JP6699633B2 - High-strength cold-rolled steel sheet excellent in corrosion resistance after painting and delayed fracture resistance and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength cold-rolled steel sheet used for automobile structural members, reinforcing members and the like, and a method for manufacturing the same.

  Cold-rolled steel sheets used in the automotive field are often coated with a coating pretreatment from the viewpoint of rust resistance, and in order to obtain excellent post-paint corrosion resistance, The rolled steel sheet is required to have good chemical conversion treatability. Further, in recent years, from the viewpoint of improving fuel efficiency, the thickness of steel sheets has been reduced in order to reduce the weight of the vehicle body, and accordingly, the cold rolled steel sheet is required to have high strength. However, there is a problem that delayed fracture resistance deteriorates due to higher strength, and each steel manufacturer has added strength of alloying elements and added various heat histories to increase strength, as well as corrosion resistance and delay after coating. We aim to achieve both breaking characteristics.

  Further, recently, in order to reduce the amount of sludge generated during chemical conversion treatment and energy cost, the temperature of the chemical conversion treatment liquid has been lowered. Since the reactivity of the steel sheet and the chemical conversion treatment liquid is lowered by lowering the temperature of the chemical conversion treatment liquid, it is required to improve the chemical conversion treatment property. In order to increase the reactivity of the chemical conversion treatment solution with the steel sheet in response to such problems, efforts have been made to control the free fluorine concentration to be high and improve the reactivity with the steel sheet.

On the other hand, examples of efforts on the steel sheet side for improving the chemical conversion treatability include the following.
(1) Technology for improving chemical conversion treatability by performing temper rolling and introducing dislocations in steel (Patent Document 1)
(2) Technology for improving chemical conversion treatability in a continuously annealed material by adding B to steel and performing temper rolling (Patent Document 2)
(3) In a continuous annealed material, after pickling a steel sheet, a metal nobler than Fe (such as Ni) is thinly attached to the surface to provide a growth site for chemical conversion crystals, so that the chemical conversion treatability can be improved regardless of the steel type. Technology for improvement (Patent Documents 3 and 4)
(4) A technology for improving chemical conversion treatability in a continuous annealed material by removing oxides which inhibit chemical conversion treatability formed during continuous annealing by pickling (Patent Document 5)

  Regarding delayed fracture resistance of high-strength steel sheets, it is known that addition of Cu is effective in improving delayed fracture resistance. For example, Cu concentration in a region from the steel sheet surface to a depth of 0.48 μm. Has been proposed to improve the delayed fracture resistance of the cold-rolled steel sheet (Patent Document 6) and the like.

JP 62-116723 A Japanese Patent Publication No. 29729/1990 Japanese Patent Publication No. 58-37391 Japanese Patent Publication No. 1-58276 JP 2012-132093A JP, 2011-246764, A

  In the continuous annealing step performed to recrystallize the cold-rolled steel sheet to impart desired structure, strength, and workability, the steel sheet contains oxides containing metal elements (Al, Si, Mn) that are more easily oxidizable than iron. Generate on the surface. This oxide containing an easily oxidizable metal inhibits the etching property of the steel sheet surface in the chemical conversion treatment (generally zinc phosphate treatment) performed as a base treatment for electrodeposition coating, and adversely affects the formation of a sound chemical conversion treatment film. Therefore, in order to remove this, pickling is performed on the continuously annealed steel sheet.

  On the other hand, recently, in order to reduce the amount of sludge generated during chemical conversion treatment and the energy cost, the chemical conversion treatment liquid has been lowered in temperature, and the chemical conversion treatment liquid has a significantly lower reactivity with the steel sheet compared to the conventional method. Processing is starting to take place. Compared with the conventional chemical conversion treatment performed at about 43°C, in the chemical conversion treatment at a temperature of about 35°C, the chemical conversion treatability is greatly influenced by the surface texture of the plate and is removed by pickling on the steel plate surface. It was revealed that the presence of metal oxides (pickling residue) and other substances that remained without chemical conversion treatment markedly lowered the corrosion resistance after coating. In particular, as a result of the study by the present inventors, in the case of a high-strength cold-rolled steel sheet to which Cu is added for improving delayed fracture resistance, even if the techniques of Patent Documents 1 to 5 described above are used, low temperature type chemical conversion is performed. It has been found difficult to improve the chemical conversion treatability of the treatment.

