CA2470042A1

CA2470042A1 - Method for manufacturing galvannealed steel sheet and galvannealed steel sheet

Info

Publication number: CA2470042A1
Application number: CA002470042A
Authority: CA
Inventors: Shoichiro Taira; Yoshiharu Sugimoto; Yoichi Miyakawa; Akira Gamou
Original assignee: JFE Steel Corp
Current assignee: JFE Steel Corp
Priority date: 2003-08-29
Filing date: 2004-06-04
Publication date: 2005-02-28
Also published as: KR20050022264A; CN1598058A

Abstract

A method for manufacturing a galvannealed steel sheet including hot dip galvanizing a steel sheet; heating the hot dip galvanized steel sheet to alloy the coating layer; temper rolling the galvannealed steel sheet; contacting the temper-rolled steel sheet with an acidic solution, and then allowing the temper-rolled steel sheet to stand for about 1 to about 30 second(s)to form an oxide layer on the surface of the temper-rolled steel sheet; and washing the temper-rolled steel sheet, on which the oxide layer is formed, with water. The acidic solution has a pH-buffering action and contains Fe ion.

Description

METHOD FOR MANUFAOTURING GALVANNEALED STEEL SHEET
A1~TD GALVANNEALED STEEL SHEET
Related Application [0001] This application claims priority of JP 2003-307072, filed August 29, 2003, and JP 2003-307073, filed August 29, 2003.
Field of the Invention [0002] This invention relates to a method for manufacturing a galvamnealed steel sheet having excellent press formability and also excellent chemical conversion treatment performance and adhesiveness in a stable manner. It also relates to a galvannealed steel sheet having excellent press formability and chemical conversion treatment performance and adhesiveness.
Background [0003] Galvannealed steel sheets are used in wide industrial fields such as automobile bodies due to the excellent weldability and paintability compared to galvanized steel sheets. Such galvannealed steel sheets are used after being subjected to press forming.
However, the galvannealed steel sheets have disadvantage of poor press formability as compared to cold-rolled steel sheets. That is because sliding resistance of the galvannealed steel sheets during press forming is larger than that of cold rolled steel sheets. The reasor,~ for such resistance is due to the fact that galvannealed steel sheets are difficult to smoothly enter into the die at a portion where the sliding resistance between the die and the bead is large. This often induces fractures.

[0004] In a galvannealed steel sheet, heat treatment is carried out after subjecting the steel sheet to zinc-coating and, as a result of an alloying reaction where Fe in the steel sheet and Zn in the coating layer are diffused, an Fe-Z.n alloy phase is formed. The Fe-Zn alloy phase usually comprises r phase, 81 phase and ~ phase. When the Fe concentration decreases, there is a tendency that hardness and melting point decrease in the order of T phase -~
81 phase ~ ~ phase.
Accordingly, from the point of sliding performance during press forming, it is effective to form an alloy phase containing a Large amount of Fe, having high hardness and high melting point.
This makes it difficult to induce adhesion. The galvannealed steel sheet where press formability is important is manufactured in such a manner that the average Fe concentration in the coating layer is made a bit high.

[0005] If, however, an alloy phase having a high Fe concentration is formed, a hard and brittle T
phase appears at the interface between the coating layer and the steel sheet.
This likely induces what is called "powdering," or a phenomenon of separation of the coating layer from the interface during press-forming. As a means to provide both the sliding performance and the anti-powdering property, Japanese CJnexamined Patent Publication No. 01/319,661 discloses a method for forming a hard iron-base alloy layer as a second layer on the coating layer using electroplating or the like.

[0006] A widely used method for improving the press formability of zinc-coated steel sheet is to apply high viscosity lubricant oil on the steel sheet. That method, however, generates painting defects during painting caused by insufficient degreasing, and causes instable press formability due to lack of oil during press forming. Accordingly, there has been a strong demand for improvement of the press formability of the galvannealed steel sheet itself.

[0007] As the methods for solving the above problems, Japanese lJnexamined Patent Publication Nos. 53/060,332 and 02/190,483 disclose methods to improve the weldability or press formability by forming an oxide layer consisting mainly of Zn0 on the surface of the zinc coated steel sheet using electrolytic treatment, immersion treatment, aplrlication and oxidation treatment or heating treatment.

[0008] Japanese Unexamined Patent Publication No. 04/088,196 discloses a method to improve the press formability and chemical conversion treatment performance by forming an oxide layer consisting mainly of phosphorus oxide on the surface of the zinc coated steel sheet by immersing the zinc coated steel sheet in an aqueous solution of pH 2 to 6 containing 5 to 60 g/1 of sodium phosphate, by conducting an electrolytic treatment or applying the aqueous solution on the surface of the zinc coated steel sheet.

[0009] Japanese Unexamined Patent Publication No. 03/191,093 discloses a method to improve the press formability and chemical conversion treatment performance by forming a nickel oxide on the surface of zinc-coated steel sheet using electrolytic treatment, immersion treatment, application treatment, application and oxidation treatment or heating treatment.
Summary of the Invention [0010] We conducted detailed studies on the causes of the failiue to attain stable and excellent press formability even when the above-mentioned methods are applied to galvannealed steel sheets and found that the reactivity at the surface is poor due to the presence of Al oxide, and that the surface irregularity is large. That is, we discovered that when the prior art methods are applied to the galvannealed steel sheets, it is difficult to form a desired layer even when electrolytic treatment, immersion treatment, application and oxidation treatment or heating treatment are carried out because the reactivity of the surface is low. The thickness of the film becomes too thin in portions where reactivity is low or, in other words, in portions where the amount of aluminum oxide is large. In addition, since the suri~ace irregularity is large, the die directly contacts the convex part of the coating layer. At that moment, the sliding resistance increases at the convex past of the coating layer having a thin oxide Layer and the desired effect for achieving good press formability is not well achieved.

