Steel sheet, plated steel sheet, press-formed product, processed member, method for manufacturing press-formed product, and method for manufacturing processed member

Abstract

The purpose of the present invention is to effectively inhibit the occurrence of delayed fracture at a shear end surface of a member in which there is a concern regarding occurrence of delayed fracture in a press-molded article obtained by using a high-strength steel sheet or a plated steel sheet including the high-strength steel sheet as a base sheet. This steel sheet has, on a surface thereof, a coating containing an organic resin and an inorganic material. In the coating, the concentration of the inorganic material in a 20%-coating region on the steel sheet side in the thickness direction is higher than the concentration of the inorganic material in a 20%-coating region on the surface layer side. The steel sheet has a tensile strength of at least 1180 MPa.

Description

Technical Field

[0001] The present invention relates to a steel sheet, a plated steel sheet, a press-formed product, and a processed member having excellent delayed fracture resistance characteristics, and mainly to a steel sheet, a plated steel sheet, a press-formed product, and a processed member suitable for strength members for automobiles and building materials. In particular, the present invention relates to a steel sheet having a tensile strength of 1180 MPa or more, a plated steel sheet having the above steel sheet as a base, a press-formed product using these, and a processed member.

Background Art

[0002] Conventionally, as steel sheets for automobiles, cold-rolled steel sheets, hot-dip galvanized steel sheets (GI), alloyed hot-dip galvanized steel sheets (GA), electro-galvanized steel sheets (EG), etc. obtained by applying zinc plating to cold-rolled steel sheets have been used due to requirements regarding sheet thickness accuracy and flatness. However, in recent years, from the viewpoints of reducing CO2 emissions and ensuring safety of automobiles, the strength of steel sheets for automobiles has been increased.

[0003] However, it is known that as the strength of steel materials increases, delayed fracture is likely to occur, and this tendency is particularly prominent in high-strength steel materials having a tensile strength of 1180 MPa or more.

[0004] Note that delayed fracture is a phenomenon in which, when a high-strength steel material is subjected to a static load stress (a load stress equal to or less than the tensile strength) and a certain period of time has elapsed, sudden brittle fracture occurs with almost no plastic deformation visually.

[0005] This delayed fracture is known to occur in the case of steel plates due to the residual tensile stress when formed into a predetermined shape by press working and the hydrogen embrittlement of steel at stress concentration sites. In particular, shear end faces in blanking and trimming processes before press working are most likely to be the starting points of delayed fracture. In most cases, the hydrogen causing this hydrogen embrittlement is considered to be hydrogen that has invaded and diffused into the steel from the external environment. Typically, it is hydrogen that has invaded and diffused into the steel during the corrosion of the steel plate. In recent years, various proposals have also been made regarding the evaluation method of delayed fracture in high-strength steel plates of 1180 MPa or more. For example, there is a method of evaluating the delayed fracture characteristics using a test piece (steel plate) that has been U-bent after shearing. Shear end faces of steel plates after shearing have strain (work hardening and residual stress due to the contact between the blade and the steel plate) and minute defects due to the strain. Due to this strain and minute defects, the crack generation frequency of the shear end face of the steel plate subjected to U-bending may differ, and the influence of the shear end face on the delayed fracture characteristics has become a problem. Also, since shear end faces exist in actual automotive components, delayed fracture due to the strain and minute defects of the shear end face can become a major problem.

[0006] In order to prevent such delayed fracture in high-strength steel plates, for example, in Patent Document 1, studies have been made to weaken the delayed fracture susceptibility by adjusting the composition.

[0007] Also, in Patent Document 2, studies have been made on high-strength alloyed hot-dip galvanized steel plates for preventing delayed fracture.

[0008] Furthermore, in Patent Document 3, a technique for suppressing delayed fracture by reducing the amount of hydrogen invading into the steel plate by applying Ni or Ni-based alloy plating to a cold-rolled steel plate is disclosed.

[0009] On the other hand, in Non-Patent Document 1, although it is not an automotive component, a technique for suppressing delayed fracture in a constant load test by shot peening S45C is disclosed.

[0010] In addition, Patent Documents 4 and 5 disclose a technique for improving the stress corrosion cracking resistance by shot peening the shear end face or the whole of a steel plate having a tensile strength of 1180 MPa or more.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Non-Patent Documents

[0012]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0013] However, in Patent Document 1, since the amount of hydrogen that penetrates from the external environment into the steel plate does not change, it is insufficient to suppress the occurrence of stress corrosion cracking.

[0014] Also, in Patent Document 2, the Fe concentration in the plating is about a dozen percent, and although corrosion resistance can be obtained, excellent stress corrosion cracking resistance cannot be expected.

[0015] Furthermore, in Patent Document 3, although it has the effect of suppressing stress corrosion cracking starting from the general surface part of the molded product, it is not sufficient to suppress stress corrosion cracking caused by the shear end face.

[0016] In addition, in Non-Patent Document 1 and Patent Documents 4 and 5, although the sheared end face can be directly processed and thus a certain degree of effect can be expected for delayed fracture, for molded products with rust preventive oil or press oil adhering thereto, since the shot material solidifies due to the oil, it cannot be applied. Further, equipment for shotting is required, and an increase in manufacturing cost due to an increase in processing steps is inevitable, and the technical and economic hurdles are high.

