JP7205664B2 - Fe electroplated steel sheet, electrodeposition coated steel sheet, automobile parts, method for manufacturing electrodeposition coated steel sheet, and method for manufacturing Fe electroplated steel sheet - Google Patents

Description

The present invention relates to a method for producing Fe-based electroplated steel sheets, electrodeposition coated steel sheets, automobile parts, and electrodeposition coated steel sheets. Fe-based electroplated steel sheet with excellent post-corrosion resistance and excellent resistance weld cracking resistance in welds, and method for producing electrodeposition coated steel sheet, automobile parts, and electrodeposition coated steel sheet using said Fe-based electroplated steel sheet Regarding.

In recent years, from the viewpoint of protecting the global environment, there is a strong demand for improving the fuel efficiency of automobiles. Also, from the viewpoint of ensuring the safety of passengers in the event of a collision, there is a strong demand for improved safety of automobiles. In order to meet these demands, it is necessary to achieve both weight reduction and high strength for automobile bodies. ing. However, since many automobile parts are manufactured by forming steel sheets, these steel sheets are required to have excellent formability in addition to high strength.

There are various methods for increasing the strength of cold-rolled steel sheets, but solid-solution strengthening by the addition of Si can be cited as a method that can increase the strength of cold-rolled steel sheets without significantly impairing the formability. However, when a large amount of Si, especially 0.5% by mass or more of Si, is added to the cold-rolled steel sheet, SiO ₂ , It is known that Si-containing oxides such as Si—Mn-based composite oxides are formed. Since this Si-containing oxide significantly lowers the chemical conversion treatability of the steel sheet, the high-strength cold-rolled steel sheet containing a large amount of Si is inferior in chemical conversion treatability. In addition, high-strength cold-rolled steel sheets containing a large amount of Si have problems in that the paint film peels more easily than ordinary steel sheets in a salt water immersion test after electrodeposition coating, and the corrosion resistance after coating is inferior.

In the manufacture of automobile parts, press-molded parts are often assembled by resistance welding (spot welding). When the parts to be resistance-welded include high-strength galvanized steel sheets, zinc in the coating layer melts and diffuses into the grain boundaries in the presence of residual stress in the vicinity of the weld during resistance welding. , Liquid Metal Embrittlement (LME) occurs, and intergranular cracking (LME cracking) occurs in the steel sheet. In particular, if welding is performed with the welding electrode angled with respect to the steel plate, residual stress may increase and cracks may occur. Since the residual stress is considered to increase as the strength of the steel sheet increases, there is concern about the occurrence of LME cracking as the strength of the steel sheet increases. Even if the high-strength steel sheet does not have a galvanized layer, if the steel sheet on the other side to be welded is a galvanized steel sheet, the galvanized layer will melt, so it does not have a galvanized layer. The problem is that LME cracking can also occur in steel sheets.

From the above, it is excellent in chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance at welds when the sheet pair is a galvanized steel sheet (hereinafter simply referred to as "resistance weld cracking resistance at welds"). There is a demand for high-strength steel sheets.

Conventionally, measures for improving the above problem have been reported. For example, in Patent Document 1, during hot rolling, a slab is heated at a temperature of 1200 ° C. or higher, descaled at a high pressure, and before pickling, the surface of the hot rolled steel plate is ground with an abrasive nylon brush, and 9% hydrochloric acid is applied. A method has been proposed for manufacturing a high-strength cold-rolled steel sheet with excellent chemical convertibility by immersing the steel sheet in a tank twice for pickling to reduce the Si concentration on the surface of the steel sheet. In addition, in Patent Document 2, from the surface of the base material to a depth of 5.0 μm or more, at least a part of the grain boundary has an internal oxide layer coated with oxide, and from the surface of the base material A steel sheet is disclosed in which the grain boundary coverage of the oxide is 60% or more in a region up to a depth of 5.0 μm. Furthermore, in Patent Document 3, after heat-annealing a cold-rolled steel sheet in a non-oxidizing atmosphere, 0.5 g/m ² or more is melted by pickling, and then a Zn-Fe alloy with an adhesion amount of 1 to 5 g/m ² is applied. An electroplating steel sheet is disclosed.

Japanese Patent No. 3990349 Japanese Patent No. 6388099 JP 2015-89946 A

However, in the high-strength cold-rolled steel sheet described in Patent Document 1, even if the Si concentration on the steel sheet surface is reduced before cold rolling, Si-containing oxides are formed on the steel sheet surface by annealing after cold rolling. Therefore, improvement in corrosion resistance after painting cannot be expected. In addition, it has been found that the steel sheet described in Patent Document 2 has weak adhesion at grain boundaries and insufficient post-coating corrosion resistance as evaluated by a salt water immersion test. Further, in the steel sheet described in Patent Document 3, although the chemical conversion treatability and corrosion resistance after painting are improved, the problem of resistance weld cracking resistance is not mentioned at all.

In this way, high-strength cold-rolled steel sheets containing a large amount of Si (hereafter also referred to as Si-containing cold-rolled steel sheets) are expected to improve their chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking at welds. It is difficult to satisfy both of these properties at a high level, and the actual situation is that no steel sheet has been developed that satisfies all of these properties at a high level.

The present invention has been made in view of the above-mentioned problems of Si-containing cold-rolled steel sheets. To provide a steel sheet which is also excellent in resistance weld cracking resistance in a part.

In order to solve the above problems, the present inventors have made extensive studies and found that, in order to satisfy all of the high levels of chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking in the weld zone, cold steel is required. After cold rolling, it is important to form an Fe-based electroplating layer on the surface of the Si-containing cold-rolled steel sheet that has been continuously annealed so that the coating amount per side is 15.0 g/m ² or more to obtain an Fe-based electroplating steel sheet. I found something. By forming an Fe-based electroplating layer on the steel sheet surface, it is possible to completely cover the Si-containing oxides formed on the steel sheet surface during annealing, thereby improving chemical conversion treatability and corrosion resistance after painting. Furthermore, the present inventors have found that by forming a soft Fe-based electroplating layer with a deposition amount of 15.0 g/m ² or more per side of the Si-containing cold-rolled steel sheet, the stress applied to the steel sheet surface during welding can be reduced. In addition to the relaxation, the Fe-based electroplating layer acts as a solid-solution Si-deficient layer to suppress the decrease in toughness due to Si solid-solution, and to improve the resistance weld cracking resistance of the weld zone. rice field.

The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.

[1] A Si-containing cold-rolled steel sheet containing 0.5% by mass or more and 3.0% by mass or less of Si,
An Fe-based electroplated steel sheet, comprising: an Fe-based electroplated layer having a coating amount of 15.0 g/m ² or more per side formed on at least one side of the Si-containing cold-rolled steel sheet.

[2] The proportion of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is 50% or less. The Fe-based electroplated steel sheet according to [1].

[3] The Fe-based electroplated steel sheet according to [1] or [2], wherein the Fe-based electroplated layer has an adhesion amount per one side of 25 g/m ² or more.

[4] The Si-containing cold-rolled steel sheet contains, in addition to the Si,
C: 0.8% or less,
Mn: 1.0% or more and 12.0% or less,
P: 0.1% or less,
S: 0.03% or less,
Fe according to any one of [1] to [3] above, which contains N: 0.010% or less and Al: 1.0% or less, with the balance being Fe and inevitable impurities. system electroplated steel sheet.

[5] Further, the component composition is mass%,
B: 0.005% or less,
Ti: 0.2% or less,
Cr: 1.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.20% or less,
V: 0.5% or less,
Sb: 0.200% or less,
Ta: 0.1% or less,
W: 0.5% or less,
Zr: 0.1% or less,
Sn: 0.20% or less,
Ca: 0.005% or less,
The Fe-based electroplated steel sheet according to [4] above, containing one or more selected from the group consisting of Mg: 0.005% or less and REM: 0.005% or less.

[6] The Fe-based electroplating layer is one or more selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co The Fe-based electroplated steel sheet according to any one of [1] to [5], having a chemical composition containing 10% by mass or less of the elements in total, with the balance being Fe and unavoidable impurities.

