JPH093534A - Method for producing ultra low carbon cold rolled steel sheet with excellent chemical conversion treatability - Google Patents

Abstract

(57) [Summary] [Structure] A solution containing a surface-active organic compound having a carboxylic acid group or a sulfonic acid group or a metal salt or ester thereof before annealing of an ultra low carbon steel sheet having a C content of 0.01 wt% or less. And the organic compound is adsorbed on the surface of the steel sheet. [Effect] The chemical conversion treatability including the zinc phosphate chemical conversion treatment at a low temperature is improved, and a uniform coating finish can be secured. After annealing, if 0.002 to 0.1 g / m ² of metal Ni is plated with Ni flash, the chemical conversion treatability is further improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having excellent chemical conversion treatability, which is an iron phosphate chemical conversion treatment or a zinc phosphate treatment at a relatively low temperature at a low temperature. It makes it possible to supply a cold-rolled steel sheet exhibiting good chemical conversion treatability even when applied.

[0002]

2. Description of the Related Art Cold-rolled steel sheets are widely used as materials for automobiles, home appliances, and building materials, and are required to have workability according to their applications. However, because of the problem of rust prevention, cold rolled steel sheets are rarely used as they are, and they are often painted. It is required to have good chemical conversion treatability.

Regarding the chemical conversion treatability of cold-rolled steel sheets, the properties of the steel sheet often have a great influence on the chemical conversion treatability as well as the improvement of the chemical conversion treatment liquid. Conventionally, various measures have been taken as countermeasures for the steel sheet side. Is proposed.

For example, as disclosed in Japanese Unexamined Patent Publication No. 62-116723, temper rolling is performed and dislocations are introduced into steel to improve the zinc phosphate treatability.
Alternatively, as disclosed in Japanese Examined Patent Publication No. 2-29729, B is added to steel and temper-rolled, which improves the zinc phosphate treatability in a continuous annealed material. There is.

Further, as disclosed in Japanese Examined Patent Publication No. 58-37391 and Japanese Examined Patent Publication No. 1-58276, in a continuously annealed material, after pickling a steel sheet, a metal nobler than Fe (eg, Ni
In some cases, the zinc phosphate treatment property is improved regardless of the steel type by providing a growth site for the chemical conversion crystal by thinly adhering () etc. to the surface.

However, such a cold rolled steel sheet is intended for a steel sheet for automobile bodies. Therefore, the chemical conversion treatment is also premised on the chemical conversion treatment of automobile coating lines, and aims to improve the chemical conversion treatment when the chemical conversion treatment is performed at a high zinc phosphate treatment temperature of 50 to 60 ° C. There is.

[0007] On the other hand, household appliances and building material manufacturers have various reasons such as the necessity of treating various parts, facility restrictions, energy cost savings, and simplification of waste liquid treatment work such as smudge. It is strongly desired to lower the temperature, and there are cases where low-temperature chemical conversion treatment in which zinc phosphate treatment is performed at a temperature of 40 ° C. or less, which is relatively close to room temperature, is performed. Such processing is disclosed in, for example, Japanese Patent Laid-Open No. 63-118078.

[0008]

In such low-temperature chemical conversion treatment or iron phosphate chemical conversion treatment, even samples taken from the same material produce chemical conversion crystals on one side and chemical conversion crystals on the other side. The phenomenon of not being formed at all may occur. As a result of investigating the occurrence rate of chemical conversion failure in which a chemical conversion film is not formed against such a phenomenon, it has been found that chemical conversion treatability has a large dependence on the steel sheet components, and especially for ultra-low carbon steel sheets. It was confirmed that the sex was significantly reduced. Also, with ultra-low carbon steel sheets, the effect of Ni flash plating for the purpose of improving chemical conversion treatability is
In some cases, it may not be recognized in the chemical conversion treatment at room temperature.

The mechanism of deterioration of chemical conversion treatability in the ultra low carbon steel sheet is that the ultra low carbon steel sheet has very few impurities in the steel, so that the formation of local batteries between the impurities and Fe in the steel is small, and the ultra low carbon steel sheet is kept at room temperature. It is considered that in the near chemical conversion treatment, the etching rate due to the acid during the chemical conversion treatment becomes extremely slow.

