WO2020241866A1 - ホットスタンプ用めっき鋼板 - Google Patents
ホットスタンプ用めっき鋼板 Download PDFInfo
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- WO2020241866A1 WO2020241866A1 PCT/JP2020/021445 JP2020021445W WO2020241866A1 WO 2020241866 A1 WO2020241866 A1 WO 2020241866A1 JP 2020021445 W JP2020021445 W JP 2020021445W WO 2020241866 A1 WO2020241866 A1 WO 2020241866A1
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- steel sheet
- plating layer
- plating
- hot
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B32—LAYERED PRODUCTS
- B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
- B32B15/00—Layered products comprising a layer of metal
- B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
- B32B15/013—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of a metal other than iron or aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D5/00—Electroplating characterised by the process; Pretreatment or after-treatment of workpieces
- C25D5/60—Electroplating characterised by the structure or texture of the layers
- C25D5/615—Microstructure of the layers, e.g. mixed structure
- C25D5/617—Crystalline layers
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D7/00—Electroplating characterised by the article coated
- C25D7/06—Wires; Strips; Foils
- C25D7/0614—Strips or foils
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/56—Electroplating: Baths therefor from solutions of alloys
- C25D3/562—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of iron or nickel or cobalt
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
- C25D3/00—Electroplating: Baths therefor
- C25D3/02—Electroplating: Baths therefor from solutions
- C25D3/56—Electroplating: Baths therefor from solutions of alloys
- C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc
Definitions
- the present invention relates to a hot stamped galvanized steel sheet, more specifically, a hot stamped galvanized steel sheet having a Zn—Ni plated layer.
- the hot stamping method is often used for forming steel sheets used for automobile parts.
- the hot stamping method is a method in which a steel sheet is press-formed while being heated to a temperature in the austenite range, and then quenched (cooled) by a press die at the same time as the forming.
- Patent Document 1 discloses a steel sheet for hot stamping having a plating layer such as a Zn—Ni plating layer having a melting point of 800 ° C. or higher and an adhesion amount of 10 to 90 g / m 2 per side on the surface of the steel sheet. Further, in Patent Document 2, the surface of the steel sheet contains 10 to 25% by mass of Ni, the balance is Zn and unavoidable impurities, and has a plating layer having an adhesion amount of 10 to 90 g / m 2. A steel sheet for hot stamping having an ⁇ phase content of 5% by mass or less is disclosed.
- Patent Document 3 the surface of the steel sheet contains 60% by mass or more of Ni in order, the balance is Zn and unavoidable impurities, and the plating layer I having an adhesion amount of 0.01 to 5 g / m 2 and 10
- a steel sheet for hot stamping is disclosed, which contains up to 25% by mass of Ni, has a balance of Zn and unavoidable impurities, and has a plating layer II having an adhesion amount of 10 to 90 g / m 2 .
- Patent Document 4 includes a base steel plate and a plating layer formed on the base steel plate, which contains 10 to 25% by mass of Ni and the balance is Zn and unavoidable impurities.
- a plated steel sheet having an adhesion amount of 10 to 90 g / m 2 per side is heated to 850 to 950 ° C., and hot press molding is started when the temperature of the plated steel sheet after heating is 650 to 800 ° C.
- a method for manufacturing a hot-pressed member is described.
- Patent Documents 5 to 7 describe that the average crystal grain size of the plating layer is adjusted to maintain a stable appearance after chromate treatment and improve press workability. It is taught to do so and to improve chemical conversion processability.
- LME liquid metal embrittlement
- the hot stamping method When the hot stamping method is applied to a steel sheet having a Zn plating layer or a Zn alloy plating layer, if the melting point of the plating layer is lower than the heating temperature of the hot stamp, the plating layer may melt on the surface of the steel sheet to form a liquid phase. Therefore, when hot stamping is performed at such a heating temperature, Zn in the Zn—Ni plating layer may penetrate into the grain boundaries of the steel sheet, and LME may occur. Then, cracks (also referred to as “LME cracks”) may occur in the obtained hot stamped molded product.
- a plating layer having a melting point of 800 ° C. or higher is provided on the steel sheet in order to obtain LME resistance.
- the melting point of the plating layer is lower than the heating temperature of the hot stamp, if the plating is performed with a refractory metal as much as possible, the contact time between the molten metal and the base steel plate is short, so that the liquid resistant metal It is stated that the brittleness is good.
- the melting point of the plating layer is 800 ° C. or higher, if the melting point of the plating layer is lower than the heating temperature at the time of hot stamping, the melting of the plating layer at the time of heating the hot stamp cannot be completely prevented. Therefore, the plating layer melted during the molding of the hot stamp may invade the grain boundaries of the steel sheet and cause LME.
- Patent Documents 2 and 3 disclose a steel sheet for hot stamping having a Zn—Ni plated layer on the steel sheet, suppression of LME has not been studied.
- Patent Document 4 teaches that hot press forming is started at a predetermined temperature to prevent LME cracking, the configuration of a Zn—Ni plated steel sheet for preventing LME cracking is sufficiently examined. It has not been.
- the present invention has been made in view of such circumstances, and a plated steel sheet for hot stamping capable of preventing melting of the Zn—Ni plated layer during hot stamping and sufficiently suppressing LME.
- the purpose is to provide.
- the present inventors have stated that in order to sufficiently suppress LME during hot stamping, it is effective to reduce the plating average crystal grain size in the Zn—Ni plating layer formed on the steel sheet. I found it.
- the Zn of the Zn—Ni plating layer becomes easy to move when the hot stamp is heated, and it becomes possible to diffuse a large amount of Zn in the plating layer to the underlying steel plate. .. Then, the Zn concentration of the plating layer is reduced, that is, the Ni concentration of the plating layer is relatively increased, and the melting point of the plating layer is increased.
- the amount of Fe diffusion of the base steel sheet into the plating layer also increases, and the melting point of the plating layer also increases due to the increase in the Fe concentration in the plating layer. Therefore, the plating layer does not exist as a liquid phase during hot stamping, and Zn diffused into the steel sheet during heating alloys with Fe in the steel sheet to form a complete solid solution, which can suppress LME.
- the present inventors set the Ni concentration in the plating layer before hot stamping to a predetermined value in order to obtain a sufficient melting point of the plating layer after Zn is diffused into the steel sheet. It has been found that it is effective to keep the above, and further, it is effective not to make the plating layer excessively thick in order to facilitate the diffusion of Zn into the steel sheet.
- the present invention has been made based on the above findings, and the gist thereof is as follows.
- (1) It has a steel plate and a Zn—Ni plating layer formed on at least one side of the steel plate.
- the Ni concentration is 8% by mass or more, and the plating adhesion amount is 10 g / m 2 per side.
