JP6610421B2 - Steel sheet and manufacturing method thereof - Google Patents

Description

  The present invention relates to a steel sheet excellent in slidability during press molding and alkali degreasing in an automobile manufacturing process, and a method for manufacturing the steel sheet.

Cold-rolled steel sheets and hot-rolled steel sheets (hereinafter sometimes simply referred to as “steel sheets”) are widely used in a wide range of fields, mainly for automobile body applications. Steel plates for such applications are subjected to press forming and coating. In particular, from the viewpoint of strengthening CO ₂ emission regulations in recent years, steel plates used for the purpose of reducing the weight of automobile bodies tend to increase the usage ratio of high-strength steel plates.

  However, a high-strength steel sheet having a strength exceeding 440 MPa has a drawback that mold galling is likely to occur. This is because the surface pressure between the mold and the steel plate during press forming increases as the strength of the steel plate increases, and the hardness of the steel plate approaches the hardness of the die. That is, in a high-strength steel sheet having a strength exceeding 440 MPa, the wear of the mold becomes severe at the time of continuous press forming, and seriously adversely affects the productivity of the automobile, such as deteriorating the appearance of the molded product. Furthermore, since such a high-strength steel sheet has a tendency that the elongation of the steel sheet is inferior as the strength increases, the steel sheet is likely to break during press forming. On the other hand, even for relatively low strength steel sheets with a strength of 440 MPa or less, there is a need for further press formability improvements that enable more complex forming in order to integrate parts and improve design. Yes.

  As a method for improving the press formability of a steel sheet, a method of performing surface treatment on a mold is widely used. However, this method has a problem that the mold cannot be adjusted after the surface treatment is performed on the mold. Therefore, there is a strong demand for improving the press formability of the steel sheet itself.

  By the way, in recent years, attempts have been made to simplify the production process and reduce environmentally hazardous substances in the production process. In particular, the shortening of the line length of the alkaline degreasing process, which is a pretreatment of the coating process, and the lowering of the working environment in the alkaline degreasing process are proceeding. There is a need for a steel sheet having excellent degreasing properties that does not adversely affect the coating process even under such severe conditions.

  As described above, there is a demand for a steel sheet that has excellent press formability and satisfies excellent degreasing properties even under severer alkaline degreasing conditions than before.

  As a method for improving press formability, a technique of forming a lubricating film on the surface of a steel sheet is known.

  Patent Document 1 discloses a technique for improving press formability by forming a lubricating film containing a mixture of an alkali metal borate and zinc stearate and a wax as a lubricant on a steel sheet.

  Patent Document 2 discloses a technique for improving press formability by forming lithium silicate as a film component and adding a wax and metal soap as a lubricant to a steel sheet.

Patent Document 3 discloses a technique for improving mold galling resistance during continuous molding by high surface pressure processing by forming a polyurethane resin having a silanol group or a hydroxyl group with a thickness of 1 to 15 μm.
Patent Document 4 discloses a technique for improving lubricity by forming an alkali-soluble organic film in which a lubricant is added in an epoxy resin on a steel plate.
As a method for improving the degreasing property, there is a technique of washing with an alkaline solution or a solution containing P (phosphorus).

  Patent Document 5 discloses a technique for improving the degreasing property by washing the surface of an alloyed hot-dip galvanized steel sheet having an oxide layer with an alkaline solution.

  Patent Document 6 discloses a technique for improving the degreasing property by washing the surface of an alloyed hot-dip galvanized steel sheet having an oxide layer with a solution containing P.

  Patent Document 7 discloses a technique for improving the degreasing property by washing the surfaces of hot-rolled and cold-rolled steel sheets having an oxide layer with a solution containing P.

JP 2007-275706 A JP 2002-307613 A JP 2001-234119 A JP 2000-167981 A JP 2007-016266 A JP 2007-016267 A JP 2007-016268 A

  In a conventional method of performing surface treatment on a metal mold, a film coated on the surface of a steel sheet reduces frictional resistance by suppressing contact between the steel sheet and the mold. However, the amount of wear of the film increases under high surface pressure conditions in press forming of a high-strength steel plate. Therefore, even if the conventional method is used, the effect of satisfying the required characteristics cannot be obtained if the sliding distance exceeds a certain amount. Further, the conventional method does not satisfy the required characteristics of the sliding property even for a steel plate having a relatively low strength. Therefore, there is a limit to the shape of parts that can be press-molded.

  In the techniques described in Patent Documents 1 to 4, the lubricity is improved by the lubrication effect of the contained lubricant or the like. However, the degreasing properties of these technologies do not satisfy the required characteristics. In the techniques described in Patent Documents 5 to 7, a degreasing improvement effect is recognized. However, under the recent severe degreasing conditions, the effect does not satisfy the required characteristics.