[7] a cold-rolled steel sheet;
An Fe-based electroplated steel sheet, comprising: an Fe-based electroplated layer formed on at least one side of the cold-rolled steel sheet and having an adhesion amount per side of 15.0 g/m ² or more.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet.

[8] A chemical conversion coating formed on the Fe-based electroplated steel sheet according to any one of [1] to [7] in contact with the Fe-based electroplating layer, and on the chemical conversion coating An electrodeposition coated steel sheet further having an electrodeposition coating film formed on the

[9] Automobile parts at least partly using the electrodeposition coated steel sheet according to [8].

[10] The Fe-based electroplated steel sheet according to any one of [1] to [7] is subjected to a chemical conversion treatment without additional plating treatment, and the Fe-based electroplated layer is contacted and chemically treated. a chemical conversion step for obtaining a chemically treated steel sheet on which a treatment film is formed;
an electrodeposition coating step of subjecting the chemically treated steel sheet to an electrodeposition coating process to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion coating;
A method for manufacturing an electrodeposition coated steel sheet, including

[11] An Si-containing cold-rolled steel sheet containing 0.5% by mass or more and 3.0% by mass or less of Si is annealed to form a Si-containing cold-rolled steel sheet,
Next, the Si-containing cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet having an Fe-based electroplating layer with a deposition amount of 15.0 g/m ² or more per side formed on at least one side. , a method for producing a Fe-based electroplated steel sheet.

[12] Annealing the pre-annealed cold-rolled steel sheet to obtain a cold-rolled steel sheet,
Then, the cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet having an Fe-based electroplating layer formed on at least one side with a deposition amount of 15.0 g/m ² or more per side. A method for producing a system electroplated steel sheet.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet.

[13] One or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co are added to the Fe-based electric The above [11] or [12], wherein the Fe-based electroplating is performed using an Fe-based electroplating bath containing such elements that the total content of these elements in the plating layer is 10% by mass or less. A method for producing a Fe-based electroplated steel sheet.

According to the present invention, in addition to being excellent in chemical conversion treatability, it is also excellent in post-painting corrosion resistance as evaluated by a salt water immersion test, and is also excellent in resistance weld cracking resistance in welds when the sheet assembly partner is a galvanized steel sheet. A Si-containing cold-rolled steel sheet and an automobile part using the Si-containing cold-rolled steel sheet can be provided.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the outline|summary of the cross section of Fe-type electroplating steel plate. Invention Example No. 43 is a diagram showing an observation image of the interface between the Fe-based electroplating layer of No. 43 and the Si-containing cold-rolled steel sheet. Invention Example No. 46 is a diagram showing an observation image of the interface between the Fe-based electroplating layer of No. 46 and the Si-containing cold-rolled steel sheet. FIG. 2 (a) is a perspective view and (b) is a cross-sectional view taken along line AA, showing an outline of an observation sample for measuring the ratio of crystal orientation integration. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram for explaining a method for evaluating the ratio of integrated crystal orientations, (a) a diagram in which a boundary line is drawn at the interface between an Fe-based electroplating layer and a Si-containing cold-rolled steel sheet in a SIM image, (b) FIG. 4C is an enlarged view of the area enclosed by the square in FIG. Invention Example No. 43 is a diagram showing an image in which a boundary line and a determination region are drawn after the binary processing of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in FIG. Invention Example No. 46 is a diagram showing an image in which a boundary line and a determination region are drawn after binary processing of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in 46. FIG. (a) is a diagram for explaining the evaluation method of resistance weld cracking resistance in the weld zone, (b) the upper diagram is the top view of the plate assembly after welding in the same evaluation, and the lower diagram is the BB cross section of the upper diagram. It is a diagram.

The above-mentioned LME cracks can be broadly classified into "cracks that occur on the surface in contact with the electrode (hereinafter referred to as surface cracks)" and "cracks that occur near the corona bond between steel sheets (hereinafter referred to as inner cracks)". It is known that surface cracks are likely to occur in resistance welding in a high current range where spatter is generated. Surface cracks can be suppressed by setting the current within an appropriate range where spatter does not occur. On the other hand, internal cracking occurs even when the current during resistance welding is set within an appropriate range where spatter does not occur. In addition, while surface cracks are easily discovered by visual inspection in the manufacturing process, internal cracks are difficult to be discovered by visual inspection. For these reasons, among LME cracks, inner cracks pose a particularly large problem. If resistance welding is performed with the welding electrode angled with respect to the steel sheet, residual stress may increase and internal cracking may occur. Residual stress is thought to increase as the strength of steel sheets increases. In the present disclosure, it is possible to improve the resistance weld cracking resistance property, especially the property to prevent this internal cracking.

An embodiment of the present invention will be described below.
In the following explanation, the content of each element in the chemical composition of the Si-containing cold-rolled steel sheet and the unit of the content of each element in the coating layer composition are both "% by mass", and unless otherwise specified, simply " %”. In addition, in this specification, a numerical range expressed using "-" means a range including the numerical values described before and after "-" as lower and upper limits. Further, in this specification, the steel sheet having "high strength" means that the steel sheet has a tensile strength TS of 590 MPa or more measured according to JIS Z 2241 (2011).

[Embodiment 1]
FIG. 1 shows an overview of a cross section of a Fe-based electroplated steel sheet 1 according to this embodiment. As shown in FIG. 1, an Fe-based electroplated steel sheet 1 has an Fe-based electroplated layer 3 on at least one side of a Si-containing cold-rolled steel sheet 2 . First, the chemical composition of the Si-containing cold-rolled steel sheet will be described.

Si: 0.5% or more and 3.0% or less Si has a large effect of increasing the strength of steel through solid solution (solution strengthening ability) without significantly impairing workability, so it achieves high strength steel sheets. is an effective element for On the other hand, Si is also an element that adversely affects chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance in welds. When Si is added to increase the strength of the steel sheet, it must be added in an amount of 0.5% or more. Also, if the Si content is less than 0.5%, there is no particular problem with the chemical conversion treatability and the resistance weld cracking resistance of the weld zone, and there is little need to apply the present invention. On the other hand, if the Si content exceeds 3.0%, the hot-rollability and cold-rollability are greatly deteriorated, adversely affecting the productivity and causing a decrease in the ductility of the steel sheet itself. Therefore, Si is added in the range of 0.5% to 3.0%. The amount of Si is preferably 0.7% or more, more preferably 0.9% or more. Also, the Si content is preferably 2.5% or less, more preferably 2.0% or less, and still more preferably 1.7% or less.

The Si-containing cold-rolled steel sheet according to the present embodiment must contain Si within the above range, but the other components may be allowed within the composition range of ordinary cold-rolled steel sheets. possible and not particularly limited. However, when the Si-containing cold-rolled steel sheet of the present embodiment is to have a high tensile strength (TS) of 590 MPa or more, the following chemical composition is preferable.

C: 0.8% or less (excluding 0%)
C improves workability by forming martensite or the like as a steel structure. When C is contained, the amount of C is preferably 0.8% or less, more preferably 0.30% or less, in order to obtain good weldability. The lower limit of C is not particularly limited, but in order to obtain good workability, the amount of C is preferably more than 0%, more preferably 0.03% or more, and 0.05% or more. is more preferred.

Mn: 1.0% or more and 12.0% or less Mn enhances the strength of the steel by solid-solution strengthening, increases hardenability, and promotes the formation of retained austenite, bainite, and martensite. is an element. Such an effect is exhibited by adding 1.0% or more of Mn. On the other hand, if the Mn content is 12.0% or less, the above effect can be obtained without increasing the cost. Therefore, the Mn content is preferably 1.0% or more and preferably 12.0% or less. The Mn content is more preferably 1.3% or more, still more preferably 1.5% or more, and most preferably 1.8% or more. Also, the Mn content is more preferably 3.5% or less, and more preferably 3.3% or less.

