JP5682366B2 - Method for producing Si-containing cold-rolled steel sheet - Google Patents

Description

TECHNICAL FIELD The present invention relates to a Si-containing cold-rolled steel sheet, a method for producing the same, and an automobile member. Specifically, it has high strength and excellent chemical conversion treatment, and is post-coating corrosion resistance evaluated by a salt warm water immersion test or a combined cycle corrosion test. those concerning the manufacturing how the Si-containing cold-rolled steel sheet excellent in. The Si-containing cold-rolled steel sheet of the present invention can be suitably used for a high-strength cold-rolled steel sheet having a tensile strength TS of 590 MPa or more.

  In recent years, there has been a strong demand for improving fuel efficiency of automobiles from the viewpoint of protecting the global environment. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, there is a strong demand for increasing the strength of automobile bodies. In order to meet these demands, the cold rolled steel sheet, which is the material of automobile parts, has been strengthened and thinned (lightened), so that it is possible to reduce the weight of the automobile body and increase the strength at the same time. It has been. However, since many automobile members are manufactured by forming a steel plate, the steel plate used as the material is required to have excellent workability in addition to high strength.

There are various methods for increasing the strength of the cold-rolled steel sheet, but an effective means for increasing the strength without greatly reducing the formability is a solution strengthening method by adding Si. However, when a large amount of Si, especially 0.5 mass% or more of Si is added to the cold-rolled steel sheet, SiO ₂ or Si on the steel sheet surface during slab heating, hot rolling, or annealing after cold rolling. It is known that a large amount of Si-containing oxides such as -Mn-based composite oxides are formed. Since this Si-containing oxide significantly lowers the chemical conversion treatment property, the high-strength cold-rolled steel sheet containing a large amount of Si is not only inferior in chemical conversion treatment property, but also after the electrodeposition coating, a salt warm water immersion test or wet-drying. When exposed to a severe corrosive environment such as a combined cycle corrosion test that repeats the above, there is a problem that the coating film peels more easily than a normal steel plate, and the post-coating corrosion resistance is inferior.

  For example, Patent Document 1 discloses a cold-rolled steel sheet containing 0.5 to 2.5 mass% of Si with a higher oxygen partial pressure than normal annealing conditions. By carrying out reduction annealing and appropriately controlling the precipitation form of the Si-containing oxide formed at the time of annealing, the ratio of the length of the Si-containing oxide in the steel sheet surface length of 10 μm is all 80% or less, A technique for obtaining a steel sheet having excellent phosphatability is disclosed.

  Patent Document 2 discloses that after annealing a cold-rolled steel sheet containing 0.70 to 1.35 mass% of Si, it is immersed in hydrochloric acid or sulfuric acid having a temperature of 50 ° C. or higher and a concentration of 10 mass% or higher for 7 seconds or more. The surface coverage of the Si-based oxide on the surface of the steel sheet is 20% or less, and the maximum circle diameter inscribed in the Si-based oxide coating region is controlled to 5 μm or less, so that the chemical conversion processability is excellent. A technique for obtaining a cold-rolled steel sheet is disclosed.

Japanese Patent No. 3840392 Japanese Patent No. 4319559

  However, as in the techniques described in Patent Documents 1 and 2, only by reducing the Si concentration on the steel sheet surface or reducing the linear oxide containing Si, the Si-containing oxide formed on the steel sheet surface layer is reduced. Since the material layer is not completely removed, the cost for improving the chemical conversion treatment is limited, and there is a basic problem that sufficient corrosion resistance cannot be obtained in a severe corrosive environment. Therefore, it is desirable to completely remove the Si-containing oxide on the steel sheet surface.

  In recent years, the temperature of chemical conversion liquid has been lowered for the purpose of reducing industrial waste (suppressing sludge generation) and reducing running costs. The sex is decreasing. The decrease in the reactivity of the chemical conversion treatment liquid is not particularly problematic in conventional plain steel sheets with a small amount of alloy addition due to improvement of the surface conditioning technique before chemical conversion treatment. However, high-strength cold-rolled steel sheets containing a large amount of Si have significantly reduced reactivity with the chemical conversion solution on the surface of the steel sheet itself due to the influence of the Si-containing oxide formed on the steel sheet surface during the annealing process. ing. Therefore, it is necessary to increase the reactivity from the steel sheet side by some means.

The present invention has been made in view of the above-mentioned problems of cold-rolled steel sheets containing a large amount of Si, and its purpose is to provide chemical conversion treatment even when a low-temperature chemical conversion treatment liquid is used. good, and is to propose an advantageous production how the Si-containing cold-rolled steel sheet excellent in corrosion resistance after coating in severe corrosive environment such as salt warm water immersion test and combined cycle corrosion test.

