JP6862316B2 - High-strength steel sheet with excellent delayed fracture resistance and its manufacturing method - Google Patents

Description

The present invention relates to a high-strength steel sheet having excellent delayed fracture resistance and a method for producing the same. More specifically, the present invention relates to a high-strength steel sheet that is mainly suitable for strength members for automobiles and building materials ^{and has a tensile strength of 1180 MPa (about 120 kgf / mm 2) or more, which is required to have delayed fracture resistance, and a method for manufacturing the same.} It is a thing.

Conventionally, cold-rolled steel sheets have been used as steel sheets for automobiles because of the requirements for accuracy of plate thickness and flatness. Further, in recent years, _{from the viewpoint of reducing CO 2} emissions of automobiles and ensuring collision safety, there is an increasing demand for reducing the thickness and increasing the strength of the vehicle body itself by increasing the strength of the vehicle body material. Therefore, the application of high-strength steel sheets to automobiles is being promoted.

On the other hand, it is known that a phenomenon called delayed fracture is likely to occur as one of the problems associated with increasing the strength of the steel sheet. Delayed fracture is a phenomenon in which a high-strength steel plate suddenly undergoes brittle fracture after a certain period of time with almost no plastic deformation in a state where a static load equal to or less than the tensile strength is applied. This phenomenon is particularly remarkable in high-strength steel having a tensile strength of 1180 MPa or more.

In the case of steel sheets for automobiles, it is known that the above-mentioned delayed fracture is caused by residual stress generated at a joint portion of a steel plate molded product pressed into a predetermined shape and hydrogen embrittlement of the steel at such a stress concentration portion. There is. In most cases, the hydrogen that causes this hydrogen embrittlement is thought to be that hydrogen generated on the surface of the steel sheet due to corrosion of the steel sheet penetrates into the steel and diffuses inside the steel sheet.

As a technique for preventing delayed fracture in such a high-strength steel sheet, Patent Document 1 discloses a technique for suppressing the occurrence of delayed fracture by adjusting the structure and composition of the steel sheet and weakening the hydrogen sensitivity of the steel sheet. There is. However, when such a method is used, although it is possible to delay the occurrence of delayed fracture, the amount of hydrogen that penetrates into the steel sheet from the external environment does not change, so it is not possible to prevent delayed fracture itself. Can not. Therefore, in order to essentially suppress delayed fracture, it is necessary to reduce the amount of hydrogen invading the inside of the steel sheet itself. From this point of view, Patent Document 2 discloses a technique of reducing the amount of hydrogen invading into the steel sheet and suppressing delayed fracture by plating the cold-rolled steel sheet with Ni or Ni-based alloy.

Japanese Unexamined Patent Publication No. 2004-231992 Japanese Unexamined Patent Publication No. 7-54194 Japanese Unexamined Patent Publication No. 2006-63364

However, as described in Patent Document 2, when Ni or a Ni-based alloy is electroplated, hydrogen generated on the surface of the steel sheet, which is the cathode during plating, may remain in the steel sheet and cause delayed fracture. Further, if the surface of the steel sheet is still plated and subjected to press working, there is a concern that the plating layer may be damaged during the processing and the desired effect may not be obtained in the exposed portion of the base steel sheet.

The present invention has been made in view of such circumstances, and is a high-strength steel sheet which is mainly suitable for strength members for automobiles and building materials, has a tensile strength of 1180 MPa or more, and has excellent delayed fracture resistance, and a high-strength steel sheet thereof. It is an object of the present invention to provide a manufacturing method.

The present inventors have conducted extensive studies and research in order to solve the above problems. As a result, it was found that by forming a film containing a conductive polymer as a main component on the surface of the steel sheet, the amount of hydrogen invading the steel sheet can be significantly suppressed, and the delayed fracture of the steel sheet can be effectively suppressed. Further, the base steel sheet before film formation is preferably pickled in advance in a strong oxidizing solution to passivate the surface of the base steel sheet and adjust the surface roughness to an appropriate range to efficiently apply the film. In addition, it was found that the film adhesion can be stabilized.