[0011] A flat part of the surface of the galvannealed steel sheet is present as a convex part as compared to the surrounding portion. The main part that actually contacts the die during gress forming is such a flat part. Therefore, when the sliding resistance in the flat part is made small, it is possible to stably improve press formability. It is effective to prevent adhesion of the coating layer on the die to make the sliding resistance at the flat part small. It is effective to form a hard and high-melting layer on the surface of the coating layer for such a purpose.
We found that control of the thickness of the oxide layer on the flat part is effective and that, when the oxide layer thickness on the flat part is controlled as such, adhesion of the coating layer with the die does not take place but a good sliding performance is available. It has been also found that, for the formation of the oxide layer as such, a method for forming an oxide layer on the surface of the coating layer by contacting the coating layer to an acid solution is effective.

[0012] Thus, in one aspect, the invention relates to a method for manufacturing a ga.lvannealed steel sheet comprising: hot dip galvanizing a steel sheet; heating the hot dip galvanized steel sheet to alloy the coating layer; temper rolling the galvannealed steel sheet to form a flat portion on the coating layer; contacting the temper rolled steel sheet with an acid solution; allowing the temper rolled steel sheet to stand for about 1 to about 30 second(s); and forming an oxide Layer on the coating layer by washing with water.

[0013] We also found that, although formation of an oxide layer effective for improving press formability is possible, there are some cases where close adhesion of the coating layer to the oxide layer is poor and, accordingly, that the method is not always excellent in terms of adhesiveness. We found that, when the method is used in a. actual manufacturing line, the thickness of the oxide layer formed on the surface changes as a result of changes in manufacturing conditions such as line speed and coating weight of the acidic solution and that, when a thick oxide layer is formed, a uniform chemical conversion layer is not formed.

[0014] Thus, in another aspect, this invention provides a method for manufacturing a galvannealed steel sheet having excellent sliding performance during press forming and also excellent chemical conversion treatment performance and adhesiveness in a stable manner. It also provides a galvannealed steel sheet having an excellent sliding performance during press forming and excellent chemical conversion treatment performance and adhesiveness.

[0015] We investigated the influence of elements added to the treating solution or the acidic solution and found that, when Fe ion is contained in the above treating solution, it has no bad influence on chemical conversion treatment performance and also provides for excellent adhesiveness of the coating layer to the oxide layer whereupon it generates an excellent adhesiveness.

(0016] Accordingly, in yet another aspect, the invention provides a method for manufacturing a galvannealed steel sheet, comprising:
(a) hot dip galvanizing a steel sheet;
(b) heating the hot dip galvanized steel sheet to alloy the coating layer, thereby forming a galvannealed steel sheet;
(c) temper rolling the galvannealed steel sheet;
(d) contacting the temper-rolled steel sheet with an acidic solution having pH-buffering action and containing Fe ion, and then allowing the temper-rolled steel sheet to stand for about 1 to about 30 second(s),to form an oxide layer on the surface of the temper-rolled steel sheet; and (e) washing the temper-rolled steel sheet, on which the oxide layer is formed, with water.

[0017] It is preferable that the acidic solution contains Fe ion where Fe3+
ion concentration is about 2 g/ liter or less and the balance is Fe2+ ion.

[0018] It is preferred that a "pH-increasing degree" of the acidic solution is within a range of about 3 to about 20. The pH-rising degree is defined as the amount (ml) of a 1 mol/liter aqueous solution of sodium hydroxide necessary for increasing the pH of one liter of the acidic solution from 2 to 5.

[0019] It is preferred that the acidic solution contains at least one of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate in an amount of within a range of about S to about SO glliter and has a pH within a range of about 1 to about 5.

[0020] It is preferred that the acidic solution contains at least one of sulfate, nitrate and chloride of Fe in an amount of within a range of about 0.1 to about 100 g/liter in terms of Fe ion cancentration.

[0021] It is preferable that the method far manufacturing a galvannealed steel sheet further comprises the step of contacting the temper-rolled steel sheet with an alkaline solution to activate the surface, before contacting with the acidic solution step (d).

[0022] It is preferable that the method for manufacturing a galvannealed steel sheet further comprises the step of contacting the temper-rolled steel sheet with an alkaline solution to conduct a neutralizing treatment for the acidic solution remaining on the surface, after contacting with the acidic solution step (d).

[0023] It is preferred that the step of contacting with the acidic solution comprises contacting the temper-rolled steel sheet with an acidic solution such that a solution film formed on the surface of the steel sheet after contacting to the acidic solution has a caating weight of about 3 g/m2 or less.

[0024] Further, this invention provides a galvannealed steel sheet having an oxide layer with a thickness of about 10 nrn or more on the surface flattened Bart of the hot dip galvanized steel sheet manufactured by the method comprising:

(a) hot dip galvanizing a steel sheet;
(b) heating the hot dip galvanized steel sheet to alloy the coating layer, thereby forming a galvannealed steel sheet;
(c) temper rolling the galvannealed steel sheet;
(d) contacting the temper-rolled steel sheet with an acidic solution having a pH-buffering action and containing Fe ion, and then allowing the temper-rolled steel sheet to stand for about 1 to about 30 second(s),to form an oxide layer on the surface of the temper-rolled steel sheet; and (e) washing the temper-rolled steel sheet, on which the oxide layer is formed, with water.
Brief Description of the Drawings [0025] Fig. 1 shows an essential part of the apparatus for forming an oxide layer used in Examples.

[0026] Fig. 2 is a rough front view which shows an apparatus for the measurement of a frictional coefficient.

