KR102666358B1 - Hot pressed member and method of producing same - Google Patents

Abstract

Provided is a hot pressed member having excellent coating film adhesion when electrodeposition coating is performed after zirconium-based chemical conversion treatment and excellent corrosion resistance after coating. The hot pressed member of the present invention is a Fe-Zn-Al-Mg system containing an α-Fe phase and a Γ phase formed on at least one side of the base steel plate and an adhesion amount per side of 40 to 400 g/m2. X has an alloy plating layer and an oxide layer containing Zn, Al, and Mg formed on the Fe-Zn-Al-Mg alloy plating layer, and uses Co-Kα (wavelength 1.79021 Å) at an incident angle of 25° as a source. By line diffraction, the intensity I _Γ of the diffraction peak of the (411) plane of the Γ phase existing at 41.5°≤2θ≤43.0° and the diffraction of the (110) plane of the α-Fe phase existing at 51.0°≤2θ≤52.0° The peak intensity I _α ratio I _Γ /I _α is 0.5 or less, and the sum of the Al concentration and Mg concentration of the oxide layer is 28 atomic% or more.

Description

Hot press member and manufacturing method thereof {HOT PRESSED MEMBER AND METHOD OF PRODUCING SAME}

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 automobile suspension members and body structural members have been manufactured by pressing steel sheets with a predetermined strength. Recently, from the viewpoint of preserving the global environment, there is a desire to reduce the weight of automobile bodies, and efforts are being made to increase the strength of the steel plates used and reduce their plate thickness. However, as the strength of the steel sheet increases, its press workability decreases, and it is increasingly difficult to process the steel sheet into a desired member shape.

In response to this problem, a processing technology called hot pressing has been proposed, which enables both ease of processing and increased strength by processing a heated steel sheet using a mold consisting of a die and a punch while simultaneously rapidly cooling it. Zn alloy-plated steel sheets are attracting attention as steel sheets for hot pressing with high rust prevention properties because a plating layer that is electrochemically inferior to the base steel sheet remains after heating, and hot press members and their production using this Zn-alloy-plated steel sheets. A method is proposed.

In Patent Document 1, the Al concentration {Al} in the plating layer is in the range of 0.2 to 1.0 g/m2, and the Mg concentration {Mg} (mass %) in the plating layer is 0.10 ≤ {Mg}/{ A hot press plated steel sheet that satisfies Al}≤5 and a hot press member obtained by heating and then hot pressing the hot press plated steel sheet are described.

Japanese Patent Publication No. 2006-265706

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

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

Additionally, the present invention aims to provide a plated steel sheet for hot pressing that is suitable as a material for obtaining such hot pressed members.

In order to solve the above problems, the present inventors conducted intensive research and obtained the following knowledge.

In the Fe-Zn-Al-Mg alloy plating layer of the hot pressed member, precipitation of the Γ phase consisting of an electrochemically inferior intermetallic compound such as the Fe3Zn10 phase is limited, and the Zn-Al-Mg formed on the plating layer is limited. In the oxide-containing layer, by increasing the total of the Al concentration and Mg concentration, the coating film adhesion when electrodeposition coating is performed after zirconium-based chemical conversion treatment and corrosion resistance after coating can be improved.

In order to manufacture a hot pressed member having a Fe-Zn-Al-Mg alloy plating layer with a small amount of precipitation of the Γ phase as described above and an oxide layer with a large sum of Al concentration and Mg concentration, predetermined amounts of Al and Mg are used. In addition, it is necessary to hot press a plated steel sheet for hot pressing having a Zn-Al-Mg alloy plating layer whose liquidus temperature is 400°C or lower after heating to a relatively low temperature.

The main structure of the present invention completed based on the above recognition is as follows.

[1] Base steel plate,

A Fe-Zn-Al-Mg-based alloy plating layer containing an α-Fe phase and a Γ phase formed on at least one side of the base steel plate at an adhesion amount of 40 to 400 g/m2 per side,

An oxide layer containing Zn, Al, and Mg formed on the Fe-Zn-Al-Mg alloy plating layer.

With

The intensity of the diffraction peak I _Γ of the (411) plane of the Γ phase existing at 41.5°≤2θ≤43.0°, 51.0°, by The ratio I _Γ /I _α of the intensity I _α of the diffraction peak of the (110) plane of the α-Fe phase existing at ≤2θ≤52.0° is 0.5 or less,

The sum of the Al concentration and Mg concentration of the oxide layer is 28 atomic% or more.

A hot press member characterized in that.

[2] Base steel plate,

A component is formed on at least one side of the base steel plate with an adhesion amount per side of 30 to 180 g/m2, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, with the balance being Zn and inevitable impurities. A Zn-Al-Mg-based alloy plating layer having a composition and a liquidus temperature of 400°C or less in an air atmosphere.

