JP2005120447A - Galvanized steel sheet having excellent press formability, and its production method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvanized steel sheet having reduced sliding resistance at the time of press forming, and having excellent press formability. <P>SOLUTION: In the galvanized steel sheet where the plating layer is composed mainly of a η phase, an oxide layer including Zn based oxide and Al based oxide with an average thickness of ≥10 nm is present on the surface of the plating, and also, the percentage of the oxide consisting essentially of Zn in which the Zn/Al ratio included in the oxide layer (the ratio at an atomic concentration in the oxide layer) is ≥4 occupied in the surface of the plating is ≥70% by area. Further, the oxide essentially consisting of Zn in which the Zn/Al ratio is ≥4 is preferably present at the projecting parts in the surface of the plating having ruggedness. Further, the oxide consisting essentially of Zn in which the Zn/Al ratio is ≥4 preferably comprises Fe by 1 to 50at% as the Fe atomic concentration defined by Fe/(Zn+Fe). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a hot-dip galvanized steel sheet excellent in slidability, adhesive bondability and chemical conversion treatment during press molding, and a method for producing the same.

In recent years, from the viewpoint of improving rust prevention, the ratio of use of galvanized steel sheets, particularly hot dip galvanized steel sheets, is increasing in automotive panel parts. Hot-dip galvanized steel sheets include those that have been subjected to alloying treatment after galvanizing and those that have not been subjected to alloying. Generally, the former is called an alloyed hot-dip galvanized steel sheet, and the latter is called a hot-dip galvanized steel sheet. Usually, hot dip galvanized steel sheets used for automotive panels are alloyed and melted by applying the alloying treatment by heating to about 500 ° C after hot dip galvanizing, taking advantage of the characteristics of excellent weldability and paintability. Galvanized steel sheet is used.
In addition, with the aim of further improving rust prevention, automobile manufacturers are increasingly demanding thick-coated galvanized steel sheets. However, when the above-mentioned alloyed hot-dip galvanized steel sheets are thickened, it is effective for alloying. Time-consuming, poor alloying, so-called burn unevenness is likely to occur, and conversely, when trying to complete alloying with the entire plating layer, it becomes overalloyed and a brittle Γ phase is generated at the plating-steel interface, and during processing Since plating peeling easily occurs, it is very difficult to produce a thick alloyed hot-dip galvanized steel sheet.

  For this reason, hot dip galvanized steel sheets are effective for thickening. However, when press-molding hot-dip galvanized steel sheets to automotive panels, adhesion is caused by a higher sliding resistance with the mold and a lower melting point of the surface compared to the alloyed hot-dip galvanized steel sheets. There is a problem that it is easy to occur and press cracks are likely to occur.

  As a technique to solve such problems, Patent Document 1 and Patent Document 2 describe a technique for controlling the surface roughness of a hot-dip galvanized steel sheet to suppress die squeezing during press forming and deep drawability. A technique has been proposed. However, a detailed examination of such a hot dip galvanized steel sheet revealed that when the sliding distance from the mold is short, there is an effect of suppressing adhesion with the mold, but the sliding distance is long. Indeed, this effect becomes smaller, and an improvement effect cannot be obtained depending on the sliding conditions. In addition, in the above proposal, as a method of imparting such roughness, a method of controlling the roll conditions and rolling conditions of skin pass rolling is mentioned, but actually, because zinc causes clogging in the roll, It is difficult to stably give a predetermined roughness to the surface of a hot dip galvanized steel sheet.

  Patent Document 3 proposes a galvanized steel sheet in which an oxide film mainly composed of ZnO is formed on the plating surface. However, it is difficult to apply this technique to hot-dip galvanized steel sheets. Normally, when manufacturing hot dip galvanized steel sheets, a small amount of Al is contained in the zinc bath to suppress excessive Fe-Zn alloying reaction and ensure plating adhesion when immersed in a zinc bath. Is added. Due to the Al contained in a small amount, an Al-based oxide is densely formed on the surface of the hot-dip galvanized steel sheet, and therefore the surface is inactive and an oxide film mainly composed of ZnO cannot be formed. Even if such an oxide film is applied to the upper layer of the densely generated Al-based oxide layer, not only does the adhesion between the applied oxide film and the base be poor, but a sufficient effect cannot be obtained, and at the time of processing There is a problem that adversely affects the press product, such as sticking to the press mold and making a scratch on the press.

  In addition, Patent Document 4 includes Mo oxide film, Patent Document 5 includes Co-based oxide film, Patent Document 6 includes Ni oxide film, and Patent Document 7 includes Ca-based oxide film. Although a galvanized steel sheet formed on the surface has been proposed, a sufficient effect cannot be obtained for the same reason as the above-described ZnO-based oxide film.

  Patent Document 8 describes a technique relating to a zinc-based plated steel sheet provided with an oxide film composed of an Fe-based oxide, a Zn-based oxide, and an Al-based oxide. Similar to the above, in the case of hot-dip galvanized steel sheet, the surface is inactive, so the Fe oxide formed initially becomes non-uniform, the amount of oxide to obtain the effect is large, and problems such as oxide peeling occur. .

The prior art document information is described below. Non-patent document 1 will be described in the section of [Best Mode for Carrying Out the Invention] for convenience of explanation.
Japanese Patent Laid-Open No. 2002-4019 Japanese Patent Laid-Open No. 2002-4020 Japanese Patent Laid-Open No. 2-190483 Japanese Patent Laid-Open No. 3-191091 Japanese Patent Laid-Open No. 3-191092 Japanese Patent Laid-Open No. 3-191093 Japanese Patent Laid-Open No. 3-191094 JP 2000-160358 A Masayasu Nagoshi and two others, "Surface of real material as seen by ultra-low acceleration scanning electron microscope", Surface Technology, 2003, 54, No. 1, p.31-34

  The present invention has been made to solve the above-mentioned problems. A hot-dip galvanized steel sheet having a small sliding resistance during press molding and excellent in press formability, adhesive bondability, and chemical conversion treatment, and a method for producing the same are provided. The purpose is to provide.

  As a result of adding various studies to solve the above-mentioned problems, the inventors have formed an oxide layer containing a specific Al-based oxide and a Zn-based oxide on the hot-dip galvanized steel sheet surface. It was found that good pressability was obtained over a wide range of sliding conditions. This is due to the following reason.

