JPH0570920A - Production of high si-containing high tensile strength galvannealed steel sheet good in low temperature chipping resistance, pitting corrosion resistance and spot weldability - Google Patents

Abstract

PURPOSE:To provide a method for manufacturing a high Si-contg. high tensile strength galvannealed steel sheet good in low temp. chipping resistance and pitting corrosion resistance. CONSTITUTION:A steel sheet contg. Si in the range of >=0.2% is oxidized by a direct fire furnace with 0.9 to 1.2 combustion air ratio and is thereafter reduced by a reducing furnace contg. hydrogen so as to regulate the thickness of an iron oxide film to the range of 200 to 1000Angstrom . Subsequently, the steel sheet is subjected to hot dipping treatment by using a zinc bath constituted of 0.01 to 2% Mn, 0.01 to 2% Al and the balance Zn and is subjected to heating and alloying treatment, and after that, at least one side of the steel sheet is furthermore coated with an organic film in the range of 0.1 to 4.0g/m<2>. In this way, a high Si-contg. high tensile strength galvannealed steel sheet good in low temp. chipping resistance, pitting corrosion resistance and spot weldability can be obtd.

Description

Detailed Description of the Invention

[0001]

The present invention relates to low temperature chipping resistance,
The present invention relates to a method for producing a high Si-containing high-strength hot-dip galvanized steel sheet having good puncture corrosion resistance and spot weldability.

[0002]

2. Description of the Related Art In recent years, the weight of automobile bodies has been actively reduced from the viewpoint of exhaust gas regulations. For this reason, the use of high-strength steel sheets containing Si is being studied.
However, when a high Si content material is used as the outer plate as the high tensile steel plate, there are the following two problems. One is the problem of poor plating property in hot dip plating. As a conventional technique for improving the plating property, JP-A-55-122865
According to the publication, after oxidation in a direct-fired furnace with a combustion air ratio of 0.9 to 1.2, the oxide film thickness is 200 to 10 in a reduction furnace containing hydrogen.
After reduction to the extent of being in the range of 00Å, the steel sheet remains after being subjected to hot dip plating treatment using a zinc plating bath consisting of 0.01 to 2% of Mn, 0.01 to 2% of Al, and the balance Zn. There is a method of reducing the oxide film of Mn and Al in the bath. In this method, since the iron oxide film exists on the surface of the steel plate until the plate is immersed in the plating bath, S
The surface concentration of i can be completely suppressed, and a steel sheet having very good plating adhesion can be obtained. Another problem is the corrosion resistance at the door of the car body and the hem of the hood.
That is, since the electrodeposition coating is not sufficiently performed on the vehicle body door and the hem portion of the hood portion, higher bare corrosion resistance is required. It is effective to increase the coating weight in order to improve the bare corrosion resistance, but increasing the coating weight increases the weight of the vehicle body and contradicts the purpose of using the P-based high-tensile steel material to reduce the vehicle body weight. It is inappropriate to do so.

[0003]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention As a result of intensive studies on plating treatment of high-strength steel sheet, the following findings were obtained. Although the steel sheet having good plating adhesion can be manufactured by the conventional technique, the bare corrosion resistance is not sufficient. In particular, there is a problem with perforation corrosion resistance caused by insufficient electrodeposition coating on the door of the vehicle body and the hem of the hood, and there is a strong demand for this improvement. However, since the weight of the steel sheet increases as the coating weight is increased in order to improve the corrosion resistance, there is no point in using a high-strength steel sheet to reduce the weight of the vehicle body. Further, Al plating can be considered as a method for improving the corrosion resistance without increasing the weight of the steel sheet, but the Al plating has a problem of poor spot weldability. Therefore, the method of the present invention has a high Si-containing high-strength alloy having good plating adhesion of the prior art and at the same time excellent corrosion resistance, and also having a good spot weldability while suppressing an increase in steel plate weight. The present invention has been completed based on this finding by discovering a method for producing a galvanized steel sheet.

