Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • 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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

• the present invention relates to plated steel sheets.
• Patent Document 1 describes a method for producing a high-strength cold-rolled steel sheet, characterized in that when a high-strength cold-rolled steel sheet is continuously annealed in a continuous annealing furnace or in a cold-rolled steel sheet/hot-dip galvanized steel sheet dual-purpose facility having a continuous annealing furnace, the cooling method in a cooling zone including part or all of the steel sheet temperature range of 600 to 250°C following heating for recrystallization is one or more of gas cooling, diffusion cooling, and cooling pipe cooling, the steel sheet surface is exposed to an atmosphere in which iron oxidizes within the above-mentioned steel sheet temperature range, pickled at the outlet of the annealing furnace, and then iron or Ni plating is applied in an amount of 1 to 50 mg/ m2 .
• Patent Document 1 teaches that while oxidation of a steel sheet is normally prevented by an extremely low concentration of oxygen and/or an inert atmospheric gas with an extremely low dew point around the steel sheet, the steel sheet is instead actively exposed to an oxidizing atmosphere to oxidize not only Si and Mn but also the iron in the steel sheet, and by pickling after the steel sheet leaves the annealing furnace, the oxide films of Si, Mn, etc. are removed together with the oxide film on the iron in the steel sheet, thereby obtaining a high-strength cold-rolled steel sheet that is free from "hollow-out" and has good chemical conversion treatability even if the contents of Si, Mn, etc. are high.
• Patent Document 2 teaches that elements such as nickel (Ni) and tin (Sn) in addition to copper (Cu) reduce the mechanical properties required of automotive steel sheets, such as strength and formability, as well as chemical stability such as corrosion resistance, and that copper compounds present on the surface of the steel sheet in particular reduce the chemical conversion treatability needed to improve corrosion resistance. Furthermore, generally, reduced chemical conversion treatability can result in the formation of areas where the chemical conversion coating, known as "whiteout,” is not formed, which can result in reduced paint film adhesion.
• a plated steel sheet comprising a base steel sheet and a plating disposed on a surface of the base steel sheet,
• the base steel plate is, in mass%, Ni: 0.010 to 1.000%, A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
• a plated steel sheet characterized in that the surface coverage with at least one of Ni, Cu, and Sn is 25% or more, and the circle-equivalent radius of an uncoated region that is not coated with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• a plated steel sheet comprises a base steel sheet and a plating disposed on a surface of the base steel sheet,
• the base steel plate is, in mass%, Ni: 0.010 to 1.000%, A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
• the surface coverage by at least one of Ni, Cu, and Sn is 25% or more, and the circle-equivalent radius of the uncoated region that is not coated with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• skid zones areas where the chemical conversion coating is not formed, known as “skid zones,” can appear, which can result in reduced paint adhesion.
• elements such as Ni, Cu, and Sn
• the potential of the steel sheet becomes more noble than when these elements are not present in solid solution, which can reduce the etching ability of Fe during chemical conversion treatment.
• the chemical conversion treatability of the steel sheet declines, resulting in reduced paint adhesion. Therefore, this reduced paint adhesion is particularly problematic when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn.
• Blast furnace steel can also contain elements such as Ni, Cu, and Sn as additives, so if these elements are present, the above-mentioned issues must be addressed appropriately.
• the inventors conducted research, focusing particularly on the element distribution on the steel sheet surface, in order to provide a plated steel sheet that can exhibit excellent paint adhesion even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn. As a result, the inventors discovered that it is effective to uniformly coat the surface of a base steel sheet containing Ni, Cu, and Sn with at least one of these elements.
• paint adhesion can be significantly improved by plating the surface of the base steel sheet so that, when measured by Auger electron spectroscopy, the surface is covered with at least one of Ni, Cu, and Sn at a predetermined surface coverage rate, more specifically, a surface coverage rate of 25% or more, and the size of the uncoated area not covered by these elements, i.e., Ni, Cu, and Sn, is limited within a predetermined range, more specifically, the circle-equivalent radius of the uncoated area is limited to 10 ⁇ m or less.
• the plated steel sheet according to the embodiment of the present invention encompasses not only electric furnace materials which inevitably contain Ni, Cu, and Sn as tramp elements, but also blast furnace materials which contain Ni, Cu, and Sn as essential elements or optional added elements. Furthermore, the plated steel sheet according to the embodiment of the present invention can achieve superior paint adhesion and, in turn, superior corrosion resistance compared to conventional plated steel sheets which simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the plated steel sheet according to the embodiment of the present invention is particularly useful in the automotive field, where superior paint adhesion and/or corrosion resistance are required. Below, each component of the plated steel sheet according to the embodiment of the present invention will be described in more detail.
• a plating is disposed on the surface of a base steel sheet, for example, on at least one, preferably both, surfaces of the base steel sheet.
• the plating may contain at least one of Ni, Cu, and Sn, and may also contain other elements such as Zn, Al, and Fe in addition to Ni, Cu, and Sn.
• the plating may consist essentially of at least one of Ni, Cu, and Sn, or may consist of at least one of Ni, Cu, and Sn, or may be composed of at least one of Ni, Cu, and Sn.
• the contents and coating weights of Ni, Cu, and Sn in the plating are not particularly limited and may be appropriately selected within ranges that satisfy the requirements for surface coverage and uncoated areas, which will be described in detail later.
• the surface coverage with at least one of Ni, Cu, and Sn is preferably 30% or more or 35% or more, more preferably 40% or more or 45% or more, and most preferably 50% or more or 55% or more.
• the surface coverage with at least one of Ni, Cu, and Sn may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.
• the circle-equivalent radius of an uncoated region not coated with at least one of Ni, Cu, and Sn is controlled to 10 ⁇ m or less. If a relatively large uncoated region not coated with at least one of Ni, Cu, and Sn exists, naturally, there are no cathodic sites in such an uncoated region, and therefore it becomes impossible to promote the anodic dissolution of Fe during chemical conversion treatment. As a result, it becomes impossible to form a chemical conversion coating uniformly over the entire steel sheet, and coating adhesion deteriorates.
• the surface of the base steel sheet can be uniformly coated with at least one of Ni, Cu, and Sn, allowing these elements to function effectively as cathode sites.
• the above-mentioned cathode reaction can proceed appropriately across the entire surface of the steel sheet, allowing a chemical conversion coating to be formed uniformly across the entire steel sheet, thereby significantly improving paint film adhesion. From the perspective of further improving paint film adhesion, the smaller the circle-equivalent radius of the above-mentioned uncoated regions, the better.
• the circle-equivalent radius of the uncoated regions not coated with at least one of Ni, Cu, and Sn is preferably 8 ⁇ m or less, more preferably 6 ⁇ m or less or 5 ⁇ m or less, and most preferably 4 ⁇ m or less or 3 ⁇ m or less.
• the circle-equivalent radius of the uncoated region that is not coated with at least one of Ni, Cu, and Sn may be, for example, 0.5 ⁇ m or more or 1 ⁇ m or more.
• the surface coverage is measured by Auger electron spectroscopy as follows. First, a sample including a plate surface is placed in an Auger electron spectrometer (e.g., AES PHI-700 (ULVAC-PHI, FE type)), and the sample surface (plate surface) is measured under conditions of an acceleration voltage of 10 kV, a current value of 10 nA, and an Auger spectrum measurement energy range of 40 to 1690 eV. The measurement area is observed at 1000x magnification or more with an SEM, and is set to an area of 60 ⁇ m ⁇ 100 ⁇ m or more. Next, a mapping is created with a lower limit of 80 cps (counts/s), and an element distribution image is obtained.
• AES PHI-700 UUV-PHI, FE type
• the calculated value is determined as the surface coverage of the plated steel sheet by at least one of Ni, Cu, and Sn.
• the uncoated regions are determined by calculating the total area and number of uncoated regions not coated with at least one of Ni, Cu, and Sn in a binarized image obtained using the image analysis software "ImageJ" in connection with the measurement of the surface coverage.
• the average area S per uncoated region is calculated by dividing the total area of the uncoated regions by the total number of uncoated regions.