  Therefore, an object of the present invention is to solve the problems of the prior art as described above, and to provide a high-strength cold-rolled steel sheet containing Cu for improving delayed fracture resistance, which has chemical conversion treatability in low-temperature type chemical conversion treatment. The object is to provide a high-strength cold-rolled steel sheet having good corrosion resistance and excellent post-painting corrosion resistance and delayed fracture resistance, and a method for producing the same.

  In order to solve the above-mentioned problem regarding the high-strength cold-rolled steel sheet containing Cu, the present inventors performed a detailed analysis of the steel sheet surface after continuous annealing-pickling, and as a result, remained on the steel sheet surface after pickling. The presence of the metal oxide Cu (pickling residue) that has been deposited on the surface of the steel sheet due to the reaction between the pickling liquid film and the steel sheet between pickling and water washing causes a great deterioration in chemical conversion treatability. Therefore, it was found that in order to improve the chemical conversion treatability (particularly, the chemical conversion treatability in the low temperature type chemical conversion treatment), it is extremely important to reduce such pickling residue and precipitated metal Cu.

The present invention has been made on the basis of such findings, and has the following gist.
[1] A steel sheet containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25 mass% and having a tensile strength of 1180 MPa or more, the steel structure of which is a recrystallized structure. There is no oxide layer containing at least one of Al, Si, and Mn on the surface (however, the absence of an oxide layer means that 5 points on the surface of a steel plate arbitrarily selected are analyzed by glow discharge emission spectroscopy. When the peaks of Al, Si, Mn, and O do not appear when analyzed in the depth direction, the coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, measured in the steel sheet cross section) A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by having a steel sheet surface coverage ratio of 30% or less.

[2] In the high-strength cold-rolled steel sheet according to [1], C: 0.08 to 0.30 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.5 mass%. , P: 0.020 mass% or less, S: 0.005 mass% or less, Sol.Al: 0.01 to 0.05 mass%, Cu: 0.05 to 0.25 mass%, and the balance A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by having a component composition consisting of iron and inevitable impurities.
[3] In the high-strength cold-rolled steel sheet according to [2], Ni: 0.05% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, B: 5 to 30% by mass. A high-strength cold-rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, which is characterized by containing at least one selected from ppm.

[4] A cold-rolled steel sheet having the component composition according to any one of the above [1] to [3] is continuously annealed, and then pickled with a pickling solution according to (1) or (2) below. A method for producing a high-strength cold-rolled steel sheet excellent in corrosion resistance after painting and delayed fracture resistance.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe ³⁺ ion concentration is ³ to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
A pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) [hydrofluoric acid/nitric acid] of 0.1 to 0.3 and Fe ³⁺ ion concentration of ³ to 30 g/L [ 5] In the production method of [4] above, pickling is performed under the conditions of a pickling solution temperature of 20 to 70° C. and a pickling time of 3 to 30 seconds, which is excellent in post-painting corrosion resistance and delayed fracture resistance. Manufacturing method of high strength cold rolled steel sheet.

  The high-strength cold-rolled steel sheet of the present invention has excellent delayed fracture resistance and, at the same time, can obtain good chemical conversion treatability even in low-temperature type chemical conversion treatment, so that it is harsh such as salt hot water immersion test or combined cycle corrosion test. It has excellent post-painting corrosion resistance even in corrosive environments. Moreover, according to the manufacturing method of the present invention, a high-strength cold-rolled steel sheet having such excellent characteristics can be stably manufactured.

The same field of view as an example of the secondary electron image (FIG. 1A) at an acceleration voltage of 1 kV and a magnification of 20000 times on the surface cross section of the steel sheet, which is imaged when obtaining the cross-sectional coverage of the metal Cu deposited on the surface of the steel sheet. Drawing showing an element mapping image (FIG. 1B) at an acceleration voltage of 15 kV in FIG. Drawing which shows typically the test piece for delayed fracture evaluation used in the Example. Explanatory drawing which shows the process of the combined cycle corrosion test performed in the Example.