  The present invention has been made in view of such circumstances, has low sliding resistance during press molding, and is excellent even under severe alkaline degreasing conditions (conditions for lowering the working environment and shortening the line length). It aims at providing the steel plate which has degreasing property, and its manufacturing method.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the surface of the steel sheet has a thin metal zinc layer as a lower layer, the upper layer contains Zn, O, S, C, P as an oxide layer, an average thickness of 20 nm or more, and solid lubrication It has been found that the above problems can be solved by forming an oxide layer containing an agent.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] The steel sheet has a metal zinc layer having an adhesion amount of 100 mg / m ² or more and 5000 mg / m ² or less as a lower layer and an oxide layer made of an oxide as an upper layer, and the oxide layer contains Zn. 30 mg / m ² or more, S 1.0 mg / m ² or more, C 0.2 mg / m ² or more, P 0.2 mg / m ² or more, containing O and a solid lubricant, and further having an average thickness Is a steel plate characterized by being 20 nm or more.
[2] The solid lubricant contains at least one of molybdenum disulfide, tungsten disulfide, boron nitride, graphite, polyethylene, and polytetrafluoroethylene in a total amount of 50 mg / m ² or more and 1000 mg / m ² or less. The steel sheet according to [1], wherein the average particle diameter is 50 to 5000 nm.
[3] The steel sheet according to [1] or [2], wherein a sulfate group, a carbonate group, a hydroxyl group, and a phosphate group are present in the oxide layer.
[4] The above-mentioned [1], wherein the oxide layer includes a crystal structure represented by Zn ₄ (SO ₄ ) _1-X (CO ₃ ) _X (OH) ₆ .nH ₂ O. The steel plate according to any one of [3]. Here, X is a real number of 0 <X <1, and n is a real number of 0 ≦ n ≦ 10.
[5] The steel sheet according to any one of [1] to [4], wherein the oxide layer contains one or more selected from the following (1) to (3). (1) PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ⁵⁻ (2) Inorganic acid of PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ⁵⁻ (3) PO ₄ ^3- , a metal compound containing at least one selected from P ₂ O ₇ ^4- and P ₃ O ₉ ^5- and at least one selected from sodium and zinc [6] above [1] to [5 And a lower layer formed in the lower layer forming step, wherein the lower layer is formed by electrogalvanizing the steel sheet with an adhesion amount of 100 mg / m ² or more and 5000 mg / m ² or less. An oxide layer forming step of contacting the formed steel sheet with an acidic solution containing 3 g / L or more of sulfate ions, 3 g / L or more of Zn ions and 0.1 g / L or more of a solid lubricant, and then washing with water; The steel sheet after forming the oxide layer formed in the oxide layer forming step is A neutralization treatment step in which the alkaline aqueous solution is kept in contact with a caustic aqueous solution for 0.5 seconds or more and then washed with water and dried, and the alkaline aqueous solution has a P ion concentration of 0.01 g / L or more, The manufacturing method of the steel plate characterized by containing carbonate ion as carbonate ion concentration 0.1g / L or more.
[7] The method for producing a steel sheet according to the above [6], wherein the acidic solution has a pH of 2 to 6 and a temperature of 20 to 80 ° C.
[8] The above-mentioned [6] or [7], wherein the alkaline aqueous solution contains one or more phosphorus compounds among phosphate, pyrophosphate and triphosphate and carbonate. Steel plate manufacturing method.
[9] The method for producing a steel sheet according to any one of [6] to [8] above, wherein the alkaline aqueous solution has a pH of 9 to 12 and a temperature of 20 to 70 ° C.

  In addition, in this invention, a steel plate is a hot rolled steel plate and a cold rolled steel plate.

  ADVANTAGE OF THE INVENTION According to this invention, the sliding resistance at the time of press molding is small, and the steel plate which has the outstanding degreasing property also in the severe arikari degreasing conditions is obtained.

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 used on the conditions 1 of the Example. It is a schematic perspective view which shows the shape and dimension of the bead used on the conditions 2 of the Example. It is the schematic diagram which showed the evaluation criteria for evaluating external appearance nonuniformity.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

In the present invention, the steel sheet surface has a thin metal zinc layer as a lower layer and an oxide layer as an upper layer, the oxide layer contains Zn, O, S, C, P and a solid lubricant, 30 mg / m ² or more, S 1.0 mg / m ² or more, C 0.2 mg / m ² or more, P 0.2 mg / m ² or more, and the average thickness of the oxide layer is 20 nm or more It is characterized by that.

  The metal zinc layer has an excellent shear deformability and has an effect of facilitating the deformation of the contact portion between the mold and the steel plate under high surface pressure. For this reason, the sliding resistance between a metal mold | die and a steel plate falls because a metal zinc layer exists. Further, the metal zinc layer has an effect of adhering to the mold surface together with an oxide in the oxide layer and a solid lubricant described later when sliding. For this reason, it is considered that the metal zinc layer and the oxide layer component adhere to the mold, thereby suppressing direct contact between the mold and the steel plate, and further reducing the frictional force.

  The lubrication mechanism of the oxide layer is not clear, but can be considered as follows. At the time of sliding, a high surface pressure is generated between the mold and the steel plate, the lubricating oil is eliminated, and a portion that directly contacts the mold and the steel plate is generated. Furthermore, a shear stress is generated on the surface of the steel sheet due to the adhesion force. In such a case, the Zn and O elements have an adhesion suppressing force that suppresses direct contact between the mold and the steel plate. S is an element used as an extreme pressure additive. S has an adhesion suppressing force that suppresses adhesion between the steel sheet and the mold by adsorbing to the surface of the steel sheet or the mold even in a high surface pressure state where oil is excluded. Furthermore, by including a solid lubricant, the solid lubricant bears part of the sliding between the mold and the steel plate, and the friction coefficient is significantly reduced. Due to the synergistic effect of these elements, it is considered possible to have excellent press formability even in high surface pressure conditions during press forming of high-strength steel sheets and complex forming of steel sheets having relatively low strength.