[0017] The present invention solves the above-described problems of the prior art, and an object thereof is to effectively suppress the occurrence of delayed fracture at the sheared end face of a member in which the occurrence of delayed fracture is a concern in a press-molded product using a high-strength steel sheet or a plated steel sheet having the high-strength steel sheet as a base.

Means for Solving the Problems

[0018] The present inventors earnestly studied and researched means for effectively suppressing the occurrence of delayed fracture starting from the sheared end face of a high-strength member for an automobile in order to solve the above problems, and as a result, obtained the following findings. (1) In a high-strength steel sheet of 1180 MPa or more or a plated steel sheet having the high-strength steel sheet as a base (hereinafter also referred to as a plated steel sheet), by applying a film containing an organic resin and an inorganic substance and having a higher inorganic substance concentration in a film region of 20% on the steel sheet side in the thickness direction than in a film region of 20% on the surface layer side to the surface of the steel sheet or the plated steel sheet, it becomes possible to reduce the strain on the surface of the steel sheet at the sheared end when subjected to shearing. (2) By reducing the strain on the surface of the steel sheet at the sheared end, that is, by suppressing the length of the sliding marks on the film surface, it becomes possible to suppress the occurrence of cracks at the sheared end face of the bent portion when subjected to processing such as press molding. (3) By suppressing the occurrence of cracks at the sheared end face of the bent portion, the occurrence of delayed fracture can be effectively suppressed.

[0019] The present invention has been completed based on the above findings, and the main configuration is as follows. [1] It has a film containing an organic resin and an inorganic substance on the surface, and in the film, the inorganic substance concentration in the 20% film region on the steel plate side in the thickness direction is higher than the inorganic substance concentration in the 20% film region on the surface layer side, a steel plate with a tensile strength of 1180 MPa or more. [2] The steel plate according to [1], wherein the film thickness of the film is 0.4 μm or more. [3] The steel plate according to [1] or [2], having one or both of the following conditions (A) and (B). (A) When subjected to shearing, the length of the sliding mark on the steel plate surface at the shearing end is 0.5 mm or less. (B) When subjected to bending, the number of cracks N1 generated on the shearing end face satisfies the formula (1). (N1 / R1)×T1≦1.0···(1) In the formula (1), N1: the number of cracks generated on the shearing end face of the bending part, R1: the bending radius (mm) of the bending part where the number of cracks is evaluated, T1: the plate thickness (mm) of the steel plate before processing. [4] It has a film containing an organic resin and an inorganic substance on the plating surface, and in the film, the inorganic substance concentration in the 20% film region on the steel plate side in the thickness direction is higher than the inorganic substance concentration in the 20% film region on the surface layer side, a plated steel plate comprising a plating base steel plate with a tensile strength of 1180 MPa or more. [5] The plated steel plate according to [4], wherein the film thickness of the film is 0.4 μm or more. [6] The plated steel plate according to [4] or [5], having one or both of the following conditions (C) and (D). (C) When subjected to shearing, the length of the sliding mark on the steel plate surface at the shearing end is 0.5 mm or less. (D) When subjected to bending, the number of cracks N2 generated on the shearing end face satisfies the formula (2). (N2 / R2)×T2≦1.0···(2) In the formula (2), N2: the number of cracks generated on the shearing end face of the bending part, R2: the bending radius (mm) of the bending part where the number of cracks is evaluated, T2: the plate thickness (mm) of the plated steel plate before processing. [7] A press-formed product press-formed using the steel sheet described in any one of [1] to [3] or the plated steel sheet described in any one of [4] to [6]. [8] A method for manufacturing a press-formed product, comprising a step of press-forming using the steel sheet described in any one of [1] to [3] or the plated steel sheet described in any one of [4] to [6]. [9] A method for manufacturing a processed member, comprising a step of shearing and / or a step of bending using the steel sheet described in any one of [1] to [3] or the plated steel sheet described in any one of [4] to [6].

[10] A processed member provided with one or both of the following (A) and (B) on the steel sheet described in [1] or [2]. (A) The length of the sliding mark on the steel sheet surface at the sheared end is 0.5 mm or less. (B) The number of cracks N1 generated on the sheared end face satisfies the formula (1). (N1 / R1)×T1≦1.0···(1) In the formula (1), N1: the number of cracks generated on the sheared end face of the bent portion, R1: the bending radius (mm) of the bent portion where the number of cracks is evaluated, T1: the plate thickness (mm) of the steel sheet before processing.

[11] A processed member provided with one or both of the following (C) and (D) on the plated steel sheet described in [4] or [5]. (C) The length of the sliding mark on the steel sheet surface at the sheared end is 0.5 mm or less. (D) The number of cracks N2 generated on the sheared end face satisfies the formula (2). (N2 / R2)×T2≦1.0···(2) In the formula (2), N2: the number of cracks generated on the sheared end face of the bent portion, R2: the bending radius (mm) of the bent portion where the number of cracks is evaluated, T2: the plate thickness (mm) of the plated steel sheet before processing.

