JP7816333B2 - High-strength steel plate with excellent resistance to delayed fracture - Google Patents

Description

The present invention relates to a high-strength steel plate suitable for strength components in automobiles and building materials, which has a tensile strength of 1180 MPa or more and excellent resistance to delayed fracture.

In recent years, steel sheets used in automobiles have been made stronger in order to reduce _CO2 emissions from automobiles and ensure safety, and the use of high-strength steel sheets with a tensile strength of 1180 MPa or more has been promoted. However, it is known that increasing the strength of steel makes it more susceptible to a phenomenon known as delayed fracture. Delayed fracture is a phenomenon in which a high-strength steel material is subjected to a static load stress (a load stress less than the tensile strength) and, after a certain period of time, a sudden brittle fracture occurs without accompanying plastic deformation. Delayed fracture becomes more likely to occur as the strength of the steel material increases, and is particularly pronounced in high-strength steels with a tensile strength of 1180 MPa or more.

It is known that delayed fracture of steel sheets is caused by residual stresses generated when the steel is pressed into a predetermined shape and by hydrogen embrittlement of the steel in areas where stress is concentrated. Non-Patent Document 1 reports that hydrogen embrittlement susceptibility increases with increasing steel strength, and is particularly pronounced in high-strength steels with a tensile strength of 1200 MPa or more, regardless of whether the amount of added alloying elements is increased. Non-Patent Document 2 also reports that hydrogen cracking occurs in high-strength cold-rolled steel sheets with a tensile strength of 1180 MPa. The hydrogen that causes this hydrogen embrittlement is thought to be, in most cases, hydrogen that has penetrated and diffused into the steel from the external environment; typically, hydrogen generated during corrosion of the steel sheet penetrates and diffuses into the steel.

In order to prevent such delayed fracture in high-strength steel sheets, for example, Patent Document 1 studies ways to reduce delayed fracture susceptibility by adjusting the structure and composition of the steel sheet. Furthermore, Patent Document 2 discloses a technology that suppresses delayed fracture by plating the steel sheet with Ni or Ni alloy, thereby reducing hydrogen penetration into the steel sheet. Furthermore, Patent Document 3 discloses a technology that improves delayed fracture resistance by forming a conductive polymer coating on the steel sheet, thereby reducing hydrogen penetration into the steel sheet.

Japanese Patent Application Laid-Open No. 2004-231992 Japanese Patent Application Publication No. 7-54194 JP 2018-44240 A

"Delayed Fracture" by Shinsaku Matsuyama, Nikkan Kogyo Shimbun, 1989 Yuki Taji and four others, "Evaluation Method for Hydrogen Embrittlement Resistance of High-Strength Thin Steel Plates," Iron and Steel, Iron and Steel Institute of Japan, 2009, Vol. 95, No. 12, pp. 887-894

However, the method of Patent Document 1 does not change the amount of hydrogen that penetrates into the steel sheet from the external environment, so while it is possible to delay the occurrence of delayed fracture, it is not effective enough in suppressing delayed fracture. Furthermore, with the method of Patent Document 2, damage to the plating occurs when the steel sheet is processed, and it is possible that hydrogen penetration from this damaged area cannot be reduced. Furthermore, the method of Patent Document 3 has issues with the adhesion between the coating and the steel sheet (hereinafter referred to as "coating adhesion"), and in order to achieve both coating adhesion and delayed fracture resistance, a process of pickling the steel sheet before the coating application process is required.

The object of the present invention is to solve the problems of the prior art described above and to provide a high-strength steel sheet with a tensile strength of 1180 MPa or more suitable for use in strength components for automobiles and building materials, which has excellent delayed fracture resistance and excellent coating adhesion, and which can be manufactured without performing special processes such as pickling before the coating process.

The present inventors have conducted extensive research and studies into means for reducing the amount of hydrogen that penetrates into steel sheets in order to prevent delayed fracture of steel sheets in their operating environments. As a result, they have found that by incorporating specific organic compounds such as azole compounds and pyridine compounds into an organic resin coating formed on the surface of steel sheets, it is possible to suppress hydrogen penetration into the steel sheets and delayed fracture of the steel sheets while ensuring good coating adhesion.
The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[1] A coating (A) is provided on the surface of a steel sheet having a tensile strength of 1180 MPa or more, the coating (A) including at least one organic compound (x) selected from an azole compound, a pyridine compound, a thiocarbonyl compound, a mercapto compound, and a sulfide compound, and an organic resin (y),
The coating (A) is a high-strength steel plate characterized in that the amount [x] of the organic compound (x) deposited is 100 to 2000 mg/m ² , the amount [y] of the organic resin (y) deposited is 200 to 3800 mg/m ² , the ratio [x]/([x]+[y]) of the amount [x] of the organic compound (x) deposited to the total amount [x]+[y] of the organic compound (x) and the organic resin (y) deposited is 0.05 to 0.5, and the coating has a thickness of 0.3 to 4.0 μm.