  The degreasing property of the oxide layer can be considered as follows. Since the oxide layer has micro unevenness, the affinity for oil is increased. However, by including P ions in the alkaline aqueous solution in the neutralization treatment step, P ions adhere to the surface of the formed oxide layer. Since P ions have a cleaning action such as conventionally used in synthetic detergents, it is considered that the degreasing property is improved by adhering to the surface of the oxide layer. On the other hand, since the oxide layer reacts with P ions and dissolves, there may be a case where a sufficient thickness of the oxide layer cannot be obtained or processing unevenness may occur, resulting in a problem that the stability of press molding is lowered. is there. However, if carbonate ions are present in the alkaline aqueous solution, the carbonate ions are taken into the oxide layer and change the crystal structure. At the same time, the physical properties of the oxide layer also change, and the affinity with oil decreases, so that the degreasing property is improved. Furthermore, the change in physical properties reduces the dissolution reaction of the oxide layer by P ions, so that the amount of dissolved oxide layer is greatly reduced. As a result, it is possible to solve problems such as uneven appearance and reduced press molding stability, which are caused by a difference in the thickness of the oxide layer due to the reaction between the P ions and the oxide layer.

The amount of Zn deposited on the metal zinc layer is 100 mg / m ² or more and 5000 mg / m ² or less. If it is less than 100 mg / m ² , it is difficult to obtain sufficient sliding characteristics, and if it exceeds 5000 mg / m ² , spot weldability that is important in automobile production is lowered.

The contents of Zn, S, C, and P in the oxide layer are as follows: Zn is 30 mg / m ² or more, S is 1.0 mg / m ² or more, C is 0.2 mg / m ² or more, and P is 0.2 mg. / M ² or more. When the Zn content of the oxide layer is less than 30 mg / m ² and the S content is less than 1.0 mg / m ² , it is difficult to obtain a sufficient sliding property improving effect, that is, a press formability improving effect. On the other hand, when the Zn content exceeds 1000 mg / m ² and the S content exceeds 100 mg / m ² , spot weldability and chemical conversion treatment, which are important in automobile production, may be deteriorated. The Zn content is preferably 1000 mg / m ² or less, and the S content is preferably 100 mg / m ² or less.

Further, if the P content is less than 0.2 mg / m ² , sufficient degreasing properties cannot be obtained. If the C content is less than 0.2 mg / m ² , it is insufficient in terms of degreasing properties, appearance irregularities, and press molding stability. On the other hand, when the P content and the C content exceed 40 mg / m ² , spot weldability and chemical conversion treatment, which are important in automobile production, may be deteriorated. P content and C content are each preferably 40 mg / m ² or less.

  The average thickness of the oxide layer is 20 nm or more. If it is less than 20 nm, sufficient press formability cannot be obtained. On the other hand, when it exceeds 200 nm, the reactivity of the surface is extremely lowered, and it may be difficult to form a chemical conversion treatment film. Accordingly, the thickness is preferably 200 nm or less.

  In order to efficiently contain the solid lubricant in the oxide layer, it is preferable to use a particulate solid lubricant having an average particle diameter of 50 nm to 5000 nm. Even if the average particle size is less than 50 nm, the effect of reducing the friction coefficient is expected. However, it is difficult to produce particles, and it is easy to cause aggregation in the liquid, which makes it difficult to manage the processing liquid. On the other hand, when the average particle diameter exceeds 5000 nm, it is difficult to be taken into the oxide layer, and the adhesion with the oxide layer tends to be inferior. A more preferable average particle diameter is 100 nm to 500 nm.

The solid lubricant in the present invention preferably contains at least one of molybdenum disulfide, tungsten disulfide, boron nitride, graphite, polyethylene (PE), and polytetrafluoroethylene (PTFE). The total content of the solid lubricant is preferably 50 mg / m ² or more and 1000 mg / m ² or less. If it is less than 50 mg / m ² , there is a possibility that a sufficient lubricating effect cannot be exhibited. On the other hand, in the case of exceeding 1000 mg / m ² , spot weldability and chemical conversion processability are lowered.

  Moreover, it is preferable from a viewpoint of oxide layer stability that a sulfate group, a carbonate group, a hydroxyl group, and a phosphate group exist in an oxide layer. Moreover, if the manufacturing conditions of bringing the steel sheet after the oxide layer is formed into contact with an alkaline aqueous solution containing carbonate ions, sulfate groups, carbonate groups and hydroxyl groups can be present in the oxide layer.

The oxide layer preferably contains a crystal structure represented by Zn ₄ (SO ₄ ) _1-X (CO ₃ ) _X (OH) ₆ .nH ₂ O. Here, X is a real number of 0 <X <1, and n is a real number of 0 ≦ n ≦ 10. By including the crystal structure, the effect of improving the sliding characteristics due to the slip deformation of the layered crystal can be obtained. Further, from the viewpoint of obtaining the above effect, the content of the crystal structure is preferably such that it can be confirmed in the examples described later. Incidentally, the steel sheet after the oxide layer formed, by adopting the manufacturing conditions that are contacted with an alkaline aqueous solution containing carbonate ions, _{Zn 4 (SO} 4) in the oxide layer _{1-X (CO 3) X} (OH) A crystal structure represented by ₆ · nH ₂ O can be contained.