The present invention is based on the above findings, and its features are as follows.
[1] Pickling treatment of a steel sheet having a tensile strength of 1180 MPa or more under the condition that the arithmetic average roughness Ra of the surface of the steel sheet after pickling is 0.5 μm or more and 5.0 μm or less. A method for producing a high-strength steel sheet having excellent delayed fracture resistance, which comprises a step and a film applying step of forming a film composed of a conductive polymer and an anion on the surface of the steel sheet after pickling.
[2] The film-imparting step is excellent in delayed fracture resistance according to the above [1], which comprises electrolyzing a steel plate as an anode in a bath in which a conductive polymer monomer and an electrolyte are dissolved. A method for manufacturing high-strength steel sheets.
[3] The conductive polymer monomer is mono-substituted pyrrole or mono-substituted aniline, and the mono-substituted pyrrole is substituted in which the substituent constant (σ value) of Hammett's rule is negative at the 1-position of the 5-membered ring. The group or the 3-position has a substituent having a positive Hammett equation σ value, and the monosubstituted aniline has a substituent having a Hammett equation σ value negative at the ortho position of the 6-membered ring. The method for producing a high-strength steel plate having excellent delayed fracture resistance according to the above [2].
[4] The conductive polymer monomer is monosubstituted pyrrole or monosubstituted aniline, and the monosubstituted pyrrole has a methyl group, an ethyl group, a propyl group, or a methoxy group at the 1-position of a 5-membered ring. Alternatively, the monosubstituted aniline has any of an acetyl group, a nitro group, a fluoro group, and a chloro group at the 3-position, and the monosubstituted aniline is any of a methyl group, an ethyl group, a propyl group, and a methoxy group at the ortho position of the 6-membered ring. The method for producing a high-strength steel plate having excellent delayed fracture resistance according to the above [2] or [3].
[5] The high strength excellent in delayed fracture resistance according to any one of the above [1] to [4], wherein the pickling bath in the pickling treatment step is composed of nitric acid and / or hydrofluoric acid. Method of manufacturing steel plate.
[6] The above [1] to [5], wherein the steel sheet is provided with a pre-plating step of plating at least one of Ni, Cu, and Sn after the pickling treatment step and before the film applying step. ], A method for producing a high-strength steel sheet having excellent delayed fracture resistance.
[7] A steel sheet having a tensile strength of 1180 MPa or more, having a film composed of a conductive polymer and an anion on at least one side, and having an arithmetic average roughness Ra of 0.5 μm or more and 5.0 μm or less after film peeling. A high-strength steel sheet with excellent delayed fracture resistance.
[8] The conductive polymer has mono-substituted pyrrole or mono-substituted aniline as a unit structure, and the mono-substituted pyrrole has a Hammett law substituent constant (σ value) at the 1-position of the 5-membered ring. The monosubstituted aniline has a negative substituent or a substituent having a positive Hammett equation σ value at the 3-position, and the monosubstituted aniline has a negative Hammett equation σ value at the ortho position of the 6-membered ring. The high-strength steel plate having an excellent delayed fracture resistance according to the above [7], which has a group.
[9] The conductive polymer has a monosubstituted pyrrole or a monosubstituted aniline as a unit structure, and the monosubstituted pyrrole has a methyl group, an ethyl group, a propyl group and a methoxy group at the 1-position of the 5-membered ring. Any of the above, or any of an acetyl group, a nitro group, a fluoro group, and a chloro group at the 3-position, the monosubstituted aniline has a methyl group, an ethyl group, and a propyl group at the ortho position of the 6-membered ring. The high-strength steel plate having excellent delayed fracture resistance according to the above [7] or [8], which has any of methoxy groups.
[10] The high-strength steel sheet having excellent delayed fracture resistance according to any one of [7] to [9] above, wherein the film thickness is 0.5 μm or more and 20 μm or less.
[11] The high-strength steel sheet having excellent delayed fracture resistance according to any one of the above [7] to [10], wherein the film satisfies the following formula.
{Film thickness (μm)}> {Arithmetic mean roughness Ra (μm) of steel sheet after film peeling}
[12] The resistance according to any one of the above [7] to [11], wherein the steel sheet has Ni, Cu or Sn in an adhesion amount of 10 mg / m ² or more and 2000 mg / m ^{2 or less.} High-strength steel sheet with excellent delayed fracture characteristics.

In the present invention, the high-strength steel sheet is a steel sheet having a tensile strength TS of 1180 MPa or more. Further, the high-strength steel sheet of the present invention includes both a cold-rolled steel sheet and a hot-rolled steel sheet.

According to the present invention, a high-strength steel sheet having a tensile strength (hereinafter, also referred to as TS) of 1180 MPa or more and excellent in delayed fracture resistance can be obtained.

FIG. 1 is a diagram showing the relationship between the energization time and the film thickness. FIG. 2 is a diagram schematically showing a test piece for evaluation of delayed fracture. FIG. 3 is an explanatory diagram showing the process of the composite cycle corrosion test.

Hereinafter, the present invention will be specifically described. In the following description, the unit of the content of each element in the steel component composition is "mass%", and hereinafter, it is simply indicated by "%" unless otherwise specified.

In the method for producing a high-strength steel sheet having excellent delayed fracture resistance of the present invention, the steel sheet is pickled under the condition that the arithmetic average roughness Ra of the surface of the steel sheet after pickling is 0.5 μm or more and 5.0 μm or less. It is characterized by having a pickling treatment step to be applied and a film applying step of forming a film containing a conductive polymer as a main component (hereinafter, referred to as a conductive polymer film) on the surface of the steel sheet after the pickling treatment. ..

