JP5338243B2 - Plated steel sheet for hot press forming and manufacturing method thereof - Google Patents

Description

  The present invention relates to a plated steel sheet for hot press forming excellent in oxidation resistance and a method for producing the same.

In recent years, efforts have been made to increase the strength of steel sheets (for example, 780 MPa class) and reduce the thickness of the steel sheets used in order to reduce the weight of automobile bodies. However, when press-working a steel plate, for example, when drawing is performed, if the strength of the steel plate is increased, the contact pressure with the mold is increased during drawing, and the steel plate is galvanized or broken. In addition, in order to alleviate such problems as much as possible, when the blank pressing pressure for increasing the inflow of the material into the mold during drawing of the steel sheet is lowered, the shape after forming varies. There is.
In addition, the problem of shape stability, so-called spring back, also occurs. There is a method to improve this by using a lubricant, but the effect is small with a high strength steel plate of 780 Mpa grade.

  In view of such a current situation, as a technique for press-forming a difficult-to-press material such as a high-strength steel plate of 780 Mpa or higher, a method in which the material to be formed is heated and formed in advance can be considered. These are so-called hot press forming and warm press forming (hereinafter, hot press forming and warm press forming are collectively referred to as hot press forming).

  However, since hot press forming is a forming method for processing a heated steel sheet, surface oxidation is unavoidable. Even if the steel sheet is heated in a non-oxidizing atmosphere, for example, for press forming from a heating furnace. When exposed to the atmosphere, iron oxide is formed on the surface. This surface iron-based oxide falls off during hot pressing and adheres to the press die, thereby reducing productivity. Or it remains in the product after pressing and causes the appearance defect. Furthermore, when it coats at the next process, the adhesiveness of a steel plate and a coating film will be inferior.

  Therefore, after hot press molding, it is necessary to perform shot blasting to remove scales made of such iron-based oxides. However, this is subject to an increase in cost.

In order to solve such a problem, Patent Document 1 proposes a steel sheet having a predetermined composition and structure, which is coated with aluminum in order to provide the oxidation resistance of the base steel sheet during hot forming. However, such a steel sheet has a significant cost increase when compared with ordinary steel.
As described above, when hot press forming is performed on a high-strength steel sheet, a step of removing the generated iron-based oxide is necessary, and the step of removing the oxide without a significant increase in cost is omitted. In addition, steel sheets that do not have a surface treatment layer such as a plating layer are inferior in rust prevention even if the oxide is removed.
JP 2000-38640 A

  In view of such circumstances, an object of the present invention is to provide a hot press-formed plated steel sheet having excellent oxidation resistance and a method for producing the same without significantly increasing costs.

  The inventors of the present invention have made extensive studies to solve the above problems. As a result, the following knowledge was obtained.

  Prior to hot pressing, hot press forming is performed by bringing a steel plate (zinc-based plating layer) into contact with an acidic solution containing Al ions and forming an oxide layer containing Zn and Al as essential components on the surface of the plating layer. It was later found that the oxidation resistance was remarkably improved without deteriorating the appearance and without significantly increasing the cost.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A zinc-based plated steel sheet is brought into contact with an acidic solution, held for 1 to 90 seconds after completion of the contact treatment, and then washed with water and dried to form an oxide layer having an average thickness of 10 nm or more on the surface of the plated steel sheet. In the manufacturing method of a plated steel plate, the said acidic solution contains Al ion, The manufacturing method of the plated steel plate for hot press forming characterized by the above-mentioned.
[2] In the above [1], the acidic solution contains at least one of Al sulfate, nitrate, chloride, and phosphate in an Al ion concentration range of 0.1 to 100 g / l. A method for producing a hot-pressed plated steel sheet, comprising:
[3] In the above [1] or [2], the acidic solution has a pH buffering action, and 1.0 mol / l hydroxylation necessary for increasing the pH of 1 liter of acidic solution from 2.0 to 5.0 A method for producing a hot-pressed plated steel sheet, characterized in that the pH increase defined by the amount (l) of sodium solution is in the range of 0.05 to 0.5.
[4] In any one of the above [1] to [3], the acidic solution is composed of acetate, phthalate, citrate, succinate, lactate, tartrate, borate, phosphate. Hot press-forming plating, characterized in that at least one of them is contained in the range of the component content of 5 to 50 g / l, the pH is in the range of 0.5 to 2.0, and the liquid temperature is in the range of 20 to 70 ° C. A method of manufacturing a steel sheet.
[5] In any one of the above [1] to [4], the acidic solution film formed on the surface of the plated steel sheet after contact with the acidic solution is 20 g / m ² or less, and the acidic solution film is A method for producing a hot-pressed plated steel sheet, characterized in that the retention time in the state of being formed on the surface of the plated steel sheet is in the range of 1 to 90 seconds.
[6] A method for producing a hot-dip galvanized steel sheet according to any one of [1] to [5], wherein an oxide layer containing Zn and Al is formed on the steel sheet surface with an average thickness of 10 nm or more. A hot-press-formed plated steel sheet.

