Classifications

• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals
  • C21D3/08—Extraction of nitrogen
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D3/00—Diffusion processes for extraction of non-metals; Furnaces therefor
  • C21D3/02—Extraction of non-metals

Definitions

• the present invention relates to a method of improving the aging characteristics of steel and, more particularly, the present invention is concerned with reducing the nitrogen content of elongated steel bodies of relatively small cross-sectional dimensions so as to improve the aging characteristics thereof.
• the solubility of nitrogen in alpha iron at room temperature is relatively low, namely less than 0.001%.
• the solubility of nitrogen in iron increases with increasing temperatures so that, for instance, at a temperature of 590 C. up to 0.10% nitrogen may be dissolved in the iron, provided that the steel melt contains such relatively high :proportion of nitrogen.
• Conventional technical steel generally contains considerably smaller proportions of nitrogen.
• basic Bessemer steel usually will contain between 0.06% and 0.015% nitrogen and open hearth steel between 0.003% and 0.008%.
• the temperature at which about 0.015% nitrogen may be dissolved in iron is in the neigh borhood of 300 C., and about 0.003% nitrogen are soluble in iron at a temperature about 150 C.
• ammonia will escape from the steel and immediately will .be diluted and distributed in the surrounding protective gas atmosphere, provided that sufiioient circulation of the gas atmosphere is maintained and fresh protective gas is admixed in suffi-cient proportions to maintain the atmosphere in which the steel body is exposed to annealing substantially free of ammonia.
• the present invention is primarily applicable to un-alloyed steel and generally to all types of steel with the exception of steel containing 0.02% or more of aluminum.
• removal of nitrogen according to the present invention is still possible with steel containing up to 0.01% aluminum and such steel containing not more than about 0.01% aluminum will be called herein substantially aluminum-free steel.
• the hydrogen gas contacts substantially the entire surface of the steel body and that the thickness of the steel bodies is within certain limitations so that the treatment will affect not only the surface portion but the entire steel body.
• annealing as described above namely in a somewhat moist, i.e. steam-containing, atmosphere which includes a major proportion of hydrogen gas and at relatively high temperatures, generally will also cause partial or complete decarbonization of the steel. Such decarbonization will occur even if the protective gas atmosphere contains only a very small proportion of steam.
• steam is decomposed into hydrogen and oxygen, and the free oxygen will oxidize the carbon located in the surface of the steel body under formation of carbon monoxide which is then removed by the circulating protective gas atmosphere. Carbon from the interior portion of the steel body will then diffuse into the surface portion thereof and again will react with oxygen, until a nearly complete decarbonization has taken place.
• Such debodies which method can carbonization may be desirable'in the case ofverysoft strips and wires, however, decarbonization or" the steel is most undesirable in those cases where the steel-must have a certain, relatively high degree of mechanical strength, for instance in the case of steel used for baling straps, spring strip, conveyor bands, Wire ropes and the like, in which the hardness and the mechanical strength is I strongly dependent on the carbon content.
• reduction of the nitrogen content of the last mentioned types of steel bodies is frequently of great importance inorder to avoidincr'ease in the brittleness thereof during aging.
• the present 7 invention contemplates a methodof improving the aging characteristics of substantially aluminum-free steel, comprising the step. oii subjecting the steel'in an atmosphere consisting essentially of nitrogen, hydrogen and steam ,to a heat treatment at a temperature between a first temperature being the temperature of incipient decarbonization of. the steel anda second temperature being 100 'C.
• the balance being nitrogena g UNKILLED 'OPEN HEARTH STEEL STRIP 0s MM.
• the annealing time will be prolonged, for instance, at a hydrogen concentration of to 8 hours; at a hydrogen concentration of 40%, to'17 hours; and ate. hydrogenconcentration of 30%, to 22 hours.
• the method of the present invention comprises the stepof subjecting the steel body in an atmosphere consisting essentially of between,75% and by volume of hydrogen gas, between 5%'and 0.5% by volume of steam, the balance being nitrogen to a heat treatment within a temperature. range of between about 500 to 600 C. and for a period-0t time sufficient to substantially reduce the nitrogen content of thesteel, whereby the aging characteristics of the steel, body are improved due to reduction of the nitrogen content thereof'without substantially affecting body.
• the nitrogen content of steel bodies is reduced by annealing the steel body (which should be substantially free of aluminum) in an atmosphere consisting of nitrogen, hydrogen and stearmwithin a temperature range suchthat the carbon content of the steel the annealing temperature is not higher than the lowest temperature at which appreciable decarbonization of the steel will take place and not more than about C. lower than, such temperature ofincipient decarbonization.
• the protective gas atmosphere should contain at least 50% hydrogen, while best results usually are achieved, with even higher. concentrations of. hydrogen gas.