  In order to solve the above-mentioned problems, the inventors repeated detailed analysis on the surface characteristics of the steel sheet after the continuous annealing, and conducted earnest research on a method for increasing the reactivity between the steel sheet surface and the chemical conversion treatment liquid. As a result, after cold rolling, the steel sheet surface that has been continuously annealed is subjected to electrolytic pickling, and the Si-containing oxide layer formed on the steel sheet surface layer is completely removed, and the iron system generated on the steel sheet surface by the electrolytic pickling. The inventors have found that it is extremely important to control the oxide, and have completed the present invention.

That is , the present invention, after cold rolling containing 0.5 to 3.0 mass% of Si, after electrolytically pickling the continuously annealed cold-rolled steel sheet to remove the Si-containing oxide layer of the steel sheet surface layer, The present invention proposes a method for producing a Si-containing cold-rolled steel sheet, characterized in that the surface coverage of the iron-based oxide on the steel sheet surface is reduced to 40% or less by re- pickling with a non-oxidizing acid.
In addition to Si, the cold-rolled steel sheet in the production method of the present invention includes C: 0.01 to 0.30 mass%, Mn: 1.0 to 7.5 mass%, P: 0.05 mass% or less, and S: 0. .01 mass% or less and Al: 0.06 mass% or less, with the balance being a component composition of Fe and inevitable impurities.

Moreover, the manufacturing method of this invention is characterized by performing the said electrolytic pickling using hydrochloric acid with a density | concentration of 50-300 g / L.

  The non-oxidizing acid in the production method of the present invention is any one of hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, and an acid obtained by mixing two or more of these. It is characterized by being an acid.

  In the production method of the present invention, the non-oxidizing acid has a concentration of 0.1 to 50 g / L hydrochloric acid, 0.1 to 150 g / L sulfuric acid, 0.1 to 20 g / L hydrochloric acid, and 0.1. It is any acid of the acid which mixed 1-60 g / L sulfuric acid.

  According to the present invention, Si is contained in a large amount of 0.5 to 3.0 mass%, and it is excellent in chemical conversion treatment property even under poor chemical conversion treatment conditions that have been lowered in temperature. A cold-rolled steel sheet having excellent post-painting corrosion resistance in a severe corrosive environment such as a test can be provided. Therefore, according to the present invention, it becomes possible to greatly improve the chemical conversion property and the corrosion resistance after painting of a high-strength cold-rolled steel sheet containing a large amount of Si and having a tensile strength TS of 590 MPa or more. Greatly contributes to the improvement of strength and strength.

Standard sample No. of cold-rolled steel sheet for determining the surface coverage of iron-based oxides The reflected-electron image of the steel plate surface of a and b is shown. Cold rolled steel sheet standard sample No. Fig. 5 shows a histogram of the number of pixels against the gray values of the reflected electron image photographs of a and b.

First, the basic technical idea of the present invention will be described.
In an annealing process using a continuous annealing furnace performed to recrystallize a cold-rolled cold-rolled steel sheet to give a desired structure, strength, and workability, it is usually a non-oxidizing or reducing gas as an atmospheric gas. Is used, and the dew point is also strictly controlled. Therefore, the oxidation of the steel plate surface is suppressed in a general cold-rolled steel plate with a small alloy addition amount. However, in steel sheets containing 0.5 mass% or more of Si, even if the atmosphere gas components and dew point during annealing are strictly controlled, Si and Mn, which are more easily oxidizable than Fe, are oxidized, and the steel sheet surface In addition, it is inevitable to form Si-containing oxides such as Si oxide (SiO ₂ ) and Si—Mn composite oxide. The structure of the Si-containing oxide varies depending on the steel plate component, the annealing atmosphere, and the like, but generally both are often mixed. In addition, since this Si-containing oxide is formed not only on the steel plate surface but also inside the iron core, the etching property of the steel plate surface in the chemical conversion treatment (zinc phosphate treatment) applied as the base treatment of electrodeposition coating It is known to greatly inhibit and adversely affect the formation of a sound chemical conversion coating.

  On the other hand, in recent years, for the purpose of reducing the amount of sludge generated during the chemical conversion treatment and the running cost, the temperature of the chemical conversion treatment liquid has been lowered, and the chemical conversion treatment is performed under a condition in which the reactivity of the treatment liquid with respect to the steel sheet is lower than conventional. It has become like this. Such a change in the chemical conversion treatment condition does not pose a particular problem due to improvement of the surface adjustment technique or the like in a conventional steel sheet with a small alloy addition amount that has been conventionally used. However, a steel sheet to which a large amount of alloy components are added, particularly a high-strength cold-rolled steel sheet in which a large amount of Si is added to increase the strength, has an extremely large influence due to the change in the chemical conversion treatment conditions. Therefore, in order to cope with the deterioration of the chemical conversion treatment conditions as described above, it is necessary to activate the steel sheet surface of the Si-containing cold-rolled steel sheet by some means to increase the reactivity with the chemical conversion liquid.

