Hot-pressed parts

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 = LZn / L, where LZn: 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

[Technical Field]

[0001] The present invention relates to a hot-pressed member, and more particularly to a hot-pressed member having excellent corrosion resistance at cut portions and excellent adhesion of a coating layer. [Background technology]

[0002] With the aim of reducing the weight of automobiles and improving collision safety, efforts are being made to increase the strength of automotive steel sheets. 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, dimensional accuracy issues arise, such as poor forming during press work and springback.

[0003] Therefore, the application of hot pressing technology, which forms steel sheets hot rather than cold, is increasing. Hot pressing is a forming method in which steel sheets are heated to the austenite temperature range, press-formed while still at high temperature, and simultaneously quenched by contact with a mold. In hot pressing, press forming is performed in a state where formability is improved by heating, and subsequent quenching increases strength, making it possible to manufacture hot-pressed parts with excellent strength and high dimensional accuracy.

[0004] Therefore, steel sheets having coatings such as Al-based plating layers, Zn-based plating layers, and Al-Zn-based plating layers on their surfaces have been proposed as hot-press steel sheets suitable for producing hot-press members (Patent Documents 1 to 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-049256 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-073774 [Patent Document 3] Japanese Patent Application Laid-Open No. 2005-113233 [Patent Document 4] International Publication No. 2017 / 195269 [Patent Document 5] International Publication No. 2019 / 180853 Summary of the Invention [Problem to be solved by the invention]

[0006] Hot-pressed members obtained by hot-pressing the above-mentioned steel sheets for hot pressing are used mainly for automotive components, particularly for frame structural members (inner panel frames) that require strength, but in recent years they have also come to be used as so-called quasi-exterior panel members, such as the members around the pillars that are visible when the door is opened. Therefore, hot-pressed members are required to be suitable for painting and to have excellent corrosion resistance in the cut parts after painting.

[0007] In addition, since hot-pressed members are usually spot-welded before use, they are also required to have excellent spot weldability.

[0008] However, the conventional techniques proposed in Patent Documents 1 to 5 have not been able to satisfy all of the above requirements, as will be explained below.

[0009] For example, in the technology proposed in Patent Document 1, a hot-dip aluminized steel sheet is used as a steel sheet for hot pressing to prevent scale formation and improve corrosion resistance. However, when an aluminized steel sheet is hot-pressed, Fe diffuses from the base steel sheet to the surface layer of the aluminized layer, forming an FeAl-based alloy layer. Generally, a chemical conversion coating is formed on a hot-pressed member before electrocoating to ensure paintability. However, since an FeAl-based alloy layer does not react with a chemical conversion treatment solution, a chemical conversion coating cannot be formed. Furthermore, since the FeAl-based alloy layer does not have sacrificial corrosion protection, corrosion resistance, such as corrosion resistance of cut parts, is insufficient.

[0010] On the other hand, in the technology proposed in Patent Document 2, a Zn-plated steel sheet is used as the steel sheet for hot pressing to ensure paint adhesion and corrosion resistance. However, when a Zn-plated steel sheet is hot pressed, a thick oxide layer is formed on the surface of the coating layer, resulting in insufficient spot weldability. In addition, because Zn has a low melting point, hot pressing a steel sheet with a Zn-based coating layer causes liquid metal embrittlement (LME) cracking, making it impossible to obtain sufficient fatigue resistance.

[0011] 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.

[0012] Therefore, it has been proposed to use Al-Zn plated steel sheets instead of Al plated steel sheets, which have problems with chemical conversion treatability, and Zn plated steel sheets, which have problems with spot weldability and LME cracking.

[0013] For example, Patent Document 4 proposes that a hot-pressed member be manufactured using an Al-Zn-plated steel sheet to form an interfacial layer of a specific composition at the interface between the base steel sheet and the plating layer. Patent Document 4 claims that providing the interfacial layer in the hot-pressed member can prevent LME cracking and improve fatigue resistance. However, this hot-pressed member still lacks sufficient corrosion resistance in the cut portion.

