JP2577246B2 - Manufacturing method of surface-treated steel sheet for coating base with excellent processing corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a drawn and ironed can (Drawn & Ironed Can, DI
Cans), drawn and redrawn cans (DRD cans)
The present invention relates to a method for producing a surface-treated steel sheet for coating undercoating, which is excellent in corrosion resistance and is suitable as a material for cans subjected to severe processing such as drawn cans and can lids.

[Prior Art] Sn-plated steel sheets, so-called tinplates, are widely used as DI can materials for beverage cans. Tinplate DI cans for beverage cans are formed by degreased lubricating oil used in molding, phosphate treatment, etc., after washing and drying, and then spray-painting the inside of the can twice. In recent years, aluminum has been remarkably advanced in the field of beverage cans, and there is a strong demand for cost reduction for tinplate DI cans. Therefore, reduction of the thickness of the steel plate used for tinplate and reduction of the amount of Sn plating have been studied,
These measures reduce the draw and ironability of tinplate, increase the exposed area of the steel plate body of the can body after the DI can forming process, and reduce the corrosion resistance to the beverage after painting. Spray painting cannot be performed once like aluminum DI cans, and the cost reduction of tin cans depends on the cost reduction of the steel plate itself. On the other hand, a so-called Tin Free Steel (TFS) coated with a double-layered coating consisting of a chromium hydrated oxide layer on the upper layer and a metal Cr layer on the lower layer
Is used as a DRD can for fish meat cans. DRD cans are TFS
After coating and printing on the inner and outer surfaces of the, it is formed by drawing and drawing again. Therefore, the TFS used is required to have excellent processing adhesion of the applied paint, and also to have excellent corrosion resistance to these foods after coating. Commercial TF
S has excellent paint adhesion, but does not have sufficient corrosion resistance after painting, and is currently used for limited foods. That is, tin can DI and TFS DRD
A common problem with cans is the corrosion resistance after painting. One method of improving corrosion resistance is to add Cr or the like during steelmaking (Japanese Patent Application Laid-Open Nos. 61-6293 and 62-3089).
6) The method of applying Ni plating on the steel sheet surface and forming a Ni diffusion layer on the steel sheet by heat treatment (JP-A-57-200592, JP-A-60-155685), Sn plating or Sn-Ni alloy plating on the steel sheet surface The steel sheet is treated with Sn or Sn by heat treatment.
Method for forming a diffusion layer of Ni (Japanese Patent Application Laid-Open
0-89594) Further, a method is considered in which Cr is plated first, Ni is plated thereon, and heat treatment is performed to diffuse Cr and Ni into the steel.

[Problems to be solved by the invention]

The method of adding Cr during steelmaking as shown in JP-A-62-30896 improves the corrosion resistance of the steel sheet itself, but not only increases the cost of the steel sheet by adding Cr, but also descaling properties after hot rolling, such as Sn Problems occur in plating properties, can manufacturing, etc.
It is difficult to say that it is an inexpensive tinplate and suitable for TFS base steel sheet. In addition, when tinning using a base steel sheet in which Ni is diffused to the surface of a steel sheet as shown in JP-A-57-200592, etc., TFS is liable to crack in the Ni diffusion layer when processed, and in this state, beverages such as fruit juice and the like are produced. When exposed to cracks in the Ni diffusion layer, the steel exposed to the cracks in the Ni diffusion layer is susceptible to anodic dissolution by the cathode of the Ni diffusion layer, and there is a risk of pitting corrosion. Furthermore, Japanese Patent Application Laid-Open Nos. 60-5894 and 60-89594, the method of forming a diffusion layer of Sn or Sn and Ni does not show sufficient corrosion resistance only with Sn, and also diffuses Sn and Ni. Although the corrosion resistance improves when unprocessed, when processed,
Similar to the steel sheet in which Ni is diffused, the steel exposed in the cracks of the Sn and Ni diffusion layers in the beverage tends to cause anode dissolution,
It is not enough as a base steel sheet for surface-treated steel sheets for cans.
Further, even if the purpose of diffusing Cr and Ni into steel is the same, as a different method, after performing Cr plating, Ni
The method of plating and thermally diffusing is a simple manufacturing method and has a high Cr diffusivity. However, because of the two-step method, in addition to the drawback that the equipment cost and the maintenance cost are high, when incorporating into a current line, it is possible that a space cannot be obtained. To solve these problems, it is inexpensive, especially effective for work corrosion resistance, and does not cause any special problems in the steps after cold rolling. As a result of various studies from the viewpoint of the base steel plate, the present invention has been achieved.

