JP7537376B2 - Hot-pressed member, its manufacturing method, and hot-pressed steel plate - Google Patents

Description

The present invention relates to a hot-pressed member, its manufacturing method, and a steel plate for hot pressing. In particular, the present invention relates to a hot-pressed member having excellent weld LME properties and corrosion resistance after painting, its manufacturing method, and a steel plate for hot pressing.

In recent years, the automotive industry has seen a promotion of weight reduction while improving the performance of steel sheets, leading to increased use of rust-resistant high-strength hot-dip galvanized steel sheets or electrolytic galvanized steel sheets. However, in many cases, as the strength of steel sheets increases, their press formability decreases, making it difficult to obtain complex part shapes. For example, in automotive applications, parts that require rust resistance and are difficult to form include chassis and other undercarriage components and frame structural components such as B-pillars.

Against this background, the manufacture of automotive parts using hot pressing, which is easier to achieve both press formability and high strength compared to cold pressing, has rapidly increased in recent years, and various technologies have been disclosed to solve the various issues with hot pressing technology.

As plated steel sheets for hot pressing, for example, Zn-based plating, such as Zn-Ni, exists, which has a high melting point for the plating layer and does not produce red rust in the early stages of corrosion, and various hot pressing members using this steel sheet and methods for manufacturing them have 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-pressed member having a Ni diffusion region, an intermetallic compound layer equivalent to the gamma phase of a Zn-Ni alloy, and a ZnO layer. The hot-pressed members disclosed in Patent Documents 1 and 2 are both hot-pressed members manufactured by heating a Zn-Ni alloy-plated steel sheet, and are described as having excellent corrosion resistance in an unpainted state and excellent corrosion resistance after painting for members that have been subjected to a zinc phosphate-based conversion treatment and then electrocoated.

Special table 2013-503254 publication JP 2011-246801 A

However, in the automotive industry, Zn-based plating has the disadvantage that it is more susceptible to LME (Liquid Metal Embrittlement) when spot welding parts than other component platings that do not contain Zn. In particular, Zn-based plating used for hot press parts is subjected to heat input twice, during hot pressing and welding, so Zn diffuses into the steel more than with Zn-based plating on cold-rolled steel sheets, making it more disadvantageous to LME.

One solution to these problems is the use of Al-Si-based plating, but since Al-Si plating produces red rust from the early stages of corrosion, it is less aesthetically pleasing, and when used on automobile bodies, an additional process is required to prevent red rust, which is disadvantageous in terms of cost and productivity.

The present invention has been made in consideration of the above problems, and aims to provide a hot-pressed member that has excellent LME resistance during welding after hot pressing and excellent corrosion resistance, and a manufacturing method thereof. It also aims to provide a hot-press steel plate for realizing the above hot-pressed member and manufacturing method thereof.

In order to achieve the above object, the present inventors have conducted extensive research and obtained the following findings.
(1) In order to obtain a hot-pressed member with good resistance to LME, it is effective to form an intermetallic compound phase containing one or more elements selected from Cu, Ni, and Zr and Zn on the surface of the hot-pressed member under conditions that result in a predetermined element concentration and melting point, and then specify the content of Zn at a predetermined amount. By providing an intermetallic compound phase with a high melting point and a predetermined intermetallic compound phase, the unevenness of the interface between the oxide layer and the steel sheet can be reduced, and as a result, the progression of cracks due to LME is suppressed, and further, stress concentration does not occur due to the small uneven structure, and the frequency of LME occurrence can be reduced.
(2) A hot-pressed member having the above-mentioned characteristics can be obtained by using a Zn-based plated steel sheet to which a pre-plated layer is applied, the pre-plated layer containing, in order from the surface of the steel sheet, one or more elements selected from Cu, Ni and Zr and unavoidable impurities in a predetermined content.

