WO2016006232A1

WO2016006232A1 - Hot-press-molded article and production method for same, and plated steel sheet for use in hot-press molding

Info

Publication number: WO2016006232A1
Application number: PCT/JP2015/003426
Authority: WO
Inventors: 泰明沖田; 池田　倫正; 公一谷口
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2014-07-10
Filing date: 2015-07-07
Publication date: 2016-01-14
Also published as: JP6146482B2; JPWO2016006232A1

Abstract

　Provided are a hot-press-molded article that has excellent resistance spot weldability, and that does not require a process such as shotblasting to remove oxides; a production method for the same; and a plated steel sheet for use in hot-press molding. The plated hot-press-molded article is obtained by heating a plated steel sheet to a temperature at or above the Ac3 transformation point of the plated steel sheet, and then simultaneously cooling while molding the sheet with a mold. An oxide layer having a network of asperities, having average roughness Ra of at least 2 μm and average thickness of no more than 3 μm, is present on the plated surface, and below the oxide layer is a plated layer having a melting point that is no higher than the aforementioned heating temperature.

Description

HOT PRESS FORMED BODY AND ITS MANUFACTURING METHOD

The present invention relates to a hot press-formed body excellent in resistance spot weldability, a manufacturing method thereof, and a plated steel sheet for a hot press-formed body.

In recent years, from the viewpoint of global environmental protection, weight reduction of automobile bodies has been eagerly awaited, and efforts are being made to increase the strength of steel sheets used in automobile bodies and reduce their thickness. However, since press formability decreases with increasing strength of the steel sheet, it is often difficult to process the steel sheet into a desired member shape.

In response to the above, there is a method in which a material to be molded is preliminarily heated and molded as a technique for pressing a difficult-to-press material such as a high-strength steel plate.

In Patent Document 1, a material (steel plate) to be formed is preheated and softened so as to facilitate press working, and then the heated steel plate is formed using a die and a die and rapidly cooled at the same time. (Hereinafter, the process of heating and then molding and cooling at the same time is referred to as hot press molding), and a molding technique that makes it easy to mold and achieve high strength is disclosed. However, with this forming technique, heating the steel plate to a high temperature of Ac3 or higher before forming is necessary to obtain high strength after press forming. For this reason, scale (iron oxide) is generated on the surface of the steel sheet, and the scale peels off during hot press forming, damaging the mold or damaging the surface of the compact after hot press forming. is there. In addition, the scale remaining on the surface of the molded body not only causes poor appearance and poor paint adhesion, but also has high electrical resistance, which makes resistance spot welding mainly used in the assembly of car bodies difficult. There's a problem. For this reason, the scale on the surface of the molded body is usually removed by a treatment such as pickling or shot blasting, but this complicates the manufacturing process and causes a decrease in productivity.

For these problems, scale formation can be suppressed during heating before molding, and the paintability and corrosion resistance of the molded product after hot press molding can be improved without treatment such as pickling or shot blasting. In addition, a steel sheet for hot pressing capable of resistance spot welding is desired, and a steel sheet having a coating such as a plating layer on the surface thereof has been proposed.

As a conventional hot-pressed plated steel sheet, for example, as described in Patent Document 2, an Al-based plated steel sheet has been often used. In the case of an Al-based plating layer, Fe rapidly diffuses into the plating layer when heated in the austenite region to form an alloy layer of Al and Fe, and resistance spot welding is performed without performing pickling or shot blasting. Is possible. However, since the Al—Fe alloy layer is hard and brittle, there are problems in that it is peeled off during processing to reduce processing productivity and shorten the mold life.

In addition to Al-based plated steel sheets, Zn-based hot-pressed plated steel sheets have also been proposed. For example, Patent Document 3 discloses a Zn—Fe-based compound that prevents corrosion and decarburization during heating when a steel sheet coated with Zn or a Zn-based alloy is formed into a hot press-formed body, and has a lubricating function. And a method for producing an alloy compound such as a Zn—Fe—Al based compound on the surface of a steel sheet. It has been shown that a member manufactured by this method, particularly a member using a steel sheet coated with Zn-50 to 55 mass% Al, can obtain an excellent corrosion prevention effect.

Further, Patent Document 4 discloses a hot pressed member in which a plated layer containing a Fe—Zn solid solution phase is formed on the surface of the member after hot pressing using an alloyed hot-dip Zn plated steel sheet. It has been shown that this hot press member can provide excellent hot press workability (coating layer adhesion), corrosion resistance, and weldability.

