DESCRIPTION
HOT DIP GALVANNEALED STEEL SHEET AND METHOD OF PRODUCTION OF THE SAME
TECHNICAL FIELD
The present invention relates to hot dip galvannealed steel sheet made using an ultra-low carbon steel sheet excellent in corrosion resistance, workability, and coatability as a sheet material and a method of production of the same. Further, the present invention relates to a method of production of hot dip galvannealed steel sheet extremely excellent in appearance.
BACKGROUND ART
Conventional hot dip galvannealed steel sheet is known as steel sheet for automobiles or buildings excellent in coating adhesion and corrosion resistance after coating. In recent years, in particular for automobile applications, deep drawability has been required, so large amounts of hot dip galvannealed steel sheet made using ultra-low carbon steel sheet as the sheet material are being used. In this case, the naked corrosion resistance and corrosion resistance of scratched parts of coatings cannot necessarily be said to be sufficient. Further, there have been the problem of the difficulty in achieving both suppression of powdering and suppression of flaking at the time of working and the problem of the ease of occurrence of flaws in appearance at the time of electrodeposition coating.
Japanese Patent Publication (A) No. 9-3417 discloses hot dip galvannealed steel sheet excellent in corrosion resistance comprised of steel sheet having a first layer made of a Zn-Fe alloy layer and a second layer made of Fe: 8 to 15%, Ni: 0.1 to 2%, and Al: 1% or less. Further, Japanese Patent No. 2783452 discloses a method of production of hot dip galvannealed steel sheet excellent

in corrosion resistance characterized by preplating the surface of the steel sheet with 0.2 to 2 g/m2 of Ni, then rapidly heating the sheet to 430 to 500°C, hot dip coating the sheet in a galvanization bath containing Al in an amount of 0.05 to 0.25%, wiping the sheet, then heat treating the sheet right over it at 470 to 550°C for 10 to 40 seconds for alloying. The above Japanese Patent Publication (A) No. 9-3417 and Japanese Patent No. 2783452 disclose hot rolled low carbon Al killed steel sheet and are not discoveries regarding the ultra-low carbon steel sheet aimed at by the present invention.
Ultra-low carbon steel sheet, compared with low carbon steel sheet, has a higher degree of cleanliness of the ferrite grain boundaries, uneven progress of alloying, and easy growth of the layer, so discoveries relating to low carbon steel sheet cannot be applied as they are. Further, said Japanese Patent Publication (A) No. 9-3417 and Japanese Patent No. 2783452 have no discoveries relating to workability and coating.
Japanese Patent No. 2804167 discloses a hot dip galvannealed steel sheet plating bath obtained by hot dip coating and alloying a sheet in a bath containing less than 0.2% of Al and 0.01 to 0.5% of Ni to give a coating containing Fe: 8 to 13%, Al: less than 0.5%, Ni: 0.02 to 1%, and the balance of Zn and having a F layer thickness of the base iron boundary of 0.5 µ or less. Japanese Patent No. 2804167 discloses low carbon steel sheet. It has no discovery regarding the ultra-low carbon steel sheet aimed at by the present invention. Even if applying the method of production disclosed here to ultra-low carbon steel sheet, the layer thickness substantially cannot be made 0.5 µ or less and the corrosion resistance, workability, coatability are also all completely insufficient.
Japanese Patent No. 2800285 discloses a method of production of hot dip galvannealed steel sheet comprising

plating ultra-low carbon steel sheet with 20 to 70 mg/m2 of Ni, then annealing, hot dip galvanizing, and galvannealing it. However, with this method, there is no effect of improvement of the corrosion resistance and, further, the workability is also not sufficient.
Japanese Patent No. 3557810 discloses a hot dip galvannealed steel sheet excellent in slidability and coatability obtained by plating in a hot dip galvanization bath containing Al: 0.1 to 0.2% and Ni: 0.04 to 0.2%, alloying it by a 10 to 20°C/s rate of temperature rise, and covering 1 to 40% of the surface with a 1 to 10 µm , layer. However, with this technology, the workability, in particular the anti-powdering property and corrosion resistance, are not sufficient.
Japanese Patent No. 3498466 discloses plating in a hot dip galvanization'bath containing Al to which Ni and further at least one type of Pb, Sb, Bi, and Sn is added and alloying under predetermined conditions to obtain hot dip galvannealed steel sheet containing Al: 0.1 to 0.25%, Fe: 6 to 18%, Ni: 0.05 to 0.3%, and 0.001 to 0.01% of at least one type of Pb, Sb, Bi, and Sn. However, with this technology, not only does the bath contain four elements and control become troublesome, but also dross comprised of Ni and Al easily is formed in the bath. When this is caught up in the plating layer, it becomes a factor behind deterioration of the corrosion resistance, so this is not preferred.
Further, ultra-low carbon steel sheet containing Ti features extremely excellent deep drawability and ductility obtained stably over a wide range of ingredients. However, when hot dip galvanizing and further alloying this steel sheet, the Ti in the steel causes the crystal grain boundaries to be cleaned, so the alloying reaction is promoted at the crystal grain boundaries. As a result, an outburst reaction occurs easily, overalloying proceeds easily, and the anti-powdering property deteriorates.

