High-strength steel member and method for manufacturing the same

Abstract

To provide a high strength steel member excellent in appearance corrosion resistance and resistance welding crack characteristics and with a little risk of delayed fracture in a corrosive environment.SOLUTION: A high strength steel member comprises a hot-pressed member and a Zn-based plating layer on at least one surface of the hot-pressed member. A thickness of an Fe-Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1.0 μm. Natural immersion potential in 0.5 mass% NaCl aqueous solution air-saturated at 25°C is -1100 to -800 mV in silver- silver chloride-saturated potassium chloride electrode standard, and Vickers hardness is 400 or above.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a high-strength steel member, particularly to a high-strength steel member that has excellent appearance corrosion resistance and resistance weld cracking resistance and has a low risk of delayed fracture in a corrosive environment. The present invention also relates to a method for manufacturing the high-strength steel member. [Background technology]

[0002] In recent years, the automotive industry has been promoting the improvement of the performance of steel sheets and the reduction of weight, leading to an increase in the use of high-strength steel sheets. However, as the strength of steel sheets increases, press formability generally decreases, making it difficult to obtain complex part shapes.

[0003] Given this background, the use of hot pressing technology, which forms steel sheets hot rather than cold, is increasingly being applied. Hot pressing is a forming method in which steel sheets are heated to the austenite single-phase temperature range (around 900°C), then press-formed while still at high temperature, and simultaneously rapidly cooled (quenched) by contact with a die. Because the steel is press-formed in a heated, softened state and then quenched to increase its strength, hot pressing allows for the production of high-strength components while maintaining press formability. Furthermore, because hot pressing generates almost no residual stress in the processed area, the risk of delayed fracture due to corrosion is significantly lower than in cold-pressed products of similar strength.

[0004] On the other hand, automotive components are also required to have high corrosion resistance. The corrosion resistance required for automotive components can be broadly divided into resistance to perforation corrosion (perforation corrosion resistance) and resistance to cosmetic corrosion (cosmetic corrosion resistance). Perforation corrosion is, as the name suggests, corrosion that forms through holes in the steel material that makes up the component. On the other hand, cosmetic corrosion is corrosion that damages the appearance, such as the formation of red rust and paint blistering due to corrosion.

[0005] In the field of hot pressing, it is common to use Al-plated steel sheets to provide rust prevention to hot pressed parts. By using Al-plated steel sheets as the material, the puncture corrosion resistance of hot pressed parts is significantly improved.

[0006] However, while hot-pressed components made from conventional Al-based plated steel sheets have excellent perforation corrosion resistance, they suffer from the problem of poor external corrosion resistance. Specifically, hot-pressed components made from Al-based plated steel sheets have intermetallic compound phases and diffusion layers on the surface, primarily composed of Al, a component of the plating, and Fe diffused from the base steel sheet. However, these phases have a small potential difference with the base material (steel), resulting in little anodic protection for the base material. Therefore, at locations where there is no plating layer, such as cut edges, and where the upper coating is thin, the base material quickly corrodes, resulting in red rust. Furthermore, because the intermetallic compounds themselves contain a high concentration of Fe, red rust can form during anodic protection, regardless of whether the base material is corroded or not.

[0007] As described above, conventional hot-pressed members using general Al-based plated steel sheets have excellent perforation corrosion resistance, but their appearance corrosion resistance is insufficient compared to pressed members manufactured by cold pressing zinc-based plated steel sheets.

[0008] One of the reasons for the insufficient external corrosion resistance is poor chemical conversion treatability. That is, in the production of general automotive components, hot-pressed components are subjected to a zinc phosphate-based chemical conversion treatment as a surface treatment, and then painted. Because a chemically stable Al oxide film is formed on the outermost surface of the Al-plated steel sheet after hot pressing, almost no zinc phosphate-based chemical conversion film is formed during the chemical conversion treatment.

[0009] Against this background, various technologies have been proposed to improve the appearance, corrosion resistance, and other properties of hot-pressed parts made of Al-based plated steel sheets.

[0010] For example, Patent Document 1 proposes a technique of using a plated steel sheet having an Al plated layer and a surface coating layer containing ZnO formed on the Al plated layer as a steel sheet for hot pressing.

[0011] Furthermore, Patent Document 2 proposes a method in which a plated steel sheet having an Al-based plating layer is heated in an atmosphere in which the hydrogen concentration and dew point are controlled, and then hot-pressed. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] International Publication No. 2009 / 131233 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-051543 Summary of the Invention [Problem to be solved by the invention]

[0013] However, as a result of the inventors' investigations, it was found that the conventional techniques proposed in the above Patent Documents 1 and 2 still provide insufficient external corrosion resistance on the painted end faces of hot-pressed parts.

[0014] For example, Patent Document 1 reports that forming a coating containing ZnO, a wurtzite compound, improves corrosion resistance after painting. However, Patent Document 1 evaluates corrosion resistance after painting based only on the width of paint blistering, and does not consider the occurrence of red rust. In actual automotive components, it is necessary to suppress not only paint blistering but also the occurrence of red rust, which has a significant impact on appearance. The technology in Patent Document 1 improves chemical conversion treatability, thereby suppressing paint blistering, but the red rust resistance was not sufficient.

[0015] On the other hand, as described in Patent Document 1, Zn-plated steel sheets are sometimes used as steel sheets for hot pressing. When Zn-plated steel sheets are subjected to hot pressing, the Zn in the plating layer and the Fe in the base material interdiffuse during the heating process, forming an intermetallic compound phase or solid solution phase containing Zn and Fe in the surface layer of the member. In this case, by adjusting the heating conditions, an intermetallic compound phase containing a high concentration of Zn can be formed on the outermost surface of the member. The hot-pressed member obtained in this manner has corrosion resistance comparable to that obtained by cold-pressing a galvannealed steel sheet.

[0016] However, since the intermetallic compound phase inevitably contains Fe, the Fe may corrode, resulting in a corroded appearance with red rust mixed in. Therefore, the hot-pressed member obtained by hot-pressing a Zn-based coated steel sheet does not have sufficient external corrosion resistance compared to an electrogalvanized steel sheet and a hot-dip galvanized steel sheet that has not been subjected to an alloying treatment.

[0017] Even when using Zn-based plated steel sheets, the appearance corrosion resistance can be improved to some extent by adjusting the heating conditions. For example, if the temperature is raised in a short time and the holding time at high temperature is very short, the Zn-rich phase that does not contain Fe remains, improving the appearance corrosion resistance.

[0018] However, if hot pressing is performed while the Zn-rich phase containing no Fe remains as described above, the risk of liquid metal embrittlement (LME) cracking increases significantly. Therefore, it is not practical to adopt the heating conditions described above when hot pressing Zn-based coated steel sheets.

[0019] Furthermore, as a result of the inventors' investigations, it was found that cracks are likely to occur in resistance welds when an intermetallic compound phase or solid solution phase containing Zn and Fe is present in the surface layer of a component, particularly directly above the base steel sheet. Although the detailed mechanism is unclear, it is speculated that Zn melts or vaporizes during the rapid heating process during resistance welding, acting on the base material, causing a phenomenon similar to liquid metal embrittlement.

[0020] As described above, it is difficult to achieve both excellent corrosion resistance and prevention of liquid metal embrittlement cracking during forming in hot-pressed members using Zn-based plated steel sheets. Also, hot-pressed members using Zn-based plated steel sheets have a high risk of resistance weld cracking.

[0021] The present invention has been made in view of the above-described circumstances, and aims to provide a high-strength steel member that has excellent appearance corrosion resistance, a low risk of resistance weld cracking, and a low risk of delayed fracture in a corrosive environment. [Means for solving the problem]

[0022] The present invention has been made to solve the above problems, and the gist of the present invention is as follows.

