JPS6314072B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention has corrosion resistance, workability, spot weldability,
The present invention relates to a surface-treated steel sheet that is excellent in all properties such as chemical conversion treatment properties, and a method for manufacturing the same. Recently, there have been many cases in which a single surface-treated steel sheet is required to have many properties at the same time. In particular, when it comes to surface-treated steel sheets used in automobiles, home appliances, building materials, etc., although most of them are originally intended to improve corrosion resistance, there are many problems such as workability, spot weldability, chemical conversion treatment properties, etc. It is well known that the following characteristics are required. However, the conventional surface-treated steel sheets used for this purpose, such as hot-dip galvanized steel sheets, galvanized steel sheets, electrogalvanized steel sheets, and zinc-rich primer-coated steel sheets, all have poor workability, spot weldability, and There are problems in one or more of the properties such as chemical conversion treatment properties, and even the corrosion resistance that should be provided is insufficient. By the way, various types of stainless steel sheets that contain a large amount of corrosion-resistant metal elements such as chromium nickel in the steel, and chromized steel sheets that have chromium diffused plated on the surface of the steel sheet are widely known for their excellent corrosion resistance. However, like the various steel sheets mentioned above, these also have some drawbacks; both steel sheets have particularly poor chemical conversion treatment properties, and chromized steel sheets also have problems in workability. In order to overcome these problems, for example,
Despite various attempts to improve zinc-rich primer-coated steel sheets, the surface treatment layer, or coating film, is simply roll-coated on the surface of the steel sheet, which has been treated with a paint base. Therefore, at present, as a complete coating surface treatment method has not yet been devised, the coating film tends to separate from the steel sheet when subjected to strong processing, which is considered to be an essential defect of zinc-rich primer-coated steel sheets. . In addition, the coating film is composed of about 70wt% or more of fine zinc particles, as well as a resin component and other additive components, and during spot welding, this large amount of zinc reacts with the electrode tip made of copper or copper alloy. The spot weldability is poor.
Similarly, hot-dip galvanized steel sheets, galvanized steel sheets, and electrogalvanized steel sheets have poor spot weldability because their surface treatment layers are mainly zinc-based. Furthermore, in chemical conversion treatment using zinc phosphate, the reaction with the treatment solution is generally insufficient, so the phosphate film formed is not complete.
That is, the chemical conversion treatment property is poor. Furthermore, various types of stainless steel sheets and chromized steel sheets boast excellent corrosion resistance because their surfaces contain large amounts of chromium and nickel, making them passivated. Therefore, there is almost no reaction with the chemical conversion treatment solution, and a complete phosphate film is not formed. Furthermore, regarding chromized steel sheets, an essential defect due to the manufacturing method is that the chromium concentration on the surface of the diffusion plated layer is 30wt.
% or more, and combined with the fact that chromium is easily oxidized, it may become hard and brittle. For this reason, chromized steel sheets are susceptible to problems such as surface cracking, powdering, and separation of the surface treatment layer during strong processing, such as press forming. In this way, conventional surface-treated steel sheets, etc.
Considerable corrosion resistance is achieved by containing large amounts of metal elements such as chromium and nickel on the surface, or by making them the main component of the surface treatment layer. It has serious drawbacks that can be attributed to this. Therefore, in view of the above-mentioned circumstances, an object of the present invention is to provide a corrosion-resistant surface-treated steel sheet with excellent workability, spot weldability, chemical conversion treatment properties, etc., and a method for manufacturing the same. A chromium-diffused steel plate layer with a thickness of 2 μm or more is placed on the steel plate base, and a chromium-diffused steel plate layer with an average chromium concentration of 10 to 35 wt% and a thickness of 1 to
55 μm iron-diffused chromium-containing layer and average chromium concentration
A surface treatment layer is provided in which a chromium diffused iron plating layer of 5 wt% or less and a thickness of 3 μm or more is sequentially formed, and the surface treatment layer has an average chromium concentration of 10 to 10.
