KR20100061374A - Chromate-free coated galvanized steel sheet superior in corrosion resistance at cut end face - Google Patents

Abstract

PURPOSE: A chromate-free coated galvanized steel sheet is provided to improve the corrosion resistance at the cut end face by adding a metal adsorption corrosion protection agent in the chromate-free coating. CONSTITUTION: A chromate-free coated galvanized steel sheet comprises a galvanized layer and a chromate-free coating layer formed on a steel base. An inclusion with a diameter of 0.1~0.5μm exists in the steel base, where the inclusion is composed of Al of 100ppm or less, Mn of 150ppm or less and S of 150ppm or less. The steel base comprises C 0.05 weight% or less, Si 0.01 weight% or less, Mn 0.05~0.30 weight%, Al 0.002~0.040 weight%, P 0.02 weight% or less, S 0.010 weight% or less, and the rest Fe and inevitable impurities.

Description

CHROMATE-FREE COATED GALVANIZED STEEL SHEET SUPERIOR IN CORROSION RESISTANCE AT CUT END FACE}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a chromate free chemical conversion galvanized steel sheet which can be suitably used as a material for household appliances, automobile parts, building materials, and the like, and particularly, having excellent corrosion resistance in cross section exposed when cutting steel sheet. Relates to a treated zinc steel sheet.

In applications such as home appliances and automobile parts, steel sheets having galvanized coatings and chemical conversion coatings have been widely used due to the demands on rust resistance.However, in consideration of the environment, galvanized steel sheets coated with chromate-free chemical conversion coatings (hereinafter referred to as non-chromate) chromate) plated steel sheet). When this chromate-plated steel sheet is cut to a predetermined dimension, the steel base material is exposed to the cut end surface. However, when the chromate-plated steel sheet having such cut end surface is stored under a high temperature and high humidity environment, corrosion progresses from the cut end portion and red rust is removed. The problem of cross-sectional corrosion which arises arises. The problem of cross-sectional corrosion is especially acute in southern China and the southeastern Asia coast, which are the main areas of use of the steel sheets.

For reference, the cut cross section at the time of cut | disconnecting a chromate plated steel plate is shown in FIG. As shown in Fig. 1, the cut section is (i) a shearing surface produced by shearing, such as a shear knife, comprising a galvanized film or chromate-free conversion film, and (ii) fracture of the base steel sheet. Is generated by the roughly divided fractured surface. In the cut end surface in which the base steel plate was exposed, since the rust prevention effect by a plating film is not acquired, red rust will arise in a few days, especially when a chromate-plated steel plate is stored in the high temperature, high humidity environment which is easy to produce dew condensation.

In view of such circumstances, the present applicant discloses a chromium-free plated steel sheet having excellent cut cross-section corrosion resistance, and the like, in Japanese Patent Application Laid-Open No. 2005-225052, Japanese Patent Application Laid-Open No. 2006-28582, and Japanese Patent Application Laid-Open No. 2004-346341. Doing. In addition, in "Control of Inclusions in Stainless Steel and High Alloy Steel" (Nishiyama Memorial Technical Lecture, Japan Iron and Steel Association, October 2004, p.247), it is a direct object to provide a chromate-plated steel sheet excellent in cutting cross-section corrosion resistance. However, it is described that corrosion resistance falls, so that content of MnS increases with respect to corrosion resistance.

This invention is made | formed in view of such a situation, and the objective is to provide the chromate-free chemical conversion galvanized steel plate excellent in the corrosion resistance of the cut | disconnected cross section which appears when cutting a steel plate.

The chromate free chemical conversion galvanized steel sheet excellent in the cut cross-section corrosion resistance which could solve the said subject is a steel plate in which the galvanized film and the chromate free chemical conversion coating film were formed on the base steel plate, and are included in the 0.1-5.0 micrometer diameter diameter existing in the said base steel plate. Al, Mn, and S contained satisfy | fills Al: 100 ppm (meaning the mass ppm. The same is below.), Mn: 150 ppm or less, S: 150 ppm or less by the ratio which occupies for the said whole base steel plate.

The base steel sheet is C: 0.05% or less (meaning of mass%. The same applies below), Si: 0.01% or less, Mn: 0.05 to 0.30%, Al: 0.002 to 0.040%, P: 0.02% or less, and S: 0.010. It is preferable to contain% or less, and remainder is iron and an unavoidable impurity.

The base steel sheet preferably further contains Ti: 0.01 to 0.10% and B: 0.0001 to 0.003% as necessary.

In addition, it is preferable that the film thickness of the chromate-free chemical conversion treatment film is 0.05 to 5 µm, and the film contains a metal adsorption corrosion inhibitor. As such a metal adsorption corrosion inhibitor, it is preferable to use a chelating agent or its salt, a nitrite, an aminocarboxylic acid derivative, a polyphosphate type compound, catechins, or an organic amine salt, for example.

In the present invention, since the Al amount, Mn amount and S amount in relatively small inclusions having a diameter of 0.1 to 5.0 µm existing in the base steel sheet are properly controlled, a chromate-free converted galvanized steel sheet excellent in cut cross-section corrosion resistance can be provided. have.

