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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
• • 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
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys

Definitions

• stainless steel sheet and strip is produced by first thoroughly surface conditioning a stainless slab and then heating the slab to a suitable hot rolling temperature usually about 2,000F, but may vary somewhat with different grades.
• the stainless slab is then hot rolled, usually to a conventional hot band gauge of about -inch, and sometimes annealed.
• the hot rolled and annealed steel Prior to cold rolling, the hot rolled and annealed steel must be thoroughly'descaled bya treatment which usually involves pickling the steel in either an acid or caustic solution and for some grades involves a scale-breaking procedure prior to the pickle such as shot blasting.
• Most typical pickling operations provide a continuous treatment first in hot hydrochloric or sulphuric acid and then in a hydrofluoric-nitric acid mixture.
• the steel may be annealed and pickled a second time, or'the defects removed in a continuous coil grinding operation.
• the stainless steel hot band is cold rolled to final gauge, typically about 0.050-inch, and then again continuous annealed, descaled and, if necessary, surface conditioned. Due to the low emissivity of conventional cold rolled stainless steel surface, the final anneal requires a relatively long holding time in order to completely recrystallize the sheet. Hence, slow line speeds are required when annealing cold rolled stainless steel.
• An object of this invention is to provide a new process for producing stainless steel sheet and strip which utilizes fewer processing steps and faster final anneal- I ing and pickling line speeds and therefore results in a cess for producing stainless steel sheet and strip which requires substantially less time on annealing and pickling facilities thereby making such facilities more readily available for processing additional steels.
• a further object of this invention is to provide a less costly process for producing stainless steel sheet and strip for industrial and automotive applications which will therefore reduce the price of such steels and enhance their utilization.
• the process of this invention is particularly applicable to the low chromium ferritic grades of stainless steel for applications not having critical surface finish requirements, specifically applications such as automobile mufflers,heat exchangers industrial piping and other such non-decorative uses.
• the inventive process will make available lower-cost ferritic stainless steels as are particularly desired by the automobile industry to manufacture low-cost engine mufflers suitable to meet future anti-pollution requirements.
• the process is not limited to ferritic stainless steels, it is for all practical purposes limited to production of stainless steels not having critical surface requirements. That is to say, the process can be used to produce austenitic or ferritic stainless steel grades with chromium contents up to about 19 percent with equal or comparable cost savings, but flawless, bright surface qualities cannot be ex pected for any grade.
• any ferritic or austenitic stainless steel containing not more than 19 percent chromium is completely processed to cold rolled sheet or strip substantially in accordance with the prior art practices, but with one notable exception, namely, the hot rolled steel is not descaled prior to cold rolling, but rather the hot band is cold rolled and subsequently annealed with the hot rolling mill scale still on the stainless steel surface. Accordingly, the inventive process first requires that a suitable stainlessslab be hot rolled to conventional hot band gauges, i.e. about 0.10-inch, cooled and then cold rolled to final gauge without descaling after hot rolling.
• Elimination of the descaling operation following hot rolling leads to the obvious advantages of reducing pro cessing time and costs naturally resulting from elimination of one processing step, and rendering the descaling equipment available for processing other materials.
• the steel is subsequently cold rolled with mill scale on the surface thereof, the final surface quality of the steel is not sufficient to meet high standards where appearance is critical. Nevertheless, if the surface appearance is seriously defective and if desired, can usually be improved to more exacting standards by in-line polishing equipment. Of more significance however is the fact that cold rolling and subsequently annealing the steel without descaling the surface does not in any way adversely affect the steels physical or chemical properties.
• the stainless steel hotband without prior descaling leads to a surprising additional advantage, in that the anneal following cold rolling may be effected at line speeds 50 percent faster than possible by prior art processes.
• the steels cold reduced surface will consist of a dull, thin and surprisingly uniform oxide layer which will more readily and easily absorb heat during the final annealing operation. Because of this substantially increased emissivity, the cold rolled steel can be subsequently annealed with shorter holding times, i.e. up to 50 percent faster line speeds.
• Those skilled in the art will recognize that it is of course not possible to give actual line speed figures because the maximum line speed necessary to effect substantially complete recrystallization is dependent upon many variable factors.
• the maximum line speed is dependent not only upon furnace length, but furnace geometry and efficiency, and upon steel grade, sheet thickness and even sheet width in some cases. Therefore, the maximum line speed will vary with different grades of steel, different sheet dimensions and different annealing furnaces. Nevertheless, whatever the maximum line speed may be at given conditions with the prior art practice, that line speed can be increased up to 50 percent when practicing this invention under the same conditions. Accordingly, the inventive process not only provides for elimination of the descaling step prior to cold rolling and the economies resulting therefrom, but further substantially reduces the annealing time after cold rolling to allow up to 50 percent faster line speeds during the anneal and pickling operation.
• the cold rolled stainless steel is annealed with a complete recrystallization, it is then pickled in accordance with conventional procedures. This usually involves continuously passing the steel from the annealing furnace into an acid or caustic pickling solution.
• This slab was hot rolled to 0.085-inch at a starting temperature of 2,250F and coiling temperature of 1,250F. After cooling the hot rolled steel was cold rolled without descaling to 0.050-inch on a 4-high cold reduction mill.
• the cold rolled strip was then continuously annealed in a 90-foot long annealing furnace, at a temperature of 1,850F and at a line speed of feet per minute. The normal line speed for this product processed by prior art practices is 55 feet per minute. Thereafter the strip was passed through a pickling solution of sulfuric acid and then a mixture of nitric and hydrofluoric acid where the cold rolled scale was readily removed.
• the stainless strip exhibited the following properties:

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

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Publications (1)

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Cited By (7)

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Citations (7)

• 1972
  • 1972-07-14 US US00271881A patent/US3776784A/en not_active Expired - Lifetime
• 1973
  • 1973-06-21 CA CA174,695A patent/CA1007966A/en not_active Expired
  • 1973-06-29 DE DE2333193A patent/DE2333193C2/de not_active Expired
  • 1973-07-06 BE BE133234A patent/BE802045A/xx unknown
  • 1973-07-12 LU LU68003A patent/LU68003A1/xx unknown
  • 1973-07-12 NL NL7309746A patent/NL7309746A/xx not_active Application Discontinuation
  • 1973-07-13 FR FR7325896A patent/FR2193087B1/fr not_active Expired
  • 1973-07-13 IT IT69115/73A patent/IT991765B/it active
  • 1973-07-13 JP JP48079192A patent/JPS4956864A/ja active Pending

Patent Citations (7)

Non-Patent Citations (1)

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