EP4382222A1 - Method for improving delayed fracture characteristics of steel sheet, method for producing blank, method for producing press-formed article, and press-formed article - Google Patents

Method for improving delayed fracture characteristics of steel sheet, method for producing blank, method for producing press-formed article, and press-formed article Download PDF

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Publication number
EP4382222A1
EP4382222A1 EP22867283.8A EP22867283A EP4382222A1 EP 4382222 A1 EP4382222 A1 EP 4382222A1 EP 22867283 A EP22867283 A EP 22867283A EP 4382222 A1 EP4382222 A1 EP 4382222A1
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EP
European Patent Office
Prior art keywords
face
bending
delayed fracture
metal sheet
unbending
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP22867283.8A
Other languages
German (de)
English (en)
French (fr)
Inventor
Yuichi Matsuki
Toyohisa Shinmiya
Kinya Nakagawa
Yuji Yamasaki
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
JFE Steel Corp
Original Assignee
JFE Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by JFE Steel Corp filed Critical JFE Steel Corp
Publication of EP4382222A1 publication Critical patent/EP4382222A1/en
Pending legal-status Critical Current

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Classifications

    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21DWORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D1/00Straightening, restoring form or removing local distortions of sheet metal or specific articles made therefrom; Stretching sheet metal combined with rolling
    • B21D1/06Removing local distortions
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21BROLLING OF METAL
    • B21B15/00Arrangements for performing additional metal-working operations specially combined with or arranged in, or specially adapted for use in connection with, metal-rolling mills
    • B21B15/0007Cutting or shearing the product
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21DWORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20Deep-drawing

