JP2020041177A - Steel plate for hot pressing - Google Patents

Abstract

To provide a steel plate for hot pressing with an excellent anti-mold adhesion characteristic by reducing a Zn adhesion amount to a mold at the time of hot-press molding.SOLUTION: A steel plate for hot pressing has a Zn alloy plating layer with a plating adhesion amount per face of 120 g/m, and further has an oxide layer with higher standard reaction Gibbs energy of oxide at 900°C than that of ZnO on a surface of the Zn alloy plating layer.SELECTED DRAWING: None

Description

  TECHNICAL FIELD The present invention relates to a steel sheet for hot pressing mainly used for a steel sheet for automobiles.

  2. Description of the Related Art In recent years, in the field of automobiles, weight reduction has been promoted along with higher performance of material steel sheets, and the use of high-strength hot-dip galvanized steel sheets or electrogalvanized steel sheets having rust prevention has been increasing. However, in many cases, the press formability of the steel sheet is reduced as the strength of the steel sheet is increased, so that it is difficult to obtain a complicated part shape. For example, in automotive applications, rustproofing is required, and examples of difficult-to-mold parts include underbody members such as chassis and structural members for skeletons such as B pillars.

  Against this background, in recent years, the production of automotive parts by hot pressing, which facilitates both press formability and high strength compared to cold pressing, has been rapidly increasing. Various techniques for solving the problem have been disclosed.

  For example, Patent Document 1 discloses a manufacturing method of obtaining a hot pressed member free from liquid metal embrittlement cracking by setting the melting point of a plating layer on a steel sheet surface to 800 ° C. or higher.

  Patent Literature 2 discloses a steel sheet for hot pressing in which a plating layer surface is provided with an oxide film mainly composed of ZnO to prevent zinc evaporation during hot press heating.

  Further, in Patent Document 3, a hot press member without micro cracks is formed by rapidly cooling a steel plate to 450 to 700 ° C. using an air jet or the like before hot pressing, and then performing die cooling by pressing. A method of making is disclosed.

Japanese Patent No. 5817479 Japanese Patent No. 3585204 Japanese Patent No. 5727037

  As described above, various solutions have been proposed from the viewpoints of steel plates, plating, and construction methods for the various problems of the hot press technology, and have supported the progress and development of the hot press technology.

  However, with the increase in the production of hot-pressed parts and the application of technology to new parts, new problems that have not existed in the past have become apparent.

  For example, when a hot pressed steel sheet having a Zn plating layer is hot pressed, a ZnO-based oxide film having irregularities is formed on the surface of the Zn plating layer. However, when a crack occurs in the plating layer due to sliding or the like, the oxygen is not sufficiently supplied because the space between the press die and the steel sheet is closed, so that Zn in the plating layer is vaporized as a metal. This causes a problem that the Zn mold adheres preferentially, that is, Zn adheres to the mold. If hot press molding is continued while the Zn mold is left attached to the mold, surface defects increase. For this reason, it is necessary to periodically remove Zn adhered from the press die by pickling or the like, which is one of the main factors for reducing productivity.

  The present invention has been made in view of such circumstances, and it is an object of the present invention to provide a hot-pressing steel sheet that reduces the amount of Zn adhered to a mold during hot press forming and has excellent die-adhesion properties. Aim.

  The present inventors have formed various oxide layers on the surface of the Zn alloy plating layer by a wet process or a dry process, and the obtained hot pressed member was pressed into a press die when hot pressed. The mold adhesion behavior was investigated. As a result, when a crack occurs in the plating layer, the oxidation reaction is performed by applying an oxygen supply source, that is, an oxide layer having a higher standard reaction Gibbs energy of the oxide at 900 ° C. than ZnO to the surface of the Zn alloy plating layer. The material layer functions as an oxygen supply source for the vaporized Zn, and as a result, even in a closed space in which oxygen is not sufficiently supplied from the outside air, such as between a mold and a steel plate, oxidation of metal Zn (mainly composed of ZnO). (Formation of an oxide film) is possible, and the mold adhesion resistance is improved.

The present invention is based on the above findings, and the features are as follows.
[1] A Zn alloy plating layer having a coating weight per surface of 120 g / m ² or less on one surface of a steel sheet, and a standard reaction Gibbs energy of an oxide at 900 ° C. is ZnO on the surface of the Zn alloy plating layer. A steel sheet for hot pressing characterized by having a higher oxide layer.
[2] The steel sheet for hot pressing according to [1], wherein the oxide layer is a metal oxide selected from one or more of Co, Mo, and Sb.
[3] The Zn alloy plating layer is a Zn-Al alloy plating layer, a Zn-Al-Mg alloy plating layer, a Zn-Al-Mg-Si alloy plating layer, a Zn-Fe alloy plating layer, a Zn-Ni alloy plating layer. The steel sheet for hot pressing according to [1] or [2], which is any one of the above.
[4] The steel sheet for hot pressing as described in [3], wherein the Zn—Ni alloy plating layer contains 10 to 25% by mass of Ni, and the balance consists of Zn and unavoidable impurities.