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 of proximity to the steel plate surface,
A hot-pressed member comprising an intermetallic compound phase containing Zn and one or more elements selected from Cu, Ni, and Zr, and an oxide layer containing Zn,
The total concentration of one or more elements selected from Cu, Ni, and Zr in the intermetallic compound phase is 40 to 70 mass%, and the melting point of the intermetallic compound phase is 800° C. or higher,
A hot-pressed member, characterized in that the sum of the Zn content contained in the intermetallic compound phase and the Zn content contained in the oxide layer is 10 to 120 g/ ^m2 .
[2] For at least one surface of the steel plate, in order of proximity to the steel plate surface,
A pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, the total content of said elements being 10 to 70 g/ ^m2 ;
A steel sheet having a Zn-based plating layer having a Zn content of 10 to 120 g/ ^m2 ,
The method for producing a hot-pressed member according to [1], characterized in that the steel sheet is heated to a temperature range of from the _Ac3 transformation point to 1000 ° C. and then hot-pressed.
[3] For at least one surface of the steel plate, in order of proximity to the steel plate surface,
A pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, the total content of said elements being 10 to 70 g/ ^m2 ;
A steel sheet for hot press use, comprising: a Zn-based plating layer having a Zn content of 10 to 120 g/ ^m2 .

The present invention makes it possible to obtain hot-pressed components that have excellent corrosion resistance and excellent LME resistance during welding after hot pressing, making a significant contribution to industry.

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

The following describes an embodiment of the present invention. Note that the following description shows one preferred embodiment of the present invention, and is not intended to be limiting in any way. The content of each element in the steel composition is expressed in units of "mass %", and hereinafter, unless otherwise specified, will be expressed simply as "%". The hot press member described below can be obtained by hot pressing the hot press steel plate described below using the method described below, so the following describes the hot press steel plate, the hot press member, and the manufacturing method of the hot press member in that order.

The steel sheet for hot press of the present invention is a steel sheet before undergoing a hot press process, and has a cross section as shown in FIG. 1. The steel sheet for hot press is characterized in that it is provided with a pre-plated layer 14 containing one or more elements of Cu, Ni and Zr, in that order from the side closest to the steel sheet surface, in a total content of the elements of 10 to 70 g/ ^m2 , and a Zn-based plated layer 16 having a Zn content of 10 to 120 g/ ^m2 , on at least one surface of the steel sheet 12.

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 The pre-plating layer is a plating layer applied between the steel sheet surface and the Zn-based plating layer before hot pressing. The components are mainly one or more elements of Cu, Ni, and Zr, but may contain other coexisting elements and unavoidable impurities. If this pre-plating is too thick, not only will it be disadvantageous in terms of cost, but it will also be impossible to obtain the desired intermetallic compound phase after hot pressing, so the total content of the elements is set to 70 g/m ² or less. Conversely, if it is too thin, the effect of suppressing the diffusion of Zn during hot pressing will decrease, and LME will be more likely to occur. For this reason, it is set to 10 g/m ² or more. It is preferably 30 g/m ² or more. The total content of the elements described here refers to the total amount of the elements attached in the pre-plating process.

The Zn content of the Zn-based plating layer is 10 to 120 g/ ^m2.
The Zn-based plating layer is a plating layer mainly composed of Zn formed on the pre-plating. It works to improve corrosion resistance by supplying Zn, which has a sacrificial anticorrosive effect, to the steel sheet surface. If this Zn-based plating layer is too thin, the corrosion resistance decreases, so the Zn content of the Zn-based plating layer is set to 10 g/ ^m2 or more. Conversely, if the Zn-plating layer is too thick, the surface smoothness and formability during pressing decrease. For this reason, the Zn content of the Zn-based plating layer is set to 120 g/ ^m2 or less.
Zn is an element supplied as the main component of the plating applied to the steel sheet. The plating method includes hot-dip plating, electroplating, electroless plating, and sputtering. In addition to pure Zn plating such as EG and GI, Zn alloy plating such as Zn-Ni, Zn-Al, Zn-Al-Mg, Zn-Sn, and GA may be used. The Zn content described here refers to the amount of Zn attached in the plating process.

