JP2012001816A - Hot-pressed member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot-pressed member which is produced without generating any scale, has excellent adhesion to coatings and excellent corrosion resistance after coating, and is inhibited from corroding and thereby suffering hydrogen penetration into the steel.SOLUTION: The hot-pressed member comprises: a steel sheet that constitutes the member and has a surface layer in which an Ni diffusion region is present; and an intermetallic-compound layer corresponding to the γ-phase present in a phase equilibrium diagram of a Zn-Ni alloy and a ZnO layer on the Ni diffusion region in order, wherein the member has a component composition containing, by mass, 0.15-0.5% of C, 0.05-2.0% of Si, 0.5-3% of Mn, ≤0.1% of P, ≤0.05% of S, ≤0.1% of Al, and ≤0.01% of N, and the balance comprising Fe and inevitable impurities. The hot-pressed member exhibits a natural immersion potential of -600 to -360 mV relative to the standard hydrogen electrode, in a 0.5 M aqueous NaCl solution saturated with air at a temperature of 25°C±0.5°C.

Description

  The present invention relates to a hot press member manufactured by pressing a heated steel plate, and more particularly to a hot press member used in an undercarriage part or a vehicle body structure part of an automobile.

  2. Description of the Related Art Conventionally, many undercarriage members and body structure members of automobiles are manufactured by pressing a steel plate having a predetermined strength. In recent years, from the viewpoint of the preservation of the global environment, there has been a strong desire to reduce the weight of automobile bodies, and efforts have been made to increase the strength of steel sheets to be used and to reduce their thickness. However, as the strength of the steel plate increases, the press workability decreases, and it is often difficult to process the steel plate into a desired member shape.

  Therefore, Patent Document 1 describes a process called a hot press that enables both easy processing and high strength by simultaneously processing a heated steel sheet using a die and punch die and simultaneously cooling it. Technology has been proposed. However, in this hot press, the steel sheet is heated to a high temperature of around 950 ° C before hot pressing, so scale (Fe oxide) is generated on the steel sheet surface and the scale peels off during hot pressing. There is a problem that the mold is damaged or the surface of the member after hot pressing is damaged. In addition, the scale remaining on the surface of the member may cause poor appearance, poor paint adhesion, and poor corrosion resistance after painting. For this reason, the scale on the surface of the member is usually removed by processing such as pickling or shot blasting, but this complicates the manufacturing process and causes a decrease in productivity.

For this reason, there is a need for a hot press technology that can suppress the formation of scales during heating before hot pressing and improve the coating adhesion and post-coating corrosion resistance of members after hot pressing. A steel plate provided with a coating such as a plating layer and a hot pressing method using the steel plate have been proposed. For example, Patent Document 2 discloses a coated steel sheet coated with Al or an Al alloy. It has been shown that by using this coated steel sheet, decarburization and oxidation are prevented during heating before hot pressing, and a hot pressed member having extremely high strength and excellent corrosion resistance can be obtained. Patent Document 3 describes a Zn-Fe base that prevents corrosion and decarburization and has a lubrication function when hot pressing a steel sheet coated with Zn or a Zn base alloy during heating before hot pressing. A hot pressing method is disclosed in which an alloy compound such as the above compound or a Zn-Fe-Al-based compound is formed on the surface of a steel sheet. It has been shown that a hot press member manufactured by this method, particularly a hot press member using a steel sheet coated with Zn-50 to 55% by mass Al, has an excellent corrosion prevention effect. Further, Patent Document 4 uses a steel plate mainly composed of Al or Zn, and has a heating temperature of Ac ₃ transformation point to 1100 ° C. in an atmosphere with a hydrogen concentration of 6% by volume or less and a dew point of 10 ° C. or less. A hot pressing method excellent in hydrogen embrittlement resistance by hot pressing after heating is disclosed. In this hot pressing method, the amount of hydrogen and water vapor in the atmosphere during heating is reduced to reduce the amount of hydrogen that penetrates into the steel, thereby avoiding hydrogen embrittlement associated with high strength exceeding 1000 MPa. .