British Patent No. 1490535 JP 2003-82436 A Japanese Patent No. 3663145 Japanese Patent No. 4039548

However, when the Zn-plated steel sheet subjected to the Zn-based plating described in Patent Documents 3 and 4 is applied to hot press forming, there are the following problems. That is, in hot press forming, the steel sheet must be heated to the Ac3 point or higher. During this heat treatment, oxide (ZnO) with high electrical resistance is formed on the plating surface, so although resistance spot welding is possible, this is a phenomenon in which a part of the locally overheated steel sheet is melted and scattered. There is a problem that scattering tends to occur and the range of appropriate welding conditions is narrow. For this reason, a step of removing oxides by shot blasting or the like is required before spot welding.

In view of such circumstances, the present invention provides a hot press-formed body excellent in resistance spot weldability that does not require a step of removing oxides by shot blasting or the like, a manufacturing method thereof, and a plated steel sheet for hot press. With the goal.

The present inventors have intensively studied to solve the above problems. As a result, the following knowledge was obtained.

In the heat treatment during hot press forming, if an oxide layer with high electrical resistance is densely formed on the plating surface, it becomes non-energized in resistance spot welding, or scattering is likely to occur. It was found that the appropriate current range is narrow or the appropriate current range cannot be obtained. On the other hand, when the oxide layer has a concavo-convex shape and has a plating layer having a melting point lower than the heat treatment temperature at the time of hot press forming below, a wide appropriate current in resistance spot welding. We found that a range was obtained.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A hot press-molded body with plating obtained by heating a plated steel sheet to a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet, and then simultaneously cooling the plated steel sheet using a mold.
On the surface of the plating, there is an oxide layer having a network-like unevenness with an average roughness Ra of 2 μm or more and an average thickness of 3 μm or less, and under the oxide layer, A hot press-formed body having a plating layer having a melting point equal to or lower than the heating temperature.
[2] The hot press forming according to the above [1], wherein the density of the plating layer before the forming and cooling is higher than the density of the oxide layer formed on the surface of the plating layer after the forming and cooling. body.
In addition, the density of a plating layer is evaluated by the density of main components constituting the plating layer. The main component means a component exceeding 50% by weight. The density of the oxide layer refers to the density of the oxide formed from the main components constituting the plating layer.
[3] A plated steel sheet for hot press-formed bodies, which has a plated layer on the surface of the steel sheet, and the density of the plated layer is higher than the density of oxides formed from the main components constituting the plated layer.
[4] A heat characterized by heating the plated steel sheet for hot press-formed bodies according to the above [3] at a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet and then simultaneously cooling with a mold. A manufacturing method of a press-formed product.
[5] The method for producing a hot press-formed body according to the above [4], wherein the heating is performed at an average temperature rising rate of 50 ° C./s or more.
[6] The method for producing a hot press-molded product according to the above [4], wherein the heating is performed at an average heating rate of 100 ° C./s or more.
[7] The method for producing a hot press-molded product according to the above [4], wherein the heating is performed at an average heating rate of 110 ° C./s or more.

In the present invention, a steel sheet having a plating layer used for a hot press-formed body is generically called a hot-press formed body plated steel sheet. Therefore, the plated steel sheet for hot press-formed bodies of the present invention has a plating layer regardless of whether or not the alloying treatment is performed after the plating treatment. That is, the plated steel sheet for hot press-formed body in the present invention is a hot dip galvanized steel sheet that has not been subjected to alloying treatment, an alloyed hot dip galvanized steel sheet that has been alloyed, It includes galvanized steel sheets, hot dip zinc-aluminum alloy plated steel sheets, aluminum plated steel sheets and the like.

In addition, “%” representing the content of a component means “mass%” unless otherwise specified.

ADVANTAGE OF THE INVENTION According to this invention, the hot press molding excellent in resistance spot weldability which does not require the process of removing an oxide by shot blasting etc. is obtained.
By performing hot press forming using the plated steel sheet for hot press-formed bodies of the present invention, processing can be performed without causing galling or breakage of the steel sheet, and it is not necessary to remove scale such as shot blasting. Cost reduction is possible.

FIG. 1 is a cross-sectional view of a plated layer of a hot press-formed body obtained by hot pressing at an inappropriate heating temperature using a Zn-based plated steel sheet. FIG. 2 is a cross-sectional view of a plated layer of a hot press-formed body using a Zn-based plated steel sheet. FIG. 3 is a cross-sectional view of a plating layer of a hot press-formed body using an Al-based plated steel sheet. FIG. 4 is a diagram used for explaining the press molding method.

