JPH0913146A - Surface treated steel sheet excellent in punching workability and formability - Google Patents

Abstract

PURPOSE: To produce a surface treated steel sheet excellent in punching workability and formability by coating at least one side of a cold rolled steel sheet having a specified componental compsn. in which the content of S is prescribed by electrogalvanizing and with a resin coating film respectively specified their coating weights in succession. CONSTITUTION: A cold rolled steel sheet having a compsn. contg., by mass, <=0.008% C, <=1.0% Si, 0.25 to 1.0% Mn, <=0.10% P, 0.010 to 0.025% S, <=0.10% Al, <=0.010% N and <=0.20% Ti, satisfying 4<=Ti*C, where Ti*=Ti-[(48/14)N +(48/32)S], and the balance Fe with inevitable impurities is prepd. At least one side of this cold rolled steel sheet is applied by electrogalvanizing of 3 to 40g/m<2> coating weight, and on this surface, a resin coating film (such as epoxy resin) of 0.1 to 2.5g/m<2> coating weight is formed Thus, the surface treated steel sheet good in corrosion resistance, fingerprint resistance and earth properties can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet excellent in punching workability and formability, and particularly excellent in formability (deep drawability and overhanging) when manufacturing a chassis or case of home electric appliances. A surface-treated steel sheet that has all the required characteristics such as corrosion resistance, fingerprint resistance, and earthing property, and has very little burr height formed on the end surface of the steel sheet when punched. It is about.

[0002]

2. Description of the Related Art Surface-treated steel sheets made of cold-rolled steel sheet as a base material, which are galvanized, and then have inorganic coating or resin coating, are used for automobile parts, household electric appliances, etc. Although it has been widely put into practical use, various characteristics are required according to each application. Above all, surface-treated steel sheets used for chassis of home electric appliances are required to have all the characteristics such as formability, punching workability, corrosion resistance, fingerprint resistance, and groundability.

Among these required characteristics, the punching workability is evaluated by the size of the burr generated in the shearing portion when the surface-treated steel sheet is punched, and the fact that the burr is small is a very important property in practical use. Becomes However, if the burr is too large, the dimensional accuracy of the sheared portion will be incorrect, and it will be difficult to align the dimensions when assembling the products. This is because it is often experienced that the surface of the steel sheet is scratched or the metal powder adhered due to the abrasion of the burr adversely affects the electric circuit. Further, there is a big problem that the electrodeposition coating becomes poorly adhered to the burrs and the corrosion resistance is deteriorated.

Further, since these surface-treated steel sheets are often used as parts for drive systems or chassis or case materials for household electric appliances and the like, corrosion resistance is also important from the viewpoint of ensuring the quality of product appearance. Characterized. Furthermore,
When assembling the product after stamping the surface-treated steel plate, a considerable amount of manual work process is added.
Fingerprint resistance is also required in order to prevent stains from being attached by fingerprints. Conventionally, a resin coating method has been mainly used as a measure for improving the corrosion resistance and fingerprint resistance. However, when the steel plate for chassis is used as a box of an electric device, conductivity is also an important required characteristic because grounding is usually performed in the box, and it is necessary to secure the conductivity, and corrosion resistance and The resin coating for improving fingerprint resistance cannot be made thick enough.

Under these circumstances, what has been put into practical use at present with the above-mentioned required characteristics is that zinc-based electroplating is applied to the surface of a cold-rolled steel sheet, and the groundability is further applied. Although this is a method of enhancing the fingerprint resistance by forming an extremely thin organic resin coating film that does not hinder the above, it is not possible to sufficiently enhance both the grounding property and the fingerprint resistance.

On the other hand, in recent years, due to the cost reduction by simplifying the process and reducing the number of parts in chassis press makers, the formability and punching workability required for the steel sheet for chassis tend to become more and more severe. Conventionally, SPCD and SPCD grade box-annealed low carbon Al-killed cold-rolled steel sheets have been mainly used for this type of application.
This is because although the workability of the low carbon Al killed steel sheet is slightly inferior, carbon coarsely precipitates as fragile cementite in the grain boundaries and in the grain during the box annealing process for 10 hours or more, and the cementite during punching is used. This is because it is a starting point of crack generation, and thus has the advantages that the amount of burrs generated is small, the wear of the mold is suppressed, and the shear load is also small.