GB 1490535 Japanese Patent No. 3931251 Japanese Patent No. 3663145 JP 2006-51543 A

  However, in the hot press members described in Patent Documents 2 to 4, hydrogen embrittlement due to hydrogen intrusion into the steel due to corrosion in the environment of use rather than hydrogen intrusion into the steel during heating before hot pressing. There's a problem.

  An object of the present invention is to provide a hot press member that can be produced without generation of scale, has excellent coating adhesion and post-coating corrosion resistance, and can suppress hydrogen intrusion into steel due to corrosion. .

  As a result of intensive studies on the above-mentioned hot press member, the present inventors have obtained the following knowledge.

  i)% by mass, C: 0.15-0.5%, Si: 0.05-2.0%, Mn: 0.5-3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less When the Ni diffusion region is present in the surface layer of the steel sheet constituting the member having a component composition containing Fe and the inevitable impurities, the hydrogen intrusion into the steel due to corrosion is suppressed.

  ii) When an intermetallic compound layer corresponding to the γ phase existing in the equilibrium diagram of the Zn—Ni alloy is provided on the Ni diffusion region, excellent post-coating corrosion resistance can be obtained.

  iii) When a ZnO layer is provided on the intermetallic compound layer, excellent coating adhesion can be obtained.

  The present invention has been made based on such findings, in mass%, C: 0.15-0.5%, Si: 0.05-2.0%, Mn: 0.5-3%, P: 0.1% or less, S: 0.05% Hereinafter, there is a Ni diffusion region in the surface layer of the steel sheet constituting the member having a component composition containing Al: 0.1% or less, N: 0.01% or less, the balance being composed of Fe and inevitable impurities, the Ni diffusion region On top of this, there is an intermetallic compound layer corresponding to the γ phase present in the equilibrium diagram of the Zn-Ni alloy, and a ZnO layer, and a natural solution shown in an air-saturated 0.5 M NaCl solution at 25 ° C ± 5 ° C. There is provided a hot press member characterized in that the immersion potential is -600 to -360 mV with respect to a standard hydrogen electrode.

  In the hot-pressed member of the present invention, the component composition of the steel plate constituting the member was further selected by mass% from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%. It is preferable that at least one kind or Sb: 0.003 to 0.03% is contained individually or simultaneously.

  In addition, the Ni diffusion region exists over 1 μm in the depth direction of the steel sheet, the intermetallic compound layer exists in an island shape, the Si-containing compound layer, the Ti-containing compound layer, the Al-containing compound layer immediately below the ZnO layer It is preferable to have at least one compound layer selected from among the Zr-containing compound layers.

  According to the present invention, it is possible to produce a hot press member that can be produced without generation of scale, has excellent coating adhesion and post-coating corrosion resistance, and can suppress hydrogen intrusion into steel due to corrosion. The hot press member of the present invention is suitable for an automobile suspension member and a vehicle body structure member having a strength of 980 MPa or more.

It is a figure which shows typically the structure | tissue of the plate | board thickness cross-section direction of the steel plate which comprises a hot press member. It is a figure which shows typically the press method used in the present Example. It is a figure which shows typically the electrochemical cell used in the present Example.

1) Hot press material
1-1) Ni diffusion region of steel plate constituting member As described above, when the Ni diffusion region is present in the surface layer of the steel plate constituting the member, hydrogen intrusion into the steel due to corrosion is suppressed. The reason for this is not necessarily clear, but can be considered as follows. In other words, the hydrogen intrusion into the steel sheet due to corrosion is related to the oxidation-reduction reaction of Fe rust in a humid environment, and it is necessary that the Fe rust is stable and difficult to change in order to suppress hydrogen intrusion. . The Ni diffusion region is effective for stabilizing Fe rust, and the presence of the Ni diffusion region suppresses hydrogen intrusion into the steel due to corrosion.