To solve this problem, a method of production of hot dip galvannealed steel sheet comprising complexly adding Nb together with Ti so as to control the alloying reaction occurring at the crystal grain boundaries and thereby improving the anti-powdering property has been disclosed (Japanese Patent Publication (B2) No. 61-32375, Japanese Patent Publication (A) No. 59-67319, Japanese Patent Publication (A) No. 59-74231, and Japanese Patent Publication (A) No. 5-106003). This further complexly adds Nb to Ti, but the addition of Nb is costly, so this has the defect of not being economical.
As technology for improving the anti-powdering property of Ti-containing ultra-low carbon steel sheet without complexly adding Nb, Japanese Patent Publication (A) No. 10-287964 discloses controlling the steam atmosphere in the cooling process after the recrystallization annealing so as to cause the crystal grain boundaries to oxidize and suppressing outburst at the time of the alloying reaction. With this method, not only is it difficult to control the oxidation, but also the plating appearance is liable to be detrimentally affected.
Japanese Patent Publication (A) No. 8-269665 discloses the method of raising the concentration of Al in the hot dip plating bath to 0.12 to 0.2% or higher than usual and creating locally high Al concentration phases at the base iron-plating boundary, but in this case the plating layer easily becomes uneven and the appearance easily deteriorates.
Further, when hot dip galvannealed steel sheet is used for automobile body panel applications, the uneven appearance of galvannealing often remains even after painting the automobile, so an extremely high quality of appearance is required. Most of this unevenness is unevenness of the oxide film of the plated sheet material, unevenness of the fine ingredients, and other unevenness arising due to the previous processes, but the

causes are almost always difficult to identify. Basic solutions were therefore difficult. The documents mentioned above do not disclose any guidelines for obtaining an extremely excellent appearance able to withstand use for automobile body panels aimed at by the present invention.
DISCLOSURE OF THE INVENTION
As explained above, the object is to provide hot dip galvannealed steel sheet using an ultra-low carbon steel sheet excellent in corrosion resistance, workability, and coatability as a sheet material and a method of production of the same. Further, in general, in production of hot dip galvannealed steel sheet, an Fe-Al-Zn alloy layer (so-called barrier layer) is formed in a hot dip galvanization bath at the base iron-plating boundary, the alloy layer is removed by later heat treatment, and an Zn-Fe alloy layer in which Al is diffused is formed. The Fe-Al-Zn alloy layer plays an extremely important role from the viewpoint of control of the subsequent Zn-Fe alloying reaction and securing the plating adhesion. However, the speed of formation of the Fe-Al-Zn alloy layer is finely affected by the surface conditions of the plated sheet material, the flow of solution in the plating bath, etc., the fine differences in thickness of the Fe-Al-Zn alloy layer directly have an effect on the alloying reaction behavior, and fine unevenness in plating appearance is induced, so it has not been easy to produce hot dip galvannealed steel sheet extremely excellent in appearance. Therefore, the present invention has as its object the provision of a method of production of hot dip galvannealed steel sheet extremely excellent in appearance.
The inventors studied hot dip galvannealed steel sheet excellent in corrosion resistance, workability, and coatability using ultra-low carbon steel sheet as a sheet material based on the discoveries of the technologies disclosed in the above-mentioned Japanese Patent

Publication (A) No. 9-3417 and Japanese Patent No. 2783452 and thereby completed the present invention. That is, the present invention provides hot dip galvannealed steel sheet excellent in corrosion resistance, workability, and coatability comprised of an ultra-low carbon steel sheet having on at least one surface a plating layer comprised of, by mass%, Fe: 8 to 13%, Ni: 0.05 to 1.0%, Al: 0.15 to 1.5%, and a balance of Zn and unavoidable impurities, having a ratio of Al/Ni of 0.5 to 5.0, having an average thickness of a F layer of the base iron boundary of 1 |o,m or less, and having a variation of the same of +0.3 jam or less.
Further, the present invention provides a method of production of hot dip galvannealed steel sheet comprising cleaning a surface of an annealed ultra-low carbon steel sheet, preplating it by 0.1 to 1.0 g/m2 of Ni, rapidly heating the sheet in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a 30°C/sec or more rate of temperature rise, then plating it in a hot dip galvanization bath containing Al: 0.1 to 0.2 mass%, wiping it, then rapidly heating it to 470 to 600°C by a 30°C/sec or more rate of temperature rise, cooling it without any soaking time or soaking and holding it for less than 15 seconds, then cooling it.
Further, the inventors engaged in studies and as a result discovered that if using an Fe-Ni-Al-Zn alloy layer instead of an Fe-Al-Zn alloy layer as the alloy layer formed at the base iron-plating in the hot dip galvanization bath, the variation in behavior for formation of the alloy layer due to the surface conditions of the plated sheet material, flow of the solution in the plating bath, etc. becomes smaller and that, further, even if the alloy layer varies in thickness, this does not have that much of an effect on the subsequent Zn-Fe alloying reaction behavior and as a result an extremely good appearance is obtained and

