JPH07310167A - Chrom-nickel diffusion treated steel sheet excellent in workability and corrosion resistance and its production - Google Patents

Abstract

PURPOSE:To produce a Cr-Ni diffusion treated steel sheet excellent in workability and corrosion resistance by subjecting a dead soft steel sheet in which a Cr plated layer and an Ni plated layer each having specified thickness are formed on the surface and which has specified C and Nb contents to heat treatment. CONSTITUTION:On one side or both sides of an Nb-added steel sheet contg. <=0.003wt.% C and in which the total content of C and N in the steel is fixed as Nb compounds, a Cr plated layer having 0.1 to 1mum thickness is formed, and on the surface, an Ni plated layer having 1 to 8mum thickness is formed. This steel sheet is subjected to heat treatment at the A3 transformation point or below in a nonoxidizing, reducing or hydrogen atmosphere, by which the diffusing treated steel sheet can be obtd. The C content is limited, since, in the case of a general low carbon steel sheet, the content of Nb increases for fixing carbon as NbC. As for the Cr plating thickness, in the case of less than the lower limit, a film having sufficient corrosion resistance can not be obtd., and in the case of more than the upper limit, the need of thickening the Ni plating layer is produced as well as the prolongation of the treatment or the like. In the case the Ni plating thickness is less than the lower limit, its decoloration can not be suppressed, and in the case of more than the upper limit, its cost is made high.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-layer plated thermal diffusion steel sheet of Cr and Ni and a manufacturing method thereof, and more particularly to a plated thermal diffusion steel sheet having good formability and corrosion resistance and a manufacturing method thereof.

[0002]

2. Description of the Related Art A variety of surface-treated steel sheets and strips are disclosed in which the plating interface is alloyed by heat treatment of a two-layer plated steel sheet of Cr and Ni to improve the formability and corrosion resistance by improving the adhesion. Has been done. For example, (1)
Japanese Unexamined Patent Publication No. 60-230995 discloses that a steel plate is first coated with 0.3 to Ni.
3g / m ² plating, and further Cr on the upper layer 0.45-
After plating at 9 g / m ² and a Cr / Ni ratio of 1.5 to 3, Cr was diffused at 900 ° C. or less over the entire surface to obtain a component having a composition similar to that of 18-8 stainless steel. ing. (2) Japanese Patent Laid-Open No. 61-41760 uses N steel plate
i: 2 to 6 μm and Cr: 4 to 15 μm by two-layer plating (N
7) after performing i and Cr plating in either order)
Disperse in a molten salt bath at 50 to 950 ° C for 1 to 4 hours,
A component similar to 18-8 stainless steel is obtained. Further, mainly for coating purposes (3) JP-A-2-274866
Then, 50 to 150 mg / m ² using the steel plate for tinplate as the original plate
Cr and 100-3000 mg / m ² Ni, Ni-F
e or Ni-Co two-layer plating, then "Ni-C
50 "in a continuous annealing furnace to form the" r "diffusion layer.
Diffusion treatment is performed at 0 to 900 ° C. for 10 to 240 seconds, and in a box annealing furnace at 500 to 700 ° C. for about 5 to 20 hours,
Further, it is disclosed that the adhesion of coating and the corrosion resistance after coating are improved by performing treatment of a chromate coating layer or the like after plating with Cr, Sn or Ni or its alloy as required.

However, in general, when Ni is plated on the underlayer and Cr is plated on the surface layer as in the method (1) and then thermal diffusion is performed, strong oxygen of Cr is present in the annealing atmosphere currently used for ordinary steel sheets and stainless steel sheets. Since it is almost impossible industrially to prevent the oxidation due to the affinity, it is essential to remove the generated strong oxide film by electrolytic pickling or buffing, which is not economical. In the method (2) described above, the thickness of the Cr plating layer is set to 4 to 15 μm, but since Cr plating has a low cathode current efficiency, such a thick plating requires a high processing current density and a long electrolytic cell. It has the disadvantage of high cost and low productivity. Further, thermal diffusion in a molten salt bath is not economical because of low productivity for steel sheets. In the method (3), the amount of Cr plating deposited is small, and the deposited Cr element preferentially combines with C in the steel, which is an interstitial element, and this hard and poorly workable compound improves the corrosion resistance of the worked part. Combined with deterioration, it has a drawback that the corrosion resistance is unsatisfactory for so-called naked applications such as unpainted use.