The conductive polymer used in the present invention is a polymer obtained by a polymerization reaction of an organic molecule (monomer) having a π-electron conjugated system, and typical examples thereof include polyacetylene, polyaniline, polypyrrole, and polythiophene. Can be mentioned. Such a conductive polymer exhibits semiconductor-metal level conductivity by doping an anion and putting it in a partially oxidized state. When a conductive polymer is applied onto a steel sheet, it has a redox potential that is noble than that of the base steel sheet (hereinafter referred to as the base steel sheet). To form. This passivation film suppresses corrosion of the base steel sheet by blocking corrosion factors such as chloride ions adhering to the surface. Further, it is known that the conductive polymer film exhibits anticorrosion ability of the base steel sheet even in a damaged part such as a pit. At the film interface at the damaged portion, a redox reaction occurs between the conductive polymer and the base steel sheet as described above, and a passivation film is formed on the base steel sheet. On the other hand, the reduced conductive polymer returns to its original state by reducing the dissolved oxygen in the water film. As described above, the conductive polymer film contributes to corrosion protection by passivating the base steel sheet based on the reversible redox reaction as a whole even in the damaged portion of the film. Therefore, in the high-strength steel sheet to which the conductive polymer is applied, the amount of hydrogen invading into the base steel sheet is reduced even in the processed portion which is easily damaged by the film, and excellent delayed fracture resistance is exhibited.

The conductive polymer film as described above can be directly deposited on the surface of the base steel sheet by electrolytic polymerization in the monomer solution. During this electrolytic polymerization, the base steel sheet is an anode and hydrogen is not generated on the surface, so that hydrogen does not enter the inside of the steel sheet, which is a concern when performing electro-Ni plating. In this respect as well, it can be said to be useful from the viewpoint of preventing delayed fracture.

On the other hand, such a conductive polymer film has a drawback that it is inferior in adhesion to a base steel sheet. If the film adhesion is inferior, when the steel sheet is processed by a press or the like, not only the film may be scratched but also a large area may be peeled off. When the base steel sheet is exposed in a large area, there may be a region where the corrosion protection ability of the above-mentioned conductive polymer does not reach.

As a factor of inferior adhesion of the conductive polymer film to the base steel sheet, it is considered that there are few functional groups capable of hydrogen bonding in the monomer molecule. However, when a large number of substituents are introduced into the monomer molecule in order to increase the number of hydrogen bond sites, not only the raw material cost increases, but also the influence of the increase in molecular weight prevails and the water solubility is impaired. There is also concern that it will become unusable.

In response to the problem of inferior adhesion, Patent Document 3 states that a silane coupling agent having a hydrogen bond and a large number of functional groups is present at the interface between the organic resin film and the base steel to form a conductive polymer film. The adhesion to the base steel plate is improved. However, in the method described in Patent Document 3, not only is it difficult to apply a film to the steel sheet by electrolytic polymerization, but also due to the presence of an interface layer made of a silane coupling agent, the conductive polymer film is in direct contact with the base steel sheet. Therefore, it may not show anticorrosion ability in the damaged part of the film.

On the other hand, in the present invention, the base steel sheet is preferably pickled in a strongly oxidizing solution before the film is applied to passivate the surface of the base steel sheet and to determine the arithmetic mean roughness Ra. Adjusting to the range. Thereby, for example, a conductive polymer film can be efficiently formed by an electric field polymerization method. In addition, the adhesion of the conductive polymer film is remarkably stabilized. It is presumed that this is because the surface of the base steel sheet has minute irregularities, so that an anchor effect occurs at the interface between the conductive polymer film and the base steel sheet, and as a result, the adhesion is improved. Since the conductive polymer film is in direct contact with the base steel sheet, it is excellent in that the adhesion can be stabilized without impairing the anticorrosion ability at the damaged portion of the film.

Next, a steel sheet that serves as a base for a high-strength steel sheet having excellent delayed fracture resistance according to the present invention will be described. The base steel sheet of the present invention is a steel sheet having a tensile strength of 1180 MPa or more. The effect of the present invention is exhibited even when a steel sheet having a tensile strength lower than 1180 MPa is used as a base steel sheet, but the effect of the invention is remarkable because a steel sheet having a tensile strength lower than 1180 MPa is essentially less likely to undergo delayed fracture. Not expressed in. As long as the tensile strength is 1180 MPa or more, the chemical composition and steel structure of the base steel sheet are not particularly limited, and the rolling method and the like are not particularly limited. The thickness of the steel plate is not particularly limited, but is preferably about 0.5 to 2.5 mm.

Next, a method for producing a high-strength steel sheet having excellent delayed fracture resistance of the present invention will be described.
A hot-rolled steel sheet or a cold-rolled steel sheet manufactured by a normal method is pickled under the condition that the arithmetic average roughness Ra of the steel sheet surface after pickling is 0.5 μm or more and 5.0 μm or less. Give. Next, a film containing a conductive polymer as a main component is formed.
The pickling treatment step and the subsequent film applying step may be performed by continuous equipment or by separate equipment. Hereinafter, a detailed description will be given.

Pickling treatment step Pickling treatment is performed for the purpose of adjusting the roughness of the surface of the base steel sheet to an appropriate range and in Passivating the surface of the steel sheet. The pickling treatment step is performed under the condition that the arithmetic average roughness Ra of the surface of the base steel sheet after pickling is 0.5 μm or more and 5.0 μm or less. When the Ra of the base steel sheet after pickling is less than 0.5 μm, the adhesion of the conductive polymer film formed after the subsequent film applying step becomes unstable, and the film is likely to peel off during processing. There is a possibility that certain delayed fracture resistance characteristics cannot be sufficiently obtained. On the other hand, even if Ra exceeds 5.0 μm, not only the effect of further improving the adhesion cannot be obtained, but also the surface roughness after applying the film may become large and the die may be easily damaged during press working. is there. From the above, the arithmetic mean roughness Ra of the base steel sheet after pickling is set to 0.5 μm or more and 5.0 μm or less. Preferably, it is 1.0 μm or more and 2.0 μm or less.

