EP0534090B1 - Lubricant for the hot rolling of steel shapes in a rolling mill - Google Patents

Abstract

A lubricant for the hot-rolling of steel shapes in a rolling mill is described. To maintain the lubricating action even under extreme rolling conditions and to provide a less polluting lubricant, the aqueous solution containing borate and phosphate also contains ethylene glycol, preferably in a proportion of 16 to 50% by weight. Moreover, the aqueous solution can additionally contain a thickener.

Description

The invention relates to a lubricant for the hot rolling of steel profiles in a roll stand consisting of an aqueous solution containing borate and phosphate.

Such lubricants, also called caliber release agents or caliber lubricants, are already known from DE-AS 1 050 486. They serve to keep the rolling stock and rolling material as free as possible of welds between the rolling stock and the rolling material of the rolling tool as the rolling stock passes through the rolling stand, and to reduce the frictional resistance between the rolling stock and the rolling tool, thereby on the one hand a smooth surface of the rolling stock and on the other hand to achieve the lowest possible tool wear and a workload that is within economically justifiable limits. The previously used term caliber designates in the rolling technology the cross-section that results between the circumferential grooves of the one roll and the grooves of the counter-roll, which the rolling stock is supposed to assume as it passes through the roll stand.

In this context, the term calibration denotes the determination of the cross-sectional shape required for the forming and its sequence.

Usually, the roll bodies are cooled by means of cooling water when the steel profiles are hot-rolled See, e.g. DE-A-1 050 486. This can cause a thinning effect of the lubricant, so that it can no longer guarantee a sufficient lubricating effect. For this reason, the known lubricant containing borate and phosphate has so far not been widely used in practice. In order to ensure the required lubricating and separating action, it is also necessary when hot rolling steel profiles that the lubricant consisting of a salt solution of the two aforementioned components has a sufficiently high salt concentration. An aqueous solution based solely on borate and phosphate cannot meet this requirement, since in such a solution only saturation concentrations of about 7% by weight can be achieved at room temperature.

In practice, mainly mineral oil-based mixtures, such as oil-bitumen mixtures, are used as caliber lubricants. Such mixtures do indeed allow a good separation between the roll body and the rolling stock, because the crack products formed during combustion prevent welds between the rolling stock and the roll material of the roll body and thereby ensure better durability of the rolls. On the other hand, mineral oil-based lubricants cause a strong smoke and smoke development when hot rolling steel profiles in a roll stand, which on the one hand can lead to a health hazard for the staff and on the other hand to a large environmental impact due to polluted wastewater and polluted exhaust air. Such smoke and smoke development can also obstruct the view of the staff to lead. In order to avoid such negative consequences and thus meet today's requirements in occupational medicine and environmental technology, it has already been proposed, see H. Matthes, Tribological Problems and Matched Newer Lubricants for Hot and Semi-Hot Forming, in Technische Akademie Esslingen, 3 International Colloquium "Lubricants in Metal Processing", 1982, Volume I, pages 28.1 to 28.5, to suction off the vapors produced and to clean them in filters. The use of such a lubricant ultimately leads to high investment and operating costs for the entire rolling mill. In addition, the mineral oil products of this lubricant can thermally decompose at the high temperatures that occur during hot rolling, so that the rolling stock is then also to be cleaned of the products formed during the thermal decomposition, for example oil carbon.

The invention has for its object to provide a lubricant of the type specified that maintains a lubricating effect even under extreme rolling conditions and is more environmentally friendly than the known lubricants based on mineral oil.

This object is achieved in a lubricant of the type mentioned above in that the aqueous solution further contains ethylene glycol (C₂H₄ (OH) ₂).

On the one hand, such a lubricant enables the environmentally friendly use of salts in the hot forming of metals and, on the other hand, ensures a sufficient lubricating effect in all temperature ranges of the steel profile to be machined from approx. 500 to 1200 ° C. To maintain the lubricating or separating effect, only small amounts are required for eutectic salt mixtures, since these mixtures do not cause hydrodynamic lubrication, but form so-called monomolecular sliding layers. The addition of ethylene glycol improves the solubility of the borate in the aqueous solution, so that it has increased cold stability. The lubricant according to the invention can thus be stored without problems even at winter temperatures of around 0 ° C. A heating to approximately 30 to 40 ° C. required in the known lubricants to maintain a suitable viscosity is therefore unnecessary. Especially with sprayed lubricants, there can be no precipitation and clogging of the nozzles as a result.

The proportion of ethylene glycol in the aqueous solution is advantageously 16 to 50% by weight. This ensures a sufficient lubricating effect even if the amounts of cooling water required to cool the rollers bring about a not inconsiderable dilution effect of the lubricant.

According to a preferred development of the subject matter of the invention, the borate is an alkali tetraborate, preferably sodium tetraborate (Na₂B₄O₇) with a proportion of 6 to 20% by weight in the aqueous solution, and the phosphate is an alkali phosphate, preferably sodium phosphate (Na₂HPO₄) with a proportion of 3 to 10% by weight in the aqueous solution. These salts are heat resistant, oil and fat free and water soluble. They do not form smoke or smoke when they are placed on the steel profiles to be formed, even at high temperatures, and are therefore less problematic than oil-based lubricants, especially for personnel from an occupational medical point of view. In addition, these mixture components can be easily collected, regenerated and recycled or disposed of.

