JP2011214029A - Galvanized steel sheet superior in slidability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a galvanized steel sheet superior in slidability.SOLUTION: The galvanized steel sheet has such a surface that when the highest point of the surface is defined as a reference surface and a histogram of the depths of the surface from the reference surface has been prepared, there is at least one peak between depths of 0.3 μm and 2 μm, and there is the maximum peak between depths of 0.3 μm and 2 μm among peaks existing between depths of 0 μm and 5 μm; and has a lubricating film at least on the surface of which the depths are within a range from 0.3 μm to 2 μm.

Description

  The present invention relates to a zinc-based plated steel sheet having excellent slidability during press forming.

  In the product field such as steel, semiconductor, and display, a surface layer film having a thickness of several tens to several hundreds of nanometers may be a dominant factor of product characteristics. One of the product characteristics related to such a super-thin film in the steel product field is press formability. In recent years, steel plates used for automobiles and home appliances are often subjected to zinc-based plating from the viewpoint of high corrosion resistance. However, when manufacturing difficult-to-form parts by pressing this plated steel plate, There is a problem that press cracking of the steel sheet is likely to occur in a severe part. A method is known to improve the press formability of this zinc-based plated steel sheet by using a high-viscosity lubricant during pressing. There is a problem that unevenness occurs.

  Various proposals have been made to improve the lubricity of galvanized steel sheets. Roughly divided, it is roughly divided into two methods: a method of controlling the shape of the plated steel sheet surface and a method of forming a lubricating film on the surface.

As a method for controlling the surface shape, Patent Document 1 discloses an alloyed hot-dip galvanized steel sheet having Ra of 0.5 to 0.8 μm, and Patent Document 2 has a surface roughness (Ra) of 0.6 μm to 1. 0.0 μm, the number of peaks (inch per inch) on the surface (PPI) is 350 or less, the waviness value (Wca) of peaks and valleys is 0.4 μm or less, and the peak amplitude from the median in the probability amplitude density distribution of surface roughness The probability of the peak portion existing in a region further upward from the position close to 2 μm in the direction is 0.05 or less, and the peak area of the virtual flat section at that position is 10 ⁻⁵ to 10 ⁻³ mm ² An alloyed hot-dip galvanized steel sheet having a density of 3 × 10 ² pieces / mm ² or less is described.

Further, in Patent Document 3, the surface roughness is 0.5 to 1.5 μm, and the height A at which the frequency to the highest peak height is 2% of the total frequency in the probability amplitude density distribution of the surface roughness, Further, the existence probability of the convex portion at a height close to 1 μm in the valley direction is 0.25 to 0.50, and from the height A, the volume of the concave portion relative to the height close to 2 μm in the valley direction is 0.5 to 0.5. It is described to be 2.5 × 10 ⁻³ mm ³ / mm ² .

  On the other hand, as a method for forming a lubricating film, for example, in Patent Documents 4 to 6, zinc oxide is formed by subjecting the surface of a zinc-based plated steel sheet to electrolytic treatment, immersion treatment, coating oxidation treatment, or heat treatment. A technique for improving weldability or workability by forming a main oxide film is disclosed. In Patent Document 7, a plated steel sheet is immersed in an aqueous solution containing 5 to 60 g / L of sodium phosphate on the surface of a zinc-based plated steel sheet and has a pH of 2 to 6, an electrolytic treatment is performed, or the above aqueous solution is applied. Discloses a technique for improving the press formability and chemical conversion property by forming an oxide film mainly composed of phosphorous oxide. Patent Document 8 discloses that press forming properties and chemical conversion properties are improved by forming Ni oxide on the surface of a zinc-based plated steel sheet by electrolytic treatment, immersion treatment, coating treatment, coating oxidation treatment, or heat treatment. Technology is disclosed.

  Further, as a method of forming a lubricating film by controlling the surface shape, Patent Document 9 discloses iron-zinc alloy plating in which the fluid retention index Svi of the flat portion on the surface is 0.120 or more. An alloyed hot-dip galvanized steel sheet having a surface height distribution skewness Ssk of 0.3 or less and a surface height distribution kurtosis Sku of 4 or more and 14 or less in a flat portion of the surface is disclosed in Patent Document 11. The average roughness (Ra) of the roughness curve on the surface of the flat part is 10 nm or more and 100 nm or less, the local average interval (S) of the fine unevenness on the surface of the flat part is 100 nm or more and 1000 nm or less, and at least the fine unevenness of the flat part An alloyed molten zinc alloy in which the convex portion is composed of a flake-like material having an average thickness of 3 nm to 50 nm and an average major axis of 50 nm to 1500 nm. Steel plate have been described.

