JP2512147B2 - Method for producing galvannealed steel sheet with excellent powdering resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a method for producing a galvannealed steel sheet for use in automobile bodies, electric appliances, etc.
By optimizing the alloying cooling conditions, which is a post-process of hot dip galvanizing, it is possible to continuously produce alloyed hot dip galvanized steel sheets with excellent so-called powdering resistance, in which peeling of the coating does not easily occur during processing or forming. It is about the method.

[Prior Art] Since galvannealed steel sheet has excellent corrosion resistance as well as paintability, paint adhesion, weldability, etc.,
Widely used in automobiles and home appliances, etc.

Such an alloyed hot-dip galvanized steel sheet is generally manufactured as follows. That is, cold-rolled steel sheet is continuously treated by passing it through a galvanizing line (hereinafter referred to as CGL), but first, it is passed through a non-oxidizing heating furnace for pretreatment to a temperature exceeding the recrystallization temperature of the steel. It is annealed by heating, then cooled to 450 ° C to 500 ° C, and then galvanized by being immersed in a bath of molten zinc. After that, adjust the amount of plating to be applied at 500 ℃ to 6 ℃.
After heating to 50 ° C to form an alloy, it is allowed to cool or forcedly cooled. In addition to this, as a method of alloying heat treatment, there is a method of using a heat treatment furnace different from the plating line and performing it in a batch method, but there is a problem that both equipment cost and running cost increase. In either method, when subjected to such alloying heat treatment, an alloying reaction occurs between the steel sheet and the zinc layer, and the ζ phase (FeZn ₁₃ ), δ ₁ phase (FeZn ₇ ), Γ phase (Fe
Fe-Zn alloy phase called ₃ Zn ₁₀ ) is formed in sequence. When these Fe-Zn alloy phases are formed on the entire coating, fine irregularities are formed on the surface to improve paintability or paint adhesion, and since iron is contained in the coating, weldability is also improved. improves. However, since the Fe-Zn alloy phase has a higher hardness than the base steel sheet and becomes weaker in the order of the ζ phase, the δ ₁ phase, and the Γ phase, the film becomes powdery when subjected to processing such as press forming. A so-called powdering phenomenon in which peeling occurs is likely to occur. When powdering occurs, not only the soundness of the film is impaired, but also peeled powder accumulates on the press molding die, which may cause flaws on the press parts.
In recent years, especially from the demand for strengthening the corrosion resistance of automobile bodies,
The adoption of heavy-weight alloyed hot-dip galvanized steel sheets is being considered, but the above-mentioned powdering phenomenon tends to occur rapidly as the coating amount increases, so there is an increasing demand for improved powdering resistance. ing.

As measures for improving such powdering resistance, a method of lowering the alloying heat treatment temperature to increase the heating time, a method of increasing the addition amount of Al in the plating bath, a method of lowering the temperature of the plating bath, etc. have been reported. There is. That is, in these methods, by delaying the alloying reaction, excessive alloying is avoided and the main components of the film are made relatively soft ζ phase and δ ₁ phase. Certainly, if the Fe content in the coating is less than 8% and the coating is softened, the powdering resistance is improved. However, such a coating is likely to cause flaking called flaking due to sliding contact with a tool during press molding, which causes the same problem as powdering.

Various properties that have been conventionally required for steel sheets for automobiles include, in addition to corrosion resistance, powdering resistance, flaking resistance, workability such as press forming including paintability, coating property such as chemical conversion treatment and paint adhesion. , Weldability, etc., and the degree of alloying that satisfies these various characteristics to some extent in a galvannealed steel sheet
It is said that the Fe content is about 8% to 14% and the plating film is mainly composed of the δ ₁ phase. Using the above method, in order to increase the Fe content to the extent that flaking does not occur, of course, when the alloying heat treatment temperature is lowered,
Even if the amount of Al added is increased or the plating bath temperature is decreased, the length of the line must be increased or the line speed must be decreased, and the degree of reduction in productivity is too large compared to the degree of improvement.

On the other hand, there is an opinion that the powdering resistance is improved by controlling the degree of alloying within a certain range. For example, in JP-A-62-290894, the Fe content in the coating is limited to 15 to 35%. A method has been proposed.