  However, in order to effectively suppress such hydrogen intrusion, it is preferable that the Ni diffusion region is present over 1 μm or more, preferably 2 μm or more, more preferably 3 μm or more in the depth direction of the steel sheet constituting the member. The upper limit of the depth is not particularly limited, but the effect is saturated at about 50 μm. The depth of the Ni diffusion region can be obtained by analyzing the cross section in the thickness direction using EPMA (Electron Probe Micro Analyzer) or analyzing the depth direction using GDS (Glow Discharge Spectroscopy).

  The Ni diffusion region in the present invention refers to a region where Ni diffused from the Ni-based plating layer into the steel during heating before hot pressing exists in a solid solution state. In addition, since the hot pressed member of the present invention is manufactured by hot pressing a Ni-based plated steel sheet having a Zn-Ni alloy layer, Zn may be contained as an impurity in the Ni diffusion region. The effect is not impaired.

1-2) Intermetallic compound layer corresponding to γ phase in the equilibrium diagram of Zn-Ni alloy on Ni diffusion region
This intermetallic compound layer provided on the Ni diffusion region is effective in improving the corrosion resistance after coating because its corrosion potential has a sacrificial anticorrosive effect on steel. In addition, the intermetallic compound layer corresponding to the γ phase existing in the equilibrium diagram of the Zn-Ni alloy is a layer made of any intermetallic compound of Ni ₂ Zn ₁₁ , NiZn ₃ , or Ni ₅ Zn _21. Say. These intermetallic compounds are detected by X-ray diffraction directly on the surface of the member, or electron beam diffraction while observing a thin piece produced by FIB (Focused Ion Beam) processing from the cross section in the thickness direction with TEM (Transmission Electron Microscopy) it can.

  In order to obtain the above effect of the intermetallic compound layer, it is necessary to control the abundance thereof as follows.

The abundance of the intermetallic compound layer can be measured by an electrochemical method, that is, by a natural immersion potential based on a standard hydrogen electrode shown in an air saturated 0.5 M NaCl solution at 25 ° C. ± 5 ° C. If there are few intermetallic compound layers and the natural immersion potential becomes nobler than -360 mV, the sacrificial anticorrosive effect on steel is lost and the corrosion resistance after painting deteriorates. On the other hand, if there are many intermetallic compound layers and the natural immersion potential becomes lower than -600 mV, the amount of hydrogen generated increases due to corrosion, and hydrogen intrusion may occur even if there is a Ni diffusion region. Accordingly, it is necessary to provide an intermetallic compound layer in such an amount that the natural immersion potential shown in an aqueous saturated 0.5 M NaCl solution at 25 ° C. ± 5 ° C. is −600 to −360 mV with respect to the standard hydrogen electrode. In order to obtain the abundance as described above, the intermetallic compound layer is preferably present in an island shape. Here, in the present invention, the island-shaped intermetallic compound layer is defined as follows by cross-sectional SEM (Scanning Electron Microscopy) observation.
(1) A specimen of 10 mm × 10 mm × thickness is cut out from the member, embedded in a resin mold and polished.
(2) Using the embedded and polished specimen of (1), a reflected electron composition image is taken with an accelerating voltage of 5 to 25 kV and a magnification of 500 times with an SEM.
(3) Take a photo with any 10 fields of view.
(4) In the photograph taken, as schematically illustrated in FIG. 1, if the intermetallic compound layer is discontinuously present on the surface of the steel sheet, the score is 1, and the intermetallic compound layer is continuously present. If there is no intermetallic compound layer in the field of view, the score is 0.
(5) If the total score of 10 photos is 7 or more, it is judged as an island.

