JP7107327B2 - Press-molded product manufacturing method and press-molded product - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for manufacturing a press-formed product and a press-formed product obtained by the method, and particularly to effectively prevent delayed fracture of a press-formed product using a high-strength steel plate. More specifically, it is suitable for application to strength members for automobiles using high-strength steel sheets having a tensile strength of 1180 MPa or more, and is intended to appropriately suppress delayed fracture.

Conventionally, as steel sheets for automobiles, cold-rolled steel sheets, hot-dip galvanized steel sheets (GI) obtained by galvanizing cold-rolled steel sheets, alloyed hot-dip galvanized steel sheets (GA), Electro-galvanized steel sheets (EG) have been used, but in recent years, efforts have been made to increase the strength of steel sheets for automobiles from the viewpoint of reducing CO ₂ emissions from automobiles and ensuring safety.
However, it is known that delayed fracture tends to occur as the strength of steel materials increases, and this tendency is particularly pronounced in high-strength steels having a tensile strength of 1180 MPa or more.
In addition, delayed fracture is a state in which a high-strength steel material is subjected to a static load stress (a load stress less than the tensile strength), and when a certain amount of time has passed, there is almost no apparent plastic deformation, and it suddenly breaks down. This is a phenomenon in which brittle fracture occurs.

In the case of a steel plate, it is known that this delayed fracture is caused by residual tensile stress when the steel plate is formed into a predetermined shape by press working and hydrogen embrittlement of the steel at the stress concentration portion. In particular, blanking before press working and sheared edge surfaces in the trimming process are the most likely starting points for delayed fracture. In most cases, the hydrogen that causes this hydrogen embrittlement is thought to be hydrogen that has penetrated and diffused into steel from the external environment. invaded and diffused into

In order to prevent such delayed fracture in high-strength steel sheets, Patent Document 1, for example, discusses weakening the delayed fracture susceptibility by adjusting the structure and components of the steel sheet.
Further, Patent Document 2 discusses a high-strength alloyed hot-dip galvanized steel sheet that prevents delayed fracture.
Furthermore, Patent Literature 3 discloses a technique of suppressing delayed fracture by suppressing the amount of hydrogen permeation into the steel sheet by applying Ni or Ni-based alloy plating to the cold-rolled steel sheet.
On the other hand, Non-Patent Document 1 discloses a technique for suppressing delayed fracture in a constant load test by subjecting S45C to shot peening, although it is not an automobile member.
In addition, in Patent Documents 4 and 5, shot peening is performed on the sheared edge surface or the entire steel plate having a tensile strength of 1180 MPa or more by centrifugal shot blasting or compressed air sand blasting, thereby improving delayed fracture resistance. disclosed.

JP-A-2004-231992 JP-A-6-145893 JP-A-6-346229 JP 2017-125229 A JP 2017-125228 A

Materials, Vol.41, No.465, pp933-938 (1992) Yoshihiro Watanabe, Toshihiko Hasegawa, Michio Inoue

However, in Patent Literature 1, the amount of hydrogen entering the inside of the steel sheet from the external environment does not change, so it is possible to delay the occurrence of delayed fracture, but the delayed fracture itself cannot be prevented.
Moreover, in Patent Document 2, the Fe concentration in the plating is about ten and several percent, and although corrosion resistance is obtained, excellent delayed fracture resistance cannot be expected.
Furthermore, in Patent Document 3, although it has the effect of suppressing delayed fracture originating from the general surface portion of the molded product, it is not sufficient to suppress delayed fracture caused by the sheared end surface.
In addition, Non-Patent Document 1 and Patent Documents 4 and 5 cannot be applied to molded products to which rust preventive oil or press oil is attached, because the oil solidifies the shot material. In centrifugal shot blasting, shot peening can be applied to the entire surface, but it is not possible to apply shot peening locally only to sheared end surfaces and tensile stress concentration areas. In compressed air sandblasting, particles are jetted radially from the nozzle, so even if shot peening is attempted only on the sheared edge surface and the tensile stress concentration area, the particles also collide with the non-edge surface area and the non-stress concentration area. Collision of particles to such unintended locations causes deterioration of appearance quality, which is considered important for steel sheets for automobiles, and damage to the plating layer of zinc-based plated steel sheets.

