JP6933197B2 - Steel plate for hot press - Google Patents

Description

The present invention relates mainly to hot press steel sheets used for automobile steel sheets.

In recent years, in the field of automobiles, the performance of the material steel sheet has been improved and the weight reduction has been promoted, and the use of high-strength hot-dip galvanized steel sheet or electrogalvanized steel sheet having rust resistance is increasing. However, in many cases, the press formability of the steel sheet decreases as the strength of the steel sheet increases, so that it becomes difficult to obtain a complicated part shape. For example, in automobile applications, rustproofing is required, and examples of difficult-to-mold parts include undercarriage members such as chassis and structural members for skeletons such as B-pillars.

Against this background, in recent years, the production of automobile parts by hot press, which makes it easier to achieve both press formability and high strength compared to cold press, is rapidly increasing, and various hot press technologies. Various technologies for solving problems are disclosed.

For example, Patent Document 1 discloses a manufacturing method for obtaining a hot pressed member without embrittlement cracking of a liquid metal by setting the melting point of the plating layer on the surface layer of a steel sheet to 800 ° C. or higher.

Further, Patent Document 2 discloses a hot-pressed steel sheet that prevents zinc evaporation during hot-press heating by providing an oxide film mainly composed of ZnO on the surface layer of the plating layer.

Further, in Patent Document 3, a hot press member without microcracks is manufactured by rapidly intermediate cooling the steel sheet to 450 to 700 ° C. using an air jet or the like before hot pressing, and then cooling the mold by pressing. The method of manufacture is disclosed.

Japanese Patent No. 5817479 Japanese Patent No. 3582504 Japanese Patent No. 5727037

As mentioned above, various solutions have been proposed from the viewpoints of steel sheet, plating, construction method, etc. to various problems of hot press technology, and have supported the progress and development of hot press technology.

However, with the increase in the production volume 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-plated layer is hot-press-formed, a ZnO-based oxide film having irregularities is formed on the Zn-plated surface layer. Such ZnO formation on the plating surface layer means that the amount of metal Zn that should ensure the corrosion resistance of the steel sheet is reduced by the sacrificial anticorrosion action. Therefore, in order to ensure the same sacrificial corrosion resistance as non-heated materials (materials that are not hot-pressed), it is necessary to thicken the plating by taking into account the amount of metal Zn lost by oxidation during the heating process of hot-pressing. This leads to an increase in cost. Further, in a general continuous plating production line, there is a problem that the productivity is lowered because the line speed is limited due to the thickening of the plating.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide a steel sheet for hot pressing having excellent corrosion resistance by reducing the amount of metallic Zn lost by oxidation during the heating process of hot pressing. do.

The present inventors have formed various metal layers on the surface of the Zn alloy plating layer by a wet process or a dry process, and investigated the corrosion resistance of the obtained hot-pressed member after hot-pressing. As a result, a metal layer highly reactive with oxygen, that is, a metal layer made of a metal having a standard reaction Gibbs energy of an oxide at 900 ° C., which is an easily oxidized metal having a high oxygen affinity, is higher than Zn. It has been found that by applying the metal layer to the surface, the metal layer is preferentially oxidized over Zn during hot press molding, the decrease in the amount of metal Zn due to Zn oxidation can be suppressed, and excellent corrosion resistance can be obtained.

The present invention is based on the above findings, and its features are as follows.
[1] A Zn alloy plating layer having ^{a plating adhesion amount of 120 g / m 2} or less per side is provided on the surface of the steel sheet.
Further, a steel sheet for hot pressing characterized by having a metal layer made of a metal having a standard reaction Gibbs energy of an oxide at 900 ° C. higher than Zn on the surface of the Zn alloy plating layer.
[2] The hot heat according to [1], wherein the metal layer is a metal or alloy selected from one or more of Al, Si, Ti, Cr, Mn, and Zr. Steel plate for pressing.
[3] 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 hot press steel plate according to [1] or [2], which is any one of the above.
[4] The steel sheet for hot pressing according to [3], wherein the Zn—Ni alloy plating layer contains 10 to 25% by mass of Ni, and the balance is Zn and unavoidable impurities.

According to the present invention, the amount of metal Zn lost by oxidation during the heating process of the hot press is reduced, and a steel sheet for hot press having excellent corrosion resistance can be obtained.

Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element in the steel component composition and the content of each element in the plating layer component composition are both "mass%", and hereinafter, unless otherwise specified, simply "%" is used. show.

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

Zn alloy plating layer with plating adhesion amount of 120 g / m ² or less per side In order to form thick plating of 120 g / m ^{2 or} more on a general plating production line, it is necessary to significantly suppress the line speed. Invite cost increase. Therefore, the amount of plating adhered to one side is 120 g / m ² or less. It should be noted that ^{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 formation during hot press heating becomes insufficient, so it is preferably ^{10 g / m 2 or more.} In addition, hot press members for automobiles are mainly applied to parts that require corrosion resistance, such as undercarriage and skeleton members. Therefore, the plating layer needs to be a Zn alloy plating layer having a sacrificial anticorrosion effect.