P: 0.1% or less (excluding 0%)
By suppressing the P content, deterioration of weldability can be prevented. Furthermore, the segregation of P at the grain boundaries can be prevented, and the deterioration of ductility, bendability and toughness can be prevented.
Also, when a large amount of P is added, the grain size increases due to the promotion of ferrite transformation. Therefore, it is preferable that the amount of P is 0.1% or less. The lower limit of P is not particularly limited, and may be more than 0% and may be 0.001% or more due to restrictions on production technology.

S: 0.03% or less (excluding 0%)
The S content is preferably 0.03% or less, more preferably 0.02% or less. By suppressing the S content, it is possible to prevent deterioration of weldability, prevent deterioration of ductility during hot working, suppress hot cracking, and significantly improve surface properties. Furthermore, by suppressing the amount of S, coarse sulfides are formed as impurity elements, thereby preventing deterioration of the ductility, bendability, and stretch-flangeability of the steel sheet. These problems become conspicuous when the S content exceeds 0.03%, and it is preferable to reduce the S content as much as possible. The lower limit of S is not particularly limited, and may be more than 0% or 0.0001% or more due to production technology restrictions.

N: 0.010% or less (excluding 0%)
The N content is preferably 0.010% or less. When the N content is 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, impairing the effect of increasing the strength of the steel sheet by adding Ti, Nb, and V. can be prevented. Further, by setting the N content to 0.010% or less, it is possible to prevent deterioration of toughness. Furthermore, by setting the N content to 0.010% or less, it is possible to prevent the occurrence of slab cracks and surface defects during hot rolling. The N content is preferably 0.005% or less, more preferably 0.003% or less, still more preferably 0.002% or less. The lower limit of the N content is not particularly limited, and may be more than 0% due to production technology restrictions, and may be 0.0005% or more.

Al: 1.0% or less (excluding 0%)
Since Al is thermodynamically most oxidizable, it oxidizes before Si and Mn, and has the effect of suppressing the oxidation of Si and Mn in the outermost layer of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is obtained when the amount of Al is 0.01% or more. On the other hand, if the amount of Al exceeds 1.0%, the cost increases. Therefore, when Al is added, the amount of Al is preferably 1.0% or less. More preferably, the Al content is 0.1% or less. The lower limit of Al is not particularly limited, and may be over 0% and may be 0.001% or more.

The component composition is further optionally B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.5% or less. 1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less obtain.

B: 0.005% or less B is an effective element for improving the hardenability of steel. In order to improve hardenability, the B content is preferably 0.0003% or more, more preferably 0.0005% or more. However, if B is added excessively, the moldability is lowered, so the B content is preferably 0.005% or less.

Ti: 0.2% or less Ti is effective for precipitation strengthening of steel. Although the lower limit of Ti is not particularly limited, it is preferably 0.005% or more in order to obtain the effect of adjusting the strength. However, if Ti is added excessively, the hard phase becomes excessively large and the formability deteriorates. is more preferred.

Cr: 1.0% or less The Cr content is preferably 0.005% or more. By setting the Cr content to 0.005% or more, the hardenability can be improved, and the balance between strength and ductility can be improved. When adding Cr, the amount of Cr is preferably 1.0% or less from the viewpoint of preventing cost increase.

Cu: 1.0% or less The amount of Cu is preferably 0.005% or more. By setting the amount of Cu to 0.005% or more, the formation of the residual γ phase can be promoted. Moreover, when Cu content is added, the Cu content is preferably 1.0% or less from the viewpoint of preventing cost increase.

Ni: 1.0% or less The Ni content is preferably 0.005% or more. By setting the Ni content to 0.005% or more, the formation of the residual γ phase can be promoted. Moreover, when Ni is added, the amount of Ni is preferably 1.0% or less from the viewpoint of preventing cost increase.

Mo: 1.0% or less Mo content is preferably 0.005% or more. By setting the Mo amount to 0.005% or more, the strength adjustment effect can be obtained. Moreover, when Mo is added, the amount of Mo is preferably 1.0% or less from the viewpoint of preventing cost increase.

Nb: 0.20% or less When the Nb content is 0.005% or more, the effect of improving the strength can be obtained. Moreover, when Nb is contained, the amount of Nb is preferably 0.20% or less from the viewpoint of preventing cost increase.

V: 0.5% or less When the V content is 0.005% or more, the effect of improving the strength can be obtained. Moreover, when V is contained, the amount of V is preferably 0.5% or less from the viewpoint of preventing cost increase.

Sb: 0.200% or less Sb can be contained from the viewpoint of suppressing nitridation, oxidation, or decarburization in a region of several tens of microns on the steel sheet surface caused by oxidation of the steel sheet surface. Sb suppresses nitridation and oxidation of the steel sheet surface, thereby preventing a decrease in the amount of martensite formed on the steel sheet surface and improving the fatigue properties and surface quality of the steel sheet. In order to obtain such effects, the Sb content is preferably 0.001% or more. On the other hand, in order to obtain good toughness, the Sb content is preferably 0.200% or less.

Ta: 0.1% or less When the Ta content is 0.001% or more, the effect of improving the strength can be obtained. Moreover, when Ta is contained, the amount of Ta is preferably 0.1% or less from the viewpoint of preventing cost increase.

W: 0.5% or less When the W content is 0.005% or more, the effect of improving the strength can be obtained. Moreover, when W is contained, the amount of W is preferably 0.5% or less from the viewpoint of preventing cost increase.

Zr: 0.1% or less When the Zr content is 0.0005% or more, the effect of improving the strength can be obtained. Moreover, when Zr is contained, the amount of Zr is preferably 0.1% or less from the viewpoint of preventing cost increase.

Sn: 0.20% or less Sn is an element that suppresses denitrification, deboronization, etc., and is effective in suppressing a decrease in strength of steel. In order to obtain such an effect, it is preferable that each content be 0.002% or more. On the other hand, in order to obtain good impact resistance, the Sn content is preferably 0.20% or less.

Ca: 0.005% or less By containing 0.0005% or more of Ca, it is possible to control the morphology of sulfides and improve ductility and toughness. Moreover, from the viewpoint of obtaining good ductility, the Ca content is preferably 0.005% or less.

Mg: 0.005% or less By containing 0.0005% or more of Mg, it is possible to control the morphology of sulfides and improve ductility and toughness. Moreover, when Mg is contained, the amount of Mg is preferably 0.005% or less from the viewpoint of preventing cost increase.

REM: 0.005% or less By containing 0.0005% or more of REM, it is possible to control the morphology of sulfides and improve ductility and toughness. Moreover, when REM is contained, the amount of REM is preferably 0.005% or less from the viewpoint of obtaining good toughness.

In the Si-containing cold-rolled steel sheet of this embodiment, the balance other than the above components is Fe and unavoidable impurities.