MEANS TO SOLVE THE PROBLEM In order to provide the chromate-free chemical conversion galvanized steel plate (Hereinafter, it may be represented by a chromium-free plated steel plate) excellent in the corrosion resistance of a cut cross section, especially the composition of the fracture surface which the base steel plate (base material steel plate) exposed was exposed to. I focused on it and studied diligently. As a result, when the Al amount, Mn amount, and S amount in the inclusions having a relatively small diameter of 0.1 to 5.0 µm among the inclusions present in the base steel sheet were properly controlled, it was found that the cut section corrosion resistance was improved to complete the present invention. Further, it was also found that it is desirable to appropriately control the chemical conversion treatment film of not only the fracture surface but also the shear surface described above for further improvement of the cut section corrosion resistance.

First, the inclusion of the base steel sheet, which is the greatest feature of the present invention, will be described in detail.

The inclusions in the present invention mean composite compounds containing elements in the steel (Al, Mn, Ti, etc.) of the base steel sheet, or precipitates formed by being precipitated or formed in the manufacturing process such as rolling or annealing. Specific examples include compounds such as oxides, nitrides, sulfides, carbides, and the like containing the above-described elements in the steel, and include, for example, inclusions containing Al (such as Al ₂ O ₃ , which is typically an oxide of Al), and Mn. Inclusions to contain (typically MnS etc. which are sulfides of Mn) etc. are mentioned.

In the present invention, the content of Al, Mn and S contained in the "micro inclusions" having a relatively small diameter of 0.1 to 5.0 µm among the inclusions of various sizes existing in the base steel sheet greatly affects the cut section corrosion resistance. The biggest feature is the findings. It was found by investigation that the composition of the "coarse inclusion" having a large size exceeding the above-mentioned diameter had little effect on the cut section corrosion resistance. Therefore, in the present invention, the upper limit of the size of the micro inclusions is set to 5.0 µm. The lower limit of the size of the micro inclusions is 0.1 µm because the inclusions are extracted by the electrolytic extraction analysis method as described below, but inclusions having a diameter of less than 0.1 µm cannot be extracted.

On the other hand, the above-mentioned document "Control of Inclusions in Stainless Steel and High Alloy Steel" discloses that MnS adversely affects the corrosion resistance, although not related to the cut section corrosion resistance, but the influence of the size of the inclusions on the cut section corrosion resistance. Nothing is taught about whether it's going crazy. Corrosion resistance here refers to corrosion resistance at the surface of the flat plate, and the surface state of SUS and high alloy steel is covered with Cr or an oxide of an alloying element, and is not at all different from the new surface of Fe such as the fracture surface at the cut section of the present application. Because they are different, their corrosion mechanisms are also completely different.

It is not clear how the difference in inclusion size affects the corrosion resistance, but it is presumed that small inclusions are finely dispersed and exposed to the cross section to promote local corrosion.

As described above, the inclusions containing the elements in the steel in the base steel sheet are contained in the micro inclusions having a diameter of 0.1 to 5.0 µm as the object of the present invention, and Al inclusions and Mn inclusions are representatively exemplified. Or when Ti is contained in a base steel plate, Ti containing inclusions are also included.

Here, the Al-containing inclusions include Al oxide (Al ₂ O ₃ ), carbides, nitrides, carbonitrides and the like. If Al is present in the micro inclusions having a diameter of 5.0 µm or less, the immersion potential (corrosion potential) in the condensation water in the cut section is lowered and is considered to cause corrosion. Therefore, in the present invention, the amount of Al in the micro inclusions It was set to 100 ppm or less. The smaller the amount of Al in the fine inclusions, the better. The content is preferably 90 ppm or less, and more preferably 80 ppm or less. On the other hand, the lower limit of the amount of Al in the micro inclusions is not particularly limited, but the lower limit that can be realized is about 1.0 ppm.

Examples of the Mn-containing inclusions include sulfides of Mn (MnS), Mn0, and the like, and MnS is representatively exemplified. When MnS exists in the micro inclusions of 5.0 micrometers or less in diameter, since cut | disconnected cross-section corrosion resistance will fall, in this invention, 150 ppm or less and M amount in the said micro inclusions were set to 150 ppm or less. The smaller the amount of Mn in the micro-inclusions, the better. The lower the amount of Mn, the more preferably 140 ppm or less, and more preferably 130 ppm or less. Similarly, the smaller the amount of S, the better. The lower the amount is, the more preferably 140 ppm or less, and more preferably 130 ppm or less. The lower limit of the amount of Mn and the amount of S in the fine inclusions is not particularly limited, but the lower limit that can be realized is about 1.0 ppm in the amount of Mn and about 1.0 ppm in the amount of S.

In order to ensure desired cut end surface corrosion resistance, content of Al, Mn, and S in the said micro inclusions made object of this invention should just be reduced as mentioned above, and components other than these are not specifically limited. For example, the micro inclusions include N, O, C, Si, etc. in addition to Al, Mn, and S, but these components may be inevitably introduced into the micro inclusions by a base steel sheet, a manufacturing process, or the like. The content of the level can be tolerated. Similarly, in the present invention, the base steel sheet may contain Ti or B as an optional component, but similarly to these, the content of a level that can be inevitably introduced into the micro inclusions can be allowed.

