JP7355083B2 - Methods for shearing and punching steel plates and methods for manufacturing press-formed products - Google Patents

Description

The present invention relates to a method for shearing and punching a steel plate and a method for manufacturing a press-formed product that can suppress strain and minute cracks occurring at sheared end faces. In particular, it is suitable for application to automobile parts using high-strength steel plates with a tensile strength of 1180 MPa or more, and is intended to appropriately suppress delayed fracture.

In recent years, with a view to reducing the weight of structural members for automobiles, efforts have been made to reduce the thickness of the steel plates used by increasing their strength. It is known that as the strength of steel plates increases, delayed fracture becomes more likely to occur, and concerns about delayed fracture, which have not been a problem with conventional automobile parts, have newly emerged.

Delayed fracture is a phenomenon in which high-strength steel parts undergo sudden brittle fracture after a certain period of time under static load stress, with almost no apparent plastic deformation. . In a broad sense, it includes liquid metal contact cracking and stress corrosion cracking, but the problem with automotive parts is hydrogen embrittlement type delayed fracture caused by hydrogen penetrating into steel as a result of corrosion. It is known that there are three factors that cause delayed fracture: material (strength), processing (strain/stress), and hydrogen. Here, possible causes of hydrogen intrusion into metal materials include intrusion from solutions and solvents that come into contact with the metal material, and intrusion of hydrogen generated as the metal material corrodes under the environment in which it is used. .

In the case of steel plates, it is known that this delayed fracture is caused by residual tensile stress when the steel plate is press-formed into a predetermined shape and hydrogen embrittlement of the steel at stress concentration areas.

In recent years, various proposals have been made regarding methods for evaluating delayed fracture in high-strength steel plates of 1180 MPa or higher. For example, there is a method of evaluating delayed fracture characteristics using a test piece (steel plate) subjected to U-bending after shearing. Strain (work hardening and residual stress due to contact between the blade and the steel plate) and minute cracks occur on the sheared end surface of the steel plate after shearing. Due to this strain and minute cracks, the frequency of occurrence of cracks on the sheared end face of a steel plate subjected to U-bending may vary, and the influence of the sheared end face on delayed fracture characteristics has become a problem. Further, since sheared end faces exist in actual automobile parts, delayed fracture due to strain and minute cracks on the sheared end faces can become a major problem.

In order to improve the delayed fracture characteristics of the sheared end surface, the techniques disclosed in Patent Document 1 and Patent Document 2 reduce the residual stress of the sheared end surface by controlling the shearing conditions or punching conditions. In addition, as in Patent Document 3, lubricant is applied to the die and the blade for shearing to suppress mold wear, and as in Patent Document 4, the blade side is coated with lubricant to improve the corrosion resistance of the sheared end surface. A technique is disclosed in which a rust preventive agent is applied.

However, with this alone, it is difficult to suppress minute cracks that occur on the sheared end face, and it is difficult to suppress delayed fracture of the sheared end face. Furthermore, not only the sheared end face but also the portion where the blade and the steel plate are in contact during shearing is affected by work hardening and residual stress due to the contact between the blade and the steel plate. Therefore, it is necessary to suppress minute cracks that occur even in the portion where the blade and the steel plate come into contact.

JP2014-223663A Japanese Patent Application Publication No. 2006-224151 Japanese Patent Application Publication No. 2003-105565 Patent No. 5239484

The present invention has been made in view of the above circumstances, and includes a method for shearing and punching a steel plate, and a press forming method that can suppress the strain and minute cracks that occur at the part where the blade and the steel plate contact each other during shearing and at the sheared end face. The purpose is to provide a method for manufacturing products.

