Fe-based electroplated steel sheets and hot-dip galvanized steel sheets and their manufacturing methods

Abstract

An Fe-based electroplating layer is formed by electroplating a substrate steel plate containing 0.50–3.00% by mass of Si using an Fe-based electroplating solution containing Fe ions and transition metal ions other than Fe ions. Following this, an annealing treatment is performed. In the aforementioned Fe-based electroplating solution, the content of Fe ions is 1.0% by mass or more, and the content of transition metal ions is 10–1000 ppm by mass. The adhesion amount of the Fe-based electroplating layer on each single side of the aforementioned substrate steel plate is 1.0 g / m². 2 The above results in a high-strength steel plate with excellent resistance to resistance welding cracks in the welded parts.

Description

Technical Field

[0001] This invention relates to Fe-based electroplated steel sheets and hot-dip galvanized steel sheets, as well as methods for manufacturing them. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, there has been a strong demand to improve the fuel efficiency of automobiles.

[0003] Furthermore, from the perspective of ensuring the safety of occupants in the event of a collision, there is a strong demand to improve the safety of automobiles.

[0004] To meet these requirements, it is necessary to balance the lightweighting and high strength of the car body. For steel sheets, which are the raw materials for car parts, the trend is to actively promote thinner walls based on high strength.

[0005] In addition, most automotive parts are manufactured by forming steel sheets (e.g., pressure forming), so the steel sheets used as raw materials for automotive parts also require excellent formability.

[0006] One method to achieve high strength without significantly impairing the formability of steel sheets is solid solution strengthening using the addition of Si.

[0007] Moreover, in the manufacturing of automotive parts, most pressure-formed components are assembled by resistance welding (spot welding) (see Patent Document 1).

[0008] Existing technical documents

[0009] Patent Literature

[0010] Patent Document 1: International Publication No. 2019 / 116531 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] When the components to be resistance welded contain hot-dip galvanized steel sheets, the following points should be noted.

[0013] That is, during resistance welding, residual stress is generated near the weld. Under this condition, the zinc in the coating melts and diffuses into the grain boundaries, resulting in liquid metal embrittlement (LME), which will generate intergranular cracks (LME cracks) in the steel plate.

[0014] In particular, if welding is performed with the welding electrode at an angle relative to the steel plate, the residual stress increases, which may easily lead to cracks.

[0015] It is believed that residual stress increases with the increase of steel plate strength, therefore, there is concern that a large number of LME cracks will be generated as the steel plate becomes stronger.

[0016] LME cracks are broadly classified into "cracks generated on the surface in contact with the electrode" (hereinafter referred to as "surface cracks") and "cracks generated between steel plates near the ductile metal ring region" (hereinafter referred to as "internal cracks").

[0017] Surface cracks are prone to occur during resistance welding in high-current regions, such as those producing sputtering, and can therefore be suppressed by setting an appropriate current range to prevent sputtering. On the other hand, internal cracks can occur even when the resistance welding current is set to a range that prevents sputtering. Furthermore, surface cracks are easily detected during visual inspection, while internal cracks are difficult to detect visually.

[0018] For these reasons, LME cracks, especially internal cracks, have become an important topic.

[0019] Even if a steel plate is not a hot-dip galvanized steel plate, such LME cracks may still occur if the object to which it is resistance welded is a hot-dip galvanized steel plate.

[0020] Therefore, when the object of resistance welding is hot-dip galvanized steel sheet or when the weld itself is hot-dip galvanized steel sheet, excellent resistance to such cracks (hereinafter referred to as "resistance welding crack resistance characteristics of the welded part") is required.

[0021] The present invention was made in view of the above circumstances, and its purpose is to provide a steel plate with high strength and excellent resistance to resistance welding cracks in the welded parts.

[0022] Methods for solving problems

[0023] The inventors discovered that the above-mentioned objectives could be achieved by adopting the following configuration, thus completing the present invention.

[0024] That is, the present invention provides the following [1] to

[11] .

[0025] [1] A method for manufacturing an Fe-based electroplated steel sheet, wherein an Fe-based electroplating treatment is performed on a substrate steel sheet containing 0.50 to 3.00% by mass of Si using an Fe-based electroplating solution containing Fe ions and transition metal ions other than Fe ions to form an Fe-based electroplating layer, followed by annealing treatment, wherein the Fe-based electroplating solution contains 1.0% by mass or more of Fe ions, the transition metal ions contain 10 to 1000 ppm by mass, and the Fe-based electroplating layer has an adhesion amount of 1.0 g / m² on each single side of the substrate steel sheet. 2 above.

[0026] [2] According to the manufacturing method of Fe-based electroplated steel sheet described in [1] above, the base steel sheet has a composition containing, by mass %, Si: 0.50% or more and 3.00% or less, C: 0.80% or less, Mn: 1.50% or more and 3.50% or less, P: 0.100% or less, S: 0.0300% or less and Al: 0.100% or less, with the balance being Fe and unavoidable impurities.

[0027] [3] According to the manufacturing method of Fe-based electroplated steel sheet described in [2] above, the above-mentioned composition, in terms of mass %, further contains at least one element selected from the group consisting of B: less than 0.0050%, Ti: less than 0.200%, N: less than 0.0100%, Cr: less than 1.000%, Cu: less than 1.000%, Ni: less than 1.000%, Mo: less than 1.000%, Nb: less than 0.200%, V: less than 0.500%, Sb: less than 0.200%, Ta: less than 0.100%, W: less than 0.500%, Zr: less than 0.1000%, Sn: less than 0.200%, Ca: less than 0.0050%, Mg: less than 0.0050%, and REM: less than 0.0050%.

[0028] [4] The method for manufacturing Fe-based electroplated steel sheet according to any one of [1] to [3] above, wherein the transition metal ions contained in the Fe-based electroplating solution are at least one transition metal ion selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo and W.

[0029] [5] The method for manufacturing Fe-based electroplated steel sheet according to any one of [1] to [4] above, wherein the amount of the Fe-based electroplated layer on each single side of the substrate steel sheet is 5.0 to 20.0 g / m 2 .

[0030] [6] A method for manufacturing hot-dip galvanized steel sheet, wherein the Fe-based electroplated steel sheet obtained by any one of the methods described in [1] to [5] above is subjected to hot-dip galvanizing treatment.

[0031] [7] According to the manufacturing method of hot-dip galvanized steel sheet described in [6] above, after the hot-dip galvanizing treatment, alloying treatment is further performed.

[0032] [8] An Fe-based electroplated steel sheet comprising a base steel sheet containing 0.50 to 3.00% by mass of Si and an Fe-based electroplated layer disposed on at least one side of the base steel sheet, wherein the Fe-based electroplated layer on each side of the base steel sheet has an adhesion amount of 1.0 g / m 2The average crystal grain size of the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the base steel plate is less than 1.00 μm, the Fe-based electroplated layer contains transition metals other than Fe, and the content of the transition metals in the Fe-based electroplated layer is 100 to 10,000 ppm by mass.

[0033] [9] According to the Fe-based electroplated steel sheet described in [8] above, wherein the transition metal is at least one transition metal selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo and W.

[0034]

[10] The Fe-based electroplated steel sheet described in [8] or [9] above is resistively welded to the hot-dip galvanized steel sheet.

[0035]

[11] A hot-dip galvanized steel sheet having the Fe-based electroplated steel sheet described above [8] or [9] and a hot-dip galvanized layer disposed on the Fe-based electroplated layer of the Fe-based electroplated steel sheet.

