TW201610235A - Cold-rolled steel sheet, method of manufacturing cold-rolled steel sheet, automobile member and facility for manufacturing cold-rolled steel sheet - Google Patents

Abstract

A cold-rolled steel sheet which has excellent chemical conversion ability, while exhibiting excellent corrosion resistance after coating; a method for producing the cold-rolled steel sheet; and an automobile member. A method for producing a cold-rolled steel sheet, wherein a steel sheet which has been subjected to continuous annealing after cold rolling is subjected to first acid pickling and then to second acid pickling, and is subsequently subjected to a neutralization process using an alkaline solution.

Description

Cold rolled steel sheet, cold rolled steel sheet manufacturing method, automobile parts, and cold rolled steel sheet manufacturing equipment

The present invention relates to a cold rolled steel sheet and a method of manufacturing the same. Further, there is a device for manufacturing the cold rolled steel sheet. In particular, there is a cold-rolled steel sheet which is excellent in chemical synthesis treatability and excellent in corrosion resistance after coating, which is evaluated by a salt warm water immersion test or a composite cyclic corrosion test, and a method for producing the same, and an automobile part. Further, the cold-rolled steel sheet of the present invention can be preferably used as a high-strength cold-rolled steel sheet having a tensile strength TS of Si of 590 MPa or more.

In recent years, in terms of protecting the global environment, there is a strong demand for improving the fuel efficiency of automobiles. In addition, from the viewpoint of ensuring the safety of the passengers during the collision, the strength of the automobile body is also strongly demanded. In order to cope with the above-mentioned requirements, the cold-rolled steel sheet, which is a raw material for automobile parts, is actively promoted to be high-strength and thin-walled, and at the same time, the weight and strength of the automobile body are achieved. In addition, since many steel parts are manufactured by forming a steel sheet, the steel sheet which is a raw material requires excellent formability in addition to high strength.

There are various methods for increasing the strength of the cold-rolled steel sheet, and an effective method for increasing the strength without significantly impairing the moldability is exemplified by a solid solution strengthening method in which Si is added. However, it is known that when a large amount of Si, particularly 0.5% by mass or more, of Si is added to the cold-rolled steel sheet, the surface of the steel sheet and the scale of the steel sheet are formed during the heating of the slab or during the hot rolling or the subsequent annealing. The interface at the surface forms a large amount of an oxide containing Si such as SiO ₂ or a Si—Mn composite oxide. The Si-containing oxide significantly reduces chemical synthesis treatability. In addition, there is the following problem: after plating, if exposed to a severe corrosive environment such as a salt spray test or a repeated cycle corrosion test of wet and dry, it is easy to cause peeling of the film and corrosion resistance after coating. Deterioration.

For example, Patent Document 1 proposes a high-strength cold-rolled steel sheet in which a slab is heated at a temperature of 1200 ° C or higher during hot rolling, and rust is removed under high pressure ( Descaling) The surface of the hot-rolled steel sheet was ground by a nylon brush to which abrasive grains were added before pickling, and immersed twice in a 9% hydrochloric acid bath for pickling to lower the Si concentration on the surface of the steel sheet.

Further, Patent Document 2 proposes a high-strength cold-rolled steel sheet which is improved by setting the line width of the linear oxide containing Si observed from the surface of the steel sheet to 1 μm to 10 μm to 300 nm or less. Corrosion resistance.

Further, Patent Document 3 proposes a technique of increasing the removal ability of an oxide by setting the iron ion concentration (divalent) in hydrochloric acid to 0.5% to 18%.

However, in the high-strength cold-rolled steel sheet described in Patent Document 1, even if the Si concentration on the surface of the steel sheet is lowered before cold rolling, since an oxide containing Si is formed on the surface of the steel sheet by annealing after cold rolling, It is also not expected to improve the corrosion resistance after painting.

In the high-strength cold-rolled steel sheet described in Patent Document 2, the corrosion resistance does not become a problem in a corrosive environment such as the salt spray test specified in JIS Z2371, but in a salt water immersion test or a composite cycle corrosion test. In the harsh environment of corrosion, sufficient corrosion resistance after painting cannot be obtained.

In other words, only the Si concentration on the surface of the steel sheet after hot rolling or the linear oxide containing Si is lowered, and a high-strength cold-rolled steel sheet excellent in corrosion resistance after coating cannot be obtained.

In the technique described in Patent Document 3, SiO _{2 is} not dissolved in hydrochloric acid, and even if the iron ion concentration is 0.5% to 18%, SiO ₂ cannot be removed.

Therefore, as a technique for solving the above problem, Patent Document 4 discloses a technique of removing an oxide containing Si concentrated on a surface of a steel sheet by pickling in an annealing step or the like, and then imparting the surface. The S-based compound improves the reactivity with the chemical synthesis treatment liquid, thereby improving the chemical synthesis treatability.

Further, Patent Document 5 discloses a technique of providing a P-based compound instead of the S-based compound in Patent Document 4.

Further, as a technique for solving the above problems, Patent Document 6 proposes a technique of removing SiO ₂ by pickling by an oxidizing acid in the first stage, and then performing non-oxidation in the second stage. The acidic acid pickling removes the Fe-based oxide formed in the pickling in the first stage, thereby improving the reactivity with the chemical synthesis treatment liquid and improving the chemical synthesis treatability. Prior art document patent document

Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-204350. Patent Document 2: Japanese Patent Laid-Open No. 2004-244698. Patent Document 3: Japanese Patent Laid-Open No. Hei. No. 64-62485. Patent Document 4: Japanese Patent Laid-Open No. 2007- Japanese Patent Laid-Open Publication No. Hei. No. 2007-246951.

[Problems to be solved by the invention]

However, in recent years, in order to reduce industrial waste (inhibition of sludge production) and to reduce operating costs, the temperature of the chemical synthesis treatment liquid has been lowered, and chemical synthesis treatment has been compared with the previous chemical synthesis treatment conditions. The reactivity of the liquid to the steel sheet is greatly reduced. In the ordinary steel sheet having a small amount of addition of the alloy used in the prior art, the lowering of the temperature of the chemical synthesis treatment liquid is not caused by the improvement of the surface adjustment technique before the chemical synthesis treatment. However, in the high-strength cold-rolled steel sheet to which a large amount of Si is added, the reactivity with the chemical synthesis treatment liquid is remarkably lowered due to the influence of the Si-containing oxide formed on the surface layer of the steel sheet in the annealing step, so it is necessary to use a certain These methods increase reactivity from the side of the steel sheet. However, in the techniques disclosed in Patent Document 4 and Patent Document 5, even if it is effective for the conventional ordinary steel sheet, it is not expected to be able to cope with the low temperature of the chemical synthesis treatment liquid for the high-strength cold-rolled steel sheet containing a large amount of Si. Fully improved results. As described above, by applying the technique disclosed in Patent Document 6, even in the case of a high-strength cold-rolled steel sheet containing a large amount of Si, the temperature of the chemical synthesis treatment liquid can be lowered. However, in the technique disclosed in Patent Document 6, when the Fe concentration is low, the pickling speed is slow and the removal ability of the oxide containing Si is insufficient, and when the Fe concentration is high, the iron-based oxide is formed, and the chemical synthesis is performed. The handleability and the corrosion resistance after coating are deteriorated. In addition, in the technique disclosed in Patent Document 6, since the chemical synthesis treatment liquid has high reactivity with the surface of the steel sheet, the rate of occurrence of spot rust during long-term storage of the cold-rolled steel sheet is increased.

In view of the above, it is an object of the present invention to provide a cold-rolled steel sheet which is excellent in chemical synthesis treatability and excellent in corrosion resistance after coating, a method for producing the same, and an automobile part. Moreover, it is an object of the invention to provide an apparatus for manufacturing the cold rolled steel sheet. [Means for solving the problem]

In order to solve the above problems, the inventors of the present invention have conducted detailed analysis on the surface characteristics of the steel sheet after annealing, and have conducted intensive studies on methods for improving the reactivity of the surface of the steel sheet with the chemical synthesis treatment liquid. As a result, it has been found that it is extremely important to subject the surface of the continuously annealed steel sheet to strong pickling after cold rolling, and to remove the Si-containing oxide layer formed on the surface layer of the steel sheet during annealing, and to reduce it by the strong pickling. The surface coverage of the steel sheet due to the iron-based oxide formed on the surface of the steel sheet, and in order to prevent the occurrence of spot rust during storage of the cold-rolled steel sheet and improve the corrosion resistance after coating, it is followed by a strong pickling followed by an alkaline solution. The residue of the acidic solution is neutralized, thereby completing the present invention.

