JP7056799B2 - Hot pressed members and their manufacturing methods, and hot pressed plated steel sheets - Google Patents

Description

The present invention relates to a hot pressed member, a method for manufacturing the same, and a plated steel sheet for hot pressing.

Conventionally, most of the undercarriage members and vehicle body structural members of automobiles are manufactured by pressing a steel plate having a predetermined strength. In recent years, from the viewpoint of preserving the global environment, there has been an eager desire to reduce the weight of automobile bodies, and efforts have been made to increase the strength of the steel sheets used and reduce their thickness. However, as the strength of the steel sheet increases, the press workability thereof decreases, so that it is often difficult to process the steel sheet into a desired member shape.

To deal with such problems, a hot press that makes it possible to achieve both ease of processing and high strength by processing a heated steel sheet using a die consisting of a die and a punch and at the same time quenching it. A processing technique called is proposed. The Zn alloy-plated steel sheet is attracting attention as a hot-pressed steel sheet having high rust resistance because a plating layer that is electrochemically lower than that of the base steel sheet remains after heating, and this Zn alloy-plated steel sheet is used. A hot-pressed member and a method for manufacturing the same have been proposed.

In Patent Document 1, the Al concentration {Al} in the plating layer is in the range of 0.2 to 1.0 g / m ² , and the Mg concentration {Mg} (mass%) in the plating layer is the same as the Al concentration. The hot-pressed plated steel sheet satisfying 0.10 ≦ {Mg} / {Al} ≦ 5 and the hot-pressed member obtained by hot-pressing after heating this hot-pressed plated steel sheet are Have been described.

Japanese Unexamined Patent Publication No. 2006-265706

It is described in Patent Document 1 that the hot press member described in Patent Document 1 is excellent in post-coating corrosion resistance when electrodeposition coating is performed after performing zinc phosphate-based chemical conversion treatment. Here, in recent years, a zirconium-based chemical conversion treatment has begun to spread in place of the conventional zinc phosphate-based chemical conversion treatment. Therefore, hot pressed members are also required to have coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment. However, as a result of studies by the present inventors, the hot press member disclosed in Patent Document 1 is excellent in post-coating corrosion resistance when electrodeposition coating is performed after performing zinc phosphate-based chemical conversion treatment. It was found that the adhesion to the coating film and the corrosion resistance after coating were insufficient when the electrodeposition coating was performed after the zirconium-based chemical conversion treatment was applied.

Therefore, in view of the above problems, the present invention provides a hot pressed member having excellent coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment, and a suitable manufacturing method thereof. The purpose is.

Another object of the present invention is to provide a hot pressed plated steel sheet suitable as a material for obtaining such a hot pressed member.

In order to solve the above problems, the present inventors conducted diligent research and obtained the following findings.
In the Fe—Zn—Al—Mg based alloy plating layer of the hot press member, the precipitation of the Γ phase composed of an electrochemically base metal compound such as Fe3Zn10 phase is restricted, and it is formed on the plating layer. By increasing the total of Al concentration and Mg concentration in the Zn-Al-Mg-containing oxide layer, the adhesion to the coating film and the corrosion resistance after coating can be improved when electrodeposition coating is performed after the zirconium-based chemical conversion treatment. Can be improved.

In order to manufacture a hot press member having a Fe—Zn—Al—Mg based alloy plating layer having a small amount of Γ phase precipitation and an oxide layer having a large total Al concentration and Mg concentration as described above. After heating a plated steel sheet for hot pressing having a predetermined Al amount and Mg amount and having a Zn—Al—Mg based alloy plating layer having a liquidus temperature of 400 ° C. or less at a relatively low temperature, Need to be hot pressed.

The abstract structure of the present invention completed based on the above findings is as follows.
[1] Base steel plate and
An Fe—Zn—Al—Mg-based alloy plating layer containing an α—Fe phase and a Γ phase formed on at least one surface of the base steel sheet with an adhesion amount of 40 to 400 g / m ² per surface.
An oxide layer containing Zn, Al and Mg formed on the Fe—Zn—Al—Mg based alloy plating layer and
Have,
The intensity of the diffraction peak of the (411) plane of the Γ phase existing at 41.5 ° ≤ 2θ ≤ 43.0 ° by X-ray diffraction using Co-Kα (wavelength 1.79021Å) with an incident angle of 25 ° as the radiation source. The ratio I _Γ / I _α between I _Γ and the intensity I _α of the diffraction peak of the (110) plane of the α—Fe phase existing at 51.0 ° ≦ 2θ ≦ 52.0 ° is 0.5 or less.
A hot pressing member characterized in that the sum of the Al concentration and the Mg concentration of the oxide layer is 28 atomic% or more.

