JP2019026864A - High strength cold rolled steel sheet excellent in corrosion resistance after coating and delayed fracture resistance, and manufacturing method therefor - Google Patents

Abstract

To provide a high strength cold rolled steel sheet to which Cu is added for improving delayed fracture resistance, good in chemical conversion treatment property in a low temperature type chemical conversion treatment and having excellent corrosion resistance after coating and delayed fracture resistance.SOLUTION: There is provided a high strength cold rolled steel sheet containing at least one kind of Si, Mn and Al, and Cu:0.05 to 0.25 mass%, in which a steel structure consists of a recrystalline structure, no oxide layer containing at least one kind of Al, Si and Mn exists on a steel sheet surface, and cross section coverage factor by a deposition metal Cu existing on the steel sheet surface is 30% or less. Since acid cleaning liquid film on the steel sheet surface is reacted with the steel sheet and Cu eluted from the steel sheet is deposited on the steel sheet surface, and the deposited metal Cu reduces the chemical conversion treatment property, the chemical conversion treatment property can be improved by suppressing cross section coverage factor by the deposited metal Cu to low.SELECTED DRAWING: None

Description

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

  Cold-rolled steel sheets used in the automotive field are often coated after applying a ground treatment for coating from the viewpoint of rust prevention, and in order to obtain excellent post-coating corrosion resistance, The rolled steel sheet is required to have good chemical conversion properties. In recent years, from the viewpoint of improving fuel efficiency, the thickness of steel sheets has been reduced for the purpose of reducing the weight of the vehicle body, and accordingly, the strength of cold-rolled steel sheets has been demanded. However, there is a problem that delayed fracture resistance decreases due to increased strength, and steel manufacturers have added alloying elements and added various thermal histories to increase corrosion resistance and delay resistance after coating. It aims to achieve both fracture characteristics.

  In recent years, the temperature of the chemical conversion liquid has been lowered for the purpose of reducing the amount of sludge generated during chemical conversion and the energy cost. Since the reactivity between the steel sheet and the chemical conversion treatment liquid decreases due to the low temperature of the chemical conversion treatment liquid, it is required to improve the chemical conversion treatment ability. In order to increase the reactivity with the steel sheet in terms of the composition of the chemical conversion solution, efforts to improve the reactivity with the steel sheet by controlling the free fluorine concentration to be higher are being performed.

On the other hand, as an approach on the steel sheet side for improving the chemical conversion property, for example, there are the following.
(1) Technology to improve chemical conversion properties by applying temper rolling and introducing dislocations in steel (Patent Document 1)
(2) Technology for improving chemical conversion processability in continuous annealing materials by adding B to steel and performing temper rolling (Patent Document 2)
(3) In a continuously annealed material, after pickling the steel sheet, a noble metal (for example, Ni) is attached to the surface thinly to provide a conversion crystal growth site, so that the chemical conversion processability can be achieved regardless of the steel type. Technology for improvement (Patent Documents 3 and 4)
(4) Technology for improving chemical conversion treatment by removing oxides that hinder chemical conversion treatment formed during continuous annealing by pickling in continuous annealing materials (Patent Document 5)

  Further, 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. A technique for improving the delayed fracture resistance of cold-rolled steel sheets (Patent Document 6) and the like has been proposed by setting Cu to 0.10% or more.

JP 62-116723 A Japanese Patent Publication No. 2-29729 Japanese Patent Publication No. 58-37391 Japanese Patent Publication No. 1-58276 JP 2012-132093 A JP 2011-246664 A

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

  On the other hand, recently, for the purpose of reducing the amount of sludge generated during chemical conversion and the energy cost, the temperature of chemical conversion liquid has been lowered, and the chemical conversion liquid has a significantly lower reactivity with respect to the steel plate than in the past. Processing is coming to be done. Compared to the conventional chemical conversion treatment performed at about 43 ° C., the chemical conversion treatment at a low temperature of about 35 ° C. greatly depends on the surface properties of the plate and is removed by pickling on the surface of the steel plate. It has been found that the presence of remaining metal oxide (pickling residue) and other product substances significantly lowers the chemical conversion property and degrades 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, the low-temperature-type chemical conversion is performed even if the techniques of Patent Documents 1 to 5 described above are used. It has been found that it is difficult to improve the chemical conversion processability in the treatment.