[0023] 1. A high-strength steel member having a hot-pressed member and a Zn-based plating layer on at least one surface of the hot-pressed member, the thickness of the Fe-Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1.0 μm; The natural immersion potential in an air-saturated 0.5 mass % NaCl aqueous solution at 25°C is -1100 to -800 mV relative to a silver-silver chloride-saturated potassium chloride electrode, and High-strength steel components with a Vickers hardness of 400 or more.

[0024] 2. The amount of Zn deposited on one side of the hot-pressed member is 5 to 100 g / m 2 2. The high-strength steel member according to 1 above, wherein

[0025] 3. The high-strength steel member according to 1 or 2 above, further comprising an Fe—Al alloy layer between the hot-pressed member and the Zn-based plating layer.

[0026] 4. The high-strength steel member according to any one of the above items 1 to 3, wherein a coverage of an Fe oxide layer having a thickness of 1 μm or more between the hot-pressed member and the Zn-based plating layer is 10% or less.

[0027] 5. A heating step of heating the steel plate for hot pressing; a hot pressing step of hot pressing the heated steel plate for hot pressing to form a hot-pressed member; and a Zn-based plating step of forming a Zn-based plating layer having a Zn content of 70 atomic % or more on at least one surface of the hot-pressed member at a temperature of 250°C or less.

[0028] 6. The method for producing a high-strength steel member according to 5 above, wherein the steel sheet for hot press use is an Al-plated steel sheet.

[0029] 7. A method for producing a high-strength steel member according to 5 or 6 above, further comprising, between the hot pressing step and the Zn-based plating step, an oxide removal step of removing oxides present on the surface of the hot-pressed member. [Effects of the Invention]

[0030] According to the present invention, it is possible to provide a high-strength steel member that has excellent appearance corrosion resistance, a low risk of resistance weld cracking, and a low risk of delayed fracture in a corrosive environment. DETAILED DESCRIPTION OF THE INVENTION

[0031] Hereinafter, embodiments for carrying out the present invention will be specifically described. Note that the following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description. Furthermore, "%" used as a unit of content represents "% by mass" unless otherwise specified.

[0032] [High strength steel parts] A high-strength steel member according to one embodiment of the present invention includes a hot-pressed member as a base material and a Zn-based plating layer on at least one surface of the hot-pressed member, and satisfies the following conditions: The thickness of the Fe—Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1 μm. The natural immersion potential in an air-saturated 0.5 mass% NaCl aqueous solution at 25°C is -1100 to -800 mV relative to a silver-silver chloride-saturated potassium chloride electrode. Vickers hardness is 400 or more.

[0033] (hot-pressed parts) The high-strength steel member of the present invention has a hot-pressed member as a base material. The hot-pressed member is a member obtained by press-forming a steel plate in a state where it is heated to a high temperature and its strength is reduced. Therefore, springback does not occur and the member has excellent shape fixability. Furthermore, the hot-pressed member generates almost no residual stress in the processed portion. Because the residual stress is small, the risk of delayed fracture is significantly lower in the hot-pressed member than in a cold-press-formed member with the same strength.

[0034] In the present invention, the above-mentioned problems are solved by controlling the thickness of the Fe-Zn alloy layer and the natural immersion potential, as will be described later. Therefore, the hot-pressed member is not particularly limited and any hot-pressed member can be used.

[0035] However, in order to make the Vickers hardness of the high-strength steel member 400 or more, it is desirable to increase the Vickers hardness of the hot-pressed member used as the base material. In order to increase the Vickers hardness of the hot-pressed member, it is preferable that the hot-pressed member have the following composition.

[0036] In mass%, C: 0.1 to 0.5%, Si: 0.1 to 5.0% Mn: 0.1 to 5.0% P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, and N: Contains 0.01% or less, The balance is Fe and unavoidable impurities.

[0037] The component composition may further be optionally Nb: 0.05% or less, Ti: 0.05% or less, B: 0.0050% or less, Cr: 20% or less, and Sb: 0.03% or less It may contain at least one selected from the group consisting of:

[0038] The effects and preferred contents of each element in the above-mentioned preferred component composition will be described below.

[0039] C: 0.1 to 0.5% C is an element that improves strength by forming structures such as martensite. From the viewpoint of increasing Vickers hardness, it is preferable that the C content be 0.1% or more. On the other hand, if the C content exceeds 0.5%, the toughness of the spot welds deteriorates. Therefore, it is preferable that the C content be 0.5% or less.

[0040] Si: 0.1 to 5.0% Silicon is an effective element for strengthening steel and obtaining good material properties. To obtain this effect, the Si content is preferably 0.1% or more. On the other hand, if the Si content exceeds 5.0%, ferrite is stabilized, resulting in a decrease in hardenability. Therefore, the Si content is preferably 5.0% or less.

[0041] Mn: 0.1 to 5.0% Mn is an element effective in increasing the strength of steel. From the viewpoint of ensuring excellent mechanical properties and strength, it is preferable that the Mn content be 0.1% or more. On the other hand, excessive Mn content increases surface segregation during annealing, which affects the adhesion of the Zn-based coating layer to the hot-pressed member. Therefore, from the viewpoint of improving adhesion, it is preferable that the Mn content be 5.0% or less.

[0042] P:0.02% or less If the P content is excessive, 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 hot-pressed member deteriorates. Therefore, from the viewpoint of improving the balance between strength and ductility, it is preferable that the P content be 0.02% or less. On the other hand, there is no particular restriction on the lower limit of the P content, and it may be 0%. However, since an excessive reduction leads to an increase in manufacturing costs, it is preferable that the P content be 0.001% or more.

[0043] 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, there is no particular restriction on the lower limit of the S content, and it may be 0%. However, since excessive reduction leads to an increase in manufacturing costs, it is preferable that the S content be 0.0001% or more.

[0044] 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.

[0045] N: 0.01% or less If the N content exceeds 0.01%, AlN is generated during heating before hot pressing, resulting in reduced hardenability. 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 and may be 0%. However, since excessive reduction leads to increased manufacturing costs, the N content is preferably 0.001% or more.

[0046] 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%.

[0047] 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%.

[0048] B: 0.0050% or less B is an element that has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries. However, the addition of excessive B significantly impairs formability. Therefore, when B is added, from the viewpoint of improving formability, the B content is preferably 0.0050% or less. On the other hand, although there is no lower limit for the B content, from the viewpoint of enhancing the effect of adding B, it is preferably 0.0002% or more.

[0049] Cr:20% or less Cr is a useful element for strengthening steel and improving hardenability. However, because Cr is an expensive element, when Cr is added, the Cr content is preferably 20% or less to reduce alloy costs. On the other hand, the lower limit of the Cr content is not particularly limited and may be 0%, but from the viewpoint of enhancing the effect of adding Cr, it is preferably 0.1% or more.

[0050] Sb: 0.03% or less Sb is an element that has the effect of preventing decarburization of the surface layer of a steel sheet during hot pressing. However, excessive Sb increases the rolling load, thereby reducing productivity. Therefore, when Sb is added, the Sb content is preferably 0.03% or less from the viewpoint of further improving productivity. On the other hand, the lower limit of the Sb content is not particularly limited and may be 0%, but from the viewpoint of enhancing the effect of adding Sb, it is preferably 0.003% or more.

[0051] The hot-pressed member can be manufactured by hot-pressing a steel plate for hot pressing, as described below. Therefore, the hot-pressed member can also be said to be a hot-pressed steel plate. The hot-pressed member having the above-mentioned component composition can be obtained by hot-pressing a steel plate for hot pressing having the same component composition.