[2] Base steel plate and
The base steel sheet is formed on at least one side with an adhesion amount of 30 to 180 g / m ² per side, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, and the balance. Is a Zn—Al—Mg based alloy plating layer having a component composition of Zn and unavoidable impurities and having a liquidus temperature of 400 ° C. or lower in an air atmosphere.
A method for manufacturing a hot-pressed member, which comprises heating a plated steel sheet for hot pressing, which comprises the above, to a temperature range of Ac ₃ transformation point to 1000 ° C., and then hot-pressing.

[3] The component composition of the Zn—Al—Mg based alloy plating layer is further selected from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B in mass%. The method for manufacturing a hot-pressed member according to the above [2], which comprises at least one type in a range of 1% or less in total.

[4] Base steel plate and
The base steel sheet is formed on at least one side with an adhesion amount of 30 to 180 g / m ² per side, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, and the balance. Is a Zn—Al—Mg based alloy plating layer having a component composition of Zn and unavoidable impurities and having a liquidus temperature of 400 ° C. or lower in an air atmosphere.
A plated steel sheet for hot pressing, characterized by having.

[5] The component composition of the Zn—Al—Mg based alloy plating layer is further selected from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B in mass%. The plated steel sheet for hot pressing according to the above [4], which contains at least one type in a total range of 1% or less.

The hot pressed member of the present invention is excellent in coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment. Further, according to the method for manufacturing a hot pressed member of the present invention, a hot pressed member having excellent coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment is performed is to be produced. Can be done.

The plated steel sheet for hot pressing of the present invention is suitable as a material for producing a hot-pressed member having excellent coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment. be.

On behalf of the invention example, No. 2 is a cross-sectional SEM image of the Fe—Zn—Al—Mg based alloy plating layer of the hot pressed member according to 2. On behalf of the comparative example, No. 8 is a cross-sectional SEM image of the Fe—Zn—Al—Mg based alloy plating layer of the hot pressed member according to 8.

(Hot pressing member)
The hot press member according to the embodiment of the present invention includes a base steel plate, a Fe—Zn—Al—Mg-based alloy plating layer formed on at least one surface of the base steel plate, and the Fe—Zn—Al—Mg-based alloy. It has an oxide layer formed on the plating layer.

[Base steel plate]
The base steel sheet in the hot-pressed member of the present embodiment is not particularly limited, but in order to make the tensile strength TS of the hot-pressed member 1470 MPa or more, the components described in the section of the hot-pressed plated steel sheet below. It is preferable to use a steel sheet having a composition.

[Fe-Zn-Al-Mg based alloy plating layer]
The Fe—Zn—Al—Mg-based alloy plating layer in the hot press member of the present embodiment contains an α—Fe phase and a Γ phase, and is preferably composed of an α—Fe phase and a Γ phase.

The α—Fe phase is a solid solution phase containing Fe as a main component and Zn, Al and Mg. When a hot-pressed plated steel sheet having a Zn—Al—Mg-based alloy plating layer is hot-pressed, Zn, Al and Mg in the plated layer diffuse into the underlying steel sheet, and Zn is mainly Fe in this diffusion region. , Al and Mg-containing solid solution phase (α-Fe phase) is formed. The α—Fe phase is formed so as to erode the surface layer portion of the base steel sheet in the plated steel sheet, but in hot press members, it is generally a Fe—Zn—Al—Mg-based alloy located on the base steel sheet. It is interpreted as forming a part of the plating layer.

The Γ phase is a phase composed of an intermetallic compound containing Zn as a main component, Al, Mg, and Fe, and is mainly composed of a Fe3Zn10 phase. Further, since the Γ1 phase has a crystal structure similar to that of the Γ phase and is difficult to discriminate by X-ray diffraction, the “Γ phase” in the present specification also includes the Γ1 phase. Examples of the intermetallic compound having another composition constituting the Γ phase include Fe4Zn9, FeZn4, and Fe5Zn21. During hot pressing, the Zn—Al—Mg-based alloy plating layer that remains without contributing to the diffusion to the base steel sheet takes in Fe diffused from the base steel sheet to form a Γ phase composed of an intermetallic compound. Then, in the hot press member, it constitutes a part of the Fe—Zn—Al—Mg based alloy plating layer.

Here, the α—Fe phase and the Γ phase can be distinguished from each other because they have clearly different contrasts in the cross-sectional SEM image of the Fe—Zn—Al—Mg-based alloy plating layer of the hot press member. With reference to FIGS. 1 and 2, the portion of the surface layer of the hot pressed member that looks relatively bright is the Γ phase, and the portion that looks relatively dark is the α—Fe phase. Further, the α-Fe phase and the Γ phase can be identified by X-ray diffraction using Co-Kα (wavelength 1.79021 Å) having an incident angle of 25 ° as a radiation source.

Since the Γ phase in the Fe—Zn—Al—Mg alloy plating layer has a significantly lower potential than the underlying steel sheet and α—Fe phase, it is preferentially corroded when exposed to a corrosive environment. .. That is, the Γ phase exhibits sacrificial anticorrosion ability against the base steel plate and the α-Fe phase.