[0027] Fig. 3 is a schematic oblique view which shows the shape and size of a bead in Fig. 2.

[0028] Fig. 4 is a schematic oblique view which shows the shape and size of a bead in Fig. 2.

[0029] Fig. 5 is a schematic oblique view which illustrates an assembling process of a test material for an adhesion test.

[0030] Fig. 6 is a schematic oblique view which shows the state of a tensile test in a test material.

[0031] Fig. 7 is a schematic scheme which shows a draw bead tester.
Detailed Description [0032] It will be appreciated that the following description is intended to refer to specific embodiments of the invention selected for illustration in the drawings and is not intended to define or limit the invention, other than in the appended claims.

[0033] In the manufacture of a galvannealed steel sheet, a steep sheet is hot dip galvanized and then heated to carry out an alloying treatment. At that time, due to the difference in the reactivity of the interface between the steel sheet and the coating layen~ during the alloying treatment, irregularities are present on the surface of the galvannealed steel sheet.
However, after the alloying treatment, temper rolling is usually carried out to secure the quality of the galvannealed steel sheet and, as a result of contact with the roll during the temper rolling, the surface of the coating layer is made smooth whereby the irregularities are relaxed/reduced.
Accordingly, upon press forming, the force necessary for flattening the convex portions of the coating layer is decreased and the sliding performance is improved.

[0034] Since the flattened part of the surface of the galvannealed steel sheet is a part to which a die directly contacts during press forming, it is important in attempting to improve the sliding performance that a hard and high-melting substance which prevents adhesion to the die is present.
The presence of an oxide layer on the coating layer is effective for improving the sliding performance since the oxide layer prevents adhesion to the die.

[0035] The oxide on the surface layer is abraded and scraped off during press forming and, therefore, the presence of a sufficiently thick oxide layer is necessary when the contacting area of the die with the material to be processed is large. Although an oxide layer is formed on the coating layer by heating during the alloying treafiment, most of the oxide layer is destroyed by the contact with the roll upon the temper rolling and a new surface is exposed. Therefore, it is necessary to form a thick oxide layer before temper rolling to achieve good sliding performance.
Even when a thick oxide layer is formed before temper rolling taking the above into consideration, it is not possible to avoid destruction of the oxide Layer during temper rolling and, therefore, an oxide layer of the flattened part is unevenly present and a good sliding performance is unable to be achieved in a stable manner.

[0036] Because of the above, a good sliding performance is stably achieved when a treatment to form a uniform oxide layer on the temper-rolled galvannealed steel sheet, particularly on the flattened part of the coating surface, is carried out.

[0037] When the galvannealed steel sheet is contacted with an acidic solution, then kept for about 1 to about 30 seconds) under such a state that a liquid film of the acidic solution is formed on the surface of the steel sheet and washed with water followed by drying, an oxide layer is able to be formed on the coated steel sheet. When the acidic solution has a pH-buffering action and is a solution containing Fe ion, an oxide layer having an excellent sliding performance is able to be stably formed on the flattened part of the coating layer. Since the oxide formed as such is very fine, it does not badly affect the formation of a chemical conversion film even if it remains immediately before the chemical conversion treatment. Further, as compared to an oxide layer formed by the use of an acidic solution containing no Fe ion, it has been found to have an excellent close contact of an oxide layer and also to have adhesiveness.

[0038] Although the mechanism for the formation of the oxide layer as such is not fully understood, we believe that it is as follows. When a galvannealed steel sheet is contacted with an acidic solution, zinc from the side of the steel sheet dissolves. Since such a dissolving of zinc results in a reaction of hydrogen generation at the same time, hydrogen ion concentration in the acidic solution decreases as dissolving of zinc proceeds and, a.s a result, the pH of the acidic solution increases and an oxide layer mainly comprising Zn is believed to be formed on the surface of the galvannealed steel sheet. When an acidic solution having no pH-buffering action is used at that time, the pH of the acidic solution immediately rises and dissolving of zinc which is sufficient for the formation of an oxide layer is not achieved whereupon formation of an oxide layer which is sufficient for an improvement of a sliding performance is not formed.

[0039] On the contrary, when an acidic solution having a pH-buffering action is used, zinc is dissolved and, even when a hydrogen generation reaction takes place, dissolving of zinc briskly proceeds since a rise in pH of the solution is mild and, as a result, formation of an oxide which is sufficient for improving a sliding performance takes place. Further, when Fe ion is contained in the acidic solution, a reduction reaction of Fe ion takes place and a very small amount of Fe is separated on the coated surface and it is presumed that, as a result, an excessive growth of the oxide layer mainly comprising Zn is suppressed and a very fine oxide layer is formed.

[0040] There are two kinds of Fe ion and they are Fe2+ ion a~ad Fe+~ ion.
Although both are effective for the formation of a fine oxide layer, much sludge is generated in the solution when Fe+3 is present and problems in terms of appearance such as formation of damage caused by pressing on the surface of the steel sheet occur. Therefore, it is better that the Fe+3 ion concentration is as small as possible. However, Fe+2 ion contained in the acidic solution is oxidized due to a change with the passage of time whereupon Fe~3 ~S present and, therefore, it is substantially impossible to conduct an operation using an acidic solution containing no Fe+3 ion.
Accordingly, it is important to control the Fe+3 ion concentration in the acidic solution and, in view of the goal of avoiding damage caused by pressing, it is effective to control the Fe+3 ion concentration to an extent of not more than about 2.0 g/l. With regard to the means for controlling the Fe+3 ion as such, a means where a treating solution is renewed when the Fe+3 ion concentration becomes more than about 2.0 g/1 and a means where Fe is dissolved in a solution to utilize a reduction reaction of Fe+~ ion may be exemplified.