[0052] (Zn-based plating layer) The high-strength steel member of the present invention has a Zn-based plating layer on the surface of a hot-pressed member serving as a base material. The Zn-based plating layer may be provided on at least one surface of the hot-pressed member, and may be provided on both surfaces. The Zn-based plating layer may be provided on the entire surface of the hot-pressed member, or on only a portion of the surface. In other words, the high-strength steel member may have a portion that is not covered with the Zn-based plating layer. In the present invention, the term "Zn-based plating layer" is defined as a plating layer containing 50 atomic % or more of Zn.

[0053] In the present invention, the desired corrosion resistance is achieved by controlling the natural immersion potential as described below. Therefore, the composition of the Zn-based plating layer is not particularly limited, and may be any composition containing 50 atomic % or more of Zn. The Zn-based plating layer may be a layer containing one or both of an intermetallic compound and an alloy (solid solution), or may be a layer consisting of one or both of an intermetallic compound and an alloy (solid solution). Examples of the intermetallic compound include Ni2Zn 11 , Co2Zn 11, MgZn2, etc. Examples of the solid solution include a solid solution of Zn in one or both of Fe and Al. The Zn-based plating layer may be a Zn plating layer.

[0054] From the viewpoint of corrosion resistance, the Zn content in the Zn-based plating layer is preferably 60 atomic % or more, more preferably 80 atomic % or more, and even more preferably 90 atomic % or more. On the other hand, the upper limit of the Zn content is not particularly limited and may be 100%. In other words, the Zn-based plating layer may be a plating layer consisting of Zn and unavoidable impurities.

[0055] If the Zn-based plating layer contains 10 atomic % or more of Fe, the corrosion products will be reddish due to Fe oxides, and the external corrosion resistance will be deteriorated. Therefore, from the viewpoint of further improving the external corrosion resistance, it is preferable that the Fe content in the Zn-based plating layer be 10 atomic % or less.

[0056] The amount of Zn coated on the high-strength steel member of the present invention is not particularly limited. However, the amount of Zn coated is preferably 5 g / m. 2 If the Zn coating weight is less than 5g / m, the period during which the effect of preventing red rust formation can be obtained will be shortened. 2 It is preferable that the content is 10 g / m or more. 2 More preferably, it is 20 g / m or more. 2 On the other hand, the Zn coating weight is more preferably 100 g / m or more. 2 If the Zn coating weight exceeds 100 g / m, cracking may occur in the weld even if no Fe-Zn alloy layer is present. Therefore, from the viewpoint of further improving resistance weld cracking resistance, it is recommended to set the Zn coating weight to 100 g / m. 2 It is preferable that the density is 80 g / m or less. 2 It is more preferable to set the following:

[0057] The Zn deposition amount can be measured by subjecting a high-strength steel member to anodic electrolysis to dissolve Zn, and quantifying the amount of dissolved Zn by inductively coupled plasma-mass spectrometry (ICP-MS). Specifically, first, constant-current anodic electrolysis is performed in a 3% sodium hydroxide-1% aluminum chloride aqueous solution, with the high-strength steel member as the working electrode and a platinum mesh electrode as the counter electrode. The current density in the anodic electrolysis is 4 mA / cm. 2 During anodic electrolysis, the electrolysis is stopped when the potential becomes increasingly noble. This allows all of the Zn to dissolve in the aqueous solution. The amount of Zn in the aqueous solution is then quantitatively analyzed using ICP-MS to determine the total amount of dissolved Zn. The Zn deposition weight can be determined by dividing the total amount of Zn obtained by the surface area of ​​the high-strength member.

[0058] [Fe-Zn alloy layer] In the present invention, it is important that the thickness of the Fe-Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1.0 μm. In other words, in the high-strength steel member of the present invention, there is no Fe-Zn alloy layer between the hot-pressed member and the Zn-based plating layer, or even if there is an Fe-Zn alloy layer, the thickness of the Fe-Zn alloy layer is 1.0 μm.

[0059] If the Fe-Zn alloy layer is present directly on the steel sheet, the Zn may melt or evaporate during resistance welding, causing resistance weld cracking in the surface layer of the steel material and degrading the strength and fatigue properties of the weld. Therefore, to avoid weld cracking, the thickness of the Fe-Zn alloy layer is set to 1.0 μm or less.

[0060] The Fe-Zn alloy layer is a layer of a phase having a Zn concentration of 15% or more and a total concentration of Fe and Zn of 80% or more, when the cross section of the high-strength steel member is observed and analyzed using SEM-EDX.

[0061] The Fe-Zn alloy layer is formed by forming an Fe-Zn-based plating layer after hot press forming. Alternatively, if a Zn-based plating layer is present on the surface of the steel sheet for hot press forming before hot press forming, i.e., directly on an alloy mainly composed of Fe, the Fe-Zn alloy layer is formed by alloying Fe and Zn during the heating process. Regardless of the method for forming the Fe-Zn alloy layer, which affects weld cracking properties, it is preferable that the method for producing a high-strength steel member of the present invention does not involve any of the above steps.

[0062] [Fe-Al alloy layer] The high-strength steel member of the present invention may further have an Fe—Al alloy layer at the interface between the hot-pressed member and the Zn-based plating layer.

[0063] The solid solubility limit of Zn in an Fe-Al alloy layer is lower than that of Zn in Fe. Therefore, during resistance welding, the Fe-Al alloy layer acts as a barrier against molten Zn that is produced when the Zn-based plating layer melts. Therefore, providing an Fe-Al alloy layer can further reduce the risk of resistance weld cracking.

[0064] Furthermore, the Fe-Al alloy layer has a low anodic dissolution rate in a corrosive environment and a low rate of the oxygen reduction reaction, which is the main cathodic reaction. Therefore, the Fe-Al alloy layer has a low corrosion rate of both the Fe-Al alloy layer itself and the galvanic corrosion rate with the upper Zn-based plating layer. Therefore, the provision of the Fe-Al alloy layer can provide even better corrosion resistance.

[0065] In the present invention, when a cross section of a high-strength steel member is observed and analyzed by SEM-EDX, a layer having a thickness of 1 μm or more and consisting of a phase having an Fe concentration of 15% or more, a total concentration of Fe and Al of 80% or more, and a Zn concentration of less than 10% is defined as an "Fe-Al alloy layer."

[0066] The Fe-Al alloy layer is not particularly limited and any layer can be used as long as it meets the above definition. That is, in the present invention, the components constituting the Fe-Al alloy layer are not limited to alloys in the strict sense, but may be intermetallic compounds or metal layers containing solid solutions. Examples of the intermetallic compounds include Fe2Al5 and Fe4Al 13 , FeAl, etc. Furthermore, examples of metal phases containing solid solutions include α-Fe with Al as a solid solution.

[0067] The thickness of the Fe-Al alloy layer is not particularly limited, but from the viewpoint of enhancing the effect of reducing resistance weld cracking, it is preferably 1 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. On the other hand, from the viewpoint of adhesion of the plating layer, the thickness of the Fe-Al alloy layer is preferably 30 μm or less, more preferably 25 μm or less, and even more preferably 20 μm or less.

[0068] The thickness of the Fe—Al alloy layer can be measured by observing and analyzing the cross section of the high-strength steel member with SEM-EDX, more specifically, by the method described in the Examples.