[0014] Patent Document 5 claims that by adding Mg to an Al-Zn-plated steel sheet, a Mg oxide layer is formed during hot pressing, preventing oxidation of Zn and resulting in a hot-pressed part in which the metallic Zn phase remains in the coating layer, resulting in high corrosion resistance. However, this hot-pressed part also lacks sufficient corrosion resistance in the cut area. Furthermore, voids formed between the granular FeAl alloy phase and the steel sheet sometimes prevented sufficient adhesion between the coating layer and the steel sheet.

[0015] As described above, even with technology using Al-Zn plated steel sheets, it has not yet been possible to realize hot-pressed components that combine high levels of cut corrosion resistance and plating adhesion.

[0016] The present invention has been made in consideration of the above-mentioned circumstances, and aims to achieve a high level of both corrosion resistance in cut areas and adhesion of the coating layer in a hot-pressed member having an Al-Zn-based coating layer that does not have the chemical conversion treatability problem of Al-plated steel sheets, or the spot weldability and LME cracking problems of Zn-based plated steel sheets. [Means for solving the problem]

[0017] As a result of research conducted to solve the above-mentioned problems, the present inventors discovered 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.

[0018] The present invention has been completed based on the above findings, and the gist of the present invention is as follows.

[0019] 1. A steel plate; A coating layer disposed on at least one surface of the steel plate; an oxide layer disposed on the coating layer, the coating layer includes a metallic Zn phase and a granular FeAl alloy phase, A hot-pressed member, wherein R, as 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 metal 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.

[0020] 2. The hot-pressed member according to 1 above, wherein the granular FeAl alloy phase has an average grain size of 5 μm or more in a cross section perpendicular to the surface of the steel plate. [Effects of the Invention]

[0021] According to the present invention, it is possible to provide a hot-pressed member having both high levels of corrosion resistance at the cut portion and high levels of adhesion of the coating layer. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, an embodiment of the present invention will be described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description. Furthermore, the unit of content, "%", represents "% by mass" unless otherwise specified.

[0023] (1) Hot-pressed parts 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.

[0024] [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 steel sheet is not particularly limited and any steel sheet can be used.

[0025] The hot-pressed member of the present invention is manufactured by hot-pressing a steel sheet for hot pressing, as described below. Therefore, the steel sheet can also be said to be a steel sheet formed by hot pressing. The steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.

[0026] From the viewpoint of use as an automobile part, etc., it is preferable that the hot-pressed part has high strength. In particular, to obtain a hot-pressed part having a strength of more than 1470 MPa, it is preferable to use a steel material having the following chemical composition.

[0027] 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: Contains 0.01% or less, The remainder consists of Fe and unavoidable impurities.

[0028] The effects and preferred contents of each element in the above-mentioned preferred component composition will be described below.

[0029] C: 0.20 to 0.35% C is an element that improves strength by forming a structure such as martensite. From the viewpoint of obtaining a strength exceeding 1470 MPa, the C content is preferably 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.

[0030] Si: 0.1 to 0.5% Silicon is an element that is effective in strengthening steel to obtain good material properties. To achieve this effect, the Si content is preferably 0.1% or more. On the other hand, if the Si content exceeds 0.5%, ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 0.5% or less.

[0031] Mn: 1.0 to 3.0% Mn is an element 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. On the other hand, 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.

[0032] P:0.02% or less If the P content is higher than 0.02%, local ductility deteriorates due to grain boundary embrittlement caused by P segregation to austenite grain boundaries during casting. As a result, the balance between strength and ductility of the steel sheet deteriorates. Therefore, from the viewpoint of improving the balance between strength and ductility of the steel sheet, it is preferable that the P content be 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 that the P content be 0.0005% or more.

[0033] S: 0.01% or less S becomes inclusions such as MnS, which can cause deterioration in impact resistance and cracking along the metal flow path of welds. Therefore, it is desirable to reduce the S content as much as possible, and specifically, it is preferable to set it to 0.01% or less. Furthermore, from the viewpoint of ensuring good stretch flangeability, it is more preferable to set it to 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 set the S content to 0.0002% or more.

[0034] Al: 0.1% or less Al is an element that acts as a deoxidizer. However, if the Al content exceeds 0.1%, 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.

