DE1803022B2 - PROTECTIVE COATING AGAINST SCALE DEVELOPMENT DURING HEAT TREATMENT OF METALS - Google Patents

Description

In the production of z. B. steel belts, steel plates or steel sheets are the steel slabs or slabs and ingots of a series of heat treatment operations subjected, the heat material usually at a temperature in the range from 900 to 1250 ° C during a suitable Period of time to be heated for tempering or tempering. This heating takes place while the metal surface is in contact with air. So there is an atmospheric oxidation. That I The resulting oxide, which is usually formed as adhering scale, must be subsequently either mechanically or removed by stripping. These treatments are time consuming.

It is known to apply protective coatings to prevent scale formation on metal surfaces prior to heat treatment apply, which prevent the access of oxygen to the metal surface. These well-known Paints generally include an organic cation modified clay and refractory Sintered or fritted material which is dissolved in an organic solvent, e.g. B. toluene, is dispersed to a to give a spreadable or sprayable mixture. These compositions form an adhesive continuous coating. By adjusting the coefficient of thermal expansion of the so formed Coating in such a way that it differs from that of the metal being treated is, it can be achieved that the coating flakes off by itself from the metal surface on cooling and the ingots, slabs or slabs in a clean condition, essentially not oxidized and for the subsequent machining process ready to be obtained.

The production of suitable protective coatings previously required the modification of clays by means of an ion or base exchange reaction in order to convert it into an organophilic state. At this Base exchange reaction is the clay generally with an "onium" compound, for example with an alkylammonium salt, reacted to form the cations on the surface of the clay particles, usually Alkali or alkaline earth cations by a long-chain organic onium cation, e.g. B. by a Dodecylammonium cation. These modified clays are for use in paints and coating compositions, particularly as suspending agents, generally known, and some types, e.g. B. Alkylammonium montmorillonite, are widely used. The modified tones are, however comparatively expensive and the paints made with them can sometimes have problems with the shelf life.

The invention is based on the object of providing protective coatings against scale formation during heat treatment of metals to create not only good shelf life but also better properties in terms of detachment after cooling.

It has now been found that in place of such modified clays as suspending agents and film-forming agents for protective coatings against scaling in the heat treatment of metals nonionic cellulose derivatives can be used.

The protective coating according to the invention against scaling during the heat treatment of metals, consisting of a suspension of a ceramic frit material in an organic solvent, wherein the frit material makes up 20 to 80 percent by weight of the paint, is characterized by that a nonionic cellulose derivative is used as a suspending agent.

According to an expedient embodiment, the cellulose derivative can consist of hydroxypropyl cellulose.

It has been found that protective paints against scale formation using such

ίο Cellulose derivatives have extremely good storage properties and a very low tendency for solid parts to settle, even when standing for a long time, own. In addition, separation or peeling of the coating from the coated metal is involved Cooling improves compared to paints in which clays are used as suspending agents.

Preferred cellulose derivatives for use

according to the invention are methyl, ethyl and propyl cellulose and hydroxypropyl cellulose.

The liquid medium of the paints according to the invention preferably consists of a lower alcohol or toluene. Isopropanol is particularly preferred.

Suitable ceramic frit materials to be used in the protective paints according to the invention can have the following composition in percent by weight:

Frit D

PbO 61.0

Al ₂ O ₃ 7.0 9.37 6.7 6.95

SiO ₂ 32.0 62.0 65.8 83.5

Na ₂ O 3.6 5.44 5.9

K ₂ O 2.73 1.8

CaO 7.31 3.68 1.58

MgO 0.15 2.2 0.08

B ₂ O ₃ 14.84 15.35 0.1

ZnO 0.9

Fe ₂ O ₃ 0.03

The paints according to the invention are applied to the metal surface before the heat treatment. During the treatment, the coating melts to form a ceramic opposite Oxygen-impermeable coating on the metal surface, which when the metal surface is cooled flakes or flakes, leaving a clean and shiny metal surface.

It is believed that the effect of such The details of the coating are as follows. Surface oxidation of the Metal occur to some extent, but at higher temperatures, e.g. B. at about At 800 ° C the coating begins to melt and forms a flowable glaze that spreads evenly over the surface spreads the metal surface and dissolves away the oxide on it. There is no further oxidation. At the The glaze bursts due to the different coefficients of thermal expansion of the metal and cooling the glaze and leaves a clean, oxide-free metal surface.