[2] In the high-strength steel plate according to [1] above, the organic resin (y) is at least one selected from the group consisting of urethane-based resins, epoxy-based resins, acrylic-based resins, and ethylene-based resins.
[3] A surface treatment solution for forming a coating on the surface of a steel plate to suppress delayed fracture of a steel plate having a tensile strength of 1180 MPa or more,
A surface treatment solution comprising an organic compound (x) consisting of at least one compound selected from the group consisting of an azole compound, a pyridine compound, a thiocarbonyl compound, a mercapto compound, and a sulfide compound, and an organic resin (y).

The high-strength steel sheet of the present invention has excellent delayed fracture resistance because hydrogen penetration into the steel sheet is suppressed, and it also has excellent coating adhesion. It can be manufactured without performing special processes such as pickling before the coating process. Therefore, the high-strength steel sheet of the present invention is particularly suitable for strength components in automobiles and building materials, making it possible to reduce the weight of strength components used in the automotive and building material fields.

Schematic drawing of a test piece for evaluating delayed fracture used in the examples. An explanatory diagram showing the steps of the combined cycle corrosion test performed in the examples. Schematic diagram of the apparatus used in the examples to measure hydrogen permeability coefficients.

The steel plate serving as the base material (substrate) of the steel plate of the present invention with excellent delayed fracture resistance is a high-strength steel plate with a tensile strength of 1180 MPa or more, and more preferably a high-strength steel plate with a tensile strength of 1470 MPa or more. Steel plates with low tensile strength are less susceptible to delayed fracture. The effects of the present invention are also exhibited with steel plates with low tensile strength, but are most pronounced with steel plates with a tensile strength of 1180 MPa or more, and even more pronounced with steel plates with a tensile strength of 1470 MPa or more. The chemical composition and steel structure are not particularly limited. However, of these, high-strength steel plates with a tensile strength of 1180 MPa or more, which are used in the automotive and building materials fields, particularly those frequently used in the automotive field, are preferred, and high-strength steel plates with a tensile strength of 1470 MPa or more are even more preferred.

The high-strength steel sheet preferably used in the present invention may have any composition and structure as long as it has the desired tensile strength. Therefore, in order to improve various properties such as mechanical properties, for example, the following structural or structural modifications may be performed alone or in combination.
-Solid solution strengthening by adding interstitial solid solution elements such as C and N, and substitutional solid solution elements such as Si, Mn, P and Cr -Precipitation strengthening by carbonitrides such as Ti, Nb, V and Al -Chemical composition modification such as adding strengthening elements such as W, Zr, Hf, Co, B, Cu and rare earth elements -Toughening by recovery annealing at a temperature where recrystallization does not occur, or partial recrystallization strengthening by leaving unrecrystallized regions without complete recrystallization -Strengthening by transformed structure such as single phase bainite or martensite, or a composite structure of ferrite and these transformed structures -Grain refinement strengthening expressed by the Hall-Petch equation: σ = σ ₀ + kd ^-1/2 (where σ is stress, σ ₀ and k are material constants) where d is the ferrite grain size -Working strengthening by rolling, etc.

Examples of the composition of such high-strength steel plates include C: 0.1 to 0.4 mass%, Si: 0 to 2.5 mass%, Mn: 1 to 3.5 mass%, P: 0 to 0.05 mass%, S: 0 to 0.01 mass%, and the remainder consisting of Fe and unavoidable impurities. Furthermore, Cu: 1.0 mass% or less, Ti: 0.2 mass% or less, V: 0.5 mass% or less, Al: 0.1 mass% or less, Cr: 1.0 mass% or less, Nb: 0.2 mass% or less, W: 0.5 mass% or less, Zr: 0.1 mass% or less, B: 0.005 mass% or less, N: 0.0005 to 0.0100 mass%, Ni: 0.01 to 2.00 mass%, Mo: 0.005 to 2.000 mass%, Ca: 0.0002 to 0.0050 mass%, Mg: 0.0001 to 0.0020%, REM: 0.0002 to 0.0050 mass%, Sb: 0.002 to 0.200 mass%, Sn: 0.002 to 0.200 mass%. It may contain one or more selected from the group consisting of. In general, it is preferable that the total amount of these elements other than C, Si, Mn, P, and S be limited to approximately 4% by mass. The lower limits of the content of each element are preferably 0.01% by mass for Si, Al, and Cr, 0.005% by mass for Cu, Ti, V, Nb, W, and Zr, 0.001% by mass for P, and 0.0001% by mass for B and S.