Advantages of the Invention

[0020] By applying the present invention to a press-formed product using a high-strength steel sheet or a plated steel sheet having a high-strength steel sheet as a base, particularly preferably a high-strength member for an automobile, the occurrence of delayed fracture starting from the sheared end face of the member can be effectively suppressed.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Figure 3

Figure 4

Embodiments for Carrying Out the Invention

[0022] Hereinafter, the present invention will be specifically described.

[0023] The steel plate of the present invention or the steel plate as the plating base (the steel plate or the steel plate as the plating base is referred to as the base steel plate) is a steel plate that serves as the substrate of a high-strength member, and is a steel plate having a tensile strength of 1180 MPa or more, more preferably 1200 MPa or more, and even more preferably 1320 MPa or more. On the other hand, for the reason that the workability decreases with the increase in strength, the tensile strength is preferably 2000 MPa or less.

[0024] A steel plate with a low tensile strength or a plated steel plate with the said steel plate as the base essentially hardly causes delayed fracture. The effect of the present invention is exhibited even in a steel plate with a low tensile strength or a plated steel plate with the said steel plate as the base, but is remarkably exhibited in a steel plate with a tensile strength of 1180 MPa or more or a plated steel plate with the said steel plate as the base, and is more remarkably exhibited in a steel plate with a tensile strength of 1320 MPa or more or a plated steel plate with the said steel plate as the base.

[0025] The high-strength steel plate preferably used in the present invention or the plated steel plate with the said high-strength steel plate as the base may have any composition and structure as long as it has the desired tensile strength, but it is more advantageous to perform the following treatments.

[0026] In order to improve various properties such as the mechanical properties of the base steel plate, for example, the following methods (1), (2), and (3) can be carried out alone or in combination of a plurality of them. (1) Chemical composition modification such as 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 carbides and nitrides such as Ti, Nb, V, and Al, and addition of strengthening elements such as W, Zr, Hf, Co, B, Cu, and rare earth elements. (2) Strengthening by recovery annealing at a temperature where recrystallization does not occur, further partial recrystallization strengthening by leaving an unrecrystallized region without completely recrystallizing, and strengthening by transformation structures such as bainite or martensite single-phasing or composite structuring of ferrite and these transformation structures. (3) Grain refinement strengthening represented by Hall-Petch's formula: σ = σ0 + kd when the ferrite grain size is d -1 / 2 (where σ: stress, σ0, k: material constants), and structural or textural modification such as work hardening by rolling, etc.

[0027] Examples of the composition of such high-strength steel plates include, for example, by mass%, C: 0.1 to 0.5%, Si: 0 to 3.0%, Mn: 1 to 10%, P: 0 to 0.05%, S: 0 to 0.005%, the balance being Fe and inevitable impurities, and further containing one or more of Cu, Ti, V, Al, Cr, Ni, etc. as appropriate according to need.

[0028] Examples of commercially available high-strength steel plates having the above tensile strength include, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (the above are manufactured by JFE Steel Corporation), etc.

[0029] In the present invention, the thickness of the steel plate (base steel plate) serving as the substrate is not particularly limited, but is preferably 0.8 mm or more. More preferably 1.2 mm or more. As the upper limit, the thickness of the steel plate (base steel plate) is preferably 2.5 mm or less, and more preferably 2.0 mm or less.

[0030] In addition, the type of plating metal to be plated on the above steel plate is not particularly limited, and general methods such as hot dip plating, electroplating, and electroless plating can be adopted. Here, the plating treatment conditions are not particularly limited, and appropriate favorable conditions may be adopted. When performing hot dip galvanizing treatment, it is preferable from the viewpoint of dross countermeasures that Al is added to the plating bath. In this case, the component of the additive element to the plating bath other than Al is not particularly limited. That is, in addition to Al, even if Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, etc. are contained or added, the effects of the present invention are not impaired.

[0031] Furthermore, an alloying treatment may be performed after the hot dip galvanizing. In the present invention, the conditions of the alloying treatment are not particularly limited, and appropriate favorable conditions may be adopted. As the steel plate, those subjected to zinc-based plating treatment, and those subjected to alloying treatment after zinc-based plating treatment can both be used.

[0032] Moreover, the processing method and shape of the high-strength member of the present invention are not particularly limited. The effect of suppressing delayed fracture is also exhibited in generally used forming processes.

[0033] In the present invention, in order to simulate high-strength automotive members processed by press forming such as lockers, B-pillars, and bumpers, members subjected to bending after shearing were fabricated, and the delayed fracture characteristics were evaluated.

[0034] In the present invention, between the steel sheet to be sheared or the plated steel sheet having the steel sheet as a base and the shearing blade, there is a film containing an organic resin and an inorganic substance, and the inorganic substance concentration in the 20% region on the base side (steel sheet side) in the thickness direction is higher than that in the 20% region on the surface layer side. By shearing through this film, the strain of the base steel sheet under the film at the portion in contact with the blade can be reduced, and delayed fracture can be suppressed. Therefore, it is necessary to have a film containing an organic resin and an inorganic substance, and the inorganic substance concentration in the 20% region on the base side (steel sheet side) in the thickness direction is higher than that in the 20% region on the surface layer side. It is more preferable if the above ratio is 1.5 or more. It is further preferable if the above ratio is 1.7 or more, and most preferable if it is 2.0 or more. On the other hand, it is preferable that the upper limit is 2.5 or less. Specifically, it is presumed that reducing the strain of the surface of the base steel sheet at the sheared end (sliding mark 10) is the following mechanism.

