SE406091B - WAY TO SURFACE STAINLESS STEEL IN AND FOR THE REPLACEMENT OF AN OXIDE FILM, WHICH CAN BE EASY TO BE REMOVED BY BET - Google Patents

Description

10 20 20 25 50 55 7i802523-6 i I '2 In addition to the fact that pickling solutions containing HF are more expensive, they cause harmful contaminants. While on the one hand platinums, billets, ingots, etc. of carbon steel and low-alloy steels can be easily treated to remove surface oxide and to eliminate roll splinters, splashes and other surface defects in preparation for further rolling, stainless steels must be ground or sandblasted to elimination of roll splinters, splashes, etc. Many of these measures must be carried out more slowly than is desirable in the treatment of stainless steel, in order to achieve complete removal of the oxide.

The present invention is based on the development of a heat treatment or annealing for stainless steels or other chromium-containing steels, which treatment gives rise to the formation of a deep oxide on the steel surface, which oxide can be easily removed, thereby eliminating the above the disadvantages. Accordingly, the present invention relates to a process for heat treatment of stainless steels, whereby an oxide is formed on the steel surface, which oxide can be more easily removed than conventionally produced oxides.

According to the invention, there is provided a process for the surface treatment of stainless steel for the production on the steel surface of an oxide layer which can be easily removed by pickling, and this process is characterized by heating the steel to a temperature of at least 950 ° C but below the steel's victory temperature. in which the logarithm .for the oxygen partial pressure is between ïêgàšëg + 5.6 and ïêzàê fi g- + 8.1, where T is the absolute temperature in degrees Kelvin, whereby an oxide film is formed, which mainly consists of the spinel FeGr204, in which the steel is cooled and the cooled steel is pickled in order to remove said spinel.

The invention will be described in more detail by means of an example with reference to the accompanying drawings, in which the device shown in fig. 1 shows graphically the critical range of the oxygen potentials in relation to the temperature, which oxygen potentials must be kept in the annealing atmosphere in the method according to the invention.

Fig. 2 shows graphically in three dimensions the oxygen potential of the annealing atmosphere, air: the gas ratio and the time at 225 ° C as a function of the peeling, ie. the weight loss of a stainless steel (Type 504), which illustrates the optimal parameters of the process of the present invention. Fig. 3 shows a number of photomicrographs with 25 times the magnification of cuts through the surface of a stainless steel (Type 504) after 'flo 15 20 25 50 55 40 5' 7802523-6 different annealing treatments.

As noted above, it is a well known fact that stainless steels preferentially oxidize under most conditions to form a thin, tightly adherent surface film of Cr 2 O 5. When this surface film is formed, it forms a thin, transparent oxide film which forms a protective barrier, which to a minimum reduces further oxidation or corrosion. The oxide film is formed quickly and easily under virtually all conditions. This means that the chromium's tendency to form Cr2O5 with oxygen is so strong that many atmospheres, which are strongly reducing compared to carbon steels and other alloy steels, in fact oxidize the chromium in stainless steels. In fact, it is very difficult to achieve technical atmospheres which are reducing relative to Cr2O5, at least within the range of normal heat treatment temperatures.

The invention is based on the discovery that if the oxygen potential of the annealing atmosphere is carefully regulated within critical limits during annealing or other heat treatment of a chromium-containing stainless steel, the ordinary, solid adhesive oxide layer is replaced by an iron-chromium spinel (FeGr 2 O 5), which does not protect. as Cr2O5 against further oxidation but is much easier to remove.

The spinel shape can be easily distinguished, because it is black and magnetic in contrast to the other chromium and iron chromium oxides. Due to the fact that the spinel is a much weaker barrier against further oxidation than Cr2O5, it is possible to easily achieve a deeper penetration oxidation, which is sufficient for the removal of roll cracks, splashes and other surface defects. In addition, the process of the present invention tends to oxidize surface peaks at greater speeds, so that rough surfaces become smoother and a flat, glossy surface is obtained under the spinel.

Fig. 1 shows the critical range of the oxygen potentials as a function of temperature. In general, the process of the present invention requires that stainless steel be heat treated in a furnace atmosphere having an oxygen potential which falls within the dashed area for a sufficient time to form or transform the desired spinel on the stainless steel surface.

The diagram in Fig. 1 is an equilibrium diagram of the oxide surface of a stainless steel. It should be noted, however, that velocity phenomena also affect the reaction mechanism. Instead of discussing in more detail mechanisms and reactions which are assumed to be involved in the process according to the invention, it should be sufficient to note that the lower limit of the belt constitutes the spinel equilibrium, i.e.

