UA63941C2 - Method for manufacturing the thin strip of stainless steel - Google Patents

Abstract

Invention relates to ferrous metallurgy, in particular - to the method for manufacturing the thin strip made of the stainless steel. The method of manufacturing the thin strip made of the stainless steel by direct hardening of liquid steel in the form of a strip with thickness less or equal to 8 mm in the pouring device having two moving and cooled walls, and the hot rolling of this strip, whose hardening to a considerable degree terminates as soon as the strip leaves the walls mentioned above. In this case, the hot rolling is performed on the rolling mill, working rolls of which have a diameter from 400 to 900 mm, and the temperature of belt at the outlet of the rolling mill is from 800 to 1100 °C. The degree of thickness reduction of the strip during hot rolling is from 15 to 50 %. Invention ensures the closure of central pores which appear in the core of the strip after its complete hardening.

Description

Description of the invention

The invention relates to the production of thin strips of stainless steel directly from liquid metal 2 by solidification inside a mold consisting of two walls that are cooled and which move at the same speed as the solidified strip, like the outer walls of two rolls rotating around horizontal axes

In this method of casting, which at this time is introduced in the industry under the name "casting between rolls", one of the main problems related to the quality of the strip is the possibility of pores in the core of the strip. 710 When such pores appear in a strip that undergoes further processing such as etching, annealing, cold rolling and other processing operations, they limit the industry, the use of products obtained from the strip due to the deterioration due to them of mechanical properties.

The reasons for the appearance of pores in the core of the strips poured between the rolls can be the same as those that cause (for large sizes) secondary seals in ingots and central pores in products poured by the traditional continuous method 19 , namely, the closing of air gaps with hard metal pockets that still contain liquid metal, as a result of which solidification of the product (which under normal conditions is largely complete when the strip leaves the walls of the mold, i.e. the core of the strip is no longer completely liquid) does not occur as a completely orderly process. Cooling and solidification of the liquid metal contained in air pockets is accompanied by shrinkage of the metal, which leads to the appearance of voids. These voids are not filled until the end of solidification, since fresh portions of liquid metal no longer enter the closed air pocket. The pores formed in this way are characterized by spherical defects called "sinks", caused by the release of dissolved gas and most often occurring near the surface of the products.

In patent EP 0 396 862, publ. 14.11.90, IPC B22011/06, B22801/46, a method designed to eliminate central pores and simultaneously internal and external defects in the process of pouring steel strips between two rolls is proposed. According to this method, the casting rolls have on their circular cross-section surfaces strictly (3) grooves of a certain size, offset relative to each other on two rolls. In this way, they try to prevent the peeling off of hardened metal crusts on the surface of the rolls, which leads to violations during the solidification of the strip. However, it turned out that the prevention of such exfoliation alone is not enough to completely eliminate the appearance of central pores. --

The UR patent 8252653 proposes a method, publ. 01.10.96, IPC B22011/06, according to which hot rolling "Its strips are carried out according to the spill under the conditions determined by the following inequality: g" (2.74x105t2 - 6.88x102T w 43.55) (Mo) F de so d - means the degree of compression during hot rolling;

T is the temperature of hot rolling, "C; i-o

Y - pore diameter in the direction of the strip thickness;

Mo is the diameter of the pore in the direction of the strip width.

Thus, according to this method, it is necessary that hot rolling is carried out with a degree of compression sufficient to close the pores, and the degree of compression depends on the rolling temperature (that is, the temperature at which the strip enters the gap between the rolls), as well as from the shape and orientation, see However, it has been found that such rolling conditions are not yet sufficient to ensure the closure of all pores and especially not sufficient to prevent the re-opening of closed pores in the process of converting the strip or manufacturing products, which leads to their destruction.

The task of this invention is to create a method that guarantees the irreversible closing of the central pores that appear in the core of the strip after its complete hardening.

With this problem, a method of manufacturing a thin strip of stainless steel is proposed by directly (95) solidifying liquid steel in the form of a strip with a thickness of less than or equal to 8 mm in a pouring device with two walls with the ability to move and cool, and hot rolling this strip, hardening which is largely completed after the strip leaves the aforementioned walls, distinguished by the fact that hot rolling

ChK" 50 is carried out on a rolling mill, the working rolls of which have a diameter from 400 to 900 mm, the temperature of the strip at the exit from the rolling mill is from 800 to 1100 "C, and also because the degree of reduction in the thickness of the strip during hot rolling is from 15 to 5095.

Cold rolling is preferably carried out immediately after the strip is poured. The filling device in this case can be of the "filling between rolls" type.

It is already clear that the purpose of the invention is achieved by a combination of features related to the diameter of the workers

GF) of the rolls, the state of hot rolling, the temperature of the strip at the exit from the rolls and the degree of reduction in the thickness of the strip in the process of 7 hot rolling.