[0004]

SUMMARY OF THE INVENTION The gist of the present invention is to provide a steel plate containing Si in a range of 0.2% or more with a combustion air ratio of 0.9.
After oxidation in an oxidation furnace of 1.2 to 1.2, reduction is performed so that the surface iron oxide film thickness is in the range of 200 to 1000Å in a reduction furnace containing hydrogen, and then the steel sheet is Mn 0.01 to 2%, Al 0.01
Hot galvanizing treatment is performed by using a galvanizing bath consisting of ˜2% and the balance Zn, and at least one surface of the steel sheet is further coated with an organic coating in the range of 0.1 to 4.0 g / m ^2. High Si with good low temperature chipping resistance, puncture corrosion resistance and spot weldability
It is a method for producing a high-strength alloyed hot-dip galvanized steel sheet.

[0005]

The present invention will be described in detail below. The method of the present invention is S
In order to suppress the surface concentration of i, an iron oxide film is formed in the direct furnace. However, the direct-fired furnace or the combustion air ratio of the direct-fired furnace needs to be 0.9 or more in order to form an iron oxide film sufficient to suppress Si. 0.9 ~ because the thickness is too thick to reduce
It should oxidize in the range of 1.2. In the reduction furnace, the oxide film is reduced to an extent of 200 to 1000Å, and the remaining oxide film is reduced by Al and Mn in the bath in the plating bath. However, both Mn and Al in the bath are required to be 0.01% or more in order to reduce the iron oxide film, and if it exceeds 2%, the alloying rate at the time of heat alloying decreases, so 0.01
Should be in the range of ~ 2%. Heating alloying temperature is 50
The range of 0 ° C to 900 ° C is preferable.

Further, an organic film is applied to at least one surface of the steel sheet. Here, the organic film is an epoxy resin, polyethylene, polystyrene, polyacetylene, polyester,
A layer formed of an organic compound such as nylon or polyimide. The presence of the organic film can suppress adhesion to the mold and improve flaking resistance. The amount of the organic film deposited is 0.1 to 4.0 g / m ² . If it is less than 0.1 g / m ² , it is difficult to completely cover the lower plating layer, and die adhesion from the exposed lower layer may occur, which is not preferable. When it exceeds 4.0 g / m ² , it is not preferable because the rust-preventive steel sheet for car bodies, which is the main application of galvannealing, tends to deteriorate the electrodeposition coatability which is an important required property. As the resin for the organic film, an epoxy resin having excellent adhesion to the steel sheet is preferable, but the surface of the galvannealed steel sheet is rich in irregularities, and any resin has no problem in adhesion. The presence of each pigment additive such as silica in the organic film, which may be either solvent-based or water-based, is effective as long as the lubricity is not impaired. Also, addition of wax is preferable because it improves lubricity.

Further, chemical conversion treatments such as phosphate treatment and chromate treatment (electrolytic type chromate, reactive type chromate, coating type chromate) as a pretreatment for improving the adhesion of the organic film are effective. It falls into the category of invention law. Furthermore, the presence of the organic film on the surface of the steel sheet increases the resistance value of the welded portion, and the welding current is dispersed appropriately, so that the appropriate welding current range is widened and the spot weldability is improved.

[0008]

Next, the method of the present invention will be described in detail with reference to Examples. Examples are shown in Tables 1 and 2. The test method is as follows. (1) Corrosion resistance test A salt water spray test according to JIS Z 2371 was continuously performed for 2000 hours to carry out a comparative investigation of sheet thickness reduction amounts. ◎: Minimal reduction in plate thickness ○: Small reduction in plate thickness △: Large reduction in plate thickness (2) Low temperature chipping resistance -20 ° C, pressure of 2 kg / cm at 100 g of No. 7 crushed stone
The peeled area when the tape was peeled off after the gravero chipping test in ² was measured and evaluated according to the following criteria. (Good) ◎, (5%)-○, (15%)-△, (30%)
-×, (50% <) (poor) (◎, ○ are virtually no problems)