Next, the Fe-based electroplating layer formed on at least one side of the Si-containing cold-rolled steel sheet described above will be described.
Fe-based electroplating layer: 15.0 g/m ² or more By having an Fe-based electroplating layer with a deposition amount of 15.0 g/m ² or more per side, a Si-containing oxide layer is formed on the surface of the steel sheet during annealing. can be coated to improve chemical conversion treatability and corrosion resistance after painting. In addition, since the Fe-based electroplating layer is soft, it can relax the stress applied to the steel sheet surface during welding. In particular, it is considered that the property of preventing inner cracks can be improved (stress relaxation effect). Furthermore, since the Fe-based electroplating layer functions as a Si-deficient layer, it is possible to suppress deterioration in the toughness of the weld zone due to Si solid solution, and to obtain a steel sheet with excellent resistance weld cracking resistance in the weld zone. Although the mechanism by which the Fe-based electroplating layer with a coating weight per side of 15.0 g/ ^m2 or more improves the resistance weld cracking resistance of welds is not clear, when the amount of solid-solution Si on the steel sheet surface is large, It is considered that the toughness of the weld zone is lowered and the resistance weld cracking resistance of the weld zone is deteriorated. On the other hand, when the steel sheet surface has a Fe-based electroplating layer of a certain amount or more, the Fe-based electroplating layer acts as a solid solution Si depleted layer, and the amount of Si solid solution in the weld decreases. It is thought that the reduction in toughness is suppressed and the resistance weld cracking resistance of the weld zone, especially the resistance to internal cracking, is improved (effect of suppressing deterioration in toughness). In addition, by applying an Fe-based electroplating layer after annealing, the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet to be integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet can be reduced. can be reduced. Therefore, it is possible to prevent the dissolved galvanized layer from entering the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated steel sheet, and the resistance weld cracking resistance at the weld zone, especially It is thought that the property of preventing inner cracks can be improved (zinc intergranular penetration suppression effect). The effects of these Fe-based electroplating layers on the stress relaxation effect, toughness reduction suppression effect, and grain boundary penetration suppression effect of zinc are not quantitatively clarified because of their complexity, but they act in combination. As a result, the resistance weld crack resistance is considered to be improved. If the deposition amount per side of the Fe-based electroplating layer is 3 g/m ² or more, excellent chemical conversion treatability and corrosion resistance after painting can be obtained, but it has the effect of improving resistance weld cracking resistance in welds. For this purpose, it is necessary to set the deposition amount per side of the Fe-based electroplating layer to 15.0 g/m ² or more. Although the upper limit of the amount of the Fe-based electroplating layer attached per side is not particularly limited, from the viewpoint of cost, the amount of the Fe-based electroplating layer attached per side is preferably 60 g/m ² or less. The adhesion amount of the Fe-based electroplating layer is preferably over 15 g/m ² , more preferably 17 g/m ² or more, still more preferably 20 g/m ² or more, most preferably 25 g/m ² or more, or 30 g/m ² . That's it. The Fe-based electroplated steel sheet preferably has an Fe-based electroplated layer on both front and back surfaces of the Si-containing cold-rolled steel sheet. By setting the adhesion amount of the Fe-based electroplating layer to 25 g/m ² or more, the resistance weld cracking resistance at the weld zone, particularly the resistance to internal cracking, becomes particularly good.

The thickness of the Fe-based electroplating layer is measured as follows. A sample having a size of 10×15 mm is taken from an Fe-based electroplated steel sheet and embedded in a resin to obtain a cross-sectional embedded sample. Any three places in the same cross section are observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and a magnification of 2000 to 10000 times depending on the thickness of the Fe-based electroplating layer. By multiplying the average value of the thickness by the specific gravity of iron, it is converted into the adhesion amount per one side of the Fe-based electroplating layer.

As the Fe-based electroplating layer, in addition to pure Fe, Fe--B alloy, Fe--C alloy, Fe--P alloy, Fe--N alloy, Fe--O alloy, Fe--Ni alloy, Fe--Mn alloy, Fe-- Alloy plating layers such as Mo alloys and Fe--W alloys can be used. The component composition of the Fe-based electroplating layer is not particularly limited, but is selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co. Alternatively, it is preferable to have a component composition containing two or more elements in a total of 10% by mass or less, with the balance being Fe and unavoidable impurities. By setting the total amount of elements other than Fe to 10% by mass or less, it is possible to prevent a decrease in electrolysis efficiency and form an Fe-based electroplating layer at low cost. In the case of Fe--C alloy, the C content is preferably 0.08% by mass or less.

In addition, it is preferable that the Fe-based electroplated steel sheet according to the present embodiment does not have a plating layer other than the Fe-based electroplating on the surface. Since the Fe-based electroplated steel sheet does not have a plating layer other than the Fe-based electroplating on the surface, it is used for parts that do not require excessive galvanized steel sheets for rust prevention purposes, or for parts that do not require excessive galvanized steel sheets for rust prevention, or where the corrosion environment is mild and excessive rust prevention is possible. Parts that are used in environments where they are not required can be provided at low cost.

The ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is not particularly limited, but may be 50% or less. In the present embodiment, since the Si-containing cold-rolled steel sheet is annealed and then Fe-based electroplated, and then not annealed, the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated. low rate of conversion. Therefore, it is expected that the melted galvanized steel sheet will be prevented from entering the grain boundary of the Si-containing cold-rolled steel sheet via the grain boundary of the Fe-based electroplated steel sheet. In particular, it can be expected that the property of preventing inner cracks will be further improved. In the Fe-based electroplated steel sheet according to the present embodiment, the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. It can be 30% or less, and can be 25% or less.

Here, the ratio at which the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is measured as follows. A sample of 10×10 mm size is taken from a Fe-based electroplated steel sheet. An arbitrary point of the sample is processed with a focused ion beam (FIB) device, and the T cross section (a cross section parallel to the rolling direction of the steel plate and perpendicular to the steel plate surface) direction is 45 °. A 45° section having a width of 30 μm in the direction perpendicular to the rolling direction and a length of 50 μm in the direction of 45° with respect to the T section direction is formed at one location to obtain a sample for observation. FIG. 4 shows an overview of the observation sample. FIG. 4(a) is a perspective view of an observation sample. FIG. 4(b) is a cross-sectional view of the observation sample shown in FIG. 4(a) taken along the line AA. Then, using a scanning ion microscope (SIM), the central part of the 45 ° cross section of the observation sample is observed at a magnification of 5000 times, and an 8-bit SIM image of width 1024 × height 943 pixels is taken. . From the SIM images taken at each 45° section prepared at three locations, based on the following formula (1), the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is Obtain the ratio of the crystal orientation integrated with the rolled steel sheet. The decimal point is rounded up.
(Percentage at which the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet) = (Fe-based electroplated layer and Si-containing cold-rolled steel sheet Of the interface with the steel sheet, the length of the portion where the crystal orientation of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated) ÷ (the length of the interface in the observation field) × 100 (1 )

Image processing is used to determine whether the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet. Using FIG. 5, a method for evaluating the ratio of integrated crystal orientations will be described. First, as shown in FIG. 5(a), the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in the SIM image described above was scanned with a scanning electron microscope (SIM), and a boundary line B was observed. to draw. Next, an image is produced by performing image processing on the SIM image separately from the image in which the front boundary line is drawn. Specifically, first, grain boundaries are emphasized using a Sobel filter for an 8-bit SIM image of width 1024×height 943 pixels. Subsequently, the image in which the grain boundaries are emphasized is smoothed by a Gaussian filter (radius (R): 10 pixels). Next, binarization processing (threshold value: 17) is performed on the image after the smoothing processing. Subsequently, the boundary line B of the image representing the interface is transferred to the binarized image. After that, as shown in FIG. 5(b), in the image after the binary processing, the determination area with a width of 40 pixels centered on the boundary line B (the area surrounded by L ₁ and L ₂ in FIG. 5(b)) is drawn along the boundary line B on the binarized image. Of the length of the boundary line B, the total length where there is no interface (black and white boundary on the binarized image) between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in the judgment area, It is regarded as the length over which the crystal orientations are integrated. Here, among the lengths of the boundary lines, the sum of the lengths in which the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet does not exist in the determination region is obtained as follows. For the sake of explanation, FIG. 5(c) shows an enlarged view of the part surrounded by a square in FIG. 5(b). First, as shown in Fig. 5(c), the two normals to boundary line B (l ₁ and l ₂ and l ₃ and l ₄ in Fig. 5(c)) include only one color, either black or white. The entire determination area is searched for locations where the determination area can be divided into substantially rectangular shapes so that the determination area can be divided. Next, the maximum distance between the intersections of the boundary line and the two normal lines at that point is totaled over the entire judgment area, and the Fe-based electroplating layer and the Si-containing cold It is the total length where there is no interface with the rolled steel sheet.

FIG. 2 shows invention example No. 2 of an embodiment to be described later. 43 shows a SIM image of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet for No. 43. FIG. 6 shows an image after the SIM image is subjected to image processing and binary processing as described above. Invention Example No. In No. 43, the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet being integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet was 6%. FIG. 3 shows a SIM image of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet for Invention Example No. 46, which will be described later. FIG. 7 shows an image after the SIM image is subjected to image processing and binary processing as described above. In Invention Example No. 46, the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet being integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet was 10%. rice field.