The aqueous solution preferably consists of 13.3% by weight of borate, 6.7% by weight of phosphate, 33.3% by weight of ethylene glycol and 46.7% by weight of water. Such a lubricant containing such an aqueous solution has proven itself in practical use for the lubrication of a roll stand, since it provides good lubrication and thus problem-free rolling, for example of a sheet pile profile without smoke and smoke development with environmentally friendly portions of the, even in the case of deeply cut profiles which are difficult in terms of rolling technology Salt components and the ethylene glycol enables.

The aqueous solution preferably contains a thickener, for example a thixotropic thickener based on xanthan. This enables a reduction in the consumption of lubricant and an improvement in the adhesiveness of the lubricant to the respective steel profile.

According to an advantageous development of the lubricant, the proportion of the thickener in the aqueous solution is approximately 0.5% by weight, the aqueous solution preferably 9.0% by weight borate, 6.0% by weight phosphate, 28.0 % By weight of ethylene glycol, 0.5% by weight of thickener and 56.5% by weight of water. With such a lubricant, it is possible to reduce the lubricant consumption by approximately 2.5 kg per ton of steel profile, the lubricant consumption in an aqueous solution without thickeners being approximately 6 to 8 kg per ton of steel profile.

The cold behavior of the lubricant containing the thickener is improved compared to a lubricant without a thickener.

Also caused due to the different peripheral speeds of the roller body in the flank area Tribologically critical areas are well wetted and lubricated by the lubricant according to the invention. The profiles obtained show a descaled and slightly phosphated surface with a uniform structure compared to the conventionally lubricated workpieces. However, it is also possible to use a more concentrated or a less concentrated aqueous solution for the lubricant.

The more concentrated solution preferably contains
20% by weight borate
10% by weight of phosphate
50% by weight ethylene glycol
20% by weight water.

A lower concentrated aqueous solution preferably contains
10% by weight borate
5% by weight phosphate
28% by weight ethylene glycol
57% by weight of water.

Even a further dilution of the aqueous solution
6.7% by weight borate
3.3% by weight of phosphate
16.7% by weight glycol
73.3 wt% water
develops a sufficient lubricating effect at least during the first passes (passes) of the rolling stock through the roll stand.

The thickener is preferably a high molecular weight polysaccharide and is sold in practice under the name Kelzan ^R. This thickener based on xanthan is particularly suitable for aqueous solutions, for example in hydraulic lubricants, cleaners and degreasers.

The particular composition of the aqueous solution without the addition of a thickening agent has only an insignificant effect on the consumption values of lubricant per ton of steel profile, as has been found on the basis of experimental studies.

Tests in practical use have shown that the lubricant according to the invention can be sprayed onto the rolling stock or the roller body (s) particularly advantageously by means of nozzles. It has been shown that the choice of spray nozzles with reduced flow cross sections compared to spray nozzles for conventional lubricants leads to better lubrication and separation results at the same spray pressure. This allows the application amount of aqueous solution to be optimized with a view to the lowest possible consumption values. It is thus possible, while maintaining the required lubricating and separating action, to reduce the amount of lubricant required, so that overall a lower environmental impact and lower manufacturing costs can be achieved.

Claims (10)

1. Lubricant for the hot rolling of steel shapes in a rolling mill, consisting of an aqueous solution comprising borate and phosphate, characterized in that the aqueous solution further comprises ethylene glycol (C₂H₄(OH)₂).
2. Lubricant according to claim 1, characterized in that the portion of the ethylene glycol in the aqueous solution is 16 to 50 weight per cent.
3. Lubricant according to claim 1 or 2, characterized in that the borate is an alkali tetraborate preferably sodium tetraborate (Na₂B₄O₇).
4. Lubricant according to one of claims 1 to 3, characterized in that the portion of the borate in the aqueous solution is 6 to 20 weight per cent.
5. Lubricant according to one of the foregoing claims, characterized in that the phosphate is an alkali phosphate preferably sodium phosphate (Na₂HPO₄).
6. Lubricant according to one of the foregoing claims, characterized in that the portion of the phosphate in the aqueous solution is 3 to 10 weight per cent.
7. Lubricant according to one of the foregoing claims, characterized in that the aqueous solution comprises
      13.3 weight per cent borate
      6.7 weight per cent phosphate
      33.3 weight per cent ethylene glycol
      46.7 weight per cent water.
8. Lubricant according to one of the foregoing claims, characterized in that the aqueous solution comprises a thickening agent preferably a thixotrope thickening agent on the basis of xanthane.
9. Lubricant according to claim 8, characterized in that the portion of the thickening agent in the aqueous solution is about 0.5 weight per cent.
10. Lubricant according to claim 8 or 9, characterized in that the aqueous solution comprises
       9.0 weight per cent borate
       6.0 weight per cent phosphate
       28.0 weight per cent ethylene glycol
       0.5 weight per cent thickening agent
       56.5 weight per cent water.