Furthermore, examples of commercially available high-strength steel sheets include, but are not limited to, JFE-CA1180, JFE-CA1320, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, and JFE-CA1180Y2 (all manufactured by JFE Steel Corporation), and SAFC1180D (manufactured by Nippon Steel & Sumitomo Metal Corporation). Furthermore, the high-strength steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.
The high-strength steel sheet may be a zinc-based plated steel sheet that has been subjected to zinc-based plating such as hot-dip galvanizing or electrogalvanizing. In the case of hot-dip galvanizing, the steel sheet may be subjected to an alloying treatment.
The thickness of the high-strength steel plate that is the substrate is not particularly limited, but is generally about 0.8 to 5 mm, and more preferably about 1.0 to 2.0 mm.

According to the inventors' research and investigation, hydrogen penetration into steel sheets during the corrosion process is believed to be largely due to salt adhering to the steel sheet surface from the environment in atmospheric corrosion environments where drying and wetting are repeated. Examples of salts that are applied to steel sheets in atmospheric environments include salt drifting from seawater and deicing salt applied to road surfaces as an anti-icing agent. This salt absorbs water due to increased humidity or adhesion of road surface water, forming a water film that corrodes the steel sheet. In the water film formed on the steel sheet surface, oxygen reduction and iron oxidation occur. The oxygen reduction reaction generates hydroxide ions, increasing the pH. However, iron ions eluted by the oxidation reaction undergo further hydrolysis, generating iron oxyhydroxide and hydrogen ions, resulting in a decrease in pH, if the chloride ion concentration is high. As humidity decreases, the water film decreases, and as the chloride concentration increases, the hydrolysis of iron ions is accelerated, decreasing the pH and increasing the amount of hydrogen that penetrates into the steel sheet. In other words, to suppress hydrogen penetration, it is important to prevent salt from adhering to the steel sheet, which causes a decrease in pH during the corrosion process.

Therefore, in this invention, an organic resin coating (A) (hereinafter sometimes referred to as "organic resin coating (A)") is formed on the surface of a steel sheet having a tensile strength of 1180 MPa or more, thereby blocking chloride ions that adhere in the atmosphere, and the organic resin coating (A) contains an organic compound (x) consisting of at least one compound selected from azole compounds, pyridine compounds, thiocarbonyl compounds, mercapto compounds, and sulfide compounds, which inhibits corrosion at paint defects, thereby providing the property of inhibiting delayed fracture. Furthermore, by retaining the organic compound (x) in the organic resin coating (A), high coating adhesion is achieved.

Here, it is believed that the organic compound (x) in the coating (A) is adsorbed onto the steel sheet by eluting into the water film formed on the steel sheet surface at the paint defect area. The nitrogen and sulfur contained in the organic compound (x) form polar groups in the organic compound and have unshared electron pairs. This unshared electron pair enters the vacant orbital of the iron atom in the steel sheet, forming a coordinate bond in which both the metal and the polar group have an electron pair, causing the organic compound to adsorb onto the metal surface and form a molecular film, which is believed to act as a barrier layer that blocks chloride ions.
It is thought that in an aqueous film with a high chloride concentration, the pH decreases due to hydrolysis of iron ions, making the aqueous film weakly acidic. Examples of azole compounds, pyridine compounds, thiocarbonyl compounds, mercapto compounds, and sulfide compounds that adsorb to the steel sheet and form a molecular film in this weakly acidic aqueous solution (pH 5 or less) include the following:
Among these, pyridine compounds are particularly preferred because they dissolve into the water film, increasing the pH of the water film and inhibiting corrosion. The reason why pyridine compounds have a corrosion-inhibiting effect is thought to be as follows: Since the lone electron pair on the nitrogen atom contained in the pyridine compound is not delocalized, even if iron ions in the water film are hydrolyzed, the pyridine compound increases the pH, keeping the water film neutral to weakly alkaline. As a result, it is thought that the cathodic reaction that generates hydroxide ions is less likely to occur.

Examples of azole compounds include 2-N,N-diethylthiobenzothiazole, 2-N-mercaptobenzothiazole, pyrazole, 3,5-dimethylpyrazole, 3-methyl-5-pyrazolone, 3-amino-5-methylpyrazole, 1,2,4-triazole, 3-amino-1,2,4-triazole, 3-mercapto-1,2,4-triazole, 5-amino-3-mercapto-1,2,4-triazole, 2,3-dihydro-3-oxo-1,2,4-triazole, 5-amino-2-mercapto-1,3,4-thiadiazole, 2,5-dimercapto-1,3,4-thiadiazole, 5-phenyl-1,2,3,4-tetrazole, 5-mercapto-1-phenyl-1,2,3,4-tetrazole, 1H-benzotriazole, 1,2,3-benzotriazole, and 1-hydroxybenzotriazole (monohydrate).

Examples of the pyridine compound include pyridine, 4-methylpyridine, 4-ethylpyridine, 4-propylpyridine, 4-butylpyridine, 4-pentylpyridine, quinoline, isoquinoline, 4-methoxypyridine, and 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine.
Examples of the thiocarbonyl compound include thiourea, thiosemicarbazide, phenylthiourea, tolylthiourea, N-methylthiourea, dimethylthiourea, diethylthiourea, diphenylthiourea, and 2-mercaptobenzimidazole.
Examples of the mercapto compound include isobutyl mercaptan, butyl mercaptan, octyl mercaptan, and 2-mercaptoimidazoline.
Examples of sulfide compounds include mercaptobenzothiazyl sulfide, tetramethylthiuram monosulfide, tetramethylthiuram disulfide, tetraethylthiuram disulfide, tetrabutylthiuram disulfide, and dipentamethylenethiuram tetrasulfide.

By incorporating the organic compound (x) into the organic resin coating (A), it is possible to retain the organic compound (x) on the steel sheet surface, thereby improving the coating adhesion. This is because chemical interactions occur between the steel sheet surface and various groups of the molecules contained in the organic resin coating (A).
In the case of automotive steel sheets, after being processed into a predetermined shape by press working, the steel sheets are assembled together by spot welding, and finally painted. In this case, if the coating (A) is too thick, the current may not flow during welding, resulting in poor welding. Therefore, the thickness of the coating (A) is set to 4.0 μm or less. On the other hand, if the coating (A) is too thin, it becomes difficult to exhibit the function of a barrier layer that blocks a corrosive environment. Therefore, the thickness is set to 0.3 μm or more. Furthermore, since hydrogen, which causes delayed fracture, is generated by corrosion, the thickness must be set to 0.3 μm or more from the viewpoint of suppressing delayed fracture.

In order to fully exhibit the effect of suppressing the occurrence of delayed fracture in an atmospheric corrosive environment, the deposition amount [x] of the organic compound (x) in the coating (A) is set to 100 mg/ ^m2 or more. On the other hand, while a large deposition amount is not a problem from the viewpoint of suppressing delayed fracture, if the deposition amount is too large, the adhesion between the coating (A) and the steel sheet decreases, which is undesirable, particularly for steel sheets used in automobiles, and therefore the deposition amount [x] of the organic compound (x) is set to 2000 mg/ ^m2 or less. Furthermore, from the viewpoint of imparting barrier properties that block corrosion factors, i.e., from the viewpoint of suppressing delayed fracture, the deposition amount [y] of the organic resin (y) is set to 200 mg/ ^m2 or more and 3800 mg/ ^m2 or less. Furthermore, from the viewpoint of weldability, the deposition amount [y] of the organic resin (y) is preferably set to 3800 mg/ ^m2 or less.
Furthermore, the ratio [x]/([x]+[y]) of the amount of organic compound (x) attached to the total amount of organic compound (x) and organic resin (y) attached, [x]+[y], is set to 0.05 to 0.5. If [x]/([x]+[y]) is less than 0.05, the effect of suppressing the occurrence of delayed fracture may be reduced. From the viewpoint of suppressing delayed fracture, it is preferable that [x]/([x]+[y]) is 0.3 or more. On the other hand, if [x]/([x]+[y]) exceeds 0.5, the coating adhesion may be reduced.

There are no particular restrictions on the type of organic resin used in the organic resin coating (A). Examples include epoxy resins, modified epoxy resins, urethane resins, alkyd resins, acrylic resins, ethylene resins (polyolefin resins), polyester resins, polybutadiene resins, amino resins, phenolic resins, fluororesins, and silicone resins, and one or more of these can be used. Among these, urethane resins, epoxy resins, acrylic resins, and ethylene resins are particularly preferred due to their high barrier effect against moisture and chlorides, which are corrosion factors.

The hydrogen penetration suppression effect of a coating on a high-strength steel sheet can be evaluated by the ratio ( _Ht / _H0 ) of the maximum hydrogen permeability coefficient Ht when a steel sheet with a _coating is corroded to the maximum hydrogen permeability coefficient _H0 when a steel sheet without a coating is corroded. This _Ht / _H0 represents the degree of hydrogen penetration suppression effect of the coating. If Ht measured on a steel sheet with a coating is half or less of _H0 measured on a steel sheet without a _coating , that is, if _Ht / _H0 satisfies the following formula (1), it can be evaluated that the coating is capable of effectively suppressing hydrogen penetration into steel. As will be shown in the examples described later, the high-strength steel sheet of the present invention has excellent properties that satisfy the following formula (1).
H _t /H ₀ ≦0.50…(1)

