Metal treatment – Stock – Ferrous
Reexamination Certificate
1999-04-22
2003-03-04
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S321000, C148S319000, C148S328000, C075S517000, C075S566000, C075S574000, C420S008000, C420S009000, C420S013000, C420S039000
Reexamination Certificate
active
06527877
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to metallurgy, and more specifically to iron-based alloys and methods of their manufacture. The present invention can be used to best advantage to produce structural elements or products, which meet higher than usual demands on strength and plasticity.
PRIOR ART
There is known an effect of carbon on properties of iron-based alloys. Its presence in metal in the form of iron carbide improves strength of steel but simultaneously deteriorates its plasticity. Such contradictory effect prevents one from obtaining an optimal combination of these properties and requires the use of thermal treatment or addition of alloying elements.
In modern technology there are known and widely used iron-based materials containing two phases: the first phase is a crystal structure of solid iron-based solution and the second phase is a non-metallic phase partially soluble in iron. Different elements such as carbon, nitrogen, sulfur, oxygen, etc. are used as said non-metallic phase, but carbon is more widespread. Examples of such materials include DRI (directly pro-reduced iron), soot iron, metallized pellets or briquettes containing free carbon and carbon partially bonded with iron, which are a simple mechanical blend of free carbon and Iron-carbon alloy. However, these materials strictly saying, cannot be alloys and have a large amount of unavoidable residuals. As a result, they have no required deformability and cannot be used as structural materials.
There are known iron-carbon alloys containing a non-metallic phase in the form of carbon, e.g., iron graphite and so called graphitized steels. Iron graphite contains a metallic base-iron-carbon alloy and 0.5-7.0% of structurally free carbon in the form of graphite (“Powder Metallurgy and Spraying Coatings”, Moscow Metallurgy, 1987, p.301-302). Unlike graphitized steels, iron graphite in addition to iron-bonded carbon contains kish, a steelmaking by product. The latter is partially a source of carbon for carburization of the initial metallic matrix and residual graphite can serve as a lubricant under conditions of friction. The main field of its use is anti-friction items.
Recently,efforts have been made to use graphitized steels as structural materials in machinery. Such steels are produced by means of annealing, which provides the transition of iron-bonded carbon to a free state and formation of graphite, a product of complete or partial decomposition of iron carbide. Therefore, in these steels, carbon is present in both a bonded and free state.
However, carbon existing in such steels is a temper graphite, i.e., a secondary carbon formed in a solid state as a result of phase changes of iron carbide. Such carbon existing in the form of dispersed particles improves cold forgeability and cuttability of steel. However, a great difference in size of graphite particles and their non-uniform distribution in the volume of metal results from different rates of diffusion and activity of particles. This diversity often prevents these products from attaining the required strength and hardness of steel. As a result, graphitized steel has nonuniform macro and microstructure, which deteriorates its mechanical and operating properties. That is why graphitized steels are not so widely used.
In the prior art, a steel material is known containing a phase that forms crystal structure of solid iron-based solution and a structurally free non-metallic phase in the form of a secondary carbon, which has a mean grain diameter of not more than 4.0 &mgr;m and a grain density of at least 3,000 pcs/mm
2
. This steel comprises by weight 0.30 to 1.0% of C, 0.4 to 1.3% of Si, 0.3 to 1.0% of Mn, 0.0003 to 0.006% of B, 0.002 to 0.010% of N and 0.05 to 0.20% of Mo, with the balance being iron and unavoidable residuals( A1, EP 0751232, 02.01.97).
The mechanical properties and the grain structure of this prior art steel material was improved by the addition of Mo. Owing to the specific composition of the steel material and method for its making, all carbon in this prior art steel is present in the form of a temper graphite, i.e., it was formed on the completion of a production process and would not act as nuclei on solidification because the oxide and nitride non-metallic inclusions retained their iron nucleating properties, thus preventing the possibility of carbon atom replacement for nucleation.
Graphite particles In this prior art steel material have a wide range of grain diameters, the size of which could be more or less than the size of a critical nucleating particle. Moreover, the share of particles that could act as nucleating centers was not controlled. Under such conditions, it was and is difficult to provide consistent metal composition and high quality.
Temper graphite of the type present in the prior art steel is characterized with a strong interatomic bond In the matrix lattice, lowered activity and relatively low rate of dissolution in Iron. The result of its use is a delayed dissolution of carbon in iron during thermal treating, non-uniform hardening and microfissures during formation, and a change for the worse of the properties of the alloy and products from it (strength, hardness, etc.).
The presence of carbon only in a free state in the prior art steel does not allow for combining strength and plasticity, requires hardening in a cold state, and limits the upper available level of carbon by the magnitude of carbon concentration.
An obligatory presence of Mo in the prior art steel providing the required conditions to form fine graphite and Rs uniform dispersion Involves difficulties in technological operation and large financial expenditures.
SUMMARY OF THE INVENTION
According to this invention, we provide an Inexpensive iron-carbon alloy for use in the production of structural elements and products, which is characterized by high strength and plasticity and good cold workability on behalf of two states of carbon present in the alloy namely, a free and a bonded and/or dissolved carbon, and more complete use of free carbon to act as nuclei for crystallization during solidification of the alloy. Said alloy comprises a constituent forming a crystal structure of a solid iron-based solution comprising a structurally free phase forming non-metallic constituent, which particle size is equal to and/or exceeds a size of a critical nuclei and which is uniformly dispersed in the volume of the constituent forming a crystal structure.
The constituent forming a crystal structure comprises iron with dissolved and bonded carbon, and the phase-forming non-metallic constituent comprises the structurally free molecular carbon in the form of carbon black.
In a preferred embodiment, particle size of carbon black is 10
−5
-10
−7
cm; the ratio between carbon in a free state, and carbon in a bonded and in a dissolved state is from 0.01 to 20.0; the constituent forming a crystal structure contains at least of one element having an affinity for carbon greater than or equal to that of iron.
The present invention further provides a method of manufacturing said alloy comprising melting out low carbon semiproduct using directly pro-reduced Iron (DRI) and/or pig iron as a charge material of which an amount of 75-100% is loaded into a melting vessel in a liquid and/or a solid state to form a melt. Said melt is overheated by 20-70° C. above a liquidus temperature, and carburized with carbon black having particle sizes of about 10
−5
-10
−7
cm, which can be introduced in an amount of 0.01-2.14% of the melt mass at tapping and/or at finishing and/or when casting. In so doing, the melt product produced is deoxidized with elements having an affinity for oxygen equal to or greater than aluminum's affinity for oxygen; these deoxidizers are taken in an amount of 0.05-3% of the melt mass to provide a desired ratio between carbon in a free state, and in a dissolved and in a bonded state as 0.01-20.0. The method preferably includes solidification of the melt obtained and casting pressure treatment. Carbon for carburization is in
Graybeal Jackson Haley LLP
Limited Liability Company “SKM”
Yee Deborah
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