1-3) ZnO layer on the intermetallic compound layer corresponding to the γ phase present in the equilibrium diagram of the Zn-Ni alloy If the ZnO layer provided on the outermost layer has only excellent adhesion to the intermetallic compound layer, In addition, the adhesion to the chemical conversion film formed during the coating base treatment is also excellent, so the coating adhesion is greatly improved. If the thickness is 0.1 μm or more, the adhesion with the chemical conversion film is sufficient, and if it is 5 μm or less, the ZnO layer itself does not cohesively break and does not impair the paint adhesion, so it is 0.1-5 μm. It is preferable.

  In addition, the ZnO layer can be confirmed by X-ray diffraction or electron diffraction by TEM observation, and the thickness can be measured as in the case of the intermetallic compound layer.

  The ZnO layer has excellent adhesion to the underlying intermetallic compound layer, but was selected from among a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer directly under the ZnO layer. When at least one kind of compound layer is provided, the adhesion is further improved, and as a result, a better coating adhesion can be obtained.

2) METHOD hot pressing member of the present invention, heating the Ni-plated steel sheet having a Zn-Ni alloy plating layer containing 13 wt% or more Ni on the surface of the steel sheet, the temperature range of Ac ₃ transformation point to 1200 ° C. It can be manufactured by post-hot pressing.

By heating the Ni-based plated steel sheet as described above to a temperature range of Ac ₃ transformation point to 1200 ° C., Ni in the plating layer diffuses into the steel sheet and forms a Ni diffusion region. In addition, the Zn-Ni alloy plating layer containing 13 mass% or more of Ni on the surface forms the intermetallic compound layer as described above, and a part of Zn diffuses to the surface, and the ZnO layer is formed on the outermost layer. Is formed.

Even if the Ni content of the Zn-Ni alloy plating layer is less than 13% by mass, the Ni content should be 10% by mass or more, and the amount of Zn-Ni alloy plating layer deposited on one side of the steel sheet exceeds 50g / m ² The hot pressed member of the present invention can be produced by hot pressing after heating in the temperature range of Ac ₃ transformation point to 1200 ° C. at an average temperature rising rate of 12 ° C./second or more. If the Ni content of the Zn-Ni alloy plating layer is less than 10% by mass or the average heating rate is less than 12 ° C / sec, not only the Ni diffusion region will be insufficiently formed, but also Zn evaporation will be active. Therefore, the intermetallic compound layer as described above cannot be formed. Further, when the adhesion amount of the Zn—Ni alloy plating layer per one surface of the steel sheet is 50 g / m ² or less, the formation of the Ni diffusion region becomes insufficient. Here, the average rate of temperature increase when heating to the temperature range of Ac ₃ transformation point to 1200 ° C is the temperature difference from room temperature to the maximum plate temperature, from room temperature to the maximum plate temperature. Define the value divided by time.

  Regardless of the Ni content, the steel sheet surface is coated with a Zn-Ni alloy plating layer, so scale does not form during heating before hot pressing.

When heating to a temperature range of Ac ₃ transformation point to 1200 ° C., it is preferable to heat at an average temperature increase rate of 85 ° C./second or more. Increasing the heating rate shortens the time during which the steel sheet is maintained at a high temperature, so that the austenite grains of the steel sheet during heating can be refined, so that the toughness of the member after hot pressing can be improved. Since the evaporation of Zn can be remarkably suppressed, the intermetallic compound layer as described above can be formed to improve the corrosion resistance after painting, and furthermore, the formation of excessive ZnO layer can be prevented, ensuring stable coating adhesion. It will be possible. Such a temperature rising rate can be realized by energization heating or high-frequency heating.

  The Ni-based plated layer of the Ni-based plated steel sheet may be a Zn-Ni alloy-plated single layer, or may be a multilayer in which the Zn-Ni-plated alloy layer is provided on the Ni layer or a Ni-based alloy layer not containing Zn. . Examples of the Ni-based alloy include those containing a total of 20% by mass or less of at least one element selected from Fe, Co, Cr, Mn, Cu, Mo and the like in Ni.