The oxide layer includes (1) PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ⁵⁻ , (2) these inorganic acids (PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ^5-inorganic acid), a (3) these metal compounds ^{_{(PO 4 3-, P 2 O}} 7 4-, P 3 O 9 5- metal compound), one or more selected from It is preferable to contain from a viewpoint of degreasing. Here, the metal compound includes one or more selected from PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ and P ₃ O ₉ ⁵⁻ and one or more selected from sodium and zinc. It is. In addition, from the viewpoint of obtaining the above effects, the content of this component is preferably such that it can be confirmed in the examples described later. If the manufacturing conditions of bringing the steel sheet after forming the oxide layer into contact with an alkaline aqueous solution containing P ions are employed, the oxide layer is made of PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ^5−. One or more selected from these inorganic acids and these metal compounds can be contained. The one or more types selected from the above (1) to (3) are, for example, when one or more types are selected from (1), when one or more types are selected from (2), When one or more types are selected from 3), when one or more types are selected from (1) and (2), one or more types are selected from (1), (2), and (3), respectively. If you mean.

The oxide layer formed in the present invention can be analyzed by the following method.
Regarding the Zn content, S content, and P content in the oxide layer, a solution in which the oxide layer is dissolved with 2% ammonium dichromate + 14% ammonia water solution (% means mass%) It is possible to quantify by analyzing using an ICP emission analyzer. About C contained in the oxide layer, the oxide layer is oxidized by rubbing the surface with a stainless brush having a diameter of 0.2 mm or less and a length of 40 mm or more and ethanol, and suction-filtering the obtained ethanol solution. It is possible to extract the physical layer component as a powder component. This can be quantified by performing temperature rising analysis using a gas chromatograph mass spectrometer.

  The thickness of the oxide layer was measured using fluorescent X-rays, and the obtained oxygen intensity was converted to a silica film thickness based on the value of a silicon wafer on which a silicon oxide film with a known thickness was formed. Can be measured. Let the average value of the value measured from this be an average thickness of an oxide layer.

  For crystal water, the powdered oxide layer components can be similarly analyzed using a suggested thermobalance, and a weight loss of 100 ° C. or less corresponds to crystal water.

  The presence forms of Zn, S, and O can be analyzed using an X-ray photoelectron spectrometer. The existence form of P can be analyzed with an X-ray absorption fine structure apparatus.

  About the presence form of C, it is possible to analyze the powdered oxide layer component using a gas chromatograph mass spectrometry similarly to the method of extracting the oxide layer component as a powder component described above.

  Further, the crystal structure can be specified based on the diffraction peak of the oxide layer obtained from the X-ray diffraction.

  The content of the solid lubricant in the oxide layer is determined by measuring the average particle size of 20 solid lubricant particles from an SEM observation image and averaging the measured particle sizes, and determining the average particle size of the solid lubricant. It is possible to calculate by measuring the number of solid lubricant particles present in and then adding the density to the total volume.

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

The method for producing a steel sheet of the present invention includes a metal zinc layer having an adhesion amount of 100 mg / m ² or more and 5000 mg / m ² or less as a lower layer and an oxide layer as an upper layer on the surface of the steel sheet. 30 mg / m ² or more, S 1.0 mg / m ² or more, C 0.2 mg / m ² or more, P 0.2 mg / m ² or more, O and a solid lubricant, and an oxide layer Is a method for producing a steel sheet having an average thickness of 20 nm or more, comprising a lower layer forming step, an oxide layer forming step, and a neutralization treatment step.

First, the lower layer forming process will be described.
The lower layer forming step is a step of forming a lower layer having an adhesion amount of 100 mg / m ² or more and 5000 mg / m ² or less, that is, a metal zinc layer, on the steel sheet surface by a plating method. As the base material steel plate, either a hot-rolled steel plate or a cold-rolled steel plate can be used.

  In the lower layer forming step, the method of performing plating is not particularly limited, and general methods such as hot dip galvanization and electrogalvanization can be employed. Further, the plating treatment conditions are not particularly limited, and preferable conditions may be adopted as appropriate. As an example, in a zinc plating bath filled with an acidic plating solution containing a predetermined amount of zinc ions, the surface of the steel plate is galvanized by electrolysis while circulating the plating solution using a steel plate as a cathode and an insoluble anode. The method of forming, ie, the method of performing electrogalvanization. Further, an alloying treatment may be performed after plating. The alloying method is not particularly limited, and a general method can be adopted.

Next, the oxide layer forming step will be described.
The oxide layer forming step means that the steel sheet after the lower layer formation formed in the lower layer forming step contains 3 g / L or more of sulfate ions, 3 g / L or more of Zn ions and 0.1 g / L or more of solid lubricant. It is the process of making it contact with the acidic solution to perform, and washing with water after that. As an example of forming an oxide layer containing Zn, O, S, and a solid lubricant as an upper layer on a steel plate, the steel plate is brought into contact with an acidic solution containing sulfate ions, Zn ions, and a solid lubricant for a predetermined time, and thereafter The method of washing with water is mentioned. In addition, you may dry after water washing as needed. Specifically, the steel sheet is brought into contact with an acidic solution containing sulfate ions: 3 g / L or more, Zn ions: 3 g / L or more, and solid lubricant: 0.1 g / L or more, and then washed with water. The acidic solution preferably has a pH of 2-6. When the pH is less than 2, the dissolution of the metal zinc layer formed in the lower layer is severe and the formation of the oxide layer is also suppressed. On the other hand, an acidic solution having a pH exceeding 6 is not preferable because Zn ions in the solution may precipitate and sludge may be generated. Therefore, the pH of the acidic solution is preferably 2 or more. The pH of the acidic solution is preferably 6 or less.

  The temperature of the acidic solution is preferably 20 to 80 ° C. When the temperature is less than 20 ° C., the oxide layer formation rate is slow. On the other hand, when the temperature exceeds 80 ° C., the amount of evaporation of the solution increases, management of the liquid concentration becomes difficult, leading to an increase in cost, which is not preferable.

  In this step, the formation mechanism of the oxide layer containing Zn, S, O and a solid lubricant is not clear, but can be considered as follows. When a steel sheet coated with a metal zinc layer is brought into contact with an acidic solution containing sulfate ions, Zn ions, and a solid lubricant, hydrogen generation occurs due to zinc dissolution reaction and hydrogen ion reduction on the steel sheet side, and the pH of the steel sheet surface rises. To do. At this time, if sulfate ions and Zn ions are present in the solution near the surface of the steel plate whose pH has been increased, Zn, O and S are precipitated as compounds. At the same time, if a solid lubricant is present in the solution in the vicinity of the steel plate, the fluororesin is taken into the Zn, O, and S compounds. And it is thought that the said oxide layer forms.

  From the viewpoint of the formation mechanism, it is preferable to contain 3 g / L or more of sulfate ions, 3 g / L or more of Zn ions, and 0.1 g / L or more of the solid lubricant in the solution in contact with the steel sheet. When each of sulfate ion and Zn ion is less than 3 g / L, the deposition rate is slow, and it may be difficult to form a sufficiently thick oxide layer. When the solid lubricant is less than 0.1 g / L, the amount taken into the oxide layer may be reduced. On the other hand, if the sulfate ion is 170 g / L or more, the Zn ion is 120 g / L or more, and the solid lubricant is 100 g / L or more, no further increase in the deposition rate can be expected. Therefore, from the viewpoint of cost, it is preferable that the sulfate ion is less than 170 g / L, the Zn ion is less than 120 g / L, and the solid lubricant is less than 100 g / L.

  The method of bringing the acidic solution into contact with the steel sheet is not particularly limited, the method of bringing the steel sheet into contact with the acidic solution, the method of bringing the steel sheet into contact with the acidic solution, and the acidic solution on the steel sheet using a coating roll There is a method of applying the coating.

  Washing with water is performed at the end of the oxide layer forming step. In addition, you may dry after that as needed. In addition, the method of water washing and drying is not specifically limited, A general method is employable.

Next, the neutralization treatment process will be described.
The neutralization treatment step is a step in which the surface of the oxide layer formed in the oxide layer formation step is held for 0.5 seconds or more in a state where it is in contact with an alkaline aqueous solution, and then washed with water and dried.

  In the present invention, the alkaline aqueous solution contains 0.01 g / L or more of P ions as a P concentration and 0.1 g / L or more of carbonate ions as a carbonate ion concentration. By contacting an alkaline aqueous solution containing P ions and carbonate ions with the oxide layer, the steel sheet exhibits excellent degreasing properties even under severe alkaline degreasing conditions where the temperature is low and the processing time is short because the line length is short. It is possible. Here, low temperature means that it is 35-40 degreeC, for example. A short line length and a short processing time means, for example, 60 to 90 seconds.

  The concentration of P ions contained in the alkaline aqueous solution is 0.01 g / in as P ions from the viewpoint of obtaining the effect of using the alkaline aqueous solution, that is, the effect of attaching P ions to the surface of the oxide layer and contributing to the degreasing property. L or more. If it is less than 0.01 g / L, P may not sufficiently adhere to the oxide layer. On the other hand, if it exceeds 20 g / L, the formed oxide layer may be dissolved, so that it is preferably 20 g / L or less.

  The kind of phosphorus compound which supplies P ion in an alkaline solution is not specifically limited. For example, it is preferable from a viewpoint of cost and procurement that it is 1 or more types chosen from phosphate, pyrophosphate, and a triphosphate.

  The concentration of carbonate ions in the alkaline aqueous solution is the effect of using carbonate ions, that is, the affinity between the oxide layer and the oil is reduced and the degreasing property is further improved, and the dissolution reaction of the oxide layer is reduced and the appearance is reduced. From the viewpoint of preventing unevenness and obtaining the effect of stabilizing the press molding, the carbonate ion is set to 0.1 g / L or more. If it is less than 0.1 g / L, the incorporation of carbonate ions into the oxide layer becomes insufficient, and the physical properties cannot be changed sufficiently. On the other hand, 100 g / L or less is preferable from the viewpoint of production cost.

  Moreover, it does not specifically limit about the substance which becomes a carbonate ion. For example, carbon dioxide blowing, sodium carbonate, manganese carbonate, nickel carbonate, potassium carbonate and hydrates thereof can be used as the carbonate ion source. The use of carbon dioxide and carbonate exemplified above is preferable from the viewpoint of cost and procurement.