  In the present invention, steel plates having a plating layer containing zinc are collectively referred to as zinc-based plated steel plates. Therefore, it is the zinc-based plated steel sheet of the present invention as long as it has a plating layer containing zinc, regardless of whether or not the alloying treatment is performed after the plating treatment. That is, the galvanized steel sheet in the present invention is a hot dip galvanized steel sheet not subjected to alloying treatment, an alloyed hot dip galvanized steel sheet subjected to alloying treatment, an electrogalvanized steel sheet, a hot dip galvanized steel sheet, and the like. Both are included.

  According to the present invention, a hot press-formed plated steel sheet having excellent oxidation resistance can be obtained. And, by performing hot press forming using the hot-press-formed plated steel sheet of the present invention, processing can be performed without causing galling or breakage of the steel sheet, and there is no need to remove scale such as shot blasting. Therefore, the cost can be reduced.

  During hot press forming, the steel plate is heated above the A3 transformation point (about 900 ° C or higher). In this case, since the melting point of Zn is 418 ° C and the boiling point is 907 ° C, in the case of zinc-based plated steel sheet, it is expected that the Zn plating on the steel sheet will evaporate, and as a result, the steel sheet substrate will be oxidized become. For this reason, it is considered that the presence of a hard and high melting point substance on the surface of the plated steel sheet is very effective from the viewpoint of preventing oxidation of the steel sheet substrate.

  Therefore, studies were made to form a hard and high melting point material on the plating surface. As a result, it has been found that forming an oxide layer having a high melting point and a predetermined thickness on the surface of a zinc-based plated steel sheet is very effective as a method for imparting excellent oxidation resistance. Furthermore, it has also been found that by containing Al having a high melting point in this oxide layer, it is possible to obtain a higher melting point oxide layer, and to dramatically improve the oxidation resistance.

  From the above, in the present invention, the plated steel sheet surface has an oxide layer containing Zn and Al.

  And the average thickness of the oxide layer formed in a plated steel plate surface shall be 10 nm or more. By setting the thickness to 10 nm or more, the oxide layer becomes a barrier layer even at the heating temperature (temperature above the Ac3 transformation point) during hot press molding, and scale generation can be suppressed. On the other hand, if the thickness is 200 nm or less, the effect of hot press formability can be obtained, and there is no need to particularly consider the line length for forming the oxide layer having this thickness on the surface of the plating layer. There will be no problems with the equipment. Therefore, the upper limit is preferably 200 nm or less.

  From the above, in the present invention, an oxide layer containing Zn and Al and having an average thickness of 10 nm or more is formed on the surface of the hot-press-formed plated steel sheet. These are the features of the present invention, and the presence of such an oxide layer on the surface of the steel sheet prevents oxidation of the steel sheet substrate and has excellent oxidation resistance.

  As a method for forming the oxide layer on the surface of the plated steel sheet, a method using a reaction of the plating layer with an aqueous solution is most effective. For example, an oxide layer having an average thickness of 10 nm or more is formed on the surface of the plated steel sheet by bringing the zinc-based plated steel sheet into contact with an acidic solution containing Al ions and holding it for 1 to 90 seconds after completion of the contact treatment, followed by washing and drying Can be formed.

