KR20210130188A - Reduction and Removal of Process Oxides on Stainless Steel - Google Patents

Abstract

Oxides formed during the annealing of the stainless steel strip are removed with an abrasive brush instead of an acid or molten salt based acid wash. In some embodiments, a stainless steel strip of the present invention is treated with a rare earth element or a related transition metal prior to annealing and then brushed after annealing to remove all oxides. The choice of brush can give a finished appearance to conventionally polished stainless steel.

Description

Reduction and Removal of Process Oxides on Stainless Steel

preference

This application is entitled "Reduction and Removal of Process Oxides on Stainless Steel," U.S. Provisional Patent Application No. 62/808,399, filed February 21, 2019. priority is claimed, the contents of which are incorporated herein by reference.

background

The present application relates to the mechanical removal of oxides formed during processing of stainless steels.

In conventional stainless steel strip processing, oxides are produced during the high temperature annealing cycle used to soften the steel after cold rolling. Typically, these oxides are removed prior to subsequent processing using conventional acid-based pickling techniques. This descaling process uses exposing the annealed stainless steel strip to a molten salt bath and/or to one or more acids in an acid bath. Descaling is slow, expensive, and can be environmentally problematic.

In aspects of the present invention, it has been found that oxides formed during annealing can be removed with an abrasive brush. In some aspects, a stainless steel strip of the present invention is treated with a rare earth element or a related transition metal prior to annealing and then brushed after annealing to remove all oxides. The brushing operation imparts a surface texture to the material, thereby eliminating the need for more conventional polishing and finishing operations.

1 shows the GDS depth profile for a 436L stainless steel sample after final annealing without brushing.
Figure 2 shows the GDS depth profile for a 436L stainless steel sample after one pass brushing and final annealing.
3 shows the GDS depth profile for a 436L stainless steel sample after final annealing with two pass brushing.
Figure 4 shows the GDS depth profile for a 436L stainless steel sample after a three pass brushed final annealing.
5 shows the GDS depth profile for a 436L stainless steel sample after final annealing without any rare earth or rare metal treatment and no brushing.
6 shows the GDS depth profile for a 436L stainless steel sample after a final annealing, Minimox treated and not brushed.
7 shows an SEM image of a 436L stainless steel surface after final annealing/acid washing, showing grain patterns.
^{8 shows Chromeshield ®} 22 stainless steel cold rolled samples after annealing, with minimox dipped samples on the right.
9A and 9B show GDS depth profiles for untreated and treated panels.
10 shows annealed ChromeShield 22 stainless steel samples after Scotch-Brite brushing.
11 shows a sample of annealed Chromeshield 22 stainless steel after brushing with a bristle type brush.

Annealing and pickling (“AP”) methods are commonly used in stainless steel processing. After work hardening occurs during rolling, annealing is necessary to soften the steel. Annealing is generally performed in air-based atmospheres at a temperature of 1,700 to 2,000° F. for several minutes. The resulting oxides formed are then removed by an acid wash step. The acid wash reaction is generally a rate controlling step during AP. Most stainless steels are sold "fully annealed" with the annealing oxide removed. If the acid wash step could be eliminated, the line speed could be increased as well as the acid and molten salt used for descaling would no longer be needed.

In some aspects, the oxide formed during annealing may be removed using mechanical brushing. In general, the oxide formed after annealing is relatively thin (<1 μm), and it has been found that modern brushing equipment can remove this surface layer.

In other aspects, the effectiveness of brushing may be enhanced if the stainless steel strip is treated with a rare earth metal or related transition metal prior to annealing. This surface treatment affects the growth of oxides on stainless steel, reduces their thickness, and alters the surface chemistry so that they are more easily removed.

The production of stainless steel strips is well known. The oxide removal process of the present invention (also known as descaling) can be implemented in a standard stainless steel manufacturing process, replacing only the acid-based acid washing and/or molten salt portion of the process.

Brushing devices, and brushes used therewith, are also well known. Brushing is often used to impart a desired surface finish to a clean (or bright) stainless steel strip, i.

In certain aspects of the method, such a brushing device is used to brush a stainless steel strip having an oxide to remove the oxide from the surface of the stainless steel strip. In one aspect, the brushing machine uses a Scotch-Bright™ nonwoven type roll containing a loose web of nylon fibers or other fibers impregnated with alumina or silicon carbide type particles. Representative roll types include 3M CB Cleaning roll and CS-CB Clean and Strip roll. Scotch-Bright is an abrasive sold by 3M Company of St. Paul, Minnesota, USA.

The rare earth element or related transition metal can be applied to the stainless steel strip in liquid form, for example in the form of an aqueous suspension or aqueous solution comprising a metal salt. Rare earth elements may include yttrium, cerium, lanthanum and their associated oxides or nitrates. And related transition metals may include zirconium and its associated oxides.

Liquids containing rare earth metals or related transition metals may be applied to the stainless steel surface by dipping, painting and spraying prior to annealing and then dried.