[0035] N: 0.01% or less If the N content exceeds 0.01%, AlN is generated during heating before hot pressing, resulting in a decrease in hardenability. Therefore, the N content is preferably 0.01% or less. On the other hand, although there is no particular lower limit for the N content, from the viewpoint of refining costs, the N content is preferably 0.001% or more.

[0036] The above component composition may further optionally include: Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0002~0.0050%, Cr: 0.1-0.3%, and Sb: 0.003 to 0.03% It may contain at least one selected from the group consisting of:

[0037] Nb: 0.05% or less Nb is an effective component for strengthening steel, but excessive Nb content reduces shape fixability. Therefore, when Nb is added, 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.

[0038] Ti: 0.05% or less Like Nb, Ti is an effective component for strengthening steel, but excessive Ti content reduces shape fixability. 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.

[0039] B: 0.0002 to 0.0050% B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. When B is added, the B content is preferably 0.0002% or more to obtain this effect. On the other hand, excessive addition of B reduces formability. Therefore, when B is added, the B content is preferably 0.0050% or less.

[0040] Cr: 0.1 to 0.3% Cr is an element useful for strengthening steel and improving its hardenability. When adding Cr, the Cr content is preferably 0.1% or more to obtain this effect. However, since Cr is an expensive element, adding excessive Cr significantly increases costs. Therefore, when adding Cr, it is more preferable to limit the Cr content to 0.3% or less.

[0041] Sb: 0.003 to 0.03% Sb is an element that has the effect of suppressing decarburization of the surface layer of a steel sheet during hot pressing. When Sb is added, the Sb content is preferably 0.003% or more to obtain this effect. On the other hand, if the Sb content exceeds 0.03%, the rolling load increases, resulting in a decrease in productivity. Therefore, when Sb is added, the Sb content is preferably 0.03% or less.

[0042] [Coating layer] The hot-pressed member of the present invention includes 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.

[0043] The coating layer includes 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 when an Al-plated steel sheet is used, the hot-pressed member of the present invention does not have problems with chemical conversion treatability, and unlike when a Zn-based plated steel sheet is used, it does not have problems with spot weldability and LME cracking.

[0044] The metallic Zn phase contained in the coating layer exerts a sacrificial corrosion protection effect in a corrosive environment, thereby achieving high corrosion resistance of the cut portion. Even if Zn is contained in the coating layer, 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.

[0045] Furthermore, even when a steel sheet having an Al-Zn-based plating layer is hot-pressed to form a hot-pressed member, depending on the composition of the plating layer, some or all of the FeAl alloy phase contained in the coating layer after hot pressing may be layered rather than granular. If the FeAl alloy phase is layered rather than granular, the desired cut corrosion resistance 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, preventing the sacrificial corrosion protection effect of the metallic Zn phase from effectively acting on the base steel sheet. Therefore, it is preferable that the FeAl alloy phase contained in the coating layer is all granular.

[0046] 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 metal 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

[0047] Said L Zn 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).

[0048] 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. Note that, if a layered FeAl alloy phase is formed on the base steel sheet side of the coating layer, the sacrificial corrosion protection effect of the metallic Zn phase does not effectively act on the base steel sheet, as mentioned above, resulting in insufficient corrosion resistance in the cut portion. For example, in the hot-pressed member of Patent Document 5, a layered FeAl alloy phase is formed between the metallic Zn phase and the base steel sheet. Therefore, sufficient corrosion resistance in 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 in the cut portion is further improved. Therefore, it is more preferable that R be 0.30 or more.

[0049] On the other hand, if R is greater than 0.80, the fluidity of the metal Zn phase during hot pressing becomes significant, resulting in a decrease in the adhesion of the coating layer. Therefore, R is set to 0.80 or less.

[0050] 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 a 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 thought to be as follows. Specifically, when an Al-Zn-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, if the average particle size of the granular FeAl alloy phase is large, such as 5 μm or more, metallic Zn can easily penetrate between the granular FeAl alloy phase, reducing the voids, thereby further improving 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 without metallic Zn increases, which may deteriorate the corrosion resistance of the cut portion. Therefore, it is preferable that the average particle size be 15 μm or less.