[0069] The method for forming the Fe-Al alloy layer is not particularly limited, but it is economically advantageous to form the layer by providing an Al-based plating layer on the surface of the steel sheet for hot press forming before hot press forming, i.e., directly on the Fe-based alloy, and allowing alloying of Fe and Al to proceed during the heating process. The method for forming the Al-based plating layer on the steel sheet for hot press forming is also not particularly limited, and examples include hot-dip plating, electroplating using a non-aqueous solvent or molten salt, physical vapor deposition, and clad rolling, but hot-dip plating is preferred from the standpoints of manufacturability and economy.

[0070] [Fe oxide layer] The high-strength steel member of the present invention may have an Fe oxide layer at the interface between the hot-pressed member and the Zn-based plating layer. In the present invention, the Fe oxide layer is defined as a layer in which, when a cross section of the high-strength steel member is observed and analyzed by SEM-EDX, an O concentration is 20% or more and the total concentration of Fe and O is 80% or more. The Fe-based oxide constituting the Fe oxide layer is not particularly limited, but examples thereof include Fe2O3, FeO, and Fe3O4.

[0071] However, if a large amount of Fe oxide exists between the hot-pressed member and the Zn-based plating layer, resistance weld cracking properties may be reduced. Specifically, Fe oxide has a higher electrical resistance than the hot-pressed member (base material), the Zn-based plating layer, and the Fe-Al alloy layer. Therefore, if a large amount of Fe oxide exists, the current path during resistance welding is restricted, causing localized heat generation. As a result, resistance weld cracking properties are reduced.

[0072] Therefore, it is desirable that the amount of Fe oxide present between the hot-pressed member and the Zn-based plating layer is small. Specifically, it is preferable that the coverage of the Fe oxide layer between the hot-pressed member and the Zn-based plating layer is 10% or less and has a thickness of 1 μm or more. On the other hand, the lower the coverage, the better, and 0% is preferable. In the following description, when simply referring to the "coverage of the Fe oxide layer," it means the "coverage of the Fe oxide layer with a thickness of 1 μm or more."

[0073] The coverage of the Fe oxide layer can be measured by observing the cross section of the high-strength steel member using SEM-EDX. Specifically, the cross section of the high-strength steel member is observed using an SEM at an acceleration voltage of 5 kV and a magnification of 1000 times to obtain a backscattered electron image. Next, the coverage can be calculated by dividing the length of the backscattered electron image where Fe oxide having a thickness of 1 μm or more exists by the total width observed.

[0074] The Fe oxide layer is formed when unplated steel sheets are hot-pressed. By adjusting the heating conditions and the chemical composition of the steel sheet, the coverage of the Fe oxide layer can be reduced to 10% or less.

[0075] A specific method for reducing the coverage of the Fe oxide layer is, for example, to shorten the heating time by direct current heating before hot pressing. In addition, using a steel sheet containing 1% or more of Cr as the steel sheet for hot pressing is also effective in reducing the coverage of the Fe oxide layer.

[0076] On the other hand, when using a typical hot-press steel sheet, such as DIN standard Mn-B steel 22MnB5, the coverage of the Fe oxide layer is nearly 100%. Even in such cases, the coverage of the Fe oxide layer can be reduced by providing an oxide removal process between the hot-pressing process and the Zn-based plating process to remove oxides present on the surface of the hot-pressed part. Specific methods for removing oxides will be described later.

[0077] [Surface oxide layer] The high-strength steel member of the present invention may further have an oxide layer on the surface of the Zn-based plating layer. Here, the oxide layer is referred to as a surface oxide layer to distinguish it from the Fe oxide layer between the hot-pressed member and the Zn-based plating layer. The oxide contained in the surface oxide layer is not particularly limited, but examples thereof include Zn oxide and composite oxides containing Zn and Al, Mg, Ti, Cr, Mn, Fe, Ni, etc.

[0078] If the surface oxide layer is too thick, there is a risk that adhesion will decrease when painted, so the thickness of the surface oxide layer is preferably 5 μm or less.

[0079] (natural immersion potential) In the present invention, it is important that the natural immersion potential of the high-strength steel member is -1100 to -800 mV based on a silver-silver chloride-saturated potassium chloride electrode (SSE). By setting the natural immersion potential within this range, the cathodic protection performance of the hot-pressed member, which is the base material, is optimal, and excellent corrosion resistance in appearance can be obtained. In addition, delayed fracture due to hydrogen generated during corrosion can be reduced.

[0080] If the natural immersion potential is more noble (positive) than -800 mV, the potential difference between the steel material and the coating layer is small, resulting in insufficient cathodic protection performance and inferior appearance corrosion resistance. Therefore, the natural immersion potential is set to -800 mV or less, preferably -900 mV or less, and more preferably -950 mV or less. On the other hand, if the natural immersion potential is more noble (negative) than -1100 mV, the effect of improving appearance corrosion resistance saturates. In addition, the amount of hydrogen generated by corrosion becomes excessive, increasing the risk of delayed fracture. Therefore, the natural immersion potential is set to -1100 mV or more, preferably -1075 mV or more, and more preferably -1050 mV or more.

[0081] In the present invention, the natural immersion potential refers to the natural immersion potential in an air-saturated 0.5 mass% NaCl aqueous solution at 25°C, expressed relative to a silver-silver chloride-saturated potassium chloride electrode. In measuring the natural immersion potential, a 10 mmφ region on any flat portion of the hot-pressed part having an area of ​​10 mmφ or more is used as the working electrode. The average value of the immersion potential measured between 60 seconds and 600 seconds after the working electrode is immersed in the NaCl aqueous solution is taken as the natural immersion potential of the hot-pressed part. In the measurement, the temperature of the NaCl aqueous solution is adjusted to 25±5°C.

[0082] Although the mechanical properties of the high-strength steel member of the present invention are not particularly limited, it is preferable that the residual stress measured by X-ray diffraction at any location of the high-strength steel member is less than 600 MPa. When trimming or piercing is performed after hot press forming, cold working generates residual stress at the location, increasing the risk of delayed fracture. To reduce the risk of delayed fracture, it is preferable not to perform cold working after hot press forming and to use a laser processing device for trimming and piercing.

[0083] (Vickers hardness) The Vickers hardness of the high-strength steel member of the present invention is 400 or more. In other words, in the present invention, "high strength" refers to a Vickers hardness of 400 or more. A high-strength steel member of the present invention having a Vickers hardness of 400 or more can be suitably used for applications such as automotive frame members. The Vickers hardness is measured on a flat portion of the cross section of the high-strength steel member. The high-strength steel member of the present invention may have a Vickers hardness of 400 or more in at least a portion thereof. For example, other hot-pressed members with different strengths may be joined to a portion of the high-strength steel member to impart functionality. Furthermore, during the cooling process after hot pressing, different portions of the hot-pressed member may be cooled with different thermal histories. In such cases, the Vickers hardness of the final high-strength steel member may be 400 or more in some portions and less than 400 in other portions.

[0084] On the other hand, if the Vickers hardness of the high-strength steel member is too high, the resistance to delayed fracture may decrease. Therefore, the Vickers hardness of the high-strength steel member is preferably 800 or less, more preferably 700 or less, and even more preferably 600 or less.

[0085] The Vickers hardness of the high-strength steel member is determined by the Vickers hardness of the hot-pressed member that is the base material. Therefore, in order to obtain a high-strength steel member having the above Vickers hardness, it is sufficient to use a hot-pressed member having the same Vickers hardness as the base material.

[0086] In the present invention, as described above, the strength of a member is defined based on Vickers hardness. Therefore, the tensile strength of the high-strength steel member is not particularly limited. However, in general, when the Vickers hardness is 400 or more, the tensile strength of the high-strength steel member is about 1180 MPa or more.

[0087] [Manufacturing method] Next, a preferred method for manufacturing the high-strength steel member of the present invention will be described.