[0035] When the shearing blade directly hits the surface of the steel sheet or the surface of the plated steel sheet having the steel sheet as a base, at the portion where the shearing blade contacts, sliding occurs, so strain is generated on the surface of the base steel sheet, work hardening occurs, and the tissue state has poor deformability. In this state, when further subjected to high processing such as press forming, a plurality of cracks of about several tens to several hundreds of μm occur at the end face of the base steel sheet in the bent portion. The cracks cause stress concentration and promote the delayed fracture of the press-formed product. Therefore, reducing the strain of the base steel sheet due to contact with the shearing blade and suppressing the generation of cracks at the end face of the base steel sheet in the bent portion such as press forming is effective in suppressing delayed fracture. In the present invention, by interposing the film containing the organic resin and the inorganic substance described above between the shearing blade and the surface of the steel sheet or the surface of the plated steel sheet having the steel sheet as a base, the film reduces the friction coefficient between the shearing blade and the surface of the steel sheet or the surface of the plated steel sheet having the steel sheet as a base. Along with the reduction of the friction coefficient, it becomes possible to disperse and relieve the compressive stress applied to the portion in contact during shearing by the inorganic substance in the film, and the strain of the base steel sheet can be reduced. As a result, the generation of cracks at the end face of the base steel sheet in the bent portion such as press forming is suppressed, and the occurrence of delayed fracture can be suppressed.

[0036] At the end of the material after general shearing, as shown in Fig. 3, sliding marks 10 are confirmed at the contact part with the blade. The sliding marks 10 in the steel plate or plated steel plate with the coating are marks formed on the surface of the steel plate at the sheared end due to contact with the blade. The surface of the steel plate or plated steel plate under the sliding marks 10 has undergone plastic processing and is a work-hardened part due to strain in the base steel plate. Since the coating is actually applied to the surface of the steel plate or plated steel plate, the sliding marks 10 formed on the surface of the steel plate at the sheared end are observed on the coating surface. Also, the surface of the steel plate at the sheared end includes not only the end itself but also the part that enters from the end into the steel plate or plated steel plate side. The fact that the sliding mark length 11 on the surface of the steel plate at the sheared end is small after shearing means that the strain during the shearing process is dispersed and the strain of the base steel plate is reduced. Therefore, in order to exhibit the effect of suppressing delayed fracture, it is preferable that the sliding mark length 11 is 0.5 mm or less. When the strain at the contact part with the blade is large, the sliding mark length 11 exceeds 0.5 mm and the effect of suppressing delayed fracture is reduced. More preferably, the sliding mark length 11 is 0.1 mm or less, and even more preferably, it is in a state where 0 mm, that is, no sliding marks are generated.

[0037] The number of cracks N1 generated on the end face of the steel plate in the bending part preferably satisfies the following formula. It is more preferable that the left side of formula (1) satisfies 0.5 or less for the number of cracks N1, and even more preferably, it is 0. The more the strain at the contact part with the blade is reduced, the more the number of cracks N1 decreases and delayed fracture can be significantly suppressed. Note that the number of cracks N1 is the number of cracks generated on the sheared end face in the bending part. (N1 / R1)×T1≦1.0···(1) In formula (1), N1: The number of cracks generated on the sheared end face 12 in the bending part, R1: The bending radius (mm) of the bending part where the number of cracks is evaluated, T1: The plate thickness (mm) of the steel plate before processing In addition, the number of cracks N2 generated on the end face of the plated steel sheet using the steel sheet as a base during bending is preferably in a range that satisfies the following formula. It is more preferable that the left side of formula (2) satisfies 0.5 or less for the number of cracks N2, and it is even more preferable that it is 0. The lower the strain at the contact part with the blade, the smaller the number of cracks N2, and delayed fracture can be significantly suppressed. Note that the number of cracks N2 is the number of cracks generated on the shear end face of the bending part. (N2 / R2)×T2≦1.0···(2) In formula (2), N2: Number of cracks generated on the shear end face 12 of the bending part, R2: Bending radius (mm) of the bending part where the number of cracks was evaluated, T2: Plate thickness (mm) of the plated steel sheet before processing Note that the above R1 and R2 are the radii inside the bending part, and the processing length in the above N1 and N2 is the outer length of the bending part 17 from the bending R stop part 18 shown in Fig. 4 to the other bending R stop part 19.

[0038] In order to reduce the strain at the shear end, the friction coefficient of the coating is preferably 0.50 or less. When the friction coefficient becomes smaller, the surface pressure between the steel sheet surface or the plated steel sheet surface and the shear blade becomes smaller, and it is easier to reduce the strain. When the friction coefficient is greater than 0.50, the strain reduction effect is difficult to appear. The friction coefficient of the coating is more preferably 0.20 or less, and even more preferably 0.15 or less. On the other hand, considering the concern that the steel sheet may slip if the friction coefficient is extremely small, the friction coefficient of the coating is preferably 0.08 or more.