- a plated steel sheet for hot stamping having an average crystal grain size of 50 nm or less and 90 g / m 2 or less.
- the steel sheet is by mass% C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.00% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, O: 0.010% or less, B: 0% or more and 0.0040% or less, Cr: 0% or more and 2.00% or less, Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 0.300% or less, Zr: 0% or more and 0.300% or less, Mo: 0% or more and 2.000% or less, Cu: 0% or more and 2.000% or less, Ni: 0% or more and 2.000% or less, Sb: 0% or more and 0.100% or less, Ca: 0% or more and 0.0100% or less,
- the steel sheet is by mass% B: 0.0005% or more and 0.0040% or less, Cr: 0.01% or more and 2.00% or less, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more and 0.300% or less, V: 0.001% or more and 0.300% or less, Zr: 0.001% or more and 0.300% or less, Mo: 0.001% or more and 2.000% or less, Cu: 0.001% or more and 2.000% or less, Ni: 0.001% or more and 2.000% or less, Sb: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and 0.0100% or less,
- the plated steel sheet for hot stamping according to (2) which contains at least one selected from the group consisting of Mg: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.1000% or less.
- a large amount of Zn in the plating layer is diffused into the underlying steel sheet when the hot stamp is heated, and the amount of Fe diffusion in the base steel sheet into the plating layer is also increased to increase Fe in the plating layer.
- the plated steel sheet for hot stamping according to the present invention has a steel sheet and a Zn—Ni plated layer formed on at least one side of the steel sheet.
- the Zn—Ni plating layer is formed on both sides of the steel sheet.
- the Zn—Ni plating layer may be formed on the steel sheet, and another plating layer may be provided between the steel sheet and the Zn—Ni plating layer.
- the composition of the steel sheet in the present invention is not particularly limited as long as the steel sheet can be used for hot stamping.
- the elements that can be contained in the steel sheet in the present invention will be described.
- "%" representing the content of each element in the component composition means mass% unless otherwise specified.
- the steel sheet in the present invention has C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less, Si: 0.05% or more and 2.00% by mass.
- Al 0.001% or more and 1.500% or less
- P 0.100% or less
- S 0.100% or less
- N 0.010% or less
- O 0.010% or less
- C (C: 0.05% or more and 0.70% or less)
- C (carbon) is an element effective for improving the strength of the steel sheet.
- Automobile members may be required to have high strength of, for example, 980 MPa or more.
- the C content is preferably 0.05% or more.
- the C content is preferably 0.70% or less.
- the lower limit of the C content is preferably 0.10%, more preferably 0.12%, still more preferably 0.15%, and most preferably 0.20%.
- the upper limit of the C content is preferably 0.65%, more preferably 0.60%, still more preferably 0.55%, and most preferably 0.50%.
- Mn manganese
- Mn manganese
- Mn manganese
- the Mn content is preferably 11.0% or less.
- the lower limit of the Mn content is preferably 1.0%, more preferably 2.0%, even more preferably 2.5%, even more preferably 3.0%, and most preferably 3.5%.
- the upper limit of the Mn content is preferably 10.0%, more preferably 9.5%, still more preferably 9.0%, even more preferably 8.5%, and most preferably 8.0%.
- Si silicon
- Si silicon
- the Si content is preferably 0.05% or more.
- the Si content is preferably 2.00% or less.
- the lower limit of the Si content is preferably 0.10%, more preferably 0.15%, still more preferably 0.20%, and most preferably 0.30%.
- the upper limit of the Si content is preferably 1.80%, more preferably 1.50%, still more preferably 1.20%, and most preferably 1.00%.
- Al (Al: 0.001% or more and 1.500% or less)
- Al (aluminum) is an element that acts as a deoxidizing element.
- the Al content is preferably 0.001% or more.
- the Al content is preferably 1.500% or less.
- the lower limit of the Al content is preferably 0.010%, more preferably 0.020%, still more preferably 0.050%, and most preferably 0.100%.
- the upper limit of the Al content is preferably 1.000%, more preferably 0.800%, still more preferably 0.700%, and most preferably 0.500%.
- the lower limit of these elements is not particularly limited. However, the content of these elements may be more than 0% or 0.001% or more. On the other hand, if these elements are excessively contained, the toughness, ductility and / or processability may be deteriorated. Therefore, the upper limits of P and S are set to 0.100%, and the upper limits of N and O are set to 0.010%. Is preferable.
- the upper limit of P and S is preferably 0.080%, more preferably 0.050%.
- the upper limit of N and O is preferably 0.008%, more preferably 0.005%.
- the basic composition of the steel sheet in the present invention is as described above. Further, the steel sheet may contain at least one of the following optional elements in place of a part of the remaining Fe, if necessary.
- the steel sheet may contain B: 0% or more and 0.0040%.
- the steel sheet may contain Cr: 0% or more and 2.00% or less.
- the steel sheet is from Ti: 0% or more and 0.300% or less, Nb: 0% or more and 0.300% or less, V: 0% or more and 0.300% or less, and Zr: 0% or more and 0.300% or less. It may contain at least one selected from the group.
- the steel sheet contains at least one selected from the group consisting of Mo: 0% or more and 2.000% or less, Cu: 0% or more and 2.000% or less, and Ni: 0% or more and 2.000% or less. You may. Further, the steel sheet may contain Sb: 0% or more and 0.100% or less. Further, the steel sheet contains at least one selected from the group consisting of Ca: 0% or more and 0.0100% or less, Mg: 0% or more and 0.0100% or less, and REM: 0% or more and 0.1000% or less. You may. Hereinafter, these optional elements will be described in detail.
- B (B: 0% or more and 0.0040% or less)
- B (boron) is an element effective for improving hardenability during hot stamping.
- the B content may be 0%, but in order to surely obtain this effect, the B content is preferably 0.0005% or more.
- the B content is preferably 0.0040% or less.
- the lower limit of the B content is preferably 0.0008%, more preferably 0.0010%, and even more preferably 0.0015%.
- the upper limit of the B content is preferably 0.0035%, more preferably 0.0030%.
- Cr 0% or more and 2.00% or less
- Cr Cr (chromium) is an element effective for improving hardenability during hot stamping.
- the Cr content may be 0%, but in order to ensure this effect, the Cr content is preferably 0.01% or more.
- the Cr content may be 0.10% or more, 0.50% or more, or 0.70% or more.
- the Cr content is preferably 2.00% or less.
- the Cr content may be 1.50% or less, 1.20% or less, or 1.00% or less.
- Ti 0% or more and 0.300% or less
- Nb 0% or more and 0.300% or less
- V 0% or more and 0.300% or less
- Zr 0% or more and 0.300% or less
- Ti (titanium), Nb (niobium), V (vanadium) and Zr (zirconium) are elements that improve tensile strength through the miniaturization of metal structures.