The hot press-formed body excellent in resistance spot weldability according to the present invention is obtained by heating a plated steel sheet to a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet, and then forming it using a mold and cooling it at the same time. It is a hot press-molded product. In addition, the surface of the plating has an oxide layer with mesh-like irregularities, and under the oxide layer, there is a plating layer whose melting point is equal to or lower than the heating temperature during hot press molding. It is characterized by. By having such a feature, the hot press-formed body of the present invention has a wide appropriate current range in resistance spot welding and is excellent in resistance spot weldability.

In oxide layer hot press forming with mesh-like irregularities, the plated steel sheet must be heated to ₃ or more points (usually 800 ° C to 1000 ° C) on the plated steel plate, and plating oxide is formed on the plated surface. Is done. In particular, in the case of Zn-based plating, ZnO having a high electric resistance value is formed thick on the surface, and this is the cause, and when the obtained hot press-molded body is resistance spot welded, scattering is likely to occur. The appropriate current range for spot welding may be narrow. For example, in a hot-pressed material in which a Zn-Ni plated steel sheet is heated to 840 ° C for 3 minutes in an air atmosphere and then immediately quenched with a die, the heating temperature is lower than the melting point of Zn-Ni plating. In addition, a flat ZnO layer is formed on the plating surface (Fig. 1), and even if resistance spot welding is performed on a plate assembly in which two hot press materials are stacked, the ZnO layer is energized between the steel plates. Path formation may be hindered and there may be no current flow.

On the other hand, even if the oxide formed on the plating surface of the hot press-molded body is an oxide exhibiting a high electrical resistance, resistance spot welding is performed when the oxide on the plating surface forms a mesh-like unevenness. It becomes possible. This is due to the following reason. A plate assembly in which a hot press-molded body in which the oxide on the plating surface forms a mesh-like unevenness is sandwiched between at least one of the two or more superposed steel sheets is sandwiched between a pair of electrodes, and the pressure is applied. Resistance spot welding is performed while adding. In this case, at the position where the electrode pressing force is applied, the conductive layer is formed by the collapse of the uneven oxide layer, and resistance spot welding becomes possible. From the above, in the present invention, the surface of the plating has an oxide layer having a mesh-like unevenness.

Furthermore, the oxide layer having mesh-like irregularities has an average roughness Ra of 2 μm or more and an average thickness of 3 μm or less. When Ra is less than 2 μm, the unevenness is small and the oxide layer does not collapse sufficiently even when pressed by an electrode, so that it is difficult to secure an energization path. Further preferable Ra is 3 μm or more. In addition, when the average thickness of the oxide layer exceeds 3 μm, even if there is an unevenness with Ra of 2 μm or more, the thickness of the oxide layer is too thick. It becomes difficult to ensure.

The oxide layer having mesh-like irregularities is formed on the surface of the plating while heating the plated steel sheet. The oxide layer formed during this heat treatment is subjected to a high load by subsequent molding. When unevenness exists on the surface of the oxide layer, the unevenness partially collapses due to this load to form a discontinuous oxide layer, and an energization path during resistance spot welding is secured. As a result, scattering is less likely to occur, a wide appropriate current range can be obtained, and resistance spot weldability is further improved. Therefore, it is preferable that a discontinuous oxide layer is present on the plating surface.

The oxide layer having such network-like irregularities uses, for example, plating in which the density of the main component constituting the plating layer is higher than the density of the oxide formed from the main component constituting the plating layer, and It can be formed by heating the heating temperature of the hot press to a temperature higher than the melting point of the plating layer.

This is because when the oxide is formed, that is, when heated, if the density of the oxide is lower than that of the plating layer, volume expansion occurs. When the plating layer under the oxide layer is melted, it is greatly deformed to absorb the volume expansion of the oxide layer, and network-like irregularities are formed. For example, in the case of a Zn-based plating, the density of Zn is the 7.14 g / cm ³ (room temperature), 600 density of 6.81 g / cm ^3, ZnO in the molten state of ℃ is 5.61 g / cm ³ (room temperature) . In this case, when the Zn-based plating surface becomes ZnO, volume expansion occurs, and when heated at a temperature higher than the melting point of the plating layer, as shown in FIG. 2, ZnO network-like irregularities are formed on the plating surface. Is done.