Here, the zinc phosphate-based chemical conversion-treated film has a function as an excellent corrosion inhibitor with respect to the Zn-based alloy. Therefore, the hot-pressed member obtained by hot-pressing a Zn-Al-Mg-based alloy-plated steel plate is subjected to zinc phosphate-based chemical conversion treatment and then electrodeposition-coated to form a coating film, a chemical conversion-treated film, and plating. The corrosion rate of the Γ phase is low, the corrosion rate under the coating film is sufficiently low, and the corrosion resistance after coating is low even in the sacrificial corrosion protection state due to the flaw that penetrates the layer and reaches the underlying steel plate. Does not matter.

On the other hand, the zirconium oxide-based chemical conversion coating does not have a corrosion inhibitor function for Zn-based alloys. Therefore, after the sacrificial anticorrosion state is reached, the corrosion rate of the Γ phase is high, and as a result, the corrosion rate under the coating film is high. When the amount of the Γ phase is large and the Γ phase is continuously present in the Fe—Zn—Al—Mg based alloy plating layer without being divided, it is under the coating film when the sacrificial anticorrosion state is reached. In the environment, corrosion of the Γ phase propagates in the plane and is visually recognized as poor appearance such as swelling of the coating film. Therefore, when applying zirconium-based chemical conversion treatment, it is important to limit the amount of Γ phase in order to ensure corrosion resistance after painting.

Therefore, in the present embodiment, as one of the necessary conditions for improving the corrosion resistance after coating when electrodeposition coating is performed after the hot press member is subjected to zirconium-based chemical conversion treatment, Fe3Zn10 phase or the like is electrochemically used. It is essential to limit the precipitation of the Γ phase, which consists of a base intermetallic compound. Specifically, the (411) plane of the Γ phase existing at 41.5 ° ≤ 2θ ≤ 43.0 ° by X-ray diffraction using Co-Kα (wavelength 1.79021 Å) with an incident angle of 25 ° as the radiation source. The ratio I _Γ / I _α of the diffraction peak intensity I _Γ to the diffraction peak intensity I _α of the (110) plane of the α-Fe phase existing at 51.0 ° ≤ 2θ ≤ 52.0 ° is 0. It is important that it is 5 or less. When I _Γ / I _α is more than 0.5, the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposition coating is performed becomes insufficient. If I _Γ / I _α is 0.5 or less, the Γ phase is sufficiently separated by the α—Fe phase in the Fe—Zn—Al—Mg-based alloy plating layer, and the hot-pressed member is subjected to zirconium-based chemical conversion treatment. When electrodeposition coating is performed after application, excellent post-coating corrosion resistance can be obtained.

The smaller the value of I _Γ / I _α , the more preferable, so the lower limit is not particularly limited, but the value of I _Γ / I _α detected when measured by X-ray diffraction as described above is usually 0.01 or more. Become.

The measurement conditions for X-ray diffraction other than the above incident angle and radiation source do not affect the ratio I _Γ / I _α , but the conditions described in Examples described later can be adopted.

Adhesion amount per side: 40-400 g / m ²
By setting the adhesion amount of the Fe—Zn—Al—Mg-based alloy plating layer of the hot pressing member to 40 to 400 g / m ² , a hot pressing member having excellent corrosion resistance can be obtained. If the adhesion amount is less than 40 g / m ² , a hot pressed member having a desired corrosion resistance cannot be obtained. When the adhesion amount exceeds 400 g / m ² , the number of cracks crossing the inside of the plating layer is remarkably increased due to the influence of solidification shrinkage of the plating layer after hot pressing, and the adhesion in the plating layer is remarkably deteriorated. The amount of adhesion of the plating layer of the hot pressed member is preferably 50 g / m ² or more, and more preferably 60 g / m ² or more. The amount of adhesion of the plating layer of the hot pressed member is preferably 350 g / m ² or less, and more preferably 300 g / m ² or less.

In the present specification, the "adhesion amount of the Fe-Zn-Al-Mg-based alloy plating layer per one side" of the hot pressed member shall be determined by the following method. The hot pressed member to be evaluated is punched and three 48 mmφ samples are collected. Then, in each sample, the non-evaluation surface on the opposite side to the one surface for which the adhesion amount is evaluated is masked. First, the oxide layer is dissolved by immersing each sample in a 20% aqueous solution of chromium oxide (VI) at room temperature for 10 minutes, and each sample is weighed. Next, the Fe—Zn—Al—Mg-based alloy plating layer was dissolved by immersing each sample in a solution prepared by adding 3.5 g of hexamethylenetetramine to 1 L of a 500 mL 35% hydrochloric acid aqueous solution for 120 minutes. Then weigh each sample again. The amount of adhesion per unit area in each sample is calculated from the mass difference before and after the dissolution of the Fe—Zn—Al—Mg based alloy plating layer. Then, the average value of the three samples is taken as the adhesion amount per one side.