  Patent Documents 9 to 11 specify the area ratio of the flat portion and the roughness of the flat portion, and do not specify where the flat portion is located in the height direction.

JP 2000-64013 A JP 2003-183801 A JP 2003-290804 A JP-A-53-60332 Japanese Patent Laid-Open No. 2-190483 JP 2004-3004 A JP-A-4-88196 Japanese Patent Laid-Open No. 3-191093 JP 2003-138361 A JP 2003-138364 A JP 2004-68141 A

  Only the control of the surface shape described in Patent Literatures 1 to 3 cannot sufficiently improve the slidability during press molding. This was thought to be because the effect of preventing zinc from adhering to the mold was inferior. Although the techniques described in Patent Documents 4 to 8 improved the slidability during press molding by forming a hard metal oxide having a melting point higher than that of zinc, it is sufficient for application to severely molded members. There was a case where performance was not demonstrated. The techniques described in Patent Documents 9 to 11 are the same as the techniques described in Patent Documents 4 to 8. Therefore, the improvement of the slidability at the time of the further press molding was calculated | required.

  It is an object of the present invention to provide a zinc-based plated steel sheet having excellent slidability, which can further improve the slidability during press forming and can exhibit excellent slidability even when applied to severely formed members. And

  In the zinc-based plated steel sheet, by flattening the high point of the plating surface that comes into contact with the mold when sliding in press molding, the load per unit area at the contact portion with the mold decreases, It has been found that the sliding resistance decreases. The present invention is based on this finding.

  Means of the present invention for solving the above-mentioned problems are as follows.

  [1] When a plane parallel to the steel plate surface passing through the highest point on the surface is used as a reference plane, and a histogram of the depth of the surface from the reference plane is created, at least 1 between a depth of 0.3 μm and 2 μm A surface having two peaks, a peak existing between 0 μm and 5 μm in depth, a maximum peak between 0.3 μm and 2 μm, and at least a depth in the range of 0.3 μm to 2 μm A zinc-based plated steel sheet characterized by having a lubricating film on the surface.

  [2] The zinc-based plated steel sheet according to [1], wherein the lubricating coating is a zinc-based oxide and the average thickness of the plating surface is 10 nm or more.

  According to the present invention, when press-molding a galvanized steel sheet, the load per unit area of the contact portion with the mold is reduced by flattening a high point on the surface in contact with the mold. By forming a lubricating film on the part, the slidability during press molding can be greatly improved.

Depth profile when the rolling reduction ratio of bright rolling is changed in alloyed hot-dip galvanized steel sheet that has been subjected to temper rolling that undergoes bright rolling after dull rolling and then treatment that forms a zinc-based oxide layer on the plated surface It is a figure which shows (the histogram of the depth from this reference surface by making a surface parallel to the steel plate surface passing the highest point of a surface into a reference surface). It is a schematic front view which shows a friction coefficient measuring apparatus. It is a schematic perspective view which shows the shape and dimension of the bead in FIG. It is a schematic perspective view which shows the shape and dimension of the bead in FIG.

  Hot-dip galvanized steel sheets are usually temper-rolled after plating. For press forming applications, temper rolling is usually temper rolling using dull rolls. This is for improving the retention of the lubricating oil by providing irregularities on the plated surface. The plated surface after temper rolling is flat because it is flattened by contact with the rolling roll of temper rolling (hereinafter referred to as the pressure adjusting region) and the rolling roll. There is no region (hereinafter referred to as an unregulated region). The pressure-regulating region does not exist only at a high point on the plating surface (a portion where the plating thickness is thick) but may exist also at a low point (a portion where the plating thickness is thin), and the unregulated region is high on the plating surface May be present at a point.

  It is known that the surface of a normal galvanized steel sheet (including an alloyed galvanized steel sheet, the same applies hereinafter) is covered with a thin Al oxide. This thin Al oxide layer is not sufficient to obtain good slidability, and it is necessary to form a thicker oxide layer. Since the Al oxide layer is relatively stable in an acidic solution, it is difficult to generate a lubricating film in a portion where the Al oxide layer exists by contacting with the acidic solution. In the pressure adjusting region, the surface is activated by physically removing the Al oxide. When the lubricating film is formed on the hot dip galvanized steel sheet, the lubricating film is formed in the pressure adjusting region, and the formation of the lubricating film is suppressed in the non-adjusted region. Since the unregulated region may exist at a high point on the plating surface, the lubricating film is not necessarily formed at a high point on the plating surface.

  As a result of actually carrying out a sliding test using galvannealed steel sheets and observing the front and back surfaces, it was found that deformation was large due to sliding in a region corresponding to a high point on the surface before sliding. all right. Further, it was found that the point on the lower surface was not deformed by sliding, and the shape before sliding remained. This is because the highest point on the surface of the material comes in contact with the die pressed at the time of press molding at the earliest point, and the load on the pressed die is concentrated at the highest point on the material surface.

  Therefore, in order to improve the slidability of the hot dip galvanized steel sheet, it is only necessary to improve the slidability of a region corresponding to a high point on the surface before sliding, which is the region most deformed by sliding. This is true not only for hot-dip galvanized steel sheets but also for galvanized steel sheets in general including electrogalvanized steel sheets.

  As a result of studying this point, the present inventors have found that the slidability of the hot-dip galvanized steel sheet can be improved by flattening a high point on the surface in contact with the mold. As a specific scale, when a plane parallel to the steel plate surface passing through the highest point on the surface is used as a reference plane and a histogram of the depth of the surface from the reference plane is created, the depth is 0.3 μm to 2 μm. There is at least one peak in between, a peak profile having a depth between 0.3 μm and 2 μm, and a depth of 0. It has been found that it suffices to have a lubricating film on the surface in the range of 3 μm to 2 μm.

  The reason why the slidability is improved by having such a depth profile is considered to be as follows.

  When the maximum peak position of the depth profile is shallow, the surface depth profile has a surface shape that is biased toward the reference plane, and when the maximum peak position is deep, the surface depth profile is far from the reference plane. It has a surface shape that is biased toward the outer side.

  The conventional plated steel sheet temper-rolled by a dull roll has a maximum peak position deeper than 3 μm, as shown by the curve of the symbol “□” in FIG. In this plated steel sheet, since the position of the maximum peak of the depth profile is too deep, there are few surfaces near the highest position of the surface that first contacts the mold. Therefore, when the steel sheet is pressed against the mold and the surface of the steel sheet is crushed, the load per unit area of the contact portion with the mold increases in contact with the mold at a small area in the initial contact with the mold, Zinc adheres to the mold. Once zinc adheres to the mold, the contact area increases thereafter, and even if the load per unit area of the contact part decreases, the presence of the adhesive part does not improve the slidability.

  On the other hand, as is well known, when the surface is completely flat, the load per unit area of the contact portion is reduced, but the amount of lubricating oil that can exist between the steel plate and the mold becomes extremely small, and rather the adhesion is caused. Is likely to occur. A completely flat surface has a depth of 0 μm at the position of the maximum peak of the depth profile.

  From the viewpoint of slidability, the position of the maximum peak of the depth profile needs to be within an appropriate depth range from the reference plane. Therefore, this point was further examined.

  As a result of investigations by the inventors, when the maximum peak depth is less than 0.3 μm or more than 2 μm from the reference surface, the slidability decreases, and good slidability is obtained in the range of 0.3 μm to 2 μm. It was.

  Next, a method for creating a histogram of the depth of the surface from the reference plane will be described. The histogram of the depth of the surface from the reference surface is obtained by measuring the surface shape using a roughness meter using a probe or an optical roughness meter using a laser beam, etc., and measuring the surface depth based on the measurement result. A histogram may be created. Preferably, in order to reduce the influence of variation, it is desirable to measure at a plurality of points of five points or more and obtain an average value. It is also desirable to measure an area of 1 mm or more with a horizontal resolution of about 2 μm or more.

  The slidability is not sufficient if the surface of the galvanized steel sheet is in the above surface state. This is because, as described above, zinc is soft and its melting point is lower than that of Fe, so that it easily adheres to the mold. In order to improve the slidability, it is necessary to form a lubricating film on the zinc surface. In order to improve the slidability, the lubricating film preferably has an average thickness of 10 nm or more.

  Lubricant films include metal oxides, metal phosphates, lubricant resins, etc. Among them, zinc-based oxides are optimal because metal components contained in the plating film can be used when forming the lubricant film. .

  The present inventors examined a method for producing a zinc-based plated steel sheet having the above-described depth profile. As a result, the temper rolling performed after plating is performed by both the dull rolling using a dull roll and the bright rolling using a bright roll, so that a high point on the plating surface is flattened, and the depth profile within the scope of the present invention is obtained. It has been found that a zinc-based plated steel sheet can be produced.

  Next, the manufacturing method of the galvanized steel sheet of this invention is demonstrated.

  The zinc-based plated steel sheet of the present invention forms a zinc-based plated film on the steel sheet. The zinc-based plating film may be any one of a hot dip galvanized film, an alloyed film that has been alloyed after hot dip galvanizing, and an electrogalvanized film. A conventional method may be used for forming the zinc-based plating film.

Temper rolling is performed on the steel sheet on which the zinc-based plating film is formed.
The temper rolling is a dull rolling that rolls at a reduction rate of 5% or less using a dull roll with Ra of 2.0 μm or more, and a reduction rate of 3% or less and 0.1% or more with a bright roll with Ra of 0.1 μm or less. Perform both bright rolling and rolling. Bright rolling may be performed after dull rolling, or dull rolling may be performed after bright rolling.

  By performing both dull rolling and bright rolling, the high surface point is flattened, the surface parallel to the steel plate surface passing through the highest surface surface is taken as the reference surface, and a histogram of the surface depth from the reference surface is obtained. When prepared, the histogram has at least one peak between a depth of 0.3 μm and 2 μm, and the maximum peak among the peaks existing between a depth of 0 μm and 5 μm is a depth of 0.3 μm to 2 μm. The surface state of the galvanized steel sheet between the two can be obtained. In the hot dip galvanized steel sheet, the flattened area as described above is physically activated from the thin Al oxide layer present on the plating surface layer, and is suitable for the formation of a zinc-based oxide layer as a lubricating film. A finished surface is obtained.

  The Ra of the dull rolled dull roll is specified to be 2.0 μm or more because when the Ra is less than 2.0 μm, the Ra of the galvanized steel sheet after the temper rolling becomes small, and the zinc-based steel is handled when handling the steel sheet coil. This is because the coefficient of friction between the plated steel sheets is reduced, the coil crushing and the like are concerned, and the handling property is remarkably reduced. The upper limit of Ra is not specified, but is preferably 5 μm or less from the viewpoint of sharpness after coating. The reason why the rolling reduction of the dull rolling is specified to be 5% or less is that when the rolling reduction exceeds 5%, the elongation of the steel sheet is lowered and adversely affects the press workability. Although the lower limit is not specified, 0.1% or more is preferable in order to eliminate the yield point elongation.

  The bright roll Ra of the bright roll is defined to be 0.1 μm or less, when the bright roll after the dull roll, when the bright roll Ra exceeds 0.1 μm, the portion that did not contact the dull roll in the dull roll, When the area ratio of the portion that comes into contact with the bright roll during the next bright rolling is reduced, and the dull rolling is performed after the bright rolling, when the Ra of the bright roll exceeds 0.1 μm, the portion that does not come into contact with the bright roll during the bright rolling However, it is because the area ratio of the part which contacts a dull roll falls at the time of the next dull rolling, and the effect | action which removes Al oxide falls. Although the minimum of Ra is not prescribed | regulated, 0.01 micrometer or more is preferable from the processing cost of a rolling roll. The rolling reduction of bright rolling is regulated to 3% or less and 0.1% or less because when the reduction ratio exceeds 3%, the position of the maximum peak of the depth profile becomes less than 0.3 μm, and the reduction ratio is 0.1 This is because the position of the maximum peak of the depth profile exceeds 2 μm when it is less than%.

  In addition, Ra is centerline average roughness Ra of JIS B0601-1994.

  The steel plate on which the zinc-based plating film is formed is brought into contact with an acidic solution having a pH buffering action after temper rolling, and after holding for 1 to 60 seconds, it is washed with water and dried to form an oxide layer on the plating surface. . If the acidic solution is a solution having a pH buffering action, a zinc-based oxide layer having excellent slidability can be stably formed on the flat portion of the plating surface.