[0036] Invention Effects

[0037] According to the present invention, a steel plate with high strength and excellent resistance to resistance welding cracks in the welded parts can be obtained. Attached Figure Description

[0038] Figure 1 This is a SIM image showing the base steel plate and Fe-based electroplating layer of No. 30.

[0039] Figure 2 This is a SIM image showing the base steel plate and Fe-based electroplating layer of No. 31.

[0040] Figure 3 This is a cross-sectional view showing the plate assembly used in resistance welding.

[0041] Figure 4 This is a top view showing the plate assembly after resistance welding.

[0042] Figure 5 yes Figure 4 AA-line cross-section diagram. Detailed Implementation

[0043] In this specification, the numerical range indicated by “~” refers to the range including the values ​​recorded before and after “~” as the lower and upper limits.

[0044] In this specification, "high strength" steel plate means that the tensile strength TS of the steel plate, as measured according to JIS Z 2241 (2011), is 590 MPa or higher.

[0045] [Insights gained by the inventor]

[0046] Previously, attempts were made to improve the resistance to welding cracks in welded sections by controlling the surface layer on the steel plate side alone.

[0047] However, the inventors believe that it is actually very difficult to achieve a high level of resistance to welding cracks in the welded part by controlling only the surface layer on the steel plate side. Therefore, they conceived of controlling the characteristics of the Fe-based electroplating layer.

[0048] The inventors conducted repeated and in-depth research. The results showed that, in order to meet the high-level resistance to welding cracking characteristics of the welded parts, it is important to use an Fe-based electroplating solution containing specific amounts of Fe ions and transition metal ions as a pre-plating agent, to form a certain amount of Fe-based electroplating layer on the surface of the steel plate (base steel plate) before continuous processing, and to micronize the grains of the Fe-based electroplating layer during subsequent annealing.

[0049] By applying a certain amount of Fe-based electroplating layer to the surface of the base steel plate, the resistance to resistance welding cracks in the welded part is improved.

[0050] The mechanism is not yet clear, but the following is a speculation.

[0051] First, the Fe-based electroplated layer functions as a soft layer, thereby reducing the stress imposed on the surface of the base steel plate during welding. This reduces residual stress in the weld, improving its resistance to resistance welding cracks (especially its ability to prevent internal cracks) (stress reduction effect).

[0052] However, it is believed that when the amount of dissolved Si on the surface of the base steel plate is high, the toughness of the welded part decreases and the resistance to welding cracks of the welded part deteriorates.

[0053] In this regard, if a certain amount of Fe-based electroplating layer is applied to the surface of the base steel plate, the Fe-based electroplating layer will act as a Si-deficient layer, reducing the Si dissolved in the weld.

[0054] Therefore, the reduction in toughness of the weld is suppressed, and the resistance to resistance welding cracks (especially the ability to prevent internal cracks) of the weld is improved (toughness reduction suppression effect).

[0055] If an annealing process is performed after the formation of an Fe-based electroplated layer, the grain size of the Fe-based electroplated layer may become coarser. In this case, the zinc molten during resistance welding can easily penetrate the grain boundaries of the base steel plate through the grain boundaries of the Fe-based electroplated layer.

[0056] Therefore, the Fe-based electroplating solution used in Fe-based electroplating processes contains a specific amount of transition metal ions other than Fe ions.

[0057] As a result, the crystal growth of Fe-based electroplated layers is hindered, and the grains of Fe-based electroplated layers become micronized.

[0058] The mechanism by which crystal growth is hindered is not yet clear. It is believed that the added transition metal ions enter the Fe-based electroplating layer and form fine precipitates at the grain boundaries, thereby hindering crystal growth.

[0059] By micronizing the grains of the Fe-based electroplating layer, numerous grain boundaries are formed. As a result, the intrusion of molten zinc is dispersed, delaying the time it takes for the zinc to reach the grain boundaries of the base steel plate. Consequently, the resistance to resistance welding cracks in the welded area becomes excellent.

[0060] This invention is based on the above insights.

[0061] [Manufacturing method of Fe-based electroplated steel sheet]

[0062] In the method for manufacturing Fe-based electroplated steel sheet according to this embodiment, an Fe-based electroplating treatment is performed on a substrate steel sheet containing 0.50 to 3.00% by mass of Si using an Fe-based electroplating solution containing Fe ions and transition metal ions other than Fe ions to form an Fe-based electroplating layer, followed by annealing. In the aforementioned Fe-based electroplating solution, the content of Fe ions is 1.0% by mass or more, and the content of transition metal ions is 10 to 1000 ppm by mass. The adhesion amount of the Fe-based electroplating layer on each single side of the aforementioned substrate steel sheet is 1.0 g / m². 2 above.

[0063] <Base Steel Plate>

[0064] The base steel plate can be, for example, a hot-rolled steel plate or a cold-rolled steel plate.

[0065] The thickness of the base steel plate is not particularly limited, for example, it is 0.6 to 3.2 mm, preferably 0.8 to 2.4 mm.

[0066] Composition

[0067] The composition of the base steel plate is explained.

[0068] The content of each element in the composition of the base steel plate is expressed in "mass%" unless otherwise specified.

[0069] (Si: 0.50% or more and 3.00% or less)

[0070] Si has a significant effect on increasing the strength of steel through solid solution (solution strengthening ability) without greatly impairing processability, making it an effective element for achieving high strength in steel sheets. Therefore, from the viewpoint of increasing the strength of steel sheets, the Si content is 0.50% or more, preferably 0.70% or more, and more preferably 0.90% or more.

[0071] On the other hand, silicon (Si) is also an element that adversely affects the resistance to welding cracks in the welded parts. If the amount of Si is too high, it may significantly reduce hot-rollability and cold-rollability, adversely affecting productivity, or leading to a decrease in the ductility of the steel sheet itself. Therefore, the Si content is 3.00% or less, preferably 2.50% or less, and more preferably 2.00% or less.

[0072] The base steel plate needs to contain Si within the above-mentioned range. Other compositions are permissible as long as they are common and are not particularly limited. However, from the viewpoint of obtaining a high-strength Fe-based electroplated steel plate, the following composition is preferred.

[0073] (C: below 0.80%)

[0074] C improves machinability by forming martensite and other structures within the steel.

[0075] Good weldability is obtained when C is added; therefore, the amount of C is preferably 0.80% or less, more preferably 0.30% or less.

[0076] There is no particular lower limit, but in order to obtain good processability, the amount of C is preferably 0.03% or more, more preferably 0.05% or more.

[0077] (Mn: 1.50% or more and 3.50% or less)

[0078] Mn strengthens steel through solid solution treatment, resulting in higher strength. Furthermore, Mn improves hardenability and promotes the formation of retained austenite, bainite, and martensite. To achieve these effects, the Mn content is preferably 1.50% or more, more preferably 1.80% or more.

[0079] On the other hand, adding excessive amounts of Mn increases costs. Therefore, from the viewpoint of achieving the above-mentioned effects without increasing costs, the amount of Mn is preferably 3.50% or less, more preferably 3.30% or less.

[0080] (P: below 0.100%)

[0081] By suppressing the amount of phosphorus (P), it is possible to prevent a decrease in weldability and, consequently, prevent P segregation at grain boundaries, thereby preventing deterioration in ductility, flexibility, and toughness. Furthermore, if the amount of P is excessive, the ferrite phase transformation is promoted, and the grain size becomes larger. Therefore, the amount of P is preferably 0.100% or less, more preferably 0.050% or less.

