KR20040038503A - Chemical composition of Zn melts for galvannealed steel sheets having excellent surface quality and method for manufacture galvannealed steel sheets to use it - Google Patents

Abstract

PURPOSE: A chemical composition of Zn melts for galvannealed steel sheets is provided which rapidly improves surface qualities such as dross defects, cratering property and surface flatness by optimizing Al concentration and Sb concentration of plating pot, and a method for manufacturing galvannealed steel sheet using the same is provided. CONSTITUTION: The chemical composition of Zn melts for galvannealed steel sheets comprises 0.15 to 0.16 wt.% of Al, 0.08 to 0.20 wt.% of Sb, 0.03 wt.% or less of Fe and a balance of Zn. The method comprises the processes of plating a steel sheet in a plating pot comprising 0.15 to 0.16 wt.% of Al, 0.08 to 0.20 wt.% of Sb, 0.03 wt.% or less of Fe and a balance of Zn; and alloying treating the plated steel sheet in the temperature range of 440 to 520 deg.C for 10 to 18 seconds.

Description

Chemical composition of Zn melts for galvannealed steel sheets having excellent surface quality and method for manufacture galvannealed steel sheets to use it}

The present invention relates to an alloyed hot-dip galvanized steel sheet used for building materials, home appliances and automobiles, and more specifically, by optimizing the Al and Sb concentrations of a plating bath, The present invention relates to an alloyed hot-dip galvanized bath with a remarkably improved surface quality and a method for producing an alloyed hot-dip galvanized steel sheet using the same.

Galvanized galvanized steel sheet (hereinafter referred to as "GA") is obtained by hot-dip galvanizing a cold rolled steel sheet, followed by alloying continuously. A Fe-Zn alloy plated steel sheet composed of Fe-Zn-based intermetallic compounds such as phase, δ phase, and Γ phase. Compared to hot-dip galvanized steel sheet, the GA material has excellent spot weldability, corrosion resistance, and corrosion resistance after painting, and thus its demand for building materials, home appliances, and automobiles is increasing. In recent years, as the range of use for home appliances and automotive exteriors has been expanded, there is a need for surface quality characteristics having no dross defects and excellent cracking resistance.

In general, when a steel sheet is deposited in a plating bath in a continuous hot dip galvanizing process, an alloy layer made of a Fe—Zn compound or a Fe—Al compound is formed on the surface of the steel sheet, and Fe is eluted by a zinc plating bath. Hot-dip galvanizing is an interaction between Fe and Zn by the alloy layer formation reaction and the Fe elution reaction. In the reaction, Fe eluted by the galvanizing bath is increased as the operation time elapses.

In particular, the GA material is alloyed heat treatment after hot-dip galvanizing to form an alloy layer by thermal diffusion of zinc in the molten zinc and the base Fe, so that the Al concentration of the plating bath is controlled to 0.13 ~ 0.14% by weight, the steel sheet is a plating bath When deposited on the Fe is eluted further increase in the plating bath. The eluted Fe reacts with the plating bath solution to generate dross mainly composed of intermetallic compounds such as Fe-Al (Zn ₂ Al ₅ ) and Fe-Zn (FeZn ₇ ). Of these dross, Fe-Al dross having a smaller specific gravity than molten zinc is easy to remove because it floats to the upper part of the plating bath, but Fe-Zn based dross having a larger specific gravity than the molten zinc should go to the bottom of the hot dip galvanizing bath. The sedimentary accumulation or the small particle diameter floats due to the progress of the steel sheet in the plating bath, and becomes floating and floating in the molten zinc plating bath.

If floating dross in the hot dip galvanizing bath adheres to the steel sheet, surface defects may occur during press molding.

In addition, one of the surface defects of the GA material is the cruttering occurs because the alloying reaction of the GA material is first nucleated at the grain boundaries of the plating material, so that the alloying proceeds. In other words, in the early stage of alloying, flower-shaped outburst tissue is formed along the grain boundary of the plating material, and it grows faster by inhaling molten zinc in the adjacent area. Since the letter (crater) is generated, the surface of the GA material is to form a fine irregularities.

When the surface defects of the GA material as described above is transferred to the surface of the coating layer after coating, the coating screenability is inhibited.