(3) Plating Adhesion Test The plating adhesion was evaluated by a powdering test by bending at 60 ° V. ◎: Peeling amount 0 mm ◯: Peeling amount more than 0 mm, 1 mm or less △: Peeling amount more than 1 mm, 3 mm or less ×: Peeling amount more than 3 mm (4) Spot weldability test Secondary weldability current: 8 KA to 12 KA Pressurizing force: 250 kg Energization Time: 10 cycles Welding interval: 1 point / 3 seconds Electrode tip: Cu-Cr alloy, C, F type tip diameter
6 mmφ Evaluation method ◎: number of welding points 10000 points or more ○: number of welding points 4000 points or more 10000 points or less ×: number of welding points 4000 points or less

In Table 1, 1 to 5 are those in which the Si concentration in the steel sheet is changed to 0.1 to 5.0%, and 6 to 9 are those in which the temperature of the inner plate of the direct furnace is changed to 500 to 800 ° C. , 10-1
1 is the one in which the air ratio in the direct furnace is changed to 0.95 to 1.2, 12 to 14 is the one in which the reduction furnace plate temperature is changed to 500 to 800 ° C., and 15 to 16 is the reduction time in 30 seconds to What was changed to 40 seconds, 17-18, the hydrogen concentration of the reduction furnace was 20
To 23%, 19 to 23 are Al concentrations in the bath, and 24 to 30 are Mn concentrations in the bath. 31
Numerals 33 to 33 are those in which the maximum plate temperature of the heating alloying furnace is changed to 500 to 700 ° C., and 34 to 39 are those to which the temperature rising rate of the heating alloying furnace is changed to 5 to 60 ° C./sec. 40 is for HOT material. 41 to 46 are those in which the organic material of the organic film is changed, 47 to 54 are those in which the amount of the organic film attached is changed, 55
.About.59 are comparative examples. Since 55 is not subjected to chemical conversion treatment, 56 is because Mn is not added to the plating bath.
Poor plating adhesion and therefore poor corrosion resistance. In No. 57, since Al was not added to the plating bath, the adhesion of the plating was poor and the corrosion resistance was also poor. In No. 58, since the combustion air ratio in the direct furnace is 0.8, iron oxide is not generated in the oxidizing furnace,
During the annealing process, Si is concentrated on the surface of the steel sheet, and the plating adhesion is poor, so the corrosion resistance is also poor. No. 59 has poor corrosion resistance because it does not have an organic film formed on the outermost surface of the steel sheet.

[0011]

[Table 1A]

[0012]

[Table 1B]

[0013]

[Table 1C]

[0014]

[Table 1D]

[0015]

According to the present invention, the following effects can be obtained. It is possible to obtain a high-Si-containing, high-strength galvannealed steel sheet having excellent corrosion resistance and weldability while maintaining good conventional plating adhesion.

Front page continuation (51) Int.Cl. ⁵ Identification code Office reference number FI Technical display location C21D 1/52 D 9269-4K 1/76 G 9269-4K 9/46 U 7356-4K P 7356-4K C23C 2 / 06 9270-4K 2/26 9270-4K 2/28 9270-4K 2/40 9270-4K (72) Inventor quote Masaji 1 Kimitsu, Kimitsu-shi, Chiba Nippon Steel Corporation Kimitsu Steel Co., Ltd.

Claims (1)

[Claims] 1. A steel sheet containing Si in an amount of 0.2% or more is oxidized in an oxidizing furnace with a combustion air ratio of 0.9 to 1.2, and then a surface iron oxide film thickness is 200 in a reducing furnace containing hydrogen. ~ 1000Å
Of the steel sheet, the steel sheet was reduced to Mn 0.01
~ 2%, Al 0.01 ~ 2%, hot-dip galvanizing bath using Zn, and heat-alloying treatment, and then an organic coating on at least one side of the steel sheet.
A method for producing a high Si-containing high-strength hot-dip galvanized steel sheet having good low-temperature chipping resistance, puncture corrosion resistance, and spot weldability, which is characterized by coating in the range of 1 to 4.0 g / m ² .