Regarding the dependence of the components in the steel on the chemical conversion treatment and the improvement of the chemical conversion treatment on the ultra low carbon steel sheet, after the cold rolled steel sheet is manufactured, the steel sheet is forcibly formed on the surface of the steel sheet to form a battery. A treatment method for applying a Ni-based, Co-based or Fe-based plated layer is disclosed in, for example, JP-A-60-131977 and JP-A-63-79996.

However, this method is not satisfactory for the following reason. That is, since the ultra low carbon steel sheet is soft, surface defects are very likely to occur in the annealing process during high temperature annealing. In the ultra low carbon steel sheet, the difference in reactivity between the surface defect portion thus generated and the portion where the defect portion does not exist is very large. It tends to be formed thick, resulting in large plating unevenness. It is known that the presence of this plating unevenness causes chemical conversion unevenness in the chemical conversion treatment. In an ultra-low carbon steel sheet, it is difficult to completely eliminate the occurrence of surface defects, and therefore the occurrence of plating unevenness is unavoidable, and the presence of this uneven plating promotes chemical conversion unevenness on the contrary. There is a limit to the improvement of chemical conversion treatability.

As described above, the chemical conversion treatability in the zinc phosphate treatment at a low temperature is greatly different from the chemical conversion treatability at a high temperature for automobile lines which has been mainly studied in the past.
Particularly for home appliances and building materials, cold-rolled steel sheets having good chemical conversion treatability even at low temperature chemical conversion treatment are required. Among them, the ultra-low carbon cold-rolled steel sheet is inferior in chemical conversion treatability, particularly low-temperature chemical conversion treatability, and therefore it is an object to improve the chemical conversion treatability so as to eliminate the poor formation.

[0013]

Means for Solving the Problems The present inventors consider that it is difficult to completely eliminate the poor formation of ultra-low carbon steel sheet by post-treatment such as plating, and the occurrence of surface defects that cause poor formation Before the annealing process in which the occurrence of heat treatment occurs, the steel sheet was subjected to some treatment to study the method of ensuring the chemical conversion treatability of the ultra low carbon steel sheet.

As a result, if a surface-active agent having a strong adsorption force is forcibly adhered to the surface of the steel sheet before annealing, C contained in the surface-active agent adhered during the subsequent annealing process is graphite or carburized. Cementite produced by the reaction is thinly and uniformly formed on the extreme surface layer of the steel sheet, and this becomes the starting point in etching with acid during chemical conversion treatment, and the chemical conversion treatability of ultra-low carbon steel sheet is significantly improved without post-treatment. Found.

The present invention provides the following method for producing an ultra low carbon cold rolled steel sheet. (1) In the production of an extremely low carbon cold-rolled steel sheet having a C content of 0.01 wt% or less, the steel sheet is selected from the group consisting of surface-active organic compounds having an acid group and metal salts and esterified derivatives thereof before annealing. A method for producing an ultra-low carbon cold-rolled steel sheet having excellent chemical conversion treatability, which is characterized by performing a dipping treatment with a solution containing at least one organic compound.

(2) The method according to (1) above, wherein the steel sheet has an intra-grain cementite number of 1 or less per crystal grain. (3) The method according to (1) or (2) above, wherein after the annealing, a Ni-based precipitation layer is applied on the surface of the steel sheet in an amount of metallic Ni of 0.002 to 0.1 g / m ² .

[0017]

The present invention is applied to an ultra low carbon cold rolled steel sheet having a C content of 0.01 wt% or less. The balance other than C is preferably Fe and unavoidable impurities, but may contain a small amount of one or more elements such as Ti, Nb, and B. Further, preferably, this ultra-low carbon cold-rolled steel sheet, the number of cementite particles in the steel sheet surface (the number of cementite existing in the crystal grains)
Is 1 or less per crystal grain.

As described above, the reason for limiting the C content and the number of cementite grains in the grain is that the ultra-low carbon cold-rolled steel sheet has a small amount of impurities in the steel, and therefore the etching during the chemical conversion treatment is performed. This is because the amount becomes insufficient and a good chemical conversion appearance cannot be obtained. In addition, such an ultra-low carbon cold-rolled steel sheet has surface defects in the annealing process after cold rolling.
There is also a problem that (scratch marks) easily occur, and this easily causes etching unevenness during chemical conversion treatment, and another reason is that improvement in chemical conversion treatability was necessary.

