JP7126093B2 - HOT PRESS MEMBER AND MANUFACTURING METHOD THEREOF - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot press member and a manufacturing method thereof. In particular, the present invention relates to a hot press member having excellent corrosion resistance and a method for manufacturing the same.

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.

For example, Patent Literature 1 discloses a manufacturing method for obtaining a hot pressed member free from liquid metal embrittlement cracking by setting the melting point of the plating layer of the surface layer of the steel sheet to 800° C. or higher.

Further, Patent Document 2 discloses a steel sheet for hot press that prevents evaporation of zinc during hot press heating by providing an oxide film mainly composed of ZnO on the surface layer of the coating layer.

In addition, in Patent Document 3, a hot pressed member without microcracks is produced by performing rapid intermediate cooling of a steel sheet to 450 to 700° C. using an air jet or the like before hot pressing, and then performing die cooling by pressing. A method of making is disclosed.

Japanese Patent No. 5817479 Japanese Patent No. 3582504 Japanese Patent No. 5727037

As described above, various solutions have been proposed from the standpoint of steel sheets, plating, construction methods, etc., to various problems in hot press technology, and have supported the progress and development of hot press technology.

However, as the production volume of hot pressed parts increases and the technology is applied to new parts, new problems that have not existed in the past have come to the fore.

For example, when chemical conversion treatment and electrodeposition coating are applied to hot pressed parts, the coverage of the electrodeposition coating may be insufficient for hot pressed parts compared to cold pressed parts. be. If the throwing power of the electrodeposition coating is insufficient, a thin film portion of the electrodeposition coating is partially formed, and there is a problem that the corrosion resistance improvement effect of the coating cannot be fully enjoyed.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot pressed member that improves the throwing power of the electrodeposition coating of the hot pressed member and has excellent corrosion resistance, and a method for manufacturing the same. .

In order to achieve the above object, the present inventors have conducted intensive research and obtained the following findings.
(1) The deterioration of the throwing power of the electrodeposition coating is correlated with the maximum height roughness Rz of the Zn-based plating layer surface of the hot pressed member, and if the maximum height roughness is 15.0 μm or less, It is possible to improve the throwing power of electrodeposition coating.
(2) The corrosion resistance of the hot pressed parts after electrodeposition coating is correlated with the ZnO coverage of the Zn-based plating layer. can be further improved.
(3) Heat treatment is performed before hot press working, the maximum temperature T of the steel sheet during heat treatment is 860 ° C. or less, and the total heating time from room temperature to the end of the heating process is (24.5-0.025 T). By setting it to min or more, it is possible to obtain a hot pressed member in which the maximum height roughness Rz of the surface of the Zn-based plating layer is 15.0 μm or less.

The present invention is based on the above findings, and has the following features.
[1] having a Zn-based plating layer on at least one surface of the steel sheet,
A hot press member, wherein the Zn-based plating layer surface has a maximum height roughness Rz of 15.0 μm or less.
[2] The hot pressed member according to [1], wherein the Zn-based plating layer has a ZnO coverage of 85% or more.
[3] The steel plate, in % by mass,
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: contains 0.01% or less,
The hot press member according to [1] or [2], characterized by having a component composition in which the balance is Fe and unavoidable impurities.
[4] Furthermore, the steel plate is mass%,
Nb: 0.05% or less,
Ti: 0.05% or less,
B: 0.0002 to 0.0050%,
Cr: 0.1-0.3%,
Sb: The hot press member according to [3], characterized by containing one or more component compositions selected from 0.003 to 0.03%.
[5] A method for manufacturing a hot pressed member according to any one of [1] to [4],
The maximum temperature T of the steel sheet is 860 ° C. or less, and the total heating time t from room temperature to the end of the heating process is (24.5-0.025 T) min or more, and then hot press working is performed. A method for manufacturing a hot pressed member.

According to the present invention, it is possible to obtain a hot pressed member with improved adhesion to the electrodeposition coating of the hot pressed member and excellent corrosion resistance. Moreover, since the hot press member of the present invention does not require rapid intermediate cooling, it can be formed by general hot press equipment.