In some embodiments, the rare earth element may be provided as a suspension. ^{For example, Minimox ®} liquid, available from Materials Interface, Inc. of Sussex, Wis., contains nanoparticles of rare earth oxides in suspension. Such materials are also described in US Pat. No. 8,568,538, the contents of which are incorporated herein by reference. Minimox is believed to contain up to about 1% by weight of yttrium as _{Y 2} O ₃ nanoparticles suspended in an aqueous medium.

One aspect of the present invention comprises an aqueous solution of a rare earth metal salt. The salt may be, for example, nitrate or acetate. The rare earth metal salt may include at least one of cerium (Ce), lanthanum (La), and yttrium (Y). Since chlorides may tolerate corrosion and carbonates may not dissolve, the rare earth metal salts in the present invention do not include carbonates or chlorides.

Rare earth metal salts are well known in the art and are commercially available. The nitrate or acetate need not be of any particular grain size, and in particular the salt or the resulting rare earth metal oxide need not be limited to nanoparticles, which are considered particles having dimensions in the range of 1 to 100 nm.

The solution comprises a rare earth metal nitrate or acetate dissolved in water, preferably in deionized water. The concentration of the rare earth metal nitrate or acetate in the aqueous solution can be extended to the solubility limit of a particular salt. In certain embodiments, the solution may have a concentration equal to about 1 to about 10 grams of rare earth metal nitrate or acetate for a total aqueous solution of about 200 grams. In other aspects, the solution may have a concentration equal to about 1 to about 20 grams of rare earth metal nitrate or acetate for a total aqueous solution of about 200 grams.

In some embodiments, the amount of rare earth metal or related transition metal applied to the stainless steel surface has a density of about 300 to 3,000 μg/m, in some embodiments a density of about 500 to 8,000 μg/m, or in other embodiments They have a density of 5,000 to 8,000 μg/m 2 .

To improve the wettability of stainless steel, a surfactant may be added to the aqueous solution or suspension. The surfactant is added at a concentration of about 0.1 to 5% by weight of the solution, and in some embodiments at a concentration of about 0.1 to 0.5% by weight of the solution. Any surfactant known to improve the wettability of the aqueous solution or suspension to the stainless steel surface can be used. The surfactant may include a detergent such as dishwashing detergent.

The resulting aqueous solution or suspension can be applied to one or both surfaces of a stainless steel strip ( or any other stainless steel product). It is then dried using all methods currently used for drying paints and other liquids on moving strips or webs, such as forced air, infrared heating, convection ovens, and the like.

It is believed that the present invention is beneficial for all types of stainless steel. Stainless steels that would benefit from the present invention include ferritic, austenitic and martensitic stainless steels.

For use of the method of the present invention, stainless steel strips are prepared using conventional and well known methods with the following exceptions. After annealing, one or both sides of the stainless steel strip are brushed at least once instead of acid washing/molten salt descaling. When a rare earth metal or related transition metal solution or suspension is applied to the stainless steel strip, the solution or suspension is applied to one or both sides of the stainless steel strip prior to annealing. When the stainless steel manufacturing process is a continuous process, the solution or suspension is applied in-line by a method known in the art, and the brushing is also performed in-line. Alternatively, one or both of the two processes may be performed in a batch operation or may be performed off-line.

Example 1

Two 436L stainless coils, each supplied by AK Steel Corporation of West Chester, Ohio, were each partially coated with a minimox nano-yttrium oxide rare earth based suspension and then annealed. In each case, about half of the strip was coated and half uncoated along the length of the strip. Annealing was performed at approximately 1,950F/1,066C in air with 3% excess oxygen. The line speed was 45 ft/min and the time in the furnace was about 3.5 minutes.

After annealing, only slight color differences were observed on the strip surface between the coated and uncoated portions of each coil. However, there was a more significant difference in appearance on the sidewall of each coil.

Scale on the treated or untreated portions of the strip was easily removed by grinding the steel strip with ultra-smooth #400 grit alumina or silicon carbide abrasive paper.

Example 2

The stainless steel coil of Example 1 was brushed using two conventional brush stands with 3M Scotch-Bright nonwoven brushes commonly used to remove red rust from carbon steel. There were two types of brushes. Two cooperating brushes in contact with the top of the strip (“Maroon” medium aggresive type CB Cleaning brush with aluminum oxide abrasive media particles) and in contact with the bottom Two brushes (“Black” heavy duty type CS-CB Clean and Strip brush with more abrasive silicon carbide abrasive media particles). It was recently dressed and balanced, brushing speed 250 to 300 surface ft/min, line speed 45 ft/min Working pressure ranged from 0.1 to 0.5 horsepower per inch of working width.

After one pass through the brushing station, the oxide removal on the top side was less than the oxide removal on the bottom side because of the use of a more abrasive brush on the bottom. The top was apparently "dark" and the bottom was "bright", indicating that there was more residual oxide on top and little oxide left on the bottom. There was also a noticeable linear pattern on the strip due to contact with the roll. No visual differences were detected between the minimox treated and untreated portions of the strip.