A low carbon steel plate (C
(Amount: 0.01 to 0.10 wt%) has good chemical conversion treatability, and therefore the effect of the treatment according to the present invention is not noticeable. However, treating such a low carbon steel sheet according to the present invention does not cause any problems.

The number of cementite particles in the grain should be discussed on the surface of the steel sheet which is originally etched by chemical conversion treatment.
In the present invention, the measurement was carried out at the cross section of the 1/2 t portion in the thickness direction of the base material. The method for measuring the number of cementite particles in the grain adopted in the present invention is an electron microscope with an observation magnification of 500.
By observing the cross-section of the t part, the number of cementite grains in the 10 base metal crystal grains randomly selected in the visual field of 0.002 × 0.002 mm was counted, and this work was repeated 5 times to obtain the average value. This is defined as the number of cementite particles in the grain.

According to the present invention, an ultra-low carbon steel sheet is treated with an organic compound having surface activity (that is, having a hydrophilic group and a hydrophobic group) (hereinafter, for simplification, The organic compound is also dipped in a solution containing a surfactant. As a result, the surfactant is adsorbed and firmly adhered to the surface of the steel sheet, and a uniform Langmuir monolayer of the surfactant is formed on the steel sheet. C contained in the adsorbed surfactant changes to cementite due to graphite or carburizing reaction during subsequent annealing of the steel sheet, and this becomes the starting point of etching during chemical conversion treatment, and a sufficient amount of etching is secured and chemical conversion treatability is improved. Remarkably improved.

As the surfactant used in the present invention, Fe
What causes a strong chemisorption reaction with is effective. The reason is that if a strong adsorption layer is not provided, the adsorption layer will easily fall off in the annealing process of the next step, and the above-mentioned effect cannot be exhibited.

In consideration of this point, in the present invention, a surface-active organic compound having an acid group which causes a strong chemisorption reaction with Fe, for example, a carboxylic acid group or a sulfonic acid group is used. Further, a metal salt or an esterified derivative of the above compound in which the acid group of such an organic compound is converted into a salt with a metal or an ester with an alcohol is also effective. Further, a phosphoric acid ester, a thiophosphoric acid ester, a phosphorous acid ester or a metal salt thereof having surface activity can be used.

An example of the surface-active organic compound having a carboxylic acid group is a higher fatty acid represented by R--CH ₂ COOH (R is a long chain aliphatic hydrocarbon group). The number of carbon atoms of R serving as a hydrophobic group is usually 10 to 20. Considering the strength of adsorption to the steel sheet and the uniformity of adsorption, it is preferable that the R group is unsaturated, and a hydrocarbon group that is branched rather than linear is preferable. The metal salt of the surfactant having a carboxylic acid group is preferably an alkali metal salt such as sodium salt.

Specific examples of the above-mentioned carboxylic acid or carboxylate type surfactants include saturated and unsaturated fatty acids such as stearic acid, lauric acid, palmitic acid and oleic acid, and their sodium salts.

On the other hand, the surface-active organic compound having a sulfone group is represented by R'-SO ₃ H. The hydrophobic group R ′ may be a long-chain aliphatic hydrocarbon group (preferably unsaturated group, more preferably branched one) similar to the above R, or an alkyl aromatic such as alkylbenzene or alkylnaphthalene. It may be a group. As the sulfonate, an alkali metal salt such as sodium salt,
And alkaline earth metal salts such as calcium salts and barium salts are preferred.

Specific examples of the sulfonic acid or sulfonate type surfactant include α-olefin sulfonic acid, dodecylbenzene sulfonic acid, dodecyl naphthalene sulfonic acid, and their sodium salts, calcium salts and barium salts. Can be mentioned.

Any of the above free carboxylic acids or sulfonic acids or metal salts thereof are anionic surfactants. Of these, metal salts (carboxylic acid salts or sulfonic acid salts) are particularly easy to use because they have high water solubility, but due to their high water solubility, Na, Ca, Ba
Metal salts such as may remain on the surface of the steel sheet and accelerate corrosion.