The present invention solves the above-described problems of the conventional technology, and is effective for target locations such as sheared end faces and stress concentration parts of members where there is concern about the occurrence of delayed fracture in press-formed products using high-strength steel plates. It is an object of the present invention to provide a technology capable of effectively suppressing the occurrence of delayed fracture by performing a shot peening treatment on the steel, and to provide a press-formed product as a result of the technology.

In order to solve the above problems, the present inventors diligently studied means for preventing delayed fracture originating from stress concentration parts of sheared end surfaces and other general surfaces of high-strength automotive members to which oil adheres. and researched.
As a result, the delayed fracture of the steel sheet can be effectively prevented by applying a wet blasting treatment using fine particles having an appropriate particle size and hardness to the sheared edge surface and/or the stress concentration portion of the general surface of the press-formed product. I got the knowledge.

The present invention has been completed based on the above findings, and the gist and configuration thereof are as follows.
1. A step of pressing a steel plate having a tensile strength of 1180 MPa or more, and a step of subjecting at least a part of the sheared edge portion or the stress concentration portion of the steel plate to shot peening by a wet blast method using fine particles. A method for manufacturing a press-molded product.

2. 2. The method for producing a press-formed product according to 1 above, wherein the steel plate is obtained by shearing.

3. After performing trimming and/or drilling by shearing on the pressed member, shot peening is performed by a wet blast method using fine particles on at least a part of the sheared end face portion or the tensile stress concentration portion of the formed member after processing. 3. The method for producing a press-formed product according to 1 or 2 above, wherein the press-formed product is treated.

4. 4. The method for producing a press-formed product according to any one of 1 to 3 above, wherein the material of the fine particles is one or more selected from stainless steel, alumina, zirconia, resin and glass. .

5. 5. The method for producing a press-formed product according to any one of 1 to 4 above, wherein the shape of the fine particles is polygonal and/or spherical.

6. 6. The method for producing a press-formed product as described in any one of 1 to 5 above, wherein the fine particles have a particle diameter of 6 to 250 μm and a hardness of 300 HV or more.

7. When the shot peening treatment by the wet blasting method is applied to the molded parts after press working, it is performed under the condition that the energy density satisfies 7.0×10 ⁻⁵ to 2.7×10 ⁻² J/mm ² . However, when applied to trimmed and/or drilled steel sheets before press working, it is carried out under conditions that satisfy an energy density of 2.5 × 10 ^-4 to 2.7 × 10 ^-2 J/mm ² 7. The method for producing a press-formed product according to any one of 1 to 6 above, characterized in that:

8. 8. Any one of 1 to 7 above, wherein the shot peening treatment by the wet blasting method is performed under conditions satisfying an energy density of 1.5×10 ⁻³ to 2.7×10 ⁻² J/mm ² . 2. A method for producing a press-molded product according to 1.

9. 9. The method for producing a press-formed product according to any one of 1 to 8 above, wherein 50% or more of microcracks observed before treatment disappear by shot peening treatment by wet blasting of the fine particles.

10. 10. The method for producing a press-formed product according to any one of 1 to 9 above, wherein the fine particles are shot peened by a wet blast method without degreasing the oil adhering to the steel plate.

11. 11. The method for producing a press-formed product as described in any one of 1 to 10 above, wherein the shot peening treatment by the wet blasting method is performed under conditions satisfying a treatment time of 0.15 seconds or less.

12. It has a tensile strength of 1180 MPa or more, a residual stress of less than 875 MPa in at least a part of the sheared end face portion or the stress concentration portion, and fine particles for shot peening treatment by a wet blast method are attached to the surface and end face. A press-molded product characterized by:

13. The fine particles are alumina or zirconia, and when the fine particles are alumina, the amount is in the range of 12.0 to 63.9 mg/m ² , and when the fine particles are zirconia, the amount is in the range of 0.38 to 4.18 mg/m ² . 13. The press-molded product as described in 12 above, wherein the adhesive is adhered to each other within a range.

The press-formed product obtained by applying the present invention to a press-formed product using a high-strength steel plate, particularly preferably to a high-strength member for automobiles, has a stress concentration at the sheared end surface and other general surfaces of the member. It is possible to effectively suppress the occurrence of delayed fracture originating from the part.
In addition, the present invention can be suitably applied even when oil such as rust preventive oil or press oil or dirt adheres to the press-molded product.
Furthermore, according to the present invention, the shot peening treatment can be efficiently applied to the target portion, so that the appearance quality required for automobile members is not impaired.