Therefore, the inventors have repeatedly studied a method for activating the steel sheet surface and improving the chemical conversion treatment property in order to cope with the deterioration of the chemical conversion treatment conditions as described above. As a result, it has been found that it is effective to perform electrolytic pickling on the surface of the cold-rolled steel sheet after continuous annealing to completely remove the Si-containing oxide layer formed on the surface layer of the steel sheet. Here, the Si-containing oxide is SiO formed along the grain boundaries on the steel sheet surface or inside the steel sheet at the time of slab heating, hot rolling, annealing after hot rolling, or cold rolling. ₂ or Si—Mn composite oxide. The thickness of the layer containing these Si-containing oxides varies depending on the steel plate components and production conditions (temperature, time, atmosphere, etc.), but is usually a region of about 1 μm from the steel plate surface (hereinafter, this region). (Also referred to as “steel sheet surface layer”).

The reason for using electrolytic pickling to remove the Si-containing oxide is that, among Si-containing oxides, Si-Mn composite oxide is easily dissolved in acid, but SiO ₂ exhibits poor solubility. This is because, in order to remove this, it is necessary to remove the Si-containing oxide on the steel sheet surface together with the base iron.

  However, according to the inventors' research, the chemical conversion treatment performance is greatly improved by removing the Si-containing oxide layer present on the steel sheet surface by electrolytic pickling after continuous annealing, but sometimes, It became clear that the chemical conversion processability might be inferior. And when the cause was further investigated, Fe dissolved from the steel sheet surface was oxidized by the above electrolytic pickling to produce a new iron-based oxide, and deposited on the steel sheet surface to cover the steel sheet surface, thereby It became clear that chemical conversion processability has deteriorated. The iron-based oxide refers to an iron-based oxide having an atomic concentration ratio of iron of 30% or more among elements other than oxygen constituting the oxide.

And as a result of further investigation, in order to reduce the adverse effect on the chemical conversion treatment of the iron-based oxide produced by the electrolytic pickling, the production of the iron-based oxide is suppressed, and the steel plate made of the iron-based oxide It is important to reduce the surface coverage to 40% or less, and as a means for achieving this, after the above electrolytic pickling, further non-oxidizing acid is used and re-pickling under proper conditions. The present inventors have found that it is extremely effective to dissolve and remove iron-based oxides deposited on the steel sheet surface.
The present invention has been completed by further studying the above-described novel findings.

Next, the component composition of the Si-containing cold-rolled steel sheet of the present invention will be described.
Si: 0.5 to 3.0 mass%
Since Si can increase the strength of steel without significantly impairing workability, Si is an effective element for achieving high strength of steel, but it is an element that adversely affects chemical conversion properties and corrosion resistance after coating. is there. In order to increase the strength by adding Si, it is necessary to add 0.5 mass% or more. Moreover, if Si is less than 0.5 mass%, since the influence by deterioration of chemical conversion treatment conditions is also small, it is not necessary to apply this invention. On the other hand, when the content of Si exceeds 3.0 mass%, the steel becomes hard and adversely affects the rollability and sheeting property (manufacturability), or the ductility of the steel plate itself is reduced. Therefore, Si is added in the range of 0.5 to 3.0 mass%. Preferably it is the range of 0.8-2.5 mass%.

  The cold-rolled steel sheet of the present invention must contain Si in the above range, but the other components can be allowed as long as they are in the composition range of a normal cold-rolled steel sheet, and are particularly limited. Is not to be done. However, when the cold-rolled steel sheet of the present invention is applied to a high-strength cold-rolled steel sheet having a tensile strength TS of 590 MPa or more used for an automobile body or the like, it preferably has the following component composition.

C: 0.01-0.30 mass%
C is an element effective for increasing the strength of steel, and is also an element effective for generating retained austenite, bainite and martensite having a TRIP (Transformation Induced Plasticity) effect. is there. The said effect is acquired by addition of 0.01 mass% or more. However, when C is added excessively, the weldability decreases, but if it is 0.30 mass% or less, no significant decrease occurs. Therefore, it is preferable to add C in the range of 0.01 to 0.30 mass%. More preferably, it is the range of 0.10-0.20 mass%.