Although the oxide layer formation mechanism is not clear, it can be considered as follows. When the 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 hydrogen generation at the same time. As the dissolution of zinc proceeds, the hydrogen ion concentration in the acidic solution decreases, and as a result, the pH of the acidic solution increases and oxides (hydroxides) are formed. When reaching a stable pH range, it is considered that a Zn-based oxide layer is formed on the surface of the galvanized steel sheet. In this case, when an acidic solution containing Al ions is used, an Al-based oxide formation reaction occurs in a pH range lower than that of the Zn-based oxide formation reaction, and when the pH further increases, a Zn-based oxide formation reaction occurs. Therefore, it can be considered that the formation reaction of the oxide layer easily occurs as compared with the formation of only the Zn-based oxide.
In order to contain Al ions in the acidic solution, it is preferable to contain at least one of Al sulfate, nitrate, chloride, and phosphate in the range of 0.1 to 100 g / l as the Al ion concentration. . When the Al ion concentration is 0.1 g / l or more, since the amount of Zn-based oxide formed is small and an oxide layer mainly composed of Al is obtained, a sufficient oxidation resistance effect at high temperature heating can be obtained. A sufficient effect of improving press formability can be obtained. On the other hand, when the amount is 100 g / l or less, the proportion of the Al-based oxide formed is moderate, and compatibility with an adhesive designed for a zinc-based plated steel plate is not deteriorated. Further, since the oxide formed is not coarse and the amount is not large, it does not adhere to a press die or the like, and the productivity is not reduced.
The acidic solution used preferably has a pH buffering action in the range of pH = 0.5 to 5.0. This is because when an acidic solution having a pH buffering action in the above pH range is used, it is maintained for a predetermined time after contact with the acidic solution, so that the dissolution of Zn and the Al-based oxide and Zn-based oxide are caused by the reaction between the acidic solution and the plating layer. This is because the oxide formation reaction occurs sufficiently, and the target oxide layer of the present invention can be stably obtained on the steel sheet surface.

  In addition, as an index of such pH buffering action, evaluated by the degree of pH increase defined by the amount (l) of 1.0 mol / l sodium hydroxide aqueous solution required to raise the pH of a 1 liter acidic solution from 2.0 to 5.0 This value is preferably in the range of 0.05 to 0.5. If the degree of pH increase is 0.05 or more, the pH will increase rapidly, and sufficient zinc dissolution for forming the oxide layer will not be obtained, so that a sufficient oxide layer will be formed. On the other hand, if it is 0.5 or less, the dissolution of zinc is not promoted too much, and not only does it take a long time to form the oxide layer, but also the plating layer is not severely damaged. It is because it is thought that the role of sacrificial protection will not be lost. Here, the pH increase degree of an acidic solution having a pH exceeding 2.0 is obtained by adding an inorganic acid having almost no buffering property in the pH range of 2.0 to 5.0 such as sulfuric acid to the acidic solution, and once reducing the pH to 2.0. We will evaluate it.

Acidic solutions with such pH buffering properties 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 and phosphate, An aqueous solution containing 5 to 50 g / l can be used. When the concentration is 5 g / l or more, since the pH of the solution does not rise relatively quickly with the dissolution of zinc, an oxide layer sufficient for improving the oxidation resistance can be formed. If it is 50 g / l or less, the dissolution of zinc is not promoted too much, the formation of the oxide layer does not have a long time, and the plating layer is not severely damaged. It is because it is thought that the role of sacrificial protection will not be lost.

  The pH of the acidic solution is desirably in the range of 0.5 to 2.0. When the pH is 2.0 or less, precipitation of Al ions (formation of hydroxide) does not occur in the solution, and Al-based oxide is taken into the oxide layer. On the other hand, when the pH is 0.5 or more, dissolution of zinc is not promoted too much, and not only the plating adhesion amount is not reduced, but also the plating film is not cracked and is not easily peeled off during processing. . When the pH of the acidic solution is higher than the range of 0.5 to 2.0, the pH can be adjusted with an inorganic acid having no pH buffering property such as sulfuric acid.