It is a schematic diagram of the shot peening process outline|summary using a wide gun. It is a schematic diagram of a shot peening treatment outline using a round gun. FIG. 3 is an explanatory diagram showing the steps of a combined cycle corrosion test performed in Examples. It is a figure which shows typically the test piece for delayed fracture evaluation used in the Example.

The present invention will be specifically described below.
The steel plate (material steel plate) that serves as the substrate of the high-strength member (press-formed product) of the present invention is a steel plate having a tensile strength of 1180 MPa or more, more preferably 1320 MPa or more.
Steel sheets with low tensile strength are inherently resistant to delayed fracture. Although the effect of the present invention is exhibited even in a steel sheet with a low tensile strength, it is remarkably exhibited in a steel sheet with a tensile strength of 1180 MPa or more, and is exhibited more remarkably in a steel sheet with a tensile strength of 1320 MPa or more.

In the present invention, the chemical composition and steel structure of the steel sheet are not particularly limited. However, it is limited to a cold-rolled steel sheet or a zinc-based plated steel sheet having a zinc-based coating applied to the surface thereof.
Among these, high-strength cold-rolled steel sheets having a tensile strength of 1180 MPa or more, which are often used in the automobile field, are preferable, and high-strength cold-rolled steel sheets having a tensile strength of 1320 MPa or more are more preferable. Moreover, the steel plate obtained by shearing can be used as the steel plate.

The high-strength cold-rolled steel sheet suitably used in the present invention may have any composition and structure as long as it has the desired tensile strength, but it is more advantageous to apply the following treatment (improvement). be.
In order to improve various properties such as mechanical properties of cold-rolled steel sheets, for example,
(1) Solid solution strengthening by addition of interstitial solid solution elements such as C and N and substitution type solid solution elements such as Si, Mn, P and Cr, precipitation strengthening by carbon and nitrides such as Ti, Nb, V and Al , strengthening by addition of strengthening elements such as W, Zr, Hf, Co, B, Cu, rare earth elements;
(2) Strengthening by recovery annealing at a temperature at which recrystallization does not occur, partial recrystallization strengthening that leaves an unrecrystallized region without completely recrystallizing, bainite or martensite single phase, or ferrite and these transformation structures Structural reform, such as strengthening of the metamorphic structure by forming a composite structure of
(3) Hall-Petch formula when the ferrite grain size is d: σ = σ ₀ + kd ^−1/2 (where σ: stress, σ ₀ , k: material constant) , strengthening by rolling, etc., can be performed singly or in combination.

The composition of such a high-strength cold-rolled steel sheet is, for example, in mass%, C: 0.1 to 0.4%, Si: 0 to 3.0%, Mn: 1 to 10%, P: 0 to 0 .05%, S: 0 to 0.005%, the balance being Fe and unavoidable impurities, to which one or more of Cu, Ti, V, Al, Cr, Ni, etc. are added , etc. can be exemplified.
Commercially available high-strength cold-rolled steel sheets having the above tensile strength include, for example, JFE-CA1180, JFE-CA1370, JFE-CA1470, JFE-CA1180SF, JFE-CA1180Y1, JFE-CA1180Y2 (above, manufactured by JFE Steel Corporation) and the like can be exemplified.

The thickness of the steel plate (steel plate material) that serves as the substrate in the present invention is not particularly limited, but is preferably about 0.8 to 2.5 mm, more preferably about 1.2 to 2.0 mm.
In addition, such a cold-rolled steel sheet may be subjected to zinc-based plating. The method of galvanizing is not particularly limited, and general methods such as hot dip galvanizing and electrogalvanizing can be employed. Here, the processing conditions for electrogalvanizing and hot-dip galvanizing are not particularly limited, and suitable conditions may be adopted as appropriate. In the case of hot-dip galvanizing, it is preferable to add Al to the plating bath from the viewpoint of dross countermeasures. In this case, the additive element components other than Al to the plating bath are not particularly limited. That is, even if Pb, Sb, Si, Sn, Mg, Mn, Ni, Ti, Li, Cu, etc. are contained or added in addition to Al, the effects of the present invention are not impaired.

Furthermore, alloying treatment may be performed after hot-dip galvanizing.
In the present invention, conditions for the alloying treatment are not particularly limited, and suitable conditions may be adopted as appropriate. As the steel sheet, a steel sheet subjected to zinc-based plating treatment or a steel sheet subjected to alloying treatment after zinc-based plating treatment can be used.