  The depth of the Ni diffusion region and the thickness of the ZnO layer can be adjusted by heating conditions (temperature, time), and the abundance of the intermetallic compound layer can be adjusted by the adhesion amount of the Ni-based plating. Note that the ZnO layer is naturally formed not only by heating in the atmosphere as usual, but also by heating in an atmosphere having an oxygen concentration of 0.1% by volume or more.

  Such a Ni-based plating layer can be formed by an electroplating method or the like.

On the Zn-Ni alloy plating layer on the surface of the steel plate, at least one compound layer selected from Si-containing compound layer, Ti-containing compound layer, Al-containing compound layer and Zr-containing compound layer is provided, and Ac ₃ When heated to a temperature range of the transformation point to 1200 ° C., a part of Zn diffuses to the surface through these compound layers, and forms a ZnO layer on the outermost layer, so the Si-containing compound layer immediately below the ZnO layer, At least one compound layer selected from a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer can be provided. At this time, if the thickness of such a compound layer provided on the Zn-Ni alloy layer is 0.1 μm or more, the coating adhesion can be sufficiently improved, and if it is 3.0 μm or less, the Si-containing compound layer does not become brittle, and the coating adhesion Therefore, 0.1 to 3.0 μm is preferable, and 0.4 to 2.0 μm is more preferable.

  As the Si-containing compound, for example, silicone resin, lithium silicate, sodium silicate, colloidal silica, silane coupling agent and the like can be applied. As the Ti-containing compound, for example, a titanate such as lithium titanate or calcium titanate, a titanium coupling agent mainly containing a titanium alkoxide or a chelate-type titanium compound can be applied. As the Al-containing compound, for example, an aluminate such as sodium aluminate or calcium aluminate, an aluminum coupling agent mainly composed of an aluminum alkoxide or a chelate-type aluminum compound can be applied. As the Zr-containing compound, for example, a zirconate salt such as lithium zirconate or calcium zirconate, a zirconium coupling agent mainly composed of a zirconium alkoxide or a chelate-type zirconium compound can be applied.

  In order to form such a compound layer on the Zn-Ni alloy plating layer, at least one compound selected from the Si-containing compound, Ti-containing compound, Al-containing compound, and Zr-containing compound is plated with Zn-Ni alloy. What is necessary is just to heat-dry, without washing with water, after carrying out the adhesion process on a layer. The adhesion treatment of these compounds may be any of a coating method, a dipping method, and a spray method, and a roll coater, a squeeze coater, a die coater, or the like may be used. At this time, it is also possible to adjust the coating amount, make the appearance uniform, and make the thickness uniform by an air knife method or roll drawing method after the coating treatment, dipping treatment, and spraying treatment with a squeeze coater or the like. Moreover, it is preferable to perform heat drying so that the maximum reached temperature of a steel plate may be 40-200 degreeC. It is more preferable to carry out at 50 to 160 ° C.

  In addition, in order to form such a compound layer on the Zn-Ni alloy plating layer, it contains at least one cation selected from Si, Ti, Al, and Zr, and includes phosphate ions, fluoric acid ions, and fluorides. After a reactive treatment in which a Ni-based plated steel sheet having a Zn-Ni alloy plating layer is immersed in an acidic aqueous solution containing at least one anion selected from ions, the substrate is heated without being washed with water or water. It is also possible by the drying method.

  In order to obtain a hot pressed member having a strength of 980 MPa or more, as a base steel plate of a Ni-based plated steel plate, for example, in mass%, C: 0.15-0.5%, Si: 0.05-2.0%, Mn: 0.5-3% , P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less, steel sheet having a component composition consisting of Fe and unavoidable impurities as the balance, and further, in mass%, It is preferable to use a steel plate that contains at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, B: 0.0005 to 0.08%, or Sb: 0.003 to 0.03% individually or simultaneously. .

  The reason for limitation of each component element is demonstrated below. Here, “%” representing the content of a component means “% by mass” unless otherwise specified.