Fe + 4/2 02 + Cr2O5 --¿> FeCr2O4 (Spinell) * (1) This equilibrium can be mathematically expressed as follows: log P02 = "2 '280 + 5.6 (2) where T is the absolute temperature in degrees Kelvin.The upper edge of the belt is the practical oxygen potential limit for spinel formation.Over this potential, velocity phenomena allow other competing reactions to form more complex oxides, which are difficult to remove from the surface of the stainless steel.This upper limit.kߧ mathematically is defined as follows: 1. log P02 Q íêïšêïg - + "B, 1 (5) where T again means the absolute temperature in degrees Kelvin.

Consequently, during heat treatment of a stainless steel within the dashed area or strip, only spinel is formed, which is easy to remove. Should the oxygen potential be such that it lies over the strip, which is typical of most atmospheres for stainless steel annealing, the surface films are also formed which are difficult to remove. Heat treatment under the belt naturally gives rise to the formation of a surface film of Cr2O5, which is also thin, tightly adherent and difficult to remove. The curve at the bottom of the diagram indicates the oxygen potential required for the atmosphere to be reducing compared to stainless steel, ie. ergo; * ___- ß 2 cr + 1 1/2 02 - (a) Although it could be concluded that annealing under this curve would also give favorable results with regard to the elimination of all surface oxides, this would be technically impractical, because so low Oxygen potentials cannot be easily maintained in technical facilities and downtime required to reduce even at higher temperatures would be inappropriately long.

Nor would a deep penetrating oxidation be obtained, which is essential for some embodiments of the present invention. Although the invention generally relates to any type of heat treatment of stainless steel in which the furnace atmosphere is controlled to provide an oxygen potential within the dashed line of Fig. 1, i.e. where log P02 lies between ïêg fi êëg + 5.6 and Zêzàêïg + 8.1, it is obvious that since the band represents equilibrium conditions, not all areas within the band are practical for technical use. Thus, for example, the desired spinel can be formed at temperatures as low as 81-15 ° C (1140 ° K) as shown in Fig. 1. However, at such low temperatures, the reactions by which the spinel is formed by the other surface oxides are too slow.

In order to achieve an acceptable reaction rate, the heat treatment must therefore be carried out at a temperature of at least 925 ° C, but preferably at higher temperatures, for example at temperatures between 1040 and 1290 ° C, depending on the desired target. Thus, if one only wishes to produce an oxide film which is easier to remove by pickling, during a continuous heat treatment of sheet, strip or wire for 75 seconds, an ideal temperature is 1040-1200 ° C. If, on the other hand, a strong oxide film is sought for oxidation of roll splinters, splashes, etc. on platinums, roll blanks or ingots, the ideal temperature is slightly higher, for example 1250-190 ° C.

In addition, the holding time should be extended to at least about 10 minutes, but preferably to about 45 minutes. Of course, the temperature should not exceed the liquefaction temperature of the treated steel.

In addition to what has been stated above regarding temperature for achieving suitable reaction rates, it is noticeable that with increasing temperature it becomes increasingly difficult to achieve and maintain lower oxygen potentials. It is therefore preferred to work with oxygen potentials just below the upper limit of the belt and at higher temperatures, because such atmospheres are easier to create and maintain.

In addition, in gas-fired furnaces, in which the combustion products are included in the furnace atmosphere, one can work more efficiently, if one uses high oxygen potentials, ie. stays closer to the stoichiometric amounts of gas and air.

As already pointed out above, the cubic iron-chromium spinel formed in the process of the present invention is completely different in nature from Cr 2 O 5 and the usual rhombic outer layers of surface oxides formed on stainless steels. These are thin, impermeable and protective, but the spinel is less protective and can therefore form faster with greater thickness, because it is more electrically conductive and ionic. The more reactive nature of the spinel is shown in Fig. 2, in which the weight loss metal for different oxygen potentials is indicated during 15 minutes of annealing of a stainless steel (AISI Type 504) at l277 ° C (l550 ° K). At this temperature, the critical value for log PO2 is between -9.8 and -10.3. As can be seen from Fig. 2, the oxidation rate is in the unit, i.e. the weight loss during the first 5 minutes of the annealing treatment is essentially the same for all oxygen potentials. At annealing times over 5 minutes, however, the oxidation rates begin to decrease for all oxides except for the spinel, ie. except when log P02 is between -9.8 and -10.5. This means that when the spinel is formed, the oxidation rate after 15 minutes is approximately the same as after 5 minutes. On the other hand, when other oxide phases are formed, the oxidation rate decreases more with time, since such oxides more effectively protect the underlying metal from further oxidation.