Definitions

  • the present invention relates to a technology for improving the delayed fracture characteristics of a metal sheet as a blank, which is used when a formed article is produced by press forming.
  • the present invention is a technology for improving the delayed fracture characteristics on a sheared end face.
  • the present invention relates to a technology for producing a formed article having favorable delayed fracture characteristics by press-forming a metal sheet made of a high-strength steel sheet.
  • an end face of a metal sheet that has been subjected to shearing is referred to as a sheared end face.
  • a steel sheet having a tensile strength of 1470 MPa or more is referred to as an ultrahigh-strength steel sheet.
  • the present invention is suitable for a high-strength steel sheet having a tensile strength of 980 MPa or more.
  • high-strength steel sheets are used for the purpose of achieving both weight reduction and passenger protection in the event of a collision.
  • ultrahigh-strength steel sheets having a tensile strength of 1470 MPa or more have been applied to vehicle bodies.
  • One problem at the time of applying high-strength steel sheets, particularly, ultrahigh-strength steel sheets, to vehicle bodies is delayed fracture.
  • measures against delayed fracture and stretch flange cracking that occur from sheared end faces, which are end faces after shearing are an important task.
  • an object of the present invention is to suppress delayed fracture from a sheared end face after forming by a simple method.
  • an object of the present invention is to make it possible to provide a formed article having favorable delayed fracture characteristics by improving the delayed fracture characteristics of a metal sheet made of a high-strength steel sheet.
  • the present disclosure is a technology for improving the delayed fracture characteristics of a metal sheet by plastic processing after shearing, which is easy to apply, even when the metal sheet is made of a high-strength steel sheet such as an ultrahigh-strength steel sheet.
  • the point of one aspect of the present invention is a method for improving the delayed fracture characteristics of a metal sheet having a sheared end face on at least a part of a sheet end portion and being made of a high-strength steel sheet, in which plastic deformation is imparted to at least a part of the sheared end face of the metal sheet.
  • the plastic deformation needs to be imparted to at least the sheared end face, for example, an end portion including the sheared end face.
  • the plastic deformation does not necessarily need to be imparted to all of the sheared end face.
  • the plastic deformation needs to be imparted to, for example, in a sheared end face, a place where at least a predetermined degree or more of delayed fracture is assumed to occur.
  • the wear of blades or the management of shearing conditions is not necessarily required even for high-strength steel sheets.
  • a method for producing a formed article of the present embodiment includes a blank production step 1 and a press forming step 2.
  • the present invention is suitable for a case where a target metal sheet is a high-strength steel sheet, particularly, a high-strength steel sheet having a tensile strength of 980 MPa or more.
  • the blank production step 1 is a step for producing a blank (metal sheet) that is used in the press forming step 2 in which the blank is press-formed in the shape of the formed article.
  • the blank production step 1 includes a shearing step 1A and an end face improvement step 1B.
  • the shearing step 1A is a step for cutting a metal sheet into a blank shape suitable for producing a formed article.
  • the end face improvement step 1B is a step of imparting plastic deformation to at least a part of an end face of the sheared end face in the metal sheet after the shearing step 1A.
  • the plastic deformation is deformation into which distortion is input along the extending direction of the end face.
  • the plastic deformation may be imparted, for example, only to a region including a place in the end face in which preset residual stress is assumed to be generated due to shearing by structural analysis such as CAE.
  • plastic deformation imparts plastic strain greater than 0 in a direction along the extending direction of the end face.
  • the upper limit of the plastic strain to be imparted is not specified, but the plastic deformation is imparted to an extent that cracking does not occur.
  • the plastic deformation is preferably imparted by bending and unbending.
  • the bending angle at each end face position to which the plastic deformation is imparted is less than 90 degrees at the time of each bending and unbending.
  • the bending angle being less than 90 degrees will be described with reference to FIG. 7 .
  • This bending angle indicates an angle formed by a straight line (tangential direction) along the extending direction of the sheared end face and the bending direction of each bending and unbending at the place in the sheared end face to which the plastic deformation is imparted.
  • the premise is that the bending is bending by which plastic strain greater than 0 is imparted in the direction along the extending direction of the end face.
  • the bending and unbending is performed by, for example, bending by press forming (refer to FIG. 5 ).
  • the bending and unbending is performed by, for example, leveling using a leveler having a plurality of rolls arranged in the conveying direction of the sheet (refer to FIG. 6 ).
  • the leveling is a processing method that is used at the time of flattening sheets.
  • bending and unbending In the bending and unbending, bending and bending by unbending (reverse bending) is executed a plurality of times on the same sheared end face in a sheet thickness direction. At that time, it is preferable to set a final bend such that the outside of the bend is on the burr side of the sheared end face.
  • the burr side is a side where burrs are formed by shearing in the sheet thickness direction.
  • the bending and unbending needs to be performed such that the plastic deformation is imparted to the end portion including the target sheared end face (for example, a range including a 1 mm range from the end face) .
  • the plastic deformation to be imparted such that the sheet end portion to which the plastic deformation has been imparted in the end face improvement step 1B becomes flat.