[Oxide layer]
The oxide layer in the hot press member of the present embodiment is formed on the Fe—Zn—Al—Mg based alloy plating layer and contains Zn, Al and Mg. When a hot-pressed plated steel plate having a Zn—Al—Mg-based alloy plating layer is hot-pressed, Zn, Al and Mg in the plating layer combine with oxygen existing in the heating atmosphere to combine Zn and Al. And an oxide layer containing Mg is formed. Although the oxide layer is mainly composed of Al oxide, it may contain Zn and Mg contained in the plating layer, and may further contain elements constituting the base steel sheet, such as Fe, Mn and Cr. ..

In the present embodiment, the Al concentration and Mg concentration of the oxide layer are another necessary conditions for improving the corrosion resistance after coating when the hot press member is subjected to zirconium-based chemical conversion treatment and then electrodeposition coating is performed. It is important that the sum of is 28 atomic% or more. When the sum of the Al concentration and the Mg concentration of the oxide layer is less than 28 atomic%, even if the above I _Γ / I _α is 0.5 or less, after the hot press member is subjected to zirconium-based chemical conversion treatment. Corrosion resistance after coating when electrodeposition coating is performed becomes insufficient. This is because when the Zn concentration constituting the oxide layer is high, the reaction between the chemical conversion treatment liquid and the oxide layer becomes non-uniform, and the thickness of the zirconium-based chemical conversion treatment film formed on the surface of the oxide layer becomes uneven. It is presumed to be due to. That is, a portion where the chemical conversion coating is thinned is likely to be formed, and the adhesion between the oxide layer and the chemical conversion coating, or the adhesion between the chemical conversion coating and the coating film is lowered, or the coating of the chemical conversion coating is incomplete. It is presumed to be due to. On the other hand, if the sum of the Al concentration and the Mg concentration of the oxide layer is 28 atomic% or more, a zirconium-based chemical conversion treatment film is formed soundly. Therefore, after the hot press member is subjected to the zirconium-based chemical conversion treatment. Excellent post-coating corrosion resistance can be obtained when electrodeposition coating is performed.

Further, when the sum of the Al concentration and the Mg concentration of the oxide layer is less than 28 atomic%, the oxide layer becomes brittle. The adhesion of the coating film becomes insufficient. On the other hand, if the sum of the Al concentration and the Mg concentration of the oxide layer is 28 atomic% or more, the oxide layer has sufficient strength. The adhesion of the coating film when the coating is applied is improved.

The upper limit of the sum of the Al concentration and the Mg concentration of the oxide layer is not particularly limited. However, the oxide layer containing an excessively high concentration of Al and Mg is chemically stable in an acidic environment such as a chemical conversion treatment liquid for coating base treatment, and may hinder the formation of a chemical conversion treatment film. .. Therefore, the sum of the Al concentration and the Mg concentration of the oxide layer is preferably 50 atomic% or less.

In the present embodiment, since the oxide layer is formed very thinly on the Fe—Zn—Al—Mg based alloy plating layer, it may not be visible in the cross-sectional SEM image as shown in FIG. However, the oxide layer is a region where oxygen is detected by measuring the cross section of the surface layer of the hot press member by energy dispersive X-ray analysis (EDX) combined with SEM and performing element mapping. Can be identified. Further, in the present specification, the "Al concentration and Mg concentration of the oxide layer" are values measured by the following method. That is, a test piece for cross-section observation is collected from the flat portion of the hot pressed member. The cross section of the test piece including the Fe-Zn-Al-Mg based alloy plating layer and the oxide layer was observed at 10000 times using a scanning electron microscope (SEM) with an acceleration voltage of 15 kV, and the oxide was observed at any three points. The composition of the layer is measured by energy dispersive X-ray analysis (EDX). The added average of the Al concentration and the Mg concentration at the three locations is defined as the "Al concentration of the oxide layer" and the "Mg concentration of the oxide layer", respectively.

(Manufacturing method of hot pressing member)
In the method for manufacturing a hot pressed member according to an embodiment of the present invention, a galvanized steel sheet for hot pressing according to an embodiment of the present invention, which will be described later, is heated to a temperature range of Ac ₃ transformation point to 1000 ° C. and then hot pressed. It is characterized by doing.

By setting the heating temperature of the hot-pressed steel sheet before hot-pressing to the Ac ₃ transformation point to 1000 ° C., the Fe—Zn—Al—Mg-based alloy plating having the α—Fe phase and the Γ phase described above was performed. A layer and an oxide layer having a predetermined Al concentration and Mg concentration can be obtained. If the heating temperature is lower than the Ac ₃ transformation point, the I _Γ / I _α of the Fe—Zn—Al—Mg based alloy plating layer exceeds 0.5 after hot pressing. As a result, the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposited is insufficient. If the heating temperature exceeds 1000 ° C, the desired oxide layer cannot be obtained, and the coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after the hot-pressed member is subjected to zirconium-based chemical conversion treatment. Is insufficient. The "heating temperature" here means the maximum temperature reached by the steel sheet. In this specification, the "Ac ₃ transformation point" is a value calculated from the following formula based on the composition of the steel sheet.
Ac ₃ transformation point (° C) = 910-203C ^1/2 + 44.7Si-4Mn + 11Cr
The element symbol on the right side of the equation indicates the content of each element, and when Cr is not contained, Cr = 0.