[0039] The film thickness of the film is preferably 0.4 μm or more. When the film thickness of the film is less than 0.4 μm, the total amount of inorganic substances in the film decreases, so it becomes difficult to obtain the effect of dispersing and relaxing the compressive stress applied to the contact portion during shearing by the inorganic substances in the film as described above, and reducing the strain of the base steel sheet. As a result, the effect of suppressing crack generation at the sheared end face 12 of the bent portion during forming becomes small. The film thickness of the film is more preferably 0.5 μm or more, and even more preferably 1.0 μm or more. On the other hand, when the film thickness of the film increases, it becomes difficult to perform predetermined forming and it causes high costs, so the film thickness of the film is preferably 2.0 mm or less. The film thickness of the film is more preferably 1.0 mm or less, and even more preferably 5.0 μm or less.

[0040] The film has not only the effect of reducing the friction coefficient between the shearing blade and the surface of the steel sheet or the surface of the plated steel sheet, but also the effect of relaxing the compressive stress during shearing and reducing the strain on the surface of the base steel sheet. Therefore, when it is outside the range of formula (1) or formula (2), the effect of suppressing delayed fracture is small.

[0041] The film in the present invention is a film containing an organic resin and an inorganic substance, and the type of the film does not particularly need to be limited, and examples thereof include a film containing an inorganic substance (inorganic film) and a film containing an organic resin (organic film). Examples of the inorganic film include Mn-P-based oxide film, Ni-based inorganic film, zinc-based oxide film, copper-based oxide film, and iron-based oxide film. Examples of the organic film include films containing polyvinyl chloride-based resin, polyethylene-based resin, polypropylene-based resin, epoxy resin, polyhydroxy polyether resin, polyester resin, urethane resin, silicon resin, and acrylic resin. Further, the film can exhibit an effect as an organic-inorganic composite film. Further, wax may be contained to improve lubricity. Examples of the wax include polyolefin wax, montan wax, paraffin wax, microcrystalline wax, carnauba wax, lanolin-based wax, silicon-based wax, and fluorine-based wax, and one or more of these can be used.

[0042] In addition, the method for forming the film in the present invention is not particularly limited, and a method is adopted in which a treatment liquid (resin solution) in which an organic resin and an inorganic substance are dissolved and / or dispersed in a solvent (water and / or an organic solvent) is coated on the surface of a steel sheet or a plated steel sheet and then dried by heating.

[0043] There is no particular limitation on the method of coating the resin solution on the surface of the steel sheet or the plated steel sheet, and any known method, for example, a coating method, a dipping method, or a spraying method may be used. In the coating method, any coating means such as a roll coater (3-roll method, 2-roll method, etc.), a squeeze coater, or a die coater may be used. Also, after coating treatment, dipping treatment, or spraying treatment using a squeeze coater or the like, it is also possible to adjust the coating amount, uniformize the appearance, and uniformize the film thickness by an air knife method or a roll squeezing method. The method of heating and drying the coated treatment liquid is arbitrary, and for example, means such as a dryer, a hot air furnace, a high-frequency induction heating furnace, or an infrared furnace can be used.

[0044] By coating in a state where the temperature of the surface of the steel sheet or the plated steel sheet is previously higher than that of the treatment liquid, convection caused by the temperature difference at the interface between the surface of the steel sheet or the plated steel sheet and the treatment liquid occurs in the coated treatment liquid, and the inorganic substance having a high affinity for the metal accumulates on the steel sheet side, forming a film having a concentration difference of the inorganic substance in the thickness direction of the film. At this time, the temperature difference between the surface of the steel sheet or the plated steel sheet and the treatment liquid is preferably 20°C or more and 40°C or less.

[0045] In the present invention, it is preferable to perform press forming after cutting out a steel plate or a plated steel plate with a clearance of the blade during shearing being 0 to 30%. When the clearance is 0%, that is, in the case of fine blanking where the clearance of the blade is 0.01 mm or less, the blade does not directly touch the steel plate or the plated steel plate due to the film, and it is possible to exert a cushioning effect to reduce strain. Therefore, it is preferable that the clearance is 0% or more. More preferably, the clearance is 5% or more, and even more preferably, it is 10% or more. On the other hand, when the clearance becomes larger than 30%, cracks due to delayed fracture are likely to occur from the burrs on the sheared end face 12. Therefore, it is preferable that the clearance is 30% or less. More preferably, the clearance is 25% or less, and even more preferably, it is 20% or less.

[0046] Moreover, the press-formed product referred to in the present invention is a press-formed product obtained by press-forming a steel plate or a plated steel plate having a film in which the inorganic substance concentration in the 20% film region on the steel plate side in the thickness direction is higher than the inorganic substance concentration in the 20% film region on the surface layer side. Examples include high-strength automotive members processed by press forming such as lockers, B-pillars, and bumpers. Some of these members have bending with a maximum bending radius R / thickness t of 4.0 or less.