The prior art for improving the dross defects on the surface of the GA material as described above is to manage the plating bath in a two-pot (2 pot) system to dualize the hot dip galvanizing material and the GA material plating bath. In other words, first, after using the plating bath for manufacturing hot-dip galvanizing material for a certain period of time, it is operated by changing to the GA material plating bath, during which the hot-dip galvanizing bath after the plating operation is suspended in the lower dross and plating bath integrated in the lower part. The floating dross is removed, and the plating process is repeated by replacing the dross increased with the GA material plating bath. At this time, a method of removing dross in the plating bath is to maintain the plating bath at a high temperature as in JP-A-99258 and accelerate the reaction of 2FeZn ₇ + 5Al → Fe ₂ Al ₃ + 14Zn by adding Al. How to separate the flotation by changing the precipitated dross FeZn ₇ to the floating dross Fe ₂ Al ₅ and attaching the dross to the bubble by blowing gas in the plating bath as in JP-A-3-183751 There is a way to separate. However, since the prior art has to install two or more plating baths, it is uneconomical because the initial equipment cost is high and the replacement of the plating baths takes a lot of time and a dummy coil is required. In addition, there is a problem that a portion of the Al component added to remove the dross is left in the plating bath, thereby exhibiting a poor alloying.

Other prior arts for improving the crutterability of the surface of the GA material include Japanese Patent Application Laid-Open Nos. 5-269505 and 4-285147. The prior art is characterized in that the roughness of the alloyed hot-dip galvanized steel sheet by temper rolling after alloying heat treatment. However, the prior art has to perform the temper rolling with a high reduction force to remove the crease, which is a fine unevenness on the surface of the GA material, the cracking occurs from the Γ phase, the weakest of the alloy phase of the GA material during the temper rolling under high pressure There is a problem that powdering (falling) is dropped.

In addition, there is another conventional technology for improving the cruttering of the surface of the GA material is Korean Patent Application No. 1999-57669. The prior art is characterized by preventing plating by adjusting plating conditions and alloying conditions such as steel sheet inlet temperature, Al concentration of the plating bath, alloying temperature and alloying time. However, the above-described conventional technique cannot fundamentally eliminate the occurrence of the creter because the alloying reaction occurs at the grain boundary of the thermodynamically unstable plating material, and also the alloying degree in consideration of the properties of the GA material such as powdering property or flaking property. There is a problem that is limited to a constant value.

In the prior arts, if the quality characteristic of one of the dross defects or the cruttering property is improved, other quality characteristics are deteriorated, and thus it is impossible to improve various surface quality characteristics required for automotive and consumer steel sheets.

The present invention is to solve the problems of the prior art as described above, to increase the concentration of Al to prevent dross defects, Sb is added to the thick inhibitor layer (inhibition layer) formed in the plating layer / iron ferrous interface in the plating bath Alloying which greatly improves the surface quality of GA material such as dross defect, crutterability, surface smoothness, etc. by neutralizing) and preventing the outburst reaction that causes local over alloying by improving the fluidity of the plating bath. The present invention provides a hot dip galvanizing bath and a method of manufacturing an alloyed hot dip galvanizing steel sheet using the same.

1 is a graph showing the change in alloying temperature according to the Sb concentration of the plating bath

The present invention for achieving the above object by weight, Al: 0.15 ~ 0.16%, Sb: 0.08 ~ 0.20%, Fe: 0.03% or less, relates to an alloying hot dip galvanizing bath comprising a composition consisting of the remaining Zn will be.

In addition, the present invention comprises plating in a plating bath composed as described above, the alloying treatment for 10 to 18 seconds at 440 ~ 520 ℃.

Hereinafter, the present invention will be described in more detail.

The present invention relates to a plating bath for a hot-dip galvanized steel sheet and a method for manufacturing a hot-dip galvanized steel sheet using the same.

[Plating bath]

The present inventors systematically studied a method for simultaneously solving dross defects due to Fe elution of the plating material and the cruttering phenomenon due to local overalloy in the conventional GA material operation of low Al concentration of 0.13 to 0.14% by weight. As a result, it was found that high concentration of Al is required to prevent Fe elution of the plating material, and improvement of the fluidity of the plating bath is required for preventing the cruttering.