[0082] There is no specific lower limit. Due to limitations in production technology, the amount of P can be greater than 0%, or even 0.001% or more.

[0083] (S: below 0.0300%)

[0084] By suppressing the sulfur content, it is possible to prevent a decrease in weldability, and consequently, a decrease in ductility during hot rolling, suppressing hot cracking and significantly improving surface properties. However, if the sulfur content is excessive, it forms coarse sulfides as an impurity element, sometimes reducing the ductility, bendability, and flange extension properties of the steel sheet. Therefore, the sulfur content is preferably reduced as much as possible. Specifically, the sulfur content is preferably 0.0300% or less, more preferably 0.0200% or less.

[0085] There is no specific lower limit. Due to limitations in production technology, the amount of sulfur (S) can be greater than 0%, or even above 0.0001%.

[0086] (Al: below 0.100%)

[0087] Al is thermodynamically the easiest to oxidize, so it oxidizes before Si and Mn, thus inhibiting the oxidation of Si and Mn on the outermost layer of the steel plate and promoting the oxidation of Si and Mn inside the steel plate.

[0088] However, excessive Al content increases costs. Therefore, when Al is added, the Al content is preferably 0.100% or less, more preferably 0.060% or less.

[0089] There is no particular limitation on the lower limit; the Al content can be greater than 0%, or it can be 0.001% or more. However, from the viewpoint of obtaining the desired effect of adding Al, the Al content is preferably 0.010% or more, and more preferably 0.020% or more.

[0090] (Other elements)

[0091] The composition of the base steel plate, expressed as a percentage by mass, may also contain at least one element selected from the group consisting of the elements listed below.

[0092] ((B: less than 0.0050%))

[0093] If the amount of B is too high, the formability may sometimes decrease. Therefore, from the viewpoint of obtaining good formability, the amount of B is preferably 0.0050% or less, more preferably 0.0030% or less.

[0094] On the other hand, B is an element effective in improving the hardenability of steel. From the viewpoint of improving hardenability, the amount of B is preferably 0.0003% or more, and more preferably 0.0005% or more.

[0095] ((Ti: below 0.200%))

[0096] If the Ti content is too high, the hard phase may become too large, reducing formability. Therefore, from the viewpoint of obtaining good formability, the Ti content is preferably 0.200% or less, more preferably 0.050% or less.

[0097] There is no particular lower limit, but from the viewpoint of achieving the effect of intensity adjustment, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more.

[0098] ((N: less than 0.0100%))

[0099] If the amount of nitrogen (N) is too high, N may sometimes form coarse nitrides with Ti, Nb, and V at high temperatures, thereby impairing the high strength effect of the steel sheet obtained by adding Ti, Nb, and V, or reducing toughness, or causing billet cracks and surface defects during hot rolling. Therefore, the amount of N is preferably 0.0100% or less, more preferably 0.0050% or less, further preferably 0.0030% or less, and particularly preferably 0.0020% or less.

[0100] There is no specific lower limit. Due to limitations in production technology, the amount of N can be greater than 0%, or even 0.0005% or more.

[0101] (Cr: less than 1.000%)

[0102] Adding Cr improves the hardenability of steel plates, thereby enhancing the balance between strength and ductility.

[0103] However, when Cr is added, from the viewpoint of preventing cost increases, the amount of Cr is preferably 1.000% or less, more preferably 0.700% or less.

[0104] On the other hand, from the viewpoint of obtaining the effect of Cr addition, the amount of Cr is preferably 0.005% or more, more preferably 0.050% or more, and even more preferably 0.200% or more.

[0105] (Cu: less than 1.000%)

[0106] The addition of Cu can promote the formation of the residual γ phase in the steel plate.

[0107] However, when Cu is added, from the viewpoint of preventing cost increases, the amount of Cu is preferably 1.000% or less, more preferably 0.700% or less.

[0108] On the other hand, from the viewpoint of obtaining the effect of Cu addition, the amount of Cu is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0109] (Ni: less than 1.000%)

[0110] Adding Ni can promote the formation of the residual γ phase in the steel plate.

[0111] However, when Ni is added, from the viewpoint of preventing cost increases, the amount of Ni is preferably 1.000% or less, more preferably 0.700% or less.

[0112] On the other hand, from the viewpoint of obtaining the effect of Ni addition, the amount of Ni is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.030% or more.

[0113] ((Mo: 1.000% or less))

[0114] The strength of steel plates can be adjusted by adding Mo.

[0115] However, when adding Mo, from the viewpoint of preventing increased costs, the amount of Mo is preferably 1.000% or less, more preferably 0.700% or less.

[0116] On the other hand, from the viewpoint of obtaining the effect of adding Mo, the amount of Mo is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.050% or more.

[0117] ((Nb: below 0.200%))

[0118] Adding Nb can improve the strength of steel plates.

[0119] However, when Nb is added, from the viewpoint of preventing cost increases, the amount of Nb is preferably 0.200% or less, more preferably 0.150% or less.

[0120] On the other hand, from the viewpoint of obtaining the effect of Nb addition, the amount of Nb is preferably 0.005% or more, and more preferably 0.010% or more.

[0121] ((V: below 0.500%))

[0122] Adding V can increase the strength of the steel plate.

[0123] However, when V is present, from the viewpoint of preventing cost increases, the amount of V is preferably 0.500% or less, more preferably 0.300% or less.

[0124] On the other hand, from the viewpoint of obtaining the effect of adding V, the amount of V is preferably 0.005% or more, and more preferably 0.010% or more.

[0125] ((Sb: below 0.200%))

[0126] To obtain good toughness, the Sb content is preferably 0.200% or less, more preferably 0.100% or less.

[0127] On the other hand, by adding Sb, nitriding and oxidation of the steel plate surface can be suppressed, or decarburization in the tens of micrometers of the steel plate surface caused by oxidation can be suppressed. Furthermore, by suppressing nitriding and oxidation of the steel plate surface, Sb prevents a reduction in the amount of martensite formed on the steel plate surface, thereby improving the fatigue characteristics and surface quality of the steel plate. To achieve this effect, the Sb content is preferably 0.001% or more, more preferably 0.010% or more.

[0128] ((Ta: below 0.100%))

[0129] Adding Ta can improve the strength of steel plates.

[0130] However, when adding Ta, from the viewpoint of preventing cost increases, the amount of Ta is preferably 0.100% or less, more preferably 0.050% or less.

[0131] On the other hand, from the viewpoint of obtaining the effect of Ta addition, the amount of Ta is preferably 0.001% or more, and more preferably 0.010% or more.

[0132] ((W: below 0.500%))

[0133] Adding W can increase the strength of the steel plate.

[0134] However, when adding W, from the viewpoint of preventing cost increases, the amount of W is preferably 0.500% or less, more preferably 0.300% or less.

[0135] On the other hand, from the viewpoint of obtaining the effect of adding W, the amount of W is preferably 0.005% or more, and more preferably 0.010% or more.

[0136] ((Zr: below 0.1000%))

[0137] Adding Zr can improve the strength of steel plates.

[0138] However, when Zr is added, from the viewpoint of preventing cost increases, the Zr content is preferably 0.1000% or less, more preferably 0.0500% or less.