When Ra exceeds 2.0 μm, it is difficult to apply it to the steel sheet by a process such as temper rolling. However, in the present invention, it is possible to impart a high roughness exceeding 2.0 μm with the dissolution of Fe by immersing the base steel sheet in a strongly oxidizing solution in the pickling treatment step.

The pickling bath in the pickling treatment step is preferably composed of nitric acid and / or hydrofluoric acid. For example, a strongly oxidizing solution containing nitric acid ions such as 10 to 30% by mass of nitric acid and nitric acid (hydrofluoric acid: about 1% by mass, nitric acid: 30% by mass, mixed aqueous solution) can be mentioned. When such a strongly oxidizing solution is used as a pickling bath, Ra of the base steel sheet is adjusted to a predetermined range, and is sufficiently passivated in a short time, and the dissolution reaction of Fe is suppressed in the subsequent film forming step. The film can be deposited immediately after energization. Economically advantageous.

When the pickling treatment is performed, it is preferable that the temperature of the pickling liquid is 20 to 30 ° C. and the pickling time is 30 to 300 seconds.

Film application step In the film application process, a film made of a conductive polymer is formed on the surface of the steel sheet after the pickling treatment. The term "conducting polymer" means that, for example, in a bath in which an electrolyte containing a conductive polymer monomer and an anion acting as a dopant (hereinafter, may be referred to as a dopant anion) is dissolved, a steel plate is used as an anode for electricity. When the film is applied to the surface of the steel plate by decomposition, the total mass ratio of the conductive polymer and the anion serving as the dopant is 50% or more of the entire film. From the viewpoint of ease of control and required time, for example, in a solution containing a conductive polymer monomer and a salt consisting of a dopant anion as an electrolyte (in a bath), electrolysis using a base steel plate as an anode (hereinafter abbreviated as electrolysis). It is preferable to do so. Electrolysis is preferably carried out by a constant current method.

Since the anodic dissolution reaction of Fe occurs preferentially over the oxidation reaction of the conductive polymer monomer on the surface of the base steel sheet which is the anode, it is difficult to efficiently form the conductive polymer film on the surface of the steel sheet. Therefore, in the present invention, as described above, by performing a pickling treatment in advance to passivate the surface of the base steel sheet, the anodic dissolution reaction of Fe in the subsequent film applying step is suppressed.

Examples of the conductive polymer that is the main component of the film include polypyrrole, polyaniline, polythiophene, polyfuran, polyparaphenylene sulfide, polyparaphenylene oxide, polyparaphenylene vinylene, polyacetylene, polyselenophene, and derivatives thereof. .. Among these, polypyrrole, polyaniline, and polyethylenedioxythiophene have high electrical conductivity and high passivation ability at the interface of the base steel sheet, and thus exhibit excellent anticorrosion ability. Since it is preferable to apply a film by electrolytic polymerization using a base steel plate as an anode in an aqueous solution, it is preferable to use monomers of polypyrrole, polyaniline and derivatives thereof having relatively high water solubility.

As a conductive polymer monomer, a substituted pyrrole (pyrrole derivative) or a monosubstituted aniline (aniline derivative) in which a substituent having a predetermined value (positive or negative) of the substituent constant σ of Hammett's law is introduced at an appropriate position ) Can be used to increase the electrode activity during electrolytic polymerization and further improve the delayed fracture resistance of the polymerized film.
The substituent constant σ is a unique value depending on the electronic effect of the substituent, and differs depending on the type of the substituent and the substitution mode such as ortho, meta, and para. It has been experienced that the substituent constant σ and the activation free energy change ΔΔG ^key of an ^{aromatic compound due to the introduction of a substituent are connected by a linear relationship of ΔΔG key} = -2.3 RTρσ (ρ is a constant depending on the nature of the reaction). If σ is positive, it shows electron attraction to aromatic conjugated systems, and if σ is negative, it shows electron donation. Therefore, it is possible to improve the electrochemical properties of the monomer during electrolytic polymerization and the film after polymerization by introducing an appropriate substituent modification.
In mono-substituted pyrroles, a substituent (electron donating group) in which the Hammett's substituent constant (σ value) is negative at the 1-position of the 5-membered ring, or a positive Hammett's σ value at the 3-position. When it has a substituent (electron-attracting group), the electrode activity is higher than that in the non-substituted case. Further, since the oxidation-reduction potential in the film after polymerization becomes more noble, the anodic reaction of the steel sheet is further suppressed and the amount of hydrogen invading into the steel is reduced, so that the delayed fracture resistance is improved.
In mono-substituted aniline, the same effect as described above can be obtained by introducing a substituent (electron donating group) having a negative Hammett equation σ value at the ortho position of the 6-membered ring.
On the other hand, when the substituent is bulky and the steric hindrance is large, the degree of freedom of structural change of the monomer and the polymerized film at the time of polymerization is reduced, and the improvement of the electrochemical properties as described above may be small. Therefore, sterically small substituents such as a methyl group, an ethyl group, and an acetyl group are more preferable regardless of the electric donating property and the electron attracting property.
From the above, as the conductive polymer monomer used in the present invention, a substituent having a negative Hammett law substituent constant (σ value) at the 1-position of the 5-membered ring, or a Hammett law σ at the 3-position. Substituent monosubstituted pyrroles having a positive value or monosubstituted aniline having a substituent having a negative Hammett equation σ value at the ortho position of the 6-membered ring are preferable. More preferably, a monosubstituted product having a methyl group, an ethyl group, a propyl group, or a methoxy group at the 1-position of the 5-membered ring, or an acetyl group, a nitro group, a fluoro group, or a chloro group at the 3-position. Pyrrole or a monosubstituted aniline having any of a methyl group, an ethyl group, a propyl group, and a methoxy group at the ortho position of a 6-membered ring.

