EP2666882B1

EP2666882B1 - Hot dipped galvanized steel sheet with excellent deep drawing properties and ultra-low temperature adhesive brittleness, and preparation method thereof

Info

Publication number: EP2666882B1
Application number: EP11856056.4A
Authority: EP
Inventors: Seung-Bok Lee; Jun-Woo Park; Hyun-Jun TARK; Moon-Hi HONG; Ju-Youn Lee
Original assignee: Posco Co Ltd
Current assignee: Posco Holdings Inc
Priority date: 2011-01-20
Filing date: 2011-01-20
Publication date: 2021-04-28
Anticipated expiration: 2031-01-20
Also published as: CN103429781A; EP2666882A1; WO2012099284A1; JP2014506626A; US20140017516A1; EP2666882A4; JP5816703B2; CN103429781B

Description

[Technical Field]

The present invention relates to a hot-dipped galvanized steel sheet, and more particularly to a hot-dipped galvanized steel sheet having improved deep drawing properties and low-temperature adhesive brittleness; and a method of manufacturing the hot-dipped galvanized steel sheet.

[Background Art]

In general, steel sheets are galvanized by passing a steel sheet through molten zinc contained in a bath and solidifying the molten zinc applied to the steel sheet. When molten zinc applied to a steel sheet solidifies, coarse dendritic crystal grains, called spangles, are formed on the surface of the molten zinc. Generation of such spangles is a characteristic of zinc solidification.
In detail, when molten zinc solidifies, crystals start to rapidly grow from solidification nuclei in the shape of dendrites to form the basic structure of a zinc plating layer, and pools of molten zinc remaining between the dendrites are subject to solidification. Due to this solidification mechanism, spangles are generated. The size of spangles may be determined by the basic structure of the zinc plating layer formed in an initial stage of plating.
Such spangles decrease the adhesive strength of paint on a zinc plating layer and the corrosion resistance of a steel sheet, and even though a zinc plating layer is painted, spangles still make the surface of the zinc plating layer uneven and spoil the appearance of a zinc-plated steel sheet because spangles can be seen through paint.
Therefore an inorganic salt solution may be sprayed on a steel sheet before molten zinc applied to the steel sheet is solidified, so as to minimize the size of spangles formed on the steel sheet. At this time, the inorganic salt solution is sprayed on the steel sheet through an electrode disposed on the front side of a nozzle. Since the inorganic salt solution is sprayed through the electrode, droplets of the inorganic salt solution are charged with static electricity and are thus easily attached to the steel sheet by electrical attraction to minify the metallographic structure of a zinc plating layer. A phosphate solution is widely used as the inorganic salt solution.
A plating layer having spangles of 150 µm or less can be formed on a steel sheet by spraying droplets of a phosphate solution charged with electricity as described above. In this case, the steel sheet can have an aestheticallypleasing appearance, improved image clarity after painting, and high corrosion resistance, and the plating layer can be prevented from breaking into flaking during a press process.
In addition, when molten zinc solidifies, spangles are formed to have different shapes depending on how hexagonal crystals of zinc are formed on the surface of a steel sheet. In other words, since hexagonal crystals of zinc grow in different angles in different regions of a steel sheet, spangles have different shapes.
FIG. 1A illustrates a hot-dipped galvanized steel sheet on which spangles having a size of 150 µm or less are formed, and FIG. 1B illustrates a hot-dipped galvanized steel sheet on which spangles having a size of 400 µm or greater are formed. Referring to the hot-dipped galvanized steel sheet of FIG. 1B on which spangles having a size of 400 µm or greater are formed, relatively large zinc crystals are randomly oriented. This is advantageous in terms of brittleness but disadvantageous in terms of the appearance of the steel sheet.
However, referring to the hot-dipped galvanized steel sheet of FIG. 1A on which spangles having a size of 150 µm or less are formed, the spangles have the same crystallographic orientation in a manner such that the basal planes of zinc, (0001) planes, are parallel to the surface of the steel sheet. The above-mentioned crystallographic orientation of zinc in which the basal planes of zinc crystals are parallel to the surface of the steel sheet is known to be most effective in preventing corrosion, a black patina, and chemical instability. Until recently, there have been many efforts to improve such properties.
For example, Japanese Patent Application Laid-open Publication No.: 1999-100653 discloses a technique for adjusting the size of spangles to be within 60 µm to 1000 µm by spraying mist through a nozzle, and Japanese Patent Application Laid-open Publication No.: 1996-188863 discloses a technique for adjusting the size of spangles to be 50 µm or less and the level of surface roughness to be within 0.4 µm to 1.0 µm. In addition, US Patent No.: 4500561 discloses a technique for decreasing the size of spangles to 1000 µm or less by using droplets passed through an electric field. A further relevant prior art document is US2008/107915 A1 .
Many automobile manufacturers have recently attempted to use structural adhesives for joining steel sheets, in addition to or instead of using common existing welding methods such as spot welding, for the purpose of reducing manufacturing costs, improving stability, reducing work time, and making processes eco-friendly.
Unlike mechanical joining methods such as spot welding, structural adhesives are used after determining whether plated steel sheets can be joined using the structural adhesives at a low temperature of -40°C for the case of using automobiles in polar regions. However, if an adhesive is used for a galvanized steel sheet in which the (0001) planes of zinc crystals are parallel to the surface of the steel sheet and thus spangles are not formed, a zinc plating layer may easily be stripped from the steel sheet at a low temperature of -40°C or during a deep drawing process.
The brittleness of a zinc plating layer increases if spangles of the zinc plating layer are small, and the (0001) planes (basal planes) of the zinc plating layer function as slip planes or cleavage planes. Therefore, if a zinc plating layer formed on a steel sheet has small spangles or the (0001) planes of zinc crystals of the zinc plating layer are parallel to the surface of the steel sheet, the zinc plating layer may easily be stripped from the steel sheet when the steel sheet is impacted.
Therefore, to deal with the recent methods of joining hot-dipped galvanized steel sheets using structural adhesives, it is necessary to develop a hot-dipped galvanized steel sheet having an improved appearance, deep drawing properties, and adhesive brittleness at a low temperature.