The holding time after raising the temperature to the heating temperature is not limited at all, but the holding time is 30 seconds or more from the viewpoint of eliminating the Γ phase and avoiding the embrittlement cracking of the liquid metal during hot pressing. It is desirable to do. From the viewpoint of avoiding hydrogen infiltration due to taking in water vapor in the furnace during the holding time, the holding time is preferably 5 minutes or less, more preferably 3 minutes or less, and further preferably 2 minutes or less.

The method for heating the hot pressed steel sheet is not limited in any way, and examples thereof include furnace heating by an electric furnace or a gas furnace, energization heating, induction heating, high frequency heating, and flame heating.

In hot pressing, a hot-pressed plated steel sheet heated as described above is simultaneously pressed and quenched using a forming die to obtain a hot-pressed member having a predetermined shape. The conditions for hot pressing are not particularly limited, and a fixed method can be adopted.

(Plated steel sheet for hot pressing)
The hot-pressed plated steel sheet according to the embodiment of the present invention is formed on the base steel sheet and at least one side of the base steel sheet with an adhesion amount of 30 to 180 g / m ² per side, in mass%, Al: Zn-Al containing 3 to 10% and Mg: 0.2 to 0.8%, having a component composition in which the balance is Zn and unavoidable impurities, and the liquidus temperature in an air atmosphere is 400 ° C. or lower. It is characterized by having a Mg-based alloy plating layer.

[Base steel plate]
In order to obtain a hot pressed member having a tensile strength TS of 1470 MPa or more, as a base steel sheet, for example, in terms of mass%, C: 0.20 to 0.35%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, and the balance is Fe and It is preferable to use a steel sheet having a component composition that is an unavoidable impurity. The base steel plate may be either a cold-rolled steel plate or a hot-rolled steel plate. The reasons for limiting each component element will be described below.

C: 0.20 to 0.35%
C improves the strength by forming martensite or the like as a steel structure. In order to obtain TS of 1470 MPa or more, the amount of C needs to be 0.20% or more. On the other hand, if the amount of C exceeds 0.35%, the toughness of the spot welded portion deteriorates. Therefore, the amount of C is preferably 0.20 to 0.35%.

Si: 0.1-0.5%
Si is an effective element for strengthening steel to obtain a good material. For that purpose, the amount of Si needs to be 0.1% or more. On the other hand, when the amount of Si 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: 1.0-3.0%
Mn is an element effective for increasing the strength of steel. In order to secure the mechanical properties and strength, it is necessary to set the Mn amount to 1.0% or more. On the other hand, when the amount of Mn exceeds 3.0%, the surface thickening at the time of annealing increases, and it becomes difficult to secure the plating adhesion. Therefore, the amount of Mn is preferably 1.0 to 3.0%.

P: 0.1% or less When the amount of P exceeds 0.1%, the balance between strength and ductility deteriorates 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 0.1% or less. Further, from the viewpoint of steelmaking cost, the amount of P is preferably 0.01% or more.

S: 0.05% or less S becomes inclusions 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 the amount of S as much as possible, and it is preferably 0.05% or less. Further, in order to secure good stretch flangeability, the amount of S is more preferably 0.01% or less. Further, from the viewpoint of steelmaking cost, the amount of S is preferably 0.002% or more.

Al: 0.1% or less If the Al amount exceeds 0.1%, the blanking workability and hardenability of the base steel sheet deteriorate. Therefore, the amount of Al is preferably 0.1% or less. Further, from the viewpoint of ensuring the effect as a deoxidizing material, the Al amount is preferably 0.01% or more.

N: 0.01% or less When the amount of N exceeds 0.01%, AlN is generated during hot rolling or heating before hot pressing, and the blanking workability and hardenability of the base steel sheet are deteriorated. Therefore, the amount of N is preferably 0.01% or less. Further, from the viewpoint of steelmaking cost, the amount of N is preferably 0.001% or more.

The rest other than the above elements are Fe and unavoidable impurities. However, for the following reasons, in mass%, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.005%, Cr: 0.1 to 0.3%, Sb. : At least one selected from 0.003 to 0.03% may be appropriately contained, if necessary.

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

Ti: 0.05% or less Ti is also effective for strengthening steel like Nb, but if it is contained in excess, the shape freezing property is lowered. Therefore, when Ti is contained, the amount of Ti is 0.05% or less.

B: 0.0002 to 0.005%
B has an action of suppressing ferrite formation and growth from austenite grain boundaries. Therefore, the amount of B is preferably 0.0002% or more. On the other hand, the addition of excess B greatly impairs moldability. Therefore, when B is contained, the amount of B is 0.005% or less.