In the present invention, in order to obtain even better cut section corrosion resistance, it is useful to control not only the base steel sheet but also the chromate-free chemical conversion treatment film as appropriate. Specifically, it is preferable to control the film thickness of the chromate-free conversion film appropriately and to contain a predetermined metal adsorption corrosion inhibitor in the conversion film.

Here, it is preferable that the film thickness of the said chromate-free conversion film is 0.05-5 micrometers. This is because if the film thickness of the chemical conversion film is less than 0.05 µm, the desired cut cross-section corrosion resistance is not effectively exhibited, while if it exceeds 5 µm, the conductivity decreases. The film thickness of the said chemical conversion treatment film is 0.5-3.0 micrometers.

The metal adsorption corrosion inhibitor means to adsorb on a metal surface (in the present invention, the surface of the base steel sheet) to prevent corrosion of the metal. The mechanism of improving the cut-off corrosion resistance by metal adsorption corrosion inhibitor is estimated as follows. That is, corrosion of the cut end surface is caused mainly by the dew condensation water, but dew condensation also occurs on the chemical conversion coating film. When the metal adsorption corrosion inhibitor is contained in the chemical conversion treatment film, the metal adsorption corrosion inhibitor is eluted by the moisture during condensation and the fracture surface is covered, thereby preventing corrosion of the base steel sheet and improving the corrosion resistance of the cut section.

Preferred metal adsorption corrosion inhibitors used in the present invention are chelating agents or salts thereof, nitrites, aminocarboxylic acid derivatives, polyphosphate compounds, catechins, or organic amine salts. Although these are well-known corrosion resistance improving agents, cutting cross-sectional corrosion resistance improves by adding in a chromate-free chemical conversion treatment film, and the outstanding cross-sectional corrosion resistance effect by the micro inclusion control of the base steel plate mentioned above is exhibited, and the outstanding characteristic is exhibited.

Hereinafter, the metal adsorption corrosion inhibitor will be described in detail.

(Chelating agent or salt thereof)

Chelating agents used in the present invention include both polymer-chelating agents and low molecular weight chelating agents.

Among the former polymer-chelating agents, for example, amines (e.g., tetraamines such as triethylenetetramine, tripropylenetetramine, tributylene tetramine; tetraethylenepentamine, tetrapropylenepentamine, tetra Polymers of pentaamines such as butylenepentamine, hexaamines such as pentaethylene hexamine, and the like, polymers of piperazine (for example, 1-aminoethyl piperazine), and cyclic imines (for example, polyethylene) Imine, methyl polyethyleneimine, polypropyleneimine, poly-3-methylpropylimine, poly-2-ethylpropylimine and the like). The chelating agent may further have a carboxyl group, whereby the adsorption effect of the metal is further enhanced. In the examples described later, carboxymethyl polyethyleneimine is used.

As the latter low molecular weight chelating agent, EDTA, DTPA, HEDP, EDTMP, NTA, HEDTA and the like are typically exemplified, and salts thereof (some or all of the plurality of acid functional groups contained in the chelating agent are neutralized). Also included. It is preferable to use the salt of the said chelating agent among these, and the solubility to the aqueous emulsion composition (described later) which is a raw material of a chromate precoat improves by this.

Examples of the salts of the low molecular weight chelating agent include alkali metal salts and alkaline earth metal salts. Preferred salts include (1) a Na salt containing Na ions, (2) a Na ion, and at least one metal ion selected from Mg, Ca, Co, Zn, Mn and Fe ions. Na-complex salts of other metals ", (3)" amine salts or ammonium salts "including amine ions or ammonium ions, (4) amine ions or ammonium ions, and further Na, Mg, Ca, Co, Zn, "Amine or the compound salt of ammonium metal" containing 1 or more types chosen from each metal ion of Mn and Fe, etc. are mentioned.

For example, when EDTA is described as an example, sodium salts (EDTA.Na), disodium salts (EDTA.2Na), trisodium salts (EDTA.3Na), and tetrasodium salts (EDTA.4Na) of EDTA are included. . In addition, a part of a plurality of acid functional groups of EDTA is neutralized with NaOH, and other acid functional groups are neutralized with amines. For example, "Kilesto M-50" manufactured by Kilesto Co., Ltd. : EDTA * 2Na * amine salt "etc. are mentioned.

Said chelating agent can also use a commercial item. For example, as an EDTA-based chelating agent manufactured by Kilesto, Inc., Kilesto Mg-40, Kilesto M-50, Kilesto OD and the like; Killest P, Killest PS, Killest PC-45, etc. as DTPA system; Killest PH-212, Killest PH-214, etc. as an HEDP system; Killesto HP-540 etc. as EDTMP system; Killest 70 as the NTA, killest NTA and the like; Killesto H etc. are mentioned as HEDTA.

(nitrite)

As said nitrite, the alkali metal salt or alkaline-earth metal salt of nitrous acid is mentioned typically. Preferred among them is one or more selected from nitrites of sodium, potassium, lithium, calcium and magnesium. Moreover, what is preferable as the nitrite of the organic amine mentioned later is the nitrite of dicyclohexylamine.

