JP2023000523A - Hot press member, production method thereof, and steel plate for hot press - Google Patents

Abstract

To provide a hot press member, which is excellent in LME resistance at the time of welding after hot press and corrosion resistance, a production method thereof, and a steel plate for hot press.SOLUTION: A hot press member, on at least one surface of a steel plate, comprises: an intermetallic compound phase including one or two or more elements of Cu, Ni, and Zr in order of from the surface of the steel plate and Zn; and an oxide layer including Zn. The hot press member also has characteristics in which: the total concentration of the one or two or more elements of Cu, Ni, and Zr in the intermetallic compound phase is 40-70 mass%; the melting point of the intermetallic compound phase is 800°C or higher; and the total content of the Zn included in the intermetallic compound phase and the Zn included in the oxide layer is 10-120 g/m2.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a hot press member, a method for manufacturing the same, and a steel plate for hot press. In particular, the present invention relates to a hot press member excellent in weld LME properties and post-coating corrosion resistance, a method for producing the same, and a steel plate for hot press.

In recent years, in the field of automobiles, material steel sheets have been promoted to have higher performance and lighter weight, and the use of high-strength hot-dip galvanized steel sheets or electro-galvanized steel sheets having antirust properties is increasing. However, in many cases, as the strength of the steel sheet increases, its press formability decreases, making it difficult to obtain a complicated part shape. For example, in automobile applications, parts that require rust resistance and are difficult to form include underbody members such as chassis and structural members for frames such as B-pillars.

Against this background, in recent years, there has been a rapid increase in the production of automobile parts by hot pressing, which is easier to achieve both press formability and higher strength than cold pressing. Various techniques for solving the problems have been disclosed.

As a plated steel sheet for hot press, for example, there is a Zn-based plating such as Zn-Ni, which has a high melting point of the plating layer and does not cause red rust in the initial stage of corrosion. A manufacturing method thereof has been proposed.

For example, Patent Document 1 discloses a hot pressed member having an α-Fe (Zn, Ni) mixed crystal, an intermetallic compound of Zn, Ni and Fe, and a layer containing Mn.

Patent document 2 discloses a hot press member having a Ni diffusion region, an intermetallic compound layer corresponding to the γ phase of a Zn—Ni alloy, and a ZnO layer. The hot pressed members disclosed in Patent Document 1 and Patent Document 2 are both hot pressed members manufactured by heating a Zn—Ni alloy plated steel sheet, and have corrosion resistance without coating, zinc phosphate-based It is also described that the post-coating corrosion resistance of a member subjected to electrodeposition coating after chemical conversion treatment is excellent.

Japanese translation of PCT publication No. 2013-503254 JP 2011-246801 A

However, in the automotive industry, Zn-based plating has the disadvantage that LME (Liquid Metal Embrittlement) is more likely to occur during spot welding of members than multi-component plating that does not contain Zn. In particular, Zn-based plating for hot pressed parts receives heat input twice during hot pressing and welding. more disadvantageous.

As a countermeasure for these problems, the use of Al-Si plating can be considered. , is disadvantageous in terms of cost and productivity.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a hot pressed member having excellent LME resistance and excellent corrosion resistance during welding after hot pressing, and a method for manufacturing the same. Another object of the present invention is to provide a steel sheet for hot pressing for realizing the hot pressing member and the manufacturing method thereof.

In order to achieve the above object, the present inventors have conducted intensive research and obtained the following findings.
(1) In order to obtain a hot press member having good LME resistance, an intermetallic compound phase containing one or more of Cu, Ni and Zr and Zn is formed on the surface of the hot press member. , it is effective to define the content of Zn at a predetermined amount after forming under the conditions of a predetermined element concentration and melting point. The intermetallic compound phase has a high melting point, and by further adding a predetermined intermetallic compound phase, it is possible to reduce the unevenness of the interface between the oxide layer and the steel sheet, and as a result, the propagation of cracks due to LME is suppressed. Furthermore, since the uneven structure is small, stress concentration does not occur, and the frequency of occurrence of LME can be reduced.
(2) The hot press member having the above characteristics contains one or more elements of Cu, Ni and Zr and unavoidable impurities in order from the steel sheet surface, and has a predetermined content Pre-plating layer. obtained by using a Zn-based plated steel sheet.