[Problems to be Solved by the Invention] However, the coating produced by this method is inferior in corrosion resistance and chemical conversion treatment property, and therefore, Fe, Zn, Ni, Co, Sn, Since a special treatment such as applying a plating film such as As is required, there is a problem that the cost is unavoidable in terms of equipment and materials.

The present invention has been made to solve such a problem, and the Fe content of the coating is kept at 8 wt% or more and 14 wt% or less, a coating mainly composed of the δ ₁ phase, and the alloyed hot-dip galvanized steel sheet has An object of the present invention is to provide a method for producing an alloyed hot dip galvanized steel sheet having excellent powdering resistance by controlling the alloying heat treatment conditions including the cooling process after alloying while maintaining various excellent properties. It is what

[Means for Solving Problems and Actions] Means for achieving this object are as follows: Ordinary plating bath, that is, Al of 0.05 wt% or more and 0.30 wt% or less, Pb of 0.20 wt% or less, and the balance Zn and inevitable impurities. In a method for producing an alloyed hot-dip galvanized steel sheet, in which the steel sheet is immersed in a hot dip galvanizing bath consisting of and then the alloying heat treatment is performed, heating and cooling conditions for the alloying heat treatment are limited as follows. That is, the maximum temperature reached during the alloying heat treatment is 450 ° C or higher and 525 ° C or lower, and after heating for a predetermined time so that the Fe content in the plating film is 8 wt% or higher and 14 wt% or lower, 350 ° C or lower and 250 ° C or higher It is important to rapidly cool to a temperature T between them at a cooling rate of 25 ° C./sec or more, and then gradually cool by spending an appropriate residence time from the temperature T to 250 ° C. t is represented by the formula (1) shown below. Then, it is a method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance, which is subjected to final cooling after passing through the step of passing through this residence time t.

8.5 × 10 ^-4 × exp {5840 / (T + 273)} ≦ t ≦ 300 ……
(1) where T: quenching end temperature (° C) t: residence time in the temperature range from the quenching end temperature T to 250 ° C (seconds) In addition, in the method for manufacturing the galvannealed steel sheet, the residence time t The method of producing a galvannealed steel sheet having excellent powdering resistance, which is a step of performing soaking at the temperature T, similarly achieves the object.

The operation will be described in detail below. In order to carry out the present invention, a pickling (hot rolling) coil or a cold rolling coil produced by a normal method is passed through CGL and galvanized, but the purpose is to decarburize or improve the material before passing the CGL. Pretreatment such as batch annealing may be performed.

Al is usually added to the galvanizing bath for the purpose of suppressing the Fe-Zn alloying reaction, and Pb is added for the purpose of adjusting the spangle of the non-alloyed material. Al content in the bath is 0.05
If it is less than wt%, the effect of suppressing the Fe-Zn alloying reaction is insufficient,
During alloying process, it becomes excessively easy to alloy, while 0.3wt
If it exceeds 0.1%, the suppression effect becomes excessive, so that other conditions such as heating temperature and heating time become strict, which promotes non-uniformity of the Fe—Zn alloying reaction and deteriorates the powdering property. Therefore, the added amount range of Al is 0.05 wt% or more and 0.3 wt%
% Or less.

Pb does not need to be added positively when manufacturing alloyed materials, but in general, the same CG is used for non-alloyed and alloyed materials.
Manufactured using L is done and inevitably included. The effect of Pb on the alloying reaction is small, but when it is contained excessively in the bath, it acts to deteriorate the powdering property, so the upper limit is made 0.2 wt%.

The plated steel strip plated with zinc in the above bath is adjusted in the amount of the coating to be applied, then introduced into an alloying furnace for Fe—Zn alloying, and subjected to heat treatment. This heat treatment is the highest temperature in a series of alloying heat treatments, and here, heating is performed at a temperature of 450 ° C. or higher and 525 ° C. or lower. In this case, the holding time is controlled according to the heating temperature, and the
Alloy the Fe content to be in the range of 8 wt% to 14 wt%. At this time, if heated to a temperature higher than 525 ° C, Γ
The phases are likely to grow and the powdering resistance is deteriorated. On the other hand, at a temperature of 450 ° C. or lower, it takes a long time to complete alloying, resulting in a decrease in productivity. Limiting the degree of alloying to a certain range is that if the Fe content is less than 8 wt%, flaking tends to occur due to sliding with a tool as described above, and if it exceeds 14 wt%, the pedaling resistance becomes poor. This is because it is significantly reduced.