Here, the maximum hydrogen permeability coefficient _Ht is calculated as follows. The steel plate surface of the test specimen having a coating (one side of the steel plate) is used as the hydrogen entry surface, and the other side of the steel plate is plated with palladium to form the hydrogen detection surface. The hydrogen entry surface is subjected to a corrosion test simulating an atmospheric corrosion environment to corrode it, and an electrochemical cell installed on the hydrogen detection surface detects a current generated due to hydrogen penetrating into the steel from the hydrogen entry surface, to calculate the hydrogen permeation current (J). The hydrogen permeation current is multiplied by the plate thickness (L) of the test specimen to calculate the hydrogen permeability coefficient (JL), and the maximum value of this hydrogen permeability coefficient during the measurement is taken as the maximum hydrogen permeability coefficient _Ht . The maximum hydrogen permeability coefficient _H0 is calculated as follows. The steel plate surface of the test specimen having no coating (one side of the steel plate) is used as the hydrogen entry surface, and the other side of the steel plate is plated with palladium to form the hydrogen detection surface. Then, this test piece is subjected to the same corrosion test as above, and the hydrogen permeability coefficient (JL) is calculated in the same manner as above, and the maximum value of this hydrogen permeability coefficient during the measurement is taken as the maximum hydrogen permeability coefficient _H0 .

The high-strength steel sheet of the present invention may have a multi-layer coating in which another coating is provided under or over the coating (A). Also, the coating (A) may have a multi-layer structure consisting of two or more layers.
The thickness of the coating (A) is determined by observing the cross section of the coating, measuring the thickness of the coating (thickness from the surface of the substrate steel sheet to the surface of the coating (A)) at multiple locations (for example, three locations) in an arbitrary field of view, and averaging these values to determine the thickness. The method for processing the cross section is not particularly limited, but examples include FIB processing.
The amount of organic compound (x) attached in coating (A) can be measured, for example, by fluorescent X-ray analysis. Specifically, the coating surface is irradiated with X-rays, the intensity of fluorescent X-rays of elements (e.g., nitrogen and sulfur) contained in organic compound (x) is measured, and the amount of attachment can be calculated by comparing the measured value with a calibration curve.
The amount of organic resin (y) attached in coating (A) can be calculated, for example, by measuring the weight of the steel sheet with and without the coating attached, dividing the difference between the measured weights by the area of the steel sheet, and then subtracting the amount of organic compound (x) attached from the result.

To form the coating (A) on the steel sheet surface, an organic compound (x) is added to a treatment liquid (resin solution) prepared by dissolving and/or dispersing an organic resin (y) in a solvent (water and/or an organic solvent), and the treatment liquid is then coated on the steel sheet surface, followed by heating and drying.
The method for coating the treatment liquid (resin solution) on the steel sheet surface is not particularly limited, and any known method, such as coating, dipping, or spraying, may be used. For coating, any coating means may be used, such as a roll coater (e.g., a three-roll method, a two-roll method), a bar coater, a squeeze coater, or a die coater. After coating with a squeeze coater or the like, dipping, or spraying, the amount of coating may be adjusted, and the appearance and film thickness may be made uniform by an air knife method or a roll squeezing method. The coated treatment liquid may be heated and dried by any method, and for example, a dryer, a hot air oven, a high-frequency induction heating oven, an infrared oven, or the like may be used.

The method for manufacturing the steel sheet used as the substrate in the present invention is not particularly limited. To facilitate understanding of the present invention, a series of processes from steelmaking will be briefly described using an example. However, the manufacturing process of the steel sheet that serves as the substrate is not limited to the following example.
Steel having a predetermined chemical composition is melted and continuously cast into a slab according to conventional methods. The resulting slab is then heated in a heating furnace at a temperature of 1100 to 1300°C, hot-rolled at a finishing temperature of 750 to 950°C, and coiled at 500 to 650°C. This is followed by pickling and cold rolling at a reduction of 30 to 70%. Thereafter, if necessary, cleaning treatment is performed according to conventional methods (i.e., cleaning with an alkali or a mixed solution of alkali and a surfactant and an organic compound containing nitrogen or sulfur that adsorbs to the steel sheet to form a molecular film, electrolytic cleaning, hot water cleaning, and drying), followed by heat treatment at 650 to 900°C, rapid cooling, and adjustment of the tensile strength of the steel sheet. Furthermore, if necessary, temper rolling of about 0.01 to 0.5% according to conventional methods is performed to obtain a cold-rolled steel sheet having the desired tensile strength.

As described above, the treatment solution (surface treatment solution) of the present invention is a surface treatment solution for forming a coating (A) on the surface of a steel sheet to suppress delayed fracture of steel sheet having a tensile strength of 1180 MPa or more. The treatment solution contains an organic compound (x) consisting of at least one compound selected from azole compounds, pyridine compounds, thiocarbonyl compounds, mercapto compounds, and sulfide compounds, and an organic resin (y). The functions, effects, and specific examples of the organic compound (x) and organic resin (y) are as described above. Furthermore, the concentrations of the organic compound (x) and organic resin (y) are adjusted so that the treatment solution, when applied to the steel sheet surface to form the coating (A), satisfies the above-described coating composition (deposition amounts [x], [y], and [x]/([x] + [y])).