Mn: 1.0 to 7.5 mass%
Mn is an element having an effect of enhancing the hardenability by solid solution strengthening of steel, enhancing hardenability, and promoting the formation of retained austenite, bainite, and martensite. Such an effect is manifested by addition of 1.0 mass% or more. On the other hand, excessive addition of Mn leads to an increase in raw material cost, but is acceptable if it is 7.5 mass% or less. Therefore, it is preferable to add Mn in the range of 1.0 to 7.5 mass%. More preferably, it is the range of 2.0-5.0 mass%.

P: 0.05 mass% or less P is an element that does not impair deep drawability for a large solid solution strengthening ability, and is an element effective for achieving high strength. In order to acquire the said effect, it is preferable to make it contain 0.005 mass% or more. On the other hand, since P is also an element which impairs spot weldability, the upper limit is preferably set to 0.05 mass%. More preferably, it is 0.02 mass% or less.

S: 0.01 mass% or less S is an impurity element inevitably mixed in steel, and is also a harmful component that precipitates as MnS and lowers the stretch flangeability of the steel sheet. In order not to lower the stretch flangeability, S is preferably limited to 0.01 mass% or less, and more preferably 0.005 mass% or less. More preferably, it is 0.003 mass% or less.

Al: 0.06 mass% or less Al is an element added as a deoxidizer in the steelmaking process, and is an element effective for separating non-metallic inclusions that reduce stretch flangeability as slag. It is preferable to contain 0.01 mass% or more. However, excessive addition causes an increase in raw material cost, so the upper limit of Al is preferably 0.06 mass%. More preferably, it is the range of 0.02-0.06 mass%.

In the Si-containing cold-rolled steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities. However, addition of other components is not rejected as long as the effects of the present invention are not impaired.
For example, Ti, Nb and V form precipitates such as carbides and nitrides, increase the strength of the steel, and further refine the structure by suppressing the growth of ferrite, improving the formability, especially stretch flangeability It is a useful element. The above effect can be obtained by adding 0.005 mass% or more of each element, and when it exceeds 0.3 mass%, the element is saturated. For this reason, Ti, Nb, and V are each preferably added in the range of 0.005 to 0.3 mass%. More preferably, it is the range of 0.005-0.2 mass%, respectively.
Mo and Cr are elements that improve the hardenability of the steel and promote the formation of bainite and martensite to contribute to high strength. The above-mentioned effects can be obtained by adding 0.005 mass% or more, and when it exceeds 0.3 mass%, it is saturated. Therefore, it is preferable to add Mo and Cr in the range of 0.005 to 0.3 mass%, respectively. More preferably, it is the range of 0.005-0.2 mass%, respectively.
Since B is an element effective for improving the hardenability of steel, 0.006 mass% or less can be added. More preferably, it is 0.002 mass% or less.
Ni and Cu are effective elements for increasing the strength of steel, and can be added in a range of 2.0 mass% or less, respectively.
N is an element that most deteriorates the aging resistance of steel. In particular, when it exceeds 0.008 mass%, the deterioration of aging resistance becomes significant. Therefore, N is preferably as low as possible and is preferably 0.008 mass% or less. More preferably, it is 0.006 mass% or less.
Ca and REM have an effect of making the sulfide form spherical, and are effective elements for improving stretch flangeability. The said effect is acquired by addition of 0.001 mass% or more, and when it exceeds 0.1 mass%, the cleanliness of steel will fall. Therefore, Ca and REM are preferably added in the range of 0.001 to 0.1 mass%, respectively.

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 of the present invention is made of SiO ₂ or Si formed on the surface of the steel sheet or the grain boundary part (steel sheet surface layer) in the vicinity of the surface at the time of manufacturing such as hot rolling or annealing after cold rolling. -It is necessary to have a steel plate surface from which a Si-containing oxide layer such as a Mn-based composite oxide has been completely removed. For that purpose, it is necessary that the Si-containing oxide formed on the steel sheet surface layer is dissolved and removed together with the base iron by electrolytic pickling.

  Furthermore, the cold-rolled steel sheet of the present invention has an area where the steel oxide surface coverage of the iron-based oxide produced on the steel sheet surface by the electrolytic pickling is in addition to that in which the Si-containing oxide layer is completely removed. It is necessary to be 40% or less. Generally, when the surface coverage of the iron-based oxide is about 85% or less, the dissolution reaction of iron in the chemical conversion treatment is suppressed, and the growth of chemical crystals such as zinc phosphate is not inhibited. However, in the case of a cold-rolled steel sheet used for an application that requires particularly severe post-coating corrosion resistance, such as a vehicle underbody member, when the chemical conversion treatment is performed using a low-temperature treatment liquid, 85 A coverage of less than or equal to% is insufficient, and needs to be further reduced to 40% or less. Preferably it is 35% or less. As a method for reducing the surface coverage of the iron-based oxide to 40% or less, as described later, it is effective to re-wash with a non-oxidizing acid after strong pickling.

