Sample steel number A 540 1.0 0 0 1 A 600 1.0 1 1 2 A 600 1.5 0 <1 3 A 700 2.0 7 5 4 B 650 1.5 8 8 5 C 650 1.5 6 7 6 D. 580 1.0 0 0 7 D. 620 1.2 <1 <1 8 E. 650 1.2 5 5 9 E. 650 1.6 <1 1 10 E. 650 1.8 0 <1 11 f 650 1.0 10 11 12 G 650 1.0 12 15 13 h 600 1.8 0 0 14 h 650 1.0 12 18 15 Table 3 embodiment and comparative example (hot-dip galvanized steel sheet) Examples and Comparative Examples Corresponding to the sample steel number in Table 2 Oxides from hot rolling cold rolling Removal of surface g/ ^m2 by pretreatment on CGL inlet side Coating weight g/m ² The amount of oxides in the coated steel sheet ppm bare point level DuPont test results Comparative example 1 1 not observed conduct <0.1 40 0 2 x Example 1 2 observed conduct <0.1 50 1 1 ○ Example 2 2 observed not carried out <0.1 50 1 1 ○ Example 3 4 observed conduct 0.5 70 5 1 ○ Comparative example 2 4 observed conduct 8.0 70 0 4 x Example 4 4 observed not carried out 0.5 70 3 1 ○ Example 5 5 observed conduct 0.1 60 7 1 ○ Example 6 6 observed conduct 3.5 60 2 1 ○ Comparative example 3 7 not observed conduct 0.1 50 0 1 x Example 7 8 observed conduct <0.1 90 <1 1 ○ Example 8 9 observed conduct 0.5 50 5 1 ○ Example 9 10 observed conduct 0.3 40 1 1 ○ Comparative example 4 10 observed conduct 1.0 40 0 3 x Example 10 12 observed conduct 0.2 50 10 1 ○ Example 11 13 observed conduct 0.2 50 11 1 ○ Comparative Example 5 14 not observed conduct 0.2 50 0 4 x Example 12 15 observed conduct 0.2 50 16 1 ○ Table 4 embodiment and comparative example (alloyed hot-dip galvanized steel sheet) Examples and Comparative Examples Corresponding to the sample steel number in Table 2 Oxides from hot rolling cold rolled Removal of surface g/ ^m2 by pretreatment on CGL inlet side Coating weight g/m ² Fe content in coating % The amount of oxides in the coated steel sheet ppm bare point level Pulverization test results Comparative example 1 1 not observed conduct 0.1 40 10.5 0 1 4 Example 1 2 observed conduct 0.1 40 10.3 1 1 1 Example 2 4 observed conduct 0.5 40 11.4 3 1 1 Example 3 6 observed conduct 0.2 70 9.6 6 1 2 Example 4 6 observed not carried out 0.2 70 9.1 5 1 2 Example 5 6 observed conduct <0.1 30 10.1 7 1 1 Comparative example 2 7 not observed not carried out 0.1 60 11.5 0 2 5 Example 6 8 observed conduct 0.1 60 11.0 <1 1 2 Example 7 10 observed not carried out <0.1 40 9.9 1 1 1 Example 8 10 observed not carried out 1.0 40 10.8 <1 1 2 Comparative example 3 10 observed not carried out 3.0 40 9.0 0 1 3 Example 9 13 observed conduct 0.2 60 10.1 14 1 2 Example 10 13 observed conduct 5.0 60 10.5 5 1 2 Example 11 13 observed conduct 15.0 60 9.2 2 1 1 Comparative example 4 13 observed conduct 25.0 60 10.5 0 3 4 Comparative Example 5 14 not observed conduct 1.0 40 11.8 0 3 5 Example 12 15 observed conduct 0.2 30 11.1 15 1 1 Example 13 15 observed conduct 0.2 60 10.4 15 1 2

Industrial Applicability

The technology disclosed in the present invention relates to hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, which have reduced bare spots and excellent coating adhesion, and are mainly applicable to automobile body steel sheets.

Claims (5)

1. A method of manufacturing hot-dip galvanized and zinc alloy steel sheets, the method comprising: Step A: In the process of coiling the hot-rolled steel strip, by setting the temperature of the steel strip to be not less than 600°C, and setting the average slow cooling rate to be cooled to 540°C to be not greater than (the temperature of the process of coiling the steel strip -540) ^0.9 ÷ 40 (°C/min), forming an oxide directly under the scale, this oxide is formed by an element that is more easily oxidized than iron, and the element is Si, Mn, P or Al; and Step B: hot-dip galvanizing and zinc alloying the steel strip. 2. The method of manufacturing hot-dip galvanized and zinc-alloyed steel sheets according to claim 1, wherein said oxide formed in said step A remains until after a pretreatment step performed after said step A, and in said step A Step B precedes the annealing treatment performed immediately in the furnace. 3. The method for manufacturing hot-dip galvanized and zinc-alloyed steel sheets according to claim 1 or 2, wherein the hot-rolled slab contains at least one element selected from Si, Mn and P as a steel component in the following ranges:   0.001≤Si≤3.0wt%,   0.05≤Mn≤2.0wt%, 0.005≤P≤0.2wt%. 4. The method for manufacturing hot-dip galvanized and zinc alloy steel sheets according to claim 1 or 2, wherein said hot-dip galvanized and zinc alloy steel sheets are subjected to thermal alloying treatment after said step B. 5. The method for manufacturing hot-dip galvanized and zinc-alloyed steel sheets according to claim 4, wherein the hot-rolled slab contains at least one element selected from Si, Mn and P as a steel component in the following ranges:   0.001≤Si≤3.0wt%,   0.05≤Mn≤2.0wt%, 0.005≤P≤0.2wt%.

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