  As described above, since the Al-based oxide layer is formed on the surface of the hot dip galvanized steel sheet, adhesion to the mold during press molding can be suppressed to some extent. Therefore, in order to further improve the sliding characteristics during pressing, it is considered effective to form a thicker Al-based oxide layer, but in order to grow the Al-based oxide layer thicker, In addition to the fact that it is necessary to oxidize at a high temperature for a long time, which is difficult in practice, there is a disadvantage that the Fe-Zn alloying reaction gradually proceeds and the plating adhesion is deteriorated. On the contrary, in order to form the Zn-based oxide layer, it is necessary to completely remove the Al-based oxide layer on the surface.

  On the other hand, if the surface is oxidized after partially destroying the Al-based oxide layer and exposing the new surface, Zn-based oxide is formed on the new surface, and Zn-based oxidation on the new surface is also performed. A physical layer can be easily provided. The oxide layer on the plating surface formed in this way coexists with Zn-based oxide and Al-based oxide, which enhances the suppression of adhesion with the press die, and therefore, under a wide range of sliding conditions. Good press formability can be obtained.

  The inventors have found that it is important in terms of improving the sliding characteristics that the average thickness and the area ratio of the Zn-based oxide contained in the oxide layer are defined with respect to the oxide layer.

  Also, the slidability of the hot dip galvanized steel sheet is different from the alloyed hot dip galvanized steel sheet, and since the plating is soft, the surface pressure dependency during sliding is large. When the surface pressure is high, the slidability is good.However, when the surface pressure is low, the tendency of the slidability to be inferior is recognized. The main part is in contact with the mold. Therefore, it has been found that in order to further improve the sliding characteristics of the hot-dip galvanized steel sheet under low surface pressure conditions, it is necessary to form the above-described oxide also on the convex portion.

  Furthermore, a larger sliding resistance reduction effect can be obtained by adding Fe to the Zn-based oxide. Although the reason for this is not clear, it is considered that by using an oxide containing Fe, the adhesion of the oxide mainly composed of Zn is improved, and the sliding resistance reduction effect is easily maintained even during sliding. Furthermore, it has been clarified that inclusion of Fe in a Zn-based oxide is also effective for controlling the amount of oxide formed and the shape (size) of the Zn-based oxide.

  Furthermore, the inventors have found that the slidability is further improved by imparting fine irregularities to the Zn-based oxide formed on the plating surface. In the present invention, the fine unevenness is formed by a convex structure and a discontinuous recess surrounded by the protrusion and has a network structure. Preferably, the average roughness Ra of the roughness curve (hereinafter referred to as Ra The surface roughness is 100 nm or less, and the average spacing S of local irregularities (hereinafter referred to as S) is 1000 nm or less. Here, the fine unevenness of the present invention is a size smaller by one digit or more than the surface roughness (Ra: around 1 μm) described in Patent Document 1 and Patent Document 2. Therefore, the roughness parameters such as Ra in the present invention are different from the general roughness parameters that define irregularities of the order of microns (μm) or higher, which are measured for a roughness curve having a length of the millimeter order or more. It is calculated from the roughness curve of micron length. The prior art document defines the roughness of the hot dip galvanized steel sheet surface, whereas the present invention defines the roughness of the oxide layer applied to the hot dip galvanized steel sheet surface.

  Based on the above knowledge, the present invention improves the slidability at a low surface pressure, realizes good press formability, and further improves the chemical conversion property and the adhesive bondability, and achieves both of them optimally. The surface state is realized, and the gist thereof is as follows.

  [1] In a hot-dip galvanized steel sheet in which the plating layer mainly consists of η phase, an oxide layer containing a Zn-based oxide and an Al-based oxide having an average thickness of 10 nm or more exists on the plating surface, and A Zn-based oxide with a Zn / Al ratio (atomic concentration in the oxide layer) contained in the oxide layer of 4 or more (hereinafter simply referred to as a Zn-based oxide with a Zn / Al ratio of 4 or more). A hot-dip galvanized steel sheet characterized by a ratio of 70% or more in terms of area ratio to the plating surface.

  [2] The hot-dip galvanized steel sheet according to [1], wherein the Zn-based oxide is present on a convex portion of a plated surface having irregularities.

  [3] In the above [1] or [2], the Zn-based oxide contains 1 to 50 at% of Fe as a Fe atom concentration defined by Fe / (Zn + Fe). Galvanized steel sheet.

  [4] In the above [1] to [3], the Zn-based oxide includes a convex part smaller than the convex part of the unevenness of the plating surface, and the unevenness of the discontinuous plating surface surrounded by the convex part A hot-dip galvanized steel sheet having fine irregularities formed of a mesh structure and having smaller recesses than the recesses.

  [5] In producing the hot dip galvanized steel sheet according to the above [1] to [4], the steel sheet is hot dip galvanized and further subjected to an activation treatment before or after temper rolling. Then, a method for producing a hot-dip galvanized steel sheet, which is brought into contact with an acidic solution having a pH buffering agent and is subjected to an oxidation treatment in which it is kept for 1 to 30 seconds and then washed with water.

  [6] The method for producing a hot-dip galvanized steel sheet according to [5] above, wherein the Al concentration contained in the oxide layer after the activation treatment is less than 20 at% by the activation treatment.

  [7] The method for producing a hot dip galvanized steel sheet according to [5] or [6] above, wherein the activation treatment is performed by contact with an alkaline solution having a pH of 11 or more and 50 ° C. or more for 1 second or more.

  [8] A method for producing a hot-dip galvanized steel sheet according to the above [5] to [7], further comprising adding 1 to 200 g / l of Fe ions to the acidic aqueous solution.

  In the present invention, the Zn-based oxide and the Al-based oxide are distinguished by the Zn / Al ratio (at ratio) in the surface layer evaluated by Auger electron spectroscopy (AES), and the Zn / Al ratio is A portion exceeding 1.0 is a Zn-based oxide, and a portion having a Zn / Al ratio of 1.0 or less is an Al-based oxide. In addition, the Zn-based oxide may include not only a Zn-based oxide but also a Zn-based hydroxide, or all may be a Zn-based hydroxide. Also, the Al-based oxides may contain not only Al-based oxides but also Al-based hydroxides as well as the Zn-based oxides, and all of them may be Zn-based hydroxides. Good.