The present invention is based on the above findings, and has the following features.
[1] For at least one surface of the steel plate, in order closer to the steel plate surface,
A hot press member comprising an intermetallic compound phase containing one or more elements selected from Cu, Ni, and Zr and Zn, and an oxide layer containing Zn,
The total concentration of one or more elements of Cu, Ni and Zr in the intermetallic compound phase is 40 to 70% by mass, and the intermetallic compound phase has a melting point of 800 ° C. or higher,
A hot press member, wherein the sum of the Zn content in the intermetallic compound phase and the Zn content in the oxide layer is 10 to 120 g/m ² .
[2] For at least one surface of the steel plate, in order closer to the steel plate surface,
a pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, and having a total content of the elements of 10 to 70 g/m ² ;
A steel plate comprising a Zn-based plating layer having a Zn content of 10 to 120 g/m ² ,
The method for producing a hot pressed member according to [1], wherein the hot pressing is performed after heating to a temperature range of Ac ₃ transformation point to 1000°C.
[3] For at least one surface of the steel plate, in order closer to the steel plate surface,
a pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, and having a total content of the elements of 10 to 70 g/m ² ;
A steel sheet for hot pressing, comprising: a Zn-based plating layer having a Zn content of 10 to 120 g/m ² .

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to obtain a hot-pressed member having excellent corrosion resistance and excellent LME resistance during welding after hot-pressing.

FIG. 1 is a schematic diagram showing a cross section of a steel sheet for hot pressing. FIG. 2 is a schematic diagram showing a cross section of a hot pressed member.

Embodiments of the present invention will be described below. The following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. In addition, the unit of content of each element in the steel component composition is "% by mass", and hereinafter, unless otherwise specified, it is simply indicated by "%". By subjecting the steel plate for hot press described below to hot pressing by the method described below, the hot press member described below is obtained. The hot pressed member and the method for manufacturing the hot pressed member will be described in this order.

1) Steel plate for hot press The steel plate for hot press of the present invention is a steel plate before undergoing a hot press process, having a cross section shown in FIG. , a pre-plating layer 14 containing one or more elements of Cu, Ni and Zr in order of proximity to the steel sheet surface, the total content of the elements being 10 to 70 g / m ² , and a Zn content and a Zn-based plating layer 16 of 10 to 120 g/m ² .

A pre-plating layer containing one or more elements of Cu, Ni and Zr and having a total content of the elements of 10 to 70 g/m ² and the Zn-based plating layer. The components are mainly one or more of Cu, Ni and Zr, but may contain other coexisting elements and unavoidable impurities. If this pre ^- plating is too thick, not only is it disadvantageous in terms of cost, but it also becomes impossible to obtain the desired intermetallic compound phase after hot pressing. . Conversely, if the thickness is too thin, the effect of suppressing the diffusion of Zn during hot pressing is reduced, and LME is likely to occur. For this reason, it should be 10 g/m ² or more. It is preferably 30 g/m ² or more. The total content of the elements described here means the total amount of the elements deposited in the pre-plating process.

The Zn content of the Zn-based plating layer is 10 to 120 g/m ²
The Zn-based plating layer is a plating layer mainly composed of Zn formed on the pre-plating. By supplying Zn, which has a sacrificial anti-corrosion effect, to the surface of the steel sheet, it works to improve corrosion resistance. If the Zn-based plating layer is too thin, the corrosion resistance is lowered, so the Zn content of the Zn-based plating layer is set to 10 g/m ² or more. Conversely, if the Zn-plated layer is too thick, the surface smoothness and formability during pressing deteriorate. Therefore, the Zn content of the Zn-based plating layer is set to 120 g/m ² or less.
Zn is an element supplied as a main component of plating applied on a steel sheet. Plating methods include hot dip plating, electroplating, electroless plating, and sputtering. In addition to pure Zn plating such as EG and GI, Zn—Ni, Zn—Al, Zn—Al—Mg, Zn—Sn, GA, etc. may be plated as a Zn alloy. The Zn content described here means the amount of Zn adhered in the plating process.

In addition, as a steel sheet for hot press, in order to obtain a hot press member having a tensile strength after hot press of 1.5 GPa or more, for example, C: 0.20% to 0.35% in terms of mass%, Si: 0.1% to 0.5%, Mn: 1.0% to 3.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002% to 0.005%, Cr: 0.1% to 0.3%, Sb: 0.05% 003% to 0.03%, the balance being unavoidable impurities.

The manufacturing method of the steel sheet for hot press is as follows. For example, after heating the steel material at 1150 ° C., hot rolling is performed, the end temperature of the hot rolling is set to 840 ° C., coiling is performed at 650 ° C., pickling is performed, cold rolling is performed, and 30 g / m ² and the Zn plating of the electroplating described above.