A method of manufacturing a hot press member, comprising heating a plated steel sheet for hot pressing to a temperature range of Ac ₃ transformation point to 1000° C. and then hot pressing it.

[3] The composition of the Zn-Al-Mg 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 [2] above, comprising at least one type in a total amount of 1% or less.

[4] Base steel plate,

A component is formed on at least one side of the base steel plate with an adhesion amount per side of 30 to 180 g/m2, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, with the balance being Zn and inevitable impurities. A Zn-Al-Mg-based alloy plating layer having a composition and a liquidus temperature of 400°C or less in an air atmosphere.

A plated steel sheet for hot pressing, characterized in that it has a.

[5] The composition of the Zn-Al-Mg 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 [4] above, which contains at least one type in a total amount of 1% or less.

The hot pressed member of the present invention is excellent in coating film adhesion when electrodeposition coating is applied after zirconium-based chemical conversion treatment and in corrosion resistance after coating. In addition, according to the method for manufacturing a hot pressed member of the present invention, a hot pressed member that is excellent in coating film adhesion when electrodeposition coating is performed after zirconium-based chemical conversion treatment and excellent corrosion resistance after coating can be manufactured.

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

Figure 1 is a cross-sectional SEM image of the Fe-Zn-Al-Mg-based alloy plating layer of the hot pressed member according to No. 2, representing the invention example.
Figure 2 is a cross-sectional SEM image of the Fe-Zn-Al-Mg-based alloy plating layer of a hot pressed member according to No. 8, representing a comparative example.

(Form for carrying out the invention)

(no hot press)

A hot pressed member according to an embodiment of the present invention includes a base steel plate, a Fe-Zn-Al-Mg-based alloy plating layer formed on at least one side of the base steel plate, and a Fe-Zn-Al-Mg-based alloy plating layer on the Fe-Zn-Al-Mg-based alloy plating layer. It has an oxide layer formed.

[Not 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, a steel sheet having the component composition described in the later section of plated steel sheet for hot pressing is used. It is desirable to use .

[Fe-Zn-Al-Mg alloy plating layer]

The Fe-Zn-Al-Mg based alloy plating layer in the hot pressed member of this embodiment contains an α-Fe phase and a Γ phase, and preferably consists of an α-Fe phase and a Γ phase.

The α-Fe phase is a solid solution phase mainly composed of Fe and containing Zn, Al, and Mg. When a hot press plated steel sheet with a Zn-Al-Mg alloy plating layer is subjected to hot pressing, Zn, Al, and Mg in the plating layer diffuse into the base steel sheet, and in this diffusion region, Fe is the main material, and Zn, Al, and A solid solution phase (α-Fe phase) containing Mg is formed. The α-Fe phase is formed to erode the surface layer of the base steel sheet in a plated steel sheet, but in a hot pressed member, it generally constitutes a part of the Fe-Zn-Al-Mg-based alloy plating layer located on the base steel sheet. It is interpreted that

The Γ phase is a phase composed of an intermetallic compound mainly composed of Zn and containing Al, Mg, and Fe, and is mainly composed of the Fe3Zn10 phase. In addition, since the Γ phase has a similar crystal structure to the Γ phase and is difficult to determine by X-ray diffraction, the term "Γ phase" in this specification shall also include the Γ phase. Examples of intermetallic compounds of different compositions 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 diffusion into the base steel sheet blows in the Fe diffused from the base steel sheet, forming a Γ phase made of an intermetallic compound, and forming a Γ phase in the hot press member. In this case, it constitutes a part of the Fe-Zn-Al-Mg alloy plating layer.

Here, the α-Fe phase and the Γ phase can each be identified because they have clearly different contrasts in the cross-sectional SEM image of the Fe-Zn-Al-Mg alloy plating layer of the hot pressed member. Referring to FIGS. 1 and 2, the portion that appears relatively bright in the surface layer portion of the hot pressed member is the Γ phase, and the portion that appears relatively dark is the α-Fe phase. In addition, the α-Fe phase and the Γ phase can be identified by X-ray diffraction using Co-Kα (wavelength 1.79021 Å) at an incident angle of 25° as a source.

Since the Γ phase in the Fe-Zn-Al-Mg alloy plating layer has a significantly inferior potential compared to the base steel sheet or the α-Fe phase, it is preferentially corroded when exposed to a corrosive environment. In other words, the Γ phase exhibits sacrificial anti-corrosion ability to the underlying steel plate or α-Fe phase.

