JP2001262302A - Method for producing galvanized steel sheet and galvannealed steel sheet excellent in hot dip metal coated property - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perfectly prevent the occurrence of non-coating portion in a galvanizing treatment and also, the development of powdering after galvannealing treatment. SOLUTION: When a galvanized steel sheet is produced, after hot-rolling under state of sticking the mill scale on the steel sheet, a heat treatment is applied in the temperature range of 650-950 deg.C in the atmosphere, in which the reduction is not substantially developed, an inner oxidized layer is formed on the iron surface layer part of the steel sheet. Further, the atmosphere in a snout for introducing the steel sheet from the heat treatment furnace into a galvanizing bath is regulated to <=70 vol. ppm oxygen content.

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 hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are suitable for use in automobile parts and the like, and particularly to an advantageous improvement of the hot-dip galvanizing property. It is. Further, the present invention is intended to improve not only hot-dipability but also powdering resistance when the base steel sheet is a high-tensile steel sheet.

[0002]

2. Description of the Related Art In recent years, the strength of automobile members has been increased from the viewpoint of reducing the weight of a vehicle body and improving reliability and safety. This tendency is no exception for hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that are frequently used as automotive steel sheets, and various methods have been proposed for increasing strength.

[0003] For example, Japanese Patent Application Laid-Open No. 59-193221 discloses Si
There has been proposed a method for increasing the strength of a steel sheet by incorporating a relatively large amount of a solid solution strengthening element such as iron or Mn. However, in this method, another problem derived from a large amount of Si or Mn is contained, that is, deterioration of hot-dip plating property due to surface concentration of Si or Mn (generation of non-plated parts, ie, generation of non-plating). And the problem of chemical conversion deterioration (a chemical conversion film such as zinc phosphate applied as a coating undercoating treatment is not formed on the cold-rolled steel sheet) occurs, and thus the steel sheet cannot withstand actual use as an automotive steel sheet.

As a solution to the above problem, a method of forcibly oxidizing a steel sheet under a high oxidizing partial pressure, and then reducing and hot-dip coating (Japanese Patent Laid-Open No. 55-122865), Although a method of performing pre-plating (Japanese Patent Laid-Open No. 58-104163) has been proposed, these methods do not provide sufficient control of surface oxides during heat treatment, and thus are not always stable depending on steel components and plating conditions. There is also a problem that hot-dipability or chemical conversion property cannot be obtained and that an extra process is added to increase the production cost.

Japanese Patent Application Laid-Open No. 9-310163 discloses a method for improving the above-mentioned deterioration in hot-dip plating properties, in which hot rolling is carried out after hot rolling to remove crystal grains on the surface layer of the steel sheet. A method has been proposed in which an oxide is formed in a boundary or a crystal grain, that is, an internal oxide layer is formed. This method of forming an internal oxide layer is extremely useful as a means for preventing the occurrence of non-plating. However, in the above method, a sufficient internal oxide layer cannot be secured depending on the type of steel and the manufacturing history, so that a problem remains in that not always satisfactory hot-dipability and chemical conversion treatment are necessarily obtained. Was.

In particular, recrystallization annealing before hot-dip plating is performed
This tendency was significant when the radiation type heating method such as a radiant tube was used. When the heating method is the direct heating method, the internal oxide layer is strengthened to some extent during this annealing, which is improved over the case of the radiant heating method. It was difficult to form a layer.

[0007] In this regard, the applicant's company has previously proposed, as an advantageous solution to the above-mentioned problem, to provide a heat treatment in an atmosphere in which substantially no reduction occurs after the hot rolling, with the black scale remaining attached. The present inventors have developed a hot-dip coated steel sheet capable of stably forming a sufficient internal oxide layer by applying the same and a method for producing the same, and disclosed it in Japanese Patent Application No. 11-322537. Due to the development of such hot-dip coated steel sheet, regardless of the composition of the steel and the manufacturing history, even if radiant heating such as a radiant tube is used for recrystallization annealing before hot-dip coating, sufficient internal By securing an oxide layer, it has become possible to stably obtain excellent hot-dip plating properties and chemical conversion treatment properties.

[0008]

However, even when a hot-dip galvanized steel sheet is manufactured according to the above-described manufacturing method, sometimes non-plating is observed. Further, especially when the material is a high-tensile steel sheet, occurrence of powdering was often observed after the alloying treatment.