On the other hand, in the case of Al-based plating, the Al density is 2.70 g / cm ³ (room temperature) and the Al ₂ O ₃ density is 3.95 g / cm ³ (room temperature). In the plating, no mesh-like irregularities like Zn-based are formed.

A plating layer whose melting point is lower than the heating temperature during hot press forming has a plating layer that is lower than the heating temperature under the oxide layer without being lost due to oxidation during the heat treatment or diffusion to the steel sheet. If the plating layer melts in the initial stage of resistance spot welding, the oxide layer with high electrical resistance formed on the plating layer collapses, and further collapses from the pressurized part with the molten plating. The oxide layer thus discharged is discharged, and a stable wide energization path is formed. As a result, scattering does not easily occur and a wide appropriate current range can be obtained. Thus, in order to prevent scattering and to obtain a wide appropriate current range, it is necessary to have a plating layer having a melting point below the heating temperature in hot press molding under the oxide layer. It is. Note that having a plating layer means observing the cross section of the plating layer of any 10 fields of view with a SEM backscattered electron image at a magnification of 500 times, and the plating layer is present on 50% or more of the steel sheet surface excluding the oxide layer. Suppose you are.

As described above, an oxide layer having irregularities can be formed by using plating in which the density of the main component constituting the plating layer is higher than the density of the oxide formed from the main component constituting the plating layer. it can. From this point, Zn plating is preferable as the plating. In the case of Zn-based plating, the sacrificial corrosion resistance of Zn to Fe can also have excellent corrosion resistance. In the case where the remainder of the plating layer other than Zn contains components such as Al and Mg, the density of Al and Mg is lower than the density of oxides such as Al and Mg, and there is a tendency to ionize Al and Mg. Since it is larger than Zn and easily oxidizes, components such as Al and Mg are oxidized first, which adversely affects the formation of irregularities on the plating surface. Therefore, it is preferable that the components such as Al and Mg are present in a small amount in the plating phase. For example, Al is preferably 10% or less. More preferably, Al is 0.1% or less. Inevitable impurities are acceptable if Mg: less than 1.0% and Si: less than 1.0%. In this way, Mg: less than 1.0%, Si: less than 1.0%, the adhesion of dross is reduced, the occurrence of cracks in the plating layer during hot press molding is reduced, and the advantage of excellent workability There is.

The plating that can be suitably used is a plating containing Al: 10% or less and Fe: 20% or less in mass%, with the balance being made of Zn and inevitable impurities.

Another plating that can be suitably used is a plating containing Ni: 10 to 25%, with the balance being Zn and inevitable impurities. The adhesion amount is preferably 10 to 90 g / m ² . Any of Ni ₂ Zn ₁₁ , ZiZn ₃ , and Zi ₅ Zn ₂₁ is contained in the component contained in the balance other than Zn by containing Ni that has an ionization tendency lower than that of Zn and does not inhibit the formation of irregularities on the surface of the plating layer. A gamma phase having a crystal structure of 1 and a very high melting point of 881 ° C. is formed. Thereby, excessive zinc oxide formation on the plating layer surface in the heating process can be suppressed. Even if the zinc oxide layer has irregularities, it will hinder the energization path in resistance spot welding, so it is preferable that the zinc oxide layer is thin, and the energization path is more effective by suppressing excessive zinc oxide formation. Secured. Furthermore, since the plating layer remains as a γ phase even after hot press forming is completed, it exhibits excellent perforated corrosion resistance due to the sacrificial anticorrosive effect of Zn, and the γ phase melts at the initial stage of energization of resistance spot welding. Contributes to the expansion and stabilization of routes. Note that the formation of the γ phase at a Ni content of 10 to 25% does not necessarily match the equilibrium diagram of the Ni—Zn alloy, but this is a non-equilibrium progress of the plating layer formation reaction performed by electroplating or the like. It is thought to do.

More preferably, in order from the steel sheet surface, it contains 60% or more of Ni, the balance is made of Zn and inevitable impurities, and the coating layer I has an adhesion amount of 0.01-5 g / m ² and 10-25% Ni. It is preferable that the plating layer has a structure in which the balance is made of Zn and inevitable impurities, and the plating layer II has an adhesion amount of 10 to 90 g / m ² . If the amount of Ni in the plating layer I is less than 60%, it is not possible to sufficiently suppress the diffusion of Zn in the plating layer into the underlying steel sheet, so that excellent perforated corrosion resistance may not be obtained. When the Ni content of the plating layer I is 60% or more, the diffusion of Zn in the plating layer to the underlying steel sheet is suppressed, and many γ phases remain after hot press forming, contributing to the expansion of the appropriate current range for resistance spot welding. To do. The amount of Ni in the plating layer I is preferably 100%. In the case of less than 100%, the balance is Zn and an unavoidable impurity having a sacrificial anticorrosive effect.