2) Hot pressed member The hot pressed member of the present invention has a cross section shown in FIG. A hot press member comprising an intermetallic compound phase 22 containing one or more elements and Zn, and an oxide layer 24 containing Zn, wherein the intermetallic phase contains Cu, Ni and Zr. The total concentration of one or more of the elements is 40 to 70% by mass, the melting point of the metal compound phase is 800 ° C. or higher, and the Zn content contained in the intermetallic compound phase and the oxide The hot press member is characterized by having a total content of Zn contained in the layer of 10 to 120 g/m ² .

It is necessary to provide an intermetallic compound phase containing one or more elements of Cu, Ni and Zr and Zn, and an oxide layer containing Zn on at least one surface of the steel sheet in order from the surface of the steel sheet. However, they may be provided on both sides of the steel plate. Below, what was provided in one surface of a steel plate is described.

The total concentration of one or more elements of Cu, Ni and Zr in the intermetallic compound phase containing one or more elements of Cu, Ni and Zr and Zn is 40 to 70% by mass.
The intermetallic compound phase is a phase in which one or more of Cu, Ni and Zr supplied as pre-plating and Zn supplied as plating form an intermetallic compound by hot pressing. be. Formed between the steel sheet and the oxide layer, it mainly retains Zn, which is a plating component, and contributes to improving the corrosion resistance of the steel sheet. The intermetallic compound phase does not need to have a single composition, and multiple intermetallic compound phases may be mixed in a lamellar or sea-island state. It may also contain a solid solution phase or a phase of an elemental metal. Compositions of main intermetallic compound phases include CuZn, NiZn, ZrZn, and the like.

When a steel sheet is coated with a normal Zn-based plating layer, Zn diffuses into the steel sheet during hot pressing, and Zn, which has a low melting point, liquefies and enters Fe grain boundaries. As a result, the interface between the plating of the hot pressed member and the steel sheet has an uneven structure, and an Fe diffusion phase having Zn-rich grain boundaries is formed. When this member is welded, Zn penetrates deeper into the Fe diffusion phase due to heat input, and the crack depth of the LME that occurs along the Zn increases. By concentrating in the depressions of the diffusion phase, the frequency of LME also increases. However, in the present invention, after forming a predetermined amount of a pre-plating layer containing one or more elements of Cu, Ni and Zr and a Zn-based plating layer on a steel sheet for hot press, the Ac ₃ transformation point ~ This can be achieved by hot pressing in the temperature range of 1000°C.

The reason why Cu, Ni, and Zr are included in the intermetallic compound phase is that each element has a function of improving LME resistance after hot pressing. Cu, Ni, and Zr, which are the constituent components of pre-plating, have a small amount of solid solution in Fe, which is the main component of the steel sheet, and have a high melting point by themselves. It is thought that there are Then, since the layer structure of the pre-plating acts as a physical barrier against Zn, the diffusion of Zn to the steel sheet side is suppressed, making it difficult to form an Fe diffused phase containing a large amount of Zn and having irregularities. . As a result, even if this member is welded, the amount of Zn present in the Fe diffusion phase and its grain boundaries is small, so the propagation of cracks due to LME is suppressed. The frequency is also expected to decrease. Furthermore, even if Zn and Cu, Ni and Zr form an intermetallic compound during hot pressing, by specifying the melting point of the compound at 800 ° C or more, can be taken longer, and as a result, it is thought that the progress of LME accompanying the diffusion of Zn can be suppressed. Among Cu, Ni and Zr, Zr is the most excellent in the above effect, followed by Ni and Cu in that order.

The total concentration of one or more of Cu, Ni and Zr is 40-70% by mass. Here, the total concentration is the sum of the mass % concentrations of Cu, Ni and Zr in the intermetallic compound. If the total concentration exceeds 70%, the ratio of Zn in the metal compound is reduced and the corrosion resistance is lowered, so the total concentration is made 70% or less. The total concentration is preferably 60% or less. On the other hand, if the total concentration is less than 40%, the effect of suppressing LME during hot pressing is reduced, so the total concentration is made 40% or more. The total concentration is preferably 50% or more.

The melting point of the intermetallic compound phase is 800° C. or higher The melting point is the temperature at which the intermetallic compound starts to melt. When the melting point is low, Zn tends to diffuse toward the steel sheet, and LME tends to occur. If the melting point of the intermetallic compound phase is less than 800°C, it is difficult to suppress the diffusion of Zn to the steel sheet side. Therefore, the melting point of the intermetallic compound phase is set to 800° C. or higher.