Furthermore, the processing method and shape of the high-strength member of the present invention are not particularly limited. It may be molded by a commonly used molding process and trimmed.

In the present invention, it is possible to suppress delayed fracture by applying shot peening treatment (hereinafter also simply referred to as wet blasting treatment) to the sheared edge portion of the trimmed steel plate by a wet blasting method. It is possible to suppress delayed fracture even in the treatment before press working, but it is remarkable by applying wet blast treatment to the stress concentration part of the sheared end face and other general surfaces after press working and becoming a member. It is possible to suppress delayed fracture.
The mechanism is thought to be that by colliding particles with a molded product that has tensile residual stress on the surface layer, it is possible to generate compressive residual stress, which relaxes the tensile residual stress. Delayed fracture occurs when tensile stress is applied to hydrogen entering from the environment, so delayed fracture does not occur in the case of compressive residual stress. Rather, delayed fracture does not occur by alleviating the residual tensile stress with the compressive residual stress.

The wet blasting treatment according to the present invention can be carried out according to conventional methods except for the items specified in the present invention.
The material of fine particles for wet blasting (also referred to as blasting particles in the present invention) is desirably one or more selected from stainless steel, alumina, zirconia, resin and glass.

As for the shape of the fine particles, polygonal particles or spherical particles generally used in the blasting method can be used. The type of particles can be appropriately selected according to the environment in which the high-strength member is used and the compressive stress to be introduced.
Here, polygonal particles are particles having sharp corners, which are generally called grids.

It is desirable to use fine particles having a size of 6 to 250 μm. This is because if the particle diameter of the fine particles is less than 6 μm, the collision energy of the fine particles is small, so that it is difficult to introduce compressive stress or relax tensile stress. On the other hand, if the particle size exceeds 250 μm, the treatment will be uneven, making it difficult to uniformly introduce compressive stress or relax tensile stress.
As the fine particles to be used, relatively hard particles are more effective in introducing residual stress and, at the same time, are superior in wear resistance, and thus are effective from the viewpoint of particle life during circulating use. Here, the suitable hardness of such fine particles is preferably 300 HV or more from the viewpoint of introduction of residual stress and particle life.

In the present invention, the conditions for shot peening treatment by the wet blast method are such that the energy density is 7.0×10 ⁻⁵ to 2.7×10 ⁻² J/mm ² when applied to molded parts after press working. A range is preferred. If the energy density is less than 7.0×10 ^-5 J/mm ² and the surface of the steel sheet is coated with rust preventive oil, cleaning oil, press oil, etc., sufficient compressive residual stress must be applied. can't On the other hand, if the energy density exceeds 2.7×10 ⁻² J/mm ² , the steel plate and equipment may wear out.

When applying the present invention to a steel plate that has been trimmed and/or drilled before press working, the energy density should be in the range of 2.5×10 ⁻⁴ to 2.7×10 ⁻² J/mm ² . preferred. If the energy density is less than 2.5×10 ^-4 J/mm ² and the surface of the steel sheet is coated with rust preventive oil, cleaning oil, press oil, etc., sufficient compressive residual stress must be applied. may not be possible. On the other hand, if the energy density exceeds 2.7×10 ⁻² J/mm ² , the steel plate and equipment may wear out.

In addition, when applying to molded parts after press working, and when applying to steel sheets that have been trimmed and/or drilled before press working, the conditions for shot peening treatment by the wet blast method are that the energy density is 1. More preferably, it is in the range of 0.5×10 ⁻³ to 2.7×10 ⁻² J/mm ² .
When the energy density is 1.5×10 ^-3 J/mm ² or more, even if rust preventive oil, cleaning oil, press oil, etc. adhere to the surface of the steel sheet, sufficient shot peening force can be obtained more stably. be able to give. On the other hand, if it exceeds 2.7×10 ⁻² J/mm ² , there is a possibility of causing abrasion of the steel plate and equipment.