In the present invention, the surface coverage of the iron-based oxide is determined as follows.
Using a scanning electron microscope (ULV-SEM) with ultra-low acceleration voltage that can detect extremely surface layer information, the steel plate surface after pickling is observed with an acceleration voltage of 2 kV, an operating distance of 3.0 mm, and a magnification of about 1000, and about 5 fields of view. Then, spectral analysis is performed using an energy dispersive X-ray spectrometer (EDX) to obtain a reflected electron image. This reflected electron image is binarized using image analysis software, for example, Image J, the area ratio of the black portion is measured, and the surface coverage of the iron-based oxide is obtained by averaging the measured values of each field of view. Can be obtained. The ultra-low acceleration voltage scanning electron microscope (ULV-SEM) is, for example, manufactured by SEISS; ULTRA55, and the energy dispersive X-ray spectrometer (EDX) is, for example, manufactured by Thermo Fisher. NSS 312E may be mentioned.

Here, the threshold used for the binarization process was set as follows.
C: 0.14 mass%, Si: 1.65 mass%, Mn: 1.33 mass%, P: 0.018 mass%, S: 0.002 mass%, Al: 0.035 mass%, the balance being substantially made of Fe The steel slab is heated to 1150 ° C. and then hot rolled to a finish finishing temperature of 850 ° C., wound at 550 ° C., pickled, cold rolled to a 1.8 mm cold rolled sheet, and then The cold-rolled sheet was annealed at 750 ° C. for 30 seconds, cooled to 400 ° C. at 20 ° C./second, over-aged for 100 seconds, and then subjected to continuous annealing to cool to room temperature at 50 ° C./second. A cold-rolled annealed plate was used.

Next, the cold-rolled steel sheet after the continuous annealing is subjected to pickling and re-acid pickling under the conditions shown in Table 1, and then subjected to 0.7% temper rolling, and the amount of iron-based oxide on the steel sheet surface With different numbers. Two types of cold-rolled steel sheets a and b were obtained.
The above-mentioned No. 5 thus obtained. The cold rolled steel sheet a is a standard sample with a lot of iron-based oxides, No. The cold rolled steel sheet b was used as a standard sample with a small amount of iron-based oxides, and a reflected electron image was obtained for each steel sheet using the scanning electron microscope under the conditions described above. FIG. The reflection electron image photograph of the steel plates a and b is shown in FIG. The histogram of the pixel number with respect to the gray value of the said reflection electron image photograph of the steel plate of a and b is shown. In the present invention, No. 1 shown in FIG. The gray value (Y point) corresponding to the intersection (X point) of the histograms a and b was used as the threshold value. Incidentally, using the above threshold, No. When the surface coverage of the iron-based oxide of the steel sheets a and b was determined, No. The steel sheet of a is 85.3%, No. The steel sheet for b was 25.8%.

Next, the manufacturing method of the Si containing cold-rolled steel plate which has the said surface characteristic is demonstrated.
The method for producing a Si-containing cold-rolled steel sheet according to the present invention comprises heating a steel material (slab) containing 0.5 to 3.0 mass% of Si, followed by hot rolling, cold rolling, continuous annealing, and then After complete removal of the Si-containing oxide layer on the surface of the steel sheet by electrolytic pickling, re-acid pickling using a non-oxidizing acid, and the iron-based oxide produced on the steel sheet surface by the electrolytic pickling This is a method of reducing the steel sheet surface coverage to 40% or less. Therefore, although it can manufacture in accordance with a conventional method from the steelmaking process to the continuous annealing process after cold rolling, it is preferable to manufacture on the following conditions after the pickling after continuous annealing.

Electrolytic pickling after continuous annealing A large amount of Si-containing oxides such as SiO ₂ and Si-Mn complex oxides are formed on the steel sheet surface layer after continuous annealing. It will drop significantly. Therefore, in the present invention, it is necessary to electrolytically pickle the surface of the cold-rolled steel sheet after annealing to remove the Si-containing oxide layer on the surface layer of the steel sheet together with the base iron.

This is because, as described above, among the Si-containing oxides, the Si—Mn composite oxide is easily dissolved in the acid, but SiO ₂ is hardly soluble in the acid. Therefore, in order to completely remove the Si-containing oxide including SiO ₂ , it is necessary to perform electrolytic pickling to remove the oxide layer together with the base iron of the steel plate. Acids such as hydrochloric acid, sulfuric acid, and nitric acid can be used as the acid that can be used for the electrolytic pickling. Among them, hydrochloric acid that is frequently used in the steel industry can be preferably used.

