Hot-pressed member, hot-pressed steel sheet, and method for manufacturing hot-pressed member

Abstract

The present invention aims to provide a hot-pressed member having excellent corrosion resistance after painting and resistance spot welding properties and a method for producing the same. In addition, the present invention aims to provide a hot-pressed steel sheet suitable for a hot-pressed member having excellent corrosion resistance after painting and resistance spot welding properties. A hot-pressed member having a Zn-based plated layer on both surfaces of a steel sheet, wherein the Zn deposition amount of the Zn-based plated layer on one surface of the steel sheet is 5 to 35 g / m 2 and the average linear roughness Ra of the surface of the Zn-based plated layer is 2.5 μm or less, and the average linear roughness Ra of the surface of the Zn-based plated layer on the other surface of the steel sheet is 3.5 μm or more.

Description

Technical Field

[0001] This invention relates to hot-pressed components, hot-pressed steel sheets, and methods for manufacturing hot-pressed components. In particular, it relates to hot-pressed components, hot-pressed steel sheets, and methods for manufacturing hot-pressed components that exhibit excellent corrosion resistance and resistance spot weldability after coating following a zirconium-based chemical conversion treatment. Background Technology

[0002] In recent years, the automotive industry has seen a promotion of high-performance and lightweight steel sheet materials, leading to an increase in the use of rust-resistant high-strength hot-dip galvanized or electro-galvanized steel sheets. However, in most cases, the increased strength of steel sheets has reduced their stamping formability, making it difficult to obtain complex part shapes. For example, in automotive applications, rust resistance is required, and these are difficult-to-form parts, such as chassis and other running gear components, and B-pillars and other skeletal structural components.

[0003] Against this backdrop, in recent years, the use of hot pressing to manufacture automotive parts, which is easier to balance stamping formability and high strength compared to cold pressing, has increased rapidly, and various technologies for solving various problems in hot pressing technology have been disclosed.

[0004] Among them, Zn-Ni alloy coated steel sheet has attracted attention as a hot pressing steel sheet due to the high melting point of the coating, and hot pressing components using this steel sheet and their manufacturing methods have been proposed.

[0005] For example, Patent Document 1 discloses a hot-pressed component having: an α-Fe(Zn, Ni) mixed crystal; an intermetallic compound of Zn, Ni and Fe; and a layer containing Mn.

[0006] In addition, Patent Document 2 discloses a hot-pressed component having: a Ni diffusion region, an intermetallic compound layer equivalent to the γ phase, and a ZnO layer.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Publication No. 2013-503254

[0010] Patent Document 2: Japanese Patent Application Publication No. 2011-246801

[0011] Patent Document 3: Japanese Patent Application Publication No. 2004-323897 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] However, in recent years, zirconium-based chemical conversion treatment has become more popular, replacing the previous zinc phosphate-based chemical conversion treatment. This has also led to a requirement for the corrosion resistance of components that have undergone this zirconium-based chemical conversion treatment and are then coated with electrodeposition coating.

[0014] Both Patent Documents 1 and 2 disclose hot-pressed components manufactured by heating Zn-Ni alloy-coated steel sheets. They exhibit excellent corrosion resistance without coating and excellent corrosion resistance after coating with zinc phosphate-based chemical conversion treatment. However, there is a problem that the corrosion resistance after coating is insufficient when using zirconium-based chemical conversion treatment.

[0015] On the other hand, resistance spot weldability is also an important characteristic required for hot-pressed components. If zinc-coated steel sheets are used for hot pressing, the Zn contained in the coating before heating is oxidized during the hot pressing process, resulting in an oxide film several μm thick, primarily composed of zinc oxide, forming on the surface. Zinc oxide is a semiconductor, but its high resistivity reduces resistance spot weldability. Therefore, for hot-pressed components using zinc-coated steel sheets, as disclosed in Patent Document 3, the oxide film is sometimes removed by shot peening or similar methods. However, the shot peening process, used to ensure resistance spot weldability, increases both time and cost, thus becoming a problem when applying zinc-coated steel sheets to hot pressing.

[0016] This invention was made in view of the above-mentioned actual situation, and its object is to provide a hot-pressed component with excellent corrosion resistance and resistance spot weldability after coating, and a method for manufacturing the same. Furthermore, its object is to provide a steel sheet suitable for hot-pressing hot-pressed components with excellent corrosion resistance and resistance spot weldability after coating.

[0017] Methods for solving problems

[0018] In order to solve the above-mentioned problems, the inventors conducted in-depth research and came to the following conclusions.

[0019] (1) In order to improve the corrosion resistance of hot-pressed components after coating, the Zn adhesion amount on the main surface of the hot-pressed components for evaluating surface corrosion is set to 5-35 g / m². 2 Setting the average linear roughness Ra of the Zn coating surface to 2.5 μm or less is effective. Furthermore, to improve the resistance spot weldability of hot-pressed components, setting the average linear roughness Ra of the Zn coating surface of the main bonding surfaces that serve as resistance spot welds of the hot-pressed components to 3.5 μm or more is effective.

