EP0094808B1

EP0094808B1 - Method of box-annealing steel sheet to minimize annealing stickers

Info

Publication number: EP0094808B1
Application number: EP83302724A
Authority: EP
Inventors: James Edward Bird; Colin Barr Hamilton; Robert Mckim Hudson
Original assignee: USS Engineers and Consultants Inc
Current assignee: USS Engineers and Consultants Inc
Priority date: 1982-05-14
Filing date: 1983-05-13
Publication date: 1985-11-21
Also published as: DE3361283D1; BR8302507A; ES522379A0; ES8404713A1; KR840004790A; JPS5953635A; CA1198708A; AU1453883A; EP0094808A1

Description

The present invention relates to the box-annealing of steel sheet.
The heat treatment of cold-reduced steel sheet and strip is accomplished either in batch operations or in continuous operations. Batch heat treatment may be divided into two categories: (a) open-coil annealing in which a tight coil is first rewound with a suitable spacer in between each wrap of the coil to permit circulation of the furnace atmosphere between the individual wraps to hasten and improve the uniformity of heating, and (b) box annealing in which a large stationary mass of steel (either cut sheet or coils) is subjected to a comparatively longer heat treatment cycle by varying the temperature within the furnace that surrounds it. In the latter, box annealing practices, the mass of steel is slowly raised to the desired annealing temperature and soaked at such temperature for a period of to 24 hours. The use of such a practice provides full recrystallization of severely cold-reduced steel and results in the softest possible finished product. However, one draw-back to such practice is the tendency of the individual wraps of sheet, because of their tightly wound nature, to pressure weld or stick together when held at annealing temperature for extended lengths of time. The tendency of sticking increases with increasing pressure between the wraps and increasing time and temperature of the anneal. When sticking of adjacent coil wraps occurs, particularly in products with critical surface finish requirements, poor yields result. Additionally, production delays are encountered at a subsequent temper mill because lower rolling speeds are required or because recoiling may be necessary before further processing. To prevent such sticking, the prior art has resorted to the use of a variety of separating media such as colloidal solutions of alumina or silica, finely divided magnesium oxide particles and the formation of thin oxidizing films on the metal surface. The latter technique includes the use of silicate containing washing liquids aids in preventing sticking. This is a practice which has been recommended for many years by manufacturers of commercial cleaning compounds. Although many steel facilities use silicated cleaners, such use has not precluded the serious incidence of coil sticking.
According to the present invention, there is provided a method of box-annealing a mass of steel sheet wherein said sheet is electrolytically cleaned in a cleaning solution consisting of silicate or phosphate cleaners, or mixtures thereof, rinsed, dried, coiled and thereafter heated to an annealing temperature in a non-oxidizing atmosphere and soaked at such temperature for a time of at least one-half hour, characterized in that the solution utilized for said rinse contains calcium formate, magnesium formate, calcium acetate or magnesium acetate, or mixtures thereof, in a total concentration 1,500 to 10,000 ppm.
Prior to the present invention, a laboratory procedure had been developed to obtain a quantitative measurement of the sticking tendency of sheet during box annealing, and which measurements were found to correlate quite well with actual mill experience. Investigations were conducted using different types of steel, both continuous cast an ingot cast steels, since the prior art had suggested that steel composition, particularly carbon and phosphorus contents, had an effect on sticking. Compositions of two of the ingot cast steels evaluated are listed in Table I.
Steel panels were cut to a size of 2;" x 8" (7 to 20 cm). After vapor degreasing, the panels were subjected to electrolytic alkaline cleaning in various solutions (indicated in Table II) maintained at a temperature of 82°C, at a current density of 10.8 Aldm² for a period of one second (either cathodic or anodic). After electrolytic cleaning, the panels were passed through rubber wringer rolls to remove excess cleaning solution. Rinse water at a temperature of 54―60°C was sprayed on the steel surface to wash off the remaining cleaner and the panels were then dried to remove unbound water.
Four 1" x 2" (2.5 to 5.1 cm) coupons were cut from each cleaned and rinsed panel to provide 24 coupons to be assembled in a test pack. Each pack consisted of 12 paired test specimens, with each pair separated by a stainless steel spacer. To simulate the pressure exerted by the wrapping of coils, a 15.2 lb. (6.9 kg) weight was used for each run. The test pack was placed in a sealed stainless steel annealing box containing a protective atmosphere of 6% H₂-94% N₂ with a dew point controlled to minus 40°C by passing the gas mixture through a column of calcium sulfate. The annealing box was placed in a furnace and heated to an annealing temperature of 1250°F (677°C) in two hours. After a three hour soak period at said annealing temperature, the furnace was cooled rapidly. Test packs were removed after the annealing box was cut open and individual tension-shear test specimen pairs were placed in a tension tester and pulled apart. The amount of force required to pull the samples apart was then utilized as a quantitative measure of the degree of sticking that occurred during annealing. For each group of 12 specimens, the average sticking force and the variance were calculated. Each average value is reported in Table II with a 95% confidence interval.
Utilizing the above test procedures, previous experiments with silicated cleaners (not utilizing the rinse additives of the present invention) had shown that significantly lower sticking force values resulted when final strip polarity was cathodic. This is a difference which was not observed when non-silicated cleaners were applied. In this regard, even when final strip polarity was anodic, lower sticking force values resulted from the use of silicated cleaners. Such results had suggested a mechanism relating to surface composition, which might involve the presence of silicates or other residue (e.g. thermal decomposition products of various constituents of the rinse water) playing a part in reducing sticking, wherein the concentration of such residues would be increased by a final cathodic pass. This was borne out by a comparative test in which sticking tendency, utilizing deionized water as against the normally employed mill water was used in the final rinse. Use of the mill water produced a significant decrease in sticking force. Since mill water is known to be made hard primarily because of magnesium and calcium ions, a number of rinse additives containing soluble salts of magnesium and calcium were evaluated to determine whether the cations of such salts would also be a factor vis-a-vis sticking tendency. When used at a concentration of 3,000 ppm in the rinse, manganese sulfate, calcium phosphate, and magnesium sulfate did not significantly change sticking force values from those resulting when hard mill water was applied. However, the formates and acetates of calcium and magnesium did provide significant changes of sticking force, and the results of same are reported in Table II. Except as indicated, all specimens were electro- cleaned utilizing a commercial silicated cleaner. For conditions 3 and 8, electro-cleaning was with a commercial phosphated cleaner.
It may be seen from Table II that when used in concentrations of 1,500 to 10,000 ppm, generally 3,000 to 8,000 ppm, and more preferably 4,000 to 6,000 ppm, the formates and acetates of magnesium and calcium can significantly improve sticking force values, irrespective of strip polarity. Use of such rinse solutions will normally result in a dried-on residue concentration of 0.3 to 2.0 mg/ft² (3.2 to 21.5 mg/m²). Such residue concentration will, of course, be a function of rinse solution concentration, but will also depend on the thickness of the drag-out film remaining on the strip. This thickness is a function of the strip speed and wringer-roll pressure. Preferably, the residue concentration of said carboxylic acid salts will be within the range 0.5 to 1.2 mg/ft2 (5.4 to 12.9 mg/m²). Although most tests were carried out with a silicated cleaner, the results show that even with a non-silicated cleaner (e.g. Condition 8), a measurable improvement in sticking force followed use of an effective rinse additive, such as a magnesium formate.
In most of the above tests, solutions containing rinse additives were used immediately after electrolytic cleaning. On commericial lines with scrubber sections that require larger volumes of water, a two-stage practice may be required to reduce chemical costs. Evaluations were therefore conducted (Conditions 12 and 13) simulating use of a two-stage rinse. The results show that a final rinse (subsequent to a mill water rinse) containing the additives of the present invention is also effective in reducing sticking.
It would therefore appear, while the complete mechanism relating to surface composition and sticking cannot yet be established, that certain soluble salts of magnesium and calcium used as rinse additives appear to fix silicon on the steel surface, over and above the silicon level resulting when either deionized water or hard mill water is used. Cathodic cleaning appears to fix more silicon than does anodic cleaning. With non-silicated cleaners containing phoshpate, cleaning polarity does not appear to be quite as important a variable. Nevertheless, both previous results and those reported above, demonstrate that sticking in the absence of silicates can also be lowered by the use of dried-on calcium or magnesium formates that degrade during annealing. Thus, while silicate levels on clean steel surfaces are important in their effect on sticking during annealing, the fact that a final rinse with an appropriate additive was effective even after an initial rinse in hard water, suggests that silicates, phosphates, and the thermal degradation products of calcium and magnesium formates and acetates all contribute to lower sticking during box annealing, as compared with a steel that does not have these residues on the surface.

