Alloys or metallic compositions – Ferrous – Lead – bismuth – selenium – tellurium or calcium containing
Reexamination Certificate
1998-07-06
2001-03-13
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Ferrous
Lead, bismuth, selenium, tellurium or calcium containing
C420S087000
Reexamination Certificate
active
06200527
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention concerns a fine-grained carbon or low-alloy steel that contains oxides and sulfides and is treated with calcium. The steel has improved machinability and is capable of being continuously cast.
2. Discussion of the Background
Carbon or low-alloy structural steels with improved machinability are well known. In these steels good machinability is obtained, notably, by the presence of relatively plastic sulfur-base inclusions and oxide inclusions having not too high a melting point, and by the absence of hard abrasive inclusions. The sulfur-base inclusions facilitate machining by favoring fragmentation of the machining cuttings, which is very desirable for low-speed machining operations such as boring. The oxide inclusions associated with calcium-rich manganese sulfides make it possible to carry out high-speed machinings by forming, on the surface of the tool which makes contact with the cutting, a layer ensuring both a lubricating function and a diffusion-barrier function which retards tool wear and makes it possible to obtain a good surface state. The absence of hard abrasive inclusions avoids tool wear caused by friction of such inclusions on the surface of the tool which makes contact with the cutting.
The sulfur-base inclusions consist of manganese sulfide, in which the sulfur can be partially substituted by selenium or tellurium, and the manganese can be partially substituted by calcium. Selenium, tellurium and calcium have the advantage of diminishing the high-temperature plasticity of manganese sulfides, which prevents the sulfur-base inclusions from becoming too elongated during high-temperature, plastic deformation shaping operations and from overly damaging the ductility of the metal. However, the calcium content of the sulfide inclusions should not be too high, because the mixed manganese and calcium sulfide inclusions may then become hard and abrasive. More precisely, the calcium content of the sulfide inclusions should be less than 75% and preferably less than 40%.
The oxide inclusions with not too high of a melting point are, notably, inclusions consisting of a lime aluminosilicate of suitable composition, in sufficient quantity and well dispersed. The oxide inclusions should not consist of pure alumina, an oxide that is, in fact, extremely hard and is found in steel in the form of very abrasive masses or alignments. In general, structural steels should contain a little aluminum to control the size of the grain and, therefore, they cannot contain pure silica inclusions, which moreover are undesirable.
The above-described steels, which are obtained by deoxidation with aluminum, resulfuration and injection with calcium or SICAL (SilicoCalciumAluminium), have a chemical composition characterized specifically by a sulfur content ranging between 0.025% and 0.3%, a manganese content exceeding 0.1%, an aluminum content ranging between 0.1% and 0.05%, an oxygen content exceeding 0.002% and a calcium content exceeding 0.002%. The rest of the composition is that of a standard carbon or low-alloy structural steel, that is, it includes of iron, carbon, alloy elements, nitrogen and impurities resulting from manufacture. Those steels can further contain supplementary addition elements, such as, for example, selenium, tellurium, bismuth or lead.
Beside the sulfur, manganese, aluminum, oxygen and calcium contents, steels with improved machinability are characterized by the nature and quantity of the inclusions. The nature of the inclusions corresponds to what has been defined above. The quantities are in direct relationship: sulfides with the sulfur content (and possibly selenium or tellurium content); and oxides with the oxygen content.
Even though they have a very good capacity for machining at both low speed and high speed, the above-identified steels inhere at least two disadvantages. First, they require a high oxygen content and consequently, a large quantity of oxide inclusions, which are unfavorable to fatigue strength. Second, they cannot be continuously cast. In fact, the injection of SICAL or calcium leads to the formation of oxides or sulfides which clog the casting nozzles. Furthermore, when the oxygen content is insufficient, the injection of calcium leads to the formation of calcium-rich sulfides, which are very hard and abrasive.
SUMMARY OF THE INVENTION
The purpose of the present invention is to solve the above-described problems by offering a carbon or low-alloy steel with improved machinability, treated with calcium, having a reduced oxygen content and capable of being continuously cast.
Accordingly, the first embodiment of the present invention relates to a carbon or low-alloy structural steel with improved machinability, with a carbon content of less than 1.5% and the sum of whose alloy elements is less than 9%, containing oxide and sulfide inclusions and having a chemical composition by weight of:
0.1%≦Mn
0.01%≦Al≦0.05%
0.025%≦S≦0.3%
0.002%≦Ca
O≦0.0015%
the state of inclusion of that steel being characterized by a “KO oxides” cleanness index below 30, by the fact that all the oxide inclusions are lime aluminates and by an average calcium content of type 2, type 3, type 4 and type 5 inclusions below 30%, and preferably below 25%.
The second embodiment of the present invention relates to a process for the manufacture of a steel metallurgical product, including:
deoxidizing a molten steel with aluminum;
degassing the molten steel under vacuum;
injecting a powder that contains oxides whose melting temperature is below 1500° C. into the molten steel; and
injecting calcium into the molten steel.
The third embodiment of the present invention relates to a core wire that contains an oxide powder having 50% to 70% SiO
2
, 5% to 25% Al
2
O
3
, 5% to 25% CaO, 0% to 10% Fe
2
O
3
and FeO, and a remainder being impurities.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description, which is not intended to be limiting unless otherwise specified.
The steel preferably does not contain type 5 inclusions. The type 3 and 4 inclusions also preferably represent less than 30% of the inclusions. Finally, it is preferable for the proportion of type 2 inclusions to be greater than 20% and for the average calcium content of those inclusions to range between 15% and 25% calcium.
The steel can be manufactured in the form of a metallurgical product of continuously cast slab, bloom, billet, bar or wire type.
Preferably, the core wire includes a tube that is fusible in the molten steel and a powder being of a mixture of oxides having a melting temperature below 1500° C., more preferably below 1400° C. and most preferably below 1300° C.
The oxide powder for the core wire preferably contains 50% to 70% SiO
2
, 5% to 25% Al
2
O
3
, 5% to 25% CaO, 0% to 10% Fe
2
O
3
and FeO, the rest being impurities.
The oxide powder for the process preferably contains 50% to 70% SiO
2
, 5% to 25% Al
2
O
3
, 5% to 25% CaO, 0% to 10% Fe
2
O
3
and FeO, the rest being impurities, and its composition is chosen so that its melting temperature will be preferably below 1500° C., more preferably below 1400° C., and most preferably below 1300° C.
Preferably, the carbon or low-alloy structural steel with improved machinability of the present invention is intended mainly for the fabrication of machine parts obtained either by machining a steel bloom cut in a rolled steel product, or by machining a blank obtained by forging. The fabrication of these machine parts generally entails a heat treatment carried out on the part, or on a steel metallurgical product, or on the forged blank. The heat treatment may be completed by a surface treatment. The chemical composition of the structural steels used for machine parts may be chosen in accordance with the mechanical properties desired, the use envisaged for the parts, and the size of the parts. The range of composition
Damie Philippe
Turrel Christian
Ascometal
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Yee Deborah
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