[0020] (2) By applying Zn with a surface thickness of 5-35 g / m 2 The Zn adhesion amount on the other side is 40-120 g / m². 2 Hot pressing of Zn-based coatings onto steel plates can produce hot-pressed components with excellent corrosion resistance and resistance spot weldability after coating.

[0021] The present invention is based on the above insights and is characterized as follows.

[0022] [1] A hot-pressed component, wherein the hot-pressed component has Zn-based coatings on both sides of a steel plate, wherein...

[0023] The Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5-35 g / m². 2 ,

[0024] Furthermore, the average linear roughness Ra of the Zn-based coating surface is below 2.5 μm.

[0025] The average linear roughness Ra of the Zn-based coating on the other side of the steel plate is above 3.5 μm.

[0026] [2] A hot-pressing steel plate, wherein the hot-pressing steel plate has a Zn-based coating on both sides of the steel plate, wherein...

[0027] The Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5-35 g / m². 2 ,

[0028] The Zn adhesion amount of the Zn-based coating on the other side of the steel plate is 40-120 g / m². 2 .

[0029] [3] A method for manufacturing a hot-pressed component, wherein a hot-pressed steel plate is heated from room temperature to a temperature range of Ac3 phase transformation point to 1000°C within a time of 5 seconds to 600 seconds, and then held within the temperature range of Ac3 phase transformation point to 1000°C for 300 seconds or less before hot pressing, wherein the hot-pressed steel plate has a Zn-based coating on both sides of the steel plate, and the Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5 to 35 g / m 2 The Zn adhesion amount of the Zn-based coating on the other side of the steel plate is 40-120 g / m². 2 .

[0030] Invention Effects

[0031] According to the present invention, hot-pressed components with excellent corrosion resistance and resistance spot weldability after coating can be obtained. Furthermore, the hot-pressing steel sheet of the present invention is suitable for hot-pressed components with excellent corrosion resistance and resistance spot weldability after coating. Detailed Implementation

[0032] The embodiments of the present invention will now be described. It should be noted that the following description illustrates a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. Furthermore, the unit for the content of each element in the steel composition is "mass%"; unless otherwise specified, it will be expressed as "%".

[0033] 1) Hot-pressed components

[0034] The hot-pressed component of the present invention has a Zn-based coating on both sides of the steel plate, and the Zn adhesion amount of the Zn-based coating on one side is 5-35 g / m. 2 Furthermore, the average linear roughness Ra of the Zn-based coating surface is less than 2.5 μm, while the average linear roughness Ra of the Zn-based coating surface on the other side is greater than 3.5 μm. The most significant feature of this invention is the intentional difference in the size of the surface irregularities on the front and back sides of the hot-pressed member.

[0035] The hot-pressed component of the present invention has a Zn-based coating on both sides of a steel plate. When the steel plate with the Zn-based coating is hot-pressed, the Zn in the coating diffuses into the base steel plate, forming a solid solution phase containing Fe and Zn in the diffusion region. It should be noted that the Zn-based coating may also contain other alloying elements. Simultaneously, sometimes the Zn in the Zn-based coating combines with oxygen present in the heating atmosphere, forming a Zn-containing oxide layer on the surface of the Zn-based coating. Furthermore, the Zn-based coating, which is an intermetallic compound that neither contributes to diffusion into the base steel plate nor to the formation of the oxide layer, remains as an intermetallic compound phase. However, due to the introduction of Fe diffused from the base steel plate, it becomes an intermetallic compound phase containing Zn, Fe, and other alloying elements contained in the coating. Both the solid solution phase and the intermetallic compound phase contain Zn, which has a sacrificial corrosion-resistant effect, thus contributing to improved corrosion resistance. As explained above, in order to satisfy the corrosion resistance after coating, which is the subject of this invention, a Zn-based coating is an essential technical feature, which contains at least one of a solid solution phase and an intermetallic compound phase.