[0088] A method for producing a high-strength steel member according to one embodiment of the present invention includes the following steps (1) to (3). (1) Heating process for heating steel sheets for hot pressing (2) A hot pressing step in which the heated steel plate for hot pressing is hot pressed to form a hot pressed member. (3) A Zn-based plating step of forming a Zn-based plating layer having a Zn content of 70 atomic % or more on at least one surface of the hot-pressed member at a temperature of 250°C or less.

[0089] The steel sheet for hot press-forming is not particularly limited and any steel sheet can be used. The suitable composition of the steel sheet is the same as the preferred composition of the steel material for the hot press-forming member described above. The steel sheet for hot press-forming is preferably a hot-rolled steel sheet or a cold-rolled steel sheet.

[0090] The steel sheet for hot press use may be a steel sheet without a plating layer, or a steel sheet with a plating layer (plated steel sheet). Examples of the plated steel sheet include an Al-plated steel sheet and a Ni-plated steel sheet.

[0091] The Al-plated steel sheet is a steel sheet having an Al-plated layer on at least one surface of the steel sheet, the Al content of the plated layer being 50 mass % or more.

[0092] An interfacial alloy layer containing 50% by mass or more of an Fe-Al-based intermetallic compound may be present between the Al-plated layer and the base steel sheet. The Al-plated layer may have any composition as long as it is an Al alloy containing 50% by mass or more of Al, but may optionally contain Si. More preferably, the Al-plated layer has a composition consisting of 0.1-13% Si, 0-10% Mg, and 0-5% Fe.

[0093] The Ni-plated steel sheet is a steel sheet having a plating layer on at least one surface thereof, the Ni content of which is 50 mass % or more. The composition of the plating layer is not limited as long as it is a Ni alloy mainly containing Ni, but it may optionally contain Ti, Cr, Fe, Co, Mo, and W.

[0094] Furthermore, if the Al plating layer or the Ni plating layer contains Zn, an Fe-Zn alloy layer may be formed after hot pressing. Therefore, from the viewpoint of suppressing the formation of an Fe-Zn alloy layer, it is preferable that the Zn concentration of the Al plating layer or the Ni plating layer be 50% or less.

[0095] The plating of the base steel sheet can be carried out by any method, but from the viewpoint of economy, it is preferably carried out by electroplating or hot-dip galvanizing. Hereinafter, the case of producing an Al-plated steel sheet by hot-dip galvanizing will be described.

[0096] First, prior to hot-dip galvanizing, the substrate steel sheet is annealed. Next, the annealed substrate steel sheet is immersed in a hot-dip galvanizing bath to produce a hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the substrate steel sheet. The hot-dip galvanizing bath may contain Al, Zn, and Fe that flows out from the base material or equipment in the bath. The hot-dip galvanizing bath may further contain Si. More preferably, the hot-dip galvanizing bath has a chemical composition consisting of 0.1-13% Si, 0-10% Mg, 0-5% Fe, with the balance being Al and unavoidable impurities.

[0097] The coating weight of the hot-dip coating layer is not particularly limited, but is preferably 20 g / m per side of the steel sheet. 2 It is preferable that the content is 30 g / m or more. 2 More preferably, it is 50 g / m or more. 2 It is more preferable that the coating weight of the hot-dip coated layer is 300 g / m2 or more per one side of the steel sheet. 2 Preferably, it is 250 g / m or less. 2 More preferably, it is 200 g / m or less. 2 It is more preferable that the coating weight of the plating layer is not more than 1000 kJ / cm2. As described above, when hot pressing is performed, the coating weight of the plating layer increases due to the diffusion of Fe from the base steel sheet. Therefore, by setting the coating weight of the hot-dip plating layer on the hot-dip plated steel sheet before hot pressing to the above-mentioned range, the coating weight of the plating layer on the hot-pressed member can be set to the above-mentioned preferred range.

[0098] The coating weight per side of the hot-dip coated layer is determined by the following method. First, the hot-dip coated steel sheet to be evaluated is punched to obtain three 48 mm diameter samples. Then, one side of each sample (the side opposite to the side on which the coating weight is measured) is masked. Each sample is immersed for 20 minutes in a 17% hydrochloric acid solution containing 1 mL of hexamethylenetetramine as an inhibitor to dissolve the hot-dip coated layer, and the weight of each sample is then measured again. The difference in mass before and after dissolution of the hot-dip coated layer is divided by the area of ​​the sample to calculate the coating weight per unit area of ​​each sample. The average coating weight of the three samples is then taken as the coating weight per side of the hot-dip coated layer on that hot-dip coated steel sheet.

[0099] (Heating process) Prior to the hot pressing step, the steel sheet for hot pressing is heated (heating step). In the heating step, it is preferable to raise the temperature from room temperature to a heating temperature between the Ac3 transformation point of the base steel sheet and 1100°C in an atmosphere with an oxygen concentration of 0.1 to 22% by volume over a temperature rise time of 10 to 300 seconds, and further hold the steel sheet for hot pressing after the temperature rise at the heating temperature for a holding time of 5 to 900 seconds in the atmosphere.

[0100] The atmosphere in the heating step preferably has an oxygen concentration of 0.1 to 22% by volume. When using an unplated steel sheet, it is preferable to reduce the thickness and coverage of the Fe oxide layer, and to achieve this, the oxygen concentration is preferably 22% by volume or more, more preferably 10% by volume or less, and even more preferably 1% by volume or less. On the other hand, an oxygen concentration below 0.1% by volume does not provide any significant effect and increases operating costs, so the oxygen concentration is preferably 0.1% by volume or more.

[0101] If the heating temperature is lower than the Ac3 transformation point, the strength required for a hot-pressed member may not be obtained, whereas if the heating temperature exceeds 1100°C, the operating costs increase.

[0102] The lower limit of the temperature-raising time for the steel sheet for hot pressing to reach the heating temperature from room temperature is not particularly limited, but is preferably 10 seconds or more from the viewpoint of operational stability of hot pressing. The upper limit of the temperature-raising time for the steel sheet for hot pressing to reach the heating temperature from room temperature is not particularly limited, but is preferably 300 seconds or less, because an excessively long temperature-raising time deteriorates productivity.

[0103] Although the lower limit of the holding time after reaching the heating temperature is not particularly limited, it is preferably 5 seconds or more from the viewpoint of operational stability of hot pressing. On the other hand, if the holding time exceeds 900 seconds, a stable oxide film is formed on the surface of the steel sheet for hot pressing, which is difficult to remove, so the holding time is preferably 900 seconds or less.

[0104] (Hot press process) Next, the steel sheet heated in the heating step is hot pressed to obtain a hot-pressed member (hot pressing step). In the hot pressing, the steel sheet for hot pressing is formed into a desired shape and cooled with a mold, water, etc. In the present invention, the hot pressing conditions are not particularly limited, but it is preferable to perform pressing at a general hot pressing temperature range of 600 to 800°C.

[0105] (Zn-based plating process) Next, a Zn-based plating layer is formed on at least one surface of the obtained hot-pressed member (Zn-based plating step). If the temperature when forming the Zn-based plating layer (hereinafter sometimes referred to as the "film formation temperature") exceeds 250°C, the hot-pressed member will be tempered, making it impossible to obtain the desired hardness. Therefore, in the step of forming the Zn-based plating layer, the film formation temperature is set to 250°C or less. Here, the temperature when forming the Zn-based plating layer (film formation temperature) refers to the maximum temperature reached by the hot-pressed member during the formation of the Zn-based plating layer. The film formation temperature is preferably 150°C or less, more preferably 100°C or less.

