JP7578118B2 - Fe-based electroplated high strength steel sheet and manufacturing method thereof - Google Patents

Description

The present invention relates to an Fe-based electroplated high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after painting, as well as excellent delayed fracture resistance, and a method for manufacturing the same.

In recent years, there has been a strong demand for improved fuel efficiency in automobiles from the standpoint of protecting the global environment. In addition, there has been a strong demand for improved automobile safety from the standpoint of ensuring the safety of passengers in the event of a collision. In order to meet these demands, it is important to simultaneously achieve a reduction in the weight and strength of automobile bodies, and there has been active progress in increasing the strength and thinning of cold-rolled steel sheets, which are the raw material for automobile parts. Furthermore, since many automobile parts are manufactured by forming steel sheets, good formability is also required in addition to high strength.

Here, one method for increasing the strength of cold-rolled steel sheets without impairing their formability is solid solution strengthening by adding Si. However, it is known that when a large amount of Si, particularly 0.5 mass% or more of Si, is added to cold-rolled steel sheets, Si-containing oxides such as SiO ₂ and Si-Mn composite oxides are formed on the steel sheet surface during slab heating or annealing after hot rolling or cold rolling. The formed Si-containing oxides significantly reduce the chemical conversion treatability, and therefore not only are the chemical conversion treatability poor, but in salt water tests after electrocoating, the coating film is more likely to peel off than in normal steel sheets, and the corrosion resistance after coating is also poor.

It is also known that increasing the strength of cold-rolled steel sheets increases the likelihood of delayed fracture. This tendency is particularly pronounced in high-strength steels with a tensile strength of 980 MPa or more.
Delayed fracture is a phenomenon in which high-strength steel undergoes sudden brittle fracture after a certain period of time under a static load stress (load stress below the tensile strength) without any apparent plastic deformation. In the case of steel plates, it is known that delayed fracture occurs due to the residual tensile stress when pressed into a specified shape and hydrogen embrittlement of the steel at the stress concentration area.

Furthermore, in addition to the above-mentioned strength and formability, improvements in the formation state of a chemical conversion coating (chemical conversion treatability) are also desired for cold-rolled steel sheets.
For example, Patent Document 1 discloses a technique for improving chemical conversion treatability by annealing a high-strength cold-rolled steel sheet containing 0.5 to 2.0 mass% Si, pickling the sheet, and then electroplating the sheet with a Zn-Fe alloy in a coating amount of 1 to 5 g/ ^m2 . Patent Document 2 discloses a technique for improving chemical conversion treatability by annealing a steel sheet in an oxidizing atmosphere, pickling the sheet, and then plating the steel sheet with Fe or Ni in a coating amount of 1 to 50 mg/ ^m2 .
Furthermore, Patent Document 3 discloses a technique for improving chemical conversion treatability by plating an annealed steel sheet with an Fe-Ni alloy, and Patent Document 4 discloses a technique for improving chemical conversion treatability by forming an Fe coating layer of 0.02 to 1.5 g/ ^m2 on an annealed steel sheet.

JP 2015-89946 A JP 2008-190030 A Japanese Unexamined Patent Publication No. 113787/1987 Japanese Patent Application Publication No. 5-320952

However, the technologies for improving chemical conversion treatability described in Patent Documents 1 to 4 all involve electroplating in the latter stage, which causes hydrogen to penetrate into the steel sheet during electroplating, and in high-strength steel sheets with tensile strengths exceeding 980 MPa, the aforementioned delayed fracture is more likely to occur.

In view of the above circumstances, the present invention provides an Fe-based electroplated high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after painting, as well as excellent delayed fracture resistance, and a method for manufacturing the same.

As a result of investigations aimed at solving the above problems, the inventors have found that in order to suppress hydrogen penetration into a steel sheet when electroplating is performed, it is effective to perform electroplating before annealing the steel sheet. Since steel sheets are usually annealed at a high temperature range of about 700 to 900°C, the diffusible hydrogen that penetrates into the steel sheet during electroplating is released into the furnace, and the content of diffusible hydrogen in the steel can be reduced.
In addition, during annealing, Si contained in the steel sheet may diffuse into the Fe plating layer and become concentrated on the surface. However, it has been found that by controlling the dew point in the annealing furnace, the crystal grain size of the Fe plating layer can be adjusted and chemical conversion treatability can be satisfied at the same time, which has led to the completion of the present invention.

The present invention has been made based on the above findings, and the gist of the present invention is as follows.
1. A Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass% Si;
and an Fe-based electroplating layer formed on at least one side of the Si-containing cold-rolled steel sheet, the Fe-based electroplating layer having a coating weight of 1.0 to 20.0 g/ ^m2 per side;
An Fe-based electroplated high-strength steel sheet, characterized in that the content of diffusible hydrogen in the steel is 0.25 mass ppm or less.

2. The Fe-based electroplated high-strength steel sheet described in 1, characterized in that the Si-containing cold-rolled steel sheet has a tensile strength of 980 MPa or more.

3. The Fe-based electroplated high-strength steel sheet according to 1 or 2, characterized in that the proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 50%.