[Disclosure]

[Technical Problem]

Aspects of the present invention provide a hot-dipped galvanized steel sheet having improved deep drawing properties and low-temperature adhesive brittleness by controlling the structure and grain size of a zinc plating layer, and a method of manufacturing the hot-dipped galvanized steel sheet.

[Technical Solution]

According to an aspect of the present invention, there is provided a hot-dipped galvanized steel sheet including a zinc plating layer, wherein grains of the zinc plating layer have an average particle diameter of 150 µm to 400 µm, and intensity of preferred orientation of (0001) planes of the zinc plating layer is from 3000 cps (count per second) to 20000 cps when the hot dipped galvanized sheet is irradiated with X-rays generated under the conditions of 20 kV and 10 mA, with a tilt angle of the sheet of 5°, and with intensity values measured at intervals of 5° in a rotational angle of 0° to 360° being averaged wherein the grains of the zinc plating layer have a diameter of 30 µm or greater and a diameter deviation equal to or less than 40% of the average particle diameter thereof , and wherein the zinc plating layer optionally includes aluminium (Al), antimony (Sb) and/or lead (Pb).
According to another aspect of the present invention, there is provided a method of manufacturing a hot-dipped galvanized steel sheet as specified above, the method including: applying molten zinc to a steel sheet; adjusting the amount of the molten zinc applied to the steel sheet; spraying an aqueous solution on the steel sheet; cooling the steel sheet; and performing a skin pass milling process on the steel sheet, wherein the spraying of the aqueous solution consists of spraying electrically charged demi-water (demineralized water) on the steel sheet; and wherein the skin pass milling process is performed at an elongation of 5% or less.