However, recent demands from customers for workability have become more and more severe, and it is the actual situation that such low carbon Al killed cold-rolled steel sheets as described above cannot satisfy such demands.

For punching workability, therefore, a method of introducing working strain into the surface layer of the steel sheet after annealing by rolling or the like to harden it (JP-A-1-255625), or nitriding or carburizing to harden the surface of the steel sheet. Methods (JP-A-1-255626, JP-A-2-133561, JP-A-3-199343, JP-A-3-202442) have been tried.

However, although the former method improves the punching workability to some extent, there is a problem that mechanical characteristics such as elongation and r-value, which are originally required characteristics of the steel sheet, are remarkably deteriorated. The method requires special equipment and processes for nitriding and carburizing, and it is hard to say that it is a good idea in terms of equipment and productivity.

Further, by adding Mn and S, M
A method for producing a large amount of nS precipitate (Japanese Patent Laid-Open No. 1-230
Nos. 748 and 6-73457) are also proposed, but this method causes hot work cracks at the time of working due to the increase in S, and has point defects such as blowholes and blisters inside. There arises a problem that dot-shaped surface defects called sliver frequently occur.

For the purpose of reducing the cost of home electric appliances, the punching process and the assembling process are being omitted for chassis parts and the like. Conventionally, the parts which were assembled and constituted by a plurality of parts are integrally molded with a press. There is also a tendency to change. For example, the pulley shaft that is attached to the chassis plate surface has been made by mechanical cutting in a separate process, and then caulked after it was fitted into the punched hole, but recently, the integral molding method of overhanging and deep drawing has been adopted. It is becoming mainstream. Therefore, excellent formability is also required for steel sheets used for applications such as chassis. From a metallurgical point of view, punching workability is affected by the presence of cementite and MnS, which are the starting points for crack initiation, and the hardness of the steel sheet itself, but these are all harmful to formability, and punching and forming It has been considered difficult to achieve both sexes.

Furthermore, for the purpose of improving the efficiency of the annealing process and improving the surface quality and internal quality, the transition from conventional box annealing to continuous annealing capable of short-time annealing is rapidly progressing. Also, there is an urgent need to develop a steel sheet that has both punching workability and excellent formability.

On the other hand, a so-called IF type cold rolled steel sheet in which carbon nitride forming elements such as Ti and Nb are added and C and N are fixed is excellent in workability such as high elongation and r value and low yield stress. The IF-type cold-rolled steel sheet exhibits its characteristics sufficiently by adopting the continuous annealing method in particular, which is widely used for difficult-to-machine parts such as automobile bodies. However, although it has good workability, it has poorer punching workability than low-carbon Al-killed cold-rolled steel sheets, and there is a problem that the burr height of the end face after punching becomes large. Not suitable.

[0014]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and its purpose is to adjust the chemical composition of the steel material well to obtain the conventional IF cold-rolled steel sheet. While maintaining excellent formability and mechanical properties such as those obtained with, the punching processability is good and burrs do not occur, and corrosion resistance and corrosion resistance do not hinder the conductivity required for grounding. An object of the present invention is to provide a surface-treated steel sheet having an improved fingerprint property.

[0015]

The structure of the surface-treated steel sheet according to the present invention which can achieve the above objects is mas
s%, C ≦ 0.008% Si ≦ 1.0% 0.25% ≦ Mn ≦ 1.0% P ≦ 0.10% 0.010% ≦ S ≦ 0.025% Al ≦ 0.10% N ≦ 0.010% Ti ≦ 0.20%, and 4 ≦ Ti * / C where Ti * = Ti − [(48/14) N + (48/3
2) S] is satisfied and at least one surface of the cold-rolled steel sheet, the balance of which is Fe and unavoidable impurities, is subjected to zinc-based electroplating with a plating adhesion amount of 3 to 40 g / m ² ,
The gist is that a resin coating film having an adhesion amount of 0.1 to 2.5 g / m ² is formed thereon.