[0036] In this invention, the Zn adhesion amount of the Zn-based coating on one surface is 5-35 g / m. 2 The average linear roughness Ra of the Zn-based coating surface is below 2.5 μm. This surface, located on the outer surface of the hot-pressed component, is the primary surface for evaluating surface corrosion performance. The Zn adhesion amount is less than 5 g / m². 2 At this time, the corrosion rate of zinc under the coating increases significantly, and the corrosion resistance decreases after coating. Therefore, the Zn adhesion amount of the Zn-based coating is set at 5 g / m. 2 The above. To further improve corrosion resistance and resistance spot weldability after coating, the Zn adhesion amount of the Zn-based coating is preferably set to 10 g / m². 2 The above is more preferably set at 15g / m 2 That's all. On the other hand, the Zn adhesion amount exceeds 35 g / m². 2 During resistance spot welding, the reaction with the electrode metal is intense, significantly increasing the risk of cracks caused by the brittleness of the liquid metal. Therefore, the Zn adhesion amount of the Zn-based coating is set to 35 g / m. 2Below, with the goal of further improving corrosion resistance and resistance spot weldability after coating, the Zn adhesion amount is preferably set to 28 g / m. 2 Hereinafter, a more preferred setting is 25g / m 2 The following refers to the amount of Zn deposited in a Zn-based coating. Furthermore, when evaluating the corrosion resistance after coating on a hot-pressed component with large irregularities and a Zn-based coating surface having an average linear roughness exceeding 2.5 μm, the formation of red rust becomes particularly noticeable on the general portions that have never been cross-cut. This is because the electrodeposited coating does not follow the irregularities of the hot-pressed component surface; on the raised portions, the electrodeposited coating film thickness becomes extremely thin, thus causing red rust in such areas. Therefore, the average linear roughness Ra of the Zn-based coating surface is set to 2.5 μm or less. The average linear roughness Ra is preferably less than 2.2 μm, more preferably less than 2.0 μm, and even more preferably less than 1.6 μm. Additionally, when the hot-pressed component has a flat irregularity with an average linear roughness of less than 0.5 μm on the Zn-based coating surface, the adhesion of the coating decreases. Therefore, the average linear roughness Ra of the Zn-based coating surface is preferably set to 0.5 μm or more, and more preferably to 1.0 μm or more.

[0037] In this invention, the average linear roughness Ra of the Zn-based coating surface on the other side is 3.5 μm or more. This side is located on the aforementioned side (having a Zn adhesion amount of 5–35 g / m). 2 The surface opposite to the Zn-based coating surface (where the average linear roughness Ra of the Zn-based coating is less than 2.5 μm) is located on the inner surface of the hot-pressed component and becomes the bonding surface during resistance spot welding (if the Zn adhesion amount is 5-35 g / m). 2Furthermore, if the surface of the Zn-based coating has an average linear roughness Ra of 2.5 μm or less, it is considered the surface of the steel plate; if the surface has an average linear roughness Ra of 3.5 μm or more, it is considered the back surface of the steel plate. As described above, an oxide film is formed on the surface of the component after hot pressing. Its resistivity is high, and therefore, the thicker and more uniformly it exists, the lower the resistance spot weldability. Specifically, when a thick oxide film exists on the surface, the current conduction path becomes narrower, leading to instability in the current conduction, resulting in spatter caused by localized current conduction with a lower welding current. Compared to metal films and electrode metals, oxide films have high hardness but poor toughness. Therefore, they are easily damaged by pressure from the electrode or the target material, the steel plate. In this case, by making the average linear roughness Ra of the Zn-based coating surface on the other side 3.5 μm or more, the destruction of the oxide film when the electrode is pressed down during resistance spot welding is promoted, and the occurrence of spatter is reduced by ensuring the current conduction point. The average linear roughness Ra is preferably 3.7 μm or more, and more preferably 4.0 μm or more. Further preferably, the linear roughness Ra is set to 4.5 μm or more. Most preferably, it is set to 5.0 μm or more. Furthermore, when the average linear roughness Ra of the Zn-based coating surface exceeds 8 μm, the coating appearance deteriorates significantly. From the viewpoint of coating appearance, the average linear roughness Ra of the Zn-based coating surface on the other side is preferably set to 8 μm or less.

[0038] The Zn adhesion amount of a Zn-based coating with an average linear roughness Ra of 3.5 μm or higher is preferably 40–120 g / m. 2 .

[0039] 2) Steel plates for hot pressing

[0040] The hot-pressing steel plate of the present invention has a Zn-based coating on both sides of the steel plate, and the Zn adhesion amount of the Zn-based coating on one side is 5 to 35 g / m. 2 The Zn deposition rate of the Zn-based coating on the other side is 40–120 g / m². 2 The metal constituting a Zn-based coating can be pure zinc or contain other alloying elements. For example, by containing 0.1% to 20% of elements selected from Mg, Al, Cr, Co, and Ni, further improvements in corrosion resistance can be expected. Additionally, a Zn-based coating can be a layer containing dispersed oxides; for example, a layer containing 0.1% to 10% SiO2 or Al2O3 nanoparticles can be used.

[0041] By making the Zn adhesion amount on one side of the hot-pressed steel plate 5-35 g / m 2 This allows for the production of hot-pressed components with excellent corrosion resistance after coating. The Zn adhesion amount is less than 5 g / m². 2During hot pressing, the heating process before pressing causes Zn to oxidize or evaporate, resulting in the disappearance of Zn in its metallic state, including its intermetallic compound state. Therefore, hot-pressed components with the desired corrosion resistance after coating cannot be obtained. In particular, coating expansion increases at the end faces and areas with coating defects, and red rust formation becomes significant at damaged areas. Therefore, the Zn adhesion amount is set at 5 g / m². 2 The above applies. Zn adhesion exceeds 35 g / m². 2 At this point, the swelling inhibition effect of the coating film reaches saturation. Therefore, the Zn adhesion amount is set to 35 g / m³. 2 The following applies. With the goal of further improving corrosion resistance after coating, the Zn adhesion amount is preferably set to 10 g / m². 2 The above is more preferably set at 15g / m 2 The above is further optimized to 17g / m 2 That's all. Additionally, the preferred Zn adhesion amount is 28 g / m³. 2 Hereinafter, a more preferred setting is 25g / m 2 The following is a further preferred setting of 20g / m 2 the following.