[0047] The manufacturing method of the press-formed product referred to in the present invention is a method for manufacturing the above-mentioned press-formed product, which includes a step of press-forming a steel plate or a plated steel plate having a film in which the inorganic substance concentration in the 20% film region on the steel plate side in the thickness direction is higher than the inorganic substance concentration in the 20% film region on the surface layer side. The step of press-forming includes shearing and bending with a maximum bending radius R / thickness t of 4.0 or less. Also, the above-mentioned shearing includes punching by a press.

[0048] In addition, the processed member referred to in the present invention is not only the above member, but also a steel sheet or a plated steel sheet having a film in which the inorganic substance concentration in the 20% film region on the steel sheet side in the above thickness direction is higher than the inorganic substance concentration in the 20% film region on the surface layer side, and is a processed member obtained by processing the steel sheet or the plated steel sheet, which is the steel sheet or the plated steel sheet after shearing, or the steel sheet or the plated steel sheet after bending after shearing. The above shearing includes punching by a press. Further, when using a steel sheet having the above film for the above processed member, it is preferable to have one or both of the following (A) and (B). (A) The length of the sliding mark on the steel sheet surface at the sheared end is 0.5 mm or less. (B) The number of cracks N1 generated on the sheared end face satisfies the formula (1). (N1 / R1)×T1≦1.0···(1) In the formula (1), N1: the number of cracks generated on the sheared end face 12 of the bent portion, R1: the bending radius (mm) of the bent portion where the number of cracks is evaluated, T1: the plate thickness (mm) of the steel sheet before processing.

[0049] Also, when using a plated steel sheet having the above film for the above processed member, it is preferable to have one or both of the following (C) and (D). (C) The length of the sliding mark on the steel sheet surface at the sheared end is 0.5 mm or less. (D) The number of cracks N2 generated on the sheared end face satisfies the formula (2). (N2 / R2)×T2≦1.0···(2) In the formula (2), N2: the number of cracks generated on the sheared end face 12 of the bent portion, R2: the bending radius (mm) of the bent portion where the number of cracks is evaluated, T2: the plate thickness (mm) of the plated steel sheet before processing.

[0050] The manufacturing method of the processed member referred to in the present invention is a method of manufacturing the above-mentioned processed member including a step of shearing and / or bending a steel sheet or a plated steel sheet having a film with a higher inorganic substance concentration in the 20% film region on the steel sheet side in the thickness direction than the inorganic substance concentration in the 20% film region on the surface layer side. Here, the step of shearing includes the above-mentioned shearing and punching by pressing. The step of bending includes bending with a maximum bending radius R / plate thickness t of 4.0 or less and bending by pressing.

Example

[0051] Hereinafter, the present invention will be specifically described based on examples. The technical scope of the present invention is not limited to the following examples, and appropriate changes within the scope of the gist configuration are included in the scope of the present invention. (Example 1) As the base steel sheet (substrate steel sheet), in mass%, C: 0.197%, Si: 0.43%, Mn: 1.56%, P: 0.01%, S: 0.0005% is contained, and the balance is composed of Fe and unavoidable impurities, or in mass%, C: 0.459%, Si: 0.20%, Mn: 0.73%, P: 0.01%, S: 0.0027% is contained, and the balance is composed of Fe and unavoidable impurities, a steel sheet with a plate thickness of 0.8 to 2.5 mm was used.

[0052] When further plating is performed, the above-mentioned base steel sheet is plated with Zn-based plating by the following hot-dip plating method or electroplating method. <Hot-dip plating method> The base steel sheet was immersed in a molten Zn-Fe-based plating bath (bath temperature 460°C) or a molten Zn-Al-Mg-based plating bath (bath temperature 460°C) by a hot-dip plating facility to produce plated steel sheets of level Nos. 41 to 44 in Table 1. Further, by directly performing an alloying treatment with an electric heating furnace after the above plating, plated steel sheets of level Nos. 28 to 40, 47 to 60 in Table 1 were produced. <Electroplating method> In a plating bath composed of an aqueous zinc sulfate solution (pH 2.0, bath temperature 50°C), the current density is 30 A / dm2 With the energization time set to 45 seconds, electroplating treatment was performed on the base steel plate to form a zinc plating layer, and the plated steel plates of level Nos. 45 and 46 shown in Table 1 were produced.

[0053] A film was formed on the above-mentioned material by the following method. The types of films are shown in Table 1. When forming the film, the above-mentioned material (steel plate or plated steel plate) was subjected to alkaline degreasing treatment. Epoxy resins and acrylic resins were used for the resin. Also, zinc-based oxides and crystalline layer-like substances were used as the inorganic substances.

[0054] For the zinc-based oxide, the steel plate was immersed in an aqueous solution of zinc sulfate heptahydrate with a concentration of 20 g / L and a temperature of 50 °C (for the immersion time, Film K: 3 seconds, Film L: 60 seconds, Film M: 100 seconds), and then after sufficient washing with water, it was dried to obtain basic zinc sulfate trihydrate to pentahydrate, which was confirmed by XRD analysis to be basic zinc sulfate trihydrate to pentahydrate.