Due to this, it is easier to increase the thickness of the spinel layer on platinums, roll blanks, etc. of stainless steel in order for efficient oxidation and removal of surface defects, such as roll splinters and splashes.

It is obvious that the substantial oxidation potential for practical application of the present invention can be achieved with any of a number of gas mixtures. An example is the HEO-Ha-OT system 002-60-02 or a combination of these two systems, which occur in combustion products when burning natural gas or other hydrocarbon fuels. In the development work, which is the basis for the present invention, the heat for the annealing has been achieved by burning natural gas in air. In order to achieve the significant oxygen potential at temperatures between 950 and 225 ° C, it has proved necessary to maintain a ratio of natural gas between 7.5: 1 and 6.5: 1. This ratio is low in oxygen, considering that the stoichiometric ratio for complete combustion of the natural gas is about 9.7: 1.

It should be noted that the composition of natural gas varies slightly from place to place. Accordingly, the above critical range of 7.5: 1 - 6.5: 1 is primarily applicable to natural gas, as commonly found in the Pittsburgh, Pennsylvania area, which natural gas has the following composition: Methane, (CH4)% .Ethane (C2H6 ) 2.9% Even (02114) 0.18% Beta (041110) 0.57% Carbon monoxide (C05) 0.97% Inert gases Residue 'Thus, it may be necessary for natural gases with a different composition to apply a slightly different ratio from the above ratio . One way to experimentally determine a suitable ratio is to anneal the stainless steel for a few minutes in a natural gas-air mixture, in which the amount of air is gradually reduced, until a suitable ratio is achieved, at which the desired spinel is easily formed. This can be determined visually, since the oxide on the stainless steel turns black when the spinel is formed. The following examples are given to better illustrate the advantages of the invention. In a series of experiments, cold-reduced stainless steel (Type 504) was machined to demonstrate the ease with which the spinel is removed by a simple pickling. For this test, test bodies of technically machined, cold-rolled steel were used. The specimens had the dimensions 76.2 x 52 mm and were cut from a 1.65 mm thick strip with the longitudinal axis parallel to the rolling direction. The specimens were ground along the edges and degrees and sharp corners were ground, whereupon they were degreased and dry-dried.

Duplicate specimens of the cold rolled strip were heated for 75 and 150 seconds at temperatures of 1127 and 1200 ° C (1400 and 1475 ° K, respectively) in atmospheres obtained by combustion of natural gas in air at an air gas ratio of 1:10; 9: 1; 8: 1; 7.5: 1 and 6: 1. At 117 ° C, these conditions gave oxygen potentials of -0.5; -9.5; -10.4; -11 ,? resp. -ll, 5. At 1200 ° C, these conditions gave oxygen potentials of -0.5; -10.6; -11.5; -11.8 resp, -12.5.

The times and temperatures were chosen to equalize the known technical parameters for this quality, ie. 125 seconds at 1121 ° C (1595-142 ° K) with an air: gas ratio greater than the stoichiometric, ie. 9.7: 1.

The oxidized specimens were pickled in 10% nitric acid at a temperature of about 70 ° C (545 ° K) for 2 minutes.

The oxide film on the specimens oxidized in a mixture with the ratio air = gee ev 10o = 1 eeh 9: 1 was dense it could not be easily removed by pickling in nitric acid. Even the oxide films produced during the first 75 seconds at the lower air gas ratios of 8: 1 to 9: 1 were difficult to remove, since all the surface oxide did not consist of spinel. In contrast, the black oxide obtained at air gas conditions of 8: 1 to 6.5: 1 and oxidation times of 150 seconds could be easily and completely removed during the first nitric acid treatment, especially the oxides formed at air gas conditions of 7.5: l to 6.5: 1.

At 1200 ° C, a dense oxide film resistant to nitric acid was formed in 75 seconds at an air: gas ratio between 10: 1 and 9: 1.

When the air gas ratio was reduced to 7.5: 1, an oxide film 10 50 ss was obtained in 40 vzoszz 0 0 in 75 seconds, which was easy to completely remove. Approximately half of the oxide formed in 150 seconds at a ratio of 10: 1 could be removed with nitric acid, whereas the main amount, but not the whole amount of the oxide formed in 150 seconds at a ratio of 9: 1 l could be removed by nitric acid. In addition, the mirror gloss of the pickled steel was reduced by a factor of at least four, when the gas ratio was lowered from 10: 1 to?, 5: 1. This gave a smooth, non-demarcating surface.