  The pH of the alkaline aqueous solution is not particularly limited as long as it is alkaline. In the present invention, the pH is preferably 9-12. If pH is 9 or more, it can fully neutralize. Moreover, if pH is 12 or less, it will be easy to prevent a metal zinc layer from melt | dissolving.

  The liquid temperature of the alkaline aqueous solution is not particularly limited. In the present invention, the temperature of the solution is preferably 20 to 70 ° C. When the liquid temperature is 20 ° C. or higher, the reaction rate increases. If the liquid temperature is 70 ° C. or lower, dissolution of the oxide layer is suppressed.

  The method for bringing the alkaline aqueous solution into contact with the oxide layer is not particularly limited. For example, a method in which a steel sheet on which an oxide layer is formed is immersed in an alkaline aqueous solution to make contact, a method in which an aqueous alkaline solution is sprayed on a steel sheet on which an oxide layer is formed, and a method in which an aqueous solution is sprayed on the oxide layer using a coating roll There is a method of applying an alkaline aqueous solution.

  In the present invention, the time for contacting the alkaline aqueous solution with the oxide layer is 0.5 seconds or more. By setting it to 0.5 seconds or more, excellent degreasing properties can be imparted to the steel sheet.

  Note that N, Pb, Na, Mn, Ba, Sr, Si, Zr, Al, Sn, Cu, Be, B, F, Ne, and the like are taken into the oxide layer due to impurities contained in the solution. However, the effect of the present invention is not impaired.

  Hereinafter, the present invention will be described with reference to examples. The present invention is not limited to the following examples.

  Electro-galvanizing was performed on a cold-rolled steel sheet having a thickness of 1.2 mm to form a metal zinc layer having an adhesion amount shown in Table 1. Subsequently, the steel sheet was immersed in an acidic solution adjusted to the conditions shown in Table 1 for a predetermined time. Next, after washing with water, a neutralization treatment was subsequently performed under the conditions shown in Table 1.

  The thickness and details of the oxide layer on the surface were measured for the steel plates obtained as described above, and press formability (sliding characteristics), degreasing properties, and appearance unevenness were evaluated. It is shown below.

(1) Analysis of oxide layer Measurement of oxide layer thickness An X-ray fluorescence analyzer was used to measure the thickness of the oxide layer formed on the steel sheet. 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 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. A calibration curve between the oxide layer thickness and the O—Kα ray intensity was created using these data, and the thickness of the oxide layer of the test material was calculated as the oxide layer thickness in terms of silicon oxide film. Let the average value of the value measured from this be an average thickness of an oxide layer.

Composition analysis of oxide layer Using a solution of 2% ammonium dichromate + 14% aqueous ammonia (% means% by mass), only the oxide layer was dissolved, and the solution was analyzed using an ICP emission spectrometer. , Zn, S and P were quantitatively analyzed.

  The oxide layer component was extracted as a powder component by rubbing the surface of the oxide layer with a stainless brush having a diameter of 0.15 mm and a length of 45 mm and ethanol, and suction-filtering the obtained ethanol solution. The oxide layer component collected as a powder was subjected to temperature analysis using a gas chromatograph mass spectrometer to perform quantitative analysis of C. A pyrolysis furnace was connected to the front stage of the gas chromatograph mass spectrometer. About 2 mg of the powder sample collected in the pyrolysis furnace was inserted, and the gas generated in the pyrolysis furnace was raised from 30 ° C to 500 ° C at a heating rate of 5 ° C / min. Helium was transported into a gas chromatograph mass spectrometer and analyzed for gas composition. The column temperature at the time of GC / MS measurement was set to 300 ° C.

Analysis of Solid Lubricant Content Using a scanning electron microscope, observe five randomly extracted visual fields at an acceleration voltage of 5 kV, a working distance of 8.5 mm, and a magnification of 5000 times to determine the average particle size and number of solid lubricants. It was. The total volume of the solid lubricant per unit area in the observation visual field was obtained, and the content was calculated by multiplying it by the density, and the average value of the five visual fields was obtained to obtain the solid lubricant content.

Presence form of C Similarly, the oxide layer components collected by pulverization were analyzed using gas chromatography mass spectrometry, and the presence form of C was analyzed.

Presence form of Zn, S, O The presence form of Zn, S, O was analyzed using an X-ray photoelectron spectrometer. Using an Al Ka monochromatic radiation source, narrow measurement of the spectrum corresponding to Zn LMM, S 2p was performed.

Presence form of P The presence form of P was analyzed using an X-ray absorption fine structure apparatus. XAFS (X-ray absorption edge fine structure *) was measured at room temperature at the beam line BL27A of Photon Factory, High Energy Accelerator Research Organization. The degreased sample surface was irradiated with monochromated synchrotron radiation, and the absorption edge XANES (X-ray absorption edge vicinity structure **) spectrum of the P-K shell was measured by the total electron yield method (TEY) based on the sample absorption current measurement. .

* X-ray Absorption Fine Structure
** X-ray Absorption Near-Edge Structure
Determination of water of crystallization The weight loss of 100 ° C. or less was measured using a suggested thermobalance. About 15 mg of powder sample was used for the measurement. After introducing the sample into the apparatus, the temperature was raised from room temperature (about 25 ° C.) to 1000 ° C. at a rate of temperature rise of 10 ° C./min, and the thermogravimetric change during temperature rise was recorded.

