MXPA01010505A - Method for continuously casting ferritic stainless steel strips free of microcracks - Google Patents

Abstract

The invention concerns a method for continuously casting a ferritic stainless steel strip with thickness not more than 10 mm directly from liquid metal between two cooled rolls with horizontal axes and driven in rotation, characterised in that:the liquid metal composition in weight percentages is as follows:%C +%N=0.12;%Mn=1;%P=0.4;%Si=1;%Mo=2.5;%Cr between 11 and 19;Al=1%;%Ti +%Nb +%Zr=1;the rest being iron and the impurities resulting from preparation;the&ggr;p index of the liquid metal ranges between 35%and 60%,&ggr;¿p ?being defined by the formula:&ggr;p=420%C + 470%N + 23%Ni + 9%Cu + 7%Mn 11.5%Cr 11.5%Si 12%Mo 23%V 47%Nb 49%Ti 52%Al + 189:the surface roughness of said rolls being more than 5&mgr;m;in the proximity of the meniscus metal liquid present between the rolls an inerting gas is used consisting of at least 60%by volume of a gas soluble in steel.

Description

CONTINUOUS BONDING PROCEDURE BETWEEN FERRITIC STAINLESS STEEL BAND CYLINDERS, MICROGRAPHY EXEMPTIONS FIELD OF THE INVENTION The invention concerns the casting of metals, and more precisely to the continuous casting, directly from liquid metal, of stainless steel bands of the ferptic type, whose thickness is of the order of a few mm, by the process called "casting between cylinders". BACKGROUND OF THE INVENTION In the last few years, significant progress has been made in the development of the casting processes of thin bands of carbon steel or stainless steel directly from said liquid metal between two cylinders cooled internally, which revolve around their horizontal e in opposite directions, and arranged in front of each other, being the minimum distance between their surfaces, substantially equal to the thickness that is desired to confer to the cast strip (for example some mm). The pouring space containing the liquid steel is defined by the lateral surfaces of the cylinders, on which the solidification of the strip begins, and by refractory side closure plates applied against the ends of the cylinders. The liquid metal begins its solidification at the contact of the outer surfaces of REF 133561 the cylinders, on which solidified "skins" form, which are formed in such a way that they meet again at the level of the "neck", that is to say, of the zone in which the distance between the cylinders is minimum. One of the main problems encountered at the time of the manufacture of thin bands of ferritic stainless steel by casting between cylinders is the important risk of an appearance on the band of surface defects called micro-cracks. These are fissures of slight dimensions that are, however, sufficient to make them unsuitable for the use of cold-processed products that originate. The micro-cracks are formed during the solidification of the steel and have a depth of the order of 40 μm and an opening of approximately 20 μm. Its appearance is related to the contact conditions, at the time of solidification, between the steel and the surface of the cylinders over the length of its contact arc. These conditions can be described as consisting of two successive stages. The first stage concerns the initial contact between the liquid steel and the surface of the cylinder, which involves the formation of a solid steel skin on the surface of the cylinders. The second stage concerns the growth of this skin up to the neck, where as it has been said, it meets with the skin formed on the other cylinder to constitute the completely solidified band. The contact between the steel of the surface of the cylinder is conditioned by the topography of the surface of the casting cylinders, conjugated with the nature of the inertial gas and with the chemical composition of the steel. All these parameters intervene in the establishment of the thermal transfers between the steel and the cylinder and the solidification conditions of the skins govern. Various attempts have been made to adjust casting procedures between cylinders that allow reliably obtaining bands free of unavoidable surface defects, such as micro-cracks. The solutions evoked in the case of carbon steels are based on the need for a good command of the thermal exchanges between the steel and the surface of the cylinders. The aim is, in particular, to increase the thermal flow extracted from the steel, the beginning of its solidification, by the casting cylinders. With this objective, EP-A-0 732 163 proposes to use cylinders of very slight roughness (Ra- less than 5 μm), associating them to a steel composition and processing conditions that favor the formation, within the metal, of liquid oxides that wet the surface interfaces of the steel / cylinder. As far as austenitic stainless steels are