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 and parts containing the same.
• 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 chemical conversion treatability required 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 layer peels off when the mold and the surface of the plated steel sheet slide against each other during press forming, the chemical conversion treatability 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 particularly on the plating layer. As a result, the inventors discovered that by coating the surface of a base steel sheet containing Ni, Cu, and Sn with a predetermined thickness of an alloy plating layer of Fe and at least one of Zn and Mn, the chemical conversion treatability of the processed area can be significantly improved, leading to the completion of the present invention.
• a plated steel sheet comprising a base steel sheet and a plating layer 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 plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more, an Fe concentration of 10 mass % or more, and a thickness of 5 ⁇ m or more.
• the plated steel sheet according to (1) above characterized in that the plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more, and has a thickness of 8 ⁇ m or more.
• 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 (3) above comprising: (5) The plated steel sheet according to any one of (1) to (4) above, characterized in that it has a Vickers hardness of 200 Hv or more. (6) A part comprising the plated steel sheet according to any one of (1) to (5) 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.
• a plated steel sheet is a plated steel sheet including a base steel sheet and a plating layer 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 plating layer has a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more, and is characterized by having a thickness of 5 ⁇ m or more.
• the plating layer may peel off when the surface of the plated steel sheet slides against the mold during press forming. If the plating layer peels off in the processed area due to 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 plating layer, 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. As a result, the inventors discovered that it is effective to coat the surface of a base steel sheet containing Ni, Cu, and Sn with an alloy plating layer of Fe and at least one of Zn and Mn to a predetermined thickness.
• the inventors discovered that, when the cross section of the plated steel sheet is measured using an electron probe microanalyzer (EPMA), by controlling the thickness of the plating layer to 5 ⁇ m or more, where the concentration of at least one of Zn and Mn is 10 mass% or more in total and the Fe concentration is 10 mass% or more, the adhesion between the plating layer and the base steel sheet is improved, and peeling of the plating layer can be sufficiently suppressed even during processing, particularly processing that involves sliding such as press working, thereby significantly improving the chemical conversion treatability of processed areas.
• EPMA electron probe microanalyzer
• Zn and Mn function as anodes during chemical conversion treatment and improve the chemical treatability of steel sheets by dissolving themselves. Therefore, even if the base steel sheet contains Ni, Cu, and Sn, if the manufacturing method can be appropriately controlled to properly coat the surface of the base steel sheet with a plating layer containing at least one of Zn and Mn, it is possible to significantly improve the chemical treatability of the plated steel sheet.
• the inventors first discovered that by appropriately controlling the manufacturing method, as will be described in detail later, it is possible to suppress the formation of oxides, particularly Mn- and/or Si-based surface oxides, on the surface of a base steel sheet containing Ni, Cu, and Sn, thereby coating the surface of the base steel sheet with a plating layer containing at least one of Zn and Mn.
• the inventors discovered that by alloying at least one of Zn and Mn in the coating layer with Fe in the base steel sheet through a subsequent annealing step, and controlling the thickness of the coating layer to 5 ⁇ m or more, where the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is 10 mass% or more when the cross section of the coating steel sheet is measured by EPMA, the chemical conversion treatability of the coating steel sheet can be significantly improved.
• the anodic dissolution (etching) of Zn and/or Mn in the coating layer can be promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved corrosion resistance.
• an alloy plating layer of at least one of Zn and Mn with Fe it is possible to improve the adhesion between the alloy plating layer and the base steel sheet compared to when simply plating at least one of Zn and Mn. Therefore, peeling of the plating layer can be sufficiently suppressed even during processes that involve sliding, such as press working, and this makes it possible to significantly improve the chemical treatability of the processed area.
• 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 layer 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 layer may contain at least one of Zn and Mn and Fe, and may have a variety of plating compositions as long as it satisfies the requirements for the concentrations of Zn, Mn, and Fe and the thickness of the plating layer, which will be described in detail later. Therefore, the plating layer may contain other elements in addition to Zn, Mn, and Fe, such as Al, Ni, Cu, Sn, and Mg. More specifically, the plating layer may contain or consist of Zn—Cu—Fe and/or Zn—Ni—Fe, etc.
• the plating layer may consist essentially of at least one of Zn and Mn and Fe, or may consist of at least one of Zn and Mn and Fe, or may consist of at least one of Zn and Mn and Fe.
• the plating weight is not particularly limited, and may be appropriately selected within a range that satisfies the requirements for the concentrations of Zn, Mn, and Fe and the thickness of the plating layer, which will be described in detail later.
• the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more is controlled to 5 ⁇ m or more.
• Zn and Mn function as an anode during chemical conversion treatment and improve the chemical treatability of the steel sheet by dissolving themselves.
• the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more to 5 ⁇ m or more it becomes possible for the Zn and Mn in the plating layer to effectively function as an anode during chemical conversion treatment.
• the anodic dissolution (etching) of Zn and/or Mn in the plating layer is promoted during chemical conversion treatment, allowing a chemical conversion coating to be formed uniformly over the entire steel sheet, resulting in significantly improved corrosion resistance.
• the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more is preferably as thick as possible, for example, 6 ⁇ m or more or 8 ⁇ m or more, more preferably 10 ⁇ m or more, and most preferably 12 ⁇ m or more.
• the upper limit is not particularly limited, and the thickness of the plating layer may be, for example, 30 ⁇ m or less, 25 ⁇ m or less, 20 ⁇ m or less, or 15 ⁇ m or less.