4. The Si-containing cold-rolled steel sheet has, in mass%,
Si: 0.5-3.0%,
C: 0.8% or less,
Mn: 1.5-3.5%,
P: 0.1% or less,
S: 0.03% or less, and
4. The Fe-based electroplated high-strength steel sheet according to any one of the above items 1 to 3, characterized in that it has a composition containing Al: 0.1% or less, with the balance being Fe and unavoidable impurities.

5. The composition of the Si-containing cold-rolled steel sheet further comprises, in mass%,
B: 0.005% or less,
Ti: 0.2% or less,
N: 0.010% 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,
Mg: 0.005% or less; and
5. The Fe-based electroplated high-strength steel sheet according to 4 above, characterized in that it contains at least one selected from the group consisting of REM: 0.005% or less.

6. The Fe-based electroplated high-strength steel sheet according to any one of claims 1 to 5, characterized in that the Fe-based electroplated layer contains 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 in a total amount of 10 mass% or less, with the remainder consisting of Fe and unavoidable impurities.

7. A process of subjecting a Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass% Si to Fe-based electroplating to obtain a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplating layer formed on at least one side with a coating weight of 1.0 to 20.0 g/ ^m2 per side;
annealing the pre-annealed Fe-based electroplated steel sheet in an atmosphere having a dew point of −30° C. or less and a hydrogen concentration of 8 vol.% or less to obtain a Fe-based electroplated steel sheet;
A method for producing an Fe-based electroplated steel sheet, comprising:

8. The method for producing an Fe-based electroplated steel sheet described in 7 above, characterized in that the atmosphere during the annealing has a dew point of -40°C or less and a hydrogen concentration of 5% by volume or less.

The present invention provides an Fe-based electroplated high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after painting, as well as excellent delayed fracture resistance, and a method for manufacturing the same.

FIG. 1A is a perspective view showing an outline of an observation sample for measuring the number of crystal grain boundaries of an Fe-based electroplating layer in contact with a Si-containing cold-rolled steel sheet at the interface between the Fe-based plating layer and the Si-containing cold-rolled steel sheet, and FIG. 1B is a cross-sectional view taken along the line A-A of FIG. 1A. 1A is an image in which a boundary line B is drawn using a scanning electron microscope at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in a SIM image, (b) is an image in which a boundary line B is drawn along the boundary line B (a region surrounded by _L1 and _L2 in a judgment region having a width of 40 pixels and centered on the boundary line B) in the image after binarization processing, and (c) is an enlarged view of the area surrounded by the square in (b). FIG. 2 is a diagram showing the steps of a combined cycle corrosion test in an embodiment. FIG. 2 is a schematic diagram showing a test piece for evaluating delayed fracture used in the examples.

The following describes an embodiment of the present invention. Note that the present invention is not limited to the following embodiment. Note that in the following description, the unit of the content of each element in the steel composition and the content of each element in the plating layer composition is "mass %", and is simply indicated as "%" unless otherwise specified.

<Fe-based electroplated high-strength steel sheet>
The Fe-based electroplated high-strength steel sheet of the present invention comprises a Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass % Si, and an Fe-based electroplated layer formed on at least one surface of the Si-containing cold-rolled steel sheet, the layer having a coating weight of 1.0 to 20.0 g/ ^m2 per surface.

(Si-containing cold rolled steel sheet)
The Fe-based electroplated high-strength steel sheet of the present invention comprises a Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass % of Si. Hereinafter, the steel components of the Si-containing cold-rolled steel sheet will be described.

Si: 0.5-3.0%
The Si-containing cold rolled steel sheet contains 0.5 to 3.0% Si. Since Si has a large effect of increasing the strength of steel (solid solution strengthening ability) without significantly impairing workability, the high strength of steel can be obtained by the above-mentioned method. Therefore, when adding Si as a means for achieving high strength, it is necessary to add 0.5% or more. If the Si content is less than 0.5%, It has little effect on chemical conversion treatability and resistance weld crack resistance. On the other hand, Si is also an element that has a negative effect on chemical conversion treatability, corrosion resistance after painting, and resistance weld crack resistance, so the Si content is 3.0%. If the Si content exceeds 3.0%, the hot rolling property and cold rolling property are significantly deteriorated, which may adversely affect productivity and may lead to a deterioration in the ductility of the steel sheet itself. From the same viewpoint, the Si content is preferably in the range of 0.7 to 2.5%.

The Si-containing cold-rolled steel sheet must contain Si in the above range, but other components are permissible as long as they are within the composition ranges of ordinary cold-rolled steel sheets, and are not particularly limited.
However, when the cold rolled steel sheet of the present invention is applied to a high strength cold rolled steel sheet having a tensile strength TS of 590 MPa or more for use in automobile bodies or the like, it is preferable that the cold rolled steel sheet has the following composition.

C: 0.05-0.3%
C forms martensite or the like as a steel structure, which makes it easier to improve workability. Therefore, it is preferable to include 0.05% or more of C. However, adding excessive C reduces weldability, so The Si content is preferably 0.3% or less. Therefore, the Si-containing cold-rolled steel sheet preferably contains C in an amount of 0.5% or more and 0.3% or less.

Mn: 1.5-3.5%
Mn is an element that strengthens steel by solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. On the other hand, if the Mn content is 3.5% or less, the above effect can be obtained without increasing the cost. It is preferable that the content be in the range of .5 to 3.5%.

