Alloys or metallic compositions – Ferrous – Over 0.05 percent sulfur – over 0.04 percent phosphorus or...
Reexamination Certificate
2002-05-31
2004-08-10
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Over 0.05 percent sulfur, over 0.04 percent phosphorus or...
C420S084000
Reexamination Certificate
active
06773661
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention concerns a free-cutting steel. More specifically, the invention concerns a free-cutting steel having good machinability both in turning and drilling.
There are various steels having good machinability as a kind of machine structural steels such as Pb(lead)-free-cutting steel, S(sulfur)-free-cutting steel and Ca(calcium)-free-cutting steel. Pb-free-cutting steel is an excellent one in that it exhibits high machinability without damaging mechanical properties of the base steel. These days, however, in view of undesirable influence by Pb to the environment, use of Pb as a machinability improving element is often avoided and efforts are made to develop Pb-free or low Pb-content free-cutting steel.
As one of the free-cutting steels which do not rely on the machinability improving effect by Pb there is a steel which is so designed to have carbosulfide inclusions of Ti and/or Zr precipitated in the steel matrix. This kind of free-cutting steel has, though machinability in turning is good, a drawback that machinability in drilling is low, when compared with the others such as S-free-cutting steel.
The inventors conducted research on the above-mentioned free-cutting steel in which the carbosulfide of Ti and/or Zr (in the following description, represented by “Ti-carbosulfide”, which mainly consists of a compound expressed as Ti
4
C
2
S
2
) precipitated to improve the machinability in drilling with maintaining the inherent good machinability in turning. The inventors' discussion is as follows.
(1) As is well known, at cutting a steel with a tool free-cutting component in the steel melts or is softened due to the heat generation caused by friction and functions as the lubricant to decrease the resistance of friction between the edge of the tool and the part of the steel which is being cut, and thus, the cutting proceeds. In the case of turning the outer round surfaces friction heat is very large and therefore, even if the free-cutting components has a relatively high melting point, the component will melt or be softened. On the other hand, at drilling, even though the rotating rate of the tool is the same as that of turning, because diameter of a drill is usually small, heat generation by friction is small and therefore, if the melting point of the free-cutting component is relatively high, it will not melt or be softened.
(2) The fact that a free-cutting steel in which the Ti-carbosulfide precipitated, when compared with an S-free-cutting steel, exhibits good machinability in turning, while low machinability in drilling is considered to be based on the fact that melting point of the free-cutting component, Ti-carbosulfide, is higher than that of MnS, free-cutting component of ordinary S-free-cutting steel.
(3) The difference of hardness of the tools is mentioned as another difference in turning and drilling. Bites used in turning is usually made of cemented carbide, and the hardness of the tool is HV 2000 or higher. On the other hand, drills are made of a high-speed steel, and the hardness of the tool is HV around 1000, while hardness of the Ti-carbosulfide is in the range of HV 800-1000. If this free-cutting component is not sufficiently softened at cutting, it is evident that the drills will be abraded.
(4) Consequently, it is not possible to improve machinability in drilling with only the Ti-carbosulfide. It is considered that the above problem could be solved by having a suitable amount of MnS coexisted with the Ti-carbosulfide to enhance the machinability in drilling by MnS. MnS is so soft (about Hv150) that it may never damages drills, even if it is not softened by heat.
Based on the above discussion the inventors conducted research on the effect of the quantitative relation between the Ti and/or Zr and S on the machinability in turning and drilling. As the results, they discovered the fact that, as the contents in weight %, Ti[%] and 0.52Zr[%] are equivalent and that a ratio of Ti+0.52Zr to S less than 2 is useful for achieving favorable balance of the amounts of the Ti-carbosulfide formed and MnS formed.
The inventors also made research on suitable composition of MnS which offers good machinability in drilling. As the results it was found that MnS in which Ca and/or Mg is dissolved therein, i.e., (Mn,Ca,Mg)S is useful for improving the machinability in drilling.