The base steel sheets used were cold-rolled steel sheets and galvannealed steel sheets (GA steel sheets). The cold-rolled steel sheets (as-cold-rolled steel sheets) had a chemical composition consisting of 0.19% by mass of C, 0.8% by mass of Si, 1.8% by mass of Mn, 0.011% by mass of P, 0.001% by mass of S, the balance being Fe and unavoidable impurities, and had a tensile strength of 1470 MPa and a thickness of 1.6 mm. The GA steel sheets had a base steel sheet consisting of 0.22% by mass of C, 1.2% by mass of Si, 3.0% by mass of Mn, 0.007% by mass of P, 0.0005% by mass of S, the balance being Fe and unavoidable impurities, and had a tensile strength of 1580 MPa and a thickness of 1.4 mm. The galvannealed steel sheet (double-sided plated steel sheet) had a coating weight of 44 g/ ^m2 per side and an Fe content of 14 mass% in the zinc plating film. Each of the above base steel sheets was immersed in toluene and subjected to ultrasonic cleaning for 5 minutes to remove the rust-preventive oil, and then an organic resin film was formed on the surface (however, in some comparative examples, no organic resin film was formed).

The following A1 to A5 were used as organic resins (y) for the organic resin coating, and a treatment liquid containing any of the organic resins (y) and a predetermined organic compound (x) (in some comparative examples, the treatment liquid contained only the organic resin) was applied to the surface of the steel sheet by any of the coating methods (bar coating), spraying, and immersion (and roll squeezing), and then the steel sheet was heated with an induction heater so that the ultimate sheet temperature was 120°C, thereby forming an organic resin coating.
A1: Acrylic resin (manufactured by DIC Corporation, product name: 40-418EF)
A2: Epoxy resin (manufactured by Japan Epoxy Resins Co., Ltd., product name: jER1009)
A3: Urethane resin (manufactured by Dai Nippon Toryo Co., Ltd., product name: V Top RC Clear)
A4: Ethylene-based resin (manufactured by Dow Mitsui Polychemicals Co., Ltd., product name: Hylamine)
A5: Fluorine-based resin (manufactured by Asahi Glass Co., Ltd., product name: Lumiflon LF552)

The high-strength steel sheets of the invention and comparative examples on which the coatings were formed as described above were evaluated for delayed fracture resistance, coating adhesion, and corrosion resistance after painting, and the maximum hydrogen permeability coefficients _Ht , _H0 , and _Ht / _H0 were calculated as follows. The results are shown in Tables 1 and 2, along with the manufacturing conditions.
In measuring the thickness of the organic resin film, the cross section obtained by FIB processing was observed with an SEM, the thickness of the organic resin film (thickness from the surface of the base steel sheet to the surface of the organic resin film) was measured at three points in an arbitrary field of view, and the average value was taken as the film thickness.
The amount [x] of the organic compound (x) attached in the organic resin coating was calculated by measuring the amounts of N and S before and after application of the coating using fluorescent X-rays and comparing the difference between the measured amounts with a calibration curve. The amount [y] of the organic resin (y) attached was calculated by measuring the weight of the steel sheet with and without the coating attached, dividing the difference between the measured amounts by the steel sheet area, and then subtracting the amount [x] of the organic compound (x) attached from the value obtained.
In addition, the pH when 1 mol/L of organic compound (x) was added to pure water was measured and used as the pH of organic compound (x).

(1) Evaluation of Delayed Fracture Resistance Steel plates of the invention and comparative examples, each 1.4 mm thick, were sheared to a width of 35 mm and a length of 100 mm, and then ground to a width of 30 mm to remove residual stresses from shearing. The prepared test specimens were bent 90° with a curvature radius of 10 mmR to obtain bending test specimens. As shown in Figure 1 , the shape of this bending test specimen 1 was fixed by restraining it with a nut 2 (size: M8), a SUS jig 3, and a bolt 4 (size: M8), thereby obtaining a test specimen for evaluating delayed fracture resistance. Note that in preparing these test specimens, the inner spacing between the flange ends was narrowed by 14 mm after restraint (after the test specimen shape was fixed by tightening with bolts and nuts) compared to before restraint (test specimen 1a in Figure 1 ).