<Method for producing Fe-based electroplated steel sheet>
Next, a method for manufacturing an Fe-based electroplated steel sheet will be described.
A method for producing an Fe-based electroplated steel sheet according to one embodiment comprises annealing a pre-annealed cold-rolled steel sheet containing Si containing 0.5% by mass or more and 3.0% by mass or less of Si to obtain a cold-rolled steel sheet containing Si,
Next, the Si-containing cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet having an Fe-based electroplating layer with a deposition amount of 15.0 g/m ² or more per side formed on at least one side. , a method for producing an Fe-based electroplated steel sheet.

First, a Si-containing pre-annealing cold-rolled steel sheet containing 0.5% by mass or more and 3.0% by mass or less of Si is produced. The manufacturing method of the Si-containing pre-annealed cold-rolled steel sheet can follow the manufacturing method of a normal cold-rolled steel sheet. In one example, the Si-containing pre-annealed cold-rolled steel sheet is obtained by hot-rolling a steel slab having the chemical composition described above to obtain a hot-rolled sheet, then pickling the hot-rolled sheet, and then cooling the hot-rolled sheet. It is produced by subjecting it to rolling to obtain a Si-containing pre-annealed cold-rolled steel sheet.

Next, the Si-containing pre-annealed cold-rolled steel sheet is annealed to obtain a Si-containing cold-rolled steel sheet before being subjected to Fe-based electroplating. The annealing conditions are not particularly limited, but in one example, in a reducing atmosphere with a dew point of 30°C or less and a hydrogen concentration of 1.0% by volume or more and 30.0% by volume or less, the temperature range is 650°C or more and 900°C or less. Annealing step of cooling is performed after holding for 30 seconds or more and 600 seconds or less to obtain a Si-containing cold-rolled steel sheet. Annealing is performed to remove the strain in the cold-rolled steel sheet caused by the rolling process and recrystallize the structure, thereby increasing the strength of the steel sheet.

Hydrogen concentration: 1.0% by volume or more and 30.0% by volume or less In one example, the annealing step can be performed in a reducing atmosphere with a hydrogen concentration of 1.0% by volume or more and 30.0% by volume or less. Hydrogen is necessary to suppress the oxidation of Fe on the surface of the Si-containing cold-rolled steel sheet before annealing during the annealing process and to activate the steel sheet surface. If the hydrogen concentration is 1.0% by volume or more, it is possible to avoid deterioration of adhesion of the Fe-based electroplating layer due to oxidation of Fe on the surface of the steel sheet. Therefore, the annealing step is preferably performed in a reducing atmosphere with a hydrogen concentration of 1.0% by volume or more, more preferably in a reducing atmosphere with a hydrogen concentration of 2.0% by volume or more. Although the upper limit of the hydrogen concentration in the annealing step is not particularly limited, from the viewpoint of cost, the hydrogen concentration is preferably 30.0% by volume or less, more preferably 20.0% by volume or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

Dew point: 30°C or less In one example, the annealing step can be performed with the dew point of the annealing atmosphere being 30°C or less. In the annealing process, by setting the dew point of the annealing atmosphere to 30°C or less, oxidation of the surface of the Si-containing cold-rolled steel sheet before annealing can be prevented and the adhesion of the Fe-based electroplating layer can be further improved. The dew point is preferably 30°C or less. The dew point of the annealing atmosphere is more preferably 20°C or less. Although the lower limit of the dew point of the annealing atmosphere is not specified, it is set to -80°C or higher because it is industrially difficult to achieve a dew point of less than -80°C. The dew point of the annealing atmosphere is preferably -55°C or higher.

Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds In the annealing step, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds to 600 seconds. . By setting the holding time in the temperature range to 30 seconds or more, the natural oxide film of Fe formed on the surface of the Si-containing cold-rolled steel sheet before annealing is suitably removed, and the Fe-based electroplating layer to be formed later is oxidized directly below. The adhesion of the Fe-based electroplating layer can be improved by preventing the formation of a film. Therefore, it is preferable to set the retention time in the temperature range to 30 seconds or longer. Although the upper limit of the holding time in the temperature range is not particularly defined, the holding time in the temperature range is preferably 600 seconds or less from the viewpoint of productivity.

Maximum temperature reached by Si-containing cold-rolled steel sheet before annealing: 650°C or higher and 900°C or lower The maximum temperature reached by the Si-containing cold-rolled steel plate before annealing is not particularly limited, but is preferably 650°C or higher and 900°C or lower. By setting the maximum temperature of the Si-containing pre-annealed cold-rolled steel sheet to 650°C or higher, recrystallization of the steel sheet structure favorably proceeds, and a suitable strength can be obtained. In addition, the natural oxide film of Fe formed on the surface of the steel sheet is preferably reduced, and the adhesion of the Fe-based electroplating layer is further improved. In addition, if the maximum temperature of the Si-containing cold-rolled steel sheet before annealing is 900 ° C or less, it prevents the diffusion rate of Si and Mn in the steel from increasing extremely, and prevents the diffusion of Si and Mn to the steel sheet surface. Therefore, the adhesion of the Fe-based electroplating layer is further improved. Moreover, if the highest temperature reached is 900° C. or less, damage to the furnace body of the heat treatment furnace can be prevented, and the cost can be reduced. Therefore, the maximum temperature reached by the Si-containing pre-annealed cold-rolled steel sheet is preferably 900° C. or less. The maximum temperature reached is based on the temperature measured on the surface of the Si-containing pre-annealed cold-rolled steel sheet.

Then, the surface of the Si-containing cold-rolled steel sheet is subjected to Fe-based electroplating. The Fe-based electroplating method is not particularly limited. For example, as the Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be applied.

The Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 0.5 mol/L or more as Fe ²⁺ . If the Fe ion content in the Fe-based electroplating bath is 0.5 mol/L or more as Fe ²⁺ , a sufficient Fe deposition amount can be obtained. Further, in order to obtain a sufficient Fe deposition amount, it is preferable that the Fe ion content in the Fe-based electroplating bath before the start of energization is 2.0 mol/L or less.

Further, in the Fe-based electroplating bath, Fe ions and at least It can contain one element. The total content of these elements in the Fe-based electroplating bath is preferably 10% by mass or less in the Fe-based electroplating layer. Metal elements may be contained as metal ions, and non-metal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, and the like. Further, the iron sulfate plating solution may contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers. In addition, after the Fe-based electroplating treatment, it is preferable not to perform additional annealing on the Fe-based electroplated steel sheet.

Other conditions of the Fe-based electroplating bath are not particularly limited. The temperature of the Fe-based electroplating solution is preferably 30.degree. C. or higher and preferably 85.degree. Although the pH of the Fe-based electroplating bath is not particularly specified, it is preferably 1.0 or more from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation. 0 or less is preferable. The current density is preferably 10 A/dm ² or more from the viewpoint of productivity, and preferably 150 A/dm ² or less from the viewpoint of facilitating control of the amount of deposition of the Fe-based electroplating layer. The sheet threading speed is preferably 5 mpm or more from the viewpoint of productivity, and preferably 150 mpm or less from the viewpoint of stably controlling the adhesion amount.

Before the Fe-based electroplating treatment, degreasing and water washing for cleaning the surface of the Si-containing cold-rolled steel sheet, and pickling and washing for activating the surface of the Si-containing cold-rolled steel sheet. It can be washed with water. Following these pretreatments, an Fe-based electroplating treatment is performed. Methods of degreasing treatment and washing with water are not particularly limited, and ordinary methods can be used. Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used in the pickling treatment. Among them, sulfuric acid, hydrochloric acid, or a mixture thereof is preferable. Although the concentration of the acid is not specified, it is preferably about 1 to 20% by mass in consideration of the ability to remove the oxide film and the prevention of surface roughness (surface defects) due to over-acid washing. In addition, the pickling treatment liquid may contain antifoaming agents, pickling accelerators, pickling inhibitors, and the like.