[Means for solving the problem]

The present invention is mainly cold-rolled, after applying a Cr-Ni alloy plating on the steel sheet surface after electrolytic cleaning, heat-treated in a non-oxidizing atmosphere of iron, a heat treatment diffusion layer of Cr, Ni on the surface layer of the steel sheet. An object of the present invention is to provide a surface-treated steel sheet for coating undercoat having excellent corrosion resistance after being formed and then subjected to Sn plating or TFS treatment. In particular, the Sn-treated surface-treated steel sheet of the present invention is a material suitable for tinplate DI cans, and the TFS-treated surface-treated steel sheet of the present invention is suitable for a DRD can material to be processed after painting.

Hereinafter, the method of the present invention will be specifically described. Cr content 3 ~ 150mg / on the steel sheet surface after cold rolling and electrolytic cleaning
m ^2, Ni amount 20 to 500 mg / m ^2, and the weight ratio of Cr / Ni is 0.05 to 0.2
0, and then heat-treated in a non-oxidizing atmosphere of iron.
% Or more and with plated Cr content of 1 mg / m known method on a steel plate to form a heat-treated diffusion layer containing ² or more 1.5~5.6g / m ²
Sn plating or TFS treatment by a known one-step method or two-step method, the upper layer is a chromium hydrated oxide layer of ^{2 to} 30 mg / m ² as Cr, and the lower layer is 30 to 200 mg / m 2
^{This is} to form a TFS treatment film composed of metal Cr of No. ² . First, the Cr-Ni alloy plating applied to the steel sheet after cold rolling and electrolytic cleaning will be described. If the Cr content in the Cr-Ni alloy plating is 3 mg / m ² or less, Sn
It does not hinder plating or TFS processing,
Cr of 1 mg / m ² or more cannot be diffused to the surface layer of the steel sheet by heat treatment, and even if the base steel sheet is subjected to Sn plating or TES treatment, the surface-treated steel sheet for coating undercoat which has excellent processing corrosion resistance aimed at by the present invention. Cannot be obtained. Metal Cr is very easily oxidized, and some of the plated Cr becomes Cr oxide by heat treatment in a non-oxidizing atmosphere of iron. Plated Cr
In this way, self-oxidation prevents oxidation of iron and prevents the surface concentration of C contained in the steel sheet, but has little effect of improving corrosion resistance as it is. It deteriorates the uniformity of the treated film and the deposition efficiency of the TFS treated film. Cr oxide can be easily removed by electrolytic cleaning before TFS treatment in the case of a small amount, and as a result contributes to the cleaning of the base steel sheet surface.
If Cr content in the i alloy plating is at 150 mg / m ² or more, if you accept, although rust resistance is improved, also increases the amount of Cr oxide which is formed during the heat treatment, remains even after the electrolytic cleaning , Sn plating or TFS treatment is not preferable because it causes trouble. Therefore, the Cr content in the Cr—Ni alloy plating is preferably in the range of 3 to 150 mg / m ² , more preferably 10 to 150 mg / m ^2.
It is in the range of 7070 mg / m ² . Ni in the Cr-Ni alloy plating is an element that easily diffuses into the steel sheet by heat treatment in a non-oxidizing atmosphere of iron, and the amount of Ni in the Cr-Ni alloy plating is 20 mg / m ^2.
If it is below, a surface-treated steel sheet for coating undercoat with excellent work corrosion resistance aimed at by the present invention cannot be obtained, and the Ni content is 500 mg /
If it is at least m ² , there is a risk that even after the heat treatment, metallic Ni remains on the surface of the steel sheet and promotes pitting corrosion of the steel sheet. Therefore, the amount of Ni in the Cr—Ni alloy plating is in the range of ^{20 to} 500 mg / m ² , more preferably 50 to 200 mg / m ^{2 in} the present invention. Further, the weight ratio of Cr / Ni in the Cr-Ni alloy plating is also a very important factor in the present invention. That is, if the weight ratio of Cr / Ni in the Cr-Ni alloy plating is 0.05 or less, after forming the heat treatment diffusion layer, Sn plating or
After TFS treatment, when painted, the corrosion resistance is not improved, and when the ratio is 0.20 or more, the corrosion resistance of the steel sheet itself is somewhat improved, but the oxide of Cr on the heat treatment diffusion layer This is not preferable because it tends to remain and lowers the uniformity of the film formed by Sn plating or TFS treatment performed thereafter. Therefore, the weight ratio of Cr / Ni in the heat treatment diffusion layer is preferably in the range of 0.05 to 0.20.

