JPH10298659A - Manufacture of hot dip galvanized steel sheet excellent in plating adhesion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve plating adhesion to a greater extent in the method of manufacture of a hot dip galvanized steel sheet by the use of a hot rolled steel sheet with oxide scale as a starting sheet for plating. SOLUTION: A steel slab is hot-rolled, finish-rolled, cooled down to <=650 deg.C at (50 to 150) deg.C/s cooling rate, and then coiled, by which a hot rolled starting sheet where the average peeling quantity of the oxide scale adhering to the surface and rear surface of hot rolled steel sheet is regulated to <=10 g/m<2> is prepared. After the hot rolled steel sheet is subjected, if necessary, to skin pass rolling, reducing treatment is applied, without acid pickling, at least to the surface of the oxide scale in a reducing-gas atmosphere of >=300 deg.C and then hot dip galvanizing is applied, or, the hot rolled steel sheet is subjected to skin pass rolling and then to reducing treatment for the oxide scale without acid pickling.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-dip galvanized steel sheet using a hot-rolled steel sheet with an oxide scale as a plating base sheet, and more particularly to a method for producing a thin or ultra-thin product having a thickness of 2.5 mm or less.

[0002]

2. Description of the Related Art Conventionally, when hot dip galvanizing is performed using a hot-rolled steel sheet manufactured in a hot rolling line as a plating base sheet,
After removing the oxide scale (also referred to as iron oxide or black scale) generated and adhered to the front and back surfaces of the steel sheet during hot rolling in an pickling line, the steel sheet is put into a hot-dip galvanizing line to perform plating. It was customary.

[0003] In recent years, the omission of the pickling process in the pickling line has been considered in order to reduce manufacturing costs and the like. For example, JP-A-6-145937 and JP-A-6-279 have been proposed.
As disclosed in Japanese Patent No. 967, there has been proposed a method for producing a hot-dip galvanized steel sheet using a hot-rolled steel sheet with an oxide scale attached as a base plate for plating. In each of these, the hot-rolled steel sheet with the oxide scale attached is passed through a hot-dip galvanizing line to perform plating.
In order to prevent non-plating and deterioration of plating adhesion due to the adhesion of the oxide scale, the amount of the oxide scale on the original plate and the conditions for reducing the hot-dip galvanizing line are specified.

[0004] The manufacturing method disclosed in the above-mentioned publication improves the wettability between the ground iron and the hot-dip galvanized sheet in a continuous hot-dip galvanizing line, and provides a plating treatment with no plating and good plating adhesion. In order to activate the surface of the original plate for plating, the original plate is heated to about 600 to 800 ° C. in a gas atmosphere containing hydrogen, and the surface thereof is subjected to a reduction treatment. Reduction temperature,
It defines the return time and so on.

[0005]

By performing the above-mentioned reduction treatment, the occurrence of non-plating is prevented, but at present the plating adhesion does not reach a satisfactory level. Improvement is strongly desired.

The present invention has been made in view of the above situation, and has as its object to further improve the plating adhesion in a method of manufacturing a hot-dip galvanized steel sheet using a hot-rolled steel sheet with an oxide scale as a plating base sheet.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have studied in detail the adhesion mechanism of the plating layer, and found that the adhesion of the plating is as follows. Properties, in particular, greatly affected by the amount of oxide scale peeling, hot rolling conditions of hot rolled steel sheet,
The present invention has been completed by defining the scale peeling amount and the plating conditions.

That is, in the method for producing a hot-dip galvanized steel sheet according to the present invention, a steel slab is hot-rolled, and 50
After cooling to 650 ° C or lower at a cooling rate of ~ 150 ° C / s, the rolled-up rolled sheet gives a hot-rolled original sheet having an average peeling amount of the oxide scale attached to the surface of the hot-rolled steel sheet of 10g / m ² or less, The hot rolled sheet is subjected to skin pass rolling, if necessary, and at least the surface of the oxide scale is subjected to a reduction treatment in a reducing gas atmosphere at 300 ° C. or higher without pickling, followed by galvanizing. It should be noted that hot-dip galvanizing means a normal hot-dip galvanizing level in a plating bath.
l or more Al, or may further contain Si, Pb, Fe, Cr, Ni, one of rare earth elements.
It means that even a kind or two or more kinds of additive baths can be applied.

In describing the present invention in detail, first, the technical background on which the present invention is made and the experiment that is the basis for limiting the manufacturing conditions will be described. The present inventors applied the reducing conditions disclosed in the above publication to the reducing step of the hot-dip galvanizing line, and manufactured a hot-dip galvanized steel sheet using a hot-rolled steel sheet with an oxide scale as a plating base sheet. The process was not completely reduced and removed, and the hot-dip galvanized steel sheet had a structure in which an oxide scale layer remained on the base iron, and the hot-dip galvanized layer was coated thereon. When a process such as bending was applied to the hot-dip galvanized steel sheet having such a configuration, peeling or cracking of the plating layer occurred depending on the properties of the oxide scale, and deterioration in plating adhesion was observed. Microscopic observation of the peeled portion revealed that peeling or cracking of the plating layer was mainly caused by peeling at the interface between the base iron and the oxide scale layer. From these results, it was found that the adhesion (peeling resistance) of the oxide scale layer to the base iron was significantly involved in the adhesion of the plating layer. Therefore, the following experiment was performed to determine conditions that affect plating adhesion.