  Accordingly, an object of the present invention is a high-strength cold-rolled steel sheet to which Cu has been added for the purpose of solving the above-described problems of the prior art and improving delayed fracture resistance, Is to provide a high-strength cold-rolled steel sheet that can provide excellent post-coating corrosion resistance and delayed fracture resistance, and a method for producing the same.

  In order to solve the above-mentioned problems related to the high-strength cold-rolled steel sheet to which Cu is added, the inventors have performed a detailed analysis on the steel sheet surface after continuous annealing and pickling, and as a result, remain on the steel sheet surface after pickling. The presence of metal Cu deposited on the surface of the steel sheet due to the reaction between the pickled oxide film (pickling residue) and the pickling liquid film and the steel sheet between pickling and washing with water is a factor that greatly deteriorates the chemical conversion processability. Therefore, the present inventors have found that it is extremely important to reduce such pickling residue and precipitated metal Cu in order to improve the chemical conversion property (especially chemical conversion property in the low temperature type chemical conversion treatment).

The present invention has been made on the basis of such knowledge and has the following gist.
[1] A steel plate containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25% by mass and having a tensile strength of 1180 Mpa or more, the steel structure being a recrystallized structure, There is no oxide layer containing at least one of Al, Si, and Mn on the surface (provided that the oxide layer does not exist, glow discharge emission spectroscopic analysis of 5 locations on the steel sheet surface selected arbitrarily) When the analysis is conducted in the depth direction, the peak of Al, Si, Mn, and O does not appear.) The coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, measured at the cross section of the steel sheet) A high-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance, characterized by having a steel sheet surface coverage of 30% or less.

[2] In the high-strength cold-rolled steel sheet of [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: 0.01-0.05 mass%, Cu: 0.05-0.25 mass%, with the balance being A high-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance, characterized by having a composition comprising iron and inevitable impurities.
[3] In the high-strength cold-rolled steel sheet of [2], Ni: 0.05% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, B: 5-30% A high-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance, characterized by containing at least one selected from ppm.

[4] The present invention is characterized in that after the cold-rolled steel sheet having the component composition according to any one of [1] to [3] is continuously annealed, it is pickled with a pickling solution of the following (1) or (2). A method for producing high-strength cold-rolled steel sheets with excellent post-coating corrosion resistance and delayed fracture resistance.
(1) The acid component is composed 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, pickling solution having Fe ³⁺ ion concentration of ³ to 30 g / L (2) The acid component is composed of nitric acid and hydrofluoric acid, and the nitric acid concentration is over 50 g / L and 110 g / L or less,
Pickling solution having a ratio R2 of hydrofluoric acid concentration (g / L) to 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, the pickling is performed under 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-coating corrosion resistance and delayed fracture resistance. Manufacturing method of high-strength cold-rolled steel sheet.

  The high-strength cold-rolled steel sheet according to the present invention has excellent delayed fracture resistance and good chemical conversion properties even in a low-temperature chemical conversion treatment, so that it is severe such as a salt warm water immersion test and a combined cycle corrosion test. Excellent post-coating corrosion resistance even in corrosive environments. Moreover, according to the manufacturing method of this invention, the high intensity | strength cold-rolled steel plate which has such an outstanding characteristic can be manufactured stably.

The same field of view as an example of a secondary electron image (FIG. 1 (A)) at an acceleration voltage of 1 kV and a magnification of 20000 times of the surface layer cross section of the steel sheet, which is imaged when obtaining the cross-sectional coverage of the metal Cu deposited on the steel sheet surface 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 having a tensile strength of 1180 Mpa or more, containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25% by mass. This high-strength steel sheet is manufactured through cold annealing followed by continuous annealing-pickling, and thus the steel structure consists of 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 as a deoxidizing element. High-strength cold-rolled steel sheets are prone to delayed fracture, and this phenomenon becomes more severe as the strength increases, and is particularly noticeable for high-strength steel sheets having a tensile strength of 1180 MPa or more. The steel plate of the present invention contains 0.05 to 0.25% by mass of Cu in order to improve delayed fracture resistance. By adding Cu to the steel sheet, it is considered that the steel sheet is less likely to corrode (anode reaction), and as a result, hydrogen generation (cathode reaction) is suppressed. Since hydrogen generation is suppressed, it is considered that the invading hydrogen is reduced and the effect of suppressing delayed fracture appears. Here, when the amount of Cu is less than 0.05% by mass, the delayed fracture resistance is not sufficiently improved. On the other hand, when Cu is excessively contained, metal Cu (as will be described later, this metal Cu hinders chemical conversion properties) is likely to be formed on the steel sheet surface immediately after pickling, so the amount of Cu is 0.25% by mass. The following.