In such a case, application of a nonionic surface-active organic compound obtained by reacting a carboxylic acid group or a sulfonic acid group with an alcohol to convert it into an ester group is also effective. Examples of such carboxylic acid ester or sulfonic acid ester type surface-active organic compounds include ester compounds of long-chain fatty acids and monohydric or preferably polyhydric alcohols, and partially sulfurized aromatic polycarboxylic acid ester compounds. Specific examples include sorbitan fatty acid ester, propylene glycol monofatty acid ester,
Glycerin fatty acid monoester etc.

Esters of phosphorus-containing acids such as phosphoric acid, thiophosphoric acid and phosphorous acid, and metal salts thereof can be used in the present invention as long as they have surface activity. Examples of such phosphorus-containing surface-active organic compounds include monoalkyl phosphates, dialkyl phosphates, dialkyl phosphites, alkyl thiophosphates, and the like. The alkyl group in this case also has a carbon number necessary for imparting hydrophobicity, and usually has 10 or more carbon atoms.

The dipping treatment of the steel sheet with the above-mentioned surfactant can be carried out in the production of the cold-rolled steel sheet by adding the above-mentioned surfactant at an arbitrary stage before annealing at an appropriate wet treatment. Before cold rolling, for example, the immersion treatment of the present invention can be carried out by adding the above-mentioned surfactant to the pickling solution in the sulfuric acid pickling step performed for scale removal during hot rolling. In the case of after cold rolling, for example, a method of adding to the alkaline degreasing liquid in the electrolytic cleaning line in the surface cleaning step performed before box annealing,
Alternatively, before the continuous annealing line, a method such as pickling or addition to an alkaline degreasing solution installed on the inlet side of this line may be considered.

In each case, the surfactant applied is
Since it is attached to the surface of the steel sheet by utilizing chemisorption, a uniform adsorption layer of Langmuir monomolecular layer is formed on the surface in a short time, so addition of a very small amount is sufficient. Although the amount of the surfactant added is not particularly limited, it is usually sufficient if the concentration of the surfactant in the treatment liquid is 10 ppm or more, preferably 50 ppm or more. It is sufficient that the treatment temperature is room temperature or higher and the immersion time is 2 seconds or longer.

Further, for the purpose of further improving the chemical conversion treatment property, it is preferable for the ultra-low carbon steel sheet to form a thin Ni-based precipitation layer made of Ni or a Ni-based alloy such as Ni-P after annealing. It is advantageous to form such a Ni-based precipitation layer by a flash plating method using electroplating.

According to the present invention, when the treatment with the above-mentioned surfactant is carried out before annealing, when the mill rolling oil is removed,
The uniform chemisorption of the surfactant on the surface of the steel sheet forms uniform graphite and cementite portions on the surface during the annealing process. Therefore, acid etching during pickling performed before flash plating occurs uniformly, and a uniform Ni-based precipitation layer is formed during flash plating to form a Ni-based precipitation layer. As a result, a uniform chemical conversion appearance can be ensured even during the chemical conversion treatment, and therefore the dipping treatment of the present invention is very effective in improving the chemical conversion treatment even when a Ni-based precipitation layer is formed.

The deposition amount of the Ni-based precipitation layer is calculated in terms of metallic Ni amount.
0.002~0.1 g / m ^2, preferably a 0.005~0.050 g / m ^2. If it is less than 0.002 g / m ² , the chemical conversion crystals become coarse, and a uniform electrodeposition appearance, that is, a coating finish appearance cannot be obtained in the electrodeposition step generally performed after the chemical conversion treatment. On the other hand, if it exceeds 0.1 g / m ² , the surface of the steel sheet is uniformly covered with the inactive Ni layer, so that the chemical conversion film is not formed.

The present invention is particularly intended to improve the chemical conversion treatability in low temperature zinc phosphate chemical conversion treatment or iron phosphate chemical conversion treatment having a weak acid etching power. Even in high temperature (> 40 ° C) zinc phosphate chemical conversion treatment with sufficient acid etching power, its effect can be expected sufficiently.

Except for the above-mentioned immersion treatment with a surfactant and formation of a Ni-based precipitation layer applied as necessary, the method for producing an ultra-low carbon cold-rolled steel sheet according to the present invention is a conventional cold-rolled steel sheet. It may be the same as the method for manufacturing a steel sheet.