The steel sheet of the present invention is a high-strength cold-rolled steel sheet containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25 mass% and having a tensile strength of 1180 MPa or more. This high-strength steel sheet is manufactured by continuous annealing-pickling after cold rolling, and therefore the steel structure is a recrystallized structure.
Generally, a high-strength cold-rolled steel sheet contains appropriate amounts of Si and Mn in order to obtain a predetermined strength, and also contains Al which is a deoxidizing element. The phenomenon of delayed fracture easily occurs in the high-strength cold-rolled steel sheet, and this phenomenon becomes more severe as the strength increases, and is particularly remarkable in the high-strength steel sheet having a tensile strength of 1180 MPa or more. The steel sheet of the present invention contains 0.05 to 0.25 mass% of Cu in order to improve delayed fracture resistance. It is considered that the addition of Cu to the steel sheet makes the steel sheet less likely to corrode (anode reaction), and consequently suppresses hydrogen generation (cathode reaction). Since the generation of hydrogen is suppressed, it is considered that the amount of invading hydrogen is reduced and the effect of suppressing delayed fracture is exhibited. Here, if the amount of Cu is less than 0.05% by mass, improvement in delayed fracture resistance is insufficient. On the other hand, if Cu is excessively contained, metal Cu (this metal Cu hinders the chemical conversion treatment property as described later) is likely to be generated on the surface of the steel sheet immediately after pickling, so the Cu content is 0.25 mass %. Below.

  In the production of a high-strength cold-rolled steel sheet, continuous annealing is performed after cold rolling, and subsequently, an oxide layer containing easily oxidizable elements (at least one of Al, Si and Mn) formed on the surface of the steel sheet is removed. However, as a result of research conducted by the present inventors, a high-strength cold-rolled steel sheet containing an appropriate amount of Cu for improving delayed fracture resistance as described above is subjected to a good chemical conversion treatment. It has been found that the surface properties of the steel sheet after pickling have to be set as follows in order to obtain good properties (particularly chemical conversion treatability in low temperature type chemical conversion treatment).

  First, the oxide layer on the surface of the steel sheet (oxide layer containing at least one of Al, Si, and Mn. The same applies hereinafter) is almost completely removed by pickling, and the oxide layer is substantially present on the surface of the steel sheet. It is necessary to stop doing it. Here, the fact that the oxide layer does not substantially exist on the surface of the steel sheet means that Al and Si are determined when five locations on the surface of the steel sheet arbitrarily selected are analyzed in the depth direction by glow discharge emission spectroscopy (GDS). , Mn, O peaks do not appear. That is, it is necessary to remove the oxide layer on the surface of the steel sheet by pickling to such a level.

  Furthermore, the pickling liquid film on the steel sheet surface continues to react with the steel sheet between the pickling and the water washing, Cu eluted from the steel sheet is deposited on the steel sheet surface, and the deposited metal Cu is the chemical conversion treatment after that. It has been found that in the step (particularly the low temperature type chemical conversion treatment step), the chemical conversion crystal forming reaction is inhibited, that is, the chemical conversion treatability is lowered. It is considered that this is because the metal Cu deposited on the surface of the steel plate inhibits the dissolution reaction of iron in the chemical conversion treatment and suppresses the growth of the chemical conversion crystals. It was found that it is necessary to suppress the coverage of the deposited metal Cu on the surface. Specifically, the coverage rate of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, the coverage rate of the steel sheet surface measured in the steel sheet cross section, hereinafter referred to as “cross section coverage rate” for convenience of description) is 30%. It was necessary to make the amount below, and it was found that good chemical conversion treatability was obtained. Therefore, in the present invention, the cross-section coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface is set to 30% or less.

Here, the cross-section coverage of the deposited metal Cu on the surface of the steel sheet is a value obtained as follows. That is, with respect to the surface layer cross section of the steel sheet, 5 fields of view are observed at an accelerating voltage of 1 kV, a working distance of 3.0 mm, and a magnification of about 20000 times by using a scanning electron microscope (ULV-SEM) with an extremely low accelerating voltage capable of detecting information on the surface layer. Then, an elemental mapping image at an acceleration voltage of 15 kV in the same field of view is obtained. FIG. 1A shows an example of a secondary electron image by ULV-SEM, and FIG. 1B shows an element mapping image in the same visual field. Then, in the obtained element mapping image, the widths A1, A2,... Of the regions where the metal Cu is present in the surface layer of the steel sheet (regions indicated by white arrows in FIG. 1B) are respectively calculated, and the sum thereof is obtained. .. The ratio of the sum obtained in the 5 fields of view to the entire 5 fields of view is defined as the cross-sectional coverage of the metallic Cu.
As a method of reducing the sectional coverage of the surface of the steel sheet by the precipitated metal Cu to 30% or less, there is a method of pickling under specific conditions as described later.