Here, the zinc phosphate-based chemical conversion film functions as an excellent corrosion inhibitor for the Zn-based alloy. Therefore, after applying zinc phosphate-based chemical conversion treatment to a hot pressed member obtained by hot pressing a Zn-Al-Mg alloy plated steel sheet, the electrodeposition-coated member penetrates the coating film, chemical conversion film, and plating layer to reach the underlying steel plate. Even if it is damaged and becomes a sacrificial corrosion state, the corrosion rate of the Γ phase is small and the corrosion rate under the coating film is sufficiently low, so that corrosion resistance after painting is not a problem in an actual use environment.

In contrast, the zirconium oxide-based chemical conversion film does not have a corrosion inhibitor function for the Zn-based alloy. Therefore, after entering the sacrificial corrosion protection state, the corrosion rate of the Γ phase is large, and as a result, the corrosion rate under the coating film becomes large. In addition, when the amount of Γ phase is large and the Γ phase exists continuously without being divided in the Fe-Zn-Al-Mg alloy plating layer, when a sacrificial corrosion state is reached, corrosion of the Γ phase propagates within the surface in the environment under the coating film. This is recognized as appearance defects 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 to ensure corrosion resistance after painting.

Therefore, in this embodiment, as one of the necessary conditions for improving post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment on a hot pressed member, an electrochemically inferior intermetallic compound such as Fe3Zn10 phase is used. It is important to limit precipitation of the Γ phase. Specifically, the intensity of the diffraction peak of the (411) plane of Γ phase at 41.5°≤2θ≤43.0°, I _Γ, by X-ray diffraction using Co-Kα (wavelength 1.79021Å) at an incident angle of 25° as a source It is important that the ratio I _Γ /I _α of 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. When I _Γ /I _α exceeds 0.5, corrosion resistance after coating becomes insufficient when electrodeposition coating is performed after zirconium-based chemical conversion treatment is performed on the hot pressed member. If I _Γ /I _α is 0.5 or less, the Γ phase in the Fe-Zn-Al-Mg alloy plating layer is sufficiently divided by the α-Fe phase, and when electrodeposition coating is performed after zirconium-based chemical conversion treatment is performed on the hot pressed member. Excellent corrosion resistance can be achieved after painting.

Since a smaller value of I _Γ /I _α is more preferable, 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.

_In addition, regarding the measurement conditions _for

Adhesion amount per side: 40∼400g/㎡

By setting the adhesion amount of the Fe-Zn-Al-Mg-based alloy plating layer on the hot pressed member to 40 to 400 g/m 2, a hot pressed member excellent in corrosion resistance can be obtained. If the adhesion amount is less than 40 g/m2, a hot pressed member having desired corrosion resistance cannot be obtained. If the adhesion amount exceeds 400 g/m 2, the number of cracks crossing the inside of the plated layer increases significantly due to the influence of solidification and shrinkage of the plated layer after hot pressing, and the adhesion within the plated layer significantly deteriorates. The adhesion amount of the plating layer on the hot pressed member is preferably 50 g/m 2 or more, and more preferably 60 g/m 2 or more. Additionally, the adhesion amount of the plating layer on the hot pressed member is preferably 350 g/m 2 or less, and more preferably 300 g/m 2 or less.

In this specification, the “adhesion amount per side of the Fe-Zn-Al-Mg alloy plating layer” of the hot pressed member is determined by the following method. The hot press member subject to evaluation is punched, and three samples of 48 mm phi are collected. Thereafter, in each sample, the non-equivalent surface on the opposite side to the surface on which the amount of adhesion is evaluated is masked. First, each sample is immersed in a 20% chromium(VI) oxide aqueous solution at room temperature for 10 minutes to dissolve the oxide layer, and each sample is weighed. Next, each sample was immersed in 1 L of 500 mL of 35% hydrochloric acid aqueous solution to which 3.5 g of hexamethylenetetramine was added for 120 minutes to dissolve the Fe-Zn-Al-Mg alloy plating layer, and each sample Weigh again. From the mass difference before and after dissolution of the Fe-Zn-Al-Mg alloy plating layer, the adhesion amount per unit area in each sample is calculated. Then, the average value of the three samples is taken as the adhesion amount per side.

[Oxide layer]

The oxide layer in the hot pressed member of this embodiment is formed on the Fe-Zn-Al-Mg alloy plating layer and contains Zn, Al, and Mg. When hot pressing is performed on a plated steel sheet for hot pressing having a Zn-Al-Mg alloy plating layer, Zn, Al, and Mg in the plating layer combine with oxygen present in the heating atmosphere, forming an oxide layer containing Zn, Al, and Mg. This is formed. In addition, the oxide layer is mainly composed of Al oxide, but may also contain Zn or Mg contained in the plating layer, and may further contain elements constituting the base steel sheet, such as Fe, Mn, Cr, etc.