Moreover, if the adhesion amount per one side of the plating layer I is less than 0.01 g / m ² , the effect of suppressing the diffusion of Zn into the underlying steel sheet is not sufficiently exhibited. If it exceeds 5 g / m ² , the effect will be saturated and the cost will increase. Therefore, 0.01 to 5 g / m ² is set.

The plating layer II is a plating layer containing 10 to 25% of Ni and the balance of Zn and inevitable impurities. By setting the Ni content of the plating layer II to 10 to 25%, a γ phase having a high melting point of 881 ° C. having a crystal structure of Ni ₂ Zn ₁₁ , NiZn ₃ , or Ni ₅ Zn ₂₁ is formed. Excessive zinc oxide formation on the plating layer surface during the heating process can be suppressed. Even if the zinc oxide layer has irregularities, it will hinder the energization path in resistance spot welding, so it is preferable that the zinc oxide layer is thin, and the energization path is more effective by suppressing excessive zinc oxide formation. Secured. Furthermore, since the plating layer II remains as a γ phase even after the hot press forming is completed, excellent perforated corrosion resistance is exhibited due to the sacrificial anticorrosive effect of Zn. The formation of the γ phase when the Ni content is 10 to 25% by mass does not necessarily match the equilibrium diagram of the Ni—Zn alloy. This is probably because the formation reaction of the plating layer performed by electroplating or the like proceeds in a non-equilibrium manner.

If the adhesion amount per one side of the plating layer II is less than 10 g / m ² , the sacrificial anticorrosive effect of Zn is not sufficiently exhibited. If it exceeds 90 g / m ² , the effect is saturated and the cost is increased. Therefore, it is set to 10 to 90 g / m ² .

The method for forming such plating layer I and plating layer II is not particularly limited. Known electroplating methods are preferred.

The γ phase of Ni ₂ Zn ₁₁ , NiZn ₃ , and Ni ₅ Zn ₂₁ can be confirmed by an X-ray diffraction method or an electron beam diffraction method using TEM (Transmission Electron Microscopy). Further, although the γ phase is formed as described above by setting the Ni content of the plating layer II to 10 to 25 mass%, some η phase may be mixed depending on the conditions of electroplating. At this time, in order to minimize the zinc oxide formation reaction on the surface of the plating layer during the heating process, the amount of η phase is preferably 5% by mass or less. The amount of the η phase is defined by the mass ratio of the η phase to the total mass of the plating layer II, and can be quantified by, for example, the anodic dissolution method.

Next, the plated steel sheet for hot press-formed bodies, which is the base steel sheet for the plating layer, will be described.

The plated steel sheet for hot press-formed bodies of the present invention preferably has a plated layer on the steel sheet surface, and the density of the plated layer is preferably higher than the density of the oxide formed from the main components constituting the plated layer.

Moreover, as described above, the plating layer preferably contains, by mass%, Al: 10% or less, Fe: 20% or less, and the balance of Zn and inevitable impurities.

Further, it is preferable that Ni: 10 to 25% is contained, the balance is made of Zn and inevitable impurities, and the adhesion amount is 10 to 90 g / m ² .

In addition, the plating layer contains, in order from the steel plate surface, in mass%, Ni: 60% or more, the balance consisting of Zn and unavoidable impurities, and an adhesion amount of 0.01 to 5 g / m ² and mass. It is preferable to have a plating layer II containing Ni: 10 to 25%, the balance being Zn and inevitable impurities, and having an adhesion amount of 10 to 90 g / m ² .

Hot-rolled steel sheets and cold-rolled steel sheets can be used as the plated steel sheets for hot press-formed bodies.

Component composition is 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, with the balance being Fe and inevitable impurities. Preferably, it contains at least one selected from Cr: 0.01 to 1%, Ti: 0.2% or less, and B: 0.0005 to 0.08%, or Sb: 0.003 to 0.03% by mass. By setting it as such a component composition, the tensile strength (henceforth TS may be called) 980 Mpa or more can be provided to a hot press molding.

The reason for limiting the suitable range of each component element will be described below.

