SK123999A3 - Manufacturing method for stainless steel thin strip - Google Patents

Abstract

Manufacturing a thin strip of stainless steel comprises direct solidification of liquid steel in a casting installation comprising two cooled moving walls; solidification is finished when the strip leaves the walls; and hot laminating this strip using a rolling mill with cylinders having a diameter of 400-900 mm, the strip temperature after leaving the rolling mill is 800-1100 degrees C; the rate of reduction of the strip thickness during hot lamination is 15-50 %.

Description

The invention relates to a method of making a thin strip of stainless steel directly from molten metal by solidification inside a mold comprising two cooled walls moving at the same speed as a solidifying steel strip, such as the outer walls of two rollers rotating about horizontal axes.

BACKGROUND OF THE INVENTION

In this casting process, known in the industry as "inter-roll casting", one of the major problems with the quality of the belt is the possible presence of pores within the belt. When these pores occur in products produced by post-treatment of the strip (such as surface cleaning, pickling, annealing, cold rolling and other processing processes), they limit the field of applicability of the products by changing the mechanical properties that they produce.

The causes of the occurrence of these pores within the casting strips between the rolls may be similar to those causing (on a larger scale) the formation of lunkers (clots and cavities in ingots) and internal center pores in products produced by conventional continuous casting, formed by closing the casings still containing liquid metal, which is surrounded by already solidified metal, because the solidification of the metal does not proceed quite regularly. The cooling and solidification of the liquid metal containing these pockets is accompanied by shrinkage of the metal, resulting in cavities. These can no longer be filled with metal before the solidification has ended because the closed cavities are no longer accessible to the liquid metal. This porosity has to be distinguished from spherical errors - “bubbles”, which are caused by dissolved gas leakage and which are most often produced under the surface of products.

EP 0 396 862 proposes a process for eliminating internal center porosity as well as other internal and surface defects during casting of steel strips between two rolls. According to this process, the casting rollers have on their surface precisely dimensioned circumferential grooves formed offset on both rollers. This is to prevent abrasion of the solidified metal bark on the surface of the rollers, which would include irregularities resulting from the belt solidification.

However, it seems that the abutment barrier alone is not sufficient to completely prevent the inner center pores from being exposed.

Japanese Patent JP 8252653 proposes a method in which, immediately after casting, the strip is hot rolled under conditions given by this inequality:

r> (2,74 x 10 ' ⁵ T ² - 6,88 x 10' ² T + 43,55) (to / w ₀ ), where r stands for strip thickness reduction by hot rolling T stands for hot rolling temperature in ° C means the porosity diameter in the strip thickness direction w ₀ means the porosity diameter in the strip width direction.

According to this process, hot rolling must be performed with a strip thickness reduction sufficient to close the pores by this rolling, and this minimum stripping depends on the rolling temperature (i.e., the temperature the strip has when it enters the roll engagement) ) and the shape and orientation of the pores. However, it has been noted that the rolling conditions are not yet sufficient to ensure that all pores are securely closed, in particular because they do not always prevent the re-closed pores from being re-opened when using the belt or when using products made from it. cause damage or breakage.

The purpose of the invention is to propose a method ensuring the final closure of the central pores formed within the strip after it has completely solidified.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a thin stainless steel strip by direct solidification of liquid steel into a strip having a thickness of not more than 8 mm in a casting machine comprising two moving cooled walls and hot rolling the strip, characterized in that hot rolling is performed on a rolling mill whose working rolls have a diameter in the range of 400 to 900 mm, the temperature of the strip at its exit from the rolling mill is in the range of 800 to 1100 ° C and the strip thickness reduction during hot rolling is in the range of 15 to 35 %.

Preferably, the hot rolling is carried out on a downstream apparatus. The casting device may be of the "roll-to-roll" type.

As mentioned above, the purpose of the invention is achieved by combining the requirements regarding the diameter of the hot rolling mill working rolls, the strip temperature at the exit of the rolls, and the strip thickness reduction during hot rolling.

The invention is suitable for use in the casting of stainless steel of all kinds having a carbon content of not more than 1%, a silicon content of not more than 1%, a manganese content of not more than 15%, a chromium content of 10 to 30%, a copper content of 5% or more , 5% (all values are expressed in weight percent). These steels may also contain considerable amounts of nickel (up to 40%) or molybdenum (up to 8%). They also contain other elements, either as impurities or as alloying additives, in particular sulfur, phosphorus, titanium, niobium, zirconium. Their total content should not exceed 2% by weight.