In this embodiment, as another necessary condition for improving post-coating corrosion resistance when electrodeposition coating is performed after zirconium-based chemical conversion treatment on a hot pressed member, the sum of the Al concentration and Mg concentration of the oxide layer is 28 atoms. It is important that it is more than ％. 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 applying zirconium-based chemical conversion treatment to the hot pressed member and then electrodeposition coating. Corrosion resistance becomes insufficient. This is presumed to be because, when the concentration of Zn constituting the oxide layer is large, the reaction between the chemical treatment liquid and the oxide layer becomes uneven, and the unevenness in the thickness of the zirconium-based chemical treatment film formed on the surface of the oxide layer increases. In other words, it is presumed that this is because areas where the chemical conversion film becomes thin tend to form, and the adhesion between the oxide layer and the chemical conversion film, or between the chemical conversion film and the coating film, decreases, or the coverage of the chemical conversion film becomes incomplete. On the other hand, when the sum of the Al concentration and Mg concentration of the oxide layer is 28 atomic% or more, a zirconium-based chemical conversion treatment film is soundly created, so excellent coating is obtained when electrodeposition coating is performed after zirconium-based chemical conversion treatment on a hot pressed member. After corrosion resistance can be obtained.

In addition, when the sum of the Al concentration and Mg concentration of the oxide layer is less than 28 atomic%, the oxide layer becomes brittle, so the coating film adhesion becomes insufficient when electrodeposition coating is performed after zirconium-based chemical conversion treatment is performed on the hot pressed member. . On the other hand, if the sum of the Al concentration and Mg concentration of the oxide layer is 28 atomic% or more, the oxide layer has sufficient strength, so the coating film adhesion when electrodeposition coating is performed after zirconium-based chemical conversion treatment on the hot pressed member is performed. It gets better.

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

In this embodiment, since the oxide layer is formed very thinly on the Fe-Zn-Al-Mg alloy plating layer, as shown in FIG. 1, it may not be visible in a cross-sectional SEM image. However, the oxide layer can be identified as a region where oxygen is detected by measuring the cross section of the surface layer of the hot pressed member using energy dispersive X-ray analysis (EDX) combined with SEM and performing elemental mapping. In addition, in this specification, “Al concentration and Mg concentration in the oxide layer” are values measured by the following method. That is, a test piece for cross-sectional observation is taken from the flat part of the hot pressed member. A cross section including the Fe-Zn-Al-Mg alloy plating layer and the oxide layer of the test piece was observed at 10,000 times using a scanning electron microscope (SEM) with an acceleration voltage of 15 kV, and the oxide layer was observed at any three locations. Composition is determined by energy dispersive X-ray analysis (EDX). The added average of the Al concentration and Mg concentration at three locations is called “Al concentration in the oxide layer” and “Mg concentration in the oxide layer,” respectively.

(Method for manufacturing hot pressed members)

A method for manufacturing a hot pressed member according to an embodiment of the present invention includes heating a plated steel sheet for hot pressing according to an embodiment of the present invention described later to a temperature range of Ac ₃ transformation point to 1000°C, and then hot pressing. It is characterized by

By setting the heating temperature of the steel sheet for hot pressing before hot pressing to Ac ₃ transformation point to 1000°C, the Fe-Zn-Al-Mg alloy plating layer having the α-Fe phase and Γ phase as described above, and the predetermined Al concentration and Mg An oxide layer with high concentration can be obtained. If the heating temperature is lower than the Ac ₃ transformation point, I _Γ /I _α of the Fe-Zn-Al-Mg alloy plating layer will exceed 0.5 after hot pressing. As a result, when electrodeposition coating is applied after zirconium-based chemical treatment is applied to a hot pressed member, post-coating corrosion resistance becomes insufficient. If the heating temperature exceeds 1000°C, the desired oxide layer cannot be obtained, and the coating film adhesion when electrodeposition coating is performed after zirconium-based chemical conversion treatment on the hot pressed member and corrosion resistance after coating become insufficient. In addition, the “heating temperature” herein refers to the highest temperature reached by the steel sheet. In addition, in this specification, “Ac ₃ transformation point” is a value calculated from the following formula based on the chemical composition of the steel sheet.

Ac ₃ transformation point (℃) = 910 - 203C ^1/2 + 44.7Si - 4Mn + 11Cr

In addition, the element symbol on the right side of the formula indicates the content of each element, and when Cr is not contained, Cr = 0.