The present invention has been made based on the above findings, and the gist thereof is as follows. [1] A method for producing a cold-rolled steel sheet, which is subjected to a first pickling of a steel sheet which is continuously annealed after cold rolling, followed by a second pickling, and further subjected to a neutralization treatment using an alkaline solution. [2] The method for producing a cold-rolled steel sheet according to [1], wherein the alkaline solution is one of sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, orthophosphate, condensed phosphate or any two or more The mixed alkaline solution having a pH of 9.5 or more. [3] The method for producing a cold-rolled steel sheet according to [1] or [2] wherein, in the neutralization treatment, the temperature of the alkaline solution is 20 ° C to 70 ° C, and the treatment time is 1 second. 30 seconds. [4] The method for producing a cold-rolled steel sheet according to any one of [1] to [3] wherein the first pickling uses nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, and two or more of the above The acid is mixed with any of the acids. [5] The method for producing a cold-rolled steel sheet according to any one of [1] to [4] wherein the first pickling is any one of the following a) or b): a) An acid solution containing nitric acid and hydrochloric acid, wherein the nitric acid concentration exceeds 50 g/L and 200 g/L or less, and the ratio of the hydrochloric acid concentration to the nitric acid concentration R1 (hydrochloric acid/nitric acid) is 0.01 to 0.25, and An acid liquid having a Fe ion concentration of 3 g/L to 50 g/L b) is an acid liquid containing nitric acid and hydrofluoric acid, and the hydrogen concentration is more than 50 g/L and 200 g/L or less. An acid solution having a ratio of the concentration of the hydrofluoric acid to the concentration of the nitric acid of R2 (hydrofluoric acid/nitric acid) of 0.01 to 0.25 and a Fe ion concentration of 3 g/L to 50 g/L. [6] The method for producing a cold-rolled steel sheet according to any one of [1] to [5] wherein the second pickling uses a non-oxidizing acid. [7] The method for producing a cold-rolled steel sheet according to [6], wherein the non-oxidizing acid is hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and Any of the acids in which two or more kinds of acids are mixed. [8] The method for producing a cold-rolled steel sheet according to [6], wherein the non-oxidizing acid is hydrochloric acid having a concentration of 0.1 g/L to 50 g/L, and 0.1 g/L to 150. G/L sulfuric acid, any of an acid obtained by mixing 0.1 g/L to 20 g/L hydrochloric acid with 0.1 g/L to 60 g/L sulfuric acid. [9] The method for producing a cold-rolled steel sheet according to any one of [1] to [8] wherein, in the second pickling, the temperature of the acid liquid is from 20 ° C to 70 ° C, and the pickling time is 1 second to 30 seconds. [10] The method for producing a cold-rolled steel sheet according to any one of [1] to [9] wherein the steel sheet contains 0.5% by mass to 3.0% by mass of Si as a component composition. [11] The method for producing a cold-rolled steel sheet according to [10], wherein the steel sheet further contains C: 0.01% by mass to 0.30% by mass, Mn: 1.0% by mass to 7.5% by mass, and P: 0.05% by mass or less. S: 0.01% by mass or less and Al: 0.06% by mass or less as a component composition, and the balance contains Fe and unavoidable impurities. [12] The method for producing a cold-rolled steel sheet according to [11], wherein the steel sheet further contains Nb: 0.3% by mass or less, Ti: 0.3% by mass or less, V: 0.3% by mass or less, and Mo: 1.0% by mass. Hereinafter, one or two or more selected from the group consisting of Cr: 1.0% by mass or less, B: 0.006% by mass or less, and N: 0.008% by mass or less are used as a component. [13] The method for producing a cold-rolled steel sheet according to the above aspect, wherein the steel sheet further contains Ni: 2.0% by mass or less, Cu: 2.0% by mass or less, Ca: 0.1% by mass or less, and REM. One or two or more selected from 0.1% by mass or less are used as a component. [14] A cold-rolled steel sheet produced by the production method according to any one of [1] to [13], wherein the Si-containing oxide layer on the surface layer of the steel sheet is removed and is present on the surface of the steel sheet. The surface coverage of the iron-based oxide is 40% or less. [15] The cold-rolled steel sheet according to [14], wherein the cold-rolled steel sheet has a maximum thickness of the iron-based oxide present on the surface of the steel sheet of 150 nm or less. [16] An automobile part using the cold rolled steel sheet according to [14] or [15]. [17] A manufacturing apparatus for a cold-rolled steel sheet, which is provided with a first pickling device, a second pickling device, an acid neutralizing treatment device, and a drying device in the subsequent stage of the continuous annealing device. [18] The apparatus for producing a cold-rolled steel sheet according to [17], wherein a water washing apparatus is provided in a rear stage of the first pickling apparatus, the second pickling apparatus, and the acid neutralization processing apparatus. [19] The apparatus for manufacturing a cold-rolled steel sheet according to [17] or [18], wherein the first pickling apparatus, the second pickling apparatus, the acid neutralization processing apparatus, the water washing A water spray device is provided on the inlet side and/or the outlet side of any one or more of the devices. [Effects of the Invention]

According to the present invention, a cold-rolled steel sheet excellent in chemical synthesis treatability and excellent in corrosion resistance after coating can be obtained. Further, according to the production method of the present invention, the cold-rolled steel sheet having good chemical synthesis treatability and corrosion resistance after coating can be easily and stably produced by merely adjusting the pickling conditions and passing through the usual cold rolling step and pickling step. .

It is possible to provide a cold-rolled steel sheet which is excellent in chemical synthesis treatability when used in a low-temperature chemical synthesis treatment liquid even when it contains 0.5% by mass to 3.0% by mass of Si, and is impregnated in a warm water such as salt. The corrosion resistance after coating is also excellent in a severe corrosive environment like a test or a composite cycle corrosion test. Therefore, according to the present invention, the chemical synthesis treatability or the post-coating corrosion resistance of the high-strength cold-rolled steel sheet containing a large amount of Si having a tensile strength TS of 590 MPa or more can be greatly improved, and thus it can be preferably used for the strength of an automobile body. Parts, etc.

The details of the present invention are explained below. In the following description, the unit of the content of each element of the steel component composition is "% by mass", and the following is only indicated by "%" unless otherwise specified.

In the annealing step of a continuous annealing furnace in which a cold rolled cold-rolled steel sheet is recrystallized to impart desired structure, strength, and workability, a non-oxidizing or reducing gas is generally used as an ambient gas. The dew point is also strictly managed. Therefore, in the ordinary normal cold-rolled steel sheet in which the amount of alloy added is small, oxidation of the surface of the steel sheet is suppressed. However, in a steel sheet containing 0.5% or more of Si or Mn, even if the composition or dew point of the ambient gas during annealing is strictly managed, oxidation of Si or Mn which is oxidizable compared with Fe is unavoidable. On the surface of the steel sheet, an oxide containing Si such as Si oxide (SiO ₂ ) or a Si—Mn composite oxide is formed. The composition of the oxide varies depending on the steel sheet component, the annealing environment, etc., but usually the steel sheet component or the annealing environment is mixed and changed. Further, it is known that the Si-containing oxide is formed not only on the surface of the steel sheet but also inside the ferrite iron, thereby hindering the surface of the steel sheet in the chemical synthesis treatment (zinc phosphate treatment) performed as the substrate treatment of the electroplating coating. Etching and adverse effects on the formation of a sound chemically synthesized film.

