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
  • C22C13/00—Alloys based on tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • 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
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper

Definitions

• the present invention relates to plated steel sheets and parts containing the same.
• 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 describes an automotive steel sheet containing 0.10% by mass or more and 0.50% by mass or less of copper (Cu), with the number of residual scales on the surface being 160,000 particles/mm2 or less , and the maximum particle size of copper compound particles exposed on the surface being 2 ⁇ m or less.
• Patent Document 2 also teaches that the above configuration makes it possible to provide a steel sheet with excellent chemical conversion treatability, since the particle size of copper compound particles exposed on the steel sheet surface, which serves as the cathode point in chemical conversion treatment, is 2 ⁇ m or less and the residual scale is kept to a predetermined amount or less.
• 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 ability to perform chemical conversion treatments to improve corrosion resistance.
• automotive steel sheets are generally often processed into the desired part shape by press forming. However, for example, if the plating peels off when the surface of the plated steel sheet slides against the mold during press forming, the ability to perform chemical conversion treatments, and therefore the corrosion resistance, of the processed area may be reduced.
• the present invention therefore aims to provide a plated steel sheet containing Ni, Cu, and Sn that exhibits improved chemical conversion treatability in processed areas, and a component that includes the same.
• the inventors conducted research focusing on the element distribution on the surface of steel sheet.
• the cross section of the plated steel sheet was measured using EPMA
• the inventors discovered that by applying plating so that the surface of the base steel sheet was covered with an alloy plating of at least one of Ni, Cu, and Sn and Fe at a predetermined surface coverage rate, and the size of the uncoated area not covered with at least one of Ni, Cu, and Sn was limited to a predetermined range, the chemical conversion treatability of the processed area could be significantly improved, leading to the completion of the present invention.
• 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: 0.010-1.000%, A chemical composition including Cu: 0.010 to 1.000% and Sn: 0.003 to 1.000%;
• the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1, and the surface coverage of the coating region in which the surface concentration of Fe is 10 mass% or more is 25% or more
• a plated steel sheet characterized in that the average length of an uncoated region that is not coated with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• [Ni], [Cu], and [Sn] are the surface concentrations [mass %] of each element.
• the chemical composition is, in mass%, Ni: 0.040-1.000%, Cu: 0.040 to 1.000%, and Sn: 0.004 to 1.000%
• the plated steel sheet according to any one of (1) to (5) above comprising: (7) The plated steel sheet according to any one of (1) to (6) above, characterized in that it has a Vickers hardness of 200 Hv or more. (8) A part comprising the plated steel sheet according to any one of (1) to (7) above.
• the present invention provides a plated steel sheet containing Ni, Cu, and Sn that exhibits improved chemical conversion treatability in processed areas, as well as a part containing the same.
• FIG. 1 is a cross-sectional schematic view of a plated steel sheet according to an embodiment of the present invention, illustrating the surface coverage of the plating.
• 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 concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1, and the surface coverage of the coating region in which the surface concentration of Fe is 10 mass% or more is 25% or more,
• the average length of the uncoated region that is not coated with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• [Ni], [Cu], and [Sn] are the surface concentrations [mass %] of each element.
• the plating may peel off when the surface of the plated steel sheet slides against the mold during press forming. If the plating peels off from the processed area during press forming or other processes, exposing the base steel sheet, this can cause a problem of reduced chemical conversion treatability in the processed area where the base steel sheet is exposed, especially if the base 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 chemical conversion treatability in processed areas, even when the steel sheet simultaneously contains the three elements Ni, Cu, and Sn.
• the inventors discovered that it is effective to appropriately coat the surface of a base steel sheet containing Ni, Cu, and Sn with an alloy plating of at least one of these elements and Fe, and to limit the size of the uncoated area that is not coated with at least one of Ni, Cu, and Sn to within a specified range.