For this Cr-Ni alloy plating, a sulfate bath, a chloride bath, and a sulfate-chloride mixed bath are suitable in terms of bath management, chemical cost, and the like. As shown in FIG. 1, the Cr-Ni alloy plating having a weight ratio of Cr / Ni of 0.05 to 0.20 is used for the concentration ratio of trivalent chromium ion to nickel ion (Cr ³⁺ / N
i ²⁺ ) is maintained at 0.45 to 1.50, boric acid, sodium citrate, etc. are added as a buffer, the pH is 1.8 to 3.5, and the bath temperature is 30.
6060 ° C., and as shown in FIG.
It is obtained by electrolysis at a current density of A / dm ² . Chromium sulfate, potassium chromium sulfate, chromium ammonium sulfate or chromium chloride may be used as the chromium salt, and nickel sulfate, nickel chloride or the like may be used as the nickel salt. The method of the present invention is subjected to a heat treatment in a non-oxidizing atmosphere of iron after the plating of the Cr-Ni alloy, but the surface-treated steel sheet obtained by the method of the present invention is a material such as DI cans and DRD cans, and is excellent. It is an essential condition that, in addition to the processed corrosion resistance, it also has good mechanical properties with good can-making properties. Therefore, it is a premise of the present invention to form a heat treatment diffusion layer of Cr and Ni on the surface layer of the steel sheet by applying the same heat treatment conditions as those of the base steel sheet of the steel sheet for cans such as normal tinplate and TFS. That is, for example, by performing a heat treatment at a temperature of 500 to 750 ° C. for 15 to 30,000 seconds in a non-oxidizing atmosphere of iron of 6% hydrogen and 94% nitrogen, 95% or more of plated Ni and plated Cr It is indispensable in the present invention to form a heat treatment diffusion layer containing 1 mg / m ² or more on the surface layer of the steel sheet. For example, when a heat treatment diffusion layer containing plated Cr and Ni is formed using equipment for continuous annealing which is a manufacturing process of a base steel sheet such as tinplate, the soaking time is as short as 15 to 60 seconds. Under these heat treatment conditions, most of the plated Ni diffuses into the surface layer of the base steel sheet, but it is difficult to diffuse 50% or more of the plated Cr, and only about 5-35% of the plated Cr is diffused. Therefore, it is necessary to apply a Cr—Ni alloy plating containing a Cr amount at least about three times the Cr amount to be diffused to the surface layer of the steel sheet. To sufficiently diffuse the plated Cr, it is preferable to perform heat treatment at a high temperature.However, if the heat treatment is performed at an excessively high temperature, the shape of the steel sheet deteriorates or a steel sheet having properties required for a material for cans is obtained. The maximum temperature is limited to about 750 ° C. When a box type annealing furnace is used and heat treatment is performed, the heating time is long, so that not only plated Ni but also plated Cr is considerably diffused even at a relatively low temperature. In the method of the present invention, after this heat treatment, temper rolling is performed in the same manner as in the production of normal tinplate and TFS.
Plating or TFS processing is performed. First, a method of Sn plating will be described. For the Sn plating, a known Sn plating bath used in the production of ordinary tinplate may be used. That is, baths such as a ferrostan bath and a halogen bath are suitable. If the amount of Sn plating is 1.5 g / m ² or less, not only the drawability is reduced, but also the corrosion resistance is reduced even when painted after the forming, which is not preferable. Moldability aperture with increasing Sn plating amount, since the corrosion resistance is improved, but there is no problem in characteristics even 5.6 g / m ² or more Sn coating weight, the upper limit of the Sn plated amount in view of economy 5.6 g / It was limited to m ^2. S
melt processing of Sn after n plating With the reduction of the metal Sn amount by the formation of Fe-Sn alloy layer is not preferable from the viewpoint of DI cans moldability, be secured about 1.0 g / m ² or more metallic Sn quantity For example, Sn
In the method of the present invention, the presence or absence of the melting treatment of Sn after Sn plating is not particularly limited because the melting process does not particularly affect the formability. Thereafter, in order to prevent oxidation of the surface Sn, the surface Sn is generally treated using a dichromate solution. However, the immersion treatment is more preferable than the cathodic electrolytic treatment from the viewpoint of DI moldability. However, when the Sn-plated surface-treated steel sheet of the present invention is used for drawing after coating, cathodic electrolysis is performed in a chromic acid solution containing a smaller amount of sulfuric acid and a fluorine compound than in the method of treating with a dichromate solution. , Metal Cr on the surface
And a method of forming a film composed of hydrated chromium oxide. Therefore, the chromic acid treatment method after Sn plating on the Sn-plated surface-treated steel sheet obtained by the method of the present invention may be appropriately selected depending on the application.