[0106] The Zn content of the Zn-based plating layer is set to 70 atomic % or more. If the Zn content is less than 70 atomic %, i.e., if the total content of elements other than Zn exceeds 30 atomic %, the natural immersion potential in an air-saturated 0.5 mass % NaCl aqueous solution at 25°C may fall outside the range of −1100 to −800 mV relative to a silver-silver chloride-saturated potassium chloride electrode, making it impossible to achieve the desired corrosion resistance. More specifically, if the potential is excessively base, the potential difference with the steel substrate may be large, increasing the rate of blistering. Furthermore, if the potential is excessively noble, red rust due to corrosion of the base material may become noticeable. By controlling the Zn content of the Zn-based plating layer to 70 atomic % or more, stable performance can be achieved against both blistering and red rust. To stabilize the corrosion resistance, the Zn content of the Zn-based plating layer is preferably set to 80 atomic % or more, more preferably 90 atomic % or more.

[0107] The method for forming the Zn-based plating layer is not particularly limited, and either wet plating or dry plating can be used. Usually, the plating method can be selected according to the type of Zn-based plating layer to be formed.

[0108] For example, when the Zn-based plating layer is pure Zn, Zn-Ni plating, Zn-Ni-Co plating, Zn-Cr plating, or Zn-Mn plating, it is preferable to form the film by electroplating. When the coating layer has a composition that is difficult to electrodeposit from an aqueous solution, such as Zn-Ti plating or Zn-Mg plating, it is preferable to form the film by PVD. Note that when forming a Zn-based plating layer by hot-dip plating, the temperature exceeds 250°C, so hot-dip plating is not used.

[0109] Regardless of the method used to form the coating layer, the conditions can be adjusted so that the coating layer on one side (front side) of the steel sheet and the other side (back side) of the steel sheet have the desired thickness. For example, in the case of electroplating, the thickness of the coating layer on each side can be adjusted by changing either or both the current density and the current flow time on each side. In addition, by adjusting the electrode arrangement and controlling the current density, it is possible to intentionally create a distribution in the plating deposition weight within the same surface.

[0110] (Surface oxide removal process) As described above, when an unplated steel sheet is hot-pressed, an Fe oxide layer is formed on the surface of the hot-pressed part. Therefore, in order to reduce the coverage of the Fe oxide layer in the final high-strength steel part, it is preferable to remove the oxides present on the surface of the hot-pressed part after hot-pressing and before Zn-based plating (oxide removal step).

[0111] The method for removing oxides in the oxide removal step is not particularly limited and any method can be used. Suitable oxide removal methods include physical methods such as shot blasting, sand blasting, and dry ice blasting, optical methods such as laser cleaning, chemical methods such as pickling, and combinations thereof. Any acidic solution can be used for the pickling without any particular limitation.

[0112] Furthermore, when an aluminized steel sheet is used as a steel sheet for hot pressing, a thin Al-based oxide is formed on the surface of the hot-pressed member. The presence of the Al-based oxide reduces the adhesion between the Zn-based plating layer and the base steel sheet. Therefore, even when an aluminized steel sheet is used, it is preferable to remove the Al-based oxide by performing the oxide removal step. There are no particular limitations on the method for removing the Al-based oxide. However, the plating layer of a hot-pressed aluminized steel sheet dissolves in an acidic solution. Therefore, in order to remove the Al-based oxide while preventing dissolution of the plating layer, it is preferable to remove the oxide by immersing the steel sheet in an alkaline solution with a pH of 11 or higher. [Example]

[0113] The functions and effects of the present invention will be described below based on examples, but the present invention is not limited to the following examples.

[0114] High-strength steel members were manufactured by hot pressing steel plates for hot pressing according to the following procedure.

[0115] As the steel sheets for hot pressing, the following steel sheets (A) to (F) were used.

[0116] (A) A cold-rolled steel sheet having a thickness of 1.4 mm, containing, by mass%, C: 0.34%, Si: 0.25%, Mn: 1.2%, Cr: 0.2%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the balance being Fe and unavoidable impurities.

[0117] (B) A cold-rolled steel sheet having a thickness of 1.4 mm, containing, by mass%, C: 0.34%, Si: 0.50%, Mn: 0.6%, Cr: 1.0%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the balance being Fe and unavoidable impurities.

[0118] (C) A cold-rolled steel sheet having a thickness of 1.4 mm and containing, by mass%, C: 0.34%, Si: 1.0%, Mn: 1.0%, Cr: 11%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the remainder being Fe and unavoidable impurities.

[0119] (D) A cold-rolled steel sheet having a thickness of 1.6 mm and containing, by mass%, C: 0.21%, Si: 0.2%, Mn: 1.3%, Cr: 0.18%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, B: 0.002%, with the remainder being Fe and unavoidable impurities.

[0120] (E) A cold-rolled steel sheet having a thickness of 1.8 mm and containing, by mass%, C: 0.42%, Si: 0.25%, Mn: 1.0%, Cr: 0.12%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the remainder being Fe and unavoidable impurities.

[0121] (F) A cold-rolled steel sheet having a thickness of 1.2 mm and containing, by mass%, C: 0.49%, Si: 0.2%, Mn: 1.2%, Cr: 0.18%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.01%, B: 0.002%, Sb: 0.01%, with the remainder being Fe and unavoidable impurities.

[0122] In some examples, plated steel sheets obtained by plating the surfaces of the above steel sheets were used as steel sheets for hot pressing. The combinations of steel sheets and plating layers used are shown in Table 1. Each plating layer was formed under the following conditions.

[0123] (Electro-galvanized zinc) Using the steel sheet as the cathode and the iridium oxide-coated titanium sheet as the anode, electroplating was carried out under the following conditions to form a Zn plating layer: The thickness of the Zn plating layer was adjusted by changing the current application time. Plating solution composition: Zinc sulfate heptahydrate 240g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0124] (Electroplated Zn-12%Ni alloy) Using the steel sheet as the cathode and the iridium oxide-coated titanium sheet as the anode, electroplating was performed under the following conditions to form a Zn-12%Ni alloy plating layer. The thickness of the Zn-12%Ni alloy plating layer was adjusted by changing the current application time. Plating solution composition: Nickel sulfate hexahydrate 240g / L Zinc sulfate heptahydrate 20g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0125] (Ni alloy electroplating) Using the steel sheet as the cathode and the iridium oxide-coated titanium sheet as the anode, electroplating was carried out under the following conditions to form a Ni plating layer. The thickness of the Ni plating layer was adjusted by changing the current application time. Plating solution composition: Nickel sulfate hexahydrate 240g / L Boric acid 30g / L pH: 3.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0126] (hot-dip plating) In some examples, hot-dip galvanized steel sheets were used for comparison. The hot-dip galvanized steel sheets were prepared by immersing the steel sheets in a hot-dip galvanizing bath. After the steel sheets were removed from the hot-dip galvanizing bath, they were wiped with N2 gas to adjust the coating weight of the coating layer. The following three types of hot-dip galvanizing baths were used. Zn-0.2%Al hot-dip plating bath (bath temperature 460℃) Al-10%Si hot-dip plating bath (bath temperature 660℃) Al-10%Si-1%Mg hot-dip plating bath (bath temperature 660℃)

[0127] ·Heating process Next, a test piece of 200 × 300 mm was taken from the obtained hot-dip plated steel sheet, and the test piece was heated to 900 ° C under the conditions (heating method and temperature rise time) shown in Table 1. After the temperature was raised to 900 ° C, it was held for 1 minute.

