JP3133189B2 - Method for producing hot-dip galvanized steel strip - Google Patents

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 steel strip obtained by subjecting a hot-rolled steel strip to hot-dip Zn plating (mainly comprising Zn or an alloy of Zn and Al).

[0002]

2. Description of the Related Art Conventionally, hot-dip galvanized hot-rolled steel strip is usually hot-dip galvanized after removing the surface oxide film of the hot-rolled steel strip. That is, the hot-dip Zn-coated hot-rolled steel strip is wound into a coil after hot rolling, is naturally cooled to a temperature of less than 100 ° C. for a long time, and then is cooled to 80 ° C. in a pickling line.
Usually, it was descaled to a degree, and then transferred to a hot-dip plating line and processed. This is because the surface oxide film formed on the surface of the hot-rolled steel strip acts as a film that inhibits hot-dip galvanizing properties, so it was necessary to remove (descaling) it in advance to ensure excellent hot-dip galvanizing properties. That is why.

[0003]

However, by performing a descaling process such as pickling (providing a descaling step), the operation cost is increased, and the initial capital investment cost is also negligible. Accordingly, the present invention provides a method for producing a hot-dip galvanized steel strip which can omit a descaling process for the purpose of reducing operating costs and initial equipment investment costs.

[0004]

SUMMARY OF THE INVENTION The present invention relates to a production method capable of securing excellent hot-dip galvanizing properties by controlling the conditions of reduction treatment without removing a surface oxide film formed on the surface of a hot-rolled steel sheet. is there. That is, the gist of the present invention is as follows. After performing a reduction treatment satisfying the following formula (1) in a reducing atmosphere gas without removing the surface oxide film of the hot-rolled steel strip, the hot-rolled steel strip is placed in a plating bath.
A method for producing a hot-dip Zn-coated hot-rolled steel strip, wherein the hot-dip Zn-coated steel strip is immersed for 1 to 8 seconds to continuously perform hot-dip Zn plating.

K × t × C−675 × h> 2970 (1) where K: reduction rate parameter K = ｛exp [(2840 + 32 × T) / (540 + R × T)]] / 474000 R 1.98719 (gas constant) T: Reduction treatment temperature (° C.) t: Reduction treatment time (second) C: Hydrogen concentration in reducing atmosphere gas (%) h: Surface oxide film thickness of hot-rolled steel strip (μm) )

[0006]

Conventionally, it has been essential to remove (descaling) a surface oxide film formed on the surface of a hot-rolled steel strip in advance in order to secure excellent hot-dip galvanizing properties. However, the present inventors ensure excellent hot-dip galvanizing properties without removing the surface oxide film formed on the surface of the hot-rolled steel strip by performing an appropriate reduction treatment according to the thickness of the surface oxide film. It was found that it could be done. FIG. 1 shows an example.

The specification conditions are as follows. Surface oxide film thickness: 2.6 μm Reduction treatment temperature: 700 ° C. Reducing atmosphere gas: N ₂ + 75% H ₂ Plating bath composition: Zn + 4.8-5.8% Al

When the reduction treatment time is short, non-plating occurs, the ball impact rating does not reach a pass level, and excellent hot-dip galvanizing property cannot be secured. However, during the reduction treatment time that satisfies the range of the present invention, there is no non-plating, the ball impact rating is at an acceptable level, and excellent hot-dip galvanizing property is secured.

[0009] The required reduction treatment time at which excellent hot-dip galvanizing properties can be ensured varies depending on the thickness of the surface oxide film on the hot-rolled steel strip, the reduction treatment temperature, and the hydrogen concentration in the reducing atmosphere gas. FIG. 2 shows an example. The specification conditions are as follows. Reduction treatment temperature: 700 ° C. Reducing atmosphere gas: N ₂ + 75% H ₂ Plating bath composition: Zn + 4.8-5.8% Al

When the thickness of the surface oxide film and the reduction treatment time satisfy the range of the present invention, the plating property is acceptable (there is no plating, and the ball impact rating is also acceptable), and excellent hot-dip galvanizing property is secured.

Further, according to the present invention, since a porous iron layer is formed in the surface oxide film by the reduction treatment, the wettability with the plating bath is improved, and in the conventional steel strip which is plated after pickling,
Even in the operation range where non-plating occurs (when the temperature of the steel sheet at the time of bath immersion is low, and when the immersion time of the steel sheet in the bath is short), good wettability is exhibited, and no non-plating occurs. In addition, it is also possible to introduce a crack into the surface oxide film of the hot-rolled steel strip by a skin pass or the like in order to ease the conditions for the reduction treatment.

[0012]

EXAMPLE A hot-rolled steel strip having the chemical components shown in Table 1 was produced, and without removing the surface oxide film, the hot-dip Zn plating heat was applied under the reduction treatment conditions and hot-dip plating conditions shown in Tables 2 and 3. Rolled steel strip was manufactured. Table 4 shows the evaluation results of the plating properties of the obtained hot-dip galvanized steel strip.

[0013]

[Table 1]

[0014]

[Table 2]

[0015]

[Table 3]

[0016]

[Table 4]

The wettability was evaluated by visually observing the presence or absence of non-plating. No plating is acceptable, and non-plating is unacceptable. After the ball impact test, the plating adhesion was visually observed for plating adhesion, and a ball impact rating was given. The rating was set to 5 grades, where the rating 1 was that the plating layer had no cracks and peeling, the rating 2 was that there was a crack but there was no peeling, the rating 3 was that there was some peeling, the rating 4 was that the peeling was remarkable, and the rating 5 Is peeled off, and the grades 1 and 2 are acceptable (ball impact test conditions: drop height and weight: 50 cm and 27 kg weight, punch shape: φ10 mm × 5 mm, die shape: φ20 mm). Further, the bare corrosion resistance was evaluated by a salt spray test, which was equivalent to the conventional material plated after pickling.

[0018]

According to the present invention, since the surface oxide film formed on the surface of the hot-rolled steel strip is not removed, there is no non-plating, good plating adhesion can be obtained, and excellent hot-dip galvanizing property can be ensured. , The descaling process can be omitted, and the operating costs and initial capital investment costs for the production of hot-dip galvanized steel strip are greatly reduced.
The effect is extremely large.

[Brief description of the drawings]

FIG. 1 is a table showing a relationship between a reduction treatment time, presence or absence of non-plating, and a ball impact score.

FIG. 2 is a table showing the relationship between reduction treatment time, surface oxide film thickness, and hot-dip galvanizing property.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-6-145937 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. After performing a reduction treatment satisfying the following formula (1) in a reducing atmosphere gas without removing a surface oxide film of a hot-rolled steel strip, the hot-rolled steel strip is plated with a plating bath. During ~
A hot-dip galvanized steel strip, wherein the hot-dip galvanized steel strip is immersed for 1 to 8 seconds in a hot-dip galvanized steel strip. K × t × C-675 × h> 2970 (1) where K: reduction rate parameter K = ｛exp [(2840 + 32 × T) / (540 + R × T)]] / 474000 R: 1. 98719 (Gas constant) T: Reduction treatment temperature (° C) t: Reduction treatment time (second) C: Hydrogen concentration in reducing atmosphere gas (%) h: Surface oxide film thickness of hot-rolled steel strip (μm)