WO2024185204A1 - Hot-pressed member - Google Patents

Abstract

Provided is a hot-pressed member having both excellent cut part corrosion resistance and coating layer adhesiveness. This hot-pressed member comprises a steel plate, a coating layer disposed on at least one side of the steel plate, and an oxide layer disposed on the coating layer, wherein the coating layer includes a metal Zn phase and a granular FeAl alloy phase; and R defined by formula (1) is 0.10-0.80 on a cross-section perpendicular to the surface of the steel plate.　(1): R = L_Zn/L, where L_Zn: total length of the metal Zn phase at the interface between the steel plate and the coating layer, and L: the length of the interface between the steel plate and the coating layer.

Description

Hot press parts

The present invention relates to hot-pressed components, and in particular to hot-pressed components that have excellent corrosion resistance in cut areas and adhesion of coating layers.

In order to reduce the weight of automobiles and improve collision safety, the strength of steel sheets for automobiles is being increased. In recent years, cold-rolled steel sheets with a tensile strength of 1500 MPa have been developed, and their application is being considered. However, as the strength of steel sheets increases, problems with forming defects and springback during pressing become an issue in terms of dimensional accuracy.

As a result, the use of hot pressing technology, which forms parts hot rather than cold, is becoming more common. Hot pressing is a forming method in which steel plate is heated to the austenite temperature range, then press-formed while still at a high temperature, and simultaneously quenched by contact with a die. With hot pressing, press forming is performed in a state where formability has been improved by heating, and the subsequent quenching increases strength, making it possible to manufacture hot-pressed parts with excellent strength and high dimensional precision.

Therefore, steel sheets with coatings such as Al-based plating layers, Zn-based plating layers, and Al-Zn-based plating layers on the surface have been proposed as hot-press steel sheets suitable for manufacturing hot-press parts (Patent Documents 1 to 5).

JP 2003-049256 A JP 2003-073774 A JP 2005-113233 A International Publication No. 2017/195269 International Publication No. 2019/180853

Hot-pressed parts obtained by hot pressing the above-mentioned hot-press steel sheets are primarily used for automotive parts, particularly frame structural parts (inner panel frames) that require strength, but in recent years they have also come to be used as semi-exterior panel parts, such as parts around the pillars that are visible when the door is opened. For this reason, hot-pressed parts are required to be suitable for painting and to have excellent corrosion resistance in cut areas after painting.

In addition, since hot-pressed components are usually spot welded before use, they are also required to have excellent spot weldability.

However, the conventional techniques proposed in Patent Documents 1 to 5 were unable to meet all of the above requirements, as explained below.

For example, in the technology proposed in Patent Document 1, hot-dip Al-plated steel sheet is used as the steel sheet for hot pressing to prevent scale and improve corrosion resistance. However, when Al-plated steel sheet is hot-pressed, Fe diffuses from the base steel sheet to the surface layer of the Al-plated layer, forming an FeAl-based alloy layer. Generally, a chemical conversion coating is formed on hot-pressed members before electrocoating to ensure paintability, but since the FeAl-based alloy layer does not react with the chemical conversion solution, a chemical conversion coating cannot be formed. In addition, since the FeAl-based alloy layer does not have sacrificial corrosion protection, corrosion resistance such as corrosion resistance of cut parts is insufficient.

On the other hand, in the technology proposed in Patent Document 2, a Zn-based plated steel sheet is used as the steel sheet for hot pressing in order to ensure paint adhesion and corrosion resistance. However, when a Zn-based plated steel sheet is hot pressed, a thick oxide layer is formed on the surface of the plated layer, resulting in insufficient spot weldability. In addition, because Zn has a low melting point, liquid metal embrittlement (LME) cracking occurs when a steel sheet with a Zn-based plated layer is hot pressed, making it impossible to obtain sufficient fatigue resistance properties.

Patent Document 3 proposes improving spot weldability by forming an oxide layer containing Mn on the surface of a Zn-based plated steel sheet. However, Patent Document 3 also uses a plated layer mainly composed of Zn, which results in insufficient fatigue resistance due to LME cracking.

Therefore, it has been proposed to use Al-Zn-based plated steel sheets instead of Al-plated steel sheets, which have problems with chemical conversion treatment, and Zn-based plated steel sheets, which have problems with spot weldability and LME cracking.

For example, Patent Document 4 proposes that an interface layer of a specific composition is formed at the interface between the base steel sheet and the plating layer by manufacturing a hot-pressed member using an Al-Zn-based plated steel sheet. According to Patent Document 4, by providing the interface layer in the hot-pressed member, it is possible to prevent LME cracking and improve fatigue resistance. However, this hot-pressed member still does not have sufficient corrosion resistance in the cut parts.