  • the press forming step 2 is a step of press-forming the blank made of the metal sheet produced in the blank production step 1 into a target component shape.
  • the press forming is executed by one pressing or multi-stage pressing.
  • plastic strain greater than 0 in the direction along the extending direction of the end face is imparted to at least a part of the sheared end face.
  • the above embodiment is an example where the present disclosure is applied to the production of a blank before a step of pressing a metal sheet into a target product shape. That is, the above embodiment exemplified a case where the method for improving the delayed fracture characteristics of a metal sheet of the present disclosure (end face improvement step 1B) is applied as a pretreatment of pressing.
  • the end face improvement step 1B of the present disclosure may be applied in the middle of pressing for forming a target product shape or after the pressing. Specifically, the end face improvement step 1B of the present disclosure may be applied to a sheared end face generated by the shearing of an end portion for shaping a sheet outer circumference.
  • the treatment of the above-described end face improvement step 1B may be applied to the sheared end face.
  • the plastic deformation in the end face improvement step 1B is different from press forming for forming a sheet into a target product shape.
  • the press forming as described below. That is, it is preferable to execute a treatment for imparting the plastic deformation in the end face improvement step 1B only to an end portion having a sheared end face (for example, only to a flange portion).
  • plastic deformation is imparted to a sheared end face by plastic processing.
  • the plastic processing of the present disclosure is performed by bending and unbending. This makes it possible to reduce residual stress in a sheared end face by a simple method even when the metal sheet (blank) is a high-strength steel sheet such as an ultrahigh-strength steel sheet. Furthermore, in the present embodiment, it is possible to obtain the above-described effect while maintaining the shape of the sheet in the same flat state as that after shearing.
  • the reduction of residual stress in the sheared end face suppresses the occurrence of delayed fracture. That is, the delayed fracture characteristics on the sheared end face of the metal sheet are improved.
  • each bending angle at the place of each sheared end face in the bending and unbending is set to less than 90 degrees, it becomes possible to introduce sufficient plastic deformation into the sheared end face.
  • the outside of a bend formed by the final bending is on the burr side of the sheared end face.
  • the burr side in the sheet thickness direction is a portion where delayed fracture is likely to occur due to the influence of burrs or rough surface texture. In this case, it becomes possible to further suppress delayed fracture occurring from the burrs as a starting point.
  • FIGS. 2A and 2B are views illustrating a state of a sheet end portion in a case where the end portion of a sheet is cut by moving a shearing blade from the upper side toward the lower side.
  • the burr side is on the lower side.
  • FIG. 3 illustrates an example of a stress distribution in a direction away from the sheared end face 10A (a direction orthogonal to the end face 10A) as indicated by an arrow in FIG. 2A .
  • the first region ARA1 is a region where strong tensile residual stress is present on the surface of the sheared end face.
  • the second region ARA2 is a region where compressive residual stress is present to counterbalance the tensile residual stress.
  • the third region ARA3 is a region inside the second region ARA2 where there is no residual stress.
  • Plastic deformation attributed to uniform tensile strain caused by bending generating the burr side on the bend outside or tensile processing is introduced mainly into the first and second regions ARA1 and ARA2 among these three regions ARA1 to ARA3.
  • the stress distribution changes from FIG. 4A to FIG. 4C . That is, in the first region ARA1, the initial tensile residual stress is relaxed by the stress-strain history.
  • the difference between stress in the first region ARA1 on the front surface side and stress in the second region ARA2 inside the first region ARA1 reduces. This is also true even in a case where plastic strain that is introduced by shearing is compressive strain.
  • the sheared end face 10A which is intended to be dealt with in the present disclosure, is, for example, a sheared end face of a metal sheet 10 having an arbitrary shape fabricated by shearing.
  • what is intended as the sheared end face 10A is an end face 10A of a drilled portion or an end face 10A configuring the contour line that specifies the outer form of a blank.
  • FIG. 3 is a case where a specimen piece made of a high-strength steel sheet having a tensile strength of 980 MPa is used.
  • the depth d of the first region ARA1 is, for example, 100 um.
  • the thickness of a sheet changes due to the introduction of the plastic strain. Furthermore, in a blank having a complicated shape, since strain concentrates in a portion having a narrow width in a direction perpendicular to the tensile axis, it is not possible to uniformly deform the blank. In addition, in a case where plastic strain is introduced by simple bend forming, a blank bends significantly as a whole after the forming. Therefore, it is impossible for the metal sheet 10 to maintain the same flat state as that after shearing.
  • the plastic deformation is preferably imparted by bending and unbending.
  • the contour shape of the end face 10A in the extending direction is a curved shape that changes in a direction orthogonal to the end face 10A
  • the plastic deformation needs to be imparted as described below. That is, bending and unbending needs to be performed such that a depth of 1 mm or less from the surface of the end face 10A can be secured in the end portion of the sheared end face 10A at the most recessed place.
  • One simple bending may be adopted, but bending and unbending is adopted in consideration of the shape returning to the original flat shape or the like.
  • the bending and unbending is performed by bending by press forming as illustrated in FIG. 5 or by leveling as illustrated in FIG. 6 .
  • bending and unbending deformation is caused on the surface of the sheared end face 10A, residual stress on the sheared end face 10A is relaxed, and delayed fracture is suppressed.