Next, the anion contained in the conductive polymer film in the present invention will be described. The conductive polymer is a semiconductor in the absence of a dopant, and in order to exhibit high conductivity, it is necessary to partially oxidize it by adding a dopant to generate holes. Therefore, in the present invention, when the film is applied by the above-mentioned electrolytic polymerization using the base steel plate as an anode, an electrolyte as an anion raw material acting as a dopant is included in the bath. As the dopant, for example, a halide ion, an oxo acid ion, a Lewis acid ion and the like can be used. In the film applying step, the conductive polymer film in the doped state can be precipitated by including the salt composed of the anion as described above in the conductive polymer monomer solution as an electrolyte. Specific examples of such salts include halide salts such _{as NaCl, KCl, NH 4} Cl, NaBr, KBr, NH ₄ Br, NaI, KI, NH _{4 I, MoCl 5} , BF ₄ , BCl ₃ , PF _{5 and the like.} Metal halide salts such as Na ₂ SO ₄ , K ₂ SO _4, NaNO ₃ , Na (CH ₃ COO), KK (CH ₃ COO) and other oxolates, H ₃ Mo ₇ O ₂₄ , H ₃ Mo ₈ Examples thereof include polyoxometallic acids such as O ₂₆ and H ₃ PW ₁₂ O _40.

Pre-plating step It is preferable to have a pre-plating step of plating at least one of Ni, Cu, and Sn on the steel sheet after the pickling treatment step and before the film applying step. The surface of the base steel sheet after pickling is adjusted to a predetermined Ra and is in a passivated state, but the portion that is not locally passivated due to the non-uniformity of the surface of the base steel sheet before the pickling treatment step. May occur. In such a non-passivated portion, the anodic dissolution reaction of Fe preferentially occurs in the subsequent film applying step, and the conductive polymer film may not be formed, which may cause a film defect. For the purpose of suppressing the occurrence of such film defects, at least one of Ni, Cu, and Sn having a standard oxidation-reduction potential higher than Fe, if necessary, with respect to the base steel sheet after the pickling treatment and before the film applying step. It is preferable to perform plating (pre-plating) treatment on the copper. In this pre-plating treatment, after the pickling treatment step, a metal plating layer having an oxidation-reduction potential noble than Fe can be preferentially formed on a locally non-passivated portion of the base steel sheet. As a result, the anodic dissolution reaction of Fe is less likely to occur in the film applying step, so that the occurrence of film defects is further suppressed.

The metal used for the pre-plating treatment needs to have an oxidation-reduction potential nobler than that of Fe, and Ni, Cu, and Sn can be preferably used. Other metals having a potential higher than Fe include metals such as Ag, Pt, and Au, which are not preferable because the cost increases significantly when used as a plating component.

As a method of pre-plating treatment, an electrolytic plating method can be mentioned. Since Ni, Cu and Sn are all metals having a redox potential noble than Fe, an electroless plating method can also be used. For example, by immersing the base steel sheet in a solution having a composition of divalent Sn ion 30 g / L, phenol sulfonic acid 5 g / L (sulfuric acid equivalent), and sulfuric acid 60 g / L, Sn plating with an adhesion amount according to the immersion time. Can be formed. Further, by electrolyzing the base steel plate as a cathode, the plating layer can be formed with higher efficiency.

As described above, a high-strength steel sheet having excellent delayed fracture resistance of the present invention can be obtained. The high-strength steel sheet having excellent fracture characteristics of the present invention preferably has a dopant anion on at least one side of the steel sheet, has a film containing a conductive polymer as a main component, and has an arithmetic mean roughness after film peeling. Ra is 0.5 or more and 5.0 μm or less. The film thickness is preferably 0.5 μm or more and 20 μm or less. Further, it is preferable that the film satisfies {film thickness (μm)}> {arithmetic mean roughness Ra (μm) of the steel sheet after the film is peeled off}. Further, the steel sheet preferably has Ni, Cu or Sn in an adhesion amount of 10 mg / m ² or more and 2000 mg / m ² or less.

The tensile strength (TS) of the steel sheet shall be 1180 MPa or more. As described above, a steel sheet having a TS of less than 1180 MPa is essentially less likely to undergo delayed fracture, the effect of the present invention is less likely to be obtained, and only cost increase is caused, which is economically disadvantageous. Therefore, the steel plate used in the present invention is TS1180 MPa or more.