The gist of the present invention is as follows.
[1] A method for shearing a steel plate, in which the coating is brought into contact with a steel plate having a tensile strength of 1180 MPa or more using an upper blade and/or a lower blade having a coating to cut out the steel plate.
[2] The method for shearing a steel plate according to [1], wherein the film has a thickness of 10 nm or more.
[3] The method for shearing a steel plate according to [1] or [2], wherein the upper blade and/or lower blade having the coating have a coefficient of friction of 0.5 or less.
[4] The method for shearing a steel plate according to any one of [1] to [3], wherein the elongation of the coating is 130% or more.
[5] The method for shearing a steel plate according to any one of [1] to [4], wherein the blade clearance during shearing is 0 to 30%.
[6] The method for shearing a steel plate according to any one of [1] to [5], wherein the steel plate is cut at a speed of the movable blade of 1 m/sec or less during shearing.
[7] A method for punching a steel plate, using a punch and/or die having a coating, and bringing the coating into contact with a steel plate having a tensile strength of 1180 MPa or more to cut out the steel plate.
[8] The method for punching a steel plate according to [7], wherein the film has a thickness of 10 nm or more.
[9] The method for punching a steel plate according to [7] or [8], wherein the punch and/or die having the coating has a coefficient of friction of 0.5 or less.
[10] The method for punching a steel plate according to any one of [7] to [9], wherein the elongation of the film is 130% or more.
[11] The method for punching a steel plate according to any one of [7] to [10], wherein the steel plate is cut out with a blade clearance of 0 to 30% during shearing.
[12] The method for punching a steel plate according to any one of [7] to [11], wherein the steel plate is cut out at a speed of the movable blade of 1 m/sec or less during shearing.
[13] A method for producing a press-formed product by press-forming a steel plate cut out by the method according to any one of [1] to [12].

According to the present invention, it is possible to suppress the occurrence of minute cracks in the portion where the blade and the steel plate contact during shearing, and the strain and minute cracks that enter the sheared end face. As a result, it is possible to improve the delayed fracture characteristics of the sheared end face. Therefore, the press-formed product obtained by the present invention is suitable for automobile parts.

FIG. 1 is a schematic diagram showing the shearing method of the present invention, and is a diagram illustrating the position of the film. FIG. 2 is a schematic front view showing the friction coefficient measuring device. FIG. 3 is a schematic perspective view showing the shape and dimensions of the bead in FIG. 2. FIG. 4 is a schematic diagram of a test piece after bending and bolting. FIG. 5 is a schematic diagram showing the observed locations of the metallic luster portions in Examples.

The present invention is a method for shearing a steel plate, and is characterized in that a steel plate having a tensile strength of 1180 MPa or more is brought into contact with the coating using an upper blade and/or a lower blade having a coating to cut the steel plate. Further, in the method of punching out a steel plate, the present invention is characterized in that a punch and/or die having a coating is used to bring the coating into contact with a steel plate having a tensile strength of 1180 MPa or more to cut out a steel plate. In the present invention, the upper and lower blades of the shearing machine and the punch and die of the punching machine may be simply referred to as blades (movable blades).

The shearing method and punching method of the present invention will be explained below.

FIG. 1 is a schematic diagram illustrating the shearing method of the present invention as an example. When shearing a steel plate using a shearing machine, as shown in FIG. 1, the steel plate fixed with a plate holder is sandwiched between the upper and lower blades, plastically deformed, and finally broken.

In the shearing method of the present invention, a steel plate having a tensile strength of 1180 MPa or more is cut using an upper blade and/or a lower blade having a coating. The above means that the steel plate is cut out using the upper blade and the lower blade, and that at least one of the upper blade and the lower blade has a coating. At this time, as shown in FIG. 1, the steel plate is cut out so that the coating and the steel plate are in contact with each other. By using the upper and/or lower blades that have a coating on the blade surface, the blade does not come into direct contact with the steel plate, and the cushioning effect of the coating reduces the shearing force applied to the contact area between the blade and the steel plate and the sheared end surface. Stress is dispersed and strain caused by contact between the blade and the steel plate can be suppressed. Normally, when a steel plate is bent, minute cracks are likely to occur on the outside of the bent portion due to strain during shearing. Hydrogen is likely to accumulate in these minute cracks, causing delayed fracture due to hydrogen embrittlement, resulting in cracks that propagate to the outside of the bent portion of the steel plate. In this way, by using the upper and/or lower blades having a coating on the blade surface in contact with the steel plate, strain can be suppressed, leading to suppression of delayed fracture.