The said nitrite is a compound which is estimated to exhibit corrosion resistance by hydrolysis by condensation water, and is a compound which shows the outstanding effect | action in all of the early rust prevention property, long-term rust prevention property, and the rust prevention property under high temperature and high humidity.

(Aminocarboxylic acid derivative)

Examples of the aminocarboxylic acid derivatives include amides of acrylamide and dimethylacrylamide, styrene, methyl acrylate, methyl methacrylate, acrylic acid, methacrylic acid, styrenesulfonic acid, itaconic acid and maleic acid. And copolymers with carboxylic acids having copolymerizable unsaturated bonds such as salts thereof.

(Polyphosphate Compound)

Typical examples of the polyphosphoric acid compound include aluminum tripolyphosphate (AlH ₂ P ₃ O ₁₀ .2H ₂ O), and as a commercially available product, for example, K-Fresh # 100P (Teika Co., Ltd. product name) Can be mentioned. Since tripolyphosphate aluminum of the said composition is a solid acid, it can also mix and use with a solid base. A commercial item can also be used for the tripolyphosphate aluminum mixed with various solid bases, For example, K-WHITE 82, K105, Ca750 (Teika Corporation make) etc. can be used as it is.

(Catechins)

Catechin is a kind of polyphenol compound, and catechins commonly used as the rust preventive agent can be used in the present invention. Specifically, tannic acid, gallic acid, catechin and the like are representatively exemplified. These may be used independently and may use 2 or more types together.

(Organic amine salt)

As said organic amine salt, ethylenediamine, propylenediamine, butylenediamine, hexamethylenediamine, diethylenetriamine, dipropylenediamine, dibutylene triamine, iminobispropylamine, mono Aliphatic polyamines such as methylaminopropylamine and methyliminobispropylamine; Aromatic polyamines such as phenylenediamine, o-, m-, p-xylylenediamine, and 3,5-diaminochlorobenzene; Polymers of unsaturated amines such as cycloalkane-based polyamine polyvinylamine and polyallylamine such as 1,3-bis (aminomethyl) cyclohexane; Salts, such as unsaturated amine, such as vinylamine and an allylamine, are mentioned. As said salt, what was demonstrated by the column of (chelating agent or its salt) mentioned above is preferably illustrated. The organic amine salt may further have a carboxyl group, whereby the adsorption effect of the metal is further enhanced. When the said polyamine is a polymer of cyclic imine, the polymer of unsaturated amine, and its copolymer, it is preferable that number average molecular weights are 300-2 million, and it is especially preferable that it is 1000-500,000.

In order to effectively exhibit the effect of improving the cross-sectional corrosion resistance by the metal adsorption corrosion inhibitor, it is preferable to appropriately control the content of the metal adsorption corrosion inhibitor in the chromate-free chemical conversion coating. Specifically, it is preferable to appropriately control the content (controlled by solid content) of the metal adsorption corrosion inhibitor in the solid content of the emulsion composition (to be described later) which is a raw material of the chromate-free chemical conversion film. In this invention, the solid content of an emulsion composition means the solid residue which remains as a film after a volatile component (for example, water or an organic solvent, etc.) evaporates by heating and drying an emulsion composition. Therefore, solid content includes resin (including solid or liquid at room temperature), inorganic solid (for example, silica, wax), metal adsorption corrosion inhibitor, and the like.

For example, preferable content of the said low molecular weight chelating agent is 0.5-10 mass% in the ratio which occupies in 100 mass% of solid content of an emulsion composition. When content of a low molecular weight chelating agent is less than 0.5 mass%, production | generation of a stable chelating compound will become inadequate, and cut surface corrosion resistance will also become inadequate. On the other hand, when the low molecular weight chelating agent exceeds 10 mass%, corrosion resistance tends to fall. Although the reason why corrosion resistance falls when adding a large quantity of low molecular weight chelating agents is not specifically known, the chelating agent reacts with the organic resin, crosslinking agent, or silane coupling agent etc. which may be contained in an emulsion composition, The influence on film formation may be considered. From the viewpoint of the film forming property of the chemical conversion treatment film and the adhesiveness with other coating films, the upper limit of the more preferable content of the low molecular weight chelating agent is 5% by mass in a proportion of 100% by mass of the solid content of the emulsion composition, and the more preferable upper limit is 3 It is mass% and the minimum of more preferable chelating agent content is 1 mass%.

It is preferable that content of the metal adsorption corrosion inhibitor except the said low molecular weight chelating agent is 1-10 mass% as solid content of a metal adsorption corrosion inhibitor with respect to the solid content total mass of a chromate-free conversion coating film composition. This is because when the amount is less than 1% by mass, the effect of improving the cross-sectional corrosion resistance is not exerted effectively, and when the amount exceeds 10% by mass, the viscosity of the chemical conversion coating is increased, and the coating property is deteriorated.