[0004]

The present invention solves the above-mentioned drawbacks, that is, "Cr-N" which is excellent in workability and corrosion resistance.
The purpose of the present invention is to provide a diffusion-treated steel sheet and its manufacturing method.

[0005]

In order to solve the above-mentioned problems, the Cr-Ni diffusion-treated steel sheet of the present invention has a carbon content of 0.003 wt% or less, and the total amount of C and N in the steel is Nb compounds. On one or both sides of the fixed Nb-added steel plate,
A Cr plating layer having a thickness of 0.1 to 1 μm is formed,
Further, by heat-treating a two-layer plated steel sheet having a Ni plating layer of 1 to 8 μm formed on the Cr plating layer, Cr
It is characterized in that the plating layer and the Ni plating layer are mutually diffused. It is desirable that the surface of such a diffusion-treated steel sheet be buffed to have gloss, workability and corrosion resistance. Further, in the method for producing a Cr—Ni diffusion-treated steel sheet of the present invention, the carbon content is 0.003 wt% or less, and C in the steel is
Using a Nb-added steel plate in which N is fixed as the total amount of Nb compound, one or both surfaces of Cr having a thickness of 0.1 to 1 μm
1 to 8 μm on the surface after plating and then Cr plating
The Ni-plated steel sheet is heat-treated at a temperature not higher than the A ₃ transformation point in a non-oxidizing atmosphere, a reducing atmosphere or a hydrogen atmosphere. In the method of manufacturing such a diffusion-treated steel sheet, it is desirable that the surface be buffed to have glossiness, workability and corrosion resistance. In addition, FIG. 1 shows a schematic cross-sectional view of a Cr—Ni diffusion-treated steel sheet of the present invention. The steel plate treated in this way can be used as it is as an original plating plate having excellent corrosion resistance without being subjected to coating treatment.

The present invention will be described in detail below.

The original plate used in the present invention is a generally used cold-rolled steel plate or a cold-rolled steel plate with a finish that has been annealed and temper-rolled and has a C content of 0.003%. In the following, an Nb-added steel plate added with Nb to fix C and N in the steel is used, but a Ti-added steel that fixes C can also be applied. The components other than these elements are not particularly limited, but the main component is commonly used for tin plating: Si: ≤0.03, M
n: 0.10 to 0.40, P: ≤ 0.02, S: ≤ 0.
02, Al: 0.03 to 0.07, N: ≤ 0.006 is preferable. The reason for making the amount of C extremely low carbon is that it is not economical when the amount of C generally used for tinplate is low and the amount of Nb added to fix C as NbC is large. .

Table 1 shows examples of the chemical composition of the Nb-added steel used in the present invention and the low carbon steel used as a general tinting material. 2 and 3 are cold-rolled steel sheets having these components, which have thicknesses: two layers of 0.4 μm Cr and 4 μm Ni, and a glow discharge emission analysis of the steel sheets diffusion-treated at 600 ° C. for 7 hours using a box-type diffusion furnace. Method (GDS = Glow Discharge Spectroscopy
2 is an example of the result of cross-section analysis in the plate thickness direction by scopy).
Is an Nb-added steel plate, and FIG. 3 is an analysis result of a plated heat diffusion steel plate using a low carbon steel as a plating base plate as a comparative example. Two
The C peak, which cannot be seen after the layer plating, is generated at the location corresponding to the Cr peak portion due to thermal diffusion.
In the Nb-added steel fixed by b, almost no C peak is found. That is, it is shown that the "Cr-C" compound is formed in the low carbon steel, but this compound is not formed in the Nb-added steel. Further, it is shown that the diffusion of plated Cr to the surface layer is slower when the Nb-added steel plate is used as the plating original plate than when the low carbon steel plate is used.

Table 1 below shows the original plate components of the sample for component distribution investigation by GDS.

[Table 1]

The above-mentioned Nb-added steel plating base plate is electrolytically degreased and pickled, and then plated with a metal Cr having a thickness of 0.1 to 1 μm by an electrolytic method. The plating bath conditions are not particularly specified. A commonly used TFS (tin-free steel) plating bath or a Cr plating salgent bath can be used.