- the content of these elements may be 0%, but in order to surely obtain this effect, the contents of Ti, Nb, V and Zr are preferably 0.001% or more, and 0.010. % Or more, 0.020% or more, or 0.030% or more.
- the Ti, Nb, V and Zr contents are preferably 0.300% or less, and may be 0.150% or less, 0.100% or less, or 0.060% or less.
- Mo molybdenum
- Cu 0% or more and 2.000% or less
- Ni 0% or more and 2.000% or less
- Mo (molybdenum), Cu (copper) and Ni (nickel) have the effect of increasing the tensile strength.
- the content of these elements may be 0%, but in order to surely obtain this effect, the contents of Mo, Cu and Ni are preferably 0.001% or more, and 0.010% or more. , 0.050% or more or 0.100% or more.
- the Mo, Cu and Ni contents are preferably 2.000% or less, and may be 1.500% or less, 1.000% or less, or 0.800% or less.
- Sb 0% or more and 0.100% or less
- Sb antimony
- the Sb content may be 0%, but in order to surely obtain this effect, the Sb content is preferably 0.001% or more.
- the Sb content may be 0.005% or more, 0.010% or more, or 0.020% or less.
- excessive content of Sb may cause a decrease in toughness. Therefore, the Sb content is preferably 0.100% or less.
- the Sb content may be 0.080% or less, 0.060% or less, or 0.050% or less.
- Ca 0% or more and 0.0100% or less
- Mg 0% or more and 0.0100% or less
- REM 0% or more and 0.1000% or less
- Ca (calcium), Mg (magnesium) and REM (rare earth metal) are elements that improve toughness after hot stamping by adjusting the shape of inclusions.
- the content of these elements may be 0%, but in order to surely obtain this effect, the Ca, Mg and REM contents are preferably 0.0001% or more, and 0.0010% or more. , 0.0020% or more or 0.0040% or more.
- Ca, Mg and REM are excessively contained, the effect is saturated and the production cost is increased.
- the Ca and Mg contents are preferably 0.0100% or less, and may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.
- the REM content is preferably 0.1000% or less, and may be 0.0800% or less, 0.0500% or less, 0.0100% or less.
- the rest other than the above elements consists of iron and impurities.
- the "impurity” is a component mixed by various factors in the manufacturing process, including raw materials such as ore and scrap, when the base steel sheet is industrially manufactured, and the present invention is carried out. It includes components that are not intentionally added to the base steel sheet according to the form. Impurities are elements other than the components described above, and are contained in the base steel sheet at a level at which the action and effect peculiar to the element do not affect the characteristics of the hot-dip galvanized steel sheet according to the embodiment of the present invention. It also includes elements.
- the steel sheet in the present invention is not particularly limited, and general steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets can be used. Further, the steel sheet in the present invention may have any thickness as long as a Zn—Ni plating layer described later can be formed on the steel sheet and hot stamping can be performed, for example, 0.1 to 3.2 mm. ..
- the Zn—Ni plating layer in the present invention is a plating layer containing at least Zn and Ni, and other components are not particularly limited.
- the Zn—Ni plating layer may be a plating layer containing Zn as a main component (that is, a Zn concentration of 50% by mass or more) and a Ni concentration of 8% by mass or more, and other components are not particularly limited.
- Zn and Ni are either solid-solved in Zn or form an intermetallic compound due to Zn and Ni.
- the plating layer may be formed by any plating method, but is preferably formed by, for example, electroplating.
- the Zn—Ni plating layer is formed on at least one side of the steel sheet, preferably on both sides of the steel sheet.
- Fe and the like are diffused from the underlying steel plate to the plating layer and Zn and the like are diffused from the plating layer to the underlying steel plate. Therefore, the composition of the plating layer after hot stamping is hot. It changes according to the heating conditions (heating temperature, holding time, etc.) at the time of stamping.
- the lower limit of the Ni concentration is 8% by mass.
- the Ni concentration of the Zn—Ni plating layer can be made sufficiently higher than the Zn concentration, and a sufficiently high melting point can be obtained.
- a plating layer having a plating layer can be obtained. As a result, the plating layer does not become a liquid phase during hot stamping, and LME can be suppressed.
- the lower limit of the Ni concentration is preferably 10% by mass, more preferably 12% by mass.
- the upper limit of the Ni concentration is not particularly limited, but it is preferably 30% by mass or less from the viewpoint of economy.
- the upper limit of the Ni concentration may be 28% by mass, 25% by mass, or 20% by mass.
- the C concentration in the Zn—Ni plating layer is preferably less than 1% by mass.
- the adhesion of the plating layer to the steel sheet can be improved.
- the C concentration exceeds 1% by mass, the plating layer becomes brittle when the hot stamp is heated, and the steel sheet after the hot stamp is likely to be peeled off. Therefore, from the viewpoint of enhancing the adhesion of the plating layer, the lower the C concentration, the more preferable, 0.8% by mass or less, 0.5% by mass or less, 0.1% by mass or less, 0.01% by mass. It may be less than or equal to 0%.
- the C concentration in the Zn—Ni plating layer is surely 0.1% by mass or less or 0.01% by mass or less. It is possible to reduce to.
- the Zn—Ni plating layer in the present invention may further contain one or more of Fe, Cr and Co. These elements may be intentionally added or may be unavoidably mixed in the production.
- the balance of the component composition of the Zn—Ni plating layer is Zn and impurities.
- the Zn—Ni plating layer is, in mass%, Ni: 8% or more and 30% or less, and one or more of Fe, Cr and Co: 0% or more and 5% or less. , And C: less than 1%, with the balance consisting of iron and impurities.
- the Zn—Ni plating layer contains Ni: 8% or more and 30% or less in mass%, and the balance is composed of iron and impurities.
- the "impurities" in the Zn—Ni plating layer refer to components and the like that are mixed due to various factors in the manufacturing process, including raw materials, when the Zn—Ni plating layer is manufactured.
- the lower limit of the plating adhesion amount per one side of the steel sheet is 10 g / m 2 .
- the amount of plating adhered to one side is less than 10 g / m 2 , the thickness of the Zn—Ni plating layer becomes insufficient, surface scale is formed when the hot stamp is heated, and scale removal by shot blasting is required before painting. Become.
- the lower limit of the amount of plating adhered to one side of the steel sheet is preferably 16 g / m 2 , more preferably 20 g / m 2 , still more preferably 24 g / m 2 , and most preferably 30 g / m 2 .
- the upper limit of the amount of plating adhered to one side of the steel sheet is 90 g / m 2 .
