JP2020041175A - Steel plate for hot pressing - Google Patents

Abstract

To provide a steel plate for hot pressing with excellent post-coating image clarity by reducing surface roughness after hot-pressing.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 a metal layer made of metal with a melting point of 600°C or less 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 steel sheet for hot pressing 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. Due to this uneven shape, that is, a rough surface, the sharpness after coating of the hot pressed member is deteriorated. For this reason, the application of the hot press member has been limited mainly to parts such as automobile undercarriage parts where the demand for sharpness after painting is low. However, in order to further expand the application of hot pressed parts, it is essential to improve the sharpness after painting.

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a steel sheet for hot press which reduces surface roughness after hot press and has excellent sharpness after coating.

  The present inventors formed various metal layers on the surface of the Zn alloy plating layer by a wet process or a dry process, and examined the surface roughness Ra after hot pressing of the obtained hot pressed member. . As a result, the surface of the Zn alloy plating layer is stretched during the hot press forming by applying a highly ductile metal layer to the surface of the Zn alloy plating layer during the hot pressing, and the surface roughness after the hot pressing is formed. Was found to be improved.

The present invention is based on the above findings, and the features are as follows.
[1] A steel plate surface having a Zn alloy plating layer having a coating weight per surface of 120 g / m ² or less and a metal layer made of a metal having a melting point of 600 ° C. or less on the surface of the Zn alloy plating layer. A steel sheet for hot pressing characterized by the following.
[2] The hot press according to [1], wherein the metal layer is made of a metal or an alloy selected from one or more of Ga, Cd, In, Sn, and Sb. steel sheet.
[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 surface roughness after a hot press is reduced, and the steel plate for hot press excellent in the sharpness after coating 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 metal layer formed on the surface of the Zn alloy plating layer will be described.

Metal layer composed of a metal having a melting point of 600 ° C. or less In the present invention, a metal layer composed of a metal having a melting point of 600 ° C. or less is provided on the surface of the Zn alloy plating layer. Generally, the hot press forming temperature is 600 to 800 ° C. In this temperature range, a metal layer made of a metal having a melting point of 600 ° C. or less exists on the surface of the Zn alloy plating layer as a liquid phase. That is, since the metal layer made of a metal having a melting point of 600 ° C. or less during hot press forming is a metal layer having high ductility during hot press forming, the surface of the Zn alloy plating layer expands during hot press forming. Thus, the surface roughness after hot pressing is improved. As a result, the sharpness after painting can be improved.

The metal layer is preferably a metal or an alloy selected from one or more of Ga, Cd, In, Sn, and Sb. Each of the metals or alloys mentioned here has a melting point of 600 ° C. or less. Regarding the lower limit of the adhesion amount of the metal layer, even if the amount is small, the effect of improving the sharpness after coating is recognized as compared with the case where there is no metal layer, but in the present invention, 0.1 g / m ² or more is preferable. Although the upper limit is not particularly defined, an excessively thick metal layer causes an increase in cost, and is therefore 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 method for forming the metal layer is not particularly limited. For example, a dry process such as PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), or cold spray, or a wet process such as electroplating, electroless plating, or hot-dip plating is used. Can be formed.

  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, various metal layers were coated on the surface of the Zn alloy plating layer by the film forming method shown in Table 1 to obtain a steel sheet for hot pressing. The conditions of the hot-dip plating treatment were such that the hot-dip plating 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. In addition, a desired metal target was used, and a desired adhesion amount was controlled by controlling a voltage and a processing time. The conditions for the electroplating of the metal layer 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.

  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.

The surface roughness Ra of the upper flat portion, the side flat portion, and the lower flat portion of the hat-shaped member obtained above was measured, and the sharpness after coating was evaluated according to the following criteria.
：: Ra average value decreased by 0.1 or more as compared with the case without metal layer Δ: Ra average value decreased by 0 or more and less than 0.1 compared with the case without metal layer x: Ra average value decreased If the evaluation was 評 価, which is an increase compared to the case without the metal layer, it was judged that the steel sheet for hot pressing was excellent in the sharpness after coating.

  Table 1 shows the evaluation results.

  According to Table 1, the steel sheet for hot pressing of the present invention has excellent post-painting sharpness.

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. Thus, it can be seen that, in addition to the sharpness after painting, 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 a metal layer made of a metal having a melting point of 600 ° C. or less on the surface of the Zn alloy plating layer.   The steel sheet for hot pressing according to claim 1, wherein the metal layer is a metal or an alloy selected from one or more of Ga, Cd, In, Sn, 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.