In the above surface-treated steel sheet according to the present invention, the r value is 1.6 or more and the burr height after punching is 20 mm or less as a standard for ensuring excellent formability and punching workability. A certain one is particularly preferable, and by using a resin coating film containing a silicon-based inorganic oxide, it is possible to obtain a surface-treated steel sheet having further excellent corrosion resistance and fingerprint resistance. The surface-treated steel sheet having the above structure can be used very effectively as a material for a chassis of electric appliances by taking advantage of its characteristics.

[0017]

In the surface-treated steel sheet according to the present invention, the type and amount of each contained element of the cold-rolled steel sheet to be used are specified, and
The content of Ti is determined in consideration of the relationship with the contents of C, N and S. This is because the present invention uses an ultra-low carbon IF steel having excellent press formability as a base composition and improves punching workability, which has been pointed out as a drawback while maintaining its characteristics. The reasons for determining the content and the content rates thereof will be described in detail later, but the most important technical features are as follows.

That is, in order to improve the punchability while maintaining the excellent formability originally possessed by the IF-type cold-rolled steel sheet, a precipitate which is a starting point of crack generation during punching, specifically, Ti The existence of ₄ C ₂ S ₂ is indispensable, and if the size and dispersion state are properly controlled, a cold-rolled steel sheet satisfying both excellent formability, punching workability, and surface properties can be obtained. It is based on the new findings confirmed by the inventors. Precipitates and inclusions existing in the ultra low carbon IF type steel sheet are TiC and Ti in the Ti addition system.
₄ C ₂ S ₂ , TiS, TiN, FeTiP, TiO ₂ etc., and Nb (C, N) etc. in the Nb added system, among these precipitates and inclusions, especially formability and punching workability , Ti ₄ C ₂ has a significant positive effect on surface properties
It was confirmed to be S ₂ . And the Ti ₄ C ₂ S ₂
It was also confirmed that other precipitates and inclusions had the following tendencies.

TiC, Nb (C, N): Since the size of the precipitate is very small, the formability tends to be significantly deteriorated, and the punchability improving effect is small. Fe
TiP: The size is about the same as Ti ₄ C ₂ S _2, and although it has an effect of improving punchability, it significantly deteriorates formability.

Although TiS, TiN: Ti ₄ C ₂ S ₂ is a little larger than Ti ₄ C ₂ S ₂ , it seems that an effect of improving punchability can be expected in terms of size, but since it is soft, there is little improvement effect. Further, when the S content is increased, Ti ₄ C ₂
The amount of S ₂ increases, but a certain amount (C is Ti ₄ C ₂ S ₂
TiS formation does not occur (during the consumption of C) and TiS formation occurs after C is consumed, but T
iS is not preferable because it causes surface defects. Further, it is difficult to control the N content in practical steel, and it is very difficult to properly control the TiN content.

TiO ₂ : The size is an order of magnitude larger than that of Ti ₄ C ₂ S ₂ , and not only the effect of improving punchability is small, but it also causes surface defects, which is not preferable. MnS: T
A punching effect improving effect similar to i ₄ C ₂ S ₂ can be expected.

For the above reasons, in the present invention, the formability and punchability are controlled by controlling the amount of Ti ₄ C ₂ S ₂ produced and the dispersed state in the ultra-low carbon Ti-added IF steel sheet.
Furthermore, they succeeded in obtaining a cold rolled steel sheet satisfying all surface properties. Incidentally, as a technique for improving the formability and punchability of an ultra-low carbon IF steel sheet, Japanese Patent Laid-Open No. 6-
Although there is a method of adding Ti and Nb in combination as described in No. 73457, the present inventors have studied and found that when Nb is added in the range of 0.003 to 0.03%, Nb (C , N) is generated and C in the steel is consumed, so that Ti ₄ C ₂ S ₂ effective for improving punchability cannot be generated. Moreover, in this method, 0.05% or more of S is indispensable for improving the punching property.
It has a problem of increasing the generation rate of surface defects due to the generation of slag, which is not practical. Hereinafter, the reason for defining the component composition of the steel material used in the present invention will be described in detail.