The strip was then passed through the line twice. Removal was marginally noticeable on the upper side, but the "dark" appearance was still present. The removal of oxide was visually complete on the underside, and the texture was smoother than after one pass. Again, no visual difference was detected between the minimox treated and non-minimoxed parts.

The strip was then turned over and the former lower side was now treated with the less abrasive upper side roll brush, and the former upper side was treated with the more abrasive roll brush above. At this point, the new top surface not only had a “bright” appearance, but also had a smoother texture similar to standard #4 Polish stainless steel. #4 Polish is a desirable short line grit pattern directional finish for stainless steels generally used in appliances and food manufacturing surfaces and equipment. #4 Polish is a standard surface finish created by polishing with a 150 grit emery polishing belt as described in the Stainless Steel Comparator published by AK Steel Corporation, West Chester, Ohio. am. After the third pass, the lower surface was also brightened. Thus, the brushing operation can replace the expensive and separate stainless steel machining step of surface polishing to achieve the desired appearance in the finished product.

Visual appearance observations of the strips were supplemented with Glow Discharge Spectroscopy (GDS) measurements to demonstrate the removal of oxides ( FIGS. 1 to 4 ). The oxygen peak (O) moves closer to the surface (zero depth position) as the number of brush passes increases. In production, it should be noted that it may not be economical for the material to go through the line several times. Instead, the number of brushes that can be used in a given process line can be increased. For example, two brushes per side were used in this study. If two passes are required to remove the oxide, the manufacturing equivalent would be 4 brushes per side. If 3 passes are required for a two brush line, 6 brushes per side may be used.

The effect of the minimox rare earth treatment is to reduce the thickness of the oxide grown during annealing. Thinner oxides require less removal and thus require less brushing. A comparison of GDS results for materials from the same annealed coil with and without oxide is shown in FIGS. 5 and 6 . A measure of the relative thickness of the oxide is the point at which the iron (Fe2) curve reaches the 50% Analyte Weight Percent point. In this case, a value of about 0.25 μm untreated vs. 0.175 μm for the Minimox treated material suggests a 30% reduction in the thickness of the oxide. This can translate to 30% faster processing speed or 30% better life for a given set of rolls before dressing is required. Note that 30% less removal can occur, indicating less waste disposal.

Brushing versus molten salt/acid wash after final annealing can be associated with two additional benefits. First, the brushing operation leaves a surface pattern due to the abrasive action of the brush. The final brushing type and operation can be selected to impart the desired finish appearance to the material. The surface of the molten salt/acid washed material has no visible pattern discernible.

Second, at the microscopic level, the acid washed material has a pattern associated with the desired etching of grain boundaries (FIG. 7). Although not visible, these patterns can lead to poorer corrosion performance of the material. Since no chemicals are used in brushing, there is no etching of grain boundaries.

Example 3

^{To simulate strip annealing after cold rolling, cold rolled samples of Chromeshield ®} 22 stainless steel strip provided by AK Steel Corporation, West Chester, Ohio, were run in air at 1,875° F. (1,024° C.) for 3.5 minutes. Annealed. Samples were washed with alkali before annealing, two were subsequently immersed in a 1% aqueous suspension of Minimox 721 yttria nanoparticles at room temperature for 5 seconds and air dried. Visual differences between treated and untreated samples were evident after annealing. See Figure 8.

The surface was investigated through depth profile using glow discharge spectroscopy (GDS). Results are shown for the untreated panel (see FIG. 9A ) and the treated panel (see FIG. 9B ). In both cases, the oxide region was found to be less than 0.5 μm thick. The profile of the treated samples could have been affected by the non-uniform application of the Minimox suspension.

The sample was then brushed against one side with a Scotch-Bright roll using a laboratory brush unit equipped with a roll impregnated with alumina particles. Inspection showed that the oxide was essentially removed after about four passes of the brush.

The two samples were also brushed against opposite sides using a silicon carbon impregnated bristle brush. In this case, the removal of the oxide was not complete, and the surface was less uniform with visible lines.

The material brushed with a Scotch-Bright roll is shown in FIG. 10 . The material brushed with a bristle brush is shown in FIG. 11 .

Example 4

436L stainless steel strips were annealed in a 3% excess oxygen air oven at 1,950°F for 3.5 minutes at 45 ft/min and then processed as follows:

The strip is treated with two to four 3M Scotch-Bright silicon carbide impregnated brushes to remove the annealing oxide from the stainless surface.

Subsequently, a smoother surface is obtained by passing the material through one or two less abrasive 3M Scotch-Bright Alumina impregnated brushes.

Brushing first with an abrasive brush, followed by a less abrasive brush, produces a more uniform surface texture than with an abrasive brush alone.

Higher line speeds require more brushing stations and/or more abrasive brushes.

Claims (3)

a. providing a stainless steel strip having at least one surface;
b. annealing the stainless steel strip;
c. and brushing said at least one surface to remove oxides formed during said annealing step. The method of claim 1 , further comprising applying a rare earth metal or a related transition metal to the at least one surface prior to the annealing step. The method of claim 1 , further comprising, after the brushing operation, using a finishing roll that imparts an appearance similar to polished stainless steel.

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