Since Al is active in the plating bath, when the steel sheet is deposited in the plating bath, it preferentially reacts with the steel sheet, and thus a thin and dense alloy layer of Fe-Al compound or Zn-Al-Fe compound at the plating layer / ferrous iron interface (inhibiting layer, Formation inhibition layer) prevents formation of weak Zn-Fe compound by reaction between base iron and molten zinc to secure plating adhesion. In addition, since the inhibitory layer serves to prevent Fe elution from the base iron, increasing the Al concentration of the plating bath can reduce the Fe elution to the plating bath, thereby reducing the dross generation.

However, when the Al concentration is increased in the GA material operation, a dense inhibitory layer formed at the interface of the plating layer acts as an alloying barrier that prevents the diffusion of iron atoms from the base iron to the zinc plating layer during the alloying process. Is generated. In order to solve this problem, the alloying temperature should be increased. However, increasing the alloying temperature will impair the crater resistance. Therefore, in the present invention, as a result of studying the alloying treatment at low temperature even at high Al concentration, it was found that Sb is effective in neutralizing the dense suppression layer formed at the interface of the plating layer and improving the surface smoothness. It is intended to solve the above problem.

First, let's look at the reason for limiting the plating bath components of the present invention.

Al: 0.15 to 0.16 wt%

The Al forms a thin and dense alloy layer (inhibition layer) of a Fe-Al compound or a Zn-Al-Fe compound at a plating layer / ferrous iron interface to inhibit Zn-Fe due to the reaction of ferrous iron and molten zinc. It is an element useful for preventing plating compound formation and securing plating adhesion.

If the content of Al is less than 0.15% by weight, the formation of a suppression layer is insufficient, so that an effect of preventing a dross defect cannot be obtained. In addition, since the Al concentration and the Fe elution amount of the plating bath are inversely proportional, it is advantageous to manage the Al concentration to prevent dross defects. However, when the Al concentration is controlled high, an appropriate amount should be added in consideration of the relationship with the Sb addition amount, which is a third element that will neutralize the thick suppression layer formed on the plating layer / ferrous iron interface during alloying treatment.

In other words, as a result of surface analysis and point analysis of the plated layer cross-section of the galvanized steel sheet plated in the Sb-doped galvanizing bath with an Electron Microbe (EPMA) analysis, Sb was mainly Fe- at the plated layer / ferrous iron interface. It was found that Al-Zn-Sb was precipitated as a quaternary alloy phase, but some of them were present as fine Fe-Al-Sb alloy phases in and on the plating layer. In particular, the structure and bonding state of the Fe-Al-Zn-Sb quaternary alloy phase precipitated in the suppression layer were observed by X-ray photoelectron spectroscopy (XPS), and Fe-Al-Zn-Sb 4 was observed. The raw alloy layer was a porous intermetallic compound, unlike the dense layered structure of the conventional Fe-Al-Zn ternary intermetallic compound.

Therefore, in the plating bath, the dense layer of Fe-Al-Zn alloy layer is changed into a porous Fe-Al-Zn-Sb quaternary alloy layer by adding Sb, thereby substantially reducing the Al effect in the hot-dip galvanized steel sheet. Will be In other words, since the Fe diffusion from the base iron to the plating layer is not prevented during the alloying process, the alloying can be performed without variation in local alloying even at a high Al concentration, thereby preventing the occurrence of creters. However, Sb improves the surface smoothness of the coated steel sheet by reducing the surface tension of the molten zinc, but when Sb addition amount is added in excess of 0.2% by weight, coarse spangles are formed on the surface of the plating layer, and even after alloying, Since the unevenness remains and inhibits the surface appearance, it is limited to 0.2% by weight or less because it is transferred to the surface of the coating layer during the coating treatment, thereby inhibiting the coating sensibility. Therefore, since the Al concentration of the plating bath capable of alloying at a low temperature of 500 ° C. or lower at 0.2 wt%, the maximum amount of Sb that can be added to the galvanizing bath, is 0.16 wt% or less, the Al concentration is limited to 0.15 to 0.16 wt%. It is preferable.

Sb: 0.08 to 0.20 wt%

The Sb prevents the occurrence of dross of the Fe-Zn compound because the Sb is precipitated in the plating layer by the Fe-Al-Sb alloy phase by reacting with Fe eluted in the plating bath as well as the alloy phase structure change of the suppression layer at the plating layer / ferrous iron interface. Has the effect.