  ADVANTAGE OF THE INVENTION According to this invention, the amount of Zn adhesion to a metal mold at the time of a hot press molding is reduced, and the steel plate for hot press excellent in die-resistant property is obtained.

  Hereinafter, the present invention will be described specifically. In the following description, the unit of the content of each element of the steel component composition and the unit of the content of each element of the plating layer component composition are “% by mass”, and hereinafter, unless otherwise specified, simply “%”. Show.

  First, the reason for limiting the Zn alloy plating layer of the steel sheet for hot pressing will be described.

Zn alloy plating layer with a coating weight per side of 120 g / m ² or less In order to form a thick plating exceeding 120 g / m ^{2 on} a general plating production line, it is necessary to significantly suppress the line speed. This leads to higher costs. Therefore, the plating adhesion amount per one side is set to 120 g / m ² or less. In addition, it is preferably 90 g / m ² or less. Further, if it is less than 10 g / m ² , the effect of suppressing Fe scale generation during hot press heating becomes insufficient, so that it is preferably 10 g / m ² or more. In addition, hot press members for automobiles are mainly applied to parts requiring corrosion resistance, such as underbody parts and skeletal members. Therefore, the plating layer needs to be a Zn alloy plating layer having a sacrificial anticorrosion action.

  From the viewpoint of rust prevention, the composition is not particularly limited as long as the main component of the plating layer is Zn, but as the Zn alloy plating layer, a Zn-Al alloy plating layer, a Zn-Al-Mg alloy plating layer, a Zn- Any one of an Al-Mg-Si alloy plating layer, a Zn-Fe alloy plating layer, and a Zn-Ni alloy plating layer is preferable.

In the present invention, the Zn alloy plating layer preferably contains 10 to 25% by mass of Ni, and the balance is preferably a Zn—Ni alloy plating layer composed of Zn and unavoidable impurities. By controlling the amount of Ni in the Zn alloy plating layer to 10 to 25% by mass, a γ phase having a crystal structure of one of Ni ₂ Zn ₁₁ , NiZn ₃ , and Ni ₅ Zn ₂₁ having a high melting point is formed. This is advantageous from the viewpoint of liquid metal embrittlement resistance as compared with other Zn alloy plating layers.

  Next, the oxide layer formed on the surface of the Zn alloy plating layer will be described.

Oxide layer whose standard reaction Gibbs energy of oxide at 900 ° C. is higher than ZnO In the present invention, the surface of the Zn alloy plating layer has an oxide layer whose standard reaction Gibbs energy of oxide at 900 ° C. is higher than ZnO . When a hot-pressed steel sheet having a Zn plating layer is hot-press-formed, a ZnO-based oxide film having irregularities is formed on the surface of the Zn plating layer. However, when cracks occur in the plating layer due to sliding or the like, since the space between the press die and the steel sheet is a closed space, oxygen is not supplied sufficiently, so that a ZnO-based oxide film is sufficiently formed. Instead, Zn in the plating layer is vaporized as a metal, and Zn-type adhesion, in which Zn adheres to the mold, occurs preferentially. In the present invention, the standard reaction Gibbs energy of the oxide at 900 ° C. is applied to the surface of the Zn alloy plating layer with an oxide layer higher than ZnO, so that the oxide layer serves as an oxygen supply source for Zn vaporized. Function. As a result, it is possible to reduce the amount of Zn adhered to the mold during hot press molding, and to improve the die-resistant adhesion properties.

The oxide layer is preferably a metal oxide selected from one or more of Co, Mo, and Sb. Each of the metal oxides listed here is an oxide layer whose standard reaction Gibbs energy of the oxide at 900 ° C. is higher than that of ZnO. The lower limit of the deposition of the oxide layer, but耐型adhesion properties is observed in comparison with the case where there is no oxide layer in trace amounts, in the present invention, 0.1 g / m ² or more. Although the upper limit is not particularly defined, an oxide layer having an excessive thickness causes an increase in cost, and therefore is preferably 30 g / m ² or less. More preferably, it is 10 g / m ² or less, still more preferably 5 g / m ² or less.

  The standard reaction Gibbs energy can be calculated using an Ellingham diagram.