There is no limitation on the preservation time after raising the temperature to the heating temperature, but from the viewpoint of eliminating the Γ phase and avoiding liquid metal embrittlement cracking during hot pressing, the preservation time is preferably 30 seconds or more. From the viewpoint of avoiding hydrogen intrusion due to blowing of water vapor in the furnace during the storage time, the storage time is preferably within 5 minutes, more preferably within 3 minutes, and even more preferably within 2 minutes.

The method of heating the steel sheet for hot pressing is not limited at all, and examples include furnace heating using an electric furnace or gas furnace, electric heating, induction heating, high-frequency heating, and flame heating.

In hot pressing, press forming and quenching are simultaneously performed on the plated steel sheet for hot pressing heated as described above using a forming mold to obtain a hot pressed member of a predetermined shape. The conditions of hot pressing are not particularly limited, and the regular method can be adopted.

(Plated steel sheet for hot pressing)

A plated steel sheet for hot pressing according to an embodiment of the present invention is formed of a base steel sheet and at least one side of the base steel sheet with an adhesion amount per side of 30 to 180 g/m2, and Al: 3 to 10% in mass%, and It is characterized by having a Zn-Al-Mg-based alloy plating layer containing Mg: 0.2 to 0.8%, with the balance being Zn and inevitable impurities, and having a liquidus temperature of 400° C. or lower in an air atmosphere.

[Not steel plate]

To obtain a hot pressed member having a tensile strength TS of 1470 MPa or more, as the base steel sheet, for example, in mass%, C: 0.20 to 0.35%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, P: It is preferable to use a steel sheet containing 0.1% or less, S: 0.05% or less, Al: 0.1% or less, and N: 0.01% or less, with the remainder being Fe and inevitable impurities. Additionally, the base steel sheet may be either a cold rolled steel sheet or a hot rolled steel sheet. The reason for limitation of each component element is explained below.

C: 0.20 to 0.35%

C improves strength by forming martensite etc. 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, when the C amount exceeds 0.35%, the toughness of the spot weld zone deteriorates. Therefore, it is preferable that the amount of C is 0.20 to 0.35%.

Si: 0.1 to 0.5%

Si is an element effective in strengthening steel and obtaining good materials. To achieve this, it is necessary to set the Si amount to 0.1% or more. On the other hand, when the Si content exceeds 0.5%, ferrite becomes stabilized and hardenability decreases. Therefore, it is preferable that the amount of Si is 0.1 to 0.5%.

Mn: 1.0 to 3.0%

Mn is an element effective in increasing the strength of steel. In order to ensure mechanical properties and strength, the Mn amount needs to be 1.0% or more. On the other hand, if the Mn amount exceeds 3.0%, surface thickening during annealing increases, making it difficult to secure plating adhesion. Therefore, it is preferable that the Mn amount is 1.0 to 3.0%.

P: 0.1% or less

If the P amount exceeds 0.1%, the balance between strength and ductility decreases through deterioration of local ductility due to grain boundary embrittlement accompanying P segregation to austenite grain boundaries during casting. Therefore, it is preferable that the amount of P is 0.1% or less. Additionally, from the viewpoint of steelmaking costs, it is preferable that the P amount is 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 in the weld zone. Therefore, it is desirable to reduce the amount of S as much as possible, and preferably 0.05% or less. Additionally, in order to ensure good stretch flangeability, the amount of S is more preferably set to 0.01% or less. Additionally, from the viewpoint of steelmaking costs, it is preferable that the amount of S is 0.002% or more.

Al: 0.1% or less

If the Al amount exceeds 0.1%, the blanking workability and quenching properties of the base steel sheet decrease. Therefore, it is preferable that the Al amount is 0.1% or less. Additionally, from the viewpoint of ensuring the effect as a deoxidizing agent, the Al amount is preferably set to 0.01% or more.

N: 0.01% or less

If the N amount exceeds 0.01%, AlN is generated during hot rolling or during heating before hot pressing, and the blanking workability and quenching properties of the base steel sheet deteriorate. Therefore, it is preferable that the amount of N is 0.01% or less. Additionally, from the viewpoint of steelmaking costs, it is preferable that the N amount is 0.001% or more.

The remainder other than the above elements is Fe and inevitable impurities. However, for the following reasons, at least one selected from among Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.005%, Cr: 0.1 to 0.3%, and Sb: 0.003 to 0.03% in mass% for the following reasons. You may contain it appropriately as needed.

Nb: 0.05% or less

Nb is an effective ingredient in strengthening steel, but if it is contained in excess, shape freezing properties decrease. Therefore, when Nb is contained, the amount of Nb is set to 0.05% or less.

Ti: 0.05% or less

Ti, like Nb, is effective in strengthening steel, but if it is included in excess, the shape freezing property deteriorates. Therefore, when Ti is contained, the amount of Ti is set to 0.05% or less.