The kinetic energy K(J) of blast particles can be calculated by the following equation.
K=mv2/ ²
Here, m is the mass of the particle (kg) and v is the velocity of the particle (m/s).
Energy per unit area received by the steel sheet: Energy density E (J/mm ² ) can be calculated by the following equation.
E = m'v' ² /2A
Here, m' is the total mass (kg) of the particles colliding with the steel plate, v' is the average velocity (m/s) of the particles at the time of collision, and A is the area (mm ² ) where the particles are irradiated. However, since v′ and A are difficult to measure precisely, in the present invention, v′ is the initial velocity of the particles (m/s): v″, and A is the injection nozzle area of the particles (mm ² ): A′. assumed to be the same as
Further, Q: flow rate of treatment liquid (m ³ /s), C: particle concentration (vol%), ρ: particle density (g/cm ³ ), t: treatment time (s), m′ (kg) can be expressed by the following equation.
m′=Q×C×ρ×t×10
Furthermore, if A' is the nozzle area (mm ² ), v″ can be expressed by the following formula.
v″=10 ⁶ ×Q/A′
Here, since it is assumed that v'=v'' and A=A', the energy density E can be calculated by the following formula.
E= ^Q3 ×C×ρ×t× ¹⁰¹³ / ^2A3
However, strictly speaking, when colliding with a steel plate, the particle velocity decreases slightly due to air resistance, etc., and the irradiated area does not always match the nozzle area. can be approximated by the formula

The compressed air pressure during wet blasting is desirably 0.05 to 1.0 MPa. Since the flow rate per unit area increases as the compressed air pressure increases, when the pressure is less than 0.05 MPa, the energy possessed by the fine particles is not sufficient, and it may not be possible to introduce sufficient compressive stress into the steel plate or relax the tensile stress. . On the other hand, if the pressure exceeds 1.0 MPa, the device tends to wear out. Moreover, the amount of wear on the steel plate side also increases.

The particle concentration of fine particles in the treatment liquid in wet blasting is preferably 1 to 30 vol %. If it is less than 1 vol%, sufficient energy cannot be obtained, and if it exceeds 30 vol%, it causes nozzle clogging and the like.
Also, the particle density of the fine particles in the treatment liquid is preferably 1.0 to 15.0 g/cm ³ . If it is less than 1.0 g/cm ³ , the time required for sufficient shot peening treatment becomes too long. On the other hand, if it exceeds 15.0 g/cm ³ , it becomes difficult to successfully disperse the fine particles in the aqueous solution.

The projection distance during wet blasting is desirably 3 to 500 mm. If it is less than 3 mm, the steel plate and the nozzle may come into contact with each other. On the other hand, if it exceeds 500 mm, the initial velocity of the particles is remarkably reduced due to air resistance and the like, and it may be impossible to sufficiently introduce compressive stress or relax tensile stress. Also, the irradiated area is too large compared to the nozzle area.

The angle of wet blasting is desirably in the range of 30 to 90° with respect to the surface on which compressive residual stress is to be introduced. Although 90° can introduce residual compressive stress or relieve tensile stress most efficiently, it may have an inclination for some reason. However, when the angle is less than 30°, the efficiency of introduction of compressive residual stress decreases.

The wet blasting time is desirably in the range of 0.15 seconds or less. If it is more than 0.15 s, the blasting particles will adhere excessively to the end face or surface of the steel sheet as a residue, so that the chemical conversion treatment property, which is a pretreatment for painting the steel sheet for automobiles, will deteriorate, and the subsequent corrosion resistance etc. may be adversely affected. There is Moreover, it is more desirable that the wet blasting time is in the range of 0.01 s or longer. If the time is less than 0.01 s, sufficient peening cannot be introduced into the sheared edge portion or the stress concentration portion of the steel plate, and there is a possibility that stable and sufficient delayed fracture resistance cannot be obtained.

The press-molded product obtained by the present invention can be obtained by the method of the present invention, but it is important that fine particles (blast particles) during shot peening treatment by the wet blast method adhere to the surface and end faces. This is because a proper adhesion amount provides stable and sufficient delayed fracture resistance.
When the blasting particles are alumina, zirconia or stainless steel, they have a tensile strength of 1180 MPa or more, a residual stress of less than 875 MPa in at least a part of the sheared edge portion or the stress concentration portion, and the surface and A press-formed product is obtained in which blast particles such as alumina, zirconia, or stainless steel adhere to the end faces.

When the blasting particles are alumina or zirconia, the composition of the blasting particles is known. , the amount of residue attached to the end face or surface of the steel plate can be calculated. When alumina is used as the blasting particles, it is more desirable that the total amount of alumina adhered to the steel plate end face and surface is in the range of 12.0 to 63.9 mg/m ² . It is more desirable that the total amount of zirconia adhered to the surface ranges from 0.38 to 4.18 mg/m ² . When alumina and zirconia are used together as blasting particles, it is sufficient that the respective fine particles adhere to each of the ranges described above.