The total Zn content of Zn contained in the intermetallic compound phase and Zn contained in the oxide layer is 10 to 120 g/m ²
The total Zn content is the total amount of Zn applied as plating on the steel sheet before hot pressing. Since Zn does not evaporate even when hot pressing is performed, the total amount of Zn applied as plating on the steel sheet before hot pressing is contained in the intermetallic compound phase of the hot pressed member as it is. It corresponds to the total Zn content of the Zn contained in the oxide layer and the Zn contained in the oxide layer. If the total Zn content is less than 10 g/m ² , the corrosion resistance deteriorates, so the total Zn content is made 10 g/m ² or more. If the total Zn content exceeds 120 g/m ² , the cost becomes disadvantageous, so the total Zn content is made 120 g/m ² or less. The Zn content is measured by ICP emission spectrometry of a chemically dissolved solution.
The oxide layer is a layer in which Zn, which is supplied as plating, forms an oxide through reaction with oxygen during hot pressing. Although the main component is ZnO, the valence of the oxide may change, or an element other than Zn may be included.

A hot pressed member is a steel plate subjected to hot pressing. The hot press conditions can be arbitrarily set as long as the conditions described later are satisfied.
3) Method for manufacturing hot pressed member The method for manufacturing a hot pressed member according to the present invention includes adding one or more of Cu, Ni, and Zr to at least one surface of a steel plate in order of proximity to the steel plate surface. A steel sheet comprising a pre _- plating layer containing elements and having a total content of the elements of 10 to 70 g/m ² and a Zn ^- based coating layer having a Zn content of 10 to 120 g/m After heating to a temperature range of up to 1000° C., hot pressing is performed.

After heating to a temperature range of Ac ₃ transformation point to 1000 ° C. _, hot pressing is performed. It is preferable to set it as ₃ or more transformation points. Conversely, if the heating temperature exceeds 1000° C., the intermetallic compound phase may disappear, so the heating temperature is preferably 1000° C. or lower. In addition, the heating time is preferably 950° C. or less from the viewpoint of cost, and the cooling method after heating can be carried out by a method such as mold cooling or oil cooling. The cooling rate is preferably 15°C/second or more up to 400°C.

EXAMPLES The present invention will be specifically described below based on examples. The following examples are not intended to limit the present invention, and appropriate modifications within the scope of the gist and configuration are included in the scope of the present invention.

As a substrate steel plate, in mass%, C: 0.33%, Si: 0.25%, Mn: 1.9%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.02%, B: 0.002%, Cr: 0.2%, Sb: 0.008%, the balance being Fe and unavoidable A cold-rolled steel sheet having a thickness of 1.4 mm and having a chemical composition consisting of organic impurities was used (Ac ₃ =730° C.). Incidentally, in the present invention, the Ac ₃ point was determined by thermodynamic calculation software Thermo-Calc.

After forming any one of Cu, Ni and Zr pre-plating layers on the base steel sheet by electroplating, any one of pure Zn plating, Zn--Ni plating, GA plating and Zn--Al plating was formed. The plating was formed by electroplating for pure Zn and Zn--Ni plating, and by hot-dip plating for GA plating and Zn--Al plating. The plating content was obtained by quantifying the chemically dissolved solution by ICP emission analysis. In the ICP emission analysis, a solution containing Cu, Ni, Zr, and Zn at known concentrations was measured, and the content was calculated from the calibration curve obtained from the correlation between concentration and emission intensity.

Plating by electroplating and pre-plating were performed using a fluid bath. The bath conditions for pre-plating are as follows: bath composition: 225 g/L of any one of copper sulfate pentahydrate, nickel sulfate hexahydrate, and zirconium sulfate tetrahydrate; sulfuric acid 50 g/L; temperature: 40°C. , the flow velocity was 1 m/s, and the current density was 10 A/dm ² . The bath conditions for plating are as follows: for pure Zn plating, the bath composition is 440 g/L of zinc sulfate heptahydrate, the pH is 1.9, the temperature is 50°C, the flow rate is 1 m/s, and the current density is 25 A/ ^dm2 . , In Zn-Ni plating, the bath composition is 1150 g/L of zinc sulfate heptahydrate, 2450 g/L of nickel sulfate hexahydrate, and 530 g/L of sodium sulfate, the pH is 1.5, the temperature is 50 ° C., and the flow rate is 1 m/s and a current density of 50 A/dm ² .