thereby reached the present invention. That is, the present invention provides a method of production of a hot dip galvannealed steel sheet comprising forming an Fe-Ni-Al-Zn alloy layer on a base iron boundary in a hot dip galvanization bath, then heat treating this to eliminate the Fe-Ni-Al-Zn alloy layer and form a Zn-Fe alloy layer in which Ni and Al are diffused.
According to the present invention, it is possible to provide hot dip galvannealed steel sheet using an ultra-low carbon steel sheet excellent in corrosion resistance, workability, and coatability as a sheet material and a method of production of the same. Further, according to the present invention, a method of production of hot dip galvannealed steel sheet with an extremely excellent appearance able to be used for automobile body panels is obtained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the results of analysis of the plating-base iron boundary alloy layer formed in a hot dip galvanization bath according to the present invention.
FIG. 2 shows the results of analysis of the plating-base iron boundary alloy layer formed in a hot dip galvanization bath in the prior art.
FIG. 3 shows the results of analysis of a hot dip galvannealed layer structure according to the present invention.
FIG. 4 shows the results of analysis of a hot dip galvannealed layer structure according to the prior art.
FIG. 5 is a view of the preferable ranges of the Al concentration in the bath and the amount of deposition of Ni preplating in the present invention.
BEST MODE FOR WORKING THE INVENTION
Below, the present invention will be explained in detail.
First, the ultra-low carbon steel sheet covered by the present invention is one to which Ti, Nb, etc. are added alone or complexly to eliminate the solute carbon,

one to which P, Mn, Si, are further added to improve the strength, etc. Further, one containing an extremely small amount of Ni, Cu, Sn, Cr, or other so-called trump element may be used.
As the ultra-low carbon steel sheet to which Ti, Nb, etc. are added alone or complexly to eliminate the solute carbon, specifically one containing, by mass%, C: 0.005% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.02% or less, S: 0.02% or less, and Ti (and/or Nb): 0.001 to 0.2% may be used. Even with Ti (or Nb) added alone, inclusion of Nb (or Ti) in an extent of 0.001% or less entering as unavoidable impurities is deemed included.
Further, as the ultra-low carbon steel sheet improved in strength through the addition of P, specifically one containing C: 0.005% or less, Si: 0.03% or less, Mn: 0.05 to 0.5%, P: 0.02 to 0.1%, and S: 0.02% may be used. This may be used as the sheet material for high strength hot dip galvannealed steel sheet good in drawability able to be applied even for 340 MPa to 390 MPa class automobile body panel applications. Further, a sheet of the above composition further containing Mn to 0.5 to 2.5% and further containing Si to 0.5% or less may be used. This may be used as the sheet material for high strength hot dip galvannealed steel sheet good in drawability able to be applied even for 390 MPa to 440 MPa class automobile body panel applications.
Next, the reasons for limitation of the composition and structure of the plating layer will be explained. Fe was made 8 to 13% because if less than the lower limit, the corrosion resistance is liable to deteriorate, while if over the upper limit, the anti-powdering property is liable to deteriorate.
Ni was made 0.05 to 1.0% because if less than the lower limit, the corrosion resistance is liable to deteriorate, while if over the upper limit, the anti-powderi'ng property is liable to deteriorate. Note that when seeking a better anti-powdering property, Ni is

preferably made 0.1 to 0.5%.
Al was made 0.15 to 1.5% because if less than the lower limit, the anti-powdering property and corrosion resistance are liable to deteriorate, while if over the upper limit, the coatability and further the corrosion resistance are liable to deteriorate. Note that when seeking a better anti-powdering property, the Al lower limit is preferably made 0.3%, while when seeking a still better coatability, the Al upper limit is preferably made 0.8%.
Further, the Al/Ni ratio was defined as 0.5 to 5.0 because if less than the lower limit, the anti-powdering property is liable to deteriorate, while if over the upper limit, the coatability and further the corrosion resistance are liable to deteriorate. When seeking a better anti-powdering property, the lower limit of the Al/Ni ratio is preferably made 1.0.
The present invention features an layer of the base iron boundary having an average thickness of 1 µm or less and having a variation of the same of ±0.3 µm or less. Here, as the means for measuring the layer thickness, for example, the electrolytic peeling method of dissolving everything except the F layer in an ammonium chloride aqueous solution by constant potential electrolysis, then quantifying the F layer by constant current electrolysis, the method of etching a cross-section of the plating by a Nital (alcohol + nitric acid) or other known etching solution and directly observing it by an optical microscope etc., the method of finding it from the X-ray diffraction strength, etc. may be used. Further, the variation of the F layer means a maximum value and minimum value of within ±0.3 µm with respect to the average value of the layer when measuring several points to tends of points of the steel sheet in the width direction. The upper limit of the average thickness of