  According to the present invention, it is possible to stably produce a hot-dip galvanized steel sheet having a small sliding resistance during press forming and stably exhibiting excellent press formability.

  Since the hot dip galvanized steel sheet is usually manufactured by dipping in a zinc bath containing a small amount of Al, the plating film is mainly composed of η phase, and the surface layer is made of Al by Al contained in the zinc bath. It is a film on which a system oxide layer is formed. The η phase is softer than the ζ phase and δ phase, which are alloy phases of the alloyed hot-dip galvanized film, and has a low melting point. Therefore, adhesion is likely to occur and the slidability during press forming is poor. However, in the case of hot-dip galvanized steel sheet, the effect of suppressing adhesion of the mold is slightly seen due to the formation of the Al-based oxide layer on the surface, so the sliding distance with the mold is particularly short. In some cases, deterioration of sliding characteristics may not be observed. However, since the Al-based oxide layer formed on the surface is thin, adhesion tends to occur when the sliding distance becomes long, and satisfactory press formability cannot be obtained over a wide range of sliding conditions. Furthermore, the hot dip galvanized steel sheet is soft and has a low sliding characteristic when it is easy to adhere to the mold and has a low surface pressure compared to other plating.

In order to suppress such adhesion between the hot dip galvanized steel sheet and the mold, it is effective to uniformly coat a thick oxide layer on the surface. That is, a part of the Al-based oxide layer present on the surface of the plated steel sheet is destroyed and oxidized to form a Zn-based oxide layer, and the oxide layer in which the Zn-based oxide and the Al-based oxide coexist. It is important to improve the sliding characteristics of the hot dip galvanized steel sheet. And, the average thickness of the oxide layer is 10 nm or more, the ratio of the Zn-based oxide contained in the oxide layer to the plating surface (coverage) is 70% or more by area ratio, A reduction in sliding resistance can be realized.
First, the average thickness will be described.

  In the present invention, the oxide layer present on the plating surface has an average thickness of 10 nm or more, more preferably 20 nm or more from the viewpoint of obtaining good slidability. With the above thickness, in the press molding process in which the contact area between the mold and the workpiece increases, the oxide on the plating surface remains even though it is partially worn, and does not cause a decrease in slidability. Because. On the other hand, there is no upper limit to the average thickness of the oxide layer from the viewpoint of slidability, but when a thick oxide layer is formed, the surface reactivity is extremely lowered and it is difficult to form a chemical conversion treatment film. Therefore, the average thickness is desirably 200 nm or less.

  The average thickness of the oxide layer on the plating surface can be obtained by Auger electron spectroscopy (AES) combined with Ar ion sputtering. In this method, after sputtering to a predetermined thickness, the composition at that depth can be obtained by correcting the relative sensitivity factor from the spectral intensity of each element to be measured. Among these, the O content caused by the oxide decreases and becomes constant after reaching a maximum value at a certain depth (this may be the outermost layer). At a position where the O content is deeper than the maximum value, a depth that is ½ of the sum of the maximum value and the constant value is defined as the thickness of the oxide layer.

  In addition, when controlling the thickness of the oxide layer on the plating surface, if you try to make it thicker, it will be thicker in the part where the Zn-based oxide exists, and conversely it will not be thicker in the part where the Al-based oxide remains, When the entire surface of the plated steel sheet is viewed, there may be a case where an oxide layer having a non-uniform thickness in which a thick portion and a thin portion of the oxide layer coexist is formed. However, even if there is a part where the oxide layer is not formed in a part of the thin part for some reason, the slidability can be improved for the same reason as described above.

  Next, the coverage of the Zn-based oxide will be described.

  In order to sufficiently coat the plating surface with the oxide mainly composed of Zn, which is a feature of the present invention, in order to prevent as much as possible the reactivity of Zn and phosphoric acid in the plating layer in the chemical conversion treatment solution during the chemical conversion treatment, and In terms of adhesive bondability, it is important to reduce the amount of Al-based oxides in order to increase the bonding strength with the adhesive. By reducing the amount of Al-based oxides, the ratio of Zn-based oxides to the plating surface ( It is necessary that the area ratio is 70% or more as the coverage ratio. If it is less than 70%, peeling between the adhesive and the surface of the plating layer tends to occur. This is because the adhesive peels locally at the portion not covered with the Zn-based oxide, and the adhesion at the adhesive / plating layer interface is reduced. The Zn-based oxide includes a case where Al is not included.

  As described later, an activation treatment method is effective as a method for reducing the amount of Al-based oxide. For example, a mechanical removal method such as rolling with a roll, shot blasting, or brush grinding, dissolution with an alkaline solution. Etc. are possible.

  The Zn / Al ratio can be evaluated by Auger electron spectroscopy (AES). Similar to the method for evaluating the thickness of the oxide layer described above, the distribution in the depth direction of the composition of the flat portion of the plating film surface is measured, and the average of Zn up to the depth corresponding to the thickness of the oxide layer estimated therefrom The Zn / Al ratio was determined from the concentration (at%) and the average concentration of Al (at%). However, the composition of the oxide formed on the actual surface is not necessarily uniform, and there may be a portion with a high or low Al concentration in a minute region of the nm level. Therefore, it is important to measure the Zn / Al ratio in a relatively wide area of about 2 μm × 2 μm or more in order to evaluate the average composition.

