JP5115154B2 - Method for producing high-strength galvannealed steel sheet - Google Patents

Description

The present invention is a steel sheet containing Si relates strength galvannealed steel sheet as a base material steel plate, more particularly a steel sheet containing Si as a base material steel plate, non-coating was excellent without powdering resistance The present invention relates to a method for producing a high-strength galvannealed steel sheet.

  In recent years, surface-treated steel sheets that have been given anti-rust properties to steel sheets, especially hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent anti-rust properties have been used in fields such as automobiles, home appliances, and building materials. ing.

  Generally, a hot dip galvanized steel sheet is a thin steel sheet obtained by hot rolling, cold rolling or further heat treating a slab as a base steel sheet. In a continuous hot dip galvanizing line or the like, the base steel sheet is degreased and / or pretreated in the pretreatment step. Alternatively, it is pickled and washed, or the pretreatment step is omitted and the oil on the surface of the base steel plate is burned and removed in the preheating furnace, and then in a non-oxidizing atmosphere or a reducing atmosphere at about 600 to 900 ° C. After heating to a temperature range and recrystallization annealing, the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere, and a slight amount of Al (0.1 to 0.2 It is manufactured by dip galvanizing by dipping in a galvanized bath to which (mass%) is added. The galvannealed steel sheet is manufactured by galvanizing and subsequently alloying the plating layer in an alloying furnace.

  By the way, in recent years, weight reduction has been promoted along with higher performance of steel sheets, and in response to this, higher strength of steel sheets has been demanded, and high strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip zinc that have rust prevention properties. The amount of plated steel used is increasing.

  Si is a very effective additive element that can increase the strength of steel while ensuring ductility, and has a cost advantage. Therefore, a steel plate containing Si is particularly promising as a high-strength steel plate. However, high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets that use Si-containing steel sheets as a base material have the following problems.

  When manufacturing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet, the base steel sheet is heat-annealed at a temperature of about 600 to 900 ° C. in a reducing atmosphere and then hot-dip galvanized. However, Si in steel is an easily oxidizable element, and is selectively surface oxidized in a reducing atmosphere generally used in the annealing process to be concentrated on the steel sheet surface to form an oxide of Si. Since the oxide of Si lowers the wettability with molten zinc and causes non-plating during the plating process, so that the wettability rapidly decreases and non-plating occurs frequently as the Si content in the steel increases. Become. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor.

  Further, when Si in the steel is selectively surface oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying treatment of the plated layer after hot dip galvanization. As a result, it is difficult to achieve both high productivity and good powdering resistance because the powdering resistance deteriorates when alloying at an excessively high temperature in order to significantly hinder productivity and secure productivity. is there.

  For such problems, it is known that the wettability with molten zinc is improved by performing reduction annealing after heating the steel sheet in an oxidizing atmosphere in advance to form iron oxide on the surface (for example, (See Patent Document 1).

Further, in Patent Document 2, sulfur or a sulfur compound is deposited as an S amount in an amount of 0.1 to 1000 mg / m ² prior to hot dipping, and then a preheating step is performed in a weakly oxidizing atmosphere, followed by non-oxidation including hydrogen. It is disclosed that a high-strength hot-dip galvanized steel sheet having a good plating appearance and a high-strength galvannealed steel sheet having no alloy unevenness and excellent powdering resistance can be obtained by annealing in a neutral atmosphere.
Japanese Patent No. 2587724 Japanese Patent Laid-Open No. 11-50223

  Oxidation does not proceed only by the oxidation method described in Patent Document 1, and it is difficult to obtain an amount of iron oxide necessary for preventing non-plating. Therefore, the suppression of the surface concentration of Si after the reduction treatment is insufficient, the wettability improvement effect with molten zinc is inferior, and the occurrence of non-plating and the reduction of powdering resistance cannot be prevented.

  The method described in Patent Document 2 has a problem of contamination in the furnace due to S and technical difficulty for uniformly attaching S to the surface, and a wettability improvement effect cannot be obtained stably. It is impossible to prevent the occurrence of non-plating and the reduction of powdering resistance.