Ra after peeling the film is 0.5 μm or more and 5.0 μm or less. As described above, in the present invention, the adhesion of the film (conductive polymer film) is improved by imparting fine irregularities to the surface of the base steel sheet. When the arithmetic mean roughness Ra after peeling the film is less than 0.5 μm, the adhesion to the film is not sufficient, the film is likely to peel off during processing, and the delayed fracture resistance, which is the object of the present invention, is sufficient. It may not be obtained. Further, when Ra of the base steel sheet exceeds 5.0 μm, not only the adhesion cannot be further improved, but also Ra in the state where the film is not peeled off becomes large, and the die may be easily damaged during press working. There is sex. Therefore, Ra after peeling the film is set to 0.5 μm or more and 5.0 μm or less. Preferably, it is 1.0 μm or more and 2.0 μm or less. In measuring Ra, the method for peeling the film is not particularly limited, but for example, in the case of a film containing polyaniline as a main component, a method of dissolving in heated toluene can be used.

The conductive polymer, which is the main component of the film, has mono-substituted pyrrole or mono-substituted aniline as a unit structure, and in the case of mono-substituted pyrrole, the substituent constant (σ) of Hammett's law is at the 1-position of the 5-membered ring. In the case of a monosubstituted aniline having a substituent having a negative value) or a substituent having a positive Hammett equation σ value at the 3-position, the Hammett equation σ value is at the ortho position of the 6-membered ring. It preferably has a negative substituent. As described above, in the case of pyrrole, an electron-donating group (σ is negative) at the 1-position, or an electron-attracting group (σ is positive) at the 3-position, and in the case of aniline, an electron-donating group (σ is positive). When σ has a negative value), the potential after the film is applied becomes more noble and the delayed fracture resistance is improved as compared with the case of no substitution. Since this effect is more remarkable as the steric hindrance of the substituent on the structural change of the film is smaller, the electron donating group is any of a methyl group, an ethyl group, a propyl group and a methoxy group, and the electron attracting group is It is more preferably any of an acetyl group, a nitro group, a fluoro group and a chloro group.

The film thickness is preferably 0.5 μm or more and 20 μm or less. In the present invention, the average value of the film thickness measured in 10 visual fields observed by an arbitrary method in the cross section of the film in the plate thickness direction is defined as the film thickness. The method for observing the film is not particularly limited as long as an image that can recognize the interface between the film surface and the base steel plate can be obtained, but considering the observation on a scale of 0.5 to 20 μm, which is the film thickness range, a scanning type is used. It is preferable to use an electron microscope (SEM). When the film thickness is less than 0.5 μm, it is difficult to uniformly coat the surface of the base steel sheet, the film is damaged or peeled off in the processed portion, and the delayed fracture resistance, which is the object of the present invention, cannot be sufficiently obtained. There is. On the other hand, even if the film thickness exceeds 20 μm, the delayed fracture resistance cannot be further improved, which is economically disadvantageous due to the increase in cost, which is not preferable. Therefore, the film thickness is set to 0.5 μm or more and 20 μm or less. It is preferably 1.0 μm or more and 10.0 μm or less.

It is preferable that the film satisfies {film thickness (μm)}> {arithmetic mean roughness Ra (μm) of the steel sheet after the film is peeled off}. The above-mentioned effect of improving adhesion is remarkable when the film thickness (μm) is larger than the arithmetic average roughness Ra (μm) of the base steel sheet.

The steel sheet preferably has Ni, Cu or Sn in an adhesion amount of 10 mg / m ² or more and 2000 mg / m ² or less. It is preferable to have Ni, Cu, and Sn at the interface between the film and the base steel sheet. By having any of Ni, Cu, and Sn at the interface between the film and the base steel sheet, it is possible to cover and protect the minute defect parts of the passive film generated in the pickling process, and the surface potential is noble even after the film is applied. As a result, the amount of hydrogen generated due to corrosion is reduced, and the delayed fracture resistance is further improved. In order to obtain such an effect, 10 mg / m ² or more is required. On the other hand, if ^{the adhesion amount exceeds 2000 mg / m 2,} no further effect can be obtained and only the cost increases. Therefore, the adhesion amount is 10 mg / m ² or more and 2000 mg / m ² or less, more preferably 50 mg / m ² or more and 1000 mg / m ^2. It is as follows.

Hereinafter, the present invention will be specifically described with reference to Examples.
It has a component composition of C: 0.191%, Si: 0.4%, Mn: 1.56%, P: 0.011%, S: 0.001%, and the balance is Fe and unavoidable impurities. A cold-rolled steel sheet having a TS: 1470 MPa and a plate thickness: 1.6 mm was used as a base steel sheet, and a conductive polymer film was formed by anodic electrolysis. However, the base steel plate that can be used is not limited to the components and the plate thickness in this embodiment.