C: 0.15-0.5%
C is an element that improves the strength of the steel. In order to increase the TS of the hot-press formed body to 980 MPa or more, the C content 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-press formed body to 980 MPa or more, the Si amount needs to be 0.05% or more. On the other hand, when the amount of Si exceeds 2.0%, the occurrence of surface defects called red scale during hot rolling significantly increases, the rolling load increases, and the ductility of the hot-rolled steel sheet deteriorates. 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: 0.5-3.0%
Mn is an element effective for suppressing the ferrite transformation and improving the hardenability. In addition, since the Ac3 transformation point is lowered, it is an effective element for lowering the heating temperature before hot pressing. In order to exhibit such an effect, the amount of Mn needs to be 0.5% or more. On the other hand, if the amount of Mn exceeds 3.0%, segregation occurs and the uniformity of the properties of the raw steel plate and hot press-formed product decreases. Therefore, the Mn content is 0.5 to 3.0%.

P: 0.1% or less When the amount of P exceeds 0.1%, segregation occurs and the uniformity of the mechanical properties of the steel sheet and the hot press-formed product decreases, and the toughness also decreases significantly. Therefore, the P content is 0.1% or less. More preferably, in order to improve the cross tensile strength of the resistance spot welded portion, the P content is 0.02% or less.

S: 0.05% or less If the amount of S exceeds 0.05%, the toughness of the hot press-formed product is lowered. Therefore, the S content is 0.05% or less.

Al: 0.1% or less When the Al content exceeds 0.1%, blanking workability and hardenability of the steel sheet are deteriorated. Therefore, the Al content is 0.1% or less.

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

The balance is Fe and inevitable impurities.

If necessary, the following elements can be appropriately contained for the following purposes.
Cr: 0.01 to 1%, Ti: 0.2% or less, B: 0.0005 to 0.08% and / or Sb: 0.003 to 0.03%
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 of the Cr content 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 and exhibiting the effect of improving the hardenability by the 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 press-formed product is reduced, so the upper limit of the amount of Ti is preferably 0.2%. .

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 a martensite phase and a bainite phase are generated after hot rolling, resulting in cracks in the steel sheet. The upper limit is preferably 0.08%.

Sb: 0.003-0.03%
Sb has an effect of suppressing a decarburized layer generated in the surface layer portion of the steel plate after the steel plate is heated until the steel plate is cooled by a series of hot press processes. 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 increases and productivity decreases. Accordingly, the upper limit of the Sb amount is preferably 0.03%.

Hot press-formed hot press with excellent resistance spot weldability by heating the plated steel sheet for hot press-formed body composed of the above at a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet and then forming using a mold. A shaped body is produced. Heating is preferably performed at an average temperature increase rate of 50 ° C./s or more. More preferably, the average rate of temperature increase is 100 ° C./s or more, and still more preferably 110 ° C./s or more. In the present invention, the average rate of temperature increase is the average rate of temperature increase from room temperature (20 ° C.) to the heating temperature, ((heating temperature) −room temperature (20 ° C.)) / (Temperature increasing time). Can be sought.

The reason why the heating is performed at a temperature higher than the Ac3 transformation point is to form a hard phase such as a martensite phase by rapid cooling during hot pressing and to increase the strength of the hot press-formed body. The reason why the plating layer is heated to the melting point or higher is to melt the plating layer and form irregularities in the oxide of the plating layer formed on the surface. On the other hand, if the heating temperature exceeds 1000 ° C, a large amount of oxide layer is formed on the surface of the plating layer, and even if unevenness is formed on the oxide layer, it is a very thick oxide that hinders the formation of a current path during resistance spot welding Layers may be formed. Therefore, the upper limit of the heating temperature is preferably 1000 ° C. In addition, the heating temperature here means the highest temperature reached of the steel sheet.

In addition, when the average heating rate during heating is 50 ° C./s or more, the formation of a thick oxide layer on the surface of the plating layer can be suppressed, and resistance spot weldability is further improved. The generation of the oxide layer on the surface of the plating layer increases as the high temperature residence time during which the steel sheet is exposed to high temperature conditions increases. Therefore, the higher the average temperature increase rate, the shorter the high temperature residence time. As a result, generation of an oxide layer on the plating layer surface can be suppressed. In addition, the holding time at the maximum plate temperature is not particularly limited. In order to suppress the formation of the oxide layer, a shorter time is preferable, preferably 300 s or less, more preferably 60 s or less, and still more preferably 10 s or less.

