Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
• • 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
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0273—Final recrystallisation annealing

Definitions

• a steel composition capable of being continuously cast and further processed to produce drawing-quality sheet steel having Substantial resistance to aging comprising: 0.01-0.08% carbon, 0.20-0.60% manganese, 0.03-0.08% silicon, up to 0.015% aluminum, less than 0.01% boron and other usual steelmaking impurities such as nitrogen and oxygen. It is essential that sutficient boron be included to provide a boron to nitrogen ratio of 1.4 to 2.5 when the oxygen content exceeds 150 p.p.m., or a ratio of 1.0 to 1.4 when the oxygen content is less than 150 ppm. In further processing the steel to sheet products, it is essential that the hot rolled product be coiled at a temperature above 1100" F.
• rimmed steels are preferred because of their superior surface quality and high degree of ductility. It is well known in the art that rimmed steel ingots are produced by casting a low-carbon, non-deoxidized steel into an ingot mold, where the decreasing temperature, and resulting decrease in oxygen solubility, causes the released oxygen to react with dissolved carbon and violently evolve carbon monoxide gas. This violent gas evolution called rimming action causes the ingot to solidify with a high purity, dense surface or rim and exceptional cleanliness and ductility throughout.
• Rimmed steels do, however, have one serious disadvantage for some applications in that the steel products rolled therefrom are subject to a high degree of strain aging. That is to say, the temper-rolled steel undergoes a spontaneous increase in hardness and decrease in ductility upon prolonged storage even at room temperatures, which is primarily due to segregation of nitrogen. atoms to dislocations producing pinning of the dislocations by solute atoms. For those applications where prolonged storage or severe cold forming are expected, non-aging steels can be produced by casting the ingot with a steel that has been deoxidized with aluminum and/or titanium.
• This invention concerns our discovery of a unique steel composition which can be continuously cast to produce a non-aging or aging-resistant steel having exceptionally good quality for further processing to thin flat rolled steel products.
• the inventive concepts not only involve the critical control of steels composition, particularly a very critical balance between the boron, nitrogen and oxygen contents, but also the critical control of the hot-rolling finishing temperature, coiling temperature, and annealing temperature when further processing the steel to thin flat rolled products.
• An object of this invention is to provide a unique steel composition which can be continuously cast and further processed to produce non-aging or aging-resistant highquality thin fiat rolled steel products with surface quality equal to that of conventionally produced rimmed steel.
• Another object of this invention is to provide a unique steel composition which can be continuously cast and further processed to produce high-quality thin flat rolled steel products having improved aging-resistance.
• a further object of this invention is to provide a process for producing a non-aging drawing-quality sheet steel from a continuous cast steel slab.
• Still another object of this inventon is to provide a process for producing a drawing-quality sheet steel having enhanced resistance to aging.
• the composition of a molten steel is determined and adjusted to provide a composition substantially as described in US. "Pat. No. 3,412,781, supra, namely, 0.01 to 0.08% carbon, 0.20 to 0.60% manganese, 0.03 to 0.08% silicon andup to-0.0l aluminum.
• this composition will provide a steel melt which can be continuously cast to provide a slab having good mechanical properties and especially suited for fiat-rolling to sheet products, the sheet products being comparable-to those previously rolled from rimmed steel ingots.'As previously noted, however, this steel'is subject to aging, so that sheet products rolled therefrom are not suitedto prolonged storage.
• the present invention overcomes the. aging problem inthe steel by further adjusting the melt composition to contain a carefully controlled amount of boron not exceeding about 0.01%.
• the boron content must be 1.4 to 2.5 times the :nitrogen content, in weight relationship, in a conventional non-degassed steel; and on the other hand, must be 1.0 to about 1.4 times the nitrogen content, in weight relationship, in a steel which has been degassed to contain less than about 150 p.p.m. of oxygen.
• the continuous cast slabs, having the above composition must be heated to a temperature within the range 2100 to 2300 F., for hot rolling, as is the customary prior art procedure.
• the hot rolling finishing temperature be above 1500 F. and preferably within the range 1550 to 1650 F., and that the hot rolled steel be coiled at a temperature above 1100 F. and preferably 1150 to 1250 F. Thereafter, the steel may be pickled, and cold rolled in accordance with conventional mill practices.
• the carbon content of the melt should not be less than 0.01%, and preferably not less than 0.03%, because otherwise the oxygen content of the steel would be too high for continuous casting.
• the lining life in the steelmaking furnace would be significantly shortened if the carbon content of steels therein are below 0.01%.
• the carbon content should not exceed 0.08% to assure sufficient ductility in the final sheet product.
• the manganese and silicon ranges for the molten steel as noted above are preferred because of the synergistic effect of these amounts in preventing pinhole porosity of a steel whose carbon content is 0.01 to 0.08%.
• the oxygen content of the steel can be more easily estimated and controlled when manganese exceeds 0.20%.
• the amount of acid soluble aluminum in the steel is preferably not greater than 0.015% because larger amounts tend to cause the formation of excessive quanticold forming operations such as press forming or deep drawing.
• the boron content of our steel must not exceed about 0.01%.
• the mere adding of .a prescribed, amount of boron in the range of 0.001 to 0.010%. is not enough, however, to assure that aging properties will even be affected, since the steels boron content must further be very carefully controlled with respect to thesteels nitrogen content, andto some extent with respect to the oxygen content as well.
• the boron content must be from -1.4 to 2.5 times the nitrogen content.
• boron to nitrogen ratios above 2.5 Nevertheless, an upper limit 2.5 on the boron to nitrogen ratio was arbitrarily chosen because excessive amounts of boron, i.e., more than about 2.5 times nitrogen when the nitrogen content is within the normal range of 0.003 to 0.005, will cause the resulting sheet steel to be quite hard, having a substantially adverse effect on the formability of the sheet.
• the boron to nitrogen ratio is maintained within the more preferred range of 1.4 to 2.0, the resulting steel will have a ductility equal to or greater than comparable prior art steels.
• the minimum boron to nitrogen ratio of 1.4 is no longer applicable, and in fact, ratios below 1.4, i.e., about 1.0 to 1.4 are equally effective at improving aging resistance.
• boron to nitrogen ratios above about 1.4 are actually detrimental, causing proportionally greater hardness at ratios thereabove.
• the boron to nitrogen weight ratio should be from about 1.0 to 1.4. Less boron than about 1.0 times nitrogen content will become exceedingly less effective in promoting aging resistance, while boron in amounts exceeding about 1.4 times nitrogen will proportionally decrease product ductility.
