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
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • 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
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working

Definitions

• the present invention relates generally to elongated semi-finished, free machining steel shapes and more particularly to such shapes which are free of surface cracking and to methods for producing such shapes.
• a semi-finished steel shape is a bloom or a billet, the latter having a smaller cross-section than the former, produced from an ingot or a continuously cast strand having a cross-section similar to that of a bloom or billet and which has been subjected to a hot deforming operation such as roll straightening.
• the machinability of steel is increased by adding alloying ingredients such as sulphur, lead, tellurium and bismuth.
• molten steel containing at least some of the machinability increasing ingredients described above, is poured into an ingot mould where the steel is cast into an elongated solid shape. After cooling, the ingot is reheated and subjected to a hot rolling operation which rolls the ingot into a bloom, and the bloom is then reheated and subjected to a further hot rolling operation to form the bloom into a billet. The billet is subsequently rolled into a bar, which is a finished steel shape.
• a semi-finished steel shape having a free machining composition the steel is continuously cast as strands having the cross-section of a billet, the strand is subjected to a roll bending operation or a roll straightening operation while the steel is at a hot deforming temperature, and, after the roll straightening operation, the strand is cut into billet lengths (which comprise semi-finished steel shapes).
• the hot rolling of the ingot into a bloom and the hot rolling of the bloom into a billet are generally performed at a hot rolling temperature in the range 920°-1150°C (1700°-2100°F).
• the roll straightening of a continuously cast strand is generally performed at a temperature in the range 920°-1100°C (1700°-2000°F).
• the hot rolling of the ingot into a bloom, the hot rolling of the bloom into a billet, and the roll straightening of the continuously cast strand are all hot deforming operations, and the temperature ranges described above are the typical, normal temperature ranges used for these hot deforming operations.
• tellurium When tellurium is present in a free machining steel, all the tellurium is in inclusion form, principally as lead telluride (PbTe) when lead is also present. Some of the tellurium may also be present as manganese telluride (MnTe).
• PbTe lead telluride
• MnTe manganese telluride
• attempts to combat surface cracking include reheating the unfinished steel shape without flame impingement on the surface of the unfinished shape (U.S. Patent No. 3,287,954 Scrader et al), reducing the moisture content in the atmosphere of the reheating furnace (U.S. Patent No. 3,365,922 Conces et al), removing the surface and first sub-layer of the unfinished steel shape, e.g. by scarfing, after reheating (U.S. Patent No. 3,382,700 Heitmann et al) and providing a blanket of non-oxidizing gas around the steel shapes as they undergo reheating (U.S. Patent No. 3,710,608 Hentz et al).
• the invention as claimed reduces or eliminates the above discussed problems of surface cracking.
• lead In conventional free machining steels, lead is normally present in the range 0.25-0.35 wt. % and tellurium is normally present in the range 0.04-0.0b wt.
• the lead content is either eliminated entirely or, if not eliminated entirely, it is reduced from 0.25-0.35 wt. % to 0.15 wt. % maximum and the tellurium is reduced from 0.04-0.06 wt. % to 0.02 wt. % maximum.
• the reduction in machinability resulting from the lower lead and tellurium contents is offset by adding bismuth in the range of 0.10-0.40 wt.
• the tellurium is present either as bismuth telluride (Bi 2 Te 3 ) or as manganese telluride (MnTe) which may be in the form of an eutectic with manganese sulphide (MnS).
• the hot deforming temperature should preferably be conducted within the range 920°-1035°C (1700°-1900°F). This is to offset the tendency of substantial amounts of manganese telluride to cause surface cracking during hot rolling at temperatures substantially exceeding 1035°C (1900°F), e.g. if the steel undergoes hot rolling at a temperature of about 1150°C (21 00 °F).
• a free machining, elongated, semi-finished steel shape devoid of surface cracking in accordance with the present invention can be produced by a method utilising. Two different casting procedures, either ingot casting or continuous casting. No matter which casting procedure is utilised, the steel composition may be essentially the same, within the broad limits set forth below, in weight percent:
• a machinability increasing amount of tellurium is generally about 0.02 wt. %, minimum. Up toO.15 wt. lead is optional.
• a bath of molten steel having a composition within the range s.et forth above is then cast into an elongated solid shape such as an ingot.