As mentioned above, the thin strip of stainless steel cast between the rollers is very susceptible to the formation of pores therein during solidification when the internally formed pocket containing the liquid metal is closed again by the surrounding solidified metal. This phenomenon occurs at the end of the solidification of the pasty zone, also called the "equiaxed zone", located between two crusts stiffened by contact with the rollers, also called the "support zones". The equiaxial zone is very difficult to influence and its thickness may vary depending on the solidification rate of the support zones. Thus, an equiaxial zone may be prematurely closed at locations where the formation of the support zones was faster than usual. As soon as the equiaxial zone is closed, the pockets can no longer be refilled with liquid metal and pores form in these pockets as the metal solidifies. However, this is a rare case and, in fact, the liquid metal pocket is usually recovered by rearranging the equiaxed crystals in the melt, forming a plug clogging the equiaxed zone. The pores formed in the even zone are made up of a plurality of gas-free channels and cavities whose maximum dimension in the sheet thickness direction corresponds to the thickness of the uniform zone (about 100 to 400 µm) and whose length in other directions may be 1 to 2 mm. As already mentioned, it is not a spherical bubble that would be caused by a gas leak nor an internal error opening on the surface of the belt.

The object of the present invention was to create hot rolled conditions such as not only to close the central pores, as already known, but also to actually seal the walls opposite the pores to which the rolling was allowed to approach. This ensures that there is no risk of pore opening when the belt is later processed or when articles made from it are used. The strip is hot rolled in two successive stages. First, the inner walls of the caries approach each other gradually as the thickness of the strip decreases until they touch them. Then, as they hardly touch each other, the erection of these walls occurs by diffusion of the steel-forming elements across the phase interface. However, an impressive erection of the walls should be achieved before the belt comes out of the roll mill rolls, otherwise the release of the belt compression that occurs when the belt exits the rollers would result in a partial elimination of the walls.

The effectiveness of wall building depends mainly on two parameters: the length of time the wall touches in the mill stand and the temperature at which this touch is made. It should therefore preferably occur after the entry of the strip between the rolls of the rolling mill and its duration essentially depends, for a given rolling speed (which in the case of rolling immediately after casting largely depends on the strip thickness before rolling), and the size of the thickness reduction that occurs between the rollers. The larger the diameter of the rollers and the thickness reduction, the faster the contact between the pore walls and the longer the contact lasts. However, it cannot be satisfied that a belt with the greatest thickness reduction and the largest diameter of the working rolls is sufficient to solve the problem in a satisfactory manner. In fact, too much removal to overcome the hot ductility of the belt leads to surface cracks on the belt, called "cracks", which must be avoided. On the other hand, the temperature at which the pore wall is applied depends not only on the temperature of the belt at which the pore wall is applied, depends not only on the temperature of the belt entering between the rollers, but also on the length of time it touches the rollers because this contact causes the belt to cool. If the rolls have too large a diameter at a given temperature with which the strip enters between the rolls, there is a danger that the cooling of the strip caused by them will result in its temperature no longer sufficient to perfectly build the pore walls. Therefore, the temperature of the strip at its exit from the rollers provides a good indication of the real reality of the pore walls to align with each other during the stay of the strip between the rollers.

The temperature of the strip at the outlet of the rollers should therefore be sufficiently high to allow pore build-up, but not too high to avoid excessive thermal loading of the rollers. This would lead to degradation of their surface causing a deterioration in the appearance of the strips, resulting in excessive roughness. Therefore, the purpose of the invention without accompanying disturbances can only be achieved if the diameter of the working rolls is appropriately combined with the strip thickness removal rate and the strip temperature at the exit of the mill stand.

DETAILED DESCRIPTION OF THE INVENTION

In order to determine how these parameters are to be combined, a series of experiments have been carried out in which the diameters of the roll mill working rolls, the strip thickness reduction and the strip temperature at the mill stand were varied using a given type of stainless steel. The rolling mill was adjusted in one line just behind the casting machine. Each of these experiments was accompanied by a characterization allowing to determine whether or not pore building had occurred. This characterization was carried out by breaking the tensile test rod and inspecting the fracture surfaces. If there were pores on these surfaces that opened during the tensile test, the pore wall construction was not sufficient. If no obvious pores appeared on the quarry surfaces, the construction was considered sufficient.

Table 1 shows the composition of the steels with which the above tests were carried out, the results of which are shown in Table 2. The content of the individual elements is given in percent by weight. Table 1 also shows the strip thickness at the exit of the casting rolls, and the stripes were tested and the casting speed measured between the casting rolls and the hot-roll rolls.

element melting tavby A ' tavby B melts C C 0.05 0.05 0.04 0.01 Mn 1.5 1.5 0.4 0.2 P <0,04 <0,04 <0,04 <0,03 WITH <0.01 <0.01 <0.01 <0.01 Are you 0.3 0.3 0.3 0.5 Ni 8.6 8.6 <0.3 <0.1 Cr 18 18 16.5 11.5 Cu <0.5 <0.5 <0.2 <0.2 Mo <0.5 <0.5 <0.1 <0.1 you <0.01 <0.01 <0.01 0.15 N 0.05 0.05 0.04 0.01 thickness waist 4 mm 2 mm 3 mm 3 mm speed casting 25 m / mn 100 m / mn 60 m / mn 60 m / mn

Table 1: Composition of steels from test melts, strip thickness and casting speed

The composition of type A and A 'melts corresponds to the composition of classic AISI 304 stainless steels. The composition of type B melts corresponds to the composition of AISI 430 ferritic stainless steels.

