Steel sheet excellent in fine blanking performance and manufacturing method of the same

The present invention is concerned with a steel sheet suitable for applications to automobile parts or the like and in particular, relates to a steel sheet excellent in fine blanking performance suitable for the uses to which fine blanking working (hereinafter also referred to as “FB working”) is applied.

In manufacturing complicated mechanical parts, from the viewpoints of an improvement in dimension precision, a reduction in manufacturing process, and the like, it is known that fine blanking working is an extremely advantageous working method as comparing with machining working.

In usual blanking working, a tool-to-tool clearance is from approximately 5 to 10% of a thickness of a metal sheet as a material to be blanked. However, the fine blanking working differs from the usual blanking working and is a blanking working method of not only setting up the tool-to-tool clearance extremely small as substantially zero (actually, not more than approximately 2% of the thickness of the metal sheet as a material to be blanked) but also making a compression stress act on a material in the vicinity of a tool cutting blade. Then, the fine blanking working has the following characteristic features.

• • (1) The generation of a crack from the tool cutting blade is inhibited, and a fracture surface seen in usual blanking working becomes substantially zero, whereby a smooth worked surface (blanked end surface) in which the worked surface is a substantially 100% shear surface is obtained.
  • (2) The dimensional precision is good.
  • (3) A complicated shape can be blanked by one process.

However, in the fine blanking working, a working ratio which the material (metal sheet) receives is extremely severe. Also, in the fine blanking working, since the working is carried out at a tool-to-tool clearance of substantially zero, there is involved a problem that a load to a mold becomes excessive so that a mold life is shortened.

For that reason, materials to which the fine blanking working is applied are required to not only have excellent fine blanking performance but also prevent a reduction in mold life.

In response to these requirements, for example, Patent Document 1 proposes a high carbon steel sheet excellent in fine blanking performance, which has a composition containing from 0.15 to 0.90% by weight of C, not more than 0.4% by weight of Si and from 0.3 to 1.0% by weight of Mn, has a microstructure with a cementite having a spheroidization ratio of 80% or more and an average grain size of from 0.4 to 1.0 μm scattered in a ferrite matrix and has a notch tensile elongation of 20% or more. According to a technology described in Patent Document 1, it is described that the fine blanking performance is improved and that the mold life is also improved.

However, the high carbon steel sheet described in Patent Document 1 involved a problem that fabrication performance after the fine blanking working is inferior.

Also, Patent Document 2 proposes a steel sheet for fine blanking prepared by applying proper hot rolling to a billet containing from 0.08 to 0.19% of C and proper amounts of Si, Mn and Al and containing from 0.05 to 0.80% of Cr and from 0.0005 to 0.005% of B into a steel sheet. It is described that the steel sheet described in Patent Document 2 is a steel sheet which is low in a yield strength, high in an impact value, excellent in fine blanking performance, high in an n-value in a low strain region, excellent in combined formability and excellent in quenching property at short-time rapid heating. However, Patent Document 2 does not show concrete evaluation regarding the fine blanking performance. Also, the steel sheet described in Patent Document 2 involved a problem that fabrication performance after the fine blanking working is inferior.

Also, Patent Document 3 proposes a high carbon steel sheet excellent in flow forming and fine blanking working, which has a composition containing from 0.15 to 0.45% of C, with the contents of Si, Mn, P, S, Al and N being adjusted at proper ranges and has a structure having a fractional ratio of (pearlite+cementite) of not more than 10% and an average grain size of ferrite grain of from 10 to 20 μm. It is described that the high carbon steel sheet described in Patent Document 3 is excellent in fine blanking performance and is improved in mold life in the fine blanking working. However, the high carbon steel sheet described in Patent Document 3 involved a problem that fabrication performance after the fine blanking working is inferior.

Furthermore, it is hard to say that all of the steel sheets described in Patent Document 1, Patent Document 2 and Patent Document 3 are not provided with satisfactory and thorough fine blanking performance in the fine blanking working under a recent severe working condition. Also, problems that the mold life is not thoroughly improved and that fabrication performance after the fine blanking working is inferior still remained.

At the beginning, the fine blanking working had been applied to parts to which working is not applied after fine blanking working even among gear parts and the like. However, recently, the application of fine blanking working to automobile parts (for example, reclining parts) tends to expand, and its application to parts which require stretch flanging working, bulging, etc. is investigated. For that reason, steel sheets which are not only excellent in fine blanking performance but also excellent in fabrication performance after fin blanking working in stretch flanging working, bulging, etc. are eagerly desired as automobile parts.