• the base steel sheet has a chemical composition containing, by mass%, Ni: 0.010 to 1.000%, Cu: 0.010 to 1.000%, and Sn: 0.003 to 1.000%.
• an object of the present invention is to provide a plated steel sheet containing Ni, Cu, and Sn that can exhibit improved paint adhesion, and this object is achieved by plating the base steel sheet so that the surface of the base steel sheet is covered with at least one of Ni, Cu, and Sn at a surface coverage of 25% or more, and the circle-equivalent radius of the uncoated region not covered with these elements is limited to 10 ⁇ m or less, as measured by Auger electron spectroscopy.
• the chemical composition of the base steel sheet is not particularly limited except that it contains, in mass %, 0.010 to 1.000% Ni, 0.010 to 1.000% Cu, and 0.003 to 1.000% Sn, and therefore it is clear that elements other than Ni, Cu, and Sn are not essential technical features for achieving the object of the present invention.
• the chemical composition of the base steel sheet may contain, in addition to Ni, Cu, and Sn, appropriate amounts of any alloying elements commonly added in the technical field of the present invention.
• C is an element that inexpensively increases strength and is an important element for controlling the strength of steel.
• the C content is preferably 0.001% or more.
• the C content may be 0.005% or more, 0.010% or more, 0.030% or more, 0.040% or more, 0.070% or more, 0.100% or more, 0.150% or more, or 0.200% or more.
• excessive C content may result in a decrease in elongation.
• the C content is preferably 0.500% or less.
• the C content may be 0.450% or less, 0.400% or less, 0.350% or less, 0.300% or less, or 0.250% or less.
• Si is an element that is effective in increasing strength as a solid solution strengthening element.
• the Si content may be 0%, but to obtain this effect, the Si content is preferably 0.01% or more.
• the Si content may be 0.05% or more, 0.10% or more, 0.30% or more, 0.50% or more, 0.80% or more, or 1.00% or more.
• excessive Si content may increase the steel strength but decrease the elongation. For this reason, the Si content is preferably 3.00% or less.
• the Si content may be 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.
• Mn is an element that improves the hardenability of steel and is effective in increasing strength. To fully obtain this effect, the Mn content is preferably 0.10% or more. The Mn content may be 0.50% or more, 1.00% or more, 1.30% or more, 1.50% or more, or 1.80% or more. On the other hand, excessive Mn content may increase the steel strength but decrease the elongation. For this reason, the Mn content is preferably 3.00% or less. The Mn content may be 2.80% or less, 2.50% or less, or 2.00% or less.
• Al acts as a deoxidizer for steel and has the effect of improving the soundness of steel.
• the Al content is preferably 0.001% or more.
• the Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.030% or more.
• excessive Al content may generate coarse Al oxides, reducing the elongation of the steel sheet. Therefore, the Al content is preferably 2.000% or less.
• the Al content may be 1.500% or less, 1.000% or less, 0.500% or less, 0.100% or less, or 0.050% or less.
• Ni and Cu are elements that contribute to improving strength through precipitation strengthening or solid solution strengthening.
• the contents of these elements are preferably 0.010% or more, and may be 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, 0.100% or more, 0.150% or more, or 0.200% or more.
• excessive content of these elements may promote the formation of oxides, particularly Mn- and/or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Ni and Cu contents are preferably 1.000% or less, and may be 0.800% or less, 0.600% or less, 0.400% or less, or 0.300% or less.
• Sn is an element effective in improving corrosion resistance.
• the Sn content is preferably 0.003% or more.
• the Sn content may be 0.004% or more, 0.008% or more, 0.010% or more, 0.020% or more, 0.030% or more, 0.040% or more, 0.050% or more, 0.080% or more, or 0.100% or more.
• excessive Sn content may promote the formation of oxides, particularly Mn- and/or Si-based surface oxides and iron oxides, on the steel sheet surface, which may impair plating adhesion in the plating process. Therefore, the Sn content is preferably 1.000% or less.
• the Sn content may be 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, or 0.200% or less.