Further, in the punching method of the present invention, a punch and/or die having a coating is used to cut out a steel plate having a tensile strength of 1180 MPa or more. The above means that a steel plate is punched using a punch and a die, and that at least one of the punch and die has a coating. At this time, the steel plate is cut out so that the coating and the steel plate are in contact with each other. Similar to the shearing method of the present invention, by using a punch and/or die that has a film on the surface that contacts the steel plate, the punch and die do not come into direct contact with the steel plate, and the film has a cushioning effect. As a result, the stress during shearing applied to the contact area between the blade and the steel plate and the sheared end face is dispersed, making it possible to suppress strain caused by contact between the punch or die and the steel plate. Normally, due to the strain during shearing, small cracks are likely to occur at the part where the blade and steel plate come into contact during bending and at the sheared end surface. Hydrogen tends to accumulate in these minute cracks, causing delayed fracture due to hydrogen embrittlement, resulting in cracks that propagate to the sheared end face. In this way, by using a punch or die that has a coating on the contact surface with the steel plate, strain can be suppressed, leading to suppression of delayed fracture.

The shearing method and punching method of the present invention target steel plates having a tensile strength of 1180 MPa or more, which is likely to cause delayed fracture. Note that the steel plate surface may have a plating layer. In order to improve the corrosion resistance of the steel plate, it is preferable to have a plating layer containing at least one of Zn, Fe, Al, Mg, Ni, and Si on the surface of the steel plate. Further, in the case of a steel plate having a plating layer, a film made of a non-metal, which will be described later, is formed on the plating layer.

Next, preferred forms of the film in the shearing method and punching method of the present invention will be explained.

The thickness of the film is 10 nm or more The thickness of the film is preferably 10 nm or more. When the film thickness of the film is less than 10 nm, the cushioning effect of the film is small. Therefore, the stress during shearing cannot be dispersed, and the effect of suppressing strain and minute cracks cannot be exhibited. Furthermore, in order to exhibit strain suppression due to the cushioning effect of the film, the thickness of the film is preferably 100 nm or more, more preferably 1000 nm or more. On the other hand, when the film thickness of the film increases, shearing or punching tends to cause sag on the sheared end face, and minute cracks are likely to occur from the sagging portion. In addition, in a blanking press or the like, the thickness of the film makes it difficult to perform predetermined press forming and increases costs, so it is preferable that the film thickness of the film is 1 mm or less.

The friction coefficient of the upper and/or lower blades with a coating is 0.5 or less The friction coefficient of the punch and/or die with a coating is 0.5 or less In order to exhibit the strain suppression effect on the sheared end face, the friction coefficient must be , preferably 0.5 or less. Preferably, it is 0.3 or less. When the coefficient of friction becomes smaller, the surface pressure between the steel plate and the shearing or punching blade becomes smaller, making it easier to suppress distortion. If the friction coefficient is larger than 0.5, the strain suppressing effect cannot be exhibited.

The elongation of the film is 130% or more The elongation of the film is preferably 130% or more. During shearing or punching, the film that exists between the blade and the steel plate stretches, and the stress applied to the area where the blade and steel plate contact due to the cushioning effect is dispersed and reduced. For this reason, the strain that enters the area where the blade and the steel plate contact is suppressed, and the occurrence of minute cracks is reduced. If it is 130% or more, a sufficient cushioning effect can be exhibited even when the film thickness is thin. If it is less than 130%, the cushioning effect of the film cannot be obtained when the film thickness is thin. Note that the elongation of the film may be the tensile elongation when the film thickness is 1 mm according to JIS-C-2151. If the film has a tensile elongation of 130% or more at a thickness of 1 mm, the cushioning effect of the film can be obtained even if the actual film thickness is thinner than 1 mm.

The type of film in the present invention does not need to be particularly limited, but examples include inorganic films and organic films. Examples of inorganic films include Mn--P oxide films, Ni-based inorganic films, zinc-based oxide films, copper-based oxide films, and iron-based oxide films. Examples of organic films include polyvinyl chloride resins, polyethylene resins, polypropylene resins, epoxy resins, polyhydroxy polyether resins, polyester resins, urethane resins, silicone resins, and acrylic resins. Note that for organic coatings and inorganic coatings, sheet-shaped coatings may be used. When using sheet-shaped coatings, the steel plate can be cut out with the sheet-shaped coating sandwiched between the steel plate and the blade surface. good. Further, even if the film is an organic-inorganic composite film, the effect can be exhibited.

In the present invention, the cushioning effect of the present invention can be obtained as long as the film is present at the location where it comes into contact with the steel plate during shearing or punching (the upper and lower surfaces of the steel plate in FIG. 1). Therefore, it is not necessary to have a coating on the entire upper blade (lower blade) or the entire punch (die).

A steel plate cut out by the shearing method or punching method of the present invention can suppress distortion and generation of minute cracks on the sheared end face. Therefore, by press-forming a steel plate cut out by the shearing method or punching method of the present invention, the obtained press-formed product can suppress delayed fracture from the sheared end surface. In the present invention, press molding conditions are not particularly limited.

In the present invention, it is preferable that the steel plate is cut out with a blade clearance of 0 to 30% during shearing or punching, and then press-formed. Note that the blade clearance refers to the clearance between the upper and lower blades during shearing, and the clearance between the punch and die during punching. Even when the clearance is 0%, that is, in the case of fine blanking where the blade clearance is 0.01 mm or less, the film prevents the blade from directly touching the steel plate and exerts a cushioning effect to suppress distortion. is possible. On the other hand, if the clearance is larger than 30%, cracks are likely to occur due to delayed fracture from burrs on the sheared end surface.

Furthermore, the speed of the movable blade during shearing or punching is preferably 1 m/sec or less. Note that the movable blade refers to the upper or lower blade for shearing, and the punch for punching. If the speed of the movable blade is faster than 1 m/sec, an unnecessarily high shearing load will be applied to the part where the blade contacts the steel plate during shearing (see Figure 5), and the cushioning effect of the film cannot be expected, resulting in distortion and small Cracks are more likely to occur.

The present invention will be explained using examples. Note that the present invention is not limited to the following examples.

The steel plates shown in Table 2 were cut out using the shearing machine shown in FIG. 1, and the strain suppression effect, microcracks, and delayed fracture characteristics of the obtained test pieces were evaluated.

The types of coatings are shown in Table 1: no coating, organic coating of polyvinyl chloride sheet and epoxy resin, organic-inorganic composite coating of epoxy resin/crystalline layered material, and inorganic coating of basic zinc sulfate. It was made into a tri- to pentahydrate. The coating was provided on the blade surfaces of the upper and/or lower blades of the shearing machine (positions in contact with the upper and lower surfaces of the steel plate in FIG. 1).

A polyvinyl chloride sheet (80,000 nm, 200,000 nm) with an organic film was in the form of an adhesive sheet, and the film was applied by pressing it against the blade surface using a roller. Further, for the epoxy resin of the organic film, an amine-modified epoxy resin/blocked isocyanate curing agent was applied to the blade surface using a roll coater, and baked at 140°C. The thickness of the epoxy resin in the organic film was appropriately controlled by changing the speed of the roll coater.

The epoxy resin/crystalline layered material of the organic-inorganic composite film is prepared by adding 113 g/L of an aqueous solution of magnesium nitrate hexahydrate and 83 g/L of an aqueous solution of aluminum nitrate nonahydrate to 31 g of an aqueous solution of sodium bicarbonate decahydrate in advance. The precipitate obtained by purification by dropping /L was filtered and dried to obtain a crystalline layered product [Mg _0.667 Al _0.333 (OH) ₂ ][CO ₃ ² ] _0.167・0.5H ₂ O is mixed with amine-modified epoxy resin/blocked isocyanate curing agent at a mass ratio of 10:2 (amine-modified epoxy resin/blocked isocyanate curing agent: crystalline layered material) and applied to the blade surface with a roller, It was baked at 140°C. It was confirmed by XRD analysis that the crystalline layered material was [Mg _0.667 Al _0.333 (OH) ₂ ][CO ₃ ² ] _0.167 ·0.5H ₂ O. The film thickness was appropriately controlled by changing the speed of the roll coater.