[0010] Mild steel (steel No. A) shown in Table 1, 390 N /
Using a steel of mm ² grade (Steel No. B) and 440 N / mm ² grade (Steel No. C), the finishing temperature (FDT) shown in Table 2 and after finishing rolling, the steel sheet temperature was cooled to 650 ° C. After cooling at an average cooling rate (CR) until winding, winding and sheet thickness 1.4
A coil of ~ 3.8 mm was obtained. Then, after performing skin pass rolling, test pieces for investigating the thickness of the oxide scale and adhesion (peeling resistance) to the base iron, and test pieces for galvanizing in a laboratory were collected. The test piece for plating was obtained by holding the sample in a reducing atmosphere gas at 700 ° C. × 30 seconds, forming a pure iron layer on the surface of the oxide scale layer, and then performing a galvanizing process at 460 ° C.

[0011] The adhesion of the oxide scale on the original plating plate was determined by applying a 10% tensile strain to the tensile test piece, peeling off the oxide scale on the front side with an adhesive tape, and measuring the peeling amount of the oxide scale by ICP analysis. Was.

The evaluation of the coating adhesion of the hot-dip galvanized steel sheet is carried out based on the mechanical properties of the hot-dip galvanized steel sheet and the steel strip; bendability. Good, slightly inferior,
Inferior).

Table 2 also shows the oxide scale thickness of the original plate, the oxide scale peeling amount, and the results of the plating adhesion test after hot-dip galvanizing. Further, FIG. 1 shows a graph in which the relationship between the evaluation of plating adhesion and the amount of oxide scale peeling of the original plating sheet is arranged.

[0014]

[Table 1]

[0015]

[Table 2]

As is clear from FIG. 1, the plating adhesion is closely related to the amount of oxide scale peeled off from the original plate.
It can be seen that when the amount of peeling is 10 g / m ² or less, preferably 6 g / m ² or less, the plating adhesion becomes good.

Based on the above results, the reasons for limiting the production conditions of the present invention will be described below. Regarding the hot rolling conditions, after finish rolling, the material is cooled to 650 ° C. or less at an average cooling rate of 50 to 150 ° C., and then wound. In order to increase the adhesion of the oxide scale to the original plate by setting the amount of the oxide scale peeled to 10 g / m ² or less, the cooling rate is set to the time required for exposure to the atmosphere, and furthermore to 65%.
The time for maintaining the temperature of 0 ° C. or higher is preferably as short as possible, and the cooling rate is preferably as fast as possible.
In the present invention, a speed exceeding 50 to 1 is not practical.
50 ° C./s, preferably 60 to 120 ° C./s.

The production process from continuous casting to hot-rolling finishing includes direct rolling (HDR), hot strip insertion rolling (HC
R), reheating rolling (HRR), etc., and the slab heating temperature is not particularly limited.
1200 ° C. is preferred. The finishing temperature of hot rolling is not particularly limited, but is preferably at least _three points in order to ensure workability and uniformity of the material, and further improve scale adhesion and workability due to grain refinement. In order to improve the temperature, it is desirable that the temperature be between Ar ₃ point and (Ar ₃ point + 50) ° C., particularly just above the Ar ₃ point.

In the reduction treatment before plating, the surface of the oxide scale layer must be made of pure iron having good wettability with the hot-dip plating metal in order to secure the adhesion between the oxide scale and the plating layer. Therefore, the reduction temperature is more than 300 ° C,
Preferably it is 500-800 degreeC. Pure iron produced by reducing the surface of the oxide scale is effective even if it is very thin, and the holding time at the reduction temperature is not particularly limited. It may be about 80 seconds. Further, the type of the reducing atmosphere gas is not particularly limited.

If necessary, a crack is introduced into the oxide scale layer by applying a means for imparting distortion to the oxide scale, such as skin pass rolling or a leveler, before plating, and the plated molten metal passes through the crack. Since it is in direct contact with iron, plating adhesion is further improved by the so-called anchor effect.

Regarding the steel composition of the hot-rolled steel sheet as the plating base sheet, since the oxide scale adheres to the surface of the steel sheet, the influence of the steel composition on the plating property is small and is not particularly limited. %) Is shown below. C is 0.
It is preferably 15% or less, and when workability of the original sheet is desired, an extremely low carbon steel is preferable. Si is an element whose component cost is relatively low and is effective for obtaining high strength without impairing ductility. When plating an acid-washed plate, it is difficult to add a large amount because the surface concentration of Si deteriorates the plating property. However, in the present invention, since the oxide scale adheres without pickling, it is 1.5% or less. It can be added according to the required strength. Since Mn enhances the hardenability of the steel sheet and easily obtains a low-temperature transformed structure with high strength, Mn can be added at 2.0% or less according to the required strength. P and S are not particularly limited, but are preferably limited to 0.005% or less when high strength and high ductility are required. Al is added for the purpose of deoxidizing molten steel. The content may be 0.10% or less, preferably 0.005 to 0.040%.