concerned, document EP-A-0 796 685 teaches to cast a steel whose Creq / Nieq ratio is higher than 1.55 in order to minimize the changes of phase at high temperature, and to perform this casting using cylinders whose surface consists of dimples joined in diameter 100- 1500 μm and depth 20-150 μm and inserting the casting space with a gas soluble in steel, or a mixture of gas composed mostly of said soluble gas. For ferritic stainless steels, JP-A-5337612 proposes casting a steel with low carbon contents (less than 0.05%) and nitrogen (less than 0.05%) and containing niobium (0.1 to 5%) and titanium. It is also necessary to cool the band at the exit of the cylinders at a high speed, and then control the winding temperature of the band. These processing and casting conditions are costly and problematic, and the particular characteristics of the required steel classes limit the fields of use of the products thus obtained. The object of the invention is to propose a method of casting thin bands of ferritic stainless steel whose surface will be free of micro-cracks. This method does not require particularly heavy casting conditions for its use, and could be applied to a wide range of grades of such steels. For this purpose, the invention has as its object a continuous casting process of a ferritic stainless steel strip with a thickness less than or equal to 10 mm directly from liquid metal between two cylinders with horizontal axes cooled and put in rotation, characterized in that: - the liquid metal has the composition in% by weight% of C +% of N <; 0.12,% of Mn = 1,% of P < 0.04, I of Si < 1,% Mo < 2.5,% Cr between 11 and 19% of Al < 1,% of Ti +% of Nb +% of Zr = 1, the rest being iron and impurities resulting from processing; - the index? p of the liquid metal is between 35% and 60%, being defined? p by the formula:? P = 420% C + 470% N + 23% Ni + 9% Cu + 7% Mn - 11.5 % Cr - 11.5% Si - 12% Mo -23% V - 47% Nb - 49% Ti - 52% Al + 189; the roughness Ra, of the surfaces of said cylinders is greater than 5 μm; a gas of inertia composed of at least 60% by volume of a gas soluble in steel is used in the vicinity of the meniscus of the liquid metal present between the cylinders. As will be understood, the invention consists in combining conditions on the composition of the metal, which govern the possibilities of forming austenite at high temperature, after the solidification of the metal, a condition on the minimum roughness of the casting surfaces and a condition on the composition of inertial gas. Respecting this combination, it is possible to avoid the formation of micro-cracks in the surface of the band without therefore having to impose too problematic limitations on the casting process and without restricting too broadly the fields of use of the products that will be manufactured from the strained bands. The invention will be better understood with the reading of the detailed description that follows: DETAILED DESCRIPTION OF THE INVENTION One of the essential parameters of the success of a casting of thin bands between cylinders is the domain of the thermal exchanges between the band in the course of solidification and the cylinders. A good mastery of these transfers requires that the adhesion conditions of the solidified skins on the walls of the cylinders are known and reproducible. Or, at the time of the casting of the ferritic stainless steel bands containing 11 to 19% chromium, the following phenomenon occurs after the complete solidification of the skin against the cylinder. The solidified skin first has a completely ferritic structure (phase d), then in the course of cooling, when it always adheres to the surface of the cylinder, undergoes a ferritic d-austenite phase transformation? in a temperature range of 1300-1400 ° C. This phase transformation causes local contractions of the metal, which result from the density differences between these two phases that are sensitive to the microscopic level. These contractions can be sufficiently important to drag local contact parts between the solidified skin and the surface of the cylinder. As it is understood, these contact losses radically modify the local conditions of the thermal transfers. Together with the state of the surface of the cylinders and with the nature of the inertial gas, present in the depressions of said surface, the amplitude of this phase transformation, together with the composition of the metal, then influences the intensity of the thermal transfers . The amplitude of the phase transformation d - > ? in ferritic stainless steels it can be described by the index of? p. This represents the maximum amount of austenite present in the metal at high temperature. This index? P is calculated, in a known way, from the composition of the metal, according to the relationship