In the method of performing Auger measurement while sputtering, the Al concentration may be higher than the value obtained by measuring the cross section with TEM or the like, but here, it is defined as the measured value by Auger.
The coverage of the Zn-based oxide can be evaluated by elemental mapping with an X-ray microanalyzer (hereinafter referred to as EPMA) or a scanning electron microscope (hereinafter referred to as SEM). With EPMA, the intensity of O, Al, Zn obtained from the oxide of interest, or their ratio, is obtained in advance, and the coverage can be estimated by performing data processing of the element map measured based on it. . Also, the area ratio can be estimated more simply by SEM image observation using an electron beam with an acceleration voltage of about 0.5 kV. In SEM image observation under the above conditions, the portion where the oxide is formed on the surface can be clearly distinguished from the portion where the oxide is not formed. The area ratio can be evaluated by converting into a value. However, it is necessary to confirm beforehand by AES, EDS, or the like whether the observed contrast matches the oxide of interest.
In hot dip galvanizing, the unevenness of the pressure adjusting roll is transferred to the plated steel sheet by temper rolling, and the unevenness is formed on the plating surface. In the concave portion, the Al-based oxide on the surface of the plated steel plate is mechanically destroyed, and the new surface is exposed, which is more active than the convex portion. On the other hand, a convex part is a part which hardly receives the deformation | transformation by a pressure-control roll, Generally the flat state as plating is maintained, and there is little destruction degree of the Al type oxide on the plated steel plate surface. Therefore, the surface of the hot dip galvanized steel sheet after temper rolling has unevenly active and inactive portions. When such a surface is subjected to an oxidation treatment, it is possible to form a Zn-based oxide in the concave portion, but the oxide is formed only in the concave portion, and a flat portion (convex portion) that is convex other than the concave portion. It is difficult to give an oxide to). Of course, even if the Al-based oxide layer is scraped off due to sliding conditions and the situation where adhesion is likely to occur, the coexisting Zn-based oxide layer can exert the effect of suppressing adhesion, so press formability The press formability can be improved without any problem, but in order to further improve the press formability, the convex part directly contacts the tool during sliding, which greatly affects the slidability of the plated steel sheet. In addition, it is preferable that a Zn-based oxide is also present in the convex portion.
In addition, hot-dip galvanized steel sheets have a Zn plating layer that is softer and has a lower melting point than other platings, so the sliding characteristics are likely to change due to surface pressure, and low slidability under low surface pressure conditions. . In order to solve this problem, it is preferable to form a Zn-based oxide also on the convex portion of the plating surface.

Furthermore, a larger sliding resistance reduction effect can be obtained by adding Fe to the Zn-based oxide. The Fe content is preferably 1 to 50% when the Fe atomic ratio calculated from the Fe / Zn atomic concentration by the formula of Fe / (Zn + Fe) is used as an index. More preferably, it is 5 to 25%. If the Fe atomic ratio exceeds 50%, the oxide easily peels off, and it is difficult to obtain a crystal form having fine irregularities as obtained in the present invention, so that sufficient characteristics cannot be obtained. The uneven shape control effect cannot be obtained. The atomic concentration of Fe and Zn in the Zn-based oxide was measured using a transmission electron microscope (TEM) and energy on a cross-sectional sample of the plating surface containing the surface oxide prepared by the FIB-μ sampling method. The most suitable method is to obtain from a spectrum measured using a dispersive X-ray analyzer (EDS). In other methods (for example, AES and EPMA), the spatial resolution of the analysis region cannot be sufficiently reduced, and it is difficult to analyze only the surface oxide.
Further, by providing fine irregularities on the Zn-based oxide, a further reduction in sliding resistance can be realized. Here, the fine unevenness is formed by a convex structure and a discontinuous concave portion surrounded by the convex portion and has a network structure, and it is sufficient that the fine concave portions are dispersed, and the convex portions around the concave portions are the same. There is no need for the height, and there may be some height fluctuation. Preferably, the fine irregularities have a surface roughness of Ra of the roughness curve of 100 nm or less and S of 1000 nm or less. As an example of the structure of the fine irregularities, the surface of the Zn-based oxide has fine irregularities, or directly on the plating surface or on the layered oxide layer and / or hydroxide layer. Alternatively, fine unevenness may be formed by the distribution of Zn-based oxides having shapes such as granular, plate-like, and flake-like shapes.

  The reason why the sliding resistance is reduced by the fine unevenness is considered to be that the concave portions of the fine unevenness work as a group of fine oil pits and can effectively hold the lubricating oil therein. That is, in addition to the above-described sliding resistance reduction effect as an oxide, a further sliding resistance reduction effect is manifested by a fine oil sump effect that can effectively hold the lubricating oil in the sliding portion. The effect of retaining lubricating oil with such fine irregularities has a relatively smooth surface from a macro point of view, making it difficult to retain lubricating oil macroscopically, and macroscopic surface roughness aiming at lubricity by rolling or the like. This is particularly effective for stable sliding resistance reduction of hot dip galvanizing, which is difficult to stably apply. The sliding condition is particularly effective under a sliding condition with a low contact surface pressure.

  The size of the fine irregularities can be represented by Ra and S as described above. In the present invention, it was confirmed that Ra is 1 nm or more and 100 nm or less, and S is 10 nm or more and 1000 nm or less, which has a sliding resistance reducing effect and is preferable. If Ra or S is made smaller than the above, it will approach a smooth surface, and the effect of a viscous oil as a sump will be reduced, which is not preferable. On the other hand, even if Ra or S is made larger than the above, no significant improvement in the sump effect is observed, and it is necessary to thicken the oxide, making it difficult to manufacture. Further, when the oxide comes into contact with the tool during sliding, a coarse Zn-based oxide has an adverse effect of increasing the fracture resistance of the oxide rather than the effect of the oil sump. From the above, more preferably, S is 500 nm or less.

  One effective method for imparting fine irregularities to a Zn-based oxide and controlling Ra and S is to include Fe in the Zn-based oxide. By including Fe in a Zn-based oxide, the Zn oxide gradually becomes finer and the number increases according to its content. Fine irregularities are formed as a collection of fine oxides. The reason why the oxide containing Zn and Fe becomes an oxide having fine irregularities is not clear, but it is estimated that the growth of Zn oxide is suppressed by Fe or the oxide of Fe. Although a suitable ratio (percentage) of Fe to the sum of Zn and Fe is not clear, Fe is effective in the range of 1 at% or more and 50 at% or less, more preferably 5 to 25 at%. Such an oxide containing Zn and Fe can be formed by adding Fe to the acidic solution in a method of forming a Zn-based oxide that is brought into contact with an acidic solution having a pH buffering action described later.