Next, a method of performing TFS processing will be described. It is known that there are two methods for TFS processing. One is a one-step method of simultaneously forming a film composed of a metal Cr layer and a chromium hydrated oxide layer using a chromic acid bath of 100 g / l or less containing an appropriate amount of sulfuric acid and a fluorine compound. This is a two-step method in which a chromium hydrated oxide layer is formed using a low-concentration chromic acid bath after Cr plating using a high-concentration chromic acid bath. In the production of the surface-treated steel sheet to be TFS-treated according to the present invention, both methods can be used. The metal Cr layer, which is the lower layer of the two-layer film formed by this TFS treatment, is thin,
For example, if the amount of Cr is 30 mg / m ² or less, even if Cr, Ni
Even if the heat treatment diffusion layer is formed on the surface layer of the steel sheet, the corrosion resistance after processing is not improved. Further, the corrosion resistance improves with an increase in the amount of metal Cr, but when the amount is 200 mg / m ² or more, the risk of cracks in the metal Cr layer during processing increases, and the effect of the increase in the amount is small. Therefore, the amount of metal Cr is 30
A range of 200200 mg / m ² is suitable in the present invention. The amount of the upper chromium hydrated oxide layer is suitably in the range of ^{2 to} 30 mg / m ² as the amount of Cr. When the amount is 2 mg / m ² or less, the processing adhesion of the coating is reduced, the coating is processed, and further retorted with steam at a temperature of 100 ° C. or higher, the coating may undesirably peel off. . If its amount is 30 mg / m ² or more, after the coloring of the hydrated chromium oxide layer becomes large, and processed after coating, the risk of peeling at the interface between the metal Cr layer and the hydrated chromium oxide layer It is not preferable because it increases. Cr-Ni alloy plating, the surface-treated steel sheet of the present invention which is subjected to Sn plating or TFS treatment after heat treatment is an excellent coated base surface-treated steel sheet as described above. Surface-treated steel sheets having Cr and Ni heat diffusion treatment layers also have excellent corrosion resistance and paint adhesion, so they can be used as materials for cans to be coated in applications that do not require severe corrosion resistance.