In addition, since the cretter generation of GA material is caused by the rapid growth of the flower-shaped outburst structure along the grain boundaries of the plating material at the initial stage of alloying, when the plating bath Al concentration is 0.14% by weight or less and the alloying temperature is 520 ° C or higher. Occurs rapidly. Therefore, in order to lower the alloying temperature to 520 ° C or less, the minimum amount of Sb to be added at 0.15 to 0.16% by weight, which is Al concentration of the present invention, is 0.08% by weight. In addition, when the Sb addition amount is added in excess of 0.2% by weight, coarse spangles are formed on the surface of the plating layer, and even after alloying treatment, unevenness of the spangles remains on the surface to inhibit the surface appearance and to the surface of the coating layer during the coating process. Since it transfers and inhibits coating selectivity, it is preferable to limit the amount of Sb added to 0.08 to 0.20% by weight.

Fe: 0.03% by weight or less

When the steel sheet is deposited in the plating bath in the hot dip galvanizing process, an alloy layer made of a Fe—Zn compound or a Fe—Al compound is formed on the surface of the steel sheet, and Fe is eluted by the zinc plating bath. In the reaction, Fe eluted by the galvanizing bath increases as the plating bath Al concentration is low and the operating time increases.

The eluted Fe reacts with the plating bath solution to generate dross mainly composed of intermetallic compounds such as Fe-Al (Zn ₂ Al ₅ ) and Fe-Zn (FeZn ₇ ). Fe-Zn-based (FeZn ₇ ) having a specific gravity greater than that of zinc and having a smaller particle diameter is floated due to the progress of the steel sheet, which floats in the hot dip galvanizing bath and adheres to the steel sheet, thereby inhibiting surface appearance and paint selectivity.

In addition, as the Fe concentration of the plating bath increases, the number of dross appearing on the surface of the steel sheet increases, thereby lowering the surface appearance. When the Fe concentration of the plating bath exceeds 0.03% by weight, dross defects occur and inhibit the surface appearance and coating stiffness, so the content is preferably limited to 0.03% by weight or less.

In addition to the above composition, the rest consists of Zn.

[Method of manufacturing molten zinc plated steel sheet]

In the plating bath, high concentration of Al is prevented to prevent Fe elution, thereby reducing dross defects, and improving the fluidity of the plating bath by adding Sb, thereby securing the cruttering property. In addition, when the alloying treatment at a low temperature (520 ℃ or less) during the manufacturing process it is possible to ensure better fluidity is better flowability.

Hereinafter, the manufacturing method of the present invention will be described.

After galvanizing in the plating bath as described above, the alloying treatment for 8-18 seconds at 440 ~ 520 ℃. In order to secure a target alloying degree (Fe%) of 9 to 12% in the plating bath composition of the present invention, the alloying treatment temperature is required to be 440 ° C or higher, and when the alloying treatment temperature exceeds 520 ° C, Since the outburst reaction which is a cause generate | occur | produces rapidly, it is preferable to limit the said alloying process temperature to 440-520 degreeC.

In addition, when the alloying treatment time is less than 10 seconds or more than 18 seconds, since the coating selectivity is lowered due to overalloying and unalloying, it is preferable to limit the alloying treatment time to 10 to 18 seconds.

Hereinafter, the present invention will be described in more detail with reference to Examples.

EXAMPLE

The Al concentration of the hot dip galvanizing bath was adjusted as shown in Table 1 below using a Zn-5 wt% Al alloy mass. In addition, Sb concentration was adjusted as Table 1 using Zn-2.5 wt% Sb mother alloy.

Hot dip galvanizing work is 0.5 ~ 0.7mm thick, 1200mm wide cold rolled steel sheet of general low carbon steel by dipping in an alkaline solution and degreased, and then annealing at 760 ℃ for 60 seconds, then the steel sheet deposition temperature and plating bath temperature After immersing in a zinc plating bath at 460 ° C. for 3 seconds and plating the cross-sectional plating deposition amount to 50 g / m 2, the alloying treatment was performed at the alloying temperature and time shown in Table 1 below.