• the plating layer may include regions that do not satisfy this requirement, such as regions where the total concentration of at least one of Zn and Mn is less than 10 mass% or regions where the total concentration of at least one of Zn and Mn is 10 mass% or more and the Fe concentration is less than 10 mass%.
• the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10% by mass or more and an Fe concentration of 10% by mass or more is measured using an EPMA as follows: First, point analysis is performed on a cross section of a plated steel sheet from the surface in the depth direction to a depth of 12 ⁇ m, with point analysis performed every 1 ⁇ m. Similar measurements are performed at five locations in the surface direction of the plated steel sheet, and the average total concentration of at least one of Zn and Mn and the average Fe concentration at each depth are calculated using the five measurements at each depth.
• 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 this object is achieved by controlling the thickness of a plating layer having a total concentration of at least one of Zn and Mn of 10 mass % or more and an Fe concentration of 10 mass % or more to 5 ⁇ m or more when a cross section of the plated steel sheet is measured by EPMA.
• 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.
• 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.
• 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%.
• 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 depletion of Mn and Si on the steel sheet surface. Therefore, it is possible to appropriately maintain the plating applied in the plating process before the annealing process, and it is possible to form a plating layer of a desired thickness in the finally obtained plated steel sheet, in which the total concentration of at least one of Zn and Mn is 10 mass % or more and the Fe concentration is 10 mass % or more.
• 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 in the coiling process is less than 520°C, 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.
• it becomes impossible to sufficiently suppress the formation of Mn- and/or Si-based surface oxides in the annealing process after the plating process and it becomes impossible to form a plating layer of the desired thickness in the final plated steel sheet having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more.
• the coiling temperature is controlled to 550°C or higher.
• the coiling temperature may be 600°C or lower, for example.
• 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.
• 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.
• 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.
• the final plated steel sheet will not be able to form a plating layer of the desired thickness having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more.
• the water rinse after the secondary pickling is performed using a water rinse with an electrical conductivity of 40 mS/m or less, which significantly suppresses the formation of iron oxides during the water rinse after the secondary pickling and enables the plating to adhere properly in the subsequent plating process.
• the formation of these oxides can be significantly suppressed by combining a 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.
• a predetermined thickness i.e., 0.3 ⁇ m or more
• the electrical conductivity of the washing solution the better; specifically, it is preferably 25 mS/m or less, and more preferably 15 mS/m or less.
• 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, such as electroplating, vapor deposition plating, thermal spraying, or cold spraying, that is effective in achieving the desired thickness of a plating layer in which the total concentration of at least one of Zn and Mn is 10% by mass or more and the Fe concentration is 10% by mass or more.
• the plating step is carried out by electroplating.
• Electroplating can be carried out using a bath containing at least one of Zn and Mn and optional additive elements at predetermined concentrations, under conditions of a current density of 5 to 20 A/dm2 and a current application time of 2.0 to 20.0 seconds.
• a current density of 5 to 20 A/dm2 and a current application time of 2.0 to 20.0 seconds.
• the current density is 8 to 15 A/ dm2 and the current application time is 3.0 to 10.0 seconds.
• the plating step by dividing the plating step into two steps, first performing the first plating treatment before the secondary pickling step and then performing the second plating treatment after the secondary pickling step, it is possible to form a plating layer of the desired thickness having a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more.
• 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 containing 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).
• 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.
• 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 metal species such as Zn and Mn at predetermined concentrations under conditions of a current density of 10 A/ dm2 and a current application time of 5.0 seconds, thereby obtaining a plated steel sheet having a plating layer attached to both sides of the base steel sheet.
• 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 an atmosphere with a dew point of 0°C and 4% hydrogen (nitrogen balance) in a furnace with an oxygen concentration of 20 ppm or less, and maintained at that temperature for 100 seconds, thereby obtaining a plated steel sheet in which at least one of Zn and Mn was alloyed with Fe and, optionally, Cu.
• the "thickness of the plating layer" in Table 2 indicates the thickness of the plating layer in which the concentration of at least one of Zn and Mn is 10 mass % or more in total and the Fe concentration is 10 mass % or more, when the cross section of the plated steel sheet is measured by EPMA.
• the plating layer may contain Cu in addition to Zn, Mn, and Fe.
• the properties of the resulting plated steel sheets were measured and evaluated using the following methods.
• the low coiling temperature resulted in insufficient formation of an internal oxide layer, making it impossible to form an Mn-Si depleted layer with a thickness of 0.3 ⁇ m or more.
• the thickness of the plating layer which had a total concentration of at least one of Zn and Mn of 10 mass% or more and an Fe concentration of 10 mass% or more, was less than 5 ⁇ m, and the chemical conversion treatability of the processed part was reduced.
• the high electrical conductivity of the water used for rinsing after the secondary pickling meant that the formation of Mn- and/or Si-based surface oxides during the annealing process could not be sufficiently suppressed, and furthermore, the formation of iron oxides during rinsing after the secondary pickling could not be sufficiently suppressed.
• the thickness of the plating layer was less than 5 ⁇ m, and the chemical conversion treatability of the processed part was reduced.
• the high electrical conductivity of the water used for rinsing after the secondary pickling presumably prevented the formation of iron oxides during rinsing after the secondary pickling.
• the thickness of the plating layer was less than 5 ⁇ m, and the chemical conversion treatability of the processed area was reduced.
• the plating was performed so that the thickness of the plating layer, in which the total concentration of at least one of Zn and Mn was 10 mass% or more and the Fe concentration was 10 mass% or more, was limited to 5 ⁇ m or more.
• 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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