- the plating layer does not become excessively thick, so that Zn in the plating layer easily diffuses into the steel sheet when the hot stamp is heated, and as a result, the hot stamp
- the melting point of the plating layer can be sufficiently improved during molding to effectively prevent LME.
- the amount of plating adhered to one side exceeds 90 g / m 2 , the thickness of the plating layer becomes too thick, and Zn diffusion to the steel sheet does not proceed sufficiently. As a result, the plating layer becomes liquid during hot stamping.
- the upper limit of the amount of plating adhered to one side of the steel sheet is preferably 80 g / m 2 , more preferably 76 g / m 2 , still more preferably 70 g / m 2 , and most preferably 60 g / m 2 .
- the Ni concentration and plating adhesion amount of the Zn—Ni plating layer in the present invention are measured by inductively coupled plasma (ICP) emission spectroscopic analysis. Specifically, the amount of plating adhered in the present invention is determined by dissolving the plating layer with 10% HCl from a plated steel sheet having a Zn—Ni plating layer and performing ICP analysis on the obtained solution. Since the plating adhesion amount in the present invention is the amount per one side, when the Zn—Ni plating layer is formed on both sides of the steel sheet, it is calculated assuming that the plating adhesion amount on both sides is the same. Further, the measurement of the C concentration in the Zn—Ni plating layer in the present invention is performed by the high frequency combustion-infrared absorption method.
- the upper limit of the average crystal grain size of Zn—Ni plating is 50 nm.
- Zn in the Zn—Ni plating layer becomes easy to move when the hot stamp is heated, so that Zn is easily diffused into the steel sheet. It should be noted that this diffusion can occur not only when Zn is in the liquid phase but also in the solid phase.
- the Ni concentration in the plating layer becomes relatively higher than the Zn concentration, and the melting point of the plating layer is improved.
- the average particle size of the plating layer is set to 50 nm or less, the diffusion of Fe in the base steel sheet into the plating layer is promoted, and the melting point of the plating layer also rises due to the increase in the Fe concentration in the plating layer. .. Therefore, the plating layer is less likely to be present in the liquid phase during hot stamping, and the generation of LME can be suppressed. In particular, when hot stamping is performed without high temperature, rapid heating, and / or holding time, LME is more likely to occur, but by making the average crystal grain size of the plating finer as in the present invention.
- the upper limit of the average crystal grain size of the plating is preferably 45 nm, more preferably 40 nm, still more preferably 35 nm, and most preferably 30 nm.
- the lower limit of the average crystal grain size of plating is not particularly limited, but since the lower limit of the average crystal grain size that can be substantially produced is 10 nm, the lower limit may be 10 nm.
- the lower limit of the average crystal grain size of the plating is preferably 12 nm, more preferably 15 nm, still more preferably 18 nm, and most preferably 20 nm.
- the average crystal grain size of Zn—Ni plating is 50 nm or less.
- Such fine Zn—Ni plating can be obtained, for example, by performing electroplating at a high current density (typically 300 A / dm 2 or more).
- the plated steel sheet having a Zn—Ni plated layer on the steel sheet as described above can be used for hot stamping under any conditions known to those skilled in the art.
- the heating method of the hot stamp is not limited, and examples thereof include furnace heating, energization heating, and induction heating.
- the heating temperature at the time of hot stamping may be any temperature as long as it is heated in the austenite region according to the composition of the steel sheet, for example, 800 ° C. or higher, 850 ° C. or higher, 900 ° C. or higher, or 950 ° C. or higher.
- molding and quenching can be performed with a press die. After heating, it may be held at the temperature for 1 to 10 minutes and then cooled, or it may not be cooled without holding. Further, quenching (cooling) can be performed at a cooling rate of 1 to 100 ° C./sec.
- Zn in the Zn—Ni plated layer can be diffused into the steel sheet when the hot stamp is heated, and the melting point of the plated layer can be raised during the molding of the hot stamp.
- the plating layer does not become a liquid phase during stamp molding, and Zn diffused into the steel sheet during heating dissolves in Fe in the steel sheet, and any of the above-mentioned conditions, especially conditions in which LME is likely to occur (high temperature, rapid heating, And / or no holding time), no LME is generated, and thus a hot stamped body without LME cracking can be obtained.
- the plated steel sheet for hot stamping according to the present invention can be obtained by forming a Zn—Ni plated layer on at least one side, preferably both sides of the steel sheet, for example, by electroplating.
- the manufacturing method of the steel sheet used for manufacturing the hot stamped plated steel sheet according to the present invention is not particularly limited.
- a steel sheet can be obtained by adjusting the composition of molten steel to a desired range, hot rolling, winding, and cold rolling.
- the thickness of the steel plate in the present invention may be, for example, 0.1 mm to 3.2 mm.
- the composition of the steel sheet used is not particularly limited, but as described above, the steel sheet has a mass% of C: 0.05% or more and 0.70% or less, Mn: 0.5% or more and 11.0% or less. Si: 0.05% or more and 2.00% or less, Al: 0.001% or more and 1.500% or less, P: 0.100% or less, S: 0.100% or less, N: 0.010% or less, And O: It is preferable that the content is 0.010% or less and the balance is composed of iron and impurities.
- the steel sheet has B: 0.0005% or more and 0.0040% or less, Cr: 0.01% or more and 2.00% or less, Ti: 0.001% or more and 0.300% or less, Nb: 0.001% or more and 0.300% or less, V: 0.001% or more and 0.300% or less, Zr: 0.001% or more and 0.300% or less, Mo: 0.001% or more and 2.000% or less, Cu: 0.001% or more and 2.000% or less, Ni: 0.001% or more and 2.000% or less, Sb: 0.001% or more and 0.100% or less, Ca: 0.0001% or more and 0.0100%
- at least one selected from the group consisting of Mg: 0.0001% or more and 0.0100% or less, and REM: 0.0001% or more and 0.1000% or less may be further contained.
- the method for forming the Zn—Ni plating layer in the present invention is not particularly limited as long as the Ni concentration, the amount of plating adhesion and the average crystal grain size according to the present invention can be obtained, but can be formed by electroplating.
- Plating can be performed.
- the composition of the bath used for forming the Zn—Ni plating layer is, for example, nickel sulfate hexahydrate: 150 to 350 g / L, zinc sulfate heptahydrate: 10 to 150 g / L, and sodium sulfate: It may be 25 to 75 g / L.
- electroplating can be performed with a high current density as described above, and a fine average crystal grain size can be obtained.
- the pH of the plating bath is set to 2.0 or less, preferably 1.5 or less, more preferably 1.0 or less by using, for example, sulfuric acid.
- the temperature of the plating bath is preferably 60 ° C. or higher, preferably 65 ° C. or higher, and more preferably 70 ° C. or higher.
- the upper limit of the current density is preferably 400 A / dm 2 or 450 A / dm 2 .