  About the temperature of an acidic solution, it is preferable that it is the range of 20-70 degreeC. If it is 20 ° C. or higher, the production reaction of the oxide layer does not take a long time, and the productivity is not lowered. On the other hand, when the temperature is 70 ° C. or lower, there is no processing unevenness on the surface of the steel sheet, and the reaction can proceed relatively quickly.

  The method for bringing the plated steel sheet into contact with the acidic solution is not particularly limited, the method of immersing the plated steel sheet in the acidic solution, the method of spraying the acidic solution onto the plated steel sheet, and the method of applying the acidic solution to the plated steel sheet via the coating roll Etc.

After the plated steel sheet is brought into contact with the acidic solution composed of the above, it is desirable that the solution exists in the form of a thin liquid film on the steel sheet surface. This is because if the amount of the solution present on the steel plate surface is too large, the pH of the solution will hardly rise even if zinc dissolution occurs, and only zinc dissolution will occur one after another, forming the desired oxide layer. This is because it takes a long time to complete. In addition, the plating layer is severely damaged, and it may be possible to lose its original role as a rust-proof steel plate. From this viewpoint, the amount of the solution film formed on the steel sheet surface is preferably adjusted to 20 g / m ² or less. More preferably, a liquid film amount of 3 g / m ² or more is suitable for the purpose of preventing the liquid film from drying. The amount of the solution film can be adjusted by a squeeze roll, air wiping or the like.
Moreover, after contact with an acidic solution, the time to water washing (holding time to water washing) is required for 1 to 90 seconds. If the time until washing with water is less than 1 second, the pH of the solution rises and the acidic solution is washed away before the Al-based oxide layer and Zn-based oxide layer are formed. The effect is not obtained. On the other hand, even if it exceeds 90 seconds, no change is observed in the amount of oxide layer formed.

  In addition, before making it contact with an acidic solution, you may perform a surface activation process, and it is preferable that the chemical | medical solution used for a surface activation process is an alkaline solution which is pH11 or more. The purpose of this treatment is, for example, in the case of a hot-dip galvanized steel sheet, removing the Zn-based oxide formed on the surface and exposing the new surface to the surface, thereby activating the reaction at the exposed portion of the new surface, This is to facilitate the formation of a new oxide layer.

  Moreover, you may perform temper rolling before this surface activation process. Furthermore, after the plating treatment, surface activation treatment may be performed, temper rolling may be performed, and then contacted with an acidic solution. It is possible to destroy part of the Zn-based oxide layer present on the surface of the plated steel sheet with a pressure-control roll used during temper rolling, and by combining surface activation treatment with a Zn-based oxide. The layer can be removed effectively.

The aqueous solution used for the surface activation treatment preferably has a pH of 11 or more, a bath temperature of 30 ° C. or more, and a contact time with the solution of 1 to 30 seconds. In the case of less than 1 second, the Zn-based oxide cannot be sufficiently dissolved, so that the reactivity with the subsequent acidic solution cannot be increased, and the oxide layer is not sufficiently formed. On the other hand, it may be longer than 30 seconds, but it is not preferable to perform the treatment for a long time because the productivity is lowered. More preferably, the pH is 11 or more and the bath temperature is 50 ° C. or more. If it is pH in the said range, there will be no restriction | limiting in the kind of solution, Sodium hydroxide, a sodium hydroxide type | system | group degreasing agent, etc. can be used.
The surface activation treatment is preferably performed before contact with the acidic solution, but may be performed either before or after the temper rolling performed after the plating treatment performed as necessary. However, if the surface activation treatment is performed after temper rolling, the Zn-based oxide is mechanically destroyed at the portion that has been crushed by the rolling roll to become a convex portion. There is a tendency that the amount of removal of objects differs. For this reason, the amount of Zn-based oxide after the surface activation treatment becomes non-uniform in the surface, and the subsequent oxidation treatment becomes non-uniform, so that sufficient characteristics may not be obtained. For this reason, more preferably, after the plating treatment, the surface activation treatment is performed, and after removing an appropriate amount of the Zn-based oxide uniformly in the surface, the temper rolling is performed, and the treatment is subsequently brought into contact with the acidic solution. Is preferred.
The method for the surface activation treatment is not particularly limited. Examples of the method include a dipping method, a spray method, and a roll coating method.
As a result, an oxide layer having an average thickness of 10 nm or more is formed on the surface of the plated steel sheet.