After ultrasonically degreasing the surface of the base steel sheet with a mixed solution of toluene and ethanol, the base steel sheet is immersed in a 14 mass% nitric acid solution at a bath temperature of 30 ° C. for 240 seconds (with pickling treatment) and not (pickled). For each of the above (without pickling treatment), anode electrolysis was performed at a current density of ^{+ 15 mA / cm 2} in an aqueous solution having a composition of 0.5 mol / L of pyrrol and 0.1 mol / L of sodium sulfate. Next, the film thickness of the obtained steel sheet film was determined as follows. A test piece cut out to a size of 15 mm × 18 mm was embedded in a resin and mechanically polished to obtain a test piece for cross-sectional observation. 10 fields of view were observed from different positions using a scanning electron microscope SEM, and the film thickness of the film measured in each field of view was observed. The value obtained by averaging the thicknesses was used as the film thickness (μm).

From the film thickness and energization time of the film obtained as described above, the film formation rates with and without the pickling treatment were compared. The results are shown in Table 1 and FIG.

From Table 1 and FIG. 1, it can be seen that the passivation treatment of the base steel sheet in the pickling treatment step is effective for improving the efficiency of film formation in the subsequent film applying step.

The base steel plate shown in Example 1 was used as a test material. The surface of the base steel sheet was ultrasonically degreased with a mixed solution of toluene and ethanol, and then immersed in the oxidizing solution shown in Table 2 for 0 to 300 seconds. Next, some of the base steel sheets were pre-plated with any of Ni, Cu, and Sn. A Ni sulfate bath was used for Ni plating, a copper pyrophosphate bath was used for Cu plating, and a phenol sulfonic acid bath was used for Sn plating. Then, in an aqueous solution in which the electrolytes and monomers shown in Table 2 were dissolved so as to have concentrations of 0.1 mol / L and 0.5 mol / L, respectively, the base steel plate was used as an anode and the stainless steel plate was used as a cathode to determine +15 mA / cm ² . By performing current electrolysis, steel sheets of the invention example and the comparative example were obtained. As the film thickness of the obtained steel sheet, the average value of the film thickness of the film measured from 10 fields of view using SEM was used as in Example 1. The amount of pre-plating adhered was determined by ICP mass analysis after dissolving the film in a 2-propanol thermal solution of potassium hydroxide in any of Ni, Cu, and Sn.

For each of the steel sheets obtained as described above, the adhesion of the film and the delayed fracture resistance were evaluated by the following methods. The arithmetic mean roughness Ra was measured using a stylus type surface roughness meter in accordance with JIS B 0601. Similar characteristics were evaluated for the steel sheets that do not form a film (Comparative Examples 1, 2 and 3) and the steel sheets that are not immersed in the oxidizing solution (Comparative Examples 4 and 5).

(1) Adhesion The obtained steel plates were sheared to a width of 35 mm and a length of 100 mm, and ground to a width of 30 mm to prepare test pieces. This test piece was bent using a 3-point bending tester. In this bending process, the bending curvature was set to 5R. The film on the inner part of the bend after bending is cut in a grid pattern with an area of 35 mm in width x 20 mm in length by a method conforming to JIS K5600, and a tape peeling test is performed to determine the peeling area ratio by the following formula.
Film peeling area ratio = {(coating area attached to tape after peeling) / (coating area before peeling: 35 mm x 20 mm)} x 100
The processability of this film peeling area ratio was evaluated according to the following criteria. In this evaluation, ○ and Δ were considered good.
〇: Film peeling area ratio less than 5% Δ: Film peeling area ratio 5% or more, less than 20% ×: Film peeling area ratio 20% or more (2) Delayed fracture resistance Specimen subjected to grinding in the same manner as above The test piece shown in FIG. 2 was bent into a V shape with a bending radius of 5.0 mm, restrained by jigs 2 to 5, and the test piece shape was fixed to obtain a test piece for delayed fracture evaluation. .. The wet and dry repeated test of the cycle shown in FIG. 3 was carried out on the test piece for delayed fracture evaluation thus prepared, and the test piece was carried out up to 28 cycles. At the start of each cycle, the number of crack cycles was determined by visually inspecting the presence or absence of cracks. In this test, three samples were evaluated for each of the steel sheets of each invention example and comparative example, and the delayed fracture resistance was evaluated according to the following criteria using the average value of the number of cracking cycles. As shown in Table 2, in the case of the comparative example in which the film was not applied, the number of cycles was 10, so ◯ and Δ, which were 15 cycles or more, were considered to be good. A cracking cycle number of 29 or more in Table 1 indicates that cracking did not occur during the test period in this example.
〇: 20 cycles or more Δ: 15 cycles or more and less than 20 cycles ×: less than 15 cycles The results obtained by the above are shown in Table 2 together with the conditions, the film thickness of the film, and the arithmetic mean roughness Ra after the film peeling.