• the inventors have found that by applying plating so that, when a cross section of a plated steel sheet is measured with an EPMA (electron probe microanalyzer), the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more, the surface coverage rate of the coated region is 25% or more, and further the average length of the uncoated region not coated with at least one of Ni, Cu, and Sn (i.e., the uncoated region not coated with any of Ni, Cu, or Sn) is 10 ⁇ m or less, the adhesion between the plating and the base steel sheet is improved and peeling of the plating can be sufficiently suppressed even during processing, particularly processing that involves sliding such as press working, and therefore the chemical conversion treatability of the processed region can be significantly improved.
• EPMA electron probe microanalyzer
• Fig. 1 is a cross-sectional schematic diagram of a plated steel sheet according to an embodiment of the present invention, illustrating the surface coverage of the plating.
• the plated steel sheet 1 comprises a base steel sheet 2 and a plating 3 (covered region) disposed on the surface of the base steel sheet 2.
• the plating 3 contains at least one of Ni, Cu, and Sn and Fe.
• the cross section of the plated steel sheet 1 is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn in the plating 3 satisfies the above formula 1, and the surface concentration of Fe is 10 mass% or more.
• these elements can function as effective cathode sites in relation to Fe during chemical conversion treatment, as described above, promoting the etching of the surrounding Fe, thereby significantly improving the chemical treatability of the steel sheet.
• the chemical conversion coating can be formed uniformly over the entire steel sheet, significantly improving corrosion resistance.
• the chemical treatability of not only the processed area but the entire plated steel sheet will be reduced. Therefore, in the plated steel sheet according to the embodiment of the present invention, it is important that the surface of the base steel sheet is uniformly coated with an alloy plating of at least one of Ni, Cu, and Sn and Fe. That is, when a cross section of the plated steel sheet is measured by EPMA, it is important that the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more, and that the average length of the uncoated region not covered with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• the plated steel sheet according to the embodiment of the present invention encompasses not only electric furnace materials that inevitably contain Ni, Cu, and Sn as tramp elements, but also blast furnace materials that 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 chemical conversion treatability in processed areas, and in turn, superior corrosion resistance in processed areas, compared to conventional plated steel sheets that 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 chemical conversion treatability and/or corrosion resistance in processed areas 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 other elements, such as Zn and Al, in addition to Ni, Cu, Sn, and Fe, as long as the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more.
• the plating may essentially consist of at least one of Ni, Cu, and Sn and Fe, or may consist of at least one of Ni, Cu, and Sn and Fe, or at least one of Ni, Cu, and Sn and Fe.
• the plating weight is not particularly limited and may be appropriately selected within a range that satisfies the requirements for surface coverage and uncoated areas, which will be described in detail later. Furthermore, as long as the plated steel sheet according to the embodiment of the present invention satisfies the requirements that the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage rate of the coated region where the surface concentration of Fe is 10 mass% or more is 25% or more, it may include plating that does not satisfy these requirements, such as plating or alloy plating in which the surface concentration of at least one of Ni, Cu, and Sn does not satisfy the above formula 1, or plating in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is less than 10 mass%.
• the surface coverage of a coating region in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the following formula 1 and the surface concentration of Fe is 10 mass% or more is controlled to be 25% or more.
• [Ni], [Cu], and [Sn] are the surface concentrations [mass %] of each element.
• Ni, Cu, and Sn in such alloy plating can function as cathode sites during chemical conversion treatment.
• the surface of the base steel sheet can be uniformly coated with the alloy plating, allowing Ni, Cu, and Sn to function effectively as cathode sites.
• the above-mentioned cathode reaction can be properly promoted over the entire surface of the steel sheet, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet and improving adhesion between the alloy plating and the base steel sheet compared to when simply plating at least one of Ni, Cu, and Sn.
• the surface coverage is preferably 30% or greater or 35% or greater, more preferably 40% or greater or 45% or greater, and most preferably 50% or greater or 55% or greater.
• the surface coverage may be, for example, 80% or less, 75% or less, 70% or less, or 65% or less.