[0139] On the other hand, from the viewpoint of obtaining the effect of adding Zr, the amount of Zr is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0050% or more.

[0140] ((Sn: below 0.200%))

[0141] To obtain good impact resistance, the Sn content is preferably 0.200% or less, more preferably 0.100% or less.

[0142] On the other hand, Sn is an effective element for suppressing denitrification and deboration, thereby inhibiting the reduction of steel strength. To achieve this effect, the Sn content is preferably 0.002% or more, more preferably 0.010% or more.

[0143] (Ca: less than 0.0050%)

[0144] From the viewpoint of ensuring good ductility of the steel sheet, the Ca content is preferably 0.0050% or less, more preferably 0.0030% or less.

[0145] On the other hand, from the perspective of being able to control the morphology of sulfides and improve the ductility and toughness of steel plates, the Ca content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0146] ((Mg: less than 0.0050%))

[0147] By adding Mg, the morphology of sulfides can be controlled, thereby improving the ductility and toughness of steel plates.

[0148] However, when Mg is added, from the viewpoint of preventing increased costs, the amount of Mg is preferably 0.0050% or less, more preferably 0.0030% or less.

[0149] On the other hand, from the viewpoint of obtaining the effect of adding Mg, the amount of Mg is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0150] ((REM: below 0.0050%))

[0151] When adding REM (rare earth metals), from the viewpoint of obtaining good toughness, the amount of REM is preferably 0.0050% or less, more preferably 0.0030% or less.

[0152] On the other hand, from the perspective of being able to control the morphology of sulfides and improve the ductility and toughness of steel plates, the REM content is preferably 0.0005% or more, and more preferably 0.0010% or more.

[0153] (margin)

[0154] The balance of the base steel plate, other than the aforementioned components (elements), consists of Fe and unavoidable impurities.

[0155] The base steel plate can be manufactured using known methods.

[0156] For example, when the base steel plate is a hot-rolled steel plate, a steel billet having the above-mentioned composition is hot-rolled to obtain a hot-rolled steel plate. The steel billet can be heated before hot rolling.

[0157] Alternatively, if the base steel plate is a cold-rolled steel plate, the obtained hot-rolled steel plate may be pickled and then cold-rolled to obtain a cold-rolled steel plate.

[0158] Before performing the Fe-based electroplating treatment described later on the substrate steel plate thus manufactured, degreasing and washing are optionally performed. This cleans the surface of the substrate steel plate. Alternatively, pickling and washing can be performed to activate the surface of the substrate steel plate.

[0159] There are no particular restrictions on the methods of degreasing and washing; any common methods can be used.

[0160] The pickling method is not particularly limited. Examples of acids used in pickling include sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof, with sulfuric acid, hydrochloric acid, or mixtures thereof being preferred. The concentration of the acid is not particularly limited, but considering the ability to remove oxide film and prevent surface roughness (surface defects) caused by over-pickling, a concentration of about 1% to about 20% by mass is preferred. Defoamers, pickling accelerators, pickling inhibitors, etc., may be added to the acid used in pickling.

[0161] <Fe-based electroplating treatment>

[0162] After optionally performing the above-mentioned degreasing and other processes on the base steel plate, Fe-based electroplating treatment is performed. As a result, an Fe-based electroplating layer is formed on at least one side of the base steel plate.

[0163] The amount of Fe-based electroplated coating applied to each side of the substrate steel plate will be explained later.

[0164] Fe-based electroplating layers are preferably formed on both the surface and back sides of the base steel plate.

[0165] Fe-based electroplating solutions

[0166] The Fe-based electroplating solution used in Fe-based electroplating processes contains a specific amount of Fe ions and transition metal ions other than Fe ions.

[0167] (Fe ions)

[0168] If the amount of Fe ions in the Fe-based electroplating solution is too low, the electrolysis efficiency will decrease, making it difficult to obtain the desired amount of Fe-based electroplated layer. Alternatively, to obtain the desired amount of Fe-based electroplated layer, a long-term or high-current-density Fe-based electroplating treatment is required, leading to a decrease in production efficiency.

[0169] Therefore, the Fe ion content in the Fe-based electroplating solution is 1.0% by mass or more, preferably 2.5% by mass or more, and more preferably 4.0% by mass or more.

[0170] On the other hand, even if the Fe-based electroplating solution contains an excessive amount of Fe ions, the effect is saturated, leading to increased manufacturing costs.

[0171] Therefore, the Fe ion content in the Fe-based electroplating solution is preferably 10.0% by mass or less, more preferably 7.5% by mass or less.

[0172] There are no particular limitations on the source of Fe ions supplied to Fe-based electroplating solutions. For example, the Fe ion content can be adjusted by dissolving iron powder in an acidic electroplating solution or by adding sulfates or chlorides.

[0173] (Transition metal ions)

[0174] As mentioned above, based on the reason that the Fe-based electroplating layer has excellent resistance to welding cracks by micronizing the grains, the content of transition metal ions in the Fe-based electroplating solution is 10 ppm by mass or more.

[0175] Based on the reason that this effect is better, although it also depends on the type of transition metal ions, the content of transition metal ions in the Fe-based electroplating solution is preferably 30 ppm by mass or more, more preferably 50 ppm by mass or more, and even more preferably 100 ppm by mass or more.

[0176] On the other hand, if the amount of transition metal ions in the Fe-based electroplating solution is too high, the current during the Fe-based electroplating process will be consumed not only by the deposition of Fe but also by the deposition of transition metals. Therefore, the electrolysis efficiency will decrease, making it difficult to obtain the desired amount of Fe-based electroplated layer. In addition, the cost of the electroplating solution will also increase.

[0177] Therefore, in Fe-based electroplating solutions, the content of transition metal ions is less than 1000 ppm by mass, preferably less than 850 ppm by mass, and more preferably less than 700 ppm by mass.

[0178] It should be noted that when the transition metal ions contain two or more types of ions, the content of transition metal ions in the Fe-based electroplating solution is the total content of these two or more ions.

[0179] The transition metal ions contained in the Fe-based electroplating solution are not particularly limited as long as they are not Fe ions. From the viewpoint of forming finer precipitates in the Fe-based electroplating layer, it is preferable to have ions of at least one transition metal selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo and W, and more preferably to have ions of at least one transition metal selected from the group consisting of Ti, V, Cr, Mn, Ni and Cu.

[0180] There are no particular limitations on the source of transition metal ions supplied to Fe-based electroplating solutions. For example, the content of transition metal ions can be adjusted by dissolving powdered transition metals in acidic electroplating solutions or by adding sulfates or chlorides.

[0181] The content of transition metal (Fe-containing) ions in Fe-based electroplating solutions was determined using an inductively coupled plasma (ICP) luminescence spectrophotometer. A standard curve was pre-prepared using solutions with known transition metal concentrations. Based on the analytical results of the Fe-based electroplating solutions and referring to the prepared standard curve, the content of transition metal ions in the Fe-based electroplating solutions was calculated.

[0182] (Other ingredients)

[0183] In addition to the Fe ions and transition metal ions mentioned above, Fe-based electroplating solutions may also contain conductivity enhancers such as sodium sulfate and potassium sulfate; chelating agents; pH buffers; etc.

[0184] Other conditions

[0185] There are no other specific restrictions on Fe-based electroplating treatment.

[0186] Regarding the temperature of Fe-based electroplating solutions, if temperature stability is considered, it is preferably above 30°C.