The present invention advantageously solves the above-mentioned problem, completely prevents the occurrence of non-plating, and, even if a high-tensile steel sheet is used as a material, powdering after alloying treatment. It is an object of the present invention to propose an advantageous method for producing a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet that are excellent in hot-dip galvanizing property and do not cause generation of cracks.

[0010]

The details of the invention will be described below. As described above, after hot rolling, heat treatment is performed in an atmosphere in which the reduction does not substantially occur while the black scale remains attached, and an internal oxide layer is actively formed on the surface layer of the steel sheet. , And the outermost layer of ground iron is Si, Mn
A purified iron layer with almost no solid solution of easily oxidizable metal elements such as Therefore, if this state can be maintained as it is until the hot-dip galvanizing step, non-plating should not occur.
Actually, after the above hot-rolled sheet heat treatment, degreasing and pickling treatments are applied, and recrystallization annealing before hot-dip galvanizing is performed in a heat treatment furnace in a reducing atmosphere, so there should be essentially no occurrence of non-plating. . However, as described above, the occurrence of non-plating is often observed even when manufactured by the above method.

Here, it is considered that the occurrence of the non-plating as described above is caused by the deterioration of the properties of the outermost surface layer of the base steel of the steel sheet after the heat treatment of the hot-rolled sheet. Therefore, the inventors re-examined all the steps after the heat treatment of the hot-rolled sheet. As a result, it was found that the cause of non-plating was a snout that led the steel sheet from a heat treatment furnace (recrystallization annealing furnace) to a hot-dip galvanizing bath.

That is, as shown in FIG.
Although the steel sheet s subjected to the recrystallization annealing in the above is guided to the hot-dip galvanizing bath 3 through the snout 2,
Since the residence time in the snout 2 is much shorter than the residence time in the heat treatment furnace 1, particularly the continuous annealing furnace, no special consideration has been given to the atmosphere in the past.
However, since the length of the snout is around 10 m, when the plate is passed at a speed of, for example, 100 mpm, the snout stays in the snout for about 6 seconds.

[0013] Then, when the surface properties of the steel sheet were examined on the entrance and exit sides of the snout, the surface concentration of easily oxidizable metal elements such as Si, Mn and P was observed on the entrance side of the snout. Although not present, on the exit side of the snout, the surface concentration of the metal element was sometimes observed, and this tendency was remarkable in a high-tensile steel sheet containing a relatively large amount of Mn or Si. In addition, it has been found that when such a high-tensile steel sheet is subjected to a heat alloying treatment after galvanizing, powdering is likely to occur.

Then, the present inventors have conducted intensive studies to solve the above-mentioned problems, and as a result, obtained the following findings. In other words, to prevent the occurrence of non-plating, (1) lower the oxygen concentration in the snout atmosphere, (2) increase the temperature of the plate entering the plating bath, (3) increase the plating bath temperature,
(4) Increase the alkali concentration during degreasing and washing, (5)
To increase the pickling concentration during pickling and to prevent the occurrence of powdering, (6) increase the concentration of dissolved Al in the plating bath, and (7) set the coating weight lower. Were found to be effective, respectively. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. After hot rolling the raw steel slab, with the black scale attached, in an atmosphere in which reduction does not substantially occur, 650-95
After a heat treatment in a temperature range of 0 ° C to form an internal oxide layer on the surface layer of the steel sheet, cold rolling is performed, followed by degreasing and pickling, followed by recrystallization annealing using a heat treatment furnace. After that, the steel sheet is guided from the heat treatment furnace to the hot dip galvanizing bath into a hot dip galvanizing bath. A method for producing a hot-dip galvanized steel sheet having excellent hot-dipability.

2. In the above item 1, the temperature of the steel sheet entering the hot-dip galvanizing bath is 475 to 515 ° C, and the temperature of the plating bath is 470 ° C.
A method for producing a hot-dip galvanized steel sheet having excellent hot-dip galvanizing properties, wherein hot-dip galvanizing treatment is performed at a temperature of up to 490 ° C.

3. In the above item 1 or 2, the degreasing / pickling treatment comprises a degreasing treatment by alkali washing with a NaOH concentration of 2 mass% or more and a pickling treatment with an HCl concentration of 5 mass% or more, which is excellent in hot-dip plating properties. Manufacturing method of hot-dip galvanized steel sheet.

4. In the above 1, 2 or 3, a method for producing an alloyed hot-dip galvanized steel sheet having excellent hot-dip galvanizing properties, wherein a hot-dip galvanizing treatment is further performed after the hot-dip galvanizing treatment.