Next, it is rapidly cooled to a temperature T between 350 ° C and 250 ° C,
Further, the temperature is gradually cooled from the temperature T to 250 ° C., but this slow cooling must be performed for a required residence time t. The inventors thoroughly investigated the relation between the alloying process and the powdering resistance of the galvannealed steel sheet, and as a result, by selecting an appropriate alloying heat treatment pattern, Fe
It has been found that the powdering resistance is further improved even if the contents are the same. This alloying heat treatment pattern will be described with reference to FIGS. 1 and 2. In Figure 2,
Figures (a) and (b) are alloying heat treatment patterns, where the vertical axis is temperature and the horizontal axis is time. In each case, the temperature was raised to 500 ° C, alloying was completed in 10 seconds, and then the cooling pattern was changed to cool to near room temperature. In Fig. (A),
Is 50 ° C / sec, is 20 ° C / sec and is 5 ° C / sec, respectively, and in Fig. (B), each is rapidly cooled to 400 ° C at 50 ° C / sec. What was held and cooled for 10 seconds, was rapidly cooled to 350 ° C at 50 ° C / second and then held at that temperature for 30 seconds, and what was cooled to 300 ° C at 50 ° C / second and then held at that temperature for 30 seconds Chilled,
Is the one that was rapidly cooled to 250 ° C at 50 ° C / sec, then held at that temperature for 200 seconds and cooled. Figure (c) shows the results of investigating the powdering resistance of these alloyed steel sheets.
The result is shown by the amount of peeling due to bead pull-out described later. The smaller the amount of peeling, the better the powdering resistance, but the and are more excellent in the powdering resistance than the and. That is, what was held in the low temperature zone after rapid cooling showed good results, but the only exception was. In the case of, it is considered that the held temperature was as high as 400 ° C. It is to be noted that and in (a) of FIG. 8 are patterns of the alloy heat treatment that has been conventionally performed, which is a rapid heating rapid cooling thermal cycle. In the rapid heating and rapid cooling, the Fe-Zn alloy phase is likely to develop inhomogeneously, and the formed alloy layer is compositionally or crystallographically out of equilibrium. When such an inhomogeneous alloy layer is processed, the plating film is likely to be partially peeled off, resulting in poor powdering resistance. On the other hand, the temperature was kept at 350 ° C to 250 ° C,
The powdering resistance was improved in and by the soaking treatment at a relatively low temperature at which these treatments do not significantly progress the heterogeneous alloying reaction. It is considered that the diffusion of atoms occurred and the film structure was homogenized. From this viewpoint, the relationship between the retention and the powdering resistance was investigated in detail, and the results are shown in FIG. The figure shows the resistance between the temperature at the end of quenching after alloying is completed and the start of slow cooling, that is, the end temperature of quenching, and the time in this temperature range from this end temperature of quenching to 250 ° C, that is, the retention time. It shows the effect on the powdering property. In the figure, the vertical axis shows the quenching end temperature, the horizontal axis shows the residence time, and the numbers show the powdering resistance improvement rate.
In this investigation, alloying heat treatment was performed at 480 ° C. for 15 seconds and rapid cooling was performed at 25 ° C./second, but the improvement rate of powdering resistance was based on the cooling rate to room temperature under this rapid cooling condition. That is, the peeling amount of the standard galvannealed steel sheet
P ₀ , the amount of peeling of the target galvannealed steel sheet P ₁ (%),
The improvement rate P is a value obtained by P = 100− (P ₁ / P ₀ ) × 100. In the figure, the points where the improvement rate is 10 or more are distributed within the range surrounded by points A, B, C, and D. In other words, if you expect an improvement of 10% or more, you can achieve this expectation by cooling under conditions within this range.
In this range, the upper limit of the quenching end temperature is 350 on the straight line AB.
C, lower limit is curve ACD. Also, similarly, the lower limit of the residence time is the curve ACD, but the upper limit is the one that was discontinued in 300 seconds from the practical viewpoint. Then, the curve ACD can be approximated by the following equation.