Identification of Crystal Structure X-ray diffraction of the oxide layer component obtained by pulverization was similarly performed to estimate the crystal structure. Cu was used as a target, and measurement was performed under the conditions of an acceleration voltage of 40 kV, a tube current of 50 mA, a scan speed of 4 deg / min, and a scan range of 2 to 90 °.
(2) Evaluation method of press formability (sliding characteristic) In order to evaluate 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 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, the washing | cleaning oil Preton R352L for press made from Sugimura Chemical Industry Co., Ltd. was apply | coated to the surface of the sample 1 as lubricating oil, and the test was done.

  2 and 3 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 shape of the bead 6 shown in FIG. 2 is 10 mm in width, 5 mm in the sliding direction length of the sample, the lower part at both ends in the sliding direction is formed by curved surfaces with a curvature of 1.0 mmR, and the bottom surface of the bead to which the sample is pressed is 10 mm in width and sliding It has a plane with a direction length of 3 mm. The bead 6 shown in FIG. 3 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 in the sliding direction is formed by a curved surface having a curvature of 4.5 mmR. It has a plane with a direction length of 50 mm.

The friction coefficient measurement test was performed under the following two conditions.
[Condition 1]
The bead shown in FIG. 2 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 2]
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 20 cm / min.

The friction coefficient μ between the test material and the bead was calculated by the formula: μ = F / N.
(3) Degreasing evaluation method Degreasing was evaluated by the water wettability after degreasing. After applying 2.0 g / m ^{2 of} cleaning oil Preton R352L for press made by Sugimura Chemical Co., Ltd. to the prepared test piece, FC-L4460 alkaline degreasing solution made by Nihon Parkerizing Co., Ltd. was used. The sample was degreased. The deterioration of the alkaline degreasing liquid in the automobile production line was simulated by adding 10 g / L of pre-cleaning oil Preton R352L for press produced by Sugimura Chemical Co., Ltd. to the degreasing liquid. Here, the degreasing time was 60 seconds, and the temperature was 37 ° C. At the time of degreasing, the degreasing solution was stirred with a propeller having a diameter of 10 cm at a speed of 150 rpm. Degreasing was evaluated by measuring the water wetting rate of the test piece 20 seconds after the completion of degreasing.
(4) Evaluation of appearance unevenness The appearance unevenness was evaluated visually. Based on the appearance sample shown in FIG. 4, 1-5 points were assigned and evaluated. In addition, 4 points | pieces or more show that it is favorable, and 5 points | pieces show that it is still more favorable.
Score 1: Clear unevenness with an area ratio of 50% or more exists.
Rating 2: The area ratio is 50% or more, but unevenness is not clear.
Score 3: There is a clear unevenness with an area ratio of 20% or more.
Score 4: The area ratio is 20% or more, but there is unevenness that is not clear.
Rating 5: There is no unevenness that can be visually confirmed.

  The results obtained from the above are shown in Tables 2 and 3.

  From Tables 1, 2, and 3, the following matters can be understood.

No. No oxide layer formation treatment or neutralization treatment was performed. The comparative examples 1 to 6 are comparative examples in that S, C, and P are not contained in the oxide layer, and are inferior in press formability. No. 7 to 12 and 19 to 21 are subjected to oxide layer formation treatment and neutralization treatment, but are comparative examples in that they do not contain a solid lubricant in the acidic solution, and are inferior in press formability. No. Nos. 13 and 14 are comparative examples in that the adhesion amount of the lower metal zinc layer is insufficient, and the press formability is inferior. No. No. 30 is a comparative example in which sulfate ions and Zn ions are not sufficiently added to the acidic solution, and the thickness of the oxide layer is insufficient and the press formability is poor. No. 33 is a comparative example in which the temperature of the acidic solution is low, and the thickness of the oxide layer is insufficient, and the press formability is poor. No. 37 and 41 are comparative examples in which the pH of the acidic solution deviates from the preferred range, and the thickness of the oxide layer is insufficient and the press formability is poor. No. Nos. 51 and 52 are comparative examples in which sufficient carbonate ions are not carbonized in the alkaline aqueous solution, the C content in the oxide layer is insufficient, and the press formability, degreasing properties, and appearance are insufficient. No. 58 is a comparative example in which sufficient P ions do not exist in the alkaline aqueous solution, and sufficient P is not contained in the oxide layer, resulting in poor degreasing properties.
No. 15-18, 22-29, 31, 32, 34-36, 38-40, 42-50, 53-57, 59-75 are invention examples which are a suitable range. Lower metal zinc layer is in the range of 100 mg / m ² or more and 5000 mg / m ² or less, and the upper oxide layer contains sufficient Zn, S, C, P, solid lubricant, press formability and degreasing property Excellent appearance.

No. 53 was analyzed in detail.
As a result of gas chromatography mass spectrometry, it was confirmed that CO ₂ was released between 150 ° C. and 500 ° C., and C was present as a carbonate.

  As a result of analysis using an X-ray photoelectron spectrometer, a peak corresponding to Zn LMM was observed in the vicinity of 987 eV, and it was found that Zn was present as zinc hydroxide. Similarly, a peak corresponding to S 2p was observed in the vicinity of 171 eV, indicating that S is present as a sulfate.

