Crack formation and propagation mechanism in the punching process of high tensile strength steel sheet

Takehiro Okano, Kota Sakumoto, Kazuhiko Yamazaki, Shunsuke Toyoda, Shinsuke Suzuki

Research output: Chapter in Book/Report/Conference proceedingConference contribution

3 Citations (Scopus)

Abstract

The microstructure (crystal grain and the center segregation of Mn) around the voids and cracks in punching process was investigated using Hot-rolled 780 MPa-grade high tensile strength steel sheets. Steel sheets without center segregation were prepared by grinding only one side, while ones with center segregation were prepared by grinding both side. Punching tests were conducted with these two kind of steel sheets. Crack length and number of cracks on the punched surface were measured and counted by an optical microscope. Steel sheets with center segregation had more cracks in total and longer average crack length than that without center segregation. The Mn mapping and the crystal orientation mapping around the crack obtained through EPMA and EBSD showed that the range larger than 70 μm in crack length tend to cause intergranular fracture along Mn segregated area. Also in the range between 10 μm and 70 μm in crack length, cracks forming both along and away from Mn segregated area were observed. Concentration of Mn around the grain boundary of the center segregation area measured by TEM/EDS revealed that grain boundary contains high Mn concentration. The calculation on distribution of equivalent stress just before the onset of crack for steel sheets with and without center segregation using finite element model showed that equivalent stress concentrates at both edges of the punch and die and the center segregation part. Next, interrupted punching tests were conducted with two kind of steel sheets. The observation around voids through SEM, EPMA, and EBSD showed that voids initiate at the ferrite-Ti precipitate interface. From these results, following tendencies were found, within and near Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates along Mn center segregation. Moreover, Mn segregates at the grain boundary, and Mn weakens grain boundary cohesion which leads to an intergranular fracture. However, without Mn center segregated area, voids initiate at the ferrite-Ti precipitate interface, and crack propagates easily into the ferrite matrix by cleavage which leads to cause a transgranular fracture.

Original languageEnglish
Title of host publicationMetal Forming 2016
EditorsDanuta Szeliga, Krzysztof Muszka
PublisherTrans Tech Publications Ltd
Pages643-651
Number of pages9
ISBN (Print)9783038357049
DOIs
Publication statusPublished - 2016 Jan 1
Event16th Metal Forming International Conference, 2016 - Krakow, Poland
Duration: 2016 Sep 182016 Sep 21

Publication series

NameKey Engineering Materials
Volume716
ISSN (Print)1013-9826
ISSN (Electronic)1662-9795

Other

Other16th Metal Forming International Conference, 2016
CountryPoland
CityKrakow
Period16/9/1816/9/21

Keywords

  • Center Segregation
  • Crack
  • High Tensile Steel Sheet
  • Punching
  • Ti Precipitate.

ASJC Scopus subject areas

  • Materials Science(all)
  • Mechanics of Materials
  • Mechanical Engineering

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  • Cite this

    Okano, T., Sakumoto, K., Yamazaki, K., Toyoda, S., & Suzuki, S. (2016). Crack formation and propagation mechanism in the punching process of high tensile strength steel sheet. In D. Szeliga, & K. Muszka (Eds.), Metal Forming 2016 (pp. 643-651). (Key Engineering Materials; Vol. 716). Trans Tech Publications Ltd. https://doi.org/10.4028/www.scientific.net/KEM.716.643