Basic zinc sulfate tri-pentahydrate for inorganic coating is prepared by immersing the upper and/or lower blade in a zinc sulfate heptahydrate aqueous solution at a concentration of 20 g/L and a temperature of 50°C, and then thoroughly soaking it. It was obtained by washing with water and then drying. It was confirmed by XRD analysis that it was basic zinc sulfate tri-pentahydrate. The film thickness was appropriately controlled by changing the immersion time.

In addition, the film thickness of the film was measured. Organic coatings (epoxy resin) and organic-inorganic composite coatings (epoxy resin/crystalline layered material) are prepared by sputtering the cross section of the coating at a 45° angle using FIB, observing the cross section using an extremely low acceleration SEM, and making arbitrary adjustments. The average value of 10 measurements was taken as the average value. For the inorganic film (basic zinc sulfate tri-pentahydrate), the film thickness was determined using a fluorescent X-ray analyzer. The measurement conditions for the fluorescent X-ray analyzer are that the voltage and current of the tube are 30 kV and 100 mA, and when measuring O-Kα rays using the spectroscopic crystal TAP, in addition to the peak position, the intensity at the background position is measured. It was also possible to calculate the net intensity of the O-Kα rays. Note that the integration times at the peak position and the background position were each 20 seconds. In addition, silicon wafers on which silicon oxide films of 96 nm, 54 nm, and 24 nm were formed were set on the sample stage, and the intensity of the O-Kα rays of these silicon oxide films could be calculated. A calibration curve with the strength was created, and the value in terms of silicon oxide film was taken as the film thickness.

Furthermore, the coefficient of friction of the upper blade and/or lower blade having each film was measured as follows. FIG. 2 is a schematic front view showing the friction coefficient measuring device. As shown in the figure, a sample 1 for friction coefficient measurement, which is a sample made of the same material as the upper or lower blade and coated with a film, is fixed to a sample stage 2, and the sample stage 2 is attached to the upper surface of a horizontally movable slide table 3. is fixed. A vertically movable slide table support 5 having a roller 4 in contact with it is provided on the lower surface of the slide table 3, and by pushing it up, the pressing load N exerted by the bead 6 on the sample 1 for friction coefficient measurement is measured. A first load cell 7 is attached to the slide table support 5. One side of the slide table 3 is moved so that the second load cell 8 moves on the rail 9 in order to measure the sliding resistance force F when the slide table 3 is moved in the horizontal direction while the pressing force is applied. attached to the end of the The test was conducted by applying Pleton R352L, a press cleaning oil manufactured by Sugimura Chemical Industry Co., Ltd., as a lubricating oil to the surface of Sample 1 for friction coefficient measurement. FIG. 3 is a schematic perspective view showing the shape and dimensions of the beads used. The lower surface of the bead 6 slides while being pressed against the surface of the sample 1. The shape of the bead 6 shown in Fig. 3 is 10 mm wide, 4 mm long in the sliding direction of the sample, the lower part of both ends in the sliding direction is composed of a curved surface with a curvature of 0.5 mm, and the lower surface of the bead against which the sample is pressed has a width of 10 mm, and the length in the sliding direction is 4 mm. It has a plane with a direction length of 3 mm. In the friction coefficient measurement test, the bead shown in FIG. 3 was used, the pressing load N was 400 kgf, and the sample withdrawal speed (horizontal movement speed of the slide table 3) was 100 cm/min. The friction coefficient μ between the sample and the bead was calculated using the formula: μ=F/N.

Regarding the elongation of the film, the tensile elongation when the film thickness was 1 mm was calculated in advance according to JIS-C-2151.

Next, each steel plate was sheared into 100 mm x 30 mm under the blade clearance conditions and movable blade speed shown in Table 2 to obtain test pieces. The resulting test piece was bent by 180° at R=10 mm so that the fractured surface of the sheared end surface was on the die side and the sheared surface was on the punch side. The following evaluations were performed on the test pieces after bending.