[0042] By making the Zn adhesion amount of the Zn-based coating on the other side of the hot-pressed steel plate 5-35 g / m 2 The Zn adhesion amount on the opposite side of the surface is 40-120 g / m². 2 This allows for the production of hot-pressed components with excellent weldability. The Zn adhesion amount is less than 40 g / m². 2 At that time, the surface roughness after heat treatment was small, and hot-pressed components with the desired resistance spot weldability could not be obtained. Therefore, the Zn adhesion amount was set to 40 g / m. 2 The above applies. Zn adhesion exceeds 120 g / m². 2 At this point, not only does the improvement in weldability saturate, but the possibility of liquid metal embrittlement cracks occurring at the weld joint also increases. Therefore, the Zn coating amount is set to 120 g / m². 2 The Zn adhesion amount is preferably set to 45 g / m³. 2 The above is more preferably set at 55g / m 2 The above is further optimized to 65g / m 2 That's all. Additionally, the Zn coating amount is set at 120 g / m³. 2 The Zn adhesion amount is preferably set to 100 g / m³. 2 Hereinafter, a preferred setting is 90g / m 2 The following is a further preferred setting: 75g / m 2 the following.

[0043] It should be noted that the Zn-based coating in the hot-pressing steel sheet of the present invention can be a single-layer Zn-based coating, but within the scope of not affecting the effect of the present invention, a lower coating or an upper coating can also be provided depending on the purpose. For example, as a lower coating, a Ni-based substrate coating can be exemplified.

[0044] In this invention, in order to obtain a hot-pressed component with a strength exceeding 1470 MPa after hot pressing, the base steel sheet used as the Zn-based coating in the hot-pressing steel sheet can, for example, be a steel sheet containing, by mass percent, C: 0.20–0.50%, Si: 0.1–0.5%, Mn: 1.0–3.0%, P: 0.02% or less, S: 0.01% or less, Al: 0.1% or less, N: 0.01% or less, with the balance being Fe and unavoidable impurities. It should be noted that the steel sheet can be either cold-rolled steel sheet or hot-rolled steel sheet. The reasons for the limitations of each component are explained below.

[0045] C: 0.20~0.50%

[0046] Carbon (C) increases strength by forming martensite and other structures within the steel. To obtain strength exceeding 1470 MPa, the C content is preferably 0.20% or more. On the other hand, when it exceeds 0.50%, the toughness of the spot weld decreases. Therefore, the C content is preferably set to 0.50% or less.

[0047] Si: 0.1-0.5%

[0048] Si is an effective element for strengthening steel and obtaining good material properties. Therefore, it is preferable to have a content of 0.1% or more. On the other hand, when the Si content exceeds 0.5%, ferrite is stabilized, thus reducing hardenability. Therefore, the Si content is preferably set to 0.5% or less.

[0049] Mn: 1.0–3.0%

[0050] Mn is an effective element for ensuring strength after cooling over a wide range of cooling rates. To ensure mechanical properties and strength, it is preferable to contain 1.0% or more Mn. On the other hand, when the Mn content exceeds 3.0%, not only does the cost increase, but the effect also saturates. Therefore, the Mn content is preferably set to 3.0% or less.

[0051] P: below 0.02%

[0052] When the phosphorus (P) content exceeds 0.02%, localized ductility deteriorates due to grain boundary embrittlement associated with P segregation towards austenite grain boundaries during casting, thus reducing the balance between strength and ductility. Therefore, the P content is preferably set to 0.02% or less. Furthermore, if the P content is set to 0.001% or less, the aforementioned improvement in the balance between strength and ductility becomes saturated, only increasing refining costs. Therefore, from the viewpoint of refining cost, the P content is preferably set to 0.001% or more.

[0053] S: below 0.01%

[0054] S forms inclusions such as MnS, which degrade impact resistance and cause cracks along the metal flow in the weld. Therefore, it is preferable to minimize the amount of S, preferably setting it to 0.01% or less. Furthermore, to ensure good elongation flange properties, the amount of S is more preferably set to 0.005% or less. Additionally, considering refining costs, the amount of S is preferably set to 0.001% or more.

[0055] Al: below 0.1%

[0056] When the Al content exceeds 0.1%, the punching processability and hardenability of the raw steel sheet decrease. Therefore, the Al content is preferably set to 0.1% or less. In addition, considering the refining cost, the Al content is preferably set to 0.0001% or more.