[Advantageous Effects]

According to the present invention, owing to the spraying of electrically charged demi-water and a high reduction ratio of the skin pass milling process, grains of the zinc plating layer of the hot-dipped galvanized steel sheet can have a reduced size deviation, the intensity of orientation of the (0001) planes of zinc crystals of the zinc plating layer can be lowered, and the volume fraction of crystallographic twins of the zinc plating layer can be increased. Therefore, the hot-dipped galvanized steel sheet can have improved properties such as deep drawing properties, bending properties, and adhesive brittleness.

[Description of Drawings]

FIG. 1A is a schematic view illustrating the crystallographic structure of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 150 µm or less are formed, and FIG. 1B is a schematic view illustrating the crystallographic structure of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 400 µm or greater are formed.
FIG. 2A is an X-ray analysis graph illustrating the crystallographic orientation of (0001) planes of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 150 µm or less are formed, and FIG. 2B is an X-ray analysis graph illustrating the crystallographic orientation of (0001) planes of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 400 µm or greater are formed.
FIGS. 3 are images illustrating evaluation results of low-temperature adhesive brittleness of Inventive Samples 1.

[Best Mode]

Hereinafter, the present invention will be described in detail.
According to embodiments of the invention, crystal grains of a zinc plating layer of a hot-dipped galvanized steel sheet have an average particle diameter of 150 µm to 400 µm.
If the average particle diameter of the crystal grains is less than 150 µm, the hot-dipped galvanized steel sheet may have a beautiful appearance owing to small spangles but the zinc plating layer may have unsatisfactory low-temperature adhesive brittleness. On the other hand, if the average particle diameter of the crystal grains is greater than 400 µm, even though the zinc plating layer may have satisfactory low-temperature adhesive brittleness, the hot-dipped galvanized steel sheet may have a poor appearance and image clarity, and the zinc plating layer may easily be separated from the hot-dipped galvanized steel sheet during a continuous press process due to coarse spangles.
In the embodiments of the invention, the crystal grains of the zinc plating layer of the hot-dipped galvanized steel sheet have a minimum diameter of 30 µm and the deviation of the diameters of the crystal grains is 40% or less of the average particle diameter of the crystal grains.
If the zinc plating layer includes crystal grains having a diameter of 30 µm or less, the crystal grains may be more brittle than surrounding crystal grains, and thus cracks may start from the crystal grains. In addition, when the hot-dipped galvanized steel sheet is bent, the zinc plating layer may be separated from the hot-dipped galvanized steel sheet, and thus the formability of the hot-dipped galvanized steel sheet may be deteriorated.
As described above, in the embodiments of the invention, the deviation of the diameters of the crystal grains of the zinc plating layer is 40% or less of the average particle diameter of the crystal grains. That is, it may be preferable that the size of spangles formed on the zinc plating layer be uniform within that range. If the deviation is greater than 40% and thus the size of zinc crystals is not uniform, when the hot-dipped galvanized steel sheet undergoes plastic deformation the zinc plating layer may receive non-uniformly applied force and may thus be partially separated from the hot-dipped galvanized steel sheet. Therefore, to prevent problems related to adhesive brittleness, the deviation of the diameters of crystal grains is 40% or less of the average particle diameter of the crystal grains.