  • a die 20 and a punch 21 for bending and a die 22 and a punch 23 for reverse bending that are used to perform unbending, which are illustrated in FIG. 5 , may be in the same die, or different dies may be used.
  • each diameter of a roll 30 for a leveler may not be the same as each other.
  • the bending and unbending can also be performed by bend deforming by press forming.
  • the tensile or compressive plastic strain with respect to the sheared end face 10A needs to be 0.003 or more.
  • the plastic strain is 0.005 or more, it is possible to significantly relax residual stress in the sheared end face 10A.
  • a bending angle ⁇ that is formed by the contour line of the sheared end face 10A (the extending direction of the end face 10A) and a bending direction of the bending and unbending as illustrated in FIG. 7 is set in a range of, for example, 0 degrees or more and 75 degrees or less. It is desirable that the bending angle ⁇ is preferably in a range of 0 degrees or more and 45 degrees or less. This is because, in a case where the angle ⁇ between the bending direction and the contour line of the sheared end face 10A is close to 90 degrees, it becomes difficult to introduce strain into the surface of the sheared end face 10A. The reason therefor is that the surface of the sheared end face 10A is open with respect to the tensile/compressive deformation direction in the direction along the end face 10A by bending.
  • contour line of the sheared end face 10A before bending is illustrated as a straight line in FIG. 7 , but the contour line of this end face 10A may be a curved line or a partially discontinuous line.
  • FIGS. 8A and 8B are a case where a final bend is present on the lower side. Therefore, after springback caused by the release of the restraint of the press, compressive residual stress as large as the tensile strain remains. Therefore, it is desirable that the outside (the lower side in FIG. 8 ) of the bend formed by the final bending is on the burr side of the sheared end face 10A.
  • the burr side is a portion where delayed fracture is likely to occur due to the influence of burrs or rough surface texture.
  • residual stress on the burr side of the sheared end face 10A is reduced by the compressive residual stress caused by forming.
  • the present disclosure may also have the following configurations.
  • Examples will be described using a test material A for which a steel sheet having a sheet thickness of 1.4 mm and a tensile strength of 1470 MPa was used.
  • the present disclosure is not limited to the steel sheet having a tensile strength of 1470 MPa.
  • the present disclosure is applicable to metal materials including steel sheets having a tensile strength of 980 MPa or more, in which delayed fracture occurs on a sheared end face.
  • test material A was sheared to fabricate a linear sheared end face having a length of 500 mm, which was to be an evaluation object.
  • the clearance during the shearing was set to 12% with respect to the sheet thickness.
  • the unbending was executed with a different angle that was formed by the contour line of the sheared end face and the bending direction of bending and unbending, which is defined in FIG. 7 , to fabricate each sample after the bending and unbending.
  • Example 1 Sample forming conditions and evaluation results in Example 1 are shown in Tables 1 and 2, respectively. In the examples shown in Table 1, the bending and unbending was performed by press forming.
  • Table 1 shows results when the angle formed by the contour line of the sheared end face and the bending direction of the bending and unbending was set to 0 degrees in the press forming. Specifically, Table 1 shows the relationship among the maximum amount of plastic strain introduced by the bending and unbending, the presence or absence of the occurrence of delayed fracture, the time taken for the occurrence of delayed fracture, and the residual stress.
  • Table 2 shows results when the angle formed by the contour line of the sheared end face and the bending direction of the bending and unbending was set to 0 degrees in the leveling. Specifically, Table 2 shows the relationship among the maximum amount of plastic strain introduced by the bending and unbending, the presence or absence of the occurrence of delayed fracture, the time taken for the occurrence of delayed fracture, and the residual stress.
  • Processing method Maximum amount of plastic strain introduced Angle formed by bending direction and sheared end face (°) Outside of final bend Time taken for occurrence of delayed fracture/h Residual stress/M Pa Leveler 0.000 0 Rollover burr side 23 1205 0.002 74 691 0.003 87 308 0.005 (No occurrence) 95 0.010 24 0.030 19 0.050 7
  • Example 2 is an example where the relationship between the maximum amount of plastic strain introduced by bending and unbending and the presence or absence of the occurrence of delayed fracture and the time taken for the occurrence of delayed fracture in a case where each bending angle of the bending and unbending was changed was examined.
  • Table 3 is an example in a case where the bending and unbending was performed by leveling.
  • the present disclosure is not limited thereto. Even by bending and unbending by press forming or even in a case where the amount of plastic strain is different from 0.005, a favorable result can be obtained in the above-described angle range.
  • Example 3 the time taken for the occurrence of delayed fracture or the presence or absence of the occurrence of delayed fracture and residual stress are shown in a case where the bending and unbending was performed by each of press forming and leveling.
  • Example 3 a case where the outside of the final bend formed by bending and unbending was on the burr side and a case where the outside of the final bend was on the rollover burr side were described.
  • the maximum amount of plastic strain was set to 0.003.
  • the angle formed by the bending direction and the sheared end face 10A was set to 0 degrees.
  • the difference was larger in the case of the bending and unbending by pressing than by leveling.
  • the reason therefor is that, in the leveling, the amount of deformation by bending and unbending gradually decreased from the start to the end of the processing, and thus the difference in stress in the sheet thickness direction was leveled.
  • delayed fracture can be further suppressed by making the outside of the final bend by bending and unbending to be on the burr side of the sheared end face.