Patent Document 5 claims that by including Mg in an Al-Zn-plated steel sheet, a Mg oxide layer is formed during hot pressing, resulting in a hot-pressed component in which the Zn is not oxidized and the metallic Zn phase remains in the coating layer, resulting in high corrosion resistance. However, this hot-pressed component also does not have sufficient corrosion resistance in the cut parts. Also, there are cases in which sufficient adhesion cannot be obtained between the coating layer and the steel sheet due to the generation of voids between the granular FeAl alloy phase and the steel sheet.

As such, even with technology that uses Al-Zn-based plated steel sheets, it was still not possible to realize hot-pressed components that combined high levels of cut corrosion resistance and plating adhesion.

The present invention was made in consideration of the above situation, and aims to achieve a high level of both cut corrosion resistance and coating layer adhesion in a hot-pressed member with an Al-Zn-based coating layer that does not have the chemical conversion treatability issues of Al-plated steel sheets, or the spot weldability and LME cracking issues of Zn-based coated steel sheets.

As a result of research conducted to solve the above-mentioned problems, the present inventors found that the above-mentioned problems can be solved when a hot-pressed part comprising a steel plate, a coating layer on the surface of the steel plate, and an oxide layer on the surface of the coating layer satisfies the following conditions (a) and (b).
(a) the coating layer contains a metallic Zn phase and a granular FeAl alloy phase;
(b) In a cross section perpendicular to the surface of the steel plate, R defined by the formula (1) described below is 0.10 to 0.80.

The present invention was completed based on the above findings, and its gist is as follows:

1. A steel plate;
A coating layer disposed on at least one surface of the steel plate;
and an oxide layer disposed on the coating layer,
The coating layer includes a metallic Zn phase and a granular FeAl alloy phase,
In a cross section perpendicular to the surface of the steel plate, R defined by the following formula (1) is 0.10 to 0.80.
R=L _Zn /L... (1)
Where:
L _Zn : the total length of the metallic Zn phase at the interface between the steel sheet and the coating layer. L: the length of the interface between the steel sheet and the coating layer.

2. The hot-pressed member described in 1 above, in which the average grain size of the granular FeAl alloy phase is 5 μm or more in a cross section perpendicular to the surface of the steel plate.

The present invention makes it possible to provide hot-pressed components that combine high levels of corrosion resistance in the cut area and adhesion of the coating layer.

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. Furthermore, the unit of content, "%", represents "mass %" unless otherwise specified.

(1) Hot-pressed member A hot-pressed member according to one embodiment of the present invention includes a steel plate, a coating layer disposed on at least one surface of the steel plate, and an oxide layer disposed on the coating layer. Each part will be described below.

[Steel plate]
In the present invention, the above-mentioned problems are solved by controlling the structure of the coating layer as described below. Therefore, the above-mentioned steel sheet is not particularly limited and any steel sheet can be used.

The hot-pressed member of the present invention is manufactured by hot pressing a steel plate for hot pressing, as described below. Therefore, the steel plate can also be said to be a steel plate formed by hot pressing. The steel plate may be either a cold-rolled steel plate or a hot-rolled steel plate.

From the viewpoint of use as automotive parts, etc., it is preferable for the hot-pressed parts to have high strength. In particular, to obtain hot-pressed parts with strength exceeding 1470 MPa, it is preferable to use a steel material with the following composition.

C: 0.20-0.35%,
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; and N: 0.01% or less;
The balance is Fe and unavoidable impurities.

The effects and preferred contents of each element in the above preferred composition are explained below.

C: 0.20-0.35%
C is an element that has the effect of improving strength by forming a structure such as martensite. From the viewpoint of obtaining a strength exceeding 1470 MPa class, it is preferable that the C content is 0.20% or more. On the other hand, if the C content exceeds 0.35%, the toughness of the spot welds deteriorates. Therefore, the C content is preferably 0.35% or less.

Si:0.1~0.5%
Silicon is an element that is effective in strengthening steel to obtain good material properties. In order to obtain the above effect, the silicon content is preferably 0.1% or more. If the Si content exceeds 0.5%, the ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 0.5% or less.

Mn: 1.0-3.0%
Mn is an element that is effective in increasing the strength of steel. From the viewpoint of ensuring excellent mechanical properties and strength, the Mn content is preferably 1.0% or more. If the Mn content exceeds 3.0%, the amount of Mn that concentrates on the steel sheet surface during annealing increases, resulting in a decrease in the adhesion of the coating layer. Therefore, the Mn content is preferably 3.0% or less.