• the average length of uncoated regions not covered with at least one of Ni, Cu, and Sn is controlled to 10 ⁇ m or less. If the uncoated regions not covered with at least one of Ni, Cu, and Sn become relatively large, naturally, no cathode sites exist in such uncoated regions, making it impossible to promote the anodic dissolution of Fe during chemical conversion treatment. As a result, the chemical conversion coating cannot be formed uniformly over the entire steel sheet, and the chemical conversion treatability of not only the processed portions but the entire plated steel sheet is reduced.
• the surface of the base steel sheet can be uniformly coated with an alloy plating of at least one of Ni, Cu, and Sn and Fe, allowing the Ni, Cu, and Sn to function effectively as cathode sites.
• the above-mentioned cathode reaction can be properly carried out over the entire surface of the steel sheet, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, thereby significantly improving corrosion resistance. From the perspective of further improving chemical conversion treatability and, in turn, corrosion resistance, the smaller the average length of the uncoated regions, the better.
• the average length 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 average length 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 using an EPMA as follows. First, five samples are taken from the plated steel sheet so that a cross section including the surface of the plated steel sheet can be observed. Next, for each sample, a rectangular area measuring 80 ⁇ m in the thickness direction and 100 ⁇ m in the direction perpendicular to the thickness direction is defined as one field of view. Images of a total of five fields of view for the five samples are taken using an EPMA (e.g., a JXA-8500 manufactured by JEOL Ltd.) at an acceleration voltage of 15 kV and a probe current of 5 ⁇ 10 ⁇ 7 A at a magnification of 1000x to obtain mapping images.
• EPMA e.g., a JXA-8500 manufactured by JEOL Ltd.
• the average of the total L i obtained for the five samples is calculated to be ⁇ L i , and ⁇ L i /L 0 ⁇ 100 is calculated from the surface length L 0 of the corresponding base steel plate (length of the long side in the field of view: 100 ⁇ m), and the calculated value is determined to be the surface coverage rate of the coating area where the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more.
• the number of uncoated regions not coated with at least one of Ni, Cu, and Sn and the length of each uncoated region are calculated in the binarized image obtained by the image analysis software "ImageJ" in connection with the measurement of the surface coverage.
• the average length per uncoated region is calculated by dividing the total length of the uncoated regions by the total number of uncoated regions (( E1 + E2 )/2 in Figure 1).
• the average value of the average lengths per uncoated region obtained for the five samples is calculated, and the calculated average value is determined as the average length of the uncoated regions not coated with at least one of Ni, Cu, and Sn.
• the base steel sheet has a chemical composition containing, in 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 chemical conversion treatability in processed portions, and the object is achieved by applying plating so that, when a cross section of the plated steel sheet is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface coverage rate of coated regions where the surface concentration of Fe is 10 mass % or more is 25% or more, and further the average length of uncoated regions that are not covered with at least one of Ni, Cu, and Sn is 10 ⁇ m or less.
• 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.
• the base steel plate contains, in mass%, C: 0.001 to 0.500%, Si: 0-3.00%, Mn: 0.10-3.00%, Al: 0.001-2.000%, Ni: 0.010 to 1.000%, Cu: 0.010-1.000%, Sn: 0.003-1.000%, P: 0.100% or less, S: 0.100% or less, N: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, B: 0 to 0.0100%, Mo: 0-1.000%, Cr: 0-1.000%, V: 0 to 0.150%, W: 0-1.000%, Hf: 0 to 0.050%, Mg: 0 to 0.050%, Zr: 0 to 0.050%, Ca: 0-0.010%, REM: 0-0.010%, As: 0 to 0.010%, It is preferable that the chemical composition be Ir: 0 to 1.000%, and the balance: Fe and impurities.
• the chemical composition be Ir: 0
• 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.