• P is an element that segregates at grain boundaries and promotes embrittlement of steel. Since a lower P content is preferable, ideally it is 0%. However, excessive reduction in the P content may result in a significant increase in costs. Therefore, the P content may be 0.0001% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content may result in embrittlement of steel due to grain boundary segregation, as described above. Therefore, the P content is preferably 0.100% or less. The P content may be 0.050% or less, 0.030% or less, 0.020% or less, or 0.010% or less.
• S is an element that generates nonmetallic inclusions such as MnS in steel, resulting in a decrease in the ductility of steel parts. Since a lower S content is preferable, ideally 0%. However, excessive reduction in the S content may result in a significant increase in costs. Therefore, the S content may be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.002% or more. On the other hand, excessive S content may cause cracks originating from nonmetallic inclusions during cold forming. Therefore, the S content is preferably 0.100% or less. The S content may be 0.050% or less, 0.020% or less, or 0.010% or less.
• Hf, Mg, Zr, Ca, and REM are elements that can control the morphology of non-metallic inclusions.
• the Hf, Mg, Zr, Ca, and REM contents may be 0%, but to obtain these effects, the contents of these elements are preferably 0.0001% or more, and may be 0.0005% or more, or 0.001% or more. However, even if these elements are contained in excess, the effects are saturated, and adding more than necessary to the steel sheet increases production costs.
• the hot-rolled steel sheet obtained in the hot rolling process is coiled in the next coiling process and then subjected to primary pickling.
• the hot-rolled steel sheet is coiled at a coiling temperature of 520°C or higher.
• a coiling temperature 520°C or higher.
• an outer oxide layer is formed on the outer (surface) side of the steel sheet, and an inner oxide layer is also formed in the inner (surface layer) side of the steel sheet.
• This inner oxide layer is mainly composed of Mn- and/or Si-based oxides.
• the coiling temperature it is preferable to control the coiling temperature to 550°C or higher.
• the coiling temperature By controlling the coiling temperature to 550°C or higher, the formation of the internal oxide layer can be further promoted, which in turn makes it possible to make the Mn-Si depleted layer thicker. As a result, the formation of Mn and/or Si-based surface oxides during the annealing process can be more significantly suppressed, making it possible to further increase the surface coverage.
• the coiling temperature There is no particular upper limit to the coiling temperature, but for example, the coiling temperature may be 600°C or lower.
• the hot-rolled steel sheet After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet.
• the reduction ratio in cold rolling can be appropriately determined depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%.
• the sheet After the cold-rolling step, the sheet may be cooled to room temperature, for example, by air-cooling.
• the secondary pickling process involves immersing the cold-rolled steel sheet in an aqueous solution having a hydrochloric acid concentration of 3 to 12% without an inhibitor that inhibits corrosion of the steel sheet at a temperature of 50 to 90°C for 2 to 100 seconds, and then rinsing the cold-rolled steel sheet with a rinse solution having an electrical conductivity of 40 mS/m or less.
• This secondary pickling process using an aqueous hydrochloric acid solution can sufficiently or completely remove Mn- and/or Si-based surface oxides formed on the surface of the cold-rolled steel sheet during the annealing process.
• the formation of Mn- and/or Si-based surface oxides in the annealing process can be sufficiently suppressed compared to when such an Mn—Si-depleted layer is not present.
• the formation of the surface oxides is not completely suppressed in the annealing process, it is important to properly perform secondary pickling even after the annealing process in order to ensure proper plating adhesion in the subsequent plating process.
• the water rinse after the secondary pickling is also extremely important.
• the electrical conductivity of the water used in the water rinse is relatively high, more specifically, if it is higher than 40 mS/m, iron oxides may form on the surface of the cold-rolled steel sheet during the water rinse after the secondary pickling.
• molten steel was cast using a continuous casting method to form a steel billet with the chemical composition shown in Table 1.
• the steel billet was cooled, reheated to 1200°C, hot-rolled, and then coiled at the coiling temperature shown in Table 2.
• Hot rolling was performed by rough rolling and finish rolling, with the finishing temperature for finish rolling being 900-1050°C and the reduction in finish rolling being 30%.
• the obtained hot-rolled steel sheet was subjected to primary pickling and then cold-rolled at a reduction of 50% to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.