P: 0.02% or less If the P content is higher than 0.02%, local ductility is deteriorated due to grain boundary embrittlement caused by P segregation to the austenite grain boundaries during casting. As a result, the balance between strength and ductility of the steel sheet is reduced. Therefore, from the viewpoint of improving the balance between strength and ductility of the steel sheet, it is preferable to set the P content to 0.02% or less. On the other hand, from the above viewpoint, the lower the P content, the better, so the lower limit of the P content is not particularly limited and may be 0%, but from the viewpoint of refining costs, it is preferable to set the P content to 0.0005% or more.

S: 0.01% or less S becomes an inclusion such as MnS, which causes deterioration of impact resistance and cracks along the metal flow of the weld. Therefore, it is desirable to reduce the S content as much as possible, specifically, it is preferable to make it 0.01% or less. In addition, from the viewpoint of ensuring good stretch flangeability, it is more preferable to make it 0.005% or less. On the other hand, from the above viewpoint, the lower the S content, the better, so the lower limit of the S content is not particularly limited and may be 0%, but from the viewpoint of refining costs, it is preferable to make the S content 0.0002% or more.

Al: 0.1% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.1%, the hardenability decreases. Therefore, the Al content is preferably 0.1% or less. On the other hand, although there is no particular lower limit for the Al content, from the viewpoint of enhancing the effect as a deoxidizer, the Al content is preferably 0.01% or more.

N: 0.01% or less If the N content exceeds 0.01%, AlN is generated during heating before hot pressing, and hardenability is reduced. Therefore, the N content is preferably 0.01% or less. On the other hand, the lower limit of the N content is not particularly limited, but from the viewpoint of refining costs, the N content is preferably 0.001% or more.

The above component composition may further include, optionally,
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%
The composition may contain at least one selected from the group consisting of:

Nb: 0.05% or less Nb is an effective component for strengthening steel, but if it is contained in excess, the shape fixability decreases. Therefore, when Nb is contained, the Nb content is preferably 0.05% or less. On the other hand, the lower limit of the Nb content is not particularly limited and may be 0%, but from the viewpoint of the strength improvement effect, the Nb content is preferably 0.005% or more.

Ti: 0.05% or less Ti is an effective component for strengthening steel, similar to Nb, but if it is contained in excess, the shape fixability decreases. Therefore, when Ti is added, the Ti content is preferably 0.05% or less. On the other hand, the lower limit of the Ti content is not particularly limited and may be 0%, but from the viewpoint of the strength improvement effect, the Ti content is preferably 0.005% or more.

B: 0.0002-0.0050%
B has the effect of suppressing the formation and growth of ferrite from the austenite grain boundaries. When B is added, in order to obtain the above effect, the B content is preferably set to 0.0002% or more. Excessive addition of B reduces formability, so when B is added, the B content is preferably 0.0050% or less.

Cr: 0.1-0.3%
Cr is an element useful for strengthening steel and improving hardenability. When Cr is added, the Cr content is preferably 0.1% or more in order to obtain the above-mentioned effects. Since Cr is an expensive element, the addition of excessive Cr leads to a significant increase in costs, and therefore, when Cr is added, the Cr content is preferably set to 0.3% or less.

Sb: 0.003-0.03%
Sb is an element that has the effect of suppressing decarburization of the surface layer of the steel sheet during hot pressing. When Sb is added, the Sb content should be 0.003% or more in order to obtain this effect. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, and the productivity decreases. Therefore, when Sb is added, the Sb content is preferably 0.03% or less. .

[Coating layer]
The hot-pressed member of the present invention has a coating layer on at least one surface of the steel plate. The coating layer may be provided on only one surface of the steel plate, but is preferably provided on both surfaces.

The coating layer contains a metallic Zn phase and a granular FeAl alloy phase. As described below, such a coating layer is obtained by hot pressing a steel sheet having an Al-Zn-based plating layer. Therefore, unlike the case where an Al-plated steel sheet is used, the hot-pressed member of the present invention does not have problems with chemical conversion treatment, and unlike the case where a Zn-based plated steel sheet is used, it does not have problems with spot weldability and LME cracking.

The metallic Zn phase contained in the coating layer exerts a sacrificial anticorrosive effect in a corrosive environment, resulting in high corrosion resistance of the cut portion. Even if the coating layer contains Zn, if the Zn exists in a solid solution state, it does not contribute to improving the corrosion resistance of the cut portion. Therefore, it is important that Zn exists in a metallic state in the coating layer. The presence or absence of a metallic Zn phase in the coating layer can be evaluated by X-ray diffraction.