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 "%".

First, the reason for limiting the Zn-based plating layer of the steel sheet for hot press will be described.

The maximum height roughness Rz of the surface of the Zn-based plating layer is 15.0 μm or less It was found that there is a correlation in Rz. Specifically, when the maximum height roughness Rz of the surface of the Zn-based plating layer was 15.0 μm or less, it was found that the film thickness minimum portion of the electrodeposition coating was thick. This is because when the maximum height roughness Rz of the surface of the Zn-based plating layer exceeds 15.0 μm in the heat treatment after the electrodeposition coating, it becomes a barrier to the heat flow of the electrodeposition coating. presumably because of the existence of If there is a part where the coating thickness is thin, even partially, corrosion of the member progresses starting from the thin film part, so the electrodeposition coating is more difficult to adhere than when the coating thickness is uniform. Turnability becomes insufficient, and as a result, corrosion resistance deteriorates. Therefore, the maximum height roughness Rz of the Zn-based plating is set to 15.0 μm or less.

The Rz of the surface of the Zn-based plating layer is measured by the stylus-type surface roughness measuring method described in JIS B 0601.

The composition of the Zn-based plating layer is not particularly limited as long as the main component of the plating layer is Zn from the viewpoint of rust prevention. Any one of a -Mg-Si alloy plated layer, a Zn-Fe alloy plated layer, and a Zn-Ni alloy plated layer is preferable.

Further, in the present invention, the Zn-based plating layer is preferably a Zn—Ni alloy plating layer containing 10 to 25% by mass of Ni, with the balance being Zn and unavoidable impurities. By controlling the amount of Ni in the Zn alloy plating layer to 10 to 25% by mass, a γ phase having a crystal structure of any one of Ni ₂ Zn ₁₁ , NiZn ₃ , and Ni ₅ Zn ₂₁ with a high melting point is formed. , is advantageous in terms of liquid metal embrittlement resistance compared to other Zn alloy plating layers.

Also, the plating amount per side of the Zn-based plating layer is preferably 120 g/m ² or less. In order to form a thick plating of more than 120 g/m ² in a general plating production line, it is necessary to greatly reduce the line speed, resulting in an increase in cost. Therefore, it is preferable that the plating amount per side is 120 g/m ² or less. In addition, it is more preferably 90 g/m ² or less. Also, if it is less than 10 g/m ² , the effect of suppressing Fe scale formation during hot press heating is insufficient, so it is preferably 10 g/m ² or more.

ZnO Coverage of 85% or More In the present invention, the ZnO coverage of the Zn-based plating layer is preferably 85% or more in order to obtain a hot pressed member with more excellent corrosion resistance. Although the mechanism by which corrosion resistance is improved by the ZnO coverage is not clear, the plating-exposed portion (that is, the portion not covered with ZnO) is inferior in chemical conversion treatability to the ZnO-coated portion, and the paint adhesion is reduced. It is possible that As for the ZnO coverage, a desired ZnO coverage can be obtained by controlling the heating time.

In the present invention, in order to obtain a hot pressed member that exceeds the 1470 MPa class, the base steel sheet for the coating layer must have, for example, C: 0.20 to 0.35% and Si: 0.1% by mass. ~0.5%, Mn: 1.0-3.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less However, a steel sheet having a chemical composition in which the balance is Fe and unavoidable impurities can be used. The steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet. The reason for limitation of each component is described below.

C: 0.20-0.35%
C improves strength by forming martensite or the like as a steel structure. 0.20% or more is necessary to obtain strength exceeding 1470 MPa class. On the other hand, if it exceeds 0.35%, the toughness of the spot welded portion deteriorates. Therefore, the C content is preferably 0.20 to 0.35%.

Si: 0.1-0.5%
Si is an effective element for strengthening steel and obtaining a good quality. For that purpose, 0.1% or more is necessary. On the other hand, if it exceeds 0.5%, ferrite is stabilized, so that hardenability is lowered. Therefore, the Si content is preferably 0.1 to 0.5%.

