Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-09-11
2002-03-26
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S603000, C148S651000, C420S126000, C420S127000, C420S128000, C420S129000
Reexamination Certificate
active
06361624
ABSTRACT:
TECHNICAL FIELD
This invention relates to a method for producing fully-stabilized steel sheet and strip for porcelain enameling applications, and particularly to a method for producing fully-stabilized extra-low carbon steel sheet and strip containing titanium, columbium and antimony, and having excellent formability and fishscale resistance, as well as improved porcelain enamel adherence. The method includes hot rolling an extra-low carbon steel slab containing titanium and columbium to sheet or strip, cold rolling, box annealing and temper rolling. The invention includes the fully stabilized steel sheet and strip produced by the process.
BACKGROUND ART
Steel sheet and strip for household appliances such as cooktops, ranges, washers and dryers are produced using one of three types of porcelain enameling steel. Type I enameling steels are made from a low carbon steel melt usually containing 0.04 to 0.06% carbon. The steel is cast, hot rolled, cold rolled, and then open coil annealed in a decarburizing atmosphere to achieve an ultra low carbon content of less than 0.008%. Type II enameling steels are made from a low carbon melt typically containing about 0.04% carbon maximum. The steel is cast, hot rolled, cold rolled and annealed in a reducing atmosphere at controlled temperatures to avoid the formation of carbides at or near the surface of the strip. Type III enameling steels are produced from a fully stabilized extra-low carbon melt, typically containing 0.01% carbon or less. Aluminum and titanium are added in amounts sufficient to combine with all the carbon, nitrogen, and sulfur present and thereby eliminate aging and yield point elongation. Several chemical elements including titanium, columbium, vanadium, and zirconium can be used to stabilize steel, but titanium is considered the most effective. However, titanium-stabilized steels in which large amounts of titanium have been added exhibit poorer enamel adherence, generally believed to be due to the excess titanium needed to assure complete stabilization, as disclosed in Japanese published patent application No. 60-110845. Titanium-stabilized steels are also noted for poor surface quality and weld area defects such as blowholes in weld zones, as disclosed in Japanese published patent application No. 61-276958.
U.S. Pat. No. 4,670,065 to Yasuda et al discloses a cold rolled steel sheet containing titanium and antimony that is suitable for enamel coating. The steel contains in weight percent up to 0.005 C, up to 0.02 P, up to 0.03 S, 0.005 to 0.012 N, up to 0.15 Ti with Ti≧(48/12 C+48/14 N+48/32 S), up to 0.08 Cu, and at least one member selected from As, Sb, and Bi in a total amount of 0.003% to 0.03% or Se and/or Te in a total amount of 0.003% to 0.05%, the balance being iron and impurities. The steel is produced by continuous casting a molten steel having the aforementioned composition, hot rolling, and cold rolling. The steel may be continuously annealed at a temperature in the range from the recrystallization temperature to the A
c3
point. Or the steel may be box annealed at a temperature in the range from the recrystallization temperature to 800° C. The reference steel is fully stabilized with titanium and not partially stabilized with titanium with the balance of stabilizing elements necessary for full stabilization being provided by columbium as in Applicant's invention.
U.S. Pat. No. 5,098,491 to Osawa et al discloses a steel sheet containing titanium and columbium for porcelain enameling applications. In one aspect the steel contains in weight percent not more than 0.005 C, not more than 0.59 Mn, 0.007 to 0.020 B, 0.01 to 0.07 Cu, not more than 0.010 Al, 0.008 to 0.020O, 0.005 to 0.020 N, not more than 0.020 P, at least one of not more than 0.050 Ti and not more than 0.050 Nb, provided the total Ti and Nb is within a range of 0.001 to 0.050, the balance iron and inevitable impurities. The steel is hot rolled, cold rolled not less than 70%, and then continuously annealed at a temperature of not lower than 800° C. but not higher than the Ac
3
temperature. The reference steel contains boron for combining with the nitrogen. The reference also places an upper limit of 0.050% on the total amount of Ti plus Cb so that the recystallization temperature in continuous annealing is not raised. In addition, the reference teaches that continuous annealing is used so that surface segregation and grain boundary segregation of certain components in the steel exerting a bad influence on enameling can be controlled to make the properties in the steel uniform. The reference does indicate that box annealing can be used in addition to continuous annealing. Applicant's steel does not contain boron, is not continuously annealed, and has higher Ti plus Cb than the steel of the reference.
Two other patents disclose columbium-containing porcelain enameling steels. U.S. Pat. No. 4,124,412 to Elias et al discloses a process for producing non-aging, vacuum degassed low carbon steel. The process includes providing a molten steel having a maximum carbon content of 0.05% and sufficient manganese to combine substantially completely with the sulfur present in the steel. The steel is vacuum degassed to a carbon content of 0.015% max., an oxygen content of about 0.010% max., and a nitrogen content of 0.012% max. Columbium is added to the molten steel in an amount at least sufficient to retard the recrystallization rate of the steel. The steel is cast and hot rolled with a finishing temperature of 1500 to 1700° F. and coiled at a temperature of 1500° F. or less. The final steel product contains in weight percent 0.002 to 0.015 C, above 0.025 to 0.30 Cb, 0.05 to 0.60 Mn, up to 0.035 S, up to 0.010 O, up to 0.012 N, up to 0.08 Al, residual P, residual Si, the remainder substantially iron. The reference steel does not provide full stabilization with a combination of titanium and columbium as in Applicant's invention.
U.S. Pat. No. 5,137,584 to Jesseman discloses a high strength steel for porcelain enameling consisting essentially of in weight percent at least 0.005 Nb, at least 0.02 C, at least 0.10 Mn, at least 0.01 Al, nitrogen as an impurity, the ratio of acid soluble Al to N being at lest 2:1, the balance iron and unavoidable impurities. The steel is produced by hot rolling with a finishing temperature at least at the Ar
3
temperature and coiling at a temperature of at least 677° C. to precipitate residual nitrogen as AlN. The steel is descaled, cold rolled and annealed without decarburization at a temperature less than 721° C. for a time sufficient to avoid formation of iron carbides on the surfaces of the sheet and to precipitate the Nb as Nb-carbide. The sheet is then temper rolled. The reference steel does not contain titanium and columbium as in Applicant's invention.
A cold rolled sheet containing boron and having excellent enamelability is disclosed in U.S. Pat. No. 3,950,191 to Ito et al. The steel is produced by providing molten steel containing in weight percent not more than 0.020 C, not more than 0.03 Si, not more than 0.50 Mn, not more than 0.010 Al, not more than 0.050 0O, B within a range of 0.003-0.020 with B ×O more than 1×10
−5
, the remainder Fe and inevitable impurities. The steel is hot rolled, cold rolled and recrystallization annealed in conventional manner. The reference steel does not contain titanium and columbium as in Applicant's invention.
A hot rolled sheet suitable for enameling on one side is disclosed in U.S. Pat. No. 4,801,341 to Itami et al. The steel consists essentially of in weight percent 0.0050-0.07 C, 0.05-1.5 Mn, 0.03-0.15 P, 0.03-0.1 Al, 0.003-0.010 N, at least 0.002 free N not bound to Al, with the ratio of Al/free N being ≧10 and the balance being Fe and incidental impurities. The reference steel does not contain titanium and columbium as in Applicant's invention.
An ultralow carbon cold rolled deep drawing steel sheet containing Ti and Nb is disclosed in U.S. Pat. No. 5,041,166 to Matsuoka et al. The steel contains up to about 0.005 C, u
Riesmeyer, III William F.
USX Corporation
USX Corporation
Yee Deborah
LandOfFree
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