Even when a steel sheet having an Al-Zn-based plating layer is hot-pressed to form a hot-pressed member, some or all of the FeAl alloy phase contained in the coating layer after hot pressing may be layered rather than granular, depending on the composition of the plating layer. If the FeAl alloy phase is layered rather than granular, the desired corrosion resistance of the cut portion cannot be obtained even if a metallic Zn phase is present. This is thought to be because the layered FeAl alloy phase is formed on the base steel sheet side of the coating layer, and the sacrificial corrosion protection effect of the metallic Zn phase does not effectively act on the base steel sheet. For this reason, it is preferable that all of the FeAl alloy phase contained in the coating layer is granular.

・R:0.10~0.80
In the present invention, it is important that R, defined by the following formula (1), is 0.10 to 0.80 in a cross section perpendicular to the surface of the steel plate.
R=L _Zn /L... (1)
Where:
L _Zn : the total length of the metallic Zn phase at the interface between the steel sheet and the coating layer L : the length of the interface between the steel sheet and the coating layer

The _LZn and L can be determined by analyzing an image obtained by observing a cross section of the hot-pressed member perpendicular to the surface of the steel plate with a scanning electron microscope (SEM).

If R is less than 0.10, the adhesive effect of the metallic Zn phase becomes insufficient, and the desired adhesion of the coating layer cannot be obtained. Therefore, R is set to 0.10 or more. If a layered FeAl alloy phase is formed on the base steel plate side of the coating layer, as mentioned above, the sacrificial corrosion protection effect of the metallic Zn phase does not work effectively on the base steel plate, and the corrosion resistance of the cut portion becomes insufficient. For example, in the hot press member of the above-mentioned Patent Document 5, a layered FeAl alloy phase is formed between the metallic Zn phase and the base steel plate. Therefore, sufficient corrosion resistance of the cut portion cannot be obtained. If R is 0.30 or more, the sacrificial corrosion protection effect of the metallic Zn phase becomes significant, and the corrosion resistance of the cut portion is further improved. Therefore, it is more preferable that R is 0.30 or more.

On the other hand, if R is greater than 0.80, the flow of the metallic Zn phase during hot pressing becomes significant, resulting in reduced adhesion of the coating layer. For this reason, R is set to 0.80 or less.

Average particle size of the granular FeAl alloy phase The size of the granular FeAl alloy phase is not particularly limited. However, from the viewpoint of further improving the corrosion resistance of the cut portion, it is preferable that the average particle size of the granular FeAl alloy phase in the cross section perpendicular to the surface of the steel sheet is 5 μm or more. The reason why the corrosion resistance of the cut portion is further improved when the average particle size is 5 μm or more is considered as follows. That is, when an Al-Zn-based plated steel sheet having a predetermined composition is hot pressed, Al and Fe are alloyed to form a granular FeAl alloy phase. At this time, if the particle size of the granular FeAl alloy phase is small, voids are formed between the granular FeAl alloy phase. However, when the average particle size of the granular FeAl alloy phase is large, such as 5 μm or more, metal Zn is easily inserted between the granular FeAl alloy phase, and the voids are reduced, resulting in further improvement of the corrosion resistance of the cut portion. Note that the upper limit of the average particle size is not particularly limited, but if the average particle size is too large, the area where metal Zn does not exist increases, and the corrosion resistance of the cut portion may deteriorate. Therefore, the average particle size is preferably 15 μm or less.

Here, the average particle size of the granular FeAl alloy phase is defined as the average value of (long axis + short axis)/2 of each granular FeAl alloy phase. The long axis and short axis can be determined by analyzing images obtained by SEM observation of a cross section of the hot-pressed member perpendicular to the surface of the steel plate.

The coating weight of the coating layer is not particularly limited, but from the viewpoint of corrosion resistance, it is preferable to set the coating weight to 60 g/ ^m2 or more per one side of the steel sheet. On the other hand, from the viewpoint of production costs, it is preferable to set the coating weight to 400 g/ ^m2 or less per one side of the steel sheet. The coating weight can be determined by dissolving and removing the coating layer from the surface of the hot-pressed member using an acid solution, and subtracting the weight after removal from the weight of the hot-pressed member before removal. An inhibitor that suppresses dissolution of the base steel sheet is added to the acid solution.

As described above, in the present invention, the desired characteristics are achieved by combining the metallic Zn phase and the granular FeAl alloy phase. Therefore, the coating layer only needs to contain the metallic Zn phase and the granular FeAl alloy phase so that R is 0.10 to 0.80, and there are no particular limitations on the other components.

However, it is preferable that the coating layer has the following component composition:
In mass percent,
Zn: 30.0-70.0%,
Si: 1.1 to 8.0%, and at least one of Sr and Ca: 0.01 to 5.0% in total;
The balance is Al and unavoidable impurities.