The composition of the chromate-free chemical conversion coating used for the present invention is not particularly limited, and a known coating can be adopted. Specifically, for example, it may be a resin film formed from an emulsion composition as described in JP 2005-264312 A, amines having a boiling point of 100 ° C. or lower, monovalent metal compounds, or ace in an ethylene-unsaturated carboxylic acid copolymer. Crosslinking agents, such as a silidine compound, the silica particle which can be used as needed, a wax, etc. may further be included. Here, the said ethylene- unsaturated unsaturated carboxylic acid copolymer is a copolymer of unsaturated carboxylic acid, such as ethylene and (meth) acrylic acid. The copolymer is most preferably a random copolymer, but may be a block copolymer or a copolymer in which an unsaturated carboxylic acid moiety is grafted. It is also possible to use olefinic monomers such as propylene or 1-butene changed into part of ethylene, and further copolymerize (about 10% by mass or less) other known vinyl monomers within a range not impairing the object of the present invention. It may be. When the copolymerization ratio of unsaturated carboxylic acid with respect to ethylene makes monomer whole quantity 100 mass%, it is preferable that unsaturated carboxylic acid is 10-40 mass%. In addition, as described in the above publication, a surface modification layer may be formed between the chemical conversion treatment film and the galvanized layer, whereby the film adhesion is improved. See the above publication for details.

Alternatively, as the chromate-free chemical conversion treatment film, for example, the film described in JP-A-2006-43913 may be used, an aqueous carboxyl group-containing polyurethane resin solution, an ethylene-unsaturated carboxylic acid copolymer aqueous dispersion, It may contain a silica particle and a silane coupling agent. The carboxyl group-containing polyurethane resin is obtained by a chain-extension reaction of a urethane prepolymer with a chain extender, and is tolylene diisocyanate and diphenylmethane, as a polyisocyanate component constituting the urethane prepolymer. 1,4-cyclohexanedimethanol, poly, as a polyol component constituting the urethane prepolymer using at least one member selected from the group consisting of isocyanate and dicyclohexyl methane isocyanate Preference is given to using both terpolyols and polyols having carboxyl groups. As the chain extender, for example, ethylenediamine or hydrazine is suitable. It is preferable that the mass ratio of the said 1, 4- cyclohexane dimethanol and the said polyether polyol is 1, 4- cyclohexane dimethanol: polyether polyol = 1: 1-1:19. As said polyether polyol, polyoxypropylene glycol or polytetramethylene ether glycol is suitable, for example. See the above publication for details. The ethylene-unsaturated carboxylic acid copolymer may also be the same as that described in JP 2005-264312 A.

In order to apply the above-mentioned chemical conversion coating on the surface of a steel plate, methods, such as roll coating, spray coating, immersion coating, and curtain flow, can be used.

Moreover, it is preferable that a chromate-free conversion film is a clear layer. This is because the white color of the galvanized film under the chemical conversion coating is reflected in the appearance of the product by using the clear layer.

The galvanizing film of the present invention is not particularly limited, and in addition to zinc plating alone, zinc-based alloy plating such as zinc-Ni, zinc-Fe, zinc-Al, etc. may be applied, and the plating method may also be a hot dip plating method or an electroplating method. , Vapor deposition plating, spraying, or the like can be used.

Next, the composition of the base steel plate used for this invention is demonstrated. On the other hand, the amounts of Al, Mn, and S in the base steel sheet are meant to include the amounts of Al, Mn, and S present in the inclusions described above.

Although the composition of the said base steel plate is not specifically limited, It is preferable to use mild steel as an object from a workability viewpoint. Specifically, it is preferable to control the steel component as follows.

C: 0.05% or less (not including 0%)

C is an element effective for improving the strength. When excessive, C is too high and the workability is deteriorated. Therefore, the amount of C is preferably made 0.05% or less. C amount is more preferably 0.04% or less, and still more preferably 0.03% or less. The lower limit of the amount of C is not particularly limited from the viewpoint of workability, but is preferably about 0.002% in consideration of steelmaking ability and the like.

Si: 0.01% or less (does not contain 0%)

Si is effective as a solid solution strengthening element, and when excessive, the strength becomes too high and workability deteriorates. Moreover, when it becomes excess, chemical conversion treatment property will deteriorate. Therefore, it is preferable to make Si amount into 0.01% or less. Si amount is more preferably 0.009% or less, and still more preferably 0.008% or less.

Mn: 0.05 to 0.30%

Mn is an element having an action of inhibiting embrittlement by solid solution S by bonding to S to form MnS. Therefore, it is preferable to make Mn amount 0.05% or more. Mn amount is more preferably 0.1% or more, and still more preferably 0.15% or more. On the other hand, when Mn amount becomes excess, intensity | strength will increase too much and workability will fall. Therefore, the amount of Mn is preferably 0.30% or less. Mn amount is more preferably 0.27% or less, and still more preferably 0.25% or less.

Al: 0.002-0.040%

Al is an element having a deoxidation action. Therefore, the amount of Al is preferably made 0.002% or more. Al amount is more preferably 0.005% or more, and still more preferably 0.01% or more. On the other hand, when the amount of Al becomes excessive, the amount of Al in the inclusions increases, thereby deteriorating the cut end corrosion resistance. Therefore, it is preferable to make Al amount into 0.040% or less. Al amount is more preferably 0.039% or less, and still more preferably 0.037% or less.