• N is an element that forms coarse nitrides in steel sheets and reduces the workability of the steel sheets. Since a lower N content is preferable, ideally it is 0%. However, excessive reduction in the N content may result in a significant increase in manufacturing costs. Therefore, the N content may be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content may form coarse nitrides as described above, reducing the workability of the steel sheets. Therefore, the N content is preferably 0.0100% or less. The N content may be 0.0080% or less, 0.0060% or less, or 0.0050% or less.
• the preferred basic chemical composition of the base steel plate is as described above. Furthermore, the base steel plate may contain at least one of the following elements in place of a portion of the remaining Fe, as necessary.
• Ti, Nb, and V form carbonitrides in steel and have the effect of improving the strength of the steel sheet through precipitation strengthening.
• the Ti, Nb, and V contents may be 0%, but to obtain such effects, the Ti, Nb, and V contents are preferably 0.001% or more, and may be 0.002% or more, 0.005% or more, or 0.010% or more.
• the Ti, Nb, and V contents are preferably 0.150% or less, and may be 0.120% or less, 0.100% or less, 0.080% or less, 0.050% or less, 0.020% or less, or 0.015% or less.
• [B: 0 to 0.0100%] B segregates at grain boundaries to increase grain boundary strength, thereby improving low-temperature toughness.
• the B content may be 0%, but to obtain this effect, the B content is preferably 0.0001% or more.
• the B content may be 0.0002% or more, 0.0005% or more, or 0.0010% or more.
• the B content is preferably 0.0100% or less.
• the B content may be 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0015% or less.
• Mo, Cr, and W are elements that improve the hardenability of steel and contribute to improving its strength.
• the Mo, Cr, and W contents may be 0%, but to achieve these effects, the Mo, Cr, and W contents are preferably 0.001% or more, and may be 0.010% or more, 0.020% or more, or 0.030% or more.
• the Mo, Cr, and W contents are preferably 1.000% or less, and may be 0.500% or less, 0.100% or less, 0.050% or less, or 0.040% 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 Hf, Mg, and Zr contents are preferably 0.050% or less, and may be 0.010% or less, 0.005% or less, or 0.003% or less.
• the Ca and REM contents are preferably 0.010% or less, and may be 0.005% or less, or 0.003% or less.
• the As content may be 0%, but to obtain this effect, the As content is preferably 0.001% or more.
• the As content may be 0.002% or more or 0.003% or more.
• the As content is preferably 0.010% or less.
• the As content may be 0.008% or less or 0.005% or less.
• Ir is an element that segregates at prior austenite grain boundaries to increase the strength of the grain boundaries.
• the Ir content may be 0%, but to obtain this effect, the Ir content is preferably 0.001% or more.
• the Ir content may be 0.003% or more, 0.005% or more, or 0.010% or more.
• the Ir content is preferably 1.000% or less.
• the Ir content may be 0.500% or less, 0.100% or less, 0.030% or less, or 0.015% or less.
• the remainder In the base steel plate, the remainder, other than the above elements, consists of Fe and impurities. Impurities in the base steel plate are components that are mixed in during the industrial production of the base steel plate due to various factors in the manufacturing process, including raw materials such as ore and scrap.
• the chemical composition of the base steel plate can be measured using standard analytical methods.
• the chemical composition of the base steel plate can be determined by first removing the plating layer by mechanical grinding, and then measuring the chips using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) in accordance with JIS G 1201:2014.
• the composition can be determined by obtaining a 35 mm square test piece from approximately half the plate thickness of the base steel plate and measuring it using a Shimadzu ICPS-8100 or similar measuring device under conditions based on a pre-created calibration curve.
• C and S which cannot be measured using ICP-AES, can be measured using the combustion-infrared absorption method, and N can be measured using the inert gas fusion-thermal conductivity method.
• the thickness of the base steel plate is not particularly limited, but is generally 0.2 to 8.0 mm.
• the thickness may be 0.3 mm or more, 0.6 mm or more, 1.0 mm or more, 1.6 mm or more, or 2.0 mm or more.
• the thickness of the base steel plate may be, for example, 7.0 mm or less, 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.