[0041] It is preferred that an acidic solution having a pH-buffering action has a pH-buffering action within a range of pH about 2.0 to about 5Ø That is because, when an acidic acid solution having a pH-buffering action is used with the above pH range, an oxide layer is stably prepared upon contact with the acidic solution followed by keeping for a predetermined time. As a yardstick for the pH-buffering action as such, an evaluation is able to be conducted by means of an increasing degree of pH which is defined as the amount (ml) of a 1.0 mol/1 aqueous solution of sodium hydroxide required to raise the pH of one liter of the acidic solution from about 2.0 to about 5.0 and, when the value is within a range of about 3.0 to about 20.0, an oxide layer having a thickness of not less than about 10 nm is able to be stably formed on a flattened part of the coating layer. The reason why the range of pH is made about 2.0 to about 5.0 here is that, in a region where pH is more than about 5.0, zinc oxide is produced and, even when it is contacted with an acidic solution and kept for a predetermined time, it is hard to form an oxide layer having a thickness of about 10 nm or more and that the pH increasing behavior when pH is less than about 2.0 does not substantially affect the easiness of production of the oxide. When the pH
rising degree is less than about 3.0, the pH increases quickly and dissolving of zinc which is sufficient for the formation of an oxide layer is not achieved whereby a sufficient production of an oxide Layer is not available. When the value is more than about 211.0, dissolving of zinc is promoted and a long time is needed far the production of an oxide layer. In addition, damage to the coating layer is significant whereupon it is likely that the role which is inherent to a rust-preventing steel sheet is lost as well. The pH increasing degree of an acidic solution where the pH is mare than about 2 is evaluated by such a manner that an inorganic acid which rarely has a pH-buffering property within a range of pH about 2 to about 5 such as sulfuric acid is added to the acidic solution to lower the pH to about 2.
[0042 With regard to the acidic solution having a pH-buffering action as such, it is possible to use an aqueous solution containing about 5 to about 50 g/1 of at least one substance selected from acetate such as sodium acetate (CH3COONa), phthalate such as potassium hydrogen phthalate ({KOOC)2C6H4), citrate such as sodium citrate (Na3C6H50~) and potassium dihydrogen citrate (KHZC6H507), succinate such as sodium succinate (Na2C4H404), lactate such as sodium lactate (NaCH3CHOHCO2), tarirate such as sodium tartrate (NaZC4H40~), borate and phosphate. When the above concentration is less than about 5 g/l, an increase ire pH of the solution takes place relatively quickly together with dissolving of zinc and, therefore, it is not possible to form an oxide layer which is sufficient for improving the sliding performance. When it is more than about SO g/l, dissolving of zinc is promoted and the result is believed to be that not only is a long time needed for formation of an oxide layer, but also damage of the coating layer is significant whereby the role inherent to the rust-preventive steel sheet is lost. When the pH of the acidic solution is too low, although dissolving of zinc is promoted, an oxide is hardly formed and, therefore, the pH is preferred to be about 1.0 or more. On the other hand, when the pH is too high, the reaction rate for dissolving of zinc becomes low and, therefore, it is preferred that pH of the acidic solution is about 5.0 or Less. When the pH of the acidic solution is higher than the range of about 1.0 to about 5.0, the pH may be adjusted using an inorganic acid having no pH-buffering property such as sulfuric acid or an acid solution of the salt used such as acetic acid, phthalic acid or citric acid.
[0043] With respect to the Fe ion contained in the acidic solution, it is preferred that at least one member selected from sulfate, nitrate and chloride of Fe is added and the range of Fe ion concentration is about 0.1 to about I00 g/l. When the Fe ion concentration is less than about 0.1 g/1, there is a possibility that an oxide is formed by an effect of a pH
buffer only and that control of the thickness of the oxide layer and making the oxide fine are difficult.
When it is more than about 100 g/l, there is a possibility that suppression of grov~th of an oxide layer becomes excessive and that formation of an oxide necessary for improving the sliding performance is not possible. Addition of Fe ion is effective for controlling the thickness of an oxide layer and making the oxide fine but, on the other hand, Fe ion in the solution promotes the dissolving of a Zn coating layer and the coating layer becomes fragile and, as a result, peeling-off of the coating or the so-called "powdering" is apt to happen by the process upon pressing.
From such a view, it is preferred that Fe ion is not mare than about IO g/1 and, when application to a site receiving severer bending and deformation upon return from the bending is taken into consideration, it is preferred to treat with a treating solution where Fe ion is not more than about 5 gll. The term "Fe ion concentration" means the sum of Fe2+ ion and Fe+3 ion concentration.
[0044] As such, according to the invention, an oxide layer having an excellent sliding performance is stably formed and it is also excellent in chemical conversion treatment performance and adhesiveness when the acidic solution used has a pH-buffering action and also contains Fe ion and, accordingly, the advantage of the invention is not deteriorated even if other metal ions and inorganic compounds are contained in the acidic solution either as impurities or intentionally. Especially because Zn ion is an ion which is eluted when the steel sheet contacts the acidic solution, an increase in Zn ion is noted in the acidic solution during the operation, but the amount of Zn ion concentration as such does not affect the advantage of the invention at all.
[0045] There is no particular limitation for the method of contacting a galvannealed steel sheet to an acidic solution and, although there are methods where a coated steel sheet is immersed in an acidic solution, an acidic solution is sprayed on a coated steel sheet, an acidic solution is applied to a coated steel sheet via an application roll, etc., it is preferred that a thin liquid film is finally present on the surface of the steel sheet. That is because, when the amount of the acidic solution present on the surface of the steel sheet is too high, the pH of the solution does not increase even when zinc is dissolved, but dissolving of zinc just takes place successively.
Thus, a long time is needed for the oxide layer to form. In addition, damage of the coating layer is significant whereupon it is likely that an inherent role as a rust-preventing steel sheet is lost as well. From such a viewpoint, it is preferred and effective when the amount of the solution membrane formed on the surface of the steel sheet is adjusted to not more than about 3 g/cm2.
Adjustment of the coating weight of the solution film may be carried out using a s~~ueezing roll, an air wiper or the like.
[0046) With regard to the time from contacting with an acidic solution until washing with water {retention time until washing with water), about 1 to about 30 seconds) is used. That is because, when the time until washing with water is less than about 1 second, the pH of the solution increases and an acidic solution is washed out before an oxide layer mainly comprising Zn is formed whereby the effect of improving the sliding performance is not achieved while, even when it is more than about 30 seconds, there is no change in the amount of the oxide layer.
[0047] When the contact with an alkaline solution is carried out to conduct an activating treatment before formation of an oxide layer by contacting with an acidic solution as mentioned above, that is more effective. That is because, although the surface oxide layer is destructed by contacting to a roll upon a temper rolling, a part thereof still remains and, therefore, reactivity of the surface is not uniform. From such a viewpoint, it is important that the oxide layer remaining on the surface layer is removed as much as possible and, when the cantact to an alkaline solution as a means therefor is conducted, it is possible to treat relatively easily.
There is no particular limitation for contacting with the alkaline solution and an effect is achieved when treated by means of immersing or spraying. In the case of an alkaline solution, the oxide layer remained on the surface layer is able to be removed as much as possible and activate the surface but, when the pH is Iow, the reaction is slow and a long time is needed for the treatment and, therefore, pH of the alkaline solution is preferably to be not less than about 10. When the pH
is within the above range, it is possible to use sodium hydroxide or the like regardless of the type of the solution.