C: 0.15-0.5%
C is an element that improves the strength of steel. In order to increase the TS of a hot-pressed member to 980 MPa or more, the amount needs to be 0.15% or more. On the other hand, if the amount of C exceeds 0.5%, the blanking workability of the steel plate as the material will be significantly reduced. Therefore, the C content is 0.15 to 0.5%.

Si: 0.05-2.0%
Si, like C, is an element that improves the strength of steel. In order to increase the TS of a hot-pressed member to 980 MPa or more, the amount needs to be 0.05% or more. On the other hand, when the Si content exceeds 2.0%, the occurrence of surface defects called red scale during hot rolling is remarkably increased, the rolling load is increased, and the ductility of the hot-rolled steel sheet is deteriorated. Furthermore, if the Si content exceeds 2.0%, the plating processability may be adversely affected when a plating process for forming a plating film mainly composed of Zn or Al on the steel sheet surface is performed. Therefore, the Si content is 0.05 to 2.0%.

Mn: 0.5-3%
Mn is an effective element for improving the hardenability by suppressing the ferrite transformation, and also an element effective for lowering the heating temperature before hot pressing because it lowers the Ac ₃ transformation point. is there. In order to exhibit such an effect, the amount needs to be 0.5% or more. On the other hand, when the amount of Mn exceeds 3%, segregation occurs and the uniformity of the characteristics of the raw steel plate and the hot press member decreases. Therefore, the amount of Mn is 0.5 to 3%.

P: 0.1% or less
When the amount of P exceeds 0.1%, segregation occurs and the uniformity of the characteristics of the raw steel plate and hot-pressed member decreases, and the toughness also decreases significantly. Therefore, the P content is 0.1% or less.

S: 0.05% or less
When the amount of S exceeds 0.05%, the toughness of the hot pressed member decreases. Therefore, the S content is 0.05% or less.

Al: 0.1% or less
If the Al content exceeds 0.1%, blanking workability and hardenability of the steel sheet will be reduced. Therefore, the Al content is 0.1% or less.

N: 0.01% or less
If the N content exceeds 0.01%, an AlN nitride is formed during hot rolling or heating before hot pressing, and the blanking workability and hardenability of the raw steel sheet are reduced. Therefore, the N content is 0.01% or less.

  The balance is Fe and inevitable impurities, but for the following reasons, at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, B: 0.0005 to 0.08%, Sb: 0.003 to It is preferable to contain 0.03% individually or simultaneously.

Cr: 0.01-1%
Cr is an element effective for strengthening steel and improving hardenability. In order to exhibit such an effect, the Cr content is preferably 0.01% or more. On the other hand, if the Cr content exceeds 1%, the cost is significantly increased, so the upper limit is preferably 1%.

Ti: 0.2% or less
Ti is an element effective for strengthening steel and improving toughness by refining. It is also an element effective for forming a nitride in preference to B, which will be described next, and exhibiting the effect of improving hardenability by solid solution B. However, if the amount of Ti exceeds 0.2%, the rolling load during hot rolling is extremely increased, and the toughness of the hot pressed member is lowered, so the upper limit is preferably 0.2% or less.

B: 0.0005-0.08%
B is an element effective for improving the hardenability during hot pressing and toughness after hot pressing. In order to exhibit such an effect, the B content is preferably 0.0005% or more. On the other hand, if the amount of B exceeds 0.08%, the rolling load during hot rolling is extremely increased, and the martensite phase and bainite phase occur after hot rolling, resulting in cracks in the steel sheet, so the upper limit is It is preferable to set it to 0.08%.

Sb: 0.003-0.03%
Sb has an effect of suppressing a decarburized layer generated in the steel sheet surface layer portion between the time when the steel plate is heated before hot pressing and the time when the steel plate is cooled by a series of processes of hot pressing. In order to exhibit such an effect, the amount needs to be 0.003% or more. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases and the productivity decreases. Therefore, the Sb amount is set to 0.003 to 0.03%.