  In the production of high-strength cold-rolled steel sheets, continuous annealing is performed after cold rolling, and subsequently the removal of oxide layers containing easily oxidizable elements (at least one of Al, Si, Mn) generated on the steel sheet surface. As a result of research by the present inventors, as a result of the study by the present inventors, good chemical conversion treatment is performed on a high-strength cold-rolled steel sheet containing an appropriate amount of Cu to improve delayed fracture resistance as described above. It has been found that the surface properties of the steel sheet after pickling must be as follows in order to obtain the properties (especially the chemical conversion properties in the low-temperature chemical conversion treatment).

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

  Furthermore, the pickling liquid film on the steel plate surface continues to react with the steel plate between the pickling and the water washing, Cu eluted from the steel plate is deposited on the steel plate surface, and this deposited metal Cu is then subjected to chemical conversion treatment. It has been found that the chemical conversion crystal formation reaction is inhibited in the process (particularly the low-temperature chemical conversion treatment step), that is, the chemical conversion treatment property is lowered. This is considered to be because the dissolution reaction of iron in the chemical conversion treatment is inhibited by the metal Cu deposited on the steel sheet surface, and the growth of chemical conversion crystals is suppressed. It has been found that it is necessary to suppress the coverage of the deposited metal Cu on the surface. Specifically, the coverage ratio of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, the coverage ratio of the steel sheet surface measured by the cross section of the steel sheet; hereinafter referred to as “cross section coverage ratio” for convenience of explanation) is 30%. It was necessary to make the following, and it was found that a good chemical conversion treatment property was obtained. For this reason, in this invention, the cross-sectional coverage of the steel plate surface by the precipitation metal Cu which exists in the steel plate surface shall be 30% or less.

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

Next, the preferable component composition of the high-strength cold-rolled steel sheet of the present invention will be described.
The steel plate 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: 0.01-0.05% by mass, Cu: 0.05-0.25% by mass, with the balance being composed of iron and inevitable impurities Further, if necessary, Ni: 0.05% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, B: 1 type selected from 5 to 30 ppm by mass The above can be contained. The reasons for limiting the contents of these elements are as follows.

C: 0.08-0.30 mass%
C is an element necessary for ensuring the strength of the steel sheet, and if the C content is less than 0.08 mass%, it may be difficult to ensure the strength, so the C content is 0.08 mass% or more. It is preferable to do. On the other hand, since the weldability deteriorates when the amount of C is excessive, the amount of C is preferably 0.30% by mass or less.
・ Si: 0.1-2.0 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. Therefore, the Si amount is preferably 0.1% by mass or more. On the other hand, when Si is excessively contained, the toughness is deteriorated, the amount of Si-based oxide formed during annealing is increased, the pickling equipment is lengthened, and the equipment cost is increased. Therefore, the Si amount is preferably 2.0% by mass or less.

Mn: 0.1 to 2.5% by mass
Mn is an element that ensures the strength of the steel sheet, and if the Mn content is less than 0.1% by mass, the strength of the steel sheet may be insufficient. Therefore, the Mn content is preferably 0.1% by mass or more. . On the other hand, when Mn is excessively contained, segregation is increased, workability is lowered, and weldability is also deteriorated. Therefore, the amount of Mn is preferably 2.5% by mass or less.
-P: 0.020% by mass or less, S: 0.005% by mass or less P and S are preferably as low as possible in view of workability. Therefore, the P content is 0.020% by mass or less. It is preferable that Moreover, since inclusion (MnS) will increase when S is contained excessively and it will have a bad influence on workability, it is preferable that S amount shall be 0.005 mass% or less.