In addition, the phosphate treatment and coating process were performed according to the conditions currently used. That is, the surface of the specimen degreased in acetone was deposited for 150 seconds to adjust the surface, phosphate treated for about 150 seconds in a phosphate solution of about 47 ° C. with acidity (acidity, free acidity) and acceleration adjusted, and the phosphate treated specimen was used as a cathode. The electrodeposition coating was adopted so that the film thickness was about 20 µm.

The alloying degree of the GA material prepared as described above, the degree of occurrence of dross defects, the crutterability and the coating stiffness are respectively evaluated and shown in Table 1 below. Here, the alloying degree of the GA material is expressed as a percentage by dissolving the plating layer in an aqueous hydrochloric acid solution containing a corrosion inhibitor and wet analysis, and considering the powdering and flaking properties, the appropriate alloying degree (Fe concentration) is 9 It was about 12%.

The dross defect and the crutterability evaluation of the alloy layer are shown in Table 1 below by examining the number of dross and the creter generation fraction exposed after press working to a sample area of 300 × 300 mm. The number of evaluations per condition was 5 and a simple average value was used.

The coating sensibility was evaluated by DOI (Distinctness Of Image) value after hot-dip galvanized steel sheet was coated with three coats of undercoat (20㎛), medium and top coat (40㎛) in the electrodeposition coating process for automobiles. The DOI value is defined as DOI = (Rs-R) when the intensity of reflected light Rs when irradiated with light from an angle of 30 degrees of the normal of the sample and the intensity of light reflected at an angle outside ± 30 degrees with respect to the specular reflection are set to R _0.3 . _0.3 ) / Rs × 100.

In Table 1, the number of dross (number / m 2) was evaluated based on the following criteria.

1st class (excellent): 3 or less dross attachments

Class 2 (good): When the number of dross is 4 to 10

Grade 3 (Poor): When the number of dross attachments is 10 to 15

Level 4 (very poor): When there are 15 or more dross attachments

In Table 1, crutterability was evaluated based on the following criteria.

1st class (excellent): less than 5%

Grade 2 (good): In case of 5 ~ 10% of creter

Grade 3 (Defective): When the percentage of occurrence of creter is 10 ~ 20%

Grade 4 (very poor): When the percentage of occurrence of creter is more than 20%

In Table 1, the coating sensibility was evaluated by the following criteria without any distortion of the image reflected on the surface of the coating, and clearly observed.

○ (excellent): When DOI value is over 90

× (bad): When the DOI value is 90 or less

Referring to Table 1, in the case of the invention materials (1 to 13) operated by the plating bath component system and the alloying treatment conditions presented in the present invention, dross defects are hardly generated, and surface quality characteristics such as cracking resistance and coating stiffness are shown. It can be seen that this is excellent.

On the other hand, in the case of the conventional materials 1 to 3 alloyed without adding Sb to the plating bath, it can be seen that surface quality is deteriorated due to the occurrence of dross defects and cretters.

In addition, in the case of the comparative materials (1 to 4) in which the amount of Sb or Al added is out of the plating bath composition of the present invention, it is possible to prevent the coating screening by the local outburst reaction, the generation of the cretter, the spangle formation, and the post-coating transfer. It can be seen that the surface quality is poor.

In addition, in the plating bath component system presented in the present invention, the alloying temperature and time is out of the range of the present invention (5 ~ 6) it can be seen that the coating stiffness due to overalloyed and unalloyed is inferior.

As described above, the present invention provides automotive and home appliances such as dross defects, crutterability, surface smoothness, and the like by reducing Fe elution and outburst reaction of the plating bath by optimizing Al concentration and antimony concentration of the plating bath. There is an effect that can simultaneously secure the various surface quality characteristics required for the GA material.

Claims (2)

An alloyed hot dip galvanizing bath having an excellent surface quality comprising, by weight%, Al: 0.15 to 0.16%, Sb: 0.08 to 0.20%, Fe: 0.03% or less, and the remaining Zn. By weight, Al: 0.15 ~ 0.16%, Sb: 0.08 ~ 0.20%, Fe: 0.03% or less, after plating in a plating bath comprising a composition consisting of the remaining Zn, 10 to 18 seconds at 440 ~ 520 ℃ A method for producing an alloyed hot dip galvanized steel sheet having excellent surface quality characteristics including alloying treatment.