[0019]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described specifically. First, a description will be given of the component composition of the material steel sheet targeted in the present invention. As the material steel sheet of the present invention, the component composition is not particularly limited, so-called low carbon steel sheet,
Conventionally known ones such as an ultra-low carbon steel sheet, a Mn-added high-strength steel sheet and a Si-Mn-added high-tensile steel sheet are suitable. Particularly preferably, Mn added with a relatively large amount of Mn to improve strength.
This is a high-strength steel sheet and a high-Si-Mn high-strength steel sheet to which Si and Mn are added. In addition, if necessary, Ti, N
b, B, Mo, Sb, P, S, C, N, Cu, Ni, Cr, V, Zr and the like can be appropriately contained.

Next, the manufacturing process of the present invention will be described. First, as a method for producing a billet, a continuous casting method is advantageously applied, but it goes without saying that an ingot-bulking method may be used. The hot rolling is not particularly limited, and may be performed according to a conventionally known method. Typical hot rolling conditions are a reduction ratio of 80 to 99%, a hot rolling end temperature of 650 to 950 ° C, and a winding temperature of 300 to 750 ° C.

The hot-rolled steel sheet obtained as described above is
Usually, after the black scale is removed by pickling, it is subjected to a cold rolling step.In the present invention, after the hot rolling, the hot-rolled steel sheet with the black scale still adhered is substantially removed. Heat treatment in an atmosphere in which reduction does not occur to positively form an internal oxide layer on the surface layer of the steel sheet, thereby stably improving the hot-dip coating properties. Here, in order to stably obtain excellent hot-dipability, the thickness of the internal oxide layer should be about 5 to 40 μm and the area ratio of the internal oxide layer in the surface layer should be 1 to 40 μm.
It is desirable to set it to about 20%. Note that this value can be easily determined as the area ratio of a portion that looks black at the time of cross-sectional observation (1000 times) in no etching.

In the above-mentioned hot-rolled sheet heat treatment step, the treatment temperature must be 650 to 950 ° C. This is because if the heat treatment temperature of the hot-rolled sheet exceeds 950 ° C, the crystal grain size becomes coarse, the surface becomes rough during the subsequent cold rolling, and the distortion of the cold rolling becomes non-uniform, leading to a decrease in the r-value. If the heat treatment temperature of the hot rolled sheet is lower than 650 ° C., a sufficient internal oxide layer cannot be formed. The heat treatment time is not particularly limited, but is preferably about 4 to 40 hours.

In the present invention, the atmosphere which does not substantially cause reduction is 100 vol% N ₂ atmosphere and H ₂ atmosphere.
H ₂ -N ₂ mixed atmosphere of content is less than 5 vol% is advantageously suited. In this regard, if the H ₂ content is 5 vol% or more, reduced iron is also generated on the surface of the black scale, which hinders removal of the remaining scale in the pickling step before cold rolling, which is not preferable. Further, in an oxidizing atmosphere such as the air, the oxidation of iron proceeds together with the internal oxidation, and the thickness of the internal oxide layer becomes insufficient, which is not preferable. However, 100 vol% N ₂ atmosphere or
O ₂ content is 1v in H ₂ -N ₂ mixed atmosphere where H ₂ content is less than 5vol%
If the content is less than ol%, the oxidation of iron is a small amount that does not cause any problem. Eliminating O ₂ completely has a significant economic disadvantage.

Next, cold rolling is performed. The conditions of the cold rolling are not particularly limited, and may be performed according to a conventional method. In order to develop the texture {111} advantageously, the rolling reduction is preferably about 50 to 95%. preferable.

Next, a degreasing and pickling treatment is performed. In this degreasing treatment, alkali washing with NaOH is generally performed, and the concentration is usually about 1 to 3 mass%, but from the viewpoint of preventing non-plating, the NaOH concentration is desirably 2 mass% or more.
Also, in the pickling treatment, the HCl concentration in the case of hydrochloric acid pickling is usually about 4 to 6 mass%, but from the viewpoint of preventing non-plating, the HCl concentration is desirably 5 mass% or more.

Thereafter, recrystallization annealing is performed. The conditions for the recrystallization annealing are not particularly limited, and the recrystallization annealing may be performed at 600 to 950 ° C. for about 0.5 to 10 minutes according to a conventional method. Further, the present invention has an advantage that a desired internal oxide layer can be secured even when radiant heating such as a radiant tube is used for recrystallization annealing before hot-dip plating. In this case, compared to the case of using a direct-fired heating method,
The amount of formation of the internal oxide layer during the heat treatment of the hot-rolled sheet may be somewhat increased.