• the steel sheet according to the embodiment of the present invention can achieve superior chemical conversion treatability in processed areas, and therefore superior corrosion resistance in processed areas, compared to conventional plated steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. Therefore, the plated steel sheet according to the embodiment of the present invention is useful for use in parts in technical fields that require superior chemical conversion treatability and/or corrosion resistance in processed areas, and is particularly useful for use in parts in the automotive field.
• an automobile part is provided that includes the plated steel sheet according to the embodiment of the present invention. Examples of automobile parts include frame parts, bumpers, and other structural and reinforcing parts that require strength, as well as exterior panel parts such as roofs, hoods, fenders, and doors that require high design quality.
• At least a portion of these parts it is sufficient for at least a portion of these parts to include the plated steel sheet according to the embodiment of the present invention, and therefore at least a portion of these parts will satisfy the characteristics of the plated steel sheet described above. In areas of the steel sheet that do not come into direct contact with a mold during forming, such as press forming, or that come into direct contact with a mold but are relatively lightly processed, the characteristics of the plated steel sheet do not change significantly before and after forming.
• the plated steel sheet according to the embodiment of the present invention may have a Vickers hardness of, for example, 90 Hv or more, but is not particularly limited thereto.
• the Vickers hardness may be 150 Hv or more, 200 Hv or more, 250 Hv or more, 300 Hv or more, 350 Hv or more, 400 Hv or more, or 450 Hv or more.
• the upper limit is not particularly limited, but the Vickers hardness may be, for example, 650 HV or less, 600 HV or less, 550 HV or less, or 500 HV or less.
• Vickers hardness is determined as follows. First, a test piece is cut out from any position of the plated steel sheet, excluding the edge, so that a cross section perpendicular to the surface (thickness cross section) can be observed. The thickness cross section of the test piece is polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a diluted solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 ⁇ m is dispersed in pure water, and this thickness cross section is used as the measurement surface. Next, the Vickers hardness is measured using a micro Vickers hardness tester at a load of 1 kgf at intervals of at least three times the indentation. Specifically, a total of 20 points are measured randomly near the half-thickness position of the plated steel sheet, and the arithmetic average of these measurements is determined as the Vickers hardness of the plated steel sheet.
• Plated steel sheets according to embodiments of the present invention can be manufactured by, for example, performing a casting process in which molten steel with an adjusted chemical composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot-rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled and then subjected to a primary pickling; a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a secondary pickling process in which the cold-rolled steel sheet is subjected to a secondary pickling; a plating process in which the secondary pickled cold-rolled steel sheet is plated; and an annealing process in which the resulting plated steel sheet is annealed.
• plated steel sheets according to embodiments of the present invention encompass not only plated steel sheets obtained by plating cold-rolled steel sheets, but also plated steel sheets obtained by plating hot-rolled steel sheets. Therefore, when manufacturing plated steel sheets obtained by plating hot-rolled steel sheets, for example, the secondary pickling process may be performed after the coiling process without performing the cold-rolling process described below. Each process is described in detail below.
• the conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.
• a hot-rolled steel plate can be obtained by hot-rolling the cast steel slab.
• the hot-rolling process is carried out by reheating the cast steel slab directly or after cooling it once, followed by hot-rolling.
• the heating temperature of the steel slab may be, for example, 1100 to 1250°C.
• rough rolling and finish rolling are usually carried out.
• the temperature and reduction ratio of each rolling step can be appropriately determined depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.
• 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.
• an Mn—Si-depleted layer is formed directly below the inner oxide layer formed on the surface layer of the steel sheet due to the consumption of Mn and/or Si in the steel caused by the formation of the inner oxide layer.
• the thickness of the Mn—Si-depleted layer can be controlled to 0.3 ⁇ m or higher. Since the outer oxide layer and inner oxide layer are removed by primary pickling after coiling, an Mn—Si-depleted layer having a thickness of 0.3 ⁇ m or higher remains on the surface of the hot-rolled steel sheet after primary pickling.