Inventive Examples 1 to 25 having the film specified in the present invention and having the arithmetic mean roughness Ra of the surface of the steel sheet after the film peeling are within the range of the present invention are all excellent in film adhesion after processing. Furthermore, the delayed fracture resistance was a good result.
On the other hand, in Comparative Examples 2 and 3 having no film, in which only the roughness was imparted to the base steel sheet by the oxidizing solution, a significant difference in the delayed fracture resistance was observed as compared with Comparative Example 1 in which the steel sheet remained. There wasn't. Further, in Comparative Examples 4 and 5 in which only the film was applied without the pickling treatment, the film adhesion and the delayed fracture resistance were inferior.
Further, in Invention Examples 1, 2, 5, 7, 10-14 and 16 to 21, the equation {film thickness (μm)}> {arithmetic mean roughness Ra (μm) of the steel sheet after film peeling} is satisfied. The film adhesion was a particularly excellent result.
In addition, Invention Examples 4 to 8 and 14 subjected to any one of Ni, Cu, and Sn pre-plating treatments are more resistant than Invention Examples 3 and 13 which are not pre-plated and have the same other conditions. The delayed fracture characteristics were even more predominant.
Further, in the present invention, Invention Examples 22 to 25 using mono-substituted pyrrole or mono-substituted aniline satisfying a more preferable substitution mode as the conductive polymer monomer have almost the same other conditions and are unsubstituted conductive polymers. Compared with the case of using the monomer, the delayed fracture resistance was further superior.

The high-strength steel plate of the present invention is suitable for strength members for automobiles and building materials because it maintains excellent delayed fracture resistance even after molding by press forming, bending, or the like.

Claims (11)

A pickling treatment step of pickling a steel sheet having a tensile strength of 1180 MPa or more under the condition that the arithmetic mean roughness Ra of the surface of the steel sheet after pickling is 0.5 μm or more and 5.0 μm or less.
Conductive polymer and anionic Tona is, the film thickness is excellent in delayed fracture resistance, characterized in that it comprises a coating applied to form a 20μm or less of the film than 0.5μm on the surface of the steel sheet after pickling process A method for manufacturing high-strength steel sheets. The high-strength steel plate having excellent delayed fracture resistance according to claim 1, wherein the film applying step electrolyzes the steel plate as an anode in a bath in which a conductive polymer monomer and an electrolyte are dissolved. Production method. The conductive polymer monomer is mono-substituted pyrrole or mono-substituted aniline.
The monosubstituted pyrrole has a substituent having a negative Hammett's substituent constant (σ value) at the 1-position of the 5-membered ring, or a substituent having a positive Hammett's σ value at the 3-position. And
The method for producing a high-strength steel plate having excellent delayed fracture resistance according to claim 2, wherein the monosubstituted aniline has a substituent having a negative Hammett equation σ value at the ortho position of the 6-membered ring. .. The conductive polymer monomer is mono-substituted pyrrole or mono-substituted aniline.
The monosubstituted pyrrole has a methyl group, an ethyl group, a propyl group, or a methoxy group at the 1-position of the 5-membered ring, or an acetyl group, a nitro group, a fluoro group, or a chloro group at the 3-position. Have and
The high degree of delayed fracture resistance according to claim 2 or 3, wherein the monosubstituted aniline has any one of a methyl group, an ethyl group, a propyl group, and a methoxy group at the ortho position of the 6-membered ring. Manufacturing method of strong steel plate. The method for producing a high-strength steel plate having excellent delayed fracture resistance according to any one of claims 1 to 4, wherein the pickling bath in the pickling treatment step is composed of nitric acid and / or nitric acid and / or nitric acid. .. Any one of claims 1 to 5, wherein the steel sheet is provided with a pre-plating step of plating at least one of Ni, Cu, and Sn after the pickling treatment step and before the film applying step. A method for manufacturing a high-strength steel sheet having excellent delayed fracture resistance. Tensile strength is a steel sheet or 1180 MPa, at least on one side, has a coating made of a conductive polymer and an anion, Ri arithmetic average roughness Ra der than 5.0μm or less 0.5μm after decapsulation, high strength steel sheet thickness of the coating is excellent in delayed fracture resistance, characterized in der Rukoto than 20μm below 0.5 [mu] m. The conductive polymer has mono-substituted pyrrole or mono-substituted aniline as a unit structure, and the mono-substituted pyrrole has a negative Hammett law substituent constant (σ value) at the 1-position of the 5-membered ring. It has a substituent or a substituent at the 3-position where the σ value of Hammett's rule is positive.
The high-strength steel plate having excellent delayed fracture resistance according to claim 7, wherein the monosubstituted aniline has a substituent having a negative Hammett equation σ value at the ortho position of the 6-membered ring. The conductive polymer has a monosubstituted pyrrole or a monosubstituted aniline as a unit structure, and the monosubstituted pyrrole has any one of a methyl group, an ethyl group, a propyl group and a methoxy group at the 1-position of the 5-membered ring. , Or has any of an acetyl group, a nitro group, a fluoro group, and a chloro group at the 3-position,
The high retardation characteristic according to claim 7 or 8, wherein the monosubstituted aniline has any one of a methyl group, an ethyl group, a propyl group, and a methoxy group at the ortho position of the 6-membered ring. Strong steel plate. The high-strength steel sheet having excellent delayed fracture resistance according to any one of claims 7 to 9, wherein the film satisfies the following formula.
{Film thickness (μm)}> {Arithmetic mean roughness Ra (μm) of steel sheet after film peeling} The delayed fracture resistance according to any one of claims 7 to 10, wherein the steel sheet has Ni, Cu or Sn in an adhesion amount of 10 mg / m ² or more and 2000 mg / m ^{2 or less.} Excellent high-strength steel sheet.

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