Mn: 1.0-3.0%
Mn is an element effective in increasing the strength of steel. In order to ensure mechanical properties and strength, it is necessary to contain 1.0% or more. On the other hand, if it exceeds 3.0%, the surface concentration increases during annealing, making it difficult to ensure plating adhesion. Therefore, the Mn content is preferably 1.0 to 3.0%.

P: 0.02% or less If the amount of P exceeds 0.02%, grain boundary embrittlement due to P segregation to austenite grain boundaries during casting deteriorates local ductility, thereby lowering the balance between strength and ductility. Therefore, it is preferable that the amount of P is 0.02% or less.

S: 0.01% or less S becomes inclusions such as MnS and causes deterioration of impact resistance and cracks along the metal flow of the weld. Therefore, it is desirable to reduce it as much as possible, preferably 0.01% or less. Further, in order to ensure good stretch flangeability, the content is more preferably 0.005% or less.

Al: 0.1% or less When the amount of Al exceeds 0.1%, the blanking workability and hardenability of the steel sheet as the material deteriorate. Therefore, the Al content is preferably 0.1% or less.

N: 0.01% or less When the amount of N exceeds 0.01%, AlN nitrides are formed during hot rolling or heating before hot pressing, which deteriorates the blanking workability and hardenability of the material steel plate. Lower. Therefore, the N content is preferably 0.01% or less.

In addition, in the present invention, in addition to the basic components described above, with the intention of further improving the properties of the steel sheet, Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002 to 0.0050% , Cr: 0.1 to 0.3%, and Sb: 0.003 to 0.03%.

Nb: 0.05% or less Nb is a component that is effective in strengthening steel, but when it is included in excess, the shape fixability deteriorates. Therefore, when Nb is contained, it is made 0.05% or less.

Ti: 0.05% or less Ti, like Nb, is also effective in strengthening steel, but there is a problem that if it is contained excessively, shape fixability is lowered. Therefore, when Ti is contained, it is made 0.05% or less.

B: 0.0002 to 0.0050%
B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries, so it is preferably added in an amount of 0.0002% or more. On the other hand, excessive addition of B greatly impairs moldability. Therefore, when B is included, it should be 0.0002 to 0.0050%.

Cr: 0.1-0.3%
Cr is useful for strengthening steel and improving hardenability. Addition of 0.1% or more is preferable in order to exhibit such an effect. On the other hand, since the alloying cost is high, addition of more than 0.3% results in a significant increase in cost. Therefore, when Cr is contained, it should be 0.1 to 0.3%.

Sb: 0.003-0.03%
Sb also has the effect of suppressing decarburization of the surface layer of the steel sheet during the hot pressing process. Addition of 0.003% or more is required to exhibit such an effect. On the other hand, if the Sb content exceeds 0.03%, the rolling load is increased, which lowers the productivity. Therefore, when Sb is contained, it should be 0.003 to 0.03%.

The balance other than the above consists of Fe and unavoidable impurities.

Next, a method for manufacturing a hot press member according to the present invention will be described. In order to manufacture the hot pressed member of the present invention, it is important to control the maximum temperature and heating time of the steel sheet during heat treatment prior to hot press working.

The highest temperature T of the steel sheet is 860 ° C. or less As a result of investigating the correlation between the uneven shape of the Zn-based coating layer and the heat treatment conditions, the Rz increases as the highest temperature T of the steel plate increases, especially 860 ° C. It has been found that Rz increases sharply when it exceeds. The authors presume the mechanism of unevenness formation during heat treatment before hot pressing as follows. That is, when a steel sheet for hot pressing is heat-treated before hot pressing, the structure of the steel sheet undergoes a phase transformation from a bcc structure to an fcc structure, thereby causing volumetric shrinkage. At this time, unevenness is formed by intrusion of the surface hot-dip coating layer into the gaps of the locally shrunk steel sheet. Therefore, in order to suppress the formation of unevenness, it is considered effective to reduce the volume of the hot-dip coating layer that enters the steel sheet gap during heating, and it is presumed that reducing the heat treatment temperature is effective. As described above, it is effective to suppress the maximum height roughness Rz of the Zn-based plating layer in order to improve the throwing power of the electrodeposition coating of the hot pressed member and ensure the corrosion resistance. Therefore, in order to obtain the hot pressed member of the present invention, the temperature T reached by the steel sheet during hot pressing is set to 860° C. or less. The lower limit of the temperature reached by the steel sheet during hot pressing differs depending on the structure of the base material, and should be equal to or higher than the Ac3 transformation point at which the steel sheet is heated to the austenite single-phase region.