P: 0.02% or less (does not include 0%)

P is a solid solution strengthening element, and when excessive, the strength becomes too high and workability is deteriorated. Therefore, the amount of P is preferably made 0.02% or less. The smaller the amount of P, the better. More preferably, it is 0.017% or less, More preferably, it is 0.015% or less.

S: 0.010% or less (not including 0%)

S is an element that causes embrittlement when present as a solid solution S, and decreases cross-sectional corrosion resistance when precipitated as MnS. Therefore, S is preferably made 0.010% or less. The smaller the amount of S, the better. More preferably, it is 0.009% or less, More preferably, it is 0.007% or less.

Preferred basic components of the steel used in the present invention are as described above, and the balance is substantially iron. However, it is naturally acceptable to include inevitable impurities in the steel due to the situation of raw materials, materials, manufacturing facilities, and the like. As an unavoidable impurity, N, Cr, etc. are mentioned, for example, N amount is about 0.008% or less, and Cr amount is about 0.03% or less.

In addition, the steel used for this invention may contain the following optional components as needed.

Ti: 0.01 to 0.10%

Ti is a carbonitride-forming element, and serves to reduce the solid solution C and the solid solution N in steel to improve workability. Therefore, it is preferable to make Ti amount into 0.01% or more. Ti amount is more preferably 0.02% or more, and still more preferably 0.03% or more. On the other hand, when the amount of Ti becomes excessive, chemical conversion processability and ductility deteriorate. Therefore, it is preferable to make Ti amount into 0.10% or less. Ti amount is more preferably 0.09% or less, and still more preferably 0.08% or less.

B: 0.0001 to 0.003%

B is an effective element for improving the cleavage resistance. Therefore, the amount of B is preferably made 0.0001% or more. B amount is more preferably 0.0005% or more, and still more preferably 0.001% or more. On the other hand, when B amount becomes excess, workability will deteriorate. Therefore, the amount of B is preferably made 0.003% or less. B amount is more preferably 0.0025% or less, and still more preferably 0.002% or less.

Next, the manufacturing method of the said chromate-free chemical conversion galvanized steel sheet is demonstrated.

In order to obtain the said steel plate which reduced the amount of Al, Si amount, and Mn amount in the micro inclusions of 0.1-5.0 micrometers in diameter which exist in a base steel plate, after hot-rolling, pickling, and cold, steel is melted and continuous casting is obtained. After rolling and annealing, it is important to pay attention to the annealing conditions after cold rolling, especially in a series of processes in which zinc plating is performed and chemical conversion is performed. Specifically, it is preferable to perform annealing for 15 to 30 hours at 600-700 degreeC, and batch annealing is preferable in order to perform such a long annealing. Conventionally, although annealing temperature was made higher than the said range, and annealing time was made shorter than the said range, it controlled in the range of 3 minutes-6 minutes at about 750-900 degreeC, According to the examination result of the present inventors, annealing time It has been found that when the reaction is carried out in a short manner as before, the amount of Al, the amount of Si and the amount of Mn in the micro inclusions exceed the scope of the present invention, and thus the desired cut section corrosion resistance cannot be obtained. This is because the number of fine inclusions decreases by annealing at a low temperature for a long time compared with the prior art, and as a result, it is thought that the total Al amount, Si amount, and Mn amount in the micro inclusions of 0.1-5.0 micrometers in diameter also decrease.

About manufacturing conditions other than the annealing conditions after rolling, it can determine suitably in the range normally performed. For example, hot rolling should just make heating temperature 1050-1250 degreeC, finishing temperature 850-1950 degreeC, winding temperature to 500 degreeC or more, and cold rolling may make cold rolling rate 50% or more.

In addition, the method of zinc plating is not specifically limited, Hot dip galvanization, an electroplating method, vapor deposition plating method, a spraying method, etc. can be employ | adopted suitably.

Hereinafter, although an Example is given and this invention is demonstrated further more concretely, this invention is not limited by the following example of course, Of course, it is also possible to change suitably and to implement in the range suitable for the meaning of the previous and later, These are all included in the technical scope of this invention.

Example 1

The steel of the component composition shown in Table 1 was melted in the vacuum melting furnace, and the ingot was produced. This ingot was heated to 1100 degreeC, it hot-rolled at the finishing temperature of 880 degreeC, and the winding temperature of 500 degreeC, and the hot rolled sheet of 3.2 mm of sheet thickness was obtained. Subsequently, the hot rolled sheet was pickled to remove the scale from the surface, and then cold rolled to have a thickness of 0.8 mm. The obtained cold rolled sheet was subjected to alkali electrolytic degreasing to remove the rolled oil adhering to the surface, and then annealed at 630 ° C. for 17 hours under N ₂ atmosphere. Then, electroplating and chromate free conversion process were performed on condition of the following.