The thus-prepared delayed fracture resistance evaluation specimens were subjected to a combined cyclic corrosion test shown in Figure 2 up to 40 cycles. This cycle test was performed at a constant test temperature (50°C), with a humidity cycle consisting of four steps: a dry step at 30% relative humidity, a wet step at 90% relative humidity, and a humidity increase/decrease step (transition period). Each step was switched every 2 hours, and the test was repeated for 8 hours. Furthermore, at the start of the dry step twice a week, the delayed fracture resistance evaluation specimens were rinsed with pure water and then sprayed with saline solution containing 8.1 mass% chloride (amount of salt deposition: 3000 mg/ ^m2 per side). Here, "twice a week" refers to a three- or four-day interval between treatments. For example, if the first treatment was performed on Monday, the second treatment would be performed on Thursday or Friday. This cycle was then repeated. At the start of the dry step of each cycle, the presence or absence of cracks in the bent portion of the test piece for evaluating delayed fracture resistance was visually confirmed, and the number of cycles until cracks occurred was counted. In addition, based on the results (presence or absence of cracks) every three cycles (every day), the number of days until cracks occurred ("days to cracks" in Tables 1 and 2) was counted.

This test was carried out on three specimens of each steel plate, and the average value was used for evaluation. Evaluation was based on the number of cycles and was made according to the following criteria. In Tables 1 and 2, the number of cycles until cracking occurred, exceeding 40, indicates that no cracking occurred in the results of this example.
◎: More than 40 cycles 〇: 30 cycles or more, 40 cycles or less △: 10 cycles or more, less than 30 cycles ×: Less than 10 cycles

(2) Calculation of maximum hydrogen permeability coefficients _Ht , _H0 and _Ht / _H0 (evaluation of hydrogen penetration amount)
The maximum hydrogen permeability coefficient _Ht was calculated as follows. The backsides of the 1.4 mm-thick steel sheets of the invention and comparative examples (test specimens with an organic resin film formed on only one side (the front side) of the steel sheet) were ground and then chemically polished to a thickness of 0.5 mm. Furthermore, metallic Pd was formed on the backsides by electroplating. That is, the front side of the steel sheet with the organic resin film formed thereon served as the hydrogen penetration surface (the surface subjected to the corrosion test), and the backside of the Pb-plated steel sheet served as the hydrogen detection surface (the surface used to detect the current generated by hydrogen penetrating into the steel due to corrosion). As shown in Figure 3 , the specimen 5 (test specimen) was fixed so that the Pd-plated surface was in contact with the current detection cells 6 (6a to 6d), and a 0.1 mol/L aqueous solution of sodium hydroxide deaerated with nitrogen gas was poured into each cell 6. A portion of the specimen 5 was covered with a protective film 9 to protect the specimen on the reference cell 6a from corrosion. Each cell 6 was subjected to the corrosion test shown in FIG. 2 while being polarized at a constant potential to +0.2 V relative to the reference electrode. The surface of the sample 5 was corroded for up to 40 cycles, and the current generated due to hydrogen penetrating into the steel was detected. The net hydrogen permeation current (J) was calculated by calculating the difference between the current value measured in each cell and the current value measured in the reference cell. Furthermore, the hydrogen permeation coefficient (JL) was calculated by multiplying the hydrogen permeation current by the thickness (L) of the test piece. The maximum value during the measurement of this hydrogen permeation coefficient was defined as the maximum hydrogen permeation coefficient _Ht . The maximum hydrogen permeation coefficient _H0 was calculated as follows. A 1.4 mm thick steel plate (test piece) without an organic resin coating was processed and Pb-plated in the same manner as above. A test piece was prepared in which the front surface of the steel plate without the organic resin coating served as the hydrogen penetration surface and the back surface of the Pb-plated steel plate served as the hydrogen detection surface. This test piece was subjected to the same corrosion test as above, and the hydrogen permeability coefficient (JL) was calculated in the same manner as above, with the maximum value during the measurement of this hydrogen permeability coefficient being designated as the maximum hydrogen permeability coefficient H _0. The maximum hydrogen permeability coefficient H _t obtained in this manner was divided by the maximum hydrogen permeability coefficient H ₀ to calculate H _t /H ₀ , and cases where H _t /H ₀ ≦0.50 were evaluated as passing.
For some comparative examples in which no organic resin film was formed on the steel sheet surface, only the maximum hydrogen permeability coefficient H ₀ was calculated.