Moreover, when using hydrochloric acid, it is preferable that the density | concentration shall be the range of 50-300 g / L. If it is less than 50 g / L, the pickling property necessary for removing the Si-containing oxide layer together with the base iron is difficult to obtain. On the other hand, if it exceeds 300 g / L, the working environment is deteriorated due to volatilization of hydrochloric acid. Moreover, when using hydrochloric acid, it is preferable to carry out as the liquid temperature of pickling liquid 20-70 degreeC, current density 1-20A / dm < ² >, and pickling time 1-30 seconds. If the temperature of the pickling solution is 20 ° C. or higher, the current density is 1 A / dm ² or higher, and the pickling time is 1 second or longer, the Si-containing oxide layer on the steel sheet surface layer can be sufficiently removed. And corrosion resistance after painting is not reduced. On the other hand, if the temperature of the pickling solution is 70 ° C. or less, the current density is 20 A / dm ² or less and 30 seconds or less, the steel plate surface becomes rough due to excessive pickling, and the chemical conversion treatment film becomes non-uniform, This is because the surface coverage by the iron-based oxide does not increase, and the chemical conversion property and the corrosion resistance after coating are not deteriorated. It is also effective to add a pickling accelerator to the hydrochloric acid to promote dissolution of the base iron.

However, in the electrolytic pickling as described above, it is difficult to stably control the surface coverage of the iron-based oxide generated on the steel sheet surface by electrolytic pickling to 40% or less. Therefore, in the present invention, in order to more reliably reduce the iron-based oxide generated on the surface of the steel plate by the electrolytic pickling to 40% or less, the surface of the pickled steel plate is further pickled again with a non-oxidizing acid. Thus, the iron-based oxide was dissolved and removed.

  Examples of the non-oxidizing acid that can be used for the re-acidification include hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid, or an acid obtained by mixing two or more of these. Yes, any of them may be used, but hydrochloric acid or sulfuric acid generally used in the steel industry can be preferably used. Among them, hydrochloric acid is a volatile acid, so that it is difficult for residues such as sulfate radicals to remain on the surface of the steel sheet after washing like sulfuric acid, and because the oxide destruction effect by chloride ions is large, Is preferred. Moreover, you may use the acid which mixed hydrochloric acid and sulfuric acid.

  When hydrochloric acid is used as the pickling solution for the re-pickling, the hydrochloric acid concentration is 0.1 to 50 g / L, and when sulfuric acid is used, the sulfuric acid concentration is 0.1 to 150 g / L. In addition, when an acid mixed with hydrochloric acid and sulfuric acid is used for re-acid pickling, an acid mixed with a hydrochloric acid concentration of 0.1 to 20 g / L and a sulfuric acid concentration of 0.1 to 60 g / L is used. Is preferred.

  In addition, the re-pickling is preferably performed in a range of 20 to 70 ° C. and a processing time of 1 to 30 seconds regardless of which re-pickling solution is used. If the concentration of the re-pickling solution is equal to or higher than the above lower limit, the liquid temperature is 20 ° C. or higher, and the treatment time is 1 second or longer, the iron-based oxide remaining on the steel plate surface can be sufficiently removed, This is because if the concentration of the liquid is not more than the above upper limit concentration, the temperature is not more than 70 ° C., and the treatment time is not more than 30 seconds, the surface of the steel sheet will not be excessively dissolved, and a new surface oxide film will not be generated. .

  After the continuous annealing as described above, electrolytic cold pickling, re-acid pickling, and the cold rolled steel sheet with the iron oxide coverage on the steel sheet surface of 40% or less is then subjected to temper rolling, leveler processing, etc. A product board can be obtained through a normal processing step.

  C: 0.125 mass%, Si: 1.5 mass%, Mn: 2.6 mass%, P: 0.019 mass%, S: 0.008 mass% and Al: 0.040 mass%, the balance being Fe and inevitable Steel consisting of mechanical impurities was melted by a normal scouring process through a converter, degassing treatment, etc., and continuously cast into a steel material (slab). Next, this slab was reheated to a temperature of 1150 to 1170 ° C., and then hot-rolled to a finish rolling finish temperature of 850 to 880 ° C., wound around a coil at a temperature of 500 to 550 ° C., and a plate thickness of 3 A hot rolled steel sheet having a thickness of ˜4 mm was obtained. Thereafter, these hot rolled steel sheets were pickled, scales were removed, and then cold rolled to obtain a cold rolled steel sheet having a thickness of 1.8 mm. Subsequently, these cold-rolled steel sheets were heated to a soaking temperature of 750 to 780 ° C. and held for 40 to 50 seconds, and then from the above soaking temperature to a cooling stop temperature of 350 to 400 ° C. at 20 to 30 ° C./sec. After cooling and continuous annealing for holding for 100 to 120 seconds in the above cooling stop temperature range, the steel sheet surface was electrolytically pickled with hydrochloric acid under the conditions shown in Table 2, and after re-acid pickling, an elongation of 0. 7% temper rolling was applied, and No. 1 shown in Table 2 was obtained. 1 to 79 cold-rolled steel sheets were obtained.

