JPH09209104A - Production of galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a galvanized steel sheet provided with a thick film excellent in low temp. impact durability and to provide a method for producing a galvannealed steel sheet. SOLUTION: (1) A steel sheet subjected to pickling treatment by a pickling soln. in which the concn. of one or two kinds of Ni and Cu is regulated to 10 to 1000ppm after hot rolling is produce or a steel sheet furthermore subjected to cold rolling is subjected to galvanizing. (2) The galvanized steel sheet obtd. by the above method (1) is moreover subjected to alloying treatment.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet (hereinafter, both are collectively referred to as hot-dip galvanized steel sheet) having a coating film excellent in low-temperature impact resistance. The present invention relates to a manufacturing method, and more particularly, to a method for manufacturing a hot dip galvanized steel sheet suitable as a material for automobiles, building materials, steelware, home appliances and the like.

[0002]

2. Description of the Related Art In recent years, as automobiles, home electric appliances and the like have become more sophisticated, there has been an increasing demand for a hot-dip galvanized steel sheet which exhibits a higher rust-preventing ability for a longer period than before.

In order to provide a hot-dip galvanized steel sheet with a high rust-preventing ability for a long period of time, it is necessary to take measures to increase the thickness of the plating film to an amount of 60 to 100 g / m ² . By this measure, under normal conditions, long-term rust preventive ability can be provided. However, when the steel sheet is exposed to a low temperature environment such as 0 ° C. to 50 ° C. below zero in a cold region, the adhesion of the plating film to an impact load, that is, the low temperature impact resistance tends to deteriorate. In particular, when hot-dip galvanized steel sheets are used as outer shells of automobiles, there is a problem that the plating film peels off or falls off due to impact at low temperatures, which deteriorates the rust preventive ability and impairs the appearance. . Therefore, there is a strong demand for the development of a hot-dip galvanized steel sheet having a thick plating film and excellent low temperature impact resistance.

Generally, the reason why the low temperature impact resistance (adhesion) of the galvanized steel sheet is low at the interface between the galvanized layer and the base steel sheet (hereinafter simply referred to as the base material). This is because the -Zn alloy phase is hard and brittle. In particular, since the Fe-Zn alloy phase becomes brittle at low temperatures, the low temperature impact resistance tends to deteriorate. The reason why the low temperature impact resistance of the galvannealed steel sheet is low because the alloying reaction proceeds excessively during the alloying treatment, and the boundary between the galvanized layer and the base metal is fragile. This is because the Γ phase (a kind of Fe-Zn alloy phase) is generated.

Therefore, in the case of hot dip galvanized steel sheet, F is used as a measure for improving the low temperature impact resistance of the plating film.
Efforts are being made to suppress the excessive generation of the e-Zn alloy phase, and for the galvannealed steel sheet, the generation of the Γ phase.

As a concrete measure against these, the following method is adopted.

[0007] Regarding the improvement of the low temperature impact resistance of the galvanized steel sheet, it is possible to suppress the formation of Fe-Zn alloy phase at the interface between the galvanized sheet and the base material by using Fe-Z.
A general measure is to increase the amount of n-Al ternary alloy phase produced. Therefore, A in the hot dip galvanizing bath is usually used.
A plating method for increasing the content rate of 1 is adopted. However, the presence of the Fe-Zn-Al-based alloy phase has a drawback in that the aging phenomenon occurs in which the adhesion of the plating film deteriorates over time, and the plating film is likely to peel off.

Regarding measures for improving the low temperature impact resistance of the galvannealed steel sheet,
A method of optimizing the chemical composition of the base material and a method of controlling the alloying process are adopted.

One of the methods for optimizing the chemical composition of the base material is to increase the P content so that Fe--
There is a method of suppressing the excessive progress of the Zn alloying reaction and preventing the formation of a brittle Γ phase. Also, as another means, T
There is also a method of limiting the content of an element that promotes the Fe—Zn alloying reaction such as i and Mn to suppress the generation of the Γ phase. However, controlling the chemical composition of the base material reduces the range of selection of mechanical properties of the steel sheet, and thus has a problem of limiting the applications of the steel sheet.

On the other hand, as a method for controlling the alloying process, for example, there is a method in which the alloying temperature is kept lower than usual so that the structure of the plating film is a δ1 single phase with rich ductility. However, in order to obtain such a phase, it is necessary to slow down the speed of the production line, and it is also necessary to optimize the control conditions of the line according to the material of the base material. Therefore, operating conditions that can obtain a δ1-phase single-phase plating film are difficult to apply to commercial production.