30 wt% and a thickness of 6 to 60 μm, and the chromium concentration increases continuously from the surface of the chromium-diffused iron plating layer to the inside of the iron-diffused chromium-containing layer, and the chromium concentration increases from the inside of the iron-diffused chromium-containing layer to the steel plate. Provided is a surface-treated steel sheet having a three-layered coating layer, which is characterized by a three-layer structure in which the coating layer decreases continuously over the substrate. According to the second aspect of the present invention, a chromium-containing layer with a thickness of 1 to 55 μm is formed on the surface of a steel plate, an iron plating layer with a thickness of 3 μm or more is formed on this chromium-containing layer, and then heat treatment is performed. The steel plate by applying
A chromium-diffused steel sheet layer with a thickness of 2 μm or more, in which a metal element is diffused between the chromium-containing layer and the iron plating layer, and chromium is diffused into the steel sheet;
An iron-diffused chromium-containing layer with an average chromium concentration of 10 to 35 wt% and a thickness of 1 to 55 μm in which iron is diffused into the chromium-containing layer, and an average chromium concentration of 5 wt% or less with a thickness of 1 to 55 μm in which chromium is diffused into the iron plating layer. Provided is a method for manufacturing a surface-treated steel sheet having a three-layer structure coating layer, characterized by forming a chromium-diffused iron plating layer with a thickness of 3 μm or more. In both of the above embodiments, the chromium-containing layer may be composed of chromium alone, or chromium and one or more metal elements selected from iron, nickel, niobium, aluminum, copper, cobalt, molybdenum, titanium, and silicon. It can also be composed of DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, a surface-treated steel sheet having a three-layered coating layer and a method for manufacturing the same according to the present invention will be described in detail with reference to the accompanying drawings. Figures 1a and 1b show the process of forming a surface treatment layer during production of the surface treated steel sheet of the present invention, and for simplicity, the case where the chromium-containing layer consists of only chromium will be described below. Figure 1a shows a state in which a layer 2 made of only chromium (hereinafter referred to as the chromium layer) has been formed on a steel sheet base 1, and an iron plating layer 3 has been formed on the layer 2, but no heat treatment has been performed yet. It shows. Figure 1b shows the state in which the coated steel plate shown in Figure 1a has been heat-treated.Due to the heat treatment of the steel plate, some of the chromium elements in the chromium layer 2 diffuse and permeate in the direction of the steel plate base 1 and the layer 3. However, on the other hand, mainly the iron element in the steel sheet base 1 and the iron plating layer 3 diffuses into the chromium layer 2, and in this way, both chromium and iron elements come into contact with the steel sheet base 1 and the chromium layer 2. They diffuse into each other at the interface and the contact interface between the chromium layer 2 and the iron plating layer 3. the result,
The two interfaces mentioned above virtually disappear in terms of chromium concentration distribution, but from the generation process of the surface treatment layer, the interface between each layer can be visually distinguished from the A layer (chromium diffused steel sheet layer, i.e., the steel sheet base layer). Layer B (a layer formed by diffusion and penetration of chromium from chromium layer 2 into chromium layer 1), B layer (iron-diffused chromium-containing layer, i.e., a layer formed by diffusion and penetration of chromium from steel plate base 1 and iron plating layer 3 into chromium layer 2) Three layers are formed: layer C (chromium diffusion iron plating layer, i.e., a layer generated by diffusion of chromium in chromium layer 2 into iron plating layer 3). . Further, in this case, the thicknesses of the B layer and the C layer are respectively equivalent to the thickness of the chromium layer and the thickness of the iron plating layer before heat treatment. Therefore, the above B layer and C layer can be said to be layers in the surface-treated steel sheet after heat treatment corresponding to the formation positions of the chromium layer and iron plating layer before heat treatment, respectively. Further, the thickness of the A layer is equal to the distance through which chromium diffuses into the steel sheet base from the chromium layer, and can be said to be the difference obtained by subtracting the thicknesses of the B and C layers from the total surface treatment layer thickness. The concentration distribution of both elements chromium and iron in the cross section of the surface treatment layer of the surface-treated steel sheet of the present invention manufactured in this manner and a conventional chromized steel sheet as a comparative material was analyzed by X-ray analysis using an X-ray microanalyzer. The results are shown in sections 2a and 2b, respectively.