P: 0.1% or less (excluding 0%)
P reduces weldability and segregates at grain boundaries to deteriorate ductility, bendability, and toughness. If added in large amounts, it also promotes ferrite transformation and increases the grain size. Therefore, the Si-containing cold-rolled steel sheet preferably contains P in an amount of 0.1% or less.

S: 0.03% or less (excluding 0%)
S reduces weldability and significantly reduces hot ductility, thereby inducing hot cracking and significantly deteriorating surface properties. Furthermore, S not only contributes very little to strength, but also forms coarse sulfides as an impurity element, thereby reducing ductility, bendability, and stretch flangeability. These problems become significant when the S content exceeds 0.030%, so it is desirable to reduce the S content as much as possible. Therefore, the S content in the Si-containing cold-rolled steel sheet is preferably 0.03% or less.

Al: 0.01~0.1%
Al is an element added as a deoxidizer in the steelmaking process, and is an effective element for separating nonmetallic inclusions that reduce stretch flangeability as slag. Therefore, the content should be 0.01% or more. On the other hand, if the content of Al is 0.1% or less, the above-mentioned effects can be obtained without increasing the raw material cost. It is preferable that the content is 0.1% or more.

Nb: 0.005-0.05%
Nb forms carbides and nitrides, suppresses the growth of ferrite during the heating stage of annealing, refines the structure, and improves formability, particularly stretch flangeability. % or more. On the other hand, if the Nb content is 0.05% or less, the above effect can be obtained without increasing the cost. Therefore, the Si-containing cold rolled steel sheet contains Nb in an amount of 0.005 to 0. It is preferable that the content is 0.5%.

Ti: 0.005-0.05%
Ti, like Nb, forms carbides and nitrides, suppresses the growth of ferrite during the heating stage of annealing, refines the structure, and improves formability, especially stretch flangeability. The effect is expressed by adding 0.005% or more of Ti. On the other hand, if the content is 0.05% or less, the above effect can be obtained without increasing the cost. It is preferable that the content be 0.005 to 0.05%.

Cu: 0.05-0.5%
The Cu content of 0.05% or more is effective in promoting the formation of the residual γ phase. On the other hand, if it exceeds 0.5%, the cost increases. Therefore, the Si-containing cold-rolled steel sheet does not contain Cu. If added, it is preferable that the content be 0.05 to 0.5%.

Ni: 0.05-0.5%
When Ni is contained in an amount of 0.05% or more, the effect of promoting the formation of the residual γ phase is easily obtained. On the other hand, if the Ni content exceeds 0.5%, the cost increases. Therefore, the Si-containing cold-rolled steel sheet is not added with Ni. If added, it is preferable that the content be 0.05 to 0.5%.

Sb: 0.001-0.2%
Sb can be contained from the viewpoint of suppressing nitridation and oxidation of the steel sheet surface, or decarburization in a region of several tens of microns on the steel sheet surface caused by oxidation. Sb suppresses nitridation and oxidation, thereby forming martensite on the steel sheet surface. This prevents the amount of Ti generated from decreasing, improving fatigue properties and surface quality. Such effects can be obtained at 0.001% or more. On the other hand, if it exceeds 0.2%, toughness deteriorates. When Sb is added to the Si-containing cold rolled steel sheet, it is preferable that the Sb content is 0.001 to 0.2%.

The Si-containing cold rolled steel sheet contains Fe and unavoidable impurities as the balance other than the above-mentioned components, but other components may be added within a range that does not impair the effects of the present invention.
For example, Mo is an element that improves the hardenability of steel and promotes the formation of bainite and martensite, and therefore may be further added in the range of 0.005 to 0.3%. Ca and REM are elements that control the morphology of sulfide-based inclusions and improve the stretch flangeability of steel sheets, and therefore one or two selected from Ca: 0.001 to 0.1% and REM: 0.001 to 0.1% may be further added.

(Fe-based electroplating layer)
The Fe-based electroplated high-strength steel sheet of the present invention comprises an Fe-based electroplated layer formed on at least one side of the Si-containing cold-rolled steel sheet, the layer having a coating weight of 1.0 to 20.0 g/ ^m2 per side.

By forming the Fe-based electroplating layer, the external oxidation of Si and Mn during annealing can be delayed, and even if the cold-rolled steel sheet contains Si, it is possible to obtain a cold-rolled steel sheet having good chemical conversion treatability.
In the present invention, an Fe-based electroplating layer having a coating weight of 1.0 g/ ^m2 or more is formed. In order to produce the effect of improving chemical conversion treatability, it is necessary that the coating weight of the Fe-based electroplating layer per side is 1.0 g/ ^m2 or more. From the same viewpoint, the coating weight of the Fe-based electroplating layer is preferably 3.0 g/m2 or ^more , more preferably 5.0 g/m2 ^{or more} . There is no particular limit to the upper limit of the coating weight of the Fe-based electroplating layer per side, but from the viewpoint of cost, the coating weight of the Fe-based electroplating layer per side is set to 20 g/ ^m2 or less.
In the Fe-based electroplated high strength steel sheet of the present invention, it is preferable to form an Fe-based electroplated layer on both sides of the Si-containing cold rolled steel sheet. Furthermore, by setting the adhesion weight of the Fe-based electroplated layer to 3.0 g/ ^m2 or more, chemical conversion treatability is particularly good.