<Electrodeposition coated steel plate>
Further, according to the present embodiment, on the above-described Fe-based electroplated steel sheet, a chemical conversion treatment film formed in contact with the Fe-based electroplating layer, and an electrodeposition coating film formed on the chemical conversion film. It is also possible to provide an electrodeposition coated steel sheet further having Since the Fe-based electroplated steel sheet according to the present embodiment is excellent in chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance in welds, the electrodeposition coated steel sheet formed using the Fe-based electroplated steel sheet is It is particularly suitable for application to automotive parts. In the electrodeposition coated steel sheet according to the present embodiment, it is preferable that a chemical conversion treatment film is formed directly on the Fe-based electroplating layer. In other words, the electrodeposition coated steel sheet according to this embodiment preferably does not have an additional plating layer in addition to the Fe-based electroplating layer. The types of the chemical conversion treatment film and the electrodeposition coating film are not particularly limited, and known chemical conversion treatment films and electrodeposition coating films can be used. A zinc phosphate film, a zirconium film, or the like can be used as the chemical conversion coating. The electrodeposition coating film is not particularly limited as long as it is an electrodeposition coating for automobiles. Although the thickness of the electrodeposition film varies depending on the application, it is preferable that the thickness of the dry film is about 10 μm to 30 μm. Further, according to this embodiment, it is possible to provide an Fe-based electroplated steel sheet for electrodeposition coating.

<Manufacturing method of electrodeposition coated steel sheet>
Next, a method for manufacturing the above-described electrodeposition coated steel sheet will be described. The above-described electrodeposition coated steel sheet is obtained by subjecting the Fe-based electroplated steel sheet to a chemical conversion treatment without performing an additional plating treatment to obtain a chemically treated steel sheet in which a chemical conversion treatment film is formed in contact with the Fe-based electroplated layer. , a chemical conversion treatment step, and an electrodeposition coating step of subjecting the chemical conversion treated steel sheet to an electrodeposition coating treatment to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion coating, It can be manufactured by a method for manufacturing a coated steel plate. Chemical conversion treatment and electrodeposition coating treatment can be carried out by known methods. Before the chemical conversion treatment, degreasing treatment for cleaning the surface of the Fe-based electroplated steel sheet, washing with water, and, if necessary, surface conditioning treatment can be performed. Following these pretreatments, a chemical conversion treatment is carried out. Methods of degreasing treatment and washing with water are not particularly limited, and ordinary methods can be used. In the surface conditioning treatment, a Ti colloid or a surface conditioning agent containing zinc phosphate colloid can be used. There is no need to provide a special process for applying these surface conditioners, and the process may be carried out according to a conventional method. For example, after dissolving the desired surface conditioner in a predetermined deionized water and sufficiently stirring it, a predetermined temperature (usually normal temperature, 25 to 30° C.) of the treatment liquid is used, and the steel sheet is placed in the treatment liquid for a predetermined time ( 20-30 seconds) to soak. Then, without drying, the next step of chemical conversion treatment is performed. The chemical conversion treatment may also be carried out according to a conventional method. For example, a desired chemical conversion treatment agent is dissolved in a prescribed deionized water, stirred sufficiently, and then treated with a treatment solution at a prescribed temperature (usually 35 to 45°C). second) to be immersed. As the chemical conversion treatment agent, for example, a zinc phosphating agent for steel, a zinc phosphating agent for both steel and aluminum, and a zirconium treatment agent can be used. Subsequently, the electrodeposition coating of the next process is performed. Electrodeposition coating may also be carried out according to a conventional method. After pretreatment such as washing with water, if necessary, the steel sheet is immersed in a sufficiently stirred electrodeposition paint to obtain an electrodeposition coating having a desired thickness. As the electrodeposition coating, not only cationic electrodeposition coating but also anionic electrodeposition coating can be used. Furthermore, depending on the application, top coating may be applied after the electrodeposition coating.

<Automotive parts>
Further, according to the present embodiment, it is possible to provide an automobile part at least partially using the above-described electrodeposition coated steel sheet. Since the Fe-based electroplated steel sheet according to the present embodiment is excellent in chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance in welds, the electrodeposition coated steel sheet using the Fe-based electroplated steel sheet is suitable for automotive parts. It is particularly suitable for application to Automobile parts using the electrodeposition coated steel sheet may include steel sheets other than the electrodeposition coated steel sheet according to the present embodiment as materials. Since the electrodeposition-coated steel sheet according to the present embodiment is excellent in resistance weld cracking resistance at the welded portion, it is possible even when the automobile parts using the Fe-based electroplated steel sheet includes a high-strength galvanized steel sheet as a welding partner. However, it is possible to suitably prevent the occurrence of secondary cracks in the weld. Although there is no particular limitation on the types of automobile parts at least partly made of the electrodeposition coated steel sheet, they can be side sill parts, pillar parts, automobile bodies, and the like.

[Embodiment 2]
Next, an Fe-based electroplated steel sheet according to Embodiment 2 of the present invention will be described.
The Fe-based electroplated steel sheet according to this embodiment is
a cold-rolled steel sheet;
An Fe-based electroplated steel sheet having an Fe-based electroplated layer having a coating amount of 15.0 g/m ² or more per side formed on at least one side of a cold-rolled steel sheet.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet.

The cold-rolled steel sheet according to the present embodiment is not particularly limited as long as the steel sheet is inferior in resistance weld cracking resistance at the weld zone when the sheet assembly partner is a galvanized steel sheet when evaluated by the following test. The chemical composition of the cold-rolled steel sheet is not particularly limited, either. We have obtained the knowledge that the weld cracking property is inferior. After forming the Fe-based electroplating layer, when evaluating the resistance weld cracking characteristics of the weld zone when the cold-rolled steel sheet is paired with a galvanized steel sheet by the test described later, the Fe-based electroplating A cold-rolled steel sheet having the same Si content without forming a layer may be separately prepared and subjected to the following test. If the Si content is the same, the same evaluation results can be obtained with respect to the resistance weld cracking resistance of the weld zone when the plate pair is a galvanized steel sheet. By using a cold-rolled steel sheet containing Si, it is possible to indirectly evaluate the resistance weld cracking resistance in the weld zone of the cold-rolled steel sheet used in the Fe-based electroplated steel sheet.

<Resistance Weld Cracking Characteristics in Welded Portion When Sheet Assembly Mate is Galvanized Steel Sheet>
A method for evaluating the resistance weld cracking resistance of welds will be described with reference to FIG. A test piece 6 cut into a size of 50 x 150 mm with the direction perpendicular to the rolling direction (TD) as the length was overlapped with a hot-dip galvanized steel sheet 5 cut into the same size and having a hot-dip galvanized layer with a coating amount of 50 g/ ^m2 per side. It will be a board assembly. The plate assembly is assembled so that the surface to be evaluated (Fe-based electroplating layer) of the test piece 6 and the galvanized layer of the alloyed hot-dip galvanized steel sheet 5 for testing face each other. The plate assembly is fixed to a fixing base 8 via a spacer 7 having a thickness of 2.0 mm. The spacer 7 is a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness. As shown in FIG. Arrange so that both ends are aligned. Therefore, the distance between the pair of steel plates is 60 mm. The fixed base 8 is a single plate with a hole in the center.

Next, using a single-phase AC (50 Hz) resistance welding machine with a servo motor pressurization, the plate assembly is pressed with a pair of electrodes 9 (tip diameter: 6 mm) while bending the plate assembly. A set of welding current and welding time at which the welding force is 3.5 kN, the hold time is 0.02 seconds or 0.1 seconds, and the nugget diameter is 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. Resistance welding is applied under certain conditions to make a plate assembly with a welded part. At this time, the pair of electrodes 9 presses the plate assembly from above and below in the vertical direction, and the lower electrode presses the test piece 6 through the hole of the fixing table 8 . When pressurizing, the lower electrode and the fixing base 8 are fixed so that the lower electrode of the pair of electrodes 9 is in contact with the plane extending the surface where the spacer 7 and the fixing base 8 contact, and the upper electrode is pressed. Make the electrodes movable. Also, the upper electrode is brought into contact with the central portion of the test alloyed hot-dip galvanized steel sheet 5 . In addition, the plate assembly is arranged 5 times in the longitudinal direction of the plate assembly with respect to the vertical plane (horizontal direction in FIG. 8(a)) to the line connecting the center axes of the electrode pairs of the resistance welder. °Perform welding in an inclined state. A space of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly is formed between the lower electrode and the test piece 6 by the above spacer. Here, the hold time refers to the time from the end of the welding current to the start of opening the electrode. Here, referring to the lower part of FIG. 8(b), the nugget diameter r means the distance between the ends of the nugget 10 in the longitudinal direction of the plate assembly. When the thickness of the cold-rolled steel sheet is t (mm) and the nugget diameter r satisfies the relationship of 4√t or more and 5√5 or less, the problem of inner cracks tends to occur particularly easily.