Next, a preferable component composition of the high strength cold rolled steel sheet of the present invention will be described.
The steel sheet of the present invention is C: 0.08 to 0.30 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.5 mass%, P: 0.020 mass% or less. , S: 0.005% by mass or less, Al: 0.01 to 0.05% by mass, Cu: 0.05 to 0.25% by mass, with the balance being a component composition consisting of iron and unavoidable impurities. Is preferable, and if necessary, one kind selected from Ni: 0.05 mass% or less, Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, and B: 5 to 30 mass ppm. The above can be contained. The reasons for limiting the contents of these elements are as follows.

-C: 0.08 to 0.30 mass%
C is an element necessary to secure the strength of the steel sheet, and if the C content is less than 0.08 mass %, it may be difficult to secure the strength, so the C content is 0.08 mass% or more. Preferably. On the other hand, if the C content becomes excessive, the weldability deteriorates, so the C content is preferably 0.30 mass% or less.
・Si: 0.1 to 2.0 mass%
Si is a solid solution strengthening element, and if the Si amount is less than 0.1% by mass, the steel sheet may be insufficiently hardened, so the Si amount is preferably 0.1% by mass or more. On the other hand, if Si is excessively contained, the toughness deteriorates, the amount of Si-based oxide formed during annealing increases, and the pickling equipment becomes long, resulting in an increase in equipment cost. Therefore, the Si amount is preferably 2.0% by mass or less.

・Mn: 0.1 to 2.5 mass%
Mn is an element that secures the strength of the steel sheet. If the Mn content is less than 0.1 mass %, the strength of the steel sheet may be insufficient, so the Mn content is preferably 0.1 mass% or more. .. On the other hand, if Mn is excessively contained, segregation is increased, workability is deteriorated, and weldability is deteriorated. Therefore, the Mn content is preferably 2.5% by mass or less.
-P: 0.020 mass% or less, S: 0.005 mass% or less P, S is preferably as low as possible in view of workability, so that the P content is 0.020 mass% or less. It is preferable that Further, when S is contained excessively, inclusions (MnS) increase, which adversely affects the workability, so the S content is preferably 0.005 mass% or less.

・Sol.Al: 0.01 to 0.05 mass%
Al is an element added for deoxidation, and if the amount of Sol.Al is less than 0.01% by mass, silicate inclusions may remain and the workability of steel may deteriorate. It is preferably 0.01% by mass or more. On the other hand, when the amount of Sol.Al is too large, surface defects are increased, so the amount of Sol.Al is preferably 0.05% by mass or less.
-Cu: 0.05 to 0.25 mass%
The amount of Cu is as described above. That is, if the amount of Cu is less than 0.05% by mass, the improvement of delayed fracture resistance is insufficient, while if the amount of Cu exceeds 0.25% by mass, metallic Cu precipitates on the surface of the steel sheet immediately after pickling. Since it becomes easy, the amount of Cu is set to 0.05 to 0.25 mass %.

-Ni: 0.05% by mass or less Ni is a solid solution strengthening element, but the effect of improving the mechanical properties of the steel sheet is saturated even if Ni is excessively contained, and the cost is rather increased. It is preferably 0.05% by mass or less.
-Ti: 0.1 mass% or less, Nb: 0.1 mass% or less Ti and Nb are elements that refine the steel structure, and are very useful elements for improving strength without impairing toughness. Is. However, even if Ti and Nb are excessively contained, the effect of improving the mechanical properties of the steel sheet is saturated and the cost is rather increased. Therefore, the Ti amount and the Nb amount are preferably 0.1% by mass or less.
・B: 5 to 30 mass ppm
B is a very useful element for enhancing the hardenability, and has the effect of strengthening the grain boundaries and improving the delayed fracture resistance. To fully exhibit these effects, B is The amount is preferably 5 mass ppm or more. On the other hand, if B is contained excessively, the hot workability is deteriorated, so the amount of B is preferably 30 mass ppm or less.