• the surface of the hot-rolled steel sheet By forming the surface of the hot-rolled steel sheet with an Mn-Si depleted layer having a thickness of 0.3 ⁇ m or more, it is possible to sufficiently suppress the formation of Mn- and/or Si-based surface oxides on the steel sheet surface during the subsequent annealing process due to the Mn and Si depletion on the steel sheet surface, thereby making it possible to appropriately maintain the plating applied in the plating process before the annealing process and achieve a desired surface coverage in the finally obtained plated steel sheet.
• the thickness of the Mn-Si depletion layer is determined as follows. First, using a high-frequency glow discharge optical emission spectrometer (GDS), the surface of the steel sheet after primary pickling is placed in an Ar atmosphere, and a voltage is applied to generate glow plasma. The surface of the steel sheet is then sputtered and analyzed in the depth direction. The elements contained in the material are then identified from the element-specific emission spectrum wavelengths emitted by excited atoms in the glow plasma, and the emission intensity of the identified elements is estimated. Depth direction data can be estimated from the sputtering time. Specifically, by determining the relationship between sputtering time and sputtering depth in advance using a standard sample, sputtering time can be converted to sputtering depth.
• GDS glow discharge optical emission spectrometer
• the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material.
• the obtained emission intensity is converted to mass % by creating a calibration curve.
• the primary pickling is not particularly limited, and may be carried out using a commonly used pickling solution under conditions suitable for removing the outer and inner oxide layers.
• the primary pickling may be carried out once, or may be carried out multiple times to ensure that the outer and inner oxide layers are completely removed.
• the coiling temperature is less than 520°C in the coiling process, the formation of an internal oxide layer is insufficient, making it impossible to form an Mn-Si depleted layer with a thickness of 0.3 ⁇ m or more.
• the formation of Mn- and/or Si-based surface oxides cannot be sufficiently suppressed in the annealing process following the plating process, and the desired surface coverage cannot be achieved in the final plated steel sheet.
• 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 in 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 step involves immersing the cold-rolled steel sheet in an aqueous solution having a hydrochloric acid concentration of 3 to 12% and not containing 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. Even if the outer and inner oxide layers are not sufficiently removed in the previous primary pickling, the secondary pickling using an aqueous hydrochloric acid solution can reliably and completely remove these oxide layers.
• the formation of Mn- and/or Si-based surface oxides in the subsequent annealing step may not be sufficiently suppressed due to this change in the surface condition and the presence of Ni, Cu, and Sn in the steel sheet. Therefore, in the present manufacturing method, secondary pickling is performed before the annealing step to condition the surface of the steel sheet, thereby making it possible to sufficiently and reliably suppress the formation of Mn- and/or Si-based surface oxides in the subsequent annealing step. Furthermore, iron oxides may also form on the surface of the steel sheet during cold rolling, which may result in poor plating performance in the subsequent plating step.
• the hydrochloric acid concentration of the aqueous hydrochloric acid solution is 4 to 8%
• the immersion temperature is 70 to 90°C
• the immersion time is 4 to 50 seconds.
• rinsing after the secondary pickling is also extremely important.
• the electrical conductivity of the rinsing solution used in rinsing 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 rinsing after the secondary pickling.
• the presence of such iron oxides on the surface of the cold-rolled steel sheet inhibits the adhesion of the plating in the subsequent plating process. In this case, the desired surface coverage and average length of the uncoated region cannot be achieved in the final plated steel sheet.
• rinsing after the secondary pickling is performed with a rinsing solution having an electrical conductivity of 40 mS/m or less, which significantly suppresses the formation of iron oxides during rinsing after the secondary pickling and enables the plating to adhere properly in the subsequent plating process.
• plating is applied to at least one, preferably both, surfaces of the cold-rolled steel sheet (base steel sheet).
• the plating step can be carried out by any appropriate plating process effective for achieving the desired surface coverage and average length of the uncoated region, such as electroplating, vapor deposition plating, thermal spraying, or cold spraying.