[0187] Regarding the pH of Fe-based electroplating solutions, considering the conductivity of Fe-based electroplating solutions, it is preferably below 3.0.

[0188] Current density is typically 10–150 A / dm 2 .

[0189] From the perspective of excellent productivity, a board speed of 5 mpm or higher is preferred. On the other hand, from the viewpoint of stably controlling the adhesion amount of the Fe-based electroplated layer, a board speed of 150 mpm or lower is preferred.

[0190] Annealing treatment

[0191] For convenience, the base steel sheet with an Fe-based electroplating layer formed by Fe-based electroplating treatment will be referred to as "Fe-based electroplated steel sheet before annealing".

[0192] Fe-based electroplated steel sheets were annealed under the following conditions. This resulted in the production of Fe-based electroplated steel sheets. The annealing process removes the strain on the base steel sheet caused by rolling, allowing the microstructure to recrystallize, thereby increasing the strength of the resulting Fe-based electroplated steel sheet.

[0193] The annealing conditions are general and not particularly limited. It is preferred to heat the Fe-based electroplated steel sheet before annealing at the following annealing temperature for the following annealing time.

[0194] Annealing temperature: above 650℃ and below 900℃

[0195] The annealing temperature is preferably 650°C or higher, more preferably 700°C or higher. This allows for proper recrystallization of the base steel sheet's microstructure, easily yielding the desired strength. Furthermore, the natural oxide film on the surface of the Fe-based electroplated layer is appropriately reduced, resulting in excellent adhesion of the hot-dip galvanized layer (described later) formed on the Fe-based electroplated layer.

[0196] The annealing temperature is preferably below 900°C, and more preferably below 850°C. This prevents an excessive increase in the diffusion rate of Si and Mn in the base steel plate. As a result, Si and Mn are prevented from diffusing to the surface of the Fe-based electroplated layer, resulting in excellent adhesion of the hot-dip galvanized layer (described later) formed on the Fe-based electroplated layer. Furthermore, damage to the annealing furnace body can be prevented.

[0197] It should be noted that the annealing temperature is the highest temperature reached by the Fe-based electroplated steel sheet before annealing during the annealing process, and it is the measured temperature of the surface of the Fe-based electroplated steel sheet before annealing.

[0198] Annealing time: 30 seconds or more but less than 600 seconds

[0199] The annealing time is preferably 30 seconds or more, and more preferably 50 seconds or more. As a result, the natural oxide film on the surface of the Fe-based electroplated layer is properly removed, and the hot-dip galvanized layer formed on the Fe-based electroplated layer exhibits excellent adhesion.

[0200] There is no particular upper limit to the annealing time, but if the time is too long, the furnace length of the annealing furnace increases, and productivity decreases. Therefore, from the viewpoint of maximizing productivity, the annealing time is preferably 600 seconds or less, and more preferably 300 seconds or less.

[0201] Annealing Atmosphere

[0202] The annealing atmosphere (the atmosphere at which the annealing treatment is performed) is preferably a reducing atmosphere containing hydrogen. The hydrogen in the annealing atmosphere inhibits the oxidation of the Fe-based electroplated layer surface and activates the surface.

[0203] The hydrogen concentration in the annealing atmosphere is preferably 1.0 vol% or more, more preferably 2.0 vol% or more. This appropriately prevents oxidation of the Fe-based electroplated layer surface, resulting in excellent adhesion of the hot-dip galvanized layer (described later) formed on the Fe-based electroplated layer.

[0204] There is no particular upper limit to the hydrogen concentration in the annealing atmosphere, but from a cost point of view, it is preferably 30.0% by volume or less, and more preferably 20.0% by volume or less.

[0205] The balance of the annealing atmosphere, other than hydrogen, is preferably nitrogen.

[0206] When the dew point of the annealing atmosphere is too low, it incurs costs to remove moisture (dehumidification), so a dew point of -50°C or higher is preferred. Furthermore, for the reason of the excellent adhesion of the hot-dip galvanized layer described later, the dew point of the annealing atmosphere is preferably -30°C or higher, and more preferably -20°C or higher.

[0207] Regarding the upper limit, the dew point of the annealing atmosphere is, for example, below +30°C, preferably below +10°C.

[0208] [Fe-based electroplated steel sheet]

[0209] The Fe-based electroplated steel sheet of this embodiment is obtained by the manufacturing method of the Fe-based electroplated steel sheet of this embodiment described above. In short, it has a base steel sheet and an Fe-based electroplating layer.

[0210] <Base Steel Plate>

[0211] The base steel plate is the same as the base steel plate described above, so the description is omitted.

[0212] <Fe-based electroplating>

[0213] The Fe-based electroplating layer is formed on at least one side of the base steel plate by the above-described Fe-based electroplating process and has undergone the above-described annealing process.

[0214] Fe-based electroplating layers are preferably applied to both the surface and back sides of the base steel plate.

[0215] Adhesion Amount

[0216] Based on the excellent resistance to resistance welding cracks in the welded parts, the adhesion amount of the Fe-based electroplating layer on each single side of the steel plate (hereinafter also referred to as "the adhesion amount of the Fe-based electroplating layer") is 1.0 g / m. 2 The above, preferably 5.0 g / m 2 above.

[0217] On the other hand, there is no specific upper limit. From a cost perspective, the preferred adhesion amount of the Fe-based electroplating layer is 20.0 g / m. 2 The following, or more preferably, is 15.0 g / m 2the following.

[0218] The adhesion amount of the Fe-based electroplated layer was measured as described below.

[0219] A 10×15mm sample was cut from an Fe-based electroplated steel sheet (or a hot-dip galvanized steel sheet as described later) and embedded in resin to obtain an embedded sample with an exposed cross-section. Using a scanning electron microscope (SEM), three arbitrary points on this cross-section were observed at an accelerating voltage of 15kV and a magnification of 2000–10000x, depending on the thickness of the Fe-based electroplating layer. The average thickness of the three fields of view was multiplied by the specific gravity of iron to calculate the amount of Fe-based electroplating layer adhered.

[0220] Average crystal grain size

[0221] At the interface between the Fe-based electroplated layer and the base steel plate, the grains of the Fe-based electroplated layer need to be sufficiently micronized. As a result, the welded joint exhibits excellent resistance to welding cracks (especially the ability to prevent internal cracks).

[0222] Specifically, the average grain size of the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the base steel plate (hereinafter also referred to as "average grain size of the Fe-based electroplated layer") is 1.00 μm or less, preferably 0.85 μm or less, and more preferably 0.70 μm or less.

[0223] There is no specific lower limit. However, if the Fe-based electroplating layer has a small grain size and a low adhesion amount, easily oxidizable elements contained in the base steel plate may diffuse to the surface of the Fe-based electroplating layer. In this case, oxides form on the surface of the Fe-based electroplating layer, which may reduce its chemical conversion treatability, wettability with hot-dip galvanizing bath, and other properties. Therefore, the average grain size of the Fe-based electroplating layer is preferably 0.05 μm or more, and more preferably 0.10 μm or more.

[0224] The average grain size of the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the base steel plate was measured as described below.

[0225] First, a 10×10mm sample is cut from the Fe-based electroplated steel sheet. It should be noted that, in the case of forming the hot-dip galvanized layer described later, according to ISO 17925, only the hot-dip galvanized layer is dissolved and removed from the sample.