(3) Evaluation of Coating Adhesion Test pieces were prepared by bending in the same manner as in the evaluation test for delayed fracture resistance described above, and the coating on the inside of the bend after bending was cut into a grid pattern with an area of 35 mm wide x 20 mm long according to a method in accordance with JIS K5600, and a tape peeling test was performed. The coating peeling area ratio in this test was calculated using the following formula (coating area before peeling: 35 mm x 20 mm).
Film peeling area ratio = (C _x /C _о ) x 100
Where, _Cx : the area of the coating that has been peeled off and adhered to the tape
C ₀ : Area of film before peeling From this peeled film area ratio, the film adhesion was evaluated according to the following criteria, with ◯ being good and × being bad.
○: Peeling area rate less than 5% ×: Peeling area rate 5% or more

(4) Evaluation of Corrosion Resistance After Painting The galvanized steel sheets of the invention and comparative examples were each sheared to a size of 150 mm x 70 mm to prepare flat test specimens for corrosion resistance testing. These corrosion resistance test specimens were subjected to a chemical conversion treatment by immersion using "Palbond" manufactured by Nippon Parkerizing Co., Ltd. under standard conditions (35°C, 120 seconds). Subsequently, electrodeposition coating and baking treatment were performed using "GT-150T" electrodeposition paint manufactured by Kansai Paint Co., Ltd. The electrodeposition coating thickness was 15 μm, and the film thickness was measured using a commercially available electromagnetic film thickness meter. After painting, the test specimens were subjected to an X-cut (crossing angle 60° to 90°) reaching the substrate using a cutter knife, and then subjected to a salt spray test specified in JIS Z2371 for 840 hours. The maximum rust width from the cross cut of the test specimens after testing was measured, and corrosion resistance was evaluated based on this maximum rust width as follows: That is, the maximum rust width A of each test piece was calculated assuming that the maximum rust width of the steel plate as is was 1, and the test piece was evaluated according to the following criteria, with ⊚ and ◯ being considered pass.
◎: A≦0.8
○: 0.8<A≦0.95
△: 0.95<A≦1.2
×: 1.2<A

In Tables 1 and 2, No. 1 is a comparative example of an as-coated cold-rolled steel sheet without an organic resin film. The maximum hydrogen permeability coefficient _H0 during corrosion was 30 μA/cm, which was relatively high, and delayed fracture occurred early, indicating poor delayed fracture resistance. No. 2 is a comparative example of an as-coated galvannealed steel sheet without an organic resin film. The maximum hydrogen permeability coefficient _H0 during corrosion was 300 μA/cm, which was significantly higher than that of the as-coated cold-rolled steel sheet of No. 1, indicating poor delayed fracture resistance. This is because, while zinc exhibits excellent sacrificial corrosion protection during the corrosion process of zinc-based coatings, it generates more hydrogen than cold-rolled steel sheets.

No. 3 is a comparative example in which a coating containing only organic resin (y) was formed on a cold-rolled steel sheet. Although the maximum hydrogen permeability coefficient _Ht during corrosion and the number of cycles to cracking were improved compared to No. 1, cracking occurred in less than 30 cycles. Nos. 4 to 6 and 13 to 15 are comparative examples in which a coating not satisfying the conditions of the present invention was formed on a cold-rolled steel sheet. Of these, Nos. 4 to 6, 13, and 14, like No. 3, cracking occurred in less than 30 cycles. No. 15 also had poor coating adhesion due to an excessive amount of organic compound (x). On the other hand, Nos. 7 to 12 and 16 to 37 are inventive examples in which a coating satisfying the conditions of the present invention was formed on a cold-rolled steel sheet or a galvannealed steel sheet. The number of cycles to cracking was 30 or more, high delayed fracture resistance was obtained, and coating adhesion and corrosion resistance after painting were also good. Among these, Nos. 7 to 12, 16 to 20, and 25 to 37, which use a pyridine compound as the organic compound (x), exhibit particularly excellent corrosion resistance after painting.

1 Test piece 1a Test piece (before stress loading)
2 Nut 3 SUS jig 4 Bolt 5 Sample (steel plate)
6a to 6d Cell 7 Reference electrode 8 Counter electrode 9 Protective film

Claims (3)

A coating (A) is provided on the surface of a steel sheet having a tensile strength of 1180 MPa or more, the coating (A) including an organic compound (x) consisting of at least one compound selected from an azole compound, a pyridine compound, a mercapto compound, and a sulfide compound, and an organic resin (y),
The coating (A) is a high-strength steel plate characterized in that the amount [x] of the organic compound (x) deposited is 100 to 2000 mg/m ² , the amount [y] of the organic resin (y) deposited is 200 to 3800 mg/m ² , the ratio [x]/([x]+[y]) of the amount [x] of the organic compound (x) deposited to the total amount [x]+[y] of the organic compound (x) and the organic resin (y) deposited is 0.05 to 0.5, and the coating has a thickness of 0.3 to 4.0 μm. The high-strength steel plate according to claim 1, characterized in that the organic resin (y) is at least one selected from the group consisting of urethane-based resins, epoxy-based resins, acrylic-based resins, and ethylene-based resins. A surface treatment solution for forming a coating on the surface of a steel sheet to suppress delayed fracture of a steel sheet having a tensile strength of 1180 MPa or more,
A surface treatment solution comprising an organic compound (x) consisting of at least one compound selected from the group consisting of an azole compound, a pyridine compound, a mercapto compound , and a sulfide compound, and an organic resin (y).