  Although the oxide layer formation mechanism is not clear, it can be considered as follows. When the steel sheet on which the zinc-based plating film is formed is brought into contact with an acidic solution, zinc is dissolved from the plating film. Since the 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, and as a result, the pH of the solution increases, and the surface of the plating film is mainly composed of Zn. It is thought that an oxide layer is formed. When an acidic solution having no pH buffering action is used, the pH of the solution rises instantaneously, and zinc cannot be dissolved sufficiently to form an oxide layer. Layer formation does not occur. On the other hand, when an acidic solution having a pH buffering action is used, even if zinc is dissolved and a hydrogen generation reaction occurs, the pH of the solution gradually increases, so that further dissolution of zinc proceeds. As a result, generation of an oxide sufficient for improving the slidability occurs.

  If the pH of the acidic solution is too low, dissolution of zinc is promoted, but it is difficult to form an oxide. Therefore, the pH is preferably 1.0 or more. On the other hand, if the pH is too high, the reaction rate of zinc dissolution becomes low, so the pH of the solution is desirably 5.0 or less.

  The acidic solution having a pH buffering action is preferably one having a pH buffering action in a pH range of 2.0 to 5.0. This is because, when an acidic solution having a pH buffering action in the above pH range is used, the oxide layer targeted by the present invention can be stably obtained by holding the acidic solution for a predetermined time after contact. is there.

Examples of such an acidic solution having a pH buffering action include acetates such as sodium acetate (CH ₃ COONa), phthalates such as potassium hydrogen phthalate ((KOOC) ₂ C ₆ H ₄ ), sodium citrate (Na Citrates such as ₃ C ₆ H ₅ O ₇ ) and potassium dihydrogen citrate (KH ₂ C ₆ H ₅ O ₇ ), succinates such as sodium succinate (Na ₂ C ₄ H ₄ O ₄ ), and lactic acid At least one of lactate such as sodium (NaCH ₃ CHOHCO ₂ ), tartrate such as sodium tartrate (Na ₂ C ₄ H ₄ O ₆ ), borate, phosphate, sulfate, An aqueous solution containing the component in a range of 5 to 50 g / L can be used. If the concentration is less than 5 g / L, the pH of the solution rises relatively quickly with the dissolution of zinc, so that an oxide layer sufficient for improving the slidability cannot be formed, and 50 g / L is reduced. If it exceeds, dissolution of zinc is promoted and not only it takes a long time to form the oxide layer, but also the plating layer is severely damaged, and it is considered that the role as an original rust-proof steel sheet is lost.

  As described above, if the acidic solution to be used has a pH buffering action, an oxide layer having excellent sliding properties can be stably formed. Therefore, metal ions, inorganic compounds, and the like are contained in the solution as impurities or intentionally. Even if it contains, the effect of this invention is not impaired.

There is no particular limitation on the method of bringing the steel sheet having the zinc-based plating film into contact with the acidic solution, the method of immersing in the acidic solution, the method of spraying the acidic solution onto the plated steel sheet, and applying the acidic solution to the plated steel sheet via the coating roll. However, it is desirable that the acidic solution finally exists in the form of a thin liquid film on the surface of the steel sheet. If the amount of liquid film present on the surface of the steel sheet is small, an oxide layer having a desired thickness cannot be formed on the plating surface. However, if the amount of the acidic solution present on the surface of the steel sheet is too 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, it is effective to adjust the liquid film amount at the end of contact with the acidic solution to 1 g / m ² or more and 15 g / m ² or less. The liquid film amount can be adjusted by a squeeze roll, air wiping or the like. The end of contact is “dipping end” in the case of the method of immersing in the acidic solution, “spray end” in the case of the method of spraying the acidic solution onto the plated steel plate, and “in the case of the method of applying the acidic solution via the coating roll” It means “end of application”.

  Moreover, after contact completion with a pickling solution, the time to water washing (holding time to water washing) needs 1 to 60 seconds. This is because if the time until washing with water is less than 1 second, the acidic solution is washed away before the pH of the solution rises and the oxide layer mainly composed of Zn is formed, so that the effect of improving the slidability is obtained. This is because no change can be seen in the amount of the oxide layer even if it is not obtained and exceeds 60 seconds.

  If the above conditions are satisfied, a zinc-based oxide layer can be efficiently and stably formed on the surface of the zinc-based plated steel sheet.