The total heating time t from room temperature to the end of the heating process is (24.5-0.025T) min or more. Low temperature is effective. On the other hand, as the steel sheet reaches a lower temperature during heat treatment, the steel sheet structure before hot pressing becomes less austenitic single-phase structure. Also, regarding the ZnO coverage of the Zn-based plating layer, it is difficult to form a sufficient amount of ZnO. However, by lengthening the heating time, it is possible to obtain an austenite single-phase structure and a desired ZnO coverage even if the steel sheet reaches a low temperature during heat treatment. Specifically, if the steel sheet reaches a temperature of T° C., the austenite single-phase structure can be obtained by heating the steel sheet for (24.5-0.025 T) min or more. Therefore, the total heating time t from room temperature to the end of the heating process should be (24.5-0.025T)min or more. As a method for performing the heating, an electric furnace, a gas furnace, or the like, which can easily secure a heating time, can be exemplified. In the present invention, the total heating time t from room temperature to the end of the heating process refers to the range of time from the start of charging the steel sheet into the heat treatment furnace until the removal thereof.

After heating, hot pressing is performed, and then cooling is performed using a mold and a coolant such as water to produce a hot pressed member. In the present invention, hot pressing conditions are not particularly limited, but pressing can be carried out at a general hot pressing temperature range of 600 to 800°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 base steel plate, in mass%, C: 0.30%, Si: 0.25%, Mn: 2.0%, P: 0.005%, S: 0.005%, Al: 0.03%, N: 0.005%, Nb: 0.005%, Ti: 0.02%, B: 0.0020%, 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 (Ac3=781° C.).
Various Zn-based plating layers were formed on the surfaces of the cold-rolled steel sheets by the plating methods shown in Table 1. Regarding the plating layer, the hot-dip plating bath composition was adjusted so as to obtain a desired composition, and the bath temperature was set to +20° C. of the melting point of each composition. As for the electroplating conditions, the metal salt ratio in the bath and the current value were adjusted so as to obtain the desired composition.

A test piece of 150 mmC×300 mmL was taken from the obtained steel sheet for hot pressing and heated in an electric furnace. Table 1 shows the heating conditions (heat treatment conditions). After heat treatment under various conditions, the test piece was taken out from the electric furnace and hot pressed at 700° C. using a hat-type mold. The shape of the molded part is 100 mm long on the upper flat portion, 50 mm long on the side flat portion, and 50 mm long on the lower flat portion. Moreover, the bending R of the mold is 7R for both shoulders of the upper surface and both shoulders of the lower surface. Next, the obtained hot pressed member was subjected to an electrodeposition coating treatment under the chemical conversion treatment conditions and the electrodeposition coating conditions shown in Table 1 to obtain an electrodeposition coated hot pressed member. PLM2100 (manufactured by Nihon Parkerizing Co., Ltd.) or PBSX-35 (manufactured by Nihon Parkerizing Co., Ltd.) was used as the chemical conversion treatment solution. GT100V (manufactured by Kansai Paint Co., Ltd.) or GT100 (manufactured by Kansai Paint Co., Ltd.) was used as the electrodeposition paint.

The maximum height roughness Rz of the surface of the Zn-based plating layer was measured for the hot pressed member obtained as described above. The measurement was carried out by the method described in JIS B 0601 using a stylus type surface roughness tester.

In addition, the ZnO coverage was observed using a surface SEM (magnification × 500) (N = 10), the area ratio of the ZnO unformed portion was measured, and the remaining portion was taken as the ZnO covered portion (average value ) was calculated.