As shown in the figure. As is clear from FIG. 2b, the conventional chromized steel sheet used as a comparison material has a high chromium concentration of about 35 wt% at the surface, but it continuously decreases toward the base of the steel sheet. Conversely, the iron concentration is low at the surface, but increases continuously toward the steel sheet base. On the other hand, as is clear from FIG. 2a, in the surface-treated steel sheet of the present invention, the chromium concentration is low at the surface, but increases continuously toward the inside of the surface-treated layer, and after reaching the maximum concentration. , decreases continuously toward the steel sheet substrate. Conversely, it can be seen that although the iron concentration is high at the surface, it continuously decreases toward the inside of the surface treatment layer, reaches its lowest concentration, and then continuously increases toward the steel sheet base. As described above, the surface treatment layer of the steel sheet of the present invention manufactured by the method of the present invention has a C layer in the area near the surface where the chromium concentration is relatively low, and a B layer in the area where the chromium concentration is relatively high inside the layer. A layer and an A layer in a region where the chromium concentration is relatively low near the steel sheet base. Regarding the surface treatment layer formed by the method of the present invention, the thickness of the layer consisting of only chromium (chromium layer) and the iron plating layer, the heat treatment conditions to promote mutual diffusion of both chromium and iron elements, etc. The thickness of each layer and the chromium concentration within each layer can be varied over a wide range by changing these functions appropriately. , processability,
We investigated in detail the relationship between properties such as spot weldability and chemical conversion treatment properties, and found that there is a deep correlation between these. FIG. 3 shows the relationship between the thickness of the total surface treatment layer, the average chromium concentration, and the corrosion resistance of the steel sheet of the present invention. In the corrosion resistance test, a surface-treated steel plate with a thickness of 1.0 mm was subjected to a salt spray test for 2000 continuous hours according to JIS Z 2371, and the amount of reduction in steel plate thickness was measured.
Those with a small amount of this were considered to have good corrosion resistance.
In Figure 3, the curves , , and have average chromium concentrations of 2, 5, 10, 30, and 2, respectively.
Indicates 40wt%. From FIG. 3, it can be seen that the thicker the total surface treatment layer and the higher the average chromium concentration, the better the corrosion resistance. Note that the dotted line in the figure indicates the lower limit aimed at by the present invention. FIG. 4 shows the relationship between the thickness of layer B, the average chromium concentration, and the corrosion resistance of the surface-treated steel sheet of the present invention. Corrosion resistance tests were conducted in accordance with JIS Z 2371 by conducting a salt spray test for 2000 continuous hours to measure the reduction in steel sheet thickness.
Those with a small amount of this were considered to have good corrosion resistance.
In Fig. 4, the curves , , and are respectively 5, 7, 10, and 35 for the average chromium concentration of the B layer.
and 40wt%. It can be seen from FIG. 4 that the thicker the B layer and the higher the average chromium concentration, the better the corrosion resistance. Note that the dotted line in this figure indicates the standard of corrosion resistance aimed at by the present invention, and it is particularly desirable that the amount of corrosion reduction is less than this. FIG. 5 shows the relationship between the thickness of layer A and press workability. According to this, it was confirmed that workability is excellent when the thickness of the A layer is 2 μm or more, and poor workability when the thickness is 2 μm or less, especially when the thickness is 1 μm or less where the A layer is hardly present. Note that the workability evaluation in FIG. 5 is as shown in Table 1, and the larger the numerical value, the better the workability is. In the figure, the dotted line indicates the lower limit aimed at by the present invention.

[Table] Figure 6 shows the relationship between the thickness of the C layer, the average chromium concentration, and spot weldability. Spot weldability was evaluated by performing spot welding continuously with a pair of Cu-Cr alloy electrode tips without dressing until nuggets could no longer be formed, and by comparing the number of welding points at which no nuggets could be formed. It was determined that the greater the number of welding points, the better the spot weldability.
In Figure 6, the curves , , and are average chromium concentrations of 1, 3, 5, 10, and 15, respectively.
and 40wt%. According to this, spot weldability is improved as the average chromium concentration of the C layer is lower, and in relation to the thickness, the average chromium concentration is approximately
It can be seen that below 10wt%, it improves as the thickness increases, and conversely, that it deteriorates as the thickness increases. In the figure, the dotted line indicates the lower limit aimed at by the present invention. FIG. 7 shows the relationship between the thickness of the C layer, the average chromium concentration, and the chemical conversion treatment properties. Chemical conversion treatment properties are determined by performing chemical conversion treatment using a chemical conversion treatment solution whose main component is zinc phosphate, then applying a coating and examining the adhesion of the coating film.Those with good adhesion are evaluated for chemical conversion treatment. The properties were considered to be good. In FIG. 7, the curve,
, , and indicate average chromium concentrations of 1, 3, 5, 10, 15, and 40 wt%, respectively.