called "Tricot and Castro" (the percentages are weighted percentages):? P = 420% C + 470% N + 23% Ni + 9% Cu + 7% Mn -11.5% Cr - 11.5% Si - 12% Mo - 23% V - 47% Nb - 49% Ti - 52% Al - 189 When, in the case of studies that have led to the invention, they realized that the value of? P constituted a good indicator of the level of thermal flow extracted by the casting cylinders at the time of solidification, all things being equal on the other hand. The thermal flow extracted from the metal by the cylinders can be quantified experimentally by a mean value, calculated from a measurement of the heating of the cooling fluid of the cylinders. Experience shows that the average heat flux extracted from the metal by the cylinders is as much lower as the index p has a high value. A necessary condition to avoid the appearance of cracks on thin bands of ferritic stainless steel cast between cylinders is that, at the moment of the initial contact between the liquid metal and the cylinders, the thermal flow extracted is high. For this purpose, it is preferable that the inertial gas that sets the surface of the liquid metal in the vicinity of the meniscus (name given to the intersection between the surfaces of the liquid metal and the cylinders) contains a gas soluble in the steel, or is completely constituted by said gas. Nitrogen is conventionally used for this purpose, but the use of hydrogen, ammonia or C02 would also be contemplated. As an insoluble gas that ensures the eventual complement to 100% of the atmosphere of inertia, argon is conventionally used, but the use of another insoluble gas, such as helium, would also be contemplated. With a gas mostly soluble in steel, a better contact between the steel and the cylinders is realized, since an insoluble gas moderates more than a soluble gas the penetration of the metal in the depressions of the surface of the cylinder. Also, a slight roughness of the surface of the cylinders provides a high thermal flow as it results in a close contact between the cylinder and the metal.

However, after the initiation of solidification, a very high average heat flux increases the risks of heterogeneity between the local values of this flow. Or, these heterogeneities can be the origin of superficial cracks on the band, since they provoke tensions between areas of the surface, which is still fragile. It would be possible, then, a compromise to find among the different imperatives to respect the casting conditions, if one wants to avoid the formation of micro-cracks at the moment of all the stages of solidification and cooling of the skins against the cylinders. . For this purpose, different casting conditions of ferritic stainless steel bands from liquid metal have been experimented. The experiments have taken place by casting bands of 2.9 to 3.4 mm thick between cylinders whose outer surfaces cooled by internal circulation of water were copper and nickel coated. Table 1, below, shows the cast metal compositions at the time of the different tests (designated from A to F), and the corresponding p values of the Index, and table 2, presents the results obtained at the time of the different tests. tests, in terms of surface quality obtained, depending on the composition of the steel, the composition of the inertial gas and the roughness of the cylinders. This last parameter is represented by the average roughness Ra, defined according to the ISO 4287-1997 standard by the arithmetic mean of the variations of the roughness profile on the middle line within the measurement path Im. The middle line is defined as being the line, produced by filtering, that cuts the profile probed in such a way that the surfaces that are superior to it are equal to those that are inferior to it. According to this definition: Table 1: Compositions of cast steels at the time of the tests Table 2: Influence of casting parameters on the presence of micro-cracks For steels A, B and F, micro-cracks are absent when the nitrogen content of the inertial gas (which is a mixture of nitrogen and argon) It is at least 60%. All these steels have an index p of 45.7 to 53.4%, and have been cast with cylinders having a Ra of 7 or 11 μm. The experience conducted on steel C, shows that, also with a Ra of 8.5 μm and a gas of inertia rich in nitrogen, micro-cracks are systematically obtained when casting a steel whose index? P is low (29.5%). The experience conducted on the steel D, whose index? P is 62%, shows that conversely, microcracks are also obtained when the cast steel has a very high? P Index. The experience conducted on the steel E, shows that also when the conditions of composition and inertia are convenient in view of the preceding tests, a slight roughness of the cylinders (Ra of 4 μm) leads to the appearance of micro-cracks. These