• the plating step is carried out by electroplating.
• Electroplating can be carried out using a bath containing at least one of Ni, Cu, and Sn at a predetermined concentration, under conditions of a current density of 0.1 to 5.0 A/ dm2 and a current application time of 0.1 to 10.0 seconds.
• the current density is 0.3 to 2.0 A/ dm2 and the current application time is 0.5 to 5.0 seconds.
• the annealing step involves heating the cold-rolled steel sheet to a temperature of 700 to 950°C in an atmosphere with a dew point of -40 to 20°C and holding the temperature for 0 to 300 seconds.
• the atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).
• the annealing step allows at least one of Ni, Cu, and Sn in the plating applied in the plating step to be alloyed with Fe in the base steel sheet, thereby forming a coating region in which the surface concentration of at least one of Ni, Cu, and Sn satisfies the above formula 1 and the surface concentration of Fe is 10 mass% or more.
• this manufacturing method in particular, enables sufficient or complete removal of Mn- and/or Si-based surface oxides and iron oxides from the surface of the base steel sheet by combining an Mn-Si depleted layer formed to a predetermined thickness, i.e., 0.3 ⁇ m or more, due to appropriate control of the coiling temperature in the coiling process with specific secondary pickling and water rinsing in the secondary pickling process.
• plated steel sheets having a coating in which, when the cross section is measured by EPMA, the surface concentration of at least one of Ni, Cu, and Sn satisfies Equation 1 above, and the coated region has an Fe surface concentration of 10 mass% or more, with a surface coverage rate of 25% or more, and the average length of the uncoated region not covered by at least one of Ni, Cu, and Sn is limited to 10 ⁇ m or less.
• the potential of the base steel sheet becomes more noble than when these elements are not present in a solid solution state.
• plated steel sheets produced using this manufacturing method can achieve superior corrosion resistance compared to conventional plated steel sheets that simultaneously contain the three elements Ni, Cu, and Sn. This can contribute to industrial development by extending the service life of plated steel sheets used in automobiles and building materials.
• Plated steel sheets according to embodiments of the present invention can be used as the various automotive parts described above, for example, after a chemical conversion coating or paint film is optionally formed on the surface.
• Whether an automotive part having a paint film or chemical conversion coating includes a plated steel sheet according to embodiments of the present invention can be determined by removing the paint film or chemical conversion coating from a sample taken from the automotive part. In this case, the location from which the sample is taken, the paint film removal process, and the chemical conversion coating removal process are as follows.
• paint film removal process The paint film is removed from a sample cut from an automobile body under the following conditions to expose the steel sheet.
• a paint remover (Neo River #160, manufactured by Sansai Kako Co., Ltd.) is applied to the surface at room temperature and allowed to stand for approximately 5 minutes.
• the paint film is then removed by rubbing with a hard sponge or similar (e.g., Kanefiel, manufactured by Aion Co., Ltd.).
• the sample is then rinsed with water and dried.
• the remaining paint film is then confirmed by SEM-EPMA measurement of the sample surface (100 ⁇ m square, 5 fields of view) after rinsing and drying.
• Image processing is performed using image analysis software "ImageJ”. After loading the C element distribution image into ImageJ, it is binarized using "Make Binary” in “Binary” under “Process” so that areas with a C concentration of 10% by mass or more are displayed as black and areas with a C concentration of less than 10% by mass are displayed as white. After binarization, “Measure” under “Analyze” is used to read the value of "Area fraction” in “Results”, and this value is determined as the area ratio of areas with a C concentration of 10% by mass or more. If peeling of the coating film is insufficient, removal of the coating film is repeated until the area ratio of areas with a C concentration of 10% by mass or more becomes less than 5%.
• regions with a P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed.
• To measure the area ratio of regions with a P concentration of 5% by mass or more first obtain an element distribution image of P using EPMA with a P concentration range of 5 to 10%. The obtained element distribution image is then image-processed to measure the area ratio. Image analysis software "ImageJ" was used for image processing.