It is preferable that at least one of Sr and Ca is contained in the coating layer, but both may be contained.

[Oxide layer]
The hot-pressed member of the present invention includes an oxide layer disposed on the coating layer. When the steel sheet for hot pressing is hot-pressed, Fe in the base steel diffuses into the plating layer to form the coating layer, and at the same time, components in the plating layer combine with oxygen present in the heating atmosphere to form an oxide layer on the surface of the coating layer.

The thickness of the oxide layer is not particularly limited. However, as the oxide layer is formed, the metallic Zn phase in the coating layer decreases, so if the oxide layer is too thick, the metallic Zn phase in the coating layer becomes insufficient, and sufficient corrosion resistance of the cut portion may not be obtained. Therefore, from the viewpoint of further improving the corrosion resistance of the cut portion, it is preferable to set the thickness of the oxide layer to 0.6 μm or less. Also, if the oxide layer is sufficiently thin, there is almost no reduction in the metallic Zn phase in the coating layer, so the corrosion resistance of the cut portion is further improved. From this viewpoint, it is more preferable to set the thickness of the oxide layer to 0.3 μm or less. On the other hand, from the viewpoint of the corrosion resistance of the cut portion, the thinner the oxide layer, the better, so the lower limit of the thickness of the oxide layer is not particularly limited and may be 0 μm.

(2) Steel plate for hot pressing The hot press member of the present invention can be manufactured by hot pressing a steel plate (steel plate for hot pressing) having a plating layer, as described later. Hereinafter, the steel plate for hot pressing that can be used for manufacturing the hot press member of the present invention will be described.

The hot press steel sheet comprises a steel sheet and a plating layer disposed on at least one surface of the steel sheet.

[Steel plate]
The steel plate is not particularly limited and any steel plate can be used. The steel plate may be either a cold-rolled steel plate or a hot-rolled steel plate. The composition of the steel plate is not particularly limited, but it is preferable to use a steel plate having the composition described in the description of the hot-pressed member.

[Plating layer]
The steel sheet for hot press use of the present invention has a plating layer on at least one surface of the steel sheet. The plating layer may be provided on only one surface of the steel sheet, but is preferably provided on both surfaces.

In order for the composition of the coating layer after hot pressing to satisfy the above-mentioned conditions, the plating layer of the steel sheet for hot pressing needs to have the following component composition.
Zn: 30.0-70.0%,
Si: 1.1 to 8.0%, and at least one of Sr and Ca: 0.01 to 5.0% in total;
The balance is Al and unavoidable impurities.

The plating layer is
Zn: 35.0 to 65.0%,
Si: 1.3 to 4.0%, and at least one of Sr and Ca: 0.1 to 1.0% in total;
It is preferable that the balance of the composition be Al and unavoidable impurities.
Each component will be described below.

Zn: 30.0-70.0%
If the Zn content in the plating layer is less than 30.0%, the metallic Zn phase in the coating layer is insufficient or absent, and the desired corrosion resistance of the cut portion cannot be obtained. When the Zn content is 35.0% or more, the metal Zn phase is present in a larger amount in the coating layer, and therefore the corrosion resistance of the cut portion is further improved. On the other hand, if the Zn content exceeds 70.0%, Zn may exist as a ZnFe alloy phase after hot pressing, and no metallic Zn phase may be contained. Even if the Zn content is 70.0%, the Zn content is 70.0% or less. Even if the content is 70.0% or less, if it exceeds 65.0%, the average grain size of the granular FeAl alloy phase in the coating layer becomes small, and voids that are not filled with the metallic Zn phase are excessively generated, resulting in deterioration of corrosion resistance. Therefore, the Zn content is preferably 65.0% or less.

Si: 1.1-8.0%
Silicon is an element that has the effect of suppressing alloying of the plating layer in the plating process and the heat treatment process before hot pressing. If the Si content in the plating layer is less than 1.1%, the alloying of the plating layer is suppressed in the hot pressing process. The granular FeAl alloy phase in the subsequent coating layer becomes thicker, and the metallic Zn phase becomes insufficient in the gap between the granular FeAl alloy phase and the steel sheet, so that the desired corrosion resistance of the cut portion cannot be obtained. On the other hand, if the Si content is excessive, the amount of Si-based oxides produced increases, which not only impairs chemical conversion treatability, but also The corrosion resistance also becomes inferior. Therefore, the Si content is set to 8.0% or less, preferably 4.0% or less.