Next, a method for manufacturing the high strength cold rolled steel sheet of the present invention will be described.
As mentioned above, in order to obtain good chemical conversion treatability (particularly low temperature type chemical conversion treatment), (i) pickling removes almost completely the oxide layer on the steel plate surface. That the oxide layer does not substantially exist on the surface of the steel sheet, and (ii) the pickling liquid film reacts with the steel sheet between after pickling and washing with water to prevent metal Cu from precipitating on the surface of the steel sheet. It is necessary to suppress it (to make the cross-section coverage by metallic Cu 30% or less).
Therefore, in the manufacturing process of the high-strength cold-rolled steel sheet of the present invention, the cold-rolled steel sheet on the surface of which an oxide containing an easily oxidizable metal element is formed on the surface by continuous annealing is pickled under the following conditions. Is preferred.

Among the easily oxidizable metals, particularly oxides containing Si include oxides that are hardly soluble in acid, such as SiO ₂ . In order to completely remove such an oxide layer containing an easily oxidizable metal element including hardly soluble SiO ₂ , it is necessary to remove the oxide layer on the surface of the steel sheet together with the base metal by pickling. Further, in the pickling, it is necessary to efficiently remove the oxide layer containing the easily oxidizable metal element in a short time.
Therefore, the pickling solution preferably comprises an acid component based on nitric acid, which is a strongly oxidizing acid. That is, in order to efficiently remove the oxide layer containing the easily oxidizable metal element in a short time, it is effective to utilize the oxidizing power of nitric acid (=the action of oxidizing (dissolving) iron). On the other hand, when such a pickling solution is used, the resurface layer of iron is easily oxidized (=Fe-based oxide is formed), and Cu is eluted in the pickling solution together with Fe, and Cu is noble than Fe. Since it is an element, it immediately precipitates as metallic Cu on the surface of the steel sheet after elution. That is, Cu in the steel is eluted together with Fe by pickling, but in order to reduce the cross-sectional coverage of metallic Cu deposited on the surface of the steel sheet, it is important to suppress the elution of Cu. Therefore, there is a trade-off relationship between "removal of the oxide layer containing an easily oxidizable metal element" and "formation of Fe-based oxide/precipitation of metal Cu in the resurface layer". In addition, it is necessary to optimize the composition and concentration of the nitric acid-based pickling solution.

Specifically, it is preferable to use a pickling solution of the following (1) or (2) in which the acid component is nitric acid as a base, and an appropriate amount of hydrochloric acid or hydrofluoric acid is added thereto.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe ³⁺ ion concentration is ³ to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
Pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) of 0.1 to 0.3 and Fe ³⁺ ion concentration of ³ to 30 g/L]

  The pickling solution of (1) above is a pickling solution in which the acid component is nitric acid as a base, and an appropriate amount of hydrochloric acid having an oxide film destroying effect is added thereto. Here, when the nitric acid concentration is 50 g/L or less, the oxide layer containing the easily oxidizable metal element cannot be removed properly, and the remaining oxide impairs the chemical conversion treatment property and causes a decrease in corrosion resistance. In addition, the surface of the steel sheet is black due to the oxide, and the appearance of the surface of the steel sheet is impaired. On the other hand, when the nitric acid concentration exceeds 110 g/L, the oxide layer containing an easily oxidizable metal element can be easily removed, but since Fe and Cu are strongly dissolved, metal Cu easily precipitates, and the steel sheet surface It is not possible to reduce the cross-sectional coverage of the metal Cu deposited on the surface. In addition, the unevenness of the steel plate surface becomes severe and the appearance of the painted surface is impaired. When the ratio R1 [hydrochloric acid/nitric acid] of the hydrochloric acid concentration (g/L) to the nitric acid concentration (g/L) is less than 0.1, a sufficient pickling rate can be obtained. Since the dissolution of base iron is promoted by the strong oxidizing power and the elution amount of Cu is increased, metal Cu is easily deposited on the surface of the steel sheet, and the cross-sectional coverage of the metal Cu deposited on the surface of the steel sheet cannot be reduced. Further, since the reaction heat accompanying pickling becomes large, it becomes very difficult to control the temperature, and a cooling facility is required, which complicates the manufacturing facility. On the other hand, if the ratio R1 [hydrochloric acid/nitric acid] exceeds 0.3, the oxidizing power of nitric acid is suppressed by hydrochloric acid, so the desired pickling rate cannot be obtained, and oxides on the surface of the steel sheet must be removed efficiently. I can't.