Cr: 0.1-0.3%
Cr is useful for strengthening steel and improving hardenability. In order to exhibit such an effect, the amount of Cr is preferably 0.1% or more. On the other hand, from the viewpoint of alloy cost, when Cr is contained, the amount of Cr is set to 0.3% or less.

Sb: 0.003 to 0.03%
Sb has the effect of suppressing decarburization of the surface layer of the steel sheet during hot pressing. In order to exhibit such an effect, the amount of Sb is preferably 0.003% or more. On the other hand, if the amount of Sb exceeds 0.03%, the rolling load is increased and the productivity is lowered. Therefore, when Sb is contained, the amount of Sb is 0.03% or less.

[Zn-Al-Mg based alloy plating layer]
In the present embodiment, the Zn—Al—Mg-based alloy plating layer of the hot-pressed plated steel sheet contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, and the balance is Zn. It is assumed that it is an unavoidable impurity and has a component composition in which the liquidus temperature in an air atmosphere is 400 ° C. or lower.

Al: 3-10%
When the Al content is less than 3%, the I _Γ / I _α of the Fe—Zn—Al—Mg based alloy plating layer exceeds 0.5 after hot pressing, and the Al concentration of the oxide layer And the sum of Mg concentrations is less than 28 atomic%. As a result, the adhesion of the coating film and the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposition coating is insufficient. Further, when the Al content is less than 3%, the liquidus temperature described later cannot be set to 400 ° C. or lower depending on the Mg content. On the other hand, when the Al content exceeds 10%, the liquidus temperature, which will be described later, cannot be lowered to 400 ° C. or lower, and after hot pressing, I _Γ / of the Fe—Zn—Al—Mg based alloy plating layer. I _α exceeds 0.5. As a result, the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposited is insufficient. Therefore, the Al content is set to 3 to 10%.

Mg: 0.2-0.8%
When the Mg content is less than 0.2%, the I _Γ / I _α of the Fe—Zn—Al—Mg based alloy plating layer exceeds 0.5 after hot pressing. As a result, the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposited is insufficient. Therefore, the Mg content is 0.2% or more, preferably 0.3% or more, and more preferably 0.4% or more. On the other hand, when the Mg content exceeds 0.8%, the sum of the Al concentration and the Mg concentration of the oxide layer becomes less than 28 atomic% after hot pressing. As a result, the adhesion of the coating film and the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposition coating is insufficient. Therefore, the Mg content is 0.8% or less, preferably 0.7% or less, and more preferably 0.6% or less.

Liquidus temperature in atmospheric atmosphere: 400 ° C. or less In this embodiment, the liquidus temperature of the Zn—Al—Mg based alloy plating layer in the atmospheric atmosphere is adjusted by appropriately controlling the Al content and Mg content. It is important to keep the temperature below 400 ° C. When the liquidus temperature exceeds 400 ° C., the I _Γ / I _α of the Fe—Zn—Al—Mg based alloy plating layer exceeds 0.5 after hot pressing. As a result, the corrosion resistance after coating when the hot pressed member is subjected to zirconium-based chemical conversion treatment and then electrodeposited is insufficient. The lower limit of the liquidus temperature is not particularly limited, but within the range of the Al content and the Mg content, the liquidus temperature is generally 380 ° C. or higher. The liquidus temperature of the Zn—Al—Mg-based alloy layer under an atmospheric atmosphere can be obtained by calculating using a database with the thermodynamic calculation software Thermo Calc.

The unavoidable impurities contained in the Zn—Al—Mg-based alloy plating layer include the base steel plate component incorporated into the plating layer by the reaction between the plating bath and the base steel plate during the plating process, and the unavoidable impurities in the plating bath. Includes impurities. Fe is contained in the plating layer in an amount of 0.01% to several% as a base steel sheet component. Examples of the types of unavoidable impurities in the plating bath include Fe, Cr, Cu, Mo, Ni, Zr and the like. The Fe in the plating layer cannot be quantified separately from the Fe taken in from the base steel sheet and the Fe taken in from the plating bath. The total content of unavoidable impurities is not particularly limited, but from the viewpoint of uniformly melting the plating layer in the hot pressing process, the total amount of unavoidable impurities excluding Fe is preferably 1% by mass or less. ..

The component composition of the Zn—Al—Mg-based alloy plating layer further comprises at least one selected from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B in mass%. It can be included in the range of 1% or less in total.

Adhesion amount per side: 30-180 g / m ²
By setting the adhesion amount of the Zn—Al—Mg based alloy plating layer to 30 to 180 g / m ² , it is possible to obtain a hot press member having excellent corrosion resistance and resistance to embrittlement cracking of liquid metal during hot pressing. .. If the adhesion amount is less than 30 g / m ² , a hot pressed member having a desired corrosion resistance cannot be obtained. If the adhesion amount exceeds 180 g / m ² , the liquid phase may remain without completing the alloying in the heating step before the hot press, and liquid metal embrittlement cracking may occur. The adhesion amount of the Zn—Al—Mg based alloy plating layer is preferably 45 g / m ² or more, and more preferably 55 g / m ² or more. The adhesion amount of the Zn—Al—Mg based alloy plating layer is preferably 120 g / m ² or less, and more preferably 100 g / m ² or less.