-Sol.Al: 0.01-0.05 mass%
Al is an element added for deoxidation. If the amount of Sol.Al is less than 0.01% by mass, silicate inclusions remain and the workability of the steel may deteriorate. It is preferable to set it as 0.01 mass% or more. On the other hand, if the amount of Sol.Al is too large, an increase in surface defects is caused. Therefore, the amount of Sol.Al is preferably 0.05% by mass or less.
Cu: 0.05 to 0.25% by mass
The amount of Cu is as described above. That is, if the amount of Cu is less than 0.05% by mass, the delayed fracture resistance is not sufficiently improved. On the other hand, if the amount of Cu exceeds 0.25% by mass, metal Cu is deposited on the surface of the steel sheet immediately after pickling. Since it becomes easy, the amount of Cu shall be 0.05-0.25 mass%.

Ni: 0.05% by mass or less Ni is a solid solution strengthening element, but even if Ni is excessively contained, the effect of improving the mechanical properties of the steel sheet is saturated and the cost is increased. It is preferable to set it as 0.05 mass% or less.
Ti: 0.1% by mass or less, Nb: 0.1% by mass or less Ti and Nb are elements that refine the steel structure and are very useful elements for improving strength without impairing toughness. It is. However, even if Ti and Nb are contained excessively, the effect of improving the mechanical properties of the steel sheet is saturated, and on the contrary, the cost is increased. Therefore, the Ti amount and the Nb amount are each preferably 0.1% by mass or less.
・ B: 5-30 mass ppm
B is an extremely useful element for enhancing the hardenability, and has an effect of strengthening the grain boundary and improving the delayed fracture resistance. In order to sufficiently exhibit these effects, B The amount is preferably 5 ppm by mass or more. On the other hand, since hot workability deteriorates when B is contained excessively, the amount of B is preferably 30 mass ppm or less.

Next, the manufacturing method of the high intensity | strength cold-rolled steel plate of this invention is demonstrated.
As mentioned earlier, in order to obtain good chemical conversion (especially chemical conversion in low-temperature chemical conversion), (i) the oxide layer on the steel sheet surface is almost completely removed by pickling. That the oxide layer is substantially absent on the surface of the steel sheet, and (ii) that the pickling liquid film reacts with the steel sheet between pickling and rinsing so that metal Cu is deposited on the surface of the steel sheet. It is necessary to suppress (the cross-sectional coverage by the metal Cu is 30% or less).
For this reason, in the manufacturing process of the high-strength cold-rolled steel sheet according to the present invention, the cold-rolled steel sheet on which the oxide containing an easily oxidizable metal element is generated by continuous annealing is pickled under the following conditions. Is preferred.

Among the oxidizable metals, particularly, oxides containing Si include oxides that are hardly soluble in acids such as SiO ₂ . In order to completely remove the oxide layer containing an easily oxidizable metal element including such hardly soluble SiO ₂ , it is necessary to remove the oxide layer on the surface of the steel sheet together with the base iron by pickling. In pickling, it is necessary to efficiently remove an oxide layer containing an easily oxidizable metal element in a short time.
For this reason, it is preferable that a pickling liquid consists of an acid component based on nitric acid which is a strong oxidizing acid. That is, in order to efficiently remove an oxide layer containing an easily oxidizable metal element in a short time, it is effective to use the oxidizing power of nitric acid (= the action of oxidizing (dissolving) iron). On the other hand, when such a pickling solution is used, the iron resurfacing layer is easily oxidized (= Fe-based oxide is formed), and Cu is also eluted into the pickling solution together with Fe, and Cu is more noble than Fe. Since it is an element, it immediately deposits as metallic Cu on the surface of the steel plate after elution. That is, Cu in steel is eluted together with Fe by pickling, but it is important to suppress elution of Cu in order to reduce the cross-sectional coverage of metal Cu deposited on the steel sheet surface. Therefore, “removal of oxide layer containing easily oxidizable metal element” and “generation of Fe-based oxide / precipitation of metal Cu in resurfaced layer” are in a trade-off relationship. In addition, it is necessary to optimize the composition and concentration of the pickling solution based on nitric acid.