The present invention has been made in view of such circumstances, and prevents the occurrence of non-plating when producing a high-strength galvannealed steel sheet using a Si- containing steel sheet as a base steel sheet, and has excellent resistance to resistance. An object is to obtain powdering properties.

  The inventors do not prevent the Si oxide from being concentrated on the surface of the steel sheet in the annealing process, but a technique for removing the oxide concentrated on the surface of the steel sheet in the annealing process in the hot dipping process. The present invention has been studied and the present invention has been made.

  Means of the present invention for solving the above-mentioned problems are as follows.

(1) When hot-dip galvanizing a steel sheet containing 1.0 % or more and 3% or less of Si in mass%, after annealing the steel sheet, Mg is adhered to the surface of the steel sheet by 50 mg / m ² or more and 500 mg / m ² or less. make performs Mg deposition process, then after hot-dip galvanized high strength method for manufacturing a galvannealed steel sheet, characterized by further plating layer to alloying treatment.

According to the present invention, when producing a high-strength galvannealed steel sheet using a Si- containing steel sheet as a base material steel sheet, non-plating is prevented and high productivity and good powdering resistance are achieved. Both can be achieved.

  Hereinafter, the present invention will be specifically described. In addition, although the unit of content of each element of a steel plate chemical component, a plating bath component, and a plating layer component is “mass%”, hereinafter, it is simply indicated by “%” unless otherwise specified.

Base steel plate:
The base steel sheet contains 0.1 to 3% of Si. The lower limit is defined as 0.1%. If the concentration is lower than this, Si concentration on the steel sheet surface is not so remarkable during the reduction treatment, and therefore, non-plating frequently occurs during hot dip galvanizing with Si oxide. This is because there is no significant alloying delay in the alloying process. The upper limit is defined as 3% because Si is an element that can increase the strength while ensuring the ductility of the steel, but if it exceeds 3%, the steel sheet itself becomes too hard.

  Other additive elements do not impede the effects of the present invention and are not particularly limited. For example, in consideration of characteristics, quality, etc. required for the steel sheet after plating, C, Mn, P, Al, etc. 1 type or 2 types or more of these elements can be contained. When these elements are contained, the contents are C: 0.0001 to 0.5%, Mn: 0.1 to 5%, P: 0.005 to 0.2%, Al: 0.01 to 5 % Is preferred. This is not limited to the effect of the present invention, and it goes without saying that it can be selected within the appropriate range from the characteristics, quality, etc. required for the steel sheet after plating.

  The base steel plate may be a cold-rolled steel plate or a hot-rolled steel plate. In the present invention, after annealing the base steel sheet, Mg adhesion treatment for adhering Mg to the steel sheet surface is performed, followed by hot dip galvanization or further alloying the plating layer. Each step will be described below.

Annealing process:
The base steel sheet is annealed before hot dip galvanizing. Annealing may be performed by a conventional method. For example, the steel sheet is heated to a temperature range of 600 to 900 ° C. in a reducing atmosphere containing hydrogen in accordance with a conventional method and subjected to reduction annealing. The annealing time is appropriately determined in consideration of the material characteristics required for the steel sheet while ensuring the time for reducing the oxide film (iron oxide) on the steel sheet surface. Usually, it is about 120 to 480 seconds.

  When the base steel plate is a Si-containing steel plate, Si in the steel is concentrated on the steel plate surface in the course of this reduction annealing and becomes Si oxide. This Si oxide reduces plating wettability during hot dip galvanizing and causes non-plating. Further, after plating, the base iron and the plating metal Zn are alloyed to form an alloyed hot dip galvanized steel sheet (Zn- During the alloying process to obtain Fe), alloying is delayed. In the present invention, in order to prevent this problem from occurring, Mg adhesion treatment is performed after annealing.

  If necessary, hot-rolled steel sheets are processed by pickling and degreasing according to conventional methods before the annealing step, and cold-rolled steel sheets are processed by degreasing and other methods according to conventional methods. May be.

