Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-08-25
2001-01-16
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S328000, C148S654000, C148S659000, C148S660000, C148S662000
Reexamination Certificate
active
06174384
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a medium-carbon steel with a dispersed fine spherical graphite structure, having high mechanical strength, good processability, and good machinability, enabling the time required for graphitization to be reduced, and lowering the temperature required for the graphitization heat treatment. The present invention also relates to a method of manufacturing a medium-carbon steel.
BACKGROUND OF THE INVENTION
Heretofore, lead steel has been used as a steel material having free-cutting properties. However, since lead free-cutting steel contains Pb, the problem of environmental contamination has arisen, and recently, the use of lead free-cutting steel has been restricted. For this reason, the development of new iron and steel materials posing no environmental problems which replace lead free-cutting steel is desirable.
Under such a situation, although grarphite-dispersed material is known as an iron and steel material having free-cutting properties, it has not been considered as a material having sufficient properties as free-cutting steel, because conventionally considered graphite steel has poor mechanical properties, such as, strength and processability. Therefore, the development of graphite steel which has fine graphite particles evenly dispersed therein and which has free-cutting properties and good mechanical properties is desirable.
In general, it is known that graphitization proceeds readily in high-carbon steel having a hyper-eutectoid composition. Specifically, it has been disclosed that the addition of 1% or more silicon significantly accelerates graphitization. For example, see: Tomo-o Sato et al., “Study on Graphitic Steel (1
st
Report) Effect of Silicon on Graphitic Steel”, J. Jpn. Inst. Met., 1956, Vol. 20, pp. 5-9.
Similarly, it has been disclosed that the graphitization of high-carbon steel is accelerated by quenching, cold working, or by the addition of Al, Si, Ni, Ti, Zr, or B. For example, see: Naomichi Yamanaka et al., “On the Mechanism of Graphitization of High-Carbon Steel”, Iron & Steel, 1962, Vol. 8, pp. 946-953.
The Yamanaka reference discloses that the addition of Ti causes acceleration of graphitization since the distribution of Ti in cementite is quite low, and special carbide, TiC, is easily formed. Thus, graphitization is accelerated by the effect of denitrification by Ti rather than the effect of stabilization of cementite.
However, all of these examples are high-carbon steels. For higher strength and better processability, a medium-carbon steel having a C concentration of 1.0% or below is required. Therefore, the above examples are not satisfactory or suitable as a free-cutting steel.
On the contrary, it is the present state that the graphitization of medium-carbon steels, especially hypo-eutectoid steel is difficult, and that the behaviors of graphitization by heat treatment or the addition of alloys have not been well known.
As the previously mentioned Sato reference discloses, although Si is added as an element for accelerating graphitization, it is considered that the quantity of Si is preferably 1.5% or less, since a large quantity of added Si significantly lowers ductility by solid solution hardening.
Under such a situation, there is a report describing the effects on a graphitization phenomenon of addition of Si and Ni, or Si and Co, together to hypo-eutectoid steel. For instance, see: Hidekazu Sueyoshi et al., “Effect of Alloying Elements on the Graphitization of Hypo-Eutectoid Steel”, J. Jpn. Inst. Met., 1979, Vol. 43, pp. 333-339.
However, considering that the materials containing Ni or Co are difficult to recycle and that such added elements are relatively expensive, the addition of these materials is not effective, and it cannot be considered that the process for manufacturing hypo-eutectoid steel in which fine graphite particles are dispersed in a stable manner and with high reproducibility has been established.
It has been known that the graphitization of high-carbon steel is accelerated by cold working before the graphitization treatment. As one of such methods there has been disclosed a method for improving softness and ductility as low-carbon steels to hypo-eutectoid steels. For instance, see: Atsuki Okamoto, “Graphite Formation in High-Purity Cold-Rolled Carbon Steels”, Metallurgical Transactions A, October 1989, Vol. 20A, pp. 1917-1925.
By this disclosed method, it is considered that cold rolling is conducted until cementite is divided, and the gaps of dividing (eg. voids) become sites of graphitization.
However, this example has problems in that stringent cold working of 20% or more is required for dividing cementite under the state of poor cold-working properties, and cannot be considered to be a stable manufacturing method.
Furthermore, a method has been proposed for accelerating graphitization by adding B, Al, or a rare-earth metal (REM) such as La and Ce to 0.53% C steel. For instance, see: Iwamoto et al., “Effects of B, Al, and REM on the Graphitization Behavior of 0.53% C Steel”, CAMP-ISIJ, 1995, Vol. 8, p. 1378.
Since it has been reported that the average particle diameter of graphite can be decreased to 2.7 &mgr;m according to this method, some effects can be considered. However, in this method, the addition of B and Al together is required for realizing the largest number of graphite particles, and the addition of either one alone cannot realize a large number of graphite particles as shown in
FIG. 1
of the previously mentioned Iwamoto reference.
As to steel, since it is not easy to control a small quantity of an added element, and the effect of multiple addition is large, there is a problem that this method is not practical.
A common problem of the above graphitization methods is that graphitization must be performed at a high temperature and takes a long time for processing. Complete graphitization requires a temperature of 700° C. and a time between 10 and 50 hours, significantly lowering the manufacturing efficiency.
OBJECTS OF THE INVENTION
For these reasons, the present invention provides a medium-carbon steel which has a dispersed fine spherical graphite structure allowing fine ZrC particles to function as graphite nucleation sites, which has high reproducibility and high productivity, which has high mechanical strength, good processability and machinability, and which enables a reduction in time required for graphitization.
It is also an object of the present invention to provide a novel and efficient method for manufacturing such a medium-carbon steel.
SUMMARY OF THE INVENTION
According to the present invention, a medium-carbon steel is provided comprising a dispersed fine spherical graphite structure containing 0.1 to 1.5% Si, 1.0% or less of C, and 0.01 to 0.5% Zr.
According to other aspects of the present invention, the fine ZrC function as graphite nucleation sites, cementite disappears, and preferably, the structure is a ferrite matrix having 30 to 6500/mm
2
of graphite particles with an average diameter of 1 to 20 &mgr;m dispersed therein.
According to another aspect of the present invention, a method for manufacturing a medium-carbon steel is provided. The method includes heat-treating a material of a medium-carbon steel composition having a matrix containing 0.1 to 1.5% Si, 1.0% or less of C, and 0.01 to 0.5% Zr at a temperature of 750 to 1300° C. for 0.5 to 10 hours and quenching the heat-treated material in water to deposit fine ZrC particles in the matrix. Preferably, the method also includes the step of heat treating the quenched material at a temperature of no greater than 740° C. for 0.5 to 100 hours to provide a graphitization treatment to the material to grow graphite using ZrC as nucleation sites. Preferably, the graphitization treatment is performed at a temperature of 450 to 700° C.
According to yet another aspect of the present invention, a method for manufacturing a medium-carbon steel having a dispersed fine spherical graphite structure is provided. The method includes hot-rolling a material of a medium-carbon steel composition contai
Abe Toshihiko
Oikawa Katsunari
Sumi Shin-Ichi
Agency of Industrial Science and Technology
Howson and Howson
Yee Deborah
LandOfFree
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