  The surface roughness parameters of Ra and S were extracted by quantifying the surface shape of the Zn-based oxide using a scanning electron microscope or a scanning probe microscope (such as an atomic force microscope) having a three-dimensional shape measurement function. From a roughness curve of several μm in length, it can be calculated according to the mathematical formula described in “Surface Roughness—Terminology” B-0660-1998 etc. of Japanese Industrial Standard. The shape of the fine irregularities can be observed using a high-resolution scanning electron microscope. Since the oxide is as thin as several tens of nanometers, it is effective to observe using a low acceleration voltage, for example, 1 kV or less. In particular, when the secondary electron image is observed except for low energy secondary electrons centered on several eV as the electron energy, the contrast caused by oxide charging can be reduced. Can be observed well (see Non-Patent Document 1).

  Next, a method for forming a Zn-based oxide on the plating surface of the present invention will be described.

  As a method for forming a Zn-based oxide, it is effective to bring a hot dip galvanized steel sheet into contact with an acidic solution having a pH buffering action, and then perform an oxidation treatment that is allowed to stand for 1 to 30 seconds, followed by washing with water and drying. is there.

  As described above, the oxide containing Zn and Fe can be formed by adding Fe to the acidic solution in the above method. A preferable concentration range of Fe is 1 to 200 g / l as divalent or trivalent Fe ions. An even more preferable range is 1 to 80 g / l. The method for adding Fe ions is not particularly specified. For example, if the Fe ion concentration is 1 to 80 g / l, ferrous sulfate (7 hydrate) can be added in the range of 5 to 400 g / l. It is.

  Although the Zn-based oxide formation mechanism is not clear, it can be considered as follows. When the hot dip galvanized steel sheet is brought into contact with an acidic solution, dissolution of zinc occurs from the steel sheet side. This dissolution of zinc causes a hydrogen generation reaction at the same time. As the dissolution of zinc proceeds, the hydrogen ion concentration in the solution decreases, resulting in an increase in the pH of the solution, and a Zn-based oxide on the surface of the hot-dip galvanized steel sheet. It is thought to form. Thus, in order to form Zn-based oxides, it is necessary to increase the pH of the solution in contact with the steel sheet along with the dissolution of zinc. It is effective to adjust the holding time. At this time, if the holding time is less than 1 second, the Zn-based oxide cannot be formed because the solution is washed away before the pH of the solution in contact with the steel plate rises, while it is left for more than 30 seconds. This is because there is no change in oxide formation. Thus, in the holding process, a Zn-based oxide having a special fine relief structure grows. A more preferable holding time is 2 to 10 seconds.

  The pH of the acidic solution used for the oxidation treatment is preferably in the range of 1-5. This is because if the pH exceeds 5, the dissolution rate of zinc is slow, whereas if it is less than 1, the dissolution of zinc is excessively accelerated and the formation rate of Zn-based oxides is slow. In addition, it is essential to add a chemical solution having a pH buffering effect to the acidic solution. This not only provides the pH stability of the treatment solution during actual production, but also prevents a local pH increase in the Zn-based oxide formation process due to the pH increase associated with the above-mentioned Zn dissolution, and provides an appropriate reaction time. This is because the Zn-based oxide growth time is secured by the application, and this acts on the formation of a Zn-based oxide having fine unevenness, which is a feature of the present invention. In addition, the anionic species of the acidic solution is not particularly defined, and examples include chlorine ions, nitrate ions, sulfate ions, and the like. More preferably, it is a sulfate ion.

As such a chemical solution (acidic solution) having pH buffering properties, there is no limitation on the type of chemical solution as long as it has pH buffering properties in the acidic region. For example, acetate salts such as sodium acetate (CH ₃ COONa), Phthalate such as potassium hydrogen phthalate ((KOOC) ₂ C ₆ H ₄ ), sodium citrate (Na ₃ C ₆ H ₅ O ₇ ), potassium dihydrogen citrate (KH ₂ C ₆ H ₅ O ₇ ), etc. Citrates, succinates such as sodium succinate (Na ₂ C ₄ H ₄ O ₄ ), lactates such as sodium lactate (NaCH ₃ CHOHCO ₂ ), sodium tartrate (Na ₂ C ₄ H ₄ O ₆ ), etc. One or more of tartrate, borate and phosphate can be used.

  In addition, the concentration is preferably in the range of 5 to 50 g / l, respectively, because if it is less than 5 g / l, the pH buffering effect is insufficient and a predetermined oxide layer cannot be formed. This is because even if it exceeds 50 g / l, not only is the effect saturated, but it takes a long time to form the Zn-based oxide. When the plated steel sheet is brought into contact with the acidic solution, Zn is eluted and mixed from the plating, but this does not significantly disturb the formation of the Zn-based oxide. Therefore, the Zn concentration in the acidic solution is not particularly specified. A more preferred pH buffering agent and its concentration are those in which sodium acetate trihydrate is in the range of 10-50 g / l, more preferably in the range of 20-50 g / l. The oxide of the present invention can be obtained.

There is no particular limitation on the method of contacting with the acidic solution, such as a method of immersing the plated steel plate in the acidic solution, a method of spraying the acidic solution onto the plated steel plate, a method of applying the acidic solution to the plated steel plate via a coating roll, and the like. It is desirable that it is finally in the form of a thin liquid film and exists on the surface of the steel sheet. This is because when the amount of acidic solution present on the steel sheet surface is large, the pH of the solution does not increase even if zinc dissolution occurs, and only zinc dissolution occurs one after another. This is because it not only has a long time but also severely damages the plating layer, and it is considered that the original role as a rust-proof steel sheet is lost. From this viewpoint, the amount of the liquid film is desirably adjusted to 3 g / m ² or less, and the liquid film amount can be adjusted by a squeeze roll, air wiping, or the like.

  Moreover, before performing the process which forms such a Zn type oxide, it is necessary to perform temper rolling to a hot dip galvanized steel plate. This is because the main purpose is usually to adjust the material, but the present invention also has an effect of destroying a part of the Al-based oxide layer existing on the surface of the steel sheet at the same time.

  When the inventors observed the surface of each of the plated steel sheets before and after the Zn-based oxide formation treatment with a scanning electron microscope, the Zn-based oxide film was mainly temper rolled. It was found that when the rolling roll was in contact with the plating surface at this time, the Al-based oxide layer was generated in a portion where the rolling roll was pressed by a dull convex part of the rolling roll and destroyed. Therefore, the coverage and distribution of the Zn-based oxide can be controlled by controlling the roughness and elongation of the roll of temper rolling and controlling the area where the Al-based oxide layer is broken. At the same time, a concave portion can be formed on the plating surface.