If the concentration of Fe ³⁺ ions in the pickling solution is less than 3 g/L, the oxidation reaction of iron by Fe ³⁺ ions is insufficient, so the desired pickling rate cannot be obtained, and the oxides on the steel plate surface are efficiently removed. Can not do it. On the other hand, when the Fe ³⁺ ion concentration exceeds 30 g/L, the iron is sufficiently oxidized by the Fe ³⁺ ion to obtain a desired pickling rate, but the Fe ³⁺ ion in the pickling solution is large. However, a large amount of Fe-based oxide is formed on the surface of the steel sheet, the Fe-based oxide newly formed by pickling cannot be completely removed, and chemical conversion treatability and corrosion resistance after coating cannot be improved.

The pickling solution of (2) above is a pickling solution in which the acid component is nitric acid as a base, and an appropriate amount of hydrofluoric acid which has an oxide film destroying effect is added thereto. Here, the reason for limiting the nitric acid concentration and the Fe ³⁺ ion concentration is the same as in the pickling solution of (1) above, and the ratio R2 [fluorine concentration (g/L) to nitric acid concentration (g/L)] The reason for limiting [acid/nitric acid] is the same as that of the pickling solution in (1) above.
Here, the Fe ³⁺ ion concentration of the pickling solution of (1) or (2) can be adjusted as follows. That is, when a steel sheet is dipped in a pickling solution, the steel sheet dissolves and becomes Fe ²⁺ ions, which are oxidized by nitric acid to become Fe ³⁺ ions, and the concentration thereof increases with time. Therefore, the Fe ³⁺ ion concentration in the pickling solution is measured and monitored, a part of the pickling solution is extracted before the control concentration (for example, 30 g/L) is exceeded, and a new solution containing no Fe ³⁺ ion is replenished. As a result, the Fe ³⁺ ion concentration of the pickling solution can be adjusted to a predetermined level.

When the above-mentioned pickling solution (1) or (2) is used, it is preferable to carry out pickling under the conditions of pickling solution temperature of 20 to 70° C. and pickling time of 3 to 30 seconds.
When the pickling solution temperature is lower than 20° C., the oxide layer containing the easily oxidizable metal element is liable to be insufficiently removed, and the remaining oxide tends to impair the chemical conversion treatment property and reduce corrosion resistance. In addition, the surface of the steel sheet is black due to the oxide, and the appearance of the surface of the steel sheet is impaired. On the other hand, when the pickling solution temperature exceeds 70° C., a sufficient pickling rate can be obtained, but the reaction heat accompanying pickling becomes large, which makes temperature control extremely difficult and requires cooling equipment. Becomes complicated. When the pickling time is less than 3 seconds, the nitric acid concentration needs to be increased in order to sufficiently remove the oxide layer containing the easily oxidizable metal element, which easily causes the above-mentioned problems. On the other hand, even if the pickling time exceeds 30 seconds, there is no problem in performance, but the equipment becomes long and the equipment cost increases.
Further, in the case of pickling, it is also effective to add a pickling accelerator to the pickling solution or use an electrolytic treatment together to accelerate the dissolution of the base iron.

  Generally, in the manufacturing process of a high-strength cold-rolled steel sheet, a cold-rolled steel sheet that has been continuously annealed is water-quenched, then pickled, and then subjected to a normal treatment step such as temper rolling to obtain a product steel sheet.