On the other hand, in recent years, in order to reduce the amount of sludge generated during the chemical synthesis treatment or the running cost, the temperature of the chemical synthesis treatment liquid has been lowered, and the reactivity of the chemical synthesis treatment liquid to the steel sheet has been compared with that of the prior art. Chemical synthesis treatment is carried out under significantly lower conditions. In the ordinary steel sheet having a small amount of addition of the alloy used in the prior art, the change of the chemical synthesis treatment conditions is not particularly problematic by the improvement of the surface adjustment technique or the like. However, in a steel sheet to which a large amount of alloy component is added, in particular, a high-strength cold-rolled steel sheet in which a large amount of Si is added and high strength is obtained, the chemical synthesis treatment liquid is lowered due to the change in the chemical synthesis treatment conditions. The impact is enormous. Therefore, in the cold-rolled steel sheet containing a large amount of Si, it is necessary to cope with the deterioration of the chemical synthesis treatment conditions, and to activate the surface of the steel sheet itself to improve the reactivity with the chemical synthesis treatment liquid.

In order to cope with the deterioration of the chemical synthesis treatment conditions as described above, the inventors have repeatedly studied the method of improving the chemical synthesis treatability of the steel sheet. As a result, it has been found that the surface of the cold-rolled steel sheet after continuous annealing is subjected to strong pickling using nitric acid or the like as an acid pickling liquid, and the Si-containing oxide layer formed on the surface layer of the steel sheet formed after continuous annealing or the like after cold rolling is removed. . Here, the Si-containing oxide refers to SiO ₂ or Si-Mn composite oxidation formed along the surface of the steel sheet or the crystal grain boundary inside the steel sheet after slab heating or hot rolling or annealing after cold rolling. The thickness of the layer in which the Si-containing oxide is present varies depending on the steel sheet component or the annealing conditions (temperature, time, and environment), and is usually about 1 μm from the surface of the steel sheet. Further, in the present invention, the removal of the Si-containing oxide layer means that the surface of the steel sheet is analyzed in the depth direction by GDS (Glow Discharge Luminescence Spectroscopy), and pickling is performed until the peak of Si or O does not occur. The oxide layer containing Si is removed.

Further, the reason why a strong acid such as nitric acid is used as the pickling liquid is that the Si-Mn composite oxide is easily dissolved in an acid in the Si-containing oxide, but SiO ₂ exhibits poor solubility, so that it is removed. The Si-containing oxide on the surface of the steel sheet must be removed together with the ferrite.

However, according to research by the inventors and the like, it is apparent that after the continuous annealing, the Si-containing oxide layer existing on the surface of the steel sheet is removed by strong pickling with nitric acid or the like, thereby greatly improving the chemical synthesis treatability, but sometimes There is a case where the chemical synthesis treatment property is poor. Further, the cause of the investigation further revealed that the Si-based oxide layer was removed by strong pickling by the nitric acid or the like, but the Fe-based oxide was formed from the surface of the steel sheet by pickling. The iron-based oxide precipitates on the surface of the steel sheet to cover the surface of the steel sheet, whereby the chemical synthesis treatability is lowered. When the residue of the pickling liquid remains, the rate of occurrence of spot rust during storage of the cold-rolled steel sheet becomes high, and coating is performed. After the corrosion resistance is poor.

Further, as a result of further research, it has been found that in order to alleviate the adverse effect on the chemical synthesis treatability, it is important to suppress the formation of iron-based oxide on the surface of the steel sheet and to make the surface coverage of the iron-based oxide present on the surface of the steel sheet. 40% or less. Further, it has been found that after the strong pickling, the non-oxidizing acid is used for pickling, whereby the iron-based oxide existing on the surface of the steel sheet can be dissolved and removed. Further, it has been found that it is important to carry out a neutralization treatment using an alkali solution after pickling using a non-oxidizing acid, thereby removing the residue of the acidic solution which is twice acid-washed.

As described above, in the present invention, the strong pickling is performed as the first pickling, and the formation of the iron-based oxide on the surface of the steel sheet is suppressed, and the Si-containing oxide layer existing on the surface of the steel sheet is removed. Then, pickling is carried out using a non-oxidizing acid as the second pickling, and the surface coverage of the iron-based oxide present on the surface of the steel sheet is 40% or less. And, the neutralization treatment is then carried out by an alkaline solution.

Furthermore, the inventors have found that when the coating ratio of the iron-based oxide formed on the surface of the steel sheet by acid pickling is 40% or less and the maximum thickness of the iron-based oxide is 150 nm or less, Further improving the chemical synthesis treatability, the corrosion resistance is further improved; and as a method for achieving the same, it is effective to change the pickling conditions (concentration, temperature, time) and non-oxidative pickling conditions (acid concentration, temperature, time) It’s appropriate.

In addition, the iron-based oxide in the present invention refers to an oxide of an iron main body having an atomic concentration ratio of iron of 30% or more in an element other than oxygen constituting the oxide. The iron-based oxide is present on the surface of the steel sheet in a non-uniform thickness, and is an oxide different from a natural oxide film having a thickness of several nanometers and being present in a layered manner. Further, according to the observation by a transmission electron microscope (TEM) or a diffraction pattern (diffraction pattern) by electron beam diffraction, it is known that the cold rolled steel sheet is The iron-based oxide formed on the surface is amorphous.

The present invention has been completed by further studying the novel findings.

Next, a method of producing the cold rolled steel sheet of the present invention will be described.

In the present invention, a steel material (slab) containing, for example, 0.5% to 3.0% of Si is heated, hot-rolled and cold-rolled, and the steel sheet subjected to continuous annealing is first pickled, followed by second. The sub-acid wash is followed by an alkaline solution for neutralization. By performing such pickling and neutralization treatment, the chemical synthesis treatability and the corrosion resistance after coating are remarkably improved.

In the first pickling condition, a large amount of oxide containing Si such as SiO ₂ or a Si—Mn composite oxide is formed in the surface layer of the steel sheet after continuous annealing, and the chemical synthesis treatment property or the corrosion resistance after coating is remarkably lowered in the above state. . Therefore, in the production method of the present invention, as the first pickling, it is preferred to strongly pickle the annealed cold-rolled steel sheet using an acid liquid containing nitric acid and hydrochloric acid or an acid liquid containing nitric acid and hydrofluoric acid. The Si-containing oxide layer on the surface of the steel sheet is removed together with the ferrite iron by the first pickling. Among the oxides containing Si, the Si—Mn composite oxide is easily dissolved in an acid, but SiO ₂ exhibits poor solubility to an acid. Therefore, in order to remove the Si-containing oxide including SiO ₂ , it is necessary to perform strong pickling to remove the oxide layer together with the ferrite iron of the steel sheet. Therefore, in the present invention, as the acid usable in the acid liquid, nitric acid which is a strong oxidizing acid can be preferably used, and if the oxide layer containing Si can be removed, it can also be hydrofluoric acid or hydrochloric acid. There is no particular limitation on the type of acid such as sulfuric acid. Further, an acid obtained by mixing the two or more acids may also be used. Further, it is also effective to add an acid pickling accelerator to the acid solution or to perform electrolytic treatment to promote dissolution of the ferrite iron.

In addition, as described above, Fe which is dissolved from the surface of the steel sheet by pickling produces an iron-based oxide which precipitates on the surface of the steel sheet and coats the surface of the steel sheet, whereby the chemical synthesis treatability may be lowered. In order to prevent this and to reduce the burden of the second pickling, it is preferred to suppress the amount of iron-based oxide formed on the surface of the steel sheet. For the above reasons, it is preferred to provide the following pickling conditions.