[0038]

EXAMPLES Next, the effects of the present invention will be illustrated by examples. (Example 1) The cold-rolled steel sheets used as test materials in this example were three types of ultra-low carbon cold-rolled steel sheets A and B shown in Table 1 and comparative low-carbon aluminum-killed cold-rolled steel sheet C. It was

[0039]

[Table 1]

The above cold-rolled steel sheets to be tested were immersed in a pickling solution containing the following surface-active carboxylic acids or sulfonic acids or salts or esters thereof. An 8% sulfuric acid aqueous solution was used as the pickling solution, and a predetermined amount of a surfactant was added to the aqueous solution, and then used for the immersion treatment of the test steel sheet.
The treatment temperature was 50 ° C. and the immersion time was 2 seconds.

Surfactant (symbol) Oleic acid (HOl) Sodium oleate (NOl) Dodecylbenzenesulfonic acid (HDBS) Dodecylbenzenesulfonic acid barium (BDBS) Sorbitan monooleate (EsSOl) Immersion treated cold rolled steel sheet The sample was annealed by continuous annealing or batch annealing under the following conditions without washing with water.

[0042]

[Table 2]

With respect to the cold rolled steel sheet test material thus treated with the surfactant and the annealing treatment, the low temperature chemical conversion treatability and the high temperature chemical conversion treatability were evaluated as follows.

(Low-temperature chemical conversion treatability) A cut sample of a steel sheet was subjected to chemical conversion treatment using a commercially available zinc phosphate chemical conversion treatment liquid (Enares-20 manufactured by Nippon Parker Rising Co., Ltd.). The chemical conversion treatment liquid has a temperature of 30 to 35 ° C, a total acidity of 15, and a free acidity of 0.
The treatment was carried out by controlling this to 5 and spraying this treatment liquid for 90 seconds. The evaluation was performed based on the number of samples which were judged by visual observation of the chemical conversion appearance that no chemical conversion treatment was formed when 20 cut samples of steel sheet were continuously subjected to chemical conversion treatment. This number is 2 or more (normal number is 18
Above) is passed.

(High temperature chemical conversion treatability) As high temperature chemical conversion treatability,
Chemical conversion treatment was carried out by immersion treatment for 120 seconds using a high temperature zinc phosphate chemical conversion treatment liquid controlled in the same manner as above except that the liquid temperature was raised to 45 to 50 ° C. In the evaluation, 10 cut steel plate samples were subjected to chemical conversion treatment, and even if one streak defect was recognized, it was rejected (x).

Table 3 shows the types of cold-rolled steel sheets tested, the annealing method,
The test results of the types of surfactants, their concentrations in the treatment liquid, and chemical conversion treatability are shown together.

[0047]

[Table 3]

Test Nos. 13 to 16 show the results of the chemical conversion treatability of the low carbon aluminum killed steel C, which originally has good chemical conversion treatability. From these results, it can be seen that the target performance is that the low rate of chemical conversion treatment has a poor chemical conversion occurrence rate of 18/20 or more, and that of the high temperature chemical conversion, no streaky defects of chemical conversion are recognized. From this, the evaluation standard of chemical conversion treatability was set as described above.

In the ultra-low carbon steel sheets A or B, in the examples of the present invention in which the cold-rolled steel sheet was treated with a surfactant before annealing, the low temperature chemical conversion treatability and the high temperature chemical conversion treatability achieved the above-mentioned target performances in all cases. It was satisfied and passed. On the other hand, in the comparative example in which the surfactant treatment was not performed, at least the low temperature chemical conversion treatability did not satisfy the target performance. From the comparison of No. 1 to 3 and No. 4, the comparison of No. 5, 6 and No. 7, and the comparison of No. 8 to 11 and No. 12, the treatment with the surfactant of the present invention shows the chemical conversion treatability, In particular, it can be seen that it is very effective in improving the low-temperature chemical conversion treatability.