  The method for forming the oxide layer is not particularly limited, and the oxide layer can be formed by a dry process such as PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), or cold spray.

  In the present invention, in order to obtain a hot pressed member exceeding 1470 MPa class, for example, as the base steel sheet of the plating layer, C: 0.20 to 0.35%, Si: 0.1 -0.5%, Mn: 1.0-3.0%, P: 0.02% or less, S: 0.01% or less, with the balance having a component composition consisting of Fe and unavoidable impurities. A rolled steel plate or a hot rolled steel plate can be used. The reasons for limiting each component are described below.

C: 0.20 to 0.35%
C improves strength by forming martensite or the like as a steel structure. To obtain a strength exceeding the 1470 MPa class, 0.20% or more is required. On the other hand, if it exceeds 0.35%, the toughness of the spot-welded portion deteriorates. Therefore, the C content is preferably set to 0.20 to 0.35%.

Si: 0.1-0.5%
Si is an element effective for strengthening steel and obtaining a good material. For that purpose, 0.1% or more is required. On the other hand, when the content exceeds 0.5%, the ferrite is stabilized, so that the hardenability decreases. Therefore, the amount of Si is preferably set to 0.1 to 0.5%.

Mn: 1.0-3.0%
Mn is an element effective for increasing the strength of steel. In order to ensure mechanical properties and strength, it is necessary to contain 1.0% or more. On the other hand, if it exceeds 3.0%, the surface concentration at the time of annealing increases, and it becomes difficult to secure plating adhesion. Therefore, the amount of Mn is preferably set to 1.0 to 3.0%.

P: 0.02% or less If the P content exceeds 0.02%, the balance between strength and ductility is lowered through deterioration of local ductility due to grain boundary embrittlement accompanying P segregation at austenite grain boundaries during casting. Therefore, the P content is preferably set to 0.02% or less.

S: 0.01% or less S becomes an inclusion such as MnS, and causes deterioration of impact resistance and cracks along a metal flow of a welded portion. Therefore, it is desirable to reduce as much as possible, and it is preferable to make it 0.01% or less. In order to secure good stretch flangeability, the content is more preferably 0.005% or less.

  In addition, in the present invention, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.0050% in order to further improve the properties of the steel sheet in addition to the above basic components. , Cr: 0.1 to 0.3% and Sb: 0.003 to 0.030%, if necessary.

Nb: 0.05% or less Nb is a component effective for strengthening steel. However, if it is contained excessively, shape freezing property decreases. Therefore, when Nb is contained, the content is set to 0.05% or less.

Ti: 0.05% or less Ti is also effective in strengthening steel like Nb, but there is a problem in that if it is contained excessively, the shape freezing property is reduced. Therefore, when Ti is contained, the content is made 0.05% or less.

B: 0.0002-0.0050%
Since B has an effect of suppressing the formation and growth of ferrite from austenite grain boundaries, the addition of B is preferably 0.0002% or more. On the other hand, excessive addition of B greatly impairs moldability. Therefore, when B is contained, the content is made 0.0002 to 0.0050%.

Cr: 0.1-0.3%
Cr is useful for strengthening steel and improving hardenability. In order to exhibit such effects, addition of 0.1% or more is preferable. On the other hand, since the alloy cost is high, adding over 0.3% leads to a significant cost increase. Therefore, when Cr is contained, the content is set to 0.1 to 0.3%.

Sb: 0.003 to 0.030%
Sb also has the effect of suppressing the decarburization of the steel sheet surface layer during the hot pressing process. In order to exhibit such an effect, it is necessary to add 0.003% or more. On the other hand, when the amount of Sb exceeds 0.030%, the rolling load is increased, so that the productivity is reduced. Therefore, when Sb is contained, the content is set to 0.003 to 0.030%.

  The remainder other than the above consists of Fe and inevitable impurities.

  The hot pressing step for manufacturing a hot pressed member using the steel sheet for hot pressing of the present invention is not limited at all, and a known hot pressing step can be applied. For example, the steel sheet for hot pressing of the present invention is heated to a temperature range from the Ac3 transformation point to 950 ° C., then hot-pressed, and subsequently cooled using a mold or a coolant such as water. Thus, a hot pressed member is manufactured. In addition, the said heating temperature means the highest attainment temperature of a steel plate. Examples of the method for performing the heating include heating using an electric furnace or a gas furnace, flame heating, electric heating, high-frequency heating, induction heating, and the like.

  Hereinafter, the present invention will be specifically described based on examples. The following examples are not intended to limit the present invention, and appropriate changes within the scope of the gist are included in the scope of the present invention.