If the Cr plating thickness is less than 0.1 μm, even if a Cr compound of “Cr—Ni—Fe” having good corrosion resistance is produced by diffusion treatment, a film having sufficient corrosion resistance cannot be obtained.
Also, if the Cr plating thickness is thicker than 1 μm, a film with excellent corrosion resistance can be obtained, but the Cr plating processing cost, the diffusion processing is prolonged, and the Ni plating thickness for preventing oxidation due to the diffusion of Cr to the surface layer. It is not economical because it is necessary to increase the thickness, and the thickness is preferably 0.1 to 1 μm, more preferably 0.2 to 0.6 μm.

After Cr plating, Ni plating is applied to increase the diffusion rate of Cr into the steel and to reduce the generation of Cr oxides caused by diffusion in the surface layer direction. Generally, the pH of the plating bath is 3 In the plating baths Nos. 5 to 5, a Ni strike plating bath having a low PH is used because a strong Cr oxide film generated in a short time after Cr plating cannot be removed. However, since this Ni plating bath has a low cathodic deposition efficiency, the total Ni plating amount is not applied only by this bath, and after performing Ni strike plating for a short time, a commonly used bath such as Watt bath or sulfamic acid bath is used. Most of the N used
It is more economical to plate the adhesion amount.

In the heat diffusion treated steel sheet plated with two layers of Cr and Ni, the Cr element is diffused and the Cr compound is "Cr-F".
Whether the "e" ferrite system, the "Cr-Ni-Fe" austenite system, or the two-phase structure of ferrite and austenite system coexist is determined by the Ni plating amount together with the diffusion conditions. Therefore, the thickness of Ni can be determined in consideration of the thickness of Cr plating, but if it is less than 1 μm, Cr cannot be suppressed from diffusing to the surface layer and discoloring. On the other hand, if it is thicker than 8 μm, the corrosion resistance is improved, but the manufacturing cost is increased and it is not economical. N
The i plating thickness of 1 to 8 μm is economical, but the thickness is more preferably 2 to 4 μm.

FIG. 4 shows SUS4 of ferritic stainless steel.
30 (18% Cr) and S of austenitic stainless
US304 (18% Cr-8% Ni) and 0.
FIG. 6 is an X-ray diffraction diagram by Cu-Kα rays of a steel plate obtained by diffusing a 6 μm Cr + 4 μm Ni-plated steel plate at 820 ° C. for 5 hours. Indicates an organization. Therefore, this alloy layer has the effect of being excellent in corrosion resistance as well as processing characteristics.

After the two-layer plating, a diffusion treatment is carried out in order to diffuse Cr and improve the workability of the Ni plating film. This diffusion treatment involves oxidation of the end surface where the Ni plating layer and the plating are not adhered and the plating pinhole portion. For prevention, generally in a non-oxidizing atmosphere or a reducing atmosphere, for example, N ₂ gas, Ar gas,
It is carried out in an atmosphere of H ₂ gas and H ₂ -N ₂ mixed gas.
It can be performed using either a box-type annealing furnace or a continuous-type annealing furnace.

The diffusion treatment conditions, that is, the heat treatment temperature and the heat treatment time depend on the type of plating original plate used, the type of thermal diffusion treatment furnace,
The diffusion rate is proportional to the square root of the diffusion coefficient, and in the case of iron, the α region with a lower temperature has a larger diffusion coefficient than the γ region with a higher temperature, and γ
When diffused in a region, there is a phase transformation associated with heating / cooling, so in the case of a thin steel sheet, plate deformation is likely to occur, and diffusion may proceed rapidly at grain boundaries, resulting in diffusion in the α region below about 900 ° C. preferable. More preferably 65 in a continuous heat treatment furnace
Approx. 900 ° C or lower at 0 ° C or higher, 550 ° C in box heat treatment furnace
The temperature is about 750 ° C or lower. If the diffusion temperature is below the above temperature, the diffusion rate will be slow even though it is in the α region where the diffusion coefficient is large.As a result, it takes a long time to diffuse and the productivity is poor. Is caused.