[0051] Here, the average particle size of the granular FeAl alloy phase is defined as the average value of (major axis + minor axis) / 2 of each granular FeAl alloy phase. The major axis and minor axis can be determined by analyzing an image obtained by SEM observation of a cross section of the hot-pressed member perpendicular to the surface of the steel plate.

[0052] The coating amount of the coating layer is not particularly limited, but from the viewpoint of corrosion resistance, the coating amount is set to 60 g / m per one side of the steel sheet. 2 On the other hand, from the viewpoint of manufacturing costs, the coating amount is preferably 400 g / m per one side of the steel sheet. 2 The coating layer adhesion weight can be determined by dissolving and removing the coating layer from the surface of the hot-pressed part using an acid solution, and subtracting the weight of the hot-pressed part after removal from the weight of the hot-pressed part before removal. An inhibitor that suppresses dissolution of the base steel sheet is added to the acid solution.

[0053] In the present invention, the desired properties are achieved by combining the metallic Zn phase and the granular FeAl alloy phase as described above, and 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 other components are not particularly limited.

[0054] However, the coating layer preferably has the following component composition: In mass%, Zn: 30.0 to 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.

[0055] It is preferable that at least one of Sr and Ca is contained in the coating layer, but both may be contained.

[0056] [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.

[0057] 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, and 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 that the thickness of the oxide layer be 0.6 μm or less. Furthermore, if the oxide layer is sufficiently thin, there is almost no decrease in the metallic Zn phase in the coating layer, and therefore the corrosion resistance of the cut portion is further improved. From this viewpoint, it is more preferable that the thickness of the oxide layer be 0.3 μm or less. On the other hand, from the viewpoint of corrosion resistance of the cut portion, the thinner the thickness of 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.

[0058] (2) Steel sheets for hot pressing The hot-pressed member of the present invention can be produced by hot-pressing a steel sheet (steel sheet for hot press) having a plating layer, as described below. Hereinafter, the steel sheet for hot press that can be used for producing the hot-pressed member of the present invention will be described.

[0059] The steel sheet for hot press use comprises a steel sheet and a plating layer disposed on at least one surface of the steel sheet.

[0060] [Steel plate] The steel sheet is not particularly limited and any steel sheet can be used. The steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet. The composition of the steel sheet is also not particularly limited, but it is preferable to use a steel sheet having the component composition described above in the description of the hot-pressed member.

[0061] [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.

[0062] 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 must have the following component composition. Zn: 30.0 to 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.

[0063] The plating layer is Zn: 35.0-65.0%, 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 be Al and unavoidable impurities. Each component will be described below.

[0064] Zn: 30.0 to 70.0% If the Zn content in the coating layer is less than 30.0%, the metallic Zn phase in the coating layer is insufficient or absent, making it impossible to achieve the desired cut-portion corrosion resistance. Therefore, the Zn content is set to 30.0% or more. If the Zn content is 35.0% or more, a larger amount of metallic Zn phase is present in the coating layer, further improving the cut-portion corrosion resistance. Therefore, the Zn content is preferably set to 35.0% or more. On the other hand, if the Zn content exceeds 70.0%, Zn may exist as a ZnFe alloy phase after hot pressing, and metallic Zn phase may not be present. Furthermore, even if metallic Zn phase is present, granular FeAl alloy phase is not present. Therefore, the desired cut-portion corrosion resistance cannot be achieved. Therefore, the Zn content is set to 70.0% or less. Even if the Zn content is 70.0% or less, if it exceeds 65.0%, the average particle size of the granular FeAl alloy phase in the coating layer becomes small, resulting in excessive voids not filled with metallic Zn phase and degrading corrosion resistance. Therefore, the Zn content is preferably set to 65.0% or less.

[0065] Si: 1.1 to 8.0% Si is an element that suppresses alloying of the coating layer during the plating process and the heat treatment process before hot pressing. If the Si content in the coating layer is less than 1.1%, the granular FeAl alloy phase in the coating layer after hot pressing will enlarge, resulting in insufficient metallic Zn phase in the gaps between the granular FeAl alloy phase and the steel sheet, making it impossible to achieve the desired corrosion resistance in the cut area. Therefore, the Si content is set to 1.1% or more, preferably 1.3% or more. On the other hand, if the Si content is excessive, the amount of Si-based oxides produced will increase, resulting in impaired chemical conversion treatability and inferior corrosion resistance. Therefore, the Si content is set to 8.0% or less, preferably 4.0% or less.