The conventional measures for improving the adhesion between the plating film and the base material at a low temperature are as described above, but the following methods have been proposed as measures for improving the adhesion at room temperature. This method is characterized in that before the plating treatment, the surface of the steel sheet is thinly coated with pure metal, alloy, nonmetal, etc., and then hot-dip galvanizing is performed. For example, Japanese Unexamined Patent Publication No. 62-139860 discloses a method of performing hot dip galvanizing after coating the surface of a steel sheet with P by a chemical plating method or the like. According to this method, an Al-P compound phase is generated at the boundary between the plating film and the base material, and this phase improves the adhesion. However, the hot-dip galvanized steel sheet obtained by this method is, compared with the steel sheet obtained by the conventional method not coated with P,
Although the adhesion of the plating film near room temperature is improved,
No effect of improving adhesion to impact under low temperature environment is observed. Moreover, the effect on the alloyed hot-dip galvanized steel sheet is similarly not recognized.

As described above, the hot dip galvanized steel sheet does not impair other properties such as the mechanical properties of the base metal, and is economical and economical on an industrial scale at low temperatures. In reality, it is extremely difficult to improve the sex.

[0013]

DISCLOSURE OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and is a hot-dip galvanized steel sheet having a large plating film thickness and excellent low-temperature impact resistance, and It is an object of the present invention to provide a method for manufacturing a galvannealed steel sheet.

[0014]

Means for Solving the Problems As a result of research and development on measures for improving low temperature impact resistance of a galvanized galvanized steel sheet, the present inventors have obtained the following findings.

"When hot dip galvanizing is performed after pickling treatment with a pickling solution containing one or two of predetermined concentrations of Ni and Cu, the low temperature impact resistance of the plating film is remarkably increased. The present invention is based on the above findings, and has the following gist (1) and (2).

(1) After hot rolling, the concentration of any one or two of Ni and Cu is 10 to 1000 ppm.
A method for producing a hot-dip galvanized steel sheet having a plating film excellent in low-temperature impact resistance, which comprises hot-dip galvanizing a steel sheet that has been pickled with the pickling solution or cold-rolled steel sheet.

(2) A method for producing an alloyed hot dip galvanized steel sheet, wherein the hot dip galvanized steel sheet obtained by the method (1) is further alloyed.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The base steel sheet used in the method of the present invention is mainly a plain steel and a plating steel containing necessary alloying elements.

When a hot-dip galvanized steel sheet is produced by the method of the present invention, a steel sheet as a base material is first produced. Processes such as refining and rolling may be carried out according to manufacturing equipment and manufacturing methods that are usually used industrially. Since the steel sheet of the base material targeted by the method of the present invention is mainly a thin steel sheet, generally, a hot rolled steel sheet is manufactured by first hot rolling a slab, annealing, and descaling treatment by pickling. After that, cold rolled steel sheet is manufactured by cold rolling, and if necessary, annealing,
It is manufactured in the process of descaling.

The hot dip galvanized steel sheet is usually prepared by preheating the above-mentioned base material in a weakly oxidizing atmosphere, and then heating or annealing it in a reducing atmosphere consisting of a mixed gas of hydrogen and nitrogen, for example, up to near the plating temperature. After cooling, it is manufactured by a method of immersing the base material in a hot dip galvanizing bath. Further, the galvannealed steel sheet is obtained by further heating the galvanized steel sheet at a temperature of 500 to 600 ° C. for 3 to 30 seconds to form an Fe—Zn alloy phase at the interface between the plating film and the base material. Manufactured.

When a thin steel sheet is manufactured by omitting cold rolling, hot-dip galvanizing may be performed immediately after hot rolling and pickling.

In the case of the method of the present invention, Ni and C are used during the pickling treatment of the hot rolled steel sheet in the above-mentioned ordinary manufacturing process.
The concentration of any one or two of u is 10 to 10
A 00 ppm pickling solution is used. The reason for setting these concentrations within the above range is as follows.

If it is less than 10 ppm, the effect of improving the low temperature impact resistance of the plating film cannot be obtained. On the other hand, 10
This is because if it exceeds 00 ppm, the pickling ability is extremely lowered, so that the scale layer remains on the surface of the steel sheet and the plating does not adhere after the hot dip galvanizing, so-called non-plating defects occur. Since Ni and Cu have almost the same action, it is sufficient that at least one of them is contained. If both are included, the total is 10-100
It should be 0 ppm.

The acid used in the pickling solution may be any acid capable of descaling the base material, but hydrochloric acid which is usually used (concentration of about 5 to 15% by weight) is suitable. Further, Ni or Cu may be added to the pickling solution by a method of adding with a salt such as a sulfate or a method of dissolving these simple metals or alloys in the pickling solution. The temperature of the pickling solution is preferably 50 ° C or higher from the viewpoint of increasing the pickling rate.