[0055] The crystalline layer-like substance is a precipitate obtained by filtering and drying the precipitate obtained by previously dropping an aqueous solution of sodium hydrogen carbonate decahydrate (31 g / L) into an aqueous solution of magnesium nitrate hexahydrate (113 g / L) and an aqueous solution of aluminum nitrate nonahydrate (83 g / L), and the crystalline layer-like substance of [Mg 0.667 Al 0.333 (OH)2][CO3 2 0.167 ·0.5H2O was used. It was confirmed by XRD analysis that the crystalline layer-like substance is [Mg 0.667 Al 0.333 (OH)2][CO3 2 0.167 ·0.5H2O.

[0056] ​​The inorganic substance and the resin thus obtained were mixed at a mass ratio of 2:10, applied to the test material using a roll coater, and baked at 140 °C. The surface temperature of the above-mentioned material (steel plate or plated steel plate) was 20 °C higher than the temperature of the treatment liquid for applying the film, and the film was applied. Regarding the film thickness, it was appropriately controlled by changing the speed of the roll coater. For each steel plate or plated steel plate, the film thickness of the film was measured. The cross-section of the film was sputtered at 45° using FIB, the cross-section was observed with a very low acceleration SEM, and the average value of arbitrarily measured 10 points was taken. In addition, the ratio of the inorganic substance concentration in the 20% film region on the steel plate side in the thickness direction of the film to the inorganic substance concentration in the 20% film region on the surface layer side was determined from the average intensity ratio of the inorganic substance by SEM-EDS mapping.

[0057] Also, the coefficient of friction of each film was measured as follows. FIG. 1 is a schematic front view showing a coefficient-of-friction measuring apparatus. As shown in the figure, a sample 1 for measuring the coefficient of friction, taken from a steel plate or a plated steel plate on which a film was formed, is fixed to a sample stage 2, and the sample stage 2 is fixed to the upper surface of a horizontally movable slide table 3. On the lower surface of the slide table 3, a vertically movable slide table support base 5 having a roller 4 in contact therewith is provided, and a first load cell 7 for measuring the pressing load N applied to the sample 1 for measuring the coefficient of friction by a bead 6 by pushing this up is attached to the slide table support base 5. A second load cell 8 is attached to one end of the slide table 3 so as to move on a rail 9 in order to measure the sliding resistance force F when the slide table 3 is moved in the horizontal direction in a state where the above pressing force is applied. Note that, as the lubricating oil, press cleaning oil Pretone R352L manufactured by Sugimura Chemical Co., Ltd. was applied to the surface of the sample 1 for measuring the coefficient of friction to conduct the test. FIG. 2 is a schematic perspective view showing the shape and dimensions of the bead used. The lower surface of the bead 6 slides while being pressed against the surface of the sample 1 for measuring the coefficient of friction. The shape of the bead 6 shown in FIG. 2 has a width of 10 mm, a length in the sliding direction of the sample 1 for measuring the coefficient of friction of 4 mm, the lower parts at both ends in the sliding direction are formed of curved surfaces with a curvature of 0.5 mmR, and the lower bead surface against which the sample 1 for measuring the coefficient of friction is pressed has a plane with a width of 10 mm and a length in the sliding direction of 3 mm. For the coefficient-of-friction measurement test, the bead 6 shown in FIG. 2 was used, and the pressing load N was 400 kgf, and the pulling-out speed of the sample 1 for measuring the coefficient of friction (the horizontal movement speed of the slide table 3) was 100 cm / min. The coefficient of friction μ between the sample 1 for measuring the coefficient of friction and the bead 6 was calculated by the formula: μ = F / N.

[0058] Also, for the steel plate or plated steel plate with a film formed thereon, with a clearance of 5% and a movable blade speed of 1 m / sec, it was sheared into a size of 100 mm × 30 mm to obtain test piece 14. The fracture surface of the sheared end face 12 of the obtained test piece 14 was bent at R = 10 mm by 180° so that the fracture surface was on the die side and the sheared surface was on the punch side. When evaluating the delayed fracture characteristics, after the bending process, the springback due to bending was tightened by bolt fastening, and stress was applied to the surface layer of the bent portion. Schematic diagrams of the test pieces after the bending process and after bolt fastening are shown in Fig. 4.

[0059] (Sliding mark length) Using a Keyence microscope, for the test piece after shearing, the sliding mark length 11 observed at a magnification of 100 times at the portion where the blade contacts the steel plate surface 13 during shearing (see Fig. 3) was measured at 10 arbitrary points, and the average value was taken as the sliding mark length. Note that the sliding mark length may be evaluated in the as-sheared state (without bending) or after bending. In this example, from the perspective of workability, it was evaluated with the test piece after shearing, but it has been confirmed that the value does not change even when evaluated after bending.