In the embodiments of the invention, the intensity of preferred orientation of the (0001) planes of the zinc plating layer of the hot-dipped galvanized steel sheet is from 3000 cps to 20000 cps (count per second). When the hot-dipped galvanized steel sheet of the embodiments of the invention was irradiated with X-rays generated under the conditions of 20 KV and 10 mA, the intensity of preferred orientation of the zinc plating layer of the hot-dipped galvanized steel sheet was measured to be from 3000 cps to 20000 cps. In detail, the maximum intensity of the (0001) planes of zinc crystals was measured to be from 3000 cps to 20000 cps (the tilt angle of a sample was 5°, and intensity values measured at intervals of 5° in a rotational angle of 0° to 360° were averaged).
Referring to FIG. 2A, the intensity of preferred orientation of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 150 µm or less are formed is greater than 20,000 cps, and referring to FIG. 2B, the intensity of preferred orientation of a hot-dipped galvanized steel sheet of the related art on which spangles having a size of 400 µm or greater is formed on is less than 3000 cps.
In the embodiments of the invention, the intensity of preferred orientation of (0001) planes is adjusted to be within the range of 3000 cps to 20000 cps. If the intensity of preferred orientation of (0001) planes is less than 3000 cps, it is advantageous in terms of the brittleness of a zinc plating layer but disadvantageous in terms of appearance due to coarse spangles. On the other hand, if the intensity of preferred orientation of (0001) planes is greater than 20000 cps, the appearance of a zinc plating layer may be good, owing to small spangles but the deep drawing properties and low-temperature brittleness of the zinc plating layer may be deteriorated.
In the embodiments of the invention, it may be preferable that the volume fraction of crystallographic twins of the zinc plating layer of the hot-dipped galvanized steel sheet be 30% or greater. Crystallographic twins may be present in the zinc plating layer when the hot-dipped galvanized steel sheet is processed through a skin pass milling process; and in zinc crystals having a hexagonal close-packed (HCP) structure, crystallographic twins function as an important plastic deformation mechanism to facilitate a deep drawing process and improve brittleness characteristics. If the volume fraction of crystallographic twins of the zinc plating layer is less than 30%, plastic deformation may be less facilitated, and the workability of the hot-dipped galvanized steel sheet may be deteriorated particularly when the size of the zinc crystals of the zinc plating layer is from 150 µm to 400 µm. A method of manufacturing a hot-dipped galvanized steel sheet will now be described in detail according to an embodiment of the invention.
In an embodiment of the invention, a method of manufacturing a hot-dipped galvanized steel sheet includes: applying molten zinc to a steel sheet; adjusting the amount of the molten zinc applied to the steel sheet; spraying an aqueous solution on the steel sheet; cooling the steel sheet; and performing a skin pass milling process on the steel sheet.
The spraying of the aqueous solution is performed by spraying electrically charged demi-water (demineralized water) on the steel sheet.
In the embodiment of the invention, the applying of the molten zinc is performed by passing the steel sheet through a zinc plating solution to attach molten zinc to the steel sheet. In the embodiment of the invention, the applying of the molten zinc is not limited to a particular method or process. That is, molten zinc may be applied to the steel sheet using any zinc plating solution and process conditions that are commonly used for manufacturing hot-dipped galvanized steel sheets in the art to which the present invention pertains. The zinc plating solution may include aluminum (Al), antimony (Sb), and/or lead (Pb). However, the embodiment of the invention is not limited thereto. The steel sheet may be any kind of steel sheet. That is, any steel sheet used for manufacturing a hot-dipped galvanized steel sheet in the related art may be used.
In the adjusting of the amount of the molten zinc after the applying of the molten zinc to the steel sheet, the steel sheet is air-wiped to remove an excessive amount of the zinc plating solution from the steel sheet. The amount of the molten zinc applied to the steel sheet may be adjusted to be any degree considered appropriate by those of skill in the art to which the present invention pertains. That is, the amount of the molten zinc applied to the steel sheet is not limited to any particular degree. For example, the amount of the molten zinc applied to the steel sheet may be adjusted according to the purpose of the steel sheet.