The steel sheet (base material) after pickling is subjected to hot dip galvanizing in the state of hot-rolled and dried hot-rolled steel sheet or the state of cold-rolled steel sheet cold-rolled to a predetermined thickness. . In this case, the annealing treatment may be performed.

The mechanism of the effect of the pickling treatment of the method of the present invention is not clear, but it is presumed as follows. In a steel containing P inevitably like a steel plate, a scale layer having a two-layer structure is formed on the surface during heating of the slab before hot rolling or during hot rolling. That is, the upper layer is a Fe oxide layer, and the lower layer (between the Fe oxide layer and the base material) is a scale layer having a two-layer structure of an Fe-P composite oxide layer. This is because the alloying element P, which is easily oxidized even under the Fe oxide layer having a low oxygen potential, is oxidized.
This reaction cannot be avoided even if the heating conditions are controlled. In the case of a normal pickling treatment, such a scale layer on the surface of the base material is almost completely dissolved and removed in the pickling solution, so that the subsequent steps are not affected.

However, since Ni or Cu is added to the pickling solution used in the method of the present invention, the descaling ability of the pickling solution is somewhat lowered. for that reason,
Even after pickling, a part of the Fe-P composite oxide layer tends to remain on the surface of the base material. This Fe-P composite oxide layer reacts with the Fe-Al alloy phase generated at the boundary between the plating film and the base material when the base material is immersed in the hot dip galvanizing bath, and Fe-Al is newly added to the interface. A P-based ternary alloy layer is formed. It is highly possible that the morphology, structure, etc. of this alloy layer have some influence on the improvement of the low temperature impact resistance of the plating film. Therefore, it can be considered that the effect of the pickling of the present invention is not the effect of so-called pre-plating treatment in which the coating layer is previously formed on the surface of the base material.

Further, the effect of the pickling treatment of the present invention for improving low-temperature impact resistance is particularly remarkable in the chemical composition of the base material and its content rate and the Al content rate in the commonly used hot dip galvanizing bath. Not affected. In addition, the effect is exhibited in both hot dip galvanizing and alloying hot dip galvanizing.

For hot dip galvanizing, it is suitable to utilize a Sendzimer type continuous hot dip galvanizing line or the like, but other processing means such as batch type conventionally used may be used. Note that the dew point of the atmospheric gas during the reduction treatment that is usually performed before the plating treatment depends on the Fe exposed on the surface of the base metal.
The temperature is preferably -40 ° C or higher so that the -P complex oxide layer can be brought into the hot dip plating bath in the form of the oxide.

As hot-dip galvanizing, in addition to normal hot-dip galvanizing, galvalume plating in which a base material is immersed in a Zn bath containing about 55% by weight of Al, and a base material is immersed in a Zn bath containing about 5% by weight of Al Galvan plating is also targeted, and the low-temperature impact resistance can be improved even for the coating films of these hot-dip galvanized steel sheets.

The amount of hot-dip galvanizing applied is not particularly limited, but the effect of the present invention becomes remarkable when it is 20 g / m ² or more per one side, so 20 g / m ² or more is desirable.

[0032]

EXAMPLES The effects of the present invention were confirmed by the following tests.

(A) Test Method Test Material Table 1 shows the chemical composition of the test material (hot rolled steel sheet) used in the test. The hot-rolled steel sheet is not descaled and has a scale and a thickness of 5 mm. This hot rolled steel sheet was subjected to pickling treatment by immersing it in a 10 wt% hydrochloric acid aqueous solution at a temperature of 60 ° C. in which a predetermined concentration of Ni or Cu was added as a chloride. The pickling was performed until no scale was visually observed. The scale was removed by this pickling, followed by washing with water and drying. Further, it was cold rolled to a thickness of 0.8 mm to obtain a test material. As a comparative example, the cold-rolled steel sheets of the test materials A and C shown in Table 1 were compared with those of JP-A-62-1398.
A test material coated with 50 mg / m ^{2 of} P by Fe-P alloy plating disclosed in Japanese Patent No. 60 was also used for the test.

[0034]

[Table 1]

Plating and Coating The above test materials were washed with water, dried, and then heated in a weakly oxidizing atmosphere at 500 ° C. for 30 seconds, and then in a mixed gas atmosphere of nitrogen and hydrogen at 760 ° C. for 60 seconds. .