- organic additives such as dextrin, diallylamine polymer, and diallyldialkylammonium salt polymer may be added to the plating bath.
- diallylamine polymer examples include a diallylamine hydrochloride polymer, a methyldiallylamine hydrochloride polymer, a diallylamine hydrochloride / sulfur dioxide copolymer, and the like
- diallyldialkylammonium salt polymer include diallyldimethylammonium chloride weight. Examples thereof include coalescing, diallylmethylethylammonium ethylsulfate polymer and the like.
- the plating layer becomes brittle when the hot stamp is heated due to the carbon component contained in the additive, and plating peeling occurs in the steel sheet after the hot stamp. In some cases. Therefore, from the viewpoint of enhancing the adhesion of the plating layer, it is necessary to reduce the C concentration in the Zn—Ni plating layer to, for example, less than 1% by mass, preferably 0.01% by mass or less. Therefore, the above electroplating is preferably performed using an additive-free plating bath.
- a cold-rolled steel sheet having a thickness of 1.4 mm is immersed in a plating bath having the following plating bath composition, and Zn—Ni plating layers are formed on both sides of the cold-rolled steel sheet by electroplating, and a sample of the plated steel sheet for hot stamping is formed. No. Obtained 1-10. All the steel sheets used were in mass%, C: 0.50%, Mn: 3.0%, Si: 0.50%, Al: 0.100%, P: 0.010%, S: It contained 0.020%, N: 0.003%, O: 0.003%, and B: 0.0010%, with the balance being iron and impurities.
- Nickel sulfate ⁇ Hexhydrate 250 g / L (fixed) ⁇ Zinc sulfate ⁇ Hexhydrate: 10 to 150 g / L (variable) ⁇ Sodium sulfate: 50 g / L (fixed) -Organic additive: None, dextrin (3 g / L) or diallylamine hydrochloride / sulfur dioxide copolymer (molecular weight 5000, 3 g / L)
- the pH of the plating bath was set to 1.0 using sulfuric acid, and the bath temperature was maintained at 70 ° C.
- the current density and energization time were adjusted in order to obtain the desired amount of Zn—Ni plating adhered and the average crystal grain size. Further, in order to obtain a desired Ni concentration, the concentration of zinc sulfate / heptahydrate was appropriately adjusted based on the set current density. Table 1 shows the current densities set to produce each sample.
- the Ni concentration of 1 to 10 and the amount of plating adhering to one side were determined by ICP analysis. Specifically, only the plating layer was dissolved from each sample with 10% HCl, and the obtained solution was subjected to ICP analysis to determine the Ni concentration and the amount of plating adhering to one side. Table 1 shows the Ni concentration of each sample and the amount of plating adhered to one side.
- the average crystal grain size of Zn—Ni plating was determined by XRD.
- the half width B of the diffraction peak was determined by XRD (tube voltage: 40 kV and tube current: 200 mA) using Co-K ⁇ rays of each sample.
- the Co—K ⁇ ray wavelength ⁇ 0.179 nm
- the Bragg angle ⁇ was the angle of the diffraction line recognized in the range of 50.1 to 50.3 °.
- Table 1 shows the average crystal grain size of each sample.
- the C concentration in the Zn—Ni plating layer was measured using a high-frequency combustion-infrared absorber CS-6000 manufactured by LECO.
- the plating adhesion was evaluated by the area ratio of the plating layer attached to the peeled tape after the tape was attached to the V-bent portion and then peeled off.
- a cellophane tape (registered trademark) CT-18 manufactured by Nichiban Co., Ltd. was used as the tape, and the tape was attached in the direction perpendicular to the V bending direction.