As is clear from FIG. 7, it can be seen that the chemical conversion treatment property is improved as the average chromium concentration is lower and the thickness of the C layer is thicker. Note that the dotted line in the figure indicates the lower limit aimed at by the present invention. As mentioned above, the surface-treated steel sheet of the present invention has a surface-treated layer composed of three regions or three layers with different average chromium concentrations, and corrosion resistance is achieved only by adding the characteristics of each of these three layers. The surface of the present invention has excellent properties such as hardness, workability, spot weldability, and chemical conversion treatability. The surface treatment layer of the treated steel sheet is
It was confirmed that the range shown in the table is particularly suitable.

[Table] The reason why the surface treatment layer is preferably formed as shown in the above table is as follows. Regarding the thickness and average chromium concentration of the entire surface treatment layer, as is clear from FIG. 3, corrosion resistance is particularly desirable when the thickness is 6 μm or more and the average chromium concentration is 10 wt% or more. In addition, in Figure 3, when the thickness of the surface treatment layer is 60 μm or more, the improvement effect due to the increase in thickness is hardly recognized, and the average chromium concentration is 30 wt%.
Above this, there is almost no improvement effect by increasing the average chromium concentration. Also, increasing the thickness and increasing the average chromium concentration increases manufacturing costs. For these reasons, the thickness of the total surface treatment layer and the average chromium concentration are each 6 μm or more.
Most preferably, the thickness is 60 μm or less, and 10 wt% or more and 30 wt% or less. Regarding layer A, as can be seen from FIG. 5, the thickness has a close relationship with press workability, and the object of the present invention can be fully achieved when the thickness of layer A is 2 μm or more. As for the B layer, as can be seen from Figure 4, there is a deep relationship between the thickness, average chromium concentration, and corrosion resistance, and the thickness and average chromium concentration of the B layer are
When the thickness is 1 μm or more and 10 wt% or more, the object of the present invention can be satisfied. Also, the thickness
If the thickness is 55 μm or more, the effect of increasing the thickness cannot be expected to be so great. The average chromium concentration is
Even at 35 wt% or more, the effect of increasing the concentration cannot be expected to be that great. For this reason, it is preferable that the thickness and average chromium concentration of the B layer be regulated to 1 μm or more and 55 μm or less, and 10 wt% or more and 35 wt% or less, respectively. As can be seen from FIGS. 6 and 7, the thickness of the C layer and the average chromium concentration have a close relationship with spot weldability and chemical conversion treatability. Spot weldability depends on the thickness of the C layer and the average chromium concentration.
It is preferable when the thickness is 2 μm or more and 5 wt% or less, but when the average chromium concentration is regulated by spot weldability to be 5 wt% or less, it is preferable that the thickness is 3 μm or more. From the purpose of the present invention, in order to satisfy both spot weldability and chemical conversion treatment properties, it is necessary to
The content is preferably 5wt% or less. The above explanation is a layer containing chromium (chromium layer)
was concerned with the case of a layer consisting of only chromium, but in the present invention, the effect of the present invention can be further improved by containing not only chromium but also other elements in the surface treatment layer. This is possible and will be explained below. Figure 8 shows the results of a corrosion resistance test according to variations in the content of elements in a steel sheet containing chromium and nickel in the surface treatment layer. When the additional value of is 25wt%, the value of (nickel weight) / (nickel weight + chromium weight) is 0 on the curve,
shows the case of 0.05, 0.10, and 0.30. Corrosion resistance was determined by conducting a salt spray test according to JIS Z 2371 for up to 2160 hours to measure the corrosion loss of the test material, and it was determined that the smaller the corrosion loss, the better the corrosion resistance. As shown in FIG. 8, it is clear that corrosion resistance is improved by adding nickel to chromium. In addition to nickel, this effect has also been observed in various metal elements such as niobium, aluminum, copper, cobalt, molybdenum, titanium, and silicon, and even when one or more of the above metal elements for which effects have been confirmed are mixed, each The effects of the elements were not impaired; in fact, there were cases where the effects appeared synergistically, as exemplified below. In the case of mixing and adding various metal elements like this, nickel, niobium, aluminum, copper, cobalt, molybdenum, titanium,