[0048] When an acidic solution remains on the surface of the steel sheet after washing with water and drying, rust is apt to be generated when the steel sheet coil is preserved for a long period. From the viewpoint of preventing the generation of the rust as such, contacting with an alkaline solution may be conducted either by immersing in the alkaline solution or by spraying the alkaline solution thereon after contacting with the acidic solution whereby the acidic solution remaining on the surface of the steel sheet may be neutralized. It is preferred that the pH of the alkaline solution is not more than about 12 for the prevention of dissolving of a Zn type oxide formed on the surface. When the pH is within the above-mentioned range, it is possible to use sodium hydroxide, sodium phosphate or the like regardless of the solution used.
[0049] The oxide layer in the invention is a layer comprising oxide and/or hydroxide containing Zn and Fe as essential elements.
[0050] In the manufacture of the galvannealed steel sheet of the invention, A1 is added to a coating bath and there is no particular limitation for the added element components which are other than Al. Thus, besides Al, the advantage of the invention i.s not deteriorated even when Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu or the like are contained therein or added thereto in addition to Al.
[0051] Further, the advantage of the invention is not deteriorated even when S, N, P, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si or the like are incorporated into an oxide layer as a result of the presence of impurities in the treating solution used for an oxidizing treatment or the like.
Example 1 [0052] The invention will now be illustrated in more detail by way of the following Examples.
[0053] A galvannealed steel sheet was produced by a common method on a cold rolled steel sheet having a thickness of 0.8 mm and then a temper rolling was further carried out. After that, an oxide layer was formed using a treating device as shown in Fig. 1.

[0054] Firstly, after a immersing in an acidic solution of pH 2.0 was carried out at 50°C in an acidic solution bath 2, a liquid filin was formed on the surface of the steel sheet using a squeezing roll 3. When the pressure of the squeezing roll was changed at that time, the coating weight of the liquid film was adjusted. After that, hot water o:F 50°C
was sprayed on the steel sheet in a washing bath 5, the steel sheet was passed through the neutralizing bath 6, hot water of 50°C was sprayed on the steel sheet in a washing bath 7 to wash and drying was conducted in a drier 8 whereupon an oxide layer was formed on a coated surface.
[0055] With regard to a solution for a immersing treatment in the acidic solution bath 2, a solution where 30 g/1 of disodium hydrogen phosphate and 20 g~T of citric acid were mixed as pH
buffers and a predetermined amount of ferrous sulfate was added for an object of addition of Fe ion was used and the pH was adjusted by addition of sulfuric acid. For the sake of comparison, a solution which was the same as above except that no pH buffer was used, but adjustment was conducted by ferrous sulfate only was used.
[0056] The retention time until the above washing with water is the time until the start of the washing in a washing bath 5 when the liquid film amount was adjusted by a squeezing roll 3 and an adjustment was conducted by changing the Iine speed and, at the same time, a thing where the steel sheet was washed immediately after squeezing using a shower washing device 4 at the outlet side of the squeezing roll 3 was partly prepared as well.
[0057] An alkaline solution of pH 10 (an aqueous solution of sodium hydroxide) was sprayed during the above treatment in a neutralizing bath 6 so that an acidic solution remaining on the surface of the steel sheet was neutralized. Also, before immersing into an acidic solution, immersing into an aqueous solution of sodium hydroxide of pH 12 in an activating bath 1 was carried out to conduct an activating treatment.