• the standard basic oxygen furnace practice for making low carbon steel may be used without modification. However, it is frequently advantageous to modify the cus .tomary .BOP furnace practice by charging enough manganese to the furnace to obtain a residual manganese content of at least 0.1% in the furnace melt. It is essential that the residual manganese content in the furnace melt beat least 0.10% when the sulfur content of the iron supplied to the furnace is in a normal range of from about 0.025% to 0.050%, in order to keep the sulfur content in the furnace melt down to an acceptable amount not greater than 0.02%.
• Residual manganese contents of over 0.1% are obtained by the addition of a manganese ore to the furnace charge, or by the addition of hot metal (iron from the blast furnace) containing enough manganese to give the residual manganese content of at least 0.10%.
• the use of manganese ore is preferred, since high manganese hot metal usually contains so much phosphorus as to raise the quantity of phosphorus in the steel casting above acceptable limits.
• the use of manganese ore makes it possible to obtain the desired residual manganese content in the furnace melt without also obtaining an excessively high phosphorus content.
• Either a high grade or low grade manganese ore may be used.
• the amount of ore added is at least about 0.1% by weight of Mn, based on the total weight of the furnace charge. Generally larger quantities are required because a large part of the manganese is lost to the furnace slag.
• the temperature in the furnace is customarily held within the range of 2850 to 3000" Temperatures above 3000 F. are to be avoided because these high temperatures cause rapid deterioration of the furnace lining, resulting in the presence of excessive quantities of refractory oxide slag in the furnace melt.
• a large portion of the manganese content of the molten steel introduced into the mold is added after tapping of the furnace melt, because it is impractical to charge enough manganese to a basic oxygen process furnace to furnish the desired manganese content in view of the excessive losses of manganese to furnace slag.
• Manganese may be added in the ladle in the form of silicomanganese, high or medium carbon ferromanganese, or electrolytic 1 manganese.
• the addition of silicomanganese also supplies the entire quantity of silicon which must be added in order to bring the molten steel composition up to the desired silicon level of 0.03-0.08%.
• Customarily about 6 to 10 lbs. per ton of silicomanganese and about 2 to 4 lbs. per ton of medium carbon ferromanganese are added in order to supply the necessary manganese and silicon to the molten steel.
• medium carbon manganese either high carbon ferromanganese or electrolytic manganese may be added. Frequently the amounts of high carbon ferromanganese required are somewhat less than the amounts of medium carbon ferromanganese normally required, being only about 1 to 2 lbs. per ton in most instances.
• the silicomanganese and the ferromanganese are most conveniently added to the molten steel during the filling of the tapping ladle with the furnace melt obtained in the steelmaking furnace. Best results are obtained when the silicomanganese and ferromanganese are added durin the filling of the middle third of the ladle.
• boron In addition to manganese and silicon, it is frequently preferably, following the aluminum addition after the aluminum has gone into solution, but early enough to prevent the boron from rising in the steel and contacting the slag and thus becoming oxidized and losing its effectiveness.
• Any practical form of boron may be used, such as ferroboron, calcium boride, silicomanganese boron, and so on. If the steel is not to be degassed or vacuum treated so that normal oxygen contents exceeding 150 p.p.m. will be present, then as explained above, the boron addition should be sufiicient to provide a boron to nitrogen weight ratio of 1.4 to 2.5 and preferably 1.4 to 2.0.
• the boron addition should be made after degassing and should be suflicient to provide a boron to nitrogen ratio of about 1.0 to 1.4.
• the steel is then poured into the upper end of an open-ended tubular water cooled continuous casting mold. solidification of the steel is initiated in the mold. A casting having a solidified skin surrounding a molten metal core is withdrawn downwardly from the mold, as entire solidification is effected by means of water sprays located below the mold, as is conventional in the art.
• the slab When the slab has been suitably conditioned for hot rolling, if such conditioning is necessary, it is heated to a hot rolling temperature within the range 2100 F. to 23-00 F. according to conventional prior art practices. Thereafter the slab is hot rolled according to conventional practices, with the finishing temperature being of course within the austenitic range, i.e., above about 1500 F., and preferably within the range 1550 to 1650 F. To eifect the aging resistance properties of this invention it is essential that the hot rolled steel be coiled at a temperature above 1100 F. and preferably Within the range 1150 to 1250 F. Although the reason for this limit is not clearly understood, the final product does show large reductions in aging resistance at coiling temperatures below about 1100 F.
• the steel may be pickled and cold rolled in accordance with conventional prior art practices.
• the steel may be pickled in either HCl or H acid solution, and then cold reduced by 50 to 75%.
• HNX gas a nitrogen containing atmosphere
• the cold rolled sheet is temper rolled in accordance with conventional prior art practices.
• the resulting cold-rolled sheet steel product obtained by practicing the above invention will have mechanical properties equal to or superior to prior art aging-resistant sheet steels produced from ingot cast steels. Of most significance is the fact that the improved results are substantially more reproduceable than experiences with prior art processes.
• conventional aging as indicated by return of yield point elongation, can be reproduceably retarded for at least days. Strain-aging index values can readily be reproduced within the range 0 to 10%.
• the aging resistance properties can be maximized to virtually an nonaging characteristic, i.e., strain-aging index values of from 0 to about 2.0% if the preferred hot rolling finishing temperatures of from 1550 to 1650 F. and coiling temperatures of 1150 to 1250 F. are provided.
• this invention is equally applicable to ingot cast steels.
• this invention ofliers the further advantage of providing an aging resistant steel which can be produced by either ingot casting or continuous casting operations.
• the R (or plastic strain ratio in the longitudinal direction) was then calculated as the ratio of the true width strain to the true thickness strain as is well known in the art.
• Table I gives the results of the tests as well as the compositions of the heats.
• eighteen heats not containing boron were identically prepared, processed and tested, with the typical results thereof shown at the bottom of Table I.
• the improved strain-aging index provided by this invention is readily apparent.
• N on-boron treated heats (typical values) 18 heats Various 0. 030/0. 046 0. 36/0. 47 0. 026/0. 052 003/0. 009 N.A. 17 1. 1
• a method of producing thin flat rolled steel products having substantial aging-resistance comprising forming a steel melt consisting of 0.0l-0.08% carbon, 0.20- 0.60% manganese, 0.0*30.08% silicon, -0.004-0.015% aluminum with a balance of iron and other usual steelmaking impurities including oxygen and nitrogen; adding up to about 0.01% boron to the steel to provide a boron to nitrogen ratio of 1.4 to 2.5 when the oxygen content is more than about ppm.
• a flat rolled steel product characterized by good surface quality, good drawability and exceptional aging resistance consisting essentially of 0.010.08% carbon, 0.200.60% manganese, 0.03-0.08% silicon up to 0.015% aluminum, other usual impurities including nitrogen and oxygen, less than about 0.01% boron but sufficient to provide a boron to nitrogen ratio of from 1.4 to 2.5 when the oxygen content is more than about 150 p.p.m.