[0057] N: less than 0.01%

[0058] When the nitrogen content exceeds 0.01%, AlN nitrides are formed during hot rolling and heating before hot pressing, which reduces the punching processability and hardenability of the raw steel sheet. Therefore, the nitrogen content is preferably set to 0.01% or less. In addition, considering the refining cost, the nitrogen content is preferably set to 0.0001% or more.

[0059] In addition, in this invention, besides the basic components mentioned above, in order to further improve the properties of the steel plate, at least one or more components selected from Nb: less than 0.05%, Ti: less than 0.05%, B: 0.0002 to 0.005%, Cr: 0.1 to 0.3%, and Sb: 0.003 to 0.03% may be appropriately included as needed.

[0060] Nb: below 0.05%

[0061] Nitrogen (Nb) is an effective strengthening component for steel, but excessive amounts reduce its shape retention. Therefore, when Nb is present, the Nb content is preferably set to 0.05% or less. Furthermore, considering refining costs, the Nb content is preferably set to 0.0001% or more.

[0062] Ti: below 0.05%

[0063] Ti, like Nb, is effective in strengthening steel, but excessive amounts can reduce its shape retention. Therefore, when Ti is present, the Ti content is preferably set to 0.05% or less. Furthermore, considering refining costs, the Ti content is preferably set to 0.0001% or more.

[0064] B: 0.0002~0.005%

[0065] Boron (B) has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries, therefore, the amount of B is preferably 0.0002% or more. On the other hand, excessive B will greatly impair formability. Therefore, when B is present, the amount of B is preferably set to 0.0002% or more. Furthermore, it is preferably set to 0.005% or less.

[0066] Cr: 0.1–0.3%

[0067] Cr is useful for improving the strengthening and hardenability of steel. To achieve this effect, the Cr content is preferably 0.1% or more. On the other hand, due to the high cost of alloys, a Cr content exceeding 0.3% would lead to a significant increase in cost. Therefore, when Cr is present, the Cr content is preferably set to 0.1% or more. Furthermore, the Cr content is preferably set to 0.3% or less.

[0068] Sb: 0.003~0.03%

[0069] Sb has the effect of suppressing decarburization of the steel surface during the annealing process of the base plate for plating. To achieve this effect, it needs to contain 0.003% or more. On the other hand, when the Sb content exceeds 0.03%, it leads to an increase in rolling load, thus reducing productivity. Therefore, when Sb is present, the Sb content is preferably 0.003% or more. Furthermore, the Sb content is preferably set to 0.03% or less.

[0070] The balance other than those mentioned above consists of Fe and unavoidable impurities.

[0071] 3) Manufacturing method of steel plates for hot pressing

[0072] There are no particular provisions regarding the manufacturing conditions for the hot-pressed steel sheet of the present invention; however, preferred manufacturing conditions will be described below. The steel with the composition described above is cast, and the resulting hot-rolled slab is hot-rolled directly, or after heating, or after reheating the cold sheet. At this time, almost no change in characteristics is observed between directly rolling the hot-rolled slab and rolling after reheating. Furthermore, the reheating temperature is not particularly limited, but considering productivity, it is preferably set to a range of 1000°C to 1300°C. Hot rolling can be either a conventional hot rolling process or a continuous hot rolling process in which slabs are joined together during finishing rolling. Considering productivity and plate thickness accuracy, the rolling end temperature during hot rolling is preferably set above the Ar3 phase transformation point. Cooling after hot rolling is performed using conventional methods, but from a productivity point of view, the coiling temperature is preferably set to 550°C or higher. Furthermore, if the coiling temperature is too high, pickling performance deteriorates; therefore, it is preferably set to 750°C or lower. Pickling and cold rolling can be performed using conventional methods.

[0073] The subsequent zinc-based coating method is not limited and can be appropriately selected according to the alloy system. Electroplating is preferred for pure zinc or zinc-nickel alloy coating, while hot-dip galvanizing is preferred for zinc-aluminum alloy coating. Vacuum evaporation is preferred for zinc-magnesium alloy coating. Furthermore, by performing alloying treatment after coating, a coating alloyed with iron can be obtained efficiently. It should be noted that the atmosphere in the coating process can be set to normal conditions regardless of whether it is a continuous coating equipment with or without an oxidation-free furnace. Since no special control is required specifically for the steel sheet, this will not hinder productivity.

[0074] Regarding the control of Zn adhesion on one side (surface) and the other side (back) of the steel plate, in the case of electroplating, different Zn adhesion amounts can be achieved by varying either or both of the current density and energizing time on each side. Alternatively, in the case of hot-dip galvanizing, different Zn adhesion amounts can be achieved by varying the flow rate of the wiping gas during gas wiping after immersion in the plating bath on each side.