In addition, as the hot press steel sheet, in order to obtain a hot press part having a tensile strength after hot pressing of TS1.5GPa or more, it is possible to use a steel sheet having a composition containing, in mass%, for example, C: 0.20% to 0.35%, 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: 0.003% to 0.03%, with the balance being unavoidable impurities.

The manufacturing method of the hot press steel sheet is as follows: For example, the steel material is heated at 1150°C, then hot rolled, the end temperature of the hot rolling is set to 840°C, the steel is coiled at 650°C, pickled, cold rolled, pre-plated with 30 g/ ^m2 Cu, and electroplated with the above-mentioned Zn.

2) Hot-pressed member The hot-pressed member of the present invention has a cross section as shown in FIG. 2, and is a hot-pressed member having, on at least one surface of a steel plate 12, an intermetallic compound phase 22 containing one or more elements selected from Cu, Ni, and Zr and Zn, and an oxide layer 24 containing Zn, in that order from the side closest to the steel plate surface, wherein the total concentration of one or more elements selected from Cu, Ni, and Zr in the intermetallic compound phase is 40 to 70 mass%, the melting point of the metallic compound phase is 800° C. or higher, and the sum of the Zn content contained in the intermetallic compound phase and the Zn content contained in the oxide layer is 10 to 120 g/m ² .

In order of decreasing proximity to the steel sheet surface, an intermetallic compound phase containing Zn and one or more elements selected from Cu, Ni, and Zr, and an oxide layer containing Zn must be provided on at least one surface of the steel sheet, but they may also be provided on both sides of the steel sheet. The following describes the provision of these on one surface of the steel sheet.

The total concentration of one or more elements selected from Cu, Ni, and Zr in an intermetallic compound phase containing Zn and one or more elements selected from Cu, Ni, and Zr is 40 to 70 mass%.
The intermetallic compound phase is a phase in which one or more elements of Cu, Ni, and Zr supplied as pre-plating and Zn supplied as plating form an intermetallic compound by hot pressing. It is formed between the steel sheet and the oxide layer, and mainly holds 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 be of a single composition, and multiple intermetallic compound phases may be mixed in a lamellar or sea-island shape. It may also contain a solid solution phase or a phase of a single metal. The main compositions of the intermetallic compound phase include CuZn, NiZn, ZrZn, etc.

In the case of a steel sheet having a normal Zn-based plating layer on its surface, the phenomenon occurs in which Zn diffuses into the steel sheet during hot pressing, and Zn, which has a low melting point, liquefies and penetrates into the Fe grain boundaries. As a result, the interface between the plating of the hot press member and the steel sheet has an uneven structure, and an Fe-diffusion phase with 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 becomes deeper. In addition, the stress generated when the electrodes are contacted or opened is concentrated in the recesses of the Fe-diffusion phase, so that the frequency of LME increases. However, the present invention can be achieved by forming a pre-plating layer containing one or more elements selected from Cu, Ni, and Zr, and a Zn-based plating layer in a predetermined amount on the steel sheet for hot pressing, and then hot pressing in a temperature range of _Ac3 transformation point to 1000°C.

The reason for including Cu, Ni, and Zr in the intermetallic compound phase is that all of these elements have the function of improving LME resistance after hot pressing. Cu, Ni, and Zr, which are components of the pre-plating, have a small amount of solid solution in Fe, the main component of the steel sheet, and have a high melting point by themselves, so it is thought that they maintain their layer structure up to a certain degree of high temperature during hot pressing. Then, the layer structure of the pre-plating acts as a physical barrier against Zn, suppressing the diffusion of Zn to the steel sheet side, making it difficult to form an Fe diffusion phase that contains a large amount of Zn and has unevenness. As a result, even if this member is welded, the development of cracks due to LME is suppressed because there is little Zn in the Fe diffusion phase and its grain boundaries, and the uneven structure is small, so stress concentration does not occur, and the frequency of LME occurrence is also thought to be reduced. Furthermore, even if Zn, Cu, Ni, and Zr form an intermetallic compound during hot pressing, by specifying the melting point of the compound at 800°C or higher, it is possible to take a longer time for the plated area to liquefy during welding than without pre-plating, and as a result, it is believed that the progression of LME associated with the diffusion of Zn can be suppressed. Among Cu, Ni, and Zr, Zr has the best effect as described above, followed by Ni and Cu.