• the P element distribution image was then loaded into ImageJ, and binarized using "Make Binary” in “Binary” under “Process” so that areas with a P concentration of 5% by mass or more were displayed as black, and areas with a P concentration of less than 5% by mass were displayed as white.
• "Measure” under “Analyze” was used to read the value for "Area fraction” in “Results,” and this value was determined as the area fraction of areas with a P concentration of 5% by mass or more. If peeling of the chemical conversion coating was insufficient, removal of the chemical conversion coating was repeated until the area fraction of areas with a P concentration of 5% by mass or more became less than 5%.
• plated steel sheets according to embodiments of the present invention were manufactured under various conditions, and the properties of the manufactured plated steel sheets were investigated.
• molten steel was cast using a continuous casting method to form a steel billet having the chemical composition shown in Table 1.
• the steel billet was reheated to 1200°C and hot rolled, and then coiled at the coiling temperature shown in Table 2.
• Hot rolling was carried out 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 resulting 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 secondary pickling.
• the secondary pickling was performed by immersing the cold-rolled steel sheet in an inhibitor-free aqueous solution having a 5% hydrochloric acid concentration 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 cold-rolled steel sheet (base steel sheet) that had been subjected to secondary pickling 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 having a plating applied to both sides of the base steel sheet.
• a metal species one of Ni, Cu, and Sn
• the obtained plated steel sheet was subjected to an annealing step in which the plated steel sheet was heated to a temperature of 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 (nitrogen balance), and maintained at that temperature for 100 seconds, thereby obtaining a plated steel sheet in which at least one of Ni, Cu, and Sn was alloyed with Fe.
• the properties of the resulting plated steel sheets were measured and evaluated using the following methods.
• the sample that had been subjected to the drawbead test was subjected to 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.
• the bead portions of the chemically treated samples were observed using SEM-EPMA, and the area ratio of the portion where the chemical conversion coating was not formed, commonly known as the "clear" area, was calculated by binarization using the image analysis software "ImageJ.”
• the chemical conversion treatability of the processed portion was evaluated according to the area ratio of the clear-cut area using the following evaluation criteria.
• the clear-cut area was defined as an area with an Fe concentration of 70% or more in a mapping image obtained by photographing an area of 80 ⁇ m x 60 ⁇ m or more at 1000x magnification using an EPMA (JXA-8500 manufactured by JEOL Ltd.) under conditions of an acceleration voltage of 15 kV and an irradiation current of 5 x 10-7 A.
• the area ratio of the clear-cut area was determined as the average value of five randomly selected fields of view.
• AAA Transparent area ratio less than 20%
• AA Transparent area ratio 20-25%
• B Skewer area ratio over 35%
• Plated steel sheets with a chemical conversion treatability rating of AAA, AA, or A in the processed area were evaluated as containing Ni, Cu, and Sn and capable of exhibiting improved chemical conversion treatability in the processed area.
• the results are shown in Table 2.
• the "surface coverage rate of the coated area" in Table 2 indicates the surface coverage rate of the coated area where the surface concentrations of the metal species shown in Table 2 satisfy the above formula 1 and the surface concentration of Fe is 10 mass% or more, when the cross section of the plated steel sheet is measured using EPMA.
• the plated steel sheets of all Examples when the cross section was measured by EPMA, the plated steel sheets were covered with coated regions in which the surface concentration of at least one of Ni, Cu, and Sn satisfied the above formula 1 and the surface concentration of Fe was 10 mass% or more at a surface coverage rate of 25% or more, and the average length of the uncoated regions not covered with at least one of Ni, Cu, and Sn was limited to 10 ⁇ m or less.
• the chemical treatability of the processed portions was evaluated as AA, further improving the chemical treatability of the processed portions.
• the chemical treatability of the processed portions was evaluated as AAA, further improving the chemical treatability of the processed portions.

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