Although Cr can be diffused within the above α region temperature range, if Cr diffuses to the surface and appears, Cr oxide is formed and discolored, which is not preferable. In order to suppress this discoloration, it is necessary to minimize the diffusion of Cr plated on the underlayer to the surface layer. Further, if the plated Cr layer is present without being alloyed, processing cracks will occur in the Cr layer and the corrosion resistance of the processed portion will be deteriorated, so it is necessary to diffuse the entire amount. If it diffuses too deep inside the steel sheet, the Cr concentration in the "Cr-Ni-Fe" alloy layer will be too low to contribute to the improvement of corrosion resistance. The preferred concentration of Cr in the "Cr-Ni-Fe" alloy layer is 20-60%.

Table 2 shows the X-ray diffraction results of the plating films obtained by diffusing Nb-added steel sheets having various amounts of Cr and Ni deposited thereon in a continuous diffusion treatment furnace and a box diffusion treatment furnace. Ni
As the coating weight increases, the structure changes from BCC structure, which is the same as ferritic stainless steel, to FCC structure, which is the same as austenitic stainless steel.
It indicates that CC and FCC exist as two layers. If there is a two-phase structure but the peak height of one is extremely low, the structure is shown in parentheses.

Table 2 below shows the surface layer structure by X-ray diffraction. In addition, a low peak is shown in () in Table 2.

[Table 2]

[0020]

【Example】

(Example 1) Table 3 shows plating plate components of the Nb-added steel plate and the low carbon steel plate used in Example 1. These three types of temper-rolled steel plates (plate thickness: 0.3 mm) are used as plating base plates, and after alkaline electrolytic degreasing and cleaning and activation treatment by sulfuric acid pickling, Cr plating is applied to the base and Ni plating is applied to the upper material. It was

Table 3 below shows the components of the original plate used in Example 1.

[Table 3]

Cr plating was performed under the electrolytic conditions shown in (A) below, and Ni plating was used in (B) and (C) below for matte Ni.
It was carried out without being continuously dried under the plating conditions.

(A) Electrolytic Cr Plating (1) Plating Bath Component--Surgent Bath Chromic Anhydride ---- 250 g / l Sulfuric Acid ------- 25 g / l Kaifuka Soda--3 g / l (2) Electrolysis Conditions Temperature --- ------- 50 ° C Current Density ------- 40 A / dm ² (B) Electrolytic Ni Plating (Ni Strike Bath) (1) Plating Bath Components Nickel Sulfate ------- 240 g / l Nickel Chloride ---- --45 g / l Boric acid ------- 30 g / l Sulfuric acid ------- 10 g / l (2) Electrolytic conditions PH -------- -1 The following temperature over over over over over over over over over 50 ° C. current densityｰｰｰｰｰｰｰｰ5A / dm ² (C) electrolytic Ni plating (1) plating bath components sulfate nickel-chromatography over over over over 240 g / l chloride Nikkeru --45 g / l Boric acid --- 30 g / l (2) Electrolysis conditions PH -------- 3.5-4.5 Temperature --------ー 50 ℃ Current Density ---------- 5A / dm ²

Table 4 shows the results of evaluation of corrosion resistance of the two-layer plated steel sheet obtained by diffusion treatment in a box-type diffusion furnace in an HNX gas atmosphere. The evaluation was performed after the diffusion-treated steel plate was bent as it was or after being bent by 90 degrees with a bending radius of 1 mm in a buffed state, and after 24 hours in a salt spray test. still,
In the table, the coating adhesion amount was measured by the fluorescent X-ray method, and the corrosion resistance was evaluated by the following method.

Visual judgment of degree of discoloration ○: Good, Δ: Slightly discolored, ×: Discolored Salt water spray test The salt water spray test according to JIS Z2371 was conducted, and the ratio of the red rust generation area after a certain period of time was measured and represented by the following symbols. "◎": 0 to 0.1% "○": 0.1
0.5% "△": 0.5 to 2.5% "x": 2.5%
that's all

Nos. 1 to 6 using the Nb-added steel plate have less degree of surface discoloration after thermal diffusion and are more excellent in corrosion resistance in a salt spray test than Nos. 7 to 11 using comparative low carbon steel plates. In Comparative Example 12, an Nb-added steel plate was used as the plating base plate, but the amount of Cr deposited was small, so the appearance after the diffusion treatment was excellent, but the corrosion resistance in the salt spray test was poor.

Table 4 below shows the evaluation results (part 1-box type diffusion furnace).