• the protective gas atmosphere under which annealing is carried out in accordance with the present invention in the indicated temperature range will contain between 40 and 99.5% by volume of hydrogen gas, preferably between 68 and 89% and most preferably between 75 and 80%.
• the steam content of the protective gas atmosphere should not exceed 5% by volume and broadly will be between 5 and about 0.5%, preferably between 1 and 2%; while the nitrogen content broadly should be maintained between 59.5 and and preferably between 10 and 30% Very good results are obtained with a protective gas atmosphere containing 75% hydrogen, a small proportion of steam as indicated above, the balance being nitrogen.
• the proportion of hydrogen and nitrogen in such mixture will be approximately the same as is obtained upon decomposition of ammonia.
• the hydrogen content can also be increased up to close to 100%, however, for economical reasons as indicated above, namely in order not to unduly lengthen the process, preferably the hydrogen content will not be reduced to less than about 50% and, as stated above, the balance will primarily consist of nitrogen.
• the protective gas atmosphere may contain up to 100% hydrogen with traces of steam, however an increase of the hydrogen content beyond 80% doesnt give an acceleration or an improvement of the reduction of the nitrogen content in the steel. Therefore the protective gas atmosphere can obtain an additional, non-decarbonizing gas, i.e., non-oxidizing gas or rare gas.
• additional, non-decarbonizing gas i.e., non-oxidizing gas or rare gas.
• nitrogen is added, non-decarbonizing gas but rare gas for instance argon may be used, too.
• the addition of non-oxidizing or rare gas to the protective gas atmosphere reduces the danger of gas explosion and therefore is opportune for security.
• nitrogen as additional,
• compositions of protective gas atmospheres according to the present invention are summarized in Table H.
• Table II PROTECTIVE GAS COMPOSITION Maximum, Preferred, Minimum, percent percent percent Table III shows 5 preferred compositions of the protective gas or atmosphere:
• EXAMPLE I A steel strip consisting of unkilled' soft basic Bessemer steel of the following analysis: 0.11% C. 0.01% Si,
• EXAMPLE II A deep drawn strip of unkilled open hearth steel of the following analysis: 0.05% C, 0.01% Si, 0.32% Mn, 0.012% P, 0.018% S and 0.004% N, and which had been cold rolled down to a thickness of 0.8 mm. was annealed for 5 hours, suspended in a protective gas atmosphere composed of 78% hydrogen, 20% nitrogen and 2% steam.
• a methodv ofuimproving the aging characteristics of an elongated substantially aluminum free nnalloyed steel body having a maximum thickness of up to about 4millimeters an d containing up to 0.01% aluminum and more than 0.001% nitrogen comprising the step of subjecting *said steelbody in an atmosphere consisting essentially of between 740% and, 9 5.5% byvolume of hydrogen gas, between 5 and 0.5% by volume of steam and up to 59.5% by volume of at least one gas selectedufrom the group consisting of nitrogen gas and rare gases toa heat treatment within a temperature rangeof between about 500 and 600"C.
• a method of improving the aging characteristics of substantially aluminum-free steel comprising the step of subjecting steel'containing up to 0.01% aluminum and a an elongated substantially aluminum-free steel body havbe' secured by ing a maximum thickness of upto about 4 millimeters and V more'than 0.001% nitrogen in an atmosphere consisting;
• a method of improving the aging characteristics of 1 an elongated substantially aluminum-free steel body having a maximum thickness of up to about 4 millimeters and containing up to 0.01% aluminum and more than 0.001% nitrogen comprising the step of subjecting said steel body in 'anratmosphere consisting essentially of be tween 68% and 89 by volume of hydrogen gas, between 10% and 30%]by volume. of nitrogen gas and. between 1% and 2% by volume of steam to a heat treatment within a temperature range ofbe'tween about 500 and 600C. and fora period of time suflicient to substantially reduce. the nitrogen content of said steel, wherebythe aging characteristics of said steel body are improved due to reduction of the nitrogen content thereof without substantially .aifecting thecarbon content 'of said steel body.
• a method of improving'the aging characteristics" of containing up to 0.01% aluminum andmore than 0.001% nitrogen comprising the'step of subjecting said steel body in an atmosphere consisting essentially of between 75% v and.80% by volume of hydrogen 'gas, between 5% and 0.5 by volume of steam, the balance being nitrogen, to a heat treatment within a temperature range of between about 500 and 600 C. and for a period of time sufiicient to. substantially reduce the nitrogen content of said steel, 7 whereby'the aging characteristics of said steel body are improved due to reduction of the nitrogen content thereof without'substantially aifeoting the carbon content of said steel body.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Thermal Sciences (AREA)
• Crystallography & Structural Chemistry (AREA)
• Mechanical Engineering (AREA)
• Physics & Mathematics (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Heat Treatment Of Strip Materials And Filament Materials (AREA)
• Heat Treatment Of Sheet Steel (AREA)
• Treatment Of Steel In Its Molten State (AREA)
• Manufacturing Of Steel Electrode Plates (AREA)

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Citations (3)

• 0
  • BE BE623982D patent/BE623982A/xx unknown
• 1962
  • 1962-02-16 DE DEH44897A patent/DE1175258B/de active Pending
  • 1962-10-15 GB GB38947/62D patent/GB956213A/en not_active Expired
• 1963
  • 1963-02-14 US US258640A patent/US3184344A/en not_active Expired - Lifetime

Patent Citations (3)

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