C contributes to the improvement of the strength of the steel material as a strengthening element, but if it is too much, the elongation, r value, etc. decrease and the workability deteriorates. Therefore, the upper limit is set to 0 to secure good workability. It was set at 008%.

Si is an effective element for increasing the strength without lowering the ductility, but if it is too much, the oxide film formed during annealing increases, which adversely affects the chemical conversion treatment often performed thereafter. Further, when electroplating is performed, there arises a problem that the plating thickness becomes uneven and uneven plating occurs. Therefore, the amount of Si should be suppressed to 1.0% or less. In particular, when hot-dip galvanizing the cold-rolled steel sheet, an unplated portion is likely to be formed due to the formation of an oxide film. Therefore, it is preferable to suppress the Si content to 0.3% or less.

Since P has a function of forming a solid solution in the steel to strengthen the steel, it is effective to contain an appropriate amount according to the required strength, but if it is too much, segregation occurs and workability is deteriorated. Since it will have an adverse effect, the upper limit is 0.10%.

When the content of N is large, it adversely affects the mechanical properties such as elongation and r value, so N must be suppressed to 0.010% or less. Al effectively acts as a deoxidizing agent, but if it is too much, it becomes a generation source of non-metallic inclusions and adversely affects the mechanical properties and workability.
Must be kept below 10%.

S is contained in the steel sheet of the present invention together with Ti described later.
Is an extremely important element. That is, excellent punchability
In order to obtain the appropriate amount of Ti in the steel for the reasons described above._Four C
_Two S _Two It is necessary to generate and disperse
Must contain at least 0.010% of S
No. However, it is excessively contained in excess of 0.025%.
Since the incidence of surface defects will increase,
I have to

Incidentally, FIG. 1 shows C: 0.0052%, S
i: 0.01%, Mn: 0.4%, P: 0.014%,
Al: 0.03%, Ti: 0.08%, N: 0.006
Fig. 2 shows punching workability and r value when the S content was changed from 0.002% to 0.037% for a cold-rolled steel sheet having a basic composition containing 10%. In this figure,
Box-annealed Al-killed cold-rolled steel sheet (C: 0.044%, A, which is said to have excellent punchability and mechanical properties
(l: 0.045%) is also shown as a comparative example.
Further, FIG. 2 shows the relationship between the S content and the surface defect occurrence rate.

The physical property test methods were as follows. That is,
120 kg of 40 kg steel ingot that was vacuum melted and cast in a laboratory
After heating to 0 ° C., rough rolling is performed to 30 mm ^t . Then
After heating this slab to 1200 ° C, the plate thickness: 4 mm
^t , finishing temperature: 920 ℃ or higher, winding temperature: 680 ℃
After hot rolling under the conditions of No. 1, grinding to 3.2 mm ^t , and then 0.
It was cold-rolled to 8 mm ^t , continuously annealed at a soaking temperature of 800 ° C. for 1 minute, and then temper-rolled at 1% to obtain a cold-rolled steel sheet for test. Then, the r value was obtained by using a 10 mm × 110 mm test piece and measuring the width and the plate thickness when a tensile strain of 0% and 15% was applied. The punching workability is 0.
The burr height of the end face of a cold rolled steel sheet of 8 mm ^t punched out to 10 mmφ with a clearance of 9% was measured and evaluated. The surface flaw was evaluated by visually observing the surface of each cold-rolled steel sheet obtained and by the surface flaw occurrence rate at a constant length.

As is clear from FIGS. 1 and 2, if the S content is less than 0.010%, the height of the burr during the punching process becomes so large that the object of the present invention cannot be achieved. On the other hand, if the S content exceeds 0.025%, the occurrence of surface defects becomes remarkable and the object of the present invention is not achieved. However, the S content is 0.010% or more and 0.025%
It can be seen that in the following ranges, surface defects and burr height can be suppressed as much as possible and a high level r value can be secured.