B: 0.0002 to 0.005%

B has the effect of suppressing the production and growth of ferrite from austenite grain boundaries. Therefore, it is preferable that the amount of B is 0.0002% or more. On the other hand, addition of excess B greatly impairs formability. Therefore, when B is contained, the amount of B is set to 0.005% or less.

Cr: 0.1 to 0.3%

Cr is useful because it strengthens steel and improves quenching properties. In order to achieve this effect, it is preferable that the Cr amount is 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 achieve this effect, it is preferable that the Sb amount is 0.003% or more. On the other hand, if the Sb amount exceeds 0.03%, the rolling load increases and productivity decreases. Therefore, when Sb is contained, the amount of Sb is set to 0.03% or less.

[Zn-Al-Mg alloy plating layer]

In this embodiment, the Zn-Al-Mg-based alloy plating layer of the plated steel sheet for hot pressing contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, with the balance being Zn and inevitable impurities. In addition, it shall have a component composition such that the liquidus temperature in an air atmosphere is 400°C or lower.

Al: 3 to 10%

When the Al content is less than 3%, I _Γ /I _α of the Fe-Zn-Al-Mg alloy plating layer exceeds 0.5 after hot pressing, and the sum of the Al concentration and Mg concentration of the oxide layer is 28. It becomes less than atomic percent. As a result, the coating film adhesion and corrosion resistance after coating become insufficient when electrodeposition coating is applied after zirconium-based chemical conversion treatment is applied to the hot pressed member. Additionally, 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 is more than 10%, the liquidus temperature described later cannot be lower than 400°C, and I _Γ /I _α of the Fe-Zn-Al-Mg alloy plating layer exceeds 0.5 after hot pressing. do it As a result, when electrodeposition coating is applied after zirconium-based chemical treatment to a hot pressed member, post-coating corrosion resistance becomes insufficient. Therefore, the Al content rate is set to 3 to 10%.

Mg: 0.2 to 0.8%

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

Liquidus temperature under atmospheric conditions: 400°C or less

In this embodiment, it is important to set the liquidus temperature of the Zn-Al-Mg alloy plating layer in an air atmosphere to 400°C or lower by appropriately controlling the Al content and Mg content. When the liquidus temperature exceeds 400°C, I _Γ /I _α of the Fe-Zn-Al-Mg alloy plating layer exceeds 0.5 after hot pressing. As a result, when electrodeposition coating is applied after zirconium-based chemical treatment to a hot pressed member, post-coating corrosion resistance becomes insufficient. The lower limit of the liquidus temperature is not particularly limited, but within the range of the Al content rate and the Mg content rate, the liquidus temperature is generally 380°C or higher. The liquidus temperature of the Zn-Al-Mg alloy layer in an air atmosphere can be obtained by calculating using a database using thermodynamic calculation software Thermo Calc.

In addition, unavoidable impurities contained in the Zn-Al-Mg alloy plating layer include components of the underlying steel sheet incorporated into the plating layer due to the reaction between the plating bath and the underlying steel sheet during plating treatment, and unavoidable impurities in the plating bath. The base steel sheet component incorporated into the plating layer contains about 0.01% to several% of Fe. Examples of inevitable impurities in the plating bath include Fe, Cr, Cu, Mo, Ni, and Zr. Regarding Fe in the plating layer, it is not possible to distinguish and quantify Fe blown in from the base steel sheet and Fe blown in from the plating bath. The total content of inevitable impurities is not particularly limited, but from the viewpoint of uniformly melting the plating layer in the hot pressing process, the total amount of inevitable impurities excluding Fe is preferably 1% by mass or less.

The component composition of the Zn-Al-Mg 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%. , may be included in a total amount of 1% or less.

Adhesion amount per side: 30∼180g/㎡

By setting the adhesion amount of the Zn-Al-Mg alloy plating layer to 30 to 180 g/m2, a hot pressed member with excellent corrosion resistance and resistance to liquid metal embrittlement cracking during hot pressing can be obtained. If the adhesion amount is less than 30 g/m2, a hot pressed member having desired corrosion resistance cannot be obtained. If the adhesion amount exceeds 180 g/m2, alloying is not completed in the heating process before hot pressing, the liquid phase remains, and liquid metal embrittlement cracks 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. Additionally, the adhesion amount of the Zn-Al-Mg alloy plating layer is preferably 120 g/m 2 or less, and more preferably 100 g/m 2 or less.