  However, although an example by temper rolling was shown here, the present invention is not limited to the above temper rolling, but the Al-based oxide layer on the plating surface can be mechanically broken to form a Zn-based oxide, Other methods may be used as long as they are effective for controlling the coverage of the Zn-based oxide. Examples thereof include metal brush processing and shot blasting.

  As described above, the surface of the hot dip galvanized steel sheet has unevenly active and inactive portions. Since the Al-based oxide is destroyed in the recess and is relatively active, an oxide is likely to be formed, but the oxide is not easily formed in the recess. Therefore, reducing the amount of Al-based oxide by an appropriate activation treatment is effective for imparting a Zn-based oxide to the convex portion. In other words, activation treatment is performed to reduce the amount of Al-based oxides to an appropriate amount, and then contact with an acidic solution having a pH buffering action, and a retention time of 1 to 30 seconds until washing with water is ensured. In addition, it is possible to form a Zn-based oxide with fine irregularities that is effective for slidability, and to achieve a significant improvement in sliding characteristics at low surface pressure.

  Specific activation treatment methods include mechanical removal methods such as rolling with a roll, shot blasting, and brush grinding, and methods of activating the surface by contacting with an alkaline solution. By the above treatment, it is possible to remove the Al-based oxide and expose the new surface on the surface.

In addition, the activation treatment is effective not only for the effect of improving the sliding property by widening the oxide coating region but also for the effect of improving the chemical conversion treatment property and the adhesive bondability. In the chemical conversion treatment, it is necessary to prevent the Al-based oxide from inhibiting the reactivity of Zn and phosphoric acid of the plating layer as much as possible in the chemical conversion treatment solution. By the activation treatment, the Al-based oxidation is difficult to dissolve in the weakly acidic chemical conversion treatment solution. There is an effect of reducing physical components. Similarly, reducing the amount of Al-based oxide is also effective for increasing the bonding strength with the adhesive.
In the chemical conversion treatment process in automobile manufacturing, depending on the state of the chemical conversion treatment liquid, the etching property may be lowered and phosphate crystals may not be formed. In the case of a hot-dip galvanized steel sheet, unevenness tends to occur when the etching property of the chemical conversion solution is insufficient due to the presence of an inactive surface layer Al-based oxide. Alkaline degreasing before chemical conversion treatment may remove Al-based oxides and chemical conversion treatment properties may not be a problem, but even in such a case, if alkaline degreasing is exposed to mild conditions, the effect cannot be obtained and it may not be possible. Uniform Al-based oxide distribution. The unevenness after the chemical conversion treatment causes unevenness and defects after the subsequent electrodeposition coating.

  In the manufacture of automobiles, an adhesive is used for the purpose of anticorrosion, vibration prevention, and improvement of bonding strength. In some adhesives applied to cold-rolled steel sheets and Zn-Fe alloy-based plating, there are cases where the compatibility with Al-based oxides is poor and sufficient adhesive strength cannot be obtained.

  Such a problem can be solved by removing the Al oxide layer on the surface of the hot dip galvanized steel sheet by alkali treatment or the like, but on the other hand, the oxide layer on the surface is removed. It is disadvantageous for the suppression of adhesion with the mold at the time, and press formability may be reduced. However, in the present invention, after the Al-based oxide layer on the surface of the hot-dip galvanized steel sheet is removed, a specific Zn-based oxide is formed and imparted to the surface. Chemical conversion treatment and adhesive strength can be obtained.

An appropriate amount of the Al-based oxide on the surface by the activation treatment, that is, a preferred form of the Al-based oxide on the surface of the plated steel sheet effective for forming the Zn-based oxide of the present invention by the oxidation treatment is as follows.
It is not necessary to completely remove the Al-based oxide on the surface of the plated steel sheet, and it may be mixed to some extent with the Zn-based oxide on the plated surface, but the average concentration of Al contained in the oxide layer is 20at% It is preferable to be in a state of less than. The Al concentration shown here is obtained by measuring the average oxide thickness and Al concentration in the depth direction in a region of about 2 μm x 2 μm by Auger electron spectroscopy (AES) and depth direction analysis by Ar sputtering. The maximum value of the Al concentration in the range up to the depth corresponding to the thickness of the oxide.
When the Al concentration is 20 at% or more, it becomes difficult to form a Zn-based oxide having a fine uneven structure locally, and a Zn-based oxide having a fine uneven structure on the plating surface with a coverage of 70% or more. It becomes difficult to coat. As a result, sliding characteristics, particularly sliding characteristics under low surface pressure conditions, chemical conversion properties, and adhesive bondability are deteriorated.
In order to obtain an appropriate amount of Al-based oxide on the surface, for example, when contacting with an alkaline solution, the pH of the solution is 11 or more, the bath temperature is 50 ° C. or more, and the contact time with the alkaline solution is 1 second or more. It is preferable that If it is in the said range, there will be no restriction | limiting in the kind of alkaline solution, Sodium hydroxide, a sodium hydroxide type | system | group degreasing agent, etc. can be used.

The activation treatment needs to be performed before the oxidation treatment, but it may be performed either before temper rolling after galvanizing or after temper rolling. However, when the activation treatment is performed after temper rolling, the Al-based oxide is mechanically destroyed at the portion that has been crushed by the rolling roll into a recess, so that the Al oxide is not formed at the recess and the protrusion other than the recess. There is a tendency for the amount of removal to differ. For this reason, the amount of Al oxide after the activation treatment is not uniform within the surface of the plated steel sheet, and the subsequent oxidation treatment is not uniform, and sufficient characteristics may not be obtained. For this reason, a process is preferred in which activation treatment is performed after hot dip galvanization and before temper rolling, and after removing an appropriate amount of Al oxide uniformly within the surface of the plated steel sheet, temper rolling is performed, followed by oxidation treatment. .
Regarding the production of the hot dip galvanized steel sheet according to the present invention, it is necessary that Al is added to the plating bath, but the additive element components other than Al are not particularly limited. That is, the effect of the present invention is not impaired even if Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu or the like is contained or added in addition to Al.