Next, the plate set with the welded part is cut in half so as to include the center of the welded part including the nugget 10, and the cross section of the welded part is observed with an optical microscope (200 times). Evaluate resistance weld crack resistance. If it is ○ or △, it is judged that the resistance weld cracking resistance of the weld zone is excellent. If it is ×, it is judged that the resistance weld crack resistance in the weld is inferior.
○: No cracks with a length of 0.1 mm or longer were observed at a hold time of 0.02 seconds. △: Cracks with a length of 0.1 mm or longer were observed with a hold time of 0.02 seconds, but the hold time was 0. 0.1 mm or longer crack is not observed in 1 second ×: 0.1 mm or longer crack is observed at hold time of 0.1 second

An example of a crack generated in the test piece 6 is schematically indicated by reference numeral 11 in the lower part of FIG. 8(b).

The Fe-based electroplated layer of the Fe-based electroplated steel sheet according to the present embodiment is the same as that of the above-described Embodiment 1, so the description is omitted here. In addition, the ratio of the crystal orientation of the Fe-based electroplating layer and the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet is not particularly limited, but is 50% as in the first embodiment. can be: The details of the ratio of the crystal orientations of the Fe-based electroplating layer and the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet are the same as in Embodiment 1 described above, so they will not be described here. omitted.

Next, a method for manufacturing an Fe-based electroplated steel sheet according to Embodiment 2 will be described.
A method for manufacturing an Fe-based electroplated steel sheet according to one embodiment includes annealing a pre-annealed cold-rolled steel sheet to obtain a cold-rolled steel sheet,
Then, the cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet having an Fe-based electroplating layer formed on at least one side with a deposition amount of 15.0 g/m ² or more per side. It can be a method for producing a system electroplated steel sheet.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet.

First, a pre-annealed cold-rolled steel sheet is manufactured. The manufacturing method of the pre-annealed cold-rolled steel sheet can follow the manufacturing method of ordinary cold-rolled steel sheets. In one example, a steel slab is hot-rolled into a hot-rolled sheet, then the hot-rolled sheet is pickled, and then the hot-rolled sheet is cold-rolled to produce a pre-annealed cold-rolled steel sheet.

Next, before the Fe-based electroplating treatment, the pre-annealed cold-rolled steel sheet is subjected to an annealing process to obtain a cold-rolled steel sheet. The conditions of the annealing step are not particularly limited, but for example, in a reducing atmosphere with a dew point of 30°C or less and a hydrogen concentration of 1.0% by volume or more and 30.0% by volume or less, in a temperature range of 650°C or more and 900°C or less. It may be an annealing step of cooling after holding for 30 seconds or more and 600 seconds or less. Since the details of the annealing process have been described above, the description is omitted here.

Then, the surface of the annealed cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet. Since the details of the Fe-based electroplating process have been described above, the description is omitted here.

When the cold-rolled steel sheet according to this embodiment is evaluated by the above-described test after annealing and before forming the Fe-based electroplating layer, resistance welding at the weld zone when the plate pair is a galvanized steel sheet. There is no particular limitation as long as the steel sheet is inferior in cracking properties. The chemical composition of the cold-rolled steel sheet is not particularly limited, and if the cold-rolled steel sheet has a Si content of 0.5% by mass or more in the steel, the resistance weld cracking resistance at the weld, which is evaluated by the following test, is inferior. . When evaluating the resistance weld cracking characteristics of the weld zone when the plate pair is a galvanized steel sheet by the above-mentioned test after forming the Fe-based electroplating layer, the Fe-based electroplating layer is not formed. By separately preparing a cold-rolled steel sheet with the same Si content and conducting the following test, the resistance weld crack resistance in the weld zone of the cold-rolled steel sheet used in the Fe-based electroplated steel sheet can be indirectly evaluated. can do.

In this embodiment, as in the above-described Embodiment 1, on the Fe-based electroplated steel sheet according to this embodiment, a chemical conversion coating formed in contact with the Fe-based electroplating layer, and on the chemical conversion coating It is also possible to provide an electrodeposition coated steel sheet further having an electrodeposition coating film formed thereon. Further, it is also possible to provide an Fe-based electroplated steel sheet for electrodeposition coating for applying electrodeposition coating. The details of the electrodeposition-coated steel sheet and the manufacturing method of the electrodeposition-coated steel sheet are the same as those of the above-described Embodiment 1, so the description is omitted here.

Further, in this embodiment as well, it is possible to provide automobile parts as in the case of the above-described first embodiment. Since the details of the automobile parts have been described above, they are omitted here.

Hereinafter, the present invention will be specifically described based on examples.

A slab obtained by melting a steel having the chemical composition shown in Table 1 was hot-rolled, pickled and cold-rolled into a cold-rolled steel sheet having a thickness of 1.4 mm.

Then, the cold-rolled steel sheet was subjected to reduction annealing under the conditions of 15% H ₂ -N ₂ , soaking zone temperature of 800°C and dew point of -40°C. Reduction annealing was performed for 100 seconds. Subsequently, the steel sheet was degreased with an alkali, and then subjected to electrolytic treatment under the following conditions using the steel sheet as a cathode. An electroplated steel sheet was produced. The adhesion amount of the Fe-based electroplating layer was controlled by the energization time.
[Electrolysis conditions]
Bath temperature: 50°C
pH: 2.0
Current density: 45A/ ^dm2
Fe-based electroplating bath: Electrode containing 1.5 mol/L of Fe ²⁺ ions (anode): Iridium oxide electrode

From the Fe-based electroplated steel sheet produced as described above, according to the method described above, the amount of adhesion per side of the Fe-based electroplated layer, and the Fe-based electroplated layer at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet The proportion of the crystal orientations of the plating layer and the Si-containing cold-rolled steel sheet that are integrated was determined.

The Fe-based electroplated steel sheets thus obtained were examined for chemical conversion treatability, post-coating corrosion resistance, and resistance weld cracking resistance at welds. The measurement method and evaluation method are shown below.

Chemical conversion treatment property and corrosion resistance after painting (1) Chemical conversion treatment A test piece taken from the above Fe-based electroplated steel sheet is subjected to degreasing treatment, surface conditioning treatment, and chemical conversion treatment, and the front and back surfaces of the test piece have a chemical conversion coating. A chemically treated test piece was produced. First, a test piece taken from the Fe-based electroplated steel sheet was immersed in a degreasing agent and degreased under the following standard conditions.
[Degreasing treatment]
・ Degreasing agent: FC-E2011 (manufactured by Nihon Parkerizing Co., Ltd.)
・Processing temperature: 43°C
・Processing time: 120 seconds

After the degreasing treatment, the test piece was sprayed with a surface conditioning agent and subjected to surface conditioning treatment under the following standard conditions.
[Surface adjustment treatment]
・ Surface conditioner: Preparen X (PL-X: manufactured by Nihon Parkerizing Co., Ltd.)
・pH: 9.5
・Processing temperature: room temperature ・Processing time: 20 seconds

Then, the test piece after the surface conditioning treatment was immersed in a chemical conversion treatment agent and subjected to chemical conversion treatment under the following standard conditions.
[Chemical treatment]
・ Chemical conversion agent: Palbond PB-SX35 (manufactured by Nihon Parkerizing Co., Ltd.)
・Temperature of chemical conversion treatment solution: 35°C
・Processing time: 90 seconds

Using the chemically treated test pieces manufactured as described above, the chemically convertible property described later was measured.