Specifically, it is preferable to use the following pickling solution (1) or (2) in which the acid component is based on nitric acid and an appropriate amount of hydrochloric acid or hydrofluoric acid is added thereto.
(1) The acid component is composed 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, pickling solution having Fe ³⁺ ion concentration of ³ to 30 g / L (2) The acid component is composed of nitric acid and hydrofluoric acid, and the nitric acid concentration is over 50 g / L and 110 g / L or less,
Pickling solution having a ratio R2 [hydrofluoric acid / nitric acid] of hydrofluoric acid concentration (g / L) to 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 (1) is a pickling solution in which an acid component is based on nitric acid and an appropriate amount of hydrochloric acid having an effect of destroying an oxide film is added thereto. Here, when the nitric acid concentration is 50 g / L or less, the oxide layer containing an easily oxidizable metal element cannot be removed properly, and the remaining oxide inhibits chemical conversion treatment and causes a decrease in corrosion resistance. Further, the steel sheet surface is black due to the oxide, and the aesthetic appearance of the steel sheet surface 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 dissolution of Fe and Cu occurs vigorously, metal Cu is likely to precipitate, and the steel sheet surface It is not possible to reduce the cross-sectional coverage of the metal Cu deposited on the surface. Moreover, the unevenness | corrugation on the surface of a steel plate becomes intense, and the beauty | look of the coating surface is spoiled. Further, when the ratio R1 [hydrochloric acid / nitric acid] of hydrochloric acid concentration (g / L) and nitric acid concentration (g / L) is less than 0.1, a sufficient pickling speed can be obtained, but when the nitric acid ratio is high, the nitric acid concentration Since the dissolution of the base iron is promoted by the strong oxidizing power and the amount of elution of Cu increases, metal Cu is likely to be deposited on the steel plate surface, and the cross-sectional coverage of the metal Cu deposited on the steel plate surface cannot be reduced. Moreover, since the reaction heat accompanying pickling increases, temperature control becomes very difficult, and cooling equipment is required, which complicates manufacturing equipment. 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 that the desired pickling rate cannot be obtained, and the oxide on the steel sheet surface is efficiently removed. I can't.

If the Fe ³⁺ ion concentration 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 oxide on the steel sheet surface is efficiently removed. Can not do it. On the other hand, when the Fe ³⁺ ion concentration exceeds 30 g / L, the iron oxidation reaction with Fe ³⁺ ions is sufficiently performed, and the desired pickling speed can be obtained, but there are many Fe ³⁺ ions in the pickling solution. A large amount of Fe-based oxides are formed on the surface of the steel sheet, and Fe-based oxides newly formed by pickling cannot be removed, so that the chemical conversion property and the corrosion resistance after coating cannot be improved.

The pickling solution (2) is a pickling solution in which an acid component is based on nitric acid and an appropriate amount of hydrofluoric acid having an effect of destroying an oxide film is added thereto. Here, the reasons for limiting the nitric acid concentration and the Fe ³⁺ ion concentration are the same as in the pickling solution of (1) above, and the ratio R2 of the hydrofluoric acid concentration (g / L) to the nitric acid concentration (g / L) [fluoride] The reason for the limitation of [acid / nitric acid] is also the same as in the pickling solution (1) above.
Here, the Fe ³⁺ ion concentration of the pickling solution (1) or (2) can be adjusted as follows. That is, when the steel sheet is immersed in the pickling solution, the steel sheet dissolves to become Fe ²⁺ ions, which are oxidized with nitric acid to become Fe ³⁺ ions, and the concentration thereof increases with time. For this reason, the Fe ³⁺ ion concentration in the pickling solution is measured and monitored, a part of the pickling solution is extracted before exceeding the control concentration (for example, 30 g / L), and a new solution not containing Fe ³⁺ ions is supplied. Thus, the Fe ³⁺ ion concentration of the pickling solution can be adjusted to a predetermined level.

Moreover, when using said pickling solution of (1) or (2), it is preferable to perform pickling on the conditions of pickling solution temperature 20-70 degreeC, and pickling time 3-30 seconds.
When the pickling solution temperature is less than 20 ° C., the removal of the oxide layer containing the easily oxidizable metal element tends to be insufficient, and the remaining oxide tends to inhibit the chemical conversion treatment and cause the corrosion resistance to decrease. Further, the steel sheet surface is black due to the oxide, and the aesthetic appearance of the steel sheet surface is impaired. On the other hand, if the pickling solution temperature exceeds 70 ° C., a sufficient pickling speed can be obtained, but since the reaction heat associated with pickling increases, temperature control becomes very difficult, and cooling equipment is required. Becomes complicated. In addition, when the pickling time is less than 3 seconds, it is necessary to increase the concentration of nitric acid in order to sufficiently remove the oxide layer containing the easily oxidizable metal element, which tends to cause the problems described above. On the other hand, even if the pickling time exceeds 30 seconds, there is no problem in performance, but the equipment becomes longer and the equipment cost increases.
Moreover, in performing pickling, it is also effective to promote dissolution of base iron by adding a pickling accelerator to the pickling solution or using an electrolytic treatment in combination.