After the adjusting of the amount of the molten zinc applied to the steel sheet, the spraying of the aqueous solution is performed by spraying electrically charged demi-water on the steel sheet to solidify the molten zinc. The electrically charged demi-water is sprayed so as to form a uniform zinc plating layer having uniformly sized spangles. If a solution is electrically charged and sprayed in the form of mist, droplets of the solution collide with a molten zinc plating layer and absorb heat from the molten zinc plating layer to facilitate solidification of the molten zinc plating layer. However, if an inorganic solution such as a phosphate solution is sprayed, regions of the molten zinc plating layer colliding with nuclear particles such as phosphate nuclear particles may lose heat much more quickly than other regions. Thus, relatively small spangles may be formed on the regions, and relatively large spangles may be formed on the other regions to increase the deviation of the sizes of the spangles.
If the deviation in the sizes of spangles is large, the zinc plating layer of the hot-dipped galvanized steel sheet may not be uniformly stressed during a deep drawing process, and thus cracks may start from relatively small spangles. In addition, when the hot-dipped galvanized steel sheet is bent, the zinc plating layer may be separated from the hot-dipped galvanized steel sheet. That is, a large deviation of the sizes of spangles may deteriorate the formability of the hot-dipped galvanized steel sheet.
In the embodiment of the invention, it may be preferable that electrically charged demi-water be sprayed through a nozzle at a demi-water injection pressure of 0.3 kgf/cm² to 5.0 kgf/cm², an air injection pressure of 0.5 kgf/cm² to 7.0 kgf/cm², (1 kgf/cm² is equal to 98066.5 Pa) and a demi-water pressure/air pressure ratio of 1/10 to 8/10.
If the demi-water is sprayed at a pressure of lower than 0.3 kgf/cm², spangles may not be minified. If the demi-water is sprayed at a pressure of greater than 5.0 kgf/cm², pitting marks may be formed on the steel sheet while the steel sheet collide with droplets of the demi-water, and thus the appearance of the steel sheet may be spoiled.
It may be preferable that the front side of the nozzle may be charged to have a voltage of -1 KV to -25 KV. If the front side of the nozzle is charged to have a voltage of less than -1 KV, electrical attraction may not be sufficient to minify droplets and spangles. On the other hand, if the front side of the nozzle is charged to a voltage of greater than -25 KV, spangles smaller than 150 µm may be formed on the zinc plating layer, and thus deep drawing properties and adhesive brittleness may be deteriorated.
In the embodiment of the invention, after the spraying of demi-water, a skin pass milling process is performed on the steel sheet. During the skin pass milling process, crystallographic twins are formed in the zinc plating layer. The skin pass milling process may be performed at an elongation of 5% or less.
In the embodiment of the invention, as described above, it may be preferable that the skin pass milling process be performed at an elongation of 5% or less. During the skin pass milling process, crystallographic twins are formed, which function as an important processing mechanism in zinc crystals having an HCP structure because the HCP structure has few deformation mechanisms. In addition, owing to physical deformation by the skin pass milling process, the intensity of preferred orientation of the (0001) planes of zinc crystals may be lowered. In other words, if the skin pass milling process is not performed, the bonding between the zinc plating layer and the steel sheet may not be firm, and the formability of the steel sheet may not be good. On the other hand, if the skin pass milling process is performed at an elongation of greater than 5%, the properties of the steel sheet may be deteriorated even though the formability and adhesiveness of the zinc plating layer are improved.