[0128] Hot pressing process As described above, after holding at 900°C for 1 minute, the test specimen was hot-pressed to obtain a hat-shaped high-strength steel member. Specifically, the test specimen was transferred to a press device installed adjacent to the heating device and immediately hot-pressed. A hat-shaped mold was used for the hot pressing, and the forming start temperature was 700°C. The shape of the obtained high-strength steel member was 100 mm long on the top surface, 50 mm long on the side surfaces, and 50 mm long on the bottom surface. The bending radius of the mold was 7R for both shoulders on the top surface and both shoulders on the bottom surface.

[0129] ·Oxide removal process In some examples, oxide removal processes were performed to remove oxides from the surfaces of the hot-pressed parts by shot blasting, pickling, or alkaline immersion. The methods used for the oxide removal processes are listed in Table 1.

[0130] In the shot blasting, steel balls with an average particle size of 0.5 mm were used as the shot material, and the shot material was sprayed at an angle of 60° against the surface of the hot-pressed part at an air pressure of 0.2 MPa. For comparison, in Example No. 5, the shot blasting treatment time was half that of the other Examples, intentionally increasing the amount of remaining oxide.

[0131] The pickling solution used was a 10% aqueous HCl solution containing 1 g / L of hexamethylenetetramine as an inhibitor. The temperature of the pickling solution was 50°C, and the immersion time was 60 seconds.

[0132] The alkali immersion was carried out using a 50% NaOH aqueous solution at 80° C. The hot-pressed part was immersed in the NaOH aqueous solution for 60 seconds to remove oxides.

[0133] Zn-based plating process Next, a Zn-based plating layer was formed on the surface of the hot-pressed member by the methods shown in Table 1. Each of the methods used is explained below. For comparison, in some examples, no Zn-based plating layer was formed.

[0134] (Electroplating) The formation of a Zn-based plating layer by electroplating was carried out under the following conditions. In each case, electrolysis was carried out using the hot-pressed member as the cathode and the iridium oxide-coated titanium plate as the anode, and the thickness of the coating layer was adjusted by changing the current flow time. The anodes had the same shape as the hot-pressed member, and were placed on both sides of the hot-pressed member to perform plating.

[0135] (1) Zn plating Plating solution composition: Zinc sulfate heptahydrate 240g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0136] (2) Zn-12%Ni alloy plating Plating solution composition: Nickel sulfate hexahydrate 240g / L Zinc sulfate heptahydrate 20g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0137] (3) Zn-18%Ni alloy plating Plating solution composition: Nickel sulfate hexahydrate 240g / L Zinc sulfate heptahydrate 20g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 60A / dm 2

[0138] (4) Zn-30%Ni alloy plating Plating solution composition: Nickel sulfate hexahydrate 360g / L Zinc sulfate heptahydrate 20g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 100A / dm 2

[0139] (5) Zn-8%Cr alloy plating Plating solution composition: Chromium(III) sulfate hexahydrate 100 g / L Zinc sulfate heptahydrate 200 g / L Sodium sulfate 100g / L Polyethylene glycol 1g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 50A / dm 2

[0140] (6) Zn-40%Mn alloy plating Plating solution composition: Manganese(II) sulfate 70g / L Zinc sulfate heptahydrate 30 g / L Sodium citrate 180 g / L pH: 5.6 ·Temperature: 50℃ ·Current density: 20A / dm 2

[0141] (7) Zn-Fe alloy plating Plating solution composition: Iron(II) sulfate heptahydrate 240g / L Zinc sulfate heptahydrate 20g / L pH: 2.0 ·Temperature: 50℃ ·Current density: 40A / dm 2

[0142] (PVD) The formation of the Zn-based plating layer by PVD was carried out by ion plating. A batch-type radio frequency (RF) excitation ion plating device manufactured by Showa Vacuum Co., Ltd. was used for the ion plating. The film formation conditions were a temperature of the hot-pressed member of 100°C, a pressure of 3 Pa, and a bias voltage of -20 V. The composition of the plating layer was controlled by adjusting the composition of the metal used as the deposition source. The thickness of the plating layer was also controlled by adjusting the deposition time.

[0143] (hot-dip plating method) In some comparative examples, the Zn-based coating layer was formed by hot-dip galvanization using a flux. The flux was a mixture of ammonium chloride and zinc chloride. The hot-dip galvanization bath temperature was 460°C, the composition was Zn-0.2%Al, and the immersion time was 60 seconds.

[0144] (natural immersion potential) The natural immersion potential of the high-strength steel member obtained by the above procedure was measured using the following procedure. First, three 16 mm diameter samples were punched from the flat portion of the top surface of the hat-shaped hot-pressed member. Using a 10 mm diameter region in the center of the sample as the working electrode and a silver-silver chloride-saturated potassium chloride electrode (SSE) as the reference electrode, the immersion potential was measured in an air-saturated 0.5 mass% NaCl aqueous solution at 25±5°C. The time average of the immersion potential from 60 seconds to 600 seconds after the working electrode was immersed in the NaCl aqueous solution was taken as the natural immersion potential of the sample. The average of the natural immersion potentials of three different samples was taken as the natural immersion potential of the hot-pressed member being evaluated. The measurement results are shown in Table 2.

[0145] Furthermore, for each of the obtained high-strength steel members, the Vickers hardness, the thickness of the Fe-Zn alloy layer, the Zn coating weight, the thickness of the Fe-Al alloy layer, and the coverage of the Fe oxide layer were measured by the following methods. The measurement results are shown in Table 2.

[0146] (Vickers hardness) Test pieces were cut out from the top plate of the obtained high-strength steel member using a laser cutting machine, and then the cross section of the test piece was mirror-polished. Thereafter, the Vickers hardness of the test piece was measured at a position 1 / 4 of the plate thickness. Measurements were made at 10 points at 200 μm intervals in a direction perpendicular to the rolling direction of the steel plate, and the average value was taken as the Vickers hardness of the high-strength steel member. The Vickers hardness measurement conditions were a test force of 100 g and a holding time of 5 seconds.

[0147] (Fe-Zn alloy layer thickness) The thickness of the Fe-Zn alloy layer was evaluated by observing and analyzing the cross section of the high-strength steel member using SEM-EDX according to the following procedure. The cross section of the mirror-finished high-strength steel member was observed using an SEM to obtain a backscattered electron image at an accelerating voltage of 5 kV and a magnification of 5000 times. If an alloy layer was observed at the interface between the Zn-based coating layer and the base steel sheet, its composition was analyzed using EDX to determine whether it was an Fe-Zn alloy layer with a Zn concentration of 15% or more and a total concentration of Fe and Zn of 80% or more. If it was an Fe-Zn alloy layer, its thickness was measured from the cross section at 10 arbitrary locations within the observation field. The above observation was performed on 10 arbitrary fields, and the average value was taken as the thickness of the Fe-Zn alloy layer.

[0148] (Zn phase adhesion amount) The amount of the Zn phase deposited was measured by subjecting the high-strength steel member to anodic electrolysis to dissolve the Zn, and quantifying the amount of dissolved Zn by inductively coupled plasma-mass spectrometry (ICP-MS). Specifically, first, constant-current anodic electrolysis was performed in a 3% sodium hydroxide-1% aluminum chloride aqueous solution, with the high-strength steel member as the working electrode and a platinum mesh electrode as the counter electrode. The current density in the anodic electrolysis was 4 mA / cm. 2 During anodic electrolysis, the electrolysis was stopped at the point where the potential became increasingly noble, thereby dissolving all of the Zn into the aqueous solution. Next, the amount of Zn in the aqueous solution was quantitatively analyzed by ICP-MS to determine the total amount of dissolved Zn. The total amount of Zn obtained was divided by the surface area of ​​the high-strength member to determine the amount of Zn deposited.