Microscopically observing the punched portion of the steel sheet, a shear fracture surface is first formed on the front and back surfaces of the steel sheet as the punch descends after the contact between the punch and the steel sheet, and the remaining portion locally undergoes strong shear deformation. Upon receiving the cracks, voids are generated and become cracks, and the cracks coalesce from the front and back surfaces to complete the processing. Therefore, in order to reduce the burr after punching, it is effective to promote the formation of voids and the development of cracks, but since Ti ₄ C ₂ S ₂ is a relatively large precipitate, It is considered that when the steel sheet is subjected to strong shear deformation, the formation of voids and cracks near the precipitates and their progress are promoted, resulting in smaller burrs.

On the other hand, Ti combines with C and S to form Ti ₄ C ₂
In addition to producing S ₂ and Ti, it also bonds with N, so that an amount capable of completely fixing N in the above range is required, and in order to impart moldability and aging resistance, it is sufficient to fix C. Amount of T
i is also required. From this point of view, in the present invention, the addition amount of Ti is 4 ≦ Ti * / C as described above, but Ti * = Ti − [(48/14) N + (48/3
2) S].

However, the effect of adding Ti as described above is about 0.2.
Saturation at 0% and addition of more than that are not economically wasteful, but also have the negative effect of increasing the recrystallization temperature, so the upper limit was set at 0.20%.

Mn is also an element that forms coarse precipitates of MnS as described above and contributes to the improvement of punching workability, and 0.25 is required to effectively exert its effect.
% Or more. But if there are too many,
Since the steel sheet becomes too hard and the workability is adversely affected, the upper limit is 1.0%.

The surface-treated steel sheet according to the present invention is produced by smelting a steel material satisfying the requirements of the above-mentioned composition and casting according to a conventional method, and then hot rolling, pickling, cold rolling, annealing temper rolling, etc. The cold-rolled steel sheet obtained by sequentially performing
At least one surface is electrogalvanized with the proper amount of coating, and the resin coating with the proper amount of coating is further applied on it, thereby improving corrosion resistance and excellent fingerprint resistance without damaging the grounding property. It has improved sexuality and appearance.

That is, a normal chassis steel plate is often used without being coated, and if a cold-rolled steel plate is used as it is, red rust is generated in a short time even in the air atmosphere, and the strength is lowered due to corrosion due to rusting. Not only that, but the rust powder that falls off during use adversely affects the drive unit and electric circuit system. Therefore, surface treatment is indispensable to prevent such rusting, and in the present invention, zinc-based electroplating is adopted as a material that exhibits an excellent rust-preventing effect without adversely affecting the formability and punching workability. To do.

The zinc-based plating includes a hot dip coating method in which a steel sheet is immersed and run in a hot dip zinc bath,
There is an electroplating method that deposits metallic zinc on the steel sheet surface by energizing it in an electrolytic solution containing zinc ions, and a vapor deposition plating method that deposits zinc etc. on the steel sheet surface in a vapor state, but with a relatively small amount of pinholes In the present invention, the electroplating method is adopted to form a uniform and beautiful plating layer without defects such as the above. In order to effectively exhibit the anticorrosive effect of the zinc-based electroplating, the amount of the electroplated layer deposited should be 3 g / m ² or more. If the amount of the plated deposit is insufficient, satisfactory corrosion resistance is guaranteed. It becomes impossible or the resin coating film formed thereon must be made excessively thick, and as a result, the groundability is impaired. From the viewpoint of corrosion resistance, there is no particular upper limit of the zinc-based electroplating adhesion amount, but even if the adhesion amount is excessively increased to more than 40 g / m ² , the corrosion resistance improvement effect cannot be further obtained. 40g / m ^{2 As} it only unnecessarily prolongs the plating treatment time, and in some cases the adhesion of the plating decreases and there is a risk of plating peeling during punching.
Was set as a temporary upper limit.

The zinc-based electroplating used in the present invention means, in addition to pure zinc plating, zinc-based plating containing metals such as Ni, Co and Fe together with zinc, or a base of these zinc-based plating or pure zinc plating. Means the same or different kinds of pre-plated plating, and includes various zinc alloy platings having a zinc content of 50% by weight or more.

