別紙４（課程博士）
Date of Submission:

平成 28 年 1 月 14 日
Department

Student ID Number

機械工学専攻Applicant’s name

氏名

学籍番号第 139107 号

Muhammad Hasnulhadi Bin Mohammad Jaafar

Supervisors

Ken-ichiro Mori

指導教員

Yohei AbeAbstract

論文内容の要旨(博士)

Title of Thesis

Punching and Trimming of Die-Quenched Steel and Ultra-High Strength Steel Sheets

博士学位論文名(ダイクエンチ鋼板と超高張力鋼板のトリミングおよび穴抜き加工)(Approx. 800 words)(要旨

1,200 字程度)
In the introduction, an overview of reduction in weight of automobiles for a less CO2 emission ispresented. The use of lightweight materials such as aluminium, magnesium and titanium alloys isattractive for the reduction. However the high cost of the alloys is disadvantageous, and thus the steel ispreferable. Hot stamping processes of ultra-high strength steel sheets are attractive as a key technique forthe reduction in weight of automobiles parts having a tensile strength of 1.5 GPa can be formed by
die-quenching. However, finishing operations such as hole-making become difficult, and the applicationof laser cutting having high investment cost and low productivity is a bottleneck. The conventional
punching process is desirable for mass production.
In Chapter 2, a cold small clearance punching process of die-quenched steel sheets by punch having asmall round edge was developed to improve the quality of the sheared edge. The sharp edge of the punchleads to the concentration of stress at the punch edge, and thus initiates the early onset of cracks.
Therefore the rough fracture surface on the sheared edge becomes large. For a small clearance and smallround edge of the punch, the concentration of deformation around the edge of the punch was relieved, andthus the onset of a crack from the edge of the punch was prevented. The burnished surfaces became
considerably large.
In Chapter 3, an automatic centring process using a moving die was developed to correct theeccentricity between the punch and die in the small clearance punching process.Although small clearance
was effective for a high quality sheared edge in punching of die-quenched steel sheets, setting of tool
becomes difficult due to the less space and complex structure of the die. For a small clearance the punchand die tend to be eccentric. By setting a gap between the moving die and holder, the die is shifted by
imbalanced force, and the punch and die become concentric after several strikes.
In Chapter 4, since the small clearance punching is effective, a repeated small clearance punchingprocess of die-quenched steel and ultra-high strength steel sheet were carried out. In the real industry
application, car parts usually consist of multiple holes for joining, attaching, painting, etc. For repeatedsmall clearance punching with fixed die, the punch was broken by an eccentricity between the punch anddie after several strikes. The combination of small clearance and automatic centring using a moving die iseffective for a high number of strikes in repeated punching of die-quenched steel and ultra-high strengthsteel sheets. However the high strength of the sheet deteriorates the punch life. By lubricating the surfaceof the punch, galling is reduced and tool life is increased.
In chapter 5, an attempt to reduce the fall velocity of the scrap and noise level during trimming of
ultra-high strength steel parts using an inclined punch was developed. Since the use of the ultra-highstrength steel sheets for automobile body-in-white parts is increased, trimming is required to remove thescrap portions. In trimming of ultra-high strength steel sheets, the flying speed is large due to the highstrength, and thus the scraps jump out from a disposal box. The trimming operation also becomes noisy
due to high trimming load. The deformation and shearing behaviours of the sheet during trimming withthe straight punch were investigated and the level of noise during trimming was measured. By theinclined punch, only a local zone of the sheet is in contact with the punch during trimming, and thus thetrimming load and the flying speed of the scrap are reduced.
In chapter 6, an approach to prevent chipping and fracture on the trimmed edge of the ultra-highstrength steel parts having curved shape was developed. Trimming of ultra-high strength steel parts
having curved shapes with the straight punch has possibilities of defect such as chipping, edge fracture,and bent which deteriorates the surface quality and dimensional accuracy of the part. In trimming of anL-shape ultra-high strength steel sheet having curved shapes, the scrap at the curved zone is bent andresults in chipping and fracture of the sheet edge. The L-shaped punch having an inclined angle wasintroduced to prevent chipping and fracture at the trimmed edge, and at the same time reduces trimming
load and energy.
The conclusion emphasises the importance of small clearance punching by punch having small roundedge to produce high quality of the sheared edge surface and prevent the delayed fracture. The applicationof automatic centring eases the small clearance punching process especially for repeated punching. By
gradually trimming ultra-high strength steel sheets with the inclined punch, the flying speed of scraps andnoise level are reduced. Moreover the problem of chipping and edge fracture was prevented.