In order to efficiently remove the oxide containing Si, in the case of an acid solution containing nitric acid and hydrochloric acid, it is preferred to set the nitric acid concentration to a range of more than 50 g/L and 200 g/L or less, and to have an oxide film destruction. The hydrochloric acid of the effect contains nitric acid and hydrochloric acid so that the ratio of the concentration of hydrochloric acid to the concentration of nitric acid of R1 (hydrochloric acid/nitric acid) is in the range of 0.01 to 0.25, and the Fe ion concentration (sum of divalent and trivalent) is 3 g/ A range of L to 50 g/L. More preferably, the nitric acid concentration is 100 g/L or more and 200 g/L or less. Further, it is more preferable that the R1 is 0.02 to 0.15. Further, it is more preferable that the Fe ion concentration is from 3 g/L to 25 g/L. In the case of an acid solution containing nitric acid and hydrofluoric acid, it is preferred to set the nitric acid concentration to a range of more than 50 g/L and 200 g/L or less, and hydrofluoric acid having an oxide film destruction effect to hydrofluoric acid. Nitric acid and hydrofluoric acid are contained in such a manner that the ratio R2 (hydrofluoric acid/nitric acid) to the concentration of nitric acid is in the range of 0.01 to 0.25, and the Fe ion concentration (sum of divalent and trivalent) is set to 3 g/L. ～50 g/L range. More preferably, the nitric acid concentration is 100 g/L or more and 200 g/L or less. Further, it is more preferable that the R2 is 0.02 to 0.15. Further, it is more preferable that the Fe ion concentration is from 3 g/L to 25 g/L. When R1 and R2 are more than 0.25 or the Fe ion concentration (sum of divalent and trivalent) is less than 3 g/L, the desired pickling speed cannot be obtained, and the Si-containing oxide cannot be efficiently removed. On the other hand, when R1 and R2 are less than 0.01 or the Fe ion concentration is more than 50 g/L, although a desired pickling speed can be obtained, since a large amount of Fe ions in the pickling liquid is formed, a large amount of surface is formed on the surface of the steel sheet. In the Fe-based oxide, the Fe-based oxide cannot be removed by the second pickling, and the chemical synthesis treatability and corrosion resistance cannot be improved. In addition, as a method of maintaining the Fe ion concentration (sum of divalent and trivalent) at 3 g/L to 50 g/L, dilution is carried out when it exceeds 50 g/L, and nitric acid and hydrochloric acid are additionally added; or A method of reducing an iron component in an acid by an iron removal device.

Further, by appropriately setting the pickling conditions (concentration, temperature, time), the maximum thickness of the iron-based oxide can be made 150 nm or less. The first pickling is carried out under the conditions that the temperature of the acid solution is from 20 ° C to 70 ° C and the pickling time is from 3 seconds to 30 seconds, and the maximum thickness of the iron-based oxide becomes 150 nm or less. The synthetic processability is further improved, and the corrosion resistance is further improved.

Second pickling condition When only strong pickling is performed as the first pickling, it is difficult to stably control the surface coverage of the iron-based oxide formed on the surface of the steel sheet to 40% or less. Therefore, in the present invention, in order to more reliably reduce the iron-based oxide formed on the surface of the steel sheet by the first pickling, the second pickling is performed. In the present invention, it is preferred to carry out pickling using an acid solution containing a non-oxidizing acid, and to dissolve and remove the iron-based oxide by the second pickling.

The non-oxidizing acid is preferably any one or two or more selected from the group consisting of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, and oxalic acid. Any non-oxidizing acid may be used, but it is preferably used if it is hydrochloric acid or sulfuric acid which is usually used in the iron industry. Among them, since hydrochloric acid is a volatile acid, it is difficult to retain a residue such as sulfate or the like on the surface of the steel sheet after washing with water, and the oxide destruction effect by chloride ions is large, and therefore it can be preferably used. Further, an acid obtained by mixing hydrochloric acid and sulfuric acid can also be used.

Among them, in order to prevent the insufficient removal of the iron-based oxide and the deterioration of the surface properties of the steel sheet due to excessive pickling, it is preferred to use hydrochloric acid having a concentration of 0.1 g/L to 50 g/L, and 0.1 g/L. Any of an acid obtained by mixing 150 g/L of sulfuric acid and 0.1 g/L to 20 g/L of hydrochloric acid with 0.1 g/L to 60 g/L of sulfuric acid.

The second pickling is preferably such that the temperature of the acid solution is from 20 ° C to 70 ° C and the pickling time is from 1 second to 30 seconds. The reason is that if the liquid temperature of the pickling liquid is 20° C. or higher and the treatment time is 1 second or longer, the removal of the iron-based oxide remaining on the surface of the steel sheet is sufficient. On the other hand, when the temperature of the pickling liquid is 70 ° C or lower and the treatment time is 30 seconds or less, the surface of the steel sheet is not excessively dissolved, and a new surface oxide film is not formed. More preferably, the temperature of the acid solution is from 30 ° C to 50 ° C. Further, it is more preferable that the pickling time is from 2 seconds to 20 seconds.

Further, in order to obtain a steel sheet having more excellent chemical synthesis treatability or corrosion resistance, it is preferable that the maximum thickness of the iron-based oxide existing on the surface of the steel sheet is surely reduced to 150 nm or less after the pickling, and therefore, It is preferred to moderately increase the concentration of the acid containing the non-oxidizing acid. For example, when hydrochloric acid is used, the hydrochloric acid concentration is preferably from 3 g/L to 50 g/L, and when sulfuric acid is used, the sulfuric acid concentration is preferably from 8 g/L to 150 g/L. Further, in the case of using an acid washing solution obtained by mixing hydrochloric acid and sulfuric acid, it is preferred to use a hydrochloric acid having a concentration of 3 g/L to 20 g/L and a sulfuric acid having a concentration of 8 g/L to 60 g/L. Made of acid. If the concentration of the acid solution is within the above concentration range, the iron-based oxide can be surely thinned to 150 nm or less, and the chemical synthesis treatability or the corrosion resistance after coating is improved. Further, if the concentration of the acid solution is within the above concentration range, the surface of the steel sheet is not dissolved excessively, and a new surface oxide film is not formed.

Neutralization Treatment Conditions In the present invention, after the second pickling, the alkaline solution is further used for the neutralization treatment.

When the oxide formed at the time of annealing is removed by pickling to improve the reactivity of the surface of the steel sheet, since the residue of the pickling liquid is present, it is feared that spot rust occurs during storage of the cold-rolled steel sheet. In order to suppress the above, it is preferred to use one of sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, orthophosphate, or condensed phosphate or any two or more of the neutralization treatment after pickling and re- pickling. The mixed alkaline solution having a pH of 9.5 or more is subjected to neutralization treatment. The reason for using the alkaline solution is that the residue of the acid washing liquid is neutralized and removed. Further, when the pH is less than 9.5, the residue of the pickling liquid cannot be completely neutralized. Examples of the condensed phosphate include sodium pyrophosphate, sodium polyphosphate, and the like. The pH is more preferably a pH of from 10.0 to 12.0.

Further, in the neutralization treatment by the alkaline solution, the temperature of the alkaline solution is preferably from 20 ° C to 70 ° C for 1 second to 30 seconds. When the liquid temperature of the alkaline solution is 20 ° C or more and the treatment time is 1 second or longer, the residue of the pickling liquid is sufficiently neutralized. On the other hand, when the temperature of the pickling liquid exceeds 70 ° C, alkaline fumes are generated, and when the treatment time exceeds 30 seconds, the length of the apparatus becomes long and a large equipment cost is required. More preferably, the temperature of the alkaline solution is from 30 ° C to 50 ° C. Further, it is more preferable that the processing time is from 2 seconds to 20 seconds.

As described above, after the continuous annealing, the first pickling, the second pickling, and then the alkaline solution are used for the neutralization treatment, and then subjected to a usual processing step such as quenching and tempering to form a product sheet ( Cold rolled steel plate).

In the present invention, the pickling method, that is, the method of contacting the acid solution described in the present invention is not particularly limited. A method of applying an acid solution or a method of immersing in an acid solution or the like is considered.

Further, it is preferred that the first pickling and the second pickling are continuously performed. By continuously performing, natural oxidation of the steel sheet after the first pickling is prevented, and the final product can be obtained at the same time, so that it can be produced at low cost.

Further, in the present invention, the water washing treatment may be performed after the first pickling, after the second pickling, and after the neutralization treatment. Further, in the case of performing the first pickling, the second pickling, the neutralization treatment, and the water washing treatment, the water can be further washed by the water spray on the inlet side and/or the outlet side of the treatment. Further, it is preferred to carry out a drying treatment by a dryer or the like after the water washing treatment.

Next, the chemical composition of the cold-rolled steel sheet of the present invention will be described.

In the present invention, it is preferable to have a composition having a high strength and an excellent chemical synthesis treatability as for an axle component or the like which can be used for an automobile.

It is preferably contained in an amount of 0.5% to 3.0% of Si. Si has an effect of increasing the strength of steel without significantly impairing the workability (solid solution strengthening energy), so it is an element that effectively achieves high strength of steel, but it is also resistant to chemical synthesis or corrosion after coating. An element that causes adverse effects. For the reasons described above, it is preferred to add 0.5% or more of Si. On the other hand, when the content of Si exceeds 3.0%, the hot rolling property or the cold rolling property is largely lowered, which may adversely affect productivity, or may cause a decrease in ductility of the steel sheet itself. Therefore, in the case of adding Si, Si is preferably in the range of 0.5% to 3.0%. Si is more preferably in the range of 0.8% to 2.5%.