The coating compositions according to the invention are particularly useful as surface protection for nickel, cobalt and

Claims (1)

3 4 their alloys and numerous steel alloys, PbO 61 ° / ₀ .Particularly stainless steel alloys, suitable. Al ₂ O ₃ 7 ° / ₀ It is important to consider the properties of the SiO _{2 used} 32% Frit or fries related to the thing to be treated. . Metal to match. In general, the middle 5 is example! coefficient of linear expansion of the above Led metals and alloys in the range between manufactured (percent by weight). see 14 and 17 · 10 ~ ⁶ each ⁰ C. The frits used lead silicate frit A 64 ° / However, generally have a substantially low hydroxypropyl cellulose '.'. '.'. '.'. '.'. '.'. '.'. '. . ' 0.25% lower expansion coefficients, e.g. m io Tcrmrr ^ imni 777 * 0 / of the order of 5 · 16 " ⁶ per ° C. (The frit A isopropanoi i ι, ο / ₀ of the table above has a linear pattern. The paint was made to a thickness of about 101.6 expansion coefficients of 5.4 · 10 ~ ^e per ⁰ C.) to 203.2μ on a sheet of stainless steel for a Oxidizable materials in the paints overall tempering or annealing at 1050 ⁰ C for measure of the invention are incorporated, is applied to the 15 periods of time up to 10 minutes. After increasing the oxide removal ability of the paint. Heat treatment peeled the paint underneath aluminum and ferrosilicon can be included for this leaving a clean, untarnished top-end. surface that does not require further treatment, such as B. a It is also important to ensure that the Vis pickling, required, viscosity of the composition at the maximum 20. The heat treatment temperature is not too low, example 2 otherwise there is a tendency to break away the over- A protective coating against the formation of scale was like Zugs produced leaving exposed metal overflows (percent by weight), areas exists. The viscosity at high temperature lead silicate frit A 40 ° / can be obtained by ingesting a particulate fiery 25 hydroxypropyl cellulose \ \ '.'. '.'. '.'. '.'. '.'. '.'. 0.25% solid material up to 35 percent by weight, for example Magesiumoxvd 20 ° / wise of silicon dioxide, magnesium oxide, feldspar, silicon dioxide 3 ° / ° Aluminum oxide, zinc oxide, andalusite, kyanite, silli-nepheline syenite 10 0 ° / manit MuIMt, talc, olivine and nepheline or syenite, _{kaolmit (present a} j _{s refractory} mate'- ' 30 rials and not as a suspending agent) ... 2% It is also important to ensure that the _{natural resin of southern} cooperage (acid number Coating has a low surface tension, around ₉₄ saponification number 165) 12 ° / the tendency to break the coating at high tem- Isopropyl alcohol 23 ^ 5 °° / keep temperatures to a minimum. For the- '" ^{/ 0} Chiomoxyd can be incorporated for this purpose. At 35 The paint indicated above was applied to an effective application of the coating composition to a stainless steel ingot which ensure, it is desirable to also have a slight heat treatment at 1250 ° C for 4 hours Incorporate amount of a wetting agent, such as. B. was subjected to the needles. The painting was done in a trium salts of alkylarylsulfonates or secondary thickness of about 381 to 508μ applied. The arrival Alkyl sulfates, or especially sodium dioctyl sulfo-coated, fell off the metal after the heat treatment. succinate. surface and left a clean, shiny surface The above-mentioned components of the surface return. The added resin served the paving compositions according to the invention can in tion of the film-forming properties of the paint,
a wide range of relative proportions or Shares are available. In general, the 45 claims: first constituent preferably 20 to 80 ° / ₀ , in particular 1. protective coating against scaling during the the latter preferably 30 to 70 percent by weight, of the heat treatment of metals, consisting of Dashed line. The nonionic cellulose derivative is a suspension of a ceramic frit material only in small proportions, e.g. B. from 0.1 to 1 ° / ₀ in an organic solvent, wherein the of painting, required. As indicated above, 20 to 80 percent by weight of the frit material the particulate refractory material, if it is paint, can be identified by it. present, up to 35 ° / ₀ draws off the composition that as a suspending agent, a non- do. The other beionic cellulose derivative listed above is used, constituents may be present in smaller proportions. 2. Protective coating according to claim 1, characterized by be, in each case the remainder to 100 ° / ₀ of the 55 indicates that the cellulose derivative from hy- organic solvents used. consists of droxypropyl cellulose. The invention is illustrated below with reference to example 3. protective coating according to claim 1 or 2 explained in more detail. This is characterized in these examples by turning up to 35 weights Frit material has the following percent of a particulate refractory mate composition. Contains 60 rials.