SUMMARY OF THE INVENTION
The object of the present invention is to solve the problem residing in the free-cutting steel containing Ti-carbosulfide as the machinability-improving-inclusion and to provide a novel free-cutting steel having good machinability both in turning and drilling.
The free-cutting steel according to the present invention consists essentially of, in mass %, C: 0.05-0.80%, Si: 0.01-2.5%, Mn: 0.1-3.5%, Ti+0.52Zr: 0.03-1.2%, S:0.015-0.6% and the balance of Fe and inevitable impurities, and is characterized in that the contents of Ti, Zr and S meet the condition of (Ti+0.52Zr)/S<2.
DETAILED EXPLANATION OF THE PREFERRED EMBODIMENTS
The free-cutting steel according to the present invention may contain, in addition to the above mentioned basic alloy components, one or more of the elements enumerated below.
(1) one or both of Ca: 0.0005-0.02% and Mg: 0.0003-0.02%,
(2) B: 0.0003-0.01%,
(3) one or more of the group consisting of Cr: up to 3.5%, Ni: up to 4.0%, Cu: up to 2.0%, and Mo: up to 2.0%,
(4) one or more of the group consisting of Se: 0.005-0.4%, Te: 0.005-0.1%, Pb: up to 0.4% and Bi: up to 0.4%, and
(5) one or more of the group consisting of V: up to 0.2%, Nb: up to 0.2% and Ta: up to 0.5%.
In the present free-cutting steel two kinds of inclusions, Ti-carbosulfide and MnS, are formed in the matrix and coexist in dispersed state. In case where Ca and/or Mg presents in the steel, these elements are dissolved in MnS and therefore, the latter inclusion is expressed as (Mn,Ca,Mg)S. As understood from the above explanation the latter inclusion, MnS or (Mn,Ca,Mg)S, is the inclusion mainly for improving the machinability in drilling and Ti-carbosulfide is the inclusion mainly for improving the machinability in turning.
In order to attain coexistence of the two kinds of inclusions it is necessary that the relation, (Ti+0.52Zr)/S<2, in the amounts of Ti and Zr, and S is established. If this relation is not established, in other words, the amount of S to the amount of (Ti+0.52Zr) is not sufficiently large, major portion of S is fixed as carbosulfide of Ti, and MnS or (Mn,Ca,Mg)S will not be formed at all or, even if formed, in an insufficient amount, and thus, the machinability in drilling can not be improved.
The coefficient “0.52” is a value obtained by dividing 47.9, atomic weight of Ti, by 91.2, atomic weight of Zr. Multiplying this coefficient by the weight percent of Zr is made because 0.52Zr is equivalent to Ti in the relation to the amount of S.
The following explains the roles of the alloy components of the present free-cutting steel and the reasons for deciding the alloy composition.
C: 0.05-0.80%
A part of carbon is dissolved in Fe to ensure strength of the steel and the rest combines with Ti, Zr and S to form Ti-carbosulfide, which improves machinability in turning. At a C-content less than 0.05% this effect is not obtained. On the other hand, if the C-content exceeds the upper limit, 0.80%, resilience and machinability of the steel in turning will decrease.
Si: 0.01-2.5%
Silicon is added as a deoxidizing agent at steel making, and further, increases hardenability of the steel. Si-content less than 0.01% will not give this merit. If Si is added to the content more than 2.5%, resilience of the steel decreases and crack formation at plastic processing tends to occur.
Mn: 0.1-3.5%
Manganese, by combining with S, and if Ca and/or Mg exists, also with these elements, forms MnS, or (Mn,Ca)S, (Mn,Mg)S or (Mn,Ca,Mg)S to improve the machinability in drilling. To obtain this merit the Mn-content must be at least 0.1%. Too much content will, however, increases hardness of the steel and decreases the machinab
Hayaishi Masakazu
Kurebayashi Yutaka
Daido Steel Co. Ltd.
Varndell & Varndell PLLC
Yee Deborah
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