The components other than the above are acceptable as long as they have a composition range of a normal cold rolled steel sheet. However, when the cold-rolled steel sheet according to the present invention is applied to a high-strength cold-rolled steel sheet having a tensile strength TS of 590 MPa or more used in an automobile body or the like, it is preferable to set a preferable component other than the above. As described below.

C: 0.01% to 0.30% C is an element effective for increasing the strength of steel, and is a residual Worthite iron, or a toughening iron or Ma Tiansan which has the effect of producing Transformation Induced Plasticity (TRIP). Iron is also an effective element. When C is 0.01% or more, the above effect can be obtained. On the other hand, when C is 0.30% or less, the decrease in the weldability does not occur. Therefore, C is preferably added in the range of 0.01% to 0.30%, more preferably in the range of 0.10% to 0.20%.

Mn: 1.0% to 7.5% Mn is an element which has a function of strengthening the steel by solid solution strengthening, increasing the smoldering property, and promoting the formation of residual Worth iron or toughened iron or granulated iron. This effect is exhibited when 1.0% or more is added. On the other hand, when Mn is 7.5% or less, the effect can be obtained without causing an increase in cost. Therefore, Mn is preferably added in the range of 1.0% to 7.5%, more preferably in the range of 2.0% to 5.0%.

P: 0.05% or less P is an element which has a large solid solution strengthening energy and which does not impair the drawability, and is an element which effectively achieves high strength. Therefore, it is preferably contained in an amount of 0.005% or more. However, P is an element which impairs the point-melting property, but if it is 0.05% or less, no problem arises. Therefore, P is preferably 0.05% or less, more preferably 0.02% or less.

S: 0.01% or less S is an impurity element which is inevitably mixed in, and is a harmful component which precipitates MnS in steel and lowers the elongational flangeability of the steel sheet. In order not to lower the stretch flangeability, S is preferably 0.01% or less. S is more preferably 0.005% or less, and particularly preferably 0.003% or less.

Al: 0.06% or less Al is an element which is added as a deacidifying agent in the steelmaking step, and is an element which is effective for separating non-metallic inclusions which reduce the elongational flangeability into slag, and therefore preferably contains 0.01%. the above. When Al is 0.06% or less, the effect of the raw material cost is not increased, and the effect can be obtained. Therefore, Al is preferably set to 0.06% or less. Al is more preferably in the range of 0.02% to 0.06%.

Further, the cold-rolled steel sheet according to the present invention may further contain, in addition to the component composition, Nb: 0.3% or less, Ti: 0.3% or less, V: 0.3% or less, Mo: 1.0% or less, and Cr: 1.0% or less, B. : one or two or more selected from the group consisting of 0.006% or less and N: 0.008% or less.

Nb, Ti, and V are elements that form carbides or nitrides, suppress the growth of ferrite during the heating phase during annealing, refine the structure, improve formability, and particularly stretch flangeability, and Mo, Cr, and B is an element which improves the ignitability of steel and promotes the formation of toughened iron or granulated iron, and therefore can be added within the above range. Further, N is an element which forms a nitride with Nb, Ti, and V or solid-solution in steel to contribute to the high strength of steel. When it is 0.008% by mass or less, a large amount of nitride is not formed, so that it can be suppressed. The effect is obtained by the fracture caused by the formation of voids at the time of press forming.

In addition to the component composition, the cold-rolled steel sheet of the present invention may further contain one or more selected from the group consisting of Ni: 2.0% or less, Cu: 2.0% or less, Ca: 0.1% or less, and REM: 0.1% or less. .

Ni and Cu have an effect of promoting the formation of a low temperature phase transformation and increasing the strength of steel, and therefore can be added within the above range. Further, since Ca and REM are elements which control the form of the sulfide-based inclusions and improve the elongational flangeability of the steel sheet, they can be added in the above range.

In the cold-rolled steel sheet of the present invention, the remainder other than the above components is Fe and unavoidable impurities. However, if it does not impair the effect of the present invention, the addition of other components is not refused.

Next, the surface characteristics of the cold-rolled steel sheet of the present invention will be described. As described above, the cold-rolled steel sheet according to the present invention is a steel sheet surface having an oxide layer containing Si such as SiO ₂ or a Si—Mn composite oxide formed on the surface layer of the steel sheet during annealing. Therefore, it is necessary to carry out neutralization treatment using an alkaline solution after the first pickling and the second pickling.

Further, in addition to removing the Si-containing oxide layer, the cold-rolled steel sheet according to the present invention must reduce the surface coverage of the iron-based oxide present on the surface of the steel sheet to 40% or less. The reason is that if the surface coverage of the iron-based oxide exceeds 40%, the dissolution reaction of iron in the chemical synthesis treatment is inhibited, and the growth of chemically synthesized crystals such as zinc phosphate is suppressed. However, when a chemically synthesized treatment liquid having a low temperature is used, as in the case of an axle component of a particularly severely corroded vehicle, 40% or less of the cold-rolled steel sheet for use in applications requiring extremely strict post-coating corrosion resistance is used. The coverage rate is not sufficient and must be reduced to a lower of 35% or less. The surface coverage of the iron-based oxide is preferably 35% or less.

In the present invention, the surface coverage of the iron-based oxide is determined as follows. Using a scanning electron microscope (ULV-SEM) with a very low acceleration voltage that can detect the surface information of the electrode, the surface of the steel plate after pickling is observed at about 5 fields with an acceleration voltage of 2 kV, an operating distance of 3.0 mm, and a magnification of about 1000 times. Spectroscopic analysis was performed using an energy dispersive X-ray spectroscope (EDX) to obtain a reflected electron image. By performing binarization processing on the reflected electron image using an image analysis software such as Image J, the area ratio of the black portion is measured, and the measured values of the respective fields of view are averaged, whereby the surface of the iron-based oxide can be obtained. Coverage rate. Further, as the scanning electron microscope (ULV-SEM) of the extremely low acceleration voltage, for example, SEISS (manufactured by SEISS); ULTRA 55; and, as an energy dispersive X-ray spectroscope (EDX), for example, List: manufactured by Thermo Fisher; NSS312E.

Here, the critical value of the binarization processing will be described. The steel slab of the steel symbol G shown in Table 3 of the examples to be described later is hot-rolled and cold-rolled under the conditions shown in No. 93 of Table 4 of the same later-described embodiment, and subjected to continuous annealing to obtain a sheet thickness. The cold-rolled steel sheet of 1.8 mm was then pickled and washed with the cold-rolled steel sheet after continuous annealing under the conditions shown in Table 1, and dried and then subjected to quenching and tempering with an elongation of 0.7%. Two kinds of cold-rolled steel sheets of No. a and No. b having different amounts of iron-based oxides on the surface of the steel sheet were obtained. Then, the cold-rolled steel sheet of No. a was used as a standard sample having a large amount of iron-based oxide, and the cold-rolled steel sheet of No. b was used as a standard sample having less iron-based oxide, and a scanning electron microscope was used. Under the conditions described, a reflected electron image was obtained for each steel sheet. Fig. 1 shows a reflection electron image of a steel sheet of No. a and No. b, and Fig. 2 shows a distribution of the number of pixels of the reflection electron image of the steel sheet of No. a and No. b with respect to a gradation value. Figure. In the present invention, the gradation value (Y point) corresponding to the intersection (X point) of the distribution maps of No. a and No. b shown in Fig. 2 is set as a critical value. In this manner, the surface coverage of the iron-based oxide of the steel sheets of No. a and No. b was determined using the above-mentioned critical value, and as a result, the steel sheet of No. a was 85.3%, and the steel sheet of No. b was 25.8%.

[Table 1]

In addition, in order to further improve the chemical synthesis treatability and the corrosion resistance after coating, the surface coverage of the iron-based oxide on the surface of the steel sheet after the second pickling is preferably 40% or less, and more preferably an iron-based oxide. The maximum thickness is below 150 nm. The reason is that if the maximum thickness of the iron-based oxide is 150 nm or less, the dissolution reaction of iron in the chemical synthesis treatment is not partially inhibited, and the precipitation of chemically synthesized crystals such as zinc phosphate is not locally suppressed. The maximum thickness of the iron-based oxide is preferably 130 nm or less.