• the amounts of manganese and bismuth, within the ranges set forth above, are sufficient so that, when the steel is in solid form and no lead is present, all of the tellurium is combined with the manganese and/or the bismuth as mico inclusions of manganese telluride and/or bismuth telluride and the bismuth is also present as micro inclusions of elemental bismuth, there being substantially no iron telluride (FeTe) present.in the solid steel. Iton telluride has a detrimental effect from the standpoint of causing surface cracking during hot deforming.
• the ingot After the steel has been cast into an ingot, and the ingot has cooled, the ingot is removed from the ingot mould, reheated (an operation known as soaking) and then subjected to a hot rolling operation at a hot rolling temperature in the range 920°-1150°C (1700°-2100°F) wherein the ingot is rolled into a bloom.
• a hot rolling temperature in the range 920°-1150°C (1700°-2100°F) wherein the ingot is rolled into a bloom.
• the resulting intermediate, hot deformed steel shape i.e. the bloom, has a surface substantially devoid of surface cracking, prior to any surface conditioning of the bloom.
• the bloom is then reheated and hot rolled at a temperature in the range 920°-1150°C (1700°-2100°F) to produce a billet having a surface substantially free of surface cracking prior to any surface conditioning thereof, and there is no need to conduct a surface removal step between the bloom reheating step and the step of hot rolling the bloom into a billet.
• the surface of the bloom or billet devoid of substantial surface cracking, but, also the surface is devoid of burning in the as-deformed condition, due to the fact that the steel shape is rolled at a temperature (1150°C maximum) (2100°F maximum), below that at which burning of the steel occurs.
• the steps comprise continuously casting molten steel (having a composition within the ranges set forth above) into a strand and then roll straightening the strand while the latter is at a temperature in the range 920°-1100°C (1700°-2000°F).
• the strand which already has the cross-section of a billet, is then cut into the usual billet lengths.
• the roll straightening step which the strand undergoes at the temperature range described above is tantamount to a hot deforming step, but the surface of the strand, and of the billets which are cut from the strand, are devoid of surface cracking and burning in the as-deformed condition.
• a billet formed from either of the above described manufacturing procedures has an oxide on its surface in the billet's as-deformed condition. This is reflective of the fact that the biller has not undergone any surface conditioning.
• as-deformed condition refers to the condition of the billet immediately after being hot rolled (or otherwise hot deformed) and before it undergoes any surface conditioning following the hot deforming step.
• Surface conditioning is a procedure conventionally utilised to remove surface imperfections or portions from semi-finished steel articles after a hot deforming step and includes grinding, chipping, scarfing, planing and the like.
• the combined lead plus bismuth content of the steel shape should be at least 0.25 wt. % to supply the desired machinability. Therefore, in that embodiment of the invention wherein lead is completely absent, the bismuth content should be at least 0.25 wt. %. In that embodiment of the invention wherein lead is present up to 0.15 wt. %, the bismuth may be less than 0.25 wt. % so long as the combined lead and bismuth content is 0.25 wt. minimum.
• the present invention may be applied to virtually all steel base compositions to which lead and tellurium have previously been added. Examples thereof are set forth in Holowaty U.S. Patent No. 3,152,889, and the disclosure therein is incorporated herein by reference. Examples of steel compositions in accordance with the present invention are contained in the table set forth below.
• the balance of the composition consists essentially of iron (plus the usual incidentals/impurities).
• the present invention may be applied to plain carbon steels having a base composition (i.e. a composition without lead, tellurium. or bismuth) in the 1000 series, 1100 series or 1200 series of steels (AISI numbers) in which the lead, tellurium and bismuth contents are controlled as described above and which, in ' their solidified form, are subjected to hot deforming procedures (including hot deforming temperatures) as described above.
• a base composition i.e. a composition without lead, tellurium. or bismuth
• AISI numbers 1100 series or 1200 series of steels
• the present invention may also be applied to certain alloy steels to which lead and tellurium have heretofore been added, such as steels having compositions corresponding to AISI steels 4140, 4142 and 8620.

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

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Citations (12)

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• 1980
  • 1980-08-11 EP EP80104708A patent/EP0045815B1/de not_active Expired
  • 1980-08-11 DE DE8080104708T patent/DE3068522D1/de not_active Expired

Patent Citations (12)

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