[0075] 4) Manufacturing method of hot-pressed components

[0076] In this invention, a hot-pressing steel sheet is heated from room temperature to a temperature range of Ac3 phase transition point to 1000°C within a time period of 5 seconds to 600 seconds, and then held within this temperature range for 300 seconds or less before hot pressing. This yields the desired hot-pressed component. The hot-pressing steel sheet has a Zn-based coating on both sides, with a Zn deposition amount of 5 to 35 g / m² on one side of the steel sheet. 2 The Zn adhesion amount on the other side of the steel plate is 40-120 g / m². 2.

[0077] By setting the heating temperature range of the hot-pressed steel sheet to the Ac3 phase transformation point ~ 1000°C, the Zn-based coating described in 1) above can be obtained. If the heating temperature is below the Ac3 phase transformation point, the required strength for the hot-pressed component may not be obtained; when the heating temperature exceeds 1000°C, Zn may sometimes disappear. It should be noted that the "Ac3 phase transformation point" is set as a value calculated based on the composition using the following formula.

[0078] Ac3 phase transition point (°C) = 910-203°C 1 / 2 +44.7Si-4Mn+11Cr

[0079] The element symbols in the above formula refer to the content (mass%) of each element. If the element is not present, it is set to zero.

[0080] To ensure the intermetallic compound phase remains and maintains corrosion resistance after coating, the time required to reach the aforementioned heating temperature from room temperature is set to 600 seconds or less. Preferably, this time is set to 450 seconds or less, and more preferably 300 seconds or less. Furthermore, if the heating rate is too high, i.e., the time from room temperature to the heating temperature is too short, not only will the amount of intermetallic compound residue become saturated, but the melting of the coating during heat treatment may also cause teardrop-like patterns, leading to a deterioration in appearance. Therefore, regarding the heating time, the time required to reach the aforementioned heating temperature from room temperature is set to 5 seconds or more, preferably 10 seconds or more, more preferably 100 seconds or more, and even more preferably 150 seconds or more.

[0081] Furthermore, regarding the holding time at the aforementioned heating temperature, from the viewpoint of ensuring that as much intermetallic compound phase as possible remains to further improve corrosion resistance after coating, and from the viewpoint of preventing hydrogen intrusion due to water vapor entrained in the furnace during the holding time, the holding time is set to 300 seconds or less. More preferably, the holding time is set to 180 seconds or less, even more preferably to 60 seconds or less, and most preferably, no holding time is used.

[0082] Furthermore, there are no limitations on the method of heating the steel plate for hot pressing. Examples include furnace heating using an electric furnace or gas furnace, electric heating, induction heating, high-frequency heating, and flame heating.

[0083] Next, heating is performed, followed by hot pressing. The hot-pressed component is manufactured by cooling it simultaneously or after the process using a mold or a cooling medium such as water. In this invention, the hot pressing conditions are not particularly limited, and pressing can be performed at a temperature of 600–800°C, which is a general hot pressing temperature range.

[0084] Example

[0085] The present invention will now be specifically described based on embodiments. These embodiments are not intended to limit the invention, and appropriate modifications within the scope of the main principles are also included within the scope of the invention.

[0086] The base steel sheet used is a cold-rolled steel sheet with a thickness of 1.4 mm (Ac3 phase transformation point = 848℃) containing, by mass %: C: 0.24%, Si: 0.25%, Mn: 1.28%, P: 0.005%, S: 0.001%, Al: 0.03%, N: 0.004%, Nb: 0.02%, Ti: 0.02%, B: 0.002%, Cr: 0.2%, Sb: 0.008%, with the balance being Fe and unavoidable impurities.

[0087] For the aforementioned base steel plate, Zn-based coatings are applied to both sides (surface and back) of the steel plate using the following electroplating or hot-dip galvanizing methods to obtain a hot-pressing steel plate.

[0088] <Electroplating method>

[0089] In a plating bath at pH 1.4 and a temperature of 50°C, consisting of 115 g / L zinc sulfate heptahydrate, 230 g / L nickel sulfate hexahydrate, and 55 g / L sodium sulfate, the current density was maintained between 10 and 100 A / dm³. 2 Electroplating was performed by varying the energizing time from 5 to 60 seconds, resulting in Zn-Ni alloy coatings with different Ni contents (12%) and Zn adhesion amounts for steel plates No. 1 to 18, as shown in Table 1-1. Additionally, Zn-based coatings for steel plates No. 19 and 20, as shown in Table 1-1, were formed by electroplating in a bath at pH 1.4 and a temperature of 50°C, consisting of 200 g / L zinc sulfate heptahydrate and 55 g / L sodium sulfate. The Zn adhesion amounts for the surface and back of the steel plates were obtained by varying the current density on each side. It should be noted that regarding the Zn adhesion amount of the Zn-based coating on the surface of the hot-pressing steel plate, three 48 mm φ samples were cut from the hot-pressing steel plate being evaluated, and measurements were taken for each sample. Then, the non-evaluation side of each sample, opposite to the side where the Zn adhesion amount was evaluated, was masked. Then, each sample was immersed in a solution of 500 mL of 35% hydrochloric acid aqueous solution containing 3.5 g of hexamethylenetetramine to a final volume of 1 L for 10 minutes to dissolve the Zn-based coating. The samples were then measured again. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to quantify the metal components in the dissolved hydrochloric acid solution samples, identifying the coating adhesion amount and Zn adhesion amount in the hot-pressed steel sheet.