[0066] Sr+Ca: 0.01 to 5.0% Sr and Ca are elements that improve weldability by suppressing the formation of an oxide layer. Specifically, Sr and Ca are preferentially oxidized to form a surface barrier, suppressing the formation of the oxide layer. This prevents the Zn contained in the coating layer from oxidizing and remains within the coating layer, thereby improving corrosion resistance. Therefore, in the present invention, it is important that the coating layer contains one or both of Sr and Ca. However, if the total content of Sr and Ca is less than 0.01%, the desired effect cannot be achieved. Therefore, the total content of Sr and Ca is set to 0.01% or more, preferably 0.1% or more. On the other hand, if the Sr and Ca content is excessive, the formation of Sr and Ca oxides themselves will be excessive, resulting in poor chemical conversion treatability and poor coating adhesion, and as a result, the expected corrosion resistance will not be achieved. Therefore, the total content of Sr and Ca is set to 5.0% or less, preferably 1.0% or less. In the present invention, the total content of Sr and Ca is expressed as "Sr + Ca."

[0067] The plating layer can be formed by any method without any particular limitation, but is preferably formed by a hot-dip plating method, in other words, the plating layer is preferably a hot-dip plated layer.

[0068] The coating weight of the plating layer is not particularly limited, but from the viewpoint of corrosion resistance, a coating weight of 30 g / m 2 On the other hand, from the viewpoint of manufacturing costs, the deposition amount is preferably 200 g / m or more. 2 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 of the steel sheet for hot press use 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.

[0069] Furthermore, in order for the R-value of the final hot-pressed member to satisfy the above-mentioned condition, annealing under specific conditions is required during the production of the steel sheet for hot pressing. Specifically, during the cooling process after the steel sheet is pulled out of the hot-dip galvanizing bath, the plated steel sheet is held at a temperature of 250 to 350°C for 5 to 30 seconds. Holding at a temperature of 250 to 350°C for 5 to 30 seconds prevents the plated layer from solidifying in a non-equilibrium state due to rapid cooling, releases strain introduced into the plated layer, and promotes two-phase separation of the α-Al phase and metallic Zn phase in the plated layer. As a result, the metallic Zn phase increases in the voids between the granular FeAl alloy phase and the steel sheet after hot pressing, enabling improved corrosion resistance and adhesion of the coating layer.

[0070] If the holding temperature is less than 250°C or the holding time is less than 5 seconds, sufficient phase separation is not achieved, 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, excessive phase separation occurs, and the R-value cannot be 0.80 or less. In this case, the average particle size of the granular FeAl alloy phase in the hot-pressed member becomes small.

[0071] (3) Manufacturing method of hot-pressed parts Next, a preferred method for producing the hot-pressed member of the present invention will be described.

[0072] The hot-pressed member of the present invention can be produced by hot-pressing a steel sheet for hot pressing that satisfies the above-mentioned conditions. In particular, as described above, it is important that the steel sheet for hot pressing that is used is one that has been annealed under specific conditions after plating.

[0073] The method of hot pressing is not particularly limited and can be carried out according to a conventional method. Typically, the steel sheet for hot pressing is heated to a predetermined heating temperature (heat treatment step), and then the steel sheet for hot pressing heated in the heat treatment step is hot pressed (hot pressing step). Preferred hot pressing conditions are described below.

[0074] [Heat treatment] In the heat treatment process, the steel sheet for hot pressing is heated to a heating temperature of not less than the Ac3 transformation point and not more than 980°C. By setting the heating temperature at or above the Ac3 transformation point, the structure of the steel sheet can be austenitized. The austenite then transforms into a hard phase, such as martensite, through rapid cooling during hot pressing, resulting in high strength of the hot-pressed part. If the heating temperature is lower than the Ac3 transformation point, the austenite fraction in the heated steel sheet decreases, resulting in an insufficient volume fraction of martensite after hot pressing, making it impossible to ensure sufficient tensile strength. 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 further enlarges, reducing the metallic Zn phase in the coating layer. As a result, the expected corrosion resistance cannot be achieved. Therefore, a hot-pressed part satisfying the requirements of the present invention cannot be obtained. Furthermore, if the heating temperature is higher than 980°C, the crystal grain size becomes excessively coarse, resulting in reduced bending crushability.