the layer of the present invention of 1 µm is a relatively large value, but for the anti-powdering property and workability, control of the above-mentioned variation is important. Further, together with the above-mentioned suitable plating composition, a good performance can be obtained.
Next, a method of production of the hot dip galvannealed steel sheet of the present invention will be explained.
In the present invention, an annealed ultra-low carbon steel sheet is used as the sheet material. First, the surface has to be cleaned. The method is not particularly limited. Alkali degreasing, brushing, acid treatment, or another known method may be performed alone or in combination in accordance with the state of dirt or oxide film on the sheet material. From the viewpoint of the uniformity of the Ni plating explained later, alkali degreasing (for example, NaOH aqueous solution treatment) and acid treatment (for example, sulfuric acid aqueous solution treatment) are preferably used in combination in that order.
In the present invention, the sheet is preplated by 0.1 to 1.0 g/m2 of Ni. While also depending on the previously explained cleaning pretreatment, if less than the lower limit, the wettability of this later hot dip coating is insufficient and, further, the corrosion resistance is also sufficient, while if over the upper limit, the anti-powdering property is liable to deteriorate. When seeking a better anti-powdering property, the upper limit of the Ni preplating is preferably made 0.8 g/m2.
After the Ni preplating, the sheet is rapidly heated in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a 30°C/sec or higher rate of temperature rise. This treatment is required for securing the wettability of the hot dip coating and further the plating adhesion. When seeking a better anti-

powdering property, the upper limit of the sheet temperature during heating is preferably made 480°C.
The hot dip galvanization bath used is a bath comprised of Al: 0.1 to 0.2%, unavoidable impurities, and the balance of Zn. This is because if the Al is less than the lower limit, the anti-powdering property and corrosion resistance easily deteriorate, while if over the upper limit, the coatability and the corrosion resistance also easily deteriorate. In the present invention, Ni is not deliberately added to the plating bath. This point differs from Patent Documents 5 and 6. As the source of Ni for the plating layer, Ni preplating is used, so the problem of the Ni-Al dross formed in the plating bath being carried to the plating layer causing the plating layer to become uneven and resulting in deterioration of performance and other problems does not occur. When seeking a better anti-powdering property, the lower limit of the bath Al concentration is preferably made 0.12%.
After plating, the sheet is wiped, then rapidly heated to 470 to 600°C by a 30°C/sec or more rate of temperature rise, then cooled without any soaking time or soaked and held for less than 15 seconds, then cooled for alloying. This is extremely important for suppressing the F layer, in particular for suppressing variation. In particular, if the rate of temperature rise is less than 30°C/seconds, both the F layer and its variation increase. After the rapid heating, cooling without any soaking time or soaking and holding for a short time (less than 15 seconds), then cooling is important. In this case, if this condition is deviated from, both the layer and its variation increase. Note that an ordinary ultra-low carbon steel sheet is preferably cooled without any soaking time. Since no soaking time is necessary, the furnace facility can be shortened and the speed does not have to be reduced for soaking. These are advantageous

from the viewpoint of the productivity. Further, an ultra-low carbon steel sheet improved in strength by the addition of P etc. tends to be slower to alloy, so it should be soaked and held for a short time in accordance with need. When seeking a better anti-powdering property, the sheet is preferably rapidly heated to 470 to 550°C by a 30°C/sec or more rate of temperature rise, cooled without any soaking time, or soaked and held for less than 10 seconds, then cooled for alloying.
Next, the method for obtaining an extremely good appearance of a hot dip galvanized steel sheet will be explained.
The plated sheet material used in the present invention may be any sheet material, but the present invention has as its object to obtain an extremely good appearance such as required for automobile body panel applications, so use of the ultra-low carbon steel sheet often used for automobile body panel applications is effective.
FIG. 1 shows the state of the alloy layer formed in the hot dip galvanization bath in the present invention. FIG. 1 shows the distribution of elements (Ni, Al, Zn, and Fe). in the plating depth direction measured by EPMA analysis of a cross-section of a sample rapidly cooled right after being lifted out from the hot dip galvanization bath and polished embedded. It is learned that an alloy layer comprised of Fe-Ni-Al-Zn is formed at the base iron-plating layer. Note that FIG. 2 shows the case, for comparison, of an ordinary Fe-Al-Zn alloy layer formed at the base iron-plating boundary as observed by a similar method.
Next, FIG. 3 shows the distribution of elements (Ni, Al, Zn, and Fe) in the plating depth direction after heating and alloying in the present invention. The Fe-Ni-Al-Zn alloy layer of the base iron-plating boundary as seen in FIG. 1 disappears and a Zn-Fe alloy layer in which Ni and Al are diffused is formed. Further, FIG. 4