Here, the maximum thickness of the iron-based oxide is determined as follows.

First, by focusing on a focused ion beam (FIB), 10 sampled copies of a cross section having a length of about 8 μm in the width direction of the steel sheet were produced from the surface of the acid-coated steel sheet. Then, using a transmission electron microscope (TEM) of an energy dispersive X-ray spectroscope (EDX) having a local information capable of investigating the cross section, the cross section of each replica was continuously photographed at an acceleration voltage of 200 kV and a magnification of 100,000 times. As an example, FIG. 3 shows a photograph of a cross section of a coating layer formed by the first pickling on the surface of the steel sheet by a transmission electron microscope (TEM), and FIG. 4 shows an EDX analysis result of the coating layer. 4, since the coating layer is an iron-based oxide, the line A indicating the steel sheet ferro-grain and the thickest portion showing the iron-based oxide layer shown in the cross-sectional photograph of FIG. 3 are measured for all 10 replicas. The interval of the line B is such that the maximum thickness thereof is taken as the maximum thickness of the iron-based oxide. In addition, the size or the number of the copies, the measurement conditions by the TEM, and the like are exemplified, and may of course be appropriately changed.

The cold-rolled steel sheet obtained as described above is excellent in chemical synthesis treatability, and excellent in corrosion resistance after coating, which is evaluated by a salt warm water immersion test or a composite cycle corrosion test, and thus can be preferably used as an automobile part. Example 1

The invention is illustrated in more detail by way of examples.

The melt contains C: 0.125%, Si: 1.5%, Mn: 2.6%, P: 0.019%, S: 0.008%, and Al: 0.040% by a usual refining process such as a converter or a degassing treatment. Steel containing Fe and unavoidable impurities is continuously cast to form a steel raw material (slab). Then, the slab is heated again to a temperature of 1150 ° C to 1170 ° C, and hot rolling is performed at a finishing temperature of 850 ° C to 880 ° C, and coiled at a temperature of 500 ° C to 550 ° C. A hot rolled steel sheet having a thickness of 3 mm to 4 mm is produced. Then, the hot-rolled steel sheet was pickled to remove the scale and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm. Then, the cold-rolled steel sheet is heated to a soaking temperature of 750 ° C to 780 ° C for 40 seconds to 50 seconds, and then cooled from a soaking temperature to 350 ° C at 20 ° C / sec to 30 ° C / sec. After continuous cooling at a cooling stop temperature of 400 ° C for 100 seconds to 120 seconds, Table 2-1 to Table 2-2 (Table 2-1 and Table 2-2 below) The surface of the steel sheet was pickled and washed with water under the conditions shown in Table 2). After drying, the temper rolling was performed at an elongation of 0.7%, and No. 1 to 2 shown in Table 2 was obtained. No. 82 cold rolled steel sheet.

A test piece was taken from each of the cold-rolled steel sheets, and a scanning electron microscope (ULV-SEM; manufactured by Sai Sai Co., Ltd.; ULTRA 55) with a very low acceleration voltage was used to accelerate the voltage by 2 kV, the operating distance was 3.0 mm, and the magnification was 1000 times. Five fields of view were observed on the surface of the steel sheet, and a reflected electron image was obtained by spectroscopic analysis using an energy dispersive X-ray spectroscope (EDX; manufactured by Thermo Fisher Scientific; NSS312E). Using the image analysis software (Image J), the gradation value (Y point) corresponding to the intersection (X point) of the distribution map of the standard sample No. a and No. b of the reflected electron image is set as a critical value. The value was subjected to binarization, and the area ratio of the black portion was measured, and the average value of the five fields of view was obtained to obtain the surface coverage of the iron-based oxide.

Further, a test piece was collected from each of the cold-rolled steel sheets, and the occurrence of spot rust of the cold-rolled steel sheet was evaluated under the following conditions. Further, after the chemical synthesis treatment and the coating treatment were carried out under the following conditions, three kinds of corrosion tests for the salt warm water immersion test, the salt water spray test, and the composite cycle corrosion test were performed, and the corrosion resistance after coating was evaluated. Further, the depth direction distribution of O, Si, Mn, and Fe was measured on the surface of the test piece collected from each of the cold-rolled steel sheets by GDS. (1) Evaluation of the occurrence of the spot rust during the storage of the cold-rolled steel sheet. After the rust-preventing oil is applied to each of the cold-rolled steel sheets, it is placed outdoors without any influence of external factors such as dust, and the point after about one month is investigated. The presence or absence of rust is set to "○" for spotless rust and "x" for some rust. (2) Chemical synthesis treatment conditions Degreaser manufactured by Nihon Parkerizing Co., Ltd.: FC-E2011, surface conditioner: PL-X and chemical synthesis treatment agent: PALBOND PB -L3065, under the conditions of the following standard conditions and comparative conditions for lowering the temperature of the chemical synthesis treatment liquid and lowering the temperature, the chemical adhesion treatment film adhesion amount is 1.7 g/m ² to 3.0 g/m ² , The test piece collected from each of the cold-rolled steel sheets was subjected to chemical synthesis treatment. <Standard conditions> · Degreasing step: treatment temperature is 40 ° C, treatment time is 120 seconds · Spray degreasing, surface adjustment step: pH value is 9.5, treatment temperature is room temperature, treatment time is 20 seconds · Chemical synthesis treatment step : The temperature of the chemical synthesis treatment liquid is 35 ° C, the treatment time is 120 seconds <low temperature conditions> The conditions for lowering the temperature of the chemical synthesis treatment liquid in the standard conditions to 33 ° C (3) Corrosion test using Nippon Electroplating coating manufactured by Nippon Paint: V-50, the surface of the test piece subjected to the chemical synthesis treatment was plated and applied to the following three types, and the film thickness was 25 μm. Corrosion test. <Salt warm water immersion test> The surface of the test piece (n=1, "n=1" means the number of test pieces is one piece) which was subjected to chemical synthesis treatment and electroplating, by a cutter After imparting a cross-cut damage of 45 mm in length, the test piece was immersed in a 5 mass% NaCl solution (60 ° C) for 360 hours, then washed with water, dried, and taped after the tape was attached to the damaged portion. In the peeling test, the total peeling total width of the cut damage portion was measured. The case where the total maximum peeling width is 6.0 mm or less is regarded as pass. When the total peeling total width is 5.0 mm or less, the corrosion resistance in the salt-tolerant warm water immersion test can be evaluated as good. <Salt Spray Test (SST)> After the cross-section damage of 45 mm in length was applied to the surface of the test piece (n=1) subjected to chemical synthesis treatment or electroplating, 5 mass% was used. The NaCl aqueous solution was subjected to a salt spray test for 1200 hours in accordance with the neutral salt spray test specified in JIS Z2371:2000, and the tape peeling test was performed on the transversely damaged portion to measure the maximum peeling of the cut damage portion. Total width. The case where the maximum peeling total width is 5.2 mm or less is set as a pass. When the total peeling total width is 4.0 mm or less, the corrosion resistance in the salt spray test can be evaluated as good. <Composite Cycling Corrosion Test (CCT)> After the cross-section damage of 45 mm in length was applied to the surface of the test piece (n=1) subjected to chemical synthesis treatment and electroplating, the salt water was sprayed. (5 mass% NaCl aqueous solution: 35 ° C, relative humidity: 98%) × 2 hours → drying (60 ° C, relative humidity: 30%) × 2 hours → wet (50 ° C, relative humidity: 95%) × 2 hours In one cycle, after the test piece was repeatedly subjected to a corrosion test for 120 cycles, it was washed with water, and after drying, a tape peeling test was performed on the cut damaged portion, and the total peeling total width of the cut damaged portion was measured. The case where the maximum peeling total width is 7.8 mm or less is set as a pass. If the total peeling total width is 6.0 mm or less, the corrosion resistance in the composite cycle corrosion test can be evaluated as good.

The results of the test are shown together with the conditions in Table 2.