[0075] The Ac3 transformation point can be determined by the following formula (1). Ac3 transformation point (℃) = 881-206C + 53Si-15Mn-20Ni-1Cr-27Cu+41Mo…(1) In formula (1), the element symbols represent the content (mass%) of each element. The content of elements that are not contained is calculated as 0.

[0076] In the heat treatment process, after heating to the heating temperature, the heating temperature can be maintained for a holding time of 5 minutes or less. If the holding time is longer than 5 minutes, excessive alloying of the plating layer will occur, resulting in an excessive Fe content in the coating layer. In addition, the granular FeAl alloy phase will enlarge, reducing the metallic Zn phase in the coating layer, resulting in an inability to obtain the desired corrosion resistance. Therefore, a hot-pressed member that meets the requirements of the present invention will not 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.

[0077] The method for heating the steel sheet for hot press use in the heat treatment step 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.

[0078] [Hot press] After the heating, the steel sheet for hot pressing is hot pressed to obtain a hot pressed member. In the hot pressing, cooling is carried out using a mold or a coolant such as water simultaneously with or immediately after the processing. In the present invention, the hot pressing conditions are not particularly limited. For example, pressing can be carried out at a general hot pressing temperature range of 600 to 800°C. [Example]

[0079] In order to confirm the effects of the present invention, a steel plate for hot press use and a hot press member using the steel plate for hot press use were produced, and their properties were evaluated.

[0080] ·Hot press steel plates A coating layer was formed on the surface of a steel sheet using the following procedure to prepare a steel sheet for hot pressing. Specifically, a coating layer was formed on both sides of a 1.4 mm thick steel sheet using continuous hot-dip galvanizing equipment. The steel sheet used was a cold-rolled steel sheet containing 0.24% C, 0.25% Si, 1.3% Mn, 0.01% P, 0.002% S, 0.03% Al, 0.005% N, 0.16% Cr, 0.03% Ti, 0.002% B, and 0.008% Sb, with the balance consisting of Fe and unavoidable impurities. The Ac3 transformation point of the cold-rolled steel sheet was 825°C. The temperature of the coating bath was 600°C, and the coating weight of the coating layer was 100 g / m per side of the steel sheet. 2 , i.e., 200 g / m on both sides 2 It was decided.

[0081] In the hot-dip galvanizing, annealing was carried out under the conditions (holding temperature, holding time) shown in Tables 1 and 2. After the steel sheet was pulled out of the hot-dip galvanizing bath, it was cooled with nitrogen gas until it reached the holding temperature. For comparison, annealing was not carried out in some examples (Comparative Example No. 2).

[0082] (Composition of plating layer) The component composition of the resulting plating layer was measured by area analysis using SEM (scanning electron microscope)-EDX (energy dispersive X-ray analysis). The SEM-EDX analysis was performed using a JEOL SEM (JSM-7200F) and a Thermo Fisher EDX detector (UltraDry) at an accelerating voltage of 15.0 kV. The results are shown in Tables 1 and 2.

[0083] Hot-pressed parts Next, each of the obtained steel plates for hot pressing was hot pressed under the conditions shown in Tables 1 and 2 to produce hot-pressed members. Specifically, the steel plates for hot pressing were first cut into pieces measuring 70 mm x 150 mm and subjected to heat treatment in an electric furnace. The heating temperature and holding time at the heating temperature in the heat treatment were as shown in Tables 1 and 2. Next, the steel plates for hot pressing were removed from the electric furnace and hot pressed using a flat die. The forming start temperature was 700°C.