shows, by comparison, the distribution of elements (Ni, Al, Zn, and Fe) in the plating depth direction of a sheet having an alloy layer in the ordinary state of FIG. 2 after heating and alloying.
In the present invention, the state of FIG. 1 is formed in a hot dip galvanization bath, then the state of FIG. 3 is changed by heating and alloying. The reason why going through these steps gives a better appearance compared with the usual steps (that is, the steps from FIG. 2 to FIG. 4) is not necessarily clear, but is believed to be because of the following reason. That is, the step of forming the boundary alloy layer of FIG. 1 is believed to involve a precipitation reaction of Ni, Al, Zn, and Fe in the bath, but since Ni is included, the Ni acts as the nucleus for crystallization. Even if there is some unevenness in the base sheet material, it is believed that this is concealed as an effect. Further, with the Fe-Ni-Al-Zn alloy layer, it is believed the barrier action on the Zn-Fe alloying reaction is less dependent on the thickness of the alloy layer compared with a Fe-Al-Zn alloy layer and therefore unevenness of the thickness of the alloy layer does not easily become unevenness after alloying.
Next, a method of production of hot dip galvannealed steel sheet from the state of FIG. 1 to FIG. 3 of the present invention explained above will be explained in greater detail. The Al of the base iron-plating boundary alloy layer of the present invention is supplied from the hot dip galvanization bath. Further, Ni can also be supplied from the hot dip galvanization bath, but in this case, a large amount of Ni has to be included in the bath and a large amount of Ni-Al dross is formed, so this is not preferred. To avoid this problem, the Ni is preferably supplied by preplating the steel sheet.
Below, a specific method in the case of applying Ni preplating will be explained.
In the present invention, first, the surface has to

be cleaned, but the method is not particularly limited. Alkali degreasing, brushing, acid treatment, or another known method may be performed alone or in combination in accordance with the state of dirt or oxide film on the sheet material. From the viewpoint of the uniformity of the Ni plating explained later, alkali degreasing (for example, NaOH aqueous solution treatment) and acid treatment (for example, sulfuric acid aqueous solution treatment) are preferably used in combination in that order.
In the present invention, the sheet is preplated by 0.05 to 1.0 g/m2 of Ni. If less than the lower limit, the wettability of this later hot dip coating is insufficient, while if over the upper limit, a boundary alloy layer as shown in FIG. 1 becomes difficult to form in the Zn bath and as a result a good appearance is hard to obtain.
After the Ni preplating, the sheet is rapidly heated in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a 30°C/sec or higher rate of temperature rise. This treatment is required for securing the wettability of the hot dip coating and further the plating adhesion.
The hot dip galvanization bath used is a bath comprised of Al: 0.07 to 0.2%, unavoidable impurities, and the balance of Zn. This is because if the Al is less than the lower limit, a boundary alloy layer as shown in FIG. 1 becomes difficult to form and as a result a good appearance is hard to obtain.
Note that formation of the boundary alloy layer as shown in FIG. 1 depends on the amount of preplating of Ni and the concentration of Al in the bath. The inventors used ultra-low carbon steel sheets, changed the amounts of Ni preplating in various ways, rapidly heated the sheets to 460°C by a 50°C/sec rate of temperature rise, dipped the sheets in 455°C hot dip galvanization baths

containing various concentrations of Al, took them out after 3 seconds, and rapidly cooled them so as to investigate if there were Fe-Ni-Al-Zn alloy layers at the base iron-plating boundaries. The results are shown in FIG. 5. In the figure, the "0" marks show samples where the Fe-Ni-Al-Zn alloy layer was confirmed. The trend was observed of the upper limit of the amount of Ni preplating dropping as the bath Al dropped. The region below the broken line in the figure (when assuming the amount of Ni preplating to be Yg/m2 and the Al concentration in the galvanization bath to be [X]%, the relationship Y=15x[X]-l stands) is the suitable region in the present invention.
In the present invention, after plating and wiping, the sheet is preferably rapidly heated to 470 to 600°C by a 30°C/sec or more rate of temperature rise, then cooled with any soaking time or is soaked and held for less than 15 seconds, then cooled for alloying. This provision is important for obtaining a good appearance and securing a suitable alloying degree and plating adhesion.
EXAMPLES
Below, examples will be used to explain the present invention in detail.
(Examples 1 to 13 and Comparative Examples 1 to 11)
Table 1 shows the ingredients of annealed ultra-low carbon steel sheets used for the tests. These were pretreated by the conditions shown in Table 2, then preplated by Ni in plating baths shown in Table 3 by electroplating (bath temperature 60°C, current density 30A/dm2) .
After this, the sheets were heated in a 3%H2+N2 atmosphere by a 50°C/sec rate of temperature rise to 450°C, then immediately dipped in a hot dip galvanization bath warmed to 450°C and held for 3 seconds, then wiped and adjusted in basis weight, and alloyed right above the wiping by a predetermined rate of temperature rise,