Thereafter, the Al content of about 0.10 wt.
The test material was dipped in a hot dip galvanizing bath maintained at 0 ° C. for 3 seconds, and hot dip galvanizing was performed so that the adhesion amount per one surface was 60 g / m ² . This adhesion amount corresponds to a thick plating film. Further, some of the test materials were subjected to hot dip galvanizing, followed by heating at 500 ° C. and alloying treatment until the plated surface had no metallic luster.

After the hot-dip galvanized and further alloyed test material was cut to a length of 150 mm and a width of 70 mm, zinc phosphate treatment (Bt3020 treatment by Nippon Parkerizing Co., Ltd.) was performed on the plating film on one side of the test material. Liquid), cationic electrodeposition coating (Nippon Paint Co., PT-U)
80, using a coating thickness of 30 μm, intermediate coating and top coating (using Kansai Paint Co., Rugerbake, coating thickness 7)
0 μm).

Regarding hot dip galvanizing, some test materials were galvanized and galvalume plated.

With respect to the test materials of the hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, which were coated in the low temperature impact resistance test, the low temperature impact resistance test was conducted by the following method.

For the test material placed on the test stand, -4
Diamond particles (diameter about 3 mm) in a low temperature environment of 0 ° C
Was impacted under the condition of colliding with 10 places at a speed corresponding to 120 km / h. After that, once a month, the test material is immersed in a 3 wt% NaCl aqueous solution for 30 minutes, and then exposed to the atmosphere in an industrial area.
It continued for a year. After the above test was completed, the maximum diameter of the coating film blister at the collision point of diamond grains was measured to evaluate the low temperature impact resistance.

The low temperature impact resistance was evaluated by the presence or absence of coating film blisters and the maximum diameter.

The evaluation categories were the following four ranks. Out of the following 4 ranks, when the occurrence of coating film blister was not recognized or the maximum diameter of the blister was less than 3 mm, that is, ∘, ∘ or Δ mark, the low temperature impact resistance was considered to be good.

⊚ (Remarkably good): No occurrence of blisters was observed at all ○ (Very good): Maximum blister diameter was less than 1 mm △ (Good): Maximum blister diameter was between 1 mm and less than 3 mm X (Inferior): The maximum diameter of the blister is 3 mm or more. (B) Test results Table 2 shows the test results. Test No. 1 to 13 are examples of the present invention and test No. 14 to 18 are comparative examples, test No. 19 and 20 are the results regarding the conventional example.

[0044]

[Table 2]

In the case of the present invention in which the concentration of either Ni or Cu in the pickling solution was 20 to 800 ppm and the total concentration of both Ni and Cu was 200 ppm, any of the test materials B, C and D was used. Also, regarding the hot dip galvanizing and the alloying hot dip galvanizing (Test Nos. 1 to 11), the occurrence of blisters was not observed at all, and the low temperature impact resistance was extremely good. In addition, among the examples of the present invention, the test No. performed by galfan plating was used. Test No. 12 for galvalume plating Regarding No. 13, the low temperature impact resistance of the plating film was good as in the case of normal hot dip galvanizing.

On the other hand, the pickling solution contains Ni or Cu.
No. was not added. In the case of Comparative Examples 14, 15 and 18, blisters having a maximum diameter of 3 mm were generated. Further, the test No. in which the concentration of Ni or the total concentration of both Ni and Cu exceeds 1000 ppm. In Comparative Examples 16 and 17, the low temperature impact resistance was good, but an unplated portion was generated during plating, which was not good as a hot-dip galvanized steel sheet.

Test No. using a test material coated with P 1
The conventional examples of Nos. 9 and 20 were inferior in low temperature impact resistance and could not satisfy the object of the present invention.

From the above results, according to the method of the present invention,
It was confirmed that a hot-dip galvanized steel sheet having a plating film excellent in low temperature impact resistance can be obtained even when the plating film thickness is as thick as about 60 g / m ² .

[0049]

The galvanized steel sheet and the galvannealed steel sheet obtained by the method of the present invention have excellent durability against impact loads at low temperatures. Further, the effect does not change even if the thickness of the plating film is increased. Therefore, the steel sheet produced by the method of the present invention has a feature that it can significantly prolong the life of the product when used as a material for outer panels of automobile bodies, home appliances, etc., and is extremely industrially useful. Is.

Claims (2)

[Claims] 1. One of Ni and Cu after hot rolling.
Steel sheet or steel sheet cold-rolled with a pickling solution having a concentration of 10 to 1000 ppm in total of two kinds or two kinds is subjected to hot dip galvanization and is excellent in low-temperature impact resistance. A method for producing a hot-dip galvanized steel sheet having a plating film. 2. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises subjecting the hot-dip galvanized steel sheet obtained by the method according to claim 1 to an alloying treatment.