Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-08-17
2003-06-17
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Ferrous
C148S320000, C148S547000, C148S548000
Reexamination Certificate
active
06579385
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a free machining steel for use in machine structures intended to be machined as components of industrial machines, automobiles and electric products and, more in particular, it intends to provide a free machining steel for use in machine structures having excellent machinability in a so-called Pb free steel, containing no substantial Pb as a machinability improving ingredient and also excellent mechanical characteristics.
2. Description of Related Art
Materials for components of industrial machines, automobiles and electric products are required to have good machinability since such components are manufactured by machining the materials. In view of the above, free machining steels for use in machine structures have usually been used as the materials and such free machining steels are often incorporated with Pb or S as a machinability improving ingredient and, particularly, it has been known that Pb provides excellent machinability with addition of a small amount.
As the technique described above, JP-A-205453/1984, for example, proposes a free machining steel for free machining low carbon sulfur steel in which all of Te, Pb and Bi are added in combination, MnS type inclusions each having a major diameter and a minor diameter of larger than a predetermined size and with a (major diameter/minor diameter) ratio of 5 or less are present by 50% or more of the entire MnS inclusions and the Al
2
O
3
content in oxide inclusions is 15% or less.
Further, JP-A-23970/1987 proposes a technique of improving the machinability of a free machining low carbon sulfur-lead steel by a continuous casting method in which each of the contents for C, Mn, P, S, Pb, O, Si and Al is defined and the average size of MnS type inclusions and the ratio of sulfide type inclusions not bonded with oxides are defined thereby improving the machinability.
Each of the techniques described above concerns free machining steel with combined addition of Pb and S. As the problem of environmental pollution caused by Pb has been highlighted, use of Pb has tended to be restricted also in iron and steel materials and a study on the technique for improving the machinability in a so-called Pb free state has been progressed positively.
In view of the situation, a study for improving the machinability by controlling the form, for example, the size or the shape of sulfide type inclusions such as MnS has been predominant in the free machining sulfur steel, but no free machining steel that can provide machinability comparable with free machining Pb steel have yet been attained. Further, in the study of improving the machinability by controlling the form of the sulfide type inclusions, it has been pointed out also a problem that the sulfide inclusions such as MnS are deformed lengthwise along with plastic deformation of the base metal upon rolling or forging the steel material, which causes anisotropy in the mechanical characteristics and the impact resistance in a certain direction.
By the way, the machinability is evaluated by the items such as (1) cutting force, (2) tool life, (3) roughness on the finished surface and (4) chip disposability. Among the items, importance has been attached so far to the tool life and the roughness on the finished surface, but the chip disposability has also become an innegligible subject in view of operation efficiency and safety along with the recent automation or man-less trend in machining operation. That is, the chip disposability is a characteristic for evaluating disconnection of chips into shorter segments during machining. If the characteristic is worsened, chips extend spirally to bring about a trouble that they twine around the cutting tool to hinder the safety operation of machining. Existent Pb-added steels can provide a relatively good machinability also in view of the chip disposability but favorable characteristics have not yet been attained in the Pb-free steel materials.
SUMMARY OF THE INVENTION
This invention has been accomplished in view of the foregoing situations and intends to provide a free machining steel for use in machine structures that can stably and reliably provide, in a Pb-free state, excellent machinability (particularly, chip disposability and tool life) and mechanical characteristics (transverse direction toughness), which are comparable with those of existent Pb-added steels.
In accordance with this invention for attaining the foregoing object, there is provided a free machining steel for use in machine structures in which sulfide type inclusions are present wherein Mg is contained by from 0.0005 to 0.02 mass % and the distribution state for the sulfide type inclusions is controlled, to thereby improve mechanical characteristics. More specifically, there is provided a free machining steel for use in machine structures in which sulfide type inclusions are present, wherein Mg is contained by from 0.0005 to 0.02% (“%” means “mass %” here and hereinafter) and a distribution index F1 for the sulfide type inclusion particles defined by the following equation (1) is from 0.4 to 0.65:
F
1
=X
1
/(
A
)
½
(1),
where
X
1
: represents an average value (&mgr;m) obtained by actually measuring the distance between each of sulfide type inclusion particle in an observed visual field and other particle nearest thereto for all of particles present in the observed visual fields, measuring the distance for five visual fields and averaging them, where
A: represents an observed area (mm
2
), and
n: represents the number of sulfide type inclusions observed within the observed area (number).
Further, the foregoing object of this invention can be attained also by a free machining steel for use in machine structures in which Mg is contained by from 0.0005 to 0.02% and a distribution index F2 for the sulfide type inclusion particles defined by the following equation (2) is from 1 to 2.5:
F
2
=&sgr;/X
2
(2),
where
&sgr;: represents a standard deviation for the number of sulfide type inclusion particles per unit area, and
X
2
: represents an average value for the number of inclusion particles per unit area.
In each of the free machining steels for use in machine structures, it is preferred to satisfy the condition that the ratio of a major diameter L1 to a minor diameter L2 (L1/L2) of the sulfide type inclusions is from 1.5 to 5, which can further improve the mechanical characteristic (transverse direction toughness) and the machinability (particularly, chip disposability and tool life).
The chemical ingredients of the free machining steel for use in the machine structures according to this invention preferably contains, in addition to Mg, C in an amount from 0.01 to 0.7%, Si in an amount from 0.01 to 2.5%, Mn in an amount from 0.1 to 3%, S in an amount from 0.01 to 0.2%, P in an amount 0.05% or less (inclusive 0%), Al in an amount of 0.1% or less (inclusive 0%) and N in an amount from 0.002 to 0.02%, respectively, in view of ensuring physical properties required as the free machining steel for use in machine structures. It is also useful to optionally incorporate at least one member selected from the group consisting of (a) Ti in an amount from 0.002 to 0.2%, Ca in an amount from 0.0005 to 0.02%, and from 0.0002 to 0.2% in total of rare earth elements and (b) Bi in an amount of 0.3% or less (exclusive 0%).
In order to solve the subjects described above, the present inventors have studied the relation, particularly, the relation between the chip disposability and the sulfide inclusions in the free machining steel with various points of view. As a result, it has been found that not only the size and the shape of the sulfide type inclusions such as MnS but also the distribution state of the sulfide type inclusions has a close concern with the chip disposability. As a result of a further study, it has been found that a free machining steel for use in machine structures having, in the Pb-free state, excellent mechanical characteristics (transverse direction toughness) and c
Kaiso Masato
Kudou Takahiro
Shindo Yosuke
Somekawa Masami
Tsuchida Takehiro
Ip Sikyin
Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel Ltd).
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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