[0058] After that, the coefficient of friction was measured as a means for evaluating the press formability for the steel sheet prepared as above, a peeling-adhesion test was conducted for evaluating adaptability of adhesion and evaluating chemical conversion treatment performance was carried out. In addition, after the steel sheet was applied with a rust-preventing oil, it was allowed to stand outside under such a condition that it was not affected by external factors such as dust and, after about six months, generation of rust spots was investigated. The case where no rust spot was evaluated as "o" and the case where rust spots were noted was evaluated as "x".
Measurement of coefficient of friction, peeling-adhesion test and chemical processing treatment test were carried out as shown below.
( 1) Test for evaluation of press formability (test by measurement of frictional coefficient) [0059] The frictional coefficient of each test material was measured to evaluate the press formability as follows.
[0060] Fig. 2 is a rough front view showing the device for the measurement of frictional coefficient. As shown in the drawing, a sample 11 for the measurement of frictional coefficient collected from the test material was f-wed on a sample stand 12 and the sample stand 12 was fixed on the upper side of a slide table 13 which is able to move in a horizontal direction. On the lower side of the slide table 13, there is installed a slide table stand 15 which is movable up and down and has a roller 14 contacting thereto and a first load cell I7 for measuring the pushed load N to a sample 11 for measuring the frictional coefficient by a bead 16 when pushed up is attached to a slide table stand I5. A second load cell 18 for measuring the slide resistance F for moving the slide table 13 in a horizontal direction under a state where the above pushing force is applied is attached to one end of the slide table 13. A test was conducted using 2 liters of Preton R 352 L which was a washing oiI for pressing manufactured by Sugimura Kagaku and which was applied on the surface of the sample 11 as a lubricant.

[0061] Fig. 3 and Fig. 4 are rough oblique views for showing the shape and the size of the bead used. Sliding takes place in such a state that the lower side of the bead 16 is pushed onto the surface of the sample 11. The shape of the bead I6 as shown in Fig. 3 is that the width is 10 mm, the length of the sample in the sliding direction is 12 mm and the Lower part of both ends in the sliding direction has a curved surface with a curvature of 4.5 mm R and the lower side of the bead to which the sample is pressed has a plane where the width is 10 mm and the length in the sliding direction is 3 mm. The shape of the bead 16 as shown in Fig. 4 is that width was 10 mm, length of the sample in the sliding direction was 6~ mm and the lower part of both ends in the sliding direction has a curved surface with a curvature of 4.5 mm R and the lower side of the bead to which the sample is pressed has a plane where the width is 10 mm and the length in the sliding direction is 60 mm.
[0062] The test for measuring the frictional coefficient was carried out under the following two conditions.
[Condition 1]
[0063] A bead as shown in Fig. 3 was used where a pushing load (N) was set to 400 kgf and a pulling-out speed for the sample (speed for movement of the slide table 13 in the horizontal direction) was set to 100 cm/min.
[Condition 2]
[0064] A bead as shown in Fig. 4 was used where a pushing load (N) was set to 400 kgf and a pulling-out speed for the sample (speed far movement of the slide table 13 in the horizontal direction) was set to 20 cm/min.
[0065] A frictional coefficient p between the test material and the bead was obtained by ~=FIN.
(2) Adhesiveness test [0066] The following test sample for the following test for adheaiveness was prepared from each sample material. Fig. 5 is a rough oblique view illustrating the assembling process therefor. As shown in the drawing, two test materials 21 having 25 mm width and 200 mm length were made into a test sample for an adhesive test 24 for an adhesive 2?'~ using a spacer 22 of 0.15 mm between them and baking was carried out at 1 SO°C for 10 minutes. The test sample 24 prepared as such was bent in a T-shape as shown in Fig. 6 and pulled at the rate of 200 mm/min using a tensile tester to conduct a peeling test. Incidentally, an adhesive of a vinyl chloride resin type for hemming was used as an adhesive.
[0067] Incidentally, the peeling is generated at the area where the strength is weakest. For example, when the close contact of the test material to the adhesive is sufficient, adhesion failure inside the adhesive happens. On the other hand, when the close contact of the test material to the adhesive is insufficient, peeling occurs at the interface between the test material and the adhesive.
Thus, adaptability of the adhesive was evaluated according to such a peeling mode and, in the case where adhesion failure inside the adhesive was resulted, :it was evaluated as "o" while, in case where peeling took place at the interface between the test material and the adhesive, it was evaluated as "x".
[0068] In the case of a ga.lvannealed steel sheet, strength of the interface between plating part and steel sheet was weak depending upon the Fe% in the coat: particularly in the coat where T
phase was formed at the interface between plating part and steel sheet and there were some cases where peeling was noted at that part. Even in those cases however, close contact of the test material to the adhesive was judged to be sufficient and was evaluated as "o".
{3) Test for chemical conversion treatment performance [0069] Each of the test materials was treated under the conventional condition using a zinc phosphate treating solution of a dipping type used for undercoating of automobiles (PBL 3080 manufactured by Nippon Parkerizing) and a zinc phosphate coat was formed on its surface.
Crystalline state of the zinc phosphate coat formed as such was observed under a scanning electron microscope (SElvl) and the case where the coat was uniformly formed was evaluated as "O" while the case where the coat was not uniform where gaps were noted was evaluated as "x".
[0070] Result of the test as above is shown in Tahle 1.
(4) Test for powdering-resistant property (0071] A draw-bead test for each test material was conducted and a peeled amount of the plating was measured to evaluate the powdering-resistant property. Fig. 7 is a rough scheme showing a draw-bead tester. The weight was measured (Wl (g)) after the coating on the side of the test material which did not contact the bead was peeled using hydrochloric acid.
After that, the test material was set at the sample part of Fig. 7, a triangular bead having an end angle of O.SR was pushed with a load of 500 kgf to make the pushing depth 4 mm and then the test material was drawn out at a constant speed of 200 mm/min. The drawn-out test material was subjected to a compulsory peeling at the area contacting the bead using a tape and the weight was measured again (W2 (g)). The difference in the weight obtained as such was divided by the drawn-out area to calculate the peeled amount per unit area_ [0072] The following were now apparent from the test result as shown in Table 1.
(1) Since No. 1 and No. 2 were not subjected to a treatment by an acidic solution, they do not form an oxide layer which is sufficient for improving a sliding performance at the flattened part and a frictional coefficient is high.
(2) Nos. 3 to 5 are comparative examples where a treatment was conducted using an acidic solution containing no pH buffer and, as compared with No. 1 and No. 2, frictional coe~cient is low but, as compared with the examples of the invention, it is high and formation of an oxide layer is insufFcient.