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples.

Example 1 A cold-rolled steel sheet having a thickness of 0.28 mm was degreased in a rolling oil in a 4% aqueous sodium orthosilicate solution at a temperature of 90 ° C., a current density of 20 A / dm ² , and an electrolysis time of 5 seconds. A Cr-Ni alloy plating was performed under the conditions shown in (a) and (b), and the plate was washed with water and dried. This sample was subjected to a heat treatment at a temperature of 640 to 680 ° C. for 30 seconds in a non-oxidizing atmosphere of iron (hydrogen 6%, nitrogen 94%). Further, after 2% temper rolling, Sn plating was applied under the conditions shown in (b), washed with water, immersed in a 3% sodium dichromate solution at a temperature of 40 ° C. for 5 seconds, washed with water and dried.

(B) Cr-Ni alloy plating condition bath composition _{Cr 2 (SO 4) 3 ·} 5H 2 O 80g / l NiCl 2 · 6HO 80g / 1 H 3 BO 3 40g / l sodium citrate 40 g / l pH 2.5 bath temperature 50 ℃ Cathode current density 40A / dm ² Cr amount in Cr-Ni alloy plating 6mg / m ² Ni amount in Cr-Ni alloy plating 55mg / m ² (b) Sn plating conditions after heat treatment and temper rolling Bath composition SnSO ₄ 60 g / l Phenolsulfonic acid (as sulfuric acid) 15 g / l ethoxylated α-naphthol 5 g / l Bath temperature 45 ° C Cathode current density 20 A / dm ² Sn plating amount 2.7 g / m ² Example 2 Same as Example 1 After the cold-rolled steel sheet was subjected to the same pretreatment as in Example 1, a Cr-Ni alloy plating was performed under the conditions shown in (a) of Example 1 with changing the electrolysis time, followed by washing and drying. Then, in the same non-oxidizing atmosphere as in Example 1, the temperature was 520 to 560.
Heat treatment was performed at 8 ° C. using a box-type annealing furnace for 8 hours. Further, passivation rolling of 2% was performed, Sn plating was performed at a Sn amount of 4.5 g / m ^{2 under} the conditions shown in (b) of Example 1 and the electrolysis time was changed. Processing was performed.

Example 3 The same cold-rolled steel sheet as in Example 1 was subjected to the same pretreatment as in Example 1, and was subjected to Cr-Ni alloy plating under the conditions shown in (a) of Example 1, and was washed with water and dried. Then, a heat treatment was performed at 640 to 680 ° C. for 20 seconds in the same non-oxidizing atmosphere as in Example 1. Further, passivation rolling of 2% was performed, Sn plating of a Sn amount of 1.8 g / m ² was performed under the conditions shown in (b) of Example 1, and the electrolytic time was changed. Chromic acid treatment was applied.

Comparative Example 1 The same cold-rolled steel sheet as in Example 1 was subjected to the same pretreatment as in Example 1, and then subjected to a heat treatment under the same conditions as in Example 1 without performing Cr-Ni alloy plating. Then, after subjecting to 2% temper rolling, Sn plating of 4.5 g / m ² of Sn amount was performed under the conditions shown in (b) of Example 1 while changing the electrolysis time.
Chromic acid treatment was performed under the same conditions as described above.

Example 4 The sample before Sn plating obtained in Example 1 was subjected to a TFS treatment under the conditions shown in (A), washed with water, and dried.