- the tape was rubbed strongly with a rubber stopper to allow it to adhere sufficiently, and then peeled off in the vertical direction.
- the evaluation range was 10 mm in the center of the sample in the longitudinal direction of the tape, and 5 mm in the width direction of the tape centered on the top of the V-bent head. The evaluation results of each sample are shown in Table 1.
- Sample No. of the plated steel sheet for hot stamping according to the present invention It was found that in Nos. 1 to 4, 7 to 9, 11 and 12, no cracks were generated in the hot stamp molded product, and LME could be sufficiently suppressed. Above all, the sample No. In 1 to 4 and 7 to 9, the C concentration in the Zn—Ni plating layer was below the lower limit of analysis (less than 0.01% by mass), and high plating adhesion could be achieved in connection with this.
- the present invention it is possible to provide a hot stamped galvanized steel sheet capable of suppressing LME and preventing LME cracking in a hot stamped molded product, thereby providing a high-strength automobile member. Therefore, it can be said that the present invention has extremely high industrial value.
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Abstract
Description
(1)
鋼板と、前記鋼板の少なくとも片面に形成されたZn-Niめっき層とを有し、前記Zn-Niめっき層において、Ni濃度が8質量%以上であり、めっき付着量が片面あたり10g/m2以上90g/m2以下であり、平均結晶粒径が50nm以下である、ホットスタンプ用めっき鋼板。
(2)
前記鋼板が、質量%で、
C:0.05%以上0.70%以下、
Mn:0.5%以上11.0%以下、
Si:0.05%以上2.00%以下、
Al:0.001%以上1.500%以下、
P:0.100%以下、
S:0.100%以下、
N:0.010%以下、
O:0.010%以下、
B:0%以上0.0040%以下、
Cr:0%以上2.00%以下、
Ti:0%以上0.300%以下、
Nb:0%以上0.300%以下、
V:0%以上0.300%以下、
Zr:0%以上0.300%以下、
Mo:0%以上2.000%以下、
Cu:0%以上2.000%以下、
Ni:0%以上2.000%以下、
Sb:0%以上0.100%以下、
Ca:0%以上0.0100%以下、
Mg:0%以上0.0100%以下、及び
REM:0%以上0.1000%以下
を含有し、残部が鉄及び不純物からなる、(1)に記載のホットスタンプ用めっき鋼板。
(3)
前記鋼板が、質量%で、
B:0.0005%以上0.0040%以下、
Cr:0.01%以上2.00%以下、
Ti:0.001%以上0.300%以下、
Nb:0.001%以上0.300%以下、
V:0.001%以上0.300%以下、
Zr:0.001%以上0.300%以下、
Mo:0.001%以上2.000%以下、
Cu:0.001%以上2.000%以下、
Ni:0.001%以上2.000%以下、
Sb:0.001%以上0.100%以下、
Ca:0.0001%以上0.0100%以下、
Mg:0.0001%以上0.0100%以下、及び
REM:0.0001%以上0.1000%以下
からなる群より選択される少なくとも一種を含有する、(2)に記載のホットスタンプ用めっき鋼板。
(4)
前記平均結晶粒径が40nm以下である、(1)~(3)のいずれかに記載のホットスタンプ用めっき鋼板。
(5)
前記Zn-Niめっき層中のC濃度が1質量%未満である、(1)~(4)のいずれかに記載のホットスタンプ用めっき鋼板。
本発明に係るホットスタンプ用めっき鋼板は、鋼板と、鋼板の少なくとも片面に形成されたZn-Niめっき層とを有する。好ましくは、Zn-Niめっき層は鋼板の両面に形成される。また、本発明においては、Zn-Niめっき層は鋼板上に形成されていればよく、鋼板とZn-Niめっき層との間に他のめっき層が設けられていてもよい。
本発明における鋼板の成分組成は、鋼板をホットスタンプに使用することができれば特に限定されない。以下では、本発明における鋼板に含まれ得る元素について説明する。なお、成分組成についての各元素の含有量を表す「%」は特に断りがない限り質量%を意味する。
C(炭素)は、鋼板の強度を向上させるのに有効な元素である。自動車用部材には、例えば980MPa以上の高強度が求められる場合がある。強度を十分に確保するためには、C含有量を0.05%以上とすることが好ましい。一方、Cを過度に含有すると鋼板の加工性が低下する場合があるため、C含有量を0.70%以下とすることが好ましい。C含有量の下限は、好ましくは0.10%、より好ましくは0.12%、さらに好ましくは0.15%、最も好ましくは0.20%である。また、C含有量の上限は、好ましくは0.65%、より好ましくは0.60%、さらに好ましくは0.55%、最も好ましくは0.50%である。
Mn(マンガン)は、ホットスタンプの際の焼入れ性を向上させるのに有効な元素である。この効果を確実に得るためには、Mn含有量を0.5%以上とすることが好ましい。一方、Mnを過度に含有すると、Mnが偏析してホットスタンプ後の成形体の強度等が不均一になるおそれがあるため、Mn含有量を11.0%以下とすることが好ましい。Mn含有量の下限は、好ましくは1.0%、より好ましくは2.0%、さらに好ましくは2.5%、さらにより好ましくは3.0%、最も好ましくは3.5%である。Mn含有量の上限は、好ましくは10.0%、より好ましくは9.5%、さらに好ましくは9.0%、さらにより好ましくは8.5%、最も好ましくは8.0%である。
Si(ケイ素)は、鋼板の強度を向上させるのに有効な元素である。強度を十分に確保するためには、Si含有量を0.05%以上とすることが好ましい。一方、Siを過度に含有すると、加工性が低下する場合があるため、Si含有量を2.00%以下とすることが好ましい。Si含有量の下限は、好ましくは0.10%、より好ましくは0.15%、さらに好ましくは0.20%、最も好ましくは0.30%である。Si含有量の上限は、好ましくは1.80%、より好ましくは1.50%、さらに好ましくは1.20%、最も好ましくは1.00%である。
Al(アルミニウム)は、脱酸元素として作用する元素である。脱酸の効果を得るためには、Al含有量を0.001%以上とすることが好ましい。一方、Alを過剰に含有すると加工性が低下するおそれがあるため、Al含有量を1.500%以下とすることが好ましい。Al含有量の下限は、好ましくは0.010%、より好ましくは0.020%、さらに好ましくは0.050%、最も好ましくは0.100%である。Al含有量の上限は、好ましくは1.000%、より好ましくは0.800%、さらに好ましくは0.700%、最も好ましくは0.500%である。