The invention is used for casting stainless steels of any class, the carbon content of which is traditionally below or equal to 195, the silicon content is below or equal to 195, the manganese content is below or equal to 1595, the chromium content is from 10 to 3095, the copper content is below or equal to 595 and nitrogen content is less than or equal to 0.595 (all percent by mass). These steels may also contain significant amounts of nickel (up to 4095) or molybdenum (up to 8905). In addition, as is usually the case, there are other elements in the metal: either as polluting impurities or as alloying elements, in particular, sulfur, phosphorus, titanium, niobium, zirconium. Their sum should not exceed 295 wt.

As already mentioned, the thin strip of stainless steel cast between the rolls is very prone to the formation of pores 65 in the core during the solidification process, when the liquid air pocket is closed by the solid metal. This phenomenon takes place at the end of the solidification of the dough-like zone, also called the "equilibrium zone", located between the two hardened crusts, which are in contact with the rolls and also called the "columnar zones". The equilibrium zone is difficult to control and its thickness can vary depending on the rate of solidification of the columnar

Zone Thus, the equilibrium zone, as it can be predicted, can be locally located in those places where the increased rate of growth of the columnar zones takes place. Downstream of the process from the point of closure of the equiaxial zone, the air pockets filled with liquid metal can no longer be filled with liquid metal as necessary, resulting in the formation of pores due to the shrinkage of the metal during solidification of these air pockets. However, such a situation occurs quite rarely and in fact, as a rule, isolation of the liquid metal in the air 7/0 Pocket occurs as a result of the rearrangement of equiaxed crystals into liquids, which form a plug that clogs the equiaxed zone. The pores formed in the equiaxial zone are formed by combinations of channels and cavities that do not contain gas, the maximum size of which in the direction of the thickness of the sheet corresponds to the thickness of the equiaxial zone (ie, from 100 to 400 µm) and the length of which in other directions can reach 1-2 mm. As already indicated, we are not talking about a spherical shell, the cause of which could be the release of gas, or about some internal defect that formed on the 75 surface of the strip.

The purpose of the invention is to create such conditions in the process of hot rolling of the hardened strip, which would not only lead, as is already known, to the closing of the central pores, but also to the actual welding of the opposite walls of the pores, which are brought together due to rolling. Thanks to this, there is a guarantee that the pores will not open during further strip forming operations or when using the 2o products produced in this way. During hot rolling of the strip, two stages alternate. First of all, as the thickness of the strip decreases, the inner walls of the pores gradually come closer together until contact occurs between them. After this contact occurs, welding of the pore walls occurs by diffusion of the elements that make up the steel through the interface. But effective welding of the pore walls should already occur before the strip leaves the gap between the rolls of the rolling mill, because otherwise the pressure drop in the strip at the exit from the rolls leads to partial separation of the pore walls.

The effectiveness of pore welding significantly depends on two parameters: the duration of forced contact with the walls in the rolling state and the temperature at which this contact is made. This forced contact must be carried out as early as possible after the strip enters the rolling mill and, at a given rolling speed (which, in the case of rolling immediately after casting, is largely determined by the thickness of the strip before Its "-- rolling") depends mainly on the diameter of the working rolls rolling state and from the degree of reduction of its thickness when acting on a strip of rolls. The larger the diameter of the rolls and the greater the degree of compaction, the faster the forced contact between the walls of the pores occurs and the longer it turns out to be. However, one cannot limit oneself to Ge"! stating that for a satisfactory solution to the problem it is enough to roll the strip with the highest possible values of the degree of compression and with the largest diameter of the rolls. Indeed, a very high degree of compression, which exceeds the limits of the possibilities of hot deformation of the strip, leads to the appearance of surface cracks on the strip (Te), called "tears", which must be avoided absolutely. .3 On the other hand, the temperature at which the forced contact of the pore walls is carried out depends not only on the temperature at the entrance of the strip to the rolling mill, but also on the duration of contact of the strip with the rolls, since this contact causes "cooling of the strip. If at a certain temperature at the entrance of the strip, the rolls have a very large diameter, their cooling of the strip can reduce its temperature to such an extent that it will be insufficient for the completion of welding of the pore walls. Thus, the value of the strip temperature at the exit from the rolls is a good indicator of the real possibility that mutual welding of the pore walls took place in the gap between the rolls. "» The temperature of the strip at the exit from the rolls should thus be sufficient to carry out pore welding, but it should not be too high, as it is necessary to avoid excessive thermal effects on the rolls.