  As the base steel sheet, in mass%, C: 0.30%, Si: 0.25%, Mn: 1.2%, P: 0.005%, S: 0.005%, Nb: 0.005%, 1.4 mm in thickness, containing 0.02% of Ti, 0.0020% of B, 0.2% of Cr, and 0.008% of Sb, with the balance being Fe and unavoidable impurities. Was used. Various Zn alloy plating layers were formed on the surface of the cold-rolled steel sheet by the plating method shown in Table 1. Next, the surface of the Zn alloy plating layer was coated with various oxide layers by the film forming method shown in Table 1 to obtain a steel sheet for hot pressing. With respect to the plating layer, the conditions of the hot-dip plating treatment were such that the hot-dip bath composition was adjusted so as to obtain a desired composition, and the bath temperature was set to the melting point of each composition + 20 ° C. Further, the amount of adhesion was controlled by adjusting the wiping pressure. The conditions of the electroplating treatment were such that the metal salt ratio in the bath and the current value were adjusted so as to obtain a desired composition, and the amount of deposition was controlled by changing the line speed. The deposition conditions of PVD for the oxide layer were such that a desired oxide target was used, and a desired amount of deposition was controlled by controlling the voltage and the processing time.

  A test piece of 150 mmC × 300 mmL was sampled from the obtained steel sheet for hot pressing, heated to 900 ° C. for 4 minutes by an electric furnace, and held at 900 ° C. for 1 minute. Then, the test piece was taken out of the electric furnace and hat-shaped. Hot pressing was performed at 700 ° C. by a mold. The shape of the part after molding is such that the length of the flat portion on the upper surface is 100 mm, the length of the flat portion on the side surface is 50 mm, and the length of the flat portion on the lower surface is 50 mm. The bending R of the mold is 7R for both upper and lower shoulders.

After pressing 50 hat-formed parts each having the above-mentioned shape, the weight increase due to the adhered Zn was measured by measuring the weight of the mold, and the die resistance was evaluated according to the following criteria.
：: Zn adhesion amount decreased by 200 mg or more compared to the case without the oxide layer Δ: Zn adhesion amount decreased from 0 mg to less than 200 mg compared to the case without the oxide layer ×: Zn adhesion amount However, if the evaluation was ○, which is an increase compared to the case without the oxide layer, it was determined that the Zn alloy plating for hot pressing was excellent in die-resistant adhesion properties.

  Table 1 shows the evaluation results.

  According to Table 1, the steel sheet for hot pressing of the present invention has excellent die-resistant property.

The steel sheet for hot pressing having the Zn-Ni alloy plating layer evaluated in Example 1 was evaluated for LME resistance (LME: Liquid Metal Embrittlement, liquid metal embrittlement). Specifically, a sample for cross-sectional SEM observation is taken from the upper shoulder R of the hat-formed part obtained in Example 1, and penetrates into the base material in a visual field having a cross-sectional length of 5 mm outside the shoulder R by observation. The crack depth was measured at a total of 20 points at a pitch of 250 μm, and the LME resistance was evaluated based on the following criteria.
：: No crack generation or average crack depth of less than 10 μm △: Average crack depth of 10 μm or more and less than 200 μm ×: Average crack depth of 200 μm or more Excellent.

  Table 2 shows the evaluation results of the LME resistance characteristics.

  From the results in Table 2, in the hot pressed steel sheet of the present invention, the Zn—Ni alloy plating layer contains 10 to 25% by mass of Ni, and the Zn—Ni alloy plating layer is composed of the balance Fe and unavoidable impurities. It can be seen from the graph that, in addition to the mold adhesion resistance, it also has excellent LME resistance.

Claims (4)

On the surface of the steel sheet, there is a Zn alloy plating layer having a plating adhesion amount per side of 120 g / m ² or less,
A steel sheet for hot pressing, further comprising an oxide layer having a standard reaction Gibbs energy of the oxide at 900 ° C. higher than that of ZnO on a surface of the Zn alloy plating layer.   The steel sheet for hot pressing according to claim 1, wherein the oxide layer is a metal oxide selected from one or more of Co, Mo, and Sb.   The Zn alloy plating layer includes a Zn-Al alloy plating layer, a Zn-Al-Mg alloy plating layer, a Zn-Al-Mg-Si alloy plating layer, a Zn-Fe alloy plating layer, and a Zn-Ni alloy plating layer. The steel sheet for hot pressing according to claim 1, wherein the steel sheet is any one type.   4. The steel sheet for hot pressing according to claim 3, wherein the Zn—Ni alloy plating layer contains 10 to 25 mass% of Ni, and the balance consists of Zn and unavoidable impurities. 5.