(S:0.100%以下)
(N:0.010%以下)
(O:0.010%以下)
P(リン)、S(硫黄)、N(窒素)及び酸素(O)は不純物であり、少ない方が好ましいため、これらの元素の下限は特に限定されない。ただし、これらの元素の含有量を0%超又は0.001%以上としてもよい。一方、これらの元素を過剰に含有すると、靭性、延性及び/又は加工性が劣化するおそれがあるため、P及びSの上限を0.100%、N及びOの上限を0.010%とすることが好ましい。P及びSの上限は、好ましくは0.080%、より好ましくは0.050%である。N及びOの上限は、好ましくは0.008%、より好ましくは0.005%である。
B(ホウ素)は、ホットスタンプの際の焼入れ性を向上させるのに有効な元素である。B含有量は0%であってもよいが、この効果を確実に得るためには、B含有量を0.0005%以上とすることが好ましい。一方、Bを過度に含有すると、鋼板の加工性が低下するおそれがあるため、B含有量を0.0040%以下とすることが好ましい。B含有量の下限は、好ましくは0.0008%、より好ましくは0.0010%、さらに好ましくは0.0015%である。また、B含有量の上限は、好ましくは0.0035%、より好ましくは0.0030%である。
Cr(クロム)は、ホットスタンプの際の焼入れ性を向上させるのに有効な元素である。Cr含有量は0%であってもよいが、この効果を確実に得るためには、Cr含有量は0.01%以上とすることが好ましい。Cr含有量は0.10%以上、0.50%以上又は0.70%以上であってもよい。一方、Crを過度に含有すると、鋼材の熱的安定性が低下する場合がある。したがって、Cr含有量は2.00%以下とすることが好ましい。Cr含有量は1.50%以下、1.20%以下又は1.00%以下であってもよい。
(Nb:0%以上0.300%以下)
(V:0%以上0.300%以下)
(Zr:0%以上0.300%以下)
Ti(チタン)、Nb(ニオブ)、V(バナジウム)及びZr(ジルコニウム)は金属組織の微細化を通じ、引張強さを向上させる元素である。これらの元素の含有量は0%であってもよいが、この効果を確実に得るためには、Ti、Nb、V及びZr含有量は0.001%以上とすることが好ましく、0.010%以上、0.020%以上又は0.030%以上であってもよい。一方、Ti、Nb、V及びZrを過度に含有すると、効果が飽和するとともに製造コストが上昇する。このため、Ti、Nb、V及びZr含有量は0.300%以下とすることが好ましく、0.150%以下、0.100%以下又は0.060%以下であってもよい。
(Cu:0%以上2.000%以下)
(Ni:0%以上2.000%以下)
Mo(モリブデン)、Cu(銅)及びNi(ニッケル)は、引張強さを高める作用を有する。これらの元素の含有量は0%であってもよいが、この効果を確実に得るためには、Mo、Cu及びNi含有量は0.001%以上とすることが好ましく、0.010%以上、0.050%以上又は0.100%以上であってもよい。一方、Mo、Cu及びNiを過度に含有すると、鋼材の熱的安定性が低下する場合がある。したがって、Mo、Cu及びNi含有量は2.000%以下とすることが好ましく、1.500%以下、1.000%以下又は0.800%以下であってもよい。
Sb(アンチモン)は、めっきの濡れ性や密着性を向上させるのに有効な元素である。Sb含有量は0%であってもよいが、この効果を確実に得るためには、Sb含有量は0.001%以上とすることが好ましい。Sb含有量は0.005%以上、0.010%以上又は0.020%以下であってもよい。一方、Sbを過度に含有すると、靭性の低下を引き起す場合がある。したがって、Sb含有量は0.100%以下とすることが好ましい。Sb含有量は0.080%以下、0.060%以下又は0.050%以下であってもよい。
(Mg:0%以上0.0100%以下)
(REM:0%以上0.1000%以下)
Ca(カルシウム)、Mg(マグネシウム)及びREM(希土類金属)は、介在物の形状を調整することによりホットスタンプ後の靭性を向上させる元素である。これらの元素の含有量は0%であってもよいが、この効果を確実に得るためには、Ca、Mg及びREM含有量は0.0001%以上とすることが好ましく、0.0010%以上、0.0020%以上又は0.0040%以上であってもよい。一方、Ca、Mg及びREMを過度に含有すると、効果が飽和するとともに製造コストが上昇する。このため、Ca及びMg含有量は0.0100%以下とすることが好ましく、0.0080%以下、0.0060%以下又は0.0050%以下であってもよい。同様に、REM含有量は0.1000%以下とすることが好ましく、0.0800%以下、0.0500%以下0.0100%以下であってもよい。
本発明におけるZn-Niめっき層は、少なくともZn及びNiを含むめっき層であり、他の成分については特に限定されない。例えば、Zn-Niめっき層は、Znを主成分(すなわちZn濃度が50質量%以上)とし、Ni濃度が8質量%以上であるめっき層であればよく、他の成分については特に限定されない。当該めっき層においてZnとNiは、ZnにNiが固溶しているか、ZnとNiによる金属間化合物を形成している。当該めっき層は、如何なるめっき方法で形成されていてもよいが、例えば、電気めっきで形成されていることが好ましい。Zn-Niめっき層は、鋼板の少なくとも片面に形成され、好ましくは鋼板の両面に形成される。当然ながら、ホットスタンプ成形を行うと、下地の鋼板からめっき層へのFe等の拡散やめっき層から下地の鋼板へのZn等の拡散が生じるため、ホットスタンプ後のめっき層の成分組成はホットスタンプの際の加熱条件(加熱温度、保持時間等)に応じて変化する。
本発明におけるZn-Niめっき層において、Ni濃度の下限は8質量%である。Ni濃度を8質量%以上とすることで、ホットスタンプ時の加熱によりZnが鋼板に拡散した後に、Zn-Niめっき層のNi濃度をZn濃度に比べて十分に高くでき、十分に高い融点を有するめっき層を得ることができる。これにより、ホットスタンプの成形時にめっき層が液相とならず、LMEを抑制することができる。Ni濃度が8質量%未満となると、Znが鋼板に拡散しても、めっき層中のZn濃度をNi濃度に比べて十分に低減することができず、したがってめっき層の融点を十分に上げられず、ホットスタンプの成形時にLMEが発生するおそれがある。例えば、Ni濃度の下限は、好ましくは10質量%、より好ましくは12質量%である。
本発明においては、Zn-Niめっき層中のC濃度は1質量%未満であることが好ましい。Zn-Niめっき層中のC濃度を1質量%未満に低減することで、鋼板に対するめっき層の密着性を高めることができる。一方で、C濃度が1質量%を超えると、ホットスタンプの加熱時にめっき層が脆化し、ホットスタンプ後の鋼板においてめっき剥離が生じやすくなる。しがって、めっき層の密着性を高めるという観点からは、C濃度は低いほど好ましく、0.8質量%以下、0.5質量%以下、0.1質量%以下、0.01質量%以下、又は0%であってもよい。例えば、有機添加剤を含まないめっき浴を用いてZn-Niめっき層を形成することにより、当該Zn-Niめっき層中のC濃度を確実に0.1質量%以下又は0.01質量%以下に低減することが可能である。
本発明におけるZn-Niめっき層において、鋼板の片面あたりのめっき付着量の下限は10g/m2である。片面あたりのめっき付着量を10g/m2以上とすることで、ホットスタンプの加熱時の表面スケール形成を十分に防止することができる。片面あたりのめっき付着量が10g/m2未満となると、Zn-Niめっき層の厚さが不十分となり、ホットスタンプの加熱時に表面スケールが形成され、塗装前にショットブラストによるスケール除去が必要になる。鋼板の片面あたりのめっき付着量の下限は、好ましくは16g/m2、より好ましくは20g/m2、さらに好ましくは24g/m2、最も好ましくは30g/m2である。