[0226] For the cut samples, a focused ion beam (FIB) device is used to form a 45° cross-section with a length of 30 μm in the rolling right-angle direction and a length of 50 μm in the direction of the T-section (a cross-section that is parallel to the rolling right-angle direction of the steel plate and perpendicular to the surface of the steel plate) at a 45° angle, thus making a sample for observation.

[0227] Next, a 45° cross-section of the sample was observed at 5000x magnification using a scanning ion microscope (SIM) to capture SIM images.

[0228] Examples of captured SIM images are shown below. Figure 1 and Figure 2 middle. Figure 1 This is a SIM image showing the base steel plate 1 and the Fe-based electroplated layer 2 of No. 30 (Comparative Example) described later. Figure 2 This is a SIM image showing the base steel plate 1 and the Fe-based electroplated layer 2 of No. 31 (Example of the Invention) described later.

[0229] At the midpoint of the thickness of the Fe-based electroplated layer 2 observed in the SIM image, draw a straight line with a length of 5 μm parallel to the surface of the substrate steel plate 1, and measure the number of grain boundaries intersecting this line. Divide this number by the length of the line, i.e., 5 μm, and take the value as the "average grain size of the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the substrate steel plate".

[0230] It should be noted that when the lines on the surface of the base steel plate 1 are not straight, lines that are approximately straight are used.

[0231] Figure 1 The Fe-based electroplating layer shown in the SIM image (No. 30: Comparative Example) was formed using an Fe-based electroplating solution that does not contain transition metal ions, and its grains grew coarsely.

[0232] In contrast, Figure 2 The Fe-based electroplating layer shown in the SIM image (No. 31: Invention Example) is formed using an Fe-based electroplating solution with a Ti ion content of 100 ppm by mass, and its grains are micronized.

[0233] Transition Metals

[0234] Fe-based electroplating layers are formed using the aforementioned Fe-based electroplating solution, and therefore contain transition metals (transition metal elements) other than Fe.

[0235] Based on the excellent resistance to resistance welding cracks of the welded part, the content of transition metal in the Fe-based electroplating layer is 100 ppm by mass or more, preferably 500 ppm by mass or more, and more preferably 1000 ppm by mass or more.

[0236] On the other hand, the content of transition metal in the Fe-based electroplating layer is less than 10,000 ppm by mass, preferably less than 8,500 ppm by mass, more preferably less than 7,000 ppm by mass, and even more preferably less than 5,000 ppm by mass.

[0237] The transition metal contained in the Fe-based electroplating layer is preferably at least one selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo and W, and more preferably at least one selected from the group consisting of Ti, V, Cr, Mn, Ni and Cu.

[0238] The content of transition metals in Fe-based electroplated layers was determined as described below.

[0239] First, using a glow discharge luminescence analyzer (GDS), the Fe intensity and the intensity of the transition metal elements were measured separately for steel plates with known contents of multiple transition metal elements. The intensity of the transition metal elements was then normalized to Fe intensity. Specifically, the ratio of the intensity of the transition metal elements to the Fe intensity (intensity of transition metal elements / Fe intensity) was determined. Furthermore, the relationship between the content of the transition metal elements and the intensity normalized to Fe intensity was pre-determined.

[0240] Next, for the transition metal elements in the Fe-based electroplated layer of the test object (Fe-based electroplated steel sheet), the strength (average strength) normalized to Fe strength is calculated in the same way. Based on this, the content of transition metal elements in the Fe-based electroplated layer is calculated according to the calculated strength and referring to the above relationship.

[0241] It should be noted that when the test object is hot-dip galvanized steel sheet, the hot-dip galvanized layer is peeled off using the test solution specified in JIS H0401 or ISO17925, and then the content of transition metal elements in the Fe-based electroplating layer is calculated in the same way.

[0242] Hot-dip galvanized steel sheet and its manufacturing method

[0243] Next, the hot-dip galvanized steel sheet and its manufacturing method will be explained.

[0244] The Fe-based electroplated steel sheet obtained after the above annealing treatment is then subjected to hot-dip galvanizing. This yields a hot-dip galvanized steel sheet.

[0245] Hot-dip galvanizing is, for example, the following process: after annealing, the Fe-based electroplated steel sheet is cooled and then immersed in a hot-dip galvanizing bath to form a hot-dip galvanized layer on the Fe-based electroplated layer.

[0246] Hot-dip galvanizing baths typically consist of Al, Zn, and unavoidable impurities.

[0247] The Al concentration in the hot-dip galvanizing bath is preferably 0.050% by mass or more, and more preferably 0.100% by mass or more. This prevents the formation of bottom slag and defects caused by slag adhesion.

[0248] On the other hand, the Al concentration in the hot-dip galvanizing bath is preferably 0.250% by mass or less, more preferably 0.200% by mass or less. This prevents the accumulation of top slag and defects caused by slag adhesion. Furthermore, it also leads to cost reduction.

[0249] There are no other specific restrictions regarding hot-dip galvanizing.

[0250] The temperature of a hot-dip galvanizing bath is typically in the range of 440–500°C. For example, Fe-based electroplated steel sheets with a plate temperature of 440–550°C are immersed in a hot-dip galvanizing bath at such a temperature.

[0251] The preferred amount of hot-dip galvanized coating on each single side of the base steel plate (hereinafter also referred to as "the amount of hot-dip galvanized coating") is 25 g / m². 2 The above, and more preferably 30g / m 2 The above explains why the corrosion resistance of the obtained hot-dip galvanized steel sheet can be improved. Furthermore, in this case, the amount of hot-dip galvanized coating is easier to control.

[0252] On the other hand, based on the good adhesion of the hot-dip galvanized layer, the preferred adhesion amount of the hot-dip galvanized layer is 80 g / m². 2 The following, or more preferably, is 75g / m 2 the following.

[0253] After hot-dip galvanizing, the amount of hot-dip galvanized coating can be adjusted appropriately.

[0254] There are no particular limitations on the adjustment method; gas wiping is commonly used. In this case, the amount of hot-dip galvanized coating is adjusted, for example, by adjusting the gas pressure during wiping or the distance between the wiping nozzle and the steel plate.

[0255] The amount of hot-dip galvanized coating is determined according to JIS H 0401 or ISO 17925.

[0256] Specifically, first, a 20×25mm sample is cut from a hot-dip galvanized steel sheet. After measuring the mass of the sample, the hot-dip galvanized layer on one side is peeled off using the test solution specified in JIS H 0401 or ISO 17925, and the mass is measured again. The mass before peeling is subtracted from the mass after peeling, and then divided by the surface area of ​​the peeled portion. The amount of hot-dip galvanized layer adhesion is then calculated.

[0257] After hot-dip galvanizing, alloying can be further carried out as needed.

[0258] In this specification, the hot-dip galvanized layer that has undergone alloying treatment (the so-called "alloyed hot-dip galvanized layer") is also simply referred to as the hot-dip galvanized layer.

[0259] Similarly, hot-dip galvanized steel sheets that have undergone alloying treatment (the so-called "alloyed hot-dip galvanized steel sheets") are also simply referred to as hot-dip galvanized steel sheets.

[0260] There are no particular limitations on the conditions for alloying treatment. From the viewpoint of enabling alloying, the alloying temperature is preferably 460°C or higher, and more preferably 480°C or higher.

[0261] On the other hand, if the alloying temperature is too high, the adhesion of the hot-dip galvanized layer may decrease, or the mechanical properties of the hot-dip galvanized steel sheet may decrease. Therefore, the alloying temperature is preferably below 600°C, and more preferably below 540°C.