The sum of the average concentration of one or more metal elements selected from the group of metal elements such as silicon in the surface treatment layer and the average chromium concentration is the average chromium concentration of single chromium shown in Table 2. The range does not exceed the value,
Moreover, it was also confirmed that (weight of elements other than chromium)/(chromium + weight of elements other than chromium) preferably does not exceed 0.4. The reason is as follows. The purpose of the present invention is to satisfy all of corrosion resistance, workability, spot weldability, and chemical conversion treatment properties, but among the above characteristics, excellent corrosion resistance is particularly important from the purpose of the present invention. be. As can be seen from FIG. 3, the chromium concentration in the surface treatment layer greatly affects corrosion resistance, and the contribution of chromium to corrosion resistance is greater than that of other elements. According to the research conducted by the present inventors, as mentioned above, elements other than chromium also contribute to improving corrosion resistance, but their contribution is generally limited to special corrosion resistance such as high temperature oxidation resistance and acid resistance. It was also found that these effects are based on the presence of chromium, and that elements other than chromium alone have little effect. When we comprehensively evaluated corrosion resistance, including salt water resistance, alkali resistance, acid resistance, high temperature oxidation resistance, etc., we confirmed that it is desirable that the weight ratio of elements other than chromium does not exceed 0.4. Each selected element of iron, nickel, niobium, aluminum, copper, cobalt, molybdenum, titanium, and silicon can bring the following effects to the present invention. Iron is the main element of the steel plate and the iron plating layer, and is naturally an essential element for the structure of the present invention. Also, its function is to facilitate the formation of each layer by diffusing and penetrating like chromium and other elements. The elements nickel, niobium, aluminum, copper, molybdenum, titanium, and silicon can be used on the surface of the present invention, excluding elements listed in the periodic table of elements that are gaseous at room temperature and atmospheric pressure. Among the elements that were individually added to the treatment and contributed to the object of the present invention, the effects were significant even when added in small amounts. Among the above metal elements, elements such as nickel and copper improved workability, and niobium, aluminum, and titanium improved weldability. Furthermore, most of the above elements had the effect of improving corrosion resistance. Table 3 shows steel sheets containing chromium at an average concentration of 20wt% in the surface treatment layer (symbol A in the table), 20wt%
Steel plate containing 10wt% nickel in addition to chromium (symbol B in the table), steel plate containing 10wt% nickel in addition to 20wt% chromium
For steel sheets containing 0.6wt% silicon (symbol C in the table) and steel sheets containing 10wt% nickel and 0.6wt% silicon in addition to 20wt% chromium (symbol D in the table), 1000 Indicates the corrosion depth when heated in air at ℃ for 100 hours. From the table, the corrosion depth is reduced by containing nickel or silicon in addition to chromium, and the effects of nickel and silicon are clear. In addition, by containing two elements, nickel and silicon, in addition to chromium, we have added nickel and silicon in addition to chromium.
It is clear that nickel and silicon act synergistically, as it is further reduced than when silicon is contained individually.

[Example 1]

The chemical composition is C: 0.04wt%, Mn: 0.25wt%,
Al: 0.033wt%, balance Fe and unavoidable lower impurities. Chromium is produced by electroplating 22g/ ^m2 of metallic chromium on one side on a 1.0mm thick cold-rolled steel sheet made of low carbon aluminum killed steel sheet as a base material. A single layer was formed, and then 75 g/m ² of iron was electroplated on one side to form an iron-plated layer. After that, it was thoroughly washed with water and dried, then introduced into a box-type annealing furnace, and heated at 970°C in an HN gas atmosphere (hydrogen gas 25vo1%, nitrogen 75vo1%).
The heat treatment was carried out at a temperature of .degree. C. for about 4 hours. After confirming that the surface treatment layer of the surface-treated steel sheet obtained in this way is within the range shown in Table 2, we compared the steel sheet according to the present invention and comparative materials such as hot-dip galvanized steel sheet, chromized steel sheet, and galvanized steel sheet. Corrosion resistance, workability, spot weldability, and chemical conversion treatment properties were tested on the steel plate and the zinc-rich primer-coated steel plate using the methods described below.