本発明のZn-Niめっき層において、Zn-Niめっきの平均結晶粒径の上限は50nmである。平均結晶粒径を50nm以下とすることで、ホットスタンプの加熱時にZn-Niめっき層中のZnが動きやすくなることで、Znが鋼板に拡散しやすくなる。なお、この拡散はZnが液相になった場合だけでなく固相であっても起こり得る。Zn-Niめっき層中のZnが鋼板に拡散すると、当該めっき層におけるNi濃度がZn濃度に比べて相対的に高くなり、めっき層の融点が向上する。また、めっき層の平均粒径を50nm以下とすることで、母材鋼板のFeのめっき層への拡散も促進され、めっき層中のFe濃度が上昇することによってもめっき層の融点が上昇する。そのため、ホットスタンプの成形時にめっき層が液相で存在しにくく、LMEの発生を抑制することが可能となる。特に、ホットスタンプが、高温、急速加熱、及び/又は保持時間なしで行われた場合にはよりLMEが発生しやすくなるが、本発明のようにめっきの平均結晶粒径を微細化することで、ホットスタンプの加熱時にZnが鋼板に拡散し、めっき層の融点が向上するため、ホットスタンプの成形時には、めっき層が溶融せず、また、加熱時に拡散したZnは鋼板中のFeと固溶体形成するため、その結果、上記のようなLMEが発生しやすい条件であってもLMEを抑制可能となる。平均結晶粒径が50nm超となると、Znの鋼板への拡散が十分に進まず、めっき層の融点を十分に高くできず、LMEを抑制できなくなるおそれがある。めっきの平均結晶粒径の上限は、好ましくは45nm、より好ましくは40nm、さらに好ましくは35nm、最も好ましくは30nmである。
平均結晶粒径(nm)=Kλ/Bcosθ ・・・(1)
により求められる(式中、K:シェラー定数、λ:Co-Kα線波長(nm)、θはブラッグ角(ラジアン)である)。なお、Kは結晶子の形状によって変化する値であるが、本発明においてはK=0.9とすればよい。
本発明に係るホットスタンプ用めっき鋼板の製造方法の例を以下で説明する。本発明に係るホットスタンプ用めっき鋼板は、鋼板の少なくとも片面、好ましくは両面に、例えば電気めっきによりZn-Niめっき層を形成することで得ることができる。
本発明に係るホットスタンプ用めっき鋼板を製造するのに使用される鋼板の製造方法は特に限定されない。例えば、溶鋼の成分組成を所望の範囲に調整し、熱間圧延し、巻取り、さらに冷間圧延を行うことで鋼板を得ることができる。本発明における鋼板の板厚は、例えば、0.1mm~3.2mmであればよい。
本発明におけるZn-Niめっき層の形成方法は、本発明に係るNi濃度、めっき付着量及び平均結晶粒径が得られれば特に限定されないが、電気めっきにより形成することができる。特に、微細な粒径のZn-Niめっきを得るために、高電流密度で電気めっきを行うことが好ましく、例えば、280A/dm2以上、300A/dm2以上、又は350A/dm2以上で電気めっきを行うことができる。特に、350A/dm2以上で電気めっきを行うと、40nm以下の平均結晶粒径を得ることが可能となる。高電流密度でめっきした場合にめっきの平均結晶粒径が微細化する理由は、析出核の発生速度が、高電流密度化により高くなる過電圧の指数関数に比例して急激に増加するためだと考えられる。なお、比較的低い電流密度(例えば約100A/dm2)では、本発明のように50nm以下のめっき平均結晶粒径を得ることはできない。また、Zn-Niめっき層の形成に用いる浴の組成は、例えば、硫酸ニッケル・6水和物:150~350g/L、硫酸亜鉛・7水和物:10~150g/L、及び硫酸ナトリウム:25~75g/Lであればよい。このような浴組成にすることで、上述したような高電流密度で電気めっきが可能となり、微細な平均結晶粒径を得ることができる。
板厚1.4mmの冷延鋼板を以下のめっき浴組成を有するめっき浴に浸漬し、電気めっきにより当該冷延鋼板上の両面にZn-Niめっき層を形成し、ホットスタンプ用めっき鋼板の試料No.1~10を得た。なお、使用した全ての鋼板は、質量%で、C:0.50%、Mn:3.0%、Si:0.50%、Al:0.100%、P:0.010%、S:0.020%、N:0.003%、O:0.003%、及びB:0.0010%を含有し、残部が鉄及び不純物であった。
めっき浴組成
・硫酸ニッケル・6水和物:250g/L(固定)
・硫酸亜鉛・7水和物:10~150g/L(可変)
・硫酸ナトリウム:50g/L(固定)
・有機添加剤:無し、デキストリン(3g/L)又はジアリルアミン塩酸塩・二酸化硫黄共重合体(分子量5000、3g/L)
平均結晶粒径(nm)=Kλ/Bcosθ ・・・(1)
により平均結晶粒径を計算した(式中、K:シェラー定数=0.9、λ:Co-Kα線波長(nm)、θはブラッグ角(ラジアン)である)。ここで、Co-Kα線波長λ=0.179nm、ブラッグ角θは50.1~50.3°の範囲に認められた回折線の角度とした。各試料の平均結晶粒径を表1に示す。
[耐LME性の評価]
上述のように得られたホットスタンプ用めっき鋼板の試料No.1~10にホットスタンプを行った。ホットスタンプは通電加熱法により各試料を950℃まで昇温し、保持せず、ただちに先端R:3mmのV曲げ金型を用いて成形及び焼入れ(冷却速度:50℃/秒)を行った。得られた各ホットスタンプ成形体のV曲げ部をエポキシ樹脂に埋め込み研磨して断面を光学顕微鏡で観察した。観察はV曲げ頭頂部周辺から無作為に選択した5か所の位置において250倍で行った。5か所の観察において、クラックが全く観察されなかったものを「割れ評価:〇」、1か所でもクラックが観察されたものを「割れ評価:×」とした。
めっき密着性の評価は、上記のV曲げ部にテープを貼り付け、次いで剥離し、剥離したテープに付着しためっき層の面積率で評価した。テープにはニチバン社製のセロテープ(登録商標)CT-18を用い、V曲げ方向に対して垂直方向に貼り付けた。そのテープをゴム栓で強くこすり付けて十分に密着させた後、垂直方向に引きはがした。評価範囲はテープの長手方向はサンプルの中心10mmとし、テープの幅方向は、V曲げ頭頂部を中心とした幅5mmとした。各試料の評価結果を表1に示す。
Claims (5)
- 鋼板と、前記鋼板の少なくとも片面に形成されたZn-Niめっき層とを有し、前記Zn-Niめっき層において、Ni濃度が8質量%以上であり、めっき付着量が片面あたり10g/m2以上90g/m2以下であり、平均結晶粒径が50nm以下である、ホットスタンプ用めっき鋼板。
- 前記鋼板が、質量%で、
C:0.05%以上0.70%以下、
Mn:0.5%以上11.0%以下、
Si:0.05%以上2.00%以下、
Al:0.001%以上1.500%以下、
P:0.100%以下、
S:0.100%以下、
N:0.010%以下、
O:0.010%以下、
B:0%以上0.0040%以下、
Cr:0%以上2.00%以下、
Ti:0%以上0.300%以下、
Nb:0%以上0.300%以下、
V:0%以上0.300%以下、
Zr:0%以上0.300%以下、
Mo:0%以上2.000%以下、
Cu:0%以上2.000%以下、
Ni:0%以上2.000%以下、
Sb:0%以上0.100%以下、
Ca:0%以上0.0100%以下、
Mg:0%以上0.0100%以下、及び
REM:0%以上0.1000%以下
を含有し、残部が鉄及び不純物からなる、請求項1に記載のホットスタンプ用めっき鋼板。 - 前記鋼板が、質量%で、
B:0.0005%以上0.0040%以下、
Cr:0.01%以上2.00%以下、
Ti:0.001%以上0.300%以下、
Nb:0.001%以上0.300%以下、
V:0.001%以上0.300%以下、
Zr:0.001%以上0.300%以下、
Mo:0.001%以上2.000%以下、
Cu:0.001%以上2.000%以下、
Ni:0.001%以上2.000%以下、
Sb:0.001%以上0.100%以下、
Ca:0.0001%以上0.0100%以下、
Mg:0.0001%以上0.0100%以下、及び
REM:0.0001%以上0.1000%以下
からなる群より選択される少なくとも一種を含有する、請求項2に記載のホットスタンプ用めっき鋼板。 - 前記平均結晶粒径が40nm以下である、請求項1~3のいずれか1項に記載のホットスタンプ用めっき鋼板。
- 前記Zn-Niめっき層中のC濃度が1質量%未満である、請求項1~4のいずれか1項に記載のホットスタンプ用めっき鋼板。
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