The total concentration of one or more of Cu, Ni, and Zr is 40 to 70 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 proportion of Zn in the metal compound decreases and corrosion resistance decreases, so the total concentration is set to 70% or less. The total concentration is preferably set to 60% or less. On the other hand, if the total concentration is less than 40%, the effect of suppressing LME during hot pressing decreases, so the total concentration is set to 40% or more. The total concentration is preferably set to 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. If the melting point is low, Zn is more likely to diffuse into the steel sheet, making LME more likely 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 into the steel sheet. For this reason, the melting point of the intermetallic compound phase is set to 800°C or higher.

The total Zn content of the Zn contained in the intermetallic compound phase and the 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 corresponds to the total Zn content of the Zn contained in the intermetallic compound phase and the Zn contained in the oxide layer in the hot pressed member. If the total Zn content is less than 10 g/m ² , the corrosion resistance decreases, so the total Zn content is set to 10 g/m ² or more. If the total Zn content exceeds 120 g/m ² , it is disadvantageous in terms of cost, so the total Zn content is set to 120 g/m ² or less. The Zn content is measured by ICP emission analysis of a chemically dissolved solution.
The oxide layer is a layer in which Zn supplied as plating reacts with oxygen during hot pressing to form an oxide. The main component is ZnO, but the valence of the oxide may change or an element other than Zn may be included.

The hot-pressed member is a steel plate that has been subjected to hot pressing. The hot pressing conditions can be set arbitrarily as long as the conditions described below are satisfied.
3) Manufacturing method of hot-pressed member The manufacturing method of the hot-pressed member of the present invention involves heating a steel sheet having a pre-plated layer containing one or more elements of Cu, Ni, and Zr, in order of proximity to the steel sheet surface, in a total content of 10 to 70 g/ ^m2 , and a Zn-based plated layer having a Zn content of 10 to 120 g/ ^m2 , on at least one surface of the steel sheet, to a temperature range of from the _Ac3 transformation point to 1000°C, and then hot pressing the steel sheet.

Heat to a temperature range of _Ac3 transformation point to 1000 ° C and then hot press. If the heating temperature is lower than the _Ac3 transformation point, the strength required for the hot press member may not be obtained, so the heating temperature is preferably the _Ac3 transformation point or higher. Conversely, if the heating temperature exceeds 1000 ° C, the intermetallic compound phase may disappear, so the heating temperature is preferably 1000 ° C or lower. From the viewpoint of cost, the heating time is preferably 950 ° C or lower, and the cooling method after heating can be performed by a method such as mold cooling or oil cooling. The cooling rate is preferably 15 ° C / sec or more up to 400 ° C.

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

The base steel sheet used was a cold-rolled steel sheet having a thickness of 1.4 mm and containing, 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%, with the balance being Fe and unavoidable impurities (Ac ₃ = 730°C). In the present invention, the Ac ₃ point was determined using thermodynamic calculation software Thermo-Calc.

After forming a Cu, Ni or Zr pre-plating layer on the above-mentioned base steel sheet by electroplating, a pure Zn plating, a Zn-Ni plating, a GA plating or a Zn-Al plating was formed. Pure Zn and Zn-Ni plating were formed by electroplating, while GA plating and Zn-Al plating were formed by hot-dip plating. The plating content was determined by quantifying the chemically dissolved solution using ICP emission spectrometry. ICP emission spectrometry involves measuring a solution containing known concentrations of Cu, Ni, Zr and Zn, and calculating the content using a calibration curve obtained from the correlation between concentration and emission intensity.