The adhesion weight of the Fe-based electroplating layer is measured as follows. A 10 x 15 mm sample is taken from the annealed Si-containing cold-rolled steel sheet and embedded in resin to obtain a cross-section embedded sample. Three arbitrary locations of the cross-section embedded sample 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, and the average thickness of the three fields of view is multiplied by the specific gravity of iron to convert it into the adhesion weight of the Fe-based electroplating layer per side.

The type of the Fe-based electroplating layer is not particularly limited and can be appropriately selected depending on the required performance. For example, as the Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be appropriately used.
The composition of the Fe-based electroplating layer is not particularly limited. For example, it is preferable that the composition contains 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 in a total amount of 10 mass% or less, with the remainder being Fe and unavoidable impurities. By making the amount of elements other than Fe 10 mass% or less in total, it is possible to prevent a decrease in electrolysis efficiency and form the Fe-based electroplating layer at low cost.

The Si-containing cold-rolled steel sheet according to the present invention preferably does not have a plating layer other than Fe-based electroplating formed on the surface. This is because the Si-containing cold-rolled steel sheet does not have a plating layer other than Fe-based electroplating on the surface, making it possible to provide at low cost parts that do not require excessive zinc-plated steel sheet for rust prevention purposes, or parts that are used in environments where the corrosive environment is mild and excessive rust prevention is not required.

(Diffusible hydrogen in steel)
The Fe-based electroplated high-strength steel sheet of the present invention is characterized in that the content of diffusible hydrogen in the steel is 0.25 mass ppm or less.
By setting the content of diffusible hydrogen in the steel to 0.25 mass ppm or less, the delayed fracture resistance can be improved.

If the content of diffusible hydrogen in the steel exceeds 0.25 ppm by mass, this leads to deterioration in bendability and delayed fracture resistance. If the content of diffusible hydrogen in the steel is 0.25 ppm by mass or less, delayed fracture resistance can be significantly improved, and it is preferable that the content of diffusible hydrogen in the steel is 0.10 ppm by mass or less. If it is 0.10 ppm by mass or less, in addition to delayed fracture resistance, mechanical properties such as bendability can also be significantly improved.

The delayed fracture is more pronounced in high-strength steel plates having a tensile strength of 980 MPa or more, and therefore the content of diffusible hydrogen in steel of 0.25 mass ppm or less is preferably applied to high-strength steel plates having a tensile strength of 980 MPa or more.
However, since hydrogen in steel also affects mechanical properties such as bendability, which are important when the tensile strength is 590 MPa or more, the technology of the present invention can also be applied to high-strength steel plate with a tensile strength of 590 MPa or more.

Here, the method for reducing the content of diffusible hydrogen in the steel to 0.25 mass ppm or less is not particularly limited. For example, the content of diffusible hydrogen in the steel can be reduced to 0.25 mass ppm or less by performing an annealing treatment described below after forming the Fe-based electroplating.

By performing an annealing treatment after the formation of the Fe-based electroplating, hydrogen that has entered the electroplating is released into the annealing furnace in a high-temperature environment, making it possible to improve chemical conversion treatability without deteriorating delayed fracture resistance.
When the electroplating is performed, the steel sheet side serves as the cathode, and Fe ions in the plating bath are reduced and electrodeposited as metallic Fe on the steel sheet surface mainly through the following reaction.
Fe ²⁺ +2e ^- →Fe
At the same time, since the plating bath is acidic, a hydrogen generating reaction occurs at a certain rate, as shown in the following reaction:
2H ⁺ +2e ^- →2H→H ₂
A certain proportion of hydrogen generated on the steel sheet penetrates into the steel sheet as diffusible hydrogen, and the remainder becomes hydrogen gas. The penetrated hydrogen remains in the steel and leads to deterioration of mechanical properties such as bendability, and is a factor in deteriorating delayed fracture resistance, which is particularly important for high-strength steel sheets of 980 MPa or more.

The amount of diffusible hydrogen in the steel can be measured, for example, by temperature rise analysis using gas chromatography. Generally, hydrogen that is released at temperatures of 250°C or less is called diffusible hydrogen.

Note that annealing after the formation of an Fe-based electroplated layer is effective for dehydrogenating the steel sheet and does not deteriorate the delayed fracture resistance. However, even if an Fe-based electroplated layer is present, external oxidation of Si and Mn occurs in the outermost layer, and sufficient chemical conversion treatability may not be obtained.
Therefore, in the present invention, it is preferable that the ratio 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 more than 50%. Si and Mn of the Si-containing cold-rolled steel sheet diffuse through the grain boundaries of the Fe electroplating layer to form oxides on the surface. Therefore, if the integration of the crystal grains does not progress sufficiently and an ultrafine structure of the Fe plating layer remains after electroplating, the diffusion of Si and Mn is promoted, and sufficient chemical conversion treatability may not be obtained. Therefore, in the present invention, by making the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet integrated more than 50%, the effect of suppressing external oxidation of Si and Mn during annealing by providing the Fe-based electroplating layer becomes significant, and more excellent chemical conversion treatability can be realized. The upper limit of the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet integrated is not particularly limited and may be 100%.