t = 8.5 × 10 ^-4 × exp {5840 / (T + 273)} …… (2) Considering based on these investigation results, when it is retained at a temperature over 350 ° C, Fe is leached from the steel sheet to the plating film. Occurs and the alloying reaction proceeds even in the cooling process, which promotes non-uniformity of the alloying degree in the thickness direction of the plating film. Therefore, after the alloying heat treatment, it is necessary to pass the temperature zone over 350 ° C in a short time, but in this quenching process, if the cooling rate is 25 ° C / sec or more, Fe elution from the steel sheet to the plating film is neglected. can do. When the cooling end temperature is 350 ° C. or lower, not only does the heterogeneous alloying reaction not proceed, but the retention also causes the diffusion of atoms in the plating film to homogenize the film structure. However, if the cooling end temperature is less than 250 ° C., the diffusion rate of atoms is small, and it takes an impractical long time for homogenization. Further, homogenization requires an appropriate time for staying in a temperature range of 350 ° C. or lower and 250 ° C. or higher. This time depends on the temperature within this range, and the lower limit of the time is represented by the formula (2). Although the upper limit of this time is considered to be very large as a natural phenomenon, it is set to 300 seconds for practical use. Therefore,
By setting the residence time within the range of formula (1), the plating film can be appropriately homogenized.

8.5 × 10 ^-4 × exp {5840 / (T + 273)} ≦ t ≦ 300 ……
(1) [Example] Example 1 A cold rolled coil having a plate thickness of 0.8 mm and a width of 200 mm was passed through a continuous hot-dip galvanizing line, and alloying heat treatment was performed under various conditions.
The powdering resistance was evaluated.

There are three types of cold-rolled coil steels and
Shown in the table.

As a pretreatment for plating, annealing was performed by heating at 820 ° C for 60 seconds. Steel strip temperature when entering the plating bath is 470 ℃,
Plating bath temperature is 460 ℃, plating bath composition is 0.12wt% Al-0.08
wt% Pb-0.04 wt% Fe-balance Zn. For the steel strip coming out of the plating bath, the deposition amount was 60 g / m ² by gas wiping.
The alloying heat treatment was subsequently performed. In alloying heat treatment, heating at 520 ℃ for 7 seconds to complete alloying, average 3
After quenching at a cooling rate of 0 sec / sec, 2 from the quenching end temperature T
The mixture was gradually cooled to 50 ° C. over a residence time t, and then cooled at a conventional speed. 30 ° C after alloying is completed
A conventional example in which the temperature was rapidly cooled to near room temperature with a cooling rate of / sec and a comparative example in which the quenching temperature end T or the residence time t was out of the range of the present invention were tested and compared under the other conditions. These alloying heat treatment patterns are shown in FIG. In the figure, the vertical axis is temperature, the horizontal axis is time, the heating process is from R point to S point, and the cooling process is from S point onward. Alloying heat treatment pattern A is a conventional pattern with no residence time, B is a pattern that is gradually cooled at a quenching end temperature of 350 ° C to 250 ° C with a sufficient residence time, and C and D are B at a quenching end temperature of 300 ° C and 250 ° C, respectively. Similarly, the pattern E is a pattern slowly cooled with a sufficient residence time, the pattern E is an insufficient retention time, and the pattern F is an excessively high quenching end temperature.