If the coating amount exceeds each range, the chemical conversion treatability, which is a pre-painting treatment of steel sheets for automobiles, is deteriorated, and there is a possibility that the subsequent corrosion resistance and the like will be adversely affected. In addition, if it is less than each adhesion amount range, sufficient peening cannot be introduced into the sheared edge portion or stress concentration portion of the steel plate, and there is a possibility that stable and sufficient delayed fracture resistance cannot be obtained. .

As for the shape of the nozzle, a wide gun capable of treating a wide area in one treatment, a circular nozzle, or the like can be used. The shape of the nozzle, the processing speed, and the number of times of processing may be appropriately determined according to the environment in which the high-strength member is used and the amount of residual stress occurring in the high-strength member.
In particular, in order to efficiently perform the shot peening treatment only on the target portions such as the sheared end face of the member and the stress concentrated portion, it is preferable to use a circular nozzle as the nozzle to be used.
Moreover, the solvent used for the treatment may be a liquid, and a solvent commonly used for wet blasting treatment can be used. Water is mainly used, but organic solvents such as ethanol can also be used.

Shot peening in accordance with the present invention with suitable particulates as described above can eliminate 50% or more of the microcracks observed prior to treatment.
The reason for this has not yet been clearly elucidated, but the inventors speculate as follows.
It is considered that wet blasting on the sheared edge surface causes the crack to disappear due to two effects: the edge surface is shaved and the area around the crack is pushed into the crack by the collision with fine particles. In particular, the use of polygonal particles enhances the former effect, and the use of spherical particles enhances the latter effect.

The wet blasting treatment may be performed manually, but it is advantageous to perform it automatically using a robot arm or the like. By using an automated machine with a robot arm, it is possible to achieve more efficient industrial production with reduced unevenness and variation.
In addition, it is not necessary to apply the wet blast treatment to the entire surface, and it is sufficient to treat only the portions where there is concern about cracks due to delayed fracture. For example, by using CAE analysis, etc., it is possible to predict locations where residual tensile stress is concentrated, and by efficiently treating those locations, the occurrence of delayed fracture can be suppressed.

In addition, wet blasting does not lose its effect even if the molded product is coated with oil (oil containing at least one of rust preventive oil, cleaning oil and press oil) or stains that are commonly used on the molded product. can be processed. In addition, since fine particles are delivered using water as a medium, even if oil is contained in the aqueous solution, the effect is not lost, which is advantageous compared to sandblasting and shotblasting. Furthermore, by selecting the optimum nozzle, it becomes possible to treat only the target portion, thereby avoiding damage to the appearance quality of the automobile and the plating layer.

The present invention will now be described with reference to examples. The technical scope of the present invention is not limited to the following examples.
(Example 1)
As the test material, an alloyed hot-dip galvanized steel sheet with a coating amount of 55 g/m ² on which a 1.6 mm thick cold-rolled steel sheet (tensile strength 1499 MPa) having the chemical composition and mechanical properties shown in Table 1 was used as the base material. board. This test material was sheared into a size of 30×100 with a rake angle of 0.8° and a clearance of 5%, and bent 90° V with a load of 15 t so that the bending R was 6 mm. At this time, the bending direction was unified so that the fractured surface was on the outside. Shot peening treatment (hereinafter simply referred to as peening treatment) was performed on the specimen after the bending test by wet blasting by the following two methods.

That is, (A) automatic processing using a wide gun (wet blast processing No. 1 to 4) and (B)
A peening treatment was performed by manual treatment (wet blast treatment Nos. 5 and 6) using a 9.5 mmφ round gun.
Detailed conditions are shown in Table 2.
In the case of the wide gun, the bent samples were arranged one on top of the other as shown in FIG. 1, and both end surfaces of multiple samples were treated simultaneously. On the other hand, in the case of the round gun, as shown in FIG. 2, each sample was individually processed on both end surfaces. The fine particles were made of six types of materials: alumina, stainless steel (SUS), and zirconia, which are polygonal and spherical.
After the treatment, the test material was washed with water and dried.