[0262] There is no particular limitation on the alloying time (heating time at the alloying temperature), for example, it is 10 to 60 seconds.

[0263] When alloying is performed, if the degree of alloying (Fe%) in the hot-dip galvanized layer is too low, uneven alloying may occur, leading to aesthetic degradation, or the formation of a so-called ζ phase, resulting in deterioration of sliding properties. Therefore, the degree of alloying is preferably 7.0% by mass or more, and more preferably 8.0% by mass or more.

[0264] On the other hand, if the alloying degree is too high, a large amount of hard and brittle Γ phase may be generated, which reduces the adhesion of the hot-dip galvanized layer. Therefore, the alloying degree is preferably 15.0% by mass or less, more preferably 13.0% by mass or less.

[0265] The degree of alloying (Fe%) was determined according to ISO 17925. Specifically, the test solution after the hot-dip galvanized layer was peeled off was analyzed using an inductively coupled plasma (ICP) luminescence analyzer. The Fe%) was calculated by dividing the Fe content by the total amount of Fe, Zn, Al, etc., contained in the test solution.

[0266] [Evaluation Test of Resistance to Resistance Welding Cracks in Welded Parts]

[0267] based on Figures 3-5 The test method for evaluating the resistance to welding cracks in welded parts is described.

[0268] Figure 3 This is a cross-sectional view showing plate assembly 5 used in resistance welding. Figure 4 This is a top view showing the plate assembly 5 after resistance welding. Figure 5 yes Figure 4 AA-line cross-section diagram.

[0269] First, test piece 4 is cut from the Fe-based electroplated steel sheet (or hot-dip galvanized steel sheet) to be evaluated. For test piece 4, the rolling right angle direction (TD) is set as the length direction and the rolling direction is set as the short side direction. Its dimensions are: the length L in the length direction is 150 mm, the length W in the short side direction is 50 mm, and the plate thickness t is 1.6 mm.

[0270] Similarly, test pieces of the same size were cut from another hot-dip galvanized steel sheet.

[0271] The evaluation surface (Fe-based electroplated layer or hot-dip galvanized layer) of test piece 4 is brought into contact with the hot-dip galvanized layer of test piece 3 to obtain plate group 5.

[0272] Plate group 5 is separated by a pair of spacers 6 (length in the longitudinal direction: 50mm, length in the short side direction: 45mm, thickness t). S The spacer 6 is fixed to the mounting platform 7 with a diameter of 2.0 mm. The spacer 6 is arranged such that its longitudinal end face is aligned with the two end faces of the short side of the plate assembly 5. Therefore, the distance D between a pair of spacers 6 is 60 mm. The mounting platform 7 is a plate with a hole 7a in the center.

[0273] Next, a single-phase AC (50Hz) resistance welding machine with a servo motor pressurization is used to perform resistance welding on the board assembly 5 while it is bent, by applying pressure with a pair of electrodes 8 (front diameter: 6mm).

[0274] More specifically, resistance welding is performed under specified conditions (pressure, holding time, and welding time) with a welding current that becomes a specified weld nugget diameter d, forming a welded part containing a weld nugget 9.

[0275] The holding time refers to the time from the end of the welding current flow to the start of opening electrode 8.

[0276] The diameter d of the melt core is the distance between the ends of the melt core 9 along the length of the plate group 5.

[0277] As a range of melt nugget diameter d that is prone to causing the aforementioned internal cracks, based on the plate thickness t of test piece 4, examples can be listed as 4√t to 5√t.

[0278] During resistance welding, a pair of electrodes 8 apply pressure to the plate assembly 5 from vertically upwards and downwards.

[0279] The lower electrode 8a applies pressure to the test piece 4 through the hole 7a of the fixing stage 7. During pressure application, the lower electrode 8a and the fixing stage 7 are fixed at a position where they meet an imaginary plane S extending from the surface where the spacer 6 contacts the fixing stage 7. The upper electrode 8b is movable at a position where it can contact the center of the test piece 3.

[0280] Resistance welding is performed with the plate assembly 5 tilted 5° towards the length direction of the plate assembly 5 relative to the horizontal direction (i.e., the angle θ relative to the horizontal direction is set to 5°).

[0281] The resistance-welded plate assembly 5 is arranged along the center of the welded portion containing the weld nugget 9. Figure 4 The AA line was cut. The cross-section of the weld was observed using an optical microscope (200x) to evaluate the resistance to welding cracks.

[0282] It should be noted that, Figure 5 The diagram schematically shows the crack 10 that occurred in test piece 4.

[0283] When cracks appeared in test piece 3, the stress in test piece 4 was dispersed, and a proper evaluation could not be obtained. Therefore, data from test piece 3 when no cracks appeared were used.

[0284] Example

[0285] The present invention will be specifically described below with reference to specific embodiments. However, the present invention is not limited to the embodiments described below.

[0286] <Manufacturing of Hot-Dip Galvanized Steel Sheets>

[0287] Preparation of the base steel plate

[0288] Steel with a composition containing the elements shown in Table 1 below, with the balance consisting of Fe and unavoidable impurities, is melted. The resulting steel billet is then hot-rolled, pickled, and cold-rolled to obtain a pre-annealed cold-rolled steel sheet with a thickness of 1.6 mm. The obtained pre-annealed cold-rolled steel sheet is used as the base steel sheet.

[0289] Fe-based electroplating treatment

[0290] Next, the base steel plate was degreased in an alkaline solution, and then electrolytically treated (Fe-based electroplating treatment) was performed using the Fe-based electroplating solution shown in Table 2 below, with the base steel plate as the cathode, under the conditions shown below. This resulted in an annealed Fe-based electroplated steel plate with Fe-based electroplating layers formed on both the front and back surfaces of the base steel plate. The amount of Fe-based electroplating layer was controlled by adjusting the energizing time.

[0291] (Conditions for Fe-based electroplating)

[0292] Temperature of Fe-based electroplating solution: 50℃

[0293] pH of Fe-based electroplating solution: 2.0

[0294] Current density: 50A / dm 2

[0295] Electrode (anode): Iridium oxide electrode

[0296] Annealing treatment

[0297] Next, the Fe-based electroplated steel sheet before annealing is subjected to annealing treatment.

[0298] Specifically, the Fe-based electroplated steel sheet before annealing is heated at 800°C for 85 seconds in a reducing atmosphere with a dew point of -35°C and a hydrogen concentration of 15% by volume (the balance being nitrogen).

[0299] This yields Fe-based electroplated steel sheets.

[0300] Hot-dip galvanizing treatment

[0301] The obtained Fe-based electroplated steel sheet is subjected to hot-dip galvanizing treatment to obtain hot-dip galvanized steel sheet.

[0302] More specifically, after cooling the Fe-based electroplated steel sheet to 440–550°C, it is immersed in a hot-dip galvanizing bath at 460°C with an Al concentration of 0.132% by mass and the balance consisting of Zn and unavoidable impurities to form a hot-dip galvanized layer. The adhesion of the hot-dip galvanized layer is adjusted by gas wiping after the hot-dip galvanizing treatment.

[0303] In some examples, alloying is then performed at an alloying temperature of 510°C. At this point, the alloying time is varied to adjust the degree of alloying. The degree of alloying (unit: mass%) is listed in Table 2 below when alloying has been performed.