• the obtained cold-rolled steel sheet was subjected to an annealing process in which it was heated to 800°C in a furnace with an oxygen concentration of 20 ppm or less in an atmosphere with a dew point of 0°C and 4% hydrogen (balance nitrogen), and held there for 100 seconds.
• the annealed cold-rolled steel sheet was subjected to secondary pickling.
• the secondary pickling was performed by immersing the cold-rolled steel sheet in an inhibitor-free aqueous solution having a hydrochloric acid concentration of 5% at a temperature of 80°C for 4.5 seconds, and then rinsing the cold-rolled steel sheet with a rinse solution having an electrical conductivity shown in Table 2.
• the obtained base steel sheet was electroplated using a bath containing a predetermined concentration of a metal species (one of Ni, Cu, and Sn) shown in Table 2 under conditions of a current density of 0.5 A/ dm2 and a current application time of 1.0 second, thereby obtaining a plated steel sheet in which a plating was applied to both sides of the base steel sheet.
• a metal species one of Ni, Cu, and Sn
• the properties of the resulting plated steel sheets were measured and evaluated using the following methods.
• the paint film adhesion was evaluated as follows: First, a 50 mm x 50 mm sample of the plated steel sheet produced above was subjected to a zinc phosphate treatment as a chemical conversion treatment under the following conditions. Degreasing: Immersed in a degreasing agent (Fine Cleaner E2083) at 40°C for 2 minutes, then rinsed with water. Surface conditioning: Immersed in a surface conditioning agent (Preparen Z) at room temperature for 30 seconds. Chemical conversion treatment: Immersed in a zinc phosphate treatment agent (Palbond L3020) at 40°C for 2 minutes, then rinsed with water and dried.
• Chemically treated plated steel sheet samples were electrocoated (Powernics Excel 1200, manufactured by Nippon Paint Industrial Coating Co., Ltd.) at an electrodeposition temperature of 30°C to a film thickness of 18 ⁇ m, followed by a baking treatment at 170°C for 30 minutes.
• the electrocoated samples were then subjected to a saltwater immersion test (SDT). Specifically, the electrocoated samples were immersed in a 5% NaCl aqueous solution at 50°C for 1000 hours. After the SDT test, the removed samples were dried and then subjected to a tape peel test on one side of the sample.
• SDT saltwater immersion test
• the peeled tape was scanned, and the area ratio of the coating peeled was calculated by binarization using the image analysis software "ImageJ.”
• the coating adhesion was evaluated as follows: AAA: Peeled area rate less than 5% AA: Peeled area rate 5 to less than 10% A: Peeled area rate 10 to 15% B: Peeling area rate over 15%
• Comparative Example 23 the low coiling temperature resulted in insufficient formation of an internal oxide layer, and as a result, it was not possible to form an Mn-Si depleted layer with a thickness of 0.3 ⁇ m or more. As a result, the Ni surface coverage was less than 25%, and coating adhesion was reduced.
• Comparative Example 25 in addition to the low coiling temperature, the high electrical conductivity of the washing solution used for rinsing after the secondary pickling presumably prevented the formation of Mn and/or Si-based surface oxides during the annealing process, and furthermore, the formation of iron oxides during rinsing after the secondary pickling presumably also failed to be sufficiently suppressed.
• the Ni surface coverage was less than 25%, and the circle-equivalent radius of the uncoated region exceeded 10 ⁇ m, resulting in reduced coating adhesion.
• the high electrical conductivity of the washing solution used for rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling.
• the surface coverage rate with at least one of Ni, Cu, and Sn was less than 25%, and the circle-equivalent radius of the uncoated area exceeded 10 ⁇ m, resulting in reduced paint film adhesion.
• the surface of the base steel sheet was covered with at least one of Ni, Cu, and Sn at a surface coverage of 25% or more, and the circle-equivalent radius of the uncoated area not covered by these elements was limited to 10 ⁇ m or less.
• the paint adhesion was rated AA, demonstrating further improvement.
• the paint adhesion was rated AAA, demonstrating further improvement.

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• 2025
  • 2025-01-27 JP JP2025536337A patent/JP7836020B2/ja active Active
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