Examples of the heating method include heating by an electric furnace or a gas furnace, flame heating, energization heating, high frequency heating, induction heating, and the like. In particular, energization heating, high-frequency heating, induction heating, and the like are suitable for setting the average temperature rising rate to 50 ° C./s or more.
In addition, after the hot press molding, the hot press molded product can be taken out from the mold and cooled using a liquid or a gas.

As a base steel plate, C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.01%, S: 0.01%, Al: 0.03%, N: 0.005%, Cr: 0.2%, Ti: A cold-rolled steel sheet containing 0.02%, B: 0.0022%, Sb: 0.008%, the balance being composed of Fe and inevitable impurities, an Ac3 transformation point of 820 ° C, and a sheet thickness of 1.2 mm was used. . On the surface of this cold-rolled steel sheet, seven types of plating listed in Table 1 (alloyed hot-dip Zn plating (GA), hot-dip Zn plating (GI), electric pure Zn plating (EG), electric Zn alloy plating (Zn—Ni ), Zn-Al plating (Zn-Al), Zn-Al-Si plating (Zn-Al-Si), Al-Si plating (Al-Si)), and during resistance spot welding after hot press forming The appropriate current range was investigated. The melting point of alloyed hot-dip Zn plating (GA) is about 670 ° C. in the main δ1 phase, the melting point of hot-dip Zn plating (GI) and electric pure Zn plating (EG) is about 420 ° C., and electric Zn alloy plating (Zn -Ni) has a melting point of about 800 ° C to 880 ° C, Zn-Al plating (Zn-Al) has a melting point of about 380 ° C to 400 ° C, and Zn-Al-Si plating (Zn-Al-Si) has a melting point of about 570 The melting point of Al—Si plating (Al—Si) is about 660 ° C., which is about 660 ° C. because it is alloyed with Fe during heating.

The above steel sheet was heated to 900 ° C. in the atmosphere for 180 seconds, removed from the furnace without being held at 900 ° C., air-cooled to 700 ° C. in the atmosphere, and immediately shown in FIG. A hot press-molded body was produced by drawing using a press molding method as shown in FIG. The punch width when drawing was 70 mm and the processing height was 30 mm.

Next, a sample is taken from the flat part of the head of the hot press-molded body obtained as described above, the presence or absence of an oxide layer having a network-like unevenness on the surface of the plating layer, the average roughness of the plating surface, the oxide layer The remaining thickness of the plating layer under the oxide layer was confirmed.

The presence or absence of mesh-like irregularities on the surface of the plating layer was determined by observing the surface with an SEM. The case where there was unevenness was marked as ◯, and the case where there was no unevenness was marked as x.

The average roughness of the plating layer was measured at 10 appropriate locations using a surface roughness measuring instrument. The average average roughness was defined as “◯” when 3 μm or more, “Δ” when 2 μm or more but less than 3 μm, and “x” when less than 2 μm.

The thickness of the oxide layer was determined by cross-sectional SEM observation of the plating layer. The reflected electron image was photographed by SEM at 10 times with 1500 magnifications, and the average value of the thickness of the oxide layer formed on the outermost surface was determined from the photographed image. The case where 3 μm or less is “◯” and the case where it exceeds 3 μm is “×”. At the same time, the presence or absence of the plating layer was confirmed from the image. The product in which the plating layer remained on 50% or more of the steel sheet surface excluding the oxide layer was indicated as “◯”, and the case where it was less than 50% was indicated as “X”.

A welding test was performed on the hot press-formed body obtained as described above. For the welding test, an inverter DC resistance spot welder was used, and welding was performed with a chrome-copper DR electrode (electrode tip diameter: 6 mm) at a pressure of 450 kgf and an energization time of 340 msec. The nugget diameter was 4√t (t: plate thickness) The appropriate current range defined from the current value (mm) to the occurrence of scattering was determined. An appropriate current range of 1 kA or more was indicated as “good”, and “1.5” or higher was indicated as “◎”, indicating an even better weldability. An appropriate current range of less than 1 kA is indicated as “x”.

The results obtained as described above are shown in Table 1 together with the conditions.

From Table 1, in the present invention examples Nos. 1 to 5, an appropriate current range of 1 kA or more is secured. In particular, No. 4 that does not contain Al has an average roughness (Ra) of 3 μm or more, and an appropriate welding current range of 1.5 kA or more, indicating a superior resistance spot weldability.