It was obvious that the oxide first formed under all conditions was a dense Cr 2 O 5 film which was impermeable to the nitric acid. At gas conditions of 10: 1 - 9: 1 and rising temperature and / or prolonged time, this phase thickened slightly and cracked and could be at least partially removed with nitric acid. At an air: gas ratio of 7.5: 1 - 6.5: 1, Cr 2 O 3 was converted to spinel, which is much easier to remove with nitric acid.

Another test series was performed to determine the leveling effect of the process of the present invention. For these tests, a strip of hot-rolled, annealed and pickled stainless steel (Type 504) with a thickness of 5.58 mm was grooved on a grinder, so that 1.02 mm deep grooves were formed therein. Then cut the strip into specimens f50.8 x 52 mm). Through the corrugation, the specimens simulated steel that was ground on the surface or that was provided with shallow surface defects.

The leveling effect of the furnace atmosphere on these was determined by oxidation treatment with air: natural gas ratios between 10: 1 and 6.5: 1.

The specimens were heated at 1250-175 ° C for 45, 90 and 180 minutes and then pickled in 10% nitric acid at 70 ° C to remove the oxide film but no metal. Sections of the specimens were photomicrographed and the results are shown in Fig. 5 for the specimens heated at 1275 ° C. The top photomicrograph in Fig. 5 shows the original grooves and the lower photomicrograph shows the remarkable effect of an air-gas ratio of 6.5: 1 for 180 minutes. It appears that the grooves were completely removed under these annealing conditions. By comparing the lowest photomicrograph with the one closest to it, for which the air-gas ratio was 10: 1, one can clearly see the advantages of the lower ratio. The specimens heated at 220 ° C show similar results, but not so drastic. Nevertheless, the results at somewhat extended times would have been just as drastic.

In addition to the above tests, a number of other tests were performed not only with Type 504 stainless steel (AISI) but also with types 450 and 409 (AISI) as well as with a patented steel, '10 15 9 ~' 7802523-6 designated as 18 -18-2 (U.S. Patent 5,525,788).

All these tests verified the above.

From the above it is clear that the method according to the present invention can be applied for different purposes. Thus, the process according to the invention can be incorporated into continuous annealing of stainless steel, whereby spinel is formed while the sheet undergoes continuous annealing, whereby an oxide film is formed, which can be removed more easily and more quickly in order to facilitate further processing. On the other hand, the process of the present invention can be incorporated into a process for reheating platinum and roll blanks or annealing hot-rolled strip to facilitate oxidation of roll splinters, splashes and other surface defects, which can then be easily removed before further heating. or cold rolling.

In addition, the method according to the invention can be applied to finished steel products for removing roller skin and improving the appearance. As pointed out above, the method can be applied to smooth rough rough surfaces on stainless steel or to provide a smooth, non-shiny silk-like surface. The method according to the present invention can be applied in all kinds of steel machining, when it is desired to remove surface oxides and / or surface defects and / or to level the surface.

Claims (1)

1. 0 15 20 25 7802523-6 im I Patent nrav p 1. Methods of surface treatment of stainless steel in order to produce an oxide surface film, which can be easily removed by pickling, can be seen by heating the steel at a temperature of at least 950 ° C but below the temperature of segregation of the steel in an atmosphere in which the logarithm of the oxygen partial pressure is between + 5.6 and + 8.1, where T is the absolute temperature in degrees Kelvin, so that a surface oxide is formed on the steel, which oxide consists mainly of spinel (FeCr2O4), whereupon the steel is cooled and pickled to remove the spinel. 2. A method according to claim 1, characterized in that the heating was carried out at a temperature between 1040 and 220 ° C. 5. A method according to claim 2 or 2, characterized in that a thin oadd film is produced by heating the steel at a temperature between 1040 and 1200 ° C for at least 75 seconds. LL. A method according to claim 2 or 2, characterized in that a thick oxide film is produced by heating the steel at a temperature between 1250 and 1290 ° C for at least 10 minutes. 5. A method according to any one of the preceding claims, characterized in that the cooled steel is pickled in a dilute acid solution. 6. A method according to claim 5, characterized in that the acid solution is diluted ïíNOš solution. 7. A method according to claim 1, characterized in that the cooled steel is pickled in an electrolytic H 2 SO 4 solution and then in a líNOš-EE solution. 8. A method according to any one of the preceding claims, characterized in that the oxygen partial pressure is produced by using the combustion products as combustion of natural gas in air at an air gas ratio of between 7.5: 1 and 6.5: 1. MENTIONED PUBLICATIONS: