Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
2000-07-20
2002-05-21
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S723000
Reexamination Certificate
active
06391187
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for electrolytically and continuously treating a hot- and/or cold-worked metal material, particularly a continuously formed one in stainless steel. The treatment method for the material involves removing continuously, in one stage, a surface layer of one or more mixed metal oxides, usually spinel, which surface layer is thicker than the passivation layer which occurs on the surface of stainless steel material, at the same time as a polished surface is obtained. According to the invention, the character of the surface obtained can also be selected by varying a number of control parameters.
PRIOR ART
When manufacturing continuously formed metal products of thin dimensions, i.e. primarily strip, from a stainless steel material, hot rolling is generally carried out followed by the final stage, cold rolling. The operation in that case is first hot rolling of the material at high temperature, at which the material softens and can be rolled to a thickness of around 5-25 mm. Owing to the high temperature, an oxide scale of a thickness of around 50-500 micrometers is formed on hot rolling, which scale consists of a mixed oxide, usually spinel, comprising at least iron and chromium. Spinel is conventionally defined as AB
2
O
4
, A being magnesium, iron(II), zinc or manganese or a combination of these, and B being aluminium, iron(III) or chromium. In the present case, the spinel formed during hot rolling is often of the type Fe(FeCr
x
O
y
), where x=1-2 and y=2-4 and the chromium content is often up to 40%. Under the oxide scale formed is a chromium-depleted zone with chromium contents down to 10-12% or even lower. Before the material is worked further, the oxide scale has to be removed. A pretreatment can be carried out for this purpose, consisting of annealing at around 1000-1200° C., which softens up the surface, followed by cooling and then blasting which breaks up the oxide scale. After this, a pickling process is normally used, which is an electrolytic treatment stage in an electrolyte consisting of one or more mineral acids or a neutral solution and an ensuing chemical stage. What happens is that the electrolyte/acids penetrate down into cracks in the oxide scale and dissolve the chromium-depleted zone, at which the oxide scale loosens. Following rinsing, the material has a dull, so-called pickled surface. To produce thinner dimensions, the treatment continues with cold rolling, at which the material becomes hard and brittle due to the formation of martensite. To restore the correct material attributes, stress-relieving annealing at around 1000-1200° C. follows, at which however a surface layer again appears of a mixed oxide of the spinel type, this time with a thickness of around 10 micrometers. Since the oxide does not have the right stainless properties, this is also pickled away in the same way as in the earlier process. Following rinsing, the material has a dull, pickled surface. In certain cases, e.g. if further forming operations are to be carried out, the pickled surface. In an advantage, but often a bright surface is desired instead, which is then produced by means of bright annealing in a reducing atmosphere, followed by smoothing rolling with only around 2% reduction of the material thickness. Blasting is not used on cold-rolled material, as the surface is destroyed by this. Instead, so-called neolyte treatment is used, which involves an electrolytic treatment with direct current, usually in sodium sulphate, at which chromium(III) is oxidized to chromium(VI), which is soluble.
A number of methods of pickling have been known for a long time. One such relatively modern method is described in SE-A-9301591-5. In this method, the electrolyte bath consists of mineral acid, for example sulphuric acid, or mixtures of mineral acids and the electrolysis is carried out by means of alternating current or alternately direct current at relatively high current densities, 150-250 A/dm
2
. The patent also covers the special equipment which is used for the method. When pickling according to SE-A-9301591-5, a dull, pickled surface is produced, as in all known pickling methods, for which reason a subsequent polishing stage consisting of bright annealing and smoothing rolling has to be carried out.
When manufacturing products which are not rolled or continuously formed, i.e. piece products, e.g. pipe parts, tapping cocks, bolts, screws or vessels, in stainless material, a polishing process called electro-polishing is usually performed. This process is a batch-wise electrolysis using direct current in e.g. sulphuric acid and phosphoric acid at relatively low current densities, around 10 A/dm
2
. The process also takes a relatively long time, often up to 10-20 minutes, in spite of the fact that the oxide layer which is being treated is very thin, around 1-5 nanometers. This layer is the passivation layer which always occurs on the surface of a stainless material and consists of chromic oxide, Cr
2
O
3
. In the process, the material is made to form an anode and a viscous film is then formed on the surface of the material. This film has a higher density, viscosity and resistance than the electrolyte remaining. As the film adjusts itself to the uneven surface of the material, it is thicker in the recesses than on the crests in the surface. In the recesses, the resistance to the passage of current is higher and the current density therefore lower. On the crests, the current is higher on the other hand and the crests thereby come to be dissolved, resulting in a levelling of the surface of the material.
A number of variants of electro-polishing are described in Sweden's Galvanotekniska Förenings (Electroplating Association's) “Lärobok i elektrolytisk och kemisk ytbehandling” (“Manual of electrolytic and chemical surface treatment”), p. 263-271, published by Ytforum Förlag. For the electro-polishing of stainless steel, electrolytes are mentioned consisting of sulphuric acid, ortho-phosphoric acid and water or orthophosphoric acid, glycerol and water. The current density is 7-25 A/dm
2
and 7-8 A/dm
2
respectively and the time is up to 10 and 15 minutes respectively.
In JP 57-101699/82, a batchwise method of electro-polishing a super-alloyed steel with 60% Ni, 23% Cr, 10% Mo, 3% Fe and 4% Nb is described. The method is used for clock parts and is said to give a figured surface, similar to that which is traditionally obtained by hand grinding. The electrolyte used is a mixture of an aqueous solution of phosphoric acid and an aqueous solution of sulphuric acid in a weight ratio of between 9:1 and 1:1. The electro-polishing takes place over 10 seconds-20 minutes, at a current density of 1-50 A/dm
2
and a temperature of 15-50° C.
No continuous method of simultaneously pickling and polishing continuously formed, stainless material is known, as far as the inventors of the present method know. A major disadvantage of the known pickling process for the removal of a surface layer of mixed oxide of the spinel type is that the surface produced remains dull and pickled, for which reason a subsequent stage with bright annealing and smoothing rolling has to be carried out for a highly bright surface to be obtained. Hitherto, however, no-one has thought it possible to obtain a highly bright surface in one and the same step as pickling in continuous treatment of continuously formed stainless products.
SOLUTION AND ADVANTAGES
The preconceived notion that highly bright surfaces cannot be obtained in one and the same stage as continuous pickling of stainless material is thwarted by the present invention.
The method according to the present invention is defined in the independent claim 1 and means that a material of stainless steel, which is preferably continuously formed, in particular strip-formed, is treated continuously, an oxide surface layer of a thickness of at least 1 micrometer being removed from the material and the method achieving a polished effect on the surface of the material in the same stage. This pickling in combinatio
Eklund Anders
Snis Malin
Avesta Sheffield Aktiebolag (PUBL)
Valentine Donald R.
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