  In general, in the manufacturing process of a high-strength cold-rolled steel sheet, the continuously annealed cold-rolled steel sheet is water-quenched and then pickled, and then processed into a product steel sheet through normal processing steps such as temper rolling.

A continuously annealed material (cold rolled steel sheet) having the component composition and tensile strength TS shown in Table 1 was used as a test material. These steel sheets are pickled under the conditions shown in Table 2 and Table 3, washed with water, dried, then subjected to temper rolling with an elongation of 0.7%, and No. 1-24 high strength cold rolling. A steel plate was produced. The Fe ³⁺ ion concentration of the ^pickling solution was adjusted by the method described above.
Specimens are collected from each manufactured steel sheet, and 5 points on the selected specimen surface are analyzed in the depth direction by glow discharge optical emission spectrometry (GDS). The peaks of Al, Si, Mn, and O are observed. Investigate whether or not one or more peaks of Al, Si, Mn, and O appear, and “Oxide layer is present (remains)”. The layer was not present (residual) ”. Moreover, the cross-sectional coverage of metal Cu was calculated | required by the method demonstrated previously about the surface layer cross section of the test piece.

Specimens are collected from each manufactured steel sheet, and these specimens are subjected to chemical conversion treatment (low-temperature chemical conversion treatment at a chemical conversion solution temperature of 35 ° C.) and coating under the following conditions, followed by a salt warm water immersion test and salt spray. The corrosion resistance after coating was evaluated by three types of corrosion tests, a test and a combined cycle corrosion test. Moreover, the test piece was extract | collected from each manufactured steel plate (steel plate which is not a chemical conversion process and coating), and delayed fracture resistance was evaluated by this test piece. In addition, 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 treatment conditions-Degreasing process Degreasing agent: "FC-E2011" manufactured by Nihon Parkerizing Co., Ltd.
Degreasing method: Spray degreasing Treatment temperature: 40 ° C
Processing time: 120 seconds ・ Surface conditioning step Surface conditioning agent: “PL-X” manufactured by Nihon Parkerizing
Surface conditioner pH: 9.5
Treatment temperature: Room temperature Treatment time: 20 seconds ・ Chemical conversion treatment process Chemical conversion treatment agent: “Palbond PB-SX” manufactured by Nihon Parkerizing Co., Ltd.
Chemical conversion solution temperature: 35 ° C
Treatment time: 90 seconds (2) Coating conditions Electrodeposition coating is applied to the surface of the test piece subjected to chemical conversion treatment using an electrodeposition paint “GT-100” manufactured by Nippon Paint Co., Ltd. so that the film thickness becomes 15 μm. gave.

(3) Corrosion test <Salt warm water immersion test>
The surface of the above-mentioned test piece (n = 1) 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 placed in a 5 mass% NaCl solution (60 ° C.). After immersing for a period of time, washed with water, dried, affixed with an adhesive tape on the crosscut collar, and then peeled off, a tape peel test was performed to measure the maximum total peel width of the crosscut collar on the left and right. 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)>
After applying a 45 mm long crosscut wrinkle to the surface of the above-mentioned test piece (n = 1) subjected to chemical conversion treatment and electrodeposition coating, a 5 mass% NaCl aqueous solution was used for this test piece. After performing a salt water spray test for 960 hours in accordance with the neutral salt spray test specified in JIS Z2371: 2000, a tape peel test is performed on the crosscut collar, and the maximum peel is obtained by combining the left and right of the crosscut collar The full width 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 test piece (n = 1) subjected to chemical conversion treatment and electrodeposition coating was applied with a 45 mm long crosscut wrinkle with a cutter, and then “salt spray (5 mass% NaCl) was applied to this test piece. “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” A corrosion test was repeated 50 cycles, washed with water, and dried, and then a tape peel test was performed on the crosscut collar, and the maximum width of the maximum peel along the crosscut 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.