  In addition, since impurities are included during the oxidation treatment, even if a small amount of P, S, N, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, Si, etc. is taken into the oxide layer, The effects of the invention are not impaired.

Next, the present invention will be described in more detail with reference to examples.
(Example 1)
A hot-dip galvanized film was formed on a cold-rolled steel sheet having a thickness of 0.8 mm, and temper rolling was further performed. In some samples, the pH was changed by appropriately changing the concentration of sodium hydroxide-based degreasing agent and FC-4370 manufactured by Nihon Parkerizing Co., Ltd. as an activation treatment before or after temper rolling. The solution was contacted for a predetermined time.

After the sample subjected to the activation treatment and the temper rolling was immersed in the treatment liquid shown in Table 1 for 2 to 5 seconds and subsequently subjected to roll drawing to adjust the liquid amount to 3 g / m ² or less. And left at room temperature in the atmosphere for a predetermined time. The standing time was changed depending on the sample.

  About the test material obtained by the said method, evaluation of sliding characteristics, chemical conversion property, and adhesive bondability were performed as a press-formability test. Moreover, the thickness, distribution, and composition of the oxide layer were measured for the test material. In order to confirm the effect of the activation treatment on some of the test materials, the surface oxide was analyzed before the oxidation treatment. The obtained results are shown in Table 2, and a characteristic evaluation method and a film analysis method are described below.

(1) Evaluation of press formability (sliding characteristics) (coefficient of friction measurement)
In order to evaluate the press formability, the friction coefficient of each test material was measured as follows. FIG. 1 is a schematic front view showing a friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measuring sample 1 collected from a test material is fixed to a sample table 2, and the sample table 2 is fixed to the upper surface of a slide table 3 that can move horizontally. On the lower surface of the slide table 3, there is provided a slide table support base 5 having a roller 4 in contact therewith and capable of moving up and down, and by pushing it up, a pressing load N on the friction coefficient measurement sample 1 by the bead 6 is applied. A first load cell 7 for measuring is attached to the slide table support 5. A second load cell 8 for measuring a sliding resistance force F for moving the slide table 3 in the horizontal direction with the pressing force applied is attached to one end of the slide table 3. In addition, the cleaning oil Preton R352L for press made by Sugimura Chemical Co., Ltd. was applied to the surface of the sample 1 as a lubricant, and the test was performed.

FIG. 2 is a schematic perspective view showing the shape and dimensions of the beads used. The bead 6 slides with its lower surface pressed against the surface of the sample 1. The bead 6 shown in FIG. 2 has a width of 10 mm, a length of 59 mm in the sliding direction of the sample, and a lower portion at both ends of the sliding direction is formed by a curved surface with a curvature of 4.5 mmR. It has a flat surface with a length of 50 mm. When this bead is used, the coefficient of friction under a long sliding distance can be evaluated. In the friction coefficient measurement test, the pressing load N was 400 kgf, and the sample drawing speed (the horizontal moving speed of the slide table 3) was 20 cm / min. The pressing surface pressure under this condition is 7.8 MPa, which is a relatively low surface pressure condition.
The coefficient of friction μ between the specimen and the bead was calculated by the formula: μ = F / N.
(2) Chemical conversion property Chemical conversion property was evaluated by the following method. Sample rust-preventive oil (Parker Kosan Co., Knoxville last 550HN) was about 1 g / m ² coating, subsequently alkaline degreasing (Nippon Parkerizing Co. FC-E2001, spraying, spray pressure 1 kgf / cm ^2), washed with water, A chemical conversion treatment film was formed by the procedure of surface tone treatment (PL-Z manufactured by Nihon Parkerizing Co., Ltd.) and chemical conversion treatment (PB-L3080 manufactured by Nihon Parkerizing Co., Ltd.). At this time, the chemical conversion treatment time was constant (2 minutes), but in alkaline degreasing, the concentration of the degreasing solution was 1/2 and the degreasing time was 30 seconds, which was milder than the standard conditions.

Evaluation was based on the appearance after the chemical conversion treatment.
○: There is no scale and the entire surface is densely covered with phosphate crystals.
Δ: Some squeal is observed ×: There is a region where phosphate crystals are not formed in a wide range.
(3) Adhesive bondability
Apply oil (Sugimura Chemical Preton R352L) to two test pieces of 25 x 100 mm size, apply a PVC resin mastic sealer to an area of 25 x 10 mm, and superimpose the parts where the adhesive is applied. It was dried and bonded in a drying oven for 1 minute to form a set of I-type test pieces. This test piece was pulled with a tensile tester at a speed of 5 mm / min until it broke at the bonding position, the maximum load at the time of pulling was measured, and the load was divided by the bonding area to obtain the bonding strength.

○ If the adhesive strength is 0.2Mpa or more
If the adhesive strength is less than 0.2Mpa ×
As evaluated.
(4) Measurement of oxide layer thickness and oxide Zn / Al ratio Using Auger Electron Spectroscopy (AES), repeated Ar ⁺ sputtering and AES spectrum measurements can be used to determine the depth of the composition on the plating film surface. The lateral distribution was measured. The conversion from the sputtering time to the depth was performed by the sputtering rate obtained by measuring a SiO ₂ film having a known film thickness. The composition (at%) was determined by correcting the relative sensitivity factor from the Auger peak intensity of each element, but C was not taken into account in order to eliminate the influence of contamination. The depth distribution of O concentration due to oxides and hydroxides is high near the surface and decreases and becomes constant as it goes inside. The depth which is 1/2 of the sum of the maximum value and the constant value was defined as the oxide thickness. The area of about 2 μm × 2 μm of the flat part was the object of analysis, and the average value of the results measured at any two to three points was taken as the average oxide film thickness. The Zn / Al ratio of the oxide was determined from the average Zn concentration (at%) and the average Al concentration (at%) up to a depth corresponding to the thickness of the oxide.
(5) Surface state measurement after activation treatment In order to confirm the effect of activation treatment, the oxide layer after activation treatment on the convex portion of the surface after activation treatment was subjected to the same method as in (4) above. The distribution of Al concentration contained in the depth direction was measured. The results are shown in FIGS. 3 shows the measurement result of the test material No1, FIG. 4 shows the measurement result of the test material No11, and FIG. 5 shows the measurement result of the test material No12. In FIG. 3, the Al concentration of the oxide is less than 20 at% at any depth. On the other hand, in FIGS. 4 and 5, the Al concentration is 20 at% or more.
(6) Measurement of Zn-based oxide coverage
In order to measure the coverage of Zn-based oxides, a scanning electron microscope (LEO LEO1530) was used, and an in-lens type secondary electron detector with an acceleration voltage of 0.5 kV was used to obtain a low-magnification secondary electron image. Observed. Under these observation conditions, the portion where the Zn-based oxide is formed has a dark contrast and can be clearly distinguished from the portion where such an oxide is not formed. Strictly speaking, the brightness distribution observed here is considered to be an oxide thickness distribution, but here we separately confirmed that the Zn-based oxide was thicker than the other oxides by AES. The dark portion was determined to be an oxide mainly composed of Zn. The obtained secondary electron image was binarized by image processing software, and the area ratio of the dark portion was obtained to obtain the coverage with the Zn-based oxide formed.
(7) Measurement of fine roughness and roughness parameters of oxide
The formation of fine unevenness of the Zn-based oxide is confirmed by using a scanning electron microscope (LEO LEO1530) and an Everhart-Thornly-type secondary electron detector installed in the sample chamber at an acceleration voltage of 0.5 kV. This was confirmed by observing a high-magnification secondary electron image.