[0304] For the Fe-based electroplated layer of the obtained hot-dip galvanized steel sheet, the adhesion amount, average crystal grain size, and transition metal content are determined.

[0305] In addition, the adhesion amount and alloying degree of the hot-dip galvanized layer of the obtained hot-dip galvanized steel sheet are calculated.

[0306] Then, the tensile strength of the obtained hot-dip galvanized steel sheet is calculated.

[0307] The results are shown in Table 2 below.

[0308] <Characteristics of resistance welding crack resistance of welded parts>

[0309] Test pieces were cut from the obtained hot-dip galvanized steel sheet, and based on... Figures 3-5 The test method described herein is used to evaluate the resistance to welding cracks in welded parts.

[0310] The test specimens were selected based on a tensile strength of 980 MPa and a hot-dip galvanized coating thickness of 50 g / m². 2 The test was conducted using alloyed hot-dip galvanized steel sheet (thickness t: 1.6mm).

[0311] With the angle θ set to 5°, resistance welding was performed on the plate assembly of the test piece and the target test piece under the conditions of a welding current and welding time of 3.5 kN pressure, 0.10 seconds or 0.16 seconds holding time, and a weld nugget diameter d of 5.9 mm to form a welded part.

[0312] The cross-section of the welded part is observed, and the resistance to welding cracks of the welded part is evaluated according to the following criteria.

[0313] If the symbol is ◎ or ○, the weld is judged to have excellent resistance to welding cracks. The results are shown in Table 2 below.

[0314] ◎: No cracks longer than 0.1 mm were found when the holding time was 0.10 seconds.

[0315] ○: Cracks longer than 0.1 mm were found when the holding time was 0.10 seconds, but no cracks longer than 0.1 mm were found when the holding time was 0.16 seconds.

[0316] ×: A crack longer than 0.1 mm was found when the holding time was 0.16 seconds.

[0317] In Tables 1 and 2 below, underlined lines indicate items outside the scope of this invention.

[0318] [Table 1]

[0319]

[0320] [Table 2]

[0321] Table 2 (1 / 2)

[0322]

[0323] Table 2 (2 / 2)

[0324]

[0325] <Summary of Evaluation Results>

[0326] As shown in Tables 1 and 2 above, the hot-dip galvanized steel sheets No.5, 7-14, 16-17, 20-22, 24-29, 31-33, 35-45 and 47-50 are all high-strength (tensile strength above 590MPa) and have excellent resistance to welding cracks in the welded parts.

[0327] The adhesion amount to the Fe-based electroplated layer is 1.0 g / m. 2 Above and below 5.0 g / m 2Compared to hot-dip galvanized steel sheets (No. 5, 7–10, 14, 17, 20, 24–25, 33, 42–43, 45, and 48) with an average crystal grain size greater than 0.70 μm and less than 1.00 μm, the Fe-based electroplating coating has an adhesion weight of 5.0 g / m². 2 The welded parts of the above-mentioned hot-dip galvanized steel sheets (No. 11~13, 16, 21~22, 26~29, 31~32, 35~41, 44, 47, 49~50) with an average crystal grain size of less than 0.70μm have better resistance to welding cracks.

[0328] In contrast, the tensile strength of hot-dip galvanized steel sheets No. 1 and No. 2 is less than 590 MPa, indicating insufficient strength.

[0329] In addition, the resistance to weld cracking characteristics of the welded parts of hot-dip galvanized steel sheets No.3-4, 6, 15, 18-19, 23, 30, 34 and 46 are insufficient.

[0330] Symbol Explanation

[0331] 1: Base steel plate

[0332] 2: Fe-based electroplating layer

[0333] 3: Test piece for the target

[0334] 4: Test piece

[0335] 5: Board assembly

[0336] 6: Spacers

[0337] 7: Fixed platform

[0338] 7a: Hole

[0339] 8, 8a, 8b: Electrodes

[0340] 9: Molten Core

[0341] 10: Cracks

Claims

1. A method for manufacturing Fe-based electroplated steel sheet, wherein, Fe-based electroplating layer is formed by performing Fe-based electroplating treatment on a substrate steel plate containing 0.50–3.00% by mass Si and 1.50–3.50% by mass Mn using an Fe-based electroplating solution containing Fe ions and transition metal ions other than Fe ions. Following this, annealing treatment is performed. In the Fe-based electroplating solution, the content of Fe ions is 1.0% by mass or more, and the content of transition metal ions is 10 to 1000 ppm by mass. The Fe-based electroplated layer has an adhesion amount of 1.0 g / m² on each single side of the base steel plate. 2 above.

2. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, The base steel plate has a composition, by mass percent, of Si: 0.50% or more and 3.00% or less, C: 0.80% or less, Mn: 1.50% or more and 3.50% or less, P: 0.100% or less, S: 0.0300% or less, and Al: 0.100% or less, with the balance being Fe and unavoidable impurities.

3. The method for manufacturing Fe-based electroplated steel sheet according to claim 2, wherein, The composition, by mass%, further contains at least one element selected from the group consisting of B: less than 0.0050%, Ti: less than 0.200%, N: less than 0.0100%, Cr: less than 1.000%, Cu: less than 1.000%, Ni: less than 1.000%, Mo: less than 1.000%, Nb: less than 0.200%, V: less than 0.500%, Sb: less than 0.200%, Ta: less than 0.100%, W: less than 0.500%, Zr: less than 0.1000%, Sn: less than 0.200%, Ca: less than 0.0050%, Mg: less than 0.0050%, and REM: less than 0.0050%.

4. The method for manufacturing Fe-based electroplated steel sheet according to any one of claims 1 to 3, wherein, The transition metal ions contained in the Fe-based electroplating solution are ions of at least one transition metal selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo, and W.

5. The method for manufacturing Fe-based electroplated steel sheet according to any one of claims 1 to 3, wherein, The Fe-based electroplated layer has an adhesion amount of 5.0–20.0 g / m² on each single side of the base steel plate. 2 .

6. A method for manufacturing hot-dip galvanized steel sheet, wherein, The Fe-based electroplated steel sheet obtained by any one of claims 1 to 3 is subjected to hot-dip galvanizing treatment.

7. The method for manufacturing hot-dip galvanized steel sheet according to claim 6, wherein, Following the hot-dip galvanizing treatment, a further alloying process is carried out.

8. An Fe-based electroplated steel sheet, comprising a base steel sheet containing 0.50 to 3.00% by mass of Si and 1.50 to 3.50% by mass of Mn, and an Fe-based electroplated layer disposed on at least one side of the base steel sheet. The Fe-based electroplated layer has an adhesion amount of 1.0 g / m² on each single side of the base steel plate. 2 above, The average grain size of the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the base steel plate is less than 1.00 μm. The Fe-based electroplating layer contains transition metals other than Fe. The content of the transition metal in the Fe-based electroplated layer is 100 to 10,000 ppm by mass.

9. The Fe-based electroplated steel sheet according to claim 8, wherein, The transition metal is at least one selected from the group consisting of Ti, V, Cr, Mn, Ni, Cu, Nb, Mo, and W.

10. The Fe-based electroplated steel sheet according to claim 8 or 9, wherein it is resistively welded to a hot-dip galvanized steel sheet.

11. A hot-dip galvanized steel sheet having the Fe-based electroplated steel sheet as described in claim 8 or 9 and a hot-dip galvanized layer disposed on the Fe-based electroplated layer of the Fe-based electroplated steel sheet.

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