Next, the steel sheet after recrystallization annealing is introduced into a hot dip galvanizing bath through a snout. In the present invention, it is important to control the oxygen concentration in the atmosphere in the snout. That is, the oxygen concentration in the snout atmosphere is
If it exceeds 70 volppm, oxidation of the easily oxidizable metal element in the steel occurs in the snout, and there is a concern that non-plating may occur in the subsequent hot-dip galvanizing treatment. The atmosphere must be a non-oxidizing atmosphere of not more than volppm, preferably not more than 40 volppm.

Next, a hot-dip galvanizing treatment is performed. In this case, in order to prevent occurrence of non-plating, the temperature of the immersed sheet into the hot-dip galvanizing bath and the temperature of the plating bath should be set higher than before. Is preferred. That is, it is preferable that the temperature of the plate immersed in the plating bath is about 475 to 515 ° C, and the temperature of the plating bath is about 470 to 490 ° C.
From the viewpoint of powdering resistance, the concentration of dissolved Al in the hot-dip galvanizing bath should be set as high as about 0.135 to 0.145 mass%.
It is preferable to set as low as about 45 g / m ² .

After the steel sheet immersed in the hot-dip galvanized bath is lifted out of the bath, the amount of plating applied is adjusted by gas wiping or the like, and becomes a hot-dip galvanized steel sheet. Also, such hot-dip galvanized steel sheet is
Thereafter, the alloyed hot-dip galvanized steel sheet can be obtained by performing a heat alloying treatment. Here, the conditions for the heat alloying treatment are preferably 460 to 520 ° C. and about 0.1 to 1.0 min.

Further, according to the present invention, the steel sheet after the hot-dip galvanizing treatment or the steel sheet after the heat alloying treatment is subjected to a temper rolling of 10% or less for the purpose of shape correction and adjustment of surface roughness. Can also be added.

[0031]

EXAMPLES Example 1 In terms of mass percentage, C: 0.0020%, Si: 0.02%, Mn: 0.15
%, P: 0.02%, S: 0.005%, Al: 0.05%, Nb: 0.00
A typical mild steel slab containing 3%, B: 0.0004% and Ti: 0.038%, with the balance substantially consisting of Fe, at 1150 ° C
After heating, a hot-rolled sheet having a thickness of 3.5 mm was formed by hot rolling, and then subjected to a heat treatment of the hot-rolled sheet under the conditions shown in Table 1, followed by cold rolling to obtain a cold-rolled sheet having a thickness of 0.8 mm. The cold-rolled sheet thus obtained was degreased and pickled, subjected to recrystallization annealing at 830 ° C. for 4 minutes, and then subjected to a hot-dip galvanizing treatment under the conditions shown in Table 1 to produce a hot-dip galvanized steel sheet. . Also,
Some of them were subsequently subjected to a heat alloying treatment to obtain an alloyed hot-dip galvanized steel sheet. Table 2 also shows the results of examining the hot-dipability and plating adhesion of the hot-dip coated steel sheet thus obtained and the alloying speed, alloying unevenness, and powdering resistance of the hot-dip galvanized steel sheet.

The evaluation method of each characteristic is as follows. <Hot-dipability> Image processing of the appearance after hot-dip plating,
The unplated area ratio was determined and evaluated according to the following criteria. 5: Unplated area ratio 0% 4: Unplated area ratio 0.1% or less 3: Unplated area ratio more than 0.1% to 0.3% or less 2: Unplated area ratio 0 More than 3% to 0.5% or less 1: Unplated area ratio More than 0.5%

<Plating Adhesion> Plating adhesion was evaluated by a DuPont impact test (diameter: 6.35 mm, weight: 1 kg, dropped onto steel plate from a height of 500 mm). The criteria are as follows. ○: No plating peeling ×: With plating peeling

<Alloying rate> Alloying conditions Heating rate: 20 ° C / s Cooling rate: 15 ° C / s Alloying temperature: 490 ° C Alloying time: 20 seconds Surface of alloyed material treated under the above conditions The alloying speed was evaluated based on whether or not the zinc η phase remained. ○: without zinc η phase ×: with zinc η phase