[table 2-1] [Table 2-2]

According to Table 2, the steel sheet of the invention example obtained by pickling under the conditions suitable for the present invention after continuous annealing can suppress the generation of spot rust, the salt warm water immersion test, the salt spray test, and the composite cycle corrosion. In any of the tests, the maximum peeling total width was small, and good chemical synthesis treatability and corrosion resistance after coating were exhibited. In particular, it is understood that the cold-rolled steel sheet having a surface coverage of 40% or less of the iron-based oxide is excellent in corrosion resistance after coating in a severe corrosive environment. In addition, the depth direction distribution of O, Si, Mn, and Fe on the surface of each steel sheet of Table 2 was measured by GDS. As a result, it was confirmed that the steel sheet obtained by pickling under the conditions suitable for the present invention did not exhibit Si or O. The peak of the oxide layer is sufficiently removed. For the reference, the depth direction distribution of O, Si, Mn, and Fe in the surface analysis of the test piece No. 2 of the invention example of Table 2 and the test piece No. 7 of the invention example by GDS is shown in Fig. 5 . Example 2

The steel having A to O having the composition shown in Table 3 is melted by a usual refining process such as a converter or a degassing treatment, and is continuously cast to obtain a slab. The slab was hot-rolled under the hot rolling conditions shown in Table 4 to obtain a hot-rolled steel sheet having a thickness of 3 mm to 4 mm, which was pickled to remove the iron oxide scale on the surface of the steel sheet, and then cold rolled. Cold rolled steel sheet with a thickness of 1.8 mm. Then, the cold-rolled steel sheets were continuously annealed under the same conditions as shown in Table 4, and then subjected to the first pickling and the second pickling under the conditions shown in Table 5, followed by washing with water to carry out neutralization. The mixture was washed with water, dried, and subjected to temper rolling at an elongation of 0.7% to obtain a cold-rolled steel sheet of No. 84 to No. 107.

The test piece of each of the cold-rolled steel sheets thus obtained was collected, and the surface coverage of the iron-based oxide on the surface of the steel sheet after pickling was measured in the same manner as in Example 1, and then subjected to the following tensile test and Corrosion resistance test after painting. Further, the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece collected from each cold-rolled steel sheet was measured by GDS. (1) Mechanical characteristics The JIS No. 5 tensile test piece (n = 1) specified in JIS Z2201:1998, which is collected at a right angle to the rolling direction (C direction), is used in accordance with JIS Z2241:1998. Test and measure tensile strength TS. (2) Evaluation of the occurrence of the spot rust during the storage of the cold-rolled steel sheet. After the rust-preventing oil is applied to each of the cold-rolled steel sheets, it is placed outdoors without any influence of external factors such as dust, and the point after about one month is investigated. The presence or absence of rust is set to "○" for spotless rust and "x" for some rust. (3) Corrosion resistance after coating The test piece collected from each cold-rolled steel sheet was subjected to chemical synthesis treatment under the same conditions as in Example 1, and a test piece subjected to electroplating coating was produced in the same manner as in Example 1. The method is applied to three kinds of corrosion tests of salt warm water immersion test, salt spray test (SST) and composite cyclic corrosion test (CCT), and the corrosion resistance after coating is evaluated.

The results of the test are shown in Table 5.

[table 3]

[Table 4]

[table 5]

According to Table 5, it is found that the surface coverage of the iron-based oxide on the surface of the steel sheet is 40% or less, which contains 0.5% or more of Si and is subjected to secondary pickling under the conditions suitable for the present invention and subjected to neutralization treatment. The high-strength cold-rolled steel sheet according to the invention is excellent not only in chemical synthesis treatability and corrosion resistance after coating, but also has high tensile strength TS of 590 MPa or more. Further, the depth direction distribution of O, Si, Mn, and Fe was measured by GDS. As a result, it was confirmed that the steel sheets obtained by pickling under the conditions suitable for the present invention did not exhibit peaks of Si or O, but were sufficiently The oxide layer containing Si is removed. Example 3

The steel containing C: 0.125%, Si: 1.5%, Mn: 2.6%, P: 0.019%, S: 0.008%, and Al: 0.040%, and the balance containing Fe and unavoidable impurities is continuously cast. Chenggang raw materials (slabs). After the slab is again heated to a temperature of 1150 ° C to 1170 ° C, hot rolling is performed at a finishing finish temperature of 850 ° C to 880 ° C, and winding is performed at a temperature of 500 ° C to 550 ° C. Hot rolled steel sheet with a thickness of 3 mm to 4 mm. The hot-rolled steel sheet was pickled to remove the scale, and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.8 mm. Then, the cold-rolled steel sheet is heated to a soaking temperature of 750 ° C to 780 ° C for 40 seconds to 50 seconds, and then cooled from the soaking temperature at 20 ° C / sec to 30 ° C / sec. After continuous cooling for 100 seconds to 120 seconds in the cooling stop temperature range until the cooling stop temperature of 350 ° C to 400 ° C, Table 6-1 to Table 6-2 (Table 6 below) 1 and Table 6-2, collectively referred to as Table 6), the surface of the steel sheet is subjected to the first pickling and the second pickling, and then washed with water, neutralized and washed, and dried. Thereafter, quenched and tempered rolling having an elongation of 0.7% was carried out, and cold-rolled steel sheets of Nos. 108 to No. 162 shown in Table 6 were obtained. A test piece was taken from each of the cold-rolled steel sheets, and the surface coverage and the maximum thickness of the iron-based oxide formed on the surface of the steel sheet by pickling were measured by the above method.

In addition, a test piece was collected from each of the cold-rolled steel sheets, and evaluation of spot rust during storage of the cold-rolled steel sheet under the following conditions; and after performing the chemical synthesis treatment and the coating treatment, the salt water immersion test and the brine were supplied. Three corrosion tests were conducted in the spray test and the composite cycle corrosion test, and the corrosion resistance after coating was evaluated. Further, the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece collected from each cold-rolled steel sheet was measured by GDS. (1) Evaluation of the occurrence of the spot rust during the storage of the cold-rolled steel sheet. After the rust-preventing oil was applied to each of the cold-rolled steel sheets, the rust-preventing oil was applied to the outside of the cold-rolled steel sheet without being affected by external factors such as dust, and the investigation was carried out after about one month. The presence or absence of rust is caused by setting "no rust" to "○" and a little rust to "x". (2) Chemical synthesis treatment conditions Degreaser manufactured by Japanese Pakajingjing Co., Ltd.: FC-E2011, surface conditioner: PL-X and chemical synthesis treatment agent: Paloban (PALBOND) PB-L3065, under Under the two conditions of the standard conditions and the comparative conditions of lowering the temperature of the chemical synthesis treatment liquid and lowering the temperature, the chemical deposition treatment film adhesion amount is 1.7 g/m ² to 3.0 g/m ² , The test piece collected on the cold rolled steel sheet was subjected to chemical synthesis treatment. <Standard conditions> · Degreasing step: treatment temperature is 40 ° C, treatment time is 120 seconds · Spray degreasing, surface adjustment step: pH value is 9.5, treatment temperature is room temperature, treatment time is 20 seconds · Chemical synthesis treatment step : The temperature of the chemical synthesis treatment liquid is 35 ° C, the treatment time is 120 seconds <low temperature conditions> The conditions for lowering the temperature of the chemical synthesis treatment liquid in the standard conditions to 33 ° C (3) Corrosion test using Nippon Electroplating coating manufactured by lacquer: V-50, the surface of the test piece subjected to the chemical synthesis treatment was subjected to electroplating coating so as to have a film thickness of 25 μm, and was provided to be more severe than Example 1. The following three corrosion tests of the conditions. <Salt temperature water immersion test> After the cross-section damage of 45 mm length was given by the dicing blade on the surface of the test piece (n=1) which carried out the chemical-synthesis process and electroplating coating, the test piece was carried out. After immersing in a 5% by mass NaCl solution (60 ° C) for 480 hours, the mixture was washed with water and dried, and a tape was attached to the damaged portion, and a peeling tape peeling test was performed to measure the total peeling total width of the cut damage portion. The case where the total maximum peeling width is 6.0 mm or less is regarded as pass. When the total peeling total width is 5.0 mm or less, the corrosion resistance in the salt-tolerant warm water immersion test can be evaluated as good. <Salt Spray Test (SST)> After the cross-section damage of 45 mm in length was applied to the surface of the test piece (n=1) subjected to chemical synthesis treatment or electroplating, 5 mass% was used. The NaCl aqueous solution was subjected to a salt spray test for 1400 hours in accordance with the neutral salt spray test specified in JIS Z2371:2000, and the tape peeling test was performed on the transversely damaged portion to measure the maximum peeling of the cut damage portion. Total width. The case where the maximum peeling total width is 5.2 mm or less is set as a pass. When the total peeling total width is 4.0 mm or less, the corrosion resistance in the salt spray test can be evaluated as good. <Composite Cycling Corrosion Test (CCT)> After the cross-section damage of 45 mm in length was applied to the surface of the test piece (n=1) subjected to chemical synthesis treatment and electroplating, the salt water was sprayed. (5 mass% NaCl aqueous solution: 35 ° C, relative humidity: 98%) × 2 hours → drying (60 ° C, relative humidity: 30%) × 2 hours → wet (50 ° C, relative humidity: 95%) × 2 hours In one cycle, after the test piece was repeatedly subjected to a corrosion test for 150 cycles, it was washed with water, and after drying, a tape peeling test was performed on the cut damaged portion, and the total peeling total width of the cut damaged portion was measured. The case where the maximum peeling total width is 7.8 mm or less is set as a pass. If the total peeling total width is 6.0 mm or less, the corrosion resistance in the composite cycle corrosion test can be evaluated as good.