Sr+Ca: 0.01~5.0%
Sr and Ca are elements that have the effect of improving weldability by suppressing the formation of an oxide layer. That is, Sr and Ca are preferentially oxidized to form a surface barrier, so that the oxide layer By suppressing the formation of Zn, the Zn contained in the coating layer is not oxidized and remains in the coating layer, improving the corrosion resistance. However, if the total content of Sr and Ca is less than 0.01%, the desired effect cannot be obtained. Therefore, the total content of Sr and Ca is set to 0.01%. On the other hand, if the Sr and Ca contents are excessive, the oxides of Sr and Ca themselves are excessively generated, which deteriorates the chemical conversion treatability. The paint film adhesion is also deteriorated, and as a result, the expected corrosion resistance cannot be obtained. Therefore, the total content of Sr and Ca is set to 5.0% or less, preferably 1.0% or less. The total content of Sr and Ca is represented as "Sr+Ca".

The plating layer can be formed by any method without particular limitations, but it is preferable to form it by a hot-dip plating method. In other words, it is preferable that the plating layer is a hot-dip plating layer.

The coating weight of the plating layer is not particularly limited, but from the viewpoint of corrosion resistance, it is preferable to set the coating weight to 30 g/ ^m2 or more. On the other hand, from the viewpoint of production costs, it is preferable to set the coating weight to 200 g/ ^m2 or less. The coating weight of the plating layer can be determined by dissolving and removing the plating layer from the surface of the steel sheet for hot press use using an acid solution, and subtracting the weight after removal from the weight of the steel sheet for hot press use before removal. An inhibitor that inhibits dissolution of the base steel sheet is added to the acid solution.

Furthermore, in order for the R-value of the final hot-pressed member to satisfy the above-mentioned conditions, it is necessary to perform annealing under specific conditions during the manufacture of the hot-press steel sheet. Specifically, in the cooling process after the steel sheet is pulled out of the hot-dip plating bath, the plated steel sheet is held at a temperature of 250 to 350°C for 5 to 30 seconds. By holding at a temperature of 250 to 350°C for 5 to 30 seconds, the plated layer does not solidify in a non-equilibrium state due to rapid cooling, the strain introduced into the plated layer is released, and two-phase separation of the α-Al phase and the metallic Zn phase in the plated layer is promoted. As a result, the metallic Zn phase increases in the gaps between the granular FeAl alloy phase and the steel sheet after hot pressing, making it possible to improve the corrosion resistance and adhesion of the coating layer.

If the holding temperature is less than 250°C or the holding time is less than 5 seconds, sufficient two-phase separation is not obtained, and the R value cannot be 0.10 or more. If the holding temperature is more than 350°C or the holding time is more than 30 seconds, two-phase separation progresses excessively, and the R value cannot be 0.80 or less. In this case, the average grain size of the granular FeAl alloy phase in the hot-pressed member becomes small.

(3) Manufacturing Method of Hot-Pressed Member Next, a preferred manufacturing method of the hot-pressed member of the present invention will be described.

The hot-pressed member of the present invention can be manufactured by hot-pressing a steel sheet for hot pressing that satisfies the above conditions. In particular, as mentioned above, it is important that the steel sheet for hot pressing used is one that has been annealed under specific conditions after plating.

The method of hot pressing is not particularly limited, and can be carried out according to a conventional method. Typically, the steel plate for hot pressing is heated to a predetermined heating temperature (heat treatment process), and then the steel plate for hot pressing heated in the heat treatment process is hot pressed (hot pressing process). Preferred hot pressing conditions are described below.

[Heat treatment]
In the heat treatment process, the hot press steel sheet is heated to a heating temperature of the Ac ₃ transformation point or higher and 980 ° C or lower. By setting the heating temperature to the Ac ₃ transformation point or higher, the structure of the steel sheet can be austenitized. The austenite becomes a hard phase such as a martensite phase by quenching during the subsequent hot pressing, and as a result, the hot press member can be made high strength. If the heating temperature is lower than the Ac ₃ transformation point, the austenite fraction in the heated steel sheet decreases, so that the volume fraction of martensite after hot pressing becomes insufficient, and sufficient tensile strength cannot be ensured. On the other hand, if the heating temperature is higher than 980 ° C, a thick oxide layer is formed, and the granular FeAl alloy phase is enlarged, so that the metal Zn phase in the coating layer is reduced, and as a result, the expected corrosion resistance cannot be obtained. Therefore, a hot press member that satisfies the conditions of the present invention cannot be obtained. In addition, if the heating temperature is higher than 980 ° C, the crystal grain size becomes excessively coarse, and the bending crushability is reduced.

The _Ac3 transformation point can be calculated by the following formula (1).
Ac ₃ transformation point (°C) = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu + 41Mo... (1)
In the formula (1), the element symbols represent the content (mass%) of each element. The content of elements that are not contained is calculated as 0.