As a test material, a continuous annealed material (cold rolled steel sheet) having the composition and tensile strength TS shown in Table 1 was used. These steel sheets were pickled under the conditions shown in Tables 2 and 3, washed with water, dried, and then temper-rolled with an elongation of 0.7% and subjected to high-strength cold rolling of Nos. 1 to 24. A steel plate was manufactured. The Fe ³⁺ ion concentration of the ^pickling solution was adjusted by the method described above.
A test piece was taken from each of the manufactured steel plates, and five spots on the surface of the arbitrarily selected test piece were analyzed in the depth direction by glow discharge emission spectroscopy (GDS), and the peaks of Al, Si, Mn, and O were found. It is checked whether or not it appears, and when one or more peaks of Al, Si, Mn, and O appear, "the oxide layer exists (remains)", and when none of the peaks appears, "oxide. There is no layer (remaining)”. The cross-sectional coverage of the metallic Cu was obtained by the method described above for the surface layer cross section of the test piece.

After collecting test pieces from each manufactured steel sheet, applying chemical conversion treatment (low temperature type chemical conversion treatment with chemical conversion treatment liquid temperature of 35° C.) and coating to these test pieces under the following conditions, salt hot water immersion test, salt water spray The test and the combined cycle corrosion test were subjected to three types of corrosion tests to evaluate the corrosion resistance after coating. In addition, test pieces were taken from each of the manufactured steel sheets (steel sheets that had not been subjected to chemical conversion treatment/painting), and the delayed fracture resistance was evaluated using the test pieces. In the chemical conversion treatment, the chemical conversion treatment time was adjusted so that the coating adhesion amount was 1.7 to 3.0 g/m ² .
(1) Chemical conversion treatment conditions-Degreasing process Degreasing agent: "FC-E2011" manufactured by Nippon Parkerizing Co., Ltd.
Degreasing method: Spray degreasing Treatment temperature: 40°C
Treatment time: 120 seconds ・Surface conditioning process Surface conditioning agent: "PL-X" manufactured by Nihon Parkerizing Co., Ltd.
Surface conditioner pH: 9.5
Treatment temperature: Room temperature Treatment time: 20 seconds ・Chemical conversion treatment step Chemical conversion treatment agent: "Palbond PB-SX" manufactured by Nippon Parkerizing Co., Ltd.
Chemical conversion treatment liquid temperature: 35°C
Treatment time: 90 seconds (2) Coating conditions Electrodeposition coating was applied to the surface of the test piece that had been subjected to chemical conversion treatment, using the electrodeposition coating "GT-100" manufactured by Nippon Paint Co., Ltd., so that the film thickness was 15 μm. gave.

(3) Corrosion test <Salt hot water immersion test>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the above-mentioned test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, this test piece was added to a 5 mass% NaCl solution (60° C.) 240 After soaking for a period of time, followed by washing with water, drying, adhering an adhesive tape to the cross-cut flaw portion, a tape peeling test for peeling off was performed, and the maximum peeling total width of the left and right cross-cut flaw portions was measured. When this maximum peeling overall width is 5.0 mm or less, the corrosion resistance in the salt hot water immersion test can be evaluated as good.

<Salt spray test (SST)>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, a 5 mass% NaCl aqueous solution was used for this test piece. After performing a salt spray test for 960 hours in accordance with the neutral salt spray test specified in JIS Z2371:2000, a tape peeling test was performed on the crosscut flaws, and maximum peeling was performed by combining the left and right crosscut flaws. Full width was measured. If this maximum peeling overall width is 4.0 mm or less, it can be evaluated that the corrosion resistance in the salt spray test is good.

<Compound Cycle Corrosion Test (CCT)>
After a cross-cut flaw having a length of 45 mm was formed by a cutter on the surface of the above-mentioned test piece (n=1) that had been subjected to chemical conversion treatment and electrodeposition coating, the test piece was subjected to "salt spray (5 mass% NaCl). Aqueous solution: 35° C., relative humidity: 98%)×2 hours→drying (60° C., relative humidity: 30%)×2 hours→wetting (50° C., relative humidity: 95%)×2 hours” as one cycle, After repeating this cycle for 50 cycles, a corrosion test was carried out, followed by washing with water and drying, and then a tape peeling test was carried out on the crosscut flaws, and the maximum peeling total width of the left and right crosscut flaws was measured. If the maximum peeling total width is 6.0 mm or less, the corrosion resistance in the combined cycle corrosion test can be evaluated as good.