The results of the test are shown in Table 6.

[Table 6-1] [Table 6-2]

According to Table 6, it is understood that the surface of the steel sheet after annealing is acidified under the condition that the surface coverage of the iron-based oxide on the surface of the steel sheet after re- pickling is 40% or less and the maximum thickness of the iron-based oxide is 150 nm or less. The maximum peeling total width of the steel sheet of the present invention washed in any of the salt warm water immersion test, the salt water spray test, and the composite cyclic corrosion test performed under conditions of long and severe test time compared with Example 1. They are all small and exhibit excellent corrosion resistance after painting. Further, the depth direction distribution of O, Si, Mn, and Fe was measured by GDS. As a result, it was confirmed that the steel sheet subjected to pickling under the conditions suitable for the present invention did not have a peak of Si or O, and was sufficiently removed. An oxide layer of Si. [Industrial availability]

The cold-rolled steel sheet produced according to the present invention is excellent not only in chemical synthesis treatability or corrosion resistance after coating, but also has high strength, and can be preferably used in fields such as home appliances or construction, in addition to raw materials for automobile parts. Raw materials for parts that require the same characteristics.

no

Fig. 1 shows a reflected electron image of the surface of a steel sheet of the cold-rolled steel sheet standard samples No. a and No. b for obtaining the surface coverage of the iron-based oxide. Fig. 2 is a view showing a histogram of the number of pixels of the reflected electron image of the cold-rolled steel sheet standard samples No. a and No. b with respect to the gradation value. 3 is a view showing a result of observing a cross section of a steel sheet surface covering material after pickling using a non-oxidizing acid by a transmission electron microscope. 4 is a graph showing the results of energy dispersive X-ray spectroscopy (EDX) analysis of the iron-based oxide observed in FIG. 3. 5 is a graph for measuring the depth direction distribution of O, Si, Mn, and Fe on the surface of the test piece of Table 2 by Glow Discharge Spectrometry (GDS).

no

Claims (19)

A method for producing a cold-rolled steel sheet, which is subjected to a first pickling of a steel sheet which is continuously annealed after cold rolling, followed by a second pickling, and further subjected to a neutralization treatment using an alkaline solution. The method for producing a cold-rolled steel sheet according to claim 1, wherein the alkaline solution is one of sodium hydroxide, sodium carbonate, sodium hydrogencarbonate, orthophosphate, or condensed phosphate, or any two or more thereof. The mixed alkaline solution having a pH of 9.5 or more. The method for producing a cold-rolled steel sheet according to the first or second aspect of the invention, wherein, in the neutralization treatment, the temperature of the alkaline solution is 20 ° C to 70 ° C, and the treatment time is 1 second. 30 seconds. The method for producing a cold-rolled steel sheet according to any one of the preceding claims, wherein the first pickling uses nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid, and two or more of the above. The acid is mixed with any of the acids. The method for producing a cold-rolled steel sheet according to any one of claims 1 to 4, wherein the first pickling is any one of the following a) or b): a) An acid solution containing nitric acid and hydrochloric acid, wherein the nitric acid concentration exceeds 50 g/L and 200 g/L or less, and the ratio of the hydrochloric acid concentration to the nitric acid concentration R1 (hydrochloric acid/nitric acid) is 0.01 to 0.25, and An acid liquid having a Fe ion concentration of 3 g/L to 50 g/L b) is an acid liquid containing nitric acid and hydrofluoric acid, and the hydrogen concentration is more than 50 g/L and 200 g/L or less. An acid solution having a ratio of the concentration of the hydrofluoric acid to the concentration of the nitric acid of R2 (hydrofluoric acid/nitric acid) of 0.01 to 0.25 and a Fe ion concentration of 3 g/L to 50 g/L. The method for producing a cold-rolled steel sheet according to any one of claims 1 to 5, wherein the second pickling uses a non-oxidizing acid. The method for producing a cold-rolled steel sheet according to claim 6, wherein the non-oxidizing acid is hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and the like. Any of the acids in which two or more kinds of acids are mixed. The method for producing a cold-rolled steel sheet according to Item 6 or 7, wherein the non-oxidizing acid is hydrochloric acid having a concentration of 0.1 g/L to 50 g/L, and 0.1 g/L to 150. G/L sulfuric acid, any of an acid obtained by mixing 0.1 g/L to 20 g/L hydrochloric acid with 0.1 g/L to 60 g/L sulfuric acid. The method for producing a cold-rolled steel sheet according to any one of the preceding claims, wherein, in the second pickling, the temperature of the acid solution is from 20 ° C to 70 ° C, and the pickling time is 1 second to 30 seconds. The method for producing a cold-rolled steel sheet according to any one of the first aspect, wherein the steel sheet contains 0.5% by mass to 3.0% by mass of Si as a component composition. The method for producing a cold-rolled steel sheet according to claim 10, wherein the steel sheet further contains C: 0.01% by mass to 0.30% by mass, Mn: 1.0% by mass to 7.5% by mass, and P: 0.05% by mass or less. S: 0.01% by mass or less and Al: 0.06% by mass or less as a component composition, and the balance contains Fe and unavoidable impurities. The method for producing a cold-rolled steel sheet according to claim 11, wherein the steel sheet further contains Nb: 0.3% by mass or less, Ti: 0.3% by mass or less, V: 0.3% by mass or less, and Mo: 1.0% by mass. Hereinafter, one or two or more selected from the group consisting of Cr: 1.0% by mass or less, B: 0.006% by mass or less, and N: 0.008% by mass or less are used as a component. The method for producing a cold-rolled steel sheet according to the above aspect, wherein the steel sheet further contains from 2.0% by mass or less, Cu: 2.0% by mass or less, and Ca: 0.1% by mass or less and REM. One or two or more selected from 0.1% by mass or less are used as a component. A cold-rolled steel sheet produced by the production method according to any one of claims 1 to 13, wherein the Si-containing oxide layer on the surface layer of the steel sheet is removed and is present on the surface of the steel sheet. The surface coverage of the iron-based oxide is 40% or less. The cold-rolled steel sheet according to claim 14, wherein the cold-rolled steel sheet has a maximum thickness of the iron-based oxide present on the surface of the steel sheet of 150 nm or less. An automobile part using the cold-rolled steel sheet as described in claim 14 or 15. A manufacturing apparatus for a cold-rolled steel sheet, which is provided with a first pickling device, a second pickling device, an acid neutralizing treatment device, and a drying device in the subsequent stage of the continuous annealing device. The apparatus for manufacturing a cold-rolled steel sheet according to claim 17, wherein a water washing device is provided in a rear stage of the first pickling device, the second pickling device, and the acid neutralizing device. The apparatus for manufacturing a cold-rolled steel sheet according to claim 17 or claim 18, wherein the first pickling device, the second pickling device, the acid neutralizing treatment device, and the water washing A water spray device is provided on the inlet side and/or the outlet side of any one or more of the devices.