[0149] (Fe-Al alloy layer thickness) The thickness of the Fe-Al alloy layer was evaluated by observing and analyzing the cross section of the high-strength steel member using SEM-EDX according to the following procedure. The cross section of the mirror-finished high-strength steel member was observed using an SEM to obtain a backscattered electron image at an accelerating voltage of 5 kV and a magnification of 5000 times. If an alloy layer was observed at the interface between the Zn-based coating layer and the base steel sheet, its composition was analyzed using EDX to determine whether it was an Fe-Al alloy layer with an Fe concentration of 15% or more and a total concentration of Fe and Al of 80% or more. If it was an Fe-Al alloy layer, its thickness was measured from the cross section at 10 arbitrary locations within the observation field. The above observation was performed on 10 arbitrary fields, and the average value was taken as the thickness of the Fe-Al alloy layer.

[0150] (Fe oxide layer thickness and coverage) The cross section of the high-strength steel member was observed with SEM-EDX to measure the coverage of the Fe oxide layer with a thickness of 1 μm or more. Specifically, the cross section of the high-strength steel member was observed with SEM at an accelerating voltage of 5 kV and a magnification of 1000 times to obtain a backscattered electron image. Next, the coverage was calculated by dividing the length of the Fe oxide layer with a thickness of 1 μm or more in the backscattered electron image by the total width observed.

[0151] When an oxide layer was observed at the interface between the Zn-based plating layer and the hot-pressed part, its composition was analyzed by EDX. If the O concentration was 20% or more and the sum of the Fe and O concentrations was 80% or more, it was determined to be an Fe oxide layer.

[0152] The coverage of the Fe oxide layer within the observed field of view was calculated by measuring the in-plane length of the area where the Fe oxide layer was 1 μm or thicker, and dividing the length of the area where the Fe oxide layer was 1 μm or thicker by the total width of the observed area. The above measurement was performed for 10 randomly selected fields of view, and the average value was taken as the coverage of the Fe oxide layer.

[0153] (Exterior corrosion resistance) Next, in order to evaluate the appearance and corrosion resistance of the obtained high strength steel members, the occurrence of paint film blisters from the painted edge and the occurrence of red rust from the painted edge were tested according to the following procedures.

[0154] First, a 70 mm wide region was cut out of the obtained hat-shaped high-strength steel member using a laser cutter, and the test piece was subjected to a zinc phosphate-based conversion treatment and electrodeposition coating to obtain a corrosion resistance test piece. The zinc phosphate-based conversion treatment was performed under standard conditions using a PB-SX35 manufactured by Nihon Parkerizing Co., Ltd. The electrodeposition coating was performed using an Electron GT-100 manufactured by Kansai Paint Co., Ltd., so that the coating film thickness was 5 μm. The baking condition for the electrodeposition coating was to reach 170°C and then hold for 20 minutes.

[0155] The obtained corrosion resistance test pieces were subjected to a combined cycle corrosion test (SAE-J2334) without masking, and the corrosion condition after 40 cycles was evaluated. The appearance corrosion resistance of the painted edge was judged based on the width of the paint blister from the edge and the occurrence of red rust on the cut edge, based on the following criteria. A rating of 1 or 2 for both the paint blister width and the red rust area ratio on the edge was considered to be pass. The evaluation results are shown in the table. Note that when the paint blister width was more than 1 mm, red rust had occurred in the cross-cut scratches. 1: Paint blister width ≦ 1mm and no red rust in cross-cut scratches 2: Paint blister width ≦ 1mm and red rust present in the cross-cut scratches 3: 1mm<paint blister width≦3mm 4: Paint blister width > 3mm

[0156] (resistance weld crack resistance) Furthermore, in order to evaluate the resistance weld cracking resistance of the obtained high strength steel members, resistance spot welding was performed in the following manner, and the occurrence of cracks in the welded portions was evaluated.

[0157] First, a test piece was cut out from the top plate of a high-strength steel member. On the other hand, the counterpart material to be welded was cut out from a galvannealed steel sheet. The galvannealed steel sheet has a tensile strength of 980 MPa and a coating weight of 50 g / m2. 2 , plate thickness t: 1.6 mm.

[0158] Next, the test piece and the mating material were resistance spot welded together. The resistance spot welding conditions were: impact angle θ: 5°, pressure: 3.5 kN, hold time: 0.12 seconds, 0.18 seconds, or 0.24 seconds. The welding current and welding time were adjusted so that the nugget diameter d was 5.9 mm.

[0159] The cross section of the weld formed by the resistance spot welding was observed, and the resistance weld crack resistance of the weld was evaluated according to the following criteria. A rating of 1 to 3 was determined to indicate that the resistance weld crack resistance of the weld was excellent. The results are shown in Table 2. 1: No cracks longer than 0.1 mm were observed with a hold time of 0.12 seconds. 2: Cracks longer than 0.1 mm were observed at a hold time of 0.12 seconds, but no cracks longer than 0.1 mm were observed at a hold time of 0.18 seconds. 3: Cracks longer than 0.1 mm were observed at a hold time of 0.18 seconds, but no cracks longer than 0.1 mm were observed at a hold time of 0.24 seconds. 4: A crack longer than 0.1 mm was observed at a hold time of 0.24 seconds.

[0160] As can be seen from the results shown in Table 2, the high-strength steel members satisfying the conditions of the present invention were excellent in appearance corrosion resistance and resistance weld cracking resistance.

[0161] [Table 1]

[0162] [Table 2]

Claims

1. A high-strength steel member having a hot-pressed member and a Zn-based plating layer on at least one surface of the hot-pressed member, the thickness of the Fe—Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1.0 μm; The natural immersion potential in an air-saturated 0.5% by mass NaCl aqueous solution at 25°C is −1100 to −800 mV versus a silver-silver chloride-saturated potassium chloride electrode, and A high-strength steel member having a Vickers hardness of 400 or more.

2. The amount of Zn deposited on one side of the hot-pressed member is 5 to 100 g / m ２ The high strength steel member according to claim 1, wherein

3. 3. The high-strength steel member according to claim 1, further comprising an Fe—Al alloy layer between the hot-pressed member and the Zn-based plating layer.

4. 3. The high-strength steel member according to claim 1, wherein a coverage of an Fe oxide layer having a thickness of 1 μm or more between the hot-pressed member and the Zn-based plating layer is 10% or less.

5. 4. The high-strength steel member according to claim 3, wherein a coverage of an Fe oxide layer having a thickness of 1 μm or more between the hot-pressed member and the Zn-based plating layer is 10% or less.

6. A method for manufacturing a high-strength steel member having a hot-pressed member and a Zn-based plating layer on at least one surface of the hot-pressed member, comprising: a heating step of heating the steel plate for hot pressing; a hot pressing step of hot pressing the heated steel plate to form the hot-pressed member; a Zn-based plating step of forming the Zn-based plating layer having a Zn content of 70 atomic % or more on at least one surface of the hot-pressed member at a temperature of 250°C or less, the thickness of the Fe—Zn alloy layer between the hot-pressed member and the Zn-based plating layer is 0 to 1.0 μm; The natural immersion potential in an air-saturated 0.5% by mass NaCl aqueous solution at 25°C is −1100 to −800 mV versus a silver-silver chloride-saturated potassium chloride electrode, and A method for manufacturing a high-strength steel member having a Vickers hardness of 400 or more.

7. The method for producing a high-strength steel member according to claim 6, wherein the steel sheet for hot press use is an Al-plated steel sheet.

8. 8. The method for producing a high-strength steel member according to claim 6, further comprising, between the hot-pressing step and the Zn-based plating step, an oxide removing step of removing oxides present on the surface of the hot-pressed member.

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