[Table 4]

Example 2 Table 5 shows N of the components shown in Table 1.
The b-added steel plate (plate thickness: 0.3 mm) was used as a plating base plate, and after performing cleaning, activation and electroplating under the same conditions as in Example 1, further diffusion treatment was performed in a box-type diffusion furnace in a hydrogen gas atmosphere. It is a result of evaluating a steel sheet. The evaluation method and the like were the same as in Example 1, but the salt spray test time was 5 hours. In each of the comparative examples, the Nb-added steel plate is used, but No. 7 is Cr
Since the diffusion temperature is high with respect to the plating thickness and it is a long time, "Cr
The peak height in the GDS analysis of the Cr concentration in the —Ni—Fe ”alloy layer was reduced to about 15%, and the corrosion resistance was poor. In Comparative Examples 8 and 9, since the diffusion treatment was performed in the γ region, the plated plate was deformed.

Table 5 below shows the evaluation results (the case of 2-box type diffusion furnace).

[Table 5]

(Example 3) Table 6 shows the same N as Example 2
It is the result of the corrosion resistance test by the salt spray test of the steel sheet which was subjected to the thermal diffusion treatment using the continuous diffusion furnace in the HNX gas atmosphere using the b-added steel sheet. The electrolytic treatment conditions and the evaluation method were in accordance with Example 2. Buffing was performed on the steel sheet that was discolored by the diffusion treatment, but the Cr plating thickness was 0.1 μm or more, N
It indicates that the i plating thickness is required to be 1 μm or more. The evaluation results are shown in Table 6 below (part 3--in case of continuous diffusion furnace).
Indicates.

[Table 6]

[0030]

As described above, the Cr-Ni diffusion treated steel sheet produced by the method of the present invention is excellent in work corrosion resistance and can be widely applied to general indoor applications and plating original sheets.

[0031]

[Brief description of drawings]

FIG. 1 is a schematic view of a coating film cross section of a Cr—Ni diffusion treated steel plate.

FIG. 2 is a schematic diagram of Cr and Ni using Nb-added steel sheet according to the method of the present invention.
2 is an example of analysis by a glow discharge emission analysis method of a section of a plated steel sheet that has been subjected to a diffusion treatment after two-layer plating.

[FIG. 3] As a comparative example, a low carbon steel is used and two of Cr and Ni are used.
It is an example of analysis by glow discharge emission spectrometry of a section of a plated steel sheet that has been subjected to diffusion treatment after layer plating.

FIG. 4 shows SUS430 of ferritic stainless steel, SUS304 of austenitic stainless steel, and SUS304 of the present invention.
FIG. 3 is an X-ray diffraction diagram by Cu-Kα rays of a steel plate obtained by annealing a 6 μm Cr + 4 μm Ni-plated steel plate at 820 ° C. for 5 hours. The structure of the plated heat diffusion steel plate is S of ferritic stainless steel.
SUS of austenitic stainless steel instead of US430
Indicates that the organization is similar to 304.

Claims (4)

[Claims] 1. A carbon content of 0.003 wt% or less,
Cr on the one or both sides of the Nb-added steel plate in which C and N in the steel are fixed as Nb compounds in the total amount.
Plating layer is formed, and 1 on Cr plating layer
By heat-treating a two-layer plated steel sheet having a Ni plating layer of up to 8 μm formed, the Cr plating layer and the Ni plating layer are diffused into each other, and Cr is excellent in workability and corrosion resistance.
-Ni diffusion treated steel plate. 2. The diffusion-treated steel sheet according to claim 1, wherein the surface of the diffusion-treated steel sheet is buffed and excellent in glossiness, workability and corrosion resistance. 3. A carbon content of 0.003 wt% or less,
Using an Nb-added steel plate in which C and N in steel are fixed as the total amount of Nb compound, a thickness of 0.1 to 1 on one side or both sides thereof
A steel sheet having a Cr plating of μm and a Cr plating surface having a Ni plating of 1 to 8 μm is heat-treated at a temperature not higher than the A ₃ transformation point in a non-oxidizing atmosphere, a reducing atmosphere or a hydrogen atmosphere. A method for producing a Cr-Ni diffusion treated steel sheet having excellent workability and corrosion resistance, which is characterized by the following. 4. A method for producing a diffusion-treated steel sheet which is excellent in gloss, workability and corrosion resistance by buffing the surface of the diffusion-treated steel sheet.