From the viewpoint of rust prevention, if the main component of the plating layer is Zn, the composition is not particularly limited, but the Zn alloy plating layer includes a Zn-Al alloy plating layer, a Zn-Al-Mg alloy plating layer, and 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.

Further, in the present invention, the Zn alloy plating layer preferably contains 10 to 25% by mass of Ni, and the balance is 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 _{Ni 2} Zn ₁₁ , Ni Zn ₃ , or Ni _{5 Zn 21 having a high melting point is formed.} , It is advantageous from the viewpoint of brittle resistance of liquid metal 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.

A metal layer made of a metal having a standard reaction Gibbs energy of an oxide at 900 ° C higher than Zn. In the present invention, the surface of a Zn alloy plating layer is made of a metal having a standard reaction Gibbs energy of an oxide higher than Zn at 900 ° C. It has a metal layer. Generally, during hot press forming, the steel sheet is heated from room temperature to around 900 ° C., which is an austenite single-phase region. In this temperature range, as a metal layer highly reactive with oxygen on the surface of the Zn plating layer, a metal layer made of a metal having a standard reaction Gibbs energy of an oxide higher than Zn at 900 ° C. is imparted to the surface of the Zn alloy plating layer. By doing so, the metal layer made of an easily oxidizing element is preferentially oxidized over Zn during hot press molding. As a result, the reduction of the amount of metal Zn due to Zn oxidation is suppressed, and further sacrificial anticorrosion action is ensured. In order to suppress the reduction of the amount of metal Zn due to Zn oxidation, an element having a higher oxygen affinity than Zn, that is, a metal having a standard reaction Gibbs energy of an oxide at 900 ° C. higher than Zn during hot press molding is used as a metal layer. It is necessary to select it as the metal of.

The metal layer is preferably a metal or alloy selected from one or more of Al, Si, Ti, Cr, Mn, and Zr. All of the metals listed here are metals in which the standard reaction Gibbs energy of oxides at 900 ° C. is higher than that of Zn. Regarding the lower limit of the adhesion amount of the metal layer, the effect of suppressing the reduction of the metal Zn amount due to Zn oxidation is observed as compared with the case where there is no metal layer even in a small amount, but in the present invention, 0.1 g / m ² or more is preferable. Further, although the upper limit is not particularly specified, an excessively thick metal layer causes an increase in cost, and therefore, it is preferably ^{30 g / m 2 or less.} It is more preferably 10 g / m ² or less, still more preferably 5 g / m ² or less.

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

The method for forming the metal layer is not particularly limited, and for example, dry processes such as PVD (Physical Vapor Deposition), CVD (Chemical Vapor Deposition), and cold spray, electroplating, non-aqueous electroplating, electroless plating, and hot-dip plating. It can be formed by using a wet process such as.

In the present invention, in order to obtain a hot-pressed member exceeding 1470 MPa class, the base steel plate of the plating layer may be, for example, by mass%, C: 0.25 to 0.35%, Si: 0.1. Cold containing ~ 0.5%, Mn: 1.0 to 3.0%, P: 0.02% or less, S: 0.01% or less, and having a component composition in which the balance is composed of Fe and unavoidable impurities. Rolled steel sheets or hot rolled steel sheets can be used. The reasons for limiting each component are described below.

C: 0.25 to 0.35%
C improves the strength by forming martensite or the like as a steel structure. 0.20% or more is required to obtain a strength exceeding 1470 MPa class. On the other hand, if it exceeds 0.35%, the toughness of the spot welded portion deteriorates. Therefore, the amount of C is preferably 0.25 to 0.35%.

Si: 0.1 to 0.5%
Si is an effective element for strengthening steel to obtain a good material. For that purpose, 0.1% or more is required. On the other hand, if it exceeds 0.5%, the ferrite is stabilized and the hardenability is lowered. Therefore, the amount of Si is preferably 0.1 to 0.5%.

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

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

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

Further, in the present invention, in addition to the above-mentioned basic components, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.0050%, with the intention of further improving the characteristics of the steel sheet. , Cr: 0.1 to 0.3%, Sb: 0.003 to 0.030%, and at least one selected from these can be appropriately contained as needed.

Nb: 0.05% or less Nb is an effective component for strengthening steel, but if it is contained in excess, the shape freezing property is lowered. Therefore, when Nb is contained, it is set to 0.05% or less.

Ti: 0.05% or less Ti is also effective for strengthening steel like Nb, but there is a problem that shape freezing property is lowered when it is contained in an excessive amount. Therefore, when Ti is contained, it is set to 0.05% or less.

B: 0.0002 to 0.0050%
Since B has an effect of suppressing ferrite formation and growth from austenite grain boundaries, it is preferable to add 0.0002% or more. On the other hand, the addition of excess B greatly impairs moldability. Therefore, when B is contained, it is set to 0.0002 to 0.0050%.

Cr: 0.1 to 0.3%
Cr is useful for strengthening steel and improving hardenability. In order to exhibit such an effect, addition of 0.1% or more is preferable. On the other hand, since the alloy cost is high, addition of more than 0.3% causes a significant cost increase. Therefore, when Cr is contained, it is set to 0.1 to 0.3%.