<Alloying unevenness> Using a salt bath, 100
A hot dip galvanized steel sheet of × 200 mm was alloyed at 490 ° C for 30 seconds, and whether or not there was uneven alloying was evaluated by observing the plating appearance after alloying. ○: No burn unevenness (uniform) ×: Burn unevenness

<Powdering resistance> The degreased test piece was bent at 60 ° V at right angles to the rolling direction, then bent back horizontally, a cello tape (registered trademark) was attached to the bent portion, and then peeled off. The amount of Zn attached to the sample was measured by a fluorescent X-ray method to evaluate powdering resistance. The criteria are as follows. ○: Zn is less than 4000 cps / 30mmφ ×: Zn is more than 4000 cps / 30mmφ

[0037]

[Table 1]

[0038]

[Table 2]

As is evident from Table 2, the hot-dip galvanized steel sheets obtained according to the present invention did not generate any non-plating, and exhibited a beautiful surface appearance even after alloying.

Example 2 In terms of mass percentage, C: 0.0015%, Si: 0.35%, Mn: 0.70
%, Nb: 0.015%, Ti: 0.07%, B: 0.001% and S
b: A high-strength steel slab containing 0.009%, with the balance being substantially Fe, was heated to 1150 ° C and then hot-rolled to 3.5 mm.
After being formed into a thick hot-rolled sheet, the sheet was heat-treated under the conditions shown in Table 3 and then cold-rolled to obtain a 0.8 mm-thick cold-rolled sheet. After degreasing and pickling the cold-rolled sheet thus obtained, 880
After performing recrystallization annealing at 4 ° C. for 4 min, a hot-dip galvanized steel sheet was manufactured by performing hot-dip galvanizing under the same conditions as shown in Table 3. Further, a part of the steel sheet was subjected to a heat alloying treatment thereafter to obtain an alloyed hot-dip galvanized steel sheet. Table 4 shows the results of examining the hot-dipability and plating adhesion of the hot-dip coated steel sheet thus obtained and the alloying speed, non-uniform alloying, and powdering resistance of the hot-dip galvanized steel sheet.

[0041]

[Table 3]

[0042]

[Table 4]

As is clear from the table, none of the hot-dip galvanized steel sheets obtained in accordance with the present invention is free from non-plating, hardly generates powder after alloying, and has a beautiful surface appearance. Was presenting.

[0044]

Thus, according to the present invention, a hot-dip galvanized steel sheet excellent in hot-dip galvanizability, free from non-plating, can be stably obtained by using the black scale mother plate annealing method. . Further, according to the present invention, even when the material is a high-tensile steel sheet, excellent powdering resistance can be obtained after alloying, and therefore, there is an effect that press working can be performed in a wide range of applications.

[Brief description of the drawings]

FIG. 1 is a diagram showing a positional relationship between a heat treatment furnace and a hot dip galvanizing bath. Reference Signs List 1 heat treatment furnace 2 snout 3 hot dip galvanizing bath s steel plate (steel strip)

Claims (4)

[Claims] 1. After hot rolling of a raw steel slab, with a black scale attached thereto, in an atmosphere in which reduction does not substantially occur.
After heat treatment in the temperature range of 650 to 950 ° C, an internal oxide layer is formed on the surface layer of the steel plate, cold rolling is performed, and after degreasing and pickling, recrystallization annealing is performed using a heat treatment furnace. After the heat treatment, the atmosphere in the snout that guides the steel sheet from the heat treatment furnace to the hot-dip galvanizing bath was changed to an atmosphere with an oxygen content of 70 volppm.
As the following non-oxidizing atmosphere, the steel sheet is introduced into a hot-dip galvanizing bath, and is subjected to a hot-dip galvanizing treatment.
A method for producing hot-dip galvanized steel sheets with excellent hot-dipability. 2. The steel sheet according to claim 1, wherein the temperature of the steel sheet entering the hot dip galvanizing bath is 475 to 515 ° C., and the temperature of the plating bath is 470 to 515 ° C.
A method for producing a hot-dip galvanized steel sheet having excellent hot-dip galvanizing properties, wherein hot-dip galvanizing is performed at 490 ° C. 3. The method according to claim 1, wherein the degreasing / pickling treatment comprises a degreasing treatment by alkali cleaning with a NaOH concentration of 2 mass% or more, and a pickling treatment with an HCl concentration of 5 mass% or more. A method for producing hot-dip galvanized steel sheets with excellent hot-dipability. 4. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanizing treatment is followed by a heat alloying treatment.