Electroplating condition

Plating solution composition: ZnSO ₄ 7H ₂ O 350 g / l

Na ₂ SO ₄ 80 g / l

H ₂ SO ₄ 20g / l

NiSO ₄ 6H ₂ O 0.9 g / l

FeSO ₄ 7H ₂ O 9 g / l

_{Fe 2 (SO 4) 3 ·} nH 2 O (n = 9.5) 1.8g / l

Na ₂ MoO ₄ 2H ₂ O 0.03 g / l

0.9 g / l of 4O% Cr ₂ (SO ₄ ) ₃ solution

Current Density: 50 A / dm ²

Plating solution temperature: 60 ℃

Electrode (anode): IrO _X electrode

Coating Weight: 20g / m ²

Chromate Free Chemical Treatment Conditions

In this embodiment, the surface modification layer and the chromate free conversion coating film were sequentially formed on the galvanized layer based on the production method described in the example of JP-A-2005-264312.

Specifically, the zinc-plated steel sheet electroplated as described above was alkali degreased, washed with water and dried, and then the surface modification layer was formed in the following procedure. As the surface treatment agent, 50% of the aqueous sodium acid aqueous solution (manufactured by Nihon Chemical Co., Ltd.) as a solid content (value when the solid content of the surface-modified layer-forming composition is 100%: the same as below) and colloidal silica (`` Snotex- O ”, manufactured by Nissan Chemical Industries, Ltd., solid content 20%) and water were prepared. The degreasing galvanized steel sheet was immersed in the surface treatment agent for 2 seconds, and then pulled out. The excess solution was removed by Ringer's Roll, washed with spray pressure at 50 kPa for 5 seconds, dried at 40 DEG C, and surface modified on the galvanized layer. Formed a layer.

Next, a polyolefin-based dispersion ("Chemal S100"; Chemipal is a registered trademark; manufactured by Mitsuga Chemical Co., Ltd.) and an epoxy-based crosslinking agent ("Likerbond AP355B"; product made by Juorika Kogyo Co., Ltd.) as a solid content (5%) The value when the solid content of the chromate-free conversion film-forming composition is 100%: The same applies below), and the silica particles having a particle diameter of 10 to 20 nm (`` Snowtex 40 ''; manufactured by Nissan Chemical Industries, Ltd.) as solids, 30%, Spherical polyethylene wax ("Chemal W700"; product made by Mitsui Chemicals) was blended and stirred so as to have a solid content of 5% to prepare a composition for forming a chromate free chemical conversion treatment film. After coating the above composition on the surface modification layer of the galvanized steel sheet by percoating, heat drying at a plate temperature of 95 ° C. for 1 minute to form a chromate free chemical conversion galvanized steel sheet having a chromate free chemical conversion coating film having an adhesion amount of 0.5 μm. Got it.

Measurement method (electrolytic runoff analysis method) of inclusion composition

The steel sheet before the above-mentioned galvanizing and chromate-free chemical conversion treatment is cut, a sample 20 mm long x 10 mm wide x 0.8 mm thick is taken from the center position in the width direction, and immersed in a non-aqueous solvent-based 10% acetylacetone electrolyte. The steel was melted by constant current electrolysis at a current value of 20 mA / cm ² . The solution was passed through a filter having a pore size of 5.0 μm, and inclusions having a diameter of 0.1 to 5.0 μm remaining on the filter having a pore size of 0.1 μm were collected. After incorporating this inclusion, it was heated with a solution of sodium carbonate: sodium tetraborate = 2: 1 to melt in a glass form, and further dissolved in a solution of hydrochloric acid: water = 1: 1 to liquefy, and ICP. Quantitative analysis of Al, Mn and S was performed with a luminescence analyzer.

According to this method, since content of each component contained in the said interference | inclusion can be computed, the composition of each component which occupies for the whole base steel plate can be calculated by calculation. On the other hand, according to the analyzer used in the present embodiment, the amount of each component in the entire base steel sheet is automatically calculated.

Evaluation of Cutting Section Corrosion Resistance

The steel sheet subjected to the chromate-free chemical conversion treatment described above was cut in the sheet thickness direction, and a sample having a length of 100 mm × width 30 mm × thickness 0.8 mm was taken from the center position in the width direction. The sample was degreased and placed in a vinyl chloride sample rack to carry out a constant temperature and humidity test. In order to remove the influence of the position in a rack, a sample was installed in each of four arbitrary places (n = 4). Before and after the sample, the dummy board of length 100mm x width 50mm x thickness 0.8mm was provided in order to avoid that the wind which circulates in a constant temperature and humidity test machine directly touches a sample. The constant temperature and humidity test was repeated for 28 days in the cycle which hold | maintains at 40 degreeC and 95% RH for 3 hours, and hold | maintains at 20 degreeC and 60% RH for 1 hour. This condition is set in consideration of the occurrence of dew condensation. In view of the fact that a cycle test under high humidity is performed, an excessively harsh condition is adopted as compared with a conventional cut cross-section corrosion resistance evaluation test. After 28 days from the start of the test, the occurrence of cyan in the cut section was observed using a magnifying glass (3x magnification), and photographs of the cut section were taken, and the area ratio of red rust occupied in the cut section was measured by image analysis. . The average value of the result (n = 4) of the area ratio thus obtained was calculated, and the cut section corrosion resistance was evaluated according to the following criteria. In this example, (circle) and (circle) were evaluated as being excellent in cutting | disconnection cross section corrosion resistance. On the other hand, dispersion analysis was performed about the area ratio of red rust obtained, and it was confirmed that the average value of the area ratio for every material is statistically significant.