(Strain suppression effect)
The strain suppression effect was determined by observing the presence or absence of a metallic luster (a smooth surface created by crushing the surface of the steel plate with the blade) in the area where the blade and the steel plate come into contact during shearing (see Figure 5). It was determined that there was no suppressing effect when the metallic shiny part existed in the entire contact area between the blade and the steel plate, and that there was a suppressing effect when there was no metallic shiny part. Moreover, when metallic luster parts were scattered, it was determined that some metallic luster parts were present. The metallic shiny part is a part crushed by a blade, and is similar to a state in which a steel plate is subjected to a rolling process and subjected to strain. From this, it was determined that there was no strain suppressing effect when there was a metallic shiny part, and there was a strain suppressing effect when there was no metallic shiny part. In addition, it was judged that the strain suppressing effect was smaller in the case where some metallic shiny parts were present, although the strain suppressing effect was smaller than that in the case where there was no metallic shiny part.

(micro crack)
Using a microscope manufactured by Keyence Corporation, the number of cracks generated on the outside of the bending portion of the bending R end portion (portion subjected to bending R processing) shown in FIG. 4 was confirmed. If the number of cracks was 10 or less, it was determined that there was a suppressing effect, and if the number of cracks was 5 or less, it was judged that there was a significant suppressing effect.

(Delayed fracture characteristics)
After the bending process, bolts were tightened to compensate for the springback caused by bending, and stress was applied to the surface layer at the apex of the bend. FIG. 4 shows a schematic diagram of the test piece after bending and bolting. The test piece shown in FIG. 4 after tightening the bolts was immersed in hydrochloric acid at pH 3, and evaluated based on the time until cracking occurred. The maximum immersion time was 100 hours. Items that did not crack after 100 hours of immersion received a rating of A, items that cracked after 50 hours or more but less than 100 hours received a rating of B, items that cracked after 10 hours or more but less than 50 hours received a rating of C, and items that cracked after immersion for less than 10 hours received a rating of C. The evaluation was d. A, b, and c were judged to be acceptable.

Table 2 shows the results obtained above.

From Table 2, it was found that the present invention suppresses strain and minute cracks, and has excellent delayed fracture characteristics. It was also confirmed that the same results as above were obtained when a steel plate was punched with a punch and die.

1 Sample for friction coefficient measurement 2 Sample stand 3 Slide table 4 Roller 5 Slide table support 6 Bead 7 First load cell 8 Second load cell 9 Rail N Pressing load F Sliding resistance force

Claims (9)

Using upper and lower blades, at least one of which has a film, the film thickness is 10 nm or more, and the tensile elongation is 130% when the film thickness is 1 mm according to JIS-C-2151. A method for shearing a steel plate, in which the steel plate having a tensile strength of 1180 MPa or more is brought into contact with the coating having a tensile strength of 200% or less, and the steel plate is cut out. The method for shearing a steel plate according to claim 1, wherein the upper blade and/or the lower blade having the coating have a friction coefficient of 0.5 or less. The method for shearing a steel plate according to claim 1 or 2, wherein the cutting is performed with a blade clearance of 0 to 30% during shearing. The method for shearing a steel plate according to any one of claims 1 to 3, wherein the steel plate is cut at a speed of the movable blade of 1 m/sec or less during shearing. Using a punch and die in which at least one of the punch and die has a film , the film thickness is 10 nm or more, and the tensile elongation when the film thickness is 1 mm is 130% or more and 200% or less according to JIS-C-2151. A method for punching out a steel plate, comprising bringing the film into contact with a steel plate having a tensile strength of 1180 MPa or more and cutting out the steel plate. The method for punching a steel plate according to claim 5 , wherein the punch and/or die having the film has a friction coefficient of 0.5 or less. The method for punching a steel plate according to claim 5 or 6, wherein the cutting is performed with a blade clearance of 0 to 30% during shearing. The method for punching a steel plate according to any one of claims 5 to 7, wherein the steel plate is cut out at a speed of the movable blade of 1 m/sec or less during shearing. A method for producing a press-formed product by press-forming a steel plate cut out by the method according to any one of claims 1 to 8 .

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