  Specimens were taken from each of the above cold-rolled steel sheets, and the surface of the steel sheet was accelerated using an ultra-low acceleration voltage scanning electron microscope (ULV-SEM; manufactured by SEISS; ULTRA55) at an acceleration voltage of 2 kV, a working distance of 3.0 mm, and a magnification. Five fields of view were observed at 1000 times, and a reflection electron image was obtained by spectroscopic analysis using an energy dispersive X-ray spectrometer (EDX; manufactured by Thermo Fisher; NSS312E). This reflected electron image is obtained by using the image analysis software (Image J) and the standard sample No. described above. The gray value (point Y) corresponding to the intersection (point X) of the histograms a and b is set as a threshold value, and binarization processing is performed to obtain the area ratio (average value of five fields of view) of the black portion. The surface coverage was determined as follows.

In addition, specimens are collected from each of the above cold-rolled steel sheets, subjected to chemical conversion treatment and coating treatment under the following conditions, and then subjected to three types of corrosion tests: a salt warm water immersion test, a salt spray test, and a combined cycle corrosion test. The corrosion resistance after coating was evaluated.
(1) Chemical conversion treatment conditions In the test piece extract | collected from each said cold-rolled steel plate, the degreasing agent: FC-E2011 made from Nippon Parkerizing Co., Ltd., surface adjustment agent: PL-X, and chemical conversion treatment agent: Palbond PB-L3065, The chemical conversion treatment was performed so that the amount of chemical conversion film coating was 1.7 to 3.0 g / m ^{2 under} the following two conditions: standard conditions and comparative conditions in which the temperature of the chemical conversion liquid was lowered to lower the temperature.
<Standard conditions>
・ Degreasing process: treatment temperature 40 ° C., treatment time 120 seconds ・ Spray degreasing, surface adjustment step: pH 9.5, treatment temperature room temperature, treatment time 20 seconds ・ Chemical conversion treatment process: temperature of chemical treatment liquid 35 ° C., treatment time 120 seconds <temperature reduction conditions>
Conditions under which the temperature of the chemical conversion solution under the standard conditions is reduced to 33 ° C

(2) Corrosion test The surface of the test piece subjected to the above chemical conversion treatment was subjected to electrodeposition coating with an electrodeposition coating material: V-50 manufactured by Nippon Paint Co., Ltd. so as to have a film thickness of 25 μm. The sample was subjected to a corrosion test.
<Salt warm water immersion test>
The surface of the above test piece subjected to chemical conversion treatment and electrodeposition coating was applied with a 45 mm long crosscut wrinkle with a cutter, and then the test piece was immersed in a 5 mass% NaCl solution (60 ° C.) for 360 hours, and thereafter After washing with water, drying, and sticking an adhesive tape on the cut collar, a tape peeling test was conducted to peel it off, and the maximum width of the entire peel that combined the left and right sides of the cut collar was measured. If this maximum peeling full width is 5.0 mm or less, it can be evaluated that the corrosion resistance in the salt warm water immersion test is good.
<Salt spray test (SST)>
The surface of the above test piece subjected to chemical conversion treatment and electrodeposition coating was applied with a 45 mm long crosscut wrinkle with a cutter, and then this test piece was specified in JIS Z2371: 2000 using a 5 mass% NaCl aqueous solution. After performing a salt water spray test for 1200 hours in accordance with the neutral salt spray test, a tape peel test was performed on the cross-cut collar part, and the maximum total peel width of the cut collar part was measured. If the maximum total peel width is 4.0 mm or less, it can be evaluated that the corrosion resistance in the salt spray test is good.
<Composite cycle corrosion test (CCT)>
The surface of the above-mentioned test piece subjected to chemical conversion treatment and electrodeposition coating was applied with a 45 mm long crosscut wrinkle with a cutter, and then the test piece was sprayed with salt water (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. After washing with water and drying, a tape peel test was performed on the cut collar, and the maximum width of the maximum peel along the left and right sides of the cut collar was measured. If this maximum peel width is 6.0 mm or less, it can be evaluated that the corrosion resistance in the combined cycle corrosion test is good.