The powdering resistance was evaluated by a bead drawing test and a deep drawing test. In the bead pull-out test, after pulling out the test piece using a tester, the plating layer was peeled off with an adhesive tape, and the difference between the weight of the test piece before molding and the weight of the test piece after peeling the tape was calculated as the peeled amount. In the deep drawing test as well, the plating layer was peeled off with an adhesive tape after the deep drawing, and the peeled amount was calculated. The essential parts of these testers are shown in FIG.
(A) is a bead pull-out tester, (b) is a deep-drawing tester, and 1 is a test piece, 2 is a die, 3 is a punch, and 4 is a wrinkle suppressor. Specimen 1 in the bead pull-out test
The sample was pulled out while being sandwiched between the die 2 and the punch 3 with a constant pressure. The tip of the punch 3 was made to have a radius of 0.5R, the test piece was wrung here, and the portion of the plating film having a small adhesion was then peeled off with an adhesive tape. In the deep drawing test, a deep drawing tester with a punch 3 diameter of 50 mm and a die shoulder of 5R was used, but a punched test piece on a disc with a diameter of 100 mm was sandwiched between the die 2 and the wrinkle restrainer 4 and then pressed. Was pressed and drawn. After processing, as in the bead pull-out test, the amount of peeling was examined by applying an adhesive tape and measuring the weight. In the deep drawing test, flaking was also observed. As for the amount of peeling, the improvement rate was calculated for each steel type with reference to the conventional example of the alloying heat treatment pattern A, and the evaluations were compared. Table 2 shows the results.

The Fe content in the plating film is almost the same as in the Examples, Comparative Examples, and Conventional Examples, but there is a clear difference in the improvement rate.
In the working example, the improvement rate exceeded 10% in both the bead pulling test and the deep drawing test, but in the comparative example, it was only a few percent.

Moreover, this effect was obtained for all of the steel types used, and it was clear that this effect was obtained by the alloying heat treatment pattern. Note that flaking did not occur in any of the examples.

Example 2 A cold-rolled coil having a plate thickness of 0.8 mm and a width of 200 mm is passed through a continuous hot-dip galvanizing line and subjected to alloying heat treatment under various conditions,
The powdering resistance was evaluated.

There are two types of steel for the cold-rolled coil, which are (i) and (b) shown in Table 1.

As a pretreatment for plating, annealing was performed by heating at 820 ° C for 70 seconds. Steel strip temperature when entering the plating bath is 465 ℃,
Plating bath temperature is 458 ℃, plating bath composition is 0.10wt% Al-0.03
wt% Pb-0.05 wt% Fe-balance Zn. The amount of adhesion of the steel strip coming out of the plating bath was adjusted to 75 g / m ² by gas wiping, and subsequently alloying treatment was performed. In the alloying treatment, the alloying was completed by heating at 475 ° C for 30 seconds, followed by rapid cooling at an average cooling rate of 30 seconds / second, and then from the quenching end temperature T to 250
The residence time t during gradual cooling to ° C was changed. In addition, the same conditions were tested for the conventional example in which it was cooled to near room temperature by quenching at 30 ° C./sec after completion of alloying and the comparative example in which the quenching temperature T or the residence time t was out of the range of the invention. And compared. These alloying heat treatment patterns are shown in FIG. In the figure, the vertical axis is temperature and the horizontal axis is time.
The process up to the point is the heating process for alloying, and the process after the point S is the cooling process. The alloying heat treatment pattern G is a conventional pattern with no residence time and therefore no soaking process, and H is the quenching end temperature 35
After soaking process with sufficient residence time at 0 ° C, I and J are patterns after rapid soaking process with quenching end temperature of 300 ° C and 250 ° C with sufficient residence time like H, and K is residence time. Is insufficient, L is too high at the end temperature of quenching, and M
Is a pattern in which the quenching end temperature is too low.

The powdering resistance was evaluated in the same manner as in Example 1 by a bead drawing test and a deep drawing test. In addition, observation of flaking and calculation of an improvement rate with respect to the amount of peeling based on the conventional example of the alloying heat treatment pattern G for each steel type were performed in the same manner. The results are shown in Table 3.

The Fe content in the plating film is almost the same as in the Examples, Comparative Examples, and Conventional Examples, but there is a clear difference in the improvement rate.
In the example, the improvement rate was more than 10% in both the bead pulling test and the deep drawing test, but in the comparative example, it was only several%. Moreover, this effect was obtained in the same manner even if the steel types were different, and it was clear that this effect was obtained by the alloying heat treatment pattern. Hooking did not occur in any of the examples.

Example 3 A cold-rolled coil having a plate thickness of 0.8 mm and a width of 200 mm was passed through a continuous hot-dip galvanizing line and subjected to alloying heat treatment under various conditions,
The powdering resistance was evaluated.