Table 2 shows the results of tests performed on strips made from the mentioned melts, together with the relevant test conditions.

Within invention Type heats average Cylinder rolling stool (Mm) Uber thickness waist (%) Strip temperature at outlet of cylinders rolling stool (° C) STA tion pore other mistakes not A 300 50 1100 not no not A 400 10 1100 not no not A 400 15 750 not no Yes A, A ', B, C 400 15 800 Yes no Yes A, A ', B, C 400 15 1100 Yes no not A 400 15 1150 Yes great roughness not A 400 50 750 not no Yes A, A ', B, C 400 50 800 Yes no Yes A, A ', B, C 400 50 1100 Yes no not A 400 50 1150 Yes great roughness not A 400 60 1100 Yes cracks not A 900 10 1100 not no not A 900 15 750 not no Yes A, A ', B, C 900 15 800 Yes no Yes A, A ', B, C 900 15 1100 Yes no not A 900 15 1150 Yes great roughness not A 900 50 750 not no Yes A, A ', B, C 900 50 800 Yes no Yes A, A ', B, C 900 50 1100 Yes no not A 900 50 1150 Yes great roughness not A 900 60 1100 Yes cracks not A 100 50 750 not no

Table 2: Results of tests performed with strips made from type A, A ', B, C heats

The results of the tests show that effective pore building is achieved without cracks and excessive roughness of the belt surface if the following three conditions are also met:

- the diameter of the working rolls of the rolling mill between 400 and 900 mm

- strip thickness reduction during rolling between 15 to 50%

- a strip temperature at the outlet of the rolling mill of at least 800 and not more than 1100 ° C.

On the other hand, no effect of the combination of strip thickness and casting speed was observed under test conditions: the results obtained with strips made of steel prepared by type A 'melt are identical to those obtained with strips made from steel prepared by type A melt with otherwise identical parameters.

As already mentioned, a hot rolling mill was used for the above tests, ranked downstream of the casting machine but before the winding machine. Regarding the objective pursued by the invention, this arrangement is not necessary and hot rolling can be carried out on a device separate from the casting and winding equipment, but after unwinding and reheating the raw cast strip. However, for various reasons, it is recommended to perform hot rolling in a row downstream of the casting machine. In particular, this solution is economically advantageous given the coherent nature of these operations. Firstly, the strip manufacturing process will be shortened, and secondly, the use of a winding device as well as a strip reheating device with relatively high throughput will be saved, since the cast strip can be warm enough to have a suitable rolling temperature, optionally by heat shroud a strip between its exit from the rollers and its entry between the rollers. If re-heating of the strip nevertheless proves necessary, it can be carried out in an induction furnace of lower power, sufficient to raise the temperature of the strip in the range of several hundred degrees. At the same time, rolling directly after casting, by not requiring the winding of the raw strip coming from the casting apparatus, also avoids the risk of damage to the strip during winding, which would occur with a relatively coarse belt with still recovered crystals. Finally, the fact that it is not necessary to heat the strip from ambient temperature to the hot rolling temperature prevents reoxidation of the strip surface, which usually occurs in this operation. This re-oxidation would produce scales which could be formed on the belt as well as on the rolls of the mill stand and thereby lead to a deterioration in the appearance of the product surface after pickling.

Industrial usability

The invention can be applied not only to the roll casting apparatus, but also to all other types of stainless steel strip casting apparatus between two cooled moving surfaces, such as the conveyor belts.

Claims (3)

PATENT CLAIMS 1. A method of making a thin stainless steel strip by solidifying directly the liquid steel into a strip having a thickness of not more than 8 mm in a casting machine comprising two moving cooled walls and hot-rolling said strip, characterized in that hot rolling is performed on a rolling mill. whose working rolls have a diameter in the range of 400 to 900 mm, the temperature of the strip at its exit from the mill stand is in the range of 800 to 1100 ° C, and the strip thickness reduction during hot rolling is in the range of 15 to 35%. Method according to claim 1, characterized in that the hot-rolling is carried out on a device located downstream of the casting device. Method according to claim 1 or 2, characterized in that the cooled walls of the casting machine are formed by the surfaces of two rollers rotating about horizontal axes.