[0084] Next, the presence or absence of a granular FeAl alloy phase and metallic Zn phase, the R value, the average particle size of the granular FeAl alloy phase, and the thickness of the oxide layer were measured for each of the obtained hot-pressed parts using the following procedures. The measurement results are shown in Tables 3 and 4.

[0085] (granular FeAl alloy phase) The presence or absence of an FeAl alloy phase in the coating layer was determined by X-ray diffraction measurement. A SmartLab X-ray diffractometer manufactured by Rigaku Corporation was used for the measurement. The measurement conditions were: X-ray: 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 on the upper surface of the hot-pressed member, and the presence or absence of a granular FeAl alloy phase in the coating layer was determined by cross-sectional observation. Note that in some examples (Comparative Examples Nos. 19, 26, and 38), no granular FeAl alloy phase was present in the coating layer, and the FeAl alloy phase was entirely layer-like. However, in the other examples, the FeAl alloy phase was entirely granular.

[0086] (metallic 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 diffractometer SmartLab manufactured by Rigaku Corporation was used. The measurement conditions were: X-ray: Cu-Kα, tube voltage: 40 kV, tube current: 30 mA, scanning speed: 4° / min.

[0087] (R value) 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 section perpendicular to the surface of the steel plate was observed to determine the total length L of the metal Zn phase on the surface of the coating layer facing the steel plate. Zn 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-pressed member was observed at a magnification of 500 times using an SEM, and L Zn and L were measured. Zn The R value was calculated from L by the formula (1). The SEM observation was carried out in 10 randomly selected fields of view. Zn and L was the average value in 10 fields of view.

[0088] (Average particle size of granular FeAl alloy phase) A test piece for cross-sectional observation was taken from the flat portion of the top surface of the hot-pressed 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-pressed member was observed at 500x magnification 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. The obtained major and minor diameters were used to calculate the grain size of each granular FeAl alloy phase, defined as (major diameter + minor diameter) / 2, and the average value of these grain sizes was used as the average grain size of the granular FeAl alloy phase.

[0089] (oxide layer thickness) A test piece for cross-sectional observation was taken from the flat part on the top surface of the hot-pressed part, 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 part was observed using an SEM at 500x magnification, and the thickness of the oxide layer was measured at 20 randomly selected locations, and the average value was taken as the oxide layer thickness.

[0090] Next, the corrosion resistance of the cut portion and the adhesion of the coating layer were evaluated for each of the hot-pressed members obtained by the following procedure. The measurement results are shown in Tables 3 and 4.

[0091] (Corrosion resistance of cut parts) Test pieces taken from the hot-pressed components were subjected to a phosphate-based chemical conversion treatment and electrodeposition coating to prepare test pieces for corrosion resistance evaluation. Cross-cuts (60° angle), each 80 mm long and measuring a total of 160 mm, were made in the center of the test pieces for corrosion resistance evaluation, and then subjected to a corrosion test (SAE-J2334). The corrosion resistance of the cut portion was evaluated based on the occurrence of red rust after 30 cycles, using the following criteria: Rating 4: No red rust on the cut area Grade 3: The length of the cut scratch where red rust has occurred is less than 2 mm Grade 2: The length of the cut scratch where red rust has occurred is 2 mm or more but less than 4 mm Grade 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 be sufficient cut corrosion resistance.

[0092] (Adhesion of coating layer) A test piece taken from the hot-pressed member was scored with 11 cuts, reaching down to the base steel sheet, in both the vertical and horizontal directions at 1 mm intervals, creating 100 grids. Cellophane tape (registered trademark) was firmly pressed onto the grids, and the edge of the tape was quickly peeled off at a 45° angle. The number of squares of plating that peeled off from the test piece surface was counted and evaluated according to the following criteria. A score of 3 or higher was considered to indicate sufficient coating layer adhesion. 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

[0093] As can be seen from the results shown in Tables 3 and 4, the hot-pressed members that satisfied the conditions of the present invention had both excellent corrosion resistance in the cut portion and adhesion of the coating layer.

[0094] [Table 1]

[0095] [Table 2]

[0096] [Table 3]

[0097] [Table 4]

Claims

1. Steel plate and A coating layer disposed on at least one surface of the steel plate; 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 metal 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 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 the surface of the steel plate.

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