Here, the ratio of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet can be measured as follows.
First, a sample of 10×10 mm size is taken from the Fe-based electroplated high-strength steel sheet. Three arbitrary locations of the sample are processed with a focused ion beam (FIB) device to form three 45° cross sections with a width of 30 μm in the direction perpendicular to the rolling and a length of 50 μm in the 45° direction relative to the T cross section direction, which are angled at 45° with respect to the T cross section direction (a cross section parallel to the rolling perpendicular direction of the steel sheet and perpendicular to the steel sheet surface) as shown in FIG. 1, to prepare a sample for observation. Next, the center of the 45° cross section of the observation sample is observed at a magnification of 5000 times using a scanning ion microscope (SIM), and a SIM image of 1024 pixels wide x 943 pixels high and 8 bits is taken. From the SIM images taken at each of the 45° cross sections created at three locations, the ratio of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is calculated based on the following formula (1), and the average value of the three locations is calculated. Note that the value is rounded up to the nearest whole number.
Formula (1): Percentage of integrated 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 = (length of a portion of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet where the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated) ÷ (length of the interface in the observation field) × 100 (%)

Whether or not the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated is determined by image processing.
A method for evaluating the rate at which the crystal orientations are integrated will be described with reference to FIG. 2. First, as shown in FIG. 2(a), a boundary line B is drawn at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in the SIM image described above using a scanning electron microscope. Next, an image is created by image processing the SIM image, separate from the image in which the boundary line is drawn. Specifically, first, the grain boundaries are emphasized by a Sobel filter for the captured SIM image of width 1024×height 943 pixels and 8 bits. Next, the image in which the grain boundaries are emphasized is smoothed by a Gaussian filter (radius (R): 10 pixels). Next, the image after the smoothing process is subjected to binarization (threshold: 17). Subsequently, the boundary line B of the image in which the interface is drawn is transferred to the binarized image. Thereafter, as shown in FIG. 2(b), in the image after the binarization process, a judgment region (region surrounded by _L1 and _L2 in FIG. 2(b)) with a width of 40 pixels and centered on the boundary line B is drawn along the boundary line B on the binarized image. The total length of the boundary line B where the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet (black and white boundary on the binarized image) does not exist within the judgment region is regarded as the length of the portion where the crystal orientation is integrated. Here, the total length of the boundary line where the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet does not exist within the judgment region is calculated as follows. First, a portion where the judgment region can be divided into an approximately rectangular shape by two normal lines of the boundary line B so that only one color, black or white, is included is found in the entire judgment region. Next, the maximum distance between the intersection points of the boundary line and the two normal lines at that portion is summed up for the entire judgment region to determine the total length of the boundary line where the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet does not exist within the judgment region. The length of the portion where the crystal orientations are integrated may be obtained by subtracting the length of the portion where the crystal orientations are not integrated from the length of the interface in the observation field. For the purpose of explanation, FIG. 2(c) shows an enlarged view of the portion surrounded by a square in FIG. 2(b). First, as shown in FIG. 2(c), a portion where the judgment region can be divided into a substantially rectangular shape by two normal lines (in FIG. 2(c), _l1 and _l2 , and _l3 and _l4 ) of the boundary line B is searched for in the entire judgment region so that the two colors, black and white, are included. Next, the maximum distances between the intersections of the boundary line and the two normal lines in the portion are summed up over the entire judgment region to obtain the total length of the boundary line where the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet exists in the judgment region. The length, i.e., the length of the portion where the crystal orientations are not integrated, is subtracted from the length of the interface in the observation field to obtain the length of the portion where the crystal orientations are integrated.

<Method for producing Fe-based electroplated high-strength steel sheet>
Next, a method for producing an Fe-based electroplated high-strength steel sheet will be described.
The method for producing an Fe-based electroplated steel sheet of the present invention includes the steps of: applying an Fe-based electroplating to a Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass% Si; and obtaining a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplating layer formed on at least one side thereof with a coating weight of 1.0 to 20.0 g/ ^m2 per side;
and annealing the pre-annealed Fe-based electroplated steel sheet in an atmosphere having a dew point of −30° C. or less and a hydrogen concentration of 8 volume % or less to obtain a Fe-based electroplated steel sheet.

(Production of Si-containing cold-rolled steel sheet)
In the step of obtaining the pre-annealed Fe-based electroplated steel sheet, first, a Si-containing cold-rolled steel sheet containing 0.5 to 3.0 mass % of Si is produced. The Si-containing cold-rolled steel sheet can be produced according to a normal method for producing a cold-rolled steel sheet. As an example, the high-strength pre-annealed cold-rolled steel sheet can be produced by hot-rolling a steel slab having the above-mentioned composition to obtain a hot-rolled sheet, pickling the hot-rolled sheet, and then cold-rolling the hot-rolled sheet, thereby producing a Si-containing cold-rolled steel sheet.

(Fe-based electroplating treatment)
In the step of obtaining the Fe-based electroplated steel sheet, the surface of the obtained Si-containing cold-rolled steel sheet is subjected to an Fe-based electroplating treatment to produce the Fe-based electroplated steel sheet. The method of the Fe-based electroplating is not particularly limited.