On the other hand, in No. 6 of the comparative example in which the net-like unevenness was not formed due to the high Al content, an appropriate current range of 1 kA could not be secured. In No. 7 Al—Si plating, which is most frequently used at present, a network-like unevenness is not formed, but an appropriate current range of 1 kA or more is obtained. This is because the plating layer becomes an Al—Fe alloy layer during heating, the amount of Al oxide formed is small, and Zn is not contained, so that highly resistive ZnO is not formed. In this case, the weldability is excellent, but since the sacrificial anticorrosive action of Zn cannot be obtained, there is a disadvantage that the corrosion resistance is poor.

As a base steel plate, C: 0.23%, Si: 0.25%, Mn: 1.2%, P: 0.01%, S: 0.01%, Al: 0.03%, N: 0.005%, Cr: 0.2%, Ti: A cold-rolled steel sheet containing 0.02%, B: 0.0022%, Sb: 0.008%, the balance being composed of Fe and inevitable impurities, an Ac3 transformation point of 820 ° C, and a sheet thickness of 1.2 mm was used. . The surface of this cold-rolled steel sheet is electroplated in a plating bath containing 200 g / L nickel sulfate hexahydrate and 0-50 g / L zinc sulfate heptahydrate at a pH of 3.0 and a temperature of 50 ° C. As a result, a plating layer I having an Ni content of 100% (mass%) and an adhesion amount of 0.05 g / m ² was formed. Next, electroplating was performed in a plating bath containing 200 g / L nickel sulfate hexahydrate and 10 to 100 g / L zinc sulfate heptahydrate at a pH of 1.5 and a temperature of 50 ° C. A plating layer II having a content of 12% and an adhesion amount of 60 g / m ² was formed. Next, the plated steel sheet was heated under the conditions shown in Table 2 by electric heating or furnace heating, and molded and cooled under the same conditions as in Example 1 to produce a hot press-formed body.

With respect to the hot press-formed body obtained as described above, in the same manner as in Example 1, the evaluation criteria, the presence or absence of network irregularities on the surface of the plating layer, the average roughness of the plating surface, the thickness of the oxide layer, The remaining plating layer under the oxide layer was confirmed.

In addition, a welding test was performed on the hot press-formed body obtained as described above under the same conditions as in Example 1. The evaluation criteria were expressed as “◯” with an appropriate current range of 1 kA or more as “good” weldability, “◎” when 1.5 kA or more was further improved as weldability, and “◎◎” when 2.0 kA or more was further improved as weldability. An appropriate current range of less than 1 kA is indicated as “x”.
The same as in the first embodiment.

The results obtained are shown in Table 2 together with the conditions.

From Table 2, it can be seen that an appropriate current range of 1 kA or more can be secured in the present invention example.
In basis weight is small and 15g / m ² No.1,6,11,16, since the Zn-Ni plating layer does not have a plated layer disappears by diffusion into the oxide and base metal, the proper current range 1kA Cannot be secured. Even with a basis weight of 60 g / m ² , No. 27, which has a long holding time, does not have a proper current range because there is no plating layer. In No. 21, where the heating temperature is as low as 840 ° C., the plating layer does not melt, so that the unevenness of the oxide layer is not formed, and an appropriate current range cannot be secured.

Claims

A hot press-molded body with plating, obtained by heating a plated steel sheet to a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet, and then simultaneously cooling the plated steel sheet with a mold,
On the surface of the plating, there is an oxide layer having a network-like unevenness with an average roughness Ra of 2 μm or more and an average thickness of 3 μm or less, and under the oxide layer, A hot press-formed body having a plating layer having a melting point equal to or lower than the heating temperature.
The hot press-formed body according to claim 1, wherein the density of the plating layer before the molding and cooling is higher than the density of the oxide layer formed on the surface of the plating layer after the molding and cooling.
A plated steel sheet for hot press-formed bodies, which has a plated layer on the surface of the steel sheet, and the density of the plated layer is higher than the density of an oxide formed from the main components constituting the plated layer.
A hot press-formed body, wherein the plated steel sheet for hot press-formed body according to claim 3 is heated at a temperature equal to or higher than the Ac3 transformation point of the plated steel sheet, and then simultaneously cooled with a mold. Manufacturing method.
The method for producing a hot press-formed body according to claim 4, wherein the heating is performed at an average temperature increase rate of 50 ° C / s or more.
The method for producing a hot press-formed body according to claim 4, wherein the heating is performed at an average temperature rising rate of 100 ° C / s or more.
The method for producing a hot press-formed body according to claim 4, wherein the heating is performed at an average temperature rising rate of 110 ° C / s or more.