(4) Delayed fracture resistance The manufactured steel sheets were each sheared to a width of 35 mm and a length of 100 mm, and subjected to grinding until the width became 30 mm, thereby producing test pieces. As shown in FIG. 2, the test piece 1 was bent into a U shape and restrained by a bolt 2 and a nut 3 to fix the test piece shape, thereby obtaining a delayed fracture evaluation test piece. For the test specimen for delayed fracture evaluation prepared in this way, a combined cycle corrosion test (see FIG. 3) consisting of drying, wetting, and salt water soaking processes defined in SAE J2334 defined by the American Society of Automotive Engineers. Up to 20 cycles were performed. The presence or absence of cracks was visually examined before the salt water immersion process in each cycle, and the number of crack generation cycles was measured. In addition, this test was performed for three specimens of each steel plate, and the average value was defined as the number of crack generation cycles.

The result of the said test is shown in Table 2 and Table 3 with pickling conditions and steel plate surface property. According to this, the steel sheet of the example of the present invention has excellent delayed fracture resistance, and the maximum total peel width is small in any of the salt warm water immersion test, the salt spray test and the combined cycle corrosion test. It turns out that it has corrosion resistance.
In addition, although the comparative example of No. 28-32 of Table 3 is inferior in the delayed fracture resistance compared with the example of this invention, this has the oxide layer in the steel plate surface, or the cross-sectional coverage of metal Cu Therefore, it is considered that the chemical conversion treatment property is poor and the post-coating corrosion resistance is insufficient, so that a lot of hydrogen is generated due to the corrosion reaction accompanying the exposure and cracking is likely to occur.

Claims (5)

A steel plate having a tensile strength of 1180 Mpa or more containing at least one of Si, Mn, and Al and Cu: 0.05 to 0.25% by mass, the steel structure being a recrystallized structure, There is no oxide layer containing at least one of Si, Mn (provided that the oxide layer does not exist is a depth determined by glow discharge emission spectroscopic analysis at five locations on the steel sheet surface selected arbitrarily. When the analysis is made in the direction, the peak of Al, Si, Mn, O does not appear.) The coverage of the steel sheet surface by the precipitated metal Cu existing on the steel sheet surface (however, the steel sheet surface measured by the steel sheet cross section) High-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance, characterized in that the coating ratio is 30% or less.   C: 0.08-0.30 mass%, Si: 0.1-2.0 mass%, Mn: 0.1-2.5 mass%, P: 0.020 mass% or less, S: 0.005 Less than mass%, Sol.Al:0.01-0.05 mass%, Cu: 0.05-0.25 mass%, The remainder has the component composition which consists of iron and an unavoidable impurity, It is characterized by the above-mentioned. The high-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance according to claim 1.   Furthermore, Ni: 0.05% by mass or less, Ti: 0.1% by mass or less, Nb: 0.1% by mass or less, B: containing one or more selected from 5 to 30 ppm by mass The high-strength cold-rolled steel sheet excellent in post-coating corrosion resistance and delayed fracture resistance according to claim 2. After continuously annealing the cold-rolled steel sheet having the component composition according to any one of claims 1 to 3, the steel sheet is pickled with the pickling solution of (1) or (2) below, and is subjected to post-coating corrosion resistance and resistance. A method for producing high-strength cold-rolled steel sheets with excellent delayed fracture characteristics.
(1) The acid component is composed 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, pickling solution having Fe ³⁺ ion concentration of ³ to 30 g / L (2) The acid component is composed of nitric acid and hydrofluoric acid, and the nitric acid concentration is over 50 g / L and 110 g / L or less,
Pickling solution having a ratio R2 [hydrofluoric acid / nitric acid] of hydrofluoric acid concentration (g / L) to nitric acid concentration (g / L) of 0.1 to 0.3 and Fe ³⁺ ion concentration of ³ to 30 g / L   The high-strength cold-rolled steel sheet with excellent post-coating corrosion resistance and delayed fracture resistance according to claim 4, wherein pickling is performed under conditions of a pickling temperature of 20 to 70 ° C and a pickling time of 3 to 30 seconds. Manufacturing method.