Mg adhesion treatment process:
After the annealing process, Mg is adhered to the base steel plate surface. The amount of Mg is desirably 5 mg / m ² or more and 500 mg / m ² or less. If the amount of Mg is less than 5 mg / m ² , the occurrence of non-plating and deterioration of powdering resistance may not be suppressed. If it exceeds 500 mg / m ² , good plating properties are ensured, but powdering characteristics deteriorate. .

  The Mg adhesion treatment on the surface of the steel base plate can be performed according to a conventional method such as a vacuum deposition method. The annealing process, the Mg adhesion treatment process, and the hot dip galvanizing process may be performed in separate lines, or an Mg adhesion process is provided in the middle of the line having the annealing process and the plating process, followed by the annealing process and the Mg adhesion process. The hot dip galvanizing process may be performed sequentially. In the Mg adhesion step, the steel plate temperature is preferably 600 ° C. or less.

Hot dip galvanizing process:
After the Mg adhesion treatment step, the base steel sheet is hot dip galvanized. When Mg adheres to the surface of the base steel plate, even if Si oxide is formed on the surface, the plating wettability is improved during hot dip galvanizing, and non-plating does not occur. The hot dip galvanization may be performed according to a conventional method. For example, the plating bath temperature is generally about 440 to 520 ° C., the plating bath immersion temperature of the steel sheet is substantially equal to the plating bath temperature, and the Al concentration in the galvanizing bath is generally 0.1 to 0.2%. There is no particular limitation.

The method of adjusting the thickness of the plated layer after plating is not particularly limited, but generally, gas wiping is used, and adjustment is performed by adjusting the gas pressure of gas wiping, the distance between the wiping nozzle and the steel plate, and the like. . At this time, the thickness of the plating layer is not particularly limited, but is preferably ²⁰ to 120 g / m ² per side. If it is less than 20 g / m ² , sufficient rust prevention properties cannot be obtained. On the other hand, 120 g / ^{m 2} by more than is rust resistance is saturated, while the workability, preferably so impair the economy of the 120 g / ^{m 2} or less. However, the difference in the thickness of the plating layer does not hinder the effect of the present invention and is not particularly limited.

Alloying process:
When producing an alloyed hot-dip galvanized steel sheet, the alloying process is subsequently performed to turn the plating layer into an Fe—Zn alloy layer. Usually, the Fe concentration is 7 to 14%.

  The alloying treatment conditions are not particularly limited. For example, the alloying treatment plate temperature is generally about 460 to 600 ° C., and the alloying holding time is about 5 to 60 seconds. The difference does not disturb the effect of the present invention. The heating method is not particularly limited. Conventionally used heating methods such as direct fire heating or radiation heating with combustion gas, direct current heating, induction heating and the like can be used.

  Hot-dip galvanized steel after Mg adhesion treatment prevents alloying delay during alloying treatment, has no unevenness in alloying, and has excellent powdering resistance without losing productivity. Can be manufactured.

  After the alloying treatment step, it is cooled to room temperature, subjected to temper rolling as necessary, and then wound.

  As described above, according to the present invention, a high-strength hot-dip galvanized steel sheet that is free from unplating and has a good appearance can be manufactured, and that is high-strength alloyed hot-dip galvanized steel sheet that has no unplating and has excellent powdering resistance. Can be manufactured without impairing productivity.

In the present invention, when Mg is deposited, non-plating is suppressed and the reason why alloying delay is suppressed is not clear, but the following two reasons are presumed.
1) The presence of Mg on the steel sheet surface improves the wettability of Mg and molten zinc (containing a small amount of Al), thereby eliminating non-plating.
2) When plating is performed by depositing Mg on the surface of the formed Si oxide, the Si oxide is reduced by Mg in the plating bath, and the Si oxide is partially or almost entirely from the steel sheet surface. It disappears and plating wettability is greatly improved.