(2) Electrodeposition coating treatment On the surface of the chemical conversion treatment test piece, using an electrodeposition paint: GT-100 manufactured by Kansai Paint Co., Ltd., electrodeposition coating was applied so that the film thickness was 15 μm, and the electrodeposition coating test was performed. cut to pieces. The electrodeposition coating test piece was subjected to a salt water immersion test described later.

<Chemical conversion treatability>
The surface of the chemically treated test piece (n=1) was observed with an SEM at a magnification of 1000 times and evaluated according to the following criteria. In addition, it was judged to be excellent in chemical convertibility if it was (double-circle) or (circle).
◎: The grain size of the chemical crystal is 5 μm or less and no unprecipitated portion is observed ○: The grain size of the chemical crystal is 5 μm or more, but no unprecipitated portion is observed ×: The grain size of the chemical crystal is 5 μm or more and no unprecipitated portion is observed Precipitation part is recognized

<Salt water immersion test>
After applying a cross-cut flaw of 45 mm in length to the surface of the electrodeposition coating test piece (n = 1) with a cutter, this test piece was immersed in a 5 mass% NaCl solution (60 ° C.) for 360 hours, and then It was washed with water and dried. Next, an adhesive tape was attached to the cross-cut flaws of the test piece, and then a tape peeling test was conducted by peeling it off, and the maximum peeling width of the electrodeposition coating film was measured on both sides of the cross-cut flaws. The maximum peeling width of the electrodeposition coating film was evaluated according to the following criteria. In addition, it was judged that the corrosion resistance after painting was excellent if it was ⊚ or ◯.
A: The maximum peel width is 3.0 mm or less. B: The maximum peel width is 5.0 mm or less. ×: The maximum peel width is over 5.0 mm.

<Resistance Weld Cracking Characteristics in Welded Portion When Sheet Assembly Mate is Galvanized Steel Sheet>
Regarding the Fe-based electroplated steel sheet, according to the method described above, the sheet pair is an alloyed hot-dip galvanized steel sheet (sheet thickness 1.4 mm) having a tensile strength of 980 MPa class and a coating amount per side of 50 g / m ² The resistance weld cracking resistance properties of the welds in certain cases were evaluated. The welding time was 18 cycles (50 Hz), and Example No. The nugget diameter was measured by changing the welding current every time, and the welding current at which the nugget diameter was 5.3 mm was used for evaluation.

The results of the above tests are also shown in Table 2. From these results, the Fe-based electroplated steel sheets of the invention examples, in which the Fe-based electroplated layer was formed under the conditions suitable for the present invention after continuous annealing, were found to have good chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance at welds. It can be seen that both are excellent. In Reference Examples 1 and 2, since the Si content was less than 0.5%, there was no particular problem with the chemical conversion treatability or the resistance weld cracking resistance of the weld zone. In each invention example in which the amount of the Fe-based electroplating layer was 25 g/m ² or more, no cracks with a length of 0.1 mm or more were observed even under the condition of a hold time of 0.02 seconds. Resistance weld crack resistance was particularly good.

The Fe-based electroplated steel sheet produced by the present invention is excellent in chemical conversion treatability, corrosion resistance after painting, and resistance weld cracking resistance at the weld zone when the plate pair is a galvanized steel sheet, especially in preventing internal cracking. In addition, it has high strength and excellent workability, so it is suitable not only as a material for automobile parts, but also as a material for applications that require similar characteristics in fields such as home appliances and building materials. can be used for

REFERENCE SIGNS LIST 1 Fe-based electroplated steel sheet 2 Si-containing cold-rolled steel sheet 3 Fe-based electroplated layer 5 Galvannealed steel sheet for testing 6 Test piece 7 Spacer 8 Fixing table 9 Electrode 10 Nugget 11 Crack

Claims (11)

a Si-containing cold-rolled steel sheet containing 0.5% by mass or more and 3.0% by mass or less of Si;
An Fe electroplated steel sheet having an Fe electroplated layer formed on at least one side of the Si-containing cold-rolled steel sheet and having an adhesion amount per side of 15.0 g/m ² or more and comprising Fe and unavoidable impurities. The ratio of the crystal orientations of the Fe electroplating layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe electroplating layer and the Si-containing cold-rolled steel sheet is 50% or less. The Fe electroplated steel sheet according to Item 1. The Fe electroplated steel sheet according to claim 1 or 2, wherein the Fe electroplating layer has a coating weight per side of 25 g/ ^m2 or more. The Si-containing cold-rolled steel sheet contains, in addition to the Si,
C: 0.8% or less,
Mn: 1.0% or more and 12.0% or less,
P: 0.1% or less,
S: 0.03% or less,
The Fe electricity according to any one of claims 1 to 3, having a component composition containing N: 0.010% or less and Al: 1.0% or less, with the balance being Fe and inevitable impurities Galvanized steel sheet. The component composition is further, in mass%,
B: 0.005% or less,
Ti: 0.2% or less,
Cr: 1.0% or less,
Cu: 1.0% or less,
Ni: 1.0% or less,
Mo: 1.0% or less,
Nb: 0.20% or less,
V: 0.5% or less,
Sb: 0.200% or less,
Ta: 0.1% or less,
W: 0.5% or less,
Zr: 0.1% or less,
Sn: 0.20% or less,
Ca: 0.005% or less,
The Fe electroplated steel sheet according to claim 4, containing one or more selected from the group consisting of Mg: 0.005% or less and REM: 0.005% or less. a cold-rolled steel sheet;
An Fe electroplated steel sheet having an Fe electroplated layer formed on at least one side of the cold-rolled steel sheet and having an adhesion amount per side of 15.0 g/m ² or more and comprising Fe and inevitable impurities.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet. A chemical conversion coating formed on the Fe electroplated steel sheet according to any one of claims 1 to 6 in contact with the Fe electroplating layer, and a chemical conversion coating formed on the chemical conversion coating An electrodeposition coated steel sheet, further comprising an electrodeposition coating film. An automobile part, at least partly of which the electrodeposition coated steel sheet according to claim 7 is used. The Fe electroplated steel sheet according to any one of claims 1 to 6 is subjected to chemical conversion treatment without additional plating treatment, and a chemical conversion coating is formed in contact with the Fe electroplated layer. a chemical conversion treatment step for obtaining a chemically treated steel sheet;
an electrodeposition coating step of subjecting the chemically treated steel sheet to an electrodeposition coating process to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion coating;
A method for manufacturing an electrodeposition coated steel sheet, including An Si-containing cold-rolled steel sheet before annealing containing 0.5% by mass or more and 3.0% by mass or less of Si is annealed to obtain a Si-containing cold-rolled steel sheet,
Next, the Si-containing cold-rolled steel sheet was subjected to Fe electroplating , and an Fe electroplating layer composed of Fe and inevitable impurities was formed on at least one side with an adhesion amount per side of 15.0 g/m ² or more. A method for producing an Fe electroplated steel sheet for obtaining an Fe electroplated steel sheet. The pre-annealed cold-rolled steel sheet is annealed to form a cold-rolled steel sheet,
Next, the cold-rolled steel sheet is subjected to Fe electroplating , and Fe having an adhesion amount per side of 15.0 g/m ² or more and an Fe electroplating layer composed of Fe and inevitable impurities formed on at least one side A method for producing an Fe electroplated steel sheet to obtain an electroplated steel sheet.
Here, the cold-rolled steel sheet is a test piece cut into a size of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and a hot-dip galvanized layer having a coating amount of 50 g / m ² per side of the hot-dip galvanized layer cut into the same size. Stacked with steel plates to form a board assembly,
Next, using a single-phase alternating current (50 Hz) resistance welder with a servo motor pressure type, the plate assembly is tilted 5° with respect to the electrode (tip diameter 6 mm) of the resistance welder. , pressure: 3.5 kN, hold time: 0.1 second, and welding current and welding time conditions that make the nugget diameter 4.5 √t mm, where t is the thickness of the cold-rolled steel sheet. and a plate assembly with a welded part,
Next, when the plate assembly with the welded part is cut in half so as to include the welded part and the cross section of the welded part is observed with an optical microscope (200 times), a crack with a length of 0.1 mm or more is observed. It is a cold-rolled steel sheet.

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