different results are explained in the following way. In order to obtain a strip free of cracks, it is first necessary that the thermal flow extracted at the time of the first contact between the metal and the cylinder be high. If the inertial gas is not sufficiently soluble in the steel, the average heat flow extracted is too slight, the steel does not solidify quite homogeneously and this favors the appearance of micro-cracks. From this point of view, it would be a priori, equally desirable to have a slight cylinder roughness. But if the roughness Ra is too light, the number and the total surface of the initiation sites of the solidification becomes very high, which leads to a too brutal cooling that causes the appearance of micro-cracks. In addition, it is also necessary to consider conditions required by the following stages of the solidification and cooling processes of the skins. Experience shows that combining a soluble gas content of at least 60% in the inertial gas and a roughness of the Ra cylinders above 5 μm, satisfactory results are obtained. In the continuation of the process of solidification and cooling of the skins against the cylinders, it is necessary, as has been said, to avoid having a flow of extraction too intense, in order to avoid thermal heterogeneities, which are also sources of micro-cracks . From this point of view, the minimum rugosity Ra of 5 μm is justified because the roughness peaks serve as sites of initiation and development of the solidification, and the parts in depression, in which the metal penetrates without necessarily going to the bottom of depression, act as contraction joints, which absorb the volume variations of the skin at the time of its solidification and its cooling. However, it is not advisable to have a roughness Ra, greater than 20 μmm, since otherwise the roughness printed "in negative" on the surface of the band is high, and will be difficult to reduce at the time of the stages. subsequent lamination and cold processing. You risk it, then, to find a final product whose surface appearance would not be satisfactory. The desired cylinder roughness can be obtained by any means known for this purpose, such as blasting, laser machining, gravure operation, EDM, etc. A strong value of the Index? P imposed by the composition of the metal, amplifies the transformation d - »? over the contact arc assembly. The solidified skins are then subjected, on said contact arc, to detachments that moderate the extracted thermal flow and maintain it at a convenient level, without therefore leading to micro-cracks that would be due to the fragility of the skin, when this is already sufficiently solidified. Experience shows that the lower limit to be set for the Index? P is 35%. Beyond an Index? P of 60%, the detachments caused by the transformation d - •? they become too important, and lead to the birth of micro-cracks by excessive fragilization of the skins. The invention thus makes a compromise between sometimes contradictory demands, dictated by the need to avoid the presence on the casting band of superficial micro-cracks, whose framing mechanisms are multiple. It allows to dispense with the mandatory presence of expensive alloying elements (stabilizing elements such as aluminum, titanium, zirconium, niobium can be present optionally). Likewise, it does not need special cooling and winding conditions of the band, after it leaves the cylinders. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (3)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property: 1) Continuous casting process of a ferritic stainless steel strip, of a thickness less than or equal to 10 mm directly from liquid metal between two cylinders of horizontal axes cooled and put into rotation, characterized in that: - the liquid metal has the composition in weighted percentages% of C +% of N < 0.12,% of Mn = 1,% of P < 0.04, from Si < 1,% Mo < 2.5,% Cr between 11 and 19, Al < 1%,% Ti +% Nb +% Zr < 1, the rest being iron and impurities resulting from the processing; - the Index? p of the liquid metal is between 35% and 60%, being? P defined by the formula:? P = 420% of C + 470% of N + 23% of. Ni + 9% Cu + 7% Mn - 11.5% Cr - 11.5% Si - 12% Mo - 23% V - 47% Nb - 49% Ti - 52% Al + 189; - the roughness Ra of the surfaces of said cylinders is greater than 5 μm; an inertial gas composed of at least 60% by volume of a gas soluble in the steel is used in the vicinity of the meniscus of the liquid metal present between the cylinders.
2. 2) Process according to claim 1 or 2, characterized in that the inertial gas is a mixture of nitrogen and argon, in respective proportions of 60-100% and 0-3%.
3. 3) Method according to any of claims 1 to 3, characterized in that the roughness Ra of the surfaces of the cylinders is between 5 and 20 μm.