  When removing this Si oxide, it is very effective in promoting the removal reaction that Mg is once melted, but the melting point of Mg is 651 ° C., and the hot dip galvanizing temperature is 500 ° C. Generally, it is the following. However, although the Mg-zinc binary alloy depends on the composition, the melting point is lowered to 400 ° C. or lower. Accordingly, it is considered that the Mg-Zn mixed composition is partially formed in the plating bath from the Mg surface, the melting point is lowered, and an active molten state is formed on the steel plate surface, thereby promoting the effect of removing Si oxides.

  Furthermore, since such Si oxide disappears entirely or partially from the steel sheet surface, the alloying reaction of Zn and Fe after plating is not suppressed by these oxides, and good powdering resistance is obtained. It is estimated that

  If the amount of Mg deposited is too large, it is possible to suppress non-plating due to the above-mentioned wettability improving effect, but on the other hand, the Mg layer remains on the steel sheet surface, and the effect of removing Si oxide by lowering the melting point of Mg Since it does not occur on the most important steel plate surface and is difficult to be removed, the powdering resistance is deteriorated.

  Hereinafter, the present invention will be specifically described based on examples.

A steel plate having the steel composition shown in Table 1 was hot-rolled, then pickled and cold-rolled to produce a 1.6 mm-thick cold-rolled steel plate. This steel plate was subjected to ultrasonic degreasing in a toluene-alcohol mixed solution to remove oil and dirt on the surface. Then, the sulfuric acid pickling was implemented and it was set as the pretreatment. For this test material, in order to reproduce the inside of the CGL annealing furnace, recrystallization annealing was performed at 830 ° C. for 120 seconds in N ₂ —H ₂ gas (H ₂ : 10 vol%, dew point: −30 ° C.). gave. Next, after depositing various amounts of Mg on the surface of the steel sheet by vacuum deposition, the steel sheet temperature was heated to 460 ° C., immersed in hot dip galvanizing, hot dip galvanized, and further alloyed. . Hot dip galvanization was performed in an Al-containing Zn bath having an Al concentration of 0.14% in the bath and a bath temperature of 460 ° C. The amount of adhesion was adjusted to 45 g to 55 g / m ² per side by gas wiping. The alloying treatment was performed at 540 ° C., and the alloying degree was changed as necessary by changing the alloying treatment time. Here, the sample size was 70 × 180 mm, and the 50 × 50 mm portion at the center was used as the evaluation surface.

  Observe the appearance of the obtained plated steel sheet, ○ if there is no unplating, △ if there is very light non-plating, if there is non-plating (except for those that are very light non-plating) X was evaluated. Appearance evaluation ○ △ is acceptable.

Plating adhesion of galvannealed alloy is determined by measuring the amount of peeling per unit length when a tape is applied to a plated steel sheet and the tape surface is bent at 90 ° C. and bent back at 90 ° C. In the light of the following criteria, ranks 1 and 2 were evaluated as good (◯ and Δ, respectively), and 3 or more were evaluated as bad (x).
X-ray fluorescence count: Rank 0 to less than 500: 1 (good)
500 or more and less than 1000: 2
1000 or more and less than 2000: 3
2000 or more and less than 3000: 4
3000 or more: 5 (poor)
The evaluation results are shown in Table 2.

  As is apparent from Table 2, the hot-dip galvanized steel sheet and the galvannealed steel sheet of the present invention have a good plating appearance and a good powdering resistance.

The present invention uses a steel plate containing Si as a base material steel plate, has a high strength alloyed hot-dip galvanized steel plate that is free from alloying unevenness due to non-plating and alloying delay, has a good appearance, and has excellent powdering resistance. It can utilize as a manufacturing method, without reducing.

Claims (1)

When hot-dip galvanizing a steel sheet containing 1.0 % or more and 3% or less of Si by mass%, the steel sheet is annealed and then Mg is deposited on the steel sheet surface by 50 mg / m ² or more and 500 mg / m ² or less. A method for producing a high-strength alloyed hot-dip galvanized steel sheet, characterized in that after the treatment, hot-dip galvanization is performed, and then the plated layer is further alloyed .