  Examples of the heating method before hot pressing include heating by an electric furnace or a gas furnace, flame heating, energization heating, high-frequency heating, induction heating, and the like, but are not limited thereto.

In mass%, C: 0.23%, Si: 0.12%, Mn: 1.5%, P: 0.01%, S: 0.01%, Al: 0.03%, N: 0.005%, Cr: 0.4%, B: 0.0022% included In addition, the balance is composed of Fe and inevitable impurities, the Ac ₃ transformation point is 818 ° C., 50 g / L (liter) sodium sulfate, 100 g / L on both sides of a 1.6 mm thick cold-rolled steel sheet In the plating bath consisting of nickel sulfate hexahydrate and 50 g / L zinc sulfate heptahydrate at a pH of 2 and a temperature of 50 ° C., the current density was changed to 10 to 50 A / dm ² and electroplating was performed. Zn—Ni alloy plating layers having different Ni contents and adhesion amounts shown in Tables 1 and 2 were formed. Then, after applying a Si-containing compound, a Ti-containing compound, an Al-containing compound, and a Zr-containing compound shown in Tables 1 and 2 except for some steel plates, it was dried under the condition that the ultimate temperature was 140 ° C, and the thickness was 0.5 μm. One of the Si-containing compound layer, the Ti-containing compound layer, the Al-containing compound layer, and the Zr-containing compound layer was formed. When heating a blank of 200 mm x 220 mm collected from a steel plate of the material thus obtained at an average heating rate of 8 ° C / sec, it is heated for 10 minutes at the heating temperature shown in Tables 1 and 2 in an electric furnace in an atmospheric atmosphere. After that, it is taken out from the furnace and immediately drawn by a pressing method as schematically shown in FIG. 2, hot press members No. 1, 4, 7 to 21, 28 to 30, 31, 34, 37, 38. Was made. Some steel plates were heated by direct current heating at an average heating rate of 12 ° C / second or 90 ° C / second, and after reaching the heating temperatures shown in Tables 1 and 2, they were taken out of the furnace and immediately Drawing was performed by the same pressing method, and hot pressed members No. 2, 3, 5, 6, 22 to 27, 32, 33, 35, and 36 were produced. The punch width when drawing was 70 mm and the processing height was 30 mm. And taking a sample from the flat part of the member head, and measuring the natural immersion potential, which is an index of the depth of the Ni diffusion region, the thickness of the ZnO layer, the abundance of the intermetallic compound layer, by the above method, The state of the intermetallic compound layer was confirmed by cross-sectional SEM observation described above. In addition, the following methods were used to investigate scale resistance, paint adhesion, post-coating corrosion resistance, and hydrogen penetration resistance.
Scale resistance: The non-punch contact surface after hot pressing was visually observed and evaluated according to the following criteria.
○: No scale adherence ×: Scale adherence Paint adhesion: A sample is taken from the flat part of the component head, and chemical conversion treatment is performed under standard conditions using PB-SX35 made by Nihon Parkerizing Co., Ltd. on the non-punch contact surface. After coating, an electrodeposition paint GT-10HT gray produced by Kansai Paint Co., Ltd. was formed into a film with a thickness of 20 μm under a baking condition of 170 ° C. for 20 minutes to prepare a coating test piece. Then, a cut that reaches the steel substrate of the grid pattern (10 x 10 pieces, 1 mm interval) with a cutter knife is put on the surface that has been subjected to chemical conversion treatment and electrodeposition coating of the prepared test piece, and is attached with an adhesive tape. A cross-cut tape peeling test was conducted. Evaluation was made according to the following criteria, and it was considered that the object of the present invention was satisfied if it was ○ or Δ.
○: No peeling △: Peeling at 1 to 10 grids ×: Peeling after peeling coating at 11 or more grids: Corrosion resistance after coating: Chemical conversion treatment and electrical treatment of coated test pieces prepared in the same manner as in the case of the above coating adhesion After cross-cutting the coated film with a cutter knife on the coated surface, a corrosion test is performed under the corrosion test cycle conditions in accordance with SAE-J2334, and the maximum one-sided film swelling width after 25 cycles is measured. The evaluation was made according to the following criteria, and it was considered that the object of the present invention was satisfied if it was ○ or Δ.
○: 0 mm ≦ swollen width <1.5 mm
Δ: 1.5 mm ≦ bulging width <3.0 mm
×: 3.0 mm ≦ swell width Hydrogen penetration resistance: A sample was taken from the flat part of the member head, and one surface (punch contact surface) was mirror-polished to a plate thickness of 1 mm. Next, the working electrode is the sample, the counter electrode is platinum, Ni is plated on the ground surface, and the hydrogen detection surface is set in the electrochemical cell schematically shown in Fig. 3, and the unground surface is corroded at room temperature in the atmosphere. The amount of hydrogen penetrating into the steel was measured by electrochemical hydrogen permeation method. That is, the hydrogen detection surface side is filled with 0.1 M NaOH aqueous solution, the reference electrode (Ag / AgCl) is set through the salt bridge, and the 0.5 M NaCl solution is dropped on the non-grinding surface (evaluation surface: non-punch contact surface) side. Corrosion is performed at room temperature in the atmosphere, and the hydrogen permeation current value is continuously set to 5 while dripping pure water into the corroded part once a day so that the potential on the hydrogen detection surface side becomes 0 VvsAg / AgCl. The measurement was conducted for one day, and the hydrogen penetration resistance associated with corrosion was evaluated from the maximum current value according to the following criteria. If ◎ or ○, the object of the present invention was satisfied. In addition, it tested after removing the surface scale by shot blasting to the member which the production | generation of the scale at the time of a hot press was remarkable.
◎: 1/10 or less when the maximum current value is cold-rolled steel sheet ○: More than 1/10 to 1/2 or less when the maximum current value is cold-rolled steel sheet ×: 1 when the maximum current value is cold-rolled steel sheet Tables 3 and 4 show the same results as those for steel sheets exceeding / 2 to cold rolled steel. It can be seen that the hot press members Nos. 1 to 27 and 30 according to the present invention are excellent not only in scale resistance, coating adhesion, and post-coating corrosion resistance, but also in hydrogen penetration resistance.