In the heat treatment process, after heating to the heating temperature, the heating temperature can be held for a holding time of 5 minutes or less. If the holding time is longer than 5 minutes, the alloying of the plating layer will proceed excessively, resulting in an excessive Fe content in the coating layer, and the granular FeAl alloy phase will enlarge, reducing the metallic Zn phase in the coating layer, resulting in the expected corrosion resistance not being obtained. Therefore, a hot-pressed member that satisfies the conditions of the present invention cannot be obtained. The lower limit of the holding time is not particularly limited, and may be 0 minutes. However, by providing a holding time, the structure of the steel sheet can be reliably austenitized, thereby increasing the strength of the hot-pressed member. Furthermore, by providing a holding time, the alloying of the plating layer can be further promoted. Therefore, the holding time is preferably 5 seconds or more.

The method of heating the hot press steel sheet in the heat treatment process is not particularly limited, and any method can be used. The heating can be performed, for example, by heating in a heating furnace, electrical heating, induction heating, high-frequency heating, flame heating, etc. Any heating furnace can be used, such as an electric furnace or a gas furnace.

[Hot press]
After the above heating, the hot press steel plate is hot pressed to obtain a hot press member. In the hot press, cooling is performed using a mold or a coolant such as water at the same time as or immediately after processing. In the present invention, the hot press conditions are not particularly limited. For example, pressing can be performed at a general hot press temperature range of 600 to 800°C.

To confirm the effects of the present invention, a hot press steel plate and a hot press member using the hot press steel plate were produced and their properties were evaluated.

・Hot press steel sheet A hot press steel sheet was prepared by forming a plating layer on the surface of the steel sheet according to the following procedure. Specifically, a plating layer was formed on both sides of a steel sheet having a thickness of 1.4 mm by a continuous hot-dip plating facility. As the steel sheet, a cold-rolled steel sheet was used having a composition containing C: 0.24%, Si: 0.25%, Mn: 1.3%, P: 0.01%, S: 0.002%, Al: 0.03%, N: 0.005%, Cr: 0.16%, Ti: 0.03%, B: 0.002%, and Sb: 0.008%, with the balance being Fe and unavoidable impurities. The Ac ₃ transformation point of the cold-rolled steel sheet was 825°C. The temperature of the plating bath was 600°C, and the coating weight of the plating layer was 100 g/ ^m2 per side of the steel sheet, i.e., 200 g/ ^m2 in total on both sides.

In addition, in the above hot-dip plating, annealing was performed under the conditions (holding temperature, holding time) shown in Tables 1 and 2. After the steel sheet was pulled out of the hot-dip plating bath, it was cooled with nitrogen gas until it reached the above-mentioned holding temperature. For comparison, annealing was not performed in some examples (Comparative Example No. 2).

(Component composition of plating layer)
The component composition of the obtained plating layer was measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). In the SEM-EDX analysis, an SEM (JSM-7200F) manufactured by JEOL Ltd. and an EDX detector (UltraDry) manufactured by Thermo Fisher were used, and the analysis was performed at an acceleration voltage of 15.0 kV. The obtained results are shown in Tables 1 and 2.

-Hot press member Next, each of the obtained hot press steel plates was hot pressed under the conditions shown in Tables 1 and 2 to obtain hot press members. Specifically, the hot press steel plates were first cut into a size of 70 mm x 150 mm and heat treated in an electric furnace. The heating temperature and the holding time at the heating temperature in the heat treatment were as shown in Tables 1 and 2. Next, the hot press steel plates were taken out of the electric furnace and hot pressed using a flat die. The forming start temperature was 700°C.

Then, for each of the obtained hot-pressed members, the presence or absence of a granular FeAl alloy phase and a metallic Zn phase, the R value, the average grain size of the granular FeAl alloy phase, and the thickness of the oxide layer were measured using the following procedure. The measurement results are shown in Tables 3 and 4.

(granular FeAl alloy phase)
The presence or absence of an FeAl alloy phase in the coating layer was determined by X-ray diffraction measurement. For the measurement, an X-ray diffraction device SmartLab manufactured by Rigaku Corporation was used. The measurement conditions were as follows: X-ray used: The conditions were Cu-Kα, tube voltage: 40 kV, tube current: 30 mA, and scanning speed: 4°/min. Furthermore, a test piece for cross-sectional observation was taken from the flat portion of the upper surface of the hot-pressed member, and the cross-sectional observation was performed. In some of the examples (Comparative Examples Nos. 19, 26, and 38), the granular FeAl alloy phase was not present in the coating layer. In the first embodiment, the FeAl alloy phase was all lamellar, but in the other embodiments, the FeAl alloy phase was all granular.