(4) Delayed fracture resistance properties Each of the manufactured steel plates was sheared to a width of 35 mm and a length of 100 mm, and ground to give a width of 30 mm to prepare test pieces. As shown in FIG. 2, this test piece 1 was bent into a U-shape, and the shape of the test piece was fixed by restraining it with a bolt 2 and a nut 3 to obtain a test piece for delayed fracture evaluation. The delayed fracture evaluation test piece thus produced was subjected to a combined cycle corrosion test (see FIG. 3) consisting of the steps of dry, wet, and salt water immersion specified in SAE J2334 defined by the American Society of Automotive Engineers. Up to 20 cycles were carried out. Before the process of immersion in salt water in each cycle, the presence or absence of cracks was visually inspected, and the number of crack generation cycles was measured. In addition, this test was carried out for each of three steel plates, and the average value was used as the number of crack generation cycles.

The results of the above tests are shown in Tables 2 and 3 together with pickling conditions and steel plate surface properties. According to this, the steel sheet of the example of the present invention has excellent delayed fracture resistance, and also has a maximum maximum peeling width in any of the salt hot water immersion test, the salt spray test and the combined cycle corrosion test, which is extremely excellent after coating. It can be seen that it has corrosion resistance.
The comparative examples of Nos. 28 to 32 in Table 3 are inferior in delayed fracture resistance to the inventive examples, but this is due to the presence of an oxide layer on the steel plate surface or the cross-section coverage of metal Cu. It is considered that this is because the chemical conversion treatment property is poor due to the high value and the corrosion resistance after coating is insufficient, so that a large amount of hydrogen is generated due to the corrosion reaction associated with exposure and cracking is likely to occur.

Claims (5)

A steel sheet containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25 mass% and having a tensile strength of 1180 MPa or more, in which the steel structure is a recrystallized structure, and Al is formed on the surface of the steel plate. , An oxide layer containing at least one of Si and Mn does not exist (provided that the oxide layer does not exist, the depths of five arbitrarily selected steel plate surfaces are determined by glow discharge emission spectroscopy analysis). When the peaks of Al, Si, Mn, and O do not appear when analyzed in the direction, the coverage of the steel plate surface by the precipitated metal Cu existing on the steel plate surface (however, the steel plate surface measured in the steel plate cross section) The high-strength cold-rolled steel sheet is characterized by having a coating rate of 30% or less. C: 0.08 to 0.30 mass%, Si: 0.1 to 2.0 mass%, Mn: 0.1 to 2.5 mass%, P: 0.020 mass% or less, S: 0.005 % Or less, Sol.Al: 0.01 to 0.05% by mass, Cu: 0.05 to 0.25% by mass, and the balance is a component composition consisting of iron and unavoidable impurities. The high-strength cold-rolled steel sheet according to claim 1. Further, it is characterized by containing at least one selected from Ni: 0.05 mass% or less, Ti: 0.1 mass% or less, Nb: 0.1 mass% or less, and B: 5 to 30 mass ppm. The high-strength cold-rolled steel sheet according to claim 2. A high-strength cold-rolled steel sheet characterized by being subjected to pickling with a pickling solution according to the following (1) or (2) after continuously annealing a cold-rolled steel sheet having the component composition according to claim 1. Manufacturing method.
(1) The acid component consists of nitric acid and hydrochloric acid, the nitric acid concentration is more than 50 g/L and 110 g/L or less, and the ratio R1 [hydrochloric acid/nitric acid] of hydrochloric acid concentration (g/L) to nitric acid concentration (g/L) is 0. 1 to 0.3, Fe ³⁺ ion concentration is ³ to 30 g/L pickling solution (2) The acid component consists of nitric acid and hydrofluoric acid, and the nitric acid concentration is more than 50 g/L and 110 g/L or less,
Pickling solution having a ratio R2 of [hydrofluoric acid concentration (g/L) and nitric acid concentration (g/L) [hydrofluoric acid/nitric acid] of 0.1 to 0.3 and Fe ³⁺ ion concentration of ³ to 30 g/L. The method for producing a high-strength cold-rolled steel sheet according to claim 4, wherein the pickling solution temperature is 20 to 70°C and the pickling time is 3 to 30 seconds.