The results are shown in Table 4. As clearly seen from Table 4, the surface-treated steel sheet according to the present invention was demonstrated to be extremely superior in all properties compared to the conventional corrosion-resistant surface-treated steel sheet. (Corrosion Resistance) A salt spray test according to JIS Z 2371 was conducted continuously for 3000 hours to determine the corrosion loss (g/m ² ) of the test material, and the weight loss values were compared and evaluated. The evaluation method is as follows. ◎...The corrosion loss is minimal. 〇...The corrosion loss is small. The degree of occurrence was comparatively evaluated. The evaluation method is as follows. ◎...Extremely good quality 〇...Almost satisfactory ×...Poor quality (spot weldability) Under the following welding conditions, one pair of electrode tips was used continuously for each test material without dressing the tip. A comparative evaluation was made based on the number of welding points until nugget formation became impossible. Welding conditions Secondary welding current…8~12KA Pressure force…250Kg Current application time…8~12 cycles Electrode tip…Cu-Cr alloy, C, F type,
Tip diameter: 5 mmφ The evaluation method is as follows. ◎...Number of welding points: 10,000 or more 〇...Number of welding points: 4,000 to 10,000 points ×...Number of welding points: 4,000 or less (chemical conversion treatment properties) As a method for evaluating chemical conversion treatment properties, the general method is to paint after chemical conversion treatment. A method is adopted to examine the adhesion of the coating film. This will be adopted in this exam as well.
Commercially available Bonderite 3112 (Nippon Parkerizing)
Co., Ltd.) for 3 minutes using a spray method, and then commercially available Power Coat 9600 (Nippon Paint Co., Ltd.).
Co., Ltd.'s anionic electrodeposition coating with bath voltage.
The coating was heated at 200V for 3 minutes to further bake the paint film.
This was subjected to an impact resistance test, which is commonly carried out as a method for evaluating the adhesion of a paint film, and comparative evaluation was performed based on the minimum height at which cracks would occur in the paint film. Paint film adhesion test conditions Impact tester: Dupont impact tester Weight: 500g Center of impact radius: 3/16 inch The evaluation method is as follows. ◎...Minimum height is 50cm or more 〇...Minimum height is 30cm or more and 50cm or less ×...Minimum height is 30cm or less [Example 2] Chemical composition: C: 0.07wt%, Mn: 0.32wt%,
A chromium-iron alloy layer (42 wt% chromium, 58 wt% iron) is applied to a cold-rolled steel sheet with a thickness of 0.8 mm, the base material of which is low-carbon rimmed steel consisting of the balance Fe and unavoidable impurities.
g/m ² and coated with an iron plating layer on one side.
Formed at a rate of 52g/ ^m2 . After this, this was led to a box-type annealing furnace, and the HN gas atmosphere (hydrogen gas 20vo1
%, nitrogen gas 80vo 1%) at a temperature of 900°C for about 7 hours. After confirming that the surface treatment layer of the surface treated steel sheet obtained in this way was within the range shown in Table 2, tests were conducted on each property of each test material using the test method explained in Example 1. Ivy. The results are shown in Table 5. From this table, it is obvious that the steel material according to the present invention is superior to conventional surface-treated steel sheets. [Example 3] Chemical composition: C: 0.05wt%, Mn: 0.30wt%,
38g of chromium-nickel alloy (62wt% chromium, 38wt% nickel) on one side is applied to a cold-rolled steel plate with a thickness of 0.7mm, which has a base material of low carbon aluminum killed steel consisting of Al: 0.042wt%, balance Fe and unavoidable impurities. /
A layer consisting of chromium and nickel is formed by electroplating at a ratio of m ² , and then an iron plating layer of 60 g/m2 is applied on each side.
m ² , it was introduced into a box-shaped annealing furnace and heat-treated at a temperature of 950° C. for about 6 hours in an HN gas atmosphere (hydrogen gas 20 vol 1%, nitrogen gas 80 vol 1 %). After confirming that the surface treatment layer of the surface-treated steel sheet obtained in this way was within the range shown in Table 2, the characteristics of each sample material were tested using the test method explained in Example 1. I went. The results are shown in Table 6. As clearly shown in this table, the steel sheet according to the present invention was superior to the conventional surface-treated steel sheet, and fully satisfied the object of the present invention. [Example 4] Chemical composition: C: 0.03wt%, Mn: 0.26wt%,
Al: 0.059wt%, balance Fe and unavoidable impurities A layer consisting of a special metal chromium-aluminum salt composite layer of 37g per side is applied to a 2.2mm thick hot rolled steel plate made of low carbon aluminum killed steel as a base material. / ^m2
(of which 31g is metallic chromium), and on top of this an iron plating layer is applied at a rate of 56g/m ² on one side.