(B) TFS treatment conditions after heat treatment and temper rolling Bath composition CrO ₃ 60 g / l NaF 3 g / l H ₂ SO ₄ 0.3 g / l Bath temperature 55 ° C. Cathode current density 30 A / dm ² Metal Cr in the formed film Amount of hydrated chromium oxide in the formed film 110 mg / m ² Amount of hydrated chromium oxide (as Cr) 18 mg / m ² Example 5 The sample before Sn plating obtained in Example 2 was subjected to the conditions shown in (a) of Example 4. TFS treatment with different electrolysis time,
Washed and dried.

The amount of metallic Cr in the formed film 75 mg / m ^{2 The} amount of chromium hydrated oxide in the formed film (as Cr) 8 mg / m ² Example 6 The sample before Sn plating obtained in Example 3 was added to (a). After chromium plating was performed under the conditions shown and washed with water, a chromium hydrated oxide film was formed under the conditions shown in (b) and washed with water and dried.

(B) Cr plating conditions after heat treatment and temper rolling Bath composition CrO ₃ 180 g / l Ns ₂ SiF ₆ 3.5 g / l H ₂ SO ₄ 1.5 g / l Bath temperature 50 ° C Cathode current density 40 A / dm ² (b) Conditions for forming chromium hydrated oxide film Bath composition CrO ₃ 30 g / l NaF 1.2 g / l Bath temperature 40 ° C. Cathode current density 25 A / dm ^{2 The} amount of metallic Cr deposited by both treatments is 135 mg / m ² , and the amount of chromium hydrated oxide Was 15 mg / m ² as the Cr content.

Comparative Example 2 The same cold-rolled steel sheet as in Example 1 was subjected to the same pretreatment as in Example 1, and then heat-treated under the same conditions as in Example 1 without performing Cr-Ni alloy plating. Next, after temper rolling of 2% was performed, TFS treatment was performed under the conditions shown in (a) of Example 4, and washing and drying were performed.

After measuring the surface coating composition of the sample obtained by the above method and the amounts of Cr and Ni in the heat-treated diffusion layer by the following methods (1) and (2), Examples 1 to 3 and Comparative Example 1 were obtained. The samples obtained in Examples 3 to 6 were obtained by the method shown in (3).
The sample subjected to TFS treatment obtained in Comparative Example 2 was examined for processing corrosion resistance by the method shown in (4). Table 1 shows the results.

(1) Measurement of Cr content and Ni content in heat-treated diffusion layer Cr in heat-treated diffusion layer of sample before Sn plating or TFS treatment
After quantity measuring the total amount of Cr sample fluorescent X-ray method, in 1N NaOH solution, at a current density of 5A / dm ^2, anode electrolysis, after the cathode electrolysis performed each 30 seconds, diffuse residual Cr amount And the amount of Cr.
The amount of diffused Ni was determined by measuring the total amount of Ni in the sample by X-ray fluorescence and then using a photoelectron spectrometer to sputter Ar from the surface of the sample. The Ni / Fe area intensity ratio was Ni / Fe> 1. The amount of sputter is defined as the undiffused Ni amount, and the remaining Ni amount is diffused. The Ni amount was determined, and the Ni diffusivity was determined from both.

(2) Measurement of amount of Sn plating, amount of Cr metal and amount of chromium hydrated oxide after heat treatment and temper rolling The amount of Sn plating of the sample was measured by a fluorescent X-ray method. Also, the amount of chromium hydrated oxide in the sample was determined by measuring the total Cr by X-ray fluorescence.
The sample was immersed in a 7.5N NaOH solution at a temperature of 95 ° C. for 1 minute, washed with water, and again measured for Cr by a fluorescent X-ray method, and the difference was defined as the amount of Cr in the hydrated chromium oxide. The amount of metallic Cr was determined from the difference between the measured amount of Cr after immersion in a 7.5N NaOH solution and the amount of Cr measured before the TFS treatment.