[Mode for Invention]

An example of the present invention will now be described in detail. However, the present invention is not limited thereto.

(Example)

Hot-dipped galvanized steel sheets were treated with a phosphate solution or demi-water under the conditions shown in Table 1 to adjust the size of spangles. Thereafter, the steel sheets were treated through a skin pass milling process at an elongation of 1.0% and a roll pressure of 200 tons to 240 tons, and the adhesive brittleness, appearance, and image clarity of the hot-dipped galvanized steel sheets were measured as shown in Table 1.
The hot-dipped galvanized steel sheets were prepared by performing a hot-dip galvanization process on soft IF steel sheets having a thickness of 0.67 mm to form zinc plating layers on the steel sheets at a plating density of 70 g/m².

Sizes and size deviations of spangles formed on the zinc plating layers were measured and analyzed using an optical microscope and an image analyzer before the hot-dipped galvanized steel sheets were treated through the skin pass milling process. The measured and analyzed results are shown in the "spangle size" and "spangle size deviation" columns of Table 1 below. Adhesive brittleness was measured by bonding two hot-dipped galvanized steel sheets with an adhesive for automotive structural parts (Sealer Terokal 5089 by Henkel Korea, Ltd.), keeping the bonded hot-dipped galvanized steel sheets at -40°C, impacting the hot-dipped galvanized steel sheets with a wedge, and observing separation of zinc plating layers of the hot-dipped galvanized steel sheets. In Table 1, ○ denotes the case where a zinc plating layer was not stripped off, Δ denotes the case where 20% or less of a zinc plating layer was stripped off, and X denotes the case where 50% or more of a zinc plating layer was stripped off. Appearance and image clarity were measured with the naked eye, and results thereof are denoted as good (○), fair (Δ) , and poor (X) in Table 1.

[Table 1]

No.	Spraying Solution	Spangle size (µm)	Spangle size deviation (µm)	Preferred orientation (cps)	Adhesive brittleness	Appearance	Image quality

** IS1	Demi-water	250	81	10190	○	○	○
IS2	Demi-water	350	126	4800	○	○	○
IS3	Demi-water	400	-	3253	○	○	Δ
* CS1	Phosphate	150	-	44214	×	○	○


CS2	Phosphate	700	-	1540	○	Δ	×
CS3	-	1000	-	954	○	×	×

*CS: Comparative Sample
**IS: Inventive Sample

Referring to Table 1, Inventive Samples treated with demi-water have spangles within a preferable size range, intensity of preferred orientation within the range of 3000 cps to 20000 cps, size deviations within a preferable range, good adhesive brittleness, and good appearance.
Comparative Samples 1 and 2 treated with a phosphate solution have unsatisfactory adhesive brittleness or appearance. Comparative Sample 3, a general hot-dipped galvanized steel sheet has poor appearance.
FIGS. 3 are images for evaluating adhesive brittleness of Inventive Samples 1. Adhesive brittleness was evaluated based on whether a blue adhesive remained. Referring to Inventive Samples 1 shown in FIG. 3, an adhesive remains owing to improved adhesive brittleness.

Claims

A hot-dipped galvanized steel sheet comprising a zinc plating layer, wherein grains of the zinc plating layer have an average particle diameter of 150 µm to 400 µm, and intensity of preferred orientation of (0001) planes of the zinc plating layer is from 3000 cps (count per second) to 20000 cps when the hot dipped galvanized sheet is irradiated with X-rays generated under the conditions of 20 kV and 10 mA, with a tilt angle of the sheet of 5°, and with intensity values measured at intervals of 5° in a rotational angle of 0° to 360° being averaged, wherein the grains of the zinc plating layer have a diameter of 30 µm or greater and a diameter deviation equal to or less than 40% of the average particle diameter thereof, and wherein the zinc plating layer optionally includes aluminium (Al), antimony (Sb) and/or lead (Pb).
The hot-dipped galvanized steel sheet of claim 1, wherein the zinc plating layer comprises 30% or more, by volume fraction, of crystallographic twins.
A method of manufacturing a hot-dipped galvanized steel sheet according to Claim 1, the method comprising:
applying molten zinc optionally including aluminium (Al), antimony (Sb) and/or lead (Pb) to a steel sheet;

adjusting the amount of the molten zinc applied to the steel sheet;

spraying an aqueous solution on the steel sheet;

cooling the steel sheet; and

performing a skin pass milling process on the steel sheet,

wherein the spraying of the aqueous solution consists of spraying electrically charged demi-water (demineralized water) on the steel sheet; and

wherein the skin pass milling process is performed at an elongation of 5% or less.
The method of claim 3, wherein the spraying of the electrically charged demi-water is performed using a nozzle at a demi-water injection pressure of 0.3 kgf/cm² to 5.0 kgf/cm² and an air injection pressure of 0.5 kgf/cm² to 7.0 kgf/cm² (1 kgf/cm² is equal to 98066.5 Pa).
The method of claim 4, wherein the spraying of the electrically charged demi-water is performed at a demi-water pressure/air pressure ratio of 1/10 to 8/10.