  When the acidic solution remains on the surface of the steel sheet after being washed and dried, rust is likely to occur when the steel sheet coil is stored for a long period of time. From the viewpoint of preventing the occurrence of rust, a treatment for neutralizing the acidic solution remaining on the surface of the steel sheet may be performed by contacting with the alkaline solution by a method such as immersion in an alkaline solution or spraying the alkaline solution. The alkaline solution preferably has a pH of 12 or less in order to prevent dissolution of the zinc-based oxide formed on the surface. If it is in the said pH range, there will be no restriction | limiting in the solution to be used, Sodium hydroxide, sodium phosphate, etc. can be used.

  The zinc-based oxide in the present invention is an oxide or hydroxide mainly composed of zinc as a metal component. When the total amount of metal components such as iron and Al is less than zinc, sulfuric acid, nitric acid In addition, a case where the total amount of anions such as chlorine is less than the number of moles of oxygen and hydroxyl groups is also included in the zinc-based oxide of the present invention. The zinc-based oxide layer may contain anionic components such as sulfate ions used to adjust the pH of the acidic solution. However, the zinc-based oxide layer has an anion component such as sulfate ions and a pH buffering action. Impurities such as S, N, P, B, Cl, Na, Mn, Ca, Mg, Ba, Sr, and Si contained in the acidic solution, S, N, P, B, Cl, Na, Mn, Ca, Mg A compound containing Ba, Sr, Si, O, C and the like can be contained in a zinc-based oxide as long as the effects of the present invention are not impaired.

  By setting the thickness of the oxide layer in the plating surface layer to 10 nm or more, a zinc-based plated steel sheet showing good slidability can be obtained, but when the thickness of the oxide layer is 20 nm or more, more preferably 30 nm or more. More effective. This is because, in the press molding process in which the contact area between the mold and the workpiece becomes large, even if the oxide layer on the surface layer is worn, it remains and does not cause a decrease in slidability. On the other hand, the upper limit of the thickness of the oxide layer is not particularly set, but if it exceeds 200 nm, the surface reactivity is lowered and the amount of oxide layer produced is lowered.

On a 0.7 mm thick steel plate annealed after cold rolling, hot dip galvanizing with a plating adhesion amount of 45 g / m ² per one side is applied by a conventional method, and then alloyed to contain Fe in the plating film. The rate was 10% by mass. Thereafter, temper rolling was performed. The temper rolling was performed by dull rolling at a reduction rate of 0.7% using a dull roll having a Ra of 2.1 μm, and subsequently bright rolling at a reduction rate of using a bright roll having a Ra of 0.1 μm. After temper rolling, the following lubricating film formation treatment was performed to form a zinc-based oxide film on the plating surface. For comparison, a dull rolling process that does not perform bright rolling and lubrication film formation treatment, and a dull rolling process that does not perform bright rolling and a lubrication film generation process were also prepared.

<Lubrication film formation treatment>
After the temper rolled steel sheet was immersed in an acidic solution containing sodium acetate 30 g / L and at 50 ° C. and pH 2.0, the amount of liquid film adhered to the steel sheet surface was adjusted with a drawing roll on the acidic solution tank outlet side. . At that time, the liquid film amount was adjusted to 5 g / m ² by changing the pressure of the squeeze roll. After adjusting the amount of liquid film, it was allowed to stand for 10 to 30 seconds, and then sprayed and washed with hot water at 50 ° C. to form an oxide layer on the plating surface.

  The surface profile of each alloyed hot-dip galvanized steel sheet prepared above, the region where the oxide layer exists, the thickness of the oxide layer, and the slidability were investigated by the following methods.

<Depth profile>
For each steel plate, a plane parallel to the steel plate surface passing through the highest point on the plating surface was used as a reference plane, the depth from the reference plane was measured, and a depth histogram was created. The measurement was carried out using an optical surface shape measuring instrument (Canon, NewView 6000), measuring area: 1.41 mm × 1.06 mm, number of measuring points: 640 × 480 points, horizontal resolution: 2.21 μm. About each steel plate, it measured by N number 5. An example of a depth histogram is shown in FIG.

  The maximum peak depth is obtained from the histogram (depth profile) of each depth, and the average value (average value of five points) is defined as the maximum peak depth. This depth is 0.3 to 2 μm in depth. It was judged whether it was between.