[0060] (Number of cracks) Using a Keyence microscope, the sheared end face 12 of the bent portion 17 of the test piece 15 after bending before bolt fastening was observed at a magnification of 100 times, and the number of cracks generated on the sheared end face 12 of the steel plate and the plated steel plate with the steel plate as the base was counted. Here, the bent portion 17 refers to the region processed with a bending radius R20, and refers to the bent test piece from the bending R stop portion 18 to the other bending R stop portion 19. Here, cracks with a length of 10 μm or more were counted as cracks affecting the delayed fracture characteristics. The counting of the number of cracks was performed over the entire range of the sheared end face 12 of the bent portion 17. Note that the number of cracks may be evaluated before or after bolt fastening. In this example, from the perspective of workability, the number of cracks was evaluated before bolt fastening, but it has been confirmed that the number of cracks does not change even when evaluated after bolt fastening.

[0061] (Delayed fracture characteristics) After bending, the springback caused by bending was tightened by bolt fastening, and stress was applied to the surface layer of the bent portion. The test piece 16 after bolt tightening shown in Fig. 4 was immersed in hydrochloric acid with a pH of 3, and evaluated by the time until cracking occurred in the bent portion 17. Cracking was determined to have occurred when it was confirmed that cracks had progressed 5 mm or more from the shear end face 12 on the steel plate surface of the bent portion 17, and the maximum immersion time was set to 100 hours. Those that did not crack even after 100 hours of immersion were evaluated as a, those that cracked between 50 hours and less than 100 hours of immersion were evaluated as b, those that cracked between 10 hours and less than 50 hours of immersion were evaluated as c, and those that cracked in less than 10 hours of immersion were evaluated as d. The ranking of the stress corrosion cracking characteristics was (excellent) a > b > c > d (poor), and a or b was judged to be qualified. Furthermore, in evaluations b, c, and d, the longer the immersion time until cracking, the better the stress corrosion cracking characteristics.

[0062] The results obtained above are shown in Tables 2-1 and 2-2.

[0063]

Table 1

[0064]

Table 2-1

[0065]

Table 2-2

[0066] From Tables 2-1 and 2-2, it can be seen that all of the present inventions are excellent in stress corrosion cracking characteristics. On the other hand, the comparative examples have poor stress corrosion cracking characteristics.

Explanation of Signs

[0067] 1 Sample for measuring friction coefficient 2 Specimen table 3 Slide table 4 Roller 5 Slide table support base 6 beads 7 First load cell 8 Second load cell 9 Rail 10 Portion where blade contacts (sliding mark) 11 Sliding mark length 12 Sheared end face 13 Steel plate surface (coated surface) 14 Test piece 15 Test piece after bending 16 Test piece after bolt tightening 17 Bending part 18 Bending R stop part 19 Other bending R stop part 20 Bending radius R N Pressing load F Sliding resistance

Claims

1. A steel sheet having a film containing an organic resin and an inorganic substance on its surface, wherein the ratio of the inorganic substance concentration in the 20% of the film region on the surface layer side to the inorganic substance concentration in the 20% of the film region on the steel sheet side in the thickness direction of the film is 1.5 or more, and having a tensile strength of 1180 MPa or more and satisfying the following conditions (A) and (B). (A) When shearing is performed, the length of the sliding mark on the steel sheet surface at the sheared end is 0.4 mm or less. (B) When bending is performed, the number of cracks N1 generated on the sheared end face satisfies the formula (1). (N1 / R1) × T1 ≤ 0.2... (1) In the formula (1), N1 is the number of cracks generated on the sheared end face of the bent portion, R1 is the bending radius (mm) of the bent portion where the number of cracks is evaluated, and T1 is the plate thickness (mm) of the steel sheet before processing.

2. The steel sheet according to claim 1, wherein the film thickness of the film is 0.4 μm or more.

3. A plated steel sheet having a film containing an organic resin and an inorganic substance on its plating surface, wherein the ratio of the inorganic substance concentration in the 20% of the film region on the surface layer side to the inorganic substance concentration in the 20% of the film region on the steel sheet side in the thickness direction of the film is 1.5 or more, and having a tensile strength of 1180 MPa or more, and satisfying the following conditions (C) and (D). (C) When shearing is performed, the length of the sliding mark on the steel sheet surface at the sheared end is 0.3 mm or less. (D) When bending is performed, the number of cracks N2 generated on the sheared end face satisfies the formula (2). (N2 / R2) × T2 = 0... (2) In the formula (2), N2 is the number of cracks generated on the sheared end face of the bent portion, R2 is the bending radius (mm) of the bent portion where the number of cracks is evaluated, and T2 is the plate thickness (mm) of the plated steel sheet before processing.

4. The plated steel sheet according to claim 3, wherein the film thickness of the film is 0.4 μm or more.

5. A press-formed product press-formed using the steel sheet according to claim 1 or 2 or the plated steel sheet according to claim 3 or 4.

6. A method for manufacturing a press-formed product, comprising a step of press-forming using the steel sheet according to claim 1 or 2 or the plated steel sheet according to claim 3 or 4.

7. A method for manufacturing a processed member, comprising a step of shearing and / or a step of bending using the steel sheet according to claim 1 or 2 or the plated steel sheet according to claim 3 or 4.

8. A processed member using the steel sheet according to claim 1 or 2.

9. A processed member using the plated steel sheet according to claim 3 or 4.

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