In the present specification, the "adhesion amount of the Zn-Al-Mg-based alloy plating layer per one side" is determined by the following method. The Zn—Al—Mg based alloy plated steel sheet to be evaluated is punched, three 48 mmφ samples are collected, and each sample is weighed. Then, in each sample, the non-evaluation surface on the opposite side to the one surface for which the adhesion amount is evaluated is masked. Then, the Zn—Al—Mg based alloy plating layer was dissolved by immersing each sample in a solution prepared by adding 3.5 g of hexamethylenetetramine to 1 L of a 500 mL 35% hydrochloric acid aqueous solution for 10 minutes. Weigh the sample again. The amount of adhesion per unit area in each sample is calculated from the mass difference before and after the dissolution of the Zn—Al—Mg based alloy plating layer. Then, the average value of the three samples is taken as the adhesion amount per one side.

In the present embodiment, a separate film may be provided on the lower layer or the upper layer of the Zn—Al—Mg based alloy plating layer according to the purpose as long as it does not affect the action and effect of the present invention. Nickel pre-plating is exemplified as the underlayer film. Examples of the upper layer film include a chemical conversion-treated film containing a zirconium oxide or a zirconium-titanium oxide.

As a base steel sheet, in terms of mass%, C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.005%, S: 0.001%, Al: 0.03%, Contains N: 0.004%, Nb: 0.02%, Ti: 0.02%, B: 0.002%, Cr: 0.2%, and Sb: 0.008%, with the balance being Fe and A cold-rolled steel sheet having a thickness of 1.4 mm and having a component composition that is an unavoidable impurity was used (Ac ₃ = 814 ° C.).

This cold-rolled steel sheet was immersed in a hot-dip Zn-Al-Mg-based plating bath having a predetermined composition and bath temperature by a hot-dip plating facility, and then N ₂ gas wiping was performed. .. Plated steel sheets for hot pressing from 1 to 14 were produced. Table 1 shows the Al content, Mg content, and other element content in the Zn—Al—Mg based alloy plating layer, and the liquidus temperature in the atmosphere. The content of each element and the liquidus temperature were controlled by adjusting the composition of the components of the plating bath. The content of each element in the plating layer was determined by a method of quantitatively analyzing each component contained in the hydrochloric acid stripping solution of the plating layer by ICP-AES. The liquidus temperature of the plating layer was determined by the method described above. Table 1 also shows the amount of adhesion per side of the Zn—Al—Mg-based alloy plating layer obtained by the method described above. The amount of plating adhesion was controlled by adjusting the flow rate of the wiping gas and the line speed.

Next, the above-mentioned steel sheet for hot pressing was subjected to hot pressing. That is, a 150 mm × 300 mm test piece was collected from the obtained hot pressed steel sheet and heat-treated by an electric furnace. Table 1 shows the heat treatment conditions (heating temperature, holding time). The test piece after the heat treatment was taken out from the electric furnace and immediately pressed by hot pressing at a molding start temperature of 700 ° C. using a hat mold to obtain a hot pressed member. The shape of the obtained hot pressed member 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.

(Evaluation of Fe-Zn-Al-Mg based alloy plating layer / oxide layer for hot pressing members)
A test piece for cross-section observation was taken from the flat portion on the upper surface of the obtained hot-pressed member, and the cross-section of the Fe—Zn—Al—Mg-based alloy plating layer was observed by SEM. At each level, the α-Fe phase and the Γ phase were distinguishable because they had distinctly different contrasts in the cross-sectional SEM image. In FIG. 1, representative of the invention example, No. A cross-sectional SEM image of the Fe—Zn—Al—Mg-based alloy plating layer of the hot pressed member according to No. 2 is shown, and FIG. 2 shows No. 2 as a representative of a comparative example. The cross-sectional SEM image of the Fe—Zn—Al—Mg based alloy plating layer of the hot pressing member according to No. 8 is shown. In FIG. 1, the precipitation of the Γ phase is suppressed, and the Γ phase is discontinuously scattered in the α—Fe phase. On the other hand, in FIG. 2, many Γ phases are precipitated, and the Γ phases exist in a continuous plane. In addition, the diffraction peak of the (411) plane of the Γ phase existing at 41.5 ° ≤ 2θ ≤ 43.0 ° by X-ray diffraction using Co-Kα (wavelength 1.79021 Å) with an incident angle of 25 ° as the radiation source. The intensity I _Γ of the above and the intensity I _α of the diffraction peak of the (110) plane of the α-Fe phase existing at 51.0 ° ≤ 2θ ≤ 52.0 ° are measured, and the ratio I _Γ / I _α is calculated. It is shown in Table 1. The X-ray diffraction was measured using a curved IPX-ray diffractometer (RINT-RAPID II-R manufactured by Rigaku Co., Ltd.), tube voltage: 45 kV, tube current: 160 mA, integration time: 600 seconds, and collimator diameter: It was performed under the condition of 3 mm.