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

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Cited By (12)

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• 1971
  • 1971-02-03 US US00112444A patent/US3725143A/en not_active Expired - Lifetime
• 1972
  • 1972-01-24 ZA ZA720471A patent/ZA72471B/xx unknown
  • 1972-01-24 CA CA133,058A patent/CA968587A/en not_active Expired
  • 1972-01-26 AU AU38338/72A patent/AU458543B2/en not_active Expired
  • 1972-01-31 DE DE19722204454 patent/DE2204454A1/de active Pending
  • 1972-01-31 BE BE778759A patent/BE778759A/fr unknown
  • 1972-02-01 PL PL1972153211A patent/PL83384B1/pl unknown
  • 1972-02-02 AT AT82272*#A patent/AT330228B/de not_active IP Right Cessation
  • 1972-02-02 IT IT67315/72A patent/IT948999B/it active
  • 1972-02-02 BR BR574/72*[A patent/BR7200574D0/pt unknown
  • 1972-02-02 FR FR7203419A patent/FR2124370B1/fr not_active Expired
  • 1972-02-03 YU YU255/72A patent/YU34717B/xx unknown
  • 1972-02-03 NL NL7201435A patent/NL7201435A/xx unknown
  • 1972-02-03 GB GB518172A patent/GB1384263A/en not_active Expired
  • 1972-02-04 JP JP47012529A patent/JPS5134807B1/ja active Pending

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