The steel type of the cold-rolled coil used has the (ro) composition shown in Table 1.

As in Example 1, the temperature of the steel strip subjected to the annealing treatment was 460 ° C.
To penetrate into the plating bath. Plating bath temperature is 455 ℃, plating bath composition is 0.15wt% Al-0.12wt% Pb-0.03wt% Fe-balance Zn
Met. Adjust the amount of coating to 60 g / m ² as in Example 1,
Subsequently, alloying heat treatment was performed. 440 ℃ in heat treatment
The effect was investigated by changing the heating temperature from 0 to 530 ℃.
The rapid cooling rate was 40 ° C / sec. However, an example of soaking at the quenching end temperature of 300 ° C, a comparative example in which the maximum temperature reached during heat treatment or the range of alloying was out, and near room temperature due to rapid cooling It compared with the conventional example cooled to. The powdering resistance was evaluated by a deep drawing test in the same manner as in Example 1, and the flaking was observed and the improvement rate of the amount of peeling was calculated based on the conventional example. Fourth of these results
Shown in the table.

In the example, an improvement rate of more than 10% was obtained, and flaking did not occur. In the test No. 5 in which the highest temperature reached in the comparative example was too high, the powdering resistance was improved,
The deep drawing peeling amount is as high as 150 mg, the heat treatment temperature for alloying is low (Test No. 6), and the heat treatment time is short (Test
No. 7), in which the Fe content in the plating film was lowered, the amount of deep drawing peeling was small, but flaking occurred.

[Effects of the Invention] As described above, according to the present invention, when the galvannealed steel sheet is alloyed, it is rapidly cooled to 350 ° C or less after obtaining a proper iron content,
The alloy plating film is made uniform over a predetermined time in the temperature range up to ℃. Therefore, the product obtained is excellent in workability without impairing corrosion resistance, paintability, and weldability, improving powdering resistance, and causing no flaring. In industrial fields such as steel sheets for automobiles that require increasingly severe workability in addition to corrosion resistance, the effect of the present invention that enables the production of alloyed hot-dip galvanized steel sheet having such excellent workability is great. .

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between residence conditions and powdering resistance improvement rate for explaining the action, and FIG. 2 is a diagram showing the relationship between alloying heat treatment pattern and powdering resistance for explaining the action. FIG. 3 is a diagram showing an alloying heat treatment pattern of Example 1, FIG. 4 is a diagram showing an alloying heat treatment pattern of Example 2, and FIG. 5 is an outline of main parts of a bead drawing tester and a deep drawing tester. It is a figure. 1 ... test piece, 2 ... die, 3 ... punch, 4 ... wrinkle suppression.

 ─────────────────────────────────────────────────── --- Continuation of the front page (56) References JP-A-2-88751 (JP, A) JP-A 61-207560 (JP, A) JP-A 61-223174 (JP, A) JP-A 53- 73431 (JP, A)

Claims (2)

(57) [Claims] 1. A1 is 0.05 wt% or more and 0.30 wt% or less Pb is 0.20 wt%
% Or less The balance is Zn and hot dip galvanizing bath consisting of unavoidable impurities, the steel sheet is immersed in a hot dip galvanized steel sheet manufacturing method in which the alloying heat treatment is continuously performed. ℃ or more and 525 ℃ or less, and after heating for a predetermined time so that the Fe content in the plating film is 8 wt% or more and 14 wt% or less, then cool at 25 ℃ / sec or more to a temperature T between 350 ℃ and 250 ℃ An alloying melt having excellent powdering resistance, which comprises rapid cooling at a speed and passing through a temperature range from the temperature T to 250 ° C. for a residence time t represented by the following formula, followed by final cooling. Manufacturing method of galvanized steel sheet. 8.5 x 10 ^-4 x exp {5840 / (T + 273)} ≤ t ≤ 300 where T: quenching end temperature (° C) t: quenching end temperature Residence time (sec) in the temperature range from T to 250 ° C 2. The method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance according to claim 1, wherein the step of allowing the material to pass through for a residence time t is a step of soaking at the temperature T.