After forming at a ratio of
HN gas atmosphere (hydrogen gas 20vo1%, nitrogen gas
Heat treatment was performed at a temperature of 950° C. for about 6 hours in 80vo 1%). After confirming that the surface treatment layer of the surface-treated steel sheet according to the present invention thus obtained is within the range shown in Table 2, the characteristics of each sample material were determined by the test method explained in Example 1. We conducted tests on The results are shown in Table 7. As is clear from this table, the surface-treated steel sheet according to the present invention was superior to the conventional surface-treated steel sheet in each characteristic, and fully satisfied the object of the present invention.

【table】

【table】

【table】

[Table] As is clear from the above, in the surface-treated steel sheet according to the present invention, the surface treatment layer mainly contains iron and chromium, and the chromium concentration is relatively low in the area from the surface direction of the treated steel sheet to the steel sheet base. Since it is composed of three regions (layers): a region with a relatively high chromium concentration and a region with a relatively low chromium concentration, the characteristics exhibited by each region (layer) are additive. In addition, it exhibits a variety of excellent properties such as corrosion resistance, workability, spot weldability, and chemical conversion treatment properties, and has a wide range of applications as a multipurpose steel sheet that has not been available before.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the generation process of the surface treatment layer in the manufacturing method of the present invention, and FIGS. 2a and 2b are diagrams showing chromium and Figure 3 shows the relationship between the thickness of the entire surface treatment layer and the average chromium concentration and corrosion resistance. Figure 4 shows the relationship between the thickness of the B layer and the average chromium concentration and the corrosion resistance. Figure 5 is a diagram showing the relationship between the thickness of the A layer and press workability, Figure 6 is a diagram showing the relationship between the thickness of the C layer, the average chromium concentration, and spot weldability, and Figure 7 is a diagram showing the relationship between the thickness of the C layer and the average chromium concentration and spot weldability. The figure shows the relationship between the thickness of the C layer, the average chromium concentration, and chemical conversion treatment properties. Figure 8 shows the relationship between the chromium weight ratio and corrosion resistance when nickel is added in addition to chromium to the surface treatment layer. It is a diagram.

Claims (1)

[Claims] 1. A chromium-diffused steel sheet layer with a thickness of 2 μm or more on a steel sheet base, and a chromium-diffused steel sheet layer with an average chromium concentration of 10 to 35 wt%.
A surface treatment layer is provided in which an iron-diffused chromium-containing layer of ~55 μm and a chromium-diffused iron plating layer with an average chromium concentration of 5 wt% or less and a thickness of 3 μm or more are sequentially formed. Chromium concentration 10-30wt%
The thickness is 6 to 60 μm, and the chromium concentration increases continuously from the surface of the chromium-diffused iron plating layer to the inside of the iron-diffused chromium-containing layer, and from the inside of the iron-diffused chromium-containing layer to the steel sheet base. A surface-treated steel sheet having a three-layered coating layer characterized by a continuously decreasing coating layer. 2. The surface-treated steel sheet according to claim 1, wherein the iron-diffused chromium-containing layer has iron diffused into the layer consisting of only chromium. 3. The iron-diffused chromium-containing layer contains chromium, iron,
The surface according to claim 1, wherein iron is diffused in a layer comprising one or more elements selected from the metal element group of nickel, niobium, aluminum, copper, cobalt, molybdenum, titanium, and silicon. Treated steel plate. 4. Forming a chromium-containing layer with a thickness of 1 to 55 μm on the surface of the steel plate, forming an iron plating layer with a thickness of 3 μm or more on this chromium-containing layer, and then heat-treating the steel plate, the chromium-containing layer. Diffuse and infiltrate the metal element between the layer and the iron plating layer,
A chromium-diffused steel plate layer with a thickness of 2 μm or more in which chromium is diffused in the steel plate, and an average chromium concentration of 10 to 35 wt% and a thickness of 1 to 35 wt% in which iron is diffused in the chromium-containing layer.
A three-layer structure characterized by forming a 55 μm iron-diffused chromium-containing layer and a chromium-diffused iron plating layer with an average chromium concentration of 5 wt% or less and a thickness of 3 μm or more in which chromium is diffused into the iron plating layer. A method for manufacturing a surface-treated steel sheet having a coating layer. 5. The method for manufacturing a surface-treated steel sheet according to claim 4, wherein the chromium-containing layer consists of only chromium. 6. Claim 4, wherein the chromium-containing layer comprises chromium and one or more elements selected from the metal element group of iron, nickel, niobium, aluminum, copper, cobalt, molybdenum, titanium, and silicon. A method for manufacturing the surface-treated steel sheet described above.