For example, as the Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two can be used.
The Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 1.0 mol/L or more in terms of Fe ²⁺ . If the Fe ion content in the Fe-based electroplating bath is 1.0 mol/L or more in terms of Fe ²⁺ , a sufficient Fe deposition amount can be obtained.
The Fe-based electroplating bath may contain Fe ions, as well as alloying elements such as B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and may also contain conductivity enhancers such as sodium sulfate and potassium sulfate as additives or impurities. The above-mentioned metal elements may be contained as metal ions, and nonmetallic elements may be contained as parts of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity enhancers such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.
Other conditions of the Fe-based electroplating bath are not particularly limited. The bath temperature is preferably 30° C. or higher in consideration of the ability to maintain a constant temperature. The pH of the Fe-based electroplating bath is not particularly specified, but is preferably 3.0 or lower in consideration of the electrical conductivity of the Fe-based electroplating bath. The current density is not particularly limited, but is usually ^about 10 to 150 A/dm2. The sheet passing speed during plating may be 5 mpm or more and 150 mpm or less. This is because a sheet passing speed of less than 5 mpm results in poor productivity, while a sheet passing speed of 150 mpm or more makes it difficult to stably control the plating adhesion weight.

As a treatment before the Fe-based electroplating, a degreasing treatment and water washing for cleaning the surface of the Si-containing cold-rolled steel sheet, and further, an acid pickling treatment and water washing for activating the surface of the Si-containing cold-rolled steel sheet may be performed. Following these pretreatments, the Fe-based electroplating treatment is performed.
The methods of the degreasing treatment and the water washing are not particularly limited, and ordinary methods can be used. For the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among these, it is preferable to use sulfuric acid, hydrochloric acid, or mixtures thereof. In addition, the concentration of the acid used in the pickling treatment is not particularly specified, but in consideration of the ability to remove the oxide film and the prevention of roughness (surface defects) due to excessive pickling, it is preferable that the concentration is about 1 to 20 mass %. In addition, the pickling treatment solution may contain an antifoaming agent, a pickling promoter, a pickling inhibitor, etc.

(Annealing)
In the method for producing a ferrous electroplated steel sheet of the present invention, the ferrous electroplated steel sheet before annealing is annealed in an atmosphere having a dew point of -30°C or less and a hydrogen concentration of 8% by volume or less.
Annealing can remove distortion of the Fe-based electroplated steel sheet before annealing that occurs during the rolling process, recrystallize the structure, and increase the strength of the steel sheet. In addition, annealing can reduce the content of diffusible hydrogen in the steel.

In the method for producing an Fe-based electroplated steel sheet of the present invention, the dew point of the annealing atmosphere in the annealing is set to a low dew point of −30° C. or less, which is a condition that does not require additional equipment such as a humidification equipment.
The present inventors have found through their own investigations that there is a correlation between the degree of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet and the dew point of the annealing atmosphere in the annealing step after the formation of the Fe-based electroplated layer. That is, when annealing the pre-annealed Fe-based electroplated steel sheet after the formation of the Fe-based electroplated layer, the lower the dew point of the annealing atmosphere, the higher the degree of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet obtained after annealing, and conversely, the higher the dew point of the annealing atmosphere, the lower the degree of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.
The reason why a correlation is found between the rate at which the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated and the dew point is not clear, but it can be speculated as follows. When the dew point is controlled to a certain level or higher, elements that diffuse from the steel sheet to the Fe-based electroplating layer during annealing form oxides inside the Fe-based electroplating layer, and these oxides inhibit the crystal growth of the Fe-plated grains, resulting in fine grains. On the other hand, when the Fe-based electroplating layer is annealed in an atmosphere with a low dew point after formation, the oxides are unlikely to be formed, and the crystal grain size of the Fe-based electroplating layer becomes coarse. Therefore, it can be considered that when annealing is performed at a low dew point, the crystal orientation of the Fe-based electroplating layer is integrated with the crystal orientation of the Si-containing cold-rolled steel sheet at a high rate. When the dew point of the annealing atmosphere in the annealing is set to −30° C. or less, the proportion of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet increases. As a result, the diffusion paths of Si and Mn are reduced, and external oxidation of Si and Mn during annealing can be efficiently suppressed. In addition, it is more preferable that the dew point of the annealing atmosphere is −40° C. or less, since integration of the crystal orientations can be promoted. Although the lower limit of the dew point of the annealing atmosphere is not particularly set, it is preferable that the dew point is −80° C. or more, since a dew point of less than −80° C. is difficult to achieve industrially. The dew point of the annealing atmosphere is more preferably −55° C. or more.

The hydrogen concentration in the atmosphere during the annealing must be 8.0% by volume or less. Hydrogen can suppress the oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing during annealing and can play a role in activating the steel sheet surface, but if the concentration is too high, it may penetrate into the steel sheet and deteriorate the delayed fracture resistance properties. Therefore, from the viewpoint of suppressing hydrogen penetration into the steel, the hydrogen concentration in the atmosphere during the annealing must be 8.0% by volume or less, and is preferably 5.0% by volume or less. Furthermore, if the hydrogen concentration in the atmosphere during the annealing is 1.0% by volume or more, the Fe on the steel sheet surface will oxidize, thereby suppressing the oxidation of the steel sheet. Therefore, the hydrogen concentration during the annealing is preferably 1.0% by volume or more, and more preferably 2.0% by volume or more. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

In addition, in the annealing, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds to 600 seconds. By holding the time in this temperature range for 30 seconds or more, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing can be suitably removed, and as described below, the chemical conversion treatability can be improved when a chemical conversion coating is provided on the surface of the Fe-based electroplated steel sheet. Therefore, the holding time in this temperature range is preferably 30 seconds or more. There is no particular upper limit to the holding time in this temperature range, but from the viewpoint of productivity, the holding time in this temperature range is preferably 600 seconds or less.