The test pieces obtained as described above were evaluated as follows.
(1) Evaluation of delayed fracture resistance Bolts are tightened so that a predetermined stress is generated at the tip of the bend, and the temperature is 10 ° C. and the amount of salt adhesion is 10000 mg / m ² for 40 days. The delayed fracture resistance was evaluated based on the presence or absence of cracking under conditions of 8 hours per cycle (repeating humidity of 30% and 90% for 2 hours each, applying salt water twice/week). Evaluation was made according to the following criteria from the number of cycles, and no cracks for 40 days were considered good.
○: No cracks for 40 days ×: Cracks are present Fig. 4 schematically shows the test piece for delayed fracture evaluation used for the evaluation.

(2) Evaluation of residual stress After stress was applied to the tip of bending by tightening with a bolt, residual stress was measured using a micro-stress measurement device with PSPC (AutoMATE) manufactured by Rigaku Corporation. By the X-ray diffraction method (parallel tilt method) using Cr-Kα rays, under the conditions of a tube voltage of 40 kV, a tube current of 40 mA, and a collimator of 0.15 mmφ, the normal of the sample surface and the α-Fe (211) surface normal The formed angle ψ was measured, and the residual stress at the sheared end surface of the bent portion was measured with a stress constant of −318.0 MPa. The results are expressed with a positive number as a tensile stress and a negative number as a compressive stress. In this example, the residual stress is far more dominated by the blasting conditions than the degree of bending. ) was evaluated.

(3) Counting of microcracks After bending, the tip of the bend was observed by SEM from the cross-sectional direction for the test material subjected to peening treatment by wet blasting. Several spots were observed with a magnification of 100, and the number of cracks in the size range of 5 to 1000 μm extending from the surface toward the center of the sheet thickness was counted. Similarly, cracks after wet blasting were also counted, and the crack disappearance rate was calculated from the following formula. Cracks smaller than the above range do not affect the breaking phenomenon of the steel plate. On the other hand, cracks larger than the above range are not preferable because they are excluded in product inspection. In addition, in this example, the crack disappearance rate is far more dominated by the blasting conditions than the degree of bending. only evaluated.
{(number of initial cracks - number of cracks after wet blasting) / (number of initial cracks)} × 100 (%)
Table 3 shows the results obtained.

In Table 3, No. Steel sheets Nos. 1 to 6 are comparative examples that were not subjected to peening treatment by wet blasting, but delayed fracture did not occur at a load stress of about 790 MPa, but delayed fracture occurred at a load stress of 1024 MPa or more. It can be seen that No. Steel sheets Nos. 7 to 42 are invention examples in which peening treatment by wet blasting was performed after bending. It can be seen that no cracks occurred in any of the steel sheets, and excellent delayed fracture resistance was obtained.

(Example 2)
In order to simulate the adhesion of particles to the sheared edge, a 1.6 mm thick cold-rolled steel sheet (tensile strength 1499 MPa) having the chemical composition and mechanical properties shown in Table 1 was sheared into 70 x 150 sizes as a test material. , Wet blasting was applied to the surface by automatic processing using a wide gun.
Detailed conditions are shown in Table 4.
The materials of the fine particles were six types in total: alumina, stainless steel (SUS), and spherical zirconia having different particle sizes.
After the treatment, the test material was washed with water and dried.
The test pieces obtained as described above were evaluated as follows.
(1) Evaluation of chemical conversion treatment The sample after treatment was degreased with a degreasing agent “FC-E2001” manufactured by Nihon Parkerizing Co., Ltd. After washing with water, the surface was treated with a surface conditioner “PL-X” manufactured by the same company for 30 seconds. After adjustment, it was then immersed in a chemical conversion treatment liquid "PB-SX35" manufactured by the same company for chemical conversion treatment at a temperature of 38°C for 90 seconds, washed with water, and dried.
◎: The crystals are denser than the cold-rolled steel sheet and good ○: The crystal size is the same as that of the cold-rolled steel sheet, and there is no skeletal part (unformed part of phosphate crystals generated during phosphate treatment) △: Inferior to the cold-rolled steel sheet (2) Measurement of Alumina Particles and Zirconia Particle Adhesion Amounts The fluorescent X-ray intensities of Al and Zr were measured for the sample after treatment by X-ray fluorescence analysis (XRF).
The amount of alumina particles deposited is calculated by converting the obtained net strength of Al into the amount of Al deposited using a sample in which a predetermined amount of Al is deposited per unit area on a steel plate as a reference, and assuming that the deposited aluminum component is Al ₂ O ₃ . and converted to the molecular weight to obtain the adhesion amount.
The amount of zirconia particles deposited was obtained by converting the obtained net strength of Zr into the amount of zirconia deposited using a sample in which a predetermined amount of zirconia per unit area was formed on a steel plate as a reference.