(3) Corrosion resistance of Sn-plated sample after heat treatment and temper rolling The sample is punched out into a 125mm diameter disk with a punch and the diameter is 53m
A DI can having a height of 134 mm and a height of m was molded, the lubricating oil was removed with a weakly alkaline solution, washed with water and dried. After repacking the beer in this DI can, it was left at room temperature for 7 days, and the amount of eluted Fe was measured by an atomic absorption method.

(4) Corrosion resistance of the sample subjected to TFS treatment after heat treatment and temper rolling. A phenol / epoxy paint of 60 mg / dm ² (dry weight) is applied to the sample surface, cured at 210 ° C. for 10 minutes, and then punched to a diameter of 80 mm. And a drawing process was performed at a drawing ratio of 2.0 with the painted surface as the inner surface. 0.4% in this cup
Add 30 ml of CH ₃ COOH solution, leave at room temperature for 2 weeks, and elute Fe
The amount was measured by the atomic absorption method.

Surface treated steel sheet was subjected to Sn plating Sn amount 1.5~5.6g / m ² on the surface treated steel sheet obtained by the method of the present invention [Effect of the Invention] The heat treatment diffusion layer especially comprising Cr, a Ni is DI cans Excellent corrosion resistance after molding,
It is effective in reducing the amount of tin plating in tinplate, and can be used as a material for DI cans instead of ordinary tinplate. Furthermore, Sn
By using in combination with chromic acid treatment with excellent paint adhesion after plating, it becomes an excellent material after coating and can be used for a wide range of uses as a can material, and is extremely useful in industry. Also, on the thermal diffusion treatment layer containing Cr and Ni
TFS-treated surface-treated steel sheet not only has excellent paint adhesion, but also has excellent corrosion resistance after painting, and is a very excellent material for cans, such as DRD cans, which are subjected to drawing and drawing after painting. Therefore, the use can be expanded.

[Brief description of the drawings]

Fig. 1 shows the Cr content in Cr-Ni alloy plating bath under certain conditions.
FIG. 3 is a diagram showing a Cr / Ni weight ratio in a Cr—Ni alloy plating with respect to a ^{3 +} / Ni ²⁺ ratio. FIG. 2 is a diagram showing the weight ratio of Cr / Ni in Cr-Ni alloy plating to the cathode current density in Cr-Ni alloy plating under certain conditions.

Claims (2)

(57) [Claims] 1. The steel sheet has a Cr content of 3 to 150 mg / per one surface.
m ^2, Ni amount is 20 to 500 mg / m ^2, and 0.05 weight ratio of Cr / Ni
Subjected to Cr-Ni alloy plating of 0.20, subsequently subjected to a heat treatment in a nonoxidizing atmosphere of an iron, a thermal diffusion treatment layer containing 1 mg / m ² or more to 95% or more and plated Cr amount of plated Ni amount Is formed, and the lower layer is formed on the heat diffusion treatment layer in an amount of 30 to 200 mg / m ^2.
A method for producing a surface-treated steel sheet for coating undercoating having excellent work corrosion resistance, comprising forming a coating comprising a chromium hydrated oxide layer having a Cr content of ^{2 to} 30 mg / m ² as a Cr layer. 2. The steel sheet has a Cr content of 3 to 150 mg / per one surface.
m ^2, Ni amount is 20 to 500 mg / m ^2, and 0.05 weight ratio of Cr / Ni
Subjected to Cr-Ni alloy plating of 0.20, subsequently subjected to a heat treatment in a nonoxidizing atmosphere of an iron, forming a heat treatment diffusion layer containing 1 mg / m ² or more than 95% of the plated Ni amount and plated Cr amount And the lower layer is 1.5 to 5.6 g / m ² on the heat-treated diffusion layer.
A method for producing a surface-treated steel sheet for coating undercoat, which is excellent in working corrosion resistance and is characterized by being subjected to Sn plating.