In this specification, the “adhesion amount per side of the Zn-Al-Mg alloy plating layer” is determined by the following method. The Zn-Al-Mg alloy plated steel sheet to be evaluated is punched, three samples of 48 mm ϕ are taken, and each sample is weighed. Thereafter, in each sample, the non-parallel surface on the opposite side to the surface on which the amount of adhesion is evaluated is masked. Afterwards, each sample was immersed in 1 L of 500 mL of 35% hydrochloric acid aqueous solution to which 3.5 g of hexamethylenetetramine was added for 10 minutes to dissolve the Zn-Al-Mg alloy plating layer, and then each sample was immersed again. Weigh. From the mass difference before and after dissolution of the Zn-Al-Mg alloy plating layer, the adhesion amount per unit area for each sample is calculated. Then, the average value of the three samples is taken as the adhesion amount per side.

In this embodiment, a separate film may be formed on the lower or upper layer of the Zn-Al-Mg alloy plating layer depending on the purpose, within the range that does not affect the effect of the present invention. As the lower layer coating, nickel-free plating is exemplified. Examples of the upper layer film include a chemical conversion film containing zirconium oxide or zirconium-titanium oxide.

Example

As a base steel plate, in mass%, C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: A cold-rolled steel sheet with a thickness of 1.4 mm containing 0.02%, B: 0.002%, Cr: 0.2%, and Sb: 0.008%, with the remainder being Fe and inevitable impurities, was used (Ac ₃ = 814°C ).

This cold-rolled steel sheet was immersed in a molten Zn-Al-Mg-based plating bath having a predetermined composition and bath temperature using a hot-dip plating equipment, and then N ₂ gas wiping was performed to obtain a level No. 1 shown in Table 1. A plated steel sheet for hot pressing of ~14 was produced. Table 1 shows the Al content, Mg content, and other element content in the Zn-Al-Mg alloy plating layer, and the liquidus temperature in an air atmosphere. The content and liquidus temperature of each element were controlled by adjusting the component composition of the plating bath. The content of each element in the plating layer was determined by quantitative analysis of each component contained in the hydrochloric acid stripper of the plating layer by ICP-AES. In addition, the liquidus temperature of the plating layer was obtained by the method already described. Table 1 also shows the adhesion amount per side of the Zn-Al-Mg alloy plating layer, which was determined by the method already described. The amount of plating adhesion was controlled by adjusting the flow rate and line speed of the wiping gas.

Next, the steel sheet for hot pressing described above was subjected to a hot press. That is, a test piece of 150 mm x 300 mm was taken from the obtained steel sheet for hot pressing, and heat treatment was performed in an electric furnace. Heat treatment conditions (heating temperature, preservation time) are shown in Table 1. The test piece after heat treatment was taken out from the electric furnace and immediately hot pressed at a molding start temperature of 700°C using a hat-shaped mold to obtain a hot pressed member. In addition, the shape of the obtained hot pressed member is 100 mm in length of the flat part of the upper surface, 50 mm in length of the flat part of the side, and 50 mm in length of the flat part of the lower surface. Additionally, the bending R of the mold is 7R for both shoulders on the upper surface and both shoulders on the lower surface.

(Evaluation of Fe-Zn-Al-Mg alloy plating layer/oxide layer of hot pressed member)

A test piece for cross-sectional observation was taken from the flat portion of 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 each distinguishable because they had clearly different contrasts in the cross-sectional SEM image. Fig. 1 shows a cross-sectional SEM image of the Fe-Zn-Al-Mg alloy plating layer of the hot pressed member No. 2, representing the invention example, and Fig. 2 shows the cross-sectional SEM image of the Fe-Zn-Al-Mg-based alloy plating layer No. 8, representing the comparative example. A cross-sectional SEM image of the Fe-Zn-Al-Mg alloy plating layer of the hot pressed member is shown. In Figure 1, precipitation of the Γ phase is suppressed, and the Γ phase is discontinuously dotted among the α-Fe phases. In contrast, in Figure 2, a large amount of the Γ phase precipitates, and the Γ phase exists in the shape of a continuous surface. In addition, the intensity I _Γ 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) at an incident angle of 25° as a source, The intensity I _α of the diffraction peak of the (110) plane of the α-Fe phase present at 51.0°≤2θ≤52.0° was measured, and the ratio _IΓ / _Iα is shown in Table 1. In addition, the measurement of , collimator diameter: 3 mm.

Additionally, at each level, the Al concentration and Mg concentration of the oxide layer were measured by the method already described, and are shown in Table 1. In addition, at each level, the adhesion amount per side of the Fe-Zn-Al-Mg alloy plating layer was measured by the method already described, and is shown in Table 1.