  The results of the above test are also shown in Table 2. From this result, after continuous annealing, the steel plate of the present invention example that was electrolytically pickled and re-washed under conditions suitable for the present invention was the maximum peel in any of the salt warm water immersion test, salt spray test, and combined cycle corrosion test. It can be seen that the overall width is small, indicating good post-coating corrosion resistance.

  Steels of steel symbols A to N having the composition shown in Table 3 were melted by a normal scouring process through a converter, degassing treatment, etc., and continuously cast to obtain a steel slab. These steel slabs were hot-rolled under the hot rolling conditions shown in Table 4 to form hot-rolled steel sheets having a thickness of 3 to 4 mm, pickled to remove the scale on the steel sheet surface, and then cold-rolled to obtain a steel sheet. A cold-rolled steel sheet having a thickness of 1.8 mm was used. Next, after these cold-rolled steel sheets were continuously annealed under the conditions shown in Table 4, they were subjected to electrolytic pickling under the conditions shown in Table 5, re-acid pickling, and then tempered with an elongation of 0.7%. After rolling, no. 1 to 23 cold-rolled steel sheets were obtained.

A test piece was collected from the cold-rolled steel sheet thus obtained, and after measuring the surface coverage of the iron-based oxide on the steel sheet surface after re-acid washing in the same manner as in Example 1, the following tensile test was performed. And subjected to a corrosion resistance test after painting.
(1) Mechanical properties Using a JIS No. 5 tensile test piece specified in JIS Z2201: 1998 collected from a direction perpendicular to the rolling direction (C direction), a tensile test was performed in accordance with the specification of JIS Z2241: 1998, The strength TS was measured.
(2) Corrosion resistance after coating A test piece sampled from each cold-rolled steel sheet was subjected to chemical conversion treatment under the same conditions as in Example 1 to produce a test piece subjected to electrodeposition coating. The corrosion resistance after coating was evaluated by three types of corrosion tests, a hot water immersion test, a salt spray test (SST), and a combined cycle corrosion test (CCT).

  The results of the above test are shown in Tables 4 and 5. From this result, the present invention example containing Si of 0.5 mass% or more, electrolytic pickling under conditions suitable for the present invention, re-acid pickling, and the steel sheet surface coverage with iron oxide was 40% or less. It can be seen that the high-strength cold-rolled steel sheet has a high strength with a tensile strength TS of 590 MPa or more and excellent corrosion resistance after coating. On the other hand, No. with Si less than 0.5 mass%. No. 22 (steel symbol M) has excellent post-coating corrosion resistance, but the tensile strength TS does not reach 590 MPa, while Si exceeds 3.0 mass%. No. 23 (steel symbol N) has high strength, and excellent post-coating corrosion resistance is not obtained even if it is processed under the conditions of the present invention.

  The Si-containing cold-rolled steel sheet produced according to the present invention not only has excellent post-painting corrosion resistance, but also has high strength and excellent workability, so not only as a material for automobile body members, It can also be suitably used as a material for applications that require similar characteristics in the field of building materials and the like.

Claims (5)

After cold rolling containing 0.5 to 3.0 mass% of Si, after electrolytically pickling the continuously annealed cold-rolled steel sheet to remove the Si-containing oxide layer on the surface layer of the steel sheet, further, with a non-oxidizing acid A method for producing a Si-containing cold-rolled steel sheet, wherein the surface coverage of the iron-based oxide on the steel sheet surface is adjusted to 40% or less by re- pickling. The cold-rolled steel sheet includes, in addition to Si, C: 0.01 to 0.30 mass%, Mn: 1.0 to 7.5 mass%, P: 0.05 mass% or less, S: 0.01 mass% or less, and Al The process for producing a Si-containing cold-rolled steel sheet according to claim 1, comprising: 0.06 mass% or less, with the balance being a component composition comprising Fe and inevitable impurities. The method for producing a Si-containing cold-rolled steel sheet according to claim 1 or 2, wherein the electrolytic pickling is performed using hydrochloric acid having a concentration of 50 to 300 g / L. The non-oxidizing acid is any acid selected from hydrochloric acid, sulfuric acid, phosphoric acid, pyrophosphoric acid, formic acid, acetic acid, citric acid, hydrofluoric acid, oxalic acid and a mixture of two or more thereof. The method for producing a Si-containing cold-rolled steel sheet according to any one of claims 1 to 3. The non-oxidizing acid has a concentration of 0.1 to 50 g / L hydrochloric acid, 0.1 to 150 g / L sulfuric acid, and 0.1 to 20 g / L hydrochloric acid and 0.1 to 60 g / L. It is any acid of the acid which mixed the sulfuric acid, The manufacturing method of Si containing cold-rolled steel plate of any one of Claims 1-4 characterized by the above-mentioned.

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