temperature, and soaking time. The sheets were cooled by gradual cooling of 2°C/sec over 10 seconds, then were rapidly cooled by 20°C/sec. After this, the sheets were temper rolled by a reduction rate of 0.5%.
Samples were produced by the various types of conditions (amount of preplating of Ni, Al concentration of plating bath, and alloying conditions). Note that the basis weight was 50 g/m2 in each case.
The inventors measured the compositions and F layer thicknesses of the plating layers of the samples of Table 4. The results are shown in Table 5. Each plating layer was dissolved in hydrochloric acid to find the concentrations of the different ingredients. Further, the layer was measured at 10 points by the electrolytic peeling method to find the average value, maximum value, and minimum value. Regarding the variation of the layer, samples with either the maximum value-average value or average value-minimum value over 0.3 µm were indicated as "Poor".
Table 6 shows the results of evaluation of the performance. The performance was evaluated as follows:
(1) Plating appearance: Visual observation with
samples with no nonplating or other defects evaluated as
"Good", some as "Fair", and remarkable amounts as "Poor".
(2) Workability (anti-powdering property): A sample
coated with rustproofing oil was pressed (drawn) by a 40
mmǿ cylinder press under conditions of a draw ratio of 2.2
and was evaluated for the degree of blackening by tape
peeling at its side surface. Samples with a degree of
blackening of 0 to less 20% were evaluated as "Good", 20
to less than 30% as "Fair", and 30% or more as "Poor".
(3) Workability (slidability): A sample coated with
rustproofing oil was subjected to a flat plate continuous
sliding test. It was slid by a compressive load of 500
kgf consecutively five times and the frictional
coefficient at the fifth time was evaluated. Samples with

a frictional coefficient of less than 0.15 were evaluated as "Good", 0.15 to less than 0.2 as "Fair", and 0.2 or . more as "Poor".
(4) Corrosion resistance (rust resistance at
scratched parts of coating): A sample of a steel sheet
was chemically converted by the trication process for
automobiles*1, cationically electrodeposition coated*2 (20
µm) , then the coating was peeled off in a 5 mm x 50 mm
slit shape to expose the plating surface and a corrosion
cycle test*3 was conducted. The resistance was evaluated
by the appearance after 10 days. Samples with no rust or
only yellow rust were evaluated as "Good", with less than
20% of red rust as "Fair", and with 20% or more of red
rust as "Poor".
(5) Corrosion resistance (pitting resistance): A
sample pressed to a U-shape with a bead was flattened,
then, while masking 40 mm x 40 mm, was chemically
converted by the trication process for automobiles*1, and
was cationically electrodeposition coated*2 (20 Jim) . A
bent plate and flat plate were joined by 0.5 mm spacers
so that the uncoated part from which the mask was removed
became the inside so as to create a chassis hem model.
This sample was then subjected to a corrosion cycle
test*3. The resistance was evaluated by the appearance
after 30 days. Samples with less than 20% red rust were
evaluated as "Good", with 20 to less than 50% red rust as
"Fair", and with 50% or more red rust as "Poor".
(6) Coatability: A sample of a steel sheet was
chemically converted by the trication process for
automobiles*1 and was cationically electrodeposition
coated*2. The electrodeposition coating was performed
under conditions of a voltage of 220V, an upslope of 0.5
minutes, and a total conduction time of 3 minutes. The
number of craters and other abnormalities in the test
piece 70 x 150 mm) were counted. Samples with no
abnormalities were evaluated as "Good", one to less than
three as "Fair", and three or more as "Poor".

*1: SD5000 made by Nippon Paint,
*2: PN120M made by Nippon Paint,
*3: SST (6 h) => dry 50°C 45% RH (3 h) => wet 50°C 95% RH (14 h) => dry 50°C 45% RH (1 h)
Table 1. Types of Test Steel
(Table Removed)
Table 2. Pretreatment Conditions
(Table Removed)
Table 3. Pre-Ni Plating Solution
(Table Removed)
Table 4. Sample Production Conditions
(Table Removed)
Table 5. Composition of Plating Layer and Layer Thickness of Test Samples
(Table Removed)
Table 6. Results of Evaluation of Performance
(Table Removed)