The maximum temperature of the Fe-based electroplated steel sheet 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 Fe-based electroplated steel sheet before annealing to 650°C or higher, the recrystallization of the steel sheet structure proceeds favorably, and the desired strength can be obtained. In addition, if the maximum temperature of the Fe-based electroplated steel sheet is 900°C or lower, the diffusion rate of Si and Mn in the steel can be prevented from increasing too much, and the diffusion of Si and Mn to the steel sheet surface can be prevented, thereby improving the chemical conversion treatability. Furthermore, if the maximum temperature is 900°C or lower, damage to the furnace body of the heat treatment furnace can be prevented, and costs can be reduced. Therefore, it is preferable that the maximum temperature of the Fe-based electroplated steel sheet before annealing is 900°C or lower. The above maximum temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet before annealing.

The other conditions for the process for obtaining the pre-annealed Fe-based electroplated steel sheet and the process for obtaining the Fe-based electroplated steel sheet are the same as those described in the Fe-based electroplated high-strength steel sheet of the present invention.

The present invention will be described in more detail below with reference to examples. However, these examples are intended to illustrate the present invention and are not intended to limit the present invention in any way.

<Samples 1 to 79>
(1) Steel plates A to O having a thickness of 1.4 mm were obtained by melting steel having the chemical composition shown in Table 1 and subjecting the resulting cast pieces to hot rolling, pickling, and cold rolling.
(2) Each of the obtained steel sheets was subjected to an alkaline degreasing treatment, and then subjected to Fe-based electroplating and annealing under the conditions shown in Table 2 to produce samples of Fe-based electroplated steel sheets.
Here, the Fe-based electroplating bath contained 1.5 mol/L of Fe2+ and 0.35 mol/L of sodium sulfate, and the pH was adjusted to 2.0 with sulfuric acid, and an iridium oxide electrode was used as the anode. The deposition amount (g/ ^m2 ) of the Fe-based electroplating layer was controlled by changing the current application time. The ratio (%) of the crystal orientations integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet was determined by processing three arbitrary points of each sample with a focused ion beam to prepare an observation sample, taking a SIM image of the observation sample using a scanning ion microscope, and calculating the ratio of the crystal orientations integrated between 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 based on the equation (1) from the SIM image taken, and calculating the average value of the three points.
Formula (1): Percentage of integrated 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 = (length of a portion of the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet where the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated) ÷ (length of the interface in the observation field) × 100 (%)
Table 2 shows the deposition amount of the Fe-based electroplated layer and the ratio of the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated for each sample.
Annealing was performed at a temperature of 800° C. in a nitrogen atmosphere having a dew point shown in Table 2 and containing hydrogen at the content shown in Table 2. Table 2 also shows which of the Fe-based electroplating and the annealing was performed first.

<Evaluation>
The following evaluations were carried out on each sample of the Fe-based electroplated steel sheet obtained as described above. The evaluation results are shown in Table 2.

(1) Chemical conversion treatability Test pieces taken from the Fe-based electroplated steel sheets of each sample were subjected to chemical conversion treatment using a degreaser FC-E2011, a surface conditioner PL-Z, and a chemical conversion treatment agent Palbond PB-SX35, all manufactured by Nihon Parkerizing Co., Ltd., under the following standard conditions so that the amount of chemical conversion coating applied was 2.0 to 3.0 g/ ^m2 .
(Conversion treatment conditions)
Degreasing process: treatment temperature 43°C, treatment time 120 seconds Spray degreasing, surface conditioning process: pH 9.5, treatment temperature room temperature, treatment time 20 seconds Chemical conversion process: chemical conversion solution temperature 35°C, treatment time 90 seconds (chemical conversion treatment performance evaluation)
The surface of each test piece (n=1) that had been subjected to the chemical conversion treatment was observed with a SEM at a magnification of 1000 times, and evaluated according to the following criteria.
◎: The grain size of the chemical crystals is 5 μm or less and no unprecipitated areas are observed. ○: The grain size of the chemical crystals is 5 μm or more, but no unprecipitated areas are observed. ×: The grain size of the chemical crystals is 5 μm or more and no unprecipitated areas are observed.