(Evaluation 1: Coat adhesion)

A test piece measuring 70 mm x 150 mm was cut out from the flat portion of the upper surface of the obtained hot pressed member, and the test piece was subjected to zirconium-based chemical conversion treatment. Specifically, chemical treatment was performed using a commercially available chemical treatment liquid (zirconium-based chemical treatment: Parmina 2100 manufactured by Nippon Parkerizing Co., Ltd.) under the conditions of bath temperature: 35°C and treatment time: 120 seconds. After that, each test piece was pressured for 30 seconds using a commercially available cationic electrodeposition paint, maintained at a constant voltage for 150 seconds, and then energized under a voltage condition where the coating film thickness after baking was 15 ㎛, and the ambient temperature Baking was performed in an electric furnace at 170°C for 20 minutes. Additionally, as the cationic electrodeposition paint, Electron GT-100V-1 Gray manufactured by Kansai Paint was used.

On the test piece after electrodeposition coating, using a cutter knife, 11 cuts reaching the base steel plate were placed at intervals of 1 mm in each of the longitudinal and transverse directions to create 100 board grooves. A cellophane tape (registered trademark) was strongly pressed against the eye portion of the substrate, and the end of the tape was peeled off at once at an angle of 45°. The number of units of the coating film peeled off from the surface of the test piece was measured, judgment was made based on the following criteria, and ◎ or ○ was considered pass. The evaluation results are shown in Table 1.

◎: Number of peeled masses = 0

○: Number of peeled masses = 1

△: Number of peeled masses = 2 to 5

×: Number of peeled masses > 5

(Evaluation 2: Corrosion resistance after painting)

A test piece subjected to electrodeposition coating was prepared in the same manner as in Evaluation 1, and the 7.5 mm end of the evaluation surface and the non-surface (back) were sealed with tape. After that, a crosscut flaw with a length of 60 mm and a center angle of 60° was added to the center of the evaluation surface using a cutter knife to a depth reaching the base steel plate. This test piece was subjected to a corrosion test (VDA 233-102) and evaluated by the corrosion status after 4 weeks.

The maximum swelling width on one side from the crosscut was measured, judgment was made based on the following criteria, and ◎ or ○ was considered pass. The evaluation results are shown in Table 1.

◎: Maximum swelling width on one side <1.5㎜

○: 1.5㎜≤maximum swelling width on one side＜3.0㎜

△: 3.0㎜≤maximum swelling width on one side＜4.0㎜

×: 4.0㎜≤maximum swelling width on one side

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

(Industrial applicability)

The hot pressed member of the present invention is suitable for automobile suspension members and car body structural members.

Claims (5)

In mass%, C: 0.20 to 0.35%, Si: 0.1 to 0.5%, Mn: 1.0 to 3.0%, P: 0.01 to 0.1%, S: 0.002 to 0.05%, Al: 0.01 to 0.1%, N: 0.001 to 0.001%. Contains 0.01%, and optionally, at least selected from Nb: more than 0.00% but not more than 0.05%, Ti: more than 0.00% and not more than 0.05%, B: 0.0002 to 0.005%, Cr: 0.1 to 0.3%, and Sb: 0.003 to 0.03%. A base steel sheet containing one type and having a composition of which the balance is Fe and inevitable impurities,
A Fe-Zn-Al-Mg-based alloy plating layer containing an α-Fe phase and a Γ phase formed on at least one side of the base steel plate at an adhesion amount of 40 to 400 g/m2 per side,
An oxide layer containing Zn, Al, and Mg formed on the Fe-Zn-Al-Mg alloy plating layer.
With
When measured from a cross section, 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) at an incident angle of 25° as a source. The ratio I _Γ /I _α of 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 0.01 or more and 0.5 or less,
A hot press member, characterized in that the sum of the Al concentration and Mg concentration of the oxide layer is 28 atomic% or more and 50 atomic% or less. Not with a grater,
A component is formed on at least one side of the base steel plate with an adhesion amount per side of 30 to 180 g/m2, and contains Al: 3 to 10% and Mg: 0.2 to 0.8% in mass%, with the balance being Zn and inevitable impurities. A Zn-Al-Mg-based alloy plating layer having a composition and a liquidus temperature of 380°C or more and 400°C or less in an air atmosphere.
A method for producing a hot press member, comprising heating a plated steel sheet for hot pressing to a temperature range of Ac ₃ transformation point to 1000° C. and then hot pressing it to produce the hot press member according to claim 1. According to paragraph 2,
The component composition of the Zn-Al-Mg-based alloy plating layer is further, in mass%, at least one selected from Ca, Sr, Mn, V, Cr, Mo, Ti, Ni, Co, Sb, Zr, and B. A method of manufacturing a hot pressed member comprising a total of more than 0% and less than or equal to 1%. delete delete