Although the purpose of rust prevention of the steel sheet itself is achieved by forming the zinc-based electroplating layer, in order to put it into practical use as a material for chassis, etc., rusting of the zinc-based plating layer itself is prevented. At the same time, it is necessary to improve fingerprint resistance, and as a means therefor, in the present invention, resin coating is further applied onto the zinc-based electroplating layer. The preferable coating amount of the resin coating film is in the range of 0.1 to 2.5 g / m ² , and when the coating amount is insufficient, satisfactory corrosion resistance and fingerprint resistance cannot be obtained, and conversely when it is too large. However, there arises a problem that the conductivity of the surface layer portion is lowered and the earthing property is extremely deteriorated.

The type of resin component constituting the resin coating film is not particularly limited, but the most common one is epoxy resin,
Ethylene copolymer resins containing ethylenically unsaturated carboxylic acid as a polymerizable component, polyester resins, polyvinyl resins, acrylic resins, urethane resins, polyamide resins, etc., which can be used alone, It is also possible to use a mixture of two or more kinds as necessary.
In addition, functional groups in these resins are modified by a conventional method or reinforced by using a cross-linking agent together. Further, for example, a silicon-based organic or inorganic compound is used in combination for the purpose of further improving fingerprint resistance. Those that have been modified by introducing a wax component into the functional group in the resin together with a small amount of wax component in order to improve the surface type galling property during pressing.
Etc. are also recommended as preferable resin components.

It is desirable that the resin coating film exhibits good corrosion resistance and fingerprint resistance as a surface protective film even after pressing, and in this sense, a particularly preferable base resin is
It is an epoxy resin, an ethylene copolymer resin, a polyester resin, a urethane resin, or a mixed resin of two or more thereof.

In addition to these base resins, the resin coating film contains 5% by weight of a silicon-based inorganic oxide such as silica particles.
It is preferable that the content thereof is not less than about since the corrosion resistance of the coating film can be further enhanced. However, if the content of silica particles etc. is too large, they will act as a lubricant, tend to increase the friction coefficient of the coating film and reduce the lubricity, and eventually deteriorate the corrosion resistance after processing. Since it appears, about 30% by weight or less, more preferably 1
It is desired to suppress the content to about 0% by weight or less.

The effect of the combined use of silica particles is most effectively exhibited by using particles having a particle size of 1 to 20 μm. That is, the smaller the silica particles are, the denser the coating film is, and the better the adhesion is. As a result, the corrosion resistance and coating property of the coating film are improved, but even if extremely fine particles of less than 1 μm are used, The effect is not obtained, and the lower limit of about 1 μm is sufficient,
On the other hand, if it exceeds 20 μm and becomes coarse, the surface of the coating film becomes rough and a dense coating film is not formed and the corrosion resistance also deteriorates. From this point of view, the particle size of the silica particles is more preferably in the range of 4 to 6 μm.
Known as colloidal silica, for example, "Snowtex-XS and SS" (both manufactured by Nissan Chemical Industries, Ltd.)
Etc. are available as commercial products.

The method for producing the surface-treated steel sheet according to the present invention is not limited at all, and one or both sides of the cold-rolled steel sheet produced by the above-mentioned method is subjected to zinc-based electroplating treatment according to a conventional method, Resin coating may be applied thereon by any method such as a roll coating method, a dip coating method and a spray coating method. At this time, in order to enhance the adhesion between the zinc-based electroplated layer and the resin coating film, it is also effective to subject the surface of the zinc-based electroplated steel sheet to chemical conversion treatment such as chromate treatment or phosphate treatment before applying resin coating. Of course, these are also included in the technical scope of the present invention.

[0045]

EXAMPLES The constitutions and effects of the present invention will be specifically described below with reference to examples, but the present invention is not limited by the following examples and can be adapted to the gist of the preceding and the following. It is also possible to make appropriate modifications within the scope of the invention, and all of them are included in the technical scope of the present invention.

Example 40 kg of the composition of components shown in Table 1 vacuum-melted in a laboratory
After heating the steel ingot to 1200 ° C, rough rolling to 30 mm ^t , then heating this slab to 1200 ° C, the plate thickness 4 m
Hot rolling was performed under the conditions of m ^t , finishing temperature of 920 ° C. or higher, and winding temperature of 680 ° C. Then pressure cold-rolled from the ground to 0.8 mm ^t to 3.2 mm ^t, was subjected to continuous annealing for 1 minute soaking at 800 ° C., it was carried out 1% temper rolling.