[0090] <Hot-dip galvanizing method>

[0091] Using hot-dip galvanizing equipment, cold-rolled steel sheets were immersed in a molten Zn-Al(-Mg) coating bath, followed by N2 gas wiping, to produce hot-pressing Zn-Al coated steel sheets of levels No. 21 to 27 in Table 1-1. The Zn coating amounts differed on the surface and back of the steel sheet by adjusting the flow rate of the wiping gas for each side. For levels No. 23 and 24 in Table 1-1, hot-pressing steel sheets with molten Zn-Al coatings were produced by alloying treatment using an electrically heated device to heat the steel sheet to 500°C. It should be noted that regarding the Zn adhesion amount of the Zn coating on the surface of the hot-pressing steel sheet, the hot-pressing steel sheet being evaluated was punched, and three 48mm φ samples were cut and measured for each sample. Then, in each sample, the non-evaluation surface opposite to the side where the Zn adhesion amount was evaluated was masked. Then, each sample was immersed in a solution of 500 mL of 35% hydrochloric acid aqueous solution containing 3.5 g of hexamethylenetetramine to a final volume of 1 L for 10 minutes to dissolve the Zn-based coating. The samples were then measured again. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was used to quantify the metal components in the dissolved coating samples, identifying the coating adhesion and Zn adhesion amounts on the hot-pressed steel sheets.

[0092] <Fabrication of Hot-Pressed Components>

[0093] Next, 100mm × 200mm test pieces were cut from the hot-pressing steel sheet obtained through the above-described plating treatment and subjected to heat treatment using an electric furnace or an electrically heated furnace. The heat treatment conditions (heating temperature, heating time, holding temperature, and holding time) are shown in Table 1-2. After heat treatment, the test pieces were removed from the electric furnace or electrically heated furnace and immediately hot-pressed using a cap-shaped mold at a forming initiation temperature of 700℃ to obtain the hot-pressed component. It should be noted that the resulting hot-pressed component has a flat portion on the upper surface with a length of 100mm, a flat portion on the side surface with a length of 50mm, and a flat portion on the lower surface with a length of 50mm. Furthermore, regarding the curvature R of the mold, both shoulders on the upper surface and both shoulders on the lower surface are 7R.

[0094] For the obtained hot-pressed components, the coating adhesion amount, Zn adhesion amount, average line roughness Ra, resistance spot welding performance, and corrosion resistance after coating were measured, and the results were evaluated.

[0095] <Determination of Coating Adhesion, Zn Adhesion, and Average Linear Roughness Ra>

[0096] For the obtained hot-pressed components, the coating adhesion amount, Zn adhesion amount, and average linear roughness Ra were measured to evaluate the coating structure. The coating adhesion amount and Zn adhesion amount of the hot-pressed components were determined by the following method. The hot-pressed components used for evaluation were punched to obtain three 48 mm φ specimens, and each specimen was measured. Then, the non-evaluation surface on the opposite side of the side where the Zn adhesion amount was evaluated was masked in each specimen. Then, each specimen was immersed in a solution of 20 g ammonium dichromate diluted to 1 L for 60 minutes, thereby dissolving only the oxide layer. Then, each specimen was immersed in a solution of 500 mL of 35% hydrochloric acid aqueous solution containing 3.5 g hexamethylenetetramine diluted to 1 L for 10 minutes, thereby dissolving the Zn-based coating, and each specimen was measured again. The metal components in the hydrochloric acid solution sample containing the coating were quantitatively analyzed using inductively coupled plasma atomic emission spectrometry (ICP-AES) to identify the coating amount and Zn amount of the hot-pressed component.

[0097] Using a Mitutoyo SURFTEST SJ-2100, and in accordance with JIS B 0601-2001, the scanning speed was set to 0.5 mm / s, the operating distance to 4 mm, and the measurement load to 0.75 mN. The arithmetic mean roughness Ra of the Zn-based coating surface was measured. Measurements were performed in any 30 intervals, and the average value was calculated as the average linear roughness Ra of this invention.