  The oxide layer in the present invention is a layer made of an oxide and / or hydroxide containing Zn and Al. The average thickness of such an oxide layer containing Zn and Al needs to be 10 nm or more. When the average thickness of the oxide layer is reduced to less than 10 nm, the effect of reducing the sliding resistance is insufficient, and the hot press formability is also inferior. On the other hand, when the average thickness of the oxide layer containing Zn and Al is 200 nm or less, the coating does not break during hot press molding, so the sliding resistance does not increase. Moreover, since weldability does not fall, it is preferable. Furthermore, since the formed oxide does not become coarse and does not have a large amount, it does not adhere to a press die or the like and productivity is not lowered, which is preferable.

  In the present invention, if the acidic solution to be used contains Al ions, it is possible to stably form an oxide layer excellent in oxidation resistance, that is, excellent in hot press formability. Even if other metal ions or inorganic compounds are contained as impurities or intentionally contained therein, the effect of the present invention is not impaired. In particular, since Zn ions are ions that elute when the plating layer and the acidic solution come into contact with each other, an increase in the Zn concentration in the acidic solution is observed during operation. The magnitude of this Zn ion concentration is effective for the effect of the present invention. Has no particular effect.

  Moreover, although the base steel material concerning this invention is not specifically limited, It is preferable to set it as the steel composition from which the plating wettability at the time of a plating process and the plating adhesiveness after plating become favorable. In consideration of the case where hot press forming is performed, a hardened steel, such as a high-tensile steel plate, which is rapidly cooled after hot press forming to become high strength and high hardness, such as a high-strength steel plate, is particularly preferable.

In the present invention, the plating method is not particularly limited, but the hot dip galvanizing method is preferable in terms of cost. Of course, as long as an oxide layer containing predetermined Zn and Al can be obtained on the plating surface, the plating layer may be provided by any other method such as electroplating, thermal spray plating, vapor deposition plating, or the like.
Moreover, it does not specifically limit about the method of hot press molding. For example, with respect to the zinc-based plated steel sheet for hot press forming of the present invention, it is heated to the Ac3 transformation point or higher at a heating rate of 1 to 100 ° C./second, and after holding for 5 to 6000 seconds, it is 400 to 800 ° C. By performing hot press molding in the temperature range, the effect of oxidation resistance of the present invention can be maximized.
In addition, during hot press molding, the member is cooled at a cooling rate of 10 to 200 ° C./s using a die and a punch, or after hot press molding, the hot press molded member is taken out from the mold, It is also possible to cool using liquid or gas.

  Next, the present invention will be described in detail with reference to examples.

For steel sheets containing C: 0.23 mass%, Si: 0.12 mass%, Mn: 1.5 mass%, Cr: 0.50 mass%, B: 0.0020 mass% as steel components Plating treatment, electrogalvanizing treatment and hot-dip Zn-Al plating treatment were performed, respectively, to produce hot-dip galvanized steel sheets, alloyed hot-dip galvanized steel sheets, electrogalvanized steel sheets and hot-dip Zn-Al-plated steel sheets with a thickness of 1.2 mm. In addition, the plating adhesion amounts of the hot dip galvanized steel sheet, the alloyed hot dip galvanized steel sheet, the electrogalvanized steel sheet, and the hot dip galvanized steel sheet are all 45 g / m ² .

  Next, an acidic solution contact treatment was performed on the hot-dip galvanized steel sheet, the alloyed hot-dip galvanized steel sheet, the electrogalvanized steel sheet, and the hot-dip Zn-Al-plated steel sheet. In the acidic solution treatment, an acidic aqueous solution of 40 g / l sodium acetate having buffering properties was prepared, and then immersed in an acidic solution prepared by adjusting pH with sulfuric acid for 3 seconds. Then, after squeezing the roll and adjusting the amount of liquid, it is left in the atmosphere at room temperature for 1 to 90 seconds, sufficiently washed with water, then dried, and the surface of the plated steel sheet is oxidized with Zn and Al. A physical layer was formed. The presence of Zn and Al was confirmed by analyzing the cross section of the formed oxide layer with an energy dispersive X-ray fluorescence analyzer. Detailed conditions are shown in Tables 1 to 4. Moreover, as a comparative example, the thing which did not perform the said acidic solution process and the thing which performed the acidic solution process which does not contain Al ion were produced.