In addition to the above main components, the following components can be contained as required. Ti, Nb, and V form carbonitrides and are effective in increasing the strength. In the case of extremely low C steel, ductility is particularly improved. Each alone or in combination,
A total range of 0.005 to 0.070% is preferable. Other elements: Cr ≦ 1.0%, Ca ≦ 0.010
%, Cu ≦ 0.5% and B ≦ 0.005% do not affect the workability, so they can be added alone or in combination as needed.

Although the present invention does not limit the thickness of the plating base plate, thin and ultra-thin plates having a thickness of not more than 2.5 mm improve the adhesion of the original plate to the oxide scale and, consequently, the plating adhesion. Is desirable. It is considered that the reason why the oxide scale adhesion is improved is that the thinner the plate thickness, the faster the average cooling rate after hot rolling, and the thinner the oxide scale thickness. Further, when the sheet thickness is reduced, it is possible to commercialize as a substitute for a hot-dip galvanized steel sheet using a cold-rolled steel sheet as a plating base sheet, which can contribute to cost reduction and improvement in productivity.

Further, after plating is completed, required characteristics,
Depending on the application, one or more surface treatments such as alloying treatment, skin pass, chromate treatment, and painting may be applied.

[0025]

EXAMPLE After steel having the chemical components shown in Table 3 was melted in a converter to form a slab, the slab was heated at a temperature of 1150 ° C. during hot rolling. The coil was 0.8 mm thick. Except for some samples, after subjecting them to skin pass rolling, samples of materials for the original plate, oxide scale thickness, oxide scale adhesion test specimens, and test specimens for galvanizing in the laboratory were collected. did. The plating conditions were as follows: after the reduction treatment shown in the table, a hot-dip galvanizing treatment (non-alloying treatment) was performed at 460 ° C. Tests and evaluation methods such as the adhesion of the oxide scale of the original plate and the plating adhesion after plating are the same as those described above. Steel type A, D, E, F are mild steel, steel grade B is 390 N / mm ² grade, steel grade C is 440 N / mm ² grade, the grades G is a ^secondary strength steel sheet 640 N / mm.

[0026]

[Table 3]

[0027]

[Table 4]

From Table 4, it can be seen that the cooling conditions after hot rolling and the reduction conditions of the oxide scale satisfy the conditions of the present invention, and that the steel sheet having an oxide scale peeling amount of the original plate of 10 g / m ² or less has a plating adhesion after plating. It turns out that it is excellent. Table 4 also shows the results of the sheet thickness change materials. For example, as is clear from samples Nos. 1 and 7, and Nos. 9 and 14, the smaller the sheet thickness, the smaller the amount of oxide scale peeling and the better the plating adhesion. It can be seen that it has improved.

On the other hand, even if the hot rolling conditions and the reducing conditions are within the range of the present invention, it is understood that the steel sheet having an oxide scale peeling amount of more than 10 g / m ² has poor plating adhesion. Also,
The scale peeling amount is 10 even with a steel plate with a thin oxide scale thickness.
It can be seen that the plating adhesion is inferior for those exceeding g / m ² (for example, Sample Nos. 4, 22).

[0030]

As described above, according to the method for producing a hot-dip galvanized steel sheet of the present invention, 50 to 15 after finish rolling is performed.
After cooling to 650 ° C. or lower at a cooling rate of 0 ° C./s, winding is performed to obtain a hot-rolled original sheet having an average peeling amount of the oxide scale adhered to the front and back surfaces of the hot-rolled steel sheet of 10 g / m ² or less.
Since this is used as a plating base plate, excellent plating adhesion is obtained, the production cost is low, and the productivity is excellent.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a graph showing the relationship between the amount of oxide scale peeling of plating base plates having different tensile strengths and the plating adhesion after plating.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masatoshi Iwai 1 Kanazawa-cho, Kakogawa-shi, Hyogo Kobe Steel, Ltd. Inside the Kakogawa Works

Claims (2)

[Claims] 1. A steel slab is hot-rolled and after finishing rolling,
After cooling to 650 ° C. or lower at a cooling rate of 150 ° C./s or less, by winding, a hot-rolled original sheet having an average peeling amount of oxide scale adhered to the front and back surfaces of the hot-rolled steel sheet of 10 g / m ² or less was obtained. Hot-rolled zinc sheet having excellent plating adhesion, comprising subjecting at least the surface of the oxide scale to a reduction treatment in a reducing gas atmosphere of 300 ° C. or more without pickling the hot-rolled original sheet, and then performing hot-dip galvanizing. Production method of base plated steel sheet. 2. The method for producing a hot-dip galvanized steel sheet having excellent plating adhesion according to claim 1, wherein the reduction treatment of the oxide scale is performed without pickling after the hot-rolled sheet is skin-pass rolled.