Claims (6)

  In mass%, C: 0.15-0.5%, Si: 0.05-2.0%, Mn: 0.5-3%, P: 0.1% or less, S: 0.05% or less, Al: 0.1% or less, N: 0.01% or less In addition, a Ni diffusion region exists in the surface layer of the steel sheet constituting the member having a component composition composed of Fe and inevitable impurities as the balance, and on the Ni diffusion region, in the equilibrium diagram of the Zn-Ni alloy in order. The natural immersion potential in an 0.5 M NaCl aqueous solution with an intermetallic compound layer corresponding to the γ phase and ZnO layer and air saturated at 25 ° C. ± 5 ° C. is −600 to −360 mV based on the standard hydrogen electrode A hot press member characterized by that.   The component composition of the steel sheet constituting the member further includes at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, B: 0.0005 to 0.08% by mass%. 2. The hot press member according to claim 1.   3. The hot pressed member according to claim 1, wherein the component composition of the steel sheet constituting the member further contains Sb: 0.003 to 0.03% by mass%.   4. The hot-pressed member according to claim 1, wherein the Ni diffusion region exists over 1 μm or more in the depth direction of the steel plate.   5. The hot press member according to any one of claims 1 to 4, wherein the intermetallic compound layer exists in an island shape.   6. The method according to claim 1, further comprising at least one compound layer selected from a Si-containing compound layer, a Ti-containing compound layer, an Al-containing compound layer, and a Zr-containing compound layer immediately below the ZnO layer. The hot press member according to one item.