About the steel plate produced by the above, while determining whether it had sufficient hot press formability (appearance after shaping | molding) as a motor vehicle outer plate, the average thickness of the oxide layer of the plating surface was measured. Details of the hot press formability evaluation method and the oxide layer thickness measurement method are as follows.
・ Hot press formability (appearance evaluation)
In a heating furnace, it was heated to 900 ° C. at a heating rate of 15 ° C./s in an air atmosphere, held at 900 ° C. for 60 seconds, taken out from the heating furnace, and subjected to hot press molding of a cylindrical drawing. Hot press molding was performed under the conditions of drawing height: 25 mm, shoulder R: 5 mm, blank diameter: 90 mm, punch diameter: 50 mm, and die diameter: 53 mm. As the plating layer adhesion state of the test piece after molding, the presence or absence of peeling of the plating layer was visually observed, and the hot press formability was evaluated in two stages.
◎: No exfoliation, x exfoliation ・ Measurement of oxide layer thickness Using a Si wafer on which a thermally oxidized SiO ₂ film with a film thickness of 96 nm is formed as a reference material, measure O / Kα X-rays with a fluorescent X-ray analyzer Thus, the average thickness of the oxide layer in terms of SiO ₂ was obtained. The analysis area is 30 mmφ.

  The results obtained as described above are shown in Tables 1 to 4 together with the conditions.

  In the example of the present invention, the appearance defect is suppressed by suppressing the evaporation of plating and the prevention of scale generation during hot press forming, and the plating layer peeling is not observed and the oxidation resistance is excellent, so the hot press formability is good. It was.

  On the other hand, in the comparative example in which the acidic solution treatment was not performed, the oxide layer of the present invention was not formed, and plating layer peeling due to scale generation was observed after hot press molding.

  The plated steel sheet for hot press forming according to the present invention is excellent in oxidation resistance, and therefore can be applied in a wide range of fields mainly for automobile body applications such as automobile undercarriages, chassis and reinforcing parts.

Claims (6)

  A galvanized steel sheet is brought into contact with an acidic solution, held for 1 to 90 seconds after completion of the contact treatment, and then washed with water and dried to form an oxide layer having an average thickness of 10 nm or more on the surface of the galvanized steel sheet. In the production method, the acidic solution contains Al ions, and the method for producing a hot-press-formed plated steel sheet.   2. The acidic solution according to claim 1, wherein at least one of Al sulfate, nitrate, chloride, and phosphate is contained in an Al ion concentration range of 0.1 to 100 g / l. The manufacturing method of the hot-press-forming plated steel plate as described in 2.   The acidic solution has a pH buffering action, and the degree of pH increase defined by the amount (l) of 1.0 mol / l sodium hydroxide solution required to increase the pH of 1 liter acidic solution from 2.0 to 5.0 The manufacturing method of the hot-press-formed plated steel sheet according to claim 1 or 2, wherein the range is from 0.05 to 0.5.   The acidic solution contains at least one of acetate, phthalate, citrate, succinate, lactate, tartrate, borate, and phosphate, and the component content is 5 to 50 g / l. The hot-press-formed plated steel sheet according to any one of claims 1 to 3, wherein the pH is in the range of 0.5 to 2.0 and the liquid temperature is in the range of 20 to 70 ° C. Method. The acidic solution film formed on the surface of the plated steel sheet after being brought into contact with the acidic solution is 20 g / m ² or less, and the holding time in a state where the acidic solution film is formed on the surface of the plated steel sheet is 1 to 90. It is the range of second, The manufacturing method of the hot-press-forming plated steel plate as described in any one of Claims 1-4 characterized by the above-mentioned.   A heat produced by the method for producing a hot-dip galvanized steel sheet according to any one of claims 1 to 5, wherein an oxide layer containing Zn and Al is formed on the steel sheet surface with an average thickness of 10 nm or more. Plated steel sheet for hot press forming.