The surface of each cold-rolled steel sheet thus obtained was electrogalvanized by an electroplating method according to a conventional method so that the amount of zinc deposited was 20 g / m ^2, and the amount of Cr deposited thereon was converted to metallic Cr. A resin obtained by applying chromate treatment to 10 mg / m ² and then using an ethylene-based, ester-based, or urethane-based resin as a base resin and adding colloidal silica having a particle size of 4 to 6 μm in an amount of 10% by weight in terms of solid content. The paint was applied so that the amount of resin adhered was 1.0 g / m ² to obtain a surface-treated steel sheet.

With respect to each of the obtained surface-treated steel sheets, the tensile properties, r value, punching workability, corrosion resistance, fingerprint resistance, and earthing property were evaluated by the following methods, and the results shown in Table 2 were obtained. Although not shown in Table 2, the corrosion resistance, fingerprint resistance, and grounding property are good for all surface-treated steel sheets because the zinc-based electroplating layer adhesion amount and the resin coating film adhesion amount are appropriate. Met.

Further, a part of the cold-rolled steel sheet shown in Table 1 was used as the steel sheet to be treated, and the surface of the cold-rolled steel sheet was electroplated by an electroplating method according to a conventional method so that the amount of zinc deposited was 1 to 50 g / m ^2. Galvanized and the amount of Cr deposited on it is metallic Cr
Chromate treatment was performed so that the conversion amount was 10 mg / m ^2, and then resin coating was performed so that the resin adhesion amount was 0.05 to 3.0 g / m ² to obtain a surface-treated steel sheet. The obtained surface-treated steel sheets were evaluated for corrosion resistance, fingerprint resistance, and earthing property, and the results shown in Table 3 were obtained.

Tensile properties : Using JIS No. 5 tensile test pieces, the yield point (YP), strength (TS), elongation (El) and work hardening index (n value) were evaluated, and 100 ° C. × 60.
The aging resistance (AI) was evaluated from the amount of increase in the yield point after treatment with min. r value : Using a test piece of 10 mm × 110 mm, the width and the plate thickness when tensile strains of 0% and 15% were applied were measured and obtained.

Punching workability : Punching was carried out in a circle having a diameter of 10 mm with a clearance of 9%, and the height of burrs formed on the punched end face was measured by a micrometer for evaluation. ◯: Burr height is less than 20 μm, ×: Burr height is 20 μm or more Corrosion resistance : A salt spray test was performed according to the method described in JIS Z 2371, and evaluated by 1% white rust occurrence time. ○: 240 hours or more, ×: less than 240 hours

Fingerprint resistance : A test piece was immersed in a 50% white petrolatum aqueous solution at 40 ° C. and then taken out, and the change in color tone between the vaseline-adhered part and the non-adhered part was measured by a color difference meter and evaluated. In addition, when the color difference (ΔE) between the vaseline-adhered portion and the non-adhered portion is “3 or less”, the fingerprint resistance is good. ◯ ... ΔE is 3 or less, × ... ΔE is 3 or more Earth property : The surface resistance was measured and evaluated by a tester while setting the conductivity of the surface of the test piece to 100 mm between terminals. ◯ ... Surface resistance less than 200Ω, × ... Surface resistance 200Ω or more Overall evaluation : ○ ... Good, × ...

[0053]

[Table 1]

[0054]

[Table 2]

[0055]

[Table 3]

From Tables 1 and 2, the following can be considered. Steel types A, B, and I are mild steel systems, and steel type E is a high-strength steel system, all of which are examples satisfying the requirements of the present invention. Strength (TS), elongation (El), r value, A. Good results were obtained on average in all of I (aging resistance), burr height, surface defects, and surface properties.

On the other hand, steel type C is a comparative example in which the amount of C is larger than that of steel type A and exceeds the specified range of the present invention, and the elongation and r value are low. There are surface defects such as blister and sliver. Further, since the steel type F has a larger amount of Si than the steel type E and exceeds the specified range, uneven plating occurs due to the formation of an oxide film when electroplating is performed.