Plating by hot-dip plating was performed using a lab-scale hot-dip plating tank. After pre-plating under the above conditions, in GA plating, plating is performed at a bath composition of 99.87% by weight of zinc and 0.13% by weight of aluminum at a bath temperature of 460 ° C., and then alloying treatment at 510 ° C. did In the Zn--Al plating, plating was performed with a bath composition of 95.0% by weight of zinc, 4.5% by weight of aluminum and 0.5% by weight of magnesium at a bath temperature of 450.degree.

After plating, the steel sheet is heated in a laboratory-scale muffle furnace for 2 minutes and 30 seconds to a maximum of 900°C, cooled in a mold to 300°C, and then air-cooled to room temperature to simulate a hot press member. was made. Tables 1-1 and 1-2 show sample levels. The intermetallic compound and oxide composition of the sample were identified by XRD, and the adhesion amount and the ratio of components other than Zn in the intermetallic compound phase were confirmed by ICP emission spectrometry. XRD measurements were performed by shearing the sample to 30 mm×30 mm and analyzing the spectrum obtained by CuKα or MoKα radiation. Also, the melting point of the metal compound phase was determined by thermodynamic calculation software Thermo-Calc.

<LME evaluation during welding>
After shearing the obtained sample to 100 mm × 35 mm, it is overlapped with 980 GA material of the same size, the hitting angle is 5 degrees, the pressure is 3.5 kN, 6.0 kA, the initial pressure time is 600 ms, the energization time is 21 cyc, and the hold time is 1 cyc. spot welding was performed. After that, the cross section in the vicinity of the nugget was polished, and the vicinity of the nugget was observed at a magnification of ×100 using an optical microscope. For evaluation, the maximum length of each LME generated was measured and judged according to the following criteria. Evaluation results are shown in Tables 1-1 and 1-2.
◎: LME not generated ○: LME length < 5 μm
×: 5 µm ≤ LME length

<Corrosion resistance after painting>
After shearing the obtained sample to 100 mm x 100 mm, it was subjected to zirconium-based chemical conversion treatment and electrodeposition coating. The zirconium chemical conversion treatment was performed under standard conditions using PLM2100 manufactured by Nihon Parkerizing Co., Ltd. Electrodeposition coating was carried out by using GT100V manufactured by Kansai Paint Co., Ltd. so that the coating film thickness was 10 μm, followed by baking at 170° C. for 20 minutes. Next, the samples subjected to the zirconium-based chemical conversion treatment and the electrodeposition coating were cross-cut, subjected to a corrosion test (SAE-J2334), and the state of corrosion after 30 cycles was evaluated.

Evaluation was performed by measuring the maximum bulging width on one side from the cross-cut and making judgments according to the following criteria. Evaluation results are shown in Tables 1-1 and 1-2.
◎: Maximum swelling width on one side <2.0 mm
○: 2.0 mm ≤ one side maximum swelling width < 4.0 mm
×: 4.0 mm ≤ maximum swelling width on one side

From the results in Tables 1-1 and 1-2, the hot pressed member of the present invention has excellent corrosion resistance and excellent LME resistance. Further, by applying the method for manufacturing a hot press member to the steel plate for hot press of the present invention, it is possible to obtain the hot press member of the present invention that is excellent in LME resistance while ensuring post-coating corrosion resistance.

10 Steel plate for hot pressing 12 Steel plate 14 Pre-plating layer 16 Zn-based plating layer 20 Hot pressing member 22 Intermetallic compound phase 24 Oxide layer

Claims (3)

For at least one surface of the steel plate, in order close to the steel plate surface,
A hot press member comprising an intermetallic compound phase containing one or more elements selected from Cu, Ni, and Zr and Zn, and an oxide layer containing Zn,
The total concentration of one or more elements of Cu, Ni and Zr in the intermetallic compound phase is 40 to 70% by mass, and the intermetallic compound phase has a melting point of 800 ° C. or higher,
A hot press member, wherein the sum of the Zn content in the intermetallic compound phase and the Zn content in the oxide layer is 10 to 120 g/m ² . For at least one surface of the steel plate, in order close to the steel plate surface,
a pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, and having a total content of the elements of 10 to 70 g/m ² ;
A steel plate comprising a Zn-based plating layer having a Zn content of 10 to 120 g/m ² ,
2. The method for producing a hot press member according to claim 1, wherein the hot pressing is performed after heating to a temperature range of Ac ₃ transformation point to 1000° C. For at least one surface of the steel plate, in order close to the steel plate surface,
a pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, and having a total content of the elements of 10 to 70 g/m ² ;
A steel sheet for hot pressing, comprising: a Zn-based plating layer having a Zn content of 10 to 120 g/m ² .

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