(3) Nos. 6 to 8 are comparative examples where a treatment was conducted using an acidic solution (aqueous solution of sulfuric acid) having a pH-t>uffering action but containing no Fe ion and, although a frictional coefficient is low, an adaptability to adhesives or a chemical conversion treatment performance is poor.
(4) Nos. 9 to 14 and Nos. 18 to 20 are the examples of the invention where a treatment was conducted using an acidic solution (an aqueous solution of sulfuric acid) having a gH-buffering action and containing Fe ion whereupon a frictional coefficient is low and both an adaptability to adhesives and a chemical conversion treatment performance are excellent.
(5) Nos. 15 to 17 are the examples of the invention where an alkaline treatment was conducted in an activating bath before the treatment with an acidic solution is performed under the same conditions as in Nos. 12 to 14 and there was achieved the effect that the frictional coefficient is far lower as compared with the examples where the retention time until washing with water is same. In addition, as a result of the use of a neutralizing bath, no rust spot was generated and a steel sheet coil forming an oxide layer was pre served for a long period before its use whereby an ability of preventing the generation of rust is excellent.
(6) There is a tendency that the peeled amount of the plating in the draw-bead test in each test material is small when treated with a treating solution where Fe concentration is not more than S glliter and a powdering-resistant property is excellent as well.

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zz [0073] The invention will now be illustrated in more detail by way of the following Examples.
[0074] A galvannealed steel sheet was produced by a common method on a cold rolled steel sheet having a thickness of 0.8 mm and then a temper rolling was further carried out. After that, an oxide layer was formed using a treating device having a constitution as shown in Fig. 1.
[0075] Firstly, after immersion in an acidic solution of pH 2.0 was carried out at 50°C in an acidic solution bath 2, a liquid film was formed on the surface of the steel sheet using a squeezing roll 3. When the pressure of the squeezing roll was changed at that time, the coating weight of the liquid film was adjusted. After that, hot water of 50°C
was sprayed on the steel sheet in a washing bath 5, the plate was passed through the neutralizing bath 6, hot water of 50°C
was sprayed on the steel sheet in a washing bath 7 to wash and drying was conducted in a drier 8 whereupon an oxide layer was formed on a coated surface.
[0076] With regard to a solution for all immersion treatment in the acidic solution bath 2, a solution where 30 g/1 of disodium hydrogen phosphate and 20 g,ll of citric acid were mixed as pH
buffers and a predetermined amount of ferrous sulfate was added far the purpose of adding Fe ion was used and the pH was adjusted by addition of sulfuric acid. For the sake of comparison, a solution which was the same as above except that no pH buffer was used, but adjustment was conducted by ferrous sulfate only was used: In addition, to check the influence of Fe+3 ion, solutions to which ferric sulfate was added were partially used.
[0077] The retention time until the above washing with water is the dune until the start of the washing in a washing bath 5 when the liquid film amount was adjusted by a squeezing roll 3 and a.n adjustment was conducted by changing the line speed and, at the same time, the steel sheet was washed immediately after squeezing using a shower washing device 4 at the outlet side of the squeezing roll 3.

[0078] Also, an alkaline solution of pH 10 (an aqueous solution of sodium hydroxide) was sprayed during the above treatment in a neutralizing bath 6 so that an acidic solution remaining on the surface of the steel sheet was neutralized. Before immersing into an acidic solution, dipging into an aqueous solution of sodium hydroxide of pH 12 in an activating bath 1 was carried out to conduct an activating treatment.
[0079] After that, the coefficient of friction was measured as a means for evaluating the press formability for the steel sheet prepared as above, a peeling-adhesion test was conducted for evaluating adaptability of adhesion and evaluating chemical conversion treatment erformance and powdering-resistant property was carried out. In addition, after the steel sheet was applied with a rust-preventing oil, it was allowed to stand outside under such a condition that it was not affected by external factors such as dust and, after about six months, generation of rust spots was investigated. The case where no rust spot was evaluated as "~~" and the case where rust spots were noted was evaluated as "X".
[0080] The results of the test obtained by the above are shown in Table 2.
[0081] The following became apparent from the test result as shown in Table 2.
(1) Since No. l and No. 2 were not subjected to a. treatment by an acidic solution, they do not form an oxide layer which is sufficient for improving sliding performance at the flattened part and the frictional coefficient is high.
(2) Nos. 3 to 5 are comparative examples where a treatment was conducted using an acidic solution containing no pH buffer and, as compared with No. 1 and No. 2, the frictional coefficient is Iow but, as compared with the examples of the invention, it is high and formation of an oxide layer is insufficient.
(3) Nos. 6 to 8 are comparative examples where a treatment was conducted using an acidic solution (aqueous solution of sulfuric acid) having a pH-buffering action, but containing no Fe ion and, although the frictional coefficient is low, an adaptability to adhesives or a chemical conversion treatment performance is poor.
(4) Nos. 9 to 14 and Nos. 24 to 26 are the examples of the invention where a treatment was conducted using an acidic solution (an aqueous solution of sulfuric acid) having a pH-buffering action and containing Fe ion whereupon the frictional coefficient is low and both the adaptability to adhesives and a chemical conversion treatment performance are excellent.
(5) Nos. 15 to 17 are the examples of the invention where an alkaline treatment was conducted in an activating bath before the treatment with an acidic solution is done under the same condition as in Nos. 12 to 14 and there was achieved the effect that the frictional coefficient is far lower as compared with the examples where the retention time until washing with water is the same. In addition, as a result of the use of a neutralizing bath, no rust spots were generated and even when a steel sheet coil forming an oxide layer was preserved for a long period before its use, an ability of preventing the generation of rust is excellent.
(6) Nos. 18 to 23 are the examples where Fe+3 ion concentration is changed by addition of ferric sulfate. All of them show lowering of the frictional coefficient and both adaptability of adhesives and chemical conversion treatment performance are excellent but, in Nos. 18 to 20, Fe+3 ion concentration is within a range of the invention whereby generation of damage caused by pressing is not noted at all while, in comparative examples of Nos. 21 to 23, Fe+3 ion concentration is out of the range of the invention whereby damage caused by pressing is resulted.
(7) There is a tendency that the peeled amount of the plating in the draw-bead test in each test material is small when treated with a treating solution where the total Fe concentration is not mare than 5 g/liter and a powdering-resistant property is excellent as well.