In the above way, sheets in the scope of the present invention exhibited superior properties.
(Examples 14 to 22 and Comparative Examples 12 and 13)
Table 7 shows the ingredients of annealed ultra-low carbon steel sheets used for the tests. The sheets were pretreated by the conditions shown in Table 2, then were preplated by Ni in the plating bath shown in Table 3 by electroplating (bath temperature 60°C, current density 30A/dm2) .
After this, the sheets were heated in a 4%H2+N2
atmosphere by a 50°C/sec rate of temperature rise to 455°C, then immediately dipped in a hot dip galvanization bath warmed to 450°C, held there for 2.5 seconds, then

wiped to adjust the basis weight, raised in temperature by 50°C/sec right above the wiping, held for 4 seconds, and rapidly cooled by 50°C/sec. After this, the sheets were temper rolled at a reduction rate of 0.5%.
(Comparative Example 14)
The ingredients of the annealed ultra-low carbon steel sheets used for the test are shown in Table 7. The sheets were pretreated by the conditions shown in Table 2, then were heated in a 4%H2+N2 atmosphere by a 20°C/sec rate of temperature rise to 650°C, held at 60 seconds, gradually cooled to 455°C, then dipped in a galvanization bath warmed to 450°C, held there for 2.5 seconds, then wiped to adjust the basis weight, raised in temperature by 50°C/sec right above the wiping, held for 4 seconds, and rapidly cooled by 50°C/sec. After this, the sheets were temper rolled at a reduction rate of 0.5%.
Samples were produced by the various types of conditions shown in Table 8 (amount of preplating of Ni, Al concentration of plating bath, alloying conditions). Note that the basis weight was 50 g/m2 in each case.
The compositions of the plating layers of the samples of Table 8 and the results of measurement of the
F layer thickness are shown in Table 9. Each plating layer was dissolved in hydrochloric acid to find the concentrations of the different ingredients. Further, the F layer was measured at 10 points by the electrolytic peeling method to find the average value, maximum value, and minimum value. Regarding the variation of the F layer, samples with either the maximum value-average value or average value-minimum value over 0.3 µm were indicated as "Poor".
Table 10 shows the results of evaluation of the performance. The performance was evaluated in the same way as above. However, the workability (anti-powdering property) was evaluated under tougher conditions (draw

ratio 2.3). The evaluation criteria etc. were the same as above. Further, in addition to the evaluation of the above examples, the low temperature chipping property was added. The low temperature chipping property was evaluated in the following way. The method of the above-mentioned evaluation item (6) was used up to the electrodeposition coating, a polyester-based midcoat was coated to 30 µm and a topcoat was coated to 40 µm, then the sample was allowed to stand for one day (size 70 mm x 150 mm). The coated sample was cooled by dry ice to -20°C, then approximately 0.4 g pebbles (10) were dropped on it vertically by an air pressure of 2 kgf/cm2. The coating raised up by the chipping was removed, then the maximum value of the peeling diameters was measured. Samples with a peeling diameter of less than 4 mm were evaluated as "Good", 4 mm to less than 6 mm as "Fair", and 6 mm or more as "Poor".
Table 7. Types of Test Steel
(Table Removed)
Table 8. Sample Production Conditions
(Table Removed)

Table 9. Composition of Plating Layer and F Layer
Thickness of Test Samples
(Table Removed)
Table 10. Results of Evaluation of Performance
(Table Removed)

In the above way, sheets in the scope of the present invention exhibited superior properties.
Next, examples for obtaining an extremely good GA appearance will be explained.
(Examples 19 to 25 and Comparative Examples 15 to 17)
The cold rolled annealed sheet materials shown in Table 1 were pretreated as shown in Table 2, then preplated by Ni in the plating bath shown in Table 3 by electroplating (bath temperature 60°C, current density 30A/dm2) . After this, the sheets were heated in a 3%H2+N2 atmosphere by a 50°C/sec rate of temperature rise to 460°C, then immediately dipped in a hot dip galvanization bath warmed to 455°C and held for 3 seconds, then wiped and adjusted in basis weight. The basis weight was 60 g/m2. After this, the sheets were heated and alloyed under predetermined conditions. After the heating, the sheets were cooled by gradual cooling of 2°C/sec over 10 seconds, then were rapidly cooled by 20°C/sec. After this, the sheets were temper rolled by a reduction rate of 0.5%. Note that the samples for observation of the boundary
alloy layer were ones dipped in the hot dip galvanization bath, held there for 3 seconds, then rapidly cooled. (Comparative Example 18)
A cold rolled unannealed sheet material of the same ingredients and same sheet thickness as the sheet material 1 of Table 1 was used as the sheet material, was just alkali degreased among the pretreatments shown in Table 2, then was annealed and reduced in a 10% hydrogen atmosphere at 800°C for 30 seconds, then cooled to 460°C, then dipped in a hot dip galvanization bath warmed to 455°C and held for 3 seconds, then wiped and adjusted in basis weight. The basis weight was 60 g/m2. After this, the sheet was heated and alloyed under predetermined conditions. After the heating, the sheet was cooled by gradual cooling of 2°C/sec over 10 seconds, then was rapidly cooled by 20°C/sec. After this, the sheet was temper rolled by a reduction rate of 0.5%. Note that the sample for observation of the boundary alloy layer was one dipped in the hot dip galvanization bath, held there for 3 seconds, then rapidly cooled.
In each of Examples 19 to 25 and Comparative Examples 15 to 18, as shown in Table 11, the hot dip galvanization bath concentration and Ni preplating amount were adjusted.
The performance was evaluated as follows:
1) Hot dip galvanized base iron-plating boundary
alloy layer: A cross-section of the sample was -polished
embedded and analyzed by EPMA to investigate the state of
the alloy layer. Samples with an Fe-Ni-Al-Zn alloy layer
were evaluated as "Good" and others as "Poor".
2) Plating appearance (visual): The sample was
irradiated with fluorescent light at a slant and the
presence of any small plating unevenness was examined.
Samples with no unevenness were evaluated as "Good".
3) Plating appearance (SEM observation): Samples
were observed under 500X power for 20 fields and the
of area of the parts crushed and smoothed by the temper rolling was found. Samples with a larger of the difference between the average value and maximum value of the area ratios or the difference of the average value and minimum value of less than 10% was evaluated as "Good", 10% to less than 20% as "Fair", and over 20% as "Poor".
4) Alloying degree: The plating layer was dissolved
in hydrochloric acid and chemically analyzed to find the
amounts of ingredients and calculate the Fe% in the
plating layer. Samples with an Fe of %9 to 12% were
evaluated as "Good" and others as "Poor".
5) Plating adhesion: A sample coated with
rustproofing oil was pressed (drawn) by a 40 mmФ cylinder press under conditions of a draw ratio of 2.2 and was evaluated for degree of blackening by tape peeling at its side surface. Samples with a degree of blackening of 0 to less 20% were evaluated as "Good", 20 to less than 30% as "Fair", and 30% or more as "Poor".
Table 11. Sample Production Conditions and Interface Alloy Layer