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  • Engineering & Computer Science (AREA)
  • Mechanical Engineering (AREA)
  • Bending Of Plates, Rods, And Pipes (AREA)
EP22867283.8A 2021-09-08 2022-09-01 Method for improving delayed fracture characteristics of steel sheet, method for producing blank, method for producing press-formed article, and press-formed article Pending EP4382222A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2021146245 2021-09-08
PCT/JP2022/032988 WO2023037961A1 (ja) 2021-09-08 2022-09-01 金属板の遅れ破壊特性改善方法、ブランク材の製造方法、プレス成形品の製造方法、及びプレス成形品

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Publication Number Publication Date
EP4382222A1 true EP4382222A1 (en) 2024-06-12

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EP22867283.8A Pending EP4382222A1 (en) 2021-09-08 2022-09-01 Method for improving delayed fracture characteristics of steel sheet, method for producing blank, method for producing press-formed article, and press-formed article

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EP (1) EP4382222A1 (ko)
JP (1) JP7435895B2 (ko)
KR (1) KR20240046543A (ko)
CN (1) CN117897239A (ko)
WO (1) WO2023037961A1 (ko)

Family Cites Families (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS5855128A (ja) * 1981-09-28 1983-04-01 Nippon Steel Metal Prod Co Ltd 溝形鋼の連続冷間ロ−ル成形方法及びその装置
JPH06530Y2 (ja) 1988-10-28 1994-01-05 新日軽株式会社 サッシの錠装置
JPH09122756A (ja) * 1995-10-31 1997-05-13 Kawasaki Steel Corp ローラレベラの長手方向反り制御方法
JP4711396B2 (ja) * 2005-02-18 2011-06-29 新日本製鐵株式会社 高強度鋼板の打ち抜き加工方法
JP6424841B2 (ja) 2016-01-13 2018-11-21 Jfeスチール株式会社 成形部材の製造方法
JP7059572B2 (ja) * 2017-11-10 2022-04-26 日本製鉄株式会社 溶接継手の製造方法及び溶接継手
JP7107327B2 (ja) 2019-01-16 2022-07-27 Jfeスチール株式会社 プレス成形品の製造方法およびプレス成形品
JP6839315B1 (ja) 2020-03-17 2021-03-03 有限会社中川商会 被処理物の表面改質方法

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JPWO2023037961A1 (ko) 2023-03-16
WO2023037961A1 (ja) 2023-03-16
JP7435895B2 (ja) 2024-02-21
CN117897239A (zh) 2024-04-16
KR20240046543A (ko) 2024-04-09

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