[0098] <Resistance Spot Welding Properties>

[0099] To evaluate the resistance spot weldability of hot-pressed components, 30mm × 50mm test pieces were cut from the flat portion of the upper surface of the hot-pressed component, and resistance spot welding was performed using two identical plates. An AC resistance spot welder was used, with DRφ16 type Cr-Cu electrodes with a tip diameter of 6mm. The applied pressure was set to 3.5kN, and the energizing time was set to 0.42 seconds. The welding current was increased from 3.0kA in increments of 0.1kA until spatter occurred, and the maximum current value without spatter was recorded. The weld nugget diameter was measured by observing the cross-section of the welded portion of the test piece after welding. The difference between the minimum current with a weld nugget diameter of 4√t (mm) or more relative to the plate thickness t (mm) and the maximum current value without spatter was defined as the appropriate welding current range. The appropriate current range was judged according to the following criteria, with ◎ or ○ indicating acceptance. The evaluation results are shown in Table 1-2.

[0100] ◎: 1.5kA ≤ appropriate current range

[0101] ○: 0.8kA ≤ appropriate current range < 1.5kA

[0102] ×: 0.8kA > Appropriate current range

[0103] Additionally, a 5° electrode angle was set, and other conditions remained the same as described above. Two identical plates were used for welding. The maximum length of the crack generated within the weld nugget was measured from the cross-section and taken as the LME crack length of the weld. The LME crack length of the weld was judged according to the following criteria, with 0 being considered acceptable. The evaluation results are shown in Table 1-2.

[0104] 〇: 20μm ≥ LME crack length of welded part

[0105] △: 100μm ≥ LME crack length in welded area > 20μm

[0106] ×: 100μm < LME crack length at the weld

[0107] <Corrosion resistance after coating>

[0108] To evaluate the corrosion resistance of the hot-pressed components after coating, 70mm × 150mm test pieces were cut from the flat portion of the upper surface of the hot-pressed components. These test pieces underwent zirconium-based chemical conversion treatment and electrodeposition coating. The zirconium-based chemical conversion treatment was performed under standard conditions using PLM2100 manufactured by Pakase Sei Co., Ltd. (Japan). The electrodeposition coating was performed using Electron GT100 cationic electrodeposition coating manufactured by Kansai Paint Co., Ltd., with a coating thickness of 10μm. Baking conditions were set at 170°C for 20 minutes. Subsequently, the hot-pressed components that underwent zirconium-based chemical conversion treatment and electrodeposition coating were subjected to corrosion testing (SAE-J2334), and the corrosion condition was evaluated after 30 cycles.

[0109] For general sections that were not subjected to cross-cutting, the following criteria are used for judgment, with ◎ or ○ indicating acceptance. The evaluation results are shown in Table 1-2.

[0110] ◎: No red rust is produced in the general parts.

[0111] ○: 1 location ≤ 3 locations where red rust has formed

[0112] △: 3 or fewer locations where red rust has formed < 10 locations

[0113] ×: 10 locations ≤ locations where red rust occurs

[0114] For the cross-section (defective section), measure the maximum expansion width on one side from the cross-section, and judge according to the following criteria, marking ◎ or ○ as acceptable. The evaluation results are shown in Table 1-2.

[0115] ◎: Maximum expansion width on one side < 1.5mm

[0116] ○: 1.5mm ≤ maximum expansion width on one side < 3.0mm

[0117] △: 3.0mm ≤ maximum expansion width on one side < 4.0mm

[0118] ×: 4.0mm ≤ Maximum expansion width on one side

[0119] [Table 1-1]

[0120]

[0121]

[0122] As can be seen from the results in Table 1-2, the hot-pressed component of the present invention exhibits excellent corrosion resistance and resistance spot weldability after coating. Furthermore, if the hot-pressed steel plate of the present invention is used, a hot-pressed component with excellent corrosion resistance and resistance spot weldability after coating can be obtained.

Claims

1. A hot-pressed component, wherein the hot-pressed component has Zn-based coatings on both sides of a steel plate, wherein, The Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5-35 g / m². 2 Furthermore, the average linear roughness Ra of the Zn-based coating surface is below 2.5 μm. The Zn adhesion amount of the Zn-based coating on the other side of the steel plate is 40-120 g / m². 2 Furthermore, the average linear roughness Ra of the Zn-based coating surface is above 3.5 μm.

2. A hot-pressing steel plate, which is used in the hot-pressing component of claim 1, and has a Zn-based coating on both sides of the steel plate, wherein, The Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5-35 g / m². 2 , The Zn adhesion amount of the Zn-based coating on the other side of the steel plate is 40-120 g / m². 2 , In addition to Zn, the metals constituting Zn-based coatings also contain 0.1% to 20% of one or more elements selected from Mg and Al.

3. A method for manufacturing a hot-pressed component, which is the method for manufacturing a hot-pressed component according to claim 1, wherein, The steel plate for hot pressing is heated from room temperature to a temperature range of Ac3 phase transition point to 1000℃ within a time of more than 5 seconds and less than 600 seconds, and then held within this temperature range for less than 300 seconds before hot pressing. The hot-pressing steel plate has a Zn-based coating on both sides, and the Zn adhesion amount of the Zn-based coating on one side of the steel plate is 5-35 g / m². 2 The Zn adhesion amount of the Zn-based coating on the other side of the steel plate is 40-120 g / m². 2 .

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