(Table Removed)

* In Comparative Example 1 occurred, so the boundary identify. For this reason, evaluated after GA.

Table 12. Results of Evaluation of Performance
(Table Removed)

As shown in Table 12, samples in the range of the present invention give superior characteristics.
INDUSTRIAL APPLICABILITY
According to the present invention, a hot dip galvannealed steel sheet having excellent corrosion resistance, workability, and coatability using an ultra-low carbon steel sheet mainly used for automobiles as a sheet material is obtain. The value of utilization in industry is tremendous. Further, according to the present invention, a method of production of hot dip galvannealed steel sheet extremely excellent in appearance able to be used for automobile body panels is obtained.

WE CLAIM :
-CLAIMS-
1. Hot dip galvannealed steel sheet excellent in
corrosion resistance, workability, and coatability
comprised of an ultra-low carbon steel sheet having on at
least one surface a plating layer comprised of, by mass%,
Fe: 8 to 13%, Ni: 0.05 to 1.0%, Al: 0.15 to 1.5%, and a
balance of Zn and unavoidable impurities, having a ratio
of Al/Ni of 0.5 to 5.0, having an average thickness of a
F layer of the base iron boundary of 1 µn or less, and
having a variation of the same of ±0.3 )µm or less.
2. A method of production of hot dip galvannealed
steel sheet comprising cleaning a surface of an annealed
ultra-low carbon steel sheet, preplating it by 0.1 to 1.0
g/m2 of Ni, rapidly heating the sheet in a nonoxidizing or
reducing atmosphere to a sheet temperature of 430 to 500°C
by a 30°C/sec or more rate of temperature rise, then
plating it in a hot dip galvanization bath containing Al:
0.1 to 0.2 mass%, wiping it, then rapidly heating it to
470 to 600°C by a 30°C/sec or more rate of temperature
rise, cooling it without any soaking time or soaking and
holding it for less than 15 seconds, then cooling it.
3. A method of production of a hot dip
galvannealed steel sheet comprising forming an Fe-Ni-Al-
Zn alloy layer on a base iron boundary in a hot dip
galvanization bath, then heat treating this to eliminate
the Fe-Ni-Al-Zn alloy layer and form a Zn-Fe alloy layer
in which Ni and Al are diffused.
4. A method of production of a hot dip
galvannealed steel sheet comprising cleaning a surface of a steel sheet, preplating it by 0.05 to 1.0 g/m2 of Ni, rapidly heating this in a nonoxidizing or reducing atmosphere to a sheet temperature of 430 to 500°C by a 30°C/sec or more rate of temperature rise, hot dip coating the sheet in a galvanization bath containing Al in a concentration of 0.07 to 0.2%, wiping it, then immediately rapid heating it to 470 to 600°C by a 30°C/sec

or more rate of temperature rise, cooling it without any soaking time or soaking and holding it for less than 15 seconds, then cooling it, wherein the amount of Ni preplating (Yg/m2) and the concentration of Al in the. galvanization bath ([X] mass%) satisfies the relationship of Y≤15x[X]-l.
5. Hot dip galvannealed steel sheet excellent in corrosion resistance, workability and coatability, and a method of production of hot dip galvannealed steel sheet, substantially as herein described, particularly with reference to, and as illustrated in the foregoing examples and the accompanying figures.