◎: less than 1.0% of red rust area ratio

○: Red rust area ratio more than 1.0% less than 2.0%

△: red rust area ratio not less than 2.0% less than 3.0%

X: Red rust area ratio more than 3.0%

The results are shown in Table 1.

From Table 1, No.1, 3, 4, 11-13 in which the amount of Al, Mn, and S in an interference | inclusion in a base steel plate are controlled within the range of this invention have said cut | disconnected corrosion resistance, and said content is this invention. In Nos. 2 and 5 to 10 other than the above-mentioned range, the cut section corrosion resistance was lowered.

[Example 2]

In the present Example, in Example 1 mentioned above, the cut-in cross section corrosion resistance effect when the predetermined | prescribed metal adsorption corrosion inhibitor was added to the chromate-free conversion film was investigated.

First, the surface modification layer was formed on the electrogalvanized steel plate similarly to Example 1 except having used steel grades A and B of the component composition shown in Table 2.

Next, 10 mass% of the various additives shown in Table 3 were mix | blended with the solid content in the composition for chromate-free conversion film formation used in Example 1, and it stirred and apply | coated on the said surface modification layer by bar coating. It is a preferable compound prescribed | regulated by this invention other than zirconium oxide among the additives of Table 3. After application | coating, it heat-dried for 1 minute at the steel plate temperature of 95 degreeC, and obtained the chromate-free chemical conversion galvanized steel plate in which the chromate-free chemical conversion coating film of 0.5 micrometer of adhesion amounts was formed.

Next, in the same manner as in Example 1, the amount of Al, the amount of Mn, and the amount of S in the inclusions occupied in the entire base steel sheet were measured, and the cut section corrosion resistance was evaluated.

These results are shown in Table 4 and Table 5.

First, consider Table 4 using steel grade A.

Steel grade A is the amount of Al, Mn, and S in the inclusions in the base steel sheet outside the scope of the present invention, and contains No. 22, which does not contain a metal adsorption corrosion inhibitor in the chemical conversion treatment film, and the metal adsorption corrosion inhibitor specified in the present invention. No. 14 which does not have inferior cutting cross-section corrosion resistance. On the other hand, in Nos. 15 to 21 containing the metal adsorption corrosion inhibitor specified in the present invention, good cut section corrosion resistance was obtained, and it was confirmed that the metal adsorption corrosion inhibitor contributed to improvement of cut section corrosion resistance.

Next, about Table 5 which used the steel grade B, it considers, comparing with Table 4.

Steel grade B is suitably controlled by Al, Mn, and S amount in the inclusions in a base steel plate. Nos. 23 to 31 of Table 5 and Nos. 14 to 22 of Table 4 described above differ only in the type of steel used in the base steel sheet, and the types of additives contained in the chemical conversion coating film are the same.

Examining the example using the preferable metal adsorption corrosion inhibitor prescribed | regulated by this invention, No.24-26 using the steel grade B is compared with No.15-17 using the steel grade A, and the cut end surface corrosion resistance improves more, respectively. I can see that there is. Similarly, Nos. 28 to 29 using steel grade B are further improved in cut section corrosion resistance compared to Nos. 19 to 20 using steel grade A, respectively. That is, it turns out that corrosion resistance of cut | disconnected cross section further improves by inclusion control of a base steel plate, and addition of the metal adsorption corrosion inhibitor in a chemical conversion treatment film.

1 is a SEM photograph showing a cut section of a steel sheet.

Claims (6)

A chromate free chemical conversion galvanized steel sheet having a galvanized film and a chromate free chemical conversion film formed on a base steel plate, Al, Mn and S contained in inclusions having a diameter of 0.1 to 5.0 μm present in the base steel sheet are in a proportion of the entire base steel sheet, Al: 100 ppm (mean of ppm by mass. The same applies hereinafter), Mn: 150 ppm or less, S: 150 ppm or less Chromate free chemical conversion galvanized steel sheet that satisfies the requirements. The method of claim 1, The base steel plate, C: 0.05% (meaning of mass%. The same applies hereinafter) or less, Si: 0.01% or less, Mn: 0.05-0.30%, Al: 0.002-0.040%, P: 0.02% or less, S: contains 0.010% or less, A chromate-free converted galvanized steel sheet whose balance is iron and unavoidable impurities. The method of claim 2, The base steel plate further Ti: Chromate free chemical conversion galvanized steel sheet containing 0.01 to 0.10%. The method of claim 2, The base steel plate further B: Chromate free chemical conversion galvanized steel sheet containing 0.0001 to 0.003%. The method according to any one of claims 1 to 4, A chromate free chemical conversion galvanized steel sheet, wherein the chromate free chemical conversion coating has a film thickness of 0.05 to 5 µm, and the chromate free chemical conversion coating contains a metal adsorption corrosion inhibitor. The method of claim 5, The said metal adsorption corrosion inhibitor is a chelating agent or its salt, a nitrite, an aminocarboxylic acid derivative, a polyphosphate type compound, catechins, or an organic amine salt.

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