  The surface roughness of the Zn-based oxide was measured by using an electron beam three-dimensional roughness analyzer (ERA-8800FE manufactured by Elionix). The measurement was performed at an acceleration voltage of 5 kV and a working distance (working distance) of 15 mm, and the sampling interval in the in-plane direction during the measurement was 5 nm or less (observation magnification was 40000 times or more). In order to avoid charging by electron beam irradiation, gold deposition was performed. More than 450 roughness curves with a length of about 3 μm were cut out from the scanning direction of the electron beam per region where the Zn-based oxide was present. There are 3 or more measured locations per sample.

  Using the analysis software attached to the apparatus, the roughness curve Ra and the roughness unevenness S of the roughness curve were calculated from the above roughness curve. Here, Ra and S are parameters for evaluating the roughness and period of the fine irregularities, respectively. These general definitions are described in Japanese Industrial Standard “Surface Roughness—Terminology” B-0660-1998 and the like. The example of the present invention is a roughness parameter for a roughness curve having a length of several μm, and Ra and S thereof are calculated according to mathematical formulas defined in the above document.

  When an electron beam is irradiated on the sample surface, carbon-based contamination grows and may appear in the measurement data. This effect is likely to be noticeable when the measurement area is small as in this case. Therefore, in the data analysis, this effect was removed by applying a Spline hyperfilter with half the length in the measurement direction (about 3 μm) as the cutoff wavelength. For calibration of this equipment, the SHS thin film step standard (step difference 18nm, 88nm, 450nm) of VLSI Standard, Inc. traceable to the national research institute NIST in the United States was used.

  According to Table 2, the inventive examples No. 1 to No. 7 have a low coefficient of friction and excellent sliding properties, and also excellent chemical conversion treatment properties and adhesive bonding properties.

  On the other hand, the comparative examples of Nos. 8 to 12 in which the average thickness of the oxide layer or the coverage ratio of the oxide mainly composed of Zn is outside the scope of the present invention also have a higher coefficient of friction than the examples of the present invention. One or more of them are inferior.

The schematic front view which shows a friction coefficient measuring apparatus. FIG. 2 is a schematic perspective view showing bead shapes and dimensions in FIG. The figure which shows the depth direction distribution of Al concentration contained in the oxide layer after the activation process in the convex-shaped part of the surface after the activation process. The figure which shows the depth direction distribution of Al concentration contained in the oxide layer after the activation process in the convex-shaped part of the surface after the activation process. The figure which shows the depth direction distribution of Al concentration contained in the oxide layer after the activation process in the convex-shaped part of the surface after the activation process.

Explanation of symbols

1 Sample for friction coefficient measurement
2 Sample stage
3 Slide table
4 Roller
5 Slide table support
6 beads
7 First load cell
8 Second load cell
N Push load
F Sliding resistance force

Claims (8)

  In the hot dip galvanized steel sheet in which the plating layer is mainly composed of η phase, an oxide layer containing Zn-based oxide and Al-based oxide having an average thickness of 10 nm or more exists on the plating surface, and the oxide layer A Zn-based oxide having a Zn / Al ratio (ratio in atomic concentration in the oxide layer) of 4 or more is contained in the alloy, and the ratio of the area occupied by the plating surface to the plating surface is 70% or more. Galvanized steel sheet.   2. The hot dip galvanized steel sheet according to claim 1, wherein the Zn-based oxide having a Zn / Al ratio of 4 or more exists on a convex portion of a plated surface having irregularities.   3. The Zn-based oxide having a Zn / Al ratio of 4 or more contains 1 to 50 at% of Fe as a Fe atom concentration defined by Fe / (Zn + Fe). Hot-dip galvanized steel sheet as described.   The Zn-based oxide having a Zn / Al ratio of 4 or more is smaller than the convexity of the unevenness on the plating surface and smaller than the concaveity of the unevenness of the discontinuous plating surface surrounded by the convexity. The hot-dip galvanized steel sheet according to claim 1, wherein the hot-dip galvanized steel sheet has fine irregularities formed of concave portions and having a network structure.   When producing the hot dip galvanized steel sheet according to claim 1 to 4, the steel sheet is hot dip galvanized, and further subjected to an activation treatment before or after temper rolling, and then a pH buffering agent. The manufacturing method of the hot dip galvanized steel sheet characterized by performing the oxidation process which makes it contact with the acidic solution which has and hold | maintains for 1 to 30 seconds, and then rinses with water.   The method for producing a hot-dip galvanized steel sheet according to claim 5, wherein an Al concentration contained in the oxide layer after the activation treatment is less than 20 at% by the activation treatment.   The method for producing a hot dip galvanized steel sheet according to claim 5 or 6, wherein the activation treatment is carried out by contact with an alkaline solution having a pH of 11 or more and 50 ° C or more for 1 second or more.   The method for producing a hot-dip galvanized steel sheet according to claim 5, wherein Fe ions are further added to the acidic aqueous solution in an amount of 1 to 200 g / l.

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