The latter would lead to damage to their surface, which, in turn, would lead to excessive roughness

Ge") of the appearance of the strip surface. The purpose of this invention can thus be achieved without obtaining unpleasant secondary effects for the overall quality of the strip only with an adequate combination of the diameter of the rolls,

Mn measures of compression and temperature of the strip at the exit from the rolls. se In order to determine how these parameters can be combined, a series of tests was conducted, during which, for a certain type of stainless steel, the diameter of the working rolls of the rolling mill, the degree of reduction e of the strip thickness and the temperature of the strip at the exit from the rolling mill were varied. The rolling mill was located on the same line as the casting device. Each test gave a characteristic that allows you to determine whether the pore welding was effective or not. This characteristic consisted in the fracture of the elongated sample and the study of the fracture surfaces. If these surfaces have pores that have opened during the pull test, it is concluded that the weld was not satisfactory. If there are no obvious pores on these surfaces, the welding is considered satisfactory. o Table 1 shows the compositions of the steels on which the tests were carried out, the results of which are given in table 2. The content of various elements is given in weight percentages. Table 1 also shows the thickness of the tested strips at the exit from the casting rolls, as well as the corresponding casting speeds measured between the casting rolls and the hot rolling condition. 00 castings | poured out | poured out | Vylyvkis vv

MP 1.5 1.5 OA 0.2

"77777777 yam | zm | zm | oo mov 1818 o

The compositions of castings of types A and A" correspond to the compositions of existing austenitic stainless steels of type AIZI 304. 75 Type B castings correspond to ferritic stainless steels of type A!5I 430. Castings of type C correspond to ferritic stainless steels of type AII5I 409, stabilized by titanium.

Table 2 shows the results of tests carried out on strips made from these castings, as well as the corresponding test conditions. according to the invention of the casting | drawn state (mm) pressing (25) | of rolling stock (C) pores cm - - y y o - roughness zo «

F and the roughness of the link under F

A 01100611 mon rel

А 0111050011000000ю00001н00 vd

Ск дав 009ю0001005000100000008ю00111000 тако відо « лак ДАВ вою 17161771 й0о1111111171тає 0 відсутні "др в с шорсткість су яю 01160111 вдут ' Ск фан 009ю0001000ю00100000008ю000000000тяако0 відо ково 09ю000100в6001000000о110010 тако відоутміо и ин шли ЛИ ШИ ВИК (о) шорсткість с А 01101111 недвяя се)

From the conducted tests, it follows that effective welding of yor occurs without the appearance of tears and excessive roughness on the surface of the strip in the insert, when the following three conditions are met: - the diameter of the working rolls of the rolling mill is from 400 to 900 mm; the degree of reduction in strip thickness during rolling is from 15 to 5090; the temperature of the strip at the exit from the rolling mill is at least 800" and does not exceed 11002C.

At the same time, under the test conditions, no effect of the combination of strip thickness and pouring speed was noted: the results of type A spills are identical to the results of type A spills, after all, under the same pouring conditions. o Tests, as mentioned above , were carried out on a hot-rolling stand located i.e.) immediately after the filling device and before the device for winding the strip. From the point of view of the invention, such a condition is not necessary and hot rolling can be carried out on a device separated from the filling device and the winding device, i.e. hot rolling can be done after unwinding and reheating the raw cast strip.However, rolling immediately after pouring is recommended for various reasons.Primarily, this solution has an economic advantage due to the continuous nature of the operations.

First of all, the strip manufacturing process is shortened. In addition, there is a saving in one winder and also in a reheater of comparatively high power, 65 because the cast strip can be hot enough to reach the necessary rolling temperatures, possibly by means of a hood, which arrests the radiation of the strip between exiting the castings. rolls and entering the rolling mill. If, however, reheating of the rolls proves necessary, it can be accomplished by means of an induction furnace with a reduced power sufficient to raise the temperature of the moving strip by several hundred degrees. On the other hand, rolling immediately after pouring, eliminating the need to wind the raw cast strip, simultaneously eliminates the risk of damage to the mud in the process of this winding, which could occur in the case of a relatively thick strip containing a non-recrystallized structure.

Finally, the elimination of reheating the strip from room temperature to hot rolling temperature also eliminates oxidation processes on the surface of the strip, typical of this operation. These processes would lead to the formation of scale, which could penetrate into the strip and rolls of the rolling mill and at the same time spoil the appearance of the surface of the Product after etching.

The invention applies not only to inter-roll casting devices, but also to any other device for casting thin strips of stainless steel between two cooling and moving surfaces, such as drawn slurries.

Claims (2)

19 Formula of the invention 1. A method of producing a thin strip of stainless steel by directly solidifying liquid steel into a strip with a thickness of less than or equal to 8 mm in a moving and cooling double-wall pouring device and hot rolling the strip, the solidification of which is largely completed after of how the strip leaves the above-mentioned walls, which is distinguished by the fact that hot rolling is carried out on a rolling mill, the working rolls of which have a diameter of 400 to 900 mm, the temperature of the strip at the exit from the rolling mill is from 800 to 1100 °C, and also by the fact that that the degree of strip thickness reduction during hot rolling is from 15 to 50905. 2. The method according to claim 1, which differs in that hot rolling is carried out in a device located directly after the filling device. at Q. The method according to claim 1 or 2, which differs in that the walls of the filling device, which are cooled, consist of the surfaces of two rolls rotating around horizontal axes. . Kk K-4. . . new Official Bulletin "Industrial Property". Book 1 "Inventions, useful models, topographies of integrated microcircuits", 2004, M 2, 15.02.2004. State Department of Intellectual Property of the Ministry of Education and Science of Ukraine. about (ze) (Se) - and? (o) (95) se) sh» - ime) 60 b5