<Area where oxide layer exists>
For the steel sheet of the invention example, oxygen mapping analysis was performed using EPMA, and the surface shape of the same region was measured with an optical surface shape measuring instrument, so that the oxide on the surface having a depth in the range of 0.3 μm to 2 μm. It was confirmed that a layer was formed.

<Thickness of oxide layer>
A fluorescent X-ray analyzer was used to measure the thickness of the oxide layer. The tube voltage and current during the measurement were 30 kV and 100 mA, the spectroscopic crystal was set to TAP, and the O-Kα ray was detected. When measuring the O-Kα line, in addition to the peak position, the intensity at the background position was also measured so that the net intensity of the O-Kα line could be calculated. The integration time at the peak position and the background position was 20 seconds, respectively.

  A silicon wafer on which a silicon oxide film having a thickness of 96 nm, 54 nm, and 24 nm, which has been cleaved to an appropriate size, is set on the sample stage together with these series of samples. The intensity of -Kα rays can be calculated. Using these data, create a calibration curve between the thickness of the oxide layer and the O-Kα line intensity, and calculate the thickness of the oxide layer of the test material as the thickness of the oxide layer in terms of silicon oxide film. I tried to do it.

<Slidability>
In order to evaluate the press formability, the friction coefficient of each test material was measured as follows.
FIG. 2 is a schematic front view showing the friction coefficient measuring apparatus. As shown in the figure, a friction coefficient measurement 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. A slide table support 5 having a roller 4 in contact with the slide table 3 is provided on the lower surface of the slide table 3, and when this is pushed up, a pressing load N applied to the friction coefficient measurement sample 1 by the bead 6. A first load cell 7 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 in a state where the pressing force is applied is attached to one end of the slide table 3. In addition, as a lubricating oil, a rust preventive cleaning oil (Preton R352L, Preton is a registered trademark) manufactured by Sugimura Chemical Industry Co., Ltd. was applied to the surface of Sample 1 and tested.

  3 and 4 are schematic perspective views 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. 3 has a width of 10 mm, a length of 12 mm in the sliding direction of the sample, and a lower portion at both ends in the sliding direction is formed of a curved surface having a curvature radius of 4.5 mmR. It has a plane with a moving direction length of 3 mm. The shape of the bead 6 shown in FIG. 4 is 10 mm wide, the length of the sliding direction of the sample is 69 mm, the lower part at both ends of the sliding direction is a curved surface with a curvature of 4.5 mmR, and the lower surface of the bead against which the sample is pressed is 10 mm wide and sliding. It has a plane with a direction length of 60 mm.

The coefficient of friction was measured under the following two conditions.
[Condition A]
The bead shown in FIG. 3 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3) was 100 cm / min.
[Condition B]
The bead shown in FIG. 4 was used, the pressing load N was 400 kgf, and the sample drawing speed (horizontal moving speed of the slide table 3) was 20 cm / min.
The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F / N.

  Table 1 shows the temper rolling conditions, surface shape profile, oxide layer thickness, and slidability investigation results.

  It turns out that the example of this invention is excellent in slidability compared with a comparative example.

  In this example, in order to see the effect of the lubrication treatment after bright skin pass according to the present invention, the result of controlling the formation time (leaving time) of the lubricant film to the same amount of lubricant film is shown. If the formation time of the lubricating film is the same, according to the present invention, the area activated by the bright skin pass increases, so that the formation area of the lubricating film increases and the amount of the lubricating film also increases. The slidability is further improved by increasing the amount of the lubricating film. Considering the manufacturing process, the present invention has an advantage that the lubricating film can be formed in a short time because the area where the lubricating film can be formed is increased as compared to the conventional temper rolling by the dull roll, so that the productivity can be improved. There is also.

  According to the present invention, the slidability at the time of press molding can be improved, so that excellent slidability can be exhibited even when applied to a more severely molded member.

Claims (2)

  When a histogram parallel to the surface of the steel sheet passing through the highest point of the surface is used as a reference plane, and a histogram of the depth of the surface from the reference plane is created, at least one peak is present between the depth of 0.3 μm and 2 μm. A lubricating film on the surface where the maximum peak is between 0.3 μm and 2 μm and the depth is at least in the range of 0.3 μm to 2 μm. A galvanized steel sheet characterized by comprising:   The zinc-based plated steel sheet according to claim 1, wherein the lubricating film is a zinc-based oxide, and the average thickness of the plating surface is 10 nm or more.

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