Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-11-21
2004-04-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S330000, C148S332000, C148S333000
Reexamination Certificate
active
06726782
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an ultra-low carbon steel sheet and a method for its manufacture. More particularly, it relates to an ultra-low carbon steel sheet having a thickness of at least 0.30 millimeters and having a low tendency to experience forming defects such as pin hole defects or press cracks originating at inclusions even when subjected to press forming of products of complicated shape with large deformation, such as during the manufacture by press forming of products such as electric motor housings or oil filter housings, and to a method for manufacturing such an ultra-low carbon steel sheet.
2. Description of the Related Art
Annealed cold rolled steel sheet has typically been used as a material for the manufacture of products by press forming. The cold rolled steel sheet for this purpose has primarily been low carbon aluminum killed steel which has been annealed by batch annealing.
In recent years, in the manufacture of cold rolled steel sheet for press forming, there has been a shift towards the use of continuous annealing because of its higher productivity. Furthermore, there has been a shift towards the use of ultra-low carbon steel sheet having good formability in applications to products formed with large deformation.
However, when ultra-low carbon steel is used to manufacture products such as motor housings or oil filter housings requiring a high degree of pressing, there are cases in which forming defects such as pin hole defects and press forming cracks occur.
Can manufacture, which is similar to the manufacture of products such as motor housings or oil filter housings, typically employs cold rolled steel sheet having a thickness of less than 0.30 millimeters. Can manufacture entails an even higher level of forming than does the manufacture of motor housings or oil filter housings, and many measures have been proposed for suppressing forming defects during can manufacture.
For example, Japanese Published Unexamined Patent Application Hei 6-172925/1994 and Hei 7-207403/1995 disclose methods for finely dispersing the amount of inclusions in a slab.
Japanese Published Unexamined Patent Application Hei 6-17111/1994 discloses a method for reducing the amount of inclusions in steel by decreasing the amounts of FeO and MnO in slag using a Ca—, or Mg-containing alloy or a reducing agent.
Japanese Published Unexamined Patent Application Hei 11-36045/1999 and Hei 11-279678/1999 also disclose controlling the composition of inclusions as a method of preventing defects.
However, the above-mentioned disclosures relate to low carbon aluminum killed steel. These steels have many aspects which make them inappropriate as cold rolled steels to be subjected to severe forming in the manufacture of products having a complicated shape such as automotive components. In this specification, severe forming for such applications will be referred to as complex deep drawing.
Japanese Published Unexamined Patent Application Hei 11-279721/1999 discloses a method of decreasing inclusions in a low carbon steel, but that steel is for use as tin plate or tin-free steel for can manufacture having a thickness of at most 0.26 millimeters.
Japanese Published Unexamined Patent Application 2000-1746 discloses a method of preventing the formation of inclusions, but that method requires the addition of Ca and/or rare earth metals, so it has the drawback that even if oxide inclusions mainly comprising FeO or MnO are reduced, Ca-containing inclusions or rare earth metal-containing inclusions are increased.
An RH vacuum treatment apparatus is usually used for secondary refining during the manufacture of ultra-low carbon steel, as described in Japanese Published Unexamined Patent Application Hei 11-36045/1999 and Japanese Published Unexamined Patent Application 2000-1746. Vacuum decarburization and deoxidation after the decarburization employing an RH vacuum treatment apparatus are typical secondary refining methods.
SUMMARY OF THE INVENTION
An object of the present invention is to provide steel sheet having a thickness of at least 0.30 millimeters and formed of an ultra-low carbon steel having a carbon content of at most 0.010% and which can be subjected to heavy but fine forming, such as during the manufacture of motor housings or oil filter housings, with reducing the occurrence of forming defects such as pin hole defects and press forming cracks.
Another object of the present invention is to provide a method of manufacturing such a steel sheet.
The present inventors performed investigations as to why cold rolled steel sheet with a thickness of at least 0.30 mm for press forming is more subject to pin holes and press cracks when the sheet is made of ultra-low carbon steel than when it is made of low carbon aluminum killed steel. As a result, they made the following discoveries concerning means for suppressing such defects.
(1) Low carbon aluminum killed steel undergoes powerful deoxidation treatment when being tapped from a converter. In addition, considerable time elapses between tapping and the start of vacuum degassing as the ladle is being moved or other operations are taking place. As a result, the majority of the deoxidation products which are formed during tapping have already floated to the top of the molten steel in the ladle during the time until the start of vacuum degassing treatment, and they are absorbed and removed by the slag on the surface of the molten steel. Inclusions are removed during vacuum degassing treatment.
In contrast, ultra-low carbon steel does not undergo any deoxidation treatment at the time of tapping from a converter, or it undergoes only mild deoxidation treatment from the addition of a small amount of aluminum, and deoxidation is carried out after decarburization by vacuum degassing treatment. For this reason, the length of time between deoxidation and casting is short, and compared to the case of low carbon aluminum killed steel, a large amount of oxide inclusions remain in the steel. Such oxide inclusions act as starting points for the generation of pin holes and press forming cracks.
(2) Defects such as pin holes at the time of deep drawing are due not only to the presence of inclusions remaining in steel in the refining step described above in (1), but are also due to the presence of inclusions which are engulfed in slag during casting. These inclusions come from slag in a ladle or powder used at the time of continuous casting.
The present inventors obtained hot rolled steel sheet using slabs which were manufactured under conditions which solve the problems described above in (1) and (2). After descaling, cold rolling was carried out, and annealing treatment was then performed to obtain cold rolled steel sheet. It was found that this steel sheet could suppress the formation of forming defects such as pin hole defects and press cracks which originate at inclusions even when subjected to press forming of products of complicated shape with large deformation.
According to one aspect of the present invention, an ultra-low carbon steel sheet is made of a steel having a chemical composition containing, in mass percent, C: at most 0.010%, Si: at most 0.5%, Mn: at most 1.5%, P: at most 0.12%, S: at most 0.030%, Al: at most 0.080%, N: at most 0.0080%, and at least one of Ti: at most 0.10% and Nb: at most 0.05%, wherein the number of non-metallic inclusions observed in sixty fields under a microscope in a sample of the steel prepared in accordance with JIS G0555 is at most 20.
The steel may further include B: at most 0.0050%, V: at most 0.05%, and Ca: at most 0.0050%.
The steel will generally include various unavoidable components. In the present invention, Cu, Cr, Sn, and Sb may be present as unavoidable impurities, each in a maximum amount of 0.1%.
The present invention also provides a method for manufacturing an ultra-low carbon steel sheet. According to this aspect of the invention, molten steel having the above-described chemical composition is produced in a converter. The molten steel undergoes secondary refining,
Higuchi Yoshihiko
Hiraki Sei
Kanai Tatsuo
Nakai Syuji
Sumitomo Metal Industries Ltd.
Yee Deborah
LandOfFree
Ultra-low carbon steel sheet does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Ultra-low carbon steel sheet, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Ultra-low carbon steel sheet will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3252303