As shown in Table 5, all the steel sheets of the present invention have no problem in chemical conversion treatability (zinc phosphate treatability). Although not shown in Table 5, when stainless steel was used as the fine particles, it was confirmed that an appropriate amount of stainless steel components adhered to the surface and end faces of the steel plate.

(Example 3)
A cold-rolled steel sheet (tensile strength: 1499 MPa) with a thickness of 1.6 mm and having the chemical composition and mechanical properties shown in Table 1 was used as a base and a galvannealed steel sheet with a coating weight of 55 g/m ² was used. This test material was sheared into a size of 30×100 with a rake angle of 0.8° and a clearance of 5%. Two types of sheared specimens were prepared: one subjected to peening treatment by wet blasting before bending, and the other subjected to peening treatment by wet blasting after bending. The bending process was performed by 90° V bending with a load of 15 t so that the bending R was 6 mm. At this time, the bending direction was unified so that the fractured surface was on the outside. The test material was subjected to peening treatment by the following method (A) by automatic treatment using a wide gun. Detailed conditions are shown in Table 6.

The materials of the fine particles were six types in total: spherical alumina, stainless steel (SUS), and zirconia with different particle diameters. After the treated specimens were washed with water and dried, evaluation of delayed fracture resistance, evaluation of residual stress, and counting of microcracks were carried out. The results obtained are shown in Tables 7 and 8.

Table 7 shows the evaluation results when peening treatment was performed by wet blasting after bending. It can be seen that excellent delayed fracture resistance is obtained.

Table 8 shows the evaluation results when peening treatment was performed by wet blasting before bending. It can be seen that delayed fracture resistance is obtained.

Claims (10)

A step of pressing a steel plate having a tensile strength of 1180 MPa or more, and a step of subjecting at least part of the sheared edge portion or stress concentration portion of the steel plate to shot peening by a wet blasting method using fine particles, wherein the wet blasting method. The shot peening treatment is performed under conditions that satisfy a treatment time of 0.15 s or less, and the steel sheet is a cold-rolled steel sheet or a zinc-based steel sheet obtained by applying zinc-based plating to the surface of the cold-rolled steel sheet. A method for manufacturing a press-molded product. The method for producing a press-formed product according to claim 1, wherein the steel plate is obtained by shearing. After trimming and/or drilling by shearing the member before press working, shot peening is performed by a wet blast method using fine particles on at least a part of the sheared end face portion or tensile stress concentration portion of the formed member after processing. 3. The method for producing a press-formed product according to claim 1, wherein the press-formed product is treated. Manufacture of the press-formed product according to any one of claims 1 to 3, wherein the material of the fine particles is one or more selected from stainless steel, alumina, zirconia, resin and glass. Method. The method for producing a press-formed product according to any one of claims 1 to 4, wherein the shape of said fine particles is polygonal and/or spherical. 6. The method for producing a press-formed product according to claim 1, wherein the fine particles have a particle diameter of 6 to 250 μm and a hardness of 300 HV or more. When the shot peening treatment by the wet blasting method is applied to the molded parts after press working, it is performed under the condition that the energy density satisfies 7.0×10 ⁻⁵ to 2.7×10 ⁻² J/mm ² . However, when applied to trimmed and/or drilled steel sheets before press working, it is carried out under conditions that satisfy an energy density of 2.5 × 10 ^-4 to 2.7 × 10 ^-2 J/mm ² The method for producing a press-formed product according to any one of claims 1 to 6, characterized in that: The shot peening treatment by the wet blast method is performed under conditions satisfying an energy density of 1.5×10 ⁻³ to 2.7×10 ⁻² J/mm ² . A method for producing a press-molded product according to any one of the above. 9. The method for producing a press-formed product according to claim 1, wherein the microcracks observed before treatment disappear by 50% or more by shot peening treatment by wet blasting of the fine particles. 10. The method for producing a press-formed product according to claim 1, wherein the fine particles are shot peened by a wet blasting method without degreasing the oil adhering to the steel plate.

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