Sb: 0.003 to 0.030%
Sb also has the effect of suppressing decarburization of the surface layer of the steel sheet 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, if the amount of Sb exceeds 0.030%, the rolling load is increased and the productivity is lowered. Therefore, when Sb is contained, it is set to 0.003 to 0.030%.

The rest other than the above consists of Fe and unavoidable impurities.

The hot pressing process for manufacturing a hot pressing member using the hot pressing steel sheet of the present invention is not limited in any way, and a known hot pressing process can be applied. For example, the hot-pressed steel plate of the present invention is heated to a temperature range of Ac3 transformation point to 950 ° C., then hot-pressed, and subsequently cooled using a refrigerant such as a die or water. As a result, the hot press member is manufactured. The heating temperature refers to the maximum temperature reached by the steel sheet. Moreover, as a method of performing the above-mentioned heating, heating by an electric furnace, a gas furnace or the like, flame heating, energization heating, high frequency heating, induction heating and the like can be exemplified.

Hereinafter, the present invention will be specifically described based on examples. The following examples do not limit the present invention, and any modification within the scope of the abstract structure shall be included in the scope of the present invention.

As a base steel sheet, in terms of mass%, C: 0.30%, Si: 0.25%, Mn: 1.2%, P: 0.005%, S: 0.005%, Nb: 0.005%, A plate thickness of 1.4 mm, containing Ti: 0.02%, B: 0.0020%, Cr: 0.2%, Sb: 0.008%, and having a component composition in which the balance is composed of Fe and unavoidable impurities. Cold-rolled steel sheet was used. Various Zn alloy plating layers were formed on the surface of this 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 metal layers by the film forming method shown in Table 1 to obtain a steel sheet for hot pressing. The hot-dip plating bath composition of the plating layer was adjusted so that a desired composition could be obtained, and the bath temperature was set to the melting point of each composition + 20 ° C. In addition, the amount of adhesion was controlled by adjusting the wiping pressure. As for the conditions of the electroplating treatment, the metal salt ratio and the current value in the bath were adjusted so that the desired composition was obtained, and the adhesion amount was controlled by changing the line speed. Further, as the PVD film forming condition of the metal layer, a desired metal target was used, and the desired adhesion amount was controlled by controlling the voltage and the processing time. As for the conditions of the electroplating treatment of the metal layer, the amount of adhesion was controlled by adjusting the metal salt ratio and the current value in the bath and changing the line speed so as to obtain a desired composition.

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

The hat molded parts obtained from the above were subjected to a corrosion test (SAE-J2334), and the corrosion resistance of the center of the flat side surface (region of 100 mmC × 30 mmL) after 60 cycles was investigated and evaluated according to the following criteria.
◯: The average corrosion depth is reduced by 100 μm or more as compared with the case without the metal layer Δ: The average corrosion depth is reduced by 0 μm or more and less than 100 μm as compared with the case without the metal layer ×: The average corrosion depth is Increased compared to the case without a metal layer If the evaluation is ○, it was judged to be a steel sheet for hot pressing with excellent corrosion resistance.

Table 1 shows the evaluation results.

According to Table 1, the steel sheet for hot pressing of the present invention has excellent corrosion resistance.

The steel sheet for hot pressing having the Zn—Ni alloy plating layer evaluated in Example 1 was evaluated for its LME resistance (LME: Liquid Metal Embrittlement, liquid metal embrittlement). Specifically, a sample for observing the cross section SEM is taken from the upper shoulder R portion of the hat molded part obtained in Example 1, and is infiltrated into the base material in the field of view having a cross section length of 5 mm outside the shoulder R by observation. The crack depth was measured at a total of 20 locations at a pitch of 250 μm, and the LME resistance characteristics were evaluated according to the following criteria.
◯: No cracks occurred or the average value of the crack depth is less than 10 μm Δ: The average value of the crack depth is 10 μm or more and less than 200 μm ×: The average value of the crack depth is 200 μm or more It was said to be excellent.

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

From the results in Table 2, in the hot pressed steel plate of the present invention, the Zn—Ni alloy plating layer may be a Zn—Ni alloy plating layer containing 10 to 25% by mass of Ni, and the balance Fe and unavoidable impurities. For example, it can be seen that it also has excellent LME resistance in addition to corrosion resistance.

Claims (3)

A Zn alloy plating layer having ^{a plating adhesion amount of 120 g / m 2} or less per side is provided on the surface of the steel sheet.
Further wherein the Zn surface of the alloy plating layer, the standard reaction Gibbs energy of oxide formation at 900 ° C. has have a metal layer made of metal having high than Zn, the metal layer, Si, Ti, Mn, of the Zr 1 hot press steel sheet, wherein a metal or alloy der Rukoto selected from species or two or more. The Zn alloy plating layer is one of 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 plate for hot pressing according to claim 1 , wherein the steel plate is any one of them. The hot press steel sheet according to claim 2 , wherein the Zn—Ni alloy plating layer contains 10 to 25% by mass of Ni, and the balance is Zn and unavoidable impurities.