Steel type F has poor surface quality because the amount of Si exceeds the specified range, and also impairs the chemical conversion treatability performed thereafter and the uniformity of electroplating. The steel type G is a comparative example in which the amount of S is insufficient with respect to the steel type A, the burr generated by punching is high and does not meet the purpose of the present invention, and the steel type H has a Ti * / C ratio of the present invention. Satisfying the elongation, r-value, and aging resistance are not obtained because the requirements of No. 1 are not satisfied. In steel type J, the N content exceeds the specified range with respect to steel type I, so the elongation and r value are poor,
Further, the steel type K has a smaller Mn content than the steel type A and is less than the prescribed amount of the present invention, so that the punching workability is poor and the purpose of reducing the burr height cannot be achieved.

Steel type L is a comparative example in which the amount of Mn is too much larger than that of the steel type E, the elongation and r value are poor, and steel type M is a comparative example in which the amount of P is too much compared to the steel type E. Elongation and r value are low. Steel type N is a comparative example in which the amount of Al is too much larger than that of steel type A, and the elongation and r value are low and surface defects are observed. It's getting worse.

The following can be considered from Table 3. No.
Nos. 1 to 5 are examples in which the composition of the steel material, the amount of the electrogalvanized layer and the amount of the resin coating film adhered satisfy the requirements of the present invention, and in any of corrosion resistance, fingerprint resistance, and groundability. Very good results have been obtained.

In contrast to these, No. 6 was poor in corrosion resistance and fingerprint resistance because both the zinc-based electroplating layer and the resin coating film were insufficient in adhesion amount. In No. 7, since the resin adhesion amount was too large, the groundability was poor, and No. No. 8 has no corrosion resistance and fingerprint resistance because it is not coated with resin. In No. 9, since the resin adhesion amount is insufficient, the arrow-tension corrosion resistance and fingerprint resistance cannot be satisfied.

[0062]

The present invention is configured as described above, and regulates the composition of the base steel material, in particular, the contents of S, Mn, Ti, etc., as well as the S, N and C amounts and the Ti amount. By properly controlling the relationship between the above, and by forming a zinc-based electroplating layer and resin coating film with an appropriate amount on the surface, excellent strength characteristics, moldability, punching workability, corrosion resistance, fingerprint resistance It is possible to provide a surface-treated steel sheet which is excellent in electrical properties and groundability and is extremely useful as a chassis material for home electric appliances.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the S content in a steel material on the burr height and r value formed during punching.

FIG. 2 is a graph showing the relationship between the S content in steel and the surface flaw generation index of the cold rolled steel sheet.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Ichiro Tsukaya 1 Kanazawa-machi, Kakogawa City, Hyogo Prefecture Kamido Steel Works, Ltd. Kakogawa Works (72) Inventor Yukihiro Utsumi Kanazawa-cho, Kakogawa City, Hyogo Prefecture Kamido Co., Ltd. Inside the Kakogawa Works

Claims (4)

[Claims] 1. Mass% C ≦ 0.008% Si ≦ 1.0% 0.25% ≦ Mn ≦ 1.0% P ≦ 0.10% 0.010% ≦ S ≦ 0.025% Al ≦ 0.10% N ≦ 0.010% Ti ≦ 0.20%, and 4 ≦ Ti * / C where Ti * = Ti − [(48/14) N + (48/3
2) S] is satisfied and at least one surface of the cold-rolled steel sheet, the balance of which is Fe and unavoidable impurities, is subjected to zinc-based electroplating with a plating adhesion amount of 3 to 40 g / m ² ,
A surface-treated steel sheet excellent in punching workability and formability, characterized in that a resin coating film having an adhesion amount of 0.1 to 2.5 g / m ² is formed thereon. 2. The surface-treated steel sheet according to claim 1, wherein the r value is 1.6 or more and the burr height after punching is 20 mm or less. 3. The surface-treated steel sheet according to claim 1, wherein the resin coating film contains a silicon-based inorganic oxide. 4. The surface-treated steel sheet according to claim 1, which is used as a chassis for electric appliances.