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Claims

1. ~A method for manufacturing a galvannealed steel sheet comprising the steps of:
(a) ~hot dip galvanizing a steel sheet;
(b) ~heating the hot dip galvanized steel sheet to alloy the coating layer, thereby forming a galvannealed steel sheet;
(c) ~temper rolling the galvannealed steel sheet;
(d) ~contacting the temper-rolled steel sheet with an acidic solution having pH-buffering action and containing Fe ion, and then allowing the temper-rolled steel sheet to stand for about 1 to about 30 second(s)to form an oxide layer on a surface of the temper-rolled steel sheet; and (e) ~washing the temper-rolled steel sheet, on which the oxide layer is formed, with water.

2. ~The method according to claim 1, wherein the acidic solution contains Fe ion where Fe3+ ion concentration is about 2 g/ liter or less and the balance is Fe2+ ion.

3. ~The method according to claim 1, wherein a pH-increasing degree of the acidic solution is within a range of about 3 to about 20, wherein the pH-increasing degree being defined as the amount (ml) of 1 mol/ liter aqueous solution of sodium hydroxide necessary to increase pH of one liter of the acidic solution from 2 to 5.~~

4. ~The method according to claim 1, wherein the acidic solution contains at least one component selected from the group consisting of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate in an amount within a range of about 5 to about 50 g/ liter and has a pH within a range of about 1 to about 5.

5. ~The method according to claim 1, wherein the acidic solution contains at least one of sulfate, nitrate and chloride of Fe in an amount within a range of about 0.1 to about 100 g/l in terms of an Fe ion concentration.

6. ~The method according to claim 1, further comprising contacting the temper-rolled steel sheet with an alkaline solution to activate the surface, before step (d).

7. ~The method according to claim 1, further comprising contacting the temper-rolled steel sheet with an alkaline solution to neutralize acidic solution remaining on the surface, after step (d).

8. ~The method according to claim 1, wherein contacting with the acidic solution comprises contacting the temper-rolled steel sheet with an acidic solution such that a solution film formed on the surface of the steel sheet after contacting with the acidic solution has a coating weight of about 3 g/m2 or less.

9. ~A galvannealed steel sheet having an oxide layer with a thickness of about

10 nm or more on a surface flattened part of a hat dip galvanized steel sheet manufactured by the method comprising:
(a) ~hot dip galvanizing a steel sheet;
(b) ~heating the hot dip galvanized steel sheet to alloy the coating layer;
(c) ~temper rolling the galvannealed steel sheet;
(d) ~contacting the temper-rolled steel sheet with an acidic solution having a pH-buffering action and containing Fe ion, and then allowing the temper-rolled steel sheet to stand for about 1 to about 30 second(s),to form an oxide layer on a surface of the temper-rolled steel sheet; and (e) ~washing the temper-rolled steel sheet, on which the oxide layer is formed, with water.

10. ~The galvannealed steel sheet according to claim 9, wherein the acidic solution contains Fe ion where Fe3+ ion concentration is about 2 g/ liter or less and the balance is Fe2+ ion.

11. ~The galvannealed steel sheet according to claim 9, wherein a pH-increasing degree of the acidic solution is within a range of about 3 to about 20, wherein the pH-increasing degree is defined as the amount (ml) of 1 mol/ liter aqueous solution of sodium hydroxide necessary to increase the pH of one liter of the acidic solution from 2 to 5.

12. ~The galvannealed steel sheet according to claim 9, wherein the acidic solution contains at least one component selected from the group consisting of acetate, phthalate, citrate, succinate, lactate, tartrate, borate and phosphate in an amount within a range of about 5 to about 50 g/ liter and has a pH within a range of about 1 to about 5.

13. ~The galvannealed steel sheet according to claim 9, wherein the acidic solution contains at least one of sulfate, nitrate and chloride of Fe in an amount of about 0.1 to about 100 g/l in terms of an Fe ion concentration.

14. ~The galvannealed steel sheet according to claim 9, further comprising contacting the temper-rolled steel sheet with an alkaline solution to activate the surface, before step (d).

15. ~The galvannealed steel sheet according to claim 9, further comprising contacting the temper-rolled steel sheet with an alkaline solution to neutralize acidic solution remaining on the surface, after step (d).

16. ~The galvannealed steel sheet according to claim 9, contacting to the acidic solution comprises contacting the temper-rolled steel sheet with an acidic solution such that a liquid film formed on the surface of the steel sheet after contacting with the acidic solution has a coating weight of about 3 g/m2 or less.