Claims

1. A method of box-annealing a mass of steel sheet wherein said sheet is electrolytically cleaned in a cleaning solution consisting of silicate or phosphate cleaners, or mixtures thereof, rinsed, dried, coiled and thereafter heated to an annealing temperature in a non-oxidizing atmosphere and soaked at such temperature for a time of at least one-half hour, characterized in that the solution utilized for said rinse contains calcium formate, magnesium formate, calcium acetate or magnesium acetate, or mixtures thereof, in a total concentration of 1,500 to 10,000 ppm.

2. A method as claimed in claim 1, wherein said mass is a coil of sheet steel, said adjacent sheets are wraps in said coil, said annealing temperature is 1,050°C to 1,400°F (566 to 760°C.) and said soak time is less than 24 hours, characterized in that said rinse is achieved by passing the sheet, at a speed of 1,000 to 3,000 ft./min (305 to 914 m/min) through a bath of said rinse solution.

3. A method as claimed in claim 2, wherein subsequent to the rinse, the drying is continued to remove essentially all the unbound water of the residue remaining on the surface of the sheet characterized in that the concentration of said salts in said rinse solution is within the range 3,000 to 8,000 ppm.

4. A method as claimed in claim 3, characterised in that said concentration is 4,000 to 6,000 ppm.

5. A method as claimed in any preceding claim, characterised in that said sheet travels in the cleaning solution through a plurality of electrode pairs and is made cathodic during passage through the last electrode pair.

6. A method as claimed in any preceding claim, characterised in that the dried sheet has a concentration of 0.3 to 2.0 mg/ft² (3.2 to 21.5 mg/m²) of said salts on the surface thereof.

7. A method as claimed in any preceding claim, characterised in that the dried sheet has a concentration of 0.5 to 1.2 mg/ft² (5.4 to 12.9 mg/m²) of said salts on the surface thereof.