Regarding the treatment concentration, for example, from the comparison of No. 1 and No. 2 and the comparison of No. 5 and No. 6, it is recognized that the higher the surfactant concentration, the greater the improvement effect. But,
For example, since No. 11 has an excellent chemical conversion treatability with a very small amount of surfactant of 5 ppm, such an extremely low concentration is sufficient depending on the type of surfactant, and other than that, 10 ppm or It was possible to obtain sufficient improvement in chemical conversion treatability at an extremely low concentration of about 50 ppm.

(Example 2) In order to investigate the effect of the Ni-based precipitate layer, sample No. 1 (steel type: ultra low carbon steel A, oleic acid treatment → continuous annealing, example of the present invention) and sample No. 1 in Table 1 were investigated. Regarding 16 (steel type: low carbon aluminum killed steel C, batch annealing without surfactant treatment, comparative example), after annealing, 5% sulfuric acid (60%
After performing pickling by immersion for 5 seconds, flash electric Ni plating was performed under the following conditions. The amount of the Ni deposition layer deposited was changed by changing the electrolysis time.

[0052]

The electrodeposition coatability of the cold-rolled steel sheet on which the Ni precipitate layer was formed was tested as follows. First, the test steel sheet was subjected to a zinc phosphate chemical conversion treatment as a pretreatment under the same conditions as the high temperature chemical conversion treatment of Example 1. Subsequent electrodeposition coating conditions were adjusted so that the electrodeposition film thickness was 20 μm when Electron 9410 manufactured by Kansai Paint Co., Ltd. was energized at 300 V. In order to evaluate the finish of the coating after applying cellophane tape to the coating surface on the test material of cold rolled steel sheet that has been electrodeposited,
ICM for image clarity (Suga Test Instruments Co., Ltd. image clarity tester)
Was evaluated. As a target value at that time, a case where the ICM value, which is an applicable level as an exterior steel sheet for automobiles, was 70% or more was regarded as acceptable.

FIG. 1 shows the results of investigating the relationship between the Ni deposition amount and the ICM value for sample Nos. 1 and 16. From FIG. 1, it can be seen from the results of the low carbon steel sheet of Sample No. 16 having good chemical conversion treatability that it is necessary to have an ICM value of 70% or more, preferably 80% or more.

From the result, sample N which is an ultra-low carbon steel plate
In the case of o.1, the Ni deposition amount is ² to 100 mg / m ² (0.002 to 0.1
g / m ² ), preferably 5 to 50 mg / m ² (0.005 to 0.050 g / m
^In the case of ² ), it can be seen that it has the same good electrodeposition coatability.

[0056]

As described above, according to the method for producing a cold-rolled steel sheet of the present invention, an extremely low carbon cold-rolled steel sheet having poor chemical conversion treatability is subjected to a favorable chemical conversion treatment without post-treatment. It is possible to impart sex. In the chemical conversion treatment at that time, it is possible to form a stable chemical conversion crystal in the low temperature chemical conversion treatment, and even in the normal high temperature chemical conversion, it is possible to prevent the streaky local etching ram due to the scratches in the furnace. Therefore, a uniform chemical conversion treatment crystal appearance can be ensured. Therefore, regarding low-temperature chemical conversion treatability, it is possible to simplify the equipment and reduce the burden of waste treatment by reducing smudge, etc., and regarding high-temperature chemical conversion treatability, a uniform coating finish can be secured, which is extremely useful in industry. The effect is brought about.

[Brief description of drawings]

FIG. 1 is a graph showing the results of an investigation of the relationship between the amount of Ni electrodeposition deposited on the surface of a steel sheet and the coating adhesion (ICM value) after electrodeposition.

Claims (3)

[Claims] 1. In the production of an ultra-low carbon cold-rolled steel sheet having a C content of 0.01 wt% or less, before annealing the steel sheet, it is selected from the group consisting of surface-active organic compounds having an acid group and metal salts and esterified derivatives thereof. A method for producing an ultra-low carbon cold-rolled steel sheet excellent in chemical conversion treatability, which comprises performing a dipping treatment with a solution containing at least one selected organic compound. 2. The method according to claim 1, wherein the steel sheet has an intra-grain cementite number of 1 or less per crystal grain. 3. After the annealing, a Ni-based precipitation layer is applied on the surface of the steel sheet in an amount of 0.002 to 0.1 g / m ^{2 in} terms of metallic Ni amount.
The described method.