(2) Corrosion Resistance The surfaces of the test pieces subjected to chemical conversion treatment under the same conditions as those in the evaluation of chemical conversion treatability in (1) above were electrocoated with an electrodeposition coating material GT-100 manufactured by Kansai Paint Co., Ltd. to a film thickness of 15 μm, and then subjected to the following corrosion test.
(Hot salt water immersion test conditions)
A cross-cut scratch having a length of 45 mm was made with a cutter on the surface of the above test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, and then the test piece was immersed in a 5 mass % NaCl solution (60° C.) for 360 hours, and then washed with water and dried.
(Corrosion resistance evaluation)
A tape peeling test was performed in which adhesive tape was applied to the cut flaw and then peeled off. The maximum total peel width of the cut flaw, including both the left and right sides, was measured and evaluated according to the following criteria.
◎: Maximum peeled width is 3.0 mm or less. ○: Maximum peeled width is 5.0 mm or less. ×: Maximum peeled width is more than 5.0 mm.

(3) Delayed fracture resistance (3-1) Hydrogen content in steel For each sample of Fe-based electroplated steel sheet, the test specimen after annealing was stored in liquid nitrogen for the purpose of suppressing the temperature rise of the test specimen and the escape of hydrogen from the steel.
Thereafter, the test piece was heated to 250° C. at a heating rate of 100° C./s in an Ar atmosphere, and the amount of released hydrogen gas was quantitatively measured by gas chromatography.

(3-2) Evaluation of delayed fracture properties For each sample of Fe-based electroplated steel sheet, in a state where it was fastened with a bolt so that a specified stress was generated at the bent tip (see Figure 4), the presence or absence of cracking was confirmed when a dry-wet cycle (a process in which a relative humidity of 30% and 90% was repeated for 2 hours each, with salt water applied twice a week under conditions of a temperature of 10°C ^and a salt deposition amount of 10,000 mg/m2, as shown in Figure 3) was carried out over a period of 40 days. Evaluation was made according to the following criteria.
○: No cracks occurred for 40 days ×: Cracks occurred

(4) Tensile strength (TS)
A JIS No. 5 tensile test piece (JIS Z 2201) was taken from each sample of Fe-based electroplated steel sheet in the direction perpendicular to the rolling direction, and a tensile test was performed at a tensile speed (crosshead speed) of 10 mm/min. The tensile strength TS was determined by dividing the maximum load in the tensile test by the initial cross-sectional area of the parallel part of the test piece. The thickness of the plated steel sheet used in calculating the cross-sectional area of the parallel part was the thickness of the plated steel sheet.

The results in Table 2 show that all of the samples of the present invention have good chemical conversion treatability and corrosion resistance, and by annealing after Fe electroplating, there is little hydrogen in the steel and the delayed fracture resistance is also excellent. In addition, in all of the examples of the present invention, the proportion of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet was 50% or more.

The present invention provides an Fe-based electroplated high-strength steel sheet that has excellent chemical conversion treatability and corrosion resistance after painting, as well as excellent delayed fracture resistance, and a method for manufacturing the same.

Claims (5)

In mass percent,
Si: 0.5-3.0%,
C: 0.8% or less,
Mn: 1.5-3.5%,
P: 0.1% or less,
S: 0.03% or less, and
A Si-containing cold-rolled steel sheet having a composition containing Al: 0.1% or less, with the balance being Fe and unavoidable impurities, and having a tensile strength of 980 MPa or more;
and an Fe-based electroplating layer having a coating weight of 1.0 to 20.0 g/ ^m2 per side formed on at least one side of the Si-containing cold-rolled steel sheet, wherein no plating layer other than the Fe-based electroplating is formed on the surface of the Si-containing cold-rolled steel sheet,
The content of diffusible hydrogen in the steel is 0.21 mass ppm or less,
A Fe-based electroplated high-strength steel sheet, characterized in that a ratio of crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at an interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 50% . The composition of the Si-containing cold-rolled steel sheet further comprises, in mass%,
B: 0.005% or less,
Ti: 0.2% or less,
N: 0.010% 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,
Mg: 0.005% or less; and
The ferroelectric plated high strength steel sheet according to claim 1 , further comprising at least one selected from the group consisting of REM: 0.005% or less. 3. The Fe-based electroplated high-strength steel sheet according to claim 1, wherein the Fe-based electroplated layer has a composition containing 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 in a total amount of 10 mass% or less, with the balance being Fe and unavoidable impurities. In mass percent,
Si: 0.5-3.0%,
C: 0.8% or less,
Mn: 1.5-3.5%,
P: 0.1% or less,
S: 0.03% or less, and
a step of subjecting a Si-containing cold-rolled steel sheet, the Si-containing cold-rolled steel sheet having a composition containing 0.1% or less of Al, with the balance being composed of Fe and unavoidable impurities, and having a tensile strength of 980 MPa or more, to Fe-based electroplating to form an Fe-based electroplating layer on at least one side with a coating weight of 1.0 to 20.0 g/ ^m2 per side, and no plating layer other than the Fe-based electroplating is formed on the surface of the Si-containing cold-rolled steel sheet;
annealing the pre-annealed Fe-based electroplated steel sheet in an atmosphere having a dew point of −30° C. or less and a hydrogen concentration of 8 vol.% or less to obtain a Fe-based electroplated steel sheet;
Including,
a rate at which crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at an interface between the formed Fe-based electroplating layer and the Si-containing cold-rolled steel sheet exceeds 50% . 5. The method for producing an Fe-based electroplated steel sheet according to claim 4 , wherein the atmosphere during the annealing has a dew point of −40° C. or less and a hydrogen concentration of 5 volume % or less.

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