In addition, the average film thickness D1 (μm) and the minimum film thickness D2 (μm) of the electrodeposition coating film thickness of the electrodeposition coating of the hot press member were measured, and the throwing power of the electrodeposition coating was evaluated according to the following criteria. .
○: D1-D2 is less than 4.0 μm △: D1-D2 is 4.0 μm or more and less than 8.0 μm ×: D1-D2 is 8.0 μm or more It was judged to be a member for hot pressing.
In addition, the obtained hat-shaped part was subjected to a corrosion test (SAE-J2334), and the maximum corrosion depth E1 and the average corrosion depth E2 at the center of the side flat part (100 mmC × 30 mmL area) after 120 cycles were investigated, Evaluation was made according to the following criteria.
◎: Average corrosion depth is less than 0.5 mm, E1-E2 is less than 0.2 mm ○: Average corrosion depth is less than 0.5 mm, E1-E2 is 0.2 mm or more △: Average corrosion depth is 0.5 mm or more and less than 1.0 mm ×: Average corrosion depth is 1.0 mm or more If the evaluation was ⊚ or ◯, it was considered to be excellent in corrosion resistance. Table 1 shows the evaluation results.

According to Table 1, the steel sheet for hot press of the present invention has excellent throwing power and corrosion resistance to electrodeposition coating.

Regarding the steel sheet for hot press having a Zn—Ni alloy plating layer containing 10 to 25% by mass of Ni and the balance being Fe and unavoidable impurities, evaluated in Example 1, the LME resistance (LME: Liquid Metal Embrittlement , liquid metal embrittlement) and corrosion resistance were evaluated. Specifically, a sample for cross-sectional SEM observation was taken from the top shoulder R portion of the hat-shaped part obtained in Example 1, and by observation, it was observed that the base material had penetrated into the base material in the field of view with a cross-sectional length of 5 mm on the outside of the shoulder R. The crack depth was measured at a total of 20 locations with a pitch of 250 μm, and the LME resistance characteristics were evaluated according to the following criteria.
○: No cracks or average crack depth of less than 10 μm △: Average crack depth of 10 μm or more and less than 200 μm ×: Average crack depth of 200 μm or more If the evaluation is ○, the LME resistance is good. I thought it was excellent.

Table 2 shows the evaluation results of LME resistance and the evaluation results of Example 1 (throwing power and corrosion resistance of electrodeposition coating).

From the results of Table 2, in the hot pressed member of the present invention, if the Zn-Ni alloy plated layer contains 10 to 25% by mass of Ni and the balance is Fe and unavoidable impurities, , In addition to the LME resistance, it is also found that the electrodeposition coating adhesion and corrosion resistance are also combined.

Claims (4)

Having a Zn-based plating layer on at least one surface of the steel sheet,
ZnO is formed on at least a partial region of the surface of the Zn-based plating layer,
A method for manufacturing a hot pressed member, wherein the Zn-based plating layer surface has a maximum height roughness Rz of 15.0 μm or less,
The maximum temperature T of the steel sheet is 860 ° C. or less, and the total heating time t from room temperature to the end of the heating process is (24.5-0.025 T) min or more, and then hot press working is performed. A method for manufacturing a hot pressed member.
Here, the maximum height roughness Rz of the surface of the Zn-based plating layer is the ZnO formed on the surface of the Zn-based plating layer (however, if the Zn-based plating layer has an exposed portion, is the maximum height roughness Rz of the surface (including the exposed portion). 2. The method for manufacturing a hot pressed member according to claim 1 , wherein the Zn-based plating layer has a ZnO coverage of 85% or more. The steel plate, in % by mass,
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: contains 0.01% or less,
3. The method for producing a hot pressed member according to claim 1 , wherein the balance has a component composition consisting of Fe and unavoidable impurities. Furthermore, the steel plate is mass%,
Nb: 0.05% or less,
Ti: 0.05% or less,
B: 0.0002 to 0.0050%,
Cr: 0.1-0.3%,
Sb: The method for producing a hot press member according to claim 3 , characterized by containing one or two or more component compositions selected from 0.003 to 0.03%.