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

Hot-dip plating was carried out using a laboratory-scale hot-dip plating tank. After pre-plating under the above conditions, GA plating was carried out with a bath composition of 99.87% zinc and 0.13% aluminum by weight at a bath temperature of 460°C, and then alloying treatment was carried out at 510°C. Zn-Al plating was carried out with a bath composition of 95.0% zinc, 4.5% aluminum and 0.5% magnesium by weight at a bath temperature of 450°C.

After plating, the steel sheet was heated to a maximum of 900°C in a laboratory-scale muffle furnace for 2 minutes and 30 seconds, cooled to 300°C in a mold, and then air-cooled to room temperature to create a sample simulating a hot-pressed part. The sample standards are shown in Tables 1-1 and 1-2. The intermetallic compound and oxide composition of the sample were identified by XRD, and the deposition amount and the proportion 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 x 30 mm and analyzing the spectrum obtained with CuKα or MoKα radiation. The melting point of the metallic compound phase was also determined using thermodynamic calculation software Thermo-Calc.

<LME evaluation during welding>
The obtained sample was sheared to 100 mm x 35 mm, and then stacked with a 980GA material of the same size, and spot welding was performed under the conditions of an impact angle of 5 degrees, a pressure of 3.5 kN, 6.0 kA, an initial pressure time of 600 ms, a current time of 21 cyc, and a hold time of 1 cyc. The cross section near the nugget was then polished, and the vicinity of the nugget was observed at a magnification of x100 using an optical microscope. The evaluation was performed by measuring the maximum length of each LME that occurred and judging according to the following criteria, with ◎ or ◯ being passed. The evaluation results are shown in Tables 1-1 and 1-2.
◎: No LME occurred ◯: LME length < 5 μm
×: 5 μm≦LME length

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

The evaluation was performed by measuring the maximum bulge width on one side from the cross cut and judging according to the following criteria, with ⊚ or ◯ being deemed to be acceptable. The evaluation results are shown in Tables 1-1 and 1-2.
◎: Maximum bulge width on one side < 2.0 mm
◯: 2.0 mm≦maximum bulge width on one side<4.0 mm
×: 4.0 mm or less maximum bulge width on one side

The results of Tables 1-1 and 1-2 show that the hot-pressed members of the present invention have excellent corrosion resistance and excellent LME resistance. In addition, by applying the manufacturing method for hot-pressed members to the hot-pressed steel plate of the present invention, it is possible to obtain hot-pressed members of the present invention that have excellent LME resistance while ensuring corrosion resistance after painting.

REFERENCE SIGNS LIST 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 the order of proximity to the steel plate surface,
A hot-pressed member comprising an intermetallic compound phase containing Zn and one or more elements selected from Cu and Zr, and an oxide layer containing Zn,
A total concentration of one or two elements selected from Cu and Zr in the intermetallic compound phase is 40 to 70 mass %, and the melting point of the intermetallic compound phase is 800° C. or higher,
A hot-pressed member, characterized in that the sum of the Zn content contained in the intermetallic compound phase and the Zn content contained in the oxide layer is 10 to 120 g/ ^m2 . For at least one surface of the steel plate, in the order of proximity to the steel plate surface,
A pre-plating layer containing one or two elements of Cu and Zr, the total content of said elements being 10 to 70 g/ ^m2 ;
A steel sheet having a Zn-based plating layer having a Zn content of 10 to 120 g/ ^m2 ,
The method for producing a hot-pressed member according to claim 1, characterized in that the hot-pressing is carried out after heating to a temperature range of from the _Ac3 transformation point to 1000°C. A hot press steel sheet used for the hot press member according to claim 1,
For at least one surface of the steel plate, in the order of proximity to the steel plate surface,
A pre-plating layer containing one or two elements of Cu and Zr, the total content of said elements being 10 to 70 g/ ^m2 ;
A steel sheet for hot press use, comprising: a Zn-based plating layer having a Zn content of 10 to 120 g/ ^m2 .