  As a result of analysis using an X-ray absorption fine structure apparatus, peaks were observed in the vicinity of 2153, 2158, 2170 eV, and it was found that P was present as pyrophosphate.

  From the result of the suggested thermobalance, it was found that a weight loss of 11.2% was observed at 100 ° C. or lower, and it contained crystal water.

  As a result of X-ray diffraction, 2θ is 8.5 °, 15.0 °, 17.4 °, 21.3 °, 23.2 °, 26.3 °, 27.7 °, 28.7 °, 32. Diffraction peaks are observed around 8 °, 34.1 °, 58.6 °, and 59.4 °.

To contain the above results and the composition ratio, the crystal structure material from a charge _balance, represented by _{Zn 4 (SO 4) 0.95 (} CO 3) 0.05 (OH) 6 · 3.3H 2 O I understand.

For other examples, the same procedure was followed to confirm the presence of zinc hydroxide, sulfate, carbonate, pyrophosphate, crystal water and Zn ₄ (SO ₄ ) _1-X (CO ₃ ) _X (OH). It was investigated whether or not the crystal structure represented by ₆ · nH ₂ O was contained. The results of the investigation are shown in Table 2 as ◯ for those whose presence and content were confirmed, and x for those whose presence was not confirmed. Examples of the present invention are No. Similar to 53, zinc hydroxide, sulfate, carbonate, pyrophosphate, and water of crystallization are present and are represented by Zn ₄ (SO ₄ ) _1-X (CO ₃ ) _X (OH) ₆ .nH ₂ O It can be seen that it contains a crystal structure.

  Since the steel sheet of the present invention is excellent in press formability, it can be applied in a wide range of fields mainly for automobile body applications.

1 Friction coefficient measurement sample 2 Sample table 3 Slide table 4 Roller 5 Slide table support table 6 Bead 7 First load cell 8 Second load cell 9 Rail N Pushing load F Pull-out load

Claims (9)

On the steel plate surface,
A metal zinc layer having an adhesion amount of 100 mg / m ² or more and 5000 mg / m ² or less as a lower layer;
It has an oxide layer made of an oxide as an upper layer,
The oxide layer, Zn and 30 mg / ^{m 2} or more, S and 1.0 mg / ^{m 2} or more, C and 0.2 mg / ^{m 2} or more, a 0.2 mg / ^{m 2} or more P, and O and solid lubricants contains further an average thickness of Ri der least 20 nm,
The solid lubricant contains at least one of molybdenum disulfide, tungsten disulfide, boron nitride, graphite, polyethylene, and polytetrafluoroethylene in a total amount of 50 mg / m ^{2 to} 1000 mg / m ^2, and has an average particle diameter steel plate but characterized by 50~5000nm der Rukoto.   The adhesion amount of the metal zinc layer is 2000 mg / m. ² The steel sheet according to claim 1, wherein:   The steel sheet according to claim 1 or 2, wherein a sulfate group, a carbonate group, a hydroxyl group, and a phosphate group are present in the oxide layer. Claim 1, characterized in that includes the oxide layer on the _{Zn 4 (SO 4) 1-} X (CO 3) X (OH) 6 · nH crystal structure represented by ₂ O The steel plate according to item 1.
Here, X is a real number of 0 <X <1, and n is a real number of 0 ≦ n ≦ 10. The steel sheet according to any one of claims 1 to 4, wherein the oxide layer contains one or more selected from the following (1) to (3).
(1) PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ⁵⁻
(2) PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ , P ₃ O ₉ ⁵⁻ inorganic acid (3) 1 selected from PO ₄ ³⁻ , P ₂ O ₇ ⁴⁻ and P ₃ O ₉ ⁵⁻ Metal compound containing at least one species and at least one selected from sodium and zinc A method of manufacturing a steel sheet according to claim 1, a lower layer forming step amount adhering to the steel plate makes a 100 mg / ^{m 2} or more 5000 mg / ^{m 2} or less of electro-galvanized,
The steel sheet after the lower layer formation formed in the lower layer forming step is brought into contact with an acidic solution containing 3 g / L or more of sulfate ions, 3 g / L or more of Zn ions and 0.1 g / L or more of a solid lubricant, and thereafter An oxide layer forming step of washing with water;
The steel sheet after forming the oxide layer formed in the oxide layer forming step is held for 0.5 seconds or more in a state where it is in contact with an alkaline aqueous solution, and then washed with water and dried to provide a neutralization step,
The said alkaline aqueous solution contains 0.01 g / L or more of P ions as P concentration, and 0.1 g / L or more of carbonate ions as carbonate ion concentration, The manufacturing method of the steel plate characterized by the above-mentioned.   The said acidic solution is pH 2-6, and temperature is 20-80 degreeC, The manufacturing method of the steel plate of Claim 6 characterized by the above-mentioned.   The method for producing a steel sheet according to claim 6 or 7, wherein the alkaline aqueous solution contains one or more phosphorus compounds of phosphate, pyrophosphate and triphosphate and carbonate.   The method for producing a steel sheet according to any one of claims 6 to 8, wherein the alkaline aqueous solution has a pH of 9 to 12 and a temperature of 20 to 70 ° C.