Further, at each level, the Al concentration and the Mg concentration of the oxide layer were measured by the method described above, and are shown in Table 1. Further, at each level, the amount of adhesion of the Fe—Zn—Al—Mg based alloy plating layer per surface was measured by the method described above, and is shown in Table 1.

(Evaluation 1: Coating film adhesion)
A 70 mm × 150 mm test piece was cut out from the flat portion on the upper surface of the obtained hot pressed member, and the test piece was subjected to zirconium-based chemical conversion treatment. Specifically, a commercially available chemical conversion treatment liquid (zirconium-based chemical conversion treatment: Palmina 2100 manufactured by Nihon Parkerizing Co., Ltd.) was used to perform the chemical conversion treatment under the conditions of a bath temperature of 35 ° C. and a treatment time of 120 seconds. After that, each test piece was boosted in 30 seconds using a commercially available cationic electrodeposition paint, held at a constant voltage for 150 seconds, energized under voltage conditions where the coating thickness after baking was 15 μm, and the ambient temperature was 170 ° C. It was baked for 20 minutes in the electric furnace of. As the cationic electrodeposition paint, Kansai Paint's Electron GT-100V-1 Gray was used.

Using a cutter knife, 11 cuts reaching the base steel plate were made on the test piece after electrodeposition coating at intervals of 1 mm in each of the vertical and horizontal directions to make 100 grids. Cellophane tape (registered trademark) was strongly crimped to the grid portion, and the end of the tape was peeled off at a stretch at an angle of 45 °. The number of cells of the coating film peeled off from the surface of the test piece was measured, and the judgment was made according to the following criteria. The evaluation results are shown in Table 1.
⊚: Number of peeled cells = 0
◯: Number of peeled cells = 1
Δ: Number of peeled cells = 2 to 5
×: Number of peeled cells> 5

(Evaluation 2: Corrosion resistance after painting)
A test piece that had been subjected to electrodeposition coating by the same method as in Evaluation 1 was prepared, and the end portion of the evaluation surface of 7.5 mm and the non-evaluation surface (back surface) were sealed with tape. Then, a cross-cut scratch with a length of 60 mm and a central angle of 60 ° was made in the center of the evaluation surface to a depth reaching the base steel plate with a cutter knife. This test piece was subjected to a corrosion test (VDA 233-102) and evaluated according to the corrosion condition after 4 weeks.

The maximum swelling width on one side from the cross cut was measured, and the judgment was made according to the following criteria, and ◎ or ○ was regarded as acceptable. The evaluation results are shown in Table 1.
⊚: Maximum swelling width on one side <1.5 mm
◯: 1.5 mm ≤ maximum swelling width on one side <3.0 mm
Δ: 3.0 mm ≤ maximum swelling width on one side <4.0 mm
×: 4.0 mm ≤ maximum swelling width on one side

From the results shown in Table 1, the hot pressed member of the example of the present invention is excellent in coating film adhesion and post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment.

The hot pressing member of the present invention is suitable for an automobile suspension member and a vehicle body structural member.

Claims (3)

With the base steel plate,
An Fe—Zn—Al—Mg-based alloy plating layer containing an α—Fe phase and a Γ phase formed on at least one surface of the base steel sheet with an adhesion amount of 40 to 400 g / m ² per surface.
An oxide layer containing Zn, Al and Mg formed on the Fe—Zn—Al—Mg based alloy plating layer and
Have,
The intensity of the diffraction peak of the (411) plane of the Γ phase existing at 41.5 ° ≤ 2θ ≤ 43.0 ° by X-ray diffraction using Co-Kα (wavelength 1.79021Å) with an incident angle of 25 ° as the radiation source. The ratio I _Γ / I _α between I _Γ and the intensity I _α of the diffraction peak of the (110) plane of the α—Fe phase existing at 51.0 ° ≦ 2θ ≦ 52.0 ° is 0.5 or less.
A hot pressing member characterized in that the sum of the Al concentration and the Mg concentration of the oxide layer is 28 atomic% or more. With the base steel plate,
The base steel sheet is formed on at least one side with an adhesion amount of 30 to 180 g / m ² per side, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, and the balance. Is a Zn—Al—Mg based alloy plating layer having a component composition of Zn and unavoidable impurities and having a liquidus temperature of 400 ° C. or lower in an air atmosphere.
The hot-pressed plated steel sheet having the above is heated to a temperature range of Ac ₃ transformation point to 1000 ° C. and then hot-pressed to produce the hot-pressed member according to claim 1. A method for manufacturing a pressed member. The component composition of the Zn—Al—Mg-based alloy plating layer further comprises at least one selected from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr and B in mass%. The method for manufacturing a hot pressed member according to claim 2, wherein the method comprises a total of 1% or less.

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