(Metal Zn Phase)
The presence or absence of a metallic Zn phase in the coating layer was determined by X-ray diffraction measurement. For the measurement, an X-ray diffraction device SmartLab manufactured by Rigaku Corporation was used. The measurement conditions were as follows: X-ray used: Cu-Kα, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4°/min.

(R value)
A test piece for cross-sectional observation was taken from the flat portion of the upper surface of the hot press member, and the cross section perpendicular to the surface of the steel plate was observed to measure the total length L _Zn of the metal Zn phase on the surface of the coating layer on the steel plate side and the length L of the surface of the coating layer on the steel plate side. Specifically, the cross section of the surface of the hot press member was observed at a magnification of 500 times using an SEM, and L _Zn and L were measured. From the obtained L _Zn and L, the R value was calculated by formula (1). The SEM observation was performed in 10 randomly selected visual fields, and the average values in the 10 visual fields were used as the L _Zn and L.

(Average particle size of granular FeAl alloy phase)
A test piece for cross-sectional observation was taken from the flat part of the upper surface of the hot press member, and the average grain size of the FeAl alloy phase was measured by observing the cross section perpendicular to the surface of the steel plate. Specifically, the cross section of the surface of the hot press member was observed at a magnification of 500 times using an SEM, and SEM images of 10 randomly selected fields were obtained. The SEM images were analyzed to measure the major and minor diameters of each granular FeAl alloy phase. Using the obtained major and minor diameters, the grain size of each granular FeAl alloy phase, defined as (major diameter + minor diameter) / 2, was calculated, and the average value of the grain size was taken as the average grain size of the granular FeAl alloy phase.

(Oxide layer thickness)
A test piece for cross-sectional observation was taken from the flat portion of the upper surface of the hot-pressed member, and the thickness of the oxide layer was measured by observing the cross section perpendicular to the surface of the steel plate. Specifically, the cross section of the surface of the hot-pressed member was observed at a magnification of 500 times using an SEM, and the thickness of the oxide layer was measured at 20 randomly selected points, and the average value was taken as the thickness of the oxide layer.

Next, the cut corrosion resistance and coating layer adhesion of each of the hot-pressed components obtained were evaluated using the following procedure. The measurement results are shown in Tables 3 and 4.

(Corrosion resistance of cut parts)
A test piece for evaluating corrosion resistance was prepared by subjecting a test piece taken from the hot press member to a phosphoric acid-based chemical conversion treatment and an electrocoating. A cross-cut scratch (angle 60°) of 80 mm in length and 160 mm in total was made in the center of the test piece for evaluating corrosion resistance, and then the test piece was subjected to a corrosion test (SAE-J2334). Based on the occurrence of red rust after 30 cycles, the corrosion resistance of the cut portion was evaluated according to the following criteria.
Rating 4: No red rust in the cut portion Rating 3: The length of the cut scratch where red rust has occurred is less than 2 mm Rating 2: The length of the cut scratch where red rust has occurred is 2 mm or more and less than 4 mm Rating 1: The length of the cut scratch where red rust has occurred is 4 mm or more Here, a rating of 3 or more was determined to have sufficient corrosion resistance in the cut portion.

(Adhesion of coating layer)
A test piece taken from the hot-pressed member was cut with 11 cuts reaching the base steel sheet in both the vertical and horizontal directions at intervals of 1 mm to create 100 grids. Cellophane tape (registered trademark) was firmly pressed onto the grids, and the end of the tape was pulled off at a 45° angle in one go. The number of squares of plating peeled off from the surface of the test piece was counted and judged according to the following criteria. A score of 3 or more was judged to have sufficient adhesion of the coating layer.
4: Number of peeled squares is 0
3: Number of peeled squares is 1
2: Number of peeled squares is 2 to 5
1: More than 5 peeled squares

As can be seen from the results shown in Tables 3 and 4, the hot-pressed parts that met the conditions of the present invention had both excellent corrosion resistance in the cut area and adhesion of the coating layer.

Claims (2)

A steel plate,
A coating layer disposed on at least one surface of the steel plate;
and an oxide layer disposed on the coating layer,
The coating layer includes a metallic Zn phase and a granular FeAl alloy phase,
In a cross section perpendicular to the surface of the steel plate, R defined by the following formula (1) is 0.10 to 0.80.
R=L _Zn /L... (1)
Where:
L _Zn : the total length of the metallic Zn phase at the interface between the steel sheet and the coating layer. L: the length of the interface between the steel sheet and the coating layer. The hot-pressed member according to claim 1 , wherein the granular FeAl alloy phase has an average grain size of 5 μm or more in a cross section perpendicular to a surface of the steel plate.